Energy demand has been increasing worldwide over the past several decades, resulting from continued industrial and economic development, and is expected to grow a further ~50% beyond 2018 demand by 2050. A transition away from fossil fuel usage to lower carbon energy technologies is needed to decarbonize our energy systems. But we must also incentivize energy efficiency and conservation to support the growth of a green economy, contribute to the protection of the environment, and improve human health. 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 geothermal energy for use in building heating and cooling, and water heating systems that can be applied across 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 underground thermal energy storage and hybrid energy systems. To achieve the zero-carbon emission goals set forth by state and local governments in the U.S. Midwest that will move society away from a reliance on fossil fuels, we propose geothermal energy and geoexchange technologies as solutions that are renewable, have low carbon emissions, are relatively inexpensive considering the system life cycles, and are reliable and safe. Here we present a comprehensive review of geothermal applications for different commercial and industrial sectors. Since their widespread inception more than 40 years ago, the efficiency of geothermal energy 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 carbon emission 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 and justice, energy efficiency, additional basic and applied research support, and the repurposing of available skills from the oil and gas industry. 


As we strive to improve the sustainability of our homes and businesses, we look to conserve energy and upgrade our heating and cooling equipment to be more energy efficient. Through these measures, we wish to improve and protect the environment, save money, and lessen our impact on the energy grid. Although many choices are available for heating and cooling, there remains a growing interest in utilizing the geothermal energy below our feet. Tapping Earth’s natural underground thermal energy is one way Illinois residents can participate in this energy transition and invest in lower cost renewable energy. 

Geothermal energy means different things to different people. In the U.S. Midwest, low-temperature resources are tapped by vertical and horizontal pipes and boreholes. At relatively shallow depths, the ground temperature remains the same year round (50°F–55°F or 10°C–14°C), providing a constant supply of energy. Combined with a temperate climate of cold winters and hot summers, the state has conditions favorable for the operation of geothermal heat pumps. In this White Paper, we discuss various system configurations and applications that serve to bring significant economic, environmental, and community benefits. They also provide security and resiliency to the electrical grid. 

To support the widespread uptake of geothermal energy systems, multiple levels of government have developed incentive programs. However, policy changes are needed in Illinois to elevate geothermal energy on par with solar and wind energy so that it benefits all stakeholders, from customers to workers and utilities. Dedicated and focused marketing and outreach for geothermal energy explaining the economic and environmental benefits would result in broader public acceptance and accelerate the deployment rate and market penetration. Public perception of renewable energy is evolving during this energy transition, and there is interest in identifying solutions for underserved and disadvantaged communities that historically have experienced vulnerability because of climate change impacts. 


In the current socioeconomic model, energy consumption will increase in the United States over the next 30 years across a variety of behavioral scenarios as population and infrastructure growth outpace the gains in efficiency of energy systems, according to the Energy Information Administration’s Annual Energy Outlook 2022 (EIA, 2022a). At present, roughly 79% of energy needs in the United States are supplied by the burning of fossil fuels, according to EIA’s April 2022 Monthly Energy Review (EIA, 2022b). Of this energy demand, 16% is for the residential sector and 12% for the commercial sector. In 2020, 55.9% of this energy consumed was for space conditioning in buildings (heating and cooling) and heating water (Figure 1.1). 

According to the U.S. Environmental Protection Agency (U.S. EPA) and National Wildlife Federation, 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 (Cross et al., 2011; L’Ecuyer et al., 1993; U.S. EPA, 2021). In 2023, the Rocky Mountain Institute reported that in the U.S. Midwest over the next 30 years, using geothermal energy would lead to an 85% and 90% reduction in greenhouse gas emissions compared with fossil fuel heating systems using natural gas and propane, respectively (Reeg et al., 2023). Furthermore, geothermal heat pumps use 25%–50% less electricity than conventional heating or cooling systems. Altogether, the lowering of energy consumption would lead to a reduction in energy costs for homeowners of up to 30%–70%. 

The combustion of fossil fuels has been shown to have adverse impacts on the environment because of their high greenhouse gas emissions (Landrigan et al., 2018), chiefly carbon dioxide (CO2) and methane (CH4), which have been identified as the main contributors to climate change (Ehhalt et al., 2001). Considering geothermal energy technologies through a full life cycle assessment, their emissions make them cost-competitive with energy from combustion of fossil fuels, solar panels, and wind turbines, and they would provide a significant long-term reduction in greenhouse gas emissions (Ball, 2020a). 

Geothermal heat pump systems tap the Earth’s inexhaustible resource of thermal energy underground, providing a reliable and flexible source for heating and cooling in the residential, commercial, agricultural, and industrial sectors (Bundschuh et al., 2017; Kurnia et al., 2022; Spitler et al., 2020; Zhou et al., 2020). Using geothermal energy also 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, 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 and hotter summers with longer dry periods. The Environmental Defense Fund has suggested these periods of extreme heat and flooding are affecting the infrastructure, human health, agriculture, transportation, and air and water quality (EDF, 2014). 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 (Melillo et al., 2014). 

In the U.S. 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 (CJEA, 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. The State will invest in training a diverse, equitable, clean energy workforce and expand Illinois’ commitments for improving energy efficiency, utilizing renewable energy, and supporting electric vehicles sales. 

Geothermal energy is one of the renewable energy sources that could be utilized to achieve the regulatory goal set forth by Illinois to become carbon neutral and meet the aforementioned signed mandates. Unlike electricity generation from solar and wind energy, which is impacted by changing atmospheric conditions, geothermal energy systems provide a constant source of heat and cooling 24 hours a day, 365 days a year, providing a level of security and resiliency to grid operations. 

Developments in geothermal technologies have motivated educational campuses in the U.S. Midwest (e.g., Ball State University) to meet climate action objectives and greenhouse gas emission targets. Here at the University of Illinois at Urbana-Champaign (UIUC), the installation of geothermal energy systems is dovetailed with renewable energy-related research studies in an effort to better understand aspects of the underground environment and the impact of groundwater, 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. Entrepreneurs will emerge to advance geothermal and sustainable energy system design, equipment manufacturing, and distribution and installation that will benefit the geothermal community by creating additional job markets. 

Furthermore, in Illinois, a specialized workforce for oil and gas exploration and drilling is available to be retrained and reskilled to construct geothermal energy systems. The specialized skills and equipment utilized by the oil and gas industry overlap those needed for geothermal energy; thus, 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 on the UIUC 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 technology demonstrations and simulations, as well as education and outreach programs. The IGC also contributes to federal and state legislation and planning documents. For example, the IGC provided technical review of the Energy Act of 2020, Division Z of the Consolidated Appropriations Act, 2021 (https://www.congress.gov/116/plaws/publ260/PLAW-116publ260.pdf) 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 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 the Illinois Climate Action Plan (iCAP), with the latest version published in 2020 (University of Illinois, 2020). The UIUC administration originally signed several climate leadership commitments, including the American College and University Presidents’ Climate Commitment in 2008 that pledged carbon neutrality on campus 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 implementation of campus geothermal energy systems will assist in meeting the 2030 goal of using clean energy sources for 15% of the total campus energy demand (University of Illinois, 2020). 

The geothermal research at the UIUC is also pertinent to the renewable energy targets mandated by the State of Illinois in the Climate and Equitable Jobs Act (CEJA) enacted in 2021 (State of Illinois, 2021). Pursuing further development and implementation of geothermal technologies that can be directly applied in Illinois will provide decision makers and stakeholders the capacity to promote equitable economic development and workforce training opportunities that will support equal access to energy-efficient techniques and renewable heating and cooling systems during this energy transition. Pairing the applied research with statewide education and outreach activities will improve the knowledge base, which 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 (American Association of Petroleum Geologists [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 funded to investigate geothermal energy and energy efficiency was active on the UIUC 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 thermal 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.illinois.edu) has conducted research on heating, ventilation, air conditioning, and refrigeration systems, creating industry-relevant technologies that contribute to building energy efficiency, including geothermal heat exchangers. The equipment operates in an environmentally friendly and sustainable way, improving energy efficiency, sustainability, and reliability such that it reduces the overall footprint, leading to significant cost savings. 

As a leader in sustainability, the UIUC campus recently earned Gold certification by the Sustainability Tracking, Assessment & Rating System (STARS) program from the Association for the Advancement of Sustainability in Higher Education (https://sustainability.illinois.edu/campus-sustainability/recognition) and was recognized for outstanding energy and resource savings made possible by installing a geothermal exchange system at the Campus Instructional Facility (https://illinoisashrae.org/downloads/Tech_Award_Reports_2021/u_of_i_campus_instructional_facility.pdf). The geothermal energy system is one of six currently being managed by the UIUC Facilities & Services (F&S) located on the main campus and at Allerton Park that services academic, administration, and residence buildings, as well as a greenhouse. Recently, F&S began supporting academic collaborations, such as studies of geothermal energy, through a new Academic Collaborations initiative. So far, 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 energy loops in four 50-foot drilled shafts at the Kavita and Lalit Bahl Smart Bridge that connects the Hydrosystems Laboratory to the Newmark Civil Engineering Laboratory (https://archive.fs.illinois.edu/services/academic-collaborations/geothermalcoalition/Geothermal-Energy-Piles).

2.1 Background

Although there is great interest in advancing the portfolio of renewable energy technologies in the power generation sector, with the installation of wind and solar energy systems continuing to increase rapidly, less attention has been paid to the decarbonization of building heating and cooling systems. To address decarbonization, there is a realization that the future energy system will include multiple energy sources and that geothermal energy will be crucial in supplying a reliable baseload to the electrical 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 legislated a “renewable energy source” (U.S. Congress, 2021) and has direct applications in all 50 states and U.S. territories for heating and cooling applications or generating electricity. Geothermal energy resources are present in a variety of underground environments that can be accessed 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 radiant solar 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) depth, beyond the impact of atmospheric temperature 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 (~30.5 m; 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 dispersed 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 currently economically viable at present in Illinois (see list below). High-temperature geothermal resources that can drive power 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 the direct use of heated water and brines are utilized for heating and cooling buildings and making domestic hot water. 

1. Hydrothermal resources: This conventional geothermal resource contains naturally occurring water, heat, and permeable bedrock that are utilized to 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. Directuse resources are commonly used for geothermal district heating applications, such as in the City of Boise, Idaho. 

4. Lowtemperature and co-production resources: Low-temperature and co-production resources are considered nonconventional hydrothermal resources that are <300°F (<150°C), and the extraction technologies are co-located within oil and natural gas fields, mineral extraction sites, and energy storage sites for generating electricity or directuse applications. Recent development of these geothermal resources has been focused on sedimentary basins (Jello et al., 2022; 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°F55°F or 10°C14°C) in the shallow subsurface (upper 500 ft or 150 m) available yearround 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. 

Because geothermal energy systems utilize the virtually untapped thermal energy resources available underground, they will play an important role in meeting our decarbonization goals. Besides their reliability and environmental benefits, geothermal energy systems 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. Unlike air source heat pumps, the performance of borehole heat geoexchangers (BHE) is not impacted by the outside air temperature and humidity. Because the underground temperatures remain relatively unchanged throughout the year and the difference between the ground and heat pump refrigerant temperature is constant, heat transfer rates are maximized and the geothermal energy system operates at much higher year-round efficiencies. Although GHPs do not replace the need for electricity (they run on electricity), in the U.S. Midwest they use up to 80% less energy annually than industry-standard fossil fuel furnaces to heat homes (Reeg et al., 2023; U.S. DOE, 2015a). In addition, GHPs use four times less electricity for heating buildings on the coldest days than do air source heat pumps..  

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 they 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. Such LCCAs 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 reduction in electricity required to operate a GHP and its coefficient of performance (COP) are the primary attributes 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 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. GSHP, ground source or geothermal heat pump.
Figure 2.4. Economic and environmental benefits of a typical 4-ton geoexchange energy system in Duluth, Minnesota, in 2012. (a) The energy cost savings per net system cost is shown for electric and fossil fuel heating systems. Ground source or geothermal heat pumps (GSHPs) 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. (b) 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. GSHP, ground source or geothermal heat pump; RH, restive heating; ASHP, air source heat pump; Nat Gas, natural gas.
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. Hot water 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 roughly 300 times more energy than 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. Hot springs served as a source of warmth and cleansing, and their minerals as a source of healing (Lund, 1995). The first European settlers to reach 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 heated waters. In 1892, Boise, Idaho, became the first city to develop a district geothermal 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 the 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 establishing WaterFurnace International, the Fort Wayne, Indiana-based heat pump manufacturer, in 1983 (Egg, 2022b). Later, Dan Ellis left WaterFurnace to lead ClimateMaster Incorporated out of a deficit to become a $200 million company within 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 GHPs (U.S. DOE, 2013). Through the American Recovery and Reinvestment Act of 2009 (U.S. Congress, 2009), the U.S. DOE’s Geothermal Technologies Office awarded funding to 149 geothermal energy 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 then, the Geothermal Technologies Office has continued investing in advanced geothermal energy 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 fossil fuel usage, alternatives have been proposed to decarbonize energy systems and electrify the building stock to reduce greenhouse gas 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. households, it could contribute to a significant reduction in greenhouse gas emissions. Furthermore, in 2019, 31% of the U.S. households used fossil fuels as the primary energy source in buildings and industrial processes, mostly for heating (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. In addition, ~41% of the U.S. total greenhouse gas emissions in 2020 came from fossil fuel heating in the residential, commercial, and industrial sectors (U.S. EPA, 2022). 

The U.S. DOE, through its 2019 report GeoVision: Harnessing the Heat Beneath Our Feet (https://www.energy.gov/eere/geothermal/downloads/geovision-harnessing-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 installing 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 minerals, drilling technologies, thermal energy storage, integrated energy systems, technical assistance, and stakeholder outreach and education. 

In addition to the federal support, state and local governments have adopted their own renewable energy 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, https://www.nysenate.gov/legislation/bills/2021/S9422). 

2.4 Geothermal Energy Technologies and Installations

The key geothermal energy technologies that can be utilized in Illinois and the U.S. Midwest are broadly grouped by the geothermal industry into conventional low-temperature geothermal technologies, which include GHPs 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 geographic location (latitude), climate, geology, project budget, complexity of installing the technology, and end use application. In the United States, 84% of the GHP systems use closed-loop borehole heat exchangers (Liu et al., 2019), which also 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). These systems 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 greenhouse gas emissions (International Ground Source Heat Pump Association [IGSHPA], 2017). Advanced geothermal systems are technologies in the conceptual, demonstration, or early commercialization 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 technologies can be used in residential, commercial, and industrial building applications, and all play an important role in our energy conservation efforts. Geothermal energy systems can be installed in new constructions or during renovations of existing buildings to improve energy efficiency. These systems essentially replace the need to own both a natural gas or propane furnace and an air conditioner by providing heating and cooling in one piece of equipment. 

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

Horizontal ground 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 the geoexchange processes (heat conduction) between the geothermal loop, which is made of high-density polyethylene (HDPE) pipe, and the ground. Water or a water and glycol (antifreeze) mixture (also known as the working fluid) is circulated through the pipe 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 fluid in the pipe. The length of pipe 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 the HDPE pipe in open trenches, excavations, and directional bored holes (Figure 2.5). The pipe is typically buried 5–6.5 ft (1.5–2 m) underground, well below the frost line. The pipes are installed in various configurations, including linear, slinky-coil, and spiral-coil arrangements (Cui et al., 2019). The performance of horizontal closed-loop systems is generally lower compared with vertical closed-loop systems because seasonal changes in soil temperature and moisture content impact the amount of thermal energy that can be transported. However, horizontal closed-loop systems cost less to install than vertical boreholes because digging trenches 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 the subsurface has a complex geology or dense network 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). Different configurations of HDPE pipe are used inside the borehole, including single, double, helical, and coaxial construction (Balaji, 2021). As mentioned, thermal energy is transferred into fluid in the pipe, and that fluid is pumped to the surface. During winter, heat is carried into the building, and colder fluids are circulated out of the building into the ground. 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 geothermal energy systems and maintain the ambient ground temperatures, preconstruction simulations are performed, especially for projects with larger borefields. These simulations require the anticipated amount and rate of thermal energy exchange with the ground (i.e., heat extraction and rejection loads), which are impacted by the (1) ground thermal properties, (2) borehole heat exchange design (e.g., borehole diameter, pipe sizes, flow rates, grout thermal properties), (3) configuration of boreholes in the borefield (e.g., number of boreholes and their spacing), and (4) depth of the boreholes (e.g., Spitler, 2000). To accurately measure the ambient ground temperature, thermocouple sensors (Gehlin & Nordell, 2003), temperature probes (Keys & Brown, 1973), or fiber-optic distributed temperature sensing (DTS) systems (Freifeld et al., 2008) are deployed in monitoring or test boreholes. To determine the thermal transport properties underground, thermal response tests (TRT) or distributed thermal response tests (DTRT) are performed in the boreholes (McDaniel et al., 2018; Raymond et al., 2011). These tests provide an estimate of the ground thermal conductivity and rates of heat transport. Alternatively, core samples taken from the boreholes can be tested in the laboratory to determine the thermal conductivity of discrete geological units (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 could potentially leak, local environmental regulations may be enacted prohibiting their use. 

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 the water leaves the GHP, it is returned to the same aquifer unit underground through the borehole or is discharged in a water body or stormwater drain at the ground 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 thermal energy with the surrounding ground (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 250–1,475 ft (75–450 m) deep, mostly constructed in hard, competent bedrock (Orio et al., 2005). The geoexchange process is enhanced by pumping water to the surface, which allows groundwater to flow into the borehole. Because of the higher rate of geoexchange, the water temperature is typically similar to the mean ground temperature. 

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 encouraged engineers and architects to design buildings with better energy efficiency. To achieve this performance, they use the building foundation to harness the thermal energy available in the shallow subsurface for heating and cooling (Adam & Markiewicz, 2009; Sani et al., 2019). When outfitted with closedloop geothermal energy systems, cast-in-place technologies (e.g., piles and drilled shafts) and other geotechnical structures in contact with the ground (i.e., shallow foundations, retaining walls, basement walls, tunnel linings, and earth anchors; Figure 2.6) are used to harness the near-surface geothermal energy (Brandl, 2006; Cui et al., 2018; Jiang et al., 2019; Liu et al., 2019). For example, thermal energy can be extracted from or injected into the ground by circulating a water and glycol mixture through HDPE pipe attached to the reinforcement cage of pile foundations, which are typically constructed to depths between 30 and 80 ft (10 and 25 m; Abdelaziz et al., 2011). The use of foundation elements for meeting thermal energy requirements is not new; they were first introduced in Austria and Switzerland in the 1980s (Katzenbach et al., 2014) and are becoming more common in energyefficient buildings (e.g., Amis, 2022). Because geothermal piles are installed as the required structural supports, there are no additional drilling costs. The cost savings for outfitted geothermal piles compared with drilling the required vertical boreholes can 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 that is applicable in the low-temperature (<200°F or <95°C) 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 region of the United States) for direct-use applications, referred to as deep direct use (DDU; U.S. DOE, 2015c). The technology comprises extraction and injection wells constructed in pairs (called “well doublets”), one for pumping heated geothermal fluid (water or brine) from porous aquifers and another for returning fluid to the geothermal reservoir once the heat has been exchanged (cf. Beckers et al., 2021; Lin et al., 2020). The thermal energy carried by fluid pumped out of the ground is exchanged directly from pipelines for various heating and cooling applications in buildings (stand-alone or in a district energy system), greenhouses, and fish farms, as well as for food drying, snow melting, and industrial processes (https://www.energy.gov/eere/geothermal/articles/energy-department-explores-deep-direct-use). Residual thermal energy can also be used in cascading heating applications (Rubio-Maya et al., 2015). Furthermore, connecting deep direct-use systems with absorption chillers would accomplish space cooling, refrigeration, and freezing. In this process, the extracted thermal energy 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 in providing geothermal energy in urban areas (Abesser et al., 2020; Dalla Longa et al., 2020). This technology offers a sustainable, zero-emissions alternative to conventional heating and cooling systems powered by fossil fuels, as well as the opportunity for significant expansion of geothermal energy to a much wider part of the United States. These geothermal energy systems are best suited for applications with large energy demands (e.g., university or college campuses, military installations, medical complexes, offices, hotels), especially in hot and humid climates where buildings are cooling dominant (Geothermal Rising, 2022). Deep direct-use geothermal energy systems provide the energy grid with a degree of reliability, reduce greenhouse gas emissions, and reduce overall water use compared with conventional evaporative cooling technologies (U.S. DOE, 2015a). Key challenges still exist for technology development, including regionally specific 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 technology that has higher efficiency and storage capacity (Lee, 2013). It is therefore the preferred choice for longduration 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 moderate climates like Illinois and the U.S. Midwest, where groundwater is abundant in the glacial deposits and in shallow and sedimentary bedrock, and where the demand for space heating and cooling alternates seasonally, the most applicable technique is ATES (Bloemendal et al., 2014). In these geothermal energy systems, thermal energy is temporarily stored underground at depths below 50 ft (15 m), and heated or cooled groundwater is circulated through injection and withdrawal wells (see the U.S. DOE news release Underground water could be the solution to green heating and cooling, https://cleantechnica.com/2023/04/09/underground-water-could-be-the-solution-to-green-heating-cooling). At these depths, the ground temperature is similar to the mean air temperature; therefore, the ground temperature will be warmer than the air in the winter and cooler during the summer (Lee, 2013). The key requirements for ATES are the availability of an aquifer and suitable hydrogeological conditions, such as a low groundwater flow, high permeability, and geochemical conditions that prevent the clogging and corrosion of wells (Fleuchaus et al., 2018). Aquifer thermal energy storage has the highest storage capacity, and is therefore the most suitable technology for large-scale applications (Bloemendal et al., 2014). The system capacity typically ranges between 0.3 and 3.4 MMBtu/h (million British thermal units per hour, or 0.1 and 0.3 MW [megawatts]) for small-scale applications and between 17.1 and 102.4 MMBtu/h (5 and 30 MW) for large-scale applications (Fleuchaus et al., 2018). 

Where groundwater is less abundant, closed-loop systems, such as BTES, would be feasible. These systems are less affected by permeability and are typically installed in hard bedrock or unconsolidated clay, sand, and soil (overburden) with high volumetric heat capacities (Gehlin, 2016). The BTES systems are commonly constructed in cylinder- and box-shaped configurations to maximize energy storage (cf. Hammock, 2018). 

According to the Intergovernmental Panel on Climate Change (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 for cities will be to increase the use of renewable energy sufficiently to meet the cooling demand, 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 and utilize waste heat common in cities (Dincer, 2002; Goetzl et al., 2023). Beernink et al. (2022) found that a 30% increase in ATES adoption would lead to a 60% maximum reduction in greenhouse gas emissions. However, with the expansion of UTES without proper management, undesirable thermal interactions between wells and geothermal energy systems could occur that would adversely impact 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 and uses a geothermal resource as a heat or cooling source and that distributes thermal energy 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 (Velvis & Buunk, 2017). 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.). This technology leverages commercially available geothermal energy components. 

Geothermal district energy systems are mature technologies in which energy 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 system, including the sizing of borefields and HVAC equipment (Gautier et al. 2022). Conventional district geothermal systems with direct-use applications take advantage of naturally heated water or brine from deep underground rock formations, which is pumped to the surface and flows across 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 multiple buildings together also allows for the use of waste thermal energy, which can be recycled and shared within 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). These systems are decentralized and bidirectional, operating as close to ground temperature as possible and using 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). 

Fifth-generation district heating and cooling systems replace 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 complexes, in residential complexes, for multi-owner districts (e.g., downtown corridors), and in data centers. When district heating and cooling systems are operated at lower or ambient temperatures, lower flow rates are possible, which leads to less distribution loss within the district (Gautier et al. 2022). When combined with large-scale heat pumps, these systems return significant energy savings. 

The economic and environmental benefits to communities that utilize geothermal energy systems are discussed by Marques et al. (2021). Integrated, smartly controlled district heating and cooling systems have the potential to deliver significant reductions in greenhouse gas emissions, improve air quality, and reduce energy costs for the end users. In addition, they have long-term, stable space-heating rates that facilities powered by fossil fuels cannot guarantee (Thorsteinsson & Tester, 2010). These 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 district geothermal systems requires an energy transition workforce that would employ engineers, petroleum and utility workers, and the building trades workforce. 

The adoption of district heating and cooling 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 operating geothermal district heating systems in the United States, mostly in the western states (Robins et al., 2021). Recently, however, a number installations in the central and eastern regions, including 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 district heating and cooling systems. The U.S. DOE is supporting research to develop innovative district geothermal 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 these systems (see https://www.nrel.gov/news/features/

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 (abandoned) or idled oil and gas wells were utilized, the total resource would increase to 11.2 million GWth (38 quads; U.S. DOE, 2019a). One issue with reusing many oil and gas fields to develop geothermal resources in sedimentary basins is that the reservoir rocks may require enhancements and stimulation (i.e., fracking), which would tie up financial resources (Kurnia et al., 2022). In addition, many states do not have regulations and incentive 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 orphaned 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 fossil fuel industries to undertake joint research, demonstration, and development activities to demonstrate the technical and economic feasibility of obtaining geothermal energy from 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 to develop the technologies needed to extract geothermal energy. This plan includes the development of open- and closed-loop geothermal energy systems (Santos et al., 2022). The open-loop systems include two or more wells for the extraction and injection of heated fluids. In addition, the open-loop technology allows for the coproduction of geothermal energy and oil and gas from wells that have experienced declining production (Erdlac et al., 2007). In closed-loop systems, water is continuously circulated through a well having a single geothermal loop or coaxial pipe structure within a closed circuit open only at the bottom to allow fluid extraction from the geologic formation. 

Repurposing oil and gas wells has several associated economic benefits, the most important being that repurposing them avoids the costs of drilling and constructing the well and reduces the overall risk of developing geothermal reservoirs because subsurface geological 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 in some places, (3) supplies thermal energy for district heating applications, (4) allows preheating or cooling industrial processes, and heating agriculture facilities and spa and thermal baths. The injection of heated water can improve oil recovery. The recovery of coproduced water also creates an additional long-term revenue stream (Duggal et al., 2022, and references therein). However, the relatively low temperature of these geothermal reservoirs (in comparison with hydrothermal systems) has often limited their development (Santos et al., 2022). 

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

Recent advancements and increased efficiency in low-temperature power conversion has renewed the interest in repurposing oil and gas wells for geothermal power generation (Santos et al., 2022). Several challenges to developing these 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 using chemicals (e.g., scale inhibitors such as acrylic acid polymers) to meet water quality standards. Advanced engineering studies are underway to determine whether these low-temperature geothermal resources can be thermally enhanced by injecting heat from various sources to develop geothermal reservoirs 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 photovoltaicthermal (PVT) technologies, referred to as solar-assisted geothermal energy systems, have gained interest for 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). These systems include different types of solar and geothermal heat pump technologies that are connected and that supply all the required thermal energy for space heating and making domestic hot water. Solar-assisted geothermal energy systems address the major problem of diurnal variation in energy supply caused by differences between electricity generation and seasonal energy demand (Knuutinen et al., 2021). In heating-dominant regions, photovoltaic thermal technologies connected with geothermal heat pumps (PVT-GHP) offer the ability to regulate the thermal balance underground and improve the performance of the photovoltaics by reducing the module temperature. They also capture some of the incident solar radiation that photovoltaic 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., the 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). This geothermal energy system involves the use of concentrating solar power collectors to heat water, which is injected into underground geologic formations to create an 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 to convert the stored heat to electricity. The stored thermal energy can be utilized at various timescales, from daily to weekly to seasonally. In district energy systems, energy distribution centers are connected in line with short-term thermal storage tanks, solar thermal collectors, and BTES systems to provide space heating and cooling, and in some places are used to make 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) to use seasonal underground thermal energy storage (Reuss, 2021). For individual buildings, the photovoltaic and photovoltaic–thermal technologies are connected to the borehole heat exchangers, which are either connected to GHPs or not (You et al., 2021). 

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

2.4.9 Dual GeothermalAir Source Heat Pump Systems 

Integrated thermal energy storage systems comprising GHPs, air source heat pumps (ASHPs) or cooling towers, and borehole thermal energy storage (BTES) offer exceptional building energy efficiency and decarbonization (Kitz, 2021; Pardo et al., 2010; Yi et al., 2008). The heat pumps take advantage of the freely available thermal energy from the ground and air, respectively, and they increase the temperature by running a compression/expansion cycle. The heat pumps are most efficient when the difference in temperature between the source (air or ground) and the output temperature is close. 

Using a dual heat pump system with thermal energy storage allows for grid management by lowering the total energy required for building heating and cooling, especially during peak demand in most climatic zones, and it reduces overall greenhouse gas emissions. Furthermore, it offers the capability for seasonal storage or for off-peak diurnal storage in winter and summer that offers a longer and greater energy storage capacity than current battery technologies (Kitz, 2021). These systems have a roundtrip efficiency of >200%, which is much higher than battery energy storage systems that have an efficiency of 90% or less. The systems comprise equipment that is not only proven in the heating and cooling industry, but also widely available (Pardo et al., 2010). They could benefit single small- and large-sized buildings as well as a group of buildings in district energy systems.  

The customized system described by Kitz (2021) operates to add either heat or cold (remove heat) to the borehole thermal energy storage system. The air source heat pump optimizes electricity consumption to produce hot or cold thermal energy to “charge” the borehole storage whenever advantageous operationally (i.e., during winter afternoons for heating, and at night for cooling). Whiting (2020) also noted the system can take advantage of changes in air temperature. For example, GHPs would only be run when the air temperature drops below 30°F (1.1°C). 

2.4.10 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. The technologies increase the system performance, which brings economic and environmental benefits to the built environment. The concept of integrating geothermal energy resources with renewable energy subsystems has been shown to mitigate and offset peak energy demands, energy consumption, and the ground thermal imbalance (Soni et al., 2016). For example, geothermal technologies are combined with air cooling (i.e., cooling towers) to effectively meet building cooling demands when they are significantly larger than the heating requirements (Kavanaugh, 1998). Furthermore, hybrid GHPs coupled with external heat sources (steam boilers, micro-gas turbines, biomass thermal energy, and waste heat), air-source heat pumps, phase-change materials, auxiliary cooling technologies (ice storage, cooling tower, 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). Less attention has been given to geothermal energy systems connected to combined heat and power (CHP) plants, combined cooling heating and power systems, ejector cooling systems, and absorption heat pumps (Xu et al., 2021). 

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). To meet these power requirements, advanced research projects are being considered that combine geothermal energy systems with underground storage of wind energy and hydrogen (Schmidt & Pettitt, 2022) or compressed air (Leetaru, 2021), particularly in traditional geothermal reservoirs. In many deep sedimentary basins in the U.S. Midwest, where wind energy is available at oil and gas extraction sites, heated fluids can be injected underground for long-duration thermal energy storage (Robertson-Tait & Hollett, 2021). Combined green hydrogen production and geothermal energy systems can advance clean fuel processes and CO2 conversion.  

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. 

2.4.11 Surface Water Heating and Cooling Systems

Heating and cooling systems utilizing surface water from lakes, ponds, rivers, and oceans (acting as thermal reservoirs) directly provide buildings with a sustainable source of renewable thermal energy. Cities, university and colleges campuses, commercial buildings, and individual homes built near water bodies have sought the great energy efficiency potential from these water sources. Surface water bodies can be connected to heat pumps, or water can be extracted directly for water heating and cooling (Mitchell & Spitler, 2013). Water bodies 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 under water 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 (10°C; e.g., Peterson, 2019). 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 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 energy 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 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 between 39°F and 41°F (4°C and 5°C) is the main source of cooling for the university campus and a nearby high school (Mitchell & Spitler, 2013). The system provides 70 MW (20,000 tons [T]) at peak capacity and has a coefficient of performance 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. 

Sea water district cooling systems have been developed across the globe in varying climates and geographic locations, ranging from Stockholm, Sweden, to Kona, Hawaii (e.g., Hunt et al., 2022). The technology is similar to lake source district cooling systems, where water from deep depths is pumped through heat exchangers and 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). 

2.4.12  Geothermal Energy Recovery from Mines and Quarries 

Abandoned open-pit and underground mines and quarries that are flooded provide the opportunity to harness the thermal energy potential of the mine water (see the BBC’s Future Planet, “How flooded coal mines could heat homes,” https://www.bbc.com/future/article/20210706-how-flooded-coal-mines-could-heat-homes). Mine water has been shown to be a reliable source of low-carbon thermal energy (Walls et al., 2021), and this geothermal technology has gained acceptance as an economically and environmentally sound method for heating and cooling buildings and energy storage through direct-use and GHP operations (Dobson et al., 2023). In mid- to high-latitude areas with the lowest average annual air temperatures and highest electricity prices, geothermal energy from mine water is the most efficient and the second most economical heating option (e.g., Bao et al., 2019). 

The feasibility of using mine water for the decarbonization of heat and achieving net zero carbon emissions goals has been determined across the globe, including Canada (Arkay, 2000), the United Kingdom (Monaghan et al., 2022), and the United States (Bao et al., 2019), by conducting detailed reviews of existing information (e.g., mine plans and features of the underground workings, hydrologic characteristics), site visits, and monitoring and measurements. The post-closure use of geothermal energy can build energy resilience into local communities for heating and cooling buildings. But these energy systems offer local and regional stakeholders additional possibilities, ranging from snow melting to maintenance of ponds to cultivating fish and microalgae (Hall et al., 2011). Using these legacy sites for redevelopment would contribute to sustainability and job creation that is likely to be regarded favorably by investors, property insurers, governments, and community leaders. Although conceptually straightforward to develop, systems that capture geothermal energy from mine water still require additional demonstrations and case studies to further characterize the complexity in the underground environment and utilize a cost-effective deployment that is optimized at the site (Monaghan et al., 2022). 

2.4.13 Earth–Air Heat Exchangers

Earth–air heat exchangers, also known as earth tubes, solar chimneys, earth-warming or earth-cooling tubes, earth-warming tubes, or air-to-soil heat exchangers, are promising passive types of thermal energy systems that utilize the constant temperature of underground soil for cooling or heating in the summer and winter seasons (Agrawal et al., 2019). They are also known commercially as GAHT® and EcoLoop™ HVAC systems (https://ceresgs.com/environmental-controls/ecoloop-sunchamber). The technology uses soil as a source or sink of heat, and air flowing through buried pipe is the exchange medium. Horizontal pipes are typically buried at least 6.5–16.5 ft (2–5 m) below the ground (Greco et al., 2022). A blower is used to move the ambient air through the buried pipes. The system can be vented to the outside or sealed, using air in the building.

Compared with GHPs, earth–air heat exchangers are very cost effective in terms of both up-front, capital costs and long-term operation and maintenance costs (Mihalakakou et al., 2022). Significant savings are achieved by not having to drill boreholes. However, their efficiency varies widely depending on the location (both latitude and altitude), ambient ground temperature, extremes in air temperature and relative humidity, solar radiation, depth to the groundwater table, soil thermal conductivity, soil moisture content, and performance of the building envelope. Integrated hybrid systems, for example, those connected with photovoltaics or air source heat pumps, can address specific issues found under different climate conditions (Greco et al., 2022).  

2.4.14 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. Presently, they have either not attained widespread adoption or are progressing toward becoming the first commercialscale 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 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 that of the ground and captures waste heat (Weber & Favrat, 2010). By reducing the temperature difference from the ambient temperature, including the difference between supply and return water, the efficiency of the system can be improved. Man Energy Solutions (https://www.man-es.com/energy-storage/solutions/energy-storage/electro-thermal-energy-storage) and Hunt et al. (2022) have proposed district energy technologies that utilize CO2 and ammonia, respectively. An international consortium of companies and research institutions is collaborating on the project CO2 IN LOOP (http://www.geothermica.eu/projects/joint-call-2021/co2inloop) to further develop the flexibility of CO2 as a heat transfer medium in district heating and cooling systems, specifically modeling the process of varying in-pipe pressure levels. Geothermal energy could be integrated into 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; https://www.cimcorefrigeration.com/alberta-community-installs-heat-pump-as-part-of-efforts-to-become-carbon-neutral).  

Advanced Borehole Heat Exchangers

Darcy Solutions Incorporated (https://darcysolutions.com) has designed a geothermal technology with the capacity to extract 40 to >80 tons of heating and cooling from a single borehole heat exchanger (Price, 2022). In comparison, the industry standard is to capture 12 tons per borehole. In a closed-loop system, a heat exchanger is installed in the borehole, which transfers thermal energy from the groundwater to the geothermal loop without pumping groundwater to the surface. This design reduces the risk of aquifer contamination and can reduce annual heating and cooling costs by 30%80%. Furthermore, because the system requires fewer boreholes than are needed for a conventional geothermal energy system with the same thermal capacity, the land required is reduced by as much as 95%. The system is all electric, which assists building owners in meeting their decarbonization goals and provides a viable heating solution where natural gas is not available. 

Wastewater Heat Recovery Energy Systems

Wastewater produced at residential, commercial, and industrial developments holds a significant amount of thermal energy that, for the most part, is harnessed. 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, https://www.sharcenergy.com/, and Huber Technology Incorporated, https://www.huber-technology.com/solutions/heating-and-cooling-with-wastewater.html) have developed wastewater heat recovery systems with integrated heat exchangers to capture waste thermal energy from water and sewer lines. The thermal energy is captured by using heat exchangers, and heat pump technologies may be deployed at different locations, whether in buildings, sewers, or wastewater treatment plants (Nagpal et al., 2021). The captured energy is sufficient to operate heat pumps cost-effectively to heat nearby buildings (e.g., schools, health facilities, swimming pools). 

It has been shown that in some installations, 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 estimates that 18 million tons of CO2 emissions could be avoided by recovering thermal energy at wastewater treatment facilities (https://www.illinoiscleanenergy.org/

Machine Learning and Geothermal Energy

The U.S. DOE and its partners are using advancements in machine-learning algorithms and artificial intelligence (AI) 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 U.S. DOE’s Geothermal Technologies Office (https://www.energy.gov/eere/geothermal/machine-learning) 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 pieces of equipment 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). 


With the new federal, state, and local tax incentives and grant programs, the uptake of geothermal energy systems is expected to accelerate. Currently in Illinois and the U.S. Midwest, policies and regulations associated with GHP systems for both with closed and open loops are focused on the use of groundwater and on environmental impacts. Throughout the United States, permits are required for drilling into the underground, particularly when such drilling may have an impact on potable water resources. Regulations for source water protection are developed and enforced primarily at the state, county, and local or municipal levels of government. Furthermore, the permits or approvals required for the design and installation of 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 address the issue of “ownership” for geothermal resources, as some European countries have proposed (e.g., United Kingdom; see Abesser & Walker, 2022). These laws are being considered to address potential adverse impacts of underground thermal energy storage systems that could alter the ambient 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 (https://www.congress.gov/bill/117th-congress/house-bill/5376/text) replaces the U.S. House-passed Build Back Better Act (BBBA; H.R. 5376) and expands on provisions in the previously enacted Infrastructure Investment and Jobs Act, which provided new funding to accelerate the growth of clean energy technologies and support consumer rebates for home electrification. The Act, signed on August 16, 2022, provides nearly $500 billions of federal funding for clean energy technologies, with the goal of substantially lowering the nation’s greenhouse gas 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 (https://www.whitehouse.gov/briefing-room/statements-releases/2021/11/01/fact-sheet-president-biden-renews-u-s-leadership-on-world-stage-at-u-n-climate-conference-cop26/). The funding is specifically programmed to make investments and provide incentives that will support national, state, and local clean energy projects, including geothermal energy. These activities move the nation’s economy away from the current emissions-based heating and cooling and toward one that utilizes lowcarbon renewables and energy storage, including GHPs and underground thermal energy storage technologies. 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. Congress.gov/bill/117th-congress/house-bill/3684/text) signed by President Biden on November 15, 2021, contains important energy provisions that invest in installing clean energy technologies that will be secure and resilient. The legislation includes funding for states and cities to help alleviate their energy burdens, implement clean energy projects that create jobs, and support the reduction of greenhouse gas emissions in communities. 

To assist low-income residents and communities dependent on fossil fuels, $3.5 billion is earmarked for U.S. DOE’s Weatherization Assistance Program. This work will expand energy efficiency programs (that include the installation of GHPs) to lower energy costs for residents vulnerable to energy price fluctuations and will 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 tribal lands 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 (https://www.energy.gov/scep/office-state-and-community-energy-programs), which will partner with state and local organizations (in Illinois, the Illinois Environmental Protection Agency is overseeing the program, https://www2.illinois.gov/epa/topics/energy/Pages/default.aspx) to increase access to clean energy technologies that will benefit 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 (https://www.congress.gov/bill/116th-congress/house-bill/133/text), which is the first comprehensive update to U.S. energy policy and the U.S. DOE’s applied energy research and development programs in more than a decade. The Energy Act authorizes up to $35 billion in investments to clean energy technologies, primarily to expand U.S. DOE research and development programs for energy storage, renewable energy, advanced nuclear energy, and carbon capture, storage, and utilization. Additional aspects to be supported are carbon removal, recovery of critical minerals and materials, fusion energy industrial decarbonization, 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 support of research for enhanced geothermal energy systems (EGS) and additional geothermal energy demonstration projects, including one specifically located 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 takes 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 companies have 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 were identified in the legislation, GHPs will be needed to decarbonize buildings to meet the renewable energy targets. 

3.2 Geothermal Energy Regulations
3.2.1 Federal

The U.S. EPA regulates Class V wells, or wells used to inject nonhazardous fluids underground (https://www.epa.gov/uic/basic-information-about-class-v-injection-wells). The Class V category also includes wells that are not already classified as Classes IIV or Class VI wells. The fluids in these wells are injected either into or above an underground source of drinking water (U.S. EPA, 1999). Class V wells are used for a variety of municipal, business, and industrial purposes, including for ATES and as DDU geothermal wells. 

3.2.2 State

The Illinois Department of Public Health (IDHP) administers and regulates the construction, modification, and sealing of all closed-loop and open-loop wells used for geoexchange. This program is conducted through agreements with the 90 local health departments. Their responsibility is to prevent contamination of groundwater from hazardous materials located at the ground surface, in shallow groundwater, from sewage disposal systems, and from other sources of contamination. To ensure the protection of groundwater, the state and local health departments review water well installation plans, issue permits for new well construction, and inspect wells. The drilling and construction of wells for geoexchange must comply with the Water Well Construction Code (https://www.ilga.gov/commission/jcar/admincode/077/07700920sections.html). 

The Illinois Environmental Protection Agency (IEPA) to date has no regulations for DDU geothermal wells in Illinois, but the IEPA is likely to regulate them as wells used to inject nonhazardous waste underground (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. 

3.2.3 Municipal (Local)

Counties, municipalities, and planning agencies in Illinois may have additional codes or ordinances that apply to the installation of geothermal energy systems (e.g., Macon County, see https://
). In the City of Chicago, geothermal energy systems are permitted by the Department of Building and require an Existing Facility Protection from the Chicago Department of Transportation’s Office of Underground Coordination. These projects are reviewed and permitted by the Green Permit Section of the Department of Building (https://www.chicago.gov/city/en/depts/bldgs/provdrs/permits/svcs/green-permits.html). The Village of Deerfield, Illinois, has specific zoning and building code compliance requirements for geothermal energy systems (https://codelibrary.amlegal.com/codes/deerfieldil/
latest/deerfield_il_zoning/0-0-0-5041). 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). 


To achieve Illinois’ 2050 renewable energy and carbon emission goals, the state’s citizens and businesses will be looking to participate in 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 pump systems. The Residential Energy Efficient Property Credit (https://www.irs.gov/pub/irs-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 GHPs. Nonbusiness Energy Property Credits were available in the tax years 2018 through 2021 for residential property owners who made energy-saving improvements. The program provided a $300 credit for energy-efficient heating and air conditioning systems. There was 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 (see https://www.irs.gov/inflation-reduction-act-of-2022). 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 (https://www.bradfordtaxinstitute.com/Endnotes/IRC_Section_48e.pdf). 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 a 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,  https://www.rd.usda.gov/programs-services/energy-programs/rural-energy-america-program-renewable-energy-systems-energy-efficiency-improvement-guaranteed-loans). 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 (https://www.irs.gov/pub/irs-pdf/p946.pdf), 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 and Intergovernmental Programs Office (WIP), 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 (https://www.energy.gov/eere/
). This U.S. DOE office partners with the Illinois Department of Commerce and Economic Opportunity, which manages the Illinois Weatherization Assistance Program (https://www2.illinois.gov/dceo/CommunityServices/
), and it establishes 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 (https://www.energy.gov/eere/slsc/property-assessed-clean-energy-programs). The improvements allowed 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, multi-family 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 in successfully establishing C-PACE programs to support greater economic development activity, as well as in contributing to the development of high-performing buildings through the installation of energy-efficient and clean energy technologies. 

4.3 County and Municipal

Since 2021, the City of Urbana has offered the Geothermal Urbana-Champaign program (https://urbanaillinois.us/geothermal), a public education and bulk purchasing program that delivers a more affordable way for Champaign, Piatt, and Vermilion County home and business owners to install geothermal energy systems that provide heating, cooling, and hot water. Through bulk purchasing, the participants are offered a lower rate on installations than would normally be available from the installers, and specific incentive rebates of the program are provided as installation targets are met. 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 (https://www.oak-park.us/village-services/housing-programs) 

New in 2023, the Jo-Carroll Energy cooperative and the Citizens Utility Board of Illinois (CUB) have developed new geothermal group-buy programs for northwestern Illinois and the Chicagoland area, respectively. The Power-Up NW Illinois (https://www.jocarroll.com/geothermal) is a program designed to help homeowners within the cooperative’s service territory make choosing geothermal HVAC more affordable. The Jo-Carroll Energy cooperative is partnering with the Geothermal Alliance of Illinois (GAOI). The Grow Geo Chicagoland group-buy program (https://growgeo.org) is a joint venture among CUB, the GAOI, and the Midwest Renewable Energy Association. This effort will help develop and scale up the GHP market in northeastern Illinois while providing consumers with education and competitive bulk pricing discounts on the most efficient technology for home heating and cooling. 

4.4 Utilities

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

Ameren Illinois: Ameren offers businesses an incentive rebate of $500 per ton (up to $25,000) for geothermal energy systems (https://amerenillinoissavings.com/business/find-incentives-on-energy-efficient 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 GHP installations, discounts of $1,500 per ton, for a maximum of six installed tons, were announced in March 2023. Up to $1,800 is available for purchasing GHP with an energy-efficiency ratio of ≥20 for replacing GHPs in residential installations (https://www.comed.com/WaysToSave/ForYourHome/Pages/HeatingCoolingRebates.aspx).

For businesses, which include existing facilities, new constructions, and the replacement of existing electric heating or cooling equipment, or both, ComEd offers a rebate of $30 per ton per energy-efficiency ratio unit above the minimum efficiency. The minimum for a water-to-air GHP (cooling mode) is 14.2 energy efficiency ratio, and for a water-to-water GHP (cooling mode) is 12.1 (https://www.comed.com/WaysToSave/ForYourBusiness/Documents/HVACWorksheet.pdf).

For builders, ComEd offers the Electric Homes New Construction financial incentive, up to $2,000 per house, for the installation of GHPs (>22 energy-efficiency ratio, >4.4 COP) in new construction and for major renovations of single-family homes, duplexes, townhomes, two to four flats, and accessory dwelling units to achieve best practice energy efficiency (https://www.comed.com/

The City of Springfield’s City Water, Light, and Power utility offers residential customers a one-time $500 per ton rebate for new GHP systems only (i.e., systems requiring installation of a new [bore] well field, https://www.cwlp.com/ServicesHome/ServicesDocuments/HPRebAppAnd
). The utility also offers commercial incentives for existing and new GHP systems (https://www.cwlp.com/ServicesHome/ServicesDocuments/FillableCOMMHPRebate
). For existing buildings, a onetime $300 per ton rebate is offered for installing a new initial GHP system. For a GHP system installed in a newly constructed commercial building, the rebate is $800 per ton of cooling capacity. The utility also offers its residential customers who have GHPs 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. 

Mt. 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 (https://mtcpu.com/files/591df074d25ce.pdf). 

4.5 Electric Distribution Cooperatives

The Association of Illinois Electric Cooperatives, which represents 25 electric distribution cooperatives in Illinois (Figure 4.1), provides its members with the advantages of a large utility operation while at the same time offering a variety of local and tailored programs within its member service territories. The association 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 (https://adamselectric.coop/member-services/energyefficiencymembers). 

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 (https://www.cmec.coop/member-services/energy-efficiency).

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 (https://www.cceci.com/ 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 GHPs for new construction projects that involve replacing electric resistance equipment and combusting fossil fuels. In addition, $750 is available for replacing an existing GHP with a new unit (https://www.cornbeltenergy.com/programs-services/
rebate-program). For commercial and industrial buildings, schools, and farm or agricultural operations, rebates are offered through the electricity supplier, Wabash Valley Power Alliance (https://www.powermoves.com/energy-efficiency/businesses-and-farms). Rebates of $500 per ton to $750 per ton are offered for the purchase of GHPs.

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 GHP systems in residential applications, a rebate of $2,000 is available for new energy installations and for GHPs that replace fossil fuel usage, air source heat pumps, and electric resistance equipment. A $250 rebate is offered for replacing old GHPs with new units (https://
). For open-loop geothermal energy systems, the rebate for new installations is $1,000, and the rebate for a new GHP is $250. 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 to $750 per ton is offered for the purchase of GHPs (https://www.powermoves.com/wp-content/uploads/2023/02/

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 (https://e-co-op.com/about-us/energy-efficiency-loan-program). 

Jo-Carroll Energy: Incentives are provided for upgrading or installing geothermal energy systems. For the replacement of a GHP, $500 per ton is offered, whereas the construction of a new borefield and installation of a GHP garners $700 per ton (https://www.jocarroll.com/sites/default/files/

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 GHP systems, a rebate of $2,000 is offered (https://www.powermoves.com/wp-content/uploads/2022/12/WVPA-344_GeothermalHeatPump
). 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 GHPs (https://mjmec.coop/2022-powermoves-rebates). For open-loop geothermal energy systems, the rebate for new installations is $1,000, and the rebate for a new GHP is the same as for 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$750 per ton is offered for the purchase of new GHPs for renovations and new construction (https://www.powermoves.com/rebates/business). 

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 GHP (https://www.menard.com/
). 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 GHP (minimum of three tons 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, https://mcec.org/ community/energy-efficiency). 

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 GHP (https://www.recc.coop/rebates). 

Tri-County Electric Cooperative, Incorporated: The cooperative provides loans for the purpose of financing the purchase and installation of GHPs. A low-interest loan of up to $13,000 is available (https://www.tricountycoop.com/geothermal-financing-program). 

Wayne-White Counties Electric Cooperative: An incentive rebate of $1,500 is available to residential customers who install GHPs in their home as a new energy system or as a replacement to electric resistance heating, a gas furnace, or a boiler (http://www.waynewhitecoop.com/pages/

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 (https://wiec.net/rebates-available). Low-interest Energy Resource Conservation (ERC) loans of up to $17,500 are also available for installing geothermal energy systems (https://wiec.net/erc-loans). For $6/month, members can be delivered electricity at a reduced rate (9¢/kWh) if they are running GHPs (https://wiec.net/rates). 

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 (https://www.illinoiscleanenergy.org) 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 GHP technologies, or underground thermal energy storage, or that 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 is the U.S. DOE through their Geothermal Technologies Office (https://www.energy.gov/eere/geothermal/geothermal-technologies
), Building Technologies Office (https://www.energy.gov/eere/buildings/building-technologies-office), Office of Fossil Energy and Carbon Management (https://www. energy.gov/fecm/office-fossil-energy-and-carbon-management), and the newly created Office of Clean Energy Demonstrations (https://www.energy.gov/oced/office-clean-energy-demonstrations). Grants are awarded to individual researchers or research teams on a competitive basis for projects proposed in response to specific funding opportunity announcements. The U.S. DOE also provides funding to qualified small businesses and research collaborators through the Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) programs (https://science.osti.gov/sbir). The U.S. Department of Defense and the National Science Foundation have also funded geothermal projects in Illinois. 


According to the Energy Information Administration’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 from the residential, commercial, industrial, and transportation sectors in Illinois. 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 usage, U.S. households require energy to power numerous electronic 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 uses, mostly seasonal and energy intensive, 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 has the largest overall thermal energy demand (Figure 5.2), and Illinois 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 the energy used in buildings 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 energy applications that use both geothermal power and direct use (Figure 5.4). Some applications can utilize the low-temperature geothermal resources at <86°F (<30°C) encountered in the shallow surface, whereas other applications require geothermal resources at higher temperatures that are accessible by drilling thousands of feet. The specific end use depends on the temperature resource. Geothermal resources with the highest temperatures (300°F or 150°C and greater) are generally used to generate electricity. In Illinois, the range in temperatures of 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 for four different economic sectors and specific applications, 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. 

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 128.8 million Btu of energy annually, 43.8% more than the U.S. average of 89.6 million Btu (Tables CE4.6 and CE4.8, EIA, 2009). Furthermore, 51% of this energy is used for space heating; 31% for running appliances, electronics, and lighting; 16% for water heating; and 2% for air conditioning, as shown in Figure 5.3. Of the amount of energy consumed for space heating, more than 80% of Illinois residents use natural gas (EIA, 2021). The cost for using electricity, natural gas, and liquid propane gas (LPG) for residential space heating, air conditioning, and water heating could be significantly reduced by using GHPs and geothermal water heaters.

Although data for the current use of geothermal energy in the residential sector of Illinois is not available, of the total energy consumed by each housing unit, 69% is needed to run heating and cooling equipment. Of the 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 0.42 quadrillion Btu, or 34.25 billion tons equivalent. It should also be noted that 2020 data for Illinois residential energy consumption was estimated as 0.94 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 of the opportunity to utilize geothermal technologies. 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 GHP 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 energy technologies in the residential and other sectors are widely recognized. Here we present several international studies documenting these 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 GHPs used in homes, small offices, and hotels that either had poor or good insulation in six European cities with varying climatic conditions. Their results indicate that the use of GHP systems 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 technologies, but could be reduced even further by either increasing the tax on purchasing gas and electricity or by implementing economic incentives and progressive policies. Yousefi et al. (2017) assessed the impacts of installing GHPs to reduce air pollution based on the projected number of installations in residential buildings of Tehran over a 5-year period after being installed. They measured an overall reduction in CO2, sulfur dioxide (SO2), and nitrogen oxide (NOx) emissions, which was attributed to using less natural gas. Finally, Han et al. (2021) monitored a geothermal energy system installed in a residential building to determine its energy performance and economic viability. Throughout the study, the system maintained a consistent temperature and humidity in the building (within the human comfort index). Using the geothermal energy system was found to avoid burning 1,131 tons of coal, which would have emitted 4,641 tons of CO2. The geothermal energy system was also highly efficient, which led to a large cost savings for the property owners.

5.2 Commercial Sector

According to the Energy Information Administration’s Illinois State Profile and Energy Estimates (EIA, 2020b), energy consumption for the commercial sector in Illinois is equal to 20.5% of the overall energy consumption in the state (3.6129 quadrillion Btus), or approximately 0.7398 quadrillion Btus. The energy is consumed in buildings that are used by (1) companies for office space, warehouses, and shopping centers; (2) federal, state, and local governments; (3) private and public organizations (e.g., churches, medical clinics and hospitals, and stadiums); (4) utilities; and (5) 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 the total commercial building floorspace (EIA, 2018). 

The first geothermal energy system installed in a commercial building in the United States was in the Commonwealth Building in Portland, Oregon, in 1946 (Hatten & Morrison, 1995). Since the 1950s, widespread acceptance of geothermal technology (by architects, engineering firms, developers, and building owners and operators) has been hampered, in part, by the lack of published case studies of their  application in commercial buildings that include detailed information about the maintenance and operational histories, equipment replacement requirements, life cycle costs, and long-term reliability of the systems (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). For both heating and cooling the building, the geothermal energy system was found to offer better operational efficiencies because heat was rejected from the building and stored in the ground to be extracted later, rather than being released into the air. From the case studies in which data have been collected, the following benefits of geothermal energy systems have been documented and compared with conventional heating and cooling systems in other commercial buildings (NREL, 1999). Overall, geothermal energy systems 

Have the lowest maintenance and life cycle costs;

Maintain better indoor environmental conditions, are able to have individual temperature controls, and are quieter than other heating and cooling systems;

Offer design flexibility in new construction and renovations;

Reduce total energy costs by 20%–40%;

Require mechanical equipment with a smaller footprint;

Eliminate the possibility of vandalism because equipment is located inside buildings;

Can be installed in historic buildings to conceal HVAC equipment;

Eliminate the need for rooftop equipment; and

Have a longer life span (30–50 years) than other energy systems that use furnaces, boilers, chillers, cooling towers, and steam radiators.

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 geothermal energy systems for commercial applications are in the City of Chicago, where they are installed in affordable housing communities, medical buildings, park district centers, libraries, and office buildings (Kearney, 2016). To promote the wider adoption of geothermal energy systems in Metropolitan Chicago for the commercial sector, Commonwealth Edison offered a pilot incentive program (https://energynews.us/2018/10/15/geothermal-and-the-city-utilities-and-industry-push-installations-in-chicago) that ran from 2018 to 2020. The goal of the program was to reduce the up-front costs of installation, making them more affordable to all types of businesses and organizations. The program provided funding for systems designed with heating and cooling capacities of 10 tons to >50 tons, 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), the total energy consumption in the industrial sector is equal to 31.3% of Illinois’ total energy or approximately 1.1306 quadrillion Btus. In Illinois, the manufacturing of chemicals, machinery, and food and beverages contributes the most to the gross domestic product, according to the U.S. Bureau of Economic Analysis (U.S. BEA, 2020). Expanding the usage of geothermal energy is one of the pathways to transitioning 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- or medium-temperature geothermal resources, offsetting 80 million tons of CO2 per year. 

Industrial processes also contribute to energy losses through the release of waste heat into the environment at different temperatures. Baresi (2023) pointed out that waste heat can be used either directly in district heating and cooling systems or during the conversion of residual, low-temperature geothermal energy into a resource sufficient for power generation that could be used in industrial processes or delivered back to the grid. Utilizing waste heat and developing its cascading use can help decarbonize hard-to-abate industries (i.e., cement, glass, petrochemical, and steelmaking) and mitigate the development of urban heat islands. 

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 energy 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

The State of 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 (https://intersectillinois.org/industries/food-processing). Food processing facilities require energy mainly for heating, cooling, drying, and to maintain specific temperatures. Geothermal energy systems could have a promising future in this sector, as illustrated by the following studies. Kinney et al. (2019) addressed the potential of geothermal energy to provide a baseload 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. They found that a geothermal energy system could supply the heating required for the greenhouse application, leading to a productive, 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 (https://geofoodproject.eu), 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 in integrating underground thermal energy storage with photovoltaic systems to optimize energy usage in the chemical, food, and brewing industries. 

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 in a way that addresses the need to manage thermal energy and heat recovery, accesses renewable sources of energy, and makes 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 a food and beverage processing system with an energy system. 

5.5 Agriculture

Worldwide, Illinois is considered a leading producer of soybeans and corn, as well as many other agricultural commodities (e.g., swine, cattle, wheat, oats, hay, sheep, and poultry). Cold winters and hot, humid summers with ample rainfall allow the land to support many kinds of crops and livestock. The agricultural products harvested are then processed into food and industrial products by nearly 27,000 manufacturing companies (EIA, 2021). Geothermal energy systems have many applications in the agricultural sector and can substitute for traditional fossil fuel resources, making the energy systems more efficient and ultimately lowering the cost of energy. Słyś et al. (2020) recommended harvesting waste heat from wastewater on livestock farms to achieve financial savings and improve air quality. Geothermal energy systems are favorable for this type of application because they can provide both heating and cooling, and GHPs can utilize waste heat generated from manure lagoons and compost piles on farms (e.g., Blázquez et al., 2021). 

Alberti et al. (2018) proposed the use of GHPs in the agrozootechnical sector to maintain an adequate indoor thermal environment and air quality to promote a healthy living environment, increase productivity in animal breeding, and reduce the intensive use of fertilizers that could contaminate surface and ground waters. Two GHP systems were proposed. The first system would be installed for efficient heating and ventilation of a piglet stable, and the second system would be composed of a groundwater heat pump coupled with an irrigation system to allow the reuse of fertilizers by recirculating the groundwater. 

Mun et al. (2022) compared the impact of heating on swine production when using a GHP versus the traditional system of heating with electrical heat lamps. They monitored the growth performance of piglets, the indoor stable temperature, greenhouse gas emissions (e.g., CO2 and noxious gas emissions such as ammonia [NH3] and hydrogen sulfide [H2S]), and energy savings. The authors determined that there was (1) no adverse effect on piglet growth, (2) an increase in the indoor temperature, (3) a significant decrease in greenhouse gas emissions, and (4) a decrease in electricity usage.  

Brzozowska et al. (2017) proposed using lakes as a heat source while limiting the impact on the lake ecosystem function. Lakes are attractive as a geothermal resource because of their limited temperature variation during the year. They determined that lakes are good heat reservoirs and that geothermal energy systems could be used for heating and ventilation. 

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 geothermal energy system compared with existing thermal energy technologies. Similarly, Brush et al. (2011) demonstrated the economic and environmental advantages of using GHPs that reduced fossil fuel usage on operating dairy farms in the United States. 

Furthermore, geothermal energy systems can provide dehumidification and increase the drying efficiency on farms. 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 GHPs improved the drying process and produced higher quality food. 

5.6 Educational Institutions

At many K-12 schools and on university and college campuses, renewable energy systems are being installed as a cost-saving measure and to improve the teaching environment. Switching from heating and air conditioning with fossil fuels to renewable energy systems (e.g., geothermal energy) provides healthier environments for students both inside and outside the classroom that have been shown to improve academic performance (Allen et al., 2016). Using geothermal energy 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 energy 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 GHP systems. For example, the McLean County Unit 5 school district had installed GHPs in 15 schools as of 2021 (Heart of Illinois ABC, 2020; Watznauer, 2021), which has significantly reduced their operating costs. For example, 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 to $1.15 per square foot. 

For colleges and universities in Illinois, geothermal energy systems are being considered as part of an aggressive strategy to meet campus sustainability goals to 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 that included 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 contribute to 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 connected to several buildings as a district heating and cooling network (Cross et al., 2011). Furthermore, college campuses can be the ideal location for installing district geothermal energy systems because such systems can be integrated into the existing district energy systems more easily. Sufficient open land is also required for constructing borefields. The geothermal energy systems have life cycles and payback periods that fit well into long-term climate action plans and on the same time horizon as the return on investment of new buildings, typically 15 to 20 years (Jossi, 2022). Examples of the use of geothermal energy in higher education instructional facilities in Illinois can be found at the University of Illinois Urbana-Champaign (Section 6), the University of Illinois Chicago (https://sustainability.uic.edu/green-campus/energy/geothermal-energy), and Southern Illinois University Carbondale (https://
, and Lake Land Community College (https://www.lakeland

Although there are no district geothermal heating and cooling systems in Illinois educational institutions, there are numerous examples at educational institutions across the United States. For example, geothermal district heating and cooling systems are utilized at 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

The State of 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 website (https://www.il.ngb.army.mil/
), there are about 50 Illinois National Guard stations across the state, including 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 (https://www.usar.army.mil/Featured/Ambassador-Program/Find-anAmbassador/Illinois) and Air Force Reserve Command websites (https://www.afrc.af.mil/Units/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 Illinois: the Rock Island Arsenal, the Great Lakes Naval Station, and Scott Air Force Base (https://installations.militaryonesource.mil/state/IL/state-installations). 

For the Guard and Reserve installations, many of these installations 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 energy for heating and cooling at individual facilities (Hammock & Sullens, 2017; U.S. Army, 2013), but the majority do not. Although geothermal energy 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 this thermal energy resource for projects. On the other hand, centralized (district) configurations of geothermal energy 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 installations (Federal Energy Management Program [FEMP], 2010; Robins et al., 2021). 

The utilization of geothermal energy at military installations 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 that 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 energy and, in turn, contribute to meeting DoD resilience requirements. 

5.8 Transportation Infrastructure

Winter in Illinois is cold and snowy, often having ice-covered roads. Traditional ice removal systems, such as rock salt and other deicing materials, 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 pre-wet rock salt for pavement application. Its usage is 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, the deicing materials accelerate corrosion of the steel reinforcing bars on the bridge deck, which can result in environmental contamination and pollution (Kelly et al., 2012; Nahvi et al., 2018). In addition, rapid fluctuations in day to nighttime temperatures from above to below freezing, common conditions during early winter and early spring, are a catalyst for aggressive freeze–thaw cycles that can further degrade the pavement. 

As an alternative to chemically treating roadways, testing is underway to use innovative geothermal systems for snow melting, through (1) the installation of direct heating in pipes, (2) heating from lake or ground water that is circulated in pipes, (3) using a heat exchanger at wellheads to condition circulating water, or (4) allowing warm water to flow across the pavement (Lund, 2005). Porakbar et al. (2021) investigated the feasibility of using a ground-coupled system that collects heat from the ground to melt ice on bridges and culverts. In North Texas, Habibzadeh-Bigdarvish et al. (2019) performed a life cycle cost–benefit analysis of a GHP system to prevent ice buildup on a bridge deck. The study identified the benefits of the GHP system that addressed the high costs of installation and concluded that beyond a threshold traffic volume, the benefits were greater than the overall costs. Balbay and Esen (2010) proposed using a borehole heat exchanger with a single loop to melt snow on pavements and bridge decks. Different geothermal loop lengths were investigated, and the coefficient of performance (COP) of the system was found to increase as boreholes were drilled deeper. In a similar study, Lai et al. (2015) proposed a snow-melting, heated pavement system that showed promise for further development. Han and Yu (2018) presented an innovative geothermal technology using energy piles to expand the viability of snow melting on bridge decks. This technology involved mixing the concrete with phase-change material to build the pile, which enhanced thermal energy extraction. Different compositions of phase-change materials in the piles were modeled and showed an overall increase in the performance of the system. 

Techno-economic and cost–benefit analyses have been done to compare the heating of roads with geothermal energy versus electrical heating. The baseline approach was applying snow-melting chemicals. Habibzadeh-Bigdarvish et al. (2019) found that the monetary benefits of using geothermal energy (excluding those associated with environmental improvements) were 2.6 times greater than the overall cost of applying road salt and deicing chemicals. The return on investment of the geothermal energy system was achieved within 25 years. Furthermore, Liu et al. (2021) evaluated the costs–benefits of a geothermal energy system in comparison with electric snow melting and found it cost three-quarters less to run the geothermal energy system. In regard to greenhouse gas emissions, Shen et al. (2016) performed a life cycle assessment comparing a traditional snow- and ice-melting system with a heated pavement system using geothermal energy. The former required more energy and produced more 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 a GHP system or heated water directly from geothermal reservoirs is mixed with cooler surface or ground water to achieve suitable temperatures for fish farming, between 68°F and 104°F (20°C and 40°C). A similar approach is being used to grow algae (mainly Spirulina; Godlewska et al., 2015), which requires water temperatures between 95°F and 99°F (35°C and 37°C; IRENA, 2019). Using geothermal energy systems can improve fish productivity, especially when aquaculture is combined with hydroponics to maintain the nutrient-rich water necessary to grow plants (Turnšek et al., 2021). In Illinois, the use of GHP systems could benefit aquaculture operations, an industry that in 2015 contributed about $3 million to the economy (Hitchens, 2018)The main fish species farmed included bass, tilapia, catfish, prawn, trout, and paddlefish. Although still being demonstrated, the cultivation of algae for biofuel production and animal feed shows great potential, especially for the use of sequestered CO2 (Schideman et al., 2019). Algae growth is much quicker when higher CO2 concentrations are found in the air, and they grow at faster rates, which shortens the harvesting life cycle, allowing an order of magnitude higher production rates per land area compared with other biofuel crops. 

Although geothermal technologies have yet to be deployed for the aquaculture industry in Illinois, much can be learned from international case studies. John and Jalilinasrabady (2021) described a hybrid geothermal energy and solar energy system developed for aquaculture in Kenya. The proximity of the geothermal energy system to the solar arrays made this coupled system economically feasible. Kuska et al. (2020) evaluated the feasibility of using geothermal energy to regulate the temperature of raceways used in the aquaculture industry. A numerical model was developed to simulate heat transfer along with the interactions among the raceway components. They determined the water temperature, flow rate, and number of boreholes and the distance between them were the most important factors impacting its feasibility. Finally, Omenikolo et al. (2020) explored the use of geothermal energy to heat water for aquaculture applications, rather than relying on solar energy. The researchers found that using geothermal energy for heating water increased the fish production because the water temperature remained nearly constant, which fish prefer. In contrast, the temperature of ponds heated by solar energy were found to fluctuate more. 


To realize the wider uptake 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, including scientific and engineering knowledge gaps, policy or market factors, and social preferences. Although the existing geothermal technologies are considered technologically mature (including GHPs and district geothermal energy systems), more education and marketing are still sought 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 a sustainable and resilient energy baseload (Ball, 2020b). Unfortunately, some potential stakeholders view geothermal technologies as immature and higher risk. In addition, the U.S. geothermal sector has been biased toward focusing primarily on power 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 pricing, 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 significantly impacted by the ambient climate, thermal conductivity of the ground, electricity rates, and temperature target (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 adding geothermal energy systems during the renovation of homes and businesses, which increases the up-front costs and extends the time to break even (Levine et al., 2007). However, if these barriers could be addressed with financial and technical solutions, installation of closed-loop geothermal energy systems could be expanded by 80% in the United States (U.S. DOE, 2019a). Furthermore, the U.S. Global Change Research Program suggests that without moving away from the present “business as usual” model to allow the uptake of geothermal technologies, the U.S. economy risks forfeiting additional financial benefits in the energy transition, which would lead to a 10% reduction in the gross domestic product by 2100 (Jay et al., 2018). However, emerging innovations in the geothermal energy sector will enable larger, more scalable, closed-loop and low-temperature direct-use heating and cooling projects that utilize the natural thermal energy or generate electricity that will develop new streams of income (Ball, 2020b). 

6.1 Technical Barriers

Much like the geothermal power sector, the adoption of non-power-generation geothermal 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. Their wider adoption is being held back by the slow advancement of new technologies or approaches that would increase the drilling efficiency and energy system performance and reduce up-front costs in a significant way. The cost of drilling still averages ~50% of the total project cost, although companies such as Dandelion Energy (https://dandelionenergy.com) and Hydra S.R.L. SEDE Hydraulic Tools (https://cheap-gshp.eu/wp-content/uploads/2019/05/
) are working to lower installation costs. 

Understanding how thermal energy is transported in the subsurface is imperative for effectively and efficiently using geothermal energy resources. Characterization of the geology and hydrogeology and the ground temperature provides a baseline for how the subsurface will respond to 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 subsurface conditions, which, under certain circumstances, can lead to overdesigning a low-temperature geothermal energy system. This issue could be addressed by involving the geothermal industry in leading demonstration projects and developing programs to collect subsurface public data, including temperature, thermophysical properties, and the direction of groundwater flow. In addition, those in the geothermal industry should publicize how they plan to mitigate environmental issues (Elliott, 2013) and what the impacts would be on drinking water supplies, particularly in closed-loop geothermal 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 (Ball, 2020a). Performance metrics addressing the levelized cost of energy (LCOE) and greenhouse gas emissions via life cycle cost analyses and building energy modeling would provide the knowledge base to 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. 

It is worth noting that the downstream impacts of using fossil fuels are not reflected in life cycle costs and are not included in the true value of all subsidies provided to the oil and gas industry, which has subsequent impacts on CO2 emissions. Several countries, such as Italy, China, Canada, Mexico, Finland, New Zealand, Ireland, Switzerland, and Sweden, have or are considering implementing carbon taxes or credits that are intended to create a revenue stream to fund mitigation strategies to offset these 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 alternative geothermal energy 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 (that were not previously eligible to claim the credit) to participate. It also addresses complex building ownership, which had been an impediment (Ball, 2020a). Within the next decade, it is predicted that closed-loop geothermal systems and district geothermal energy heating and cooling technologies will become cost 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 advanced geothermal technologies, particularly for deep direct-use and underground thermal energy storage (e.g., Alkhwildi et al., 2020; Nyborg & Røpke, 2015). It has been suggested that the geothermal 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 these energy systems (ASHRAE, 2019; American National Standards Institute [ANSI]/Canadian Standards Association [CSA]/IGSHPA, 2016). Furthermore, the U.S. Midwest, particularly Illinois, faces a shortage of qualified geothermal drillers to install borehole heat exchangers, which could potentially slow the future of expansion and be partially responsible for the recent increases in drilling costs, which the geothermal industry has estimated to be roughly 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). Performing grid-scale and demand-side modeling, demonstration projects, and data collection and analysis would illustrate how adopting geothermal 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 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 public did not fully understand what geothermal energy systems are and that such systems have important energy efficiency and environmental benefits (Innergex Renewable Energy, 2019), that their expansion can create jobs, and most important, that they can save people money in the long term (Jay et al., 2018). The systems would 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 hinders the installation of geothermal energy systems or discourages communities from investing in them. 

Accompanying the expansion of geothermal energy systems is the need to develop a larger workforce adequately trained to design and install these systems. This will require significant and sustained funding from the federal and state governments to provide the required training, mentorships, and educational programs focused on geothermal energy and underground thermal energy storage. This energy 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, and 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 promoted as part of Illinois’ Climate and Equitable Jobs Act (CEJA) may be a model for job creation. 

Ball (2020a) reiterated the need to accurately report the levelized cost of energy (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), to correctly report the true energy mitigation value (i.e., energy avoidance) of geothermal heating and cooling. 


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 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 unrealized 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 effective 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 restoration, 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) allows the incorporation of advanced building energy management technologies that can optimize the demand profile response of the system. Furthermore, the approach will allow bidirectional simulations of flexible energy-storage options and analyses of the cost-reduction impacts of technology improvements (i.e., lowering drilling costs and improved reservoir characterization; U.S. DOE, 2019b). 

Machine learning and artificial intelligence (AI) techniques offer new opportunities to improve and optimize (1) geothermal resource assessments, (2) drilling and well testing performance, and (3) 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 parameters (e.g., temperature, ground thermal conductivity, groundwater flow) can reduce resource uncertainty and development costs. 

7.2 Financial Incentive Programs

Using previous experience in other countries (Rosenow et al., 2022), policy makers should ensure that financial incentives are available to property 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 currently available in Europe and China as well as from agencies in several U.S. states (e.g., New York, https://www.nyserda.ny.gov/ny/PutEnergyToWork/Energy-Program-and-Incentives/Renewable-Technology-Programs-and-Incentives, and Colorado, https://energyoffice
). Increasing taxes on fossil fuels may reduce the cost differential between fossil fuels and renewable energy sources. Sweden, Norway, and Finland have already 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 SBIR and STTR programs. Current funding opportunity announcements are available at https://eere-exchange.energy.gov. The U.S. DOE also provides funding through their Weatherization and Intergovernmental Programs Office (https://
) and the State Energy Program (https://www.energy.gov/scep/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, Renewable Energy Systems and Energy Efficiency Loans and Grants (https://www.rd.usda.gov/programs-services/energy-programs/rural-energy-america-program-renewable-energy-systems-energy-efficiency-improvement-guaranteed-loans). 

The U.S. Economic Development Administration (EDA) within the U.S. Department of Commerce offers a number of funding programs for renewable energy technologies that can be accessed through their six regional offices (https://www.eda.gov/funding/programs). One program in particular, the American Rescue Plan (https://eda.gov/arpa), provides specific funding, through the Coal Communities Commitment (https://eda.gov/arpa/coal-communities), 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” (https://www.whitehouse.gov/briefing-room/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 https://energycommunities.gov/funding-opportunities. Supporting “energy communities” in the Illinois Basin is a priority of the working group, and a rapid response team was recently established for the region (https://energycommunities.gov/federal-working-group-launches-rapid-response-team-to-support-illinois-basin-energy-communities). 

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, https://dnr.mo.gov/energy/business-industry/financial-opportunities, and Massachusetts Clean Energy Center, https://goclean.masscec.com/clean-energy-solutions/ground-source-heat-pumps). 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 (https://www.nyserda.ny.gov/all-programs/ community-heat-pump-systems). 

Locally, cities and counties provide specific support for renewable energy technologies, including GHP systems (http://www.usmayors.org/wp-content/uploads/2018/10/Cities-with-Policies-to-Incentivize-Renewable-Energy.pdf). This may be through tax incentives (i.e., Tax Increment Financing, City of Sheboygan, Wisconsin, https://www.sheboyganwi.gov/wp-content/uploads/
), rebates (e.g., City of Ames, Iowa, https://www.cityofames.org/
), or other funding programs listed in the Database of State Incentives for Renewables & Efficiency (https://www.dsireusa.org). In addition, utilities and energy providers may offer their own incentive programs (Eversource Energy, https://www.eversource.com/content/nh/residential/ about/transmission-distribution/projects/
), along with financing and rebates from the GHP manufacturers (e.g., WaterFurnace International Incorporated, https://www.waterfurnace.com/
). Furthermore, some nonprofit organizations, foundations, and company sustainability or green energy programs provide funding for renewable energy projects (e.g., Abell Foundation [Baltimore], https://abell.org/publication/geothermal-potential). Candid’s GuideStar (https://
) is the most complete and up-to-date nonprofit database available. 

7.3 Outreach and Education

Achieving the broader adoption of geothermal technologies will require a dedicated and more focused marketing and outreach campaign that involves diverse groups of engineers, designers, architects, energy analysts, financial investors, and decision makers to introduce these 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 accelerate the deployment rate and market penetration, especially for innovative and emerging technologies such as deep direct use, underground thermal energy storage, and repurposing orphaned oil and gas wells. For the latter technology, reusing existing wells would eliminate the high up-front drilling costs. In Illinois, there are >44,000 orphaned oil and gas production wells, according to the IOGCC (2021). Moreover, co-generation of the reservoir could extend oil production and be used to store thermal energy and waste heat to develop a geothermal resource for supporting district heating, aquaculture, and greenhouse agriculture. 

To address the higher up-front costs, the investment capital and entrepreneur and incubator community in the U.S. Midwest could be leveraged to develop the geothermal technologies, expand GHP manufacturing capacity, and create a supply chain in the region. The presence of companies that exclusively work on these projects would also improve the geothermal energy sector. Furthermore, the State of Illinois could develop an Energy Center similar to Wisconsin’s Energy Institute (https://energy.wisc.edu), whose focus is on education and outreach for renewable energy throughout the state, including geothermal energy. The Center’s first activity could be to develop a statewide plan for expanding the usage of geothermal technologies by leveraging the region’s unique climatic conditions, favorable geology and hydrogeology, engineering and entrepreneurial talent, manufacturing capacities, and the advanced agricultural research and food processing sector. Expanding and enhancing international collaboration and partnerships in the geothermal energy sector would allow for sharing of successes, particularly with European partners 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 reach of the geothermal energy sector is a long-term effort. It would be gratifying to see positive impacts relatively quickly, but it will likely take several years of consistent effort to see significant growth. Nevertheless, it is encouraging to see the new federal tax incentives made available for the next decade, even as the sector is clearly at the optimal time to start this effort. Public perceptions about renewable energy are evolving, and there is now a better opportunity to steer the preferences of property owners and lead the public conversation toward decarbonizing 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). 


Installing geothermal energy systems 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) estimates that with the installation of 50–100 direct-use geothermal energy systems per year, it adds ~10,000 new jobs annually while reducing greenhouse gas emissions, which would prevent dozens of premature deaths related to exposure to air pollution. 

The current energy transition, although only recently becoming a public concern, has in fact 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. The 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, which will ensure a secure and reliable source of heating and cooling. It will also support the training and employment of community members, which will ensure environmental justice and the inclusion of underserved communities. 

8.1 Environmental Benefits

Increased deployment of geothermal technologies would 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 in the “breakthrough” scenario, which equates to the expansion of GHPs at levels prescribed in the U.S. DOE’s GeoVision Report (U.S. DOE, 2019a). In this scenario, aggressive cost reductions (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 energy use by replacing fossil fuels and lowering the overall electricity demand. Traditional residential and commercial heating and cooling systems that contain furnaces, portable space heaters, and air conditioners would be replaced by GHPs. Although GHPs require electricity to pump fluids and run fans, in most locations on-site electricity use would be reduced. The fossil fuels displaced by these technologies include natural gas, fuel oil, and liquified propane gas. The combustion of fossil fuels is associated with emissions of SO2, NOx, and PM2.5. 

The expansion of GHP systems under the breakthrough scenario would reduce the 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 of these systems, 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 CO2 equivalent (CO2e), representing an 8.3% reduction in on-site emissions from buildings relative to a scenario that holds the number of new GHP systems at 2012 deployment levels. Millstein et al. (2019) documented an annual avoidance of 90 MMT of CO2e, equivalent to removing almost 20 million cars from the road. The benefits of reducing SO2 emissions were concentrated in the U.S. Midwest, where GHPs offset the residential use of fuel oil (Figure 8.1). Reductions in NOx emissions were most noted in several states in the eastern Midwest and Northeast, including Illinois and New York. The spatial distribution of PM2.5 emission reductions was similar to that of NOx, with the largest reductions in the eastern Midwest and the Northeast. The PM2.5 emissions were significantly lower than emissions 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 boilers 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 the breakthrough scenario (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 increased deployment of geothermal energy systems is expected to deliver both direct and indirect benefits that result in additional employment opportunities at the location of the installation. The jobs created would be local, in the service, hospitality, and lodging sectors, and companies that sell supplies or rent equipment would drive economic development in the community. Additional benefits would be associated with geothermal-related manufacturing and deployment activities. Several studies have examined the economic impacts of geothermal plants in the United States (e.g., Battocletti & Glassley, 2013). The direct and indirect job creation data represent the dynamics of resource utilization and 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 (GEA) established an economic multiplier of 2.5 times for geothermal energy investment, meaning that each US$1.00 invested in tapping the geothermal resource would result 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 targets for clean energy usage, which are needed 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), and development will be beneficial not only for energy conservation and a reduction in GHG emissions, but also for greater cost efficiency. Technologies that are scalable on the community level, such as GHP systems, are a key component in achieving these goals. For example, integrated underground thermal energy storage systems (UTES) that connect multiple buildings into a single thermal network or multiple coordinated energy networks will reduce the overall capital costs by allowing fixed costs to be shared by the community members and allowing excess thermal energy to be transferred 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 specialized expertise. Achieving these sustainability goals will require communities to (1) reduce energy consumption, (2) use energy more efficiently, and (3) integrate energy efficiency and renewable energy technologies into integrated systems; a situation that emphasizes the importance of holistic building design. 

Clean energy development can bolster the economy of communities through job creation, local tax revenues, and reduced energy costs; however, those communities most in need of support and employment often have a 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 residents, such as energy affordability, job creation, and climate resilience. Developing clean energy projects should involve meaningful engagement from the community that draws residents together to bring projects to fruition. Thinking holistically about establishing such a relationship requires communities to identify, develop, and finance impactful and investable projects (Hangen, 2022). 

8.4 Workforce Development

The wider adoption of geothermal technologies would spur economic growth and job creation in the U.S. Midwest. The accelerated 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 the systems online. They would also create a direct benefit to GHP 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 technologies would take advantage of the favorable geology and hydrogeology conditions. Additionally, high-paying jobs at drilling companies that service the petroleum industry could be retained because geothermal energy systems require industry-standard drilling and well-development methodologies. Together, these factors would lead to a more consistent market for geothermal energy systems that would allow equitable deployment for a range of stakeholders. 

The recently enacted Climate Equity and Jobs Act in Illinois will support programs for training a diverse clean energy workforce that builds wealth and creates capacity and employment opportunities in diverse businesses. Although the Act does not encourage specific workforce programs for geothermal energy, subsequent legislation is expected to support the development of the geothermal energy sector, which will be ready to install community geothermal systems, underground thermal energy storage, and deep direct-use technologies. 

Nationally, the expansion of GHP systems under DOE’s 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 sector would rely on a cumulative investment of $112 billion through 2050. Much of the main growth in the workforce would result  from the creation of on-site construction jobs and expansion of the supply chain that would last over the lifetime of construction, typically 1 to 3 years. 

8.5 Technical Assistance and Training Programs

Like other disciplines in the physical sciences and engineering, geothermal energy programs are affected by the shortage of professionals, consultants, and supporting businesses, along with a general aging of the existing 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 companies to operators of GHP systems. Training a 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 programmatic structure to offer relevant hands-on learning and apprenticeship programs. The technical assistance may also include educating the stakeholders on how to assess the potential and feasibility of geothermal energy use  (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, https://gaoi.org) 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 IGSHPA (https://igshpa.org) 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 (https://www
). 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 (https://www.asiplocal150.org), offers apprenticeship programs for heavy equipment operators, machinery repair technicians, and geothermal contractors or well drillers. Apprentices learn about drilling methods and their applications, safety practices, and the 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 (https://erc.uic.edu) provides public service, research, and special projects organization to improve energy efficiency and protect the environment. The Illinois Green Economy Network (IGEN, http://www.igencc.org) is a collaborative initiative that develops and promotes sustainable programs across all Illinois’ 48 community colleges to provide statewide clean energy jobs training and workforce placement. 

Nonprofit organizations and industry–public partnership consortiums were formed to incentivize the development of clean energy technologies and promote energy efficiency, which lowers greenhouse gas emissions, increases profitability, and reduces peak energy demand. Evergreen Climate Innovations (formerly Clean Energy Trust, https://evergreeninno.org) 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 the 360 Energy Group (https://www.360eg.com) is seeking to lower energy costs by reducing electricity and fossil fuel consumption. The National Renewable Energy Laboratory (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., Local Energy Action Program [LEAP] communities, https://www.energy.gov/communitiesLEAP/leap-communities). This approach leverages the experience and expertise of local community leaders, residents, and organizations. The Office of Energy in Illinois (https://epa.illinois.gov/topics/energy.html), administered by the IEPA, supports clean energy education and training for community college students. Through a collaboration with IGEN and SEDAC (https://smartenergy.illinois.edu/bee_fundamentals), Illinois’ energy efficiency workforce is being expanded through living laboratories and hands-on training as well as through support for energy infrastructure improvements. 

The Climate and Equitable Jobs Act (CEJA) created a statewide Clean Jobs Workforce Network Program (https://dceo.illinois.gov/climateandequitablejobs/clean-jobs-workforce-network-program.html) 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 legislation also creates a Clean Energy Contractor Incubator Program (https://dceo.illinois.gov/climateandequitablejobs/
) to provide access to low-cost capital and financial support for small clean energy businesses and contractors. 


The extraction and combustion 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 technologies (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 the wider adoption of geothermal technologies as an alternative renewable energy source. Geothermal energy systems provide a constant supply of thermal energy that can be extracted for various applications, including heating and cooling buildings, meeting water heating demands, and dehumidification, which are the predominant societal needs in Illinois and the U.S. Midwest. To meet local and global challenges related to the emission of carbon and greenhouse gases, 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 the supporting technologies. We are optimistic this White Paper will provide a reference for future legislation targeting the utilization of the natural thermal energy underground and developing the associated underground energy storage technologies. 

Generally speaking, geothermal energy systems are environmentally beneficial, have a high efficiency and reliability, and have low greenhouse gas emissions, low maintenance requirements, and the additional benefits of energy security and resilience. The geothermal technologies applied across various sectors have a proven level of efficiency and reliability (Spitler et al., 2014). Furthermore, the advantages of using geothermal energy (which include, but are not limited to, environmental, economic, geographic, and local community benefits) outweigh the higher initial costs of installation, compared with conventional 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 four 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, (3) advanced geothermal systems, such as underground thermal energy storage, and (4) energy foundations that are integrated into buildings and other infrastructure. Geothermal heat pumps can be used in different configurations to meet a range of heating and cooling demands, resulting in considerable mitigation of greenhouse gas emissions and greater cost savings. A range of federal, state, county, and municipal programs have been developed to support and offset the cost of these efforts. Furthermore, experimental geothermal technologies, as well as some hybrid systems, are now being considered in the residential, commercial, industrial, manufacturing, education, military, transportation, and agricultural sectors. 

The University of Illinois at Urbana-Champaign is well positioned to perform applied research and technology development and to host technical demonstrations of various geothermal technologies, as its ongoing “Living Laboratory” program has actively engaged faculty, staff, and students on numerous projects. Active geothermal energy projects include a multi-organization partnership to develop more accurate models, which will help expand the deployment of geothermal technologies at federal sites. Historical geothermal research projects include a feasibility study using deep direct-use geothermal energy in agricultural research facilities on the UIUC campus to exploit low-temperature sedimentary basins, such as the Illinois Basin. Recently, educational facilities constructed on the UIUC campus have incorporated novel geothermal heating and cooling technologies that will also serve as ongoing test beds for research by 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. As the economy shifts away from fossil fuels, a sociocultural barrier also exists, given that 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 technologies, employment opportunities and economic impacts must also be addressed. Although the environmental benefits of installing geothermal energy systems are widely known to researchers and energy professionals, the economic, energy security, and resilience benefits have yet to be fully demonstrated to the public. These benefits can be enhanced as inefficient energy systems — such as conventional natural gas and propane furnaces and heating oil boilers — are upgraded. Furthermore, efforts to adopt community-scale energy policies and retrain workers in this energy transition will encourage equitable economic growth and workforce development in Illinois and the U.S. Midwest. 


Andrew Stumpf 
Principal Research Scientist, ISGS
University of Illinois at Urbana-Champaign 
615 E. Peabody Drive 
Champaign, IL 61820 

Frank Holcomb
Associate Director, U.S. Army Engineer Research & Development Center 
Construction Engineering Research Laboratory (ERDC-CERL) 
2902 Newmark Drive 
Champaign, IL 61822 

Josiane Jello
Doctoral Candidate, Department of Civil and Environmental Engineering 
University of Illinois at Urbana-Champaign 
205 N. Mathews Avenue 
Urbana, IL 61801 


Tugce Baser
Assistant Professor, Department of Civil and Environmental Engineering 
University of Illinois at Urbana-Champaign 
205 N. Mathews Avenue 
Urbana, IL 61801 

Ryan Dougherty
President, Geothermal Exchange Organization 
312 South 4th Street, Suite 100 
Springfield, IL 62701 

Yu-Feng Lin
Director, Illinois Water Resources Center 
University of Illinois at Urbana-Champaign 
607 E. Peabody Drive 
Champaign, IL 61820 

Nancy Ouedraogo
Extension State Specialist, Community Economic Development, University of Illinois Extension 
University of Illinois at Urbana-Champaign 
905 S. Goodwin Avenue 
Urbana, IL 61801 

Jay Solomon
Extension Educator Natural Resources, Environment & Energy, University of Illinois Extension 
University of Illinois at Urbana-Champaign 
Highland Community College, 2998 W. Pearl City Road 
Freeport, IL 61032 


Sharon Irish, Community leader, Urbana, IL 
Dave Handeen, College of Design, University of Minnesota, Minneapolis, MN 

Illinois Water Resources Center Support Staff

Amy Weckle, Assistant Director 
Susan Krusemark, Editor 
Joey Hartz, Web Content Developer 
Sarah Dendy, Web Content Developer 


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°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
CH4 Methane (molecular formula)
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)
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
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)
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
Air Source Heat Pump (ASHP): The most common type of heat pump, widely used in residential and commercial applications. Similar to a refrigerator, this type of heat pump uses electricity to take advantage of the difference in temperature between the outdoors and inside the building for cooling and heating. In other words, the heat pump extracts heat from the outdoors into the building during the winter. During cooling mode (summer), the heat pump operates in reverse, pulling heat out of the building. 

Ambient ground temperatures: The natural temperature of the Earth in a specific location. The ground temperature is typically constant below a depth of 30 ft (9 m). In Illinois, the temperature at this depth is between 50°F and 55°F (10°C and 14°C). Above this depth, the ground temperature is usually close to the annual average air temperature; the ground is primarily impacted by the average air temperature and secondarily by the thermal energy absorbed from the sun. 

Antifreeze: Typically, methanol alcohol or propylene glycol is added to water to make antifreeze solutions, which are circulated in closed-loop systems to lower their freezing temperature. Both antifreeze solutions are nontoxic and environmentally friendly, and their use is approved in the United States by state and county or local health departments. Like the antifreeze in your car, the composition of the antifreeze solution in geothermal energy systems needs to be monitored, tested, and adjusted at regular intervals. 

Aquifer: A geologic layer containing sufficient permeable material (sand, gravel, or both) to conduct groundwater and to yield significant quantities of water to wells and springs. 

Aquifer thermal energy storage (ATES): An open-loop geothermal technology for the long-term storage of thermal energy underground by using groundwater (aquifer) as the heat-transfer medium. The storage and recovery of thermal energy seasonally is achieved by extracting and injecting groundwater by using well doublets. Suitable hydrogeological conditions are required for storing heat, including a highly permeable aquifer and low groundwater flow velocities, among others. The storage system takes advantage of the high heat capacity of groundwater and the large volumes available. Typically, ATES systems are used for large-scale heating and cooling applications, such as office buildings, hospitals, airports, or universities, and in district thermal networks. 

Aquitard: A hydrogeologic unit (or confining unit) composed of geologic materials of low permeability (e.g., clay, silt, unfractured bedrock, etc.) that does not transmit significant quantities of groundwater on a regional scale or over geologic time. 

Bedrock: A general term for rock, usually solid, that underlies soil, unconsolidated material, and overburden, or that outcrops at ground surface. 

Borefield: A collection of boreholes in the ground that contain closed-loop ground heat exchange U-bend piping loops. 

Borehole: A hole, typically drilled, bored, cored, driven, hydraulically advanced, or otherwise constructed into the ground to extract water, oil and gas, and geothermal energy. 

Borehole heat exchanger (BHE): A ground heat exchanger installed in a borehole. 

Borehole thermal energy storage (BTES): A geothermal technology used to store heat seasonally underground for later use. It is the most common method for seasonal thermal energy storage around the world and uses heat-transfer fluid in a borehole heat exchanger (BHE) to move the heat into the ground. The system exploits the high volumetric heat capacity of rock-forming minerals and pore water to store large quantities of heat (or cold) on a seasonal basis in the ground. 

British thermal unit (Btu): A measurement of the heat content of fuels or energy sources — the quantity of heat required to raise the temperature of one pound of liquid water by 1°F (0.56°C) at the temperature that water has its greatest density (~39°F, 3.89°C). 

Clean energy: Energy that comes from renewable, zero emission energy sources that do not produce any kind of pollution, notably greenhouse gases such as carbon dioxide, as well as energy saved by energy efficiency measures. 

Closed-loop (source) system: A continuous, sealed, underground, or submerged ground or borehole heat exchanger system through which a heat-transfer fluid passes that extracts heat or cold from the ground by circulating a heat carrier fluid around an array of closed-pipe loops (borehole heat exchanger). These systems are typically installed vertically or horizontally at depths up to 500 ft (152.4 m). 

Coefficient of performance (COP): A rating system for heating efficiency — the ratio between the power (kW) drawn out of the heat pump for heating and the power (kW) that is supplied to the compressor. 

Combined cooling heating and power: A trigeneration process by which some of the heat produced by a cogeneration plant is used to generate chilled water for air conditioning or refrigeration. An absorption chiller is connected to the combined heat and power (CHP) to provide this added functionality. 

Combined heat and power (CHP): Also known as cogeneration, it is the concurrent production of electricity and thermal energy (heating, cooling, or both) from a single source of energy. 

Commercial sector: An energy-consuming sector that includes the service-providing facilities and equipment of businesses; federal, state, and local governments; and other private and public organizations, such as religious and social groups. Institutional living quarters and sewage treatment facilities are also considered part of the commercial sector. 

Decarbonization: The process of stopping or reducing the emission of greenhouse gases, especially carbon dioxide, into the atmosphere from the burning of fossil fuels. The process also includes removing carbon or material containing carbon during the manufacture of substances or objects. 

Development (or developed): A term used for a groundwater well. It is the act of cleaning out clay and silt introduced during the drilling process and the finer part of the aquifer materials entering the well directly around screen or long bedrock fractures prior to pumping the well. 

Direct-use: The use of geothermal energy without first converting it to electricity, such as for space heating and cooling, food preparation, and industrial processes. May include the heating and cooling uses in cascading applications. 

Distributed thermal response test (DTRT): An extension of the conventional thermal response test (TRT) with the addition of fiber-optic cable to make continuous temperature measurements along the borehole heat exchanger (BHE) or geothermal loop. The ground thermal conductivity and borehole thermal resistance are determined at different vertical sections along the borehole. 

Energy avoidance: The amount of energy resources (e.g., heating and cooling) not used because of initiatives for energy conservation. It is the difference between the baseline without a plan and actual consumption. 

Energy efficiency: The use of less energy to meet the same heating and cooling needs. 

Energy-efficiency ratio (EER): A rating system for cooling efficiency — the 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 (35.0°C) outside temperature and an inside (return air) temperature of 80°F (26.7°C) and 50% relative humidity. 

Energy foundation: An alternative means of harvesting and storing the natural thermal energy underground for heating and cooling purposes. Ground heat exchange loops are installed within underground foundations and structures (e.g., retaining walls, basement walls, and tunnels) or vertical support systems (e.g., piles) to transfer heat between the ground and overlying infrastructure. Possible applications would be to heat and cool an overlying building and for bridge deck deicing. 

Energy resiliency: The ability of thermal energy systems to continue meeting the heating and cooling demands despite the connection to the energy grid being disrupted. 

Energy security: The uninterrupted availability of energy sources, and the ability to protect and improve the resiliency of the energy system. Geothermal energy provides an alternative, and usually local, source of thermal energy that replaces the use of fossil fuels or displaces electricity usage. Geothermal energy contributes to increasing (1) the diversity of energy sources and (2) the flexibility of the energy system to grid outages, increasing costs of fossil fuels and electricity, and inflation. 

Environmental justice: The fair treatment and meaningful involvement of all people, regardless of race, color, national origin, or income, with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies. 

Equality: A quality or state in which each individual or group of people is given the same access or opportunity for access to energy services, energy technologies, and energy consumption, and one that embodies the energy to satisfy personal needs and holding capabilities (i.e., achieving future career goals and establishing significant social ties). 

Equity: A quality or state in which each person is recognized as having different circumstances, and one that allocates the exact resources and opportunities needed to reach an equal outcome. 

Evaporative cooling technologies: Cooling equipment (e.g., cooling towers, “swamp” coolers, and other air-handling systems) that uses the heat and mass transfer process, in which water evaporation is used for air cooling: heat is transferred from air to water, and consequently, the air temperature decreases. The air is not cooled by use of a refrigeration unit. Evaporative cooling systems are more energy efficient because they consume less energy and their performance improves as the air temperature increases and humidity decreases. 

Fossil fuels: Energy sources formed in the Earth’s crust from decayed organic material. The common fossil fuels are petroleum, coal, natural gas, and liquified petroleum gas (propane). 

Geoexchange: The process whereby heat is transferred between the ground and the heat exchange loop. This geothermal technology takes advantage of the Earth’s energy storage capability, whereby heat always flows from areas of higher temperature to areas of lower temperature. The greater the difference in temperature between two adjacent areas, the higher the rate of heat transfer between them. The process allows the ground to act as a sink for excess heat energy during the summer and a source of heat energy during the winter. A typical geothermal exchange system consists of a geothermal heat pump, a borehole heat exchanger (BHE), and a distribution circuit (loop) in which a heat-transfer fluid is circulated. 

GEOthermal Energy for Production of Heat and Electricity (“IR”) Economically Stimulated (GEOPHIRES): A free and open-source computer code to perform techno-economic simulations of geothermal energy systems. 

Geothermal gradient: The rate of increase or decrease in the ground temperature relative to depth. In Illinois, the geothermal gradient ranges from 1.0°F/100 ft (0.56°C/100 ft) to 1.56°F/100 ft (0.87°C/100 ft). 

Geothermal heat pump system: Any heat pump system that employs a heat pump unit that is connected to a ground closed-loop, ground open-loop, or standing column well system. 

Geothermal reservoir: A subsurface layer of rock that is an aquifer containing heated water and for which the exploitation of thermal energy is economically profitable. 

Geothermal utility: A company that designs, builds, owns, and operates geothermal energy systems for city districts and neighborhoods, and multi-family, office, and institutional buildings. Through developing optimal geothermal energy systems, the project costs are reduced, which maximizes the economic benefits and eliminates the risk. 

Greenhouse gases (GHG): Gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit infrared radiation emitted by the Earth’s surface, the atmosphere, and the clouds. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), ozone (O3), and inhalable particles (PM2.5) are the primary greenhouse gases. 

Ground heat exchanger, vertical borehole heat exchanger, horizontal ground heat exchanger: Continuous, sealed, underground heat exchanger consisting of a closed loop through which a heat-transfer fluid passes to and returns from a heat pump or manifold. The ground heat exchangers may be vertically or horizontally configured or submerged in surface water. 

Ground loop heat pump (GLHP): A ground source heat pump system connected to a closed-loop system. Other terms used in the industry include “ground source heat pump system,” “geothermal heat pump,” “geoexchange,” “ground coupled heat pump,” and “water source heat pump.” 

Ground thermal imbalance: The thermal imbalance between sites of heat injection and extraction to and from the ground. This phenomenon has a significant impact on the natural thermal gradient and will consequently affect the operational performance of geothermal heat pumps. 

Ground thermal properties: The thermal regime underground that is impacted by various thermophysical properties, including thermal conductivity, thermal diffusivity, heat capacity, and ground temperature. Of these properties, thermal conductivity is of paramount importance for the design of geothermal heat pump (GHP) systems. Thermal conductivity is the parameter that governs the steady state in heat transfer, whereas thermal diffusivity is applied to cases of transient heat transfer. Both ground thermal properties are input data for analytical and numerical models used in the design and performance analysis of GHP systems. 

 Ground water heat pump (GWHP): A ground source heat pump system connected to an open-loop system. Other terms used in the industry include “open-loop system,” “geothermal heat pump,” and “water source heat pump.” 

High-density polyethylene (HDPE): A tough, durable piping material with unique performance properties (density, before additives or pigments, is >0.941 g/cm) used by the geothermal heat pump industry and approved for geothermal loops in all model codes across the United States and Canada. 

High-temperature geothermal resources: Geothermal resources that have temperatures above 300°F (150°C) and that are typically found in areas of high tectonic activity, such as volcanic belts. The hydrothermal resource is used directly in the form of steam to drive a turbine and thereby generate electricity. 

Horizontal drilling: Drilling that involves advancing a near-horizontal borehole, which is drilled at a very low angle to penetrate a formation that does not directly underlie the drilling rig. 

Horizontal loop: A ground heat exchange loop that is inserted into a trench or open excavation, or installed by a directional drilling method, and that is typically made to a depth of less than 5 ft (1.5 m). The loop does not penetrate an aquifer. 

Industrial sector: An energy-consuming sector that consists of all facilities and equipment used for producing, processing, or assembling goods. The sector encompasses some types of manufacturing, agriculture, forestry, mining, and construction. In this sector, the overall energy use is largely for process heat and cooling and powering machinery, with lesser amounts used for facility heating, air conditioning, and lighting. 

Injection well: A well that is used to return spent geothermal fluids to the geothermal reservoirs. The process is sometimes referred to as reinjection. 

Leadership in Energy and Environmental Design (LEED®): A rating system devised by the United States Green Building Council to evaluate the environmental performance of a building and encourage market transformation toward sustainable design. The system is credit based, allowing projects to earn points for environmentally beneficial strategies taken during the construction and use of a building. LEED Certification is the most widely used global standard; it recognizes buildings that are efficient, cost-effective, and better for occupants and the environment. LEED Certification can be applied to new or existing buildings and homes looking to become sustainable. 

Levelized cost of energy (LCOE): Also referred to as the levelized cost of electricity, it is a measurement used to assess and compare alternative methods of energy production. For an energy-generating facility, it is thought of as the average total cost of building and operating the asset per unit of total electricity generated over an assumed lifetime. This requires future costs to be expressed in present value terms by discounting. It is one of the utility industry’s primary metrics for the cost of electricity produced by an electricity generating facility. 

Life cycle costs: An important economic analysis used in the selection of alternatives that impact both pending and future costs. It compares initial investment options and identifies the least cost alternatives over a specific period (typically 20 years). 

Local requirements: Laws or statutes that regulate the use of products, materials, or services within a jurisdiction. Examples of local requirements may include those requirements from authorities having jurisdiction in various municipalities, cities, counties, provinces, and states. 

Loop or U-bend loop: The heat exchanger conduit of a ground loop heat pump system through which the heat-transfer fluid is circulated and thermal energy transfer takes place. 

Low-temperature geothermal resources: Resources generally considered those geothermal resources below 300°F (150°C). Low-temperature geothermal uses include geothermal heat pumps (GHPs) for individual homes and businesses as well as direct-use applications, where water from the geothermal resource is piped through heat exchangers or directly into commercial or residential buildings to meet heating and hot water demands. 

Measured undisturbed ground temperature: The mean temperature over the entire depth of the borefield, as measured from one or more test boreholes and without any external influence. 

Nonconventional energy resources: Generally regarded as renewable sources of energy that are continuously produced in nature and are limitless (e.g., geothermal energy, solar energy, bioenergy, tidal energy, and wind energy). 

Open-loop (groundwater) system: A geothermal energy system designed to use groundwater for the purpose of extracting or injecting heat. The loop is open at the bottom in an aquifer, and water is pumped to the ground surface and circulated through a geothermal heat pump. 

Orphan well: A subset of any abandoned oil or gas well for which no owner can be determined that is responsible to conduct proper plugging and site restoration. These wells include modern wells associated with company bankruptcies and legacy wells abandoned prior to contemporary regulatory standards for plugging and for which there is no current responsible party. Orphaned wells can be found on federal, state, private, and tribal land in every oil- and gas-producing state in the United States. 

Overburden: Unconsolidated soil, sediments, and weathered bedrock (regolith) that is variable in thickness and may be discontinuous, and material overlying the bedrock. 

Owner: An individual, group of individuals, or legal entity who owns rights to the natural resources found at the surface and in the subsurface. In some states, rights to the surface and subsurface may be held by different entities. According to the British Geological Survey, one key challenge with ownership and regulation of geothermal energy resources is that in some jurisdictions, it is regarded as a physical property, not a recoverable (raw) material, such as minerals or aggregates. As such, “heat” is not a legally defined entity, which causes some difficulties when assigning legal ownership and regulating it. 

Permeability: A measure of the relative ease with which a fluid moves through the pore spaces of a bedrock or unconsolidated sediment. 

Photovoltaic (PV): Materials and devices that convert sunlight into electrical energy. 

Photovoltaic–thermal (PVT): Materials and devices that convert sunlight into both electricity and hot water. 

Power purchasing agreement (PPA): An arrangement in which a third-party developer installs, owns, and operates an energy system on a customer’s property. The customer then purchases the system’s electric output for a predetermined period. The agreement allows the customer to receive stable and often low-cost electricity with no upfront cost while also enabling the owner of the system to take advantage of tax credits and receive income from the sale of electricity. Although most commonly used for renewable energy systems, PPAs can also be applied to other energy technologies, such as combined heat and power. 

Public–private partnership (P3): A long-term contract between the public sector and the private sector for the purpose of delivering a project or a service traditionally provided by the public sector, in which the private party bears significant risk and management responsibility. 

Radiant solar energy: Radiant energy, also known as electromagnetic radiation, is energy transmitted without a mass movement. Radiant energy is the energy of electromagnetic waves. An example of radiant energy is the heat from direct sunshine. 

Residential sector: An energy-consuming sector that comprises the living quarters for private households. Common uses of energy associated with this sector include space heating and cooling, water heating, lighting, refrigeration, cooking, and powering a variety of other appliances. The residential sector excludes institutional living quarters. 

Renewable energy resources: Energy resources that are naturally replenishing but flow limited. They are virtually inexhaustible in duration but limited in the amount of energy that is available per unit of time. Renewable energy resources include biomass, hydropower, geothermal, solar, wind, ocean thermal, wave action, and tidal action. 

Sedimentary basins: Low areas in the Earth’s crust, of tectonic origin, in which thick deposits of sediments accumulate over geological time periods. Sedimentary basins range in size from as small as hundreds of meters to large parts of ocean basins. The essential element of the concept is tectonic creation of relief, to provide both a source of sediment and a relatively low place for the deposition of that sediment. 

Septic system: An arrangement for disintegrating the organic matter in sewage; this includes any piping supplying the holding tank and the tile bed. 

Standing column well (SCW): In this variation of an open-loop geothermal system, one or more deep vertical boreholes (typically ~245 ft or ~75 m deep) are drilled. Groundwater is drawn from the bottom of a standing column and returned to the top after the heat is exchanged at the surface. During periods of peak heating and cooling, the system can bleed a portion of the return water rather than reinjecting it all, causing water inflow to the column from the surrounding aquifer. The bleed cycle cools the column during heat rejection, heats it during heat extraction, and reduces the required borehole depth. Standing column wells are commonly used in areas where groundwater is found in smaller quantities and competent bedrock is fractured. 

Subsurface: The zone below the surface where soil, sediment, and rock are present, and whose geologic features, principally stratigraphic and structural, are interpreted on the basis of drill records and various kinds of geophysical evidence. 

Sustainability Tracking, Assessment & Rating System (STARS): A transparent, self-reporting framework administered by the Association for the Advancement of Sustainability in Higher Education (AASHE) for colleges and universities to measure their sustainability performance. STARS is intended to engage and recognize the full spectrum of higher education institutions, from community colleges to research universities. The framework encompasses long-term sustainability goals for already high-achieving institutions, as well as entry points of recognition for institutions that are taking first steps toward sustainability. 

Thermal conductivity: A measure of the ability of a material to conduct heat. Heat transfer takes place at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, whereas the opposite is true for insulating materials such as Rockwool or Styrofoam. The defining equation for thermal conductivity is q = −λT (Fourier’s law of heat conduction), where q is the local heat flux density, l is the thermal conductivity (expressed as W/(m·K) [watts per meter and kelvin]), and T is the temperature gradient. 

Thermal energy storage: Storage of heat or heat sinks (coldness) for later heating or cooling. Examples are the storage of solar energy for night heating; the storage of summer heat for winter use; the storage of winter ice for space cooling in the summer; and the storage of electrically generated heat or coolness when electricity is less expensive, to be released in order to avoid using electricity when the rates are higher. There are four basic types of thermal storage systems: ice storage; water storage; storage in soil, sediment, and bedrock or other types of solid thermal mass; and storage in other materials, such as glycol (antifreeze). 

Thermal response test (TRT): Also known as a thermal conductivity (TC) test, this activity involves measuring the ability of the soil, sediment, or bedrock in which a heat exchanger is buried to transfer energy. To conduct a TRT of a vertical borehole, HDPE pipe is installed in a borehole to the depth that is most appropriate for the site and building loads. Heated water is circulated through the pipe. It is typically heated by using electric elements powered by a generator. The flow rate and temperature of the water are measured as it enters and leaves the borehole. The test is typically operated for at least 48 hours. Flow rate and temperature data are recorded usually every 2 minutes. The data collected are used to calculate the thermal properties of the borehole to determine how much heat can be transferred to and from the ground. The thermal property data are used in conjunction with the building energy loads to calculate the number, spacing, and depth of the boreholes for a GHP system. 

TRaNsient SYstem Simulation Tool (TRNSYS): A flexible graphically based energy simulation software environment used to assess the performance of thermal and electrical energy systems. 

Underground Injection Control: A 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 liquids and gas. 

Underground thermal energy storage (UTES): The seasonal storage of heat, cold, or both underground in geological strata ranging from soil, sediment, and bedrock to aquifers that is generally performed using a shallow geothermal system. Underground thermal energy storage is a sensible thermal energy storage method (i.e., the stored energy does not require a phase change) and is characterized by high storage efficiencies and high storage capacities. Therefore, UTES is a preferred choice for long-term thermal energy storage. The UTES techniques can be subdivided into open-loop or closed-loop systems. The most popular seasonal UTES techniques are aquifer thermal energy storage (ATES), borehole thermal energy storage (BTES), tank thermal energy storage (TTES), pit thermal energy storage (PTES), and cavern thermal energy storage (CTES). 

Vertical borehole: Any vertical hole that is drilled, bored, cored, driven, hydraulically advanced, or otherwise constructed into the ground. 

Vertical borehole heat exchanger: A subsystem of the ground source heat exchanger resulting from the drilling of the vertical borehole, placement of the loop piping to the bottom of the vertical borehole with the grout tremie, and grouting of the vertical borehole from the bottom of the vertical borehole to the Earth’s surface at the drill site. 

Water well: An excavation that is drilled, cored, bored, washed, driven, dug, jetted, or otherwise constructed for the purposes of extracting groundwater, monitoring groundwater, using the geothermal properties of the ground, or injecting water into an aquifer or subsurface reservoir. 

Well doublet: A set of two hydraulically coupled wells used for the extraction (production well) and injection (reinjection well) of groundwater. 


ASHRAE Handbook Online (HVAC Systems and Equipment): Chapter 9 Applied Heat Pump and Heat Recovery Systems. https://handbook.ashrae.org/Handbook.aspx 

Geothermal Glossary. Geothermal Technologies Office, U.S. Department of Energy. https://www.

Glossary of Geology. American Geosciences Institute. https://glossary.americangeosciences.org 

Glossary of Geothermal Heating and Cooling.  https://en.wikipedia.org/wiki/Glossary_of

Glossary of Geologic Terms. National Park Service, U.S. Department of the Interior. https://www.nps.

Glossary. U.S. Energy Information Administration. https://www.eia.gov/tools/glossary 

Third Assessment Report: Annex B. Glossary of Terms. The Intergovernmental Panel on Climate Change (IPCC). https://archive.ipcc.ch/pdf/glossary/tar-ipcc-terms-en.pdf 

ANSI/CSA C448 Series-2016 – Design and Installation of Ground Source Heat Pump Systems for Commercial and Residential Buildings. CSA Group. Preface available at https://webstore.ansi.org/