Performance indexes of an air-water heat pump versus the capacity ratio: Analysis by means of experimental data

The spread of near-Zero Energy Buildings (nZEB) involves the employment of high performant air-conditioning plants where renewable sources can be integrated easily. In this context, heat pumps appear as a promising solution given their ability to exploit aerothermal, hydrothermal and geothermal sources and to supply both heating and cooling loads with the same device. In order to evaluate the energy performances in transient conditions, the actual winter (COP) and summer (EER) performance indexes, in the function of the sources temperatures and the capacity ratio (CR), have to be available. Nevertheless, heat pump manufactures often provide the trend of the performance indexes in the function of the temperatures of the sources specifically for nominal conditions, whereas the dependence of the performance indexes in the function of CR, that takes into account the part-load operation, is almost always not provided. Alternatively, specific technical standards suggest the use of a correction factor to modify nominal COP and EER for the attainment of the real performance indexes. In this paper, by using data from an experimental set-up equipped with air-water heat pumps, these correlations were tested and tuned. Winter results showed that correction factors suggested by standards have to be modified in the presence of a storage system. In summer, instead, a new correlation was developed to find a function between nominal and actual EERs in the function of CR by exploiting a similar approach employed for the COP calculation.