Hydro schemes require a combination of a height difference in water levels (called a ‘head’) and a flow rate of water. The energy extracted is from the mass of water falling a distance, under the influence of gravity, i.e. the energy extracted is its change in potential energy. This is given as:
Potential Energy = Mass × 9.81 × Head Difference
Energy per second is power, so we can say:
Power = Mass Per Second × 9.81 × Head Difference
This is the maximum possible power available, and the aim of the hydro system is to extract as much as possible of this power, and use it for some purpose. In the natural water course all this energy is dissipated (wasted) in the turbulence that is usually seen in a fast flowing river.
The precise type of hydro scheme can vary widely, the most obvious may be like the Three Gorges dam project in China. These are the large end of the technology, and can tend to draw criticisms for environmental reasons. In the UK nearly all large sites with potential have already been exploited where practicable. They are very capital intensive, but provide an excellent long term option, and of course do not involve burning fossil fuels. Other large uses of hydro are notably in North Wales, at Ffestiniog and Dinorwic, where there are pumped storage power stations, which are operated like a battery, in order to manage demands on the national grid.
Currently in the UK, most development opportunities lie in the area of ‘Micro-Hydro’, broadly those schemes below 100kW in capacity. As a general guide, one household uses about 1kW continously, so a scheme of this size would run around 100 homes. Schemes of this size often require no reservoir, and as such are termed ‘run of river’. In this case a proportion of the flow is abtracted, and run through a turbine. Power can be generated when there is water available in the watercourse, and with no reservoir, its environmental footprint is small. A well designed system therefore represents an excellent return, in energy terms, for a small environmental cost. Indeed often works can be engineered to have a positive net effect.