Basic Hydro Power - part 2

We already know that flow of water and the height of water fall (or height difference of water flowing from point A to point B or to be short "head") are the important things one needs to know correctly before deciding to go for hydro power or not. Measuring those two parameters needs special skill. Special skill means one has to understand how to use the method and its weaknesses. Let's discuss about measuring flow of water. There are many ways or methods of flow measurement: current meter method, float method, tracer method (salt dilution/water conductivity for example), sharp crested weir method, gauging station method etc.

The choice of methods depends on the stage of hydro power development. In the early stage one can use simple and easy method such as current meter, float or tracer. Those methods can provide on site direct flow data. The data will only represent that current condition and not representing all year round condition. However using those simple methods at the right time of the year, one might get the safest flow data assumption. For example someone is measuring flow of a river in a dry year in dry season. At that time of the year the flow is assumed to be the least (maybe even the base flow of the river) so it is quite safe to design a hydro power plant based on that flow measurement since the power plant might be able to operate all year round.

From this point, we then understand one other simple thing that is necessary to know before measuring flow. One has to know the records of rainfall in the area so he/she can determine whether the year of flow measurement is dry year or wet and also can determine which time of the year to measure the flow. Beside those purposes, rainfall data can be used to simulate flow based on mathematical equations (combined with map). Other methods are very useful for the design of the power plant. Sharp crested weir and also gauging station will need all year long measurement efforts. Everyday one has to record certain parameter that relates positively with flow of water (i.e. height of water surface). 

Combining flow simulation (based on rainfall data and map) and year long flow recording (weir or gauging station), one can have a more realistic understanding of the stream flow. One can then produce what is called "flow duration curve". A curve that shows duration of certain flow in a year. for example the base flow of a stream will have a 100% duration in a year. It means the base flow will be available all year round. Flood flow might only available for 5% of a year (or any percentage). Based on this flow duration curve one can decide whether the power plant will operate on base flow or certain higher flow. If it goes to higher flow then, the optimal operation of power plant will be less than 100% (i.e. a turbine is usually works at best efficiency at certain designed flow). The rest of the year it will operate at lower efficiency level.

For more information about how to measure flow, you can visit my docstoc profile and check the Good & Bad of Mini Hydro Power vol. 1.