Experimental study on the effect of compressor frequency on the performance of transcritical CO2 heat pump system with regenerator

Highlights: • Effects of the compressor frequency on the transcritical CO₂ heat pump system have been experimentally investigated. • Variations of the CO₂ gas-cooler outlet status under different compressor frequencies have been presented. • The CO₂ gas-cooler outlet temperature has indirect influence on the compressor discharge temperature. • The CO₂ gas-cooler outlet status and the water inlet temperature greatly affect the Q and the COP. • The system has a suitable operating range. -- Abstract: To clarify main factors that affect the discharge pressure and compressor frequency on air-source transcritical CO₂ heat pump water heater, an experimental research is designed in which the discharge pressure is controlled by varying the expansion valve opening and the plant is tested at three different compressor frequencies (25, 30 and 35 Hz). The results show that both the gas-cooler outlet status of CO₂ and the water-inlet temperature have significant effects on Q and COP. Q_max (the maximum heating capacity) and COP_opt (the optimal COP) exist in the system. With the compressor frequency increasing from 25 Hz to 35 Hz, Q_max shows a rise of 30.0% and COP_opt is decreased by 25.1%. Compared to COP_opt, the COP corresponding to Q_max drops in a mild way. Thus, the system should be operated in a suitable situation between COP_opt and Q_max under the precondition to satisfy the actual heating needs. This paper not only provides reference for designing and further study on transcritical CO₂ cycle, but also an operational strategy of air-source transcritical CO₂ heat pump.