Grid tie/SW [RE-wrenches]

[Rob Wills]

Advanced Energy strongly supports the work to get inverter efficiency
measurement on a level playing field.

A few points:

1/ Correct and accurate measurement of efficiency is a $10,000+ proposition.
We had fuel cell inverters measured at Plug Power and at GE Research labs
using the same, expensive equipment.  Result - about 4% difference in
measurements.  The reason turned out to be that one setup's current probes
were loaded by the Yokogawa power meter too much, but getting to better than
about 2% is still questionable.  The overall accuracy is a function of the
accuracy of all four (Vin, Iin, Vout and Iout) measurements. A complicating
factor is that AC ripple current either on the PV input, or on batteries (if
any), results in reduced system efficiency but is not part of the inverter
loss per se.  Average measurements (i.e. the multimeter approach) can give
wrong system numbers as they do not catch the i2r losses due to ripple
currents.  This is especially so for battery power measurement.

2/ The bottom line - why efficiency is important - is that efficiency
effects our total energy harvest.  The year average efficiency is what we
are looking for.  This can be calculated as the sum of the efficiency at
each power output level, times the percent of time that that power is
running during the year.

3/ The best measure of efficiency to my mind is something like the European
rating - a weighted sum of the efficiency at various power levels that
reflects the annual operation at various power levels.

4/ The final resolution of this issue will probably be that an independent
lab (UL or Sandia or Enertech) will measure efficiency at 10-100% of output
power in 10% steps, and that these numbers will go into a formula that
represents average operation for a "typical" site.  This would become the US
standardized efficiency rating.

5/ A real or really good simulated PV source must be used for the
measurement else input ripple current losses will not be correctly allowed
for.  Results may be different for different types of PV (with different
fill factors)

6/ The accuracy of the MPPT algorithm is also important to the bottom line
efficiency.  This should also be tested.

Rob Wills

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