Examples of how hydropower is used

This page explains hydropower types and applications.


Torrs hydro community energy project

Hydropower is a proven technology that has been practiced worldwide for hundreds of years. The principle is to harness the energy from flowing water to generate electricity, using a turbine or other devices. Precisely how much electricity can be generated depends entirely on both the site's specific flowing water volume and the height from which the water falls.

Impressively, hydropower is able to convert between 70 and 90 per cent of the potential energy stored in the water into actual electricity. Moreover, hydropower also boasts a higher capacity factor (typically greater than 50 per cent) than that for solar (10 per cent) and wind (30 per cent). The capacity factor is the ratio between actual annual output and theoretical annual output.

Image provided courtesy of Peak Digital

Scale and classifications

Hydropower schemes vary greatly in size across the world, from the Three Gorges Dam in China with a capacity of 18.2 gigawatts (GW) down to small domestic systems of a few kilowatts (kW). The size of hydropower schemes can be classified in the following different ways:

  • head of water: the vertical distance through which the water descends (broken down into high, medium and low heads)
  • capacity factor: ratio between actual output and potential output
  • type of turbine: francis turbines, propellers and impulse turbines
  • location and type of hydropower plant.

A further category that is recognised but has no agreed formal definition is small-scale hydroelectricity. Within in the UK, a small-scale hydropower scheme is deemed to have a turbine that is smaller than five megawatts (MW) of power.

Broadly speaking, it is the head and required power outputs that are critical in determining the most suitable turbine type. Propellers tend to be suited to low heads and impulsive to high heads of water.

Storing electricity

Hydropower is currently the only technology that can be used to store large quantities of electricity in a clean way. To do this, the generator and turbine must be able to make water flow in either direction within a hydropower plant. If surplus energy is available (for example, at night), water can be pumped from a lower reservoir to an upper reservoir. Water can then be released from the upper reservoir back down to the lower reservoir to meet peak demands in electricity. There will always be losses in the conversion process but the efficiencies of modern turbines and generators allow for nearly 80 per cent of the energy to be retrieved.

Small-scale hydroelectricity

Close up of Archimedies screw that is used by the Torrs Hydro community energy project

With most of the potential UK sites for large scale hydroelectric plant lying in areas of natural beauty, it has been recognised for some time that few if any major new developments are likely to prove acceptable. As a result, attention has shifted towards small-scale hydroelectricity. Indeed, this has been focused on two main areas: firstly the upgrading of extant hydroelectricity plants and secondly the use of existing dams for new plants.

Small-scale projects tend to be developed using a low dam or barrage, known as ‘run-of-river'. These types of systems have little storage capacity and are dependent on prevailing flow rate at any given time. At around 35 per cent, the average capacity of run-of-river plants tends to be lower than that of the average for hydropower. However this is still competitive when compared against alternative renewable energies.

Image provided courtesy of Peak Digital

7 February 2011

Average (0 Votes)
The average rating is 0.0 stars out of 5.