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Date:         Sat, 13 Jun 2015 02:35:01 -0700
Reply-To:     Keith Hughes <keithahughes@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Keith Hughes <keithahughes@YAHOO.COM>
Subject:      Re: vanagon Digest - 11 Jun 2015 to 12 Jun 2015 - Special issue
              (#2015-126)
Content-Type: text/plain; charset=iso-8859-1

Hi Patti,

That's pretty close, but there's a bit more to it. The color does not affect the thermal conductance, it affects the absorption of insolation. The thermal emissivity of the absorber material (in this case the fiberglass top) and the conductance of the material determine how much of the absorbed heat is transmitted to the inside. The product of absorbance and conductance is the term FsubR UsubL, which is a measure of how effective the material is as a solar absorber and heat transfer device. A good reference is John A. Duffie "Solar Energy Thermal Processes". Given the properties of fiberglass as a conductor, I wouldn't expect the color to make a *huge* difference (but it'll be more than a couple of degrees - depending on your location), but the hotter the exterior surface, the higher the deltaT will be between the interior surface of the top, and the interior air temp, and the higher the heat transfer rate you get for any given material-specific thermal conduction rate. The emissivity is constant for the specific material. But for any wavelength, the higher the absorbance, the higher the emissivity, so if you have highly absorbant surface (e.g. black paint) it's also a good radiator, so if you can't conduct the heat away from the surface fast enough (to whatever you're transferring to, whether air or water) the black surface will re-radiate a great deal of the energy back outward as lower energy IR radiation. That may well be the context in which your seminar was discussing not being able to conduct heat fast enough - its a matter of loss of efficiency if you re-radiate IR to space.

Just to give an example of the magnitude of the problem, I spent a few years testing solar thermal devices North of Phoenix some decades ago. I've measured the insolation as high as 360 BTU/ft^2 per hour (measured normal to the surface, with the surface perpendicular to the light - i.e. tracking the sun). Obviously a worse case situation, but if you figure an 8' x 4' top, it would require just under 1 ton of A/C to remove that amount of heat if it were all transmitted to the interior. And that's *just* from the poptop.

You also need to look at absorbance and emissivity in the context of the wavelength of energy you're interested in. While Black has high absorbance and emissivity at higher wavelengths (eg. visible light) and white does not, white does have quite high absorbance/emissivity at low wavelengths - i.e. IR. So white is more efficient on the interior of solar walls since they radiate IR very well, while the external collection surface is more efficient when it's black to absorb the most high energy wavelengths.

Same concern relative to glass windows. Very, very high transmittance of high energy light waves (why we use it after all), but very low transmittance of low frequency IR light. So the heat gets in via visible light, but gets trapped when re-radiated from interior surfaces as IR. The more you let in, and the more absorbant the interior surfaces, the higher the interior temp will be. We did some testing back in the day for several auto manufacturers. The sent us basically a "center-clip" (just the cab and interior) in several colors and materials, and we mapped the interior temps under stagnant (sealed) conditions, with the windshields tilted to the optimal (highest insolation input) angle. IIRC (it has been decades), interior colors made up to about a 50F temp difference due to colors and materials, with interior dashboard temps reaching >300F.

So, blocking light through the windows is the biggest bang for the buck, and will maintain much lower interior temps. Relative to insulation, remember that the amount of heat the poptop absorbs will be pretty much constant whether it's insulated on the interior or not. So if that heat isn't radiating inward, the result will be much higher fiberglass temps, and more thermal degradation. That's why they don't insulate the underside of roofs in climates like Phoenix. While it would keep the attic, and house, much cooler, it bakes the shingles and they die rapidly.

Keith Hughes '86 Westy Tiico (Marvin) ------------------------------ Date:    Fri, 12 Jun 2015 13:18:15 -0400 From:    Marc Perdue <mcperdue@GMAIL.COM> Subject: Re: how to cool a poptop? Alistair, The surface of a dark poptop will be hotter. A darker surface can't conduct the heat absorbed by the dark surface fast enough so it reradiates it back to the exterior and feels hotter on the surface. How much of the heat gets through to the interior depends on the conductivity of the fiberglass. This strikes me as odd too, and I didn't get it till I attended a solar engineering conference years ago. A presentation by the masonry institute showed that lighter colors for mass storage walls were more efficient because their ability to conduct the heat collected on the surface to the interior of the mass was better matched to the ability of their surface to absorb incoming solar radiation, or insolation. What they also found was that amount of mass wasn't as important as the surface area exposed to the incoming solar radiation. What that means to us is park in the shade. As others have noted, your windows are the bigger problem. Their high transmissivity allows much of the insolation to pass through, where it gets converted to heat, which doesn't pass back out of the window as easily. If you further insulate the inside of your van without reducing the insolation, you're basically creating a passive solar collector or oven. Patti,


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