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Refrigerators and air conditioners generate heating because they consume fossil energies and they use greenhouse gases. In turn, warming requires more cooling to safeguard the health of people and products.

Get out of this vicious circle is not easy. Magazine ‘Science’ Today dedicates a series of articles to the proposals of science to cool without polluting. There are promising avenues, but no magic bullets. In the short term, it is inevitable to drastically change the use of refrigeration.

What is at stake is your own human survival. In 2017 in the Persian Gulf already was promptly exceeded the threshold of 35 degrees in the shade (technically, “wet bulb temperature”). These degrees are incompatible with human life, since it prevents the body from expelling heat out. Climate change statistics predict that, from the middle of this century, this threshold will be exceeded with increasing frequency.

Refrigeration appears to be a matter of life and death. But it consumes fossil energy. In addition, the gases it uses, HFC (hidrofluorocarburos), are between 1,000 and 9,000 times more powerful than the famous CO2 as a cause of warming. Good recycling can transform them into harmless substances, but it is not the norm around the world.

De HFC a HFO

The Kigali protocol of 2016 requires the phase-out of HFCs in 2028. However, there is no clear replacement. HFOs (hydrofluoroolefins) they are the best candidates, according to the studies of ‘Science’. Other contenders are ammonia, carbon dioxide, propane, and isobutane. But none of them are optimal. The ideal replacement should be efficient and cheap, neither toxic nor flammable. Nor should it pollute the air or water. At the moment, none meets all these characteristics.

While industry improves gases, science explores the solid. In standard refrigeration, a substance is compressed and expanded, transforming it from a gas to a liquid and vice versa. As a result of one of these transitions, the material absorbs energy from the environment and cools it. The same thing happens with some solid materials, with the difference that they remain solid throughout the process, changing their microscopic structure.

“The first to notice was the blind scientist John Gough, at the beginning of the XIX century “, explains Xavier moya, researcher at the University of Cambridge and co-author of one of the articles from ‘Science’. “They brought him a piece of rubber and when he brought it to his lips he noticed that it got hot if he stretched it, “he says.

Mechanocaloric materials

Since then, many mechanocaloric materials have been found or manufactured. The nitinolFor example, it is a nickel-titanium alloy that cools down to 20 degrees if force is applied to it. In 2019, researchers from the University of Barcelona added manganese, reaching up to 30 degrees.

Moya works with plastic crystals, a class of materials that drop as low as 40 degrees when applied to large pressures. There are also materials that cool when a magnetic field (As the gadolinium, a rare earth that cools under the effect of a powerful magnet) or a electric current (like some ceramics). There are prototypes of air conditioners or refrigerators based on caloric materials, but “any technology that is not software has development times of a decade”, Moya points out.

There are many difficulties. Some materials are rare or expensive. Others need strong magnets. Others break from effort or current. Others are losing their cooling power. “There is a long list of other materials that are being investigated, but calories are the ones that are best placed,” they agree Lluís Mañosa Y Antoni Planes, UB researchers and inventors with other scientists of the nickel-titanium-manganese alloy that is giving such good results.

Reflective film

Perhaps the most surprising proposal is a kind of reflective film developed by researchers at the University of Colorado at Boulder. It’s about a super mirror that bounces the sun’s heat towards the cold parts of the atmosphere, producing drops of up to 8 degree. Its inventors believe that the roof of buildings and power plants could be lined with this material to reduce the requirement for cooling inside.

The effect is based on weaving the material with microscopic structures of a size comparable to the wavelength of light, allowing it to be directed upwards, with frequencies that easily cross the atmosphere. In nature, silver ants from the Sahara have similarly structured skin.

“We have been using the same technology for cooling for more than a century. The gases are getting a lot better, but we need a jump like the one that happened between vacuum tubes and semiconductors, which gave way to the computer revolution “, concludes Moya.

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