What is Hydro-electric power station?
A generating station which utilizes the P.E. of water at a high level for the generation of electricity is understood as a hydro-electric power station.
Hydro-electric power stations are generally located in hilly areas where dams are often built conveniently and enormous water reservoirs are often obtained.
In a hydro-electric power plant, a water head is made by constructing a dam across a river or lake. From the dam, water is led to a water turbine.
The water turbine captures the energy within the falling water and changes the hydraulic energy (i.e., a product of head and flow of water) into energy at the turbine shaft.
The turbine drives the alternator which converts energy into electricity.
Hydro-electric power stations are getting very fashionable because the reserves of fuels (i.e., coal and oil) are depleting day by day.
They have the added importance for control, storage of water for irrigation, and water for drinking purposes.
What are the Advantages of Hydro-electric power station?
The most important advantages are-
(i) It requires no fuel as water is employed for the generation of electricity.
(ii) it's quite neat and clean as no smoke or ash is produced.
(iii) It requires very small running charges because water is that the source of energy which is out there freed from cost.
(iv) it's comparatively simple in construction and requires less maintenance.
(v) It doesn't require an extended beginning sort of a steam power plant. In fact, such plants can put into service instantly.
(vi) it's robust and features a longer life.
(vii) Such plants serve many purposes. additionally, to the generation of electricity, they also help in irrigation and controlling floods.
(viii) Although such plants require the eye of highly skilled persons at the time of construction, yet for operation, a couple of experienced persons may do the work well.
What are the Disadvantages of Hydro-electric power station?
(i) It involves high capital costs thanks to the development of the dam.
(ii) there's uncertainty about the supply of an enormous amount of water thanks to dependence on weather.
(iii) Skilled and experienced hands are required to create the plant.
(iv) It requires a high cost of transmission lines because the plant is found in hilly areas which are quite far away from the consumers.
Schematic Arrangement of Hydro-electric power plant
Although a hydro-electric power plant simply involves the conversion of hydraulic energy into electricity, yet it embraces many arrangements for correct working and efficiency.
The dam constructed across a river or lake and water from the catchment basin collects at the rear of the dam to make a reservoir.
A pressure tunnel is beginning from the reservoir and water delivered to the valve house at the beginning of the penstock. The valve house contains the most sluice valves and automatic isolating valves.
The former controls the water flow to the powerhouse and therefore the latter cuts off the availability of water when the penstock bursts.
From the valve house, water is taken to the water turbine through an enormous steel pipe referred to as a penstock.
The water turbine converts hydraulic energy into energy. The turbine drives the alternator which converts energy into electricity.
A surge tank (open from top) build just before the valve house and protects the penstock from bursting just in case the turbine gates suddenly close thanks to the electrical load being thrown off.
When the gates close, there's a sudden stopping of water at the lower end of the penstock and consequently, the penstock can burst sort of a paper log. The surge tank absorbs this pressure swing by a rise in its level of water.
Note: The governor opens or closes the turbine gates in accordance with the changes in electrical load. If the electrical load increases, the governor opens the turbine gates to permit more water and vice-versa.
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Choice of Site for Hydro-electric Power Stations
The following points should take into consideration while selecting the location for a hydro-electric power plant :
(i) Availability of water: Since the first requirement of a hydro-electric power plant is that the availability of a giant quantity of water, such plants should be built in an area (e.g., river, canal) where adequate water is out there at an honest head.
(ii) Storage of water: There are wide variations in the water system from a river or canal during the year.
This makes it necessary to store water by constructing a dam so as to make sure the generation of power throughout the year.
The storage helps in equalizing the flow of water in order that any excess quantity of water at a particular period of the year is often made available during times of very low flow within the river.
This results in the conclusion that the location selected for a hydro-electric plant should provide adequate facilities for erecting a dam and storage of water.
(iii) Cost and sort of land: The land for the development of the plant should be available at an inexpensive price.
Further, the bearing capacity of the bottom should be capable of withstanding a load of heavy equipment to be installed.
(iv) Transportation facilities: the location selected for a hydro-electric plant should be accessible by rail and road in order that necessary equipment and machinery might be easily transported.
It is clear from the above-mentioned factors that the perfect choice of site for such a plant is near a river in hilly areas where a dam can conveniently construct and enormous reservoirs are often obtained.
Constituents of Hydro-electric power station
The constituents of a hydro-electric power station are-
(1) hydraulic structures
(2) water turbines
(3) electrical equipment.
We shall discuss these things successively.
Hydraulic structures during a hydro-electric power station include a dam, spillways, headworks, surge tank, penstock, and accessory works.
A dam may be a barrier that stores water and creates a water head. Dams are built of concrete or stone masonry, earth or rock fill.
The type and arrangement depending upon the topography of the location. A masonry dam may form during a narrow canyon. An earth dam could also be best fitted to a good valley.
The type of dam also depends upon the inspiration conditions, local materials and transportation available, the occurrence of earthquakes, and other hazards.
At most of the sites, quite one sort of dam may suit. and therefore the one which is most economical is chosen.
There are times when the river flow exceeds the storage capacity of the reservoir. Such a situation arises during heavy rainfall within the catchment basin.
In order to discharge the excess water from the storage reservoir into the river on the downstream side of the dam, spillways may use.
Spillways are constructed of concrete piers on the highest of the dam. Gates provided between these piers and surplus water discharged over the crest of the dam by opening these gates.
The headworks consist of the diversion structures at the top of the intake.
They generally include booms and racks for diverting floating debris, sluices for by-passing debris and sediments, and valves for controlling the flow of water to the turbine.
The flow of water into and through headworks should be as smooth as possible to avoid heat loss and cavitation.
For this purpose, it's necessary to avoid sharp corners and abrupt contractions or enlargements.
(iv) Surge tank:
Open conduits leading water to the turbine require no protection. However, when closed conduits used, protection becomes necessary to limit the abnormal pressure within the conduit.
For this reason, closed conduits are always given a surge tank. A surge tank may be a small reservoir or tank (open at the top) during which water level rises or falls to scale back the pressure swings within the conduit.
A surge tank is found near the start of the conduit. When the turbine is running at a gentle load, there are not any surges within the flow of water through the conduit i.e., the number of water flowing within the conduit is simply sufficient to satisfy the turbine requirements.
However, when the load on the turbine decreases, the governor closes the gates of the turbine, reducing water system to the turbine.
The excess water at the lower end of the conduit rushes back to the surge tank and increases its water level. Thus the conduit prevented from bursting.
On the other hand, when the load on the turbine increases, additional water drawn from the surge tank to satisfy the increased load requirement.
Hence, a surge tank overcomes the abnormal pressure within the conduit when the load on the turbine falls and acts as a reservoir during a rise of load on the turbine.
Penstocks may open or closed conduits that carry water to the turbines. they're generally made from ferroconcrete or steel. Concrete penstocks are suitable for low heads (< 30 m) as greater pressure causes rapid deterioration of concrete. The steel penstocks can design for any head; the thickness of the penstock increases with the top or working pressure.
Note: Because just in case of open conduits, regulating gates control the inflow at the headworks and therefore the spillway discharges the excess water.
Various devices like an automatic valve, air valve, and surge tank provided for the protection of penstocks. the automated valve shuts off water flow through the penstock promptly if it ruptures.
The Air valve maintains the air pressure inside the penstock adequate to outside atmospheric pressure.
When water runs out of a penstock faster than it enters, a vacuum created ultimately which can cause the penstock to collapse.
Under such situations, the air valve opens and admits air within the penstock to take care of inside atmospheric pressure adequate to the surface atmospheric pressure.
The energy of falling water converts water turbines into energy.
The principal sorts of water turbines are:
(i) Impulse turbines
(ii) Reaction turbines
(i) Impulse turbines:
Such turbines are used for top heads. In a turbine, the whole pressure of water converted into K.E. during a nozzle, and therefore the velocity of the jet drives the wheel.
The example of this sort of turbine is that the Pelton wheel. It consists of a wheel fitted with elliptical buckets along its periphery.
The force of water jet striking the buckets on the wheel drives the turbine. the number of water jet falling on the turbine controlled by means of a needle or spear placed within the tip of the nozzle.
The governor controls the movement of the needle. If the load on the turbine decreases, the governor pushes the needle into the nozzle, thereby reducing the number of water striking the buckets. Reverse action takes place if the load on the turbine increases.
(ii) Reaction turbines:
For low and medium heads use the reaction turbines. during a turbine, water enters the runner partly with pressure energy and partly with velocity head.
The important sorts of reaction turbines are:
(a) Francis turbines
(b) Kaplan turbines
For low to medium heads use a Francis turbine. It consists of an outer ring of stationary guide blades fixed to the turbine casing and an inner ring of rotating blades forming the runner.
The guide blades control the flow of water to the turbine. Water flows radially inwards and changes during a downward direction while passing through the runner.
As the water passes over the “rotating blades” of the runner, both pressure and velocity of water are reduced. This causes a reaction force which drives the turbine.
For low heads and enormous quantities of water uses a Kaplan turbine. it's almost like Francis's turbine except that the runner of the Kaplan turbine receives water axially.
Water flows radially inwards Bhakra Dam through regulating gates all around the sides, changing direction within the runner to axial flow. This causes a reaction force which drives the turbine.
The electrical equipment of a hydro-electric power station includes alternators, transformers, circuit breakers, and other switching and protective devices.
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