Energy-saving roof tiles

A team of MIT graduates has developed roof tiles that change colour based on 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 only when it is required.

The team's laboratory measurements showed that in their white state, the tiles reflected about 80 per cent of the sunlight falling on them, while when black they reflected about 30 per cent. That means in their white state, they could save as much as 20 per cent of present cooling costs. Savings from the black state in winter have yet to be quantified.

The team, which the students call Thermeleon, was one of the competitors in this year's Making and Designing Materials Engineering Contest (MADMEC), a competition for teams of MIT students (or 2009 graduates). Now in its third year, the contest this year was specifically devoted to projects aimed at improving energy efficiency through innovative uses of materials. The Thermeleon team took first place, earning $5,000 (£3,155).

Dr Nick Orf, a member of the Thermeleon team, explained that he and his teammates originally tried to develop a colour-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 decided 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.

They are 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 colour-changing property, but the surface could easily be applied over an existing black roof, much more inexpensively than installing new roofing material.

Although they have not yet made specific plans for forming a business to commercialise their concept, Dr Orf said that the team members are determined to pursue the idea and develop it into a marketable product.

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 on accelerated testing by running the material through repeated hot-cold cycles.

Hashem Akbari, leader of the Heat Island Group at Lawrence Berkeley National Laboratory in California, is a long-time advocate of white roofs as an energy-saving measure. He said that some other groups, including a team at the University of Athens, have done research on the use of colour-changing materials for roofs, but that in those tests, the cost and durability was a serious issue.

The Thermeleon team hopes to address those concerns. 'It's got to stand up to very harsh conditions,' Dr Orf said. 'Those sorts of tests will have to be done before we will know if we have a viable product.'