Clean and unlimited energy from hydrogen and lightning?

Okay this sounds really crazy and not very likely to work but what if it does? My idea is to get a long conductors(Mainly a field somewhere where the occurrences of lightning are high)  which lightning will strike to separate water in a dam into hydrogen and oxygen to store the energy in chemical form. The stored hydrogen and oxygen will then be passed into a piston or turbine of some-sort and would cause a combustion, this combustion will then cause the turbine or piston to run. The product of this is simple and clean water which then can be passed back into the dam to repeat the process. This is not only a safe clean way of producing energy it might also be away of producing unlimited energy since water is abundant the only problem is the lightning its self. This idea is totally open for criticism and improvement and my only request is to give me credit if used and to at least consider the name given to the generator.
Likely problems:
1.The need for lightning
2.Giving the circuit a positive and negative terminal
3.Cost

A diagram which shows the idea. Please point out any errors.