 |
||
|
||
| OPNION |
|
||
|
||
| OPNION |
Hydropower And West Rapti Dam By AB Thapa The hydropower component
of the West Rapti dam project would not
be insignificant despite the fact that the main
focus of this project would be on
irrigation. The West Rapti project could be
planned to supply water for irrigation mostly
in dry season when there is water
scarcity almost everywhere in Nepal. It would
be necessary to raise the Bhalubang dam
height to provide such extensive dry
season irrigation. According to the pre-feasibility
study the volume of the storage reservoir would
be more or less equal to the annual
flow of the West Rapti if the dam
height is raised to 120 meters. The
electricity generation would be significantly
increased after such revision of the dam
height and storage volume Moreover, it
would be possible to generate electricity
mostly in dry season when the demand for
electricity in Nepal is the highest. The West Rapti project
would have hydropower stations at two
different places. The hydropower station
at the foot of the West Rapti dam
would generate electricity using the
water to be supplied to Deokhuri and
Banke for irrigation. The next hydropower
station would be in Kapilabastu district
at a distance of about 15 km from
the West Rapti storage reservoir. The operation
of this hydropower station would be
directly linked with the supply of water
stored in the West Rapti reservoir for
irrigation in Kapilabastu, Rupandehi and
Nawalparasi districts. Hydropower Planning The planning of the
hydropower station at Bhalubang would not
be too complicated. The preliminary design
concept adopted in the past
pre-feasibility study of this project is
relatively simple and it can be right
away applied with few changes. It is
explained in the study report that the
power station would be located near the
dam toe on the left abutment. The diversion
tunnel could be used as power tunnel.
The installed capacity of the hydropower
station would be determined by the volume
of water needed to irrigate lands in
Deokhuri and Banke. It need not be
explained that the installed capacity of
this hydropower station, along with the annual
electricity generation, would be increased if
the dam height is raised. The planning of
the hydropower station to be located at
the Surai Naka in Kapilbastu is rather
complicated. The design concept adopted
in the pre-feasibility study of this
power station would have to be significantly
modified. The shortest route of
the power tunnel from the Bhalubang storage
reservoir to the hydropower station site at
Surai Naka crosses two such places where
the ground elevation is too low. In
the pre-feasibility study, an open channel has
been provided to deliver the water across
the depressions, as a result, the water
head between the full supply level of
the storage reservoir and the water level
in the open channel crossing the depressions
would be completely lost and we can not
make use of it in generation of electricity .
Thus this type of hydropower planning could
not be acceptable. The Surai Naka hydropower
station planning needs to be carefully revised.
The revision is all the more
important now because there is a
need to raise the West Rapti dam height
by a substantial margin. Design Options The layout of the
power tunnel and the location of the
hydropower station should be afresh fixed
to utilize the full power generation potential
of the West Rapti high dam. There are
three ways to circumvent the problems that
forced to limit the capacity of the
hydropower station in course of the
pre-feasibility study. The underground hydropower
station in the immediate proximity of the
storage reservoir could be the first
alternative. The hydropower station would be
at a considerable depth and it
would discharge into a long tailrace
tunnel that would carry the water
into the Surai Naka river. The top level
of the tailrace tunnel would be well
below the bottom of the depressions.
Thus it would not be necessary to change
the route of the tunnel that would
carry the water stored in the Bhalubang
reservoir after being used for power generation
into the Surai Naka river in Kapilbastu
if this type of hydropower station layout
is selected. Hydropower stations built
in the past that fall under this type
of layout planning are the Harspranget in
Sweden, Soupiti in Western Africa, Salamonde in
Portugal, Irafoss in Iceland, Ambuklao in
Philippines etc. The way the Harspranget
hydropower project was developed in Sweden
could be of great interest to us.
Development of a 105 MW power station at
Harspranget was planned and construction work started
in 1919. However, the severe economic crisis following
the war reduced the power demand, the works
slowed down and all works were discontinued in 1922.
Twenty years later the plans were resumed, resulting
in completion of the project in 1952. The power
station was given a completely new layout, and
a three times greater installed capacity than
had been planned in 1919. In 1974
construction started on further extension of the
power station increasing the capacity nearly
three times to 945 MW. Locating the power
station somewhere in between the storage
reservoir and the exit of the power
tunnel could be the next option. In
this type of layout like in the Santa
Giustina underground hydropower project in Italy,
most often, very expensive surge chambers would
have to be provided at the end of
the headrace tunnel and also at the
beginning of the tailrace tunnel carrying
the water discharged from the power house into
the river. Sometimes it could be possible
to dispense with such costly surge
chambers as in case of the Kilforsen
underground hydropower project in Sweden. In
the Kilforsen project non pressurized headrace
tunnel and a forebay have been provided
in place of pressurized headrace tunnel
and a surge chamber. Unfortunately it appears
that the topography of the project
site might not allow us to
adopt straightaway this type of
design option without changing the route of
the power tunnel. In future the possibility
of such alternative layout should also be
carefully investigated if it would allow to
cut down the project cost. Locating the hydropower
station at the end of the power tunnel
could be the third option. Generally
this type of hydropower project layout
planning is preferable because it is
often cheaper to build the hydropower
project that would have the power station
at the end of the power tunnel. Most
of the underground hydropower projects built
all over the world fall under this
category. The underground hydropower projects
built so far in Nepal like the Kulekhani
1&2, Marshyangdi and Kali-Gandaki also fall under
this category. A preliminary rechecking
of the topographical maps shows that it
might be possible to locate the
hydropower station of the West Rapti
project delivering water to the Kapilbastu at
the end of the power tunnel without
sacrificing the full hydropower generation
potential of the West Rapti storage reservoir
if the alignment of the power tunnel
running more or less from north to south is
shifted towards the east. Revised Tunnel Layout
It appears based on
the map study that it would be possible
to provide a power tunnel, operating at
a reasonable pressure, from the West
Rapti storage reservoir to the hydropower
station at Surai Naka if the tunnel route
is shifted to the east by few kilometers.
Such revision in tunnel route would allow
to bypass the depressions. The intake
structure of the power tunnel would also
be shifted. It could be about four
kilometers away further upstream from its
location proposed in the pre-feasibility study.
The total length of the headrace power
tunnel from the storage reservoir to the
hydropower station located on the left bank
of the Surai Naka river could be about
fifteen kilometers. The headrace power tunnel
after crossing the Ransing Khola would
veer more towards the south. It would
pass through Shiwapur on the left bank
of the Surai Naka river before it
reaches the point where the surge
regulating structure would have to be provided.
The surge regulating structure would possibly
have double horizontal chambers connected by
a vertical shaft. The West Rapti water
after being used for electricity generation would
be discharged into the Surai Naka river
to be diverted for irrigation in Kapilabastu
and other districts in Terai. Electricity Generation The power station would
be located at the foot of the
mountain in the vicinity of the Surai
Naka riverbed. The maximum gross head would
be about 230 meters if it is presumed
that the height of the West Rapti dam
would be raised to 120 meters. If it
is presumed furthermore that about 70% of
the regulated flow of the West Rapti
river is held back in the storage
reservoir during the monsoon period of 3 to
4 months to use in dry season for
cultivation and power generation, the total
water supply available for irrigation and power
generation in dry season months could go
up to 130 cumecs or even more.
The maximum gross head of the hydropower
station at the dam site would, however,
be only about 100 meters. The installed capacity of
the dam site hydropower station and the
hydropower station at Surai Naka could be
in the range of 50 MW and 150 MW
respectively. The capacity of Surai Naka power
station would have to be significantly raised
if that power station is to be operated
at considerably lower capacity factor. The
total annual firm energy generation of
both the hydropower stations could be in
the range of 1200 GWh. In dry
season months, when on the one hand the
demand for electricity in Nepal is the
highest and on the other the
electricity generation of the existing hydropower
stations is greatly reduced due to steep fall
in river discharge, the monthly electricity
generation of the West Rapti hydropower
stations could reach 150 GWh if it is
decided to hold back the monsoon season
water in the storage reservoir to increase
the electricity generation as well as the
coverage of irrigation area in dry season
months. The significance of the West Rapti
project supplying electricity mostly in
dry season months would be too great to
our country suffering from severe power
shortages in dry seasons and at the same
time running into enormously large seasonal
electricity surpluses during the wet seasons. High Value of
West Rapti Power The value of the
West Rapti power would be very high because
it would be possible to produce bulk
of it at a time when the demand for
power in Nepal is very high. Relevant
text on this matter from the design
guidelines prepared by ASCE of the United
States has been presented hereinafter. "Both the demand for
power and the generation available from a hydro plant
vary with season. Hydropower is most valuable
if it can be produced when it is most
needed. For example, a hydro plantís output may
be highly marketable if a substantial portion
of its output is produced in the peak load
months, even though little or no power is
produced during the remainder of the year. Conversely,
a hydro project may have little value as a
peaking project if its output is limited
during the high demand period, even though the
capacity is dependable throughout the remainder
of the year. A project of the latter type
might best be evaluated as an energy displacement
project." The feasibility study of the
Upper Karnali project carried out under the
aegis of the World Bank has determined
that the value of seasonal energy as
thermal displacement would be only about 10%
of the value of firm energy. |
Send your feedback to the
editor: spotligh@mos.com.np |
