Title: Thermal Performance of Passive Radiative Cooling Strategies on Building Envelopes
Description: Passive radiative cooling has been extensively studied as a means to cool the exterior surfaces of buildings and reduce space cooling loads. This phenomenon is caused by thermal radiation that is continuously emitted from surfaces on Earth, and transmitted through the atmosphere to outer space (at approximately 3-4 Kelvin temperature scale). To gain a deeper understanding of how terrestrial objects can access this extraterrestrial cold reservoir, I use a theoretical framework derived from classical radiative heat transfer to investigate the radiative properties of surfaces and the atmosphere over a spectrum of wavelengths. In this dissertation, I demonstrate the theoretical cooling potential that can be achieved by surfaces with idealized radiative properties under various atmospheric conditions. While several researchers have optimized the optical properties of their surfaces to emit strongly in wavelength bands corresponding to high atmospheric transparency, I show that a high degree of spectral tailoring is only benefcial when humidity in the atmosphere is low or when a surface can minimize its absorption of solar radiation.

Additionally, I prescribe appropriate sets of surface radiative properties that are required to achieve cooling under various solar and atmospheric loads. An evaluation of passive strategies on building envelopes would be incomplete without considering green facades. To that end, I propose a theoretical model to calculate the heat flux reduction offered by green facades. Unlike previously reported works that use the Pennman-Monteith approach to calculate evapotranspiration in a leaf canopy, my model takes a simpler approach in calculating the sensible and latent heat loss from a layer of leaves while preserving prediction accuracy. By extending the theoretical models for passive radiative cooling and green facades to building envelopes, my work provides insights into the appropriate passive strategy suitable for a particular climate. In dry conditions, surface coatings with optically-tuned radiative properties can perform better than green facades by maximizing their thermal emission through the atmosphere.

However, the additional evaporative cooling benefits, insulation and aesthetic value offered by green facades may make them more favorable in cooler and more humid climates. Since the cooling performance of all passive strategies is strongly correlated to the local climate, my work indicates that variations in ambient air temperature, solar radiation and humidity must be considered when choosing an appropriate strategy for a building envelope.