Exergy-based sizing of a R290 air-to-water reversible heat pump for space heating and cooling purposes

This works aims at evaluating the impact of different ratios of heating design loads to cooling ones on the thermodynamic efficiency of a R290 air-to-water reversible heat pump with the aid of an exergy analysis. The potential improvements of the investigated solution were also assessed. The heat pump was sized for a two-pipe fan-coil system designed with supply/return water temperatures of 45/40 °C in winter and 7/12 °C in summer. The outdoor air was cooled from -7 ℃ to -12 ℃ in heating mode and heated from 30 ℃ up to 35 ℃ in cooling mode, respectively. The heating loads were varied in a range from 8 kW to 13 kW, whereas the cooling loads were ranged between 6 kW and 15 kW, respectively. The results obtained showed that heat exchanger sizing plays a significant impact on the distribution of exergy destruction within the system components. As the system was sized based on the cooling load, the air-based heat exchanger was found to be oversized and the water-based heat exchanger was observed to be undersized for covering the heating loads and vice versa. It was also found that for the ratios of heating design loads and cooling ones less than 1.2 the system should be sized based on the cooling design loads. In this case lower system exergy destruction could be obtained. Furthermore, in this case the thermodynamic improvement of the air-based heat exchanger, the water-based heat exchanger and the compressor had approximately the same potential of increasing the efficiency of the system. As the ratios of heating design loads and cooling ones were higher than 1.2, the heating load should have been used for sizing the system components due to higher thermodynamic efficiency of the system compared to the previous case and technical possibilities to be operated. Removing the avoidable irreversibilities within the air-based heat exchanger offered the biggest decrease of the exergy destruction within the system.