It appears to be like a regular roof, but the top of the Packard Electrical Engineering Building at Stanford University has indeed been the setting of a number of milestones in the development of a unique cooling technology capable of someday being a part of an individual’s daily life.
Since 2013, this roof has been used by Shanhui Fan, Professor of Electrical Engineering, and his Students and Research Associates as a testbed for a high-tech mirror-like optical surface that has the potential to become the future of lower-energy air conditioning and refrigeration.
Research published in 2014 initially revealed the cooling capabilities of the optical surface on its own. Fan and former Research Associates Aaswath Raman and Eli Goldstein have now demonstrated that a system involving these surfaces is capable of cooling flowing water to a temperature less than that of the surrounding air. Electricity is not used for the entire cooling process.
This research builds on our previous work with radiative sky cooling but takes it to the next level. It provides for the first time a high-fidelity technology demonstration of how you can use radiative sky cooling to passively cool a fluid and, in doing so, connect it with cooling systems to save electricity.
Aaswath Raman, Co-lead Author and Former Research Associate
The paper detailing this research was published in Nature Energy on 4th September.
Fan, Goldstein and Raman have together established the company SkyCool Systems, which focuses on further testing and commercializing this technology.
Sending our heat to space
Radiative sky cooling is considered to be a natural process performed by everyone and everything due to the moments of molecules discharging heat. Individuals can witness this for themselves in the heat that comes off a road as it cools after sunset. This phenomenon is mainly visible on a cloudless night, because, in the absence of clouds, the heat radiated by humans and everything around them can more effortlessly make it through the Earth’s atmosphere, all the way to the cold, vast reaches of space.
If you have something that is very cold – like space – and you can dissipate heat into it, then you can do cooling without any electricity or work. The heat just flows. For this reason, the amount of heat flow off the Earth that goes to the universe is enormous.
Shanhui Fan, Senior Author and Professor of Electrical Engineering, Stanford University
Even though the body of an individual releases heat through radiative cooling to both the surroundings and the sky, it is an established fact that on a sunny, hot day, radiative sky cooling is not going to live up to its name. The reason being that sunlight will warm an individual more than the cooling provided by radiative sky cooling. In order to solve this problem, the team’s surface employs a multilayer optical film capable of reflecting around 97% of the sunlight while simultaneously having the potential to emit the surface’s thermal energy via the atmosphere. The radiative sky cooling effect will be able to enable cooling below the air temperature even on a sunny day without heat from sunlight.
With this technology, we’re no longer limited by what the air temperature is, we’re limited by something much colder: the sky and space.
Eli Goldstein, Co-lead Author of the paper
The experiments published in 2014 were executed using tiny wafers of a multilayer optical surface, about 8″ in diameter and only revealed how the surface itself cooled. The next step was to obviously scale up the technology and observe how it functions as part of a larger cooling system.
Putting radiative sky cooling to work
The Researchers, for their most recent paper, developed a system where panels covered in the particular optical surfaces rested on pipes of running water and tested it on the roof of the Packard Building in September 2015. These panels were to some extent more than 2" in length on every single side and the Researchers ran as many as four at a time.
With the water flowing at a comparatively fast rate, the Researchers discovered that the panels were able to steadily reduce the temperature of the water 3 to 5 oC below ambient air temperature over a period of three days.
Data from this experiment was also applied to a simulation where their panels covered the roof of a two-story commercial office building located in Las Vegas – a dry, hot location where their panels would function best – and contributed to its cooling system.
The Researchers calculated how much electricity they would be able to save if they employed a vapor-compression system with a condenser cooled by their panels instead of a conventional air-cooled chiller. They studied that in the summer months, the panel-cooled system will be capable of saving 14.3 megawatt-hours of electricity, a 21% reduction in the electricity used to cool the building. The daily electricity savings fluctuated from 18% to 50% over the entire period.
The future is now
SkyCool Systems is presently measuring the energy saved when panels are incorporated with conventional air conditioning and refrigeration systems at a test facility. Additionally, Fan, Goldstein and Raman are hopeful that this technology will find wide-ranging applications in the future. The Researchers concentrate on making their panels incorporate effortlessly with standard air conditioning and refrigeration systems and they are specifically excited at the prospect of using their technology to the serious task of cooling data centers.
Fan has also performed research on several other aspects of radiative cooling technology. He and Raman have used the concept of radiative sky cooling to the development of an efficiency-boosting coating for solar cells. Fan developed a cooling fabric along with Yi Cui, a Professor of Materials Science and Engineering at Stanford and of photon science at SLAC National Accelerator Laboratory.
“It’s very intriguing to think about the universe as such an immense resource for cooling and all the many interesting, creative ideas that one could come up with to take advantage of this,” he said.
Fan is also Director of the Edward L. Ginzton Laboratory, a Professor, by courtesy, of applied physics and an affiliate of the Stanford Precourt Institute for Energy.
The Advanced Research Projects Agency – Energy (ARPA-E) of the Department of Energy funded this work.