Anomalous Photon Transport induced Asymmetric Electromagnetic Transmission for use in Daytime Passive Radiative Cooling in a Humid Climate

Project: Research

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Space cooling consumes significant amounts of energy and is a major driver of peak electricity demand. Passive radiative cooling requires no electricity input, making it a solution for smartgreen buildings. By selectively reflecting and emitting photons at different wavelengths, net cooling can be achieved if the emission of infrared to outer space (where the temperature is ~2.7K) exceeds the absorption of sunlight and environmental thermal radiation. Recently, a radiative cooler has been shown to reduce the ambient temperature by almost 5°C under sunlight. We have examined an identical cooler in Hong Kong’s climate and found that although the cooler’s surface temperature dropped below ambient at night, it remained higher than ambient in the daytime since weather conditions (humidity) play a crucial role. When the water vapor level is high (humid environment), the 8–13μm atmospheric window loses transparency, leading to a low transmission and high thermal emission in that wavelength range from the atmosphere. As a radiative cooler strongly absorbs heat emitted by water vapor, the cooling capacity of a radiative cooler drops significantly under humid conditions. We therefore propose to study the possibility of using an asymmetric electromagnetic transmission (AEMT) window that permits outgoing transmission of radiation from the radiative cooler but reflects incoming radiation of the same wavelengths from the atmosphere to improve the cooler’s power. This study will attempt to formulate a theoretical framework for designing AEMT-enhanced radiative coolers which can realize daytime radiative cooling in humid climates. Preliminary simulation results show that the cooling power of a radiative cooler using the AEMT window can be improved by 140% (from ~21W/m2 to ~50W/m2). However, the origin and nature of asymmetric transmission are not fully understood and an ultra-broadband (>5μm) AEMT device with high forward transmittance remains undiscovered. This study will develop a multi-reflection model for identifying key influential parameters (e.g. AEMT bandwidth, forward and backward transmittances) on cooling performance, providing design guidance for AEMT-enhanced radiative coolers. Experiments will be carried out to verify the model. Fundamental understanding of asymmetric transmission will also be revealed by studying the photonic band-structure. The AEMT window will be characterized using Finite-Difference Time-Domain simulations and Fourier-transform infrared spectrometry. Ultimately, an AEMT-enhanced radiative cooler will be designed, fabricated, assembled, optimized and tested under different weather conditions in Hong Kong. This study will motivate research interest in passive radiative cooling, enhance knowledge of asymmetric electromagnetic transmission and benefit the built environment in subtropical cities. 


Project number9042758
Effective start/end date1/01/19 → …