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% ?

A conundrum solved, - possibly.

Throughout the first year of operation, there had been something niggling me and needing an explanation:  I had noticed there was a difference between the power generated by the top nozzle on its own compared with when the same nozzle was in the bottom position.  In the top position, the power from the inverter into the grid was about 20% less.

The factors influencing the efficiency of a pelton in converting energy in its water jet into rotational energy in its shaft, constitute a complicated science, but one of the key factors is getting the jet to hit the buckets of the wheel (also called a runner) in exactly the right spot.  

What I discovered on dismantling the Powerspout for its summer service was that the positioning of the centre of the shaft was not at the midpoint between the two tangents which the jets form with the 'pitch circle diameter' (pcd) of the pelton runner.

To be sure of the line taken by each jet to make the above measurements, each jet was given two nozzles, one inside, the other outside the casing, each having small diameter holes. In this way parallax error was minimized when sighting through them.

I have come to the conclusion that this asymmetry accounts for the power discrepancy I had seen, yet just as soon as reaching it, I doubt it, simply because the folk at EcoInnovation who designed the turbine know their stuff and are unlikely to have allowed such an error of geometry.  In part it could be explained by there being some play in the nozzle holders where they pass through the casing: by positioning the holders at the extremes of their play, the difference in the dimensions as given above can be made smaller, though never eliminated.

As can be seen above, it is not obvious that the dimensional difference is visible from the splash pattern of the two jets.  Where it does become obvious is when changing nozzles: the clearance between nozzle and runner for the upper is less than for the lower, making it more fiddly to do.

It would have been a simple matter, before having the new stainless bulkhead cut (see last post), to adjust the CAD drawing so the shaft and bearing housing were better centred, - but I hadn't solved the conundrum before having it cut.  

So it is going to have to remain as one of those efficiency losses which are an inevitable part of all machines.  To minimise the effect of it, I make sure that I always have the bigger of the two nozzles on the bottom where it will produce better output.  

In the next post, we'll look at the merits of one nozzle vs two nozzle operation.

Summer maintenance

The hoped for rain has not materialised !  Flow from the spring is stuck at 0.85 lps.  So with the start of generation for this 'water year' not looking imminent, I'm going to back-track to  August to say something about the yearly maintenance done then.

It was interesting to see what effects one year of operation had had. Impressively there was no wear detectable on any of the nozzles or runner buckets and I put that down to silt mostly dropping out in the header tank, from which about half a cubic metre had to be bucketed out.

I had been concerned that silt might also collect at the bottoms of the upstand pipes leading to each Powerspout nozzle.  To clear the pipes through, the following arrangement was used but from the colour of the discharge, it didn't look as if much silt had collected.

The only serious concern evident on dismantling the turbine was galvanic corrosion going on between the aluminium bulkhead and the stainless steel dump load element, and also where the stainless self tapping screws secure the plastic shell to the bulkhead:

Whilst none of this corrosion would likely have caused failure in the near future, certainly it would have needed attention eventually and I decided to replace the bulkhead with a stainless version.

Michael Lawley in New Zealand was immensely helpful as ever and sent me the CAD drawings to get the replacement cut.  I had never had any experience of laser cutting before so the accuracy which is possible, - to 0.2 mm, astounded me.  

Here is the finished article after refitting in the casing, using M5, socket-head machine screws rather than the original self tapping screws, and a 1" BSP stainless nut, which has a 'captive' O-ring behind it, to hold the heater element: (the bulkhead cost £60 plus £12 VAT)

To complete the annual overhaul, a new set of bearings was put into the bearing block.  Ever helpful again, the EcoInnovation instructional video on this was first class.

That's enough for this post.  I'll add a few other snippets discovered during maintenance in the next.