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Ask Slashdot: What Stands In the Way of a Truly Solar-Powered Airliner?
centre21 writes "I've been reading about solar-powered aircraft all over the Internet, as well as solar power in general. But I'm wondering: is it more than just solar cell efficiency that's preventing the creation of a solar-powered airliner? Conspiracy views aside (which may be valid), it seems to me that if I were running an airline the size of United or American, eliminating the need for jet fuel as a cost would be highly appealing. So, I'm asking: what stands in the way of creating true solar-powered airliners?"
Clouds?
Night?
No boom today. Boom tomorrow. There's always a boom tomorrow. - Cmdr. Susan Ivanova
Nothing a good kickstarter campaign cannot solve...
Just saying...
The capacity and weight as well as power delivery, for taking off (with clouds above) and night flights.
I'm a huge solar fan but to make an aircraft that could carry a 100 or more passengers the surface area would be massive. No current airport could handle a plane that size and it'd never be cost effective. Better to run a plane off biodiesel. Even battery powered makes no sense. Large aircraft need a dense power source.
If you had 100% efficient solar panel, you'd have to make a solar panel the size of a small town to capture enough energy to power a passenger jet.
I always thought that heavy-lifting solar-powered airships would make excellent replacements for long-haul trucks.
No boom today. Boom tomorrow. There's always a boom tomorrow. - Cmdr. Susan Ivanova
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I want to delete my account but Slashdot doesn't allow it.
In simple terms, Physics.
I don't think that the entire surface area, even with a truly 100% efficient panel, would produce the power needed to propel the aircraft.
So, I guess that you could say that physics gets in the way.
yes, there are solar-powered flying wings. They are not man-rated, they fly very slowly, they are very fragile, and they carry only the most minimal payload/cargo, usually a miniaturized electronics package for a very specific purpose. They're analogous to the folding two-wheel luggage dolly as compared to the crew-cab pickup truck.
Do not look into laser with remaining eye.
Assuming 100% conversion efficiency, zero solar panel weight and an access to ideal tropical daylight during the flight you'd have to have a collector size of a couple football fields to power typical airliner.
Why? It is simply not practical application of technology, you hair-brained hippie.
Physics, mostly. Take 1200W/m^2, then imagine the upper surface area biggest plane you can practically create - that'd be ~1200m2 for a 787 dreamliner, or 1.44MW. That's the limit of power you will have on a sunny day with 100% efficient solar panels. Buy really expensive cells, and divide that number by 5. Then multiply by 0.7 for really efficient conversion to a form you can use. Your now at 202kW, or 271HP. That's probably around 10% of the cruising HP of an actual jetliner.
Assuming that actually works...
Speed - you're probably looking at a prop or fan flying at maximum efficiency, which probably means relatively slow.
Overall cost efficiency - solar panels cost, in power, as much or more than the electricity used to make them.
Is it just my observation, or are there way too many stupid people in the world?
- A 747 consumes 140MW. [ http://en.wikipedia.org/wiki/Orders_of_magnitude_(power) ]
- Nevada Solar One, a 400 acre solar generating station, generates 64MW. [ http://en.wikipedia.org/wiki/Nevada_Solar_One ]
Hmmmm...
A solar powered air ship is probably more the go. Greater surface area, less power required. But it would need to fly above the weather, and the low speed combined with daylight operation would yield a very low range. Probably in the same category as a solar powered submarine.
what stands in the way of creating true solar-powered airliners?
Nothing.
Oh, you meant airplanes? Yea, sorry, can't help you there.
An enigma, wrapped in a riddle, shrouded in bacon and cheese
This is not true, you can make a solar aircraft that will fly. The problem is it will be slow, Solar Impulse, for example, only goes about 40 miles per hour.
In general Darkness would stand in the way of solar powered anything...
Look, let's try to keep Microsoft out of this discussion, OK?
Faster! Faster! Faster would be better!
This is why: http://what-if.xkcd.com/17/
There simply isn't enough solar power delivered to the surface of the aircraft, even at 100% conversion efficiency, to move people and luggage using only available sunlight.
Google tells me direct illumination to a surface perpendicular to incoming full intensity sunlight is about 1.4 kW per square meter. Google also tells me that the wing surface area of a 747 is around 5500 square feet. Only half of the 747 wing is directly illuminated by sunlight at any given moment, but the surface of the fuselage could be covered with photocells as well, so 5500 square feet overall is probably a decent estimate for the directly illuminated surface area of the aircraft as a whole. And for hand-wavy purposes lets assume that the entire surface of the 747 is perpendicular to the incoming sunlight (i.e. a planar plane... pun totally intended). And that we have perfectly efficient photocells giving us 100% conversion efficiency. Running the math, this gives us around 715kW under bright direct sunlight, or about 959 horsepower -- the equivalent of 1.5 2012 Ford Shelby GT500's.
Each engine of a 747 generates around 15,000 horsepower at cruise, and around 30,000 at takeoff, and a 747 has four engines. So you need around 125 times the power generated by a perfectly efficient perfectly illuminated solar-powered 747 to get said plane off the ground, and around 65 times the power for cruising. And then you could only fly it in the middle of the day near the equator.
Cyrano de Maniac
Aha! So all we need to do is build solar-powered helicopters that sits perfectly still while the Earth rotates under it until you're where you need to be!
Use the Sun's energy to vaporize water to ultra-high pressure steam that is then directed as thrust and everything else works like a petrol jet engine?
Or use the Sun's energy to separate water into hydrogen and Oxygen and then burn them both in a modified petrol jet engine?
Wild ideas?!? Absolutely! But that's what we need. Let's think outrageously and go from there.
Sure, but the weight to energy ratio of either of these solutions would be prohibitive, unless you're talking airship instead of airplane, and maybe not even then. You'd have to do the energy collection on the ground and then somehow get it into the airplane. Something like a hydrogen plant on the ground that produces liquified hydrogen which is then used for fuel. (Which may still not work because even liquified hydrogen has much less energy per volume than jet fuel.)
As to using heat to vaporized water... unless your hydrogen fusion source is very local (as opposed to 92M miles away) I don't think you'll ever approach enough thrust to be noticeable. Heinlein used to write about torch ships that were propelled by superheated seawater, but the heat source was a nuclear fusion reactor in the vehicle.
Niven wrote about a lifting body propelled by air compressed to nearly-degenerate matter, but I don't know if the math works out for that one either.
Some "solutions" (like a steam powered airplane using a solar collector) aren't worth trying because they just don't pencil out. Heavier than air craft need a lot of energy to stay airborne and move about, and replacements for jet fuel have to have at least vaguely similar energy density.
Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
Actually, that's not a bad idea. Might work better as a replacement for cargo ships
Boy oh boy, this is where industry knowledge separates the men from the boys. I just worked a file for a ship that had 180 million cargo pounds handled at one port, and it can carry about 250 million. There are also ships almost twice its size in operation today, and these are on a weekly rotation all over the world. There's some interesting calculations here for the mathematically inclined on how big the blimp would need to be. On the bright side, the bouyancy needed to airlift that kind of weight might solve our albedo issues though, what with the entire ocean being blotted out by blimps an all :)
Yep. approx 30 kWh per gallon of fuel, a 747 is burning approx 1 gal/second, so 100K kW (3600x30) needed. Solar gives us approx 1kW / square meter, so we need about 100K square meters of solar panels on our 747
But you forget that as you have to increase surface area for more energy, you also get more wingspan, reducing the need for energy!
By my calculations* the new needs cross over each other at around a wingspan of 200K square meters of solar panel with 10K kW provided. Simple!
* Calculations actually made up figures for humorous effect, writer does not guarantee a 747 with 200K square foot wingspan will fly or not collapse in on itself.
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Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
To inject some math into the discussion:
ThrustToKeepFlying = FlyingMass / LiftToDragRatio
PowerToKeepFlying = ThrustToKeepFlying * Velocity = Velocity * FlyingMass / LiftToDragRatio
Typically LiftToDragRatio is about 20 or so. Airplanes don't really make sense unless they are faster than other vehicles, so Velocity needs to be 100-300 m/s. (Typically, jets fly just under Mach 1, where they have the least drag/greatest power)
FlyingMass = AircraftMass + PayloadMass + EngineMass + PowersourceMass
Since we are using unobtainium to build our aircraft, it doesn't weigh anything. And we'll just say that we can fly arbitrarily large airplanes for a single passenger, so PayloadMass is essentially zero as well.
The best solar cells are about 300W/kg (http://en.wikipedia.org/wiki/Solar_panels_on_spacecraft), and the best electric engines are about 6 kW/kg. So
FlyingMass = OtherStuff + PowerToKeepFlying / 300 + PowerToKeepFlying / 6000 = OtherStuff + 0.0035 * PowerToKeepFlying
FlyingMass = OtherStuff + 0.0035 * ( 300 * FlyingMass / 20 )
FlyingMass = OtherStuff + 0.0525 * FlyingMass
OtherStuff = 0.9475 * FlyingMass
So this says that as long as your airplane and payload are under about 95% of the engine / power source mass, it is at least possible. Structures that light are not really an issue - the real issue is only flying during the day and in good weather. (And, of course, it would cost an arm and a leg!)
while (sig==sig) sig=!sig;
That's right.
A 737-300 burns about 5500 lbs/hour at cruise (~2500 kg/hour).
Jet-A contains 43 MJ/kg (lower heating value). So energy to cruise is about 107,500 MJ/hour = 29,800 kWh per hour
The terrestrial solar maximum (insolation on a hot sunny day at noon at the equator) is +/- 1000 watts/m^2. It's actually a bit higher at the equator, and will be higher still at cruising altitude. Call it 2000 watts/m^2.
So, just to maintain cruise speed (which is its most efficient operating mode, vs, say, takeoff or landing) you would need 15,000 m^2 of 100% efficient collector area. (Commercial PV is 15-25% efficient). A 737-300 is about 28m (wingspan) x 33m (length). So even if the airplane were a solid square of 100% efficient collector, it would still be an order of magnitude too small to power the plane at cruise.
The fundamental problem is that people do not understand the relative energy density of fossil fuels relative to renewable sources. Renewable sources are inexhaustible, but they are sparse. Fossil fuels are distilled sunlight - very dense. If solar energy is beer, petroleum is whiskey.