A review on the development of cool pavements to mitigate urban heat island effect

Conventional impervious pavements have dark surface and large thermal inertia. During summertime they tend to absorb and store solar radiation but negate the evaporative cooling, contributing to the development of urban heat island (UHI). The idea of using cool pavements to mitigate the UHI has gained momentum recently. This review synthesizes the existing definition, physical mechanism, and typical cooling techniques of cool pavements, presenting the influence of cool pavements on the urban thermal environment. Benefits, penalties, costs and policies for the applications of cool pavements are presented with special emphasis on reflective pavements and evaporative pavements. The review suggests that the definition of cool pavements remain incomplete; that the influence of cool pavements on the air temperature in the urban canopy layer is unknown; and that the impact of cool pavements on the thermal conditions of adjacent buildings and pedestrians remains unknown. Many speculations of using cool pavements to battle the UHI effect need refinements and validations. Heat-harvesting pavements seem interesting because they not only stay cool but harness renewable energy. However, the results from the heat-harvesting pavement prototype require scrutiny on the power output, durability, and lifetime of the pavement system. Future studies are expected to understanding the impacts of cool pavements on pedestrian thermal stress, on adjacent building’s energy loads, and on the air temperature in the urban canopy layer.