Experimental Performance Study of a Transcritical CO2 Heat Pump Equipped with a Passive Ejector

This study is dedicated to an experimental investigation of a passive two-phase ejector used as an expander in a transcritical CO2 heat pump. The investigation focused on the impact of the evaporating temperature (T-evap) and the CO2 gas cooler outlet temperature (Tgc-out) on the ejector and the overall cycle performance. The basic cycle without an ejector was also tested as a baseline for comparison. Two ejectors designed with different modeling approaches were tested and compared. The ejector with an enlarged mixing section diameter was selected for subsequent testing due to its improved pressure lift. The optimum primary nozzle position was found to be 4 times the mixing section diameter (D-mix). Although the ejector was designed for specific conditions, the results demonstrate its ability to remain operational under varying conditions with some changes in performance. The ejector's performance was observed to be dependent on the T-evap, and particularly on the Tgc-out. The pressure lift recorded was in the range of 3.7-6.5 bar, and the lowest value was obtained with the low Tgc-out value (29 degrees C). Under the tested conditions, the integration of the ejector enhances the performance and the capacity of the heat pump. The ejector cycle improvement is primarily based on improved mass flow rates due to increased compressor suction pressure, reduced compression ratio, and consequently, improved compressor operating conditions. Improvements of up to 18% in heating COP and 20.5% in heating capacity were observed. The study provides valuable insights into enhancing the performance of transcritical CO2 heat pump system by refining ejector design. It explores the behavior of the system across varying conditions, highlighting the significant impact of the ejector-compressor interaction on overall performance.