Cooling vertical surfaces

Abstract

Radiative cooling is a promising passive technology that dissipates heat throughout the Earth’s atmosphere and into space without electrical or mechanical input. It therefore offers an appealing solution to the rising demand for cooling in buildings. Radiative coolers can achieve temperatures below ambient air when their surface faces the sky, such as on rooftops. Yet, reaching sub-ambient temperatures on vertical surfaces, which make up most external building surfaces, is challenging as radiative coolers with conventional designs tend to absorb heat from the ground and nearby buildings, failing to cool effectively. Now, Andrea Alu, Shanhui Fan, Wei Li and colleagues across China and the USA have optimized the design of these coolers, achieving a temperature 2.5 °C below ambient air for a cooler tested outdoors at noon.

The researchers employ a sawtooth design: the flat surface of each ‘tooth’ faces the sky while the slanted surface faces the ground. These two surfaces are made of different materials to control the angular and spectral emission of radiation. The slanted surface reflects radiation over the ultraviolet, visible, and near-infrared ranges, including thermal radiation coming from the ground and adjacent buildings. The horizontal surface selectively emits in the mid-infrared wavelength range, maximizing heat dissipation into space. The research team shows that their system outperforms conventional radiative coolers and commercial white paint. This demonstration of daytime radiative cooling below ambient air temperature on vertical surfaces extends the practical potential of the technology for real-world applications.