Forum: aviation.stackexchange.com
Tags: <aircraft-design><electrical-system><solar-power>
Title: Has there ever been an airliner design involving reducing generator load by installing solar panels?
Question: 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?

Answers:
sents: text: If no, why not?
text: While I can't say categorically that it's not happened, I'm pretty sure.
text: Solar radiance is approx.
text: 1kW/m^2.
text: A 737 has approximately 100m^2 wing area .
text: Solar cells are approximately 20% effective .
text: If you covered the entire wings in solar panels, that would work out to 20kW of electrical power at best.
text: At night, it would be close to zero extra power.
text: Jet fuel contains ~43MJ/kg of energy.
text: 20kW is 20kJ/s.
text: For a 2 hour flight, the total energy produced would be 144MJ, or comparable to energy in 3-4 kilograms of jet fuel.
text: 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.
text: That means you'd need 12kg of fuel to provide the same amount of electricity as the solar panels.
text: 12kg of fuel.
text: That's probably far less than the solar cells will weigh, probably by a factor of at least ten.
text: In addition, the you don't have to carry around already burnt fuel, unlike solar cells, which you will have to carry around.
text: Edit: I found another answer on this site, that claims extra fuel use is on the order of 0.125kg/kWh.
text: I don't know if that's correct or not, nor do I really care.
text: It doesn't change the conclusion, it only makes jet fuel even more favorable.
text: In short the amount of power provided by solar cells is tiny compared to the energy contained in jet fuel.
text: And that doesn't even touch on the mechanical requirements of a wing...
answer_details: author: aviation.stackexchange.com/users/27894/vidarlo
score: 57
sents: text: No, there are several reasons: Fragility v
text: Efficiency
text: v Weight:
text: the most efficient solar panels are rigid and heavy, which is bad for a wing structure.
text: Flexible and light panels do exist, but they are half the efficiency.
text: 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.
text: 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:
text: 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.
text: If a panel breaks you'd have to take apart the wing to get at it
text: Cost: you'd need solar panels that are efficient, flexible, durable and light.
text: 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.
text: 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
text: 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.
answer_details: author: aviation.stackexchange.com/users/1791/GdD
score: 11
sents: text: Not purely on topic
text: but there is a solar airplane.
text: Solar supported airliner isn't out of the realm of possibilities, just solar tech isn't there yet.
text: Also it would have to be economically feasible to even be considered.
text: Here's an article from 2016 about a solar airplane that traveled the globe.
text: https://www.theguardian.com/environment/2016/jul/26/solar-impulse-plane-makes-history-completing-round-the-world-trip
answer_details: author: aviation.stackexchange.com/users/38524/Mertyman
score: 2
sents: text: I once worked for a company that made electronics for commercial aircraft (flight deck printers, Ethernet switches, digital chart recorders).
text: 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.
text: 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.
answer_details: author: aviation.stackexchange.com/users/9084/Jerry Sweeton
score: 2
sents: text: 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.
text: Unless you're flying in the tropics at noon, an aircraft's wings won't meet that.
text: Their insolation (the amount of power from sunlight) drops with the cosine of the angle from vertical incidence.
text: 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)?
answer_details: author: aviation.stackexchange.com/users/22435/Jim Horn
score: 1

Summary of the answers:
Solar panels on plane wings are impractical and inefficient. Firstly, they are heavy and unlike jet fuel the weight does not reduce during the flight. Secondly, the amount of energy they would generate wouldnot be sufficient for most flights and does not compare to the energy generated by jet fuel.
The efficiency of the solar cells is easily compromised by weather conditions and the amount of power generated is more often than not insignificant, thus the more an aicraft relied on it, the slower it would fly. Also, the weight of solar panels is proportionate to their efficiency but even the efficiency of rigid solar panels does not justify the extra stress on the aircraft due to the weight of the solar cells and their equipment. Jet fuel is by far more efficient. The cost would be too high as well, and the results not that impressive. Finally, whether the aircraft is manufactured in the US or the EU, there are multiple requirements and standards with which it must comply, which makes this even more challenging.