Light transfer through windows with external condensation. (March 2018)

• Record Type: Journal Article
• Authors: Zhu, Keyong; Li, Shaoling; Pilon, Laurent
• Is Part Of: Journal of quantitative spectroscopy & radiative transfer. Volume 208(2018)
• Page Start: 164
• Page End: 171
• Publication Date: 2018-03
• Subjects: External condensation -- Dew -- Dropwise condensation -- Energy efficient windows -- Visibility reduction -- Solar cell efficiency; Spectrum analysis -- Periodicals; Radiation -- Periodicals; Analyse spectrale -- Périodiques; Rayonnement -- Périodiques; Radiation; Spectrum analysis; Periodicals; 543.0858
• DOI: 10.1016/j.jqsrt.2018.01.019
• Languages: English
• ISSNs: 0022-4073

Abstract

Highlights: Light transfer through windows covered with droplets on the outside was simulated. Transmittance was nearly independent of droplet mean diameter and size distribution. Total internal reflection at window/air and droplet/air interfaces played a major role. Expressions for normal-hemispherical transmittance were derived in limiting cases. Abstract: This study investigates systematically light transfer through windows supporting cap-shaped droplets on their external face. The presence of such droplets may have negative effects on the conversion efficiency of solar cells, distorts image quality of lenses, or hinders visibility through windows and windshields. Here, the directional-hemispherical transmittance was predicted by the Monte Carlo ray-tracing method. The droplets were monodisperse or polydisperse randomly distributed on the outside face of optically smooth windows. For nonabsorbing droplets, the diameter and size distribution did not have a significant effect on the window directional-hemispherical transmittance. The latter was nearly independent of contact angle for incident angle θi ≤ 30°. However, the directional-hemispherical transmittance decreased monotonously with increasing incident angle and droplet contact angle for contact angle θc ≤ 70° to reach a minimum at a contact angle θc, min beyond which it increased with increasing contact angle before reaching a plateau at large contact angles. This was attributed to total internal reflection at the back window/air and droplet/air interfaces. For absorbing droplets, the normal-hemispherical transmittance decreased significantly with increasing droplet contact angle, mean diameter, polydispersity, and projected surface area coverage due to strong absorption within the droplets. Moreover, the normal-hemispherical transmittance decreased with increasing contact angle for θc < 90° and remained constant and independent of the droplets' absorption index, mean diameter, and contact angle for θc ≥ 90°. Finally, Analytical expressions for the upper and lower bounds of the normal-hemispherical transmittance as a function of droplet contact angle, optical properties, and projected surface area coverage were derived.