System Characteristics of Direct and Secondary Loop Heat Pump for Electrical Vehicles

The electricity energy consumption for passenger cabin heating can drastically shorten the driving range for electric vehicles in cold climates. Mobile heat pump system is considered as an effective method to improve heating efficiency. This study investigates the system characteristics of mobile heat pump systems for electrical vehicle application. Based on KULI thermal management software, simulation models including HFC-R134a direct heat pump (DHP) and secondary loop heat pump (SLHP) were developed. The secondary loop employed in the SLHP includes a coolant pump, an indoor heater core and a plate heat exchanger, instead of an indoor condenser in the DHP. The use of a secondary loop has advantages to improve air outlet temperature uniformity. The simulation models were verified by measured data obtained from calorimeter experiments. By adopting simulation models, the effects of indoor and outdoor temperatures on system performance and cycle characteristics were discussed. Results show that the increase of indoor temperature will largely decrease the system efficiency, and varied outdoor temperature has a big impact on heating capacity. Then the comparison simulations between DHP and SLHP were conducted, to determine the effect of the secondary loop on heating performance. Results show that the use of a secondary loop is severely harmful to system efficiency, but has little effect on heating capacity. Furthermore, the impact of coolant flow rate on the capacity and COP was discussed, and heat transfer effectiveness of heater core and plate heat exchanger were also evaluated.