Radiation Transfer through Droplet-Covered Substrates: Simulations, Experiments, and Applications
Water droplets are commonly observed on the inner or outer surface of covers of solar energy conversion systems such as greenhouses, solar collectors, and solar stills as a result of dropwise condensation and/or rain. These systems have an operating temperature ranging from 280 K to 320 K and emit infrared thermal radiation while being exposed to solar radiation. The presence of such droplets can reduce the solar transmittance and alter the thermal load on the solar energy conversion systems. In addition, radiative cooling surfaces used for building cooling and water harvesting applications also collect dew and these condensed droplets can alter the emittance and selectivity of radiative cooling surfaces. This dissertation aims (i) to investigate both numerically and experimentally visible and infrared radiation transfer through surfaces supporting polydisperse droplets on their front or back side and (ii) to assess experimentally the effect of droplets in important applications including photovoltaic solar cells and water production using radiative cooling surfaces.First, light transfer through soda-lime glass windows supporting acrylic droplets on its back side was investigated. For contact angle θc smaller than the critical angle θcr for total internal reflection at the droplet/air interface, the presence of droplets did not significantly affect the transmittance and reflectance. However, for droplet contact angle θcr ≤ θc < 90�, the transmittance decreased significantly with increasing droplet contact angle and/or surface area coverage while the reflectance increased. Second, infrared radiation transfer through glass windows covered with droplets on their front or back sides was examined. The transmittance of windows with slightly absorbing droplets on the front increased while the reflectance decreased with increasing contact angle and surface coverage due to antireflection effects. For droplets on the back with contact angles larger than the critical angle for total internal reflection at the droplet/air interface, the transmittance decreased with increasing contact angle and surface coverage. For strongly absorbing droplets, the transmittance decreased with increasing surface coverage for droplets on either the front or back sides. Experimental measurements for both visible and infrared parts were in good agreement with numerical predictions obtained using the Monte Carlo ray-tracing method. Moreover, the effect of droplets on the performance of solar photovoltaic (PV) cells was quantified experimentally. The current vs. voltage curves of polycrystalline silicon solar cells with dry and droplet-covered glass covers were measured under simulated solar irradiation under different incidence angles. For incident angles θi ≤ 30�, the droplets did not affect the performance of the PV cells. However, for incident angles θi > 30�, the presence of droplets caused the maximum power and energy conversion efficiency of the PV cells to decrease significantly. Such performance degradation was attributed to the fact that the incident light was back-scattered through the droplets instead of being trapped by total internal reflection at the cover/air interface. Finally, the effect of droplets on the emittance and selectivity of radiative cooling surfaces was investigated experimentally. The spectral directional-hemispherical reflectance of a moderately selective emitter supporting acrylic droplets with different contact angles and surface area coverages was measured. The spectral emittance of the radiative cooling surfaces was found to increase significantly across the infrared spectrum in presence of droplets and with increasing surface area coverage. In fact, the droplets transformed the spectrally selective surface into a broadband emitter. This was attributed to the absorption by the acrylic droplets. In addition, outdoor nighttime experiments under different relative humidity and cloud coverage established that the temperature of the radiative cooling surface increased when covered by droplets due to the increase in the radiative heat gain from the atmosphere.