Suppose that we have a tower with two compartments. The upper one is filled with mercury and the bottom one with water. The floor can be opened (and so the compartments are abolished), and mercury could flow down to the bottom of the tower. I have calculated the potential energies of the two setup (A and B) and came to the conclusion that there is an energy difference of 14755 kilojoules. My question is that if we modify the setup (C) by adding a channel for the flowing mercury and a waterwheel (or a similar device) then how much energy could be collected of that 14 755 kilojoules? If no exact calculation can be made, then any estimate? I have found in Wikipedia that a waterwheel can have an efficiency of 60%, but I suppose that a setup with mercury and water has a different efficiency than a setup with water and air. I'm not a professional physicist, nor an engineer, so please no offense if I'm completely wrong at some point.