(Cont.)

3. Magnetocaloric refrigerant : Gadolinium

The choice of the magnetocaloric refrigerant is of great importance as it influences strongly the thermodynamic performances of the cooling machine. Pure gadolinium is the only material used in most magnetic refrigeration prototypes. This is attributed essentially to its important MCE, its ability to answer the several engineering requirements and the availability in the market.

Firstly, we have used Gd flat plates refrigerants in our machine. The hermomagnetic properties of Gd such magnetization, entropy, adiabatic temperature change and specific heat were widely studied and reported in the literature [19]. However, before placing the material in the machine, we have measured the magnetocaloric performances in practical running conditions using a set-up developed in our Laboratory. This system allows the measurement of temperature change close to room temperature in a magnetic induction of about 2 Tesla.

The measurement results are given in figure 5 (Gd: 2 mm). The obtained normalized _T with respect to the magnetic field is about 2 K/ T which is comparable with that reported in the literature [19].

In order to study the demagnetization effect on the magnetocaloric properties of Gd, several _T measurements were performed on gadolinium sheets with different thickness and the magnetic field was applied perpendicularly to the surface of plates. The temperature change for three plates with a thickness of 0.3 mm, 1 mm and 2 mm are compared in figure 5. We can observe that the MCE of Gd decreases drastically when decreasing the sheet thickness from 2 to 0.3 mm.

Figure 5: Effect of the demagnetization field on the Gd magnetocaloric properties (under 2 T).

This difference is attributed to the demagnetization effect due to the shape of the sample. The application of a perpendicular field to the material surface induces an internal field in the inverse direction called the demagnetization field. The latter cancels out a part of the applied field which reduces the total internal field of the magnetocaloric material and decreasing as a consequence the magnetocaloric performances. To avoid the demagnetization effect in our machine, the plates were placed parallel to the applied field.

4. Magnetic refrigerator description and preliminary results

A general view of the designed magnetic cooling machine is presented in figure 6. The experimental apparatus is composed of two permanent magnetic sources producing about 1.45 Tesla, two regenerators with Gd plates, four heat exchangers.

The regenerator is divided in to two parts; each part contains Gd flat plates with 1 mm thickness and 100 mm length, corresponding to about 400 grams of Gadolinium.

The magnetic work constitutes a large part of the fully absorbed energy by the magnetic cooling system. Furthermore, the reduction of the magnetic forces is of great importance for the development of machines with high efficiency. For this purpose and aiming to compensate magnetic forces, the regenerator was divided into two parts separated by a distance of about 30 mm. Figure 7 demonstrates the difference between magnetic forces calculated numerically for 1 and 2 blocs of Gd.

As shown in figure 7, the magnetic force can be decreased drastically when using a bed constituted of 2 blocs of Gd. The numerical calculations developed in figure 7 were confirmed experimentally by the measurements performed in the here reported machine. A detailed study of the magnetic forces in magnetic cooling systems will be published in a forthcoming communications.

The temperature span between hot and cold ends was amplified using special thermodynamic cycles called active magnetic refrigeration regeneration (AMR) [19]. Such cycles break up into four steps:

• magnetization of the magnetic materials inducing heating;
• flow of a fluid from the cold source to hot source to evacuate heat: the temperature of the fluid increases and the heat generated by MC material is removed and evacuated in direction of the hot end;
• demagnetization of the material when it is removed from magnetic field, thus leading to potential increase of magnetic entropy, decreasing the refrigerant temperature;
• flow of the heat transfer fluid from hot to cold source in order to evacuate cooling energy.

The operating process of the AMR can be controlled by adjusting the movement of the actuator and the valve. The operation frequency of the cycle was 0.5 Hz.

Figure 8 shows an example of an experimental data results. At each heat source, the temperature changes progressively to a limit value at steady state. After several AMR cycles, the maximum temperature span achieved between cold and hot ends is about 12 °C.

The relatively low temperature span is attributed essentially to the bad thermal properties of the heat carrying fluid. The Basylon was used especially to protect Gd bed from corrosion and oxidation.

Figure 8: Temperature span: experimental results for f = 0.5 Hz and Basylon as heat transfer fluid.

However, the preliminary results show that by using water or Zitrec as heat transfer fluids, a temperature difference of about 22 °C can be reached. More details about machine with optimized parameters will be communicated in the future.

5. Conclusion and future work

A linear reciprocating permanent magnet cooling system has been designed and built. Gadolinium was used as the first magnetocaloric test material, but other materials are considered for test in particular NaZn13 based compounds. However, much effort was dedicated in order to make the developed machine more compact, to obtain sufficient magnetic induction in the air gap (1.45 Tesla) and to reduce the magnetic forces acting on the magnetocaloric refrigerant during the magnetization-demagnetization process. Preliminary tests of the machine were made and encouraging results were obtained. To investigate the device, more experimental runs will be carried out and a detailed report about the machine with optimized parameters will be communicated in the future.

Acknowledgements

We further are grateful to Direction Générale de l’Enseignement Supérieur du canton de Vaud- Switzerland, the Office Fédéral de l’Energie (OFEN) – Switzerland, INTERREG Iva and the Haute Ecole de Suisse Occidentale (HES-SO). We acknowledge their financial support.

Source: University of Applied Sciences of Western Switzerland

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