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Hydroelectric Power Plant Prepared by
Dr. Sikder Sunbeam Islam
Department of EEE
IIUC
Hydropower & Hydroelectric Power • Hydropower, hydraulic power or water power is
power that is derived from the force or energy of moving water, which may be controlled for useful purposes.
• A Hydroelectric power plant is aimed at generating power from water flowing under pressure.
World distribution of hydropower
• Hydropower is the most important and widely-used renewable source of energy.
• Hydropower represents 19% of total electricity production.
• China is the largest producer of hydroelectricity 2
Basic Principle
• In a hydro-electric power station, water head is
created 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 in the
falling water and changes the hydraulic energy
into mechanical energy at the turbine shaft.
• The turbine drives the alternator which
converts mechanical energy into electrical
energy. 3
Advantages of Hydroelectric Power Plants • 1) No fuel required: One of the major advantages of the
hydroelectric power plants is that they don’t require any fuel for producing power. The hydroelectric power plants utilize renewable energy of water to generating electricity.
• 2) Cost of electricity is constant: Since no fuel is required for the hydroelectric power plants, the cost of electricity produced by them is more or less constant. It does not depend on the cost of fuels like coal, oil and natural gas in the international market. The country doesn’t even have to import the fuel for running the hydroelectric power plant thus saving lots of local currency.
• 3) No air-pollution is created: Since the hydroelectric power plants don’t burn any fuel no pollution is caused by them. It does not emit harmful gases and particulate matter, thus keeps the surrounding atmosphere clean and healthy for living.
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• 4) Long life: The life of hydroelectric power plants is longer than the life of thermal power plants. There are some hydroelectric power plants that were built more than 50-100 years ago and are still running.
• 5) Cost of generation of electricity: For the working of hydroelectric power plant very few people are required since most of the operations are automated, thus operating costs of hydroelectric power plants are low. Further, as the hydroelectric power plants become older, the cost of generation of electricity from it becomes cheaper since initial capital cost invested in the plant is recovered over the long period of operations.
• 6) Can easily work during high peak daily loads: The daily demand of power is not constant throughout the day. The peak power occurs at night. It is very difficult to start and stop the thermal and nuclear power plants on daily basis. The hydroelectric power plants can be easily started and stopped without consuming much time. Water can be collected in the dam throughout the day and this can be used to generate electricity during peak periods.
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• 7) Irrigation of farms: Water from the dams can also be used for the irrigation of farm lands thus producing the agriculture outputs throughout the year even in the areas where there is scanty or no rainfall.
• 8) Water sports and gardens: In vicinity of the dams the water from reservoir can be utilized to develop public recreational facilities like water parks for water sports and gardens.
• 9) No disposal problem: The steam power plant has ash disposal problem whereas hydroelectric power plant has no comparable problem.
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Drawbacks of Hydroelectric Power Plants
• Disrupts the Aquatic Ecosystems: The dams developed across the rivers can disturb the aquatic life and lead to their large scale destruction. There are chances that the fishes and other water animals may enter the penstock and ultimately the power generation turbines where they may get crashed. The dams can also disturb the mating seasons and mating areas of the water animals. In some cases the water animals have to swim against the water stream during breeding seasons, in such cases the dams create hindrances on their paths. Such animals can also get trapped in the turbines and loose lives on large scale.
• Requires large areas: The construction of dam, the power generation unit and the transformers and their connection to the national grid acquires large areas of the forest. The larger the land acquired for the dam, more is the disturbance to the natural ecosystem in the surrounding forest areas.
• High Transmission Cost: It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.
• Large Capital Cost: It involves high capital cost due to construction of dam.
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Layout of Hydroelectric Power
Plant(HEPP)
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Fig.1
Water cycle in the hydraulic power plant: • The water in rivers posses two types of energies: kinetic energy
and potential energy. The energy of water due to its motion is called as kinetic energy. The energy of water due to its high level is called as potential energy. Depending on the type of hydraulic power plants kinetic and/or potential energy of water is used to generate electricity.
• The most commonly used method of production of electricity from hydropower is by dams, which are constructed across the large rivers. The large quantities of water from river are diverted by pipelines also called as penstock, towards the main plant where large turbines are located. The water from penstock is allowed to fall on the large turbine blades that start rotating. Thus the water turbine converts the hydraulic energy into mechanical energy. The shaft of the turbine rotates in the electric generators where electricity is generated. Thus the mechanical energy is converted into electrical energy. This electricity is then passed to the transformers from where it is connected to the main national grid.
• The water leaving the turbine flows back to the river at the lower levels. In almost all the plants where water is used to generate electricity, the motion of water is used to rotate the turbine which generates the electricity in the generators and the water flows back to the river or ocean. 9
Choice of Site for Hydroelectric power station • Water Availability: Since the primary requirement of a hydroelectric
power station is the availability of huge quantity water, such plant should be built at a place (e.g; river, canal) where adequate water is available at a good head.
• Storage of water: There are wide variation in water supply from a river or canal throughout the year. This makes it necessary to store water for by constructing a dam in order to ensure the generation of power throughout the year. The storage help in equalizing the flow of water so that any excess quantity of water at a certain period of the year can be made available during times of very low flow in the river.
• Cost and Type of Land: The land for the construction of the plant should be available of the reasonable price. Further the bearing capacity of the ground should be adequate to withstand weight of the heavy equipment to be installed in the plant.
• Distance from the load centre: It is desired that the power plant should be set up near the load centers, so that the cost of erection of the transmission lines and their maintenance are low.
• Transport facility: The site selected for the plant should be accessible by rail and road so that, necessary machinery could be easily transported.
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Types of HEPP
• The hydro-power plants can be classified as below:
• (a) High head plants (b) Low head plants (c) Medium
head plants.
• (a) High head plants: About 100 m and above.
• (b) Medium head plants: about 30 to 500 m.
• (c) Low head plants: Upto about 50 m.
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Components of Hydroelectric Power Plant
There are eight important components of the hydroelectric power plant. All
these components and their working have been described below:
1) Dam
A dam (See in Fig.2) is a barrier which stores water and
creates water head. Dams are built of concrete or stone
work, earth or rock fill.
The dam is built on a large river that has abundant
quantity of water throughout the year.
The dam is built at location where the height of the river is
sufficiently high so as to get maximum possible potential
energy from water.
2) Water reservoir
Water reservoir is the place behind the dam where
water is stored.
The height of water in the reservoir decides how much
potential energy water possesses. Higher the height of
water more is its potential energy. The high position of
water in the reservoir also enables it to move
downwards effortlessly due to gravity.
The height of water in the reservoir is higher than the
natural height of water flowing in the river.
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Fig.2
3) Intake or control gates
These are the gates built on the inside of the dam. The water from
reservoir is released and controlled through these gates. These are called
inlet gates because water enters the power generation unit through
these gates.
When the control gates are opened the water flows due to gravity
through the penstock and towards the turbines. The water flowing
through the gates possesses potential as well as kinetic energy.
4) The penstock
The penstock is the long pipe or the shaft that carries the water flowing
from the reservoir towards the power generation unit that comprises of
the turbines and generator. The water in penstock possesses kinetic
energy due to its motion and potential energy due to its height.
The total amount of power generated in the hydroelectric power plant
depends on the height of the water reservoir and the amount of water
flowing through the penstock. The amount of water flowing through the
penstock is controlled by the control gates.
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5) Water turbines
The water flowing from the penstock is allowed to enter the power
generation unit that comprises of the turbines and generator. When
water falls on the blades of the turbine the kinetic and potential
energy of water is converted into the rotational motion of the blades
of the turbine. Due to rotation of blades the shaft of the turbine also
rotates. The turbine shaft is enclosed inside the generator. In most
of the hydroelectric power plants there are more than one power
generation units comprising of the turbine and generator.
There is large difference in height between the level of turbine and
level of water in the water reservoir. This difference in height, also
called as head of water, decides the total amount of power that can
be generated in the hydroelectric power plant.
There are various types of water turbines such as Kaplan turbine,
Francis turbine, Pelton wheels etc. The type of turbine used in the
hydroelectric power plant depends on the height of the reservoir,
quantity of water and the total power generation capacity.
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A water turbine is a rotary engine that takes energy from
moving water. Here, flowing water is directed on to the blades
of a turbine runner, creating a force on the blades.
Hydraulic turbines have the following advantages-
• Simple in construction
• Easily controllable
• Efficient
• Ability to work at peak load or stand by plant
• They can start from cold and peak up load in a short time.
Types of turbine:
• Water turbines are divided into two groups; reaction turbine
and impulse turbines.
5.1.Water Turbine
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5.2. Impulse Turbine: In impulse turbine the wheel
passages are not completely filled.
Water coming out of the nozzle at
the end of penstock is made to
strike a series of buckets fitted on
the periphery of a wheel or runner.
The resulting change in
momentum causes a force on the
turbine blades.
Water acting on wheel buckets is
at atmospheric pressure and is
supplied at few points at the
periphery of wheels and kinetic
energy is supplied to wheel.
Pelton turbine is an impulse
turbine.
Fig.4. Impulse turbine 17
Pelton Wheel in Plant
18 Fig.5. Pelton Wheel Plant
5.3. Reaction Turbine Reaction turbines are acted on by
water, which changes pressure as it
moves through the turbine and
gives up its energy. They must be
encased to contain the water
pressure (or suction), or they must
be fully submerged in the water
flow.
Here the turbine water passages
are completely filled with water,
water acting on wheel vanes is
under pressure greater than
atmospheric, water enter all round
the periphery of wheel and energy
in the form of both pressure and
kinetic energy is utilized by the
wheel. Francis wheel is a
reaction turbine. Fig.6. Francis Wheel 19
Difference between Reaction and Impulse Turbine.
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5.4. Comparison between Reaction and Impulse Turbine.
There are difference between impulse and reaction turbine as following :
In impulse turbine, at inlet , only kinetic energy available. But in
reaction, at inlet kinetic energy as well as pressure energy both
available.
In impulse, water may allow to enter part of circumference. But in
reaction, water is admitted over the circumference of the wheel.
In impulse, blades are only in action when they are in front of the
nozzle. But in reaction, blades are in action all the time.
In case of impulse, unit is installed above the tail race. But in reaction,
unit is kept entirely submerged in water below tail race.
In impulse, relative velocity either remain constant or reduce slightly
due to friction. But in reaction, there is continuous drop of pressure
during flow thus relative velocity increased. 21
5.5. Kaplan Turbine The Kaplan turbine is a propeller-type water turbine which
has adjustable blades. It was developed in 1913 by the Austrian
professor Viktor Kaplan.
Provide efficient power production in low-head
applications that was not possible with Francis turbines.
Kaplan turbines are now widely used throughout the world in
high-flow, low-head power production.
Unlike the Francis Turbine which has guide vanes at the
periphery of the turbine rotor (called as runner in the case of
Francis Turbine), there is a passage between the guide vanes
and the rotor of the Kaplan Turbine. The shape of the passage is
such that the flow which enters the passage in the radial
direction is forced to flow in axial direction. The rotor of the
Kaplan Turbine is similar to the propeller of a ship.
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Kaplan Turbine
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Fig.7.Kaplan turbine in plant
6) Generators
It is in the generator where the electricity is produced.
The shaft of the water turbine rotates in the generator,
which produces alternating current in the coils of the
generator. It is the rotation of the shaft inside the
generator that produces magnetic field which is
converted into electricity by electromagnetic field
induction. 8) Tailrace
The water that has been used to rotate the turbine blades and turbines shaft leaves the power generation unit entering the pipeline called as the Tailrace. From here the water flows into the main river. The height of water in the tailrace is much below the height of water in the water reservoir behind the dam. The potential energy of water in the tailrace has been used to generate electricity.
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Performance of Water Turbine Performance of turbine means working of turbine
under different loading condition. Although turbine is
required to run at constant speed but it is difficult
achieve because of fluctuation of load. To meet up the
varying load the water discharge is regulated.
The important parameters for the performance of any
turbine are-(i)Discharge,(ii)Head,(iii)Speed, (iv)Power.
It is difficult to compare performances of different
turbines as they work in different conditions.
Therefore, the studies are carried out under unit
characteristics like, unit discharge, unit head, unit
speed, unit power.
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Performance of Water Turbine (Continues.)
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Unit Power (𝑷𝒖):
It is defined as the power produced by the turbine when running under
Unit head (1 meter).
We know, P=𝜔𝐻φ
75, where, P=power produced by a turbine,
φ =Discharge in Cubic-m/s.
H=Head in meter
ω=Unit weight of water
D=Diameter of the runner
V=Velocity.
For, Unit Power (𝑃𝑢): 𝑃𝑢=𝑃
𝐻3/2 ----------------------- (1)
Unit Speed (𝑵𝒖):
It is the speed developed by a turbine when operating under a
unit head.
We know, V=𝜋𝐷𝑁
60, where, N=Speed of a turbine in R.P.M.,
For, Unit Speed (𝑁𝑢): 𝑁𝑢=𝑁
𝐻1/2 ----------------------- (2)
Performance of Water Turbine (Continues.)
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Unit Discharge(φ𝒖):
It is defined as the discharge of a turbine working under a unit
head.
We know, φ =AV where, A=Area
For, Unit Discharge (φ𝒖):φ𝒖=φ
𝐻1/2 -----------------(3)
Specific Speed (𝑵𝒔):
It is the speed of a geometrically similar turbine running under
a unit head and producing unit power.
It is useful because,
i. It helps in selecting type of turbine
ii. It permits to visualize the performance of turbine
iii. Normal running speed can be determined from it.
For, Specific Speed (𝑁𝑠): 𝑁𝑠=𝑁 𝑃
𝐻5/4 ----------------------- (2)
Turbine Classification Specific Speed
Pelton turbine Impulse 10-45
Francis Turbine Reaction 45-450
Kaplan Turbine Reaction 450-1000
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Table: Specific Speed (𝑁𝑠) for different turbine
Example-1. Calculate the specific speed of a turbine and suggest the type of
turbine required for a river having a discharge of 250 li/sec with an available
head of 50 meter. Assume efficiency of turbine as 80% and speed 450 RPM.
Given, φ =250 li/sec
H=50 m
η=80%
N=450 RPM
P=η𝜔𝐻φ
75=
0.8×1×250×50
75=133.3 W
(Taking ω=1kg/li)
𝑁𝑜𝑤, 𝑁𝑠=𝑁 𝑃
𝐻5/4=450 133.3
505/4 =39.9 rpm
For this specific speed Pelton turbine is required.
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Example-3. A hydro-electric generating station is supplied from a reservoir
of capacity 5 × 106 cubic meters at a head of 200 meters. Find the total
energy available in kWh if the overall efficiency is 75%.
Solution.
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References:
1. Principles of Power Systems,V.K.Mehta&RohitMehta
2. Power Plant Engineering, G.R.Nagpal
3. Power Station Engineering &Economy, William A Vopat
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