4 ms·

Serious question: If a plane runs on electricity, would it ever make sense to put solar panels on top of it to try and get extra range? Or is the scale so off

Serious question: If a plane runs on electricity, would it ever make sense to put solar panels on top of it to try and get extra range? Or is the scale so off that solar panels can't meaningfully contribute?

DiffEq 9y ago
They would not meaningfully contribute.
- Kelbit 9y ago
  Quick back of the envelope calculation: Wing area of a Cessna 172 (representative GA aircraft) is 16 m^2. Best-case insolation on a sunny day with no clouds at high noon is around 1000 W/m^2. Best commercially available solar cells are the triple-junction cells used on commercial satellites, at about 30% efficient. They are horrendously expensive (around $50-100k per square meter), but we will ignore economics for now. 16 m^2 * 1000 W m^2 * 0.3 = 4.8 kW. Powerplant for a Cessna 173 is a 160 hp piston engine, or 120 kW. So, we're about an order of magnitude and a half off. The cells also aren't massless, so they will add weight and reduce range... you'd need to do a cost/benefit there. There are a couple solar UAVs in existence (example: https://en.wikipedia.org/wiki/Qinetiq_Zephyr https://en.wikipedia.org/wiki/Qinetiq_Zephyr) which use super lightweight materials and have very large wing areas to support solar cells. Even then, they are really on the hairy edge of where physics works in your favor.
  - mrfusion 9y ago
    Does it use the full 160hp while cruising though? I’d imagine most of that peak power is for take off.
    - plantain 9y ago
      Most of it is still used while cruising. 100hp+ depending on cruise speed. The excess power for climb is very small, which is why weight/loading is so critical in aviation.
      - Gibbon1 9y ago
        > The excess power for climb is very small, which is why weight/loading is so critical in aviation. This is one of the reasons in favor hybrid electric aircraft. The marginal hp to weigh ratio of an electric motor is about 3 hp/lb or ~5.5 kw/kg. And partial load efficiency is high. Means you potentially have a lot more hp available on take off. For commecial passenger aircraft hybrid turbofans wouldn't have spooling up lag like straight turbofans. Turbine engines take seconds to spool up. This big big problem with jet aircraft during landing. If you hit wind shear and/or need to go around you need to apply more power and sometimes the lag is fatal.
  - AshleyGrant 9y ago
    Aviation is an expensive market, so in some future world where a 75% efficiency of solar cells was available, it would be possible. And maybe even economically feasible at some point. Not any time soon though. And the great thing about flying is that it's pretty much always a sunny day when you're in cruise.
ythn 9y ago
I wonder though if you could do "regenerative braking" in electric planes, i.e. spin up a flywheel anytime you descend, and then tap that energy next time you want to ascend. Doesn't really make sense for planes that cruise at altitude, but could make sense for hobby craft.
- AshleyGrant 9y ago
  The aircraft in the article uses the propeller as a windmill during descents to charge the battery.
- skykooler 9y ago
  The problem is that you'd have to descend rather steeply for any sort of "braking" to work. In practice, you're unlikely to get more range by descending steeply and then using the power gained from that to fly level than you would get by descending at best glide angle.
- bdamm 9y ago
  This very same airplane mentioned in the top article does in fact capture energy from descent, and it's only viable because the application is assumed to be training, where the point of the airplane is to do a lot of takeoffs and landings. Another reference: https://newsline.kitplanes.com/2017/07/26/alpha-electro-an-electric-pipistrel/ https://newsline.kitplanes.com/2017/07/26/alpha-electro-an-e...
  - AshleyGrant 9y ago
    I think there's plenty of viability for recovering energy during a descent. Any aircraft that has spoilers for use during descent could instead use the energy extraction from the prop windmilling to reduce the need for spoilers and thus get some "free" energy during descent. It isn't going to be a ton of energy, but it might be enough to get a "free" taxi to the gate/hangar/tie-down out of the deal.
    - bdamm 9y ago
      It's not quite that direct. Many aicraft use flaps to boost lift during descent (better low-speed control) and steepen the fuselage pitch relative to the direction of flight (better visibility of the ground). Putting that energy into the propeller doesn't help with either of those. Also, some of the speed brakes are deployed so that the engine can stay warm by producing power instead of being "shock cooled". Otherwise stress builds in the engine and results in the engine wearing out earlier than it otherwise would if it were consistently cooled slowly. So this also doesn't directly translate; the pilot of the electric plane simply pulls back all the power and is coasting, which for many airplanes (not gliders!) results in a satisfactory sink rate and no need for additional braking. The pilot could engage more braking, but then the sink rate would be "emergency" class.
agumonkey 9y ago
It probably helps, since solar impulse works with that. But if it makes a difference for heavy planes .. good question.
- 24gttghh 9y ago
  Was about to say this: https://en.wikipedia.org/wiki/Solar_Impulse https://en.wikipedia.org/wiki/Solar_Impulse :)
elihu 9y ago
It might. Wings have a lot of surface area, so it makes more sense than putting solar on cars. Also, if a plane isn't being constantly used it could be useful to have it charge while parked (which is also a good argument for putting solar on cars even if it doesn't noticeably improve range). It would also provide a charging option if you have to land at an airstrip that doesn't have an electrical outlet handy.
marcosdumay 9y ago
The slower your plane move, the more difference those would make. As people already commented, it wouldn't make much difference for the usual models, but you can design a plane that benefits from it. It may have some good uses (agriculture comes to mind), but probably won't be good for transportation.
xkcd-sucks 9y ago
http://aroundtheworld.solarimpulse.com/ http://aroundtheworld.solarimpulse.com/
prostoalex 9y ago
People are experimenting with this https://www.facebook.com/groups/138721826332626/permalink/668465850024885/ https://www.facebook.com/groups/138721826332626/permalink/66...
dredmorbius 9y ago
Not for substantial cargo or human transport, the areal energy density is simply too low. For surveillance, comms, and very small-scale payload delivery (possibly drugs, far more likely munitions), ultralight drones with battery + solar could offer loiter / time-in-air and / or modest speed longer-distance capabilities.
gruturo 9y ago
Not in any meaningful way with just the sun's light hitting the panels. On the other hands, if you were to beam a few hundred kilowatts by (carefully!!) aiming at the panels with a massive laser, you could power it this way, yes. Bonus if you are doing it from a satellite: you can power a plane even across an ocean and don't need many ground stations. On the other hand you just built an orbital death ray so expect protests from, well, everybody.

DiffEq 9y ago

They would not meaningfully contribute.

Kelbit 9y ago
Quick back of the envelope calculation: Wing area of a Cessna 172 (representative GA aircraft) is 16 m^2. Best-case insolation on a sunny day with no clouds at high noon is around 1000 W/m^2. Best commercially available solar cells are the triple-junction cells used on commercial satellites, at about 30% efficient. They are horrendously expensive (around $50-100k per square meter), but we will ignore economics for now. 16 m^2 * 1000 W m^2 * 0.3 = 4.8 kW. Powerplant for a Cessna 173 is a 160 hp piston engine, or 120 kW. So, we're about an order of magnitude and a half off. The cells also aren't massless, so they will add weight and reduce range... you'd need to do a cost/benefit there. There are a couple solar UAVs in existence (example: https://en.wikipedia.org/wiki/Qinetiq_Zephyr https://en.wikipedia.org/wiki/Qinetiq_Zephyr) which use super lightweight materials and have very large wing areas to support solar cells. Even then, they are really on the hairy edge of where physics works in your favor.
- mrfusion 9y ago
  Does it use the full 160hp while cruising though? I’d imagine most of that peak power is for take off.
  - plantain 9y ago
    Most of it is still used while cruising. 100hp+ depending on cruise speed. The excess power for climb is very small, which is why weight/loading is so critical in aviation.
    - Gibbon1 9y ago
      > The excess power for climb is very small, which is why weight/loading is so critical in aviation. This is one of the reasons in favor hybrid electric aircraft. The marginal hp to weigh ratio of an electric motor is about 3 hp/lb or ~5.5 kw/kg. And partial load efficiency is high. Means you potentially have a lot more hp available on take off. For commecial passenger aircraft hybrid turbofans wouldn't have spooling up lag like straight turbofans. Turbine engines take seconds to spool up. This big big problem with jet aircraft during landing. If you hit wind shear and/or need to go around you need to apply more power and sometimes the lag is fatal.
- AshleyGrant 9y ago
  Aviation is an expensive market, so in some future world where a 75% efficiency of solar cells was available, it would be possible. And maybe even economically feasible at some point. Not any time soon though. And the great thing about flying is that it's pretty much always a sunny day when you're in cruise.

ythn 9y ago

I wonder though if you could do "regenerative braking" in electric planes, i.e. spin up a flywheel anytime you descend, and then tap that energy next time you want to ascend. Doesn't really make sense for planes that cruise at altitude, but could make sense for hobby craft.

AshleyGrant 9y ago
The aircraft in the article uses the propeller as a windmill during descents to charge the battery.
skykooler 9y ago
The problem is that you'd have to descend rather steeply for any sort of "braking" to work. In practice, you're unlikely to get more range by descending steeply and then using the power gained from that to fly level than you would get by descending at best glide angle.
bdamm 9y ago
This very same airplane mentioned in the top article does in fact capture energy from descent, and it's only viable because the application is assumed to be training, where the point of the airplane is to do a lot of takeoffs and landings. Another reference: https://newsline.kitplanes.com/2017/07/26/alpha-electro-an-electric-pipistrel/ https://newsline.kitplanes.com/2017/07/26/alpha-electro-an-e...
- AshleyGrant 9y ago
  I think there's plenty of viability for recovering energy during a descent. Any aircraft that has spoilers for use during descent could instead use the energy extraction from the prop windmilling to reduce the need for spoilers and thus get some "free" energy during descent. It isn't going to be a ton of energy, but it might be enough to get a "free" taxi to the gate/hangar/tie-down out of the deal.
  - bdamm 9y ago
    It's not quite that direct. Many aicraft use flaps to boost lift during descent (better low-speed control) and steepen the fuselage pitch relative to the direction of flight (better visibility of the ground). Putting that energy into the propeller doesn't help with either of those. Also, some of the speed brakes are deployed so that the engine can stay warm by producing power instead of being "shock cooled". Otherwise stress builds in the engine and results in the engine wearing out earlier than it otherwise would if it were consistently cooled slowly. So this also doesn't directly translate; the pilot of the electric plane simply pulls back all the power and is coasting, which for many airplanes (not gliders!) results in a satisfactory sink rate and no need for additional braking. The pilot could engage more braking, but then the sink rate would be "emergency" class.

agumonkey 9y ago

It probably helps, since solar impulse works with that. But if it makes a difference for heavy planes .. good question.

24gttghh 9y ago
Was about to say this: https://en.wikipedia.org/wiki/Solar_Impulse https://en.wikipedia.org/wiki/Solar_Impulse :)

elihu 9y ago

It might. Wings have a lot of surface area, so it makes more sense than putting solar on cars. Also, if a plane isn't being constantly used it could be useful to have it charge while parked (which is also a good argument for putting solar on cars even if it doesn't noticeably improve range). It would also provide a charging option if you have to land at an airstrip that doesn't have an electrical outlet handy.

marcosdumay 9y ago

The slower your plane move, the more difference those would make. As people already commented, it wouldn't make much difference for the usual models, but you can design a plane that benefits from it. It may have some good uses (agriculture comes to mind), but probably won't be good for transportation.

xkcd-sucks 9y ago

http://aroundtheworld.solarimpulse.com/ http://aroundtheworld.solarimpulse.com/

prostoalex 9y ago

People are experimenting with this https://www.facebook.com/groups/138721826332626/permalink/668465850024885/ https://www.facebook.com/groups/138721826332626/permalink/66...

dredmorbius 9y ago

Not for substantial cargo or human transport, the areal energy density is simply too low. For surveillance, comms, and very small-scale payload delivery (possibly drugs, far more likely munitions), ultralight drones with battery + solar could offer loiter / time-in-air and / or modest speed longer-distance capabilities.

gruturo 9y ago

Not in any meaningful way with just the sun's light hitting the panels. On the other hands, if you were to beam a few hundred kilowatts by (carefully!!) aiming at the panels with a massive laser, you could power it this way, yes. Bonus if you are doing it from a satellite: you can power a plane even across an ocean and don't need many ground stations. On the other hand you just built an orbital death ray so expect protests from, well, everybody.