The B2B platform for full-electric and plug-in hybrid electric vehicles: Industry News

Battery cooling and thermal management are important aspects in designing advanced electric drive systems such as hybrids, plug-in hybrids and battery electric vehicles and are critical in terms of safety, reliability, performance, and passenger comfort. Two researchers from the US Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) advance system integration as a possible solution.

The operation of a vehicle requires a careful balance between different systems in the vehicle that might need depending on their mode of operation either heating or cooling or specific temperatures for optimal efficiency. With increasing variety in vehicle propulsion, thermal management systems cannot longer be tailored for vehicles with combustion engines only but must be flexible enough to serve different vehicle packages. However, an increasing number of components that need active thermal management, also augments cost, weight and size.

Thermal management systems impact on the vehicle's overall efficiency through the reduction of thermal load, efficient heat transfer and reuse of available waste heat. Kevin Bennion and Matthew Thornton, researchers at NREL, examine the integrated heat loads of combined systems and apply the elaborated techniques to electric drive systems that operate over transient thermal duty cycles in which peak thermal loads can occur (in contrast to steady-state duty cycles where peak loads normally do not occur).

How much heat is in the system?

Bennion & Thornton propose in their paper that they are going to present on the SAE 2010 World Congress in April, to use the generated heat load curve to evaluate the heat loads in integrated electric vehicle systems (the electric drive system included) that function mainly with transient duty cycles. The work compares opportunities to create an integrated low temperature coolant loop combining the power electronics and electric machine with the air conditioning system in contrast to a high temperature system integrated with the internal combustion engine (ICE) cooling system. They find the low-temperature integrated system to be a promising approach.

The researchers selected representative vehicle configurations with similar performance characteristics in order to ensure fair comparison across different power-train configurations. This selection also allows the comparison of heat loads across components and vehicle propulsion types. In a second step, the techniques for evaluating the transient and continuous heat loads were applied to the electric drive thermal management system. Finally, thermal and fluid system models were developed along with a heat exchanger sizing model for a louver fin heat exchanger to evaluate the viability of alternative concepts.

Bennion & Thornton identified two crucial key components for an integrated thermal management system to work:

• a similar coolant temperature specification;
• a misalignment of peak heat loads of the combined or integrated systems;

Heat load calculations

The heat load for integrated thermal management systems does not forcibly correspond to the sum of the peak or continuous heat loads from the combined systems. Depending on their use, different components experience peak heat loads at different times. Misalignment of the peak heat loads can potentially cause a decrease in the net heat exchanger weight and volume. The ability of the heat load curve to illustrate both the transient and continuous heat loads was estimated useful for evaluating the impact of combining multiple systems onto the same thermal management system where transient and continuous loading conditions are important.

The authors applied their techniques to the electric drive thermal management system in order to explore the possibility of integrating it with the ICE and air-conditioning thermal management systems. The high temperature thermal management system integrating the electric drive and ICE systems, for example, seemed a promising application for PHEV configurations. High temperature systems are, however, generally problematic for electric vehicles.

The low temperature thermal management system, which integrated the electric drive system with the vehicle air conditioning system, showed similar coolant temperature requirements. Furthermore, the authors found that the misalignment of the peak heat loads was also possible through control of the AC system operation and the lack of electric drive system heat when the vehicle was idle. Bennion & Thornton conclude that while there appear to be synergies possible related to temperature and heat loading, further analyses are required.