At best, this is a good way of storing energy. It's no more renewable that petrol in reality. They said that there was enough aluminium out there to meet the US electricity demand for 35 years, which would be cool, but after that you're left with a shitload of alumina. In the aluminium extraction process, alumina must be electrolised to split it - and at a very high temperature, in order to keep it molten. This would use more energy than you would get out the aluminium in the form of hydrogen gas.
It's like this with all hydrogen technologies I've seen so far. When you split water, you impart chemical energy to the hydrogen atoms, because hydrogen gas is less stable than hydrogen bonded to oxygen. When you recombine the hydrogen with oxygen (i.e. burn it) that energy is released as heat. Since the energy you get from burning the hydrogen is effectively the same energy you gave it when you split it, the most energy you can get back will be the same amount you put in. Otherwise, energy is coming from nowhere. It depends where the energy you impart to the hydrogen comes from. If you burn fossil fuels in a power station and use that energy to electrolyse water, that is obviously economically unviable. You're just converting one fuel into another, and you'll lose some of the energy in the process. If however, you had a solar cell powering the electrolysis, that could be economically viable in the long run, because you're effectively taking solar energy and storing it in hydrogen.
This process of using aluminium to split water is unviable. The energy you're giving to the hydrogen in this case is coming from the aluminium. When the aluminium reacts with the water and displaces the hydrogen from the oxygen, it must be imparting energy to the hydrogen. I'm not sure how it does this, but it must be doing, otherwise where is the energy in the hydrogen coming from? Eventually you'll run out of aluminium, and you'll just have alumina, which is more stable than aluminium, and therefore doesn't have the energy to split water. It's just like getting energy from a fossil fuel, in principal. You take the chemical energy from the substance, but in the process you convert the substance into something useless. Like the article said, you can convert alumina back to aluminium, but to do so you have to give it energy, because aluminium has more energy than alumina and it has to come from somewhere.
Hope that made sense...
