Rotor packing

The weather has been cool and cloudy here in Wales for what seems far too long. There has been rainfall too, more than usual for this time of year, but none of this has stopped flow to the turbine steadily diminishing.  Today was a day for reducing the size of nozzle to keep flow delivered matched to what is available.

Doing this gave an opportunity.  I'm working in previously uncharted territory trying to make the most of these smaller summer-time flows whilst using a non-MPPT inverter**, and today's nozzle change gave scope to experiment.  

In previous diary posts, I've outlined what the aim is, - to keep pelton speed up in order to have it operate near its optimum rpm where most energy is extracted from the head and flow available. Keeping speed up can be achieved by making the rotor stand off from the stator coils by placing packing washers on the shaft. But the method is one of finding by experiment how much packing produces the best output: too little makes rpm and power output low; too much makes rpm better but reduces alternator efficiency so the end result is just as bad.  Somewhere in the middle lies a best compromise and the challenge is to find it. These pictures show how it can be done:

For these photos, the camera was carefully aligned with the back of the turbine casing so the amount of rotor stand off would be captured along with the rpm on the tachometer.  As can be seen, 8 mm of stand off produced most watts at a speed of 876 rpm, so that was how I left it, but I can't help feeling a tad more, say 10 mm taking rpm just into the 900's, could be better.  

That'll be a job for tomorrow perhaps...***

** It is worth stressing that I am only experimenting with rotor packing because this inverter does NOT use Maximum Power Point Tracking (MPPT) to optimise the power fed to it.  Inverters that use MPPT should find optimum speed automatically and with greater accuracy. They should therefore always give maximum possible power so long as voltage is within the range specified for MPPT tracking.  My own installation probably proved this last year: then I was using an MPPT inverter and with the turbine set up precisely as it was today except for having ceramic bearings and a reduced core stator, the output to grid was 299 watts.  Allowing 10 watts extra output for the decreased rolling resistance of ceramics compared to standard greased bearings, this was 14 watts more than the best I could achieve today (275 watts) with experimental rotor packing.

***... and indeed it was a job done later: an extra 1.75 mm (one of EcoInnovation's special packing washers, specially made for the job) was added to the existing packing of two rubber O-rings. It meant that the 'knob' only turned 9 full turns instead of the previous 10 needed to fully push home the rotor. The effect was to raise rpm to 974 and watts output to 281.  OK, that's a tiny gain over 275 w.  What's important though is that it proves, and calibrates, the method. Very satisfying !

Harvesting energy

Harvesting is a seasonal affair.  Whether the crop be potatoes, apples or kWh's, there is a season when the bulk of the crop is gathered in.  For my Powerspout the season for harvesting most kWh's is now showing signs of tailing off.

In the past seven days, I have seen the energy generated fall from its maximum of 18.86 to 13.82 kWh's per day, these figures representing the energy yield when instantaneous power is 786 and 576 watts respectively.

To track the diminishing water as tightly as possible requires having a series of nozzles of different sizes: - the available flow is steadily diminishing in a linear fashion but reducing the flow to the turbine necessarily has to follow a step-wise pattern as successively smaller nozzles are employed. 

The way I operate my Powerspout is to have a small nozzle at the top position and a large one at the bottom.  The top one delivers just 0.3 l/s and I never change this one, only turning it on or off.  The bottom nozzle delivers most of the flow and is always on. It has to be changed when in-flow to the header tank falls to be less than the nozzle's delivery rate.  

The difference in size of the orifices is really quite small, - just a difference in diameter amounting to fractions of a millimetre as the picture below shows.  

The nozzle I'm using at the moment is the one missing from the line up above: nozzle X; it's in the bottom position with the top, small nozzle turned off. Here it is in operation this morning, delivering 2.13 l/s and putting 576 watts into the grid:

As I go down through the nozzles, I'm measuring the speed of the turbine at each flow to see how far off the 'sweet spot' speed ( i.e. the optimum speed) the pelton is operating at:

The theoretical optimum speed* for my installation is 1200 rpm near enough, so with the rpm being 924, the operating rpm is 23% below optimum speed.  As discussed in an earlier post, whilst this is not ideal, neither is it as bad as it might seem.  The efficiency of the pelton in converting pressure energy into rotational energy is probably only diminished by 5% by operating at this slower than optimum speed, and this 5% loss of efficiency translates into a loss of about 30 watts, or 0.72 kWh in a day.  I think I can live with that though it would be nice to think of a work around to improve things.

Whilst the harvest of kWh's from hydro generation seems to be ending its season, the good news is that the harvest from solar panels is just starting.  I look forward to seeing how well the two blend their respective outputs and will report the outcome at the end of the year.

* to calculate theoretical optimum speed: 
1. calculate jet velocity (m/s): Vjet = 0.96√(2g x Hnet)
2. calculate optimum runner velocity at pcd (m/s): Vpcd = 0.46 x Vjet
3. optimum speed (rpm) = (Vpcd / 0.69**) x 60

So for my installation: 
Vjet = 0.96 x √(2 x 9.81 x 53) = 30.96 m/s
Vpcd = 0.46 x 30.96 = 14.24 m/s
optimum speed = (14.24 / 0.69) x 60 = 1238 rpm

** the pcd (pitch circle diameter) of a Powerspout pelton is 220mm; 0.69 is the circumference, in metres, of the circle having a diameter of 0.22m.

Tachometer installed

Against a background of fast reducing flows, it's been a busy time with my Powerspout.  Much of the 'busy-ness' of course has been the need to downsize nozzles to keep matched to the falling flow (3 changes in a week), but of greater excitement has been planning for other developments:

• installing a tachometer to measure turbine rpm
• running a test with a 'de-finned' rotor to see if power output was improved (it was but only by 1 watt).
• receiving a reduced core stator from NZ which will allow the turbine to operate at a lower output voltage, and so, hopefully, allow operation for more weeks per year.  The new stator has only 18 poles rather than 42. More about it in a later post.
• planning with Michael Lawley at EcoInnovation to run tests using ceramic bearings (no grease) and SKF E2 Energy Efficient bearings (greased but more free rolling) to see if they enhance energy output.  Again, more info in a future post. 

The past week also saw the end of the 'abstraction year' and for the first time I was able to complete my return to Natural Resources Wales by doing it on-line.  This was helpful: I had made a mistake in my version of the annual record and this became immediately evident when 'in-putting' the figures to their on-line record because my total was different from theirs.

The final tally was therefore less than I expected it to be: instead of going over my limit, the total was 3,870 cubic metres (8%) under. No reprimand for me this year then !

The project to install a tachometer was born out of a curiosity to know exactly what the 'loaded' rpm of the turbine actually is (see here for previous post where this was investigated using sound frequency), and also to investigate how rpm alters at different levels of power output.  The fundamental issue I'm trying to sort out is: how much of the voltage rise I see at low flow / low power times of the year is due to the shaft turning faster, and how much is due to the inverter imposing less load on the Smart Drive alternator.

In looking for a suitable tachometer, ebay again came to the rescue:  a British-made, 1988 vintage, opto-reflective type of tachometer having a remote sensor. I only needed to devise a way of mounting the sensor tidily in the Smart-Drive housing: 

The sensor needed something to hold it which could be easily bent to get a good position: annealed, thick, copper wire 

A piece of foil stuck on to the rotor rim, which has been painted matt black

Sensor positioned in casing and held in place with Blu Tack

Lead and plug brought out through other vent hole, and stored inside when not in use.

And now the true speed of the turbine is revealed: 968 rpm (+/- 4; at 441 w; 1.79 lps), a figure which is considerably lower than the theoretical optimum speed for the pelton, 1260 rpm, but a figure, nonetheless, which is supported by the frequency of sound coming from the turbine, as mentioned above.

It'll be interesting from now on to record rpm at each level of power output, starting soon with the newly acquired 18 pole stator.  I can't wait !