Based on changes in temperature, the tile becomes white when hot, reflecting about 80 percent of the sunlight falling on them (and saving as much as 20 percent of cooling costs). When cold, they turn black to absorb heat, with only about 30 percent of sunlight reflected.
How is this done? With a common commercial polymer in a water solution, between layers of flexible plastic. At the back, there is a dark layer that is revealed when the temperature is below a certain point, at which the polymer stays dissolved. As it gets warmer, the tile becomes a white surface when the polymer condenses to form droplets that scatter light.
At this time, the inventors, MIT's Thermeleon team (rhymes with chameleon, ha) from the Materials Science Department, are still testing out the product's durability. We've seen heat-activated color tiles before (at Moving Color), but this seems to be the first to apply the technology for external building use. Hopefully, costs will be competitive with established building materials, and prove to be a viable roofing alternative that is chock full of energy-saving possibilities.