A Review of the Energy-Saving Potential of Phase Change Material-Based Cascaded Refrigeration Systems in Chinese Food Cold Chain Industry

Abstract

As the global demand for food increases, the efficiency and environmental sustainability of refrigeration systems have become increasingly critical issues. Cascaded refrigeration systems (CRSs) are widely used in the Chinese food cold chain due to their capacity to meet a wide range of temperature requirements. However, energy consumption of these systems is always high. If phase change materials (PCMs) are combined with the refrigeration systems, the energy-saving effect is remarkable. The paper reviews the integration of PCMs within CRS, focusing on their potential to reduce energy consumption, thereby improving food safety and reducing reliance on conventional, electricity-intensive refrigeration methods. The study categorizes and explores the low-temperature applications of PCMs in CRS, providing novel insights into enhancing energy efficiency in food cold chain logistics. Despite most PCM research focusing on single-stage systems, this review innovates by introducing PCM integration in multistage cascade systems, which is particularly relevant for low-temperature requirements. The discussion encompasses the structure, working fluids, and applications of CRSs in the cold chain, emphasizing the role of PCMs in sustainable cold chain management. The review concludes by highlighting the need for further research on PCMs in CRS, especially regarding their economic viability and large-scale implementation potential.

1. Introduction

The continuous rise in global energy consumption is a significant worry for the global scientific community, pushing researchers to find ways to ensure sustainable energy use [1]. The energy crisis, combined with environmental pollution from the dependence on traditional fossil fuels, has become a major obstacle to human development [2]. The European Union and its member states have committed to decarbonizing the European economy by 2050 [3]. According to the Paris Agreement, the government of China has pledged to peak carbon emissions by 2030 and achieve carbon neutrality by 2060 [4]. Heating, Ventilation and Air Conditioning (HVAC), refrigeration, and heat pump systems contribute approximately 60% of total energy consumption in buildings, leading to 39% of energy-related greenhouse gas emissions. It shows the need for efficient energy management and sustainable technologies in these areas [5,6,7]. The development of renewable energy is key to reducing greenhouse gas emissions and addressing the scarcity of traditional energy sources. Utilizing innovative technologies that use clean, abundant, and renewable energy can decrease the reliance on finite fossil fuels and remove the adverse effects of climate change [8]. However, renewable energy sources, for example, solar energy, wind energy, and tidal energy, are affected by the environment, resulting in unpredictable yields. Hence, the future development of renewable energy will require substantial, long-term energy storage technologies.

By the year 2050, the world’s population is projected to exceed 10 billion, posing a formidable challenge that necessitates a staggering 70% increase in food production to meet the demands of this growing populace [9]. According to forecasts by the Food and Agriculture Organization of the United Nations (FAO), an alarming one-third of the global food supply destined for human consumption is either lost or wasted, amounting to approximately 1.3 billion tons annually. Consequently, as we strive to boost food production to cater to the projected growth in the global population, it is imperative that we concurrently focus on minimizing food loss and waste [10]. China has traditionally been an agricultural country, which produces the largest amount of fruits and vegetables in the world that are perishable. According to the data released by the National Bureau of Statistics from 2015 to 2019, the total production of perishable food in China is huge and increasing yearly. As presented in

Table 1. 2015–2019 total production of major perishable foods in China (million tons) [11].

Year	Fruits	Vegetables	Meat Aquatic	Products	Poultry and Eggs	Milk
2015	24,524.6	66,425.1	8749.5	6211.0	3046.1	3297.6
2016	24,405.2	67,434.2	8628.3	6379.5	3160.5	3176.8
2017	25,241.9	69,192.7	8654.4	6445.3	3096.3	3148.6
2018	25,688.4	70,346.7	8624.6	6457.7	3128.3	3173.9
2019	27,400.8	72,102.6	7758.8	6480.4	3309.0	3295.5

, the total production of perishable food has exceeded 1.2 billion tons [11]. Global food losses exhibit significant regional variations, with countries like China experiencing notably higher rates. In China, despite the necessity for refrigeration, a mere 15% of fresh produce is actually shipped under refrigerated conditions, contributing to the elevated levels of food loss. Ensuring a comprehensive and efficient cold chain system is imperative for both economic and public health reasons [12]. Cold chain logistics is not only an important means to maintain the quality and safety of fresh agricultural products but also a support for national economic development. An effective cold chain will ensure good food quality and reduce losses and pollution, while substantially enhancing the food supply system.

As defined by the International Dictionary of Refrigeration (IIFIIR), the cold chain represents a comprehensive term that encapsulates the seamless integration of various methods employed to ensure the continuous refrigerated preservation of perishable food products, spanning from the point of production all through to the ultimate stage of consumption. Cold chain logistics represents an integrated low-temperature supply chain system, marrying the realms of refrigeration and logistics industries to ensure seamless management of temperature-sensitive products [13]. The equipment and facilities in the cold chain may include precooling and freezing facilities, cold storage warehouses, refrigerated trucks, freezers, display cabinets, and home refrigerators. In contrast to normal temperature logistics, cold chain logistics imposes stricter standards on factors such as promptness, precise temperature regulation, energy efficiency, and equipment quality. It serves as a vital instrument not only in preserving the quality and safety of perishable agricultural goods but also as a cornerstone supporting national economic growth and development [14]. However, China’s entry into the realm of cold chain logistics was relatively belated. When juxtaposed with other advanced nations, it becomes evident that our country’s cold chain infrastructure, the expansive logistics network dedicated to preserving perishables, and the overall efficiency in managing these commodities, all necessitate significant enhancements and upgrades [15]. Cold chain standards on agricultural products and food are a crucial aspect of ensuring the quality, safety, and sustainability of the food supply chain. These standards cover various aspects, from production to consumption, and involve a range of regulations and guidelines aimed at minimizing food loss, preventing contamination, and maintaining optimal conditions throughout the entire process. There exist over 200 comprehensive cold chain standards, encompassing intricate temperature regulations and diverse procedures specific to agricultural products and food items. Figure 1 [16] outlines the comprehensive structure of cold chain standards, which are systematically organized into three key segments: (1) Fundamentals, which establish the foundational principles and guidelines; (2) Equipment and Facilities; and (3) Technology, Operations, and Management, encompassing the technological advancements, operational procedures, and managerial practices essential for effective cold chain management. In China, the rapid growth of the food industry has led to an increased demand for cold storage facilities. The food cold chain, which includes the transportation, storage, and distribution of perishable food products, is a major consumer of energy. Food cold chain applications consume roughly 8% of the world’s electricity, thereby contributing to an elevated consumption of fossil fuels and accounting for 2.5% of global carbon emissions [17]. The evolving global food safety landscape across the entire supply chain poses fresh challenges to the existing cold chain storage and transportation infrastructure. The swift advancements in diverse sectors, including medicine, electronics, and cold chain logistics, necessitate the use of lower refrigeration temperatures for product storage and scientific research endeavors [18].

However, the energy consumption of traditional refrigeration systems is substantial, leading to elevated operational costs and posing environmental concerns. According to statistical data, refrigeration accounts for more than 15% of the total energy consumption in China [19]. Therefore, exploring energy-saving solutions in refrigeration systems is of utmost importance. A variety of cooling technologies have been investigated by researchers, encompassing forced-air cooling, hydrocooling, vacuum cooling, and others. Among these, PCMs exhibit the capability to absorb or release energy from their surrounding environment during the phase transition process, thereby facilitating energy transfer. In recent years, PCMs have demonstrated significant potential in the field of energy saving, thanks to their unique characteristics of energy storage and release. The phase change process can be viewed as an isothermal process that boasts reusability, thereby enabling significant cost savings through repetitive utilization [20]. This presents an appealing approach to tackling some of the aforementioned key challenges. Harnessing the advantages of PCMs’ substantial latent heat and virtually constant temperature during phase transitions, this methodology has been explored as a means to minimize energy consumption and mitigate environmental impact in refrigeration systems [21]. The present research endeavor centers around the post-harvest preservation of perishable goods, with a particular emphasis on the implementation of cooling techniques within cold chains. The incorporation of PCMs into these cold chains serves a dual purpose: first, it ensures that the products are maintained under optimal thermal conditions, thereby extending their shelf life; second, it enhances the overall efficiency of the cold storage process by more effectively regulating temperature fluctuations. In the realm of incorporating PCMs into refrigeration systems, the preponderance of prior research endeavors has been narrowly centered on single-stage refrigeration systems. Conversely, there has been a notable lack of research investigating the application of PCM in cascade refrigeration systems. When the evaporating temperature descends below −30 °C, the application of single-stage refrigeration systems becomes economically impractical, primarily due to the substantial increase in pressure differential that adversely impacts the compressor’s efficiency and overall performance. Thus, a viable solution involves implementing a CRS that utilizes distinct refrigerants for its two compression cycles. This paper endeavors to bridge the existing research gap by delving into the influence of harnessing the energy-saving potential of PCMs within cascade refrigeration systems. It places a spotlight on the application of PCMs in cascade refrigeration systems specifically tailored for the cold chain logistics industry, with a particular emphasis on maintaining low-temperature environments across diverse applications. Furthermore, cold chain logistics systems based on phase change cascaded refrigeration systems are well-positioned to proactively respond to the pressing global demand for reduced or even zero carbon emissions.

2. Cascade Refrigeration Systems

2.1. The Structure of Cascade Refrigeration System

Given the country’s heightened focus on energy conservation efforts coupled with the advancement of cold chain logistics, refrigeration technology has garnered substantial attention and significance. For applications requiring low temperatures, such as rapid freezing systems and frozen food storage, the evaporator’s evaporation temperature ranges from −30 °C to −85 °C. Given the significant temperature difference between the heat source and heat sink, employing a single-stage refrigeration system becomes economically unfeasible. Utilizing a refrigerant with a wide temperature range can result in reduced evaporator pressure, increased suction volume, and higher condenser pressure, thereby compromising the system’s efficiency and cost-effectiveness [22,23]. Therefore, cascade refrigeration systems, which consist of a series of single-stage refrigeration systems thermally coupled via cascade condensers, have emerged as the preferred solution due to their exceptional performance under conditions of large temperature differences and low temperatures. These systems offer an advantage over standalone single-stage systems by efficiently handling the varying temperature requirements, enabling them to maintain optimal cooling performance across a broader temperature range. Generally, achieving a low temperature of ≤−80 °C can be accomplished through two primary methods: utilizing Stirling refrigerators and employing CRSs [24]. The CRS holds extensive application prospects within both domestic and commercial refrigeration sectors. This extends to specialized domains like hypothermic medicine, cryogenic preservation of instruments, and cryogenics, where it finds usage in applications, including the liquefaction of gases [25]. It is also widely used in the storage and distribution of food, supermarkets, small refrigeration devices, air conditioning, etc. However, the performance of the CRS undergoes significant variations with changes in the intermediate pressure within the cascade condenser. As a result, accurately predicting and optimizing the system’s performance becomes paramount for effective energy management and reducing emissions.

CRSs typically consist of two or more independent refrigeration cycles, each utilizing distinct refrigerants and equipped with their own condensers, evaporators, and compressors. Its structural characteristic lies in the serial connection of multiple refrigeration cycles, enabling a gradual decrease in temperature to achieve the desired low-temperature target. The schematic diagram of a typical CRS with a high-temperature cycle (HTC) positioned above and a low-temperature cycle (LTC) positioned in Figure 2 [26]. Both cycles, comprising a compressor, condenser, expansion valve, and evaporator, are interconnected via a heat exchanger that serves a dual purpose: functioning as the condenser in the HTC while simultaneously acting as the evaporator in the LTC. CRS commonly comprises two primary configurations: an Auto-Cascade Refrigeration System (ACRS) and a typical two-stage CRS. The standard external cascade cycle encompasses two main types: the two-stage vapor compression cascade refrigeration cycle (CRC) and the two-stage absorption cascade refrigeration cycle (CARC) [27]. CRSs achieve efficient heat transfer and extremely low refrigeration temperatures through the series connection of multiple refrigeration cycles and the coordinated use of various refrigerants. Despite their complex structure, they are powerful in functionality and serve as an essential component in modern refrigeration technology.

Typically, the two-stage CRS has a limitation in terms of its lowest achievable evaporation temperature, reaching only up to −80 °C. When the required evaporation temperature necessitates a further drop, such as within the range of −80 °C to −100 °C, a multistage cascade refrigeration system becomes a feasible alternative. This system comprises three or more sequentially arranged individual refrigeration cycles, capable of meeting the lower temperature demands. Given the escalating costs associated with multistage CRS, it is generally deemed uneconomical. To address additional requirements and enhance efficiency, various studies have focused on structural optimization of CRSs. Incorporating ejectors into CRS has emerged as a notable approach, significantly boosting the performance of CRSs [28,29]. Furthermore, substituting traditional expansion devices with two-phase ejectors has also proven to significantly improve the performance of CRSs [30]. The advantage of incorporating phase change materials into refrigeration systems lies in their inherent ability to significantly reduce temperature fluctuations and thereby enhance the overall system performance. Over the years, researchers have embarked on a myriad of theoretical and experimental endeavors, meticulously examining the performance assessments of refrigeration systems that utilize PCMs. Wang et al. delved into the potential positions of shell-and-tube PCM heat exchangers within a refrigeration system [31]. They positioned the PCM heat exchanger after the compressor, as well as subsequent to both the condenser and the evaporator. These placements, respectively, resulted in a decrease in condenser pressure, an increase in evaporator capacity, and reduced superheat at the compressor inlet. Cheng et al. [32] embarked on a comprehensive study, exploring the potential of a particular shape-stabilized PCM in the fabrication of heat storage condensers. Their rigorous experiments conclusively revealed that the integration of this cutting-edge PCM into the refrigerator design resulted in a remarkable boost in energy efficiency by approximately 12%. Furthermore, this integration significantly enhanced the overall heat-transfer performance of the condensers, marked by a reduction in condensation temperature, an elevation in evaporation temperature, and a notably larger subcooling degree at the condenser outlet.

2.2. The Work-Fluid of Cascade Refrigeration System

Refrigerants play a pivotal role across various segments of the cold chain, including household refrigerators and freezers, supermarket display cabinets, cold storage facilities, and refrigerated trucks. However, amidst escalating environmental concerns, the selection of refrigerants has become increasingly stringent, with more restrictions imposed to ensure environmental sustainability. For energy-saving and environmental problems, much attention has been devoted to the optimization of CRS performance and the selection of refrigerant couples [33,34]. PCMs utilized in refrigeration systems can be categorized into three distinct types: organic, inorganic, and eutectic. Organic PCMs consist of carbon-based compounds, whereas inorganic PCMs encompass metals and hydrated salts. The phase change temperature is a crucial factor in designing refrigeration systems. The essential properties of PCMs, such as their enthalpy of fusion and phase change temperature, can be meticulously analyzed using differential scanning calorimetry (DSC), where variations in heating and cooling rates offer further insights into their behavior [35]. Ambient temperature exerts a notable influence on the performance of PCMs in refrigeration systems [36]. When ambient temperatures drop significantly, the performance of PCMs undergoes a decline. In scenarios involving low thermal loads, the compartmental temperature falls at a quicker rate, leading to inadequate time for the PCM to fully solidify before the compressor halts operation. This results in a reduced effectiveness of the PCM in such conditions.

The phenomenon of global warming triggers significant climate shifts and has profound impacts on biodiversity, ultimately leading to devastating natural disasters, including hurricanes, droughts, and floods. Consequently, in the foreseeable future, Hydro fluorocarbons (HFCs)—potent replacements for hydrochlorofluorocarbons (HCFCs) that exert a climate influence thousands of times greater than carbon dioxide due to their high GWP—will be gradually phased out. These high-GWP HFCs will be substituted with novel, low-GWP alternatives and natural hydrocarbon refrigerants, exemplified by R32, HFO-1234yf, R600a, R290, and other similar products [37]. Based on the fundamental principles of achieving zero Ozone Depletion Potential (ODP) and low Global Warming Potential (GWP) for refrigerant substitution, the environmentally friendly refrigerants R290 and R170 are utilized in the high-temperature cycle and low-temperature cycle of CRS. A selection of potential alternative refrigerants has been identified and presented in

Table 2. Basic physical properties of some refrigerants that are quoted in the article [39].

Refrigerant		Formula Weight/(kg/k mol)	Normal Boiling Point/C	Critical Temperature/C	Critical Pressure/MPa	ODP	GWP	Security Classification
HTC	R32	52.02	−51.7	78.2	5.8	0	550	A2
	R1234yf	114.04	−29.45	94.7	3.38	0	4	A2
	R1234ze	114.04	9.745	109.37	3.64	0	4	A1
	R161	48.06	−37.1	102.2	4.7	0	12	A2
	R1270	42.08	−47.7	92.4	4.67	0	−20	A3
	R290	44.1	−42.2	96.7	4.25	0	−20	A3
	R717	17.03	−33.3	132.3	11.34	0	<1	B2
LTC	R23	70.01	−82.1	25.9	4.84	0	12,000	A1
	R170	30.07	−88.9	32.2	4.87	0	−20	A3
	R41	34.0	−78.1	44.1	5.9	0	97	A2

. The high-temperature cycle (HTC) of CRS can normally be charged as intermediate-temperature refrigerants whose normal boiling point usually ranges from 0 °C to −60 °C, such as R32, R404A, propane (R290), NH₃ (R717), propylene (R1270), R507A, R134a, R410A, and so on [38]. The normal boiling points of refrigerants used in low-temperature (LTC) are usually lower than −60 °C, such as R23, R41, ethane (R170), and N₂O (R744a).

In response, the Montreal Protocol and its subsequent Kigali Amendment have further propelled the global elimination and substitution of these harmful substances. The international community is gradually reducing the use of ozone-depleting substances like CFCs [40]. Besides, seven Chinese ministries and commissions jointly issued the “Green and Efficient Refrigeration Action Plan” in June 2019. CO₂ emerges as a highly suitable and environmentally benign refrigerant option. CO₂ stands as one of the safest natural refrigerants, classified under the esteemed ‘A1’ level of safety by ASHRAE. According to projections, the global refrigerant landscape will evolve to encompass light commercial refrigeration systems (ranging from 0.15 KW to 5 KW) in the future, with CO₂ regularly featured and hydrocarbon refrigerants expected to hold a significant market share by 2025. The CO₂/NH₃ cascade refrigeration system is extensively utilized across diverse CRSs due to the environmentally benign nature of both CO₂ and NH₃.

Therefore, in order to fulfill the unique demand for ultra-low temperatures, such as temperatures at or below −100 °C, and to enhance the operational efficiency of low-temperature refrigeration systems, conducting relevant experimental research is imperative. Given the myriad benefits of mixed refrigerants, coupled with the imperatives of environmental protection, safety, and optimizing system performance, HFC/HFO blends are emerging as a promising substitute, not just in the immediate future but also with a long-term outlook. Sun et al. [39] conducted an exhaustive study on 28 refrigerant groups, focusing on the energy and exergy efficiency of low-GWP refrigerants employed in CRS. Their findings revealed that the combination of R161 for the HTC and either R41 or R170 for the LTC of CRS resulted in superior performance compared to other refrigerant options. Llopis et al. [41] conducted a comparative analysis of the operational performance between four direct two-stage systems and a CRS, utilizing various low-GWP refrigerants. Specifically, they employed R744 as the low-temperature cycle fluid and explored the impact of various high-temperature cycle fluids, including R744, R152A, R134A, R404A, R290, and R1234ze. Their study revealed that the CRS demonstrated outstanding performance, with its efficacy slightly influenced by the choice of HTC refrigerant [42]. The key attributes of the aforementioned fluids are outlined in

Table 3. Physical, environmental, and safety characteristics of the considered refrigerants [41].

Refrigerant	R717	R744	R290	R152a	R1234ze(E)	R404A	R134a
Molecular weight (g mol−1)	17.03	44.01	44.10	114.04	108.4	97.60	102.03
Critical temperature (°C)	−33.3	−56.6	−42.1	−24	−19.0	−26.1	−46.2
hfg at −30 °C (KJ Kg−1)	132.3	31.0	96.7	113.3	109.4	101.1	72.0
hfg at 40 °C (KJ Kg−1)	1099.3	_	307.1	260.0	154.6	163.0	120.3
ʋv at −30 °C (m3 Kg−1)	0.9640	0.0269	0.2586	0.3824	0.2817	0.2259	0.0948
GWP100years	<1	1	~20	133	6	1370	3700
Safety group (ASHRAE standard 34-2010) [43]	B2 (B2L)	A1	A3	A2	A2	A1	A1

.

2.3. CARS

CARS, or Cascade Absorption Refrigeration Systems, typically comprise a combination of an NH₃-H₂O (ammonia–water) system and a LiBr-H₂O (lithium bromide–water) system. In this configuration, the evaporator element of the LiBr-H₂O high-temperature cycle (HTC) fulfills a dual function, offering cooling to the condenser segment of the cycle simultaneously. Figure 3 presents a schematic diagram that illustrates the interconnected nature of these two systems within the CARS configuration [44]. Absorption refrigeration operates on the principle of utilizing heat to drive the refrigeration process instead of mechanical work, as in traditional compression refrigeration. The absorption refrigeration system is an effective way to recover waste heat, which helps to reduce the energy consumption. It relies heavily on the availability of heat sources and can offer significant economic benefits when sustainable or waste heat sources are utilized, making it a suitable choice for scenarios where reliable access to such resources is guaranteed.

In CARS, NH₃-H₂O and LiBr-H₂O are commonly used as refrigerants. However, the LiBr-H₂O absorption system faces challenges when the temperature of the LiBr solution drops too low or its concentration becomes excessively high. This can lead to crystallization, which impedes efficient heat and mass transfer between the absorber and generator. Given the low evaporating pressure, maintaining high airtightness becomes crucial. Consequently, operating the LiBr-H₂O system at a condensation temperature exceeding −40 °C proves difficult [45]. The primary advantages of the NH₃/H₂O working fluid pair lie in their high miscibility, allowing for seamless integration, their substantial latent heat of evaporation, ensuring efficient heat transfer, and their capability to attain cooling temperatures below 0 °C, broadening the range of potential applications. However, the application of ammonia is significantly limited owing to its toxicity and flammability hazards, coupled with the narrow boiling point margin between ammonia and water, which necessitates the use of distillation equipment [46]. Moreover, ammonia’s pronounced corrosivity towards non-ferrous metals poses additional challenges in terms of material selection and equipment durability.

Another prevalent option among cascade refrigeration system (CRS) designs is the two-stage vapor compression cascade refrigeration system (CCRS). In response to emerging applications, the development of novel materials, and the absorption of innovative working fluids, it becomes feasible to propose novel absorption cycles that exhibit enhanced efficiency and cater to a wider range of heat source-driven temperatures and solution concentrations. Wang et al. [47] proposed integrating phase change microcapsules within the absorbing working pairs as a means to enhance the effective heat capacity. This strategy aims to reduce the need for solution recirculation and consequently lower the power consumption associated with atomization. Their investigation, employing an adiabatic spray absorption model specifically tailored for lithium bromide solutions, revealed that the incorporation of PCMs leads to a decrease in atomization power consumption while maintaining equivalent cooling capacities. However, this advantage comes at the cost of an extended absorption time.

2.4. CCRS

The vapor-compression cooling system has garnered widespread adoption for cooling fruits and vegetables in cold storage facilities, owing to its proficiency in sustaining a broad spectrum of temperature values and maintaining relative humidity levels within the optimal range of 80% to 90% [48]. Compression refrigeration represents a cost-effective and highly efficient approach, making it the perfect choice for a diverse range of food products that necessitate meticulous temperature regulation. A cascade vapor compression refrigeration system comprises two distinct single-stage vapor compression refrigeration cycles, interconnected via a cascade heat exchanger (CHE) [49]. CCRS is composed of two distinct subsystems. One is a HTC, which usually uses NH₃ as the refrigerant, and the other is a LTC, usually with CO₂ as refrigerant (see Figure 4) [50]. The system functions by harnessing the phase transformations of the refrigerant fluid, undergoing successive pressure variations to enable the cooling process.

A preponderance of vapor compression refrigeration systems have historically relied on HFCs as refrigerants, which are fluorinated greenhouse gases that collectively exert a significant environmental impact. CO₂ emerges as a promising substitute for HFCs as a refrigerant. Its application in transcritical and cascade systems underscores its potential as a sustainable, long-term solution. CO₂ boasts inherent thermodynamic attributes that distinguish it from other refrigerants, notably its low critical temperature [51]. Numerous studies have conclusively demonstrated that CARS outperform two-stage CRS. Consequently, an analysis of CARS’ performance, employing CO₂ and NH₃ as working fluids, has been conducted to facilitate the generation of cold energy at reduced temperatures. Hence, the CO₂/NH₃ cascade refrigeration system has garnered widespread adoption in the food storage and distribution sector. Wang et al. [52] delved into the performance of two-stage Carbon Dioxide Cascade Refrigeration Systems (CCRSs) utilizing CO₂ and NH₃ as refrigerants, elucidating the influence of various parameters on system efficacy. Notably, as the evaporation temperature rises, the COP of CCRS improves. Li et al. [53] conducted an experimental study on a modified version of the CCRS, which incorporated an ejector technology within the LTC environment. To evaluate the performance of this innovative system, they designed an ultra-low-temperature freezer prototype equipped with the proposed modifications. The test results revealed that the prototype, incorporating the ejector-based system, demonstrated a significant reduction in energy consumption compared to a conventional baseline freezer, highlighting the potential energy-saving benefits of this novel approach.

Figure 4. The schematic of the CO₂/NH₃ cascade refrigeration system [54].

Figure 4. The schematic of the CO₂/NH₃ cascade refrigeration system [54].

Despite its commendable efficiency and versatility, the vapor-compression refrigeration system confronts significant constraints, with its dependency on electricity being a pivotal limitation. This reliance renders the system impractical for implementation in regions plagued by limited or unreliable power supply. The implementation of CCRS involves a multitude of considerations beyond merely the efficiency of the cycling process. A crucial factor is the ever-rising cost of power supply, which exacerbates as the number of cascade systems increases. Therefore, it is absolutely necessary to not only evaluate the efficiency improvements but also tackle the comprehensive range of economic challenges that stem from these systems. This includes managing operational expenses, evaluating long-term financial viability, and ensuring cost-effectiveness in the face of potential energy consumption hikes. Keshtkar et al. [55] conducted a thermoeconomic optimization study of CCRS operating with R717/R744 refrigerant pairs. This investigation employed a Multi-Objective Optimization Strategy (MOS) to achieve an optimal balance between thermodynamic efficiency and economic cost in CCRS. Additionally, two Single-Objective Optimization Strategies (SOSs) were implemented, focusing on exergetic optimization and cost optimization, respectively, to further refine the system’s performance. As per Sholahudin et al. [56], there are two primary objective functions requiring optimization: the overall annual cost, encompassing both capital and operational expenses, and the total exergy destruction. The findings indicate a clear inverse relationship between the total cost and both exergy destruction and the coefficient of performance, with the latter two notably improving as the total cost rises.

2.5. ACRS

Auto-Cascade Refrigeration System (ACRS), an innovative approach, harnesses a blend of refrigerants while employing a single compressor to effectively implement cascade refrigeration, thereby affording a more compact and streamlined design. In comparison to two-stage CRS, ACRS boasts lower manufacturing costs, a smaller device footprint, and a simplified system operation. Implementing automatic cascade techniques within compression refrigeration systems leads to substantial benefits, including a notable reduction in the required number of compressors, an enhancement of the heat transfer and coupling capabilities within the circulation system, and a lowering of the evaporating temperature, thereby enhancing overall system efficiency and performance. However, ACRS’s reliance on a blend of two or three refrigerants with varying boiling points poses challenges in maintaining precise refrigerant mass ratios, operational stability, and adapting to load variability, all of which are crucial factors impacting its performance. Due to its design and working reliability with high-level performance, ACRS has a wide application area that obtains a low temperature of −60 °C [57]. Another advantage of the ACRS lies in its enhanced flexibility in regulation. The composition distribution characteristic of ACRS utilizing mixed refrigerants is notably more intricate compared to refrigeration cycles that solely employ pure refrigerants.

Furthermore, the utilization of mixed refrigerants mitigates irreversible losses during heat transfer by reducing the temperature slip. Given the numerous limitations and suboptimal performance associated with pure refrigerants, the concept of mixed refrigerants has emerged as a promising solution. These blends, consisting of two or more pure refrigerants in specific proportions, aim to enhance the performance of refrigeration systems while mitigating the drawbacks of using single refrigerants. Depending on the azeotropic properties of the mixed solution, these blends can be categorized into azeotropic refrigerants and zeotropic refrigerants. The selection of the working fluid holds a pivotal role in determining the overall performance of the thermodynamic system. Research efforts in this regard have been primarily directed towards elucidating the shift relationship between the circulating and charging concentrations of the mixed refrigerant, along with exploring the impacts of system configuration [58], operating conditions [59], and two-phase interactions [60]. As a result, ACRS possesses distinct advantages for applications in low-temperature environments, making research into this technology both significant and meaningful.

Despite its numerous advantages, the ACRS is recognized for having a comparatively low COP. Wang et al. [61] directed their attention towards the utilization of ethane blends (specifically, R170 combined with R290, R600, and R600a) for applications requiring temperatures down to −60 °C in a two-stage air conditioning system featuring dual separators. Du et al. [62] conducted a study on an ACR system that operated using a blend of R23 and R134a refrigerants. Their research delved into the effects of varying cooling-water temperatures and cycle fluxes on the system’s performance. The three-stage auto-refrigerating cascade system has employed the zeotropic mixture comprising R290/R23/R14 and R1270/R170/R14 to conduct an exhaustive analysis of both exergy and energy performance. The incorporation of an ejector device has been identified as an effective means of enhancing the overall performance of the system. The ejector boasts numerous advantages, including its low cost and the absence of moving parts, making it an appealing choice for the advancement of high-performance refrigeration systems. Yan et al. [63] developed and examined a novel ejector-enhanced advanced compression-refrigeration cycle (EARC) utilizing an R134a/R23 refrigerant blend, aimed at recovering work losses incurred during the throttling process. As shown in Figure 5, the schematic diagram of the basic ARC system that is widely employed in low-temperature refrigeration fields is presented. As shown in Figure 6, a modified ejector-enhanced auto-cascade refrigeration cycle is proposed. They conducted a performance comparison between this proposed system and a conventional ACRC, leveraging the principles of the first and second laws of thermodynamics. Additionally, they evaluated the influence of key parameters on the system’s overall performance. Tan and his team [64] developed an innovative auto-cascade ejector refrigeration cycle (ACERC), building upon the fundamentals of traditional ejector refrigeration and auto-cascade refrigeration technologies, with the aim of achieving even lower refrigeration temperatures. Their study conducted a comprehensive analysis of the exergetic efficiency, economic feasibility, and environmental footprint of both the auto-cascade refrigeration system (ACRS) and the ejector-enhanced internal auto-cascade refrigeration system (EACRS). The findings underscored the superiority of the EACRS over the ACRS in terms of its holistic performance, highlighting its advantages across key performance indicators.

3. Cascaded Refrigeration Systems Applications in the Cold Chain

3.1. Precooling

The maintenance of optimal temperature levels is crucial for preserving the quality of fresh produce when stored in refrigerated conditions. Precooling, which refers to the process of reducing the temperature of an object, material, or environment to a predetermined value through physical or chemical methods prior to its entry into the next processing stage, is widely applied in various fields such as food preservation, industrial manufacturing, and energy utilization. As Elansari’s research [65] highlights, the delay of precooling highly perishable produce by a mere hour can result in a substantial shortening of its shelf life by an entire day. There are many precooling methods, such as air cooling, cold warehouse precooling, cold water precooling, refrigerant precooling, vacuum precooling, and pressure precooling [66]. Air cooling utilizes the flow of air to carry away heat, achieving precooling of objects or environments. This method is simple and easy to implement but is greatly influenced by factors such as ambient temperature and wind speed. Cold water cooling involves circulating water to effectively dissipate heat, resulting in rapid temperature reduction. This method boasts advantages like high efficiency and even cooling distribution, albeit at the cost of substantial water consumption. Vacuum precooling, which operates under vacuum conditions, relies on the evaporation of water to swiftly remove heat, facilitating rapid temperature reduction. This method is particularly well-suited for high-moisture foods, including fruits and vegetables, due to their natural abundance of water content. Decreasing the storage temperature not only diminishes the respiration rate of fruits and vegetables but also slows down their metabolic and water loss rates. Refrigerant precooling harnesses the unique properties of refrigerants, namely their low boiling points and high latent heat capacities, to achieve precooling through the process of evaporation and subsequent heat absorption. This method is ubiquitous in refrigeration systems, demonstrating its efficacy and widespread applicability. Pressure or forced air precooling stands as a pivotal technique for the treatment of fruits and vegetables, offering an effective means of preserving their freshness and extending their shelf life [67]. This technique involves creating pressure differentials across packaging boxes as cold air is forcibly circulated through stacks or boxes. As a result, this method is gradually being adopted in China and is projected to gain widespread usage in the future, especially given the extensive research conducted by Chinese researchers. Despite their numerous benefits, these cooling technologies are not without limitations. Forced-air cooling, for instance, can result in water loss in certain fruits, whereas hydrocooling necessitates the use of significant amounts of water. Each method has its own advantages and disadvantages, and the choice of method should be made based on a comprehensive consideration of factors such as the type of product, quantity, and packaging conditions.

Table 4. Classification of chilling-sensitive food products with respect to their lowest safe temperatures for storage [68].

Food Products	Lowest Safe Temperature (°C)
Cranberry and asparagus	3
Orange, durian, cowpeas, cactus pear, and guava	5
Pepper, pomegranate, okra, pineapple, and olive	7
Papaya, lime, cucumber, passion fruit, eggplant, watermelon, tomato (ripe), grapefruit, plantains, and mango (ripe),	10

presents a classification of chilling-sensitive food products, grouped according to their respective minimum safe storage temperatures. This classification serves as a valuable reference for the development of suitable cooling technologies tailored to each product’s unique requirements.

Precooling stands as the initial and crucial step in the preservation of food, playing a pivotal role in ensuring its quality and longevity [69]. This process involves exposing vegetables and fruits to a carefully controlled, relatively low-temperature medium or device, with the objective of swiftly reducing their temperature to the optimal level required for preservation. The perishability of fruits and vegetables, as agricultural commodities, is heavily influenced by several factors, including crop water content, tissue softness, and metabolic activity, among others. By rapidly achieving this desired temperature, precooling effectively inhibits microbial growth and enzymatic activity, thereby safeguarding the freshness and nutritional value of the produce. CRSs play a pivotal role in the intricate workings of the cold chain. They are specifically designed to cater to the diverse temperature requirements that arise across various stages of the cold chain process, ensuring the preservation and quality maintenance of perishable goods such as food and pharmaceuticals.

In refrigeration systems, precooling treatment can enhance the system’s energy efficiency ratio (EER) and environmental friendliness. By more fully utilizing the cooling capacity of refrigerants and reducing unnecessary energy losses, it achieves the goals of energy conservation and emission reduction. CRSs are capable of providing efficient cooling effects across a wide range of temperatures. In the precooling stage of the cold chain, such systems can rapidly reduce the temperature of food or goods, minimizing the risk of spoilage and deterioration. By adjusting the flow rate and temperature of refrigerants in different cycles, CRS can precisely control the temperature during the precooling process, ensuring the quality and safety of food or goods. In practical applications, CRSs have been widely utilized in various segments of the cold chain, encompassing precooling, refrigerated storage, freezing, and transportation. With the continuous development of the cold chain industry and technological advancements, the application prospects of cascaded refrigeration systems in cold chain precooling will become even broader. In the future, these systems are likely to evolve towards higher efficiency, environmental friendliness, and intelligence, catering to the ever-growing refrigeration demands of the cold chain industry.

3.2. Subcooling

Compared to precooling, supercooling refers to the phenomenon where a liquid remains in its liquid state at a temperature below its theoretical freezing point or ice point; whereas precooling is the process of rapidly reducing the temperature of a product to an appropriate level. Subcooling refers to the procedure where a refrigerant liquid is cooled down to a temperature that is lower than its saturation point at a specific pressure level [70]. This means that the refrigerant liquid is cooled further after it has condensed from a vapor state, releasing additional heat and lowering its temperature. In refrigeration and air conditioning systems, subcooling is often used to improve system performance and efficiency. Subcooling reduces the temperature difference between the refrigerant entering the evaporator and the temperature of the medium being cooled. This reduces the required refrigerant flow rate and compressor work, leading to energy savings and improved overall system efficiency. By lowering the temperature of the refrigerant fluid prior to its entry into the evaporator, a heightened capacity to extract heat from the environment (such as from food items) during the evaporation process is achieved, resulting in an enhanced cooling performance.

In refrigeration systems, subcooling is often employed in the intermediate or lower-temperature stages to maximize the system’s performance. Refrigeration systems with subcooling can achieve a wider range of temperatures, making them suitable for a variety of applications in the cold chain, from chilled storage to deep-freezing. Subcooling the refrigerant prior to its expansion stage is a well-established and efficient technique for enhancing the COP of refrigeration systems. Optimizing the refrigerant’s thermodynamic cycle, subcooling can contribute to reducing energy consumption and improving the overall efficiency of the refrigeration system. Yan et al. [71] conducted a study on a modified vapor-compression refrigeration cycle (MVRC) equipped with a PCM subcooler. In this cycle, the utilization of PCM for cold storage serves to enhance the subcooling degree, thereby improving the overall performance of the refrigeration cycle. Specifically, the coefficient of performance increased by 3.3% to 10.5%, while the volumetric cooling capacity surged by 5.5% to 25.4% as compared to a conventional vapor-compression cycle. In the research conducted by Nandanwar et al. [72], PCMs were employed as an innovative approach to extract heat from the refrigerant at the condenser outlet. This innovative process entails the PCM undergoing melting, which in turn leads to subcooling of the refrigerant. The outcome of this subcooling process, along with a decrease in the net work required, resulted in a notable 7% increase in the COP of the refrigeration system. Riahi et al. [73] have undertaken a parametric analysis of a vapor compression refrigeration system that incorporates a PCM storage tank, with the objective of enhancing the condenser’s subcooling temperature. The results indicate that modification significantly enhances the availability of cooling energy during the phase of cold energy discharge, achieving an augmentation from 39.67% to 54.60% over the conventional system. Thus, the system’s cooling capacity and efficiency during periods of peak demand are significantly boosted.

In the cold chain, maintaining stable and optimal temperatures for perishable goods is crucial. Subcooling plays a pivotal role in ensuring that products are consistently maintained at their optimal temperature throughout the entire distribution process. This not only extends their shelf life significantly but also safeguards their quality, ensuring that they retain their freshness, flavor, and nutritional value until they reach their intended destination [74]. By optimizing the refrigerant’s thermodynamic cycle through the application of subcooling, it becomes possible to minimize energy consumption and enhance the overall operational efficiency of the refrigeration system. This optimization allows for more efficient heat transfer and cooling processes, resulting in a more energy-efficient system that meets the demands of the cold chain effectively. CRSs have found extensive applications in subcooling, enabling the achievement of very low temperatures and improved system performance and efficiency.

4. Phase Change Material

4.1. Characteristics of Phase Change Material

4.1.1. Classification of Phase Change Materials

In order to enhance the quality of food, maintaining an appropriate temperature during both storage and transportation is crucial, as this practice significantly contributes to minimizing food waste. PCMs have surfaced as a promising approach for ensuring optimal temperature maintenance within the cold chain, especially crucial for safeguarding temperature-sensitive commodities like vaccines, pharmaceuticals, and perishable food items [75]. PCMs are substances that exhibit remarkable thermal stability during their phase transition, maintaining a consistent temperature throughout the process. This unique property allows PCMs to be effectively utilized as thermal energy storage media, balancing the heat load and reducing the energy consumption of refrigeration systems. These materials can be grouped based on the specific phase transition they exhibit. Solid–solid PCMs primarily encompass polyols, metal compounds, and polymer materials. However, solid–solid PCMs are often overlooked for practical usage due to their relatively low latent heat of fusion. Conversely, solid–liquid PCMs, which can be further categorized into organic, inorganic, and eutectic materials, are more commonly utilized [76]. Despite the evident advantages of solid–liquid phase change material, its transformation between solid and liquid phases poses a significant challenge. During these transitions, there is a risk of liquid leakage, which not only restricts its widespread adoption as an energy storage medium but also poses environmental hazards and escalates costs.

Inorganic PCMs encompass a wide array of materials, such as crystalline hydrated salts, molten salts, metal alloys, and more. Among these, crystalline hydrated salts are particularly popular due to their advantageous properties. The merits of inorganic PCMs lie in their broad applicability, cost-effectiveness, high thermal conductivity, and substantial volumetric heat storage density, making them attractive options for various applications [77]. Given the inherent drawbacks of both inorganic and organic PCMs for cold storage, researchers have devised a solution by integrating these two types of materials to create composite PCMs for cold storage. The development of composite PCMs for cold storage effectively addresses the limitations of both inorganic and organic PCMs, enhancing their overall performance and broadening their range of applications. Figure 7 clearly illustrates the comprehensive classification of PCMs, providing a detailed overview of the various types within this important group of materials.

4.1.2. Selection of Phase Change Materials

The selection of PCM depends on the phase change temperature, thickness, thermal loads, and the specific cooling and heating demands of the intended application. Additionally, PCMs exert considerable influence on various other parameters within the refrigeration system, all of which must be given equal weight and consideration in the selection process. The kinetics of the phase change process, including the rate of melting and solidification, can impact the performance of the system. Faster phase change rates can improve response times and efficiency. The critical parameter in the phase transition process is the latent heat of fusion or solidification, representing the quantity of heat that is either absorbed or released during this transformation [79]. PCM must be able to absorb and release sufficient heat to meet these demands. A higher latent heat capacity allows for greater energy storage and improved system efficiency. It is imperative that the latent heat capacity and nucleation properties of the PCM surpass those required by the selected application. The key selection criteria encompass a multitude of factors, including thermal, physical, kinetic, and chemical attributes, as outlined in

Table 5. The properties of phase change cold storage materials used in cold chain logistics [80].

Category	Property
Thermal	Suitable phase transition temperature and latent heat with good heat transfer characteristics.
Physical	High density, small volume change, with favorable phase equilibrium
Kinetic	Sufficient crystallization rate; no supercooling.
Chemical	Long-lasting stability; no toxicity; nonflammable

[80]. When incorporating PCMs into the cold chain transportation of food, it is crucial to not only evaluate their traditional properties but also to meticulously consider their safety and potential corrosiveness. In cases where it is deemed necessary, the aspects of leakage prevention and flame retardancy must also be addressed. Ensuring the non-toxic and harmless nature of PCMs is paramount to safety, safeguarding against contamination of fruits, vegetables, and any detrimental effects on human health. However, it is noteworthy that many hydrated salt melts exhibit corrosive properties towards metals, posing a threat to the longevity of container materials used for PCM storage and transportation. Consequently, addressing and mitigating these corrosive effects is equally crucial [81].

4.1.3. Cold Thermal Energy Storage

Employing cold thermal energy storage represents a practical and effective strategy for enhancing energy efficiency, operational flexibility, and the overall robustness of refrigeration processes [82]. The cold thermal energy storage (CTES) process involves the deposition of cold thermal energy into a predetermined medium, facilitating its subsequent retrieval on demand. During the charging phase, this methodology enables the sequestration of thermal energy at reduced temperatures within the chosen medium. Subsequently, when required, the accumulated low-temperature thermal energy is extracted and dispensed to the end user. The storage of frigid energy can be accomplished by either modifying the internal energy state of the storage medium or eliciting a phase change within it. The occurrence of phase transitions during the storage and retrieval of energy significantly enhances the energy storage capacity of the system when utilizing PCMs [36]. The practice of storing thermal energy at low temperatures often incorporates both sensible and latent storage methodologies. Sensible heat storage (SHS) constitutes the most direct approach to energy storage, fundamentally leveraging the distinct heat capacity and density properties of the material in both the charging and discharging phases. However, it is inherent in this process that heat losses occur, resulting in the storage of only a limited amount of energy [83]. In contrast, latent heat storage (LHS) proves to be more efficient than SHS due to its heat transfer mechanism being intricately linked to phase transitions, such as solid to liquid, liquid to gas, or even solid to solid, allowing for a more substantial energy storage capacity. PCM utilization currently holds a pivotal position in the promising landscape of sustainable energy technologies, facilitating the efficient accumulation of thermal energy.

4.2. PCM Applications on a Refrigeration System

Refrigeration and air-conditioning systems exert a substantial environmental influence and actively contribute to the phenomenon of global warming [84]. The escalating energy consumption of refrigeration and air-conditioning systems results in substantial indirect emissions, underscoring the urgency for a frugal and sustainable solution to energy conservation. Enhancing the efficiency of refrigeration systems is pivotal, and this improvement largely hinges on factors such as the compressor’s efficiency, thermal load, ambient temperature conditions, and the refrigerant employed. A notable approach to minimizing the energy consumption of cascaded systems involves the utilization of PCMs. The heat energy harnessed from PCM mimics a natural process, earning it the designation of ‘green energy’ due to its environmentally friendly nature.

Minimizing the compressor’s running time emerges as a key energy-saving strategy, aiming to decrease the overall power consumption of the system. PCMs possess the capability to absorb heat, known as the enthalpy of fusion, through undergoing a phase transition from a solid state to a liquid state. This process occurs while maintaining a constant temperature within the enclosed space as the PCM melts. Consequently, during a specific duration when the PCM is fully undergoing its phase change during melting, the temperature inside the cabinet remains stable, thereby prolonging the compressor’s off-cycle period [85]. Improving the evaporator’s efficiency emerges as the primary objective in enhancing the performance of refrigeration systems. This optimization is contingent upon the specific thermal load and the type of PCM utilized; this improvement can lead to a notable reduction in operating temperatures, with a range of approximately 2 °C to 5 °C. Visek et al. [86] conducted an experimental study on domestic refrigerators/freezers, incorporating PCMs placed in direct contact with the visible radiant cooling (RC) evaporator. Their research demonstrated that this innovative approach achieved a noteworthy 5.6% overall energy savings for the appliance, underscoring the potential benefits of PCM integration in enhancing energy efficiency. One of the primary motivations for employing PCM in the condenser is to minimize its temperature. By incorporating PCM into the condenser, it enables the extension of heat rejection beyond the compressor’s operational period, thereby achieving a lower condensation temperature through enhanced heat transfer within the condenser [87].

Table 6. Advantages and disadvantages of using PCM in various applications on a refrigeration system [88].

Component	Advantages	Disadvantages
Evaporator	Enhancement of system performance	Higher condensation temperature
	Supports in case of power outages	Longer compression operating time during a single cycle
	Decrease in refrigerator noise
	Shorter compressor overall “On-time” ratio
	Reduction in electricity consumption in peak hours
	A decrease in overall costs of the refrigeration system
Condenser	Higher COP	More refrigerant displacement losses
	Shorter compressor global “On/Off” ratio	Heat accumulation
	Lower condensation temperature and pressure
	Higher subcooling degree
	Faster, stable condition of the refrigerator

comprehensively outlines the advantages and limitations of PCM applications on the key components of a refrigeration system.

The application of PCM in refrigeration systems has demonstrated notable enhancements in system performance, particularly by optimizing compressor cycling and significantly reducing electricity consumption. Numerous researchers examined the consequences of incorporating PCMs within the evaporator, condenser, and compartment space of refrigeration systems. Ko et al. [26] investigated the energy-saving capabilities of PCMs when integrated into a cascade refrigeration system that employs CO₂ as the refrigerant. The model of the studied cascade system is presented in Figure 8. The system comprises two distinct R-744 refrigeration cycles: the high-pressure system (HPS), operating as a transcritical cycle, and the low-pressure system (LPS), functioning as a subcritical cycle. These systems are seamlessly interconnected through an intermediate heat exchanger (IHX), facilitating the efficient transfer of heat between the LPS and HPS. Within the evaporator, the refrigerant effectively absorbs heat from the compartment air, while in the gas cooler, it proficiently rejects heat to the ambient environment. The PCM is positioned between the evaporator and the compartment space, serving as an intermediary heat transfer medium. It absorbs the heat from the air within the compartment, effectively bridging the gap between the refrigerated space and the refrigerant circulating through the evaporator. The current findings distinctly underscore the advantageous effects of PCMs in terms of power consumption reduction for low-temperature refrigeration systems utilizing CO₂ as the refrigerant. Liu et al. [89] conducted a rigorous evaluation of an air-cooled household refrigerator’s performance enhancement through the incorporation of cold PCMs. Their study thoroughly examined various control strategies and revealed that integrating PCMs into the refrigerator led to a substantial decrease in energy consumption by 18.6% compared to the standard scenario. Additionally, the use of PCMs significantly reduced the compressor’s operating time ratio by 13.6%, emphasizing their potential to significantly boost the refrigerator’s energy efficiency. Wang et al. [90] devised and fabricated a prototype showcasing the effective utilization of PCMs within refrigeration systems, and underwent rigorous testing. The findings indicate that by decreasing the temperature of the sub-cooled refrigerant, energy savings of up to 8% can be realized under the climatic conditions prevalent in the United Kingdom.

4.3. Use of Phase Change Materials in Cold Chain

4.3.1. PCM in Refrigerated Transportation

As the government’s focus on food safety and waste issues intensifies, food transportation has emerged as a pivotal step within the realm of cold chain management. Common cold chain transportation solutions encompass a range of equipment, including incubators and refrigerated trucks [91]. Innovative methodologies, including the integration of hybrid technology alongside the utilization of PCMs within traditional refrigeration systems, have been proven to enhance both the quality and economic efficiency of transportation processes. By incorporating PCMs, we can significantly decrease carbon emissions and contribute to energy conservation efforts. During the cold chain transportation of these perishables, PCMs predominantly function as refrigerants, providing heat insulation, maintaining precise temperature control, and facilitating energy savings. In the realm of cold chain transportation, PCMs assume pivotal roles, encompassing refrigeration, precise temperature control, effective heat insulation, and substantial energy savings.

Burgess et al. [92] embarked on a meticulous research endeavor focused on optimizing a PCM storage system tailored for integration into a portable cold chain delivery system, designed specifically to ensure the safe and efficient transportation of perishable food items. In the context of food cold chain transportation, PCMs primarily function as refrigerants, providing heat insulation, enabling precise temperature control, and contributing to energy savings. Their integration into this process is instrumental in maintaining the integrity and freshness of perishable goods during transportation. Hubbard has triumphantly integrated the commercial utilization of thermal energy storage (TES) systems employing PCMs within the cold chain transportation sector. These refrigerated vehicles are now equipped with eutectic plates, which enable the efficient loading of PCMs during stationary intervals. The innovative setup interconnects the PCM unit with the cargo compartment, facilitated by a thermostat-controlled fan, ensuring seamless air circulation and the efficient transfer of coolness from the PCMs. This technology has proven to be highly effective in the transportation of deep-frozen goods and ice cream products, consistently maintaining the required temperatures throughout the journey, thereby enhancing product quality and safety [93]. Liu et al. [94] conceived of an innovative refrigeration system utilizing PCM to sustain optimal thermal conditions within refrigerated trucks. A key advantage of employing PCM for maintaining low temperatures lies in its ability to obviate the need for a conventional, on-board refrigeration system. Furthermore, this innovative approach boasts reduced energy consumption and significantly lower local greenhouse gas (GHG) emissions, making it a more environmentally friendly and efficient solution for food transportation. The configuration of the refrigeration system is shown in Figure 9. The PCM is encapsulated into thin, flat containers, which are contained in a well-insulated case (referred to as the phase change thermal storage unit). The research outcomes underscore that incorporating phase change thermal storage unit into the thermally insulated cargo space significantly enhances temperature control efficacy. Remarkably, the phase change thermal storage unit sustains a wide temperature range from −12.3 °C to −16.5 °C for prolonged periods of 16.6 h and 10 h, respectively, while simultaneously achieving substantial cost savings in energy consumption, ranging from 15.4% to 91.4% in comparison to traditional refrigeration methods.

4.3.2. PCM in Cold Storage Boxes

Traditional refrigeration systems are susceptible to cold chain disruptions in the event of sudden equipment failure. In stark contrast, cold storage refrigeration systems proficiently counteract sudden temperature fluctuations, significantly diminishing the chances of cold chain disruptions and ensuring the integrity of perishable goods. Compared to traditional refrigeration, cold storage refrigeration offers: mechanical wear-free operation, simplicity, ease of maintenance, diesel-free usage, reducing greenhouse gas emissions, and promoting environmental sustainability. The cold storage box, an innovative and highly efficient piece of technology within the cold chain logistics realm, primarily consists of two integral components: cold storage units and an insulation box. This configuration is clearly illustrated in Figure 10 [95]. The cold storage box boasts substantial advantages over the conventional mechanical refrigeration insulation box, primarily in its heightened environmental friendliness and energy efficiency. By strategically positioning cold storage panels atop the facility, it proficiently dampens temperature variations and minimizes energy expenditure during defrosting cycles, ultimately elevating the quality preservation of frozen foods housed within. Xu et al. [96] have achieved a groundbreaking innovation by encapsulating PCM within a polyethylene-based cold storage panel. This cutting-edge panel was seamlessly integrated into both expanded polyurethane (EPU) and vacuum insulation panel (VIP) insulation boxes, resulting in the development of advanced cold chain transportation equipment that offers unparalleled performance. Kozak et al. [97] undertook a meticulous and multifaceted examination, encompassing both experimental and theoretical approaches, of two distinct cold storage systems. To guarantee comprehensive solidification, the PCM underwent rigorous cooling procedures, ultimately achieving a temperature of −70 °C. Paquette et al. [98] enhanced the thermal insulation of a food transport box by incorporating aluminum foil onto its inner surface, effectively mitigating thermal radiation and achieving a reduction in food temperature rise by over 10%. Based on the aforementioned research, it is evident that installing cold storage panels on all surfaces of the box results in the most even temperature distribution within the box’s interior. However, the literature available to date offers limited discourse on the structural dimensions of the cold storage box. Devrani et al. [99] undertook a research endeavor wherein they altered the shape of a cold storage container, transitioning from a cuboid to a cylindrical form, and subsequently simulated the distribution of temperatures within its interior. Their observations pointed to a noteworthy finding: the cuboid configuration consistently demonstrated higher thermal permeability compared to the cylindrical design. Ray and his colleagues conducted a numerical investigation into the cooling efficacy of portable cold storage boxes utilizing PCM across a temperature spectrum ranging from −55 °C to −40 °C. Their comparative study focused on cuboidal and cylindrical configurations, both sharing an equal volume. The findings revealed that under both environmental conditions tested, the cylindrical structure outperformed the cuboid in terms of thermal performance. This superiority can be attributed to the cylinder’s lower surface area-to-volume ratio, which enhances heat management efficiency [100].

These materials effectively sustain the desired low-temperature environment for stored items, simultaneously functioning as a protective barrier against external influences, thereby enhancing the safety and quality of the cold chain transport process. However, it is crucial to factor in the associated costs during practical implementation. The research endeavors in this field can be summarized and categorized into two main avenues: reducing external convection and radiation (where altering the box’s shape is the primary strategy employed at this juncture) and enhancing internal cold insulation (which predominantly explores thermal insulation materials, PCMs, and the strategic positioning of cold plates). Additional research can be undertaken to enhance the performance of cold storage boxes to meet diverse requirements.

4.3.3. PCM in Domestic Refrigerators and Freezers

The primary function of refrigerators is to ensure the preservation of food by maintaining a designated low-temperature environment. It is estimated that China possesses approximately 0.2 billion refrigerators, collectively consuming between 30% and 40% of the country’s residential electrical power demand [32]. Given the alarming surge in refrigerator usage and their corresponding total electrical consumption globally, the incorporation of PCM in refrigerators emerges as a promising approach to enhance their efficiency and curtail energy consumption. Commonly, PCMs are applied to the walls, top, and bottom of the storage compartment in refrigerators. This stored thermal energy can be harnessed in domestic refrigerators, enhancing their operational efficiency and elevating the quality of food preservation within them [101].

Additionally, in the event of a power outage, the PCMs offer an added benefit by providing a sustained cooling effect, helping to preserve the food. Khan and Afroz [102] conducted an experimental study on a PCM-based refrigerator equipped with a single evaporator and a single door. Their findings revealed that, contingent upon the thermal loads and the specific PCM employed, the evaporator temperatures were observed to be approximately 2–5 °C higher compared to conventional refrigerators. Liu’s et al. [94] conducted an experiment to investigate the impact of incorporating a cold storage panel on both frost-free and frost-prone air coolers. Their findings revealed that the cold storage temperature was notably lower in the presence of the cold storage panel compared to its absence. Furthermore, during the defrost cycle, when the refrigerator fan accumulated frost, the inclusion of the cold storage panel led to a substantial reduction of 47.20% in the temperature fluctuations within the cold storage area. Lu et al. [103] utilized differential scanning calorimetry (DSC) to test ternary salt aqueous solutions with varying salt combinations and concentrations, resulting in a phase change refrigeration material suitable for use in refrigerators. This material is capable of maintaining temperatures below −17 °C, ensuring effective temperature control for refrigerated storage. It was discovered that the HCO₂Na/KCl/H₂O ternary solution exhibited a phase transition temperature of −23.5 °C. Notably, the properties of this solution remained largely unchanged after undergoing multiple freezing-thawing cycles, indicating its potential for reliable use in frozen food storage and transportation applications. Azzouz et al. [104] performed a comprehensive numerical analysis to assess the effectiveness of affixing a PCM slab to the exterior surface of a household freezer’s evaporator. Their findings underscored the profound impact of the PCM on augmenting the evaporator’s heat transfer capabilities, facilitating a notable rise in the evaporating temperature. Consequently, this enhancement translated into a significant boost in the system’s overall energy efficiency. The research conducted by Gin et al. thoroughly analyzed the effects of door openings, defrosting cycles, and power outages on the operational efficiency of a vertical freezer. In their study, the researchers innovatively added a PCM, specifically an aqueous solution of ammonium chloride maintained at a temperature of −15.7 °C, onto aluminum plates that were securely fixed to the interior walls of the freezer. This strategic implementation of PCM led to a notable reduction of approximately 7 to 8% in the overall energy consumption during instances of door opening, demonstrating its potential to enhance energy efficiency and sustainability in freezer operation [105]. Sonnenrein et al. [106] conducted a comprehensive evaluation of the influence exerted by latent heat storage elements on the condenser temperature of a commercial household refrigerator. Their findings clearly demonstrated that the implementation of PCM significantly lowered the condenser temperature, subsequently resulting in a notable reduction in power consumption.

5. Discussion

In China’s food cold chain industry, promoting and applying PCM technology holds significant importance for promoting energy conservation and emission reduction, as well as achieving sustainable development [107]. The use of phase change materials in the cold chain industry represents a significant advancement in temperature control and energy efficiency. The high latent heat, exceptional stability, and reusability of PCM can compensate for the limitations and drawbacks of conventional cold chain logistics, enabling efficient and secure cold chain transportation. In recent years, a plethora of review articles authored by experts have been published, focusing on the application of PCMs in cold chain logistics.

Table 7. Abstracts of the reviews on phase change materials for cold chain logistics [95].

Author	Key Content	Year	Ref.
Raju R. et al.	The use of phase change materials (PCMs) in conjunction with portable cold storage units	2024	[69]
Qi et al.	The application and research of phase change cold storage technology in cold chain transportation.	2023	[77]
Ben Taher et al.	Computational and experimental studies on refrigerated trucks	2022	[108]
Leungtongkum et al.	Insulated box and refrigerated equipment with PCM for food preservation	2021	[109]
Zhang et al.	Phase change cold storage materials in cold chain logistics	2021	[110]
Liu et al.	PCCSM used in cold chain logistics	2021	[111]
You et al.	Low-temperature phase change materials and their applications in cold chain	2021	[112]
Sun et al.	Phase change cold materials used in cold storage	2021	[113]
Selvnes et al.	Cold thermal energy storage applied to refrigeration systems	2020	[114]
Zhao et al.	Cold storage technology in cold chain transportation and distribution	2020	[115]
Ning et al.	Phase change cool storage technology in food cold storage transport	2020	[116]
Xu et al.	Energy-saving optimization of cold storage plate refrigerators	2020	[117]

presents a summary of the key concepts outlined in the review articles that have been published in recent years. PCMs play a crucial role in this process by absorbing or releasing large amounts of heat during phase transitions (such as melting or solidification), thereby helping to stabilize temperatures and minimize temperature fluctuations. By harnessing the unique properties of PCMs, as technology advances and costs decrease, the adoption of PCMs in the cold chain is likely to increase, transforming the way we manage temperature-sensitive goods from production to consumption.

Amidst China’s rapid economic growth and steady enhancement of citizens’ living standards, the safe and efficient transportation of food through cold chain systems has emerged as a pressing concern of the current era. Therefore, maintaining a consistent low-temperature environment throughout the food cold chain’s transportation phase is paramount to ensuring product integrity, while simultaneously minimizing carbon emissions and energy consumption is crucial for sustainable development. Energy storage technology holds the key to achieving these goals, fostering a more environmentally responsible and energy-efficient approach to food transportation. PCMs are utilized in cold chain transportation owing to their exceptional cold storage capabilities, significantly contributing to minimizing the loss of food, preserving their quality, and reducing energy consumption. The heat storage capacity per unit volume of PCMs surpasses that of traditional energy storage methods by an impressive factor of 5 to 14 times, offering a substantial advantage through their exceptionally high heat storage value. PCM application plays an energy-saving role in the whole cold chain transport process. Their exceptional latent heat capacity, unparalleled stability, and remarkable reusability render them adept at mitigating the shortcomings and constraints inherent in traditional cold chain logistics, ultimately facilitating the seamless execution of efficient and secure cold chain transportation.

PCMs find application in refrigeration systems as a form of passive refrigeration, offering advantages such as the absence of wearing parts, low noise levels, energy efficiency, and environmental friendliness. CRSs possess the capability to boost energy efficiency significantly. Furthermore, the employment of advanced insulating materials not only enhances energy efficiency but also contributes to the cost-effectiveness of all cold storage solutions. PCMs can be seamlessly incorporated into diverse components of a refrigeration system, encompassing the evaporator, condenser, or as a standalone thermal energy storage unit. Creating environmentally sustainable solutions that minimize energy usage, cut down on greenhouse gas emissions, and employ renewable materials represents a major challenge within this domain. Their application can yield substantial energy savings by mitigating the reliance on mechanical cooling during peak demand hours and harnessing waste heat for cooling purposes. The use of PCMs in refrigeration systems not only leads to energy savings but also has economic and environmental benefits. The reduced energy consumption can lower operational costs, and the reduced reliance on conventional cooling methods can decrease greenhouse gas emissions.

Despite the potential benefits, there are challenges associated with the implementation of PCMs in refrigeration systems. One of the main challenges is the selection of suitable PCMs that meet the specific requirements of the refrigeration system. PCMs initially entail higher expenses, the installation of PCMs requires modifications to the system design and may involve additional costs, yet they offer substantial long-term cost savings by reducing energy consumption, making them ideal for applications that necessitate energy efficiency and precise temperature control. Orchestrating and administering the cold chain effectively to maintain product integrity and quality amidst the intricacies of the entire supply chain poses formidable challenges, particularly when dealing with multiple stakeholders and a myriad of varying environmental conditions. The seamless integration of PCMs into existing refrigeration systems necessitates meticulous design and engineering endeavors to guarantee peak performance and efficiency. Ideally, PCMs with high latent heat capacity, a low melting point, and excellent thermal stability are preferred for refrigeration applications. Nevertheless, identifying PCMs that simultaneously fulfill all these criteria can present a formidable task. Future research endeavors should prioritize the development of cost-effective PCMs, enhance the integration of PCMs within refrigeration systems, and delve into innovative applications, including leveraging waste heat from industrial processes for PCM charging.

6. Conclusions

Cascade refrigeration systems have emerged as the preferred option in the realm of cold storage, owing to their exceptional performance when operating under conditions of low evaporation temperature. Hence, the quest for energy-saving solutions within refrigeration systems holds paramount significance. Among these, PCMs have surfaced as a promising innovation that significantly enhances the energy efficiency of refrigeration systems. Regarding the placement of PCMs within refrigeration systems, the majority of previous research endeavors have concentrated on single-stage refrigeration systems. In contrast, the investigation into cascade systems for refrigeration applications remains relatively limited. For applications requiring low temperatures, employing a conventional system may prove economically unfeasible due to the substantial pressure increase that adversely impacts the compressor’s performance. Consequently, a cascade system emerges as a potential solution to address this challenge.

Based on the aforementioned literature, it is evident that the majority of articles have overlooked the application of PCMs in cascade refrigeration systems and their subsequent impact on system performance. The present study endeavors to comprehensively integrate existing research findings and delve deeper into the application and energy-saving potential of PCMs in cascade refrigeration systems within the food cold chain, ultimately offering valuable insights and reference points for this burgeoning field. Notably, what sets this research apart is its focus on an area that has been largely overlooked by previous works: the integration of PCMs into cascade refrigeration systems and the subsequent implications for the performance of refrigeration and cooling systems. Therefore, this can be considered the novelty of this study.

Currently, the design of refrigeration systems utilizing phase change materials in the food cold chain remains an understudied area. This paper provides a comprehensive overview of cascade refrigeration systems utilizing PCMs in the food cold chain logistics and their various applications. Additionally, it summarizes the research progress of PCMs in cold chain transportation and storage, cold storage containers (including the placement of cold storage units, insulation materials, and insulated containers of diverse shapes), as well as their integration into household refrigerators and freezers. Looking ahead, the following are the key research directions anticipated for future investigation: In practical applications, the temperature control range of a refrigeration system must incorporate considerations for both the phase change range of the PCM in use and the permissible temperature limits for stored food products. On the contrary, the cascade systems currently under study exhibit a simplistic layout. Hence, future research will delve deeply into the application of PCMs in low-temperature refrigeration systems. To guarantee compatibility with the optimal preservation temperatures of a wide range of cold chain products, it is imperative to continuously pursue the development of advanced cold storage materials that exhibit lower melting points and increased energy density in the foreseeable future.