Energy Physics
Lecture 8
B V Kheswa
University of Johannesburg
15 April 2025
Lecture 8 Content
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Introduction to Hydroelectric Power
Principle of operation of Hydroelectric Power
Types of Hydroelectric turbines
Pelton Impulse Turbine
Water Power Extracted by One Cup
Maximum Water Power Extracted by One Cup
Maximum Water Power Extracted by n Cups
Hydroelectric Power
• It is well established and contributes 20% of world´s electricity
• Hydroelectric Power: Power obtained from water that falls through
vertical distance from reservoir to drive a turbine
Hydroelectric Power
Principle of Operation
• The water reservoir is connected to a
turbine by a large pipe called a penstock.
• The filter keeps debris from entering the
turbine
• Valve controls the water flow and hence
the rotational speed of the turbine and
the electrical generator
Hydroelectric Power
Principle of Operation
• Vertical distance h between the surface of
the water in the reservoir and the turbine
is called the head.
• Turbine drives a generator that produces
electrical power at an output voltage of
typically 400 volts AC power
Types of Hydroelectric Turbines
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There are two types of hydroelectric turbines
i) Reaction turbine
ii) impulse turbine
Reaction turbine is complete immersed in flowing water powered from pressure drop
It works exactly the same as the wind
turbine
Types of Hydroelectric Turbines
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There are two types of hydroelectric turbines
i) Reaction turbine
ii) impulse turbine
Reaction turbine is complete immersed in flowing water powered from pressure drop
It works exactly the same as the wind
turbine
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impulse turbine is not enclosed in the water.
Instead, a jet of water hits the turbine and the power is derived from the rate of loss
of momentum of the water. It is discussed in details in the next slide.
Types of Hydroelectric Turbines
Pelton Impulse Turbine
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The type of impulse turbine used in hydroelectric power is called pelton impulse turbine
• The turbine has a
series of cups attached to a
revolving wheel
• The jet of water impinges on
the cups and is deflected so that
The water suffers a change in
momentum.
• This produces a
tangential force on the wheel
that causes it to rotate
Pelton Impulse Turbine
Water Power Extracted by One Cup
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Water power extracted by one impulse turbine cup is calculated according to
Water density
Volume flow rate
Hitting the cup
Velocity of
the jet
Tangential velocity of
The cup
Pelton Impulse Turbine
Water Power Extracted by One Cup
Classwork
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Show that the power extracted by one cup is maximum if tangential speed of the cup is half
of the jet velocity, and hence given by
, where
Pelton Impulse Turbine
Water Power Extracted by One Cup
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The maximum power corresponds to the ideal case where all water power would be
extracted
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That would happen if velocity of the water jet would be reduced to zero at the cup
In that case the turbine is said to be 100% efficient
In practice, commercial impulse turbines have efficiency of 90%
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The maximum power equation,
due to viscosity
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, does not account for energy losses
Pelton Impulse Turbine
Available Head
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Water falls a vertical distance and its potential energy is transformed into kinetic energy
This kinetic energy is then converted into electrical energy by the turbine
Some of this potential energy is NOT converted to kinetic energy
It is lost due to internal friction between flowing water and penstock
• To account for these losses, head providing
potential and kinetic energy is thought to be reduced.
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The reduction in the water head is called head loss,
net head is called available head and given by
Available head
head
Head loss
Pelton Impulse Turbine
Available Head
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Water falls a vertical distance and its potential energy is transformed into kinetic energy
This kinetic energy is then converted into electrical energy by the turbine
Some of this potential energy is NOT converted to kinetic energy
It is lost due to internal friction between flowing water and penstock
• To account for these losses, head providing
potential and kinetic energy is thought to be reduced.
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The reduction in the water head is called head loss,
net head is called available head and given by
Available head
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head
Head loss
Available head is the one used to compute maximum power extracted by impulse turbine
cups. In a typical power plat it is 90% of the head due to careful design
Pelton Impulse Turbine
Maximum Water Power Extracted by n Cups
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Taking viscosity into account, the maximum power extracted from n jets is
Available water head
Number of jets
Water
density
Gravitational acceleration
Cross-sectional area of nozzle
that makes the jet
Pelton Impulse Turbine
Maximum Water Power Extracted by n Cups
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Taking viscosity into account, the maximum power extracted from n jets is
Available water head
Number of jets
Water
density
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Gravitational acceleration
Cross-sectional area of nozzle
that makes the jet
The overall efficiency of a hydroelectric plant is product of the turbine efficiency and the
efficiency of the electrical generator and the available loss
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Typical efficiency of turbine and generator is 90%, available head may be 90%. Hence, giving
73% overall efficiency of a typical hydroelectric plant
Maximum Water Power Extracted by n Cups
Classwork
Taking viscosity into account, show that the maximum power extracted from n jets is
Available water head
Number of jets
Water
density
Gravitational acceleration
Cross-sectional area of nozzle
that makes the jet
Example
Problem
Things to know
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Discuss Hydroelectric Power
Name Types of Hydroelectric turbines
Discuss Pelton Impulse Turbine
Derive and Compute Water Power Extracted by One Cup
Derive and Compute Maximum Water Power Extracted by
One Cup
• Derive and Compute Maximum Water Power Extracted by n
Cups
Self-Directed Learning
• Read up on sections 7.2 and 7.3
(Wave Power and Tidal Power)
End of Hydroelectric Power