An Experimental Study on the Effect of Electronic Expansion Valve Opening on the Performance of a Closed-Loop Transcritical CO2 Heat Pump Drying System

The opening of an electronic expansion valve (EEV) directly affects the operating characteristics, heating capacity, and dehumidification performance of a closed-loop CO2 heat pump drying (HPD) system. To clarify the underlying mechanisms governing these effects, experiments were conducted on a closed-loop CO2 HPD test platform to examine the variations in system operating parameters and performance metrics under different EEV openings. With an increase in EEV opening from 20% to 50%, the heating and refrigeration capacities, together with the total power consumption, decreased continuously. In contrast, the heat exchange capacity of the internal heat exchanger increased progressively. The discharge pressures of the compressor and EEV inlet declined continuously, whereas the compressor suction pressure and EEV outlet pressure showed a gradual increase. Both the coefficient of performance (COP) and specific moisture extraction rate (SMER) increased initially and subsequently declined, whereas the moisture extraction rate (MER) decreased progressively. The SMER exhibited a maximum variation of 5.32% between its highest and lowest values.