Re: EXERCISE: Optimum emissivity for hot-weather house

timo@xxxxxxxxxxxxxxxxx wrote:
Andy Resnick wrote:

<snip>

I wonder how much performance can be extracted by fine-tuning the
emissivity in the 8-12 micron range: 300K blackbodies peak around 9.6
microns, 325K is 8.9 microns.


That's the main point of the question, how much performance can be
extracted by fine-tuning. A reasonable guess is to make the emissivity
look like a step function, going from 0.9 to 0.1 at a sharp cut-off.
Exactly where should the cut-off be?

<snip>

The main wrinkle in this problem is that it's dynamic- if you put an object into a blackbody cavity, and say that object had an emissivity of 0 everywhere except for a thin spectral window, the object would still be in thermal equilibrium with the cavity. However, the sun is incident only part of the time and the ambient temperature changes.

The other important complication is that the thermal flow is from many sources: the sun, the rest of the sky, the ground, surrounding objects, and the air- which has a variable thermal capacity.

I can imagine a simple experiment: build a small shack and try different roofing materials: reflective, absorptive, etc. Simply monitor the internal and external temp. Not a high-budget experiment, but one that requires a lot of time to correct for various weather conditions. Probably a great undergraduate civil engineering project.


--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
.