A lot of pressure is being placed on storage technologies that, for the past century or more, have primarily been used to power small appliances and electronics in order to fit enough energy into a battery to power an automobile.
They are showing signs of stress, like malfunctions, poor performance, and even meltdowns. By removing some of the physical heat from batteries and planning a more sustainable path for their use in electric vehicles, researchers at Drexel University are attempting to assist.
In a recently published paper in the journal Composites Part B: Engineering, researchers led by Drexel’s Ahmad Najafi, PhD, an assistant professor in the College of Engineering, revealed a design optimization system for incorporating a blood vessel-like cooling network into the packaging of a new generation of carbon-fiber based batteries used in electric vehicles.
Their approach provides the ideal battery package parameters for each electric vehicle design by balancing performance-improving elements like battery capacity and conductivity against problematic elements like weight and thermal activity that can degrade performance and lead to malfunctions.
“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 authors wrote.
Multifaceted Batteries
The market has been restrained by a number of high-profile electric vehicle recalls over the last year that have called into question the longevity and safety of their batteries, even as the demand for electric vehicles has been stoked by increasingly pressing concerns about air quality, climate change, and rising gas prices.
Because solid batteries can be cleverly incorporated into the physical structure of the vehicle chassis as a way to cut weight, more businesses are considering using them in place of the heavier, thicker lithium-ion batteries that are frequently used in electric vehicles. These solid batteries are thin, carbon fiber-based versions of those batteries.
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. Our system strives to integrate improvements in both of these areas, which could play an important role in the progress of electric vehicles.
Ahmad Najafi
According to some estimates, reducing a car’s weight by just 10% can increase its range between charges by as much as 6-8%. Therefore, replacing some of the car’s frame with a carbon-fiber composite that serves as both a structural element and a battery could both lower the overall weight of the car and increase its energy storage capacity.
Heating Up
The use of a solid polymer as the medium for electron transit rather than a liquid electrolyte solution presents a difficulty that designers of these structural, or “mass-less,” batteries must overcome in order for them to be successful.
“Heat generation will be substantially higher in structural batteries in comparison with standard lithium-ion batteries,” Najafi explained, because the conductivity of the polymer electrolyte is much smaller than that of the liquid electrolytes used in lithium-ion batteries. This means that electrons face more of a bottleneck as they move through the polymer; they’re forced to move slower 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.”
Staying Cool
Najafi’s research group has been developing special composite materials for heat management for a number of years. Their work draws on nature’s own cooling method the vascular system to dissipate heat.
The researchers were able to create cooling composites that would function as a component of the structural battery packaging now being evaluated by businesses like Tesla, Volvo, and VW by modifying a design tool they created to draw the ideal “microvascular” network.
The design approach, which Najafi’s group has now demonstrated, can determine the ideal pattern, size, and number of microvascular channels to quickly drain heat from the batteries while also optimizing the design for coolant flow efficiency.
“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.”
The time and power range in which structural batteries can operate can be increased by sandwiching the batteries between layers of cooling microvascular composites.
The Right Fit
According to the paper, the team’s structural battery optimization process takes into account a number of design factors, including the volume fraction of fibers in the active materials, the thickness and fiber directions in each layer of carbon fiber, and the quantity of microvascular composite panels needed for thermal regulation.
The team assessed the stiffness of each structural battery-cooling composite laminate before testing each combination to make sure it complied with vehicle structural integrity criteria. The energy requirement of a car was then simulated over a period of several minutes at various speeds, while the temperature of the battery and the anticipated range of the vehicle were being recorded.
According to the research, computer models of one optimized system showed that it could improve the driving range of a Tesla Model S by as much as 23%. The actual significance of the team’s study, however, is in being able to determine the ideal balance between battery size and weight, as well as the sufficient cooling capacity to keep the battery functional for any electric car currently in production and 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.”
