About two thirds of the Earth's surface is covered with water. Water flows down the valleys in streams and rivers until it finally reaches the oceans, condensates through the power of the sun, ascends and descends again from the skies as precipitation in the form of rain or snow. This cycle is endless and is therefore an inexhaustible source of renewable energy that human beings have been using for their purposes for thousands of years. With the invention of the electrical generator, it became possible for the first time to convert the power of water directly into electricity. Depending on local conditions, the usable fall height (or head) and the volume of water available, different types of power stations are used. The underlying operating principle is always the same: water passing through a turbine causes the turbine to rotate. This rotational movement produces electricity in the generator. Afterwards the water is returned to the natural cycle and is not consumed.