Micro-Inverters in the Attic

GoldDigger

Moderator
Staff member
we have a very good design for sealing each point of entry but thank you for the concern, and as in an above reply, a string inverter is too uneconomical to consider for our particular system.
1. What factors (partial shading, multiple orientations, rapid shutdown requirements, plans for incremental additions later, etc.) do you see as making a string inverter uneconomical in your situation?
2. I have, in the past, not heard many good things about thin film systems (efficiency, longevity among other things.) Have there been qualitative improvements recently?
 

Adamjamma

Senior Member
My only comment about it is that you should look at cooling options in that attic area now... between the extra heat build up due to the black panels on the roof, and the extra heat from the inverters, you now have added to the heat buildup already there from the building itself.
But... the cooling can be as simple as old computer fans hooked up directly to a solar panel so they only operate when it is sunny... which is when they are needed..lol...
 

Adamjamma

Senior Member
the thin film system on a crimped metal roof with piping under the roof to provide water heating is what I am looking into for installation over the sips panels... but was designing to use a cupola style roof system to have the cupola enclose both the water tanks for the solar water system and the extra power generation from using the waste building heat exhaust.. but still theoretical stages so interested in hearing the actual reports on thin film as most people have pushed the framed panels as the way to go but have never seen any actual tests either way.
 

jaggedben

Senior Member
1. What factors (partial shading, multiple orientations, rapid shutdown requirements, plans for incremental additions later, etc.) do you see as making a string inverter uneconomical in your situation?
2. I have, in the past, not heard many good things about thin film systems (efficiency, longevity among other things.) Have there been qualitative improvements recently?
My guess would be this is a really small system.
Why did you bring up thin film?
 

MikeNZ

Member
Location
Burleson, Texas
Our system is an asphalt tile replacement system, lots of small modules hooked up in parallel strings enough to run a microinverter. rapid shutdown rules dictate the size of the strings, also the most of the roof will be covered so none of these strings (we figure about 8-10 on a small house) will have the same voltage.
we know about the efficiency values of our thin film cells, we have other non-PV aspects to the system to boost the total efficiency.
As for cooling the attic there will be attic ventilation installed if not already present, potentially even forced ventilation (we haven't decided on that yet)
This forum has been an incredibly useful source of expertise, keep up the questions, we want to make sure we have all concerns addressed before making final decisions on this system.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Our system is an asphalt tile replacement system, lots of small modules hooked up in parallel strings enough to run a microinverter. rapid shutdown rules dictate the size of the strings, also the most of the roof will be covered so none of these strings (we figure about 8-10 on a small house) will have the same voltage.
we know about the efficiency values of our thin film cells, we have other non-PV aspects to the system to boost the total efficiency.
As for cooling the attic there will be attic ventilation installed if not already present, potentially even forced ventilation (we haven't decided on that yet)
This forum has been an incredibly useful source of expertise, keep up the questions, we want to make sure we have all concerns addressed before making final decisions on this system.
What has rapid shutdown to do with string length?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Voltage. Especiallly if the 'modules' are under 30V.
Oh, right. I remember now; I had to reduce string length on a SolarEdge system because I had more than 30 modules in a string. I haven't dealt with thin films in a while; do any of them have Voc high enough that it would be a problem for a microinverter system?
 

pv_n00b

Senior Member
Location
CA, USA
Oh, right. I remember now; I had to reduce string length on a SolarEdge system because I had more than 30 modules in a string. I haven't dealt with thin films in a while; do any of them have Voc high enough that it would be a problem for a microinverter system?
Full-size thin-film modules are usually too high a voltage for current micro-inverters, which are set up for 60 or 72 cell modules. Module voltage is basically about 0.7V multiplied by the number of cells and thin film has a lot of small cells in series that give high voltage and low current per module. But the OP is describing a system that uses many shingle size modules that will not have a very high voltage per module, probably similar to the old DOW Powerhouse system if you remember that. They are most likely wiring enough of these small shingles together in series and parallel so they get the equivalent voltage and power of a 60 or 72 cell module and connecting that sub-array to a micro-inverter.

I can only hope the OP is getting cost savings from using micro-inverters over string inverters because either they have a secret sauce no one else has or they are going down a rathole. I can see how the variability in the string length can be worked out with micro-inverters but it introduces a few problems that look to be costly to deal with.
 
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jaggedben

Senior Member
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I can only hope the OP is getting cost savings from using micro-inverters over string inverters because either they have a secret sauce no one else has or they are going down a rathole. I can see how the variability in the string length can be worked out with micro-inverters but it introduces a few problems that look to be costly to deal with.
They are going down a rat hole. Maybe Tesla will eventually get their roof tile system well organized enough to be profitable for that affluent niche market. They have the name recognition. For everyone else, this thread shows exactly why the concept is so cumbersome and will never take off.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
They are going down a rat hole. Maybe Tesla will eventually get their roof tile system well organized enough to be profitable for that affluent niche market. They have the name recognition. For everyone else, this thread shows exactly why the concept is so cumbersome and will never take off.
That's not to mention the possible listing violation using microinverters that way. They are made to bolt to the back of modules.
 

MikeNZ

Member
Location
Burleson, Texas
Full-size thin-film modules are usually too high a voltage for current micro-inverters, which are set up for 60 or 72 cell modules. Module voltage is basically about 0.7V multiplied by the number of cells and thin film has a lot of small cells in series that give high voltage and low current per module. But the OP is describing a system that uses many shingle size modules that will not have a very high voltage per module, probably similar to the old DOW Powerhouse system if you remember that. They are most likely wiring enough of these small shingles together in series and parallel so they get the equivalent voltage and power of a 60 or 72 cell module and connecting that sub-array to a micro-inverter.

I can only hope the OP is getting cost savings from using micro-inverters over string inverters because either they have a secret sauce no one else has or they are going down a rathole. I can see how the variability in the string length can be worked out with micro-inverters but it introduces a few problems that look to be costly to deal with.
our module voltage is about 70Voc max, this means every module is wired in parallel to add up the current, we could have our module voltage higher but would then require rapid shutdown switched on the roof, where there is no space for them. the only way we can abide by the rapid shutdown rule is by keeping all voltages on the roof below 80V, and below 30v (zero in our case) from 3' from the point of entry for each string. so either we run each string (8-10 for a small house as i mentioned before) through a RSS then outside in metal conduit then invert them outside somehow, or we install our micros within 3' from the point of entry and RSS them with those, only having to deal with AC from that point on. We did look at a string inverter option for our modules but that added about $12 per module (aka per asphalt tile exposure size) with the string inverter and rapid shutdown switches on top of that, hopefully this makes our decisions easier to understand.
 

jaggedben

Senior Member
Well, I think you're solution is code compliant with 2014 NEC and just barely with the 2017 NEC. For 2017 it's probably better if the micros are within 1ft of the roof so that they are within the 'array boundary.' Less debatable.

To answer your original question, yes you can use romex for the output, if romex would be compliant for other purposes where you're routing it.

Good luck.
 

pv_n00b

Senior Member
Location
CA, USA
That's not to mention the possible listing violation using microinverters that way. They are made to bolt to the back of modules.

I agree, unless the OP has found a microinverter that has instructions for remote mounting then this installation will not comply with the UL listing. All the microinverters I know of have instructions that say to mount them to the racking under the module. Remote mounting would not be in compliance.
 

MikeNZ

Member
Location
Burleson, Texas
Roof surfaces get really hot too. The Enphase IQ7 has a max temp rating of 65C. There are a bunch of reasons I think it's a bad idea but that's not high on the list.
Thanks for your response, one more question regarding temperature, this inverter is going to be in a metal enclosure, can this enclosure be made from perforated steel?
trying to maximise ventilation here for obvious reasons.
We intend on there being no exposed live parts to contact with the lid off, all MC4 connectors(DC) or covered terminals (AC).
 

jaggedben

Senior Member
I know of no microinverters that involve exposed live parts, so I don't know why you'd do that. As others have pointed out, what you're proposing is already dubious if the micro instructions call for mounting on the roof. I wasn't totally convinced of that being a problem, but now you propose an additional aspect that presumably isn't in line with the intended use. Would you put them in an enclosure if they were on the roof? No. I think it's even worse if you put them in an enclosure in the attic.

Are we misunderstanding? Are you designing the inverter as well here?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Long story short, IMO you have three options:

1. Use microinverters the way they are designed and listed to be used, i.e., mounted on the undersides of 60 or 72 cell PV modules.
2. Get the microinverter manufacturer to design and/or approve your alternative way of using their product.
3. Don't use microinverters.
 

MikeNZ

Member
Location
Burleson, Texas
I know of no microinverters that involve exposed live parts, so I don't know why you'd do that. As others have pointed out, what you're proposing is already dubious if the micro instructions call for mounting on the roof. I wasn't totally convinced of that being a problem, but now you propose an additional aspect that presumably isn't in line with the intended use. Would you put them in an enclosure if they were on the roof? No. I think it's even worse if you put them in an enclosure in the attic.

Are we misunderstanding? Are you designing the inverter as well here?
the enclosure is to comply with 690.31 (G), where PV system DC circuits are run inside a building they must be concealed in raceway or metal enclosures. We have our source circuit coming through the roof deck in FMC, and this connects to the enclosure to ensure there is always metal between the DC components and the environment while inside a building. this enclosure will serve to house the Romex AC connections as well as provide a much larger than normal surface area for the inverter to conduct heat into which will actually help keep it cooler, however this enclosure will need to be well ventilated to keep the internal air as close to ambient as possible. Hence the suggestion of using perforated steel for as much as practical.
 

MikeNZ

Member
Location
Burleson, Texas
Long story short, IMO you have three options:

1. Use microinverters the way they are designed and listed to be used, i.e., mounted on the undersides of 60 or 72 cell PV modules.
2. Get the microinverter manufacturer to design and/or approve your alternative way of using their product.
3. Don't use microinverters.
Ultimately we plan on working with an inverter manufacturer to make a product optimised for us, but in the mean time we are working with what we've got.
 
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