Ground source heat pumps (GSHPs) extract heat from fluid passing under the ground and absorbing heat from solar radiation transferred to the soil. This warmed fluid , known as brine (actually an antifreeze mixture) changes the state of the refrigerant from liquid to gas and as it does so absorbs heat energy.
When this gas is compressed it gets hot. The heated gas is passed through a second heat exchanger where heat can be taken away as heated water.
Ground source heat pumps will work at any ambient temperatures providing the source fluid from the ground is above about -5 degrees.
These heat pumps have a much more consistent power output across their operating range than the air source version.
Ground source units are slowly getting less obtrusive and quieter. There are important issues which must be considered before purchasing. These include:
- Size of ground loop collectors or boreholes. The cost of a borehole is about £10,000 and the length that needs to be dug for ground loop trenches can be anywhere between 400 and 1500 metres.
- Noise levels. Modern heat pumps employ much better noise and vibration proofing than older types but if placed badly or not properly installed can still be heard in teh middle of the night and the low frequency drone can be really annoying if precautions are not taken to acoustically separate the heat pump from the building.
- Start current. When the compressor is required it can take a massive amount of power to start. This can cause lights to dim momentarilly and causes real headaches for the electricity supply companies who are required to keep supplies to minimum standards. An inverter drive compressor may require only 1 or 2 amps to start and is much more gentle both on the ears and on the power supply. Not many GSHPs employ inverter technology yet.
- Size. A basic compressor unit is quite small. Multiple compressors may be required however and there is also the cylinder and heating buffer tank to consider. Unless you are buying a Compact type of heat pump there will also be a lot [of additional plumbing required.
Site your heat pump as close to the house as possible. It is okay to site them in the house but keep them well away from sleeping areas as they tend to work hardest at night as that is when the heat load is highest so they often run for long periods after dark.
Closed loop systems require one or more lengths of collector to be laid into the ground. This is quite a large engineering task and will require the movement of lots of earth. If multiple loops are laid, then there will also be a requirement for a manifold chamber where the loops are all connected together.