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Cool Roof Material Eases Energy Demands

Photonics Spectra
Aug 2015

The sun over Australia is notoriously brutal, with consistently high UV index levels. This demands air conditioners and fans, heightening most buildings’ power needs at a time when saving energy is a top priority for many. A new roofing material in development could help cool that demand and reduce energy consumption; and what better place to test it out than under the fierce Aussie sun?

The new material, developed by a team at the University of Technology, Sydney (UTS), with funding from an Australian Research Council Discovery Project grant, comprises a coated polymer stack on a silver thin film. It was tested on the roof of UTS’s Faculty of Science building, which offers a clear view of the sky with no shadowing. The material exhibited nearly 100 percent solar reflectance and thermal emittance at IR wavelengths from 7.9 to 13 μm, allowing it to stay cooler than ambient temperatures.

“Cool roofing reduces the severity of the urban heat island problem in towns and cities, and helps eliminate peak power demand problems from the operation of many air conditioners,” said UTS emeritus professor Geoff Smith.

The angular spectral response of the new reflective roofing material.

The angular spectral response of the new reflective roofing material. Photo courtesy of UTS.



In order to assess the impact of the buildup of dust and grime, data was collected from an unprotected new surface and from one aged over several days in a polluted, outdoor summer environment.

“Extensive dew formation is inevitable for a supercool roof, and dew drops precipitate dirt,” Smith said, noting that the surface maintained its high performance in all conditions. “This roof site, being 25 m above a busy city transit road, was a stern test. Results show that excellent thermal performance can be maintained.”

Even white roofs absorb enough sunlight to warm up by 9 to 12 °C, according to Smith. The new surface “stayed 11 degrees or more colder than an existing state-of-the-art white roof nearby because it absorbs only 3 percent of incident sunlight, while simultaneously strongly radiating heat at IR wavelengths that are not absorbed by the atmosphere.”

Although the materials used in the demonstration are commercially available and potentially suited for use in basic roofing, the idea of installing such a roof is not yet widespread.

“The added feedback benefits from cool roofs are not yet widely appreciated, but recent reports have shown they are substantial,” Smith said. “Examples include ventilation with cooler air and higher performance of rooftop air-conditioning installations.”

 



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