How Exactly Does Geothermal Work?

What is geothermal energy?

Geothermal energy is thermal energy generated and stored in the Earth (EIA.gov). It arises from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface (EIA.gov). Geothermal energy is a renewable energy source because heat is continuously produced inside the Earth (EIA.gov).

There are several types of geothermal resources that can be utilized:

• Hydrothermal systems utilize both hot water and steam from reservoirs of porous rock (Energy.gov). The heated water can be piped directly into buildings for heating or be used to generate electricity.
• Enhanced geothermal systems (EGS) extract heat by fracturing hot dry rock and circulating fluid through it (Energy.gov). The heated fluid is used to generate electricity.
• Geo-pressured systems produce thermal energy from hot water and methane gas trapped under pressure (Energy.gov). The methane can be extracted and burned to generate electricity.

Overall, geothermal energy harnesses the Earth’s internal thermal energy as a clean, renewable energy source for heating and electricity generation.

Geothermal Resources

There are several types of geothermal resources that can be harnessed for energy production:

Hydrothermal Systems

Hydrothermal resources involve hot water reservoirs located deep underground. The high temperature water (150-700°F) is pumped to the surface and the steam is used to turn electricity-generating turbines. Hydrothermal systems are concentrated in the western U.S., Alaska, and Hawaii where hot spots are located near tectonic plate boundaries.

Enhanced Geothermal Systems (EGS)

EGS involves injecting fluid into hot dry rock reservoirs to create an artificial geothermal reservoir. The fluid absorbs heat from the rock as it travels through fractures and pores. The heated fluid is extracted and used to produce electricity at the surface. EGS has the potential to expand geothermal energy production to most areas of the U.S.

Geo-Pressured Systems

Geo-pressured reservoirs contain hot salty water under high pressure. This hot water can be used directly for heat, or the methane dissolved in it can be extracted and used to generate power. Geo-pressured resources are found along the Gulf Coast of Texas and Louisiana.

Hydrothermal systems

Hydrothermal systems are reservoirs of geothermal water stored in deep cracks and porous rock to depths of around 10km (Energy.gov). These reservoirs contain naturally heated water or steam with temperatures above 150 °C that can be used for geothermal power generation and other direct applications.

The three main components of a hydrothermal system are (Greenfireenergy.com):

• A heat source – This provides heat from below that warms the water.
• Permeability – This allows water to flow through fractures and porous rock.
• Water – Heated water fills the cracks and pores in the rock at depth.

Hot water and steam can be extracted from hydrothermal reservoirs through production wells that pump the geothermal fluid to the surface. The type of power plant depends on the state of the fluid (DOE):

• Flash plants – Used when fluid is pumped under high pressure of 182-374°C. The drop in pressure flash vaporizes some of the water to steam that can drive a turbine.
• Dry steam plants – Used directly with underground steam at 235°C and above. Steam rises through the well and powers a turbine.
• Binary cycle plants – Used for lower temperature reservoirs of 57-182°C. Hot water is passed through a heat exchanger to heat a secondary fluid with a lower boiling point that flashes to vapor.

In all three plant types, the geothermal steam or vapor from the turbine powers a generator to produce electricity. The fluid is either injected back into the reservoir or used for direct heating applications.

Enhanced geothermal systems

Enhanced geothermal systems (EGS) are engineered reservoirs created to produce energy from geothermal resources that are otherwise not economical due to lack of water and/or permeability. EGS expands the potential for geothermal energy production by accessing the earth’s heat through creation of reservoirs in hot dry rock (HDR).

EGS involves pumping fluid into naturally occurring, hot but impermeable rock to extract heat, allowing thermal energy to be captured from areas not previously deemed suitable. The fluid travels through artificially created networks of cracks and pores in the rock, absorbs heat, and returns to the surface where the heat is converted into electricity.

Creating an EGS reservoir involves:

• Drilling one or more wells into hot rock
• Creating cracks and pores by hydraulic fracturing or other means
• Circulating fluid through the reservoir to absorb heat
• Extracting the heated fluid and generating electricity at the surface

EGS technology has the potential to produce 10 times more energy than conventional geothermal systems. Ongoing research seeks to improve reservoir creation techniques, subsurface engineering, and process sustainability. However, EGS is still an emerging technology with challenges related to induced seismicity, water loss, and economics that must be overcome before it can provide a major source of renewable baseload power.

Sources:

https://www.energy.gov/eere/geothermal/enhanced-geothermal-systems


https://www.energy.gov/sites/default/files/2016/05/f31/EGS%20Fact%20Sheet%20May%202016.pdf

Geo-pressured systems

Geo-pressured resources represent a specialized form of geothermal energy that exists in deep, pressurized sedimentary formations containing hot, saline brines (National Energy Technology Laboratory, 2021). These systems utilize the heat, pressure, and methane gas trapped in fluid-filled rock pores or fractures located at depths of 10,000 to more than 30,000 feet (Energy.gov, 2022).

The high pressure is caused by the weight of overlying sediment and geological processes. It keeps fluids with temperatures higher than 300°F from boiling. The methane is generated by thermally driven chemical alteration of indigenous organic matter in the rock (National Energy Technology Laboratory, 2021).

To extract energy, wells are drilled into the geo-pressured zones. The hot brines are brought to the surface under controlled conditions where the thermal energy is converted to electricity by flashing the brine to steam to drive turbines. The dissolved methane is separated from the brines and can also be used as fuel (Energy.gov, 2022).

Therefore, geo-pressured systems provide a unique opportunity to produce electricity as well as capture methane for use as a fuel source or chemical feedstock. The combination of heat, pressure, and methane make these resources an attractive potential energy source.

Direct use applications

Geothermal energy can be used directly for a variety of applications, including heating buildings, greenhouses, fish farms, and industrial uses. One of the most common direct uses is through geothermal heat pumps, which can provide heating and cooling for buildings (NREL, 2004).

Geothermal resources with temperatures below 150°C are suitable for direct heating applications (WBDG, 2016). The hot water can be piped directly into buildings to provide space heating. Schools, offices, and residential buildings near geothermal resources often use this direct heating. Greenhouses also frequently use geothermal heating to maintain optimal temperatures for plant growth. Additional agricultural applications include heating water for aquaculture and fish farming.

On an industrial scale, geothermal heat can be used for drying fruits, vegetables, and timber. The heat can also be used in industrial processes requiring temperatures below 150°C. Geothermal fluids provide process heat for operations like milk pasteurization, washing and drying of food products, and paper production.

Geothermal heat pumps are used worldwide for heating and cooling of both residential and commercial buildings. These systems operate by exchanging heat between the relatively stable ground or ground water and buildings. In winter, the pumps draw heat from the ground and transfer it inside. In summer, the process reverses to transfer heat from indoors to the ground (NREL, 2004). Geothermal heat pumps significantly increase efficiency compared to conventional heating and cooling systems.

Environmental impacts

Geothermal power plants can have various environmental impacts that need to be managed and mitigated. Three key areas of concern are land use, emissions, induced seismicity, and water use.

In terms of land use, geothermal plants require between 1-8 acres per megawatt of installed capacity, more land than solar or wind plants (UCSUSA). The actual footprint depends on the type of geothermal plant, with binary plants generally requiring less land. Roads may need to be constructed to bring equipment to remote geothermal sites.

Geothermal plants emit substantially lower emissions of greenhouse gases and air pollutants than fossil fuel plants. However, geothermal reservoirs can contain gases like carbon dioxide, hydrogen sulfide, ammonia, methane, and boron that may be released into the atmosphere. Proper reservoir management and use of abatement technology can minimize this (EIA).

Enhanced geothermal systems that inject fluids into the earth to create reservoirs may increase the risk of small, localized earthquakes. Seismic activity and ground deformation needs to be closely monitored. Projects can be shut down if seismic activity exceeds preset thresholds (USFWS).

Finally, geothermal plants use substantial amounts of water for cooling and geothermal fluid replacement. Binary plants use less water but water use still needs to be managed, especially in arid regions. Water can be recycled and reinjected into the reservoir.

Costs

The costs of geothermal power plants vary significantly depending on the resource quality and plant configuration. According to the U.S. Department of Energy, capital costs for geothermal power plants in 2019 ranged from $2800 to $6500 per kW of capacity (source). Operating costs are estimated to be around $0.01 to $0.03 per kWh.

When considering the levelized cost, geothermal is competitive with other energy sources. A 2020 analysis by the International Renewable Energy Agency found the global weighted average levelized cost of electricity (LCOE) for geothermal power to be $0.047 per kWh. This is lower than the LCOE for fossil fuels like coal and natural gas and other renewables like solar PV and onshore wind (source). However, there is significant regional variation in geothermal LCOE based on resource quality.

Future advancements

There are several promising areas of advancement for geothermal technology:

Enhanced geothermal systems (EGS) are expected to greatly expand the potential of geothermal energy. EGS involves drilling deep wells and pumping water into hot dry rock formations to create new subterranean water networks, allowing more geothermal reservoirs to be tapped.¹ This has the potential to provide access to geothermal energy almost anywhere.

Co-produced fluids from oil and gas wells, such as hot wastewater, can potentially be used for geothermal electricity generation and direct use applications. Using these co-produced fluids takes advantage of existing resources and infrastructure.¹

Hybrid power plants that combine geothermal with solar photovoltaic or biomass sources are also being developed. These hybrid systems maximize efficiency by using geothermal for consistent base load power and renewables for variable power.¹ They could enable broader use of geothermal energy worldwide.

Conclusion

In summary, geothermal energy holds great potential as a renewable, sustainable energy source that can provide constant base-load power. By utilizing the heat beneath the earth’s surface, geothermal plants can operate 24/7, independent of weather conditions. The three main types of geothermal resources each have their own advantages and challenges. Hydrothermal systems, which utilize hot water reservoirs, are the most common but are limited to certain geographic locations. Enhanced geothermal systems can be developed in more areas by injecting fluid to create an artificial geothermal reservoir, but this approach is still in its early stages. Geo-pressured systems make use of highly pressurized hot water but have even more technical hurdles to overcome. In addition to power generation, geothermal energy can also be used directly for heating, agriculture, and industrial applications.

With its ability to provide constant baseline power, geothermal has the potential to play a major role as renewable energy scales up to meet modern electricity demands. Ongoing research and development can help make geothermal accessible in more locations worldwide. Though upfront costs are high, geothermal offers a sustainable long-term energy solution. With thoughtful implementation and environmental protections, geothermal can be part of the global transition to a cleaner energy future.