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Optimizing retro-reflective surfaces to untrap radiation and cool cities

Nature Cities volume 1, pages 275–285 (2024)Cite this article

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Abstract

Extreme heat and its various impacts are a growing threat to cities and their residents, and it is increasingly clear that portfolios of solutions are needed to mitigate the resulting risks. Here we comprehensively evaluate and optimize the application of existing retro-reflective (RR) materials, which reflect incoming solar radiation back to the sky, on urban surfaces to cool them. Using detailed energy budget models, we show that RR walls and pavements decrease urban canyon surface temperatures by up to 20 °C and canyon air temperatures by up to 2.6 °C, outperforming highly reflective surfaces, with a notable improvement in pedestrian thermal comfort (up to 0.55 °C and 153 W m−2 reductions in human skin temperature and net radiative gain, respectively). We then develop optimized RR design guidelines for diverse climatic conditions, latitudes, seasons and urban geometries. On the basis of our analysis, we recommend RR pavements for open, low-rise areas and propose specific RR wall design strategies for compact, high-rise areas.

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Fig. 1: Total reflectance and retro-reflectance of three surfaces in urban environments.
Fig. 2: Maximum surface temperature reductions and maximum canyon air temperature reductions by RR walls and RR ground.
Fig. 3: Effects of albedo and retro-reflectivity on temperature and thermal comfort in urban canyons.
Fig. 4: Effect of different aspect ratios on the maximum canyon air temperature reduction by HR and RR surfaces.
Fig. 5: Maximum reduction in street canyon net shortwave radiation.
Fig. 6: Optimization matrix of the most effective RR surfaces.

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Data availability

The simulation data for this study are available in the open-access Zenodo repository56 at https://doi.org/10.5281/zenodo.10638146.

Code availability

The core MATLAB codes used for incorporating retro-reflectivity in shortwave radiation can be accessed via the open-access Zenodo repository56 at https://doi.org/10.5281/zenodo.10638146. The whole UCM codes used in this study are available upon request from the corresponding author or X.H. (e-mail: xjhuang@princeton.edu).

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Acknowledgments

X.H. and E.B.-Z. acknowledge support from Princeton University’s School of Engineering Innovation Funds and the Dean of Research Innovation Fund for Exploratory Energy Research. A.L.P. thanks the European Research Council for supporting her research in the framework of the Horizon Europe Programme (ERC, HELIOS, G.A. 101041255). I.P. thanks the Italian funding program Fondo Sociale Europeo REACT EU – Programma Operativo Nazionale Ricerca e Innovazione 2014–2020 (D.M. n.1062 del 10 agosto 2021) for supporting her research through the “Red-To-Green” project. J.M. acknowledges support from Princeton University’s SEAS start-up funds.

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Authors and Affiliations

  1. Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, USA

    Xinjie Huang, Elie Bou-Zeid & Jyotirmoy Mandal

  2. CIRIAF - Interuniversity Research Center, University of Perugia, Perugia, Italy

    Ilaria Pigliautile & Anna Laura Pisello

  3. Department of Engineering, University of Perugia, Perugia, Italy

    Ilaria Pigliautile & Anna Laura Pisello

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  1. Xinjie Huang
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Contributions

X.H. and E.B.-Z. designed the methodology and scientific questions. E.B.-Z. and A.L.P. conceived the idea of using RR materials for cooling. X.H. developed the RR model, and I.P. developed the human thermal comfort model. X.H. analyzed and visualized the data. J.M. contributed to refining the methodology and presentation. X.H. and E.B.-Z. led the writing. All authors contributed to the writing and presentation of the results and gave final approval for publication.

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Correspondence to Elie Bou-Zeid.

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Nature Cities thanks Mattia Manni, Chaoqun Zhuang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Huang, X., Bou-Zeid, E., Pigliautile, I. et al. Optimizing retro-reflective surfaces to untrap radiation and cool cities. Nat Cities 1, 275–285 (2024). https://doi.org/10.1038/s44284-024-00047-3

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