WO2005093343A1

WO2005093343A1 - Magnetic refrigeration system and use method thereof

Info

Publication number: WO2005093343A1
Application number: PCT/FR2005/050190
Authority: WO
Inventors: Jean-Marc Fournier; Jean-Paul Yonnet; Farid Allab; Afef Lebouc
Original assignee: Peugeot Citroen Automobiles Sa; Cnrs
Priority date: 2004-03-25
Filing date: 2005-03-24
Publication date: 2005-10-06
Also published as: FR2868153B1; EP1759150A1; FR2868153A1

Abstract

The invention relates to a magnetic refrigeration system which is intended to cool a fluid. The inventive system comprises an active magnetocaloric element (1) which is subjected to a magnetisation/demagnetisation cycle, known as the magnetic cycle. For said purpose, the system uses a heat-transfer fluid which passes alternatively through the active element (1) in synchronism with the magnetic cycle. According to the invention, the fluid to be cooled constitutes the heat-transfer fluid. The invention can be used for the air-conditioning of motor vehicles.

Description

MAGNETIC REFRIGERATION SYSTEM AND IMPLEMENTATION METHOD

The present invention relates to a magnetic refrigeration system for refrigerating a fluid, as well as an implementation method. The invention finds a particularly advantageous application in the field of air conditioning, in particular automobile air conditioning. The application to refrigeration of the magnetocaloric effect of certain magnetic materials has been the subject of several articles published in particular in the French publication Revue de l'Electricite et de l'Elronique (REE) and in the American journal IEEE Transactions on Magnetism (IEEE Trans. Mag.): - “Permanent magnet device for the study of active refrigeration” by F. Allab, P. Clôt, D. Viallet, A. Lebouc, JM Foumier and JP Yonnet, REE, n ° 9, October 2003, pages 43 to 46,

- "A magnetic device for active magnetic regeneration refrigeration" by P. Clôt, D. Viallet, A. Kedous-Lebouc, JM. Fournier, JP. Yonnet, F. Allab, IEEE. Trans. Mag., Vol. 39, n ° 5, pp. 3349-3351. In essence, magnetic refrigeration is based on the magnetocaloric effect which is an intrinsic property of magnetic materials. This effect results in heating or cooling of the material when it is adiabatically magnetized or demagnetized. It can be used to perform a cycle similar to the conventional refrigeration cycle based on the compression and expansion of a two-phase gas. In the vicinity of the Curie temperature, the variations in the magnetization of the magnetic materials are significant and result in significant corresponding variations in the internal energy of the crystal lattice leading to a modification of the temperature of the sample, hence the name of magnetocaloric effect which was given this phenomenon. This intrinsic property of para- and ferromagnetic materials is the consequence of a magnetothermal coupling between the network entropy and the magnetic entropy. However, applying the magnetocaloric effect to magnetic refrigeration poses a number of difficulties. On the one hand, if it has to be used in a temperature range close to ambient temperature, this considerably reduces the choice of possible materials, all the more so as the candidate materials must also have a magnetocaloric effect known as giant, resulting in a large temperature variation for a given magnetization variation. Currently, the only material available on the market that meets these requirements is gadolinium. In addition to gadolinium, there are other materials with a giant magnetocaloric effect at room temperature; these are, for example, the compounds GdSiGe, MnFePAs and MnAS, which offer a greater magnetocaloric effect than gadolinium with relatively reduced costs. On the other hand, the experimental studies carried out on these materials have shown that the direct exploitation of the magnetocaloric effect according to a process analogous to that of a compression (magnetization) / expansion (demagnetization) of a gas offers variations in relatively low temperatures (2 ° C to 1T and 14 ° C to 7T) compared to those produced by traditional air conditioning systems which can reach 50 to 70 degrees between the hot point and the cold point of the cooling circuit. This limitation in the dynamics of the temperature variations obtained obviously constitutes an obstacle to the application of the magnetocaloric effect to refrigeration. This is all the more so since, in practical terms, small magnetic refrigeration systems must preferably use permanent magnets to magnetize the materials with magnetocaloric effect used, which limits the magnetic fields applied to values of the Tesla order. On the other hand, for large systems, it is possible to use superconductive coils creating several teslas. To solve this last difficulty, the aforementioned articles propose an amplification of the temperature difference obtained by a particular thermodynamic cycle (Stirling cycle), the principle being, by a game heat exchange, to create a temperature gradient along the material. This gradient increases with each cycle to reach the temperatures of the hot and cold sources at each end. This principle is called active magnetic recovery refrigeration (AMRR). These articles also describe an active element of elongated shape consisting of a succession of thin gadolinium plates which is subjected to a magnetization / demagnetization cycle by alternating displacement in a fixed magnetic field generated by a permanent magnet. More specifically, inside the active element, each plate sees a temperature excursion limited to a few degrees which corresponds to the intrinsic magnetocaloric effect of the material. For the whole of the active element, there is a thermal gradient between a cold end and a hot end of the element which can be of the order of ten or even twenty degrees. To produce a magnetic refrigeration system, there is at each end of the active element a heat exchanger for transferring calories via a heat transfer fluid. These exchangers must operate with excellent efficiency despite the fairly small differences between the sources. In summary, a magnetic refrigeration system is composed of three sub-assemblies: - the heart of the system, or active element, with its magnetocaloric material which functions according to a thermal cycle making it possible to create an internal temperature gradient associated with the alternative circulation d '' a heat transfer fluid, - a system with permanent magnets or with superconductive coils which makes it possible to create the magnetization and the demagnetization of the magnetocaloric material, - heat exchangers capable of transferring the calories around the cold and hot ends of the active element and to make them usable outside. The orders of magnitude which emerge from studies carried out on magnetocaloric effect systems of this type show that they are indeed suitable for powers of the order of a few hundred watts, or even a few kilowatts. They also work with excellent performance. Magnetic refrigeration systems therefore operate with a heat transfer fluid and storage tanks for this fluid. One of these tanks is at low temperature and the other is at high temperature. This fluid is generally a liquid to have good heat capacity and sufficient heat transfer power with the magnetocaloric material. The heat transfer fluid may for example be water. A disadvantage of these magnetic refrigeration systems when one wants to cool air mainly lies in the need to have a first exchanger between the reserve of cold fluid and the useful cold air, on the one hand, and a second exchanger between the reserve of hot fluid and the hot air to be evacuated, on the other hand. These exchangers must operate with relatively small temperature differences because, as we saw above, the magnetic refrigeration system gives only limited differences between hot source and cold source. In addition, these exchangers must have an excellent efficiency in order not to penalize the efficiency of the refrigeration system. Also, the technical problem to be solved by the object of the present invention is to provide a magnetic refrigeration system intended to refrigerate a fluid, comprising an active magnetocaloric element subjected to a magnetization-demagnetization cycle, called magnetic cycle, and using a heat transfer fluid circulating alternately through said active element in synchronism with said magnetic cycle, which would make it possible to significantly simplify current systems and facilitate their integration into industrial products such as motor vehicles. The solution to the technical problem posed consists, according to the invention, in that the fluid to be refrigerated constitutes said heat transfer fluid. Preferably, the fluid to be refrigerated is air and constitutes the heat-transfer fluid which circulates alternately through the active element. Thus, the fluid to be cooled, air in general for air conditioning type applications, is directly used as the heat transfer fluid. The main resulting advantage is the elimination of heat exchangers, which considerably simplifies the construction of magnetic refrigeration in accordance with the invention while ensuring excellent performance. To operate the magnetic refrigeration system according to the invention, it is necessary to make the active element with a magnetocaloric effect pass through the fluid with an alternative circulation, that is to say that the fluid sometimes passes in one direction sometimes in direction reverses the magnetocaloric zone. This alternative circulation is necessary to obtain the thermal cycle. To this end, the invention provides that said system includes:

- a magnetocaloric active element subjected to a magnetic cycle, - a first valve able to put in communication the active element with, as desired, a hot fluid discharge outlet or a first ambient fluid (air) suction inlet ,

a second valve able to put the active element in communication with, as desired, an outlet for discharging cold fluid (air) or a second inlet for suctioning ambient fluid (air), the alternative fluid circulations being in synchronism with the magnetic cycle of the active element. This simple system is well suited to large-scale installations for which the magnetic field creation system can be produced by superconductive coils, which makes it possible to have a large temperature difference during the magnetic cycle. For more compact and smaller systems, such as those suitable for automotive air conditioning, the magnetic field creation system is preferably made with permanent magnets. To amplify the temperature difference of the magnetic cycle, a thermal cycle must be used to generate a temperature gradient in the magnetocaloric active element. To this end, the invention provides that said system includes:

- a magnetocaloric active element subjected to a magnetic cycle and having a hot end and a cold end,

a first valve capable of bringing the hot end of the active element into communication with, as desired, a first variable volume tank, a hot fluid discharge outlet or a first ambient fluid (air) suction inlet,

- a second valve capable of bringing the cold end of the active element into communication with, as desired, a second variable volume reservoir, a cold fluid (air) discharge outlet or a second fluid suction inlet (air) ambient, the volumes of the first and second reservoirs being able to vary in synchronism with the magnetic cycle of the active element. The magnetic gradient magnetic refrigeration system according to the invention operates according to a method which, according to the invention, comprises the steps consisting:

- in a start-up phase, circulating the fluid (air) between the two variable volume tanks through the active element in synchronism with the magnetic cycle until a given temperature difference is obtained between the two tanks,

- in steady state, in a first half-cycle: * to partially empty the first tank by circulation of hot fluid (air) through the active element then towards the cold fluid (air) discharge outlet, the second tank being kept empty, * completely emptying the first tank and partially filling the second tank in a synchronized manner through the active element, * completely filling the second tank by circulation of ambient fluid (air) from the first suction inlet of ambient fluid (air) through the active element, and, in a second half-cycle, performing the preceding steps by reversing the first and second reservoirs. The description which follows with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be implemented. Figure 1 is a diagram of an embodiment of a magnetic refrigeration system according to the invention. Figure 2 is a diagram of an embodiment of a magnetic refrigeration system according to the invention with amplification of temperature differences by thermal gradient in the magnetocaloric active element. Figure 3 is a diagram of an alternative embodiment of the system of Figure 2. In Figure 1 is shown schematically a magnetic refrigeration system for cooling a fluid. The system of FIG. 1 comprises an active magnetocaloric element 1. This active element is subjected to a magnetic cycle consisting of magnetizations and successive demagnetizations under the action of a source of magnetic field not shown. This magnetic cycle increases and then decreases the temperature in the active element 1. A first valve 2 is able to put the element 1 into communication with either an outlet 2C for evacuating hot air, or a first inlet 2A d ambient air intake. Likewise, a second valve 3 is able to put the element 1 into communication with either an outlet 3F for evacuating cold air, or a second inlet 3A for aspirating ambient air. The magnetic refrigeration system of Figure 1 operates as follows. The fluid to be cooled, for example air, circulates by passing through the active element 1 in a reciprocating movement. The thermal cycle operates over 4 times: - initially, the active element 1 is magnetized by a source of magnetic field, which causes an increase in its internal temperature,

- In a second step, a volume of air circulates through the inlet 3A of suction, the valve 3, the element 1, the valve 2 and the outlet 2C of evacuation. This volume of air is heated by passing through the active element 1,

- in a third step, the magnetic field around the active element 1 is eliminated, which causes a decrease in its internal temperature, - In a fourth step, an air volume equivalent to the previous one circulates in the opposite direction through the suction inlet 2A, the valve 2, the element 1, the valve 3 and the outlet outlet 3F. This volume of air is cooled by passing through the active element 1. In practice, the temperature difference which is obtained is less than the temperature difference Δθ in the element 1, because of the partial contributions of air at room temperature. These intakes correspond to the cold air flow of the system, it is therefore necessary to find a compromise between the drop in temperature of the refreshed air and its flow. In Figure 2 is shown schematically a magnetic refrigeration system operating with a thermal gradient intended to cool a fluid which, in the example described below will be air, such as the air contained in the passenger compartment of a motor vehicle that you want to air condition. The system of FIG. 2 is of the same type as the gadolinium plate device presented above with reference to the above-mentioned articles. It comprises a magnetocaloric active element 1. This active element is subjected to a magnetic cycle consisting of magnetizations and successive demagnetizations under the action of a permanent magnet not shown. This magnetic cycle shows in the active element 1 a hot end, for example the upper end in FIG. 2, and a cold end, the lower end in FIG. 2. A first valve 2 is able to put in communication the hot end of the element 1 with either a first variable volume reservoir 4C constituting the hot source, either an outlet 2C for evacuating hot air, or a first inlet 2A for aspirating ambient air. Similarly, a second valve 3 is able to put the cold end of the element 1 into communication with either a second variable volume reservoir 4F constituting the cold source, or a cold air discharge outlet 3F, or a second inlet 3A for ambient air suction. A balance type link 5 acting on pistons allows the need to vary the volumes of the first 4C and second 4F reservoirs in synchronism with each other and with the magnetic cycle of the active element 1. The magnetic refrigeration system of Figure 2 operates as follows. In a start-up phase, the system operates empty, the two valves 2, 3 being open so as to circulate the air only between the two reservoirs 4F and 4C by crossing the active element 1 in a back-and-forth movement. -Comes. Synchronization of this movement with the magnetic cycle is ensured by link 5 of the balance type. During this start-up phase, the temperature of the reservoir 4C increases while that of the reservoir 4F decreases. As a first approximation, compared to the ambient temperature, the temperature of the hot source is higher by + Δθ and that of the cold source is lower by less than - Δθ. When the temperature difference between cold source and hot source has reached a given value, the magnetic refrigeration system of FIG. 2 enters a phase of operation in steady state, the principle of which is as follows. When the air rises from the reservoir 4F, it heats up in the active element 1, and it comes out hot. Thanks to the valve 2, part of this hot air is sent to the tank 4C, and the other part is sent to the outside 2C. For the opposite path, when the hot air comes from the reservoir 4C, it cools in the active element 1 and comes out cold. This time, thanks to the valve 3, part of this cold air is sent to the tank 4F, and the other part is sent to the use of cold air 3F. For continuous operation, the cyclic volumes of the two tanks 4C and 4F must remain identical. During a first half-cycle, when for example the air circulates from the hot source to the cold source, that is to say from the reservoir 4C to the reservoir 4F, the volume of air which has been used by the 3F output must be compensated by a suction of an equivalent volume on the 2A input. This can be done in the following stages, consisting of:

- firstly, to partially empty the first tank 4C by a partial movement of the piston of the tank 4C which sends the hot air to the active element 1 by the valve 2, then to the outlet 3F for the use of the 'air cold, the piston of the second tank 4F being stationary, keeping the tank 4F empty,

- in a second step, to completely empty the first tank 4C and, by a synchronized movement through the active element 1, to partially fill the second tank 4F,

- in a third step, completely fill the second tank 4F by suction on the inlet 2A of the valve 2 towards the tank 4F by the valve 3 while crossing the active element 1. Symmetrically, during the next second half-cycle , we carry out: - firstly, a partial emptying movement of the second tank 4F which returns the hot air through the outlet 2C,

- in a second step, a synchronized movement at the end of emptying the reservoir 4F and at the start of filling the reservoir 4C,

- thirdly, the end of filling of the tank 4C and the suction of ambient air 3A. In this type of operation, part of the air from the hot tank 4C is sent to the cold tank 4F and vice versa. This alternative circulation makes it possible to operate the thermal cycle. In practice, in the two tanks, the temperature difference that is obtained with respect to the ambient temperature is less than the initial temperature difference Δθ, because of the partial inflows of air at ambient temperature. These intakes correspond to the cold air flow of the system, it is therefore necessary to find a compromise between the drop in temperature of the refreshed air and its flow. When the fluid to be cooled is air, the heat capacity of the air being limited, it is necessary to have a very large exchange surface inside the active element 1. The magnetocaloric material can be produced under a form where the ratio between its volume and its exchange surface is very large. It can be put for example in the form of powder or foam. In Figures 1 and 2, the active element 1 has a linear shape. This element can be produced in several parts arranged linearly. The field at the level of the active elements must be successively a field of magnetization and a null field. This can be obtained either by a variable field source, or by a permanent field source having a movement back and forth. In the latter case, the active elements can also be in movement, and it is the relative displacement between the source of magnetic field and these magnetocaloric elements which is an alternating movement. FIG. 3 proposes another arrangement of the active element 1 with a magnetocaloric effect, in the form of toric sectors, a shape better suited to operation in continuous rotation. The geometric shape of the element 1 then allows a relative movement of continuous rotation between the magnetic field creation system, on the one hand, and the magnetocaloric elements in the form of toric sectors, on the other hand. This rotation also causes the movements in the tanks 4C and 4F as well as the switching in the valves 2 and 3. The present invention is very well suited to the air conditioning of motor vehicles. The fluid to be refrigerated is then the air inside the passenger compartment. Still with air as the refrigerant, the invention can also be used in systems for generating cold air, for cold rooms or for buildings, for example. The present invention is also very well suited for cooling water, for example in chilled water sources.

Claims

1. Magnetic refrigeration system for refrigerating a fluid, comprising a magnetocaloric active element (1) subjected to a magnetization-demagnetization cycle, called magnetic cycle, and using a heat transfer fluid circulating alternately through said active element (1) in synchronism with said magnetization cycle, characterized in that the fluid to be refrigerated is air and constitutes the heat transfer fluid which circulates alternately through the active element.

2. Magnetic refrigeration system according to claim 1, characterized in that said system comprises: - an active magnetocaloric element (1) subjected to a magnetic cycle, - a first valve (2) capable of placing the active element in communication ( 1) with, as desired, an outlet (2C) for evacuating hot air or a first inlet (2A) for aspirating ambient air, - a second valve (3) capable of placing the active element in communication (1) with, as desired, an outlet (3F) for evacuating cold air or a second inlet (3A) for aspirating ambient air.

3. Magnetic refrigeration system according to claim 1 or 2, characterized in that said system comprises: - an active magnetocaloric element (1) subjected to a magnetic cycle and having a hot end and a cold end, - a first valve (2 ) capable of bringing the hot end of the active element (1) into communication with, as desired, a first reservoir (4C) with variable volume, an outlet (2C) for evacuating hot air or a first inlet ( 2A) for ambient air suction, - a second valve (3) capable of bringing the cold end of the active element (1) into communication with, as desired, a second reservoir 4F) with variable volume, an outlet (3F) for evacuating cold air or a second inlet (3A) for ambient air suction, the volumes of the first (4C) and second (4F) tanks being able to vary in synchronism with the magnetic cycle of the active element (1).

4. Magnetic refrigeration system according to claim 3, characterized in that said synchronism is achieved by a link (5) of the balance type between two pistons disposed, each, in a variable volume tank.

5. Magnetic refrigeration system according to any one of the preceding claims, characterized in that the magnetocaloric active element or elements (1) are arranged linearly and that the relative movement between the magnetic field source and these magnetocaloric elements is an alternating movement .

6. Magnetic refrigeration system according to any one of claims 1 to 4, characterized in that the magnetocaloric active element or elements (1) are arranged in toric sectors and that the relative movement between the magnetic field source and these elements magnetocaloriques is a rotational movement.

7. Magnetic refrigeration method using a magnetic refrigeration system according to any one of claims 2 to 4, characterized in that said method comprises the steps consisting: - in a start-up phase, in circulating the fluid between the two tanks (4C.4F) with variable volume through the active element (1) in synchronism with the magnetic cycle until a given temperature difference is obtained between the two tanks, - in steady state, in a first half-cycle : * partially emptying the first reservoir (4C) by circulation of hot fluid through the active element (1) then towards the outlet (3F) for evacuating cold fluid, the second reservoir (4F) being kept empty, * to completely empty the first tank (4C) and to partially fill the second tank (4F) synchronously through the active element (1), * to completely fill the second tank (4F) by circulation of ambient fluid from the first inlet (2A) for suction of ambient fluid through the active element (1), and, in a second half-cycle, performing the preceding steps by reversing first (4C ) and second (4F) tanks.

8. Application of the magnetic refrigeration system according to one of claims 1 to 6 and of the method according to claim 7 to the air conditioning of motor vehicles.

9. Application of the magnetic refrigeration system according to one of claims 1 to 6 and of the method according to claim 7 to the generation of cold air, in particular for cold rooms or for buildings.