Air gap beneath flexible panels

[PhilC]

I see a lot of people suggesting the use of Coreflute/corrugated plastic/plastic cardboard beneath flexible solar panels to provide an 'air gap' to aid in cooling.

Anyone out there with actual measurements of temperature with and without this behind the panel?

I am a little skeptical that sufficient air would flow through the channels to convect more heat away than conduction through a hard substrate. Looking for some science (rather than youtube) based evidence if anyone has it.

[Uncle Bob]

Originally Posted by PhilC

I see a lot of people suggesting the use of Coreflute/corrugated plastic/plastic cardboard beneath flexible solar panels to provide an 'air gap' to aid in cooling.

Anyone out there with actual measurements of temperature with and without this behind the panel?

I am a little skeptical that sufficient air would flow through the channels to convect more heat away than conduction through a hard substrate. Looking for some science (rather than youtube) based evidence if anyone has it.

Hi, no experience with corflute etc but have seen a paneling section that consisted of translucent plastic, two walls with regularly spaced ribs between them approx 10 to 12 mm between the skins, the temp difference between the sunny side and the other side was markedly different so yes they can work.

[BAD ORCA]

Originally Posted by Uncle Bob

Hi, no experience with corflute etc but have seen a paneling section that consisted of translucent plastic, two walls with regularly spaced ribs between them approx 10 to 12 mm between the skins, the temp difference between the sunny side and the other side was markedly different so yes they can work.

That stuff is called polycarbonate twinwall.

I came here to ask a similar question this morning. What about a solid piece of 1/8" aluminum? I am considering using it for a small 26 x 26 panel mounted on 1/4" aluminum standoffs. The panel would be mounted directly to the aluminum and the standoffs would obviously go between that and the deck.

I guess the aluminum could be a weight issue if its a lot of panels though but wonder how much better it would cool -vs- the corrugated plastic.

[Rumrace]

I’ve printed pin an used some thin ss strapping to connect them. The four panels which burnt my canvas are now 70mm above the canvas at the stern and 45mm towards the front of the Bimini.
Can’t say they are running cooler because I never measured them in the stupid position. The only thing which got too hot was the connector and I’m printing heat sinks for them I’ve also gone from 3-4 panels. Great results

[thinwater]

Originally Posted by Uncle Bob

Hi, no experience with corflute etc but have seen a paneling section that consisted of translucent plastic, two walls with regularly spaced ribs between them approx 10 to 12 mm between the skins, the temp difference between the sunny side and the other side was markedly different so yes they can work.

That could as easily indicate insulation value as cooling value.


A better question might be the difference in temperature between Coreflute, cored hull, and solid glass, each painted black. The solid glass will conduct heat through to the inside, the cored hull is a good insulator, and the core flute I am unsure of without testing. My feeling is that without a good breeze and without one end much higher than the other (to start a thermosiphon) the cooling effect is probably more myth than science. It's probably just an insulator, not that different from a cored deck.

Yeah, data would be nice. But we can crunch some numbers.



Solar irradience is about 300 BTU/hr, depending on latitude and angle. Assuming about 5 BTU/F ft^2 convention from the top and perfect insulation below, the panel might equilibrate out at about 150 F. They run a little cooler than that in practice, because some heat does go through the back (these numbers match field experience). Dropping the temperature 10F would require removing about 50 BTU/hr*ft^2, a 50W panel is about 3 square feet, so about 150 BTU/hr. The heat capacity of air is about 0.24 BTU/#, so assuming a 20F air temp rise through the grid, about 31 pounds of air per hour. That's about 450 cubic feet per hour, or about 8 cfm. Since the gap is only about 1/2-inch, that is an air flow rate of about 3 ft/sec air flow.


That would require a breeze to drive it (narrow restricted space), which will also cool the top, of course, making the need less. In still air, I bet it acts more as insulator than vent, possibly making the panel hotter. Unless the panel were at a steep angle to drive thermosiphon, the math says you are wasting your time. The only reason for the mounting is mechanical support... which is a good reason, of course. But in that case an open grid would be much better. By which time you are better off with a glass panel mounted on stand-offs.


[I've had both glass and semi-flex panels and like them both, in the correct application.]

[mcarthur]

Trying to follow, @thinwater...

3ft/sec is less than 2kn of air flow, or wind/breeze. If the open gap under the panels is bow-stern, there seems to be an extremely high likelihood of airflow you devised being able to cool the panels by the 10F as the bow tends towards the wind (dependent on current/tide of course).

Cooling the top is of course different to cooling the bottom - there's clear research on the benefits of cooling the bottom of the panel, separate to the ambient conditions of what's happening on the top; ie. a comparison of ambient top+ambient under versus ambient top+cooled under.

The question is whether an air gap is worth it under flexible panels. The research would indicate it is "worth it" in that there is a measurable benefit. The question is whether the cost is worth the benefit. The low cost of 10-12mm polycarb twinwall makes a pretty compelling story, as long as both the front and back of the polycarb permit wind to enter and exit easily.