A double tube-type counter flow heat exchanger was used in the experiment. In the heating mode, the heat sink water temperatures at the inlet and the outlet of the condenser were set at 20 °C and 45 °C, respectively, and the corresponding inlet and outlet water temperatures in the evaporator were kept constant at 15 °C and 9 °C, respectively. In the cooling mode, the heat sink water temperature at the inlet and the outlet of the condenser was maintained at 30 °C and 45 °C, respectively, and the inlet and outlet water temperatures in the evaporator were maintained at 20 °C and 10 °C, respectively.
The heat transfer rate varied from 1.4 kW to 2.6 kW by adjusting the refrigerant flow, compressor rotational speed, and water flow. The degree of superheat was kept constant at 3–4 °C over the entire range of test conditions.
R152a (1,1 difluoroethane, C2
) has been considered as a refrigerant blend for a long time, but not as a pure refrigerant. The refrigerant’s critical pressure and temperature are 4.52 MPa and 115 °C, respectively 
. It does not have any ODP. Compared to the HFC and HFO refrigerants, R152a has a low global warming potential (138 
) and a lower price. The volumetric capacity and pressure of R152a are quite close to those of R134a. On the other hand, the energy efficiency, mass flow, and vapor density are much better than those of R134a 
. The system performance in a vapor compression system was found to be higher than that of R134a 
. Test results demonstrated a 4.7–13% improvement in the COP, with a decrease in the cooling capacity of about 0–10%. Figure 3
shows the comparison of the (a) COP and (b) energy consumption between R134a, R152a, and R32 in 120 L refrigerators for different charge amounts. It was found the COP increases with the increase in refrigerant amount, but the minimum power consumption reached 100 g. R152a is commonly used as an aerosol spray propellant and foam-blowing agent. It is also used as a component in a variety of refrigerant blends (R401A, R415A, R430A, R500, etc.) 
. However, its flammability is rated as A2 by the ASHRAE 
, and hence flammable dangers may have been the primary factor preventing its use as a pure refrigerant up until now 
. The minimum ignition energy for this refrigerant is 0.38 MJ, which is slightly higher than that for flammable hydrocarbons. Cabello et al. 
also studied cascade refrigeration plants using R152a/R744 and R134a/R744 pairs for industrial and commercial applications. Though the comparative energy results did not show a great difference, its low GWP can be a great advantage from the environmental point of view.
) Coefficient of performance (COP) and (b
) energy consumption as a function of the refrigerant’s charged amount 
R744 (carbon dioxide, CO2
) has been utilized in various types of vapor compressor systems for over 130 years 
. It is a low-cost and natural refrigerant. However, it works at an exceptionally high-pressure level and operates in a transcritical cycle. The decline of using R744 started in the mid-1950s after the invention of high-performance CFC refrigerants. When the adverse effect of synthetic refrigerants was found in the late 1980s, there was a renewed interest in R744 as it is not toxic and flammable 
. In the early 1990s, Gustav Lorentzen revived R744 as a refrigerant 
. To assess the COP of R744 and R12, Lorentzen and Pettersen 
designed a laboratory prototype considering its application in vehicle air conditioning systems. The researchers speculated that the increased energy density might have a high cost but provide practical benefits due to their smaller size and weight. Koyama et al. 
and Xue et al. 
constructed a steady-state model where they assessed the heating and cooling performance of R744. Many researchers have studied theoretically and empirically the performance of R744. The performance has also been compared with other refrigerants as a feasible alternative to synthetic fluids 
. Hwang et al. 
tested the performance of several two-stage compressor R744 cycles and observed an 18–35% improvement in the COP over the basic cycles. Girotto et al. 
calculated the COP of all-CO2
systems for a supermarket in northern Italy, estimated the annual energy consumption and the cost of CO2
installation. Even though the installation cost was 20% greater, the researchers discovered a way to raise the efficiency of the CO2
system to match that of the R404A system. According to Maina and Huan 
and Nekså 
, this refrigerant has been used for different purposes, including hot water generation, commercial refrigeration, heat pump dryers, etc.
New refrigerant R1338mzz(Z) has been found very promising for high-temperature heat pump cycles. It is non-flammable, and the GWP of this refrigerant is only 2 
. The boiling and critical temperatures of R1336mzz-Z are 33.5 and 171.3 °C, respectively. Therefore, it can be potentially considered as an alternative to refrigerants HFC-245fa and HCFC-123. The pool boiling performance of R1336mzz(Z) on a flattened horizontal Turbo-ESP surface has been studied for air conditioning and industrial heat recovery applications 
. The potential application of these refrigerants has also been studied for transcritical Rankine power cycles 
2.1.6. HC (Propane R290, N-Butane R600, Isobutane R600a)
In general, the natural refrigerant known as HC (hydrocarbon) has great efficiency, good miscibility with mineral oils, a lower compressor discharge pressure, and strong heat transfer properties. However, because of its significant flammability, this refrigerant cannot be used on a larger scale. Because many other nations have outlawed using flammable gases in the public, HCs are only used as refrigerants in Europe following some regulations. The theoretical and practical performance of R290 in contrast to R22 was examined by Lampugnani and Zgliczynski 
. According to the experimental findings, R290 is a superb candidate to replace R22 from the thermodynamic standpoint. Granryd 
discussed the advantages and disadvantages of employing HC as a refrigerant for refrigeration and heat pump applications. Numerous HCs have been discovered to exhibit advantageous properties as refrigerants from the thermodynamic and heat transport perspective. In their investigation, Halimic et al. 
came to a conclusion that, after addressing technical operation and safety concerns, R290 is a desirable replacement for R12 in small residential refrigerators. Bjerre and Larsen 
assessed the potential usage of R600 for residential applications and discovered that it performed 10% better than R134a. Palm and Somchai and Chimres 
found the excellent performance of hydrocarbon compared with R134a, R22, and ammonia. According to the researchers, the safety risk may be decreased by constructing systems hermetically with the least amount of connections and refrigerant charge or by using an indirect system. The accepted practice of employing HC in a vapor compression system was examined by Corberán et al. 
. According to the IEC355.2.20 standard, up to 150 g of HC can be sealed in a regular refrigerator, and tiny freezers can be placed anywhere, regardless of the size of the room, with a few additional safety precautions. Some European refrigerator producers now have access to flammable HCs to make residential refrigerators. However, the failure rate of the compressor is increased with the vast commercialization of such refrigerants.
2.1.7. Other Natural Refrigerants
In home and automotive heat pump applications, water, ethanol, ammonia, and methanol, known as natural refrigerants, are receiving attention 
. The thermodynamic properties of ammonia (R717) are excellent, and it is energy-efficient when applied in large industrial systems and commercial buildings, but its toxic behavior limits its usage as a refrigerant in domestic and automobile applications 
. Due to their low volumetric efficiency, water (R718) and ethanol are not common refrigerants in conventional vapor compression systems, but they can be regarded as common refrigerants in sorption-based systems 
2.2. Refrigerant Mixtures (Blended)
Due to their azeotropic character and generally well-developed thermophysical properties, pure refrigerants are a good choice for vapor compression systems. HFOs are reasonable alternatives regarding their toxicity, flammability, and ultra-low GWP. According to Bella and Kaemmer 
, pure HFOs are inefficient substitutes for R410A from the perspective of system performance. For drop-in replacement, rebuilding the system with a larger compressor, pipe, and heat exchangers is necessary. It has also been found that the volumetric capacity of the refrigerant is low, which leads to larger mass flow rates, larger pressure drops in the heat exchangers and connection pipes, and, ultimately, lower coefficients of performance (COP) 
. The volumetric capacity as well as the COP can be improved by adding some other refrigerants with HFOs. In China, R32 has been investigated due to its lower cost as compared to R410A, while blends of R32 with HFOs have been studied in Japan and the USA 
. A blend is often defined as a combination of two or more single-component refrigerants. Blends can be either azeotropic or zeotropic. Azeotropic blends act like pure refrigerants in such a way that they boil and condense at consistent temperatures at any given pressure. As opposed to azeotropic blends, zeotropic blends boil and condense at a variety of temperatures at a given pressure (see Figure 4
a). This temperature range is known as temperature glide, which is essentially the difference between the refrigerant compound’s bubble point and dew point. Figure 4
b shows the formation of temperate glides of the R32/R1234ze(E) and R32/R1234yf blends at a constant pressure. The maximum temperature glides (△T) of the R32/R1234yf and R32/R1234ze(E) mixtures at 0.5 MPa are 7.6 and 12.2 °C, respectively (source of thermophysical properties—REFPROP V9.1) 
. Despite the vast possibilities of the mixing strategy, a few mixtures, such as binary and ternary mixtures, remain popular due to the possible complexity involved in the thermophysical properties of the mixtures.
Figure 4. Temperature glides of a binary mixture. (a) Temperature vs. mass fraction diagram of a zeotropic mixture, (b) temperate glides of the R32/R1234ze(E) and R32/R1234yf binary blends.
2.2.1. Binary Mixtures
Numerous studies have demonstrated that R1234ze(E) has a poorer coefficient of performance (COP) than other widely used refrigerants like R410A or R134a, mostly because of its limited volumetric capacity and greater pressure drop 
. R32 is typically blended in a variety of ratios to improve the performance of R1234ze(E) and the vapor density 
. The cycle performance analysis showed that the inclusion of R32 (50% mass) with R1234ze(E) boosted the volumetric capacity while maintaining the COP greater than that of R410A, suggesting this mixture is a strong contender to replace R410A 
. The thermodynamic properties of the mixture developed by Akasaka 
were used in the study.
Due to its extremely low GWP and performance that is equivalent to that of R134a, refrigerant R1234yf can be regarded as a promising next-generation refrigerant. Prior to now, R1234yf has had a few drawbacks, most notably a volumetric capacity that was much smaller than that of R134a. However, pure R32 exhibits a very high temperature at the compressor output and a significantly greater GWP, making it a poor candidate for home air conditioning. In order to address their respective weaknesses, these two refrigerants are combined with different ratios. . The experimental result show the performance of the R32/R1234yf binary mixture of 42/58 (by mass) is higher than R410A.
AGC (Asahi Glass Co., Ltd., Japan) developed a novel refrigerant for air conditioning systems in 2014 that uses hydrofluoroolefin R1123 as its primary component and can cut GWP even further. Trifluoroethylene, or R1123, has a performance that is comparable to that of common refrigerants and a very low GWP (0.3) 
. However, the R1123 refrigerant has disproportionate properties at a higher temperature. To make the system safe, the blend of R1123 and R32 was proposed to replace R410A. Higashi and Akasaka 
measured the thermodynamic properties of the 60/40 and 40/60 R32/R1123 mixtures (by mass). When R410A is replaced in residential and commercial air conditioning systems, the new refrigerant blend, which is azeotropic, can provide good performance, according to AGC 
. The performance of this mixture is being tested at many laboratories in Japan; however, cycle performance results are not available in the open literature.
HC blends are ecofriendly refrigerants that may be employed in already-built systems. Due to their high flammability (A3), HC blends are recommended over halogenated refrigerants for usage in small systems with low charge amounts. Many studies investigated the R290/R600a combination as a replacement for R12 and discovered that it has a greater COP and refrigeration capacity than R12. R290/R600a mixtures have been investigated to replace R134a; it was found the mixtures are an appropriate alternative to R134a in terms of power consumption, charge amount, and cooling rate 
. When the temperature glide matches the fluid temperature change, the 75/25 mixture (by mass) of R744/R290 increases the COP 12.8% more than R744 
. The performance of R32/R290 mixtures (68/32) as a drop-in replacement for R410A was found to be 6–7% lower than that of R410A, but the charge amount is reduced by 30–35%.
2.2.2. Ternary Mixtures
For the case of binary blends of R32/R1234yf and R32/R1234ze(E), more than 50% R32 always shows a better performance than R410A. To maintain a GWP less than 300, the mass fraction of R32 must be below 50%, resulting in a lower COP, which is a consequence of limited volumetric capacity. When R744 added in the binary mixture, the performance of R744/R32/R1234ze(E) mixtures having GWP 300, found the COP is comparable to that of R410A in both heating and cooling modes.
Two ternary blends were formed by adding R744 into binary blend R32/R1234yf to enhance the volumetric capacity of the binary blend. The coefficient of performance of the ternary blends was experimentally studied, and their performance was compared with R410A 
. For the blends, the researchers considered the GWP value of 200 and 300. Temperature glide was a concern during the condensation and evaporation procedures since these combinations are zeotropic. The inclusion of R744 lowers the GWP but increases the temperature glide. The irreversible loss is minimized by matching the gliding temperature with the temperature changes in the heat sink and the heat source.
Other Ternary Mixtures
Ternary blend R744/R32/R134a (7/31/62) was studied theoretically and experimentally and compared with R32 
. This blend was found as a promising alternative to R22 as a drop-in replacement. It shows a 10% better COP. The blend is recommended for the use of low-temperature heat pumps because the condensing pressure is too high. The main problem with this blend is its GWP value (~1100). The performance of R410A was compared with R744/R32/propane (10/80/10, GWP ~540) 
, and the heating capacity of the blend was found to be higher than that of R410A. Two low-GWP refrigerant mixtures R32/R152a/R1234ze(E) (12/5/83) and R134a/R152a/R1234yf (7/11/82) have also been chosen as a promising alternative for next-generation heat pump systems