From Open Energy Information
Geothermal energy can be utilized for electricity or heating in more than one way. Regardless of the energy conversion, geothermal energy requires heat(in the form of rock), water, and flow; and every resources will have different values for each. Some resources have very high temperature rock with high porosity (allowing for flow) but little to know water (see Enhanced Geothermal Systems (EGS). Some resources have plenty of water, great flow, but the temperatures are not very high which are commonly used for direct use. Any combination of those 3 things can be found in nature, and for that reason there are different classifications of geothermal energy. It is possible for a resource to be technically capable of both electricity production and heating purposes, but the basic classifications of geothermal energy are listed below:
- Hydrothermal Systems
- Hydrothermal Systems take advantage of the coincidence of heat, water, and permeable rock at shallow depths (typically less than 5 km) to produce electricity.
- Enhanced Geothermal Systems (EGS)
- Enhanced Geothermal Systems (EGS) are human-engineered hydrothermal reservoirs developed for commercial use as an alternative to naturally occurring hydrothermal reservoirs, which are not known to be abundant.
- Sedimentary Geothermal Systems
- Sedimentary Geothermal Systems produce electricity from medium temperature, high flow rate geothermal reservoirs in large-basin, sedimentary deposits.
- Co-Produced Geothermal Systems
- Co-Production refers to the production of electricity from hot geothermal fluid as a byproduct of oil and gas wells. Historically this hot water has been an inconvenience and a disposal issue; however, it is now being looked at as an electricity resource for field use or to be sold to the grid.
- Direct-Use Geothermal Systems
- Low- to moderate-temperature water from geothermal reservoirs can be used to provide heat for a number of applications, such as pools and spas, greenhouses, fish farms, and mining operations.
- Ground Source Heat Pump Systems
- Ground Source Heat Pumps are used for space heating and cooling in buildings, using the ground as a heat source in the winter and a heat sink in the summer.