Assessment of Thermal Control Technologies for Cooling Electric Vehicle Power Electronics

The U.S. Department of Energy (DOE) FreedomCAR Program's technical targets for the electric traction system (power electronics and electric machines) of advanced vehicles require significant reductions in volume, weight, and cost while also meeting performance and 15 year life requirements [1]. The performance of the semiconductor switches and diodes, the ripple-current capability of the capacitors, and the life of the electronics all decrease as the operating temperature is increased. The current approach for cooling hybrid electric power inverters uses a separate cooling loop with water ethylene glycol coolant at approximately 70°C. This cooling system is used in hybrid electric vehicles (HEVs) in the market such as the Toyota Prius and Ford Escape. However, this approach is thought to be too large and costly to meet future vehicle requirements and achieve the kind of market penetration that would substantially reduce the United States' dependence on oil imports. A second approach that is being considered is to reduce the coolant system's cost by eliminating the separate cooling loop and cool the power electronics using engine coolant at approximately 105°C. This will require devices, such as trench or silicon-carbide (SiC) switches, that can operate at higher temperatures, as the ability to remove heat decreases as the temperature difference between the coolant and the electronic components decreases. While this solution is plausible, an optimized thermal control system design is required to overcome a number of technical barriers and meet the combined program requirements of performance, weight, volume, cost, life and reliability. The National Renewable Energy Laboratory (NREL) is conducting research and development on a number of thermal control technologies aimed at improving thermal performance while reducing the cost, weight, and volume of the system. These research areas include: Advanced thermal interface materials Direct backside cooling Single and two-phase jet and spray cooling Air cooling Thermal system modeling. This paper presents an overview of the research being conducted, with results from laboratory experiments and numerical modeling in each area.