Insulation under solar panels

[RobinTH]

I am planning to cover every inch of my travel trailer roof with panels. I read someone make the point that making the roof turn from white to black could add a lot of heat inside the trailer. That could be a valid point, so I was thinkig of leaving a gap for ventilation directly below the panel ( maybe adding a series of vent holes around the edge of the panel frame) ( not enough in quantity or size to weaken the frame), Then placing a high r value slab of foam directly under the panel tight to the roof. The edges would have to be sheilded from the sun to prevent deterioration. Any thoughts?

 

[Ex-Calif]

On boats we would generally install panels with a 1 inch standoffs and just let air circulate.

 

[Jayflight]

In most fixed framed solar panels you already have an air space between the panels and the roof with the panel mounting feet around the perimeter. So I would not think that the blacking out issue really come into play.

 

[RobinTH]

On boats we would generally install panels with a 1 inch standoffs and just let air circulate.

Thanks for commenting. That is the norm. I guess the only way to know if the insulation would be of practical benefit, would be to set up a temp sensor under the roof in a side by side test of both methods. If I have a chance to do that I will post my results.

 

[tlmgcamp]

Wouldn't the panels effectively be creating shade for the roof underneath them? I guess the panels will hot but heat wants to go up, not down so the air gap should take care of the heat that is in excess of what direct sunlight on the roof would create. Probably wouldn't hurt to leave a couple inches between panels to aid ventilation

 

[Rene T]

I think adding the foam panels would allow the roof to stay wet possibly a source for mold to start growing. I would just leave a air gap

 

[RobinTH]

I think adding the foam panels would allow the roof to stay wet possibly a source for mold to start growing. I would just leave a air gap

That could be a valid point if the edges were not sealed, and we all know there is already enough things up there to keep sealed.

 

[Utclmjmpr]

Not necessary as heat rises,, and with the stand off brackets under the panels will allow ventilation...>>>Dan

 

[RobinTH]

Wouldn't the panels effectively be creating shade for the roof underneath them? I guess the panels will hot but heat wants to go up, not down so the air gap should take care of the heat that is in excess of what direct sunlight on the roof would create. Probably wouldn't hurt to leave a couple inches between panels to aid ventilation

Just wanting to add a bit of science to the discussion. "Hot air" does in fact rise. Having worked with radiant heating taught me that any matter at above zero degrees Kelvin emits radiant heat energy equally in all directions. When the sun peeks out from behind a cloud you immediately feel the radiant energy, not hot air. So if the underside of the panel gets heated to a higher temp because of the upper side being black, could it radiate more energy to the roof, than an area of the roof subjected to direct sunlight? I don't have an answer currently, and only side by side tests would prove one way or the other. I haven't ordered my panels yet, so has anyone ever put their hand on the underside of a panel to feel the heat on that surface?

 

[Ex-Calif]

Just wanting to add a bit of science to the discussion. "Hot air" does in fact rise. Having worked with radiant heating taught me that any matter at above zero degrees Kelvin emits radiant heat energy equally in all directions. When the sun peeks out from behind a cloud you immediately feel the radiant energy, not hot air. So if the underside of the panel gets heated to a higher temp because of the upper side being black, could it radiate more energy to the roof, than an area of the roof subjected to direct sunlight? I don't have an answer currently, and only side by side tests would prove one way or the other. I haven't ordered my panels yet, so has anyone ever put their hand on the underside of a panel to feel the heat on that surface?

Agree 100% but IMO the idea is that ventilation around the panel keeps the bulk panel temperature lower, therefor less radiated heat.

 

[Gary RV_Wizard]

Yes, the radiant heat will go down as well as up. And as it heats the roof below the panel, that surface also begins to radiate heat in all directions. The air in between gets warmer and quickly begins to rise, but trapped by the panel above, it gets squeezed out the sides. Meanwhile, that "extra" heat is at least partially heat that would have been generated from direct sunlight on the roof if there were no panels. So several factors are in play, with varying degrees of impact that are hard to assess in net effect.

I think the overall heat transfer into the roof from the panels is going to be modest, or maybe even near zero. I side with those who think that a simple stand-off to allow some ventilation under the panel is all that is desirable.

 

[Kevin Means]

"It may seem counter-intuitive, but Solar panel efficiency is affected negatively by temperature increases. Photovoltaic modules are tested at a temperature of 25 degrees C (STC) – about 77 degrees F., and depending on their installed location, heat can reduce output efficiency by 10-25%. As the temperature of the solar panel increases, its output current increases exponentially, while the voltage output is reduced linearly. In fact, the voltage reduction is so predictable, that it can be used to accurately measure temperature."

That's a quote from Greentech Renewables. The entire article is here How Does Heat Affect Solar Panel Efficiencies?.

There are many other articles that talk about this - it's nothing new. The only reason I mention it is because you said you're going to cover your roof with panels - maybe provide ventilation holes, then maybe some insulation. Ventilation of some type, that allows for good airflow under the panels is a good idea, but I recommend that you not place insulation under the panels. Keep them as cool as possible.

Kev

 

[RobinTH]

When I get my panels I will do some tests and update this thread. Thanks for the opinions.

 

[Len and Jo]

I am planning to cover every inch of my travel trailer roof with panels. I read someone make the point that making the roof turn from white to black could add a lot of heat inside the trailer. That could be a valid point, so I was thinkig of leaving a gap for ventilation directly below the panel ( maybe adding a series of vent holes around the edge of the panel frame) ( not enough in quantity or size to weaken the frame), Then placing a high r value slab of foam directly under the panel tight to the roof. The edges would have to be sheilded from the sun to prevent deterioration. Any thoughts?

Temperature: High temperatures will directly reduce the efficiency of a photovoltaic panel. Insulating the bottom of the panel, could reduce its efficiency. Temperature—Solar cells generally work best at low temperatures. Higher temperatures cause the semiconductor properties to shift, resulting in a slight increase in current, but a much larger decrease in voltage. Extreme increases in temperature can also damage the cell and other module materials, leading to shorter operating lifetimes. Since much of the sunlight shining on cells becomes heat, proper thermal management improves both efficiency and lifetime.

 

[RobinTH]

Temperature: High temperatures will directly reduce the efficiency of a photovoltaic panel. Insulating the bottom of the panel, could reduce its efficiency. Temperature—Solar cells generally work best at low temperatures. Higher temperatures cause the semiconductor properties to shift, resulting in a slight increase in current, but a much larger decrease in voltage. Extreme increases in temperature can also damage the cell and other module materials, leading to shorter operating lifetimes. Since much of the sunlight shining on cells becomes heat, proper thermal management improves both efficiency and lifetime.

You missed the part about the air gap between the panel and insulation. The stand offs would have to be taller to give the standard gap. The panel would be no hotter than the un-insulated scenario. The issue which we don't currently have an answer to, is whether or not the roof surface is hotter or cooler under the panel compared to the no panel areas.

 

[Jayflight]

There are thousands of rigid panels installed on low and high end rvs . They are used in the southern most part of the U.S. I don't recall reading lengthy descriptions about changes of temperature inside their rvs. If you look at some of the you tube videos and the overhead drone shots there are a lot of units with many panels on them.

But of course since you say that you are in Nova Scotia, your warm weather season are fairly short to generate the same extremes as the southern borders will generate for longer periods of times . I think you may be over thinking this a wee bit unless you are planning on relocating and needing full solar to operate your unit. Of course your maximum battery capacity needs to be proportionate too. So what are planning to use on the storage end? And just how much total solar are you thinking on using when covering your entire roof?

Of course you can buy one solar panel with the capacity of four panels, for an example, reducing the room needed. I think we need more details of your unit.

 

[Tulecreeper]

Jayflight has a good point. Are you planning on quadrupling your battery bank size, or running more loads directly from the panels?

 

[RobinTH]

There are thousands of rigid panels installed on low and high end rvs . They are used in the southern most part of the U.S. I don't recall reading lengthy descriptions about changes of temperature inside their rvs. If you look at some of the you tube videos and the overhead drone shots there are a lot of units with many panels on them.

But of course since you say that you are in Nova Scotia, your warm weather season are fairly short to generate the same extremes as the southern borders will generate for longer periods of times . I think you may be over thinking this a wee bit unless you are planning on relocating and needing full solar to operate your unit. Of course your maximum battery capacity needs to be proportionate too. So what are planning to use on the storage end? And just how much total solar are you thinking on using when covering your entire roof?

I'm embarking on full time rving, north in summer and south in winter. This is more a theoretical discussion, than a perceived need. I merely read this statement and was pondering its validity. I would imagine it would be more true if you were using flexible panels bonded directly to the roof surface. I will be using framed panels with an air gap.

 

[HueyPilotVN]

I have eight 200 watt panels on my roof. I am not at all worried about heat transfer and we have extreme summer temps here in southern Arizona.

First, I think that the shade created by the panel would help mitigate any heat transfer.

I also used insulated plastic mounts that both raise the panels for air flow and prevent any direct heat transfer thru the traditional metal mounts.

https://www.amazon.com/Brackets-Mounting-Drill-Free-Bracket-Caravans/dp/B087CMF8SW/ref=sr_1_13?crid=2ABS18CQ04OY9&keywords=solar+panel+mounts+for+roof&qid=1685826816&sprefix=solar+panel+mounts%2Caps%2C150&sr=8-13

 

[RobinTH]

Jayflight has a good point. Are you planning on quadrupling your battery bank size, or running more loads directly from the panels?

I'm building 4 LiFePo4 batteries totaling 1120 amphours, about 2000 watts of solar feeding 2 mppt controllers (1 for each side of the domed roof). 4000 watt inverter and all led lights, to facilitate long term boondocking. I will seldom see a campground.

There are thousands of rigid panels installed on low and high end rvs . They are used in the southern most part of the U.S. I don't recall reading lengthy descriptions about changes of temperature inside their rvs. If you look at some of the you tube videos and the overhead drone shots there are a lot of units with many panels on them.

But of course since you say that you are in Nova Scotia, your warm weather season are fairly short to generate the same extremes as the southern borders will generate for longer periods of times . I think you may be over thinking this a wee bit unless you are planning on relocating and needing full solar to operate your unit. Of course your maximum battery capacity needs to be proportionate too. So what are planning to use on the storage end? And just how much total solar are you thinking on using when covering your entire roof?

Of course you can buy one solar panel with the capacity of four panels, for an example, reducing the room needed. I think we need more details of your unit.

I have measured the roof but haven't plotted out the panels yet. I'm thinking the batwing tv antenna is useless now, so removing that will leave room for 400 watts.