Besides superfluous features like touchscreens and internal cameras, basic refrigerator technology hasn't changed much in decades. They still chill your milk by way of chemical refrigerants and compressors, and are notorious drains on your electricity bill. Now researchers in Europe have shown promising early results with an experimental cooling system that uses magnetic fields and shape-shifting memory alloys.
Magnetic cooling systems work by exploiting the magnetocaloric effect – which basically means that certain materials will change temperature when exposed to a magnetic field. The technology has been around almost as long as conventional fridges, but has never really taken off because device complexity can ruin energy efficiency. The problem is often the superconducting magnets used, which require their own cooling system.
To get around that problem, researchers from Technical University Darmstadt and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany used a unique combination of magnets and special alloys. The magnets contain the rare-Earth metal neodymium, as well as iron and boron. The alloy is a mixture of nickel, manganese and indium.
That combination is key to making the system practical. Those magnets are the strongest permanent magnets currently known, capable of generating magnetic fields 40,000 times stronger than that of the Earth. That particular alloy, meanwhile, will cool down when exposed to a magnetic field and, in addition, it can return to its original shape after being deformed.
Using this combination, the researchers on the project developed a six-step refrigeration cycle. First, the cooling element (the alloy) is exposed to the magnetic field – just a millisecond is long enough for it to become magnetized and cool down. Then the alloy is removed from the magnetic field, and a heat sink cools whatever is needed. As the alloy warms back up, it will stay magnetized. Next the alloy is compressed by a roller, which makes it switch forms to become denser, lose its magnetism and heat up. When the roller is removed, the alloy returns to its original shape as it returns to its regular temperature, ready for the cycle to begin over again.
The project was mostly a feasibility study, to demonstrate how shape-memory alloys can help reduce the amount of permanent magnets needed in this kind of setup. Those magnets, the team says, are the most expensive part.
"We have been able to show that shape-memory alloys are highly suitable for cooling cycles," says Oliver Gutfleisch, an author of the study. "We need far fewer neodymium magnets but can nevertheless generate stronger fields and a correspondingly greater cooling effect."
The team plans to build a demonstrator unit by 2022 to get a better understanding of how well the system can cool things, as well as how energy efficient it is.
The research was published in the journal Nature Materials.
Source: TU Darmstradt
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