One jet or two ?

In their literature, EcoInnovation recommend that Powerspout peltons are not operated on one jet if the power output from the Smart Drive exceeds 400 W.  This is presumably because of mechanical stresses on the root of each runner cup.

For a GE 400, (or the later Powerspouts which are directly grid connected using an Enasolar inverter, without the V-clamp board which is found in the GE 400), 400 W DC output from the Smart Drive equates to about 330 - 350 W AC from the inverter to the grid. The actual figure will be dependent on transmission losses and inverter efficiency at this power level.

The plot below is taken from the literature of a long established UK manufacturer of water turbines based in the Lake District and shows how, for their turbines, there is an efficiency benefit to be gained from delivering the flow via two jets rather than one when the flow is toward the low end of design flow.

Assuming what is true for their twin jet peltons is also true for a Powerspout  and applying the information from this plot to my installation, it would mean that at 40% of full flow (1.2 lps / 3 lps) I would gain 3 % in efficiency.  In power terms this equates to seeing 250 W rather than 243 W into the grid.

So the message would seem to be: at those levels of flow where EcoInnovation say it is OK to run on one jet, it is actually more productive to still operate with two.
Correction added 28 May 2015: this is an erroneous conclusion. Please refer to later addendum "One jet or two - the bigger picture"

I have to admit though, - I haven't tested this experimentally.  It would be very difficult to cut 3 nozzles sufficiently accurately to ensure that the sum of the flow of two of them was exactly equal to the flow of the third.

In the next post: since the above plot shows that a pelton remains reasonably efficient down to a part flow of 20%, why can't I operate mine below a part flow of 40% ?