The common scientific armour against alternative energy systems, if we take the energy from water concept, for that matter, is the first law of thermodynamics (which leads to conservation of energy). It says "The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings". In layman terms, it says energy cannot be created and input energy should be greater than output considering all systems are less than 100% efficient