CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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10. Innovations
10.1 General
A number of innovative ideas, research, applications and pilot
projects relevant to micro-hydro technology that have not yet
been fully field tested, especially in the Nepalese context, are
discussed in this chapter. Some applications are in the research
and development stage, others have been successfully
implemented in other countries or carried out as “pilot
projects” in Nepal.
10.2 Coanda intake
A Coanda intake has a special screen that utilises the tendency
of fluids to follow a surface. This is known as the “Coanda
effect”. As shown in Figure 10.1, the Coanda screen is installed
along the crest of the diversion weir and is shaped in the
ogee curve configuration. A curve acceleration plate at the
top of the screen stabilises and accelerates the flow. As the
flow passes over the screen surface, the shearing action of the
bars combined with the Coanda effect separates the flow.
Clean water passes down through the screen whereas
sediment and debris pass over the screen to rejoin the water
course below the weir.
On rivers carrying cobbles and boulders during flood, the
Coanda intake must be carefully located so that heavy bedload
does not pass over the screen and damage it.
The potential advantages of the Coanda intake are on
particular sites which suffer from exposure to high silt load or
which offer scope for cost savings in the headrace. In the first
case, the intake can reduce the need for large or multiple
settling basins. In the second case, where a site layout is
suitable, it may be possible to commence the penstock run
directly from the Coanda, gaining head and avoiding the
need for a headrace canal. Of course, this might imply that
the penstock is longer than other potential layouts or that it
runs close to the river. A thorough financial and technical
analysis of the options is required before making a decision
on the suitability of the Coanda for a particular site.
The Coanda screens are fabricated to a high tolerance from
stainless steel. The supplier of the Coanda screens (also called
“Aqua Shear Screens”) in Europe, Dulas Limited, Wales UK,
claims that screens can be produced with 0.5 mm to even 0.2
mm clear spacings, which eliminate 90% of 0.25 mm and 0.1
mm particles respectively. Both types of screen eliminate all 1
mm particles. In most micro-hydro systems this would also
eliminate the need for a settling basin.
The flow capacity of these screens is 1401/s per metre of weir
length. A screen with a flow capacity of 401/s (0.3 m width)
costs about US$ 1380 (1997 price).
A Coanda intake has been tested at a micro-hydro site in
Wales by Dulas Ltd., in conjunction with Practical Action, UK.
Figure 10.1 A Coanda intake screen
Photo 10.1 Coanda intake of a micro-hydro scheme in Wales, UK