Refrigeration systems have changed the way we live, but they are also contributing to the deterioration of the planet. At least a quarter of the electricity we use is dedicated to cooling things and to achieve this miracle of the technique gases with an intense greenhouse effect are used. The emission into the atmosphere of one kilo of these gases is equivalent to the carbon dioxide emitted by a car circulating without rest for half a year.
Recently, a team led by Bing Li of the Chinese Academy of Sciences has presented in the magazine Nature a system that can take advantage of the physical principle that makes refrigeration systems with gases possible, but with less environmental impact and the possibility of miniaturizing it and having a great impact in the world of electronics. In this case, the material used to extract heat from hot objects would be plastic crystals.
More than a quarter of the electricity that humanity spends is used in cooling systems
To understand the proposal of the Chinese scientists, it is first necessary to know how standard refrigeration systems like refrigerators work. Basically, they require four steps. In the first, the gas contracts, which increases in temperature. Then, that heat is extracted from the compressed gas and when decompressing it loses temperature and it is colder than the environment. That cold gas can then be used to cool the food in the fridge, for example. "The idea is that initially the gas molecules are very messy and with the increase in pressure the volume of the gas and its degree of disorder is reduced," explains Claudio Cazorla, a researcher at the School of Materials Science and Engineering at the University from New South Wales (Australia). "Reducing the level of disorder in a reversible way, you can cool a system and that is what is done in the case of plastic crystals," he continues.
The basic idea, continues Cazorla, "is to be able, by applying an external change, to generate a change in a substance, be it a gas or some crystals, that involves a great change of entropy or, to put it more simply, of order within the system. " In the case of plastic crystals, a pressure or an electric or magnetic field could be applied that produces a variation in the molecular order of the crystal. In principle, the changes that could occur in some crystals would not be as great as with a gas, but the crystals proposed in the article of Nature they are special and allow to induce a great change in the level of disorder by applying small pressures on them.
These plastic crystals, which are routinely used in industries such as pharmaceuticals, are made up of simple organic molecules that are rotating. "They have what is called rotational disorder and you can not tell what the order of the molecules is," says Cazorla. "But when you apply a small pressure, you cancel those free rotations of the molecules and you happen to have a more ordered system, as in the case of the gas when we compress it, because all the molecules have a certain orientation," he says.
Although the technology used in refrigeration is old and has been optimized, its energy efficiency, which is around 60%, can be improved and some of these solid refrigerants could achieve higher efficiencies. In addition, miniature cooling systems could be designed to increase the potential of microelectronics. Until now, it is impossible to introduce a gas cooling system in a mobile, because on a microscopic scale it would not be effective, but you could put sheets of these materials that cooled the circuits on a microscopic scale and improve efficiency.
Another possible application would come as support for the development of batteries for electric cars. One of the limitations of these devices is the loading speed. The faster it is done, the more heat the process generates and that heat is difficult to eliminate.
Along with the advantages of plastic crystals as refrigerants, Li's team also recognizes some important limitations. The same malleability that makes them sensitive to slight pressures makes them mechanically very soft and does not convert them into ideal materials to create a durable cooling system.