Paint that cools your home in the heat of the summer and warms it up in winter has been developed by researchers in China. Not only will the paint reduce your heating bills, it will also change the colour of your home to match the season.
Developed by Yiping Ma and his colleagues at Tongji University in Shanghai, the coating absorbs heat from the Sun when the temperature drops below 20 掳C, helping to warm the building. But when the temperature rises above 20 掳C the coating automatically starts to reflect sunlight to keep the building cool.
The coating also changes colour with the seasons. 鈥淭here are different seasons in nature, so that cool tones are preferred in summer and warm tones in winter,鈥� explains Ma. He thinks people will want blue colours when the weather is hot and reds in the depths of winter.
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Pigment and polymer
To develop pigments with the right properties Ma looked at a number of thermochromic substances. Most of them changed colour at too high a temperature to be useful for domestic paint, but a substance called crystal violet lactone produces a variety of hues, from red to green to blue, at close to room temperature.
In order to prevent crystal violet lactone from reacting with chemicals in the paint, Ma encapsulated the pigment in a transparent polymer and then stirred it into a standard wall paint. The paint has a noticeable effect on the internal temperature of a typical house, says Ma.
鈥淭he coating can increase the temperature by about 4 掳C in winter, and can decrease the temperature by about 8 掳C in summer,鈥� explains Ma.
Black and white
鈥淚 haven鈥檛 come across anything that adapts in this way,鈥� says Mike Wilson, director of the Low Energy Architecture Research Unit at the University of North London.
鈥淚deally though, the colours of the coating would change from black to white, as far as solar absorption is concerned.鈥� But, he adds, colour is important in designing buildings and people might not like buildings that turn from black to white.
鈥淢y final goal is to use this sort of coating in buildings to create a thermally comfortable environment without consuming any fossil fuel,鈥� says Ma.
In the meantime he is working to improve the durability of the coating. He currently estimates that buildings will have to be repainted every four years for the coating to remain effective.