Development of cool pavement with dark colored high albedo coating

It is considered that the dark surface of buildings and pavements is one of the major heat source causing the urban heat islands, as it absorbs more heat from the sun. In Japan, several kinds of cool pavement have been developed and their cooling effects were investigated. However, none of them holds the effect for longer periods, as it depends on the available water content in the surface layer.

Some of the heat island mitigation studies deal with the albedo increase of road and building surfaces. It is not difficult to raise the albedo of the building roof as it can be attained by bright colored painting. For pavements, however, the brighter surface is not allowable for reasons of driving safety and visibility of white line, unless the brightness is less than that of the conventional asphalt and concrete pavements.

In this study, a new type of pavement is developed to satisfy both high albedo and low brightness with the innovative paint coating technology. The function of this pavement is based on a thin paint coating on the surface of the conventional dark asphalt pavement, which has quite high reflectivity for the near infrared and low reflectivity for the visible. This results in the dark colored pavement surface while achieving much higher albedo. The fine hollow ceramic particles included in the paint have additional effect on reducing thermal conduction and heating of the coat.

In the laboratory experiments, the reflectivity is measured for a number of metal plates coated with different types of paint. The highest albedo was 50% with the L* value (a brightness index) around 40. The measurement results indicated that the weighted average reflectivity was 83% for the near infrared (750-2100nm wavelength) and 20% for the visible (350-750nm). The temperature of asphalt coated by selected paints was measured in the field yards located in Tsukuba and Okinawa, the latter has been exposed to larger solar radiation. The maximum reduction of surface temperature from the conventional asphalt pavement was about 15°C in Tsukuba and more than 17°C in Okinawa.

The high albedo pavement reflects more solar radiation back to the sky. If it is widely used in the urban canopy floor, surrounding buildings absorb part of the reflected solar radiation, which in turn could increase the wall temperature. Thus, the impact of introducing high albedo pavement on the overall canopy albedo and building wall temperature was investigated by applying three-dimensional radiation transfer model developed by Kanda et al. The impact depends on the aspect ratio, surface albedo and the meteorological condition of upper atmosphere. With the case of aspect ratio 1.5, for example, the area-averaged street surface temperature for the high albedo pavement case is reduced as much as 10°C from that of the conventional pavement case under the summer fine weather condition. The change in average temperature of four vertical walls is insignificant and the overall canopy albedo is doubled due to higher street albedo. Changes in the overall canopy energy balance are also quantified.