Question about panel mounting on crowned roof

chevymike

Solar Enthusiast
Joined
Nov 18, 2019
Messages
89
I am looking to mount 2 panels onto my ambulance project and the roof has a crown. From center line to edge it drops about 1.5".

My questions are, should I mount the panels flat across the roof, almost touching the center but being up almost an 1.5" at the edges or can I mount them "flat" to each side of the center line? Keeping them flat on each side of the center line will have each panel angled slight away from each other. Are there any pros or cons to doing this or should the panels be flat across the full span of the roof?

It would be much simpler and cleaner looking to keep them flat on each side of the center line but if I am going to greatly decrease performance, that would be more of an issue.

Thanks
 

snoobler

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Jul 10, 2020
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HBR, AZ
Flat panels suck in general when compared to properly tilted panels. Panels at a slight angle will suck less in one direction and more in the other.

The panels tilted slightly more towards the sun would perform better than those tilted away, but they would offset each other still performing very comparably to a completely flat array.

Any difference in performance would be very difficult to measure between your two scenarios.

Mounting in a structurally sound manner is far more important.
 

chevymike

Solar Enthusiast
Joined
Nov 18, 2019
Messages
89
Flat panels suck in general when compared to properly tilted panels. Panels at a slight angle will suck less in one direction and more in the other.

The panels tilted slightly more towards the sun would perform better than those tilted away, but they would offset each other still performing very comparably to a completely flat array.

Any difference in performance would be very difficult to measure between your two scenarios.

Mounting in a structurally sound manner is far more important.

Thanks for that reply. That was my thinking but my knowledge is limited. I know panels angled towards the sun always do better but wasn't sure if the one away might really decrease performance. Sounds like it's likely to be a wash. Definitely makes securely mounting them much easier.
 

Screwball

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Apr 3, 2021
Messages
53
My aluminum roof (commercial truck body) has about a 1.5" crown in it ... went with large, flat panels (panel frame about 1.5" over the crown) for good airflow to take advantage of the permanent shading of the roof without the panels too close for heat transfer ... pv panels are cheap, so i added more wattage then really needed (2000w) to compensate for the loss of flat panels ... also thinking of "overpaneling" twin 60a chargers ; probably never going to see the full output to justify 80a chargers
20201020_180706.jpg
 

Screwball

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Apr 3, 2021
Messages
53
Btw used twin 21' unistrut sections on custom Z 's ... all bolted with 3/8" hardware into each roof bow ... was painful to drill the one-piece roof, but with the crazy winds here in the west I rather be safe then sorry (plus the bolted together panels virtually shed all water from hitting the roof anyway)20201017_115152.jpg
 

time2roll

Photon Sorcerer
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Mar 20, 2021
Messages
2,217
Some tilt is preferred to let the dust wash off better with a light rain. I would want to align them the same if on the same controller.
 

danielsjoseph891

New Member
Joined
Mar 30, 2021
Messages
31
Flat panels suck in general when compared to properly tilted panels. Panels at a slight angle will suck less in one direction and more in the other.

The panels tilted slightly more towards the sun would perform better than those tilted away, but they would offset each other still performing very comparably to a completely flat array.

Any difference in performance would be very difficult to measure between your two scenarios.

Mounting in a structurally sound manner is far more important.
My situation is similar to Chevy Mike's only it's just one panel, and instead of the angle being width-wise on either side of a center line, one long side of the panel is angled down toward the front edge of the roof. It's slight. 5 degrees -or- not that drastic. Would you venture to have the same consensus as with Chevy Mike?
 

fratermus

Solar Addict
Joined
Mar 19, 2021
Messages
225
panels angled towards the sun always do better

In direct light, yes, all other things being equal. In diffuse light (like overcast) there may be minimal difference between flat and daily-optimal tilts. Sample observation in this blog post made in mid-winter when tilt would be most advantageous.


My questions are, should I mount the panels flat across the roof, almost touching the center but being up almost an 1.5" at the edges or can I mount them "flat" to each side of the center line? Keeping them flat on each side of the center line will have each panel angled slight away from each other. Are there any pros or cons to doing this or should the panels be flat across the full span of the roof?

Much good input on this thread, and I agree with others that flat vs subtle opposite tilt won't be a significant difference in most situations.

My setup: the roof of my van is crowned. I mounted the panels on a rack that cleared the crown and provided ample airgap (pic from a previous version). Even with a 5" gap in that configuration the temp gun says it was hotter on the roof in the shade of the re-radiating panels than in direct sunlight where there was no panel shade. If you are in a cold climate the more closely-mounted mild tilt arrangement might help donate heat to the ambo. I camp in the desert SW so the airgap is welcome respite from superheated black panels.


just one panel, and instead of the angle being width-wise on either side of a center line, one long side of the panel is angled down toward the front edge of the roof. It's slight. 5 degrees -or- not that drastic

In winter or high latitudes it might be worth it to park facing south to make use of that marginal tilt, all other factors being equal. A few free watts, maybe.


I would want to align them the same if on the same controller.

I think if the differently-aligned panels (or panel strings) were paralleled into the controller it'd be fine. I've experimented with 2x 12v 100w portable panels paralleled on one MPPT controller and set in slightly different orientations (let's say "mid-morning" + "mid-afternoon" orientations).

With both panels facing the same daily-optimal direction power production peaked noticeably at solar noon then fell off, as one would expect. With the panels rotated away from each other production seemed to averaged out across the day; much lower peak but more evenly distributed throughout the day. I reached Vabs a little earlier and held Vfloat a little longer than with the same-facing panels.
 
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