Cool surface as urban heat island effect mitigation strategy for tropical climate

Abstract

Due to retention of solar radiation heat, reduced ventilation and concentrated anthropogenic heat activities, many urban areas experience higher ambient temperatures than their less developed rural surroundings. This phenomenon is often referred to as the urban heat island (UHI) effect. The UHI effect can cause heat stress to urban population, which could lead to health problems or even deaths, in cases that the effect is exacerbated by heatwaves. The UHI effect is also associated with adverse secondary effects such as increased energy consumptions on cooling, worsened urban air quality, etc., Urban fabric materials play an important role in the urban thermal balance as they absorb incident solar radiation and exchange heat with the urban environment. It is hypothesised that increasing solar reflectance and thermal emittance of urban surface (or ‘cool surface’) can reduce solar radiation absorption by urban fabric helps and subsequently reduce the UHI effect intensity and improve urban microclimate. This study presented in this thesis investigates the potential impact of cool coating applied on surfaces of urban fabric on urban microclimate through experimental and numerical methods. Reduced-scale experiments using a typical street canyon model are conducted to test the hypothesis on the effect of change in surface albedo on the canyon air temperature, net radiation and mean radiant temperature. The experimental investigation is extended to a real-scale field experiment involving two side-by-side street canyons at an industrial site in Singapore. Projected impact of cool surfaces in the district scale is obtained by numerical microclimate modelling using a model tool jointly developed with an overseas research partner (EnviBatE). Using the numerical results, the integration of pedestrian comfort simulation with EnviBatE in terms of the Universal Thermal Climate Index (UTCI) is proposed. Results suggest that apply cool surfaces can lead to 30% reduction in sensible heat absorption by urban fabrics. Air temperature reduction in the street canyon is observed up to 2°C (in the afternoon) with the application of cool coating on canyon surfaces. As a result, pedestrian thermal comfort is improved by having up to 1.5°C reduction in terms of UTCI. Overall, the results of this research demonstrate the effectiveness of cool surface coating under different parametric scenarios such as canyon aspect ratio, canyon orientation as well as the individual impact of canyon surfaces towards developing strategies for cool surface deployment in tropical climate of Singapore.