Experimental study on the heating performance of a variable speed CO ₂ heat pump with a variation of operating conditions

The applications of a transcritical CO₂ cycle into water heaters show advantages over conventional systems in the respect of power consumption and heating efficiency because the CO₂ cycle has a high compressor discharge temperature. Besides, the heating performance of the transcritical CO₂ cycle can be improved by optimizing operating conditions. In this study, the heating performance of a variable speed CO₂ heat pump was measured and analyzed by varying refrigerant charge amount, EEV opening, compressor frequency and outdoor temperature. As a result, the optimum normalized charge for heating was 0.226. The COPs at the compressor frequencies of 40, 50 and 60 Hz were 2.94, 2.75 and 2.25, respectively. The heating performance of the CO₂ cycle with charge amount was more sensitive than the cooling performance. Moreover, the heating performance was improved significantly by optimizing of compressor frequency and EEV opening.