Measurement of hourly urban albedo of building façades with different reflective directional characteristics by a simple urban model

Abstract

In order to mitigate urban heat island (UHI) phenomenon, diffuse highly reflective (DHR) materials like highreflective paint have been applied to the building facades as exterior wall materials, because it can greatly reflect the solar radiation incident on the exterior wall. However, it has been pointed out by some studies that the DHR materials as building facade surface materials cannot completely solve the problem of UHI effect, because most of the solar radiation heat will continue to remain in the urban canyon. Thus, retro-reflective (RR) materials have been suggested by scholars worldwide to replace the DHR materials for application of building facades to deal with the UHI effect in recent years. Urban albedo is a measure for reflectivity of the urban coverage, and appropriately increasing urban albedo is regarded as a verifiable and repeatable UHI mitigation strategy. There is still little research on evaluation and comparison of urban albedo between the DHR and RR building facades. Therefore, this study proposed a simple urban model with different incident angles of light beam, and used it to evaluate the urban albedo, with consideration of two different reflective directional characteristics including DHR and RR facade surface materials. In addition, in order to evaluate the effect of incident angles of sunlight on the upward to downward ratio of reflection under two conditions of DHR and RR building facades, this study also used the proposed simple urban model experiment to evaluate it. The results obtained by the proposed simple urban model experiment are compared with that evaluated by an analytic simulation of urban canyon proposed in previous study. It is shown that the RR building facade will increase the urban albedo and upward to downward ratios of reflection from building facades, thus it is considered that RR building facades can potentially contribute to UHI mitigation, while compared to the DHR building facades.