Thermal conductivity enhancement of phase change materials for thermal energy storage: A review

Thermal conductivity enhancement of phase change materials for thermal energy storage: A review

Renewable and Sustainable Energy Reviews 62 (2016) 305–317 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journa...

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Renewable and Sustainable Energy Reviews 62 (2016) 305–317

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Thermal conductivity enhancement of phase change materials for thermal energy storage: A review Lingkun Liu, Di Su, Yaojie Tang, Guiyin Fang n School of Physics, Nanjing University, Nanjing 210093, China

art ic l e i nf o

a b s t r a c t

Article history: Received 9 September 2015 Received in revised form 17 February 2016 Accepted 26 April 2016

Thermal energy storage systems have been recognized as one of the most efficient ways to enhance the energy efficiency and sustainability, and have received a growing attention in recent years. The use of phase change materials (PCMs) in building applications can not only improve the indoor thermal comfort but also enhance the energy efficiency. The necessity to enhance thermal conductivity of the PCMs is evident due to its low energy charging/discharging rates. Therefore, the high thermal conductivity additives or inserts to enhance thermal conductivity or to form the composite PCM are sought to achieve high energy charging/discharging rates. In this paper, the experimental and theoretical methods to enhance the thermal conductivity of the PCMs are summarized, and the thermal conductivity inserts/ additives in recent investigations are listed and summarized. The evaluation of each thermal conductivity enhancement method is discussed. & 2016 Elsevier Ltd. All rights reserved.

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

Contents 1.

2.

3.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 1.1. Phase change materials (PCMs) for thermal energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 1.2. Thermal conductivity enhancement of phase change material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 1.3. Scope and coverage of the present review. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 Theoretical study on thermal conductivity enhancement of phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 2.1. Microstructure of different kinds of additives or its composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 2.2. Application of effective medium models in describing thermal performance of composite phase change materials . . . . . . . . . . . . . . . 308 2.3. Effect of thermal resistance on thermal conductivity phase change materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 2.4. Effect of diffusion coefficient on thermal conductivity of phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 2.5. Other approaches to calculate the enhanced effective thermal conductivity of composite phase change materials . . . . . . . . . . . . . . . . 309 Experimental study on thermal conductivity enhancement of phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1. Thermal conductivity enhancement of phase change materials: carbon materials as additives/inserts . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1.1. (Exfoliated/expanded) graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1.2. Graphite powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1.3. Carbon fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1.4. Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 3.1.5. Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 3.1.6. Comparison of different types of carbon additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 3.2. Thermal conductivity enhancement of phase change materials: metal or their oxides as additives/inserts. . . . . . . . . . . . . . . . . . . . . . . 313 3.2.1. Metal foam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 3.2.2. Metal nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 3.2.3. Metal salt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 3.3. Thermal conductivity enhancement of phase change materials: fins and other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 3.4. Comparison of different ways to enhance thermal conductivity of phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

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

http://dx.doi.org/10.1016/j.rser.2016.04.057 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

1. Introduction Recently greater energy demand of stable supply of fossil fuels and growing awareness of environmental issues have contributed to a serious attention on various renewable sources of energy. The unpredictability of the endothermic and exothermic process demands the strong, reliable and efficient storage units in the thermal system. Among various forms of energy, thermal energy is widely spread in nature as in solar radiation, geothermal energy and building applications. Thermal energy is the model in the energy field in correcting the gap between supply and demand and improving the performance and reliability of the thermal system. Because of its abundance, thermal energy is generally categorized as a low-grade form of energy and is associated with waste in industrial processes. Storage of thermal energy can efficiently improve the industrial processes, which significantly decreases the consumption of thermal energy. 1.1. Phase change materials (PCMs) for thermal energy storage Thermal energy can be stored as latent energy by heating and cooling the material without much visible temperature change. The stored energy can be retrieved when the process is reversed. Phase change materials are widely used to store such thermal energy due to their high latent heat during phase change process. A number of articles [1–5] introduced the PCMs with large phase change temperature range, including their thermophysical behavior, encapsulation, thermostability, heat transfer enhancement and system-related issues. Thermal conductivity enhancement is the point of focus in this review.

thermal conductivity enhancement. The large coverage of thermal conductivity graphite based PCMs are not completely summarized in the former review. On the other hand, mathematical modeling for porous structure and models to calculate effective thermal conductivity of nanofluids has been investigated [9,10]. It can be stated that the pertinent parameters influencing the thermal conductivity are concentration, particle size, viscosity, shape, temperature and the material properties, etc. In this work, the extra information contained is included as follows: (1) Tables containing the relative thermal conductivity enhancement of different PCMs are listed in detail, giving a clear overview of different categories of conductivity-enhancement technologies. Thus it is easier to compare different methods to enhance thermal conductivity. (2) Different effective medium models for many featured situations are introduced in order to calculate the effective thermal conductivity of the PCMs with complex nanostructures. (3) Nano-stucture, porous structure, nanofluids and alignedmolecule structure enhanced PCMs are all included in this work. Therefore, the objective of the present work is to provide a comprehensive review of the various kinds of enhanced composite PCM including simple mixture, foam, nanofluids, fibrous aggregates composed by the additives and PCM. The models to calculate the thermal conductivity of the nanostructures of the newly developed composite PCMs are elaborate in this work, and the influential factors are discussed as well. By enumerating the thermal conductivity enhancement data of different lately works and making comparison, this work presents a clear overview of different categories of conductivity-enhancement technologies.

1.2. Thermal conductivity enhancement of phase change material The necessity to enhance thermal conductivity of the PCMs is evident due to its low energy charging/discharging rates. Therefore, the additives to enhance thermal conductivity or to form the composite PCMs are searched to achieve high energy charging/ discharging rates. Conventional additives are categorized into carbon type and metal type in terms of the composing element. The heat transport mechanisms involved in phase change systems are drawn attention, and existing studies are mainly performed by thermodynamics and molecular dynamics [6–8]. The efforts are helpful in deciding the employment of the additives and anticipating the effective thermal conductivity of the composite PCM. In addition, different methods to enhance the thermal conductivity by experiment are also presented. 1.3. Scope and coverage of the present review There are already some reviews focusing on the fixed highconductivity inserts and free-form, particle-dispersed systems [2,5] presented to enhance the thermal conductivity of PCMs. Metallic fins, foams, wools are considered as conventional stationary inserts and metallic foam and graphite based PCM systems are newly developed methods in the last few years. Lately, the approaches using nanotechnology introduce nanostructures (nanoparticles, nanotubes, nanofibers, etc.) into PCMs, which has shown great extra functionalities and improvements including

2. Theoretical study on thermal conductivity enhancement of phase change materials 2.1. Microstructure of different kinds of additives or its composite Since the thermal conductivity of most thermal conductivity promoter can be known from the manufacturer, it is not intriguing to obtain the value of thermal conductivity of different types of additives themselves. Thermal conductivity of some common additive materials is summarized in Table 1, from where an overview of the thermalphysical properties of some common additives can be gotten. On the other hand, some approaches come out to theoretically deduce thermal conductivity of some featured material with aligned structure. Bagchi and Nomura [24] concluded several equivalent continuum models for a multi-walled carbon nanotube (MWCNT) among the past work, as shown in Fig. 1. The MWCNT can be replaced by a solid fiber with the same shape, thus the equivalent conductivity can be calculated from the corresponding equation. Huang et al. [25] conducted the effective thermal conductivity of salt/expanded graphite (EG) composite material by a fractal approach. By use of this approach, the microstructure of the composite is presented, then the ratio of volumes between graphite and the porous EG matrix can be expressed. Moreover, with the help of turning these fractal units into corresponding thermal

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Table 1 Thermal conductivity of some common additive materials. Additives Graphite Carbon fiber Carbon foam Carbon nanatubes

Expanded graphite Metal

Graphene Graphite powder

Graphite matrix [11] Spherical graphite [12] CF, XN-100 [13] CF-20 [14] MWCNTs [15] MWCNTs [7] SW carbon nanohorns [16] S-SWNTs [17] L-SWNTs [17] MWNTs [17] EG [18] Expanded graphite [12] Aluminum container [14] AuNPs [15] CuNPs [15] Nickel particle [12] Copper foam [19] Nickel foam [19] Graphene aerogel (GA) [20] Graphene [21] Graphite [22] Graphite powder [23]

Density (kg/m3)

Specific heat (J/kg K)

Thermal conductivity (W/m K)

 2270 2220 250(apparent) 2600  1100 2100 2100 2100 – – 2719 19,300 8960 8910 8930 8900 227(apparent) 2200  –

 710 690 750 – – – – – – – – 871 – – 440 – – 1121 – – –

10–70 80 900 3.10 3000 41000 27 (using the EMT model)  4000  4000  2000 129 200(average) 182 318 400 90.3 398 91.4 2.183 3000 25–500 130

Fig. 1. Development of a continuum model for an MWNT: (a) schematic diagram of an MWNT showing concentric graphene layers, (b) equivalent continuum model, (c) effective solid fiber and (d) a prolate spheroidal inclusion [24].

Fig. 2. Two physical domains with the same volume fraction of EG: (a) the geometry (10  10  10 μm3) of EG with big sheets and (b) the geometry (10  10  10 μm3) of EG with small sheets [26].

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resistance networks, the effective thermal conductivities can be predicted. Li et al. [26] proposed a novel two-level scale model from micro-scale to macro-scale to compute the thermal conductivity of the composite. The micro-scale model is significant in describing the porous structure of the composite material, as shown in Fig. 2. Agrawal and Satapathy [27] proposed a model to estimate the effective thermal conductivity of the hybrid polymer composites (Fig. 3). 2.2. Application of effective medium models in describing thermal performance of composite phase change materials Different models using effective medium approach are successively proposed in a long time, listed in Table 2, where keff is the effective thermal conductivity of the suspension, n ¼ 3=ψ and ψ is the sphericity of the particle, kp and kf are the thermal conductivities of the suspending medium and additive particle respectively. The models based on effective medium theory successively take into account the additive volume fraction, the ratio of thermal conductivity of the additive to that of the PCM, the sphericity of the additives, the alignment of the additives and the particle aggregation effect. In most cases, the experiment results can match some certain model listed above. However, none of these models indicates that thermal conductivity has any particle size dependence, while it has been confirmed in numerous studies [17,22,35,36]. And these aforementioned models can be summarized as an expression to a certain extent [37].

2.3. Effect of thermal resistance on thermal conductivity phase change materials Since experiments with high ratio of thermal conductivity of the additive to that of the PCM has a mismatching result from the model estimation and most works have focused on the idealized case of perfect interface contact, novel models considering thermal resistance between the additives and the fluid/matrix have come out. Nan et al. [38] incorporated the Kapitza resistance (the inverse of the interfacial resistance) into the effective medium theory and calculated the thermal conductivity. Effective thermal conductivity under four limiting cases of interest can be derived with an expression, which contain aligned continuous fibers, laminated flat plates, spheres and completely misoriented ellipsoidal particles. It solves problems of most cases with different kinds of thermal conductivity enhancement additives. Bagchi and Nomura [24] solved the Laplace equation in his model's system. By applying the boundary conditions and given some parameters including the interfacial conductance, the thermal conductivity of the “effective fiber” can be calculated. Chen et al. [39] developed an effective medium model with the unit cell shown in Fig. 4, and the corresponding thermal resistances are herein indicated. After the thermal conductivity of the unit cell is calculated, the nonbridging single-wall carbon nanotubes (SWCNTs) are treated as dispersions distributed in the medium, and the equations from last section can be applied to calculate the effective thermal conductivity of the real material. Thermal resistances have several express forms, researchers found different ways to integrate it into the expressions for deciding the effective thermal conductivity of the composite material. With this effect taken into account, the mismatching results are signally amended.

2.4. Effect of diffusion coefficient on thermal conductivity of phase change materials

Fig. 3. 3-Dimensional view of (a) particulate filled composite and (b) single element [27].

In order to incorporate the effect of temperature on the thermal conductivity enhancement, the contribution of Brownian particles in the suspension must be considered. Wasvekar et al. [40] used the Einstein relation for obtaining diffusivity. Li and Peterson [41] considered a local periodic motion within the suspension. The expressions are listed to determine the velocity and the range of influence for this motion. As a result, the governing equations are established to simulate the convection caused by the Brownian motion of the nanoparticles.

Table 2 List of thermal conductivity models using effective medium approach. Model

Expression

Maxwell [28]

keff ¼ kp þ 2kf  2

Hamilton&Crosser [29]

keff ¼

Jeffrey [30]

Krisher[31] Yu and Choi[32] Kunii and Smith [33] Xue [34]

k þ 2k þ 2ðkp  kf Þ∅ k ðkp  kf Þ∅ f p f kp þ ðn  1Þkf  ðn  1Þ∅ðkf  kpÞ kf kp þ ðn  1Þkf þ ∅ðkf  kp Þ

0

keff ¼ kf @1 þ keff ¼ keff kf

¼





3∅

kp kf

kp kf



1

þ2

1f þf ð1  ∅Þkf þ ∅kp

þ 3∅2 

1∅ þ k∅p kf

kpe þ 2kf þ 2ð1 þ β1 Þ ðkpe  kf Þ∅

(considering the shape of particles) 1 !2 1 þ …A (considering the effect of interactions between particles)

kp kf kp kf

þ2

  1

(considering an array of elements of specific resistance)

3

kpe þ 2kf  ð1 þ β1 Þ3 ðkpe  kf Þ∅

(nanofluid situation, where is the ratio of the ordered liquid layer thickness to the nanoparticle radius)

ð1  ∅Þ  1=3 kkeff ¼ ðð1 ∅Þ  1=3  ð1  ∅Þ1=3 Þnkp þ 1 (fibrous additive situation) f f p ffiffiffiffiffiffiffiffiffi π 1  ∅ þ 4∅ kp =kf arctan pffiffiffiffiffiffiffi π kp =kf 4 keff pffiffiffiffiffiffiffiffiffi (large axial ratio situation) ¼ π kf 1  ∅ þ 4∅ kf =kp arctan pffiffiffiffiffiffiffi π k

4

kf =kp

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Fig. 4. Schematic diagram of the structure of unit cell and the corresponding thermal resistances: (a) overall geometric pattern of the unit cell, (b) sketches for the vertical profile and (c) series and parallel thermal resistance network [39].

2.5. Other approaches to calculate the enhanced effective thermal conductivity of composite phase change materials Although a complete set of model procedure based on effective medium theory has been developed to predict the value of thermal conductivity of composite material, other approaches by using governing equation, molecular dynamics, Lattice Boltzmann algorithm, etc. are also proposed to simulate or calculate the effective thermal conductivity. Han et al. [42] proposed a phase field model to solve the problems in the latent thermal energy storage system. By setting the governing equation, an Effectiveness Map is produced to decide whether incorporating metal foam can enhance the thermal conductivity greatly. The range of effective thermal conductivity can also be determined. Mesalhy et al. [43] discretized the governing equations using a finite volume approach. Then the high thermal conductivity porous matrix is added into the model, the total effective thermal conductivity is derived. Sankar et al. [44] proposed a theroretical approach based on molecular dynamics (MD) modeling for estimating the enhancement of the thermal conductivity of metallic nanofluid. Four major interactions among the nanofluid are taken into accout. Next the molecular dynamics simulations are performed. The simulate result is in good agreement with Maxwell's model (Fig. 5), which confirms MD method is a practicable way to calculate the thermal

Fig. 5. Variation of thermal conductivity ratio with volume fraction at different temperatures [44].

conductivity in many cases. Sun et al. [45] also used equilibrium molecular dynamics simulation to calculate the thermal conductivity of Ar-Cu nanofluid confined between two parallel walls (nanochannel). It is proved both theoretically and experimentally that thermal conductivity can be enhanced by solid walls in a nanochannel. Wang et al. [6] proposed a three-dimensional numerical method in investigating the effective thermal conductivity enhancement. It

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is also a numerical method other than MD method based on computational fluid dynamics simulation. This method has the advantage of not depending on empirical parameters, so it is more suited in design and optimization for new materials.

3. Experimental study on thermal conductivity enhancement of phase change materials Experimental study on heat transfer enhancement of the PCMs is a long term research. The researches are divided into several categories with different kinds of thermal conductivity enhancement promoter. The merits and shortcomings are demonstrated and compatibility in different applications is discussed. 3.1. Thermal conductivity enhancement of phase change materials: carbon materials as additives/inserts As the most common high thermal conductivity material, carbon-based material are long used and widely spread. Novel allotropes of carbon have continued to arise in recent years. Each carbon material is discussed and its thermal parameters measured are listed. 3.1.1. (Exfoliated/expanded) graphite The most regular form of graphite is flake graphite, which shows great thermal properties with high thermal conductivity. Expanded graphite is made by natural flake graphite. The crystal lattice planes of flake graphite are crackled apart (Fig. 6) [46] and shows better thermal performance than flake graphite. Jeong et al. [47] prepared form stable Bio-based PCMs with exfoliated graphite nanaplatelets (xGnP). The latent heats and thermal conductivity showed a large increase. It is analyzed that

the thermal conductivity of Bio-based PCM and Bio-based PCM with xGnP shows 0.154 W/mK and 0.557 W/mK. The thermal conductivity has a 375% increase. Shi et al. [48] compared the improvements in the thermal conductivity of the PCM between adding xGnP and graphene. He found that although graphene has huge thermal conductivity, its composite possess a much lower thermal conductivity than xGnP's. The paraffin with a 10 wt% loading of xGnP produced a more than 10-fold increase in thermal conductivity of 2.7 W/mK. Xiang and Drzal [49] prepared the paraffin composite with different kinds of xGnP. It is found that higher thermal conductivity can be achieved with xGnP of larger aspect ratio, better orientation and lower interface density. Similar experiments [18,26,50–52] of preparing xGnP/paraffin composite are successively come out, shows the effective thermal conductivity of the EG fraction, listed here: 0.82 W/mK of 10 wt%, 0.452 W/mK of 20 wt%, 14.7 W/mK of 25 wt% and 7.654 W/mK, respectively. There is a large discrepancy among those enhanced composite, indicating the preparation of thermal conductivity enhanced PCM composite is quite important in deciding whether it possesses the high thermal conductivity. In the case of other PCMs, Huang et al. [53] prepared a LiNO3/ KCL-EG composite PCM for solar thermal energy storage application. The thermal conductivity of the PCM can be improved by 1.85 times using 10% EG and 6.65 times using 30% EG. And thermal conductivity can be significantly influenced by its density. Tian et al. [54] also prepared a metal salt/EG composite using a ternary eutectic chloride as PCM. Thermal conductivity of eutectic chloride with 5 wt% EG is 2.084 W/mK. And the addition of less than 1 wt% EG significantly increased the specific heat of the composite in liquid state. Wang et al. [55] prepared a form-stable PCM using microencapsulated PCM as latent heat storage material, highdensity polyethylene (HDPE) as matrix and EG as thermal conductivity promoting agent. Thermal conductivity of the composite with 20 wt% EG loaded could be enhanced from 0.43 W/mK to 4.59 W/mK. It is also experimented that thermal conductivity of the composite only increased to 0.98 W/mK when same ratio of regular graphite powder was used as promoting agent. 3.1.2. Graphite powder The microstructure of graphite powder is shown in Fig. 7, from where it is proved that graphite powder is of various shapes. Azeem and Zain–ul–Abdein [23] used P.A Hilton Heat Conduction Unit H-940 to test the thermal behavior of bakelite-graphite made with graphite powder. The results showed that the maximum thermal conductivity (kc ) value (for 55 vol% graphite) is 12.28 W/ mK. He also made a comparison with the theoretical models, indicating the models where the particle shape morphology was considered suited his experiment results best. Li [56] conducted the experiment measuring the heat performances of the carbon powder/paraffin composite by hot disk thermal constant analyzers with a Type 5464 probe, giving the maximum kc (for 10 wt%) with 0.9362 W/mK. Johansen et al. [22] prepared sodium acetate trihydrate and graphite powder mixtures and tested the thermal conductivity enhancing properties. The composite with 5 wt% graphite powder owns the thermal conductivity of 0.746 W/mK.

Fig. 6. SEM images of (a) the expandable graphite and (b) the EG [46].

3.1.3. Carbon fiber Since carbon fiber (CF) takes shape only when mixed with the PCM by melting-dispersion method, hot-press method, etc, there is no microstructure of single carbon fiber. The EDS mapping of the PCM/carbon fiber composite prepared by the hot-press method is shown in Fig. 8 [13]. Fukai et al. [33] investigated two types of fibers that is randomly oriented or brushed directional. The carbon fibers essentially enhance the effective thermal conductivity of the composite. The thermal conductivity of the carbon fiber/ Na2SO4  10 H2O mixture (2 vol.% carbon fiber) is 0.8 W/mK.

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Fig. 7. SEM image of graphite powder [23].

Fig. 8. EDS mapping from the cross-section of the PCCs: (a) a lower packing ratio of 0.59 and (b) a higher packing ratio of 0.71 [13].

Nomura et al. [13] compared two method of preparing PCM/carbon fiber composite and measured the effective thermal conductivity of the phase change composite by the laser flash method. It is confirmed that the novel hot-press method is a better preparation method than the conventional melt-dispersion method. The value of effective thermal conductivity of the composite (about 25 vol.% carbon fiber) is about 30 W/mK, where the thermal conductivity of PCM (erythritol) is 0.733 W/mK. It can be seen the well-prepared carbon fiber composite can dramatically enhance the thermal conductivity. Lately, Tian et al. [57] used EG and CF as hybrid thermally conductive fillers to enhance the thermal conductivity of form-stable paraffin. The utilization of EG and CF developed a synergistic enhancement to thermal conductivity. The slight orientation of molecular chains of CF and randomly dispersed EG both contributed to the thermal conduction, which have been discussed from the experiment results in detail. 3.1.4. Carbon nanotubes Carbon nanotubes (CNTs) are the promising nanomaterials with superior thermal features. CNTs with a very high aspect ratio have high thermal conductivity in axial. The microstructure of

three kinds of CNTs (short single-wall CNTs, long single-wall CNTs and multi-wall CNTs) with different aspect ratios observed by TEM method is shown in Fig. 9 [17]. As for the most common used multi-wall CNTs (MWCNTs), it is observed that the thermal conductivity of water/CNTs composite increased 3.0-3.1% for CNTs with a concentration of 0.01-0.1 wt% [40], indicating the MWCNTs are helpful in thermal conductivity enhancement. Similar investigation [58] to enhance the thermal conductivity of water showed a highest value of thermal conductivity enhancement ratio of 26%. For organics as PCM/medium, Li et al. [59] prepared the stearic acid/MWCNTs composite and the thermal conductivity can be improved by 5.7% only using 3.0 wt% of MWCNTs. It is also found that the dispersing addictive of poly vinyl pyrrolidone (PVP) can further promote the uniform distribution of MWCNTs in the nanocomposite and thus promote the thermal conductivity more. Li et al. [60] prepared the composite PCM with grafted CNTs and paraffin. It is proved that the grafted CNTs with polyhydric alcohols have the better dispersibility than CNTs’. The grafted CNTs with stearyl alcohol enhanced the thermal conductivity of paraffin from 0.2312 W/mK to 0.7903 W/mK.

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reaction with ball milling method. Then thermal conductivity of palmitic acid/treated CNTs was measured in both liquid state and solid state. It is found that the thermal conductivity is about 30% higher than the pure palmitic acid. Zhang et al. [64] treated the CNTs by surface modification with the polyethylene glycol. Then the nanofluid composite was prepared with the as-synthesized CNTs and ethylene glycol. It is proved that surface modification can both improve the suspension stability and the thermal conductivity of the CNTs-based nanofluids. And in general, thermal conductivity can be further enhanced by increasing the volume fraction of CNTs or increasing the temperature. For using single-wall carbon nanoparticles as promoters, Harish et al. [16] prepared lauric acid/chemically functionalized singlewalled carbon nanohorns composite and measured its thermal performance. Maximum thermal conductivity enhancement in solid and liquid phase at 2 vol.% is 37% and 11%, respectively. Xing et al. [17] designed an experiment using both SWCNTs and MWCNTs added into de-ionized water to measure the thermal conductivity enhancement. The thermal conductivity value of short-SWCNTs/water nanofluids varies from 0.604 to 0.707 W/mK for the temperature increasing from 10 to 60 °C, in the case of pure water which varies from 0.580 to 0.654 W/mK. Correspondingly, the thermal conductivity of L-SWCNTs and MWCNTs nanofluids are 0.627-0.760 W/mK and 0.598-0.687 W/mK. It can be seen that the long SWCNTs can produce the best effect in promoting thermal conductivity.

Fig. 9. The TEM images of CNTs: (a) S-SWNTs, (b) L-SWNTs and (c) MWNTs [15].

There are studies using other kinds of CNTs like SWCNTs and functionalized CNTs. Ji et al. [61] confirmed that the functionalized MWCNTs with more oxygen-containing groups have more hydrogen bonding interactions with the PCM molecules, so that the nanotubes are better dispersed in the solution and the thermal conductivity of the composite is significantly enhanced. Wang et al. [62] used four different methods, acid oxidation, mechanochemical reaction, ball milling, and grafting following acid oxidation to functionalize the MWCNTs with three groups, hydroxyl groups, carboxlic groups, and amidocyanogen introduced onto the surfaces of the MWCNTs. Then they are dispersed into palmitic acid respectively. It is found that the one containing MWCNTs with hydroxyl groups, treated by a mechanochemical reaction, has the best effect of enhancing thermal conductivity of the composite at room temperature. The composite with a MWCNTs addition of 1.0% has a thermal conductivity increase of 51.6%. Wang et al. [63] treated the CNTs and potassium hydroxide by mechanochemical

3.1.5. Graphene Graphene as a single layer structure of two-dimensional new carbon material, similar to graphite nanoflake in shape (Fig. 10), whereas has its unique physical properties like strong mechanical stability, excellent thermal conductivities and large specific surface area. Zhong et al. [20] prepared a PCM composite consisting of graphene aerogel (GA) and octadecanoic acid (OA). The GA enhanced composite had a thermal conductivity of about 2.635 W/ mK for 20 vol%, about 14 times that of the OA (0.184 W/mK). Dao and Jeong [66] used graphene to encapsulate stearic acid (SA) to form a core-shell composite microcapsule. The maximum thermal conductivity using solution mixing method with 5 phr graphene increases to 126% that of SA. Fu et al. [65] used few-layer graphene sheets as additives to obtain the epoxy/graphene composite. The results showed that graphene sheets can effectively enhance the thermal conductivity of epoxy matrix, with 4.01 W/mK at a loading of 10.10 wt%, more than 22 times of the pure epoxy resin. And the thermal conductivity enhancement produced by graphite nanoflakes and graphite are much lower compared to graphene sheets. Functionlized graphene are also studied in recent years. Thermal conductivity enhancement exists when reduced graphene oxide (rGO) sheets are added to silica microencapsulated neicosane [67], nitric acid functionalized graphene nanoplatelets are added to lauric acid (LA) [68] and graphene oxide nanosheets are added to water [69]. Moreover, Park and Kim [21] compared the thermal conductivity of three kinds of graphene water-based nanofluid. The results showed that fabricated by oxidized graphene have the better thermal conductivity compared with the other graphene nanofluids. 3.1.6. Comparison of different types of carbon additives The comparison of the thermal conductivity enhancement made by different types of carbon additives are listed in Table 3. As the true density of different kinds of carbon material is not quite different, it is convictive to claim that the additive with larger thermal conductivity increase has its advantage in practical application. It can be concluded that the carbon additives with higher aspect ratio have the better effect of thermal conductivity

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Fig. 10. SEM, TEM and HRTEM images of graphite nanoflake and graphene sheet (A, B and C are SEM, TEM and HRTEM images of graphite nanoflake respectively; D, E and F are SEM, TEM and HRTEM images of graphene sheet respectively) [65].

Table 3 Comparison of the thermal conductivity enhancement with different types of carbon additives. PCM

kf (W/m K)

Graphite

kp (W/mK)

Fraction

keff (W/m K)

Increase

Paraffin (solid) [48]

0.25

RT44HC [18] Eutectic LiNO3/KCl [53] Ternary eutectic chloride [54] Form-stable PCM [55] Erythritol [13] Paraffin [60] Palmitic acid [63] Water [17]

0.22 1.749 1.174 0.43 0.733 0.2312 0.22/0.16(solid/liquid) 0.580 (10 °C)

Bakelite [23] OA [20] Erythritol (solid) [12]

1.4 0.184 0.733

xGnP Graphene Expanded graphite Expanded graphite Expanded graphite Expanded graphite Carbon fiber Grafted CNTs Treated CNTs Short SWCNT Long SWCNT MWCNT Graphite powder Graphene aerogel (GA) Spherical graphite Expanded graphite

– – 129 – – – – – – – – – 130 2.183 80 200

10 wt% 10 wt% 25 wt% 30 wt% 5 wt% 20 wt% 25 vol% 4 wt% 1 wt% 0.48 vol% 0.48 vol% 0.48 vol% 30 vol% 20 vol% 17 vol% 15 vol%

2.7 0.5 14.7(42°c) 11.63 2.084 4.59 About 30 0.7903 0.33/0.21(solid/liquid) 0.604 0.627 0.598 4.84 2.635 2.126 4.691

1080% 200% 668% 665% 178% 1067% About 4000% 342% 150%/131% 104% 108% 103% 346% 1400% 290% 640%

enhancement. Carbon fiber with its largest aspect ratio can rebuild the microstructure of PCM and produce the significant thermal conductivity increase, but the preparation process is challenging. CNTs of a small amount of addition can make an impressive enhancement in thermal conductivity, due to its alike large aspect ratio. And the most commonly used EG and graphite powder remain the competitive promote additives. Graphene, as the advanced carbon material with special structure, has showed unusual chemical, electronic and thermal properties, but it is differently reported whether graphene has the comparable thermal conductivity enhancement ability to EG/CNTs [48,70]. Because of the dissimilar preparation methods they used, further research and novel synthesize process are awaited.

compared to carbon method. Table 4 shows some of the experiment results.

3.2. Thermal conductivity enhancement of phase change materials: metal or their oxides as additives/inserts

3.2.1. Metal foam Vacuum impregnation has been used to prepare composite PCM in the experiment conducted by Xiao et al. [19]. The thermal conductivities of the paraffin based composite PCMs incorporated by the copper foam with porosities of 96.95%, 92.31%, 88.89% and pore sizes of 25 PPI (Pores Per Linear Inch) were about 13, 31, 44 times larger than that of pure paraffin, respectively. The nickel foam exhibited weaker enhancement in thermal conductivity than copper, yet still about 3-5 times larger than that of paraffin. Thapa et al. [74] also achieved a conductivity of 3.7 W/mK in experiment testing via copper foam enhancement using an icosane wax as PCM. And there are researches [42,75] that confirmed the addition of metal foam can largely increase the overall heat transfer rate of PCM without thermal conductivity tested.

Metal along with its high thermal conductivity properties and strong mixing capability has long been an alternative material in this research. Several forms of metal are applied in the thermal properties enhancement process. Literatures are listed to show what degree they can achieve to enhance the thermal conductivity

3.2.2. Metal nanoparticles Metal is known as susceptible to oxidation, and considering the metal oxide still has much larger thermal conductivity than that of common PCM, the regular metal nanofluids refer to the both fluids with metal and metal oxide as dispersion nanoparticles. The

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Table 4 Comparison of the thermal conductivity enhancement with different types of metal additives. PCM

kf (W/mK)

Metal

kp (W/mK)

Fraction

keff (W/mK)

Increase

Paraffin [19]

0.354

Erythritol (solid) [12] Paraffin [71] Deionized water [72] Polyethylene glycol [73] Polyethylene glycol [86]

0.733 0.25 0.618 0.2985 0.2985

Copper foam Nickel foam Nickel particle Al Al2O3 β-AIN powder Silver nitrate

398.0 91.4 90.3 167   

88.89% (foam porosities) 90.61% (foam porosities) 35 vol% 9 wt% 3.0 wt% 30 wt% 1 wt%

16.01 2.33 4 5 About 6.61 0.7661 0.3522

4500% 658% 546% 2000% 107% 257% 118%

experiments using metal nanoparticles as thermal conductivity enhancement additives must be conducted in well-controlled environment. For instance, the metal nanoparticle suspensions prepared by Jana et al. [15] were composed with Cu/Au nanoparticle, laurate salt and deionized water, and the thermal measuring process was at room temperature of 25 °C. And the results showed the greatest thermal conductivity enhancement for water was produced by Cu nanoparticles, 74% increase with 0.3 vol%. Oya et al. [12] found that nickel particles of 14 vol% did not spread throughout the PCM, and nickels particles (NPs) of 24 vol% created many clusters. The value of effective thermal conductivity of the PCM composite with 17 vol% NPs was increased by 290% compared to that for pure PCM, almost the same as that of spherical graphite. It can be seen that metal additives regardless of metal foam or metal particles can increase the thermal conductivity a large amount. However the high density and weak thermal/chemical stability must be taken into account. For metal oxide nanoparticles, CuO, Al2O3, TiO2, CO3O4, Fe3O4, et al. [72,76–79] have been studied in thermal conductivity and heat transfer enhancement of nanofluids. For the convenience of browsing, here lists the thermal conductivity enhancement of these metal oxides in order. CuO incorporated in liquid cyclohexane (4 wt%) and solid cyclohexane (2 wt%) has the thermal conductivity of 0.124 and 0.34 W/mK [80]. Al2O3 incorporated in methanol (0.15 vol%), silicone rubber (0.6 vol%), deionized water (3.0 wt%) and ethylene glycol/water mixture (1.5 vol% at 60 °C) has the thermal conductivity of 0.26, 2.1, 0.661 and 0.70 W/mK, respectively [36,72,77,81]. TiO2 incorporated in methanol (0.15 vol%) has the thermal conductivity of 0.25 W/mK [77]. And Fe3O4 incorporated in kerosene (1.0 vol%) and paraffin (20 wt%) has the thermal conductivity increase of 134% and 60% (0.40 W/ mK), respectively [79,82]. To better improve the dispersion of the nanoparticles in PCM, Nourani et al. [83] used sodium stearoyl lactylate as a surfactant. During the sample preparation, a vacuum pump was utilized to prevent the formation of air bubbles, and ultrasonic vibration was used to ensure uniform distribution of the nanoparticles in the base fluids. The results showed that effective thermal conductivity enhancement for the sample with 10 wt% nano-Al2O3 were 31% and 13% in the solid and liquid states. Sahan et al. [82] used the cost-effective sol-gel method to mix the paraffin and nano magnetite (Fe3O4) as a composite, obtaining 48% and 60% of thermal conductivity enhancement for 10 wt% and 20 wt% of magnetite. Furthermore, Nemade et al. [84] reported a novel approach in thermal conductivity enhancement of CuO based nanofluids via probe sonication time. CuO nanoparticles were grown by spray pyrolysis technique. The experiment results show that thermal conductivity increased smoothly with probe sonication time. This is attributed to tinnier particle sizes and better dispersion of CuO nanoparticles in longer probe sonication time. CuO based nanofluid achieved 18% of enhancement in thermal conductivity over base fluid for 60 min of probe sonication time.

3.2.3. Metal salt Metal salt is another form of metal additives with high thermal conductivity and reliable stability. Fauzi et al. [85] tended to incorporate an acid-based surfactant (sodium laurate) to increase the latent heat storage capacity and decrease the subcooling of PCM material. Then it was found that the addition of 10% sodium laurate to Myristic acid (MA)/palmitic acid (PA) eutectic mixture had also increased the thermal conductivity from 0.225 to 0.235 W/mK. Wang et al. [73] blended polyethylene glycol, silica gel and β-Aluminum nitride (β-AIN) powder to produce a high thermal conductivity form-stable PCM. The thermal conductivity increased from 0.2985 W/mK (pure polyethylene glycol) to 0.7661 W/mK when the amount of β-AIN were 30 wt%. Sharma et al. [86] also used ethylene glycol as a solvent PCM and merged silver nitrate nanoparticles into it to synthesize the thermal conductivity enhanced nanofluids. Thermal conducitivity of silver nanofluids increased to 18% with 1 wt% of silver particles, compared to about 157% increase with 30 wt% of AIN. Thermal stability of the metal salt nanofluids was also found to be very reliable. 3.3. Thermal conductivity enhancement of phase change materials: fins and other methods Fins are another way to enhance the thermal conductivity other than inserting or adding additives into PCMs. Liu et al. [87] conducted an experiment in a vertical annulus energy storage system to investigate the thermal characteristics. A new copper fin was designed to connect the electrical heating rod to enhance the thermal conductivity of the stearic acid. The conduction and the natural convection heat transfer of the PCM have been enhanced to be as high as 250% in solidification. And their serial experiment [88] showed that the equivalent thermal conductivity of the PCM fixed with a new type of fin can be augmented to 300% in melting process. Mat et al. [89] investigated the melting process in a triplex-tube heat exchanger with a PCM numerically. The results showed that the melting time was reduced to 43.3% than that without fins. Kiho Kim and Jooheon Kim [90] used a magnetic field to vertically align the anisotropic boron nitride (BN) along the direction of heat transport in epoxy. Iron oxide nanoparticles are deposited onto the BN surface to achieve this. The results showed that the thermal conductivity of the synthesized vertically aligned composite increased from 1.765 W/mK to 3.445 W/mK with 30 vol.% filler loading compared with a randomly oriented compostie. Later Kim et al. [91] replaced the BN with BN/SiC binary filler. The SiC nanoparticles hindered BN particles aggregation and led to the formation of a three-dimensional heat conduction path, resulting in the maximum thermal conductivity (5.77 W/mK) of 3.08-fold and 1.1-fold higher than that of randomly mixed BN and vertically aligned BN composites, respectively. Other approaches like mescoporous silica (MPSiO2) nanopaticles [92] of 5 wt% in the noctadecane PCM enhanced the thermal conductivity of 5% for 3 wt% MPSiO2 at 5 °C and 6% for 5 wt% MPSiO2 at 55 °C. Hexagonal

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boron nitride (H-BN) nanosheets [93] of 10 wt% in a paraffin-based PCM enhanced the thermal conductivity of up to 60%. The microcapsule technique, like silica encapsulation of n-octadecane [94] and calcium carbonate (CaCO3) encapsulation of n-octadecane [95], has the effect of enhance the thermal conductivity as well. Other ways to enhance the thermal conductivity include interfacial modification [96], the fluid adsorption process [97] and the situ doping [98], etc. 3.4. Comparison of different ways to enhance thermal conductivity of phase change materials Overall the methods to enhance thermal conductivity of PCM can be divided into two categories: fixed and stationary high conductivity inserts/additives, and extrinsic enhancement methods like fins and PCM encapsulation, etc. As for the stationary inserts/ additives, carbon material with the characteristics of diversity of allotrope, low density, thermal stability, has quite a few candidates for choice. Metal inserts/additives have the even higher thermal conductivity enhancement, yet their incorporation in PCM is limited in many circumstances due to the chemical activity and high density. Fins are normally used to directly enhance the heat transfer, the equivalent thermal conductivity can be very impressive. Selection of these enhancement methods are based on the factor of PCM, price, preparation, operation condition, stability, etc.

4. Conclusions and outlook A review of theoretical and experimental work to promote the thermal conductivity of the PCM was presented. The need to promote the energy charging/discharging rates has been the impetus to find a better way to enhance the thermal conductivity of PCM. Focusing on placement of stationary high conductivity additives/ inserts, the thermal conductivity promoter materials have been carbon materials, copper, aluminum, nickel, metal salt, etc. The reviewed research work encompassed a variety of approaches to enhance the thermal conductivity, from effective medium method to MD simulation in theory and from carbon additives to metal nanoparticles in experiments. The tradition theoretical method using effective medium theory has long been established and developed, numerous research has used it to see if the experimental results can be fit for the theoretical results. A variety of thermal conductivity inserts/additives have been discussed. Models suited for each application circumstances must be brought out to predict the thermal behavior. In practical applications, significant advances of thermal conductivity are still needed to make it sufficiently efficient in meeting the requirements of most market applications. Except for carbon/metal promoters, other novel thermal conductivity enhancement materials are meant to be discovered. And systematic evaluation for different promote materials and manufacture processes are looking forward to be presented to help with the heat transfer of the PCM for thermal storage.

Acknowledgments This project is supported by the National Natural Science Foundation of China (Grant no. 51376087) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also wish to thank the reviewers for kindly giving revising suggestions.

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