Magnetic Refrigeration Advances

Hot on the news about nano solar we have this item about advances in magnetic refrigeration. I draw your attention to two points made in this report. The first is that the energy efficiency is at the 60% level as compared to 40% for current technology which has been extant for almost a hundred years. The fact that mechanical devices such as valves and compressors and the like also disappear is a bonus. This means that a refrigerator will consist as always of an insulated box with a much simpler cooling panel doing the work. The redesign and conversion will be swift.

The other point made is that an operating range of 100 degrees is claimed. When I reviewed work in this field around two years ago, the best operating range for the best magnetics was around 10 to 20 degrees and represented major stumbling block. This is even more important than efficiency.

This makes completely new applications of refrigeration completely feasible and the reengineering of old applications a must.

The first and most obvious fix is the creation of solar powered magnetic air conditioning units. This would eliminate that component of grid load and make most of the benefits freely available during the hottest hours. The resulting units should be cheap enough to sell world wide. I can almost envisage a third world residence with its obligatory TV been run of the solar panel and air conditioning unit.

The second important fix is the manufacture of stand alone combined units that simply produce atmospheric water for adjacent crops. These last two innovations permit the cost to drop down to below the thousand dollar point. And once mass production kicks in the cost will continue to come down.

The point I am making is that there are now no remaining technology road blocks.

May 19, 2008

Magnetic refrigeration moves on

Nanocomposites produced from metallic glasses could make promising magnetic refrigeration materials according to new work by scientists in France. The materials are as good as the best currently available magnetic refrigerants with some added advantages. Refrigerators using such materials would be environmentally friendly and more efficient than existing devices that rely on a vapour cycle.

"Magnetic refrigeration is an environmentally friendly cooling technology, unlike the gas-compression refrigerators used today," team member Stéphane Gorsse of the Institute of Condensed Matter Chemistry of Bordeaux (ICMCB-CNRS) told It uses no ozone-depleting, hazardous chemicals or greenhouse gases (such as hydrofluorocarbons used in conventional refrigeration systems). Moreover, the energy efficiency can reach up to 60%, compared to just 40% for the best gas-compression refrigerators.

Original material

And that's not all: current magnetic refrigerants are only efficient in a narrow temperature range of a few degrees above and below their transition temperature. The new material is the first to efficiently perform in a wide temperature range of about 100 K. Moreover, the working temperature and operating range can be tailored by tuning the composition and manipulating the microstructure.

"Our material is original because its properties combine advantages of crystallized and amorphous materials due to its unique microstructure: it is a nanocomposite made of gadolinium nanocrystallites embedded in a gadolinium-aluminium-manganese (Gd60Al10Mn30) metallic glass matrix," explained Gorsse.

Unique properties

Metallic glasses are still relatively "immature" materials and have few applications – mainly in sports equipment (zirconium-based metallic glasses in golf clubs, for example). But these materials exhibit unique properties thanks to their disordered atomic structure.

Gorsse and colleagues made their nanocomposite by rapid quenching of melt to avoid crystallization and to form a metastable disordered amorphous solid (the metallic glass). The glass was then subjected to a heat treatment, which needs to be stopped early to prevent the glass from fully crystallizing.

"Our material is as good as the best currently available materials that are crystallized and which exhibit first-order transitions and strong magnetocrystalline coupling," said Gorsse. "These materials present several disadvantages compared to ours – they have highly hysteric and hard magnetic behaviour, which reduces the efficiency of the cooling process since it leads to energy losses. Also, structural changes in these materials promote crack nucleation and propagation that can cause severe damage to the refrigerant material during cycling."

The microstructure of the nanocomposite (volume fraction, size and composition of the nanocrystallites formed in situ), and thus the resulting magnetocaloric properties and refrigeration capacity, depends on the heat treatment temperature and time. The researchers therefore plan to study and model how the microstructure of their material evolves during heat treatment and how the glass composition affects crystallization. "We also hope to identify and produce the ideal microstructure that gives the best material with improved magnetic refrigeration and working temperature," revealed Gorsse.

The work was published in Appl. Phys. Lett.

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