Plant Services.com writes that a typical flat, tar-black roof can get to 170 F in summer. Further they point out that these hot roofs contribute to the “heat island effect” which is said to be connected to increased carbon dioxide levels in urban areas. The heat build-up also requires extra fossil-fuel use to generate power for the additional cooling loads.
The relative reflectivity of the Earth itself is of interest in the nature of cooling. A black earth somehow denuded of clouds, ice caps, glaciers, deserts would be a hot place indeed. The Business Week article compared a dark Earth to a black leather car interior on a hot day and the Earth with those light-colored features as a car with a beige leather interior. In the jargon of planetary scientists the “albedo” of the planet – its relative level of reflectivity – is 30%.
The new element affecting the albedo of Terra is the presence of greenhouse gases in the atmosphere which contain additional heat. The sun gives Earth 1350 constant watts of energy, of illumination. Some is absorbed, some reflected by the light-colored features. In the modern, industrial world an additional 2 watts per square meter is retained over the retained levels in the world before it became industrialized.
The new generation of solar, energy-producing panels are not reflective. They are black and designed to collect and use those watts of illumination beamed down to us. The panels receive an average of 300 watts per day in a square meter panel. Most of that is still lost due to inefficiency but the 20% that is utilized is enough to offset the 2% loss to greenhouse gases.
Therein lies the hot rub. The meter (3.1 foot) square photo-voltaic panel costs about $1000. That thousand bucks would buy a lot of white paint. Enough, Business Week estimated, to cover 2000 square meters with high-quality white paint.
Should alternative electricity-producing products be replaced by white roofs, lighter roads and parking lots? No. You cannot use the white roof to produce electricity to power your refrigerator, water pump or even charge your new iPhone. But, some experts have put the estimate of savings by painting roofs light colors at $750 million.
Article comments
1 - Todd Miller
There are numerous new exciting technologies being developed pertaining to roofing because the roof represents a huge opportunity for energy production as well as for keeping homes cooler.
New coatings available on metal roofing systems (and potentially on tile and even standard shingles) have decent reflectivity rates even in dark colors due to special pigmentation.
Additionally new "thin film" photovoltaics will be easily applied to smooth roof surfaces such as metal roofing or may even be incorporated into the coating on the metal itself.
Todd Miller
2 - Nathan Libbey
Generally the whiter the better. - Good information for those building or reroofing