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Solar Planes Aren't the Green Future Of Air Travel (vox.com)
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."
They dump the fuel before landing.
If construction was anything like programming, an incorrectly fitted lock would bring down the entire building...
"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
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.
Exactly. I ran some back-of-the-envelope calculations on this 3.5 years ago in another Slashdot thread. https://slashdot.org/comments....
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]
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.
Cyrano de Maniac
On a non emergency, such as a mild tail strike on takeoff, where the plane appears safe but they would not risk a transatlantic crossing, they'll fly around in small circles for a few hours with full flaps, full airbrakes and high thrust to burn off the fuel as fast as possible in order to safely land at the same airport. Dumping fuel is messy, dangerous and nasty, so it's only done in a real emergency.
SJW n. One who posts facts.
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.