It’s no small matter. In fact, Chu says that turning all the world’s roofs white would eliminate as much greenhouse gas emissions in 20 years as the whole world produces in a year. But some critics point out that in northern cities, the gain in summer could be outweighed by the loss in winter. The ideal situation, then, would be to get the advantage of white roofs when it’s hot and black roofs when it’s cold.
Now, there may be a way to have both. A team of recent MIT graduates has developed roof tiles that change color based on the temperature. The tiles become white when it’s hot, allowing them to reflect away most of the sun’s heat. When it’s cold they turn black and absorb heat just when it’s needed.
The polymer gel is sealed in a container (usually made out of weather-resistant plastic) with a transparent top and black bottom. When the polymer gel is above the transition temperature the scattering it produces reflects over 80% of the incident light whereas below the transition temperature (when it’s cool outside) the gel is transparent and the 70% of the sun’s energy is absorbed by the black backing.
Our roofing system is based on a polymer gel which phase separates at a pre-determined temperature. Although most of our testing has been done with a gel that transitions around room temperature, we are able to select from a wide variety of transition temperatures ranging from approximately 0 to 100 degrees Celsius and beyond. When the polymer phase separates from the gel, the solution becomes a mixture of polymer and solvent and because the polymer and solvent have different refractive indices the mixture becomes strongly scattering (white colored). When the mixture cools below the transition temperature the polymer re-dissolves in the liquid and the solution is clear and colorless. The pictures below illustrate the change in color when the tile is subjected to hot and cold temperatures.