Comprehensive evaluation of the transcritical CO2 ejector-expansion heat pump water heater

Nowadays, the transcritical CO₂ heat pump water heater is widely used for hot water production. However, the larger pressure difference of the basic system leads to the huge throttling loss, which will significantly worsen the system performance. The ejector-expansion method accompanied by the internal heat exchanger is an effective way to reduce the throttling irreversibility. Nevertheless, the CO₂ ejector-expansion system is seldom studied with the exergy analysis, including the exergo-economic and exergo-environmental analysis. In this paper, this system is theoretically evaluated from comprehensive perspectives with emphasis on the exergy analysis and the sensitivity analysis of ejector cost and electricity price is also conducted. Results show that with the ejector and internal heat exchanger, the coefficient of performance is improved by 7.38% and pressure ratio will reduce to 2.66 with the ejector efficiencies of 0.90. Furthermore, the exergy destruction of the throttling value could be reduced by 95.38%-98.76%, leading to the increased exergy efficiency. The largest temperature matching factor is achieved by the basic system with the internal heat exchanger. The cost rate of the throttling valve and cost per unit of exergy production is reduced by 85.65% and 0.42 CNY⚫MJ⁻¹, respectively. Moreover, the maximum pollutant emission reduction is 115.41 tons, which results in the lowest specific exergo-environmental cost of 1.95 CNY⋅(kW⋅h)⁻¹ in the proposed system.