Tuesday, August 16, 2011

Hydro-electric Power Station : Part 1

       A power generation station which uses the potential or kinetic energy of water for the generation of an electrical energy is called hydro-electric power station.
       Water has a kinetic energy when it is in motion. While the water stored at high level has a potential energy. The difference in level of water between the two points is called head. Such a water head is practically created by constructing reservoirs across river or lake. Generally a dam is constructed at high altitudes, which can be used as a continuous source of the water for the hydro-electric power stations. The water from the dam is taken through pipes and canals to the water turbine, which is at lower level. The turbine obtains the energy from the falling water and changes it into a mechanical energy. This mechanical energy of the turbine is then used to drive the alternator, which converts the mechanical energy into an electrical energy. The energy conversion involved in hydro-electric power generation is shown in the Fig. 1.
Fig. 1 Energy conversion

1.1 Factors for Selection of Site
       The water reservoir like dam can not be constructed anywhere. There are number of factors of affecting the choice of site for the hydroelectric power station.
1. Availability of water : As the basic requirement of hydro-electric plant is the water, the availability of huge quantity of water is the main consideration. The plant must be constructed where sufficient quantity of water is available at a good head. The previous rainfall records are studied and the maximum and minimum quantity of water available during the year estimated. Considering the losses such as evaporation, the water necessary for the plant is calculated. Then by comparing both the estimations, the choice of the site is done.
2. Storage of water : The rainfall is not consistent every year. Hence the available water should be stored. This makes necessary to construct dams. The storage helps in equalizing the flow of water throughout the year. So site should be provide sufficient facilities for erecting dam and the storage of water.
3. Head of water : For getting sufficient head, the dam or reservoir should be constructed at a height in a hilly area. The availability of the head directly affects the cost and economy of the power generation. So site should be selected in proper geographical area, which can give sufficient water head.
4. Cost and type of land : The initial cost of the project includes the cost of the land. Hence land must be available at a reasonable price. Similarly the type of the land must be such that it should able to withstand the weight of the heavy equipments to be installed.
5. Transportation facilities : For transporting the equipments and the machinery, the site selected must be easily accessible by rail and road.
6. Distance from load centers : The load center is connected to the site by the transmission lines. Hence to keep the cost of the transmission lines minimum and the losses occurring in the line minimum, the distance of the site from the load centers must be less. Otherwise the overall cost increases considerably.
       All these factors affect the selection of site for the hydro-electric power station.
1.2 General Arrangement of Hydro-electric Plant
       Though hydro-electric power station simply involves the conversion of hydraulic energy to the mechanical energy, it requires many types of supporting arrangements. The Fig. 2 shows the schematic arrangement of hydro-electric power station which uses water supply from an artificially constructed dam.
Fig. 2  Schematic arrangement of hydro-electric power plant

       The dam is constructed across the river and water from catchment area is collected behind the wall of the dam, in high mountains. A pressure channel is taken from such a water reservoir which takes water to a surge tank. The surge tank is a controlling room which controls the flow of water i.e. adjusts the discharge of water according to the need of the turbine and load on it. Trash rack does not allow floating and other impurities to pass to the turbine. The pressure channels plays a very important role. It relieves the pressure on the penstocks when the turbine valves are open or closed suddenly. The water is then taken to a valve house from where the penstocks start. The valve house contains main sluice valve and the automatic isolating valves. These valves also regulate the flow of water to the power house and isolates the supply of water if there is any emergency such as bursting of a penstock. Through the penstocks, the water is taken to the power house which consists of turbine and the alternator. The penstock are nothing but the steel pipes which are arranged in the form of open or closed conduits, supported by the anchor blocks.
       When the water from the penstock is hammered through a nozzle, on the turbine blades, the turbine starts rotating. At this stage the hydraulic energy is converted to a mechanical energy. The turbine drives the alternator which is coupled to the shaft of the turbine. The alternator converts the mechanical energy  into an electrical energy. This electrical energy is then transmitted to the load centers. The water collected from the turbine is called tail race. This tail race is then taken off to the river.

See Hydro-electric Power Station Part 2 

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