Are there any commercial aircraft that use solar energy in terms of auxiliary (not as main power source, as of now) power source ?

If Yes, In what forms is the solar power used?

If No, Why isn't solar energy used in commercial aircraft as an auxiliary power source?

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    $\begingroup$ Probably not on a commercial airliner, because solar cells don't have a good energy/weight ratio, they need a battery to store energy, and are fragile at Mach .7. In addition they don't work well within clouds and rain. On Earth, solar power is limited to 250 W/m² at 100% efficiency (so maybe a mean 50 W/m² in operational conditions). $\endgroup$ – mins Aug 16 '16 at 8:59
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    $\begingroup$ Yes. When the aircraft is sitting at the gate on a bright, sunny day, it uses solar energy to overheat the interior! $\endgroup$ – FreeMan Jan 16 at 14:14

The sun's energy is a max of about 1000 W/m2 at sea level (sun directly overhead, clear day). A solar power installation may have around 15% efficiency, yielding a max of 150 W/m2. Let's just see roughly what that gives us for a 737, to see if it's even that much.

The wing area of a 737 is 124 m^2, and the fuselage is 42 m long and 3.76 m wide. This gives a total area of 280 m2 if the sun is directly overhead. Covering all of this with solar panels gives a total (extremely optimistic) output of 42,375 W. The fuselage tapers at both ends though, and needs to have openings for the windows, antennas, etc. The wings will probably only have the fixed panels covered in solar panels (not flaps, slats, etc) so you would be looking at significantly less for a realistic installation. The Solar Impulse 1 was in the same ballpark (200 m2 for a peak of 45 kW).

The APU on a 737 is rated at 90 kVA. Of course you will usually use less than that, but we are looking at solar only providing maybe 20% of the capacity for any considerable part of the day. Since solar is not always available, you cannot even downsize the APU.

Another important function of the APU is to provide bleed air. This can be used to run the air conditioning packs and to start the engines. Photovoltaic panels would be a very inefficient way to provide bleed air.

An airplane will probably look strange being covered in solar panels, but covering so much area also causes some issues. Inspecting the aircraft for damage will be difficult. This means the solar installation should be easily removable (also for repairing damage to the solar cells), and the structure may need to be more damage tolerant. The panels would also need to be attached well so that they would not depart the plane even if damaged. This all adds more weight.

You would be adding a lot of weight to install solar that only helps a little bit sometimes. You may get a small benefit by reducing the generator load on the engines, but probably not nearly enough to make up for the increase in weight.

Airlines are big on reducing fuel usage, and are willing to add weight to save fuel overall (such as with winglets), but we haven't seen solar put to use on any airliners yet. The most efficient way overall to provide regular power is to extract it from the engines that are already running. For auxiliary power, a small turbine generator, which can provide both bleed air and electricity. And for emergencies, a RAT.


No. The APU has to work on night flights as well as it works during the day. (Granted, it can be inoperative and deferred for either, but the general expectation is that the equipment, when operative, is available 24x7, rather than 12x7.) Anything that's always inoperative from sunset to sunrise is a novelty at best, and excess weight for sure.

  • $\begingroup$ This may be misleading, aircraft fly quite regularly with inop APU's, or they are sometimes powered down in flight. $\endgroup$ – Ron Beyer Aug 16 '16 at 13:10
  • $\begingroup$ While an aircraft certainly CAN be dispatched with an APU inop, it would be wasteful to build one knowing in advance that it will be inop essentially half of the time at best. The purpose of an APU isn't to be run continuously during flying (ETOPS excepted), but to be available as a backup power supply if a generator dies. Generators die at night just like during the day. $\endgroup$ – Ralph J Aug 16 '16 at 13:14
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    $\begingroup$ Right, but consider the APU is not on the MEL for most commercial (non-ETOPS) aircraft and is a deferrable maintenance item. The actual back-up for an "all-generator-out" scenario is the RAT, some aircraft are even equipped with squat switches that prevent APU operation in flight (typically smaller aircraft). Some aircraft won't allow an APU start when the engines are dead (DC-9) because the batteries are required equipment (30 minutes without generators). Either way though, having an inop APU is more common than you think. $\endgroup$ – Ron Beyer Aug 16 '16 at 13:26
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    $\begingroup$ Who wants a RAT replaced with something that can't be used at night?!? The OP asked why solar power isn't used in commercial aircraft, and the answer is that adding weight + complexity for something that doesn't work at night, isn't viewed is a positive addition to the aircraft. All the discussions of DC-9 systems and Boeing APU's and whatever else doesn't change that central fact. ((BTW, if an APU is "not on the MEL" for a given aircraft, then it would have to be operative for the aircraft to fly... not on the MEL = no MEL relief = can't defer it. Think you meant, not REQUIRED by the MEL.)) $\endgroup$ – Ralph J Aug 16 '16 at 13:36
  • $\begingroup$ Yeah, I worded that wrong about the MEL. I wasn't suggesting replacing the RAT or the APU, I guess I was just trying to say that when you said "The APU has to work on night flights..." it sounds like there is some rule about an operational APU, which could be misleading. I'd say it would be better if you just said "The APU works on night flights as well..." to clarify. I think we are agreeing but I'm just not wording things right, its early :) $\endgroup$ – Ron Beyer Aug 16 '16 at 14:05

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