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To demonstrate that the technology of magnetic refrigeration works as well as to study the design and physics of a magnetic refrigeration device we have constructed a magnetic refrigeration test machine that can perform continuous magnetic refrigeration and produce a temperature difference of up to 8 degrees. The central part of the test machine is the Active Magnetic Regenerator (AMR) which is the part that produces the cooling effect. The AMR consists of a tube containing water (or some other fluid) and a magnetocaloric material. Two pistons are placed in each end of the AMR and can push the water from side to side. An illustration of the AMR can be seen below.
The AMR produce cooling through a four step regenerator cycle. This cycle is illustrated and documented below:
Step 1:
1) the magnetocaloric material is magnetised, thus decreasing the entropy and thereby increasing the temperature of the material.
Step 2:
2) the water is pushed to the hot end of the AMR.
Step 3:
3) the magnetocaloric material is demagnetised, decreasing the temperature.
Step 4:
4) the water is pushed to the cold end of the AMR.
In this way, a temperature gradient is build up across the AMR, and by using heat exchangers in the ends of the AMR, heat can be transported from the cold end to the hot end.
Currently, the magnetocaloric material used is Gadolinium and the heat transfer fluid is a water/ethanol mixture. This, however, can be changed depending on the experiment.
To apply a magnetic field to the magnetocaloric material, a 1.1 T permanent magnet is used. The AMR can be moved vertically in and out of the magnet with a motor. The magnet consists of 16 trapezoid-shaped NdFeB magnets, arranged in a so-called Hallbach array. This arrangement of the magnets provides a large and homogeneous field inside the magnet but only a small field outside the magnet.
A picture of the test machine as well as graph showing the temperature difference between the hot end and the cold end of the machine as a function of time is shown below.
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