Okay, so energy density is low - but what are the costs like? If you can build one of these cheaply, and the running costs are trivial, then it's worth doing.

Okay, so energy density is low - but what are the costs like? If you can build one of these cheaply, and the running costs are trivial, then it's worth doing. How many of them would you need to smooth out the energy of a wind farm, for instance?