What is geothermal energy?

Geothermal energy involves the exploitation of different kinds of thermal energy stored within the earth.

In certain places, heat from deep inside the earth can rise to the surface. This heat is called geothermal energy. When water enters fissures in this hot rock, it is heated and can emerge as hot springs, or even as steam. This creates features such as steam vents, geysers and hot mud springs. Alternatively, hot water can be trapped below the earth’s surface as a geothermal reservoir. This heat can reach temperatures of 400°C and can be accessed by drilling to depths of over two miles.

Ground-source heat is a different kind of geothermal energy. It is extracted from the low-temperature heat (10–20°C) that is found at relatively shallow depths within the earth’s crust. This heat source stays at a relatively constant temperature and can be taken from the ground itself or from groundwater. Heat pumps can raise the temperature of this heat to a more useful 40–50°C which is ideal for low-temperature heating systems like underfloor systems and radiant panels.

 

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How does it work?

Geothermal power plants extract steam, heat or hot water from geothermal reservoirs to provide a force to turn generators and produce electricity. The water can then be recycled by returning it to the reservoir for reheating.

Ground-source heat pumps are not strictly a source of renewable energy, as they need electricity to extract and use low-grade heat. However, heat pumps can be very energy efficient, producing four or five times the amount of heat energy for every unit of electrical energy needed. A heat pump takes the heat from a refrigerant fluid (or water) that is in contact with the ground, extracts the heat from this source and transfers it to a heat sink where it can then be circulated through a heating system. This process cools the refrigerant fluid but it can be re-circulated through the ground where it will absorb more heat before being passed through the heat pump again. Heat pumps can also work in reverse to provide cooling instead of heating.

 

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Are there any examples of geothermal energy in use in the UK?

Only one geothermal power plant is working in the UK, at Southampton. Ground-source heat pumps were first popular in the early 1980s, when electricity was cheaper than gas. They are now becoming increasingly popular due to their energy efficiency and the fact that gas prices are beginning to rise again relative to electricity prices. There are an increasing number of heat pumps installed throughout the UK everywhere from homes to commercial buildings and swimming pools.

Around 250 ground-source heat schemes are being installed in the UK every year. Since 1992 around 3,000 heat pumps have been installed in single family homes.

 

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What is its future potential?

It has been estimated that there are 1,550 large industrial sites in the UK where heat-pump systems could be installed, with an average size of 800 kilowatts of thermal power. There are unlikely to be more geothermal plants in the UK, since sites need hot rock near to the surface and which is sufficiently fractured to allow the (heated) water to pass. The North Pennines, parts of southern England and Derbyshire all have areas of hot rocks.

 

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What are its most likely applications?

Ground-source heat pumps are especially suitable for buildings that have high heating and cooling needs, such as hotels and hospitals. The more a heat pump system is used, the more efficient it becomes in comparison to a conventional fossil-fuel system.

Electricity production from geothermal sources can be supplied to the main electricity network.

 

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Can geothermal energy be developed in any part of the country?

Not every area is suitable for geothermal power plants. They need sites with hot rock near to the surface and which is sufficiently fractured to allow the (heated) water to pass. There are areas of hot rocks in the North Pennines, parts of southern England and Derbyshire, but the UK has a far more limited resource compared to other countries such as Iceland or New Zealand.

Heat pumps need access to enough ground surrounding the development or to suitable bodies of water and groundwater nearby. Lakes, canals and tidal mudflats are potential heat sources for the pumps. Areas with aquifers are good for large ground-source heat schemes.

 

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What is the environmental impact of geothermal energy?

Very low. Geothermal fluids can contain dissolved gases such as carbon dioxide (CO2) and hydrogen sulphide (H2S). These gases can be released when they are brought to the surface. However, geothermal plants do not produce any oxides of nitrogen (NOx) or sulphur dioxide (SO2). They also produce much less carbon dioxide (CO2) than gas, coal or oil-fired plants.

Heat-pump installations are unobtrusive and noise- and pollution-free. They do release some carbon dioxide (from the electricity used to drive the compressor) but they have lower emissions than conventional gas or oil boilers. The power needed to drive the heat pump could come from renewable technologies.

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