Question about: Solar Panel install on whole roof of an RV

[HookEm]

I'm considering covering the whole rooftop on a Sprinter-based RV, with (4 * 425W) LG panels (75" x 41" each), as shown.
- The panels will be mounted on a pair of 1-foot tall structures ((1-5/8" Aluminum Strut Channel, shown in Dark-Grey) with 2 panels per structure, and the (Mabru RV, 12V) A/C unit is 6" tall.
- The A/C unit will (of course) be covered by the panels, keeping the unit cooler, but covering its top-mounted Vent fan.
Question: What minimum clearance should be allowed above the A/C's Vent fan? (my plan is to allow for 6" of Clearance... is that OK?)

I did talk to the folks at Mabru, but didn't get much help (link): Mabru RV, 12V A/C

 

[sunshine_eggo]

That fan would blow hot air from the condenser directly onto the back of the panel and would likely 1) decrease its performance significantly and 2) likely shorten its life.

I don't see a better option for getting that much solar on a small area. If you could leave the 3rd panel out, that would be the best, but you lose 25% of your planned solar.

If you can't figure something better out, I would install some sort of backing, like a thin aluminum sheet to ensure the fan doesn't directly impinge on the panel:

 

[Zil]

I would worry about the panels being subject to wind lift mounted 12 inches above the roof. The fan blowing on the panels would not effect the measurable output. In sunshine the panel would be much hotter than the exhaust fan.

 

[eXodus]

I'm considering covering the whole rooftop on a Sprinter-based RV, with (4 * 425W) LG panels (75" x 41" each), as shown.
- The panels will be mounted on a pair of 1-foot tall structures ((1-5/8" Aluminum Strut Channel, shown in Dark-Grey) with 2 panels per structure, and the (Mabru RV, 12V) A/C unit is 6" tall.
- The A/C unit will (of course) be covered by the panels, keeping the unit cooler, but covering its top-mounted Vent fan.
Question: What minimum clearance should be allowed above the A/C's Vent fan? (my plan is to allow for 6" of Clearance... is that OK?)

I did talk to the folks at Mabru, but didn't get much help (link): Mabru RV, 12V A/C
View attachment 78286

did you already buy the A/C Unit?

Might shop around for a low profile unit with a rear fan exhaust.
1 foot distance is mighty tall. I'm mounting my large panels with 3 inches distance to the roof - and I'm still afraid that wind might lift them of.

Since you are already cutting holes in the Van - you can also install Mini Split or Window Unit in the rear door.

 

[HRTKD]

Install panels up to the AC unit and stop there. Mount the panels low, not high. If you need more PV, consider a set of ground deployed panels.

 

[OffGridInTheCity]

My home roof 300w panels are installed over the bathroom vent - about 3". 3 years and no problems. Mount them a few inches above. The 4 on my cargo trailer are up off the roof by 5" on rounded edge and 2" in center - no problems except maybe 'drag'.

Panels are pretty darn hardy and an RV is not usually 365days/year use like my home / bathroom vent. And if if corrodes in 5 or 10yrs - so what? - its not a danger situation / just replace it.

I worried about about the bathroom vent being covered and I also worried about the external mini-split unit being only 3" off the front wall of my trailer instead of 6 or 9 they recommend. Both turned out to be nothing burgers....

 

[sunshine_eggo]

I would worry about the panels being subject to wind lift mounted 12 inches above the roof. The fan blowing on the panels would not effect the measurable output. In sunshine the panel would be much hotter than the exhaust fan.

Condensers run >120°F. Adding a heat source to the back of the panel will absolutely impact its performance measurably.

 

[OffGridInTheCity]

Condensers run >120°F. Adding a heat source to the back of the panel will absolutely impact its performance measurably.

Maybe you could measure this. If it's 'only' 120F - my panels on the roof get this hot when it's over a 100F during summer days. It not so much that PV power is affected other than the small % loss due to heat but rather will it destroy the weather-proof sealant on the back of the panel over time. But measuring the temp would actually give 'data' instead of speculation.

Maybe cover that area (between panel and AC) with a galvanized sheet of metal from side-to-side of the panel screwed into the back side panel frame so the heat does not directly reach the panel backside material + leaving a 1/2" gap for air-flow.

 

[sunshine_eggo]

I've seen installations of this nature. You have to design for wind speed.

Maybe you could measure this. If it's 'only' 120F - my panels on the roof get this hot when it's over a 100F during summer days. It not so much that PV power is affected other than the small % loss due to heat but rather will it destroy the weather-proof sealant on the back of the panel over time. But measuring the temp would actually give 'data' instead of speculation.

Your panels hit the temperature they hit because of 1) the input energy and 2) radiating excess heat into the ambient air.

If you raise the ambient air from 100°F to 120°F with the cells receiving the same amount of solar energy, the cells will get hotter

 

[HookEm]

did you already buy the A/C Unit?

Might shop around for a low profile unit with a rear fan exhaust.
1 foot distance is mighty tall. I'm mounting my large panels with 3 inches distance to the roof - and I'm still afraid that wind might lift them of.

Since you are already cutting holes in the Van - you can also install Mini Split or Window Unit in the rear door.

Yes, the A/C unit is on hand... And I don't plan to cut holes on the roof: VHB tape and Lap Sealant along the Strut Channel.
(So getting as low as possible would be ideal)

 

[HookEm]

Maybe cover that area (between panel and AC) with a galvanized sheet of metal from side-to-side of the panel screwed into the back side panel frame so the heat does not directly reach the panel backside material + leaving a 1/2" gap for air-flow.

Not sure about the short gap, but I like the "shield" idea... Thanks!

 

[MisterSandals]

It looks like you might be able to move the back 2 panels a foot aft. Then maybe you could have a 1ft gap between the front 2 panels by moving 2nd from front back 1ft or however much room you can create.

I love VHB but too have mine about 3" above the roof. I would be afraid of having them a foot high. Make sure you tether them to something in case you have VHB failure (rare but happens.)

 

[HookEm]

It looks like you might be able to move the back 2 panels a foot aft. Then maybe you could have a 1ft gap between the front 2 panels by moving 2nd from front back 1ft or however much room you can create.

I love VHB but too have mine about 3" above the roof. I would be afraid of having them a foot high. Make sure you tether them to something in case you have VHB failure (rare but happens.)

Yes, I do have about a foot to play with, but the Vent fan is 14" in diameter... I'll redraw based on that.
And will definitely consider shortening the "vertical" gap... getting as close to the A/C as possible.

 

[rcrracer]

spit-balling one of these type of things: https://www.floorregisters-n-vents.com/White-Round-Register-8-Inch.html

 

[HookEm]

spit-balling one of these type of things: https://www.floorregisters-n-vents.com/White-Round-Register-8-Inch.html

Ummh... Sorry, but I'm not following the idea here.
(The Vent is question pushes "hot air" from the Condenser out through the top)

 

[fratermus]

Condensers run >120°F. Adding a heat source to the back of the panel will absolutely impact its performance measurably.

One would think so. But I think mythbusters (?) did a segment where a cup of hot coffee cooled faster when a blow dryer blew warm air across the top of the cup. The superheated boundary layer was being continually moved away from the surface of the coffee.

I should probably try to find pictures of Kari that episode. For science.<tm>

 

[eXodus]

Yes, the A/C unit is on hand... And I don't plan to cut holes on the roof: VHB tape and Lap Sealant along the Strut Channel.
(So getting as low as possible would be ideal)

aren't you cutting a hole for the A/C Unit? Is the location fixed?

Here my take on aerodynamics the solar hump:

Solar "Hump" for aerodynamics for Class A RV

I'm currently fitting large 435W on my RV. (82x 40 inches) My idea is to fit the long edge (82 inches) across the roof and create a kind of spoiler hump in the front of the RV. (Primary use - is to clear above the vent) This is recently done in Bus design and I saw it on some newer Class C...

diysolarforum.com

assuming that the right side of the picture is forward of your camper. You could angle the first panel to protect the following panels and the remaining panels are flat on the roof. So basically - the first panel is a spoiler while driving down the road and directing the wind up.
Move the A/C unit further forward - so that is only underneath one panel - the first one.

 

[sunshine_eggo]

One would think so. But I think mythbusters (?) did a segment where a cup of hot coffee cooled faster when a blow dryer blew warm air across the top of the cup. The superheated boundary layer was being continually moved away from the surface of the coffee.

There's a difference between "across" and "against". Across means there's a Venturi effect that draws ambient air between the coffee and the hot airstream. This will cool it.

Assuming they had a means of containing it, had they directed the hairdryer directly against the surface of the liquid, it would have heated it or at least maintained some temperature well above ambient.

I should probably try to find pictures of Kari that episode. For science.<tm>

She can science me any time.

 

[HookEm]

aren't you cutting a hole for the A/C Unit? Is the location fixed?

Nope... I'm replacing the factory-installed A/C.
(The motorhome comes with a rooftop A/C that runs on 120VAC and is *not* as energy-efficient)

 

[HookEm]

aren't you cutting a hole for the A/C Unit? Is the location fixed?

Here my take on aerodynamics the solar hump:

Solar "Hump" for aerodynamics for Class A RV

I'm currently fitting large 435W on my RV. (82x 40 inches) My idea is to fit the long edge (82 inches) across the roof and create a kind of spoiler hump in the front of the RV. (Primary use - is to clear above the vent) This is recently done in Bus design and I saw it on some newer Class C...

diysolarforum.com

assuming that the right side of the picture is forward of your camper. You could angle the first panel to protect the following panels and the remaining panels are flat on the roof. So basically - the first panel is a spoiler while driving down the road and directing the wind up.
Move the A/C unit further forward - so that is only underneath one panel - the first one.
View attachment 78325

Clever concept... Not sure I'll be able to do this since the A/C location is fixed.