Date of Award
Doctor of Philosophy (PhD)
Global energy demand for cooling has increased rapidly in the past decades and will increase by another tenfold by 2050. A cooling technique with high energy efficiency is desirable to reduce energy consumption globally. Radiative cooling is a passive cooling technique, which emits heat to the ultra-cold universe in the form of thermal radiation with zero energy consumption. Particularly, sub-ambient radiative cooling could generate substantial impacts in countries with arid and hot weather. However, day-time sub-ambient radiative cooling could be overwhelmed by the solar heating effect and has been a giant challenge for decades. Until very recently, the first day-time radiative cooler has been realized with a multilayer photonic structure on a silicon wafer, but is limited in applications due to its high cost and low output rate as the result of fabrication processes with nanometer scale precision. Since cooling demands in real applications are usually from Kilo-Watts to hundreds of Mega-Watts, a large area of radiative cooling material is required to provide enough cooling power. Thus, the ideal radiative cooling material with high cooling power must be able to be cost effectively manufactured on a large scale. In my thesis, a scalable-manufactured optical metamaterial is developed for effective day-time sub-ambient radiative cooling. The optical metamaterial consists of a polymeric matrix with randomized dielectric particles and a solar reflective coating. The metamaterial is engineered as both a strong thermal emitter with an average emissivity of ~ 0.94 over the long-wave infrared region, as well as a solar light reflector with 0.05 average light absorptivity over the entire solar spectrum. The metamaterial is manufactured via a roll-to-roll based manufacturing process, including roll-to-roll extrusion and web coating, exhibiting that the metamaterial is compatible with large scale manufacturing processes for high throughput. The scalable-manufactured radiative cooling metamaterial achieves 8 oC and 15 oC sub-ambient temperate differences during day- and night-time, respectively, and also demonstrates 93 W/m2 cooling power at noon and 110 W/m2 through a consecutive 72 hour measurement. The durability of the metamaterial is investigated through accelerated ageing tests, showing reliable spectroscopic performance, including solar absorptivity and infrared emissivity after 6 months accelerated ageing test. The radiative cooling performance affected by weather conditions and its reliability are also discussed.
Zhai, Yao, "Scalable Manufactured Random Metamaterial for Sub-Ambient Day-Time Radiative Cooling" (2023). Mechanical Engineering Graduate Theses & Dissertations. 158.
Available for download on Sunday, October 10, 2021