[Greenbuilding] Question about a window solar heater

nick pine nick at early.com
Sat Dec 3 09:31:24 PST 2011

"LarenCorie" <LarenCorie at axilar.net> writes:

>The net absorptance, of a flat black collector absorber, and that of most 
>rooms, is roughly the same...

The room geometry helps. Page 210 of the 1991 2nd edition of Duffie and 
Beckman's Solar Engineering of Thermal Processes says:

   ALPHAeff = ALPHAi/(ALPHAi+(1-ALPHAi)Aa/Ai), where

    ALPHAeff is the effective absorptance of an cavity opening,
        ALPHAi is the absorptance of the inner surface of the cavity,
        Aa is the area of the aperture of the cavity, and
        Ai is the area of the inner surface.

A very large window in front of a shallow room has an Aa/Ai of about 1, so 
ALPHAeff is close to Ai. A small window in front of a deep room (like a crab 
trap) has Aa/Ai close to zero, so ALPHAeff is close to 1.

A 10' cube with a 1 ft^2 solar window (about twice what's needed for good 
indoor illumination in full sun) has Aa/Ai = 1/600. If the cube is covered 
inside with bright white paint with absorptance ALPHAi = 0.2, the window has 
an effective absorptance of 0.2/(0.2+(1-0.2)/600) = 0.9934, so less than 
0.7% of the incoming sun is reflected back out of the window.


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