Experimental investigation of the optimal heat rejection pressure for a transcritical CO2 heat pump water heater

• Experimental investigation of the optimal heat rejection pressure has been presented.
• The optimal heat rejection pressure influence factor has been introduced.
• Obtaining high COP at the lowest refrigerant outlet temperature has been addressed.
• New correlation to predict the optimal high pressure was obtained.

The system performance of a transcritical CO2 heat pump is significantly influenced by the heat rejection pressure due to the nature of the transcritical refrigeration cycle. It has received wide attention in the scientific community. In this article, an experimental investigation of the optimal heat rejection pressure for a transcritical CO2 heat pump water heater is presented. It is found that the optimal heat rejection pressure varies with gas-cooler outlet refrigeration temperature at different ambient temperatures. The further experimental results show that the Coefficient of Performance (COP) at the optimal heat rejection pressure decreases substantially with increasing gas-cooler outlet refrigeration temperature in a range from 25 to 45 °C. Based on the experimental data, a simple correlation of the optimal heat rejection pressure in terms of gas-cooler outlet refrigeration temperature is obtained. The analysis shows that the deviation of the correlation is within ±5%, and the predicted COP at the optimal heat rejection pressure is within 6%.