Development, optimization, and evaluation of silica sphere embedded polymethylpentene (TPX) radiative cooler for heat mitigation purposes

Abstract

In the context of urban heat island, radiative coolers with high solar reflectance and strong emissivity in the atmospheric window can cool the substrate as well as the ambient air. Silica (SiO2) at its nano or micro-scale being randomly dispersed into a uniform transparent polymer can form scalable radiative coolers for large-scale application. Promising cooling performance has been reported for SiO2-polymers compared with conventional cooling materials, but their performance can be largely influenced by various fabrication parameters. So far, how fabrication parameters influence the emissivity and the cooling performance has not been experimentally demonstrated. Also, the cooling capacity of SiO2-polymers reported is not substantial compared to other superior radiative coolers. In this thesis, random SiO2-polymer has been optimized theoretically and experimentally. Using OptiFDTD and Mie theory, together with a well-validated thermal model, the cooling power of SiO2-polymer samples under various climates was predicted. Lab measurement and experimental testing of six fabricated SiO2-polymers under subtropical and desert climates indicated that samples with the lowest emissivity in either 8-13 microns or the whole infrared range have the best cooling performance under both climates, conflicting with most existing conclusions. If combined with superior reflectors with higher solar reflectance and especially the emissivity in 8-13 microns enhancing the heat dissipation ability, substantial cooling capacity can be achieved: when peak solar radiation reached over 1100 Wm-2, the combination presented a sub-ambient temperature of maximum 4.7 C when air temperature reached its peak, and the maximum daytime and night-time sub-ambient temperatures were 12.5 C and 15.9 C respectively. The application of radiative cooling roof can generate positive net annual energy savings under both humid subtropical climate and desert climate, with more savings observed for the latter. Positive net annual energy savings were also observed for both insulated and uninsulated buildings, with the advantage over conventional roofs more obvious in uninsulated buildings. On the city scale, we showed that broadband coolers can significantly mitigate urban heat but are not free of problems as the cooling island effect they generate would rise side effects such as decreases in the mixing layer of the atmosphere and increases of pollutants concentration.