Electricity-free refrigeration possible with heat-dissipating optical surface

Air conditioning and refrigeration could be achieved without electricity using a high-tech, mirror-like optical surface that naturally dissipates heat even on hot, sunny days.

Stanford University’s Aaswath Raman and Eli Goldstein have demonstrated a system involving these surfaces that can cool flowing water to a temperature below that of the surrounding air without requiring any 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 research.

Radiative sky cooling is a natural process resulting from the moments of molecules releasing heat and can typically be seen in the mirage effect produced from heat dissipating off a hot road as it cools after sunset.

This phenomenon is particularly noticeable on a cloudless night because without clouds the heat radiates more easily through the Earth’s atmosphere and out into 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,” said Shanhui Fan, senior author of the paper. “For this reason, the amount of heat-flow off the Earth that goes to the universe is enormous.”

However, on hot, sunny days radiative sky cooling typically won’t work because the amount of energy expressed is lower than the amount absorbed from the surroundings.

To overcome this problem, the team’s surface uses a multilayer optical film that reflects about 97 per cent 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.

“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.

In 2014, experiments were performed using small wafers of a multilayer optical surface, approximately 20cm in diameter, but they only showed how the surface itself cooled. The next step was to scale up the technology and see how it 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 and tested it on the roof of a building.

These panels were slightly more than 60cm 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° Celsius below ambient air temperature over a period of three days.

The researchers also applied data from this experiment to a simulation where their panels covered the roof of a two-story commercial office building in Las Vegas - a hot, dry location where their panels would work best - 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 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 megawatt hours of electricity, a 21 per cent reduction in the electricity used to cool the building. Over the entire period, the daily electricity savings fluctuated from 18 per cent to 50 per cent.

Fan, Goldstein and Raman are optimistic that this technology will find broad applications in the years to come. The researchers are now focused on making their panels integrate easily with standard air conditioning and refrigeration systems and are looking into using their technology to cool data centres.

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