An investigation of cool roofing on urban street canyon air quality

Abstract

The installation of cool roofing in urban areas is a practical mitigation measure to reduce the Urban Heat Island (UHI) effect and is currently being implemented in many cities around the world to reduce building cooling load demands. The UHI effect results from surface and air temperatures of urban areas being higher than rural areas due to the thermal properties of buildings and reduced vegetative cover. Examples of cool roofs are white vinyl roofs and roofs with reflective coatings. Cool roofs have high albedo (also known as solar reflectivity or solar reflectance) which helps to reflect sunlight and therefore, reduce roof temperatures. Lower roof temperatures result in lower internal temperatures within buildings and this significantly reduces their cooling load. A street canyon is an representative unit of an urban area whereby a street is flanked by two tall buildings on both sides. The aim of this study was to investigate the effect cool roofing has on the air quality within a street canyon. In this project, an idealised street canyon with a street canyon height-to-width ratio of one was created in a Computational Fluid Dynamics (CFD) model which utilises the Renormalization Group (RNG) k-ε turbulence model. In order to verify the model's ability to simulate thermal effects, the CFD model was first validated against experimental results from an atmospheric diffusion wind tunnel in a study carried out by the Japanese National Institute for Environmental Studies. Then, the validated CFD model was run with two different roof temperatures to investigate the effect of a cool roofing product's cooling intensity on the street canyon. In the model, a constant area source of pollutant mass flow was generated at the bottom of the street canyon to simulate road traffic emissions. Selected air quality indicators such as Air Exchange Rate (ACH), Pollutant Exchange Rate (PCH) and Average Volume Mass Fraction (Θ) were then used to analyse the modelling results. The analysis indicated that the air quality within the street canyon is improved marginally as a result of cooler air entering the street canyon due to negative buoyancy generated by cool roofing.