|Black surfaces absorb the sun's heat very efficiently, producing a toe-singeing surface. In the winter, a dark roof heats up and helps reduce heating bill. But in the summer months, it makes the house even hotter, increasing pressure on the air conditioning bills. In most places, the summertime penalty is greater than the wintertime gain, it turns out, so that's why many people, including U.S. Secretary of Energy Steven Chu, strongly advocate switching to white roofs. 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. However, in the northern cities, the gain in summer could be outweighed by the loss in winter. The ideal situation therefore would be to get the advantage of white roofs when it's hot and black roofs when it's cold.
This has led to the innovation of color-changing roof tiles that 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% of the sunlight falling on them, while when black they reflect only about 30%. That means in their white state, they could save as much as 20% of present cooling costs. Savings from the black state in winter have yet to be quantified. The students call the team Thermeleon. The team uses a common commercial polymer 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.
The team is now working on an even simpler version in which the polymer solution would be micro-encapsulated and the tiny capsules carried in a clear paint material that could be brushed or sprayed onto any existing surface. The tiny capsules would still have the color-changing property, but the surface could easily be applied over an existing black roof, much more inexpensively than installing new roofing material.
Because the materials are common and inexpensive, team members think the tiles could be manufactured at a price comparable to that of conventional roofing materials � although that won't be known for sure until they determine the exact materials and construction of their final version. The biggest remaining question is over durability, and answering it will require spending some time to do accelerated testing by running the material through repeated hot-cold cycles.