Electric vehicles help keep cool during the Battery Weight Balancing Act

Electric vehicles help keep cool during the Battery Weight Balancing Act

Drexel researchers have developed a system for optimizing battery capacity, weight and heat management in electric vehicles.

Putting enough energy into a battery to power a car puts a lot of strain on the storage devices that for the past century have been primarily tasked with running small appliances and electronics. The stress starts to get to them – manifested in breakdowns, decreased performance and even meltdowns. Drexel University researchers are trying to help by taking some of the literal heat from batteries and charting a more sustainable route for use in electric vehicles.

In a recently published article in the magazine Composites Part B: Engineeringresearchers led by Drexel’s Ahmad Najafi, PhDan assistant professor in the Technical University, unveiled 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 method balances performance enhancing factors, such as battery capacity and conductivity, against problematic variables such as weight and thermal activity, which can degrade performance and cause failure, to provide the best battery pack specifications for any electric vehicle design.

“One of the major barriers to developing EVs, and consequently increasing 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.

Versatile batteries

While demand for electric vehicles has been fueled by increasingly looming air quality and climate change concerns and rising gas prices, the market has been dampened in the past year by a number of high-profile electric vehicle recalls that have cited durability and safety. of their batteries in doubt.

As a result, more companies are looking to use solid batteries — a thin, carbon fiber-based version of the larger lithium-ion batteries commonly used in electric vehicles — because they can get smart. integrated into the physical structure of the vehicle chassis as a way to save weight.

Reducing the weight of a car by as little as 10% can increase the efficiency of the mileage between charges by as much as 6-8% by some estimatesso replacing parts of the car frame with a carbon fiber composite that works both as a structural part and as a battery, could reduce the overall weight of the vehicle and improve its energy storage capacity.

Heat up

For these structural or “massless” batteries to succeed, designers must meet a challenge arising from using a solid polymer — rather than a liquid electrolyte solution — as the medium for electron transit.

“The heat generation will be significantly higher in structural batteries compared to standard lithium-ion batteries,” explains Najafi, 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 are forced to move more slowly and therefore generate more heat as the battery discharges its energy.

“While structural battery composites are a promising technology for reducing weight in electric vehicles, their design could certainly benefit from the addition of a thermal management system,” said Najafi. “Not only would this improve the range of the EV, but it would also significantly reduce the chance of a thermal runaway response.”

stay cool

Najafi’s research group has been developing special composite materials for heat management for a number of years. Their work relies on nature’s own cooling method – the vascular system – to dissipate heat. By adapting a design tool they invented to map 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 like Tesla, Volvo and Volkswagen.

The design system, presented by Najafi’s team in their latest study, can calculate the best pattern, size and number of microvascular channels to quickly dissipate heat from the batteries, and optimize the design for the flow efficiency of the refrigerant passing through the channels. moves.

“These composites function much like a radiator in a vehicle with an internal combustion engine,” Najafi said. “The refrigerant attracts the heat and pulls it away from the battery composite as it moves through the network of microchannels.”

Placing the structural batteries between layers of cooling microvascular composites can stabilize their temperature during use and extend the time and power range in which they can function.

The right fit

As mentioned in the article, the team’s structural battery optimization process takes into account several design parameters, such as thickness and fiber directions in each layer of carbon fiber, volume fraction of fibers 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 the vehicle’s structural integrity standards. They then simulated the energy demand of a vehicle at different speeds over a period of several minutes, while recording the temperature of the battery, the predicted range of the vehicle.

According to the study, computer models of one optimized system showed that it could improve the driving range of a Tesla Model S by as much as 23%. But the team notes that the real value of their work is the ability to obtain the best combination of battery size and weight — including enough cooling capacity to run it — for any electric vehicle in production now and for future designs.

“While we know that every bit of weight savings can help improve the performance of an EV, thermal management can be just as important — perhaps even more important, when it comes to making people feel comfortable while driving,” Najafi said. “Our system aims to integrate improvements in both areas, which can play an important role in the advancement of electric vehicles.”

In addition to Najafi, Reza Pejman and Jonathan Gorman, graduate students of the College of Engineering, took part in this study. Read the full paper here: