Re: EXERCISE: Optimum emissivity for hot-weather house

Timo Nieminen wrote:
Incident solar radiation and thermal emission from a house and its surroundings differ have significantly different spectra. This can be exploited to minimise heating by incoming solar radiation and maximising radiative cooling.

Assuming that the surface of the house will have a reflectivity of between 0.1 and 0.9 at any particular wavelength, what is an ideal emissivity, as a function of wavelength, to minimise the daytime temperature increase, assuming that radiative equilibrium is reached?

Reasonable assumptions are OK, but should be stated.

Supplementary questions:

(a) Neglecting convection and conduction, what would the equilibrium temperature be?

(b) How much difference would it make if the reflectivity could vary from 0.05 to 0.95? From 0.01 to 0.99?

(c) Estimate the effectiveness of a canopy or fly over the house. What should the emissivities of the upper and lower surfaces of the canopy be?

Presumably to goal is to keep the interior of the house (with closed windows) at some constant (and comfortable) temperature: say 300K. Most solar radiation is in the visible, but most thermal flow is from the air, also around 300 K (maybe 325 K on a hot day).

I would think reflection in the visible and high absorption/emission in the 8-12 micron range would be ideal. That, plus good thermal conductivity to the ground. Ditto the canopy (reflection on the upper, absorption on the lower).

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. So maybe having a high reflectivity everywhere except 9.6 microns?

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