Optical Scattering Properties of Pigmented Foils for Radiative Cooling and Water Condensation: Theory and Experiment
Doctoral thesis, 1994
Light scattering properties of inhomogeneous materials were investigated in this thesis. Optical properties of pigmented polyethylene foils were optimized to take advantage of radiative cooling and for water condensation. This thesis reports on a unique combination of scattering theory, materials modeling, experiments and a prospect for future applications.
The first part of the thesis deals with the theory of scattering and absorption in a pigmented matrix material. Lorenz-Mie theory was used for calculating the extinction of light by a single sphere. Maxwell-Garnett effective medium theory predicted the surface reflections of a foil. A four-flux theory was investigated for moderately low volume fractions of pigments in a matrix material. The total transmittance, reflectance and absorptance spectra for pigmented foils were calculated and compared with the optical properties of ZnS pigmented foils of different thicknesses and pigment volume fractions. The overall agreement was good, though the highest degree of correlation was obtained for low pigment volume fractions. The results indicated that multipolar interactions between the particles as well as agglomeration, cannot be neglected in calculations regarding foils with close packed particles.
The second topic is optimization and experiments on pigmented foils acting as convection shields. The day-time radiative cooling for an underlying material is exploited if the cover foil combines high solar reflectance and high transmittance of thermal radiation in the infrared. Different pigment materials were studied in order to optimize cover foils for cooling a black body surface at noon with the sun in its zenith. A 400 µm thick ZnS pigmented foil with a pigment volume fraction of 0.15 was prepared and tested in Tanzania. The heating power at noon was 7.2 W m-2 and the temperature of the radiator was 1.5 K above the air temperature. The field experiments agreed well with simulations.
A third part of the thesis examines the potential of water condensation. The heat transfer equations were studied, as well as the condensation rate as a function of climatic factors, dew collector design, and chemical properties of the foil's surface. The literature on dew observations was reviewed. A 390 µm thick foil with TiO2 and BaSO4 pigments was produced and was found to have a fairly high emittance in the thermal infrared; it was therefore used in outdoor experiments. During drought months in Tanzania, the foil condensed 1.19 litre/m2 in a semi-desert. This is an encouraging result that agrees with theory.