Geothermal

Geothermal is the extraction of heat from below the Earth's surface for energy. Hot reservoirs can be used for electricity production, while warm reservoirs can be harvested directly for heat for industrial processes or district heating.

We recommend a research and development program into enhanced geothermal systems, but not into hydrothermal sea vents. These geothermal methods are detailed below, with hydrothermal sea vents discussed in the context of ocean energy.

Potential Cost

Geothermal power has the following current and future project costs.

Sources: Beckers et al. ¹, IEA et al. ², International Renewable Energy Agency ³, Lazard ⁴, Logan et al. ⁵, OpenEI ⁶, Salvatore et al. ⁷, Shnell et al. ⁸, Energy Information Administration ⁹. Note that costs are highly site-specific.

The current high costs for enhanced geothermal reflects the fact that it is still an emerging technology, and mature EGS technology has the potential to be competitive with mainstream power generation.

Recovery of valuable minerals from geothermal brine, such as gold, cesium, rubidium, lithium, and silicates, could further improve the economics, though this is not done presently ¹⁰. Recovery of rare earth elements does not appear to be economically feasible ¹¹.

Resource Potential and Environmental Considerations

The following shows potential annual production from several forms of geothermal energy.

Sources: Hiriart et al. for hydrothermal ¹², Krewitt et al. ¹³ for world geothermal heat and electricity, and NREL ¹⁴ for US enhanced geothermal electricity. Current world geothermal production is given by BP ¹⁵ and the EIA ¹⁶, and current primary energy by BP ¹⁵.

The lifecycle greenhouse gas emissions of geothermal power depends on the technology.

Sources: Matek ¹⁷ and Schlömer et al. ¹⁸.

Enhanced Geothermal

An enhanced geothermal system (EGS) extracts heat from deep within the Earth and in environments that are not naturally porous. Unlike traditional hydrothermal geothermal energy, EGS resources are accessible from almost everywhere on Earth, though at varying cost and difficulty. Medium-grade EGS resources are widespread throughout the Western United States and available in other parts of the country, while low-grade resources are available almost everywhere ¹. Estimated EGS reserves are 266,000 EJ ¹⁹, over 400 years of world primary energy consumption.

EGS functions by first drilling a deep well, known as an injection well, into the rock and injecting water at high pressure to create a network of fractures. After the reservoir has been created, another well, known as the production well, is drilled. This well is used to extract the hot water that has been injected in the previous step. The fluid can then be recycled through the system ²⁰.

Following are estimates of potential EGS costs. Enhanced geothermal is a particularly attractive option for low-temperature industrial processes such as paper processing and biomass drying ¹.

Electricity is reported by Tester et al. ¹⁹ and assumes a cumulative $1 billion of investment and learning-by-doing improvement. Beckers et al. ¹ report cost estimates for EGS for direct heat. NREL ¹⁴ estimates the cost for district heating, which is higher since not all extracted heat in a district system would be useful ²¹.

Even at a higher price, EGS for district heating should be competitive with natural gas prices observed since 2005 ²². Further cost reductions may be possible through combined district heating and cooling systems ²³.

EGS technology does not face any obvious technical barrier and is currently undergoing demonstration deployment, notably at Cooper Basin in Australia ²⁴. Nevertheless, additional deployment is needed to reduce costs.

Geothermal will be needed to reach 2050 emission goals.

In addition, in the United States, permitting requirements under the National Environmental Policy Act (NEPA) can cause significant delay to geothermal projects and may greatly slow the development of enhanced geothermal resources.

Induced Seismicity

Earthquakes and increased seismic activity have hampered EGS development ²⁵. Studies in Finland have shown limited interaction between seismic activity and fluid injection, with a proportionate amount of seismic energy and hydraulic energy input, and seismic activity is not time-delayed ²⁶. However, an earthquake in South Korea that injured about 70 people and caused extensive damage ²⁷ has been associated with a nearby EGS project. Researchers believe that the fluid injection added pressure to an existing at-risk fault line, pushing it over the tipping point and triggering the seismic event ²⁸.

Assessing seismic risk should be part of EGS deployment. Switzerland’s mitigation plan for induced seismicity involves limiting the areas of hydraulically stimulated fracture planes and avoiding densely populated areas and at-risk faultlines ²⁹. Japan’s Beyond-Brittle Project seeks to utilize ductile basements, which are rock formations characterized by their ability to deform in response to stress without fracturing ³⁰. Deep closed-loop geothermal wells could also provide a solution to seismic risks as well as broaden the range of potential sites to global levels ³¹.

Supervolcano Risk

A supervolcanic eruption, such as threatened at Yellowstone National Park, would be a catastrophic event. Reducing the buildup of heat in a potentially supervolcanic caldera may reduce the risk. This could be done by drilling holes over the top of the magma chamber, drilling holes around the perimeter of a supervolcano, and managing the water supply within a caldera ³². The second of these solutions would allow energy production via enhanced geothermal systems ³². However, such an approach could take thousands of years to safely cool a caldera, and there remain unknown risks of inducing an eruption and generally the mechanics of how supervolcanoes work ³².