Question: what is geothermal power?

Hazel Gibson answered on 5 Mar 2018:

I’m glad you asked! Geothermal power is when we use the natural heat of the Earth to get energy. It is a renewable source of energy that uses water heated up by the rocks to make power and, because of this, lots of countries are trying to use it to make electricity to reduce how much we depend on coal, oil and gas. Basically it can be broken up into three types.

The first kind is the kind that most people think of when we say geothermal – that is places like Iceland, which is a volcanic country, with lots of water already in the rocks. The volcanic rocks heat the water, which is pushed up to the surface because of natural pressure and that steam drives a turbine which makes electricity. Think of the geysers, if you could put a watermill above that, the steam and hot water being pushed up from underground would drive the mill, which would drive a turbine and make electricity. These are also places with a lot of heat, so that heat can be used to heat people’s houses. This kind of geothermal is called ‘high enthalpy’ which basically means high temperature. We don’t have any of this kind of geothermal in the UK, because we don’t have any volcanoes.

The second kind of geothermal is one that we have a lot of in the UK: shallow geothermal, captured using Ground Source Heat Pumps (GSHPs). These are only used to heat people’s houses – not make electricity as they use the lower temperatures only found in the soils near the surface. This is not traditional geothermal because it actually uses the heat from the sun stored in the soil (think of the way that rocks, or pavements feel warm in the evening after a really hot sunny day). These are great because you can put them in when building new buildings and they are very cheap to heat your house – no expensive gas bills here.

The third kind of geothermal is the one I am working on, Enhanced (or Engineered) Geothermal Systems (EGS). EGS uses rocks that are really hot, but don’t have any water in them or any way for water in them to escape to the surface. The rocks that this involves are rocks that are really deep (though not deep enough to be in the mantle) and in Cornwall (where my study is) that means granite. Granite is a rock that deals with high temperatures really well – which is why you can have it for your counter top; no matter how hot your pan is, the rock won’t burn – so it’s great for geothermal! But in Cornwall there is no way for water in the granite to escape, so we have to drill really deep into the granite to get at the hot water, then build the power station at the top. When the hot water has helped run the turbine and cooled back down, it is pumped back into the rock to heat up and start the process all over again. If this project works it will be the first geothermal power station in the UK making heat AND electricity.