Geothermal energy — what is it?

Simplified diagram of geothermal power plant

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.

The BGS Geothermal map of the UK shows the geothermal potential in the United Kingdom and a new collaboration — BritGeothermal — will help develop a greater understanding of UK geothermal resources and research their exploitation so that geothermal energy can become part of the energy mix.

Deep geothermal energy in operation

Locations of existing geothermal 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.

Shallow geothermal energy in the UK

Although the UK is not actively volcanic, there is still a substantial resource of geothermal energy at shallow depths but it is exploited in different ways. The upper 10–15 m of the ground is heated by solar radiation and acts a heat store.

This heat can be utilised by ground source heat pumps that can substantially reduce heating bills and reduce the associated carbon footprint. The heat from the sun is conducted downwards into the ground.

At a depth of about 15 metres, ground temperatures are not influenced by seasonal air temperature changes and tend to remain stable all year around at about the mean annual air temperature (9–13°C in the UK). Hence, the ground at this depth is cooler than the air in summer and warmer than the air in winter.

This temperature difference is exploited by ground source heat pumps that are used for heating and/or cooling of homes and office buildings. There are different types of systems which can be broadly grouped into closed-loop systems and open-loop systems.

Various geological factors have to be considered when selecting and designing any ground source heat pump system — see BGS guidance: Initial geological considerations before installing ground source heat pump.

Open-loop ground source heat pumps

Open-loop systems, for example, exchange heat with subsurface water (groundwater) and require the presence of water-bearing rocks (aquifer) within a suitable distance from the surface.

The BGS (in collaboration with the Environment Agency) have mapped where these conditions are fulfilled in England and Wales and have produced Open-loop GSHP screening tools.

Closed-loop ground source heat pumps

Closed-loop systems extract the heat from the ground via heat exchangers installed in boreholes (vertical systems) or shallow trenches within unconsolidated ground (horizontal systems).

The BGS collaborates with European partners to map this very shallow geothermal heat potential in the UK and Europe (See Thermomap project).

Mine water systems

Mine water systems make use of the enhanced permeability in previously mined areas where rock and minerals/coal were removed, creating artificial void space. Collapsed shafts, remnant roadways and subsidence fractures provide storage and pathways for the flow of underground water.

The heat energy contained within these waters (which may be enhanced in deep mining systems) can be extracted using ground source heat pumps.

The BGS are investigating the potential of these resources and how the heat can be utilised by local communities such as the heat energy from mine waters beneath Glasgow.

Deeper geothermal energy in the UK

With increasing depth, the ground temperatures are also affected by the heat conducted upwards from the Earth's core and mantle, known as the geothermal heat flow.

When combined with the thermal conductivities of the rocks this allows the prediction of subsurface 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 contain free flowing water (groundwater) and so at depth this water will be warm and can be extracted for use in district heating schemes or for industrial uses such as heating green houses.

'Hot rocks'

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:
Heat flow map of the UK
  1. A borehole is drilled into the fractured rock to a depth where high temperatures will be found (~150–200°C).
  2. 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.
  3. 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

Contact

For further information please contact Dr Jonathan Busby.