Exploiting selective angular properties of retro-reflective coatings to mitigate solar irradiation within the urban canyon

Abstract

The combination of urban sprawl and densification in built areas increases the number of interbuilding solar reflections. This contributes to over warming in urban environments – a phenomenon known as the urban heat island (UHI) effect. During the last decades, the exploitation of cool materials, such as high-reflective (HR) and retro-reflective (RR) coatings, has been proposed to mitigate UHIs. In this context, this work is a part of the wider research focused on the investigation of a new concept for surface treatments, which consists of RR materials enhanced with an angular-selective behavior. This research study aims at evaluating the application of the selective angular properties to assess solar irradiance impinging urban canyons’ surfaces in different climatic contexts, from the Arctic to the Persian Gulf area. First, the geometry configurations receiving the largest solar energy contribution were defined by varying the canyon's orientation and height-to-width ratio. Second, yearly and seasonal solar analyses were conducted considering traditional and angular-selective RR materials applied to the façade and the street. The outcomes show that angular-selective RR materials can activate the mitigation potential of these materials only during the warm season, when the risk of UHI is higher without hindering solar energy contribution in cold months. In fact, optimized RR turns out to be efficient at all latitudes considered and reduced the yearly solar irradiation impinging canyon's surfaces by up to 20%, 30%, and 50% in Oslo, Milan, and Cairo, respectively.