Parametric analysis of the defrosting process of a reversible heat pump system for electric vehicles

• A transient simulation model of the reverse-cycle defrosting process was set up.
• A parametric analysis for the expansion valve opening was conducted.
• A visualization of the simulated frost-thicknesses on the heat exchanger was created.
• There is an optimum throttle opening regarding defrost time and defrost efficiency.
• A too low refrigerant mass flow causes a significantly higher defrost time.

The use of resistance heaters with PTC elements to cover the heat demand of electric vehicles reduces significantly the cruising range. Reversible cooling and heating systems are one of the most promising solutions for this problem. However, in heat pump mode the frost formation on the exterior heat exchanger reduces the performance and efficiency of the system. Therefore, an efficient defrosting method is crucial to benefit from the heat pump also under frosting conditions.In the present paper, measurement results of a reverse cycle defrosting process on a reversible cooling and heating system using CO2 as working fluid for an electric city car at chosen operating condition are discussed. Further, a transient Modelica simulation model was set up in order to assess the impact of different parameters during defrosting and to optimize the process. The simulation results showed the effect of different throttle valve openings on the defrosting process and a best performing valve-opening, where the defrost efficiency was higher and the defrost time was shorter compared to the other cases.