Effects of refrigerant charge on performances of transcritical CO₂ heat pump water heater

The performance parameters such as heating COP (coefficient of performance), compression ratio, suction and discharge pressures, and so on, of a transcritical CO₂ heat pump water heater were experimentally investigated under various refrigerant charging conditions to study the effects of refrigerant charge on the heating COP. Experiments were conducted under the condition of constant heating capacity and compressor inlet superheat by simultaneous control of the compressor frequency and the pulse of electronic expansion valve. Experimental results showed that the compressor discharge pressure rose and the refrigerant outlet temperature from the gas cooler dropped as the refrigerant charge was increased, the suction pressure increased while the compression ratio and discharge temperature decreased as the inlet temperature of heat source fluid increased, and the maximum heating COP increased with an increase in heat source fluid inlet temperature. The maximum heating COP was 3.25 at the heat source fluid inlet temperature of 15°C, the compressor inlet superheat of 5°C, and the optimum refrigerant charge of 1.7 kg.