Inorganic Salt Hydrate for Thermal Energy Storage

Using phase change materials (PCMs) for thermal energy storage has always been a hot topic within the research community due to their excellent performance on energy conservation such as energy efficiency in buildings, solar domestic hot water systems, textile industry, biomedical and food agroindustry. Several literatures have reported phase change materials concerning various aspects. Among these materials, salt hydrates are worthy of exploring due to their high-energy storage density, rational price, multiple sources and relatively good thermal conductivity. This paper reviews the present state of salt hydrates PCMs targeting classification, properties, defects, possible solutions as well as their idiographic features which are suitable for applications. In addition, new trends of future research are also indicated.


Introduction
With the growing consumption of fossil fuels and increasing emission of carbon dioxide, thermal energy storage (TES) has received a great deal of attention.Latent heat storage and sensible heat storage are two main forms of TES.Using phase change materials (PCMs) for latent heat storage, which can storage and release energy by melting and solidification, is becoming an effective way to solve the contradiction of supply and demand of energy, such as peak difference of power load and gap of solar energy [1,2].High storage density, stable performance, wide phase change temperature range and reasonable price make PCMs play a significant role in energy conserving areas.
In the field of phase change energy storage technology, it is a fundamental step to develop high latent heat, stable, and cost-effective materials.Different types of PCMs have been studied after petroleum crisis in 1970s, including solid-solid PCMs (polyalcohols and other polymers), solid-gas PCMs and solid-liquid PCMs (salt and salt hydrates, clathrate hydrates, paraffins, fatty acids and their mixtures, alloys and ect) [3,4].Among these, salt hydrates, which account for a large proportion in inorganic PCMs, have always been attracted attention owing to their reasonable price, wide sources, good thermal conductivity and high volumetric energy storage density.However, some defects, such as leakage, causticity, supercooling and phase separation impose restrictions on their practical applications [5][6][7].In order to reduce such adverse effects, form-stable PCMs are studied recent years by incorporating PCMs into porous materials or through the method of microencapsulation to prevent leakage and phase separation [8][9][10].Moreover, different kinds of nucleating agents have also been developed to reduce supercooling degree.
It is a promising direction to apply low-cost inorganic PCMs to energy conservation areas such as electric peak-shaving, industrial waste heat utilization, aerospace field, air conditioning cold storage and so on.This paper reviews the research progress of salt hydrates for TES including properties, improvements as well as their specific characteristics, which are suitable for practical application in different fields in recent years.Research trends and potentials of these PCMs are also prospected.

Properties of Inorganic Salt Hydrated PCMs
Salt hydrates can be generally described as formula of AB•nH 2 O, where n represents number of water molecules and represents salt composition.During phase transformation dehydration of the salt happens, the process can be expressed as (1) and (2).
According to the phase transition performance, the salt hydrates can be divided into three categories: salt hydrates with congruent melting behavior, incongruent behavior and semi-congruent behavior.
There is a good deal of salt hydrates that can be regarded as available candidates for PCMs.However, a large proportion of salt hydrates which process large transition latent heat and moderate phase change temperature cannot melt congruently.The most meritorious ones are salt hydrates with congruent melting behavior, low supercooling degree and competitive profitability.Thermophysical, kinetic and chemical properties of such salt hydrates, which can possibly meet the requirements, are presented in Table 1.The most representative salt hydrates are sodium sulfate decahydrate, calcium chloride hexahydrate, sodium phosphate, dodecahydrate and magnesium chloride hexahydrate [5,11].However, the phase transition temperature range of single salt hydrate cannot always meet the requirements of practical application.Therefore, two or more salt hydrates can be mixed to adjust the melting point.Salt hydrate mixtures can be classified as eutectics with congruent behavior and non-eutectics with incongruent melting points.The eutectics do not tend to segregate during melting and freezing process because they freeze to a friendly blend of crystals [12][13][14][15].Binary or multicomponent eutectics with a common melting point and large latent heat can reduce the melting point of single constituent salt hydrate and then they can be utilized in wider fields [16].Besides, the eutectics can also make up for the deficiencies of single salt hydrates such as supercooling and phase separation.Liu [17,18] respectively added Na 2 HPO   2.
Generally speaking, it is difficult to determine the thermo-physical properties of PCMs due to the limitation of the conventional devices such as differential scanning calorimetry (DSC), modulated differential scanning calorimetry (MDSC) and differential thermal analysis (DTA).DSC method is common for measuring enthalpy, specific heat and phase change temperature for PCMs.However, only a small amount of samples can be measured once, which lead to possible errors of results.In addition to this, the measuring results are largely influenced by environmental factors.Consequently, the investigations on thermo-physical properties of salt hydrates are inadequate in literature.However, they are vital for heat flow transformation process, which is a decisive factor for effective heat storage system.In recent years, Temperature-history (T-history) method has been mentioned by Yin et al. [20].As a simple, accurate, and economic way for determining the main thermophysical properties such as enthalpy, specific heat, supercooling degree as well as process of phase transition, T-history method represents an advance to obtain average properties of PCMs especially for sensitive salt hydrate PCMs which suffer supercooling.

Supercooling of Salt Hydrates
Supercooling is a common phenomenon among salt hydrates due to their poor nucleating ability [20].Some salt hydrates without a nucleating agent do not crystalize even when supercooling temperature is decreased under tens of degrees.This always brings undesirable effects to practical application [21].For instance, when we impregnated salt hydrates into building materials using as wall boards, the phenomenon of supercooling impedes the solidification process of PCMs and then the storing heat in PCMs cannot release even when the surrounding temperature has reached their freezing point.Consequently, the PCM with high supercooling degree will not have functions to regulate the indoor temperature fluctuation and this lead to the lost of the meaning of latent thermal energy storage.Supercooling degree depends on several kinds of parameters, such as nucleating agents, cooling rate, stirring rate, stirring time, solution composition.To overcome the supercooling problems, a number of methods have been suggested.One of the most effective and economical ways is adding nucleating agents.Theoretically, the choice of nucleating agents should be close to the lattice parameter of the target materials.For instance, Borax is always regarded as the most suitable nucleating agents for Na 2 SO   [23] investigated the surface effects of interaction between salt hydrates and nano-particles.Three nano-particles of aerosol SiO 2 , RNS-A SiO 2 and liquid phase SiO 2 were compared by adding them to Na 2 SO 4 •10H 2 O. Results showed that aerosol SiO 2 can easily suppress the supercooling.CaCl 2 •6H 2 O is also mentioned frequently as a kind of fine PCMs.However, its supercooling degree is up to 20 • C. Sutjahja [24] used SrCl 2 •6H 2 O (1.0 wt %), Ba 2 CO 3 (0.5 wt %), and K 2 CO 3 (0.5 wt %) as additives to CaCl 2 •6H 2 O.It was found that SrCl 2 •6H 2 O exerted the best effect on CaCl 2 •6H 2 O, this result was consistence with paper [25].Palittin [26] adopted sonocrystallization method which used ultrasound radiation for promoting CaCl 2 •6H 2 O to crystallize and results indicated this technique made crystals have uniform distribution and dense arrangement.Analogously, Miyasaka et al. [27] observed that ultrasound irradiation exceled in controlling primary nucleation of Na 2 HPO 4 •12H 2 O and suggested that nucleation induced by ultrasound was probably the main cause of temperature rise.Application of nanomaterials has recently put forward to give notable effect on reducing supercooling degree.Li et al. [28] focused on nanocomposite PCMs and found that the degree of supercooling is within the range of 0.3-1.1 • C when CaCl 2 •6H 2 O composite contained with 1 wt % nanomaterials and the complete solidification times were reduced by 17.84%.Fine graphite powder less than 50 µm [29], borax [30], disodium phosphate dodecahydrate [31] and nano silver particles [32] [34], Sr(OH) 2 [34,35] SrCO 3 [35] and Na 3 AlF 6 [36] are both good nucleating agents for bischofite (MgC1 2 •6H 2 O).For decreasing supercooling degree of Mg(NO 3 ) 6 , both Mg(OH) 2 and BaO are good choice to promote its crystallization [37].Ushak et al. [38] reports that supercooling of CaCl 2 •4H 2 O was slightly reduced by adding 1 wt % NaCl.Another paper [39] demonstrated that black carbon was good for crystallization of Ca(NO 3 ) 2 •4H 2 O and CaF 2 was beneficial for NH 4 Al(SO 4 ) 2 •12H 2 O.In conclusion, graphite nanoparticles, metal nanoparticles, nano-silica molecules, borax are relatively effective nucleating agents, ultrasonic can also be regarded as good methods for reduce supercooling degree.Further research may pay attention on this.

Phase Separation of Salt Hydrates
A number of salt hydrates with large latent heat which can be regarded as PCMs cannot melt congruently [11].As a result, the amount of water released by salt hydrates is no enough to dissolve the crystalline salt formed during the process of dehydration.Residual salts will deposit on the bottom due to a larger density and then lead to phase separation.With the continuous cold-thermal cycle, the sediments will continue to increase, resulting in gradually declining of thermal storage capacity.Thus, phase separation is another major problem of salt hydrate PCMs [40].
Adding thickener agents into salt hydrates is one of common methods to minimize or avoid phase separation.Many researchers used hydroxyethyl cellulose, nanocellulose, cellulose [41][42][43][44], super absorbent polymer [45], polyethylene glycol [46] and some silicone derivatives [47] as thickeners.Porous materials such as expanded graphite [29,48], diatomite [49][50][51] and nano-titanium dioxide [52,53] etc. were also reported as fine supporting materials for inorganic materials.They increase the viscosity of solution so that the solid particles can be evenly distributed.In addition to this, microencapsulation, macro encapsulation (hermetically packed in vessel) and impregnation with porous materials to prepare composite PCMs are effective methods.This cannot only solve problems of phase separation, but can also effectively prevent leakage problem attributing to the melting process of PCMs.
Microencapsulation phase change materials (MEPCMs) usually consist of a polymeric material as shell and a desired PCM as core [54].Microencapsulated salt hydrates can reduce the reactivity with the outside surroundings, which can be utilized in powder form or dispersed into a carrier fluid, and then prevent PCMs from phase separation and leakage.Physical and chemical processes are two major methods to prepare MEPCMs.Physical methods typically consist of spray drying, fluidized bed and complex coacervation.Interfacial polymerization, in situ polymerization, suspension-like polymerization, and sol-gel methods make up chemical method [55,56].Hitherto, vast researches have accomplished to formulate MEPCMs, whereas most of them focused on organic materials as cores because it is more troublesome to formulate water-solubility salt hydrate MEPCMs.Consequently, only a few literatures have reported hydrated salt microencapsulation.Streufert [57] outlined a novel microencapsulated PCM which used sodium acetate trihydrate as core materials.Its ability of dispersions into a matrix and variation of mechanical properties were investigated.The study performed two different emulsions: emulsion by using microfluidic device with acrylate as shell material and emulsions preparing by mechanically creating with polytrimethylene carbonate (PLA) as shell.Figures 1 and 2 shows SEM image and optical microscope image of the PLA microcapsules and microfluidic device as microcapsules, respectively.From the results we can clearly see more stable and less dispersive microcapsules by microfluidic device.The microcapsules showed outstanding performance without leakage and phase separation.Fully loaded microencapsulated PCM with Na 2 HPO 4 •7H 2 O as core and methyl methacrylate as shell was prepared by Huang et al. [58], which was exploited suspension copolymerization-solvent volatile method.During the procedure of polymerization reaction, the original salt hydrate Na 2 HPO 4 •12H 2 O lost 5 crystal waters through heating process and the remainder was successfully scattered into organic solvent.It was worthy noted that the thermal conductivity did not seem to change after microencapsulated Na 2 HPO 4 •7H 2 O formed.This excellent PMMA/Na 2 HPO 4 •7H 2 O MEPCM can be deemed as a potential material for solar space heating.
Appl.Sci.2017, 7, 1317 6 of 17 see more stable and less dispersive microcapsules by microfluidic device.The microcapsules showed outstanding performance without leakage and phase separation.Fully loaded microencapsulated PCM with Na2HPO4•7H2O as core and methyl methacrylate as shell was prepared by Huang et al. [58], which was exploited suspension copolymerization-solvent volatile method.During the procedure of polymerization reaction, the original salt hydrate Na2HPO4•12H2O lost 5 crystal waters through heating process and the remainder was successfully scattered into organic solvent.It was worthy noted that the thermal conductivity did not seem to change after microencapsulated Na2HPO4•7H2O formed.This excellent PMMA/Na2HPO4•7H2O MEPCM can be deemed as a potential material for solar space heating.
(a)  Porous materials like carbon foam, diatomite, expanded graphite and expanded perlite used as supporting materials can also effectively overcome leakage, phase separation and corrosion problems of PCMs.Such composite materials are often synthesized by blending or impregnating PCMs with porous matrix [59].The supporting materials with large specific surface area make liquid materials be adsorbed by their internal pores and then shortcomings of phase separation and leakage of PCMs can be overcame.EG (expanded graphite) is a sort of carbon materials by heating graphite at high temperature through microwave irradiation.EG/PCM composites have good performance with high thermal conductivity, light weight, no phase separation leakage phenomenon and slight supercooling degree owing to the capillary and surface tension of EG [60][61][62][63].Duan et al. [48] prepared CaCl2•6H2O/EG composites by adsorbing liquid salt hydrate into EG.The shape-stable composite was obtained by putting EG into fused CaCl2•6H2O.Alkylphenols polyoxyethylene-10 (Op-10) as emulsifier was also added to enhance binding force between EG and salt hydrate.Thermal conductivity also improved by 82% compared with neat CaCl2•6H2O.Another composite salt hydrate/EG composite was prepared by Wu et al. [64].Na2SO4•10H2O and Na2HPO4•12H2O mixture was impregnated into air voids by vacuum impregnation methods and then coated by paraffin.In addition, the wax addictive did not affect the thermal conductive at all.Phase segregation and supercooling of salt hydrates mixtures were also almost inhibited.Li [28] investigated the influence of oxidation expandable graphite (OEG) on thermal behaviors of CaCl2•6H2O, and results showed that OEG can be served as nucleation to promote crystallization process thanks to their uniform distribution in salt hydrates.Similar research was carried out by Mao [31].Latent storage, supercooling degree and other properties of sodium acetate trihydrate-disodium-dodecahydratecarboxyl methyl cellulose/expanded graphite composite PCMs were investigated.With the increasing mass proportion of EG, lower enthalpy and higher transfer efficient emerged.The maximum heating temperature was determined not higher than 78 °C, which is beneficial for refraining from undercooling.Ultimately, the system with 3 wt.% EG was known as the most suitable selection.Liu [65] combined graphene oxide-modified hydrate salt with poly copolymer hydrogel.The results indicated that the obtained composite was a promising material for achieving further Porous materials like carbon foam, diatomite, expanded graphite and expanded perlite used as supporting materials can also effectively overcome leakage, phase separation and corrosion problems of PCMs.Such composite materials are often synthesized by blending or impregnating PCMs with porous matrix [59].The supporting materials with large specific surface area make liquid materials be adsorbed by their internal pores and then shortcomings of phase separation and leakage of PCMs can be overcame.EG (expanded graphite) is a sort of carbon materials by heating graphite at high temperature through microwave irradiation.EG/PCM composites have good performance with high thermal conductivity, light weight, no phase separation leakage phenomenon and slight supercooling degree owing to the capillary and surface tension of EG [60][61][62][63] In addition, the wax addictive did not affect the thermal conductive at all.Phase segregation and supercooling of salt hydrates mixtures were also almost inhibited.Li [28] investigated the influence of oxidation expandable graphite (OEG) on thermal behaviors of CaCl 2 •6H 2 O, and results showed that OEG can be served as nucleation to promote crystallization process thanks to their uniform distribution in salt hydrates.Similar research was carried out by Mao [31].Latent storage, supercooling degree and other properties of sodium acetate trihydrate-disodium-dodecahydrate-carboxyl methyl cellulose/expanded graphite composite PCMs were investigated.With the increasing mass proportion of EG, lower enthalpy and higher transfer efficient emerged.The maximum heating temperature was determined not higher than 78 • C, which is beneficial for refraining from undercooling.Ultimately, the system with 3 wt % EG was known as the most suitable selection.Liu [65] combined graphene oxide-modified hydrate salt with poly copolymer hydrogel.The results indicated that the obtained composite was a promising material for achieving further phase transition behavior targets, which can be used for thermal storage under a wide scope of ambient temperatures.Except for EG, other composite salt hydrate PCMs were studied as well.Cui [66] used vacuum impregnation method to prepare CaCl 2 •6H 2 O and sepiolite compound.Sepiolite, as a fibrous clay mineral with superior capability, possesses a good flare retardance property, can be widely applied to buildings.The obtained composite with 70 wt % CaCl 2 •6H 2 O resulted in good phase change energy storage properties.However, sepiolite leads to a decrease in enthalpy and thermal stability.Extensive studies have been developed with respect to porous silica, which owns large surface area and porosity.Wu et al. [67] adopted sol-gel method by impregnating mixture of Na 2 SO 4 •10H 2 O and Na 2 HPO 4 •12H 2 O into porous silica matrix.Polymethylene was furthermore coated on the surface of silica for preventing the water vapor from evaporating through the pores during phase transition.No leakage and phase separation was observed.Meanwhile, depressing melting pointing was observed by DSC, which was attributed to the difference of wettability between coated and uncoated composite.
In a word, in spite of some disadvantages, salt hydrates are generally considered as suitable materials for thermal energy storage applications due to more and more effective methods are developing to overcome the problems [68,69].Except for nucleating agents and thickening agents, preparing composite salt hydrate PCMs are also worthy noting due to their excellent abilities for solving phase separation and eliminating supercooling degree.Future trends of salt hydrates materials will most likely be related to the area of microencapsulated PCMs and shape-stabilized materials prepared by PCMs and supporting materials.Nanomaterials which can improve thermal conductivity of PCMs are also supposed to be paid attention.

Application
Application of salt hydrate PCMs emerged among various areas such as energy-effective construction industry, smart electronics, concentrated solar domestic hot water systems, functional textile industry and food industry in recent years [14,70].Limited by temperature range for most salt hydrates, using salt hydrates for thermal storage in buildings have always attracted interests of researchers due to their abilities for changing the increasingly indifferent energy generation [71].It can be deduced the trends in decreasing the energy demand of buildings is necessary [72].Salt hydrates combining with active and passive systems can minimize variations between energy consumption and demanding in extremely cold and hot areas.There are two main methods to combine salt hydrates with building materials: integrating salt hydrates into light weight construction materials and using packaging materials for encapsulating.However, corrosion with mild metal containers is also a universal behavior of salt hydrates.This phenomenon causes severe effects related to safety and economic in actual applications.As a result, the compatibility between PCMs and metal containers has been widely studied and valuable suggestions have been proposed.Sole [68] focused on immersion corrosion test of metal vessel (Cu, Al, stainless steel, carbon steel) with solid salt hydrates, aiming to searching for optimized container.Metal specimens immersed in salt hydrates were weighted after experiments.Results demonstrated that stainless steel can be considered as the most corrosion-resistant for all experimental salt hydrates.

Salt Hydrates for Hot Water Tanks
Presently, the most widely used heat storage systems for solar thermal energy are hot water tanks, which process an energy storage density of 69.4 kW•h•m −3 (∆T = 60 K) but a finite storage time [73].Salt hydrates with melting temperature of 50~60 • C can be incorporated into hot water tanks of solar domestic to reinforce stratification.Salt hydrate PCMs put off the time of reaching thermal equilibrium between top layer with higher temperature and bottom layer with low temperature.An experimental solar stand was constructed by Cabeza [74] with two identical thermal solar collectors.The picture of the installation is shown in Figure 3.One of them was inserted by sodium acetate trihydrate.It was found that the inclusion of a PCM module in water tanks for domestic hot-water supply is a very promising technology.It would allow to have hot-water for longer periods of time even without exterior energy supply, or to use smaller tanks for the same purpose.Three types of encapsulated commercial salt hydrates (DC58, PT58 and HD60) are tested inside tanks through continuous temperature measurements with four sensors installing at various heights [75].The percentage of liquid state of PCMs remaining after tests was obtained by energetic analysis, from which the solidification processes could be clearly knew.even without exterior energy supply, or to use smaller tanks for the same purpose.Three types of encapsulated commercial salt hydrates (DC58, PT58 and HD60) are tested inside tanks through continuous temperature measurements with four sensors installing at various heights [75].The percentage of liquid state of PCMs remaining after tests was obtained by energetic analysis, from which the solidification processes could be clearly knew.

Salt Hydrates for Wallboards
With the increasing emission of carbon dioxide, blending PCMs in building materials or directly using PCMs as wall boards, is able to store considerable energy during the daytime at high temperature and release energy at night when temperature is low by melting and solidifying processing.The combination of PCMs with building materials has showed superior performance which can contribute to energy saving, electricity shifting and human comfort [70,[76][77][78][79]. Figure 4a showed a schematic of a kind of full-scale room with PCM concrete wallboards.As seen in Figure 4b, the effects of alleviating temperature fluctuation of a kind of PCM wall room compared with ordinary room are apparent [80].However, PCMs used in construction field has certain requirements in terms of phase change temperature, stability, strength and price.Consequently, it is a significant step to choose suitable materials and methods of building envelope enclosures, as it is a major parameter to decide by what extent the thermal mass is activated.High fusion heat, odorless and nontoxic properties make salt hydrates be good candidates for construction energy saving.

Salt Hydrates for Wallboards
With the increasing emission of carbon dioxide, blending PCMs in building materials or directly using PCMs as wall boards, is able to store considerable energy during the daytime at high temperature and release energy at night when temperature is low by melting and solidifying processing.The combination of PCMs with building materials has showed superior performance which can contribute to energy saving, electricity shifting and human comfort [70,[76][77][78][79]. Figure 4a showed a schematic of a kind of full-scale room with PCM concrete wallboards.As seen in Figure 4b, the effects of alleviating temperature fluctuation of a kind of PCM wall room compared with ordinary room are apparent [80].However, PCMs used in construction field has certain requirements in terms of phase change temperature, stability, strength and price.Consequently, it is a significant step to choose suitable materials and methods of building envelope enclosures, as it is a major parameter to decide by what extent the thermal mass is activated.High fusion heat, odorless and nontoxic properties make salt hydrates be good candidates for construction energy saving.even without exterior energy supply, or to use smaller tanks for the same purpose.Three types of encapsulated commercial salt hydrates (DC58, PT58 and HD60) are tested inside tanks through continuous temperature measurements with four sensors installing at various heights [75].The percentage of liquid state of PCMs remaining after tests was obtained by energetic analysis, from which the solidification processes could be clearly knew.

Salt Hydrates for Wallboards
With the increasing emission of carbon dioxide, blending PCMs in building materials or directly using PCMs as wall boards, is able to store considerable energy during the daytime at high temperature and release energy at night when temperature is low by melting and solidifying processing.The combination of PCMs with building materials has showed superior performance which can contribute to energy saving, electricity shifting and human comfort [70,[76][77][78][79]. Figure 4a showed a schematic of a kind of full-scale room with PCM concrete wallboards.As seen in Figure 4b, the effects of alleviating temperature fluctuation of a kind of PCM wall room compared with ordinary room are apparent [80].However, PCMs used in construction field has certain requirements in terms of phase change temperature, stability, strength and price.Consequently, it is a significant step to choose suitable materials and methods of building envelope enclosures, as it is a major parameter to decide by what extent the thermal mass is activated.High fusion heat, odorless and nontoxic properties make salt hydrates be good candidates for construction energy saving.In early years, Hadjieva [81] mixed Na2SO4•10H2O with porous concrete.Due to the formation of strong hydrogen bond, composite PCMs concrete system showed high stability and it could be further applied to floor construction.Large absorption area of autoclaved porous concrete was also observed.Ye et al. [82] carried out a detailed study on thermal properties of artificial climatic chamber with CaCl2•6H2O/expanded graphite composite panels at different position.Figure 5 showed the PCM panel packed by polyvinyl chloride (PVC) panel followed by compacting.Four pieces of the PCM panels were fabricated by the same process to use as wallboards.The thermal performance of the test room equipped with the PCM panels at each position was evaluated by placing the room in an artificial climatic chamber.It was revealed that both elements of position and quantity could influence thermal performance of the testing rooms.Moreover, numerical modeling verified the experimental investigations, and better performance of CaCl2•6H2O was found comparing with organic (RT27/EG composite PCM) panel.This success model showed that radiative effects of salt hydrate wallboards could evidently enhance thermal comfort of residents.Recently, Fu et al. [83] developed a novel nonflammable CaCl2•6H2O/expanded perlite composite and then fabricated into a board for replacing the foam board to be employed as the core in a foam insulating brick to obtain a PCM board.It is found that when applied PCM brick as the roof of a test room, the indoor peak temperature decreased more apparently compared with foam brick.In early years, Hadjieva [81] mixed Na 2 SO 4 •10H 2 O with porous concrete.Due to the formation of strong hydrogen bond, composite PCMs concrete system showed high stability and it could be further applied to floor construction.Large absorption area of autoclaved porous concrete was also observed.Ye et al. [82] carried out a detailed study on thermal properties of artificial climatic chamber with CaCl 2 •6H 2 O/expanded graphite composite panels at different position.Figure 5 showed the PCM panel packed by polyvinyl chloride (PVC) panel followed by compacting.Four pieces of the PCM panels were fabricated by the same process to use as wallboards.The thermal performance of the test room equipped with the PCM panels at each position was evaluated by placing the room in an artificial climatic chamber.It was revealed that both elements of position and quantity could influence thermal performance of the testing rooms.Moreover, numerical modeling verified the experimental investigations, and better performance of CaCl 2 •6H 2 O was found comparing with organic (RT27/EG composite PCM) panel.This success model showed that radiative effects of salt hydrate wallboards could evidently enhance thermal comfort of residents.Recently, Fu et al. [83] developed a novel nonflammable CaCl 2 •6H 2 O/expanded perlite composite and then fabricated into a board for replacing the foam board to be employed as the core in a foam insulating brick to obtain a PCM board.It is found that when applied PCM brick as the roof of a test room, the indoor peak temperature decreased more apparently compared with foam brick.In early years, Hadjieva [81] mixed Na2SO4•10H2O with porous concrete.Due to the formation of strong hydrogen bond, composite PCMs concrete system showed high stability and it could be further applied to floor construction.Large absorption area of autoclaved porous concrete was also observed.Ye et al. [82] carried out a detailed study on thermal properties of artificial climatic chamber with CaCl2•6H2O/expanded graphite composite panels at different position.Figure 5 showed the PCM panel packed by polyvinyl chloride (PVC) panel followed by compacting.Four pieces of the PCM panels were fabricated by the same process to use as wallboards.The thermal performance of the test room equipped with the PCM panels at each position was evaluated by placing the room in an artificial climatic chamber.It was revealed that both elements of position and quantity could influence thermal performance of the testing rooms.Moreover, numerical modeling verified the experimental investigations, and better performance of CaCl2•6H2O was found comparing with organic (RT27/EG composite PCM) panel.This success model showed that radiative effects of salt hydrate wallboards could evidently enhance thermal comfort of residents.Recently, Fu et al. [83] developed a novel nonflammable CaCl2•6H2O/expanded perlite composite and then fabricated into a board for replacing the foam board to be employed as the core in a foam insulating brick to obtain a PCM board.It is found that when applied PCM brick as the roof of a test room, the indoor peak temperature decreased more apparently compared with foam brick.

Salt Hydrates for Refrigeration System
Table 3 presented a variety of salt solutions with large enthalpy values that can apply to low temperature fields.NH 4 Cl, NaCl, KCl and NH 4 Br etc are mainly additional salt that can add to other PCMs to decrease melting point according to their abilities to weaken the hydrogen bond among water molecule [84,85].An investigation of PCM panels with NH 4 Cl and water placing against internal wall in a refrigeration during 24 h for 2 weeks was carried out by Gin [86].Performance of this system was evaluated by drip loss and ice crystal sizes of beef muscle and ice cream in the commercial freezer.Results showed that PCM panels can assist to lessen the rate of temperature fluctuations within the freezer.Little drip loss has been found in meat and minimal crystal sizes were observed on the surface of ice cream by microscopy and image analysis tool.Analogously, another experiment of improving thermal performance of freezer containing with sodium nitrate and water was conducted in [87].The commercial salt hydrate PCM with melting point of minus 18 • C was placed in 10 mm thick stainless panels at diverse position of refrigerated trucks.Test packages with chemical components were used to simulate frozen food.The result of door opening tests demonstrated that the salt evidently maintained interior temperature constantly for more than 3 h of power loss.This paper also stated that it is significantly to choose appropriate phase change temperature which is supposed to be near the storage temperature of freezer.

Salt Hydrates for Air Conditioning System
Apart from applications in freezers, eutectic salt hydrates are also popularly applied to air condition system due to their similar temperature with the evaporation of the mainly refrigeration host and high density compared with water.There are following characteristics of air conditioning cold energy storage technologies with eutectic salts.
(1) Transfer the electricity peak and time of power consumption (2) Capacity and power of refrigeration equipment can be reduced by 30~50% (3) Improve efficiency of operation and utilization Figure 6 [90] showed several air conditioning system by various media with chilled water, ice and PCMs.Nanoparticles like TiO 2 and Al 2 O 3 [88][89][90][91] are often added into PCMs to increase melting rate and reduce problems of refrigerating mediums such as environmental pollution, low thermal conductivity, large size and high installing and running cost.Hui et al. [92] found that LiCl/H 2 O and NH 3 /H 2 O pairs have good performance among several absorbent/refrigerant pairs, which can be used for air conditioning application and subzero applications, respectively.

Conclusions
The available information of salt hydrate PCMs has been concluded on the basic of thermalphysical classification, properties, problems, possible solutions and applications in this paper.It is generally agreed that salt hydrates have significant merits among phase change materials for latent heat storage.In spite of some disadvantages may limit their application in certain aspects, corresponding solutions are developed continuously.It is worthy noting that the excellent performance of combination PCMs with porous materials to prepare shape-stabilized PCMs can successfully solve phase separation and leakage problems when phase transition happens.Nanoparticles are also predominant additions which can ameliorate thermal conductivity and reduce supercooling degree of most hydrates.Highly stable PCMs are being increasingly applied to different sectors.The low phase change temperature range and rational cost of salt hydrates makes them be paid high attention in constructions, air condition, and solar heating systems.However, careful designation should be taking into account of temperature and enthalpy decreasing caused by operational thermal cycling and technical performance of salt hydrates.
For the prospect, surface properties of pore materials which can be regarded as inorganic supporting materials are needed to be regulated furthermore, low cost and anti-corrosion microencapsulation or encapsulation techniques should be studied as well.Any critical testing standards for PCMs should try to be confirmed at international level, this will contribute to directly measure properties of different kinds of PCMs.

Conclusions
The available information of salt hydrate PCMs has been concluded on the basic of thermal-physical classification, properties, problems, possible solutions and applications in this paper.It is generally agreed that salt hydrates have significant merits among phase change materials for latent heat storage.In spite of some disadvantages may limit their application in certain aspects, corresponding solutions are developed continuously.It is worthy noting that the excellent performance of combination PCMs with porous materials to prepare shape-stabilized PCMs can successfully solve phase separation and leakage problems when phase transition happens.Nanoparticles are also predominant additions which can ameliorate thermal conductivity and reduce supercooling degree of most hydrates.Highly stable PCMs are being increasingly applied to different sectors.The low phase change temperature range and rational cost of salt hydrates makes them be paid high attention in constructions, air condition, and solar heating systems.However, careful designation should be taking into account of temperature and enthalpy decreasing caused by operational thermal cycling and technical performance of salt hydrates.
For the prospect, surface properties of pore materials which can be regarded as inorganic supporting materials are needed to be regulated furthermore, low cost and anti-corrosion microencapsulation or encapsulation techniques should be studied as well.Any critical testing standards for PCMs should try to be confirmed at international level, this will contribute to directly measure properties of different kinds of PCMs.

Figure 2 .
Figure 2. (a) Optical microscope images and (b) SEM image of microcapsules synthesized in a microfluidic device.Reproduced with permission from [57], IDEALS, copyright 2015.
. Duan et al. [48] prepared CaCl 2 •6H 2 O/EG composites by adsorbing liquid salt hydrate into EG.The shape-stable composite was obtained by putting EG into fused CaCl 2 •6H 2 O. Alkylphenols polyoxyethylene-10 (Op-10) as emulsifier was also added to enhance binding force between EG and salt hydrate.Thermal conductivity also improved by 82% compared with neat CaCl 2 •6H 2 O. Another composite salt hydrate/EG composite was prepared by Wu et al. [64].Na 2 SO 4 •10H 2 O and Na 2 HPO 4 •12H 2 O mixture was impregnated into air voids by vacuum impregnation methods and then coated by paraffin.

Figure 3 .
Figure 3. (a) Solar thermal collectors (b) Hot-water tanks with PCM modules.Reproduced with permission from [74].Elsevier: Solar Energy Materials and Solar Cells, copyright 2006.

Figure 3 .
Figure 3. (a) Solar thermal collectors (b) Hot-water tanks with PCM modules.Reproduced with permission from [74].Elsevier: Solar Energy Materials and Solar Cells, copyright 2006.

Figure 3 .
Figure 3. (a) Solar thermal collectors (b) Hot-water tanks with PCM modules.Reproduced with permission from [74].Elsevier: Solar Energy Materials and Solar Cells, copyright 2006.

Figure 4 .
Figure 4. (a) The schematic of a full-scale experimental room with PCM; (b) Temperature comparison of ordinary wall room and PCM room.Reproduced with permission from [80].Elsevier: Energy Procedia, copyright 2017.

Figure 4 .
Figure 4. (a) The schematic of a full-scale experimental room with PCM; (b) Temperature comparison of ordinary wall room and PCM room.Reproduced with permission from [80].Elsevier: Energy Procedia, copyright 2017.

Figure 4 .
Figure 4. (a) The schematic of a full-scale experimental room with PCM; (b) Temperature comparison of ordinary wall room and PCM room.Reproduced with permission from [80].Elsevier: Energy Procedia, copyright 2017.

Figure 6 .
Figure 6.(a) Chilled water tank for air conditioning; (b) Ice storage tank for air conditioning (c) PCM storage tank for air conditioning.Reproduced with permission from [90].Springer: Energy Solutions to Combat Global Warming, copyright 2016.

Figure 6 .
Figure 6.(a) Chilled water tank for air conditioning; (b) Ice storage tank for air conditioning (c) PCM storage tank for air conditioning.Reproduced with permission from [90].Springer: Energy Solutions to Combat Global Warming, copyright 2016. 4

Table 1 .
Thermo-physical properties of salt hydrates.

Table 2 .
Melting temperatures and heat of fusion of some eutectic mixtures.

Table 3 .
Melting temperatures and heat of fusion of salt solutions for refrigeration systems.