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Solid-state refrigerators based on “plastic crystals” could one-day revolutionize cooling

Credit: Pexels.

Credit: Pexels.

The refrigerator is one of the most useful and valuable modern inventions. It forever changed our way of life, allowing food to be stored for much longer without having to change its taste or texture by pickling, potting, drying or salting it (the traditional way of preserving food before mechanical refrigeration appeared). But despite the fact that they’ve been around for more almost a century, refrigerators haven’t changed that much. They’re still essentially bulky boxes powered by compressors that move environmentally-taxing chemicals — but this seemingly timeless design may soon change.

Researchers at the University of Cambridge are experimenting with a new kind of refrigerating agent called “plastic crystals” which switch from a disordered phase into an ordered phased when pressed by a mechanical force. During the phase switch, the temperature rises and this extra heat is absorbed by the environment. This leads to a drop in pressure that forces the crystals back into an ordered state, causing the temperature to drop.

This pressure and temperature cycle is the same as the one used by a conventional refrigerator. However, your kitchen fridge cools things with a compressor (which is very bulky, constraining the product’s design) that raises the pressure of hydrofluorocarbons (HFCs). The gases — also used to cool cars and buildings — pose a rapidly growing climate threat, trapping heat in the atmosphere. They’re nearly 10,000 times as potent as carbon dioxide and unless their growth is checked, their emissions could double by 2020 and triple by 2030, according to U.S. data.

“If we can do the same job with a solid, it will be better for the environment,” Xavier Moya, a materials scientist at Cambridge University in the U.K., told Inside Science.

In a recent study published in Nature Communications, Moya and colleagues showed that plastic crystals of neopentylglycol (CH3)2C(CH2OH)2 display thermal changes comparable to HFCs.

In the future, solid-state refrigeration could lead to more compact and flexible devices compared to traditional ones. Vapor compression systems generally occupy a lot of space. Without the need for a bulky compressor and extensive pipes for vapor, solid refrigerators could theoretically be small enough to cool microchips.

Before solid-state fridges become a reality, there are some engineering challenges that need to be solved. One is hysteresis, which occurs when the system’s output depends not only on its present inputs but also on past inputs (when the system exhibits memory so to speak). In this particular case, the temperature of the plastic crystals doesn’t return all the way back down to its original temperature, so each cooling cycle becomes less efficient. Another downside is that plastic crystals require a huge pressure to operate, hovering at around 2,500 bars, compared to only 50-100 bars in traditional refrigerators.

According to the Cambridge researchers, it might a decade before plastic crystal refrigerators enter the consumer market. They are currently testing the technology under a spinoff which is already collaborating with manufacturers.

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