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Ground source heat pumps

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).Heat pump housing

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

horizontal heat exchangerWhere 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 cost of ground source systems varies with the type of house, location, and size of the system, but typically the equipment cost is £7500 to £15,000 for an 8-12 kW system; this excludes the cost of installation and 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.

Renewable Heat Incentive (RHI)

Heating systems fired by ground source heat pumps are eligible for incentive payments under the Renewable Heat Incentive (RHI) scheme, launched in spring 2014. Only systems installed after 15 July 2009 are eligible for the incentive payments. Click here to find out more, or visit the Ofgem website for full details.

The tariff payments are currently 19.86 pence per kilowatt hour of heat produced, based on estimated annual heat use calculated from the property's Energy Performance Certificate, and the seasonal performance factor. A system suitable for a four-bedroom house would get RHI payments typically between £2500 and £4000 per annum, according to the Energy Saving Trust. The tariff lifetime is 7 years. Any Renewable Heat Premium payments already received are clawed back through reduced RHI payments over the tariff lifetime.See Incentives for renewables.

You can find out how much money you are likely to receive by using the government's own Renewable Heat Incentive Calculator.

Advantages

  • GSHP systems have few moving parts, require maintenance only every 3-5 years, 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
  • Most suitable for underfloor heating system in a well insulated building
  • Conventional radiators will be warm rather than hot