Research on the Influence of High-Low Pressure-Volume Ratio and Discharge Pressure on the Heating Performance of Transcritical CO2 Single- Machine Two-Stage Compression Heat Pump Water Heater

Abstract

Background: In order to reduce global carbon emissions, a cross-critical CO2 singlemachine two-stage compression system was proposed. Due to the characteristics of cooling on the high-pressure side of the cross-critical cycle and bipolar compression on the single-stage, the discharge pressure and the high-low pressure volume ratio have a great influence on its heating performance. Objective: The purpose of this paper is to explore the influence of high-low pressure-volume ratio and discharge pressure on the heating performance of the system, so as to obtain the optimal operating conditions of the system. Methods: By constructing theoretical calculation formula of discharge pressure, high-low pressurevolume ratio and intermediate pressure, combined with experiments. Results: Under certain working conditions, with the increase of the volume ratio of high-low pressure, the intermediate pressure increases, the heating capacity of the system increases due to the increase of the discharge temperature and the increase of the heat exchange temperature difference on the hot water side, while the heating performance coefficient decreases slowly because the rate of the increase of the power consumption of the compressor is greater than the rate of the increase of heating capacity. With the increase of discharge pressure, the increasing rate of CO2 discharge temperature will gradually decrease at the high-pressure side, so the heating capacity of the system increases rapidly first and then tends to slow down, and the heating performance coefficient increases first and then decreases. There is an optimal value, that is, when the evaporation temperature is -20 °C, the maximum heating capacity can reach 3 kW. The system heat production performance coefficient has an optimal value of 3.29, and the optimal discharge pressure is 8.5 MPa. The optimal discharge pressure will increase with the increase of evaporation temperature. Conclusion: The selection of reasonable high-low pressure volume ratio and the control of discharge pressure can effectively improve the heating performance of cross-critical CO2 single-machine twostage compression heat pump water heater.