The top of the Packard Electrical Engineering Building at Stanford University has been the scene of milestones in the development of an innovative cooling technology. Shanhui Fan, professor of electrical engineering and his research team have employed the roof as a test area for a new high tech, a mirror-like optical surface that could develop to be the future of low-energy air conditioning and refrigeration.
Fan and former research associates Aaswath Raman and Eli Goldstein have shown that a system with these surfaced can cool flowing water to a temperature below the temperature of the surrounding air. The cooling process is done without electricity.
"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, co-lead author of the paper. SkyCool Systems, founded by Fan, Goldstein and Raman, is working on further testing and commercializing the technology.
Radiative sky cooling is a natural process that everything does. It results from the moments of molecules releasing heat.
"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. "For this reason, the amount of heat flow off the Earth that goes to the universe is enormous."
The radiative sky cooling isn’t going to live up to its name when it is hot and sunny outside. This is because the sunlight will warm you more than radiative sky cooling will cool you. In order to overcome this problem, the team’s surface has a multilayer optical film that reflects 97 percent of the sunlight while simultaneously being able to emit the surface’s thermal energy through the atmosphere. Without heat from the sun, the radiative sky cooling effect can enable cooling below the air temperature, even on a sunny day.
In the latest paper, the researchers created a system where the panels that were covered in the specialized optical surfaces sat on top of pipes of running water and tested it on the Packard Building. These panels were a bit more than 2 feet in length on each side and the researchers ran up to four at a time. The water moved at a fairly fast rate and researchers found the panels were able to consistently reduce the temperature of water 3 to 5 degrees Celsius below ambient air temperature over three days.
The researchers applied data from this experiment to a simulation where the panels covered the roof of a two-story commercial office building in Las Vegas and contributed to the cooling system. They calculated how much electricity would be saved if in place of a conventional air-cooled chiller. They also used a vapor-compression system with a condenser cooled by the panels. The researchers found that the panel-cooled system would save 14.3 megawatt-hours of electricity in the summer, a 21 percent reduction in the electricity used to cool the building. Over the whole period, the daily electricity savings fluctuated from 18 percent to 50 percent.
SkyCool Sytems is measuring the energy saved when panels are integrated with traditional air conditioning and refrigeration systems at a test facility. The team is hopeful that the technology will find broad applicability in years to come. The researchers are currently focused on making the panels easily integrate with standard air conditioning and refrigeration systems. They are excited at the idea of applying their technology to the cooling data centers.
The paper on this research was published in Nature Energy on September 4.