To address problems such as greenhouse gas emissions and energy security, the US is considering renewable resources, such as geothermal energy. Hydrothermal systems produce about 3000 MWe domestically; however, electricity may also come from engineered geothermal systems (EGS) with siting issues such as low-permeability rock, limited water, and deep wells. Water use can be reduced
with a power cycle that works efficiently with air cooling, using refrigerant mixtures. Heat extraction from the subsurface represents another aspect of geothermal water use because complex fluid-rock interactions affect heat transport and lifetime performance. These factors all contribute to the viability of EGS.