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Civil work components
Figure 3 shows the major components of a typical micro hydropower scheme.
The water in the river is
diverted by the weir through an opening in the river side (the `intake')
into an open channel. A settling basin is used to remove sand particles
from the water. The channel follows the contour of the hillside so as to
preserve the elevation of the diverted water. The water then enters a tank
known as the `forebay' and passes into a closed pipe known as the `penstock'.
This is connected at a lower elevation to a waterwheel, known as a turbine.
The turning shaft of the wheel can be used to rotate a mechanical device
(such as a grinding mill, oil expeller, wood lathe and so on), or to operate
an electricity generator. The machinery or appliances which are energised
by the hydro scheme are called the `load'.
Figure 3 Major components
of a micro hydro scheme
Various possibilities exist for the general lay-out of a hydro scheme, depending on the local situation:
1) use of available head
The design of the system has effects on the net head delivered to the turbine. Components such as the channel and penstock cannot be perfectly efficient. Inefficiencies appear as losses of useful head of pressure.
2) flow variations
The river flow varies during the year but the hydro installation is designed to take a constant flow. If the channel overflows there will be serious damage to the surroundings. The weir and intake must therefore divert the correct flow wether the river is in low or in high flow. The main function of the weir is to ensure that the channel flow is maintained when the river is low. The intake structure is designed to regulate the flow to within reasonable limits when the river is in high flow. Further regulation of the channel flow is provided by the spillways.
Flowing water in the river may carry small particles of hard abrasive matter (sediment); these can cause wear to the turbine if they are not removed before the water enters the penstock. Sediment may also block the intake or cause the channel to clog up if adequate precautions are not taken.
Flood water will carry larger suspended particles and will even cause large stones to roll along the stream bed. Unless careful design principles are applied, the diversion weir, the intake structure and the embankment walls of the river may be damaged.
In all parts of the water supply line,
including the weir, the intake and the channel, sudden alterations to the
flow direction will create turbulence which erodes structures and causes
A hydropower station has to divert water from the river. To perform this function civil structures are necessary. Figure 3 shows the different element the civil works consist of.
A hydro system must extract water from the river in a reliable and controllable way. The water flowing in the channel must be regulated during high river flow and low flow conditions. A weir can be used to raise the water level and ensure a constant supply to the intake. Sometimes it is possible to avoid building a weir by using natural features of the river. A permanent pool in de river may provide the same function as a weir.
Another condition in siting the weir is to protect it from damage.
Usually it is sensible to adopt traditional water management techniques known to local people. Temporary weir construction might be one of these techniques. The principle of the temporary weir is to construct a simple structure at low cost using local labour, skills and materials. It is expected to be destroyed by annual or bi-annual flooding. Advanced planning is made for rebuilding of the weir whenever necessary.
The intake of a hydro scheme is designed to divert a certain part of the river flow. This part can go up to 100 % as the total flow of the river is diverted via the hydro installation.
The following points are required for an intake:
Different types of intakes
are characterised by the method used to divert the water into the intake.
For micro hydro schemes only the smaller intakes will be suitable. The
following three types of intakes will be discussed here: the side intake
with and without a weir and the bottom intake. For these types the advantages
and disadvantages will be mentioned.
Side intake without weir
Side intake with weir
If floating debris is a problem,
a steel or wooden bar (`skimmer'), can be positioned on the water surface
at an angle to the flow as to stop the debris and protect the intake.
The channel conducts the water from the intake to the forebay tank. The length of a channel can be considerably. In Nepal channels exist with a length of a few kilometres to create a head of 10 to 30 metres.
The length of the channel depends on local conditions. In one case a long channel combined with a short penstock can be cheaper or necessary, while in other cases a combination of short channel with long penstock suits better.
Most channels are excavated, while sometimes structures like aqueducts are necessary. To reduce friction and prevent leakages channels are often sealed with cement, clay or polythene sheet.
Size and shape of a channel are often a compromise between costs and reduced head. As water flows in the channel, it loses energy in the process of sliding past the walls and bed material. The rougher the material, the greater the friction loss and the higher the head drop needed between channel entry and exit.
Where small streams cross
the path of the channel very great care must be taken to protect the channel.
A heavy storm may create a torrent easily capable of washing the channel
away. Provision of a drain running under the channel is usually not adequate
protection. It will tend to block with mud or rocks when needed the most.
In the long term it is economic to build a complete crossing over the channel.
Incorporated in the channel are the following elements, which will be discussed here:
The water drawn from the river and fed to the turbine will usually carry a suspension of small particles. This sediment will be composed of hard abrasive materials such as sand which can cause expensive damage and rapid wear to turbine runners. To remove this material the water flow must be slowed down in settling basins so that the silt particles will settle on the basin floor. The deposit formed is then periodically flushed away.
From the size of the smallest
particle allowed into the penstock the maximum speed of the water in the
settling basin can be calculated as the slower the water flows the lower
the carrying capacity of the water for particles. The water speed in the
settling basin can be slowed down by increasing the cross section area
of the channel. For each maximum size of the particles the optimum size
of the settling tank can be calculated.
Spillways are designed to permit controlled overflow at certain points along the channel. Figure 20 depicts a flood spillway in detail, including flow control and channel emptying gates. Flood flows through the intake can be twice the normal channel flow, so the spillway must be large enough for diverting this excess flow.
The spillway is a flow regulator for the channel. In addition it can be combined with control gates to provide a means of emptying the channel.
The spill flow must be fed back to the river in a controlled way so that it does not damage the foundations of the channel.
The forebay tank forms the connection between the channel and the penstock. The main purpose is to allow the last particles to settle down before the water enters the penstock. Depending on its size it can also serve as a reservoir to store water.
A sluice will make it possible to close the entrance to the penstock. In front of the penstock a trashrack need to be installed to prevent large particles to enter the penstock.
A spillway completes the
The penstock is the pipe which conveys water under pressure from the forebay tank to the turbine. The major components of the penstock are shown in figure 8. The penstock often constitutes a major expense in the total micro hydro budget, as much as 40 % is not uncommon in high head installations, and it is therefore worthwhile optimising the design. The trade-off is between head loss and capital cost. Head loss due to friction in the pipe decrease dramatically with increasing pipe diameter. Conversely, pipe costs increase steeply with diameter. Therefore a compromise between cost and performance is required.
The design philosophy is
first to identify available pipe options, then to select a target head
loss, 5 % of the gross head being a good starting point. The details of
the pipes with losses close to this target are then tabulated and compared
for cost effectiveness. A smaller penstock may save on capital costs, but
the extra head loss may account for lost revenue from generated electricity
The following factors have to be considered when deciding which material to use for a particular penstock:
* = poor ***** = excellent
Table 3 Comparison penstock materials
Pipes are generally supplied in standard lengths and have to be joined together on site. There are several ways of doing this and the following factors should be considered when choosing the best joint system for a particular scheme:
Penstock pipelines can either be surface mounted or buried underground. The decision will depend on the pipe material, the nature of the terrain and environmental considerations.
Buried pipelines should be ideally be at least 750 mm below ground level, specially when heavy vehicle are likely to cross it. Burying a pipe line removes the biggest eyesore of a hydro scheme and greatly reduces its visual impact. However, it is vital to ensure a buried penstock is properly and meticulously installed because any subsequent problems such as leaks are much harder to detect and rectify.
Where the nature of the ground renders burying the penstock impossible there is sometimes no option but to run the line above the ground, in which case piers, anchors and thrust blocks will be needed to counteract the forces which can cause undesired pipeline movement.
The three types of forces that need to be designed against are:
The different support structures can usually be built of rubble masonry or plain concrete. Anchor blocks may need steel reinforcement and triangulated steel frames are sometimes used for support piers.
The size and cost of support structures for a given penstock are minimised by: