INDUSTRIAL & COMMERCIAL HEATING HEAT PUMPS
Heat pumps are becoming increasingly popular for meeting larger- scale needs. But evidence suggests that the performance of many air- source installations are falling short of the claims made for them. John Cantor and Kate de Selincourt consider the issues
conventional heating methods. Since their conception, a coeffi cient of performance (COP) of 3 has been considered the norm for a heat pump (3kW of heat for 1kW of power input). Perhaps due to the optimism of sales people, a COP of 4 is sometimes claimed. However, it is not the COP but the seasonal performance factor (SPF) that we really need to consider (see ‘Defi nitions’ box). An SPF of 4 is certainly achievable for space heating with a good ground- source heat pump (GSHP), connected to a well-designed underfl oor heating system in a well-insulated building. But what performance are UK air-source heat pump (ASHP) users generally getting overall? Last year the Energy Saving Trust (EST)
T
published the long-awaited fi ndings of one of the few fi eld trials of heat pump
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he selling point for heat pumps is the fact that, because they can use relatively little energy, they are greener and cheaper to run than
installations in the UK, and this appeared to be something of a reality check (www.
energysavingtrust.org.uk/Media/ node_1422/Getting-warmer-a-fi eld-trial-of- heat-pumps-PDF). The results, expressed as ‘system effi ciencies’, showed that for almost all the ASHP installations (heating and hot water), the performance was well below 3. Of the 22 ASHP installations evaluated,
50% fell within the range of 1.6 to 2.2, with one as low as 1.2, and only one achieving a measured effi ciency of 3. While GSHPs performed better, the middle 50% still fell between 2.0 and 2.6. Studies in Germany and Switzerland,
where the technology is better established, have shown SPFs for ASHP installations generally clustering around the 2.5-2.9 mark, with some as low as 2.2, and GSHP installations usually above 3. (See Figure 2.) The EST results were taken by some
commentators as ‘proving’ what a disappointment heat pumps in general, and
October 2011 CIBSE Journal 51
The way many heat pumps have been installed and set up in the UK falls short of the ideal, so it is little wonder that some performances have been disappointing
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