Asymptotically. We have three problems: power capacity, power on demand, and power portability. Renewable energy sources can solve the capacity problem, if our energy consumption goes down through efficiency and changing habits (in particular, if we stop flying everywhere...). The main thing we must optimize for is energy per day versus cost.
Power on demand is another major problem: we need to supply a 'peak' of power when people demand it. Renewable energy wavers during its production, so we need to store some proportion of its energy somehow. The main thing to optimize for here is energy capacity versus cost, assuming you can get the power out fast enough. Efficiency also factors in, though not hugely. Anything over 50% is ok.
Portable power is most problematic. We need an energy source that's clean, but that has a high energy density, measured relative to both weight and volume. Solving each problem leads to different possibilities.
For example, if you solve the weight problem, but it takes up tons of room (hydrogen), you might be able to power airplanes. You can also power trains.
If you solve the volume problem, you can easily power automobiles and two wheeled vehicles. (for example, hydrocarbon fuel cells work pretty well here. We're trying to make compressed air vehicles that will work well here)
If you have solved the low cost and efficiency problems, and it doesn't weigh too much, you can power ships, in particular, container ships. If the fuel is buoyant you can make a double hulled vehicle and store the fuel in it. (compressed air works very well here).
If ... it doesn't weigh too much, you can power ships, in particular, container ships. If the fuel is buoyant you can make a double hulled vehicle and store the fuel in it. (compressed air works very well here).
Why would the fuel need to be buoyant? Ships use steel hulls, steel is denser than seawater, and, despite that, ships float. Some ships are even powered by lead-acid batteries (http://en.wikipedia.org/wiki/Submarine#The_snorkel) -- though these batteries are recharged by onboard diesel generators -- and yet they float. Aircraft carriers and icebreakers are even powered by uranium -- 18 times the density of water.
Pressurizing the space between the two hulls of a double-hulled boat would require thicker steel than normal. (For a single curve, the tensile PSI needed is the air-pressure in PSI multiplied by the radius in inches -- for a sphere, divide by two.) Since steel is denser than seawater, this would tend to make the vessel sink (so the fuel would hardly buoyant unless it were further being contained in-between the hulls in carbon-fiber cells -- but that would obviate the need for two hulls). It would also impact the shape of the boat, since containing pressure is easier with a smaller radius-of-curvature (e.g. the usual flat-sides of containter ships would be counter-efficient for this task of retaining air-pressure).
An efficient design of such a container-ship might have two or three long, parallel tubes as hulls, with a container-platform mounted above on blade-pylons (to slice through the water). Optimally, the tubes would remain entirely submerged during cruising, so as not to interact with surface waves (which interaction normally causes efficiency losses).
The principle was to make the air stored in the volume surrounded by the hull, not to store the air in between the hull cases.
One of the major costs in storing air at high pressures is the impact shell. This can be merged with a thick hull, which are already designed with this purpose in mind.
True, there are other alternatives. One can have a heavy fuel and a much larger hull. But if the fuel and tank and hull combo are buoyant you minimize the required material, lowering cost.
Power on demand is another major problem: we need to supply a 'peak' of power when people demand it. Renewable energy wavers during its production, so we need to store some proportion of its energy somehow. The main thing to optimize for here is energy capacity versus cost, assuming you can get the power out fast enough. Efficiency also factors in, though not hugely. Anything over 50% is ok.
Portable power is most problematic. We need an energy source that's clean, but that has a high energy density, measured relative to both weight and volume. Solving each problem leads to different possibilities.
For example, if you solve the weight problem, but it takes up tons of room (hydrogen), you might be able to power airplanes. You can also power trains.
If you solve the volume problem, you can easily power automobiles and two wheeled vehicles. (for example, hydrocarbon fuel cells work pretty well here. We're trying to make compressed air vehicles that will work well here)
If you have solved the low cost and efficiency problems, and it doesn't weigh too much, you can power ships, in particular, container ships. If the fuel is buoyant you can make a double hulled vehicle and store the fuel in it. (compressed air works very well here).