Radiative Cooling and Thermoregulation in the Earth's Glow (2006.11931v3)

Abstract: Passive radiative cooling involves a net radiative heat loss into the cold outer space through the atmospheric transmission windows. Due to its passive nature and net cooling effect, it is a promising alternative or complement to electrical cooling. For efficient radiative cooling of objects, an unimpeded view of the sky is ideal. However, the view of the sky is usually limited - for instance, the walls of buildings have >50% of their field of view subtended by the earth. Moreover, objects on earth become sources of heat under sunlight. Therefore, building walls with hot terrestrial objects in view experience reduced cooling or heating, even with materials optimized for heat loss into the sky. We show that by using materials with selective long-wavelength infrared (LWIR) emittances, vertical building facades experience higher cooling than achievable by using broadband thermal emitters like typical building envelopes. Intriguingly, this effect is pronounced in the summer and diminishes or even reverses during the winter, indicating a thermoregulation effect. The findings highlight a major opportunity to harness untapped energy savings in buildings.

• The paper shows that selective LWIR emitters on vertical facades achieve up to 0.46˚C cooling advantage over traditional broadband emitters.
• The authors use a robust theoretical model and multi-location experiments in summer and winter to validate the cooling performance.
• The study highlights practical applications for building energy efficiency, offering scalable and sustainable passive thermoregulation solutions.

Radiative Cooling and Thermoregulation in the Earth's Glow

The paper conducted by Mandal et al., focuses on the potential of selective long-wave infrared (LWIR) emitters for radiative cooling and thermoregulation on vertical building facades. The context of this research is rooted in the escalating energy demands globally, particularly for cooling and heating systems in buildings, leading to substantial greenhouse gas emissions. While passive radiative cooling, exploiting the atmospheric transparency in the LWIR spectrum, has been extensively explored for horizontal, sky-facing surfaces, the application for vertical facades has remained under-utilized until now.

Study Context and Methodology

The authors propose a novel utilization of selective LWIR materials on vertical building surfaces, which could significantly reduce the need for energy-intensive cooling systems. The research aims to address a critical gap in the optimization of building envelopes by exploring the benefits of LWIR emissive materials, which contrast with traditional broadband emitters. These materials selectively emit in the atmospheric transmission window (approximately 8-13 µm), allowing for efficient radiative cooling that other broadband approaches fail to harness due to interference from terrestrial objects.

Mandal et al., provide a robust theoretical model illustrating how selective LWIR emitters can enhance cooling by minimizing broadband heat gain from terrestrial sources while maximizing heat loss to the sky. This paper focuses on vertically oriented facades, which are often exposed to the thermal emissions of the built environment. The research undergoes experimental validation in varying environmental conditions (summer and winter) across different geographical locations, such as Los Angeles and Nashville.

Key Findings and Results

The empirical data corroborate the theoretical predictions, demonstrating a tangible cooling effect from selective LWIR materials. During summer, the selective emitters achieved cooling advantages of approximately 0.43 to 0.46˚C over broadband emitters. Remarkably, in winter, these materials effectively reversed the cooling benefits, thereby providing a controlled passive thermoregulation mechanism.

Furthermore, the paper suggests practical applications of selective LWIR materials, which are produced from widely available polymers and ceramics. This research emphasizes the adaptability of these materials for various building components, such as walls and windows, and highlights their scalability for widespread implementation.

Implications and Future Directions

The findings challenge the conventional approach to building energy efficiency, proposing an innovative strategy that leverages natural atmospheric properties for thermoregulation, with potential energy savings comparable to existing "cool roof" technologies. Theoretical estimates forecast possible energy savings, reaching values similar to traditional cool roofs, demonstrating the substantial impact these materials could have on reducing energy consumption and enhancing building energy performance.

The broader implications of this research suggest substantial contributions to both applied and theoretical domains. Theoretically, it advances the understanding of radiative cooling dynamics in complex urban environments. Practically, this paper signifies a step towards sustainable urban development. It proposes a solution that not only reduces dependence on active cooling systems but also integrates seamlessly into existing architectural designs without significant aesthetic compromises.

In summation, the research by Mandal et al., highlights significant potential in the use of selective LWIR emitters. The advancement presents preliminary evidence for large-scale adoption, which promises to redefine energy strategies in the construction sector. Future research could extend this work to evaluate longevity, economic impacts, and feasibility across diverse climatic conditions, thereby solidifying its place in the broader context of sustainable urban energy solutions.