The good, bad, and ugly of electric aviation
Hi there! Some weeks ago, I finished a consulting engagement for a start-up company developing an electric aircraft. Rather than boring you with the details, I wrote a general overview on battery-powered flight.
It won’t work hurr durrr
Before anything else, let’s get the obvious objection out of the way. Are you active, or even observant, on LinkedIn (let’s connect by the way)? If so, you’ve probably seen the picture illustrating the amount of batteries needed for an Airbus A380. It’s huge. And the picture is kinda funny too, with its (photoshopped, duh) plane-sized AA battery and all.
The point is, batteries are heavy. And to replace the kerosene fuel of a jetliner with batteries of equal energy would require an insane amount of them. Kerosene has an energy density of 12 kWh/kg, while the best lithium batteries at the moment have maybe 0.2 kWh/kg. (Edit: seems specific energy is the correct term, but this’ll have to do.)
So who would be stupid enough to try this?
Well, nobody. Nobody that can be taken seriously, at least.
Replacing a jet-powered intercontinental cruise airliner with a battery powered cruise airliner does seem very, very infeasible, given the current technology. Nobody’s trying to do that, as far as I know.
However, there are other kind of planes that could very well be electrified. Siemens has built a prototype small plane, and other companies are working on short-to-medium haul airliner as well as personal VTOL planes / air-taxis, to name but a few. Plus of course there are a million (or some dozens, at least) hobbyists building their own multi-multirotors in their carages, with varying levels of success.
All of these examples are characterized by shorter flight distances – at least compared to crossing the Atlantic. Correspondingly, the equivalent kerosene-powered plane is not a flying fuel tank either. Alright, replacing that fuel quantity with batteries does increase the weight, but it’s not an insane increase. The wing area may have to be increased a little or whatnot, but technologically it’s quite alright.
Anyways, now that we have the main complaint out of the way, let’s move on to what makes electric propulsion an attractive option.
Benefits of electric aircraft
Electric motors can be more suitable
Electric motors are a million times better than combustion engines in some applications. For example, think of a typical multirotor. It has exactly N moving parts, N being the number of rotors. The rotor (meaning the propeller-thingy) is rigidly connected to a shaft, which is directly connected to the electrical machine rotor. That’s it. (Of course, bearings might have some extra balls or rolls, but let’s not count them.)
Now, replace each electric motor by a gasoline-powered one. Suddenly you have pistons and crankshafts, fuel flow control and injection, plus a million other components. Even a starter of some kind is needed. Not to mention about vibrations, which will be drastically bigger compared to an electric motor. Plus the torque/power curves of an ICE can be just plain nasty.
The same applies to any kind of flying apparatus with several small motors. For example, a company called Lilium is building a personal aircar with dozens of ducted fans placed inside the wings. If I understood correctly, they are using air-cooling for their motors, i.e. sacrificing some power density for the convenience of not needing a separate cooling system. I can understand the decision, it makes perfect sense in the application.
Now, imagine implementing the same system with dozens of small gasoline engines.
Doesn’t sound very practical, does it?
The take-away point here is that electric motors are very compact, and very easy to control. Only dirt-cheap power electronics needed, and you have a very wide, very nice speed/power range available.
So for any application that needs that, or otherwise has several small power units, electric motors rule.
Engine failure on an airplane sounds scary as hell. Luckily, an electric motor can be segmented in such a way that it behaves like a collection of several almost-independent motors. This means that even if one of them fails, you’ll still be able to use a nice fraction of the maximum engine power.
Nowadays, almost everybody knows how much more energy-efficient electric motors can be compared to combustion engines. Indeed, efficiencies in the 90-95 % are common today. By contrast, a combustion engine is lucky to reach 40 %.
Thus, a propeller can be rotated much more efficiently by an electric motor, compared to an ICE. That much is clear.
Of course, there are some complications, like the weight issue discussed earlier. A battery-powered aircraft will most likely by heavier, after all.
Furthermore, not all aircraft utilize propellers. Instead, turbofans are typically used for higher-speed craft. Replacing that by a ducted fan may not be as straigthforward as it sounds, with all the thermal expansion of air etc. going on.
But, as mentioned earlier, nobody is trying to do that in the first place. Building a barely-subsonic electric plane, I mean.
Another benefit of electric propulsion is energy recovery. An electric plane slowing down its velocity, or decreasing its altitude, can use some of its kinetic/potential energy to recharge its batteries. This may not be a huge figure, but hey, every bit counts.
See above. A significant portion of an aircraft’s lifetime costs is spent on fuel.
Alright, this might be something of an inflammatory topic. So I’ll try to keep this as neutral as I can. Alright?
Burning fossil fuels releases nitric (and possibly sulphuric) oxides as well as microparticulates. Not good for living beings. I guess everyone on this.
It also releases a ton of carbon dioxide. However, since your political allegiance pretty much 1-on-1 predicts your views on that topic, I’ll skip it for now.
Also, know another crappy thing about fossil fuels? They will run out.
Electric propulsion suffers from none of these issues. (Lithium / rare-earth mining is another topic.)
Luckily for electric motors, your personal views on climate change don’t matter that much. There is immense political pressure towards cleaner, more-renewable propulsion. Something with smaller carbon emissions. There has been for years, and there most likely will be even more of that in the future.
And having government backing for a particular technology is almost certainly beneficial for that technology.
Alright, electric aircraft seem to have several benefits. What about problems? Well, there aren’t many major pains in the backside.
The batteries are heavy.
That’s pretty much it, and this problem was already covered earlier.
So, what might help with those colossal battery sizes? Well, obviously not needing a huge energy storage is one. But this was already discussed in the very beginning. And it doesn’t really eliminate the problem (low energy density), it only makes it less significant.
However, there are a couple of technologies that might help.
Fuel cells are one. They convert chemical energy directly into electricity, after all. They are certainly being researched a lot. However, their efficiencies are lowish, falling in the 40-60 % range. Even if you didn’t care about energy expenditure, 40 % losses translate to a lot of waste heat to be managed.
Lithium-air batteries present another reaaaally interesting option. Although still experimental, they offer a theoretical energy density of 12 kWh/kg. Which, incidentally, is pretty much equal to that of kerosene. Of course, a complete battery pack with all the accessories will be much heavier. But, that extra mass is then again compensated by the high efficiency of an electric drivetrain, compared to a combustion engine.
At the moment, Li-air batteries aren’t ready for commercial use. However, with the increasing interest in electric everything, I wouldn’t be surprised if we saw some exponential jumps on that front. Because that’s how technology development usually happens. For years, there’s only incremental development, until boom! Something else comes along, and everything suddenly gets twice or thrice or four times as good. Development of permanent magnet materials is a good example, illustrated below.
Lots of buzz about electric aviation, all around. Long distance cruiseliners probably won’t be able to ditch kerosene any time soon, but for other purposes batteries are already good enough. Furthermore, electric motors themselves beat gas-burners (jets included) in many applications. Finally, new energy storage technologies may change the game altogether.
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