Raise north-facing panels to make them south-facing on low-angle roof

Leomania

Now that I have my panels nice and clean again I'm looking at the amperage readings on my three Trace C40 charge controllers when all panel groups aren't shaded and no surprise, my north-facing panel group is 1/2 to 2/3 the power output of the south-facing group. Note that my roof pitch is 1 in 10 so while it's not flat, it's not like the north-facing group gets no sun. We recognized the compromise at the time the installation was done.

Still, I don't recall if I even asked the installers whether the north-facing panels could be tilted so they'd face south at the same angle as the south-facing group. I'm estimating this would mean raising the panels near the edge of the roof about 4-5 feet. While I'm guessing the cost to do this would probably outweigh any long-term economic benefits, I'm curious if there are downsides to doing this I'm not aware of.

Thanks!

BB.

I think you are in Belmont Ca (If I recall correctly)? So no snow issues. Leaves wind--The reverse tilt mounting may not pass a building permit (I think I read here somewhere a while ago, that "reverse tilt" mounts were not available (or not recommended) from typical rack system producers.

Assuming you are not in a valley that gets focused winds from a strong storm (out of the south)--It probably is not the worst thing in the world (unless your wife hates what it does to the look of your home--Then watch out ).

-Bill

Leomania

Hi Bill. Yeah, that's me.

Hmmm... come to think of it there was a crazy funneling of wind that happened, oh, a year back that blew a small but not lightweight ladder that had been laying on the roof (to get to the second level roof where two panel groups are) off onto our deck. Scared the crap out of us. I couldn't believe the wind was concentrated enough to do that. So I think I'd have to enclose the space if I did that.

Sure wouldn't make it any cheaper...

Thanks,

- Leo

BB.

Our little rancher in Belmont just west of the 101, with a south facing roof and "cheap" asphalt shingles (from the previous owner)--Blew off (unzipped) a 1/3rd of the south facing roof after a wind storm a decade or so ago--Had to reshingle (low pitch roof thankfully) in not fun weather--And this time I used asphalt cement under every tab. That problem never happened again (cross my fingers).

No focused winds, but still did amazing things to the roof (did not have too many shingles I could find to dispose of).

More more--Hope for the best and prepare for the worst.

-Bill

mike95490

I hear there are going to be lots of shingles loose in Florida tonight. 

softdown

I can't hardly imagine a wind that a properly constructed rack made up of 1/8" angle iron could not handle. Angle iron must be primed before painting.

It would be interesting to see what an aeronautical engineer had to say about wind potentials regarding roof slopes and panel angles.

Leomania

softdown said:

I can't hardly imagine a wind that a properly constructed rack made up of 1/8" angle iron could not handle. Angle iron must be primed before painting.

That's exactly what is in use now. I see plenty of installations on flat roofs with angled panels, but usually in wide open spaces. If Bill hadn't mentioned it I wouldn't have given it a second thought. I think done properly it would probably be okay.

I think it's really not financially viable to pay someone to do it, but I'm going to ponder it. Bugs me to not be producing as much as I could be.

softdown

Especially with pvwatts.gov to tell you "exactly" what will produce what.
I got almost everything right the first time. The 45 degree panel tilt was the ideal choice, in my opinion, with a 37 degree latitude. The winter time production is very good. 

I would like to see the real world result of someone placing panels on both the east and west facing roofs. I am actually doing this on a shed. Just been busy with other projects. Figure about half the homes have no usable southern roof mount.

BB.

Even if the racks+panels will survive--There is the question about the roof itself. Not everywhere uses hurricane (and earthquake) ties like Florida (and newer California?) homes.

https://www.strongtie.com/strapsandties_woodconnectors/category

I am not saying it cannot or should not be done--But hanging "sails" in high winds needs to be well designed and implemented.

This stuff adds up pretty quickly--Say 285 Watt panel:
https://www.solar-electric.com/lib/wind-sun/SolarWorld_Sunmodule_280-290_black_datasheet.pdf

• 178 psf downward, 64 psf upward
• 39.4" x 65.95" * 1/144 sqin per sqft = 18 sqft per panel
• 18 sqft 8* 178 lbs per sqft = 3,204 lbs down force (maximum panel ratings) per panel
• 18 sqft 8* 64 lbs per sqft =1,152 lbs per sqft uplift (maximum panel rating) per panel
  

http://www.ameriluxinternational.com/SideBarLinks/Technical Info/Wind Load Vs PSF.pdf
Wind Pressure of Basic Speed is: q= (V^2)/1.6 { V to the power of 2
divided by 1.6} Where q is the pressure in Pa and V is the wind velocity
in m/sec.

Transferring this to American unit’s yields:
q= (V^2)/383.6 where q is the pressure in PSF and V is the velocity in
mph.

For Example, Speed of 100 mph gives 26.07 PSF.
Important Note:

This is the wind pressure calculated for the Basic Speed. The
actual pressure that operates on the building depends on many
other factors, and may be much higher than this value.
Always include this important note when you quote this formula.

Wind (MPH) Load (PSF)
25   1.63
50   6.52
75   14.66
100  26.07
125  40.73
150  58.65
175  79.84
200  104.28

I do not know what San Mateo area building codes requirements are... Perhaps gusts to 70-95 MPH?

That is why folks pay the big bucks to civil engineers to design and sign off on designs like this.

-Bill

mike95490

The racks hold up OK, but when does the mounting hardware pull out ?   or the rafters pop off ?

softdown

The rafters are not going to "pop out".  It is, of course, important that the mounting hardware be quite sturdy. Galvanized lag screws are generally hard to beat. Hitting the rafters must be confirmed of course. I would think bolts may be the way to go at times.

littleharbor2

Interesting topic. We have a beach house south of San Felipe. Northern Baja. The area is known to be very windy at times. There is no grid available so consequently everybody has off grid solar setups. There are some quite old and questionable tilted arrays.  (you see lots of old, bronzed Arco panels for example). Anyway my point is that I'm constantly amazed at the feeble looking , adjustable tilt array supports and how well they seem to hold up in the wind. That being said, imho, any well thought out and properly executed tilted array ought to hold up well in all but the heaviest of wind. Properly engineered and executed should be bombproof. Aesthetics, on the other hand, are another issue altogether. 

softdown

littleharbor2 said:

Interesting topic. We have a beach house south of San Felipe. Northern Baja. The area is known to be very windy at times. There is no grid available so consequently everybody has off grid solar setups. There are some quite old and questionable tilted arrays.  (you see lots of old, bronzed Arco panels for example). Anyway my point is that I'm constantly amazed at the feeble looking , adjustable tilt array supports and how well they seem to hold up in the wind. That being said, imho, any well thought out and properly executed tilted array ought to hold up well in all but the heaviest of wind. Properly engineered and executed should be bombproof. Aesthetics, on the other hand, are another issue altogether. 

Yes....I believe that conditions in that type of geography are conducive to high winds due to various pressure systems colliding.....perhaps just strong coastal winds at times.

I think the thinness and the angle of the panels makes them less susceptible to catching the full brunt of high winds. Unless the panel angle exceeds, lets say, 50 degrees or more....the wind has an inherent tendency to "skim over" the panels. This is why modern airplanes and aerodynamically designed automobiles almost never have ~90 degree angles for the wind to slam into.

Imagine an automobile shaped like a solar panel flowing down the road at a 20 degree angle. That is pretty darned aerodynamic.

I have trailered materials that presented a 90 degree angle for the wind to catch on. It was like pulling a sail. Have found that aerodynamics are far more important than weight unless the climb angle is significant.

BB.

Wind tunnel testing in hurricane force winds:
https://www.youtube.com/watch?v=L-qzOF3E_DU
https://www.youtube.com/watch?v=DZyjMbMHbTU
It is not just the solar panels that need to be well anchored.

-Bill

softdown

My late father grew up in tornado country.....Kansas. He explained it like this: Wind lifts/tears off the roof first, then the house loses almost all of its strength." Because of him, I always use hurricane ties...even with sheds.

BTW...that house looked like it was put together with staples.