Low-energy cooling system sends heat into space

US researchers have designed panels with the ability to cool buildings by 3–5°C below ambient air temperature, which could save up to 20% on a commercial building’s summer power bill. Their breakthrough has been published in the journal Nature Energy.

Typical air-conditioning systems use a condenser system to cool circulating air, which consumes electricity and heats the local environment. Finding ways to lower the energy usage of air-conditioning systems could save money, reduce greenhouse gas emissions and reduce the impact of buildings on the local microclimate.

Since 2013, Stanford University researchers led by Professor Shanhui Fan have been developing an alternative system that uses the sky to accept heat and induce cooling without requiring energy beyond the circulation system. Their system uses a specially designed panel with a mirror-like optical surface that emits infrared radiation directly into the atmosphere and outer space through a mechanism known as radiative sky cooling — a natural process that results from 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,” said Professor 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.”

Although our own bodies release heat through radiative cooling, radiative sky cooling is not particularly effective on a hot, sunny day, as the sunlight will warm you more than radiative sky cooling will cool you. To overcome this problem, the team created a multilayer optical film that reflects about 97% of the sunlight while simultaneously being able to emit the surface’s thermal energy through the atmosphere. Without heat from sunlight, the radiative sky cooling effect can enable cooling below the air temperature even on a sunny day.

The team’s original experiments, published in 2014, were performed using small wafers of a multilayer optical surface, about 8″ in diameter, and only showed how the surface itself cooled. The next step was to see how the technology worked as part of a larger cooling system.

For their latest paper, the researchers created a system where panels covered in the specialised optical surfaces sat atop pipes of running water. These panels were slightly more than 0.6 m in length on each side and the researchers ran as many as four at a time. With the water moving at a relatively fast rate, they found the panels were able to consistently reduce the temperature of the water 3–5°C below ambient air temperature over a period of three days.

A fluid-cooling panel designed by Professor Shanhui Fan, Aaswath Raman and Eli Goldstein being tested on the roof of the Packard Electrical Engineering Building at Stanford University. This is an updated version of the panels used in the research published in Nature Energy. Image credit: Aaswath Raman.

The researchers also applied their data to a simulation where their panels covered the roof of a two-storey commercial office building in Las Vegas and contributed to its cooling system. They calculated how much electricity they could save if, in place of a conventional air-cooled chiller, they used a vapour-compression system with a condenser cooled by their panels. They found that, in the summer months, the panel-cooled system would save 14.3 MWh of electricity — a 21% reduction in the electricity used to cool the building. Over the entire period, the daily electricity savings fluctuated from 18 to 50%.

Fan, Raman and Goldstein have gone on to found the company SkyCool Systems, which is working on further testing to optimise and eventually commercialise the technology. The researchers are currently focused on making their panels integrate easily with standard air-conditioning and refrigeration systems, and they are particularly excited at the prospect of applying their technology to the task of cooling data centres.

Top image caption: This photo from 2014 shows the reflectivity of the mirror-like optical surface Fan, Raman and Goldstein have been researching, which allows for daytime radiative sky cooling by sending thermal energy into the sky while also blocking sunlight. The people in this photo are Linxiano Zhu (co-author of the 2014 paper), Fan and Raman. Image credit: Norbert von der Groeben.