Factors in selecting sites for hydroelectric power plants

UNIT 4
POWER PLANT ENGG.
HYDRO ELECTRIC POWER PLANT
S.PALANIVEL ASSOCIATE PROF./MECH ENGG.
KAMARAJ COLLEGE OF ENGG. & TECH. VIRUDHUNAGAR(NEAR)

HYDRO ELEC. POWER PLANT
• HYDRO : BASED ON AVAILABLITY OF WATER AND
UTILISABLE WATER HEAD
• INITIAL COST WILL BE VERY HIGH
• RUNNING COST WILL BE LESS
• TAKES MORE TIME TO CONSTRUCT
• DIFFICULT TO CONVINCE PEOPLE
• POWER CAN BE PRODUCED BY TAKING LESS TIME
• MOSTLY MULTI PURPOSE LIKE … IRRIGATION, FLOOD
CONTROL , POWER GENERATION, PUMPING
STATION.

Selection of site for Hydroelectric
Plant
Following factors should be considered while selecting the site for hydro-
electric plant
1.Availability of water
2.Water storage Capacity
3.Available water head
4.Accessibility of the site
5.Distance from Load centre
6.Type of land of the site.
Availability of water : Design and capacity of the hydro plant greatly depends
on the amount of water available at the site. The run – off data along with
precipitation at the proposed site with maximum and minimum quantity of
water available in a year should be made available to
i)Decide capacity of the plant
ii)Set up the peak load plant or base load plant
iii)Provide adequate spillways or gate relief during flood period.
Water storage capacity :It is required to store the water for continuous
generation of power, since there is a wide variation in rain fall all round the
Year. Storage capacity can be estimated with help of mass curve.

• Available water head : In order to generate the desired quantity of
power it is necessary that a large quantity of water at sufficient head
should be available. An increase in head for a given output , reduces
the quantity of water required to be supplied to turbines.
• Accessibility of the site : The site should be easily accessible by rail
and road to facilitate movement of men and material for
construction and operation of hydro plant. An inaccessible terrain
will create problems and disturb movement of men and material.
• Distance from the load centre : If the site is close to the load centre,
the cost of transmission lines and transmission losses will be
reduced.
• Type of the land of the site :The land of the site should be cheap
and rocky. The dam constructed at the site should have large
catchment area to store water at high head. The foundation rocks of
the masonry dam should be strong enough to withstand the
stresses in the structure and the thrust of water when the reservoir
is full.

• Storage : Restoration of a considerable amount of excess run off
during seasons of surplus flow for use in dry seasons. This is
accomplished by constructing a dam across the stream at a
suitable site and building a storage reservoir in the upstream of
the dam.
• Pondage : A regulating body of water in the form relatively
small pond provided at the plant. The pondage is used to
regulate the variable water flow to meet power demand.
It caters for short term fluctuations which may occur due to
(i)sudden increase or decrease of load on the turbine,
(ii)sudden changes in the inflow of water, say by breaches in the
conveyance channel
(iii) Change of water demand by turbine
• Pondage increases the capacity of river over a short time, such
as a week, where as storage increases the capacity of river over
an extended period of 6 months to as much as 2 yearsS.PALANIVEL ASSOCIATE PROF./MECH ENGG.

Essential elements of a Hydro electrical plant
• Catchment area.
• Reservoir
• Dam
• Spillways
• Conduits
• Surge tanks
• Draft tubes
• Power House
• Switch yard for Power
transmission.

• Catchment area : The whole area behind the dam draining
into a stream or river across which dam has been
constructed.
• Characteristics of this are its size, shape, surface,
orientation, altitude, topography and geology
• Bigger the catchment area steeper is the slope, higher its
altitude and greater is the total run of water
• Reservoir : Storage during plenty for subsequent use in
times of scarcity. Water is stored not only for power
generation but also for irrigation, flood control, water
supply.
• Water held in upstream reservoir is called storage, where as
water behind the plant is called pondage

Dam
• Two basic functions of dam are
• (i) It develops a reservoir of the desired capacity to store water
• (ii) builds up head for power generation
• Dams can be classified in the following various ways
• Function, shape, material of construction, hydraulic and
structural design
• Based on their functions dams can be classified into storage
dams, diversion dams and detention dams.
• Storage dams are mainly for storing water for subsequent use
such as, for power generation, irrigation and water supply
• Diversion dams are constructed to raise the water level and
divert the river flow in another direction. This type may not
have large capacities.
• Detention dams are constructed primarily to store flood water.
S.PALANIVEL ASSOCIATE PROF./MECH ENGG
. KAMARAJ COLLEGE OF ENGG. & TECH. VIRUDHUNAGAR(NEAR)

• On the basis of shape, there can be trapezoidal, arch dams
to suit the structural function.
• On the basis of materials of construction, dams can be
constructed of earth, rock pieces, stone masonry,
concrete, RCC and even of timber and rubber.
• The concrete dams, plain as well as steel reinforced,
earthen and rock fill dams are popular.
• Base on hydraulic design dams can be of no- overflow
type, in which water is not allowed to flow over the top
the dam and the over flow type which allows water to
over flow over it.
• As per structural design there can be gravity dam, arch
dam and buttress dam where water thrust is resisted by
gravity, arch and buttress.

SPILL WAYS
• When the water level in the reservoir basin rises,
the stability of the dam structure is endangered.
To relieve the reservoir of this excess water, a
structure is provided in the body of the dam or
close to it. This safe guarding structure is called a
spillway.
• It provides structural stability to the dam under
conditions of floods without raising reservoir level
above H.F.L. ( High flood level)
• Types of spill ways are- overall spill ways, Chute or
Trough spill way, side channel spill way, saddle
spill way, shaft spillway, siphon spill way

CONDUITS
• A head race is a channel which leads water to a turbine and a
tail race is a channel which carries water from the turbine.
• A canal is an open water way excavated in natural ground
following its contour.
• A flume is an open channel erected on a surface above the
ground supported on a trestle.
• A tunnel is a closed channel excavated through an obstruction
such a ridge of high land between the dam and power house.
• A pipe line is a closed conduit supported on or above the
surface of the ground.
• A PENSTOCK is a closed conduit for supplying water under
pressure from the head pond or forebay to the turbine.
• When the distance between forebay and power house are
short, a separate penstock for each turbine is preferred.
• For moderate heads and long distances, a single penstock is
used to feed two or more turbines.S.PALANIVEL ASSOCIATE PROF./MECH ENGG.

Surge tanks & Draft tubes
• A surge tank is a small reservoir in which the water level rises or
falls to reduce the pressure swings so that they are not
transmitted to the close conduit.
• If the power house is located within the short distance of the
head works, medium head schemes & run off plants surge tanks
are not required.
• Surge tanks are required for high head plants where water is
taken to the power house through tunnels and penstocks.
• Draft tubes allow the turbine to be set above the tailrace to
facilitate inspection and maintenance and by diffuser action in
the draft tube regains the major portion of the kinetic energy or
velocity head at runner outlet, which would otherwise go waste
as an exit loss. Draft tube can be straight conical or an elbow
type.
• Conical type is used for low power units, in elbow type energy is
regained in the vertical portion which flattens in the elbow
section to discharge water horizontally to the tailrace.S.PALANIVEL ASSOCIATE PROF./MECH ENGG.

Hydro electric power house
• Power House should have a stable structure and its layout
should be such that adequate space is provided around the
equipment for convenient dismantling and repair.
• The following equipments are provided in the power house
 Hydraulic Turbines
 Electric Generators
 Governors
 Gate valves
 Relief valves
 Water circulation pumps
 Air duct
 Switch board and Instruments
 Storage batteries
 Cranes

Classification
• A) According to the availability of Head :
(i) High Head Power plants – Above 100 m.
(ii) Medium Head Power Plants – 30 m to 100 m
(iii) Low Head Power plants –
B)According to nature of load
(i) Base load plants
(ii) Peak load plants
C) According to quantity of water available
(i) Run- of river plant without pondage
(ii) Run – of river plant with pondage
(iii) Hydro electric plants with storage reservoir
(iv) pump storage plants
(v) Mini and micro hydel plantsS.PALANIVEL ASSOCIATE PROF./MECH ENGG.

Pumped storage Plants

• Pumped-storage hydroelectricity (PSH), or pumped
hydroelectric energy storage (PHES), is a type of hydroelectric
energy storage used by electric power systems for load
balancing. The method stores energy in the form of
gravitational potential energy of water, pumped from a lower
elevation reservoir to a higher elevation.
• Low-cost off-peak electric power is used to run the pumps.
During periods of high electrical demand, the stored water is
released through turbines to produce electric power.
• Although the losses of the pumping process makes the plant a
net consumer of energy overall, the system increases revenue
by selling more electricity during periods of peak demand,
when electricity prices are highest.
• Pumped-storage hydroelectricity allows energy from time-
inflexible sources (such as nuclear, solar and wind) to be saved
for periods of higher demand.

Classification of Turbines
• Hydraulic Turbines convert the potential energy of water
into a shaft work, which in turn rotates the electric
generator coupled to it in producing electric power.
• Classified according to
• (i) head and quantity of water available
• (ii) name of the originator
• (iii) nature of working on the blades
• (iv) direction of flow of water
• (v) axis of the turbine shaft
• (vi) specific speed

• The difference in elevation of water surface between upstream and
downstream of the turbine is the HEAD under which turbine acts.
• Turbine works between 2 to 2000 m
• LOW HEAD – 2 TO 15 M
• Medium head – 16 to 70 m
• High head – 71 to 500 m
• Vey high head – above 500 m
• According to originator :
• Pelton Turbine – impulse , used for high head & low discharge
• Francis Turbine _ Reaction , used for medium head & discharge
• Kaplan turbine – Reaction , used for low head & large discharge
• Deriaz Turbine – Reversible , used to a head of 300 m.

According to nature of working on blades
Turbines are classified as impulse and reaction turbines
depending on the mode of energy conversion of potential
energy of water into shaft work.
In an impulse turbine all the available head of water is
converted into kinetic energy in a nozzle.
The water shoots out of the nozzle in a free jet into a
bucket revolves round a shaft.
During this action , the water is in contact with air all the
time and water discharged from buckets falls freely
through discharge passage into the tailwater.
The free jet is at atmospheric pressure before and after
striking the vanes. These are pressure less or impulse
turbnies.
Pelton wheel belongs to this category

• In reaction turbines, the entire flow from the head water
to the tailwater takes place in a closed conduit system
which is not open to the atmosphere at any point in its
passage.
• At the entrance to the runner, only a part of potential
energy is converted into kinetic energy and the remaining
into pressure energy.
• The runner converts both kinetic energy and pressure
energy into mechanical energy.
• Such turbines are called reaction or pressure turbines.
• Francis, Propeller, Kaplan and Deriaz turbines belong to
this category.

According to the direction of flow of water
• The three orthogonal directions in the turbine flow can be
referred to as ‘radial’, ‘axial’ and ‘tangential’, with respect
to the wheel. The shaft axis denotes the axial direction.
• Sometimes, the flow direction can change between inlet
and outlet. It can be radial flow at the inlet and axial flow
at the outlet. Such flow is termed as a “mixed flow”.
• If the flow is neither parallel to axis nor perpendicular to it,
but is in an angular direction with respect to axis, it is
called as “diagonal flow”
Turbine types Flow direction
Francis Turbine Radial inward flow or mixed flow
Pelton Turbine Tangential flow
Propellar or Kaplan turbine Axial flow
Deriaz turbine Diagonal flow

• According to the axis of turbine shaft : Turbine shaft can be
either vertical or horizontal. Pelton Turbines have horizontal
shafts , where as other turbines have vertical shafts.
• According to specific speed : Specific speed of turbine Ns is
the speed of geometrically similar turbine which produces 1
kW under 1 m head, it is given by Ns = Nx P1/2
• H5/4
(N= working speed in rpm P = Power output in kW, H =net head in m)
Specific speed
Runner Slow Medium Fast
Pelton 5 - 15 16 - 30 31 – 70
Francis 60 - 150 151- 250 251 – 400
Kaplan 300 - 450 451 - 700 700 - 1100

PELTON WHEEL
• It is an Impulse turbines used extensively for high head installation
• The runner consists of a large circular disc on the periphery of
which a no. of two-lobe ellipsoidal buckets are evenly mounted.
Each bucket has a ridge or splitter in the middle which divides the
jet into two equal streams. The symmetry ensures that there is no
momentum in the axial direction and hence there is no axial force
on the shaft bearings.
• The nozzle directs the flow on the wheel. It also govern the
quantity of flow with the help of a spear valve controlled by the
governor action. In the simple arrangement there is a single nozzle
feeding water to the turbine. However for large discharge there
are turbines having upto six jets, all symmetrically arranged and
causing rotation in the same direction.
• Specific speed of a given wheel can be increased by using multi-jet
arrangement. Max. no. of jets is six. (Specific speed of multi jet, NsMJ
=n1/2
NsSJ , where n= no. of jets, Ns SJ = Specific speed of single jet .
Mini. No. of buckets ‘Z’ = m/2 +15 : m=jet ratio: D/d .

FRANCIS TURBINE
• These are reaction Turbines ie. During energy transfer from
water to the runner there is a drop in static pressure as well as
drop in velocity head.
• Water from the penstock enters a spiral or scroll casing which
surrounds the runner. The cross section of the spiral diminishes
uniformly along the circumference to keep the water velocity
constant along its path.
• The water then enters the guide vanes or wicket gates which
are pivoted and can be tuned suitably to regulate the flow and
output . The guide vanes impart a tangential velocity or angular
momentum to the water before entering the runner. Runner
has a no. of curved blades, welded to the shoruds.
• Velocity of water gradually change from radial to axial. After
flowing past runner, the water leaves through the draft tube,
either straight or elbow type, increasing the pressure and
reducing velocity before falling into tailrace.S.PALANIVEL ASSOCIATE PROF./MECH ENGG.

• In Francis turbine the pressure of water at the inlet is more
than that at the outlet. Thus water in the turbine must
flow in closed conduit.
• Unlike in Pelton wheel water strikes only a few of the
runner buckets at a time, in the Francis turbine the runner
is always full of water. After doing work, the water is
discharged to the tail race through the closed tube of
gradually enlarging section, the draft tube. Which does
not allow water to fall freely to tailrace as in Pelton
turbine.

Degree of Reaction
• By applying Bernoulli’s equation to inlet & outlet of a turbine
• P1/ g + V1ƿ 2
/2g = E + P2/ g + V2ƿ 2
/2g , where E= energy transferred from
fluid to rotor
• E= (p1-p2)/ g + (V1ƿ 2
– V22
)/2g ,
• the first term on RHS is the energy transfer due to drop in static pressure
and the second term relates to energy transfer due to drop in velocity head
• If p1=p2 if the pressure is constant; E = (V12
– V22
)/2g , this happens in
Impulse turbine where pressure is atmosphere
• If V1=V2; E = (p1-p2)/ g , this happens only in reaction turbine, whereƿ
wheel rotates due to pressure drop across it exerting a reaction.
• Degree of reaction , R = Energy transfer due to pressure drop /Energy
transfer
• R= ((p1-p2)/ g )/ E ; R= (E – ((V1ƿ 2
– V22
)/2g)) /E = 1- (V12
– V22
)/2gE
• Substituting from Euler equation, E = Vw1Vb1 /g
• R= 1 – (V12
- V22
) / (2 Vw1Vb1 )

Points on Hydro elec. plant
• Speed ratio : The model of a turbine and its proto type are in
definite geometric ratio depending on their respective heads and
rotating speeds. The ratio of the blade velocity Vb and the water
velocity V is called speed ratio.
Speed ratio for Pelton turbine 0.42 to 0.47, 0.55 to 1 or more for
a Francis turbine and 1.5 to 3.0 or more for a propeller turbine
• Scale ratio : Ratio of the diameters of the model turbine and the
prototype turbine is called as Scale ratio
V b ἀ V
DN ἀ (H)0.5
DmNm / DPNP= (Hm/ Hp)0.5
Dm/ Dp= (Hm/ Hp)0.5
Np/ Nm
• Three factors influencing the output of Hydro electirc plants are
Specific Speed, head and flow

• Three parameters to be considered for designing
the turbine are speed, power and head of the
turbine.
• Cavitation : When the velocity of a fluid increases
its pressure falls. In any turbine part if the pressure
drops below the vapor pressure at that
temperature , some of the liquid flashes into
vapor. Bubbles formed during vaporization are
carried by water stream to higher pressure zones,
where the bubbles condense into liquid forming a
cavity or vaccum. The surrounding liquid rushes
towards the cavity giving rise to very high local
pressure as high as 7000 atm, this happens
repeatedly 100 times in a second. This
phenomenon is known as cavitation.

Governing of Hydraulic turbine
• Generators are required to run at a constant speed irrespective
variations in the load according to Grid.
• Speed of the turbines are required to be maintained as per
grid frequency. If there is a sudden drop in load, turbine speed
will tend to increase, this will be controlled by regulating the
flow to turbine either by deflector, diverting the flow or
reducing the flow by spear or needle valve.
• When there is sudden fall of load, the spear has to move
rapidly to close the nozzle, this rapid closing may cause water
hammer. It is quite serious in large capacity plants with long
penstocks. To avoid the water hammer effects during a sudden
fall of load, a deflector is introduced in the system, which
deflect some water the jet advancing to turbine runner. The
quantity of nozzle flowing thro nozzle will be the same, but a
certain water coming out nozzle is deflected and not to allow
strike the buckets. Deflected water goes waste into tailrace
level. In reaction turbine this is achieved by relief valveS.PALANIVEL ASSOCIATE PROF./MECH ENGG.

Selection of Turbines
• The choice depends on the Head available, Power to be developed and speed
at which it has to run. The following factors basically govern the selection of a
suitable type of turbine.
• Operating Head : Kaplan and Propeller type turbines are used for heads up to
50 m. For heads from 50 m to 400 m Francis turbines are used. For heads
above 400 m, impulse or Pelton turbines are used. The range of heads as
mentioned is not rigid and may change if other conditions dominate to
achieve economy.
• Specific Speed : It is better to choose turbines of high specific speeds. High
specific speeds mean small sizes of turbines, generators, power house etc.,
and therefore more economical. The range of specific speeds of the turbines
should correspond to the synchronous speed of generator N = 120xf/p where,
f = frequency and p = no. of poles.
• Height of Installation : it is better to install the turbines as high above tail
water level as possible. This saves the cost of excavation for the draft tube.
Care should be taken to ensure that cavitation does not occur.
• Size of the turbine : It is better to go in for as large a size of turbine as possible
since this results in economy of size of the power house, the no. of penstocks,
the generator etc. Bigger size means less no. of runners. However no. of
runners should be less than two so that at least one unit is always available
for service

Propeller and Kaplan Turbine
• The Propeller Turbine is a reaction turbine used for low loads (4m-8m)
and high specific speeds (300 – 1000).
• It is an axial flow device providing large flow area utilizing a large volume
flow of water with low flow velocity. It consists of an axial flow runner
usually with four to six blades of airfoil shape. The spiral casing and guide
blades are similar to Francis turbine. In propeller turbines as in Francis
turbines the runner blades are fixed and non- adjustable.
• A special type of propeller turbine is the Kaplan turbine in which the
individual runner blades are pivoted on the hub so that their inclination
may be adjusted automatically rotating about pivots with the help of
governor servo- mechanism.
• Efficiency of a reaction turbine depends on the inlet angle. In a fixed
blade runners, it is not possible to vary the inlet blade angle for varying
demands of power (load). So such turbines are designed for max.
efficiency only for a particular load. At all other loads their efficiency is
less than this.
• in Kaplan turbine, because of arrangement for automatic variation of inlet
blade angle with variation in load, the turbine can be run at max.
efficiency at all loads. S.PALANIVEL ASSOCIATE PROF./MECH ENGG.

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