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Ask Slashdot: What Stands In the Way of a Truly Solar-Powered Airliner?
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?"
Clouds?
Night?
No boom today. Boom tomorrow. There's always a boom tomorrow. - Cmdr. Susan Ivanova
Nothing a good kickstarter campaign cannot solve...
Just saying...
The capacity and weight as well as power delivery, for taking off (with clouds above) and night flights.
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.
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.
I always thought that heavy-lifting solar-powered airships would make excellent replacements for long-haul trucks.
No boom today. Boom tomorrow. There's always a boom tomorrow. - Cmdr. Susan Ivanova
Dear Slashdot, this is not a highschool paper.
Also, Roland Piquipaille is dead - please stop with the sensationalist, page-hit-generating crap.
I want to delete my account but Slashdot doesn't allow it.
In simple terms, Physics.
I don't think that the entire surface area, even with a truly 100% efficient panel, would produce the power needed to propel the aircraft.
So, I guess that you could say that physics gets in the way.
yes, there are solar-powered flying wings. They are not man-rated, they fly very slowly, they are very fragile, and they carry only the most minimal payload/cargo, usually a miniaturized electronics package for a very specific purpose. They're analogous to the folding two-wheel luggage dolly as compared to the crew-cab pickup truck.
Do not look into laser with remaining eye.
The man with the paddles or flags.
There was an unknown error in the submission.
I recommend looking at the current solar powered aircraft - they're extremely light, look fragile, and barely carry anything. They generally look like gliders with some efficient propellers. Seems like it's a matter of efficiency and getting enough power from solar panels. I'm also betting they don't travel very fast (commercial aircraft travel above 500 MPH).
http://www.google.com/search?q=solar+power+aircraft
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.
Physics, mostly. Take 1200W/m^2, then imagine the upper surface area biggest plane you can practically create - that'd be ~1200m2 for a 787 dreamliner, or 1.44MW. That's the limit of power you will have on a sunny day with 100% efficient solar panels. Buy really expensive cells, and divide that number by 5. Then multiply by 0.7 for really efficient conversion to a form you can use. Your now at 202kW, or 271HP. That's probably around 10% of the cruising HP of an actual jetliner.
Assuming that actually works...
Speed - you're probably looking at a prop or fan flying at maximum efficiency, which probably means relatively slow.
Overall cost efficiency - solar panels cost, in power, as much or more than the electricity used to make them.
Is it just my observation, or are there way too many stupid people in the world?
- 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...
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.
As much as we can try to legislate and wish technology into existence, you have to let things run their course. The hard politically incorrect reality is that things like battery technology and solar panel technology are years away from being production ready.
By way of point look at where they are actually being used in alternative fuel vehicles like the Fiskar Kharma. The car has a small solar panel on the roof and a battery to run the vehicle. Since it doesn't carry passengers for hire it has far lower requirements for regulatory purposes than a plane. It is made by a company that in principal is fully dedicated to having vehicles that don't run on fossil fuels. I think you can safely say they are not in on any conspiracy theories your tinfoil hatters can come up with.
The solar panel on this car is rated only for minimal charging for accessories and to help keep the battery from going completely flat (it is very expensive if this happens to your Tesla). The car still has trouble with batteries catching fire which led to a recall not that long ago. It's a beautiful car that is the bleeding edge of technology and arguably was produced before it was ready.
If they are having this level of problems with a car, just imagine the hurdles that need to be overcome with an airplane. Your weight to thrust ratio is much, much more critical on a jet or plane than a car. Your fire that burned down a garage could instead burn alive hundreds of people. You have regulations from all over the world to pass and they can take years for certification to clear.
Carbon fiber is just now hitting the market with the Boeing dreamliner, yet it's been in consumer cars for at least a decade and military jets for even longer. It will likely be decades before the technology /could/ power something like a commercial aircraft. It will then take at least another decade after that for it be proven well enough to be considered for passenger use. If you want to get real about energy usage for commercial aviation that help with finding fuels that can be used at a commercial scale (algae etc).
Yeah, but they're not using *solar* jets.
So I did the math and I'm getting something a lot closer to 1000 miles per hour at the equator. I can't do the math for greater latitudes except to say it approaches 0 mph at the North Pole.
what stands in the way of creating true solar-powered airliners?
Nothing.
Oh, you meant airplanes? Yea, sorry, can't help you there.
An enigma, wrapped in a riddle, shrouded in bacon and cheese
This is not true, you can make a solar aircraft that will fly. The problem is it will be slow, Solar Impulse, for example, only goes about 40 miles per hour.
In general Darkness would stand in the way of solar powered anything...
Look, let's try to keep Microsoft out of this discussion, OK?
Faster! Faster! Faster would be better!
This is why: http://what-if.xkcd.com/17/
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
Weight - solar cells add weight. The huge electric motors to generate >10,000 horsepower would weigh a lot.
Nighttime - they'd sit on the ground all night.
Density of solar power - ~1 KW per square meter at the earth's surface, ~750 watts per horsepower to get 10,000 horsepower you'd need a square array ~85 meters on a side. That assumes 100% efficiency of cells and motors.
Not happening any time soon.
The fact that this question is even being asked shows how poorly people understand the practicalities of solar power. Cover something with PV cells and you've got power. Problem solved, right?
There are two problems here. First, small-scale solar power generation is just not very efficient. If you spend a lot of money and cover your roof with PV cells, not only will you not make back your cost, you probably won't even prevent enough greenhouse gas emissions to offset those emited by manufacturing and installation
Second is storage. There's just no way to store electrical energy that comes even close to the energy storage provided by hydrocarbons. And you have to have storage, because you can't count on the sun being out when you need juice.
These problems can be solved but without some fundamental breakthroughs they can't be solved on a small scale. So the future of solar power is huge generation and storage facilities, not vehicles covered with solar cells,
Aha! So all we need to do is build solar-powered helicopters that sits perfectly still while the Earth rotates under it until you're where you need to be!
and the number of European or African sparrows you can capture.
SJW: a person who perceives an injustice, and while correcting it, commits a greater injustice.
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.
Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
1. Lift
2. Weight
3. Thrust
4. Drag
I'm sorry, but your opinion seems to be wrong.
So Seattle-Tacoma International Airport would have to basically take up the entire Vancouver, BC to Portland, OR metropolitan corridor for the panels alone. While this would solve several problems inherent to the constantly dripping wet environment (this summer excepting) it's probably not an especially practical land use scenario.
Faster! Faster! Faster would be better!
Truthfully, I think the only viable solar powered option is a huge giant blimp wing semi-dirigible. And that having a light weight spray on solar conductor.
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 :)
A typical trans-country flight in a Boeing 767 uses around 50,000 lbs of fuel or around 7500 gallons.
Each gallon of jet fuel contains 34 KWh of energy, so the 5 hour flight uses 255KWh worth of fuel.
A jet engine is only around 35% efficient, so 89,250kWh of energy is needed to power the plane for the 5 hour trip
Assuming your electrical drivetrain is 100% efficient, you need "only" an average of 18,000kW to power the plane. In full sun, during the peak of the day, you'll get around 1300W/m^2 of solar power, let's use 75% efficient solar cells (which do not exist) and assume 1kW/m^2 of usable power.
So, your mythical solar powered jetliner will need 18,000 m^2 of surface area as long as you don't mind flying only in peak sun.
A B747 is around 80m long with a 70m wingspan. If you constructed a huge rectangular solar array above the plane that's as long and as wide as the plane, you'd have 5600 square meters, you'd need at least 3 of these giant solar arrays to power the plane (ignoring the extra drag caused by the huge solar array).
This only looks at average power and ignores the huge amount of power used in takeoff to get up to cruising altitude, for that you'll need some pretty serious batteries or other power source (fuel cells?). I'd like to see what the numbers look like if you use conventional jet engines (or even something more like one-time-use JATO rockets) to take off and get up to cruising altitude and wanted to rely on solar for the rest of the trip.
What the hell is electric landing gear? The wheels on the plan are unpowered and spin freely. All of the propulsion for moving around is provided by the engines. You can't keep the engines off until you're on the runway unless you're being towed. Also the engines need to be started using an external device so you'd need to drag that along so that it could spin up the engine and then start it.
The fact that the wheels are unpowered and free spinning is the issue, the only propulsion comes from the engines. They've been working on electric nose wheels that drive the aircraft as opposed to using the engines: http://www.theengineer.co.uk/news/electric-nose-wheel-could-reduce-aircraft-emissions/1007378.article
You don't need an external device to start the engines, aircraft have their own APU.
-- toolie
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
But you forget that as you have to increase surface area for more energy, you also get more wingspan, reducing the need for energy!
By my calculations* the new needs cross over each other at around a wingspan of 200K square meters of solar panel with 10K kW provided. Simple!
* Calculations actually made up figures for humorous effect, writer does not guarantee a 747 with 200K square foot wingspan will fly or not collapse in on itself.
"There is more worth loving than we have strength to love." - Brian Jay Stanley
You just need to fly it out of the also-rotating atmosphere...
Most fuel consumed by airliners is done while rolling around the airport on the ground. A jet engine burns almost the same amount of fuel at idle as it does while in cruise.
Wrong on both counts.
Most fuel in burned in the flying. This why we have long and short range aircraft, load fuel based on the flight plan, etc.
Idle burns much less fuel. What do you think that loud noise is when a jet powers up just before take-off? It's the engines doing a lot more work (generating thrust,) and the power for that work comes from dumping a lot more fuel/second into the engines.
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?
Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
164 Watts per Square meter is all you will get from the sun. THIS is the number 1 reason why you will never ever see a Solar powered airliner.
Do not look at laser with remaining good eye.
To inject some math into the discussion:
ThrustToKeepFlying = FlyingMass / LiftToDragRatio
PowerToKeepFlying = ThrustToKeepFlying * Velocity = Velocity * FlyingMass / LiftToDragRatio
Typically LiftToDragRatio is about 20 or so. Airplanes don't really make sense unless they are faster than other vehicles, so Velocity needs to be 100-300 m/s. (Typically, jets fly just under Mach 1, where they have the least drag/greatest power)
FlyingMass = AircraftMass + PayloadMass + EngineMass + PowersourceMass
Since we are using unobtainium to build our aircraft, it doesn't weigh anything. And we'll just say that we can fly arbitrarily large airplanes for a single passenger, so PayloadMass is essentially zero as well.
The best solar cells are about 300W/kg (http://en.wikipedia.org/wiki/Solar_panels_on_spacecraft), and the best electric engines are about 6 kW/kg. So
FlyingMass = OtherStuff + PowerToKeepFlying / 300 + PowerToKeepFlying / 6000 = OtherStuff + 0.0035 * PowerToKeepFlying
FlyingMass = OtherStuff + 0.0035 * ( 300 * FlyingMass / 20 )
FlyingMass = OtherStuff + 0.0525 * FlyingMass
OtherStuff = 0.9475 * FlyingMass
So this says that as long as your airplane and payload are under about 95% of the engine / power source mass, it is at least possible. Structures that light are not really an issue - the real issue is only flying during the day and in good weather. (And, of course, it would cost an arm and a leg!)
while (sig==sig) sig=!sig;
In principle, beamed power to power airliners is not impossible.
Tricky - certaintly.
http://authors.library.caltech.edu/3303/1/PARaipcp04a.pdf for example is a paper on doing this for vehicles to launch into orbit.
However, airliners are rather easier in some ways.
The 275 megawatts needed to boost the space vehicle are moderately less for the airliner, a 10m diameter, not 3m beam receptor is plausible for aircraft, making the frequency and/or dishes lots easier.
Range could also be considerably lower than the assumed 150km.
In use, it would involve multiple chains of dish stations, and microwave transmitters, perhaps 90km apart.
On the plus side, this can save _lots_ of power, as the airliners have to carry almost no fuel. (some for emergencies perhaps)
The problem is not "Airplanes are not solar powered!" the problem is "Moving large numbers of people and cargo around at almost Mach 1 is pretty energy intensive".
If you're seriously interested in what engineers are actually doing about this problem, start reading about NASA's SUGAR research:
http://www.nasa.gov/topics/aeronautics/features/future_airplanes.html
I can't help you with your conspiracy theories. Anybody who could make such an airplane as you imagine would become instantly, vastly, wealthy.
Why yes, I AM a rocket scientist!
Something like a hydrogen plant on the ground that produces liquified hydrogen which is then used for fuel.
The on-the-ground solar power collection mechanism that's currently most workable is algae. We've already had 747s fly on fuel produced from algae and algae are much more efficient than solar panels at harvesting energy from the sun. From what I've read, the only reason algae-based fuel isn't used commercially today is cost.
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.
Photons will go out of style in coming decades, as they start to piss people off by becoming too few or too numerous. The future is neutrinos. Just switch from catching solar photons to solar neutrinos. We have no excuses.
First of all they have millions of times more energy per particle than these little photons that barely get through the atmosphere much less the earth. Anytime at night, a certain number of neutrinos will still go straight through the earth and emerge from the ground on parallel paths aimed directly at your jet in the stratosphere. And there are three different kinds of neutrinos to choose from. I could believe not being able to harness one or two, but all three? Come on. I don't believe it. Let's get real. Let's get off our butts and get to work. Here is my five point plan:
Now some of you are going to say this is impossible, and pull a bunch of numbers out of your ass including a neutrino flux that is one third of its true value. If you insist on incorrect numbers to start then I will say haha those numbers are stupid.
Well yes and no. You don't care as much about the Mass, you care about gravity, i.e. downward acceleration. That *is* essentially Thrust, just in the wrong direction. His equation where he divides the FlyingMass by the LiftToDragRatio accommodates that issue seamlessly.
---jstlook ---For that is the way of Elves, for they say both yes AND no, and mean every word of it. --- J.R.R.T.
There's not enough energy in sunlight to push a plane. If you'll pardon some algebra:
Drag force on plane = (1/2) Cd * air density * wing area * speed^2
where Cd is the drag coefficient, which is fairly constant (about 0.03) for typical aircraft ranging from Cessnas to 747s.
Power needed to push through the air = Drag force * speed
Let's suppose the wings are entirely covered with solar panels, producing power:
Solar power = wing area * solar intensity
Suppose these panels are 100% efficient, and the electric engines are 100% efficient too: then solar power in = drag power out.
wing area * solar intensity = (1/2) Cd * air density * wing area * speed^3
Good news: wing area cancels out. It doesn't matter how big our plane is. Solve for speed:
speed = (2 solar intensity / (Cd * air density)^(1/3)
For a typical high-altitude airliner flying at 30,000 ft in daylight in mid-latitudes,
solar intensity = 300 W/m2
Cd = 0.03
air density = 0.4
speed = 37 m/s (or about 80 mph).
Bad news: your plane can go no faster than highway speed. You might as well drive. Worse news: at this altitude, at this speed, your airplane is sure to stall. To maintain enough lift to stay in the air, you're going to have to fly at low altitude. Where the air density is greater. And you're beneath the clouds. Crap.
Flying near sea level, let's suppose
solar intensity = 250 W/m2
air density = 1.3
our equation gives a top speed of 23 m/s, or 50 mph. Still tough to design a cargo plane that can stay aloft at that speed, and once again, you're definitely better off driving.
Keep in mind that I assumed absolutely perfect solar cells and engines, which are impossible. And you can't fly at night. Or at high latitude. And if it gets too cloudy you'll crash. And...
Practically* every jet engine in use has an internal starter. The J58 didn't at first, but I don't think too many airliners use that engine. ;)
The J58 (known for its use in the Blackbird) did use external starter motors - either large Buick V8s, or a pneumatic starter. However, even that engine can be started without external assistance by combusting triethylborane but that was probably never done on the ground outside of testing, since the number of times the engines could be restarted or afterburners ignited in flight during a mission was extremely limited. The triethylborane reserve was saved for in-flight use in the event that the engines stalled, or to relight the afterburners.
*I would say every one in a certified aircraft, but I'm not 100% certain of that.
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