22 November/December 2023 | E-Mobility Engineering between the inlet and the outlet is an order of magnitude lower than that in normal sidewall cooling ribbons or cold plates. That enables much higher flow rates than typical cooling systems, and hence more even temperature gradients across modules and packs, which in turn improves performance and lifetimes while reducing total cost of ownership. Dr Flannery also notes that conventional aluminium cooling plates use a lot of thermal interface materials (TIMs), which while more conductive than some other plastics have around 1/100th the conductivity of aluminium. That is partly down to the thickness of the TIM, which is a necessary evil for preventing the aluminium from shorting the battery cells. “Skip the metal entirely and you save considerable weight, and if you use a plastic that’s incredibly thin, rather than the very thick TIM plastics, you maximise thermal conductivity, because thickness is the dominant factor in preventing heat transfer,” Dr Flannery says. “And our ducts are also inflatable and pliable, so once they’re pressurised during assembly and in normal operation, they fill against the curved cylindrical cell walls for perfect angular contact with every cell in the pack. Achieving consistently close physical contact between coolant and cell is a key design challenge – even Tesla’s approach of glueing 4680 cells to an epoxied ‘bandolier’ has struggled to achieve that kind of contact in the Model 3, Y and Cybertruck packs.” The ducts’ mechanically simple nature and close cell contact also make for an inherently open structure inside the modules. No ribs or multi-lumen tubing extrusions are needed, as with cooling systems based on metal pipes and cold plates; this contributes to the minimal pressure drop. “The rate of pressure drop in conventional systems increases as a quintic function of coolant inlet and outlet sizing,” Dr Flannery comments. “Halve your orifice diameters and you can get 32x the pressure drop, so having an open structure is critical from a thermal management perspective. “Your goal isn’t turbulent flow and high heat transfer, it’s a high flow rate with even heat transfer, so you hold the temperature across a pack within a 1 oC envelope for a given discharge rate. So maximising mass flow rate and minimising pumping head losses is what we believe to be the best design trade-off for a pack.” The larger Hibernium packs therefore achieve water-glycol flow rates of up to 300 litres/minute, around 10 times that of comparable 290 kWh packs, owing to the minimal pressure drops of Xerotherm. Xerotherm was originally inspired in about late 2017 by a computer antistatic bag, and Dr Flannery began prototyping it with flat polyester sheets, their edges welded lengthwise before being joined to additional plastic assemblies. Eventually, he and his team moved to a blown film extrusion process, a more conventional and technologically mature approach for making thin-film plastic ducts, and gradually optimised the material’s thickness, doping, welding and other qualities. “Development has generally been experimentally led, but the entire cooling system has been replicated in CFD, where we’ve used a combination of tools to optimise the duct design,” Dr Flannery says. “A lot of the original work was done in Ansys Fluent, with structural design simulated in Ansys Mechanical as well as Altair SimSolid. We’d validate that back against a great deal of physical testing, along with some 1D and 2D modelling in MATLAB Simulink as well as straightforward spreadsheets.” Fire and electrical safety The structural, insulating and fireretardant foam encapsulating the cells is a polyurethane that expands following extrusion, enabling wide volume filling with a material density equivalent to about 15% that of Xerotech uses only 21700 cylindrical cells for their thermal runaway safety advantage over pouch and prismatic cells, but it might also use 4680 cylindrical cells in the future An overhead illustration showing a thin-film water-glycol duct between two rows of cells, and the polyurethane foam encapsulating them
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