Investigations on a Heat Pump Using Two-Phase Refrigerant Compressions

Abstract

Two-phase compression in volumetric machines shows several advantages: sealing effects brought by the liquid blocking the gap between two working chambers and it allows to get closer to an isothermal process, reducing the thermal stress on moving parts and minimizing compressor work. However, such compression also comes with some disadvantages: the mechanical reliability of the machine is questioned due to the presence of liquid during the compression, moreover, literature has shown that compressing in the two-phase region usually tends to decrease the isentropic efficiency of the machine. The irreversibility creation in cycles such as ORC's or vapor compression cycles comes from two sources: the deviation of compression/expansion processes from internally reversible processes and the temperature difference between the hot/cold sources/sinks and the working fluid along heat exchanges (external irreversibility). Therefore, two-phase refrigerant compression/expansion can be integrated to pursue a beneficial trade-off between internal and external irreversibility, searching to increase the performance of the cycle by allowing to match as close as possible the temperature profile of the hot/cold sources/sinks. This paper presents an investigation on vapor compression heat pump cycles where the pressure increase is performed by a two-phase compressor. To this aim, a validated semi-empirical model of a scroll compressor tested with two-phase refrigerant is integrated into the cycle model. Moreover, a moving boundary model is used to model the cycle heat exchangers. The overall model is used to investigate the performance of the heat pump cycle and to quantify both internal and external irreversibility. The results show that the exergy destruction rate of the compressor at low vapor qualities are too high to be counterbalanced by the optimal heat exchange in the condenser. Therefore, the maximum coeffient of performance (COP) is located at saturated vapor compressor inlet condition. Nevertheless, the analysis is strongly dependent on the compressor used and a more optimized compressor for the application could enhance the heat pump COP at low qualities.