Mode of operation of geothermal heat pumps

In the cooling mode a GHP is transformed into a refrigerator. Water circulating in the earth loop is warmer than the surrounding ground. It therefore releases heat to the ground, cooling in the process. The cooled water then passes through a heat exchanger in the heat pump. Within the heat exchanger refrigerant, gas heated by a compressor releases its heat to the water, which then begins its travel to release heat to the ground. The refrigerant, having released its heat energy, becomes a cold gas after passing through an expansion valve which is used to cool air or water. In a ducted air system the heat pump's fan circulates warm air from the building through the coils containing the cold refrigerant. The resultant cooled air is then blown through the ductwork of the building. The cold refrigerant in the air coil picks up heat energy from the building and then travels to the compressor where it again becomes a hot gas and the cycle starts again.

A reversing valve linked to the compressor enables the heat pump to revert to a heating mode. In this case the water in the earth loop is colder than the surrounding ground and draws warmth from it. This heat is conveyed to the heat exchanger in the heat pump where the refrigerant absorbs heat from the water. The water leaves the heat exchanger to circulate through the earth and pick up more heat. The refrigerant, now converted to a gas having absorbed the heat from the earth loop, travels from the heat exchanger to the compressor. After compression the temperature of the refrigerant rises to about 65°C. It then passes to the building's heating distribution system, which frequently comprises an underfloor hot water circuit.

After the refrigerant has released its heat it returns to the earth loop heat exchanger to start the cycle once more (see Fig. 4.3).

Whilst heat pump technology has proved popular in the USA it still has to establish itself in the UK. To help with this process a business park in Cornwall has just been completed that is exclusively heated by heat pumps supplied by Kensa Engineering of Falmouth. At present this is one of the few companies constructing heat pumps as opposed to importing them (see Fig. 4.4).

It is the Tolvaddon Energy Park which exploits geothermal energy with nineteen heat pumps which pump water around boreholes to a depth of 70 m. The existence of this project is due to the support of the Regional Development Agency (RDA) for the South West and its insistence on the use of geothermal energy. The heat pumps were supplied by Kensa Engineering.

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