Geothermal Energy

While most renewable energies are, directly or indirectly, derived from the sun, geothermal energy originates in the interior of the earth. Geothermal energy is the most stable of the renewable energies because it can be utilized constantly, regardless of weather or season. Geothermal energy can be used not only for power generation, but also for direct heat application. The development of geothermal power generation entered a phase of rapid growth in 2005, and its total-installed capacity worldwide reached 10.7 GW_e in 2010. The capacity of 10.7 GW_e appears small when compared with solar and wind power generation; however, the high-capacity factor of geothermal power plants, which is 0.7–0.9, provides several times greater electricity from the same installed capacity than photovoltaic and wind plants. Direct heat application can be used almost anywhere on land. Geothermal resources are classified into two categories: hydrothermal convection resources and thermal conduction resources. Today’s geothermal power capacity is mainly hydrothermal-based and unevenly distributed in volcanic countries. As a borehole is drilled into deeper formations, formation temperature becomes higher but permeability becomes lower. Hydrothermal convection resources have a limit depth. Rock’s brittle-plastic transition gives a bottom depth to permeability, and it is the absolute limit depth for the hydrothermal convection resources. Enhanced or engineered geothermal systems (EGS), in which fractures are artificially created in less-permeable rocks and heat is extracted by artificially circulating water through the fractures, are still at a demonstration stage, but they will extend geothermal power generation to thermal conduction resources and to depths even deeper than the brittle-plastic transition. Assessment of worldwide geothermal resource potential is still under study. However, an estimate shows that potential is 312 GW_e for hydrothermal resources for electric power generation to a depth of 4 km, 1,500 GW_e for EGS resources to a depth of 10 km, and 4,400 GW_th for direct geothermal use resources. Were 70% of hydrothermal resources, 20% of EGS resources, and 20% of direct use resources to be developed by 2050, it could reduce carbon dioxide emission by 3.17 Gton/year, which is 11% of the present worldwide emission.