Would it not be simpler to use existing hydro electric technology to capture energy from the outfall points of storm drain systems in all populated areas where annual rainfall might justify the required capital investment? Using rainfall statistics combined with surface capture area such as roof and road surface area should enable fairly detailed potential energy forcasts to be produced. For example 1cm of rainfall over a 6m x 6m area roof will produce .36 cubic metres of water x 100,000 dwellings is 36,000 cubic metres or 36,000 tonnes.