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A New Cooling Technique

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Cooling is a significant end-use of energy globally and a major driver
of peak electricity demand. Air conditioning, for example, accounts for
nearly fifteen per cent of the primary energy used by buildings in the
United States. A passive cooling strategy that cools without any electricity input
could therefore have a significant impact on global energy consumption.
A New Cooling Technique
Modern strategies to keep homes cool by
reflecting the sunlight have been presented by researchers in California. They’ve created a new multilayer coating that cools
using a one-two punch. 
First, it reflects nearly all the incoming energy in
sunlight. Second, it absorbs heat (say, from a building) and radiates it
away without warming the nearby air. Because the new technology doesn’t require
any electricity, it could slash the cost of air conditioning buildings – an
application that consumes nearly 15% of all electricity used in the United
States.
Shanhui Fan, An Electrical Engineer from Stanford University, California said, ” There is no natural material that reflects solar wavelengths but is
also radiative in thermal wavelengths” just the right length to escape
the atmosphere”. He gives an idea to create a mirrored coating that was also good at radiating energy at long wavelengths.
So with the use of Thermal Photonic Approach, Fan and his team build up a seven-layer stack of alternating silicon dioxide (SiO2) and hafnium dioxide (HfO2) with integration of photonic solar reflector and thermal emitter. They grew the layers on the top of pie plate sized silicon wafer, topped with a thin silver layer that acted as an initial good reflector. 
Out of the seven layers, first four layers of SiO2 and HfO2 designed to reflect nearly all the incoming light not reflected by the silver. These layers are ultrathin in size. Together those five layers reflected 97% of the incoming energy. The top three layers – two thicker SiO2 layers separated by a thick HfO2
layer—acted as the radiator, absorbing heat from below and re-emitting
the energy at wavelengths between 8 and 13 micrometers. In various
tests, the coating cooled surfaces below it by 5°C, even in full
sunlight. The research is published online in Nature by Fan and his team.

A Photonics engineer, Eli Yablonovitch  at University of California, Berkeley says,”This idea  is a very novel and simple. But whether the coating can be applied cheaply enough over large areas, this has to be seen. Fan replied to that comment,” For this existing technology can do the job.”

Source: ScienceMag, Nature.

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