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 and a ground source heat pump follows this basic principle by circulating a cold fluid around the ground arrays, which absorbs 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.
Every home has a heat pump in the form of a refrigerator which acts just like a ground source heat pump but operates in reverse by moving heat from the inside of the fridge to the exterior.
Ground source heat pumps compared to 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.
Ground source heat pumps compared to air source heat pumps
Ground source heat pumps extract heat from the ground or water, as opposed to the air in the case of air source heat pumps. 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 in 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 usually less efficient in colder weather when you need heating most. Air source heat pumps are significantly lower cost, however, and are usually better suited to provision of heating in single dwellings. Air source heat pumps will normally be noisier as they incorporate a fan. They also have a somewhat higher maintenance requirement. Ground source heat pumps will normally be used in situations where many dwellings are to be heated.
How efficient are ground source heat pumps?
Ground source heat pumps extract heat energy from the ground which is free. The only energy required is for the compressor in the heat pump and the recirculation pumps. Ground source heat pumps typically deliver three times more energy than they consume, significantly reducing running costs – for every 1kWh of energy the heat pump uses to power itself, you will get typically 3kWh of heat back
1kW of electricity = 3kW of heat
This means heating costs are effectively reduced by two thirds when compared to a simple electric heater.
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 (for 20 years if you have a commercial property or two or more properties sharing the same ground array). (note that the incentive finishes in 2021).
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 does a heat pump work?
- A cold water and anti-freeze mix is pumped through the ground inside 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 then 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.
- The only energy used is the energy to drive the pumps and the compressor.
- There are two key types of system: 1. Those with one large single heat pump which then delivers the hot water to the individual properties in a district heating type of arrangement 2. Those where each individual property has its own mini heat pump.
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 do. This will therefore make it even more efficient, and possibly even reduce the size of pump required and lower how much the heat pump costs. If the flow temperature is lower, the system will produce less heat for heating, so it is important that the building is insulated as well as possible. The better insulated the property, the less heat is needed to heat it and therefore a lower flow temperature can be used. This is worthwhile as the cost savings that are achieved for the heat pump system are greater than the cost of the added insulation, and on-going savings in electricity will be achieved.
The Non-Domestic Renewable Heat Incentive
The Non-Domestic Renewable Heat Incentive (RHI) is a government environmental programme that provides financial incentives to increase the uptake of renewable heat by businesses, the public sector and non-profit organisations. The installation must comply with certain requirements It is applicable to ground source heat pumps.
Eligible installations receive quarterly payments over 20 years based on the amount of heat generated. The scheme covers England, Scotland, and Wales. Further information, including the tariffs for the RHI can be found on OFGEM’s website.
Council saves £230,000 in running costs using heat pumps in council buildings