Urban Canyon

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Cooling Urban Canyons by Redirecting Radiation

Cities do not just get hot because they absorb sunlight; they also get hot because heat has a hard time escaping. In dense urban canyons, sunlight can bounce between façades and pavements, while thermal radiation emitted by the hot ground is repeatedly absorbed by surrounding walls. This radiative trapping makes streets hotter, pedestrians less comfortable, and buildings harder to cool.

Design and fabricated prototype of SRR-TLE façade tiles
SRR–TLE façade design. A layered tile combines a solar-transmissive low-E film, a retro-reflective board, and a reflective backing layer to control solar and thermal radiation simultaneously.

We asked a simple question: instead of only making building façades more reflective or more emissive, can we design them to send radiation in the right direction? Our answer is a solar retro-reflective and thermal low-emissivity façade, abbreviated as SRR–TLE. In the solar spectrum, the façade sends incoming sunlight back toward the sky, reducing shortwave trapping inside the canyon. In the thermal infrared, the low-emissivity specular surface acts like an infrared mirror, redirecting heat emitted by the hot pavement toward the cold sky instead of letting it be absorbed by cooler walls.

Infrared image of a hot urban canyon in Hong Kong
The problem: trapped heat in urban canyons. Infrared imaging shows that pavements can become much hotter than surrounding façades, making ground-emitted thermal radiation a key contributor to street-level heat stress.
Longwave radiation control by low-emissivity specular walls
Longwave strategy. Conventional high-emissivity walls absorb thermal radiation from the hot ground, while low-E specular walls redirect it toward the sky, helping the canyon release heat more efficiently.

This gives an interesting twist to conventional cooling intuition: a low-emissivity wall can help an urban canyon release more heat, as long as it reflects ground-emitted thermal radiation out of the canyon. In outdoor scale-model experiments, SRR–TLE façades cooled the canyon ground by up to 10 °C during the day and 5 °C at night, reduced peak solar heat gain by about 150 W m⁻², and lowered total daytime energy uptake by 43%. Simulations in a real high-density district of Hong Kong further suggest that this directional-radiation strategy can remain effective under realistic urban geometry.

Shortwave radiation control by retro-reflective façades
Shortwave strategy. Diffuse reflective walls scatter sunlight deeper into the canyon, whereas retro-reflective walls send sunlight back toward its source and reduce solar trapping.

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