Immersion cooling has emerged as a highly promising technology for efficiently managing the thermal demands of battery packs in electric vehicles. In principle, this approach involves the use of a fluid in direct contact, leading to reduced surface-to-surface thermal resistance and improved heat exchange performance. While water-based coolants are known to possess superior thermal conductivity and heat capacity, they cannot be applied in immersion cooling due to their electrical conductivity. Dielectric fluids can help address safety concerns, specifically those related to thermal runaway events and overheating. Moreover, immersion technology usually has a simpler design than indirect solutions. The scope of this study involves conducting a series of experimental tests to evaluate the cooling capability and thermal runaway prevention effectiveness of different dielectric fluids under various scenarios. In order to evaluate the cooling capabilities of the dielectric fluids under various conditions, we collaborated with battery systems providers to devise a comprehensive test plan. This test plan covered parameters such as charging and discharging rates, various fluid inlet temperature, variable flow rate, different initial temperatures, as well as different fluid formulations. Two distinct battery modules were used, one featuring prismatic cells and the other employing 21700 cylindrical cells. Moreover, to assess the thermal runaway prevention performance of the fluids, we conducted nail penetration tests. Temperature sensors were installed on the battery modules to measure the temperatures of the affected cell, its surroundings, and the fluid temperatures around the module. The testing program included a range of cell configurations, including single-cell, 6-cell cluster, and full module behavior testing. Interesting results have been obtained from the cooling and abusive tests conducted. Various fluids with viscosity levels ranging from 2.5 to 8.2 cst (measured at 40°C) and density levels ranging from 770 to 850 kg/L were tested. The cooling performance tests revealed that variations in fluid formulations had a clear impact on the pressure drop within the system. Of particular interest was the fact that identical formulations displayed different factor pressure drop disparities when tested across different modules. When compared at the same flow rate, different formulations resulted in similar temperatures throughout the battery module. We can explain those behavior relying on fluid dynamics and heat transfer principles. Continuous cycles of fast charge and discharge were achievable with great steady-state repeatability. The thermal runaway tests produced results indicating that utilizing fluids is an efficient means of dissipating heat in comparison to a non-immersive setup. Despite the obvious benefits of using fluids for heat dissipation, there is still some ambiguity regarding how to compare the performance of various fluid formulations. This study has also attempted to identify the underlying reasons for this difficulty. This study includes various configurations and architectures, but its validity is limited to this scope. Battery systems currently don’t have a standardized design, and each battery system designer employs its own technology that can significantly differ from one another. The diverse cell chemistry used can also result in different behaviors during thermal events. We have taken a further step in thermal runaway testing. Instead of utilizing it solely to validate the safety of one fluid for immersion cooling purposes, we have employed it as a tool for research. This approach has led also to the development of customized immersive rig testing, as well as an instrumented and monitored testing electric vehicle. In conclusion, this study shows the performance of immersion cooling and how the thermal runaway test alone should not be employed for fluid certification purposes even if no thermal propagation is observed. Rather, they should be utilized as a means of understanding the behavior of an immersive system and how different fluids react under those circumstances. Given the wide range of systems currently available, it isn’t easy to generalize trends. However, by undertaking such a study, we can identify differences within the same framework and choose the best fit. We intend to advance this research by testing instrumented cells to monitor core-cell temperatures, more severe thermal events, fluids with more pronounced physicochemical differences, and increasingly controlled conditions.

Mr. Rodrigo AMORIM DiaS, Dielectric Fluids Engineer Formulator, Motul