Geology And Geophysics

Types of Geothermal Energy Systems



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Geothermal energy is a term used to describe the extraction of thermal energy from the ground by heating water. This heat energy can then be used for a number of applications, usually either heating a building or generating electricity. However, the source of the geothermal energy is different depending on the depth used.

It is becoming more popular these days for new homes to use geothermal energy as part of a heating system. This usually comprises of a long length of pipe, buried within the grounds of the house, to a depth of a few feet. Water is then pumped through the pipe, where it is heated, until it returns to the heating system and a heat exchanger, where the energy is used. Despite common belief, this type of system does not utilise the heat from deep within the earth, instead it is using solar energy.

The heat from deep within the earth is constantly being conducted upwards to the surface. However, these surface layers are very good heat insulators. The resulting energy flow is not enough to maintain the ground temperature when heat is taken by the circulating water. However, the energy from the sun is far greater (here in the UK it's about the same as a 1 kw electric fire for every square metre - on a sunny day). Through out a warm summer, when heating is not required, the ground captures the solar heat and stores it. This heat reservoir is then used during winters. It is therefore quite possible for such a system to fail if there are repeated cold or cloudy summers.

The other type of geothermal energy does indeed take heat from the earth itself. These systems generally fall into 2 main types - hot dry rock, and hot wet rock.

Hot dry rock systems rely on drilling at least 2 deep boreholes into hot bedrock. These boreholes may be several miles deep, often using the same drilling rigs as used in the oil industry. Water is then pumped down one and retrieved by the other. There have been several experimental sites developing this type of technology over the years. Perhaps the most extensive was the Geothermal Energy Project conducted by the Camborne School of Mine in Cornwall, UK during the 1980s. However, these projects have always encountered problems, most notably the cost of borehole drilling, and that a site will 'cool' down over a period of 30 years or so. None are economically viable.

Hot wet rock is used extensively in volcanic regions such as Iceland. Here the surface rock temperatures are extremely high, and it is a comparatively simple task to use superheated ground water to generate electricity and for heating buildings. If ground water is not sufficient, the water can be pumped down boreholes. Whilst extremely efficient, most countries cannot utilise such systems.

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