E-Mobility Engineering | January/February 2024 59 Immersive cooling | Deep insight Unlike oil or refrigerant fluids, the dielectric fluid is recyclable and can be filtered and reused for the lifetime of the pack. A key part of the design is to simulate the best pressure release valve. If the cells vent gas then the size of the valve needs to allow the gas to escape without letting the dielectric out. The pressure is used to drive the flow of the dielectric through the battery pack at around 3 Pa (three atmospheres). The pressure depends on the size of the module and the form of the cells, whether they are pouch, cylindrical or prismatic. “You want a minimum cell to cell distance but the best performance and not to destroy the thermal exchange and stop the flow of the fluid and you need to consider cell swelling in pouch,” said Ennever. Using pouch cells needs a support framework for the cells with channels between the cells for the fluid to flow, but this is lighter than a cold plate system that would have to be implemented every two cells. Another advantage is that immersion cooling also cools the busbars directly. Tab welding is the main hot spot, and the immersive cooling fluid can completely surround the busbars. This in turn allows for smaller, lighter busbars to be used. Ricardo has completed several immersion cooled battery projects for customers in both passenger car and heavy-duty truck markets. Dual phase immersive cooling Carrar and Röchling Automotive are developing the first EV battery modules with Two-Phase immersion thermal management. The partnership results in lightweight, fully-sealed battery modules built in plastic for simplified manufacturing. These can be built to fit any battery cell and every chemistry. “The Carrar thermal management solution prevents thermal runaway and greatly increases the safety of the batteries for electric vehicles,” said Marco Barbolini, global product manager, Röchling Automotive. “Keeping the battery temperature at the optimal range, Carrar also enables batteries to last twice as long, helping to cut back on battery production and its environmental impact.” “We treat everything in the system as an evaporator so the battery cells are the evaporator,” said Eitam Friedman, chief technology officer at Carrar. He explains that Carrar is using commercial refrigerant liquids as the cooling fluid, ones which have a low boiling point, and is adjusting that boiling point to match the ideal temperature of cells and between the cells, which cause slower degradation rates and increased pack life. The advantage of the immersion cooling is bringing components closer together to improve the volumetric density over other cooling approaches as the cooling fluid can be used with the battery pack, the power electronics and to cool the motor. “We can achieve increased volumetric power density because the battery cells can be packaged more closely together”, explained Bahareh Yazdani, Ricardo’s Technical Authority for batteries. “This is possible because the dielectric cooling fluid, passing between the cells, improves cooling performance. The improved cooling performance also enables downsizing of components like the busbar and removal of complex components like the cold plate. “These weight savings offset the increased mass of the cooling fluid and pump, meaning we can achieve a similar, or slightly improved, gravimetric power density too. It also helps us maintain or reduce the overall battery pack cost.” Using immersion cooling also extends equipment lifetime as it eliminates hot spots in the batteries since the dielectric fluid surrounds the cells. This allows higher performance with higher discharge and charging rates and avoids the risk of thermal runaway. That happens when a cell overheats and causes the neighbouring cells to heat up, potentially leading to a fire. “The dielectric is a good shield between the cells,” said Yazdani. “We have tried to instigate thermal runaway and we haven’t been able to.” The dielectric is an organic fluid based on esters and Ricardo worked with the fluid suppliers for compatibility of the fluid for the immersion system. “The viscosity, heat capacity and thermal conductivity all need to be optimised to give the best performance,” said Yazdani. “The viscosity is key for the definition of the pump.” The immersive cooling system developed for the IncoBat project (Image courtesy of Ricardo)
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