Technology and applications

There are three basic types of hydroelectric power plant: the run-of-the river, reservoir and pumped storage power plants.

The most common type used worldwide is the run-of-the-river power plant, also known as a river power plant, which uses a river’s flow energy. These plants achieve an efficiency factor of almost 94 percent and are usually used to cover base load. These plants’ output is determined by the river’s flow velocity and water level. Some run-of-the river power plants can also store water when energy demand is low, using it as reserve water at times of increased demand for electricity. One particular type of run-of-the river power plant is the diversion hydropower plant. Here, the water is dammed in a weir and redirected through a separate intake canal to drive the turbines. With a standard run-of-the river power plant, there is only a slight difference in altitude between the upper and lower water levels, but a diversion hydropower plant exploits the greater difference in altitude (or higher head) created by the damming.

A reservoir power plant stores water in a natural or artificial lake, from where it is fed via penstock into the lower-lying power station. Since reservoir hydropower plants can function independently of natural water flows, they are ideally suited to balancing fluctuations in regional and national electricity and consumption. In contrast to the reservoir power plant, a pumped storage power plant uses two water reservoirs to store water, with the greatest possible difference in altitude between the upper and lower reservoirs. If the amount of power generated exceeds the amount required (e.g.at night), the resulting overcapacity is used to pump water from the lower into the upper reservoir, where it is available for electricity generation in peak load periods. The generator is driven by impulse turbines, one type of which is the so-called Pelton turbine.

The type of turbine used depends on the flow rate and head height (pressure) of the water. One of the oldest types of turbine is the conventional Francis turbine, which is used mainly by small hydropower plants with low head heights and medium flow rates. Archimedean screw turbines, which work on the principle of the Archimedean screw, can also be used in plants with low head heights and small outputs. Kaplan turbines and in-pipe turbines are used in plants with low head heights and high-volume flow rates and are suitable for fluctuating amounts of water. This type of turbine is commonly used in large run-of-the river power plants with low head heights ranging from six up to 15 metres. The Pelton turbine mentioned above is used in plants with high head heights and small amounts of water. Cross flow turbines are used in plants with low head heights and small amounts of water that generally have small outputs.