Thermal Performance Improvement of Composite Phase-Change Storage Material of Octanoic Acid–Tetradecanol by Modified Expanded Graphite

Abstract

Phase-change cold storage technology is recommended as a solution for energy conservation and carbon neutrality in air conditioning systems of buildings. This study focuses on the development of binary composite phase-change materials comprising octanoic acid–tetradecanol (OA-TD). To enhance its thermal conductivity, expanded graphite (EG) was employed as an additive carrier, and the surface modification of EG particles using hexadecyltrimethoxysilane (HDTMOS) was attempted to make up for the instability and further to improve the performance of OA-TD/EG CPCMs. The OA-TD/EG-HDTMOS CPCMs were synthesized by EG mixed with EG-HDTMOS at a 1:1 mass ratio. The thermal performance and stability of the OA-TD/EG-HDTMOS CPCMs were thoroughly evaluated by multi-cycle melting–solidification and thermal conductivity measurements. The results revealed that the OA-TD mixture, when at a mass ratio of 77:23, exhibited a phase-transition temperature of 11.4 °C and a latent heat ranging from 150 to 155 J/g. Then, the OA-TD/EG-HDTMOS composite material, at a 12:1 mass ratio of OA-TD to EG-HDTMOS, solidified and melted at temperatures of 9.2 °C and 11.2 °C, with a latent heat ranging from 138 to 143 J/g, and significantly improved the thermal conductivity to 0.7 W/(m·K), representing a remarkable 133% increase compared to that of OA-TD alone. Even after undergoing 100 melting–solidification cycles, the OA-TD/EG-HDTMOS maintained superior phase-change thermal performance and stability, making it suitable for cold storage and energy conservation in air conditioning.

1. Introduction

With the improvement of people’s living standards and the stimulus of economic growth, the demand for cooling comfort is continuously surging. Statistical data have revealed that CO₂ emissions from building energy consumption account for about 28% of the total consumption, of which HVAC systems account for more than 60% [1]. A phase-change energy storage material has the capacity to release stored energy during peak hours while accumulating it during off-peak periods, thus effectively balancing the power grid’s supply and demand equilibrium. Simultaneously, it enhances the economy of air conditioning operations by capitalizing on the fluctuation in electricity prices between peak and off-peak hours. Furthermore, it can store renewable energy to promote the realization of achieving carbon neutrality [2].

Organic phase-change materials have been widely utilized across multiple fields, encompassing building energy efficiency [3], air conditioning [4], and cold chain transportation [5] for the advantage of minimal subcooling, the absence of phase separation, and robust stability. In conventional phase-change materials utilized in special air conditioning systems, such as ice, the preparation of the material undergoes a cold storage cycle by the media operating at temperatures below 0 °C. Employing phase-change materials with transition temperatures ranging from 5 to 12 °C [6], it becomes feasible to produce cold water by cycling refrigeration media with a phase change above 0 °C. However, as fatty acids and alcohols exhibit a narrow range of phase-transition temperatures, they are unsuitable for low-temperature energy storage systems [7]. A few scholars have addressed this problem by blending two or more phase-change materials, creating binary or polycrystalline eutectic mixtures. For instance, Ayaz et al. [8] crafted a binary eutectic mixture of octanoic acid and hexadecanol (at a molar mass ratio of 85:15) with an initial melting–freezing point of 10/8.9 ± 0.1 °C and a latent heat of 154.1/153.3 ± 1% J/g, but the solid-phase thermal conductivity was only 0.288 W/(m·K) and the liquid thermal conductivity was only 0.160 W/(m·K). The material displayed remarkable stability even after 500 cycles, but its thermal conductivity warrants further improvement. Jebasingh et al. [9] developed a decanoic acid–myristic acid phase-change storage material tailored for building cooling with a mass ratio of 85:15, which exhibited a phase-change temperature of 20.86 °C, a latent heat of 156.99 J/g, and a thermal conductivity of 0.152 W/(m·K). Thus, the organic phase-change materials have limitations, including relatively low thermal conductivity as well as sluggish cold storage and release rates, making it difficult to flexibly adjust the cold release rate during practical applications.

Researchers have explored various ways to improve the thermal conductivity of organic phase-change materials, such as via introducing nanoparticles [10], utilizing porous media for adsorption [11], and even utilizing microencapsulation techniques [12]. However, nanoparticles often tend to settle or aggregate, while the microencapsulation to improve the stability has a complex preparation. The alternative approach involves loading the phase-change materials within porous media, offering improved thermal conductivity while also mitigating the risk of material leakage. EG is widely used in shaping the adsorption and thermal performance improvement of PCMs due to its large specific surface area, good adsorption performance, and high thermal conductivity [13,14]. Wang et al. [15] prepared a CPCM by blending octanoic acid and myristic acid (MA) into expanded graphite. This composite demonstrated a phase-change temperature of 6.8 °C, a latent heat amounting to 136.3 J/g, and an impressive thermal conductivity of 0.998 W/(m·K). Undergoing 100 freeze–thaw cycles, the properties of the OA-MA/EG did not change significantly. Furthermore, in order to increase surface compatibility and prevent liquid leakage, appropriate surface modification should be carried out [16]. Golnoosh Abdeali [17] found that for PEG-PCM/modified EG nanocomposites, due to the increase in surface adsorption capacity and sufficient compatibility, when the modified EG filler was of 5 wt%, shape stability was achieved and the melt leakage of all the prepared samples was less than 10%. Blackley [18] presented the fabrication of a PCM composite consisting of CCH, the nucleating agent SCH, and surface-modified EG using a nonionic surfactant, addressing the concerns related to salt hydrates while providing excellent cycling stability.

Though the enhancement of phase-change materials by introducing EG has been extensively explored, liquid leakage and performance degradation remain somewhat challenging for the EG carrier’s surface wettability and aperture. In this study, expanded graphite particles are not only used to improve thermal conductivity, but their surface is also partly modified by hexadecyltrimethoxysilane (a kind of coupling agent for organic and inorganic material compatibility; HDTMOS) in order to enhance the adhesion and adsorption between the EG and the organic components and to reduce the leakage and undercooling of the CPCM. Therefore, a kind of novel and harmless composite phase-change material composed of octanoic acid–tetradecanol/modified expanded graphite is prepared here with a suitable phase-change temperature, higher thermal conductivity, and cycling stability for air conditioning cold storage systems. This work is the first to investigate the improvement effect of partial EG surface modification on the thermal performance of CPCMs.

2. Experiment and Method

2.1. Materials and Instruments

The reagents in the experiment, octanoic acid and tetradecanol, are analytically pure. The expandable graphite has a particle size of 80 meshes and an expended volume of 200–300 mL/g. Detailed specifications of the reagents for CPCM preparation, as well as the experimental apparatus with accuracy information, can be found in Tables S1 and S2 in the Supplementary Material.

2.2. Preparation of CPCMs

2.2.1. Preparation of OA-TD Binary Low Eutectic Mixture

Octanoic acid and tetradecanol with different mass ratios (79:21, 78:22, 77:23, 76:24, and 75:25) were accurately weighed to 100 g and then poured into a 200 mL conical flask. Subsequently, the conical flask was positioned in a water bath at 50 °C with a magnetic stirrer stirring for 40 min; at the same time, the flask autorotating at a speed of 300 rpm to ensure uniform mixing of OA and TD. Ultimately, the desired OA-TD binary eutectic compound was yielded as shown in step 1 in Figure 1.

2.2.2. Preparation of OA-TD/EG

First, a quantity of expandable graphite was weighed and placed flat in a cupel. It was then heat treated in a muffle furnace at 800 °C for 30 s until the graphite was fully expanded to obtain the loose and porous EG, as shown in step 2 in Figure 1.

Second, the EG was modified with HDTMOS. HDTMOS was added into ethanol and stirred evenly. The heat-treated EG was mixed into the solution at a mass ratio of 25:1 of EG to HDTMOS. Ultrasonic treatment was applied for 15 min followed by stirring for an additional 2 h; the solution was then filtered, and the particles were washed using ethanol at room temperature. Subsequently, the particles were dried under 80 °C until constant weight; thus, the modified expanded graphite was prepared as shown in step 3 in Figure 1. Evaluating the heat transfer rate and stability across multiple cycles for various blending ratios, it was determined that a 1:1 mass ratio of HDTMOS-modified expanded graphite to unmodified EG should be utilized.

The OA-TD/EG CPCMs were composed using the vacuum adsorption method as shown in step 4 in Figure 1. Different masses of the OA-TD eutectic substance were accurately weighed and mixed within a conical flask. Then, 1 g of EG was added, and the mixture was stirred uniformly at a rotational speed of 300 r/min for 15 min to ensure full contact between the EG and OA-TD. The mixture then underwent vacuum treatment using a circulating water vacuum pump over a 3-h period under a water bath temperature of 50 °C and a vacuum degree of 0.09 MPa to facilitate thorough adsorption of the OA-TD solution into the EG and ensure that the EG particles were suspended evenly in the solution. A similar process was followed for the preparation of the OA-TD/EG-HDTMOS CPCMs.

2.3. Characterization of CPCMs

2.3.1. Latent Heat

The phase-change temperature of the OA-TD mixture is always lower than the ambient temperature, and the CPCM heat/cooling loss to the environment has a clear influence on a latent heat test. Therefore, a “thermos”-type latent heat measurement device was ingeniously self-designed to insulate the environment and reduce heat loss in this study. The experimental model is depicted in Figure 2. The container outermost layer is an insulating annular tube, the annular tube and the space outside of test tube are full of air, and the two layers of air in the test tube are isolated from their surroundings. During the testing process, the water temperature is stabilized and the testing errors are reduced. In the insulated container, a certain amount of distilled water is put into the test tube, within which two thermocouples are securely inserted and positioned to measure temperature changes at the upper and lower sections of the water. The CPCM is full of a small glass test tube, equipped with a lid, which is centrally positioned within the water, ensuring that the water level completely envelops the CPCM in the glass test tube; then, a thermocouple is affixed for phase-change temperature measurements. Subsequently, after sealing the lid onto the water tube, gentle agitation is applied to expedite the uniformization of water temperature. This rapid thermal equilibrium allows for the swift absorption and release of heat during the phase-change process to the tube water. The latent heat of CPCM is calculated using the following Equation (1).

$$\mathsf{\Delta }{H}_m=C_w{m}_w\mathsf{\Delta }{T}_w-C_m{m}_m\mathsf{\Delta }T-C_g{m}_g\mathsf{\Delta }T$$

(1)

where ΔH_m is the latent heat of the phase-change material, J/g; m_m, m_g, and m_w are the mass of the phase-change material, glass, and water, respectively; C_m is the process average specific heat capacity of the CPCM; C_g and C_w are the specific heat capacity of the glass and water, respectively; ΔT is the difference in temperature between the CPCM and the glass bottle before and after the experiment; ΔT_w is the temperature of the water at the beginning and end of the test; and C_m = 2.2865 J/(g·K), C_g = 0.82 J/(g·K), and C_w = 4.2 J/(g·K).

Multiple repeated experiments in the self-designed device were conducted to minimize experimental errors. When compared with the DSC measurement results of tetradecanol, the phase-change temperature of which is a little higher, the relative deviation of the latent heat data is approximately 5%, and thus the testing method is relatively accurate.

2.3.2. Thermal Conductivity Testing

The thermal conductivity of CPCMs plays a pivotal role in their utilization for building energy-saving. If the thermal conductivity is too low, the heat transfer rate between CPCMs and the heat resource or room environment are also too low. The thermal conductivity of the liquid phase is difficult. In this experiment, Fourier’s law of thermal conductivity (Equation (2)) serves as the fundamental principle to design another model analyzing the solid-phase thermal conductivity coefficient of the phase-change materials. The experimental setup and model are depicted in Figure 3.

$$\varphi =-\lambda {\displaystyle \frac{dt}{dx}}$$

(2)

where ϕ is the heat flow, W/m²; λ is the thermal conductivity, W/(m·K); dt is the temperature difference between the two thermocouples, °C; and dx is the distance between the two thermocouples, m.

The CPCM liquid was first laid flat within a sealed bag to ensure even thickness when it solidifies. After cooling and solidification, the sheet thickness was approximately 7 mm. Then, a heat flow meter and T-type thermocouples were carefully affixed to both sides of the sheet. Importantly, the heat flow meter probe and thermocouples were positioned at the same height and eliminate any potential airflow over the surface of device. The heat flow density and temperature of the phase-change process were continuously recorded. On one side of the material sheet, a cold source (such as ice) was introduced to cause a temperature difference between the two sides of the material sheet, the heat flow occurred and was recorded by the heat flow meter, and then the thermal conductivity was calculated. In our previous studies, this testing method was used to measure the thermal conductivity of phase-change materials, and the test results are reliable [19,20].

The thermal performance, phase-change behavior, and solid-phase thermal conductivity coefficient were used for the evaluation of the CPCMs.

3. Results and Discussion

3.1. Characteristics of EG and EG-HDTMOS

The absorption capacity of EG and EG-HDTMOS was firstly analyzed via nitrogen adsorption–desorption, as presented in

Table 1. BET analysis of EG before and after modification.

	Specific Surface Area/m2·g−1	Mean Pore Size/nm	Pore Volume/cm3·g−1
EG	19.1941	15.1601	0.0727464
EG-HDTMOS	7.2841	29.9451	0.0545309

. The modified EG exhibited a reduction in specific surface area and pore volume, possibly due to HDTMOS molecules adsorbing onto the EG surface and occupying some adsorption sites. From the point of view of the sorption effect, the modified EG cannot be used accordingly, but the modified EG showed an increase in hydrophilic forces within the pores and less recrystallizable phase-change material. Figure 4 shows the SEM images of the EG and EG-HDTMOS. In Figure 2, it can be seen that the isothermal adsorption capacities of the EG and EG-HDTMOS slowly increase with the increase in P/P0, and there is a hysteresis loop, indicating that the EG and EG-HDTMOS do not have a microporous structure, only a mesoporous structure. From Figure 5a, it can be seen that the EG has a worm-like microporous structure, the uneven deformation of each lamella after high-temperature expansion makes the lamellae of the EG curled, and the pores between the lamellae are favorable for infiltration of the phase-change material in the molten state, so that EG achieves a better adsorption effect. From Figure 5b, it can be seen that the HDTMOS-modified material was uniformly adhered to the surface of the EG, and the EG remained loose and porous after adsorption.

Equal masses of EG and EG-HDTMOS were mixed and added to an OA-TD solution to prepare the CPCM. When only EG was added, the CPCM solidification could not completely fill the test tubes, causing the presence of air gaps in the CPCM and subsequently affecting the latent heat and thermal conductivity. When only EG-HDTMOS was added, the EG-HDTMOS particles first remained suspended in the OA-TD solution, but after multiple cycles, the EG-HDTMOS particles gradually settled at the bottom, leading to phase separation and a decrease in cycling stability. However, when both the EG and EG-HDTMOS mixture were added, the CPCM exhibited a paste-like consistency, offering a degree of flowability that helps prevent air bubble formation. It also possessed a certain viscosity, which is beneficial in preventing phase separation in the CPCM, resulting in excellent cycling stability.

3.2. Determination of OA-TD Low Eutectic Mixture Ratios

Known from the phase equilibrium theory and laws of thermodynamics, the eutectic temperature and enthalpy value of the eutectic mixture can be predicted by Equations (3) and (4) [21]. When the mixture is at its lowest eutectic temperature, it exhibits perfect thermal stability. The phase diagram of the OA-TD binary system is shown in Figure 6.

$$T_m={\left[{\displaystyle \frac{1}{T_i}}-{\displaystyle \frac{Rln{X}_i}{H_i}}\right]}^{-1}$$

(3)

$$H_m=T_m\sum _{i=1}^n {\displaystyle \frac{X_i{H}_i}{T_i}}.$$

(4)

where T_i and H_i are the phase-transition temperature and latent heat of the phase transition of component i, respectively, T_m and H_m are the phase-transition temperature and latent heat of the eutectic mixture, X_i is the molar percentage of component i in the eutectic mixture, ΣX_i = 1, and R is the gas constant of 8.314 J/(mol·K).

The melting point of n-octanoic acid stands at 16.5 °C, with a latent heat of melting measuring 149.8 J/g. Tetradecanol, on the other hand, has a melting point of 38 °C and a latent heat of 222.93 J/g. The theoretical calculation can be used to conclude that the mass ratio of n-octanoic acid and tetradecanol at the eutectic point is 77:23, with a melting point of 10.19 °C and a latent heat of 159.34 J/g. It is of note that the material composition and inherent chemical properties could cause some variance between theoretical and measurement data. To validate the theoretical eutectic ratio, an OA-TD binary CPCM with different mass ratios was prepared, as shown in

Table 2. The phase-transition temperature and latent heat of OA-TD with different mass ratios.

Mass Ratio of OA-TD	79:21	78:22	77:23	76:24	75:25
Phase-transition temperature/°C	12.2	11.6	11.4	11.4	11.8
Latent heat/J/g	145–150	150–155	150–155	150–155	155–160

. It is known that the lowest melting point occurs at a mass ratio of 77:23, with a latent heat falling within the range of 150–155 J/g. The step-cooling curve of the lowest eutectic point, 77:23, illustrated in Figure 7, reveals that the phase-transition temperature of OA-TD is 11.4 °C, exhibiting a slight degree of supercooling and a stable phase-transition platform. Furthermore, the experimental assessment of OA-TD’s thermal conductivity yielded a value of 0.3 W/(m·K). Such latent heat values of OA-TD with a mass ratio of 77:23 are highly promising for applications in air conditioning cold storage, showcasing its potential in the field.

3.3. Optimization of the Mass Ratio of OA-TD to EG

A higher latent heat is crucial to the application of CPCMs. However, for OA-TD/EG CPCMs, increasing the quantity of EG to enhance the thermal conductivity has an impact on the latent heat. Therefore, it is necessary to evaluate the adsorption properties of CPCMs with varying ratios of EG, ultimately to determine the optimal mass ratio (OA-TD:EG) [22]. The impregnation capacity or the leakage mass percentage of OA-TD was calculated, as determined by Equation (5).

$$E={\displaystyle \frac{m-m_0}{m_0}}\times 100\%$$

(5)

where E is the mass percentage of OA-TD leakage and m₀ and m are the sample masses before and after holding for an hour, respectively.

The leakage test of OA-TD/EG CPCMs for different mass ratios is presented in Figure 8. In this test, approximately 1 g of fluid OA-TD/EG samples was placed on filter paper at 35 °C for 1 h, and any leakage out of the carrier of EG was observed to be adsorbed by the filter paper. The quantity and rate of leakage are detailed in

Table 3. Mass changes of OA-TD/EG CPCMs before and after standing for an hour.

Mass Ratio of OA-TD to EG	Quality before Standing/g	Quality after Standing/g	Leakage/g	Leakage Rate/%
10:1	1.0257	0.9837	0.0420	4.09
12:1	0.9810	0.8978	0.0832	8.48
14:1	1.0998	0.9546	0.1452	13.20
16:1	1.0543	0.8849	0.1694	16.07

. It was found that the OA-TD/EG materials with mass ratios of 14:1 and 16:1 displayed noticeable “liquid separated” marks on the filter paper, with leakage rates of 13.2% and 16.07%, respectively. As the mass fraction of OA-TD increased, the surface tension of the EG became insufficient in absorbing OA-TD, resulting in phase-change material leakage. Additionally, an increased ratio of EG led to a decrease in the latent heat of the CPCM. Overall, it was determined that the optimal mass ratio for OA-TD/EG was 12:1. Consequently, both OA-TD/EG and OA-TD/EG-HDTMOS with a mass ratio of 12:1 were selected in subsequent studies.

3.4. Thermal Conductivity Analysis of Materials

As depicted in Figure 9, the thermal conductivity of the OA-TD ranged between 0.26 and 0.33 W/(m·K), with an average value of 0.30 W/(m·K). For the OA-TD/EG, the thermal conductivity ranged from 0.63 to 0.72 W/(m·K), with a mean value of 0.67 W/(m·K). Meanwhile, the OA-TD/EG-HDTMOS demonstrated a thermal conductivity spanning from 0.65 to 0.75 W/(m·K), with an average value of 0.7 W/(m·K), slightly surpassing that of the unmodified EG, and increased by 2.23 times to that without an EG carrier. As shown in the step-cooling curves of the melting process in Figure 10, it is evident that the addition of EG substantially cut down the phase-change time of the CPCM by almost 66%, as the heat transfer rate increased significantly. Natural convection typically occurs to cause the different density distribution in the liquid phase; however, the introduction of EG in the CPCM constructs an effective porous heat transfer lattice, diminishing the influence of convection and enhancing heat conductivity. Additionally, the EG addition results in an increased contact surface area for organic system materials, thereby elevating the thermal conductivity and nucleation rate of OA-TD/EG and OA-TD/EG-HDTMOS. The improved thermal conductivity almost meets the requirement of various applications.

It is worth noting that, as seen in Figure 10, the CPCM with the modified EG maintains a melting temperature in alignment with the theoretical phase-change temperature of the organic system, which could be evidence of stable performance.

3.5. Melting–Solidification Multi-Cycle Test

Cycling stability is a key factor in evaluating the practical applications of phase-change materials. The OA-TD/EG and OA-TD/EG-HDTMOS CPCMs were subjected to multiple cycles of melting–solidification testing. A thermocouple was fixed at the center of the PCMs in the test tube, and a data acquisition instrument was used to record temperature changes of the PCMs during the cold energy storage/releasing processes to compare the temperature variation and the thermal performance variation of the PCM with the number of phase-change cycles.

The melting–solidification cycle curves of the CPCMs are illustrated in Figure 11. For the OA-TD/EG, the solidification temperature stays at 7.5 °C, the melting temperature is 9.2 °C, the maximum subcooling degree is 1.4 °C, and the latent heat falls within the range of 138 to 143 J/g. Meanwhile, the OA-TD/EG-HDTMOS exhibits a solidification temperature of 9.2 °C, a melting temperature of 11.2 °C, and a maximum subcooling degree of 1.8 °C. The latent heat of the OA-TD/EG-HDTMOS is also within the range of 138 to 143 J/g. Comparing the two materials, the step-cooling curves of the OA-TD/EG show a lower degree of overlap at the end of the solidification process, more fluctuation in the melting transition, and relatively lower stability. Although the maximum subcooling of the OA-TD/EG-HDTMOS is 0.4 °C higher than that of the OA-TD/EG, its phase-transition temperature is higher. Specifically, it is about 1.7 °C higher than that of the OA-TD/EG in the cold storage period, which still better meets the demand of the cold storage temperature range of 10 to 15 °C. The OA-TD/EG-HDTMOS shows excellent thermal performance when applied for air cooling, so it is a more practical and efficient choice.

Table 4. Comparison of the performance of OA-TD/EG-HTDMOS with materials reported in the literature.

PCMS	Mass Ratio of EG	Melting Process		Thermal Conductivity (W/(m·K))	Ref.
		Tm (°C)	Hm (J/g)
OA-MA	7 wt%	6.8	136	0.998	[15]
CA-SA-OD	8.33%	27.06	142.09	3.136	[23]
CA-SA	10 wt%	24.47	150.42	0.528	[24]
RT62HC	6 wt%	62.2	174 ± 3	0.518	[25]
OA-TD	7.69 wt%	11.2	138–143	0.7	This work

compares the properties of the OA-TD/EG-HDTMOS prepared in this work with other organic CPCMs. The phase-change temperature range of 5 to 12 °C makes it more suitable for utilization in air cooling conditioners. Though Fei [23] reports that the thermal conductivity of CPCMs with an 8.33 wt% EG addition reached 3.136 W/(m·K), their phase-change temperatures are not suitable for energy storage in air conditioners. Different researchers have adopted treatment methods for expanded graphite. Though the thermal performance of OA-MA is similar to that of OA-TD, the corrosivity of OA-TD is clearly lower than that of OA-MA, which is composed of two kinds of organic acids, while OA-TD is composed one organic acid and one alcohol. The addition of HTDMOS-modified EG regulated the temperature range from 5 to 12 °C and resulted in a more suitable phase-change temperature along with a higher latent heat and thermal conductivity. In comparison with existing PCMs, OA-TD/EG-HTDMOS has some potential for application.

3.6. Mechanism Analysis and Cycle Stability

During multiple cycles, both the OA-TD/EG and OA-TD/EG-HDTMOS experienced a temperature reduction during the phase transition, about 0.8 °C compared to their pre-cycling temperatures. Additionally, the latent heat exhibited a decrease state, ranging from 133 to 140 J/g. The latent heat reduction can be attributed to that the proportion of EG in the CPCMs reached its maximum, and during the heating stage in multiple cycles, some of the OA-TD that had adhered to the surface of the EG volatilized, resulting in a slight decrease in the latent heat of the CPCM. According to Figure 11, the OA-TD/EG-HDTMOS multi-cycle curves remained close to the cooling rate at the solidification end, the phase-change platform stayed stable, and there were minimal alterations in the cold storage–release periods and thermal conductivity. Overall, the thermophysical properties parameters of the CPCM exhibited minor variations before and after cycling, demonstrating excellent stability.

When the EG and EG-HDTMOS were blended, the CPCM showed more stable features. Figure 12 illustrates the role of HDTMOS in modified materials. The hydrophilic group lies on one side of HDTMOS as the circle part, while the hydrophobic group is on the other side. The modification process causes the HDTMOS to attach to the expanded graphite, and hydrophobic organic groups are attracted to the layers on the surface of EG to change its surface properties, reduce its surface energy, and enhance the compatibility and dispersion of EG within the OA-TD solution, hence enhancing its adsorption capacity for OA-TD. However, it should be noted that, due to the addition of a certain amount of HDTMOS, which occupied some pores in the EG, the entry of OA-TD into those pores was thereby restricted. Therefore, co-using EG and EG-HDTMOS not only improved the stability and decreased the leakage of the CPCM via surface modification but also endowed enough specific area to fasten the OA-TD to achieve a higher latent heat.

4. Conclusions

A binary organic phase-change material of OA-TD was prepared according to the theoretical analysis and experimental studies of energy storage in air conditioners. The organic CPCM features a mass ratio of 77:23 OA-TD, a phase-change temperature of 11.4 °C, a latent heat ranging from 150 to 155 J/g, and a thermal conductivity of 0.3 W/(m·K). Expanded graphite was added to enhance the CPCM’s thermal conductivity, in which a part of the EG was surface-modified with hexadecyltrimethoxysilane. The modified EG also acted as the surface-active agent, reducing the undercooling of the CPCM and reinforcing its stability. When the EG and modified EG (EG-HDTMOS) were mixed at a 1:1 ratio and added into OA-TD at a ratio of 1:12, the best CPCM was prepared. Compared to OA-TD, the thermal conductivity was improved, and a 66% reduction in the phase-change time contributed to the cold storage performance. Even after undergoing 100 melting–solidification cycles, both the OA-TD/EG and OA-TD/EG-HDTMOS CPCMs retained their excellent stability. The phase-change temperature of the OA-TD/EG-HDTMOS is more suitable for cold storage in air cooling applications.