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How to estimate the water resource potential for a site if considering hydroelectric power?

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There are many factors that need to be considered. Primarily hydroelectric power (HEP) works by using the kinetic energy of falling water. The greater the fall or head, the more power that is potentially available. Therefore, mountainous areas, such as Nepal, have potential for HEP. In addition to head height, catchment characteristics need to be known, for example area of the river basin, valley slope, rainfall and river discharge.

Figure 1 shows an example of head height.

The power generated from a hydropower scheme is given by the following formula

Power = ηρgQH Where:

η is the ‘water to wire’ efficiency – typically (70-80% for a small scheme).

ρ is the density of water (1,000 kg/m3).

g is acceleration due to gravity (9.81 m/s2).

Q is the flow through the turbine (m3/s).

H is the Net Head (m) i.e. including head losses.

Power is proportional to the head and the flow. In general, the physical size and, hence, to a large extent, cost of the turbine is governed by its design flow, rather than the hydraulic head. For example, a 100kW scheme at 300m head will require about 40 litres/s, whilst a 3m head will require a flow of 4,200 litres/s (assuming 80% efficiency).

Therefore, a high head scheme is preferable to a low head scheme and head is more important than flow (although clearly flow is also important!). But in both cased hydroelectric turbines are available for high head or low head situations. From the equation Power = ηρgQH there are two critical parameters in determining the potential power from a site; the hydraulic head and the flow in the river. However, the flow in the river will tend to vary, and often substantially vary, on a day by day basis. It is of great importance to know what kind of variation there is over a period of a year shown in Figure 2.

Figure 2 example of flow variation

  1. For a couple of months, the flow dropped to very low values of less than 1 m3/s.

  2. The river exceeded over 20 m3/s on several occasions.

  3. The river level tends to rise fairly quickly but this level reduces gradually, i.e. there is a sharp ‘leading edge’ followed by a slow decay. This is a general characteristic of all rivers.

This data is often shown in the form of a Flow Duration Curve (FDC). This is a probability model of the frequency that a particular flow rate will occur.
Taking the example from Figure 2.

  1. A flow of 0.5 m3/s is exceeded about 95% of the time;

  2. A flow of 2.41 m3/s is exceeded 50% of the time;

  3. The mean flow is 4.83m3/s, which is exceeded just under 30% of the time; and

  4. Flows of more than 20m3/s are exceeded about 1% of the time.

Why is this important? Since the flow in a river is continuously varying, the determination of the optimum size ... (more)

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