Renewables - Reverse Cycle Heat Pumps

Heat pumps use refrigerant gases and an electrical compressor to take heat from a source and deliver it to an output. Chillers and refrigerators are examples of systems that remove heat, but other types of system use the heat removed from a source to heat a building. Traditional heat pumps use air as the source of heat. However, the ideal source for maximum efficiency would be one having a stable temperature, and the ground provides such a source. The ground acts as a huge solar collector and thermal store. The surface is warmed by the sun and the adjacent air during daytime and in the summer. Similarly it is cooled during night-time and in the winter. Fluctuations in ground temperature reduce with depth and stabilise at the annual mean for the location by about 12 m below the surface. Typically in the UK this temperature lies between 9 °C and 12 °C.

Ground-source heat pumps (GSHPs) make use of the heat stored in the ground at this relatively stable temperature of around 9 °C and 12 °C and raise it to a more useful output temperature of around 40–50 °C for use in heating buildings. These output temperatures are ideal for low temperature systems, e.g. underfloor heating coils and radiant panels in most types of building.

Heat can be extracted from the ground either by a buried loop of pipework through which a refrigerant fluid (or water) is circulated, or directly by abstraction of ground water. With correct design, the depletion of the heat source is matched by the rate of heat flow back from the surrounding earth and under these circumstances the technology is a renewable source of energy. Ground-source heat pumps require an input of energy, usually electricity, but they can be very energy efficient, ‘moving’ up to 4 or 5 kW of heat output for every kW of electrical input. This ratio is known as the seasonal performance factor (SPF). Heat pump performance is better when the difference between source and sink temperatures is lower. The seasonal performance of ground source heat pumps will be better than for an air source heat pump because air temperatures will often be below 12 ºC over the course of the heating season.

Heat pumps can also be operated in reverse to provide cooling. Other types of heat pumps are available which extract heat from other sources (e.g. air or water). They save carbon when compared to other forms of electrical heating, but are not as efficient.

GSHPs are suitable for soil and rock types in most locations. Final output depends on the size of the loop, controls and the heat pump itself rather than on location. The availability of ground-source heat is not confined to certain times of the day or year but some locations exhibit more seasonal behaviours than others according to local geology. GSHPs are most efficient when they operate continuously, and so are best suited to buildings which are in constant use such as housing developments, hotels and hospitals.

Please get in touch if Callidus Design can be of assistance with the design of your new Air or Ground Source Heat Pump Installation.

 (extracted from CIBSE Guide L - Sustainability and TM38 – Renewable energy sources for buildings)