Low flow matters.

I measured available flow again this morning.  There hasn't been any significant rain, so I wasn't expecting it to be any better, and it wasn't, - only 0.9 lps.

0.9 lps is 30% of the design flow for my scheme (3 lps) and from the graph in my last post, it is clear that a pelton is still pretty efficient at this part flow.  So why can't I be generating with the 0.9 lps available ?  Why is it I'm waiting until flow increases to 1.2 lps ?

The answer lies in what happens to the output voltage of the SmartDrive at low flows.

There are two determinants* of the output voltage of any permanent magnet alternator (pma): rotational speed (rpm) and load. Thus:

• a decrease in rpm will lower the voltage
• a decrease in load will raise the voltage

Now the first of these, rotational speed, we can discount as causing voltage change: the rpm at which the pelton and alternator turn are more or less constant and determined by the characteristics of the hydro half of the installation.  Ultimately it is net head which fixes the rotational speed and because net head won't change, rpm won't either. For my set up, it is 1,200 rpm.

So any change in voltage has to result from the second determinant: a change in load, and the only load on the Smart Drive is the inverter.

Recognising that the inverter and pma interact with each other is a key understanding. The inverter, far from being a fixed load of so many ohms, is instead a smart device which can change the load it puts on the system. This is seen in two ways:

1.  a steady 'hunting' of voltage as the MPP tracking function of the inverter continually seeks the optimum voltage and current combination.  This can be seen in the following plot where a SmartDrive with nominal output voltage of 220v dc is seen to hunt up as far as 285 v and down as far as 200 v over the period of time the record was made.

2.  a step change in voltage when nozzles are changed to generate either more or less power according to available flow.  Smaller nozzles generate less power and cause the system voltage to rise. This occurs because the inverter, working in tandem with the pma, presents itself to the pma as a reduced load.  

Thus in my scheme, when power out to the grid is high: 713 W, system voltage is low: 295 ~ 315 v dc (the variability being due to MPP hunting as above), but when ac power out is down: 240 W, system voltage rises to 360 ~ 379 v dc.

The effects of these higher operating voltages are several:

• higher transmission voltage so less line loss - GOOD
• higher dc input voltage to inverter reduces its efficiency - BAD
• dc input voltage rises above inverter MPPT voltage range (100 v ~ 320 v) - BAD
• Powerspout's V-clamp starts to dump some of the generated power - VERY BAD

So, with the final point, we have at last arrived at the main reason why 1.2 lps is the lowest flow I seem to be able to operate at, - any less and the system voltage rises to be too high, too much of the time: the V clamp spends so much time dumping power that only a small amount (150 W at 0.83 lps) ever gets into the grid through the inverter.

Here is a record of the power output into the grid at two flow levels: 1 lps (237 W) and 0.83 lps (150 W), showing some dumping at the higher level, evidenced by the occasional, random, downward spikes, and at the lower level, the pattern created by more frequent dumping:

Perhaps I should consider buying a different Smart Drive core for use at low flow times of the year: a core wound so that its output voltage stays around 300 v dc when power generated is in the range 100 ~ 260 W.  

They're available from EcoInnovation at $US 199 plus freight.

And that's what I like about this game, - always something new to consider !  Always a new reason to spend !

*There are others, eg magnet field strength, width of air gap between stator coils and rotating magnets, number of poles and whether connected delta or star, but these are not variables in a Smart Drive pma which is installed and functioning.

One jet or two ?

In their literature, EcoInnovation recommend that Powerspout peltons are not operated on one jet if the power output from the Smart Drive exceeds 400 W.  This is presumably because of mechanical stresses on the root of each runner cup.

For a GE 400, (or the later Powerspouts which are directly grid connected using an Enasolar inverter, without the V-clamp board which is found in the GE 400), 400 W DC output from the Smart Drive equates to about 330 - 350 W AC from the inverter to the grid. The actual figure will be dependent on transmission losses and inverter efficiency at this power level.

The plot below is taken from the literature of a long established UK manufacturer of water turbines based in the Lake District and shows how, for their turbines, there is an efficiency benefit to be gained from delivering the flow via two jets rather than one when the flow is toward the low end of design flow.

Assuming what is true for their twin jet peltons is also true for a Powerspout  and applying the information from this plot to my installation, it would mean that at 40% of full flow (1.2 lps / 3 lps) I would gain 3 % in efficiency.  In power terms this equates to seeing 250 W rather than 243 W into the grid.

So the message would seem to be: at those levels of flow where EcoInnovation say it is OK to run on one jet, it is actually more productive to still operate with two.
Correction added 28 May 2015: this is an erroneous conclusion. Please refer to later addendum "One jet or two - the bigger picture"

I have to admit though, - I haven't tested this experimentally.  It would be very difficult to cut 3 nozzles sufficiently accurately to ensure that the sum of the flow of two of them was exactly equal to the flow of the third.

In the next post: since the above plot shows that a pelton remains reasonably efficient down to a part flow of 20%, why can't I operate mine below a part flow of 40% ?

A conundrum solved, - possibly.

Throughout the first year of operation, there had been something niggling me and needing an explanation:  I had noticed there was a difference between the power generated by the top nozzle on its own compared with when the same nozzle was in the bottom position.  In the top position, the power from the inverter into the grid was about 20% less.

The factors influencing the efficiency of a pelton in converting energy in its water jet into rotational energy in its shaft, constitute a complicated science, but one of the key factors is getting the jet to hit the buckets of the wheel (also called a runner) in exactly the right spot.  

What I discovered on dismantling the Powerspout for its summer service was that the positioning of the centre of the shaft was not at the midpoint between the two tangents which the jets form with the 'pitch circle diameter' (pcd) of the pelton runner.

To be sure of the line taken by each jet to make the above measurements, each jet was given two nozzles, one inside, the other outside the casing, each having small diameter holes. In this way parallax error was minimized when sighting through them.

I have come to the conclusion that this asymmetry accounts for the power discrepancy I had seen, yet just as soon as reaching it, I doubt it, simply because the folk at EcoInnovation who designed the turbine know their stuff and are unlikely to have allowed such an error of geometry.  In part it could be explained by there being some play in the nozzle holders where they pass through the casing: by positioning the holders at the extremes of their play, the difference in the dimensions as given above can be made smaller, though never eliminated.

As can be seen above, it is not obvious that the dimensional difference is visible from the splash pattern of the two jets.  Where it does become obvious is when changing nozzles: the clearance between nozzle and runner for the upper is less than for the lower, making it more fiddly to do.

It would have been a simple matter, before having the new stainless bulkhead cut (see last post), to adjust the CAD drawing so the shaft and bearing housing were better centred, - but I hadn't solved the conundrum before having it cut.  

So it is going to have to remain as one of those efficiency losses which are an inevitable part of all machines.  To minimise the effect of it, I make sure that I always have the bigger of the two nozzles on the bottom where it will produce better output.  

In the next post, we'll look at the merits of one nozzle vs two nozzle operation.