What is Geothermal Energy?

“Potentially the largest – and presently most misunderstood – source of energy in the world”

geothermal world cross section earth core

Geothermal Energy means harnessing the natural heat within the Earth’s crust to generate power or for heating and cooling homes, swimming pools, greenhouses and any number of industrial processes.

Conventionally, geothermal developments have been associated with tectonically active regions where the Earth’s crust is thin and surface heat flow is high. This provides high temperatures at depths of only 1-3km. However, as drilling and power plant technology becomes more specialised and efficient, new geothermal prospects at greater depths or lower temperatures are opening up for commercial development.


Worldwide surface heat flow, Limberger et al (2018)
Worldwide surface heat flow, Limberger et al (2018)

What are the Benefits of Geothermal?

Geothermal offers a low carbon, sustainable energy source, generating less than 3% of the CO2 per kilowatt hour compared to the UK National Grid.

Geothermal Carbon Comparison

This equates to a saving of more than 713,000 tonnes of Carbon Dioxide equivalent per 5MW power project!

Geothermal carbon footprint

It also has the smallest surface footprint of any land-based energy source and generates minimal waste products.

AND it provides 24/7 baseload power, so it’s not dependent on the weather or the time of day!

Any time of the day clock

What can we do with Geothermal Energy?

Geothermal energy can be used for any process which requires heating; even cooling

Different processes are most efficient at different temperatures, so the ideal use of a particular geothermal reservoir can be selected dependent on the temperature of the resource. Not only this, but a single resource can actually be utilised for multiple purposes as each process uses only some of the thermal energy available in the water. This means a single 200oC resource may generate electricity, then dry timber, then provide heating to a village and still have enough heat leftover to provide warm water to a fish farm!

Below are just some of the possible uses for geothermal energy.

Multiple applications of geothermal energy

How can we Harness Geothermal Energy?

Different types of geothermal energy can be harnessed in different ways, but in general it will require drilling into the Earth to reach natural reservoirs of hot water.

To the right is a schematic of a geothermal doublet into a natural fault zone in Cornwall. A doublet is made up of two wells: one production well and one injection well. To harness the geothermal energy, the hot water at depth must be pumped up the production well, utilised at the surface and then reinjected into the reservoir through the injection well. This ensures the hot reservoir is recharged over time.

The exact design of a geothermal doublet will depend on the geological environment. In Cornwall, GEL have designed a system with a shallower injection well to allow the reinjected fluid to percolate down through the rock over time and heat back up before being pumped back to the surface through the production well.

Fault zone cross section

If you’d like to learn about using the hot water at the surface to generate electricity, you can visit our Geothermal Power Plants page.