Morphological characterization and applications of phase change materials in thermal energy storage: A review

Renewable and Sustainable Energy Reviews 72 (2017) 128–145

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Morphological characterization and applications of phase change materials in thermal energy storage: A review

MARK

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Xiang Huang, Guruprasad Alva, Yuting Jia, Guiyin Fang School of Physics, Nanjing University, Nanjing 210093, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Phase change materials (PCMs) Morphological characterization Applications of the PCMs Thermal energy storage

Phase change material (PCM), stores and releases heat at a particular required temperature as it undergoes phase change at that temperature. Because of their large latent heat and constant temperature during the phase change process, the PCMs are extensively used in latent thermal energy storage system (LTES) and thermal management system (TM). Due to its virtue of competitive performance, PCM has been used extensively in heat recovery, solar energy, aerospace industry, buildings, textile industry etc. However, the PCM has limitations like low thermal conductivity and low heat transfer rate, which decrease the performance of LTES and TM systems. In this work, ways of improving thermal conductivity and heat transfer rate of the composite PCMs (CPCMs) are summarized from perspective of three kinds of CPCMs’ morphologies (fiber, porosity and sphere). This review paper presents morphological characterization of the CPCMs, and several fabrication methods of the CPCMs with enhanced thermal properties.

1. Introduction Faced with energy shortage and environmental pollution, renewable energy has gained increasing attention, due to its role in energy conservation and environmental protection. Developing new sources of renewable energy and improving the efficiency of energy utilization have become an important issue to the industry. During phase change process, PCMs absorb heat from environment or release heat to environment, so as to achieve the purpose of energy storage and release. Some investigations [1–5] in the past revealed that the PCMs have advantages like simple equipment, small size and flexible phase change temperature. Currently, solid–liquid PCMs are widely used due to their high compactness and small change in volume during phase change. Based on their chemical properties, the PCMs can be divided into organic and inorganic ones. Organic PCMs which include sugar alcohol, paraffin and fatty acid etc, have merits like low corrosiveness, but at the same time have demerits like phase separation is not easily achieved. Water, salt hydrate and molten salt are typical inorganic PCMs, more details about inorganic PCMs can be found in references [4,6–8]. Mixtures of two or more miscible components are called eutectics, which can melt and solidify simultaneously without separation of substances. Various review articles have been presented in the past decade regarding PCMs, the classification and applications of PCMs were introduced fully in [4,10]. Khudhair and Farid [7] reviewed the

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encapsulation of PCMs. Naphon and Wongwises [11] described flow and heat transfer characteristics in curved tubes. The usage of PCMs in building fields were summarized in [14,21]. Verma et.al [16] gave mathematical modeling on latent heat thermal systems. Fan and Khodadadi [20] noted ways for improving thermal conductivity of PCMs. Form–stable latent heat storage system were introduced in [23,26]. Rathod and Banerjee [24] focused on the thermal stability of PCMs. Su et al. [28] and Liu et al. [30] paid attention to the latest encapsulation technologies of PCMs. The summary of review articles on PCMs are listed in Table 1. Up to now, PCMs have been reviewed from above respects. However, little review articles emphasis on the links of thermal properties with the morphologies of PCMs. The objective of this work is to analyze PCMs in three aspects according to their morphologies: fiber, porosity and sphere. The contents of this review article are shown in Fig. 1. Extensive research has been conducted on applications of the PCMs. In addition to applications in buildings, the PCMs are also widely used in solar energy, industrial heat recovery, liquefied natural gas, electric power peaking regulation, green house agriculture, textiles, health care and aerospace. Rabin et al. [34] summarized the thermal storage of solar energy. Ismail and Henriquez [35] investigated passive thermal energy storage in buildings. Buick et al. [36] presented the use of off–peak rates and reduction of installed power. Moreover, the investigation of spacecraft thermal systems has been carried out by

Corresponding author. E-mail address: [email protected] (G. Fang).

http://dx.doi.org/10.1016/j.rser.2017.01.048 Received 6 April 2016; Received in revised form 16 November 2016; Accepted 9 January 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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Table 1 Summary of review articles on PCMs. Author

Year

Topic of review article

Reference

Zalba et.al Papadopoulos Farid et.al Khudhair and Farid Naphon and Wongwises Smyth et.al Kenisarin and Mahkamov Tyagi and Buddhi Regin et.al Verma et.al Sharma et.al Jegadheeswaran and Pohekar Paul et.al Kenisarin Fan and Khodadadi Cabeza et.al Liu et.al Kenisarin and Kenisarina Rathod and Banerjee Khodadadi et.al Fang et.al Yuan et.al Su et.al Browne Liu et.al Bose and Amirtham Alva et.al Ahmed et.al

2003 2003 2004 2004 2006 2006 2007 2007 2008 2008 2009 2009 2010 2010 2011 2011 2012 2012 2013 2013 2014 2014 2015 2015 2016 2016 2017 2017

Classification, heat transfer and applications of PCMs Solar cooling Classification and applications of PCMs Encapsulation of PCMs Flow and heat transfer characteristics in curved tubes Solar water heaters Solar heating systems PCMs in building fields Heat transfer characteristics of thermal systems using PCM capsules Mathematical modeling on latent heat thermal systems Thermal energy storage systems Performance enhancement in latent heat thermal storage system Measurements of the thermal conductivity of nanofluids High-temperature phase change materials Thermal conductivity enhancement of PCMs PCMs used in buildings thermal performance enhancement techniques for high temperature systems Form–stable latent heat storage system Thermal stability of PCMs Introduction of nanostructures and enhancement of thermal conductivity of PCMs Preparation, thermal properties and applications of shape-stabilized composite PCMs Fatty acids as phase change materials Encapsulation technologies of solid–liquid PCMs PCMs for photovoltaic thermal management Preparation, heat transfer and flow properties of microencapsulated PCMs Thermal conductivity enhancement of paraffin wax Thermal energy storage materials and systems for solar energy applications Nanomaterials used in solar thermal energy storage

[5] [9] [10] [7] [11] [12] [13] [14] [15] [16] [4] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

affected by various factors, such as experiment temperature, the magnitude of applied voltage, air velocity, the polymer type and surface tension. Morphological images of the composites are shown in Fig. 2. The fibers are distributed randomly in a three–dimensional web. Each fiber looks like a long cylinder with a smooth surface. The diameter of the fiber was reduced as the nanoparticle loading was raised. The minimum diameter occurred in Fe–4 composite, of which the diameter was 59 nm. There were some nanoparticles on the fiber surface in Fig. 2c and d, while most particles were embedded within the fiber. Conclusions were made that Al and Fe fiber composites possessed higher properties, the thermal conductivity of Al–4 composite increased by 41.75% comparing to original PEG. Besides, results proved that the presence of PA6 had a good effect of preventing PEG leakage. Cai et al. [43] used SiO2 nanofibers as raw material and a mixture of capric–lauric–palmitic acid as PCM to fabricate a novel composite. SiO2 nanofibers were prepared under a high temperature annealing procedure to support the CA–LA–PA mixture via electrospinning. The CA–LA–PA mixture was placed at 60 °C until completely melted, then 12 h was required for electro–spun SiO2 nanofibers immersed into the molten CA–LA–PA mixture, finally the composite was hung in the oven at 60 ℃ for 10 h. Morphological images of the composites are shown in Fig. 3. Experiments found that SiO2 nanofibers were capable of

Mulligan et al. [37]. 2. Morphological characterization of the PCMs There is convincing evidence of a link between morphological characterizations and thermal properties of the PCMs. In some cases, the PCMs differ in shape such as fiber, porosity and sphere [38]. 2.1. PCMs with the fibers structure 2.1.1. Nanomaterial PCMs PCM fibers can be prepared by sol–gel, microencapsulation and electrospinning [39,40]. Electrospinning is a simple and convenient technique to fabricate nanomaterials such as poly fibers embodied with nanoparticles, ceramics and metal fibers. Babapoor et al. [41] used polyamid 6 (PA6) and some nanoparticles such as SiO2, Al2O3, Fe2O3 and ZnO as raw materials and polyethylene glycol (PEG) as PCM to fabricate a novel composites via electrospinning. The PEG has the advantages of high thermal storage capacity and low supercooling. The experiment data showed that the diameter of the fiber lessened as the electrical conductivity of the solution increased. Thompson et al. [42] explained that the diameter of the fiber was

Fig. 1. Flow chart of this work.

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Fig. 2. Morphological images of nanofibers: (a) Al–2, (b) Zn–4, (c) Si–2, (d) Fe–1, (e) Fe–2 and (f) Fe–4 [41].

was used as the PCM and CA acted as the supporting material. Chen et al. [45] fabricated PA6/PEG400 nanofibers via electrospinning and explored the feasibility of the nanofibers for thermoregulation function. The Ag nanoparticles were embedded into the poly (vinyl alcohol) and poly (vinyl pyrrolidone) to fabricate nanofibers at room temperature via electrospinning [46]. Cai et al. [47] used PA6 as the raw material and PEG 4000 as the PCM to fabricate electro–spun fibers, the results showed that the melting enthalpies of the PEG/PA6 fibers increased as the PEG content increased, but was lower than that of pure PEG.

absorbing plenty of CA–LA–PA mixture, the maximum absorption capacity was close to 81.3 wt% (weight percent). The composite PCM possessed high thermal reliability with results showing little variations in phase transition temperatures after 50 thermal cycles. The fibrous structure ensured the mechanical strength of the composite and the surface tension forces helped prevent the leakage of fatty acids. The phase transition temperature of the mixture was lower than that of their individual one but still had high enthalpy value. It was concluded that the phase transition temperature and enthalpy of melting/crystallization of fatty acid eutectic mixture can be controlled, making the fatty acid eutectic mixture best suited to indoor thermal controlling and thermal–regulating fibers. Many researches had discussed the properties of nanofibers. Chen et al. [44] made combinations of polyethylene glycol (PEG) and cellulose acetate (CA) to prepare fibers via electrospinning, the PEG

2.1.2. Carbon material PCMs Some fillers with certain structure and property were added to form thermally enhanced composites. Carbon fiber/graphene nanoplatelets [48], single–walled carbon, nanotubes/graphite nanoplatelets [49], multiwalled carbon nanotube/graphene [50] and boron nitride/multi130

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Fig. 3. Morphological images of (a) nanofibers and (b) composite PCM [43].

Fig. 4. SEM images of the composite PCMs: (a) CF0, (b) CF20, (c) CF20 and (d) CF40 [53].

walled carbon nanotube [51,52] have been fabricated. Carbon fiber is compatible with the majority of the PCM ascribing to its non– corrosiveness and low density. Tian et al. [53] used paraffin as the PCM, ethylene–vinyl acetate (EVA), expanded graphite (EG) and carbon fiber (CF) were used as raw materials. 70 wt% of the paraffin was adopted to ensure high enthalpy, then it was impregnated with EVA under vacuum conditions. The mixture of the paraffin and EVA was heated in a jar for 5 min, then EG/CF mixture was added with different content. The melted paraffin/EVA/EG–CF composites were heated for 20 min and poured into a mold at 40 ℃. The SEM images of the composite PCM are shown in Fig. 4, the EG and CF can be seen from the images, Fig. 4b clearly shows the orientation of the EG. There are three contact types of the EG and CF shown in Fig. 4c: α, β and γ, namely point contact, line contact and face contact. The EG and CF possessed well compatibility in composites. The EG presents a worm

structure and CF displays a cylindrical structure. The content of the CF increasing from CF 0% to CF 40% resulted in a decline of the leakage rate and a rise in thermal conductivity, the lowest leakage rate was lower than 2%. Nevertheless, thermal conductivity fell drastically when the content of the CF was added up to and over 40%. All samples had the same performance in thermal conductivity before 30 ℃, linear increases were observed from 30 to 40 ℃, faster changes had taken place when the temperature was over 40 ℃. Two alternative methods for usage of carbon fibers were compared [54]. First method– the distribution of fibers in PCM was completely random, the length of fibers varied from 5 to 200 mm; the second method – a radial fiber brush was used to match the directions of the heat flow. Mixtures of fibers and paraffin wax were separately packed into two transparent cylindrical capsules with 50 mm diameter and 130 mm height. Experiments showed that the fiber brush type was 131

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Fig. 5. SEM images and skeleton structures of porous nickel foam with pore size of 0.1, 0.3 and 0.5 mm [59].

submersion was 75%, less than 100% of vacuum impregnation. Vacuum impregnation is also available in fabricating metal composite PCMs [57–60]. Erythritol was used as the PCM and porous nickel acted as a support material to prepare metal composite PCM [59]. The experimental results showed that vacuum impregnation had competitive edge in immersing liquid erythritol into metal foam when the pore size was about 100–150 µm, and the impregnation ratio was expected to be 98– 100%. Porous nickel was prepared into cylindrical structure with a 10 mm diameter and 10 mm thickness, and then cylindrical disks were made into 2 mm thickness with 85% ( ± 1%) porosity. Three comparative experiments were made with pore size of porous nickel 100, 300 and 500 µm respectively. The SEM photograph of erythritol/nickel foam is shown in Fig. 5. As the pore sizes increased, less damage could be seen in the bone structure, the framework turned to be stronger as well. Higher thermal conductivity was obtained for increased connectivity of the skeleton. Thermal conductivity rose as the pore size increased, the thermal conductivity of the composite achieved 11.6 W/(m K), which was 16 times higher than that of original erythritol. The melting point and the latent heat were not affected by the pore size. Xiao et al. [57] used nickel foam and copper foam to fabricate paraffin/metal foams, no air entered the micro–pores of metal foam composite on account of 50 Pa being maintained during the impregnation process. As a result, the impregnation ratio reached up to 95%. Morphological characterizations of paraffin/metal foams are shown in Fig. 6, Fig. 6a and c refer to morphologies of original nickel and copper foam before the experiment, Fig. 6b and d show paraffin/nickel and paraffin/copper foam respectively. The surface of composites gradually

more sensitive to temperature change, and the effective thermal conductivity was as much as three times higher than that of the first type. Both methods performed thermal conductivity enhancement. For the first method, the length of fibers had little effect on enhancement while the second type contributed to thermal enhancement significantly. In spite of brush type excelling at many aspects over the random type, the practical choice between these two types still depends on the equipment and the costs. Two methods also have been carried out by Nomura et al. [55], erythritol was used as the PCM and carbon fiber acted as the high thermal conductivity filler. Erythritol (C4H8O4) melts at 118 ℃, the latent heat is 473 MJ/m3, the thermal conductivity of the erythritol is about 0.733 W/(m K) at room temperature. The composite PCMs with percolated network of carbon fibers were prepared, the composite PCMs were fabricated via melt–dispersion (MD) method and hot– press (HP) method separately. As to the first method, fillers were mixed into liquid PCM. Fillers were mixed with solid PCM particles in the second type. The results revealed that the effective thermal conductivity of composite PCM made by HP ascended dramatically and was much higher than that made by MD. Two main reasons were accounted for this phenomenon: (1) carbon fibers differed in the orientations; (2) carbon fibers differed in their network structures in composite PCMs. 2.2. PCMs with the porosity structure 2.2.1. Metal foam PCMs The advantages and disadvantages of simple submersion and vacuum impregnation were compared by Warzoha et al. [56], the results pointed out that the average impregnation ratio of simple 132

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Fig. 6. SEM images of composite PCMs with different pore sizes: (a) metal, (b) paraffin/nickel, (c) copper and (d) paraffin/cooper foam (I: 5PPI, II: 10PPI, III: 25PPI) [57].

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Table 2 Thermal property of several metal foams. Composite PCM

Paraffin/copper Paraffin/nickel Paraffn/copper Paraffin/nickel Paraffin/copper Paraffin/nickel Water/copper Water/aluminum Erythritol/nickel Paraffin/aluminum

Porosity (%)

95 95 > 95 > 95 96.8 97.4 96 94.5 85 93

Pore size (PPI)

5 5 5 5 8 40 0.5 0.5

Melting point of PCM/ composite

52.1/53.32 52.1/52.70

118/118

Latent heat of PCM/composite (kJ/kg)

189.4/132.5 189.4/144.4

354.7/291.7

Thermal conductivity of PCM/composite (W/m ∙ K) liquid

solid

0.335/7.334 0.335/3.172 0.305/4.9 0.305/1.3 0.354/4.98 0.354/1.22 0.56/3.47 0.56/1.8 0.733/11.6 –/21.3

0.558/7.467 0.558/5.618

1.9/3.92 1.9/5.1 0.52/16.2

Reference

[63] [63] [57] [57] [58] [58] [64] [62] [59] [61]

PPI: pore per inch.

Fig. 7. SEM images of composite PCM: (a) G–NF and (b) Paraffin–PDMS–G–NF [71].

ties [65–70]. In addition, it helps in improving the heat transfer performance of composite PCMs due to its high thermal conductivity. Graphene is considered to be a promising material as a new carbon material, the thermal conductivity of the composite PCMs can dramatically increase when porous graphene–block is used, which guarantees efficiency of the heat transfer network. Liang et al. [71] made combinations of graphene and nickel to fabricate composite PCM. Graphene was prepared according to the method of chemical reduction of graphene oxide (GO) in N–dimethylformamide (DMF) [72,73]. After being dried at 100 ℃, nickel foam (NF) was infiltrated into graphene/DMF suspension. Continually doing this for each step was required to ensure the coating of graphene on the substrate, then the foam was heated at the temperature of 234 ℃ together with polydimethylsiloxane (PDMS), which was used to complete the modifications of the surface of the nickel foam. The final foam was called as PDMS–G–NF, making liquid PCM (n–carboxylic acids) easily self–impregnate into the foam without leakage. Fig. 7 shows the morphological characterizations of the G–NF and paraffin– PDMS–G–NF. The surface of G–NF was rough with multi–layer structure (shown in Fig. 7a inset), which exhibits more clearly in Fig. 7b with wrinkled surface and folded edges. The melting and freezing latent heat was 126.34 and 123.41 kJ/kg of PDMS–G–NF/PA respectively, which revealed that the thermal capacity of PDMS–G– NF/PA was 46% and 35% higher than that of original PA. Experiments showed that the melting latent heat was 113.81 kJ/kg of PDMS–G– NF/PA comparing to 126.34 kJ/kg of the pure PA after 200 melting/ freezing cycles, which proved the thermal stability of the composite PCM. Zhong et al. [74] used graphene aerogel (GA) as the support

turns non–smooth structure as shown in Fig. 6b–I to b–II and b–III due to the lack of strong support of the skeleton. The same happens in Fig. 6d–I, Fig. 6d–II and d–III. The effective thermal conductivity of paraffin/nickel composite rose to 1.2 W/(m K) from 0.305 W/(m K) of pure paraffin, and effective thermal conductivity of paraffin/copper composite was 0.305–4.9 W/(m K). The melting temperature of paraffin/metal foam was a little higher than that of pure paraffin, while the freezing temperature was a little lower than that of pure paraffin. Limited by lack of phase change in metal skeleton, the overall latent heat of paraffin/nickel and paraffin/copper decreased by 22–30% and 14–24%, respectively. Xiao et al. [58] prepared metal foams of paraffin/nickel and paraffin/copper with different porosities and pore sizes, and the fabrication was carried out under vacuum conditions. The results revealed that the thermal conductivity of the composites was greatly increased, the highest effective thermal conductivity was 2.33 W/(m K) for paraffin/nickel foam and 16.01 W/(m K) for paraffin/copper foam with 91% and 89% porosity respectively, and the pore size was both 1.0 mm. Attributing to the high porosity and strong skeleton structure, metal foam such as aluminum foam [61,62], nickel foam [57–59,63] and copper foam [57,58,63,64] possesses high thermal conductivity. Larger the porosity of metal foam, larger the amount of PCM that can be impregnated into. Thermo–physical properties of several metal foams are listed in Table 2. Comparing with pure PCM, there is a significant improvement in thermal conductivity. However the latent heat decreases owing to use of metal material. 2.2.2. Carbon material PCMs Graphene has superior electrical, heat transfer and optical proper134

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Fig. 8. SEM images of (a), (c) and (e) unfilled GA, (b), (d) and (f) the surface of GA/OA composites [72].

the GA remains well. Heat was transferred rapidly throughout the OA. The thermal conductivity of the composites was 2.635 W/m K, nearly 14 times than that of pure OA (0.184 W/m K). The GA needed 150 s to reach thermal balance at 60 ℃, while GA/OA composite needed 200 s to reach thermal balance at 70 ℃, ascribing to its large heat storage capacity. Another experiment conducted by Zhong et al. [75] studied thermal

material and octadecanoic acid (OA) as the PCM. After being heated for 1 h in a vacuum oven, GA and OA reached 80 ℃, then the molten OA was impregnated into GA. The system started to cool when OA was completely solidified. The original GA had a structure with an interconnected three–dimensional porosity. The pore walls existed numerous thin layers, shown in Fig. 8a, c and e, enabling GA contain plenty of OA. Fig. 8b, d and f show images of GA/OA composites, the structure of 135

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Fig. 9. SEM images of (a) unfilled graphene foam and (b) alloy filled graphene foam [73]. Table 3 Thermal conductivity models of the metal foam PCM. Models

Thermal conductivity

Bhattacharya et al. [174]

keff =Ak +(1 − A) k⊥

Notes A=0.35

k⊥= (

1 1−ε ε + ) kp kf

k =εkp+(1 − ε ) k f where k⊥ is the effective thermal conductivity of series models, k is the effective thermal conductivity of parallel models, ε is the bulk porosity, kp is thermal conductivity of pure PCM and k f is thermal conductivity of the support material Boomsma and Poulikakos [175]

keff =

2 2(RA + RB + RC + RD )

RA= RB=

4C (2e2 +πC (1 − e)) k f +(4 − 2e2 −πC (1 − e)) kp (e − 2C )2 (e − 2C ) e2k f +(2e − 4C − (e − 2C) e2) kp

RC = 2 ( 2 −2e) 2πC2 (1 − 2e 2 ) k f +2( 2 −2e − πC2 (1 − 2e 2 )) kp

RD=

2e e2k f +(4 − e2) kp

where kp is thermal conductivity of pure PCM, k f is thermal conductivity of the support material, e is a parameter, which equals to 0.339, C is length ratio, C =

⎛ 5 ⎞ 2 ⎜2 − e3⎟ 2 −2ε ⎝ 8 ⎠ π (3 − 4e 2 −e)

and RA,RB, RC , RD is thermal

resistance Singh and Kasana [176]

keff =k Bk⊥1− B

kf

B=1.0647(0.3031+0.0623 ln(ε ) ) kp

k⊥=

1 1−ε ε ( + ) kp kf

k =εkp+(1 − ε ) k f where k⊥ is the effective thermal conductivity of series models, k is the effective thermal conductivity of parallel models, ε is the bulk porosity, kp is thermal conductivity of pure PCM and k f is thermal Xu et al. [63]

keff =kp ((1 −

ξ )2xs +2ξ (1

−

ξ 2 (ξ 2 + ξ 2 ) ξ )+ 2 2 s 3 ξ − ξs + ξs

conductivity of the support material ξ is length ratio (liquid), ξs is length ratio (solid), xs is thermal conductivity ratio k f /kp , kp is thermal conductivity of pure PCM and k f is thermal conductivity of the support material

for its high thermal conductivity and large contact area and volume. Tian and Zhao [78] created a two–equation model to analyze heat transfer of metal foam in phase change process. Zhao and Wu [79] made comparisons of metal foam and carbon foam to investigate thermal performance of sodium nitrate (PCM). The results revealed that the thermal performance of metal foam was 3 wt% better than that of carbon foam, while in some cases the metal was inferior to carbon material in respect of the compatibility with the PCM. The porous structure also took effect in shortening charge/discharge time and temperature difference. Conclusions could be made that the combination of porous structure and PCM can significantly increase energy storage abilities than the pure PCM. Table 3 shows some thermal conductivity models of the PCM. The formulations apply to the effective

properties of graphite foam. Wood's alloy was used as the PCM owing to its high thermal conductivity and favorable chemical stability [4,76,77]. Graphite foam was employed as the support due to its low density and high thermal conductivity. After being heated for 5 h in a vacuum oven, graphite foam and wood's alloy reached 80 ℃, then the molten alloy was impregnated into the graphite foam. The system started to cool when the alloy was completely solidified. Morphological images of graphite foam and composites are shown in Fig. 9a, b, respectively, thermal conductivity of the composites was 193.74 W/ m K, twice more than that of pure alloy (58.88 W/m K). The combination of graphite and wood's alloy showed enhancement in thermal conductivity and mechanical toughness. A number of researchers are attracted to the usage of porous foam

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Fig. 10. SEM images of (a) and (b) PUT micro–PCMs, (c) and (d) PUI micro–PCMs [91].

environment problems caused by organic cosolvent such as acetone and cyclohexane, Li et al. [91] prepared a novel composite micro–PCM by applying reactive monomer divinybenzene and styrene to cosolvent and shell monomer. The n–octadecane was used as core. Styrene, diethylenetriamine (DETA), 2,4–Diisocyanatotoluene (TDI), isophorone diisocyanate (IPDI) and divinylbenzene were employed as the shell monomer. 2, 2–azobisisobutyronitrile (AIBN) was used as initiator. Sodium salt of styrene–maleic anhydride copolymer (SMA) was used as dispersant. Gum arabic, gelatin, glutaraldehyde and sodium alginate were also obtained. Morphological characteristics of micro–PCM with polyurethane shell are shown in Fig. 10, DETA acted as chain extender reacting along with TDI in micro–PCM. The surfaces of microcapsules are smooth in Fig. 10a, the diameter varied between 5 µm and 20 µm, the thickness was estimated between 0.5 µm and 1.5 µm according to the broken shell shown in Fig. 10b. However, the micro–PCM exhibited low fusion heat, and authors replaced TDI with IPDI to overcome this problem. The SEM images of modified micro–PCM are shown in Fig. 10c and d, the strength was destroyed and the thickness of shell was lower than 0.5 µm mainly due to damage arisen in filtration process. PSD micro–PCM (styrene–divinybenzene was used as shell) and PUSD micro–PCM (shell was composed of polyurethane and styrene–divinybenzene) were fabricated as well. There were dimples on the surface of the PSD micro–PCM, for one thing, the dimples decreased the PCM content; for another, the dimples expanded the surface space thus thermal enhancement was achieved [92]. Styrene– divinybenzene was occupied as not only cosolvent but also materials in forming the shell in PUSD micro–PCM. In this case, the PCM content and strength of the shell were guaranteed. No dimples occurred on the surface of PUSD micro–PCM. Nevertheless, the surface was unusually rough and displayed a broken structure. The average enthalpy of micro–PCM was 57.1 kJ/kg, and the octadecane leakage happened in microcapsules. The average enthalpy of PSD micro–PCM was nearly

conductivity prediction of random porous metal medium. The model [63] can be used when the porosity of the metal foam is bigger than 70%. The formulation [174] is an empirical correlation considering the heat flow path. With the relative thickness of the ligaments taken into consideration, the formulation [175] was obtained. 2.3. PCMs with the sphere structure Encapsulated phase change material (EPCM) comes in a core–shell form. The PCM is employed as the core and the polymer or inorganic material act as the shell. The EPCM is outstanding in improving thermal conductivity of the PCM and reducing leakage during phase change process. The EPCM is classified as nano–PCM (diameter smaller than 1 µm), micro–PCM (diameter varies from 1 µm to 1 mm) and macro–PCM (diameter larger than 1 mm). The micro– PCM has several advantages: (1) Enlarged specific surface areas, improved thermal conductivity. (2) Phase change process is carried out in the capsule, which helps eliminate phase separations and supercooling. (3) Improved stability and lower the toxicity of some PCMs [80,81], frost on the surface of building materials also can be eliminated. (4) Extended life of the PCM. (5) Easier to be encapsulated, which meets the demand of using green environmental protection materials. Therefore, the EPCM is widely applied in building materials, textiles, spacesuits, dyes, medicine etc. Complex coagulation [82,83], suspension polymerization [84,85], interfacial polymerization [86,87] and in situ polymerization [88,89] are available to prepare micro– PCM. Interfacial polycondensation is regarded as a promising method to fabricate micro–PCM for its high reaction rate [90]. Because of the 137

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Fig. 11. SEM images with different emulsifiers (a) SDBS, (b) OP–10 and (c) Tween60 [93].

the micro–PCM made with OP–10 had better emulsifying effect than that of SDBS. However, the sizes are not regular. Smooth surface and spherical structure are maintained in Fig. 11c, which indicates that the lipophilicity of Tween60 and polyuria excelled the other two. The particle size of the micro–PCM was determined by the diameter of the emulsion droplets, and identical particle sizes could be obtained with stirring speed maintaining at 3000–4000 rpm. Micro–PCMs with three different shells were made by a suspension polymerization method [94]. The n–octadecane was used as the core. The n–butyl acrylate (BA), n–butyl methacrylate (BMA), methacrylic acid (MAA) and acrylic acid (AA) were employed as the shell monomer respectively. 2, 2’–azobisisobutyronitrile (AIBN) was used as an initiator. Pentaerythritol triacrylate (PETA) was employed as the cross–linking agent. Sodium salt of styrene–maleic anhydride polymer (SMA) acted as a stabilizer. Morphologies of micro–PCMs are shown in Fig. 12. There is a strong sign that aggregations are severe in Fig. 12a and b, while the images are regular in Fig. 12c and d. The phenomenon is relevant to the hydrophilicity of BMA–co–MAA and BMA–co–AA, which strengthened the stability of droplets in fabrication process [95,96] and resulted in decrease of coalescence and increase of dispersion of the droplets. The usage of MAA or AA can intensify the polarity of shells, and the higher the polarity was, the stronger strength of shells could be achieved. The micro–PCMs made with BMA–co– MAA, BMA–co–BMA–co–MAA, BMA–co–MAA and BMA–co–AA had melting points of 20.9, 20.6, 21.6, 20.8 ℃ and crystallization points of 21.7, 19.9, 21.9, 22.1 ℃ respectively. In contrast, the pure n–octadecane melts at 25.5 ℃ and crystallizes at 23.7 ℃. It is obvious that the

160.1 kJ/kg, and the experimental results showed that the fusion heat of PSD micro–PCM was higher than the calculated value. With all samples taken into account, PSD micro–PCM will have good prospect in many fields. Paraffin was used as core material, isophorone diisocyanate (IPDI) and ethylene diamine (EDA) were employed as the shell monomer [93]. Cyclohexane (CHX) was used as an assistant reagent. Sodium dodecylbenzenesulfonate (SDBS), polyethylene glycol sorbitan monostearate and polyoxyethylene nonyl phenyl ether (OP–10) were used as the emulsifiers. Petroleum ether, anhydrous ethanol and acetone were used as extracting solvent. Micro–PCM was prepared by interfacial polycondensation. The procedure was as follows: 2.2g IPDI, 5.7g paraffin and 6 ml CHX were required to form oil solution in a flask. After being stirred for a few minutes, the oil solution was dropped into 70 ml emulsifier aqueous solution at room temperature, the oil–in–water (O/ W) emulsion was stirred for 10 min at a constant rate. EDA was diluted in 15 ml distilled water, then the solution was dropped into paraffin with the stirring rate at 400 rpm (round per minute). The optimal mass ratio of IPDI/EDA was 3.4:1, which corresponded with theoretical value. The mixture was heated to 60 ℃ and was stirred continuously. After 4 h, the suspension was cooled down to room temperature. Finally, filtering and washing were required. After being dried for 24 h in a vacuum oven, the micro–PCM was successfully produced. Three types of micro–PCM were examined with SDBS, OP–10 and Tween60 as the emulsifier. Fig. 11 shows the morphological images respectively. It is clearly shown in Fig. 11a that the sample made with SDBS does not exhibit a spherical structure. Coarse surface can be seen from Fig. 11b, 138

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Fig. 12. SEM images of composite PCMs with different particles (a) BMA–co–BA, (b) BMA–co–BA–co–MAA, (c) BMA–co–MAA and (d) BMA–co–AA [94]. Table 4 Comparison of the thermal conductivity enhancement with different support materials. PCM

Thermal conductivity of the PCM, kp (W/m K)

Support material

Thermal conductivity of support material, k f (W/m K)

Thermal conductivity of composite PCM, keff (W/m K)

Percentage of improvement

Erythritol [59] Paraffin [57]

0.733 0.305

Bakelite [65] Octadecanoic acid (OA) [64]

1.4 0.184

Nickel foam Nickel foam Copper foam Graphite powder Graphene aerogel (GA)

90.3 91.4 398 130 2.183

1.2 1.2 4.9 4.84 2.635

163% 393% 1607% 346% 1432%

monomer and C18. Finally, The residue was dried in the oven at 50 ℃. The micro–PCMs presented a spherical structure, but the surface showed some concaves and wrinkles which was in consonance with previous reports [98–100]. Besides, the situation of concaves was more severe on micro–PCMs with DVB as shell monomer than that of micro–PCMs with AMA. The micro–C18 with diameter of 1.60– 1.68 µm had a low supercooling degree.

melting point and crystallization point of the pure n–octadecane are higher than that of micro–PCMs, hence the BMA–based copolymer will receive desirable applications in buildings for its ideal melting points at 19–26 ℃ which are close to normal temperatures. The micro–PCMs with BMA–co–MAA are superior to those made with BMA–co–AA in aspects of heat capacity and thermal stability. There was almost no difference in phase change temperatures and phase change enthalpies after all samples being cycled for 1000 times. Based on researches involving micro–PCM with a low supercooling degree, Tang et al. [97] developed a novel microencapsulated n– octadecane (micro–C18) using n–octadecyl methacrylate (ODMA) and methacrylic acid (MAA) as shell monomer by suspension polymerization. The n–octadecane (C18) was used as the core. The ODMA and MAA were used as shell monomer. Benzoyl peroxide (BPO) was used as the initiator. Divinylbenzene (DVB) and allyl methacrylate (AMA) acted as cross–linking agents. Sodium salt emulsion of styrene– maleic anhydride copolymer (SMA) was applied as surfactants. The ODMA, MAA, C18 and BPO were mixed with SMA, and then the mixture was added to 100g deionized water with a stirring rate of 9000 rpm for few minutes. The emulsion was put into a non–oxygen reactor with a stirring rate of 400 rpm at temperature of 85 ℃ for 5 h in nitrogen. Repeatedly washing was necessary to remove the unreacted

2.4. Comparisons of the PCMs with different structures Fiber PCMs have been used for their strengths in small diameter, longer length and large surface. The diameter of the fiber is affected by the electrical conductivity to a certain degree. The specific surface of the fiber can absorb plenty of PCM. Generally, nanoparticles have a significant impact on nanofiber PCM in improving heat transfer and thermal storage capacity. Electrospinning is a simple technique in fabricating ultrafine fibers. The properties of composite PCM are hardly influenced even after repeated storage/retrieval cycles. This indicates that the composites have a long life length on account of its thermal stability. While the thermal conductivity of the PCM will increase, the latent heat of the PCM will decrease if too much mass fraction nanomaterial is added, which indicates that effective improvement in 139

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Table 5 Applications of the PCMs. Application fields of the PCMs

Content

Reference

Solar energy

solar air heating system solar water heater photovoltaic thermal system

[101] [102–105] [106–108]

Buildings

building components passive and active technology seasonal thermal systems thermal glass systems microcapsules in thermal systems nanocomposites in thermal systems

[109,110,116] [111] [112] [114,115] [113,117]

cooling technology in buildings solar cooling evaporative cooling passive cooling methods thermoelectric refrigeration systems

[120], [123] [121] [122] [119,124,125] [126,127]

Heating

hot water systems heat pump solar heating systems heat exchanger material

[128,129,132,133] [130] [131] [134]

Thermal protection of food Medical uses Spacecraft thermal systems

superchilling of food food processing and production drug delivery spacecraft thermal control

[135,137,138,140] [136,139] [141,142] [143]

Solar power plants

internal combustion engines heat transfer optimization concentrated solar thermal power

[144] [145,146] [147]

Cooling

[118]

Fig. 14. The structure of the floor [156].

Fig. 15. The front image of the roof and its boundary conditions [159].

structure contribute to the thermal conductivity, which makes porous PCM attractive in thermal energy storage systems. Table 4 shows the thermal conductivity enhancement with different support materials. Heat transfer area is considerably increased in composite PCM with sphere structure. High reaction speed and lower penetrability are guaranteed by the core–shell structure. The material applied in the shell is crucial in controlling the morphology of the surface, thermal storage capacity and thermal conductivity. The ideal shell material can offer good protection for the core, thus the leakage can be prevented. The thermal stability is increased and the thickness of the shell is thickened when the content of the monomer rises. The higher mechanical strength of shell, the higher pressure can be endured on external surface.

Fig. 13. Classification of the PCMs in buildings.

thermal conductivity of the composite PCM relies heavily on an optimal amount of additive nanomaterial. The phenomenon of leakage can also be lowered by the usage of carbon fiber. The heat conduction rate is enhanced in pace with the increase of the temperature. Impregnating PCM into porous material is a promising method to improve the effective thermal conductivity of the composite PCM. Porous structure is capable of enhancing heat transfer rate and adapting to high temperature applications. The capillary and the surface tension form the stable shape of the composite PCM, thus the leakage is prevented. Additional thermal conduction paths are provided attributing to the good mechanical and thermo–physical properties of porous PCM, consequently, every portion of the PCM is heated and cooled uniformly. The high strength, stiffness and lasting skeleton

3. The applications of the PCMs Table 5 displays a set of applications of the PCMs. Since the PCMs usually turn out to be almost thermostatic during phase change process, they can be used to control the temperature of the systems. Using PCM to store energy is an effective method to improve the efficiency of energy utilization and benefit environmental protection. In addition, PCM helps in smoothening of fluctuations in electricity's 140

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containing composite, the experiment lasted one year long. Temperature profiles of the room air with the PCM wallboards and regular ones were recorded respectively in summer, mid–season and winter. Conclusions were made that PCM wallboards outstripped regular ones in reducing air temperature, the decrement factor of the air temperature amplitude varied between 0.73 and 0.78. The temperature of the room was decreased by 4.2 ℃ to the maximum extent. 3.1.2. Floors Recently, efforts have been done to develop PCM in radiant floor heating system, which has advantages like better use of place and not requiring cleaning at all. The system does help to get rid of noise and ducts. A uniform temperature distribution is also provided [154]. Lin et al. [155] set up a prototype room to testify its thermal performance and feasibility of under–floor electric heating system with shape–stabilized PCM plates. Attributing to shift in electric heating requirement from the peak period to the off–peak period, economic benefits were obtained. The indoor temperature could be raised effectively by the system during work hours. Besides, the system was examined to be capable of applications in different climate regions. The study carried out by Jin et al. [156] was aimed to research the influence of the thermal resistance of pipe as well as water velocity on the performance of the radiant floor cooling system. The experimental house with a physical model of the floor was located in Nanjing. The structure of the floor is shown in Fig. 14. When the thermal conductivity of pipe was low, the pipe had impact on the performance of the radiant floor cooling system. Although the flow was laminar, the water velocity affected radiant floor cooling system. Reducing water velocity made contributions to save energy.

Fig. 16. A typical heat recovery system [164].

demand and production. 3.1. PCM in buildings PCM has good qualities like delaying action period, smoothing out indoor and outdoor temperature fluctuations. Thus it improves the comfort and saves energy. Fig. 13 shows some applications for the PCMs in buildings, Rodriguez–Ubinas et al. also made classification of the PCM in buildings [148]. Zhu et al. [149], Zhang et al. [150] and Pasupathy et al. [151] presented reviews on high performance of the PCMs in building applications.

3.1.3. Concrete Ling and Poon [157] pointed out that adding PCM into concrete led to lower fire resistance, lower strength and uncertain long–term stability. Cabeza et al. [158] devoted themselves into studying a new innovative PCM concrete to develop a product capable of attaining effective energy savings in buildings. They found that concrete with PCM improved thermal inertia and lowered indoor temperatures in warm seasons. An idea of building concrete roof with vertical cylindrical holes filled with PCM was proposed by Hashem et al. [159]. Fig. 15 shows the front image of the roof and its boundary conditions. They intended to employ PCM into the concrete roof to add the value of thermal mass of the roof. The research results indicated that larger the diameter of the PCM holes, more the reduction in heat. Furthermore, the heat gain was affected by the melting temperature of the PCM to a great extent.

3.1.1. Wallboards Wallboards refer to those walls containing PCMs, which have high latent heat, stable chemical properties, non–leakage, long–term cycling, incorporation and compatibility with building materials. Biswas et al. [152] investigated numerical investigations on nano–PCM wallboards, and they used COMSOL to test simulations. With massive data worked out, the model gained validation. Kuznika and Virgoneb [153] used a kind of PCM that could absorb optical ray by 77% with 1 cm thickness to analyze its comparative thermal performances. For the purpose of finding differences between wallboards with and without

Fig. 17. The structure of (1) air energy–flow and (2) energy recovery ventilator's schematic diagram [160].

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Fig. 18. The solar thermal systems: (a) PCM integrated into the tank, (b) PCM node between the collector and the tank and (c) PCM integrated into the solar collector [173].

3.2. PCM in heat recovery

4. Conclusions and outlook

As a result of global warming and environment issue, demand for energy saving technologies increases. This directly results in researchers’ interest in investigations of heat recovery systems. In the year 2000, the heat recovery demand in Germany increased by 60% [160]. Confronted with the rise of urbanization and industrialization in China, heat recovery systems are used to decrease energy consumption [161]. The system not only offers energy saving, but also reduces heat loss instead of generating heat [162]. The recovery rate of the heat in exhaust air reaches to 60–95%, which makes the energy efficiency of buildings significantly improve [163]. A typical heat recovery system is shown in Fig. 16 [164]. The system comprises of ducts and a heat exchanger. In warm seasons, the system lowered the enthalpy of buildings and raised it in cold seasons via transferring heat between ventilation air supplement and exhaust air streams. Based on a new energy simulation program named EnergyPlus, Zhou et al. [160] presented a model shown in Fig. 17. Fig. 17 (1) represents the structure and air energy–flow and Fig. 17 (2) represents energy recovery ventilator's schematic diagram. Conclusions were made that heat recovery devices could be used in passive and mechanical ventilation. Cuce et al. [165] viewed heat recovery system as one of the most promising means to relieve fuel consumption in buildings, which should gain attention worldwide for its potential in energy saving and bringing down concentration of greenhouse gases in the atmosphere.

Three common morphologies of composite PCM are reviewed in this paper, namely fibrous structure, porosity and sphere. The morphologies together with their thermal properties are introduced. The PCM fiber presents a cylinder shape that constructs a valid foundation for heat transfer. The nanofiber and carbon fiber are utilized for their high thermal properties. The virtue of porous composite PCM resides in its skeleton structure with capillarity. Metal foam composite PCM and carbon material PCM are frequently used types. Composite PCM with spherical structure is competitive with enlarged surface, and the PCM in the core is protected from damage. Hence micro–PCM has got tremendous attention and research is in progress to improve its thermal performance. Applications of the PCMs in aspects like buildings, heat recovery and solar energy are listed as well. More assessments and experiments are required with economic and environment conditions taken into consideration. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant no. 51376087, 51676095) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also wish to thank the reviewers and editor for kindly giving revising suggestions. References [1] Li LP, Wang G, Guo CG. Influence of intumescent flame retardant on thermal and flame retardancy of eutectic mixed paraffin/polypropylene form–stable phase change materials. Appl Energy 2016;162:428–34. [2] Esen M, Esen H. Experimental investigation of a two–phase closed thermosyphon solar water heater. Sol Energy 2005;79:459–68. [3] Esen M, Yuksel T. Experimental evaluation of using various renewable energy sources for heating a greenhouse. Energy Build 2013;65:340–51. [4] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energy Rev 2009;13:318–45. [5] Zalba B, Marin JM, Cabeza LF. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251–83. [6] Giro–Paloma J, Martínez M, Cabeza LF, Fernández AI. Types, methods, techniques and applications for microencapsulated phase change materials (MPCM): a review. Renew Sustain Energy Rev 2016;53:1059–75. [7] Khudhair AM, Farid MM, Zalba B. A review on energy conservation in buildingapplications with thermal storage by latent heat using phase change materials. Energy Convers Manag 2004;45:263–75. [8] Kim DW, Jung JY, Kim YN, Lee MJ, Seo JC, Khan SB. Structure and thermal properties of octadecane/expanded graphite composites as shape–stabilized phase change materials. Int J Heat Mass Transf 2016;95:735–41. [9] Papadopoulos AM, Oxizidis S, Kyriakis N. Perspectives of solar cooling in view of the developments in the air-conditioning sector. Renew Sustain Energy Rev 2003;7:419–38. [10] Farid MM, Khudhair AM, Razack SAK. A review on phase change energy storage: materials and applications. Energy Convers Manag 2004;45:1597–615. [11] Naphon P, Wongwises S. A review of flow and heat transfer characteristics in curved tubes. Renew Sustain Energy Rev 2006;10:463–90. [12] Smyth M, Eames PC, Norton B. Integrated collector storage solar water heaters. Renew Sustain Energy Rev 2006;10:503–38.

3.3. PCM in solar energy PCM is intensively used in solar energy fields as it has good properties like high thermal capacity and constant temperature during phase change process [166]. Solar energy can be applied to solar water heating, solar photovoltaic and solar lighting [167–169]. Sensible heat storage material has smaller thermal energy storage density and large heating loss [170], latent heat storage becomes an efficient way to store heat by using PCM for its large heat capacity. There are three kinds of solar thermal systems. As shown in Fig. 18a, it was the first kind of collectors, and the PCM was used to store solar energy. Most collectors replaced conventional tanks with PCM–integrated ones, thus reduced costs and saved space. A new system was put forward (shown in Fig. 18b), the configuration was called solar domestic hot water (SDHW) including a PCM node in the heat transfer fluid (HTF) primary solar loop [171,172], the node was between the collector and the storage tank. Compared with the traditional water–based solar thermal system, the configuration produced an increase in solar fraction with 8% in summer and 4% in winter. Sodium thiosulfate pentahydrate was used as PCM in the tank by Canbazoglu et al. [173], shown in Fig. 18c, the experimental results indicated that the charging rate of this kind of solar thermal system was nearly 2.59–3.45 times faster than that of conventional ones. 142

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