By Paul Grana, co-founder of Folsom Labs
Everyone knows that optimizers and micro-inverters solve mismatch. But this leads to a couple follow-up questions: are these module-level power electronics (MLPE) the only way to solve mismatch? And is mismatch a binary thing where you either have it or you don’t? In particular, you might wonder how much benefit you get from string-level (but not module-level) optimization. Luckily, we have the tools to figure out the answers to these questions.
It’s best to start by getting a better sense of how mismatch behaves. We can take two scenarios with “hard” shade from obstructions: one with a pole (that will cast a longer, skinny shadow), and one with a nearby building (which will cast a shorter, fatter shadow). Then, we change the size of the maximum power point tracking (MPPT) zone: from a central inverter (300kW), to zone-level MPPT (30kW), to string-level MPPT (3kW), and finally module-level MPPT (300W). These have the nice benefit of each being 10 times larger than the other—so for the nerds out there, this gives us a chance to use a log-scale chart!
String-level MPPT reduces mismatch by 60% compared to a central inverter with wall shading, and by 70% for the pole shading. And even compared to the zone-level 30-kW mismatch, a string-level MPPT reduces 50% of the mismatch from shade in both scenarios.
In fact, each change in size of the MPPT tracking zone results in an improvement to the mismatch losses—there isn’t a ‘magic’ MPPT level that is clearly better than others.
It’s worth mentioning that there is a component of mismatch that can’t be addressed by string-level MPPT. “Soft” mismatch is statistical mismatch that is applied to modules for small things like soiling (e.g. bird droppings) or vegetation (e.g. a weed). Because these are unplanned, they are applied as statistical randomness across all modules in the array evenly. This creates series mismatch but not parallel mismatch—and it’s the series mismatch that the string-level MPPT can’t fix. As a result, this soft mismatch persists until the array has module-level MPPT:
Module-level optimization is great, and keep in mind that optimizers and microinverters have a ton of additional benefits beyond mismatch mitigation (including safety, design flexibility, electrical BOM benefits and data visibility). But it is striking that a string-level MPPT approach can mitigate a significant amount of an array’s mismatch from shade.