Are electric VTOL aircraft the future of urban mobility? It all depends on the batteries
“The transportation sector is responsible for a significant portion of greenhouse gas emissions,” said Partha Mukherjee, professor of mechanical engineering, whose Energy and Transport Sciences Lab studies the degradation, safety, and performance of lithium-ion batteries and advanced battery chemistries and energy storage solutions. “A large portion of this comes from medium-range flights, which account for the highest emissions per kilometer of distance traveled. If we could replace some of these with electrified urban aviation options, that would be a huge step forward to reducing our carbon footprint. The eVTOL has the potential to be that sweet spot of urban mobility.”
But you can’t just take a normal drone and supersize it. Every increase in size or weight of the vehicle also increases demand on the batteries, which in turn affects its speed, range, efficiency, and safety.
“Lithium-ion batteries are a mature technology, but their use in electric aviation can be challenging,” said Abhinand Ayyaswamy, a Ph.D. student working with Mukherjee. “Many companies have produced promising prototypes, but they will need to undergo a rigorous verification phase to ensure the performance and safety of their batteries.”
The Purdue study focuses on three potential scenarios where eVTOLs may be most useful: short trips for urban mobility (less than 25 miles); medium trips for regional mobility (up to 125 miles); and long-haul flights (beyond 125 miles).
“Each of these scenarios requires a different kind of aircraft to meet the mission requirements,” Ayyaswamy said. “For example, short trips are easily accomplished with rotors only. However, higher range trips utilize a combination of rotors, ducted fans, and aerodynamic wings to achieve greater performance.”
The team modeled a myriad of mission scenarios including variation in range, vehicle type, payload, altitude, speed, and other factors. As a result, they inferred that batteries respond very differently depending on the demand across different stages of the mission.
Their research has been published in Joule, as a featured cover story.
Lithium-ion batteries require a balance between performance, efficiency, and safety. But for eVTOLs, the safety margins must be multiplied more than usual. Why? Because you’re in the air!
“More than 95% of an eVTOL trip takes place above the ground,” Ayyaswamy said. “If something catastrophic happens in an electric car, you can at least stop the car and get out. But when the vehicle is in flight, this could become more challenging. That’s why we must go to great lengths to ensure that eVTOL batteries do not cross thermal limits or otherwise catastrophically fail.”
With this in mind, the team analyzed the electrochemical performance through the five phases of a typical eVTOL journey: liftoff, climbing, cruising, descending, and landing. But they also determined there is a sixth phase, the “balked” phase, which requires critical attention.
“If you’ve ever been on a commercial flight, sometimes you have to circle the runway because there is so much traffic trying to land at the same time,” Ayyaswamy said. “The same will be true of eVTOLs. Once they become popular, you might arrive at your destination and have no place to land, forcing you to hover until a spot opens up. Hovering during the end of the trip requires the most power, and it comes at the end of a trip where the batteries have already discharged almost all their capacity.”
Their models indicate a direct correlation between power demand and thermal safety. For a typical eVTOL mission, the hovering of a balked phase at the end of a theoretical journey caused battery temperatures to shoot up, often resulting in potential risk of thermal runaway and catastrophic failure. This led to the first major finding of their study: safety analysis for eVTOL batteries must always include the “balked” phase, since they reflect the maximum battery temperatures during any mission.
Their second finding also dealt with temperature — not just of the battery, but of the geographic location of the flight. Lithium-ion batteries require precise thermal management to operate properly, and flights expose these batteries to rapidly changing ambient temperatures as the vehicle ascends and descends through the atmosphere. The more thermal management is required, the less efficient the batteries become.
“Let’s say you’re flying in Minnesota,” Ayyaswamy said. “The temperature variations are so great between the ground and the air, you would need much more thermal management than you would in, say, Texas. That doesn’t mean they can’t operate at all; it just means that in Texas, eVTOLs can deliver higher range and payload thanks to simpler thermal management systems. Every situation has numerous variables to consider.”
“The most important result of this study is that there is not a one-size-fits-all solution,” said Bairav Vishnugopi, a research scientist at the Energy and Transport Sciences Lab. “Every application dictates an entirely different set of requirements – distance, payload, trip time. That’s why we encourage mission-specific battery design. If you are designing an eVTOL for first responders to navigate a dense city, the batteries will be different than if you are building a passenger vehicle for a one-way 100-mile trip.”
Mukherjee's team continues to conduct research at the forefront of battery safety. They recently joined forces with UL Research Institutes to establish the Center for Advances in Resilient Energy Storage (CARES), a research hub that will explore the design and operation of batteries and energy-storage systems in general, as well as their impact on safety and sustainability.
“There is growing interest in both private and public entities, because it’s an exciting area with tremendous potential in decarbonization and energy transition,” Mukherjee said. “Our job is to think outside the box and develop solutions. More importantly, the design guidelines we derive from this research should serve as a crucial starting point toward rapid growth in urban air electric mobility.”
Financial support for this research comes in part from the Office of Naval Research (ONR) grants: N00014-23-1-2608 and N00014-18-1-2397.
Writer: Jared Pike, jaredpike@purdue.edu, 765-496-0374
Source: Partha Mukherjee, pmukherjee@purdue.edu
Revealing hidden predicaments to lithium-ion battery dynamics for electric vertical take-off and landing aircraft
Abhinand Ayyaswamy, Bairav S. Vishnugopi, and Partha P. Mukherjee
https://doi.org/10.1016/j.joule.2023.07.014
ABSTRACT: The future of urbanization engulfs the trident of electrification, increased accessibility, and enhanced productivity. Although electric vertical take-off and landing (eVTOL) aircrafts provide cleaner, faster, and more efficient mobility solutions, they exhibit stringent phase-disparate demands on Li-ion batteries (LIBs). Through our mechanistic modeling framework, we demonstrate that eVTOL architecture, its mission constraints, and electrode design portray complex electrochemical implications in LIBs. Accrescent current densities distinctive to eVTOLs signify landing/balked phases as critical pathways to trigger thermal safety. During cold starts, we identify key limitations arising from the union of initial energy consumption and thermal convection from altitude variation. Cognizant of the mission-specific thermo-electrochemical interactions in LIBs, practical insights into the dynamic response of battery thermal management systems are discussed. The confluence of eVTOL power requirements with its concomitant battery response conveys mechanistic trade-offs pertinent to a spectrum of target applications, including passenger mobility, cargo, and emergency medical services.