UCLA

Depleting fossil fuels and climate change have necessitated development of novel energy technologies for a sustainable future. Many of these technologies involve interaction of solar radiation with scattering particles embedded in an absorbing medium, albeit at different length scales. For example, photoelectrochemical water splitting involves direct conversion of water into H2 and O2 gases using semiconductor photoelectrodes exposed to sunlight. However, the gases are released in the form of bubbles of about 1 mm in diameter that scatter the incident light. Moreover, some of the near-infrared radiation is absorbed in the aqueous electrolyte, resulting in optical losses. Similarly, geoengineering entails large-scale modification of Earth's natural systems to reflect enough solar energy to counter the energy imbalance (known as ``radiative forcing") caused by greenhouse gases. Notably, microbubbles entrained in ship wakes scatter sunlight, and thus can be harnessed to reflect a significant amount of solar radiation, thereby mitigating global warming. Likewise, radiative cooling is a passive cooling technology that involves radiating heat to the outer space (at 3 K) via thermal emission primarily in the long-wavelength infrared (LWIR) atmospheric transparency window between 8 to 13 μm. Mesoporous aerogels made up of silica nanoparticles around 4 nm in diameter could be promising radiative cooling materials because of their high transmittance (> 90%) in the visible and near-unity LWIR emittance.

This dissertation aims to (i) systematically quantify the optical losses caused by gas bubbles during photoelectrochemical water splitting, (ii) critically review and validate different radiative transfer models for semitransparent media containing scatterers and use the most accurate model to predict albedo of ship wakes containing microbubbles, and (iii) demonstrate the use of mesoporous silica aerogels as optically transparent radiative cooling materials.

First, state-of-the-art Monte Carlo ray-tracing models were developed to simulate interaction of solar radiation with gas bubbles attached to a large horizontal Si photoelectrode surface as well as with those dispersed in the semitransparent aqueous electrolyte. The optical losses were quantified by comparing the photoelectrode absorptance with and without the presence of bubbles for a wide range of bubble diameters, volume fractions, plume thicknesses, and contact angles. Overall, the results suggested that in order to minimize the optical losses, the bubble departure diameter D should be large, while the bubble volume fraction fv and plume thickness H should be as small as possible. The effect of bubble contact angle θc on the optical losses was explained by identifying three different optical regimes based on the interplay of total internal reflection at the electrolyte/bubble interface and reflection at the bubble/photoelectrode interface. Accordingly, design guidelines were provided, such as using hydrophilic photoelectrodes to minimize the bubble contact surface area coverage and using convection to decrease bubble plume thickness.

Second, the relevant models predicting radiation transfer through semitransparent media containing scatterers were critically reviewed. A new hybrid model was proposed that predicts the effective scattering coefficient and asymmetry factor using the Lorenz–Mie theory and the effective absorption coefficient as the volume-weighted sum of the bubbles and medium absorption coefficients and solves the radiative transfer equation using the Monte Carlo method. Its predictions showed excellent agreement with those by the Monte Carlo ray-tracing method based on geometric optics for a wide range of bubble volume fractions, slab thicknesses, and medium absorption coefficients. For experimental validation, microcomputed X-ray tomography scans were performed on a fused silica sample containing bubbles to retrieve the exact locations, diameters, and total volume fraction of bubbles. Here also, predictions of the hybrid model using the retrieved data agreed well with experimental measurements of the spectral normal-hemispherical reflectance and transmittance of the sample for wavelengths between 0.4 and 3 μm when silica ranges from weakly absorbing to absorbing. Finally, simulations were performed over solar spectrum throughout the day using hybrid model to predict the albedo of ship wakes containing microbubbles for a wide range of bubble diameters and seafoam thicknesses. The results indicated that albedo as high as 0.9 could be achieved with bubble volume fraction fv = 74%, diameter D = 20 μm, and seafoam thickness H = 20 mm.

Lastly, mesoporous aerogels composed of silica nanoparticles and having porosities φ = 72.5%, 80.9%, or 87.5% and thickness around 1 mm were prepared and characterized for passive radiative cooling applications with or without aluminum substrates. The samples’ normal-hemispherical reflectance over the solar spectrum and normal emittance over mid-IR wavelengths were measured experimentally using UV-Visible and FTIR spectrophotometers. Their radiative cooling power was quantified across different aerogel thicknesses by performing spectral simulations using the Transfer Matrix method. Overall, it was found that increasing the porosity of silica aerogels eliminated the characteristic decrease in emittance of silica around 9 μm, enabling significantly larger radiative cooling power.