A team of Canadian-Bulgarian researchers has developed a promising novel approach for magnetic cooling that’s far more efficient and ‘greener’ than today’s standard fluidcompression form of refrigeration.

WASHINGTON D.C., June 10, 2014 – Magnetic cooling is a promising new refrigeration technology boasting several advantages – ranging from lower energy consumption to eliminating the use of hazardous fluids – that combine to make it a much more environmentally friendly option than today’s standard fluid-compression form of refrigeration.

One novel magnetic cooling approach, developed by a team of Canadian-Bulgarian researchers, relies on solid magnetic substances called magneto caloric materials to act as the refrigerant in miniaturized magnetic refrigerators. As the team describes in Applied Physics Letters, these materials are the key to the development of a ‘green’ cooling technology whose efficiency is able to scale directly with the generated magneto caloric effect.’

The magneto caloric effect is ‘the thermal response of a magnetic material to the change of an external magnetic field, which manifests as a change in its temperature,’ explained Mohamed Balli, a researcher in the physics department at the Université de Sherbrooke in Quebec, Canada.

Ferromagnetic materials, for example, are known to heat up when magnetized and to cool down when the magnetic field is removed.

“The presence of a magnetic field makes ferromagnetic materials become more ordered. This is accompanied by disorder within the atomic lattice, which causes an increase in the material’s temperature,” Balli said.

“Inversely, the absence of a magnetic field means that the atomic lattice is more ordered and results in a temperature decrease. Magnetic refrigeration essentially works by recapturing produced cooling energy via a heat transfer fluid, such as water,” he added.

The researchers originally set out to measure the standard magneto caloric effect in the multiferroic compound HoMn2O5, because this material possesses an insulating behaviour that prevents energy losses associated with electric currents passing through it when altering its magnetic field.

But, much to their surprise, they discovered that a giant magneto caloric effect can be obtained by simply rotating a crystal of HoMn2O5 within a constant magnetic field – without requiring moving it in and out of the magnetic field zone (which is the case for materials exhibiting standard magneto caloric effects).

This discovery is an important step toward the development of magnetic cooling technology, and will likely lead to efficient, ‘green’ cooling systems for both domestic and industrial applications.

“Using the rotating magneto caloric effect means that the energy absorbed by the cooling machine can be largely reduced,” Balli noted. “It also opens the door to building simplified, efficient, and compact magnetic cooling systems in the future,” he added further.

These prototypes work like rotary heat recovery machines applied with success for decades in air conditioning. A first step is the magnetization of a porous solid magneto caloric structure in a magnetic field, followed by a simultaneous heating up of the material [see (A)]. By a fluid flow this structure is cooled [also in region (A)], and after that it turns out of the magnetic field and shows a demagnetization process (B). Here the magneto caloric alloy becomes cold and is heated by a fluid flow, which preferable has the opposite direction to the first flow [also in region (B)]. If the hot fluid on side (A) is used it’s a heat pump application, if the cold fluid is applied then the machine is a cooler or a refrigerator.

Magnetic refrigeration background

Magnetic refrigeration is based on the magneto caloric effect, discovered by E. Warburg in 1881. Similarly to mechanical compression and expansion of gases, there are some materials that raise their temperatures when adiabatically magnetised, and drop their temperature when adiabatically demagnetised. This refrigeration effect has been routinely used since mid 20th c, in the liquefaction of hydrogen and helium, but it is not yet competitive for room refrigeration because of the small cooling effect (it is difficult to drop more than 10ºC even with a strong field of several tesla).

The magneto caloric effect depends on the pyromagnetic coefficient, M/T (the variation of magnetization with temperature), which is maximum near the Curie point, i.e., the temperature at which a magnetic substance loses its coercive force and transforms from ferromagnetic to paramagnetic, near which the sensitivity of magnetization with temperature is highest in ferromagnetic materials (it is near 0 K for paramagnetic materials). A strong magnetic field applied to a solid material near its Curie point forces the magnetic moments of its atoms to become aligned with the field; the thermal energy that was distributed between the vibration and spin levels is suddenly concentrated in less vibration levels, with a consequent temperature rise (again similarly to adiabatic gas compression: forcing more order, without allowing for entropy to escape, raises the temperature; compare the magneto caloric cycle, Fig. 8, with the inverse Brayton cycle in gas refrigerators, Fig. 5); other type of cycles may be applied, as for instance the Ericson cycle using heat regenerators (see Power). The farther away from the Curie point, the weaker the magneto caloric effect (the useful portion of the magneto caloric effect usually spans about 25 ºC on either side of that point.

Materials with advanced magnetic and super conductive properties have been developed to improve magnetic refrigeration efficiency. Materials are magnetized to several tesla using superconductors and electromagnets, and cooled by contact with the high-temperature sink region, then suddenly demagnetized (adiabatically), reaching low temperatures and cooling the load while returning to the initial state.

Most magneto caloric materials are rare earths; usually gadolinium compounds [Gd and notably Gd5(SixGe1−x)4), with densities in the range 6000..8000 kg/m3, thermal capacities around 200 J/(kg∙K), thermal conductivities around 10 W/m∙K), and Curie temperatures around 300 K. A large magnetic entropy change has been found to occur in MnFeP0.45As0.55 at room temperature, making it an attractive candidate for commercial applications in magnetic refrigeration. Energy efficiency may approach 50 % of Carnot limit, against some 10 % for typical mechanical compression refrigerators, without moving parts and associated noise and maintenance burden.

Another application of the magneto caloric effect is to drive a magnetic fluid in a cooling fluid loop. A magnetic fluid (a kind of the new substances known as nanofluids) is a normal fluid (usually a hydrocarbon) seeded with magnetic particles (e.g. Mn–Zn ferrites) of nanometric size (of about 10 nm in diameter, coated with a surfactant layer); with typical low concentrations say 5 % in volume) the colloidal fluid has nearly the same flow properties than the base liquid. The driving force is proportional to H(M/T)T, i.e. to the magnetic field, H (usually achieved with a permanent NdFeB magnet), times the pyromagnetic coefficient, times the temperature gradient).

Typical velocities achieved are small, say a few mm/s, but the absence of moving parts, the positive response (the speed is directly proportional to the thermal gradient), and the controllability (H is usually achieved with a permanent NdFeB magnet, but), makes this pumping mechanism ideal for thermal control, particularly aboard spacecraft.

Principle of magnetic refrigeration

Description of Technology: A magnetic refrigeration cycle employs a solid-state magnetic material as the working refrigerant, and exploits the Magneto Caloric Effect (MCE), or the ability of a material to warmup in the presence of a magnetic field and cool down when the field is removed.

Heat absorption and heat rejection are facilitated by thermally linking the magnetic material with the cold source and hot sink respectively, using an environmentally benign heat transfer fluid such as water, anti-freeze mixture or a gas, depending on the operating temperature range. The forces involved in applying and removing the magnetic field provide the necessary net work input to the cycle for heat pumping from the source to the sink.

Magnetization and demagnetization of a magnetic refrigerant can be viewed as analogous to compression and expansion in a vapour compression refrigeration cycle, but in contrast these magnetic processes are virtually loss free and reversible for soft ferromagnetic materials. Further advantages associated with the solidstate nature of magnetic refrigerants are the absence of vapour pressure, resulting in zero ODP and zero GWP, and a large magnetic entropy density, which is the key thermodynamic property determining the magnitude of the MCE. Magnetic refrigeration, therefore offers the prospect of efficient, environmentally friendly and compact cooling.

State of development of magnetic refrigeration technology for operating temperatures near to room temperature, including both magnetic materials and systems design, is under active development by several teams in North America, the Far East and Europe and a number of prototype systems (including both reciprocating and rotary designs) have been announced. Cooling capacities of prototypes are low, maximum reported to date is 540 W, with a COP of 1.8 at room temperature.

Potential application to the food sector Considerable research and development is still required for the successful commercialisation of magnetic refrigeration systems. The most important challenge is the development of materials with high magnetocaloric effect, to reduce the size, weight and cost of the system. Other important areas of research are the development of effective methods of heat transfer between the refrigerant and secondary heat transfer fluid and overall thermal management and control.

Magnetic refrigeration has the potential for use across the whole refrigeration temperature range, down to cryogenic temperatures. It is anticipated that the first commercial applications will be for low capacity stationary and mobile refrigeration systems. Time to commercialisation is estimated to be greater than ten years.


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