Innovations in thermal management systems for EVs
Electric vehicles (EVs) are rapidly emerging as a central part of a future greener and more economical transportation ecosystem. The benefits of EVs include emission-free driving, cheaper maintenance, and enhanced energy security. According to a report from the Argonne National Laboratory, more than 1 million plug-in electric vehicles (hybrid and battery EVs) were sold from 2010 to 2018 in the United States, with a reduction in gasoline use by 950 million gallons and CO2 emissions reduction of 4.5 million metric tons.
Today, EVs use large packs of Li-ion batteries due to their reasonable energy density, power density, and efficiency metrics. During the battery charge/discharge process, current flows within the cells; and depending on the internal resistance, different levels of heat can be generated (especially during rapid charging or extreme driving conditions). Although there is a significant development in the range per charge (up to 180 miles in 15 minutes in the Tesla Model 3), recharging the battery pack could take several hours to replenish fully.
Moreover, Li-ion batteries require an optimum temperature to perform efficiently without any internal and external thermal instability. Temperatures less than 5°C or more than 50°C can lead to significant battery degradation and thermal runaway. A publication in Energy Reports stated that the best operating temperature for Li-ion batteries is 25-40°C, with an internal temperature difference within 5°C. Therefore, a proper battery thermal management system (BTMS) is crucial.
BTMS development for EVs accounts for several aspects: cell temperature difference, rise in temperature, energy consumption by the coolant, and the additional weight and size of the cooling system. Different well-studied cooling methods include phase change materials (PCMs), cooling fins, air cooling, and liquid coolants.
PCMs are typically solids that function by absorbing the generated heat and thereby changing the phase to liquid. Because of the significant volume change of the materials and their inability to transfer the heat, PCMs are not favorable for EVs. A detailed comparison study showed that indirect liquid cooling systems display the lowest maximum temperature rise and are therefore more practical to use in EVs. While fin cooling systems make the battery pack heavier (up to 40% extra weight), air cooling systems consume 2-3 times more energy to maintain the same average temperature.
Liquid coolants are typically mixtures of water and petroleum-based products (such as propylene glycol), which have higher heat capacity, low freezing points, and better thermal conductivity. Notably, extensive research on the direction of cooling with liquid coolant showed that tab cooling is more efficient and can extend the battery life by three times that of surface cooling. However, the engineering of tab cooling is complex.
The liquid cooling system can be direct or indirect. Indirect liquid cooling typically involves circulating a water-glycol mixture through tubing around the battery pack. Several current models (such as Tesla, Chevy Volt, BMW i-3 and i-8, Jaguar I-pace) use this system. The Tesla Model 3 uses a machine assembled tubing (also called “bandolero”), which has significant improvements over the previous models. Audi e-tron has a more sophisticated cooling system for maintaining an optimum battery temperature and cooling the motor parts. It uses 5.8 gallons of coolant circulating through a 40 meter tube in combination with a typical heat pump. The Chevy volt cooling system comprises rectangular aluminum cooling plates (containing coolant paths), which are stacked between the battery packs as shown in the animation below.
In the direct cooling system, the battery is in direct contact with the liquid coolant. Therefore, the coolant must be nonconducting to prevent any electrical hazard from the cell current flow. Although there are no EVs on the market that use the direct cooling system, innovative approaches are emerging rapidly with practical testing. XING mobility has launched an immersion cooling technology by sinking the battery pack into a nonconducting liquid that has a high boiling point (3M Novec 7200 engineered fluid). Additionally, their research team is developing modular battery systems for different manufacturers.
M&I Materials, operating under the UK government’s Faraday Battery Challenge, is leading the i-CoBat project that features cooling through immersion into a biodegradable dielectric fluid named MIVOLT. Extensive optimization research is ongoing in collaboration with the Warwick Manufacturing Group (WMG). Prof. David Greenwood of WMG says, “It’s no longer just a matter of keeping the battery cool: it’s about optimizing the temperature for any given operation.”
While robust battery development for diverse conditions is rapidly surging, coolant technology requires further attention to employ effective thermal management with higher efficiency. Besides, simulation techniques could also be highly useful for engineering integrated EV car battery packs with an advanced cooling system.
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