Experimental investigation on a capillary tube based transcritical CO₂ heat pump system

In this study, a transcritical CO₂ heat pump system using a capillary tube as an expansion device was experimentally investigated. A prototype was developed for this purpose. Experimental study was conducted under different outdoor and indoor temperatures. The results showed that the system cooling capacity and coefficient of performance (COP) dropped by 22% and 24%, respectively as the outdoor air temperature increased from 30 to 40 °C and increased by 12% and 15%, respectively as the indoor air temperature increased from 22 to 32 °C. The system performance was further compared with the transcritical CO₂ heat pump using an electronic expansion valve (EEV) at different gas-cooler outlet temperatures. The comparison results showed that the transcritical CO₂ heat pump using the capillary tube is promising and could achieve performance close to the heat pump using EEV over a wide range of gas-cooler outlet temperature.