Our new energy report, Energy Independence 2025, identifies the options for increasing renewable energy generation within the Exeter region. We’ve already looked at how Solar, Wind and Hydro power is generated, in this Energy Explainers blog we will be introducing Geothermal energy.

Geothermal energy comes from heat that is made by and stored in the Earth. At its core the Earth is incredibly hot – over 4000°C! – but at the surface it is much cooler. This huge difference causes heat to travel from the Earth’s core to its surface, heat which can be captured and turned into energy.

Whether it’s by simply transferring the heat from one place to another to warm up our homes, or to use it to boil fluids and create steam which will drive turbines, geothermal is a source of renewable and sustainable energy that’s right beneath our feet – so put yours up and join us for a journey from the centre of the Earth to Exeter.


Exeter Eruptions

A common example of a geothermal energy source that everyone knows is a volcano. Water in volcanic areas is heated naturally underground, rising to the surface as hot water or steam which can be used to drive turbines and create electricity. There are no active volcanoes in the UK now, but there were 250 to 400 million years ago. Exeter even had its own, which is now the site of Rougemont Castle.




From Rome to Renewable

Much later the Romans realised the potential of geothermal energy, harnessing hot springs for their bath houses like the famous example in Bath. Water was fed into the baths from natural springs, starting out at temperatures between 69 and 96°C, so the Romans could enjoy a good soak.

Although its remains are buried beneath the Cathedral green Exeter once had its own Roman bath house. It was one of the most advanced in the country and even had underfloor heating. The springs that fed the bath house weren’t heated by geothermal energy, as there are no hot springs in Exeter, but they were heated by biomass which we’ll be talking about in our next Energy Explainers blog.




Not much has changed since Roman times and geothermal energy is still harnessed by making use of water that is heated underground. Today much more advanced methods are used to capture that energy.

For more information about Exeter’s Roman bath house, and to find out more about its Roman history, visit Exeter’s Royal Albert Memorial Museum. Entry is free and it’s open from 10am – 5pm, Tuesday to Sunday.


Exchanging Energy

Ground source heat pumps are one way of reducing our energy demand, and pump fluids around a pipe which is buried several meters underground. Heat pump systems can also transfer heat from underground and into our radiators, underfloor heating systems, and hot water. It’s also possible to get ones that exchange heat, warming up homes in the winter and taking heat away to cool them down in the summer.

While many people consider these pumps to be a source of geothermal energy, the heat they transfer is actually heat from the sun which is stored underground. So technically they are classed as solar power. To harness the full power of geothermal we need to dig a little deeper.


Heat Rocks!

There’s a huge potential for generating energy from hot rock granite sources. The technology for capturing this energy still needs to be validated, but solutions are being tested including a geothermal power plant at the Eden Project in Cornwall.

To access the heat that’s trapped in gaps between granite in the ground two holes are drilled. Each hole is very narrow, but goes several kilometers below the ground. Cold water is pumped down one hole into the gap where it’s super hot – around 180°C. The hot water is then pumped back up the other hole where it runs through a turbine to make electricity.




In the turbine the hot water from underground travels through a heat exchanger, where a fluid with a low boiling point is evaporated. This steam is then directed through the turbine to produce electricity.

Barriers exist to truly tapping into the potential of geothermal energy but if harnessed correctly could make a significant contribution to reducing our energy demand and increasing generation.


You can find out more about the Exeter region’s geothermal energy potential in Energy Independence 2025 which is available to download now. This provides a detailed analysis of how geothermal power can be generated in the region, in addition to other forms of renewable energy, and offers key recommendations for becoming energy independent.