The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Tuesday, 21 October 2014

"Capacity factor" and "Water Year"

Capacity factor and water year are two useful concepts.

The capacity factor for a scheme is a measure of its productivity over time.  It is calculated from the actual energy generated over a year as a proportion of how much energy could have been generated if the scheme had run at full capacity "24/365".  Like overall efficiency, it is one of those things which would be nice to know for a scheme before it is implemented, - but cannot be with certain accuracy.

No scheme can run 24/365. Most 'run-of-river' schemes have to reduce generation or shut down in the drier months for lack of flow. Then there is down-time for maintenance, breakdowns and grid-out spells.

For a Powerspout installation there is the added factor that nozzles have to be changed manually. As the flow changes with the seasons, so the size of the nozzle has to change.  It is somewhat of a "hands-on" business doing this, with quite a learning curve for reading the signs as to when to make a change, but the assiduousness of the operator in doing it  will improve the capacity factor. 

For my scheme in 2013/14, the total generation was 3,672 kWh.  The peak power output possible in that year was 0.713 kW which makes for a maximum possible energy generation of 6,245.9 kWh. So the capacity factor was 0.59.  My hope is to better that in this next year.

Which leads on to what period of 12 months is best taken as the period of time over which to base such a study: a calendar year ?  a financial year ? an Environment Agency year (which runs 1 April to 31 March) ? - or a "Water year" ?

Since the operation of any hydro site falls into a natural rhythm governed by the seasons and rainfall, it makes sense to use such a natural pattern and call it a "Water year".  It should begin and end in the middle of the driest season.  Here, that runs from August to October, so my water year starts 1st October and ends 30th September.

One advantage of doing things this way is that if there is a particularly severe dry season, and they happen every few years, its effects on the output record will be spread over two water years, rather than giving one apparently very bad year.  It only makes the figures appear to be better !

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