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Transportation Power

Solar Planes Aren't the Green Future Of Air Travel (vox.com) 286

An anonymous reader writes: By any standard, the Solar Impulse 2 is a marvel of engineering. This solar-powered plane didn't use a drop of kerosene on its epic trip across the Pacific Ocean. It's a real testament to how far solar technology has advanced. Unfortunately, for anyone hoping that we'll all be puttering around in solar planes soon -- well, that's pretty unlikely. From a Vox report, "Consider: The Solar Impulse 2 features 17,000 solar cells crammed onto its jumbo jet "size wings, along with four lithium-polymer batteries to store electricity for nighttime. Yet that's still only enough power to carry 2 tons of weight, including a single passenger, at a top speed of just 43 miles per hour. By contrast, a Boeing 747-400 running on jet fuel can transport some 400 people at a time, at top speeds of 570 miles per hour. Unless we see some truly shocking advances in module efficiency, it'll be impossible to cram enough solar panels onto a 747's wings to lift that much weight -- some 370 tons in all. Nor is it enough to load up on batteries charged by solar on the ground, since that would add even more weight to the plane, vastly increasing the energy needed for takeoff. A gallon of jet fuel packs about 15 to 30 times as much energy as a lithium-ion battery of similar weight. That fundamental difference in energy density is a big reason we're unlikely to see large commercial airliners powered by batteries fill the skies."
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Solar Planes Aren't the Green Future Of Air Travel

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  • Still (Score:5, Interesting)

    by NotInHere ( 3654617 ) on Friday May 06, 2016 @01:04PM (#52062269)

    Having a solar driven plane circle the world is still cool.

    • The problem is people are always thinking progress need to equate to a practical consumer level solution.

      Many times the process of doing it just because you can, comes up with many side effect results.
      I am wondering if the lessons learned to make an airplane fly around the world with solar, can have factors brought to the next generation fuel planes that are more efficient.

      • The problem is people are always thinking progress need to equate to a practical consumer level solution.

        Seriously, what a strange summary. "This technology demonstrator that is capable of flying around the planet using only solar power cannot replace a Boeing 747 with today's technology." Yeah, no kidding. Why did someone spend the time to write this? Is this an article paid for by Exxon-Mobile or something?

        • Re:Still (Score:5, Informative)

          by legRoom ( 4450027 ) on Friday May 06, 2016 @04:24PM (#52063595)

          A solar-powered direct replacement for something like a Boeing 747 is impossible, period. Incremental technological development cannot get us from here to there.

          Boeing 747-8I maximum fuel = [240 kL]
          Energy density of Kerosene = [37 MJ / L]
          Thermal efficiency of a modern turbofan engine = [40+%]
          Flight duration = [16+ hours]

          Energy required = ([240 kL] * [37 MJ / L] * [40%]) / [16 h] = [220 GJ/h] = [62 MW]

          So, even a hypothetical 100% efficient solar-powered 747-8I replacement would require about 62MW of average (not peak!) power to operate. The maximum power that can be collected by a solar energy system (no matter how efficient) is limited by its surface area: it cannot gather more energy than what is present in the sunlight hitting it.

          Maximum solar irradiance at Earth's Orbit: [less than 1.4 kW/m^2]
          Upper surface area of a Boeing 747-8I: [less than 1000 m^2]
          Maximum solar power available to a solar 747-sized object: [less than 1.4 kW/m^2] * [less than 1000 m^2] = [less than 1.4 MW]

          Even an ultra-high-tech solar 747-8I replacement could not possibly generate more than about 2% of the power required to perform the same mission. It would inevitably need to fly much slower and lower (probably low enough for cloud shadowing to cause major problems), and/or carry a far smaller payload.

          Barring a major (read: not foreseeable) physics or engineering breakthrough, true solar-powered jet replacements are not possible. Electric planes might happen eventually, but they will require refuelling or recharging on the ground, just like today's hydrocarbon-powered designs.

          • Yes and no. Your math looks OK. However, I think a lighter than air craft with solar power might work out as a cargo carrier and possibly even as passenger vehicle. The Graf Zeppelin did a round the world trip sometime back around 1930. It took them something like a week and a half. They didn't use a lot of power and I'd expect that a modern design might be a bit faster. I think the zeppelins maxed out around 100kph (60mph in American).

            I can't see them replacing jets at current fuel prices, but who k

          • A solar-powered direct replacement for something like a Boeing 747 is impossible, period. Incremental technological development cannot get us from here to there.

            A battery-powered direct replacement, however, is possible. It simply requires a battery chemistry that can match jet fuel for energy density. There's nothing impossible in that; your own cells accumulate electric charge as an intermediate step in the process of "burning" your food, and plant cells can run the process in reverse (photosynthesis).

            Or

          • Re: (Score:3, Insightful)

            by Suomi-Poika ( 453539 )

            LZ-129 (airship Hindenburg) surface area: 27,299 square meters. If upper half is covered with solar panels there would be ~4MW of power if we use current off the shelf 30% efficient solar panels. That power is actually more than the four 16 cylinder Daimler Benz diesels provided for that airship.

            However when flying over water the reflected sunlight would provide energy too. It would be probably wiser to wrap the entire airship with solar panels. Remove diesel engine weight three times (one left for emergenc

      • Re:Still (Score:4, Insightful)

        by tlhIngan ( 30335 ) <slashdot AT worf DOT net> on Friday May 06, 2016 @02:17PM (#52062867)

        Many times the process of doing it just because you can, comes up with many side effect results.
        I am wondering if the lessons learned to make an airplane fly around the world with solar, can have factors brought to the next generation fuel planes that are more efficient.

        Exactly - it's how progress is done. The innovation isn't that it'll replace a 747 immediately, but with R&D, it might. Or it might fit itself in a new niche.

        I mean, it's like saying airplanes are stupid when you see the Wright brother's 1903 example. The thing only flew a few meters. What, airplanes are completely pointless because anyone can walk farther than they can fly?

        No, progress is made by refining the process. It flies a few meters first, then as you learn from it, you fly farther and farther until you can go halfway around the world.

        Likewise, solar planes will likely not replace a 747, but they may replace balloons and satellites (which are extremely expensive).

        There's a lot of research going on airships too - while not as fast as a plane, they have enormous cargo carrying capacity and can be launched inland, so if you have cargo that's not required to be there within a day, but can take a week or two, it's competitive with regular shipping (which usually takes a month), plus you don't need a port and trains/trucks to bring it inland.

        Just because something isn't a perfect replacement for an existing piece of technology, doesn't make the development pointless.

        • Likewise, solar planes will likely not replace a 747, but they may replace balloons and satellites (which are extremely expensive).

          Just because something isn't a perfect replacement for an existing piece of technology, doesn't make the development pointless.

          Also, a solar plane would not need to be able to fly at night or around the world to be useful. A solar plane that could take off and land in fair weather without using fuel would still be useful. It would still likely need a small battery for emergency landing but not for 8+ hours of flying at night.

        • by perpenso ( 1613749 ) on Friday May 06, 2016 @03:07PM (#52063153)

          ... progress is made by refining the process ...

          Yes, and the bias against internal combustion and jets and the bias towards solar are causing people to miss a major piece of that process. The fuel. There is nothing wrong with internal combustion and jets, the problem is only their current petroleum based fuels. Switch to bio fuels that are carbon neutral and we have no problem. Carbon is not the problem if it is taken from and returned to the current environment, as with bio fuels. Carbon is only a problem when we mine ancient sequestered carbon and reintroduce it to the current environment, as with petroleum.

          Liquid fuels have incredible energy density. We would probably need a Back-to-the-Future-like "Mr Fusion" reactor, not improved batteries, to make electrically powered fixed/rotary wing aircraft practical.

          • We would probably need a Back-to-the-Future-like "Mr Fusion" reactor, not improved batteries, to make electrically powered fixed/rotary wing aircraft practical.

            Technically, we only need 1960's "let's make it nuclear" optimism [wikipedia.org] and it doesn't even need to be electric. That thing is that's so awesome it needs a use. Maybe a supersonic atmospheric flyer on Jupiter for planetary research?

      • Exactly. Look at the Apollo moon landings. We haven't made traveling to the moon a reality for consumers, and we basically did it just because we could. But in the process, we came up with all kinds of spin-off technologies that were a huge boost to the economy.

        The lessons learned in flying a solar airplane around the world might not just help with other aircraft, they could have benefits for many other not-that-related industries as well. The Apollo program provided a big boost to the electronics indus

    • Hmmmm. And only five centuries after Magellan circled the world powered only by wind.

      Making progress by leaps and bounds.

  • Battery weight (Score:5, Interesting)

    by Anonymous Coward on Friday May 06, 2016 @01:06PM (#52062291)

    A gallon of jet fuel packs about 15 to 30 times as much energy as a lithium-ion battery of similar weight. That fundamental difference in energy density is a big reason we're unlikely to see large commercial airliners powered by batteries fill the skies."

    This isn't even the whole story. As a plane flies, it burns fuel, essentially throwing mass out the engines for thrust. Getting lighter allows the plane to climb to a higher altitude where it is more efficient.

    • Re:Battery weight (Score:5, Insightful)

      by MightyYar ( 622222 ) on Friday May 06, 2016 @01:12PM (#52062351)

      In addition, a plane would have to be made stronger to support landing at full-takeoff weight. Current planes cannot land safely when fully loaded with fuel.

    • The wall street journal estimates that for a 100 seat airliner, 30% of the cost of a flight is fuel. And another 14% is govt fees and taxes. Thus if batteries let you get cheaper fuel, don't know if thats true, then there's a large margin for cost savings. And if the gov't were willing to kick back taxes for not polluting the upper atmosphere. Then there's an even greater margin. So there's a powerful incentive to come up with electric power aviation if the total cost of ownership for electric power can

      • Obviously I ignored that takeoffs use a lot of fuel. But that's factors of 2 and I was just roughing it out to orders of magnitude.

  • only enough power to carry 2 tons of weight, including a single passenger, at a top speed of just 43 miles per hour

    May as well go back to sail ships [thepeoplescube.com]... Maybe, not as fast, but certainly much more capable.

  • Just over 100 years ago man figured out heavier-than-air flight. About 70 years after that, we had flying buses and a man on the moon. To say anything is "not the future" in air travel is stupid.
  • Efficiency (Score:5, Insightful)

    by thinkwaitfast ( 4150389 ) on Friday May 06, 2016 @01:21PM (#52062407)

    Unless we see some truly shocking advances in module efficiency

    It wouldn't work with 100% efficiency, so why would increase in efficiency matter as far as making it practical? What is happening to critical thinking skills?

    • Just put a magnifying glass over the solar cells. I can't believe you guys fail to see the obvious solution to these things (now if you excuse me, I have a thesis to write on time travel and to prepare for my concert at carnegie hall).
    • Unless we see some truly shocking advances in module efficiency

      It wouldn't work with 100% efficiency, so why would increase in efficiency matter as far as making it practical? What is happening to critical thinking skills?

      Regarding critical thinking, why couldn't we just use solar panels on the ground to make jet fuel(*)?

      Jet fuel in this instance is just an energy carrier, and has a much higher energy density than lithium. While Lithium batteries may be appropriate in some cases (portable devices, ground transportation), for air flight it's more appropriate to use something else.

      (*) Or perhaps a biological method such as GM modified algae or a bio-yielding plant. The Wikipedia page [wikipedia.org] of crop yields indicates that Algae can yie

      • by fnj ( 64210 )

        A quick calculation shows that a 747 holds around 183,000 kg of fuel, so 3ha of open-pond algae could supply enough fuel for one tank each year.

        I doubt your 3 ha figure, but let's run with it just for fun.

        And how many hectares of algae ponds would it take to supply one tank of fuel several times a day for each of thousands of 747-equivalents? Several MILLION hectares? That's several tens of billions of square metres, or several tens of thousands of square km. How about the colossal energy input you would ne

  • by SumDog ( 466607 ) on Friday May 06, 2016 @01:22PM (#52062423) Homepage Journal

    I think it's pretty apparent that the solar concepts are just concepts. Obviously there's not enough energy output to replace passenger flight.

    But the question is, can solar panels on a plane offset the energy consumption enough to make a difference? That's probably also a no, but that's where the question should start.

    Keep in mind, some cargo ships have been experimenting with massive kites/sails that help offset the energy needed for their engines:

    http://www.skysails.info/english/skysails-marine/skysails-propulsion-for-cargo-ships/

    • >But the question is, can solar panels on a plane offset the energy consumption enough to make a difference? That's probably also a no, but that's where the question should start.

      There's just not enough energy in sunlight per square foot to make it worth it. You put all you effort into creating a huge receptor area and keeping weight down that the other needs become impracticable (comfort, speed, luggage, etc). Storage density and associated weight are the things that matter. If we have a very big leap in storage capability, then short hop flights with small planes might become practical. Until then, we might see some recreational crafts that get used for a very short duration thril

  • I thought the energy density gap between batteries and jet fuel was greater than a factor 15-30. Batteries are still being improved, and getting the energy density into the same ballpark doesn't seem completely impossible. Of course there's also the actual propulsion to consider; you need something able to put out as much power as a jet engine and at the same efficiency. But all in all it seems a lot more feasible than it did only a few years ago.

    Not sure if solar panels on the airplane make sense tho
  • by roc97007 ( 608802 ) on Friday May 06, 2016 @01:24PM (#52062455) Journal

    > Unless we see some truly shocking advances in module efficiency, it'll be impossible to cram enough solar panels onto a 747's wings to lift that much weight [...]

    Besides that, I strongly suspect there isn't enough power in the form of sunlight falling on a surface the size of a 747's wings to achieve the objective. In other words, it's not just a matter of solar panel efficiency, it's also a matter of total energy available for capture.

    • It's a matter of efficiency. That's how solar energy is generated.
    • Re:module efficiency (Score:5, Informative)

      by Cyrano de Maniac ( 60961 ) on Friday May 06, 2016 @03:15PM (#52063211)

      Exactly. I ran some back-of-the-envelope calculations on this 3.5 years ago in another Slashdot thread. https://slashdot.org/comments.... [slashdot.org]

      And because we're presumably too lazy to click that link, I'll paste it below for your reading pleasure...

      This is why: http://what-if.xkcd.com/17/ [xkcd.com] [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.

    • by fnj ( 64210 )

      I strongly suspect there isn't enough power in the form of sunlight falling on a surface the size of a 747's wings to achieve the objective

      Master of the understatement. At high noon, there is only a peak on the order of 1/100 of the required power impinging on the wings in the form of sunlight - in the DAYTIME. And zero power after the sun sets. The idea of solar powered jetliners is no more than fantasy for the feeble-minded.

  • to increase efficiency of solar cells - Sun radiation is not increasing, pretty much constant, I would say. Probably need a football-field size surface constant to generate enough oompf to compare to kerosene-propelled turbines.

    Generating fuel from solar/wind/water and propel airplane traffic will probably show the discrepancy between necessity and actual use - no one will even touch restricting air traffic, so....

    Compared fuel efficiency/use per person from airplanes to cars, airplanes are winners, hands d

  • by Gravis Zero ( 934156 ) on Friday May 06, 2016 @01:39PM (#52062553)

    yeah, this whole airplane thing reminds me of that fool who thought people would have computers in their houses. pfft! how can you fit a giant electromechanical machine that fills a warehouse into your living room? some people, am i right?

    • It's not that no one can imagine such a future -- a little digging into science fiction stories should turn up a variety of examples of battery- or solar-powered air vehicles. One that I can think of is Robert Heinlein's novel Friday, with vehicles powered by Shipstones, an energy-storage device. What causes battery- or solar-powered air vehicles to be dismissed is an awareness of the energy density limitations of current generation and storage systems, and the progress of improvement in these systems; eith

    • Yeah, and those people who say perpetual motion machines will never work and that we can't make a transistor out of something smaller than a single atom... we'll show them.

      Computers in every house were solved through steady miniaturisation and were far from being impossible as defined by laws of physics.
      By comparison here we're talking about a situation where a theoretically perfect solar energy source driving theoretically perfect engines on a theoretically perfect wing still won't have the ability to do e

  • They are lightweight relative to their power output and are approaching 60% efficiency.

    They also are very durable requiring little maintenance over their lifetime.

    Fuel density and turbine efficiency are pretty hard to beat when it comes to aircraft.

  • Er, okay (Score:4, Insightful)

    by wonkey_monkey ( 2592601 ) on Friday May 06, 2016 @01:55PM (#52062687) Homepage

    Solar Planes Aren't the Green Future Of Air Travel

    I never thought they were. Jeez, why always looking for the negative?

    Next: The LHC can't solve global warming.

  • I wonder if there's any potential to beam power to a plane by microwave, either from a satellite or from a network of ground stations?

    (I'd read a proposal to do space ship launches this way, to save on fuel weight.)

  • by hey! ( 33014 ) on Friday May 06, 2016 @02:53PM (#52063067) Homepage Journal

    To slashdot's new masters: your readers aren't so ignorant that we think that Solar Impulse 2 means we'll be seeing solar powered 747s. Sheesh.

  • ... In order to make commercial solar, one has to abandon the wing, not solar power per se. Imagine a blimp whose entire bag is made up of ultrathin solar cells. Now lift is "free" -- all one needs is enough power to run a pusher that can exceed the drag force of the prevailing wind, and surface (of the blimp) to volume works in your favor, as increasing the surface area increases net buoyancy and hence the total weight of storage batteries one can lift. The Chinese are building a prototype already, as a

  • There are lots of small savings to be made

    For a short commuter flight, say between LA and SFO. the speed of the plane is just one consideration, there are lots of on-the-ground delays that add to the total travel time

    Here's my plan...

    Step 1 - abolish the TSA, they're worthless and that saves an hour off the trip for a start

    Prior to flight, batteries loaded, fully charged, into plane on the runway. The batteries here have a different function, they are a power boost for a daylight flight not a reserve for o

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