And we certainly aren't going to use solar to power commercial human air transport any time this millenium; it will always be better, for people, to compress energy before a flight and travel faster, than try to make the flight itself self-sustaining and travel slower. The only exception could be recreational flyers.
Bonus, you now have an orbital death ray if you decide to go Bond villain.
http://phys.org/news/2014-11-artificial-photosynthesis-fuel-...
I'm not at all sure that that's true for large jet aircraft though. Storing liquid hydrogen is a non-trivial exercise. On the other hand the larger the tank you're storing it in, the easier the problem becomes -- and large passenger jets would by necessity be using very large tanks. These tanks would be so large that they would impact jet performance in terms of increased drag. However, even with the larger tanks, liquid hydrogen will still provide a much better energy-to-weight ratio than you can get with jet propellent. This means you need less lift to fly the airplane and you have lower energy losses to lift-induced drag. I don't know that this would get you to break-even, but it seems likely that a hydrogen powered jet can get comparable performance to a jet using conventional jet fuel.
Of course just handling liquid hydrogen is no easy task. But a large airport should benefit from considerable economies of scale. It wouldn't be like trying to provide liquid hydrogen to cars at your local gas station.
There has been at least one experimental liquid hydrogen fueled aircraft, the Tupolev Tu-155 [2].
I don't necessarily disagree with you. However, weight is a really, really big deal in aircraft. And the volume becomes less of an issue as aircraft get bigger, thanks to the square-cube law. Are modern day jet aircraft big enough to largely offset the costs of fueling them with liquid hydrogen? I don't know, but they're pretty damn big.
Now the 787 isn't the largest craft out there, but spitballing the 747 gave similar numbers. I also don't have numbers on how much less fuel you'd need for the lighter weight (you'd reduce the weight of the aircraft by ~⅓ with equivalent amounts of fuel). If you assume it's more or less linear, you're talking about using ⅔ the fuel, which would be cutting out "only" 5-10% of revenue space or cutting your range by ⅔ and keeping revenue space the same.
Given the amount that airlines are trying to squeeze out of the existing revenue space already, it doesn't look like a particularly appealing trade-off.
I'm also not claiming that existing airframes could be easily retrofitted to fly on hydrogen. Not only do you need a much larger fuel volume, you can't practically store it in wing tanks. On the other hand if you took an existing design and stretched the fuselage to accommodate larger tanks, or attached large external tanks to the wings, you could get something that's economical. That's because you have an aircraft that's larger (more expensive, all other things being equal) but also lighter (cheaper, all other things being equal).
Solar impulse is large, but light and can carry two people plus stuff. Keep scaling it up, ditch the pilot, and you can have something between big slow boats and fast, but expencive jet aircraft. A full 50,000 lb 20' shipping container and 1 week China to US is probably viable. (Boats take 2 week to 1 month and unload at ports.)
The scaling depends on how much lift you generate, the lift is a question of speed and your the available lift surface the nominal lift produced is then divided by surface area to give you your wing loading.
Solar is pretty much the most inefficient way of powering virtually any aircraft because you needlessly increase the weight of the lifting surface and batteries weight considerably more than fuel and that weight is dead weight during the entire flight while a fueled aircraft becomes lighter.
That said the scaling between the volume and the "surface area" isn't even in question here because you are going to be hitting a wall much quicker and that's takeoff weight as mentioned before lift is a factor of the surface area that can provide lift and the speed of the aircraft (as well as air density and some other things but lets ignore it) the heavier the aircraft the more energy you need to actually get to takeoff speeds, and when you make your lift surface bigger you increase drag as well as weight.
The speed you can reach is going to be more or less limited because even a 100% efficiency solar powered engines are going to be limited to the max cell efficiency * cell surface area, the 2nd part of the equation is your wing surface and eventually there is a limit to how big your wings can be and still support themselves and most importantly not flutter. When you take all that and more into account there is absolutely no fucking way that you'll make an aircraft capable of lifting a 50,000lbs of crago, heck you won't build a solar aircraft capable of lifting 50,000 lbs of anything including itself. To put that into perspective 50,000lbs is 1/6th of the maximum structural load that a 747-8F (freighter) can carry, and about 1/4th of the nominal cargo load.
To put it into even more perspective Solar Impulse weights 3000 lbs, it's maximum takeoff weight is about 4000 lbs and it has a max speed of 80km/h with a wing span greater than that of a 747. To put it simply there is no freaking way to scale this up, there is a good reason why nature haven't developed heavy flyers the heaviest bird capable of flight weights 35 lbs, the rule of thumb is that when you double the weight you need a 4th time as much wing area and spend 4 times more calories to achieve flight and while mechanical flight isn't exactly the same it's not that different - the amount of energy you can produce more or less limits how well you can fly because at the end if you stick 2 SRB's to a cargo container you don't even need wings.
P.S. And that's before we getting into finding a material light enough to be viable for solar flight while still being able to bare the load of 50,000 lbs without collapsing, but even if you'll take perfect carbon nano tubes or any other super material you still won't be getting off the ground.
If you want solar cargo air transport blimps are probably the way to go...
The issue with overly large wings is mechanical stress let's try and get around that.
<This is a dumb design> As a thought experiment, take 3 Solar impulse aircraft fly them next to each other with a simple non weight baring cable to share power between them. The outer two can clearly provide more power than they need and supply that extra power to the center one. Now the center one has extra power for 'free'. So, it can have a higher angle of attack and more weight.
As to scaling birds, a Quetzalcoatlus had a wing span of 9 to 12 meters. That's huge for a land animal. https://en.wikipedia.org/wiki/Quetzalcoatlus Birds are limited by biology not so much aerodynamic issues.
And again you are missing the point Solar Impulse can lift about 1000 lbs of w/e if you want to scale it up either increase the thrust by 50 or increase your lift surface by 50 to get to that 50,000 lbs cargo you want. If we ignore load baring and drag than 3 Solar Impulse aircraft combined could lift 3000 lbs that's it.
We can argue about this till the sun doesn't shine it won't change the fact that solar flight will never be a viable option for cargo transport. But it can replace other things, things that for example are well in range of the 1000 or lbs of payload that any reasonable designed solar aircraft could carry - surveillance, communications, weather and atmospheric monitoring equipment etc. They could potentially replace satellites, and increase the endurance of both military and civilian drones considerably but no one is looking at them to carry cargo because it's simply silly.
Also if you want to move 50,000 lbs today by air it usually won't take less than 6 days, the only thing you can really ship overnight are small things that end up going on commercial flights not cargo flights. And while it's also true that a container ship can take 21-30 days from say China to the US about 14-15 of those days are spent in local transit and loading, the sailing it self a cargo ship can do within 10 days of good weather with the average being probably closer to 12 or so days due to weather and fuel economy.
As a final point on energy larger birds can do generally have a slower metabolism. They simply need less energy per lb to maintain flight and can travel longer distances on smaller per pound energy budgets.
Not that I think anything could convince you, but look into glide ratios and optimum glide speeds. If A and B have 15:1 ratios and B glides at 2x the speed that's more efficient.
Amount of thrust (limited by solar power) * lift surface limited by materials and physics, it all boils down to math. http://www.ajdesigner.com/phpwinglift/wing_lift_equation_for...
So I really don't see any way of a solar power aircraft which uses aerodynamic lift getting of the ground even in the most perfect unattainable conditions.
Second wing area and lift is related to velocity (not power) right before you stall outside of that it's a looser connection. Velocity is indirectly related to power in steady level flight based on drag. Drag in level flight is a function of weight, shape, altitude, and velocity.
Economicly I don't think this is a good idea. Physically it seems possible with cheaper and more efficemt solar cells it might make this more reasonable up to a point. Especially if they can boost altitude and thus speed.
50x the power takes 50x the surface area and 7.1x the length / width. Or less than that with a lifting body approach. Or significantly less than that if it's daytime only.
Solar Impulse is a marvel of engineering, and the fact that during on of the legs of the flight a single person flew the damn thing for 6 days straight is an attestment to human ingenuity and courage but there is a very very big difference between flying 50 people using electric aircraft (again electric not specifically solar) in 50 years to flying 50,000 of lbs on solar power.
We already have electric aircraft capable of 1.5-2h of flight scaling those up on electric power alone without solar might actually be easier as you do not inherit the weight of the solar panels. Efficient fuel cells and ultra light batteries / nano super caps could potentially make electric flight possible within 50 years but again relying on solar to transport cargo long haul no sorry not happening.
You would probably want a thicker coating to protect the cell, but in 50 years solar can add very minimal weight.
Anything that will be dependent on aerodynamic lift alone literally won't be getting of the ground with any significant payload.
Imagine three aircraft flying the same distance.
The first is a large airplane flying at 20 m/s. Solar impulse speed. The second is a large airplane flying at 200 m/s. Basic propeller plane. The third is a small airplane flying at 200 m/s.
Slow plane:
speed: 1x
area: 1x
lift: 1x
payload: 1x
drag: 1x
thrust: 1x
power: 1x
distance: 1x
energy: 1x
Big fast plane:
speed: 10x
area: 1x
lift: 100x
payload: 100x
drag: 100x
thrust: 100x
power: 1000x
duration: 0.1x
distance: 1x
energy: 100x
Small fast plane:
speed: 10x
area: 0.01x
lift: 1x
payload: 1x
drag: 1x
thrust: 1x
power: 10x
duration: 0.1x
distance: 1x
energy: 1x
(This didn't take into account altitude. That means you can go even faster for the same energy expenditure.)
I accept that Solar does not work to supplement the onboard generator used for electrical power on jets due to weight and night time flights. At the same time solar is ~1% the fuel cost for that amount of power for 20 years. They say they can scale this up 50x and carry 50 people. IMO, it would be better to build cargo only craft in that size range as this is far to slow outside of sight seeing in VFR.
Not to mention wind. That said if you can say double the solar power and add significant altitude and speed that's much more viable. I am assuming this is significantly worse than the best possible solar aircraft we could build in say 30 years.
Much better to have the solar panels on land and do synthetic kerosene for planes.
Ships go half the speed of Solar Impulse, but they have a lot more payload per dollar invested.