Gigantic amounts of heat energy exist at depth in the Earth’s crust, caused by volcanic activity or the slow decay of radioactive elements. The ease with which this energy can be accessed varies hugely from place to place. In some locations (e.g. Iceland, New Zealand, California) high temperatures are found at shallow depths or even at the surface. Here electricity generation is often technically unchallenging and can be very cheap. Direct exploitation of heat production is also possible.
- First geothermal power plant – Larderello, Italy, 1911 (still producing)
- Earliest geothermal heat network in US – Boise, Idaho, 1892 (still working).
- The 'Geysers' geothermal plant in California provides power for > 1 million people.
- 87% of space and water heating in Iceland comes from geothermal sources
Where high temperatures are found at greater depths - e.g. the UK - it is still possible to exploit them for power and/or heat, though it is more complicated, riskier (cost of failed boreholes), and therefore more expensive.
The basic idea is simple: cold water is pumped down one borehole, heats up as it moves through hot rocks, and returns to the surface as steam to run a turbine. The main challenge for a plant of this kind is establishing a flow of water at depth. This can be aided by fracturing rocks with high pressure water (known as frac’ing). This technology is often termed "enhanced geothermal systems" (EGS).
A few small demonstrator deep geothermal power plants already exist (Soultz-sous-Forets in France, Landau in Germany). These are a few MW in size.
Heat-only plants are shallower, cheaper, less risky, and require lower temperatures. Where hot aquifers exist (e.g. Paris basin, Wessex basin) exploitation is easier.
Deep geothermal energy generation is low carbon in nature. ‘Heat-only’ geothermal plants have an extremely low carbon intensity: less than 1 KgCO2/MWhth, while closed loop plants – like the ones proposed for construction in Cornwall - are expected to have almost zero direct carbon emissions.
Though there will be embodied emissions in the plant fabric, deep geothermal energy is therefore a very low carbon energy source.