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Optical Design & Engineering

Cool-colored coatings fight the urban heat-island effect

Replacing conventional pigmented coatings with specialized ones that reflect more near-infrared radiation lowers surface temperatures.
26 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200706.0777

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, 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 NIR with novel ones that have the desired properties.11,12

We have developed ten prototype coatings using NIR-reflecting pigments and compared them with color-matched, conventional counterparts. The difference in the solar reflectance observed between old and new coatings ranges, according to color, from 0.06 to 0.22, the highest being for the black coating (440% increase in SR, see Figure 1). The reflectance curves for each conventional coating and its corresponding cool equivalent coincide in the visible range. These results indicate that although the colors are similar, the conventional coatings exhibit low or modest reflectance in the NIR range, while the cool-colored coatings display a more selective absorption band and significantly reflect NIR radiation.

Figure 1. Spectral reflectance of cool and conventional black coatings.

Measurements for the summer period show that during the day, the cool-colored coatings consistently had surface temperatures lower than the matched ordinary ones. The highest difference was observed between cool and standard black and was equal to 10.2°C, corresponding to a difference in solar reflectance of 0.22 (see Figure 2).

Figure 2. Visible and IR images of cool (1, 3) and conventional (2, 4) black coatings.

Although both coatings absorb in the visible range, the cool version reflects NIR radiation. Consequently, less solar energy is absorbed and converted to heat energy, resulting in a smaller increase in surface temperature for the cool coating. During the night, when there is no solar radiation, the surface temperature of the samples was found to be quite uniform: all the coatings have an emissivity of about 0.88 (see Figure 3).

Figure 3. Forty-eight-hour distribution of surface temperatures of cool and standard black coatings, and ambient temperature (Tamb).

Cool-colored coatings are an inexpensive and passive solution that can help to reduce cooling loads and improve indoor comfort at the level of buildings. At the city level, the materials could provide a means to alleviate air temperature due to heat-transfer phenomena, ultimately improving outdoor thermal comfort and reducing the heat-island effect. Aside from these applications, the pigments could also be used to create other cool-colored materials, as well as automotive topcoats and coatings for oil rigs and military applications.

We would like to thank H. Akbari and the other members of the Heat Island Group from Lawrence Berkeley National Laboratory for their valuable advice and discussions, K. Apostolakis for his help in preparing the cool-colored coatings, and Ferro Corporation and Shepherd Color Company for providing the NIR reflective pigments.

Afroditi Synnefa, Mat Santamouris
Physics Department Applied Physics Section
University of Athens
Athens, Greece

Afroditi Synnefa is a physicist specialized in building and environmental physics. She has a DEA (so-called first doctoral degree) in buildings and environment from the Institut National des Sciences Appliquéfes de Lyon, France. She is currently a PhD student focusing on heat-island mitigation strategies. She has worked as a research scientist and has participated in national and European research projects in energy conservation, integration of renewable energies in buildings, indoor air quality, thermal comfort, and passive cooling.

Mat Santamouris is an associate professor of energy physics at the University of Athens. He is the editor in chief of the Advances in Building Energy Research and associate editor of Solar Energy. He sits on the editorial boards of the International Journal of Solar Energy, Energy and Buildings, the International Journal of Sustainable Energy, and the International Journal of Ventilation. He is editor of the book series Buildings, Energy and Solar Technologies published by James and James in London. He has published nine international books on topics related to solar energy and energy conservation in buildings. He is guest editor of eight special issues of various scientific journals. He has coordinated many international research programs and is author of almost 135 scientific papers published in international scientific journals. He is currently visiting professor at London Metropolitan University.