, called a cascade heat exchanger. A substantial number of studies have been presented in the literature regarding the potential performance improvement of cascade refrigeration cycles via several means and the selection of a more suitable refrigerant pair to achieve energy-saving and address environmental concerns.
For example, Bingming et al. [9] examined the performance of NH 3 /CO 2 cascade refrigeration system using an experimental study. They conducted a parametric study to investigate the performance of several parameters on the system performance. The cascade system performance was compared with a two-stage NH 3 system and a single-stage NH 3 system with and without an economizer. The results showed the superiority of the cascade cycle for an evaporating temperature of lower than 40 • C. Yari and Mahmoudi [10], proposed two CO 2 cascade refrigeration cycles and evaluated their performance using a thermodynamic model. While both cycles consisted of an ejector-expansion transcritical cycle as the topping cycle and a sub-critical CO 2 cycle as the bottoming cycle, in one of the cycles, a supercritical CO 2 power cycle was used to recover the waste heat from the gas cooler of the topping cycle.
The results of this study showed that both cycles showed a COP improvement over a conventional cascade cycle and the cycle that used supercritical CO 2 power cycle for waste energy recovery had a better performance. Sarkar et al. [11] performed a thermodynamic analysis and optimization to identify the most suitable refrigerant pairs for a cascade refrigeration system. They evaluated the suitability of eight refrigerants and 56 possible combinations of refrigerant pairs using COP as a criterion. They found that propane-ammonia and propylene-ammonia are the best refrigerant pairs for higher evaporator temperatures and ethane-propylene is the best refrigerant pair for lower evaporator temperatures.
Aghazadeh Dokandari et al. [12] evaluated the use of ejectors in a cascade refrigeration cycle that used ammonia-carbon dioxide (NH 3 -CO 2 ) refrigerant pair from a thermodynamic perspective. It was shown that using ejectors in the cycle can improve the COP of the cascade refrigeration cycle up to 7%. The results showed that the proposed ejector-based cascade refrigeration cycle has a promising application. Yilmaz et al. [13] carried out a thermodynamic analysis to evaluate the performance of a cascade refrigeration cycle that used CO 2 -R404a refrigerant pair. The authors performed a parametric study to identify the optimum operating conditions of the cycle and maximize the COP. They showed that the maximum COP was obtained when the cascade condenser and primary condenser temperatures were equal to −5 • C and 25 • C, respectively. Parmar and Kapadia [14] thermodynamically evaluated the performance of a cascade refrigeration system for a supermarket application and examined the potential of using six refrigerants in the hightemperature cycle and R744 in the low-temperature cycle. They studied the influence of several operating parameters on the COP of the system and found R717-R744 as the most suitable refrigerant pair. Llopis et al. [15] evaluated five two-stage cascade refrigeration systems and examined the potential of using different low GWP refrigerants in these systems. The authors analyzed the systems' performance thermodynamically over a wide range of evaporator and ambient temperature conditions. They concluded that the cascade refrigeration systems that used CO 2 as a low-temperature refrigerant have a promising application. In addition, the application of direct CO 2 transcritical systems is not appealing for warm climates. Llopis et al. [16] evaluated the performance of a cascade refrigeration cycle that used two internal cascade heat exchangers and used R134a/CO 2 as a refrigerant pair. They conducted an evaluation based on experimental results for evaporator and condenser temperatures ranging from −40 • C to −30 • C and from 30 • C to 50 • C, respectively. The authors concluded that using the second cascade heat exchanger caused a slight reduction in the cooling capacity of the system, but it increased the COP up to 3.7%. Mosaffa et al. [17] carried out exergoeconomic and environmental analyses to evaluate two CO 2 /NH 3 cascade refrigeration cycles: the first one was equipped with two flash tanks and the second one was equipped with a flash tank and a flash intercooler. A comparative analysis was conducted to examine and compare the performance of the systems and identify the optimum operating parameters. The results showed that the annual cost rate of the first one could be over 11% lower than the second system. In addition, the results showed the potential benefits of these cascade refrigeration systems.
Megdouli et al. [18] proposed a novel ejector expansion transcritical cascade refrigeration cycle and evaluated its performance thermodynamically. In this cycle, two ejectors were used in the low and high-temperature cycles and a transcritical CO 2 Rankine cycle was used to recover the available heat in the condensation stage of the high-temperature cycle of the refrigeration system. CO 2 and nitrous oxide (N 2 O) were used as refrigerants in the high-temperature and low-temperature cycles of the system. A COP improvement of over 9% was reported for the novel system compared to a typical ejector expansion cascade refrigeration cycle. Sun et al. [19] evaluated the potential of 28 low GWP refrigerant pairs for a typical cascade refrigeration cycle from a thermodynamic viewpoint. They showed that, among the examined refrigerant pairs, the R41/R161 and R170/R161 refrigerant pairs are the best options. The authors also recommended the use of R41/R161 as the most suitable pair for the evaporator temperature of higher than −60 • C. Sun et al. [20] conducted a thermodynamic analysis to evaluate the potential of using different low GWP potential refrigerants for a three-stage cascade refrigeration system. The results of their study showed that R1150 and R170 are suitable refrigerants for the low-temperature and medium-temperature cycles. For the high-temperature cycle of the cascade refrigeration system, R717, R152a, and R161 refrigerants were identified as the most suitable options. Overall, the authors recommended the use of a combination of these refrigerants for a three-stage cascade refrigeration system.
Patel et al. [21] conducted a thermo-economic analysis and optimization to evaluate and compare the performance of using NH 3 /CO 2 and propane (R290)-CO 2. The authors evaluated the influence of several design parameters on the performance of the cascade refrigeration cycle. The results showed that the R290-CO 2 refrigerant pair provided a 5.33% lower cost rate than the NH 3 /CO 2 refrigerant pair. Kumar Singh et al. [22] proposed a cascade refrigeration cycle equipped with a flash tank and a flash intercooler and evaluated its performance from energy, exergy, and economic viewpoints. The authors conducted a comparative study of using different natural refrigerants to determine the most suitable pair. It was found that R717-R290 is the most suitable refrigerant pair from both thermodynamic and economic views. For the system with a refrigeration capacity of 500 kW that used the R717-R290 refrigerant pair, the maximum COP was found to be 1.917, which corresponded to the total annualized cost of $836,395 $/yr. Dual-evaporator or dual-temperature refrigeration systems have attracted much attention recently because of the growing need for two different evaporator temperature ranges in the industry. Dual-evaporator refrigeration systems are appealing because they provide two evaporating temperature ranges using only one system, resulting in reduced capital and operating cost by avoiding the construction of two separate systems [23]. A dual-evaporator cascade vapor compression refrigeration system is a promising concept, evaluated and studies in some previous studies such as the studies of Sánchez et al. [24] and Mohammadi et al. [25]. The primary potential benefit of this system is the ability to provide a high COP in low evaporating temperatures, which is not normally achievable by other types of dual-evaporator refrigeration systems, including the dual-temperature absorption refrigeration systems.
As noted, the proper selection of refrigerants is an important research area to maximize the performance of refrigeration systems and minimize their negative environmental impacts. There is no comprehensive study in the literature to explore the use of different refrigerant pairs in a dual-evaporator cascade vapor compression refrigeration system. Therefore, in this study, a thermodynamic and economic analysis is carried out to assess the potential of using several low GWP refrigerant pairs and identify the most suitable pair. The goal of this work is to fulfill the current need of determining the most suitable environmentally friendly refrigerant couple for a dual-evaporator cascade refrigeration system through conducting a comprehensive comparative thermodynamic and economic analysis. Overall, 18 refrigerant pairs, including three refrigerants for the low-temperature cycle and six refrigerants for the high-temperature cycle of the cascade cycle, were carefully chosen from the available refrigerants because of their minimal GWP, are examined in this study. A parametric study is performed to evaluate the performance of the cascade refrigeration cycle and the selected refrigerant pairs over a wide range of design and operating parameters. The refrigerant pair that gives the lowest unit production cost of cooling (UPCC) for the cascade refrigeration system is identified as the most suitable refrigeration pair. An optimization process using a particle swarm optimization (PSO) algorithm is performed to minimize the UPCC of the cascade refrigeration system. Figure 1 shows the simplified schematic layout of the dual-evaporator cascade vapor compression refrigeration cycle evaluated in this study. As illustrated by Figure 1, the dual-evaporator cascade cycle consists of a topping cycle or high-temperature (HT) cycle and a bottoming cycle or a low