On average, hydroelectric power (HEP) stations deliver one-fifth of the world's electricity. Within mountainous countries such as Switzerland and Norway, these HEP stations provide almost all of the electricity. Power stations of this type are extremely expensive to build, but the great benefit is that their `fuel,' water, is free.
To produce hydro electricity a `head' of water is required. This means that there should be a difference in height and therefore pressure between two water levels. The power station is erected at the lower level and the water at the higher level can then flow down to it. The water gushes at high velocity through water turbines. This causes the turbines to rotate and as they are connected to generators, they produce electricity.
When a hydroelectric plant is running the level of water at the highest point will obviously fall. With a customary hydroelectric design, the level can only be re-established by natural means, such as rainfall. In the summer this level may become very low. For this reason, many hydroelectric works are devised for pumped storage. A pumped storage system utilises two reservoirs. They are arranged so that one reservoir is at high level, while the other is at a low level. Throughout the daytime when the need for power is great, water is allowed to stream from the high to the low reservoir through the turbines, thus generating electricity. During the nighttime, when power requirements are less, these turbines are changed into pumps that can pump the water back from the low to the high level. The original head will then be recovered and maximum power will then be attainable the following day.
