Synthesis of microencapsulated stearic acid with amorphous TiO2 as shape-stabilized PCMs for thermal energy storage

Synthesis of microencapsulated stearic acid with amorphous TiO2 as shape-stabilized PCMs for thermal energy storage

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Applied Energy Symposium andSymposium Forum 2018: Low carbon cities andcarbon urbancities energy systems, CUE2018-Applied Energy and Forum 2018: Low and Applied Energy Symposium and Forum 2018: LowShanghai, carbon cities and urban energy systems, CUE2018, 5–7 June 2018, China urban CUE2018, energy systems, 5–7 2018, June 2018, Shanghai, 5–7 June Shanghai, ChinaChina

Synthesis ofThe microencapsulated stearic acid with amorphous 15th International Symposium on District Heating and Cooling TiO2 as Synthesis of microencapsulated stearic acid with amorphous TiO2 as shape-stabilized PCMs for thermal energy storage shape-stabilized PCMs thermal energy storage Assessing the feasibility offor using the heat demand-outdoor a b a Chaoen Li , Guixiong Heabb, long-term Huaguang Yan , Hang Yu *, Yuan Songaaforecast temperature function for district heat demand a b a Chaoen Li , Guixiong He , Huaguang Yan , Hang Yu *, Yuan Song a*Tongji University, 1239 Siping Road, Shanghai, P.R. China

a University, a Siping b 1239 Road, Shanghai, P.R. China China c b*Statea*Tongji Grid China Power Research Institute,100192,Beijing, I. Andrića,b,c*,b*State A. Pina , P.Electric Ferrão , J. Fournier ., B. Lacarrière , O. Le Correc Grid China Electric Power Research Institute,100192,Beijing, China a

IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal

b Abstract Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Abstract Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Stearic acid/amorphous TiO2 composites were synthesized by a Sol-gel way with different pH values. The as-prepared PCMs were Stearic acid/amorphous were synthesized by a Sol-gel way with The PCMs were 2 composites characterized by XRD, TiO SEM, FTIR to determine the chemical compositions anddifferent structurepHtovalues. find out theas-prepared influence of synthesis characterized XRD,The SEM, FTIRperformance to determineand thethermal chemical compositions and structure find out the influence of synthesis condition (pH by valves). thermal stability of the composite phasetochange material were determined by Abstract condition (pH valves). thermal performance thermal stability of acidic the composite phase change material determined by DSC and TGA. It was The found that the composite and PCMs prepared in the condition exhibited better thanwere that under alkaline DSC and TGA. It was found that the composite PCMs prepared in theofacidic condition exhibited better than that under alkaline condition. The addition of CNTs enhanced the thermal conductivity the as-prepared PCMs. The purpose of our study is to Districta sample heating networks are commonly addressed in conductivity the literature oneas-prepared of the most effective decreasing the condition. The addition CNTs enhanced the PCM thermal ofasstorage. the PCMs. The solutions purpose offorour study is to provide way to of synthesis of composite for thermal energy greenhouse gas way emissions from the building sector. systems require high investments which are returned through the heat provide a sample to synthesis of composite PCM These for thermal energy storage. sales. Due to theElsevier changed conditions Copyright © 2018 Ltd.climate All rights reserved.and building renovation policies, heat demand in the future could decrease, Copyright ©the 2018 Elsevier Ltd. Ltd. Allperiod. rights reserved. reserved. prolonging investment return Copyright © 2018 Elsevier All rights Selection and peer-review under responsibility of the scientific committee of Applied Energy Symposium and Forum 2018: Low Selection and peer-review under responsibility of the scientific committee of the– outdoor CUE2018-Applied Energy Symposium and The main scope of thisenergy paper is responsibility to assess the feasibility of using the heat demand for heat Selection and peer-review under of the scientific committee of Applied Energytemperature Symposiumfunction and Forum 2018:demand Low carbon cities and urban systems, CUE2018. Forum 2018: Low carbon cities and urban energy systems. forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 carbon cities and urban energy systems, CUE2018. buildingsPCM; that Amorphous vary in both period and typology. Three weather scenarios (low, medium, high) and three district Keywords: TiOconstruction ; Sol-gel synthesis 2 Keywords: PCM; Amorphous TiO renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were 2; Sol-gel synthesis compared with results from a dynamic heat demand model, previously developed and validated by the authors. results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1.The Introduction error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1.(the Introduction scenarios, error the valueworldwide increased up to 59.5% (depending on the weather scenarios considered). In recenttheyears, energy demand was mainly satisfiedand byrenovation fossil fuels, whichcombination made the fossil fuel The value ofyears, slope the coefficient increased on demand average within the range of 3.8% upfossil to 8% per decade, that corresponds to the [1] In recent worldwide energy was mainly satisfied by fuels, which made the fossil has continuously exhaust, and the environmental problem has become urgently . Therefore, more and more attentionfuel decrease in the numberand of heating hours of 22-139h during the heating season (depending on the combination of weatherhas and [1] continuously exhaust, the environmental problem has become urgently . Therefore, more and are more attention taken to use the renewable-energy resources. However, some renewable energies like solar energy intermittent by renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the taken toand userequire the renewable-energy resources. someheat renewable energies like solarmore energy intermittent by nature storage systems. PCMscould as aHowever, kind of latent storage system attracted andare more attention by coupled scenarios). The values suggested be used to modify the function parameters for the scenarios considered, and [2] nature and require storage systems. PCMs as a kind of latent heat storage system attracted more and more attention by . of heat demand estimations. many researchers improve the accuracy [2]

many researchers Commonly, based. on chemical composition, there are two principal categories of PCMs: inorganic and organic [3]. Commonly, based on chemical composition, twohas principal of PCMs: and because organic [3] Among such paraffin fatty are acids, drawn categories much attention in theinorganic last decades of. © 2017the Theorganic Authors.PCMs, Published by as Elsevier Ltd.andthere Among the organic PCMs, such as paraffin and fatty acids, has drawn much attention in the last decades because of Peer-review under storage responsibility of thenarrow Scientific Committee of change The 15thtemperature, International Symposium on Districtstability, Heating and their high-energy capacity, range of phase excellent chemical and low their high-energy capacity,isnarrow range ofwill phase change temperature, excellent stability, and low Cooling. cost. However, thestorage main drawback liquid leakage happen when the solid-liquid PCMchemical undergoes phase transition cost. However, the main drawback is liquid leakage will happen when the solid-liquid PCM undergoes phase transition Keywords: Heat demand; Forecast; Climate change 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the Applied Energy Symposium and Forum 2018: Low carbon cities Selection and peer-review under responsibility the scientific Selection peer-review responsibility of the scientific committee of the Applied Energy Symposium and Forum 2018: Low carbon cities and urbanand energy systems, under CUE2018. and urban energy systems, CUE2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the CUE2018-Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems. 10.1016/j.egypro.2018.09.162

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process, which restricts its applications. In order to solve this problem, shape-stabilized composites PCM, which composed of a working substance and supporting materials has been developed [4]. Another potential way is encapsulation of PCMs into polymeric or inorganic materials [5]. Titanium dioxide (TiO2) is environmental friendliness, low-cost, fire resistant and high thermal conductivity, which make it be a potential shell material for micro-encapsulation. The different preparation process may cause differences within the phase structure of TiO2 [6], including crystalline phase TiO2 and amorphous phase TiO2. In this study, we synthesized amorphous phase TiO2 to encapsulate stearic acid (SA) by Sol-gel method. At the same time, we studied the influence of the pH values during the synthesis process. We also adopted carbon nanotubes (CNTs) into the composite PCMs to enhance the thermal conductivity. The synthesis process in the present study is simple, highefficiency, environmentally friendly. The aim of this study is to develop a simple way to synthesize of composite PCMs with amorphous phase TiO2 and CNTs for energy storage. 2. Experimental methods 2.1. Materials Stearic acid(SA) (Reagent grade), Anhydrous ethanol, Sodium dodecyl sulfate (SDS), tetrabutyl titanate (TBT), hydrochloric acid and ammonia were obtained from Sinopharm Chemical Reagent Company. Carbon nanotubes (CNTs) were purchased from Jiangsu Hengqiu technology companies. 2.2. Preparation of SA/TiO2 composites Firstly, stearic acid (9g), SDS (0.3g) and deionized water (50ml) were mixed together in a three-necked flask to form the O/W emulsion using a magnetic stirrer at 800 rpm for 1hour and keep the solution at 75℃. Then the pH of the mixture was adjusted to 2.5 (sample MPCM-1) or 11 (sample MPCM-2) by adding little hydrochloric acid and ammonia respectively. In another breaker, the precursor solution was prepared by a mixture of TBT (9g) and anhydrous ethanol (50ml) to form the precursor solution. Then the precursor was added into above O/W emulsion drop by drop. Keep stirring the solution at 800 rpm and the temperature was controlled at 75℃ for 1 hour. In one prescription, we adopted 4% CNTs into the solution to enhance the conductivity of MPCMs-1. Finally, the mixture was cooled to room temperature, the obtained MPCMs were washed by distilled water three times, then put the samples into the constant temperature oven dried at 60℃ for 24h. 2.3. Characterization of materials The morphology of microcapsule PCMs were determined by a scanning electron microscope (SEM, Nova NanoSEM 450). Infrared spectra of the PCMs were obtained by FT-IR (Jasco 430 model). Powder XRD patterns were taken on a BRUKER D8 ADVANCE Diffractometer. The thermal stability was determined by a thermal gravimetric (TGA) analyzer at a heating rate of 10℃/min and temperature of 50-600℃. Thermal properties were measured by a DSC instrument (Q100 model) and carried out at 10℃/min constant heating and cooling rate in purified nitrogen atmosphere. 3. Results and discussion 3.1. Morphology of the MPCMs The appearance, morphology, and microstructure of the samples were shown in Fig. 1. From the Fig. 1(b and c), the sample prepared in alkali conditions (a) was rougher than that prepared under acidic condition (c). It is obvious that under acidic condition, easier to form microcapsules. With the CNTs adopted, there are a lot of carbon nanotubes attached to the surface of microcapsules uniformly. Carbon nanotubes (CNTs) has high thermal conductivity (20006000 W/m·K) which will enhance the composites thermal conductivity [7].

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Fig. 1 (a) Digital picture of samples, (b, c, d) SEM images of MPCM-1, MPCM-2, MPCM-1-CNTs

3.2. FT-IR analysis of the MPCMs The FTIR spectrums of SA, CNTs, TiO2, MPCM-1, MPCM-1+CNTs were shown in Fig. 2(a). The bonds at 2916.5 cm-1 and 2850.6 cm-1 represent the symmetrical stretching vibration peaks of -CH2 of SA. The peak appearing in 1705.0 cm-1 stands for the stretching vibration of the carboxy group. The stretching vibration of Ti-O was found in both two microcapsule samples and pure TiO2 shells at 630 and 480 cm-1. Above discuss confirmed that TiO2 shell encapsulation of stearic acid successful. Comparing with the spectrums of MPCM-1 and MPCM-1+CNTs, there was no difference, which means there was chemical reaction happen with the CNTs adopted.

Fig. 2(a) FT-IR spectra of SA,MPCM-1,MPCM-1+CNTs,CNTs,TiO2, (b)XRD spectra of SA,MPCM-1,MPCM-2,MPCM1+CNTs,CNTs, PDF of TiO2

3.3. XRD analysis of the MPCMs The XRD patterns of the stearic acid, TiO2, MPCMs, and CNTs were shown in Fig. 2(b). The characteristic peaks at 21.6° and 23.8° from SA patterns represent the crystal structure of SA. There was no obviously TiO2 characteristic

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peak found on MPCMs, which means the TiO 2 that form by hydrolysis and condensation was non-crystalline and amorphous. The spectrums of MPCM-1 and MPCM-2 consist of SA typical peaks and TiO2 characteristic peaks. It is means that the during the synthesis process the form of TiO 2 shells didn’t make any effect on the crystal structure of the SA. Comparing with the patterns of MPCM-1 and MPCM-1+CNTs, there was no characteristic CNTs peak found in the MPCM-1+CNTs, which means the adoption of CNTs didn’t influence the crystal form of the PCMs. 3.4. DSC analysis of the MPCMs The DSC curves of SA and MPCM-1, MPCM-2, MPCM-1+CNTs were shown in Fig .3 and the characteristic temperatures were summarized in Table 1. As shown in Fig. 3 (a), pH valves extremely influence the thermal performance of the encapsulated PCMs. The thermal performance of the as-prepared MPCMs under acidic conditions was better than which prepared under alkaline conditions. The latent heats of MPCM-2 was much larger than that of MPCM-1. At the same time, amorphous TiO2 greatly influence the performance of SA which different with other’s research. Higher content of the SA in the microcapsules results in higher enthalpy values. The encapsulation ratio of the SA is calculated by the following equation: = 

H MPCM 100% H PCM

(0) Where η presents the encapsulation ratio of the SA, ΔHMPCM, and ΔHPCM stands for the melting enthalpy values of the MPCMs and SA, respectively. The calculated results were listed in table 1. Acidic conditions will lead to a higher encapsulation ratio, which means synthesis under acidic conditions benefits for preparing the SA/TiO2. With the TiO2 adopted, making the super-cooling degree of PCMs larger compared with that of SA. This results also diffident with other’s research. The acidic condition is benefits to decrease the super-cooling degree of PCMs in this study. We test the DSC curves for three times to study the thermal history influence on the thermal performance of PCM, the results shown in Fig .3 (b and c). To the pure SA and MPCM-1+CNTs, during the melting process, the DSC curves of the second cycle and third cycle almost keep consistent which much diffident with the first cycle. During the solidification, the curves of three cycles almost keep consistent. Above discuss means that in order to get current data about PCMs thermal performance by DSC, we should test the sample for at least three times to decrease the influence of thermal history.

Fig. 3. SA and MPCMs: (a) DSC curves of pure SA and MPCMs (b) DSC curves of the SA; (c) DSC curves of the MPCM-1+CNTs; (d) TGA curves of SA and the MPCMs

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Table 1 DSC results of SA and composite PCMs Melting process

Solidification process

SA

68.80

70.8

235.0

67.7

66.82

237.9

Encapsulation ratio of SA (%) (%) 100

MPCM-1

68.70

70.17

81.67

55.58

53.10

63.97

34.75

MPCM-2

65.87

68.61

131.5

61.66

61.78

124.4

55.74

MPCM-1+CNTs

68.44

71.05

105.1

56.68

53.65

83.82

44.7

Samples

Tm (℃)

Tpeak-m (℃)

ΔHm (J/g)

Ts (℃)

Tpeak-s(℃)

ΔHs (J/g)

3.5. Thermal stability of the composites PCMs For the application of composites PCMs, the thermal stability index is necessary. As shown in Fig. 3(d), pure SA and as-prepared samples exhibited exceptional thermal stability at temperatures below 200 ℃ which much higher than its phase transition temperature. The sharp weight loss at the temperature of about 250 ℃ is due to the decomposition of the PEG chains. The encapsulation ratio of MPCM-2 is better than that of MPCM-1. However, the thermal stability of MPCM-2 is lower than that of MPCM-1. 4. Conclusions Stearic acid/ amorphous TiO2 micro-PCMs as form-stable PCMs were prepared by Sol-gel method. The influence of pH valves on the synthesis process was discussed. We found that the thermal performance of as-prepared composites under acidic conditions is better than that of in alkaline conditions. However, the thermal stability of the as-prepared samples under acidic conditions is lower than that of under alkalic conditions. Though two kind MPCMs can keep thermal stability under their working temperature, we recommend choosing acidic conditions as a successful synthesis condition. Acknowledgments The study has been supported by the China National Key R&D Program "Solutions to heating and cooling of building in the Yangtze river region" (Grant No.2016YFC0700305-02). State Grid Science and Technology Program “Optimization models for community intelligent heating and cooling networks” (YDB17201700095) References [1] Zeng C, Liu S, Shukla A. Adaptability research on phase change materials based technologies in China. Renewable & Sustainable Energy Reviews. 2017; 73:145-58. [2] Ma Y, Chu X, Tang G, Yao Y. Synthesis and thermal properties of acrylate-based polymer shell microcapsules with binary core as phase change materials. Materials Letters. 2013; 91:133-5. [3] Barreneche C, Solé A, Miró L, Martorell I, Fernández AI, Cabeza LF. Study on differential scanning calorimetry analysis with two operation modes and organic and inorganic phase change material (PCM). Thermochimica Acta. 2013; 553:23-6. [4] Deng Y, Li J, Nian H, Li Y, Yin X. Design and preparation of shape-stabilized composite phase change material with high thermal reliability via encapsulating polyethylene glycol into flower-like TiO2 nanostructure for thermal energy storage. Applied Thermal Engineering. 2017; 114:328-36. [5] Ma Y, Chu X, Tang G, Yao Y. Adjusting phase change temperature of microcapsules by regulating their core compositions. Materials Letters. 2012; 82:39–41. [6] Yu J, Yu H, Cheng B, Trapalis C. Effects of calcination temperature on the microstructures and photocatalytic activity of titanate nanotubes. Journal of Molecular Catalysis A Chemical. 2006; 249:135-42. [7] Karaipekli A, Biçer A, Sarı A, Tyagi VV. Thermal characteristics of expanded perlite/paraffin composite phase change material with enhanced thermal conductivity using carbon nanotubes. Energy Conversion & Management. 2017; 134:373-81.