Ground source heat pump
Ground source heat pumps (GSHP) transfer heat from the ground and into a building to provide spatial heating - and, in some cases, to pre-heat domestic hot water.
How does it work?
There are three important elements to a GSHP:
1. Ground loop
This is comprised of 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 around the pipe; absorbing heat from the ground.
2. Heat pump
This has three main parts:
- The evaporator - takes the heat from the water in the ground loop
- The compressor - moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit
- The condenser - gives up heat to a hot water tank which feeds the distribution system.
3. Heat distribution system
This consists of underfloor heating or radiators for spatial heating, and, in some cases, water storage for hot water supply.
How much does it cost?
A typical 8kW system costs £6,400-£9,600, plus the price of connection to the distribution system. This can vary according property and location.
The efficiency of a GSHP system is measured by the coefficient of performance (CoP). This is the ratio of units of heat output for each unit of electricity used to drive the compressor and pump for the ground loop. Typical CoPs range from 2.5 to 4. The higher end of this range is for underfloor heating, because it works at a lower temperature (30-35ºC) than radiators.
Based on current fuel prices, and assuming a CoP of 3-4, a GSHP can be a cheaper form of spatial 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.
Ground source heat pumps and your home
There are some general factors you will need to bear in mind, and questions you may need to ask yourself when considering a GSHP:
- The type of heat distribution system you use. GSHPs can be combined with radiators, but underfloor heating is usually 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 suitable for digging a trench or borehole?
- What fuel is being replaced? If it's electricity, oil, LPG or any other conventional fossil fuel, the payback will be more favourable. Heat pumps are a good option where gas is unavailable.
- Want to be 100% renewable? Buy green electricity, or install solar PV or some other form of renewable electricity generating system to power the compressor and pump.
- Do you need a back-up heating system?
- Is the system for a new building development? Combining the installation with other building works can reduce costs.
- Can you incorporate insulation measures? Solid wall, cavity wall, floor and loft insulation will lower your heat demand.
It is important to consider all of the above factors in detail in order to decide if a GSHP is the right option – both financially and logistically – for your property.
Source: Energy Savings Trust 2007
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