In the UK, the ground a few metres below our feet keeps a constant temperature of about 11OC throughout the year. This is approximately the average air temperature, derived from solar energy, which it absorbs and holds steady because of its high thermal mass. Ground source heat pumps (GSHP) take advantage of this steady temperature.
Although electricity drives a compressor to upgrade the steady low level heat to a higher usable temperature, and pumps which circulate fluid around a closed loop of pipe in the ground, heat pumps are considered to generate renewable heat because each unit of electricity can deliver several units of solar-derived heat.
GSHPs usually work by chilling an antifreeze liquid and pumping it through underground pipes where, being colder than the ground, it absorbs the warmth and heats up a few degrees. This warms a chilled refrigerant in the heat pump enough to evaporate it. The warmed vapour is then compressed, greatly increasing its temperature.
This heat is then transferred, via a heat exchanger, into your home’s heating and hot water systems causing the refrigerant to cool and condense back into a liquid. Its temperature drops again to well below zero when it passes through an expansion valve, ready to start the process again.
There are three important elements to a GSHP system:
Three options are available for the groundloop: borehole, straight horizontal and spiral horizontal (or 'slinky') or a combination. Each has different characteristics allowing you to choose the most suitable for your property.
Horizontal trenches can cost a lot less than boreholes but require greater land area. For slinky coil, a trench of about 10m length can provide for about 1kW of heating load, compared to 15-20m of borehole or 20-30m of trench with two passes of straight pipe, all depending on ground conditions.
Correct sizing of the heat pump and the ground loop is crucial to the effective and efficient operation of the system and will depend on your building’s heat requirements and the ground conditions. At the outset it is important to implement all possible energy efficiency measures such as roof, wall and floor insulation and draught-proofing to minimise your heat demand.
When sizing a system it’s important to consult a professional installer for expert advice. A heat pump can be designed to meet 100% of space heating requirements but often they are sized to provide less, needing some form of supplementary heating for the few coldest days of the year, either immersion heaters in the heat pump system, or some other form of heating such as a wood-burning stove. Heat pumps will usually only partially heat domestic hot water so secondary heating (ie. an immersion heater) must be programmed to top it up.
Heat pumps operate most effectively in very energy-efficient buildings, ideally new builds. Whilst they will provide heat in any situation, their efficiency is very sensitive to the flow temperature demanded of them, which is a function of the size of emitter (radiator or underfloor pipework) and the rate of heat loss.
You should consider the following issues if you are considering a ground source heat pump. An accredited installer will be able to provide more detailed advice regarding suitability.
The cost of a professional GSHP installation is dependent on property and location and ranges from about £1,200 to £2,000 per kW of peak heat output, excluding the cost of the heat distribution system (eg. underfloor heating).
Trench systems are cheaper so tend to be at the lower end of this range. The installed cost of a typical 8kW system, for example, would vary between £10,000 and £16,000 plus the cost of the distribution system. The price per kW gets lower as systems get larger.
By harnessing an inexhaustible resource, a heat pump uses much less energy than a typical heating system to provide the same benefit. In a well-designed system, up to four units of heat are delivered for each unit of electricity consumed. Because of this, it also has lower CO2 emissions. A good heat pump system (running on mains electricity) causes 40% fewer CO2 emissions than an equivalent oil boiler.
Ground source heat pumps are low-maintenance, low noise solutions. The underground elements can be expected to last up to 50 years while the other parts of the system have a reasonable life expectancy of 15-25 years.
Ground-source heat pump systems are supported under the Government’s Renewable Heat Incentive, the full domestic phase of which is expected in 2013 as part of the forthcoming ‘Green Deal’, which would eventually pay off your capital cost or even provide some of the cost in the form of a loan recouped through your electricity bills.
A well-designed GSHP system in an energy-efficient dwelling will cost less to run than an oil, LPG or direct electric (eg. storage heater) system, and potentially mains gas. Approximate running costs per kWh might be as follows:
|Per Kwh||Per Year|
|GSHP||4.6 – 6.5p||£963 - £1261|
|Nights Storage Electricity||8.1p||£1748|
(Source: Sutherland Tables April ’13)
*Price for radiator use only, not peak tariff backup or hot water heating
In 2013-14 the RHI pays non-domestic users 4.8p/kWh for each unit of ‘green heat’ generated by accredited non-domestic GSHP systems, and is similar to the Feed-In Tariffs already in place for renewable electricity generators.
For information about the existing non-domestic RHI, the Renewable Heat Premium Payment and proposed domestic RHI schemes, see www.decc.gov.uk/rhi .