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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?"
Just saying...
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 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.
Valid point. A solar powered fleet would limit travel to daylight hours - not just twilight but sun-overhead daylight. East-bound travel would have to start and finish within a short time-span mid-winter. Adding a fuel backup means adding all the infrastructure necessary to convert fuel into motion in addition to electrical systems used for solar energy. By the time you add all that and the fuel, you've exceeded the weight limit that would allow solar-powered flight.
There may be no "I" in team, but there's also no "F" in way.
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...
Actually, that's not a bad idea. Might work better as a replacement for cargo ships, not trucks, though.
With Hydrogen/Helium providing the lift, the engines only have to provide thrust. And cargo rarely needs to go as quickly as people - it currently takes what, weeks, to cross the Pacific? So you can get by with much less power demands.
And you also get much more power to work with. Dirigibles are pretty bulky, lots of surface area, so you have nice big expanses to cover in photovoltaics.
And you even have less potential damage from wave motion or humidity compared to container ships. That might be enough of an advantage for getting electronics from the factory in China to the stores in US/Europe.
Someone get Apple on this - it makes a good stunt, at the very least. "iPhone 7 - now delivered by dirigible".
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.
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
The fact that this question is even being asked shows how poorly people understand the practicalities of solar power. Cover something with PV cells and you've got power. Problem solved, right?
There are two problems here. First, small-scale solar power generation is just not very efficient. If you spend a lot of money and cover your roof with PV cells, not only will you not make back your cost, you probably won't even prevent enough greenhouse gas emissions to offset those emited by manufacturing and installation
Second is storage. There's just no way to store electrical energy that comes even close to the energy storage provided by hydrocarbons. And you have to have storage, because you can't count on the sun being out when you need juice.
These problems can be solved but without some fundamental breakthroughs they can't be solved on a small scale. So the future of solar power is huge generation and storage facilities, not vehicles covered with solar cells,
Clouds?
I think a bigger problem is that the surface area of an 'airliner' can never provide enough energy to keep it in the air even with 100% conversion efficiency at noon.
Blimps might work, but they're slow and helium supply is a problem.
Hydrogen is too scary for passenger blimps. People wouldn't like them even if they had ejectors and parachutes.
No sig today...
Physics.
If Slashdot were chemistry it would look like this:Cadaverine
Jet engines are quite inefficient. The main problem being power I would start with props.
To get an idea of the power needed, just take a regional aircraft, for example an ATR equiped with two PW100. That makes roughly 7000kW of energy provided to propellers. Lets suppose you have 100%-efficient electrical motors, and let aside aircraft internal consumption.
Now this paper suggest that high density power solar cells provides about 1kW/sqm. The only challenge is to find 7000sqm of surface exposed to the sun on a 22m long and 24m wide aircraft.
So far, the only electrical plane that I have been able to see were ultra light aircraft which could barely support they own weight and a pilot. Still a long way until commercial exploitation.
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!)
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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.