On a blazing summer day, a black roof gets miserably hot, while a white roof reflects the sun and keeps a home cooler. In winter, the warmth generated by a solar-radiation-absorbing black roof can save energy.

That's well-known and simple enough. Unfortunately, you can't have it both ways. Well ...

A team of recent MIT graduates has developed roof tiles that change color based on the temperature. The tiles become white on a hot day and turn black when it's cold outside.

When white, the tiles reflect about 80 percent of the sunlight that hits them. When they are black, they reflect only about 30 percent. The white state could save up to 20 percent of present cooling costs, according to other recent studies on the theory of all this. Savings from the black state in winter have yet to be quantified.

The tiles rely on a polymer similar to that used in hair gels and water, in a solution encapsulated between layers of flexible plastic. When cool, the polymer stays dissolved, letting a black background show through. When warmed, the polymer condenses to form tiny droplets, whose small sizes scatter light and thus produce a white surface, reflecting the sun's heat.

More work is needed to make the setup commercial-ready.

"It's got to stand up to very harsh conditions," said Nick Orf, a member of the team that calls itself Thermeleon (get it?). "Those sorts of tests would have to be done before we'll know if we have a viable product."

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