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

a0128958

Here's an illustration of a grid-tie (GT) only solar PV system installation for a residence in an urban area (Plano, TX (Dallas suburb), 1/4 mil pop.), designed to take advantage of all available rebates and incentives (Oncor is my utility Transmission Delivery & Service Provider). Construction started this month, Nov.

Day 0 - Design

System Sizing:

• Electric service panel (circuit breaker) box is 200 A. Max circuit breaker size for inverter-to-panel circuit is 200 A * 20% = 40 A (AC).
• Properly derate 40 A at 65% = 26 A max, which, at 240 VAC, is 6.2 KW (AC).
• Assuming a 0.77 default PVWatts DC-to-AC Derate Factor, multiplying the inverse of 0.77 times 6.2 kW, you get max 8.1 KW (DC) for total solar PV panels output.

Equipment Selected:

• Panels - Astronergy CHSM 6610M mono-crystalline, 225 Wp, 37 Voc, 8.5 A Ioc, UL Listed.
• Inverter - SMA Solar Technology Sunny Boy 7000US, 8750 W max PV power, 30 A (DC) max input current.
• DC Disconnect Switch - SMA Solar Technology - integrated with above inverter, accommodates up to 4 x 20 A (DC) strings.

Resulting System Size:

• 8.1 KW / 225 W/panel = 36 panels.
• Each panel is 5.4' x 3.3,' or 17.7 sf in area; 36 panels * 17.7 sf/panel = 636 sf min. roof area (not counting area needed for fire department 'walk around')

Array Design:

• 3 strings, 12 panels each string (later changed).
• 2 strings (24 panels) on the house roof (later changed).
• 1 string (12 panels) on the garage roof (later changed).

Notes:

Using PVWatts Ver 2 and my specific cell location (0218386), and assuming all defaults (77% DC-to-AC derate, panel tilt at Lat. (33.156°), all panels at 180° azimuth, and no shade ever), system should produce 11.1 kWh annually.

During some periods of solar kWh production I'll be able to totally consume it. The rest of the time I'll have to 'bank' into the grid as best as possible. Best available plan appears to be to sell at 7.5 cents to TXU, my utility Retail Electric Provider.

My last-12-month's average rate at the moment is 10.3 cents. My last-12-month's electricity consumption is 29,039 kWh, which is 2420 kWh/mo, or 80 kWh/day.

Here are 2 images showing roof and electric service entrance areas, before installation begins (8:45 AM CST).





Here are 3 images showing the solar PV system equipment as it arrived.





Next: Day 1 - Installation starts.

Best regards,

Bill

AntronX

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

How much did you pay per watt for those panels?

a0128958

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

Day 1: Installation starts

The design was changed after it was noticed that there's more shade on the garage than expected. Eight panels were moved from the southern most end of the garage to the far western edge of the house (left side of picture below). Another 7 panels were moved to an expanded house roof area. This leaves 9 panels on the garage roof. On the house roof (image below) will be 27 panels - 3 strings of 9 each.

The 36 panels will be divided into 4 strings of 9 each. With only 8 of 9 panels on the western most roof area, a conduit will need to be installed on the roof to the large area of house roof panels, to pick up the 9th panel for the string (and to avoid a 3rd roof penetration for wiring) (image shot at 8:35 AM CST).



The Sunny Boy inverter is mounted inside the garage, near the electric service panel. It weighs 141 pounds.

The panel is near the inverter. Room inside the panel will be needed, at the bottom, for a 40 A 2-pole breaker. The low voltage wiring attached to the 3 sets of current transformers, that are part of an existing power monitoring system, separate from the solar PV installation, will need to be installed inside the panel so that the cover can be put back on properly.



Here's the finished rail installation on the garage roof where a string of 9 panels will go. The rail supports are screwed into the roof rafters. Each obstruction installed into or onto the roof has an upper slope deflecting sheet metal water screen. I don't know how these screens are permanently attached to the roof. (Image shot at 8:35 AM CT)



Next: Day 1 continued.

Best regards,

Bill

a0128958

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

Day 1: Installation Starts (Continued)

The mounting rails and hardware are all from Unirac.



The panels are wired in series for each string, then connected to #10 USE-2 wires (same as what's attached to panels) via mating connectors like what are already part of panel wires. The USE-2 wires go to roof top junction boxes (shown below). There is EMT conduit that runs from the bottom of these boxes into the attic. This is the first of 2 roof penetrations.



The panels will be elevated approximately 6" off of the roof surface. This should reduce attic temperatures. Each attachment to the roof and penetration of the roof is sealed.



Polyurethane construction adhesive is what's used to do all sealing of rail attachments and roof penetrations.



Directly connecting to the roof-top junction box, via roof penetration, is flexible conduit.



Next: Day 1 - Installation starts (continued).

Best regards,

Bill

a0128958

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

Day 1 - Installation Starts (Final Segment)

The flex conduit connects to an attic junction box where #10 THWN-2 wires carrying DC current come together (2 each for 4 strings, plus ground). Since this is a 1-story structure, the attic is large, making all locations in the attic easily accessible to run conduit for DC wiring.



Each rail has an interconnecting #8 bare copper ground wire connected to appropriate grounding fasteners. This ground makes its way to the inverter. No separate ground rod will be installed at the inverter.



The second roof top junction box, for the remaining 3 panel strings' wires, along with the associated roof penetration and attached flexible conduit, is shown here.



Here's an overall summary view of the first day's work. (Image shot at 10:40 AM CST).



Next: Day 2 - 2nd day of installation.

Best regards,

Bill

a0128958

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

AntronX wrote: »

How much did you pay per watt for those panels?

The general rate in the Dallas area is $6/watt, installed and complete. It could be a little more for much smaller than 8 KW (DC) systems.

Rarely, if ever, is the panel pricing broken out separately. As is the case for the inverter, too.

In Texas, an Oncor (Utility Transmission Service & Delivery Provider) rebate is only available when a certified installer is used. DIY installations are not able to get the rebate.

The combination of the rebate plus tax incentives cuts the project cost about in half. I.e., an 8.1 KW (DC) system, at $6/watt, is $49K gross. Out of pocket cost, after rebates and tax credits, is about $24K.

In my case, I'm doing a lease purchase with a monthly payment set by the system's design output. For the kWh that I consume from the solar PV system I'll save 30% over what I would have otherwise paid w/o having the system.

Hope this longer explanation helps. The short answer to your question is 'I don't know.'

Best regards,

Bill

icarus

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

I don't know what time of day the pictures were taken, but it sure seem like there is lots of shadowing, even shading over great huge expanses of your roof. Unless I am seeing it wrong I would worry about how much harvest you are really going to get.

It concerns me that after the hardware was purchased, the shading was discovered and design changes were made. I would hope that folks look carefully before purchasing hardware so they are not stuck with a system that doesn't work as well as expected.

I hope I am wrong with my assumptions,

tony

a0128958

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

Day 2 - Second Day of Installation

Today was spent doing 3 things: installing mounting rails on the roof to support 19 panels, finishing installing conduit in the attic for all DC wiring, and installing remaining DC carrying conduit to the inverter and AC carrying conduit outside of the garage.

Here is shown the flex conduit (left) connection to the DC Disconnect switch (9 wires total, each #10 THWN), and the conduit (right) that goes from the inverter to the AC (fused) Disconnect switch outside (four #8 THWN wires - 3-wire single phase 240 VAC and ground).



Up in the attic EMT conduit runs to the right to pick up the 7 wires from the remaining 3 panel array strings. 1" flex conduit runs to the DC Disconnect switch.



It was mostly an all day effort to install the mounting rails for 19 more panels. (Image shot at 8:35 AM CT)



Overall, after two days, lots of progress. Mounting rails are complete now for 28 panels, and all conduit runs up to the outside AC Disconnect switch are complete. (Image shot at 8:40 AM CST)



Next: Day 3 - Third day of Installation.

Best regards,

Bill

a0128958

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

icarus wrote: »

... what time of day the pictures were taken, but it sure seem like there is lots of shadowing, even shading over great huge expanses of your roof.

... It concerns me that after the hardware was purchased, the shading was discovered and design changes were made. I would hope that folks look carefully before purchasing hardware so they are not stuck with a system that doesn't work as well as expected.

Tony, thanks for the note.

All of the images so far, except for one, were shot between 8:30 - 9:00 AM Dallas time. All of the roof is in shade at this point. The last shot for Day 1, was shot at about 10:40 AM, showing all of the house in full sun except for the garage (where there's one string of 9 panels).

I went back and added the time of day an image was shot for appropriate pictures.

We have had so much rain this Fall season that it's been hard to find sunny days to check measurements. The installing company came out at least 5 times to check measurements. They found the shading problem and modified the design appropriately.

Best regards,

Bill

niel

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

do note that grounding the rails is not usually required, but grounding the pv frames are required and only in the approved spot(s) on the frames with something like this,
http://store.solar-electric.com/gbdbtsopagrl.html

icarus

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

That's what I was hoping!

Tony

mike95490

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

So, all sections of array, feed 1 inverter? I guess this will test the theory of MPPT does not care about different strings orientation.

Solar Guppy

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

panels 90 degrees off each other but not full strings?, shading into 10:30 am? a single Sunnyboy inverter? not a good combination and I suspect you will find very subpar performance. If the system doesn't perform as expected ( contracted lease ) who pays the difference?

This whole things smells of one of those lease scams where they don't really care about performance as your locked into a fixed payment for what 20 years and regardless if the system actually works well your paying for it, then what happens if you need to sell the home and no one wants the PV?

a0128958

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

niel wrote: »

do note that grounding the rails is not usually required, but grounding the pv frames are required and only in the approved spot(s) on the frames with something like this,
http://store.solar-electric.com/gbdbtsopagrl.html

Niel, thanks for the note.

Panel grounding will be via bonding the panels to the rails using WEEB technology parts. The reference you provided has a nice link to the technology/products. Thanks.

I found a nice overview at http://www.we-llc.com/WEEB_howitworks.html that helped me learn.

Best regards,

Bill

a0128958

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

mike90045 wrote: »

... all sections of array, feed 1 inverter? I guess this will test the theory of MPPT does not care about different strings orientation.

Mike, yes. All 4 strings (9 panels each) feed to a Sunny Boy SB7000 inverter. The 4 strings are combined in an attached Sunny Boy DC Disconnect switch (fused).

The inverter's PV start voltage is 300, and its MPPT voltage range is 250-480.

Each panel's Voc is 37.3, and their Voltage at Pmax (Vmp) is 28.7. Thus, I think each string's Voc is 336 volts, and each string's Vmp is 259 volts.

I don't have enough expertise to know if this is going to be a problem. Thanks for bringing up the subject.

Best regards,

Bill

Brock

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

Bill good to see you over here

Looks like it is moving along! I am guessing you will put all the data online via welserver?

a0128958

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

Solar Guppy wrote: »

panels 90 degrees off each other but not full strings?, shading into 10:30 am? a single Sunnyboy inverter? not a good combination and I suspect you will find very subpar performance. If the system doesn't perform as expected ( contracted lease ) who pays the difference?

This whole things smells of one of those lease scams where they don't really care about performance as your locked into a fixed payment for what 20 years and regardless if the system actually works well your paying for it, then what happens if you need to sell the home and no one wants the PV?

Solar Guppy, thanks for the note. Three of the 4 strings (9 panels each) are all facing predominantly directly South. One string (9 panels total), on the garage roof surface, are facing about SW. No strings have panels with varying orientation.

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.

The mission is to maximize the solar PV system's output, enabling 'win-win' for everyone.

Best regards,

Bill

a0128958

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

Brock wrote: »

Bill good to see you over here

Looks like it is moving along! I am guessing you will put all the data online via welserver?

Brock, yes. Thank you for refering me here. I see from your heading that you're a 'not so new guy' 'solar superstar.' I look forward to learning from you.

I sure am learning a lot from the experts here. Hopefully I'll at some point be able to return the favors here, like I can at the geothermal HVAC forum.

Yes, I am working on changing my energy monitoring site ( http://welserver.com/WEL0043/ ) to include my solar PV system's performance. I'll remove some of the very complex and detailed geothermal stuff to make room for the solar information.

Here are some examples of what other solar PV system owners have done with the same monitoring system I have:

http://welserver.com/WEL0032/ (City of Boulder's first net zero home; 6.8 KW GT solar PV system)

http://welserver.com/WEL0075/ (CO example, 10 KW GT solar PV system)

http://welserver.com/WEL0183/ (CO example, 4.5 KW GT solar PV system)

http://welserver.com/WEL0093/ (CO example, 10 KW GT system)

http://welserver.com/WEL0030/ (VA example, 4KV solar PV panels (24 Sharp panels at 167 W each)

Best regards,

Bill

icarus

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

Once again, with no offense to anyone, but understand that while there may be some brusqueness in his replies, Solar Guppy is a recognized world class expert on the subject of grid tie PV, and he makes his living testing and evaluating various Pv hardware/installations I believe. That said, one discounts his comments at ones peril. As I suggested in one of my earlier posts, substantial shading as late as 10:30, and partial shading of panels at other times is a area of GREAT concern in terms of net energy harvest, which after all is what is the bottom line. By using only one inverter for various faceted strings the efficiency of the entire string will suffer, PLUS any shadow or shading on any one portion of any one PV cell could drop the effective output of the entire string effectively to zero, depending on diodes, series parallel wiring etc.

Good luck,

Tony

mike95490

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

a0128958 wrote: »

The inverter's PV start voltage is 300, and its MPPT voltage range is 250-480.

Each panel's Voc is 37.3, and their Voltage at Pmax (Vmp) is 28.7. Thus, I think each string's Voc is 336 volts, and each string's Vmp is 259 volts.

Sounds iffy to me. On a hot day, you may fall below your start voltage, and if there is a grid glitch that reboots the inverter, I think you may have a real chance of not restarting. Does Sunny Boy have a on-line calculator to determine array configurations?

Are are your panels 4" or 6" off the roof, 6" airspace would be better

stephendv

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

mike90045 wrote: »

Does Sunny Boy have a on-line calculator to determine array configurations?

Download: http://www.sma-america.com/en_US/products/software/sunny-design.html

Solar Guppy

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

icarus wrote: »

Once again, with no offense to anyone, but understand that while there may be some brusqueness in his replies, Solar Guppy ...
Tony

I'll try and be more cordial with my responses, I do cut to the chase a bit quick on my posts and having used the internet since its inception I tend to have a glass half empty view on the intentions of some situations I see posted.

I'm constantly impressed with how others can write posts that would take me 30 minutes or more to write on an repetitive basis here. I look to fill in with some technical facts that add value and prospective beyond the common questions but I'll give more thought to my responses

mike95490

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

stephendv wrote: »

Download: http://www.sma-america.com/en_US/products/software/sunny-design.html

AS this is blocked from my worksite, I'd like the OP (Bill) to consider consider d/l this and run the calcs for himself. I tried the XW site, just to see, but they don't list Astronergy Panels, and my evil restrictive workplace blocks their site too!!

Mike

a0128958

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

Day 3 - Last set of rails finished.

The last set of rails are installed. These rails took some time because of the lack of room to walk around the roof. Eight panels will go here. Since each of the 4 strings is 9 panels each, 1 more panel from the rails directly to the right will be used to complete the string. A conduit is required for the string series wiring to extend over to the other panels - it can be seen in the upper right corner of the image below.


The ground wire for each rail in the middle section (19 panels) is installed. The panels will be grounded to the rails using grounding 'weebs.' (Image shot at 8:41 AM CT)


All of the rails are now installed, with a grounding wire interconnecting each rail. Eight panels in the left section, 19 panels in the middle section, and 9 panels in the right section, for a total of 36 panels (4 strings at 9 panels each). (Image shot at 8:45 AM CT)


Next: Day 3 Continued - All inside wiring completed.

Best regards,

Bill

strawbale

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

a0128958, I am interested in your system since we have a similar system on order: SMA 7kw inverter and 33 Sharp 235w panels. Total gross cost $51,000. or $6.58/watt. Ours will be 3 strings of 11. Our Pathfinder report is just over 90%. Our shade will come late in the afternoon, so the strings will be in blocks so the shade doesn't take out all three strings at the same time. We are hoping to produce about 8 megawatts/year, which is about double of what our household of four consumes. Since I am a newbie I'd be glad to hear if any of the concerns raised on your system would also be a concern with ours.

Strawbale

a0128958

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

Day 3 Continued - Interior Wiring Completed

All wires from the 4 sets of 9 panels come to this junction box in the attic - 4 sets of 2 pair, plus the ground wire to the rails - 9 wires total (#10 THWN). These 9 wires head to the fused DC disconnect switch through the flex conduit on the left.


Inside the fused DC disconnect switch, on the left side the 9 wires come in from the panels. Each string is fused at 15 A (upper left corner). Out on the right side is the inverter's AC output. 240 VAC, 3-wire single phase, plus ground - 4 wires total (#8 THWN). Note that the DC side ground is not direct connected to the AC side ground. These 4 wires head to the outside AC fused disconnect.


The fused AC disconnect switch and the inverter's kWh meter are mounted on the exterior of the garage. The 4 wires from the DC disconnect switch are connected to first the AC disconnect switch and then the kWh meter base. The remaining connection to the service panel is left for the electrician to wire at a future date.


Next: Day 4 - Finish up remaining details before panels installation.

Best regards,

Bill

a0128958

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

icarus wrote: »

Once again, with no offense to anyone, but understand that while there may be some brusqueness in his replies, Solar Guppy is a recognized world class expert on the subject of grid tie PV, and he makes his living testing and evaluating various Pv hardware/installations I believe. That said, one discounts his comments at ones peril. As I suggested in one of my earlier posts, substantial shading as late as 10:30, and partial shading of panels at other times is a area of GREAT concern in terms of net energy harvest, which after all is what is the bottom line. By using only one inverter for various faceted strings the efficiency of the entire string will suffer, PLUS any shadow or shading on any one portion of any one PV cell could drop the effective output of the entire string effectively to zero, depending on diodes, series parallel wiring etc.

Tony, thanks for the comments. I'm a technical kind of guy, and I'm pretty good sticking to facts. I'm OK with however anyone wants to help, and am immensely appreciative. Hopefully I can eventually do some good for others here.

Earlier in this thread Mike(90045) made a comment about all 4 strings being connected to one inverter, and thus questioned whether or not MPPT would work. Since I didn't know anything about MPPT I went and researched it. In my course of study, I ran across and studied 2002 publications by Mr. Guppy, writing about his experience comparing Xantrex and SB2500 inverters.

I'm highly respectful of Mr. Guppy and am appreciative, and feel priviledged, that Mr. Guppy wanted to contribute to help to this thread here.

Best regards,

Bill

a0128958

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

mike90045 wrote: »

... On a hot day, you may fall below your start voltage, and if there is a grid glitch that reboots the inverter, I think you may have a real chance of not restarting. Does Sunny Boy have a on-line calculator to determine array configurations?

Are are your panels 4" or 6" off the roof, 6" airspace would be better

I found the Sunny Boy on-line string calculator, and it does have Astronergy panel data in it. I'll report the findings in a future posting.

It's 4.5" from the roof surface to the top of the rails. It's another 1.5" from the top of the rails to the bottom of the panels. That's 6" total.

Thanks.

Best regards,

Bill

icarus

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

Bill, As you say, I'm sure you will do some good here. I hope that your project exceeds your expectations, and that the information that you glean from this site helps in that endeavor. The whole point of this forum, as I see it, is to let each succeeding project benefit from all the experiences that have come before, both good and bad. (Not to imply that yours is likely to bad)

As I have always said, there are some very sharp folks here, some who have forgotten more about RE/PV than most of us will ever know. They have made the mistakes that so that perhaps we don't have to.

Keep up with the postings as they continue to be interesting,

Tony

stephendv

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

a0128958 wrote: »

I found the Sunny Boy on-line string calculator, and it does have Astronergy panel data in it. I'll report the findings in a future posting.
Bill

Hi Bill, it will be interesting to see what you find when using the tool. The question isn't whether MPPT "works" or not, it's a question of how well it will work with your strings facing different directions. So when you do the simulation in the tool it will be good to see a comparison between using 1 SB7000 versus 2 x SB4000 (or whatever), 1 for each panel orientation.

a0128958

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

mike90045 wrote: »

... On a hot day, you may fall below your start voltage, and if there is a grid glitch that reboots the inverter, I think you may have a real chance of not restarting. Does Sunny Boy have a on-line calculator to determine array configurations?

stephendv wrote: »

Download: http://www.sma-america.com/en_US/products/software/sunny-design.html

mike90045 wrote: »

... I'd like the OP (Bill) to consider consider d/l this and run the calcs for himself.

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.

The online version is a little simpler, and thus for me, it's probably a better starting point. (It's at http://america.sma.de/newstringsizing.aspx ).

Before I publish a summary of the performance concerns it raises, I'd like to make sure I get the temperature input numbers correct. I could use some help here.

It asks for 'Coldest expected temp when sunlight will be on the modules.' This one's easy. Using weather.com, I see that my location's 'Lowest Record Low' is -7°F (Jan.). And thus is what I put in.

It then asks for 'Hottest expected temperature when sunlight will be on the modules.' The s/w cautions to use 'Highest Average High,' which (again using weather.com) is 94°F (Aug.) for my Dallas location.

Is 94° really the temperature to put in? Is the temperature 6" above the roof much higher? If so, does the s/w make an automatic adjustment for this?

Many thanks!

Best regards,

Bill