Magnetocaloric effect: From materials research to refrigeration devices

Magnetic refrigeration (MR) at room temperature is an emerging technology and shows real potential to enter conventional markets. The principle of MR obeys the magnetocaloric effect (MCE), which is based on the effect caused by a magnetic field on the materials that bear the property of varying the magnetic entropy, as well as its temperature, when varying the magnetic field. This article revises the MCE, its theory and thermodynamics. It discusses the properties that must be met by the magnetocaloric materials as well as those most suitable for room temperature MR and those displaying a promising future. Finally, a presentation and description of the Carnot, Brayton, Ericsson, AMR and cascading MR cycles is given with a view to establishing selection criteria based on their performances.

Magnetocaloric effect: A review of the thermodynamic cycles in magnetic refrigeration

The active magnetic regenerator (AMR) is an alternative refrigeration cycle with a potential gain of energy efficiency compared to conventional refrigeration techniques. The AMR poses a complex problem of heat transfer, fluid dynamics and magnetic field, which requires detailed and robust modeling. This paper reviews the existing numerical modeling of room temperature AMR to date. The governing equations, implementation of the magnetocaloric effect (MCE), fluid flow and magnetic field profiles, thermal conduction etc. are discussed in detail as is their impact on the AMR cycle. Flow channeling effects, hysteresis, thermal losses and demagnetizing fields are discussed and it is concluded that more detailed modeling of these phenomena is required to obtain a better understanding of the AMR cycle.

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Review on numerical modeling of active magnetic regenerators for room temperature applications

Magnetic heating, refrigeration and energy conversion have caused awareness of being promising future environmentally benign technologies with much potential to enter some existing markets. An increased research activity is observed, which to the end of the year 2010 lead to approximately forty-two prototypes. As most of these machines contain solid linear moving or rotating beds – consisting of magnetocaloric material - through static magnetic fields or moving magnets, which sweep a magnetic field over a static bed, all researchers face more or less the same problems. These concern materials, fluids, design and the operation characteristics. In this article innovative new system ideas are presented, which may have the potential to overcome existing barriers. The article is a proposal to the research community in the field of magnetocaloric technologies for future R&D activities. Taking into account all the limits and problems that researchers are facing at present, the best chance to bring magnetocaloric technologies to the market is to try some substantially new approaches that go beyond the present state-of-the-art.

Innovative ideas for future research on magnetocaloric technologies

One of the key issues in magnetic refrigeration is generating the magnetic field that the magnetocaloric material must be subjected to. The magnet constitutes a major part of the expense of a complete magnetic refrigeration system and a large effort should therefore be invested in improving the magnet design. A detailed analysis of the efficiency of different published permanent magnet designs used in magnetic refrigeration applications is presented in this paper. Each design is analyzed based on the generated magnetic flux density, the volume of the region where this flux is generated and the amount of magnet material used. This is done by characterizing each design by a figure of merit magnet design efficiency parameter, Λcool. The designs are then compared and the best design found. Finally recommendations for designing the ideal magnet design are presented based on the analysis of the reviewed designs.

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Review and comparison of magnet designs for magnetic refrigeration

The operation behaviour of an active magnetic regenerator (AMR) with a wavy-structure, or a honeycomb-like regenerator bed was numerically investigated. The thermodynamic model was applied to a static regenerator and – in a generalized version – to a rotary type. The models take two-dimensional unsteady heat conduction in the magnetic material during the four basic processes of the AMR cycle into account. The numerical results were used to determine optimal arrangements of different magnetic materials in order to obtain larger temperature spans between both ends of the porous beds. Furthermore, a first study of magnetic flux lines in a porous rotary heat exchanger was performed.

Static and rotating active magnetic regenerators with porous heat exchangers for magnetic cooling

The present invention concerns a continuously rotary magnetic refrigerator or heat pump able to operate in a continuously working process, having a high efficiency and a low cost of manufacturing. This rotary magnetic refrigerator or heat pump (10) comprises a cylindrical structure (11) provided with at least one stationary external cylindrical housing (11a) containing at least one rotating internal magnetocaloric cylinder (12) containing at least one working fluid. The rotating internal magnetocaloric cylinder (12) rotates around a longitudinal axis (13) through a magnetic field generated by at least one permanent magnet located at one half portion of the internal magnetocaloric cylinder (12) and generating flux lines crossing the rotating internal magnetocaloric cylinder (12) in a direction perpendicular to the longitudinal axis (13). One half portion of the rotating internal magnetocaloric cylinder (12) is connected to a cold fluid duct (17) and the other half portion of the rotating internal magnetocaloric cylinder (12) is connected to a hot fluid duct (18). This rotating internal magnetocaloric cylinder (12) is rotated by a gear or belt drive pulley (15) moved by a belt (15a) and a driving motor (15b) mounted on its periphery.

Continuously rotary magnetic refrigerator and heat pump and process for magnetic heating and/or cooling with such a refrigerator or heat pump

The invention concerns a rotary magnetic refrigerator/heat pump able to operate in a continuously working process, able to be applied in industrial thermal processes and having a high efficiency and a low cost manufacturing. The rotary magnetic refrigerator (10) comprises a magnet (11), a magnetocaloric ring (12) and a fluid conducting subassembly (13). The fluid conducting subassembly (13) comprises a horizontally positioned rotating disc (14), frame-suspended with bearings (15 and 16) in stationary ducts above (17) and below (18), which serve to supply and evacuate a working fluid (19, 20, 21, 22). The rotating disc (14) has two circular arrays of hollow sectors (23 and 24), one close to each of its planar faces. Every hollow sector (23 and 24) comprises a slot (25 and 26) to the adjacent planar surface and to the circumferential surface of the rotating disc (14). The working fluid is introduced axially into the lower hollow sectors through the openings on the lower planar surface, in order to flow out in radial direction through the circumferential openings of the lower array of hollow sectors and back into the disc through the circumferential openings of the upper array of hollow sectors, from where it finally exits the disc axially through the openings of each hollow sector on the upper planar surface.

Continuously rotary magnetic refrigerator or heat pump

A magnetic refrigerator with a cylindrical heat exchanger of porous periodic structure has been designed and numerical system performance calculations have been performed. This refrigerating machine, respectively heat pump device, can be operated with air or water as working fluid. In the article the design of the new refrigerator is outlined and the coupled basic equations of magnetism and thermodynamics are presented, which contain three independent characteristic geometrical quantities characterizing the heat exchanger structure. Magnetic fields, magnetic flux densities, and COP, etc. were investigated for magnetic inductions between 1 and 5 T. The obtained COPs are promising high compared to conventional refrigeration equipment working with gas compression and expansion.

A rotary heat exchanger magnetic refrigerator

At present magnetic refrigeration shows a realistic potential to penetrate into some niche markets in the field of cold and hot "production". It is well known that a refrigerator for cooling and a heat pump for heating are machines based on the same principle. A small review on magnetic heat pump studies is outlined and a simple engineering calculation scheme to determine the coefficient of performance for a magnetic heat pump with a rotary porous structure heat exchanger is presented. Magnetic heat flux line distributions are calculated by a 2-d finite element method numerical simulation program with the objective to obtain high field's mu H-0 with not too heavy permanent magnets assemblies. The porous structures are geometrically optimised to obtain maximal magnetic field inductions. It is shown that for a water/ethylene-glycol magnetic heat pump the coefficient of performance is higher than that of a conventional heat pump. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fabrice Gendre

Papers

Static and rotating active magnetic regenerators with porous heat exchangers for magnetic cooling

Continuously rotary magnetic refrigerator and heat pump and process for magnetic heating and/or cooling with such a refrigerator or heat pump

Continuously rotary magnetic refrigerator or heat pump

A rotary heat exchanger magnetic refrigerator

A magnetic heat pump with porous magneto caloric material