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Review

Recent Advances in Nanoencapsulated and Nano-Enhanced Phase-Change Materials for Thermal Energy Storage: A Review

by
Faïçal Khlissa
1,
Mohsen Mhadhbi
2,
Walid Aich
3,4,
Ahmed Kadhim Hussein
5,
Muapper Alhadri
3,
Fatih Selimefendigil
6,7,
Hakan F. Öztop
8,9 and
Lioua Kolsi
3,4,*
1
Laboratory of Physical Chemistry of Mineral Materials and Their Applications, National Center for Research in Materials Science, Technopole Borj Cedria BP 73, Soliman 8027, Tunisia
2
Laboratory of Useful Materials, National Institute of Research and Physicochemical Analysis, Technopole Sidi Thabet, Ariana 2020, Tunisia
3
Department of Mechanical Engineering, College of Engineering, University of Ha’il, Ha’il City 81451, Saudi Arabia
4
Research Laboratory of Metrology and Energy Systems LR18ES21, National Engineering School, University of Monastir, Monastir 5000, Tunisia
5
Mechanical Engineering Department, College of Engineering, University of Babylon, Hilla 51002, Iraq
6
Department of Mechanical Engineering, College of Engineering, King Faisal University, Al Ahsa 31982, Saudi Arabia
7
Department of Mechanical Engineering, Celal Bayar University, 45140 Manisa, Türkiye
8
Department of Mechanical and Nuclear Engineering, College of Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates
9
Department of Mechanical Engineering, Technology Faculty, Fırat University, 23119 Elazığ, Türkiye
*
Author to whom correspondence should be addressed.
Processes 2023, 11(11), 3219; https://doi.org/10.3390/pr11113219
Submission received: 29 August 2023 / Revised: 7 November 2023 / Accepted: 8 November 2023 / Published: 13 November 2023
(This article belongs to the Special Issue State-of-the-Art Thermal Energy Storage Systems)
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Abstract

:
Phase-change materials (PCMs) are becoming more widely acknowledged as essential elements in thermal energy storage, greatly aiding the pursuit of lower building energy consumption and the achievement of net-zero energy goals. PCMs are frequently constrained by their subpar heat conductivity, despite their expanding importance. This in-depth research includes a thorough categorization and close examination of PCM features. The most current developments in nanoencapsulated PCM (NEPCMs) techniques are also highlighted, along with recent developments in thermal energy storage technology. The assessment also emphasizes how diligently researchers have worked to advance the subject of PCMs, including the creation of devices with improved thermal performance using nano-enhanced PCMs (NEnPCMs). This review intends to highlight the progress made in improving the efficiency and efficacy of PCMs by providing a critical overview of these improvements. The paper concludes by discussing current challenges and proposing future directions for the continued advancement of PCMs and their diverse applications.

1. Introduction

In the realm of thermal energy storage, significant progress has been achieved in the creation of nanoencapsulated and nano-enhanced phase-change materials (PCMs) in recent years. These developments have made it possible to improve energy efficiency and sustainability across a number of industries. Researchers have unlocked a variety of innovative features and uses by fusing nanotechnology with phase-change materials, paving the way for improved energy savings and long-lasting thermal regulation in various industrial, commercial, and residential settings. This study intends to investigate the most recent developments in the creation and use of nanoencapsulated and nano-enhanced PCMs, casting light on their potential to change the way thermal energy is stored and used.
Rising world-wide energy demand in the building industry will jump 50% by 2050 [1]. In fact, according to the international energy agency (IEA), the building industry is the primary source for energy consumption, with about 40% [2,3,4]. A phase-change material (PCM) is the most useful method for storing thermal energy [5]. During phase changes, thermal energy is either absorbed (melting, evaporation) by the considered material, or released (solidification, condensation), at a constant temperature. Thermal energy storage (TES) involves storing energy by heating, freezing, solidifying, melting, or evaporating a specific material [6]. The energy is available to use when the process is repeated in the reversed mode. Latent heat storage (LHS) stocks energy in TES because of its efficiency in ensuring an elevated energy-storage density factor and its heat storage properties at a regular temperature [7]. Positive advances in cold-storage applications in air conditioning with PCMs have also been reported [8]. Moreover, a considerable number of investigations have been carried out to study heat transfer in numerous LHS systems [9]. While significant strides have been made in the utilization of PCMs, recent research and review papers have delved into further improving their efficiency and safety for thermal energy storage applications.
In 2020, several review papers were published on PCMs for TES applications [10,11,12,13,14,15,16,17,18,19,20]. Also, in 2021, Zhang et al. [21] presented a review paper on heat transfer in porous-based stabilized PCMs for TES. Pathak et al. [22] published a review of microencapsulated PCMs as slurries for TES. They summarized the different routes for slurry synthesis, discussed the techniques of characterization, and also presented the applications of slurries for TES. Noël et al. [23] used different freeze-cast matrices, including different PCMs, to prepare various forms of stable PCMs. They concluded that the obtained PCMs had better thermal conductivity, and good hardness and mechanical stability after 1000 cycles. Wu et al. [24] synthesized a thermal shape memory composite PCM including paraffin, triblock copolymer, and expanded graphite. They revealed that the obtained material exhibited greater shape and thermal stability. Peng et al. [25] reviewed the development process of nanoencapsulated PCMs, including nanoencapsulation processes, characterization, and applications. They summarized the structure, principles, and functions of nanoencapsulated PCMs. They also discussed the thermal and physicochemical properties of nano capsules and their various applications. Afghan and Bing [26] recently published a review on TES cement-based composites. They revealed that just a few countries are actively funding this new research field. In their study, Veismoradi et al. [27] focused on simulating the performance of energy transport in a shell-and-tube TES system involving the insertion of copper foam or paraffin wax with wax paraffin. They showed that the phase-change intensity was particularly significant in the case of porous metal foam. Recently, Kumar et al. [28] undertook a study on PCMs for TES application with the intention of developing a novel PCM with improved properties and safety. However, the aim of these research works was to improve heat transfer, avoid leakage, and inhibit corrosion.
This paper provides a comprehensive review of the various approaches used for PCM nanoencapsulation, emphasizing the complex methodologies and procedures used to insert nanoparticles into PCMs. The numerous uses of nanoencapsulated PCMs are examined in Section 2, which focuses on their employment in a variety of fields, including building construction, electronics, textiles, and automobile thermal-management systems. Additionally, in Section 3, this paper provides a thorough examination of the use of nano-enhanced PCMs in thermal energy storage systems, illuminating their function in improving the effectiveness and performance of energy-storage applications. This section focuses on how nanoparticles affect the improved PCMs’ thermal conductivity and stability. The extensive reviews’ major results and insights are then painstakingly summarized in Section 4, with particular attention paid to the theoretical implications and real-world applications that have been covered throughout the work. PCMs may be classified into three major families, as shown in Figure 1. According to their chemical compositions, phase-change materials (PCMs) can be classified into the following categories:
  • Organic PCMs: These are typically made up of paraffins, fatty acids, and esters, among other organic substances. They have benefits including inexpensive price, high latent heat, and adaptability to different applications.
  • Inorganic PCMs: These are made of inorganic components such metals, non-metals, and salt hydrates. Inorganic PCMs are renowned for their stability and strong heat-storage capacity and frequently exhibit high thermal conductivity.
  • Eutectic mixtures are made up of two or more materials that have a melting point that is lower than the sum of their parts. Eutectic PCMs are advantageous due to their high heat-storage capacity and sharp melting point.
Understanding the many characteristics and uses of PCMs in various thermal energy storage systems is made easier by this classification.

2. Nanoencapsulated PCMs (NEPCMs)

Moving from the comprehensive literature exploration to the in-depth analysis of nano-enhanced PCMs, in this section, we delve into the encapsulation of phase-change materials (PCMs), which has garnered significant attention due to its effectiveness in addressing leakage issues, enhancing structural stability, improving fracture resistance, and precisely controlling particle size [29]. On the other hand, numerous studies demonstrate that reducing capsule size contributes significantly to the enhancement of the PCM’s performance [30,31,32,33]. Indeed, nano capsules (NEPCM) are considered to be better than microcapsules (MePCM) for the following reasons [34]:
  • Applications related to heat transfer require the use of suspensions that are stable during pumping and flowing. Therefore, it is more interesting to use nanoencapsulated PCMs, which have higher suspension stability, super-high specific surface area, minimal pump breakage, and promising structures in terms of management and storage of energy.
  • PCMs can release or store heat by exchanging it with the surrounding environment. Indeed, these exchanges are more important and faster when the size of the PCM particles is reduced (high surface area to volume ratio).
  • Compared to the microencapsulation process, the nanoencapsulation of PCMs is expected to result in the fabrication of energy-storage systems with higher characteristics, and the generation of more heat transfer in the system.

2.1. Preparation of NEPCMs

The utilization, storage, and transportation of PCMs are easier when they are encapsulated. Therefore, many processes, such as polymerization and physicochemical or physicomechanical processes have been adopted [34,35,36,37] in order to produce nanoencapsulated PCMs. These techniques allow the fabrication of different NEPCMs with different shapes (regular or irregular), different numbers of cores within the capsules (single or several), and different numbers of walls [38]. As illustrated in Figure 2, if the particle size is in the range 1–1000 nm, we can say that we have a nanoencapsulated PCM (NEPCM) [32].
Shell materials used for encapsulation should be more thermally stable than the core materials, with a melting point higher than that of the PCM, and should not be affected by the variation in volume and pressure caused by the melting/solidification cycles.

2.1.1. In-Situ Polymerization

In-situ polymerization involves the process whereby chemical reaction takes place between two immiscible liquids (water-soluble phase and oil-soluble phase) in a continuous phase, such as emulsion/mini-emulsion, suspension, and interfacial polycondensation [39].
Figure 3 shows the encapsulation process, using in-situ polymerization, of n-octadecane with a resorcinol-modified melamine–formaldehyde shell.

Suspension Polymerization

A suspension polymerization-based process (Figure 4) can be used to prepare PCM micro and nano capsules with a polymer shell [41]. Indeed, suspension polymerization is a heterogeneous radical polymerization process, usually initiated by a monomer-soluble initiator, which is responsible for the generation of free radicals at the water–monomer droplet interface [42].

Emulsion/Mini-Emulsion Polymerization

It was Harkins who first proposed a mechanism of emulsion polymerization [44]. An emulsion-polymerization system consists of a monomer that is slightly soluble in the dispersing medium (water), an emulsifier, an initiator, and a modifier, if necessary. Tiny amounts of monomers form droplets, which are suspended and stabilized by the association of the emulsifier molecules to form micelles surrounding the monomers [45]. The mini-emulsion method is the most employed method to synthesize encapsulated PCMs with nanometric sizes because of its efficiency and simplicity [32]. Emulsion polymerization was adopted by Zhang et al. [31] to fabricate a series of NEPCMs formed of paraffin wax (core) and melamine–formaldehyde (shell) with a regular shape and an average diameter of 260–450 nm (Figure 5). In their study, the morphology and the thermal properties of the prepared NEPCMs were studied. The findings demonstrated that the paraffin wax was successfully encapsulated in the melamine–formaldehyde without chemical contact, and the nanoencapsulated phase-change materials (NEPCMs) exhibited a regular spherical form with an average diameter of 260–450 nm. The amount of supplied core material had to be increased to increase the NEPCMs’ encapsulation effectiveness. The NEPCMs’ highest encapsulation efficiency reached up to about 75%. After 2000 thermal cycles, the NEPCMs could still operate with good thermal stability and dependability. Due to their exceptional encapsulation efficiency and thermal properties, the produced NEPCMs were able to be effectively used in latent-heat thermal energy storage and thermal management systems.

Interfacial Polymerization

Interfacial polycondensation is an in-situ polymerization method which leads to the formation of a polymer shell at the interface between the two immiscible phases (water and oil) [46]. Park et al. [47] demonstrated that the incorporation of Fe3O4 nanoparticles in the polyurea shell (see Figure 6) was able to improve the thermal properties of NEPCM by increasing its thermal conductivity and reducing the supercooling of the paraffin (core PCM).
Nikpourian et al. [30] and Barlak et al. [48] used the interfacial polymerization method to encapsulate paraffin wax within a polyurethane shell. They obtained spherical nano capsules, with diameters between 25 and 185 nm and high thermal resistance, even after 100 melting/solidifying cycles. Solid fractions of the NEPCMs, prepared by Barlak et al. [48] were added to water and ethylene glycol to improve the thermal properties of the corresponding nanofluids with increasing temperature. The same synthesis method was adopted by Liang et al. [49] to encapsulate paraffin within a silica shell. The prepared nano capsules (169 to 563 nm) were thermally stable and had high heat-storage capability with the melting enthalpy and encapsulation ratios as high as 109.5 J g−1 and 51.5%, respectively.

2.1.2. Physicochemical Techniques

Coacervation

Coacervation is a physicochemical encapsulation technique which consists in the deposition of a single polyelectrolyte or a mixture of polyelectrolytes, initially in the solution around the core PCM material [32]. Figure 7 illustrates the flow diagram, which is an example of complex coacervation in which gum arabic encapsulates gelatin. This method has the advantage of allowing good control of NEPCM’s particle size, but it cannot prevent agglomeration and difficulty in scale-up.

Sol–Gel

Using the sol–gel method for the encapsulation of PCM-core materials with inorganic shell materials such as silica is a well-investigated method, since silica shell materials have better mechanical and thermal properties than polymer shell materials [41,51,52,53]. The sol–gel process refers traditionally to the hydrolysis and condensation of alkoxide-based precursors such as tetraethyl orthosilicate (Si(OEt)4, or TEOS) [54]. The sol–gel process can be described as illustrated in Figure 8.

Supercritical CO2-Assisted Encapsulation

Supercritical carbon dioxide (scCO2) has attracted much interest as an alternative solvent for materials synthesis and processing because it is non-toxic, non-flammable, and naturally abundant [55]. Haldorai et al. [56] summarized the synthesis of polymer-inorganic filler nanocomposites in scCO2 and concluded that although there are three methods for the preparation of nanocomposites (blending, sol–gel and in-situ polymerization), scCO2 has been demonstrated (Figure 9) to be a viable alternative to the conventional solvents.

2.1.3. Physicomechanical Techniques

Spray-Drying Techniques

Spray-drying techniques (Figure 10) were first developed in the 1930s. The spray-drying process uses relatively less energy and is ideal for temperature-sensitive materials with significant encapsulation efficiency [29].
The spray-drying encapsulation process usually gives polynuclear or matrix type microcapsules. However, the common problems of this rapid microencapsulation method remain its low coating ratio and the agglomeration of microcapsules [57]. The spray-drying method has mainly been employed to encapsulate PCM core materials with polymers [58,59,60].

Electrohydrodynamic Process

With the electrohydrodynamic process (encompassing electrospinning and electrospraying), it is possible to control the morphology of PCM capsules, and reduce their size to micro-, sub micro-, and nano-sized scales [61,62]. Figure 11 depicts the schematic diagram of the electrospraying method for producing silk fibroin nanoparticles. In electrospraying, the electric field forces the liquid pouring out of a capillary nozzle, which is kept at a high electric potential, to be scattered into microscopic droplets.
This technique was used by Chalco-Sandoval et al. [64,65,66] to prepare encapsulated PCM in different polymers, in order to prepare new TES systems for smart food packaging. The prepared composite materials had good energy storage capacity, and were dedicated to thermal insulation applications.

2.2. Applications of NEPCMs

Currently, there is an increase in the usage of NEPCM materials in various fields of modern technology as well as in engineering applications, ranging from drug delivery to thermal energy storage in buildings [67,68]. In general, NEPCM materials offer the four following noteworthy and distinctive properties [69]:
  • Solid-to-liquid phase transition;
  • Large amount of energetic changes;
  • Stabilization of temperature;
  • Variation in thermal conduction during phase transition.
However, it remains very difficult to classify NEPCMs because of the variety and specificity of their potential applications, which are illustrated in Figure 12.

2.2.1. Thermal Management of Electronic Devices

As the miniaturization of electronic devices increases, the problems of heat dissipation become more important. More efficient thermal management often leads to increased reliability and lifespan of devices. Indeed, the heat loads may be dissipated through a microchannel heat sink (MCHS) [70], which often uses liquid coolants (Figure 13) able to store considerable amounts of heat and with suitable thermophysical properties. A uniformly fixed temperature was applied to the model’s bottom surface in order to take into account the source of heat generation. The cover plate was totally insulated, thus there was no heat transfer through it. All the setups under study had the same fluid inlet temperature (296.15 K), which was purposefully chosen to be below the PCM particles’ melting point to ensure that the PCM particles entering the fluid were all in the solid state. The edges of computational domain borders were ignored in calculations and were thought to be symmetric. Two specialized 50- and 70-mm inlet and outlet blocks were considered to enable a fully developed flow along the channels, which had a 100 mm length. According to the scenarios examined, the configuration using nanofluid/NEPCM slurry coolants with 0.04/0.2 volumetric concentrations in upper/lower layers, respectively, were able to be used to achieve the system’s optimum flow and cooling performance. By increasing the NEPCM concentration to 0.3, the system’s cooling performance was improved, but the system’s overall efficiency decreased significantly. Nanofluid and NEPCMs are coolants of great interest in thermal management systems for high-tech cooling applications in microelectronics. Therefore, paraffins are frequently used in systems where the temperature must be kept below 40–45 °C, to melt, absorb, and dissipate the released heat [71,72,73].
Ho et al. [75] mention that the presence of working fluid (water/NEPCM particles) improves heat dissipation and the index of performance of the microchannel walls up to 70% and 45%, respectively. Krishna et al. [76] confirmed that the use of NEPCM formed using a mixture Al2O3 nanoparticles and tricosane as the energy storage medium for electronic cooling purposes contributed to the reduction of the evaporator temperature of the heat pipes by 25.75% and therefore was able to save approximately 53% of the fan power.

2.2.2. Food Industry

To preserve the cold chain, extend food products’ shelf-life and reduce microbial activity, some food products have to be marketed, distributed, and sold under freezing, chilled, or refrigerated conditions. NEPCMs have been used in the food industry for applications such as heat processing, chilled storage, and packaging [77,78,79]. Chalco-Sandoval et al. [65] synthesized a smart food package by incorporating a phase-change material formed of a commercial blend of paraffin with a transition temperature of 5 °C into the packaging structures (polystyrene film). The measured latent heat of the prepared smart package was about 88–119 J/g. McCann et al. [80] used a coaxial electrospinning-based method to fabricate phase-change nanofibers formed of long-chain hydrocarbon cores and composite sheaths. The large heat of fusion of the long-chain hydrocarbons endows the fabricated phase-change nanofibers with the ability to absorb, hold, and release large amounts of thermal energy over a certain range of temperature.

2.2.3. Thermal Storage in Buildings

According to a report by the international energy agency (IEA), in 2019, the global shares of final energy consumption and CO2 emissions by buildings and the construction industry were equal, 35% and 38%, respectively, with growing need for space heating and cooling [1]. A recent review article classified thermal energy storage (TES) applications in buildings using PCMs in two classes: active and passive systems.
  • Active TES systems: heat transfer is generated by forced convection and, in some cases, also by mass transfer such as with a heat exchanger [81,82]. Active PCM-based systems require mechanical equipment or an additional power source for their operation, such as electricity for pumps or fans. These systems are best suited to situations where greater heat-transfer performance or better application control is required.
  • Passive TES systems: the employed PCMs exploit naturally available energy sources (for example, solar power or wind) as well as the architecture of building components to minimize energy requirements [83]. Passive systems reduce the use of mechanical heating or cooling systems. There is no need for additional energy input as heat is charged or discharged when the temperature of the environment rises or falls beyond the phase-change temperature of the PCM. These PCMs can be used in building ceilings; floors; walls; cooling, heating, and hot water systems; etc. [84,85].

2.2.4. Solar Energy Storage

Solar energy is the most abundant source of energy on the earth. Indeed, average annual solar energy received represents 6000 times the current annual world energy consumption [86]. It is therefore essential to fully benefit from this inexhaustible source of renewable energy and find scientific and technological solutions to overcome the characteristic drawbacks of solar energy, which are:
  • It is intermittent (day/night);
  • It is random (thunderstorms and cloud passages);
  • It is diluted and shifted in relation to daily or seasonal energy demands.
One of the most interesting solutions proposed to remediate these inconveniences is the use of PCMs to store the excess of thermal energy given off by the sun using latent heat storage (LHS), then releasing it during peak hours to meet demand. Some studies have proved that the use of nanoparticles raises the heat transfer rate [87,88]. Xu and Zhou [89] synthesized NEPCMs with a size of 600–900 nm which had Cu and Cu2O as a shell and paraffin as the core. They reported excellent LHS capacity and thermal conductivity, with about 127.55 J g−1 and 0.92 Wm−1K−1, respectively. Hammou and Lacroix [90] have proposed hybrid thermal–electrical energy storage. Their results showed that energy consumption was considerably reduced.

2.2.5. Heat Exchangers

The main selection characteristics of heat exchangers are their capacity of storing heat energy and the rates at which they release and absorb this heat. To improve these characteristics, PCMs can be incorporated into heat exchangers. They contribute to decreasing the pipeline’s size, the heat exchanger’s size, and the transport energy consumption [33,71]. As shown in Section 3, it has been proved that the addition of nanoparticles to PCM leads to the enhancement of the stored thermal energy [91].

2.2.6. Smarts: Textiles, Clothes, and Footwear

The need for humans to have thermal comfort has pushed them to develop “smart” textiles, which can acclimatize automatically to environmental changes. Textiles containing PCMs are “smart” materials, since they react immediately to changes in environmental temperatures, and the temperatures in different areas of the body, by absorbing heat and storing it in the liquified PCM when a rise in temperature occurs. However, when the temperature falls again, the PCM releases the stored heat energy and solidifies again [92]. Fiber technology, coating, lamination, and micro and nano encapsulation methods have been found suitable for incorporating PCMs in textiles. When choosing the PCM type, importance should be given to the quantity of PCM to be used, its latent heat, its melting temperature, its crystallization temperature, and the area covered by the PCM on the human body [93,94,95]. Karthikeyan et al. [96] used the pad-dry-cure method to apply coated nano capsules (average size of 260 nm) over cotton fabric. These thermally stable nano capsules have a latent heat of fusion of 74.2 J g−1 and a melting temperature of 64.30 °C. For cotton fabric with a 20 wt% and a 40 wt% coating of nano capsules, the heat storage capacities were 1.52 and 1.91 J g−1, respectively. The tensile strength, water absorption, and abrasion resistance of the coated cotton fabric were improved. In the early 1980s, NASA aimed to reduce the influence of variations in extreme temperature encountered by astronauts during space missions by encapsulating PCMs in the textile fibers of the space suits [97,98].

3. Nano-Enhanced PCMs for Thermal Energy Storage Systems

This section is dedicated to the most important recent studies on the use of NEnPCMs to enhance the performance of thermal storage energy systems with mathematical, analytical and numerical modeling. Expanding on these critical recent studies concerning the utilization of NEnPCMs to optimize the performance of thermal energy storage systems through experimental, mathematical, analytical, and numerical modeling, this section provides an overview of their key findings, which are summarized in Table 1.
Karaağaç et al. [99] undertook on experimental study on a concentrated photovoltaic–thermal solar dryer (CPV/TSD) using a nano-enhanced PCM (Al2O3- paraffin wax). The authors presented a thermodynamic study to explain the rate of overall thermal energy in the system and the overall thermal energy efficiency of the system. Moreover, they calculated the exergy efficiency for the CPV/TSD system and the drying mass was estimated based on the difference between the initial and final mass. The moisture ratio (MR) and drying rate (DR), during drying experiments, were also calculated based on previous works [100,101,102,103,104,105]. The authors observed an increase in the efficiency values after the 450th minute; therefore, they recommended the use of NEnPCM to prevent the decrease of the greenhouse temperature during the drop in the amount of solar radiation after 450 min and the drop in the environmental temperature. In addition to the experimental work, the authors predicted the dryers’ parameters via ANN and SVM. The use of Al2O3- paraffin wax for thermal energy storage considerably improved the performance of the dryer and energy and exergy efficiencies were found to be 20% and 8%, respectively. Aqib et al. [106] performed the preparation and characterization of PCM enhanced by alumina (Al2O3), metallic nanoparticles (NPs), and multiwall carbon nanotubes (MWCNTs), nonmetallic NPs. The purpose of the investigation was to increase the thermal conductivity of the PCM (paraffin wax) by adding Al2O3 and MWCNTs NPs, at three different concentrations: 2 wt%, 4 wt% and 6 wt%. It was deduced that the maximum peak temperature was enhanced with the increase of the NPs’ concentration, resulting in better PCM charging and discharging. Indeed, the recorded maximum peak temperatures were 65 °C and 73.99 °C for the samples prepared by adding 6 wt% of Al2O3 and MWCNTs NPs, respectively. Thus, samples containing MWCNTs can be used for the improvement of heat transfer in a thermal management system. Elarem et al. [107] undertook a numerical study on the effect of using NEnPCMs and fins on the performance of a solar collector’s evacuation tube. It was found that both fins and nanoparticles contributed to the enhancement of the thermal efficiency of the system. Chen et al. [108] studied the melting performance of a PCM-filled enclosure with triangular double fins using a 2D configuration. This 2D enclosure was employed as the latent thermal energy storage (LTES) unit to study the effect of porosity and nanoparticles on the melting performance of lauric acid, used as a PCM material. Different cases were considered in this study, including pure PCM, pure PCM + NPs, pure PCM + Porosity, and pure PCM + NPs + porosity. The enthalpy porosity method was employed to analyze the melting performance of the considered system. It was found that adding a porous medium would lead to significant improvement in the melting process. Indeed, the best performance (93% reduction in the total melting time) was recorded with a porosity of 95%. However, the simultaneous addition of NPs and porosity to the PCM contributed to the deterioration of the melting performance of the LTES system. Saeed et al. [109] presented the progress of form-stable eutectic mixtures for enhanced thermal performance. These additives may lead to the next generation of PCMs. A thickening agent was added to the PCM to decrease its fluidity and to overcome the liquid-leakage problem. Graphene nano-platelets (GNPs) were added to increase thermal and heat transfer properties. The authors found that adding 10% NGPs to the PCM enhanced thermal performance as follows:
Thermal conductivity was enhanced approximately 97.7% for liquid phase and 102.2% for solid phase. It was found that the specific heat capacity was enhanced by 64% for liquid phase and 52% for solid phase. In addition, thermal diffusivity was raised by 54% for liquid phase and 47% for solid phase.
Alomair et al. [110] undertook a numerical and experimental analysis of the effect of using a bio-nano-PCM on the performance of concentric cylindrical TESS. They investigated the melting process numerically by calculating the heat transfer rate and the melt fraction. In addition to the numerical study, they developed an experimental setup working under the same conditions. The experimental and numerical findings revealed that the addition of the CuO-nanoparticles provided a faster melting rate and high heat transfer. An experimental study on the effect of adding GNP to laden microencapsulated PCM on the thermal performance of a heat sink was performed by Praveen et al. [111]. The results showed that the use of GNP led to an enhancement in thermal conductivity and the reduction of the temperature rise rate. Ho et al. [112] used a water-based nano-PCM with the aim of improving the cooling performance of a heat sink Several governing parameters such as the volume–flow rate and nanoparticle concentration were varied. It was found that the addition of nanoparticles at higher flow rates led to better cooling performance. A numerical study based on the finite element method was conducted by Hosseinzadeh et al. [100] to investigate the solidification process in an LTES. The effects of adding hybrid nanoparticles and using fins were studied. It was observed that the combination of these techniques caused a reduction in the solidification time by about 78%. Kumar and Mylsamy [113] studied the performance of an evacuated tube solar water heater with a nano-CeO2 embedded PCM system. By varying the governing parameters and conducting an optimization study, it was found that the NEnPCMs with 1.0% of CeO2 performed best. Similar conclusions regarding the enhancement of the performance were obtained by Hosseinzadeh et al. [114], using hybrid nanoparticles and various shapes of fins. Al-Jethelah et al. [115] performed a CFD study and an experimental investigation of the heat transfer and the flow field inside a nano-PCM-filled TESS. The experimental and numerical results indicated that the addition of nanoparticles caused a considerable enhancement of the melting process due to the increase in the effective thermal conductivity. Parameshwaran and Kalaiselvam [116] developed an air-conditioning system coupled to a silver nano-based PCM-TESS. The results indicated that the new air-conditioning system achieved an on-peak and per-day average energy savings potential of 36–58% and 24–51%, respectively, compared to the conventional system. Zadeh et al. [117] used partially filled copper foam and Cu/GO nanoparticles to improve the performance of an LHTES. The authors mention that the energy needed for the charging phase was enhanced by about four times when compared to pure PCM. Khan and Khan [118] correlated the effects of extending fins and adding graphene nanoparticles to enhancement of LHTES performance. It was found that the optimal nanoparticles volume fraction was 1% for all the considered fin lengths. Ebadi et al. [119] carried out a numerical and experimental study of the melting process inside a TESS filled with NEnPCMs. They concluded that the addition of nanoparticles had no tangible effect on the melting process. Ebadi et al. [120] studied several configurations of TESSs filled with NEnPCMs. They concluded that the melting process was faster for NEnPCM than for pure PCM, especially at the end. Hosseinzadeh et al. [114] investigated the effect of using hybrid nanoparticles on hexagonal triplex LHTESS numerically. They used the FEM to solve the governing equations and concluded that using nanoparticles reduces the solidification time by about 12%. Hajizadeh et al. [121] modeled the heat transfer, flow field and entropy generation during the discharging phase inside a NEnPCM-filled TESS. The authors investigated the effect of nanoparticle shapes and found that, for all the cases, the solidification time was reduced by increasing the nanoparticle volume fraction.
Table 1. Summary of the analytical and numerical studies of NEPCM in TESS.
Table 1. Summary of the analytical and numerical studies of NEPCM in TESS.
AuthorsConfigurationUsed PCM(s)Used Nanoparticle(s)
Karaagaç et al. [99]Processes 11 03219 i001paraffin waxAl2O3
Aqib et al. [106]Processes 11 03219 i002paraffin waxAl2O3 and MWCNTs
Elarem et al. [107]Processes 11 03219 i003paraffin waxcopper (Cu)
Chen et al. [108]Processes 11 03219 i004lauric acidAl2O3
Saeed et al. [109]Processes 11 03219 i0052-hydroxypropyl ether cellulose is introduced to stabilize the PCMnano-graphene platelets (NGPs)
Alomair et al. [110]Processes 11 03219 i006coconut oil biobased in PCMcopper oxide
Ali et al. [122]Processes 11 03219 i007paraffin waxnano graphene
composite
Gong et al. [123]Processes 11 03219 i0081-octadecanol (OD)nano-silicon carbide (SiC),
expanded graphite (EG)
SiC/EG composite
Raj et al. [124]NA: Characterizationmanganese organo-metallic SS-PCMliquid metal gallium
Hosseinzadeh et al. [100,101]Processes 11 03219 i009waterhybrid nanoparticles (HNP) (MoS2–Fe3O4) and (TiO2-Go)
Ebadi et al. [119,120]Processes 11 03219 i010coconut oilcopper oxide CuO
Khan and Khan [118]Processes 11 03219 i011paraffingraphene nano-platelets (GNP)
Parameshwaran, and Kalaiselvam
[116]		Processes 11 03219 i012not specifiedsilver nanoparticles
Al-Jethelah et al. [115]Processes 11 03219 i013coconut oilCuO
Hosseinzadeh et al. [114]Processes 11 03219 i014waterMoS2–TiO2
Kumar and Mylsamy [113]Processes 11 03219 i015paraffin and NEPCMsnano-CeO2
Ho et al. [112]Processes 11 03219 i016watern-eicosane
Praveen et al. [111]Processes 11 03219 i017paraffin
(ME/GnP PCM)		graphene nano-platelets (GnP)
Li et al. [125]Processes 11 03219 i018 ternary carbonate salt without nanoparticles:
Encapsulated PCM
Elbahjaoui et al. [126]Processes 11 03219 i019paraffin wax P116copper
Kumar et al. [127]Processes 11 03219 i020PCMcalcium carbonate, silicon carbide, copper
Ben Khedher et al. [128]Processes 11 03219 i021paraffin waxCNT, Al2O3
Kolsi et al. [129]Processes 11 03219 i022Encapsulated ParaffinCNT

4. Conclusions

With a focus on recent developments and advances in the field, the current paper offers a thorough overview of phase-change materials (PCMs) in the context of thermal energy storage. It presents a thorough classification of the three primary PCM types—organic, inorganic, and eutectic PCMs—and provides an in-depth explanation of the various preparation processes used in their synthesis. The emphasis is on PCMs’ uses in thermal energy systems, with a special emphasis on the newly developed field of nano-enhanced phase-change materials (NEnPCMs). When nanoparticles (NPs) are added to PCMs, stable NEnPCMs with much better thermal conductivity are created. This paper explains that the interface polymerization method, in-situ polymerization method, sol–gel method, and emulsion polymerization method are the common methods used for the creation of NEPCMs. Additionally, it emphasizes how the amount of nanoparticles and operation temperature have a significant impact on the thermophysical characteristics of NEnPCMs. The majority of study results point to the role that PCM integration, especially when enhanced by nanotechnology, plays in the creation of high-efficiency thermal energy systems. The fact that NEnPCMs are used in a variety of industries, including waste heat recovery, electronic thermal management, solar heat collecting, and building insulation, highlights how versatile they are. Additionally, it has been demonstrated that adding fins to NEnPCMs significantly improves their thermal performance. Together, these results highlight the potential of NEPCMs and NEnPCMs for overcoming significant obstacles in energy-efficient applications and open the door for further breakthroughs in PCM use across a range of industries.

Funding

This research has been funded by Scientific Research Deanship at University of Ha’il, Saudi Arabia through project number RG-23 006.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

ANNArtificial Neural Network
CFDComputational Fluid Dynamics
CPV/TSDConcentrated Photovoltaic–Thermal Solar Dryer
DRDrying Rate
FEMFinite Element Method
GNPsGraphene Nano-Platelets
IEAInternational Energy Agency
LHSLatent Heat Storage
LHTESLatent Heat Thermal Energy Storage
LTESLatent Thermal Energy Storage
MCHSMicrochannel Heat Sink
MePCMMicro-Encapsulated Phase-Change Material
MRMoisture Ratio
MWCNTsMultiwall Carbon Nanotubes
NEPCMNanoencapsulated Phase-Change Material
NEnPCMNano-Enhanced Phase-Change Material
NPsNanoparticles
PCMPhase-Change Material
SEMScanning Electron Microscope
SFSilk Fibroin
SVMSupport Vector Machines
TESThermal Energy Storage
TESSThermal Energy Storage System

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Figure 1. Classification of PCMs based on their chemical compositions.
Figure 1. Classification of PCMs based on their chemical compositions.
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Figure 2. Classification of encapsulated phase-change materials as a function of their size.
Figure 2. Classification of encapsulated phase-change materials as a function of their size.
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Figure 3. Illustration of the in-situ polymerization process for the formation of the encapsulated PCM, based on n-octadecane core and resorcinol-modified melamine–formaldehyde shell [40].
Figure 3. Illustration of the in-situ polymerization process for the formation of the encapsulated PCM, based on n-octadecane core and resorcinol-modified melamine–formaldehyde shell [40].
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Figure 4. The suspension polymerization process used for the preparation of NEPCM [43].
Figure 4. The suspension polymerization process used for the preparation of NEPCM [43].
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Figure 5. SEM micrographs of the NEPCMs prepared by Zhang et al. [31] for various Average Particle Size: (a): 449.6 nm, (b): 295.5 nm, (c): 261.3 nm, (d): 397.8 nm, (e): 352.3 nm, (f): 295.5 nm, (g): 339.4 nm.
Figure 5. SEM micrographs of the NEPCMs prepared by Zhang et al. [31] for various Average Particle Size: (a): 449.6 nm, (b): 295.5 nm, (c): 261.3 nm, (d): 397.8 nm, (e): 352.3 nm, (f): 295.5 nm, (g): 339.4 nm.
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Figure 6. Interfacial polycondensation synthesis of Fe3O4 nanoparticle-embedded paraffin/polyurea NEPCM [47].
Figure 6. Interfacial polycondensation synthesis of Fe3O4 nanoparticle-embedded paraffin/polyurea NEPCM [47].
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Figure 7. Flow diagram of a complex coacervation of gelatin with gum arabic [50].
Figure 7. Flow diagram of a complex coacervation of gelatin with gum arabic [50].
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Figure 8. Schematic representation of a sol–gel process [29].
Figure 8. Schematic representation of a sol–gel process [29].
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Figure 9. The three general approaches for preparation of polymer/inorganic filler nanocomposites in scCO2.
Figure 9. The three general approaches for preparation of polymer/inorganic filler nanocomposites in scCO2.
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Figure 10. Flow diagram of a typical spray-drying encapsulation process [50].
Figure 10. Flow diagram of a typical spray-drying encapsulation process [50].
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Figure 11. Preparation of silk fibroin (SF) nanoparticles via electrospraying [63].
Figure 11. Preparation of silk fibroin (SF) nanoparticles via electrospraying [63].
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Figure 12. Main fields of NEPCMs applications.
Figure 12. Main fields of NEPCMs applications.
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Figure 13. Microelectronics cooling using PCM in MCHS [74].
Figure 13. Microelectronics cooling using PCM in MCHS [74].
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Khlissa, F.; Mhadhbi, M.; Aich, W.; Hussein, A.K.; Alhadri, M.; Selimefendigil, F.; Öztop, H.F.; Kolsi, L. Recent Advances in Nanoencapsulated and Nano-Enhanced Phase-Change Materials for Thermal Energy Storage: A Review. Processes 2023, 11, 3219. https://doi.org/10.3390/pr11113219

AMA Style

Khlissa F, Mhadhbi M, Aich W, Hussein AK, Alhadri M, Selimefendigil F, Öztop HF, Kolsi L. Recent Advances in Nanoencapsulated and Nano-Enhanced Phase-Change Materials for Thermal Energy Storage: A Review. Processes. 2023; 11(11):3219. https://doi.org/10.3390/pr11113219

Chicago/Turabian Style

Khlissa, Faïçal, Mohsen Mhadhbi, Walid Aich, Ahmed Kadhim Hussein, Muapper Alhadri, Fatih Selimefendigil, Hakan F. Öztop, and Lioua Kolsi. 2023. "Recent Advances in Nanoencapsulated and Nano-Enhanced Phase-Change Materials for Thermal Energy Storage: A Review" Processes 11, no. 11: 3219. https://doi.org/10.3390/pr11113219

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