What is a heat pump?

How does a ground source heat pump work? Learn the basic principles of ground source heat pumps and why they are recognised as the most efficient and lowest carbon heating technology.

Ground source heat pump diagram

How does a ground source heat pump work? Illustration

What is a ground source heat pump?

A ground source heat pump extracts solar energy stored in the ground or water via submerged or buried pipework (ground arrays) and converts this to a higher temperature to meet 100% of a building’s heating and hot water needs, all year round.

Heat naturally flows from warmer to cooler places, a ground source heat pump follows this basic principle by circulating a cold fluid around the ground arrays, which absorb and attract the ground’s low-grade heat energy.

The ground source heat pump then compresses and condenses this heat energy and transfers it to the property’s heating and hot water system. Having surrendered the heat energy from the ground to the heat pump, the fluid from the ground arrays continues its circuit back to commence the cycle all over again.

We all have heat pumps in our home already – standard refrigerators act like heat pumps in reverse by moving heat out of a fridge.

 

What is a ground source heat pump? It is not geothermal heating!

Geothermal heat comes from the earth’s core. In the UK you have to go down 500m – 2,500m before there is any appreciable input from the earth’s core. This form of heat tends to be used directly by very large-scale applications. Ground source heat pumps tend to go to 200m (if using borehole ground arrays – vertically drilled pipework), or ‘soil‘, coiled pipework buried in trenches to a depth of 1m – 2m.

What is a ground source heat pump? It is not an air source heat pump, either!

Ground source heat pumps extract heat from the ground or water, as opposed to the air; the temperature in the ground remains a stable 8-10C all year round, making ground source heat pumps an extremely reliable and efficient heat source, especially at winter when the air temperature is colder than the ground temperature (and when you need heat most!); this is why air source heat pumps are less efficient when you need your heating most.

How efficient is a ground source heat pump?

By using freely available heat energy from the ground, the electrically powered ground source heat pump delivers three times more energy than it consumes, significantly reducing running costs – for every 1kWh of energy your heat pump uses to power itself, you will get typically 3kWh of heat.

1kW of electricity = 3kW of heat

This means heating costs are effectively reduced by two thirds.

Better still, to offset the additional cost of installing a ground source heat pump over a fossil fuelled system, the Government’s Renewable Heat Incentive pays a quarterly income for ground source heat pump installations (or 20 years if you have a commercial property or two or more properties sharing the same ground array).

The bigger picture?

Due to the high efficiency of a heat pump, CO2 emissions are significantly lower than traditional fossil fuelled systems (up to 43% lower than gas).

Ground source heat pumps emit no point of use carbon emissions or air pollution, helping to significantly improve air quality and aid the UK’s ambition to achieve net zero carbon emissions by 2050.

With gas being ruled out for new builds from 2025, ground source heat pumps offer a low-carbon heating solution to rival and replace the gas network and aid the UK’s transition to the mass electrification and decarbonisation of heat.

How efficient is a ground source heat pump? Illustration

How does a heat pump work?

This short video by Kensa Heat Pumps explains what a heat pump is and how a ground source heat pump works.

For a more detailed technical explanation an extended version is available here, with commentary from Kensa’s Technical Director, and co-founder, Guy Cashmore.

Heat pump process

  • A cold water anti-freeze mix is pumped through the ground within a series of energy absorbing pipes, known as ground arrays. As heat naturally flows from warmer to cooler places, the anti-freeze mix circulating around the array is constantly warmed by the ground’s low grade heat.
  • Having increased in temperature, the anti-freeze mixture is fed into a heat exchanger called the evaporator.
  • Within the secondary sealed side of the evaporator heat exchanger is a refrigerant which acts as a heat transfer fluid. When the water anti-freeze mixture enters the evaporator, the energy absorbed from the ground is transferred into the refrigerant which begins to boil and turn into a gas. The refrigerant never physically mixes with the water anti-freeze mixture. They are separated like sandwich layers by the plates of the heat exchanger which permit the heat transfer.
  • This gas is then fed into a compressor. The pressure of the refrigerant gas is increased in the compressor, which makes the gas temperature rise. The hot refrigerant gas then flows into a second heat exchanger, called the condenser, which features an identical set of heat transfer plates.
  • The condenser delivers water hot enough to serve the space heating system and, if required, the property’s hot water needs.  Having transferred its heat, the refrigerant gas reverts to a liquid.
  • This liquid is then passed through an expansion valve at the end of the cycle to reduce its pressure and temperature, ready to commence the cycle all over again.
Watch the video

Improving heat pump efficiency

In order to improve heat pump efficiency it is worth considering that the lower the flow temperature required in your heating distribution system, the less work your ground source heat pump will need to make. This will therefore make it even more efficient, and possibly even reduce the size and of pump required and lower how much a heat pump costs.

To achieve as low flow temperatures as possible, insulation is the first place to look.

better insulation = lower flow temperature = less work & more efficient heat pump

I think ground source heat pumps are the future of energy generation and everyone should embrace this technology.

Alistair Mackintosh, Selfbuilder