MIT researchers have developed roof tiles that change color based on the temperature, turning white when it's hot to deflect heat and turning black when it's cold to absorb it.
According to the team's lab measurements, the tiles can reflect about 80 percent of sunlight when they're white and 30 percent of sunlight when they're black. In their white state, the tiles could save as much as 20 percent of present cooling costs, according to the researchers.
That's a smart move. U.S. Secretary of Energy Steven Chu has urged Americans switch to white roofs to curb global warming and eliminate greenhouse gas emissions -- as much in 20 years as the entire world produces in a year.
Tellingly, officials in cities north of Washington, D.C. say the tradeoff isn't worth it for them.
The MIT tiles aim to please both sides.
The team introduced the tiles at the Making and Designing Materials Engineering Contest for MIT students and earned $5,000 for their efforts.
According to team member Nick Orf, the researchers originally tried to develop a color-shifting roof tile using a system of mixed fluids, one dark and one light, whose density would change with temperature (the dark fluid would float to the top when it was cold, and the white fluid would surface when it was hot).
Proving too complicated, that system was exchanged for one made of a common commercial polymer in a water solution. That solution is encapsulated between flexible plastic layers, with a dark layer at the back. When the temperature is above a certain level -- the number depending on how the solution's formula is tailored -- the polymer condenses to form tiny droplets, whose small sizes produce a white surface and scatter light.
Below that temperature, the polymer stays dissolved, revealing the tile's black backing and absorbing the sun's heat.
The researchers are now working on a more inexpensive version that can be retrofit to houses with existing black roofs. That version containes micro-encapsulated polymer solution that's mixed with a clear paint material that could be brushed or sprayed onto an existing surface.
The team has yet to commercialize the product. The next step? Testing its durability in the elements.