Electric aircraft—the future of aviation or wishful thinking?

August 25, 2015 by Peter Wilson, The Conversation
Electric aircraft – the future of aviation or wishful thinking?
Solar Impulse landing at Brussels Airport. Credit: Brussels Airport, CC BY-SA

Since the dawn of aviation, planes have primarily been powered by carbon-based fuels such as gasoline or kerosene. These contain a lot of energy for their weight, providing the vast power required to lift large commercial airliners on journeys across the globe. But with oil resources declining and penalties on greenhouse gas emissions increasing, the future of aviation is dependent on finding an alternative power source. Is electricity the answer?

A first step is to develop "more electric aircraft" – jet-powered planes that maximise the use of electricity for all the other systems. The idea is to significantly reduce fuel consumption by improving overall energy efficiency. In practice, this means reducing the weight of the aircraft, reducing drag with improved aerodynamics and optimising the flight profile to use less fuel.

But though these improvements can save on fuel, that alone isn't enough. The shift to more sustainable aircraft requires major, longer-term solutions.

Such significant innovations have often been driven by military requirements. The jet turbine engine was developed during World War II and the US Air Force's Chuck Yaeger first broke the sound barrier in the Bell X-1 as part of the Cold War race to achieve supersonic speeds. The drive for new technologies led to massive improvements in performance and reliability, which has since filtered through to and made mass intercontinental air travel a reality.

Concorde was the ultimate expression of this transformation from military to high-performance , but despite its phenomenal performance it was plagued by complaints of excessive noise and pollution. Modern jet air travel still consistently raises such environmental concerns and, while the military has an obvious incentive to design the fastest aircraft, its motivation to go green is less obvious. We may need to look elsewhere for the next big innovation.

Electric aircraft – the future of aviation or wishful thinking?
Left: the Bell X-1, the first supersonic aircraft. Right: a British Airways Concorde jet, the only commercial supersonic plane. Credit: Left: US Air Force. Right: Aero Icarus via Wikimedia Commons
Cleaning up the skies?

Solar-powered endurance aircraft have received a lot of attention recently, with the Solar Impulse team attempting to make the first round-the-world flight. But solar power, while an interesting technical challenge, is not a particularly realistic option for mass transit of passengers. As can be seen from the Solar Impulse aircraft, the power output from the Solar Panels on a very wide wingspan is able to transport only the aircraft and the pilot for any significant distance.

Battery storage is the key limiting factor for electric aircraft. If electric aircraft are held back by either weight or fuel restrictions, it's probably down to the battery. Aircraft typically have a longer fuelling time than a car, so rapid recharging is possible and effective, as current jet aircraft take about the same time to refuel (and also for passenger and cargo turnaround) so electric charging of about 1hr is reasonable, however the critical problem is energy density – how much energy does the battery provide for its weight?

Typical lithium-ion batteries in use today have a maximum energy density of around 1,000,000 joules of energy per kilogram, and while newer research promises the possibility of higher densities, these are not available commercially. A million joules sounds like a lot. However, compare this with 43 million joules per kilogram for aviation fuel. Swapping the fuel tanks for a battery weighing 43 times as much isn't a viable option – clearly there's a significant storage problem to be solved before electricity can power large aircraft over long distances.

The future for electric air travel

So where does electric power fit in the long-term vision for consumer ? Despite the obvious technical challenges, The Airbus prototype E-Fan aircraft is due to be put into production by 2017. The E-fan is a very light two-seater plane powered by two electric motors, with a relative speed and carrying capacity far lower than those required by commercial carriers. However,

Within the next decade, this technology may extend to short-range commuter and business aircraft – especially targeting routes that still use conventional propeller propulsion. Airbus has medium-term plans for such an aircraft, with a target capacity of perhaps 60 passengers – making it a suitable platform for short-haul commuter flights.

Safety and reliability must be addressed before are adopted by commercial airlines. Much as the electric car still has to achieve a critical level of public confidence, perceived reliability will have a significant impact on consumer trust in new aircraft.

If prototypes such as the E-Fan can build public confidence, this may mark a "tipping point" in overcoming the technical challenges inherent in any new form of transportation, especially in aviation which has a track record of rapid innovation. Advances – particularly in new materials, storage and power electronics technology – may offer the prospect of purely electric commercial aircraft within the next two decades.

Explore further: Airbus CEO talks about emissions, innovation and more

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17 comments

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antialias_physorg
5 / 5 (2) Aug 25, 2015
But solar power, while an interesting technical challenge, is not a particularly realistic option for mass transit of passengers.

Unless you add in some orbital (or ground based) concentrator scheme. It's one of those weird ideas that gets kicked around every so often that would eliminate (or at least seriously reduce) the need for heavy batteries.
Urgelt
5 / 5 (1) Aug 25, 2015
Unfortunately, battery advances come very slowly, despite high levels of investment by governments and corporations. We'll be lucky to see a doubling of energy density per kilogram in 15 years.

Still, electric aircraft are inevitable, if our civilization doesn't crash first. But I wouldn't look for commercial passenger aircraft to go electric in this century. Smaller recreational aircraft, yes.

I don't think it's wise to discount military designers, as the author does. 'Going faster' isn't the only design goal they have in mind. Long loiter times, range, payload and stealth are examples of other design considerations. And there's reliability, too. Jet fighters have tremendous maintenance needs. Electrical motors have fewer moving parts and require servicing less often.

Electric aircraft may get a boost from Lockheed's small fusion plant project, if it works. That's a big 'if,' of course.
jeremy_h
not rated yet Aug 26, 2015
The issue with such electric aircraft is that actually no batteries can do the job alone for the commercial sector. Even the lithium-air Holy Grail is not able to do it, despite having a record energy density of 10 times lithium-ion.
Recreational aircraft are the first logical step but the E-Fan has a maximum 40 minutes flight time which some may find too short, however you could circumvent this problem by using a fuel cell for example.

As said by Lex Talonis, we have to move beyond the black and white "all electric or not" fixation.
If you look at the Airbus medium-term aircraft, it is electric but has still a turbine to do the heavy-lifting.
IMO, the future of aviation lies in hybridization, for example, the turbo-electric system is interesting: a turbine fed by hydrogen, natural gas, bio or synfuel drives a generator which drives the electric fan. Because there is one turbine it is easier to maintain, cheaper and the noise can be muffled more easily but it has the same range.
antialias_physorg
not rated yet Aug 26, 2015
Unfortunately, battery advances come very slowly

I'm wondering if battery materials (unlike liquid fuels) could not also form the structure of the aircraft. In that case the added weight may well be OK if it directly replaces weight for otherwise non-functional structural components.

'Going faster' isn't the only design goal they have in mind. Long loiter times, range, payload and stealth are examples of other design considerations.

A major consideration is: availability of fuel in the field. If your craft is dependent on having jet fuel then you become inoperable as soon as supplies of that one commodity don't come through. Electricity can be generated from any number sources available and so would make the system a lot more robust in actual conflict scenarios.
Urgelt
not rated yet Aug 26, 2015
Auntie, I agree, it may be possible to form batteries into structural components. Tough engineering challenge, though. And you're right, military designers are always concerned about fuel. Fuel and ammunition make up the largest bulk of supplies that must be moved to troops, and require tremendous effort, manpower and infrastructure. Easing that burden is always a consideration in trade-off decisions.

And yep, batteries are only one option for electric propulsion aircraft. I mentioned Lockheed's small fusion reactor project; they're explicitly hoping to put those on electric aircraft. There are other possibilities, but they're even further out - like Meta, a theoretical nonoxidizing fuel based on a nitrogen atom and 64 helium atoms placed into a metastable energy configuration (half the electrons are in an excited state). You use electricity to make the Meta, then take energy out by letting metastability lapse (temperature-dependent). It *should* work; but the details...
Urgelt
not rated yet Aug 26, 2015
(Continuing)

The expected specific impulse of Meta as a fuel is much greater than any oxidizing rocket fuel. But thus far metastable helium atoms can only be kept in that state for an average of 2.5 hours (and each atom must be kept isolated to achieve that much), and nobody has made the theoretically-more-stable nitrogen-helium Meta atom, let alone devised an industrial-scale manufacturing and handling process. It's way, way out there, if it can be done at all.

So, for aviation, we'll limp along with fossil fuels and hybrid solutions for a good long while.

The prospects for ground-based electrical propulsion look much rosier. At the rate batteries and electric cars are improving, we should see ranges of 600 miles or more for commercially-available vehicles within 30 years.

Unless our civilization crashes and burns, that is. Our endlessly growing population and resultant species extinctions and climate change makes our civilization look kind of shaky.
antialias_physorg
not rated yet Aug 26, 2015
Tough engineering challenge, though

Maybe not so much. Batteries can already be 3D printed (albeit not at the same quality as manufactured ones - so there's still a bit of a gap). E.g. from here:
http://www.techti...ries.htm

However, even less-than-optimal batteries as structural components with the internal ones being state-of-the-art sounds like a beneficial mix to me.

What I would really like to see is either catapult (railgun) or ground based energy transmissions to get electric planes up to speed/altitude (for shorter flights currently up to 25% of fuel is used just for takeoff).

With good sail characteristics the internal batteris would be used just to maintain speed and altitude this would increase operational ranges substantially (as the weight factor is even worse for battery operated planes).
jeremy_h
not rated yet Aug 26, 2015
either catapult (railgun) or ground based energy transmissions to get electric planes up

It's a great idea for space launch but I think not so much when it comes to conventional aviation.
Airports are already a nightmare when it comes to logistics and maintenance, adding a railgun, microwave antenna, laser or whatever will just complicate things.
When a plane is grounded because of whatever reason, you can quickly call for a replacement and only one flight is impeded.
For the railgun (or else), not so much. If one stops working, then you have all or a portion of the airport that shuts down because you can't get the birds in the air and on the other end, you have planes landing and piling up waiting to take off again.
This can be manageable for small airports but imagine that for Heathrow where a plane takes off every 45 seconds. Also, the power for that venture would not be trivial.
jeremy_h
not rated yet Aug 26, 2015
For curiosity, if you replaced all the fuel and operating empty weight (includes structure but also hydraulics, flight controls, seats, etc) of a Boeing 787 Dreamliner by conventional lithium-ion battery (160 Wh/kg), you would get an average of 26 MWh (factoring an efficiency of 85%) of power or 30 minutes of flight at cruise speed.

https://en.wikipe...ications
Urgelt
not rated yet Aug 26, 2015
I'll side with Auntie on this point, Jeremy: directed energy sufficient to launch a spacecraft is orders of magnitude greater (read: hotter and harder to control precisely) than directed energy to launch subsonic aircraft - which have no need to race away from the source of the directed energy and can even choose a circular rising orbit while accepting directed energy and converting it to propulsion. Once up, you turn off the beam and drive away on internal power. None of those advantages pertain to spacecraft launches. I don't know if either technology will ever be commercially feasible, but if so, it'll work for aircraft far sooner than for launch to orbit.
Urgelt
not rated yet Aug 26, 2015
Auntie wrote, "Batteries can already be 3D printed (albeit not at the same quality as manufactured ones - so there's still a bit of a gap)..."

It's a lot more than 'a bit of a gap.' We can 3-D print batteries, but we can't 3-D print *good* batteries, nor can we 3-D print batteries that pass the sanity test as structural components in an aircraft.

But I agree, the way 3-D printing has been accelerating, it's within our imagination, if not our immediate reach, to print structural batteries. I think we'll see that technology arrive, eventually, if the civilization which sustains our technological advancement holds together long enough.

Jeffrey makes a good point, too - at L-ion levels of performance, you can't pack in enough energy to sustain long flights even if you convert every kilogram of aircraft mass into batteries. So... hybrids, for a really long time to come, unless we give up on the idea of fast air transportation entirely.
Urgelt
not rated yet Aug 26, 2015
Interestingly, Hyperloop and similar concepts might deliver fast ground transportation without reliance on any fossil fuels at all, and do so long before any of the speculative aircraft technologies we're talking about. But there's a clear disadvantage: however you design them, fast ground transportation systems can only deliver passengers where the infrastructure has been built. With an aircraft, you have range restrictions, but you can take off and land on appropriate surfaces anywhere. Much more flexible.

If civilization is still a going concern in a couple of centuries, I wonder if we might see an inverse of the relationship between air and ground transportation: to go long distances fast, you go on the ground. To get from hubs to local destinations, you use buoyant (slow) aircraft supplemented by ordinary ground vehicles, both propelled by electricity.
jeremy_h
not rated yet Aug 26, 2015
Thanks for your feedback, I actually had this in mind when I wrote this comment: http://www.nasa.g...nch.html which is a rail propelling the spacecraft up to a certain speed (Mach 2 and not orbital speed) and then coasting on its own power, similar to the system described above.
Urgelt
not rated yet Aug 26, 2015
A 'bouyant' aircraft isn't necessarily lighter than air. Controlling lighter-than-air aircraft in all weather conditions is a challenge not easily solved. But if you embed vacuum vacuoles (heh) into structural materials, you can reduce the energy required to propel an aircraft and extend its range. This idea is an extension of the 'honeycomb' concept, only instead of air-filled honeycombs, you have massless spaces that reduce the weight of the aircraft more than air-filled spaces can. Bulk up a bit while reducing mass and you have a buoyant aircraft - still heavier than air, but reduced weight. It'll be somewhat bulky and slow, as aircraft go, which is okay for local transportation.

Not that this will be easy to do. Forming perfect vacuums in any material and preserving them for extended periods of time is a trick that's well beyond our materials science at present.
Urgelt
not rated yet Aug 26, 2015
I agree with both Auntie and Jeremy that rail propulsion is a terrific concept. I think we should concentrate on launching cargo to orbit with that technology; it would greatly reduce the need for fossil fuel-powered launches, which could be restricted to manned flights.

Why concentrate on cargo? Because it'll be far cheaper to design and build if you can eliminate G force restrictions needed for human launches. Rail launch for human passengers will require much longer runs.

For aircraft, the technical challenge is much reduced - what we are looking at there are catapults. Steam catapults would work. Or you could build rail-driven catapults. The problem there is the expense of infrastructure might not pay for itself. With space launches, the cost per kilogram to orbit (even with Space-X trouncing prices) is so very high, infrastructure investment makes a lot of sense. Whereas jet fuel is still cheap.
Urgelt
not rated yet Aug 27, 2015
Lex, your concept for catapult-launched aircraft is technologically feasible, I believe. But I sense that your cost analysis (50% lower prices, 30% slower flights) falls into the category of assumption, not calculation.

The only seriously developed catapult technology on-hand exists in US aircraft carriers. It's *very* expensive, and it imposes design constraints (penalties) on aircraft which take advantage, so that's another cost. The cost of developing a new fleet of special-purpose aircraft and developing and installing this new catapult infrastructure will either be borne by taxpayers or through air fares. Either way, those costs must be considered in any cost-benefit analysis of the concept.

A rail system isn't cheap. Steam catapults aren't cheap, either, and are notoriously maintenance-intensive. It's not obvious which would better serve, or if either will serve to reduce costs of air travel over short-to-mid-range flights.

So, a feasibility study would be good.
Urgelt
not rated yet Aug 28, 2015
Eh, I didn't say it would be infeasible. I said the infrastructure and operating costs aren't as negligible as you'd have us believe, and I said your 50% cost/30% slower figures are assumed, not calculated.

The technology won't work for any aircraft now flying. None of them have attachment points and structural stiffening they'/d need to repeatedly survive catapulting from the ground, and no commercial aircraft are optimized for a high glide ratio, so you're talking about redesigning and developing an entirely new commercial air fleet - I can't even imagine how many trillions that will cost.. You'll need to establish standards before investment will make any sense at all. Even US Navy planes are unlikely to work with whatever catapult system ends up as standard for commercial; they won't have the right attachment/release gear.

Don't trivialize the cost of this solution, Lex. It's feasible, possibly even desirable. But it's not as simple or cheap as you pretend.

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