Since ancient times, people living in hot climates learned if they paint their rooftops white, then their quarters would stay cooler during the scorching heat. In an attempt to curve energy consumed on air conditioning, which accounts for 15% of all electricity consumed in the US, scientists have devised a multi-layered surface that acts in two ways to expel heat: a traditional heat reflecting mechanism and a high-tech, innovative heat transfer system which expels radiation from inside the house and into space.

Expelling heat into space

A multilayer stack cools surfaces below it by radiating heat into space. Credit: FAN LAB, STANFORD ENGINEERING

A multilayer stack cools surfaces below it by radiating heat into space. Credit: FAN LAB, STANFORD ENGINEERING

The first step you need to take in order to cool a surface is make sure as much incoming heat from the sun is reflected. White surfaces and mirrors are great for this. Cooling rooms without air conditioning is a bit trickier, and to understand how the engineers at  Stanford University in Palo Alto devised their system we first need to grasp how heat is transferred.

All matter dissipates heat under the form of far infrared waves, be it rocks, trees or cells. Humans, at normal body temperature, radiate most strongly in the infrared at a wavelength of about 10 microns, which can be viewed with special thermal vision goggles. Heat is dissipated in wavelengths between 6 and 30 micrometers, but air molecules can only absorb, and thus emit, heat in the lower and upper range. Anything that’s between 8 and 13 micrometers passes right through the air and into space. So the trick lies in building a surface that reflects lower and upper ranges, while radiating microwaves that can’t be absorbed by air.

A white roof reflects heat away and keeps your home cool. Image: vtecoliving.blogspot.com

A white roof reflects heat away and keeps your home cool. Image: vtecoliving.blogspot.com

Shanhui Fan placed a surface made up of seven layers alternating between silicon dioxide (SiO2) and hafnium dioxide (HfO2) onto a silicon wafer. At the very top, a thin silver coating was applied to act as a first line of reflection. The first four ultra thin layers of SiO2 and HfO2 reflect nearly all the rest of the energy that wasn’t reflected in the first place by the silver layer. Together, this stack reflects 97% of incoming radiation. The bottom three layers – two thicker SiO2 layers separated by a thick HfO2 layer – absorbed heat from below and radiated it wavelengths between 8 and 13 micrometers.

When tested, even during full sunlight, the coating cooled surfaces below it by 5 degrees Celsius.

This is definitely a most interesting solution, considering the high cooling potential without any energy input whatsoever. It remains to be seen if the system can be scale efficiently enough to be cost effective. The space radiator was described in a paper published in Nature.

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