**5. Low-temperature technology**

Low-temperature technology is achievable through the refrigeration processes, particularly in the vapor compression refrigeration system (VCRS). This process requires heat transfer from the refrigerating space through a heat exchanger known as the condenser. The heat moves to the immediate surroundings, thereby lowering the heating space's temperature and increasing the surrounding temperature [24, 25]. However, the application of VCRS has become a worldwide phenomenon. The VCRS is used to protect perishable items and provide human comfort [26–29]. In designing a VCRS, the refrigerant (working fluid) choice is crucial as it preempts the system's cost, dependability, and safety. Considering various environmental challenges such as global warming and ozone layer depletion, the appropriate selection of refrigerant is required due to the atmospheric greenhouse effect and stratospheric ozone depletion caused by refrigerant emission [21]. The refrigerant is selected based on their safety, thermo-physical and thermodynamics properties, and economic factor [30].

In 1850, ethyl ether became the most prominent refrigerant used in a commercial refrigeration system. Other natural refrigerants such as ammonia, water, carbon dioxide, and gasoline were exploited as heat transfer mediums. Still, they were replaced with sulfur dioxide and methyl chloride because of incompatibility with the system materials. Moreover, at a low cost and good thermodynamic properties, ammonia has an increased coefficient of performance. However, it is toxic, preventing it from being used for domestic purposes [31–33]. Therefore, the search to have better-working fluid with excellent thermal properties used in a refrigeration system has prompted the discovery of halocarbon refrigerants. The halocarbons are synthetic refrigerants, which include chlorofluorocarbon and hydrochlorofluorocarbon refrigerants (HCFCs). They were better alternatives for natural refrigerants because of their high thermal efficiency [34–37]. The chlorofluorocarbon (CFC) refrigerant was discovered in the 1930s with zero explosion risk but later disadvantaged the ozone layer [30]. In 1996, the developed nations successfully phased out the CFC, while the developing countries agreed to cease its application in 2010 [38]. The HCFCs were found as the best short-term alternative replacement for chlorofluorocarbon, but with a global warming potential of 1810. The use of HCFCs will be limited to 2020 and 2030 in developed and underdeveloped countries. Likewise, Du Pont's chemical manufacturer declared that the HCFC substances' production should be ceased but would persist in making it available for existing equipment until their expiry period [39–41].

The emergence of hydrofluorocarbon (HFCs) refrigerants with appropriate thermo-physical and thermodynamic properties have provided opportunities for extensive scale usage at the consumer and commercial levels since the 1990s. This rapid growth and application of HFCs in domestic and mobile refrigeration systems require prompt attention due to their negative effect on global climate [42, 43]. However, hydrofluorocarbon refrigerant (R134a) has zero ODP with a high GWP of 1430, which suggests it is a working fluid that threatens the immediate

surroundings. The international regulatory bodies (Kyoto and Montreal protocols) have called for the banning of pure fluid, posing a threat to climate and environment due to high global warming and ozone depletion [44–46]. But the reduction rate of the pure fluid is subject to review. More so, some limiting factors disannulled halocarbon refrigerants' application in the heating and air-conditioning industries, such as the enormous energy consumption rate and increased global warming [47]. Furthermore, the Kigali amendment (KA) adoption in 2016 created a platform to generate a phase-out schedule for HFC refrigerants towards the next decades, and if this is achieved, it will contribute positively to the Paris agreement (PA) and to the United Nation Framework Convention on Climate Change (UNFCCC) adopted in 2015, which focuses on keeping the global temperature to less than 2 oC with the enforcement by Nationally Determined Contribution (NDC) for greenhouse gas (GHG) [48–51]. The consistent climate change prompted research towards discovering and applications of eco-friendly refrigerants in the heating and airconditioning systems that reduce GHG and enhance energy saving [52].

Halogen-free refrigerants (HFRs) are found naturally and have been extensively discovered as an alternative to halocarbon refrigerants in the refrigeration system. They are also referred to as eco-friendly refrigerants with an organic composition of hydrogen and carbon atoms [21]. The European nations use HFRs instead of halocarbon refrigerants because it possesses good thermodynamic properties. Hydrocarbon refrigerant is miscible with mineral oil, which provides a smooth running of the single hermetic reciprocating compressor (SHRC). The working fluid is compatible with the refrigeration system's elastomeric materials with zero ozone depletion and negligible global warming potential. Also, the HCR refrigerant has a high critical temperature that enhances the domestic refrigerator's efficiency [53–59]. The HFR is for preserving the environment, but it serves as an energy reduction substance with high energy efficiency in the refrigeration system. Although HFRs was reported flammable [60], this proved to be invisible because of various factors that must be attained before the explosion takes place, which includes:

