Magnetic cooling

Perhaps the refrigerators of the future will be based on magnetic compounds?

Conventional refrigeration

The first refrigeration machines were developed because of the need to preserve foodstuffs. Previously, snow or ice was collected in winter and stored during the summer in adobe dwellings or in cavities dug out of cool, shaded areas.

It was not until 1913 that the first domestic refrigerator appeared, which was operated by hand, and not until 1918 that the first refrigerator with an electric motor appeared.

The refrigerator

The first apparatus capable of producing ice was built by the Scottish physician and chemist William Cullen in 1748. He used ethyl ether as a refrigerant, which was highly explosive and caused many accidents. Subsequently, other compounds such as propane, butane or ammonia were used until 1931, when DuPont introduced CFCs, also known as freons, which were non-flammable and initially assumed to be harmless. CFCs were later found to be the cause of the ozone hole and were banned in 1995.

Two refrigerants are currently used:

  • R134a, a hydrofluorocarbon, HFC, which does little harm to the ozone layer, but is an air pollutant that acts as an agent of climate change. Its production is to be phased out by 2026.
  • R600a, isobutane, which is flammable, but can be used in small quantities so that the danger is minimised and there is no toxic waste.

This is how it works…

Conventional refrigerators or fridges are devices capable of cooling, i.e. extracting heat from food and expelling it to the outside.

The operation of the refrigerator is based on the compression of gases and the evaporation of liquids, i.e. on the heat involved in the changes of state. The process followed by these cooling systems is based on the Carnot Cycle.

In the refrigerator there is a circuit through which the coolant circulates as long as the motor, compressor, is running. The motor connected to the mains produces work which is used to extract heat from a cold source (the refrigerator cavity) and which gives up heat to the hot source (the atmosphere).

Part of the circuit is inside and liquid refrigerant circulates through it, which absorbs heat from the environment when it evaporates, cooling the inside of the refrigerator. This refrigerant condenses again as it passes through the coil, which is located at the back of the refrigerator, giving up heat to the environment, i.e. the room. Therefore, if we touch the back of the refrigerator, we can see that it is warm.

Magnetic refrigeration

This new form of refrigeration is intended to replace the use of noisy and bulky refrigerants and compressors with a device based on the physics of a different phenomenon, the magnetocaloric effect.

This is based on the heat associated with certain changes that certain substances undergo when subjected to the action of a magnetic field.

The magnetocaloric effect

It was discovered by the German physicist Emil Warburg when, in 1881, he observed that a piece of iron heated up when placed near a strong magnet.

Matter is made up of atoms, and some of them, such as iron, nickel or cobalt, behave like tiny magnets, so they are said to have an associated magnetic moment that depends on the number of single or unpaired electrons in the atom. Normally these are randomly oriented, known as paramagnetism, but when a magnetic field capable of overcoming thermal agitation is applied, the moments align and there can be a transition to magnetic order known as ferromagnetism. This transition involves an increase in internal order, associated with a change in energy which is recorded in the form of heat released.

In some materials ferromagnetic order is reached, even without the presence of an external field, below a critical temperature called the Curie temperature Tc.

Magnetic refrigerators

Instead of using gases, magnetic solids are used, and instead of compression-expansion cycles, magnetisation- demagnetisation cycles are used.

Giant magnetocaloric effect

If we use materials with an abrupt transition to the ferromagnetic state, accompanied by a change in volume, a sudden release of heat occurs. We can force this transition by applying an external magnetic field so that the heat produced is the sum of the heat due to the alignment of the magnetic moments and the latent heat in a transition with a change of volume. Removing the magnetic field produces the opposite cooling process.

The first compound that was used for magnetic cooling was gadolinium, later compounds of gadolinium, silicon and germanium were discovered with higher cooling power than pure gadolinium.

How does it work?

  1. The ferromagnetic material is introduced into the electromagnetic field, the dipole moments are aligned in favour of the field by increasing the temperature of the alloy.
  2. The compound is cooled to room temperature by contact with a liquid that brings the material into contact with the outside, in magnetic refrigerators water is used.
  3. When the material is removed from the magnetic field, the moments are randomly oriented due to the thermal agitation of the atoms, the temperature decreases.
  4. Once the material is cooled, it is brought into contact with the medium to be cooled, usually by means of a liquid that passes through the ferromagnetic material.

Although the purpose of magnetic refrigeration is to avoid the use of fluids that are harmful to the atmosphere, the efficiency is also better than with traditional systems, i.e. it saves energy. In a traditional system based on the compression-expansion of a fluid, the efficiency rarely exceeds 20% of the theoretical limit, obtained in a Carnot cycle. In the tested prototypes of magnetic refrigerators, efficiencies of up to 60% are obtained, which means that a magnetic refrigerator consumes a third of the electricity of a conventional one.

In addition, the mechanics of a magnetic refrigerator are simpler and more robust than conventional refrigerators, as they do not use high-pressure fluids that can leak into the atmosphere.

The technology required is really simple and all that is needed is to find materials with sufficient cooling capacity in the temperature range of each application.

At the moment the tested prototypes already outperform the traditional method in all respects, but the limit of what is possible in the optimisation of materials has not been reached.

What do we do at INMA?

Determining the magnetocaloric properties of a material is not easy, which is why indirect measurements of magnetisation or heat capacity as a function of the applied field were used until now, since direct measurements were only of very low accuracy.

At the INMA, an adiabatic system has been developed that has been adapted to directly determine the magnetocaloric effect and that allows the temperature increase produced by applying a field to a thermally insulated material to be measured with an error of less than 1%.

Several groups at the Materials Science Institute of Aragon study materials with a magnetocaloric effect, with special emphasis on thermal aspects, magnetic and structural problems or very low temperature systems.

Instituto de Nanociencia y Materiales de Aragón