SolarEdge question

[ggunn]

I have a design on my plate using a SolarEdge inverter and bifacial modules, specifically the Sirius ELNSM54M-HC-410. Maximum string lengths (STRMAX) for a given optimizer is a maximum connected DC power divided by a single module power. The module data sheet shows FRONT STC as 410Wp and BACK STC as 287Wp. Incidentally, this system will be a low slope flush mounted array, so I do not anticipate much contribution from the back side of the modules.

Here's the question: for STRMAX calculation purposes are these 410W modules, 697W modules, or something in between?

 

[jaggedben]

That's a good one. I think if you want to be sure the Solaredge warranty isn't affected they are 697W modules. Unless they tell you otherwise.

 

[ggunn]

That's a good one. I think if you want to be sure the Solaredge warranty isn't affected they are 697W modules. Unless they tell you otherwise.

I haven't run the numbers yet but I am concerned that doing that may drive STRMAX below the minimum string length.

 

[Zee]

All I know is to expect 5-30% extra for bifacial, per Google AI.
The rated 287W seems very high at 287W/410W = 70% .

My guess , either find Code (doubt it addresses this) or find the absolute maximum gain if it were on a white roof tilted way the hell up.

Maybe use micro-inverters instead? Then it limits the issue to clipping at worst.
Not to mention avoiding solaredge.

Or maybe another RSD solution. I used SMA inverters plus APsmart or something on a big bifacial job tilt racked over white metal roofs.

What pitch are they at?

 

[ggunn]

All I know is to expect 5-30% extra for bifacial, per Google AI.
The rated 287W seems very high at 287W/410W = 70% .

My guess , either find Code (doubt it addresses this) or find the absolute maximum gain if it were on a white roof tilted way the hell up.

Maybe use micro-inverters instead? Then it limits the issue to clipping at worst.
Not to mention avoiding solaredge.

Or maybe another RSD solution. I used SMA inverters plus APsmart or something on a big bifacial job tilt racked over white metal roofs.

What pitch are they at?

The array will be flush mounted on a roof with 5-10 degree pitch. Modules and inverter are set - not my decision. I agree that the backside Wp seems very high. There is nothing that I can find in 690 which references bifacial modules.

 

[jaggedben]

Maybe try to get the module manufacturer to stipulate an appropriate STC to use given the mounting specs?

 

[solarken]

I have a design on my plate using a SolarEdge inverter and bifacial modules, specifically the Sirius ELNSM54M-HC-410. Maximum string lengths (STRMAX) for a given optimizer is a maximum connected DC power divided by a single module power. The module data sheet shows FRONT STC as 410Wp and BACK STC as 287Wp. Incidentally, this system will be a low slope flush mounted array, so I do not anticipate much contribution from the back side of the modules.

Here's the question: for STRMAX calculation purposes are these 410W modules, 697W modules, or something in between?

Use the Solaredge designer tool and if that module is not in the library pick list, add it manually using the data sheet info. If this is flush mounted on roof I would not worry about any bifacial increase from the back side. I would use 410W.

 

[ggunn]

Use the Solaredge designer tool and if that module is not in the library pick list, add it manually using the data sheet info. If this is flush mounted on roof I would not worry about any bifacial increase from the back side. I would use 410W.

It's not that I am concerned that the back side would contribute much if any additional current to Isc given the nature of the array design, it's that I want to make sure that I don't put the SolarEdge inverter and optimizer warranties in jeopardy. Your point is well taken, however; if the SE design tool passes a design it would be something that could be used to defend it should the necessity arise.

 

[steve66]

The back side would never get 1000W/m2 unless the panel was mounted upside down, so I would also ignore the 287 watt back STC.

That brings up the question - which locations in the US actually get 1000W/m2 even on the best day? My guess is not many. So in reality, your peak output will probably be less than the 410W.

So using the full front rating with a little backside gain seems like it would be safe.

 

[ggunn]

True, STC conditions rarely if ever exist in the real world and the back side of the modules won't get much if anything in this array, but that isn't what I am concerned about. I want to make sure that I don't design a system that will put the equipment warranties in danger.

 

[steve66]

True, STC conditions rarely if ever exist in the real world and the back side of the modules won't get much if anything in this array, but that isn't what I am concerned about. I want to make sure that I don't design a system that will put the equipment warranties in danger.

Understood, but I would assume (maybe verify is a better word) any solar inverter worth its salt would simply not produce enough power to put it at risk of failing.

It seems like undersizing compared to the PV rated power is a common thing;

Lesson 5: Solar inverter oversizing vs. undersizing

Undersizing a solar system inverter is a smart choice when building a solar system because that actually increases the daily amount of power produced.

freedomforever.com

 

[jaggedben]

Understood, but I would assume (maybe verify is a better word) any solar inverter worth its salt would simply not produce enough power to put it at risk of failing.

...

We're talking about an optimizer. (From a company that, imho, strains to meet the criteria you describe.)

 

[ggunn]

Understood, but I would assume (maybe verify is a better word) any solar inverter worth its salt would simply not produce enough power to put it at risk of failing.

SolarEdge inverters are different, but regular string inverters will do that by moving the max power point to a higher voltage to limit the current input (clipping), although most of them have a limit to how much they can clip.

 

[pv_n00b]

Dealing with bifacial modules introduces a lot more moving parts than monofacial modules.

Comparing modules
Compare modules using the STC ratings, that's really what STC was developed for.

Design
The NEC does not provide specific instructions for bifacial modules other that to say follow the manufacturer's documentation. The manufacturer's documentation up to now has been less than helpful on this point due to the battle between marketing wanting to show large backside gains to sell modules and engineering wanting to show reasonable gains to help people design reasonable systems. Leaving it up to our imagination how to do it. Do the best you can and be ready to defend it with an AHJ who asks. There is currently a push happening in UL to update the module standard to require better design information in the module documentation.

Inverters
Inverter manufacturers can have their own requirements for design that don't follow the NEC or the module manufacturer. It's important to get the inverter manufacturer's guidelines on designing with bifacial modules and follow them. If the inverter manufacturer has design software use that to size the strings and make a copy of the results in the event of a warranty claim. If their design tool said it is a valid design then they won't use the module and stringing as an excuse to refuse a warranty claim.