Improving the Heat Transfer of Phase Change Composites for Thermal Energy Storage by Adding Copper: Preparation and Thermal Properties

Abstract

Phase change materials (PCMs), as an effective thermal energy storage technology, provide a viable approach to harness solar heat, a green energy source, and optimize energy consumption in buildings. However, the obstacle preventing widespread practical use of PCM is its poor performance in terms of heat transfer and shape stabilization. This article focuses on the application of the shape stabilization method. To improve the thermal conductivity of organic PCMs (hexadecane), copper microparticles are added to form phase change composites (PCC). This process allows an enhanced PCM (75 wt%) that distributes effective thermal storage capabilities while maintaining low cost. SEM, FTIR, ATG, infrared thermography (IRT), and DSC were used to characterize the composites’ micromorphology, chemical composition, thermal degradation stability, and thermal energy storage capabilities. DSC results showed that a proportion of 75 wt% phase change material with 15 wt% Cu had excellent thermal stability and high energy storage density per unit mass. In light of its high latent heat storage capacity of 201.32 J/g as well as its ability to prevent Hexadecane exudation, PCC ensures higher thermal conductivity and shape stability during phase transition than ordinary PCM. The incorporation of Cu to paraffin causes delay in PCM phase transformation, leading it to respond to rapid charging and discharging rates and, consequentially, to challenges in temperature control, as shown by IRT. The new PCCs had favorable thermal stability below 100 °C, which was advantageous for practical application for thermal energy storage and management, and notably for solar thermal energy storage.

1. Introduction

In response to the increasing global need for a sustainable and low carbon energy, more and more attention has been paid to increase the exploitation of renewable energy sources (RES), especially if integrated with the built environment. However, solar and industrial waste heat are discontinuous and have different cycles which vary from generation to generation. The energy storage techniques, such as Hydrogen Energy Storage System (HESS), Thermal Energy Storage (TES) Battery Energy Storage (BES), and Electric Vehicles (EVs) have been suggested to establish any country’s environmental and economic sustainability. The stored energy can then be used for thermal regulation or to generate electricity [1]. Due to its high storage density and relatively low cost, TES provides sustainable, reliable, clean, non-intermittent energy. TES thermal storage capacity is typically 1000 times larger than hydroelectric storage and 2 times larger than electrical storage [2]. According to the thermal mechanism used to store energy, TES can be classified in three types: latent heat energy storage (LHTES), sensible heat energy storage (SHTES), and thermochemical energy storage. Particularly promising is LHTES using latent heat phase change materials (PCMs), where thermal energy is stored or released during the melting or solidification phase transition of the PCM [3,4,5].

The use of PCMs can also have significant impacts on environmental health. For example, in sustainable buildings, PCM has been identified as an effective way to keep indoor temperatures constant for long periods of time close to thermal comfort [6]. This reduces heating and ventilation [7], which account for more than half comfort in a building ‘s thermal energy consumption, and therefore reduces energy consumption and greenhouse gas emissions. This can help drive decarbonization and combat climate change. Likewise, PCMs are involved in various intelligent engineering processes in terms of medicine and vaccine transportation [8,9], electronic thermal management [10], smart clothing systems [11], etc.

PCM constitute an essential field of research and development in geophysics, where a precise understanding of the mechanical and thermal properties of partially molten rocks near phase transitions is crucial for the correct modeling of igneous crustal systems [12]. Most organic materials such as alcohol and polyalcohol [13], fatty acids [13], PEG [14,15], erythritol [7], and PW [8] melt in the range of useful purposes of human comfort, chemically stable, non-toxic, non-corrosive, and readily available. Despite such advantages, organic phase change materials have some inherent problems like low thermal conductivity, leakage during melting, and instability over the charging and discharging of cycles. To prevent leakage and loss during phase transition, PCMs are usually encapsulated in a support matrix [16,17,18,19,20]. The TES system is prepared by melting and mixing a polymer with suitable organic PCM (such as paraffin wax) through a simple impregnation method, forming a three-dimensional network of macroscopic polymer chains and trapping the PCM within them like a net. To meet these criteria, shape-stable phase change composites (PCCs) overcome the disadvantage of high leakage, outperform conventional solid–liquid PCMs, and are more practical in applications. Based on the literature review, polyethylene seems to be the most commonly used component/matrix to obtain shape-stable PCCs. This is mainly thanks to its availability, low cost, and ease of processing. Moreover, the thermoplastic polymer is the most extensively used polymer for paraffinic PCC applications because of their similar chemical structures [21]. Moreover, a large number of researchers have studied the thermal properties of the elastomers improved with the PCM content.

Juarez et al. [22] relate to the incorporation of PCM on an elastomeric matrix of styrene-ethylene/butylene-styrene (SEBS) matrix to enhance the thermoregulating capacity of elastomers while maintaining good strength and ductile properties for PCM concentrations of 1–5% [22]. PCMs still have limitations due to their low thermal conductivity which slows down the rate of charge and discharge [3]. Therefore, finding a way to improve the thermal properties of PCMs is imperative. To improve the thermal conductivity of organic PCMs, the use of pre-built porous support structures made of highly thermally conductive materials is a good strategy to fabricate shape-stable PCCs. PCM additives like meatal powder, carbon nanotube (CNT), metal screens, BN foams, metal spheres or utilizing metal fins and PCM absorption in porous metal foam or expanded graphite matrix are used to form composites which are an ideal choice to enhance the performance of PCCs [23,24].

Adding highly thermally conductive particles in micro and nano sizes is the second way to increase the overall conductivity of the PCM. The ability to improve the thermal conductivity of micro/nanoparticles varies depending on the addition ratio, the congruence between PCM and micro/nanoparticles, and physical properties (e.g., particle size and shape).

Besides improving thermal conductivity, micro/nano-particles are quite effective in eliminating supercooling. Due to the uniform heat dissipation through the molten PCM, the degree of supercooling decreases considerably. Metallic particles, including copper [25,26], nickel [27], silver [28,29], Al₂O₃ [30] and TiO₂ [31,32] carbon-based materials (CNT) [33,34], graphene [35], graphite [36,37], GO/rGO [38,39], CFs [40,41], EG [42,43] and graphene aerogels (GAs), [44,45] and materials-based ceramics (boron nitride (BN) [46,47] and aluminum nitride (AlN) [48,49] are added to form phase change composites (PCC). The addition of nano-Cu particles into sodium acetate trihydrate was assayed by Cui et al. [50], improving the supercooling and thermal conductivity. The melt and freeze experiment revealed that the composite PCM’s thermal conductivity increased by almost 20%, and adding 0.5% nano-copper as the recommended amount decreased the supercooling degree by roughly 0. 5. Furthermore, copper nanoparticles with an average particle size of 20 nm have been dispersed in paraffin to synthesize Cu-PCM nanocomposites [51].

The addition of up to 2 wt% Cu has been found to increase the thermal conductivity of paraffin by 4.3%. Moreover, the presence of nanoparticles also showed a significant reduction of the supercooling in the charging and discharging processes. Akram Fadhl Al-Mahmodi et al. [52] experimentally investigated the enrichment of PCM paraffin with non-added metals of different types and concentrations and analyzed their thermal behavior. Adding 2.5% Cu nanoparticles was found to increase the thermal conductivity of paraffin by 39%. Overall, it was seen that the thermal conductivity of nano-Cu led to good heat transfer performance, specifically causing less longer discharging times compared to PCM and good thermal reliability in the proposed applications, even after 150 thermal cycles at different temperatures. Another beneficial feature of the dispersion of nanoparticles in PCMs apart from enhancing thermal conductivity is to reduce charging/discharging times.

It also leads to longer melting times due to increased viscosity [53]. Wu et al. [54] also added Cu nanoparticles for thermal conductivity enhancement of paraffin. PCM material with metallic nanoparticles has good thermal reliability for 100 cycles. However, most studies have shown that the addition of micro/nanoparticles leads to a significant reduction of the latent heat capacity with increasing nanoparticle content [55,56]. Although there are many researchers involved in this field, there is almost no complete summary of the statement “what is the effect of adding micro/nanoparticles on latent heat”. The addition of particles can increase or decrease the latent heat. Thus, this setback affects the storage of micro/nanoparticles as well as improving thermal conductivity. Supercooling is the major drawback of PCMs, making it difficult to recover the stored energy at the desired temperature, limiting the applications of PCMs. The micro/nano-additives restrain the supercooling by acting as nucleating agents, thus helping crystallization [57].

Enhancing PCM properties through micro/nano-additives is recognized as an effective method to improve heat storage/release performance in TES applications, thereby expanding the use of solar thermal technology applications. Considering this presentation and the gaps identified in the literature, the present study experimentally investigated the enhancement of SEBS/Hex/LDPE using micro-additive copper of different concentrations and analyzed their thermal behavior. The (PCCs) containing Cu were prepared by a physical impregnation process.

SEM, FT-IR, TGA, IRT, and DSC techniques were used to characterize these composites. The enhancement of heat transfer efficiency was assessed and compared. The proposed PCC can be used in a variety of applications, including building materials to lessen indoor temperature fluctuations, improve thermal comfort, and reduce electricity consumption.

2. Materials and Methods

Hexadecane (C₁₄H₃₄, Sigma Aldrich with 99% purity) is an organic paraffin with a phase change temperature of 18–20 °C, chosen because it is stable, homogeneous, and has a high latent heat (above at 224 kJ/kg). Styrene-ethylene-butylene-styrene, also known as SEBS, is a linear triblock copolymer (Kraton G1650 M); this and low-density polyethylene (LDPE) were chosen as support materials because of their excellent compatibility with hexadecane. Microparticle copper (Cu 99.9% pure) with particle size <120 μm was used. Toluene (C₇H₈, analytical reagent, Sigma Aldrich) was used as solvent. The three-step fabrication process as shown in (Figure 1) is adopted for the synthesis of the PCC composite material (

Table 1. Summary of PCC composition (wt.%).

PCCs	SEBS	Hex	LDPE	Cu
PCC1	0	75	25	0
PCC2	25	75	0	0
PCC3	20	75	5	0
PCC4	15	75	5	5
PCC5	10	75	5	10
PCC6	5	75	5	15

). The steps are as follows: (1) SEBS and LDPE were added to a solution of PCM in toluene (T = 80 °C, t = 45 min and speed 800 rpm). (2) The copper microparticles were added to the SEBS/Hex/LDPE solution at 80 °C for 30 min under vibration by an electric ultrasound at a frequency of 40 kHz. The vibrational energy generated at this frequency aids in the uniform dispersion of the microparticles in the base liquid PCM. (3) After vaporization of the solvent, the composite was placed in a Teflon mold having a size of 45 mm in diameter for hot pressing. The same process produced parallelepipedal phase change composites (PCCs) with different mass fractions such as 5%, 10%, and 15% Cu (Table 1).

Chemical and Microstructure Characterization

Using an environmental scanning electron microscope (Hitachi S-3400N), the PCCs’ surface morphology and microstructure were examined without seepages or leaks. Chemical stability analysis of the composites PCM was carried out using a Perkin Elmer FT-IR system spectrum BX using an ATR mode under the wave range 4000 cm⁻¹–400 cm⁻¹ with 4 cm⁻¹ resolution, at an operating voltage of 3 kV outfitted with an X-Ray energy dispersion spectrometer dot.

Thermal characterization: For the practical application of organic PCMs, researchers have paid close attention to the shape stability and strong mixing ability. Shape-stabilization is the process to integrate PCM into matrix could potentially avoid PCM leakage. (LDPE/SEBS) matrix, formed stable via cross-linking, has been employed as a PCM supporting material due to its high structural strength.

The filter paper was weighed before the test, and then a known mass of composite material was added on top. Compared to the first weight, it can be determined whether there is a loss in phase change processing. The leak rate can be calculated as follows from Equation (1):

$$L\left(\%\right) = \frac{\left(M_0-M_n\right)}{M_0}\ast 100\hspace{1em}$$

(1)

Differential scanning calorimetry (DSC Q1000) is used to study the thermal transitions of materials such as melting temperatures and enthalpies (fusion/crystallization) of Hex and PCC. DSC measurements were performed at 10 °C/min of heating and rates and temperature ranges of [(−80 °C)–(160 °C)] and [(160 °C)–(−80 °C)], respectively.

Thermal stability was an essential parameter for form-stable phase change materials in latent heat storage applications and could be measured by thermogravimetric analysis.

Thermogravimetric analysis (TGA) Q5000 was used to measure PCCs over the temperature range of 25–600 °C at a heating rate of 10 °C/min and under a nitrogen flow of 20 mL/min on samples weighing between 2 and 4 mg. Using an infrared thermographic camera that creates images based on the apparent radiant temperature of the target surface, thermography of samples is performed. Figure 2 depicts the IRT test setup used for this investigation, which also includes a thermal imaging camera (FLIR) and a hot plate. IR imaging devices measure IR radiation emitted by objects in the field of view in order to estimate temperature. The devices are calibrated to estimate the temperature of objects in the field of view based on the IR signal and, in the case of the FLIR One Pro, are accurate to ±3 °C according to the manufacturer’s specifications. One of the factors that can affect accurate temperature estimation is dependent on an object’s emissivity, which is a parameter that describes how effectively an object emits infrared radiation. Perfect blackbody emitters (flat, nonreflective surfaces) have an emissivity of 1.0, whereas the opposite extreme includes perfect reflectors (emissivity of 0.0).

The heating plate was preheated to 30 °C before starting the IRT test. The temperature at the center of the samples was then measured at 60 s intervals for 38 min in order to create a thermal image (thermogram) from the infrared irradiance that was emitted by the surfaces and the data. The relative intensity of thermal radiation emitted from various regions of the surface is depicted in this thermogram. The temperature and distribution of the surface, its properties, the environmental conditions, and the sensor’s performance all have an impact on how much radiation is detected by the image.

3. Results

3.1. Morphologies and Microstructures of PCCs

The morphology and microstructure of PCC3, PCC4, PCC5 and PCC6 as shown in Figure 3 were investigated by scanning electron microscopy (SEM) to verify whether the Cu-based PCM was successfully incorporated into its matrix. Figure 3 shows the SEM images of PCCs before and after embedding different Cu weight fractions (5%, 10%, and 15%) into their matrix for each magnification. These micrographs show that the paraffin dispersed in the solid LDPE/SEBS network allowing the composites to retain their shape in the solid state without seepage of the molten paraffin [58,59,60,61]. A similar morphology was observed for all composites because shape-stable PCMs are made of Hex and LDPE presented a similar chemical and structural properties, ensuring good compatibility between the two components. As can be seen in the SEM photograph of PCC4, PCC5, and PCC6, the appearance of Cu increased significantly after the incorporation of Cu microparticles fillers. We can see more clearly that the Cu microparticles seem to be well dispersed into the SEBS/Hex matrix. This is representative of good integration of Cu in the SEBS/Hex mixture without formation of clusters. This type of SEBS/Hexadecane/LDPE/Cu composite microstructure proved to be valid in solving the leakage problem through the leakage test.

3.2. Chemical Compatibility of PCCs

The chemical analysis was conducted by FT-IR spectra of 400 to 4000 cm⁻¹ for Hex (PCM), LDPE, SEBS, and PCCS (Figure 4). In the LDPE spectrum, four bands observed at about 293 cm⁻¹, 2850 cm⁻¹, 1458 cm⁻¹ and 717 cm⁻¹ [62] characterize the vibrational modes of the C-H bonds in CH₃ and -CH₂ groups. In the pure n-hexadecane spectra, the symmetrical stretch vibration of the CH₃ and CH₂ group led to peaks at 2956 cm⁻¹ 2916 cm⁻¹ and 2848 cm ⁻¹. The bands at 1471, 1461, 1376 cm ⁻¹ are linked to the deformation mode of CH₃ and CH₂ vibration groups. In the SEBS spectrum, besides the characteristic bands of the C-H bond vibration of the alkyl groups, we observed a strong band at 757 cm⁻¹ associated with the stretching mode of the monosubstituted benzene ring [63,64,65]. The peaks observed on the IR spectrum of PCCs at about 2800–3000 cm⁻¹ for C-H stretching and 1350–1500 cm⁻¹ for CH₂ deformation is similar to all previous reports [66]. IR spectra of PCCs show specific absorption regions at about 2800–3000 cm⁻¹ and 1350–1500 cm⁻¹ attributed to C-H stretching and CH₂ group stretching vibrations respectively. The FT-IR spectrum graph showed that each characteristic peak recorded on the PCC spectrum reappeared clearly with no shifts or wavenumber changes. These results confirm that the relationship between the hexagon and its matrix is purely physical, showing that the PCCs form an immiscible mixture. Thus, in the presence of Cu, the PCC shows good compatibility with its matrix, confirming that the dispersion of Cu throughout the Hex was uniform and without any chemical reaction.

3.3. Thermal Stability Analysis of PCCs

Thermal stability is a crucial parameter of PCCs applications. For this, we performed TGA tests from 25 °C to 600 °C at a heating rate of 10 °C/min (Figure 5 and Figure 6). For all PCC samples, the thermal degradation is carried out according to a two-step process corresponding to the degradation of hexadecane and polymers [67]. The weight loss processes of LDPE and SEBS occurred in one step and no residue was observed in a temperature range of 500–600 °C and 400–500 °C, respectively [68]. TGA and DTG curves of PCCs show two stages of weight loss. The first step in the temperature range of 150 to 250 °C is due to the decomposition of the Hex. It should be noted that Hex decomposes more slowly from the PCC samples than from the pure Hex sample, indicating a good affinity between the polymer matrix (LDPE or SEBS) and Hex (PCM). The second stage at more than 400°C represents the degradation of the polymer matrix. For PCC3, the percentage mass loss during the first degradation step correlated well with the amount of Hex initially added to the mixture (approximately 75 wt%) [69]. Figure 6 (DTG curves) confirmed that the addition of Cu increased the decomposition onset temperature and the maximum weight loss temperature. PCC6 sample synthesized with 15% weight ratio of Cu reached the highest weight loss temperature confirming that the maximum weight loss temperature of the microcapsules depends on the Cu weight ratio. Moreover, the addition of Cu improved the thermal stability of the PCCs composite by the fact that the decomposition onset temperature is related to the specific heat capacity of the PCCs, which can be increased by the specific heat capacity of the microparticles. As a result, the phase change temperature of PCCs was less than 25 °C, and the application temperature range was generally less than 100 °C. Therefore, the PCC produced is readily achievable as long as the wax of interest is thermally stable at melt processing temperatures, in this case, LDPE (~120 °C), which may be beneficial for practical application in solar thermal energy storage and has excellent thermal storage performance.

3.4. The Shape Stability of PCCs

Leakage during the solid–liquid phase change is inescapable in pure solid–liquid PCMs. As a result, solid–liquid PCMs must first be prepared as shape-stable phase change composites (PCCs) by melt impregnation in supporting matrix, which prevents the leakage of PCMs, and thus enhances reliability during the heat charging/discharging process.

The maximum non-exudative PCM retention capacities of shape stabilizers have been determined by conducting a leak test at high temperatures, as impregnation of shape stabilizers with the highest amount of lossless PCM is a highly desirable property to obtain optimal latent heat storage properties [70]. By contrasting PCCs heated above the melting point of hex (PCM), the leak test was carried out. Pictures of PCC composites taken before and after various periods of heating at 50 °C are shown in Figure 7. Figure 7 illustrates some imprints that develop after heat treatment on the PCC3. Figure 8 illustrates the relationships between PCM composite leakage rates and time for both PCM composites with and without Cu. PCC leak rates increased significantly during the 12 h thermal cycle due to residual melting on the surface of the samples. After 15 h, the overall leakage rates of PCC3~PCC6 reached 6.97%, 6.045%, 5.23%, 4.17%, respectively. Adding Cu can help stabilize the shape of samples. The leak rate decreases with increasing Cu. This is already initiated in Figure 8; the leak rate is less than 5.5% after 16 heating cycles. It can be concluded that Cu helps to improve the shape stability of PCCs mixed with large amount of Hex. The liquid leakage problem of Hex can be effectively avoided by using the compatibility between (LDPE/SEBS) and Hex; just like the case with the polymer matrix, fix Hex by strong intermolecular force, thus suppressing Hex leaching [71,72].

This should primarily be attributed to the robust capillary force and intermolecular hydrogen bonding interactions between Hex and (LDPE/SEBS/Cu) in composite PCMs. Due to the confinement effect of strong intermolecular hydrogen bonds, the hexagonal molecules are confined to the surface of (LDPE/SEBS/Cu), losing the freedom of movement. The results showed that even during tests conducted at temperatures well above the melting point, no significant hexagonal leakage occurred; thus, the matrix is able to restrict the liquid flow of Hex, leading to a weakening of the convective heat transfer effect on the liquid phase, showing good shape stability and properties of improved heat transfer.

3.5. TES Performance of PCCs

Determining the enthalpy of a PCM as a function of temperature, the melting point, and specific heat capacity is critical for thermal energy storage systems. The thermal analysis of the PCCs with and without the addition of Cu will be performed through a heating and cooling cycle monitored by the DSC device.

It is one of the most powerful techniques for the thermophysical analysis of a PCM as it provides enthalpies of fusion/solidification, temperatures of fusion/solidification and specific heats of the materials under study. Figure 9 displays an overlay of DSC results from PCM and PCC samples with different mass fractions of Cu microparticles.

Table 2. Thermal properties of PCM and PCCs.

Sample		Onset (°C)		Peak (°C)		Endset (°C)		Supercooling
		To,m	To,s	Tp,m	Tp,s	Te,m	Te,s	ΔT = Tp,m−Tp,s
PCM	Hex	21.15	16.96	25.57	14.04	13.5	28.38	11.53
PCCs	PCC1	7.71	8.7	14.95	6.81	16.88	−2.4	8.14
	PCC2	7.37	6.44	13.89	4.63	15.83	−2.48	9.26
	PCC3	6.61	5.91	13.93	3.01	16.3	−3.07	10.92
	PCC4	10.63	7.85	14.74	5.80	17.89	2.81	8.94
	PCC5	9.21	7.5	15.13	4.94	16	1.76	10.19
	PCC6	8.44	7.5	16.21	5.49	16.44	0.92	10.72

T_o,m: Onset melting temperature of DSC curve. T_o,s: Onset solidification temperature of DSC curve. T_p,m: Melting peak temperature of DSC curve. T_p,s: Solidification peak temperature of DSC curve. T_e,m: Endset melting temperature of DSC curve. T_e,s: Endset solidification temperature of DSC curve.

presents the measured values of the thermal phase transition properties of PCM (Hex) and PCC prepared by the impregnation method as determined by DSC analysis. Hex’s mass fraction is found to be 75%, which nearly exceeds all mass fraction values reported in the literature and has good dimensional stability in order to achieve the highest effective energy storage capacity [43].

For PCMs where the solid-to-liquid transition occurs, this is equivalent to the specific heat (C_p) which is the typical output of a measurement from a colorimeter [73]. Enthalpy is obtained by integrating the heat stored at a given temperature, as follows from Equation (2):

$$\mathsf{\Delta }\mathrm{H}=\underset{{\mathrm{T}}_1}{\overset{{\mathrm{T}}_2}{{\displaystyle \int }}}{\mathrm{C}}_{\mathrm{p}}\left(\mathrm{T}\right)\cdot \mathrm{dT}=\underset{{\mathrm{T}}_1}{\overset{{\mathrm{T}}_2}{{\displaystyle \int }}}\frac{\frac{\delta \mathrm{Q}}{\mathrm{dt}}}{\frac{\mathrm{dT}}{\mathrm{dt}}}\cdot \mathrm{dT}=\underset{{\mathrm{T}}_1}{\overset{{\mathrm{T}}_2}{{\displaystyle \int }}}\frac{\delta \mathrm{Q}}{\mathrm{dT}}\cdot \mathrm{dT}$$

(2)

where:

• C_p is the specific heat capacity at constant pressure in units of J/(g·K),
• ΔH is the latent heat of fusion in units of J/g.
• The operating temperature range of the storage is represented by T₁ and T₂.
• $\frac{\delta \mathrm{Q}}{\mathrm{dt}}$ is the heat flow measured in W/g,
• $\frac{\mathrm{dT}}{\mathrm{dt}}$ is the rate of DSC heating measured in °C/s.

Sensible heat is used to store additional added heat as the temperature rises steadily at a rate proportional to the PCM’s specific heat. This heat is then slowly released as the temperature rises again. Thus, the total amount of energy stored for a TES system is given by Equation (3):

$$\mathrm{Q}=\mathrm{m}\left[\underset{{\mathrm{T}}_1}{\overset{{\mathrm{T}}_{\mathrm{m}}}{{\displaystyle \int }}}{\mathrm{C}}_{\mathrm{p}-\mathrm{Solid} }\cdot \mathrm{dT}+\mathsf{\Delta }\mathrm{h}+\underset{{\mathrm{T}}_{\mathrm{m}}}{\overset{{\mathrm{T}}_2}{{\displaystyle \int }}}{\mathrm{C}}_{\mathrm{p}-\mathrm{Liquid} }\cdot \mathrm{dT}\right]$$

(3)

where the first term and last term represent the contributions of sensible heat of the solid and liquid phases, m: mass of PCM, T₁ and T₂ are the temperature range in which the TES process operates, T_m: melting temperature, and ΔH: latent heat of fusion.

The addition of Cu microparticles has an expected effect on melting/solidification points in all graphs. The peak melting temperature was found to increase, as shown in Table 2, as a result of additives penetrating the PCC microstructure. The peak melting and pouring temperatures were determined at 25.57 °C and 14.04 °C for Hex, respectively. This corresponded to the values in the literature [74]. After the copper microparticles are introduced into the PCC3 at concentrations of 5, 10, and 15 wt% T_p,m ratio increased by 5.8%, 8.61%, and 16.36% and its T_p,s also increased by 92.69%, 64.11%, and 82.39%, respectively.

The peak intensity associated with the melting of the composite is higher than that without Cu and delayed the melting end of the phase change. Adding such additives to the paraffin resulted in a higher T_p,s and allowed the solidification phase of the paraffin to change to higher temperatures. This situation makes it possible to use phase transition heat at higher temperatures to improve the heat release rate of the paraffin. The obtained results on the enhancement of conductive heat transfer through the addition of Cu into the PCMs are compared in cases of different concentrations of microparticles. The comparison of the enthalpies of melting (ΔH_m) and solidification (ΔH_s) of Hex and PCC is shown in Figure 10. It is obvious that Cu microparticles were very effective in improving the thermal properties of the PCC compared with the pure paraffin sample. For Hex, ΔH_m and ΔH_s values, were recorded to be 223.66 and 220.86 J/g, respectively.

The latent heat of fusion increased by 8%, 8.9%, and 23.8% for PCC4, PCC5, and PCC6, respectively. Another phase transition occurs during cooling and an increase in ΔH_s of PCC4, PCC5, and PCC6 of 7.4%, 8.6%, and 20.11% compared to PCC3 was obtained.

As the rate of heat transfer between the PCM and the environment increases, the experimental results demonstrated that the latent heat of PCCs increases with the addition of Cu. The liquid–solid phase stratification, the microparticles’ movement mechanism, and their surface charge state could all be contributing factors to the favorable effect. It can be determined by comparison that the unit energy storage capacity of PCC6 turned out to be a promising material for thermal energy storage, which not only provided the highest latent heat storage capacity, namely, 201.32 J/g, but also prevented Hex from exudating and kept its shape stable. An important parameter to consider in the practical application of phase change materials is supercooling. Therefore, minimizing supercooling is one of the goals of PCC synthesis. As shown in Figure 11, the extent of supercooling (ΔT) was calculated based on the differences between the melting and solidification temperatures of the hexagon and the PCCs. A remarkable fact of Figure 11 is that the degree of supercooling of all PCCs is less than that of PCM during the melting/solidification process.

Composites (SEBS/Hex/LDPE/Cu) are suitable for use in thermal storage systems due to their higher thermal conductivity. In addition, composite materials can be used economically in the event of rapid thermal load changes in the charging and discharging process.

3.6. Infrared Thermography (IRT) Analysis

PCM suffers mainly from low rate of response to demand that is mainly due to low thermal conductivity of PCMs. This drawback could be improved by mixing the PCMs with matrix of high thermal conductive materials for accelerating thermal charging and discharging in practical thermal energy storage and thermal management systems.

Figure 12 shows the surface temperature behavior images captured by an infrared camera to examine the thermal absorption-release properties of materials at different times in 60 s intervals. For melting and solidification experiments to study the thermal storage capacity, infrared images enable a precise determination of the phase change process, as shown for PCC3 and PCC6, respectively. Scale bar, black and dark blue denote the coldest temperatures, then through light blue, yellow and red for the hottest temperatures. As seen in the thermal image, a transient, uniform thermal zone gradually extends from this apex towards the center, with the ventral apex at the center. As shown in Figure 12, PCC6 exhibits faster thermal response to load (melting) in just 420 s. Sample PCC3 has accumulated heat; it slowed down to a darker color by 660 s. This point occurred when all the Hex had melted.

While local natural convection is hindered by the presence of Cu, hex can melt faster and have better temperature uniformity. The temperature–time response curves required for Hex melting and solidification processes in PCCs without and with Cu were collected (Figure 13). The heating time of PCC6 is 36.36% shorter than that of PCC3. Moreover, the composite PCC3 leads to a slower response time. In the cooling process, unlike the faster temperature drop of PCC3, the temperature of PCC6 dropped more slowly and stretched.

The reduction in charging and enhancement of discharging time does not only depend on thermal conductivity (conduction heat transfer), but also on surface area of additives that cause to increase convection heat transfer according to Newton’s law of cooling.

The shorter melting time suggests that by incorporating Cu into PCM, TES’s on-demand response time can be accelerated. Since the biphasic structure of the PS and EB blocks created a physically cross-linked network, allowing SEBS to strongly absorb Hex as a potential support material, this increase was primarily attributed to the SEBS content [75].

4. Conclusions

Since strong economic growth is not possible without a sustainable energy conservation strategy, storing renewable energy has emerged as a global issue today. It plays a crucial role in the transition from fossil fuels to renewable energy, paving the way for a cleaner and brighter future for future generations.

Besides the long-term environmental impact, a well-managed energy consumption strategy can really improve short-term financial performance. Phase change materials (PCMs), with their ability to store thermal energy as latent heat, are a viable approach to harnessing solar heat, a green energy source, and optimizing energy consumption in the buildings. Advanced form-stable phase change composites (PCC) are being developed and produced by melt dispersion and hot-pressing methods in order to address the drawbacks of PCMs, such as issues with low thermal conductivity problems that led to poor heat transfer performance, specifically causing much longer discharging times compared to charging times, liquid leakage, and supercooling problem.

The incorporation of Cu particulate additives is of great significance to overcome these shortcomings and promote the wide application of PCMs. Higher PCM content (75%) can be easily incorporated into structures and requires lower cost. Furthermore, there were no chemical interactions between Cu and all ingredients as demonstrated by SEM and FTIR results. All composites were quite stable at their working temperatures, according to the TGA results, ensuring their practical applicability. DSC results showed high heat transfer capacity during phase transition for heat storage and phase transition temperatures suitable for human comfort state. The latent heats of PCC4, PCC5, and PCC6 in the melting process were 175.71 J/g, 177.21 J/g, and 201.32 J/g, respectively, which compared to PCC3 increased by 8%, 8.9%, and 23.8%, indicating an increased rate of heat transfer between PCM and the environment. Overall, it was seen that the high thermal conductivity of Cu led to improved heat transfer performance of the composites in the two phases.

The degree of supercooling of all PCCs is less than Hex (PCM) during the melting/solidification process. Infrared thermography analysis showed that PCC with Cu microparticles has higher thermal conductivity performance and allows PCM to respond to rapid changes in thermal load in the process of charging and discharging.

The addition of metallic microparticles to PCM for weight fractions 15% showed enhancement of discharging time and reduction of the charging time compared with the PCC 3 (without Cu). The phase change composite, such as (PCC6), melts at 16.21 °C and solidifies at 5.49 °C. This phase change property makes its application suitable in buildings of hot and dry climates where temperatures are usually very high (above 40 °C) during the day and drop brutally during the night by around 1–10 °C. These weather conditions lead to activate the potential of the PCC to switch repeatedly between the two phases in order to limit the indoor thermal fluctuations.

All these results strongly suggest that the produced composite (PCC) can be recognized as potential candidates for thermal energy storage as phase change composite materials are shape stable.