A Review on Phase-Change Materials (PCMs) in Solar-Powered Refrigeration Systems

Abstract

Over the past few years, the combination of solar power with refrigeration technology has matured, providing a promising solution for sustainable cooling. However, a key challenge remains, namely the inherent intermittency of solar energy. Due to its uneven temporal distribution, it is difficult to ensure continuous 24 h operation when relying solely on solar energy. To address this issue, thermal energy storage technology has emerged as a viable solution. This paper presents a comprehensive systematic review of phase-change material (PCM) applications in solar refrigeration systems. It systematically categorizes solar energy conversion methodologies and refrigeration system configurations while elucidating the fundamental operational principles of each solar refrigeration system. A detailed examination of system components is provided, encompassing photovoltaic panels, condensers, evaporators, solar collectors, absorbers, and generators. The analysis further investigates PCM integration strategies with these components, evaluating integration effectiveness and criteria for PCM selection. The critical physical parameters of PCMs are comparatively analyzed, including phase transition temperature, latent heat capacity, specific heat, density, and thermal conductivity. Through conducting a critical analysis of existing studies, this review comprehensively evaluates current research progress within PCM integration techniques, methodological classification frameworks, performance enhancement approaches, and system-level implementation within solar refrigeration systems. The investigation concludes by presenting strategic recommendations for future research priorities based on a comprehensive systematic evaluation of technological challenges and knowledge gaps within the domain.

1. Introduction

Energy is undoubtedly one of the most critical drivers for the advancement of nations and human societies [1]. Currently, fossil fuels dominate the global energy supply, accounting for 75% of total consumption [2]. However, their use has led to substantial environmental pollution over the past several decades. In light of this, there has been a significant shift in focus toward cleaner, renewable energy sources such as solar and wind power. These energy alternatives are expected to play a crucial role in aligning societal progress with ecological preservation. At present, renewable energy contributes to approximately 30% of global energy consumption [3], and nations worldwide are actively working toward reshaping their energy infrastructures to increase the share of renewables.

As societal aspirations for elevated quality of life continue to advance, refrigeration systems have evolved from discretionary luxury goods to indispensable household necessities [4]. Statistics reveal that 40% of global food production requires refrigeration or freezing [5], making the food refrigeration sector a major consumer of energy, responsible for around 15% of global electricity consumption [6]. However, nearly 800 million people across the globe still lack reliable access to electricity [7], and many others experience inconsistent power supply. For instance, in Bangladesh, unstable voltage levels and power shortages can prevent the use of essential high-power devices, including refrigerators [8]. In this context, solar-powered refrigeration systems present an ideal solution. These systems are not only low-cost but also independent of traditional electrical grid infrastructures, making them particularly suitable for regions with unreliable power supply. Furthermore, they offer higher efficiency in utilizing renewable energy, effectively addressing these challenges. However, the intermittent nature of solar energy—its uneven distribution over time and space—poses a significant challenge, potentially leading to voltage fluctuations that could damage electrical components. By integrating energy storage technologies, such as phase-change materials (PCMs), with solar refrigeration systems, this issue can be substantially mitigated. PCMs are a cost-effective and convenient energy storage solution, making them a popular choice in the development of solar refrigeration technologies.

In recent years, the integration of PCMs into solar refrigeration systems has rapidly progressed, with a growing body of the literature exploring the subject. Notably, a 2017 review by Khan et al. provided an in-depth evaluation of the suitable PCM types, integration techniques, performance-enhancing features, and the temperature-related challenges faced by each component of a single-effect solar absorption system [9]. More recently, Patil et al. [10], in their 2024 review, focused on the stability of PCMs used in solar thermal storage systems for absorption refrigeration, offering a comprehensive overview of PCM applications within these systems. While the literature on PCM integration in solar refrigeration systems is expanding, there is still a need for further research into the specific roles and benefits of PCMs across different components of these systems.

This study endeavors to address this research deficiency through a systematic examination of contemporary solar-powered refrigeration systems and their corresponding phase-change material (PCM) applications. The investigation methodologically examines PCM integration protocols across multiple system architectures, with particular emphasis on establishing selection criteria for optimal material specifications based on thermodynamic requirements. Through comparative performance analysis, the research quantitatively evaluates energy conservation capabilities and operational efficiency metrics in PCM-enhanced solar refrigeration configurations. Furthermore, the analytical framework systematically identifies critical implementation barriers within current technological paradigms while proposing targeted optimization strategies to improve thermodynamic performance and mitigate operational constraints in practical applications.

2. Classification of Solar Refrigeration Systems

Solar refrigeration systems leverage solar energy, transforming it into thermal or electrical energy to power refrigeration processes, thereby providing cooling services to end users. System configurations are determined by the specific power input requirements and target temperature parameters of the particular cooling application. This section presents a comprehensive classification of solar refrigeration systems based on their operational principles and energy conversion mechanisms.

From a functional perspective, solar refrigeration systems can be divided into three main subsystems, which are solar energy conversion devices, refrigeration systems, and cooling load endpoints. Notably, the type of cooling load endpoint (i.e., the end user) does not have a fundamental impact on the core principles of solar refrigeration. Therefore, this paper will present a detailed and systematic classification of solar refrigeration systems based on the principles governing both solar energy conversion devices and refrigeration systems.

The solar energy conversion devices can be categorized into two primary types, namely solar thermal conversion devices and solar electric conversion devices. Refrigeration systems can further be classified into two main types, which are thermally driven refrigeration systems and electrically driven refrigeration systems. As depicted in Figure 1, the classification of solar refrigeration systems is primarily determined by the type of solar energy conversion device used, resulting in three key categories of solar thermal refrigeration systems, solar electric refrigeration systems, and solar combined refrigeration systems.

2.1. Solar Thermal Refrigeration

Solar thermal refrigeration systems function by absorbing solar energy through solar thermal collectors and converting it into heat, which is then stored in a thermal storage tank. The stored heat is used to power devices that require thermal energy, and these devices utilize thermochemical or thermophysical methods to perform refrigeration. This in turn drives the operation of the refrigeration system. Based on their operational principles, solar thermal refrigeration systems can be classified into three main types, namely open adsorption cycles, closed absorption cycles, and thermomechanical refrigeration cycles.

2.1.1. Open Adsorption Cycle

The open adsorption cycle refrigeration system operates by utilizing liquid or solid desiccants to dehumidify or humidify air, effectively providing refrigeration. The system operates via two fundamental processes that are integral to its functionality. The first process, known as adsorption, involves the desiccant absorbing moisture from one airflow to another by leveraging the vapor pressure differential between the humid air and the desiccant material. When the desiccant material is dry and cold, its surface vapor pressure is lower than that of the humid air, and moisture is adsorbed onto the desiccant. During the adsorption phase, the latent heat of water vapor is released, causing an increase in temperature. As a result, the desiccant becomes wet and progresses to the second phase. The second phase, known as desorption or regeneration, involves raising the desiccant’s temperature to release the absorbed moisture into the airflow. After cooling, the desiccant material can absorb moisture again. These two processes form a continuous cycle, enabling the system to transfer moisture effectively, as shown in Figure 2. The open-type adsorption cycle refrigeration system utilizing liquid desiccant consists primarily of a regulator and a regenerator. In the regulator, sprayed liquid desiccant absorbs atmospheric moisture, reducing the hot–humid air’s dry-bulb temperature by 5–8 °C through dehumidification cooling. The diluted desiccant subsequently accumulates in the collection pit, corresponding to the 1→2 process in Figure 2. The regeneration phase transports this solution to the regenerator—structurally analogous to the regulator—where desiccant spraying facilitates moisture transfer to ambient air (process 2→3 in Figure 2). Finally, the reconcentrated desiccant undergoes temperature reduction via a cooling tower or heat exchanger (process 3→1), completing the thermodynamic cycle. To drive the cycle, external heat is required during the moisture release phase (from point 2 to point 3 in the figure), with heating being provided by solar energy.

2.1.2. Closed Absorption Cycle

The closed absorption cycle can be classified into two types based on the nature of the absorption material, namely the liquid absorption cycle and solid adsorption cycle. The liquid absorption cycle is the most widely used absorption refrigeration cycle, while the solid adsorption cycle refers to the adsorption-based cycle.

In the liquid absorption refrigeration cycle, the system differs from traditional vapor compression refrigeration. It employs water–lithium bromide or ammonia–water as the working fluids. At low pressures, the vaporized refrigerant absorbs heat to enable cooling, while the absorbent material captures the vaporized refrigerant. In the generator, external heat sources (e.g., solar energy or industrial waste heat) drive desorption, releasing the refrigerant and preparing it for subsequent condensation and reuse. The schematic of absorption refrigeration cycle is shown in Figure 3.

Solid adsorption solar refrigeration cycles involve the use of solid adsorbents (such as zeolite, activated carbon, silica gel, etc.) combined with suitable refrigerants to form a functional refrigeration system. These adsorbents have a porous structure and, at low temperatures, absorb (or adsorb) refrigerants like a sponge. When heated, the adsorbents release (or desorb) the refrigerant. This makes the adsorption refrigeration cycle entirely driven by thermal energy, meaning the system can be powered by solar energy [11]. A comparison of solid adsorption refrigeration and liquid absorption refrigeration is shown in

Table 1. Comparison of solid adsorption refrigeration and liquid absorption refrigeration [11,12,13,14,15,16].

Comparison Dimension	Solid Adsorption Refrigeration Systems	Liquid Absorption Refrigeration Systems
Temperature of the heat source	20–90 °C	80–150 °C
COP	0.3–0.6	0.6–1.2
Structural complexity	Simple (no pump)	Complex (requires pump)
Operating mode	Intermittent	Contiguity
Applicable scenarios	Small, decentralized	Large-scale central air conditioning
Environmental	Using natural refrigerants such as water, methanol, ammonia, etc., which are not destructive to the ozone layer	Ammonia is environmentally friendly; lithium bromide needs to be corrosion resistant
Maintenance cost	Lower	Higher (pumps, corrosion maintenance)
Technology maturity	In the pipeline	Mature and standardized

.

2.1.3. Thermomechanical Refrigeration System

The thermomechanical refrigeration system, commonly termed the steam ejector refrigeration system, harnesses solar energy as thermal energy using solar collectors. This thermal energy is subsequently transformed into mechanical energy via a nozzle, ultimately generating the desired cooling effect.

In a solar steam ejector refrigeration system, the solar collectors absorb solar radiation and convert it into heat energy. This thermal energy is used to heat a low-boiling-point working fluid, such as water or another suitable refrigerant, transforming it into high-pressure steam. The high-pressure steam is then expanded adiabatically through the ejector’s nozzle, causing the steam’s velocity to increase and its pressure to decrease. This high-speed steam creates a low-pressure region in the intake chamber of the ejector, which draws low-pressure steam from the evaporator into the ejector. The low-pressure steam mixes with the high-speed steam in the mixing chamber. The resulting mixture of gases enters the diffuser, where kinetic energy is gradually converted into pressure energy, leading to an increase in pressure. Finally, the pressurized steam enters the condenser, where it is cooled by the cooling medium and condensed back into liquid form. The liquid is partially directed through a throttling valve into the evaporator, where it absorbs heat and vaporizes, completing the refrigeration cycle. The remaining liquid is pumped back to the generator, where it is reheated and vaporized. The system is depicted in Figure 4 [17].

In an ejector refrigeration system, the circulating pump is the only moving component. The system is simpler than absorption-based systems, operates more stably, and is highly reliable. However, its main drawback is the lower refrigeration efficiency. The ejector, which is the core of this system, was invented by Sir Charles Parsons around 1901 to remove air from the condenser of a steam engine. In 1910, the ejector was used in the first steam ejector refrigeration system [18].

2.2. Solar Electric Refrigeration

Solar electric refrigeration systems operate by using solar photovoltaic panels to capture solar energy and convert it into electrical energy, which is then stored in a battery bank. The stored energy is subsequently utilized to operate refrigeration systems, including thermoelectric cooling systems and vapor compression refrigeration systems. These systems leverage the stored energy to achieve cooling through distinct thermodynamic mechanisms. As a result, solar electric refrigeration systems can be categorized into solar semiconductor refrigeration systems and solar vapor compression refrigeration systems.

Semiconductor refrigeration technology is based on the Peltier effect. When DC flows through a thermocouple composed of N-type and P-type semiconductor materials, the direction of the current entering the coupling varies, causing heat absorption and heat generation. When the current flows from N to P, the end of the P side absorbs heat, making it the cold end. Conversely, when the current flows from P to N, the end of the N side generates heat, creating the hot end [19,20].

With the development of semiconductor cooling technologies, new materials, and related advancements, scientists worldwide have been pushing for the adoption of solar semiconductor refrigeration systems in fields such as defense, industry, agriculture, and everyday life [21,22,23]. Ma et al. [24] designed an integrated solar semiconductor refrigeration unit for scenic areas to meet the demand for cold beverages in tourist spots. Mirand et al. [25] applied solar semiconductor refrigeration technology to design an air conditioning system for electric vehicles, achieving an efficiency of 0.5 in cooling mode and 1.72 in heating mode. Wahab et al. developed a solar semiconductor refrigeration system, finding that in comparison with solar photovoltaic vapor compression air conditioning systems, the solar semiconductor refrigeration system does not require intermediate circuits for energy conversion, has no moving or rotating parts, and does not need refrigerants. This makes it more stable, environmentally friendly, and nearly silent [26]. Song et al. developed a solar-powered dual-function container employing thermoelectric modules, demonstrating effective temperature regulation capabilities suitable for pharmaceutical preservation in disease-endemic regions like Africa. This system serves both electrified and off-grid environments, with applications spanning automotive systems, tourism infrastructure, and medical cold chain management for vaccines and temperature-sensitive medications [27]. Solar vapor compression refrigeration systems will be detailed in the next sections.

2.3. Solar Combined Refrigeration

Solar combined refrigeration systems utilize solar energy to generate both electrical and thermal energy, which are then harnessed to power refrigeration systems driven by electricity and heat. This dual-source approach offers a more versatile solution for refrigeration, combining the benefits of solar electricity with thermal energy conversion. For instance, in 1996, Goswami introduced a novel combined system that integrates the Rankine cycle with an absorption cooling cycle, using an NH₃/H₂O mixture as the working fluid. In this system, the turbine replaces the traditional expansion valve found in absorption cycles. By expanding highly concentrated ammonia vapor to a low temperature without condensation, the turbine generates mechanical power. The low-temperature ammonia then enters the evaporator, where it vaporizes and undergoes processing in a typical absorption cycle. Despite its innovative design, the complexity of the system has limited its widespread adoption and research in the field of solar combined cooling systems [28]. This highlights the challenges faced in developing efficient, reliable, and economically viable systems that combine both thermal and electrical energy sources for refrigeration.

3. PCMs

PCMs are innovative substances that store and release thermal energy through the absorption or release of heat during phase changes. This ability to absorb or release large quantities of heat makes PCMs ideal for regulating temperatures, reducing energy consumption, and managing energy loads effectively [29]. Current energy storage technologies extend beyond PCMs to encompass electrochemical batteries, pumped hydro storage (converting electrical energy into gravitational potential energy through water elevation), flywheel systems (storing rotational kinetic energy in high-speed rotors), supercapacitors (electrostatic energy storage through surface charge adsorption on electrodes), and compressed air energy storage (CAES) (pressurized air containment in subterranean reservoirs). These systems predominantly utilize mechanical or electrochemical energy conversion mechanisms. Distinctively, PCM-based thermal energy storage employs reversible phase transitions to manage heat absorption/release cycles, characterized by non-toxic composition, ecological compatibility, simplified operational architecture, and exceptional recyclability. PCM technology remains unparalleled for thermal regulation applications, demonstrating particular efficacy in scenarios requiring low-energy-density storage, extended-duration operation, and stringent environmental compliance. The versatility of PCMs allows them to be utilized in a wide range of applications, from improving energy efficiency in buildings to enhancing cooling systems. PCMs are broadly categorized into three types, namely inorganic, organic, and eutectic phase-change materials.

Inorganic PCMs include materials such as crystallized hydrated salts, molten salts, and metals or alloys. Organic PCMs primarily consist of paraffin, acetic acid, and other organic substances, which offer certain advantages in terms of safety and ease of handling. Eutectic phase-change materials (PCMs) comprise composites of two or more organic or inorganic constituents, which can be engineered to achieve precise thermal properties through compositional adjustments. This tunability enables their application in diverse thermal management systems, ranging from energy storage to temperature regulation. This flexibility allows for precise control over important properties such as phase-change latent heat and phase-change temperature, making them highly adaptable to various energy storage needs.

The development of thermal energy storage technologies can be traced back to the Industrial Revolution, where early methods were based on relatively simple systems that relied on storing sensible heat, typically using hot water or air. These methods were limited in efficiency but provided a foundation for future advancements. Over time, as the demand for energy increased and technology advanced, thermal energy storage systems evolved, becoming more sophisticated and efficient. The introduction of PCM was a breakthrough, transforming thermal storage from sensible heat storage to latent heat storage, which is far more efficient.

Phase-change materials operate by absorbing or releasing latent heat during the phase-change process, allowing for much higher energy density compared to sensible heat storage. As a result, PCM-based thermal storage systems are capable of storing significantly more energy in the same volume. By the end of the 20th century, significant research began to explore both organic and inorganic PCMs, driven by the need for better, more efficient materials for energy storage. For example, paraffin-based organic PCMs are well-regarded for their excellent thermal stability, wide phase-change temperature range, and relatively low cost, making them a popular choice for a variety of applications. On the other hand, inorganic salts, such as nitrates, offer high latent heat capacity and are more cost-effective, making them highly attractive for large-scale thermal storage applications [30].

In recent years, PCM research has gained significant momentum. A literature review by Yataganbaba et al. found that between 1990 and 2015, there were 34,626 publications that referenced “phase change material” and 1034 that discussed “encapsulation”. The research trend has become increasingly focused on improving the performance and application of PCM encapsulation, which has seen a sharp rise in the number of publications since 2000 [31]. Recent innovations in nanoparticle-enhanced phase-change materials (PCMs) have achieved significant milestones, particularly in enhancing thermal conductivity, stability, and energy storage efficiency. For instance, the integration of high-conductivity nanoparticles (e.g., silica or metal oxide nanoparticles) into PCM matrices has addressed long-standing challenges in heat transfer lag and phase segregation. Shchegolkov et al. demonstrated that incorporating carbon nanotubes and graphene into PCMs achieved over 30% thermal conductivity enhancement while enabling magnetic field-assisted thermal flow regulation. This engineered composite exhibits superior thermal modulation in critical applications, such as emergency cooling systems, effectively reducing temperatures from 100 °C to approximately 40 °C. Such innovations expand the frontiers of intelligent thermal energy storage and precision thermal management technologies [32]. Shchegolkov et al. developed a magnetically responsive thermal storage composite comprising paraffin, multi-walled carbon nanotubes, and nickel–zinc ferrite. Through ultrasonic irradiation and magnetic field processing, they systematically enhanced material properties while investigating temperature field dynamics during thermal cycling. Their experimental studies elucidated mechanisms underlying thermal storage efficiency improvements and clarified the interrelation between magnetic moment orientation and thermal transport behavior, establishing a magnetization model for predictive control. This work advances functional thermal management systems with field-programmable energy regulation capabilities [33]. Due to their high heat storage capacity—up to eight times greater than sensible heat storage [34]—PCM-based systems are now being widely adopted in various cooling applications [35]. For instance, Elsayed et al. [36] conducted experiments combining PCM with frozen products in both winter and summer. They found that under different environmental conditions, the optimal energy-saving effect was achieved when the mass ratio of frozen products to PCM was 5.0, leading to a 12.5% increase in energy saving in summer and 36.5% in winter. Rahimi et al. [37] also demonstrated that integrating phase-change cooling materials into evaporators can significantly reduce power consumption, lower the compressor’s start–stop frequency, and minimize temperature fluctuations inside refrigeration units. These advancements highlight the growing potential of PCM-based systems in enhancing energy efficiency and reducing the environmental impact of cooling technologies.

Solar vapor compression refrigeration systems and solar thermal absorption refrigeration systems are two of the most widely studied and utilized solar refrigeration technologies. Given the prevalence and significance of these systems, this study focuses on two representative cases to examine the integration mechanisms of PCMs within key system components, assessing their impacts on performance metrics, energy efficiency, and thermal management optimization. This analysis aims to elucidate the role of PCMs in enhancing operational stability and scalability across diverse thermal applications.

4. PCM-Integrated Solar Vapor Compression Refrigeration System

The solar-driven vapor compression refrigeration system is a high-efficiency technology that synthesizes solar energy harvesting with conventional vapor compression cycles. By utilizing dual solar energy inputs (thermal and photovoltaic), the system optimizes renewable energy utilization for cooling and has seen substantial progress in experimental research and industrial implementation, emerging as a sustainable alternative to fossil fuel-dependent systems. For example, Wang et al. developed a thermodynamic model to optimize solar vapor compression refrigeration system performance, including the selection of suitable working fluids. They used Engineering Equation Solver (EES) software to model a low-temperature solar organic Rankine vapor compression refrigeration cycle and found that the refrigerant R123 demonstrated the best performance in terms of thermal efficiency and coefficient of performance (COP) [38]. This highlights the importance of selecting the right refrigerant to improve overall system efficiency.

In another innovative approach, Aung et al. proposed integrating solar energy directly with vapor compression systems. By using solar thermal energy to drive the vapor compression air conditioning system and incorporating components such as vacuum collectors and hot water storage tanks, the system’s COP was significantly enhanced [39]. This approach not only improves efficiency but also reduces overall power consumption, making it more sustainable and cost-effective. Similarly, Hans et al. [40] designed a solar photovoltaic-driven refrigeration system that can maintain an internal temperature of 10–15 °C with a COP of 0.34, demonstrating the growing potential for solar-powered refrigeration solutions in real-world applications. Today, solar vapor compression refrigeration systems have moved beyond theoretical research and are now being applied in practical household refrigeration systems. For instance, Dhawan et al. [41] introduced a solar thermoelectric refrigeration system that uses clean solar energy to drive the compressor, resulting in a substantial reduction in power consumption—by 44–63%—for a 1.6 L household refrigerator.

Typically, these systems include key components such as photovoltaic panels, compressors, condensers, expansion valves, and evaporators. The operation follows the basic vapor compression cycle, in which the refrigerant absorbs heat in the evaporator, turning into low-temperature, low-pressure vapor, which is then compressed adiabatically in the compressor. The compression process elevates the temperature and pressure of the refrigerant vapor. The high-pressure, high-temperature vapor enters the condenser, releasing heat to the ambient environment and condensing into a high-pressure liquid state. This liquid refrigerant is subsequently expanded through the throttling valve, causing its pressure and temperature to be significantly reduced prior to entering the evaporator. After expansion, the refrigerant evaporates and absorbs heat, completing the cycle. The principle of the solar vapor compression refrigeration cycle is shown in Figure 5. However, this idealized description of the cycle is based on the Carnot cycle. In real-world systems, various irreversible factors, such as non-ideal compressor behavior and the inefficiencies associated with the expansion valve, cause deviations from the ideal cycle. These factors lead to a decrease in system efficiency. Reducing these irreversible losses is a key focus for researchers and engineers working on improving the performance of solar vapor compression refrigeration systems. By minimizing inefficiencies, we can move closer to optimizing the system for both practical and large-scale applications, making solar refrigeration systems an increasingly viable solution for sustainable cooling [42].

4.1. Photovoltaic Panels

Photovoltaic (PV) panels lie at the heart of modern renewable energy solutions, directly converting sunlight into electricity through the photovoltaic effect. Structurally, a PV panel consists of multiple interconnected solar cells, often made from high-efficiency materials such as monocrystalline silicon, polycrystalline silicon, or thin-film semiconductors like cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). When photons in sunlight possess enough energy to surpass the bandgap of these semiconductor materials, electrons are excited from the valence band to the conduction band, resulting in the generation of direct current (DC) in an external circuit. As PV technology evolved, it became common to classify these panels into three generations based on core materials and production techniques. The first generation features traditional silicon-based solar cells; the second generation includes thin-film technologies; and the third generation encompasses emerging innovations such as organic photovoltaics, perovskite photovoltaics, and dye-sensitized solar cells. Each generation aims to enhance conversion efficiency, reduce manufacturing costs, and broaden the range of practical applications [43].

Although photovoltaic efficiency has steadily improved over the past few decades, thermal management remains a pivotal challenge. High operating temperatures can degrade cell performance, shorten component lifespans, and lower overall energy output. During the 1990s and early 2000s, researchers began paying closer attention to managing heat within PV systems [44]. As efficiency targets rose, the need to regulate temperature became even more pressing. Around the 2000s, integrating PCMs into PV panels emerged as a promising solution; by absorbing and storing excess heat when solar irradiance is high, PCMs can help stabilize cell temperature. These efforts primarily focused on placing PCMs at the back or even within the layers of the PV panel, aiming to maintain optimal operating conditions and further boost overall energy conversion performance [45,46].

4.1.1. Effect of Photovoltaic Panel Temperature on COP

The performance of photovoltaic panels is highly temperature-sensitive, with efficiency decreasing as the surface temperature increases. This phenomenon is primarily driven by the impact on the open-circuit voltage of the panel. In practice, current photovoltaic panels only manage to convert about 10–16% of incident solar radiation into usable electrical energy. The remainder of the radiation that is not converted into electricity is absorbed by the panel, which heats it up and reduces overall efficiency [47]. On a sunny day, the surface temperature of photovoltaic panels can be as much as 40 °C higher than the surrounding ambient temperature [48]. Research has shown that for crystalline silicon photovoltaic cells, every 1 °C increase in panel temperature above 25 °C results in a 0.4–0.65% drop in efficiency [49,50,51,52]. Boudhina et al. (2019) systematically analyzed PV module performance across varying ambient temperatures, establishing comparative temperature coefficient benchmarks. Their findings confirm that PV cells exhibit a negative temperature coefficient—quantifying a percentage power loss per 1 °C rise—typically ranging from −0.2% to −0.5%. This parameter indicates that the PV module output decreases by 0.2–0.5% per 1 °C temperature elevation, critically informing photovoltaic system efficiency optimization under thermal stress conditions [53]. Given that solar panels typically operate at temperatures between 60 and 80 °C, this results in a power generation efficiency of only about 10% [54].

These factors highlight the critical need for effective temperature management strategies to enhance the efficiency of photovoltaic systems. As the photovoltaic industry continues to evolve, managing heat within solar panels has become a priority. Cooling technologies engineered to mitigate these challenges are broadly classified into active and passive cooling systems. In contrast to active systems—which depend on external energy inputs and typically incorporate mechanical components—passive cooling systems operate without auxiliary energy input and require minimal maintenance. This distinction underscores the growing preference for passive systems in sustainable thermal management applications. Among passive cooling methods, the integration of PCMs into photovoltaic systems—known as PV-PCM systems—has proven to offer superior cooling performance and more stable temperature regulation. Xiao et al. (2022) demonstrated that a stearic acid–lauric acid composite (phase-change temperature: 42.9 °C) applied as a PCM to photovoltaic panels reduced peak operational temperatures from 61.6 °C to 48.9 °C, significantly enhancing thermal regulation efficiency [55]. The most significant advantage of PV-PCM systems lies in the latent heat storage capabilities of PCMs. These materials can absorb large amounts of heat without experiencing a significant increase in temperature, making it possible to maintain the photovoltaic panel at an optimal operational temperature range for extended periods. This passive method of cooling ensures that the panel operates efficiently without requiring external energy or mechanical components, which not only improves the longevity and performance of the system but also makes it more environmentally friendly and cost-effective.

4.1.2. Methods of Integrating PCM with Photovoltaic Panels

Phase-change material (PCM)-integrated photovoltaic panels leverage latent heat absorption to stabilize module temperatures within the 25–40 °C high-efficiency conversion range, effectively curbing power loss from thermal degradation. This integration achieves 20–30% peak temperature reduction, mitigating encapsulant aging while extending module lifespan by 10–15%. The system’s passive thermal regulation operates energy-autonomously via solid–liquid phase transitions, providing diurnal thermal buffering that suppresses microcrack propagation induced by thermal cycling. Such thermal stress management enhances operational stability and reliability under extreme temperature differentials. Efficiently managing the heat generated by PV panels is a key challenge in maintaining high energy output and prolonging panel lifespan. One promising approach involves incorporating PCMs directly onto the back surface of PV panels, enabling a more effective transfer of heat away from the solar cells. One of the methods is where the PCM is encapsulated and placed between the PV cells and the collection back panel. In this arrangement, the PCM absorbs excess heat during periods of high solar irradiance, preventing the PV panel from reaching excessively high temperatures that would otherwise reduce conversion efficiency [56,57].

A distinctive feature of the PCM-PV setup is its ability to maintain a near-constant temperature at the phase-change point while the material melts. As soon as the PV panel’s temperature drops below the melting point, the PCM begins to solidify, gradually releasing stored heat in the process. The phase transition thermal inertia of PCM maintains near-constant thermal equilibrium during solidification exothermic processes, preserving its characteristic phase-change temperature plateau. This thermal stabilization mechanism prevents the photovoltaic efficiency losses typically associated with temperature fluctuations, as the PCM’s phase transition plateau avoids detrimental temperature elevation during energy discharge. This cyclical absorption and release of heat helps stabilize the panel temperature, ultimately boosting the system’s overall power output [58]. Beyond that, the stored latent heat can be harnessed for other thermal management purposes within the system, offering additional flexibility in design [59,60,61].

Recent studies have further validated the benefits of integrating PCMs into PV systems. Hasan et al. [62] performed a parametric investigation examining the influence of various PCM types and thicknesses on temperature regulation. Their findings revealed that this technique could reduce the panel’s temperature by up to 18 °C within just 30 min. In a more recent study, Ahmadi et al. embedded PCMs within a thermally conductive foam layer attached to a PV panel. This design successfully lowered panel temperatures by 6.8%, resulting in a 14% increase in energy conversion efficiency [63]. Similarly, Fayaz et al. [61] demonstrated—using both numerical simulations and experiments—that a PVT system employing polyethylene glycol as its phase-change material exhibited a 12.91% and 12.75% performance improvement over a conventional PVT setup without PCM. Refaey et al. (2024) demonstrated that integrating aluminum fins into phase-change material (PCM) encapsulation significantly improves photovoltaic thermal management. Their experimental framework evaluated three PCM compositions and fin geometries, identifying RT42 (a Rubitherm Technologies organic phase-change material) paired with triangular fin arrays as the optimal configuration. This combination reduced PV module peak operating temperatures by 14.5 °C (from 74.7 °C to 60.2 °C) while elevating power conversion efficiency from 17.1% to 18.5%. The triangular fin design enhanced heat dissipation through an increased surface-area-to-volume ratio, facilitating efficient thermal energy transfer from PV modules to the PCM during phase transition cycles [64]. Al Arni et al. (2024) engineered a gradient concentration three-layer PCM system incorporating graphene nanoparticles, where particle density decreased progressively from the photovoltaic interface layer to the base substrate. Encapsulated within high-thermal-conductivity aluminum shells, this configuration achieved a 5 °C operational temperature reduction (46 °C to 41 °C) in photovoltaic modules alongside a 0.4% efficiency enhancement (16.2% to 16.6%). Experimental validation demonstrated that the graded nanoparticle distribution optimized interfacial heat flux while maintaining material stability during phase transition cycles [65]. Collectively, these approaches highlight the growing appeal of PCM-based heat management strategies in PV technology. By effectively flattening temperature spikes and leveraging latent heat storage, PCM integration contributes not only to enhanced efficiency but also to improved system stability and a potentially longer operational life for photovoltaic installations.

4.1.3. Selection of PCM

Selecting an appropriate PCM is crucial for effective thermal management, particularly in solar-powered applications such as photovoltaic (PV) cooling. Broadly, PCMs fall into three categories, namely organic, inorganic, and eutectic. Organic PCMs, like paraffin, typically offer stable chemical properties and a flexible range of melting points but can have lower latent heat capacities and undergo significant volume changes during phase transitions [58]. Inorganic PCMs, often in the form of salt hydrates (e.g., CaCl₂·6H₂O), boast higher thermal conductivity and greater latent heat but can suffer from stability issues, including incongruent melting and supercooling. Eutectic PCMs combine two or more substances, aiming to fine-tune phase-change temperatures and latent heats; however, their application in PV panel cooling remains relatively unexplored [66,67].

Table 2. Comparison of thermal properties of three types of PCMs [68,69].

Thermophysical Parameters	Organic PCMs	Inorganic PCMs	Eutectic PCMs
Latent heat (kJ/kg)	120–250	150–400	100–300
Thermal conductivity (W/m·K)	0.1–0.3	0.5–1.5	0.5–8.5
Phase transition temperature (°C)	−20–150	20–800	Can be adjusted by changing the ratio of components

summarizes and compares the typical thermophysical properties of the three types of PCMs.

Historically, researchers have focused on two key properties when selecting a PCM, namely phase-change temperature and latent heat. The phase-change temperature must align with the operating range of the system—60–80 °C for typical solar PV panels [54]—to ensure that the PCM effectively stores and releases energy under real-world conditions. Latent heat capacity determines how much thermal energy can be stored or released during the phase transition, directly influencing the effectiveness of temperature regulation. In laboratory settings, differential scanning calorimetry (DSC) is commonly used to evaluate these parameters, providing insights into a material’s suitability for integration into thermal storage systems.

Inorganic PCMs, particularly those involving water and salts, offer high latent heat and robust thermal conductivity but face two persistent challenges. First, incongruent melting can lead to irreversible melt–freeze cycles if the salt hydrate lacks sufficient water of crystallization. Thickeners can mitigate this to some extent, but complete prevention is difficult [70]. Second, pronounced supercooling occurs when nucleation during freezing is hindered. Although adding nucleating agents alleviates this, the phenomenon is not fully eliminated [71]. These issues have largely deterred researchers from incorporating inorganic salts into PV-PCM systems, with most investigations focusing only on fundamental melting–freezing behaviors rather than practical integration.

In contrast, organic PCMs—notably paraffin-based compounds—have gained widespread adoption in photovoltaic cooling applications owing to their chemical inertness, negligible supercooling tendencies, and design flexibility. These attributes enable reliable thermal regulation while maintaining long-term operational consistency in solar energy systems. Paraffin is a petroleum-derived mixture of saturated hydrocarbons with phase-change temperatures that can be tailored by varying chain length. Studies have consistently shown paraffin’s effectiveness in cooling PV panels and boosting efficiency. For example, Ahmadi et al. integrated paraffin (melting at 37 °C) into a PV system, reducing the panel’s temperature by 6.8% and improving power output by 14% [63].

Table 3. Stability comparison of three types of PCMs [72,73].

Parameters	Organic PCMs	Inorganic PCMs	Eutectic PCMs
Cycling stability	Latent heat decay <5% after 500 cycles	Susceptible to phase separation and subcooling, up to 20% degradation after 100 cycles	Phase separation inhibited by encapsulation or thickening agents, <10% decay over 300 cycles
Chemical stability	Non-corrosive, acid- and alkali-resistant	Partially hydrated salts tend to corrode metal containers	Reduced corrosion risk through cladding
Thermal stability	Decomposition temperature >200 °C	Decompose easily at high temperatures	Can be adjusted by changing the ratio of components

compares the stability of the three types of PCMs.

Nonetheless, paraffin’s low thermal conductivity remains a notable limitation, prompting extensive research into thermal conductivity enhancements. Wu et al. [74,75] investigated the addition of copper nanoparticles at various weight percentages, finding improvements of up to 14.2% (solid phase) and 18.1% (liquid phase) in thermal conductivity at 2 wt% copper. A subsequent study comparing copper and aluminum nanoparticles identified copper as more effective; a 1 wt% copper addition reduced heating and cooling times by over 30%, with negligible impacts on latent heat or phase-change temperature. These findings underscore the potential for nanoparticle doping to address paraffin’s inherent conductivity shortcomings, making organic PCMs an even more viable option for real-world, large-scale PV cooling applications.

Table 4. PCMs integrated onto PV panels.

Research Study	Type	PCM	Density (Solid)	Density (Liquid)	Specific Heat Capacity (Solid)	Specific Heat Capacity (Liquid)	Thermal Conductivity (Solid)	Thermal Conductivity (Liquid)	Latent Heat of Fusion	Melting Point	Performance
			[kg/m3]	[kg/m3]	[kJ/kg·K]	[kJ/kg·K]	[W/m·K]	[W/m·K]	[kJ/kg]	[°C]
[55]	Organic	Mixture of stearic acid and lauric acid	/	/	/	/	/	/	/	42.9 °C	Reduced maximum temperature of PV plate from 61.6 °C to 48.9 °C
[61]	Organic	A44	805	805	2.15	/	/	0.18	242.0	44.0	12.91% (numerical simulation) and 12.75% (experimental) increase in maximum electrical efficiency with 1000 W/m2 radiation.
[63]	Organic	PA	805	805	/	/	/	0.4	124.6	47.0	A 6.8% lower temperature and 14% higher power generation efficiency at 1100 W/m2 radiation.
[64]	Organic	Wax (RT42)	880	770	/	2	0.2	0.2	165.0	38–43	At a radiation intensity of 850 W/m2, the power generation is increased to 737.4 W-h/m2 and the PV plate temperature is reduced to 60.2 °C, which is 19.4% lower than that of the uncooled plate.
	Organic	Wax	860	/	/	2	/	0.2	160.0	32–38
	Organic	Wax (RT31)	/	760	/	2	0.2	0.2	165.0	29–34
[65]	Organic	Wax (RT26)	880	760	/	2	/	0.2	180	25–26	The electrical efficiency of the PV plate increased to 16.1% within 100 min, and the temperature of the PV plate reduced by 5 °C compared to the single-layer PCM.
[62]	Organic	Wax (RT20)	880	/	2.0	/	0.2	0.2	140.3	21.2	Maximum temperature reduction of 14 °C at 1000 W/m2 radiation for 30 min.
	Organic	Mixture of capric–lauric acid	880	863	/	/	0.139	0.139	171.9	20.8	Maximum temperature reduction of 16.5 °C at 1000 W/m2 radiation for 30 min.
	Organic	Mixture of capric–palmitic acid	883	840	/	/	0.143	0.143	196.1	22.3	Maximum temperature reduction of 16.5 °C at 1000 W/m2 radiation for 30 min.
	Organic	Commercial blend (SP22)	1490	1430	2.5	/	0.6	0.6	182.0	23.0	Maximum temperature reduction of 14 °C at 1000 W/m2 radiation for 30 min.
	Inorganic	CaCl2·6H2O	1710	/	1.4	/	1.090	1.090	213.1	29.2	Maximum temperature reduction of 18 °C at 1000 W/m2 radiation for 30 min.

summarizes and compares the PCM types, compositions, and their thermophysical parameters used in the above articles.

4.2. Condenser

The condenser is a critical component in vapor compression refrigeration systems, designed to transform a high-temperature, high-pressure refrigerant vapor into a high-pressure liquid while releasing heat to the surroundings. In most setups, air serves as the cooling medium. Without forced convection, the rising air temperature around the condenser lowers the available temperature gradient, diminishing overall heat exchange efficiency. Furthermore, lowering the condensing temperature can significantly improve the system’s COP, as a reduced discharge temperature and pressure translate into lower compressor power consumption. Studies show that each 1 °C reduction in condenser inlet water temperature can lift a chiller’s COP by about 2–5% [76]. By integrating PCMs into the condenser, the refrigerant exchanges heat with a substance capable of absorbing and releasing thermal energy at a (relatively) fixed temperature, thereby stabilizing the heat absorption side and helping maintain a beneficial temperature differential for condensation.

One challenge, however, is that embedding PCMs within a heat exchanger inherently adds thermal resistance, initially hindering heat transfer. Yet, a PCM with a suitably low phase-change temperature can offset these losses by creating a larger temperature difference, and any accumulated heat can dissipate when the compressor is idle. However, to solve the problem of additional thermal resistance caused by PCMs, the most direct and effective way is to improve the thermal conductivity of PCMs and PCM packaging or to combine heat pipe technology to improve the heat transfer rate and also to reduce the thermal resistance by changing the PCM integration method. In practice, combining PCMs with the condenser often leads to marked gains in system performance. For instance, Wang et al. [77,78,79] conducted comprehensive optimization studies on PCM integration in a vapor compression system. In their initial phase, they compared the placement of the PCM (latent heat of 220 kJ/kg, phase-change temperature of 21 °C) between the compressor and condenser versus between the condenser and expansion valve. The latter arrangement raised the system’s COP by 8% and delivered a 10 °C temperature drop at the expansion valve—superior to the 5 °C cooling achieved in the former setup. Building on these insights, they developed a dynamic model featuring a liquid refrigerant flash mechanism to predict COP gains, which correlated well with experimental data but showed limited accuracy when significant subcooling or superheating occurred. To augment thermal stability, the researchers integrated a PCM-based storage tank strategically positioned between the evaporator and compressor units, thereby enhancing operational reliability. By carefully regulating refrigerant inlet temperatures, this configuration reduced overall operating temperatures and shaved peak heat loads. Notably, employing a composite PCM arrangement (linking heat exchangers in both the main and bypass loops) enabled the active control of condenser pressure/temperature, even in low-ambience conditions. This strategy effectively maintained minimal condensing temperatures, significantly improving operational stability and achieving meaningful energy savings—a testament to the importance of PCM spatial optimization in dynamic load scenarios. Cheng et al. took a different approach by engineering a PCM that blends paraffin/high-density polyethylene with graphite powder (GP) or expanded graphite (EG). The addition of EG multiplied the material’s thermal conductivity fourfold [80]. Incorporating this PCM into a household refrigerator condenser allowed it to absorb heat during compressor operation and release that heat when the compressor was off [81]. Experimental measurements recorded a 6.3 °C drop in outlet temperature and a 2.3 °C reduction at the condenser’s center, boosting the COP by 12%. However, the system’s compressor start–stop frequency increased, raising concerns about potential compressor wear. A subsequent dynamic numerical model validated these observations, predicting up to a 19% improvement in the COP [82]. Abdulaziz et al. (2025) demonstrated that integrating a phase-change material (PCM) unit as a pre-cooling module upstream of air conditioning condensers significantly enhances system efficiency. Their hybrid methodology combining numerical simulations and experimental validation under varied summer conditions revealed an 8.6% annual energy consumption reduction and a peak coefficient of performance (COP) of 5.63 compared to conventional configurations. This innovation leverages the PCM’s thermal buffering capacity to precondition condenser inlet air, thereby optimizing heat rejection processes in HVAC systems [83]. Ismail et al. (2024) developed a dual-PCM configuration incorporating distinct phase-change materials on air conditioning evaporator and condenser components. Their methodology combined ANSYS (ANSYS fluent 2020 R2)-based 2D transient numerical modeling with experimental validation, revealing seasonal energy savings of 11.8% (summer) and 12.8% (winter) compared to conventional systems. This hybrid thermal management approach optimized heat exchange dynamics through targeted PCM deployment at critical refrigeration cycle interfaces [84]. While this condenser–PCM integration has already been practically applied in small domestic refrigerators, its scalability to larger systems remains uncertain. Recent research on larger-scale implementations is sparse, likely due to the risk of intensified compressor cycling. Moving forward, deeper investigations into modeling, PCM property enhancement, and full-scale system validation are essential to unlock the potential of PCM-integrated condensers in diverse refrigeration environments, ensuring both efficiency gains and operational reliability.

Table 5. PCMs integrated in the condenser.

Research Study	Type	PCM	Density (Solid)	Density (Liquid)	Specific Heat Capacity (Solid)	Specific Heat Capacity (Liquid)	Thermal Conductivity (Solid)	Thermal Conductivity (Liquid)	Latent Heat of Fusion	Melting Point	Performance
			[kg/m3]	[kg/m3]	[kJ/kg·K]	[kJ/kg·K]	[W/m·K]	[W/m·K]	[kJ/kg]	[°C]
[81]	Organic	SSPCM	957	/	/	/	0.31	/	133.1	44.0	A 12% higher COP and 6.5 °C lower condensing temperature
[85]	Organic	RT35HC	880	/	2000	/	0.2	/	240.0	35.0	Reduced temperature fluctuations; the COP of the system averages about 1.82
[86]	Organic	SP24E	1500	1400	/	/	0.6	/	180.0	25.0	The COP was increased by 50% with a power saving of 6.85%
[83]	Inorganic	SP31	1350	1250	2	2	0.5	0.5	210.0	30.0	Annual energy consumption reduced by up to 8.6% with a maximum COP of 5.63
[84]	Inorganic	SP24E (summer)	1500	1400	2	2	0.5	0.5	180.0	25.0	An 11.8% reduction in energy consumption
	Inorganic	SP11gel (winter)	1330	1320	2	2	0.5	/	155.0	12.5	Reduction in energy consumption by 12.8 per cent
[87]	Inorganic	SP24E	1500	1400	/	/	0.6	/	180.0	25.0	Energy consumption is reduced by about 9.8% to 11.2%

systematically presents a comprehensive summary of the PCM types incorporated into condenser components, detailing their key thermophysical characteristics along with their corresponding thermophysical properties documented in existing studies.

4.3. Evaporator

The evaporator plays a central role in refrigeration systems, where it serves as the primary site for heat absorption. As low-pressure liquid refrigerant flows through the evaporator, it transitions into a vapor phase, extracting heat from the surrounding environment and thus lowering the ambient temperature to achieve cooling [88]. The evaporation temperature is a key determinant of both the system’s power consumption and its COP. If the evaporator runs at excessively high temperatures, the cooling effect diminishes. Conversely, if it runs at too low a temperature, the compressor’s pressure ratio increases, heightening energy consumption and undermining efficiency. Crucially, small increases in the evaporation temperature can enhance the COP by reducing the compressor’s workload and boosting the refrigerant’s heat absorption capacity—studies indicate that each 1 °C rise in evaporator outlet temperature can raise a chiller’s COP by 1.5–3% [76]. Moreover, in applications like cold cabinets, the evaporation temperature can significantly influence both temperature gradients and fluctuations within the cabinet itself.

A viable strategy for stabilizing evaporator operation entails the integration of phase-change materials (PCMs). These materials absorb and release latent heat during phase transitions, effectively mitigating thermal fluctuations. Researchers have investigated diverse PCM configurations—including plates, granular beds, and tubular geometries—to optimize interfacial surface area and enhance effective heat exchange. Such innovations aim to balance energy storage capacity with thermal responsiveness in dynamic operating conditions. Bista et al. [89] found that adding PCMs to the evaporator steadied temperature fluctuations and provided a buffer against changing thermal loads, leading to improved operational stability. By slowing the rate at which heat is transferred to the refrigerant, the PCM also tempers the evaporator’s temperature, allowing for a steadier and more continuous evaporation process. This, in turn, reduces temperature swings throughout the system, enhancing response time and increasing control precision [90].

From the viewpoint of solar-driven refrigeration, using PCMs in the evaporator can help capture and store more heat consistently, thus supplying greater cooling potential. An experimental study by Maiorino et al. [91] demonstrated that introducing PCMs into an evaporator significantly reduced internal temperature gradients and product temperature fluctuations in a cooled cabinet while extending the compressor’s off cycle. In various conditions—ranging from ambient temperatures and heat loads to differing PCMs—household refrigerators equipped with PCM-based evaporators have shown COP improvements between 2% and 74% [92]. Msell et al. integrated a PCM with a phase-change temperature of 2–4 degrees Celsius into an evaporator and experimentally tested the performance of the evaporator in charging and discharging modes of operation, which showed that the evaporator was able to efficiently store and release the cooling capacity in the PCM, which significantly improved the performance of the system and resulted in energy savings [93].

Table 6. PCMs integrated into the evaporator plate.

Research Study	Type	PCM	Density (Solid)	Density (Liquid)	Specific Heat Capacity (Solid)	Specific Heat Capacity (Liquid)	Thermal Conductivity (Solid)	Thermal Conductivity (Liquid)	Latent Heat of Fusion	Melting Point	Performance
			[kg/m3]	[kg/m3]	[kJ/kg·K]	[kJ/kg·K]	[W/m·K]	[W/m·K]	[kJ/kg]	[°C]
[93]	Organic	Wax	880	770	/	/	0.20	0.20	/	5.0	COP increased by 8–10%.
[94]	Organic	Corn oil esters	/	/	/	/	/	/	/	−15.0	CCOP increased by 6% and compressor power consumption reduced by 4%.
[86]	Organic	PCM10HC	/	/	/	/	0.2	/	200.0	12.0	The COP increased by 13.5% and the power saving was 3.9%.
[95]	Organic	Diethylene glycol	1200	1320	4.86	/	/	/	247.0	−10.0	PCM stores cold during the day and releases it at night.
[96]	Inorganic	H2O	917	1000	2.01	4.18	2.2	0.58	334.0	0	When the compressor stops working, the PCM is able to utilize its latent heat of phase change to maintain a low cabinet temperature.
[97]	Inorganic	H2O	917	1000	2.01	4.18	2.2	0.56	333.6	0	Cooling capacity is stored during the nighttime trough hours and released during the daytime peak hours, thereby taking advantage of peak and valley tariff differences and reducing operating costs.

summarizes information including the types of PCMs used in the literature mentioned above and their thermophysical parameters.

As a rule of thumb, the PCM’s phase-change temperature should be set 5–10 °C above the evaporation temperature, especially in sub-zero applications where solid–liquid PCMs are favored for their high latent heat, minimal supercooling, adequate thermal conductivity, and stable chemistry. Li et al. offered a comparative analysis of several PCMs suitable for air conditioning systems; the graph compares four common phase-change materials (PCMs) across six critical parameters, namely thermal conductivity, volumetric latent heat, density, corrosion potential, supercooling tendency, and phase separation behavior. Salt hydrates and co-crystals exhibit superior volumetric energy storage capacity, enhanced thermal transfer properties, inherent flame resistance, and cost-effectiveness. However, these materials demonstrate the three following significant limitations: pronounced phase separation during thermal cycling, substantial supercooling effects, and corrosive interactions with metallic containment systems, collectively resulting in compromised cyclic stability. Paraffin waxes and fatty acids represent organic phase-change materials that demonstrate distinct advantages over inorganic counterparts, including high chemical inertness, structural stability, and recyclability. These organic compounds exhibit minimal supercooling tendency while maintaining freedom from phase separation and corrosive behavior. However, their practical application is constrained by relatively low thermal conductivity and reduced phase-change enthalpy. Refrigerant-based hydrates present unique thermodynamic characteristics, existing in gaseous states under atmospheric conditions and requiring pressurized containment for hydrate formation. Notably, their fusion enthalpy significantly exceeds that of low-temperature eutectic salts, paraffinic compounds, and fatty acid systems. (refer to Figure 6) [98].

Despite these gains, PCM integration in evaporators faces some hurdles. Chief among them is the slow charging and discharging inherent to many PCMs, largely due to their low thermal conductivity, which can lead to delayed system responses. Proposed solutions involve the incorporation of microchannel arrays or porous architectures to augment the effective heat transfer surface area, as well as the integration of thermally conductive nanostructured additives (e.g., metallic nanoparticles or graphene) into the PCM matrix. Recent findings suggest that composite PCM/metal systems can markedly enhance thermal conductivity and expedite heat transfer [99,100]. Another concern is the volumetric change that occurs during phase transitions, potentially causing structural stress or deformation over extended operation. Advanced encapsulation—utilizing metallic encapsulation shells or polymeric enclosures—effectively mitigates leakage and thermal stress, whereas tailoring the PCM’s intrinsic material properties to minimize volume expansion provides a foundational solution. Looking ahead, refining these approaches and exploring new PCM formulations will be pivotal to harnessing the full potential of PCM-enhanced evaporators, particularly in the context of dynamic load environments and solar-driven cooling applications [101,102].

5. PCM-Integrated Solar Thermal Absorption Refrigeration System

Absorption refrigeration cycles hinge on four key components—an evaporator, an absorber, a generator, and a condenser—and typically unfold in five distinct stages. First, in the evaporator, the refrigerant (often water or ammonia) absorbs heat and evaporates, thereby cooling the surrounding environment. The resultant refrigerant vapor is then drawn into the absorber, where it is taken up by a weaker solution—commonly water–lithium bromide or ammonia–water—raising the system pressure. This diluted solution is subsequently pumped to the generator, where external heat (which may be waste heat or solar-derived) drives out the refrigerant, forming high-pressure vapor. That vapor progresses to the condenser, where it discharges its latent heat and liquefies, and finally, the liquid refrigerant undergoes throttling through an expansion valve before returning to the evaporator to repeat the cycle [103,104].

Among the strengths of absorption refrigeration systems is a relatively high COP, spurring considerable research interest. Xu et al. [105] devised a solar absorption refrigeration system that employed variable mass energy transformation and storage (VMETS) technology, attaining COPs of 0.7525 (air-cooled condenser) and 0.7555 (water-cooled condenser). Others have compared the fundamental performance of vapor compression and absorption cycles. One notable advantage of photovoltaic vapor compression systems lies in their reduced land footprint, cutting initial capital investment. By contrast, solar thermal absorption systems rely on solar collectors, whose required area is shaped by collector efficiency and attainable high temperatures. If concentrated solar collectors are adopted, higher collector outlet temperatures become feasible, minimizing the physical footprint and boosting practicality. Moreover, absorption refrigeration often pairs well with solar energy, and single-effect solar absorption systems have attracted growing attention in recent years for their relatively high efficiency and cost-effectiveness [16,106].

In such setups, the generator’s external heat source can stem from waste heat or solar-heated fluids. Once heated, the low-pressure refrigerant solution frees high-pressure refrigerant vapor that passes to the condenser [107]. Notably, each component in a solar absorption refrigeration system—the solar collector, generator, absorber, condenser, and evaporator—stands to benefit from PCM integration. By absorbing and releasing latent heat, PCMs can buffer thermal fluctuations and elevate system performance. In the following sections, we explore how PCMs can be integrated with each component, discuss suitable material selections, detail the impact on individual component efficiency, and evaluate the overall effect on system performance.

5.1. Solar Collector

Within solar-driven absorption refrigeration systems, the solar collector constitutes the critical component, distinguishing the system through its unique energy conversion role. The collector harnesses incident solar radiation, converting it into usable thermal energy that subsequently powers the generator to initiate the refrigeration cycle. Because solar energy supply fluctuates throughout the day, it is difficult to provide a continuous stable heat source for the generator solely from direct sunlight. Consequently, integrating thermal storage is imperative. By embedding PCMs into the collector, any surplus thermal energy can be stored during periods of strong sunlight. Later, when solar irradiance weakens or after sunset, the PCM can release the stored heat to sustain the generator’s operation. This strategy not only improves system performance but also maximizes the utilization of available solar energy. As previously discussed, we will not repeat the basic principles and classifications of solar collectors here.

The concept of combining PCMs with solar thermal collectors dates back to the late 1980s. Serale et al. [108] investigated a flat-plate collector enhanced with a PCM, reporting an 8% gain in efficiency compared to traditional heat storage methods and a 20–40% rise in annual average thermal conversion efficiency. As with other PCM-integrated systems, a careful selection of phase-change temperature and latent heat is key. The solar collector’s working temperature range dictates which PCM is most suitable, and references often provide typical output temperatures for different collector types [109].

Brancato et al. [110] studied PCMs with melting points of 80–100 °C in solar thermal absorption systems. Although these materials offered high latent heat, they also encountered issues like supercooling and incongruent melting. Agyenim et al. [111,112] explored phase-change temperatures in the 100–130 °C range for use in a LiBr-H₂O absorption system operating at 120 °C. Erythritol, with a 117 °C phase-change temperature, emerged as their choice due to its stable phase transition and high latent heat. Because its melting point was 3 °C lower than the generator temperature, it provided a sufficient temperature differential for heat exchange. Their subsequent tests, examining four container designs (no fins, circular fins, longitudinal fins, multi-tube) and three different PCMs (erythritol, RT100, and magnesium chloride hexahydrate), highlighted that a high latent heat and a phase-change temperature aligned with the system requirements, which were the primary selection criteria—supercooling concerns were secondary if latent heat was sufficiently high. Chopra, K. et al. integrated phase-change materials (PCMs) into heat pipe evacuated tube solar collectors (ETCs) by charging each tube with 2.25 kg of SA-67. Their experimental results demonstrated a 32.73–37.56% increase in daily thermal efficiency for PCM-enhanced ETCs, along with sustained high water temperature maintenance during nocturnal periods or low solar radiation intervals [113]. Naghavi et al. and Abokersh et al. incorporated phase-change materials (PCMs) into evacuated tube collector (ETC) arrays, demonstrating significant heat loss reduction, elevated system thermal efficiency, and enhanced operational stability under fluctuating irradiance conditions [114,115].

Currently, one key limitation for PCM-integrated solar collectors is limited adaptability to diverse or rapidly changing weather conditions. Researchers recommend several optimization strategies. For example, advanced thermal regulation strategies and adaptive control systems enable coordinated heat collection and storage, ensuring stable system operation. Innovative heat exchanger configurations enhance thermal coupling between the PCM and solar collector, thereby improving overall energy conversion efficiency. These advancements collectively address transient thermal mismatches and elevate system reliability under variable solar irradiance conditions. Additionally, modular system designs allow for easier adaptation and scaling in different climates, ensuring that the solar collector’s integration with PCMs remains robust and practical [116,117,118].

Table 7. PCMs integrated into solar collector.

Research Study	Type	PCM	Density (Solid)	Density (Liquid)	Specific Heat Capacity (Solid)	Specific Heat Capacity (Liquid)	Thermal Conductivity (Solid)	Thermal Conductivity (Liquid)	Latent Heat of Fusion	Melting Point	Performance
			[kg/m3]	[kg/m3]	[kJ/kg·K]	[kJ/kg·K]	[W/m·K]	[W/m·K]	[kJ/kg]	[°C]
[112]	Organic	Erythritol	1480	1300	1.4	1.4	0.7	0.3	339.8	117.7	The performance of the heat collector can be significantly improved.
[113]	Organic	SA-67	1100	1190	2.01	2.47	1.1	1.2	244.2	67.1	Thermal efficiency upgraded from 55.46% to 87.80.
[114]	Organic	Wax	990	916	2.76	2.48	0,35	0.17	174.0	64.0	Systems with integrated PCMs exhibit higher thermal efficiency at different water flow rates and are insensitive to changes in water flow rate.
[115]	Organic	Wax	920	795	2.3	2.3	0.21	0.21	189.0	58–62	System efficiency increased by approximately 14%.
[116]	Organic	Caprylic acid and lauric acid	2000	2000	2	2	0.2	0.3	130.0	21.0	In the active heat release mode, the ratio of heat release to heat storage increased by 30.12%.

summarizes the information including the types of PCMs integrated on solar collectors and their thermophysical parameters used in the literature mentioned above.

5.2. Absorber

Absorption refrigeration systems rely on external heat sources to drive the cooling process, utilizing the interaction between the absorbent and refrigerant to achieve the desired cooling effect. However, because these systems often operate with fluctuating heat inputs, incorporating PCMs can help buffer these variations, significantly improving both thermal storage and release efficiency. The absorber plays a critical role in the absorption refrigeration cycle, with the falling film absorber being the most widely used design. In this type of absorber, refrigerant vapor is absorbed by a thin film of solution flowing over horizontal tubes [119], and the heat transfer rate is directly proportional to the flow rate of the solution [120,121].

The absorption process in the absorber is a combination of chemical and physical heat exchange, and it is an exothermic process. As the refrigerant vapor is absorbed, the temperature of the absorber increases. Therefore, to prevent overheating, heat must be released to the environment. In the early designs of absorbers, this heat dissipation was addressed by using cooling water to absorb the excess heat. However, PCM integration within the absorber enables the capture and storage of surplus thermal energy, which can subsequently be redirected to auxiliary heat-dependent equipment, thereby enhancing overall solar energy utilization efficiency. This approach aligns with circular energy management principles, reducing waste and maximizing renewable resource exploitation.

Given the inherent instability of the heat source in absorption refrigeration systems, integrating PCMs allows the system to better cope with fluctuations in thermal load. The PCM helps to absorb the excess heat when the system experiences high load and releases stored heat when the load decreases, ensuring the system remains stable and operational [122]. Ji et al. [123] found that by incorporating PCMs into the absorber, temperature fluctuations in the absorbent are significantly reduced, allowing the system to operate efficiently across a wider temperature range and improving the overall reliability of the system.

5.3. Generator

In an absorption refrigeration system, the generator’s principal job is to heat the absorbent solution so that refrigerant vapor can be separated. In a LiBr-H₂O absorption refrigeration cycle, for instance, hot water or steam elevates the temperature of the diluted lithium bromide solution, progressively vaporizing the aqueous refrigerant. This phase-change process is critical for regenerating the concentrated absorbent solution and sustaining the refrigeration cycle. This process must also build up enough pressure for the vapor to flow into the condenser, functioning similarly to a compressor in traditional vapor compression systems. With advances in solar thermal technology, more efficient heat exchangers are now employed to drive this step in solar absorption systems. Common strategies for supplying heat include using thermal storage tanks, which store the necessary heat and release it as needed.

As the generator functions inherently as a heat exchanger, integrating PCMs can offer distinct advantages for thermal energy storage in solar-driven absorption systems. This integration facilitates load shifting and stabilizes thermal output during intermittent solar availability, enhancing system reliability. For instance, Ponshanmugakumar et al. [124] designed a vertically oriented generator with an integrated PCM section for an NH₃-H₂O solar absorption refrigeration system, facilitating forced convective boiling. They built a TRNSYS model to simulate a 1 m long coaxial generator, interlinked with a hot water storage tank and a boiler. Their research showed that when cooling loads are high or solar irradiance is low, the PCM substantially reduces the reliance on auxiliary heat sources. In a separate study, Lorente et al. conducted numerical simulations of a cylindrical storage tank filled with a paraffin PCM, featuring a vertically arranged spiral heating tube. By adjusting the spiral’s radius, they assessed how geometry affected the melting fraction and temperature distribution. Lorente et al. [125] underscored the value of combining numerical modeling with experimental work to advance generator design.

Although the concept of using PCMs in heat exchangers is not new [126], applying it specifically to the generator in solar absorption systems remains relatively novel. In theory, selecting a phase-change material (PCM) with a phase-change temperature and latent heat capacity well aligned with the generator’s operational parameters enables the PCM to act as an auxiliary thermal buffer, thereby stabilizing the generator’s thermal output. Prior investigations have primarily focused on structural innovations; going forward, researchers should also seek or develop new PCMs and explore ways to enhance the thermal properties of existing ones. This dual approach—combining structural optimization with material innovation—promises further improvements in the overall efficiency and reliability of solar absorption refrigeration systems.

Table 8. PCMs integrated into generator.

Research Study	Type	PCM	Density (Solid)	Density (Liquid)	Specific Heat Capacity (Solid)	Specific Heat Capacity (Liquid)	Thermal Conductivity (Solid)	Thermal Conductivity (Liquid)	Latent Heat of Fusion	Melting Point	Performance
			[kg/m3]	[kg/m3]	[kJ/kg·K]	[kJ/kg·K]	[W/m·K]	[W/m·K]	[kJ/kg]	[°C]
[124]	Organic	Erythritol	1480	1300	1.4	1.4	0.7	0.3	339.8	117.7	By optimizing the use of PCMs, the overall performance of the system can be improved and the system can achieve an average annual solar utilization of 0.58.
[125]	Organic	Wax	800	800	1.25	/	0.2	0.2	125	30	The use of vertical spiral heaters significantly improves the melting efficiency of PCMs.
[127]	Organic	RT90HC	850	/	2.0	/	0.2	/	170.0	91.0	Electricity consumption reduced by 6.2%, and heat delivery time increased by 27.8%.
[128]	Organic	RT82	950	/	2.0	2.0	0.2	0.2	176.0	82.0	A 170% increase in COP.
	Organic	RT100	/	/	1.8	2.4	0.2	0.2	168.0	105.0
	Inorganic	MgCl2·6H2O	1570	1450	2.3	2.6	0.70	0.57	167.0	118.0

summarizes the information including the types of PCMs integrated on the generator and their thermophysical parameters used in the literature mentioned above.

6. Conclusions

This paper has thoroughly surveyed the use of PCMs in solar refrigeration systems, discussing the criteria for selecting PCMs, techniques for integrating PCMs with various system components, and the resulting effects on overall performance. By highlighting both the hurdles and promising avenues in this field, it provides a roadmap for future research and practical solutions to enhance system reliability, efficiency, and cost-effectiveness. From the analysis presented, the following primary conclusions emerge:

• Phase-change temperature and latent heat are the most critical parameters for PCM selection. Ideally, the PCM should have a phase-change temperature that aligns with the system’s operating range and a high latent heat to maximize energy storage.
• Choosing the right PCM depends on whether a component is intended to absorb or release heat. For evaporators, using a PCM with a phase-change temperature above the evaporation temperature ensures sufficient heat absorption; for condensers, a phase-change temperature below the condensation temperature aids effective heat release.
• In addition to phase-change temperature and latent heat, supercooling can significantly affect a system’s performance and must be considered during PCM selection and design.
• Microencapsulation technology enhances the heat transfer surface area of inorganic phase-change materials (PCMs) while mitigating their inherent phase separation challenges. Concurrently, the incorporation of nucleating agents effectively addresses the pronounced supercooling phenomenon in these materials by promoting controlled crystallization during phase transition processes.
• Organic PCMs, often chosen for their low supercooling, remain prevalent despite lower thermal conductivity. Inorganic PCMs feature higher thermal conductivity but may suffer from more pronounced supercooling. Currently, organic PCMs are still predominant in practical applications.
• Much of the existing literature focuses on improving the thermal conductivity of PCMs by introducing nanoparticles or employing microencapsulation techniques. However, these methods can be expensive, limiting their economic feasibility.
• PCMs experience volumetric changes during phase transitions, potentially causing deformation or structural stress on storage tanks and containers, thereby reducing their service life.
• Although a direct contact integration of PCMs is broadly validated in research, actual integration strategies remain relatively narrow, suggesting the need for innovative designs to optimize heat transfer and encapsulation.
• While adding PCMs raises initial costs, it confers significant advantages, including enhanced energy efficiency, better performance under fluctuating weather conditions, and partial mitigation of the inherent intermittency of solar energy.
• Experimental and numerical investigations on phase-change material integration across multiple components within the same solar cooling system remain insufficiently explored in the current literature.
• Although integrating PCMs in both the condenser and evaporator enhances the temperature gradient for heat transfer and elevates system performance, inherent limitations in existing integration approaches frequently amplify thermal resistance, leading to significant energy dissipation.
• The integration of phase-change materials with solar collectors remains relatively uncommon in current practice, with existing implementations often necessitating solution pump operation that introduces additional electrical power consumption.
• The incorporation of phase-change materials (PCMs) in generator units presents significant theoretical advantages, as their superior thermal exchange properties effectively mitigate energy losses, thereby enhancing overall system efficiency.

7. Outlook

• While organic PCMs dominate current applications, their widespread use stems largely from the stability challenges that plague inorganic counterparts—particularly phase separation. Nonetheless, many inorganic salt solutions outshine organics in terms of thermal performance, thanks to their higher thermal conductivity. Bridging the gap in phase separation and other drawbacks could unlock significant potential for inorganic PCMs.
• Research on eutectic materials remains comparatively sparse, presenting a promising area for innovation. Further investigation into eutectic mixtures could broaden the range of operating temperatures and provide specialized solutions for diverse refrigeration and thermal storage needs.
• Embedding nanoparticles in PCMs can markedly boost thermal conductivity but often at a high cost. Future studies should aim to develop more cost-effective additives, balancing affordability with the desired performance gains.
• Current PCM integration methods across various systems are relatively limited and uniform. Exploring novel designs—potentially involving new heat exchanger geometries or hybrid approaches—could further improve heat transfer efficiency and reliability.
• The existing one-size-fits-all integration strategies may not accommodate varying operational or climatic demands. Modular or detachable PCM systems could adapt to shifting loads or seasonal changes, enhancing both system resilience and practical utility. A representative application involves integrating phase-change material (PCM) behind photovoltaic panels, where automated regulation enables heat absorption during peak operating temperatures, followed by detachment upon complete phase transition to facilitate thermal dissipation.
• Phase transition in PCMs is typically accompanied by volumetric changes that can induce mechanical stress on storage containers, shortening their lifespan. Ongoing research should focus on reducing or managing this expansion, whether through advanced encapsulation techniques or by modifying the PCM’s intrinsic properties.
• The majority of current experimental and modeling efforts address small-scale solar refrigeration units, such as household refrigerators or commercial freezers. Extending PCM integration to large-scale or industrial refrigeration systems represents an important frontier with the potential to substantially increase energy savings and system stability on a bigger stage.
• Future research should prioritize experimental and numerical investigations into concurrent PCM integration across multiple system components to enhance integration methodology flexibility while simultaneously advancing solar cooling system performance.