What is geothermal energy and how does it work?

21 January 2021 | Stephen Marcus

There’s something magical about geothermal energy.  A renewable way of generating energy, it builds on an idea going back centuries – using the heat beneath the ground to keep us warm. Widely used in many parts of the world today, it could be expanded in the UK, as we look for more ways to move away from fossil fuels and carbon

But how does it actually work? In this article, we explain the different ways in which geothermal energy is generated, and weigh up its advantages and disadvantages. 

How does geothermal energy work?

The basic idea behind geothermal energy is to tap into underground reservoirs of steam and hot water. This means drilling underground wells of up to a mile deep, and then harnessing the heat to drive turbines and generate electricity. 

The first time geothermal heat was used to produce electricity was in Larderello, Italy, in 19041. But the idea of harnessing geothermal warmth goes back much further – with the Lardarello region known for its hot springs since at least the Roman times.

So where’s all this heat coming from? Well, the earth is a bit like an enormous gobstopper. It’s made up of layers that get hotter and hotter as you go down. 1,864 miles deep is the earth’s core2 – a solid 5,430°C ball (not magma as people often think). It’s as hot as the surface of the sun4!

And why is it so hot down there? Some of the heat was created more than 4 billion years ago when the earth began, and it’s still there today. And new heat is also being created by radioactive elements in the earth’s core5.

How is geothermal energy produced?

It all starts with a geothermal power plant, where the power is harnessed and converted into electricity. There are a few different ways of doing this, but each of them involves driving turbines, turning the energy from steam or vapour into electricity.

How is geothermal energy generated? 

There are 3 main types of geothermal energy plant:

  • Dry steam plants – the oldest method, they use steam drawn directly from fractures in the ground to drive a turbine.
  • Flash plants – they draw high-pressure hot water from underground, and blend it with cooler low pressure water, creating steam that’s then used to drive a turbine.
  • Binary plants – they draw hot water from underground and pass it through a secondary fluid with a lower boiling point than water. The secondary fluid is turned into vapour, which then drives a turbine. This is the method used by most new geothermal power plants.

What are the advantages and disadvantages of geothermal energy?

Advantages of geothermal energy

There are many great advantages of geothermal energy: 

  • It’s low carbon – it doesn’t burn any fossil fuels, like oil, coal or gas6
  • It’s renewable – the earth’s hot underground reservoirs are naturally replenished, so there’s no risk of it being used up as a source of energy
  • It’s steady – unlike wind or solar power, geothermal energy isn’t affected by changes in the weather, and so won’t change its output from one day to the next

Disadvantages of geothermal energy

As with anything, it’s not perfect. Here are the disadvantages:

  • It’s location-specific – only certain areas have conditions suitable for generating geothermal energy, so it’s limited to particular locations.
  • Greenhouse gas emissions – greenhouse gases are released in the process – albeit at a much lower rate than those released through burning fossil fuels. But the good news is that these emissions are avoided in newer binary power plants7.
  • Local environmental impact – the digging and extraction involved in geothermal energy can, in some cases, lead to earthquakes. But this risk can be minimised by doing surveys of the area before any work begins8.

So is geothermal energy renewable? 

Yes, it is. Unlike fossil fuels such as coal or gas, geothermal energy draws on a resource that won’t run out. This is because, like with solar or wind power, the earth’s reservoirs of hot water will naturally keep on replenishing themselves.

To learn more about renewable energy, and how it can help you cut your carbon footprint, check out some of our other guides:

Geothermal heating systems

While geothermal power stations can be used to generate energy for the grid, it’s also possible to harness the heat directly, rather than convert it to electricity.

What types of geothermal systems are there?

There are two main types of geothermal heating systems:

  • Direct-use geothermal energy – here, geothermal heat from deep underground reservoirs is pumped directly into buildings or infrastructure, such as swimming pools or greenhouses.
  • Ground source heat pumps – a method for individual households, this is like a scaled-down version of geothermal energy, digging down to 2 metres, rather than 1,000! Here, the temperature is around 11°C, and pipes are installed which circulate a mixture of water and antifreeze. They absorb heat from the ground, which then passes through the liquid, to then go into a heat exchanger in the pump – and then it warms up your home!

How much does a geothermal energy system cost?

Installing a ground source heat pump typically costs between £11,000 and £15,0009. After the initial upfront cost, it can help to cut your carbon and energy bills in the long-run.

Want to learn more? Head to our complete guide to ground source heat pumps, to find out how they work, how to get yours installed, and how they could cut your carbon even further.

Can I get help through the Domestic Renewable Heat Incentive (DHRI) scheme?

It’s likely that you could get payments from the government to help with the cost of installing a ground source heat pump. Under the DHRI, homeowners or landlords who install heat pumps can earn payments based on the amount of energy their heat pump generates. For more information, head to the DHRI site.

How and where to install a geothermal system?

If you want to install a ground source heat pump, you need to lay down pipes in an outdoor area beside your home. This means you’ll need a decent amount of space – and then you need to do a lot of digging! 

A typical heat pump needs a 200-metre area of trenches dug, with an area of 400 square metres needed in total. Usually, the trenches are dug by your home, but if you have a big area of land neighbouring your house, then it’s possible to go slightly farther afield, to seek out a patch that’s better suited.

Want to know more? Head to our complete guide to ground source heat pumps.  

How much of UK energy is geothermal?

When it comes to the national grid, the answer is – none of it! At least, for now that is. There have been several efforts to research how much energy the UK could generate from geothermal sources, with estimates ranging from 4% to 20% . 

Want to find out more about alternative energy sources? Check out some of our other guides:

Sources and references:

1  Luhr, J.F., “Earth” (Doring Kindersly)


4  Phillips, K.J.H., Guide to the Sun (Cambridge University Press), pp. 47–53