63 packs are thermally managed using water-glycol-based coolants housed within channels in a baseplate that regulate the temperature of the cells. The general belief in the industry is that immersion fluids are much more viscous than water-glycol. That was true for the first generation of Castrol’s ON EV thermal fluids. However this has evolved with the secondgeneration immersion fluids exhibiting similar viscosity to water-glycol fluids at 30 C at 2.9cSt compared with 2.7cSt for 50:50 water-glycol. The SI unit for kinematic viscosity is one meter squared per second and is equivalent to 10,000 stoke (St). Usually, centistokes (cSt) is used, where 1 cSt = 1 mm2/s. This decrease in viscosity reduces the pump power required in an immersioncooled battery system, increasing efficiency. Below 30 C, which is key for immersive cooling, the secondgeneration EV thermal fluid has even lower viscosity than water-glycol, for even higher pump efficiency. The analysis in Graz focused on identifying the optimal cell cooling. That included whether it is better to have an increased cooling area but lower thermal properties of the fluid with immersion cooling, or a lower cooling area and higher fluid thermal properties with indirect cooling. VVR also looked at the extra benefits that can be derived from cooling busbars and how the thermal environment affects the electrical performance and aging of the cells, and whether fluid flashpoint is related to safety performance. To support this research, immersion and indirect mini-modules were designed and constructed with seven nickel-cobalt-aluminium (NCA) 21700 cylindrical cells. The immersioncooled mini-module used direct busbar cooling and was designed and optimised with computational fluid dynamics (CFD) for heat transfer and minimised pressure drop. The mini-modules were then put through performance and durability testing to analyse thermal and electrical behaviour, as well as being subjected to thermal propagation testing. Performance and durability test conditions were created to replicate realistic fast-charging scenarios, representing a 10-90% charge at a 3C charging rate, equivalent to just under 20 minutes of charge time. The analysis found that immersion cooling improved the thermal gradient and peak temperature of cells under fast charging, minimising the surface cell thermal gradient to under 1.5 C. Evaluation of durability identified that immersion cooling can extract more performance from cells. While the immersion-cooled module received 25% higher capacity through constant current charging than the indirectly cooled module, it had a 16% lower degradation rate despite the higher electrical stress. Significantly, Castrol and VVR expect this difference to increase alongside an increase in charge rate. To gauge the safety of immersion and indirectly cooled systems, Castrol and VVR designed a series of thermal runaway tests. These tests were performed in a sealed chamber with limited oxygen to enable the observation of any failure, as well as to ensure that the module was not fully destroyed. A nail penetration method was used to initiate the thermal runaway of the central cell. Thermal runaway can lead to thermal propagation, where heat E-Mobility Engineering | January/February 2024 Immersive cooling | Deep insight A single phase immersive cooling system (Image courtesy of Mersiv) A dual phase cooling module for commercial vehicles (Image courtesy of Carrar)
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