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With carmakers increasingly investigating carbon fibre-based structural batteries as lithium-ion alternatives, the researchers at Drexel University in Pennsylvania developed the design optimisation system to maximise vehicle range and reliability.
Structural, or ‘massless’, batteries can be incorporated into the structure of a vehicle chassis to save weight, but they bring new challenges – “heat generation will be substantially higher in structural batteries in comparison with standard lithium-ion batteries,” said research leader Ahmad Najafi.
The conductivity of the polymer electrolyte used in structural batteries is much smaller than that of the liquid electrolytes in lithium-ion batteries, so electrons face more of a bottleneck as they move through the polymer, moving slower and generating more heat.
Drawing on years of experience developing composite materials for heat management, Najafi’s team modified a design tool to plot optimal ‘microvascular’ networks for cooling composites that could be used in structural battery packaging.
The system can calculate the best pattern, size and number of microvascular channels to quickly dissipate heat from batteries, the researchers said, as well as optimising the design for flow efficiency of the coolant moving through the channels. The method balances performance-enhancing factors such as battery capacity and conductivity against problematic variables including weight and thermal activity.
“These composites function something like a radiator in an internal combustion engine vehicle,” Najafi said. “The coolant draws in the heat and pulls it away from the battery composite as it moves through the network of microchannels.”
Sandwiching the structural batteries between layers of cooling microvascular composites could stabilise their temperature during use and extend their operational life, the team said.
The optimisation process considers several design parameters, such as thickness and fibre directions in each layer of carbon fibre, the volume fraction of fibres in the active materials, and the number of microvascular composite panels required for thermal regulation.
To test each combination, the group measured the stiffness of each structural battery-cooling composite laminate, to ensure they met vehicle structural integrity standards. Then they simulated the energy demand of a vehicle at various speeds over several minutes, while recording the temperature of the battery and the predicted range of the vehicle.
Computer models of one optimised system showed it could improve the range of a Tesla Model S by as much as 23%, the researchers said. But, they added, the real value of the work is its ability to determine the best combination of battery size and weight – including enough cooling capacity to keep it functioning – for any EV.
“While we know that every bit of weight saving can help improve the performance of an EV, thermal management can be just as important – perhaps more, when it comes to making people feel comfortable driving them,” said Najafi. “Our system strives to integrate improvements in both of these areas, which could play an important role in the progress of EVs.”
The research was published in Composites Part B: Engineering.
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