phase change materials for latent heat thermal energy storage

Stearic acid/polymethylmethacrylate composite as form-stable phase change materials for latent heat thermal energy storageYi Wang a ,b ,c ,*,Tian Dong Xia a ,b ,Hui Xia Feng c ,Han Zhang caState Key Laboratory of Gansu Advanced Non-ferrous Metal Materials,Lanzhou University of Technology,Lanzhou 730050,China bKey Laboratory of Non-ferrous Metal Alloys,Ministry of Education,Lanzhou University of Technology,Lanzhou 730050,China cCollege of Petrochemical Technology,Lanzhou University of Technology,Lanzhou 730050,Chinaa r t i c l e i n f oArticle history:Received 15September 2010Accepted 20December 2010Available online 8January 2011Keywords:Form-stable composite PCM LHTESEncapsulationThermal properties Thermal reliability PMMAa b s t r a c tThe aim of this research is to prepare of a novel form-stable composite phase change material (PCM)for the latent heat thermal energy storage (LHTES)in buildings,passive solar space heating or functional fluid by entrapping of SA into PMMA cell through ultraviolet curing dispersion polymerization.The composite PCM was characterized using scanning electron microscope (SEM)and Fourier transformation infrared (FT-IR)analysis technique.The results show that the form-stable microencapsulated PCM with core/shell structure was formed and the maximum encapsulated proportion of SA in the composite was 51.8wt.%without melted PCM seepage from the composite.In the shape stabilized microencapsulated PCM,the polymer acts as supporting material to form the microcapsule cell preventing the leakage of PCM from the composite and the SA acts as a PCM encapsulated in the cell of PMMA resin.The oxygen atom of carbonyl group of skeleton is interacted with the hydrogen atom of hydroxyl group of SA.Thermal properties,thermal reliability and heat storage/release performance of the composite PCM were determined by differential scanning calorimetry (DSC),FT-IR and thermal cycling test analysis.The melting and freezing temperatures and the latent heats of the composite PCM were measured as 60.4 C,50.6 C and 92.1J/g,95.9J/g,respectively.The results of DSC,FT-IR and thermal cycling test are all show that the thermal reliability of the composite PCM has an imperceptible change.This conclusion indicates that the composite has a good thermal and chemical stability.Ó2010Elsevier Ltd.All rights reserved.1.IntroductionThe direct solar radiation is considered to be one of the most prospective sources of energy to avert the growing concern about greenhouse gas emissions and the climb in fuel prices.However,the large-scale utilization of this form of energy is possible only if the effective technology for its storage can be developed with acceptable capital and running costs.The storage of energy in suitable forms,which can conventionally be converted into the required form,is a present day challenge to the technologists.The scientists all over the world are in search of new technologies to develop solar energy storage devices,which are as important as developing new sources of energy.Energy storage not only reduces the mismatch between supply and demand,decreases the excess energy wasted,but also improves the performance and reliabilityof energy systems and plays an important role in conserving the energy.Among all ways of energy storage,latent heat thermal energy storage (LHTES)using phase change material (PCM)is particularly attractive technique because the PCM can absorb or release large latent heat during the melting or solidifying process when surrounding temperature increases and decreases [1,2].When compared to conventional sensible heat energy storage systems,the LHTES provides much higher energy storage density with a smaller temperature difference between storing and releasing heat.Therefore,these features make them more func-tional and widely used than the traditional PCM in heat pumps,spacecraft thermal control,functional fibers thermal insulation,heat transfer,solar energy utilization,building materials and shifting peak energy demand applications.Many kind of inorganic and organic compounds such as salt hydrates,paraf fins,fatty acids,fatty acid esters and their binary and ternary mixtures are de fined as PCM and they can be poten-tially used for LHTES in many fields due to their different phase change intervals.Unfortunately,prior to the large-scale practical application of this technology,it is necessary to resolve numerous problems.For their employment as latent heat storage materials,*Corresponding author.State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials,Lanzhou University of Technology,Lanzhou,Gansu 730050,China.Tel.:þ8613919809076;fax:þ869312973648.E-mail addresses:haoyunwangyi1977@ ,wangyi@ (Y.Wang).Contents lists available at ScienceDirectRenewable Energyjournal ho me page:www.elsevier.co m/locate/renene0960-1481/$e see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.renene.2010.12.022Renewable Energy 36(2011)1814e 1820these materials must exhibit certain desirable thermodynamic, kinetic and chemical properties.Moreover,economic consider-ations and easy availability of these materials has to be kept in mind.The major drawbacks such as leakage,increases not only the heat resistance but the cost of the LHTES system,and lower thermal conductivity,which limits the power that can be extracted from the energy storage system,have restricted their extensive application. Various methods have been developed for overcoming these problems.The most potential and actually useful method is preparing form-stable PCM.The form-stable PCM can be prepared by encapsulation of PCM into a polymeric structure,some articles defined as microencapsulated phase change material(MEPCM),by physical-mixing or chemical synthesis.Its main advantages of these materials is it can maintain the shape and prevent PCM leakage during the phase change process and it can be directly used in thermal energy storage system without extra encapsulation due to the PCM has been encapsulated in three-dimensional net structure formed by polymer materials[3].In addition,thermal conductivity of the form-stable PCM composites can be increased remarkably by the loading of carbonfibers,carbonfiber brushes,graphite and etc due to their higher thermal conductivity[4,5].There are many literatures have concentrated on form-stable PCM.A literature survey on form-stable PCM indicates that mela-mine-formaldehyde(MF)resin[6],urea e formaldehyde(UF)resin [7],polyurethane(PU)[8],high density polyethylene(HDPE) [9e11],styrene-butadiene-styrene(SBS)copolymer[12]and pol-ymethyl methacrylate(PMMA)[3,13]are usually selected as microcapsule shell materials for the PCM protection.PMMA is a thermoplastic,transparent and commercially available acrylic polymer.It has moderate properties,easy handling and processing, high impact strength,relatively good chemical resistance and low cost.From this point of view,PMMA is a versatile material and the promising polymer as shell material containing PCM for thermal energy storage applications.Alkan et al.[3]prepared stearic acid (SA),palmitic acid(PA),myristic acid(MA),and lauric acid(LA)/ PMMA composite as form-stable PCM by encapsulation of fatty acids into PMMA which acts as supporting material though solution casting method.Wang and Meng[13]synthesized a series of PMMA microcapsules containing CA-LA,CA-MA,CA-SA and LA-MA eutectics acting as the heat-absorbing materials using the method of self-polymerization.Alkan and their colleagues fabricated microencapsulated PCM by emulsion polymerization using doco-sane[14],n-octacosane[15]and n-heptadecane[16]as core material and PMMA resin as shell materials.Stearic acid(SA)is known as linear chain fatty acid and widely utilized as PCM.It is chemically stable,showing no phase segre-gation during phase change,odorless,nontoxic,easily available and large thermal storage capacity[17e19].Therefore,it may be used as energy storage material for passive solar space heating,functional fluid or decreasing indoor temperature swing in buildings.As can be seen from the literature survey given above,there are some studies on microencapsulation or form-stable PCM with PMMA.However,as far as the authors are aware,there is no investigation reported in literature on preparation and comprehensive characterization of PMMA blends as PCM by UV photoinitiated dispersion polymerization.Ultraviolet photo-polymerization is a high efficiency,wide enabling,energy saving, environmental friendly and environmental protection tech-nology.It has been widely used in chemical industry,machinery, electronics,light industry and communication[20,21].In this regard,the objective of this study is to prepare microcapsules made of PMMA as shell material and SA as core by UV photoinitiated dispersion polymerization.Fourier transfor-mation infrared spectroscope(FT-IR)and scanning electronic microscope(SEM)were used to determine the chemical characterization and microstructure of composites,respectively. Differential scanning calorimetry(DSC)and thermal perfor-mance test were employed to investigate the thermal properties and thermal stability of the prepared composite.In addition, thermal cycling test was conducted to determine the thermal reliability of the PCM.2.Experimental2.1.MaterialsStearic acid(SA,commercial grade,T e¼66.5 C,ΔH¼177.8J/g) was used as phase change material(core material)and supplied by Beijing Chemical Co.Ltd.Methyl methacrylate(MMA,AR) purchased from Tianjin Kermel Chemical Reagent Co.Ltd.was used as shell-forming monomers.MMA was washed by5%NaOH solu-tion(V MMA:V NaOH¼5:1)in a separatory funnel for several times to remove the polymerization inhibitor hydroquinone and then double distilled before use.The polyethylene glycol octyl phenyl-ether and poly(vinyl alcohol)(PVA)were obtained from Shanghai Chemical Reagent Co.Ltd.and used as surfactant and dispersing agent,respectively.Ethylene glycol dimethacrylate(EGDMA)was bought from Shanghai Chuanghe Chemical Reagent Company and used as crosslinking agent.Photosensitive solicitation reagent 2-hydroxyl-2-methyl-1-phenyl acetone(1173)which chemical formula are showing in Fig.1and deionized water was self-made by our laboratory.2.2.Preparation of the form-stable SA/PMMA composite PCMIn preparation of the SA/PMMA form-stable PCM,dispersion polymerization through UV photoinitiated method was conducted. Reactionflask with100mL deionized water,5g SA,0.01g PVA and 0.02g polyethylene glycol octyl phenylether was heated above melting point of SA under nitrogen atmosphere.Then the mixture was violent mixed for half an hour before polymerization.Then 4mL methyl methacrylate monomer and EGDMA photoinitiator were added and stirred vigorously at the same temperature for1h. The mixture contained the monomer was reacted at UV radiation for30min and the precipitate was separated form the solution by casting water.The particles was dried under vacuum at80 C for 24h.This process was recognized as leakage test and the composite which did not leak melted PCM from the microcapsules was defined as form-stable PCM.2.3.Characteristic of the form-stable SA/PMMA composite PCMPhase change properties,melting and freezing temperatures and latent heats,of the SA and SA/PMMA composite were deter-mined by differential scanning calorimeter(DSC,NETZSCH STA-449C model)at a heating rate of5 C per minute and temperature range from room temperature to100 C in a purified nitrogen atmosphere.The specimens for DSC analysis with the mass of 5e10mg were taken from the brittle failure surface of the form-stable PCM.The extrapolated onset temperature(T S),peakFig.1.Chemical structure of1173.Y.Wang et al./Renewable Energy36(2011)1814e18201815temperature(T P )and extrapolated end temperature (T E )of the SA and SA/PMMA microcapsules were obtained by the special soft-ware of the DSC.The phase change latent heats were determined by numerical integration of the area under the peaks.In order to prove the compatibility between the components of composite,the form-stable PCM was characterized by spectroscopy methods.The spectroscopic analysis after and before thermal cycling was per-formed on a KBr disk in the range of 4000e 400cm À1by using a Nicolet AVTAR 360FT-IR.The scanning electron microscope (SEM)images of form-stable composite were taken by JSM-6701model SEM.The form-stable PCM specimens were broken and the frac-tured surfaces were coated with gold before SEM imaging.2.4.Thermal performance and thermal stability analysisTo con firm the heat storage/release performance of PCM composite,thermal performance test was conducted using the experimental set-up in Fig.2.In this experimental set-up,the water was circulated throughout the surrounding of test cell for the melting process of the composite (5g)at a constant temperature of 80 C [17].After melting process,the sample were immediately subjected to solidi fication process in the refrigerator (cooling facility)at 30 C.The temperature variations of the PCM and reference substance during these periods were automatically recorded to a PC via data logger with a temperature measuring accuracy of Æ0.1 C at time intervals of 2s.The thermal cycling test was also used to determine thermal reliability of form-stable composite PCM in terms of the change in phase change temperatures and latent heats with respect to the thermal cycling number.A thermal cycling test consisted of exposing composite PCM to a melting and freezing process.The tests were performed consecutively up to 500thermal cycling using a thermal cycler.DSC and FT-IR analysis were also performed to determine the thermal and chemical stability of the composite PCM after thermal cycling.3.Results and discussion3.1.Characterization of the form-stable SA/PMMA composite PCM The morphologies of the microcapsules,particle size and shape especially,are crucial for achieving the desired properties.The solubility,stability,chemical reactivity,opacity,flow ability,and strength of the materials are affected by the size and characteristics of the microcapsules.The surface morphology of the SA/PMMA form-stable PCM was investigated by SEM analysis and the image was shown in Fig.3.As shown in Fig.3(a),the SA/PMMA micro-capsules have a single phase surface,regular spherical shape,relatively uniform sizes and smooth surface without edges or sharp dents.The surface of the microcapsules is adhered with small PMMA resin particles.The similar appearance was coincided indifferent microcapsules appears as quite homogenous and the diameters of the microcapsules vary within the narrow range,2e 3m m.As can be seen in the image of Fig.3(b),the dark colored area is pore which entraps the core material and the light colored region is microcapsule shell materials.This result veri fied strongly the core/shell structure of microcapsules was formed and the skeleton was stable against the leakage of SA from the blends.The formation of microcapsule,the PMMA shell fabricated on the surface of SA droplets through monomers dispersion polymeriza-tion,is implemented by ultraviolet curing dispersion polymeriza-tion.Schematic fabrication of the microcapsules has been described in Fig.4.FT-IR spectroscopy can be used to reveal speci fic interactions in PCM/polymer composite [22].Fig.5displays the FT-IR absorption spectra of the characteristic peaks form SA,PMMA and SA/PMMA composite.In the pure SA spectrum,there is an adsorption peak at the wave number of 3017cm À1and the 2848cm À1which usually overlaps with the absorption band of aliphatic C e H vibration caused by stretching vibration of O e H group.The peak at 1702cm À1is the characteristic absorption peak for the stretching vibration of carbonyl group.The peak at 1463cm À1is the e CH 2bending peaks,1389cm À1represents C e H and C e C bending and 729cm À1and 718cm À1corresponds to rocking vibration and bending,which are all characteristic for aliphatic chain of SA.In the spectrum of PMMA,the peak at 1731cm À1is the most intensive absorption band representing the carbonyl group.The peaks at 1076cm À1and 1198cm À1can be assigned to the C e O stretching of the ester paring the FT-IR spectra of SA and PMMA,it is clearly seen that the FT-IR spectrum of the microcapsules consti-tutes of both the peaks of SA and PMMA.The only difference is the peak positions in the SA/PMMA spectrum slightly deviates from the positions in their pure spectra.This changes are attributed to the interactions between oxygen atom of carbonyl group (C ]O)of PMMA and hydrogen atom of hydroxyl group (O e H)of SA,likely.Fig.2.Experimental set-up for studying the thermal behavior ofPCM.Fig.3.The SEM images of the composite.Y.Wang et al./Renewable Energy 36(2011)1814e 18201816The infrared spectrum has no signi ficant new peaks indicates there are no chemical reaction between fatty acid with PMMA.The FT-IR analysis conclusions assumed as another evidence of successful formation of bined with the result of SEM images,it can be see clearly that the SA and PMMA have good compatibility.The DSC results of the form-stable composite and pure SA samples are shown in Fig.6.Form the curves,the T S ,T P ,and T E values of SA can be determined as 54.2 C,59.9 C and 66.5 C for melting phase change and 54.9 C,50.9 C and 48.3 C for freezing phase change.And the result for the composite can be determined as 54.8 C,60.4 C and 66.1 C for melting and 53.8 C,50.6 C and 48.1 C for crystallization pared the phase change temperatures value of SA and composite,it can be seen that the change in melting point was 0.5 C and 0.3 C in freezing point,respectively.These results can be considered as an indicator of the interaction to be occurred between the functional groups of PMMA and the SA.In the curve of DSC,the latent heats of melting and freezing were found to be 177.8J/g and 185.6J/g for SA and 92.1J/g and 95.9J/g for form-stable composite PCM,respectively.The percentage of PCM in microcapsules is the key factor for form-table PCM.It determines directly enthalpy and thus energy storage ef ficiency of the PCM.The encapsulation ratio can becalculated with the equation of SA (wt.%)¼D H(SA/PMMA composite PCM)/D H(pure SA)based on enthalpy values as 51.8%in the experiments.Where D H (SA/PMMA composite PCM)and D H (pure SA)are measured enthalpies of form-stable SA/PMMA composite PCM and SA itself,respectively.These latent heat values were slightly lower than the calculated values based on the mass ratio of the SA in parison of LHTES properties of form-stable composite PCM prepared in this study with that of the different composite PCMs in literature [17],it can remarkably be noted that the form-stable composite PCM has an important LHTES potential.3.2.Thermal reliability of the composite PCMA high-Quality composite PCM must be thermally and chemi-cally stable over a large number of melting and freezing cycling.The superior composite should be shown no or less change in its thermal properties and chemical structure after long term utility period.Therefore,thermal cycling test was performed to determine the change in thermal properties of the composite PCM and their DSC curves and the results before and after 500cycling are given in Fig.7and Table 1,respectively.As seen form the Fig.7and Table 1,Fig.4.The formation process of SA/PMMAmicrocapsules.Fig.5.FT-IR spectra of PMMA,SA and SA/PMMA compositePCM.Fig.6.DSC thermogram of SA and SA/PMMA composite.Y.Wang et al./Renewable Energy 36(2011)1814e 18201817the thermal properties (melting and freezing temperature,latent heat)of SA/PMMA after 500cycling have irregular and reasonable changes compared with the SA/PMMA composite.In other words,the reasonable changes of temperature or enthalpy indicated the composite has good thermal reliability in terms of the changes in its melting and freezing temperatures.Fig.8shows the TG curves of before and after 500cycled SA/PMMA composite and pure SA.The variations of weight loss rate about SA/PMMA composite before and after 500cycling are nearly constant at intervals of 30e 90 C.It is also known that the weight loss rates of SA is larger than that of SA/PMMA composite which indicate the SA/PMMA has good thermal stability under used temperature.On the other hand,the chemical stability of composite after repeated thermal cycling was also investigated by FT-IR.Fig.9shows the FT-IR spectra of form-stable composite PCM before andafter 500thermal cycling.The shape and frequency value of all peaks are consistent in the samples before and after thermal cycling indicates that the chemical structure of the composite was not affected by repeated melting/freezing cycling.3.3.The heat storage and release performance of PCM composites The heat storage or release performance was veri fied by temperature curves for melting and freezing time of the form-stable SA/PMMA composite PCM which are shown in Fig.10(a)and (b),respectively.The melting and solidi fication time was estimated from the temperature curves by constructiontheirFig.7.DSC curves of form-stable SA/PMMA composite PCM before and after thermal cycling.Table 1Thermal properties of form-stable SA/PMMA composites before and after thermal cycling.SampleMelting FreezingT S ( C)T P ( C)T E ( C)Latent heat (J/g)T S ( C)T P ( C)T E ( C)Latent heat(J/g)SA/PMMA composite before thermal cycling54.860.466.192.153.850.648.195.9SA/PMMA composite after thermal cycling 55.159.365.590.953.150.947.797.8Changes 0.03À1.1À0.6À1.2À0.70.3À0.41.9Fig.8.TG curves of pure SA and SA/PMMA composite PCM before and after thermalcycling.Fig.9.FT-IR spectra of SA/PMMA composite PCM before and after 500cycling.Fig.10.Melting and solidi fication temperature curves of SA,PMMA and SA/PMMA composite.Y.Wang et al./Renewable Energy 36(2011)1814e 18201818outer tangent lines.The interval time between two points of intersection is de fined as melting and solidi fication time.As seen from Fig.10(a),the melting times of pure SA and the composite PCM were determined as 580s and 600s,respectively,showing SA and SA/PMMA have the approximate melting times.This indicated that the composite PCM have an almost equal heat absorbtion velocity supplied from a heat source.As show in Fig.10(b),the solidi fication time estimated as 940s for pure SA and 620s for the composite PCM.This result also suggested that the SA/PMMA has an excellent heat storage property.The total times of heating process were less than those of cooling process because of the heat transfer rate of heating process in the hot water were higher than that of cooling process in the standing cold air of the refrigerator.The melting and freezing peak posi-tion of samples were evidently less than those obtained from DSC.This because of DSC measurement is an express method and differs from classical thermo-physical methods.The thermal performances of a PCM in a thermal cycling with a slow heating/cooling rate are very different from those in a rapid heating/cooling process [23].The heat storage and release reliability of the composite PCM after 500cycling were also determined and the result are show in Fig.11.According to Fig.11,it can be obviously observed that the composite has heat storage and release process.Moreover,the heat storage and release process curves before and after 500cycling of the form-stable SA/PMMA composite PCM have less change.This result indicated that the composite can be implemented to transfer energy in different time and space.4.ConclusionA novel form-stable SA/PMMA composite PCM for LHTES applications was prepared by entrapping of SA into PMMA cell by ultraviolet curing dispersion polymerization.The structure,morphology,thermal properties,thermal reliability and heat storage/release performance were determined by SEM,FT-IR,DSC and melting/freezing cycling test respectively.The form-stable composite PCM can be formed by 51.8wt.%SA encapsulated in three-dimensional net structure of PMMA supporting material.The melting and freezing temperatures and the latent heats of form-stable SA/PMMA composite PCM were measured as 60.4 C,50.6 C and 92.1J/g,95.9J/g,respectively.The results of DSC,FT-IR and thermal cycling test are all showed that the form-stable composite PCM had good thermal reliability and chemical stability although it was subjected to 500melting/freezing cycling.So,the composite can be used as energy storage material for passive solar space heating or decreasing indoor temperature swing in buildings.AcknowledgmentsAuthor would like to acknowledge the financial support of National Natural Science Foundation of China (Grant No.51063003)and outstanding young teachers ’project by Lanzhou University of Technology (Grant No.2008012).References[1]Bayés-GarcíL,Ventol L,Cordobilla R,Benages R,Calvet 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