roof mounted panels and re-radiated heat

[frater secessus]

At 4pm in my location the sun was 41.4 deg above the horizon, so it was well past full sun.

• ambient temp was 98F.
• underside of panels were 117F even though the sun was low-ish and I have a generous 5" standoff between the roof and bottom of the cells.
• interior temp of uninsulated section of roof was 103-105F where the roof was shaded by the panels.
• interior temp of insulated section of roof was 100F where the roof was shaded by the panels.

This appears to be a function of heat re-radiated from the underside of the solar panels.  I would expect the phenomenon to be more pronounced in full sun conditions (hotter panels), and more pronounced for folks who have less of a gap between panels and roof.  

Takeaway:  roof insulation may be useful with roof-mounted panels even if living in very mild climates.   

Bonus observation:  the white body of the van was 8F-15F cooler than the black plastic moldings around the wheelwells, etc.  I don't know how much color affects the temp vs. shiny paint / matte plastic.

 

[Spaceman Spiff]

 . . . Takeaway:  roof insulation may be useful with roof-mounted panels even if living in very mild climates . . .   

Interestingly, even though the roof temperature of a black vehicle is 40º - 50ºF hotter than a white roof in full sun, the interior temperature is only about 10º - 15ºF warmer.  The little bit of insulation in the headliner has that much effect.

 

[Gideon33w]

I'd call that temperature difference pretty minor considering it's a metal box sitting in the sun. Coat the rest of your roof in elastomeric reflective roof coating.
It will absorb less heat from the sun and then act as a heat sink for the area under the panels. Not that insulation is a bad idea though.

I did the roof coating on mine. In an Arizona summer when the body panels would burn you instantly you could leave your hand on the roof no problem.

 

[frater secessus]

The sun was low and filtered through trees. Panel temps were 50F+ less than at full sun in my area. Will test today again under full sun and update the thread.

Most folks have less than 5" gap and so will be more affected.

Sent from my BLU_STUDIO_XL using Tapatalk

 

[highdesertranger]

this time get a temp reading on the floor in the inside too. remember hot air rises. the 2 degree difference between ambient and the inside ceiling is pretty normal. highdesertranger

 

[John61CT]

For the sake of panel efficiency and longevity you want to get the radiated heat below away asap.

Reflective paint there will keep the roof cooler but put the heat back up into the solar cells, not good.

In theory you'd want a black membrane sitting on the roof under panels, with water circulating within off to a heat sink, aircooled radiator whatever.

Or just active airflow in the space between would help.

 

[frater secessus]

Watercooling the panels could help with their heat AND get water warmed up for human use.  

I do have some additional readings to post later today.  I did check the floor, which was the same as ambient.  I didn't mention it because I have a ~3/4" vanrug thing on the floor.  

 

[user 15722]

Thanks to all participating !
I had considered roof mounted panels might shade the coach. Did not consider them radiating heat downwards...
What height bracket is best ? We'll fabricate them ourselves when I figger a simple way to tilt them for winter use.
Tnx, wheels

 

[John61CT]

Watercooling the panels could help with their heat AND get water warmed up for human use

Wow, too true!

An aluminum sheet design purpose-built, come up with a size that fits most common panels.

Thermal bonding pastes the flat panel backing directly to the flat heatsink surface. Water circulates within, not in channels, would create uneven temps, but as a sheet between flat sheets, spacers maybe more thickness where the flow would otherwise be less.

Need to iterate that part of the design using infrared video until the top cells' surface is pretty evenly cooled in 100+° conditions.

Then comes the biggest challenge, getting the cycling liquid cool using a minimum of energy.

I guess a truck radiator's the way to go.

And super-efficient high CFM fan(s).

Hmm, I bet this all has been done before, at least academic tests must be publicly available.

Fun!

 

[tx2sturgis]

Solar panels that do BOTH:

Solar PV AND Solar Thermal.

Might be time to apply for a patent!

 

[Gideon33w]

There's still the trade off in fuel economy. The higher the array the greater the frontal area. Increasing drag substantially.
In a van getting 15mpg highway a 2mpg decrease is a pretty large loss.
Fairings will decrease the added drag but also decrease the cooling airflow around the panel.
Everything is a compromise.

 

[highdesertranger]

I say 2 degrees above ambient on the inside is nothing. any heat on the inside is going to rise. so I don't see a problem here. highdesertranger

 

[John61CT]

Roof rack that can be lowered for travel, raised when panels in use.

 

[Boyntonstu]

At 4pm in my location the sun was 41.4 deg above the horizon, so it was well past full sun.

• ambient temp was 98F.
• underside of panels were 117F even though the sun was low-ish and I have a generous 5" standoff between the roof and bottom of the cells.
• interior temp of uninsulated section of roof was 103-105F where the roof was shaded by the panels.
• interior temp of insulated section of roof was 100F where the roof was shaded by the panels.

This appears to be a function of heat re-radiated from the underside of the solar panels.  I would expect the phenomenon to be more pronounced in full sun conditions (hotter panels), and more pronounced for folks who have less of a gap  between panels and roof.  

Takeaway:  roof insulation may be useful with roof-mounted panels even if living in very mild climates.   

Bonus observation:  the white body of the van was 8F-15F cooler than the black plastic moldings around the wheelwells, etc.  I don't know how much color affects the temp vs. shiny paint / matte plastic.

"interior temp of insulated section of roof was 100F where the roof was shaded by the panels."

Perfect!

Measure the paint where there was no shade.  160* F is not unusual.
Do you remember burning your hand on your sunny car paint?

 

[John61CT]

Watercooling the panels could help with their heat AND get water warmed up for human use.  

Nothing new under the sun

http://www.custommarineproducts.com/solar-water-heating-system.html

 

[frater secessus]

What height bracket is best ?  We'll fabricate them ourselves when I figger a simple way to tilt them for winter use.

Dunno about best but some is better than none, both for underside breeze cooling and decrease in reception of radiated heat at a distance (1/r^2).  A couple inches at least?  If the cable gland or box is mounted under the panels for additional rain protection they'd have to clear that anyhow. 

My panel height ( 5") is taller than I would have otherwise chosen;  it was a function of flush-mounting with Vantech rack crossbars.   All else being equal I might have gone for something like 3".

 

[frater secessus]

Measure the paint where there was no shade.  160* F is not unusual.

I have done so previously.  Solar panel temp was higher than unshaded body panels. 

Hottest to coolest:
solar panel top
unshaded body panel
shaded body panel
shaded body panel (under solar panel)

I have not measured the top and bottom of the solar panel in the same conditions.