11.9 Geothermal Energy
11.9 Geothermal Energy
Geothermal energy refers to heat energy stored in the ground. Heat is supplied to the ground from two sources, namely the hot core of the planet and the sun. It can be classified as either ‘deep’ or ‘shallow’ depending on the depths involved. For deep geothermal energy extraction, developments drill into the earth’s crust to reach ‘hot rocks’, such as granite whose radioactive elements generate heat at great depth. The second source of heat in the ground is from radiation from the sun. Solar thermal radiation is absorbed by the surface of the earth each day. This energy can be regarded as stored energy which stays relatively warm throughout the year. This type of solar energy is extracted from the first few metres of the earth’s crust and can be harvested using heat pumps.
Heat pump systems extract the heat stored in the ground (ground source heat pumps), bodies of water (water source heat pumps) or air (air source heat pumps). This heat can then be used to heat the spaces in buildings, heat water or enable a building to be cooled. Through compression, heat pumps can ‘pump up’ heat at low temperature and release it at a higher temperature so that it may be used again. A heat pump looks similar and can perform the same functions as a conventional gas or oil boiler, i.e. space heating and sanitary hot water production. For every unit of electricity used to operate the heat pump, up to four units of heat are generated. Therefore, for every unit of electricity used to pump the heat, 3-4 units of heat are produced. The collector can be “closed loop” where the same fluid (usually water and antifreeze) always flows through the collector pipes or “open loop” where new water (e.g. from a well) flows through the heat pump.
Ground source heat pumps (GSHP) operate on the fact that the ground maintains a constant temperature between 11oC and 13oC, several metres below the surface. Ground source heat pumps use a 'closed loop' system of water/anti-freeze to collect the soil heat. In general terms the ground area required for the collector is approximately equal to that of the foot-print of the house or building to be heated.
Air source heat pumps (ASHP) use the surrounding air as a heat source to heat a building. Air source heat pumps can be located in the roof space or on the side of the building. They are similar in appearance to air conditioning boxes. Water source heat pumps (WSHP) extract heat from large bodies of water or rivers. These are generally 'open loop' collectors, i.e. the water is passed through and discarded, unlike the 'closed loop’ systems.