Effects of Superheat and Secondary-Fluid Inlet Temperature on a Water-Cooled Transcritical CO2 Heat Pump

This study aims to provide essential data for the optimal design of a transcritical CO2 water-cooled heat pump system utilizing an internal heat exchanger (IHX). To investigate cycle and application characteristics, a heat pump test rig consisting of a compressor, gas cooler, expansion valve, evaporator, IHX, and liquid receiver was fabricated. All heat exchangers were counterflow, concentric dual copper tubes. System performance was evaluated by varying the IHX and evaporator outlet superheat degrees (10–30 °C and 0–20 °C, respectively) via expansion valve opening control, alongside changes in the secondary fluid inlet temperature. The experimental results indicate that optimal high-pressure conditions exist to maximize both capacity and coefficient of performance (COP) in cooling and heating modes. Furthermore, changes in the secondary fluid inlet temperatures of the evaporator and gas cooler significantly impact the COP, showing trends similar to conventional Freon-based systems. Ultimately, precise control of the expansion valve opening, using the IHX outlet superheat as a control variable, is a key factor in operating the CO2 heat pump system near its maximum COP.