Researchers at Drexel University in Philadelphia have created a system for optimising the battery capacity, weight and heat-management demands of electric vehicles.
Packing enough energy into a battery to power a car puts a lot of pressure on storage devices that, for the past century or so, have mainly been used to run small appliances and electronics, leading to malfunctions, diminished performance and even meltdowns.
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In response, the Drexel team has created a design-optimisation system for incorporating a blood-vessel-like cooling network into the packaging of a new generation of carbon-fibre-based batteries used in EVs. Their method balances performance-enhancing factors, such as battery capacity and conductivity, against problematic variables, including weight and thermal activity, that can sap performance and cause malfunctions, to provide the best battery package specifications for any EV design.
‘One of the primary hindering factors in the development of EVs, and consequently expanding their market share, is that the specific energy of batteries is low, which makes EVs heavy, especially for a long-range design,’ the researchers wrote in apaper published recently in Composites Part B: Engineering.
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Recent high-profile EV recalls have called into question the durability and safety of their batteries. Consequently, companies are increasingly looking to use solid batteries – thin, carbon-fibre-based versions of the larger lithium-ion batteries widely used in EVs – because they can be cleverly incorporated into the physical structure of the vehicle chassis as a way to cut weight.
Trimming the weight of a car by just ten per cent can boost its mileage efficiency between charges by as much as six to eight per cent according to some estimates, so replacing portions of the car frame with a carbon-fibre composite that functions both as a structural component and as a battery, could reduce the overall weight of the vehicle while also improving its energy-storage capacity.
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In order for these structural, or ‘mass-less,’ batteries to succeed, designers must confront a challenge that arises from their use of a solid polymer, rather than a liquid electrolyte solution, as the medium for electron transit. Because the conductivity of the polymer electrolyte is much smaller than that of the liquid electrolytes used in lithium-ion batteries, ‘Heat generation will be substantially higher in structural batteries in comparison with standard lithium-ion batteries,’ said Ahmad Najafi, an assistant professor in Drexel’s College of Engineering. This means that electrons face more of a bottleneck as they move through the polymer; they’re forced to move more slowly and, as a result, generate more heat as the battery discharges its energy.
‘While structural battery composites are a promising technology for reducing weight in electrical vehicles, their design could certainly benefit from the addition of a thermal-management system,’ Najafi said. ‘Not only could this improve the range of the EV, but it would also greatly reduce the chances of a thermal runaway reaction.’
The work of Najafi’s research group draws on nature’s own cooling method – the vascular system – to dissipate heat. Modifying a design tool that they invented to plot the optimal ‘microvascular’ network, the researchers were able to design cooling composites that would work as part of the structural battery packaging currently being tested by companies such as Tesla, Volvo and Volkswagen.
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The design system can calculate the best pattern, size and number of microvascular channels to quickly dissipate heat from the batteries, as well as optimising the design for flow efficiency of the coolant moving through the channels. ‘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 can stabilise their temperature during use and extend the time and power range in which they can function.
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The structural-battery-optimisation process considers several design parameters, such as thickness and fibre directions in each layer of carbon fibre, 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 in order to ensure that they met standards for vehicle structural integrity. They then simulated the energy demand of a vehicle at various speeds over several minutes while recording the temperature of the battery, then predicted range of the vehicle.
The results suggested that the optimisation system could improve the driving range of a Tesla model S by as much as 23 per cent. However, the team noted that the real value of their work is its ability to glean the best combination of battery size and weight – including enough cooling capacity to keep it functioning – for any EV currently in production or any future designs.
‘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,’ Najafi said. ‘Our system strives to integrate improvements in both of these areas, which could play an important role in the progress of electric vehicles.’