Given is a diagram illustrating the whole process of how a hydroelectric power station generates electricity, which consists of 2 operating modes of day and night.

From the information suppiled, it is evident that river, during the daytime, will be blocked by dam as it flows to downhill in order to create a high-level reservoir in upstream. Beyond this, under the dam, there will be a series of pipes, which are used for intaking water from the high-level reservoir. Then, after going through these pipes, the water from the higher reservoir will arrive the power station, which includes generator, powerline and reversible turbines. Next to it, flow of water will spin the blades of reversible turbines below the generator by hit them strongly. The kinetic energy of these blades will be converted into electricity after being transmitted to the generator through the shaft and send to the national grid through wires. Finally, after across the whole power station, water will reach the low-level reservoir.

The diurnal situation is the same as I said before, let’s see how it works at night.

As I said above, the turbines in the hydroelectric power station are reversible, which means that they can change its direction. Thus, at night, the whole situation will be completely contrary to the diurnal situation. During the night, Turbines will pump water from the lower reservoir into high-level reservoir in order to maintain the water level of higher reservoir, which, of course, will consume energy.

To sum up, a hydroelectric power station mainly take advantage of height difference of water level to generate electricity during the day and at night, store energy for the next day back sending water back up.