Recent Investigations of Phase Change Materials Use in Solar Thermal Energy Storage System

Solar thermal energy storage (TES) is an efficient way to solve the conflict between unsteady input energy and steady output energy in concentrating solar power plant. +e latent heat thermal energy storage (LHTES) system is a main method of storing thermal energy using phase change materials (PCMs).+ermal properties, that is, melting points and latent heat, are the key parameters of PCMs for the TES system. In this paper, the PCMs are classified into inorganic and organic by the chemical composition, and according to the melting point, the inorganic PCMs can be divided into three contributions: low-temperature heat storage (less than 120°C), medium-temperature heat storage (120–300°C), and high-temperature heat storage (more than 300°C). +e present article focuses mainly on the recent investigations on the melting point and latent heat of PCMs via DSC setup in the solar TES systems. +e results can provide a good reference for the selection and utilization of PCMs in the solar TES systems.


Introduction
With the essential increasing in energy consumption, energy shortage and environmental problem including tremendous demand and insufficient supply have been vital points that affect the economic development all over the world.e governments of many countries stimulate development of energy-saving technologies based on utilization of renewable energy sources [1].Among various forms of energy, widespread thermal energy is abundantly available, such as solar radiation and geothermal energy, due to its convenience, high efficiency, and the value of direct utilization compared with other resources [2][3][4].And the unpredictability of the output of renewable energy systems demands reliable and efficient energy storage units.erefore, thermal energy storage (TES) technology plays a significant role in utilizing the renewable resources [5,6].ere are mainly two types of TES systems of sensible storage systems and latent storage systems, and the latent heat thermal energy storage (LHTES) system is an effective way of storing thermal energy [7].Remarkably, phase change materials (PCMs) are latent heat storage materials, and the thermal energy transfer occurs when a material changes from solid to liquid or liquid to solid which is called the phase transition process.Initially, the thermal performances of PCMs are similar to conventional (sensible) storage materials which absorb heat with the increase of temperature in PCMs, and then PCMs absorb and release heat at a nearly constant temperature compared with the conventional storage materials [8,9].erefore, the PCM is currently seen as one of the most promising to research the TES system.
In past four decades, there were wide researches on the TES system.Some review articles have a summary in the open literatures, which focus on as follows: TES system design methodologies and the factors [10]; the materials and heat transfer performance enhancement techniques of LHTES [11,12]; the materials, heat transfer, and phase change problem for LHTES [13]; residential scale systems for seasonal storage of solar thermal energy [14]; TES technologies using PCM capsules [5]; CFD (Fluent) software applications in LHTES [15]; PCMs for high-temperature TES system [1]; microencapsulation PCMs for TES [16]; thermal stability of PCM [17]; the preparation, thermal properties, and applications of shape-stabilized thermal energy storage materials [18]; mathematical modeling on LHTES [19]; passive LHTES system to building's energy performance [20]; storage materials and thermal performance enhancement for high-temperature phase change thermal storage systems [21]; thermal conductivity enhancement of PCM [2]; thermal conductivity enhancement of nanostructure of PCM [3]; the PCMs and different applications of TES [8]; metal hydride material-based thermochemical hydrogen storage [22]; the TES for power generation [7,23]; chemical heat storage [24]; three available technologies for seasonal heat storage (sensible heat storage, latent heat storage, and chemical storage) [25]; the types of solar collectors [26]; thermochemical heat storage [27,28]; the thermal reliability, heat storage durability, and thermal stability of cross-linked form-stable PCMs [29]; nanomaterials used base fluid to enhance thermophysical properties [30]; SrBr 2 •6H 2 O as a potential material for lowtemperature energy storage systems and building applications [31]; molten alkali nitrates used as heat transfer fluid (HTF) [32]; various solar collectors and parabolic tough collector (PTC) [33]; specific heat of nanofluids [34]; the HTF for concentrating solar power (CSP) systems [35]; the practical applications, thermal properties, and configurations of PCMs [36]; the geometrical configurations of TES for CSP plant [37]; and inorganic PCMs for TES systems [38].
However, the review of thermal properties (melting point and latent heat) of PCMs with DSC is limited in the previous works.In order to gain the detailed summary of the melting point and latent heat with PCMs in the TES systems, the present paper focuses mainly on the recent investigation of the melting point and latent heat via DSC in the TES systems.According to the chemical composition, the PCMs are classified into inorganic and organic.And based on melting point, the PCMs can be divided into three contributions: low-temperature heat storage (less than 120 °C), medium-temperature heat storage (120-300 °C), and hightemperature heat storage (more than 300 °C).
is can provide a theoretical guidance in future for selecting the more suitable PCMs in the investigations of the thermal performance for TES applications.

Thermal Properties of PCMs with DSC
2.1.DSC Setup.Differential scanning calorimetry (DSC) is an analytical method for measuring the energy difference between a sample and a reference with temperature at a programmed temperature.erefore, the DSC system is the most commonly used approach for the study on the thermal properties of the PCMs such as the melting and freezing temperatures, the specific heat, and the latent heat [39].According to the measuring principle, it can be divided into heat-flux DSC and power compensation DSC.For the heat-flux DSC, the temperature difference of the sample and reference can be measured accurately, which can be translated into heat-flux signals [40].And compared with differential thermal analysis (DTA), the determination of DSC is more accurate and the reproducibility and resolution are better.erefore, DSC has been widely used in the field of polymers, and some DAT applications can be replaced by DSC measurement with the high accuracy and sensitivity and less sample dosage.According to the DSC curves, the thermal properties including melting point and latent heat of PCMs can be investigated.

ermal Properties of PCM.
e selection of PCM for TES application depends on different factors, and the PCMs must present desirable properties that are associated with the thermodynamic, kinetic, chemical, and economics.Suitable PCMs need some thermal properties such as suitable melting point, high latent heat, high thermal conductivity, high heat capacity, and high specific heat [38].Especially, the melting point of the material must be equal or similar to the operating temperature on some applications.e latent heat should be as high as possible, in particular, on a volumetric basis to minimize the size of the thermal energy storage device.Consequently, it is extremely significant to study the melting point and the latent heat of PCMs.
Based on the large quantity investigations, there were a lot of works to research the thermal properties of PCMs with DSC.In general, PCMs are classified into organic and inorganic materials.Organic materials consist of paraffin and nonparaffin such as fatty acids, while inorganic materials comprise salt hydrates, saline composites, and metallic alloys [11].In this paper, according to the melting point and latent heat, the PCMs including inorganic material, organic material, and the three types of inorganic PCM based on the phase change temperature have been reviewed.

Recent Investigations of PCMs with DSC
3.1.Inorganic Materials 3.1.1.Low-Temperature PCMs.Low-temperature PCMs have the low-melting point (<120 °C), which was appropriate for TES in building' application and CSP plant.Many researchers have adopted a lot of methods to decrease the melting point of mixed nitrates, such as introducing additives (Ca(NO 3 ) 2 •4H 2 O or LiNO 3 ) and changing the proportion of solar salt (60% NaNO 3 /40% KNO 3 ) and HITEC salt (53% KNO 3 /40% NaNO 2 /7% NaNO 3 ).And some researchers also make full use of the low-melting point materials as the PCMs, such as salt hydrates.
In order to obtain the nitrate salts with excellent thermal characteristics and substantial cost, there are a lot of options for low-temperature applications.Wu et al. [41] researched a new kind of binary molten salts KNO 3 /Ca(NO 3 ) 2 •4H 2 O.
e mixture had a regular melting peak and reached the eutectic point when the mass ratio of KNO 3 and Ca (NO 3 ) 2 •4H 2 O was 0.57 : 0.43, and its melting point was lowest.From the DSC curve, the melting point was 116.9 °C, which was lower than solar salt and HITEC salt, and the latent heat of this PCM was 78.79 J/g.Jiang et al. [42], Wu et al. [43], and Ren et al. [44] selected the quaternary molten salts LiNO 3 /KNO 3 /NaNO 3 /Ca(NO 3 ) 2 as the PCMs to study their thermal properties by DSC. e results showed that the composite of 27% LiNO 3 /47% KNO 3 /16% NaNO 3 /10% Ca (NO 3 ) 2 as the PCM and calcium silicate as structural supporting material had a low-melting point (103.5 [54] studied the thermal properties of three inorganic salt PCMs (NaCl, KCl, and LiCl) by adding CaCl 2 •4H 2 O.For CaCl 2 •4H 2 O, the melting point and heat of fusion were 44.2 °C and 99.6 J/g, respectively.And the addition reduced the melting point of alkaline salts; on the contrary, the heat of fusion of composite had slightly increased compared with the ones without additives.Wu and Wang [55] studied the shape-stabilized PCMs, the mixture of 50% Na 2 SO 4 •10H 2 O/50% Na 2 HPO 4 •12H 2 O as PCM, and EG as the supporting material.According to the DSC curves, the latent heat of the composite material can reach 172.3 J/g with a melting temperature of 32.05 °C.Schmit et al. [56] proposed the three-step method to test the eutectic composition of 75% Zn(NO 3 ) 2 •6H 2 O/25% NH 4 NO 3 and 73% Mn(NO 3 ) 2 •6H 2 O/27% NH 4 NO 3 .And the latent heat of two composites was 135 J/g and 133 J/g with the melting points of 12.4 °C and 3.9 °C, respectively.Huang et al. [57] prepared PMMA/Na 2 HPO 4 •7H 2 O by means of the suspension copolymerization solvent volatile method.According to the DSC curves, the melting point and latent heat were about 51 °C and 150 J/g, respectively.Wu and Wang [58] and Liu and Yang [59] selected EHS Na 2 SO 4 •10H 2 O/Na 2 HPO 4 •12H 2 O to study, and the melting point and latent heat of this EHS were 31.2°C and 280.1 J/g. e latent heat was higher than that of most of lowtemperature phase change materials showing a prominent heat storage capacity.en Na 2 HPO 4 •12H 2 O/Na 2 SO 4 •10H 2 O-SiO 2 as a novel shape-stabilized PCM was prepared, which selected SiO 2 as the supporting material via sol-gel process.e DSC plots showed that the latent heat of the composite was 106.2 J/g with its melting temperature of 30.13 °C, which were suitable for lowtemperature TES applications.
In the above literatures, the main works focus on the investigation of melting point and latent heat of lowtemperature inorganic PCMs, which indicated the salt hydrates are the most excellent materials as the lowtemperature PCM for low-temperature TES applications.And Tables 1 and 2 show the summary of the melting point and latent heat of mentioned low-temperature inorganic eutectic composites.

Medium-Temperature PCMs.
In the applications of solar energy and industrial waste heat thermal energy storage system, PCMs with a melting temperature between 120 °C and 300 °C were also of great advantage.For example, a large amount of heat energy was required to generate process steam in this temperature range for applications in production of paper, food processing, textile industry, and so on.And a lot of investigations on the thermal performance of molten salts in the temperature range have been carried out by many researchers [60].
Based on the solar salts (40% KNO 3 /60% NaNO 3 ), a large number of investigates were studied by DSC.Xiao et al. [61] investigated the KNO 3 /NaNO 3 -EG composite PCM, which the EG as the additive.According to the DSC curves, the peak temperature (223.2 °C) of composite was similar to that of NaNO 3 /KNO 3 , and the latent heat of composite was 128 J/g, while that of nitrates was 142.2 J/g.And Li et al. [62] also selected nitrate mixture KNO 3 /NaNO 3 (50 : 50 mol.%)as the PCM and SiC ceramic honeycomb (SCH) as the heat transfer promoter to make a new composite KNO 3 /NaNO 3 -SiC.Compared with that of the pure PCM, the melting point of this composite shifted slightly, and its latent heat was 72.8 J/g while the pure PCM was 106.3 J/g.Luo et al. [63] and Myers et al. [64] researched the thermal properties of pure binary salt KNO 3 /NaNO 3 , and the mixtures of KNO 3 /NaNO 3 -CuO by DSC.It was found that the melting point of these mixtures is about 222 °C, which closely agreed with that of the pure salts KNO 3 /-NaNO 3 , and the latent heat of these mixtures was approximately the same as that of the pure salts.Xu et al. [65] made use of diatomite to form stabilized NaNO 3 , which found that the diatomite and NaNO 3 have an excellent chemical compatibility and were suitable for formulating composite PCM.And DSC measurements showed that the melting point of the PCM was approximately 307.8 °C with a latent heat of 115.79 J/g.ere are some researches on the thermal properties with nitrates including LiNO 3 , Ca(NO 3 ) 2 , and CsNO 3 .Zhou and Eames [60] studied the two binary salts (46% NaNO 3 /54% LiNO 3 and 40% NaNO 3 /60% LiNO 3 ) to Advances in Materials Science and Engineering estimate their suitability for the medium-temperature TES system.e average melting points of the binary systems were 193.87 °C and 193.27 °C, respectively.e average latent heat of the samples was 261.7 J/g and 244.1 J/g, respectively.Fernández et al. [66,67] found that the melting point and latent heat of the LiNO 3 /KNO 3 /NaNO 3 /Ca(NO 3 ) 2 is 130.61 °C and 157.5 J/g, respectively, and the melting point of KNO3/NaNO 3 /Ca(NO 3 ) 2 And some studies utilize chloride to satisfy the demand of medium-temperature TES system.Huang et al. [71,72] selected the system with 50% LiNO 3 /50% KCl as the PCM and expanded graphite as the heat transfer promoter to prepare the composite of LiNO 3 /KCl-EG.e experimental results revealed that the melting temperature of this composite was 165.60 °C, and the latent heat ranging from 142.41 J/g to 178.10 J/g was dependent on the mass fraction of EG.Zhong et al. [73] selected three kinds of composites (LiNO 3 /KCl-EG, LiNO 3 /NaNO 3 -EG, and LiNO 3 /NaCl-EG) as the PCM.
e DSC plots indicated that the melting temperatures of them were 171.0 °C, 199.3 °C, and 229.7 °C, respectively, and the latent heat was 193.5 J/g, 232.7 J/g, and 305.3 J/g, respectively.Zhou and Eames [74] investigated the eutectic salt of 87% LiNO 3 /13% NaCl to evaluate its potential for latent heat storage with medium-temperature applications (<300 °C).From the DSC curves, the average phase change temperature measured was 220 °C.And the measured average latent heat value was about 310 J/g.Peng et al. [75][76][77] investigated that the eutectic properties of the NaCl/NaF/NaNO 3 .It was found that there was a sharp endothermic peak around 293.5 °C and absorbing heat quantity of 155.5 J/g in the DSC curve, which represented the melting point and latent heat.And the molten salts KNO 3 /NaNO 3 /NaNO 2 with 5% additive A were prepared by the statical mixing method.Compared with ternary nitrate salt, the melting point and latent heat of the composite were decreased from 138.79 °C to 137.62 °C and from 59.14 J/g to 56.57J/g, respectively.And the DSC curve showed that the melting point of KCl/NaNO 3 /NaCl/NaNO 3 was 140 °C.
In the above literatures, the main works focus on the investigation of melting point and latent heat of mediumtemperature inorganic phase change materials (PCMs).It is found that a majority of nitrates and a small amount of chlorides can achieve the demand of medium-temperature thermal energy storage applications.Meanwhile, the summary of the melting point and latent heat of mentioned medium-temperature inorganic eutectic composites is displayed in Tables 3 and 4, respectively.And some commercial salts have been presented and applied in concentrating solar power (CSP) plants [78].Especially, inorganic salts were promising candidates for hightemperature PCMs because they produce a large latent heat without substantial density change during the process of solidliquid phase transformation [79].
In this section, we will summarize and analyze the present investigations of high-temperature inorganic PCMs in terms of thermal properties, such as melting point and latent heat, via DSC.Generally, the high-temperature inorganic PCMs can be divided into carbonates, metal halogenide, the mixtures of them, alloy, and so on.
e DSC curve shows that the eutectic salt has a melting point of 498.3 °C, with the latent heat of 330.8 J/g.Omran et al. [83] investigated the influence of the moisture on the latent heat of a binary mixture (Li 2 CO 3 /K 2 CO 3 ).e DSC result showed the onset temperature of 502.43 °C, the peak temperature of 502.42 °C, and the latent heat of 314.29 J/g.Tao et al. [84] proposed four kinds of carbon nanomaterials mixed into binary carbonate eutectic salts (Li 2 CO 3 /K 2 CO 3 ) as the composite PCM.According to the DSC curves, the pure PCM has the melting point and melting enthalpy of 486.40 °C and 331.50 J/g, respectively.However, the melting point and melting enthalpy of composition decreased with the increase of nanomaterial.Zhang et al. [85] prepared the composites of 32.1% Li 2 CO 3 /33.4% Na 2 CO 3 /34.5% K 2 CO 3 with T-ZnOw by the solution-evaporation method.ere was a slight change with regard to latent heat and melting point of ternary carbonate/T-ZnOw, and the value ranged from 403 to 406 °C.
It is well known that the alkali halides are the materials with high-melting point (the temperature more than 600 °C), such as fluorides, chlorides, and their eutectic salts.In order to meet the needs of solar energy storage, Wei et al. [86] and Du et al. [87] et al. chose the ternary chloride salt NaCl/-CaCl 2 /MgCl 2 to study the thermal properties.And the lowest eutectic temperature was 424.05 °C which was in good agreement with experimental results of 424 °C measured by DSC.And the latent heat of the PCM is about 201.50 J/g, which was so large that this ternary eutectic chloride salt was suitable for the TES system.Tian et al. [78] chose binary eutectic chloride (NaCl/CaCl 2 ) as the high-temperature (>400 °C) heat storage medium, and the EG was adopted to enhance the heat transfer performance of this system.From the DSC plot, the melting point and the phase change latent heat of binary eutectic chloride were 506.2 °C and 164.2 J/g.Khokhlov et al. [88] studied the key thermal properties of some molten salt eutectics.e latent heat of mixture (LiF/NaF/KF) measured was about 385 J/g with the melting point of 731 K, and there were two kinds of eutectics LiF/NaF/ZrF 4 with the melting point and latent heat of 842.7 K, 727.9 K and 106.4 J/g, 40.7 J/g, respectively.
And the melting points of the mixtures including carbonates, halogenated salts, and nitrates are relatively higher.
e research on the melting point and latent heat of this mixture and other materials has been investigated by many researchers.Li et al. [89,90] analyzed the thermophysical properties of pure NaNO 3 with the melting point and latent  [64] Advances in Materials Science and Engineering heat of about 306 °C and 178 J/g, respectively.Sang et al. [91] researched that the average melting point of the studied ternary carbonates, the nitrate-mixed molten salts, and the hydroxide-mixed molten salts were 402.1 °C, 394.7 °C, and 323.2 °C.NaOH as the additive has more significant effect on lowering the melting point of ternary carbonate mixtures.Wang et al. [92] studied the melting point and latent heat of the novel eutectic composition in the LiF/Na 2 CO 3 /K 2 CO 3 ternary system via DSC.From the DSC curve, the average melting point was 420.9 °C and the latent heat was 202.46 J/g.Jiang et al. [93] selected the eutectic molten salt (Na 2 CO 3 /NaCl) as a new high-temperature PCM for solar thermal energy storage.From the DSC plot, the melting point and latent heat of the eutectic composition were 637 °C and 283.3 J/g, respectively.
In addition, the metal and alloy, as a high-temperature material and high thermal conductivity, have attracted much attention.Wei et al. [94] prepared ten aluminum alloy samples using a casting method by designing compositions of Al/Si, Al/Cu, Al/Mg, and Al/Cu/Zn alloys.Additions of Cu, Zn, and Si in the samples all helped in reducing the melting point of the aluminum alloys.e Al/19.7Sialloy sample had the highest latent heat of 372.1 J/g.Yolshina and Kvashinchev [95] investigated the synthesis of nanocomposites of Al/Al 2 O 3 with improved physical properties.
e melting point of the nanocomposite (668 °C) was only more than 6 °C compared with the initial aluminum (662 °C).
e aim of this study was to investigate high thermal conductive materials such as metallic alloys for latent heat energy storage in CSP applications.Rwasueño and Faik [96] investigated two ternary eutectic alloys, Mg 70 Zn 24.9 Al 5.1 and Zn 85.8 Al 8.2 Mg 6 .e DSC curves indicated that the melting point and the latent heat of the two alloys were 339.5 °C and 157 J/g and 343.5 °C and 104 J/g, respectively.
In the above literatures, the main works focus on the research of melting point and latent heat of high-temperature inorganic PCMs via DSC.It can be found that the carbonates (Li 2 CO 3 , Na 2 CO 3 , and K 2 CO 3 ), alkali halides (NaCl, CaCl 2 , MgCl 2 , LiF, NaF, and KF), the mixtures of them (such as Na 2 CO 3 /NaCl), metal, alloy (such as Al/Cu/Zn), and so on have been the focus of high-temperature PCMs.erefore, the summary of the melting point and latent heat of mentioned high-temperature inorganic eutectic composites was demonstrated in Tables 5 and 6.
According to the inorganic PCMs mentioned above, the rough relationship of phase change temperature and inorganic PCMs is displayed in Figure 1, where the melting point of the material increases gradually from the bottom to the top.

Organic Materials.
According to the phase change temperature and heat storage capacities of PCMs, a wide variety of them are utilized in the different applications.A majority of materials including inorganic compounds, organic compounds, and their mixtures were used as PCMs, which can be further classified into inorganic PCMs and organic PCMs, such as paraffin waxes, fatty acids, polyhydric alcohol, and n-alkanes [17].In general, inorganic PCMs have some unavoidable shortages used in the TES systems, such as their supercooling trend, toxicity, corrosivity, and ecologically harmful properties.On the contrary, most of the organic PCMs are safe, noncorrosive, inexpensive, and chemically inert and have no phase segregation.Among them, the paraffin waxes are the most popular due to the remarkable properties, such as high energy storage density, low vapor pressure, small degree of supercooling, nontoxicity, and commercial availability at a relatively low cost [97].6 Advances in Materials Science and Engineering Meanwhile, fatty acids have also many advantages for the TES system due to the high latent heat, little volume change during phase change process, little supercooling, nontoxicity, and moderate price.Some study of thermal properties focus on the para n wax as PCM by researchers.In this study, Yang et al. [97] selected the para n wax with the melting temperature of 50-52 °C and the latent heat of 220.3 J/g as the PCM for preparing the composites.As presented in DSC plots, the latent heat and melting point were found to be 110.15J/g and 46-48 °C for para n/rude-Pal; 124.52 J/g and 50-52 °C for para n/rinsed-Pal; 132.18 J/g and 54-56 °C for para n/ H − -Pal; and 120.16J/g and 50-52 °C for para n/organic-Pal.
e composites of graphite-para n with di erent mass fraction were elaborated by Fethi et al. [98].And the latent heat and melting point were measured using the DSC.It can be seen that the melting point of the composite 20% graphite-para n (57.21 °C) was slightly di erent compared with the pure para n (58 °C).
Recently, the fatty acids as the very popular PCMs have been investigated.Ke [99] selected fatty acids including capric acid (CA), lauric acid (LA), myristic acid (MA), palmitic acid (PA), and stearic acid (SA) as the PCMs.According to the DSC curves, the developed binary and multiple fatty acid eutectics exhibited desired latent heat ranging from about 130 J/g to 200 J/g, and their onset melting temperatures were in the range of about 14-53 °C.Fauzi et al. [100] researched the thermal properties of both binary mixtures of fatty acid, myristic acid/palmitic acid/sodium myristate, and myristic acid/palmitic acid/sodium palmitate (MA/PA/SP) as the PCMs.From the DSC plots, the melting points of two kinds of binary mixtures were 41.36 °C and 41.58 °C, respectively, with the latent heat of 179.12 J/g and 184.14 J/g. e researchers have studied the e ect of a lot of supporting materials (such as diatomite, expanded vermiculite, and bentonite) on fatty acids.In this study, Tang et al. [101] prepared the shape-stabilized fatty acid eutectics, such as palmitic acid (PA), capric acid (CA), and diatomite (as the supporting material) composites.
e melting points and latent heat of the PA and CA were 62.89 °C, 32.19 °C and 208.78 J/g, 159.97 J/g, respectively.e DSC results showed that the composite (PA/CA-diatomite) melts at 26.69 °C with a latent heat of 98.26 J/g.Jiesheng et al. [102] and Wen et al. [103] investigated the thermal properties of the composite PCM lauric acid-expanded perlite (LA-EP) with expanded vermiculite (EVM) as the supporting material.e results of DSC analysis showed that the melting point and latent heat of the composites of LA-EP and LA-EVM were 43.2 °C, 41.88 °C and 105.58 J/g, 126.8 J/g, respectively.Fauzi et al. [104] studied the fatty acid of myristic acid/palmitic acid/sodium palmitate (MA/PA/SP) and myristic acid/palmitic acid/sodium stearate (MA/PA/SS), and Shorea javanica (SJ) was used as a porous material.e latent heat and melting point were 169.71J/g, 172.72 J/g and 40.13 °C, 41.65 °C, respectively.In this work, capric acid-expanded Advances in Materials Science and Engineering perlite (CA-EP) and capric/stearic acid-expanded perlite (CA/SA-EP) composite PCMs were prepared.Wei et al. [105] found that the latent heat of CA-EP composites was 87.3 J/g with the melting point at 33.6 °C.And the corresponding value of the CA/SA-EP was 82.1 J/g.Huang et al. [106] investigated the thermal properties of palmitic acid/stearic acid (PA/SA) eutectic mixtures as PCM and the bentonite as the supporting material.And the melting temperature of the eutectic was 53.64 °C with the latent heat of 189.47 J/g.
And n-alkanes and polyhydric alcohol as the PCMs utilized in the TES system have been studied in recent years.Xiong et al. [107] prepared the composite of polyethylene glycol-graphene oxide (PEG4000-GO).For pure PEG4000, the melting point and latent heat are 65 °C and 182.8 J/g during the melting process.And the melting point and latent heat of this composite were 62.2 °C and 174.5 J/g, which were slightly lower compared with the pure PEG.Zhang et al. [108] studied the thermal properties of polyethylene glycolexpanded perlite (PEG-EP) composite with carbon layer PCMs.e results indicated that the values for the latent heat and melting point of the composite during melting were 134.93 J/g and 55.19 °C, respectively.Zhang et al. [109] selected the PEG as the latent heat storage material, and graphene oxide-based mesoporous silica sheets (GOS) were prepared as supporting materials.Combined with DSC results, it was found that the melting point of composite PCMs was slightly higher than that of the pure PEG (46.26 °C).When the PEG mass fractions were between 60% and 80%, the latent heat of composites were 61.32-136.30J/g.Tang et al. [110] studied the thermal properties of the stable composite Ti 4 O 7 /PEG/SiO 2 by DSC. e results indicated that the melting point of Ti 4 O 7 /PEG/SiO 2 was close to the pure PEG6000 (61.4 °C).
e latent heat was 129.8 J/g for Ti 4 O 7 (3%)/PEG/SiO 2 with the melting point of 59.8 °C.In this research, Yang et al. [111] selected the highdensity polyethylene (HDPE) as the PCM for mediumtemperature TES application.
e DSC curve results show that the composite of HDPE-graphite melts at 139.1 °C with the latent heat of 130.7 J/g while the pure HDPE melts at 137 °C with the latent heat of 172.7 J/g.Tang et al. [112] selected the PA/HDPE and graphene nanoplatelets (GNP) as solid-liquid PCM and nanoadditives.
e DSC result showed that the PA/HDPE-GNP had the high latent heat of 157.82 J/g and constant melting point of around 62 °C.
A shape-stabilized octadecane and SiO 2 microcapsules were prepared by using the sol-gel method, and the octadecane was selected as PCM for TES.Tang et al. [113] adopted DSC to measure the thermal properties of the microcapsules.
e results showed that the satisfactory microencapsulated phase change material (MPCM) sample melts at 28.32 °C with a latent heat of 227.66 J/g.Xu et al. [114] selected a composite of pure D-mannitol (C 6 H 14 O 6 ) with a melting point of 166-169 °C as PCM and EG as encapsulation heat transfer promoter.From the DSC plots, the melting point slightly shifts from 164.87 °C of pure D-mannitol to 151.82 °C of composite PCM.And the latent heat of pure D-mannitol and composite was 319.0 J/g and 267.7 J/g.Rodríguez and López [115] selected pluronic F127 as a PCM, and the DSC results indicated that the melting point of F127 is 54 °C with the latent heat of 102 J/g.Han et al. [116] chose the eutectic mixture of 1,6-hexanediol/lauric acid (HE/LA) as PCM.And the DSC curve displayed the melting point of 36.92 °C as well as a heat of fusion of 177.11J/g.
In recent years, the sugar alcohols as the PCMs were very promising in the application of the TES system due to their high storage capacity, safety, and economic cost.Solé et al. [117] analyzed two samples of each sugar alcohol (D-mannitol and myoinositol).e DSC results indicated that the melting point and latent heat for both samples were 179.9 °C, 180.34 °C and 246.4 J/g, 267.9 J/g, respectively.8 Advances in Materials Science and Engineering And Diarce et al. [118] investigated the new binary eutectic mixtures of sugar alcohols in the temperature range of 50-90 °C.e mixtures were composed of erythritol-xylitol, erythritol-sorbitol, and xylitol-sorbitol.According to the DSC plots, the melting points of these mixtures were 83.9 °C, 86.6 °C, and 74.9 °C, respectively.And the erythritol-xylitol eutectic PCM showed the largest latent heat of 248.7 J/g.Meanwhile, the latent heat for the erythritol-sorbitol and xylitol-sorbitol eutectic compositions was found to be 172.8J/g and 169.6 J/g, respectively.
In the above literatures, the main works focus on the research of phase change temperature and latent heat of organic PCMs via DSC.It can be seen that the organic PCMs, such as paraffin waxes, fatty acids, polyhydric alcohol, and n-alkanes, are the very promising materials with a lot of advantages for satisfying the demand of low-medium temperature applications.In addition, the summary of the melting point and latent heat of involved organic eutectic composites is displayed in Tables 7 and 8, respectively, and generally, it can be seen that the organic materials as the PCMs are used in low-medium temperature TES applications.

Conclusions
Investigations on thermal energy storage (TES) system have developed over the past four decades and are ongoing.Especially, the PCM is currently one of the most promising elements to research the TES system, and the desirable PCMs need some thermal properties, such as suitable melting point, high latent heat, high thermal conductivity, and high specific heat.
erefore, this paper reviews the melting point and latent heat of PCMs via the differential scanning calorimetry (DSC) in order to obtain the suitable PCM for TES applications.ere is a series of PCMs with different phase change temperatures via DSC to assist the selection of PCM in the TES system which has been investigated and reviewed.e following conclusions can be reported: (1) According to the above analysis of the literatures in this paper, it can be seen that the PCMs can be classified into organic materials (such as paraffin waxes, fatty acids, polyhydric alcohol, and n-alkanes) and inorganic materials based on the chemical composition.(2) On the basis of melting point of the inorganic PCMs, they can be divided into three groups: low-temperature heat storage (<120 °C), medium-temperature heat storage (120-300 °C), and high-temperature heat storage (>300 °C).For the solar TES system, the medium-high temperature PCMs are the most popular material.
(3) According to the phase change temperature of the organic PCMs, it is found that the organic PCM is very promising materials with many advantages for satisfying the demand of low-temperature applications.(4) e melting point and latent heat of the PCM have been analyzed and summarized, which are the key to select and utilize the suitable PCM in the TES applications.However, the types of PCMs are not enough to satisfy the optional utilization in the TES applications at present.In later work, in order to supply perfect thermal properties of PCMs, the better characteristics of PCMs need to be investigated.And it will provide a more significant reference and guidance for the TES applications.

Nomenclature
DSC: Differential scanning calorimetry CSP: Concentrating solar power LHTES: Latent heat thermal energy storage PCMs: Phase change materials TES: ermal energy storage HTF: Heat transfer fluid DTA: Differential thermal analysis PTC: Parabolic tough collector EHS: Eutectic hydrated salt.

3 TFigure 1 :
Figure 1: e relationship of the melting point and inorganic PCMs.
•4H 2 O is 132.08 °C, which was significantly lower than the binary salt KNO 3 /NaNO 3 with the melting point of 223 °C.Mohammad et al.
[70]from FactSage) and the latent heat was 126.9 J/g.Pramod et al.[69]prepared the mixtures of LiNO 3 , KNO 3 , and Ca(NO 3 ) 2 as the ternary molten salts with the melting points of 137 °C.And Li et al.[70]chose a lower percentage of CsNO 3 to add into the ternary nitrate salts in order to obtain three kinds of quandary salts (LiNO 3 /KNO 3 /Ca(NO 3 ) 2 /CsNO 3 ).And the DSC plot results indicated that the phase transitions of the samples occurred at 113.0-113.1 °C.e fusion heat of them was 95.61 J/g, 86.70 J/g, and 87.95 J/g, respectively.

Table 1 :
e melting point of low-temperature inorganic eutectic composites.

Table 4 :
e latent heat of medium-temperature inorganic eutectic composites.

Table 5 :
e melting point of high-temperature inorganic eutectic composites.

Table 6 :
e latent heat of high-temperature inorganic eutectic composites.

Table 7 :
e melting point of organic eutectic composites.

Table 8 :
e latent heat of organic eutectic composites.