How do they work?

If you have ever pumped up a bicycle tyre, you probably remember the pump becoming hot. This is a quite general rule: if you increase the pressure of a gas, like the air in the pump, it becomes hotter. The reverse is also true: reduce the pressure, it gets colder.

This is the science behind the operation of a heat pump. At its heart is a compressor which pressurises a refrigerant gas to make it hot. The hot gas is piped to a heat exchanger, where it heats up the water. As the gas gives up its heat, it turns back into a liquid. This liquid, which is still under pressure, passes through a tiny nozzle and into a second heat exchanger, or evaporator. The evaporator connects to the compressor input, completing the circuit. The low pressure in the evaporator causes the liquid refrigerant that is being forced through the expansion valve, to fall in temperature to around 2°C to 6°C.

In the Soleco heat pump, the cold refrigerant in the evaporator is warmed by the ground water pumped up from the borehole at around 8°C to 12°C. Because of the low pressure (the evaporator is connected to the suction side of the compressor) the refrigerant boils, and turns back into a gas. This is re-pressurised by the compressor to repeat the cycle. The borehole water, which is cooled by around 5°C during the process, is simply returned to the underground water table via a soak-away.