If we are able to make small hydro power generators that we can easily generate power from very small streams in our neighborhood and even from a water tank. Let the water, being pumped into the tank, flow on a fan (fixed inside the tank) which can produce electricity. Thus we can generate almost the same energy spent for pumping water into the tank. That means energy used is generated the next moment.
Reward: I would be happy if I get such a product for my own use. Please let me know if it is already in use.