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

Heat pump systems have been used for more than a century 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. These use a cheap renewable heat source and electricity, and the system purchase is motivated by an attractive coefficient of performance (COP), besides the option to reverse the heat pump operation from heating during winter to cooling during summer. Also, industry is increasingly implementing heat pump technology that circumvents the production of CO₂ when producing heat, although reaching sufficiently high temperatures may be challenging. Developments are accelerated by the increasing availability of (cheap) zero-CO₂ electricity. One complication for the assessment of the energy efficiency of heat pump systems using exergy analysis is that besides temperature levels, features of the heat reservoirs, such as humidity of air in a building envelope, become important. In modern buildings, this is affected by exhaust air heat recovery (EAHR) systems that typically replace the air inside a building every few hours. A side-effect of this is a net in- or outflux of humidity. 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 an significantly 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.