A more efficient, energy-saving, green, and portable cooling method is the direction of human unremitting exploration. Recently, an online article in the journal Science reported on a new flexible refrigeration strategy discovered by a joint research team of Chinese and American scientists – “torsional heat refrigeration”. The research team found that changing the twist inside the fibers can achieve cooling. Due to higher refrigeration efficiency, smaller size, and applicability to various ordinary materials, the “twisted heat refrigerator” made based on this technology has also become promising.
This achievement comes from the cooperative research of Professor Liu Zunfeng’s team from the State Key Laboratory of Medicinal chemistry Biology, School of Pharmacy, and the Key Laboratory of Functional Polymer of the Ministry of Education of Nankai University, and the team of Ray H. Baugman, professor of Texas State University, Dallas Branch, and Yang Shixian, Docent of Nankai University.
Just lower the temperature and twist it
According to data from the International Refrigeration Research Institute, the electricity consumption of air conditioners and refrigerators in the world currently accounts for about 20% of the global electricity consumption. The widely used principle of air compression refrigeration nowadays generally has a Carnot efficiency of less than 60%, and the gases released by traditional refrigeration processes are exacerbating global warming. With the increasing demand for refrigeration by humans, exploring new refrigeration theories and solutions to further improve refrigeration efficiency, reduce costs, and reduce the size of refrigeration equipment has become an urgent task.
Natural rubber will generate heat when stretched, but the temperature will decrease after retraction. This phenomenon is called “elastic thermal refrigeration”, which has been discovered as early as the early 19th century. However, to achieve good cooling effect, the rubber needs to be pre stretched to 6-7 times its own length and then retracted. This means that refrigeration requires a large volume. Moreover, the current Carnot efficiency of “thermal refrigeration” is relatively low, usually only about 32%.
Through the “torsional cooling” technology, the researchers stretched the fibrous rubber elastomer twice (100% strain), then fixed both ends and twisted it from one end to form a Superhelix structure. Subsequently, rapid untwisting occurred, and the temperature of the rubber fibers decreased by 15.5 degrees Celsius.
This result is higher than the cooling effect using ‘elastic thermal refrigeration’ technology: the rubber that is stretched 7 times longer contracts and cools down to 12.2 degrees Celsius. However, if the rubber is twisted and extended, and then released simultaneously, the ‘torsional thermal refrigeration’ can cool down to 16.4 degrees Celsius. Liu Zunfeng said that under the same cooling effect, the rubber volume of ‘torsional thermal refrigeration’ is only two-thirds of that of ‘elastic thermal refrigeration’ rubber, and its Carnot efficiency can reach 67%, Far superior to the principle of air compression refrigeration.
Fishing line and textile line can also be cooled
Researchers have introduced that there is still much room for improvement in rubber as a “torsional heat refrigeration” material. For example, rubber has a soft texture and requires many twists to achieve significant cooling. Its heat transfer speed is slow, and issues such as repeated use and durability of the material need to be considered. Therefore, exploring other “torsional refrigeration” materials has become an important breakthrough direction for the research team.
Interestingly, we have found that the ‘torsional heat cooling’ scheme is also applicable to fishing and textile lines. Previously, people did not realize that these ordinary materials could be used for cooling, “Liu Zunfeng said.
The researchers first twisted these rigid polymer fibers and formed a helical structure. Stretching the helix can raise the temperature, but after retracting the helix, the temperature decreases.
The experiment found that using the “torsional heat cooling” technology, polyethylene braided wire can generate a temperature drop of 5.1 degrees Celsius, while the material is directly stretched and released with almost no temperature change observed. The principle of ‘torsional heat cooling’ of this type of polyethylene fiber is that during the stretching contraction process, the internal twist of the helix decreases, leading to changes in energy. Liu Zunfeng said that these relatively hard materials are more durable than rubber fibers, and the cooling rate exceeds that of rubber even when stretched very short.
Researchers also found that applying the “torsional heat cooling” technology to nickel titanium shape memory alloys with higher strength and faster heat transfer results in better cooling performance, and only a lower twist is required to achieve a greater cooling effect.
For example, by twisting four nickel titanium alloy wires together, the maximum temperature drop after untwisting can reach 20.8 degrees Celsius, and the overall average temperature drop can also reach 18.2 degrees Celsius. This is slightly higher than the 17.0 degrees Celsius cooling achieved using ‘thermal refrigeration’ technology. One refrigeration cycle only takes about 30 seconds, “Liu Zunfeng said.
New technology can be used in refrigerators in the future
Based on the “torsional heat refrigeration” technology, researchers have created a refrigerator model that can cool flowing water. They used three nickel titanium alloy wires as cooling materials, rotating 0.87 revolutions per centimeter to achieve a cooling of 7.7 degrees Celsius.
This discovery still has a long way to go before the commercialization of ‘twisted heat refrigerators’, with both opportunities and challenges, “said Ray Bowman. Liu Zunfeng believes that the new refrigeration technology discovered in this study has expanded a new sector in the refrigeration field. It will provide a new way to reduce energy consumption in the refrigeration field.
Another special phenomenon in “torsional heat refrigeration” is that different parts of the fiber exhibit different temperatures, which is caused by the periodic distribution of the helix generated by twisting the fiber along the fiber length direction. The researchers coated the surface of nickel titanium alloy wire with Thermochromism coating to make “torsional cooling” color changing fiber. During the twisting and untwisting process, the fiber undergoes reversible color changes. It can be used as a new type of sensing element for remote optical measurement of fiber twist. For example, by observing color changes with the naked eye, one can know how many revolutions the material has made in the distance, which is a very simple sensor. “Liu Zunfeng said that based on the principle of” torsional heat cooling “, some fibers can also be used for intelligent color changing fabrics.
Post time: Jul-13-2023