The Collaborative Optimization of the Discharge Pressure and Heat Recovery Rate in a Transcritical CO₂ Heat Pump Used in Extremely Low Temperature Environment

Considering the excellent environmental properties and heating capability under wide running conditions of the natural fluid CO₂, the transcritical CO₂ heat pump system has widely been used in the application of water heaters, commercial heating and cooling, electric vehicle thermal management, etc. Since the performance was highly affected by the discharge pressure and heat recovery rate in a transcritical CO₂ system, the collaborative optimization of these two parameters was analyzed in detail in this study. The results showed that the optimal value of the system heating COP, which was the ration of heating capacity to power consumption, was better under a higher heat recovery rate and relatively lower discharge pressure, which is why these kinds of operating conditions are highly recommended from the perspective of collaborative optimization. Additionally, the heat recovery rate had a positive effect on the system performance when the discharge pressure was lower than its optimal value, while the heat recovery rate would present a passive effect on the system performance when the discharge pressure was higher than its optimal value. The relevant conclusions of this study provide a good theoretical basis for the efficient and stable operation of the transcritical CO₂ heat pump technology under the conditions of a wide ambient temperature range.