The engineers of Stanford University discovered an ultrathin multilayered material which can reflect sunlight from buildings and thereby can lower the temperature of the building. The findings were published in the Journal Nature on 26 November 2014.
Integrated Photonic Solar Reflector and Thermal Emitter
Stanford University engineers experimentally demonstrated the radiative cooling to nearly 5 degrees Celsius below the ambient air temperature under direct sunlight.
Using a thermal photonic approach, they introduced a material named integrated photonic solar reflector and thermal emitter. The material consists of seven layers of hafnium oxide and silicon dioxide.
At a thickness of about 1.8 microns, the material is thinner than an aluminium foil. It is designed to reflect both infrared light and visible sunlight. The material reflects 97 percent of incident sunlight at the frequency which is not absorbed by atmospheric gases. This is known as the atmospheric window.
When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature and has a cooling power of 40.1 watts per square metre at ambient air temperature.
Need of this ultrathin multilayered material
Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning accounts for nearly fifteen percent of the primary energy used by buildings in the United States.
A passive cooling strategy that cools without any electricity input could have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below that of the ambient air.
At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling in which a device exposed to the sky is used to radiate heat to outer space through a transparency window in the atmosphere between 8 and 13 micrometres.
Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling to a temperature below ambient of a surface under direct sunlight has not been achieved because sky access during the day results in heating of the radiative cooler by the Sun.
Conclusion of the study
• These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency.
• Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource even during the hottest hours of the day.
• The entire goal of the discovery is to harness the effect on the surface specifically during the day and not to cool the atmosphere itself in anyway.
When: 26 November 2014
Why: for cooling surfaces
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