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Description
Received October 6, 2006; accepted February 23, 2007
Active magnetic regenerative refrigeration (AMRR) systems represent an environmentally attractive alternative to vapor compression systems that do not use a fluorocarbon working fluid. The AMRR concept has previously been demonstrated using superconducting solenoid magnets that are not practical for small-scale commercial applications. However, recent AMRR prototypes that use more practical permanent magnets have proved that AMRR systems can produce cooling over a useful temperature range with a relatively low magnetic field. In addition, families of materials with large magnetocaloric effects and adjustable Curie temperatures have been developed; these materials may be used to construct layered regenerator beds that may have lower cost and provide higher performance than current materials. This paper reviews recent developments in the field of room temperature magnetic refrigeration and discusses some design issues that may affect practical systems.
INTRODUCTION
Active magnetic regenerative refrigeration (AMRR) systems represent an attractive alternative to vapor compression refrigeration and air-conditioning systems. AMRR systems do not use a fluorocarbon working fluid; instead, a solid refrigerant is used. The solid refrigerant, a magnetocaloric material, communicates with the environment via a heat transfer fluid. Because the solid refrigerant has essentially zero vapor pressure, AMRR systems have no ozone depletion potential (ODP) and no direct global warming potential (GWP). The heat transfer fluid will likely be aqueous and will therefore have minimal environmental impact. In theory, a well-designed AMRR system can be competitive with or even more efficient than vapor compression systems, provided that the volume of the active magnetic regenerator is sufficiently large.
The Thermodynamics of the AMRR Cycle
The temperature and magnetic field of a magnetocaloric material are highly coupled over certain, typically limited, operating ranges; this characteristic allows them to be used within energy conversion systems. A thermodynamic substance can change its internal energy (U) as a result of either work or heat, leading to the differential energy balance:
dU = TdS + dW (1)
The first term in Equation 1 corresponds to an inflow of heat (TdS) and the second to an inflow of work (dW). In general, work can flow in many forms (e.g., mechanical, electrical, etc.). The familiar fundamental property relationships that describe most fluids result when only volumetric compression work (P-V) is considered; however, for magnetocaloric materials, the [[mu].sup.0]H) and magnetic moment (M) form the work term in Equation 1 hysteresis effects are ignored (Guggenheim 1967).
dU = TdS + [[mu].sup.0]HdM. (2)
Increasing the applied field for magnetic materials tends to align the magnetic dipoles, which requires work and reduces entropy. Using this relationship between entropy, internal energy, and magnetic field, it becomes possible to apply all of the typical thermodynamic results and identities that are ordinarily used in the context of a pure compressible substance to a magnetocaloric material. For example, Maxwell's relations (Guggenheim 1967) can be used to describe relationships between the partial derivatives of properties, and a magnetocaloric material will be characterized by an equation of state that describes the magnetization as a function of temperature and applied field. A temperature-entropy diagram for a magnetic material will include lines of constant applied magnetic field rather than isobars; however, the diagram is otherwise analogous to a more familiar diagram characterizing a compressible working fluid. For example, Figure 1 illustrates the temperature-entropy diagram for an alloy of 94% Gadolinium and 6% Erbium, G[d.sup.[0.94]]E[r.sup.[0.06]] (Zimm et al. 2003).
Closer examination of Figure 1 reveals that it is possible to change the temperature of a magnetic material in an adiabatic process by changing the applied magnetic field. Figure 2 illustrates the adiabatic temperature change of G[d.sup.0.94]E[r.sup.[0.06]], when the magnetic field is increased from to 2 Tesla and from to 5 Tesla. Figure 2 shows that the adiabatic temperature change (which is a direct indicator of the magnetocaloric effect) depends on the initial temperature of the material and that a large magnetocaloric effect is only exhibited for a relatively limited temperature span. In a material such as G[d.sup.[0.94]]E[r.sup.[0.06]] that exhibits a second-order phase transition above the magnetic ordering temperature, magnetic hysteresis does not exist. In general, magnetic anisotropy goes to zero when approaching the magnetic ordering temperature. In this case, adiabatic magnetization and demagnetization are isentropic processes; therefore, when the material is subsequently demagnetized, its temperature will return to its original, zero-field value.
[FIGURE 1 OMITTED]
Figure 2 reveals several details that are relevant to practical AMRR systems. First, the adiabatic temperature change is relatively small compared to the temperature span required for most practical cooling systems. This characteristic necessitates the use of a regenerative cycle in order to provide a cooling load over a useful temperature span. Second, the magnetocaloric effect is largest over a relatively narrow temperature range. In order to maximize the magnetocaloric effect and therefore the performance of the AMRR system, it is desirable to construct a regenerator bed from several materials that have Curie temperatures that are tailored to the local regenerator temperature.
[FIGURE 2 OMITTED]
Magnetic Cooling System Configurations
Early magnetic coolers were used to achieve extreme cryogenic temperatures and used an adiabatic demagnetization refrigeration (ADR) cycle. Giauque and MacDougall (1933) used an ADR system to reach temperatures below 1 K, breaking the temperature barrier that had previously been imposed by the properties of compressible fluids. The ADR system that they and other researchers used consisted of a solid piece of magnetocaloric alloy that utilized an isothermal magnetization in which the material is placed into contact with a hot reservoir followed by an adiabatic demagnetization. All of the material in an ADR cycle undergoes the same thermodynamic cycle and therefore the temperature lift is limited to the adiabatic magnetization temperature change exhibited by the material. ADR cycles also require complex heat switches with limited capacities. For these reasons, the ADR cycle is not a practical alternative for near room temperature, commercial devices.
The technical barriers associated with the ADR cycle have been overcome by the use of a regenerator within the active magnetic regenerative refrigeration (AMRR) cycle. Brown (1976) first constructed a regenerative magnetic refrigerator and showed that the use of a regenerative configuration can provide a no-load temperature span that is much greater than the adiabatic temperature change of the magnetocaloric material that is used to construct the regenerator. Green et al (1986) constructed the first successful AMRR, which achieved a 40 K temperature span. In an AMRR system, a porous bed of magnetic material is exposed to a time-varying magnetic field and a time-varying flow of heat transfer fluid. Each segment of the bed undergoes a unique refrigeration cycle and interacts with the adjacent material via the heat transfer fluid. The net result of these cascaded refrigeration cycles is a temperature lift that is much larger than can be achieved by an ADR cycle.
The AMRR cycle consists of four processes. A conceptual drawing illustrating the processes that make up the operation of a rotary AMRR, such as is described by Zimm et al. (2006), is shown in Figure 3. A regenerator consisting of six individual beds is discussed here; one of the six beds is highlighted in Figure 3 and is considered in the following discussion. The bed is magnetized by rotating it into the field of a permanent magnet, Figure 3a. The magnetocaloric effect causes the material in the bed to increase in temperature when it is magnetized. While the bed is in the magnetic field, it experiences a flow of heat transfer fluid from its cold end to its hot end; this flow causes a heat rejection in the hot heat exchanger (Figure 3b) because the temperature of the fluid leaving the hot end is hotter than the ambient temperature. The bed is demagnetized as it rotates out of the permanent magnet (Figure 3c), causing the temperature of the bed to decrease. The regenerator then experiences a flow of heat transfer fluid from its hot end to its cold end while it is out of the magnetic field (Figure 3d), which causes a cooling load to be accepted at the cold heat exchanger because the temperature of the fluid leaving the bed is less than the refrigeration load temperature.
[FIGURE 3 OMITTED]
DEVELOPMENTS IN MAGNETOCALORIC MATERIALS
The properties of the magnetocaloric material that is used in an AMRR system are primarily responsible for the system performance that can be achieved. A review of recently developed materials for room-temperature refrigeration is given by Brueck (2005). Recently, researchers have developed several promising materials with large magnetocaloric effects and tunable Curie temperatures that may be suitable for room-temperature applications (Gschneidner et al. 2005). Magnetocaloric materials generally have nonlinear properties that are highly dependent on temperature; therefore, evaluating the relative performance of one material compared to another is not straightforward. A rigorous comparison of materials would require that the properties be integrated with a detailed model of the AMRR system, and even then, the results would depend on the regenerator geometry, operating temperatures, heat transfer fluid properties, and several other... |
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