An experimental investigation of secondary fluid parameters on heat pumps with higher temperature lift and zeotropic working fluid

Abstract

Heat pumps are also discussed for replacing conventional house heating systems at higher temperatures. Their performance depends on the heat demand, heating system return temperature, as well as the heat source temperature, which determine the reversibly achievable coefficient of performance (COP). To approach reversibility in the heat exchangers, zeotropic mixtures are often investigated, but the mean temperature difference between both fluids also varies with the secondary fluid parameters and the heat load, which is less considered in the literature. Thus, this study experimentally investigates the impact of secondary fluid parameters, especially secondary fluid mass flow rate and temperature levels, on the performance of a water-water compression heat pump system with sink temperatures of 60 to 75 °C and source temperatures of 8 to 29 °C. The heat pump operates with a piston compressor and an isobutane-propane (R600a/R290) working fluid mixture (mole fractions: 0.25:0.75), while the system is adapted by variable throttling. COPs and exergy destruction rates of the individual components are evaluated, revealing the influence of secondary fluid parameters on the process, especially the cycle mass flow rate and the compressor efficiency. The COP rises by 63.4 %, if the heat flow demand rises from 1.17 to 3.17 kW. The variation in the working fluid throttling for matching the heat demand, influences pressure levels, mass flow rates, pressure ratios, and compressor efficiencies, often neglected in simple cycle calculations. Varying the evaporator water inlet temperature showed no significant change in COP, due to counteracting changes in compressor efficiency and evaporator exergy destruction.