Researchers from the Risoe National Laboratory – Technical University of Denmark, in Roskilde, have successfully achieved an 8.7ºC drop in magnetic cooling, bringing environmentally friendly refrigeration closer to development. Magnetic cooling technology exploits materials such as gadolinium that heat up when exposed to a magnetic field and cool down when that field is removed.

‘Other groups have reported larger drops [using magnetic cooling]. However, our result was achieved with a small and versatile machine,’ says Nini Pryds, Senior Research Scientist at Risoe.

‘Our system is contrary to others in that the configuration of the regenerator is in the form of plates and not phorous/spherical particles. Using plates [gives us the] advantage [of] controlling the flow characteristics inside the regenerator as well as obtaining a low pressure drop along [it], which reduces the consumption of energy.’

The research group ultimately aims to build a prototype refrigerator by 2010, using ceramic plates made of lanthanum, strontium, calcium and manganese, to replace gadolinium.

‘Ceramic materials are much cheaper than gadolinium. Also, they do not corrode in water, and tuning [their] Curie temperature (the temperature with the maximum magnetocaloric effect) is possible,’ says Pryds.

In the prototype, the magnetised plates will transfer their heat to water which will be pumped to a hot heat exchanger located outside the machine. When the magnetic field is removed, the plates will become cooler, drawing heat from the water, which will transfer its low temperatures to the cold heat exchanger located in the fridge.

‘This is the same principle as the cold and hot loops of a compressor-based refrigerator,’ explains Pryds.

But while traditional fridges can use up to 150W to induce temperature changes, the magnetic refrigerator will only require energy to circulate the water. And no damaging hydrofluorocarbons will be used.

The initial drop of 8.7ºC was obtained using a large electromagnet which requires high power consumption, making it unsuitable for practical use.

‘We are currently implementing a permanent magnet field source of the Halbach type (a cylinder assembly of neodymium iron boron magnet blocks with a diameter of around 20cm)’, adds Pryds.

There is still some fine-tuning that needs to be done before the system can achieve the 40ºC temperature range required for refrigeration. Pryds expects the technology will initially be used in industry, but would like to see it one day used in household refrigeration.