1. Radiation: Processes and Properties - Environmental Radiation - Chapter 12 Section 12.8

2. Solar Radiation The sun is a nearly spherical source of radiation whose outer diameter is 1.39 x 10 9 m and whose emissive power approximates that of a blackbody at 5800K. The distance from the center of the sun to the center of the earth varies with time of year from a minimum of 1.471 x 10 11 m to a maximum of 1.521 x 10 11 m, with an annual average of 1.496 x 10 11 m. Due to the large sun-to-earth distance, the sun’s rays are nearly parallel at the outer edge of the earth’s atmosphere, and the corresponding radiation flux is

3. Solar Radiation (cont). Extraterrestrial irradiation of a surface whose normal is at a zenith angle relative to the sun’s rays is Interaction of solar radiation with earth’s atmosphere:  Absorption by aerosols over the entire spectrum.  Absorption by gases (CO 2, H 2 O ( ), O 3 ) in discrete wavelength bands.  Scattering by gas molecules and aerosols.

4. Solar Radiation (cont). Effect of Atmosphere on Spectral Distribution of Solar Radiation:  Attenuation over the entire spectrum but more pronounced in spectral bands associated with polar molecules.  Note concentration of all radiation in the spectral region and peak at  Why is the assumption of graybody behavior often inappropriate for surfaces experiencing solar irradiation?

5. Solar Radiation (cont). Effect of Atmosphere on Directional Distribution of Solar Radiation:  Rayleigh scattering is approximately uniform in all directions (isotropic scattering), while Mie scattering is primarily in the direction of the sun’s rays (forward peaked).  Directional distribution of radiation at the earth’s surface has two components. – Direct radiation: Unscattered and in the direction of the sun’s rays. – Diffuse radiation: Scattered radiation strongly peaked in the forward direction.  Calculation of solar irradiation for a horizontal surface often presumes the scattered component to be isotropic. Clear skies Completely overcast

6. Terrestrial Radiation Emission by Earth’s Surface:  Emissivities are typically large. For example, from Table A.11:  Emission is typically from surfaces with temperatures in the range of 250 < T < 320K and hence concentrated in the spectral region with peak emission at Atmospheric Emission:  Largely due to emission from CO 2 and H 2 O (v) and concentrated in the spectral regions

7. Terrestrial Radiation (cont).  Although far from exhibiting the spectral characteristics of blackbody emission, earth irradiation due to atmospheric emission is often approximated by a blackbody emissive power of the form Cold, clear sky Warm, overcast sky Can water in the natural environment freeze if the ambient air temperature exceeds 273K? If so, what environmental conditions (wind and sky) favor ice formation?

8. Surface Properties Surface Radiative Properties Concentration of solar and terrestrial in different spectral regions often precludes use of the gray surface approximation.  Note significant differences in for the two spectral regions: snow, human skin, white paint.  In terms of net radiation transfer to a surface with solar irradiation, the parameter has special significance. Why?

9. Surface Properties (cont). Surface Snow0.29 Human skin0.64 White paint0.22 Black paint1.0 Evaporated Al film3.0 Rejection Collection

10. Problem: Heat Load on Food Delivery Truck Problem 12.119: Determination of preferred roof coating (Parsons Black, Acrylic White, or Zinc Oxide White) and corresponding heat load for prescribed operating conditions.

11. Problem: Heat Load on Food Delivery Truck (cont) SCHEMATIC:

12. Problem: Heat Load on Food Delivery Truck (cont)