It's a fabulous time of year here. The arrival of swallows at exactly their usual time and the carpets of bluebells in the wood through which the penstock runs, they both herald spring and the coming summer. And, of course with summer comes a lack of water and this year we are experiencing that prematurely.
The upside of this relative drought has been the opportunity to put the new reduced core stator through its paces at low flows. What I have found is that the 18 pole core has made it possible for me to continue generating at flows lower than I was previously able to use, as this graph shows:
But also as the graph shows, the benefit in terms of extra power into the grid over the range of flows where either stator could have been used, is not great: at best probably just 7 watts.
This marginal benefit results from an improvement in "whole system efficiency" with the 18 pole core, amounting at best to about 3% over the efficiency when using the 42 pole core:
The increased efficiency only becomes apparent at flows below 1.4 lps, so for my scheme, that is the point at which I will have to swap over cores.
It also seems, though the few data points scarcely support this, that at flows below about 0.8 lps this improvement in efficiency is lost. Since the power generated at these very low flows is so small, less than 180 w or 4 kWh/day, it is probably not going to be worthwhile trying to harvest this level of output.
As I write, the turbine is generating 259 w on 1.13 lps. In spite of the lack of rain, this flow will probably diminish only slowly so there could be another 2 months before flow hits the 0.8 lps when stopping generation makes sense.
As ever, the anticipation of what actually works out is what makes it all worthwhile.