Small hydropower projects are those under 10 MW in size, though this classification can vary from country to country. (In China, for example, any project under 25 MW is considered small.) While small projects operate on essentially the same principles as large projects and use similar components, there are differences that need to be considered separately.
There are three types of small hydropower project, designated small, mini and micro. According to a United Nations Development Programme (UNDP)/World Bank definition, a project in the range 1-100 kW is classified as a micro project while 100 kW to 1 MW is a mini project. The small project range will stretch from 1 MW to between 5 and 30 MW depending on who is defining it.
Small hydropower potential is often assessed separately from large-scale hydro potential. A 1996 estimate4 put the global small hydro capacity at 47,000 MW with a further 180,000 MW remaining to be exploited. In Europe there is around 9000 MW of installed small hydro capacity and sites exist for 18,000 MW more. China claims an exploitable potential for sites with capacities under 25 MW of 70,000 MW. Madagascar maintains it has a gross theoretical small hydro potential of 20,000 GWh each year. Clearly there is enormous potential for future development in may corners of the world.
Small hydropower projects can be developed anywhere, but mountainous terrain often offers the best potential. Thus Austria and Switzerland are both big users of small hydropower in Europe. This represents a valuable resource since communities located in mountainous terrain often cannot be connected to a national grid.
Small hydropower plants are conceptually similar to their larger siblings but the level of investment involved will affect the way a small project is developed. The turbines used in small plants are the same types as those employed in large projects but whereas the large plants will use turbines designed specifically for the site being developed, a small plant will normally have to use off-the-shelf turbine designs and generators in order to keep costs down.
In addition to the standard Pelton, Francis and propeller turbines, there are a number of special small hydro turbines. These include Mitchell Banki turbines, Turgo impulse turbines, Osberger crossflow turbines and Gorlov turbines. Energy efficiency tends to be lower for small hydro projects. A study by the UNDP and the World Bank in Ecuador5 found that systems under 50 kW had a maximum efficiency of 66% rising to 70% for units in the 50-500 kW range and 74% for units between 500 kW and 5 MW.
Head height is an important factor in determining small hydro economics with higher head sites generally cheaper to develop. An impulse turbine is the best choice where the head height is above 30 m, a reaction turbine below for lower heads. A head height of less than 2.5 m is difficult to exploit.
Dam and barrage structures are also similar in small and large projects but many small schemes use simpler designs. Run-of-river designs are popular since they involve the minimum of civil works. Novel designs, such as inflatable and rubber barrages have also been employed.
A key cost factor in a small hydro project is the feasibility study. Any hydro project must involve a pre-feasibility study to determine if the site is suitable for development and a feasibility study to prepare design details. The studies will look at the hydrological and geological conditions at the site. For a large scheme the feasibility study normally accounts for 1-2% of the total cost. In a small scheme it has been known to consume 50% of the budget.
Small hydro budgets are squeezed from other directions too, because capital costs do not necessarily fall in proportion to the size of the scheme. Control system costs, for example, escalate as the project size falls. The cost of grid connection may also make smaller projects uneconomical as grid-connected public power providers, although they can still provide an economic supply to a small isolated village or hamlet.
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