Cool-colored coatings fight the urban heat-island effect

Abstract

The urban heat island effect is becoming increasingly more intense. Among its causative factors are the lower albedo (reflective power) of urban surfaces and the replacement of vegetation by buildings.1 Absorption of solar energy by concrete and paved structures raises their surface temperature several degrees over that of the ambient air, which also then increases. Apart from causing thermal discomfort, heat islands precipitate greater consumption of cooling energy. They also drive up energy demand and costs,2–4 and accelerate the formation of harmful smog.5 Mitigating the urban heat island phenomenon requires cool materials that are characterized by high solar reflectance (SR) and IR emittance values.6 Increasing either reflectance or emittance lowers the surface temperature of the building envelope, which in turn decreases the heat penetrating the structure. These techniques also work by lowering the temperature of the ambient air, because the cooler the surface, the lower the heat convection intensity. A number of cool materials are currently available commercially for buildings and other surfaces of the urban environment, These including cool surface coatings, reflective tiles, and so on: however, they are all white or light colored.7–10 There is a need for cool, nonwhite products for situations where darker colors are preferred or where white products introduce glare. For this reason, new materials that are dark but still reflect more sunlight and stay cooler are being developed using specialized pigments. A cool non-white coating absorbs in the visible range to give the appearance of a specific color. Considering that about half of all solar power arrives as invisible near-IR (NIR) radiation, such a coating should be highly reflective in the NIR part of the spectrum to maintain high solar reflectance. This is achieved by replacing conventional pigments that absorb in the NIRwith novel ones that have the desired properties.11, 12 Figure 1. Spectral reflectance of cool and conventional black coatings.