Theoretical Hydropower Output

“Work volume per 1 second (J/s)” when the water of discharge Q (m3 /sec) drops from
the height of H(m) is shown as the following equation.

The above-mentioned equation shows that the lager “Q” (discharge) and the higher “H” (head)
is, the bigger “P” (power output) is. “Q” (discharge) mainly depends on catchment area at intake
point and an amount of rainfall in the catchment, while “H” (head) depends on topographical
conditions such as riverbed slope.

Practical Hydropower Output
No power conversion system can deliver as much useful power as it absorbs –some power is
lost by the system itself in the form of friction, heating, noise, etc.

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What’s Catchment Area

The volume of the river flow or discharge depends on the catchment area and the volume of
rainfall. Figure 1.5-1 shows how the rainfall is divided on both sides (A and B) of the watershed.
For example, there is an existing Hydropower Plant at A-side, the rainfall at B-side cannot be
used for power generation at this Hydropower Plant. Therefore, the catchment area of a
proposed hydropower plant should be known at the first step of the study of hydro scheme.

The dashed-dotted lines in Figure 1.5-2 indicate the watershed of Point-A. The catchment area
is the area enclosed by dashed-dotted lines.

In Figure 1.5-2, the arrows indicate the flows of rainwater. The rainwater in the catchment area
at point A, such as blue arrows in the figure, will eventually flow down through point A, while
the rainwater outside of the catchment area, such as red arrows in the figure, will not pass
through point A.

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Concept of Hydro Power

Energy of a falling stone depends on a height and the weight of the stone. The higher the
position of the stone is and the heavier the stone is, the larger its energy is. Just like the energy
of a falling stone, energy of hydropower depends on the height or a drop in height (referred to as
‘Head’) and weight of the water or “discharge” as shown in Figure 1.2-1.

A hydro scheme requires both water flow and a drop in height or ‘Head’ to produce useful
power. The power conversion absorbs power in the form of head and discharge, and delivering
power in the form of electricity or mechanical shaft power as shown in Figure 1.2-2

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