Design Con 2015

Dreamlining Boeing and batteries

-January 22, 2013

You've undoubtedly seen the news that various fleets of Boeing's 787 Dreamliner, which recently went into service, have been temporarily grounded while an apparent problem with some of its battery sets is checked out. (There have also been some fuel leaks, but that's another story.) Among the battery issues was a smoke-filled locker and fire damage that occurred when a plane was parked on the runway.

I'm not going to jump to conclusions about the problem or the cause. Any engineer with any experience knows that what appears to be "the problem" often is just a misleading manifestation of the real problem elsewhere, while the actual root cause (or causes) is often buried somewhere down the line in a long chain of events. Just because you see smoke doesn't mean that there's a fire where you think it is, so to speak.

But there's no doubt that there's a lot of advanced battery technology in the 787 - as well as many other of the latest devices, ranging from laptops to EVs/HEVs to aircraft. An excellent article from The Wall Street Journal - "Dreamliner-Like Batteries Raised Concerns" - and its associated graphic here (sorry, they may be behind a paywall) show how the Dreamliner 787 uses lots of electrical power for actuators and more, compared to earlier aircraft.

Why? That's easy to answer: in order to reduce weight and get the benefits of electronics and advanced algorithms, much of the conventional hydraulic power and control of the aircraft has been replaced by electrically-based power and control, and with advanced electronics and software, of course. (Plus, the airframe is also largely made of up composites for reduced weight, instead of conventional aluminum.)

These electrical systems require lots of batteries for start-up, backup, and even operation. The preferred rechargeable energy-storage technology for the batteries is based on lithium-based chemistry. This makes sense, since lithium offers much, much higher energy density (by weight and volume) than any other available battery chemistry.

But much more so than with older chemistries such as nickel cadmium, lithium cells also need lots of careful attention when charging and discharging: rate, temperature, internal external potential failures, and more. Plus, their energy density is so high that there's a lot of stored energy in that small volume, and a failure such as an internal short can result in huge current flows and subsequent fires or even explosions. (There are well-documented instances of laptops catching fire even though they were "off" and not even plugged into their AC line charger.)

So we have the typical engineering situation of a tricky tradeoff. We want the benefits of lower weight, greater efficiency, higher energy density, and so on, but we also have to accommodate and anticipate all the implications of the technology that makes it possible. In the case of energy and power, you have to have layers of protection, management, allowance for single and double faults, and planning for various failure sequences and scenarios - it's a lot of failure-mode planning to work through and anticipate.

And as all engineers know, you can't plan for everything, so you also have to have some independent, overall watchdogs in case something happens which you didn’t think of, or didn't plan for. It's also true that while lithium chemistry has such high density and subsequent danger, just remember that hydrocarbons such as gasoline have far greater energy density than the best batteries, yet we have managed to make that explosive technology into a safe energy-storage medium and power source. [Note that gasoline rates 46 MJ/kg and 36 MJ/liter; lithium-based batteries are around 1 MJ/kg and 2 MJ/liter, depending on the specific chemistry and design.]

Of course, the same politicians and activists who want engineers to perform miracles and deliver it all - better efficiency, lower weight, better products - will also be the first to scream and point fingers (and maybe lawsuits) when they don't get everything they wanted, and perhaps were promised. Once again, no good deed goes unpunished.

Have you ever been is a challenging design situation, where you were expected to deliver on a set of aggressive, conflicting goals by using advanced techniques and technologies? What sobering lessons did the experience teach you about both engineering and associated "politics?"

Loading comments...

Write a Comment

To comment please Log In

FEATURED RESOURCES