Analysis of a modified transcritical CO₂ two-stage ejector-compression cycle for domestic hot water production

High-efficiency domestic hot water (DHW) production has gained more attention currently, and a modified transcritical CO₂ ejector enhanced two-stage compression cycle (METC) for the DWH production is proposed in this study. To investigate the applied potential of the modified cycle, the validated theoretical models are constructed to compare and analyze the performance between the METC system and the basic transcritical CO₂ two-stage vapor compression cycle (BTC) under different operating conditions. The results show that the system METC outperforms the system BTC in terms of the heating coefficient of performance (COP_H), system total heating capacity (Q_tot), and the exergy efficiency (η_II) under different conditions. Compared with the system BTC, the COP_H, Q_tot, and η_II of the system METC are improved by 12.1%, 15.7%, and 15.3% under a typical operating condition. The ejector improves the refrigerant mass flow rate and absorption of heat in the evaporator. The low-grade working fluids are converted to a mid-grade state via the ejector, which is condensed in a mid-temperature heat exchanger to preheat the domestic hot water. Besides, the ejector reduces the exergy consumption and improves the system's exergy efficiency. The proposed cycle promotes the further improvement of the transcritical CO₂ two-stage compression cycle for DHW production.