Radiative Cooling as a Radiative Heat Transfer Problem: Optical and Climate-related Considerations

ABSTRACT

In recent years, radiative cooling has received increasing attention as a promising and sustainable way to cool buildings, cities and even larger environments. The process involves the emission of longwave infrared (LWIR, λ~8-13 μm) and reflection of solar (λ~0.3-2.5 μm) heat to space through the atmosphere’s optical transmission windows, and has a “zero-energy, zero-carbon” functionality that has seen radiative cooling technologies proceed rapidly from conception to commercialization. The built environment has been a major area of application for these designs.

Much of the research and development related to radiative cooling technologies has been focused on surfaces that face the heat sink of the sky. In reality, a significant fraction of surfaces in the built environment are facades (e.g. walls and windows) that have both the sky and the ground in view. This complicates cooling, as terrestrial environments act as broadband radiant heat sources that overwhelm skywards cooling in the summer, and heat sinks that overcool buildings in the winter. In this talk, I will discuss how facades can be engineered to have spectrally selective emittances that enable a passive all-season thermoregulation where traditional designs fall short. Specifically, we will explore a scattering based design paradigm that achieves ultrabroadband thermal reflectance and selective LWIR emittance without metal mirrors, and which can yield selective emitters at very large scales. The potential impacts of the design on buildings are comparable and complementary to the cooling benefit of painting dark facades white.

The challenge of thermoregulating buildings across weather conditions has also motivated intense research on dynamic designs (e.g. electro- or thermochromic designs) that adapt their optical properties to cooling and heating needs. These designs have been proposed as the next evolutionary step for sustainable thermal control of buildings, based on their energy savings potential. However, a consideration of their radiative impact on the Earth indicates that they may actually be a less sustainable option for cooling buildings than ordinary white roofs. This aspect will also be discussed.

BIOGRAPHY

Jyotirmoy Mandal is an assistant professor in the Department of Civil and Environmental Engineering at Princeton University, where they lead the Optical and Thermal Design research group. Their research involves understanding and controlling nano-to-macro-scale radiative heat flows in both natural environments and artificial surfaces, with characterizing and mitigating ambient heat in a warming world as a guiding theme. Before they joined Princeton, they received their PhD in Applied Physics at Columbia University, and was a Schmidt Science Fellow at UCLA.