[janlifevanlife]

Ahh, but the shelf is not in the cab. It is above and completely separate from the cab. It is only accessible from, and therefore an extension of, the cargo area. And, unlike the cab, it’s only purpose is storage.

HA! that's the term I was trying to use. the SHELF!

btw I ended up gluing two furing strips (2x1) together. They're each technically about 3/4 inch thick. It doesn't seem to be hanging to low from the liner on top of the shelf . Ill taking photos so help give an idea.

 

[MsNomer]

That 1.5" should be perfect.

 

[gdpetti]

I began like akarmy but used no plywood, just headliner fabric over the 1” of polyiso. If your ceiling is not up yet you might rethink the 2”? Have you calculated the heat loss/gain? If I had used 2” my Espar heater would short cycle even at freezing. Same if I installed the smallest practical A/C unit.

See: https://www.builditsolar.com/References/Calculators/VanHeatLoss/VanHeatLoss.htm

Everyone should be using this.

I was wondering if anyone has seen or done analysis on the difference of the roof insulated with solar panels above them or not? The shading should be helpful if it's a large panel covering a large area of the roof, and if you load up and cover almost the whole roof except the fan area/section, then the heat loss/gain should be even better, no? The solar panels need ventilation, but that would only help dissipate any heat buildup so again, helpful to keeping the roof cooler... right? Has Gary @buildagreenrv done this analysis? I didn't see anything like that there. Hmmm... then if you built in the panels to tilt... it would affect the issue as well... assuming you kept them tilting well and the sun off the roof, but on the panels as well as possible during the day like that Lee Blake guy with his build.

 

[GaryBIS]

I was wondering if anyone has seen or done analysis on the difference of the roof insulated with solar panels above them or not? The shading should be helpful if it's a large panel covering a large area of the roof, and if you load up and cover almost the whole roof except the fan area/section, then the heat loss/gain should be even better, no? The solar panels need ventilation, but that would only help dissipate any heat buildup so again, helpful to keeping the roof cooler... right? Has Gary @buildagreenrv done this analysis? I didn't see anything like that there. Hmmm... then if you built in the panels to tilt... it would affect the issue as well... assuming you kept them tilting well and the sun off the roof, but on the panels as well as possible during the day like that Lee Blake guy with his build.

Hi,
I went out and took a couple pictures with the thermal camera.


Full sun.
Ambient temperature 73F
Very light breeze.


The IR pics (for this case) show a PV panel surface temp of 139F, and a white roof temp around 113F.


These may not be right on as the camera assumes that emisivity of the surfaces you are taking a picture of is 0.95, but both white paint and the glass surface of the PV panel should be close to 0.95.


Because my PV panel is mounted so close to the roof, I could not get a temperature of the roof under the PV panel (which is really whats wanted) -- maybe someone with PV panels mounted further above the roof could get some temps with an IR temperature gun.


But, I think that it likely that the back of the PV panel runs about the same temp as the front, and also likely that the roof surface under the PV panel (with the very light breeze) is also pretty close. On my home PV array, the front and back temps of the PV panels are close.



So, seems likely that the heat gain on a sunny day is going to be greater for the part of the roof under the PV panel. If you assume the roof surface under the PV panel is the 139F that I measured on top the PV panel, and that the roof would be at 113F without the PV panel, and that you are trying to maintain 70F inside, then the heat gain without the PV panel would be about 40% less that with the PV panel.
Going from 1 inch polyiso to 1.5 inch polyiso would take care of it.



All just kind of guesses at this point -- it would be nice to get the actual temps of the roof under the PV panel to get a better estimate.



Gary

 

[RDinNHandAZ]

And if you have a non-white van that roof is going to be MUCH hotter. Probably the panels do not contribute extra heat to non-white vans. WAG

 

[joylesshusband]

So, seems likely that the heat gain on a sunny day is going to be greater for the part of the roof under the PV panel. If you assume the roof surface under the PV panel is the 139F that I measured on top the PV panel...

//snip

Bolding mine.

This would be a mostly correct assumption only if the bottom of the PV panel was in full contact with the roof surface.
Otherwise, it would be an entirely wrong assumption, leading to wrong conclusions.

 

[RDinNHandAZ]

OK that is easy to say. What would it be?

It seems if there is a flat plate heat source giving off heat at 139º suspended an inch or two above a sheet of metal the metal would tend towards the temperature of the radiating heat source especially in calm air and more so if the sheet metal was insulated on the non-radiating side.. If that is wrong please elaborate.

(Stefan–Boltzmann law?)

 

[GaryBIS]

//snip

Bolding mine.

This would be a mostly correct assumption only if the bottom of the PV panel was in full contact with the roof surface.
Otherwise, it would be an entirely wrong assumption, leading to wrong conclusions.

Hi,
I think the back of the PV panel is likely to be close to the same temperature as the measured temperature of the front of the panel -- this is based on the PV panel basically being a fairly thin sheet of glass (with high thermal conductivity) with a very thin coating of plastic on the back, and its also what I measured on front and back of my home PV panels (nearly the same temperature front and back).



The roof of the van is insulated inside, so it is not going to get any meaningful cooling from the inside of the van, and, because the wind velocity was near zero, there is not going to be much heat transfer via air currents between the PV glass and van roof.



There is some heat exchange between the PV panel and the roof surface via radiation and conduction/convection through the thin layer of air, so it seems like this heat exchange will drive their temperatures to about the same level?



But, I think I do have a surface mount thermocouple around somewhere -- I'll try to find it and get an actual reading on the roof surface temp. Might be a couple days til we get some sun.



Gary

 

[joylesshusband]

There is some heat exchange between the PV panel and the roof surface via radiation and conduction/convection through the thin layer of air, so it seems like this heat exchange will drive their temperatures to about the same level?

Gary,

The predominant mode of heat transfer from back of PV panel to roof surface is radiation. Convection in between is nearly zero due to the open space in even the slightest of breezes.
Conduction is non-existent as these surfaces are _NOT_ in contact.

You seem to be forgetting that the roof, and the entire van body, is in fact a huge steel heat sink. A radiator if you will. Any spot anywhere on the surface being heated (by whatever source and heat transfer mode) rapidly conducts this heat to the rest of the steel body of the van, and thus its locally increased temperature quickly reaches an equilibrium with its vicinity, as heat conduction in steel works extremely well.
Meaning that the roof surface under the PV panel will read just about the same temperature as the one exhibited over the rest of the roof.

Go ahead with your thermocouples if you wish (one - under the PV panel, and another - elsewhere on the roof). Their readings will only confirm what I stated.

 

[RDinNHandAZ]

Ah a testable hypothesis, great. One factor not accounted for yet is the time factor in heat dissipation of the conduction. I used to hold a nail by it’s head and heat the point in a lab burner to demonstrate this to my physics students. You can get the heated end glowing before you need to drop he nail! I predict a gradient in the temperature of the roof under the panel from very near the radiating panel’s rear temperature a foot under the panel to a lower temperature a foot from the panel consistent with the rest of the van top being heated only by the Sun.
Essentially the roof acts as a "black body" (but white I know) absorbing the radiation emitted by the rear of the panel in one place and radiating away that heat from the surrounding roof.

 

[GaryBIS]

Hi,
Nice sunny day here, so did the test with surface mount thermocouple to measure actual temperature of the roof below the PV panel.


I did several runs, but this one is representative:

• Ambient temperature: 73F
• Full sun: 790 watts/sqmeter on PV panel surface
• Inside van temperature: 78F
• Wind speed: 0 to 4 mph (near calm)
• PV panel top surface temperature: 157F
• Roof temperature under the PV panel: 136F
• Roof temperature outside the PV panel: 119F

So, the roof is about 17 F warmer under the PV panel than away from it. If you were trying to maintain them temperature inside the van at 70F, then the heat gain for the part of the roof under the PV panel would be about 35% greater per sqft than the gain from rest of roof.
This is with a white roof van, as RD pointed out, the difference would likely be less with a dark roof as the roof runs hotter.


The roof temperature under the PV panel ends up being about half way between the PV panel temperature and the roof temperature.

Our van.

Surface mount thermocouple on roof under PV panel (PV panel edge unscrewed and raised up a bit to allow placement of the thermocouple)

surface mount thermocouple on top surface of PV panel

Measuring solar radiation

Measuring wind velocity





A little more on the web page for this test: https://www.buildagreenrv.com/desig...tion-for-camper-vans/install-electrical/effect-of-rv-solar-panels-on-heat-gain/



JH -- on your comments...



"The predominant mode of heat transfer from back of PV panel to roof surface is radiation. Convection in between is nearly zero due to the open space in even the slightest of breezes.
Conduction is non-existent as these surfaces are _NOT_ in contact."


There is contact and conduction between the two surfaces and the air and then heat transfer from convection currents. You don't have to have contact between the two final surface to have heat transfer that is in part conduction -- almost all heat exchangers use conduction/convection to a heat exchange media (like water) with no contact between the final surfaces.




"You seem to be forgetting that the roof, and the entire van body, is in fact a huge steel heat sink. A radiator if you will. Any spot anywhere on the surface being heated (by whatever source and heat transfer mode) rapidly conducts this heat to the rest of the steel body of the van, and thus its locally increased temperature quickly reaches an equilibrium with its vicinity, as heat conduction in steel works extremely well.
Meaning that the roof surface under the PV panel will read just about the same temperature as the one exhibited over the rest of the roof."


The thin sheet metal of the (0.036 inch) does not hold much heat and is not really that effective in conducting heat away.
The whole roof weights about 145 lbs and with a specific heat of 0.122 BTU/lb-F, it only holds/sinks about 18 BTU per deg F of temperature increase -- almost nothing compared solar heat flux coming onto the van.



Its also not that effective in conducting heat away from the hotter PV panel area - its just to thin to provide an effective heat transfer area. The perimeter of the panel is 19.5 ft, and this whole perimeter has a heat transfer area of only (19.5ft)(0.036 inch/12 in/ft) = 0.059 sqft. If you consider the heat transfer through a 6 inch wide strip of sheet metal all the way around the perimeter of the PV panel for a 1F temperature difference, it comes out:

Q = (k/S)(A) (dT) = ((25 BTU/hr-F-ft)/(0.5ft)) (0.059ft^2)(1F) = 3 BTU/hr --- pretty darn small compared to the more 5400 BTU/hr coming onto the PV panel.



If the van roof/skin were such an effective at transferring and equalizing temperature differences as you suggest, you would not see such large differences in skin temperature around the van. In the test this morning, walking around the van and measuring skin temperatures, they ranged from 72F to 120F -- sometimes with only small distances between markedly different temperatures.



It would be nice if you provided some numbers to go with the strong assertions you make.


Gary

 

[phil]

This should be it's own thread so it doesn't get buried.

 

[RDinNHandAZ]

You the man Gary! Everyone gets all a flutter when you bring out the instrumentation and we get to do research! Thanks.

 

[GaryBIS]

I do love a good test -- no idea why

 

[travelvanvan]

Awesome, Gary!

Not to hijack, but you proved why I, in a different thread (my "Don't laugh" thread) had been ruminating about putting 1-inch of polyiso on TOP of my roof between the solar panels and my roof.

I had done my own cruder testing, using a simple IR laser pointer type thermometer. There was a noticeable difference in temps by touch (hand on the uninsulated roof from inside) and by the IR thermometer (I can't remember if it was 4C or 8C difference).

Also, I'm not sure if it matters, but my battery was topped up at the time and so the solar panels were not pumping out a lot of power (they were in full sun, but the charge controller wasn't pushing much current into the battery).

 

[joylesshusband]

The thin sheet metal of the (0.036 inch) does not hold much heat and is not really that effective in conducting heat away.
The whole roof weights about 145 lbs and with a specific heat of 0.122 BTU/lb-F, it only holds/sinks about 18 BTU per deg F of temperature increase -- almost nothing compared solar heat flux coming onto the van.

Gary,

Thank you for the data you collected.

I may concede a few points, but not this one (quoted above).
You can not seriously consider the roof as a separate body in isolation. It may be only 145 lbs, but is firmly attached to the van body, whose mass is in the vicinity of 1,200 lbs. Heat gets conducted throughout the whole body and while its speed of dissipation is slow, it still propagates. That's how heat sinks work.

At any rate, I am glad you demonstrated that the heat emitted by the PV panel is far from having any monstrous effects on van's indoor temps, and the ceiling insulation does not to be beefed up under the panels.
What is the distance between roof and panel underside on your van?

 

[smdub]

You can not seriously consider the roof as a separate body in isolation. It may be only 145 lbs, but is firmly attached to the van body, whose mass is in the vicinity of 1,200 lbs. Heat gets conducted throughout the whole body and while its speed of dissipation is slow, it still propagates. That's how heat sinks work.

Yes and no. Heat sinks can only conduct heat to their extremities if they have higher thermal conductivity than the surface area for convection/radiation heat removal. A very thin steel sheet (a promaster body) is a terrible heat sink design. The rest of the van will never attain the same temperature as the roof. You can prove this to yourself w/ the van in the sun. Does the bottom of the shady side wall ever reach the same temp as the roof? No. Gary could prove this by adding a few thermocouples radiating out (or just moving one.) The rest of the van will NEVER reach the same temperature as that under the panel as there is heat loss from the entire surface trying to cool the van to ambient. Steel is a miserable heat conductor (about an order of magnitude worse than aluminum.) All materials have about the same radiation & convection effectiveness however. It simply can't carry the heat vary far before the energy is lost.



We design our own heat sinks using FEA. I don't have one of those handy so I grabbed the first pic I found on the net. You can see in the design below the heat flux is being introduced in the channel. The farther fins are cooler because the heatsink doesn't have infinite thermal conductivity. This is a typical looking heatsink pattern.

 

[GaryBIS]

Hi,
My PV panel is mounted as close to the van roof as I could get it without touching -- its about 1/4 inch off the roof at the center and about 5/8 inch at the outer edges. I had to undo the anchor screws on one side to get the thermocouple in toward the center of the panel, and then lower the panel back down.
I did this thinking it would minimize the frontal area increase and reduce drag.



Gary

 

[joylesshusband]

My PV panel is mounted as close to the van roof as I could get it without touching -- its about 1/4 inch off the roof at the center and about 5/8 inch at the outer edges.

That's what I thought when I saw your thermocouple readings.
I doubt anyone else has placed their PV panel so close to the roof, which means the roof temps under the panel would be much lower than what you measured.

 

[RDinNHandAZ]

Mine are near Gary’s. I’d think many of us who used tabs from the roof are also similar. Why don’t you test your idea and post the data? Without that it’s just a WAG.