An energy-efficient composite by using expanded graphite stabilized paraffin as phase change material

Accepted Manuscript An energy-efficient composite by using expanded graphite stabilized paraffin as phase change material Xi Guo, Shaodi Zhang, Jinzhen Cao PII: DOI: Reference:

S1359-835X(17)30472-4 https://doi.org/10.1016/j.compositesa.2017.12.032 JCOMA 4881

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

7 November 2017 10 December 2017 31 December 2017

Please cite this article as: Guo, X., Zhang, S., Cao, J., An energy-efficient composite by using expanded graphite stabilized paraffin as phase change material, Composites: Part A (2017), doi: https://doi.org/10.1016/j.compositesa. 2017.12.032

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An energy-efficient composite by using expanded graphite stabilized paraffin as phase change material Xi Guo, Shaodi Zhang, Jinzhen Cao* MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Qinghua East Road 35, Haidian 100083, Beijing, China

Abstract: Thermal energy storage (TES) composites were prepared by employing expanded graphite (EG) stabilized paraffin as phase change material (PCM) and wood flour/ high density polyethylene (WF/HDPE) as matrix. The morphology and structure of EG and form-stable phase change material (FSPCM) were investigated by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and mercury intrusion porosimetry. The fabricated TES composites with different FSPCM types and contents were characterized by differential scanning calorimetry (DSC), thermogravimetric (TG), infrared thermography and laserflash thermal analysis. Physical and mechanical strength were also evaluated. The results showed that: (1) the EG had abundant pores and most of the pores were below 26 μm, the EG stabilized paraffin material showed perfect stability without any chemical reactions; (2) thermal performance indicated that the TES composites had efficient temperature-regulated ability, but thermal durability need to be further enhanced; (3) addition of paraffin and EG destroyed the interface bonding of the TES composites and mechanical properties appeared slight decrease; (4) the satisfying thermal performance and acceptable mechanical property indicating the TES composites can be used as building material for temperature conditioning. Keywords: phase change materials (PCMs), expanded graphite (EG), thermal energy storage (TES), temperature regulation, wood-plastic composites (WPC)

1. Introduction With the rapid development of modern society, buildings has been one of the biggest energy

*

Correspondence to: Jinzhen Cao ([email protected]). Tel/Fax: 86 010 62337381.

consumers in most countries [1-3]. In order to solve the increasing energy demands in buildings, the concept of thermal energy storage (TES) was introduced to achieve the indoor comfortableness at low energy consumption in buildings. Phase change materials (PCMs) are a kind of TES materials which can adsorb or release heat during the phase change process [4-8]. PCMs as TES materials is one of the most efficient methods to save thermal energy, based on its high thermal capacity, good thermal stability and low cost [9-12]. However, most solar energy utilizations are in medium- (80 ~ 250 ℃) and high-temperature (>250 ℃), such as solar drying, cooking, water heating and solar power generating [13]. The investigation on low-temperature solar energy utilizations is less developed. Therefore, it is important to develop a specially designed PCM which can store thermal energy at low temperatures. Among the numerous chemical substances, paraffin wax is a typical organic PCM which has wide phase change temperature range, since its carbon atom number determines the different melting and cooling temperatures [14]. Alkanes like n-hexadecane, n-heptadecane, n-octadecane, n-nonadecane and n-eicosane have low phase change temperatures within the suitable temperature range of human which can be used in low-temperature solar energy applications. While drawbacks including volume expansion, low thermal conductivity and liquid leakage during phase change process limit their applications. So various methods has been carried out to prevent leakage and enhance thermal conductivity, either by blending PCM with polymers [15, 16], partly encapsulating PCM with porous materials [17, 18], or forming microencapsulated PCMs (MicroPCMs) [19, 20], etc. The latest studies show that combination of PCMs with porous materials to prepare form-stable PCMs is an efficient and simple method [21-25]. Among the abundant porous materials, expanded graphite (EG) is a commonly used material which has low density, abundant pores and low chemical activity, the most important is that it has high thermal conductivity. Merlin et al. [26] fabricated a latent heat thermal energy storage system by using paraffin wax as active component and EG as support material. The results were encouraging for an industrial use, but stability of the composites still needs to be investigated. Xu et al. [27] prepared and investigated a new high temperature composite PCM by using D-Mannitol and EG. The results showed that the composite can be suggested as a potential alternative for solar energy storage. Although EG had

improved the encapsulation ability, however, the liquid leakage and decrease of energy storage capacity still occurred after hundreds of heating-cooling cycles. Here, we propose to combine high-density polyethylene (HDPE) and wood flour (WF) with EG to further prevent the liquid leakage of paraffin. As reported in previous studies [15, 28-30], the blends of pure HDPE and paraffin had been evaluated as form-stable material for TES application. As a thermoplastic resin, HDPE can act as a film which encapsulates the whole surface of EG based form-stable PCMs (FSPCMs) at melting temperature and keep stable shape at low temperatures. However, mechanical deterioration may occur for HDPE coated FSPCMs, so WF is also introduced into the system to improve the mechanical strength. As a natural sustainable material, wood has abundant pores, low thermal conductivity and high ratio of strength to weight, which makes it an excellent building material to create comfortable living environment at low energy consumption [31, 32]. The WF/HDPE composite, as one type of the wood-plastic composites (WPC), has also been widely used in building owing to its excellent physical and mechanical properties, as well as recyclability [33-35]. Therefore, by combining WPC with FSPCMs, the liquid leakage problem might be completely solved by not only the encapsulation of HDPE but the stabilization of abundant porous structure established by EG and WF. In this study, we prepared FSPCMs by using paraffin as the PCM and EG as porous supporting material. The morphology and porosity of EG and FSPCM were characterized by scanning electron microscopy (SEM) and mercury intrusion porosimetry. Then the HDPE and WF were combined with FSPCMs to further improve the stability of this TES system. Thermal performance of the TES composites was analyzed by differential scanning calorimetry (DSC), thermogravimetric (TG), infrared thermography and laserflash thermal analysis. Physical and mechanical properties were also tested in order to evaluate the possibility of the composites as building materials.

2. Materials and methods 2.1. Materials Paraffin wax with a melting range of 20 ~ 30 oC was purchased from Fushun wen'ai new technology Co., Ltd., Dalian, China. EG with an expansion ratio of 300 ml/g was supplied by the Qingdao Herita Graphite products Co. Ltd., Qingdao, China. WF (Populus tomentasa Carr.) with

a size of 60 ~ 80 mesh was kindly donated by Xingda Wood Flour Company, Hebei, China. HDPE with a melt flow index of 10 g/10 min and a density of 0.952 g/cm3 was supplied by Dongguan Ying Sheng plastic chemical Co., Ltd., Guangdong, China. 2.2. Preparation of the FSPCM The biggest encapsulation ratio (k) of EG loaded paraffin can be calculated by the below formula:

Where V is the total pore volume of EG, ρ is the density of paraffin. For the EG and paraffin we used in this study, V and ρ are 16.95 ml/g and 0.83 g/cm3, respectively. So the biggest encapsulation ratio is around 14. The different mass ratios of paraffin and EG were set as 6:1 (FSPCM1), 8:1 (FSPCM2), 10:1 (FSPCM3), 12:1 (FSPCM4) and 14:1(FSPCM5), respectively. Firstly, the EG was dried in an oven at 103 oC for 4 h. Then, a beaker containing EG was put into a water bath and heated at 60 oC constantly. The liquid paraffin was added into the EG powders drop by drop with constant mechanical stirring (120 rpm). Further 10 min stirring was needed after the addition of paraffin. And after 2 h heat preservation (60 oC) in a vacuum oven to ensure the sufficient mix of paraffin and EG, the FSPCM was obtained. 2.3. Preparation of the TES composites The TES composites based on the prepared paraffin/EG FSPCM, HDPE and WF were fabricated by extruding followed by a hot-pressing process. The compositions are shown in Table 1. The composites with different FSPCM kinds are named P1WPC-20, P2WPC-20, P3WPC-20, P4WPC-20 and P5WPC-20, respectively. The mass fractions of FSPCM are all 20 wt %. And the composites with different FSPCM mass fractions are labeled P3WPC-10, P3WPC-20 and P3WPC-30, respectively. The WF/HDPE ration of WPC is kept constant at 2/3. The details for preparation can be found in our previous publication [11]. Summarily, FSPCM, HDPE and WF with certain proportions were mixed thoroughly before put into a co-rotating twin-screw extruder (KESUN KS20, China). The screw speed of the extruder barrel was 180 rpm and the corresponding temperature were 140/150/160/160/115 oC. The extrudates were crushed into particles (smaller than 10 mm) and then processed by a hot press (SYSMEN-Ⅱ, China Academy of Forestry) at 160 oC with pressure of 4 MPa. Further 6 min cold pressing at room temperature was needed. The appearances of the control group and TES composite (P3WPC-20) are shown in

Fig. 1. 2.4. Characterization Surface morphology and structure of EG and FSPCM were observed by an S-3400 SEM (Hitachi, Japan) with an acceleration voltage of 5 kV. All samples were gold-coated and glued to the sample stage before observation. Mercury intrusion porosimetry was carried out by using an AutoPore ΙV 9500 (Micromeritics Instruments, USA) to analyze the porosity of the EG. The contact angle was 130°. The samples were vacuated for 5 min at a vacuum pressure about 50 mm Hg. The mercury filling pressure was about 0.5 psia with an equilibration of 10 secs. Chemical compositions were examined by an X-ray diffraction (XRD) (X-ray 6000, Shimadzu, Japan) instrument. Diffraction patterns were collected in the ranges from 5 to 60° at the scanning rate of 5° per min with Cu Kα (λ=0.1540 nm) radiation. Continuous scanning mode was operated at 40 kV with 30 mA current. Thermal energy storage capacity was characterized by DSC using a TA Q2000 (USA) analyzer at a heating or cooling temperature rate of 5 oC/min in the range of -10 ~ 50 oC under nitrogen atmosphere. Thermal stability was measured using a Q5000IR TG instrument (TA, USA) from room temperature to 600 oC at a heating rate of 10 oC/min under nitrogen atmosphere. 2.5. Thermal performance test Temperature changes of the composites during constant heating and cooling process were recorded using a temperature detector (SH-X, Shenzhen, China). The composites powder was put into a beaker to experience heating process in an oven at 60 oC and cooling process in a refrigerator at -20 oC. In order to exhibit the temperature changes graphically, infrared thermography (G90, SAT Infrared Technology Co. Ltd, China) was used to show the surface temperature distributions of the composites. All samples were heated at 80 oC and at a 60 cm distance from the camera lens. The surface images of the composites at different intervals were analyzed by its own SatIrReport software. A laserflash thermal analyzer (LFA 447, Netzsch, Germany) was used to measure the thermal conductivity of the composites. The tests were carried out at 20, 30 and 40 oC under nitrogen

atmosphere by using a disk-like samples with a diameter of 12 mm and a thickness of 3 mm. Thermal durability of the composites was performed using boiled and ice water for 500 melting-cooling cycles. Specimens (50×50 mm) covered hermetically by tinfoil were soaked into the water more than 30 s to make sure the internal paraffin melt or crystallize completely. Wipe off the leaked paraffin for further tests. 2.6. Physical and mechanical properties Hygroscopicity of the composites under the relative humidity from 5 % to 95 % was carried out by using a dynamic vapor sorption instrument (IGAsorp, Hiden Isochema, UK). A small basket containing 5.0 mg oven-dried sample was tested at 25 oC under nitrogen atmosphere. The equilibrium moisture content at every relative humidity was recorded. Impact strength was characterized according to the standard ISO 179-1:2000 [36] by using six replicates with the size of 80×10×3 mm for each group. Flexural properties was measured in terms of the standard ISO14125:1998 [37] by using the three-point bending test (60×25×3 mm).

3. Results and discussion 3.1. Characterization of EG The surface morphology and pore diameter distribution of EG are shown in Fig. 2. Typical curving worm-like structure and hundreds of volume expansion of EG can be obtained after high temperature heating (Fig. 2a). Meanwhile, rapid heating led to a roughly microstructure which exhibited like an irregular honeycomb network (Fig. 2b). The pore diameter distribution of EG is shown in Fig. 2c. The pore size range varies from 0.003 to 362.6 μm, with a volumetric average diameter of 16.3 μm. The largest quantity of pores concentrates around 9.05 μm and the total pore volume reaches 61.3 % for diameter below 26.0 μm. This abundant crevices and net-like pores with small diameters make the encapsulation of various PCMs possible [38]. After full impregnation of paraffin, plenty of small pores disappeared as shown in Fig. 2d. The pores below 30.29 μm were all filled by paraffin. The porosity of EG decreased from 81.21 % to 27.16 % after the encapsulation. But there were still some pores remained even after the vacuum impregnation, this may be related to the complex structure of EG. It should be noted that the large quantity of

pores appeared around 369.58 μm were resulted by the re-formation of EG structure rather than the original pores in pure EG. The solidified paraffin formed new crevices at the surface of EG, which increased the pore numbers in this size region. 3.2. XRD analysis of the TES composites The XRD patterns of paraffin, EG, WF, HDPE and P3WPC-20 are shown in Fig. 3. Because all TES composites had the same compositions, so only the pattern of P3WPC-20 group with moderate paraffin and EG content was recorded to represent the other groups. The paraffin was regularly crystallized according to the peaks at 2θ=21.16° and 24.05° which are attributed to the planes of (110) and (200), respectively [39]. EG showed a strong diffraction peak located at 26.45° (2θ), corresponding to the feature peak (002) formed on the basis of hexagonal graphite [40]. WF showed amorphous area in the range of 11 ~ 25° (2θ), and as main composition, cellulose was crystallized according to the peaks at 2θ=15.46° and 22.14°. HDPE had two diffraction peaks at 2θ=21.57° and 23.91°, which are related to the (110) and (200) crystal planes, respectively [39]. As shown in the curve of P3WPC-20, the XRD patterns include three peaks, which are just the combination of the characteristic peaks from paraffin, EG, WF and HDPE. No new diffraction peak was found in the XRD pattern of the composite, which indicates that the TES composite was just the integration of paraffin, EG, WF and HDPE. 3.3. Thermal properties of the TES composites 3.3.1. Heat storage capacity Specific heat capacity curves obtained by the DSC measurements are used to evaluate the heat storage capacity of TES composites (Fig. 4). The corresponding phase change characteristics are presented in Table 2. It was clear that the TES composites showed little difference at various paraffin/EG ratios (Fig. 4a). But as shown in Table 2, the onset melting temperature increased slightly with increasing paraffin/EG ratio. The endothermic enthalpies of P1WPC-20 and P5WPC-20 were 17.79 and 21.93 J/g, respectively, which improved by about 23.3 % with increasing paraffin/EG ratio from 6/1 to 14/1. Almost the same result can be found in freezing process. The TES composites with different FSPCM contents performed very differently as shown in Fig. 4b. The endothermic enthalpies of P3WPC-10, P3WPC-20 and P3WPC-30 were 6.96, 20.70 and 39.04 J/g, respectively. The exothermic enthalpies of them were 7.55, 20.56 and 40.29 J/g, respectively. Apparently, P3WPC-30 had a much higher enthalpy than P3WPC-10, which

indicated a better heat storage capacity. It was interesting that the P3WPC-10 had relatively lower onset melting (16.4 oC) temperature than the other groups. The better heat transfer may benefit from the low content of paraffin with poor thermal conductivity. 3.3.2. Thermal performance In order to evaluate the practical temperature regulation ability of the TES composites, the temperature change curves were recorded in Fig. 5. The composites with various FSPCM had obvious temperature delay started from the phase change stage as shown in Fig. 5a. A moderate constant temperature range (14 ~ 34 oC) was observed during the heating process, which was attributed to the thermal energy storage ability of paraffin. Two constant temperature ranges appeared due to the special crystallization of paraffin [41]. There was no evident distinction between the various TES composites except a slight curve hysteresis for the groups with high paraffin/EG ratios. However, the thermal performance of the TES composites was greatly influenced by the mass fraction of FSPCM (Fig. 5b). It was obvious that the P3WPC-10 had a faster temperature change speed than the other groups before 14 min. The reason can be attributed to the high thermal conductivity of the composites containing more EG but less paraffin. The P3WPC-10 just showed slow temperature changes after the phase change stage of paraffin, while the P3WPC-30 with 30 % FSPCM content exhibited excellent temperature-regulated performance in both heating and cooling processes. To display the temperature changes more directly, infrared thermal images of the TES composites with different FSPCM types and contents are shown in Fig. 6 and Fig. 7. Surface temperature distributions of the composites were recorded every 120 s and the average surface temperatures were calculated. All TES composites had lower temperatures than the control group at every moment, indicating the outstanding temperature regulating ability during the heating process. Besides, the better thermal energy storage capacity performed with increasing paraffin/EG ratio as shown in Fig. 6 at 480 s, the average temperature of the P1WPC-20 was 42.5 o

C, which was 5.1 oC higher than the P5WPC-20. Fig. 7 shows the effects of FSPCM content for

the thermal energy storage capacity. Likewise, the composites with higher FSPCM content had better temperature regulating ability, which showed darker color on surface. The paraffin inside the composites adsorbed thermal energy during the heating process, while desorbed the energy during the cooling process. Therefore, the TES composites could regulate temperatures depending

on the phase change character of paraffin. 3.3.3. Thermal stability Fig. 8 shows the thermal stability of the TES composites investigated by TG analysis. Three weight loss stages could be easily observed from the TG curves of all TES composites, representing the degradations of paraffin, WF and HDPE, respectively. There was no obvious distinction for the composites prepared with different types of FSPCM at the first stage, but P5WPC-20 with paraffin/EG ratio of 14/1 showed the maximal thermal decomposition at 134 oC as shown in Fig. 8c. However, the difference was evident for composites prepared at various FSPCM contents (Fig. 8b). P3WPC-30 with 30 % FSPCM content showed sharp decrease at 119 o

C, resulting from the degradation of paraffin. All the composites showed little weight loss below

about 75 oC, which indicated that they could meet the thermal stability requests as a building material used at room temperature. 3.3.4. Thermal conductivity Thermal conductivity of various TES composites are shown in Fig. 9. Generally, lower conductivity is expected for composites while used as a building material, which can hinder the heat transfer between indoor and outdoor air. However, it is not determined for a composite containing phase change material. Phase change material needs a suitable thermal conductivity to guarantee the sorption and release of heat energy during the phase change process [42]. In Fig. 9a, thermal conductivity of P1WPC-20 decreased to 0.258 W/mK (20 oC) after the incorporation of paraffin. Furthermore, thermal conductivity decreased gradually with increasing paraffin/EG ratios. Thermal conductivity of TES composites with various FSPCM contents were obviously different from each other as shown in Fig. 9b. The thermal conductivity of P3WPC-30 at 20, 30 and 40 oC were 0.265, 0.252 and 0.235 W/mK, respectively. Approximately the same high value was found compared with the control group due to the high EG mass fraction. Thus, a filler with high thermal conductivity or chemical modification are necessary to make sure the heat transfer of TES composites [43-45]. 3.3.5. Thermal cycling Heat enthalpy of the TES composites after 500 heating-cooling cycles are presented in Table 2. Obviously, latent heat of all groups had different reductions after thermal cycling test. Endothermic enthalpies of the TES composites prepared with different types of FSPCM all

decreased by about 23 % and so as the cooling process. The latent heat values of P3WPC-10, P3WPC-20 and P3WPC-30 during the melting and cooling process were decreased by 30.6, 23.4, 25.0 % and 36.0, 25.3, 23.0 %, respectively. Thermal durability need to be further enhanced to avoid the evaporation of paraffin in TES composites. 3.4. Physical and mechanical properties 3.4.1. Hygroscopicity Hygroscopicity is an important index for a composite used as a building material, which may be related to the dimensional stability and durability. The equilibrium moisture contents (EMC) of the composites at various RHs are shown in Fig. 10. P3WPC-20 had the strongest hygroscopicity as shown in Fig. 10a. The highest EMC appeared for the TES composites with different paraffin/EG ratios was below 5 % at 95 % RH environment (Fig. 10a). P3WPC-20 had the worst anti-moisture property among all groups, which can be attributed to the poor interface bonding produced by the FSPCM incorporation. Though P5WPC-20 also existed the same deterioration, the large amount of long-chain paraffin had much strong anti-moisture property which favored its lower hygroscopicity. For the TES composites with different FSPCM contents, it was clearly that high FSPCM content led to strong hygroscopicity, resulting from the non-ideal interface bonding (Fig. 10b). 3.4.2. Mechanical strength Flexural properties of the TES composites are shown in Fig. 11. It is clearly that flexural strength and modulus of the composites decreased after the incorporation of FSPCM and high content led to lower value (Fig. 11b). The deterioration can be attributed to the poor interface caused by FSPCM addition. Besides, liquid paraffin during phase change process also promoted the decrease by filling the space between wood fiber and HDPE. Different FSPCM types did no distinct influence to the flexural properties (Fig. 11a). Impact strength of the TES composites are shown in Fig. 12. The impact strength increased slightly with increasing paraffin/EG ratio, which benefited by the addition of flexible paraffin (Fig. 12a). There was also a little increase for P3WPC-10 with 10% FSPCM, but impact strength decreased with the increasing FSPCM content. This indicated that the degraded interface could not be rescued though with high paraffin content.

4. Conclusion

Paraffin can be encapsulated by porous EG and used for TES composites fabrication with WF/HDPE as matrix. EG has abundant pores for encapsulation and stay stable with paraffin. Thermal enthalpy of the TES composites can be controlled by paraffin/EG ratio and mass fraction of FSPCM. Thermal properties were investigated by DSC, TG, thermal cycling etc. and the results showed that the TES composites had excellent thermal energy storage capacity and efficiency for preventing temperature change. EG promoted the heat transfer of the composites thanks to its high thermal conductivity. Incorporation of paraffin in favor of the superior moisture resistance of the TES composites, while slightly decreased the mechanical properties. The prepared TES composites were advised to be applied as building materials for temperature conditioning according to their satisfying heat storage capacity and acceptable mechanical strength.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No. 31570542) and the Fundamental Research Funds for the Central Universities in China (2015ZCQ-CL-01).

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Table(s)

Table 1 Mass percentages of FSPCM, WF, HDPE in different groups of TES composites. Labeling

FSPCM (wt %)

WF (wt %)

HDPE (wt %)

Control

-

40

60

P1WPC-20

20 (FSPCM1)

32

48

P2WPC-20

20 (FSPCM2)

32

48

P3WPC-20

20 (FSPCM3)

32

48

P4WPC-20

20 (FSPCM4)

32

48

P5WPC-20

20 (FSPCM5)

32

48

P3WPC-10

10 (FSPCM3)

36

54

P3WPC-30

30 (FSPCM3)

28

42

The ratio of WF/HDPE was kept constant at 2/3.

Table 2 Parameters obtained from DSC measurements of TES composites. Melting Labeling

Solidifying

Onset temperature

Peak temperature

Latent heat

Onset temperature

Peak temperature

Latent heat

(oC)

(oC)

(J/g)

(oC)

(oC)

(J/g)

P1WPC-20

18.7

22.7

17.79 (13.69)

19.4

5.4

18.82 (14.30)

P2WPC-20

19.0

23.0

20.66 (15.98)

19.0

5.1

20.36 (15.97)

P3WPC-20

20.7

23.2

20.70 (15.85)

19.0

5.2

20.56 (15.35)

P4WPC-20

20.7

23.6

20.25 (14.77)

19.3

5.3

21.58 (15.64)

P5WPC-20

20.8

23.8

21.93 (15.64)

19.5

5.4

22.47 (16.50)

P3WPC-10

16.4

22.5

6.96 (4.84)

20.00

4.9

7.55 (4.83)

P3WPC-30

20.6

23.9

39.04 (29.28)

20.00

6.5

40.29 (31.02)

Note: Values in the parentheses represent the latent heats of the TES composites after experiencing 500 heating-cooling cycles.

Figure caption Click here to view linked References

Fig. 1 Appearances of the control group and TES composite (P3WPC-20). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 2 SEM images and pore diameter distribution of expanded graphite (EG) and form-stable PCM (FSPCM), (SEM images of EG (a, b), diameter distribution of EG (c), diameter distribution of EG and FSPCM (d)). Fig. 3 XRD patterns of paraffin, EG, WF, HDPE and P3WPC-20. Fig. 4 Specific heat capacity curves of the TES composites prepared by different FSPCM types (a) and contents (b). Fig. 5 Temperature change curves of TES composites prepared by different FSPCM types (a) and contents (b). Fig. 6 Infrared thermal images of TES composites prepared with different types of FSPCM ((a) Control, (b) P1WPC-20, (c) P2WPC-20, (d) P3WPC-20, (e) P4WPC-20, (f) P5WPC-20). Fig. 7 Infrared thermal images of TES composites prepared at different FSPCM contents ((a) Control, (b) P3WPC-10, (c) P3WPC-20, (d) P3WPC-30). Fig. 8 TG and DTG curves of TES composites prepared by different FSPCM types (a), (c) and contents (b), (d). Fig. 9 Thermal conductivity of TES composites. Fig. 10 Hygroscopicity of TES composites prepared by different FSPCM types (a) and contents (b). Fig. 11 Flexure properties of TES composites prepared by different FSPCM types (a) and contents (b). Fig. 12 Impact strength of TES composites prepared by different FSPCM types (a) and contents (b).