Run-of-the-river hydroelectricity is a type of hydroelectric generation whereby the natural flow and elevation drop of a river are used to generate electricity. Power stations of this type are installed on rivers with a consistent and steady flow, either natural or through the use of a large reservoir at the head of the river.
Power stations on rivers with great seasonal fluctuations require a large reservoir in order to operate during the dry season, resulting in the necessity to impound and flood large tracts of land. In contrast, run of river projects do not require impoundment of water. Instead, some of the water is diverted from a river, and sent into a pipe called a penstock. The penstock feeds the water downhill to the power station's turbines. Due to the difference in altitude, potential energy from the water up river is transformed into kinetic energy while it flows downriver through the penstock, giving it the speed required to spin the turbines that in turn transform this kinetic energy into electrical energy. The water leaves the generating station and is returned to the river without altering the existing flow or water levels.
Most run-of-river power plants will have a dam across the full width of the river in order to utilize all the river's water for electricity generation. Such installations will have a reservoir behind the dam but since flooding is minimal, they can be considered "run-of-river".
Another type of run-of-river power plants is floating in the water.
Flooding the upper part of the river is not required as it doesn't need a reservoir. As a result, people living at or near the river don't need to be relocated and natural habitats are preserved, reducing the environmental impact as compared to reservoirs.
Disadvantages and Problems
The output and consequently the cost efficiency are highly dependent on natural run-off. Spring melts will create a lot of energy while dry seasons will create relatively little energy. Though this disadvantage is negated if a site with consistent flow is chosen. It has little or no capacity for energy storage and can't co-ordinate the output of electricity generation to match consumer demand.
Major challenges of floating power plants are the anchoring, the cable routing without affecting the shipping traffic, floodwaters with heavy flotsam (tree trunks).