The headlines have been filled with the Boeing 787 Dreamliner, a revolutionary vehicle hailed as the airliner of the future. The Dreamliner uses a phenomenal amount of electricity to power many of the systems on board — and that power is supplied by huge and complex lithium-ion (li-ion) batteries. This week, aviation authorities around the world ordered airlines to stop flying their Boeing 787 Dreamliners due to the fire risk associated with battery failure.
While experts everywhere are weighing in on the possible causes, solutions and consequences, we at ANSYS have confidence that Boeing’s engineering, research and technology teams will rapidly and thoroughly resolve the issues, and history will show that this was merely a blip in the overarching aircraft development process.
An airplane’s design, by necessity, must satisfy all safety standards to the highest degree. Then the process becomes a tradespace of many critical and often competing design parameters, such as weight, range, propulsion, aerodynamic drag, power consumption, electronic capabilities. Aircraft today are such complex interdependent systems that it is a major engineering challenge to understand the performance of all systems and components across all potential operating conditions and to understand the safety factor in all cases.
The risk in making design decisions is that one small change can have consequences, sometimes unintended, sometimes grave. The software that ANSYS develops gives engineers the tools they need to predict, with confidence, how a change will affect performance. And the airline industry has long recognized how effective simulation can be for studying design tradeoffs.
For example, consider the use of composites in the Dreamliner, which is often referred to as the first composite airliner. Weight is the enemy of fuel efficiency; composite materials give the aircraft designer the option to reduce weight while retaining required strength, improve the aerodynamics of the aircraft, and provide a better traveling experience for the passenger. Boeing’s approach offered weight savings on average of 20 percent compared to more conventional aluminum designs. However, composites add to design complexity because they act as insulators. “When you build an aircraft out of 50 percent composites (insulators), you have a huge challenge getting rid of the heat from the electronics. And in the 787, there are a lot more electronics than ever before,” says our aerospace expert Rob Harwood.
With the development of the more electric aircraft, the number of electronic control units on board increases, while the space available for housing those components does not. Additionally, increasing usage of composite materials in aircraft design reduces the opportunity to utilize the airframe surrounding electronics boxes to dissipate heat as heat sinks. At the same time, energy spent to power fans that cool electronics components must be as low as possible to reduce the impact on overall system efficiencies. “You can’t make a decision to simply put in a fan to cool the electronics, because it draws more power, and this impacts the efficiency gain from reducing weight. These issues become system-level decisions. Because of the combination of factors, electric components may run hotter than ever before, and accurate predictive tools are needed to manage the huge thermal output and guarantee their proper function even before flight,” Harwood adds.
Harwood says that though we don’t yet know the exact cause of the airliner’s battery problems, his industry experience tells him it is either a discrete component-level failure or a fault related to how the batteries are integrated within the whole system.
Lithium-ion batteries power products we use every day — laptops, smartphones and electric vehicles, to name a few, and now airplanes. In the past, there have been cases of battery packs in these products smoking and catching fire. “A battery that is smoking is probably being used beyond the operating conditions for which it was tested. But product developers know that people use devices well beyond their expected ranges — and sometimes that failure mode hasn’t been captured — so virtual testing becomes a critical tool,” says Sandeep Sovani, our automotive expert. The batteries that power today’s electric cars are similar in design to those that Boeing developed for its aircraft. During high power extraction, these large batteries may experience a significant temperature increase — which can lead to safety concerns. A properly designed thermal management system is crucial to prevent overheating and uneven heating across a large battery pack, which can lead to degradation, mismatch in cell capacity and thermal runaway.
Li-ion is particularly quirky, adds Sovani, in the sense that it is extremely complicated. It is influenced by a lot of internal parameters, so its behavior is harder to nail down. Its charging―discharging profiles and numerous other factors can combine in unforeseen ways and lead to thermal runaway, a catastrophic failure mode. These factors include manufacturing variables (such as material impurities), which can cause an internal short. All this is extremely difficult to pretest due to overwhelming complexity.
It’s not up to ANSYS to second guess what the exact issue is. Instead, we intend to focus on making the best simulation solutions available so that our customers can get a better handle on complexity and deliver the products they intended.
If you’re interested in reading more about some of these concepts, you can download these ANSYS papers from our Resource Library: