How does a dam’s water produce electricity?

A reservoir above a power station stores water at a higher elevation than the river downstream. When water passes through the station, this height difference can supply energy. The useful difference in water level is called head; it is not simply the thickness or total height of the concrete dam.

Water travels through a large pipe called a penstock. Its pressure and motion act on the turbine’s blades, making a shaft rotate. That shaft drives a generator, where changing magnetic fields and conducting coils convert mechanical energy into electrical energy. Water then leaves toward the downstream river. It is not consumed as a fuel.

Both head and flow matter. The pictured flow is 2 cubic meters each second: about 2,000 kilograms of water descending through a usable head of 20 meters. That transfers about 392 kilojoules each second, or 392 kilowatts. With a combined turbine-and-generator efficiency of 80 percent, about 314 kilowatts becomes electrical output. The remainder goes into other energy transfers, including turbulence and heating.

A high reservoir with little flowing water can therefore deliver little power at that moment, while a greater flow can transfer more energy each second within the equipment's limits. As water levels and river supply change, the available head and flow change too. The dam holds water; the moving water and connected machinery make the electricity. Dams may also have purposes other than power generation.