Study on optimal exhaust pressure in transcritical CO2parking heat pump air conditioning systems

Based on a transcritical CO2heat pump air conditioning system equipped with a regenerator, an experimental platform for the transcritical CO2parking heat pump air conditioning system is designed and constructed. By controlling the parameters such as the cabin return air temperature, compressor frequency, and cabin air supply rate, the optimal exhaust pressure of the transcritical CO2parking heat pump air conditioning system is investigated. The results show that the transcritical CO2system is more sensitive to suboptimal exhaust pressure. When the outside ambient temperature is 7 ℃, the cabin return air temperature is 20 ℃, and the cabin and outside air supply rates are 400 m3/h and 1 000 m3/h, respectively, the system’s optimal exhaust pressure is 7.77 MPa. Under this condition, increasing the exhaust pressure from 7.42 MPa to 7.77 MPa results in a 1.1 enhancement in COP, while further increasing the exhaust pressure from 7.77 MPa to 9.41 MPa demonstrates only a COP degradation of 0.3. Increasing the cabin air supply rate can effectively reduce both the gas cooler outlet temperature and the optimal exhaust pressure. When the cabin air supply rate increases from 300 m3/h to 400 m3/h, the gas cooler outlet temperature decreases by 4 ℃, and the optimal exhaust pressure decreases by 1.15 MPa. The gas cooler outlet temperature demonstrates the most significant influence on the optimal exhaust pressure. When operating conditions change, these two parameters exhibit synchronous variations. Based on the experimental data, a novel correlation for predicting the optimal exhaust pressure is fitted, with a maximum deviation of 3.5% between the predicted and experimental values, demonstrating high accuracy. This correlation provides a reference for exhaust pressure control for transcritical CO2heat pump air conditioning systems, ensuring that the system always operates at the optimal COP.