Geothermal power is energy generated by heat stored in the earth.  Geothermal power generation refers to the collection of absorbed heat derived from underground resources.  Geothermal resources range from shallow ground to hot water and rock several miles below the Earth's surface, and even further down to the extremely hot molten rock. Wells over a mile deep can be drilled into underground reservoirs to tap steam and very hot water that can be brought to the surface for use in a variety of applications. As of 2008, geothermal power supplies less than 1% of the world's energy.

The United Stated has the greatest geothermal production in the world.  Its largest group of power plants in the world is located in The Geysers, a geothermal field in California. This plant is recharged by injecting treated sewage effluent from the City of Santa Rosa and the Lake County sewage treatment plant, which used to be dumped into rivers and streams.

Geothermal steam and hot springs have been used for centuries for bathing and heating, but it was not until the 20th century that geothermal power started being used to make electricity.  The first geothermal power generator was tested on July 4th, 1904 in Italy.

Twenty-four countries generated a total of 56,786 GWh (204 PJ) of electricity from geothermal power in 2005, accounting for 0.3% of worldwide electricity consumption. This output is growing by 3% annually, thanks to a growing number of plants as well as improvements in their capacity factors.

Because a geothermal power station does not rely on transient sources of energy, unlike, for example, wind turbines or solar panels, its capacity factor can be quite large.  In fact, a capacity factor of up to 90% has been demonstrated. The global average was 73% in 2005. The global capacity was 10 GW in 2007.

The thermal efficiency of geothermal electric plants is typically low because geothermal fluids are at a low temperature compared to steam from boilers. Constrained by the laws of thermodynamics, the low temperatures limit the efficiency of heat engines in extracting useful energy during the generation of electricity. The exhaust heat is wasted, unless it can be diverted and used directly and for other purposes. The efficiency of the system does not affect operational costs as it would for a coal or other fossil fuel plant, but it does factor into the viability of the plant. To produce more energy than the pumps consume, electricity generation requires high temperature geothermal fields and specialized heat cycles.  The plant types are as follows:

1. Dry steam plants - the most basic design and also the oldest type. They use geothermal steam of 150°C or more to turn turbines.
2. Flash steam plants -  The most common type of plant operating today. These plants siphon deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines. They require fluid temperatures of at least 180°C, usually more.
3. Binary cycle power plants - These are the newest technology, and can accept fluid temperatures as low as 57°C. The moderately hot geothermal water is passed by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to vapor, which then drives the turbines. This is now the most common type of geothermal plant being built today. This type of plant uses both Organic Rankine and Kalina cycles. The thermal efficiency for this type of geothermal plant is typically about 10%.

Until fairly recently, geothermal plants have been built exclusively on the edges of tectonic plates where high temperature geothermal resources are available near the surface. The development of binary cycle power plants and improvements in drilling and extraction technology has opened the hope that they might be viable over a much greater geographical range. A demonstration project has recently been completed in Landau-Pfalz, Germany, and others are under construction in Soultz-sous-Forêts, France and Australia.

Investing in Geothermal

Investing in geothermal power can come from direct investing in geothermal power facilities.  New technologies for geothermal capture can present opportunities for speculative investors looking for investments in geothermal power.  Geothermal power investment can also come in the form of investment in geothermal equipment producers.

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This article is licensed under the GNU Free Documentation License. It uses material in part or in whole from the Wikipedia article "Geothermal Power"