67 replies to this topic

[PaulT]

My guess, as a retired engineer, is that instead of heat flow from the panel to the roof, you would get heat flow from the large roof area, exposed to the sun, to the panel (plus whatever the panel absorbs from the sun.)

+1 on the undesired heat flow direction. However, efficiency of heat flow may not be that good without appropriate heat sink compound. Haven't heard of heat sink compound that resists a stream of rain water blasting it at 55+ mph.

Sounds like an experiment is in order. Mount one on corrugated plastic panel oriented so that air flow is allowed by vehicle motion, another directly mounted to roof, another mounted 1 inch above roof with air gap. Then record amps produced by panel, vehicle speed, panel temp, roof temp and chart the results. It may be that one approach is always superior or that one is better at speed while another is better with vehicle stationary or that it depends on some combination of all the parameters.

Of course, one could just overwhelm the variations with enough panel watts for the conditions that are commonly encountered while spending one's time enjoying the experience of boondocking.

But that wouldn't be a guy thing to do, would it?

Man, I love this forum.
Paul

[MarkBC]

Personally, I don't worry much about the temperature effect.  The effect of cloudy days and partial shade and low-sun/short-days in winter are all a lot bigger than the temperature.  And those other factors all reduce the current, while an increase in temperature reduces the voltage, typically ~ 0.3 - 0.5 %/°C.  So, there are conditions where you could lose up to 20%...but that's still a small loss compared to the shade/sun-angle losses.

Unless you're using an MPPT charge controller you're probably throwing away excess voltage anyway.  Panels may be putting out 17+ volts but your battery never needs much more than 14+ volts.

 

All things being equal, I'm sure it's a good idea to reduce the temperature effect -- but not necessary...not for me, anyway.

[Boonie]

So far the consensus is to screw it direct to the roof and have a beer while we watch the experimenters try to figure out that an air gap does very little.

[JaSAn]

My guess, as a retired engineer, is that instead of heat flow from the panel to the roof, you would get heat flow from the large roof area, exposed to the sun, to the panel (plus whatever the panel absorbs from the sun.)

 

It's not my specialty but from my remembrance from my heat transfer classes black gets significantly hotter in the sun than white.  One study showed the interior of a black vehicle in the sun gets ~ 25º hotter than a white one.  I think that would hold true for solar panels/camper roof also.

 

Reason I put an air gap in is insulation; air is a poor heat conductor.  I don't want the interior any hotter than it already gets in the sun.

 

jim

[Esus]

My guess, as a retired engineer, is that instead of heat flow from the panel to the roof, you would get heat flow from the large roof area, exposed to the sun, to the panel (plus whatever the panel absorbs from the sun.)

 

This is exactly where I'm coming from. The corrugated plastic isn't to stream air through the panels while driving. In fact, the direction of the corrugation isn't relevant in my mind - it could be side-to-side rather than front-to-rear. We're obviously not driving 100% of the time and solar, for me, is for not driving and hanging out off grid. The roof transfers heat to the panel when directly mounted. Any medium that prevents heat transfer to the panel will help keep it within its operating range (I think the max temp for the Solar Cynergy panel is something like 180 F). My thought with adding the corrugation is that it's essentially 3 layers of insulation; polycarbonate, air, polycarbonate - with the air gap being the most efficient part of the trio. 

 

There are a coupe good reasons for not mounting the panel directly to the studs. Even with a nut under the panel to create an air gap, the panel is very flexible and would bow in the middle allowing it to come in contact with roof. Also, with wind speeds while driving, the flex in the panel and the gap would create a lot of movement in the panel. I can't imagine that would be good for it. The corrugated polycarbonate is significantly stiffer. Making it one long piece with the panels glued to it adds to its structural integrity. Rather that each panel having 4 to 6 mount points and flex, this one, single reinforced unit it mounted with 16 total studs.Having built and experienced it first hand, this not just theoretical. It really is a lot stronger and I'm not concerned with airflow getting under it like I would have been with a raised panel. 

 

I agree with MarkBC that efficiency isn't a big deal when compared to, say, the angle of the panel, but directly mounting it to the roof isn't just an efficiency issue. People routinely report the cells cupping and warping from the excessive heat. I'm trying to avoid physically damaging and ultimately breaking the panel. I oversized my setup to overcome the efficiency issues. 290 watts of solar can suffer from a number of efficiency issues and still charge me up quickly. Especially with panels wired in series which is desirable for the nice MPPT controller I have. Not to mention a LiFePO4 battery that can charge much faster than conventional lead acid.

 

I love these conversations, This forum kicks ass.

[rando]

I may be way off base here - but if you want to keep your panels cooler, I would think you would want to mount them directly to the roof.  The panels get so hot because they are essentially black, and absorb a lot of solar radiation (a small amount of which they convert into electricity).   The roof is bright white aluminium, and reflects most of the incoming solar radiation and as a result is much cooler than the panels.   Insulating the panel from the roof is going to keep all that heat in the panel, whereas thermally bonding it to the roof would allow that large area of roof to act as a heatsink of sorts - spreading the heat out over a much larger area and radiating it back to the air.   I would guess that the twinwall would act more like an insulator than allowing airflow.  I think the idea of airflow under panels comes from mounting panels to asphalt shingle roofs, which get super hot themselves and are poor thermal conductors. 

 

That said, I certainly wouldn't reccomend actually bonding these panels to the roof as they have a pretty high failure rate, but my guess (and it is only a guess) would be that panels in good contact with the roof would likely be cooler than those insulated from the roof.   This would be pretty easy to test.

[nikonron]

Airflow under the panels will keep them cooler, the built up or produced heat needs to escape not be trapped.  Ron

Edited by nikonron, 20 April 2017 - 07:29 PM.

[Boonie]

Rando, Have a beer and while you are up would you get one for me?