- is there a difference in power output between a nozzle employed in the top position compared with the same nozzle in the bottom position ?
- 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:
- 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.
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:
- to keep within EcoInnovation's recommendation not to exceed a power output of 400 W on a single jet.
- 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.
- '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.
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).
- 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.