The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Wednesday, 29 October 2014

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.

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