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280: Radiative passive cooling system

Problem:

With the melting of Arctic ice and mountain glaciers which had previously reflected back solar heat and maintained global cooling, alternatives must be found, particularly for the air conditioning of buildings which traditionally use chemicals and electricity.

Solution:

Qiaoqiang Gan, Electrical Engineering Associate Professor at the University at Buffalo School of Engineering and Applied Sciences, working with staff from King Abdullah’s Saudi Arabian University have has developed an air conditioning system using a unique plastic on a roof that allows heat to pass back into the sky.


The ideal material for radiant cooling is such as a mirror or white paint. They will scatter or reflect most of the solar light. Therefore, the solar light will not heat the object. Then it’s easier to cool it down. Gan’s Newtact system, going beyond simple white paint, consists of an inexpensive polymer/aluminium film that is installed inside a box at the bottom of a specially designed solar “shelter.”

Taken together, the shelter-and-box system the engineers designed measures about 18 in. tall (45.72 cm), 10 in. (25. cm) wide and 10 in. long (25.4 cm). As a commercially viable alternative, the researchers fabricated their thermal emitter from polydimethylsiloxane (PDMS) and either silver or aluminium.

The PDMS film absorbs heat from the environment and then transmits the heat to cool down its surroundings. The metal reflects the solar light to prevent the transmission of sunlight to materials under the emitter, such as a roof.

The film helps keep its surroundings cool by absorbing heat from the air inside the box and transmitting that energy into outer space. The shelter serves a dual purpose, helping to block incoming sunlight, while also beaming thermal radiation emitted from the film into the sky.

Outdoor experiments performed in Buffalo, NY, to test the device provided cooling of up to 9 °C. To cool a building, numerous units of the system would need to be installed to cover its roof.

America
Working in Stanford University, Shanhui Fan, Eli A; Goldstein, and Aaswath Pattabhi Raman have also developed a flat rectangular metal panel covered in a sheet of the material: a high-tech film which reflects the light and heat of the sun so effectively that the temperature beneath the film can drop 5 to 10° C (9 to 18° F) lower than the air around it.

A system of pipes behind the RC panel is exposed to that colder temperature, cooling the fluid inside before it is sent out to current-day refrigeration systems. More efficient than any vapour-compression based cooling system, the panel can also prevent the emissions of CO2 and other harmful greenhouse gases. It can be roof-mounted as a simple add-on to new and existing cooling systems

To commercialise their innovation, Fan, Goldstein and Raman started up a company, SkyCool Systems in Davis, California. As a pilot study, they installed an array of RC panels on the roof a supermarket as a subcooler and were pleased to observe a 10% to 15% efficiency improvement target, although subcooling could be as much as 20°F (11°C) below the outlet of the condenser. Other studies have followed.

Discover Solution 281: Sea grass

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