Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

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  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    icarus wrote: »
    ... Keep up with the postings as they continue to be interesting.

    Tony, thanks. I'm learning a lot, and, hopefully am able to contribute to others' learning too and future project considerations.

    Best regards,

    Bill
  • BB.BB. Super Moderators Posts: 26,873 admin
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    I don't know about other calculators, but the Xantrex GT/XW ones assume a 35 degee C rise (63F) rise over ambient temperatures due to panel heating by the sun (and minimum wind conditions?).

    The Xantrex GT string sizer shows for a:
    CHSM-225M (I think this for for your panel):
    Max Power Voltage - Vmp 28.66 Vdc
    Open Circuit Voltage - Voc 37.33 Vdc
    Voltage Temp Coeff - Vtoc -0.1426 V/°C
    STC Rating - Pmp 225 Wstcdc
    Max Power Current - Imp 7.86 Adc
    PTC Rating 207.81 Wptc
    Ambient Temps:
    Tmax=94F (assume temp rise of 63F/full sun no wind?)
    Tmin=-4F
    [FONT=Fixedsys]
    Number of Mods Voc Max / Vmp Min Temp
    =====================================
     8 Modules    349.98    178.58
    [B] 9 Modules    393.72    200.9[/B]
    10 Modules    437.47    223.22
    11 Modules    481.22    245.54
    12 Modules    524.96    267.87
    13 Modules    568.71    290.19[/FONT]
    
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Day 4 - Panel rails and wiring complete.

    One pair of pigtail wires (#10, THWN) are installed, from the junction box, for the 9 panel array on top of the garage. Mating connectors that will fit the connectors on the panels are installed on the ends of the wires.
    original.jpg

    Three pairs of pigtail wires (#10, THWN) are installed, from the second junction box, for the 3 sets of 9 panel arrays on top of the house. One pair goes low, another pair goes to the upper right, and the last pair goes to the far left to pick up the 8+1 panel array on the western most edge of the house. Mating connectors are again installed on the ends of the wires.
    original.jpg

    Here's an image for those interested in what's inside a Sunny Boy 7000 inverter. The back half (not visibile) is a huge transformer. The front half shows a lot of capacitors at the top, and a lot of electronics elsewhere. If I had the serial port option, an interface card would be installed just to the left of the LCD screen. At the bottom left is a 2 wire pair for solar PV current coming in to the inverter from the fused DC disconnect switch. And at the bottom right are the 4 wires (hot, hot, neutral and ground) for the single phase 3-wire 240 VAC circuit that goes first to the fused AC disconnect, then to the kWh meter, and then to the 40 A 2-pole circuit breaker in the service panel.
    original.jpg

    At the end of this day, one work week is complete.

    I'll publish the summary from SMA's String Sizing Program after I can get some help on the temperature input questions (posted earlier in the thread).

    Best regards,

    Bill
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    The temperatures you are putting in the string calculator are not the panel temperatures but the min/max for ambient. Most calculators don't adjust for roof mounting so using even 94F is going to give optimistic results, meaning in warm to hot weather the Inverter will only track as low as the Mppt range allowed, so you could have substantial reduction in harvest ... maybe 30-40% loss if your inverter is limited to 250 VDC when the VMP of the array is 200VDC. We have seen this posted here before with Sunnyboy systems with the exact same mistakes / oversight.

    Using 0F/90F for the min max looks like you need a minimum of 11 to a maximum of 13 panels in series, the Xantrex Calculator has shown to be very accurate in my experience ... 9 panels even in cool weather the vmp of the panels will be below the Sunnyboys 250V minimum tracking window ... for sure your installer goofed.

    Panels roof mounted run hotter than ground mount, typically I see 60-65C on my panels in the Summer

    If Xantrex calculator is correct, I would be very concerned that your contractor either used the 25C values only and or didn't run the string calculator. For your benefit you should do this before the panels get installed as now would be a better time to come up with a string configuration that will match the Sunnyboy's mppt window.

    UPDATE:


    I didn't realize you were in Texas, checked the numbers and your high is 106F, which what I would expect as typically Texas is a good 10F hotter then central FL ( I have family in college station so I know what your summers are like )

    You only have one option, 12 panels per string .. less and its not enough for VMP the warm/hot weather and more cold weather will be to high for VOC
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    It seems to me that the j-box method used here for tieing all the wires together is a bit shaky for a 400-600 VDC system. We always use one of the Midnite Solar high voltage combiners. One of several problems with DC is that electrolysis is much more to worry about than in AC, and wire nuts + twisted wires are not reliable.

    Or am I missing something here?
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    I agree, pretty sure the wire count isn't NEC compliant either with all those wire nuts in that one work box, to lazy to go pull out my NEC book to sight for reference ... looks amateurish in many ways ( plastic work box for the roof penetration ), footing torqued right onto the shingles with goober, not booted and flashed.

    UPDATE:

    Looked it up, for sure the box with all the wire nuts is not to code

    NEC 314.16(A) Metal Boxes and Maximum Number of Conductors

    Box looks like a standard 4x4x2.125 maximum number of #10 conductors is 16 ... there are 8 conductors plus 3 grounds = 11 but wire nuts themselves count for 1 ( the blue one probably 2 ) so you have 11 + 12 = 23 for the fill count were code allows only 16 ... so not to code.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    i also think that polyurethane sealant may not be tolerant of uv rays so a protective coating on it of some sort may be in order.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Yikes, missed that one ... yes, they are using a construction adhesive for the seal, that a big issue, you will get leaks for sure, it is not intended for roofing!

    Companys web site says that's for panels!

    http://www.stickwithpl.com/Products.aspx?ID=fef65cc9-47bf-4802-aaa9-a343f2ef9458

    They should be using

    http://www.stickwithpl.com/Products.aspx?ID=Polyurethane-Roof-Flashing-Sealant
  • icarusicarus Solar Expert Posts: 5,090 ✭✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Personally, I wouldn't allow any penetration that is not done with a proper flashing. Even roof grade silicone or tar is not a long term solution. Between temperature swings and expansion and contraction they will ALL leak, it is only a matter of when.

    I too was going to comment on the box fill numbers. That said, even it it were legal, the idea of having so many conductors with such short wires is another failure waiting to happen. While the wire nuts may make a connection, the strain is taken up by the wire nut rather than a few twists of wire, so that any little movement might lead to failure.

    Tony
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Don't know about local code where the Op is having this done but ...
    Pretty sure NEC limits you to four (pairs) wire connections per box.
    PL Premium - polyurethane construction adhesive - is definitely NOT suitable for roofing or exposed areas. I'd put some 'ice dam' membrane under each shingle where the mounts are bolted (or are they lagged?) and not slobber PL all over hoping it will provide a seal.

    What's with all those loose wires running will-nilly?

    Frankly, it looks worse than one of my own notoriously sloppy temporary it-will-do-for-now jobs. I'd be wanting my money back already.

    The project itself has the potential for being a really fine set-up. It just looks like the installer doesn't really know what he's doing. :confused:

    I could be completely wrong: I'm not on site and I don't know what's required/allowed there.

    BTW: those 'turbine' vents on the roof are useless. Looks like the builder didn't want to do proper venting.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    a0128958 wrote: »
    The lease terms motivate everyone to do the best possible job, different from well publicized scams of the past. In this case, I always pay only for what the solar PV system produces (at 70% of whatever my kWh rate happens to be). I.e., my payments are variable. I'm thus guaranteed a 30% savings on PV produced kWh usage. The terms are styled after very popular commercial terms.

    A few questions:

    So, If I understand this, the PV system has a dedicated meter, power company pays you 7.5 cents kWr , you pay the lessee 70% of this ... is this correct? How do they know what your being paid from the power company?

    Who pays the taxes on your income selling power? as your technically a power producer and there is usually some taxes on this ( income taxes as well ? ) ... this came up with someone looking into a similar deal in MA and when the posted the contact it appeared they we on the hook for this but not told about it

    What happens say in 15 years when you have to re-roof? does the lessee come out and pull the panels for you?

    Who is your installer? there was a picture with the name but looks like it was pulled?
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    I could use some help with the 'Design Irradiance' value that significantly affects the results produced from SMA's String Sizing Program.

    This is an input parameter. Default is 1000 w/m2. This value, as SMA explains, is the average value for the world, near sea level, and is a per second value.

    My question is, how do I arrive at a more representative Design Irradiance number, to put into the String Sizing program, for my specific location (75074, Dallas area, TX, etc.)?

    I checked PVWatts, but, it gives me Solar Radiation in kWh/m2/day (for my location average is 5.46 kWh/m2/day).

    Thanks!

    Best regards,

    Bill
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    Windsun wrote: »
    It seems to me that the j-box method used here for tieing all the wires together is a bit shaky for a 400-600 VDC system. We always use one of the Midnite Solar high voltage combiners. One of several problems with DC is that electrolysis is much more to worry about than in AC, and wire nuts + twisted wires are not reliable.

    Or am I missing something here?

    Agreed.

    My first impression when I looked at the pic was that those wire leads are mighty short. I normally leave at least 6" to work with.

    As to the wire fill violation that SG mentioned - that can be solved by sticking on a 4-S extension box in order to double the volume of the j-box...which then makes the short leads even more of an issue.

    I also notice that the split-bolt is situated so that the open end is in position to rub the current carrying wires. I would have A) flipped it over so the more rounded end is toward the wire, and B) pushed it to the back where it can't possibly touch a current carrying wire.

    My other first impression, when looking at the underside of the roof - where the flex goes into the bottom of the plastic pullbox - is that the bend in that flex is a pretty tight radius. Legal probably, but stuffing a fishtape through flex with a bend like that can be a pain. I generally try to keep the bends in a flex as smooth (straight) as possible. I think that one could have been straighter while still keeping the strap within 12" of the connector.

    Nit-picky I know, but having done Beverly Hills/Bel Air work for 8 years (without a single inspector correction for the last 5 years I worked there), I'm well known for my raging perfectionism...I prefer to think of it as a "craftsmanlike" attitude. :D


    Overall though, the electrical work looks pretty clean.

    However, if one of my apprentices stuck a plastic box on a roof like that...with the wrong goop and cock-eyed to boot...I'd've put his butt on ditch digging detail for a week as penance. AFTER he fixed that goofy looking box.
  • stephendvstephendv Solar Expert Posts: 1,571
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Bill, the downloadable version of the SMA tool doesn't ask for an irradience value, it lets you choose specific cities within the US and uses more accurate weather data. Unfortunately it doesn't list Dallas specifically, but you could pick a city with a similar climate...
    I downloaded this version: http://www.sma.de/en/products/software/sunny-design.html

    "Sunny Design 1.51Komplettversion inklusive aller Standorte / complete package including all locations"

    The choose custom installation and select all the countries you'd like to include.
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    a0128958 wrote: »
    I found 2 string sizing calculators on SMA's U.S. web site: a downloadable s/w package, and an online version requiring no download. ...

    OK, thanks for encouraging me to do this. I learned a lot.

    Here are the results for an SB 7000US inverter operating at 240 VAC, and Astronergy CHSM 6610M 225 W panels.

    Both programs provided meaningful and similar information, just in different formats.

    I used these temperature values for my Dallas location (75074 ZIP), looked up from weather.com:
    • -4°F coldest expected temp when sunlight will be on modules (SMA says this value should be the record low)
    • 95° hottest expected temp when sunlight will be on panels (SMA says this value should be the highest avg high on record)
    Summary key points:
    1. In the event of a power glitch, the inverter won't restart for a 9 panel array at temps above 60°F because the array's min open circuit voltage will be below the inverter's 300 V Start Voltage.
    2. The inverter will run inefficiently or not at all for a 9 panel array at temps above 40° because the array's min peak power voltage will be below the inverter's min operating voltage and/or min peak power tracking voltage (both 250 V).
    3. Even if the default peak irradiance value (1000 W/m2/sec) is lowered, to reflect non-optimum azimuth orientation and mounting angle, still, the hot Dallas summer temps prevent a 9 panel array from generating enough voltage to be deemed 'acceptable' by the string sizing program.
    4. An 11 panel array string size works well within the inverter's operating parameters.
    5. A 12 panel array will work just as well, in spite of some output power related 'cautionary' notes. These note concerns won't be realized, though, because one of the strings is in a different plane to the other two.
    6. Even if the assumed 'hottest expected temperature ...' is increased from 95°F to 104° (next highest 'step'), nothing changes for the conclusions or analysis. Array qty needs to be 11 or 12 panels.
    Detail results for 9 panels per array:
    • OK -> Array Max Open Circuit Voltage @ low temp = 394
    • Too low (below 300 VDC Start Voltage) -> Array Min Open Circuit Voltage @ high temp = 278
    • Too low (below 250 VDC Min. Operating Voltage and Min. Peak Power Tracking Voltage) -> Array Min Peak Power Voltage @ high temp = 214
    Detail results for 11 panels / array, 3 strings:
    • Cautionary -> Array Max Open Circuit Voltage @ low temp = 481
    • OK -> Array Min Open Circuit Voltage @ high temp = 340
    • OK -> Array Min Peak Power Voltage @ high temp = 261
    • OK up to 3 strings -> Inverter Max Output Pwr @ high temp = 7028
    • OK up to 3 strings -> Inverter Max Output Pwr @ low temp = 7250
    • OK up to 3 strings -> Array Max Output Pwr @ high temp = 7238
    Detail results for 12 panels / array, 3 strings:
    • Cautionary -> Array Max Open Circuit Voltage @ low temp = 525
    • OK -> Array Min Open Circuit Voltage @ high temp = 371
    • OK -> Array Min Peak Power Voltage @ high temp = 285
    • Cautionary @ 3 strings; inverter will limit to 7.1 KW -> Inverter Max Output Pwr @ high temp = 7.7 KW
    • Cautionary @ 3 strings; inverter will limit to 7.1 KW -> Inverter Max Output Pwr @ low temp = 7.9 KW
    • Cautionary @ 3 strings; inverter will limit to 7.4 KW -> Array Max Output Pwr @ high temp = 7.9 KW
    Very interesting analysis. I hope I got it right. Learned a lot in the process. Thanks for the encouragement to do this.

    Now I'll try out the downloadable SMA string sizing program, to see what it says.

    Best regards,

    Bill
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    You should use 106F, which is what your last year high was I found online, this will also help compensate for the fact you have a dark roof and roof mounted panels, both which add 10-15F to the panels actual operating temperatures as your ambient at 4" over the roof is not the open air ambient.

    The Xantrex Site is very easy to use, it shows 12 panels is your only option.

    The watt/meter squared should have no effect as that effects current mostly and very minor voltage which is what you verifying.
  • BB.BB. Super Moderators Posts: 26,873 admin
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    The BIG issue is how do you approach your installer...

    Even on an "average" summer day, the system is pretty much running in a "gray" area (nice way of saying I would not pay to have my system run in that area).

    I have compared the numbers of my system with Xantrex's high temp / low Vmp ratings/estimates are pretty close to what I see on my Hot Days with my 3.5 kW GT system...

    -5 volts on a very hot day--you might let that slide. -40 volts on an average hot day--may not be a good idea.

    If you are feeling that you need backup--you might call SMA's US support center and say you have a question about their installer's configuration. They may be willing to help (normally the installers are trained by the manufacturers of the equipment they sell/install).

    -Bill

    PS: You have 36 panels... If you could get 3x12 strings (instead of 9x4) and get one facet of 2x12=24; and the other as 1x9 -- that would be nice,

    If that does not work--then you are left with one string that has 3 panels on one facet, and 9 on the other--Is this better than what you have now? Don't know).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    ... You should use 106F

    ... The watt/meter squared should have no effect as that effects current mostly and very minor voltage which is what you verifying.

    Thanks.

    Looking further at the charts, the conclusions and analysis don't change going from assuming 95°F to the next step 104°.

    And yes, looking at it further, changing the irrandiance value around doesn't materially change the fundamental conclusions.

    I went back and edited my conclusions posting to reflect these further observations, and, I added some 11 and 12 panel analysis numbers to make the posting clearer and more comprehensive.

    Best regards,

    Bill
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Keep in mind you already have 36 panels but these strings are not all in the same orientation, so the peak power caution doesn't really apply because you can't have all the strings at max power at the same time. Good luck explaining this to your installer
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    Keep in mind you already have 36 panels but these strings are not all in the same orientation, so the peak power caution doesn't really apply because you can't have all the strings at max power at the same time. ...

    Thanks. This is a very helpful comment.

    It increasingly appears that a change from 9x4 to 12x3 will put all operating values in a 'good' range. Perhaps not perfect, or optimal, but at least 'good.' Versus some operating values for 9x4 that clearly 'won't work.'

    While I haven't been able to show it yet, since I'm not yet caught up to real time posting my illustrated chronology, in fact, the panels are not installed on the roof yet.

    Thus, a key question is, why can't the DC wiring be replaced at this point to support a 12x3 installation? Beyond cost, is there a good reason not to do this?

    (And if the DC wiring is changed, the problems many of you have identified with the wiring could be fixed.)

    Many thanks.

    Best regards,

    Bill
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Yes, by all means having the wiring modified to 3x12

    It would be in your best interest to not let the installer finish the installation as 4x9 as the performance will be well below expectations

    You need to keep each string in the same plane so you would have 24 facing south and 12 facing west if I understand you roof layout
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    I finished looking at SMA's second program to design strings matched up with their inverters. This one is a downloable s/w program that runs on your PC.

    While presentation of format is very different from the online program I first used, I found the 2nd program 'painted' the same picture: there are better alternatives than my planned 4 strings / 9 panels each design.

    Instead of putting another long summary and analysis here, I put minor edits back into my posting on the page earlier than this one.

    I'll repeat the summary here since it's where I added / clarified the most:

    Summary key points:
    • In the event of a power glitch, the inverter won't restart for a 9 panel array at temps above 60°F because the array's min open circuit voltage will be below the inverter's 300 V Start Voltage.
    • The inverter will run inefficiently or not at all for a 9 panel array at temps above 40° because the array's min peak power voltage will be below the inverter's min operating voltage and/or min peak power tracking voltage (both 250 V).
    • Even if the default peak irradiance value (1000 W/m2/sec) is lowered, to reflect non-optimum azimuth orientation and mounting angle, still, the hot Dallas summer temps prevent a 9 panel array from generating enough voltage to be deemed 'acceptable' by the string sizing program.
    • An 11 panel array string size works well within the inverter's operating parameters. A 12 panel array will work but some output power amounts are 'cautionary.'
    • An 8000 W inverter may be needed to maximally harvest the power from a 3 string / 12 panel system.
    • Even if the assumed 'hottest expected temperature ...' is increased from 95°F to 104° (next highest 'step'), nothing changes for the conclusions or analysis. Panel array qty needs to be 11 or 12.
    I'll catch up soon to the helpful posting on other aspects of this project. I spent today learning and understanding the concepts of string design via SMA's 2 programs.

    Thanks for all of the comments and suggestions so far. This should end up as a pretty good illustrated tutorial for someone who want to understand the 'why' behind the design and installation of an efficient GT system.

    Best regards,

    Bill
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    ... It would be in your best interest to not let the installer finish the installation as 4x9 as the performance will be well below expectations

    You need to keep each string in the same plane so you would have 24 facing south and 12 facing west if I understand you roof layout

    I have asked the installer for a design review.

    I don't know how to accomplish keeping each string in the same plane. 27 panels total are in the same plane facing South. 9 are in the same plane facing SW. Minimum # of panels per string has to be 11, per analysis done today and published.

    What challenges will arise with this arrangement (Alternative 1):
    • String 1: 12 panels facing South in same plane; 8 are in one place and 4 in another (with about 10 feet between the 2 sections).
    • String 2: 12 panels facing South in same plane, all together.
    • String 3: 9 panels facing South in same plane, and 3 panels facing SW in same plane (with about 15 feet between the 2 sections).
    Another option (Alternative 2) is to reduce down to 24 panels (reduce from 8.1 to 5.4 KW (DC) system), simply omitting String 3 (and removing rails, etc. from garage). Is this a better alternative than Alternative (1)?

    Is there an Alternative 3?

    Many thanks!

    Best regards,

    Bill
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    a0128958 wrote: »
    [*]An 11 panel array string size works well within the inverter's operating parameters. A 12 panel array will work but some output power amounts are 'cautionary.'
    [*]An 8000 W inverter may be needed to maximally harvest the power from a 3 string / 12 panel system.
    Bill


    Since you left out our earlier exchange, I don't want you to overlook this important point:

    The SMA string program has all the panels in the same orientation, it doesn't have an options to have your configuration where some face south and others west. It also doesn't take into account the angle difference from perpendicular to the Sun ( this means only two days a year are the panel 100% true to the Sun angle ) This is important in that the peak production for these strings will be hours off from each other. This means when some are peak, others will be somewhere in the 70-80% range of peak.

    And even if they were are pointing south, its a couple a day a year at best ( cold clear weather, sun angle perfect ) and all that would happen is the inverter will power limit at its 7000 watt AC limit ( about 7400 DC ) for maybe an hour or so.

    Here is my real world example ... I have 14kW name plate on my roof, 6kW SE and 8kW SW ... SE peaks about 11am , SW peaks closer to 2pm and combined never breaks much above 10.5kW AC

    Point is, in your case there is no issue with 3 strings of 12 panels even though some piece of software has a "cautionary" alert. This is where the installer is suppose to earn is income and "engineer" your system to be proper for your specific installation.

    Hope this is explained well enough so you understand ... you have a though Job ahead to educate your installer and that may well be the hardest part of this task
  • icarusicarus Solar Expert Posts: 5,090 ✭✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    I believe that we are seeing living proof of my comment about "ignoring SG's advice at our peril".

    I think this thread is a pretty good illustration of the value of this whole forum. While the OP may find himself in a box in this installation, he and all of us will have learned from it, and indeed the next guy coming along will have the benefit this wisdom, and won't have to ,,,reinvent tie wheel.

    Keep it up guys, and thanks to SG especially,

    Tony

    Or why it pays to do your homework!
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    a0128958 wrote: »
    I have asked the installer for a design review.

    I don't know how to accomplish keeping each string in the same plane. 27 panels total are in the same plane facing South. 9 are in the same plane facing SW. Minimum # of panels per string has to be 11, per analysis done today and published.
    Is there an Alternative 3?

    Many thanks!

    Best regards,

    Bill

    Yes, move 3 panels to the west facing roof, there is plenty of space in the photos to add three panels at the V of the south and west roofs. Put two to the left, you may have to rotate to fit two there and one one the left will align with the lower row.

    You should not mix panels in different planes ... the string will be limited to the lower current, will probably have a good 20% hit in production for that string ( best case ) if you mix them

    Good Luck!
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

    Alternative 3 - got it! Move 3 panels to the SW facing panels on the garage to total 12 (String 1), all in the same plane. Remaining 24 panels are on the house (Strings 2 and 3), split with 8 in one physical location and 16 in another physical location, but all panels in the same plane (South).

    Some early morning shading will exist on some of the 12 panels on the garage.

    Because of mixing strings with 2 different planes (but not panels within strings), the cautionary notes from the String Sizing Program, for 12 panel strings, related to array output power vs inverter max input power, can be explained and are not material.

    I think this is the summary, with plenty of help here, and plenty of study on my part today.

    Many thanks!

    Best regards,

    Bill
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    icarus wrote: »
    ... I think this thread is a pretty good illustration of the value of this whole forum.

    While the OP may find himself in a box in this installation, he and all of us will have learned from it, and indeed the next guy coming along will have the benefit this wisdom, and won't have to ,,,reinvent tie wheel.

    ... thanks to SG especially,

    Agreed.

    Working to improve the situation, thanks to the counsel being received here. Panels aren't up yet (sitting in garage) so there's still time.

    Hopeful that the postings here contributes to future projects.

    Much appreciate Mr. Guppy's help particularly!

    Best regards,

    Bill
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    Since you left out our earlier exchange, I don't want you to overlook this important point ...

    The SMA string program has all the panels in the same orientation, it doesn't have an options to have your configuration where some face south and others west. ... This is important in that the peak production for these strings will be hours off from each other. This means when some are peak, others will be somewhere in the 70-80% range of peak.

    ... in your case there is no issue with 3 strings of 12 panels even though some piece of software has a "cautionary" alert. ...

    Hope this is explained well enough so you understand ...

    I think I got it. I'm a pretty technical person. You're help is immensely appreciated. Your posting here was particularly helpful to broadly summarize the 'picture.'

    Thanks!

    Best regards,

    Bill
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area
    BB. wrote: »
    ... if you are feeling that you need backup--you might call SMA's US support center and say you have a question about their installer's configuration. They may be willing to help (normally the installers are trained by the manufacturers of the equipment they sell/install).

    Bill/Mod, thanks. I found SMA's forum ( http://forums.sma-america.com/ubbthreads.php?ubb=postlist&Board=2&page=1 ). I have a question posted on the minimum start voltage as this is a key consideration for 4x9 versus 3x12, and, my installer was advised by SMA that it's 250 V, not 300 V.

    Best regards,

    Bill
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