Ground source heat pumps (GSHP) transfer heat from the ground into a building to provide space heating and, in some cases, pre-heating domestic hot water. For every unit of electricity used to pump the heat, 3-4 units of heat are produced. As well as ground source heat pumps, air source and water source heat pumps are also possible.
The main elements of a GSHP are:
Ground loop - comprises lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground.
Heat pump - although we may not know it heat pumps are very familiar to us - fridges and air conditioners are both examples. A heat pump has three main components:
1. Evaporator - (e.g. the squiggly thing in the cold part of your fridge) takes the heat from the water in the ground loop;
2. Compressor - (this is what makes the noise in a fridge) moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit. Condenser, (the hot thing at the back of your fridge) gives up heat to a hot water tank that feeds the distribution system;
3. Heat distribution system - consists of under floor heating or radiators for space heating and in some cases water storage for hot water supply.
Three options are available for the ground loop: borehole, straight horizontal and spiral horizontal (or ’slinky’). Each has different characteristics allowing you to choose the most suitable for your property. Horizontal trenches can cost less than boreholes, but require greater land area. For slinky coil, a trench of about 10m in length will provide for about 1kW of heating load.
The installed cost of a typical 8kW system varies between £6,000 and £10,000 plus the cost of the distribution system. Naturally costs are dependent on property and location so the cost for a system for your home may different to the estimate above.
The efficiency of a GSHP system is measured by the Coefficient of Performance (CoP). This is the ratio of the number of units of heat output for each unit of electricity input used to drive the compressor and pump for the ground loop. Typical CoPs range between 2.5 to 4. The higher end of this range is for under floor heating, because it works at a lower temperature (30-35C) than conventional radiators.
Based on current fuel prices, assuming a CoP of 3-4, a GSHP can be a cheaper form of space heating than oil, LPG and electric storage heaters. It is however more expensive than mains gas. If grid electricity is used for the compressor and pump, then an economy 7 tariff usually gives the lowest running costs.
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 type of heat distribution system. GSHPs can be combined with radiators but under floor heating is better as it works at a lower temperature.
· Is there space available for a trench or borehole to accommodate a ground loop?
· Is the ground material suitable for digging a trench or borehole?
· What fuel is being replaced? If it is electricity, oil, LPG or any other conventional fossil fuel the payback will be more favourable. This makes heat pumps a good option for off gas grid areas.
· Do you want to be 100% renewable? If so, purchase green electricity, or install solar PV or some other form of renewable electricity generating system to power the compressor and pump.
· Do you require a back up heating system?
· Is there also a cooling requirement?
· Is the system for a new building development? Combining the installation with other building works can reduce costs.
· Can you incorporate insulation measures? Including wall, floor and loft insulation will reduce your heat demand.
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