Ground source heat pumps extract heat from the ground and 'upgrade' it to a temperature at which it can be used for space heating of buildings or to preheat hot water. Like all other types of heat pumps, they require a certain amount of energy, usually in the form of electricity, to drive the system, and use this to tap into free renewable energy from the environment. Typically they deliver 2.5 to 4 times more energy than is supplied, depending on their coefficient of performance (COP).
Typical heat pump housing for a domestic system
In the domestic market, these systems are best suited to houses in rural locations that are off the gas grid. They are a financially and environmentally attractive alternative to oil- and solid fuel-fired heating systems, especially with new-build developments and extensive renovation projects.
Basic designs
At depths of 1-2 metres below the surface, the soil temperature fluctuates much less than the air temperature above the ground, and in the UK only very rarely approaches zero celsius. In effect, the earth beneath our feet acts as an energy store, which ground source heat pumps exploit. This heat is removed by a heat exchanger, typically consisting of sealed lengths or coils of pipe buried in the ground. Fluid is pumped through the heat exchanger to transfer heat from the source to the heat pump circuit.
System with a vertical ground heat exchanger
Most modern domestic systems are indirect systems, in which the heat exchanger is made of high-density polyethylene piping and forms a separate closed circuit containing a mixture of water and antifreeze. This antifreeze mixture circulates and transfers heat to the coolant in the heat pump circuit via a second heat exchanger.
In a direct expansion (DX) system, the refrigerant itself circulates through the ground heat exchanger, which is usually made of copper for better heat conduction. Although more efficient than indirect systems, greater volumes of refrigerant fluid are needed, and there is a higher risk of refrigerant leakage.
Horizontal or vertical heat exchanger?
The cheapest option it to bury the ground heat exchanger in a horizontal trench at a depth of 1-2 metres. This system is made more efficient by coiling the pipe into a 'slinky', to maximize the length of pipework for a given length of trench. The extent of the pipework, and hence the dimensions of the trench to bury it, depend on the system's operating temperatures. But an average-sized house would need a loop of around 200 m, equivalent to a slinky in a trench 50-70 m long, . This then requires sufficient suitable land adjacent to the building
Where land is limited, the heat exchanger can be placed in a vertical borehole. This requires a specialist drilling contractor, and is more expensive than a horizontal trench. However, efficiency of the system might be better because of the slightly higher temperatures at greater depths. Also, the cost can be spread by designing a single borehole to serve several buildings. Several vertical holes, between 15 and 60 metres deep, may be required, especially for larger commercial projects and institutions.
'Slinky' heat exchanger in a horizontal trench
How much will it cost?
The total installed cost of ground source systems varies with the type of house, location, and size of the system, but typically is £6000 to £12,000 for an 8-12 kW system, excluding the cost of the distribution system (e.g. underfloor heating). Horizontal trench systems are cheaper than ones using a vertical borehole.
What will I save?
The savings on energy bills depend on what fuel a ground source heat pump system displaces. When installed in an electrically heated house, such a system could save up to £1000 per year, and prevent emissions of up to 7 tonnes of carbon dioxide annually, according to the Energy Saving Trust. These savings, and corresponding carbon benefits, are lower when the system displaces an existing oil or gas-fired system.
Can I get a grant?
Yes. Grant funding for ground source heat pumps is available from the Low Carbon Buildings Programme run by the Department for Business Enterprise and Regulatory Reform (BERR). Currently this is £1200 or 30% of the eligible costs, whichever is the lower.
Advantages
- GSHP systems have few moving parts, require little maintenance, and have a long life expectancy of 40+ years
- System is hidden underground and within the building, so no problems with planning consent
- Low noise levels
- Some systems can operate in reverse, to cool the building in hot weather
- Electricity to drive the compressor can be supplied using economy tariffs
Drawbacks
- High initial cost of installation
- Requires electricity to drive the system
- Needs secondary heating system for hot water
- Not suitable for heating systems with conventional high-temperature radiators