Magnetic Refrigerator Operating Research Articles

Experimental study on refrigeration performance optimisation of reciprocating room temperature magnetic refrigerator

In this paper, the effects of operating conditions on the energy efficiency of a reciprocating room temperature magnetic refrigerator were experimentally investigated in order to optimise its refrigeration performance. An experimental setup of a magnetic refrigerator allowing the thermal characterisation of an active magnetic regenerator, with gadolinium particles (diameter 0.3 mm) used as a magnetocaloric material and water used as a working fluid, was developed. The effects of the magnet velocity (0.04–0.16 m s −1 ), the water filling ratio (2.98–5.96) and the room temperature (14.5–18.1 °C) on the cold end temperature of the active magnetic regenerator were determined and analysed. An optimal operating point of the magnetic refrigerator was found for a magnet velocity of 0.16 m s −1 , a water filling ratio of 5.21 and a room temperature of 18.1 °C, for which a characteristic time of 70 s, a maximum cold end temperature variation of 12.4 °C, a cold power of 1450 W and a coefficient of performance of 6.7 were obtained. A synthesis on the suitable choice of the magnetic refrigerator operating conditions is given in order to optimise its refrigeration performance.

Experimental investigation of two-stage active magnetic regenerative refrigerator operating between 77 K and 20 K

This paper presents an experimental investigation on a magnetic refrigerator operating between 77 K and 20 K. Since the magneto-caloric effect of typical magnetic refrigerant is concentrated in a narrow temperature region, four different rare-earth intermetallic compounds (GdNi2, Dy0.85Er0.15Al2, Dy0.5Dr0.5Al2, and Gd0.1Dy0.9Ni2) are utilized according to their favorable temperature regions. The magnetic refrigerator is composed of two stages each of that is made of two different porous active magnetic regenerators. Helium gas as the heat transfer medium in the magnetic refrigerator is shuttled similar to a GM type pulse tube refrigerator so that the overall cooling capacity is enhanced by gas expansion effect when both gas and magnetic expansions are properly coordinated. The approximate peak magnetic field of 4 T is provided for the internal region of the regenerators by the AC superconducting magnet which can fast ramp up to the maximum field for 3 s. The fabricated magnetic refrigerator reached the lowest no-load temperature of 24 K and the whole temperature span was approximately 56 K rather than 57 K. The cooling performance of the system is sensitive according to the load match between the first and the second stages in conjunction with helium mass flow rate distribution.

Percolation processes and spin-reorientation ofPrNi5−xCox

In the present work we report on the structural and magnetic behaviors of the PrNi5�xCox intermetallic compounds. Due to the competition between the anisotropy energies of both Co and Pr sublattices, this series has a spin-reorientation phenomenon at low temperature 140 K. The Curie temperature, as a function of Co content, has a sudden increase above a critical concentration xc 1.9 and this feature is assigned as a percolation of geometrically spaced Co clusters. This assumption is explained based on the critical exponent of percolation theory. The series presents therefore a rich magnetic phase diagram, which could be established over a full doping range, i.e., from x =0 tox = 5. We have also studied these compounds on the magnetocaloric point of view and found a quite large full width at half maximum TFWHM of the magnetic entropy change curves for some of the compositions, due to the merging of the S peaks associated with the spin-reorientation process and the Curie temperature TC. In addition, the series has an appreciable relative cooling power, which is therefore suitable to be used in a magnetic refrigerator operating in a large range of temperature.

A pulsed superconducting magnet for a static magnetic refrigerator operating between 1.8 K and 4.5 K

Superfluid helium is increasingly used as a technical cooling medium in large-scale research projects involving high-field superconducting magnets, such as the CERN Large Hadron Collider (LHC). For refrigeration cycles operating below the normal boiling point of helium, a potentially interesting alternative to vapour compression is magnetic refrigeration, pioneered decades ago. In the framework of development for an industrial-size 1.8 K magnetic refrigerator using a static active material, we have designed, built and tested a pulsed superconducting magnet producing up to 3 T at a ramping rate of 6.5 T/s. The solenoid-type coil, wound with low-loss superconducting wire (small-filament NbTi in a composite Cu/Cu-Ni matrix), is internally cooled by normal boiling helium, and installed inside a toroidal yoke, made of tapered laminations of commercially pure iron. After describing design and construction of the magnet, we present test results and measurements of performance, field quality and pulsed-operation losses. >

Carnot magnetic refrigerator operating between 1.4 and 10 K

A magnetic refrigerator operating directly from 10 to 1.4 K has been constructed and tested. The refrigerator is a static system which operates following the Carnot cycle using a pulsed magnet and thermal switches. Typical experimental results were 550 mW at 4.5 K using a Dy 3 Al 5 O 12 refrigerant and 100 mW at 1.8 K using Gd 3 Ga 5 O 12 . Without a cooling load, the lowest temperature obtainable was 1.36 K. The Carnot efficiencies were 30% at 2.0 K and 38% at 4.4 K. A cooling test for a superconducting magnet was also performed.

A regenerative magnetic refrigerator operating between liquid helium and liquid hydrogen reservoirs

A regenerative refrigerator with a GGG (Gd/sub 3/Ga/sub 5/O/sub 12/) salt core built and tested in the Cryogenics Engineering Laboratory at MIT is discussed. The core remains stationary as supercritical helium is forced through it via the action of a displacer system. At the same time, the magnetic field is varied by an AC superconducting magnet. Preliminary data obtained from this system suggested the need for modification of (1) the helium flow system, (2) the reservoir temperature control system, (3) the cycle control system, and (4) the instrumentation. A heat exchanger has been constructed which enables the heat of rejection to be deposited in a liquid-hydrogen bath. The refrigeration load is provided by an electric heater. Real-time feedback is used to synchronize the helium flow with the magnetic-field variation. The modifications are described and various schemes for producing the required helium flow without the use of moving mechanical parts are noted.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Conceptual Design of a New Magnetic Refrigerator Operating between 4 K and 20 K

A thermodynamic cycle analysis has been performed on a new magnetic refringeration system for liquid helium. This system features the use of two heat switches and a small helium-gas refrigerator to establish a high-temperature heat sink. As a result of cycle analysis, it is clarified that cooling power of 1 W at 4.5 K can be attained.

Theory for the Operation of a Magnetic Refrigerator at Temperatures below 1°K

A theory for determining the optimum cycle of a magnetic refrigerator is developed. The refrigerator consists of a helium bath, a paramagnetic working substance, and an isothermal reservoir. The working substance is connected to the bath and to the reservoir via thermal valves. In the theory the valves are presumed to be superconducting wires, but no assumptions are made concerning the characteristics of the working substance. The expected behavior of a refrigerator utilizing both He4 and He3 baths is investigated. It is predicted that a magnetic refrigerator operating from a bath temperature of 0.3° or 0.4°K will be able to maintain an isothermal reservoir temperature of 0.05°K or lower with heat inputs of 10–20 ergs/sec, providing the effects of thermal contact resistance can be kept small. The effect of thermal resistance on refrigerator performance is investigated. The same theoretical techniques have been used to predict the behavior of a magnetic refrigerator utilizing a mechanical switch. In the paper the theoretical and experimental results are compared. Moderately good agreement is obtained.

The Design and Operation of a Magnetic Refrigerator for Maintaining Temperatures below 1°K

The problems involved in the construction of a magnetic refrigerator operating below 1°K, using superconducting metallic links for the thermal valves and a paramagnetic salt as the working substance, are discussed. A cycle for the operation of the refrigerator is suggested and then examined for the optimum operating conditions with readily available laboratory facilities. Design equations are obtained and compared with our more recent experimental model. The constructional details of a completely automatic experimental model of the magnetic refrigerator using lead for the thermal valves, iron ammonium alum as the working substance, a helium bath at 1°K as the high-temperature reservoir, a maximum magnetic field of 7000 gauss, a 2 minute cycle of operation, and capable of extracting 120 ergs/second from a reservoir at 0.3°K are presented in full. Temperatures as low as 0.2°K are obtained with this model. Improvements in this this design, its use either for the production of an isothermal reservoir or as a heat sink for adiabatic demagnetization work below 1°K, and the extension to a two-engine refrigerator for the production of a reservoir at temperatures as low as 0.05°K are also discussed.