Geothermal energy represents one of the cleanest and most plentiful energy sources available for both direct heating and for electric power generation. To date most geothermal electricity production has been from permeable hydrothermal reservoirs where magmatic heated steam or very hot water is pumped to the surface and used to turn a conventional steam turbine. However the introduction of organic Rankine binary generators has recently made it possible to utilize much lower enthalpy reservoirs to generate geothermal electricity, including hot water from existing oil and gas wells. This is leading to a rapid proliferation of new geothermal installations worldwide. In addition, there is considerable excitement surrounding the concept of enhanced geothermal systems (EGS). In enhanced geothermal systems wells are drilled into hot, impermeable bedrock and hydraulic fracturing is used to connect injection and recovery wells. Water is then pumped into the reservoir to produce an engineered closed-loop geothermal system. Prototype EGS power plants have been built worldwide including in Australia, the United States, and Europe. EGS is exciting because the heat being tapped is due to radioactive decay in widely dispersed basement rock rather than from geographically limited near surface magma bodies. This means EGS has the potential to be used almost anywhere in the world and could make geothermal one of the largest sources of future electrical power. However there are significant exploration and drilling challenges to be overcome in order to realize the full potential of enhanced geothermal systems.

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