Solar Powered Air Transport

[Zolty]

It occurred to me that if we can phase out fuel cost as a concern for airlines flying would become much cheaper (somewhere from 20-50% cheaper based on my out of ass estimate). The most abundant energy source on the planet is the sun and we have solar technology although it's far from mature. I propose the following questions to you highly technical forum:


First do you think it is possible to have solar powered air transport? (Why?)

What obstacles do we face in development ?
Battery weight, solar cell effectiveness, engine efficiency, ect...

How far are we away from replacing Jet transportation?


I will concede that Jets will always exist in some form, just because I cannot envision an electrically bound system which can approach Mach. But what about for shipping or cheap flights?

 

[silverpig]

Well you'd need a prop plane to run off an electric motor. You'd need to cover the thing in solar cells to get enough thrust. You wouldn't be able to take off.

My guess is that you'd have enough juice to keep the plane flying, but not enough to takeoff. You'd need some kind of booster for that.

 

[Cogman]

Originally posted by: silverpig
Well you'd need a prop plane to run off an electric motor. You'd need to cover the thing in solar cells to get enough thrust. You wouldn't be able to take off.

My guess is that you'd have enough juice to keep the plane flying, but not enough to takeoff. You'd need some kind of booster for that.

yep, if you want indefinite flight then a nuclear powered plane is where it is at (and they WERE actually being build by the government at one point).

The biggest problem being that the plane has to have be propeller driven, and it has to be fairly big.

For solar, at least, the takeoff juice needed could be overcome with some powerful batteries (or even rather large capacitors might be able to do it).

 

[KillerCharlie]

If you covered every inch of an airplane with 100% efficient solar cells and it was a perfectly clear day, you would have nowhere near enough power to even get off the ground.

If you're going to put batteries in it (which are heavy and would dog the aircraft performance), why even bother with solar cells in the first place?

 

[silverpig]

Originally posted by: Cogman

Originally posted by: silverpig
Well you'd need a prop plane to run off an electric motor. You'd need to cover the thing in solar cells to get enough thrust. You wouldn't be able to take off.

My guess is that you'd have enough juice to keep the plane flying, but not enough to takeoff. You'd need some kind of booster for that.

yep, if you want indefinite flight then a nuclear powered plane is where it is at (and they WERE actually being build by the government at one point).

The biggest problem being that the plane has to have be propeller driven, and it has to be fairly big.

For solar, at least, the takeoff juice needed could be overcome with some powerful batteries (or even rather large capacitors might be able to do it).

You wouldn't want to haul batteries around.

If you had some kind of launching system similar to the steam catapult on aircraft carriers, or maybe some kind of electric boost drive to power the engines directly (linear induction or something), then you could leave all that stuff behind on the ground.

 

[BrownTown]

The physics make this more or less an impossibility fort anything less than a proof of concept vehicle. Even if every square inch was covered in 100% efficient solar panels you still couldn't go anywhere fast (if at all). Solar power is a very diffuse power source and not feasible on any moving object (save a satellite which is in space with no gravity or air resistance to contend with).

 

[silverpig]

Originally posted by: BrownTown
The physics make this more or less an impossibility fort anything less than a proof of concept vehicle. Even if every square inch was covered in 100% efficient solar panels you still couldn't go anywhere fast (if at all). Solar power is a very diffuse power source and not feasible on any moving object (save a satellite which is in space with no gravity or air resistance to contend with).

Quick calcs:

100% efficient solar cells would produce about 1000 hp worth of energy on a 747 sized plane.

A 747 requires about 55000 lbs of thrust to maintain cruising speed of ~600 mph.

Some very rough googling says that a P51D could put out about 3000 lbs of thrust with a 1200 hp engine.

So you'd really need something quite drastic in order for it to work. Either take a glider, plaster it with thin film solar cells and power a small motor to help keep it aloft longer, or do the only thing that would ACTUALLY work: Cover a blimp with solar cells and just those to run the propeller. A solar powered airship would probably be feasible, but long trips would be multi-day journeys, and the sun's not out at night. You could store energy in batteries etc I suppose.

 

[Gibsons]

There are solar powered airplanes. Here's one that reached 100,000 feet. Text Not sure how it was launched. Looks like it couldn't lift more than a few pounds of cargo. It's pretty slow too...

someone mentioned nuclear powered airplanes, the U.S. successfully tested a nuclear powered ramjet in the 50s. It was meant for a cruise missile/unmanned bomber. Project Pluto Scary stuff.

 

[Paperdoc]

To get even close, I'm sure you would need solar cells nearly 100% efficient (we're no where close on this now) plus big-time batteries (big weight here). Batteries for huge power surge for take-off, and for continuing to fly smoothly in clouds. Imagine flying into a cloud bank and having your engines suddenly drop to 60% of power! Oh yeah, how about red-eye flights at night!

 

[Farmer]

Aside from spinning compressors using electric motors, technically you can use a purely electric source to drive a conventional Brayton-cycle jet turbine: instead of exploding fuel in the combustor, heat via huge electric heaters (like your electric oven). You can also take advantage of some resonance with certain components of air, hence your combustor becomes something akeen to a microwave oven for air. This would eliminate point-emissions, but don't know how efficient that would be though, it all depends: if you can get energy/mass of a battery to approach or exceed that of jet fuel, then there's some way to make it work. So an electric-drive jet is possible at least in principle, and hence the possibility of supersonic flight with electric aircraft is also plausible.

But since many have already calculated, running modern commercial aircraft on only solar power is impossible. The Sun is immensely powerful, sure, but at Earth, the energy flux isn't high enough to do that. That is the major draw of fossil fuel: extremely high energy density. Maybe if we lived on Mercury, but not on Earth. If you insisted on running aircraft only on solar power as directly as possible, it'd make much more sense to focus on development of better batteries, and leave the collection of solar energy to large scale-ground based stations.

 

[deputc26]

As a pilot and a solar power/ battery enthusiast I can tell you with confidence that the problems are almost certainly insurmountable; but before I address them I first want to point out that many (or at least half a dozen different, operating designs) solar powered planes exist right now using solar cells that are at best 30% efficient.

1. Power density: Helios, NASA's highest performance solar plane was a flying wing. It had to be a flying wing because that is the most efficient shape for an aircraft. A plane that carries passengers at a useful speed must due so at a high altitude which means that the aircraft would have to have a pressure vessel large enough for the pilot/pax. Pressure vessels do not fit in a flying wing shape and they are heavy structures. All current solar aircraft are made with cutting edge material and are on the absolute bleeding edge of low mass design. A solar plane currently could not fly at all with a pressure vessel of useful size and batteries if you stay within a reasonable scale. Batteries are required for all these aircraft because... well... the sun ain't always shining. Qinetiq's record holding solar powered plane used Lithium Sulfur batteries which have amazing energy density but... contain elemental lithium (ya they're not approved for public sale) these batteries are considered "exotic" which brings me to my next point.


2. Cost of materials: With the best solar tech, batteries and materials/structures available today a solar powered plane with the wing span of a 747 could carry at most two adults. The cost of just the materials involved (not including the cost of bringing them all together) would be well north of $15 million; I cannot, give you anywhere near a solid number but I am confident that it could not be cheaper. This is, of course, economically impractical.

3. Safety/the feds: The current aircraft have all sacrificed strength for low mass to alarming degrees, Helios (which I mentioned before) came apart in the air because its low mass design was also (as with all extremely low mass designs) very low strength. A Solar aircraft that was strong enough to comply with all Federal Aviation Regulations (FARs) would certainly be too heavy to fly.

Hypothetically if solar cells could be made that were 100% efficient (this is considered completely impossible) and batteries with twice the energy density of the Li-S cells existed then t would start to look almost possible for non-commercial purposes (I can see some billionaire flying around in one).

In the mean time useful (for transporting humans anyway) solar aircraft are just a fairy tale.

 

[WildW]

If you're not in such a hurry, what about an airship?

 

[Gibsons]

Originally posted by: WildW
If you're not in such a hurry, what about an airship?

Text

Pretty cool, but I wonder what happens when it encounters a bad storm, either in the air or on the ground.

 

[pm]

There are several model airplanes that run on solar power that I have seen fly in videos. Standard-looking sailplane type planes with brushless motors and large props. They were expensive to build though - they use surplus space-grade GaAs solar cells (www.spectrolab.com sells surplus space-grade cells... about 25-29% efficiency. you have to call them directly).

I can provide links if anyone is curious - I'd have to go look them up.

Still if it can be done with a scale model, it should be scalable to larger planes. It would just scale in cost.

I disagree with the idea that you couldn't get enough thrust for take-off - I'm certain you could. You just wouldn't be flying very fast when you are airborne. And it would cost a lot.

 

[Gibsons]

Originally posted by: pm
There are several model airplanes that run on solar power that I have seen fly in videos. Standard-looking sailplane type planes with brushless motors and large props. They were expensive to build though - they use surplus space-grade GaAs solar cells (www.spectrolab.com sells surplus space-grade cells... about 25-29% efficiency. you have to call them directly).

I can provide links if anyone is curious - I'd have to go look them up.

Still if it can be done with a scale model, it should be scalable to larger planes. It would just scale in cost.

I disagree with the idea that you couldn't get enough thrust for take-off - I'm certain you could. You just wouldn't be flying very fast when you are airborne. And it would cost a lot.

I'm not so sure about the scalable part. The old surface area:volume ratio thing.

 

[BrownTown]

Originally posted by: pm
I disagree with the idea that you couldn't get enough thrust for take-off - I'm certain you could. You just wouldn't be flying very fast when you are airborne. And it would cost a lot.

Getting enough thrust to take off I don't think is the issue, you could just sit around all day charging a capacitor and then discharge it to takeoff. The problem is AFTER you waste all that stored energy to get in the air you have to reach an equilibrium where the energy from the solar panels exceeds the energy required to counteract gravity. Even on a cloudless day on the equator that is still an impossible task for anything other than an impractical concept plan. Not to mention the first hint of bad weather KILLS any hope. Not just because the clouds would cut off your pwoer supply, but also because a storm would put stresses on the plan that such a weak frame could never withstand.

This really isn't a question of not having advanced enough technology, we are talking about the laws of physics imposing a limit on the abilities of such a plane.