Propane-isobutane mixtures in heat pumps with higher temperature lift: an experimental investigation

Abstract

The shift towards renewable energies leads to an increased interest in heat pumps, not only for floor heating at temperatures around 40 °C, but also for providing hot water at higher temperatures (> 60 °C). To reach the goal of a climate neutral process, it is important to use refrigerants with low global warming potentials. Zeotropic mixtures are discussed to potentially increase the coefficient of performance (COP) of a heat pump compared to pure fluids. The present work experimentally analyses a compression heat pump process of such fluids; five mixtures of propane and isobutane are investigated here. The influence of composition, compressor frequency, inlet temperature of the working fluid into the evaporator and the inlet temperature of the secondary fluid water into the condenser are researched. Besides the COP, the destroyed exergy of the components are evaluated, finding the most relevant losses.  It is shown that the COPs for a temperature lift of 33 K are above 3 for the mixtures, while at a higher temperature lift of 53 K the COP is around 2. Furthermore, a strong impact of the compressor efficiency on the process was found, that changes with the composition, due to composition dependent pressure and density variations. The results show that the advantage of the zeotropic mixture within the heat exchangers is limited by their low contribution to the total specific exergy destruction compared to the compressor. The often-neglected changes in electro-mechanical losses get important, when the pressure ratios are changing with composition, as shown experimentally.