Optimization of Secondary Coolant Circuit in a Small Battery Electric Vehicle Heat Pump System

Heat pump systems combined with secondary coolant circuits offer significant advantages when used in battery electric vehicles. The systems can be designed as very compact which allows integration even into the smallest vehicles. The small refrigerant volume and possible separation of the refrigerant circuit from the passenger compartment gives the opportunity to investigate wider range of refrigerant fluids better suited to the heat pump operation. The resulting improved system efficiency directly contributes to higher BEV driving ranges; however, the additional thermal inertia of the coolant circuit potentially diminishes the advantage. To limit the negative impact, the additional components need to be considered during the system design phase. This study presents a workflow where numerical tools are used to optimize the system design in order to minimize the impact of increased system inertia due to the secondary coolant circuit. For the optimization, short city drive-cycles are considered to represent the typical use case of a small electric vehicle. Different representations of the heat pump model have been tested to evaluate the impact on the dynamic model behavior.