Hydroelectric discharge

Figure 1. Run-of-the-river systems like the one shown above tend to have larger flow rates than hydro dams that use reservoirs.

Hydroelectric discharge, also referred to as flow rate, is usually represented by Q. It is the volume of water that passes through a hydroelectric power plant per unit of time. Thus, the units for flow rate are meters cubed per second ($m^3/s$).

Understanding the flow rate of the stream or river used for a hydropower generation plant is vital to predict the available amount of hydropower. Flow rate, along with the head loss of the stream or river, are the two values necessary to calculate the available power.

Flow rate simply represents the volume of water that can be captured and then re-directed by a dam to flow across a turbine generator, move it, and generate electricity. The larger the flow rate value the more energy is available to be converted to electricity.

Having both a high flow rate and a high hydraulic head isn't necessary for a hydroelectric power plant to be viable. If there is a high flow rate, the head doesn't need to be as high: the sheer amount of water flowing through the dam is sufficient to move the turbines. This is the case for run-of-river systems, as seen in Figure 1 above. Conversely, if the water is falling from a large height it isn't as important for a large volume of water to flow through the turbines.

For Further Reading

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• Energy from water
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