Preparation and Properties of Capric–Myristic Acid / Expanded Graphite Composite Phase Change Materials for Latent Heat Thermal Energy Storage

: A novel composite phase change material (CPCM), capric–myristic acid / expanded graphite (CA–MA / EG) CPCM, was prepared by absorbing liquid CA–MA (as the phase change material (PCM)) into EG (as the substrate material) for heat storage in the backﬁll materials of soil-source heat pump systems. The thermal characteristics and microstructure of the novel CPCM were analyzed using di ﬀ erential scanning calorimetry (DSC) and scanning electronic microscopy (SEM). The thermal conductivities of CA–MA / EG CPCM were surveyed. The thermal stability of the CA–MA / EG was analyzed using thermogravimetric analysis (TGA) and thermal cycle tests. The results showed that the optimal mass content of CA–MA in CPCM was approximately 92.4% and the CA–MA was uniformly distributed in the vesicular structure of EG; the CA–MA / EG CPCM had an appropriate phase change temperature ( T m : 19.78 ◦ C, T f : 18.85 ◦ C), high latent heat ( H m : 137.3 J / g, H f : 139.9 J / g), and excellent thermostability and thermal reliability. The thermal conductivity of the CPCM was remarkably enhanced after adding EG. Therefore, the CPCMs demonstrated outstanding thermal performance and can be utilized in low-temperature latent heat thermal energy storage (LHTES) systems, such as soil-source heat pump systems.


Introduction
Phase change materials (PCMs) can be utilized to store thermal energy temporarily and release it when necessary, which can alleviate the contradiction of energy supply-demand and energy saving. Thermal energy storage by virtue of latent heat is a powerful method to enhance energy efficiency [1]. To date, PCMs for thermal energy storage have been widely employed for solar energy utilization [2], building energy savings [3], industrial waste heat recovery [4], and air conditioning condensation heat recovery systems [5].
In accordance with the chemical composition, PCMs can be classified as organic phase change materials [6], inorganic phase change materials [7], and composite phase change materials [8]. Organic phase change materials are mainly divided into esters, sugar alcohols, paraffins [9], and aliphatic acids [10]. Fatty acid PCMs have been generally utilized for their positive characteristics, including large latent heat of phase change, no supercooling, non-toxicity, and their low price [11]. When aliphatic acids are utilized as PCMs, the choice of the phase transition temperature is very flexible; not only can Table 1. Heat performances of some phase change materials (PCMs) and composite phase change materials (CPCMs). Capric acid (CA), myristic acid (MA), expanded graphite (EG), stearic acid (SA), lauric acid (LA), palmitic acid (PA), octadecane (OC). Most of the existing studies only focused on mono-or poly-fatty acid eutectic mixtures in the temperature range of 30-60 • C, which are mostly used in energy-efficient buildings and solar energy utilization. To the best of our knowledge, few studies focused on the backfill well energy storage materials for a buried pipe heat-exchanger of a soil-source heat pump with a phase transition temperature of around 20 • C. The key to stable and efficient operation of the soil-source heat pump system is the heat transfer between the underground heat exchanger and the soil. The temperature of shallow soil is essentially constant below 15 m. For example, it is 19 • C in Shanghai, China [41].

PCMs/CPCMs
The temperature of the soil around the buried pipe will vary when the system runs for a long time, resulting in a decrease in the speed of the system. Therefore, the capability of the backfill material around the buried tube heat exchanger is especially crucial. We prepared CA-MA PCMs with a phase transition temperature of 19.4 • C and studied the thermal performance. The latent heat of phase change reached 150.9 J/g, which is an appropriate PCM for the backfilling well of a soil-source heat pump buried tube heat exchanger, but the heat transfer performance of the composite PCMs was not studied and improved [42].
This study concentrated on novel latent heat thermal energy storage PCM for the backfill materials of soil-source heat pump systems. A capric-myristic acid ternary eutectic mixture (CA-MA) was prepared with melting and freezing temperatures of 19.4 • C and 18.3 • C, respectively, and then the CA-MA/EG composite PCM was prepared for soil-source heat pump systems. The thermal characteristics, microstructure, thermal conductivity, thermal reliability, and thermal stability of the composite PCM were studied carefully.

Preparation of Capric-Myristic Acid Binary Eutectic Solution
The CA and MA were weighed according to the mass ratio of 72:28 and then mixed in a beaker. The admixture was heated with separated water in a magnetic agitator to 50 • C and stirred for 1 h to obtain the CA-MA phase change material.

Preparation of CA-MA/EG CPCMs
Expanded graphite was obtained by heating the dried expandable graphite in a porcelain crucible for 40 s in a 900 • C muffle furnace. A quantity of prepared expanded graphite and the CA-MA binary eutectic were fully mixed in a beaker, and then the beaker was placed into an 80 • C constant temperature water bath for 48 h. After the CA-MA binary eutectic solution melted, the mixture was stirred every 2 h to ensure that the eutectic solution was fully absorbed by the EG. Then, the prepared CA-MA/EG CPCMs were cooled to ambient temperature for the next characterization.

Experiment of Maximum Mass Absorption
Ratio of the CA-MA in the CA-MA/EG CPCMs About 5 g of CA-MA/EG composite PCMs with six ratios were weighed using an electronic balance and placed in the beaker. This was placed into a 50 • C incubator to keep it warm for 48 h and shaken every 2 h. Six kinds of CA-MA/EG composite phase change material samples were prepared after adsorption with a sufficient mixture of CA-MA and EG. The mass ratios of CA-MA were 90%, 91%, 92%, 93%, 94%, and 95%, respectively. About 0.5 g of CA-MA/EG composite phase change material was weighed for each sample and placed on filter paper. This was placed into the incubator at 50 • C and kept warm for 30 min. This was then removed and we observed the adsorbed CA-MA mixture solution on the filter paper.

Thermal Conductivities Test of CA-MA/EG CPCMs
Five groups of different quality CPCMs were heated to 50 • C, and then were placed in a cake model with a diameter of 3 cm, respectively. The cake samples with smooth surfaces and different densities were prepared by pressing, grinding, and cooling. Due to the different quality and compression strengths of the samples, the packing densities of the samples were different. The thermal conductivities of CA-MA/EG CPCMs were surveyed by a Hot Disk thermal constants analyzer (TPS2500, Hot Disk AB Company Sweden, Gothenburg, Sweden) at ambient temperature with an accuracy of ±3%.

Characterization
The phase change temperature (melting and freezing temperatures) and latent heat (melting and freezing latent heat) of the CA-MA and the CA-MA/EG before and after thermal cycling were determined by differential scanning calorimetry (DSC, TA Q20, TA Instruments, New Castle, USA), adjusted with an indium standard at a temperature range of 0-60 • C. The heating/cooling rate of DSC measurement was 5 • C /min, and the samples were cooled by liquid nitrogen. The measurement of the same sample DSC measurements was performed three times. The accuracy of the phase change temperature was ±0.05 • C and ±0.1% for latent heats. The extrapolated onset temperatures on the DSC curves were the transformation temperature of the PCMs.
The thermal stability of the CA-MA/EG was analyzed by the thermogravimetric analysis technique (TGA, TA Q50, TA Instruments, New Castle, USA) at a temperature range of 25-500 • C under a nitrogen gas atmosphere at a heating rate of 10 • C/min with an accuracy of ±0.2%. We used a scanning electronic microscope (SEM, Phenom LE, Phenom-World, Eindhoven, the Netherlands) to inspect the microscopic structure of the EG and CA-MA/EG composite phase change materials.
The effect of the thermal cycles on the thermal properties was investigated by heating the CA-MA eutectic mixtures from a solid to a liquid state and then cooling from a liquid to a solid state using a High Low Temperature Test Box (XF/GDW, Shanghai Xiangfeng test instrument and equipment Co. LTD., Shanghai, China). A test tube with a quantity of CA-MA/EG composite PCMs samples was heated in the test box until the center temperature of the samples reached 30 • C, indicating that the CA-MA had completely melted. Then, the samples were cooled in the test box until the center temperature of samples reached 10 • C, indicating that the CA-MA had completely frozen. The above heating/cooling cycle process was stopped until the thermal cycles reached 500 and 1000 times. The heating/cooling rate of the High Low Temperature Test Box was 5 • C /min, and the samples were cooled by a mechanical compression refrigeration system. We used DSC to measure the changes in performance.

Maximum Mass Absorption Ratio of the CA-MA in the CA-MA/EG CPCMs
The main factors influencing the performance of PCMs are the phase change temperature, latent heat, and thermal conductivity. When selecting PCMs, an appropriate phase transition temperature should be selected and PCMs with higher latent heat and thermal conductivity should be selected at this temperature. Generally, the higher the mass of fatty acids in composite PCMs, the larger the phase change latent heat value of the composite materials. In order to obtain more CA-MA per unit mass without leakage, the ratio of CA-MA to EG must be measured.
The experimental results are shown in Figure 1. As shown in Figure 1a, for samples with 94% and 95% mass ratios, the filter paper around the material was completely soaked with obvious leakage, which did not meet the requirements of the optimal ratio. For samples with mass ratios of 90% and 91%, the samples were too dry, indicating that the content of the CA-MA/EG eutectic mixture was too Energies 2020, 13, 2462 5 of 12 small; this would lead to an excessively low latent heat of phase transition, which did not meet the requirements of the phase change materials. The leakage of samples with mass ratios of 92% and 93% was not obvious. After brushing the samples (see Figure 1b), the sample with a mass ratio of 92% had no leakage on the filter paper, while the sample with a mass ratio of 93% had some micro-leakage on the filter paper, indicating that the optimal quality was between 92% and 93%.
did not meet the requirements of the phase change materials. The leakage of samples with mass ratios of 92% and 93% was not obvious. After brushing the samples (see Figure 1b), the sample with a mass ratio of 92% had no leakage on the filter paper, while the sample with a mass ratio of 93% had some micro-leakage on the filter paper, indicating that the optimal quality was between 92% and 93%.
To further fix the accurate mass ratio, 5 g CA-MA/EG composite phase change materials with four different ratios were weighed again for preparation, with mass ratios of 92.2%, 92.4%, 92.6%, and 92.8%. We repeated the same steps and observed the leakage of the sample. As shown in Figure  1c, there was no leakage of the CA-MA solution around the four samples, indicating that there was little difference. After the samples were brushed off (In Figure 1d), it was found that samples with a mass ratio of 92.2% and 92.4% were not adsorbed, while those with a mass ratio of 92.6% and 92.8% of the sample filter paper demonstrated slight leakage. In conclusion, it can be determined that the optimal ratio of CA-MA in CA-MA /EG was 92.4%.  To further fix the accurate mass ratio, 5 g CA-MA/EG composite phase change materials with four different ratios were weighed again for preparation, with mass ratios of 92.2%, 92.4%, 92.6%, and 92.8%. We repeated the same steps and observed the leakage of the sample. As shown in Figure 1c, there was no leakage of the CA-MA solution around the four samples, indicating that there was little difference. After the samples were brushed off (In Figure 1d), it was found that samples with a mass ratio of 92.2% and 92.4% were not adsorbed, while those with a mass ratio of 92.6% and 92.8% of the sample filter paper demonstrated slight leakage. In conclusion, it can be determined that the optimal ratio of CA-MA in CA-MA /EG was 92.4%.

Thermal Properties of the CPCMs
The heat-storage properties of phase change composites can be measured by DSC and the effects are indicated in Table 2 and Figure 2. The phase transition temperature of the CA-MA eutectic mixture Energies 2020, 13, 2462 6 of 12 was 19.45 • C and the latent heat of phase transition was 150.9 J/g. The phase transition temperature of the CA-MA/EG composite PCM was 19.78 • C, and the latent heat of phase transition was 137.3 J/g. It follows that the initial phase transition temperature of the endothermic curve of CA-MA/EG was slightly delayed compared with that of CA-MA, which occurred in previous studies [34,37,39,40]. The heat-storage properties of phase change composites can be measured by DSC and the effects are indicated in Table 2 and Figure 2. The phase transition temperature of the CA-MA eutectic mixture was 19.45 °C and the latent heat of phase transition was 150.9 J/g. The phase transition temperature of the CA-MA/EG composite PCM was 19.78 °C, and the latent heat of phase transition was 137.3 J/g. It follows that the initial phase transition temperature of the endothermic curve of CA-MA/EG was slightly delayed compared with that of CA-MA, which occurred in previous studies [34,37,39,40].
Three reasons for this phenomenon are probable. First, there is a weak interaction between fatty acids and EG pores, and EG has good thermal conductivity; therefore, the phase transition temperature of CPCM changes slightly. Secondly, there is an error in the instrument and experiment. Finally, the purity of the experimental materials is different. However, the shape and position of the two absorption peaks are basically the same, indicating that it has no new material produced in the preparation process of the composites. EG only plays a role with adsorbed material in composites and has little influence on the phase transition temperature and the latent heat of the composite PCM.
As shown in Table 2, there is a certain degree of undercooling for PCM and the undercooling decreases to a certain extent after recombination. Although EG can improve the thermal properties of PCM, there is no evidence that EG can reduce the undercooling of fatty acids. The change in supercooling may be caused by impurities. Impurities may be inserted in the preparation of EG and CPCM. Impurities have a strong influence on the dissolution and solidification of PCM. In some cases, impurities may also be a good nucleating agent, which can reduce the undercooling of PCM.   Three reasons for this phenomenon are probable. First, there is a weak interaction between fatty acids and EG pores, and EG has good thermal conductivity; therefore, the phase transition temperature of CPCM changes slightly. Secondly, there is an error in the instrument and experiment. Finally, the purity of the experimental materials is different. However, the shape and position of the two absorption peaks are basically the same, indicating that it has no new material produced in the preparation process of the composites. EG only plays a role with adsorbed material in composites and has little influence on the phase transition temperature and the latent heat of the composite PCM.
As shown in Table 2, there is a certain degree of undercooling for PCM and the undercooling decreases to a certain extent after recombination. Although EG can improve the thermal properties of PCM, there is no evidence that EG can reduce the undercooling of fatty acids. The change in supercooling may be caused by impurities. Impurities may be inserted in the preparation of EG and CPCM. Impurities have a strong influence on the dissolution and solidification of PCM. In some cases, impurities may also be a good nucleating agent, which can reduce the undercooling of PCM.

Microstructure of EG and CA-MA/EG Composite PCMs
The microscopic structure of the EG and CA-MA/EG composite PCMs was determined using SEM and the effects are indicated in Figure 3. Figure 3a shows that the EG has a large number of loose and folded porous structures, which have a strong adsorption capacity and can absorb the CA-MA Energies 2020, 13, 2462 7 of 12 eutectic melt mixture. The SEM diagram of CA-MA/EG (Figure 3b) indicates that, due to the high adsorption of the EG mesoporous structure, the eutectic mixture of CA-MA is adsorbed, and the pores are filled with solution and become full. The porous structure provides surface tension for the eutectic melt mixture, making the solution less leaky. Therefore, EG is a good adsorption material for CA-MA phase change materials, due to its superior adsorption performance, and as a base material, it can provide certain mechanical strengths and prevent the leakage of molten materials.
Energies 2020, 13, x FOR PEER REVIEW 7 of 13 The microscopic structure of the EG and CA-MA/EG composite PCMs was determined using SEM and the effects are indicated in Figure 3. Figure 3a shows that the EG has a large number of loose and folded porous structures, which have a strong adsorption capacity and can absorb the CA-MA eutectic melt mixture. The SEM diagram of CA-MA/EG (Figure 3b) indicates that, due to the high adsorption of the EG mesoporous structure, the eutectic mixture of CA-MA is adsorbed, and the pores are filled with solution and become full. The porous structure provides surface tension for the eutectic melt mixture, making the solution less leaky. Therefore, EG is a good adsorption material for CA-MA phase change materials, due to its superior adsorption performance, and as a base material, it can provide certain mechanical strengths and prevent the leakage of molten materials.

Improvement of the Thermal Performance of the CPCMs
The high thermal conductivity of aliphatic acids as PCMs is one of the most important requirements. The thermal conductivity of a single aliphatic acid or polyfatty acid eutectic mixture is relatively poor; thus, it is necessary to add a substrate with better thermal conductivity to improve the thermal conductivity. EG has a good adsorption capacity and high thermal conductivity due to its porous properties. To test the thermal conductivity of PCMs, the heat storage and release experiment and the transient plane heat source technique were used.
In order to test the accuracy, the temperature probe was placed in the center of the CA-MA and CA-MA/EG phase change materials, and the two materials were placed in a stable temperature environment. A test began when the temperature of the material reached the set value and was stable. The temperature of the storage process was set at 48 °C, and that of the release process was set at 0 °C. The test effects are indicated in Figure 4. Figure 4a shows the storage curves of the two materials. It can be clearly seen that the time taken for CA-MA/EG to reach the set point is faster than CA-MA. It takes 115 minutes at 48 °C for the CA-MA, while it takes only 63 minutes for the CA-MA/EG to be heated to 48 °C. Figure 4b shows the release curves of the two materials. It takes 81 min for the CA-MA to complete the exothermic process, while it takes only 25 minutes for CA-MA/EG to complete the release process. The effects indicate that the thermal conductivity of CA-MA/EG is higher than that of CA-MA for the porous structure of EG.

Improvement of the Thermal Performance of the CPCMs
The high thermal conductivity of aliphatic acids as PCMs is one of the most important requirements. The thermal conductivity of a single aliphatic acid or polyfatty acid eutectic mixture is relatively poor; thus, it is necessary to add a substrate with better thermal conductivity to improve the thermal conductivity. EG has a good adsorption capacity and high thermal conductivity due to its porous properties. To test the thermal conductivity of PCMs, the heat storage and release experiment and the transient plane heat source technique were used.
In order to test the accuracy, the temperature probe was placed in the center of the CA-MA and CA-MA/EG phase change materials, and the two materials were placed in a stable temperature environment. A test began when the temperature of the material reached the set value and was stable. The temperature of the storage process was set at 48 • C, and that of the release process was set at 0 • C. The test effects are indicated in Figure 4. Figure 4a shows the storage curves of the two materials. It can be clearly seen that the time taken for CA-MA/EG to reach the set point is faster than CA-MA. It takes 115 min at 48 • C for the CA-MA, while it takes only 63 min for the CA-MA/EG to be heated to 48 • C. Figure 4b shows the release curves of the two materials. It takes 81 min for the CA-MA to complete the exothermic process, while it takes only 25 min for CA-MA/EG to complete the release process. The effects indicate that the thermal conductivity of CA-MA/EG is higher than that of CA-MA for the porous structure of EG.
Although EG can greatly ameliorate the thermal conductivity of CA-MA, EG is a porous structure with a loose texture, which determines its thermal conductivity concerning the packing density of composite PCMs. When the packing density is higher, the thermal conductivity is also higher. To ascertain the relationship between the thermal conductivity of CA-MA/EG with different packing densities, the thermal conductivity of several composite phase change energy storage materials with different packing densities was measured. As shown in Table 3 and Figure 5, the packing densities of PCM were 764.25, 625.08, 910.02, 850.80, and 604.41 kg/m 3 , and the thermal conductivities were 1.725, 1.594, 2.168, 1.958, and 1.458 W/m·K. Through fitting analysis, the thermal conductivity of CA-MA/EG had a linear relationship with the bulk density. The fitting formula is y = -0.27542 + 0.00205x (R 2 = 0.92455).  Although EG can greatly ameliorate the thermal conductivity of CA-MA, EG is a porous structure with a loose texture, which determines its thermal conductivity concerning the packing density of composite PCMs. When the packing density is higher, the thermal conductivity is also higher. To ascertain the relationship between the thermal conductivity of CA-MA/EG with different packing densities, the thermal conductivity of several composite phase change energy storage materials with different packing densities was measured. As shown in Table 3 and Figure 5, the packing densities of PCM were 764.25, 625.08, 910.02, 850.80, and 604.41 kg/m 3 , and the thermal conductivities were 1.725, 1.594, 2.168, 1.958, and 1.458 W/m·K. Through fitting analysis, the thermal conductivity of CA-MA/EG had a linear relationship with the bulk density. The fitting formula is y = -0.27542 + 0.00205x (R 2 = 0.92455).     Although EG can greatly ameliorate the thermal conductivity of CA-MA, EG is a porous structure with a loose texture, which determines its thermal conductivity concerning the packing density of composite PCMs. When the packing density is higher, the thermal conductivity is also higher. To ascertain the relationship between the thermal conductivity of CA-MA/EG with different packing densities, the thermal conductivity of several composite phase change energy storage materials with different packing densities was measured. As shown in Table 3 and Figure 5

Thermo-Stability and Thermo-Reliability of the CPCMs
A PCM must be sufficiently stable and cannot be volatile during multiple repeated exothermic phase changes. The thermostability of CA-MA/EG was tested using thermogravimetric analysis. The TGA curves are shown in Figure 6. The CA-MA /EG began to lose weight at approximately 140 • C. After the first derivative of the TG curve, the maximum weight loss rate of the CA-MA/EG was obtained at 194.4 • C. Finally, it evaporated completely at approximately 230 • C.
The thermophysical properties of a good phase change material should not change much after many phase change cycles. To determine the thermophysiological properties of CA-MA/EG after many phase transition thermal cycles, the DSC test was conducted again on the material after 500 and 1000 cycles of CA-MA/EG. The test results are shown in Table 4 and Figure 7. Through comparison of Energies 2020, 13, 2462 9 of 12 the curves, the peak shape of the DSC curve of CA-MA/EG after multiple cycles was almost exactly the same, indicating that the phase transition temperature and the latent heat before and after the cycle were not changed much. After analyzing the data, we found that after 500 and 1000 cycles, the phase transition temperature of CA-MA/EG changed by 0.1 • C and 0.3 • C, respectively, the latent heat of phase transition decreased by 2.03% and 10.6%, respectively, and the change values were relatively small. This demonstrates that CA-MA/EG composite phase change materials show great thermal reliability.

Thermo-Stability and Thermo-Reliability of the CPCMs
A PCM must be sufficiently stable and cannot be volatile during multiple repeated exothermic phase changes. The thermostability of CA-MA/EG was tested using thermogravimetric analysis. The TGA curves are shown in Figure 6. The CA-MA /EG began to lose weight at approximately 140 °C. After the first derivative of the TG curve, the maximum weight loss rate of the CA-MA/EG was obtained at 194.4 °C. Finally, it evaporated completely at approximately 230 °C. The thermophysical properties of a good phase change material should not change much after many phase change cycles. To determine the thermophysiological properties of CA-MA/EG after many phase transition thermal cycles, the DSC test was conducted again on the material after 500 and 1000 cycles of CA-MA/EG. The test results are shown in Table 4 and Figure 7. Through comparison of the curves, the peak shape of the DSC curve of CA-MA/EG after multiple cycles was almost exactly the same, indicating that the phase transition temperature and the latent heat before and after the cycle were not changed much. After analyzing the data, we found that after 500 and 1000 cycles, the phase transition temperature of CA-MA/EG changed by 0.1 °C and 0.3 °C, respectively, the latent heat of phase transition decreased by 2.03% and 10.6%, respectively, and the change values were relatively small. This demonstrates that CA-MA/EG composite phase change materials show great thermal reliability.

Thermo-Stability and Thermo-Reliability of the CPCMs
A PCM must be sufficiently stable and cannot be volatile during multiple repeated exothermic phase changes. The thermostability of CA-MA/EG was tested using thermogravimetric analysis. The TGA curves are shown in Figure 6. The CA-MA /EG began to lose weight at approximately 140 °C. After the first derivative of the TG curve, the maximum weight loss rate of the CA-MA/EG was obtained at 194.4 °C. Finally, it evaporated completely at approximately 230 °C. The thermophysical properties of a good phase change material should not change much after many phase change cycles. To determine the thermophysiological properties of CA-MA/EG after many phase transition thermal cycles, the DSC test was conducted again on the material after 500 and 1000 cycles of CA-MA/EG. The test results are shown in Table 4 and Figure 7. Through comparison of the curves, the peak shape of the DSC curve of CA-MA/EG after multiple cycles was almost exactly the same, indicating that the phase transition temperature and the latent heat before and after the cycle were not changed much. After analyzing the data, we found that after 500 and 1000 cycles, the phase transition temperature of CA-MA/EG changed by 0.1 °C and 0.3 °C, respectively, the latent heat of phase transition decreased by 2.03% and 10.6%, respectively, and the change values were relatively small. This demonstrates that CA-MA/EG composite phase change materials show great thermal reliability.

Conclusions
The form-stable CA-MA/EG CPCMs with the largest CA-MA quality content of 92.4% is suitable for latent heat thermal energy storage (LHTES) systems, such as backfill materials of soil-source heat pump systems. The SEM characterization shows that CA-MA can equably disperse in the network structure of EG, and no seepage of liquid CA-MA was found. The DSC effects indicated that the CPCMs had appropriate phase change temperatures (T m : 19.78 • C, T f : 18.85 • C) and high latent heat (H m : 137.3 J/g, H f : 139.9 J/g). By the thermal storage/release experiment, we concluded that adding the EG into CA-MA led to a notable advancement in thermal conductivity. The TGA test proved that the CA-MA/EG CPCMs had excellent stability within the operating temperature range. The thermal cycling test results after 1000 thermal cycles showed that the CA-MA/EG CPCMs had positive thermal reliability. Consequently, the prepared CA-MA/EG composite PCM can only be utilized in low-temperature LHTES systems, for instance, as backfill materials of ground-source heat-pump systems.