25 H2D2 snow groomer | Dossier snow groomer has the added advantage of regenerative braking. “If the snow groomer is moving down a 45° slope, and it happens to be a long slope, we must have a large and open bank of electrical energy storage, such that the regenerative braking has somewhere to put that recovered energy. We calculated the best way to maximise the autonomy of the vehicle is to have those 50 kg of H2 for powering travel up difficult uphill slopes, and a pack with space for at least 80 kW/h of recovered energy,” says Santiano. Lastly, having a large battery makes for a useful source of backup energy if the fuel cell suffers a fault mid-operation, so the driver can return for maintenance and repair on battery energy alone. Immersion-cooled cells To achieve the energy density needed for a pack of at least 80 kW/h onboard, GCK Battery chose cylindrical NMC 21700 cells, and to aid the power efficiency needed for fast recharging of the pack (either by regenerative braking down slopes or via the fuel cell), an immersion-cooling system has been engineered to enhance the battery’s thermal stability. “We wanted cylindrical cells so that the modules and pack could integrate our immersion-cooling system; pouch or prismatic cells would not have functioned so well in our dielectric oil,” Santiano says. “The 21700 cell is the most common and standard type, so it made the most sense to pick that one. We also found NMC to be the most energy-dense of the commercially available cathode chemistries, and we use a nickel-rich cathode – NMC 811, specifically – to maximise that energy density. We can’t go too much into the design of the module, but I can tell you they’re constructed in our automated facilities and connected using automatic resistance welding.” Each module is a 12s16p system enclosed in a housing that is IP68-rated for protection against dust and water ingress, while being IP2X-rated against penetration by solid foreign objects of 12.5 mm diameter or greater. The housing has also achieved UN ECE R100 certification for use in road vehicles. Immersion cooling uses a bath of oil that surrounds every exposed part of every cell and flows through the modules (at an undisclosed flow rate, but significantly lower than in typical water-glycol heat-extraction systems). The oil is a dielectric fluid, blended and engineered by GCK Battery through close collaboration with Motul to achieve a few critical performance characteristics. “The first characteristic is that it must be electrically insulative, otherwise it will represent a short-circuiting hazard, and it must have a high coefficient of thermal conductivity, oriented around keeping the cells at their optimal 20-25 C window for recharging and discharging, and moreover preventing extremes of temperature that will decrease their lifespan,” Santiano says. “The oil needs to have a very high flashpoint, as stable performance depends on it always being a liquid and never any other state of matter. A pump sits outside the battery for circulating the oil, and a radiator and fan are installed outside the vehicle chassis – typically out of the way, above the fuel cell – to cool the oil as it flows outside of the pack.” As to the flow rate of oil through the modules, Santiano says the oil must not move too quickly as effective heat extraction requires some prolonged exposure between the oil and the walls of the cells, and considerable CFD simulation went into studying the ideal oil-flow dynamics and flux for optimising thermal transfer. The pack has been designed for C-Rates up to 10 in continuous charging and discharging, which immersion cooling is critical towards. It is also E-Mobility Engineering | May/June 2024 21700 NMC cells have been integrated into the modules; specifically, cells with the NMC 811 cathode type for high energy density (Image courtesy of Groupe GCK)
RkJQdWJsaXNoZXIy MjI2Mzk4