Novel optimized operating strategies of two-phase injection heat pumps for achieving best performance with safe compression

Two-phase injection (TPI) is known to be an effective technique in reducing the discharge temperature and increasing the heating performance of heat pumps. However, for actual applications of TPI heat pumps, optimized operating strategies to guarantee increased energy saving potential with safe compression are still not available. The objectives of this study are to analyze the optimum operating conditions and propose novel optimized operating strategies of TPI heat pumps for achieving best performance with safe compression. The performance of the TPI heat pump is measured as a function of the injection quality, outdoor temperature, and compressor frequency. The optimum and safe injection qualities for achieving maximum performance and high reliability are determined in various operating conditions. Moreover, three novel optimized operating strategies for injection quality in the TPI heat pump are proposed based on the empirical correlations for the electronic expansion valve, injection pressure, and discharge superheat, respectively. In addition, the operating characteristics of these optimized operating strategies are compared to provide a selection guide for actual applications.