Prototype roof tiles that turn white to reflect heat when they get hot seem like a pretty cool idea, as do tiles that turn black to absorb heat when it’s cold. That’s why a team of Massachusetts Institute of Technology (MIT) graduates has won the third annual MIT MADMEC (Making and Designing Materials Engineering Contest) by demonstrating their thermally-activated, color-changing, roofing material called “Thermeleon” (as in chameleon).

Using phase-change polymer gel-filled tiles, the team is able to control the light energy transmission properties of the roofing material, meaning that on a hot day the roofing material turns white to reflect heat, but on a cold day the material becomes transparent and absorbs heat back into the home.

Many modern homes with black roofing shingles absorb energy from the summer sun leading to higher cooling bills and wasting much of that solar energy. While this heat absorption factor may be offset in the winter with the home taking advantage of the sun's energy reducing heating bills (often only slightly) the problem remains what to do about the intense summer sun that turns into interior heat.

White roofs are an obvious solution – but permanent white roofs, such as factory-painted materials, repel much of the advantage of winter heat absorption. While this solves the problem of summer heat it doesn’t provide any advantage in winter. Having a tile that automatically changes color depending on the temperature is a great solution all year round.

While the team says most of its testing has been conducted on a gel that transitions at around room temperature, it is able to select from a wide variety of transition temperatures ranging from approximately 0-100°C (32-212°F) and beyond.

“When the polymer phase separates from the gel, the solution becomes a mixture of polymer and solvent and because the polymer and solvent have different refractive indices the mixture becomes strongly scattering (white colored). When the mixture cools below the transition temperature, the polymer re-dissolves in the liquid and the solution is clear (exposing the black backing) and colorless,” says the team on its website.

Using commodity materials to keep the price down, the team has experimented with different products including a rigid tile with a glass front and plastic back, a flexible tile with plastic front and back, and a paint-based system for existing black roof tiles or similar materials.

“We are currently developing our system and are looking forward to doing some long-term environmental testing (heat/UV/humidity) to determine the anticipated lifespan of our design,” says the Thermeleon creators.

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