On Earth, water is neither created nor destroyed, but is constantly moved around. Water evaporates from the oceans and other freshwater bodies such as rivers and lakes, and is stored by the atmosphere. Under certain weather conditions water vapour in air condenses to form clouds, and eventually precipitates out as rain, snow, sleet and hail. This precipitation collects into streams and rivers, and flows back to the sea. This is known as the water cycle.
All this movement provides an enormous opportunity to create useful energy. Hydroelectric power (HEP) uses the force of moving water to create electricity. However, HEP stations often require large dams, which can disrupt ecosystems and displace people.
Large-scale HEP currently accounts for about 20% of the world's electricity supply. In the UK this percentage is much smaller, but there are a number of large-scale HEP stations currently in operation. In Scotland they are responsible for providing a significant amount of energy. Unfortunately there is little room for the future development of large scale HEP stations in the UK and so the potential of small-scale HEP stations is now being investigated.