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

Posted by msmash on from the affinity-for-all-things-solar dept.

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."

8 of 286 comments (clear)

  1. Still by NotInHere · · Score: 5, Interesting

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

    1. Re:Still by chaboud · · Score: 5, Insightful

      Limitless in-theater dwell time, controllable deployability (at 43mph). If the military isn't already all over this, something is horribly out of whack.

    2. Re:Still by legRoom · · Score: 5, Informative

      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.

  2. Battery weight by Anonymous Coward · · Score: 5, Interesting

    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.

    1. Re:Battery weight by MightyYar · · Score: 5, Insightful

      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.

      --
      W..w..W - Willy Waterloo washes Warren Wiggins who is washing Waldo Woo.
    2. Re: Battery weight by JaredOfEuropa · · Score: 5, Informative

      They dump the fuel before landing.

      --
      If construction was anything like programming, an incorrectly fitted lock would bring down the entire building...
    3. Re: Battery weight by ryanmetcalf · · Score: 5, Informative

      "Aircraft without fuel dumping capability are certified for overweight landing if necessary. Should an event occur requiring an immediate return for landing, the crew executes the landing and notes in the maintenance log that an overweight landing was made. The maintenance department conducts an inspection, and if there was no damage the airplane is released back into service."
      http://www.usatoday.com/story/travel/columnist/cox/2014/07/13/fuel-dumping-emergency-landings/12530075/


      "It all comes down to the fact that certain planes are designed to be significantly lighter when landing than when taking off -- in some cases more than 200,000 pounds (90,909.1 kilograms) lighter [source: Boeing]. This may sound backwards; one might think that taking off at a heavy weight would necessarily be harder than landing with that same weight. But landing can put more stress on a plane. When a plane lands heavy, it's very easy to hit the ground too hard and cause damage to the aircraft."
      http://science.howstuffworks.com/transport/flight/modern/planes-dump-fuel-before-landing.htm

  3. Batteries need similar engines for thrust by raymorris · · Score: 5, Informative

    Batteries, or solar cells, don't make thrust by themselves either. You'd still need the same turbofans, just with heavy electrical windings in the middle rather hollow combustion cavities. Electric motors tend to be quite a bit heavier than empty cavities are, so I'd expect the electric engine to be probably a bit heavier.

    You could put a prop on an electric motor. That limits maximum speed, and still electric motors are heavy.

    > The fuel system itself

    Such as the fuel lines and fittings , the hollow copper tubes? Compare with the copper required to carry thousands of amps safely, with heavy-duty insulation. The fuel line and fittings are probably lighter than electrical lines and fittings capable of transmitting the same amount of power.

    > If wing didn't have to carry fuel could it be more efficient?

    Wing efficiency is determined by shape and surface smoothness. What's inside doesn't matter, except a snall effectbthat carrying load in the wing is slightly more efficient than carrying the same load inside the fuselage, by eliminating the bending moment on the wing root. Putting solar panels on the SURFACE of the wing, where it's right in the critical boundary layer airstream , is a much bigger design constraint than putting something IN the wing.