Reduced core stator: Part 3 (and final !)

It's a fabulous time of year here.  The arrival of swallows at exactly their usual time and the carpets of bluebells in the wood through which the penstock runs, they both herald spring and the coming summer.  And, of course with summer comes a lack of water and this year we are experiencing that prematurely.

The upside of this relative drought has been the opportunity to put the new reduced core stator through its paces at low flows.  What I have found is that the 18 pole core has made it possible for me to continue generating at flows lower than I was previously able to use, as this graph shows:

But also as the graph shows, the benefit in terms of extra power into the grid over the range of flows where either stator could have been used, is not great: at best probably just 7 watts.

This marginal benefit results from an improvement in "whole system efficiency" with the 18 pole core, amounting at best to about 3% over the efficiency when using the 42 pole core:

The increased efficiency only becomes apparent at flows below 1.4 lps, so for my scheme, that is the point at which I will have to swap over cores.

It also seems, though the few data points scarcely support this, that at flows below about 0.8 lps this improvement in efficiency is lost.  Since the power generated at these very low flows is so small, less than 180 w or 4 kWh/day, it is probably not going to be worthwhile trying to harvest this level of output.

As I write, the turbine is generating 259 w on 1.13 lps.  In spite of the lack of rain, this flow will probably diminish only slowly so there could be another 2 months before flow hits the 0.8 lps when stopping generation makes sense.  

As ever, the anticipation of what actually works out is what makes it all worthwhile.

In search of extra watts

Extracting as much energy as possible from a small turbine is part of the fun of operating one.  

As described in the last post, installing the reduced core stator actually had the opposite effect: power output decreased.  Disappointing as that was, I'm anticipating the time, which hasn't come yet as water flows are still holding up quite well through this past week, when the reduced core will generate more productively than the full core. 

Another investigation to see if more power could be extracted involved using a rotor from which the cooling fins had been removed.  It was supplied as a 'one-off' by EcoInnovation in New Zealand:

The idea was that some energy must be consumed in rotating the vanes of the rotor against the resistance imposed by air, and that by removing them, increased electrical output might be seen. 

The degree of benefit was never going to be very great so a really sensitive way of measuring power / energy output was needed.  

The study involved running the turbine without interruption at constant flow and head for several days, first with the usual, finned, rotor and then with the new, de-finned rotor.  For each rotor, energy output was measured with an Elster A100C kWh meter over a very precise period of time, precise enough to be measured to the nearest second after a minimum of 2 days of recording.  By converting kWh to watt-seconds and dividing this figure by the duration of the recording period in seconds, the mean power output for each rotor was obtained.

Well, the findings were not very impressive:  the study was done on two separate occasions, the first demonstrated a gain of 0.96 W and the second 1.51 W.  So the conclusion has to be that removing the fins is not a worthwhile thing to do.

As a side investigation, the temperature inside the Smart Drive housing was measured with each of the two rotors in use:

This demonstrated that when the fins have been removed the lack of circulating cooling air raises the differential temperature between inside and outside by a factor of 2 or more.  So the fins do achieve their desired effect and removing them could make the Smart Drive compartment too warm, especially if high power outputs are being generated, if ambient temperature is high or if the turbine is situated in the direct glare of the sun.  It has to be remembered that LDPE, the plastic of the Powerspout casing, is a thermo-plastic !

The next investigation into how more power can be squeezed out of a Powerspout will look at using different bearings, starting with SKF's Energy Efficient type of deep groove ball bearings.  Watch this space.

Reduced core stator: Part 2

*For latest news, see note added at end, added 4 years after original blog entry was written.

The performance of the new stator has been almost wholly pleasing.  I made no alterations to the pelton nozzles between changing stators so comparison of one with the other was possible. The flow into the turbine was therefore a constant, namely 1.51 lps. Here are the things I observed:

1. Power: rather than being better, was 8% worse. This was the only disappointment.  I attribute it to the efficiency of the alternator being poorer when 24 of its poles have been cut off:

2. The quality of the power was markedly smoother, presumably because at the lower MPPV, the inverter was better able to fix on a maximum power point and hold to it: (note the scale in the lower trace, which is from the 18 pole stator, is different from the trace above, hiding the extent to which the trace is better.)

3. MPPV settled at about 145 v; before it was around 370 v; current rose proportionately from about 1.1 to about 2.4 amps:

4. Shaft rpm, shown here at 1079, later settled at about 1100 ± 4 once the grease had warmed up and been spread about, higher than the 968 ± 4 seen with the former stator:

5. In keeping with this higher measured shaft speed, the splash pattern from the pelton showed the changes expected. The bottom jet has moved to the left of the centre line whereas before, it was to the right; the top jet doesn't seem to have changed at all:

6. During a 24 hour period, there is still a slow undulation in the trace of power output, being just 7w peak to trough. What the cause of this is, I don't yet understand:

My big hope now after installing this new stator is that as summer approaches and water diminishes, I will be able to continue generation for more weeks than was previously possible.  Before, once flow got down to below 1.2 lps, I had to stop generating.  The excitement will be in discovering how low a flow I can go to.  Bring on the dry weather !

*Update note added 5 years later (21 Jan 2019)

• The hope of being able to generate year round and not stop generating in the drier months has been completely fulfilled: when flow gets to below 1.23 lps that is the time when I change to the reduced pole stator, changing back again as the flows of winter pick up.  For me these changes usually happen in June and November.
• With the reduced pole stator the lowest flow I can generate at is 0.53 lps.
• When the above entry was written, I was using an SMA SunnyBoy which operated in MPPV mode.  The algorithm the inverter follows in MPPV caused the operating voltage to rise at lower levels of generation and, as stated, this was the reason for introducing the cut down stator so operating voltage would be lower.
• Since that time, I've been using an SMA WindyBoy which operates in table mode. This algorithm works differently from MPPV and instead of the voltage being too high at low levels of generation it became too low: the inverter placed such a load on the SmartDrive that rpm (and hence volts) were too low.  It was the low rpm I needed to correct - it was causing the pelton to be operating away from its 'sweet spot' speed thus causing a loss of efficiency in the transformation of 'water jet energy' to shaft rotational energy.
• The good thing I have discovered is that the reduced stator corrects this problem with the WindyBoy just as well as it corrected the different problem with the SunnyBoy. It does this because the voltage put out from the cut down stator is less, and in consequence the look-up table in the WindyBoy commands a lesser current (i.e. imposes a lesser load) on the SmartDrive, and that allows the shaft speed to settle at a higher rpm.
• Final tuning of shaft rpm can then be done by standing off the rotor with packing washers. I aim for 900 to 1050 rpm.

Reduced core stator: Part 1

As mentioned in the last diary entry, I have recently received a reduced core stator from EcoInnovation in NZ. It's now been installed and has been generating for the past 5 days.  In this post, I want to say a bit about it and in part 2, give some observations about how it performs.

This new stator has just 18 poles rather than 42 and it looks a bit odd at first sight.  The unused poles have simply been cut off and the remaining ones connected in three strings of 6. The wire terminations from both ends of each 6 pole string are available with connectors attached, thus enabling the three strings to be connected in either delta or star configuration. The core carries EcoInnovation's designation 60-6s-1p-6wire whilst the original stator was 60-7s-2p-star.

The reasoning behind replacing the original stator with this reduced version is that at those times of year when water is less and power output consequently less, operating voltage (MPPV) rises to be too high if the full 42 pole core is used. For an account of why the voltage rises, and how generation is curtailed because of it, see this previous post. By taking away some of the poles, the voltage output will be lower.

To know by how much lower, EcoInnovation ran the core on their test bed and supplied the following information about its performance at different speeds and with different connection configurations:

From this, I chose to connect it in delta configuration so as to get the lowest MPPV I could (150 v at 1000 rpm).  I chose this option to improve the efficiency of the inverter: as can be seen from SMA's data for their SB1200 inverter, shown below, reducing the MPPV from 320 down to 110 volts should add about 2 % to the inverter's efficiency.  Since I would be reducing from 370 down to 150 v, I was hoping for about the same improvement.

I didn't have any worries about increased transmission losses at this lower voltage because the cable is generously over spec (5 mm² copper) and this stator will only be used when power generation is at its lowest limit, with only small currents flowing.

Installation of the reduced core went like clockwork and within a couple of hours. everything was ready to run:

Initially, I had a few anxious moments as it seemed as if the inverter didn't want to connect to the grid in spite of receiving an open circuit voltage registering 370 v. The unloaded shaft rpm at this voltage was a previously unseen high of 1487 rpm.  And this was just with the bottom jet open, delivering 0.83 lps.  I'm left wondering what unloaded rpm would rise to with greater flow, but that, I decided, was an investigation for another day.

Scratching my brain as to what could be not right, I walked up to the house, which is where the inverter lives and just as I got there, the green winking "waiting" l.e.d. changed to constant illumination.  Hooray, - grid connection ! - and then I had to hurry back down to the turbine to turn on the other jet before the load of the inverter caused the pelton to stall. 

And since that moment, it hasn't stopped...

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 !