Geothermal energy is the energy stored in the form of heat beneath the earth's surface. Geothermal energy is a carbon free, renewable, sustainable form of energy that provides a continuous, uninterrupted supply of heat that can be used to heat homes and office buildings and to generate electricity.
Our planet is a huge source of energy. In fact 99.9 per cent of the planet is at a temperature greater than 100°C; so geothermal energy is a significant renewable resource.
Geothermal energy has been used to provide heat for as long as people have been around to take advantage of it. For example, in some places the natural groundwater, heated by this geothermal energy, finds its way to surface and emerges in hot springs or steam geysers, which have been used by humans for bathing and agriculture since pre-history.
Geothermal energy in operation
Geothermal energy plants are normally located in regions where there is volcanic activity, such as in Iceland and New Zealand.
The first electricity to be generated from geothermal was at Larderello in northern Italy in 1904. There are now geothermal energy plants in 24 countries throughout the world and there are deep geothermal energy systems currently being developed and tested in France, Australia, Japan, Germany, the USA and Switzerland as well as the UK.
In Iceland, which has abundant geothermal energy resources, geothermal energy is used to provide the majority of the electricity and heating demands of the country.
Many other countries also obtain significant amounts (> 10 per cent) of their electricity from geothermal sources including El Salvador, Kenya, the Philippines, Costa Rica and New Zealand.
Geothermal energy in the UK
Although the UK is not volcanic, geothermal energy is still a substantial resource but it is exploited in different ways. The upper 10–15 m of the ground is heated by solar radiation and acts a thermal heat store. This heat can be utilised by ground source heat pumps that can substantially reduce heating bills and reduce the associated carbon footprint. Geological factors have to be considered when designing the ground collector loop of a ground source heat pump (GSHP) system; see BGS guidance Initial geological considerations before installing ground source heat pump.
Below 15 metres temperatures are also affected by the heat conducted upwards from the crust, known as the heat flow. When combined with the thermal conductivities of the rocks this allows the prediction of sub-surface temperatures.The UK's geothermal gradient, the rate at which the Earth's temperature increases with depth, has an average value of 26°C per km. Some rocks have free flowing water and so at depth this water will be hot and can be extracted for use in district heating schemes or for industrial uses such as heating green houses. The BGS are investigating the potential of these resources and how the heat can be utilised by local communities.
There are also regions in the UK where the rocks at depth are hotter than expected. This occurs in granitic areas because some granite generates internal heat through the radioactive decay of the naturally occurring elements potassium, uranium and thorium. Granites have very little free flowing water, but it is possible to engineer the fracture system such that water can be made to flow from one borehole to another through the granite. The extracted hot water is at a sufficiently high temperature to drive an electricity generating turbine. Parts of Cornwall have geothermal gradients that are significantly higher than the UK average due to the presence of granite and have potential for geothermal power generation. These systems are known as engineered geothermal systems (EGS) and are described below.
Engineered geothermal systems (EGS)
Exploiting the geothermal potential of rocks with poor natural permeability involves enhancing or engineering the permeability of the hot rocks at depth.
An EGS typically involves the following processes:
- A borehole is drilled into the fractured rock to a depth where high temperatures will be found (~150–200°C).
- Water is then injected at sufficient pressure to ensure fracturing, or to open existing fractures within the developing reservoir and hot basement rock. This hydro fracturing generates shear along existing fractures. When the pressure is removed the fracture closes but due to shearing the two surfaces are now slightly offset and so the fracture is propped open enhancing the permeability.
- The developing fractured reservoir is monitored with micro seismicity. A second borehole is then drilled into the reservoir so that cold water pumped down the original borehole is heated by heat exchange in the reservoir and is abstracted to the surface where it is used for electricity generation and district heating schemes.
Factors that are required for a successful EGS include:
- a large interconnected fracture network that will enhance heat exchange
- no preferential pathways, such as fracture zones in the reservoir, that take most of the water and lead to poor heat exchange (known as short circuits)
- low water losses so that most of the pumped water is returned via the 2nd borehole and is not lost to the natural fracture network
- low impedance so that resistance to flow is not too high and mitigates the need for high injection water pressures
Related research
Contact
For further information please contact Dr Jonathan Busby.
