Researchers have demonstrated that a unique system can cool flowing fluid to a temperature below that of the surrounding air, without using electricity.
Since 2013 Shanhui Fan, professor of electrical engineering, and his colleagues, have been using the roof of Stanford University’s Packard Building to test a high-tech mirror-like optical surface that they believe could be the future of lower energy air-conditioning and refrigeration.
In 2014, the team published research showing the cooling capabilities of the optical surface. Now, Fan and former research associates Aaswath Raman and Eli Goldstein have shown that a system involving these surfaces can cool flowing water to a temperature below that of the surrounding air.
"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," said Raman.
Applying radiative sky cooling
At the heart of the team’s research is radiative sky cooling, the natural process that everything and everyone does as molecules release heat. An obvious process of this is the heat that comes off a road as it cools after the sun goes down.
“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," explained Fan, who is senior author of the paper. "For this reason, the amount of heat flow off the Earth that goes to the universe is enormous."
The main obstruction to radiative sky cooling is hot weather, where sunlight will warm faster than radiative sky cooling can cool. Fan and colleagues overcame this problem with their multilayer optical film which reflects about 97% of sunlight while simultaneously being able to emit a surface’s thermal energy through the atmosphere.
The material can enable cooling below air temperature, even on a sunny day.
"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," said Goldstein, co-lead author of the paper.
For their latest research, the team created a system where panels covered in the specialised optical surfaces sat atop pipes of running water. The test was performed on top of the Packard Building in 2015.
With the water moving at a relatively fast rate, the team found that the 2-foot-long panels were able to consistently reduce the temperature 3-5 degrees below ambient air temperature over a period of three days.
Data from the Stanford experiment was applied to a simulation where their panels covered the roof and contributed to the heating system of a two-story commercial office building in Las Vegas - a hot, dry location where the team believed their panels would be most effective.
It was revealed that in the summer months the panel cooled system would save 14.3 megawatt hours of electricity – a 21% reduction in energy used to cool the building, compared to a conventional air-cooled chiller. Electricity savings fluctuated from 18 to 50% across the entire period.
Fan, Goldstein and Raman have now founded the company SkyCool Systems, which is working on further testing and commercialising their panel technology.
The study detailing their research has been published in the journal Nature Energy.