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Power Transportation

Ask Slashdot: What Stands In the Way of a Truly Solar-Powered Airliner? 590

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

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  • Um... (Score:5, Insightful)

    by mbstone (457308) on Tuesday October 30, 2012 @03:53PM (#41821811)

    Clouds?

  • Also (Score:5, Insightful)

    by Flounder (42112) on Tuesday October 30, 2012 @03:54PM (#41821823)
    Night?
  • Batteries. (Score:5, Insightful)

    by sbrown7792 (2027476) on Tuesday October 30, 2012 @03:56PM (#41821869)
    The capacity and weight as well as power delivery, for taking off (with clouds above) and night flights.
  • by Gothmolly (148874) on Tuesday October 30, 2012 @03:58PM (#41821891)

    Dear Slashdot, this is not a highschool paper.

    Also, Roland Piquipaille is dead - please stop with the sensationalist, page-hit-generating crap.

  • Physics. (Score:5, Insightful)

    by Anonymous Coward on Tuesday October 30, 2012 @03:58PM (#41821897)

    In simple terms, Physics.

  • Size (Score:5, Insightful)

    by sinij (911942) on Tuesday October 30, 2012 @04:02PM (#41821989) Journal
    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.
  • by jesseck (942036) on Tuesday October 30, 2012 @04:05PM (#41822043)
    Horses are a truly renewable resource- when one wears out, sell it for meat in a foreign country (or maybe our own some day), and buy a new one. Add a buggy, and a whip to go fast, and you are green.
  • Air Ship (Score:5, Insightful)

    by waimate (147056) on Tuesday October 30, 2012 @04:07PM (#41822075) Homepage

    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.

  • Re:Um... (Score:5, Insightful)

    by roc97007 (608802) on Tuesday October 30, 2012 @04:10PM (#41822131) Journal

    I'm pretty sure "solar powered" and "jet engine" do not belong together.

  • by vlm (69642) on Tuesday October 30, 2012 @04:17PM (#41822261)

    Nobody on /. got the correct answer although a couple got close. It all boils down to cost of capital.

    There is no reason a solar-glider airfleet, or a solar powered fleet of blimps or whatever could not exist other than the existence of jetfuel powered competitors.

    Your competitor rents $100M from the bank and keeps his planes in the air 20 hours out of every 24 hour day and during that 20 hours makes lets say 5 revenue generating flights. So five butts in seats per day or more importantly they sold at least 5 tickets per seat per day (probably a hair more).

    You rent $100M from the bank and your planes sit on the ground on average 12 hours per day and you're lucky at the slower speeds involved to get maybe 1 revenue generating flight per day. So you sold one ticket per seat per day. Whoops.

    Your competitors gross 5 times the cash you do to pay the bank loan... who cares if 30% of your expenses are fuel or fuel related (engine maint, whatever), leaving you with a mere 70% of previous expense, if you only get to keep 20% of your previous revenue... That's going to ruin your profit margin, in fact you'll be unable to pay the bank back unless or until your competitors go out of business.

    The "world" could run on giant solar gliders, or solar powered helium blimps, or whatever, if and only if the cost of capital dropped way more than a factor of 5 and/or ticket prices floated up about 5 times what they are today. Frankly we will inevitably end up there sooner or later, maybe in 20 or 30 years. But its not going to work today.

  • Re:Um... (Score:5, Insightful)

    by rtfa-troll (1340807) on Tuesday October 30, 2012 @04:19PM (#41822297)

    But if you had a solar powered jet engine you could chase the sun. And you would never need to waste energy on landing lights.

    Why stop at that? If you had a solar powered transporter you could just go straight from wherever you are to wherever you want to be in two simple steps. In fact, once we can completely ignore basic laws of energy conservation and so on, why not just use a solar powered magic wand and will yourself to already be wherever you want to be?

  • by vlm (69642) on Tuesday October 30, 2012 @04:30PM (#41822499)

    And cargo rarely needs to go as quickly as people

    Bzzzzt

    You can BS passengers into paying much more because you have comfier seats or serve cookies or have non-stop / non-transfer flights to their home town or some goofy "air miles" deal. None of which cost much but people will pay thru the nose for... nearly pure profit.

    Cargo doesn't care. You've got a $1M/month mortgage to pay on that $100M plane and the ONLY thing that matters is making as many trips as possible to haul cargo to make that payment. You go quick you make tons of profit and pay your mortgage. You glide around like drunken seagull and make one revenue generating glide per week, the plane gets repo'd.

    Even worse, its always going to cost more to fly than swim in a boat or drive a truck. Some cargo is time sensitive enough that they'll pay 50 times as much to go 5 times as fast as a semi trailer. But there's no way in hell they'll pay 50 times as much to go slower than the alternatives.

  • by roc97007 (608802) on Tuesday October 30, 2012 @04:38PM (#41822673) Journal

    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.

  • Big Numbers time (Score:5, Insightful)

    by PPalmgren (1009823) on Tuesday October 30, 2012 @04:47PM (#41822881)

    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 :)

  • Re:Um... (Score:5, Insightful)

    by Gorobei (127755) on Tuesday October 30, 2012 @05:08PM (#41823319)

    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.

    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. Our 747 has approx 1000 sq m top surface, so solar would provide 1% of the power needed even in optimal conditions.

  • Re:Size. (Score:4, Insightful)

    by gman003 (1693318) on Tuesday October 30, 2012 @05:18PM (#41823519)

    All power is either solar, nuclear, or in one case gravitational.

    Photovoltaic is obviously solar. Any hydrocarbon (oil, gas, peat, even garbage incinerators) is just solar energy gathered by long-dead plants, or just plants in the case of biofuels. Wind is just solar energy heating one area, and we run a dynamo off it. Hydro is like wind, only we're running it off water, not air, being moved around by the sun's energy. Even having a hamster run in a wheel uses solar, as the hamster is powered by plants, which are powered by the sun.

    Nuclear power is obviously nuclear. Geothermal is tapping energy from natural nuclear decay in the Earth's interior.

    The only exception is tidal - pulling the energy of the Moon's orbit. Try as I may, I cannot justify it as nuclear-powered.

    If you want to get technical, you can merge solar and nuclear as well, as the sun is just a Big Thermonuclear Fusion Reactor In The Sky. Or if you want to get technical in a different way, you can rename solar to "fusion" and "nuclear" to "fission", so when we finally get fusion working we can file it properly.

  • by roc97007 (608802) on Tuesday October 30, 2012 @05:25PM (#41823663) Journal

    You're going to have to change your signature. George is selling Lucasfilms to Disney.

    Yes, I read that. But if Disney makes a watchable Star Wars film without Lucas, that'll prove the rule, won't it?

  • Re:Um... (Score:4, Insightful)

    by Rich0 (548339) on Tuesday October 30, 2012 @06:06PM (#41824289) Homepage

    Ok, let's take a CRJ-200. That consumes 1200 lb/hr of fuel in cruise, or about 170 gal/hr. That's 5MW of power - still a huge amount for a solar plant to generate, and that neglects the power needed to reach cruise altitude which is MUCH higher.

    A little Cessna 172 on 50% power uses 5gal/hr, which is 150kW of power. One of those probably could carry a single passenger and the pilot at typical passenger+baggage limits. If you stretched the thing out into the size of a small airliner using balsa wood you might be able to power it in cruise with top-mounted solar panels, assuming it could be towed up to cruise altitude, still carrying only two people.

    We can't even build solar-powered cars - forget planes with any kind of payload and cruise speed.

  • Re:Um... (Score:2, Insightful)

    by PermacultureEngineer (2712673) on Tuesday October 30, 2012 @06:52PM (#41824843)
    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.
  • by ScentCone (795499) on Tuesday October 30, 2012 @06:58PM (#41824897)
    Nah, I just make observations about people shouting shrilly about how horrible they think I am for being realistic. Thank you for being one of them, and making my point. You can't process even a whiff of satire or sarcasm, which is characteristic of the whole mindset.

    On a more specific note:

    Nuclear disaster happens, people lose their shit, governments respond.

    Great example. The reality: earthquake and tsunami happen, and plant is damaged ... though actual injuries and damage to population pale compared to the coal burned in the new coal-fired plants that places like China are putting online every week, or the coal that's now being burned for Germans to make up for shutting down nukes. The real story is that the "governments respond" thing you mention is really a bunch of un-informed, emotional nutjobs reacting irrationally, and a bunch of politicians pandering to them while switching back to dirtier energy that's more dangerous.

    Of course you know all that, but I realize you need to vent in order to distract from it. Go ahead, have a tantrum.

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