Hydro-electric Power Station

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.

1.3 Constituents of Hydro-electric Power Station 
       Let us discuss the constituent and their functions in the operation of the hydroelectric power station.

1.3.1 Dam 

       The water reservoir in the form of a dam is the main part of the power station. It stores the water, provides the continuous supply of water and maintains the necessary water head. The dams are built up of stones and concrete. The design and type of the dam us selected according to the topography of the site and economical aspects.

1.3.2 Spillways

       There are certain times when the river flow exceeds the storage capacity of the dam, due to the heavy rainfall. The spillways are provided to discharge this surplus water and maintain safe water level in the dam.

1.3.3 Surge Tank

       This is an important projecting device in a hydro-electric power plant. It is built just before the valve house. It protects the penstocks from bursting due to sudden pressure changes.

       If the load on the turbine is thrown off suddenly then by the governing action, the turbine input gates get suddenly closed. Thus there is sudden stopping of water at the lower end of the penstock. This time the excess water at the lower end of the penstock, rushes back to the surge tank. The surge tank water level increases. Thus the penstock is protected from bursting due to high pressure. The surge tank absorbs this high pressure swing by increasing its water level.

       On the other hand, when the load on the turbine suddenly increases, the additional water required is drawn from the surge tank. This satisfies the increased water demand instantly.

        Thus the surge tank controls the pressure changes created due to rapid changes in the water flow in penstock and hence protects the penstock from water hammer effects which might burst the penstock.

1.3.4 Penstocks

       The penstocks are made up of steel or concrete and arranged in the form of conduits, supported by the anchor blocks. The penstocks are used to carry water to the turbine. For the low head (less than 30 m) power stations, the concrete penstocks are used. The steel penstocks are suitable for any head.

Fig. 3  Protecting devices of penstock

       There are certain protective devices attached to the penstocks. These devices are shown in the Fig.3.

       The automatic butterfly valve completely shuts off the water flow if the penstock bursts.

       The air valve maintains the air pressure inside the penstock equal to the outside atmospheric pressure.

       The anchor block supports the penstock and holds it in the proper position.

       The surge tank also protects the penstock from sudden pressure changes.

1.3.5 Water Turbines

       The main two types of water turbines are,

i) Impulse      and    ii) Reaction

       In an impulse turbine, the entire pressure of water is converted into a kinetic energy in a nozzle. Then the water jet is forced on the turbine which a large velocity which drives the wheel. The pelton wheel is an example of impulse turbine which is shown in the Fig. 4.

Fig. 4  Impulse turbine

       It contains elliptical buckets mounted on the periphery of a wheel. The force of water jet on the buckets, drives the wheel and the turbine. There is a needle or spare at the tip of the nozzle. The governor controls the needle which controls the force of the jet, according to the load demand. The impulse turbines are used for the high head power stations.

       In the reaction turbines, the water enters the runner, partly with pressure and partly with velocity head. There are two type of reaction turbines.

i) Francis          and     ii) Kalpan

       The Fig. 5 shows the basic principle of reaction turbine. The reaction turbine consists of an outer ring of stationary guided blades and an inner ring of rotating blades. The guided blades control the flow of water to the turbine. Water flows radially inwards and changes to a downward direction when it passes through the rotating blades. While passing over the rotating blades, the pressure and velocity of water are decreased. This causes reaction force to exist which drives the turbine. For large variation of head, Kalpan is used as its efficiency does not vary with change in load. For fairly constant head, a Francis or propeller turbine is used.

       The reaction turbines are used for the low head power stations.

Fig. 5   Reaction turbine

1.4 Advantages

1. If the proper site is selected, the continuous water supply is available.

2. Requires no fuel as water is used.

3. No burning of fuel hence neat and clean site as no smoke or ash is produced.

4. It does not pollute the atmosphere.

5. The operating cost is very low as free water supply is available.

6. The turbines in this plants can be switched on and off in a very short period of time.
7. It is relatively simple in construction, self contained in operation and requires less maintenance.
8. It is robust and has very long life.
9. It gives high efficiency over a considerable range of load. This improves the overall system efficiency.
10. It provides the additional benefits like irrigation, food control, afforestation etc.
11. Being simple in design and operation, highly skilled workers are not necessary for the daily operation. Thus man power requirements is low.

1.5 Disadvantaes

1. Due to the construction of dam, very high capital cost.
2. The low rate of return.
3. Uncertainity of availability of water due to unpredictable rainfall.
4. As its location is in hilly areas and mountains, the long transmission lines are necessary for the transmission of generated electrical energy. This requires high cost.
5. The large power stations disturb the ecology of the area by the way of disforestation, destroying vegetation and uprooting people.
6. Highly skilled and experienced persons are necessary at the time of construction.