Preparation and Characterization of CPCM for Thermal Energy Storage Unit

Abstract

The efficiency and economy of an ASHP (air source heat pump) can be significantly improved in a cold area by combining it with a TESU (thermal energy storage unit). The work of looking for a phase change material with a suitable temperature range, a large thermal capacity, and high conductivity has been always on the road. This paper prepared 10 CPCMs (composites of a phase change material) by using the vacuum adsorption method, which consists of LA (lauric acid) as the phase change material and EG (expanded graphite) as the skeleton matrix for its high thermal conductivity and porous characteristics. By characterizing and analyzing their basic properties, surface morp\hology, and stability, a suitable CPCM for the TESU coupled with the ASHP heating system is found. The leakage experiment results show that the maximum effective content of LA absorbed in 100-mesh EG of a CPCM is 80%, and that in 200-mesh EG, it is 90%. Among 10 CPCMs, CPCM3, with a mass ratio of LA:EG/8:2, is considered the more satisfactory one for the TESU proposed by this paper, because its performance shows good stability, its phase transition temperature is 40.98–41.94 °C, its latent heat is 164.34–168.28 kJ/kg, and especially, its thermal conductivity is 8.15–8.33 W/(m·K). This paper will be followed by performance research on TESUs coupled with ASHP heating systems.

1. Introduction

The huge degree of energy consumption in building heating systems is a challenge in achieving the goals of “carbon peak and carbon neutrality”. ASHPs (air source heat pumps) could offer a promising solution to this challenge if the contradiction between their energy efficiency and users’ heating demands in cold areas could be effectively solved. When the outdoor temperature is lower, the heating ability and corresponding energy efficiency of an ASHP will be much lower, but the heating demand of the user is contrary. The heating system of an ASHP combined with a TESU (thermal energy storage unit), shown as Figure 1, could effectively solve this issue, in which the make-up water pump P1 will be started when the system water pressure is less than the set value. When the air temperature is not very low, when solar power and wind power can be obtained, or when the power cost is charged at the off-peak power price, the ASHP will work to heat the house and charge heat energy to the TESU. During this period, valves V₁ and V₃ are opened, valve V₂ is closed, and pump P₁ drives the heating water to circulate along 1-2-3-4-5-6-1. When the outdoor air temperature lowers, the TESU is used to heat the house, and the ASHP is off. During this period, valves V₁ and V₃ are closed, valve V₂ is opened, and pump P₁ drives the heating water to circulate along 2-3-4-5-2. Whenever the heating water temperature is lower than its set value, the ASHP is started. In this case, the energy efficiency and economy of the heating system can be significantly improved because the ASHP does not work when the outdoor temperature is too low. It is clear that the performance coefficient of the heating system will mainly depend on the TESU, which is expected to have suitable operating temperatures, high thermal conductivity, a large heat capacity, and good cyclic thermal stability.

Compared to sensible heat storage materials, such as water, and chemical heat storage materials, phase change materials (PCMs) demonstrate a superior energy storage density and constant temperature during the phase transition process, but before a PCM with a suitable phase change temperature can be used in a TESU, its poor thermal conductivity must be improved [1]. Expanded graphite (EG) could be an excellent candidate to solve these issues because its thermal conductivity is up to 100 W/(m·K) [2] and its porous structure can accommodate and constrain a large amount of PCM. Chen Limei et al. [3] prepared a novel paraffin/EG composite of a phase change material (CPCM) with stable properties by a direct impregnation method whose thermal conductivity increased by 113% (from 0.276 W/m·K to 0.589 W/m·K) when the mass fraction 10%EG was added. Hou Junying et al. [4] proposed a TESU filled with a CPCM of ZIF-8 metal–organic skeleton-carried octadecyl alcohol (OD) and numerically studied the effects of the material ratio and the arrangement of the tube bundle and fins on its performance. Guo Yanqin et al. [5] numerically studied the effects of the natural convection and thermal conductivity of a CPCM on the TESU performance of a double-helix coil filled with EG.

Compared with paraffin, the thermal conductivity of fatty acids is higher in the same packaging mode. Fang Guihua et al. [6] prepared a CPCM by using EG to absorb the eutectic mixture of palmitic acid and stearic acid, which showed excellent thermal stability and thermal conductivity during 1000 thermal cycles. Tang X et al. [7] impregnated sebacic acid and LA (lauric acid) into wormlike EG to obtain a stable CPCM (1:5) whose enthalpy is very close to that of the original acid, but whose thermal conductivity is improved from 0.14 W/(m·K) to 3.15 W/(m·K). Wang Z et al. [8] obtained a CPCM of LA and SA (stearic acid) encapsulated in EG (10–15 wt.%) whose phase change temperature and latent heat are stable, but whose thermal conductivity and thermal diffusivity are 2.4–2.6 times and 3.2–3.7 times that of LA, respectively. Zhou Weibing et al. [9] obtained a CPCM by modifying expanded vermiculite as an adsorption material and lauric acid and stearic acid as phase change materials. This CPCM has a phase change enthalpy of 167.6 kJ/kg, high thermal conductivity, and good thermal stability after 1000 cycles, and was applied in the construction field. By using the sol–gel method of encapsulating an LA and tetradecanol matrix in silicon dioxide, Wang Sai et al. [10] obtained a CPCM with a phase change temperature of 24.81 °C and a latent heat of 47.96 J/g. Zeng Yuan et al. [11] used an LA–tetradecanoic acid binary eutectic mixture as the phase change material, nano-SiO₂ as the support material, and activated carbon as the agent to prepare a CPCM whose thermal conductivity is improved by 41.95% when the mass fraction of activated carbon is 5%. Chen et al. [12] and Li Xianghui et al. [13] prepared a CPCM with good thermal conductivity and thermal stability by adsorbing liquid LA with activated carbon and impregnating capric acid–LA with expanded vermiculite.

There are many more studies on the preparation of CPCMs using fatty acids and EG [14,15,16,17], but there is still a gap between their results and the ones expected from TESUs used in ASHP heating systems, especially with high thermal conductivity. As a type of fatty acid, LA can be absorbed in EG to become potentially suitable for the TESU proposed in this paper, but there are few studies on this work. This paper will firstly prepare 10 CPCMs with LA as the phase change material and EG as the absorbing material, and then characterize these CPCMs to select a suitable one for the TESU described in this paper. This work will lay a foundation for following studies on the performance of TESUs used in air source heat pump systems.

2. CPCM Preparation and Characterizing Instruments

2.1. Preparation of LA/EG CPCMs

Firstly, analytically pure LA made by Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China) and both kinds of EGs (carbon content 99%) with 100 mesh and 200 mesh made by Henan Liugong Graphite Co., Ltd. (Zhengzhou, China) are prepared. Because the temperature of the heating water from an ASHP is usually about 50 °C, LA with a phase transition temperature of 42.0~44.0 °C and a latent heat of 180 kJ/kg is selected (these characteristics are provided by the manufacturers), which agrees with the results of the literature [12,18].

Then, 10 kinds of CPCM samples are prepared by absorbing LA in EG. The mass ratios of LA:EG (100-mesh) of 6:4, 7:3, 8:2, 9:1, and 9.5:0.5 are named CPCM1 to CPCM5, respectively, and the same mass ratios of LA:EG (200-mesh) are named CPCM6 to CPCM10, respectively. The CPCM preparation process is shown as Figure 2.

(1)

Weighing LA and EG by a HUAZHI 10,000-digit electronic balance (HUAZHI, Qingdao, China) with an accuracy of 0.0001 g and mixing them together.

(2)

Putting the mixture into a 60 °C constant temperature water bath (HH-6) and stirring it at a speed of 250 r/min for 5 min to melt evenly.

(3)

Transferring the mixture into a conical flask and adsorbing it for 10 min in the constant temperature water bath under a vacuum condition of −0.68 MPa.

Finally, the CPCM is obtained by naturally cooling the mixture to room temperature. By the same process, 10 CPCMs are prepared.

2.2. Characterization Instruments

In order to select a suitable CPCM for the TESU used in the ASHP heating system, the following tests are carried out to assess the phase transformation temperature, latent heat of phase transformation, thermal conductivity, thermal stability, chemical groups, microstructure, and density of CPCMs.

SEM analysis: A Mira4 scanning electron microscope with an accuracy of 5–10 nm is first used. The LA/EG composite phase change material is ground into powder, a small amount is placed on the conductive glue made by surface gold spraying treatment, and the surface topography is observed with the scanning electron microscope. The topography and microstructure of EGs with different meshes and CPCM3 are observed with the transmitted scanning electron microscope with a magnification of 6000 times.

Hot disk thermal conductivity test: The transient plane heat source method and a 3500S type thermal conductivity tester (Hot Disk Ltd., Uppsala, Sweden) with an accuracy of 3% is used in this paper whose probe, consisting of a temperature sensor and a heating source, is a continuous double-helix structure sheet formed by the conductive metal nickel after an etching treatment. Conductivities of 10 solid CPCMs and LA are obtained in 30 °C conditions, and those of 11 liquid samples are obtained in 50 °C conditions.

DSC test: A TA DSC25 differential scanning calorimeter (TA Inc., New Castle, DE, USA) with an E type thermocouple of 0.1% accuracy is used. Each sample in an aluminum crucible is 5 mg. In turn, 11 samples are heated from 30 °C to 85 °C at a rate of 5 °C/min with nitrogen protection and then cooled from 85 °C to 30 °C for a cycle. Based on the test results, the phase transformation temperature and latent heat of 10 CPCMs and LA are obtained.

TG thermogravimetric analysis: A TA Q600 type thermogravimetric analyzer (TA Corporation, New Castle, DE, USA) with an accuracy of 2% is used to indicate the temperature–mass change relationship of the CPCMs in this paper. Based on the tests and analysis above, LA, CPCM3, and CPCM9 are selected for this test. The sample powder is placed in the furnace cavity first, and then heated from 20 °C to 500 °C at a rate of 10 °C/min with nitrogen protection.

FT-IR test: A THERMO 50 Fourier transform infrared spectrometer (Thermo Scientific™, Waltham, MA, USA) with an accuracy of 0.005 cm⁻¹ is used. Based on the tests and analysis above, LA, CPCM3, and CPCM9 are selected for this test. A total of 1 mg powder of each sample is added into a potassium bromide diluent and ground evenly, and then, the spectrum is collected by the Fourier infrared spectrometer, in which an infrared light source is used, whose beam splitter is XT-KBr, wave number is 500–4000 cm⁻¹, and resolution is 2 cm⁻¹.

Density test: Solid density is determined by measuring its mass and volume at a room temperature of 20 °C. Based on Archimedes’ law and the equilibrium state of floating objects on a liquid surface at a constant temperature of 60 °C in a water bath, the liquid density of the CPCMs is obtained by using a hydrometer of ET-120D (Beijing ETuno Electronic Technology Co., Ltd., Beijing, China).

2.3. Uncertainty Analysis

When parameter y is obtained based on the independent parameters x₁, x₂, x₃, …, and x_n involved in Equation (1), its absolute error δy can be calculated according to Equation (2) based on the absolute errors δx₁, δx₂, δx₃, …, and δx_n, which mainly depend on instrument accuracies. The uncertainty of y can be indicated by δy/y [19]. The uncertainty of each parameter involved in this paper is shown as follows.

$$y=\int \left(x_1,x_2,\cdots x_n\right)$$

(1)

$$\delta y=\sqrt{{\left({\displaystyle \frac{\partial y}{\partial x_1}}\delta x_1\right)}^2+{\left({\displaystyle \frac{\partial y}{\partial x_2}}\delta x_2\right)}^2+\cdots +{\left({\displaystyle \frac{\partial y}{\partial x_n}}\delta x_n\right)}^2}$$

(2)

3. Results and Discussion

3.1. SEM Test Results

To show the EG porous structure before and after LA absorption, Figure 3a gives a 6000-times SEM image of EG with 100 mesh, Figure 3b gives that of EG with 200 mesh, and Figure 3c gives that of CPCM3 synthesized by 100-mesh EG and LA. EG shows a worm-like porous structure with many cavities in μm sizes to house LA [20,21,22]. Compared to the EG with 100 mesh, the hole arrangement of the EG with 200 mesh is more uniform, and the hole diameter is smaller, which results in a larger specific surface area and porosity. After LA is adsorbed, the porous structure of EG is not changed, and cavities are mostly filled by LA accompanied by some spaces.

3.2. Maximum Content of LA

The larger content of LA in the CPCMs means larger heat storage, but the leakage risk of LA also increases. In order to determine the maximum content of LA in the CPCMs, some CPCM samples on filter papers are heated at 60 °C for 24 h in a DZF-6012 Vacuum Drying Oven (Shanghai bluepard instruments Co., Ltd., Shanghai, China) with an accuracy of 1 °C. Four results are shown in Figure 4. Because some holes in depth are not effectively used, the maximum content of LA in the CPCM with EG of 100 mesh is 80%, which corresponds to CPCM3, and the maximum content of LA in the CPCM with EG of 200 mesh is 90%, which corresponds to CPCM9. The reason for the difference between the LA maximum content in the two EGs could be the larger specific surface area and the greater porosity of the EG with 200 mesh than those of the EG with 100 mesh. The relative error of the LA mass content is 0.01%.

3.3. Thermal Conductivity

A larger thermal conductivity can quicken the heat store/release speed of CPCMs, which is expected for a TESU. Figure 5 shows the thermal conductivities of LA and 10 CPCMs tested in a liquid state and a solid state. The thermal conductivity of LA is the smallest one, 0.27 W/(m·K) in liquid and 0.22 W/(m·K) in solid, which is the same as that of the literature [12]. The conductivity of a CPCM increases when its EG mass content increases from 5% to 40%, which is between 1.68 and 16.82 W/(m·K) in the liquid state and between 1.55 and 16.64 W/(m·K) in the solid state. When an EG mass content of 40% is introduced, the conductivity of CPCM1 with 100-mesh EG is improved by 56 times more than LA in the melt process and 67 times in the solidification process, and that of CPCM6 with 200-mesh EG is improved by 62/76 times more than LA, respectively.

Because the particles of 200-mesh EG are smaller and the heat transfer distance in a particle is shorter, their thermal conductivities are always larger than those of CPCMs based on 100-mesh EG in the same LA mass content condition. Because the heat convection of LA in cavities could happen in the liquid state, which can improve heat transfer, the thermal conductivity of a CPCM in the liquid state is greater than that of the same one in the solid state. CPCM3’s conductivity is 8.33 W/(m·K) in the liquid state and 8.15 W/(m·K) in the solid state, and that of CPCM9 is 5.44 W/(m·K) and 5.32 W/(m·K), respectively.

3.4. DSC Test Results

Because more energy is required by LA from an ordered state (solid state) to a disordered state (liquid state) [23], the melting temperature T_onset of a CPCM in the melting process is always higher than the solidification temperature T_onset of a CPCM in the solidifying process, and their difference is defined as supercooling. According to the onset definitions of the melting process (T_onset) and supercooling [24,25], Figure 6a shows the DSC results of LA and how to determine the supercooling degree, Figure 6b shows the DSC results of 10 CPCMs, and

Table 1. DSC test results of CPCMs and LA.

Sample Name	M(LA)/M(EG)	EG (Mesh)	Melting Process		Solidifying Process		Degree of Supercooling
			Temperature (°C)	Latent Heat (kJ/kg)	Temperature (°C)	Latent Heat (kJ/kg)	(°C)
CPCM1	6:4	100	41.38	107.04	40.32	102.25	1.06
CPCM2	7:3	100	41.71	134.67	40.53	130.62	1.18
CPCM3	8:2	100	41.94	168.28	40.98	164.34	0.96
CPCM4	9:1	100	42.00	168.13	41.64	164.56	0.36
CPCM5	9.5:0.5	100	41.96	173.08	41.57	168.77	0.39
CPCM6	6:4	200	41.68	86.56	40.43	83.01	1.25
CPCM7	7:3	200	41.85	144.60	40.62	140.60	1.23
CPCM8	8:2	200	40.41	160.83	39.4	157.76	1.01
CPCM9	9:1	200	41.80	170.65	41.23	167.70	0.57
CPCM10	9.5:0.5	200	41.75	178.49	40.8	175.34	0.95
LA	/	/	42.52	180.40	40.73	175.04	1.79
Relative error (%)		/	0.1%	0.15%	0.1%	0.15%	0.14%

gives their phase transition temperature, latent heat, and supercooling degree.

The phase transition temperature of CPCMs should increase with LA content in the melt process, and that of LA should be the highest one. The situation should be contrary in the solidification process, but the change rule is not as obvious as expected. This reason could be indicated by the Clausius-Clapeyron equation shown in Equation (3). The absorbed processes of LA in EG cannot be completely identical among CPCMs, which results in the different fullness of LA in each cavity. When the temperature of a CPCM is changed, the air pressure in the cavity space of a CPCM will also change, which leads to different pressure effects on the phase transition temperature. In addition, the phase transition temperature may be affected by the thermal conductivity and specific heat of a CPCM.

$${\displaystyle \frac{dp}{dT}}={\displaystyle \frac{{∆H}_c}{T∆V}}$$

(3)

where dp/dT is the rate of change in pressure with temperature; ΔH_c is the phase transition enthalpy; T is the phase transition equilibrium temperature; and ΔV is the specific volume change during the phase transition.

The supercooling degree of LA is the largest one, 1.79 °C, but the CPCM supercooling degree increases when the LA mass content decreases and the EG content increases, which could be caused by more air-filled cavities of the CPCM with more content of EG. When the CPCM with more air is heated, the air pressure in the cavities increases, which leads to a larger Tonset in the melt process, and it is contrary in the solidification process. Because the porosity of CPCMs with 200-mesh EG is greater,, as shown in Figure 3, their supercooling degree is larger than that of CPCMs based on 100-mesh EG in the same condition.

The relation between the latent heat of the CPCMs, ΔH_c, the latent heat of LA, ΔH_m, and the mass fraction of EG, φ, can be expressed by Equation (4). The latent heat of the CPCMs gradually increases with the LA mass content [23,26,27]. The latent heat of the CPCMs shown in the melt process is larger than that shown in the solidification process. The mesh effect of EG on the latent heat of the CPCMs is not obvious.

$$∆H_c=\left(1-\phi \right)∆H_m$$

(4)

The phase transition temperature of the CPCMs and the latent heat are between 40.41 and 42 °C and between 86.56 and 178.49 kJ/kg, respectively, in the melt process, and in the solidification process, they are between 39.4 and 41.64 °C and between 83.01 and 175.34 kJ/kg, respectively. The supercooling degree of a CPCM is between 0.36 and 1.25 °C. Based on the above analysis, CPCM3 and CPCM9 are selected as the subjects for subsequent research due to their better performance. The latent heat and supercooling degree of CPCM3 are 164.34–168.28 kJ/kg and 0.96 °C, and those of CPCM9 are 167.70–170.65 kJ/kg and 0.57 °C.

3.5. FT-IR Analysis on CPCMs

The chemical composition of LA (C₁₂H₂₄O₂) contains C, H, and O. FT-IR testing and analysis on LA and CPCMs can indicate if the chemical reaction happens in the preparation process of the CPCMs. Figure 7 shows the FT-IR test curves of LA, CPCM3, and CPCM9.

Table 2. Different chemical groups and their corresponding absorption peaks.

Chemical Groups	Absorption Peaks (cm−1)
Hydroxy-oh stretching vibration peak	3449.13, 3432.30, 3425.60
Methylene C-H antisymmetric stretching vibration and symmetric stretching vibration peaks	2953.86, 2917.41, 2871.09, 2955.31, 2917.61, 2955.01, 2916.91, 2850.45, 2848.64
Carbonyl C=O stretching vibration peak	1701.83, 1698.54, 1700.24
C-H bending vibration peak of saturated alkyl group	1464.87, 1410.92, 1303.91, 1249.02, 1465.57, 1302.46, 1466.21, 1326.79
C-O stretching vibration peak	1114.23, 1082.21, 1109.59, 1194.54
Unsaturated hydrocarbon group =C-H out-of-plane rocking vibration peak	939.23, 935.90, 938.02
C-H out-of-plane bending vibration peak	777.94, 720.68, 718.27, 723.94

gives the chemical groups and their corresponding absorption peaks, whose relative error is 0.5%.

The test curves of the CPCMs are consistent with that of LA, and no peak shift or new characteristic peaks are found during the preparation process of the CPCMs, which indicates no new substance is generated. Because the LA mass content reduces in the CPCMs, the spectral band intensity and shape of the CPCMs have some changes, and the peak height, sharpness, and intensity of the FT-IR spectrum decrease a little. This also indicates that the process of LA absorbed in EG is a physical one under the capillary force and surface tension [28,29], in which the heat storage characteristics of LA are not changed, and that LA is highly compatible with EG.

3.6. Thermal Stability of CPCMs

To indicate their thermal stability, thermal cycling tests for CPCM3 and CPCM9 are carried out. Figure 8 shows their DSC results after 50 cycle tests and 100 cycle tests. Before/after 100 cycle tests, the melting temperature of CPCM3 and its corresponding latent heat are 41.94 °C/42.03 °C and 168.28 kJ/kg/167.59 kJ/kg, respectively, and the solidification temperature and its corresponding latent heat are 40.93 °C/41.44 °C and 164.34 kJ/kg/163.47 kJ/kg, respectively. For CPCM9, they are 41.80 °C/42.03 °C and 170.65 kJ/kg/169.78 kJ/kg, respectively, in the melting process, and 41.44 °C/41.44 °C and 167.70 kJ/kg/166.67 kJ/kg, respectively, in the solidification process. It can be concluded that the phase change temperature and latent heat of CPCM3 and CPCM9 are almost constant during the thermal cycling test of 100 times, and their thermal stability is good.

Figure 9 shows the TG images of LA, CPCM3, and CPCM9 to indicate their thermal stability more. Below 130 °C, the mass loss rate of the CPCMs is less than 1% due to free-water evaporation [30,31]. When the heating temperature is increased to more than 130 °C, the volatilization and thermal decomposition of LA gradually occur. When the heating temperature is about 200–250 °C, the mass loss rate of the CPCMs reaches the maximum and the thermal decomposition of EG occurs. The weight loss rate of pure LA almost reaches 100%, that of CPCM3 is 74%, and that of CPCM9 is 84%. This indicates that EG can form a physical protective barrier on the LA absorbed in EG [32,33,34] to improve the thermal stability of a CPCM, which is expected by the TESU of the ASHP heating system.

When a CPCM is used in the TESU, its volumetric expansion rate should also be considered, and a smaller volumetric expansion rate is expected during the solid–liquid transition process [35,36]. The volume expansion rate is defined by Equation (5).

$$\beta =\left(V_2-V_1\right)/V_1$$

(5)

where β represents the volume expansion rate (%), V₁ represents the initial volume (m³), and V₂ represents the volume after the temperature change (m³).

Figure 10 shows the densities of the CPCMs and LA in solid and liquid states. Because some air fills in loose particles of a solid CPCM, the CPCM density in the solid state is less than that in the liquid state. When the phase change material is changed from a liquid state to a solid state, the volumetric expansion rate of LA is 2.480%, and the volumetric expansion rates of the CPCMs are all less than that of pure LA. Among them, the volumetric expansion rate of CPCM3 is the smallest at 0.714%. The relative error of the density measurement in the solid state and liquid state is 1% and 0.1%, respectively.

To indicate the suitability of CPCM3 for a TESU,

Table 3. Performance comparison between CPCM3 and CPCMs commercially available.

Name	CPCM Source	Base Material	Latent Heat (kJ/kg)	Thermal Conductivity (W/(m·K))	Phase Transition Temperature (°C)	Density (kg/m³)	Degree of Supercooling (°C)
CPCM3	This paper	LA+EG	164.34–168.28	8.15–8.33	40.98–41.94	417–420	0.96
PCM-A-42	Dongguan Donglin	PA	198–200	0.2–0.25	42–43.5	780–850	1.5
PCM-M-40	Hebei Ruosen	n-Docosane + PMMA	134.3–136.5	0.2–0.21	39.17–42.97	770–800	3.8

shows the performance of two CPCMs commercially available in China, of which PCM-A-42 is made by Dongguan Donglin Polymer Materials Co., Ltd. (Dongguan, China), and PCM-M-40 is made by Hebei Ruosen Technology Co., Ltd. (Shijiazhuang, China).

4. Conclusions

By introducing a TESU, the performance of an ASHP heating system will be obviously improved. In order to identify a CPCM that can meet the TESU requirements, 10 CPCMs are prepared by the vacuum adsorption method, with the phase material LA absorbed in EG, and characterized. Conclusions can be drawn as follows.

• The preparation of a CPCM is a physical process, and no chemical reaction happens. LA is effectively encapsulated in EG due to its capillary and surface tension, and the thermal stability of the CPCM is satisfied.
• Compared to LA, the phase change temperature of the CPCM changes little. Although the latent heat of the CPCM decreases due to the EG introduced, its thermal conductivity, supercooling degree, and volume expansion rate are significantly improved.
• Of the 10 CPCMs, the thermal performance of CPCM3 is better and more suitable for the TESU combined with an ASHP heating system.