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Hydropower basics

Civil work components


Other documents in this series:
Introduction
Civil work components
Turbines
Drive systems
Electrical power
Measurement of head
Measurement of flow
further reading
 
Within this document:
Components of a scheme
Civil works
Weir and intake
Channels
Settling basin
Spillways
Forebay tank
Penstock
Components of a scheme

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
 
 

Civil works

Various possibilities exist for the general lay-out of a hydro scheme, depending on the local situation:

  1. low head with a river barrage
  2. low head with a channel
  3. high head with no channel
  4. high head with channel
A number of essential factors must be kept in mind when designing a micro hydropower system. Those are:

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.

3) sediment

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.

4) floods

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.

5) turbulence

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 energy losses.
 
 

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.

Weir and intake

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:

  • the desired flow must be diverted,
  • the peak flow of the river must be able to pass the intake and weir without causing damage to them,
  • as less as possible maintenance and repairs,
  • it must prevent large quantities of loose material from entering the channel,
  • it must have the possibility to remove piled up sediment.
From these points follow that the positioning and shape of the weir and intake are very important.

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
relatively cheap
no complex machinery required for construction
asks for regular maintenance and repairs
at low flows very little water will be diverted and therefore this type of intake is not suitable for rivers with great fluctuations in flow.

Side intake with weir
The weir used in this configuration can be partly or completely submerged into the water.
control waterlevel
little maintenance necessary (if well designed)
low flow can not be diverted properly
modern materials like concrete necessary

Bottom intake
At a bottom intake the whole weir is submerged into the water. Excess water will pass the intake by flowing over the weir.
very useful at fluctuating flows. Even the lowest flow can be diverted
no maintenance required (if well designed)
expensive
local materials not useable
good design required to prevent blockage by sediment.

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.
 
 

Channels

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:

  • settling basin,
  • spillways and
  • forebay tank.


Settling basin

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

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.



Forebay tank

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 forebay tank.
 
 

Penstock

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 each year.
 


Materials

The following factors have to be considered when deciding which material to use for a particular penstock:

  • surface roughness,
  • design pressure,
  • method of jointing,
  • weight and ease of installation,
  • accessibility of the site,
  • terrain,
  • soil type,
  • design life and maintenance,
  • weather conditions,
  • availability,
  • relative cost,
  • likelihood of structural damage.
The following materials can be considered for use as penstock pipes in micro hydro schemes:
  • mild steel,
  • unplastified polyvinyl chloride (uPVC),
  • high density polyethylene (HDPE),
  • spun ductile iron,
  • asbestos cement,
  • prestressed concrete,
  • wood stave,
  • glass reinforced plastic (GRP).
Mild steel, uPVC and HDPE are the most common used materials. In table 3 the different materials are compared on its merits.
 
 
material friction  weight corrosion cost jointing pressure
ductile iron **** * **** ** **** ****
asbestos cement *** **** **** *** *** *
concrete * * ***** *** *** *
wood stave *** *** **** ** **** ***
GRP ***** ***** *** * **** *****
uPVC ***** ***** **** **** **** *****
mild steel *** *** *** **** **** *****
HDPE ***** ***** ***** ** ** *****

* = poor ***** = excellent

 Table 3 Comparison penstock materials







Penstock jointing

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:

  • suitability for chosen pipe material,
  • skill level of personnel installing the pipe,
  • wether any degree of joint flexibility is required,
  • relatively costs,
  • ease of installation.
Methods of pipe jointing fall roughly into four categories:
  • flanged,
  • spigot and socket,
  • mechanical,
  • welded.


Burying or supporting the penstock

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 weight of the pipes plus water,
  • expansion and contraction of the pipe,
  • fluid pressure (both static and dynamic).
Support piers are used primarily to carry the weight of the pipes and enclosed water. Anchors are large structures which represent the fixed points along a penstock, restraining all movements by anchoring the penstock to the ground. A thrust block is used to oppose a specific force, for example at a bend or contraction.

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:

  • keeping the penstock closer to the ground,
  • avoiding tight joints,
  • avoiding soft and unstable ground.
     
Next document: turbines

 


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