On-off cycling losses of reversible air-to-water heat pump systems as a function of the unit power modulation capacity

In this paper, the impact of on-off cycling losses on the seasonal energy performances of three reversible air-to-water heat pumps having equivalent thermal performance at full load but different power modulation capacity is discussed by means of a series of dynamic simulation models. The analysis of the heat pump performance at partial load, the development and comparison of different control algorithms for heat pumps and, finally, the modeling and evaluation of the degradation effect due to the heat pump on-off cycles at partial load conditions are numerically investigated by means of a series of innovative TRNSYS models not available in the software standard library. The models are able to take into account the variation of the heat pump performance at partial load conditions for multi-stage, inverter-driven and single-stage devices. The findings of this paper point out that the energy losses introduced during the start-up transient of the heat pump has a significant impact on the overall energy performance of the system only for single-stage units, which perform on-off cycles during the whole season, with a decrease of the seasonal performance factor (SPF) up to 12%. On the other hand, modulating heat pumps (i.e. multi-stage and inverter-driven units) avoid carrying out on-off cycles and, of course, cycling losses for a significant part of the season according to their enhanced modulation capacity. It is shown how the accurate dynamic simulation of heat pump-based systems is a useful tool for HVAC designers in order to optimize their control system parameters, with the aim to reduce the compressor maximum hourly number of cycles.