Temperature of the roof metal under solar panels - Ram Promaster Forum
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post #1 of 45 Old 07-02-2019, 12:09 PM Thread Starter
phil
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Temperature of the roof metal under solar panels

I don't know the protocol or etiquette for using someone else's post to start a thread, but I didn't want this data point to get lost or buried in a thread somewhere.

All data is provided by GaryBIS

Quote:
Originally Posted by GaryBIS View Post
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/design...-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

Last edited by phil; 07-02-2019 at 12:35 PM.
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post #2 of 45 Old 07-02-2019, 12:23 PM Thread Starter
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Does the color of the roof metal under the solar panel make a difference?

How much space is needed under the solar panel to negate the heat gain?
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post #3 of 45 Old 07-02-2019, 12:58 PM
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Hi Phil,
Thanks for moving the post.


One thought I had was that if the bottom of PV panel or top of roof under the PV (or both) were highly reflective in the IR (like aluminum foil) that heat transfer to the roof would be reduced. Adding something like alum foil to the bottom of PV panel makes it a poor emitter of heat, so it sends less heat down to the roof. Covering the roof under the PV panel with something like alum foil means it reflects more of the heat radiation coming from the bottom of PV panel. But (I think) it does have to be a finish that is reflective in the IR as these are the wavelengths the PV panel will be radiating heat. Its sort of like a thermos bottle with two highly reflective surfaces facing each other --but the thermos has the additional advantage of the vacuum between.



But, with the alum foil, the PV panel would run hotter, and there might be warranty or other issues with adhering alum foil to the back of your PV panel.



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post #4 of 45 Old 07-02-2019, 03:27 PM
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Thanks for creating this thread, Phil. Very valuable empirical evidence by Gary!

I had been pondering in my "Don't laugh" thread recently, about putting 1-inch of polyiso on TOP of my roof between the solar panels and my roof.

Could that make a difference? It's foil backed on both sides. So like you said, it should reflect heat back towards the panel and any heat that makes it through the polyiso would be radiated through to the roof less effectively.

My own tests with a simple IR thermometer gun showed a few degrees of difference under the panels versus in direct sun; 4C to 8C (can't remember) with the area "shaded" by the panels being hotter.

Also, my battery was topped up at the time and so the solar panels were not putting out a lot of power (they were in full sun, but the charge controller wasn't pushing much current into the battery). I'm not sure if the charge controller restricting the current coming out of the panels made them less hot or not.

I'd test it today, but it's cloudy here.

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post #5 of 45 Old 07-02-2019, 04:00 PM Thread Starter
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Crazy idea.

Underneath the panels maybe a good spot for solar water heater tubing.

Depending on the type of climate you travel in.
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post #6 of 45 Old 07-02-2019, 04:52 PM
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Yes, thanks phil for reviving this topic. It's a timely issue for me, having just installed PV panels on mounts that have 3/4" of vertical flexibility. I've been wondering do I mount them high to reduce downward heat transfer, or mount them low to maybe reduce some wind resistance. Reducing heat transfer does seem like more of a sure thing.

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post #7 of 45 Old 07-02-2019, 07:30 PM
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Hi,
That's an interesting idea to have solar water heating tubes between the bottom of PV panel and the roof.


There were several attempts at doing home PV panels with solar water heating tubes integrated into the back of the PV panel. The idea is that at most only about 15% of the incoming solar energy gets converted to electricity, so that leaves a lot of remaining energy to heat water. These panels (I think) have not been a commercial success yet, but I think this has more to do with the complexity and cost of making such a panel and not that they don't make sense thermally. You could potentially get quite a bit more water heating energy out of the PV panel than you get electrical energy as solar water heating panels are up around 50 to 60% efficient, and, by taking heat out of the PV panel, the electrical efficiency would also be improved.


It would be a great thing to try. I think the most effective approach might be to figure out some kind of adhesive to mechanically and thermally bind the hot water tubes to the bottom of the PV panel. Anyone have an idea on what to use for an adhesive? Copper tubing might be ideal, and even though its expensive, there would not be much of it. PEX might be a possibility -- its good for 200F at 80 psi, so it might just squeak by?


A nice thing is that it would provide solar heated water with less junk and with no aerodynamic penalty



I did a PEX tubing solar water heating collector some time back, and it did work fine: https://www.builditsolar.com/Experim...W/Overview.htm


Gary

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Last edited by GaryBIS; 07-02-2019 at 07:35 PM.
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post #8 of 45 Old 07-02-2019, 08:22 PM
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One other nice option for such a system would be a 12 volt low lift pump powered from an additional 10 or 20 watt panel to circulate water only when the panels are being heated and a check valve to keep the less dense hot water from thermo-syphoning back up to the roof once the sun is not hitting the panels. I have one 4’X8’ glazed copper production panel in AZ that works on that principal and heats 90% of our water most of the winter. In a van with 5-6 gal of water in an insulated tank you would have shower water every day the sun shone.

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post #9 of 45 Old 07-02-2019, 09:00 PM
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Hi,
Couple links on hybrid solar electric and thermal panels...


https://dualsun.com/en/product/panels/


https://www.instructables.com/id/Hyb...c-and-thermal/


The 2nd one is a DIY version in which the guy builds the PV panel from individual cells as well as the thermal back to heat water. With todays PV panel prices, I don't think its worthwhile building PV panels (except as learning execise), but there are some ideas on how to extract heat from the PV panels.


I'm thinking there may be a simpler way to couple the water tubes to the back of the PV panel than in the DIY version above -- any ideas?


The first link apparently uses a water collector on the back of the PV panel made from polypropylene. This is the same material used to make solar pool heating collectors, which are cheap. Maybe a pool heating collector could be adapted for this?





Another slant on what RD suggests is to do a drainback system in which a small pump is on only when the collector is hot, and when the collector goes cold, water drains back down to the tank. I have two of these and they work find in -30F Montana with no antifreeze. As RD suggests, the pump can be run by a small solar panel (which provides automatic control), or via a differential controller that compares temp of storage tank to collector.




With a drainback, the hot water storage tank (which is not pressurized) could gravity feed to the galley sink hot water faucet -- or, there could be a simple submersible 12 volt pump could pump hot water to galley or shower -- nice and simple.



Gary

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post #10 of 45 Old 07-02-2019, 10:28 PM Thread Starter
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and all I was thinking was coiling up a black garden hose and sticking it under the panel or a https://www.intexcorp.com/products/a...7in-solar-mat/
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