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, 24 May 2015

One jet or two: the bigger picture

In my last posting, I mentioned that experimentally I have found more power is produced by one jet than by putting the same flow through two jets, and that this was contrary to what I had previously supposed, - a supposition based on this graph from the advertising literature of a long established UK manufacturer of pelton turbines:




Here is another graph addressing the same matter but which shows a completely different picture: (it is reproduced with permission from Practical Action Publishing and is taken from Jeremy Thake's book "The Pelton Turbine Manual"




Although the axes of the two plots are labelled slightly differently this should make no difference to where the '1 jet' line lies in respect to the '2 jets' line, and yet the graphs display the lines in such a way that the two plots do not at all concur with each other in the story they set out to tell.

So wherein lies the truth? So far my experimental findings support the second graph.  I am continuing to take measurements to see if more readings will confirm this but at this stage Jeremy Thake's graph is the one which seems correctly to illustrate how a Powerspout performs in a low power / low flow situation.

Addendum written 28th May 2015: following correspondence with Gilkes, the long established UK manufacturer of pelton turbines referred to above, their Hydro Sales Manager has kindly explained that their graph is attempting to illustrate the benefits of installing a smaller runnered twin jet turbine vs. a larger runnered single jet turbine.  It is not looking at jet performance of single vs twin on the same runner.

Therefore, I misinterpreted their graph and used it to draw the wrong conclusion. The truth is that, when considering a turbine where either one or two jets can be employed,  at low flows single jet operation is better than two. 

Flows are still holding up surprisingly well here giving a daily output of 11.44 kWh on a flow of 1.9 lps.  This means I am operating around the 60% point in terms of 'Output Power' or 'Percent of full Flow' (depending on which graph you refer to).  Whichever graph one uses, at this 60% point, there is little difference between one jet or two and so I am not getting much opportunity to obtain further readings to confirm one jet is better than two at the present level of flow.  I happen to be operating with two jets at the moment, with most (1.6 lps) going through the bottom jet because it is the more productive, with just a little (0.3 lps) through the top jet.

During this past week it was meter reading day again, necessary in order to claim the next quarter yearly payment of Feed in Tariff. Rather a low number of kWh's have accrued since the last reading submitted in February: just 838 kWh, representing 53% of what could have been generated had there been sufficient water for full flow.

But one can't complain: if it doesn't rain, there isn't anything you can do about it !  What one can do, and this is what I have been working on recently and will report later, is to maximise the efficiency of the turbine.  Checking the balance of the pelton runner (a spare one) was one aspect of this, using this crude but effective set up:










The runner proved to be very well balanced !

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