Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage

Renewable and Sustainable Energy Reviews 82 (2018) 2730–2742

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Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage Yaxue Lin, Yuting Jia, Guruprasad Alva, Guiyin Fang

T

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School of Physics, Nanjing University, Nanjing 210093, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Thermal energy storage Phase change materials (PCMs) Thermal conductivity enhancement Thermal properties PCMs applications

In recent years, energy conservation and environmental protection have become most important issues for humanity. Phase change materials (PCMs) for thermal energy storage can solve the issues of energy and environment to a certain extent, as PCMs can increase the efficiency and sustainability of energy. PCMs possess large latent heat, and they store and release energy at a constant temperature during the phase change process. Thereby PCMs have gained a wide range of applications in various fields, such as buildings, solar energy systems, power systems and military industry. However, low thermal conductivity of PCMs leads to low heat transfer rate, thus, numerous studies have been carried out to improve thermal conductivity of PCMs. The main purpose of this paper is to review the methods for enhancing thermal conductivity of PCMs, which include adding additives with high thermal conductivity and encapsulating phase change materials. It is found that addition of thermal conductivity enhancement fillers is a more effective method to improve thermal conductivity of PCMs, where carbon-based material additives possess a more promising application prospect. Finally, the applications of PCMs in solar energy system, buildings, cooling system, textiles and heat recovery system are also analyzed.

1. Introduction There are many forms of energy in nature. Among these forms, thermal energy is extensively distributed in solar radiation, geothermal energy, etc. Thermal energy is regarded as a low–grade type of energy and treated as waste in industrial production in general [1]. On the other hand, solar radiation continues to supply abundant solar energy during day time. However, large quantity of energy is often wasted. If large amount of thermal energy can be stored and released when it is supplied and demanded, fossil fuels consumption will be reduced, which plays an important role in overcoming the troubles of energy crisis and environmental pollution. Hence, thermal energy storage has been gaining great attention and undergoing rapid development. There are three types of thermal energy storage: latent heat, sensible heat and reversible thermochemical reaction [2]. Among different types of thermal energy storage, latent heat storage type plays a vital role. Particularly, phase change materials (PCMs) absorb or release heat from the environment by changes in phase or structure, so as to realize the storage and release of thermal energy. Some studies [3–5] have pointed out that the advantages of PCMs are high heat storage density, huge latent heat storage capacity, low cost, excellent chemical stability, etc. PCMs have a wide range applications due to continuous research, such as industrial waste heat recovery, comfort applications in

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buildings, electric peak–shaving, solar energy systems, etc. [6–8]. In addition, PCMs have a prominent feature that the temperature nearly remains constant during phase change process, which can be used in temperature control system [9]. There are many kinds of PCMs and they can be classified according to different criteria. According to the state of substances before and after phase change, PCMs can be divided into solid–solid PCMs, solid–liquid PCMs, solid–gas PCMs and liquid–gas PCMs. At present, the most commonly used is solid–liquid PCMs, because of their high latent heat capacity and low volume change during phase change process as compared with the others [3,9]. Temperature of solid–liquid PCMs rises until the temperature reaches phase change temperature, then PCMs absorb massive heat as latent heat storage while phase change occurs from solid to liquid [10]. On the basis of their chemical nature, they are classified into organic PCMs, inorganic PCMs and eutectic PCMs. Organic PCMs can be further divided into paraffin and non–paraffin [11], where non-paraffin contains fatty acid, polybasic alcohol etc. Noncorrosive, non-toxic, congruent melting, chemical stability, almost no supercooling and so on are the advantages of the organic PCMs [5,11,12]. Inorganic PCMs, which commonly refer to water, hydrated salt, molten salt and metal or alloy [13], possess the merits of high latent heat per unit mass, non-flammable and low cost with the same volume as compared with organic PCMs. Eutectics are two or more

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

http://dx.doi.org/10.1016/j.rser.2017.10.002 Received 29 May 2017; Received in revised form 5 September 2017; Accepted 2 October 2017 Available online 09 October 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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Fig. 1. The classification of PCMs.

used in solar absorption refrigeration systems, Kenisarin and Mahkamov [22] reviewed PCMs employed in residential buildings with passive thermal control. As PCMs can maintain constant temperature and store (or release) massive latent heat during the phase transition process, they are widely employed in the field of solar energy system, buildings, cooling system, textile and heat recovery, and the applications with PCMs in recent years will be summarized in this paper.

soluble ingredients mixed together, which have the feature of simultaneous melting and solidification without material's separation [14]. Fig. 1 shows the classification of PCMs. Not all PCMs can be used in practical application. The selection of PCMs has the following requirements [9,15–17]: (1) Phase transition temperature is within the range of practical operation. (2) High latent heat storage capacity. (3) High thermal conductivity. (4) Stable chemical and thermal properties. (5) Non–toxic, non–corrosive and harmless to environment. (6) Low cost and easily available. (7) Small volume change. (8) No or little supercooling. No PCMs can fully meet the above requirements. Always there are few defects, like supercooling, phase separation, low heat transfer rate, leakage in the molten state, instability of performance [8,18,19]. However except metallic-based PCMs, all other kinds of pure PCMs have a common defect of low thermal conductivity. Organic PCMs have the lowest thermal conductivity and thermal conductivity of inorganic PCMs is slightly higher than that of organic PCMs [20]. Hence, improving thermal conductivity is one of the focuses in the field of PCMs. This paper reviews the reported studies in recent years about thermal conductivity enhancement of PCMs by adding fillers with high thermal conductivity and encapsulating PCMs, where the fillers mainly include carbon-based materials and metal-based materials. Fig. 2 shows the approaches of thermal conductivity enhancement. Furthermore, the advantages and disadvantages of each method for improving the thermal conductivity of PCMs are summarized and compared, with the aim of providing the reader with a relatively systematic and detailed awareness of them. PCMs are applied in various fields, Khan et al. [21] reviewed PCMs

2. Thermal conductivity enhancement and thermal properties of phase change materials in thermal energy storage Thermal conductivity enhancement can increase the rate of charging and discharging heat, thereby improving the efficiency of thermal energy storage systems [23]. The ways of enhancing thermal conductivity are roughly divided into two types: adding substances with high thermal conductivity and encapsulated phase change materials, which will be discussed in the following sections.

2.1. Adding substances with high thermal conductivity As we know, pure PCMs suffer from deficiency of low thermal conductivity. Therefore, there are numerous researches about adding additives with high thermal conductivity to improve thermal conductivity of PCMs. This review paper focuses on the additives of carbon–based materials and metal-based materials, and their thermal conductivity parameters are listed and compared with each other. 2731

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Fig. 2. The approaches of thermal conductivity enhancement.

compression density of 1.83 g/cm3 has a thermal conductivity of 7.31 W/m. K, which increased by approximately 12 times compared with the thermal conductivity of pure D-Mannitol (0.60 W/m. K). Tang et al. [27] blended together the palmitic acid and capric acid to formulate eutectic mixtures as PCM, and utilized the diatomite as supporting material, further more added EG to improve the thermal conductivity of composite PCM. The results showed that the thermal conductivity of the composite PCM without EG is 0.119 W/m. K in freezing state and 0.190 W/m. K in melting state. The thermal conductivity of the composite PCM with 3 wt% and 5 wt% EG was improved by 15.1% and 25.2% in freezing state, and the thermal conductivity of the composite PCM with 3 wt% and 5 wt% EG was improved by 26.3% and 53.7% in the melting state. As the latent heat of the composite PCM reduces slightly with the increase in mass fraction of EG, the composite PCM with 5 wt% EG is selected as the optimal one. Carbon fiber (CF) is an inorganic fibrous carbon compound possessing thermal conductivity of 900 W/m. K along the in-plane direction [28]. Besides, features like excellent corrosion resistance and chemical attack resistance etc., make CF compatible with a great variety of PCMs. Nomura et al. [29] used two methods, routine melt–dispersion and novel hot-press, to prepare a good thermally conductive composite PCM and utilized the laser flash method to measure the effective thermal conductivity of the composite PCM. Erythritol and CF were PCM and thermal conductivity enhancement additive, respectively. Experimental results indicated that the effective thermal conductivity of composite PCM prepared by novel hot-press method is better than that of composite PCM prepared by routine melt-dispersion method, what is more, the novel hot-press method need less additive as compared with melt-dispersion method. The measurement results

2.1.1. Carbon-based materials Carbon based materials are one of the most popular additives due to their high thermal conductivity, stable chemical nature, extensive usability and low density [24]. They have been extensively studied and applied. Carbon based materials have a variety of morphological structure, such as expended graphite (EG), carbon fiber (CF), graphene, carbon nanotube (CNT) [4]. Expended graphite (EG) possesses a porous structure. Ling et al. [25] used RT44HC and EG to formulate RT44HC/EG composite, where RT44HC as organic PCM has phase transition temperature at 44 °C and good latent heat capacity (above 200 kJ/kg). The composites were formulated with 25% and 35% content of EG (mass fraction), respectively. Experimental results revealed that the factors affecting thermal conductivity include the mass fraction of EG and packing density of composites. For the same density, the thermal conductivity of 35% EG loading in composites is 30% greater comparing with 25% EG loading. On the other hand, for the same mass fraction, the thermal conductivity of composites increases with density, and is 20–60 times the thermal conductivity of pure RT44HC corresponding to a lower density. Likewise, they created a model with the function that can forecast the effective thermal conductivity of EG based compound material according to packing density and EG mass fraction at room temperature. Dmannitol (organic material)/EG composite PCM was manufactured by Xu et al. [26], which is dedicated to applications like solar heat storage system or waste heat recovery system. EG plays two significant roles in composite PCM. It is used as supporting material to prevent D-mannitol from leaking and is also used as an additive to enhance the thermal conductivity of composite PCM. It indicated that the D-mannitol/EG composite PCM with a loading of 15 wt% EG and with a best 2732

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aerogels composite PCM, which increases the thermal conductivity from 0.31 W/m. K of pure polyethylene to 1.43 W/m. K of the composite PCM, and the enhancement of thermal conductivity is about 361%. Mehrali et al. [39] used nitrogen–doped graphene as additive to mix with palmitic acid, and found that the thermal conductivity improvement is over 500% at 35 °C when the mass fraction of nitrogen–doped graphene is 5%. Carbon nanotubes (CNTs) have the merits of high thermal conductivity, low density and large surface area–to–volume ratios, meanwhile they are composed of carbon atoms and the density is close to the density of organic matter, which is easy to form stable mixture with organic–based matrix [40,41]. At present, CNTs include two primary kinds, which are single–walled carbon nanotubes (SWCNTs) and multi–walled carbon nanotubes (MWCNTs) [42,43], where MWCNT is relatively more frequently used. Ye et al. [44] used Na2CO3, MgO and MWCNTs as raw materials of the composite PCMs, where Na2CO3 and MgO were treated as PCM and supporting material, respectively. A series of composite PCMs were prepared with the mass fraction of MWCNTs of 0.1%, 0.2%, 0.3% and 0.5%, and their thermal conductivity were determined by the thermal property analyzer. It reveals that the thermal conductivity increases with the increase of the mass fraction of MWCNTs, and that thermal conductivity also increases with the increase in test temperature. From the experimental values, the thermal conductivity of composite PCM with 0.5 wt% MWCNTs increases by 69% compared with the composite PCM without MWCNTs at 120 °C. There is a similar case, Xu and Li [45] also utilized MWCTs as additive in order to enhance thermal conductivity. Compared to paraffin/diatomite mixtures, the thermal conductivity of paraffin/diatomite/MWCNTs composite PCM is greatly enhanced by MWCNTs, which improves by 42.45%. Tao et al. [43] utilized four types of carbon nanomaterials as fillers in order to improve the properties of high temperature salt PCM, where carbon nanomaterials were SWCNT, MWCNT, graphene and C60. The ability to improve thermal conductivity is sorted from large to small, which in descending order is SWCNT, MWCNT and grapheme. However, the thermal conductivity of composite PCM with C60 decreases. It reveals that the columnar structure facilitates the effective connection of heat conduction route, hence SWCNT and MWCNT are more beneficial on thermal conductivity improvement. For SWCNT and MWCNT, the thermal conductivity of the composite PCM with the loading of 1.5 wt% improved by approximately 56.98% and 50.05%, respectively. Besides, other types of CNTs are researched, such as grafted CNTs. Li et al. [46] grated the acidified CNTs with three kinds of polyalcohol, then obtained three kinds of grated CNTs with octanol, tetradecyl alcohol and stearyl alcohol respectively, and the corresponding graft ratio is 11%, 32% and 38%. It is found that CNTs grafted with polyhydric alcohols possess greater dispersibility than pristine CNTs. The grafted CNTs are more beneficial for improving the thermal conductivity of paraffin than pristine CNTs, and among three kinds of grafted CNTs, CNTs grafted with stearyl alcohol for the thermal conductivity enhancement of paraffin is the best. Li et al. [47] grafted CNTs with stearyl alcohol for improving the thermal conductivity of microcapsule PCMs. It indicated that the grated CNTs possess great dispersibility and heat conductivity in microcapsule PCMs. The thermal conductivity of microcapsule PCM with 4 wt% CNTs increased by 79.2%. Xiao et al. [48] prepared CNTs, oxidized CNTs and grafted CNTs as fillers to add into palmitic acid (PA), where the grafted CNTs was grafted on the γ –propyltrimethoxysilane on the basis of oxidized CNTs. The thermal conductivity of composite PCMs with CNTs, oxidized CNTs and grafted CNTs increase by 36.4%, 39.3% and 34.1% respectively compared with that of PA. Nevertheless, it should be pointed out that the latent heat of PA/grafted CNTs is higher than that of PA, while the latent heat of PA/CNTs and PA/oxidized CNTs are reduced compared with that of PA. In addition to the carbon–based additives mentioned above, there are some other forms of carbon–based additives. Nano–graphite (NG) was used as filler to enhance the thermal conductivity of paraffin by Li

showed that the effective thermal conductivity of erythritol is 0.73 W/ m. K, and the thermal conductivity of composite PCM with approximately 25 vol% CF has a tremendous enhancement, which is approximately 30 W/m.K. In other words, the composite PCM with CF additives for improving thermal conductivity is effective. Zhang et al. [30] chose two types of short carbon fibers (SCFs) with diverse lengths as fillers and erythritol as PCM, and prepared a series of composite PCMs with different mass fraction of two types of SCFs, respectively. As the aspect ratio of longer SCF is 25 and that of another one is 5, for the convenience of narration, longer SCF and another one are named C25 and C5. X–ray diffractometer results showed that both types of SCFs have almost the same structure. Furthermore, the thermal conductivity of composite PCMs can be promoted by SCFs, and the C25 for thermal conductivity enhancement is better than that of C5, which is proved by experimental result: 3.91 W/m. K of 10 wt% C25 and 2.46 W/m. K of 10 wt% C5. It indicated that the geometry of SCFs have an effect on the thermal conductivity, which have been explained by thermal resistance theory and percolation theory in detail. Tian et al. [31] used EG and CF with different mass ratio as thermal conductivity enhancement fillers. The fillers were added to the form–stable phase change materials, where paraffin was used as PCM and ethylene–vinyl acetate (EVA) was used as supporting material. According to experimental results, the application of EG and CF has a synergistic effect on thermal conductivity improvement, and the thermal conductivity of longitudinal orientation is lower compared to horizontal orientation. Graphene with a single layer two-dimensional structure has gained great attention because of its distinctive chemical and physical nature, large aspect ratio, and outstanding thermal conductivity [32,33]. Mehrali et al. [34] used impregnation method to prepare palmitic acid (PA)/graphene nanoplatelets (GNPs) form–stable PCM, where three kinds of GNPs with specific surface areas of 300, 500 and 750 m2/g were added to composite PCM samples S1,S2 and S3, respectively. In this form–stable PCM, GNPs are not only fillers to enhance thermal conductivity but also supporting materials to prevent PCM leakage. It indicated that the amount of PCM absorbed by GNPs is greatly increases with the increase in specific surface area of GNPs. Thermal conductivity of form–stable PCM is much higher than that of pure PA, and that of S1, S2 and S3 are 2.75/2.54, 2.43/2.17 and 2.11/1.84 W/m. K in solid/ molten state respectively, which shows that the smaller specific surface area of GNPs is more effective on thermal conductivity compared to bigger ones. Hence, according to the specific needs of practical applications, suitable specific surface area of GNPs can be selected. Amin et al. [35] prepared beeswax/graphene composite PCM for thermal energy storage in building applications. Experimental results indicated that both thermal conductivity and latent heat are significantly enhanced. Initially the thermal conductivity of composite PCM rises as the mass fraction of graphene rises, which reveals that the thermal conductivity increases linearly with the mass fraction of graphene. Nevertheless, after reaching an optimum value if the filler is added further, because of the agglomeration of nanoparticles, the thermal conductivity does not increase linearly. The value of thermal conductivity of the composite PCM with 0.3 wt% graphene is 2.89 W/m. K, which is about 11 times that of pure beeswax. Liu and Rao [36] compared the ability of thermal conductivity enhancement of graphene with that of exfoliated graphite sheet by preparing paraffin/graphene and paraffin/exfoliated graphite sheet composite PCMs. According to the experimental results, as compared with paraffin, the thermal conductivity of the composite PCM with graphene of 2.0 wt% increases by 58.6% and that of with exfoliated graphite sheet of 2.0 wt% increases by 41.4%. Thereby, it illustrates that graphene in thermal conductivity enhancement is more effective compared with exfoliated graphite sheet. In addition, other kinds of graphene used as thermal conductivity enhancement additives are researched, too. Spongy graphene with a low concentration was added into docosane by Li et al. [37], and it indicated that both latent heat and thermal conductivity are promoted greatly. Yang et al. [38] synthesized polyethylene/hybrid graphene 2733

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icosane wax/copper foam composite PCM is 3.78 W/m. K, which is much higher than that of pure icosane wax. Chen et al. [56] used aluminum foams to increase heat transfer of paraffin wax. They studied the temperature field and melting evolution to obtain a conclusion that aluminum foams are able to improve heat transfer of solid–liquid PCM. Furthermore, they found that influence of the metal foam structure on the heat transfer in the molten state is particularly pronounced. Likewise, Wang et al. [57] also proved that aluminum foams are able to enhance the heat transfer rate of paraffin by experimental study. Metal particles are a type of metal additives which are commonly used to promote the thermal conductivity of PCMs. Ghossein et al. [58] took three kinds of solidification approaches (ice–water bath, room temperature and oven solidification) to prepare three types of eicosane/ silver nanoparticles composite PCMs with various mass fraction of silver nanoparticles. Thermal conductivities of composite PCMs are all increased, regardless of the solidification approaches, where oven solidification approach showed the highest improvement and the ice–water bath approach showed the lowest improvement. Likewise, the thermal conductivity increases as the temperature rises, and when approaches the melting temperature, a sharp increase in thermal conductivity appears. On the other hand, when the loading of silver nanoparticles is more than 2 wt%, thermal conductivity of composite PCM would firstly decrease and then rise until the loading of silver is increased to 10 wt% achieving the highest value. For the same loading of additives (10 wt%), thermal conductivity of composite PCMs obtained by three solidification approaches is 0.8319, 0.8534 and 0.8754 W/m. K, respectively. Oya et al. [59] observed the SEM–EDS pictures of erythritol/nickel particles (NPs) composite PCM with the loading of NPs of 14 vol%, 24 vol% and 34 vol%. It was found that 14 vol% NPs produce a few clusters and do not spread throughout the composite PCM, while 24 vol% NPs produce a lot of clusters, 34 vol% NPs produce bigger clusters and percolation clusters, which revealed that the formation of percolation clusters is attributed to thermal conductivity enhancement of the composite PCM. As the loading of NPs is 17 vol%, thermal conductivity of composite PCM is 290% higher than that of pure erythritol, whose values were similar to that of spherical graphite. There are other metal particles used as additives, like copper particles. Cui et al. [60] found that the content of copper nanoparticles of 0.5 wt % added into sodium acetate trihydrate would increase the thermal conductivity from 0.777 to 0.936 W/m. K, which is increased by 20.5%. Meanwhile, the heat transfer rate was improved by 20%. It is known that metals are easily oxidized into metal oxides. Thermal conductivity of metal oxide is lower than that of metal, but much higher than that of most PCMs, thereby, metal oxides are also used as thermal conductivity enhancement additives. Sahan et al. [61] formulated paraffin–nanomagnetite composites consisted of paraffin and Fe3O4 by a dispersion technique. The thermal conductivity was measured by “DICO” approach, whose results indicated that the thermal conductivity of paraffin–nanomagnetite composites with 10 wt % and 20 wt% Fe3O4 content is improved by 48% and 60%, respectively. SiO2, Al2O3, Fe2O3, ZnO and their mixtures were incorporated into paraffin by direct–synthesis approach [62] to enhance thermal conductivity of PCM. All kinds of additives are able to enhance thermal conductivity of paraffin. Thermal conductivity can be improved significantly when the content of additives increases. For the loading of 2 wt%, the composite PCM with the mixtures of additives showed the highest thermal conductivity (0.724 W/m. K). For the loading of 4 and 6 wt%, the composite PCM with Al2O3 showed the greatest thermal conductivity (0.919 W/m. K). And for the loading of 8 wt%, the composite PCM with Fe2O3 in thermal conductivity is slightly higher than that with Al2O3. It can be said that Al2O3 is the optimal nanoparticle in this research. Other studies on Al2O3 are listed here: Al2O3 were added into paraffin (10 wt%), lauric–myristic–stearic acid eutectics/expanded vermiculite composites, PEG/SiO2 (12.6 wt%) and myristic acid/high density polyethylene composites, and they possess thermal conductivity of 0.259, 0.671, 0.435 and 0.3972 W/m. K, respectively, which is

[49]. The experimental results showed that NG is randomly dispersed in paraffin and thermal conductivity of composite PCM increases with the increase of the NG loading. The NG (10 wt%) promotes thermal conductivity of paraffin from 0.1264 W/m. K to 0.9362 W/m. K, and the latter is 7.41 times the former. Seki et al. [50] formulated binary eutectic mixture containing adipic acid and sebacic acid as PCM, and prepared the eutectics/graphite nanoplates composite PCM. It is found that supercooling degree of eutectics can be effectively reduced and the thermal conductivity of eutectics can be improved by graphite nanoplates. When the content of graphite nanoplates is 0.5 wt%, the thermal conductivity of composite PCM is 0.131 W/m. K, which increases by 19% compared with eutectics. Wang et al. [51] dispersed graphite nanoparticles into stable OP10E/water emulsion in order to remove supercooling degree and enhance thermal conductivity. The results indicated that supercooling is almost eliminated and the thermal conductivity is improved by 88.9% in graphite nanoparticles–dispersed OP10E/water emulsions with the graphite loading of 2 wt%, compared with emulsions without graphite. In summary, carbon based materials as additives possess the advantages of high thermal conductivity, stable chemical property and low density, etc. There is no doubt that, the high thermal conductivity of additives is beneficial to improve the thermal conductivity of PCMs, however, the most important factor is the aspect ratio, large aspect ratio of additives results in good thermal conductivity enhancement. CFs, graphene and CNTs have large aspect ratio, where CFs possess corrosion resistance and chemical attack resistance, graphene with a special structure of a single layer structure of two–dimension shows distinctive chemical and physical properties. Secondly, geometry of additives also has influence on the thermal conductivity enhancement. What is more, different preparation techniques have influence on thermal conductivity enhancement. The densities of carbon–based additives are low (commonly less than 2.26 g/cm3) [30], which provide a great convenience for the practical applications that have a greater limit to the mass of system. Various carbon–based additives for thermal conductivity enhancement are listed in Table 1. 2.1.2. Metal-based materials It is known that high thermal conductivity is a remarkable characteristic of metals, and they have a strong mixing ability. Thereby, several types of metals are also commonly used as additives for thermal conductivity enhancement of thermal energy storage system. Metal foam with a porous structure is composed of a metal, which includes a great volume fraction of gas–charged pores [52]. Xiao et al. [53] used the way of vacuum impregnation to formulate paraffin/metal foam composite PCM, where metal foam include nickel foam and copper foam. Measurement results revealed that the thermal conductivity of paraffin with nickel foam (1.2 W/m. K) increases approximately three times compared with that of pure paraffin (0.305 W/m. K). And 5PPI (pore per inch) copper foam promotes the thermal conductivity of paraffin from 0.305 W/m. K to 4.9 W/m. K, which improves about 15 times. Xiao et al. [54] continued to study the thermal conductivity enhancement of paraffin/metal foam. They utilized pure paraffin, nickel foam and copper foam with a variety of pore sizes and porosities to formulate composite PCMs. Compared with pure paraffin, thermal conductivity of paraffin/copper foam composite PCM with porosities of 96.95%, 92.31%, 88.89% and pore size of 25PPI increases by about 13, 31 and 44 times, respectively. While nickel foam is weaker on thermal conductivity enhancement than copper foam, they can enhance thermal conductivity of pure paraffin 3, 4 and 5 times corresponding to the foam porosities of 97.45%, 94.24%, 90.61% and pore size of 25 PPI of nickel foam. It can be concluded that reducing the foam porosity leads to an increase in thermal conductivity, and for the same porosity, changing the size of the pore does not have any significant influence on thermal conductivity. Thapa et al. [55] used copper foam as filler and icosane wax as PCM to prepare composite PCM for small–scale thermal energy storage. The thermal conductivity of 2734

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Table 1 Comparison of thermal conductivity with different carbon material additives. PCM

Thermal conductivity of PCM kp (W/m. K)

Carbon material additive

Thermal conductivity of additive ka (W/m. K)

Fraction of additive

Thermal conductivity of composite PCM kc (W/m. K)

Magnification

RT44HC [25]

0.22

Expended graphite

129

25 wt%

–

Expended graphite Expended graphite

– –

15 wt% 5 wt%

7.31 (1.83 g/cm ) 0.149/0.292 (solid/liquid)

Erythritol [29]

0.60 0.119/0.190 (solid/ liquid) 0.73

Carbon fiber

900

30

Erythritol [30]

0.77

900

Palmitic acid [34]

0.29/0.21 (solid/liquid) 0.25

0.3 wt%

2.75/2.54 (solid/liquid) (300 m2/g) 2.89

Paraffin [36] Paraffin [36]

– –

Parallel to surface: 3000 Perpendicular to surface:6 Parallel to surface: 3000 Perpendicular to surface:6 3000–5000 300–400

5 wt%

Beeswax [35]

2.0 wt% 2.0 wt%

0.46 0.41

58.6% 41.4%

N-eicosane [23] Binary carbonate eutectic salts [43] Binary carbonate eutectic salts [43] Binary carbonate eutectic salts [43] Na2CO3/MgO [44] Paraffin [46] Palmitic acid [48] Palmitic acid [48] Palmitic acid [48] Paraffin [49] Eutectic mixture (adipic acid and sebacic acid) [50] OP10E/water emulsions [51]

0.22 –

Short carbon fiber (SCF) Graphene nanoplatelets Graphene nanoplatelets Graphene Exfoliated graphite sheet Cabon nanotube SWCNT

About 25 vol % 10 wt%

20–60 times (depend on its packing density) 12 times 1.25/1.54 times (solid/ liquid) 41times

2000–6000 –

1 wt% 1.5 wt%

0.32 –

1.45 times 56.98%

–

MWCNT

–

1.5 wt%

–

50.05%

–

Graphene

–

1.5 wt%

–

26.11%

0.881 0.2312 0.214 0.214 0.214 0.1264 0.110

MWCNTs Grafted CNTs CNTs Oxidized CNTs Grafted CNTs Nano-graphite Graphite nanoplates

– – – – – – –

0.5 wt% 4 wt% 1 wt% 1 wt% 1 wt% 10 wt% 0.5 wt%

1.489 (120 °C) 0.7903 0.292 0.298 0.287 0.9362 0.131

1.69 3.42 1.36 1.39 1.34 7.41 1.19

0.306

Graphite nanoparticles

–

4 wt%

0.648

2.12 times

D–Mannitol [26] Fatty acid eutectics [27]

3

3.92/2.46 (long SCF/ SCF)

5.1/3.2 times (long SCF/ SCF) 10 times 11.56 times

times times times times times times times

porous structure and large aspect ratio, and metal foam has a smaller density, hence, it has a better prospect employed as additive. However, common defects of metal–based materials, like high density, inferior chemical stability and thermal stability, also need to be considered. Besides, when metal particles are used as additives, it is also necessary to consider whether their dispersion is uniform.

31.47%, 37.5%, 20.80% and 94.90% higher than that of composite PCM without Al2O3 [63–66]. There are other forms of metal–based material additives. For instance, silver nanowire was used as thermal conductivity improvement additive to formulate the form–stable composite PCMs consisting of polyethylene glycol (PEG), expanded vermiculite (EVM) and silver nanowire (Ag NW) [67]. When the composite PCMs contain the Ag NW of 7.1 wt%, 13.7 wt% and 19.3 wt%, thermal conductivity is 0.36, 0.51 and 0.68 W/m. K, which is 6.0 times, 8.5 times and 11.3 times that of pure PEG (0.06 W/m. K) and is 1.44 times, 2.04 times and 2.71 times that of PEG/EVM composite PCMs (0.25 W/m. K). It indicated that both EVM and Ag NW are beneficial for enhancing the thermal conductivity of PEG. Reyes et al. [68] made 8% w/w of aluminum foils mixed with paraffin wax, where aluminum foil consists of three configurations. Experimental results showed that all configurations of aluminum foils have positive effects on thermal conductivity enhancement, where paraffin wax with aluminum in horizontal perforated disks configuration possesses the highest thermal conductivity (0.63 W/m. K), which is about two times that of pure paraffin wax (0.31 W/m. K). Li et al. [69] researched the PCM with aluminum powder inside a sphere. By observing the shortening of the melting and solidification time, it can be proved that the aluminum powder can improve thermal conductivity of PCM. Furthermore, as compared with uniformly diffused aluminum powder, deposition of aluminum powder is more effective on the thermal conductivity enhancement of the sphere during melting. Thermal conductivity enhancement of multiform metal–based additives is listed in Table 2, which indicates that metal–based materials are excellent additives in term of improving thermal conductivity of PCMs. Compared with other metal–based materials, metal foam has a better effect on improving thermal conductivity of PCMs due to its

2.1.3. Other materials In addition to carbon-based material and metal–based material additives, some other additives such as boron nitride and silica are employed to improve thermal conductivity of PCMs as well. Hexagonal boron nitride (HBN) was employed to promote thermal conductivity of n-octadecane and stearic acid eutectics PCM by Su et al. [70]. The measurement results revealed that HBN improves thermal conductivity of eutectics. By analyzing the results, the composite PCM with 10 wt% HBN was identified as the optimal one and thermal conductivity of composite PCM was 0.3220 W/m. K in solid state and 0.1764 W/m. K in liquid state. That is to say, compared with pristine eutectics (0.2982 and 0.1512 W/m. K in solid and liquid state respectively), the thermal conductivity is increased by 8.0% and 16.7% in solid and liquid state respectively. This reveals that the HBN is more effective in enhancing the thermal conductivity of the molten state. Fang et al. [71] prepared the paraffin/hexagonal boron nitride (h–BN) nanosheets composite PCM, where the h–BN nanosheets was used as fillers with loadings of 0, 1, 2, 5 and 10 wt%. It can be determined by experimental results that temperature has a significant effect on thermal conductivity in solid state and has little effect on that in liquid state. The more h–BN is added, the higher thermal conductivity will be achieved, and the optimum enhancement is approximately 60% when the composite PCM contains the largest loading of h–BN nanosheets of 10 wt%. And the 2735

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Table 2 Comparison of thermal conductivity with different metal material additives. PCM

Thermal conductivity of PCM kp (W/m. K)

Metal material additive

Thermal conductivity of additive ka (W/m. K)

Fraction of additive

Thermal conductivity of composite PCM kc (W/m. K)

Magnification

Paraffin [53]

0.305

91.4

–

1.2

3.93 times

Paraffin [53]

0.305

398.0

–

4.9

16.06 times

Paraffin [54]

0.354

91.4

–

2.33

6.58 times

Paraffin [54]

0.354

398.0

–

16.01

45.23 times

Icosane wax [55] Paraffin [57] Sodium acetate trihydrate [60] Paraffin [61] Paraffin [62] Paraffin [62] Polyethylene glycol [67]

0.20 0.21/0.29 (liquid/solid) 0.777 (30 ℃ )

Nickel foam (foam porosities > 95%) Copper foam (foam porosities > 95%) Nickel foam (foam porosities: 90.61%) Copper foam (foam porosities: 88.89%) Copper foam Aluminum foam Copper nanoparticles

– 218 –

7 vol% – 0.5 wt%

3.78 46.04/46.12 (liquid/solid) 0.936

18.9 times 218 times 1.20 times

0.25 0.418 0.418 0.06

Fe3O4 Al2O3 Fe2O3 Silver nanowire

9.7 41.1 6.4 429

20 wt% 4 wt% 8 wt% 19.3 wt%

0.40 0.919 1.020 0.68

1.6 times 2.199 times 2.44 times 11.3 times

consisting n-octadecane core and calcium carbonate (CaCO3) shell. The CaCl2 is a CaCO3 precursor used to form a CaCO3 shell. The CaCO3 is used as the shell to enhance thermal conductivity and increase the servicing life. The experimental results showed that thermal conductivity of EPCM increases as the mass fraction of CaCO3 shell increases, where the smallest value of thermal conductivity of EPCM (1.264 W/m. K) is much higher than that of pure n-cotadecane (0.153 W/m. K). It is indicated that the n-cotadecane encapsulated with a thicker CaCO3 shell reaches a better thermal conductivity, due to continuous phase acting like a fictitious thermal transfer meshwork created by CaCO3, increasing the thermal transfer rate of the entire microcapsules. Similarly, Wang et al. [75] also employed CaCO3 as shell, while paraffin–based binary mixtures (RT 28 and RT 42) were used as core. For the binary cores weight ratios of RT 28 to RT 42 of 10:0, 5:5 and 0:10, thermal conductivity of the EPCM is 0.759, 0.739 and 0.936 W/m. K respectively when the CaCl2/paraffin weight ratio is 1/1, and thermal conductivity of the EPCM is 0.714, 0.701 and 0.817 W/m. K when the CaCl2/paraffin weight ratio is 1/2, where the CaCl2 is a CaCO3 precursor used to form a CaCO3 shell. Thermal conductivity of the EPCM are about 2–3 times that of pure RT 28 (0.289 W/ m. K) and RT 42 (0.388 W/m. K), which indicated that CaCO3 shell has an outstanding effect on thermal conductivity enhancement. Meanwhile, it is found that the more content of CaCO3 results in the better enhancement of thermal conductivity. A kind of EPCM consisting of noctadecane core and inorganic silica shell was fabricated by using a sol–gel method [76]. Thermal conductivity was determined by thermal conductivity apparatus at an ordinary temperature. It is found that the mass fraction increase of silica leads to the improvement of thermal conductivity of EPCM. When noctadecane/TEOS weight ratio increases from 50/50 to 70/30, thermal conductivity of EPCM corresponds to 0.6213 and 0.4568 W/m. K at pH 2.45 and room temperature, where the TEOS is the precursor of silica. Nevertheless, no matter which EPCM, their thermal conductivity are higher than that of pure n–octadecane (0.1505 W/m. K). Zirconia was employed as shell by Zhang et al. [77] to encapsulate n–dodecane core, and the in–situ polycondensation method was utilized to synthesize the EPCM. The zirconia shell plays an outstanding role in promoting thermal conductivity, and thermal conductivity of pristine n–dodecane is enhanced from 0.152 W/m. K to 0.906 W/m. K, which is increased by about 5 times. Furthermore, thermal conductivity improvement of n–dodecane has an inhibitory effect on supercooling degree. Peng et al. [78] synthesized EPCM with montmorillonite as shell and stearic acid as core. It can be confirmed that the montmorillonite shell has the capacity to promote thermal conductivity by measuring heat storage rate. In order to further increase thermal conductivity, the EPCMs with

value of the highest thermal conductivity of composite PCM with 10 wt % fillers is 0.53 W/m. K at 50 °C. What is more, the phase transition rate is accelerated due to thermal conductivity enhancement. Motahar et al. [72] dispersed the mesoporous silica (MPSiO2) into n-octadecane, aiming to formulate a kind of new composite PCM, whose content of MPSiO2 is 1 wt%, 3 wt% and 5 wt%, respectively. Both in solidifying and melting state, thermal conductivity of composite PCM was determined via transient plane source technique between 5–55 °C. In solidifying state, thermal conductivity of composite PCM drops from 5 to 20 °C, and the thermal conductivity increases by 5.1% at 5 °C compared with pure PCM when the loading of MPSiO2 is 3 wt%. In melting state, thermal conductivity is inversely proportional to the temperature and is proportional to the MPSiO2 content, moreover, at 55 °C and MPSiO2 loading of 5 wt%, the thermal conductivity enhances by 5.5%. 2.1.4. Comparison of the additives with different materials Carbon–based and metal–based additives are two main additives used to enhance the thermal conductivity of PCMs. For carbon–based additives, they are characterized by manifold allotropes with high thermal conductivity, stable thermal and chemical properties, low density and good compatibility. However, in case of some carbon–based additives like carbon fibers, preparation and processing have a few challenges. Metal–based additives are very beneficial for thermal conductivity enhancement. Nevertheless, they are subject to many restrictions in practical applications such as: (1) most of them have high density, (2) they cannot be easily dispersed uniformly which results in unstable heat transfer, (3) their chemical natures are lively, leading them to being highly reactive with other substances. Hence, carbon–based additives possess better application prospects. For other material additives, the current researches are still relatively less, and the more materials and their properties are expected to be further developed. 2.2. Encapsulated phase change material The encapsulated phase change material (EPCM) is also referred to as microencapsulated phase change material (MPCM), and is made up of core and shell by emulsion polymerization, interfacial polymerization, mini-emulsion polymerization and other methods. The PCM and the polymer (or inorganic material) is utilized as core and shell, respectively. EPCM has a remarkable ability to enhance thermal conductivity of PCM [9,73]. Thermal conductivity enhancement of various encapsulated phase change materials is listed in Table 3. Possessing a relatively high thermal conductivity is one of the reasons why inorganic materials are often employed as shells for EPCMs. Yu et al. [74] utilized a self–assembly method to fabricate the EPCM 2736

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Table 3 Comparison of thermal conductivity with various encapsulated phase change materials. Core (PCM)

Thermal conductivity of PCM kp (W/m. K)

Shell

Thermal conductivity of shell ka (W/m. K)

Encapsulation efficiency (%)

Thermal conductivity of encapsulated PCM kc (W/m. K)

Magnification

n–octadecane [71] n–octadecane [73] n–octadecane [74] RT 42 [76]

0.153 (solid) 0.1505 0.152 0.369

CaCO3 Silica Zirconia CaCO3

2.467 1.296 2.560 –

40.04 57.7 64.52 –

Paraffin [77]

About 0.265

Silica

–

50.8 49.6

8.26 times 4.13 times 5.96 times 2.21 times 24 times 3.89 times 4.38 times

RT 21 [78]

0.15

Polymethyl methacrylate

0.192

–

1.264 0.6213 0.906 0.814 8.855 (with 24 wt% EG) 1.031 1.162 (graft with graphene oxide) 0.189 2.41 (coated with silver)

1.26 times 16 times

conductivity of slurry enhances as the temperature rises and the required mass fraction of NEPCM reduces. Thermal conductivity values of 1 wt%, 3 wt%, 5 wt% NEPCM slurry with silica and 5 wt% NEPCM slurry without silica is 0.4341, 0.4128, 0.4035 and 0.3721 W/m. K, respectively, at 5 °C. It is revealed that the shell of NEPCM grafted with silica helps to increase thermal conductivity of the slurry. A type of EPCM with n–octadecane core and polymethylmethacrylate shell added into silicon nitride powders was fabricated by Yang et al. [84]. It is found that adding silicon nitride powders has an excellent effect on increasing thermal conductivity and has little effect on latent heat. The thermal conductivity of EPCM with 10 g silicon nitride is 0.3630 W/m. K, increasing by 56.8% compared with the pure EPCM.

inorganic material shells are further optimized. Wang et al. [79] prepared EPCM used paraffin (RT42) as core and CaCO3 as shell, which was blended with EG, and then the mixtures were pressed by 10 kN. It should be illustrated that surface tension and pressure occur between EPCM and EG instead of any chemical reaction. The porosity of mixture blocks, the loading and the connection state of EG have direct influences on thermal conductivity of the mixtures consisted of EPCM and EG. According to the experimental results, it is observed that when the mixture contains EG loading of 24 wt%, it will build a more compact carbon network structure, and thermal conductivity of the mixture with 24 wt% EG is 24 times that of RT42. Graphene oxide (GO) was grafted on the EPCM with paraffin core and silica shell [80]. The values of the experiment revealed that thermal conductivity of EPCM is 1.031 W/m. K which is much higher than that of pure paraffin, and thermal conductivity has been further improved as the EPCM is grafted with GO, the value is 1.162 W/m.K. Polymers are also common materials employed as shells of EPCMs, however, due to their low thermal conductivity, polymers are usually modified to improve thermal conductivity. Al–Shannaq et al. [81] used RT21 as core and polymethyl methacrylate (PMMA) as shell to synthesize EPCM, then the EPCM was coated with silver, aiming to promote thermal conductivity. It is focused on the impact of two aspects on improving thermal conductivity, diverse particle size and diverse silver coating coverage of coated EPCM. The conclusions are as follows: (1) the apparent thermal conductivity will reduce when the size of non–coated EPCM decreases, (2) the apparent thermal conductivity of coated EPCM increase with the increase of particle size, especially when the diameter is more than 9.4 µm, the apparent thermal conductivity improvement of coated EPCM is outstanding, (3) the apparent thermal conductivity of all EPCM increases with the increase in the concentration of silver nitrate, but the degree of the increase is different. For the EPCM with different diameter, the larger the diameter is, the more obvious the thermal conductivity increases with the concentration of silver nitrate. For the EPCM with the same diameter, thermal conductivity increases rapidly when the concentration of silver nitrate increases from 10 g/L to 20 g/L, and the enhancement rate of thermal conductivity is greatly reduced when concentration increases from 20 g/L to 30 g/L, which is related to the coverage of the silver coating on the EPCM surface. Jiang et al. [82] used emulsion polymerization method to prepare EPCM with paraffin wax core and poly (methyl methacrylate–co–methyl acrylate) shell and modified it by embedding nano alumina in the shell. As expected, thermal conductivity of EPCM increases with the increase of nano alumina content, however, adding too much nano alumina will result in a significant reduction in phase change enthalpy. Finally, the EPCM with 16% mass ratio of nano alumina was selected as the optimal one, whose thermal conductivity is 0.3104 W/m. K, and is 1.271 times that of EPCM without nano alumina. Fu et al. [83] synthesized a kind of nano EPCM (NEPCM), where core is n–tetradecane and shell is polystyrene grafted with silica. The NEPCM was dispersed into base fluid and a new kind of NEPCM slurry was synthetized in order to store cold energy. It is found that thermal

2.3. Comparison of two ways for improving thermal conductivity of PCMs Adding additives with high thermal conductivity into PCM for enhancing thermal conductivity is an effective method. Generally speaking, supporting materials like high–density polyethylene were employed to improve the stability of the PCM and prevent leakage during melting process, but for the additives of pore structure, such as expended graphite and metal foam, they enable to adsorb PCM well, in other words, they can also play a supporting role in composite PCM. There are many types of additives to improve thermal conductivity of PCMs, and most of them are carbon–based and metal–based materials, where the carbon–based additives possess outstanding properties like low density, excellent stability and so on, thereby they are applied extensively. Moreover, with the development of nanotechnology, nano–additives have a considerable application prospects. Nevertheless, there are some problems brought about by additives, which cannot be ignored. Firstly, it is difficult to ensure that the additive is uniformly distributed in the PCM, and long–term operation results in undesirable consequences such as aggregation and precipitation, so as to reduce the thermal conductivity and deteriorate the temperature uniformity of the thermal energy storage system. Secondly, the additives increase the weight of the PCM, especially certain denser additives, which are limited in applications. What is more, additives will decrease the latent heat storage capacity in general. The PCM is encapsulated in a spherical capsule, which separates the PCM from the external environment, and solves the problems of leakage, phase separation and corrosiveness. That not only prolongs the use period of the PCM, but also increases the thermal contact area of the PCM, improving heat transfer rate of thermal energy storage system. In addition, the capsule shell is able to control the volume change of PCM greatly during the phase transition process, greatly improving application performance of EPCM. Besides, some shell materials like organic polymer shells, suffer from low thermal conductivity, so it needs to be modified with additives of high thermal conductivity, such as grafted with graphene oxide [80], and coated with metal silver [81], in order to further promote thermal conductivity of EPCM. In other words, the two methods of improving thermal conductivity can also be combined to achieve the 2737

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3.1.2. Solar thermal power generation system In a solar power tower, linear Fresnel reflectors or parabolic trough collectors of a concentrated solar thermal power (CSP) can be utilized to collect sunlight and produce strong heat, and then the heat transfer fluid will transport it to thermal power plant aiming at power generation [90]. Due to the large latent heat storage capacity of PCMs, the CPS which combines with PCMs is the most effective approach to provide flexible power to the grid and provide large–scale power services [91]. A solar thermal power plant located in Shiraz, Iran, whose thermodynamic properties including energy and exergy were studied by Mahfuz et al. [92]. The properties of the solar thermal power plant with PCM storage were analyzed based on second law of thermodynamics. The performances of PCM storage system integrated with solar power plant were investigated [93]. The results of analysis indicated that the overall efficiency of energy and exergy of the system without PCM is 30% and 10% respectively, however, after adding PCM into solar collector, the overall efficiency of exergy increased to 30%, due to the high latent heat storage capacity of the PCM. What is more, the higher melting point temperature of the PCM leads to higher efficiency of exergy. Bhagat and Saha [94] investigated an organic Rankine cycle–based solar thermal power plant with the packed bed containing EPCM. Five factors were studied, which were mass flow rate, entrance charging temperature, storage system size, EPCM shell diameter and porosity. The results of analysis indicated that the increases in mass flow rate and entrance charging temperature lead to the improvement in the thermal property during storage and recovery process. And the fluctuation in heat transfer fluid (HTF) temperature at the storage exit decreases as the EPCM shell diameter decreases. The storage system size and porosity have little influence on the efficiency of the system, however they will enlarge the fluctuation in HTF temperature.

goal of thermal conductivity enhancement. 3. Applications of PCMs in thermal energy storage For the goal of environmental protection and energy conservation, new renewable energy sources have been developed and utilized, such as solar energy, wind energy. However, these renewable energy sources are intermittent and fluctuant. PCMs can bridge the gap between energy supply and demand, thus the shortcomings of renewable energy can be overcome. In addition, PCMs can maintain constant temperature during the phase transition process, making them widely employed in the field of solar energy, building, textile, industrial heat recovery, etc. 3.1. PCMs in solar energy system Solar radiation is regarded as being rich in renewable energy, but it only exists during clear days. PCMs can store solar energy during day and supply at night, overcast and rainy days, so they are utilized extensively in solar energy system [85]. There are two popular types of solar energy system, namely solar water heating system and solar thermal power generation system, respectively. 3.1.1. Solar water heating system Solar water heating (SWH) system is a kind of environmental protection and energy saving facility. Nevertheless, the conventional SWH suffers from inefficiency in cold weather, severe heat losses at night and inability to efficiently capture solar energy, leading to the low utilization rate, etc. The PCMs can solve these problems to a certain extent [86]. Narayanan et al. [87] prepared nanocomposite PCM with eutectic gel PCM (paraffin and oleic acid) and 0.5 wt% nanographite supporting material, which is applied to SWH. The SWH system mainly contains three parts: a solar collector unit, a thermal energy storage unit with nanocomposite PCM and a water storage tank with heat insulation. The consequences of the solar illumination experiment indicated that the melting of nanocomposite PCM with 0.5 wt% nanographite needs only 3 min, which decreases by 93% compared with that of eutectic gel PCM (45 min). Furthermore, compared with the melting rate of conventional heating route, melting rate of solar illumination is much faster. The merits of nanocomposite PCM that can be listed are significant solar energy capture capability, efficient light to heat conversion, high thermal conductivity, ultra–fast thermal storage and high heat transfer rate, which result in improving the overall efficiency of SWH system. Su et al. [88] formulated microencapsulated PCM (MPCM) for SHW storage system, where the core is paraffin wax and the shell is melamine–formaldehyde resins. It is found that the type of emulsifier employed for formulating MPCM has an influence on the morphology of MPCM, and a high enthalpy of 126 kJ/kg can be achieved. The thermal properties of MPCM with the highest enthalpy were assessed by theoretical evaluation in an integrated compacted bed unit. And the results revealed that the SWH system with MPCM has larger thermal energy storage density and correspondingly less physical storage dimension than water–based system. Compared with the water–based system, the thermal conductivity of the MPCM is a little lower, but it still is about two times that of most common PCM storage units. Khalifa et al. [89] studied a storage solar energy collector for SHW, which is made up of the paraffin wax as PCM heat storage media and six copper pipes with 80 mm diameter linked to a string. Clear and semi–cloudy days in January, February and March were chosen to carry out outdoor experiments. It is noticed that the change of the temperature between the system with PCM and the system without PCM is different, where the temperature of the system with PCM increases from the entrance to 2.5 m length, and that of remaining 7.6 m length almost remains constant. During the absence or lack of sunshine, the system is cooled down and the liquid PCM becomes a heat source to transfer heat to circulating water until it is completely frozen.

3.2. PCMs in buildings With the continuous improvement of living standards, the demand for thermal comfort inside buildings is increasing, resulting in more and more energy consumption, especially in the summer and winter. Integrating PCM into buildings not only decreases energy consumption, but also improves the thermal comfort of buildings. Thereby, wallboards, floors, concrete, gypsum and other parts are integrated with PCMs in order to improve their performance [85,95]. Sharifi et al. [96] assessed the efficiency of the gypsum boards integrated with PCMs for increasing the thermal properties of buildings. They investigated the properties of the gypsum boards with PCMs by utilizing computational simulation of real temperature distribution in diverse cities. It is found that the efficiency of PCMs is directly related to the external temperature and the large temperature fluctuation brings about low efficiency of PCMs. It can be confirmed that the gypsum boards with PCMs have an excellent effect on energy conservation, and the gypsum boards can be used in both new buildings and old buildings. It is worth noting that the efficiency is not proportional to the number of PCMs. Ye et al. [97] prepared the PCM panels consisting of CaCl2·6H2O/expanded graphite composite PCM for application in buildings. The thermal properties of the test room with PCM panels were compared with the reference room without PCM panels, and they were placed in the simulated climate chamber. The experimental results revealed that PCM panels are able to reduce the temperature fluctuation of the test room, showing smaller range of temperature compared with the conventional room. In addition, the thermal performance of the test room is related to the position of PCM panels. The simulation results are similar to the experimental one and indicate that the best thickness of the PCM panels is between 8 mm and 10 mm. Xia and Zhang [98] developed a novel double-layer radiant floor consisting of PCMs, which is propitious to electric peak–shaving, due to the fact that PCMs possess the ability of storing heat or cold energy during low–peak period. Two experimental setups were established to study the temperature field of the double–layer radiant floor 2738

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results show that the textiles coated with microparticles have an excellent thermal comfort property. What is more, the microparticles possess outstanding thermal stability, eicosane does not leak even at the temperature that is two times higher than melting point of pure eicosane, hence, textiles coated with the microparticles can be employed as high temperature exposure suits. Nejman et al. [107] investigated the influence of different integration methods of microcapsule PCMs on the thermal properties and air permeability of knitted fabric, where three integration methods, printing, coating and padding were studied. It was found that among three types of improved knitted fabrics, printed fabrics have the strongest thermal conditioning ability and weakest air permeability, while the case of padded fabrics is just the opposite. Kazemi and Mortazavi [108] developed a new method to prepare PCM without microencapsulation for textiles, where Na2SO4·10H2O was used as PCM. Besides, nano montmorillonite as thickening and sodium tetra borate as nucleating agent were added to maintain the thermodynamic stability of PCM and to ensure that the thermal storage capacity does not decrease after diverse thermal cycles. Silicone rubber with elastomeric structure was also added to maintain the thickness and bending properties of the modified textiles. Nevertheless, the modified textiles have weaker air transfer and water vapor permeability, which can be improved by using screen printing technique. Carreira et al. [109] used octadecane as PCM to fabricated acrylic based microcapsules by suspension polymerization. It was found that the microcapsules exhibit excellent potential for combining with the textiles, making the textiles with heat regulating function.

system. Compared with box B in the same circumstances, the temperature and temperature fluctuation of box A are lower, and the range of temperature of box A is only 2 °C in the whole process of heat dissipation. It indicated that the double-layer radiant floor can meet the heat demand and the energy consumption reduces as the temperature of thermal storage PCM rises during storage process. Johra and Heiselberg [99] pointed out in a review article that furniture has a significant impact on the thermal performance of PCMs in building applications, but there are still too few investigations on them. 3.3. PCMs in cooling system Cooling system is widely utilized in many fields, such as cooling electronic equipment, food preservation and cooling buildings. The cooling system integration with PCMs can effectively improve its performance. Krishna et al. [100] integrated heat pipe with nano enhanced PCM, with target application in electronic cooling system. And the PCM stores or releases thermal energy according to the input power of the evaporator and the fan speeds of the condenser. Three types of energy storage materials, such as water, PCM (Tricosane) and nano enhanced PCM (Tricosane) with nano alumina particles, were used to verify the performance of the nano enhanced PCM. The experimental results revealed that thermal conductivity of nano enhanced PCM is higher than the pure PCM and the maximum increment is 32%. What is more, compared with the conventional heat pipe, the heat pipe integrated with nano enhanced PCM reduces the evaporator temperature by approximately 25.75%, leading to cutting down on 53% fan power consumption. Besides, 30% energy from evaporator will be absorbed by nano enhanced PCM, which can reduce fan power consumption as well. For improving the comfort of human body in hot weather, Itani et al. [101] studied a type of cooling vest with PCM. The main target is to find the best position and number of the PCM packets in the vest, so as to achieve the optimal cooling effect and maximize the comfort of the human body, and the effect is determined by a synthetic fabric–PCM and bio–heat simulation model. It indicated that the number of PCM packets are needed by cooling vest were 8, 18 and 20 when the environmental temperature is 28 °C, 35 °C and 45 °C, respectively, which can provide comfortable cooling for workers who work outside. In other words, different environmental conditions require different amounts of PCM packets. Food preservation and storage cannot be separated from the refrigerator in daily life, Yusufoglu et al. [102] integrated PCMs into the domestic refrigerator aiming at improving the performance. Two types of refrigerator were employed to test the performance of four kinds of PCMs. The results revealed that the opening and closing time of compressor is optimized and just 0.95 kg PCMs can save up to 9.4% energy, besides, the effect of PCM can be enhanced by enlarging the surface area of condenser. In order to respond to the requirement of energy saving and emission reduction, cooling systems incorporated with PCMs have also played a great role in the field of buildings, and Akeiber et al. [103] and Souayfane et al. [104] made reviews on it in detail.

3.5. PCMs in heat recovery system Heat recovery is of great importance for energy saving and emission reduction, however, in general there is a time–gap and geography–gap between heat release and heat demand points. This problem can be solved significantly by heat recovery system with PCMs, as PCMs possess an outstanding latent heat storage capacity. Xia et al. [110] designed a heat recovery system with PCM to recover heat from refrigerator condensation heat and release heat for preparation of household or industrial hot water. The experimental results indicated that the composite PCM containing carnauba wax (melting point: 81.98 °C and latent heat: 150.9 J/g) and expanded graphite (mass ratio of 10:1) possesses great potential as thermal storage medium in the heat recovery system. Jia et al. [111] compared the performances of storage–enhanced heat recovery room air–conditioner (SEHRAC) with PCM and without PCM, which was carried out by control variable experiment. The results indicated that the space cooling and water heating abilities of SEHRAC with PCM increase by 5.4% and 16.1%, respectively, compared with those of SEHRAC without PCM, what is more, PCM greatly improves the insulation ability of water tank of SEHRAC, the insulation time is extended by 21.1%. In other words, the performance of SEHRAC is improved by PCM. Bertrand et al. [112] pointed out that the household hot water energy consumption accounted for 16% of household heating energy consumption at EU in 2013, and the proportion would be further increased. Hence, it is vital to establish waste water heat recovery systems in buildings, in order to reduce energy consumption. So a way was presented for determining building energy consumption and energy saving potentials, which was based on pinch analysis, and was based on building waste water heat recovery system of a city scale. Building energy–saving technology for environmental protection and energy conservation is of great significance, so heat recovery technology in buildings has been gaining more and more attentions. Cuce and Riffat [113] considered that heat recovery systems have great potentials to reduce the fossil fuel consumption of buildings, resulting in saving energy and decreasing greenhouse gas discharges in the air. Besides, Mardiana–Idayu and Riffat [114] summarized and analyzed the technologies of heat recovery for buildings and found that the forms of heat recovery include fixed plate, heat pipe, rotary wheel, etc.

3.4. PCMs in textiles The basic function of the textiles is to provide a relatively stable ambient temperature to human body. If PCMs are integrated into the textiles, the insulation capacity of the textiles can be greatly improved, due to large thermal storage capacity and almost constant phase change temperature of PCMs. Currently, the main methods of fabricating thermally stable textiles with PCMs are infiltration of microencapsulated PCMs to fibers, fabrics and foams [105]. Besides, the PCMs with a phase change temperature range from 18 °C to 35 °C are the most suitable for application in clothing [105]. Shaid et al. [106] prepared eicosane/aerogel microparticles as coating additive on textiles to regulate body temperature. The test 2739

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PCMs are widely applied in various fields, as they possess remarkable features, such as large latent heat, almost constant temperature during phase change process and so on. It is worth noting that in order to maximize the effect of the PCMs, it is necessary to select a PCM with a suitable phase transition temperature. PCMs in solar energy systems, recovery heat systems are mainly used for their large latent heat storage capacity to adjust the imbalance between heat supply and demand. PCMs in buildings, textiles and cooling systems are mainly used for their constant phase change temperature to maintain an optimal environmental temperature. In summary, PCMs have a prominent role in protecting environment and saving resources. They are indispensable materials in the future trend of energy development.

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4. Conclusions and outlook Heat transfer rate is a vital factor to determine the efficiency of thermal energy storage system, and enhancing thermal conductivity is an effective approach to improve it. Hence, this review focuses on two methods for improving thermal conductivity of PCMs, which include adding fillers with high thermal conductivity and encapsulated PCMs. The fillers mainly include carbon–based additives and metal–based additives, namely expended graphite, carbon fiber, graphene, carbon nanotube, metal foam, metal particles and metal oxides, etc. Both carbon-based additives and metal–based ones possess excellent thermal conductivity, however, carbon-based additives are better than metalbased additives in terms of density and stability. For encapsulated PCMs, shells prevent the PCM from damage and leakage, and also improve the thermal conductivity. The applications of PCMs in solar energy system, buildings, cooling system, textiles and heat recovery system are introduced as well. In addition to the above mentioned methods to improve thermal conductivity of PCMs, new methods and new materials need to be developed and investigated for meeting the requirements of thermal energy storage applications. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant no. 51376087, 51676095). The authors also wish to thank the reviewers and editor for kindly giving revising suggestions. References [1] Liu LK, Su D, Tang YJ, Fang GY. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energy Rev 2016;62:305–17. [2] Safari A, Saidur R, Sulaiman FA, Xu Y, Dong J. A review on supercooling of Phase Change Materials in thermal energy storage systems. Renew Sustain Energy Rev 2017;70:905–19. [3] Khan Z, Khan Z, Ghafoor A. A review of performance enhancement of PCM based latent heat storage system within the context of materials, thermal stability and compatibility. Energy Convers Manag 2016;115:132–58. [4] Meng ZN, Zhang P. Experimental and numerical investigation of a tube–in–tank latent thermal energy storage unit using composite PCM. Appl Energy 2017;190:524–39. [5] Ma GX, Liu S, Xie SL, Jing Y, Zhang QY, Sun JH, Jia YZ. Binary eutectic mixtures of stearic acid-n-butyramide/n-octanamide as phase change materials for low temperature solar heat storage. Appl Therm Eng 2017;111:1052–9. [6] Miró L, Gasia J, Cabeza LF. Thermal energy storage (TES) for industrial waste heat (IWH) recovery: a review. Appl Energy 2016;179:284–301. [7] Liu YS, Yang YZ. Preparation and thermal properties of Na2CO3·10H2O–Na2HPO4.12H2O eutectic hydrate salt as a novel phase change material for energy storage. Appl Therm Eng 2017;112:606–9. [8] Sharif MKA, Al–Abidi AA, Mat S, Sopian K, Ruslan MH, Sulaiman MY, Rosli MAM. Review of the application of phase change material for heating and domestic hot water systems. Renew Sustain Energy Rev 2015;42:557–68. [9] Huang X, Alva G, Jia YT, Fang GY. Morphological characterization and applications of phase change materials in thermal energy storage: a review. Renew Sustain Energy Rev 2017;72:128–45. [10] Mohamed SA, Al–Sulaiman FA, Ibrahim NI, Zahir MH, Al–Ahmed A, Saidur R, Yılbaş BS, Sahin AZ. A review on current status and challenges of inorganic phase change materials for thermal energy storage systems. Renew Sustain Energy Rev

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