Passive cooling to cut electricity bills

BENGALURU: Air-conditioning your building without electricity? Researchers at Stanford University in the US report it is potentially possible through the technology of "radiative sky cooling" using a new coating material they have developed.

"Radiative sky cooling exploits a natural property of our atmosphere," Mumbai-born Aaswath Raman, co-lead author of the paper, told this IANS correspondent in an email. "If you can dissipate heat as infrared radiation into something that is very cold -- like outer space -- you can cool a building without any electricity. This then provides a completely passive, non-evaporative way to cool below the ambient air temperature."

The heart of the invention is an ultra-thin multi-layered material Raman and co-workers Eli Goldstein and Shanhui Fan had developed and first tested in 2014.

The material, made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver, does two things at the same time. It beams invisible infrared heat from within a building into the cold outer space (using it as a heat sink), while simultaneously reflecting virtually all of the incoming sunlight that would otherwise warm up the building.

According to the authors, the material thus acts both "as a radiator and an excellent mirror" and the net result is cooler buildings that require less air conditioning. "The internal structure of the material is tuned to radiate infrared rays at a frequency that lets them pass into space without warming the air near the building."

"In 2014, we showed that optical surfaces could be designed to enable this cooling effect even on a sunny day," Raman said. In the recent work, the researchers tested a system -- with panels coated with the specialised material laid atop pipes of running water -- on the roof of a Stanford University building. They found the panels were able to consistently reduce the temperature of the water three to five degrees Celsius below ambient air temperature over a period of three days.

"When connected to refrigeration or air conditioning systems, they can improve efficiency 20 per cent or more. We are now commercialising the technology as a start-up, SkyCool Systems, and have a pilot demonstration active in California," said Raman, who has already partnered with a manufacturer that can produce large sheets of the cooling material for further development.

"For Indian buildings, our fluid cooling panels can have a major impact in commercial refrigeration in supermarkets, cold storage facilities, data centres, office buildings, malls and other commercial buildings," Raman said. "Also, there is the remarkable opportunity to use this technology to enable completely electricity-free, low-grade cooling in rural scenarios."

At least two technical problems remain to be solved before the technology is put to practical use. The engineers must first figure out how to efficiently deliver the building's heat to the coating material and then create fabrication facilities that can make the panels on the scales needed.