An experimental study of trans-critical CO2 water–water heat pump using compact tube-in-tube heat exchangers

• Thermodynamic analyses of transcritical CO2 cycle with and without IHX are provided.
• A transcritical CO2 heat pump system adopts compact tube-in-tube heat exchangers.
• Experiment results of systems with and without IHX have been analyzed and compared.
• IHX can improve the performance of the transcritical CO2 heat pump system.

A transcritical CO2 water–water heat pump system is introduced in this study, which employs compact tube-in-tube evaporator and gas cooler. Its primary test standards and operating conditions are introduced. Under test conditions, experiments have been carried out with compression cycles with and without internal heat exchanger (IHX). Experiment results have been analyzed and compared, showing that IHX can improve the coefficient of performance of the system. The analyses are done mainly on the variations of outlet CO2 temperature of the gas cooler, compressor discharge pressure, compressor lubricant temperature, hot water mass flow rate, etc. When the inlet water temperature of the gas cooler is 15 °C, 20 °C, 25 °C respectively, the hot water temperature ranges from 45 °C to 70 °C, the relative COPh (coefficient of performance when heating) change index (RCICOP) of the heat pump system with IHX is about 3.5–8% higher than that without IHX. The relative capacity change index (RCIQ) of the heat pump system with IHX is about 5–10% higher than that without IHX. Temperature of CO2 increases at the outlet of the gas cooler when the outlet water temperature of the gas cooler increases. Lowering the outlet CO2 temperature of the gas cooler is an important way to improve the performance of the system.