Need help calculating the wind and snow loading of my array

[D0T-C0M]

Hi everyone,

I am in the research and design phase of my solar project. My plan is to design and fabricate a grid tied 2 axis tracking array mount to accommodate 16-18 panels to start off with. I can weld and have access to a CNC machine. I can definitely do a lot of the fabrication work myself in regards to the mount itself and I think I want to go with a dual axis slew/worm gear setup. I live in Canada at 47.5 degrees latitude where we can get high winds and snow. I need help determining the wind and snow loading in order to properly size the slew gears.

Are there any structural or mechanical engineers here that could give me a recommendation on max axial and radial load ratings and other relevant minimum specs I should consider. I would need to spec in order to be safe in all weather conditions but not go way overboard unnecessarily in order to keep costs as low as possible? The attached image is the specs of a slew gear I am looking at, Its SE7 model dual axis slew gear, is it overkill or am I in the ballpark? Its very reason for just over $500 for the slew gear. I can make this mount for under $2000 and probably around $1000 for the cement pad no problem. My only issue would be to properly size the slew gear to handle the load.

Thanks for any help you can provide.

[Yet another Yeti]

60 cell panels weight how much ? approx. 40 pounds ?

How many 2-axis knees do you think would be deployed -- 1 or more ?


My guess is , it would need literature .

In former times people acted and had chosen more intuitively , not so much by intelligence ....

There are softwares out , even as trial versions , that could probably "calculate snow load" ....


HTH

[Mike90250]

That's going to need a mechanical engineer and maybe a soil engineer to make sure the base will perform as expected.

That's a LOT of sail you have there.

[J.P.M.]

You do indeed need the services of some engineering professionals, but they'll need a lot more information than I've seen so far. Hope you have deep pockets.

As for external loadings for wind, see ASCE 7-13.

[D0T-C0M]

Thanks for expressing all your concerns. Your right its a huge sail. I have also considered splitting the array and going with two 3by4 (12 panels) arrays. I would be easier installation and also easier to remove snow from in the winter. The pole could be lowered to 10' and much less of a sail per array. As in any project there is some amount of assumed risk. I'm in a rural part of the country and worst scenario is the array(s) falls down in the field and I lose $13k. I have been planning this for a long time. As part of my job I have directly overseen large substation jobs that involve alot of civil work including erecting large cement piers that include structural steel reinforced reebar grids and provisions for grounding on tall transmission structures. I'm quite confident in this respect and will be going well overboard on the pad and upright structures. Just wanted a ballpark value I can expect for wind loading so I can size the slew gear or if someone has direct knowledge of the slew gear specs installed on similar sized arrays.

[Yet another Yeti]

These solar panels have usually a Snow load resistance of 5400 Pa , and Wind load resistance of 2400 Pa -- assuming Pascal -- in their datasheets .
How these values apply on areas like squarefeet and squaremeters I would need to figure out ....

But there are a few panels , that have higer mechanic load values .

A Corean manufacturer , that also produces smartphones comes in mind .


Recently I had even read " Designed Load / Test Load " on one datasheet , not sure what that would mean ...

[J.P.M.]

Thanks for expressing all your concerns.

As part of my job I have directly overseen large substation jobs that involve alot of civil work including erecting large cement piers that include structural steel reinforced reebar grids and provisions for grounding on tall transmission structures. I'm quite confident in this respect and will be going well overboard on the pad and upright structures. Just wanted a ballpark value I can expect for wind loading so I can size the slew gear or if someone has direct knowledge of the slew gear specs installed on similar sized arrays.

Then you know that what you are planning will probably take a P.E. or two.

As for your wind load concerns, as a retired M.E. P.E who, among other things did structural design that included external loading considerations, even with all the site particulars, and in spite of what you may want to think, there is no ballpark. See ASCE 7-10 latest ed , now, ASCE 7-16.

[D0T-C0M]

JPM thanks for your links. That hazard tool was quite interesting.

Wind Speed
110 Vmph

10-year MRI
70 Vmph

25-year MRI
79 Vmph

50-year MRI
81 Vmph

100-year MRI
87 Vmph


Value provided is 3-second gust wind speeds at 33 ft above ground for Exposure C Category, based on linear interpolation between contours. Wind speeds are interpolated in accordance with the 7-16 Standard. Wind speeds correspond to approximately a 3% probability of exceedance in 50 years (annual exceedance probability = 0.000588, MRI = 1,700 years).


Site is not in a hurricane-prone region as defined in ASCE/SEI 7-16 Section 26.2.

[J.P.M.]

JPM thanks for your links. That hazard tool was quite interesting.

Wind Speed
110 Vmph

10-year MRI
70 Vmph

25-year MRI
79 Vmph

50-year MRI
81 Vmph

100-year MRI
87 Vmph


Value provided is 3-second gust wind speeds at 33 ft above ground for Exposure C Category, based on linear interpolation between contours. Wind speeds are interpolated in accordance with the 7-16 Standard. Wind speeds correspond to approximately a 3% probability of exceedance in 50 years (annual exceedance probability = 0.000588, MRI = 1,700 years).


Site is not in a hurricane-prone region as defined in ASCE/SEI 7-16 Section 26.2.

So, keep going and get a set of design wind forces on the structure and its component assemblies.

There will, in all likelihood, be several conditions with different forces applied to different components. Several checks for wind from different directions may be necessary, depending on severity and how far you want to take it.

Note: The highest wind speed may not necessarily produce the highest stress on any component or several components, and also the foundation which will possibly/likely also need design.

Also, depending on the design, know that some wind scenarios may produce "coupling" between the structure and the wind. If so, that may result in flow (wind) induced vibration in the structure. That most often and usually is a big consideration in the mechanical design of structures like distillation towers, but it's not unusual to find flow induced vibration with large, flat surfaces on what become, for wind design purposes, cantilevered supports (like PV arrays on posts).

I'd not consider vibration, or secondary/synergistic loading between/among different parts of the assembly at this time, first of all as it may not be required. secondly, the design probably isn't complete enough to give meaningful estimates at this time.

The best advice I have at this time : In any or all events, after you find out any building code requirements from the AHJ and you assess the liability caused by things flying through the air, I'd take what you've done so far to a P.E. who claims proficiency in wind design of structures and contract for a consult.

You may not need any engineering at all, but if you do, my guess is you may find you'll need more than you think you do at this time.

That may not be what you want to hear, but given what I think I may know about mechanical design and what you describe of your design so far, it's the best opinion I have at this time. Other's equally or more knowledgeable in the discipline than me may have other opinions.

Good luck.

[D0T-C0M]

If anyone else in Canada is interested doing some of the solar installation work themselves, I just checked with the Canadian and New Brunswick provincial government agencies regarding solar panel regulations and the only requirement in my area is I need an electrical permit and a licensed electrician to certify the electrical side of it and a public safety officer will come by and check the electrician's work. This is done to make sure in large part that I install approved CSA panels and proper grid tie inverters to protect the grid and the utility workers. There is no regulatory inspection of the mechanical aspect such as the mount and I am free to perform that work without regulatory compliance issues.

[J.P.M.]

If anyone else in Canada is interested doing some of the solar installation work themselves, I just checked with the Canadian and New Brunswick provincial government agencies regarding solar panel regulations and the only requirement in my area is I need an electrical permit and a licensed electrician to certify the electrical side of it and a public safety officer will come by and check the electrician's work. This is done to make sure in large part that I install approved CSA panels and proper grid tie inverters to protect the grid and the utility workers. There is no regulatory inspection of the mechanical aspect such as the mount and I am free to perform that work without regulatory compliance issues.

That's for letting us all know the installation is in Canada.

I don't doubt you're being truthful, but I'd double check on those building code requirements. I've done a fair amount of mechanical design of power equipment for operation and use in Canada (but no PV installations). My experience is that codes and standards in Canada are often similar to those in the U.S., but certainly not identical. In any case, I've found Canadian codes and standards every bit as rigorous as those in the U.S. and sometimes more so.

[ICPete]

Hi D0TC0M,
I'm not a PE but a degreed mechanical engineer. I'm also designing my own tracker (in this case, four single-axis trackers each holding 10 60-cell panels). A reasonable estimate of wind load on a large flat object such as a highway sign, for 90 MPH wind, is 15 PSF. At 110 MPH that increases to 22.4 PSF. In my area the official design snow load is 20-30 PSF (I'm at an elevation just on the boundary between those two official numbers). So I'm designing for 30 PSF load in any direction; up, down, and lateral in any orientation. The structure I've come up with, for a tilted single-axis system (the axis of rotation runs north-south but elevated at 35 degrees from horizontal), is far beefier than any commercially available tracker I've seen online, except maybe the utility-scale single-horizontal-axis trackers that rotate hugely long lines of panels. FWIW, my structure is designed around 3-inch schedule-40 pipe joined by brackets welded up from 1/4-inch steel plate. The end of each pipe is captured by two 5/8-inch steel bolts. So just to run through the math, in my case with 10 panels per tracker the total area is nearly 180 SF, for a net load of up to 5400 lbf. The rotation axis is supported by a pair of spherical roller bearings, each of which has a dynamic and static radial load rating of about 20,000 lbs. Their axial load rating is about 9000 lbs. In this design all axial load is taken by just one of the bearings. But given the cross-section of the array, as seen in the direction parallel to the axis, is extremely small, the load in the direction is reliably not going to be much different than the calculated gravity load (about 500 lbs). So I'm comfortable with the bearing design, and I've also done quite a bit of FEA on the various critical portions of the structure. And let's remember those bearing ratings are for 10% failure rate at 1 million revolutions.

Finally I'm using IronRidge XR1000 rails connected to the 3-inch piping, exactly as they do in their recommended ground-mount racking designs, with similar member lengths, spacings, and loadings. So I'm confident about the connection of the panels to the tilting pipe frame.

BTW, my net anticipated cost will be roughly a wash compared to simply buying more panels, charge controllers, and fixed mounts. So yes, as has been amply pointed out, it would be more sensible from a purely economic standpoint to avoid the motion, for my particular location and latitude. But there's something about trackers that has always interested me, it's a very compelling design-build challenge, and I suppose I'm a bit like Don Quixote and his windmills.

[ICPete]

Oh, and to be clear, in my jurisdiction I WILL be required to get a mechanical PE sign-off of the tracker design, a step that would NOT be necessary if I used pre-engineered ground-mount designs from IronRidge or others. So yes, I need to look into that and establish the pathway before I start spending any money on constructing my design.

[J.P.M.]

Hi D0TC0M,
I'm not a PE but a degreed mechanical engineer. I'm also designing my own tracker (in this case, four single-axis trackers each holding 10 60-cell panels). A reasonable estimate of wind load on a large flat object such as a highway sign, for 90 MPH wind, is 15 PSF. At 110 MPH that increases to 22.4 PSF. In my area the official design snow load is 20-30 PSF (I'm at an elevation just on the boundary between those two official numbers). So I'm designing for 30 PSF load in any direction; up, down, and lateral in any orientation. The structure I've come up with, for a tilted single-axis system (the axis of rotation runs north-south but elevated at 35 degrees from horizontal), is far beefier than any commercially available tracker I've seen online, except maybe the utility-scale single-horizontal-axis trackers that rotate hugely long lines of panels. FWIW, my structure is designed around 3-inch schedule-40 pipe joined by brackets welded up from 1/4-inch steel plate. The end of each pipe is captured by two 5/8-inch steel bolts. So just to run through the math, in my case with 10 panels per tracker the total area is nearly 180 SF, for a net load of up to 5400 lbf. The rotation axis is supported by a pair of spherical roller bearings, each of which has a dynamic and static radial load rating of about 20,000 lbs. Their axial load rating is about 9000 lbs. In this design all axial load is taken by just one of the bearings. But given the cross-section of the array, as seen in the direction parallel to the axis, is extremely small, the load in the direction is reliably not going to be much different than the calculated gravity load (about 500 lbs). So I'm comfortable with the bearing design, and I've also done quite a bit of FEA on the various critical portions of the structure. And let's remember those bearing ratings are for 10% failure rate at 1 million revolutions.

Finally I'm using IronRidge XR1000 rails connected to the 3-inch piping, exactly as they do in their recommended ground-mount racking designs, with similar member lengths, spacings, and loadings. So I'm confident about the connection of the panels to the tilting pipe frame.

BTW, my net anticipated cost will be roughly a wash compared to simply buying more panels, charge controllers, and fixed mounts. So yes, as has been amply pointed out, it would be more sensible from a purely economic standpoint to avoid the motion, for my particular location and latitude. But there's something about trackers that has always interested me, it's a very compelling design-build challenge, and I suppose I'm a bit like Don Quixote and his windmills.

Have you done wind loading and design for structures in the past ? If so, What did you use to determine the wind loads and conditions ? Did any of your work get checked/signed off by a P.E.?

[bcroe]

it would be more sensible from a purely economic standpoint to avoid the motion, for my particular location and latitude. But there's something about trackers that has always interested me, it's a very compelling design-build challenge, and I suppose I'm a bit like Don Quixote and his windmills.

Do keep us posted, pictures are great. One thing to consider, a tracker will not help under clouds, but an
increased number of panels can, as happens here at 42 lat. Bruce (cloud central) Roe