Does wearing a white T-shirt really help you keep cool in the summer?
Absolutely. White objects tend to reflect most of the light that falls upon them no matter what the wavelength in the visible spectrum. Objects are often coloured because they selectively absorb certain wavelengths of light preferentially compared with other wavelenghs that are then reflected. But what happens to the light that is absorbed? Well, in most cases dye or pigment molecules absorb the energy and go to a higher energy state. But as every physicist knows, energy must be preserved. Eventually, the energy is released, usually in the form of heat. Now an object which is black is a very strong absorber and often absorbs much of the light at all wavelengths in the visible spectrum. Therefore a black T-shirt will heat up when exposed to the sun when compared with a white T-shirt.
This idea has been taken further by a team of MIT graduates who have developed roof tiles that change colour 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 when it’s needed.
In their white state the tiles reflect about 80% of the sunlight falling on them, while black they reflect about 30%. That means in their white state, they could save as much as 20% of present cooling costs, according to recent studies.
The tiles use 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 the polymer stays dissolved and the black backing shows through, absorbing the sun’s heat. 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 was one of the competitors in this year’s Making and Designing Materials Engineering Contest (MADMEC) a competition for teams of MIT students.
For further information see http://thermeleon.com/