Geothermal energy comes from reservoirs of hot water underneath the Earth's surface. This hot water makes its way to the surface naturally in the form of hot springs or geysers. The water and steam from these have been used throughout history for bathing and cooking. More recently, geothermal heat that exists near the surface can be used to heat buildings, and is used in Idaho, northern California, Iceland, and New Zealand.1 Technologies similar to those found in the oil and gas industries are used to drill wells to access additional hot water reservoirs. The steam from the hot water is used via a variety of methods, such as flash, dry steam, and binary, to turn turbines that generate electricity.2 Unlike other sources of renewable energy that rely on weather conditions, geothermal energy can produce electricity consistently.3
Conventional geothermal reservoirs are identified by places where pressurized hot water reaches the surface, such as steam vents or geysers. Wells are drilled through the caprock at the surface near these fissures to access the natural hot water reservoir and deliver it to a geothermal power plant. Some issues with conventional geothermal reservoirs are that they are geographically limited to places near tectonic plates, and they are difficult to standardize and scale. Enhanced geothermal systems (EGS) are when a hot water reservoir is created by drilling through solid rock, injecting water, and collecting heated water through another well. The difficulty increases the deeper the need to drill and the harder the rock.4 EGS uses similar fracking technology from the shale gas industry, but the geothermal industry is working to distance itself from the term fracking, because of negative public reactions.5 Some advanced closed loop systems are being developed to work like EGS, but without fracking. Instead of fluids injected into the ground, they have pipes with water running underground that is heated by the Earth through conductive heat.6 Supercritical geothermal systems are also being investigated, which would enable energy companies to tap into super hot rock located deep within Earth's surface. At these extremely hot temperatures fluid is supercritical, which means it has more energy per unit mass, and it could provide up to 10 times more energy than a conventional geothermal reservoir.7
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The following links are to government and industry websites and documents related to geothermal energy.