Camel-inspired cooling material

Side view of the two layers of the camel-inspired double-layer material. (Image: Zhengmao Lu and Ningxin Chen)

Animal experts were the godfathers: Researchers present a new cooling technology that mimics how camels protect themselves from overheating in the desert. It is a layer material made up of two components, which enables water-saving evaporative cooling without the use of energy. The base forms a hydrogel that takes on the effect of the sweat glands. Above it sits an airgel layer that looks like the fur of desert animals: heat is kept out, but water vapor is let through. This two-component design can keep objects cool about five times longer than previous materials. For example, products sent by post could be kept fresh longer in this way, say the developers.

In some regions of the world it is always hot, in others the summer sun causes extreme values ​​and in the course of climate change the situation is likely to worsen. Innovative cooling systems are therefore increasingly in demand. For some time now, scientists have also been working on concepts for passive cooling, which, in contrast to active cooling using air conditioning systems, does not require an external energy supply. Nature has equipped us and many animal species with such a concept: The release of moisture through sweating ensures that the body is cooled by evaporation.

Another look at the nature patents

This concept has already been implemented technically in the form of water-containing hydrogels that release moisture and thus provide evaporative cooling. But they need a lot of water and their service life leaves a lot to be desired. In order to look for optimization possibilities, the researchers led by Jeffrey Grossman from the Massachusetts Institute of Technology in Cambridge have now looked around the animal kingdom again. When looking at the physiology of camels, the scientists realized that a key component was missing in previous evaporative cooling technologies: the fur.

Biological studies gave the decisive indication: “Zoologists have reported that a shorn camel has to increase the water consumption for sweating during the day by 50 percent compared to a camel with natural fur,” says Grossman. Conversely, this means: The fur provides a cooling effect that enables water to be saved. “While previous research on passive cooling focused on mimicking evaporation from sweat glands in mammals, we have now turned to the role of fur insulation,” says Grossman.

In order to imitate the camel’s fur layer, the researchers synthesized highly porous, hydrophobic silica aerogels that have low thermal conductivity and at the same time allow moisture to pass through. They then placed a layer of this material on a layer of hydrogel, which can release moisture – similar to the sweat gland-covered skin of animals. “In this way, we created an evaporation-isolation double layer that corresponds to the fur-gland system of the camels,” says Grossman. In order to find out to what extent the concept delivers what it promises, the researchers carried out tests: They enclosed a chamber with the double layer and recorded the temperature inside at controlled ambient temperature and relative humidity.

Cooled to save water

It turned out that the material was able to guarantee a temperature inside the chamber that was seven degrees Celsius lower than the external values. The highlight was the duration of the service. A hydrogel layer alone could also reduce the temperature and even a little more – but only for 40 hours. Then the moisture was exhausted and the gel had to be recharged with water. A five-millimeter hydrogel layer covered by a five-millimeter airgel, on the other hand, was able to reduce the temperature for 200 hours before its moisture was depleted, the tests showed. “The concept can thus enable a considerable extension of the passive evaporative cooling time with the same water consumption”, summarizes Grossman.

The scientists therefore now see considerable potential for applications in their camel-inspired cooling material. Due to the ability to keep objects cool over a longer period of time without the need for energy or maintenance, the cooling process could be used, for example, when shipping sensitive goods such as medicines. The effects would be particularly helpful, especially when transporting in warm regions of the world, where continuous cooling by air conditioning is problematic. Using it to cool large objects does not seem to make sense, say the researchers. “The material costs for our airgel are low, but so far the manufacturing costs have been a critical aspect for scalability due to the drying step,” says Grossman. However, according to the researchers, further developments may still be able to eliminate this problem.

Source: Cell Press, technical article: Joule, doi: 10.1016 / j.joule.2020.10.005

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