HYDROELECTRIC POWER PLANTS
Hydroelectric power is the power obtained from the energy of falling water where as hydroelectric power plant is the power plant utilising the potential energy of water at a high level for the generation of electrical energy.
Hydro electric power plants, however, cannot be located everywhere. Firstly there must be an ample quantity of water at sufficient head and secondly a suitable site must be available. The amount of power that can be developed depends on the quantity of water available, the rate at which it is available, the head etc. The electrical power, P developed is given by the equation.
P=wQHη (9.81 )× 10-³ kW
where
- w is the specific weight of water in kg/m³,
- Q is the rate of flow of water in m³/s,
- H is the height of fall or head in m .
- η is the generation efficiency.
In a hydroelectric power station, water head is created by constructing a dam across a river or lake. The pressure head of water or kinetic energy of water is utilised to drive the water turbines coupled to alternators and, therefore, generation of electrical power.
India is blessed with immense amount of hydro electric potential and ranks fifth in terms of exploitable hydro potential on global scenario. According to the assessment made by CEA, India is endowed with economically exploitable hydropower potential to the tune of 148,700 MW of installed capacity.
Table Of contents
MERITS AND DEMERITS.
Merits.
Such plants are neat and clean, robust, highly reliable, cheapest in operation and maintenance and have got longer life. Such piants do not need any fuel, can be run up and synchronised in few minutes, and have no standby losses. Very acute governing is possible with water turbines resulting in constant frequency and efficiency of such plants does not fall with the age.
Demerits.
It needs long area, enormously high construction cost, long time for erection and long transmission lines. Reservoir of such a plant submerges huge areas, uproots large population and creates social and other problems. Long dry season may affect the power supply.
SELECTION OF SITE.
While selecting a suitable site for a hydro- electric power plant, if a good system of natural storage-lakes at high altitudes and with large catchment areas can be located. the plant will be comparatively economical.
Anyhow the essential characteristics of a good site are large catchment area, high average rainfall and a favourable place for constructing the storage or reservoir. The land should be cheap in cost and rocky in order to withstand the weight of large building and heavy machinery.
There should be possibility of providing adequate transportation facilities so that the necessary equipment and machinery could be easily transported.
HYDROLOGY
Hydrology is a science which deals with the depletion and replenishment of water resources.
Hydrograph indicates the power available from the stream at different times of a day, week or year. Precipitation includes all the water that falls from atmosphere to the earth.
Run-off can also be named as discharge or stream flow. It may be defined as the part of the precipitation which is available as stream flow.
Flow duration curve is the plot of flow available during a period versus the fraction of time. Mass curve is the graph of the cumulative values of water quantity (run-off) against time.
Storage is provided to make more water available during deficient flow times and thus increase the firm capacity of the power plant and also increase the energy generated.
Pondage is required near the power plant, in case power plant is away from the storage reservoir, in order to meet the hourly changes in power demand.
ESSENTIAL ELEMENTS.
A hydroelectric plant consists of a reservoir for storage of water, a diversion dam, an intake structure for controlling and regulating the flow of water, a conduit system to carry the water from the intake to the waterwheel, the turbines coupled with generators, the draft tube for conveying water from waterwheel to the tailrace, the tailrace and a powerhouse i.e. the building to contain the turbines, generators, the accessories and other miscellaneous items.
The size, location, and type of each of these essential elements depend upon the topography and geological conditions and the amount of water to be used. The height to which the dam may be built is usually limited by the extent of flowage damage.
Pondage may have great value. particularly for peak load power plants, warranting the purchase of extensive flowage rights. The spillway section of the dam must be long enough to pass safely the maximum amount of water to be expected. Likewise the abutments and other short structures must be built to withstand successfully the greatest freshet conceivable on the river.
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CLASSIFICATION.
The hydroelectric power plants may be classified according to
- (i) extent of water flow regulation available
- (ii) the availability of water head and
- (iii) the type of load they supply.
According to the extent of water flow regulation available the hydroelectric power plants may be classified into
- (a) run-off river power plants without pondage
- (b) run-off river power plants with pondage and
- (C) reservoir power plants.
Run-off river power plants without pondage, as the name indicates, do not store water and uses the water as it comes. Such plants can be built at a considerably low cost but the head available and the amount of power generated are usually very low. During the high flow periods such plants can be employed to supply a substantial portion of base load.
Run-off river power plants with pondage have increased usefulness because of pondage which usually refers to the collection of water behind a dam at the plant and increases the firm capacity for a short period, say a week or more depending on the size of pondage. Such power plants are comparatively more reliable and its generating capacity is less dependent on available rate of flow of water. Such power plants can serve as base load or peak load power plants depending on the flow of stream (during high flow periods-as base load plants and during low flow periods-as peak load plants).
Reservoir power plants are with reservoirs of sufficiently large size to permit carry-over storage from the wet season to the dry season, and thus to supply firm flow substantially more than the minimum natural flow. Such plants can be used as base load plants or peak load plants as per requirement. Most of the hydro- electric power plants everywhere in the world are of this type.
According to availability of water head the hydroelectric power plants may be classified into
- (a) low head
- (b) medium head and
- (c) high head power plants.
Though there is no definite line of demarcation for low, medium and high heads but the head below 60 m is considered low head, the head above 60 m and below 300 m is considered as medium head and above 300 m is considered as high head, Low head power plant usually consists of a dam across a river. A sideway stream diverges from the river at the dam and over this stream the powerhouse is constructed. Later this channel joins the river further down stream. Such a plant uses vertical shaft Francis, propeller or Kaplan turbine. Structure of such a plant is extensive and expensive. Generators used in such plants are of low speed and large diameter:
Medium head power plant uses horizontal shaft Francis, propeller or Kaplan turbines. In such a plant water is carried from main reservoir to forebay through open channel and then to turbines through the penstock. The forebay itself serves as the surge tank in this case.
High head power plant uses Pelton wheels or jet impulse turbines as a prime movers. In such a plant, the water from the reservoir is carried through tunnel up to the surge tank and from surge tank to the power- house through the penstock. The generators used in such plants are of high speed and small diameter. Penstocks used are of large lengths and comparatively smaller x-section.
According to the load supplied hydroelectric power plants may be classified into
- (a) base load
- (b) peak load and
- (c) pumped storage plants.
The base load plants cater the base load of the system. Such plants have high load factors and continue to run for longer durations. Such plants are usually of large capacity. Run-off river plants without pondage and reservoir plants are used as base load plants. Plants having large storage can best be used as base load plants and particularly in rainy seasons when the water level of the reservoir will be raised by rain water.
Peak load plants are used only when the power demand exceeds the limits of other power plants in the interconnected system. Diesel engine plant, gas turbine plant or even steam power plant is used as a peak load plant.
Pumped storage power plants use reversible turbines which operate as turbines for power generation during peak load hours and as pumps for pumping water during peak-off hours. Pumped storage plant in India exists at Kadamparai (Tamil Nadu). This plant has four reversible Francis turbines of 100 MW each. Net available head is 350 m and live storage of dimensions of about 30 x 10⁶ m³.
WATER TURBINES.
In hydroelectric power plants water turbines are used to convert the energy of falling water into mechanical energy, which is further utilised for driving the electric generators.
The water turbines are simple in construction, highly efficient in operation (about 90% on full load), easily controllable and pick up the load in a very short time. They are built in various size up to 1000,000 hp with speeds varying from 80 rpm to 1,275 rpm depending on their size. Hydraulic turbines may be vertical or horizontal.
The water turbines used as prime movers in hydroelectric power stations, according to the type of flow of water, are of four types, namely
(i) axial flow turbines having flow of water along the shaft axis such as propeller and Kaplan turbine
(ii) inward radial flow turbines having flow of water along the radius such as Francis turbine
(iii) tangential flow turbine having flow of water along the tangential directions such as Pelton wheel and
(iv) mixed flow (radial inlet and axial outlet) such as Francis turbine.
According to the action of water on moving blades, water turbines are of two type, namely impulse and reaction type turbines.
When the entire pressure of water is converted into kinetic energy in a nozzle and the jet thus formed drives the wheel, the turbine is of impulse type, whereas if the water pressure combined with its velocity work on the runner the turbine is known as reaction type turbine.
According to the name of the originator water turbines are of three types, namely Pelton wheel, Francis turbine and Kaplan turbine. Pelton wheel is an impulse turbine and is used for high head power plants and their speed is about 1,275 rpm.
Francis turbines are used for medium head power plants and their speed range is 80 to 425 rpm. Kaplan turbines are used for low head power plants and for variable load. Their speed range is 320 to 1,000 rpm.
In case of reaction turbines vertical shaft arrangement has proved better and is, therefore universally adopted. In case of large sized impulse turbines, horizontal shaft arrangement is mostly adopted.
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