Polarization-Induced Infrared Emissivity Control System for Precise Temperature Adjustment

The global energy and climate challenges require energy-saving solutions to achieve net-zero emissions worldwide. As a result, eco-friendly thermal management has become crucial in various sectors, including residential buildings and mobile devices. In this context, passive radiative cooling has emerged as a promising solution to address the global energy and climate crises by providing energy-efficient and zero-emission cooling. The high emissivity of objects in the mid-infrared (MIR) atmospheric window allows for radiative cooling, where heat is drawn from the objects and emitted to the cold outer space. This unique feature has sparked renewed interest in extensive research on passive radiative cooling, with successful experimental demonstrations in various applications such as wearable devices, solar cells, buildings, displays, clothing, roofs, and vehicles [1].<br/>However, conventional radiative cooling technologies face limitations, such as undesired cooling in cold weather due to strong thermal emissions in a single state. Recent research has introduced dual-state emitters capable of switching emission spectra for cooling and heating purposes [2]. However, achieving precise thermal comfort temperatures, especially in moderate weather conditions like spring and autumn, requires continuous emissivity adjustment rather than simple on/off cooling switching. In this context, multi-state emitters with continuously adjustable emission spectra offer promising solutions for all weather conditions, although they have received limited attention in scientific investigations. Additionally, active emitters utilizing phase change materials provide thermoregulation functionality for all seasons but have constraints in customizing target temperatures due to fixed transition points.<br/>Here, we propose a dynamic temperature-adjustable system with an infrared (IR) polarization valve as an energy-balancing channel. The system is composed of a one-dimensional grating emitter and an IR polarizer. A simple rotation of the polarizer allows continuous temperature modulation. Optical simulations are implemented to optimize the design of the thermal emitter, which comprises a one-dimensional silver grid on quartz. Our system achieves a wide range of emissivity, from 2 to 80%, by simple adjustment of the polarizer. This enables precise cooling/heating capabilities (-17 to 51 W/m²). Theoretical calculations estimate the potential energy saving of over 20 GJ/year compared to conventional emitters across different climate zones when applied to roofs of buildings. Outdoor experiments validate the precise temperature regulation within a target range of 17-18 degrees (Celsius). The successful outdoor proof-of-concept demonstration highlights the reliability and applicability of our approach in diverse situations such as residential buildings, farms, and wearable devices. Furthermore, we have confirmed the suitability of phase change materials, particularly GST, in our system using a glancing angle deposition technique. This result proves the versatility of our system and the potential for various applications, including thermal camouflage patches. Our findings contribute to advancing thermal management technologies for achieving net-zero emissions and sustainable energy savings in the face of global energy and climate challenges.<br/><br/>References<br/>D. H. Kim, S. -Y. Heo, Y. -W. Oh, S. Jung, M. H. Kang, I.-S. Kang, G. J. Lee, and Y. M. Song, <i>APL Photonics</i> 8, 030801 <b>(2023).</b><br/>X. Li, B. Sun, C. Sui, A. Nandi, H. Fang, Y. Peng, G. Tan, and P.-C. Hsu, <i>Nat. Commun.</i> 11, 1 <b>(2020).</b>