**2.5 The Active Magnetic Regenerative Refrigeration (AMRR)**

The direct exploitation of the giant MCE around the room temperature is limited by the fact that existing MCE materials do not achieve high temperature differences (Lebouc, 2005). For example, a sample of gadolinium around room temperature produces an MCE of approximately 10 K in a magnetic field of 5 T.

Magnetic Refrigeration Technology at Room Temperature 233

This technical barrier has been overcome by the application of the Active Magnetic Regenerative Refrigeration (AMRR) (Engelbrecht, 2005; Lebouc, 2005; Tura, 2002). Regeneration in magnetic refrigeration systems allows the heat rejected by the network in any step of the cycle to be restored and returned to the network in another step in the same cycle (Yu, 2003). Thus, the capacity used for cooling the network load can be used effectively to increase the actual change of entropy and the obtained temperature difference (Yu, 2003). AMRR cycles are illustrated in Fig. 4. The regenerative bed consists of plates of MCE material that initially have a quasi-linear temperature profile between the hot and cold

The bed itself acts as a regenerator. The different solid parts of the regenerator are connected by the fluid, so the heat does not need to be transferred between two solid parts separated,

Each particle of the bed undergoes a regenerative Brayton cycle and the entire bed undergoes a cascade Brayton cycle (Yu, 2003). This cycle is repeated 'n' times and the Δ*T* generated is amplified at each cycle to reach the temperatures limits of hot and cold sources (steady state). This Δ*T* is higher than the adiabatic temperature change of refrigerant material (MCE). In addition, the regenerator bed can be achieved by superposing different materials of different composition to expand the temperature's range of variation

Since the first magnetic refrigeration system manufactured by Brown in 1976, many researchers around the world have paid considerable attention to the magnetic refrigeration around room temperature and consecutively developed some interesting systems (Bouchekara, 2008) (Yu, 2010) (Bjørk, 2010). This section reviews – in detail – some of the

The system of Brown is a rotating system and employs an Ericsson cycle (Yu, 2003). The magnetic field is produced by an electromagnet (water cooled) with a maximum magnetic field of 7 T. The MCE material used is the Gd in the form of plates with 1 mm thickness, separated by stainless steel wires with 1 mm intervals to allow the regenerator fluid to flow vertically. The fluid is composed of 80% of water and 20% of alcohol. Without load and after 50 cycles, the temperatures reached were 46 ° C for the heat source and -1 ° C for the cold source, thus 47 C Δ= ° *T* . However, the cooling power obtained was not rely important, this is due to the large Δ*T* obtained. Moreover, the cycle can operate only at low frequencies; the temperature gradient is reduced because both warm and cold sides have time to interact.

An alternative system with a rotating refrigerant, implementing a Brayton's cycle has been designed by Steyert (Yu, 2003). In this system, the porous magnetocaloric material has a form of rings. This wheel (the regenerator with a ring form) rotates through a first area of

tanks.

but on the same block.

and thus to extend the utilization range of the system.

magnetic refrigeration systems available until now.

**3. Magnetic refrigeration systems** 

**3.1 The magnetic system of Brown** 

**3.2 The magnetic system of Steyert** 

Step 1: Magnetization of the material from an initial state where the entire system is at temperature Ta. Each point of the regenerator material sees its temperature increase by ΔT following the application of the magnetic field.

Step 2: Flow of the fluid from the cold source to the hot source. The heat produced by the magnetization step is removed by the fluid flowing from the cold source Tc to the hot source Th. This creates a temperature gradient along the bed.

Step 3: demagnetization of the material. The temperature of Each point of the regenerator decreases by Δ T due to the demagnetization.

Step 4: Flow of fluid from the hot source to cold source. The flow of the fluid from the hot source Th to the cold source Tf transfers its heat to the regenerator. The temperature gradient is amplified.

Fig. 4. Representation of AMRR cycle and temperature profile along the MCE material.

Since the gadolinium is considered as one of the best magnetocaloric materials currently available (Lebouc, 2005), the MCE corresponds to the absolute maximum value that can be obtained between the hot tank and cold tank. Thus it is obviously hard to imagine the exploitation of the MCE in most refrigeration applications (Engelbrecht, 2005).

232 Trends in Electromagnetism – From Fundamentals to Applications

Step 1: Magnetization of the material from an initial state where the entire system is at temperature Ta. Each point of the regenerator material sees its temperature increase by ΔT

Step 2: Flow of the fluid from the cold source to the hot source. The heat produced by the magnetization step is removed by the fluid flowing from the cold source Tc to the hot

Step 3: demagnetization of the material. The temperature of Each point of the regenerator decreases

Step 4: Flow of fluid from the hot source to cold source. The flow of the fluid from the hot source Th to the cold source Tf transfers its heat to the regenerator. The temperature gradient is amplified.

Since the gadolinium is considered as one of the best magnetocaloric materials currently available (Lebouc, 2005), the MCE corresponds to the absolute maximum value that can be obtained between the hot tank and cold tank. Thus it is obviously hard to imagine the

Fig. 4. Representation of AMRR cycle and temperature profile along the MCE material.

exploitation of the MCE in most refrigeration applications (Engelbrecht, 2005).

source Th. This creates a temperature gradient along the bed.

following the application of the magnetic field.

by Δ T due to the demagnetization.

This technical barrier has been overcome by the application of the Active Magnetic Regenerative Refrigeration (AMRR) (Engelbrecht, 2005; Lebouc, 2005; Tura, 2002). Regeneration in magnetic refrigeration systems allows the heat rejected by the network in any step of the cycle to be restored and returned to the network in another step in the same cycle (Yu, 2003). Thus, the capacity used for cooling the network load can be used effectively to increase the actual change of entropy and the obtained temperature difference (Yu, 2003).

AMRR cycles are illustrated in Fig. 4. The regenerative bed consists of plates of MCE material that initially have a quasi-linear temperature profile between the hot and cold tanks.

The bed itself acts as a regenerator. The different solid parts of the regenerator are connected by the fluid, so the heat does not need to be transferred between two solid parts separated, but on the same block.

Each particle of the bed undergoes a regenerative Brayton cycle and the entire bed undergoes a cascade Brayton cycle (Yu, 2003). This cycle is repeated 'n' times and the Δ*T* generated is amplified at each cycle to reach the temperatures limits of hot and cold sources (steady state). This Δ*T* is higher than the adiabatic temperature change of refrigerant material (MCE). In addition, the regenerator bed can be achieved by superposing different materials of different composition to expand the temperature's range of variation and thus to extend the utilization range of the system.
