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Has there ever been a trial/project/design, in which solar panels were incorporated in the design of a commerical airliner (e.g. on the wings), with the aim to reduce fuel consumption by lowering the generator workload?

If no, why not?

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    $\begingroup$ I'm not aware of any. And I can see several important reasons why not: solar panels are heavy, they're fragile, they require a lot of maintenance, they require a lot of wiring. $\endgroup$ – jwenting Apr 4 '19 at 6:59
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    $\begingroup$ @jwenting the actual solar panels aren't heavy at all. Most of the weight comes from structural reinforcements, which you don't need if you implement it into an existing structure such as the fuselage or wings. Nevertheless I don't think it would be worth it. $\endgroup$ – GittingGud Apr 4 '19 at 12:36
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    $\begingroup$ Solar energy is just a religion. The amount of energy gatherable per unit area is ... totally trivial. Indeed, the example of aircraft points out how utterly useless solar energy is. Solar cells are a fantastically ingenious invention for, say, calculators. $\endgroup$ – Fattie Apr 4 '19 at 12:57
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    $\begingroup$ As you include trail options, there have been at least two planes which have been powered completely by photovoltaic panels, Solar Impulse: en.wikipedia.org/wiki/Solar_Impulse it isn't commercial, isn't in anything like mass production, and is very slow, but they did do a round-the-world trip in the second one. $\endgroup$ – Puffafish Apr 4 '19 at 14:39
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    $\begingroup$ @Fattie Solar energy is perfectly workable on the ground, where getting 200W per square meter is fine. $\endgroup$ – pjc50 Apr 4 '19 at 15:20
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If no, why not?

While I can't say categorically that it's not happened, I'm pretty sure.

Solar radiance is approx. 1kW/m^2. A 737 has approximately 100m^2 wing area. Solar cells are approximately 20% effective.

If you covered the entire wings in solar panels, that would work out to 20kW of electrical power at best. At night, it would be close to zero extra power.

Jet fuel contains ~43MJ/kg of energy. 20kW is 20kJ/s. For a 2 hour flight, the total energy produced would be 144MJ, or comparable to energy in 3-4 kilograms of jet fuel.

Turbines is not 100% efficient, so let's say that with all losses in engine, 25% of the power in the fuel is available as electricity. That means you'd need 12kg of fuel to provide the same amount of electricity as the solar panels.

12kg of fuel. That's probably far less than the solar cells will weigh, probably by a factor of at least ten. In addition, the you don't have to carry around already burnt fuel, unlike solar cells, which you will have to carry around.

Edit: I found another answer on this site, that claims extra fuel use is on the order of 0.125kg/kWh. I don't know if that's correct or not, nor do I really care. It doesn't change the conclusion, it only makes jet fuel even more favorable.

In short the amount of power provided by solar cells is tiny compared to the energy contained in jet fuel. And that doesn't even touch on the mechanical requirements of a wing...

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    $\begingroup$ Right, although a not completely unconceivable setup would be to have thin-film cells on the wings instead of paint. Solar cells can be made very light. The more fundamental problem is that there's just not really much area available, especially with reasonably high aspect ratio wings – and lower aspect ratio would mean higher drag, which would again defeat the point. $\endgroup$ – leftaroundabout Apr 4 '19 at 12:34
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    $\begingroup$ The problem with generating "only" 20kW by solar panels is not only that it is a tiny proportion of the total, but it's more electrical power than you actually need most of the time. For example the battery backup system on a 737-800, designed to provide 30 minutes emergency power if all other power generators fail, is rated at less than 2kW. Adding yet another system to convert the tiny amount of "excess" solar energy into mechanical thrust would make the concept even less practical. $\endgroup$ – alephzero Apr 4 '19 at 16:20
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    $\begingroup$ @leftaroundabout the quality of the paint job is actually critical for the aerodynamics of the plane. Any "roughness" on the wing surface from the installation of solar cells (e.g. at the joints between sections of panel) would potentially mess up the boundary layer behaviour and reduce the stall margin. $\endgroup$ – alephzero Apr 4 '19 at 16:28
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    $\begingroup$ Sure, there's lot's of small reasons why it's difficult. But those could probably be solved if it was economically viable. But it's not. It's technically difficult, and there damn close to zero reason for it. $\endgroup$ – vidarlo Apr 4 '19 at 16:50
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    $\begingroup$ @alephzero I imagine that this power would be used mainly for non-critical systems, like passenger entertainment, WiFi, and power outlets. $\endgroup$ – Barmar Apr 4 '19 at 17:44
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No, there are several reasons:

  • Fragility v Efficiency v Weight: the most efficient solar panels are rigid and heavy, which is bad for a wing structure. Flexible and light panels do exist, but they are half the efficiency. They also have limitations to how much flexing they really can take, the constant flexing of a wing, vibrations, cycles between hot and extreme cold at altitude all make it a punishing environment for that kind of technology. Covering the fuselage would mean less flexing, but then you'd only have a few panels positioned right at any one time to create electricity
  • Weight: In addition to the weight of the panels themselves you have all the other technology to make them work, like regulators, power conditioners, power storage, delivery wiring
  • Complexity: This is yet another system to maintain, and it would be complicated to do so. If a panel breaks you'd have to take apart the wing to get at it
  • Cost: you'd need solar panels that are efficient, flexible, durable and light. That all adds up to expensive panels, far more than is worth it
  • Limited window of use: Obviously solar panels are no good at night, but they are also only generate electricity when they are oriented at least partly towards the sun. If you're going to cover the wing then the sun must be a good 30-40° up before you'll get appreciable power from them

So it's a lot of weight and cost for a technology that isn't going to generate power for much of the time the airplane is in use.

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    $\begingroup$ This answer is not really correct. (1) the actual answer is that solar cells provide what can only be described as "no" power, within rounding error. (2) the difficulties mentioned (cost, engineering difficulty etc) would, indeed, be instantly overcome if solar energy was 10,000x more powerful than it is (indeed everything on an aircraft is very expensive, difficult to make). $\endgroup$ – Fattie Apr 4 '19 at 13:00
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    $\begingroup$ I don't follow you @Fattie. Solar cells do provide power, it's why they're being installed on homes all over the place.... $\endgroup$ – GdD Apr 4 '19 at 13:02
  • $\begingroup$ They provide a minuscul amount of power, in terms of the question asked though. Consider vid's answer. At the absolute theoretical max it would provide the equivalent of "a few KG" of jet fuel. (!) That's why I said it is zero within rounding error. $\endgroup$ – Fattie Apr 4 '19 at 13:08
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    $\begingroup$ There's more than one aspect to this @Fattie, and there was no point in repeating vidarlo's answer. $\endgroup$ – GdD Apr 4 '19 at 15:05
  • $\begingroup$ "Flexible and light panels do exist, but they are half the efficiency" - are you sure? The absolute thinnest and lightest "thin film" photovoltaics may be less than half the efficiency of crystalline cells (~10.5% vs ~26.7%), but panels using multicrystalline cells (~22.3%) are cheap and easily available and are as light and flexible as a laminated lunch menu. I use a folding panel like this with around 22% efficiency to charge my laptop when I'm away from power sources. $\endgroup$ – user56reinstatemonica8 Apr 6 '19 at 10:23
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Not purely on topic but there is a solar airplane. Solar supported airliner isn't out of the realm of possibilities, just solar tech isn't there yet. Also it would have to be economically feasible to even be considered.

Here's an article from 2016 about a solar airplane that traveled the globe.

https://www.theguardian.com/environment/2016/jul/26/solar-impulse-plane-makes-history-completing-round-the-world-trip

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    $\begingroup$ Solar Impulse 2 is basically covered in solar panels of probably at least 20% efficiency. This gives it enough power to fly with just a single pilot, at about 60 mph during the day, or 30 mph on batteries at night. Even with 100% efficient panels, that's still nowhere near the power to carry a cabin full of people at much higher speeds. Modern high-speed air travel uses a ridiculous amount of energy; that's why it creates so much greenhouse gas emissions. It would be nice if solar flight were viable for airliners, but it doesn't look like it ever will be. $\endgroup$ – Peter Cordes Apr 4 '19 at 19:44
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    $\begingroup$ "just solar tech isn't there yet" - that's not the problem. Even if solar panels were 100% efficient, they (and all the wiring, the motors they drive, etc.) would still weight much more than the tiny amount of fuel holding the same amount of energy. Solar panels make sense on the ground, where you have a lot of area and weight is not a big problem. Even if technology advances by ludicrous amounts to make solar powered airliners possible, the panels won't be on the plane, the solar power will be generated on the ground, and stored in batteries or fuel cells to be loaded unto the aircraft. $\endgroup$ – vsz Apr 5 '19 at 6:20
  • $\begingroup$ It is on topic enough! And still, if a plane with a wingspan of a 747 can carry one person, then a plane with a double that wingspan can carry 2 persons. The thing is how to combine wingspan with lift. Is there a limit to wingspan? One possible solution is unfoldable wings or a kind of sail that doesn't drag. In 100 years or less oil will be gone, and I guess people will still wanna fly. $\endgroup$ – GwenKillerby Apr 5 '19 at 17:53
  • $\begingroup$ @GwenKillerby : No, that's not how it works. Oil will never be completely gone. The oil reserves of the planet are not like the fuel in your car, that you drive and then at one point the car suddenly stops because fuel just ran out. As we use up the more economically extractable oil, it will become progressively more and more expensive. When it gets expensive enough, we will use less and less of it. There are many alternatives to oil, it's just that oil is so much cheaper. Once it gets expensive enough, it will be gradually replaced, first where it is easier to do so. $\endgroup$ – vsz Apr 5 '19 at 21:29
  • $\begingroup$ For air travel oil is much more difficult (with current technology nearly impossible) to replace, so oil will be probably used for longer, and it will last longer because other industries will use less of it. And after that, either some currently inconceivable technology will be discovered, or we will continue to use oils extracted by other means, maybe from crops. $\endgroup$ – vsz Apr 5 '19 at 21:33
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I once worked for a company that made electronics for commercial aircraft (flight deck printers, Ethernet switches, digital chart recorders).

In addition to what others have mentioned on this thread, you also have to account for the fact that if a product is manufactured for aircraft in the US, it must comply AS9100 and FAR, and whatever standard the EU is using nowadays. This includes rigorous testing to ensure that, not only is the device safe, but also that the device will not interfere with any of the critical systems of the aircraft.

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    $\begingroup$ How is this related to the question? Solar power is a pretty simple source of electricity to deal with compared to everything else, especially compared to a generator. $\endgroup$ – pipe Apr 5 '19 at 8:10
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One consideration that makes solar aircraft less feasible is that the figure of 1kw/m2 is for sunlight striking the solar panel square on - i.e. perpendicular to the panel. Unless you're flying in the tropics at noon, an aircraft's wings won't meet that. Their insolation (the amount of power from sunlight) drops with the cosine of the angle from vertical incidence.

Regarding doubling 747 wingspan for two seats - how many passengers will settle for a 45 MPH / 39 knot / 72 kph flight speed (i.e. New York to London in 77 hours if no headwind)?

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  • $\begingroup$ but Jim - EVERYONE would get a window seat $\endgroup$ – Fattie May 11 '19 at 19:25

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