Continous Supplies of Heat
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.

Blue Sky

Air
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.

fancoil unit
In the fan coil unit the refrigerant will evaporate at, typically, 10 degrees below the air temperature, i.e zero degrees C for a 10°C air temperature. This temperature difference is needed to drive the heat at a sufficient rate (downhill) from the air to boil the refrigerant. The fins are used to collect the heat from the air. The air is light and a large volume has to be driven across the coil by the fan in order to make it work. Another factor is that when the weather is really cold the amount of heat pumped reduces, and this is just when more heat is needed for the building. Also the outside coil can become covered in frost which further reduces efficiency. Heat pumps which take heat from the air are called Air Source Heat Pumps (ASHP). Small air conditioners which act as heaters in winter are ASHPs.

The Ground could be a lawn or garden
Ground
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.

The Ground could be a lawn or garden
Water
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.

 

Slinky Coil Buried in the Ground 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.