Published on September 28th, 2016 | by Steve Hanley1
Axial Stack Battery May Make The Electric Airplane Possible
People roll their eyes when they hear talk about an electric airplane, especially if that talk is about large passenger planes, the ones that move millions of people around the world every day. The sad truth is that commercial aircraft are some of the worst polluters in the transportation sector, even though improvements have been made in recent year and more are coming.
We can envision a small electric airplane flying on going a short distance — a few miles, perhaps — but a 747 loaded with 300+ passengers and all their luggage flying from Boston to Barcelona on battery power? That’s the stuff of fantasy, isn’t it? Not if your name is Luke Workman, a mad scientist type of inventor/experimenter/innovator. Workman is the father of such things as the Death Bike, a high powered bicycle with an electric motor designed for drag racing. He has also designed some of the most rugged, high density lithium battery packs available. He is regarded as one of the world’s best lithium battery designers.
The issue is not power units. Electric motors are more efficient that fossil fueled engines. They also are unaffected by altitude. They make as much power at 40,000 feet as they do sitting on the runway. The issue that is vital to an electric airplane is energy density. The most efficient batteries available today have 161 watt-hours per kilogram of energy density. Workman says the batteries needed to make electric airplanes possible will need an energy density of 400 to 500 watt-hours per kilogram.
Workman says he has knows how to make that happen and it won’t require a breakthrough in battery chemistry. Instead, it means making batteries that weigh far less than today’s products. “About 35 percent of the weight in the cell is current collection — sheets of aluminum and copper foil that are just there to get the energy in and out of the cell,” says Workman. “That’s a lot of weight that’s not active material. It’s a lot of weight a plane has to carry that’s not storing energy.”
“But here’s the trick,” says Workman, “I’ve come up with a battery design that weighs far less and handles heat insanely well — it just requires a gigantic, flat surface area. Like, say, the oversized wings of a supersonic plane.”This isn’t about using the area inside the wing or fuselage to store battery cells. His idea is to use the whole wing surface as part of a giant battery. “I wanna use the full available wing area for electrode plate surface, and conduct through the axis with the cross section of the full wing area.”
“Composite structures, to be strong, need a middle portion there to support the skin on the outside,” says Workman, “We can use the battery as the middle layer of the wing, and we can use the aluminum skin of the wing as a current collector to get power from the ends of that battery sandwich out to the motors. There’s a way to conduct that’s the hard way, and a way that’s incredibly easy, with abundant conductivity. In today’s small batteries, we conduct the hard way, because it’s the only way we can make high capacity, high power cells at that scale. This sandwich idea couldn’t work at the small scale, because you couldn’t get it up to a decent capacity.
“But give me an entire supersonic aircraft wing, and that problem just goes away. That huge surface area of conductive material would conduct extremely efficiently while generating almost no heat. And because there’s almost no heat to deal with, we could use higher density active materials that we can’t use in the automotive world. From quick estimations based off existing cell materials that are safe and have high cycle life, you can get around 13,300 amp hours per 0.2 mm of thickness for each foil layer. Nine-hundred layers would give us 3.3 kV nominal and around 44 megawatt hours of battery storage.
“Total weight would be roughly 104,000 kg, with an extraordinarily high percentage of that mass being active material and lower conduction losses than any currently existing topology despite its high charge/discharge rate capabilities. That gives us 423 watt-hours per kilo, well and truly in the ballpark, using proven materials we can get off the shelf today. And that’s if we’ve got 300 square meters of wing area, with a foil core about 20 centimeter thick and 1 centimeter current conductor plates on the top and bottom. The bigger this battery gets, the more efficient it becomes.”
That means the bigger the electric airplane gets, the further it will be able to fly and the more efficient it wlll get at carrying cargo or passengers. If there’s enough surface area, it could also be useful on large scale electric boats or for grid level power storage. Getting rid of packaging and the thick tabs that interconnect the cells in a regular battery can save a meaningful amount of weight on a large scale battery. But it’s the low impedance, low resistance conductivity path and reduction in voltage sag and heat production that lets you really ramp up the capacity of the thing and increase charge rates.
“I’m not going to build this personally,” Workman says, “not unless it was the only project I was working on. If any company wants to use it, they’re very welcome to and I’m happy to help.” Workman is not in it for the money. He is far more interested in making decisions that benefit the planet. “The reason I’d never restrict anyone from using this is that we’re all sharing the same spaceship here. We either do the things we need to do to keep an atmosphere on this planet, or we fail the Darwin test as a species.”
The prospect of a zero emissions supersonic transport is certainly appealing. Replacing all those jet airliners that make hundreds of thousands of flights a day worldwide with a zero emissions electric airplane would be a giant step forward in the quest to reduce the amount of carbon emissions the atmosphere is required to absorb. That will benefit every person on the planet, whether or not they ever travel by air.
Source: New Atlas Image credit: Luke Workman