Illinois Geothermal Coalition

Executive Summary

Energy demand has been increasing worldwide over the past decades, exacerbated by industrial and economic development, and is expected to grow by approximately 50% between 2018 and 2050. A transition from fossil fuels is needed to decarbonize energy systems, incentivize energy efficiency and conservation, promote economic growth in a green economy, and protect the environment. State and federal policy makers in the U.S. Midwest, and particularly in Illinois, are committed to implementing operational and technological solutions pursuant to these goals. In this White Paper, we examine possible technological solutions, with a focus on building heating and cooling, and water heating systems for the residential, commercial, and educational sectors. We also examine applications appropriate for the industrial, agricultural, and aquaculture sectors and propose advanced technologies suitable for the U.S. Midwest, such as thermal energy storage and hybrid energy systems.

To achieve the proposed zero-carbon emission goals set forth by U.S. Midwest governments to move beyond economies reliant on fossil fuels, we propose geothermal (geoexchange) energy as a solution that is renewable, low carbon, relatively inexpensive, reliable, and safe. Here we present a comprehensive review of geothermal applications for different economic sectors. Over time, the efficiency of geothermal systems has been improved and enhanced, making them stronger competitors in the renewable energy marketplace. Within a policy framework that would incentivize the deployment of geothermal energy technologies that meet the proposed targets, the transition to a decarbonized economy would be faster. The wider deployment of geothermal energy systems would have positive impacts on the U.S. Midwest, including in terms of environmental awareness, economic development, energy efficiency, additional basic and applied research support, and the repurposing of available skills from the oil and gas industry.

1. Introduction

Energy consumption will increase in the United States over the next 30 years across a variety of economic scenarios as population and economic growth outpace energy efficiency gains, according to the Energy Information Administration (EIA) Annual Energy Outlook 2022 (EIA, 2022a) Currently, roughly 79% of the United States’ energy needs are met by fossil energy resources, according to a 2022 EIA report (EIA, 2022b). Of this percentage, 16% is for the residential sector and 12% for the commercial sector. In 2020, 55.9% of the energy used was for space conditioning (heating and cooling) and making hot water (Figure 1.1).

According to the U.S. Environmental Protection Agency (U.S. EPA), the use of geothermal heat pumps (ground source heat pumps) could reduce residential energy consumption—and the corresponding greenhouse gas emissions—by 72% compared with the standard electric resistance heating and standard air-conditioning equipment, and by up to 44% compared with air-source heat pumps (U.S. EPA, 2021). Furthermore, geothermal heat pumps use 25%–50% less electricity than conventional heating or cooling systems. Altogether, the reduction in energy consumption would result in an 80% reduction in energy costs for homeowners.

The use of fossil fuel resources has been shown to have detrimental impacts on the environment because of their high greenhouse gas emissions (Landrigan et al., 2018), chiefly carbon dioxide (CO2) and methane (CH4), which are the main contributors to climate change (Ehhalt et al., 2001). Three major endeavors are needed in response to these climate change impacts: prediction, adaptation, and mitigation. Regarding the first aspect, prediction, the main global climatic concern at present is the projected temperature increase of 2.9°F–4.3°F (1.6°C–2.4°C) by 2060 if no preventive measures are taken, according to a working group of the Intergovernmental Panel on Climate Change (IPCC; Masson-Delmotte et al., 2021), which will profoundly affect societies’ capacity for coping and adaptation, hence increasing their level of vulnerability. To address mitigation, it is vital to decarbonize the energy sector and reduce the dependence on fossil fuels (Miglani et al., 2018).

Geothermal energy systems tap the inexhaustible resource of thermal energy in the Earth underground, providing a reliable and flexible source of thermal energy that can be utilized in the residential, commercial, agricultural, and industrial sectors (Bundschuh et al., 2017; Kurnia et al., 2022; Spitler et al., 2020; Zhou et al., 2020). Geothermal energy offers an affordable and efficient heating and cooling option (Koon et al., 2020; Lin et al., 2020a; Soltani et al., 2019; Van Horn et al., 2020; Van Nguyen et al., 2015; Yang et al., 2016). The U.S. Department of Energy’s GeoVision analysis (U.S. DOE, 2019a) illustrates that with technological improvements nationally by 2050, geothermal energy systems could increase power generation by 26 times from today (~60 GWe [gigawatt electrical]) and that the number of geothermal heating and cooling installations could grow by 14 times, servicing another 73 million homes.

Figure 1.1. Residential site energy consumption by end use (U.S. Energy Information Administration, 2021). Figure courtesy of the U.S. Department of Energy.
1.1 Motivation for Developing the White Paper

In the U.S. Midwest, climate change has significantly impacted our livelihoods. According to the U.S. Environmental Protection Agency (U.S. EPA, 2017), annual average temperatures are projected to increase over the next few decades, and the region is expected to experience warmer but wetter winters, hotter summers with longer dry periods, and springs with heavy precipitation. The Environmental Defense Fund (EDF, 2014) has suggested these periods of extreme heat and flooding are affecting the infrastructure, human health, agriculture, transportation, and air and water quality. Health-related impacts are becoming more frequent, and it is estimated that more than 20 million people in the region live where the air quality does not meet the national standard.

In the Midwest, many environmental policies take the form of regulations that are focused on decarbonizing the electrical grid to reduce associated greenhouse gas emissions. Illinois is emerging as a global leader in addressing climate change. Specifically, on January 23, 2019, Governor Pritzker signed Executive Order 2019-06 entering the State of Illinois into the U.S. Climate Alliance (State of Illinois, 2019). Governor Pritzker also signed into law the Climate and Equitable Jobs Act (S.B. 2408) on September 15, 2021 (State of Illinois, 2021), which puts Illinois on a path toward 100% decarbonization of the electrical grid by 2045, invests in training a diverse, equitable clean energy workforce, and expands the State’s commitments for energy efficiency, renewable energy, and electric vehicles.

Geothermal energy is one of the renewable energy sources that could be utilized to achieve the regulatory goal set forth by Illinois to becoming carbon neutral and meet the signed mandates. Unlike solar and wind electricity generation, which are impacted by atmospheric conditions, geothermal energy systems can provide a constant source of heat and cooling 24 hours a day, 365 days a year. Renewable geothermal energy is clean and reliable, and it provides a level of security and resiliency to energy systems.

Developments in geothermal systems have motivated educational campuses in the region (e.g., Ball State University) to meet climate action objectives and greenhouse gas emission targets. Here at the University of Illinois at Urbana-Champaign, the installation of geothermal energy systems is dove-tailed with research studies on them to better understand the impacts of subsurface thermal transport and hydrogeology, which has led to better and more cost-effective designs.

Increasing the deployment of geothermal energy systems will also lead to economic development by creating opportunities for new enterprises to form and expand. New businesses will emerge, such as in geothermal and sustainable energy system design, equipment manufacturing, and distribution and installation, which will benefit the community by creating additional job markets.

Furthermore, in Illinois, a specialized workforce for oil and gas exploration and drilling can be retrained and reskilled for geothermal energy projects. The skills and equipment used for oil and gas operations overlap those needed for geothermal energy, The energy transition will open new opportunities for the workers so their skills can be leveraged and used effectively.

1.2 Illinois Geothermal Coalition

The Illinois Geothermal Coalition (IGC) was founded in 2020 at the University of Illinois, Urbana-Champaign campus and represents a group of research scientists and engineers, industry professionals and regulators, corporations, nonprofit organizations, and community leaders who wish to establish Illinois as a leader in geothermal energy (IGC, 2022). The mission of the IGC is to support the advancement and wider implementation of geothermal energy systems in Illinois and the U.S. Midwest through collaborative study and demonstrations of technologies and system designs. Part of this mission involves providing education and outreach opportunities on geothermal energy to our members and other affiliates in the U.S. Midwest. The IGC also contributes to federal and state legislation and planning documents. For example, the IGC provided technical review for the Energy Act of 2020, Division Z of the Consolidated Appropriations Act, 2021 ( that was signed by President Trump. Communications such as this White Paper also fall under the mission and jurisdiction of the IGC.

1.3 Geothermal Research at the University of Illinois at Urbana-Champaign

The University of Illinois at Urbana-Champaign (UIUC) has an important role in supporting the transition to using renewable, low-carbon-emission energy sources, particularly geothermal energy. The research and the associated education and outreach undertaken have directly addressed the objectives of Illinois’ Climate Action Plan (iCAP; University of Illinois, 2020). The UIUC administration originally signed climate leadership commitments, including the American College and University Presidents’ Climate Commitment in 2008 that pledged to achieve carbon neutrality no later than 2050, and the Resilience Commitment in 2016 that sought to support the resilience to climate change in the campus community. These objectives are described in detail in the iCAP, which has been renewed twice since it was first drafted in 2010 (University of Illinois, 2010). The development of campus geothermal energy systems will assist in meeting the goal of using clean energy sources for 15% of total campus energy demand by 2030 (University of Illinois, 2020).

The geothermal research at the UIUC is also pertinent to the renewable energy targets mandated in the State of Illinois’ Climate and Equitable Jobs Act (CEJA; State of Illinois, 2021). Pursuing the development and further advancement of geothermal technologies that can be directly applied in Illinois will provide decision makers and stakeholders the capability to promote economic development and workforce training opportunities that will support the equitable access to energy-efficient techniques and renewable heating and cooling systems during the energy transition. Supporting the research with education and outreach activities throughout the state will improve the knowledge base that is critical for the wider adoption of geothermal energy systems.

Geothermal research at the UIUC and affiliated Illinois State Geological Survey (ISGS) began in the late 1960s and early 1970s when Dr. Keros Cartwright at the ISGS led research on heat flow in alluvial and glacial deposits (Cartwright, 1968) and sandstone reservoirs in the Illinois Basin (Cartwright, 1970). He later contributed to the first U.S. national geothermal survey (AAPG, 1994). His colleagues at the ISGS, Tim Larson, Ed Mehnert, and David Larson, also published studies on geothermal energy and geothermal heat pumps (Holm et al., 2015; Larson, 1990; Mehnert, 2004). Furthermore, the Energy Research Group, which was funded to investigate geothermal energy and energy efficiency, was active on campus through the 1970s and early 1980s (Hannon, 2013). Recently, contributors to this White Paper, Yu-Feng Lin, Andrew Stumpf, Tugce Baser, and their students and colleagues, have published findings from the following studies:

Modeling of ground heat transfer (Liu et al., 2021; McDaniel et al., 2018);

Efficiency of borehole heat exchangers (Zhao et al., 2022; Zong et al., 2021);

Designed geoexchange energy systems (Stumpf et al., 2021);

Feasibility of deep direct-use geothermal energy systems (Lin et al., 2020a);

Energy foundations (Reiter et al., 2020);

Underground energy storage (Jello et al., 2022); and

Sensor development (Lin et al., 2020b).

Since 1988, the Air Conditioning and Refrigeration Center at the UIUC (https://acrc.mechse. has conducted research on heating, ventilating, air conditioning, and refrigerating (HVAC&R) systems, creating industry-relevant technologies that contribute to the development of energy-efficient equipment (including geothermal heat exchangers) that operates in an environmentally friendly and sustainable way; improving energy efficiency, sustainability, and reliability; and reducing their overall footprint, leading to cost savings.

As a leader in sustainability, the UIUC recently earned the Sustainability Tracking, Assessment & Rating System (STARS) Gold status ( and was recognized for outstanding energy and resource savings made possible by installing a geothermal energy system for the Campus Instructional Facility ( Reports_2021/u_of_i_campus_instructional_facility.pdf). The geothermal energy system is one of six currently being managed by Facilities & Services (F&S) on campus and at Allerton Park, which services academic, administration, and residence buildings, and a greenhouse (internal report). Recently, F&S began supporting academic collaborations, such as studies of geothermal energy, through a new Academic Collaborations initiative. The collaboration has involved monitoring ground temperatures at two sites on campus (McDaniel et al., 2018; Stumpf et al., 2021) and monitoring the performance of geothermal loops in four drilled shafts at the Ven Te Chow Hydrosystems Laboratory ( academic-collaborations/geothermalcoalition/Geothermal-Energy-Piles).

2. Geothermal Energy Technologies
2.1 Background

Although there is great interest in advancing the renewable energy portfolio, and the power generation sector (wind and solar energy systems) continues to increase rapidly, less attention has been paid to the decarbonization of heating and cooling systems. To address decarbonization in the buildings, there is a realization that the energy system of the future will include multiple energy sources, and that geothermal energy will be crucial in supplying a reliable baseload to the energy grid.

The term geothermal energy refers to the natural thermal energy found underground from depths of a few feet to several miles. Geothermal energy has recently been pronounced a “renewable energy” (U.S. Congress, 2021), which has applications in all 50 states and U.S. territories for heating and cooling applications or generating electricity. The thermal energy in geothermal resources is present in a variety of underground environments and can be accessed by using different techniques and technologies to recover this energy for beneficial use. These concepts are discussed in more detail in the subsequent sections.

Conceptually, the Earth acts as a thermal battery, storing low-grade energy in the shallow underground (<655 ft or <200 m) that is absorbed from solar radiation and transported in the subsurface by the groundwater system (Figure 2.1). Because of the Earth’s ability to store thermal energy, the groundwater temperatures at a depth of 20 ft (6 m) remain constant throughout the year, in Illinois between 50°F and 55°F (10°C and -14°C; Figure 2.2). Below ~40 ft (~12 m) beyond the thermal impacts of atmospheric changes (e.g., Tyler et al., 2009), under natural conditions the ground temperature in Illinois increases at a rate of 1°F (0.6°C) per 100 ft (Frailey et al., 2004). But the thermal gradient was found to be altered (e.g., ground temperatures decrease with depth) where heat below urban areas has been conducted into the ground (cf. Stumpf et al., 2021).

Geothermal resources can be grouped into five main categories (Figure 2.3), with only the latter three economically viable at present in Illinois (see list below). High-temperature geothermal resources that can drive electricity generation are typically found in the volcanic terrain of the western United States, whereas in low-temperature subsurface environments common to the central and eastern regions of the United States, geoexchange technologies and heated water and brines are utilized for heating and cooling buildings and making hot water.

1. Hydrothermal resources: This conventional geothermal resource contains naturally occurring water, heat, and permeable bedrock that generate electricity. Most of the resources that can be extracted economically are present in the western United States.

2. Enhanced geothermal systems (EGS): Enhanced geothermal systems contain man-made geothermal reservoirs that require stimulation (opening fracture networks in the bedrock) to circulate hot water in sufficient volumes to generate electricity that will support commercial rates of energy extraction.

3. Direct use: Where electricity generation has not historically been cost effective, heated water at temperatures of <300°F (<150°C) is pumped from geothermal reservoirs and piped through heat exchangers to directly harness thermal energy to heat and cool commercial and residential buildings or to produce hot water directly. Direct use resources are commonly used for geothermal district heating applications, such as in the City of Boise, Idaho.

4. Low-temperature and co-production resources: Considered nonconventional hydrothermal resources that are <300°F (<150°C), the extraction technologies are co-located within oil and natural gas fields, mineral extraction sites, and energy storage sites for generating electricity or direct use applications. Recent development of these geothermal resources has been focused on sedimentary basins (Wang et al., 2018). The quality of the geothermal resource depends on the fluid volumes and temperature.

5. Very low temperature, shallow geothermal resources: With the application of geoexchange technologies (i.e., geothermal heat pumps [GHPs], ground source heat pumps, borehole heat exchangers, etc.), the relatively constant ground temperatures (50°F-55°F or 10°C-14°C) in the shallow subsurface (500 ft or 150 m) available year-round can be used for heating and cooling buildings. These systems use the ground for seasonal heat energy storage. In the summer, buildings are cooled by exchanging excess heat with the ground, typically with the help of an electric heat pump. The process is reversed in the winter when that heat is exchanged and provided back to the building. There are many different ways to design a geothermal exchange system, as discussed in Section 2.4.

Geothermal energy systems utilize the virtually untapped thermal energy resources available underground, and they will play an important role in meeting our decarbonization goals. These systems are very reliable and operate around the clock, 24 hours a day and 365 days of the year, providing clean, low-carbon, renewable thermal energy that emits few or no greenhouse gases. They have very small physical and environmental footprints and are hidden from sight underground. The borefields required are usually developed under buildings, athletic fields, park lands, and parking lots, and the mechanical equipment is found in basements, crawl spaces, and utility closets, which makes the systems highly resilient to natural hazards and adverse impacts from storm events.

According to the U.S. EPA, geoexchange energy systems are the most energy-efficient, environmentally friendly, and cost-effective technology for heating and cooling systems (U.S. EPA, 2009). These systems offer the ability to lower utility bills because for every unit of electricity used, 3 to 5 units of thermal energy (heat and cool) is released. Furthermore, geoexchange energy systems are up to 44% more efficient than air source heat pumps, up to 72% more efficient than electric resistance heaters with standard air-conditioning equipment, up to 48% more efficient than gas furnaces, and up to 75% more efficient than oil furnaces, as also reported by the U.S. EPA (L’Ecuyer et al., 1993). Unlike air source heat pumps, the performance of geoexchange energy systems is independent of changes in the outside air temperature and humidity. Because the underground temperatures remain relatively unchanged throughout the year and the difference between the refrigerant temperature is constant, heat transfer rates are maximized and the geoexchange energy system operates at much higher year-round efficiencies. Although geothermal energy systems do not replace the need for electricity (they run on electricity), 25%–50% less electricity is required than for conventional heating or cooling systems (U.S. DOE, 2015a).

In addition to the operating cost benefits of geothermal energy systems, they provide additional societal benefits:

1. The elimination of fossil fuels: Heating does not require the combustion of fossil fuels.

2. A single energy system: GHPs provide both heating and cooling. There is no need for an air conditioner outdoors, which can be unsightly and noisy.

3. Indoor air quality: Geothermal energy systems do not emit carbon monoxide or carbon dioxide, and they provide dehumidification.

4. Lower operating costs: All equipment is installed indoors or underground, requiring less maintenance.

5. Design flexibility: Geothermal energy systems can easily and inexpensively be made modular or expanded to fit building remodels or expansion.

To evaluate the financial viability of installing geothermal energy systems, metrics such as the payback period, return on investment (ROI), and net present value (NPV) are often calculated. The National Renewable Energy Laboratory (NREL) has developed several technoeconomic models (https://www.nrel. gov/geothermal/data-tools.html, e.g., GEOPHIRES; Durga et al., 2021) to estimate the levelized cost of energy (LCOE) for electricity generation and heat production for geothermal energy projects. Typical payback periods for geoexchange energy systems are 5–10 years (U.S. DOE, 2015b). The long-term operating costs are much lower for geoexchange energy systems compared with other heating and cooling technologies (Figure 2.4a). Once the system is paid off, the owner begins to realize the significant annual cost savings that geoexchange systems provide. Furthermore, because these systems are powered by a relatively small amount of electricity, the operating cost year to year can be estimated and is not dependent on the unpredictability of fossil fuel costs.

Life cycle cost analyses (LCCA) are also performed to evaluate the mid- and long-term sustainability metric impacts of geothermal energy systems (Saner et al., 2010). Because GHPs continually use electricity to produce heating or cooling through the extraction of thermal energy from a carrier fluid by compressing and evaporating a refrigerant, the environmental costs extend beyond their manufacturing and disposal. Life cycle analyses are increasingly recognized as an important tool for conducting a holistic evaluation of the environmental impacts of the whole value chain of a product (e.g., Smith et al., 2021). The electricity supplied for the operation of a GHP and the coefficient of performance (COP) of the heat pumps are the keys influencing the environmental impact of geothermal energy systems (Saner et al., 2010). Secondary contributions include the type of heat pump refrigerant, the production of the heat pump, transport of the heat carrier fluid, the borehole, and the borehole heat exchanger (BHE).

Figure 2.1. Cross-section view showing the average geothermal gradient and magnitude of geothermal heat flux, the average solar insolation for Illinois, and the seasonal fluctuations in temperature in the near surface. Modified from Vangkilde-Pedersen et al. (2012). Figure reproduced courtesy of the Geological Survey of Denmark and Greenland.
Figure 2.2. Groundwater temperatures at 20 ft (6 m) depth across the United States. From IGSHPA Ground Source Heat Pump Residential and Light Commercial Design and Installation Guide (November 26, 2021 revision), International Ground Source Heat Pump Association, Springfield, Illinois. Figure reproduced courtesy of the International Ground Source Heat Pump Association.
Figure 2.3. Geothermal energy resource types and settings. Figure reproduced courtesy of the British Geological Survey.
Figure 2.4. Economic and environment benefits of a typical 4-ton geoexchange energy system in Duluth, Minnesota in 2012. In (a) the energy cost savings per net system cost is shown for electric and fossil fuel heating systems. GHPs have much lower annual costs, which leads to a relatively short 5- to 10-year payback period. Once the system is paid off, the owner begins to realize the substantial cost savings over the next 20–30 years. The geoexchange energy system also provides significant environmental benefits. Because the GHP is powered by a small amount of electricity, the total annual CO2 emissions are much lower compared with other heating technologies. The cost–benefit graphs are reproduced courtesy of Northern GroundSource Incorporated, Brimson, Minnesota.
2.2 History

The term geothermal has its root in the Greek language, originating from the words geo (earth) and therme (heat). Geothermal energy has been used for thousands of years from naturally occurring hot springs and seeps. Geothermal heating has been used for bathing since Paleolithic times (Cataldi, 1993) and for space heating by the Romans. The world’s oldest geothermal district heating system is in Chaudes-Aigues, France, and has been operating since the 14th century (Lund, 2007). It is estimated that more than 10 billion quads of thermal energy are contained in the Earth’s hydrothermal resources at accessible depths (Tester, et al., 1989). This is ~300 times more energy than is contained in all fossil fuel resources.

In North America, the Paleo-Indians were the first humans to use geothermal resources more than 10,000 years ago. The hot springs they visited served as a source of warmth and cleansing, and their minerals as a source of healing (Lund, 1995). The first European settlers that reached the central and western states were also attracted to the natural hot springs and geysers (U.S. DOE, 2013). In 1807, settlers founded the city of Hot Springs, Arkansas, and during the 1850s and 1860s, hotels and resorts were developed near the warmer waters. In 1892, Boise, Idaho, became the first city to develop a geothermal district heating system (Wood and Burnham, 1983).

The first heat pump was conceptualized by Lord Kelvin in 1852 around the theory of using an air-cycle heat pump to absorb air from the atmosphere and deliver it at a higher temperature to a building (Banks, 2012). Lord Kelvin’s work was followed by that of H. Zoelly, who obtained a Swiss patent in 1912 on a heat pump that could extract heat from the earth (Wirth, 1955). It was not until 1945 that Robert C. Webber built the first direct exchange GHP at his home in Indianapolis, Indiana (Crandall, 1946). In North America, the years between 1945 and the early 1950s were a heyday of GHP development. The first successful commercial installation of a GHP was in the Commonwealth Building in Portland, Oregon, in 1948 (American Society of Mechanical Engineers [ASME], 1980). Professor Carl Nielsen of Ohio State University built the first residential open-loop geoexchange energy system in his home in 1948 (Gannon, 1978). The installation of geothermal energy systems waxed and waned during the 1970s and 1980s with the rise and fall in global oil prices (Sanner, 2017). A chance meeting between employees of a Canadian water well company and an Indianapolis-based heating, ventilating, and air conditioning (HVAC) contractor eventually led to the creation of Fort Wayne, Indiana-based heat pump manufacturer WaterFurnace International in 1983 (Egg, 2022b). Later, Dan Ellis left WaterFurnace to lead ClimateMaster Incorporated from the red to a $200 million company in 10 years.

During the 1990s, several geothermal power plants were constructed in the western United States. In 1994, the U.S. DOE created programs that would (1) reduce the cost of producing geothermal electricity and make it more competitive with other power sources, and (2) participate in a collaborative effort with industry stakeholders to promote a significant increase in the use of GHP (U.S. DOE, 2013). Through the American Recovery and Reinvestment Act (ARRA) of 2009, the U.S. DOE’s Geothermal Technologies Office (GTO) awarded funding to 149 geothermal projects across the United States, including a preliminary study to convert the heating and cooling system at Ball State University to a district geoexchange energy system (Im et al., 2016). Since these studies, the GTO has continued investing in advanced geothermal systems, including the development of hydrothermal and EGS geothermal resources in the western United States and deep direct-use feasibility studies in the eastern region.

2.3 Current Status

Because of the growing awareness of climate change and environmental issues related to burning fossil fuels, alternatives have been proposed to decarbonize energy systems and electrify the building stock to reduce greenhouse gas (GHG) emissions. The sustainability and resilience of the electrical grid and heating and cooling systems are important considerations, although geothermal energy is currently only a marginal component of the U.S. energy portfolio (0.4% of U.S. energy generation in 2021; EIA, 2022b). There is a realization it will be vital in our “energy transition” to provide affordable, safe, and accessible energy supplies (Tester et al., 2021). To put this into context, a recent study by Deetjen et al. (2021) suggested that 32% of U.S. residents would benefit economically from installing heat pumps, and if installed in 70% of U.S. homes, it could contribute to a significant reduction in GHG emissions. In 2019, 31% of the United States’ primary energy consumption was for direct fossil fuel combustion (mostly for heating) in buildings and industry (EIA, 2020c). Fox et al. (2013) estimated that >25% of the U.S. total primary energy demand is for low-temperature (<200°F or <95°C) heating for residential and commercial buildings and industrial processes. Furthermore, in 2020, ~41% of the United States’ total GHG emissions came from heating with fossil fuels in the residential, commercial, and industrial sectors (U.S. EPA, 2022).

The U.S. DOE, through its recent 2019 report GeoVision: Harnessing the Heat Beneath Our Feet (, has set forth a pathway to increase geothermal energy use across the United States to accelerate the market potential for both electricity generation and installing GHPs (U.S. DOE, 2019a). For example, the U.S. DOE predicts that the technology improvement scenario would lead to supplying 28 million GHPs and the installation of up to 17,500 geothermal district heating systems. The GHPs would account for 25% of the entire U.S. heating and cooling market (Liu et al. 2019) and would be crucial for decarbonizing the energy systems for U.S. households, university and college campuses, and cities and towns. The GeoVision report also presents the U.S. DOE’s research and development strategy over the next 30 years, which is currently focused on hydrothermal resources, EGS, and low-temperature applications, as well as work in critical materials, drilling technologies, thermal energy storage, integrated energy systems, technical assistance, and stakeholder outreach and education.

In addition to the federal support, states and local governments have adopted their own aggressive legislation to decarbonize the heating and cooling sector, some including geothermal energy (e.g., New York State, Senate Bill S9422—Utility Thermal Energy Network and Jobs Act, legislation/bills/2021/S9422).

2.4 Geothermal Energy Technologies and Installations

The key geothermal energy technologies found in Illinois and the U.S. Midwest can be broadly grouped into conventional low-temperature geothermal technologies, which include geothermal heat pumps and community or district energy systems, and advanced low-enthalpy geothermal technologies (cf. Ball, 2020a). The technology application is determined by a combination of factors, such as latitude, climate, geology, budget for and project complexity of the enabling technology, and end use application. In the United States, 84% of the GHP systems are closed-loop borehole heat exchangers (Liu et al., 2019), and include energy (geothermal) piles or shafts (energy foundations) constructed in the structural supports or foundations of buildings, bridges, and other infrastructure (Sani et al., 2019). They have made a measurable impact on the U.S. energy system by providing baseload heating and cooling, which has brought security and resiliency to the electrical grid while significantly reducing GHG emissions (IGSHPA, 2017). Advanced geothermal systems are technologies in the conceptual, demonstration, or early commercial phases that have the potential to provide baseload heating and cooling or electricity (e.g., Jello et al., 2022; Lin et al., 2020a).

In the following subsections, each technology is described in detail and the most common applications are highlighted, including state-of-the-art applications. All the approaches can be used in residential, commercial, and industrial building applications, and all play an important role in energy conservation efforts. Geothermal systems can be installed during the construction of new homes or during retrofitting of an existing home to improve the efficiency of heating and cooling. Geothermal energy systems essentially replace the need to own both a furnace and an air conditioner by providing heating and cooling in one unit.

2.4.1 Closed-Loop Geothermal Energy Systems: Horizontal and Vertical Borehole Fields

Horizontal and vertical borehole heat exchangers are the most common geothermal energy systems, with more than 1.7 million systems installed in the United States (International Energy Agency [IEA], 2022). Closed-loop systems take advantage of geoexchange processes (heat conduction) between the high-density plastic (HDPE) geothermal loop and the ground. Water or a water–antifreeze (glycol) mixture is circulated through the loop to transport the thermal energy to the geothermal heat pump. A heat exchanger transfers the energy between the refrigerant in the heat pump and the antifreeze solution in the closed loop. The length of loop installed depends on the energy demand required.

Horizontal closed-loop systems are the most common in rural areas and in cities where sufficient land is available to bury pipe in open trenches, excavations, and directional bored holes (Figure 2.5). The closed loops are buried 5–6.5 ft (1.5–2 m) underground below the frost line. The loops are installed in various configurations, including linear, slinky-coil, and spiral-coil arrangements (Cui et al., 2019). The performance of the horizontal closed-loop systems is lower compared with vertical closed-loop systems because seasonal soil temperature and moisture content variations affect the transport of thermal energy. However, horizontal closed-loop systems are more cost-effective than systems with vertical boreholes because digging trenches and excavations is much cheaper than drilling boreholes.

Vertical closed-loop systems are commonly used in residential, commercial, and public buildings where the land area required for horizontal loops is not available or where trenching or excavating at the surface is not feasible because of the complex geology or the presence of utility lines. In Illinois, vertical closed loops are typically installed in boreholes 300–500 ft (90–150 m) deep (Figure 2.5) that are arranged in grid or circular arrangements (cf. Park et al., 2018). There are different configurations of the geothermal loop inside the borehole, including single, double, helical, and coaxial shapes (Balaji, 2021). As mentioned, thermal energy is transferred into the loop and transported to the surface by the working fluid. During winter, heat is carried to the GHP, and cold water is returned to the ground for conduction. The process is reversed in the summer: Excess heat is transferred to the geothermal borefield and cooler fluid is brought to the surface.

To effectively manage the geothermal system to maintain the ambient ground temperatures, especially for projects with larger borefields, simulations are performed that require anticipated ground heat extraction and rejection loads, ground thermal properties, details about the borehole heat exchange design (e.g., borehole diameter, pipe sizes, flow rates, grout thermal properties), the configuration (e.g., number of boreholes and their configuration), and depth of the boreholes (e.g., Spitler, 2000). To accurately measure the ambient ground temperatures, thermocouple sensors (Gehlin & Nordell, 2003), temperature probes (Keys & Brown, 1973), or fiber-optic distributed temperature sensing (DTS) systems (Freifeld et al., 2008) are used in monitoring wells. To determine the thermal transport properties underground, thermal response tests (TRT) or distributed thermal response tests (DTRT) are performed in boreholes (Raymond et al., 2011; McDaniel et al., 2018). These tests provide an estimate of the ground thermal conductivity and heat transport. Alternatively, core samples taken from the boreholes can be tested in the laboratory to determine thermal conductivity (e.g., Stumpf et al., 2021).

Direct exchange (DX) systems are alternative closed-loop horizontal loop systems that do not require a heat exchanger because the heat pump circulates refrigerant directly into the ground through copper tubing (Waterless, 2022). Because DX systems circulate refrigerant through the ground and potentially could leak, local environmental regulations may be enacted prohibiting their use in some locations.

2.4.2 Open-Loop Geothermal Energy Systems: Shallow Aquifers

Open-loop geothermal energy systems use groundwater or local surface bodies of water (e.g., lakes and ponds) as the geoexchange fluid that circulates directly through the GHP (Figure 2.5). Once it has circulated through the system, the water is returned to the same aquifer unit underground through the well or is discharged in a water body or stormwater drain at the surface. Open-loop systems are practical only where an adequate supply of relatively clean water is available and all local codes and regulations regarding groundwater discharge are met. Aquifers act as both a heat source and heat sink, as well as a medium to exchange energy with the surrounding earth (Casasso & Sethi, 2014).

Standing-column well systems utilize the thermal energy from groundwater for geoexchange energy systems (Pasquier et al., 2016). This semi-open-well system includes a single deep, open borehole, typically 250–1475 ft (75–450 m) long, mostly constructed in competent bedrock (Orio et. al., 2005). The geoexchange process is enhanced by water pumping, which causes the groundwater to enter the borehole. Because of the higher heat geoexchange rate, the water temperature is similar to the mean ground temperature measured in closed-loop geothermal energy systems.

Figure 2.5. Geothermal heat pump systems are adaptable to a number of different configurations. From Hughes (2008). Figure reproduced courtesy of the U.S. Department of Energy.
2.4.3 Energy Foundations: Geothermal (Energy) Piles, Shafts, and Walls

Recent advances in the building sector to achieve higher sustainability ratings (e.g., LEED [Leadership in Energy and Environmental Design] Gold and Platinum certification) have placed the focus on achieving better building performance. Developers are considering using the foundation elements of buildings to harness the thermal energy available in the shallow subsurface for heating and cooling (Sani et al., 2019; Adam & Markiewicz, 2009). When outfitted with closed-loop geothermal energy systems, cast-in-place technologies, such as piles and drilled shafts, along with other geotechnical structures in contact with the ground (i.e., shallow foundations, retaining walls, basement walls, tunnel linings, and earth anchors) (Figure 2.6), can harness the near-surface geothermal energy (Brandl, 2006; Cui et al, 2018; Jiang et al., 2019; Liu et al., 2019). For example, heat can be extracted from or injected into the ground by circulating a water and glycol mixture in geothermal loops attached to the reinforcement cage of pile foundations constructed at depths of 30–80 ft (10–25 m; Abdelaziz et al., 2011). The application of using foundation elements for meeting energy requirements is not new; they were first introduced in Austria and Switzerland in the 1980s (Katzenbach et al., 2014). Because geothermal piles are installed as the required structural supports, there are no additional drilling costs. The energy cost savings for typical buildings outfitted with geothermal piles could be as much as 70% (Olgun et al., 2012).

Figure 2.6 Conceptual representation of geothermal energy piles: (a) heat extraction during winter for heating; (a) heat injection during summer.
2.4.4 Deep Direct-Use Geothermal Energy

Geothermal direct-use is an emerging technology in the geothermal sector that is suitable for low-temperature applications (<200°F or <95°C) in sedimentary basins in Illinois and the U.S. Midwest. In the past, geothermal direct-use systems were developed primarily in the western United States, where the resources are located at shallow depths. More recently, the U.S. DOE has supported the exploration of deeper geothermal resources in regions with lower geothermal gradients (e.g., in the eastern United States) for direct-use applications, referred to as deep direct-use (DDU; U.S. DOE, 2015c). The technology includes extraction and injection wells constructed in pairs (called “doublets”), one for pumping geothermal fluid (water or brine) from porous aquifers and another for returning fluid to the geothermal reservoir once the heat has been exchanged (cf. Lin et al., 2020). The heat in fluid pumped out of the ground is exchanged directly from pipelines for various applications, such as buildings (stand-alone or in a district energy system), greenhouses, fish farming, food drying, snow melting, and industrial processes (Beckers et al., 2021). Residual energy can also be used in cascading heating applications (Rubio-Maya et al., 2015). With absorption chillers, space cooling, refrigeration, and freezing can be accomplished. The extracted heat is used to evaporate the refrigerant vapor out of the fluid pumped through the heat exchanger (Liu et al., 2015).

Although the geothermal direct-use market is relatively small in the United States, there is growing interest in Europe to provide geothermal energy in cities (Abesser et al., 2020; Dalla Longa et al., 2020). The DDU technology offers a sustainable, zero-emissions alternative to conventional heating and cooling systems mostly supplied by fossil fuels, which could be part of a significant expansion of geothermal energy to a much wider part of the United States. The DDU geothermal energy systems are best suited for applications with large energy demands (e.g., university or college campuses, military installations, hospital complexes, offices, hotels), especially in hot and humid climates where buildings are cooling dominant (Geothermal Rising, 2022). They provide reliability to the energy grid, reduce GHG emissions, and have lower water use compared with conventional evaporative cooling technologies (U.S. DOE, 2015a). Key challenges still exist for DDU development, including a lack of suitable geothermal resources, high upfront costs, especially for drilling, and somewhat longer development timelines.

2.4.5 Underground Thermal Energy Storage

Underground thermal energy storage, known as UTES, is a sensible thermal energy storage method characterized by high storage efficiencies (Lee, 2012) and high storage capacities. It is therefore the preferred choice for long-duration thermal energy storage (Fleuchaus et al., 2018). Thermal energy is exchanged with fluids pumped underground in one or more wells that cool the ground in the winter and raise the ground temperature in the summer. This concept is known as seasonal thermal energy storage (Yang et al., 2021). The most popular UTES techniques are both open-loop or closed-loop systems and aquifer thermal energy storage (ATES), borehole thermal energy storage (BTES), mine shaft thermal energy storage, tank thermal energy storage, pit thermal energy storage, and cavern thermal energy storage (Fleuchaus et al., 2018).

In the moderate climates of Illinois and the U.S. Midwest, where groundwater is abundant in the glacial deposits and shallow and sedimentary bedrock and the demand for space heating and cooling alternates seasonally, the most applicable technique is ATES (Bloemendal et al., 2014). In these systems, thermal energy is temporarily stored underground through injection and withdrawal of heated or cooled groundwater to depths below 50 ft (15 m). At these depths, the ground temperature roughly equals the mean air temperature, so the ground temperature will be higher than the air temperature in the winter and lower during the summer (Lee, 2012). The key requirements for ATES are the availability of an aquifer and suitable hydrogeological conditions, such as a low groundwater flow, high permeabilities, and geochemical conditions that prevent the clogging and corrosion of wells (Fleuchaus et al., 2018). Aquifer thermal energy storage has the highest storage capacities and is therefore the most suitable for large-scale applications (Bloemendal et al., 2014). Their capacity typically ranges between 0.3 and 3.4 million British thermal units per hour (MMBtu/h) (0.1 and 0.3 megawatts [MW]) for small-scale systems and between 17.1 and 102.4 MMBtu/h (5 and 30 MW) for large-scale systems (Fleuchaus et al., 2018).

Where groundwater is less abundant, closed-loop systems, such as BTES, would be feasible. Borehole thermal energy storage systems are less affected by permeability and are installed in hard rock and unconsolidated clay, sand, and soil with a high volumetric heat capacity (Gehlin, 2016). The BTES systems are commonly constructed in cylinder and box shapes to maximize energy storage (cf. Hammock, 2018).

According to the IPCC, the amount of electricity required for air conditioning in cities is expected to increase 33 times by 2100 (Arent et al., 2014). The challenge will be to increase the share of renewables to address the needs of the cooling sector, which is attributed to the seasonal offset between thermal energy demand and supply. Underground thermal energy storage will be a key technology to support energy conservation programs (Dincer, 2002). Beernink et al. (2022) found that a 30% increase in ATES adoption would lead to a 60% maximum reduction in GHG emissions. However, with the expansion of UTES without proper management, thermal interactions could occur between wells and systems that could affect the efficiency of thermal recovery (Duijff et al., 2021).

2.4.6 Geothermal District (Community) Heating and Cooling Systems

A geothermal district or community heating and cooling (GDHC) system is an energy system that connects multiple buildings or facilities within a shared energy network that uses a geothermal energy resource as a heat or cooling source and that distributes heat through a connected distribution network (Dincer & Ozcan, 2018). The geothermal energy is used for space heating and cooling, making domestic hot water, and preheating or cooling industrial processes (Dincer & Ozcan, 2018). Geothermal district or community heating and cooling systems include energy centers, underground distribution systems, central pumping stations, and in-building equipment (heat exchangers, circulation pumps, etc.). They leverage commercially available geothermal energy components.

Geothermal district energy systems are a mature technology in which heat is used directly from the geothermal resource (e.g., in Iceland, see Karlsdóttir et al., 2014). However, where a district energy system includes GHPs, further innovation is needed to optimize the performance of the energy system, including the sizing of borefields and HVAC equipment (Gautier et al. 2022). Conventional GDHC systems with direct-use technology take advantage of naturally heated water or brine from deep underground rock formations, which is pumped to the surface and through a heat exchanger to capture the thermal energy. In contrast, GHP-connected systems utilize the temperature difference between the air temperature and the shallow underground to develop a heat sink in the summer and a heat source in the winter. Connecting many buildings together also allows the use of waste heat, which can be recycled and shared in the district.

Geothermal energy is an important component in advanced, fifth-generation district heating and cooling (5GDHC) systems that provide heat and cold in urban environments (Boesten et al., 2019). The 5GDHC are decentralized and bidirectional energy systems operating at close to ground temperature that use the direct exchange of warm and cold return flows and thermal storage to effectively balance seasonal energy demands. This is the same advantage GHPs have with respect to air source heat pumps (Li et al., 2014).

The 5GDHC systems replace the previous generations of district energy systems (1G to 4G) that required fossil fuels or operated at higher temperatures (>120°F or >50°C), which are commonly used on college and university campuses, in medical centers, in residential complexes, for multi-owner districts (e.g., downtown corridors), and in data centers. When GDHC systems are operated at lower or ambient temperatures, lower flow rates are needed, which leads to less distribution loss in the district (Gautier et al. 2022). When combined with large-scale heat pumps, GDHC systems result in significant energy savings.

The economic and environmental benefits to communities that utilize GDHC systems are discussed by Marques et al. (2021). Integrated, smartly controlled GDHC systems have the potential to deliver significant reductions in GHG emissions, improve air quality, and reduce energy costs for end-users. They have long-term, stable space-heating rates that facilities powered by fossil fuels cannot guarantee (Thorsteinsson & Tester, 2010). Geothermal district or community heating and cooling systems also have relatively lower upfront costs because the cost is spread out over multiple end users and may introduce renewable thermal energy to residents who cannot easily install geothermal energy on their own property. Building and maintaining a GDHC system requires an energy transition workforce that would employ engineers, petroleum and utility workers, and the building trades workforce.

The adoption of GDHC systems has so far been slow, partially because of the lower market price of competing heat sources (oil and natural gas). As of 2020, there were 23 active geothermal district heating systems in the United States (mostly in the western states; Robins et al., 2021), but recently a number of college and university campuses (Jossi, 2002), computing and data facilities (e.g., Epic Systems in Verona, Wisconsin; Massey, 2018), and communities (Framingham, Massachusetts, see Buro Happold Engineering, 2019; Whisper Valley, Austin Texas, see Wolfson & Mapel, 2020) have installed or are installing GDHC systems. The U.S. DOE is supporting research to develop innovative GDHC systems in communities to reduce energy costs, provide secure and reliable heating and cooling, improve environmental conditions, and train a diverse workforce to build, operate, and maintain GDHC systems.

2.4.7 Geothermal Energy from Abandoned Oil and Gas Wells

The U.S. DOE has estimated that the total low-temperature geothermal resource (<300°F or <150°C) through direct-use (nonelectric sector) technologies in the United States is equivalent to about 3.6 million gigawatt-hours-thermal (GWth) or 12 quadrillion Btu (12 quads). Almost 70% of the total geothermal resource in the United States is low temperature and largely untapped for direct use (Franco & Vaccaro, 2014). If orphaned or idled oil and gas wells were utilized, the total resource would increase to 11.2 million GWth (38 quads; U.S. DOE, 2019a). The problem with developing these resources in sedimentary basins is that reusing many oil and gas fields would require enhancements and stimulation (i.e., fracking) of the reservoir rocks, which would tie up financial resources (Kurnia et al., 2022). In addition, many states do not have regulations and royalty programs for repurposing oil and gas wells, which hinders geothermal energy development. Furthermore, geothermal energy competes with cheaper renewable energy resources (e.g., solar and wind energy) and lower cost natural gas (Ball, 2021a).

To support the repurposing of oil and gas wells in this energy transition, the U.S. DOE’s Geothermal Technologies Office has developed the ReAmplify and GEODE programs, which will spur collaboration between the geothermal and hydrocarbon communities on geothermal systems research, demonstration, and development (RD&D) by demonstrating the technical and economic feasibility of geothermal energy production using existing oil and gas wells. These programs provide the opportunity to transition the oil and gas workforce to jobs in the clean energy economy, and they set forth methodologies to repurpose hydrocarbon wellfields to extract geothermal energy. This plan includes the development of open and closed geothermal systems (Santos et al., 2022). The open-loop systems include two or more wells for extraction and reinjection that are primarily used for electricity generation. The programs also provide the option to coproduce geothermal energy and oil and gas from the increasing number of abandoned wells that experience declining petroleum reserves (Erdlac et al., 2007). In closed-loop systems, fluid is continuously circulated through a well having a single geothermal loop or coaxial well design (BHE) in a closed circuit open only at the bottom to allow fluid extraction. Shallow BHEs are widely used as a reliable source of heating.

Repurposing wells has several associated economic benefits, the most important being that repurposing avoids the costs of drilling and well completion and reduces the risk of developing geothermal reservoirs because subsurface and pumping data are already available (e.g., Guo et al., 2017). The practice also (1) alleviates the issue in mature hydrocarbon fields of requiring coproduced water to be separated and disposed of, (2) allows hot water to be used to generate electricity, (3) supplies thermal energy for district heating applications, and (4) allows preheating or cooling industrial processes, which provide heating for agriculture and spa and thermal baths, for example, improve oil recovery, and create an additional revenue stream over the long term (Duggal et al., 2022, and references therein). However, the low temperature of the geothermal reservoir (in comparison with hydrothermal systems) has often limited further development (Santos et al., 2022).

In the 23 states surveyed in the United States, nearly 1.75 million idle and orphan oil and gas wells are known (Interstate Oil and Gas Compact Commission [IOGCC], 2021). The wells are a source of unwanted and uncontrolled emission of methane, which is the second most important greenhouse gas and the most harmful because it is 86 times more effective in trapping solar heat in the atmosphere than CO2 (Riddick et al., 2019). The federal government has committed $4.7 billion to plug a limited number of these wells over the next 8 years (U.S. Department of the Interior [U.S. DOI], 2022).

Recent advancements and increased efficiency in low-temperature power conversion have piqued interest in attempting to repurpose oil and gas wells (Santos et al., 2022). Several challenges to developing the geothermal reservoirs remain, including controlling the pressure decline of the reservoir over time and undertaking the simultaneous extraction of oil and gas with heated water; the extracted water requires specific treatments to meet water quality standards. Advanced engineering studies are underway to determine whether these low-temperature geothermal resources can be altered by injecting heat from various sources to develop enhanced geothermal reservoirs containing fluids hot enough to generate electricity (e.g., Jello et al., 2022).

2.4.8 Solar-Assisted Geothermal Energy Systems

Hybrid geothermal energy systems that connect a geothermal resource with photovoltaic (PV) or photovoltaic–thermal (PVT) technologies, referred to as solar-assisted geothermal energy systems, have gained traction through the practice of integrating building energy systems and the design of low-carbon buildings, especially for the purpose of developing energy-efficient HVAC systems, and as renewable energy systems (You et al., 2021). The systems include different types of solar and geothermal heat pump technologies that are coupled together, which supply all the required heat for domestic hot water and space heating. Solar-assisted geothermal energy systems address the major problem of diurnal variation caused by differences between electricity production and seasonal demand (Knuutinen et al., 2021). In heating-dominant regions, PVT with geothermal heat pumps (PVT-GHP) offer the ability to regulate the thermal balance underground and improve the performance of the PV by reducing the module temperature. They also capture some of the incident solar radiation that PV cells cannot convert into electricity (Dupeyrat et al., 2014).

Advanced solar-assisted geothermal heat pump (SAGHP) systems have been designed for large-scale underground thermal energy storage in sedimentary basins (e.g., Geologic Thermal Energy Storage project, known as GeoTES; Wendt et al., 2019), district energy systems (Reed et al., 2018), and heating and cooling single- and multi-family homes (Nouri et al., 2019) and greenhouses (Yin et al., 2022). The GeoTES system involves the use of concentrating solar power collectors to heat water, which is injected into underground geologic formations to create a synthetic or engineered geothermal resource (Wendt et al., 2019). The water, at temperatures between 375°F and 445°F (190°C and 230°C), can be used in conventional power cycles to convert the stored heat to electricity. The stored thermal energy can be dispatched at various timescales, from daily to weekly to seasonally. In district energy systems, energy centers are connected in line with short-term thermal storage tanks, solar thermal collectors, and borehole thermal energy storage systems (BTES) to provide space heating and cooling, and in some places domestic hot water (Reed et al., 2018). The Drake Landing Solar Community located in Okotoks, Alberta, Canada, was the first solar district heating system in North America, serving 52 homes with a high solar fraction (up to 90%) through the use of seasonal underground thermal energy storage (Reuss, 2021). For individual buildings, PVs or PVTs are connected to the borehole heat exchangers with and without GHPs (You et al., 2021).

The integration of solar energy with geothermal energy systems contributes to reducing the consumption of conventional energy resources and lowers the associated environmental impacts (Nouri et al., 2019). Because solar-assisted geothermal energy systems store thermal energy underground, a stable ground temperature is maintained that increases the COP of the GHP (Chiasson & Yavuzturk, 2014). Exchanging heat with the ground in the summer may allow for shorter boreholes, which reduces the overall cost of installation. Storing thermal energy underground supports grid stability, reliability, and flexibility (Wendt et al., 2019).

2.4.9 Hybrid Geothermal Energy Systems

Hybrid geothermal energy systems include several different geothermal energy technologies that are combined with other renewable energy and fossil fuel resources to advance opportunities for space and water heating applications and thermal energy storage. Hybrid systems increase the performance of the energy systems and bring economic and environmental benefits to the built environment. The concept of integrating geothermal energy systems with renewable energy subsystems has been shown to temper and offset peak energy demands, energy consumption, and the ground thermal imbalance (GTI; Soni et al., 2016). For example, geothermal energy technologies are combined with air cooling (i.e., cooling towers) to effectively meet building cooling demands when they are significantly larger than the heating needs (Kavanaugh, 1998). Furthermore, hybrid GHPs coupled with external heat sources (fossil fuel boilers, micro-gas turbines, biomass thermal energy, and waste heat), air-source heat pumps, phase-change materials, auxiliary cooling technologies (ice storage, cooling towers, and cooling ponds), and sensible, latent thermal energy storage systems (soil storage, thermal storage tanks, and ice storage) can considerably reduce the peak energy demand, the size of the GHP borehole length, and the installation cost, all of which impact the economic feasibility and optimization of the system (Kim et al., 2016; Kumar & Sharma, 2021). Fewer studies have been undertaken to determine the feasibility of geothermal energy systems with combined heat and power (CHP) plants, combined cooling heating and power (CCHP) systems, ejector cooling, and absorption heat pumps (Xu et al., 2021).

To meet the baseload power requirements, advanced research projects are being considered that combine geothermal energy systems with the underground storage of wind energy and hydrogen (Schmidt & Pettitt, 2022) or compressed air energy storage (Leetaru, 2021), particularly in traditional geothermal domains. In many deep sedimentary basins in the U.S. Midwest, where wind energy is available at sites of oil and gas extraction, heated fluids can be injected underground for long-duration thermal energy storage (Robertson-Tait & Hollett, 2021). Green hydrogen plus geothermal energy systems can advance clean fuel processes and CO2 conversion. Significant interest exists in utilizing hybrid renewable energy systems to meet the baseload power demand at electricity-dominant facilities (e.g., data centers and cryptocurrency mining farms).

Additional synergies could be realized by developing cascading uses from hybrid geothermal energy systems that address technological gaps and offset the electricity demand but that also improve the feasibility of these renewable energy technologies (Robertson-Tait & Hollett, 2021). For example, critical minerals (e.g., lithium and rare earth elements) can be extracted from brine from geothermal reservoirs, yielding additional economic benefits.

2.4.10 Surface Water Heating and Cooling Systems

Surface water heating and cooling systems utilize lakes, ponds, rivers, and oceans (acting as thermal reservoirs) for direct heating and cooling applications and heat sinks for heat pumps. The systems provide buildings with sustainable energy-efficient heating and cooling. Cities, university and colleges campuses, commercial buildings, and individual homes built near water bodies have sought the great energy efficiency potential these water sources offer. Surface water bodies can be connected to water source heat pumps, or water can be extracted directly for water heating and cooling (Mitchell & Spitler, 2013). Systems connected to heat pumps have a supply water line that is run underground from the building into the water body and connected to a linear or coiled HDPE pipe or fabricated plate exchangers, which are installed at a depth of at least 8 ft (2.5 m; Figure 2.5). According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), these systems have a thermal energy capacity of 20 tons/acre in the cooling mode and 10 tons/acre in the heating mode (Kavanaugh & Rafferty, 2014). For direct surface water cooling, water is pumped from oceans or deep lakes that have a temperature below 50°F or 10°C. The water has a temperature similar to the water returned from absorption chillers. Direct-use cooling systems can save up to 90% of the energy required for conventional heat pump systems (Mitchell & Spitler, 2013).

In eastern North America, surface water is being used to supply year-round heating and cooling at several installations. At the Nashville International Airport, a former 43-acre quarry that holds storm water runoff is being used to heat and cool buildings (Egg, 2017). The lake water where the plate heat exchangers are installed remains at 50°F or 10°C year-round. Although the geothermal system requires electricity to run a mechanical plant, the lake replaces the use of cooling towers, which saves the City of Nashville 30 million gallons of potable water and 1.3 million kilowatt-hours (kWh) of electricity annually. The geothermal systems also simplify the maintenance of the heating and cooling system, which reduces plumbing and mechanical costs significantly. At Cornell University in Ithaca, New York, a direct surface water district cooling system draws water from Cayuga Lake at a depth of ~245 ft (~75 m). Cold water pumped from the lake at 39°F to 41°F, or 4°C to 5°C, is the main source of cooling for the university campus and the nearby high school (Mitchell & Spitler, 2013). The system provides 70 MW (20,000 tons) at peak capacity and has a COP of 30 (Tian et al., 2022). In Toronto, Ontario, Canada, water from Lake Ontario is used to cool downtown buildings (Newman & Herbert, 2009). Since 2004, the hybrid surface water cooling system has provided 260 MW of cooling annually. The system is connected to an auxiliary chiller and the city’s potable water supply.

Seawater district cooling systems have been developed across the globe in varying climates and geographic locations, ranging from Stockholm, Sweden, to Kona, Hawaii. The technology is similar to lake source district cooling systems, where water from deep depths is pumped through heat exchangers, where the cooling load is drawn off. The water is then discharged at a warmer temperature back to the water body. These systems contribute significantly to energy conservation and are a viable alternative to conventional central air conditioning systems. With escalating and volatile energy prices, these systems provide a sustainable and reliable energy source, stabilizing energy costs for space cooling in buildings (Looney & Oney, 2007).

2.4.11 Emerging Geothermal Technologies

In addition to the geothermal technologies discussed, several emerging technologies are applicable to the geological and energy demand needs of Illinois and the U.S. Midwest. They have either not attained widespread adoption or are progressing toward becoming the first commercial-scale projects. The development of new and innovative technologies is needed to make geothermal energy scalable by reducing costs and lowering the risk to accessing the geothermal energy resource.

CO2 and Ammonia-based District Energy Systems

The operation of district heating and cooling systems has attracted interest in reducing CO2 emissions where the heat-transport medium is at a temperature close to the ground and captures waste heat (Weber & Favrat, 2010). By reducing the temperature difference from the ambient temperature, including that of the supply and return water, the efficiency of the system can be improved. Man Energy Solutions ( and Hunt et al. (2022) have proposed district energy technologies that utilize CO2 and ammonia, respectively. Geothermal energy could be integrated into the district energy systems for preheating or cooling. Heat pumps with CO2 and ammonia refrigerants have been installed in district energy systems (e.g., the Blatchford development in Edmonton, Alberta, Canada; see

Advanced Borehole Heat Exchangers

Darcy Solutions Incorporated ( has designed a downhole technology with the capacity to extract 40 to >80 tons of heating and cooling from a single borehole (Price, 2022). In comparison, the industry standard is to capture 1–2 tons per borehole. In a closed-loop geothermal energy system, a heat exchanger is installed in the borehole, which transfers thermal energy from the groundwater to the geothermal loop without pumping water to the surface. This design reduces the risk of groundwater contamination and cuts annual heating and cooling costs by 30%–80%. Furthermore, because the system requires fewer boreholes to be drilled to meet the heating or cooling demand of conventional geoexchange energy systems, a smaller borefield footprint is required; the land required can be reduced by as much as 95%. The system is all electric, which assists building owners in meeting carbon emission goals and provides a viable heating solution where natural gas is not available.

Wastewater Heat Recovery Energy Systems

Wastewater produced at domestic, industrial, commercial, or residential developments holds a significant amount of thermal energy that, for the most part, is wasted. Egg (2022a) estimated that in the United States, 1.3 billion kWh of thermal energy is lost per day, or more than 480 billion kWh a year, and that wastewater extraction could provide equivalent kilowatt-hours (in thermal energy) for nearly 45 million households. This should not be a surprise, as Egg (2022a) explained, because in the summer in the southern United States, it is not uncommon for water entering residential and commercial buildings to be at 80°F (27°C) or above. In contrast, in winter, water temperatures in the northeastern United States average 40°F–49°F (4°C–10°C). However, no matter what the season, water leaves buildings at room temperature or above, between 65°F and 75°F (18°C and 24°C). The important point is that energy is required to condition the water to room temperature.

Several companies (e.g., SHARC Energy Systems,, and Huber Technology Incorporated, water/sewers-sources-of-energy.html) are developing wastewater heat recovery technologies with integrated heat exchangers to capture waste thermal energy from water lines and sewer systems. The energy is captured by using heat exchangers, and heat pump technologies may be deployed at different locations, whether in buildings, in public sewers, or at wastewater treatment plants (Nagpal et al., 2021). The captured energy is sufficient for the economical operation of heat pumps to heat nearby buildings (e.g., schools, health facilities, swimming pools).

It has been shown that in some situations, 50% of the thermal energy can be stored or recovered, which would return significant economic and environmental benefits (Wehbi et al., 2022). The Water Environment Research Foundation ( Energy-from-Wastewater-ENER6C13-Factsheet.pdf) estimates that 18 million tons of CO2 emissions could be avoided by recovering thermal energy at wastewater treatment facilities.

Machine Learning and Geothermal Energy

Advancements in machine-learning algorithms and artificial intelligence are being used by the U.S. DOE and its partners to improve geothermal reservoir characterization, increase the drilling success rate, and optimize geothermal facility operations. These successes will result in cost reductions throughout the project life cycles, from resource exploration to commercial operations. Since 2018, the Geothermal Technologies Office ( has funded early-stage research and development applications in machine learning to develop technological improvements in exploration and operational improvements for geothermal resources.

For shallow geothermal energy systems, predictive models based on physical laws or inferences from data sets simulate the operating environment so that maximum performance can be achieved (Noye et al., 2022). Geothermal heat pumps are complex systems whose operation is affected by variations in external conditions, such as weather and energy demand, and by long-term variations in their performance. Assouline et al. (2019) used geographic information systems (GIS), machine learning, and numerical models to estimate the geothermal potential in the shallow subsurface. Regarding deep direct-use geothermal energy systems, machine-learning models have been developed to predict temperature profiles and geothermal gradients (e.g., Shahdi et al., 2021). Machine-learning algorithms have also been used to predict energy demand in district heating and cooling systems (Saloux & Candanedo, 2018).

3. Policies and Regulations

With the new federal, state, and local tax incentives and funding programs, the uptake of geothermal energy systems is expected to accelerate. Currently in Illinois and the U.S. Midwest, policies and environmental regulations associated with geothermal energy systems with GHPs, with both closed and open loops, are focused on groundwater use and environmental impacts. Throughout the United States, permits are required for drilling into the underground, particularly when such drilling may have an impact on water resources. Regulations for source water protection are developed and enforced primarily at the state and local municipal levels of government. In addition, the design and installation permits or approvals for underground thermal energy storage systems (i.e., ATES and BTES) are not handled consistently by the same agency in every state.

It is possible that future regulations will be needed to control “ownership” of geothermal resources, as some European countries have proposed (e.g., United Kingdom; see Abesser & Walker, 2022). These regulations are being considered to address potential adverse impacts from changing the ground temperature (e.g., Bourne-Webb et al., 2009; Fry, 2009; Wood et al., 2009). Because ATES systems use groundwater directly, there is a risk of contamination (thermal or otherwise; Hähnlein et al., 2013).

3.1 Renewable (Geothermal) Energy Legislation
3.1.1 Federal

Inflation Reduction Act of 2022 (H.R. 5376)

This legislation ( replaces the U.S. House-passed Build Back Better Act (BBBA; H.R. 5376) and builds on provisions in the previously enacted Infrastructure Investment and Jobs Act, which provided new funding to accelerate the growth of clean energy and support consumer rebates for home electrification. The Act, signed on August 16, 2022, provides nearly $500 billion of federal funding for clean energy, with the goal of substantially lowering the nation’s GHG emissions to meet the Biden administration’s goal of a 100% carbon pollution-free power sector by 2035 and a net-zero emissions economy by 2050 ( The funding is specifically earmarked to make investments and provide incentives that support national, state, and local geothermal energy projects. This support moves the nation’s economy away from emissions-based heating and cooling, and toward one that utilizes low-carbon renewables and energy storage, which includes GHPs and underground thermal energy storage. Moreover, it will create a more sustainable, resilient, and equitable economy and grow a skilled workforce that will deliver economic, environmental, and social justice advancements.

Infrastructure Investment and Jobs Act (H.R. 3684)

Also known as the Bipartisan Infrastructure Law, this $1.2 trillion legislation (https://www. signed by President Biden on November 15, 2021, contains important energy provisions that invest in building a clean energy system that will be secure and resilient. The legislation includes funding for cities and states to help alleviate their energy burden, implement clean energy projects that create jobs, and support the reduction of GHG emissions in communities.

To assist low-income residents and energy communities, $3.5 billion is earmarked for U.S. DOE’s Weatherization Assistance Program. This work will expand energy efficiency (including the installation of GHPs), lower energy costs for residents vulnerable to energy price fluctuations, and provide energy assistance funding for low-income households. An additional $500 million is available to increase energy efficiency and the use of renewable energy technologies at public schools. A further $500 million is available to states, cities, and tribes to implement clean energy programs and projects through the U.S. DOE’s Energy Efficiency and Conservation Block Grant program. The U.S. DOE has also created the Office of State and Community Energy Programs (, which will partner with state energy offices (for Illinois, see to increase access to clean energy systems for low-income households, businesses, schools, nonprofit organizations, and communities. This U.S. DOE office has nearly $6 billion in grants for states, tribal nations, territories, local governments, school districts, and nonprofit organizations.

Consolidated Appropriations Act, 2021 (H.R. 133)

The stimulus package signed by President Trump on December 27, 2020, includes the Division Z–Energy Act of 2020 (, which is the first comprehensive update to U.S. energy policy and U.S. DOE’s applied energy research and development programs in more than a decade. The Energy Act authorizes up to $35 billion in clean-energy investments, primarily to expand U.S. DOE research and development programs for energy storage; renewable energy; advanced nuclear energy; carbon capture, storage, and utilization; carbon removal; critical minerals and materials; fusion; industrial technologies; smart manufacturing; and grid modernization, among other areas. Specifically for the geothermal energy sector, Section 3002 provides funding to reauthorize the U.S. DOE’s geothermal energy Research, Development, Demonstration and Commercialization Application (RDD/CA) program, which includes enhanced geothermal research and additional geothermal demonstration projects, including one specifically in the eastern United States. The Act also expands the definition of renewable energy to include thermal energy and directs the U.S. Geological Survey to update its geothermal resource assessments.

3.1.2 State

The Climate and Equitable Jobs Act (CEJA) signed on September 15, 2021, advances Governor J.B. Pritzker’s vision to transition Illinois away from fossil fuels onto a path toward a 100% clean energy system by 2050 (State of Illinois, 2021). The legislation invests $80 million per year for training a diverse workforce that creates jobs and business opportunities in a green economy. It also establishes a $40 million grant program to support energy communities where power plants have closed and the fossil fuel industry has left. The Act creates a “Green Bank” to finance clean energy projects to address inequity and energy insecurity in historically low-income communities, which are disproportionally affected by climate change and environment pollution. Although no specific programs for geothermal energy systems were identified in the legislation, GHPs will be needed to electrify buildings to meet clean energy targets.

3.2 Geothermal Energy Regulations
3.2.1 Federal

The U.S. Environmental Protection Agency (U.S. EPA) regulates Class V wells, meaning wells used to inject nonhazardous fluids underground ( Class V includes any wells that are not already classified as Classes I–IV or Class VI wells. The fluids are injected either into or above an underground source of drinking water (U.S. EPA, 1999). In addition to shallow wells, Class V wells are used for a variety of municipal, business, and industry purposes, including for aquifer thermal storage (ATES) and as deep direct-use geothermal wells.

3.2.2 State

The Illinois Environmental Protection Agency (IEPA) to date has no regulations for deep direct-use geothermal wells in Illinois, but the IEPA is likely to regulate them as injection wells of nonhazardous waste (Class I) under the Underground Injection Control (UIC) program (Illinois Pollution Control Board [IPCB], 2018). The State of Illinois has primacy for administering UIC regulations for Class V wells, which the U.S. EPA regulates in other states.

The Illinois Department of Public Health (IDHP) administers and regulates the construction, modification, and sealing of all closed-loop and open-loop geoexchange energy wells. This program is conducted through agreements with 90 local health departments. Their work prevents contamination of groundwater from hazardous materials at the ground surface, in shallow ground water, from sewage disposal systems, and from other sources of contamination. To ensure the protection of groundwater, the IDPH and local health departments review water well installation plans, issue permits for new well construction, and inspect wells. The drilling of geoexchange wells must comply with the Water Well Construction Code (

3.2.3 Municipal (Local)

Counties, municipalities, and planning agencies may have additional codes of ordinances for installing geothermal energy systems (e.g., in Macon County, see county/latest/maconcounty_il/0-0-0-5315). In the City of Chicago, geothermal energy systems are regulated by the Department of Building and require an Existing Facility Protection (EFP) from the Chicago Department of Transportation’s Office of Underground Coordination. The projects are reviewed and permitted by the Green Permit Section of the Department of Building ( /en/depts/bldgs/provdrs/permits/svcs/green-permits.html). The Village of Deerfield has specific zoning and building code compliance requirements ( _il_zoning/0-0-0-684). Municipal governments are in the best position to incentivize the development of clean energy technologies because they can leverage their planning processes and programs and their municipal codification to enact clean energy policies (Cook et al., 2016).

4. Programs Funding Geothermal Energy

To achieve Illinois’ 2050 renewable energy and carbon emission goals, the state’s citizens and businesses will be looking to the incentive and loan programs offered by federal, state, county, and municipal governments; electric utilities; and nongovernmental organizations to support deploying geothermal energy systems that will replace existing electric resistance and conventional fossil fuel energy systems. The programs being offered make investing in geothermal energy more cost effective for homeowners, businesses, and property developers, which is vital to growing any additional deployment. Below is a list of programs currently being offered.

4.1 Federal

The U.S. Internal Revenue Service (IRS) offers two different tax incentives for making energy-efficient improvements and major infrastructure overhauls that include the installation of geothermal energy pumps systems. The Residential Energy Efficient Property Credit ( -pdf/f5695.pdf) allows for a credit based on a percentage of the cost paid for eligible energy-saving home improvements. The recently signed Inflation Reduction Act of 2022 increases the rate of the residential tax credit (Internal Revenue Code Section 25D [IRC §25D]) from its current 26% to 30% and extends the credit for an additional decade to include projects begun before January 1, 2033. The tax credit rate then decreases to 26% in 2033 and to 22% in 2034. Eligible property has been expanded to include battery storage in addition to solar photovoltaics, wind turbines, and geothermal heat pumps. The Nonbusiness Energy Property Credits were available in taxable years 2018 through 2021 for residential property owners who made energy-saving improvements. The program provides $300 credit for energy-efficient heating and air conditioning systems. There is an overall lifetime credit limit of $500.

The Inflation Reduction Act of 2022 also extends and increases commercial investment tax credits (ITCs) for geothermal heat pump systems. Under Internal Revenue Code Section 48 (IRC §48), the Business ITC rate is either 6% (base rate) or 30% (bonus rate) for most qualifying geothermal property constructed before the end of 2032. The bonus rate is offered if the project meets prevailing wage and apprenticeship requirements. The bonus rate decreases to 26% in 2033 and to 22% in 2034. In addition, the commercial tax credit is increased a further 10% if domestic steel, iron, and manufactured products are used or if the project is in an “energy community” (typically a brownfield site or a census area with high unemployment or historical coal, oil, or natural gas industry). A separate 10% bonus is also available for work in low-income areas. Altogether, the effective commercial tax credit is between 6% and 50%, depending on the project location, labor requirements, and content of domestic materials. Furthermore, the Act introduces direct pay and transfer options for many of the tax credits. The direct pay option is available only to tax-exempt entities (e.g., state and local governments, tribal governments, universities, and nonprofit organizations). These entities are not eligible to transfer credits. Those entities eligible to transfer tax credits may sell them for cash, which is not required to be reported as taxable income. Several administrative rules apply, but this benefit alleviates complicated tax equity partnerships that normally are required to monetize tax credits by companies other than the property owner.

In addition to the well-publicized extensions of tax credits in the Inflation Reduction Act of 2022, significant new funding ($2 billion over the next 10 years) has been allocated for renewable energy (including geothermal energy systems) as part of the Rural Energy for America Program (REAP). The grants and loans support the use of renewable energy and making energy efficiency improvements in rural areas (mostly downstate) that, over time, will help lower the cost of energy for small businesses and agricultural producers. Through the federal government’s Modified Accelerated Cost Recovery System (, businesses and individuals with income-producing property having geothermal heat pumps can recover the cost of depreciation from the IRS through annual deductions.

The Weatherization Assistance Program (WAP), part of U.S. DOE’s Office of Energy Efficiency and Renewable Energy, provides funding and technical assistance to states to enhance energy security, advance state-led energy initiatives, maximize the benefits of decreasing energy waste, and reduce energy costs for low-income households ( The WAP partners with the Illinois Department of Commerce & Economic Opportunity, which manages the Illinois Weatherization Assistance Program (https://dceo.illinois .gov/communityservices/homeweatherization.html), which contracts with local community action agencies and nonprofits to install weatherization improvements in low-income households.

4.2 State

The Illinois Property Assessed Clean Energy Act, 50 ILCS 50/1 et. seq. (“PACE Act”), authorizes governing bodies to create commercial property assessed clean energy (C-PACE) programs. The C-PACE program is an innovative mechanism for allowing commercial property owners in 14 counties of Illinois to obtain up to 100% long-term energy conservation, energy efficiency, and grid resiliency improvements ( The renewable energy improvements include the installation of geothermal energy systems. The property owners access fixed-rate financing from private capital providers for eligible improvements in both existing buildings and new construction. The C-PACE financing terms may extend up to 20–30 years, beyond the useful life of the improvement, and the resulting energy cost savings may exceed the amount of the initial C-PACE financing. Eligible properties include commercial and industrial buildings, multifamily apartment buildings consisting of five or more residential units, and cooperative housing buildings.

The Illinois Energy Conservation Authority NFP (IECA) is an Illinois tax-exempt 501(c)(3) nonprofit corporation formed to bring innovative PACE program administration to Illinois, and it currently administers programs in most of northern Illinois, along with other urban centers across the State. The IECA is uniquely qualified to assist municipal and county governments to successfully establish C-PACE programs to support greater economic development activity, as well as contribute to developing high-performing buildings through the installation of energy efficiency and clean energy technologies.

4.3 County and Municipal

Since 2021, the City of Urbana has run the Geothermal Urbana-Champaign program (https://urbanaillinois. us/geothermal), a public education and bulk purchasing program that offers a more affordable way for Champaign, Piatt, and Vermilion County home and business owners to install geothermal energy systems to provide heating, cooling, and hot water. Through bulk purchasing, the participants are offered a lower rate for installation than would normally be available, and incentive rebates are provided as the program reaches installation targets.

The Village of Oak Park, through the Energy Efficiency Grant Program, provides its very low- and low-income residents up to $10,000 for the installation of geothermal energy systems (

4.4 Utilities

In Illinois, the primary electric utilities, Commonwealth Edison Company (ComEd) and Ameren Illinois, provide several programs for their residential and commercial customers to promote energy efficiently and reduce peak-time usage impacts on the electrical grid. The smaller municipal-owned utilities are seeking similar objectives, but also reward their customers by providing preferred electrical rates to those installing energy-saving technologies.

Ameren Illinois: Residential electric customers who install a new geothermal heat pump may receive an incentive rebate of $600 ( 20821#:~:text=Residential%20electric%20customers%20who%20install,operating%20as%20a%20secondary%20unit). For businesses, Ameren offers an incentive rebate of $500/ton (up to $25,000) for geothermal energy systems ( equipment/heat-pump-systems).

Commonwealth Edison Company (ComEd): ComEd heating and cooling rebates make it easier to invest in efficient heating and cooling equipment for your home and save on your energy costs in the process. For residential geothermal heat pump installations, discounts of up to $6,000/home are available for new geothermal energy systems (including the loop), and discounts of $850 to $1,200 are available for the replacement of geothermal energy systems ( WaysToSave/ForYourHome/Pages/HeatingCoolingRebates.aspx.

For businesses, which include existing facilities, new construction, and replacement of existing electric heating or cooling equipment, or both, ComEd offers a rebate of $30/ton per energy-efficiency ratio (EER) unit above the minimum efficiency. The minimum for a water-to-air geothermal heat pump (cooling mode) is 14.2 EER, and for a water-to-water geothermal heat pump (cooling mode), it is 12.1 EER ( HVACWorksheet.pdf).            

For builders, ComEd offers the Electric Homes New Construction financial incentive, up to $2,000 per home, for the installation of geothermal heat pumps (>22 EER, >4.4 COP) in new construction and for major renovations of single-family homes, duplexes, townhomes, 2–4 flats, and accessory dwelling units to achieve best practice energy efficiency ( ForYourBusiness/Pages/ NewConstructionBusiness.aspx).

The City of Springfield’s City Water, Light, and Power utility offers residential customers a one-time $500/ton rebate on new initial installations of geothermal energy systems that require a new well field ( 1012.pdf). The utility also offers its residential customers who have geothermal heat pumps as their primary heating source the Electric Heat Rate, which provides a 9% savings over the regular residential electric rate between September 16 and May 15.

Carmel Public Utility Company: Residential customers who have permanently installed and are using a heat pump are eligible for the Residential Electric Space Heating Service Rate of 5.9240¢/kWh by paying a monthly fee of $12 (

4.5 Electric Distribution Cooperatives

The Association of Illinois Electric Cooperatives (AIEC), which represents 25 electric distribution cooperatives (Figure 4.1), provides its members with the advantages of a large utility operation at the same time offering a variety of local and tailored services and programs within its member systems. The AIEC is working at the local, state, and national levels to support increased investment in renewable energy and energy conservation. The following geothermal energy incentive programs support these objectives:

Adams Electric Cooperative: Through the EnergyWyse Loan Program, low-interest loans of up to $30,000 are available to install high-efficiency electrical heating and cooling systems (

Coles-Moultrie Electric Cooperative: The cooperative offers an Energy Efficiency and Conservation Rebate to homeowners of up to $250, or no more than 25% of the actual cost of purchasing and installing energy-saving technologies and equipment. Commercial projects may qualify for a rebate of up to $500 (

Clinton County Electric Cooperative: The cooperative offers a Non-Controlled All-Electric Space Conditioning Rate to lower your heating and cooling costs. A monthly credit of $7.50 is applied to members with a geothermal heating and cooling system ( member-benefits-and-programs).

Corn Belt Energy Corporation: Residential members are offered two rebates for geothermal energy systems. A total of $2,000 is available for installing geothermal energy systems in new construction that replaces electric resistance equipment and combusting fossil fuels. In addition, $750 is available for replacing an old geothermal heat pump with a new unit ( For commercial and industrial buildings, schools, and farm or agricultural operations, rebates are through the electricity supplier, Wabash Valley Power Alliance ( A rebate of $500/ton to $750/ton is offered for the purchase of geothermal heat pumps.

EnerStar Electric Cooperative: Through the electricity supplier, Wabash Valley Power Alliance, incentive payments for geothermal energy systems are offered through the Power Moves program. For closed-loop systems in residential applications, a rebate of $2,000 is available for new construction and for systems that replace fossil fuel usage, air source heat pumps, and electric resistance equipment. A $250 rebate is offered for replacing old geothermal heat pumps with new ones ( For open-loop geo-thermal energy systems, the rebate for new installations is $1,000, and the rebate for a new heat pump is the same. The residential incentive payment is $10,000 per year, and the rebate cannot exceed 75% of the total project cost. For businesses, a rebate of $500/ton to $750/ton is offered for the purchase of geothermal heat pumps.

Illinois Electric Cooperative: Through the cooperative’s Energy Efficiency Loan Program, low-interest loans are available for energy efficiency projects, including the installation of geothermal energy systems (

Jo-Carroll Energy: Incentives are provided for upgrading or installing geothermal energy systems. For the replacement of indoor equipment, $500/ton is offered, whereas the construction of a new borefield and installation of geothermal heat pumps garners $700/ton ( A geothermal group buyer incentives program will also be offered in late 2022 or early 2023.

MJM Electric Cooperative: Through the cooperative’s energy supplier, Wabash Valley Power Alliance, members with geothermal energy systems receive incentive payments through the Power Moves program. For residential closed-loop geothermal energy systems, a rebate of $2,000 is offered. This incentive is for new construction and to replace fossil fuel usage, air source heat pumps, and electric resistance equipment. A $250 rebate is offered for replacing old geothermal heat pumps ( For open-loop geothermal energy systems, the rebate for new installations is $1,000, and the rebate for a new geothermal heat pump is the same as open-loop systems. The residential incentive payment is $10,000 per year, and the rebate cannot exceed 75% of the total project cost. For businesses, a rebate of $500/ton to $750/ton is offered for the purchase of new geothermal heat pumps.

Menard Electric Cooperative: The cooperative promotes the efficient use of electricity and the wise use of energy by offering a $300 rebate on the purchase of a geothermal heat pump ( Members may also qualify for other programs that have a reduced cost per kilowatt hour for winter and summer rates.

Monroe County Electric Cooperative, Incorporated: A rebate of $1,000 is provided to members who install a geothermal system (minimum of 3-ton capacity) and qualify for an “all-electric” account, which has a minimum requirement that electricity be the main source of heat and that the system have an electric water heater (minimum of 50 gallons) (

Rural Electric Convenience Cooperative: The cooperative has always encouraged the wise use of electricity and promotes energy efficiency. To support its members, a $250 rebate is offered for the purchase of a geothermal heat pump (

Tri-County Electric Cooperative, Incorporated: The cooperative provides loans for the purpose of financing the purchase and installation of geothermal heat pumps. A low-interest loan of up to $13,000 is available (

Wayne-White Counties Electric Cooperative: An incentive rebate of $1,500 is available to residential customers who install geothermal in their home or replace electric resistance heating, a gas furnace, or a boiler (

Western Illinois Electrical Cooperative: The cooperative offers a $1,000 bill credit for the installation of a new geothermal energy system and a $500 bill credit for replacing a geothermal energy system ( Low-interest Energy Resource Conservation (ERC) loans of up to $17,500 are also available for installing geothermal energy systems ( For $6/month, members can be delivered electricity at a reduced rate (9¢/kWh) if they are running geothermal heat pumps (

Figure 4.1. Electric cooperatives in Illinois. Figure reproduced courtesy of the Association of Illinois Electric Cooperatives.
4.6 Nongovernmental Organizations

The Illinois Clean Energy Community Foundation ( is an independent foundation with a $225 million endowment provided by Commonwealth Edison. The Foundation’s mission is to improve energy efficiency and advance the development and use of renewable energy resources. Although the foundation does not fund the installation of traditional geothermal energy systems, projects that include innovative borefield designs, emerging geothermal heat pump technologies, underground thermal energy storage, or are part of net zero energy buildings may be considered for funding. Eligible applicants include 501(c)(3) nonprofit organizations, local government agencies serving Illinois residents, public schools that provide continuous educational services to K-12 students, and colleges and universities.

4.7 Research and Innovation Grants and Programs

The primary funder of geothermal energy research in Illinois has been the U.S. DOE through the Geothermal Technologies Office ( geothermal-technologies-office), Building Technologies Office ( building-technologies-office), and Office of Fossil Energy and Carbon Management (https://www. Grants are awarded to researchers or research teams on a competitive basis for projects proposed in response to specific funding opportunity announcements. Funding is also provided by the U.S. DOE to qualified small businesses and research collaborators through the Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) programs ( The U.S. Department of Defense and the National Science Foundation have also funded geothermal research in Illinois.

5. Applications of Geothermal Energy Systems in Illinois

According to the EIA’s Illinois State Profile and Energy Estimates (EIA, 2020b), Illinois is the fifth-largest energy-consuming state in the country. Figure 5.1 shows the total energy consumed (percentage) by end users in Illinois’ residential, commercial, industrial, and transportation sectors. The aforementioned sectors are listed in decreasing order of total energy usage, with the industrial sector having the highest consumption of 31.3% and the commercial sector being the lowest at 20.5%. Of this, U.S. households require energy to power numerous home devices and various types of equipment, but on average, more than half (51% in 2015) of a household’s annual energy consumption is for just two purposes: space heating and air conditioning (EIA, 2015). These mostly seasonal and energy-intensive uses vary significantly by geographic location, home size and structure, and equipment and fuels used.

If we look more broadly at the U.S. energy data, according to the U.S. DOE, heating and cooling accounts for more than 25% of the total energy used and is responsible for 39% of the carbon dioxide emissions from buildings. In Illinois, Cook County overall has the largest thermal energy demand (Figure 5.2) and leads all other states in thermal energy demand in the residential and commercial sectors (McCabe et al., 2016). Nationally, approximately 80% (25.4 quads) of this energy is for end uses that require temperatures less than 300°F (150°C), specifically space and water heating, process heating, and refrigeration and cooling (Figure 5.3). Illinois’ low-temperature 60°F–300°F (15°C –150°C) geothermal energy resources could supply much of this energy demand in the state. The U.S. DOE provides an overview of geothermal energy applications for electric and non-electric energy production (Figure 5.4). Some applications can utilize the lower ground temperatures 86°F (<30°C) encountered near the ground surface, whereas other applications require higher temperatures that can only be obtained by drilling thousands of feet. The specific use depends on the temperature resource. Geothermal resources with the highest temperatures (300°F or 150°C and greater) are generally used to produce electricity. In Illinois, the range in temperatures of natural geothermal resources that can economically be accessed is between 50°F and 140°F (10°C and 60°C).

The following subsections introduce the application of geothermal energy systems in the four sectors, which currently or may in the future benefit from the use of geothermal energy. The impacts of using geothermal energy are discussed, along with an analysis of the possible benefits, based on the available data. Finally, each subsection concludes with a short discussion of the potential opportunities for increasing the usage of geothermal energy systems in each sector.

Figure 5.1. Illinois energy consumption by end-use sector, 2020 (EIA, 2020b). Figure reproduced courtesy of the U.S. Energy Information Administration.
Figure 5.2. Illinois thermal energy demand, by county (EIA, 2009, 2010, 2012). Figure reproduced courtesy of the U.S. Energy Information Administration.
Figure 5.3. Illinois residential energy consumption by end use (EIA, 2009). Figure recreated courtesy of the U.S. Energy Information Administration.
Figure 5.4. Range of applications of geothermal fluid as a function of temperature (U.S. DOE, 2019). Figure reproduced courtesy of the U.S. Department of Energy.
5.1 Residential Sector

Illinois households use 129 million Btu of energy per home, 44% more than the U.S. average (EIA, 2009, Residential Energy Consumption Survey, Furthermore, 51% of this energy was reported as used for space heating; 31% was used for appliances, electronics, and lighting; 16% was used for water heating; and 2% was used for air conditioning, as shown in Figure 5.2. Of the amount of energy consumed for space heating, more than 80% of Illinois residents use natural gas (EIA, 2021). Geothermal energy could be used to offset electrical, natural gas, or liquid propane gas (LPG) use and the costs for residential space heating, air conditioning, and water heating by using GHPs or geothermal water heaters.

Although data for the current use of geothermal energy in Illinois’ residential sector is not available, of the 129 million Btu of energy consumed by each housing unit, 69% is used for heating and cooling applications (space heating, air conditioning, and water heating), and the total annual heating and cooling load per household in 2009 was approximately 89 million Btu. Subsequently, with 4.8 million households reported in 2009, the total heating and cooling load that could be served by geothermal energy systems is equal to approximately 0.423 quadrillion Btu, or 34.248 billion tons equivalent. It should also be noted that 2020 data for Illinois residential energy consumption was estimated as 0.940 quadrillion Btu (EIA, 2020a, Table C11), which is more than double the amount reported in 2009.

Equipment manufacturing statistics can also be used as another indicator for the opportunity to utilize a geothermal technology. As published in the Geothermal Heat Pump Manufacturing Activities 2009 report (EIA, 2010a), domestic shipments of geothermal heat pumps to the residential sector accounted for 172,559 tons of capacity, or almost 51% of the entire domestic geothermal heat pump market. According to the 2009 total energy consumption, the total capacity delivered was less than 0.0001% of the potential capacity for geothermal heating and cooling in the residential sector of Illinois alone. Geothermal heat pump shipments across all sectors totaled 338,689 tons of capacity, which included shipments to all 50 states and the District of Columbia. Total shipments to Illinois were 26,599 tons of capacity in 2008 and 18,795 tons of capacity in 2009 (EIA, 2010a).

The expected benefits of installing geothermal technologies in residential and the other sectors are widely recognized. Here we present several international studies documenting the benefits. First, Rivoire et al. (2018) performed numerical simulations with the Transient System Simulation Tool (TRNSYS) modeling software to assess the energy efficiency, economic, and environmental benefits of geothermal heat pumps used in homes, small offices, and hotels that either had poor or good insulation in six European cities with varying climatic conditions. Their results indicated that the use of geothermal heat pumps would have a highly positive environmental impact. As for the economic performance, payments on the principal during the payback period would be higher than for other renewable energy systems but could be reduced by either increasing the tax on gas and electricity usage or by implementing economic incentives and progressive policies. Yousefi et al. (2017) assessed the impacts of installing geothermal heat pumps on reducing air pollution based on the projected number of installs in residential buildings of Tehran in the subsequent 5 years. The study showed an overall reduction in CO2, sulfur oxide (SOx), and nitrogen oxide (NOx) emissions, which was attributed to the reduction in the consumption of natural gas. Finally, Han et al. (2021) monitored a geothermal energy system for a residential building to determine its energy performance and economic liability. The study showed the temperature and humidity in the building remained consistently within the human comfort index and the system would offset 1,131 tons of coal burned, which would have resulted in 4,641 tons of CO2 emissions. The geothermal energy system was also highly efficient, which led to large cost savings.

5.2 Commerical Sector

According to the EIA’s Illinois State Profile and Energy Estimates (EIA, 2020b), commercial sector energy consumption in Illinois is equal to 20.5% of the overall energy consumption in the state (3.612.9 quadrillion Btus), or approximately 0.7398 quadrillion Btus. The energy is consumed in buildings owned by businesses (offices, warehouses, and shopping centers); federal, state, and local governments; other private and public organizations (e.g., churches, medical clinics and hospitals, and stadiums); utilities; and combined heat and power (CHP) operators (NREL, 1999). Warehouse and storage, office, and service buildings together account for 48% of all commercial buildings and 42% of total commercial building floorspace (EIA, 2018).

The first geothermal energy system installed in a commercial building in the United States was at the Commonwealth Building in Portland, Oregon, in 1946 (Hatten & Morrison, 1995). Since the 1950s, widespread acceptance of the technology by architects, engineering firms, developers, and building owners and operators has been hampered, in part, by the lack of published studies on geothermal energy systems in commercial buildings that include information on maintenance and operation histories, equipment replacement requirements, life-cycle costs, and long-term system reliability (Bloomquist, 2000). The most cited study is the recent performance comparison between a variable refrigerant flow system and a geothermal energy system at the ASHRAE headquarters (Spitler et al., 2014). They found for both heating and cooling that the geothermal energy system offered better operational efficiencies because heat was rejected or extracted from the ground rather than through the air. From the completed studies, the following benefits of geothermal energy systems were found when they were compared with conventional heating and cooling systems in commercial buildings (NREL, 1999). A geothermal energy system:

Has the lowest maintenance and life-cycle costs;

Delivers better indoor environmental conditions; is able to have individual temperature controls and is quieter than other heating and cooling systems;

Offers design flexibility in new construction and retrofit applications;

Reduces energy costs by 20% to 40%;

Has a smaller footprint for mechanical equipment;

Eliminates the risk of vandalism because equipment is located inside buildings;

Can be used in historic buildings to conceal heating, ventilation, and air conditioning (HVAC) equipment;

Eliminates the need for rooftop equipment; and

Has a longer life span (30–50 years) than systems with furnaces, boilers, chillers, cooling towers, and steam.

Often, the initial cost of a geothermal energy system is competitive with conventional heating and cooling systems, and the typical payback period is 5 years or less.

In Illinois, most commercial geothermal energy systems are in the City of Chicago, installed in affordable housing communities, medical buildings, park district buildings, libraries, and commercial buildings (Kearney, 2016). To promote the wider adoption of commercial geothermal energy systems in Metropolitan Chicago, Commonwealth Edison offered a pilot incentive program between 2018 and 2020 ( The aim was to reduce up-front costs of installation, making them more affordable. The program targeted systems with 10–50 or more tons of heating and cooling capacity, particularly in commercial, light industrial, and public sector buildings. Following the pilot, a standard prescriptive incentive for commercial geothermal installations was added to their Energy Efficiency Program portfolio.

5.3 Industrial Sector

According to the Energy Information Administration’s Illinois State Profile and Energy Estimates (EIA, 2020b), Illinois commercial sector energy consumption is equal to 31.3% of the overall energy consumption in the state (3.6129 quadrillion Btus), or approximately 1.1306 quadrillion Btus. In Illinois, chemical machinery and food and beverage manufacturing contribute the most to the state’s manufacturing gross domestic product, according to the U.S. Bureau of Economic Analysis (U.S. BEA, 2020). Geothermal energy is one of the pathways to transition these industries to a sustainable energy future with clean, affordable energy. Palomo et al. (2022) presented a study in Europe, particularly in Spain, in which they highlighted the role of geothermal energy in the industrial sector. Their results showed that almost 85% of the industrial processes in all the industries in Spain could be supplied by very low temperature to low-medium temperature geothermal resources, offsetting 80 million tons of CO2 per year.

Industrial processes also contribute to energy losses through heat streams released into the environment at different temperatures. Many researchers have investigated waste heat recovery. Christodoulides et al. (2022) proposed geothermal heat pipes as one technology to tackle that challenge. Guelpa and Verda (2019) discussed implementing thermal energy storage for district heating and cooling. They investigated both short- and long-term storage and conducted a performance analysis that yielded encouraging results. Gadd and Werner (2021) likewise discussed the use of thermal energy storage within contemporary district heating and cooling systems. Their results showed that this technology could be implemented for daily, weekly, or seasonal storage, depending on the need based on the expected heat load variation. Sáez Blázquez et al. (2018) performed an economic and environmental analysis of various geothermal district heating systems. They compared three alternatives: a geothermal district heating system, a geothermal system coupled with existing natural gas heaters, and a geothermal system coupled with natural gas boilers. They found the geothermal district heating system to be the ideal solution from both an environmental and an economic standpoint. The industrial sector in Illinois could clearly benefit from the increased use or adoption of geothermal energy systems.

5.4 Manufacturing Sector

Illinois encompasses significant expertise in food processing. The state employs more than 78,000 people in the industry and ranks third nationally, with a gross rating product of $12.3 billion, as reported by Intersect Illinois ( Food processing facilities need energy mainly for heating, cooling, drying, and to maintain specific temperatures. Geothermal energy systems have a promising future in this sector. Sovacool et al. (2021) presented a comprehensive overview of greenhouse gas emissions from the food and beverage industry, from agriculture to manufacturing, distribution, consumption, and use. As one of the emerging technologies, geothermal energy systems can be applied to decarbonize the food and beverage industry and address the needs to manage thermal energy and heat recovery, access renewable sources of energy, and make the processing more energy efficient. Tuncer et al. (2018) also emphasized the need for a sustainable energy system for food processing. They stressed that when renewable energy, such as geothermal energy, is used in hybrid systems and waste heat recovery applications, the per unit energy consumption significantly decreases. Additional energy efficiency benefits are found when coupling and layering the food and beverage processing system with the energy system.

Kinney et al. (2019) addressed the potential of geothermal energy to provide a base-load source of energy that could support food production, consequently mitigating the impacts of food insecurity on communities, particularly in cold areas. Their focus was on evaluating the technical and economic feasibility of producing vegetables in a controlled environment. Their results showed that the system could supply the needed heat for the greenhouse application, leading to a robust, reliable, cost-competitive, and sustainable energy source, thus improving food security and community empowerment. In another project, Thorarinsdottir et al. (2021) showcased the GeoFood Project, which utilizes geothermal energy to improve sustainability in food production. The project coupled different food processes together so that the effluent from one production is used as a resource for the succeeding one, thus reducing the environmental impact. Furthermore, Crespo et al. (2019) discussed the great potential of integrating underground thermal energy storage with photovoltaic systems to optimize energy usage in the chemical, food, and brewing industries.

5.5 Agriculture

Illinois is considered a leading producer of swine, soybeans, and corn, along with many other agricultural commodities (e.g., cattle, wheat, oats, hay, sheep, and poultry). Cold winters and warm, dry summers with ample rainfall allow the land to support many kinds of livestock and crops. The state’s grain crops and livestock are then processed into food and industrial products by 26,940 food manufacturing companies (EIA, 2021). Geothermal systems have many applications in the agricultural sector that can substitute for conventional practices more efficiently and at a lower energy cost. Słyś et al. (2020) recommended harvesting waste heat from wastewater on livestock farms to achieve financial savings and decrease air pollution. Geothermal heat pumps are favorable for this application because they can alternate between heating and cooling. Their results proved that geothermal heat pumps could efficiently collect heat from manure on farms.

Alberti et al. (2018) proposed the use of geothermal heat pumps in the agrozootechnical sector to address the need to maintain an adequate indoor thermal environment and air quality to preserve a healthy environment, increase productivity for animal breeding, and reduce the intensive use of fertilizers that contaminate surface water and migrate to the groundwater. Two systems were proposed. The first was a GHP system for efficient heating and ventilation of a piglet stable, and the second was a groundwater heat pump coupled with an irrigation system to promote the reuse of fertilizers by recirculating the groundwater. Both systems showed promise.

Mun et al. (2022) compared the heating effects on pig production of using a GHSP versus traditional system of heating with electrical heat lamps. They monitored the growth performance of piglets, the stable temperature, greenhouse gas emissions (e.g., CO2 and noxious gas emissions such as NH3 and hydrogen sulfide), and energy savings. Their results showed no adverse effects on piglet growth, an increase in the indoor temperature, a significant decrease in greenhouse gas emissions, and a decrease in electricity consumption. Thus, the geothermal heat pump provided an eco-friendly energy source for swine production.

Brzozowska et al. (2017) proposed using lakes as a heat source while limiting the impact on the functioning of the lake ecosystem. Lakes are attractive because of their limited temperature variation during the year. Their results showed that this system could be implemented for heating, ventilation, and the heating of substrates for vegetable production.

Strpić et al. (2020) studied a borehole heat exchanger with a double circuit in cow barns to enhance milk cooling and water heating and storage. A dynamic simulation showed the potential of the system compared with existing applications. Similarly, Brush et al. (2011) showed the economic and environmental advantages of using geothermal heat pumps that reduced fossil fuel consumption in dairy operations in the United States.

Geothermal heat pumps can also operate as dehumidifiers and increase the drying efficiency of the system. Kumoro and Kristanto (2003) presented a preliminary study on the utilization of geothermal energy for drying agricultural products. They showed that geothermal steam could be applied, and that the drying performance could be enhanced by increasing the steam flow rate. Similarly, Qu et al. (2021) presented a comprehensive overview of food-drying techniques that use renewable energy sources. They found that coupling thermal energy storage with geothermal heat pumps made the system more efficient and produced higher quality food.

5.6 Educational Institutions

At many elementary and high schools and on university and college campuses, renewable energy systems are being installed as a cost savings and to improve the teaching environment. Switching from fossil fuel heating and air conditioning to renewable energy systems (e.g., geothermal energy) provides healthier environments for students both inside and outside of the classroom that have been shown to improve academic performance (Allen et al., 2016). Using geothermal systems allows for continuous control of the indoor temperature and humidity, which is important for students’ ability to concentrate, and it regulates ventilation, which improves the air quality and prevents the spread of illness (Deng et al., 2018). The ability to install geothermal systems in basements or mechanical closets reduces unnecessary noise in the classroom and frees up space for other uses.

School districts and educational campuses in Illinois have been very proactive in reducing their energy costs and improving air quality through the installation of geothermal energy systems. For example, the McLean County Unit 5 school district has installed geothermal heat pumps in 15 schools as of 2021 (Heart of Illinois ABC, 2020; Watznauer, 2021), which has significantly reduced operating costs. At one school, Parkside Junior High, the district has reduced usage of natural gas and electricity by 98% and 68%, respectively, resulting in a $15,000/month cost savings and a reduction in building utility costs from $3.02/square foot to $1.15/square foot.

For colleges and universities in Illinois, geothermal energy systems are being considered as part of an aggressive strategy to meet campus sustainability objectives and significantly reduce their carbon footprints. These institutions are also finding that the use of geothermal energy has significant economic advantages, provides new educational opportunities, supports energy security by enhancing operational resiliency, and creates jobs in the “green” renewable energy industry (Stumpf et al., 2020). For example, at Loyola University in Chicago, a geothermal energy system was constructed to heat and cool a mixed-use space including student residences, state-of-the-art classrooms and laboratory space, and an urban agricultural greenhouse for the Institute for Environmental Sustainability, now the School of Environmental Sustainability (Roewe, 2020). The geothermal energy system has not only reduced heating and cooling costs by 30% but is a focal point for the university’s sustainability research, demonstrating how forward-thinking and innovative technologies make a more sustainable campus and community.

On educational campuses, geothermal energy systems serve individual buildings as stand-alone sources of heating and cooling or are utilized to serve several buildings in district heating and cooling configurations (Cross et al., 2011). Further, college campuses can be ideal for geothermal applications because they often have existing district energy systems, the space required for drilling boreholes into the Earth’s surface, a climate action plan, and the time horizon needed for a return on investment that may take 15 to 20 years (Jossi, 2022). Examples of the use of geothermal technology serving individual Illinois educational facilities can be found at the University of Illinois Urbana-Champaign (see Section 6), the University of Illinois Chicago ( and Southern Illinois University Carbondale ( There are no known district heating and cooling applications of geothermal technology in Illinois educational institutions. However, prominent examples of educational institutions outside of Illinois that have or will utilize geothermal district heating and cooling systems include Ball State University in Indiana (Lowe et al., 2010), Epic Systems in Wisconsin (Massey, 2018) Carleton College in Minnesota (Larson, 2022), Microsoft Corporation in Washington State (Berg, 2021), Google in California (Bergen, 2021), Cornell University in New York (Tester et al., 2020), and Princeton University in New Jersey (Bonette, 2022).

5.7 Military Installations

Illinois is home to a number of military installations, including the Illinois National Guard units, the Illinois Reserve units, and major Department of Defense (DoD) installations. The Illinois National Guard is made up of the Illinois Army National Guard and the Illinois Air National Guard units. According to the Illinois National Guard (, there are about 50 Illinois National Guard stations across the state, which include five Army Guard Brigades and four Air Guard Wings. The Illinois Reserves include the Illinois Army Reserve and Illinois Air Force Reserve Command units. According to the U.S. Army Reserve ( Ambassador-Program/Find-anAmbassador/ Illinois/) and Air Force Reserve Command ( Units-by-State), there are 23 Illinois Army Reserve facilities and one Air Force Reserve Wing in Illinois. Additionally, there are three major DoD installations in the State of Illinois ( the Rock Island Arsenal, the Great Lakes Naval Station, and Scott Air Force Base.

For the Guard and Reserve installations, many of these facilities are armories or reserve centers that are used as training centers and can include classrooms, storage, and maintenance areas (Hewitt, 2017). Some of the Guard and Reserve installations designated as Brigades or Wings consist of many facilities that all support a specialized mission, such as a Headquarters mission (with command and control and other supporting facilities) or a mission supporting aircraft (with fueling, runway, maintenance, and other facilities). In contrast, major DoD installations may contain hundreds if not thousands of facilities and, depending on the particular mission served, may resemble a large university campus or a small city. One commonality is that all these military installations require heating and cooling for the various facilities at their locations.

Some military installations utilize geothermal systems for heating and cooling at individual facilities (U.S. Army, 2013; Hammock & Sullens, 2017), but the majority do not. Although geothermal systems can have a cost-effective life cycle in many applications, the relatively high first costs associated with drilling and installation of underground infrastructure are often impediments to utilizing geothermal systems for these projects. On the other hand, centralized (district) configurations of geothermal systems can provide economies of scale to larger numbers of facilities that are in relatively close proximity to each other and may represent an opportunity for some of the larger military installation sites (Federal Energy Management Program [FEMP], 2010; Robins et al., 2021).

Utilization of geothermal systems at military facilities can have advantages beyond economics, especially in terms of resilience. The DoD defines resilience as the “Ability to anticipate, prepare for, and adapt to changing conditions and withstand, respond to, and recover rapidly from disruptions” (Office of the Under Secretary of Defense for Acquisition and Sustainment [OSD], 2018). The DoD also defines the requirements (metrics) for energy resilience as follows: “Planning and programming for energy resilience and energy security shall provide for a minimum of 14 days of energy disruption, unless otherwise prescribed by the military department or other departmental guidance” (Office of the Undersecretary of Defense [OUD], 2021). In 2017, the State of Texas implemented a geothermal heating and cooling system at one of its National Guard Readiness Centers, which was designed to substantially reduce demand and allow operation off the grid in case of an emergency (Edwards & Veracruz, 2019). With all the military installations located in Illinois, ample opportunities exist to utilize geothermal heating and cooling at these facilities and, in turn, contribute to meeting DoD resilience requirements at the facilities.

5.8 Transportation Infrastructure

Winter in Illinois is snowy, with ice covering the roads. Conventional ice removal systems, such as rock salt and deicing chemicals, can cause environmental damage, put the safety of motorists at risk, delay travel, and incur a financial burden. Moreover, according to the Illinois Department of Transportation, ice removal requires an average of 500,000 tons of rock salt per year (Panno et al., 2005). Calcium chloride (CaCl2) is commonly used in Illinois and is applied as a brine either directly on bridge decks or to prewet rock salt for pavement application. Its usage should be reserved for periods of cold temperatures, chiefly <40°F (<5°C), because of its detrimental effect on concrete (Gombeda et al., 2022). For both pavements and bridge decks, ice removal systems during winter that involve the application of chemicals also accelerate corrosion of the steel reinforcing bars on the bridge deck. Conventional deicing applications of such high-salinity materials also result in environmental contamination and pollution (Nahvi et al., 2018; Kelly et al., 2012). In addition, rapid fluctuations in day to nighttime temperatures from above to below freezing in early winter and spring drive an aggressive freeze–thaw cycle that can further degrade the pavement.

As an alternative to treating roadways, testing is underway to use innovative geothermal systems that can be used for snow melting (1) through the installation of direct heating in pipes, (2) through heat from well water that is circulated in pipes, (3) by using a heat exchanger at the well head to condition circulating water, or (4) by allowing warm water to flow across pavement (Lund, 2005). Porakbar et al. (2021) investigated the feasibility of using a ground-coupled system that employs heat collected from the ground to deice bridges and culverts. In North Texas, Habibzadeh-Bigdarvish et al. (2019) performed a life-cycle cost–benefit analysis of a geothermal heat pump system to deice a bridge deck. Results showed that the benefits of the system balanced the high costs incurred for installation and that beyond a threshold traffic volume, the benefits became greater than the costs. Balbay and Esen (2010) proposed using a GHP system consisting of a single vertical U-type borehole heat exchanger for snow melting on pavements and bridge decks. Different geothermal loop lengths were investigated, and the results showed that the COP increased with deeper drilling. In a similar study, Lai et al. (2015) suggested a snow-melting heated pavement system. Their results showed promise for further applications. Han and Yu (2018) presented an innovative technology using energy piles to expand the viability of snow melting on geothermal bridge decks. Their technology involved mixing the concrete with phase-change material within the pile to enhance thermal energy extraction. Different mass fractions of phase-change materials were simulated, and the results showed an overall increase in the performance of the system.

Techno-economic and cost–benefit analyses have been done to compare heating of roads with geothermal energy versus electrical heating and the baseline approach of applying snow-melting chemicals. Habibzadeh-Bigdarvish et al. (2019) found the monetary benefits (excluding the environmental improvements) of using geothermal energy were 2.6 times greater than the overall costs of applying road salt and deicing chemicals. The return on investment of the geothermal system is achieved within 25 years. Liu et al. (2021) evaluated the cost–benefit of a geothermal energy system and electrical based snow melting system and found the geothermal energy system cost $22,000 CDN to run annually versus $85,000 CDN. In addition, Shen et al. (2016) performed a life-cycle assessment comparing a traditional snow and ice removal system with heated pavement system using geothermal. The former required more energy and produced more greenhouse gas (GHG) emissions.

5.9 Aquaculture

Geothermal energy systems are currently being used in 21 countries for aquaculture and raceway pond heating (International Renewable Energy Agency [IRENA], 2019). At these sites either fresh or brackish water is being heated with heat exchangers or heated water is mixed with freshwater to obtain suitable temperatures of 68°F–104°F (20°C–40°C) for fish farming. A similar application is being used to grow algae (mainly spirulina; Godlewska et al., 2015), which requires temperatures between 95°F and 99°F, or 35°C and 37°C (IRENA, 2019). Using geothermal energy systems can improve fish productivity, especially when aquaculture is combined with hydroponics, whereby nutrient-rich aquaculture water is used to grow plants (Turnšek et al., 2021).

In Illinois, using geothermal energy systems could benefit aquaculture operations, an industry that in 2015 contributed about $3 million to the state’s economy (Hitchens, 2018). The main fish species farmed included bass, tilapia, catfish, prawn, trout, and paddlefish. Although still at the pilot scale, the cultivation of algae for biofuel production and animal feed shows great promise, especially when using captured CO2 (Schideman et al., 2019). Algae can effectively use high CO2 concentrations and grow at faster rates, which shortens the harvesting cycle and supports an order of magnitude production per land area compared with traditional biofuel crops.

Although geothermal technologies have not yet been developed for aquaculture in Illinois, we can learn much from the applications installed internationally. John and Jalilinasrabady (2021) describe a hybrid geothermal energy and solar energy system developed for aquaculture in Kenya. The proximity of the geothermal system to the resource makes this coupled system economically feasible. Kuska et al. (2020) evaluated the feasibility of using geothermal energy to regulate the temperature for raceways used in the aquaculture industry. A numerical model was developed to simulate the heat transfer of the systems as well as the interaction between components. They determined the water temperature, flow rate, and number of boreholes and spacing were the most important factors. Finally, Omenikolo et al. (2020) explored the use of geothermal energy to heat water for aquaculture applications, rather than relying solar energy. They found results using geothermal energy led to increased fish production because the water temperature remained nearly constant, whereas the water temperature in ponds heated by solar energy fluctuated more.

6. Barriers to the Deployment of Geothermal Energy in Illinois

To realize the wider development and deployment of low-temperature geothermal energy technologies in Illinois and the U.S. Midwest, as discussed in the U.S. DOE’s GeoVision report (U.S. DOE, 2019a), members of the geothermal energy sector must overcome significant technical and nontechnical barriers that involve scientific and engineering knowledge gaps, policy or market factors, and social preferences. Although the existing geothermal energy technologies are considered technologically mature (including GHPs and district energy systems), more education and marketing are still needed to inform potential stakeholders (individuals, companies, policy makers, and regulators) that geothermal energy is a renewable, cost-competitive, low-carbon energy resource that is important for securing sustainable and resilient baseload energy (Ball, 2020b). Unfortunately, some potential stakeholders view geothermal energy technologies as immature and high risk. In addition, the U.S. geothermal sector has been biased toward focusing primarily on electricity generation (Kahan, 2019).

Above and beyond all the barriers identified is the high initial up-front capital costs compared with other renewable energy technologies (i.e., solar and wind energy and air-source heat pumps). These up-front costs, including drilling, piping, grout, GHP, and installation activities, contribute to the cost difference (Beckers, 2016; Beckers & Young, 2016; Hughes, 2008; Lu et al. 2017). Studies have shown that up-front and operational costs are sensitive to prevailing climatic conditions, ground thermal conductivity, electricity rates, and set-point temperatures (Aditya et al., 2020; Tan and Fathollazadeh, 2021; Yousefi et al., 2018). The cost difference is also linked to economies of scale and difficulties in making retrofits to homes and businesses, which increases the cost and extends the time to break even (Levine et al., 2007). However, if these barriers were addressed, closed-loop geothermal energy technologies could be expanded to 80% of the United States (U.S. DOE, 2019a). The U.S. Global Change Research Program (Jay et al., 2018) suggests that without moving away from the present “business as usual” model to allow the uptake of geothermal energy technologies, the U.S. economy risks forfeiting additional financial benefits in the energy transition that could lead to a 10% reduction in the gross domestic product by 2100. Rather, the emerging innovations in the geothermal energy sector could enable larger, scalable, closed-loop conduction and low-temperature heating and cooling projects that harvest direct heat or electricity and develop new streams of income (Ball, 2020b).

6.1 Technical Barriers

Much like the geothermal power sector, the adoption of non-electric-generating geothermal energy technologies (e.g., GHPs) is affected by several technical barriers. The U.S. DOE (2019b) suggests these challenges are less technically complex than conventional high-temperature geothermal energy technologies. The slow advancement of new technologies or methods that would increase the drilling efficiency and energy system performance and would reduce upfront costs in a significant way is holding back wider adoption. The cost of drilling still averages ~50% of the total cost, although companies such as Dandelion Energy ( and Hydra S.R.L. SEDE Hydraulic Tools ( are working to lower installation costs.

Having a well-calibrated understanding of the subsurface is imperative in effectively and efficiently utilizing geothermal resources. Characterization of the geology, material properties, ground temperature, groundwater flow, and thermal energy transport processes allows for prediction of how the subsurface will respond when operating geothermal energy systems. The perceived gap between industry practices and research efforts has been noted in the literature (e.g., Schincariol & Raymond, 2021). This incongruity has delayed technological advancements as some in the industry continue to present an image of the underground with generic, fixed analytical codes that consider uniform ground conditions, which, under certain conditions, can lead to overdesigning a low-temperature geothermal system. This issue could be addressed by involving geothermal industry members in leading demonstration projects and developing programs to collect public data, including temperature, thermophysical properties, and groundwater flow distributions in the subsurface. In addition, those in the geothermal industry should publicize how they plan to mitigate environmental issues (Elliott, 2013) and what the impacts are on drinking water supplies, particularly in closed-loop geothermal energy systems. These interactions could be demonstrated by how geothermal energy might be integrated into food production and the significance of that result (Ball, 2020a).

To better engage potential stakeholders and end users, those in the geothermal energy sector should provide more case studies that illustrate the economic and environmental benefits that can be attained (Ball, 2020a). Performance metrics addressing the levelized cost of energy (LCOE) and GHG emissions via life cycle analyses and building energy modeling would provide a knowledge base that could emphasize potential energy savings and the positive impacts on grid stability and societal livability. Additional metrics, such as energy density or kilowatts per acre, could be used to advertise the limited land footprint required, which, in these terms, makes geothermal energy the most efficient renewable resource compared with solar and wind energy systems.

It is worth noting that the impacts of using fossil fuels are not reflected in life cycle costs and do not consider the true value of all subsidies provided to the oil and gas industry that have subsequent impacts on CO2 emissions. Several countries, such as Italy, China, Canada, Mexico, Finland, New Zealand, Ireland, Switzerland, and Sweden, have considered implementing or have implemented carbon taxes or credits that are supposed to create a revenue stream to fund mitigation strategies to offset the impacts (Belausteguigoitia et al., 2022; Bowley and Evins, 2021; Haites et al., 2018; Kohlscheen et al., 2021; Martelli et al., 2020; Olsen et al., 2018; Zhou et al., 2019).

6.2 Policy and Economic Barriers

In the United States, the expansion of the geothermal energy sector has been hampered to some extent by the long-standing tradition of supporting fossil fuel production and the policy bias that has supported the growth of solar and wind energy and the conversion of natural gas to electricity (Ball, 2020a). In the past, energy systems that operated on natural gas and fossil fuels were less expensive than the proposed geothermal systems. In addition, the lack of long-term regulations and incentives to reduce CO2 emissions plays a role in limiting the development of geothermal energy and underground thermal storage systems.

With the enactment of the Inflation Reduction Act of 2022 and the Infrastructure Investment and Jobs Act, incentives for the geothermal energy sector have been extended and improved. The previous tax credits for residential and commercial projects were to sunset in 2023 and were restricted to taxpayers. It is hoped the Inflation Reduction Act will provide the stimulus needed to bring geothermal on par with solar and wind energy. Extending the tax credits out to 2034 and introducing a new direct payment option allows residents of multifamily dwellings, government agencies, and nonprofit organizations, which were not previously eligible to claim the credit, to participate. It also addressed complex building ownership, which had been an impediment (Ball, 2020a). It is predicted that within the next decade, closed-loop geothermal energy systems and district geothermal energy heating and cooling technologies will become competitive with other renewable energy sources (U.S. DOE, 2019a, 2019b).

Uncertainty in policy combined with a lack of well-defined pathways to decarbonization and electrification of the building sector have been cited as impediments to the widespread adoption of geothermal energy technologies, particularly for deep direct-use and underground thermal energy storage (e.g., Alkhwildi et al., 2020; Nyborg & Røpke, 2015). The sector generally lacks consistent rules, regulations, and procedures for developing potential resources and completing deep geothermal wells to develop standards of practice. The GHP industry in the United States is farther ahead, as standards have been developed for the design and installation of GHP systems (ASHRAE, 2019; American National Standards Institute/Canadian Standards Association [ANSI/CSA/IGSHPA], 2016). In addition, the U.S. Midwest, particularly Illinois, faces a shortage of qualified geothermal drillers to install GHP systems, which potentially slows the rate of expansion and further increases drilling costs by approximately 30%.

The development of deep direct-use and district or community geothermal energy systems in the United States faces additional challenges because business models and technology uptake pathways have not been developed. Studies of consumer behaviors may help quantify the market potential for such systems in the United States (U.S. DOE, 2019a). Grid-scale and demand-side modeling, demonstration projects, and data collection and analysis will illustrate how adopting geothermal energy technologies at a larger scale is integral to an economy-wide reduction in GHG emissions that delivers economic and environmental justice in this energy transition.

6.3 Knowledge Gaps

The public’s lack of knowledge about and awareness of geothermal energy technologies and underground thermal energy storage is the most important factor hindering wider adoption. In several surveys, most stakeholders, investors, decision makers, and members of the general public did not fully understand what geothermal energy is and that it has economic and environmental benefits (Innergex Renewable Energy, 2019), can create jobs, and most important, can save people money in the long term (Jay et al., 2018). It could provide energy security and resilience, particularly for lower income communities (ORMAT Technologies, 2019), and could even increase the value of a property (Ball, 2020a). The lack of knowledge in the public service sector inhibits introducing geothermal energy systems and encouraging communities to invest in them.

Accompanying the expansion of geothermal energy will be the need to develop a larger, qualified workforce that is adequately trained to design and install these systems. This will require significant and sustained funding from the federal and state governments to provide specific training, mentorships, and educational programs focused on geothermal energy systems and underground thermal energy storage. This transition will also involve engaging three diverse groups simultaneously: (1) the implementers (architects, city or town planners, nonprofit organizations, and members of the business and financial sectors), (2) the designers (engineers, contractors, and equipment manufacturers), and (3) the innovators (universities, community colleges, government agencies). The workforce development programs being developed by the U.S. DOE and other federal agencies in support of geothermal energy are encouraging. The equitable workforce development programs and initiatives being developed as part of Illinois’ Climate and Equitable Jobs Act may be a model for job creation for this energy transition.

Ball (2020a) reiterates the need to accurately report the LCOE of energy technologies by region and by energy technology to market and promote the economic and environmental benefits of geothermal energy systems effectively. Both heat and power need to be considered interchangeably, which may be most effectively conveyed by developing a best practice for an electricity-equivalent metric for megawatts of heat output (MWhe) so as to correctly report the true energy mitigation value for the direct use of geothermal heating and cooling required for “cradle to grave” life cycle assessments.

7. Methods Available for Wider Adoption

Advancing the adoption of geothermal energy systems will require the development of creative financial and business models. In addition to the enacted tax incentives and the funding commitments by federal and state governments, new business models involving owners and operators and local communities will be needed. This will involve securing sufficient financing either by establishing public–private partnerships (P3s) and power purchasing agreements (PPAs) or by contracting third-party geothermal utilities to develop, install, and operate the geothermal energy systems. Such agreements will help eliminate the overall project risk to the property owner because the capital cost can be either spread out over a longer time period or converted to an off-balance sheet operating expense.

The vision of geothermal heating and cooling to deliver deep decarbonization is best achieved by pairing demonstrations and deployments at scale in multiple locations, which will validate the system performance and underscore the value of geothermal energy (U.S. DOE, 2022). If deployment pathways are tailored to specific community needs and if innovative outreach and engagement strategies are used, it will help build public awareness and consumer acceptance. Emphasizing the communal benefits and how geothermal energy systems can support urban renewal, energy diversity and security, employment opportunities, and improved resilience may lead to greater public acceptance (Tester et al., 2021).

7.1 Technology Development

Determining the effectiveness of district-scale geothermal energy systems will require new modeling procedures that consider thermo-hydro-mechanical properties coupled with building energy loads. Linking the subsurface with surface infrastructure (e.g., commercial buildings or campuses) provides the opportunity to incorporate advanced building energy management technologies that will optimize the demand profile response of the system. This will allow systems to be simulated for bidirectional flexible energy-storage options and analyses of the cost-reduction impacts of technology improvements (i.e., lowering drilling costs and improving reservoir characterization; U.S. DOE, 2019b).

Machine-learning and artificial intelligence (AI) techniques offer substantial opportunities to improve and optimize geothermal resource assessments, drilling and well testing performance, and the design and operational efficiencies of low-temperature geothermal resources (e.g., predictive behavior adaptation by GHP systems; U.S. DOE, 2022). Using machine learning to characterize and correlate key geothermal resource parameters (e.g., temperature, ground thermal conductivity, groundwater flow) can reduce geothermal resource uncertainty and development costs.

7.2 Financial Incentive Programs

From previous experience in other countries, policy makers should ensure that financial incentives are available to building owners to cover the cost of installing GHP systems. This could include tax credits, direct payments, grants and subsidies, or low-interest loans. Such financial programs are available in Europe and China (Rosenow et al., 2022), as well as several states in the United States (e.g., New York State: logy-Programs-and-Incentives; Colorado: https://energyoffice. Increasing taxes on fossil fuels may reduce the cost differential between fossil fuel and renewable energy sources. Sweden, Norway, and Finland have taken this approach (Lilliestam et al., 2020). Economic incentives are one element of a supportive policy to expand the geothermal energy sector.

The U.S. federal government, through the U.S. DOE, offers funding for geothermal energy systems through cost-shared research for development, demonstration, and deployment projects, prizes and competitions, loans, and the Small Business Innovation Research and Small Business Technology Transfer Programs. Current funding opportunity announcements are available at The U.S. DOE also provides funding through the Weatherization Assistance Program and State Energy Program, which work in partnership with state and local organizations and community-based nonprofits to provide strategic investments for low-income households that increase the energy efficiency of their homes while ensuring their health and safety. The U.S. Department of Agriculture provides guaranteed loan financing and grant funding to agricultural producers and rural small businesses for renewable energy systems (including geothermal energy systems and waste heat recovery) and to make energy efficiency improvements through the Rural Energy for America Program’s Renewable Energy Systems and Energy Efficiency Improvement Guaranteed Loans and Grants (

The U.S. Economic Development Administration (EDA) within the U.S. Department of Commerce offers a number of funding programs for renewable energy systems that can be accessed through their six regional offices ( One program in particular, the American Rescue Plan (, provides specific funding, through the Coal Communities Commitment (, to assist communities affected by the declining use of coal through activities and programs that support economic diversification, job creation, capital investment, workforce development, and re-employment opportunities. President Biden, through Executive Order 14008, “Tackling the Climate Crisis at Home and Abroad” ( presidential-actions/2021/01/27/executive-order-on-tackling-the-climate-crisis-at-home-and-abroad/), established the Interagency Working Group on Coal and Power Plant Communities and Economic Revitalization; specific funding opportunities can be found at Supporting “Energy Communities” in the Illinois Basin is a priority of the group, and they recently established a rapid response team (

At the state level, renewable energy and environmental agencies offer grants, incentives, and rebates for geothermal energy systems, GHPs, and underground thermal energy storage (e.g., Missouri Department of Natural Resources:; Colorado Energy Office: clean-energy-grants/geothermal-energy-grant-program; and Massachusetts Clean Energy Center: The New York State Energy Research and Development Authority was the first state agency to offer funding specifically for district or community geothermal energy systems that are connected to GHPs ( community-heat-pump-systems).

Locally, cities and counties provide specific support for renewable energy technologies, including GHP systems ( This may be through tax incentives (i.e., Tax Increment Financing; City of Sheboygan, Wisconsin: 2021/07/TIF-Policy-2021-06-21.pdf), rebates (e.g., City of Ames, Iowa: government/departments-divisions-a-h/electric/smart-energy), or other funding programs listed in the Database of State Incentives for Renewables & Efficiency ( In addition, utilities and energy providers may offer their own incentive programs (Eversource Energy: massachusetts-projects/geothermal-pilot-project), along with financing and rebates from the geothermal heat pump manufacturers (e.g., WaterFurnace International Incorporated: https:// Furthermore, some nonprofit organizations, foundations, and company sustainability or green energy programs provide funding for renewable energy projects (e.g., Abell Foundation [Baltimore]: ermal-Energy-Report.pdf). Candid’s GuideStar ( is the most complete and up-to-date nonprofit database available.

7.3 Outreach and Education

Achieving broader adoption of geothermal energy technologies will require a dedicated and more focused marketing and outreach program that involves diverse groups (engineers, designers, architects, energy analysts, financial investors, and decision makers) to introduce them to the emerging technologies and explain what economic and environmental benefits can be achieved and why this is important for decarbonization of the building sector. Increasing the level of awareness for geothermal energy would result in broader public acceptance and would accelerate their deployment rate and market penetration, especially for innovative and emerging technologies such as deep direct-use, underground thermal energy storage, and repurposing abandoned oil and gas wells. For the latter, reusing existing wells would eliminate the high up-front drilling costs. In Illinois, there are >44,000 abandoned oil and gas production wells (IOGCC, 2021). Moreover, co-generation with oil and gas wells could extend oil production and could be used to store thermal energy and waste heat, developing a geothermal resource to support district heating, aquaculture, and greenhouse agriculture.

To address the higher upfront costs, the investment capital and entrepreneur and incubator communities in the U.S. Midwest could be leveraged to develop geothermal energy technologies, expand geothermal heat pump manufacturing capabilities, and create a supply chain in the region. The presence of companies that exclusively undertake geothermal projects would also improve the geothermal energy sector. Furthermore, the State of Illinois could develop an Energy Center similar to Wisconsin’s Energy Institute (, whose focus is on education and outreach for energy systems throughout the state, including geothermal energy. The Center’s first activity could be to develop a statewide plan for expanding the usage of geothermal energy technologies by leveraging the region’s unique climatic conditions, favorable geology and groundwater systems, engineering and entrepreneurial talent, manufacturing capacities, and advanced agriculture research and food processing sector. Expanding and enhancing international collaboration and partnerships in geothermal energy would allow us to share successes, particularly with our European colleagues who have demonstrated that geothermal energy plays an important role in decarbonizing energy systems (García-Gil et al., 2020).

It is important to recognize that the effort to expand the footprint of the geothermal energy sector is a long-term effort. We would like to see positive impacts relatively quickly, but it will likely take several years of consistent effort to see significant growth. Nevertheless, we are encouraged that federal tax incentives are available for the next decade even as we are clearly at the optimal time to start this effort. Public perceptions about renewable energy are evolving, and we now have a better chance to steer property owners’ preference and lead the public conversation toward decarbonization of energy systems (Twait, 2021). Furthermore, consumers should be engaged and encouraged to participate through demand-side responses (i.e., changing how and when energy is used). There is extensive academic literature around demand-side engagement in future energy systems (Skjølsvold et al., 2018).

8. Jobs and Economic Development Impacts

Developing geothermal energy in the U.S. Midwest, and particularly in Illinois, could have beneficial impacts on the regional and local economies, the geothermal manufacturing sector, and national and state employment statistics. Millstein et al. (2019) estimated that the installation of 50–100 direct-use geothermal energy systems per year could annually provide ~10,000 new jobs while reducing emissions and preventing dozens of premature mortalities related to exposure to air pollution.

The current energy transition, although only recently coming to the public’s attention, has been underway for some time and is increasingly becoming visible to (and driven by) the consumer. Twait (2021) pointed out that energy-related decisions now involve aspirational, emotional, and noneconomic factors. Public awareness of future climate change impacts is accelerating the introduction of renewable energy technologies in the energy generation and conservation sectors, to fully electrify and decarbonize the energy system. Associated with this transition are new opportunities for governments, businesses, and stakeholders to develop the supporting clean energy workforce. The increased deployment of geothermal energy systems will produce a range of environmental and economic benefits across the United States, as well as in local communities. This work will improve the resilience of the energy system in communities that will ensure a secure and reliable source of heating and cooling. It will also support training and employing community members, which will ensure environmental justice and the inclusion of underserved communities.

8.1 Environmental Benefits

Increased geothermal deployment could improve the air quality and reduce CO2 emissions in addition to reducing emissions of sulfur dioxide (SO2), methane (CH4), nitrogen oxides (NOx), and fine particulate matter (PM2.5). The following metrics are referenced to the ‘”breakthrough” scenario, which equates to primary expanded uptake of GHPs at levels prescribed in the U.S. DOE’s GeoVision Report (U.S. DOE, 2019a). In this scenario, an aggressive cost reduction (30% by 2030) and increases in operational efficiency (50% by 2050) are achieved through technological improvements. The benefits of installing GHP systems result from reducing on-site fuel use and from lowering the electricity demand. Common residential and commercial heating and cooling systems, such as furnaces, portable space heaters, air conditioners, and central air systems, are the technologies displaced by GHP systems. Geothermal heat pump systems do require electricity to pump cooling and heating fluids and run fans, and in some areas, they would increase the total electricity demand, but in most locations, GHP systems reduce both on-site fuel use and electricity use. The fuels displaced by these technologies include natural gas, fuel oil, propane, electricity, district steam, and district hot water. The combustion of fossil fuels is associated with emissions of SO2, NOx, and PM2.5.

The expansion of GHP systems under the breakthrough scenario reduces cumulative building heating emissions of SO2, NOx, and PM2.5 by an additional 232,000, 711,000, and 57,000 metric tons, respectively, providing $28–$61 billion of value based on avoiding up to 8,700 premature deaths (Millstein et al., 2019). When considering the full life cycle, the projected cumulative reductions in GHG emissions from 2015 to 2050 following the breakthrough scenario would offset a cumulative total of 1,281 million metric tons (MMT) of carbon dioxide equivalent (CO2e), representing an 8.3% reduction in on-site emissions from buildings relative to a scenario that holds the number of GHP installations constant at 2012 deployment levels. By the end of the study period, Millstein et al. (2019) calculated that 90 MMT (CO2e) of annual emissions are avoided from GHP deployment, equivalent to removing almost 20 million cars from the road. The SO2 emission benefits are concentrated in the U.S. Midwest, where GHPs offset residential fuel oil use (Figure 8.1). Nitrogen oxide emission benefits are concentrated in several states in the eastern Midwest and Northeast, including Illinois and New York. The spatial distribution of PM2.5 emission reductions is similar to that of NOx, with the largest reductions in the eastern Midwest and the Northeast. The PM2.5 emissions are significantly lower than those of NOx and SO2.

Figure 8.1. Cumulative (2015–2050) emission reductions for the geothermal heat pump fuel use sector by state based on the “breakthrough” scenario. From Millstein et al. (2019). Figure reproduced courtesy of the U.S. Department of Energy.
8.2 Economic Benefits

Tapping geothermal energy resources and expanding their applications would have direct positive impacts on local economies. New business opportunities would be created as the existing furnaces and boiler systems are replaced to accommodate the new GHP systems. Figure 8.2 shows the geographic distributions of GHP system capacities in 2050 if the future uptake follows either the business-as-usual or breakthrough scenarios (Liu et al., 2019). Expenditures for GHP systems are expected to grow from $2.9 billion in 2030 to $4.3 billion in 2050, with maximum expenditures coinciding with peak employment in 2043 (Millstein et al., 2019). In addition, nearly half of the increase in expenditures from 2030 to 2050 are expected to take place in only six states: New Jersey, New York, California, Massachusetts, Michigan, and North Carolina (ranked in order from greatest to least change).

The expansion of geothermal energy systems is expected to deliver both direct and indirect benefits that increase employment at the location of the installation. The additional jobs in communities will be in the service, hospitality, and lodging sectors, and companies that sell supplies or rent equipment will drive the local economic development. Additional benefits will be associated with geothermal-related manufacturing and deployment activities. Several studies have looked at the economic impacts of geothermal plants in the United States (e.g., Battocletti & Glassley, 2013). The direct and indirect job creation figures indicate the dynamics of resource utilization and reflect the impact of this renewable resource on the economy (Wei et al., 2010). For the development and operation of geothermal power plants, the Geothermal Energy Association established an economic multiplier of 2.5 times for geothermal energy investment, meaning that each US$1.00 invested in the resource results in US$2.50 of output growth in the local economy (Hance, 2005).

Figure 8.2. Installed GHP capacities in 2050 by county under the business-as-usual (top) and breakthrough (bottom) scenarios. The State of Illinois is outlined in light blue. From Liu et al. (2019). Figure reproduced courtesy of the U.S. Department of Energy.
8.3 Clean Energy Community Development

The energy transition has accelerated in recent years because of the combined actions of local, state, and national governments, which are driven in part by external forces and personal preferences of the residents. In Illinois, legislation has set aggressive clean energy targets to meet carbon neutrality within the next several decades. The only way to achieve such goals is by strategic planning on the community scale (cf. Mahlmann & Escobedo, 2012). Community-scale developments will be beneficial for energy conservation and reduction in GHG emissions, and will also be more cost efficient. Technologies that are scalable on the community level, such as GHP systems, are a key component to achieving the goals. For example, integrated underground thermal energy storage systems that connect multiple buildings into a single network or multiple networks reduce capital costs by allowing fixed costs to be shared by the community and transferring excess thermal energy from one zone to another (Bloemendal et al., 2014).

To support these efforts, communities increasingly need technical expertise and assistance to undertake comprehensive and equitable energy planning (Ross & Day, 2022). Furthermore, the energy transition is complex and will require technical, policy, and other expertise. To achieve sustainability requires communities to reduce energy consumption, using energy more efficiently, and it integrates energy efficiency and renewable energy technologies into integrated systems, a situation that emphasizes the importance of holistic design.

Clean energy development can bolster the communities’ economies through job creation, local tax revenues, and reduced energy costs; however, communities most in need of support and employment often have lower uptake of renewable energy technologies (Ross et al., 2022). The clean energy projects serving low-income communities require significant supplemental funding, especially to create the co-benefits that matter to the residents—such as energy affordability, job creation, and climate resilience. Developing clean energy projects should involve deep engagement from the community that brings together residents to bring projects to fruition. Thinking holistically about building the ecosystem requires communities to identify, develop, and finance impactful and investable projects (Hangen, 2022).

8.4 Workforce Development

The expansion of geothermal energy systems would spur economic growth and job creation in the U.S. Midwest. The installation of geothermal energy systems would provide long-term income for people with a diversity of job skills, including mechanics, pipe fitters, plumbers, machinists, electricians, carpenters, construction and drilling equipment operators, surveyors, architects and system designers, geologists, engineers (electrical, mechanical, and structural), HVAC technicians, regulatory and environmental consultants, accountants, computer technicians, and government employees, all of whom play an important role in bringing geothermal energy systems online. They would also create a direct benefit to geothermal heat pump installers, drilling companies, and consulting engineers and would indirectly benefit supply chain industries (e.g., heat pump manufacturers, loop manufacturers, and drilling supply companies).

The development of these geothermal energy technologies takes advantage of the favorable geology and groundwater conditions in this region. Additionally, high-paying drilling jobs in the petroleum industry could be retained because geothermal installation requires standard drilling and well-development methodologies. Together, these factors would lead to a more consistent market for geothermal energy that would allow the profitable deployment of these systems for a range of stakeholders.

The recently enacted Climate Equity and Jobs Act will help Illinois build a diverse clean energy workforce that builds wealth and creates capacity and employment opportunities in diverse businesses. Although the Act does not contain specific workforce programs for geothermal energy, it is expected that subsequent legislation will support the development of GHP systems, community geothermal energy systems, underground thermal energy storage, and deep direct-use geothermal energy technologies.

Nationally, the expansion of GHP systems under the breakthrough scenario would support an additional 36,300 employees at the peak uptake in 2043 (Millstein et al., 2019). Under the breakthrough scenario, growth in geothermal energy will rely on a cumulative investment of $112 billion through 2050. Much of the main workforce growth will come from the creation of on-site construction jobs and supply chain manufacturing jobs that will last over the lifetime of construction (1–3 years).

8.5 Technical Assistance and Training Programs

Like other sectors in the sciences and engineering disciplines, the geothermal energy sector is affected by a shortage of professionals, consultants, and businesses, along with a general aging of the existing geothermal workforce. Anticipated growth in the geothermal energy sector will drive demand for a larger knowledgeable and skilled workforce across a diverse business community—from drilling operators to GHP installers. Training this new workforce effectively will require coordination among universities, community colleges, trade unions, energy efficiency educators, and other clean energy organizations to identify the specific needs and structure relevant to hands-on learning and apprenticeship programs. Technical assistance efforts may also include educating stakeholders on how to assess the potential and feasibility of geothermal energy use cases, potential, and/or feasibility (U.S. DOE, 2022).

In Illinois, several organizations stand ready to provide training and mentorship to this new workforce. The Geothermal Alliance of Illinois (GAOI, is a not-for-profit association that works to advance the GHP industry in Illinois. The GAOI offers in-person accredited training to geothermal contractors and loop installers or drilling contractors. The International Ground Source Heat Pump Association (IGSHPA, is a nonprofit, member-driven organization established in 1987 to advance GHP technology on the local, state, national, and international levels. The IGSHPA encourages the adoption of geothermal energy by increasing understanding, awareness, and adoption through federal and state government advocacy (in coordination with the Geoexchange Organization, It also provides training and certification and develops industry standards and specifications for the geothermal heating and cooling industry. The Local Chapter 150, based in Wilmington, Illinois (, offers apprenticeship programs for heavy equipment operators, heavy equipment repair technicians, and geothermal contractors or well drillers. Apprentices learn drilling method applications, safety practices, and the safe and efficient operation of various drilling equipment.

In support of their industry partners, interdisciplinary groups at academic institutions provide applied education and training programs that focus on energy efficiency and environmental mitigation efforts to identify opportunities to save energy and money while reducing overhead and operational costs. The Smart Energy Design Assistance Center (SEDAC, https://smartenergy at the University of Illinois at Urbana-Champaign provides fee-free design assistance to businesses and municipalities throughout Illinois and supports building design, materials selection, and construction practices that foster energy efficiency. At the University of Illinois at Chicago, the Energy Resources Center ( provides public service, research, and special projects organization that works to improve energy efficiency and the environment. The Illinois Green Economy Network (IGEN, is a collaborative initiative to develop and promote sustainable programs across all Illinois’ 48 community colleges to provide statewide green jobs training and creation.

Nonprofit organizations and industry–public partnership consortiums were formed to incentivize the development of clean energy technologies, promote energy efficiency that lowers greenhouse gas emissions, increases profitability, and reduces peak energy demand. Evergreen Climate Innovations (formerly Clean Energy Trust, provides hands-on support to entrepreneurs bringing impactful renewable energy technologies to market. In the City of Chicago, a partnership with SEDAC, the Metropolitan Mayors Caucus, and 360 Energy Group ( is seeking to lower energy costs by reducing electricity and fossil fuel consumption. The NREL partners with communities to identify and assess community energy needs and explore energy technologies that can provide resilience and other community benefits (e.g., LEAP communities, gov/communitiesLEAP/leap-communities). This approach leverages the experience and expertise of local community leaders, residents, and organizations. The Illinois Environmental Protection Agency Office of Energy supports clean energy education and training for community college students. Through a collaboration with the IGEN and SEDAC, Illinois’ energy efficiency workforce is being expanded through living laboratories and hands-on training as well as the support of energy infrastructure improvements (

The Climate and Equitable Jobs Act will create a statewide Workforce Network Hubs Program that leverages community-based organizations to ensure members of disadvantaged communities have dedicated and sustained support to develop a workforce for clean energy and related sector jobs. This plan makes Illinois a national leader in fighting climate change and creates thousands of high-paying jobs in the clean energy economy. The Act also creates a clean energy contractor incubator program to provide access to low-cost capital and financial support for small clean energy businesses and contractors.

9. Summary

The extraction and use of fossil fuels are major sources of greenhouse gas emissions, and such emissions have been shown to contribute to global warming and climate change. The level of greenhouse gases continues to rise as energy demand increases. The urgent need to meet emission reduction goals has encouraged a move toward decarbonizing the energy sector by implementing clean, renewable, and sustainable energy sources (e.g., Denholm et al., 2022). Changing climate conditions, such as longer and more frequent periods of extreme heat and cold, are affecting environmental health, agricultural production, the transportation infrastructure, and water and air quality. Energy security and the need for resilience prompt the use of local sources of energy, which are less susceptible to commodity price fluctuations, and which can withstand disruptions from natural or technological threats and hazards.

This White Paper was developed as an introduction to geothermal energy and to promote it as an alternative renewable energy source. Geothermal technologies provide a constant energy source that can be extracted for various applications, including heating, cooling, water heating demands, and dehumidification, which are the predominant societal needs in the U.S. Midwest. To meet local and global challenges for carbon emissions, the State of Illinois has set regulatory goals for using renewable energy, specifically solar and wind (State of Illinois, 2021). However, Illinois lacks a dedicated program for promoting the development of geothermal energy resources and supporting technologies, and we are optimistic this White Paper will provide a reference for future legislation toward utilizing the available thermal energy in the ground and the development of associated energy storage systems.

Generally speaking, geothermal systems are environmentally sound, have proven efficiency and reliability, have low carbon emissions, have low maintenance requirements, and offer the additional benefits of energy security and resilience. The application of geothermal technologies across various sectors has demonstrated their efficiency and reliability. Furthermore, the advantages of using geothermal energy (which include but are not limited to environmental, economic, geographic, and local community benefits) outweigh the initial costs for implementation, which are higher than for traditional electric resistance and fossil fuel energy systems. First costs for the installation of geothermal energy systems can be reduced as technological advances lead to more efficient designs. In addition, unemployed workers in the oil and gas industry can be cross-trained to both install and maintain geothermal energy systems.

The Illinois Geothermal Coalition is dedicated to advancing Illinois as a leader in geothermal energy. The work of the Coalition is disseminated primarily through educational seminars and documents that present a comprehensive treatment of geothermal, such as this White Paper. In the U.S. Midwest, low-temperature geothermal has three main applications: (1) shallow geothermal exchange for heating and cooling and to meet hot water demands, (2) deep direct use for district-scale heating and cooling and cascading applications, and (3) advanced geothermal systems, such as underground thermal energy storage. Geothermal heat pumps in different configurations can be used to meet a range of heating and cooling demands, resulting in considerable mitigation of greenhouse gas emissions and cost savings. A range of federal, state, and county programs have been developed to support and offset the cost of these efforts. Experimental geothermal applications, as well as some hybrid systems, are now being used in the residential, commercial, industrial, manufacturing, education, military, transportation, and agricultural sectors. Geothermal heat pumps and underground thermal storage systems have great economic and environmental advantages compared with fossil fuel consumption.

The University of Illinois at Urbana-Champaign is well positioned to perform R&D and host technical demonstrations for various geothermal technologies because of the ongoing “Living Laboratory” program, which has actively engaged faculty, staff, and students on numerous geothermal energy projects. Active geothermal R&D projects include a multi-organization partnership to develop more accurate models to help expand the deployment of geothermal technologies at federal sites. Historical geothermal R&D projects include a feasibility study using DDU geothermal energy in agricultural research facilities on the UIUC campus to exploit low-temperature sedimentary basins, such as the Illinois Basin. Recent educational facilities on the UIUC campus have been constructed incorporating novel geothermal heating and cooling technologies, which will also serve as ongoing testbeds for faculty, staff, and students.

However, technical, financial, policy, and economic barriers to the implementation of geothermal technologies remain to be addressed, primarily the high cost of drilling. A sociocultural barrier toward a shift away from fossil fuels also exists, as large segments of the public remain uninformed about the benefits of geothermal energy. Technological developments, financial incentive programs, and outreach and education will help overcome this barrier. With the wider adoption of geothermal energy systems, employment opportunities and economic impacts must also be addressed. Although the environmental benefits of installing geothermal energy systems are widely known to researchers, the economic, energy security, and resilience benefits have yet to be fully demonstrated to the public. These benefits can be enhanced by upgrading inefficient energy systems, such as aging natural gas and propane furnaces and heating oil boilers. Furthermore, adopting community-scale energy policies and retraining the unemployed will encourage equitable economic growth and workforce development in the U.S. Midwest.

10. Credits and Acknowledgments

This document was initiated by the research project “Integrating Groundwater Resources and Geothermal Energy for Water–Energy Security and Resilience” at the Illinois Water Resources Center (IWRC) and the Illinois State Geological Survey (ISGS), and received funding from the Institute for Sustainability, Energy, and Environment (iSEE) at the University of Illinois at Urbana-Champaign and the University of Illinois Extension, as well as State of Illinois General Revenue funds to the Illinois State Geological Survey (ISGS) at Prairie Research Institute (PRI).

10.1 Project Team

Yu-Feng F. Lin (Principal Investigator, Director of IWRC, Principal Research Scientist at ISGS,

Tugce Baser (Assistant Professor, Department of Civil and Environmental Engineering,

Mohamed Attalla (Executive Director, Illinois Facilities & Services, now at City University of New York)

Praveen Kumar (Lovell Professor, Department of Civil and Environmental Engineering, now Executive Director of PRI,

Franklin H. Holcomb (Associate Technical Director, Construction Engineering Research Laboratory, U.S. Army Corps of Engineers,

10.2 Collaborators

Andrew Stumpf (Principal Research Scientist, ISGS,

Morgan White (Associate Director for Sustainability and Interim Director of Capital Programs, Illinois Facilities & Services,

Ryan Dougherty (President, Geothermal Exchange Organization,

Nancy Ouedraogo (Extension State Specialist, Community Economic Development, University of Illinois Extension,

Jay Solomon (Extension Educator Natural Resources, Environment & Energy, University of Illinois Extension,

10.3 Authorship

The following are initial authors of this document for the first release in February 2023:

Andrew Stumpf (Principal Research Scientist, ISGS,

Franklin H. Holcomb (Associate Technical Director, Construction Engineering Research Laboratory, U.S. Army Corps of Engineers,

Josiane Jello (Ph.D. Candidate, Department of Civil and Environmental Engineering,

10.4 Reviewers

Awaiting Reviews

10.5 Support

This document can be found at the website, which is supported by the IWRC/PRI at the University of Illinois at Urbana-Champaign (


Abdelaziz, S. L., Olgun, C. G., & Martin, J. R., II. (2011). Design and operational considerations of geothermal energy piles. In J. Han & D. E. Alzamora (Eds.), Proceedings of Geo-Frontiers Congress 2011. American Society of Civil Engineers.

Abesser, C., Busby, J. P., Pharaoh, T. C., Bloodworth, A. J., & Ward, R. S. (2020). Unlocking the potential of geothermal energy in the UK (Open Report OR/20/049, 22 pp.). British Geological Survey.

Abesser, C., & Walker, A. (2022). Geothermal energy (POSTbrief 46, 70 pp.). UK House of Parliament, Parliamentary Office of Science and Technology.

Adam, D., & Markiewicz, R. (2009). Energy from earth-coupled structures, foundations, tunnels and sewers. Géotechnique, 59, 229–236.

Aditya, G. R., Mikhaylova, O., Narsilio, G. A., & Johnston, I. W. (2020). Comparative costs of ground source heat pump systems against other forms of heating and cooling for different climatic conditions. Sustainable Energy Technologies and Assessments, 42, Article 100824. 10.1016/j.seta.2020.100824

Alberti, L., Antelmi, M., Angelotti, A., & Formentin, G. (2018). Geothermal heat pumps for sustainable farm climatization and field irrigation. Agricultural Water Management, 195, 187–200.

Alkhwildi, A., Elhashmi, R., & Chiasson, A. (2020). Parametric modeling and simulation of low temperature energy storage for cold-climate multi-family residences using a geothermal heat pump system with integrated phase change material storage tank. Geothermics, 86, 101864, 1–11.

Allen, J. G., MacNaughton, P., Satish, U., Santanam, S., Vallarino, J., & Spengler, J. D. (2016). Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic compound exposures in office workers: A controlled exposure study of green and conventional office environments. Environmental Health Perspectives, 124, 805–812;

American Association of Petroleum Geologists (AAPG). (1994). CSDE, COSUNA, and Geothermal Survey. Data CD-ROM.

American National Standards Institute/Canadian Standards Association (ANSI/CSA/IGSHPA). (2016). Design and installation of ground source heat pump systems for commercial and residential buildings (C448 Series-16, 206 pp.).

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). (2019). Geothermal energy (pp. 35.1–35.51). In 2019 ASHRAE handbook—HVAC applications.

American Society of Mechanical Engineers (ASME). (1980). The Equitable Building: Heat pump system.

Arent, D. J., Tol, R. S. J., Faust, E., Hella, J. P., Kumar, S., Strzepek, K. M., Tóth, F. L., & Yan, D. (2014). Key economic sectors and . In C. B. Field, V. R. Barros, D. J. Dokken, K. J. Mach, M. D. Mastrandrea, T. E. Bilir, M. Chatterjee, K. L. Ebi, Y. O. Estrada, R. C. Genova, B. Girma, E. S. Kissel, A. N. Levy, S. MacCracken, P. R. Mastrandrea, & L. L. White (Eds.), Climate Change 2014: Impacts, adaptation, and vulnerability. Part A: Global and sectoral aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (pp. 659–708). Cambridge University Press.

Assouline, D., Mohajeri, N., Gudmundsson, A., & Scartezzini, J.-L. (2019). A machine learning approach for mapping the very shallow theoretical geothermal potential. Geothermal Energy, 7, Article 19.

Balaji, K. (2021). Energy demand reduction in the built environment using shallow geothermal integrated energy systems—A comprehensive review: Part I. Design consideration of ground heat exchanger. ASME Journal of Engineering for Sustainable Buildings and Cities, 2(3). Article 030801.

Ball, P. J. (2020a). A review of geothermal technologies and their role in reducing greenhouse gas emissions in the USA. Journal of Energy Resources Technology, 143(1), Article 010903.

Ball, P. J. (2020b). Macro energy trends and the future of geothermal within the low-carbon energy portfolio. Journal of Energy Resources Technology, 143(1), Article 010904.

Balbay, A., & Esen, M. (2010). Experimental investigation of using ground source heat pump system for snow melting on pavements and bridge decks. Scientific Research and Essays, 5(24), 3955–3966.

Banks, D. (2012). Heat pumps and thermogeology: A brief history and international perspective. In D. Banks (Ed.), An introduction to thermogeology: Ground source heating and cooling (2nd ed.). Wiley.

Battocletti, E. C., & Glassley, W. E. (2013). Measuring the costs & benefits of nationwide geothermal heat deployment (DOE-DE-EE0002741 Final Report, 306 pp.). University of California Davis.

Beckers, K. F., Kolker, A., Pauling, H., McTigue, J. D., & Kesseli, D. (2021). Evaluating the feasibility of geothermal deep direct-use in the United States. Energy Conversion and Management, 243, Article 114335.

Beckers, K. F., & Young, K. R. (2016). Performance, cost, and financial parameters of geothermal district heating systems for market penetration modeling under various scenarios. In Proceedings of the 42nd Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA.

Beckers, K. J. H. F. (2016). Low-temperature geothermal energy: Systems modeling, reservoir simulation, and economic analysis [Ph.D. thesis, Cornell University, Ithaca, NY]. Cornell University Library eCommons.

Beernink, S., Bloemendal, M., Kleinlugtenbelt, R., & Hartog, N. (2022). Maximizing the use of aquifer thermal energy storage systems in urban areas: Effects on individual system primary energy use and overall GHG emissions. Applied Energy, 311, Article 118587.

Belausteguigoitia, J. C., Romero, V., & Simpser, A. (2022). The political economy of carbon pricing: Lessons from the Mexican carbon tax experience for the Mexican cap-and-trade system. In S. Lucatello (Ed.) Towards an emissions trading system in Mexico: Rationale, design and connections with the global climate agenda (pp. 133–150). Springer Climate.

Berg, N. (2021, August 16). The centerpiece of Microsoft’s massive new expansion is 550 feet underground. Fast Company.

Bergen, M. (2021, October 29). Google’s biggest moonshot is its search for a carbon-free future. Bloomberg Green, 5(Fall).

Bloemendal, M., Olsthoorn, T., & Boons, F. (2014). How to achieve optimal and sustainable use of the subsurface for Aquifer Thermal Energy Storage. Energy Policy, 66, 104–114.

Bloomquist, R. G. (2000, May 28–June 10). Geothermal heat pumps five plus decades of experience in the United States. In Proceedings of the World Geothermal Congress 2000.

Bloomquist, R. G. (2001). Geothermal heat pumps: Concerns over operation, maintenance, and reliability unfounded (pp. 183–190). International Geothermal Association, International Summer School.

Boesten, S., Ivens, W., Dekker, S. C., & Eijdems, H. (2019). 5th generation district heating and cooling systems as a solution for renewable urban thermal energy supply. Advances in Geosciences, 49, 129–136.

Bonette, J. (2022, March 24). Major construction underway for new geo-exchange energy system. Princeton Alumni Weekly.

Bourne-Webb, P. J., Amatya, B., Soga, K., Amis, T., Davidson, C., & Payne, P. (2009). Energy pile test at Lambeth College, London: Geotechnical and thermodynamic aspects of pile response to heat cycles. Géotechnique, 59(3), 237–248.

Bowley, W., & Evins, R. (2021). Energy system optimization including carbon-negative technologies for a high-density mixed-use development. International Journal of Sustainable Energy Planning and Management, 31, 211–225.

Brandl, H. (2006). Energy foundations and other thermo-active ground structures. Géotechnique, 56(2), 81–122.

Bundschuh, J., Chen, G., Chandrasekharam, D., & Piechocki, J. (Eds.). (2017). Geothermal, wind and solar energy applications in agriculture and aquaculture. Taylor Francis Group.

Buro Happold Engineering. (2019). Geo Micro District feasibility study.

Brush, A., Masanet, E. & Worrel, E. (2011). Energy efficiency improvement and cost saving opportunities for the dairy processing industry: An Energy Star guide for energy and plant managers [Report No. LBNL-6261E]. U.S. Department of Energy, Lawrence Berkeley National Laboratory.

Brzozowska, R., Neugebauer, M., & Piechocki, J. (2017). Lakes as a heat source for heat pumps—A model study to determine the ecological impact of summer heat transfer. In J. Bundschuh, C. Guangnan, D. Chandrasekharam, and J. Piechocki (Eds.), Geothermal, wind and solar energy applications in agriculture and aquaculture: Volume 13. Sustainable energy developments (pp. 301–348). CRC Press/Balkema.

Cartwright, K. (1970), Groundwater discharge in the Illinois Basin as suggested by temperature anomalies. Water Resources Research, 6(3), 912–918.

Casasso, A., & Sethi, R. (2014). Efficiency of closed loop geothermal heat pumps: A sensitivity analysis. Renewable Energy, 62, 737–746.

Cataldi, R. (1993). Review of historiographic aspects of geothermal energy in the Mediterranean and Mesoamerican areas prior to the Modern Age. Geo-Heat Centre Bulletin (Oregon Institute of Technology Klamath Falls), 18(1), 13–16.

Chiasson, A. D., & Yavuzturk, C. (2014). Simulation of hybrid solar-geothermal heat pump systems [Paper SGP-TR-20]. Proceedings of the 39th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA.

Christodoulides, P., Agathokleous, R., Aresti, L., Kalogirou, S. A., Tassou, S. A., & Florides, G. A. (2022). Waste heat recovery technologies revisited with emphasis on new solutions, including heat pipes, and case studies. Energies, 15(1), 384.

Cook, J. J., Aznar, A., Dane, A., Day, M., Mathur, S., & Doris, E. (2016). Clean energy in city codes: A baseline analysis of municipal codification across the United States (NREL TP-6A70-66120, 38 pp.). U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Energy Policy and Systems Analysis.

Crandall, A. C. (1946). House heating with earth heat pump. Electrical World, 126(19), 94–95.

Crespo, A., Barreneche, C., Ibarra, M., & Platzer, W. (2019). Latent thermal energy storage for solar process heat applications at medium-high temperatures—A review. Solar Energy, 192, 3–34.

Cross, S., Eagan, D., Tolmé, P., Keniry, J., Foreword, J., & Koester, R. (2011). Going underground on campus: Tapping the earth for clean, efficient heating and cooling. FINAL% 203-1-11.ashx

Cui, Y. L., Zhu, J., & Meng, F. R. (2018). Techno-economic evaluation of multiple energy piles for a ground-coupled heat pump system. Energy Conversion and Management, 178, 200–216.

Cui, Y., Zhu, J., Twaha, S., Chu, J., Bai, H., Huang, K., Chen, X., Zoras, S., & Soleimani, Z. (2019). Techno-economic assessment of the horizontal geothermal heat pump systems: A comprehensive review. Energy Conversion and Management, 191, 208–236.

Dalla Longa, F., Nogueira, L. P., Limberger, J., van Wees, J.-D., & van der Zwaan, B. (2020). Scenarios for geothermal energy deployment in Europe. Energy, 206, Article 118060.

Deetjen, T. A., Walsh, L., & Vaishnav, P. (2021). US residential heat pumps: The private economic potential and its emissions, health, and grid impacts. Environmental Research Letters, 16(8), Article 084024.

Deng, Y., Feng, Z., Fang, J., & Cao, S. J. (2018). Impact of ventilation rates on indoor thermal comfort and energy efficiency of ground-source heat pump system. Sustainable Cities and Society, 37, 154–163.

Denholm, P., Brown, P., Cole, W., Mai, T., Sergi, B., Brown, M., Jadun, P., Ho, J., Mayernik, J., McMillan, C., & Sreenath, R. (2022). Examining supply-side options to achieve 100% clean electricity by 2035 (NREL/TP-6A40-81644). National Renewable Energy Laboratory.

Dincer, I. (2002). Thermal energy storage systems as a key technology in energy conservation. International Journal of Energy Research, 26(7), 567–588.

Dincer, I., & Ozcan, H. (2018). 1.17 Geothermal energy. Comprehensive Energy Systems, 1, 702–732.

Duggal, R., Rayudu, R., Hinkley, J., Burnell, J., Wieland, C., & Keim, M. (2022). A comprehensive review of energy extraction from low-temperature geothermal resources in hydrocarbon fields. Renewable and Sustainable Energy Reviews, 154, Article 111865.

Duijff, R., Bloemendal, M., & Bakker, M. (2021). Interaction effects between aquifer thermal energy storage systems. Groundwater.

Dupeyrat, P., Ménézo, C., & Fortuin, S. (2014). Study of the thermal and electrical performances of PVT solar hot water system. Energy and Buildings, 68(Part C), 751–755.

Durga, S., Beckers, K. F., Taam, M., Horowitz, F., Cathles, L. M., & Tester, J. W. (2021). Techno-economic analysis of decarbonizing building heating in Upstate New York using seasonal borehole thermal energy storage. Energy and Buildings, 241, Article 110890.

Edwards, R., & Veracruz, J. (2019). Sustaining guardsman with geothermal power. The Military Engineer, 111(723), 56–57.

Egg, J. (2017, May 8). Nashville Airport’s geothermal legacy. PHC News, 17(5), 156, 158.

Egg, J. (2022a, April 15). Wastewater energy exchange is making sense to energy utilities. PHC News, 22(4), 60–61.

Egg, J. (2022b, April 25). The billion dollar boys of the geothermal heat pump industry. Mechanical Hub.

Ehhalt, D., Prather, P., Dentener, F., Derwent, E., Dlugokemcky, E., Holland, E., Isaksen, I., Katima, J., Kirchhoff, V., Matson, P., Midgley, P., & Wang, M. (2001). Atmospheric chemistry and greenhouse gases. In J. T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, & C.A. Johnson (Eds.), Climate change 2001: The scientific basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (pp. 241–288). Cambridge University Press.

Elliott, E. D. (2013). Why the U.S. does not have a renewable energy policy. Environmental Law Reporter, 43, 10094–10101.

Environmental Defense Fund (EDF). (2014). How will climate change affect the Midwest?

Erdlac, R. J., Jr., Armour, L. K., Lee, R., Snyder, S. M., Sorensen, M., Matteucci, M., & Horton, J. (2007). Ongoing resource assessment of geothermal energy from sedimentary basins in Texas [SGP-TR-183]. In Proceedings of the 32nd Workshop on Geothermal Reservoir Engineering.

Federal Energy Management Program (FEMP). (2010). U.S. Army Fort Knox: Using the earth for space heating and cooling (Fact Sheet). U.S. Department of Energy.

Fleuchaus, P., Godschalk, B., Stober, I., & Blum, P. (2018). Worldwide application of aquifer thermal energy storage—A review. Renewable and Sustainable Energy Reviews, 94, 861–876.

Fox, D. B., Sutter, D., Beckers, K. F., Lukawski, M. Z., Koch, D. L., Anderson, B. J., & Tester, J. W. (2013). Sustainable heat farming: Modeling extraction and recovery in discretely fractured geothermal reservoirs. Geothermics, 46, 42–54.

Frailey, S. M., Grube, J. P., Seyler, B., & Finley R. J. (2004). Investigation of liquid CO2 sequestration and EOR in low temperature oil reservoirs in the Illinois Basin [SPE-89342-MS]. Society of Petroleum Engineers/Department of Energy 14th Symposium on Improved Oil Recovery, Tulsa, OK, United States.

Franco, A., & Vaccaro, M. (2014). Numerical simulation of geothermal reservoirs for the sustainable design of energy plants: A review. Renewable and Sustainable Energy Reviews, 30, 987–1002.

Freifeld, B. M., Finsterle, S., Onstott, T. C., Toole, P., & Pratt, L. M. (2008). Ground surface temperature reconstructions: Using in situ estimates for thermal conductivity acquired with a fiber-optic distributed thermal perturbation sensor. Geophysical Research Letters, 35(14).

Fry, V. A. (2009). Lessons from London: Regulation of open-loop ground source heat pumps in central London. Quarterly Journal of Engineering Geology and Hydrogeology, 42(3), 325–334.

Gadd, H., & Werner, S. (2021). Thermal energy storage systems for district heating and cooling. In L. F. Cabeza (Ed.), Advances in thermal energy storage systems: Methods and applications (2nd ed., pp. 625– 638). Woodhead Publishing.

Gannon, R. (1978). Ground-water heat pumps—Home heating and cooling from your own well. Popular Science, 212(2), 78–82.

García-Gil, A., Goetzl, G., Kłonowski, M. R., Borovic, S., Boon, D. P., Abesser, C., Janza, M., Herms, I., Petitclerc, E., Erlström, M., Holecek, J., Hunter, T., Vandeweijer, V. P., Cernak, R., Mejías Moreno, M., & Epting, J. (2020). Governance of shallow geothermal energy resources. Energy Policy, 138, Article 111283.

Gautier, A., Wetter, M., & Sulzer, M. (2022). Resilient cooling through geothermal district energy system. Applied Energy, 325, Article 119880.

Gehlin, S. (2016). Borehole thermal energy storage. In S. J. Rees (Ed.), Advances in ground-source heat pump systems (pp. 295–327). Woodhead Publishing.

Gehlin, S. E. A., & Nordell, B. (2003). Determining undisturbed ground temperature for thermal response test. ASHRAE Transactions, 109, 151–156.

Geothermal Rising. (2022). Geothermal heat pumps.

Godlewska, K., Tomaszewska, B., Michalak, I., Bujakowski, W. & Chojnacka, K. (2015). Prospects of geothermal water use in cultivation of spirulina. Open Chemistry, 13(1), 1218–1227.

Gombeda, M. J., Rivera, E. Jr, & Lallas, Z. N. (2022). Optimal approach for addressing reinforcement corrosion for concrete bridge decks in Illinois [Research Report No. FHWA-ICT-22-005]. Illinois Center for Transportation.

Guelpa, E., & Verda, V. (2019). Thermal energy storage in district heating and cooling systems: A review. Applied Energy, 252, 113474.

Guo, H., Meggers, F., & Kim, M.-g. (2017). Geothermal district heating investigation of retired oil/gas wells as higher-temperature renewable heat sources. In Proceedings of World Sustainable Built Environment Conference 2017 (pp. 364–369). Construction Industry Council and Hong Kong Green Building Council.

Habibzadeh-Bigdarvish, O., Yu, X., Lei, G., Li, T., & Puppala, A. J. (2019). Life-cycle cost–benefit analysis of bridge deck de-icing using geothermal heat pump system: A case study of North Texas. Sustainable Cities and Society, 47, 101492.

Hähnlein, S., Bayer, P., Ferguson, G., & Blum, P. (2013). Sustainability and policy for the thermal use of shallow geothermal energy. Energy Policy, 59, 914–925.

Haites, E., Maosheng, D., Gallagher, K. S., Mascher, S., Narassimhan, E., Richards, K. R., & Wakabayashi, M. (2018). Experience with carbon taxes and greenhouse gas emissions trading systems. Duke Environmental Law & Policy Forum, 29, 109–182. vol29/iss1/3

Hammock, C. W., Jr. (2018). Borehole thermal energy storage. Plumbing Engineer, 46(3).

Hammock, C.W., & Sullens, S. (2017). Coupling geothermal heat pumps with underground seasonal thermal energy storage [Final report to the Department of Defense Environmental Security Technology Certification Program, EW-201135]. Andrews, Hammock, & Powell, Inc.

Han, C., & Yu, X. B. (2018). An innovative energy pile technology to expand the viability of geothermal bridge deck snow melting for different United States regions: Computational assisted feasibility analyses. Renewable Energy, 123, 417–427.

Han, J., Cui, M., Chen, J., & Lv, W. (2021). Analysis of thermal performance and economy of ground source heat pump system: A case study of the large building. Geothermics, 89, 101929.

Hance, C. N. (2005). Employment involved in the US geothermal industry. Geothermal Resources Council Transactions, 29, 445–448.

Hangen, E. (2022). Clean energy project development for low-income communities: Strengthening the ecosystem for delivering solar energy and deep efficiency retrofits.

Hannon, B. (2013). Energy and materials conservation: Applying pioneering research and techniques to current non-energy materials conservation issues. Philosophical Transactions, Series A: Mathematical, Physical, and Engineering Sciences, 371(1986), Article 20120005.

Hatten, M. J., & Morrison, W. B. (1995). The Commonwealth Building: Groundbreaking history with a groundwater heat pump. ASHRAE Journal, 37(7).

Heart of Illinois ABC. (2020, March 6). New geothermal system added to Kingsley Junior High.

Hewett, O. L. (2017). Armories. Naval Facilities Engineering Systems Command, Engineering Criteria and Programs Office.

Hitchens, P. L. (2018). Live haul aquaculture marketing & advantages of collective sales [Paper presentation]. 2018 North Central Aquaculture Conference, Kansas City, MO. files/presentation/file/live_haul_aquaculture_marketing_advantages_of.pdf

Holm, T. R., Lu, X., & Larson, D. R. (2015). Feasibility of groundwater source heat pumps for space heating and cooling in Mason County and the American Bottoms Area, Illinois (RR-127, 77 pp.). Illinois Sustainable Technology Center.

Hughes, P. (2008). Geothermal (ground-source) heat pumps: Market status, barriers to adoption, and actions to overcome barriers (ORNL/TM-2008/232, 40 pp.). U.S. Department of Energy.

Hunt, J. D., Nascimento, A., Zakeri, B., Barbosa, P. S. F., & Costalonga, L. (2022). Seawater air-conditioning and ammonia district cooling: A solution for warm coastal regions. Energy, 254(Part B), Article 124359.

Illinois Geothermal Coalition (IGC). (2022). Partner with Illinois Geothermal Coalition. University of Illinois at Urbana-Champaign.

Illinois Pollution Control Board (IPCB). (2018). Title 35: Environmental Protection, Subtitle G: Waste Disposal, Chapter I: Pollution Control Board, Subchapter d: Underground Injection Control and Underground Storage Tank Programs. Part 730: Underground Injection Control Operating Requirements, Subpart A: General, Section 730.105 Classification of Injection Wells.

Im, P., Liu, X., and Henderson, H., Jr. (2016). Case study for the ARRA-funded ground source heat pump demonstration at Ball State University (ORNL/TM-2016/644, 42 pp.). U.S. Department of Energy, Oak Ridge National Laboratory.

Innergex Renewable Energy Inc. (2019). Annual report 2018.

International Energy Agency (IEA), Geothermal Technologies Office. (2022, October). 21. United States of America (2020). In 2021 annual report (pp. 173–182).

International Ground Source Heat Pump Association (IGSHPA). (2017, September). Action plan to expand the market for ground source heat pumps in North America.

International Renewable Energy Agency (IRENA). (2019). Accelerating geothermal heat adoption in the agri-food sector.

Interstate Oil and Gas Compact Commission (IOGCC). (2021). Idle and orphan oil and gas wells 2021.

Jay, A., Reidmiller, D. R., Avery, C. W., Barrie, D., DeAngelo, B. J., Dave, A., Dzaugis, M., Kolian, M., Lewis, K. L. M., Reeves, K., & Winner, D. A. (2018). Overview (pp. 33–71). In Impacts, risks, and adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Jello, J., Khan, M., Malkewicz, N., Whittaker, S., & Baser, T. (2022). Advanced geothermal energy storage systems by repurposing existing oil and gas wells: A full-scale experimental and numerical investigation. Renewable Energy, 199, 852–865.

Jiang, Q. Q., Jiao, Y. Y., Luo, J., & Wang, H. (2019). Review and prospect on heat transfer and bearing performance of energy piles. Rock and Soil Mechanics, 40, 3351–3362. doi:10.16285/j.rsm.2018.0949

John, N., & Jalilinasrabady, S. (2021). Potential of cascaded use of geothermal energy to uplift fish production in Kenyan aquaculture. Transactions Geothermal Resources Council, 45, 486–498.

Jossi, F. (2022, February 7). Colleges see untapped potential in geothermal district energy systems. Energy News Network.

Kahan, A. (2019). EIA projects nearly 50% increase in world energy usage by 2050, led by growth in Asia. U.S. Energy Information Administration.

Karlsdóttir, M. R., Feracor, J. A., Pálsson, H., & Pálsson, O. P. (2014). Geothermal district heating system in Iceland: A life cycle perspective with focus on primary energy efficiency and CO2 emissions. In Proceedings of the 14th International Symposium on District Heating and Cooling (pp. 7–9). IEA Bioenergy.

Katzenbach, R., Olgun, C. G., Loveridge, F. A., Sutman, M., Bowers, G. A., McCartney, J. S., Laloui, L., Mimouni, T., Dupray, F., Spitler, J. D., Clauss, F., Meyer, L. L., & Akrouch, G. (2014). New technologies and applications: Materials and equipment in near surface geothermal systems. DFI Journal—The Journal of the Deep Foundations Institute, 8(2), 93–107.

Kavanaugh, S. P. (1998). A design method for hybrid ground-source heat pumps [Paper TO-98-07-3]. ASHRAE Transactions, 104(2), 691–698.

Kavanaugh, S. P., & Rafferty, K. D. (2014). Geothermal heating and cooling: Design of ground-source heat pump systems. American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Kearney, S. (2016). Peak geothermal. Chicago Cityscape.

Kelly, W. R., Panno, S. V., & Hackley, K. C. (2012). Impacts of road salt runoff on water quality of the Chicago, Illinois, region. Environmental & Engineering Geoscience, 18(1), 65–81.

Keys, W. S., & Brown, R. F. (1978). The use of temperature logs to trace the movement of injected water. Ground Water, 16(1), 32–48.

Kim, Y., Lee, J. S., & Jeon, S. W. (2016). Hybrid ground-source heat pump systems. In S. J. Rees (Ed.), Advances in ground-source heat pump systems (pp. 331–357). Woodhead Publishing.

Kinney, C., Dehghani-Sanij, A., Mahbaz, S., Dusseault, M. B., Nathwani, J. S., & Fraser, R. A. (2019). Geothermal energy for sustainable food production in Canada’s remote northern communities. Energies, 12(21), 4058.

Knuutinen, J., Böök, H., Ruuskanen, V., Kosonen, A., Immonen, P., & Ahola, J. (2021). Ground source heat pump control methods for solar photovoltaic-assisted domestic hot water heating. Renewable Energy, 177, 732–742.

Kohlscheen, E., Moessner, R., & Takáts, E. (2021). Effects of carbon pricing and other climate policies on CO2 emissions [CESifo Working Paper No. 9347]. Munich Society for the Promotion of Economic Research—CESifo GmbH. cesifo1_wp9347.pdf

Koon, R. K., Marshall, S., Morna, D., McCallum, R., & Ashtine, M. (2020). A review of Caribbean geothermal energy resource potential. West Indian Journal of Engineering, 42(2), 37–43.

Kumar, B., & Sharma, V. (2021). Energy demand reduction in the built environment using shallow geothermal integrated energy systems: Part II—Hybrid ground source heat pump for building heating. ASME Journal of Engineering for Sustainable Buildings and Cities, 2(3), Article 030802.

Kumoro, A. C., & Kristanto, D. (2003). Preliminary study on the utilization of geothermal energy for drying of agricultural products [S14 Paper00]. International Geothermal Conference, Reykjavík, Iceland.

Kurnia, J. C., Shatri, M. S., Putra, Z. A., Zaini, J., Caesarendra, W., & Sasmito, A. P. (2022). Geothermal energy extraction using abandoned oil and gas wells: Techno‐economic and policy review. International Journal of Energy Research, 46(1), 28–60.

Kuska, M. H., DeGroot, C. T. & Siddiqui, K. (2020) Investigation of geothermal energy utilization for thermal regulation of aquaculture raceway. International Journal of Green Energy, 17(9), 540–553,

Lai, J., Qiu, J., Chen, J., Fan, H., & Wang, K. (2015). New technology and experimental study on snow-melting heated pavement system in tunnel portal. Advances in Materials Science and Engineering, 2015, 706536.

Laloui, L., & Di Donna, A. (2013). Energy geostructures: Innovation in underground engineering. John Wiley & Sons.

Landrigan, P. J., Fuller, R., Acosta, N. J., Adeyi, O., Arnold, R., Baldé, A. B., … & Zhong, M. (2018). The Lancet Commission on pollution and health. The Lancet, 391(10119), 462–512.

Larson, P. (2022). Steam heat. Voice [Carleton College], Winter.

Larson, T. H. (1990). Interpretation of shallow geothermal surveys [Ph.D. thesis, University of Illinois at Urbana-Champaign, 164 pp.]

L’Ecuyer, M., Zoi, C., & Hoffman, J. S. (1993). Space conditioning: The next frontier—The potential of advanced residential space conditioning technologies for reducing pollution and saving consumers money (EPA 430-R-93-004, 245 pp.). U.S. Environmental Protection Agency, Office of Air and Radiation.

Lee, K. S. (2012). Underground thermal energy storage. In Underground thermal energy storage (pp. 15–26). Green Energy and Technology Series. Springer.

Leetaru, H. (2021, April 6). U.S. Department of Energy Kickoff Meeting: Illinois compressed air energy storage (DE-FE0032019).

Levine, M., Ürge-Vorsatz, D., Blok, K., Geng, L., Harvey, D., Lang, S., Levermore, G., Mongameli Mehlwana, A., Mirasgedis, S., Novikova, A., Rilling, J., & Yoshino, H. (2007). Residential and commercial buildings. In B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (Eds.), Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press.

Li, R., Ooka, R., & Shukuya, M. (2014). Theoretical analysis on ground source heat pump and air source heat pump systems by the concepts of cool and warm exergy. Energy and Buildings, 75, 447–455.

Lilliestam, J., Patt, A., & Bersalli, G. (2020). The effect of carbon pricing on technological change for full energy decarbonization: A review of empirical ex‐post evidence. WIREs Climate Change, 12(1), Article e681.

Lin, Y. F., Stumpf, A., Frailey, S., Okwen, R., Lu, Y., Holcomb, F., Tinjum, J., Stark, T., Damico, J., Elrick, S., Fisher, K., Fu, W., Garner, D., Hammock, C., Kirksey, J., Korose, C., Lin, J., Lin, Z., McKaskle, R., Nelson, J.. . . . Yang, F. (2020a). Geothermal heat recovery complex: Large-scale, deep direct-use system in a low-temperature sedimentary basin [DOE-DE-EE0008106 Final Report]. Illinois State Geological Survey.

Lin, Y.-F., Tseng, C.-Y., & Sargent, S. L. (2020b). User’s manual for the portable thermal response test device (Circular 603). Illinois State Geological Survey.

Liu, H., Maghoul, P., Bahari, A., & Kavgic, M. (2019). Feasibility study of snow melting system for bridge decks using geothermal energy piles integrated with heat pump in Canada. Renewable Energy, 136, 1266–1280.

Liu, H., Stumpf, A. J., Lin, Y.-F. F., & Liu, X. (2021). Distributed thermal response multi-source modeling to evaluate heterogeneous subsurface properties. Groundwater.

Liu, X., Gluesenkamp, K. R., & Mehdizadeh Momen, A. (2015). Overview of available low-temperature/coproduced geothermal resources in the United States and the state of the art in utilizing geothermal resources for space conditioning in commercial buildings (ORNL/TM-2015/131). U.S. Department of Energy, Oak Ridge National Laboratory.

Liu, X., Hughes, P., McCabe, K., Spitler, J., & Southard, L. (2019). GeoVision Analysis Supporting Task Force report: Thermal applications—Geothermal heat pumps (ORNL/TM-2019/502). U.S. Department of Energy, Oak Ridge National Laboratory.

Looney, C. M., & Oney, S. K. (2007). Seawater district cooling and lake source district cooling. Energy Engineering, 104(5), 34–45.

Lowe, J., Koester, R., & Sachtleben, P. (2010). Embracing the future: The Ball State University Geothermal Project. In In W. L. Filho (Ed.) Universities and climate change: Introducing climate change to university programmes (pp. 205–220). Springer-Verlag.

Lu, Q., Narsilio, G. A., Aditya, G. R. & Johnston, I. W. (2017). Economic analysis of vertical ground source heat pump systems in Melbourne. Energy 125, 107–117,

Lund, J. W. (1995). Historical impacts of geothermal resources on the people of North America. Geo-Heat Centre Bulletin (Oregon Institute of Technology, Klamath Falls), 16(4), 7–14.

Lund, J. W. (2005). Pavement snow melting. Geo-Heat Center, Oregon Institute of Technology, 1–13,

Lund, J. W. (2007). Characteristics, development and utilization of geothermal resources. Geo-Heat Centre Quarterly Bulletin (Oregon Institute of Technology, Klamath Falls), 28(2), 1–9.

Mahlmann, J., & Escobedo, A. (2012). Systems for strategic planning on the community scale geothermal heat pump. The American Council for an Energy-Efficient Economy.

Marques, C., Dunham, C., Jones, P., Matabuena, R., Revesz, A., Lagoeiro, H., & Maidment, G. (2021). Integration of high temperature heat networks with low carbon ambient loop systems. ASHRAE Transactions, 127(1), 547–556. _id=2221913#jumps

Massey, J. (2018, June 18). Epic takes sustainability efforts to new heights. The County Today.

Masson-Delmotte, V., Zhan, P., Pirani, A., Connors, S. L., P’ean, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., & Gomis, M. (2021). Climate change 2021 the physical science basis. In Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press. SPM_final.pdf

Martelli, E., Freschini, M., & Zatti, M. (2020). Optimization of renewable energy subsidy and carbon tax for multi energy systems using bilevel programming. Applied Energy, 267, 115089.

McDaniel, A., Tinjum, J., Hart, D. J., Lin, Y. F., Stumpf, A., & Thomas, L. (2018). Distributed thermal response test to analyze thermal properties in heterogeneous lithology. Geothermics, 76, 116–124.

Mehnert, E. (2004). The environmental effects of ground-source heat pumps—A preliminary overview. Illinois State Geological Survey (Open-File Series Report 2004-2, 9 pp.).

Miglani, S., Orehounig, K., & Carmeliet, J. (2018). A methodology to calculate long-term shallow geothermal energy potential for an urban neighbourhood. Energy and Buildings, 159, 462–473.

Millstein, D., McCall, J. D., Macknick, J. E., Nicholson, S. R., Keyser, D. J., Jeong, S., & Heath, G. A. (2019). GeoVision analysis supporting task force report: Impacts. The employment opportunities, water impacts, emission reductions, and air quality improvements of achieving high penetrations of geothermal power in the United States (NREL/TP-6A20-71933, 81 pp.). Lawrence Berkeley National Laboratory and National Renewable Energy Laboratory.

Mitchell, M. S., & Spitler, J. D. (2013). Open-loop direct surface water cooling and surface water heat pump systems—A review. HVAC&R Research, 19(2), 125–140.

Mun, H. S., Dilawar, M. A., Kim, C. D., Ryu, S. B., Moon, J. P., & Yang, C. J. (2022). Geothermal heat pump as an environmental friendly, renewable energy source for sustainable pig production. EnvironmentAsia, 15(1), 166–173.

Nagpal, H., Spriet, J., Murali, M. K., & McNabola, A. (2021). Heat recovery from wastewater—A review of available resource. Water, 13(9), Article 1274.

Nahvi, A., Daghighi, A., & Nazif, S. (2018). The environmental impact assessment of drainage systems: A case study of the Karun River sugarcane development project. Archives of Agronomy and Soil Science, 64(2), 185–195.

National Renewable Energy Laboratory (NREL). (1999). Ground source heat pumps for commercial buildings (6 pp.). U.S. Department of Energy.

Newman, L., & Herbert, Y. (2009). The use of deep water cooling systems: Two Canadian examples. Renewable Energy, 34(3), 727–730.

Nouri, G., Noorollahi, Y., & Yousefi, H. (2019). Designing and optimization of solar assisted ground source heat pump system to supply heating, cooling and hot water demands. Geothermics, 82, 212–231.

Noye, S., Mulero Martinez, R., Carnieletto, L., De Carli, M., & Castelruiz Aguirre, A. (2022). A review of advanced ground source heat pump control: Artificial intelligence for autonomous and adaptive control. Renewable and Sustainable Energy Reviews, 153, Article 111685.

Nyborg, S., & Røpke, I. (2015). Heat pumps in Denmark—From ugly duckling to white swan. Energy Research & Social Science, 9, 166–177.

Office of the Undersecretary of Defense (OUD). (2021). Metrics and standards for energy resilience at military installations. U.S. Department of Defense.

Office of the Under Secretary of Defense for Acquisition and Sustainment (OSD). (2018). Climate change adaptation and resilience [DoD Directive 4715.21]. U.S. Department of Defense.

Olgun, C. G., Martin, J. R., & Bowers, G. A., Jr. (2012). Energy piles: Using deep foundations as heat exchangers. American Society of Civil Engineers Geo-Strata, 13(2), 46–51.

Olsen, D. J., Dvorkin, Y., Fernandez-Blanco, R., & Ortega-Vazquez, M. A. (2018). Optimal carbon taxes for emissions targets in the electricity sector. IEEE Transactions on Power Systems, 33(6), 5892–5901.

Omenikolo, A. I., Obasi, I. B., & Nleonu, E. C. (2020). Application of geothermal energy in agriculture: A review. International Journal of Engineering Applied Sciences and Technology, 5(1), 666–673.

Orio, C. D., Johnson, C. N., Rees, S. J., Chiasson, A., Deng, Z., & Spitler, J. D. (2005). A survey of standing column well installations in North America. ASHRAE Transactions, 109–121.

ORMAT Technologies Inc. (2019). 2018 annual report (244 pp.).

Palomo, E., Colmenar-Santos, A., & Rosales-Asensio, E. (2022). Potential of low-medium enthalpy geothermal energy—Hybridization and application in industry. Springer Nature Switzerland AG.

Panno, S. V., Hackley, K. C., Hwang, H. H., Greenberg, S., Krapac, I. G., Landsberger, S., & O’Kelly, D. J. (2005). Database for the characterization and identification of the sources of sodium and chloride in natural waters of Illinois (Open File 2055-1). Illinois State Geological Survey.

Park, S.-H., Jang, Y.-S., & Kim, E.-J. (2018). Using duct storage (DST) model for irregular arrangements of borehole heat exchangers. Energy, 142, 851–861.

Pasquier, P., Nguyen, A., Eppner, F., Marcotte, D., & Baudron, P. (2016). 10 – Standing column wells (pp. 269–294). In S. J. Rees (Ed.), Advances in ground-source heat pump systems. Woodhead Publishing.

Price, M. (2022, June 20). The Twin Cities, Minnesota-based company is pioneering geothermal heating and cooling in a more cost-effective, practical, energy-efficient, and environmentally friendly way. Water Well Journal, 24–27.

Qu, H., Masud, M. H., Islam, M., Khan, M. I. H., Ananno, A. A., & Karim, A. (2021). Sustainable food drying technologies based on renewable energy sources. Critical Reviews in Food Science and Nutrition.

Raymond, J., Therrien, R., Gosselin, L., & Lefebvre, R. (2011). A review of thermal response test analysis using pumping test concepts. Ground Water, 49(6), 932–945.

Reed, A. L., Novelli, A. P., Doran, K. L., Ge, S., Lu, N., & McCartney, J. S. (2018). Solar district heating with underground thermal energy storage: Pathways to commercial viability in North America. Renewable Energy, 126, 1–13.

Reiter, M. B., Kurtz, L., Attala, M. M., & Baser, T. (2020). Changes in shaft resistance and pore water pressures during heating of an energy foundation. E3S Web of Conferences, 205, 05022.

Reuss, M. (2021). The use of borehole thermal energy storage systems. In L. F. Cabeza (Ed.), Advances in thermal energy storage systems: Methods and applications (2nd ed., pp. 139–171). Woodhead Publishing.

Riddick, S. N., Mauzerall, D. L., Celia, M. A., Kang, M., Bressler, K., Chu, C., & Gum, C. D. (2019). Measuring methane emissions from abandoned and active oil and gas wells in West Virginia. Science of the Total Environment, 651(Part 2), 1849–1856.

Rivoire, M., Casasso, A., Piga, B., & Sethi, R. (2018). Assessment of energetic, economic and environmental performance of ground-coupled heat pumps. Energies, 11(8), 1941.

Robertson-Tait, A., & Hollett, D. (2021). Better together: New synergies and opportunities from hybrid geothermal projects. Geothermal Resources Council Transactions, 45, 1182–1200.

Robins, J. C., Kolker, A., Flores-Espino, F., Pettitt, W., Schmidt, B., Beckers, K., Pauling, H., & Anderson, B. (2021). 2021 US geothermal power production and district heating market report (NREL/TP-5700-78291, 111 pp.). U.S. Department of Energy, National Renewable Energy Laboratory.

Roewe, B. (2020, Dec. 15). Loyola Chicago launches School of Environmental Sustainability as center for research, action. EarthBeat (National Catholic Reporter).

Rosenow, J., Gibb, D., Nowak, T., & Lowes, R. (2022). Heating up the global heat pump market. Nature Energy, 7(10), 901–904.

Ross, L., & Day, M. (2022). Community energy planning: Best practices and lessons learned in NREL’s work with communities (Technical Report NREL/TP-6A50-82937, 16 pp.). U.S. Department of Energy.

Ross, E., Day, M., Ivanova, C., McLeod, A., & Lockshin, J. (2022). Intersections of disadvantaged communities and renewable energy potential: Data set and analysis to inform equitable investment prioritization in the United States. Renewable Energy Focus, 41(1), 1–14.

Rubio-Maya, C., Ambríz Díaz, V. M., Pastor Martínez, E., & Belman-Flores, J. M. (2015). Cascade utilization of low and medium enthalpy geothermal resources—A review. Renewable and Sustainable Energy Reviews, 52, 689–716.

Sáez Blázquez, C., Farfán Martín, A., Nieto, I. M., & González-Aguilera, D. (2018). Economic and environmental analysis of different district heating systems aided by geothermal energy. Energies, 11(5), 1265.

Saloux, E., & Candanedo, J. A. (2018). Forecasting district heating demand using machine learning algorithms. Energy Procedia, 149, 59–68.

Saner, D., Juraske, R., Kübert, M., Blum, P., Hellweg, S., & Bayer, P. (2010). Is it only CO2 that matters? A life cycle perspective on shallow geothermal systems. Renewable and Sustainable Energy Reviews, 14(7), 1798–1813.

Sani, A. K., Singh, R. M., Amis, T., & Cavarretta, I. (2019). A review on the performance of geothermal energy pile foundation, its design process and applications. Renewable and Sustainable Energy Reviews, 106, 54–78.

Sanner, B. (2017). Ground source heat pumps—History, development, current status, and future prospects. In Proceedings of the 12th International Energy Agency Heat Pump Conference (pp. 1–14). International Energy Agency.

Santos, L., Dahi Taleghani, A., & Elsworth, D. (2022). Repurposing abandoned wells for geothermal energy: Current status and future prospects. Renewable Energy, 194, 1288–1302.

Schideman, L., Martin-Ryals, A., & Kuo, C. T. (2019). Advancing pilot-scale integrated systems for algal carbon capture and biofuel production (Technical Report 73, 36 pp.). Illinois Sustainable Technology Center.

Schincariol, R. A., & Raymond, J. (2021). Borehole heat exchangers—Addressing the application gap with groundwater science. Groundwater.

Schmidt, B., & Pettitt, W. (2022, January 27). Geothermal in hybrid renewable systems. Geothermal Rising.

Shahdi, A., Lee, S., Karpatne, A., & Nojabaei, B. (2021). Exploratory analysis of machine learning methods in predicting subsurface temperature and geothermal gradient of Northeastern United States. Geothermal Energy, 9(1), Article 18.

Shen, W., Ceylan, H., Gopalakrishnan, K., Kim, S., Taylor, P. C., & Rehmann, C. R. (2016). Life cycle assessment of heated apron pavement system operations. Transportation Research Part D: Transport and Environment, 48, 316–331.

Skjølsvold, T. M., Throndsen, W., Ryghaug, M., Fjellså, I. F., & Koksvik, G. H. (2018). Orchestrating households as collectives of participation in the distributed energy transition: New empirical and conceptual insights. Energy Research & Social Science, 46, 252–261.

Słyś, D., Pochwat, K., & Czarniecki, D. (2020). An analysis of waste heat recovery from wastewater on livestock and agriculture farms. Resources, 9(1), 3.

Smith, M., Bevacqua, A., Tembe, S., & Lal, P. (2021). Life cycle analysis (LCA) of residential ground source heat pump systems: A comparative analysis of energy efficiency in New Jersey. Sustainable Energy Technologies and Assessments, 47, Article 101364.

Soltani, M., Kashkooli, F. M., Dehghani-Sanij, A. R., Kazemi, A. R., Bordbar, N., Farshchi, M. J., Elmi, M., Gharali, K., & Dusseault, M. B. (2019). A comprehensive study of geothermal heating and cooling systems. Sustainable Cities and Society, 44, 793–818.

Soni, S. K., Pandey, M., & Bartaria, V. N. (2016). Hybrid ground coupled heat exchanger systems for space heating/cooling applications: A review. Renewable and Sustainable Energy Reviews, 60, 724–738.

Sovacool, B. K., Bazilian, M., Griffiths, S., Kim, J., Foley, A., & Rooney, D. (2021). Decarbonizing the food and beverages industry: A critical and systematic review of developments, sociotechnical systems and policy options. Renewable and Sustainable Energy Reviews, 143, 110856.

Spitler, J., Cook, J., & Liu, X. (2020). A preliminary investigation on the cost reduction potential of optimizing bore fields for commercial ground source heat pump systems [SGP-TR-216]. Proceedings of the 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA.

Spitler, J. D. (2000). GLHEPRO—A design tool for commercial building ground loop heat exchangers. Fourth International Heat Pumps in Cold Climates Conference, Aylmer, Quebec, Canada.

Spitler, J. D., Southard, L. E., & Liu, X. (2014). Performance of the HVAC systems at the ASHRAE headquarters building (Final Report, 110 pp.). The Geothermal Exchange Organization.

State of Illinois. (2021). Climate and Equitable Jobs Act. State of Illinois, Public Act 102-0662, signed September 15, 2021, PDF/102-0662.pdf

State of Illinois. (2019). Executive order joining the U.S. Climate Alliance and committing to the principles of the Paris Climate Agreement. Executive Department, State of Illinois, Executive Order 2019-06.

Strpić, K., Barbaresi, A., Tinti, F., Bovo, M., Benni, S., Torreggiani, D., Macini, P., & Tassinari, P. (2020). Application of ground heat exchangers in cow barns to enhance milk cooling and water heating and storage. Energy and Buildings, 224, 110213

Stumpf, A. J., Frailey, S. M., Okwen, R. T., Lu, Y., Holcomb, F. H., Tinjum, J. M., & Lin, Y. F. (2020). Feasibility of deep direct-use for district-scale applications in a low-temperature sedimentary basin [SGP-TR-216]. In Proceedings of the 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA.

Stumpf, A. J., Lin, Y.-F., & Stark, T. D. (2021). Subsurface characterization, monitoring, and modeling of a geothermal exchange borefield for the Campus Instructional Facility at the University of Illinois at Urbana-Champaign (Circular 606). Illinois State Geological Survey.

Tan, L., & Fathollahzadeh, M. H. (2021). Why heat pumps are the answer to heat waves. Rocky Mountain Institute.

Tester, J., Brown, D. W., & Potter, R. M. (1989). Hot Dry Rock geothermal energy—A new energy agenda for the 21st century (LA-11514-MS). Los Alamos National Laboratory.

Tester, J. W., Beckers, K. F., Hawkins, A. J., & Lukawski, M. Z. (2021). The evolving role of geothermal energy for decarbonizing the United States. Energy & Environmental Science, 14(12), 6211–6241.

Tester, J. W., Beyers, S., Gustafson, J. O., Jordan, T. E., Smith, J. D., Aswad, J. A., … & Pritchard, M. (2020). District geothermal heating using EGS technology to meet carbon neutrality goals: A case study of earth source heat for the Cornell University campus. Proceedings of the World Geothermal Congress, 1(2020), 1–23.

Thorarinsdottir, R., Olafsdottir, K., Unnthorsson, R., Boedijn, A., Romero, E. B., van de Ven, R., et al. (2020). Direct use of geothermal resources for sustainable circular food production—Results from the GeoFood Project. Proceedings of the World Geothermal Congress, 1(2020), 1–6.

Thorsteinsson, H. H., & Tester, J. W. (2010). Barriers and enablers to geothermal district heating system development in the United States. Energy Policy, 38(2), 803–813.

Tian, X., Zhou, Y., Morris, B., & You, F. (2022). Sustainable design of Cornell University campus energy systems toward climate neutrality and 100% renewables. Renewable and Sustainable Energy Reviews, 161, Article 112383.

Tuncer, A. D., Mavuş, R., Gökçe Celikten, C., Koşan, M., & Aktaş, M. (2018). Energy efficient industrial buildings: Sustainable energy systems for food process. In Proceedings of International Conference on Technology and Science (pp. 73–79). energy_systems_for_food_process/links/5ce92e9e92851c4eabbcb3c9/Energy-efficient-industrial-buildings-sustainable-energy-systems-for-food-process.pdf

Turnšek, M., Pavlakovič, B., & Boedijn, A. (2021). A feasibility study for the usage of geothermal energy for aquaponics circular food production in Municipality of Brežice [Report; Project deliverable D 6.2]. Faculty of Tourism, University of Maribor, Brežice, Slovenia.

Twait, P. (2021, September). The case for consumer-side demand generation for geothermal (Version 1.2, 23 pp.). Customer Engines.

Tyler, S. W., Selker, J. S., Hausner, M. B., Hatch, C. E., Torgersen, T., Thodal, C. E., & Schladow, S. G. (2009). Environmental temperature sensing using Raman spectra DTS fiber-optic methods. Water Resources Research, 45(4), Article W00D23.

University of Illinois. (2020). iCAP—Illinois climate action plan: Climate action plan for the University of Illinois at Urbana-Champaign (222 pp.).

University of Illinois. (2010). A climate action plan for the University of Illinois at Urbana-Champaign (62 pp.).

U.S. Army. (2013). Going green: Tapping the earth to heat, cool buildings.

U.S. Bureau of Economic Analysis (U.S. BEA). (2020). Annual gross domestic product (GDP) by state: GDP in current dollars (SAGDP2), Illinois. U.S. Department of Commerce.

U.S. Congress. (2021). Consolidated Appropriations Act, 2021 (Division Z–Energy Act of 2020). 116th Congress, 1st session, House Report 116-113.

U.S. Department of Energy (U.S. DOE), Geothermal Technologies Office. (2013). A history of geothermal energy in America.

U.S. Department of Energy (U.S. DOE). (2015a). Energy saver—Choosing and installing geothermal heat pumps.

U.S. Department of Energy (U.S. DOE). (2015b). Energy saver—Geothermal heat pumps.

U.S. Department of Energy (U.S. DOE), Geothermal Technologies Office. (2015c). Low temperature deep direct-use program draft white paper.

U.S. Department of Energy (U.S. DOE). (2019a). GeoVision: Harnessing the heat beneath our feet. 2019/06/f63/GeoVision-full-report-opt.pdf

U.S. Department of Energy (U.S. DOE). (2019b). Energy efficiency and energy security benefits of district energy: Report to Congress, July 2019 (56 pp.).

U.S. Department of Energy (U.S. DOE), Geothermal Technologies Office (2022). Fiscal years 2022–2026: Multi-year program plan (DOE/EE-25578832).

U.S. Department of the Interior (U.S. DOI). (2022, April 12). Biden–Harris Administration releases final guidance on new orphaned well program.

U.S. Energy Information Administration (EIA). (2009). Residential energy consumption survey 2009. U.S. Department of Energy.

U.S. Energy Information Administration (EIA). (2010a). Geothermal heat pump manufacturing activities 2009.

U.S. Energy Information Administration (EIA). (2010b). Manufacturing energy consumption survey.

U.S. Energy Information Administration (EIA). (2015). Residential energy consumption survey 2015.

U.S. Energy Information Administration (EIA). (2018). 2018 commercial buildings energy consumption survey.

U.S. Energy Information Administration (EIA). (2020a). U.S. states: State profiles and energy estimates.

U.S. Energy Information Administration (EIA). (2020b). U.S. states: State profiles and energy estimates, Illinois energy consumption by end-use sector, 2020.

U.S. Energy Information Administration (EIA). (2020c). U.S. energy facts explained.

U.S. Energy Information Administration (EIA). (2021). Annual energy outlook 2021.

U.S. Energy Information Administration (EIA). (2022a). Annual energy outlook 2022.

U.S. Energy Information Administration (EIA). (2022b). Monthly energy review (April 2022).

U.S. Environmental Protection Agency (U.S. EPA). (1999). Geothermal direct heat return flow wells. In Class V underground injection control study (Vol. 18).

U.S. Environmental Protection Agency (U.S. EPA). (2009, December 14). New category of geothermal heat pumps can now earn the Energy Star.

U.S. Environmental Protection Agency (U.S. EPA). (2017). Climate change impacts in the Midwest. climate-impacts/climate-impacts-midwest_.html

U.S. Environmental Protection Agency (U.S. EPA). (2021). Choosing and installing geothermal heat pumps.

U.S. Environmental Protection (U.S. EPA). (2022). Inventory of U.S. greenhouse gas emissions and sinks: 1990–2020 (EPA 430-R-22-003).

Vangkilde-Pedersen, T., Ditlefsen, C., & Lajer Højberg, A. (2012). Shallow geothermal energy in Denmark. GEUS Bulletin, 26, 37–40.

Van Horn, A., Amaya, A., Higgins, B., Muir, J., Scherer, J., Pilko, R., & Ross, M. (2020). New opportunities and applications for closed-loop geothermal energy systems. GRC Transactions, 44, 1123–1143.

Van Nguyen, M., Arason, S., Gissurarson, M., & Pálsson, P. G. (2015). Uses of geothermal energy in food and agriculture: Opportunities for developing countries. Food and Agriculture Organization of the United Nations.

Wang, K., Yuan, B., Ji, G., & Wu, X. (2018). A comprehensive review of geothermal energy extraction and utilization in oilfields. Journal of Petroleum Science and Engineering, 168, 465–477.

Watznauer, K. (2021, February 11). Unit 5 to begin geothermal projects at Chiddix this summer. The Pantagraph (Bloomington, IL).

Weber, C., & Favrat, D. (2010). Conventional and advanced CO2 based district energy systems. Energy, 35(12), 5070–5081.

Wehbi, Z., Taher, R., Faraj, J., Ramadan, M., Castelain, C., & Khaled, M. (2022). A short review of recent studies on wastewater heat recovery systems: Types and applications. Energy Reports, 8(Supplement 9), 896–907.

Wei, M., Patadia, S., & Kammen, D. M. (2010). Putting renewables and energy efficiency to work: How many jobs can the clean energy industry generate in the US? Energy Policy, 38(2), 919–931.

Wendt, D., Huang, H., Zhu, G., Sharan, P., McTigue, J., Kitz, K., Green, S., & McLennan, J. (2019). Geologic thermal energy storage of solar heat to provide a source of dispatchable renewable power and seasonal energy storage capacity. Geothermal Resources Council Transactions, 43, 73–92.

Wirth, E. (1955). Aus der Entwicklungsgeschichte der Wärmepumpe. Schweizerische Bauzeitung, 73(42), 647–651.

Wolfson, G., & Mapel, M. (2020, September 29). The importance of nanogrids in low-carbon residential communities. Taurus Investment Holdings, LLC.

Wood, C. J., Liu, H., & Riffat, S. B. (2009). Use of energy piles in a residential building, and effects on ground temperature and heat pump efficiency. Géotechnique, 59(3), 287–290.

Wood, S. H., & Burnham, W. L. (1983). Boise, Idaho geothermal system. Geothermal Resources Council Transactions, 7, 215–223.

Xu, L., Pu, L., Zhang, S., & Li, Y. (2021). Hybrid ground source heat pump system for overcoming soil thermal imbalance: A review. Sustainable Energy Technologies and Assessments, 44, 101098.

Yang, T., Liu, W., Kramer, G. J., & Sun, Q. (2021). Seasonal thermal energy storage: A techno-economic literature review. Renewable and Sustainable Energy Reviews, 139, Article 110732.

Yang, Z., Liu, X., Gluesenkamp, K. R., Mehdizadeh Momen, A., & Li, J. M. (2016). Transported low-temperature geothermal energy for thermal end uses—Final report (ORNL/TM-2016/658). Oak Ridge National Laboratory.

Yin, H., Zadshir, M., & Pao, F. (2022). 9 – BIPVT applications in farms. In Building integrated photovoltaic thermal systems: Fundamentals, designs and applications (pp. 459–503). Elsevier.

Yoon, S., Lee, S. R., & Go, G. H. (2015). Evaluation of thermal efficiency in different types of horizontal ground heat exchangers. Energy and Buildings, 105, 100–105. j.enbuild.2015.07.054

You, T., Wu, W., Yang, H., Liu, J., & Li, X. (2021). Hybrid photovoltaic/thermal and ground source heat pump: Review and perspective. Renewable and Sustainable Energy Reviews, 151, 111569.

Yousefi, H., Ármannsson, H., Roumi, S., Tabasi, S., Mansoori, H., & Hosseinzadeh, M. (2018). Feasibility study and economical evaluations of geothermal heat pumps in Iran. Geothermics, 72, 64–73.

Yousefi, H., Noorollahi, Y., Mohammadi, M., Bigdelou, P., & Taheri Bavil Oliaei, M. (2017). Assessment and deployment of ground source heat pump for air pollution reduction in Tehran, Iran. Environmental Energy and Economic Research, 1(3), 269–278. eeer.2017.87106.1007

Zhao, Z., Lin, Y.-F., Stumpf, A. J., & Wang, X. (2022). Assessing impacts of groundwater on geothermal heat exchangers: A review of methodology and modeling. Renewable Energy, 190, 121–147.

Zhou, K., Mao, J., Li, Y., & Zhang, H. (2020). Performance assessment and techno-economic optimization of ground source heat pump for residential heating and cooling: A case study of Nanjing, China. Sustainable Energy Technologies and Assessments, 40, 100782. j.seta.2020.100782

Zhou, N., Price, L., Yande, D., Creyts, J., Khanna, N., Fridley, D., Lu, H., Feng, W., Liu, X., Hasanbeigi, A., Tian, Z., Yang, H., Bai, Q., Zhu, Y., Xiong, H., Zhang, J., Chrisman, K., Agenbroad, J., … Franconie, E. (2019). A roadmap for China to peak carbon dioxide emissions and achieve a 20% share of non-fossil fuels in primary energy by 2030. Applied Energy, 239, 793–819.

Zong, Y., Valocchi, A. J., & Lin, Y.-F. F. (2021). Coupling a borehole thermal model and MT3DMS to simulate dynamic ground source heat pump efficiency. Groundwater.

List of Acronyms and Abbreviations
°C Degrees Celsius
°F Degrees Fahrenheit
AAPG American Association of Petroleum Geologists
AI Artificial intelligence
AIEC Association of Illinois Electric Cooperatives
ANSI American National Standards Institute
ARRA American Recovery and Reinvestment Act
ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers
ASME American Society of Mechanical Engineers
ATES Aquifer thermal energy storage
BBB Build Back Better Act (bill introduced by the 117th Congress and later spun off from the American Jobs Plan, alongside the Infrastructure Investment and Jobs Act; some climate change, health care, and tax reform proposals were included in the Inflation Reductions Act of 2022)
BHE Borehole heat exchanger
BTES Borehole thermal energy storage
Btu British thermal unit (measure of the heat content of fuels or energy sources—quantity of heat required to raise the temperature of one pound of liquid water by 1°F at the temperature that water has its greatest density [~39°F])
CaCl2 Calcium chloride
CCHP Combined cooling heating and power (process by which some of the heat produced by an energy system is used to generate chilled water for air conditioning or refrigeration)
$CDN Canadian dollars
CHP Combined heat and power (energy-efficient technology that generates electricity and captures the heat)
CH4 Methane (molecular formula)
CJEA Climate and Equitable Jobs Act (bill passed in Illinois legislature in 2021)
CO2 Carbon dioxide (molecular formula)
CO2e Carbon dioxide equivalent
ComEd Commonwealth Edison Company, an Exelon Company
COP Coefficient of performance (rating system for heating efficiency—ratio between the power [kW] drawn out of the heat pump for heating, and the power [kW] that is supplied to the compressor)
C-PACE Commercial property assessed clean energy
CSA Canadian Standards Association
DDU Deep direct-use (type of geothermal energy technology)
DoD United States Department of Defense
DTRT Distributed temperature response tests
DTS Distributed temperature sensing
DX Direct exchange systems
EDA Economic Development Administration of the United States
EER Energy-efficiency ratio (rating system for cooling efficiency—ratio of output cooling energy [in BTU] to input electrical energy [in watts] at a given operating point, and normally calculated with a 95°F outside temperature and an inside [return air] temperature of 80°F and 50% relative humidity)
EFP Existing Facility Protection (part of the Chicago Department of Transportation’s Planned Work Permit process and required during the design stage to notify agencies with facilities within the adjacent infrastructure of upcoming work)
EGS Enhanced geothermal systems
EIA United States Energy Information Administration
ERC Energy Resource Conservation
F&S Facilities & Services at the University of Illinois at Urbana-Champaign
FEMP Federal Energy Management Program
ft Feet
GAOI Geothermal Alliance of Illinois
GDHC Geothermal district or community heating and cooling
GEOPHIRES GEOthermal Energy for Production of Heat and Electricity (“IR”) Economically Stimulated (free and open-source computer code to perform techno-economic simulations of geothermal energy systems)
GeoTES Geologic Thermal Energy Storage project being led by Lawrence Berkeley National Laboratory
GHG Greenhouse gases (includes CO2, CH4, SO2, and NOx, and PM2.5)
GHP Geothermal heat pump
GIS Geographic information systems
GTI Ground thermal imbalance (thermal imbalance between heat injection and extraction to/from the ground)
GTO Geothermal Technologies Office of the United States Department of Energy
GWe Gigawatt electrical (refers to electric power)
GWth Gigawatt-hours-thermal (power available directly from heat or thermal energy)
HDPE High-density polyethylene (tough, durable piping material with unique performance properties [density, before additives or pigments, is >0.941 g/cm] used by the GHP industry and approved for geothermal loops in all model codes across the United States and Canada)
H.R. House of Representatives of the U.S. Congress
HVAC Heating, ventilation, and air conditioning
iCAP Illinois Climate Action Plan
IEA International Energy Agency
IECA Illinois Energy Conservation Authority NFP (not for profit)
IEPA Illinois Environmental Protection Agency
IDPH Illinois Department of Public Health
IGC Illinois Geothermal Coalition
IGEN Illinois Green Economy Network
IGSHPA International Ground Source Heat Pump Association
IOGCC Interstate Oil and Gas Compact Commission
IPCB Illinois Pollution Control Board
IPCC Intergovernmental Panel on Climate Change
IRENA International Renewable Energy Agency
IRS Internal Revenue Service of the United States
ISGS Illinois State Geological Survey
ITC Investment tax credits
kWh Kilowatt-hour (unit of energy—one kilowatt of power for one hour)
LCCA Life cycle cost analysis
LCOE Levelized cost of electricity
LEAP Local Energy Action Program
LEED Leadership in Energy and Environmental Design (globally recognized holistic green building project and performance management rating system devised by the U.S. Green Building Council)
LPG Liquefied petroleum gas (also known as propane)
m Meters
MMt Million metric tons
MW Megawatts (measure of the total amount of energy consumed)
MWhe Megawatts of heat output
NOx Nitrogen oxides
NPV Net present value
NREL National Renewable Energy Laboratory
OSD Office of the Under Secretary of Defense for Acquisition and Sustainment
OUD Office of the Under Secretary of Defense
P3 Public–private partnership
PM2.5 Fine particulate matter (inhalable particles with diameters that are generally 2.5 micrometers and smaller)
PPA Purchased “Power” (Thermal) Agreement (PPA)
PV Photovoltaic (materials and devices that convert sunlight into electrical energy)
PVT Photovoltaic–thermal (materials and devices that convert sunlight into both electricity and hot water)
RD&D Research, demonstration, and development
RDD/CA Research, Development, Demonstration, and Commercialization Application
REAP Rural Energy for America Program of the U.S. Department of Agriculture
ROI Return on investment (ratio of net profit to cost of investment)
SAGHP Solar-assisted geothermal heat pump
S.B. Senate bill (Illinois and New York legislatures)
SBIR Small Business Innovation Research (program of the U.S. Department of Energy)
SEDAC Smart Energy Design Assistance Center
SO2 Sulfur dioxide
STARS Sustainability Tracking, Assessment & Rating System (transparent, self-reporting framework for colleges/universities to measure sustainability performance administered by the Association for the Advancement of Sustainability in Higher Education [AASHE])
STTR Small Business Technology Transfer (program of the U.S. Department of Energy)
TRNSYS TRaNsient SYstem Simulation Tool (energy simulation software used to assess the performance of thermal and electrical energy systems)
TRT Thermal response tests
UIC Underground Injection Control (federal program established under the provision of the Safe Drinking Water Act of 1974 that regulates the underground injection or discharge of six classes of hazardous and nonhazardous liquid and gas)
UIUC University of Illinois at Urbana-Champaign
U.S. United States
US$ United States dollar
U.S. DOE United States Department of Energy
U.S. DOI United States Department of the Interior
U.S. EPA United States Environmental Protection Agency
UTES Underground thermal energy storage
WAP Weatherization Assistance Program Office of the U.S. Department of Energy