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The experiments that researchers from the Polytechnic of Catalonia and the University of Cambridge did a few years ago were quite simple. But they could prove extremely important for artificial cold and cooling.
The researchers placed plastic crystals made from neopentyl glycol – a chemical used in the manufacture of paints and lubricants – in a chamber, added oil, and inserted a plunger. When the liquid was compressed, the temperature of the crystals increased by about 40 degrees.
40 degrees in one step
This was the largest temperature rise ever achieved by putting a material under pressure. At least that holds true for the time when these findings were published in Nature Communications last year. And if the pressure is reduced, it has the opposite effect: the crystals cool dramatically.
According to the research team, the results show a promising approach for replacing traditional refrigerants; it could potentially enable “green cooling without any loss of performance”. Such advances are hugely important because, without significant technical improvements, the energy needed to cool indoor spaces could triple by 2050, as global prosperity, population numbers and temperatures increase, according to the International Agency for L ‘power.
The temperature variation in the materials was comparable to that of the hydrofluorocarbons, as they are used for cooling in normal air conditioning and refrigeration systems. But these are also strong greenhouse gases.
Magnetic refrigerators
The work on alternatives is based on a long-known phenomenon that you should know if you’ve already pulled out a balloon and touched your lips with it: so-called caloric materials emit heat when pressurized or otherwise stressed to want. For some, this effect can also be achieved with magnetic or electric fields or a combination of these.
Based on these principles, scientists have been trying to develop magnetic refrigerators for decades, but until now they have mainly required large, powerful and expensive magnets. But according to an early November analysis in Science, there has been significant progress in this area. It was written by Xavier Moya and ND Mathur, materials scientists at the University of Cambridge, who were also involved in the experiments described above.
The researchers are now familiar with numerous caloric materials with severe temperature changes and are using them in prototypes for heating and cooling devices, according to the study. Systems based on electricity, voltage or pressure have only been explored most intensively for just over a decade. In terms of performance, however, they are already close to magnetic devices that have been worked on much longer.
Releases latent heat
Furthermore, the technology could not only reduce the need for hydrofluorocarbons, but also become more energy efficient than conventional cooling devices. A key difference is that the materials remain in the solid state, while traditional refrigerants such as hydrofluorocarbons alternate between the gas and liquid phase.
Under certain forces, there are small temperature changes in many materials. However, researchers are looking for substances with which this effect is particularly pronounced, ideally with as little energy as possible. Some metal alloys, among others, have shown promise when ironed. Some ceramic and polymeric materials react strongly to electric fields, and inorganic salts and rubber show potential under pressure.
The forces or fields acting on the materials ensure that the atoms or molecules they contain are brought into a more rigid order. This corresponds to a phase change similar to when liquid water molecules become solid ice crystals. But the caloric materials still remain in the solid state, which only becomes more rigid. During this process, latent heat is released, which corresponds to the energy difference between the two states. And when the materials return to their original state when the forces are removed, a decrease in temperature occurs which can be used for cooling.
On the way to commercialization
This is not fundamentally different from the way coolers work today: they decompress hydrofluorocarbons until they transform from liquids into gases. But the solid-state-only cooling approach can be significantly more energy efficient. One reason is that you don’t have to move molecules that much to get the phase change, says Jun Cui, a senior scientist at Ames Laboratory.
Crucial to the development of competitive commercial devices are materials that undergo large temperature changes, easily return to their original state, survive many of these cycles without breaking (commercial coolers can run for millions of cycles), and are inexpensive. . Some materials and applications are gradually emerging, says Ichiro Takeuchi, a materials scientist at the University of Maryland. Ten years ago, he founded Maryland Energy & Sensor Technology, to produce cooling devices with materials that respond to stretching.
His research group has developed a prototype that compresses nickel-titanium tubes and relieves them again to generate heat or cold. In the first phase, the water flowing in the pipes absorbs heat and releases it, then the process proceeds in the opposite direction to obtain cold water for cooling containers or living spaces. The company wants to offer a wine cellar as the first product because it requires less energy than a large refrigerator or air conditioning. An unspecified material is used for this purpose, which is said to be relatively inexpensive.
Prototype for a heat pump
Materials scientist Moya, one of the authors of the Science paper, was also involved in founding a start-up a year and a half ago: Barocal, based in Cambridge, UK, developed a prototype for a pump heat. It uses plastic crystals that are “bound to neopentyl glycol, but better,” he explains.
In total, about a dozen start-ups have been created to commercialize the technology. Several established companies, including Chinese home automation giant Haier and Astronautics Corporation of America, have also looked into its potential.
Cui predicts that the first finished products based on materials that change temperature in response to forces will hit the market in the next five to ten years. It will take a few more years before they can compete with conventional air conditioning and cooling units in terms of price.
(High school)
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