Sustainable Cooling and Refrigeration

Nowadays, one of the main present challenges of the industry is to reduce its environmental impact. In this context, refrigeration is an industry branch that must be taken into account, too. Indeed, refrigeration has a great variety of applications, from household to industrial refrigerators and freezers, but also concerning cryogenics, pharmaceutics and air conditioning. Its impact on industry, lifestyle, agriculture, and settlement patterns is continuously increasing, due to the related growth of technology and to the requirements of human well-being. Worldwide, about 15% of the overall energy consumption originates from refrigeration, and only to mention one among the multitude of refrigeration applications and its relative impact, it has been estimated that the environmental impact of refrigeration food industry in terms of CO_2e in the United Kingdom amounted to 12.9 Mt_CO2e, equal to approximately 3.2% of the total emissions of the same country.

Refrigeration is the technique of cooling a control volume and its content to a temperature below the environmental one, but also to maintain, by removing heat towards the environment from the low-temperature control volume. Of course, this heat outflow cannot spontaneously occur due to the Second Law of Thermodynamics; consequently, this process requires work from the outside. In this process, the refrigerant (the working substance which transforms the cycle) plays a crucial role due to its thermo-physical and chemical properties concerning the thermodynamic inverse cycle. In order to determine if a substance is suitable as a refrigerant, some fundamental characteristics of the thermal fluid, its toxicity, its flammability, and its impact on ozone depletion and climate change must be considered. The main categories of refrigerants that have been commonly used in vapour compression cycles and absorption cycles are the following: i. chlorofluorocarbons (CFCs), ii. hydrochlorofluorocarbons (HCFCs), iii. hydrofluorocarbons (HFCs), iv. natural refrigerants and v. refrigerant blends, which can be zeotropic or azeotropic. The main criteria that determine suitable refrigerant substances for vapour-compression cycles are the following ones:

• From a chemical viewpoint they should be stable and inert;

• From a safety and environmental viewpoint they should be non-toxic, non-flammable and do not degrade the atmosphere;

• From a thermodynamic viewpoint they should have a critical point and boiling point temperature suitable for the desired application, low vapour heat capacity, low viscosity, low freezing point, high thermal conductivity;

• They should have a sufficient oil solubility, a high dielectric constant of vapour.

Many efforts have been done by regulation agencies to rule out harmful refrigerants for the environment, that have brought to look for new refrigerant substances. Thus, as concerns refrigerants regulation, in 1987, with the Montreal Protocol, the phase out of ozone depleting substances was set and improved during the decades (beginning with CFCs, passing to HCFCs). In 2019, with the Kigali Amendment, also the reduction in the use of HFCs (e.g., R-134a, R-245fa, R-125, R-32) was established, because of their high impact on global warming potential (GWP), mainly due to their fluorine content. In particular, the ratified commitment of the industrialised countries consists in the reduction in the use of HFCs by 85% compared with their use during the period 2011-2013, by 2036 (with the intermediate target of 45% reduction by 2024). Other countries, such as China, and Brazil have set an 80% HFCs reduction by 2045, while the remaining countries, including the Middel-East ones, should arrive at the same reduction target by 2047. Furthermore, in 2006, the European Union (EU) adopted a Regulation on fluorinated greenhouse gases (GHGs) to induce the use of natural refrigerants, phasing out refrigerants with highest GWPs, since 2011.

To address a technical answer to the EU requirements, natural refrigerants are being adopted, together with other possible new technologies (magnetocaloric refrigeration, electrocaloric refrigeration, etc.). However, some technical difficulties exist concerning the use of natural fluids and alternative technological systems. So, thermal physicists and engineers develop new approaches to these solutions. This special issue aims to collect new technological and sustainable proposals on the subject of refrigeration and cooling.

Keywords: Refrigeration; Cooling; Sustainable refrigeration; Sustainable cooling; Sustainable refrigerant; New technologies in refrigeration and cooling; Magnetocaloric and electrocaloric effects; Second law analysis; Energy, exergy and emergy; Adsorption plants.

This Collection supports and amplifies research related to SDG7, SDG9, SDG11, SDG13.