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.

Sunday, 26 October 2014

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.

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