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1 Hydroelectric Power Plants
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Hydroelectric Power Plants
Hydropower to Electric Power
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Potential Energy
Kinetic Energy
Electrical Energy
Mechanical Energy
Electricity
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One of the fundamental laws of hydraulics, Bernoulli's theorem says that for a perfect fluid in steady state is constant the sum of the position, pressure energy and kinetic energy at each point of the same fluid thread . The trio of constant sum:
z+ p!+v2
2grepresents the energy that the unit of weight of the liquid in motion has in point of height z, having there velocity v and pressure p. Taken an horizontal plane of reference, and examining a vertical section of the current in the sense of movement, we have:
• the line of an ordered set of points z is the trajectory of the fluid particle
• the line of all points of ordered is the piezometric line
• the line of points of ordered , which is a Bernoulli's theorem,
is a line parallel to the reference plane and is the line of hydraulic loads or total.
z+ p!
z+ p!+v2
2g
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We define the power of a current in a broad cross-section of the current energy that passes through that section in unit time.
Consider an infinitesimal section of pipe flow dA, is: the extent of the pipe flow and H is also the total load. dP said the power supply section of the thread, then by definition you have:
dAdQ != vdA
dP = (! !dQ) !H
= [weight / (unity of time)]![energy / (unit of weight)]
To go to the entire section it is sufficient to integrate over the entire cross-section section
!! "#
$%&
'++==
AQdA
gpzHdQP v2v2
(((
mass
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Q is constant so the power is constant in all the section of the current
P = ! H dQ =Q! ! z+ p
!+v2
2g"
#$
%
&'vdA
A! =
P = ! z+ p!+v2
2g"
#$
%
&'Q
In real fluids, which are viscous, the power does not remain constant along the path, but will decrease in the sense of motion as a result of the actions of dissipating energy, due to internal friction that will transform a part into heat.
The expression of power can be simplified in the case of hydroelectric power stations, given a section at the upstream reservoir. In this case the kinetic term is negligible and the term p is zero because the pressure is the atmospheric reference.
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If H is the height difference (called the jump) between the reservoir and the turbine (taken as a reference plane) the theoretical power available will be:
P = !HQ
By measuring the flow in m3/s the jump in meters, since the specific weight of water is 1000 kg/m3, the power is given by:
P [W]= 9.81[m / s]!1000[kg /m3] !H[m] !Q[m3 / s]P [kW]= 9.81[m / s]!H[m] !Q[m3 / s]
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HYDROELECTRIC PLANT: defines the set of hydroelectric facilities that: • allow to change the natural flow of waters from a river or a
stream, in order to divert it for some distance on a new path, with minimal slope and minimum losses, the end of which is concentrated around the jump;
• use the jump to power a hydraulic motor with its electric generator;
• download the discharge in the channel of the same river downstream of the divert point (or into a different river)
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Defintions: Gross jump: it is the difference between the surface of the water in the tank load (or in the well surge) and the surface of the water in the channel to return immediately downstream the turbines Net jump (or motor) is that part of the gross jump that is actually used by hydraulic motors, is therefore the difference between the total load current entry and the total load current output of the turbine.
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Maximum derivable flow of a hydroelectric plant (in m3/s) is the total payable by all of the central hydraulic motors can run simultaneously. To that value are commensurate the tunnels under pressure, the penstock and the return channel. If the penstock is a free surface, it may be commensurate with a lower flow rate, where a storage capacity at its end (loading reservoir). flow used (or withdrawable) at a given time interval T: is the amount of water, in m3, which can be used (or derived) during the time interval considered, in relation to the maximum derivable flow. average flow rate used at a given time interval T: is the ratio between the flow used during that time and the time in seconds. legal power (or concession) of use: it is the average hydraulic power, in kW, theoretically available in the year related to the flow and the jump in granting the hydroelectric plant in question:
P = 9.81QH!n
where Q and H are respectively the flow rates used and the average jumps in single days or months year and n is the number of days or months to which the summation is extended.
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effective power corresponding to a flow rate Q and a jump H: is the power actually developable by the generators for that scope and that jump. Expressed in kW, it is equal to:
P = 9.81!! !Q !H
Types of Hydropower Plants
Hydroelectric plants can be schematically classified according to various parameters, which are: 1. TYPE 2. JUMP 3. FLOW 4. POWER 5. SERVICE 6. TYPE OF TANK (IF ANY)
Class Parameter
Impound The diverted water is guaranteed by a reservoir
Diversion The diverted water is a function of the river discharge
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Classification following the type
Class Parameter
Very small and small fall 0 < H < 20 m
Medium Fall 20 < H < 200 m
High Fall 200 < H < 1000 m
Very High Fall H > 1000 m
Classification following the jump
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Classification following the discharge
Class Parameter
Micro P < 0.1 MW
Mini 0.1 < P < 1 MW
Small 1 < P < 10 MW
Big P > 10 MW
Classification following the power
Class Parameter
Small discharge 0 < Q < 5 m3/s
Medium discharge 5 < Q < 25 m3/s
High discharge 25 < Q < 1000 m3/s
Very High discharge Q > 1000 m3/s
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Classification following the service
Class Parameter
Daily or weakly Discharge available within few days
Partial Discharge available within few days
Seasonal Able to shift the resources among seasons
Annual Able to shift the resources among years
Classification following the reservoir
Class
Continuous service
Power service
Regulation service
Energy transfer (pumping)
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Types of Hydropower Plants Diversion
The Tazimina project in Alaska
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Pumped Storage System
Boyle, Renewable Energy, 2nd edition, Oxford University Press, 2003
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Pumped Storage Power Spectrum
Glossary of Hydropower Terms Alternating current (AC) Electric current that reverses direction many times per second.
Ancillary service Operations provided by hydroelectric plants that ensure stable electricity delivery and optimize transmission system efficiency.
Cavitation Noise or vibration causing damage to the turbine blades as a results of bubbles that form in the water as it goes through the turbine which causes a loss in capacity, head loss, efficiency loss, and the cavity or bubble collapses when they pass into higher regions of pressure.
Direct current (DC) Electric current which flows in one direction.
Draft tube A water conduit, which can be straight or curved depending upon the turbine installation, that maintains a column of water from the turbine outlet and the downstream water level.
Efficiency A percentage obtained by dividing the actual power or energy by the theoretical power or energy. It represents how well the hydropower plant converts the energy of the water into electrical energy.
Head Vertical change in elevation, expressed in either feet or meters, between the head water level and the tailwater level.
Flow Volume of water, expressed as cubic feet or cubic meters per second, passing a point in a given amount of time.
Headwater The water level above the powerhouse.
Low Head Head of 66 feet or less.
Penstock A closed conduit or pipe for conducting water to the powerhouse.
Runner The rotating part of the turbine that converts the energy of falling water into mechanical energy.
Scroll case A spiral-shaped steel intake guiding the flow into the wicket gates located just prior to the turbine.
Small hydro Projects that produce 30 MW or less.
Tailrace The channel that carries water away from a dam.
Tailwater The water downstream of the powerhouse.
Ultra low head Head of 10 feet or less.
Wicket gates Adjustable elements that control the flow of water to the turbine passage.
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