Hydropower is the generating of electricity by harnessing the energy in water as it falls. The key factors in determining the amount of electricity are the amount of water falling during a given second (“Flow”) and how far it falls (the vertical distance, “Head”). Consequently, a site on a large river with water falling 5 meters at a weir might produce as much electricity as a site on a small river where the fall is 100m. In either case, a water turbine is used to extract the energy from the water. The turbine drives a generator that converts the mechanical power into electrical power. This electricity can be used on site (whether or not the system is connected to a national electricity network) or it can be “exported” through a network to consumers in other locations.

Small-scale hydropower is one of the most cost-effective and reliable energy technologies for providing clean electricity generation. In particular, the key advantages that small hydro has over wind, wave and solar power are:

• A high efficiency (70 - 90%), by far the best of all energy technologies.
• A high capacity factor (typically >50%), compared with 10% for solar and 30% for wind.
• A high level of predictability, varying with annual rainfall patterns.
• Slow rate of change; the output power varies only gradually from day to day (not from minute to minute).
• It is a long-lasting and robust technology; systems can readily be engineered to last for 50 years or more.

It is also environmentally benign. Small hydro is in most cases 'run-of-river'; in other words any dam or barrage is quite small, usually just a weir, and little or no water is stored. Therefore run-of-river installations do not have the same kinds of adverse effect on the local environment as large-scale hydro.

Some of this text has been taken from the website of the British Hydropower Association (BHA): www.british-hydro.org, Derwent Hydro is a member of the BHA’s council.