Color-changing roof tiles

Color-changing roof tiles absorb heat in winter, reflect it in summer

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 team's lab measurements show that in their white state, the tiles reflect about 80 percent of the sunlight falling on them, while when black they reflect only about 30 percent. That means in their white state, they could save as much as 20 percent of present cooling costs, according to recent studies. Savings from the black state in winter have yet to be quantified.

The team, which the students call Thermeleon (rhymes with chameleon, because of its color-changing property), was one of the competitors in this year's Making and Designing Materials Engineering Contest (MADMEC). Nick Orf PhD ’09, a member of the Thermeleon team, explains that he and his teammates 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 substance would float to the top when it was cold, and white would float when it was hot. But the system proved too complicated, and instead they hit on a simpler, less expensive method.
Now, they use a common commercial polymer (in one version, one that is commonly used in hair gels) in a water solution. That solution is encapsulated — between layers of glass and plastic in their original prototype, and between flexible plastic layers in their latest version — with a dark layer at the back.

When the temperature is below a certain level (which they can choose by varying the exact formulation), the polymer stays dissolved, and the black backing shows through, absorbing the sun's heat. But when the temperature climbs, the polymer condenses to form tiny droplets, whose small sizes scatter light and thus produce a white surface, reflecting the sun's heat.
Read more at: http://www.physorg.com/news174209373.html