map of UK showing geothermal energy
Source | British Geological Survey (NERC)

An exciting new project has been launched to extract geothermal energy from beneath the rocks in Cornwall. The project has secured significant funding through ERDF, the local council and private investors. The objective of the project at this stage is start drilling the wells for the installation of a commercial scale geothermal power plant.

Geothermal power

Simplifying somewhat, a geothermal power plant works much like a coal or gas fired power plant, except in this case the heat is not generated by burning fossil fuels, but obtained from within the Earth. This heat vaporises water into steam, which is used to drive a steam turbine. Sounds pretty easy right? Well, although it does take advantage of many well established and reliable technologies, it's the task of getting to that heat in the Earth's crust that's the challenge. In the proposed Cornish site it is estimated that two wells will be required; one at 2500m depth and the other at 4500m depth. For most of their depth they will be steel lined too, so this is quite a significant construction challenge. There are also potential challenges with the permeability of the rocks and whether enough water can be heated to produce the desired power. However, once you get to the required depth you have a massive, non-polluting heat source to tap into.

Cornwall hasn't been chosen on a whim, you can see from the figure above that there is significant thermal resource located in the south-western corner of the UK. This is not to suggest that Cornwall is sat on top of a volcano, it is not (don't panic); it's to to with the geology. At the centre of the Earth is a molten core that is emitting huge amounts of heat energy (estimated at 44.2 TW). Through the principle of heat conduction, this heat is transferred to ward the surface of the surface of the Earth. Due to this immense heat, most of the Earth is molten and only the top ~18 miles of crust is solid. The heat can be felt as soon as you start to dig to significant depths; for every kilometre the temperature rise is ~25°C on average. The large quantities of granite found in Cornwall conduct the heat from the Earth's core much more effectively than other rocks, so the temperature even relatively near the surface can be very high.

The abundance of granite and therefore high temperatures relatively near the surface makes Cornwall the ideal location of geothermal power plants. Looking at the resource map the area has the potential to support multiple commercial scale projects. For more information on the current proposed project see the United Downs Deep Geothermal Power project.