... What we need is a way of harvesting solar energy without using arable land. Giant energy ships out at sea perhaps?
Interesting, but not original, observation that much of the sunlight absorbed by the sea immediately is converted into heat. (All not reflected back into space eventually will, but it could be used to make food and other products man needs first as it is very high quality energy.) Perhaps even electrolsis of water with simple discard of the O2 and storage inside the gas bag of a blimp is economically feasible? I.e. leave land base with load of H2O ballast and as O2 mass is discharged, take on more water from the sea, effectively converting the initial mass of the water load into an equal mass of H2 before returning to land base to deflate most of the H2 in to some chemical plant steel storage tanks, which can hold it in much smaller volume at higher pressure than the blimp can.
There are now flexible solar cells that could cover one top/side third of the blimp to catch energy, some of which would drive the electric motors, which at least in low wind conditions keep the axis of the blimp orthogonal to the sun for maximum collection efficiency. (Perhaps even in high winds, if they are blowing the blimp westward for more hours of sunlight in the "day." - Just be convected along, get back later or go all the way round the Earth.)
When the autonomous, un-manned, (GPS as ref to return to home base) blimp leaves with its initial H2O load, the gas bags* are slightly above atmospheric pressure with H2 and when it returns they are full of H2 to near the rupture pressure.
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*There would be a multitude of internal bags. The one on the axis would have the highest pressure when coming home. The skin of that bag would contain only a small fraction of that pressure. Most would be counter balanced by the adjoining hexagonal cross section surrounding bags. Etc all the way to the outer skin. You might naively think such internal subdivision would add too much weight, but it does not. - It actually reduces the weight of the blimp for a given "near rupture" load of H2 returned to the base. Also if internally divided the computer controlling the pressure can "roll" the blimp as the sun sets so that the solar cells are always orthogonal to the sunlight via shifting any water on board or slight pressure differences in the gas bags when the H2 mass is much greater than the mass of H2O.
The mass of a long cylinder, or set of smaller cylinders, required to contain a given mass of gas, at "rupture pressure," does not depend up the radius of the cylinder. Proof: The wall thickness required by the pressure stress increases linearly with the cylinder radius.* The circumference of the wall is also proportional to the radius. Thus, the mass of the wall required
for an isolated cylinder, such as an undivided blimp, is proportional to r^2. Likewise, the volume of gas contained in the cylinder is proportional to r^2. Thus, the ratio of gas contained to wall mass (and the cost of containment cylinder is approximately proportional to that mass.) does NOT depend upon the radius.
There are several advantages to having many smaller tubes compared to one large diameter tube. - This is why trucks delivering compressed gasses (H2 and He mainly, as they will not liquefy at reasonable temperatures) always use many small steel tubes.
Note that the inner hexagonally tube walls can be much thinner than if they were isolated and trying to hold the same high internal pressure. This is why your intuitions that subdivision of the blimp adds weight is exactly backwards!
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*If this is not obvious to you, imagine the cylinder as two "half cylinders" glued together. - What force is acting on the glue seam? How does it depend upon the radius?
PS1: A more cost effective idea may be to seed some parts of the ocean with iron, I think it is, that is deficient and limiting the growth of photo-plankton. (To get more fish at the end of the food chain) This also will remove more CO2 from the air and becomes more attractive economically if "carbon credits" are allowed.
PS2: The great Von Nueman, of computer fame, thought along these same lines about the energy wasted in sunlight producing heat directly in the sea. He like to think about the limts of automation. He made a rather complete study of the feasibility of making autonomous, un-manned, self-replicating boats, made of mainly alumimium, which they extracted from the sea. Periodically men would go and harvest a few of them as a source of Aluminium! Perhaps that is a better idea - more economical than harvesting H2via blimps?
PS3: Some of my old idle thoughts about these Al boats:
If those boats did exist, and mankind became extinct, what would aliens later visiting the Earth think? "Metalic life form evolved?" Even more interesting as the sea became crowded with these boats, would some evolve guns to sink the competition for the decreasing Al concentrations in the sea? Or perhaps they would notice the sunlight falling "wasted" on the land and evolve legs to go and capture it? Eventually, with legs, even mine the much richer ore deposits of the land and evolve different, more specialized forms, so they could kill the Earth invading aliens when they arrive and try to destroy the complex of metalic life forms and their society?
Perhaps you are correct. I.e. it is not the sub-prime loans, but the fact houses last for 30 years that has the US economy in or near a recession. 