Ask Slashdot: What Stands In the Way of a Truly Solar-Powered Airliner? 590
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?"
A solid grasp of reality? (Score:5, Informative)
Just saying...
The math doesn't work (Score:5, Informative)
Size. (Score:5, Informative)
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.
Uh, surface area? (Score:5, Informative)
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.
Re:Um... (Score:4, Informative)
Physics? (Score:5, Informative)
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.
power requirements (Score:5, Informative)
- Nevada Solar One, a 400 acre solar generating station, generates 64MW. [ http://en.wikipedia.org/wiki/Nevada_Solar_One [wikipedia.org] ]
Hmmmm...
Re:Let's go retro... (Score:5, Informative)
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".
Re:Uh, surface area? (Score:4, Informative)
A) Please get into the habit of using metric. Pretty please? we look like the scientific laughing stock.
B) It's not the energy passing through a sqr meter of the sun, it's how much of that energy is there in a sqr meter on earth. Unless you are planning on flying a meter above the surface of the sun~
1KW per sqr Meter on the earth. Altitude will change that, but not nearly enough.
Re:weight. power (Score:4, Informative)
This is not true, you can make a solar aircraft [wikipedia.org] that will fly. The problem is it will be slow, Solar Impulse, for example, only goes about 40 miles per hour.
Green Cows (Score:4, Informative)
This is why: http://what-if.xkcd.com/17/ [xkcd.com]
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.
Re:Size. (Score:5, Informative)
The key is to separate the solar panel from the phone. Leave the panel, with a battery, in a sunny place as you go about your business, and charge from it when convenient. ThinkGeek has them for $40 or thereabouts [thinkgeek.com]
Some basic literacy (Score:3, Informative)
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,
Re:Um... (Score:5, Informative)
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.
Re:Um... (Score:5, Informative)
Re:Size. (Score:5, Informative)
Passenger plane fuel consumption is on the order of 5-7 gallons per mile [airliners.net]. Call it 6 gal/mi.
Airspeed is about 550 mph, or 0.153 miles per second.
Fuel consumption is thus (6 gal/mi) * (0.153 mi/s) = 0.918 gal/s.
Jet fuel has about 35 MJ/L of energy [wikipedia.org], or 132.5 MJ/gal.
At 0.918 gal/s, that's 121.6 MJ/s or 121.6 megawatts.
Solar constant [wikipedia.org] in space is about 1361 Watts/m^2. On Earth it's about 750 Watts/m^2.
Air pressure at 35,000 ft is about 25% that of sea level [engineeringtoolbox.com].
So figure with only 25% of the atmosphere intercepting sunlight, you get 1208 Watts/m^2 at cruising altitude.
To generate 121.6 MW with 100% efficient panels producing 1208 Watts/m^2, you need 100,660 m^2, or about a tenth of a square km. A roughly 320x320 meter patch, or about 5-10 city blocks. I suppose a really small town could fit in that area.
You could quibble about gas turbines only being 40%-50% efficient, but then real-world commercial solar panels are only 15%-20% efficient. And we're ignoring clouds, night, and angle to the sun (all the above assumes the sun is directly overhead). So more realistically you're probably looking anywhere from a quarter of square km to over 1 square km of solar panels needed to propel a passenger plane.
Re:Um... (Score:5, Informative)
To get an idea of the power needed, just take a regional aircraft, for example an ATR [wikipedia.org] equiped with two PW100 [www.pwc.ca]. 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 [energysavingtrust.org.uk] 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.
Re:Solar powered jet engine (Score:5, Informative)
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!)
Yep, physics (Score:5, Informative)
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.