To make the beer cold in the fridge, it needs a coolant. It is important that it vaporizes even at low temperatures, because at the transition from liquid to gaseous, it absorbs heat. Inside the fridge it gets colder. Even freezers and air conditioners work on this principle.
But coolants often have negative properties. The formerly used and now banned chlorofluorocarbons (CFCs) damage the ozone layer. Newer refrigerants such as propane, butane or R134a act as greenhouse gases and have low energy efficiency.
Researchers are therefore working on cleaner and more efficient alternatives and have now tried out a seemingly unusual approach: a team led by Run Wang of China's Nankai University has twisted natural rubber fibers, fishing lines and nickel-titanium wires. The materials absorb heat, dissolve the twist, release the substances, write the scientists in the journal "Science".
Cool down by 16.4 degrees
Since the early 19th century, researchers know that natural rubber gets warm when pulled apart. If the tension dissipates, it cools down. "To lower the temperature significantly, however, one has to release the rubber from a very large strain," explains Ray Baughman of the University of Texas, who was also involved in the study. "Or you just unravel him."
University of Texas at Dallas
The color indicates the temperature of the rubber: when it is rolled up, it becomes warm (yellow) and cools down when relaxing (orange and brown)
In the current trial, the researchers combined both techniques. At first, they pulled out rubber fibers and then twisted them until a ball of twine formed. The researchers twirled the rotation, the temperature of the material dropped by 15.5 degrees, they also solved the elongation, it was 16.4 degrees.
The same principle also worked with polymer fibers used, for example, as fishing lines. Here, researchers discovered a peculiarity: twisting the material in one direction and tangling it in the other, the fibers cooled when pulled apart. "This is unusual, because materials usually warm when you stretch them," says Baughman.
Refrigerators and air conditioners consume 20 percent of electricity
Nickel-titanium wires were most easily cooled using the technique. They are considered to be particularly promising materials because they have a shape memory: when their shape changes, they move back to their original position.
This changes their state of aggregation. However, unlike conventional coolants, it does not become liquid gaseous, but the alloy alternates between two crystal structures. In the phase transitions heat is consumed or released - the wire heats up when pulled apart or twisted and cools in the opposite movement.
In the current experiment, the temperature of such a wire sank by 17 degrees during untwisting. When researchers used a bundle of four wires, they reached a temperature drop of 20.8 degrees. The scientists also used the material to build a small test refrigerator.
They integrated a three-layer nickel-titanium wire cable into a purpose-built device. By unwinding the wire, they were able to cool down a stream of water by 7.7 degrees. If you add more cycles of winding and unwinding, you can cool the device even more, so the researchers.
In order for the technology to work, the wire in the system has to be unrolled again and again - and then contracts independently. The compressor, which liquefies the coolant in the refrigerator again and again, is thus at least partially integrated in the material.
Researchers from Germany work on similar devices
Refrigerators, refrigerators and air conditioners consume about 20 percent of the world's electricity, according to the intergovernmental science organization International Institute of Refrigeration. In the course of industrialization in developing countries, it is likely that the value will increase significantly.
Scientists at the Saarland University are also researching nickel-titanium wires. They too have already developed an air-conditioning prototype with the help of the materials and presented it at the Hanover fair at the beginning of the year.
However, neither the test refrigerator of the Chinese-American research team nor the air conditioning of the German scientists are ready for the market. The engineers from Saarland are still working on improving the heat transfer of their machine.
Scientist Baughman says, "One of the challenges is to find materials that have a long life and at the same time are able to recover part of the mechanical energy from the rotation."
