Solar heating of materials is not a "black and white" issue. You have
to think about wavelengths. The input from the Sun (after atmospheric
effects) is primarily in the visible wavelengths. (That's why our
eyes are adapted for it.) So the heat energy absorbed by your
aluminum plate depends on its absorption (low albedo) in the visible
and near infrared range.
On the other side of the equation, how does the plate _lose_ heat? If
there is not much conduction or convection (low surface area, low
wind, etc.), it can only lose by radiation. The peak radiation of a
body in the range of few hundred degrees or less is in the far
infrared. So, to stay cool, the infrared albedo needs to be low and
the visible albedo to be high. Things that are reflective (white)
can't radiate their own heat well. In other words, you want "black"
in infrared and "white" in visible. (That's the greenhouse effect in
reverse.)
The temperature rises until the heat power lost (proportional to
temp**4 in a black body) equals the power received.
White paint doesn't always have good IR properties. We had to specify
titanium dioxide paint for painting radio telescopes. It was
expensive, but had the best properties. Everything stayed safe to
touch even in the summer desert.
More than you wanted to know?
73 Martin AA6E
On 5/3/05, Roger K8RI on Tower <k8ri-tower@charter.net> wrote:
>
>
> One could think of an object here on Earth as an antenna receiving energy
> from the sun. Since the sun is transmitting about 4 × 10**26 watts, it is
> not difficult to understand why that Al plate got so hot.
>
> Not just think of, but they actually work that way.
> I've forgotten the number of watts per square foot/meter/yard, but it's a
> fair amount.
>
> The reason a white object does not get as hot as a dark one is simply that
> it reflects a large portion of light energy instead of absorbing it. Also,
> an object with more mass stores more energy.
>
> It stores more jouls, just as in capacitance, but capacity does not
> translate into higher temperature. That is a function of the color. OTOH
> that capacity represents energy so it takes longer to heat the massive
> object to a stable temperature and it takes longer for the temperature to
> come down. That is a good reason for concrete walls in solar heated homes.
> Thermal stability. Surprisningly, Forrest Green seems to absorb more heat
> than black.
>
> Imagine the temperature of that huge Aluminum plate had it been painted
> black or forrest green.
> Even weating leather gloves I burnt my hands, but they provided enough
> protection I was able to drop the edge of the plate before the burns were
> serious. 32 square feet, 1" thick is 32/12=2.66 Cu feet of Aluminum.
> What's that weigh?
>
> Roger Halstead (K8RI and ARRL 40 year Life Member)
> N833R - World's oldest Debonair CD-2
> www.rogerhalstead.com
>
--
martin.ewing@gmail.com
http://blog.aa6e.net
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