'Low flow matters' revisited

Owners of small water turbines usually get excited in the wet months of a year. That's when their turbines start to perform at maximum capacity.  Yet last December, just when my turbine was increasing its output like everybody else's, I found myself writing in this blog that I couldn't wait for the next dry season.  What on earth was I on about ? - and now that the next dry season is happening, why was it that I was so keen for these dry months to arrive?

The development that prompted my comment was when I started using a WindyBoy instead of a SunnyBoy inverter and came to see that the WindyBoy, quite unlike the SunnyBoy, caused the turbine to operate at a lower voltage at times when flows were small.

This was a startling difference and the plot below shows it.  As can be seen, with diminishing power the SunnyBoy's operating voltage progressively rises, whilst the WindyBoy's progressively falls.

The significance of this was great: up until this time I had always operated with a SunnyBoy using MPPT mode and it had caused me problems in the dry months.

In the first summer, from beyond the middle of July, the operating voltage rose to be too high to keep the turbine running. In the following year, having obtained an 18 pole stator to use in place of the 42 pole one, voltage was kept down and this enabled generation to continue all through the second summer.  But there was the hassle of needing to change stators. My excitement about the WindyBoy came from anticipation that I might in future be able to keep operating without need of having to change the stator.

So how are things working out ? - not quite as promisingly as hoped is the short answer!

The issue centres on speed.  Whilst WindyBoy does keep voltage lower it does so by imposing such load on the SmartDrive that shaft speed is reduced significantly. This effect can be seen in the plot below:

The problem with this reduction in speed is that it makes the pelton operate at an rpm where maximum energy is not extracted from the head and flow available.

One way around the problem is to re-program the 'table' in the inverter, by which I mean re-program the algorithm it calls upon to work, to make it draw less current from the pma* for a given voltage when it is working at the lower end of its range.  This can be done, I think, and I'm beginning to explore how to do it.

Another way which is simpler is to tinker with the SmartDrive.  By placing a packing washer on the shaft so that the rotor is made to stand off a bit when the knob is fully tightened, the operating rpm is lifted back toward the speed at which the pelton functions most efficiently.



Doing this is not, strictly speaking, increasing the air gap as is commonly done with axial flux pma's to adjust their output.  Rather, in a radial flux, "brake drum" type pma which the SmartDrive is, it is degrading the flux linkage between the rotor field and the poles of the stator**. But it has the same effect as increasing the air gap, which is to reduce the voltage generated at a given rpm, or, as happens when feeding the output to an inverter in turbine mode, giving the same voltage at a raised rpm.  

A few days ago when I put in the O-ring illustrated, the rpm was lifted from 829 to 897 and with this, the ac watts output from 449 to 462.  No other change was made which might have confounded these results, and in particular the hydraulic parameters powering the pelton were identical before and after fitting the O-ring.

When I went on to put two such O-rings in, the effect was not beneficial at all: ac watts became worse than with no O-ring despite rpm having risen to almost exactly the theoretical optimum speed for a Powerspout pelton on my site. The explanation for this is that the gain from the pelton operating at its 'sweet spot' speed was outweighed by poorer performance from the SmartDrive.   So the method is not straightforward.  It involves trial and error to find how much packing will benefit pelton performance before that benefit is cancelled by degraded performance from the SmartDrive.

Summer is still in its early stages here in Wales.  It's still too early to say whether my excitement 6 months ago about operating with WindyBoy in the dry months was premature.  I'll see how things go as flows diminish further and continue to play around with rotor packing. 

The experiment I have running in parallel, of operating with variable head, seems to be going OK. It allows a much more 'hands off' approach to changing nozzles as flow diminishes, and significantly, it allows use of all the flow.

The stimulus behind all these efforts is to squeeze as many kWh's out of this 'low flow' time of year as I can, - all in the hope I'll hit my 4 MWh total for the year!

*pma = permanent magnet alternator
** more exactly it is reducing the area of the gap field

Monitoring productivity

Those who own and those who aspire to own a water turbine are equally interested in how much electricity can be expected. This concept of electrical productivity must not be understood as being the same as the installed capacity of a scheme, which is the maximum power it can produce from the head and flow available at the site.  Whilst not the same as installed capacity, clearly installed capacity is a major determinant of productivity. 

Rather, productivity is a measure of how well the installation performs over time and many are the factors beyond installed capacity which contribute to it.  To give an answer to how productive a scheme is requires some way of capturing energy output with respect to time and then a way of presenting it.  In this diary entry I want to outline how I record and display such information for my scheme.

The two plots I find most useful are these:

1. Cumulative energy output displayed by year:

2. Daily energy output displayed by year:

These two plots both use the same data. It is captured automatically from the inverter via a Bluetooth feed to a desk-top display device.  The inverter I use is a German SMA WindyBoy and the desk-top device is an SMA Sunny Beam. The latter is now not marketed and because it is no longer supported by SMA with updates, the latest versions of some computer operating systems are not able to download data from it.  I have to keep alive an old laptop using an out-of-date operating system just for the purpose of downloading data from the Sunny Beam.

Generally though, the system has proved very robust. It seamlessly accepted energy data from a new solar installation I had put in last year, capturing and displaying this on Sunny Beam alongside the hydro data.  The Sunny Beam has memory capacity to store several months of hydro and solar generation before it starts to over-write the oldest data.  My habit is to download data to a computer each month where it can be displayed in an MS Excel spreadsheet and then used to construct graphs such as those above.

As data from successive years is added, so the usefulness and interest value of keeping these records increases.  Each year is different depending on how much rainfall there is, yet also there are regular patterns to each year as can be seen in the daily energy output graph above.

In the cumulative graph, it is the slope of the plot which reveals the rate at which energy is being harvested: the steeper the slope in comparison to previous years, the more productive that year is proving to be.  As can be seen, the present year is, so far, better than the two previous ones but the daily energy plot shows that as this month of June starts, the daily yield is dipping below what it was in 2013-14.  The next four months, which are the drier months of the year, will determine how the whole year works out. Whether I can keep generating through August and September will determine if I reach my target of 4000 kWh in the year.

Collecting this data adds greatly to the enjoyment of operating a Powerspout and the technology required, though sophisticated, is not expensive.  Market competition has kept prices down because it has been developed for the mass solar market. It is certainly something worth considering.