The earth’s crust contains a virtually limitless amount of thermal energy. The temperature of the rocks under our feet increases with depth at an average rate of about 25ºC per km, which means that very useful temperatures can be reached at quite shallow depths.
In volcanic areas and at some tectonic plate boundaries the temperature increases much more quickly and where there is also suitable geology, underground reservoirs of steam or hot water can develop. These can be tapped by drilling to bring the energy to surface where it can be used for electricity generation. Where the temperatures are lower, the produced water can be used for a variety of direct heating applications.
Geothermal energy has been commercially harnessed in this way for decades and today there are power or heat installations in 90 countries.
The UK is not generally regarded as a geothermal country but it does have geothermal resources in the form of sedimentary aquifers and high heat production granites, as well as low temperature resources at shallow depths that can be harnessed using ground source heat pumps.
Even these shallow resources can offer significant carbon savings compared to fossil fuel alternatives. For example in a 50kW GSHP installation, CO2 savings of several hundred tonnes could easily be achieved over the system’s lifetime at a cost of less than £400 per tonne. If a warm water feed is available from boreholes or flooded mines, the carbon savings would increase by a third and the cost per tonne of CO2 saved would be approximately halved.