As an owner/developer of a potential hydro site, you may wonder how much power your site will produce. A more exact question is how much energy it will produce – it is energy in kilowatt hours (kWh) that we buy from or sell to the electricity supplier.
Energy is a measure of the length of time we have used or produced a given amount of power. For example, if you use 1 kW (1000 W) of electricity for one hour, you have used 1 kWh of electrical energy.
A site on a stream or river that has a highly variable flow (i.e., a wide range of flows with many highs and lows) may not produce as much energy as a river that has a smaller range of flows but that is more consistent on average.
A hydrologist or professional consultant can produce a flow duration curve (FDC) for a river or stream by ordering the recorded water flows from maximum to minimum flow.
This is a way to show the probability in graph form of how many days in a year a particular flow will be exceeded. (The area below the curve is a measure of the energy potential of the river or stream.)
The FDC is used to assess the expected availability of flow over time and the power and energy at a
site and to decide on the “design flow” in order to select the turbine. Decisions can also be made on how large a generating unit should be.
If a system is to be independent of any other energy or utility backup, the design flow should be the flow that is available 95 percent of the time or more. Therefore, a stand-alone system such as a micro-hydropower system should be designed according to the flow that is available year-round; this is usually the flow during the dry season.
It is possible that some streams could dry up completely at that time. Remember that for any water source, be it a river, stream or creek, there will be a difference in flow between winter and summer, and this will affect the power output produced by a micro-hydropower system.
Flow in the stream changes continually (sometimes daily) if precipitation has occurred; however, some generalizations can be made. In southern Ontario, rivers and streams are at their highest levels in early spring and are at their lowest levels in late summer. In northern Ontario and Quebec, smaller rivers and streams are usually at their lowest levels in mid-winter and at their highest in spring.
British Columbia and Newfoundland and Labrador generally have low flows in late winter and high flows in the spring, except for the south coast of British Columbia, which has low flows in summer and high flows in winter. These variations must be considered in the estimated total energy generation expected from a site.
Ideally, minimum flow over the year should be taken to calculate the design flow to ensure that
power is available year-round. Normally, only a fraction of the available flow in the stream is used for power generation.
Therefore, FDC is less important as the size of system decreases. If the system’s generating capacity is less than 10 kW or so, FDC may not be relevant at all.
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