40 January/February 2024 | E-Mobility Engineering The system is engineered to reduce the temperature difference between the boiling point and the cell, by changing the pressure to adjust the boiling point. The aim is to achieve a boiling point within one or two degrees of the operating temperature of the cell of around 25 C. The heat from the cells boils the PCM, generating gas to remove the thermal energy from the cells. This gas circulates through the system to a condenser, where the heat is removed and the gas liquifies and is recirculated through the battery pack. The condenser is typically four times more efficient than a radiator, so a 30 to 40 kW condenser supports high C rates of 2C or 3C continuously. Battery pack The biggest reservoir is the battery modules, with a volume of typically 20 litres for a 100 kW pack and 2 litres in the rest of the system. A condenser measuring 60 x 40 cm and 30 mm capacity thick can dissipate 25 kW while keeping the batteries at 25 C. This is typically, and deliberately, the same size as a radiator for a traditional liquid cooled system so there is no additional space required to implement the liquid PCM system. Keeping the cells at a constant temperature also extends the lifetime and the usable capacity of the battery pack. In tests with cylindrical NMC lithium ion cells charging and discharging at 2C using a liquid phase change system, a 100 kW pack took 2400 cycles before reaching 80% capacity, compared to 600 cycles for a liquid cooled pack. The pack also stays above 90% capacity for 2200 cycles rather than dropping below 90% at 400 cycles. This is important for the secondhand market as the battery pack with PCM cooling would last several years longer than one without. The system has also been tested with prismatic cells to over 1000 cycles and with lithium iron phosphate (LFP) cells. Next generation dual phase liquids However the dual phase liquid suppliers are targeting a temperature of around 50 C as the sweet spot to cover the rest of the system, such as inverters and power modules. The current dielectric fluids in development such as 2P50 have a 50 C boiling point and a low global warming potential figure of 10 for the environmental impact with zero ozone depletion potential (ODP). The proprietary hydrofluoroolefin (HFO) dielectric fluid is intended for commercial launch in 2025, doesn’t have a flash point and can be classified as a flame suppressant. The aim is that this can be used for a lot of Focus | Phase change materials Testing different phase change materials in thermal runaway for an electric boat (Image courtesy of Beam Global) Testing PCM cooling cells amperemeter K-thermocouples PCE-T1200 T data logger PC/laptop Arduino Uno V data logger INA219 21700 battery module dummy load
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