Building energy consumption is an important part of energy consumption. Popularizing latent heat storage technology in building is beneficial to reducing building energy consumption. Phase change materials (PCMs) are important carriers of latent heat energy storage technology. The application of PCMs in building materials is helpful in increasing the latent heat storage capacity of the building. The leakage of PCMs can be prevented and the thermal conductivity of PCMs can be improved by incorporation of PCMs into inorganic porous media. Among various types of inorganic porous materials, the materials containing mainly micropores (0.1 lm–100 lm) such as expanded perlite (EP), expanded vermiculite (EV), diatomite and expanded graphite (EG) have characteristics of high porosity, moderate pore diameter, low price and wide sources. The four kinds of inorganic porous medium based composite PCMs are suitable for largescale usage in cement mortar. In this paper, the preparation, thermal properties, and performance improvement of the four composite PCMs are reviewed. The effects of them on the properties of cement mortar are also summarized.
Trang 1Review on micropore grade inorganic porous medium based form stable
composite phase change materials: Preparation, performance
improvement and effects on the properties of cement mortar
Min Li⇑, Junbing Shi
Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing 211189, China
h i g h l i g h t s
The micropore grade inorganic porous medium are appropriate for the adsorption of PCMs
The performances of the inorganic porous medium based form-stable CPCMs were reviewed
The effects of the CPCMs on cement mortar were summarized
a r t i c l e i n f o
Article history:
Received 6 May 2018
Received in revised form 22 October 2018
Accepted 29 October 2018
Keywords:
Phase change materials
Micropore
Inorganic porous media
Thermal energy storage
Cement mortar
a b s t r a c t
Building energy consumption is an important part of energy consumption Popularizing latent heat stor-age technology in building is beneficial to reducing building energy consumption Phase change materials (PCMs) are important carriers of latent heat energy storage technology The application of PCMs in build-ing materials is helpful in increasbuild-ing the latent heat storage capacity of the buildbuild-ing The leakage of PCMs can be prevented and the thermal conductivity of PCMs can be improved by incorporation of PCMs into inorganic porous media Among various types of inorganic porous materials, the materials containing mainly micropores (0.1lm–100lm) such as expanded perlite (EP), expanded vermiculite (EV), diatomite and expanded graphite (EG) have characteristics of high porosity, moderate pore diameter, low price and wide sources The four kinds of inorganic porous medium based composite PCMs are suitable for large-scale usage in cement mortar In this paper, the preparation, thermal properties, and performance improvement of the four composite PCMs are reviewed The effects of them on the properties of cement mortar are also summarized
Ó 2018 Elsevier Ltd All rights reserved
Contents
1 Introduction 288
2 The preparation method and characterization of micropore grade inorganic porous medium based FSCPCMs 288
3 Expanded vermiculite based FSCPCM 290
3.1 Thermal properties of EV based FSCPCM 290
3.2 Performance improvement of the EV based FSCPCM 293
3.2.1 Improvement of the adsorption performance and heat storage performance 294
3.2.2 Improvement of the thermal conductivity 294
4 Expanded perlite based FSCPCM 295
4.1 Thermal properties of EP based FSCPCM 296
4.2 Performance improvement of the EP based FSCPCM 298
4.2.1 Improvement of the adsorption performance and heat storage performance 298
4.2.2 Improvement of the thermal conductivity 298
5 Diatomite based FSCPCM 299
https://doi.org/10.1016/j.conbuildmat.2018.10.222
0950-0618/Ó 2018 Elsevier Ltd All rights reserved.
⇑ Corresponding author.
E-mail address: limin.li@163.com (M Li).
Contents lists available atScienceDirect
Construction and Building Materials
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c o n b u i l d m a t
Trang 25.1 Thermal properties of the diatomite based FSCPCM 300
5.2 Performance improvement of the diatomite based FSCPCM 302
5.2.1 Improvement of the adsorption performance and heat storage performance 302
5.2.2 Improvement of the thermal conductivity 302
6 Expanded graphite based FSCPCM 302
6.1 Thermal properties of the EG based FSCPCM 303
6.2 Thermal conductivity of the EG based FSCPCM 304
7 The effect of the micropore grade inorganic porous medium based FSCPCMs on cement mortar 304
7.1 Effect of the micropore grade inorganic porous medium based FSCPCMs on the heat storage performance of cement 306
7.2 The influence of the micropore grade inorganic porous medium based FSCPCM on the strength of cement mortar 307
8 Conclusions and outlook 308
Conflict of interest 308
Acknowledgements 308
References 308
1 Introduction
With the development of economy, the demand for energy is
increasing in all walks of life However, due to the decrease of
tra-ditional fossil energy reserves and inefficient usage of existing
energy sources, the contradiction between supply and demand of
energy is becoming tense Improving energy utilization and
devel-oping new energy have become an important way to ease this
contradiction
At present, the thermal energy storage (TES) is regarded as an
important method to increase energy efficiency[1] It includes
sen-sible heat storage and latent heat storage The sensen-sible heat storage
can be implemented easily through heating or cooling the medium
However, its utilization is limited by the low heat storage capacity
and large volume requirement[2] The latent heat storage can be
realized through the phase change process (solid–solid,
solid–liq-uid and gas–liqsolid–liq-uid) of the materials during which large amount
of heat can be utilized without a significant change of temperature
[3] Because of the advantages of latent heat storage, the related
materials and technologies have become research hotspots in the
field of energy saving[4,5]
PCM is a kind of important latent heat storage material The
PCMs can be divided into solid–solid PCMs, solid–liquid PCMs,
gas–solid PCMs and liquid–gas PCMs according to the type of phase
change Besides, PCMs can be divided into inorganic PCMs, organic
PCMs and composite PCMs according to the chemical composition
For solid–gas PCMs and liquid–gas PCMs, due to the generation of
gaseous substances during the phase change process, the volume
change of them is larger than that of the other kinds of PCMs As
a result, even though the heat storage density of these PCMs is
higher than the other PCMs, the application of them was limited
Solid-solid PCMs and solid–liquid PCMs were proved to have good
application potential in building energy conservation, solar energy
utilization, heat recovery, temperature control and other fields
[6–9] Although PCMs have high thermal storage capacity, there
are still some problems in practical applications The leakage
dur-ing the solid–liquid phase change process and low thermal
conduc-tivity of the PCMS are considered to be the two outstanding issues
[10,11] At present, incorporation techniques and
macro/microen-capsulating method are effective methods to prevent leakage
The incorporation technique is to prepare composite PCMs by
com-bining pure PCM with layered materials or porous materials
Microencapsulation is a process of encapsulating pure PCM with
polymer or organic shell.[12,13] Adding high thermal
conductiv-ity materials such as expanded graphite and nanometal can
improve the thermal conductivity of PCMs[14]
Inorganic porous medium, which has large surface area and
abundant pore structure, is an ideal supporting material to prepare
form stable composite phase change materials (FSCPCMs)[15,16]
In the inorganic porous medium based FSCPCM, the leakage issue
can be effectively solved due to the micro capillary force and the interfacial adsorption effect of inorganic porous medium More-over, the heat conduction of the PCMs can be improved because
of the high thermal conductivity of the inorganic materials[17] The pore sizes of some inorganic porous materials[18–23]such
as zeolite, molecular sieve and porous silica are nanoscale (0–
100 nm) Such nanopores will hinder the phase transformation of PCMs during adsorbing the PCMs As a result, the adsorption capac-ity and heat storage capaccapac-ity of the composite PCMs are decreased [24] However, the pore sizes of expanded perlite, expanded vermi-culite, diatomite and expanded graphite are mainly microscale (0.1lm–100lm) These micron scale pores have little interference
on the phase transition behavior of PCMs[25] Besides, the four porous materials have the advantages of wide sources and low cost.[26,27]
Some reviewers have presented classifications, applications and performance, however few reviewers focused on the micron pore grade inorganic porous medium based FSCPCMs The preparation, characterization, thermal properties, and performance improve-ment of the EP based FSCPCM, EV based FSCPCM, diatomite based FSCPCM and EG based FSCPCM are reviewed in this paper The effects of the four FSCMs on cement mortar are summarized
2 The preparation method and characterization of micropore grade inorganic porous medium based FSCPCMs
The preparation method of micropore grade inorganic porous medium based FSCPCMs included direct impregnation method and vacuum adsorption method[28,29] The schematic diagram
of preparing FSCPCMs by direct impregnation method was shown
inFig 1 [28] It included two kinds of mixing process One was
Trang 3heating the PCMs to a molten state and then mixing them with the
supporting materials, the other was mixing the PCMs and
support-ing materials at solid state and then heatsupport-ing to a molten state The
first mixing process was usually used to prepare composite
mate-rials with low phase change temperature and the second mixing
process was suitable for composites with high phase change
tem-perature The vacuum adsorption method was more complex than
the direct immersion method The schematic diagram of preparing
PCMs by vacuum adsorption method was shown inFig 2 [29] For
vacuum adsorption, except the capillary force and the surface
ten-sion of the porous materials, the pressure difference of the
environ-ment was also helpful to the adsorption of the liquid PCMs
The characterization methods of micropore grade inorganic
porous medium based FSCPCMs mainly included differential
ther-mal analysis (DSC), thermogravimetric analyzer (TG), Fourier
transform infrared spectroscopy (FTIR), scanning electron
micro-scopy (SEM), X-ray diffraction analysis (XRD), thermal conductivity
analysis and thermal cycling test Among them, DSC was used to
analyze the heat storage capacity of composite PCMs The melting
temperature, freezing temperature and the latent heat of the
com-posite PCMs could be calculated through the DSC curve The DSC
curves of paraffin/expanded vermiculite based FSCPCM were
shown in Fig 3 [30] The FTIR was mainly used to determine
whether there were chemical reactions between PCMs and the
inorganic porous medium The FTIR testing curves of capric acid/
expanded perlite based FSCPCM were revealed inFig 4 [31] The
distribution of PCMs in supporting materials could be observed
by SEM[32] The XRD was used to analyze the crystallinity of
the PCMs[33] The thermal stability of composite PCMs was often
analyzed by TG and thermal cycling The TG curves of Stearic acid/
Expanded vermiculite based FSCPCM were shown inFig 5 [34]
The leakage of PCMs could be estimated by observing the oilstains on filter papers after FSCPCMs were put on filter papersand heated to the melting point, as shown in Fig 6 [35] Thethermal conductivity of FSCPCMs was characterized by thermalconductivity analysis The methods of thermal conductivity analy-
Fig 2 Schematic diagram of vacuum adsorption method [29]
Fig 4 FTIR testing curves of capric acid/expandead perlite based FSCPCM [31]
Fig 5 TG curve of Stearic acid/expanded vermiculite based FSCPCM [34]
Trang 4sis in literatures were different Apart from the directly testing,
some researchers investigate the thermal conductivity by heating
and cooling rate The pore structure of the inorganic porous
mate-rials was characterized by mercury intrusion method and low
tem-perature nitrogen adsorption method The mercury intrusion test
was more accurate for micropore grade inorganic porous medium
The pore distribution of EG tested by mercury intrusion method
was shown inFig 7 [25]
3 Expanded vermiculite based FSCPCM
The expanded vermiculite is obtained by calcination of mineral
vermiculite at high temperature [36] The mineral vermiculite
belongs to the trioctahedron structure Most of them are formed
by hydrothermal alteration or weathering of biotite, mica and
chlo-rite Vermiculite crystal consists of three structural unit layers The
silicon oxygen skeleton exists between the structural unit layers
The tetrahedron is formed by the combination of silicon oxygen
skeleton and the magnesium hydroxide layer or the hydrogen gen aluminum layer Moreover, there is a large amount of boundwater and free water existing among the unit layers of vermiculite.The structure of vermiculite is shown inFig 8 [37] The vermiculitecan expand more than ten times in the vertical direction duringcalcination due to the loss of internal moisture The structure var-ied to be layer structure of 2:1
oxy-The bulk density of the vermiculite decreases and the porosityincreases after expansion The EV mainly consists of two dimen-sional lamellar pores between 0.1lm and 10lm.Figs 9 and 10pre-sent the appearance of vermiculite and expanded vermiculite,respectively[38].Fig 11shows the pore distribution of EV[39] EVhas many advantages such as wide source, high fire resistance,non-toxic, sound absorption and low cost[40] Especially, EV hasgood adsorption properties because of the well-developed porestructures.Fig 12showed the adsorption of mercury ions by EV[41].3.1 Thermal properties of EV based FSCPCM
The EV has been selected as the supporting material to preparethe EV based FSCPCM by many researchers because of its good por-ous structure The direct impregnation and vacuum adsorptionmethod were common preparation method The preparedFSCPCMs showed good thermal reliability and chemical stability.Chung et al.[39]fabricated n-octadecane/EV FSCPCM via vac-uum incorporation method and investigated the thermal proper-ties and chemical stability of it The highest mass percentage ofn-octadecane in the FSCPCM was 80.65% Under this condition,the melting temperature and latent heats of the FSCPCM were26.1°C and 142 J/g The solidification temperature and latent heatswere 24.9°C and 126.5 J/g Moreover, the TG analysis and FIIRanalysis showed that the FSCPCM has good thermal stability andchemical stability The lauric acid/EV FSCPCM was prepared byWen et al.[42] The content of the lauric acid (LA) in the compositeFSCPCM without leakage reached 70 wt% The melting temperature
of the composite phase change material was 41.88°C and the ing latent heat was 126.8 J/g Thermal cycling test showed that theFSCPCM still had good thermal reliability and chemical stabilityafter 200 times of melting/freezing cycling The SEM morphology
melt-inFig 13showed that the EV empty interlayer spaces were largelyoccupied by the impregnated LA The results from FIIR analysisshowed that there was no chemical interaction between the EVand the lauric acid
The phase change temperatures can be adjusted to a propertemperature by blending different PCMs when the phase changetemperature of pure PCMs are too high for building applications.The Capric–myristic acid/EV FSCPCM was prepared by Karaipekli[43]via vacuum incorporation The capric acid (CA)–myristic acidFig 7 Pores distribution of expanded graphite [25]
Fig 6 Observation on leakage of the FSCPCM [35]
Trang 5(MA) eutectic mixture was selected as PCMs The phase change
temperature of the eutectic mixture was adjusted to 25°C by
con-trolling the mass ratio between capric acid and myristic acid at 3:7
The eutectic acid was absorbed into the pore structures of the EV
The maximum content of eutectic acid was 20% under the
condi-tion of no leakage The melting and solidificacondi-tion temperature of
this FSCPCM were 19.8°C and 17.1 °C and the latent heat was
27 J/g The FSCPCM still had good thermal reliability and chemical
stability after 3000 thermal cycles Wen et al.[44]used capric acid
(CA) – lauric acid (LA)eutectics mixture as PCM to prepare EVbased FSCPCM The highest mass percentage of this eutectics mix-ture was 57.48% Only physical combination existed between theeutectics and the EV The phase change temperature and latentheat of this FSCPCM are 21–23°C and 81.34 J/g In addition, the
TG analysis and the 200 times melting/freezing cycling testshowed that this FSCPCM has good thermal stability In the study
of Karaipekli et al [45], a series of FSCPCMs were prepared byincorporation of eutectic mixtures of fatty acids (capric–lauric,Fig 9 The appearance of vermiculite [38]
Fig 10 The appearance of expanded vermiculite [38]
Trang 6capric–palmitic and capric–stearic acids) and EV by vacuum
impregnation method In order to meet the requirements of indoor
temperature controlling, the mass ratios of capric–lauric acids,
capric–palmitic acids and capric–stearic acids were adjusted to
64:36, 76.5:23.5 and 83:17, respectively The maximum content
of these eutectic mixtures in the FSCPCMs were 40 wt% The
melt-ing temperatures and latent heats of these FSCPCMs are in the
range of 19.09–25.64 8°C and 61.03–72.05 J/g, respectively
More-over, the results of the 5000 times heating and cooling cycle tests
showed that these FSCPCMs had good thermal reliability and
chemical stability The capric(CA)–palmitic(PA)–stearic acid(SA)/
EV FSCPCM was prepared by Zhang et al.[29]via vacuum nation method When the mass ratio of CA:PA:SA was 79.3:14.7:6,the melting and freezing temperature was 19.3°C and 17.1 °C,respectively The research showed that the CA–PA–SA wassufficiently absorbed in the porous network of EV There was nochemical interaction between the expanded vermiculite and theCA–PA–SA The 70 wt% CA–PA–SA/EV sample melted at 19.3°Cwith a latent heat of 117.6 J/g and solidified at 17.1°C with a latentheat of 118.3 J/g Moreover, the FSPCMs exhibited adequate stabil-ity even after being subjected to 200 melting–freezing cycles.These literatures showed that the eutectic mixture was combinedwith EV physically, which was similar to the pure PCM The eutec-tic mixture/EV FSCPCM also showed good thermal reliability andchemical stability
impreg-Compared with organic PCMs, inorganic PCMs have larger mal capacity, higher thermal conductivity, higher operating tem-peratures and the better compatibility with the micropore gradeinorganic porous media The phase change temperature of theinorganic PCMs is higher than that of the organic PCMs Thesodium nitrate/EVFSCPCM was prepared by the incorporation ofsodium nitrate into EV with directing impregnation method[28].The results showed that sodium nitrate and expanded vermiculite
ther-in the composites only undergo physical combther-ination, not a ical reaction The adsorptive capacity of the EV to sodium nitratewas about 88% The phase change temperature of the FSCPCMwas 300.9°C and the latent heat was 157.2 J/g In addition, after
chem-200 h of heat treatment, the supercooling degree of the FSCPCMwere between 0.1°C and 3.9 °C and the thermal enthalpy changerate was lower than 5.0% The thermal properties of the EV basedFSCPCMs were summarized and listed inTable 1
Fig 11 The pore distribution of the expanded vermiculite [39]
Fig 12 Adsorption of mercury ions [41]
Trang 73.2 Performance improvement of the EV based FSCPCM
Until now, many kinds of EV based FSCPCM has been
developed Although they exhibited good thermal stability
and chemical stability, the disadvantages such as limited
adsorption capacity and low thermal conductivity hinderedtheir application in thermal energy storage To solve theseproblems, some researchers have devoted to improve theadsorption capacity, heat storage density and thermalconductivity
Fig 13 SEM images of (a), (b) EV; (c) composite PCM (70 wt% LA/EV); (d) SEM image and EDS spectra of composite PCM(70 wt% LA/EV); Appearance photo of 70 wt% LA/EV (e) at room temperature (f) after heating at 80 ◦C for 30 min [42]
Latent heat
of melting
Freezing temperature
Latent heat
of freezing
Thermal cycling
Decrease percentage
of the latent heat
Trang 83.2.1 Improvement of the adsorption performance and heat storage
performance
The latent heats of EV based FSCPCMs are mainly decided by
PCMs in the EV based FSCPCM because the expanded vermiculite
has no phase change ability With the increase of the content of
PCMs, the adsorption capacity of the EV based FSCPCMs increased
As a result, the thermal storage density is increased Wei et al.[46]
conducted an experimental study of the performance
improve-ment of the EV based FSCPCMs The EV was modified by means
of acid treating method followed by loading Al2O3 particles as
shown inFig 14 Then, the Al2O3-loaded EV (aEV/AO) was used
as supporting matrix to absorb the lauric(LA)-myristic(MA)-stea
ric(SA) acid eutectic mixture (LA-MA-SA acid) Compared to the
EV, the adsorption capacity of aEV/AO to LA-MA-SA acid was
increased by 32.3 wt% Due to the modification, the melting and
freezing latent heats of the FSCPM were increased by 51 J/g and
50.6 J/g, respectively According to their study, the Si-OH groups
on the surface of EV were exposed after EV was partially
delami-nated and corroded by acid treatment, which was the reason for
the improvement of the adsorption capacity Similar to the study
of Wei et al., Li et al.[47]treated titanium dioxide-loaded EV with
nitric acid firstly Then the modified EV was used as stearic acid
(SA) supporting matrix to prepare a SA/modified EV FSCPCM via
vacuum impregnation method The results showed that the
melting latent heat of the SA/modified expanded vermiculiteFSCPCM were increased by 69.2 J/g compared to the unmodifiedFSCPCM
Wei et al.[48]used methyl ammonium bromide and nitric acid
to modify the EV and obtained in-situ carbonation expanded miculite (EVC) The modification method was shown in Fig 15.The EVC was used as a carrier to adsorb capric acid(CA)–myristicacid(MA)–stearic acid(SA) ternary eutectic acid to prepare FSCPCM
ver-It was found that the melting latent heat of the CA-MA-SA/modified expanded vermiculite FSCPCM was 86.4 J/g, which was39.1% higher than the unmodified EV based FSCPCM The reasonfor this improvement is the same with the explanation in Ref.[46].3.2.2 Improvement of the thermal conductivity
Low thermal conductivity is a major drawback of the EV basedFSCPCM because it will lead to low heat transfer rate and heat stor-age efficiency Researchers have done a great deal of work toenhance the thermal conductivity The thermal conductivity ofthe EV based FSCPCM was increased by modifying the EV and add-ing high thermal conductivity components Guan et al.[30]modi-fied the EV with sucrose solution to form carbide film between thelayers of the EV The EV was altered to the EV/carbon (EVC)through this method Then the paraffin/EMVC FSCPCM was pre-pared by vacuum impregnation method The schematic diagram
Fig 14 Schematic diagram of the preparation process of aEV/AO [46]
Trang 9of the preparation process of the FSCPCM was shown inFig 16 The
results showed that the maximum content of paraffin in paraffin/
EMVC FSCPCM was 53.2 wt% The thermal conductivity of the
paraffin/EMVC FSCPCM was increased by 193% The authors
believed that the improvement was attributed to implanting high
thermal conductivity carbon network in vermiculite layers
Simi-larly, Zhang et al.[34]modified the EV by in situ carbonation with
starch solution The result indicated that the maximum content of
stearic acid in the stearic/modified EV FSCPCM could reach
63.12 wt% The thermal conductivity of FSCPCM prepared with
the modified EV was increased by 52.9% compared to unmodified
EV based FSCPCM
In addition to the above modification methods, it was often
adopted to add thermal conductivity enhancement components
during the preparation process of the FSCPCM to improve the
ther-mal conductivity Deng et al.[49]developed a thermal
conductiv-ity enhanced FSCPCM with polyethylene glycol(PEG) as the phase
change material and the EV as the supporting material The
nano-silicon carbide was used as the thermal conductivity
reinforcement material The results revealed that the thermal
conductivity of this FSCPCM was 0.53 W/mK when the addition
of nano-silicon carbide was 3.29 wt% The thermal conductivity
was increased by 96.2% compared to the FSCPCM without
nano-silicon carbide The authors contributed the enhancements to the
rapid heat transfer of nano-silicon and their effective dispersion
in the pore structures of EV The same research group also studied
the effects of nano-silver wire on the thermal conductivity of the
PEG/EV FSCPCM [50] The diameter of the nano-silver was 50–
100 nm and the length was 5–20lm It was founded that thenano-silver wire could be well dispersed in the pore of expandedvermiculite After being added with 19.3 wt% of nano-silver wire,the thermal conductivity of the PEG/EV FSCPCM was 0.68 W/mK,which was increased by 172% Deng et al.[51]used the Alumina
to enhance the thermal conductivity of the Na2HPO412H2O/EVFSCPCM The thermal conductivity of the Na2HPO412H2O/EVFSCPCM was increased by 45.6%” after adding 5.3 wt% alumina.Besides, the EG was commonly used as thermal conductivityenhancement components in EV based FSCPCMs The thermal con-ductivities of the EV based FSCPCM were provided inTable 2
4 Expanded perlite based FSCPCMPerlite is the acidic vitreous lava with a structure of pearlcracks About 95% of the perlite ore is glassy phase, in which theamorphous quartz accounts for 65–75% and alkali metal oxidesaccounts for about 8–9%.The internal moisture in the perlite isabout 2–6% A porous EP with a low bulk density can be formedafter perlite is heated rapidly at the temperature of 700–1200°C[27,52].The pores in the EP range from 1lm to 100lm The micro-scopic appearance of perlite before and after expansion was shown
inFig 17 [53] The structure and distribution of the pores insidethe EP were shown in Figs 18 and 19, respectively [39,54] The
EP displayed good adsorption property due to the developed pores
Fig 16 Schematic diagram of the preparation process of EVC composite materials [30]
Table 2
The thermal conductivities of the EV based FSCPCM.
EV based FSCPCMs Thermal conductivity W/(mK) Adding amount (wt%) Increase ratio of the
Capric Acid – Lauric Acid/EV/EG 0.253
Capric Acid – Palmitic Acid – Stearic Acid/EV/Copper 0.362
Trang 10and moderate pore size The adsorption effect of EP onp-nitrophenol was shown inFig 20 [55].
4.1 Thermal properties of EP based FSCPCM
A series of form-stable EP based FSCPCMs have been prepared.Organic materials with low phase change temperature are oftenused as the phase change substance in the EP based FSCPCM Thepreparation methods of EP based FSCPCM mainly included vacuumadsorption and melt impregnation According to the research ofTakahiro et al.[56], the latent heat of the FSCPCM prepared by vac-uuming was larger than the FSCPCM prepared without vacuuming,
as shown inFig 21 In the preparation process of EP based FSCPCM,the liquid PCM was impregnated through capillary forces in EP.However, the air pressure within the pores prevented theFig 17 The micro-morphology of perlite before (a) and after expansion (b) [53]
Fig 18 Internal pore structure of expanded perlite [54]
Trang 11impregnation It was difficult to evacuate the air within the pores
without vacuuming As a result, the adsorption effect of the direct
impregnation was worse than the vacuum adsorption method
Lu et al.[57]prepared a EP based FSCPCM in which paraffin was
used as PCM The maximum content of paraffin can reach 60%
without leakage occurring The phase change temperature is
27.56°C and the latent heat of transformation is 80.9 J/g The
results showed that paraffin and EP were physically combined,
which was the same as the EV based FSCPCM Paraffin was
dis-tributed in the pores of the EP after impregnation The FSCPCM still
maintain stable after 2000 times of heating and cooling cycles The
thermal stability of the FSCPCM was shown inFig 22 Lauric acid/
EP FSCPCM was prepared by Sari et al.[58]via vacuum adsorption
method The melting and solidification temperatures of the
composite were 44.13°C and 40.97 °C, respectively The latent heat
of fusion and latent heat of solidification were 93.36 J/g and
94.87 J/g, respectively It was found that no chemical reaction
occurred between lauric acid and EP In addition, the latent heat
value of melting reduced by 1.2% and the latent heat of freezing
reduced by 4.1% after 1000 times of thermal cycles The decreases
in the latent heat capacity of the composite PCM were in a
reason-able level for TES applications in buildings The same authors[28]
also prepared capric acid/EP FSCPCM The maximum adsorption
capacity of capric acid was up to 55% The melting point was
31.8°C and the latent heat of FSCPCM was 98.1 J/g The latent heat
of melting decreased by 2.6% and the latent heat of freezing
chan-ged by 0.6% after 5000 times of thermal cycles Liu et al [59]
prepared a lauric acid/EP FSCPCM by the method of meltingimpregnation The maximum adsorption amount of lauric acid inthe EP can reach 70 wt% The phase change temperature and latentheat were 43.2°C and 105.58 J/g, respectively
In order to meet the demand of phase change temperature ofthe PCMs, binary or multiple eutectic acids has been developed
by adjusting the content ratio of them The preparing and thermalproperties of the EP based FSCPCMs with binary or multiple eutec-tic acids as PCM were investigated by some researchers They drewsome similar conclusions with the EP based FSCPCMs with purePCM Zhang et al [60] prepared a capric acid-palmitic acid/EPFSCPCM via vacuum adsorption The maximum adsorption capac-ity of the EP to the eutectic acid was 65 wt% The FSCPCM has amelting temperature of 24.1°C and a latent heat of 88.39 J/g Jiao
et al.[61]used binary eutectic acid of capric acid and stearic acid
as the phase change material to prepare the EP based FSCPCM byvacuum adsorption method The content of the eutectic acid was43.4% The eutectic acid and the expanded perlite were simplyphysically bonded The melting temperature of the FSCPCM was
33°C and the latent heat of phase transformation was 131.3 J/g.The melting temperature and the latent heat of the specimen were33.5°C and 131.1 J/g after 1000 thermal cycles Zhang et al.[62]prepared lauric-palmitic-stearic acid/EP FSPCM using vacuumimpregnation method The maximum adsorption amount of the
EP to the eutectic acid was 55 wt% The melting temperature ofthe FSCPCM was 31.8°C, and the latent heat of melting was81.5 J/g The solidification temperature of the FSCPCM was30.3°C and the latent heat of solidification was 81.3 J/g Moreover,the melting and freezing latent heats of the FSCPCM droppedslightly by 4.29% and 5.54%, respectively after 1000 times of ther-mal cycles The FITR curves of the FSCPCM were shown inFig 23.The infrared spectrum without significant new peaks indicatedthat there were no chemical reactions between the lauric-palmitic-stearic acid and EP The TG curves were shown inFig 24 It can be seen that the 5% weight loss temperature of theLA–PA–SA and LA–PA–SA/EP form-stable PCM were higher than
180°C, which means that the LA–PA–SA/EP form-stable PCM had
a good thermal stability in the working temperature range whichwas always designed as below 80°C
Considering the application of the EP based FSPCM under hightemperature, some researchers have attempted to prepare FSCPCM
by adsorbing inorganic PCMs into the pores of EP Li et al.[63]usedsodium nitrate as the PCM to prepare sodium nitrate/EP FSCPCM.The phase change temperature of the FSCPCM was 300°C andthe latent heat of the FSCPCM increased with the content of
Fig 20 Effect of pH on the adsorption of Co and Pb onto EP [55]
Fig 21 The Comparison between vacuum impregnation treatment and
impregna-tion treatment for each porous material [56]
Fig 22 DSC curves of paraffin/expanded perlite composites before and after hot and cold cycles [57]
Trang 12sodium nitrate The results indicated that there was only a physical
combination between sodium nitrate and EP The EP can absorb
90 wt% of sodium nitrate without leakage occurring
In the EP based FSCPCMs, there was no chemical reactions
between EP and PCMs including pure organic PCMs, binary or
mul-tiple eutectic acid and inorganic PCMs The vacuum adsorption was
beneficial for the adsorption of PCMs in EP Under the
impregna-tion treatment, the actual relaimpregna-tionship was expressed by the
por-The prepared EP based FSCPCM have good thermal stability andchemical stability The thermal properties of the EP based FSCPCMswere summarized and listed inTable 3
4.2 Performance improvement of the EP based FSCPCMAlthough the EP based FSCPM has excellent thermal properties,some problems still should be solved to meet the requirement ofapplication On the on hand, the thermal conductivity of the EPbased FSCPM is low due to the low thermal conductivity of EP(0.07 W/(mK)) On the other hand, the leakage of the PCM would
be increased when the EP based FSCPM was used in cement Untilnow, the researchers have taken some measures to solve the twoproblems
4.2.1 Improvement of the adsorption performance and heat storageperformance
Undesired adverse effects of the form-stable PCMs with thecementitious composites have been reported when the PCM melt-ing temperature was lower than the ambient temperature Thatwas PCM leakage occurring during the mixing process with cemen-titious materials when water was added Some measured had beentaken to avoid the leakage Ramakrishnan et al.[64]covered thesurface of EP with a hydrophobic coating and then preparedFSCPCM by vacuum adsorption using the hydrophobic EP as a sup-port material The maximum adsorption capacity of paraffin in thehydrophobic EP was 50% The paraffin adsorption ratio of thehydrophobic EP was increased about 43% compared to theuncoated EP The latent heat of melting of the FSCPCM wasincreased from 35.5 J/g to 60.9 J/g The leakage of the FSCPCMbefore and after modification was shown in Fig 25 The reasonfor the improvement was that the hydrophobic coated EP can pre-vent the contact between water molecules and porous EP due tothe hydrophobicity of EP
4.2.2 Improvement of the thermal conductivity
In the field of latent thermal energy storage, the heat transfertechnology that has to be employed to achieve high enough heatcharging/retrieval rates was the major cost Therefore, it was akey point in both energy and economic aspects to enhance the heattransfer performance of the EP based FSCPCM In order to increasethe thermal conductivity of EP based FSCPCM, researchers havedone a great deal of work Zhang et al.[65]adopted in-situ car-Fig 24 TG curves of the LA–PA–SA and LA–PA–SA/EP [62]
Fig 23 FT-IR spectra of LA–PA–SA, EP and LA–PA–SA/EP [62]
Latent heat
of melting
Freezing temperature
Latent heat
of freezing
Thermal cycling
Decrease percentage
of the latent heat