This article was linked to on Digg recently, and there has been some misunderstanding of its content. It concerns a discovery of a new catalyst that could make it possible to use sunlight to directly split water into hydrogen and oxygen. This might turn out to be quite important in the future (if we start using hydrogen as fuel, and if this turns out to be more efficient than other ways of producing hydrogen, etc.), but you can’t mention “water” and “energy” in the same sentence without certain people assuming that the useable energy is in the water itself. I know I’ve said this before, but it bears repeating: water is already in its lowest chemical energy state. You cannot extract energy from water using chemical* means. That means no water-powered car engines, no water-powered batteries, etc.

Of course this discovery is interesting enough in itself. Storing solar energy for later use has always been a bottleneck in this particular form of alternative energy, so if you could store it as chemical energy in hydrogen, it could potentially surpass the standard lead-acid or lithium-polymer battery solution. However, this remains to be seen. For the time being, this technology is still in experimental stages.

*Other forms of energy are routinely extracted from water in order to generate electrical power, for example kinetic/potential energy in a hydroelectic station, or thermal energy at a geothermal plant. The hydrogen in water could also be used as fusion fuel**, thus releasing nuclear energy.

**Note that a fusion reaction would far surpass (by several orders of magnitude) the energy required to split the hydrogen and oxygen apart, thus avoiding the problem that makes it impossible to get a surplus of chemical energy from water.