Geothermal energy (in Greek geo means Earth and thermal means heat) is an underground heat of the Earth used for heating purposes or electricity source. The internal heat occurs in the form of steam and hot water or hot rock and molten rock, called magma.
The ground of the upper 3 meters of the Earth's surface has constant temperature between 10° and 16°C. Few miles down, the temperature can be as much as 250 °C. In general, the temperature rises one degree Celsius for every 30 - 50 meters down, but this varies in different locations. Hot dry rock resources occur at depths of 5 to 8 kilometers almost everywhere beneath the Earth's surface. Down deeper, magma reaches extremely high temperatures. The centre of the Earth, over 6500 kilometers below, is about as hot as the surface of the sun, around 6000°C.
All this energy can be used in two ways: directly and indirectly. Hot water near the surface of Earth can be used directly to heat buildings and for hot water systems. Indirectly, it has vast potential to generate large amounts of electricity.
Humans have used geothermal sources since Paleolithic times, using hot springs, for cooking and bathing. Romans were building reservoirs around hot springs to create bathing complexes and to heat the rooms. In Chaudes-Aiges, France the world's oldest geothermal district heating system has been operating since the 14th century. The idea of using geothermal heat pump to capture heat from the ground was patented in Switzerland in 1912.Attempts to generate electricity from this energy were made in the early the 20th century. The first geothermal electricity generator was tested and built in Ladarello in Italy in 1904. Later, in 1911, the world's first commercial geothermal power plant was built there. New Zealand build the next one, in 1958. This technology was widely adapted in Sweden in 1970s, and since then, it has been growing with worldwide interests.
Geothermal power has been limited previously to areas bordering with tectonic plates. However many new technologies have been developed and they expanded the range and size of viable applications, especially for home heating systems. As mentioned before geothermal power can be used in two different ways - to produce heat or to produce electricity and there are three main areas where this energy can be used effectively: Geothermal Direct Use, producing heat directly from hot water within the earth; Geothermal Heat Pumps, using the shallow ground to heat and cool buildings; and Geothermal Electricity Production, generating electricity from the earth's heat. (source: http://www.bp.com/)
In geothermal direct use, naturally heated water can be piped directly into radiators where hot springs are available. If the ground is hot but dry, earth tubes can be used to deliver the heat, without a heat pump. Water is usually put through one end of a lengthy piping system, and by the time it reaches the other end the water has significantly higher temperature than at the start. At present, geothermal wells are rarely more than 3 km deep. Over 70 countries use this kind of geothermal power.
Geothermal heat pumps are more effective - they can be used essentially everywhere. Even in snowy regions the results are efficient. The shallow ground is still warmer in the winter than the ground above, so the water absorbs heat from the underground, which is then transferred to the buildings. The heat can be extracted with a geothermal heat pump and provide a house with a source of low cost hot water and heating system. Such system consists of heat pump, air delivery pipes, and pipes buried in the shallow ground near the house. The initial cost are high, yet the investment will pay off in a few years. The area to lay the piping system has to be quite big, so suitable land access is needed.In addition to that, geothermal heat pumps can tap into this resource not only to heat but also to cool buildings in the summer time. The pump can suck the unnecessary heat from the house back down.
Direct application for heating is far more efficient than electricity generation and has less demanding temperature requirements. It is very economic as it covers much greater geographical range. Global geothermal heat pump capacity grows by 10% annually.
Electricity harnessed from the geothermal power requires much higher temperatures, coming from deep underground. Holes and wells need to be drilled down to the hot areas. Away from tectonic plate boundaries, wells have to be several kilometers deep to permit electricity generation. The coming up heat is used to drive turbines and electric generators. Some geothermal power plants, dependent on location, use the natural steam to power a turbine, while others use hot water to boil a liquid, such as oil, which vaporizes and then turns a turbine and generator.
The production of electricity from geothermal energy sources is estimated to be highly powerful and effective. Large geothermal power station can deliver energy to entire cities, yet location is the key. Most geothermal reservoirs are without outside visible clues as to their existence, unfortunately. As the result, in 2007 less than 1% of the worlds electricity supply came from geothermal resources.
Several countries located on the edges of tectonic plates (Iceland, and Costa Rica, El Salvador, the Philippines) generate around 20% of their electricity from this source. Worldwide output is growing by around 3% per year (in 2008 over 4%) and is generated in 24 countries around the world including: the USA, Mexico, El Salvador, Nicaragua, Costa Rica, New Zealand, the Philippines, Indonesia, Japan, China, Russia, Lithuania, Turkey, Germany, The Netherlands, Italy, France, Iceland and Kenya.
There are three main designs for power stations that can convert geothermal energy to electricity: "dry steam", "flash steam", and "binary-cycle". The improvement of the last one may be promising to use of the geothermal energy on much greater scale. Geothermal power stations need no fossil fuel to run and may take up considerably less land compared to fossil-fuel power stations. The construction work can affect the land stability, though, and can trigger small scale earthquakes. For many, the concerns are the possible emissions of hazardous gases that may come up from underground, difficult to be disposed safely.
The electricity in geothermal power stations is produced up to 90% of the time compared to fossil-fuelled power plants producing electricity 65-75% of the time, 20% for solar and 25% for wind energy generation.
The cost of electricity from geothermal power plants is becoming competitive with that from traditional power plants. With technology improvements, the growing potential to extract 2,000 zetajoules (ZJ) will be sufficient to provide all the world's energy consumpiton for several thousand years, according to a report from the Massachusetts Institute of Technology.
It is very reliable, cost effective, and suistainable source of energy, according to experts, but initial capital costs, especially for drilling, tend to be high. The exploration of the resources inside the Earth entails some risks which has to be professionlly assessed, as well.
Pictures:
geysers - www.inhabitat.com
power plant - image credit: Gretar Ívarsson - The Nesjavellir Geothermal Power Plant in Iceland
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