I have a friend who insists in an idea and I want to prove to him that it cannot work because it would violate basic physical laws. But somehow I don't find the right words. He says if he drilled a well of 2km (or any other depth) into the earth and at the bottom there was a pump that pumped water through a pipe back into to the head of the well he could create more electric energy than the pump would be using. He would install a few, not just one, turbines into the well at different heights, where the sum of energy generated by all turbines is greater than the energy needed to pump all the water up. Now according to this: A simple formula for approximating electric power production at a hydroelectric plant is: where P is Power in kilowatts, h is height in meters, r is flow rate in cubic meters per second, g is acceleration due to gravity of 9.8 m/s2, and k is a coefficient of efficiency ranging from 0 to 1. the only parameter that is different for all turbines is the height. Unless the flow rate is changing. This formula is similiar for pumps. Leaving acceleration (fixed) and water density and losses aside the only parameters ar flow rate and height. It looks like very easy to explain why it cannot work, yet... Does anyone have a calculation example?