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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?

Steve V.
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avsol
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    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. – jwenting Apr 04 '19 at 06:59
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    @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. – GittingGud Apr 04 '19 at 12:36
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    Airliners are ridiculous energy annihilators(1). It's like every single passenger driving in their own car. It's like asking why Warren Buffett does not cut his empty toothpaste tube open to not waste the remaining toothpaste.-- (1) Of course the energy is not annihilated in the strict sense -- energy is always only transformed. But it is transformed into an unusable state (from dense chemical energy to dispersed heat) by increasing the entropy. – Peter - Reinstate Monica Apr 04 '19 at 12:43
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    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. – Fattie Apr 04 '19 at 12:57
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    As you include trail options, there have been at least two planes which have been powered completely by photovoltaic panels, Solar Impulse: https://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. – Puffafish Apr 04 '19 at 14:39
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    @Fattie Solar energy is perfectly workable on the ground, where getting 200W per square meter is fine. – pjc50 Apr 04 '19 at 15:20
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    The course of action that could be viable would be to use ground-mounted solar arrays to generate power, and then store it either as a synthetic fuel, or in batteries for use in flight. Worth noting that the navy is working on the synthetic fuel idea, albeit not solar, as they operate airplanes off of floating nuclear power plants and would find it convenient not to have to replenish hydrocarbon fuel stores for the air wing and assorted accessories as often. – Chris Stratton Apr 04 '19 at 15:59
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    I hav heard that the wind is pretty strong up there. Why not use a wind turbine instead of solar panels? – TonioElGringo Apr 05 '19 at 12:54
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    @TonioElGringo I LOLed! – barbecue Apr 05 '19 at 14:20
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    @Fattie The actual religion is believing that solar panels don't work. An area the size of Spain covered in solar panels would generate enough power for the entire world. Or 0,34 % of the LAND AREA to solar panels. The windmill parks needed for that would be 10 times as big in area, and using windmills to power your laptop & phone requires way more infrastructure. PS Trumps recent rambling, completely incoherent rant against windmills ("wind sounds cause cancer") is because of his ideology of GOLF COURSES over breathing and oxygen, and had nothing to do with cancer. Or TV. – GwenKillerby Apr 05 '19 at 17:30
  • @TonioElGringo a more serious answer would be that [A]. Windmills on planes is not a bad idea, it would recoup SOME of the spent energy. But present day cultural capitalism is against you. And the airflow IS TOO used for airconditioning, saving energy. Airplanes don't drag a fullblown AC system up there. But [B] The winddrag would slow down the plane (but not to the point it would cause the plane to lose lift capability. and [C] A windturbine on a moving vehicle is almost a pull up by your bootstraps kind of paradox, if you ignore point [A]. Which you should not. – GwenKillerby Apr 05 '19 at 17:42
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    @GwenKillerby Windmills on planes is a bad idea because you're using fuel to spin a fan to move the plane to spin a windmill (that slows the plane down again) to spin a generator. Why not just connect the generator to the engine directly and skip all the wind stuff? And in fact, that is what they do. Excluding RATs for emergencies. – user253751 Apr 05 '19 at 23:44
  • @immibis why not? because using wind to generate AC/cooling is skips the step of using extra machine power to power an AC system. Literal windmills on planes would be bad because it messes with the streamlined aerodynamics etc. There's nothing wrong with using clever air intakes, thats being done now already. – GwenKillerby May 11 '19 at 14:09
  • If we used windmills on the plane, it would be wise to additionally cover the windmill blades with solar panels. – Fattie May 11 '19 at 16:00

5 Answers5

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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...

vidarlo
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    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. – leftaroundabout Apr 04 '19 at 12:34
  • Black solar cell paint could also heat up the underlying wing structure. If aluminum, that's not good as heated fuel expands. If composites, that's not that good as heat is bad - that's why composite structure are painted white to help keep them cooler. – CrossRoads Apr 04 '19 at 15:36
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    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. – alephzero Apr 04 '19 at 16:20
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    @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. – alephzero Apr 04 '19 at 16:28
  • Also, having solar panels at different angles to the sunlight introduces complexity. And an airfoil, by definition, is a curved surface. – Acccumulation Apr 04 '19 at 16:28
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    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. – vidarlo Apr 04 '19 at 16:50
  • There is this though: https://interestingengineering.com/boeings-solar-autonomous-aircraft-can-fly-forever-and-its-due-in-2019 and https://en.wikipedia.org/wiki/Solar_Impulse#Solar_Impulse_2_(HB-SIB) The latter being already operational, the first being technically more relevant. – paul23 Apr 04 '19 at 17:33
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    @alephzero I imagine that this power would be used mainly for non-critical systems, like passenger entertainment, WiFi, and power outlets. – Barmar Apr 04 '19 at 17:44
  • @paul23 that's not an commercial jetliner. As a proof of concept it's been done. For some applications, such as surveillance drones, it may make sense. – vidarlo Apr 04 '19 at 18:07
  • That's probably far less than the solar cells will weigh, probably by a factor of at least ten try another order of magnitude. 20kg/m^2 is not uncommon, and even light solar panels work out to about 10kg/m^2 so figure on maybe 1000kg for such an installation - just panels. Now add the inverters and auxiliary equipment to feed it into the existing power grid, plus the extra fuel to carry that extra load. No way this is coming out energy positive. – J... Apr 04 '19 at 20:52
  • Or... to put it another way, you would burn much more fuel just carrying the extra weight of the solar generation equipment than said equipment could displace in fuel required for electrical generation. – J... Apr 05 '19 at 14:48
  • @J can you show an actual calculation for that, or are you just guessing? – GwenKillerby Apr 05 '19 at 17:44
  • @GwenKillerby A 737 is about 25% fuel when loaded. Add 1000kg of payload, add 250kg fuel (~310L). Back of the envelope. – J... Apr 05 '19 at 20:52
  • @Joey Please don't edit to put units into that fractional format. Nobody writes units like that and, IMO, it makes the post harder to read by making the text bounce up and down. – David Richerby Apr 06 '19 at 16:51
  • @DavidRicherby: Then reject the edit. I don't have edit privileges here. But at least leave the non-breaking spaces before the units and the ², which do make the units easier to read (along with being just sensible typography). – Joey Apr 08 '19 at 07:20
  • @Joey I did reject the edit but others voted to accept. I commented to advise you for the future. I agree that the non-breaking spaces are good but it takes so long to edit out all the <sup> and <sub> tags that whoever decided to roll back the edit (which is just one click) presumably didn't feel it was worth their time. – David Richerby Apr 08 '19 at 08:33
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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.

GdD
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    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). – Fattie Apr 04 '19 at 13:00
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    I don't follow you @Fattie. Solar cells do provide power, it's why they're being installed on homes all over the place.... – GdD Apr 04 '19 at 13:02
  • 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. – Fattie Apr 04 '19 at 13:08
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    There's more than one aspect to this @Fattie, and there was no point in repeating vidarlo's answer. – GdD Apr 04 '19 at 15:05
  • "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. – user56reinstatemonica8 Apr 06 '19 at 10:23
  • @Fattie At the risk of sounding risque, perhaps coarse, (but it is outmatched by your senseless remarks) ............... if you REALLY think solar panels provide no power would you be willing to hook your private parts up to a solar panel and ...... switch it on? It would just tingle, so it's not a problem, right? That would be just a case of putting your money (or rather..... johnson) where your mouth is, no pun intended. – GwenKillerby May 11 '19 at 14:13
  • @GwenKillerby, I appreciate the humour (ouch ! :) ), when I said "no power !" I was partially being hyperbolic, but do note that quoting the actual sentence - the immediately following words - [in an ariliner milieu] solar cells provide "no" power, within rounding error. Please note that that sentence is literally precise and accurate in an engineering/basic math sense. If you simply entirely covered a 787 with hypothetical 100% solar panels, the total power produced would be zero within rounding error compared to the two jet engines. – Fattie May 11 '19 at 15:46
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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

Mertyman
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    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. – Peter Cordes Apr 04 '19 at 19:44
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    "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. – vsz Apr 05 '19 at 06:20
  • 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. – GwenKillerby Apr 05 '19 at 17:53
  • @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. – vsz Apr 05 '19 at 21:29
  • 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. – vsz Apr 05 '19 at 21:33
  • FYI, if you start a post with "Not purely on topic" it should probably be a comment not an answer. I get that you don't have the reputation to make a comment though. – Sam Apr 06 '19 at 01:31
  • @vsz Yours might be the most Trumpian comment I ever read on stack exchange. To use your own claims against you: If oil will never be completely gone...... then why would it need to be replaced???? Riddle me that. Stuff is finite. And oil reserves will OF COURSE be gone. Even coal, if we were stupid enough to use it will only last 400 years. Your dangerous fossil fuel religion is the one ideology that will genocide us. Starting with the poor, of course. – GwenKillerby May 11 '19 at 14:26
  • @GwenKillerby I just think you are not aware of the scale of numbers involved. A big airliner produces on the order of 300,000 horsepower. You can look up or calculate yourself how big an area of solar collector one would need to generate that much power. – Fattie May 11 '19 at 15:53
  • @FATTIE y I just think you are not aware of the third or fourth answer, which clearly says that there is already a solar panel powered plane. And NOWHERE do i say that you can fly a 747 on solar panels alone. Please read better. Also, learn to think better. You are denying that solar panels produce electricity. That is ... INSANE, even in this era of Trump. That's some #Antivaxxer level of denying. In 10 years time, poor, 3rd world Morocco will get 20-33% of all its power from solar panels. While richer countries play stupid. – GwenKillerby May 11 '19 at 17:23
  • hi @GwenKillerby ! - need to get worked up. (1) the SolarImpulse experiment is a fantastic example of how ....... absolutely tiny/slow a solar aircraft has to be! (2) let's say the discussion is only about supplementing *just the electric generators* on an airliner, using solar power. Again, it's just utterly, totally out of scale. Solar Panels are incredibly clever technology for certain things, but utterly out of the question for airliners. – Fattie May 11 '19 at 19:27
  • @GwenKillerby : Before throwing personal insults around first please learn to read comprehensively, instead of focusing only on two words out of context. The main problem with oil is NOT that it will one day be just used up completely (and it won't, because as easy-to-access supplies will get used up, it will become more and more expensive, up to the point it will no longer be economical to extract). Actually, if there were more oil reserves, we would be in much deeper trouble, as it would get more expensive much later. – vsz May 12 '19 at 18:31
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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.

Jerry Sweeton
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    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. – pipe Apr 05 '19 at 08:10
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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)?

Jim Horn
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