Hydroelectric systems work by transferring the energy of moving water into electricity using a turbine and a generator. They do this by exploiting the water’s potential energy (its fall or 'head') along with some kinetic energy (its 'flow').
Systems vary in size from the small ones of about 5 megawatts – the equivalent to two large wind turbines and enough to supply the electricity demand of around 8,000 homes – through to the industrial-scale schemes of up to 1,000 megawatts or more that can involve the damming of rivers and flooding of entire river valleys.
The video below explains how it works.
Small and community-scale
The smallest hydro systems commercially available are about 600W and one of these would provide most of the electricity for a single, average house over the course of a year. A fairly typical size for a community hydro system is 30 kilowatts, which would supply the electricity needs of around 50 average homes.
Small hydro systems work by taking some water out of a water-course and diverting it through the turbine (which generates the electricity), before the water re-enters the river lower down. They don’t have high dams or reservoirs and don’t alter the nature of the river in the way large hydro schemes do.
Although expensive, hydro is a low-maintenance technology and has a longer working life than other renewable technologies, lasting between 30-50 years. It also generates energy almost all the time, varying slightly with the seasons. Over a year, a typical 1MW hydro installation generates more electricity than a 1MW wind turbine or solar array.
All hydro systems need to be designed to make sure there is adequate flow for fish and other aquatic life, and includes a tank where silt and small stones are caught to prevent them entering the machinery. They will also have a grid structure here that sieves out floating debris, like branches and rubbish. The system will also need an electricity sub-station situated nearby that will feed the energy generated by the hydro plant into the grid.
Hydro turbines come in many forms, such as archimides screw, pelton wheel and crossflow, and different designs suit different circumstances. Archimedes screws, for example, are more suitable in rivers with a low head. And not all rivers are suitable for hydro; large rivers with significant falls are the best sites for hydro because they have high head and high flow.
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