Seasonal performance assessment of large size air-source heat pumps with multiple evaporators affected by frosting and defrosting cycles
Large-scale heat pumps are considered a mature technology for integration into new district heating networks and for decarbonizing existing ones that currently rely on fossil fuels. Among the available heat sources, airsource heat pumps offer a universally applicable solution. However, in cold and humid climates, frost formation becomes a critical issue, affecting the performance of the multi air-to-refrigerant evaporator system. To ensure uninterrupted service to the district heating network, defrosting of the evaporators must be nonsimultaneous, but this necessary operation inevitably impacts overall system performance. This study involves a novel model designed to estimate the seasonal performance of this kind of systems, based on a detailed dynamic model of the evaporator field, capturing the behaviour of each individual heat exchanger, including their frosting and defrosting cycles. The aim of this work is to quantify the impact of frost formation on seasonal efficiency, exploring a domain of 20 evaporator field configurations and 4 time-based defrosting strategy settings for a case study located in Harbin, China. Using total costs as performance parameter, the configurations analysed resulted in a maximum of about 16% difference between the best and worst solutions. Configurations with a wide fin pitch proved to be the most convenient regardless defrosting strategy. Instead, the best defrosting strategy on average was the one with the lowest frequency, due to the low mean ambient temperature and humidity of the location. Nevertheless, it is the combination of field configuration and defrosting strategy to define the proper optimal solution, valid in the chosen case study.