Geothermal in general
Almost everywhere, the shallow ground or upper 10 feet of the earth's surface maintains a nearly constant temperature between 50° and 60°F (10° and 16°C).
As you go deeper into the earth’s crust, the temperature rises. Depending on what type of rock you are in, the temperature will actually rise quicker. For a crystalline rock like granite, the temperature increases quickest – up to 30°C for every kilometre that you descend.
It is this heat energy at great depth that an engineered geothermal system is seeking to exploit, by way of creating an underground heat exchanger, or reservoir, from which the energy can be brought to the surface.
Geothermal energy is the name given to the extraction process of this heat stored in the earth’s crust. This natural thermal phenomenon results from two primary sources:
- Convection and conduction of heat from the earth’s core; and
- The heat from the decomposition of substances in that crust - substances like potassium and thorium.
It is already used for smaller scale applications, such as heat pumps, and on a larger scale, conventional hydrothermal (wet rock) technology is a proven commercial concept where there are natural sources of steam close to the surface.
Geothermal heat pumps can tap into this resource to heat and cool buildings. A geothermal heat pump system consists of a heat pump, an air delivery system (ductwork) and a heat exchanger-a system of pipes buried in the shallow ground near the building.
In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide a free source of hot water.
In areas where there are tectonic fault lines, the heat in the earth comes much closer to the surface. Precipitation filters through the rock and is heated by it, returning to close to the surface as steam.
This steam can be used to drive a turbine and thus produce electricity. There are 500 of these geothermal plants worldwide in 30 countries (United States, Philippines, Mexico, Italy, Indonesia and Iceland) and there is around 10,000MW of generating capacity worldwide. Hydrothermal systems use the steam or hot water trapped in the rocks to drive turbines to produce electricity.
Maps with geothermal data
Example of 20 Mw Geothermal powerplant in Reno, Nevada
This video (from YouTube) shows an example of an Ormat surface plant in operation. It generates more than 6 times the output of the plant proposed for the initial EGS developments in the UK but utilises the same technology.