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"Modern string inverter"? Isn't that an oxymoron?First of all, the test procedure DID imitate a "Global MPPT" algorithm. All data points where the Fronius inverter was off-MPPT (i.e. when it was causing the "Christmas light effect") were excluded from the results (even though it's arguable that these are valid data points). NREL noted this in "Inverter MPPT Error" section of the study, stating that there would be another 2% energy harvest gain for the microinverter system in the moderate shade scenario, if they had included these MPPT errors.
Regardless, that "Global MPPT" algorithms or dual-MPPT channels address shade mismatch are misleading for two reasons:
(1) The string-level MPPT will still "turn-off" any shaded sections of the array (using a bypass diode), even though there is a lot of diffuse light still reaching the shaded section. The result is that the string-level MPPT is ALWAYS exacerbating the impact of shade.
(2) The "Global MPPT" is still limited to the input voltage range of the inverter, so the term "Global" is really just marketing spin. And, this has a further consequence that the effectiveness of "Global MPPT" algorithms will be more limited in situations where the string is short or the weather is hot.
To summarize, if you rely on bypass diodes and "Global MPPT" algorithms to address mismatch effects, you will ALWAYS be at a disadvantage to micro inverters. It's only a question of how much.
And, let's not forget that shade is only ONE source of mismatch. In fact, there are many forms of mismatch within a string (i.e. series mismatch) and across an array (i.e. parallel mismatch):
- series and parallel shade mismatch
- series and parallel temperature mismatch
- series and parallel manufacturing mismatch
- series and parallel degradation mismatch
- series and parallel soiling mismatch
- parallel mismatch due to voltage gradient across an arrays (i.e. string voltage varies depending on array layout)
In ALL cases, microinverters would reduce or eliminate these effects. Again, it's only a question of how much of an effect is it?
And one other point about "Global MPPT" algorithms-- By constantly "sweeping" across the I-V curve, they are going off MPPT on a regular basis, which is an inherent imperfection in the way they track. For systems with ideal solar access (which is where string-based inverters are supposed to shine), "sweeping" will drag down conversion efficiency. One last time, the only question is HOW MUCH?