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.

Monday, 20 October 2014

Down hill all the way.

I've just measured available flow this morning and it's up to 0.85 lps, so getting closer to the minimum of 1.2.  

Whilst the turbine remains inop therefore, let me continue the discussion about efficiency:

Having determined by experiment the overall efficiency to be 0.5 (see last post), one can speculate as to what the component efficiencies contributing to this overall figure might be.  Bear in mind though: these are either estimates, culled from the literature as being 'typical figures', or calculated efficiencies specific to my installation.  Their only justification is that collectively, when multiplied together, they give the overall efficiency established by experimental means: 0.5.
  • penstock efficiency @ 3 lps                      0.95
  • manifold efficiency                                    0.98
  • nozzle efficiency                                        0.98
  • Pelton turbine efficiency                           0.77
  • turbine / alternator drive efficiency         1.00
  • SmartDrive PMA alternator efficiency    0.80
  • transmission line efficiency                     0.98
  • SMA 1200 inverter efficiency                  0.90
What happens to these different efficiencies at flows below the design flow, which for my scheme is 3 lps ?
Some will improve, such as the hydraulic related components and the transmission efficiency, but the pelton efficiency and the inverter efficiency will suffer, most especially the latter.  Let us see why:

Below is a copy of SMA's efficiency plot for the Sunny Boy 1200 and on it I have annotated real data taken from my scheme's first year of operation.  What can be seen is that when PAC[W] is low, the efficiency drops off markedly.  In fact at all points, the inverter comes nowhere close to operating at its best efficiency because the input voltage, which is between 300 and 379 V DC depending on power generated, is not the optimum voltage which the inverter likes to see.  

SMA SunnyBoy efficiency curve:

























So the message is: at those times of the year when there is low flow / low power generation, the efficiency of the scheme slides down hill rather fast.  Just when you want to get as much power from your diminished flow as you can, you end up getting proportionately even less.  

To put it another way, for my scheme it takes 18 m³ of water to generate one kilo-watt-hour when the flow is 1.2 lps but only 13  when the flow is 2.7 lps.

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