Innovative design of a magnetocaloric system

In the present paper we consider the problem of optimizing the cooling of a magnetocaloric refrigerator. In this work we first theoretically and then experimentally study the performance of a single material regenerator under different operating conditions. The basic principles of the design and implementation of our magnetic refrigerator prototype are presented as well as a new magnetic assembly of NdFeB permanent magnets.

The design of the equipment uses a movement of relative displacement optimized for the phases of activation and inactivation of the magnetic field. Each part of the equipment is implemented in order to be controlled separately and to allow a large variety of the tests: gear pumps with individual control, sequence of programmable magnetocaloric cycle, unit control by programmable controller and application programming interface by color LCD touch screen, real-time processing data acquisition using a National Instruments System implemented on Independent PC, expelled heat using different standard heat exchangers.     

Experimental study on the performance of a room temperature magnetic refrigerator using permanent magnets

An investigation of a room temperature active magnetic refrigerator was carried out in this work. An experimental rig was built, in which two reciprocating regenerative beds packed with 1167.4 g of gadolinium were used, helium gas was used as a heat transfer fluid, and an average 1.5 T magnetic field was supplied by permanent magnets. With this apparatus, the influence of the gas pressure, the operating frequency and the temperature range were studied systematically. The lowest no heat load temperature of −2.79 °C at the cold end heat exchanger and a maximum no heat load temperature span of 42.28 °C were obtained. A maximum cooling power of 51.3 W was achieved over a temperature span of 18.16 °C. The results in this study provide useful data for future design and development of room temperature magnetic refrigerators.     

Performance of a room-temperature rotary magnetic refrigerator

We have designed and operated a rotating-magnet type AMR (active magnetic regeneration) refrigerator that uses water as a heat transfer fluid. Four kinds of gadolinium-based alloy are used as magnetic materials. A magnetic field of 0.77 T is applied by neodymium permanent magnets. The refrigerator produces a maximum cooling power of 60 W around 10 °C. An optimal time for one cycle exists, and it depends on the water flow rate and the frequency of magnetization and demagnetization. Enhancement of the water flow rate and the frequency is known to be essential for increasing the cooling power of this refrigerator.     

Optimization of a magnetic refrigerator at room temperature for air cooling systems

The purpose of this study is to understand the optimum operating condition of magnetic refrigerator at room temperature for direct air-cooling. The basic components of the target system are a magnetic circuit including two permanent magnets, a test section, an air blower, and an associated instrumentation. The test section consists of 10 test cells which enclose gadolinium chips as a magnetic working substance in a prescribed packing rate. In order to change the applied magnetic field from 0 to 0.9 T, the magnetic circuit is installed on an electric slider which generates reciprocating motion. The system performances are widely investigated both experimentally and analytically for the variety of conditions such as a volumetric flow rate of air, a packing length of magnetic working substance, and a heat exchange cycle. The results reveal that the present magnetic refrigerator has a maximum value of the cooling rate in an appropriate operating condition.     

Experimental investigation on refrigeration performance of a reciprocating active magnetic regenerator of room temperature magnetic refrigeration

Room temperature magnetic refrigeration has been proved to be a feasible refrigerating technology and has a prosperous application potential. In this research, magnetocaloric effect (MCE) of metal gadolinium is measured and the metal is prepared from ingot to granular state by method of hydriding–ball milling–dehydriding. The other compound, Gd₅Si₂Ge₂ alloy, is also prepared into grains by mechanical comminuting and its magnetocaloric property is obtained. An experimental system of room temperature magnetic refrigeration is established, and three kinds of magnetic refrigerant (MR I: 0.3 mm mean diameter gadolinium particle, MR II: 0.55 mm mean diameter gadolinium particle and MR III: 0.3–0.75 mm Gd₅Si₂Ge₂ alloy particle) are employed in AMR. Performance experiments of AMR system under various temperature range, temperature span, flow rate, and flow period conditions are investigated. The results indicate that AMR adopting MR I, II, III can generate a maximum refrigerating capacity of 18.7 W, 17.8 W, and 10.3 W, respectively, under a 3 K temperature span. With the increasing temperature span, the capacity decreases. MR I and MR II have an equivalent refrigerating ability higher than MR III.     

Performance analysis of a room temperature rotary magnetic refrigerator for two different gadolinium compounds

A thermodynamic performance analysis is developed for a Steyert-like rotary magnetic refrigeration (RMR) system operating in the near-room temperature range with two possible, alternative, gadolinium compounds. The first magnetocaloric material (MCM) is an alloy (Gd₇Pd₃) with a well defined Curie temperature (around 318 K), while the second MCM (Gd₇₆Pd₂₄) is an eutectic compound with a smoothed double Curie transition (at 298 and 318 K, respectively).

The main issues linked to the thermodynamic properties of the magnetic material are outlined and the influence of the magnetocaloric properties on the global performance (useful effect, coefficient of performance, and so on) of the refrigeration system is discussed.     

Research on performance of regenerative room temperature magnetic refrigeration cycle

On the basis of classical Langevin theory along with statistical mechanics, thermodynamics and magnetism, a new expression of magnetocaloric parameters used for room temperature magnetic refrigeration is proposed, which is briefer and more accurate than the existing one, providing a new way for studying performance of regenerative room temperature magnetic Ericsson refrigeration cycle. Influences of temperature of heat reservoirs and magnetic intensity on cycle refrigeration capacity and coefficient of performance are analyzed. The results show that the maximal temperature span of the cycle increases but its increasing rate decreases with the increase of magnetic field strength. In addition, there exists only one maximum value of effective refrigerating capacity. Two cycles with the same COP can reach a same temperature span under a certain magnetic field strength. A large magnetic field strength can improve COP but the increase rate of COP decreases.     

Two-dimensional mathematical model of a reciprocating room-temperature Active Magnetic Regenerator

A time-dependent, two-dimensional mathematical model of a reciprocating Active Magnetic Regenerator (AMR) operating at room-temperature has been developed. The model geometry comprises a regenerator made of parallel plates separated by channels of a heat transfer fluid and a hot as well as a cold heat exchanger. The model simulates the different steps of the AMR refrigeration cycle and evaluates the performance in terms of refrigeration capacity and temperature span between the two heat exchangers. The model was used to perform an analysis of an AMR with a regenerator made of gadolinium and water as the heat transfer fluid. The results show that the AMR is able to obtain a no-load temperature span of 10.9 K in a 1 T magnetic field with a corresponding work input of 93.0 kJ m⁻³ of gadolinium per cycle. The model shows significant temperature differences between the regenerator and the heat transfer fluid during the AMR cycle. This indicates that it is necessary to use two-dimensional models when a parallel-plate regenerator geometry is used.     

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

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.     

Investigation on a Stirling refrigerator with thermal buffer tube driven by an oil-lubricated compressor

In this article, a special configuration of Stirling refrigerator for domestic refrigeration purpose is introduced. A thermal buffer tube is installed between the refrigerator cold-end and the expansion piston to improve the system reliability by moving the expansion piston from low temperature to ambient temperature. Furthermore, a commercial oil-lubricated dual-piston compressor is modified to drive the refrigerator, inside which an elastic membrane is used to transfer acoustic work and separate the working gas of the refrigerator from that of the compressor. Experimental investigations on the refrigerator are performed using helium as the working fluid and a cooling power of 200 W at −78 °C is achieved at 15 Hz working frequency and 2.5 MPa mean pressure. Meanwhile, a rough estimation of the refrigerator COP in terms of cooling power divided by input acoustic power gives the value of 0.64. It gives the possibility of building a low-cost, high efficiency domestic refrigerator.     

Experimental investigation of a three-material layered active magnetic regenerator

This work reports on experimental studies using an active magnetic regenerative test apparatus (AMRTA) in near room-temperature refrigeration cycles. Experiments using regenerator beds composed of three different magnetocaloric materials combined in a layered configuration with applied fields of 2 T have produced no-load temperature spans in excess of 50 K. The test apparatus uses two active magnetic regenerators each containing approximately 135 g of refrigerant. An overview of the test apparatus, operating parameters, and performance is described. The impacts of operation at varying heat rejection temperatures, applied fields of 1.5 T and frequencies between 0.65 and 1.0 Hz are presented. In addition, the impacts of operating pressure and applied load on temperature spans are discussed.     

Matériaux magnétocaloriques de pointe: le futur que réserve-t-il?

Recent achievements in the design of robust near room temperature magnetic cooling devices signify paradigm shift in refrigeration, liquefaction and freezing technologies, and call for a much broader base of advanced magnetocaloric materials to support quick materialization of this environmentally friendly, energy efficient technology in a variety of markets. The latest material discoveries are reviewed and current trends in engineering of advanced magnetocaloric compounds have been identified.     

Potential for cost effective magnetocaloric air conditioning systems

Magnetic refrigeration is an emerging technology that exploits the magnetocaloric effect found in solid-state refrigerants. The combination of solid-state refrigerants, water-based heat transfer fluids, and high efficiency will lead to environmentally desirable products with minimal contributions to global warming. Among the numerous applications of refrigeration technology, air conditioning applications provide the largest aggregate cooling power and use the greatest quantity of electric energy. The primacy of the air conditioning application makes the establishment of cost targets for this application an essential feature of the R&D plan for magnetic refrigeration technology. A preliminary assessment of the permanent magnet costs and magnetocaloric material costs indicates that, for suitably chosen materials and operating conditions, these costs lay well below the total manufactured costs for vapor compression based air conditioners.     

A practical model for analysis of active magnetic regenerative refrigerators for room temperature applications

In this paper, a practical model for predicting the performance and efficiency of active magnetic regenerative refrigerators (AMRRs) has been developed. With this model, the refrigeration capacity, the power consumption (including the power required to move regenerator cylinder and drive heat transfer fluid) and consequently the coefficient of performance (COP) of a real AMRR system can be predicted with different heat transfer fluids. A dimensionless parameter, utilization at maximum refrigeration capacity (UMRC), is used to numerically characterize the performance of an AMRR. The numerical results indicate that the UMRC increases with increasing number of transfer units (NTU) and eventually reaches its maximum. Increasing operating frequency increases the refrigeration capacity of the AMRR while causes a reduction in COP. The influences of the physical properties of transfer fluids on the AMRR performance are also studied. Liquid is more favorable than gas for being used as heat transfer fluid in AMRR systems.     

A magnetic field source system for magnetic refrigeration and its interaction with magnetocaloric material

The magnetic field source system constitutes an important component of magnetic refrigeration. A specific methodology for its' dimensioning is proposed in this paper. It is based on analytical calculation models and takes into account the geometry of the system, the magnetic properties of the magnetocaloric material and the magnetothermal cycle (direct or active magnetic regenerative refrigeration). The analytical calculation of the field is first developed and applied to usual permanent magnet-based field sources with and without soft magnetic materials. Then the forces generated by the interaction between the field and the magnetocaloric effect material are analytically evaluated considering the real field distribution. All calculations are validated thanks to two- or three-dimensional finite element method simulations.     

Development of a novel multi-capillary, multi-temperature commercial refrigerator cabinet with common low-pressure receiver

A multi-temperature 4 drawer catering cabinet was designed to operate using a low-pressure receiver with capillary expansion to the separate evaporator in each drawer. Low-pressure receivers have been shown to be an effective way of allowing evaporators to operate in a fully flooded mode thus enabling more efficient use of the evaporator surface for heat transfer. If a low-pressure receiver is used in a refrigeration circuit the control of refrigerant flow into the evaporator is less critical as the expansion device is not responsible for preventing liquid returning to the compressor. Therefore, a capillary expansion device can be used effectively over a range of operating pressures. The system was shown to be effective at maintaining temperatures in the storage drawers during chilled, frozen and mixed storage temperature tests carried out to the EN441 test standard. The cabinet operated successfully at all conditions except when the heat load in each drawer was excessive (>400 W above base level heat load). In this case, refrigerant was found to back up in the condenser and the low-pressure receiver was empty of liquid refrigerant. A solution to this would be to allow controlled flow of refrigerant from the condenser to the low-pressure receiver at high condensing pressures.     

Experimental investigation of a CO2 automotive air conditioner

In this study, a CO₂ automotive air conditioner prototype was designed and constructed. The compressor was of swash plate design; the gas cooler and evaporator were made of fin-tubes; a manual expansion valve and an internal heat exchanger accumulator were used. The lubricant, the CO₂ charge, the evaporator outlet pressure, the compressor speed, the air inlet temperature and flow rate of the gas cooler and the air flow rate of the evaporator were varied and the performance of the prototype was experimentally investigated in detail. The cooling capacity, compressor power consumption, CO₂ mass flow rate, and COP value were analyzed. The experimental results showed that the CO₂ system performance was greatly affected by different lubricants; the CO₂ system performance was sensitive to the mass charge; the high side pressure affected the system performance greatly and a high side pressure controller was needed.     

Ice slurry: Pressure drop and deposition velocity

Ice slurries are used in indirect refrigeration systems. The storage tank of ice slurry systems decreases the maximum required cooling capacity of the equipment and smooths down the installation consumption. An advantage of ice slurry systems compared to chilled water systems is the higher cooling energy that can be delivered to customers using the same installed pipes. This paper gives a complete investigation of ice slurry pressure drops and deposition velocity. A model for the friction factor, obtained by empirical and semi-empirical approximation, is discussed. A new simple and efficient method is introduced to find the deposition velocity in an ice slurry fluid flow. Finally a variety of experimental results and some theoretical calculations of ice slurry flow patterns are shown.     

Modeling of integral-type Stirling refrigerator using system dynamics approachModélisation d''un réfrigérateur Stirling de type intégré à l''aide d''une approche par dynamique du système

A modeling of the integral-type Stirling refrigerator (ITSR) using a system dynamics approach is developed in the present study. The system dynamics models are derived to describe the input/output relation of each component of an ITSR. Connecting the equivalent circuits of the components together, we obtain a flow network diagram and derive a transfer function to represent system dynamics behavior of an ITSR. The performance analysis of an ITSR is then easily carried out by use of the transfer function with sinusoidal signal assumption. The performance calculation for a test refrigerator has shown that the analysis is accurate. An empirical correlation for a correction coefficient r in the expansion space was also derived for better results. The present study has demonstrated how a linear system theory or system dynamics approach can be applied to the design of an ITSR.