Light transfer through semi-transparent glass panes supporting pendant droplets

This paper quantifies experimentally the effect of pendant droplets condensed on the back of semitransparent glass panes on their normal-hemispherical transmittance and reflectance in the visible and near-infrared. To enable sample characterization and ensure repeatability, acrylic droplets were deposited on the back side of 3 mm-thick soda-lime silicate glass slabs with or without hydrophobic surface treatment including perfluorinated silane, perfluorinated silane-coated silica nanoparticle monolayer, or Teflon coatings. The droplet contact angle.c was varied between 26 degrees and 76 degrees and the projected surface area coverage reached up to 60%. For contact angle theta(c) smaller than the critical angle.cr for total internal reflection at the droplet/air interface, the presence of droplets did not significantly affect the normalhemispherical transmittance and reflectance. However, for droplet contact angle theta(cr) = theta(c) < 90 degrees, the normal-hemispherical transmittance decreased significantly with increasing droplet contact angle and/or surface area coverage while the normal-hemispherical reflectance increased. The measurements of the normal-hemispherical transmittance were in excellent agreement with numerical predictions obtained from Monte Carlo Ray Tracing method. These results further validate our previous numerical simulations and the different optical regimes identified. The results of this study can provide guidelines for the design and operation of energy efficient flat-plate solar collectors, outdoor photobioreactors, greenhouses, solar desalination systems, and other solar energy conversion systems. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

This study quantified the effect of pendant droplets on the transmittance and reflectance of semitransparent glass panes, demonstrating that droplets with a contact angle smaller than the critical angle did not significantly impact the transmittance and reflectance, while droplets with a contact angle less than 90 degrees led to a significant decrease in transmittance and increase in reflectance.