Radiation: Processes and Properties - Environmental Radiation -

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

2. Solar Radiation Solar Radiation • The sun is a nearly spherical source of radiation whose outer diameter is • 1.39 x 109 m and whose emissivepower 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 1011 m to a maximum of 1.521 x 1011 m, with • an annual average of 1.496 x 1011 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. Extraterrestrial irradiation of a surface whose normal is at a zenith angle • relative to the sun’s rays is • Absorption by gases (CO2, H2O ( ), O3) in discrete wavelength bands. Solar Radiation (cont). • Interaction of solar radiation with earth’s atmosphere: • Absorption by aerosols over the entire spectrum. • Scattering by gas molecules and aerosols.

4. Note concentration of all radiation in the spectral region and peak at 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. • Why is the assumption of graybody behavior often inappropriate for surfaces experiencing solar irradiation?

5. Direct radiation: Unscattered and in the direction of the sun’s rays. Clear skies Completely overcast 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. • 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.

6. 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 • Largely due to emission from CO2 and H2O (v) and concentrated in the spectral regions Terrestrial Radiation Terrestrial Radiation • Emission by Earth’s Surface: • Emissivities are typically large. For example, from Table A.11: • Atmospheric Emission:

7. Cold, clear sky Warm, overcast sky 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 • 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. 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? Surface Radiative Properties Surface Properties

9. Surface Properties (cont). 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. SCHEMATIC: Problem: Heat Load on Food Delivery Truck (cont)

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

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