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 !

Definitely for Powerspout aficionados only.

Two aspects of the performance of my Powerspout have intrigued me over the two years it has been operational:

1. is there a difference in power output between a nozzle employed in the top position compared with the same nozzle in the bottom position ?
2. do you produce more power with the flow going through two nozzles compared with the same flow going through one nozzle ?

Both of these curiosities have been addressed in earlier posts: the first here and the second here, but I had never got around to answering either definitively.

So yesterday I conducted a series of tests which used the record of power output provided by Watson Anywhere:

Explaining this:

• starting at left with 'Two jets, total flow 0.98 l/s', the power out to the grid was 185 W
• jumping to the end trace on the right 'Single jet, bottom, 0.96 l/s', power output was 211 W.

Conclusion: even though flow was marginally less through the single jet (0.96 vs 0.98) the power produced was 26 W greater, ie 14 % greater.  
So it is better to operate with one jet rather than two, but this will only apply at low flow times of year for two reasons: 

1. to keep within EcoInnovation's recommendation not to exceed a power output of 400 W on a single jet.  
2. because a jet cannot be too big: if it is, some water will miss hitting the pelton cups.  As a general rule, a jet shouldn't be bigger than 11% of the runner pcd .  For a Powerspout pelton, the pcd is 220 mm, so 11% is a jet having a  diameter of 24 mm.  

...moving to:

• 'Single jet, bottom, 0.83 l/s', the power there was 191 W.
• ...but with 'Single jet, top, 0.83 l/s' the power drops to 174 W.

Conclusion: the bottom jet position is significantly more productive: 17 W (nearly 10%) more at this flow. 

These differences in power generation, though only measured in watts, amount to much more significant differences than were evident when looking to see what might be gained by removing the fins from the rotor (see here) or operating with a reduced core stator (see here).  I suspect they will also be more significant than will be revealed by any change in the type of bearings.

As to explanations for what has been observed, the factors relevant to the bottom jet being more effective than the top include:

• the lower jet has a little more head
• water from the lower jet is cleared by gravity away from the runner
• in my turbine at least, top and bottom jets have slightly different pitch circle radiuses (pcr's) ie: where, in relation to the shaft centre line, they strike the pelton runner (see earlier ref).  

The factors relevant to one jet being more productive than two include:

•  there is more spray and splash within the casing with two jets operating, causing drag on the runner and also causing the jets to break up as they leave the nozzle orifice
• there are losses intrinsic to a nozzle and if there are two nozzles the sum of these losses will be greater than if there is one

Well, doing the study was interesting and the results will now guide me for the future operation of the turbine so power output is maximised: - at the moment, a single jet in the bottom position.