Step outside and you are looking at sources of heat. Upwards the air, downwards the ground, or maybe if you are lucky, a lake or river.
The outside air is the most obvious source. Why not pump heat out of the air and into the building? This can certainly work well. Most of the time the air in the UK is not particularly cold. It is maybe cooler than we would like to be indoors. It could be at say 10°C. If we want the indoor temperature to be 20°C, then the heat only needs to be lifted by 10 degrees. This would seem to be quite easy. But there are one or two snags. To extract the heat from the air a fan coil unit is needed. This is quite a simple device such as may be seen on the walls of many buildings.
By digging down we find that the temperature beneath the surface is roughly constant all through the year. If we could take the heat from the ground the supply would be constant even in times of very cold weather. This is certainly true and a Ground Source Heat Pump (GSHP) can offer a very good solution. To extract the heat from the ground it is necessary to circulate water through a coil beneath the surface. Actually the water will need to contain "anti-freeze". The water, or brine, gives up heat in the evaporator of the heat pump and becomes cooler. The evaporator in the heat pump will be quite small because a small volume of water contains a lot more heat than air. But of course, the water has to return to its "slinky" coil in the ground again to warm up. Care must be taken to make this coil large enough and deep enough to ensure that the garden is not frozen! Sometimes a deep drilling is used instead of a "slinky" coil. This takes up far less space.
For those fortunate enough to live close enough to water which is flowing or where there is a large enough lake to support some removal of some heat, this is an ideal source. Technically it is still a Ground Source Heat Pump (GSHP) but the "slinky" coil may be smaller than one which us buried in the ground. Needless to say it is only possible to extract heat from water with permission from the appropriate water authority.
The Slinky Coil
For any GSHP a coil or drilling into the ground is needed to gather the heat. Water or brine is circulated around this coil and back to the heat pump unit in the building. The GSHP operates very well and thousands have been installed in Sweden. But the cost of the underground gatherer makes it an expensive option.