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Ask Slashdot: What Stands In the Way of a Truly Solar-Powered Airliner?

Posted by Soulskill on from the i-blame-gravity dept.

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?"

9 of 590 comments (clear)

  1. A solid grasp of reality? by Anonymous Coward · · Score: 5, Informative

    Just saying...

  2. The math doesn't work by Grayhand · · Score: 5, Informative

    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.

  3. Size. by Gordonjcp · · 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.

    1. Re:Size. by Solandri · · Score: 5, Informative

      Dunno why it would be unbelievable. Solar-powered aircraft have been made. They're ultralight, are covered in solar panels, have practically no payload, and fly at about 20 mph. But if you insist...

      Passenger plane fuel consumption is on the order of 5-7 gallons per mile. 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, or 132.5 MJ/gal.
      At 0.918 gal/s, that's 121.6 MJ/s or 121.6 megawatts.

      Solar constant 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.
      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.

  4. power requirements by goertzenator · · Score: 5, Informative

    - 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...

  5. Re:Let's go retro... by gman003 · · 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".

  6. Re:Um... by paiute · · Score: 5, Informative

    Physics.

    --
    If Slashdot were chemistry it would look like this:Cadaverine
  7. Re:Um... by Capitaine · · Score: 5, Informative

    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.

  8. Yep, physics by Anonymous Coward · · 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.