A method to keep things cool without burning energy is getting a practical look, says S.Ananthanarayanan.

It takes energy to pump out heat and cool a thing colder than its surroundings. This is the energy that is used in refrigeration and the air conditioning. Air conditioning costs account for 15%, in the US, to 70%, in the Middle East, of the total energy consumption, and are set to quadruple

There has hence been the greatest interest in finding ways to bring about cooling without the use of energy. While this appears to be an impossible quest, as heat always flows from hot things to cooler, not the other way about, as in refrigeration, hope has been held out with the discovery of a channel where the flow of heat is not to the surroundings, but to the extreme cold of outer space! In recent years, a number of devices have shown that it is indeed possible for an object to radiate and lose heat despite the environment being warmer than it

Lyu Zhou, Haomin Song, Jianwei Liang, Matthew Singer, Ming Zhou, Edgars Stegenburgs, Nan Zhang, Chen Xu, Tien Ng, Zongfu Yu, Boon Ooi and Qiaoqiang Gan, from the State University of New York at Buffalo, King Abdullay University of Science and Tech, Saudi Arabia, University of Wisconsin and Hangzou Dianzi University, China, describe in the journal, Nature Sustainability, an improvement that makes this idea, of 'passive cooling,' both economical and scalable.

The reason why things on the surface of the earth stay warm is that the earth is enveloped by the atmosphere, which absorbs the heat the earth radiates and radiates most of it right back. Scientists, however, have noted that not all the heat that is radiated back, there is still a portion, a specific frequency band that passes clean through the atmosphere. If we could manage that a larger part of the radiation from objects is at this 'window' frequency of radiation, they could cool by themselves, without the heat lost being radiated back.

Warm objects radiate heat over a range of frequencies, distributed around a middle frequency, at which the radiation is the greatest. This peak frequency is usually the infrared region, but can rise to the red region with objects that are 'red hot'. Only a fraction of the energy, however, is at the 'window' of interest. The rule, fortunately, is not followed in all cases and there are specific nano-structured materials that have greater emission in given frequency bands. Creating structures like this, however, takes complex fabrication of very small dimension components and the solution is not viable. A development of this method uses layers of specific materials, whose atomic structure ensure emission at the desired frequency band. An arrangement with a seven-layer stack of these materials, mounted on a silver reflector, and insulated from external heat, was found to drop by 4-5 °C, as soon as it was moved from the shade and into sunshine. The cooling was some 40 W for a square meter, which was comparable to solar panels, and was expected to improve.

A further development was a material, fabricated from silicon crystal, that emits heat at the correct frequency 'window', and is transparent to the frequencies of light that are useful for solar cells. Such a material used as a cover for solar cells would allow the cells to continue working, but radiate heat and keep the cells cool. As rise in temperature is a factor that affects the efficiency of solar cells, this kind of cover would have great value.

The current paper in Nature Sustainability recounts these developments, and others, but says these are effective at night, and their manufacture is complex and they are not economical or capable of being widely used. In contrast, the authors of the paper present a low cost material, a film of which can be easily laid on a reflecting metal surface, and which enables substantial, daytime cooling of large structures like buildings, even in crowded, urban regions.

The emitting material is a thin film of polydimethylsiloxane (PDMS), a common silicone, a transparent, soft and pliable material that is found in contact lenses, medical devices, as a lubricant, sealing agent, water repellant coating, even in shampoos. In 2017, PDMS was reported to selectively emit at the frequency range, which can get through the atmosphere, with potential for passive cooling by 12°C, under the night sky.

A thin film of this inexpensive material is coated on a plate of aluminium or silver, using a simple process that can be adapted for mass production. A layer that is 100 microns thick, and hence a hardy layer, is found to be an effective selective emitter. The graph in the picture shows that the emissivity is high in the relevant region of 8 to 13 micrometers. As it is low in the visible and other regions of solar energy, the absorption of energy at these wavelengths is low, which makes the material suitable for use in the daytime.

Whether the arrangement is effective was tested, both in the laboratory and in the field. For the laboratory test, the extreme cold of outer space was simulated by a liquid nitrogen bath, with a black, aluminium base, while the PDMS/plate was fixed facing down, towards the cold bath. Temperature gauges helped confirm that the presence of the cold bath did not affect the temperature around the emitter.

Another thing the gauges confirmed was that the emitter sent out the radiation not as a beam, but in all directions. As this would limit the effective loss of energy by the emitter, to the cold absorber, the mirrors of aluminium foil were fitted to the sides of the container, to act as guides, to beam the radiation. The measurement using the temperature gauges showed that with guiding the radiation, the arrangement could attain a cooling of 9.5°C.

In the outdoor trial, the effect of surrounding buildings was simulated using a window placed above the emitter, which was narrowed or widened, to limit or extend the exposure to the clear sky. The result, the paper reports, was cooling of 11°C. Even without the window that blocked external influences, there was cooling of up to 9°C, using a vertical, reflecting sunshade

In both, the indoor and outdoor trials, the theoretical cooling expected was calculated, and it was found that the cooling in practice was in close agreement.

"In summary," the paper says, "we developed a highly efficient and low-cost passive cooling technology by exploiting the sky as the cold source...with disruptive potentials in transforming cooling solutions in a wide range of industrial and residential applications."

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