An inverse approach to identify selective angular properties of retro-reflective materials for urban heat island mitigation

This work presents the preliminary stages of a wider study aiming at assessing the potentials of retro-reflective (RR) materials to mitigate urban heat island effects. Th study herewith reported is based on an inverse approach, which originates from the evaluation of the solar irradiation incident on urban surfaces (i.e. façade, roof, and paving) and leads to the identification of the optimal angular properties required to activate such a material. The solar radiation geometry and the solar irradiation collected by the south-exposed vertical and the horizontal surfaces, were assessed by solar dynamic simulation tools. Furthermore, the angular distribution of the solar direct irradiation component and the direct to global solar irradiation ratio were estimated. The analyses were carried out for nine locations between Oulu (Finland) and Doha (Qatar), with an increment of 5° latitude between two locations. The results demonstrate that the application of RR materials to horizontal surfaces can always be effective, whereas when applied on the vertical surface, the solar geometry influences to a much greater extent the performance of these materials. The main findings of this study show that the selective angular properties of an ideal RR material should be in the angular interval between 25° and 55° and between 30° and 90°, in case of vertical surfaces and horizontal surfaces, respectively. Best practices related to the application of RR materials and the activation of their selective angular properties in different climate zones are also reported.