Investigations on the dynamic characteristic and its influence factors of a transcritical CO₂ automobile heat pump

The objective of this paper is to develop a more efficient control logic via investigations of the dynamic characteristics of a CO₂ heat pump for heating vehicles. A transient thermodynamic model was set up using Modelica language, and the PID controller was used for a stable cabin environment. The influence of the discharge pressure and airflow rate on the dynamic characteristics, including the dynamic COP, the heating capacity and the return air temperature, were investigated and analyzed. Moreover, the comprehensive COP during the dynamic process was calculated and analyzed. Experimental data from a sample vehicle were used to validate the accuracy of the model. Results showed that the real-time COP decreased quickly at the initial starting process but then increased gradually to the stable value. The heating capacity and the return air temperature had the reverse tendency. With the increase of the discharge pressure and airflow rate, the parameters of COP, the heating capacity and the return air temperature fluctuated less heavily and reached a stable value more quickly. Improved understanding of the dynamic performance can support development of more efficient control logic.