A review of phase change material and performance enhancement method for latent heat storage system

Renewable and Sustainable Energy Reviews 93 (2018) 245–259

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

A review of phase change material and performance enhancement method for latent heat storage system

T

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Y.B. Tao , Ya-Ling He Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Latent heat storage Phase change material Performance enhancement Optimization

Latent heat storage (LHS) is considered as the most promising technique for thermal energy storage, due to its high energy storage density and nearly constant working temperature. However, the lower thermal conductivity of the phase change material (PCM) used in LHS system seriously weakens thermal energy charging and discharging rates. In order to improve the thermal performance of LHS system, a lot of research on performance enhancement have been carried out. This review paper will concern on the development of PCMs and performance enhancement methods for LHS system in the last decade. The available enhancement methods can be classified into three categories: using high thermal conductivity additives and porous media to enhance PCM thermal conductivity, using finned tubes and encapsulated PCMs to extend heat transfer surface, using multistage or cascaded LHS technique and thermodynamic optimization to improving the heat transfer uniformity. The comparative reviews on PCMs, corresponding performance enhancement methods and their characteristics are presented in present paper. That will help in selecting reliable PCMs and matching suitable performance enhancement method to achieve the best thermal performance for PCM based LHS system. In addition, the research gaps in performance enhancement techniques for LHS systems are also discussed and some recommendations for future research are proposed.

1. Introduction New energy development and industrial waste heat recovery are becoming more and more important to solve the increasingly serious problems of energy crisis and environmental pollution. However, both new energies such as solar energy and wind energy, and waste heat in conventional industries are typically unsteady with periodic and intermittent. Thus, thermal energy storage (TES) has been a key technology to ensure new energy and industrial waste heat recovery systems operation with high efficiency and high stability. Studies on high efficient TES technologies have been a scientific concern over the past few decades. There are three kinds of TES technologies, including sensible heat storage (SHS), latent heat storage (LHS) and thermochemical heat storage (TCHS). LHS system uses phase change material (PCM) as thermal energy storage medium, where thermal energy is stored or retrieved during the phase transition process of PCM, melted from solid to liquid or solidified from liquid to solid. Comparing to SHS system, LHS has larger energy storage density and smaller temperature variation. In addition, LHS has excellent repeatability and controllability

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Corresponding author. E-mail address: [email protected] (Y.B. Tao).

https://doi.org/10.1016/j.rser.2018.05.028 Received 9 December 2016; Received in revised form 17 April 2018; Accepted 13 May 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.

compared to TCHS system. So, LHS is considered as the most promising technique for TES in the present stage. LHS has been successfully applied in solar thermal utilization, industrial waste heat recovery, building energy saving, electronics cooling, etc. With the applications of LHS, many investigations on the performances of PCMs and LHS systems have also been performed. Sharma et al. [1] summarizes the investigation and analysis of the available thermal energy storage systems incorporating PCMs for different applications. Kenisarin [2] reviews the investigations and developments of high-temperature PCMs perspective for storage thermal in the range of temperatures from 120 to 1000 ℃. Cardenas and Leon [3] summarizes the comprehensive thermophysical properties of inorganic salt compositions and metallic alloys, which could potentially be used as storage media in a high temperature (above 300 °C) LHS system. During the applications and investigations, the PCM's low thermal conductivity as the most significant drawback of LHS is becoming more and more obvious, which seriously slows down thermal energy charging and discharging rates. In order to improve the thermal performance of LHS system, a lot of research on PCMs and the performance

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PCM side thermal resistance. Therefore, the first performance enhancement method for LHS system is to decrease PCM side thermal resistance by enhancing PCM thermal conductivity. The high thermal conductivity porous media and nanoparticles are commonly used to form stable composite PCM (CPCM), which can efficiently enhance PCM thermal conductivity [7–9].

enhancement techniques have been carried out over the past decades. Those research results provide valuable reference for the performance enhancement and optimization of LHS system. Jegadheeswaran and Pohekar [4] summarizes the performance enhancement techniques reported in the literature before 2009, where the influence of enhancement techniques on the thermal response of PCM in terms of phase change rate and amount of latent heat stored/retrieved has been addressed. Fan and Khodadadi [5] reviewed the experimental/computational studies to enhance the thermal conductivity of PCM that were conducted before 2010. Liu et al. [6] also reviews the experimental and theoretical methods to enhance PCM thermal conductivity and the thermal conductivity inserts/ additives in recent investigations are listed and summarized. This review paper will concern on the development of phase change material and performance enhancement methods for LHS system in the last decade (from 2007 to present). The previous works can be found in Ref. [1–6]. The comparative reviews on different kinds of PCMs, the corresponding enhancement methods and their characteristics will be presented in present paper. That will help in selecting reliable PCM and performance enhancement method to achieve the best thermal performance for PCM based LHS system. In addition, the research gaps in the present performance enhancement methods for LHT systems will be discussed and some recommendations for the future research will be proposed.

2.1.1. Porous media When porous media is used, PCM is filled into the porous media to form CPCM. The porous media must have high thermal conductivity to efficiently enhance PCM thermal conductivity, high porosity to ensure enough PCM filled and keep high energy storage density. Metal foams such as copper [10–14], nickel [11,15] and aluminum foams [16], expanded graphite foams [17–29] and some expanded rocks such as perlite [30] and vermiculite [31] are used as the porous supporting material. The schematic diagrams for metal foams and metal foams CPCM are shown in Fig. 2. 2.1.2. Nanomaterial additives When nanomaterial additives are used to enhance PCM thermal conductivity, the nanomaterial will be dispersed into PCM to form uniform CPCM. The additives must have high thermal conductivity and chemical stability to ensure the PCM thermal conductivity can be efficient enhanced and no chemical reaction occurs. The carbon nanomaterials such as multi-walled carbon nanotubes (MWCNT) [32–40], single-walled carbon nanotubes (SWCNT) [32], graphite [34,36,38,41–43], graphene [36,38,39,44,45] and metal oxide nanoparticles [46–48] or metal nanoparticles [49] are commonly used as additives to enhance PCM thermal conductivity. The microstructures for the carbon nanomaterials/eutectic carbonate CPCMs are shown in Fig. 3.

2. Classification of performance enhancement methods for LHS system The basic heat transfer equation for an arbitrary heat transfer process can be expressed as (1)

Φ = KAΔt

2.2. Extending heat transfer surface

where, Φ is heat transfer rate, W; K is heat transfer coefficient between cold and heat objects, W m−2 K−1; A is heat transfer area, m2; Δt is heat transfer temperature difference. According the equation, it can be found that there are three key parameters (K, A and Δt ) which are positive correlation with heat transfer rate. So, although an extensive research is carried out in performance enhancement of LHS system, totally speaking, the performance enhancement methods can be classified into three categories as shown in Fig. 1: enhancing PCM thermal conductivity, extending heat transfer surface and improving uniformity of heat transfer process.

According to, Eq. (1), the second way to improve heat transfer performance is increasing heat transfer area. Extended heat transfer surface, including finned tubes and encapsulated PCMs are widely used to increase heat transfer area between HTF and PCM and enhance LHS performance. 2.2.1. Finned tube For shell-and-tube LHS system, the finned tubes are used to enhance the heat transfer performance for both the PCM side and heat transfer fluid (HTF) side. The axially fins [49–54], radially fins [55–60] are commonly used to enhance PCM side heat transfer performance. Some other finned tubes such as dimpled tube, cone-finned tube and helically finned tube [61] are used for the HTF side enhancement. At the same time, some heat pipes and finned heat pipes are also used to enhance PCM thermal performance [62–69]. The fin material must have high thermal conductivity to achieve the better enhancement effect. Fig. 4 shows the structure of the axially finned tube and radially finned tube used for LHS. Recently, Dhaidan and Khodadadi [70] reviewed the analytical, computational and experimental studies on the LHS performance enhancement with high thermal conductivity fins, where some interesting conclusions were derived.

2.1. Enhancing PCM thermal conductivity According to Eq. (1), increasing heat transfer coefficient (K) is an efficient way to enhance LHS rate. K is the reciprocal of total thermal resistance, including HTF side thermal resistance, PCM side thermal resistance and wall thermal resistance. In order to increase the heat transfer coefficient, the total thermal resistance must be decreased. For most of the LHS process, the total thermal resistance is dominated by Porous media Enhancing conductivity

LHS Performance enhancement methods

PCM Nanomaterial additive

Extending heat transfer surface

2.2.2. Encapsulated PCM Another method to extend the heat transfer surface of LHS is using encapsulated PCM, where the PCM are encapsulated to form stable capsules. The capsules are accumulated in LHS container and HTF flows through the gaps among different capsules, as shown in Fig. 5. The heat transfer area can be efficiently augmented by this method. According to the dimensions of the capsules, it can be divided into microcapsules [71–76] and macrocapsules [77–89]. For the microcapsules, the dimensions of the microcapsules are usually in micron level, and the dimensions for the macrocapsules are in millimeter level.

Finned tube Encapsulated PCM

Improving uniformity

process

Multiple PCMs Process optimization

Fig. 1. Classification of performance enhancement methods for LHS system. 246

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Fig. 2. Schematic for metal foams and metal foams CPCM.

2.3.1. Multiple PCM Using multiple PCMs to form cascaded or multi-stage LHS system is an efficient method to improve the uniformity of heat transfer process [90–102]. The schematic for cascaded LHS system is shown in Fig. 6. Based on HTF temperature variation when it flows through the LHS system, the PCMs with different melting temperatures are arranged according to the melting temperature successively increasing or decreasing. For charging process, the HTF temperature gradually decreases, so the PCMs melting temperature should gradually decrease along HTF flow direction. The PCM with high melting temperature should be arranged in the HTF inlet region and low melting temperature PCM is arranged in the HTF outlet region. For discharging process, because the HTF temperature gradually increases, the HTF should flows from the lower melting temperature PCM region to the high melting temperature PCM region. With this method, the heat transfer

2.3. Improving heat transfer process uniformity According to Eq. (1), the third way to improve heat transfer performance is increasing heat transfer temperature difference. For most of LHS applications, HTF inlet temperature and PCM initial temperature are given. It is difficult to increasing the total temperature difference between HTF and PCM. However, the temperature difference in different location of the LHS equipment is controllable. The heat transfer process can be enhanced by controlling a nearly uniform temperature difference in different time and location of the whole heat transfer process. Therefore, the third commonly used performance enhancement method for LHS system is to improve the uniformity of the heat transfer process between HTF and PCM. Cascaded LHS system with multiple PCMs and thermodynamic optimization are widely used to improve the uniformity of LHS process.

Fig. 3. SEM images of carbon nanomaterials/eutectic carbonate CPCM [39]. 247

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Fig. 4. Schematic of finned tube LHS system.

copper foam, nickel foam and aluminum foam are the mostly used porous media to enhance the thermal conductivity of organic PCMs with low melting temperature, such as paraffin [10–13,16], succinonitrile [14] and n-carboxylic acids [15]. This is because metal foam has excellent thermal conduction performance (e.g. the thermal conductivity for copper foam is about 400 W m−1 K−1, which is much higher than PCM), higher porosity (up to 90%) and chemical stability in lower temperature region. For high temperature PCMs (e.g. salts [24–29]), expanded graphite is mostly used to enhance PCM thermal conductivity. This is because expanded graphite has high thermal conductivity (up to 300 W m−1 K−1, high porosity (up to 90%)) and chemical stability even in high temperature region. In addition, there are some other kinds of porous media are used such as expanded perlite and expanded vermiculite, but due to its lower thermal conductivity, the enhancement effect is weak. 3.1.1. Metal foam With metal foams, the thermal conductivity of PCM can be enhanced dozens of times [11,12,15]. Xiao et al. [11] prepared the CPCM with copper/nickel metal foams and paraffin PCM, experimental results show that the thermal conductivities of the CPCMs are drastically enhanced, e.g., the thermal conductivities of the CPCMs fabricated by the copper foams with the porosities of 96.95%, 92.31%, 88.89% are about 13, 31, 44 times larger than that of pure paraffin. Wang et al. [12] also prepared copper metal foams/paraffin CPCM and found that the thermal conductivity of the prepared CPCM is increased up to 48 times compared with pure paraffin. In addition, copper foam can effectively improve the internal heat transfer uniformity of paraffin, which reduces the heat storage time of paraffin wax by 40%. Liang et al. [15] prepared form-stable CPCMs with graphene-nickel foam/n-carboxylic acids. Thermal performance testing results show the thermal conductivity of the CPCM can be increased about 14 times as compared with that of pure PCM. The geometric parameters of metal foam have important influences on the thermal performance of metal foam CPCM [10,13,14]. Although the high thermal conductivity of metal foam is very efficient to enhance PCM thermal conductivity, it also restricts the natural convection of liquid PCM, which causes complex effects on total thermal performance of LHS system. Liu et al. [10] numerically investigated the thermal performance of a shell-and-tube LHS unit with copper foams/paraffin CPCM. The results show that compared with the pure PCM, the phase change heat transfer can be enhanced by more than 7 times. Moreover, the metal foam structure characteristics and the HTF inlet conditions have significant influences on LHS performance. Tao et al. [13] numerically investigated the LHS performance of copper metal foams/ paraffin CPCM with lattice Boltzmann method. The effects of porosity and pore density on PCM melting rate, heat storage capacity and heat storage density were investigated. The results show that the CPCM heat transfer performance is determined by both heat conduction and natural convection. The geometric parameters of metal foams have great effects on heat transfer performance, e.g., increasing pore density can enhance heat conduction, but weaken natural convection. Based on the results, an improved metal foam structure was presented. Zhao et al. [14] numerically investigated the solidification process of succinonitrile in open-cell metal foams by phase field method. Effects of Rayleigh number, porosity and pore density on the solid-liquid phase change were examined. Wang et al. [16] successfully applied the paraffin/

Fig. 5. Schematic of LHS unit with encapsulated PCM.

Fig. 6. Schematic of multi-stage LHS system.

temperature difference in the whole LHS system can be controlled in a nearly equal value, which is beneficial to improve the total heat transfer performance of LHS system. 2.3.2. Thermodynamic analysis and optimization Some thermodynamic analysis and optimizations were also performed both for the single and multi-stage LHS systems to further improve the thermal performance of the LHS system, which had been summarized in Ref. [103]. The analysis and optimization methods include energy analysis [104–108,110,112,114,118], exergy or entropy analysis [105,108–114] and entransy analysis [115–118]. The concept of entransy was introduced by Guo et al. [119] to characterize the heat transfer ability of an arbitrary object, and the entransy dissipation can be used to evaluate the heat transfer ability loss during the irreversible heat transfer process. The entransy dissipation theory had been successfully used in the optimization of heat conduction process [120], convection heat transfer process [121], and radiation heat transfer process [122]. 3. Recent development on PCM thermal conductivity enhancement 3.1. Porous media The research works carried out on PCM thermal conductivity enhancement with porous media in the last decade are summarized in Table 1. From the table, it can be seen that metal foams including 248

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Table 1 Summary of PCM thermal conductivity enhancement with porous media. porous supporting material

Phase change material

Melting temperature /°C

Reference

Copper foam Copper and nickel foam Copper foam Copper foam Copper foam Graphene–nickel foam Aluminum foam Expanded graphite Graphite foam Expanded graphite Expanded graphite Expanded graphite Expanded graphite Expanded graphite Graphite foam Expanded graphite Expanded graphite Expanded graphite Graphite foam Graphite foam Expanded perlite Expanded vermiculite

Paraffin Paraffin Paraffin Paraffin Succinonitrile n-carboxylic acids Paraffin Paraffin Paraffin Acetamide Neopentyl glycol Polyethylene glycol Stearic acid D-Mannitol MgCl2 LiNO3/KCl KNO3/NaNO3 NaNO3/KNO3 NaCl MgCl2 Capric-myristic acid Stearic acid

48–62 60–62 42.24 54.0 58.09 50.6–62.8 46–52 41.6 58 66.95 45 41.8–50.9 68.88 151.82 714 165.60 223 220 800 714 21.70 67.2

Liu et al. [10] Xiao et al. [11] Wang et al. [12] Tao et al. [13] Zhao et al. [14] Liang et al. [15] Wang et al. [16] Sarı and Karaipekli [17] Zhong et al. [18] Xia and Zhang [19] Wang et al. [20] Lv et al. [21] Wu et al. [22] Xu et al. [23] Zhao et al. [24] Huang et al. [25] Zhao et al. [26] Xiao et al. [27] Singh et al. [28] Singh et al. [29] Karaipekli and Sarı [30] Li et al. [31]

For high temperature PCM, Zhao et al. [24] numerically investigated the LHS performance of graphite foam/MgCl2 CPCM. Results show that the graphite foam can help to improve the heat transfer performance as well as the exergy efficiency significantly in the LHS system. Huang et al. [25] prepared LiNO3/KCl-EG CPCM for thermal energy storage application at high temperature (~ 200 °C). The tested results show that the thermal conductivities of the composites were 1.85–7.56 times higher compared with the eutectic LiNO3/KCl. Zhao et al. [26] prepared the expanded natural graphite-KNO3/NaNO3 CPCM. The thermal conductivities, phase transition properties, thermal stability and microstructures of different samples were evaluated. The results show that the highest effective thermal conductivity of CPCM was 50.78 W m−1 K−1, which was 110 times higher than that of salt powder. Xiao et al. [27] prepared and investigated the heat transfer performance of nitrate/EG CPCM. Results show that the addition of EG significantly enhances thermal conductivities, e.g., the thermal conductivity of sodium nitrate with 20 wt% EG is about 7 times larger than that of pure sodium nitrate. Singh et al. [28] numerically investigated the thermal performance and exergy efficiency of a graphite foam/NaCl LHS storage system for supercritical CO2 power cycles. The results show that the graphite foam can improve the heat transfer performance of the LHS system significantly. After that, Singh et al. [29] prepared graphite foam/MgCl2 CPCM and characterized its thermal performance. Experimental results show that the CPCM thermal conductivity is about 200 times higher than that of pure MgCl2.

aluminum foam CPCM to the thermal management of Li-ion battery. The experimental results show that the use of aluminum foam can speed up the melting process and improve the temperature uniformity of the PCM. 3.1.2. Expanded graphite Expanded graphite (EG) has excellent thermal conductivity and chemical stability even in high temperature region. It can be used to enhance the thermal conductivity not only for the low temperature organic PCMs (e.g. paraffin [17,18], acetamide [19], glycol [20,21], stearic acid [22] and mannitol [23]), but also for the high temperature salts PCMs (e.g. chloride [24,25,28,29] and nitrate [25–27]). In the low temperature field, Sarı and Karaipekli [17] prepared paraffin/EG CPCMs with different EG mass fractions, and experimentally tested their thermal performance. The results show that the thermal conductivity can be increased from 0.22 to 0.82 W m−1 K−1 with 10 wt% EG. Zhong et al. [18] prepared the CPCMs with paraffin wax and graphite foam and characterized their thermal performance. The results show that CPCM thermal diffusivity can be enhanced up to 570 times as compared with that of pure paraffin wax. Xia and Zhang [19] prepared acetamide/EG CPCM and found that the addition of 10 wt% EG results in a more than five-fold increase in the thermal conductivity. Wang et al. [20] prepared the CPCM of neopentyl glycol/ compressed expanded natural graphite and found that the thermal conductivities can be enhanced 11–88 times as compared with that of the pure PCM. Lv et al. [21] prepared the CPCM of polyethylene glycol (PEG)/EG with different mass fraction. Experimental results show that the addition of EG can enhance the thermal conductivity of PEG significantly. The thermal conductivity increases as the EG mass fraction increases. For the CPCM with 10 wt% EG, the thermal conductivity can be enhanced about 20 times of the pure PCM. Wu et al. [22] proposed a new synthesis method to prepare formstable EG/stearic acid CPCM and found the CPCM has obvious anisotropic thermal conductivity and the largest difference between axial and radial thermal conductivity reaches up to 4 times. Both axial and radial thermal conductivities can be enhanced significantly by using the additive of EG, and the radial thermal conductivity is as high as 23.27 W m−1 K−1, which is about 130 times higher than that of pure PCM. Xu et al. [23] prepared D-Mannitol/EG CPCM for thermal energy storage operated at 180–240 °C. The results show that the thermal conductivity of pure D-Mannitol is improved by about 12 times with EG mass fraction of 15%.

3.1.3. Other porous media Besides the metal foam and expanded graphite or graphite foam, some other porous media are also be used to form stable CPCM. Karaipekli and Sarı [30] prepared capric-myristic acid/expanded perlite CPCM. Experimental results show the prepared CPCM has a low thermal conductivity (0.048 W m−1 K−1) due to low thermal conductivity of the expanded perlite. In order to improve the thermal conductivity, the EG was added to the composite in mass fraction of 10%. The further experimental results show thermal conductivity of the form-stable CPCM was increased about 58% by adding 10 wt% EG. Li et al. [31] prepared stearic acid/modified expanded vermiculite CPCM to enhance thermal conductivity and latent heat simultaneously. The results show that the thermal conductivity is enhanced from 0.25 to 0.58 W m−1 K−1. From the above literature review, it can be concluded that for the low temperature PCMs (e.g. paraffin, fatty acids, Aliphatic 249

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Table 2 Summary of PCM thermal conductivity enhancement with nanomaterial additives. Nanomaterial additives

Phase change material

Melting temperature /°C

Reference

MWCNTs MWCNTs MWCNTs, graphite MWCNTs MWCNTs, graphene, graphite MWCNTs MWCNTs, Graphene, Graphite SWCNTs, MWCNTs, graphene, C60 MWCNTs Exfoliated graphite nanoplatelets Nano-graphite Graphite Nitrogen-doped graphene Graphene SiO2, Al2O3, Fe2O3, ZnO nanoparticles CuO nanoparticles TiO2 nanoparticles Copper particles

Palmitic acid Palmitic acid Paraffin Stearic acid Stearic acid Paraffin Stearic acid Li2CO3/K2CO3 Li2CO3/K2CO3 Paraffin Paraffin Polyethylene Palmitic acid Stearic acid Paraffin KNO3, NaNO3, KNO3/NaNO3 Palmitic acid Water

NA 61.6–62.7 60.74 62.0–63.2 53.83–60.39 NA 64.0–71.0 488 488 55.2 30.45 130 64–67 71.1–72.4 53–57 334, 306, 222 62 0

Wang et al. [32] Ji [33] Teng et al. [34] Li et al. [35] Li et al. [36] Li et al. [37] Choi et al. [38] Tao et al. [39] Tao et al. [40] Kim and Drzal [41] Li [42] Yang et al. [43] Mehrali et al. [44] Dao et al. [45] Babapoor and Karimi [46] Jr et al. [47] Sharma et al. [48] Lohrasbi et al. [49]

and it can be further improved by 11.4% when both the additives of 3.0 wt% MWCNTs and dispersing additive are incorporated with the PCM. Tao et al. [40] experimentally investigated the effects of nanomaterial dispersion and surface active agent (SAA) on thermal performance of carbonate salt/MWCNTs NCPCM. The results show that the effect of SAA on NCPCM thermal performance has duality: on the positive side, SAA can improve nanomaterial dispersion and enhance NCPCM thermal performance; on the negative side, SAA decomposition products may weaken NCPCM thermal performance. So, SAA and its mass fraction should be selected carefully. Sodium dodecyl sulfate (SDS) is a better SAA for high temperature NCPCM and a high mass ratio of SDS to nanomaterial is recommended. With mass ratio of SAA to nanomaterial 10:1, PCM thermal conductivity can be enhanced up to 58.75% by adding 1 wt% MWCNTs. The adding of nonmaterial can enhance the thermal conduction performance of PCM, but also weakens the natural convection of liquid PCM. Li et al. [36] prepared three CPCMs with stearic acid and different carbon additives (MWCNTs, graphene, graphite). The experimental results indicate that the addition of carbon additives can improve the heat conduction of stearic acid effectively, but it also weakens the natural convection of stearic acid in liquid state. The microstructure of nanomaterials also has great effects on CPCM thermal performance [28–30]. Li et al. [37] prepared the NCPCM with grafted MWCNTs and paraffin. The comparative study was performed for the PCM/MWCNTs and PCM/grafted MWCNTs. The results show that the grafted MWCNTs were shorter than MWCNTs and the dispersibility of grafted MWCNTs was better than that of MWCNTs. The thermal conductivity of grafted MWCNTs /paraffin CPCMs was higher than that of pristine MWCNTs /paraffin CPCMs. Choi et al. [38] adopted three kinds of carbon nanomaterials (MWCNTs, graphite and graphene) to enhance the thermal energy storage performance of stearic acid. It is found that the heat transfer rate enhances up to 3.35 times in the case of graphite at 5.0 vol%. It is finally concluded that graphite is the most promising candidate for heat transfer enhancement of stearic acid among three carbon additives. To enhance the performance of high temperature salt PCM and reveal the effects of nanomaterial microstructures on PCM thermal properties, Tao et al. [39] mixed four kinds of carbon nanomaterials with different microstructures (SCWCNTs, MWCNTs, graphene and fullerene C60) into binary carbonate eutectic salts to prepare carbonate salt/nanomaterial NCPCM. The thermal properties such as melting point, melting enthalpy, specific heat, thermal conductivity and total thermal energy storage capacity were characterized. The results show that the nanomaterial microstructure has great effects on CPCM thermal properties, as shown in Fig. 7. The graphene with sheet structure is the

hydrocarbon, alcohol etc.), the metal foams are most efficient to enhance PCM thermal conductivity; for the high temperature PCMs (e.g. salts), the expanded graphite is suitable to enhance PCM thermal conductivity. However, it should be pointed that although the high conductivity porous media can efficiently enhance PCM thermal conductivity and improve the thermal performance of LHS system, the geometric parameters of porous media such as porosity and pore density have great effects on CPCM thermal performance. Some deeply investigations on the interactions between the porous skeletons and PCM should be further performed to optimize the configuration of the porous media and obtain the best thermal performance. 3.2. Nanomaterial additive The CPCM with nanomaterial additives is named nanocomposite PCM (NCPCM). The research progress on PCM thermal conductivity enhancement with high thermal conductivity nanomaterial additives in the last decade is drawn in Table 2. From the table, it can be seen that carbon nanomaterials including MWCNT, graphite, graphene are the mostly used additives to enhance PCM thermal conductivity both for the low temperature organic PCMs (e.g. acid [32,33,35,36,38,44,45] and paraffin [34,37,41,42,46]) and high temperature salts PCMs (e.g. carbonate [39,40]). Besides that, some metal oxide or metal particles are also used for the performance enhancement of low temperature PCMs [46,48,49]. 3.2.1. Carbon nanomaterial MWCNT is the most commonly used additives for PCM thermal conductivity enhancement. Wang et al. [32] added MWCNTs into palmitic acid to enhance its thermal conductivity and found the thermal conductivity is enhanced about 30% with 1 wt% MWCNTs. Ji et al. [33] also prepared the NCPCM of palmitic acid/MWNTs. Experimental results show the thermal conductivity enhancement increases with MWCNTs mass fraction increase. When the mass fraction of MWCNTs is 7%, the thermal conductivity can be enhanced about 65%. Teng et al. [34] prepared NCPCM with mixing MWCNTs and graphite into paraffin. The results show that adding the MWCNTs is more effective than graphite in modifying the thermal storage performance of paraffin. The carbon nanomaterial additives can enhance PCM thermal conductivity, but the enhancement effect greatly depends on the dispersion of the nanomaterial additives in PCM [35,40]. Li et al. [35] prepared stearic acid/MWCNTs NCPCM to investigate the effects of MWCNTs mass fraction and dispersing additives on PCM thermal properties. Experimental results show that the thermal conductivity of stearic acid can be improved by 5.7% only using the additive of 3.0 wt% MWCNTs, 250

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conductivity of the CPCM prepared using solution mixing method with 5% graphene increases to 126% of SA. However, the conductivity of a composite prepared by latex mixing with 5% graphene increases to 163% of SA. 3.2.2. Metal and metal oxide nanoparticle The thermal conductivity and diffusivity of PCM can be enhanced significantly by adding metal oxide nanoparticles [46–48]. Babapoor and Karimi [46] used various nanoparticles including SiO2, Al2O3, Fe2O3, ZnO and their combinations as thermal conductivity promoter to produce modified paraffin PCM. The thermal conductivity and diffusivity of the composites are enhanced by increasing the nanoparticles mass fractions and Al2O3 nanoparticle has significant potential for enhancing the thermal storage characteristics of the paraffin mixture. Jr et al. [47] experimentally investigated the effect of CuO nanoparticle on the thermal performance enhancement of nitrate salts LHS system. The results show the improvements in diffusivity and conductivity are clearly demonstrated for the inclusion of small amounts (2 vol%) CuO nanoparticles in nitrate salts. Sharma et al. [48] investigated the phase change behavior of prepared NCPCM with palmitic acid and TiO2 nanoparticles for thermal energy storage. Results show the thermal conductivity of palmitic acid increased by 12.7%, 20.6%, 46.6%, and 80% for the nanoparticle weight fractions of 0.5%, 1%, 3%, and 5% respectively. Lohrasbi et al. [49] used copper particles to enhance thermal performance of water as PCM and numerically investigated the effect of nanoparticles volume fractions on solidification rate. Results indicate that adding nanoparticles has considerable effect on LHS performance during solidification process and when the volume fraction of copper nanoparticles is 2.5% and 5%, full solidification time decreases 8.8% and 16.9% respectively. From the above literature review, it can be concluded that the high conductivity nanomaterial additives can also efficiently enhance PCM thermal conductivity and improve the thermal performance of LHS system. However, the dispersion of nanomaterial in PCM directly affects the CPCM thermal performance. In order to improve nanomaterial dispersion, SAA is usually used during CPCM preparation process. Therefore, the actual NCPCM is a composite of nanomaterial/PCM/ SAA. The interactions among them are very complicated, which results in the performance enhancement effects are different in different references. Some further studies should be performed to investigate the interaction mechanism among nanomaterial, SAA and PCM, which is very important for performance optimization of nanomaterial CPCMs.

Fig. 7. Effects of nanomaterial microstructure on CPCM thermal properties [39].

best additive to enhance specific heat, which could be enhanced 18.57% with adding 1.5 wt% graphene. The SWCNTs with columnar structure is the best additive to enhance thermal conductivity, which could be enhanced 56.98% with adding 1.5 wt% SWCNTs. Graphite is another effective additive to PCM thermal conductivity enhancement. Kim and Drzal [41] prepared exfoliated graphite nanoplatelets (xGnP)/paraffin CPCM. The results show that when the thermal conductivity of pure paraffin is 0.26 W m−1 K−1, the thermal conductivity of the composite PCM including 7 wt% xGnP is found to be 0.8 W m−1 K−1. Li [42] prepared nano-graphite (NG)/paraffin CPCM. The results indicated that the NG layers were randomly dispersed in the paraffin, and the thermal conductivity increased gradually with the content of NG. Thermal conductivity of PCM can be enhanced from 0.1264 to 0.9362 W m−1 K−1 with 10 wt% NG. A recycled high-density polyethylene/graphite mixture, for medium temperature thermal energy storage application has been formulated and characterized by Yang et al. [43]. The results show that when the graphite content was in the ratio of 20 wt%, the thermal conductivity of the PCM increased from 0.51 up to 1.31 W m−1 K−1. Graphene is also used to enhance PCM thermal conductivity. Mehrali et al. [44] prepared nitrogen-doped graphene (NDG)/palmitic acid (PA) CPCM. Experimental results show that the thermal conductivity of the PCM containing 3 wt% NDG is 3.5 times higher than that of PA. Dao and Jeong [45] prepared stearic acid (SA)/graphene composite microcapsule PCM using a latex mixing method. The thermal properties of the prepared CPCM were compared with the CPCM prepared by solution mixing method. The results show that the

4. Recent development on extended heat transfer surface 4.1. Finned tube Finned tube is an efficient performance enhancement method for conventional heat exchanger. It is also effective to LHS system. The research progress on adopting finned tube to enhance performance of LHS system in the last decade is drawn in Table 3. From the table, it can be seen that both the axially and radially fins are used to enhance LHS performance not only for shell-and-tube LHS system [49–61] but also for the heat pipe assisted LHS [62–69], the corresponding PCMs are mostly lower temperature organic PCM. The materials for the fins are usually the high thermal conductivity metals such as aluminum [49,50,57,58,60,64,66], copper [50,51,53–56,60,66], steel [50,60], nickel [67–69], and so on. 4.1.1. Shell-and-tube LHS with fins Lohrasbi et al. [49] investigated the effects of adding Y-shaped fins and nanoparticles dispersion on PCM solidification process. Comparison between these two methods was carried out from the viewpoint of solidification acceleration and maximum energy storage capacity. The results show that applying fin with suitable array increases full 251

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Table 3 Summary of LHS performance enhancement with finned tube. Fin pattern

Fin material

Phase change material

melting temperature /°C

Reference

Axially finned tube Axially finned tube Axially finned tube Axially finned tube Axially finned tube Axially finned tube Radially finned tube Radially finned tube Radially finned tube Radially finned tube Radially finned tube Radially finned tube

Aluminum Copper, aluminum, and steel Copper NA Copper Brass Copper Copper Aluminum Aluminum NA Aluminum, copper, stainless steel and glass NA

Water RT82 Dodecanoic acid LiF/CaF2 RT50 Stearic acid Xylitol RT22 Polyethylene Dodecanoic acid, Sodium acetate trihydrate Methyl palmitate, paraffin, octadecane, polyglycol, etc LiF/CaF2

0 77–85 42.5 ± 0.5 767 45–51 55.7–64.1 93–94.5 19–23 NA 43 ± 0.5 NA 22–30

Lohrasbi et al. [49] Al-Abidi et al. [50] Murray and Groulx [51] Tao and He [52] Hosseini et al. [53] Rathod and Banerjee [54] Shon et al. [55] Zhao and Tan [56] Zauner et al. [57] Kabbara et al. [58] Dannemand et al. [59] Wang et al. [60]

767

Tao et al. [61]

No fin

KNO3

335

Heat pipe

No fin

NaNO3/KNO3

503

Axially finned heat pipes Axially finned heat pipe Radially finned heat pipe Radially finned heat pipe Radially finned heat pipe Radially finned heat pipe

Aluminum NA Copper, anodized aluminum Nickel Nickel Nickel

KNO3 KNO3 RT60, LiCl/KCl KNO3 KNO3 NaNO3/KNO3

335 335 60, NA 335 335 220

Nithyanandam and Pitchumani [62] Nithyanandam and Pitchumani [63] Khalifa et al. [64] Almsater et al. [65] Khalifa et al. [66] Tiari et al. [67] Tiari et al. [68] Tiari and Qiu [69]

Dimpled, cone-finned and helicallyfinned tubes Heat pipe

tube LHS unit with finned tube than HTF mass flow rate [54,58]. Rathod and Banerjee [54] analyzed the enhancement of total melting/ solidification time for shell-and-tube LHS unit with longitudinal fins. Experimental results show that the heat transfer augmentation is more sensitive to increase in HTF inlet temperature as compared to increase in mass flow rate of HTF. Kabbara et al. [58] experimentally investigated the effects of HTF inlet parameters on the charging and discharging performance of a LHS unit with finned tube heat exchanger. The results show that increasing the HTF inlet temperature during charging results in significantly faster melting time, however, the increase of flow rate did not have a significant impact during the discharge process. The fined tube has been used successfully in LHS unit for waste heat recovery and air-conditioning systems. Shon et al. [55] experimentally investigated the actual heat transfer coefficient of PCM in a fin-tube heat exchanger for an automobile coolant waste heat recovery system and found that improving convective heat transfer inside the tube is the most effective way to increase heat absorption efficiency. Zhao and Tan [56] numerical studied the performance of a shell-and-tube LHS unit with radially fins for air-conditioning application. The effects of HTF inlet temperature, mass flow rate and conductive fin height on the system performance were examined. In above literatures, the fins are usually used in PCM side to enhance the heat transfer performance, because in most of the studies the HTF are liquid with excellent convective heat transfer performance. Therefore, the thermal resistance in the LHS unit is dominated by the PCM side. However, when gas is used as HTF, the heat transfer performance of the HTF side is also poor, so enhancing the heat transfer performance of HTF side would be another efficient method to improve the total LHS performance. Tao et al. [61] established a shell and fintube LHS model with the gas mixture (He/Xe) as HTF and eutectic salts (LiF/CaF2) as PCM and the fins were placed in HTF side only. In order to investigate the effects of fin-tube patterns on LHS performance, three enhanced finned tubes were adopted which are dimpled tube, conefinned tube and helically-finned tube respectively. The results show that compared with the smooth tube, all of the three internal enhanced tubes can obviously reduce the PCM melting time and not decrease the maximum heat storage capacity, as shown in Fig. 8.

solidification rate much more than the case of adding nanoparticles. Moreover, it does not decrease the maximum energy storage capacity significantly. Al-Abidi et al. [50] numerically investigated the heat transfer enhancement by using axially internal and external fins for PCM melting in a triplex tube heat exchanger. The computational results show that the total melting time is decreased to 34.7%. The fin geometric parameters have great effects on the total performance of LHS system [51–53,57,60]. Murray and Groulx [51] experimentally investigated the heat transfer and phase change behavior of a PCM inside a vertical shell-and-tube LHS with axially fins. The effect of natural convection was found to be significant during melting and to be negligible during solidification. So, in a system which uses fins for enhancing heat transfer, it would be ideal to design the fins to induce convection cells where desired. Tao and He [52] numerically investigated the effects of liquid PCM natural convection on LHS performance of shell-and-tube system and found that although the natural convection can enhance the heat transfer performance of liquid PCM, it also causes larger non-uniformity for the solid-liquid interface and temperature distribution during PCM melting process. Based on that result, a local enhanced fin-tube was designed to improve the uniformity of the melting process. The effects of fin geometric parameters on the LHS performance were investigated and the optimum fin parameters were recommended. Hosseini [53] numerically investigated the effect of longitudinal fins in a double pipe heat exchanger containing PCM during charging process. Results show that fins extension leads to the less melting time and deeper penetration of heat. In addition, heat absorption power is a function of fins height at initial steps of the charging process. Zauner et al. [57] experimentally and numerically investigated the heat storage performance of a fin-tube LHS unit with high density polyethylene as PCM, the optimization on fin configuration were also performed. Wang et al. [60] numerically studied the thermal performance of a LHS unit with circular finned tube. The effects of fin parameters on LHS performance were examined. The results show that when the fin pitch is greater than 4 times of the inner tube radius, the fin height and the fin thickness have little effect on energy efficiency ratio and heat storage rate. When the fin pitch is small, the performance of LHS unit becomes better using large fin height and width. HTF inlet temperature has more significant influence on shell-and252

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extracted can be increased by 140%. Tiari and Qiu [67–69] performed three-dimensional simulation of high temperature LHS system assisted by finned heat pipes. It is found that the heat pipes network configuration and heat pipe quantities play a crucial role in the thermal response of the thermal energy storage system. In addition, the natural convection has considerable effect on the PCM melting process; however, it has little effect on the PCM solidification process. Based on the above literatures review, it can be found that using finned tube is a simple and efficient method to enhance the heat transfer performance of a shell-and-tube LHS system, because the fins can increase the heat transfer surface and PCM effective thermal conductivity. However, the PCM volume is reduced by introducing of fins, which will reduce the thermal storage capacity. Moreover, the existing of fins weakens the natural convection of liquid PCM. From the cost point of view, the fins will cause the augmentation of LHS unit cost and weight. Therefore, more attentions should be paid to the fin pattern, fin geometric parameter and fin numbers to get the optimum comprehensive performance. 4.2. Encapsulated PCM Another commonly used method to extend the heat transfer surface of LHS is adopting encapsulated PCMs, which was summarized in Table 4. According to the size of the PCM capsules, it can be divided into two categories: microcapsules and macrocapsules. Most of the microcapsules are spherical with diameter from tens of nanometers to hundreds of microns. The macrocapsules are spherical or cylindrical with dimensions ranging from several millimeters to several hundred millimeters. The encapsulated PCM technique, especially the microcapsules can significantly increase the heat transfer surface between HTF and PCM, so it can efficiently increase the LHS performance. 4.2.1. Microcapsule The microcapsules are usually applied for the low temperature organic PCMs (e.g., paraffin, fatty acids, etc.), where the organic materials (e.g., methyl methacrylate [71,73], polystyrene [72,74,75], alginate [76]) are used as the packaging materials. Sarı et al. [71–75] prepared a series of micro/nano encapsulated PCMs with poly or polystyrene shell and low temperature organic PCMs such as paraffin, n-heptadecane, n-nonadecane, fatty acids, n-tetracosane and n-octadecane. The detailed preparation, characterization and latent heat thermal energy storage properties were reported. The results show the encapsulated PCMs have good thermal conductivity and phase change reversibility, which can be considered as promising PCMs for low-temperature LHS applications. Nemeth et al. [76] prepared a new double-shell alginate microcapsules containing paraffin PCM. Experimental results show the prepared PCM capsules have uniform sizes, core/shell structure, doublewalled non-porous alginate coating, tunable void space inside the core, and suitably high paraffin content at properly selected process conditions and good thermal stability.

Fig. 8. Effects of enhanced tubes on PCM melting time and heat storage capacity [61].

4.1.2. Heat pipe with fins Another approach to reducing the PCM side thermal resistance is embedding heat pipes to augment the energy transfer between HTF and PCM, especially for high temperature PCMs, such as nitrate [62–65,67–69], chloride [66]. Fins are also be used in the heat pipes assisted LHS system to enhance the heat transfer performance. Nithyanandam and Pitchumani [62,63] carried out detailed parametric studies to assess the influence of the heat pipes and the LHS geometric and operational parameters on the performance of a shelland-tube LHS unit with embedded heat pipes. The optimizations were also performed to maximize energy transferred, energy transfer rate and effectiveness. In those studies, the heat pipe is smooth tube. After that, in order to further enhance the heat transfer performance, finned heat pipes were introduced to the LHS system. Khalifa et al. [64] conducted a numerical and experimental investigation on the thermal performance of LHS systems with axially finned heat pipes. The results show that finned heat pipes can efficiently increase the energy extraction rate and heat pipes effectiveness. The energy extracted increased by 86% and the effectiveness increased by 24% with finned heat pipes. Almsater et al. [65] also investigated the effects of axially finned heat pipes on the thermal performance of a LHS unit. The results show that by adding four axial fins, the overall thermal performance of the storage system is enhanced significantly compared to having bare heat pipes. The radially finned heat pipes were also used to enhance the LHS performance [66–69]. Khalifa et al. [66] numerically investigated the performance of a finned heat pipes used in high-temperature LHS system. Results show that the proposed finned heat pipes can improve the reliability and functionality of LHS system. Moreover, the energy

4.2.2. Macrocapsule The macrocapsules are usually used for the high temperature PCMs (e.g., nitrate, chloride, and carbonate) with metal [78,79,84,86], rocks [80], polymer [84–86] and ceramic [88] as the packaging materials. The operation parameters have great effects on the heat transfer performance of the encapsulated PCMs [77–79,89]. Regin et al. [77] numerically investigated the performance of a packed bed LHS system with spherical capsules filled with paraffin wax as PCM. The effects of inlet HTF temperature, mass flow rate and phase change temperature range on thermal performance of the capsules with various radii were investigated. Zheng et al. [78,79] developed high temperature encapsulated PCM for concentrating solar power systems with stainless steel capsules and NaNO3 as PCM. The LHS performances were studied experimentally and numerically. Bhagat and Saha [89] numerically 253

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Table 4 Summary of LHS performance enhancement with capsulated PCM. Capsule shape

Capsule dimension (D: diameter; H: height)

Capsual material

Phase change material

Melting temperature /°C

Reference

Spherical Spherical Spherical spherical Spherical Spherical Spherical Cylindrical

D: D: D: D: D: D: D: D:

Paraffin n-heptadecane n-nonadecane Fatty acids Tetracosane/octadecane Paraffin Paraffin NaNO3, NaCl/MgCl2

20–36 21.48 31.23 22–48 25.9 55–57 59.9 308, 444

Sarı et al. [71] Sarı et al. [72] Sarı et al. [73] Sarı et al. [74] Sarı et al. [75] Nemeth et al. [76] Regin et al. [77] Zheng et al. [78,79]

Spherical

D: 2–15 mm

Li2CO3/Na2CO3/K2CO3

550

Nithyanandam et al. [80]

Spherical Spherical Spherical Spherical Spherical Spherical

D: D: D: D: D: D:

Methyl methacrylate Polystyrene Methyl methacrylate Polystyrene Polystyrene Alginate Polyethylene Stainless steel, carbon steel Granite rocks, quartzite rocks NA NA Stainless steel Polymer and nickel Polymer Polymers, nickel

58 290 0 306.8 305–307 306, 334, 222, 122

Narasimhan et al. [81] Peng et al. [82] Chandrasekaran et al. [83] Bellan et al. [84] Bellan et al. [85] Alam et al. [86]

Spherical Cylindrical Spherical

D: 50 mm D: 22.2 mm; H: 17.99 mm D: 1 cm, 5 cm

Myristic acid NaNO2 Deionized water Sodium nitrate Sodium nitrate NaNO3, KNO3, NaNO3/ KNO3, NaNO3/ KNO3/ LiNO3 n-octadecane Al/Si A164

28 577 171

Fan et al. [87] Fukahori et al. [88] Bhagat and Saha [89]

0.01–100 µm 1–20 µm 0.01–80 µm 7.7–42 µm 0.01–115 µm 4.32–6.08 mm 40–120 mm 25.4–50.8 mm; H: 46.4–127 mm

50 mm 5–45 mm 74 mm, 86 mm, 100 mm 10 mm, 25 mm 27.5 mm 12.5–25 mm

Plexiglas Ceramic (Al2O3) NA

shortened up to nearly 30% at the highest fin height, because the presence of fin enhanced the heat conduction by the extended heat transfer area and local natural convection induced near the fin. Recently, Alam et al. [86] presented an innovative technique to encapsulate high temperature PCM, which does not require a sacrificial layer to accommodate the volumetric expansion of the PCMs on melting. Experimental results show that the PCM in the macro-capsules melts and solidifies in a significantly shorter time, which can satisfy the need for a quick response time of power plant. The use of ceramic containers for macro-encapsulation of metallic PCMs, and the sealing method to endure the thermal stress from volume expansion during phase change were proposed by Fukahori et al. [88]. The prepared PCM capsule has excellent corrosive resistance and cycling performance. From the above literatures review, it can be demonstrated that the LHS performance can be enhanced by encapsulated PCM, and with the capsule size decreasing, the thermal performance can be further improved. Therefore, the PCM microcapsule is a promising method for LHS system performance enhancement. However, due to the fabrication problem, the microcapsule is commonly used for low temperature PCM. There is an urgent need to develop high efficient encapsulation methods for high temperature PCM.

studied the transient response of packed bed LHS system with spherical shaped encapsulated PCMs for removing fluctuations in HTF temperature during charging and discharging period in an organic Rankine cycle-based solar thermal power plant. It is found that the ability of the latent heat thermal energy storage system to store and release energy is improved significantly by increasing mass flow rate and inlet charging temperature. The thermal performance of a LHS system with encapsulated PCMs can be enhanced with decreasing the capsule size [80,82–85]. Nithyanandam et al. [80] analyzed the dynamic behavior of a packed bed thermal energy storage system with encapsulated PCMs. The influence of the design configuration and operating parameters on dynamic storage and delivery performance of the system was studied. The results show that smaller radii capsules yield higher latent utilization. Peng et al. [82] numerically analyzed the behavior of a packed bed latent heat thermal energy storage system with molten slat as HTF and NaNO2 as PCM. The results indicated that decreasing PCM capsule size and fluid inlet velocity, or increasing storage height, results in an increase in charge efficiency. Chandrasekaran et al. [83] investigated the influence of spherical capsule size on the solidification characteristics of water as the PCM. It was observed that the capsule size had a significant influence on subcooling at lower temperature driving potential, but it was totally eliminated at higher temperature potential. The shell properties also have directly influence on encapsulated PCMs thermal performance. Bellan et al. [84,85] numerically and experimentally analyzed the charging and discharging performance of a LHS system with encapsulated PCM. The results show that decreasing the capsule size can significantly improve charging and discharging rates. Moreover, shell properties of the capsule significantly influence thermal performance of the system; the influence of the shell thickness increases when thermal conductivity of the shell is low. Some other performance enhancement methods can be integrated with encapsulated PCMs to achieve better performance enhancement effect, such as nanomaterial additives [81] and fins [87]. Narasimhan et al. [81] numerically investigated the performance of an encapsulated LHS unit containing a PCM dispersed with high conductivity particles. The results show the time taken for PCM complete solidification can be reduced by about 26% with a particle fraction of about 30% due to the high thermal conductivity of the particles. Fan et al. [87] investigated the constrained melting heat transfer of PCM in a circumferentially finned spherical capsule and found that the TES performance is enhanced with increasing fin height. The melting duration time is

5. Recent development on uniformity enhancement of heat transfer process 5.1. Multiple PCMs For LHS system, the HTF temperature decreases/increases along the flow direction during the charging/discharging process. The multi-stage or cascaded LHS technique was proposed based on the principle of energy cascade utilization, which uses multiple PCMs with different melting temperatures allocated along the fluid flow direction to ensure the heat transfer process has a good uniformity. The research progress on adopting multiple PCMs to enhance the LHS system performance in the last decade is drawn in Table 5. A lot of comparative studies on the performances of multi-stage and single stage LHS were performed to validate the advantages of multistage LHS system [90,91,99,101,102]. Michels and Pitz-Paal [90] numerically and experimentally investigated the performance of a cascaded LHS system for parabolic trough solar power plants with different kinds of PCMs. The study affirms the positive effect of a cascaded LHS compared to a non-cascaded LHS with respect to a higher utilization of 254

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Table 5 Summary of LHS performance enhancement with multiple PCM. Stage number

Phase change materials (melting temperature /°C)

Reference

n = 3, 5 n=3 n=5 n=3 n = 1,3 n = 2,3 n=2 n=3 n=3 n = 1–3 n = 1, 2 n=2 n = 1, 3, 5

MgCl2/KCl/NaCl (380), KOH (360), KNO3 (335), KNO3/KCl (320), NaNO3(306) Paraffin waxes (20–80) LiF/CaF2 (767), LiF/MgF2 (735), NA(700), NA(650), NA(600) NaOH/NaCl (370), KCl/MnCl2/NaCl (350), NaOH/NaCl/Na2CO3 (318) RT31 (31), RT50 (50), RT82 (82), RT55 (55) CaCl2·6H2O (29), paraffin C18 (27.5,), RT25 (26.6) CaCl2·6H2O (29), RT25 (26.6) Paraffin blends (7, 9, 11) K2CO3/Na2CO3 (710), Li2CO3/Na2CO3/K2CO3 (550), Li2CO3/K2CO3/Na2CO3 (397) Paraffins (43, 51, 61) LiF/CaF2 (767), K2CO3/Na2CO3 (710), LiF/MgF2 (746) Hydroquinone, D-mannitol (150–200) NA (375), NA (360), NA (340), NA (320), NA (305)

Michels and Pitz-Paal [90] Fang and Chen [91] Seeniraj and Narasimhan [92] Shabgard et al. [93] Tian and Zhao [94] Mosaffa et al. [95] Mosaffa et al. [96] Chiu and Martin [97] Li et al. [98] Aldoss and Rahman [99] Tao and He [100] Peiro et al. [101] Wu et al. [102]

results show that heat exchange rate of STES is improved up to 30% by CTES, and is further improved about 2–7 times by MF-CTES. Tao and He [100] proposed a compound enhancement method to improve the LHS performance of a shell-and-tube LHS unit, which consists of internal enhanced tube (ET) and multiple PCMs. A comparative study on the LHS performances with smooth tube (ST), simple enhancement method (ET), and the presented compound enhancement method (ET & multiple PCMs) were performed. The results show that the compound enhancement method can further reduce the PCM melting time and total charging time compared with simple enhancement method, as shown in Fig. 9. Although multiple PCM can enhance the LHS performance, the enhancement effect is greatly influenced by the selected PCM thermal properties. With multiple PCMs, the uniformity of the heat transfer process in LHS system can be improved and the heat transfer temperature difference can be controlled in a nearly equal value, which is beneficial to improve the total heat transfer performance of a LHS system. However, it also should be pointed out that the performance of a cascaded LHS system greatly depends on PCM ratio and thermal properties such as melting temperature, thermal conductivity, melting enthalpy etc. It is vital for cascaded LHS system to carefully select and arrange the multiple PCMs. Thermodynamic analysis and optimization is an efficient tool to improve thermal performance of LHS system, especially for the cascaded LHS system.

the possible phase change, and a more uniform outlet temperature over time. Chiu and Martin [97] performed performance analysis of a threestage cascaded LHS unit and a single stage LHS unit. The results show that three-stage multi-PCM storage unit reaches 40% higher performance than single-PCM unit. Aldoss and Rahman [99] performed comparatively study on the single-PCM and multi-PCM thermal energy storage unit. The results indicate that as the number of stages increases the multi-PCM TES performance increases. However, using more than three stages does not add any appreciable improvement. Peiro et al. [101] performed an experimental evaluation on the advantages of using multiple PCMs configuration instead of single PCM configuration in TES systems at pilot plant. Results showed that the multiple PCMs configuration introduced an effectiveness enhancement of 19.36% if compared with single PCM configuration as well as a higher uniformity on the HTF temperature difference between the inlet and outlet. Wu et al. [102] numerically investigated the transient performance of cascaded molten salt packed-bed TES system. The results show that the noncascaded system suffers from a low charging ratio and a long charging time while the cascaded systems especially with 5 cascaded PCMs are found to have both a fast discharging rate and a fast charging rate. However, the performance of cascaded LHS system greatly depends on the melting temperature of the selected PCMs and their arrangements [91,95,96,98]. Fang and Chen [91] numerically investigated the performance of a shell-and-tube LHS unit using multiple PCMs and found that PCMs’ fractions and melting temperatures play important roles in the performance of the LHS unit. Appropriate choosing of multiple PCMs is very significant for the performance improvement of the LHS unit. Mosaffa et al. [95,96] numerically investigated the performance enhancement of a free cooling system using a LHS unit employing multiple PCMs, where the energy and exergy analyses were also performed. Li et al. [98] numerically investigated the performance of a three-stage LHS unit. The instantaneous solid-liquid interface positions and liquid fractions of PCMs were analyzed and the optimum lengths for different stages are recommended. The multiple PCMs can also be integrated with other performance enhancement methods, such as finned tube [92,100], heat pipes [93], and metal foam [94]. Seeniraj and Narasimhan [92] numerically studied the performance of a LHS unit with both finned-tube and multiple PCMs. The results show that the unit can obtain appreciable energy storage in the form of latent heat and a nearly uniform HTF exit temperature as compared to a single PCM unit. Shabgard et al. [93] performed heat transfer and exergy analysis of cascaded LHS system with gravity-assisted heat pipes for concentrating solar power applications. The results show that cascaded LHS recovers about 10% more exergy during a 24 h charging-discharging cycle compared to the best noncascaded LHS. Tian and Zhao [94] performed theoretical studies to examine the overall thermal performances of single-stage thermal energy storage (STES), cascaded thermal energy storage (CTES) and metal foam-enhanced cascaded thermal energy storage (MF-CTES). The

5.2. Thermodynamic analysis and optimization Thermodynamic analysis and optimization for LHS system were performed to improve the thermal performance of LHS system in the last decade, which is summarized as in Table 6.

Fig. 9. Effects of enhancement methods on PCM melting fraction [100]. 255

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Table 6 Summary of thermodynamic analysis and optimization on LHS system. Stage number

Phase change material (melting point /°C)

Performance evaluation method

Reference

n = arbitrary n=1 n=1 n=1 n=1 n=1 n=1 n=1 n=1 n=3 n=1 n = 1, 2 n=∞ n=∞ n=1

NA NA Aluminum/Parafol 22–95 (41.6) NaNO3 (306) Paraffin (18–38) NA NA NA NA NA Paraffin (60) LiF/CaF2 (767), LiF/MgF2 (746) NA NA NaNO3 (306)

Energy Energy and exergy Energy Energy Energy and exergy Entropy and exergy Energy and exergy Entropy generation Energy and exergy Exergy Energy and exergy Entransy Entransy Entransy Entansy

Ezra et al. [103] Tao and Carey [105] Veelken and Schmitz [106] Hubner et al. [107] Kousksou et al. [108] Erek and Dincer [109] Koca et al. [110] Guelpa et al. [111] Shabgard et al. [112] Xu et al. [113] Manfrida et al. [114] Tao et al. [115] Xu and Zhao [116] Xu and Zhao [117] Guo and Huai [118]

mathematical model of a LHS system for a solar-powered Organic Rankine Cycle and the energy and exergy analysis was performed.

5.2.1. Energy analysis The energy analysis is a mature thermodynamic analysis method, which has been widely used in all kinds of energy systems including the LHS system. Ezra et al. [103] performed numerical analysis and optimization of melting temperature for a multiple PCM LHS unit with arbitrary number of PCMs. An optimal way to choose the melting temperatures of PCMs is established. Tao and Carey [105] investigated the effects of PCM thermal properties on LHS performance with orthogonal experiment method. Based on the range analysis and regression analysis, the optimization design and validation were performed and the PCM selection criteria for a shell-and-tube LHS unit were established. Veelken and Schmitz [106] performed optimization of a composite LHS with non-uniform heat fluxes using a genetic algorithm. The fin positions were optimized and the results showed a relative improvement of up to 3% compared to the system with equally distributed fins. Hubner et al. [107] discussed the systematic development and optimization of heat transfer structures in LHS from a technological and economic point of view.

5.2.3. Entransy analysis Recently, the entransy dissipation analysis was also be used to optimize the performance of the LHS system. Tao et al. [115] numerically analyzed the effects of PCM melting temperatures on LHS performance of a two-stage LHS unit. The optimization for the match of two-stage PCMs melting temperatures was performed based on the entransy theory. The results show that there is an optimal match of the two-stage PCMs melting temperatures to achieve the maximum heat transfer rate or the minimum entransy dissipation rate. In addition, the formulas for the optimum two-stage PCMs temperatures were presented. Xu and Zhao [116,117] presented a thermodynamic modeling of cascaded LHS system for direct thermal utilization purpose. The optimization of HTF temperatures and multistage PCMs is performed based on the entransy theory. The results show that cascaded LHS can extend applicable temperature scope for multi-graded thermal energies and the increase of stage number can increase thermal efficiency. Guo and Huai [118] performed heat transfer mechanism study of three-tank LHS system based on entransy theory and found that there exists an optimal mass flow rate ratio to maximize the performance of storage system.

5.2.2. Exergy or entropy analysis The entropy analysis method is also be used to analyze and optimize the thermal performance of LHS system. Kousksou et al. [108] developed a theoretical model for analysis and optimization of a solar system using PCM. Energy and exergy analyses were carried out to understand the behavior of the system using single PCM or multiple PCMs. Numerical results show the LHS system performance can be enhanced by the judicious choice of PCM melting temperature. Erek and Dincer [109] performed an entropy and exergy efficiency analysis of a LHS system during charging process. The results show that entropy generation is crucial in such systems and should be minimized in order to increase the exergy efficiency and hence system performance. Koca et al. [110] performed energy and exergy analysis for a LHS system with PCM for a flat-plate solar collector. The results show that there is a significant difference between the results of energy and exergy. The average net energy and exergy efficiencies are 45% and 2.2%, respectively. Guelpa et al. [111] investigated the design improvements of a shell-and-tube LHS unit using an approach based on the analysis of entropy generation. The results show that the improved system allows to reduce PCM solidification time and to increase second-law efficiency. Shabgard et al. [112] developed a combined heat transfer and exergy analysis model to investigate the performance of a LHS system for solar power generation. The results show that increasing the PCM side surface area by a factor of 10 results in a six-fold increase in the exergy extracted from the LHS system. Xu et al. [113] performed exergy analysis and optimization of charging-discharging processes of a LHS system with three phase change materials. The optimum PCM melting temperatures were recommended. Manfrida et al. [114] developed a

6. Future development areas From the above literature review, it can been found that although a lot of research works on the performance enhancement of LHS system have been performed and many valuable results have been obtained, there are still many works need to be performed. Some of the future development areas in this topic are listed as follows: (1) Porous composite phase change materials: The available research results show that the high conductivity porous media can efficiently enhance PCM thermal conductivity and improve the thermal performance of LHS system. However, the geometric parameters of porous media such as porosity and pore density have great effects on CPCM thermal performance. Moreover, the porous media will restrict the natural convection of liquid PCM. Some deeply investigations on the interactions between porous skeletons and PCM should be further performed to optimize the porous media configuration and obtain the best thermal performance. (2) Nanocomposite phase change materials: The high thermal conductivity nanomaterial additives can efficiently enhance PCM thermal conductivity and improve the thermal performance of LHS system, but it is significantly affected by the dispersion of nanomaterial in PCM. In order to improve nanomaterial dispersion, SAA

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(3)

(4)

(5)

(6)

Thermodynamic analysis and optimization is an efficient tool to improve thermal performance of LHS system, especially for cascaded LHS system. (6) Although many research works on performance enhancement of LHS system have been performed and valuable results have been obtained, many problems still need to be solved to further improve the LHS performance and promote large-scale applications of LHS technology.

is usually used during CPCM preparation process. Therefore, the actual NCPCM is a composite of nanomaterial/PCM/SAA. The interactions among them are very complicated, which results in the lack of performance regulation mechanism. Some further studies should be performed to investigate the interaction mechanism among nanomaterial, SAA and PCM, which is very important for performance regulation of NCPCM. Finned tube: Finned tube is a simple and efficient heat transfer performance enhancement for LHS system. However, the PCM volume is reduced by introducing of fins, which causes the reduction of thermal storage capacity. In addition, the existing of fins weakens the natural convection of liquid PCM. Therefore, more attentions should be paid to fin pattern, fin geometric parameter and fin numbers with comprehensively considering the heat storage rate and capacity. Encapsulated PCM: Encapsulated PCM can greatly extend the heat transfer surface between PCM and HTF and improve LHS thermal performance efficiently, especially the microcapsules. However, due to the fabrication problem, the microcapsule is commonly used for low temperature PCMs. There is an urgent need to develop high efficient encapsulation methods for high temperature PCMs. Cascaded LHS: The cascaded technique with multiple PCMs can improve the uniformity of the heat transfer process in LHS system. However, the enhancement effects deeply depend on PCM ratio and thermal properties such as melting temperature, thermal conductivity, melting enthalpy etc. It is vital for cascaded LHS system to carefully select and arrange multiple PCMs. The studies about the selection and matching criteria of multi-PCMs are very important for cascaded LHS system. There is only a few works considering compound performance enhancement method for LHS system. The use of compound enhancement techniques such as the combination of finned tube and multiple PCMs, or the finned tube and high conductivity porous media can get more excellent performance for LHS system. Therefore, the studies on compound performance enhancement techniques are also recommended for future research.

Acknowledgement The present work was supported by the National Natural Science Foundation of China (No. 51376146) and the KeyProject of National Natural Science Foundation of China (No. 51436007). The authors would also like to thank the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 51721004). The authors declare that the permissions for all the figures, graphics or images presented in the paper have been obtained from both the publisher (Elsevier) and the authors. References [1] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energy Rev 2009;13:318–45. [2] Kenisarin MM. High-temperature phase change materials for thermal energy storage. Renew Sustain Energy Rev 2010;14:955–70. [3] Leon N, Cardenas B. High temperature latent heat thermal energy storage: phase change materials, design considerations and performance enhancement techniques. Renew Sustain Energy Rev 2013;27:724–37. [4] Jegadheeswaran S, Pohekar SD. Performance enhancement in latent heat thermal storage system: a review. Renew Sustain Energy Rev 2009;13:2225–44. [5] Fan L, Khodadadi JM. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energy Rev 2011;15:24–46. [6] Liu L, Su D, Tang Y, Fang G. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energy Rev 2016;62:305–17. [7] Ibrahim NI, Al-Sulaiman FA, Rahman S, Yilbas BS, Sahin AZ. Heat transfer enhancement of phase change materials for thermal energy storage applications: a critical review. Renew Sustain Energy Rev 2017;74:26–50. [8] Ma Z, Lin W, Sohel MI. Nano-enhanced phase change materials for improved building performance. Renew Sustain Energy Rev 2016;58:1256–68. [9] Amaral C, Vicente R, Marques PAAP, Barros-Timmons. Phase change materials and carbon nanostructures for thermal energy storage: a literature review. Renew Sustain Energy Rev 2017;79:1212–28. [10] Liu Z, Yao Y, Wu H. Numerical modeling for solid-liquid phase change phenomena in porous media: shell-and-tube type latent heat thermal energy storage. Appl Energy 2013;112:1222–32. [11] Xiao X, Zhang P, Li M. Effective thermal conductivity of open-cell metal foams impregnated with pure paraffin for latent heat storage. Int J Therm Sci 2014;81:94–105. [12] Wang C, Lin T, Li N, Zheng H. Heat transfer enhancement of phase change composite material: copper foam/paraffin. Renew Energy 2016;96:960–5. [13] Tao YB, You Y, He YL. Lattice Boltzmann simulation on phase change heat transfer in metal foams/paraffin composite phase change material. Appl Therm Eng 2016;93:476–85. [14] Zhao Y, Zhao CY, Xu ZG, Xu HJ. Modeling metal foam enhanced phase change heat transfer in thermal energy storage by using phase field method. Int J Heat Mass Transf 2016;99:170–81. [15] Liang W, Zhang G, Sun H, Chen P, Zhu Z, Li A. Graphene–nickel/n-carboxylic acids composites as form-stable phase change materials for thermal energy storage. Sol Energy Mater Sol Cells 2015;132:425–30. [16] Wang Z, Zhang Z, Jia L, Yang L. Paraffin and paraffin/aluminum foam composite phase change material heat storage experimental study based on thermal management of Li-ion battery. Appl Therm Eng 2015;78:428–36. [17] Sarı A, Karaipekli A. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl Therm Eng 2007;27:1271–7. [18] Zhong Y, Guo Q, Li S, Shi J, Liu L. Heat transfer enhancement of paraffin wax using graphite foam for thermal energy storage. Sol Energy Mater Sol Cells 2010;94:1011–4. [19] Xia L, Zhang P. Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage. Sol Energy Mater Sol Cells 2011;95:2246–54. [20] Wang X, Guo Q, Zhong Y, Wei X, Liu Y. Heat transfer enhancement of neopentyl glycol using compressed expanded natural graphite for thermal energy storage. Renew Energy 2013;51:241–6. [21] Lv Y, Zhou W, Jin W. Experimental and numerical study on thermal energy storage

7. Conclusion This review paper presents a detail literature survey focused on PCMs, the corresponding thermal performance enhancement methods and their characteristics used in LHS system. This review will help in selecting reliable PCMs and matching suitable performance enhancement method to achieve better thermal performance for PCM based LHS system. Based on the review, the following conclusions can be derived. (1) Metal foam and expanded graphite are very efficient to enhance PCM thermal conductivity. Metal foam is usually used for low temperature organic PCMs. Expanded graphite can be used both for low temperature PCMs and high temperature salts PCMs. (2) Carbon nanomaterial and metal oxide particles are the commonly used additives to enhance PCM thermal conductivity and improve the thermal performance of LHS system. Carbon nanomaterial is applicable both for low and high temperature PCMs. Metal oxide particles is mostly used for low temperature organic PCM. (3) Using finned tube is a simple and efficient method to enhance the heat transfer performance of a shell-and-tube LHS system. The fins can be arranged in both the PCM side and HTF side, which is determined by the dominant thermal resistance. When it was used in PCM side, the PCMs are mostly lower temperature organic PCM. (4) Encapsulated PCM can greatly extend the heat transfer surface and efficiently improve LHS thermal performance, especially the microcapsules. However, due to the fabrication problem, the microcapsule is commonly used for low temperature organic PCM. (5) The uniformity of the heat transfer process in LHS system can be improved by the cascaded technique with multiple PCMs. 257

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