Photovoltaics: String vs. Microinverters
CZ-4. Southern Indiana. New Construction Ranch. Garage Roof faces 30 deg west of south (210 deg). NO shading from trees now or in future. About 36 Grid-Tied Panels. No battery back-up. A purchase arrangement, not a lease type transaction. Three major installers in the metropolitan Louisville, KY market.
Two competing PV installers. One for Microinverters ; one for Strings with single inverter/optimizer.
Of course each touts the advantages of what they sell and the disadvantages of what the other guy sells; I’m left confused. I’ve listed some plusses and minuses of each system below, reflecting my understanding or lack thereof.
Pluses for the Microinverter:
Each panel performs separately so that any shading on one panel does not affect the performance of the other panels (including clouds).
Can monitor the performance of individual panels.
Minuses for the Microinverter:
More complex with one micro inverter per panel and more prone to failure.
That’s lots of electronic equipment mounted on the roof and any failures require much unwarranted labor to disassemble much of the system to get to the failed panel.
The vast majority of failures are of the microinverter, not the panel itself.
Pluses for the String Inverter/Optimizer:
Simpler, less prone to failures, failed equipment is more readily accessible (not on the roof)
Minuses for the String Inverter/Optimizer:
The performance of the entire string is restricted to the performance to the weakest panel in the system.
Any shading of one panel (including clouds) thereby reduces the output of all panels.
I’m not sure how one monitors the performance of the system or individual panels.
Would someone please compare and contrast the two different systems, perhaps correcting misconceptions and providing additional information where needed to help me decide which system to go with.
Also, What type (wavelengths) of light do PV panels convert into electricity ? Or more particularly… Does the shading caused by clouds cause decreased performance of PV panels ?
I have read that the cost/watt of PV systems has been falling and have seen several suggestions of current pricing [Scott Gibson’s “Cost of Installed Solar Continues To Fall”, 8/28/15]. I’d say that southern Indiana is NOT a developed market. Projecting his graph ahead leads me to believe that I should be able to get installed PV for about $3.50/watt. Is that a reasonable conclusion ?
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Replies
Ted,
Your analysis of the pluses and minuses of microinverters seems accurate to me.
Q. "What type (wavelengths) of light do PV panels convert into electricity?"
A. See the graph on this page:
http://www.pveducation.org/pvcdrom/characterisation/spectral-response
Q. "Does the shading caused by clouds cause decreased performance of PV panels?"
A. Of course. You'll get the highest output on a cold day, when sunlight is unobstructed. Cumulus clouds that aren't blocking the sun can sometimes increase the output of the PV array, because sunlight bounces off the cumulus clouds (which act like mirrors) and increase the amount of solar radiation hitting the array.
These links seem like good additional reading:
https://www.energysage.com/solar/101/string-inverters-microinverters-power-optimizers
http://news.energysage.com/microinverters-and-power-optimizers-future-of-home-solar-power/
https://www.altestore.com/blog/2015/08/micro-inverters-optimizers-and-string-inverters-oh-my/
Other good factors to consider would the reputation of each installer and the costs of each system.
Our system was originally spec'd with a SMA string inverter, but our installer switched to a system with SolarEdge optimizers/inverter...with rapid shutdown on the inverter to meet new electrical codes. While I don't have numerical evidence, I think the power optimizers have been helpful with our small amount of tree shading and snowfall on panels.
Cost was $3.66/watt with 280W LG panels and SolarEdge optimizers/inverter. This is in NH, contract with price was signed March 2015.
Shading of individual panels by a cloud is a red herring. The edge of a cloud will not stay stationary for any length of time. Shading by a tree or neighboring house might be an issue, but not individual clouds.
Generally, I give some weight to simpler designs.
The capabilities of optimizers and microinverters to deal with partial shading are almost the same. The advantage of a microinverter in that regard is with respect to a single central inverter with no optimizers. If I understand right, neither installer is proposing a central inverter without optimizers. And you don't have any partial shading anyway. So there is no significant difference there.
It is true that having the whole microinverter on the roof rather than just the optimizer means there is more stuff up there. But either way, you have some circuitry up there, and you are counting on it being reliable. I would compare the microinverters vs. the optimizers by looking at:
-Length and terms of the warranty.
-Experience of the company making the microinverter or optimizer---have they have their products in the field for long enough to have learned what the failure modes are and to have fixed them?
-Financial stability of that company--if they have a 25 year warranty, will they still be around in 25 years? It's hard to be sure any company will be around that long, so the likely longevity of the company might matter more than the number of years the warranty says.
In thinking about the long term reliability, I think that one advantage of microinverters is that each one is independent. If one fails in 15 years, and the company that made it no longer exists, you could swap in a new mircoinverter made by someone else, and run a mixed system. That's not possible with the optimizer system--if that company doesn't exist anymore, and you can't find a replacement for a failed optimizer, you might not be able to get something compatible to get the system running again--worst case you'd need to replace all the electronics and keep only the panels.
To be fair, I should disclose that I have worked with a microinverter company, so I might be biased toward them.
An additional wrinkle to my question:
In the longer term I would like to add an Electric Vehicle to the mix. There is another section of roof which faces precisely the same direction as the panels above and at the same roof pitch but is somewhat smaller. From reading all the references above, and others, I gather that a system using microinverters simplifies that process. It also appears that microinverters coax a bit more kwh out of a system than Optimizers, possibly reducing some equipment costs; However, a system using microinverters remains the most expensive of the options.
Does anyone else have any additional comments or advice ?
We chose microinverters for our last PV array for a couple of reasons. First, we didn't have a good place near the electrical breaker box for an inverter and wanted it outside. Central inverters that should work in minus 30 F or colder weren't readily available. Second, the microinverters (Enphase) have a 25 year warranty, rather than the 5 or 10 year warranty of most central inverters. And third, it is easy to add a few more PV panels at a time with the microinverters, instead of having to grossly oversize a central inverter for possible future additions. (Our previously installed PVs and wind turbine are in a battery based system so that was a different situation.)