Numerical investigation of the transcritical CO2 heat pump system employing different expansion devices.

It is well known that the high irreversibility in the throttling process degrades the performance of the transcritical CO2 heat pump system that employs expansion valve. To tackle this problem, different expansion devices have been proposed and investigated including expansion turbines and two-phase ejectors. A numerical analysis has been conducted in this study to compare the performance of these expansion devices. A numerical model has been developed using MATLAB and the thermophysical properties are provided by a database REFPROP. The working parameters and correlations were taken from verified experimental and theoretical studies in literatures. In the present study, the results show that the two phase ejector has a better coefficient of performances (COP). The cycle with an ejector can achieve a COPh about 5% and 15 % higher than that of the cycles with a turbine and a valve as expansion device, respectively. The findings showed that the advantages of the ejector cycle include the reduction
of irreversibility and the energy recovery in the throttling process of the transcritical CO2 heat pump system.