Mumbai-born Aaswath Raman, who grew up in Canada and is now a Canadian citizen, researches unique new ways of harnessing a largely unexploited renewable source of energy — the cold of the universe. Raman moved to the USA for his bachelor’s degree in 2002 and is currently a research associate with the Ginzton Laboratory at Stanford University, where he investigates “radiative or sky cooling” to develop prototype systems for cooling, refrigeration and beyond.
K. S. Jayaraman spoke to Aaswath Raman to find out how you can air-condition your building without electricity.
Here’s Jayaraman’s guest blog.
Aaswath Raman and his colleagues at Stanford University recently reported1 that it is potentially possible to air-condition a building through the technology of “radiative sky cooling” using a new coating material they have developed.
I wrote to Raman, co-lead author of this paper, and he replied explaining the mechanism: “Radiative sky cooling exploits a natural property of our atmosphere. 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 ultrathin multilayered 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, these researchers showed that optical surfaces could be designed to enable this cooling effect even on a sunny day. In their recent work, they 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 3 to 5 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% or more. Raman and his colleagues are now commercialising the technology as a startup, SkyCool Systems, and have a pilot demonstration active in California, USA. They have already partnered with a manufacturer that can produce large sheets of the cooling material for further development.
And how does it apply to a country like India? “For Indian buildings our fluid cooling panels can have a major impact in commercial refrigeration in supermarkets, cold storage facilities, data centers, office buildings, malls and other commercial buildings,” he offers. “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 secondly, create fabrication facilities that can make the panels at the scales needed.
Goldstein, E. A. et al. Sub-ambient non-evaporative fluid cooling with the sky. Nat. Energy (2017) doi: 10.1038/nenergy.2017.143