Sharing his expertise with Power Today, Dr Sunil Kumar Singal, Principal Scientific Officer, Alternate Hydro Energy Centre discusses the various technologies used in SHPs development.
Currently what new technologies are being applied internationally for SHPs and are they being used in India?
Listed below are some new and innovative technologies for weirs, intakes, turbines, controls and materials which may improve the functionality of SHPs.
Inflated Weir: In this weir, the rubber bag is bolted on the concrete floor laid at the river bed level or the weir crest. The rubber bag comprises of multiple interwoven layers of chloroprene rubber and nylon, which has excellent friction and weather resistant properties. Inflation and deflation are controlled automatically with a monitoring and control pumping/valve system. The rubber bag is filled either with air or water. Rubber weir is earthquake resistant and its other advantages include rapid installation, low maintenance, easy installation on existing weirs and dams, silt flushing, passing the floods safely and precise flow regulation.
Coanda Weir: Involves the tendency of fluids to follow a surface over a row of horizontal wedge wire bars (1 mm apart) perpendicular to the flow. The screen is installed along the crest of a diversion weir and is shaped in the ogee spillway configuration. There are no moving parts and no electricity supply is required. These screens are self-cleaning requiring little maintenance. Cost is competitive over trench type intake in the Himalayan range.
Siphon Intakes: Siphon intakes on penstock/closed conduits have several advantages like ease of operation, elimination of intake gate/valve, ice formation in cold climate, improved run away conditions and silt entry reduction as also cost reduction. Its application however is limited to diversion type plants and reservoir plants having head race water level fluctuation less then 56 m. Use of vertical axis semi-Kaplan units in a siphon elbow for heads upto 4.0 m and unit capacities upto 500 kW, provides above water access to turbine runners, eliminating the need for costly intake and draft tube gates.
Serpent Sediment Sluicing System: Simply called `S4´, its first two commercial installations began operation in Nepal during the 1994 monsoon season. The serpent floats in the basin while sand and silt are settling out. To flush the basin, the serpent is filled with water and the flushing gate is opened. The silt opening will then move in the opposite direction, thus moving the sluicing area through the entire basin. An S4 installation can be operated automatically or manually.
Siphon Spillways: A siphon is essentially a conduit located above the hydraulic grade line. The existence of sub-atmospheric pressure enables water to be sucked up above the upstream free surface level before it is discharged at a lower level. The discharge through the siphon produces a fall in upstream level, but siphon continues to operate even though the level may have fallen below the crest of the siphon. In fact, it continues to do so until the inlet is uncovered, where upon the entry of air breaks the siphon´s action. Since it is the pressure of the atmosphere acting on the free surface that forces the water up over the siphon, it follows that it will only continue to operate, provided that the minimum pressure which occurs at the throat does not fall below that exerted by the vapour. The minimum permissible pressure head is usually taken as 3 m.
Roofless Power House Building: In low head small hydro power schemes, the size of machines are bigger, thus height of the building are more to facilitate the movement of crane for maintenance. By providing removable roof, the height of the power house building can be reduced substantially.
All the above technologies except Coanda Weir have been used in SHPs in India. Other innovations include use of new material for penstock like high density polyethylene (HDPE), poly vinyl chloride (PVC) and glass reinforced pipe (GRP). Most of these materials are being manufactured in India too. Though these materials are not cost effective compared to conventional steel penstocks, which are commonly used in India, they offer higher hydraulic efficiency, ease of installation and transportation. Other innovations include use of plastic or HDPE for trash racks with high hydraulic efficiency, geotextile and HDPE lining in canals, hydrokinetic turbines to harness power potential of flowing streams, supervisory control and data acquisition (SCADA) system for automatic control of the power station.
What factors determine the technology to be used in SHPs?
SHPs are categorized into three types, based on whether they are canal based, dam based or run of river. Based on head, they are defined as high, medium and low head schemes. While low head schemes could be canal or dam based or run of river, medium and high head schemes are either dam based or run of river. SHPs in the hills, where small streams are available, are mostly medium and high head schemes utilising small discharges.
Most of these schemes are in the Himalayan region, with steep hilly terrain, no firm geology and some areas prone to avalanches and land slides. Due to these uncertainties, medium and high head schemes are considered more site specific and these problems are countered by using suitable intake structure, construction materials, choosing between open channel/pipe or tunnel as per site requirement etc. Low head schemes in the canals meant for irrigation system have established hydrology and are free from geological and discharge uncertainties. In construction of civil works, since blasting can affect stability of adjoining areas, drilling or boring and controlled blasting can be used. For pipes and penstocks, lighter weight materials like PVC and HDPE can be used instead of the conventional steel.