Ground source heat pump

How can we get free heating for our homes and other buildings? What do a fridge and a warm, comfortable building have in common?

To heat a building and make it suitable for people to live or work in usually means spending money on fuel which, depending on the fuel, emits carbon into the environment and contributes to climate change. But what if we could get free heating by looking underground?

Illustration of house in winter and summer with arrows underground. Text reads deeper than six meters under the ground. There’s a stable temperature all year round.

As you go underground and into the Earth, at around six metres or so, the ground is a stable temperature of about 12⁰C all year round. Whilst this doesn’t sound very warm, we can use technology called a ground source heat pump to make use of the temperature of the soil to keep the inside of buildings at a comfortable temperature.

What is a ground source heat pump?

A ground source heat pump is much like a refrigerator. In a fridge, fluid passing through pipes that run both in and outside the fridge is compressed using an electric motor and pumped around so that it changes from liquid to gas and back again. This process keeps the pipes outside the fridge hot and inside cold; cooling the inside and warming the outside.

In summer, a heat pump works in the same way. Pipes running inside the building cool it, and pipes running outside the building warm the ground. In winter, the heat pump works in reverse. The ground is cooled and the inside of the building is warmed.

Illustration of 2 buildings. Text reads Winter, building warmed, 72m, ground cooled. Summer, building cooled, ground warmed.

Sustainable energy from underground

In London, the temperature of the clay and soil a few metres below ground remains steady at 12⁰C all year round, while above ground the temperature may vary from below 0⁰C in winter to above 30⁰C in summer. The constant temperature below ground allows a ground source heat pump to work predictably and effectively all year round.

Although heat pumps need electricity to operate, tapping into the energy stored underground allows them to be more efficient and environmentally friendly than more traditional heating and cooling systems. In winter, a heat pump can warm a building as effectively as an electric heater, but only requires a quarter of the power.

In summer, on a long sunny day, it is possible to operate the heat pump using only electricity generated by the solar panels on the roof, which reduces the building's overall carbon dioxide emissions by reducing reliance on electricity generated by power stations.

Find out more

Advice for homeowners on ground source heat pumps from the Energy Saving Trust.
Discover more about the physics behind the IOP’s King’s Cross home in a feature from Physics Review (PDF, 1MB). (Originally published November 2019 and reproduced with kind permission of Hodder Education)