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Hydroelectric
Power
Hydroelectric Power
Hydro-electric power, using the potential energy of rivers, now
supplies 17.5% of the world's electricity (99% in Norway, 57% in Canada, 55% in
Switzerland, 40% in Sweden, 7% in USA). Apart from a few countries with an
abundance of it, hydro capacity is normally applied to peak-load demand,
because it could be readily stopped and started. It is not a major option for
the future in the developed countries because most major sites in these
countries having potential for harnessing gravity in this way are either being
exploited already or are unavailable for other reasons such as environmental
considerations. Growth to 2030 is expected mostly in China and Latin America.
Hydro energy is available in many forms, potential energy from
high heads of water retained in dams, kinetic energy from current flow in
rivers and tidal barrages, and kinetic energy also from the movement of waves
on relatively static water masses. Many resourceful ways have been developed
for harnessing this energy but most of them involve continuous passage of water
flow through a turbine to generate electricity.
Like steam turbines, water turbines also depend on
the impulse of the working fluid on the turbine blades or the reaction between
the working fluid and the blades to turn the turbine shaft which in turn drives
the generator. Several different families of turbines have been developed to optimize
performance for particular water supply conditions.
Turbine Power Output
In general, the turbine converts the kinetic energy of the working
fluid, in this case water, into rotational motion of the turbine shaft.
Swiss mathematician Leonhard
Euler showed in 1754 that the torque on the shaft is equal to
the change in angular momentum of the water flow as it is deflected by the
turbine blades and the power generated is equal to the torque on the shaft
multiplied by the rotational speed of the shaft as per diagram below.
This result does not depend on the turbine configuration or what
happens inside the turbine. All that matters is the change in angular momentum
of the fluid between the input and output of the turbine.
Hydroelectric Power Generation Efficiency
Hydroelectric power generation is by far the most efficient method
of large scale electric power generation. Flow of energy through running water is
concentrated and can be controlled. The conversion process captures kinetic
energy and converts it directly into electric energy with the help of rotating water
turbines as prime movers coupled with generators. There are no inefficient
intermediate thermodynamic or chemical processes and no heat losses. The
overall efficiency can never be 100%; since extracting 100% kinetic energy from
the flowing water means the flow would have to stop.
The conversion efficiency of a hydroelectric power plant depends
mainly on the type of water turbine employed and can be as high as 95% for
large installations. Smaller hydroelectric power plants with output power less
than 5 MW may have efficiencies between 80 and 85 %.
It is, however, difficult to generate electric power when flow
rate of water is low to drive the prime mover.
Note: The theoretical Betz conversion efficiency limit of 59.3%
which represents the maximum efficiency that can be obtained from a wind
turbine, does not apply to hydraulic turbines since there are many variations
in turbine designs and more control devices for water flowing through turbine
blades. This means that there are equivalent variations in potential turbine
efficiency, many of which can exceed the Betz limit.
Turbine Types
The most appropriate turbine to use depends on the rate of water
flow and the head or pressure of water.
·
Impulse Turbines
Impulse turbines require tangential
water flow on one side of the turbine runner (rotor) and must therefore operate
when only partly submerged. They are best suited to applications with a high
head but a low volume flow rate such as fast flowing shallow water courses through
it is used in a wide range of situations with heads from as low as 15 meters up
to almost 2000 meters.
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The Pelton turbine
is an example of an impulse turbine. High pressure heads give rise to very
fast water jets impinging in the blades resulting in very high rotational
speeds of the turbine. The split bucket pairs divide the water flow ensuring
balanced axial forces on the turbine runner.
Pelton wheels are
ideal for low power installations with outputs of 10kW or less but they have
also been used in installations with power outputs of up to 200 MW.
Efficiency up to 95%
is possible.
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·
Reaction Turbines
Reaction turbines are designed to
operate with the turbine runner fully submerged or enclosed in a casing to
contain the water pressure. They are suitable for lower heads of water of 500 meters
or less and they are the most commonly used high power turbines.
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The Francis turbine
is an example of a reaction turbine. Water flow enters in a radial direction
towards the axis and exits in the direction of the axis.
Large scale
turbines used in dams are capable of delivering over 500 MW of power from a
head of water of around 100 meters with efficiency of up to 95%
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§ Propeller and Kaplan
Turbines
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The propeller turbine
is another example of a reaction turbine. Designed to work fully submerged,
it is similar in form to a ship's propeller and is the most suitable design
for low head water sources with a high flow rate such as those in slow
running rivers. Designs are optimized for a particular flow rate and the efficiency
drops rapidly if the flow rate falls below the design rating. The Kaplan
version has variable pitch vanes to enable it to work efficiently over a
range of flow rates.
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Power from Dams
(Potential Energy)
·
Supply Characteristics
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A hydroelectric dam
installation uses the potential energy of the water retained in the dam to
drive a water turbine which in turn drives an electric generator. The
available energy therefore depends on the head of the water above the turbine
and the volume of water flowing through it. Turbines are usually reaction
types whose blades are fully submerged in the water flow.
The diagram
opposite shows a typical turbine and generator configuration as used in a
dam.
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Source U.S. Army
Corps of Engineers
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Source: TVA
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The civil works
involved in providing hydro-power from a dam will usually be many times the
cost of the turbines and the associated electricity generating equipment.
Dams however provide a large water reservoir from which the flow of water,
and hence the power output of the generator, can be controlled. The reservoir
also serves as a supply buffer storing excess water during rainy periods and
releasing it during dry spells.
The buildup of huge
silt along the bed of the dam during a considerable period of time can cause
maintenance problems.
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·
Available Power
·
Potential energy per unit volume = ρgh
·
Where ρ is he density of the water (103 Kg/m3 ), h is
the head of water and g is the gravitational constant (10
m/sec2)
·
The power P from a dam is given by
·
P = ηρghQ
·
Where Q is the volume of water flowing per second
(the flow rate in m3/second) and η is the
efficiency of the turbine.
·
·
For water flowing at one cubic meter per second from a head of one
meter, the power generated is equivalent to 10 kW assuming an energy conversion
efficiency of 100% or just over 9 kW with a turbine efficiency of between 90%
and 95%.
·
Supply Characteristics
"Run of river" installations
are typically used for smaller schemes generating less than 10 MW output. Water
from a fast flowing river or stream is diverted through a turbine, often a
Pelton wheel which drives the electric generator. The head of water is
essentially zero and the turbine converts the kinetic energy of the flowing
water into the rotational energy of the turbine and the generator. The
available energy therefore depends on the quantity of water flowing through the
turbine and the square of its velocity. Impulse turbines which are only
partially submerged are more commonly employed in fast flowing run of river
installations while in deeper, slower flowing rivers, submerged Kaplan turbines
may be used to extract the energy from the water flow.
Run of river projects are much less
costly than dams because of the simpler civil works requirements. They are
however susceptible to variations in the rainfall or water flow which reduce or
even cut off potential power output during periods of drought. During flood
conditions the installation may not be able to accommodate the higher flow
rates and water must be diverted around the turbine losing the potential
generating capacity of the increased water flow. Because of these limitations,
if the construction of a dam is not possible, run of river installations may
need to incorporate some form of supply back-up such as battery storage,
emergency generators or even a grid connection.
The maximum power output from a turbine
used in a run of river application is equal to the kinetic energy (½mv2)
of the water impinging on the blades. Taking the efficiency η of
the turbine and its installation into account, the maximum output power Pmax is
given by
Pmax =½ηρQv2
where v is the
velocity of the water flow and Q is the volume of water
flowing through the turbine per second.
Q is given by
Q = A v
where A is the swept
area of the turbine blades.
Thus
Pmax =½ηρAv3
This relationship also applies to
shrouded turbines used to capture the energy of tidal flows (as shown below)
and is directly analogous to the equation for the theoretical power generated
by wind turbines. Here, the power output is
proportional to the cube of the velocity of the water.
Thus the power generated by 1.0 cubic meter
of water flowing at 1.0 meter per second rate through a turbine with 100%
efficiency will be 0.5 kW or slightly less taking into account the losses in
the system. This is only one twentieth of the power generated by the same
volume flow from the dam above. To generate the same power with the same volume
of water from a run of river installation the speed of the water flow should be
√20 meters per second (4.5 m/sec).
·
Supply Characteristics
Harnessing the power of the tides can
be achieved by placing bi-directional turbines in the path of the tidal water
flow in bays and river estuaries. To be viable, it needs a large tidal range
and involves creating a barrier across the bay or estuary to funnel the water
through the turbines as the tide comes in and goes out. Although tidal energy
captured in tidal ponds have been used since Roman
times to power mills, there are few modern installations. The first plant to utilize
tidal energy on a large scale for electricity generation was built at Rance
in France in1966. Others followed in Canada and Russia.
Tidal power comes closest of all the
intermittent renewable sources to being able to provide an unlimited,
continuous and predictable power output but unfortunately there are few
suitable sites in the world and environmental constraints have so far prevented
their general acceptance.
Shrouded water turbines placed in deep
water tidal currents show better potential for exploitation, though the
associated civil works are more complicated, and several projects are under
development.
Power is available for only six to
twelve hours per day depending on the ebb and flow of the tides.
Available Power
The maximum power output from a
shrouded water turbine used in tidal energy applications is equal to the
kinetic energy of the water impinging on the blades, similar to the "run
of river" calculation above. Taking the efficiency η of
the turbine and its installation into account, the maximum output power Pmax is
given by
Pmax =½ηρAv3
where v is the
velocity of the water flow and A is the swept area of the
blades.
A turbine 1.0 meter in diameter with a
water current of 1.0 meter per second flow rate through it would generate 0.4
kW of electricity assuming 100% efficiency. Similarly a 3.0 meter diameter
turbine with a water current of 3.0 meters per second flow rate would produce
32 kW of power.
·
Supply Characteristics
The energy available from the ocean's
surface wave motion is almost limited, but it has proved extremely difficult to
capture. Many resourceful systems have been proposed but, except for very small
installations, very few are generating electricity commercially and most have
been let down by practical problems. Some of these proposals, as below, are
still in an experimental phase and many are not scalable into high capacity
systems.
§ Energy Conversion
Systems
§ Oscillating Float
System
One of the simplest and most common
solutions is the oscillating float system in which a float is housed inside a
cylinder shaped buoy which is open at the bottom and moored to the seabed.
Inside the cylinder the float moves up and down on the surface of the waves as
they pass through the buoy.
Various methods have been employed to
turn the motion of the float into electrical energy. These include:-
§ Hydraulic systems in
which air is compressed in a pneumatic reservoir above the float during its
upward movement on the crests of the waves. After the crests have passed, the
air expands and forces the float downwards into the underneath troughs of the
waves. A hydraulic system then uses the reciprocating movement of the float to
pump water through a water turbine which drives a rotary electrical generator.
§ Pneumatic systems in
which the air displaced in the cylinder is used to power an air turbine which
drives the generator.
§ Linear generators to
turn the reciprocating motion of the float directly into electrical power.
§ Instead of generating
the electricity on board the buoy, some systems pump the hydraulic fluid ashore
to power shore based generators.
§ Oscillating Paddle
System
This system uses large paddles moored
to the ocean floor to mimic the swaying motion of sea plants in the presence of
ocean waves. The paddles are fixed to special hinged joints at the base which
use the swaying motion of the paddles to pump water through a turbine
generator.
§ Oscillating Snake
System
The snake system uses a series of
floating cylindrical sections linked by hinged joints. The floating snake is
tethered to the sea bed and maintains a position head on into the waves. The
wave-induced motion at the hinges is used to pump high-pressure oil through
hydraulic motors via smoothing accumulators. The hydraulic motors in turn drive
electrical generators to produce the electrical power.
§ Oscillating Water
Column
Water columns are formed within large
concrete structures built on the shore line or on rafts. The structure is open
at both the top and the bottom. The lower end is submerged in the sea and an
air turbine fills the aperture at the top. The rising and falling of the water
column inside the structure moves the air column above it driving the air
through the turbine generator. The turbine has movable vanes which rotate to
maintain unidirectional rotation when the movement of the air column reverses.
§ Pressure Transducer
System
The hydraulic pump system uses a
submerged gas-filled tank with rigid sides and base and a flexible,
bellows-like, top. The gas in the tank compresses and expands in response to
pressure changes from the waves passing overhead causing the top to rise and fall.
A lever attached to centre of the top drives pistons, which pump pressurized
water ashore for driving hydraulic generators.
§ Wave Capture Systems
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Wave capture
systems use a narrowing ramp to funnel waves into an elevated reservoir.
Waves entering the funnel over a wide front are concentrated into a narrowing
channel which causes the amplitude of the wave to increase. The increased
wave height coupled with the momentum of the water is sufficient to raise a
quantity of water up a ramp and into a reservoir situated above the sea
level. Water from the reservoir can then be released through a hydroelectric
turbine located below the reservoir to generate electricity.
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 |
§ Overtopping Wave
Systems
These are floating systems similar to
the above mentioned land based system. They focus waves onto a tapered ramp
which causes their amplitude to increase. The crests of the waves overtop the
ramp and spill into a low dam. Water from the low dam then flows through
hydroelectric turbines back into the sea beneath the floating structure.
§ Lever Systems
Various lever based energy capture
systems have been developed. Long levers may be mounted on steel piles or on
floating platforms. Large floats or buoys are attached to the extremities of
the levers which move up and down with the waves.
Movement of the lever arms forces fluid
to a central hydraulic accumulator and through a generator turbine.
Alternatively high-pressure water can be pumped ashore to power shore based
generators.
§ Technical Challenges
Formidable technical challenges are
involved in designing practical systems for capturing wave energy.
§ Variability of the
sea conditions
Sea conditions are extremely variable
and the system must be able to cope with a wide range of wave amplitudes and
frequencies as well as changes in the directions of currents.
§ Matching the
generating equipment the wave characteristics
Mechanisms are required to convert the
power of the irregular oscillating mechanical forces induced by the waves into
electrical power synchronized with the grid. This could involve some expensive
power electronics.
Typical rotating machines used for
power generation operate at a synchronous speed of 1200 r.p.m. (20 revolutions
per second) whereas the frequency of waves driving the generator is likely to
be between 5 and 10 seconds per cycle. A mechanical gearing system is needed to
match this 200:1 ratio in operating speeds, possibly combined with special
purpose, slow speed generators, incorporating a large number of pole pairs.
One way around all of these problems is
to use hydraulic accumulators either in situ or on shore to smooth out the
energy delivery to the generator.
§ Equipment
construction
For reasonable sized systems, very high
mechanical forces will be involved converting the wave energy into mechanical
energy for driving the electrical generator.
§ Housing and mooring
the equipment
Substantial housings must be provided
to protect the generating equipment from the harsh environment. Holding the
installation in place is also particularly difficult in deep water.
§ Energy transmission
Low loss armored and insulated cables
or high pressure pipes must be developed for delivering the electrical or
hydraulic energy back to the shore.
§ Resistance to storm
damage
Storm damage is a major threat. The
frequency of occurrence of waves of any particular amplitude follows a Rayleigh
distribution similar to that which applies to wind speeds.
Though the frequency of serious storms may be rather small, a wave of ten times
the average amplitude may be expected once every 50 years. From the power
calculation below, the wave power is proportional to the square of the wave
amplitude. This means that the installation must be designed to withstand
forces one hundred times greater than the normal working level. This adds
considerably to the costs.
Available Power
The wave power per unit length of the
wave front PL is given by (Twiddel & Weir.
Renewable Energy Resources) as
PL =ρga2λ/4T
Where ρ is the density
of the water (103 Kg/m3), a is the
wave amplitude (half of the wave height), g is the
gravitational constant (10 m/sec2), λ is the wave
length of the oscillation and T the period of the wave.
Thus for a wave with amplitude 1.5 meters,
length 100 meters and period 5 seconds, the power per meter of wavefront will
be 75 kW.
Ocean Thermal Energy
The boundless thermal energy of the warmer oceans of the world can
also be used to generate electricity in much the same way as geothermal heat is
used for electrical energy generation. Unfortunately conversion efficiencies
are very low and economic viability is difficult to justify with current energy
prices.
posted by Debabrata Dey @ 2:37 AM
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