.
Clas Cl assi sifi fica cati tion on of Hy Hydr drop opow ower er Pl Plan ants ts
H dr dro o ow ower er la lant nts s ex exhi hibi bitt a re reat at de deal al of va vari riet et .
Almost Alm ost eve every ry hyd hydrop ropowe owerr pro projec jectt has som some e spe specia ciall fea featur tures es unc uncomm ommon on wit with h other projects of the same type.
us, y rop ropower p ants cou
e c ass e on t e as s o :
• The hydraulic features of the plant • O erati eratin n featu features res of the lant • Plant Capacity • Construction Features (layout) • Location & topographical features • Presence or absence of storage •
e rang range e o oper opera a ng ea s
A
complete understanding of the types of hydropower
developments
requires
information
under
all
such
categories. An
im or orta tant nt oi oint nt wh whic ich h sho shoul uld d be be bor borne ne in mi mind nd is th that at
all those classifications are not are not mutually exclusive. exclusive.
us,
e presence or a sence o s orage a so o some
extent determines the hydraulic the hydraulic features of the plant. plant. The
b
operational features of the plant are determined rese re senc nce e or ab abse senc nce e of st stor ora a e.
. The ba The basi sic c hy hydr drau auli lic c pr prin inci cipl ple e go gove vern rns s th the e ty type pe of cl clas assi sifi fica cati tion on in th this is category.
.
Use normally available available hydraulic energy of the flowing water of the the rivers. e.g. Run-of river plant, diversion diversion plant, storage plant
ii. Pumped storage plants
Use the concept of recycling the same water by using pumping selectively. .
It
gene ge nera rate tes s en ener ergy gy fo forr pe peak ak lo load ad,, an and d at of offf-pe peak ak pe peri riod ods s wa wate terr is
pumped back for future use. A
pumped storage plant is an economical addition to a system which
increases the load factor of other systems and also provides additional capac y o mee
e pea
oa .
.
a) Tidal power plant
se
e
a energy o
e sea wa er.
Very few have been constructed due to structural complication.
b) Wave power plant c) Depression power plant
Hydropower generated by diverting an ample source of water (e.g. sea water) in the natural depression which provides operating head for the plant
Water level in the depression is controlled by natural evaporation
2. Classification based on actual operation in meeting the demand
isolated plant (SCS)- operating independently
mini and small hydropower scheme serving small community
interconnected into rids ICS
Thus in a grid system, a power station may be distinguished as a base load plant or peak load plant.
Hydropower plants are best suited as peak load plants, because hydropower plants can start relatively quickly and can thus accept load quickly
.
over a year.
Construction of a dam usually implies a much more efficient and controlled use of the available water.
Without
storage, the plant uses only the natural flow as
best as it can. In
such cases, only a mini-reservoir or a pondage which
a es care o
ay- o- ay uc ua ons may e necessary.
4. Classification based on location and topography
dams;
whereas plants in plain areas may have only weirs for the main structure.
For
plants situated far in the interior and away from load
centers the transmission costs are relativel more.
Thus the knowledge about the location and topography of a .
5. Classification based on plant capacity
Classification of hydropower plants on the basis of plant capacity changes with time as technology improves.
Classification according to Mosonyi, and present day trend are:
According to Mosonyi: Mosonyi:
ii) Low capacity plant
< 1 MW
iii) Medium capacity plant
< 10 MW
iv) High capacity plant
> 10 MW
Present day classification: classification : < ii) Medium capacity plants
5 to 100 MW
iii) High capacity plants
101 to 1,000 MW
iv) Super plants
above 1,000 MW
. While
any of the above classifications could be used to describe a
the head operating on the turbine.
n
s as as s: i) Low head plants
< 15m
ii) Medium head plants
15-50m
iii) High head plants
50-250m
iv) Very high head plants
> 250m
. (layouts)
classified based on the dominant the dominant construction features of features of e p an as: Run-off-river Valley
plants (low to medium head plants)
dam plants (Medium to high head plants)
Diversion Canal Canal Plants
High head diversion diversion plants
umpe s orage p an s
Site Selection, Layouts & Arrangements o o ediu Hi h Head Pla ts 1.Run-off-river plants (low to medium head plants)
The normal flow of the river is not materiall
disturbed due to the
construction of the plant; They
have small ponds to provide the necessary pondage in order to
balance day-to-day fluctuation;
Such plants neither have a significant storage nor do they have a
A weir or barrage is built across a river & the low head created is used to generate power.
It also acts as a controlled spilling device
Preferred
in perennial rivers with moderate to high discharge, flat
slo e with low sediment and stable reach of a river.
Water enters the power house through an intake structure incorporating some or all of the following: Entrance
flume separated by piers and walls for each machine
unit.
The appurtenances of the entrance structure are the sill, fine rack or screen and gate;
ur ne c am er o scro case w Concrete or steel draft tube; ower ouse u
ng
ur ne;
Depending on different arrangements, Mosonyi proposed the following groupings of the run-of-river plants: i.
Block power plant,
ii.
Twin power plant,
iii.
Pier-head power plant,
iv.
Submersible power plant
These groupings are mainly on the basis of constructional arrangements of the power house vis-à-vis the weir.
is the most widely used arrangement among the above mentioned layouts.
Power house provided along one bank adjacent to weir and separated only by a divide wall;
ac ua w
s sma
may
e en arge
y excava ng a
ay w c
may offer many advantages.
Amon the advanta es is havin
sufficient weir len th which can
ass
the flood without obstruction.
However, one side bay gives rise to curvilinear flow and adversely affects the turbine efficiency.
The eddies and vortices developed in the bay may also move the bed load sediment which eventuall enters the machines.
The consideration of the choice a ba is based on the oint that the width of the river left after the accommodation of the plant should be sufficient to pass the design flood without creating unfavorable flow conditions.
,
,
,
.
Twin Power Station:
is similar in arran ement with block ower station exce t that instead of a single power house, two power houses at the two banks are provided.
If the plant discharge capacity is large at a low head, then it becomes difficult to achieve satisfactory flow conditions in a single bay and uniform normal flow to the racks, because of an unusually long power station and a arge ay.
Under such circumstances a twin power house is preferred by dividing the .
the straight short cut of a bend or in bends where the bed load is not heavy. more uniform current in all operating conditions compared to block power stations. A
variation of this type of power house is the island-type arrangement in which a block type of power station is located centrally and on both sides of it are the portions of the weir.
The twin power station presents some difficulties which outnumber the advantages of it.
,
higher maintenance and supervision costs and
the practical difficulty of carrying the cables with high voltage are
.
is one which houses a turbine generator set in each pier (hollowed out iers of the weir.
Under
special requirements, a pier can also house two generating
sets. A
pier head power station is suitable when there is no possibility of
widening the river bed and the river stretch is straight or slightly curved. This
is one of the advantages of this layout where the valley is
comparatively narrow. This
layout gives the most uniform current distribution in all flow
conditions.
u mers
e power s a on:
In this type of plants, the machine hall is provided under the body of the weir.
The weir floor serves
racticall
as the roof of the
machine hall.
s ype o ayou s se ec e
or ow ea s
–
rivers with little bed load transport and large floods.
m n
2. Valley dam plants (medium to high head plants)
The dominant feature is the dam that creates the required storage (to balance seasonal fluctuations) and necessary head for the power house;
Power house is located at the toe of the dam;
No diversion of water away from the main river is involved;
a er ows
roug
e pens oc em e
e
n
e am or
ver e
no
a cannel/tunnel system to deliver flow to the power house; - -
spillway location. If the spillway is in the central portion of the dam, then the power house may be located on one of the banks or as twin power house, one on each bank.
Single power hous e Twin power hou se
Important components of a valley dam plant:
The dam with its appurtenance structures like spillway, energy dissipation arrangements, etc; ,
The
,
penstock conveying water to the turbine with inlet
valve & anchorage; The
main ower house with its com onents.
HW
TW
, immediately at the toe of the dam but at some distance ownstream. Such
arrangement is costlier than the more general dam-
and-power house-together-arrangement and is adopted onl when it offers some s ecial advanta es like
achieving extra head (e. g. Melka Wakena HP).
The
arrangement, however, needs longer conveyances
with consequent losses
Storage plant (remote development):
a considerable distance over which the water is conveyed, generally by a tunnel and pipeline, so as to achieve medium and high heads at the plants ; The
reservoir storage upstream of the dam increases the
firm ca acit of the lant substantiall
and de endin on
the annual run-off and power requirements, the plant may -
.
. The
vers on cana p an s distinguishing feature is the presence of power
channel; The power house is provided at suitable location along the stretch of the canal; The water often flowing through the turbine is
Diversion canal plants are generally low head or medium head plants; They don't have storage reservoir; Pondage requirement is met through a pool called
The development of the required head in diversion canal plants may be achieved by:
The head ma
be made available due to the flatter bed slo es of
ower
canal (as compared with the river);
besides, due the river meanders, the length of the river between two points may be much greater as compared to that of the relatively straight reach of the channel , & locating the power house at the downstream side of the fall provide the required head;
In inter-basin diversion, water may be diverted from a higher level river to a lower river through a diversion canal to the power house located at the lower r ver;
most suitable on rivers either of steep slopes or meandering reaches. ,
,
have to be moderate. - -
,
large discharge development.
,
1) Diversion weir with its appurtenant structures; 2) Diversion canal intake with its ancillary works such as sills, trash racks, skimmer wall, sluices, settling basin, de-silting canal, and silt exclusion arrangement is
3) Bridges or culverts of the diversion canal; 4) Forebay & its appurtenant structures.
. g The
ea
vers on p an s
features of such a plant the development of high
head resulting from the diversion of water, which could be achieved by: Diverting
the river water through a system of canals and
tunnels to a downstream point of the same river;
Diverting the water through canals and tunnels to a point on .
arrangement: 1)
2)
A diversion weir to create pondage (and no storage). Here like run-of-river lant the ower roduction is overned b the natural flow in the river. Storage may be provided on the main river at the point of diversion which feeds into the diversion system. This second situation is advantageous since the fluctuation in reservoir level does not materially affect the head and the power output can be adjusted by the controlled flow release from the reservoir. . .
.
This advantage is not available to the valley dam plant in which the power house is built on the downstream face of the dam. Under such cases, a change in reservoir level also changes the head propor ona e y. If the length of the pressure tunnel is considerable, a surge tank may be provided upstream of the power station, which may smoothen the fluctuation of flow demand. This purpose was served in the canal plants by the forebay.
Canals
follow the contours of the terrain and thus
may not have the shortest route from the intake to the power house.
unne s,
owever, can ma e
roug
e r way
y
the shortest distance and thus create enormous heads apart from enabling to divert water of one basin to another
Main Com onents of hi h head diversion lants:
Diversion weir with appurtenant structures; ana unne ; Head race either open cut or tunnels with its structures;
Penstock;
Power house;
Tail race.
to the low head diversion canal plants. e ma n po n o
erence s,
owever,
e e a ora e
conveyance system for the high head plants (diversion tunnel plants).
In the diversion tunnel type plant; the
dam replaces a diversion weir,
reservoir intake is used instead of a canal intake and
a surge tank is employed in place of a forebay
-
. Pumped
plants
storage plants are special types of power which
work
as
ordinary
conventional
h dro ower stations for art of the time.
Pumped storage plant is suitable where:
the natural annual run-off is insufficient to justify a conventional hydroelectric installation;
It
is possible to have reservoir at head & tail water .
This
kind of plant generates energy for peak load, & at off peak period water is pumped back for future use.
During
off peak periods excess power available from some
the lower reservoir.
Various arrangements are possible for higher and lower 1)
Both reservoirs in a single river;
2)
Two reservoirs on two separate rivers close to each other and flowing at different elevations;
3)
Higher reservoir on artificially constructed pool on a high level plateau or on a leveled hilltop and the lower reservoir on natural river;
4)
The lower reservoir in a natural lake while the higher one is on artificially created reservoir.
classify them as pure or mixed operation.
A
ure
um ed stora e
lant is a closed c cle
lant with the
volume of water flowing to the lower reservoir being equal to the volume pumped to the higher reservoir in one cycle of operation. In such a system, same water is circulated again and again and thus except for make-up quantity of water for seepage and evaporation losses, the plant does not need any fresh water flow. In
mixed plants the total generation in one cycle is greater than
the total pumping during that period. In mixed type of plants, the higher reservoir has to be necessarily on a natural stream so as to provide greater flow during generation.
-
on the basis of cycle of operations. Some
lants are o erated on a dail c cle of um in
and generation;
ome are p anne
on a wee y cyc e w ere
e
pumping is confined to slack weekend periods only; A
few pumped storage plants have been built on a
seasonal cycle where the pumping is done during
seasons of lean demand and generation during high .
relative arrangements of turbines and pumps.
Four-unit installation - pump, motor, generator, turbine;
Three-unit installation - pump, turbine and generator which can also function as a motor – both the pump and .
In this case, when the turbine runs, the unit operates as a generator and when the pump is operated the same unit operates
Two-unit installation - generator, turbine or reversible pump-turbine installation.
The modern trend is to use only a two-unit installation, namely, a generator which operates as a motor coupled to a turbine which in turn also operates as a pump when rotating in reverse direction.
This arrangement is called reversible pump-turbin e installation.
-
locked together and the pump can be coupled during the pumping phase Reversible
pump-turbines: Any reaction turbine can, technically speaking, work as a pump if the direction of rotation is reversed. , , runners and the versatile Francis turbines, all can be used as reversible machines. The
salient design features of reversible pump-turbines are not markedly different from those of conventional turbines.
Lar
e ca acit units are usuall Francis t e reversible um turbines. For low head developments, propeller/Kaplan turbines are suitable
e opera ng c arac er s cs o
e revers
e mac nes
are different when it runs as a turbine and as a pump. If
the rotational speed is kept constant during both modes,
the discharge during the pumping phase is less than the discharge during the turbine operation. The
maximum efficiency of the pump-turbine as a pump
occurs at a different speed as compared to its running as a turbine.
In order to obtain good efficiencies at the same head, some .
any
es gns,
owever, rom
e s mp c y po n o
view, keep the same rotational speed during both phases. , different heads. Problems
of operation: The main problem of a high head
pump is cavitation. Cavitation
is the phenomenon which manifests in the flow when
the pressures are nearing vapour pressure of water.
Thoma
has su pumps as;
ested a cavitation arameter
σ
for turbines and
Where hb, hs, and h are the barometric head, the suction head (or head on the pump, respectively.
According to Thoma, for cavitation free running, σ, has to be greater
For high values of head h, hs comes out to be negative and hence it becomes necessary to provide the pump with negative suction head.
, fixed that the pump operates under submerged condition.
The magnitude of submergence depends upon the specific speed and the net head.
If
the submergence required is high, the power house .
As
a result, many of the pumped-storage plants have
underground power houses.
Desirable site characteristics
1. In order to be cost-effective, an off-stream pumped storage site should have most or all of the following characteristics:
eolo ic conditions should be suitable for water-ti ht reservoirs head should be as high as possible; ,
,
tunnel) should be as short as possible;
reservo r s es s ou embankment
requ re m n mum excava on an
use
existing reservoir for lower reservoir, if possible;
characteristics;
site should be suitable for a large power installation;
centers or transmission corridors;
source(s) of relatively low cost pumping energy
.
ea : eservo r storage requ rements are nverse y proportional to head (Figure below),
so reservoir costs can be minimized b selectin a site with a high head.
Hydraulic capacity is also inversely proportional to head.
so pens oc ame er, an ence pens oc cos s, can a so be minimized by maximizing head.
For
a given plant capacity, powerhouse costs are lower for high head plants.
This
is because the units run at higher speeds and highs eed machines are smaller than low-s eed machines.
Because smaller water volumes are required at high head plants, reservoir drawdowns are usually smaller at both .
Figure. Reservoir storage required vs. head for 1000 MW plant with 14 hours of storage
.
, penstocks, and discharge tunnels) can represent one-quarter or more of a um ed-stora e ro ect’s costs so
sites should be sought which will require minimum penstock
and dischar e tunnel len ths. This
is particularly important at the lower head sites, because of
the lar er enstock and tunnel diameters involved. The
economic limits to length of water conduits is a function of
head and can be ex ressed in terms of the len th between the two pools along the water passage to head (L/H) ratios.
e
ess
e va ue o
s ra o,
e more
economic is the pumped-storage project.
Recent
experience
suggests
that
maximum
acceptable L/H ratios range
from 10 to 12 for high-head (370-460 m.) projects down to 4 to 5 for low-head (150-180 m.) sites.
. either with a dam across a natural valley or with an enclosure , To
.
minimize costs, sites should be sought where minimum
excava on an em an men vo umes are requ re , an
sites having natural depressions are particularly desirable
Large so
drawdown may cause slope instability,
sites with large, relatively shallow reservoirs are usually
preferred to narrow, steep reservoirs.
.
ower
eservo rs :
ro ec cos s can o en
e re uce
by using existing reservoirs as lower reservoirs.
However, care should be taken to insure that sufficient storage is available to handle fluctuations due to pumped-storage operation in addition to fluctuations resulting from existing reservoir operations.
Because of the limited head range for efficient pump-turbine operation and submergence requirements, caution should be exercised when considering the use of existing multipleur ose reservoirs with lar e fluctuation ran es.
It
is customary to state that for every 3 kW input, you .
The normally attainable overall plant efficiency is around 70%.
It
should be worked out as below, for closed cycle operation:
, Then,
Where = the overall efficienc of eneration includin turbine, generator and transformer efficiency). And
.
p
Then,
=
then
Average values of ηt,
ηp,
and k are respectively 0.88,
0.85 and 0.02 to 0.03. With these values the overall efficiency comes out to be 72%.
Example A closed cycle pumping-storage plant with a gross head of 350 m, has a head race tunnel 4 m diameter and 700 m . reservoir. The flow velocity is 6.5 m/s and the friction factor =
.
.
e overa e c enc es o
e pump ng an
generation are 85% and 88%, respectively, estimate the plant efficiency.
o u on Friction head loss
= . m Therefore,
hf = kH . K = 0.0194
0.02
≈
