Appendix B - Hydro Plant Factor Commentary
One of the parameters for a hydro power station that reflects its performance, and a key input into the design process, is the plant factor (sometimes called the capacity factor).
The plant factor is the ratio of mean annual output (over a number of years of operation) of a power station to its maximum annual output if it operates at full capacity for the whole year. For a hydro power station it may also be considered to be the ratio of the mean annual flow to the maximum flow through the turbines of the power station.
The instantaneous output of a power station is directly related to the flow through its turbines. If the turbines' maximum capability was set to only match the mean annual flow then, for any period where the flow is higher than the mean (assuming little or no water storage is available) some of the flow will be "spilt" and the overall annual output will be less than it would have been if the penstocks, turbines, generators etc were larger. Flows that are lower than the mean will be fully utilised.
To ensure reasonable economics of the investment in the power station, including dams, spillways etc it is usual to provide for the capacity of the penstocks, turbines, generators etc to be capable of using a higher flow than the annual mean flow. Historically, hydro power station components have usually been sized to give an overall plant factor of about 50 to 55%.
Some schemes, especially those where there is an out-of-river canal, may have a greater plant factor than 50% (e.g. Project Aqua with approximately 65%). While this will usually result in less water passing through the canal/power station system to generate electricity than would be the case if a lower plant factor was selected, it allows for more cost-effective sizing and use of the intake, canal, power station equipment etc.
For the purpose of this study we have generally carried through the plant factors that were assumed or required to be met during the investigations that gave rise to the data and information provided, unless more up-to-date information is available. It is quite possible that re-assessment of a particular scheme during further investigations may lead to a more cost-effective option that will have a higher plant factor which will usually be associated with a lower MW level but a lesser reduction in energy (GWh per annum) output.
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