3.4.1 Design Flow
It is unlikely that schemes using significantly more than the mean river flow (Qmean) will be either environmentally acceptable or economically attractive. Therefore the turbine design flow for a run-of-river scheme (a scheme operating with no appreciable water storage) will not normally be greater than Qmean. The exception would be a scheme specifically designed to capture very high winter flows, which is very rare in mini-hydro applications.
The greater the chosen value of the design flow, the smaller proportion of the year that the system will be operating on full power, i.e. it will have a lower ‘load factor’.
3.4.2 Load Factor
The ‘load factor’ is a ratio summarising how hard a turbine is working, expressed as follows:
Load factor (%) = Energy generated per year (kWh/year)
Installed capacity (kW) x 8760 hours/year
A first estimate of how load factor varies with design flow is given as follows:
|
Design Flow Qo |
Load Factor |
|
Qmean |
40% |
|
0.75 Qmean |
50% |
|
0.5 Qmean |
60% |
|
0.33 Qmean |
70% |
3.4.3 Rated Power
The peak power P can be estimated from the design flow Q0 and head H as follows:
P(kW) = 7 ´ Qo(m3/s) ´ H(m)
3.4.4 Energy Output
The annual energy output is then estimated using the Load Factor (LF) as follows:
Energy (kWh/year) = P (kW) ´ LF ´ 8760
There is clearly a balance to be struck between choosing a larger, more expensive turbine which takes a high flow but operates at a low load factor, and selecting a smaller turbine which will generate less energy over the year, but will be working flat out for more of the time i.e. a higher load factor. The load factor for most mini-hydro schemes would normally fall within the range 50% to 70% in order to give a satisfactory return on the investment.
Most turbines can operate over a range of flows (typically down to 20-40% of their rated flow) in order to increase their energy capture and sustain a reduced output during the drier months.
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