New panel can make buildings reflect heat, cut cooling cost
COME summer, air-conditioners and coolers are plugged into electrical sockets once more. Though effective against hot Indian summers, the accompanying high electricity bills and energy consumption make them a cause of worry for consumers and environmentalists.
A team of scientists, led by Shanhui Fan of Stanford University, US, have now developed a material that can cool buildings in direct sunlight, thereby reducing energy consumed by cooling devices. In their paper, published on March 5 in Nano Letters, they have described how molecules of the photonic material are so arranged at a nano scale that it can passively reflect sunlight at wavelengths that do not get absorbed in the atmosphere and go to outer space. It can therefore, theoretically, ensure that heat gets radiated away from a surface.
The new material acts as a substitute for rooftop solar panels and can help reduce dependence on rooftop cooling. It consists of two-layers of photonic crystals of quartz and silicon carbide. These materials are very weak absorbers of sunlight. In combination, they act as thermal emitter and solar reflector, making the device highly efficient.
The study shows that if just 10 per cent of the roof of a single-story single-family house was to be fitted with the new radiative cooling panels, it could offset 35 per cent of its entire air-conditioning needs during the hottest summer hours. This kind of radiative cooling is a passive technology that needs no energy. It has no moving parts and is easy to maintain. It can be installed on the roof or the sides of buildings and starts working immediately.
Photonic materials have a unique structure that allows certain wavelengths of light to pass through them while disallowing others. This ability makes them the perfect candidate for any application where light needs to be manipulated. Though scientists have been fabricating photonic structures for a long time now, this new design is the first robust structure capable of providing substantial cooling power without the need for any electricity even during the hottest hours of the day.
“Our structure accomplishes this by doing two things simultaneously: it radiates thermal heat away to outer space at a specific set of wavelengths where the earth’s atmosphere is transparent, and it strongly reflects, thus minimally absorbs, sunlight,” says Aswath Raman, a doctoral student associated with the work.
The technology also holds great potential in reducing carbon emissions and decelerating global warming. The researchers are now making efforts to move the technology from the lab to prototype stage. The biggest challenge is fabrication. Photonic materials are fabricated in special facilities like clean rooms. Such facilities are expensive to create. “In India we still do not have any clean room facilities, though the IITs are in the process of building them,” says G Vijay Prakash a professor in the Department of Physics at IIT, Delhi.
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