6.2.2 Site selection and basic layout

As adequate head and flow are necessary requirements for hydro power generation, site selection is conditioned by the existence of both requirements. Before a site is selected, a preliminary assessment study should include definition of power potential, estimation of power output, identification of physical works needed, identification of critical issues (environmental and social constraints), and a preliminary study of economic feasibility.

 

Source: Ref 7.7

Figure 6.2 High head small hydropower scheme

SHP schemes can be high-head or low-head depending on the geographical characteristics of the available site. In general high-head sites are less expensive to develop than low-head sites, because for the same power output the flow through the turbine and required hydraulic structures will be smaller. In a river with a steep gradient over part of its course, the level difference can be utilised by diverting all or part of the flow and returning it to the river once it has passed through the turbine. The water can be brought from the intake directly to the turbine through a pressure pipe.

A cheaper alternative is the one shown in figure 6.2. The scheme comprises a dam or a weir, a river intake, and an almost?level open canal running along the side of a river valley ending in a forebay (local open basin), from where a pressure pipe conducts the water down to the turbine in the powerhouse. If the topographical or geotechnical characteristics of the ground are unfavourable the open canal may not be the best solution. A low pressure pipe, the "penstock", may provide a more economic solution in such circumstances.

In low-head schemes two configurations are possible. One uses a diversion weir with a layout very similar to the above, although the canal is usually short and the penstock short or non-existent (figure 6.3). The other involves a dam with an integral intake and powerhouse.

 

Source Ref 6.1
Figure 6.3 Low head small hydropower configuration

Another possibility is to install a power plant on an existing dam built for multiple purposes (flow control, irrigation, etc.). Water enters the turbine through a penstock constructed as an integral part of the dam structure or, if the dam is not too high, through a syphon intake (figure 6.4).

 

Source Ref 6.1
Figure 6.4 Power plant scheme on an existing conventional dam

In the syphon solution the penstock runs over the dam before sloping down to the turbine, which can be located either on top of the dam or on the downstream side. Although the head in most syphon installations varies from 1.8 to 11 m, there are a few examples with heads as large as 30 m.