The diagram illustrates how electricity is produced in a hydroelectric power station. The process is shown for both daytime and nighttime.
Overall, during the day water flows down from the high-level reservoir to generate electricity, while at night it is pumped back to the upper reservoir.
Initially, water from the river is stored in a high-level reservoir behind a dam. Below the dam there is an intake which directs water to the generator turbines. During the daytime, water flows through the intake and spins the turbines, which drive the generator to produce electricity. The generated electricity is then transmitted through power lines to the national grid. After passing through the turbines, the water is stored in a low-level reservoir.
At night, the process is reversed. The intake is closed and the turbines operate in the opposite direction, acting as pumps. As a result, water is pumped from the low-level reservoir back to the high-level reservoir, ready to be used again the next day.