Micro Inverters VS String Inverter (The Shading Debate is Over)

robby

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Most of us know the Pluses and Minuses of Micro Inverters. The one Major selling points of Micro Inverters has recently been busted as a lie. I think most of us knew this, but here it is being proven in a properly controlled environment.


 
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meetyg

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Modern solar panels have at 2-3 internal strings with bypass diodes to mitigate shading issues, so I'm not that surprised that there wasn't much of a difference.

They are designed not to drop voltage if a cell or cells of a certain string are shaded.
 

meetyg

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What exactly is the debate? What are the major selling points?
It is common to say that the use of micro-inverters causes less power drop if a panel (or part of it) is shaded, as opposed to multiple panels in a string using a string inverter.
Again, with modern panels with proper bypass diodes, this is less of a problem, from my understanding.
 

DerpsyDoodler

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It is common to say that the use of micro-inverters causes less power drop if a panel (or part of it) is shaded, as opposed to multiple panels in a string using a string inverter.
Again, with modern panels with proper bypass diodes, this is less of a problem, from my understanding.
Thanks for that. Being new to this space, I'm not fully up to speed.
 

Bluedog225

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I thought a major selling point of microinverters was the avoidance of DC arc and the ability to take advantage of the well-established safety features offered by alternating current.
 

meetyg

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One real advantage of using microinverters is that they output household AC voltage, and not high DC voltages usually paired with string inverters. This is considered safer, and you can use cheaper wiring. But cost wise you will need a microinverter for each panel (or each 2-4 panels, as some have multiple inputs) so I'm not sure cost is the real issue here.
 

Hedges

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Two parallel strings of PV panels into an SCC (or GT inverter), and the algorithm matters. It may or may not do a good job of finding max power when some panels in one string are shaded. It may stop at full voltage, very low current because that is a local maxima.

One user observed excessively low output under those conditions. I proposed he test completely covering some panels with an opaque material, see if MPPT algorithm then finds the lower voltage, higher current maximum power point.

With string inverters it is easier to select an appropriate amount of over-paneling, and to use multiple angle panels in parallel.
These days, many economical panels are so much higher wattage than the microinverters that output is clipped for several hours. And two panels in parallel (oriented for morning and evening sun) would be way over-paneled.
 

wattmatters

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Putting aside relative safety concerns with household AC v high voltage DC (it's there but not a big deal if a system is well installed), the two main benefits of micro inverters are:
i. dealing with tricky rooftops
ii. expandability

1. Tricky Rooftops or similar scenarios
There are scenarios where micros make more sense, for example when putting solar PV on a rooftop with these "modern" home designs with multi faceted roofs, multiple orientations etc with different amount of space available for panels. Such rooftops can be PITA for string sizing with string inverters, e.g. you might have space for 4 panels west, 9 south and 5 east. That's a bugger to set up with a string inverter, even with a model with 3 MPPTs the string voltages are barely going to be high enough on the smaller strings and you can't really parallel them either unless you make the strings of the same size requiring a reduction in the available production capacity.

2. Expandability.
It's generally much easier to expand a system later on with micros than with a string system. Add another panel or three? No issue. Use a different panel because the old ones are no longer available? No problem. Use a later model of micro inverter? No problem. Have a bit of space on another orientation you'd like to add panels onto? No problem. These sort of expandability options are not always so simple with a string inverter, especially if your string is already close to MPPT voltage limits, or you just can't make the string MPPT maths work.

The primary downside is the additional cost of micros per kW and being tied to that platform.

Micros are a good solution and quality units like Enphase work very well but they are more expensive. Quality string systems are also very good. It's just a matter of assessing which is more suitable for the individual application.

Installer simplicity
There are installers here which have made the call to ditch string inverters altogether. Instead of having to stock a wide range of inverters of different sizes and sales rates, they now only need to have stock of micros and panels as they can be deployed in every situation.
 

robby

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Two parallel strings of PV panels into an SCC (or GT inverter), and the algorithm matters. It may or may not do a good job of finding max power when some panels in one string are shaded. It may stop at full voltage, very low current because that is a local maxima.
At the very end of the Video he says he is doing a video on that next and from what he hinted at it seems like it is not going to have a big effect.
 

wattmatters

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For additional reference, here are two more videos (8-9 minutes each) showing results from a similar kind of controlled testing comparing Enphase micro inverter and Fronius string systems. This is from a high quality installer of solar PV systems in Australia who installs both:

Part I

Part II
 

svetz

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Both have their places. If you know-how, you can get definitely prove either system works better with shading. But the fact is, per panel MPPT is always going to outperform a string MPPT when the panels are properly matched to the microinverter and there is variable shading throughout the day. If you have a lot of single-edge shading you might be interested in replacing the panel's diodes with maximizer chips if you have string or microinverters.

Enphase Microinverters are more expensive per watt than some inverters (but possibly less the Sol-Ark), but when you start adding in the MPPTs and other stuff it equalizes the price somewhat. But there are cheaper vendors that cost less than Enphase too.

IMO, Microinverters are simple and easy to install, have low roof voltage, no figuring out how to design strings for the MPPT, allow for different panels of different ages/outputs to work together without losses, usually have far longer warranties, they usually have the highest of inverter efficiencies, have the RSD built-in, give per panel diagnostics making problems easier to find, the system is easier to expand, they don't take up additional space like a string inverter does, are silent, decreased shipping costs, if one dies all the rest keep working, and yes they are worth it even if your panels don't have any shade.

They also have unique problems if you want to add a local ESS.
 

RCinFLA

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I don't care to get involved with the debate over benefit of panel independance of micro inverters.

But I do not like degradation/reliability prospects of electrolytic capacitors being baked in a box behind hot panels. Enphase fell flat on their face on their first versions microinverter's reliability. They have since increased the voltage rating and ripple current capability of the electrolytic capacitors used to improve reliability.

Pushing power to single phase AC has a sine squared power profile. PV panels must have a constant I_mp load current based on given illumination to produce their maximum output power capability.

It is the job of the storage capacitors to smooth out the 120 Hz ripple current from inverter grid power push from being seen by PV panels.

When electrolytic caps electrolyte paste dries out their capacitance drops. As capacitance drops, more 120 Hz ripple current shows up on panels reducing their effective output yield.
 

MJSullivan

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And then there is the issue of the per module rapid shutdown requirement of NEC 2020….
 

GVSolar

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Since I am fortunate enough to have a Schneider SW4024 system which is both AC coupled (12 Enphase m215s, 3kw REC panels) and DC coupled (2 Conext SCCs, 3kw Renesola panels), I can say with some conviction (IMHO) that both have their place - and that a combined system has advantages over either type alone. Between the two, I have over 6kw power available from what would ordinarily be a 3.5kw inverter, higher surge, plus battery backup - along with multi-directional panel orientation. One reason Mickey Mantle hit so many home runs - he was a switch hitter (in its original context).
 

Hedges

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But I do not like degradation/reliability prospects of electrolytic capacitors being baked in a box behind hot panels. Enphase fell flat on their face on their first versions microinverter's reliability. They have since increased the voltage rating and ripple current capability of the electrolytic capacitors used to improve reliability.

I thought they did the design with only ceramic. Apparently not.
In this paper, they say 50 year expected life with the electrolytics they chose.


Any chance the first versions you mention had capacitors with counterfeit/alternate vendor electrolyte? Enphase "founded in 2006", may have been back when those capacitors were affecting all the tier-1 electronics companies.
 

robby

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Another point about Micro Inverters that I think is down played is Clipping.
I would love to see that same rig put through a test during the summer and see how much effect clipping really has on monthly generation.
 

Ampster

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Another point about Micro Inverters that I think is down played is Clipping.
Clipping is a function of the DC to AC ratio which is not unique to micro inverters. It is not as easy to manage that ratio with micro inverters because there are a lot more combinations of string sizes and inverter sizes available with string inverters. Enphase IQ7 micros only have 3 sizes so matching panel and micros is more about picking the panel micro combo for the AC to DC ratio you want.
Strings of panels can be increased to subtlety increase the DC to AC ratio.
 

robby

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Clipping is a function of the DC to AC ratio which is not unique to micro inverters. It is not as easy to manage that ratio with micro inverters because there are a lot more combinations of string sizes and inverter sizes available with string inverters. Enphase IQ7 micros only have 3 sizes so matching panel and micros is more about picking the panel micro combo for the AC to DC ratio you want.
Strings of panels can be increased to subtlety increase the DC to AC ratio.
I know, but since enphase and others are strapping bigger panels on the same sized Inverters I would like to see how this plays out. Enphase likes to make it sound like it's a wash. I just don't believe that, just like I never believed the shading issue was real. It may have been valid at one point in time but certainly not in the last 15 years.
 
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