Preparation of capric acid/halloysite nanotube composite as form-stable pha change material for thermal energy storage
Dandan Mei,Bing Zhang n ,Ruichao Liu,Yatao Zhang,Jindun Liu
School of Chemical Engineering,Zhengzhou University,Zhengzhou 450001,PR China
a r t i c l e i n f o
Article history:
Received 29November 2010Received in revid form 18April 2011
Accepted 12May 2011
Available online 14June 2011Keywords:Capric acid
Halloysite nanotube Pha change material Thermal properties
a b s t r a c t
A novel form-stable composite as pha change material (PCM)for thermal energy storage was prepared by absorbing capric acid (CA)into halloysite nanotube (HNT).The composite PCM was characterized by TEM,FT-IR and DSC analysis techniques.The composite can contain capric acid as high as 60wt%and maintain its original shape perfectly without any CA leakage after subjected to 50melt-freeze cycles.The melting temperature and latent heat of composite (CA/HNT:60/40wt%)were determined as 29.341C and 75.52J/g by DSC.Graphite (G)was added into the composite to improve thermal storage performance and the thermal storage and relea rates were incread by 1.8times and 1.7times compared with the composite without graphite,respectively.Due to its high adsorption capacity of CA,high heat storage capacity,good thermal stability,low cost and simple preparation method,the composite can be considered as cost-effective latent heat storage material for practical applications such as solar energy storage,building
energy conrvation and agricultural greenhou in the near future.
&2011Elvier B.V.All rights rerved.
1.Introduction
Latent heat thermal energy storage (LHTES)using PCM has attracted interest in solar storage and utilization due to its ability to provide a high storage density at nearly isothermal conditions [1,2].So far,many PCMs such as salt hydrates,paraffins,fatty acids and their mixtures have been widely investigated for LHTES [3–6].Among them,capric acid is taken as a promising PCM becau of its proper melting temperature range,high latent heat capacity,good chemical and thermal stability,little or no supercooling during the pha transition,nontoxicity and noncorrosivity against metal containers [7].However,it is clear that the melted capric acid has to be kept in a clod tank or container to prevent leaching during the pha transition [8,9].Therefore,special latent storage device or elements such as a heat exchanger or lots of containers to encapsulate the PCM are needed,which increa the cost.Moreover,the low thermal conductivity of CA leads to low heat transfer rate during the heat discharging process,which also limits its utility areas.This problem can be solved by dispersing graphite due to its excellent thermal conductivities in the range of 10–70W/mK [10,11].
Recently,a new type PCM called form-stable composite PCM bad on high density polyethylene was developed for their attractive advantages such as direct u without an outer container and easy preparation with desirable dimensions [12–14].However,its applica-tion was hampered by high cost of encapsulation.There were very few literatures that aimed to u clays as absorbents to prepare
form-stable composite PCMs for thermal energy storage [15–17].Compared with high density polyethylene,clays are readily obtain-able and much cheaper.Moreover,veral advantages like high adsorption capacity,high heat storage capacity,good thermal stability and direct usability without extra encapsulation render the composite potential heat storage material for practical application.
HNT is a two-layered aluminosilicate clay mineral,which is available in abundance in China as well as other locations around the world.It is chemically similar to kaolin,differing mainly in the morphology of crystals [18].HNTs posss hollow nanotubular structure and large specific surface area.Their novel physical and chemical properties have provided opportunity as low-cost adsor-bents and have been reported in literatures [19–22].However,there is not any literature that aims to u halloysite as adsorbent to prepare form-stable composite PCM.wind万点
In this study,a new form-stable PCM was prepared by absorbing CA into the pores of halloysite nanotubes by the capillary and surface tension forces.In order to improve the thermal conductivity of the form-stable composite,we also introduced high thermal conductivity of graphite into the composite.The results indicated that the composite had a high adsorption capacity of CA,high heat storage capacity and good thermal stability.
2.Experimental 2.1.Materials
Capric acid was ud as a latent heat storage material.It was supplied by Chemical Reagent Co.,Ltd.(Tianjin,China).Halloysite
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Solar Energy Materials &Solar Cells
0927-0248/$-e front matter &2011Elvier B.V.All rights rerved.doi:10.1016/j.solmat.2011.05.024
n
Corresponding author.Tel./fax:þ8637167781724.E-mail address:zhangb@zzu.edu (B.Zhang).
Solar Energy Materials &Solar Cells 95(2011)2772–2777
clay from Henan Province (China)was milled and sieved followed by oven dried at 373K for 24h.Graphite was supplied from the Grenada Development Carbon Materials Co.,Ltd.(Nanjing,China).2.2.Preparation of the form-stable composite PCM
The HNT and CA were mixed in ethanol solution.The mixture was irradiated ultrasonically for 90min,and then stirred and refluxed under 601C for 2h in a water bath.After the recovery of ethanol by simple distillation,the mixture was dried at 701C.In order to find the maximum adsorption capacity of CA,the CA/HNT composite were prepared using different weight ratios of 50:50,
55:45,60:40and 65:35,respectively.In order to increa thermal conductivity of the form-stable composite PCM,graphite (5wt%)was added in the mixture at the beginning.The following preparation was consistent with the same method mentioned above.Thermal conductivities of the form-stable CA/HNT and CA/HNT/G composite PCMs were measured by a thermal property analyzer (TC 3010,Xiatech Electronic Technology Co.,Ltd.).
2.3.Characterizations of HNTs and the CA/HNTs composite PCM The Brunauer–Emmet–Teller (BET)surface area and pore-size distribution of the HNT and the composite were measured by a specific surface area analyzer (Quantachrome NOVA4200e).The microstructures of the HNT and the composite PCM were obrved using a transmission electron microscope (TEM,FEITEC-NAIG2).The form-stable composite PCM was characterized using the FT-IR spectroscopy (NEXUS FT-IR).The pha change tem-perature and latent heat of the composite PCM were measured using a differential scanning calorimeter (STA449C,NETZSCH)at a heating rate of 21C/min in a purified nitrogen atmosphere.In order to verify the form-stable performance of the prepared composite PCMs,thin slices (12mm diameter,11mm thickness)were prepared by compressing the CA,CA/HNT (60/40wt%)and CA/HNT/G (60/35/5wt%)composite powder in a tablet machine,respectively.
2.4.Test of thermal storage and relea rates
Thermal performance test was conducted using the constant temperature water bath method [16,23].Two glass test tubes with the same shell thickness and diameter were ud:one containing the form-stable CA/HNT composite as reference and the other containing the CA/HNT composite with 5wt%graphite (CA/HNT/G).Two thermocouples were placed in the centers of the test tubes,respectively.The test tubes were put into a water bath at a constant temperature of 101C.After
the temperature reached balance,the two tubes were rapidly placed into another water bath at a constant temperature of 401C,where the composite PCM performed process of heat storage.After the heat storage was finished,the composite were immediately subjected to solidification process at a constant temperature of 101C,where the composite PCM performed process of heat extraction.The temperature variations of the composite were automatically recorded by a PC via data logger (Agilent 34970A)with a temperature measuring accuracy of 71.51C at time intervals of ten
conds.
Fig.1.Images of the pure CA (a),CA/HNT (b)and CA/HNT/G (c)samples at room temperature (A)and after heated at 401C
(B).
Fig.2.The TEM images:(a)HNT and (b)form-stable CA/HNT composite
PCM.
Fig.3.Isothermal adsorption–desorption curves (a)and pore distribution curves (b)of HNT and CA/HNT composite PCM.
D.Mei et al./Solar Energy Materials &Solar Cells 95(2011)2772–27772773
3.Results and discussion
3.1.Morphology characterization of the form-stable PCM
The photograph of the pure CA,CA/HNT (60/40wt%)and CA/HNT/G (60/35/5wt%)slices at room temperature is shown in Fig.1A(a)–(c).The original samples have cylindrical shape with smooth surface.When heated to 401C above the pha change temperature of the capric acid (291C),the pure capric acid melt in Fig.1B(a).However,the form-stable composite samples still kept in shape perfectly and had no liquid leakage on the surface after subjected to 50melt-freeze cycles,shown in Fig.1B(b)and (c).Therefore,the composite PCMs are thermally stable after melt-freeze cycles.
Fig.2shows the TEM images of the original HNTs and the composite prepared in this work.The original HNTs have an average length of 0.5–1m m,a diameter in the range of 30–40nm (Fig.2(a)).Th
e inner diameter is more or less 30nm,while the thickness of the wall is about 8–10nm.Compared with natural nanotubes,the pores of the HNT in the composite are partly filled with capric acid (Fig.2(b)).Due to its large and smooth unhin-dered pores in the nm range,HNT could keep the melted CA in pores even over the melting temperature of the CA.Hence,the composite maintains its solid shape without any epage of the melted CA.
Fig.3depicts the adsorption–desorption curves and the pore distribution curves of original HNT and CA/HNT (60/40wt%)composite.Although the isotherm shapes of the two samples are somewhat different from each other,each isotherm ems to be of the type IV indicating both meso-and micro-pores (IUPAC classification)[24,25].Unlikely the sharp increa for HNT,the composite has a significant decrea in the mesopore volume at relative pressures between 0.2and 0.9,which is attributed to the adsorption of CA into the mesopores (Fig.3(a)).Compared with the specific surface area of original HNT(57.76m 2/g),the specific surface area of CA/HNT composite is drastically decread to 4.95m 2/g.Moreover,the peaks between 6and 10nm are obrved in the pore distribution of the original HNT.However,for HNT/CA composite,the peaks between 6and 10nm are completely disappeared (Fig.3(b)).The dramatic decrea of surface area and the disappearance of the peaks between 6nm and 10nm were caud by the filling of the nanotubes with CA,which is consistent with our TEM obrvation result.
The HNTs have a high adsorption capacity of CA due to its nanotubular structure and large specific surface area and can absorb CA by the capillary and surface tension force [19–21].In addition,the abundant hydroxyl groups existing in inner surface of HNT can form hydrogen bond with carboxyl group of CA,which is helpful to absorb CA [22].
3.2.FT-IR spectroscopy
The composite PCM was characterized by FT-IR spectroscopy to investigate the interactions between CA and HNT.The FT-IR spectra of halloysite nanotubes,capric acid and the form-stable CA/HNT composite are shown in Fig.4.The FT-IR spectra of halloysite nanotubes exhibits two Al–OH stretching bands at 3699and 3625cm À1(Fig.4(a))and a single Al–OH bending band at 910cm À1[26,27].The 1094cm À1peak is assigned to stretch-ing mode of apical Si–O,while the band at 1033cm À1is caud by the stretching vibrations of Si–O–Si.Moreover,the band obrved at 534cm À1is deformation vibration of Al–O–Si.In FT-IR spectra of capric acid (Fig.4(b)),the peaks at 2926and 2856cm À1are caud by stretching vibration of C–H and the peaks at 1710and 1461cm À1are attributed to carboxylic acid (C ¼O)stretching vibration and symmetric carboxylate (COO–)stretching vibration,respectively.For the composite (Fig.4(c)),the peaks assigned to halloysite at 3699,3625,1094,910,534cm À1and the peaks assigned to CA at 2926,2856,1710,1461cm À1are still
英语故事表演existed and no significant new peak is obrved,which indicate that the composite is just a physical interaction between CA and
aftermathHNT.
Fig. 4.FT-IR spectra of HNT (a),CA (b)and form-stable CA/HNT composite PCMturnto
(c).
Fig.5.DSC curves for heating (a)and cooling (b)of CA and form-stable composite PCM.
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3.3.Thermal properties of the form-stable composite PCMs The DSC curves of CA and CA/HNT composite are shown in Fig.5.The pha change temperatures and latent heat are listed in Table 1.As indicated in Table 1,pha change temperatures for melting and freezing were determined as 2
9.62and 25.571C for CA,and 29.34and 25.281C for CA/HNT (60/40wt%)composite,respectively.The results showed that the CA had a small degree of supercooling (À
4.051C).In addition,the pha change tempera-tures of CA/HNT composite with different weight ratio of CA only decread by about 11C.The decrea in the pha change temperatures can be attributed to the interaction between the capric acid molecules and the pore wall of HNT.Some rearchers have found similar results in other areas [28–30].The latent heat of CA for melting and freezing was found to be 139.77and 140.12,and 7
5.52and 75.81J/g for CA/HNT (60/40wt%)composite,respectively.However,the composite had slightly liquid leakage increasing the content of CA more than 60%when it was heated above the melting point of CA.Therefore,the maximum mass fraction of CA retained in HNT was 60wt%.Previous studies have suggested graphite surface had better ability to absorb some organic molecules,such as alcohols,benzene,aromatic acids,octylamine,etc.[31–33].Graphite could also absorb extra amounts of CA and prevent its leakage when it was added into the composite.
The effect of graphite additive on thermal properties of the composite was also investigated by DSC analysis.As en in Table 1,pha change temperatures for melting and freezing were determined a
s 29.34and 25.281C for CA/HNT (60/40wt%)composite,and 29.56and 25.361C for CA/HNT/G (60/35/5wt%)composite,respectively.In addition,the latent heat of CA/HNT (60/40wt%)composite for melting and freezing was found to be 75.52and 75.81,and 75.40and 75.35J/g for CA/HNT/G (60/35/5wt%)composite,respectively.The results showed that
graphite additive had no significant effect on pha change temperatures and latent heat of the composite PCM.
The thermal energy storage properties of the synthesized composite PCM were compared with that of the different compo-site PCMs reported,as listed in Table 2.It was obrved that the adsorption capacity of HNTs was similar to that of the montmor-illonite and expanded perlite,while higher than that of gypsum,wallboard,diatomite,vermiculite and expanded vermiculite,etc.[7,16,34–40].This confirmed that halloysite had a comparably higher adsorption capacity for adsorbing pha change materials.3.4.Thermal conductivity improvement of the form-stable composite PCM
Thermal conductivity of PCM is one of the important para-meters in thermal heat storage applications.The thermal con-ductivity of PCM was measured by hot-wire technique.The prepared CA/HNT composite PCM has a low thermal conductivity (0.479W/mK)due to low thermal conductivity
of the capric acid (0.2–0.3W/mK)[41].In order to improve the thermal conductiv-ity,the graphite was added to the composite in mass fraction of 5%.The thermal conductivity of CA/HNT/G (60/35/5wt%)compo-site in the solid state was measured as 0.758W/mK.The thermal conductivity of the composite PCM is incread by about 58%.As reported in literatures,the expanded graphite and carbon fiber had been ud to improve the thermal conductivity of the
Table 1
eumendiesThermal properties of the capric acid and composite PCMs.CA:HNT:G (wt%)Melting point (1C)Melting latent heat (J/g)Freezing point (1C)Freezing latent heat (J/g)50:50:028.6456.2725.0856.8555:45:029.2366.7625.1367.4360:40:029.3475.5225.2875.8160:35:529.5675.4025.3675.3565:35:029.5885.8925.3186.24100:0:0
29.62
139.77
25.57
140.12
Table 2
Comparison of thermal properties of the composite prepared with that of some composite PCMs in literatures.Composite PCM
Meltingm point (1C)Freezing point (1C)Latent heat (J/g)Reference Butyl stearate(25–30wt%)/gypsum
18.021.030.0[34]Methyl palmitate-stearate (26.6wt%)/wallboard 22.523.841.1[35]Dodecanol(25–30wt%)/gypsum
20.021.017.0[35]Lauric-stearic acid (38wt%)/gypsum 34.0–50.3[36]RT20(58wt%)/montmorillonite
23.0–
79.25[37]Capric-myristic acid(55wt%)/expanded perlite 21.7020.7085.40[16]lauric acid(60wt%)/expanded perlite 44.1340.9793.36[38]Capric-myristic acid(20wt%)/vermiculite 23.3514.5427.46[7]Polyethylene glycol(50wt%)/diatomite
27.7032.1987.09[39]Capric-lauric acid(40wt%)/expanded vermiculite 19.0919.1561.03[40]Capric-palmitic(40wt%)expanded vermiculite
23.5121.4072.05[40]Capric-stearic acids(40wt%)expanded vermiculite 25.6424.9071.53[40]
CA/HNT(40:60wt%)/HNTs
29.3425.2875.52Prent study CA/HNT/G(35:60:5wt%)/HNTs
29.56
25.36
76.40
Prent
study
Fig.6.Melting temperature curves of CA/HNT and CA/HNT/G composite PCM.
D.Mei et al./Solar Energy Materials &Solar Cells 95(2011)2772–27772775in ca
composite PCMs,which ranged from 0.26to 1.0W/mK with different amounts of carbon [33,42–44].Compared with the values of thermal conductivity,the CA/HNT/G displayed a higher level of thermal conductivity (0.758W/mK)when the same amount of 5%carbon was added into the composite PCMs.
3.5.The improvement of the thermal storage and relea rates The improvement of thermal conductivity of the CA/HNT composite PCM was also verified by comparing its melting and freezing performances before and after graphite addition.Melting and freezing temperature curves of the composite PCM with and without graphite additive were shown in Figs.6and 7.As en from Fig.6,it took 700s for CA/HNT composite by heating the samples from 10to 281C,however,for the CA/HNT/G (5wt%)composite only 250s.The freezing time was also determined from the freezing curves in Fig.7.For CA/HNT composite,it took 350s by cooling the samples from 40to 281C and for the CA/HNT/G (5wt%)composite only 130s.When comparing the melt-ing and freezing time of CA/HNT c
omposite with that of CA/HNT/G (5wt%)composite,it was obvious that the thermal storage and relea rates were incread by 1.8and 1.7times,respectively,which indicated that the thermal storage and relea rates were greatly incread by introducing graphite.
4.Conclusion
The CA/HNT composite was prepared as a new kind of form-stable composite PCM.The composite can contain capric acid as high as 60wt%and maintain its original shape perfectly without any leakage of capric acid after subjected to 50melt-freeze cycles.The melting temperature and latent heat of composite (CA/HNT:60/40wt%)were determined as 29.341C and 75.52J/g by DSC.Graphite additive can improve the thermal storage performance of the composite and the thermal storage and relea rates were incread by 1.8and 1.7times,respectively.Bad on the results,it was concluded that the form-stable CA/HNT composite can be considered as candidate PCMs for thermal energy storage appli-cations such as solar energy storage,building energy conrvation and agricultural greenhou due to having good thermal proper-ties,simple preparation,low cost and direct usability without needing an extra encapsulation.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No.20871105)and Henan Out-standing Youth Science Fund (No.0612002400).
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Fig.7.Freezing temperature curves of CA/HNT and CA/HNT/G composite PCM.
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