What is the fundamental problem with this?
Cost. Wind/wave/solar energy is more expensive than fossil-fuel or nuclear energy to start with, and adding not-too-efficient storage mechanisms to even out the supply does not help it at all.
Really, the answer to most questions of this kind is "cost". It is the default and usually correct answer.
But "cost" isn't very specific. Is the major problem that it's too inefficient? That it takes too much to build? Too much to maintain?
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.