The energy in water has been harnessed for thousands of years. In prehistoric times, for example, it is probable that felled timber was moved by floating it down rivers. At sea, too, early mariners knew how to exploit the tides to help their passage. Hydro schemes helped power industrialisation in the 19th century, while over the last hundred years, hydroelectric projects, mainly in Scotland and Wales, have been an important source of the UK's electricity.
Water is fed through a turbine to generate power before being channelled back to the waterway.
The amount of electricity that can be produced is determined by how much water is available and how fast it flows is determined by the vertical distance between the point where water from a river or reservoir and the turbines below. The energy in waves can be turned into electricity by devices placed on the shoreline or out to sea. Wave motion can be used to compress air to drive a turbine or hydraulic pump. Underwater currents, caused by tides, can also be used to drive turbines. Exploiting water to generate power produces virtually no carbon dioxode or other harmful emissions.