**3.5 Japanese system**

234 Trends in Electromagnetism – From Fundamentals to Applications

low magnetic field and a second area of high magnetic field as shown in Fig. 5. The exchange fluid enters the wheel (regenerator) at the temperature *Thot* and exits at the temperature*Tcold* , having transferred its heat to the coolant located in the area of weak field. After receiving the heat of the load to cool *Qcold* the fluid enters the wheel again at a temperature *Tcold* + Δ due to heat exchange with the wheel which is at this instant at the temperature *Thot* + Δ . The temperature of the fluid increases to *Thot* + Δ . Finally, the fluid transfers heat *Qhot* to the reservoir of the hot source completing one cycle at the same time.

Fig. 5 describes schematically the magnetic system of Steyert.

Fig. 5. Schematic representation of the Steyert's magnetic system.

thermodynamic cycles are operated and a Δ*T* of 11 K is obtained.

This system was designed by Kirol (Yu, 2003) on the principle of a rotating machine and Ericsson's cycle. The magnetic field is produced by permanent magnets NdFeB and reaches a maximum value of 0.9 T in the air-gap. The refrigerant rotor is composed of a flat disk of 270 g of gadolinium as magnetocaloric material. During one rotation of the rotor, the four

The device shown in Fig. 6 was developed by the team of the Polytechnic University of Catalonia in Barcelona (Allab, 2008). The magnetocaloric material is a ribbon of gadolinium (Gd 99.9%) fixed on a plastic disc and immersed in a fluid (olive oil). The magnetic cycle of magnetization / demagnetization is provided by the rotation of the plastic disc and its interaction with a magnet. The temperature span is obtained respectively: 1.6 and 5 K for a magnetic field of 0.3 T and 0.95 T. This corresponds to 2.5 times the MCE of Gd. Even if obtained performances of this system were weak, this device is the first that has shown the

feasibility of magnetic refrigeration with fields accessible by permanent magnets.

**3.3 The magnetic system of Kirol** 

**3.4 The Spanish device** 

Okamura et al. have constructed a magnetic refrigeration system, as shown in Fig. 7-a (Okamura, 2006). The yoke has an outer diameter of 27 cm and a length of 40 cm. The magnetic field is produced by rotating permanent magnets, producing a maximum field of 0.77 T. The bed regenerator is composed of 4 blocks. Each block is composed of a different alloy GdDy (sphere shaped) to enhance the range of variation of temperature. The fluid circulation is ensured by a pump and a rotary valve. The power obtained is about 60 W. The initial system has been improved as shown in Fig. 7-b (Okamura, 2007). The stator used was a laminated yoke and the magnetic field source was improved (the maximum field is 0.9 T). This helped to obtain a power of 100 W (using Gd as MCE material).

Magnetic Refrigeration Technology at Room Temperature 237

At the University of Victoria in Canada, Tura and Rowe (2007) constructed a magnetic refrigerator containing permanent magnets for a testing of all sorts of magnetic refrigerants in different configurations. This machine is shown in Fig. 9. A nested Halbach array of NbFeB permanent magnets was applied and led to a magnetic field of 0.1-1.47 T strength. Water was the heat transfer fluid with a heat rejection temperature range of 253-311 K, and the operation frequency was between 0 Hz and 4 Hz. The prototype showed cylindrical magnetocaloric regenerators (with a porosity of 57%) whose volume, diameter and length were 20 cm3, 16 mm, and 110 mm, respectively. The void in the regenerator of the hot heat exchanger and the cold heat exchanger was 0.83 cm3 and 0.4 cm3, and the parallel flow paths in the heat exchangers were optimized with a computational fluid dynamics (CFD) approach. The system which is designed to be flexible showed many advantages: for example, a simple design, easy accessibility to all the components and very low heat

leakages. This machine reached a maximum temperature span of 13.2 K (Yu, 2010).

Fig. 9. Rotary magnetic refrigerator with permanent magnets as presented by researchers of

The company Cooltech Applications in France built a rotary magnetic refrigerator composed of eight pieces of supporting discs positioned in synthetic material (see Fig. 10), which were mechanically stable and thermally isolated (Vasile and Muller, 2005, 2006).. These inserts were interchangeable for the test of different magnetocaloric materials, different sensors for temperature, pressure, air velocity, hydrometry and electrical power. Each insert was packed with 165 g Gd. The rotating axes were made of stainless steel, where four pieces of NdFeB permanent magnets were rotating to provide a magnetic field of 1 T. However, the authors reported on a new type (open Halbach) magnetic assembly, which yielded a magnetic field between 1 to 2.4 T. The flow of fluid was controlled to improve the cooling

the University of Victoria in Canada (Tura and Rowe, 2007).

capacity, which was obtained in the range of 100W to 360 W (Yu, 2010).

**3.8 Cooltech systems** 

**3.7 Canadian system** 

Fig. 7. The Japanese Device: (a) The initial one, (b) The improved one (Hirano et al., 2007; Okamura et al., 2006, 2007).
