The set up

The set up
5.36mm jet delivering 0.63 l/s to the pelton which is rotating at 870 rpm and generating 127 watts into the grid.

Saturday, 3 August 2019

Fine tuning

Summer dryness is making itself felt here and today I've made the change I make every year at about this time, - changing the 42 pole stator to the reduced version which has only 18 poles*.  The effects of doing this were two:

  1. the rpm of the turbine increased from 793 to 885; this makes the efficiency at which the pelton converts hydraulic power to shaft power rather better (my optimum pelton speed is about 1000 rpm)
  2. the rotor, which had to be packed off maximally to keep the rpm up to 793 with the 42 pole stator, could now not be packed off at all; this makes magnetic flux linkage between stator poles and rotating magnets better and so improves the efficiency at which the shaft converts shaft power to electrical power.

The benefit of these two efficiency improvements are apparent in the record of the turbine's output to the grid.  The output can be seen to have been lifted from 206 to 227 W.

OK, so it's not a huge increase in output, - just half a kWh per day.  But I needed to do the change so that as flows decrease further, I have the Smart drive set up for the coming weeks as I go down through my nozzle sizes.  As you can see from the output record, it only took 30 mins to do.

*see here and here to read about the reduced core stator

Saturday, 13 April 2019

Measuring nozzle Cd

EcoInnovation, the company in New Zealand making Powerspout turbines, has developed a new jet nozzle.  It's longer and more tapered than the original and the change in design slightly changes the way the nozzle performs.  There is a theoretical flow an ideal nozzle will deliver which is dependant on the orifice size and the pressure head.  However in the real world the actual flow is less than the ideal by what is called the Discharge Coefficient (CD), and it was the C for the new nozzle which needed to be measured.  This is how it was done.

1. The formula* for determining CD requires net head to be known, - net head being the water pressure at the entry to the nozzle when it is operating.  The very large pressure gauge in the picture, calibrated in metres head of water, provided the means for measuring it.  Plumbing it to the manifold required the temporary removal of the upper pelton jet.

2. The formula also requires the flow through the nozzle to be known. This was determined by measuring with a stop watch the time it took for a defined volume of water to pass through the orifice.  This defined volume was 429.5 litres. It was known to be this because a drop in water level in the header tank of 104 mm could be calculated to be equal to this volume.  The height drop of 104 mm was determined by two knitting needles set exactly with their points that distance apart. The stop watch was started when the surface tension 'grab' to the upper needle was broken and was stopped when broken again at the bottom needle. The inflow of water to the header tank was diverted while each measurement was made.  The measurement time varied between 131s for the nozzle with the largest orifice and 20m 11s for the smallest.

3. For 13 nozzles with different sized orifices, paired sets of net head and flow measurements were obtained. The Discharge Coefficient for each nozzle was then calculated and the results, with the flow for each nozzle, plotted in MS Excel:

4. As can be seen, the Discharge Coefficient is not the same for all sizes of nozzle.  For the smallest size, nozzle I, where the flow is least, the CD is close to unity** indicating that actual flow is the same as would be expected from an ideal nozzle.  But as orifice size increases, the CD falls.  

5. The new type of nozzle performs better than the old.  Old nozzles had a mean CD of 0.85 whereas the new 0.90.  

6.The value of Discharge Coefficient for a nozzle depends on the pressure difference between the pressure of water entering the nozzle and the pressure once it has left through the orifice.  So although on my site (which has a pressure head of between 51.5 and 53.6 m depending on flow) the CD has been measured as indicated above, the same nozzle on another site with a different pressure head will have a different CD, though it won't be greatly different .

* Q = CD x Anoz x Sqrt (2g x Hnet) whence CD = Q / (Anoz x Sqrt (2g Hnet))

** The result obtained was 1.01, i.e. better than unity, which is not possible and represents experimental error.

Sunday, 6 January 2019

Stopping water entry

I've been experimenting recently with trying to stop wetness from the pelton side of the turbine creating dampness on the alternator side. There's a seal around the shaft which should prevent water in any quantity getting across but an investigation I've done using bags of silica gel indicates that in spite of the seal about 300mls per month still gets across.
These photos tell the story of the sequence of steps I've taken:

1. Limescale deposits on the shaft indicated that a considerable amount of water enters the top-hat labyrinth chamber

2. A V-ring seal (purchased here) was mounted on the shaft; the seal turns with the shaft and its lip seals against the plastic face of the top-hat, with the idea of preventing water tracking alongside the shaft

3. Inspection after 3 weeks running showed the seal had badly scored the plastic face of the top-hat, presumably from softening of the plastic by the heat of frictional contact, - I must have applied it too tight to the face.

4. A friend who is skilled on his metal lathe kindly turned a stainless steel cap to fit over the plastic top-hat so the seal rubbed on metal; the cap is held on only by being a tight fit.

5. Suspecting that water might also enter the top-hat via its drain hole, a deflector was devised to shield the hole from the upward direction of water leaving the pelton from the lower jet.

6. So the complete arrangement as it is at the moment looks like this:

Only time will tell if it makes any difference.  The early signs are that the silica gel bags do seem to be taking up less water but I'm yet to be convinced this is a genuine observation.

Whilst I was working through these stages of development, EcoInnovation have come up with a slightly different approach:

Theirs is a neater solution but care will be needed not to apply the V ring seal too tightly against the face of the top-hat.  The seal only needs to just touch. After observing the scoring illustrated above, a new top hat with the seal just touching ran for 3 weeks with not even a mark being caused.  A smear of grease is also a good idea.