Magnetocaloric as Solid-State Cooling Technique for Energy Saving

Introduction

Nowadays, the refrigeration is responsible of more than 20% of the overall energy consumption all over the world and most modern refrigeration units are based on Vapor Compression Plants (VCP). The traditional refrigerant fluids employing VCP, i.e. ChloroFluoroCarbons (CFCs) and HydroChloroFluoroCarbons (HCFCs), have been banned by the Montreal Protocol in 1987 (Montreal Protocol, 1987), because of their contribution to the disruption of the stratospheric ozone layer (Ozone-Depleting Potential substances - ODPs). Over the time, periodical meetings among the Parties to the Montreal Protocol have been succeeded. Since 2000, the usage of HCFCs in new refrigerating systems is forbidden, letting HydroFluoroCarbons (HFCs) the only fluorinated refrigerants allowed because of their zero ODP characteristic. Since 2009, each meeting related to Montreal protocol, initially dedicated to the phase-out of the substances depleting the stratospheric ozone layer, namely CFCs and HCFCs, had been leading to conflicting exchanges on high Global Warming Potential HFCs which replace CFCs and HCFCs most of the time. Year after year, human activities have been increasing the concentration of greenhouse gases in the atmosphere, thus resulting in a substantial warming of both earth surface and atmosphere. The impact of greenhouse gases on global warming is quantified by their GWP (Global Warming Potential). As a result, over the year measures to reduce global warming have been taken, beginning with the Kyoto Protocol (Kyoto Protocol, 1997) and consequently with the EU regulations applying the prescriptions derived from it, like EU regulation 517/2014 (EU No 517/2014) on fluorinated greenhouse gases.

The above described general frameworks led scientific community to studying and applying solutions with environmentally friendly gases, with small GWP and zero ODP (Aprea & Greco, 1998; Greco & Vanoli, 2005): one of the most focused classes of new generation refrigerants is hydrofluoroolefins (HFO) (Aprea et al., 2016a; 2016b; 2018a; Mota-Babiloni et al., 2016), descending of olefins rather than alkanes (paraffins) and they are known as unsaturated HFCs, with environmentally friendly behavior, quite low costs. Despite all these advances, it is essential to underline that a vapor compression plant produces both a direct and an indirect contribution to global warming. The former depends on the GWP of refrigerant fluids and on the fraction of refrigerant charge which is either directly released in the atmosphere during operation and maintenance or is not recovered when the system is scrapped. The indirect contribution is related to energy-consumption of the plant. In fact, a vapor compression refrigerator requires electrical energy produced by a power plant that typically burns a fossil fuel, thus releasing CO₂ into the atmosphere. The amount of CO₂ emitted is a strong function of the COP of the vapor compression plant.

For all these reason, in the last decades the interest of scientific community has oriented itself in studying and developing new refrigeration technologies of low impact in our ecosystem: a class of them is composed by solid-state cooling (Kitanovski et al., 2015; Aprea et al., 2018b), which are gaining more and more attention, due to their potential in being performing and ecological methodologies. Recent discoveries of giant caloric effects (Pecharsky and Gschneidner, 1997; Lu et al., 2010; Liu et al., 2014) in some ferric materials opened the door to the use of solid-state materials for caloric cooling as an alternative to gases for conventional and cryogenic refrigeration.