Geothermal Education

Geothermal power (from the Greek roots geo, meaning earth, and thermos, meaning heat) is power extracted from heat stored in the earth. This geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. It has been used for space heating and bathing since ancient roman times, but is now better known for generating electricity. About 10 GW of geothermal electric capacity is installed around the world as of 2007, generating 0.3% of global electricity demand. An additional 28 GW of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications.

Geothermal power is cost effective, reliable, and environmentally friendly, but has previously been geographically limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for direct applications such as home heating. Geothermal wells tend to release greenhouse gases trapped deep within the earth, but these emissions are much lower than those of conventional fossil fuels. As a result, this technology has the potential to help mitigate global warming if widely deployed.

Prince Piero Ginori Conti tested the first geothermal generator on 4 July 1904, at the Larderello dry steam field in Italy. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California, United States. As of 2004, five countries (El Salvador, Kenya, the Philippines, Iceland, and Costa Rica) generate more than 15% of their electricity from geothermal sources.

Electricity

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; up to 90% has been demonstrated. Their global average was 73% in 2005. The global capacity was 10 GW in 2007.

Geothermal electric power plants have been limited to the edges of tectonic plates until recently.The thermal efficiency of geothermal electric plants is low because geothermal fluids are at a low temperature compared to steam from boilers. By the laws of thermodynamics this low temperature limits the efficiency of heat engines in extracting useful energy during the generation of electricity. Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses, timber mills, and district heating. The efficiency of the system does not impact operational costs as it would for a coal or other fossil fuel plant, but it does factor into the viability of the plant. In order to produce more energy than the pumps consume, electricity generation requires high temperature geothermal fields and specialized heat cycles:

  • Dry steam plants are the simplest and oldest design. They directly use geothermal steam of 150°C or more to turn turbines.
  • Flash steam plants pull 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. This is the most common type of plant in operation today.
  • Binary cycle power plants are the most recent development, and can accept fluid temperatures as low as 74°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 the most common type of geothermal electricity plant being built today. Both Organic Rankine and Kalina cycles are used. The thermal efficiency is typically about 10%. Geothermal electric plants have until recently 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.

Direct Application

Approximately seventy countries made direct use of a total of 270 PJ of geothermal heating in 2004. More than half of this energy was used for space heating, and a third was used for heated pools. The remainder was used for industrial and agricultural applications. The global installed capacity was 28 GW, but capacity factors tend to be low (around 20%) since the heat is mostly needed in the winter. The above figures include 88 PJ of space heating extracted by an estimated million geothermal heat pumps with a total capacity of 15 GW. Global geothermal heat pump capacity is growing by 10% annually.

Direct application of geothermal heat for space heating is far more efficient than electricity generation and has less demanding temperature requirements. It may come from waste heat supplied by co-generation from a geothermal electrical plant or from smaller wells or heat exchangers buried in the shallow ground. As a result it is viable over a much greater geographical range than electricity generation. Where natural hot springs are available, the water may be piped directly into radiators. If the shallow ground is hot but dry, earth tubes or downhole heat exchangers may be used without a heat pump. But even in areas where the shallow ground is too cold to provide comfort directly, it is still warmer than the winter air. Seasonal variations in ground temperature diminish and disappear completely below 10m of depth. That heat can be extracted with a geothermal heat pump more efficiently than it can be generated by conventional furnaces. Geothermal heat pumps can be used essentially anywhere.

There are a wide variety of applications for cheap geothermal heat. The cities of Reykjavìk and Akureyri pipe hot water from geothermal plants under roads and pavements to melt snow. District heating applications use networks of piped hot water to heat buildings in whole communities. Geothermal desalination has been demonstrated.

 

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