Experimental investigation on the performance of transcritical CO2 ejector-expansion heat pump water heater system

The effects of hot-water outlet temperature, compressor discharge pressure, compressor rotation speed, and expansion valve opening on the ejector and overall system performance were investigated for tap water outlet temperatures ranging from 50 to 90 degrees C. The coefficient of performance (COP) of the ejector-expansion heat pump system reaches 4.6 when the tap water outlet temperature is 70 degrees C, which is 10.3% higher than the corresponding basic cycle. Compared to the basic system, adding an ejector is more effective for the generation of high-temperature hot water. The COP of the ejector-expansion heat pump system increases; however, the COP improvement ratio MOP decreases with the increase in the discharge pressure because of lower pressure lift ratio and ejector efficiency. The COP and COP increase when the compressor rotation speed decreases under our test conditions; simultaneously, the heat capacity decreases.