The effect of dropwise condensation on glass solar properties

The antireflective potential of dropwise condensation

“…5. Total internal reflection, which occurs when light hits the water-air interface above its critical angle of 48.8°, has been identified as a factor in reducing the transmittance of condensate-covered glass [3,9]. However, at angles of incidence greater than the complement of the contact angle, shadowing of the most sharply angled part of the droplet occurs, reducing the occurrence of total internal reflection.…”

“…Visible light is represented by 500 nm radiation because there is no significant variation in reflectance over the visible wavelength range [3,9]. Dry air at STP (n 1.00028 [16]) has a refractive index very close to that of vacuum, but the actual refractive index of air depends on temperature and humidity.…”

“…The contact line diameter of the droplets simulated corresponds to a Bond number of unity, D σρg 1∕2 ≈ 2.66 mm. However, for constant coverage, drop diameter has a negligible effect on reflectance [3,9]. The glass thickness was 3 mm.…”

“…The presence of condensed water on glass has generally been found to reduce the total transmissivity of solar radiation [1][2][3][4][5][6][7][8], but its effect on the spectral, directional-hemispherical transmissivity of visible light wavelengths has been explored in less depth [3,5,9]. The decrease in total transmissivity of solar radiation has been attributed to absorption by the water, especially of the near-IR, and in some cases, to increased reflection off the angled droplet walls [9].…”

“…Hsieh and Rajvanshi [3] identified the effects of absorption in a fixed geometry by analytically modeling transmittance and reflectance at normal incidence on hemispherical droplets at various wavelengths in the solar spectrum. Their calculated transmittance is low due to both absorption of IR radiation and the 90°contact angle, which has highest reflectance [4].…”