In the realm of energy sources, geothermal energy stands as an embodiment of the ancient parable of the blind men and an elephant. Just as the blind men each touched a different part of the elephant and experienced only a fraction of its true nature, so do we sometimes perceive geothermal energy through a narrow lens. But geothermal energy, like the majestic elephant, reveals itself in diverse and multifaceted forms. In this article, we unravel the multifaceted nature of geothermal energy, drawing inspiration from the parable to reveal its versatility.
Geothermal energy is a sustainable renewable energy resource that taps the heat stored within the earth to generate electricity and provide heating and cooling solutions (Stober and Bucher 2013). Its extensive advantages make it popular across multiple sectors, from residential to industrial applications.
We will explore its primary applications, namely electricity, direct use, and ground source heat pumps (GSHPs). In the following section, we delve into the details of these diverse applications and also share some unconventional applications.
Electricity Generation
Geothermal energy presents an attractive option for electricity generation, primarily because of its consistent baseload characteristics and reliable production capabilities. The high-capacity geothermal power plants can generate electricity around the clock, leveraging the earth’s continuous heat production. When properly managed, geothermal energy is sustainable and renewable. Examples of geothermal power plants include the longstanding Larderello Field in Italy, operational since 1914, the Wairakei plant in New Zealand, and The Geysers plant in the US, which recently celebrated its 50th anniversary. Another compelling aspect of geothermal electricity generation is its minimal emissions and waste production. The hot water used in the process can be reinjected back into the reservoir, further enhancing its appeal for electric generation applications. The geothermal energy for electricity generation is harnessed through power plants, depending on the state and enthalpy of the subsurface resource. The three most common power plants are:
● Dry steam geothermal power plants: High-pressure steam is directly extracted from underground geothermal reservoirs and used to drive a turbine, producing electricity.
● Flash steam geothermal power plants: High-pressure geothermal fluid is rapidly depressurized to cause some of it to flash into steam and be separated and used to power a turbine to produce electricity.
● Binary geothermal power plants: These use low-temperature geothermal fluids to heat a secondary working fluid, expanding and driving a turbine generator to produce electricity.
Direct Use
Direct use refers to using geothermal heat directly for industrial and residential applications without the need for conversion into electricity (Lund 2010). The geothermal resource does not have to be very hot. Geothermal energy has gained significant recognition as a highly utilized source for both heating and cooling various buildings worldwide (e.g., Paris; Reykjavik, Iceland; and Reno, Nevada). Currently, the global use of geothermal energy for direct use applications exceeds 107,727 MWth and 283,580 GWh per year (Lund and Toth 2021). Direct use applications include:
● Agricultural applications—Direct heat from geothermal energy is used for soil warming, greenhouse and soil sterilization, fruit and vegetable drying, lumber drying, and beet sugar evaporation and pulp drying. With geothermal heating, farmers can extend the growing season by having a controlled and stable heat source. Geothermal fluids, which contain minerals and nutrients that can enrich the soil, provide crops with additional nutrients when used in irrigation systems.
● Aquaculture—Geothermal energy provides a heat source that can be used in cultivating fish and aquatic plants in geothermally active regions such as Iceland, New Zealand, and some areas in the US (e.g., Idaho).
● Geothermal hot spring bathing—This application is popular among tourists for relaxation and therapeutic experiences in destinations such as Blue Lagoon in Iceland, Beppu in Japan, and Rotorua in New Zealand.
● Industrial processes—Several industrial processes use direct heat from a geothermal resource to increase efficiency, including concrete block curing, food processing, refrigeration and ice making, cement and aggregate drying, ethanol and biofuel production, fabric dyeing, pulp and paper industry, chemical production, mineral recovery, metal smelting, glass production, ceramic production, snow melting and deicing, hydrogen production, and soft drink carbonation.
● Mineral production—Some geothermal brines contain commercially high concentrations of various minerals such as lithium, silica, and zinc. Geothermal energy is then used to precipitate minerals out of geofluids, reducing energy consumption and environmental impact compared with traditional methods. For instance, the Salton Sea in California has the potential to produce over 600,000 tons of commercial lithium annually, which could double the world’s current supply.
Ground Source Heat Pumps (GSHPs)
GSHPs take advantage of the stable temperature conditions found within the top layers of soil to heat and cool buildings, using underground pipes buried at 5°C and 30°C to transport warm and cool air directly into buildings during winter and summer, respectively. Widely used across North America, Europe, and East Asia, GSHP systems are highly energy-efficient, consuming 25–50% less electricity than air-source heat pumps or conventional air conditioning systems while emitting less greenhouse-gas pollution.
Further to the primary applications of geothermal energy, there are growing applications of the resource such as the following.
Geothermal Energy Storage
Geothermal energy can be used as a storage system. Geothermal storage uses the earth's natural heat reserves to store and retrieve energy by capturing the excess heat from the geothermal sources at times when energy demands are low and storing it underground in rock or water reservoirs. This stored energy can be accessed later for electricity generation or heating. These systems consist of wells injecting and extracting fluids from the subsurface, creating a closed system for heat transfer. Using the earth's constant temperature makes geothermal systems a more efficient and sustainable way to store thermal energy.
Geothermal energy storage options include:
● Aquifer thermal energy storage—This method injects excess heat from geothermal sources into underground aquifers for later retrieval. When geothermal sources provide excess heat, hot water or steam is injected into an aquifer to store thermal energy until needed to heat buildings or generate electricity.
● Thermal ponds—Geothermal heat can be stored in shallow surface ponds and reservoirs on shallow terrain, acting as thermal storage units during high geothermal heat production periods. Hot water or steam from geothermal heat sources is directed into these reservoirs for thermal storage and later used directly or converted into electricity through an exchanger and power generation system.
● Underground thermal energy storage (UTES)—UTES stores heat in underground rock formations or geological structures during periods of high geothermal heat production, transferring excess energy from geothermal heat production into this storage medium and later retrieving it by passing fluid through an underground system to extract its thermal energy.
Geothermal Hybrid System
Hybrid geothermal energy systems couple geothermal energy with other renewable sources, such as wind or solar facilities, for added reliability and efficiency.
Geothermal provides a stable baseload power supply while solar/wind energy contributes peak availability energy generation. This combination ensures more reliable electricity delivery while reducing systemic vulnerability because of overreliance on one energy source.
Another type of hybrid system couples geothermal heating and cooling technologies with other energy technologies. Geothermal heat pumps, which use the earth's stable temperatures to deliver heating and cooling services, can be integrated with solar thermal or biomass systems for more-efficient and flexible operation, reducing dependence on fossil-fuel-based heating methods and optimizing energy use.
Complementing Low-Carbon Technologies
Geothermal energy is also being explored in the production of green hydrogen, produced through the process of electrolysis, where electricity is used to split water into hydrogen and oxygen. It can provide a renewable and sustainable source of electricity to power the electrolysis process, significantly reducing the overall carbon footprint of hydrogen production. This makes geothermal-powered hydrogen a promising option for decarbonizing various industries, including transportation and energy storage.
Geothermal energy is being investigated as a source of required heat and power for direct air capture (DAC) technology, a process that captures carbon dioxide directly from the atmosphere.
Flexible Geothermal Systems
The increasing global demand for clean and sustainable energy has sparked a renewed interest in geothermal power generation. To enhance economic viability and grid integration, there is growing recognition of the need for flexible operation strategies in geothermal power systems (Aljubran et al. 2023; Ricks et al. 2022). Flexibility in power generation refers to the ability of a power plant to adapt its output to meet varying electricity demand patterns and grid conditions. With the integration of intermittent renewable sources such as wind and solar, flexible high-capacity geothermal power generation becomes essential to accommodate their inherent baseload variability and to maintain grid stability. Various initiatives such as the US Department of Energy’s Beyond Batteries project and the recognition of flexible geothermal generation as an emerging technology (Millstein et al. 2021; Robins et al. 2021) aim to exploit the added value of flexibility. Not all geothermal plants may have the capability or economic incentives for flexibility, however, so each project should be assessed individually, considering specific circumstances and requirements.
In summary, geothermal energy is a versatile and multifaceted resource that offers numerous benefits across various sectors. It is not limited to electricity generation but extends to direct use applications and ground source heat pumps. It demonstrates its potential in unconventional applications, including geothermal energy storage and hybrid systems that combine it with other renewable energy sources for increased reliability and efficiency. It complements low-carbon technologies such as hydrogen production and DAC, contributing to decarbonization efforts. The flexibility of geothermal power systems is gaining recognition, allowing them to adapt to varying electricity demand patterns and grid conditions, supporting the integration of intermittent renewables and enhancing grid stability.
For Further Reading
Techno-Economic Modeling and Optimization of Flexible Geothermal Operations Coupled with Energy Storage by M.J. Aljubran, O. Volkov, and R.N. Home, Stanford Geothermal Program. Proceedings, 48th Workshop on Geothermal Reservoir Engineering, Stanford University, SGP-TR-224.
Geothermal Energy Use in Hydrogen Production: A Review by M. Ghazvini and M. Sadeghzadeh, University of Tehran, and M.H. Ahmadi, Shahrood (Iran) University of Technology, et al. International Journal of Energy Research.
Direct Utilization of Geothermal Energy by J.W. Lund, Oregon Institute of Technology and National Renewable Energy Laboratory. Energies.
Direct Utilization of Geothermal Energy 2020 Worldwide Review by J.W. Lund, Oregon Institute of Technology, and A.N. Toth, Ana-Geo Ltd., Miskolc, Hungary. Geothermics.
A Review of Geothermal Energy-Driven Hydrogen Production Systems by M. Mahmoud, University of London; M. Ramadan, International University of Beirut; and S. Naher, University of London, et al. Thermal Science and Engineering Progress.
The Potential To Improve the Value of US Geothermal Electricity Generation Through Flexible Operations by D. Millstein, P. Dobson, and S. Jeong, Lawrence Berkeley National Laboratory. ASME Journal of Energy Resources Technology.
A Green Hydrogen Economy for a Renewable Energy Society by A.M. Oliveira, R.R. Beswick, and Y. Yan, University of Delaware. Current Opinion in Chemical Engineering.
The Value of In-Reservoir Energy Storage for Flexible Dispatch of Geothermal Power by W. Ricks, Princeton University; J. Norbeck, Fervo Energy; and J. Jenkins, Princeton. Applied Energy.
2021 US Geothermal Power Production and District Heating Market Report (No. NREL/TP-5700-78291) by J.C. Robins, A. Kolker, and F. Flores-Espino, National Renewable Energy Lab, et al.
Geothermal Energy: From Theoretical Models to Exploration and Development by I. Stober and K. Bucher, University of Freiburg (Germany). Springer Science & Business Media.
Geothermal Explained—Use of Geothermal Energy. US Energy Information Administration.
