Experimental investigation on micro-scale phase change material based on sodium acetate trihydrate for thermal storage

Experimental investigation on micro-scale phase change material based on sodium acetate trihydrate for thermal storage

Solar Energy 193 (2019) 413–421 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Experiment...

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Solar Energy 193 (2019) 413–421

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Experimental investigation on micro-scale phase change material based on sodium acetate trihydrate for thermal storage

T

Jianwei Liua, Chenhao Zhua, Wenzheng Lianga, Yanhui Lia, Hongcun Baib, Qingjie Guob, ⁎ Cuiping Wanga,c, a

College of Mechanical & Electrical Engineering, Qingdao University, Qingdao 266071, China State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, China c College of Civil Engineering and Architecture, Shandong University of Science and Technology, Qingdao 266590, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Sodium acetate trihydrate Vacuum impregnation Porous micro-structure Composite phase change material Heat storage density

A new composite phase change material (CPCM) for heat storage was prepared via the vacuum impregnation method. The material was based on sodium acetate trihydrate (SAT) adsorbed in micro-porous expanded vermiculite (EV), to form the micro-scale phase change of SAT in each EV micro cell acting as an independent phase change unit. The adsorptive capacity of SAT in EV is up to 600 wt%, while with 2 wt% borax added as a nucleating agent in the CPCM. The micro-structure of EV occupied by SAT was characterized by scanning electron microscopy (SEM) images. X-ray diffraction (XRD) tests indicated good chemical compatibility among the different components of the CPCM. The phase change temperature and heat storage density of the CPCM were 57.6 °C and 270.6 kJ/kg respectively, and the thermal conductivity was higher than that of each component in the compound. Thermogravimetry (TG-DSC) results showed that the insurmountable super-cooling and phase separation problems were almost overcome, and the thermal performance test system results showed that the CPCM was stable over 150 melting-solidification cycles. The performance improvement of SAT is attributed to the micro-scale phase change and heat transfer in EV cells.

1. Introduction Since 2019, the policy of “peak-valley electricity price” has been expanded it’s application field from industrial to civilian electricity consumption in China, which is aimed at solving the mismatch between power production and supply in a timely manner and encouraging electricity consumption at night when the price is lower (Alva et al., 2018). This policy encourages electrical energy to be converted into thermal energy during the night and stored in a thermal storage material, for subsequent release during the day when the electrical energy demand is higher (Anisur et al., 2013). Thus, the heating costs in winter would be largely reduced by using the “valley price” of electricity to store heat for daytime heating instead of the “peak price”, which has driven significant research interest into the technology and materials for thermal storage in recent years (Wang et al., 2019; Niu et al., 2019; Arcos-Vargas et al., 2018; Lv et al., 2017). Solar cells have been actively researched for improved efficiencies, such as the use of multiple junctions or back surface field designs (Chee and Hu, 2018). However, a significant part of the solar spectrum does not convert into electrical energy but may be wasted as heat, and increased junction temperatures ⁎

also decrease efficiencies. Therefore PCMs could convert solar energy to thermal energy, and so could be used for practical devices in passive cooling and as solar energy storage media. The desirable thermophysical properties of thermal storage materials typically include storage and release temperatures, high energy storage density and thermal conductivity, along with the chemical stability of non-toxicity, noncorrosivity, and non-flammability (Alva et al., 2017). Compared with the conventional thermal storage materials of sensible thermal storage, a promising alternative is the phase change thermal storage material (PCM), including sodium acetate trihydrate and sodium sulfate decahydrate, which release its large latent heat and keep approximately constant temperatures throughout the storaged energy releasing process (Farid et al., 2004). The most widely studied PCM are divided into organic and inorganic two categories (Cunha and Eames, 2016). Organic PCM such as paraffin and stearic acid (Khudhair and Farid, 2004) maintain their phase stable over the liquid-solid phase transformation process, cause less prone of super-cooling and phase separation. In addition, Kahwaji et al. (Kahwaji et al., 2018) showed that good thermal cycle performance of paraffin with different carbon atoms was maintained over

Corresponding author at: College of Mechanical & Electrical Engineering, Qingdao University, Qingdao 266071, China. E-mail address: [email protected] (C. Wang).

https://doi.org/10.1016/j.solener.2019.09.050 Received 30 June 2019; Received in revised form 1 September 2019; Accepted 12 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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Fashandi and Leung (2018) proposed to reduce the super-cooling of SAT using bio-derived chitin nanowhiskers (CNW) as nucleating agents. Fang et al. (2013) found that using CNW may be effective, the latent heats of the composite materials may be decreased through the use of too many auxiliary additives, such as CMC and sodium lauryl sulfate as surfactants and borazon and graphene as thermal conductive fillers. Analogously, Xiao et al. (2018) partially sacrificed the latent heat of SAT in order to prepare a shape-stabilized PCM for photothermal conversion, wherein the leakage of the material was prevented by adding 17.1 wt% expanded graphite during the cycles, resulting in an enthalpy decrease of only 5.9% after 150 cycles. The literature cited so far shows that the issues faced when using SAT-based materials, such as super-cooling and phase separation phenomena and low thermal conductivity have not been solved to date to fulfil its excellent potential towards CPCM applications. Against this backdrop therefore, the present study aims to improve the performance of SAT-based CPCM. Expanded vermiculite (EV) was used to load SAT stably. EV was chosen for its adsorption capacity towards sodium nitrate of as high as 734.6% (Li et al., 2016) as well as the ability to prevent the leakage of crystalliferous water via its hydrophilic properties (Xie et al., 2019) (though in which the latent heats decreased clearly and the phase change temperature varied great). The SAT/EV CPCM used in this study was prepared via the vacuum impregnation method. The mechanism behind the improvements in the super-cooling and phase separation of SAT were investigated in detail via micromorphology, chemical compatibility, and thermal performance analyses, and the experiments were performed to characterize the multicycle thermal reliability of the CPCM. Also, the CPCM is suitable for clean solar energy storage to absorb the excessive heat during peak sun hours and release the stored energy when heat needed. (Simon-Allue et al., 2019).

3,000 melt-solidification cycles, meaning that it can be used for solar thermal storage for a period of eight or more years. However, pure organic materials do suffer from the drawback of relatively poor thermal conductivity, which may be nearly overcome by the use of additives; for example, Chen et al. (Chen et al., 2019) found that the heat absorption rate of paraffin towards solar heat was increased by the addition of CuO nanopowder. Asmaa et al. (Asmaa et al., 2018) found that the heat conductivity property of paraffin was also improved by the addition of graphite or graphene, which caused the heating and cooling time to shorten by 54% owing to the high thermal conductivity of the carbon filler. However, the applications of organic PCM are mainly confined to small-scale thermal storage and still be focused on the heat conductivity improvement (Vasu et al., 2019). Recently, a shape-stable composite phase change material (CPCM) was synthesized by Li et al. (Li et al., 2019), wherein stearic acid was impregnated into the microstructure of expanded graphite, thereby improving its thermal conductivity by a factor of more than 8. But the CPCM had some drawbacks as well, as its latent heat per cubic meter was reduced by about 20% due to the low-density of the expanded graphite, even when the graphite content was only 6%. Wen et al. (Wen et al., 2018) used straw with a natural porous structure as a supporting material compounded with stearic acid, with the resultant CPCM showing excellent thermal stability and an increased thermal conductivity (by a factor of two) compared with pure stearic acid. Although the heat transfer performance of organic materials can be significantly improved through combination with high thermal conductivity materials, the overall latent heats of the composite materials are significantly reduced (Zhang et al., 2018), resulting in markedly lower heat storage densities compared to inorganic PCM. Inorganic PCM include a variety of substances such as hydrated salts, molten salts, and metals, which encompass a wide range of available melting temperatures (Mohamed et al., 2017). Of these, the hydrated salts are mostly restricted to heating temperatures below 100 °C, as shown in a recent study (Lin et al., 2018). While these materials do offer the advantages of good thermal conductivity, high heat storage density, and low price, super-cooling and phase separation problems are ubiquitous. Some commonly used hydrated salts are potassium alum dodecahydrate (KAl(SO4)2·12H2O), barium hydroxide octahydrate (Ba(OH)2·8H2O) and sodium acetate trihydrate (a.k.a SAT, CH3COONa·3H2O), chosen for their phase change temperatures within 50–80 °C which are appropriate for as heat source of heating applications in winter. Zhang et al. (2018) added expanded graphite to potassium alum dodecahydrate and found that the thermal conductivity was increased with no liquid flow, while the latent heat value was slightly lowered; However, potassium alum does suffer the drawback of being severely corrosive towards stainless steel 304L and aluminum, necessitating the use of brass which is more costly. Barium hydroxide octahydrate is an ideal medium of low temperature phase change with melting temperature of 78 °C, a high thermal conductivity and a heat storage density of 572 MJ/m3 (Sharma et al., 2009); nevertheless, as it is a strong base, it also causes corrosion on brass, carbon steel, and stainless steel 304L in its molten state (Wang et al., 2019). The third of the aforementioned hydrated salts, SAT, has the advantages of being non-toxic and non-corrosive and having a relatively high latent heat of up to 270.0 kJ/kg; however, SAT does suffer from serious super-cooling and phase separation issues (Wang et al., 2019). Shin et al. (2015) tried to modify SAT to improve its performance using sodium carboxymethylcellulose (CMC) as a suspending agent and expanded graphite as a nucleating agent, finding that the latent heat was reduced by 10 to 15%, but the thermal performance of the composite material might not be convincing only by five testing cycles. A more recent study by Mao et al. (2017) used disodium hydrogen phosphate dodecahydrate (DHPD) and CMC additives to weaken the super-cooling and suppress the phase separation of SAT via the disintegrated melt method; however, the addition of thickener increased the viscosity of the liquid and prevented the water molecules from escaping the hydrated salt.

2. Experimental 2.1. Materials and instruments of CPCM preparation The SAT (analytically pure) and EV (particle size of 14–18 mesh) used in the experiments were purchased from Shanghai Aibi Chemistry Preparation Co., Ltd.. The basic physical properties of the SAT include a melting temperature of about 58 °C, a latent heat of over 270.0 kJ/kg, and a density of about 1.5 g/cm3. And the nucleating agent borax (Na2B4O7·10H2O) and the flocculant CMC were also purchased in analytically pure from Tianjin Fengchuan Chemical Reagent Technology co., Ltd. for use as additives. As shown in Fig. 1, the PCM was melted using an HWS-24 electric thermostatic bath (Shanghai Yiheng Instrument Co., Ltd.). The composite material was mechanically mixed using a JQ-88 temperature controlled magnetic stirrer (Shanghai Junqi Instrument Equipment Co., Ltd.) to ensure a uniform composition throughout. A SHZ-D(III) circulating water vacuum pump (Shanghai Baoling Instrument Equipment Co., Ltd.) was used to promote the adsorption of SAT by EV. Temperature data were recorded using a 34972A data acquisition system (Agilent, USA) with an accuracy of 0.001 °C and a measurement rate of 1 data point per second. The thermocouples wires used to obtain the temperature readings were fixed at different positions in the sample beaker. 2.2. Nucleating agent addition In the second step in Fig. 1, the pure SAT was distributed evenly by weight around 10 g into six beakers and heated to a fully molten state in the 70 °C water bath for about 30 min. The beakers were sealed to prevent the loss of crystal water. Following this, different mass proportions of borax were dissolved in each beaker and the mixtures were stirred at 500 rpm for about 15 min until the temperature of the solution phase was uniform at 70 °C. The mixtures were then allowed to 414

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Fig. 1. A schematic of experimental procedure of CPCM study.

solidify and the temperature data during solidification were collected to find the optimum borax concentration of 2 wt% to achieve the supercooling of SAT minimized.

experiments using this ratio.

2.3. Preparation of CPCM

The pore volume and surface area of EV were measured by BET method (America Quantachrome Autosorb, iQ3). The micro-porous structure of EV was investigated using a JSM-6390LV (Japan Electronics Co., Ltd) scanning electron microscope (SEM) to evaluate the ability of EV to adsorb the SAT. The prepared CPCM was also studied with SEM to estimate the amount of SAT in the micro-pores of EV. The CPCM and its components (EV, SAT, borax) were characterized using a D8 ADVANCE (Bruker, Germany) X-ray diffractometer (XRD), and the chemical compositions were tested before and after compositing. Differential scanning calorimetry (DSC) and thermogravimetric (TG) analyses of the CPCM were conducted using an HCT-4 integrated thermal analyzer (Beijing Hengjiu Scientific Instrument Factory). The specific heat capacities of the SAT and CPCM were compared to ascertain whether addition of EV increased the heat storage density.

2.4. Characterization of EV, SAT and CPCM

In the third step in Fig. 1,the SAT mixed with the optimum ratio of borax was poured into a flask, and mechanically mixed with different proportions of EV at 500 rpm for about 30 min to ensure a uniform concentration. The system was then evacuated using a vacuum pump in preparation for the impregnation process. In order to remove the absorbed air in the micro-pores and to ensure the micro-pores of EV were filled with SAT without any crystal water evaporation, the vacuum pressure was calculated based on the relationship between the saturated water pressure and temperature, as shown in Eq. (1). (1)

ln(P ) = 9.3876 − 3826.36/(T − 45.47)

where T is in the range 290–500 K and P is in MPa. For the evaporation pressure of saturated water corresponding to a temperature of 70 °C, 0.032 MPa, the degree of vacuum was controlled in the range of 0.05–0.06 MPa during the preparation of the CPCM. The process of vacuum impregnation was carried out for about 5 h, until the SAT had been completely filled into the EV. Different CPCMs were prepared, with mass ratios of EV to SAT of 1:4, 1:5, 1:6 and 1:7, the appearances were shown in Fig. 2. When the mass ratios were 1:4 and 1:5, the crystal water did not ooze out from the EV particles. When the mass ratio was 1:6, the CPCM was tightly polymerized with almost no gaps present, allowing it to maintain the gel state. However, when the mass ratio was 1:7, the material form of the CPCM was fluid and unstable, with obvious crystal water leakage. Given these results, the EV to SAT mass ratio of 1:6 was selected as the optimum value for ensuring the stable morphology of the prepared CPCM and ensuring a high heat storage density, with subsequent

a. 1:4

2.5. Thermal conductivity analysis of SAT and CPCM The thermal conductivity of materials is mainly affected by their temperature, particle size, porosity, chemical composition, and heat flow direction (Palacios et al., 2019). However, variations in the thermal conductivity cannot be measured during phase change using transient methods (Hoque et al., 2018). As the main components of EV include SiO2, Al2O3, CaO, MgO, and Fe2O3, they might be beneficial for increasing the thermal conductivity of composites. Hence, the thermal conductivities of the SAT and CPCM were qualitatively analyzed to determine whether the addition of EV was favorable for heat conduction. A schematic of the experimental apparatus designed for this purpose is shown in Fig. 3. Some simplifying assumptions were used in the analysis process, as follows:

b. 1:5

c. 1:6

Fig. 2. Photographs of SAT/EV CPCM with different mass ratios. 415

d. 1:7

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3.1.2. Micro-morphology of EV The layered micro-structure of EV was characterized using SEM, as shown in Fig. 6. The pore lengths were 20–200 μm in the visible range, meaning that the micro-space was suitable for the filling of hydrate salt. In addition, the close connection of the layers ensures the mechanical strength of EV, which is also favorable for the adsorption of SAT. The micro-porous structure of EV can be very clearly observed in the images with 500x and 1000x magnification.

Thermocouples

Water

a bc PCM

3.1.3. Micro-morphology of CPCM The micro-morphology of the incision section of the CPCM particles was characterized using SEM, as shown in Fig. 7. The layered structure of EV and the crystals of the SAT can be observed in the image with 200x magnification (Fig. 7a). When the image is further enlarged to 500x (Fig. 7b), it can be clearly seen that the SAT was filled in the layered micro-pores of EV and the filling rate was over 95%, meaning that the adsorption of SAT on EV was good. Increasing the magnification further, to 1000x (Fig. 7c), allows the crystal forms in the individual micro-pores to be observed. The SEM images in Fig. 7 showed that the crystals of the SAT were split into multiple independent micro-cells. In general, the morphology of the prepared CPCM possessed the desired properties when the mass ratio of EV to SAT was 1:6. And the micro-scale phase change cells of SAT has been formed by the numerous micro-porous structures of EV.

d

Fig. 3. A schematic of analyzing thermal conductivity of PCM.

a. A one-dimensional steady state was reached, with no internal heat source present in the thermocouple wires; b. The heat transfer along the horizontal plane does not involve gravity or convection; c. The heat dissipation in the constant temperature water bath may be neglected. In the cylindrical coordinate system, when heat transfer is at a steady state, the heat flux passing through the unit axial length, ϕl, is equal at any radius (r), and may be calculated as follows:

φl =

2πλPCM (Ta − Tb) 2πλ water (Tc − Td ) = ln(rb/ ra) ln(rd/ rc )

3.1.4. Chemical compatibility among the components of CPCM Detailed XRD analysis was performed on the SAT, EV, borax, and CPCM, respectively, to determine the component stability and chemical compatibility after compounding. The characteristic diffraction angles of SAT were 17.13°, 19.11°, 22.84°, 32.72°, and 47.08°, whereas those of borax were 12.75°, 30.62°, 40.08°, and 47.96°. The corresponding components are shown in Fig. 8. The diffraction peaks of the different components did not change in the CPCM, indicating that no chemical reactions occurred among the components, although the intensities of some peaks did vary with the concentration varying in the compounds. These results indicate that the different components had good chemical compatibility.

(2)

where λPCM and λwater are the thermal conductivities of the PCM and water, respectively, T is the temperature, and the subscripts a–d refer to the corresponding positions in Fig. 1. The values of ra, rb, rc, and rd correspond to 4 mm, 8.65 mm, 9.85 mm, and 31 mm, respectively. The value of λwater is known, as are the temperatures at each measured point. Hence, rearranging Eq. (2) allows λPCM to be calculated, as follows:

3.1. Characterization analysis of materials

3.1.5. Thermal performance of SAT and CPCM The key factors in evaluating the heat storage performance of CPCM are the melting/solidification temperature and latent heat. The results for the thermal performance analysis of SAT and CPCM using TG-DSC are shown in Fig. 9. From Fig. 9a, it can be seen that the melting temperature and latent heat of pure SAT were 58.1 °C and 277.7 kJ/kg, respectively, while those of CPCM were 57.6 °C and 238.8 kJ/kg, respectively. Both the latent heat as well as the melting temperature of SAT were reduced by the addition of EV. In addition, the process of SAT melting is accompanied by the crystallization water becoming free, and there may be evaporation of low-temperature water causing the variation of the heat storage capacity and melting temperature. The results of the TG analyses for pure SAT and the CPCM are shown in Fig. 9b, from which it can be seen that the weight losses of the two materials did not exceed 0.2% during the phase change. Therefore, the addition of EV almost completely eliminates crystal water loss in the CPCM, thus ensuring high heat storage capacity and performance stability.

3.1.1. Pore volume and surface area of EV Pore volume and surface area are important indicators for porous materials which are used as supporting materials for PCM. This can be characterized by the N2 adsorption quantity of samples. Fig. 5 showed the nitrogen adsorption quantity of EV. It is distinct that there was a drastic improvement on the N2 adsorption quantity of EV under higher relative pressure. The BET surface area of EV was 17.092 m2/g and the pore volume was 0.073 cm3/g, indicating that the pore micro-structure of EV is beneficial for adsorbing PCM.

3.1.6. Specific heat capacity of SAT and CPCM The specific heat capacities of the EV, pure SAT, and CPCM were analyzed, as shown as Fig. 10. The specific heat capacity of the CPCM was significantly increased compared to that of pure SAT. However, the theoretical specific heat capacity value calculated according to the mass ratio weighting of EV to SAT in CPCM was only slightly higher than that of the SAT and lower than that of CPCM in test. So the heat storage capacity of CPCM was increased after the composite of EV, even close to the latent heat of 277.7 kJ/kg of pure SAT in test.

λPCM =

φl ln(rb/ ra) 2(Ta − Tb)

(3)

2.6. Thermal performance cycle testing of CPCM The experimental apparatus used for testing the thermal cycle performance of the CPCM was constructed as shown in Fig. 4. Multiple melting/solidification cycles were used to evaluate the stability of the temperature and super-cooling degree of CPCM. The red arrows in the figure indicate the fluid circulation directions. A water reservoir was built in the system in order to ensure that the temperature of the water flowing around the CPCM pipe in the heat storage device was constant during heat storage and release. 3. Results and discussion

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Fig. 4. A diagram of the thermal performance evaluation system.

Quantity adsorbed / (cm3/g STP)

50

experimental apparatus shown in Fig. 3. As shown in Fig. 11a, the thermal conductivities of the two materials decreased with decreasing temperature prior to solidification, and the thermal conductivity was sharp dropping during the solidification phase change, whereas the thermal conductivity of both materials was almost the same in the solidification phase. The thermal conductivity of SAT varied from 0.10 to 0.60 W/m·K, which is consistent with the literature (Dannemand et al., 2016); however, the thermal conductivity of the CPCM, about 0.12–0.80 W/m·K, was improved by the addition of EV. Similarly, the EV also enhanced the heat transfer of the CPCM over the melting process, as shown in Fig. 11b. Before and after melting, however, both curves basically overlap. Overall, during the two phase transformation processes (i.e. melting and solidification), the thermal conductivity of the melting was clearly greater; that is, the heat absorption of the CPCM was faster than the heat release over solidification, which satisfies the requirement for winter heating applications of quick heat storage at night and slow heat release during the day.

Adsorption Desorption

40 30 20 10 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure / (P/Po) Fig. 5. N2 adsorption and desorption isotherms of EV.

3.2. Thermal reliability of phase change materials

3.1.7. Thermal conductivity of SAT and CPCM Generally, the thermal conductivity of SAT ranges between 0.17 and 0.70 W/m·K (Dannemand et al., 2016). The temperature evolutions of the thermal conductivities of SAT and CPCM were qualitatively analyzed during the processes of solidification and melting using the

a. 200 times

3.2.1. Thermal stability of SAT and CPCM The thermal stability performance of CPCM is always characterized by the multiple cycles. The cyclic thermal performances of SAT and CPCM were evaluated using the experimental system shown in Fig. 4,

b. 500 times Fig. 6. SEM images of EV. 417

c. 1000 times

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a. 200 times

b. 500 times

c. 1000 times

Fig. 7. SEM images of CPCM.

3.5

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

CPCM

Intensity / (a.u.)

BORAX

Cp / (J/g·K)

3.0

—Na2B4O7·10H20

EV pure SAT CPCM (1/7)EV+(6/7)SAT

2.5 2.0 1.5

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1.0

—CH3COONa·10H20

40

45

50

55

60

65

70

Temperature / (°C) Fig. 10. Heat capacity of EV / pure SAT / CPCM.

EV

— SiO2

— Al2O3

— MgO

—CaO

with the results shown in Fig. 10a and b, respectively. As Fig. 12a shows, there was no super-cooling of SAT during the first two cycles and the melting temperature was about 55.5 °C for the suspension agent inhibited the phase separation as well as prevented the crystals from undergoing phase change. As the number of cycles increased, the constant temperature cooling section became shorter, causing the latent heat release from the SAT to gradually decrease. In addition, the supercooling phenomenon becomes more and more evident, with the maximum degree of super-cooling being close to 8 °C in cycle 7, and the melting temperature increased a little with a concurrent reduction in the stored heat. In comparison, the thermal cycle stability of CPCM (as shown in Fig. 12b) was improved overall compared to the pure SAT. Initially, the

— Fe2O3

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2 / (°) Fig. 8. XRD pattern of EV, SAT, Borax and CPCM.

10

0.2

0

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Heat flow /

mW/mg

TG

DSC

-10 -20 pure SAT CPCM

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a. DSC curves of pure SAT and CPCM

b. TG curves of pure SAT and CPCM

Fig. 9. Thermal properties of pure SAT and CPCM. 418

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CPCM pure SAT

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°C

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W/ m·K

°C

CPCM pure SAT

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/

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melting

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Temperature / (°C)

Temperature / (°C)

a. Thermal Conductivity of solidification process

b. Thermal Conductivity of melting process

Fig. 11. Thermal Conductivity comparison of SAT and CPCM.

80

80

CPCM

70

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475s

155s

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melting

solidification

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cycle 1 cycle 2 cycle 3 cycle 4 cycle 5 cycle 6 cycle 7

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melting

cycle 1 cycle 2 cycle 3 cycle 4 cycle 5 cycle 6 cycle 7

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800 1000 1200 1400 1600

145s

0

200

400

600

800

1000

1200

Time / s

Time / s

b. Thermal cycles of CPCM

a. Thermal cycles of SAT

Fig. 12. Thermal stability of SAT and CPCM.

from the theoretical value, calculated based on mass fraction, by −5.6 kJ/kg to 9.5 kJ/kg, with an error range of −2.4% to 4.0%. The thermal analysis results for the prepared CPCM over 150 melting and solidification cycles showed that it is thermally stable and maintains its thermal properties of phase change temperature and latent heat.

results for the two materials were similar, as there was no super-cooling during the first two cycles of CPCM and the melting temperature was about 57.5 °C, which is in accordance with the DSC results in Section 3.1.4. Although super-cooling did begin to occur in the third cycle, its degree was less than 2 °C, eventually stabilizing at 3 °C as the number of cycles increased. It is worth noting that in the solidification sections, the constant temperatures were very similar between the pure SAT and the CPCM, but the phase change period is clearly shorter for the higher thermal conductivity discussed above. In addition, the cyclic performance was very stable, as evidenced by the fact that the melting temperature was almost invariant and the lower super-cooling temperatures. The improvement is attributed with the micro-scale phase change.

3.2.3. Chemical compatibility after 150 thermal cycles Fig. 14 showed XRD pattern of CPCM after 150 melting and solidification cycles. The composites with the EV to SAT mass ratio of 1:6 were characterized by XRD pattern to investigate the chemical compatibility. The XRD pattern of the diffraction peaks after 150 cycles were consistent with that of first cycle. The results indicated that CPCM has good thermal compatibility and chemical stability. 4. Conclusions

3.2.2. Long-term thermal performance of CPCM In order to observe the long-term thermal performance of CPCM, the prepared CPCM was tested in the thermal cycling system, and a small amount of sample after every 20 melting-solidification cycles was tested DSC to evaluate the evolution of the melting temperature and latent heat of CPCM as a function of the number of cycles (Fig. 12). As shown in Fig. 13a, there are a little variation between curves of different samples. Hence, the heat absorption performance of CPCM was stable over 150 thermal cycles. There was no correlation between the variations of melting temperature and latent heat of CPCM, as Fig. 13b shows. The melting point of CPCM was lower than that of the pure SAT by −3.6 °C to −0.8 °C. Furthermore, the CPCM latent heat differed

The experimental work conducted in this study aimed to improve the super-cooling and phase separation performance of SAT while maintaining its high energy storage density. To achieve this, a microscale phase change material of SAT/EV CPCM was prepared via the vacuum impregnation method, with 2 wt% borax added as a nucleating agent. The main conclusions drawn from the results presented herein are as follows: (1) The micro-porous of EV can provide micro-spaces for SAT phase change, as shown by SEM images. XRD results showed that there 419

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a. DSC curves of CPCM

b. Melting temperature and latent heat of CPCM Fig. 13. 150 thermal cycles of CPCM.

References

first cycle after 150 cycles

Intensity / (a.u.)

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5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ / (°) Fig. 14. XRD pattern of CPCM after first cycle and 150 cycles.

was good chemical compatibility between the different components of the CPCM and it was also found that the best absorptive mass ratio of EV to SAT was 1:6. (2) The melting temperature of the CPCM was 57.6 °C and its latent heat was 238.8 kJ/kg. The CPCM had good thermal stability and no crystal water was lost before or after the phase change. (3) The thermal conductivities of pure SAT and CPCM were qualitatively analyzed, and it was found that that the thermal conductivity of the CPCM was higher than that of pure SAT during phase change process, meaning that the heat transfer performance was enhanced. (4) The super-cooling and phase separation problems were effectively improved during the phase transition of CPCM, with good performance maintained even after 150 cycles of melting and solidification. The mechanism of the thermal performance improvement is thought to be due to micro-scale phase change occurring in the prepared CPCM whereby each independent SAT cell undergoes a stable phase change without crystal water loss and phase separation.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51676102). The authors also greatly acknowledge the support from the Foundation of State Key Laboratory of Coal Clean Utilization and Ecological Chemical Engineering (Grant No. 2016-07) and the Taishan Scholar Program of Shandong Province (201511029). 420

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