Experimental investigation of the impact of urban fabric on canyon albedo using a 1:10 scaled physical model

Alkis Kotopouleas,Renganathan Giridharan,Marialena Nikolopoulou,Richard Watkins,Muhammed Yeninarcilar

doi:10.1016/j.solener.2021.09.074

Abstract

This paper presents findings from a large-scale experiment on the impact of urban fabric on canyon albedo, aimed at informing the development of an urban albedo calculator for London, an empirical model to predict changes in urban albedo in relation to changes in urban fabric and solar altitude. Through different material applications on a 1:10 scaled physical model of an actual residential area, the study assessed the effect of street-level materials and three common façade types on canyon albedo alongside the canyon’s reflective power in the infrared, and the impact of rain. The results showed that the addition of concrete paving at street level increased canyon albedo by 9%, while the substitution of tarmac with grass by 70%. Brickwork and aluminium cladding façades were seen to contribute to significantly higher canyon albedo than curtain wall across the measured irradiance spectrum. The results also revealed the highly dynamic nature of canyon albedo and a consistent rainfall-induced reduction of albedo in the range 22–36% throughout the experimental phases. The findings from this study can inform urban planning and policy making directed at tackling the urban heat island effect.

Highlights

Urban street canyons are fundamental elements of urban settings shaping our cities
Focussed on the impact of urban fabric on street canyon albedo, this paper presents findings from an experimental investigation that used a large physical model of an actual residential area to study the effect of common urban materials on canyon albedo and inform the development of an urban albedo calcu lator that predicts changes in urban albedo in relation to changes in urban fabric and solar altitude
The investigation of albedo in the infrared revealed that brickwork façades resulted in 43% and 82% higher canyon albedo than curtain wall and aluminium cladding façades respectively (Fig. 14), in agreement with the measured surface reflectance of the individual ma terials (Table 6), making curtain wall systems less preferable ap plications in terms of avoiding thermal energy trapped within street canyons

Summary

Introduction

Urban street canyons are fundamental elements of urban settings shaping our cities. Canyon albedo, the capacity of urban surfaces within a canyon to reflect solar radiation out of the canyon, is defined as the dimensionless fraction of reflected to incident radiation measured on a scale from 0 to 1. The reduction of urban albedo is directly associated with the intensification of the urban heat island (UHI) phe nomenon where temperatures in urban centres are higher than the surrounding rural areas. It is an important determinant of changes in outdoor ambient temperature (Oke, 1988) and has implica tions for outdoor thermal comfort (Mishra & Ramgopal, 2013; Pantavou et al, 2011; Steemers et al, 1998) and building energy demand for cooling (Hassid et al, 2000; Kolokotroni et al, 2007). A review and meta-analysis of 14 studies on the impact of increased urban albedo on ambient air tem peratures and heat related mortality found that an albedo rise of 0.1 could reduce the afternoon temperature by 0.10–0.38 ◦C and the heat related mortality by an average of 1.8% (Santamouris & Fiorito, 2021)

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