Can commercial flights go electric?

30 July 2019

Interview with 

Paul Robertson, University of Cambridge

PLANE-WING

Image of a plane wing flying above clouds

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We’ve been talking about the future of aviation, and that future might be electric given that batteries are overall a cleaner energy source than fuel. Fully electric cars are everywhere now and batteries are constantly improving their capacity by about 5% each year. But that being said, why is there still no Tesla equivalent for aeroplanes? To find out, Izzie spoke with Paul Robertson, a senior lecturer in electrical engineering from Cambridge University.

Paul - There are some leisure aircraft, which are just one or two seaters, which are purely electric but they're very much limited by their range and flight time. So typically up to about an hour’s flight time would be what's currently achieved in these sorts of aircraft. It's a 'hot' research topic at the moment, looking at moving up to larger aircraft from your two seater, four seater, nine seaters up to airlines sort of scale

But one of the major issues which limits electric aircraft is the power density in the batteries. By that I mean for a given weight of battery, how much energy can you store compared to burning fuel using a conventional engine? And at the current time, equivalent weight-for-weight batteries or fuel, then burning fuel gives you about 20 times the energy that you'll get from batteries.

Izzie - Wow. And what about the mechanics going on, say, with a car compared to a plane, does that impact anything at all?

Paul - In cars, when weight is an important issue in cars, for performance and to a lesser extent for range, but in an aircraft it's absolutely critical because the total weight of the aircraft determines how much power you need to fly and so if you're storing a certain amount of energy that tells you how far or how long you can fly for. By increasing the weight of batteries, for example, in an aircraft, you make the overall aircraft heavier, it therefore requires more power. So if you just double the number of batteries, the weight of batteries, that does not double your range because you’ve paid a penalty for the extra weight that you're carrying.

Izzie - If we were in a situation where we could get rid of all of the fuel, with the batteries we've got at the moment, how far could a plane go?

Paul - If you're to take out of say an airliner, you took the engines off, you take the fuel tanks out and you replace that equivalent mass, that weight with batteries and electric motors, if you could do that your flight time would probably be around half-an-hour.

Izzie - That's not very much at all!

Paul - So it is rather limited. However, there are some applications where even short flights of that sort of endurance are useful. City to city hopping is a possibility. We still though have the issue, how do we recharge the batteries or do we swap them out, what do we do in order to refuel the aircraft? Because the infrastructure to do that is very different to what we currently have which is just piping liquid fuels around. So it isn't just the technology of the batteries in the aircraft themselves, in any sort of commercial application we have to think about the infrastructure which goes around it.

Izzie - And what are some of the other major issues associated with say an all electric commercial aircraft?

Paul - Okay. So we've talked about the batteries which is the main one. There are lots of other things which concern us if we're going to commercial aviation. We need to look at legislation and certification, so that's what makes the aircraft safe, how do we prove it safe, and how do we operate it. If we change the propulsion system from our normal aviation fuel to electric then, as I say we need the ground infrastructure, we need all the testing and qualification work to be done. We need to rewrite the manuals on how the pilots are trained for these propulsion systems, we need change to simulators.

So there's a lot behind commercial aviation and, in fact, the regulations are really only just beginning to be looked at now for how do you certify these sorts of aircraft?  It's not so much in issue at the very small-scale, leisure aircraft, there are experimental categories we can work in there. But when you move to passenger carrying the general public, then it's a very different regime and everything needs to change basically as we move from our current fuel burning approach to a purely electric approach.

Izzie - Could we see a combination of fuel and batteries? Is that something that would be possible?

Paul - Yes that is. That's what we call a hybrid electric propulsion system. What we're doing in that case is we are assisting the conventional fuel burning engine, whether it be a turbine or a piston type of engine, we're assisting that with an electrical boost, or we’re combining an electrical machine with that fuel burning engine so that we can improve the overall efficiency of the system.

Izzie - And  so would it be that you'd have the batteries to get off the ground but you'd then use fuel for once you reach your correct altitude?

Paul - Yes, that's the basic idea. Burning fuel, because of the large amount of energy it stores in a given mass, then that's a good way to propel yourself a very large distance. But the addition of an electric boost means that you can optimise the size of that fuel burning engine for that long cruise condition and the extra power you need for takeoff and climb is provided by the electrical system.

Izzie - And what about going fully electric, could that be a possibility?

Paul - It's a possibility, but I think it's quite a long way off. The density of the energy storage in batteries is not high enough at the moment and it'll probably be some decades before we can store enough energy to take an aircraft to match the sort of ranges which we can get now. So going transatlantic, transcontinental, it would be very difficult for a purely electric aircraft to do that for some time yet.

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