Most of us have seen a dam built for hydroelectric power plants. But some hydroelectric power plants have lower power generation capacity as they run on the natural flow of the water body while some can generate power for the whole city. The classification of hydroelectric power plants can be of various types.
Based on the type of flow, the first type is Run-Of-River plants. As the name suggests, a run-of-river plant works according to the flow of the river. The power generated depends on the quantity of the flow. Thus, such plants are suitable for rivers having a minimum flow. If the water is not stored by the plant then it is called run-of-river without pondage. On the other hand, if the river flow exceeds the requirements of the plant then the water can be stored in a pond for later use. Such a plant is called run-of-river with pondage.
The next type is the conventional hydroelectric plant with a storage reservoir. Such plants are also called impoundment plants. Impoundment plants have reservoirs that store the water for later use. Due to the large size of such plants, the water from the rainy season can be stored for use in times of shortage. Thus, the power generation of impoundment plants is less variable than run-of-river plants.
A pumped storage plant is used at places where the water flow is inadequate. In such plants, a tailwater reservoir is used to store the water after it passes through the turbine. The tailwater pond stores the water from the turbine outlet. When the electricity demand is low, the water is pumped back to the headwater reservoir. This stored water can be used to generate electricity at the time of peak loads.
Hydroelectric power plants classified according to head are of three types.
A high head power plant will have a head of more than 100 m. Such power plants employ Francis turbines or Pelton wheels for power production. Francis turbine is effective in working at huge pressure created by the head. While in a medium head power plant, the height of the head ranges between 30 m to 100 m. In medium head power plants, the water from the main reservoir is channelled by a canal to the forebay from where it is sent to the turbine for power production. Another type called the low head power plants has a dam constructed across the river to provide the necessary water head. The water head is used for power generation while the excess flow is discharged through the dam.
Lastly, hydroelectric power plants can also be classified based on size. A large-hydro power plant has large dams and reservoirs. Such power plants can generate power exceeding 100 megawatts and supply power to large electricity grids. Small-hydro power plants are capable of producing power of fewer than 10 megawatts. Furthermore, another type called mini-hydropower plants operate as standalone units and have a water head of 5 to 20 meters. Mini-hydro power plants can generate power ranging from 1 megawatt to 5 megawatts. Even smaller power plants are possible. They are called micro-hydropower plants. Micro-hydropower plants have a water head at a height of less than 5 meters and generate power ranging from 0.1 to 1 megawatt.
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