The effect of air humidity on the exergy efficiency of domestic heat pumps

Heat pump systems have been used for decades in refrigeration and the upgrading of heat to temperature levels demanded by consumers. Private housing in Northern Europe and other countries is moving away from direct electric heating, combustion-based heating and even district heating in favour of heat pumps that use a cheap renewable heat source and electricity. The system purchase is motivated by an attractive coefficient of performance (COP), the option to reverse the heat pump operation from heating during winter to cooling during summer, and the increasing availability of (cheap) zero-CO₂ electricity. Assessment of the energy efficiency of heat pump systems using exergy analysis is complicated by features of the heat reservoirs, such as humidity of air in a building envelope, becoming important besides temperature levels. In modern buildings, exhaust air heat recovery (EAHR) systems replace the air inside a building every few hours, with a net in- or outflux of humidity as side-effect. In this paper, an exergy analysis is presented that quantifies energy efficiency of a heat pump system as partly determined by the humidity of a building envelope being heated using a ground source or air source heat pump. Humidity control introduces a significant energy penalty. As shown, an EAHR unit can result in a significant increased exergy efficiency, adding to the benefits offered by the heat pump, depending on indoor versus outdoor temperature and humidity, and whether a ground source or air source heat pump is used. Using weather data for 2018, during the coldest months a ground heat source heat pump is clearly more efficient than an air source heat pump for maintaining a modern housing space in Finland at 22 °C and 50% relative humidity year-round. Exergy (in practice electricity) consumption is 20% higher for an air source heat pump compared to a ground source heat pump, with maximum exergy efficiencies of 14% and 11.5%, respectively. Integration with an EAHR system can add up to 20%-points to the exergetic efficiency, irrespective of the type of heat pump making also that a powerful investment.