Ibai Mugica, PhD

gases have been identified as the direct cause of the ozone depletion observed in earth's atmosphere. Furthermore, 20% of the global-warming produced by refrigerator machines is associated to the use of fluorocarbons (Coulomb et al. 2015).

2. The reduced noise during the operation of the refrigerator machine, due to the absence of compression and expansion stages.

3. The potential of an increased efficiency. This could come from the more isentropic phase change of the solid refrigerant, and the absence of inefficient processes like the compression and expansion stages of a vapor-compressor machines.

The first practical use of magnetic refrigeration was related to hydrogen liquefaction in the mid-70s (see Gschneidner and Pecharsky 2008). Then, research extended to refrigerators that worked at room temperature. In the mid-90s the US department of Energy funded an AMR laboratory to extend the knowledge to industrial room temperature refrigeration. Some of the publications yielded by that project boosted the research on magnetocaloric room temperature refrigerators. Among those are the discovery of a "giant" Magneto Caloric Effect (MCE) by Pecharsky and Gschneidner 1997, and a prototype that ran for 5000h over an 18 month period (Zimm et al. 1998). Since then, prototypes with increasingly higher cooling power and efficiency have been reported. Yu et al. 2010 reviewed many of the prototypes developed in the last decade. In their review a clear tendency towards the use of permanent magnets and rotatory prototypes was shown. Despite the recent developments, magnetic refrigeration still needs to face some challenges before becoming commercially viable**:

**as of 2019

1. Price of the raw materials to produce these refrigerators is still elevated**. This is because of the relatively high amount of rare earths the prototypes have (i.e. kilograms of Gadolinium (Gd), Neodymium (Nd)).

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2. The physical phase transition of the solid refrigerant needs of a relatively strong magnetic field (around 1-2T). This is usually costly to produce, so the magnetized airgap tends to be relatively small (in the 10s of mm).

3. During the regular operation of a magnetic regenerator, multiple physical phenomena coexist in a porous medium. Fluid flow, heat transfer, and magnetic field dene the performance of the machine and are difficult to measure without being intrusive. Some efforts have been made to lower the cost of raw materials by searching for more earth abundant Magneto Caloric Materials (MCMs, see Tan et al. 2013). Other scientists have work optimizing the design of the necessary permanent magnet (Teyber et al. 2017). Also, some results have been published on the inner temperature measurement of Active Magnetic Regenerators*.

*Christiaanse et al., Measurement of adiabatic temperature change in a porous regenerator  using fibre bragg grating , volume 2, pages 315318. Canadian Congress of Applied Mechanics, Victoria, British Columbia, Canada, May 29-June 1, 2017).

The central piece of magnetic refrigeration is the magnetic phase change MCMs undergo in the vicinity of their Curie temperature (Tc). Similar to the latent heat of a fluid, the solid refrigerant suddenly experiences an atomic structural change, which modifies the entropy balance of the MCM. The total entropy of a MCM is often expressed by the following equation:

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