Impact of urban albedo on microclimate: Computational investigation in London

The urban albedo (UA), defined as the ratio of the reflected to the incoming shortwave radiation at the upper edge of urban canyons, quantifies their ability to reflect solar radiation towards the sky. This research investigates the impact of real-world urban geometries and optical properties of facades and roads materials on the UA and street level microclimate in London. The Indexed Sphere (IVS) algorithm of ENVI-met 4.4.4 is used to compute the UA of several canyon configurations. The accuracy of the IVS algorithm is evaluated against measurements on a 1:10 physical model reproducing the geometry and materials of the case study area. The simulation results show that reflective materials applied to the canyon surfaces are more effective in increasing the UA of canyons with low aspect ratios. The use of reflective materials in urban canyons always increases the amount of reflections at the street level, increasing the mean radiant temperature in most cases. Air temperature is not affected by the canyon’s façades reflectivity while it shows a significant daytime reduction for increased roads’ reflectivity. The results provide preliminary guidelines for the control of UA and the improvement of microclimate in London.EPSRC UK under the project ‘Urban albedo computation in high latitude locations: An experimental approach’ (EP/P02517X/1).https://www.conftool.org/plea2020/index.php/SC-5-4-Impact_Of_Urban_Albedo_On_Microclimate_Salvati_1751_b.pdf?page=downloadPaper&filename=SC-5-4-Impact_Of_Urban_Albedo_On_Microclimate_Salvati_1751_b.pdf&form_id=1751&form_index=2&form_version=fina

Impact of reflective materials on urban canyon albedo, outdoor and indoor microclimates

The urban canyon albedo (UCA) quantifies the ability of street canyons to reflect solar radiation back to the sky. The UCA is controlled by the solar reflectance of road and façades and the street geometry. This study investigates the variability of UCA in a typical residential area of London and its impact on outdoor and indoor microclimates. The results are based on radiation measurements in real urban canyons and on a 1:10 physical model and simulations using ENVImet v 4.4.6 and EnergyPlus. Different scenarios with increased solar reflectance of roads and façades were simulated to investigate the impact on UCA and street level microclimate. The results showed that increasing the road reflectance has high absolute and relative impact on UCA in wide canyons. In deeper canyons, the absolute impact of the road reflectance is reduced while the relative impact of the walls' reflectance is increased. Results also showed that increasing surface reflectance in urban canyons has a detrimental impact on outdoor thermal comfort, due to increased interreflections between surfaces leading to higher mean radiant temperatures. Increasing the road reflectance also increases the incident diffuse radiation on adjacent buildings, producing a small increase in indoor operative temperatures. The findings were used to discuss the best design strategies to improve the urban thermal environment by using reflective materials in urban canyons without compromising outdoor thermal comfort or indoor thermal environments.This work was funded by EPSRC UK under the project ‘Urban albedo computation in high latitude locations: An experimental approach’ (EP/ P02517X/1)