64 January/February 2024 | E-Mobility Engineering from a damaged cell is transferred via convection and conduction to adjacent cells, a process that can potentially lead to catastrophic failure of the entire battery pack. The immersion-cooled module proved effective in mitigating thermal propagation even though it was not optimised for safety. Heat dissipation and not the flashpoint of the fluid determines safety in immersioncooled systems. The effective heat dissipation comes from a combination of vent-gas management and the fluid conducting heat away from the damaged cell. The higher performance enabled by the immersion cooling does not come at the cost of capacity. The research suggests that in extrapolating the data to higher energy density cells and higher charging rates, the differences would become even more pronounced between immersion and indirect systems. Dual phase immersive cooling tests Tests at the Technical University of Munich with cylindrical NMC lithium ion battery cells with 2C charging and discharging rates (2C/2C) showed that the dual phase immersive cooling system developed by Carrar allowed the pack to operate for 2400 cycles until it reached the 80% of capacity that is usually regarded as the end of life for such a pack. This compares to 600 cycles for the NMC cells using liquid cooling. The cells also stayed above 90% for 2200 cycles. That compares to dropping below at 400 cycles with a liquid cooled system, which is important for the second hand market notes Friedman. He is less concerned about outgassing from the cells. “If we have outgassing from the cell it’s an extreme event and you need to take the car to the garage. We have shown in tests we can prevent thermal runaway as we can dissipate more heat than the cell is generating. And if there is runaway we don’t get thermal propagation. That data comes from the voltage data from the BMS.” Hydrogen The use of hydrogen for power systems also opens up opportunities for more integrated cooling systems. The cryogenically cooled hydrogen can be used as a coolant for the power electronics, the battery pack and the motor before being used in a fuel cell to generate power. Ricardo also has a research project on cryogenic cooling. “We see the opportunity of using this as a way to achieve high efficiency in component performance,” said Ennever. Hydrogen fuel cells are more particular about leakage in the hose portion, so hard tubing tends to be used, says St John at Danfoss. “Again, for hydrogen it is cleanliness that is the issue and that means we will use stainless steel.” Conclusion Single and dual phase immersive cooling can provide advantages for e-mobility powertrain designs in many ways. With a similar level of complexity as today’s liquid cooled systems, immersive cooling can keep battery cells within one or two degrees of the optimum operating temperature of 25 C. This enables faster charging – at 3C and above – removing the heat generated and ensuring the charging does not damage the cells. This also supports higher discharge rates to provide more power for construction and mining equipment. The stable temperature of immersive cooling supports safer operation, eliminating the issue of thermal runaway. It also leads to a significantly longer lifetime for the cells, boosting the secondhand value of vehicles, while reducing the number and weight of the components for the cooling system. Deep insight | Immersive cooling Testing thermal runaway in an immersion cooling system (Image courtesy of Castrol)
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