Solar Pergola Racking Recommenation

[khanh dam]

looks like the steel goes into the concrete foundation and sticks out of it, correct. Concrete guys have told me that the exposed steel will expand faster than the cool 60 degree ground concrete and can cause cracks around the poles? not sure if it really is an issue. maybe a thin foam wrap and then caulked on top to prevent water from getting in latter on or just diamond saw cut a control cut around each post.

[J.P.M.]

Where you steel penetrates the 4" concrete pad, you need to leave an expansion space, or the microflexing of the steel and thermal effects will create a gap in an uncontrolled manner.

I don't claim competence in reinforced concrete design but concrete and steel have coefficients of thermal expansion that are within 5 to 10 % of one another, that is, quite small. That usually makes the relative lateral or short span thermally induced axial movement quite small and so too the resulting thermally induced stresses over commonly seen temp. ranges and conditions.

Induced stresses from usually local galvanic corrosion of the steel within the concrete (the resulting rust = iron oxide) has a greater volume than the steel) are usually a greater concern than the thermally induced stresses and can create (depending on length/thickness) a much larger tensile stress in the concrete than any thermal expansion. Most of those concerns deal with rebar, but structural members can be affected as well. Usually, a competent designer addresses those and other such concerns.

[heimdm]

If I am following correctly, Mike90250, there is a need to have a gap between the concrete and the base plate. Based on this schematic, I believe that is addressed.
.baseplate.png

[Mike90250]

Where you steel penetrates the 4" concrete pad, you need to leave an expansion space, or the microflexing of the steel and thermal effects will create a gap in an uncontrolled manner.

[heimdm]

ss-prelim.png
Received preliminary drawings/plans for the structure. The foundations are essentially a 5' x 12' x 1' pad with a 6" pier extending up, which is where the base plate and anchor will connect. The entire foundation will require just over 18 yards of concrete. My soil type has lots of limestone chunks, so a shallow foundation is preferred. At the moment the concrete anchors will be recessed. A concrete pad will be poured over the majority of the area covered by the panels. Hopefully, next week we will get finalized drawings and I can meet with my excavator/concrete contractor and get this moving forward.ss-prelim.png
ss-foundation.png

[heimdm]

This week saw the arrival of 92 panels arrive by freight carrier. The panels came stacked 2 pallets high. Luckily, a local business with a skidsteer with forks was able to put all the pallets on the ground. They were kind enough not to charge. I have to pick up a pallet jack tomorrow to move them inside the garage, so we can inspect all the panels for damage. Next week, the temperatures are supposed to be near 10 degrees or less, so definitely want to have the panels in the garage for inspection.

As far as the steel structure goes, the company that is building it is doing the design work, which hopefully will be completed in the next 2 weeks.

[heimdm]

sounds dumb. I put commercial sunpower panels on my picnic shelter roof. main difference is they are bigger than residential panels. Why the heck LG cares is anyone's guess.
I spoke to a residential engineer today they said pergola rafters needed to be 2x8" steel tube with 1/8" wall spaced 40" on center to span 160" from south header to North header. OR two 2x12 wood rafters coupled together or two 2x8" with steel filch plate between the two or 2 aluminum self mating 2x10 beams. Designed for 115mph wind load and 25 degree tilt. I'm guesisng the 25 degree tilt is what makes it have to be so strong. Most pergolas are not tilted that much.

The tilt is up to each project for sure. The guidance, I was given is a minimum of 5 degrees so that rain water does not pool on the panels. I would have desired a higher tilt, but settled at 15 at that prevents the panels on the high end from being above the raised deck on the back of the house. Our high side (north end) is about 14' above ground.

My spacing is expected to be:
North-South: 21'
East-West: 22'8"

[khanh dam]

sounds dumb. I put commercial sunpower panels on my picnic shelter roof. main difference is they are bigger than residential panels. Why the heck LG cares is anyone's guess.
I spoke to a residential engineer today they said pergola rafters needed to be 2x8" steel tube with 1/8" wall spaced 40" on center to span 160" from south header to North header. OR two 2x12 wood rafters coupled together or two 2x8" with steel filch plate between the two or 2 aluminum self mating 2x10 beams. Designed for 115mph wind load and 25 degree tilt. I'm guesisng the 25 degree tilt is what makes it have to be so strong. Most pergolas are not tilted that much.

[heimdm]

Update to yesterday setback. I ended up talking with the regional sales manager from LG for the Midwest. The deal with "commercial" Solar Panels is you can not put those on a primary residence (house). If you do ground mount or put them on a solar pergola/canopy that is perfectly. fine. LG will honor the 25/25 warranty for the product. However, they will not cover the labor cost. For the most part anytime you see LG panels and "commercial only" that is what is going on.

[heimdm]

A little setback today. Thought I had the LG bifacials all squared away. Distributor went to order them, and LG will only ship those modules for commercial projects. My backup module at this point is the REC Alpha Black module.

[bcroe]

We are fed by 50 kvA transformer, and we are the only ones on it. Our angle will be 15 degree's, which optimally would be about 2x that. I am in southern indiana, which for the past 5 years, we have not got much snow. So, if there is an uptick in snow in the area in the next couple of years, you can blame me.

We did add extra panels to make the winter performance not as bad. My intent is that we will underproduce during the winter months (Nov-Feb). Until 2032, we have net-metering. Once we get past that, we might add some panels at a steep angle on the ground, to offset winter (December, January). Bruce, what kind of performance do you get during the winter months? Trying to determine if the energy is worth hunting during a Midwest winter.

The distance between the inverters and grid interconnect is about 75ft. Current design calls for 4/0 wire. Always appreciate your insights and feedback.

75 feet with 25 more for both end terminations (mine took more than that), gives about a
200 foot loop, 0.049 ohms per 1000 feet of copper 4/0 wire. With 36.8KW (92 400W panels),
a lower angle Cos of .95, and an inverter + DC wire efficiency of 0.95, an inverter AC output
of 33.2KW is possible. At 250VAC line that is 133A, causing around 1.3VAC drop in the loop,
or 0.5% power loss, a pretty good number. Increase that by about 1.6 if the wire is aluminum.

For my 42 Lat midwest, mid winter KWH production is about 1/3 my mid summer production.
The main factors are much shorter days, much more prevalent clouding, and a poor seasonal
elevation angle on fixed sections of the arrays. The seasonal increased clouding is the
biggest thing, with cloud dispersed light the angle matters less. A few days ago the array
under excellent sun produced about 57% of the best summer day ever, but 10% on the
worst day.

I clear snow after every storm, not so hard with my design. That will be nearly impossible for
you, so a good part of winter production may be lost. That is a smaller part of the picture,
but it is large enough to get me out pushing snow (manually), in between cleaning paths
(with a tractor).

I see no practical way to bring winter production up to summer levels, so net metering is
the essential bridge here. It might be possible to reduce the need by building a new super
insulated house.

With a given ground area, the same energy may be collected by covering it with flat
panels, or by a considerably reduced number of panels elevated to the angle to
intercept that sunlight on a perpendicular. With optimal mounts costing more than
panels these days, I see nothing wrong with using lots of flat mounted panels on
that existing pergola.

If you do add ground mount panels later, you might consider something like I built
here after half a decade of experiments. The elevation may easily be changed
seasonally, both for production and to minimize snow accumulation. This winter,
this array has stayed snow free, while my conventional array required 5 cleanings.
Note the high ground clearance to avoid snow accumulation.

The next of these arrays will be 2 sided, to replace a decade old wooden support
section. Going this route with string converters, you could arrange to collect a lot
more energy with a lot more panels, while still using your original inverter plant
and AC wiring. That is done here by east directly facing the rising sun, west
facing the setting sun, with the other panels taking care of mid day and boosting
output under clouds. Peak energy will not increase, but the number of production
peak hours a day will jump. My best unclouded day produced 10.5 sun hours.
Since only a fraction of the panels are at peak at any one time, the same inverters
will handle near the same output, over a longer day.

Expansion here is based on panel strings totaling 720 cells, about 360 VDC in
summer, 420 in winter. I am less concerned with the exact panel arrangement
or voltage specs, all my varied 720 cell strings play nicely together. Just need
a combiner box with more inputs. Mine has 12 strings, will take 14.

I see arrays of large X by Y panel numbers. These DO NOT save ground space,
but they make it impossible to change the tilt or clear the snow. Designs here
are for 2 high landscape mounted panels, set at a steep angle for winter, any
convenient length, arrays. More panels = more length, NOT height. What snow
does collect, is easily reached with a pusher. Yes more foundation and support
structure is needed, my back and snow pushing mid winter dictate my tradeoff.
good luck, Bruce Roe


Array20Rear1.JPG

[heimdm]

Will do for sure. I should have design specs for the steel structure in the next couple of weeks. With it finally starting to feel like winter, not too much will be happening from a construction perspective until March would be my guess, as concrete foundation work is next.

[khanh dam]

Jinko makes crap panels with white backsheets that crack and fail. I wouldn't trust their data on a transparent back sheet at all.
I'm pretty sure transparent backsheets have just come out in the last few years, there is no 20 to 30 year samples we can point to as a success. hopefully they work and lg is around that long if warranty is needed.

Anyways that will be amazing once done. What are you using for the rafters? I assume it will have to be metal since the span is so long? I'm guessing 2x6 steel with 1/8" wall. please let me know as I'd love to make a similar project latter on.

[heimdm]

Hopefully the backsheet holds up. Found this as a good comparison between dual glass and transparent backsheet.
JinkoSolar: Transparent backsheet vs dual glass--- Advantages and disadvantages | PV Tech (pv-tech.org)

The modules have a 25 year warranty.


In the account -> privacy settings there is an option to enable private messaging.

[khanh dam]

Purchased LG Bifacial Panels LG405N2T-J5 (transparent backsheet).

I'm surprised you got a plastic backsheet, glass is going to be much more durable. Hopefully they last 25+ years.

If anyone is interested, I worked with a company that supplied me with alumawood patio cover, and they got me the structural stuff sorted out with a cover letter stamped by a PE. I can supply the details if its needed by anyone.

Yes I would love that option, but this forum does not allow private messaging, please post link? Never mind I see it was already blocked. this place is locked down more than a maximum security prison.