Selection principles and thermophysical properties of high temperature phase change materials for thermal energy storage: A review

Selection principles and thermophysical properties of high temperature phase change materials for thermal energy storage: A review

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Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx

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Selection principles and thermophysical properties of high temperature phase change materials for thermal energy storage: A review ⁎

Gaosheng Wei , Gang Wang, Chao Xu, Xing Ju, Lijing Xing, Xiaoze Du, Yongping Yang School of Energy, Power and Mechanical Engineering, Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of Education, North China Electric Power University, Beijing 102206, China



Keywords: Thermal energy storage (TES) Phase change material (PCM) Thermophysical properties Material selection Heat transfer enhancement

Phase change thermal energy storage (TES) is a promising technology due to the large heat capacity of phase change materials (PCM) during the phase change process and their potential thermal energy storage at nearly constant temperature. Although a considerable amount of research has been conducted on medium and low temperature PCMs in recent years, there has been a lack of a similar systematic and integrated study on high temperature PCMs and high temperature thermal energy storage processes. Analyzing the available literature, this review evaluates the selection principles of PCMs and introduces and compares the available popular material selection software options. The thermophysical property data of high temperature PCMs is comprehensively summarized, including high temperature molten salts and metal alloys. Several heat transfer and performance enhancement techniques are summarized and discussed as potential alternative methods to overcome poor thermal conductivity when using high temperature molten salt as the PCM. The common thermophysical property measurement methods used in literature are also summarized and compared. This review gives a broad overview of material selection, innovation and investigation of thermophysical properties for high temperature PCM development, and will be a helpful reference for the design of high temperature phase change TES systems.

1. Introduction As science and technology rapidly develop and living standards around the world advance, global primary energy consumption has increased dramatically. Excessive exploitation and use of fossil energy has caused serious environmental pollution and ecological damage, and in recent years these issues have attracted the attention of governments and research institutions around the world. Actively promoting and developing renewable energy has certainly become an important means of solving these ecological issues. Concentrated solar power (CSP), which uses a solar collector to produce high temperature and pressure steam that can drive a turbine to generate electric power, is one of the most promising forms of renewable energy. The technology has numerous advantages, such as emitting no pollution or greenhouse gases and possessing huge energy reserves. Many believe that actively developing CSP technology is one of the most effective ways to solve current global energy supply problems. Exploitation of a cost-effective thermal energy storage (TES) system is a crucial part of CSP technology development. Since there is typically a mismatch between available solar energy supply and electrical energy

demand, heat energy storage systems play a very important role in CSP technology. An effective TES unit can improve the thermal management level of a CSP system, and ensure that the system can safely provide a given load even during overcast days and at night. Thus, an efficient TES system is critical for large-scale switching to CSP technology and for improving system efficiency by improving the initial steam parameters [1,2]. High temperature TES systems, which can be used with current CSPs, can be classified into three types: sensible heat storage systems, latent heat storage systems, and thermal chemical storage systems. Sensible heat storage technology stores and releases thermal energy by raising and lowering the material’s temperature. Latent heat storage technologies realize thermal energy storage and release through endothermic and exothermic phase change processes of the medium (e.g., solid to liquid or liquid to gas and vice versa). Thermal chemical storage achieves thermal energy storage by relying on completely reversible chemical reactions of the medium, in which the molecular bonds are damaged and reorganized repeatedly while accompanied by endothermic and exothermic processes. To date, only sensible heat storage systems have been used in commercial CSP systems. A typical

Corresponding author. E-mail address: [email protected] (G. Wei). Received 28 March 2016; Received in revised form 15 March 2017; Accepted 29 May 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Wei, G., Renewable and Sustainable Energy Reviews (2017),

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Cambridge engineering selector Carbon nanofiber Carbon nanotube Concentrated solar power Differential scanning calorimeter Differential thermal analysis Ente Nazionale per l'Energia eLettrica Guarded hot plate

Heat transfer fluid Laser flash analysis Latent heat storage Phase Change Materials Thermal energy storage Thermogravimetry Transient hot-strip Transient hot-wire Transient plane source Uniform resource locator

heat transfer fluid (HTF) and the storage medium, very high mechanical and chemical stability of the storage materials, good compatibility between the HTF, heat exchanger and/or storage medium (safety), complete reversibility over many charge/discharge cycles (lifetime), low thermal loss, ease of control and operation strategies, maximum load, nominal temperature and specific enthalpy drop in load and rational integration with thermal power plants [6]. In addition, low initial investment and controllable maintenance cost is also very important for thermal energy storage system. The whole system must be considered when selecting prospective heat storage materials.

example is the Andasol solar power station in Spain [3,4], which uses a molten salt mixture consisting of 60% sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3) as the heat storage medium. Latent heat storage has many advantages over sensible and chemical thermal storage [3–6]. Latent heat storage can achieve heat storage and release even when there is almost no temperature variation, and its storage capacity per unit volume is 5–14 times higher than sensible heat storage (e.g. when using water, refractory brick or rock). Researchers have studied latent heat storage extensively, since the beginning of the 21st century, and remarkable achievements have been made in this field in terms of material selection, basic performance and application. Although high-temperature phase change TES has huge potential in CSP systems, it remains an immature technology yet to see commercial application. The main drawbacks of such systems include high investment costs to develop and implement the technology, and non-ideal performance of the energy storage material since most phase change materials have a relatively low thermal conductivity that seriously affects the speed of heat adsorption and release. Therefore, developing phase change materials that possess dramatically improved thermophysical properties and stable performance while being low cost, optimizing heat exchanger design and expanding operations management research are the most likely research topics for advancing TES systems. Current research indicates that a large number of materials can experience a phase change at a specific temperature simultaneously with heat release or absorption [7–9]. However, if used as a thermal energy storage medium, many other factors must be comprehensively evaluated, including thermophysical properties, corrosion, economical efficiency and so on. In the face of the sheer number of potential phase change materials (more than 160,000 species) and the fact that various factors must be considered simultaneously, determining the most optimal storage material is a complicated and time-consuming process. However, this is a crucial issue that must be solved to effectively exploit phase change TES systems. In this review, the selection principles for phase change TES materials are evaluated through a related literature summary and analysis, mainly focused on the high temperature PCM which can be widely used in CSP project and whose phase change temperatures are above 300 ℃. The research and development status and future trends of the high-temperature molten salts and metal alloys with the most potential are analyzed, and we introduce the digital tools available for phase change TES material selection. This work also summarizes the thermophysical properties of some potential high temperature molten salts and discusses the results from some thermophysical property enhancement studies. The goal of this work is to provide an in-depth discussion of the key problems currently facing the exploitation of high-temperature phase change TES.

2.1. Selection principles The selection of phase change materials for TES systems depends on many factors: material properties, storage capacity of the system, operating temperature, the performance of the HTFs and the design considerations of the heat exchangers [7]. The performance of the selected materials in various aspects will directly affect the heat storage ability and thermal storage/release efficiency. Fig. 1 illustrates the relationship between the performance of the heat storage device and the properties of the heat storage materials. The figure clearly shows that the performance of the heat storage device and the properties of the heat storage materials are intertwined and significantly influence one another. Clearly, a large number of factors must be considered when measuring the overall performance of a heat storage system. Kemick [8] indicated that any selection of PCMs must comprehensively consider the integrated performance of the materials on the basis of thermodynamics, kinetics, chemistry and economics. Many PCMs Performance of thermal energy storage device

The properties of thermal energy storage material

Input/output temperature

Phase change temperature

Heat accumulation density Input/output density Load responsiveness (dynamic response) Thermal efficiency

Latent heat Volumetric change phase change process


Dynamic characteristics in phase change Heat transfer characteristic

Exergy efficiency Price Manufacturing expenses Lifetime Reliability System security Construction cycle

2. Selection of high temperature phase change materials The requirements for a thermal storage system include: high energy storage capacity per unit volume, good heat transfer ability between the

Quantity of resources Vapor pressure, volatility Thermal stability Corrosion of the container Toxicity Chemical reactivity (explode, ignition)

Fig. 1. Relationship between heat storage device performance and the heat storage material properties [5].


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ii. Minimal corrosiveness and anti-oxidation – This means that the PCM is compatible with a variety of packaging materials, making the container material selection easy and lower cost. iii. Meet the requirements of green chemistry – Material should be non-toxic, non-flammable, non-explosive and safe to use.

suitable for high temperature TES, and their basic thermophysical properties, were presented by the author for comparison. Combining these selection criteria with the criteria from other literature sources [5,8–17] and the relationships in Fig. 1, we can summarize the selection principles for PCMs is TES systems into six major requirement types.

2.1.5. Meeting economic performance requirements 2.1.1. Meeting thermal performance requirements i. Readily available raw materials that are low cost and possess good industrial utility.

i. Appropriate phase change temperature – Should make the best match of the material’s ideal operating temperature with its application temperature. This allows the phase change material to operate at maximum efficiency. ii. Large latent heat and specific heat – The first characteristic means the PCM can store more latent heat at the same phase change temperature, while the latter means it can store more sensible heat for the same temperature difference. High latent and specific heat makes the thermal storage density as great as possible. iii. Relatively large thermal conductivity – This characteristic allows for heat absorption or release at a relatively low temperature gradient, and makes for a faster heat storage/release rate of the system. Therefore, this characteristic can improve system efficiency. iv. Congruent melting – The phase change material should have a consistent chemical composition in different phase states, which can help avoid phase separation phenomena caused by density differences between the solid and liquid states.

2.1.6. Meeting technical performance requirements i. The selected material should be technically efficient, compact, reliable and applicable as far as possible. In actual development processes, it is difficult to find an ideal material that meets all of the above conditions simultaneously. Appropriate phase change temperature, high latent heat and low cost are the first considerations, with a variety of other factors then considered comprehensively. The phase change temperature is primary because the PCM can storage mass of heat during phase change process only if its phase change temperature matches with the design conditions, otherwise, the PCM may lose its function. Latent heat is the direct embodiment of ideal thermal storage capacity of PCMs. The PCM with high latent heat usually can decrease the whole volume of the TES system. Low cost can increase the revenue ability of whole project, and make the technology generalization more easily in the market. Furthermore, some technical measures can be adopted to remedy some of the deficiencies of the PCM. For example, adding a nucleating agent and a thickening agent can improve the supercooling property of a crystalline hydrated salt. Some heat transfer enhancement techniques [9,18–21] can also be adopted to improve the heat transfer ability of the PCM. These techniques are summarized in a later section of this review.

2.1.2. Meeting physical performance requirements i. Good phase equilibrium – The phase change process should be completely reversible and only temperature dependent. ii. Low vapor pressure – A low vapor pressure corresponding to its operating temperature means that the material is not likely to evaporate or deteriorate. iii. High density – For PCMs with considerable latent heat and specific heat, a higher density means that the heat storage capacity per unit volume is high, which contributes to reducing container costs. iv. Small volume change in the phase change process – A small volume expansion means that a simpler type of thermal storage heat exchanger can be adopted for the system, and a large heat transfer rate can be guaranteed. If we assume that the medium in a horizontally placed tube has a 10% reduction in volume, for example, then a gap will appear at the top of the pipe when the medium solidifies, which means the effective heat transfer area will be reduced by 25%. Moreover, the gap may also be dispersed throughout the heat storage medium due to the combined effect of solidification rate, viscosity, surface tension and other factors of the medium. This can result in a remarkable reduction in thermal conductivity of the heat storage medium, thus significantly lowering the heat transfer rate [14].

2.2. Selection methods Many different methods are available to measure or analyze the thermal storage characteristics of high-temperature PCMs, including experimental methods [22–25], theoretical calculations and numerical simulations [26–28]. However, no matter which method is used, the obtained storage characteristics of the investigated PCMs should be comprehensively compared with as large a number of properties of PCMs as possible that have been obtained by other scholars. Historically, this has meant comparison by means of manual diagram methods [29]. However, there are many drawbacks to this traditional practice for selecting PCMs. First, the traditional literature retrieval methods usually have limitations and are incomplete in terms of access to relevant data. For example, most works only provide the thermophysical properties of the studied materials, with few simultaneously providing the data in terms of costs, environmental impact evaluation and other analogous issues. Secondly, it is a very complex and timeconsuming task to pick out a material with an optimal combination of properties for a specific application due to the increasing number of commercially available PCMs (over 160,000) [7] and the continuous development of new materials. Thus, it is unlikely that using only manual collections and comparisons will meet the needs of most researchers. On the other hand, using a digital selection tool such as the CES software package can improve the comprehensiveness of any comparison of available information and make the selection process more efficient. Most of the related databases and software cover a variety of materials, including polymers, metals and alloys. In terms of selection method, the process can be limited according to material name, type, composition, structures, physical/chemical properties, manufacturing

2.1.3. Meeting dynamic performance requirements i. Subcooled temperatures – As low as possible during solidification, and no supersaturation during melting. ii. Fast crystallization – Thermal storage medium has fast exothermic speed, and this can raise the dynamic response ability of the whole thermal storage system. 2.1.4. Meeting chemical performance requirements i. Good chemical stability – There should be no segregation, chemical decomposition, or other side effects when repeatedly cycling through the heat absorption and release processes. This ensures long-term minimal attenuation of the heat storage capacity and improves reliability. 3

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database, and indicated that binary and multicomponent alloys based on Al, Mg, Si and Zn can be used as high-temperature (400–750 °C) latent heat storage materials. The authors also made a note of pointing out that the performance of Al-12Si and Al-34Mg-6Zn is superior. The EcoAudit tool of CES Selector was simultaneously used to evaluate environmental impact, and the results showed that Al-12Si had the lowest CO2 emissions, followed by pure metal aluminum. The efficiency and intuitiveness of conducting material selections by using digital tools is self-evident. However, note that the data stored in the digital tools must be accurate enough to ensure accuracy of the selection. Because of the continuous development of new materials, software companies must update and improve their materials databases in a timely manner. Therefore, in order to promote further development of phase change thermal storage technology, related researchers should make these database developers aware of the importance of basic thermophysical property measurements and data to make the related database more complete for the new materials development process. The standard and some methods for PCM selecting are introduced and summarized in above, and how to select certain PCMs in engineering or researching, we may learn something or develop a new thought from CES software. CES software uses a strategy to select PCM based on concrete conditions of engineering or research, and this strategy is scientific and reasonable. In order to select the material with the highest performance for a given application, a design-led approach strategy is developed. The first step is that of translating the design requirements into a specification for materials selection. It is followed by a screening step, where those candidates that do not meet the specifications previously established are rejected. From a criterion of excellence, the remaining materials are ranked, and finally, more detailed information about the best material is needed to ensure that the selection is successful. The procedure is as follows:

processes, available manufacturers and many other options. The results usually include the technical name, physical properties, mechanical properties and related manufacturing technology of each material. Some additional information is occasionally given as well, such as material cost, pictures, environmental impact and so on. In terms of usage charges, most online databases are free. Some databases require an annual fee if you need to browse or analyze detailed properties of the materials. Additionally, the software package itself must be purchased, and the price level depends on the number of licenses and the authorization period. Ramalhete et al. [30] investigated and analyzed 87 databases and associated software for material selection. It should be noted that most of the databases cover the thermal properties of the materials, which are the most relevant properties for thermal storage applications. The authors also indicated that the online database MatWeb provided the greatest variety of related materials and more comprehensive material properties, allowing users to analyze the materials and material properties in various defined ways. Table 1 gives the URL access to several databases or software that contains thermal property information for many different materials. Ashby et al. [31] developed the CES (Cambridge engineering selector) series of software, which integrates a variety of information in a single tool and achieves a progressive method to select materials. The software allows users to select materials using a diagram method based on design process parameters. CES Selector can help corporate users to select materials, and gives support for materials machining analysis. In addition, CES EduPack is an excellent material engineering toolbox, which can be used in conjunction with the material selection textbooks written by Professor Ashby. More than 550 colleges all over the world, including the University of Cambridge, use the software. Using the CES database and basing material selection on cost minimization, Fernandezetal et al. [32] successfully evaluated and optimized several sensible heat storage materials for temperatures ranging between 150 and 200 °C, confirming that these types of digital tools can be used for TES material selection. The authors also pointed out that this method can achieve multiple optimization objectives in conjunction, although with some use restrictions. In addition to thermophysical properties, material cost, availability and environmental impact (CO2 emissions) were also considered. Khare et al. [7] discussed the selection method for high-temperature phase change TES materials for CSP systems when using the CES

1. Function of the component for which the material is sought; 2. List of the constraints it must meet—satisfy limits on thermal or electrical properties and so forth; 3. List of objectives, the criteria by which the excellence of choice is to be judged, for example minimizing cost, minimizing mass and so on; 4. List of free variables—those that the designer is free to change; usually dimensions or shape, and, of course, the choice of material.

Table 1 Several databases and software for material selection. Names














Software Software

Chargeable Chargeable

Physical property data of metal, plastics, ceramic and composites, more than 115,000 kinds of materials. Information of innovative materials,more than 2600 exciting materials. Environmental impact data of materials. Professional standard and data of different materials.




Ravara Database AZOM.COM

Free Free


Online Online/ Software Software Software Software


Software Software

Chargeable Chargeable

Eco Materials Adviser



ASM Alloy Center Database



Properties of energy-efficient and eco-friendly materials used for construction and furniture manufacture. Property database of many materials. Properties, analysis methods, detection equipment and information of materials in industry. Property data of abundant materials. Temperature dependent material property data. Property data, processing technic and performance comparison of metals, plastics, composites, and ceramics. Education resources of materials, processes, and sustainability. Management, design, simulation and analysis of materials, data can be used by other design software. Toolkit of Autodesk Inventor, including property and environmental impact of Eco materials. Materials property and corrosion performance data of metal and alloy.

Chargeable Chargeable Chargeable




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In actual heat transfer or storage applications, a mixed molten salt formed of binary and/or ternary eutectic salts is used rather than one single kind of salt. The advantages of using mixed molten salts include [17]: The desired melting point can be obtained by appropriately changing the ratio of its components to accommodate a wider application temperature range; and compared with a single salt, a mixed salt can provide a higher energy storage density when melting at a similar temperature. Furthermore, expensive salt and salt compositions with good heat storage performance can be used together with inexpensive salts in order to reduce the cost while maintaining approximately the same heat storage capacity. Many researchers have concentrated their studies on high-temperature molten salt and salt compositions in applications where the material is mainly used for sensible heat storage as well as applications where the salt is simultaneously used as the heat transfer fluid [45]. Currently, only the sensible heat storage application has obtained large-scale use in practical engineering, with commercially available compositions such as Hitec (40%NaNO2-7%NaNO3-53% KNO3), HitecXL (48%Ca(NO3)2-45%KNO3-7%NaNO3), Solar Salt (40%KNO360%NaNO3), and so on. Eurelios station in Italy and the CESA-1 power plant in Spain both use Hitec as their sensible heat storage medium [3], while the Spanish Andasol power plant, Solar Tres and the U.S. Solar Two station use Solar Salt as their sensible heat storage material [45– 47]. ENEL constructed Archimedes station in Sicily in July 2008, and the plant was formally put into operation in July 2010 with an installed capacity of 5 MW. Archimedes is the world’s first solar thermal power station that uses molten salt (Solar Salt) as the heat storage medium and heat transfer fluid simultaneously [48]. Table 2 shows the main performance characteristics and costs of the above mentioned three molten salts. We can see from the above table that the heat capacity values of these molten salts are limited, which leads to a relatively low heat storage density. As a result, practical applications require hundreds of tons of the molten salts and huge storage container volumes. Additionally, the freezing points of these molten salts are much higher than the ambient temperature; while these can be reduced to some degree by properly optimizing composition and salt proportions [12,13], protection measures are still necessary. This leads to an increase in cost and complexity of operations. Moreover, these salts cannot meet the demand of large-scale commercial applications solely through sensible heat storage because the thermal storage performance and cost cannot be guaranteed simultaneously. At the same time, it should be noted that high-temperature molten salt and salt compositions not only have a high sensible heat capacity but also a large latent heat capacity. When heated, these materials can store both sensible heat and latent heat simultaneously, greatly improving the thermal storage capacity. Thus, the development of high-temperature molten salt PCMs of low cost and with excellent properties is extremely important. Research on thermophysical properties is a fundamental and extremely important part of PCM development. However, it is also an extremely time-consuming task to accumulate the relevant thermo-

The performance of an engineering component depends on the properties of materials with which it is made of. It usually depends not only on one property but also on a combination of two or more expressed as a criterion of excellence, called material index, which maximizes the performance for a given design and is the result of the translation step. An example of objective is to minimize cost. In this case, the cheapest solution that meets all constraints is the best choice. It is rare that a design has only one objective, and when there are two objectives to meet, a conflict arises—the choice that minimizes one metric does not generally minimize the other, and then a compromise must be sought. A multi-objective optimization should be used for reaching a compromise between conflicting objectives. 3. Thermophysical properties of PCMs There are several solar power generation plants in use today with TES systems within their facilities: Andasol I–III in Guadix Spain [33– 36], PS10 and PS20 in Seville, Spain [37,38], and Solar I and II in California, USA [39]. Different storage media and HTFs are used in these CSP plants. However, none of these commercial plants use latent heat as their TES method, despite the many attractive advantages of the method. This is mainly due to the previously discussed comprehensive requirements for PCMs and their associated limitations [40]. Zalba et al. [15] and other researchers such as Zuo [4], Kenisarin [17], Peikeman [41] and Guo [42] have all indicated in their works that the thermophysical properties of PCMs have not yet been studied enough to ensure a clear recommendation for the design of a commercial latent heat storage (LHS) unit, and so far there is no sufficiently comprehensive database of thermophysical properties to facilitate the material selection. The authors also point out that there are also notable discrepancies in the available data for the melting temperature, latent heat of fusion, thermal conductivity and density (both solid and liquid state) for many PCMs at present. This inconsistency in information is largely because there are no national or international standard methods for PCM testing. Thus, it is very difficult to make a comparison when evaluating a PCM for a particular application. A standard platform should be developed to ensure that researchers apply the same test procedures and analyses when carrying out various PCM experimental measurements to allow subsequent comparisons and ensure that the knowledge gained from a particular test can be applied to others [43]. Despite this limitation, many investigations have been done on PCMs such as molten salts and alloys by researchers around the world, resulting in the accumulation of many discoveries and a significant amount of thermophysical property data for these materials. Some theoretical and experimental studies have also been completed on thermophysical properties, enhancing the ability to find further PCMs. These studies indicate that molten salts and alloys are two primary categories of prospective PCMs. A summary of the studies are given here. 3.1. Molten salt and salt compositions 3.1.1. Thermophysical properties High-temperature molten salt and salt compositions generally refers to nitrates, chlorides, carbonates and their eutectics. The advantages of these materials include high working temperature, high thermal stability, high specific heat, a high convective heat transfer coefficient at liquid state, low viscosity, low saturated vapor pressure and low price [3]. Meanwhile, this class of material can also absorb or release latent heat during the phase change process. Therefore, the heat transfer ability and thermal energy storage capacity for molten salts are both excellent, and the material is regarded as a potentially ideal heat storage medium for CSP generation systems. In terms of cost, in general lithium salts are most expensive followed by potassium salts and sodium salts,and calcium salts usually has the lowest price [44].

Table 2 Physical characteristics and cost of three commercial high-temperature molten salts [49].



Solar Salt



Freezing point (°C) Decomposition temperature (°C) Density (kg/m3), 300 °C Viscosity (cp), 300 °C Specific heat capacity (J/kg K), 300 °C Cost ($/kg), ΔT = 200 °C Thermal storage cost ($/kWht)

220 600 1899 3.26 1495 0.49 5.8

142 535 1640 3.16 1560 0.93 10.7

120 500 1992 6.37 1447 1.19 15.2 (ΔT = 200 °C) 20.1 (ΔT = 150 °C) 30.0 (ΔT = 100 °C)

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physical data. In this field, Marianovski and Maru [50] nicely summarized the thermophysical properties of some molten salts with a phase change temperature above 450 °C. Unfortunately, for many data the specific measuring temperature and the measurement method were not given. Venkatesetti and LeFrois [51] summarized the thermophysical properties of nine inorganic eutectic compounds between 220 and 290 °C in their work. Birchenall and Riechman [52] also collected the thermophysical properties of several salt mixtures provided in literature. Japanese scholar Kamimoto [53] measured the melting heat of 50 mol%NaNO3 and 50%KNO3(HTS2) using a differential scanning calorimeter (DSC), while Takahashi [54] measured the melting heat of NaNO3 and KNO3 and their mixtures. Tufen et al. [55] experimentally measured the thermal conductivity of several nitrates at atmospheric pressure, using the coaxial cylinder method. Nagasaka et al. [56] measured the thermal diffusivity of liquid alkali metal chlorides using the forced scattering method, with the same method used to measure the thermal diffusivity and thermal conductivity of alkali metal bromides in another work [57]. Araki el al. [58] measured the density, specific heat and thermal diffusivity of several alkali metal carbonates. Bauer et al. [59] measured the specific heat of a NaNO3-KNO3 mixture using DSC and its thermal diffusivity by the laser flash method. Kenisarin et al. [17] summarized the thermophysical properties of many molten salt and salt compositions in their review article. Wu et al. [60] studied the thermodynamic properties of a mixed chloride molten salt using DSC, and in his experiment, the mixed salt has good thermal stability in a number of thermal cycles. Hu et al [61] tested the phase transition properties and thermal stability of a chloride-molten salt mixture using the thermogravimetric (TG)-DSC method,and after manifold thermal cycles, there is no obvious change in the XRD results of this chloride-molten salt mixture, this proves that the chloridemolten salt mixture has good thermal stability and chemical stability.

Liu [19] also gathered data on melting point, latent heat and specific heat of a large number of high-temperature molten salts in a review article, and Zhang et al. [62] tested thermal stability and chemical stability, operating temperature range and latent heat of purified nitrite, and their results show that purified nitrite has good thermal stability and with no corrosion with container materials, this means purified nitrite has good chemical stability. In the study of thermal stability and chemical stability of PCM, there is no reliable and widely acceptable theoretical method to use until now. The main approach is to measure the thermophysical property variations of PCM before and after cycles. If the difference before and after the manifold cycles is within an acceptable range, and there is no obvious change in XRD spectrum, it is verified that the PCM has reliable thermal stability and chemical stability performance. If thermal stability and chemical stability can be explained in microscopic scale and these explanations can form a certain principle, this will effectively speed the development of PCM. However, there is not research or analysis according to the material's microstructure or microscopic calculations to predict the thermal stability and chemical stability of PCM, and there is no microscopic principle which can explain why the material has good thermal stability and chemical stability until now. A summary of the thermophysical properties of some excellent high-temperature molten salts is provided in Table 3. And in this table, the prices of some readily available salts are listed. The above investigations indicate that the available data on the melting point and latent heat of molten salts and their mixtures is comprehensive while thermal conductivity data is less so, with large gaps in the available data still extant. Data is especially scarce for temperature dependent thermal conductivity values. The biggest deficiency of high-temperature molten salts is their

Table 3 Melting temperatures and fusion heats of some excellent high-temperature molten salts [4,17,41,42]. Composition (wt%)

NaNO3 NaNO2 NaOH KNO3 KOH NaNO3/28NaOHc 27NaNO3/73NaOH NaNO2/73NaOHc NaOH/7.2Na2CO3 NaOH/26.8NaCl NaCl/52MgCl2 NaCl/67CaCl2 45NaBr/55MgBr2 NaCl/6.4Na2CO3/85.5NaOH NaCl/32.4KCl/32.8LiCl Na2SO4/5.7NaCl/85.5NaNO3 NaNO3/3.6NaCl/78.1NaOHc NaCl/5.0NaNO3 NaNO3/5.0NaCl MgCl2/24.5NaCl/20.5KCl NaNO3/10KNO3 KNO3/4.5KCl KNO3/4.7KBr/7.3KCl KCl/46ZnCl2 Na2CO3/BaCO3/MgO NaF/21KF/62K2CO3 LiNO3/2.6Ba(NO3)2c LiNO3/6.4NaCl Li2CO3/53K2CO3 Li2CO3/33Na2CO3/35K2CO3 a b c

Melting point (°C)

310 282 318 337 380 247 240 237 283 370 450 500 431 282 346 287 242 284 282 385–393 290 320 342 432 500–850 520 253 255 488 397

Latent heat of fusion (kJ/kg)

174 212 160 167 149.7 237 244.3 294 340 369 431 281 212 316 281 176 242 171 212 410 170 150 140 218 415.4 274 368 354 342 277

Densitya kg/m3

Specific heata kJ/kg K

Conductivitya W/m K










0.5 1780




0.92 0.5


2225b 2160b 3490b

1610 1900 –

0.92 0.84 0.50

1.0 1.0 0.59

– – –

– 1.02 0.90















@melting point. @25 °C. Mol%.


Price $/ton

377 362 508 508 1391 377/508 377/508 362/508 508/405 508/110 110/290 110/405 -/110/405/508 110/580/145/110/377 377/110/508 110/377 377/110 290/110/580 377/508 508/580 508/-/580 580/1300 405/362/247 880/1378/870 -/940 -/870 -/870 -/405/870

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using PCMs embedded in a copper foam.

small thermal conductivity in the solid state, which can seriously reduce the charge/release rate of the thermal storage system. In addition, smaller specific heat and/or smaller latent heat can result in a lower heat storage capacity. Therefore, enhancing such properties is key to further development of high-temperature PCMs.

Some non-metallic materials can also be adopted as a heat transfer enhancement medium, with porous graphite currently the most widely used [93–104]. The experimental results of Sedeh and others [84,94,95,100,101,104] show that the thermal conductivity of pure PCM can be increased by 200 times, 30–120 times, 5 times and 6.9 times, respectively, by embedding the PCM in porous graphite. Zhong [104] pointed out that composite salts produced through a solution impregnation method are more uniform compared with other methods. Additionally, there is a critical porosity value that exists when enhancing a PCM with porous carbon graphite foams. Yin et al. [97] discovered in their paraffin enhancement experiments that the critical mass fraction of porous graphite was 6.25%. However, the reinforcement effect decreased for mass fractions exceeding this value. Sari [96] showed that the total heat capacity of a PCM decreased when the mass fraction of the porous graphite was high. Elgafy [105] discovered that the pore volume of porous graphite was a key factor influencing heat reinforcement results. Mesalhy and Krishnan [106,107] indicated that the PCM might leak when the pore density is greater than the critical value. Wu [83] compared the heat transfer enhancement effects of porous metal and porous graphite, as well as their mixture, showing that a mixed porous base is more effective than a single porous base. There are other non-metallic materials that can be used as a strengthening medium other than graphite, such as ceramics. Porous ceramics are a prospective heat enhancement material that may be investigated in future. Li [108] indicated in their study that ceramic honeycombs have a strong heat transfer enhancement effect on PCMs.

3.1.2. Methods of thermal conductivity enhancement Considerable achievements have been made in enhancing the thermophysical properties of PCMs at medium and low temperatures [18,19,63–65]. However, There remains insufficient study on hightemperature molten salts. The thermophysical properties of hightemperature PCMs, and how those properties change with temperature, are undoubtedly the most fundamental and important pieces of information for describing the charge/discharge process of a TES system. However, there is a lack of systematic research on the enhancement of the thermophysical properties of high-temperature molten salts. Therefore, accumulating more data on the temperature dependent thermophysical properties of high-temperature PCMs and their composites is very critical. Different techniques for improving heat transfer and performance include using extended surfaces, multiple PCMs, high conductivity particle dispersion and porous matrices embedded within the PCM. This review provides an overview of these different techniques, their limitations, and describes any results obtained from study into each enhancement method so as to promote further research. Fins and extended surfaces. Utilization of fins is the most popular method to extend the heat transfer surface and to further increase heat transfer efficiency. Readers can refer to Incropera and other researchers such as Sharifi, Gharebaghi, Hosseinizadeh etc., for additional information [66–72]. Incropera and Stritih [66,67], through their experimental studies, proposed that the fin interval is the primary factor influencing heat transfer efficiency. Hosseini, Rozenfeld and Rathod [73–75] arranged the ribbing vertically instead of horizontally in their experiments, and discovered that the solidification time of the PCM could be reduced by 43.6% using only 3 horizontal fins. Pakrouh and Saha [76,77] studied needle ribs and disc ribs, respectively, for enhancing heat transfer efficiency. Sciacovelli [78] studied Y-type fins, and showed a 24% increase in heat transfer efficiency. Fukai et al. [79] used carbon brushes in their experimental study, and successfully achieved a 30% increase in rate of heat accumulation and a 20% increase in heat release rate. Wang [80] achieved an apparent heat transfer enhancement by combining different fins in the heat storage system, while Khalifa [81] studied heat transfer enhancement at high temperatures from fins in the storage system. Dispersion of highly conductive particles within the PCM. Dispersing highly conductive particles within the PCM can reduce the internal resistance and enhance the internal heat transfer rate of PCM, so as to increase the thermal conductivity of the material. Recently, Kibria [109] gave a comprehensive review on the addition of highly heat conductive particles in PCMs to strengthen heat transfer. Some new investigations in this field beyond that review are described here. Adding different types of carbon particles in a PCM is currently the most popular method for this approach. The experimental results of AlMaadeed and other researchers [110–116] have shown that carbon particles have an apparent enhancement on PCM heat transfer performance. Fan and others [117–121] have studied heat transfer enhancement using unconventional carbon particles, showing that disk carbon nanoparticles can improve the thermal conductivity of PCMs by a factor of 10 or more while simultaneously increasing the phase change temperature. Some researchers such as Qi, Tao and others [105,122–126] have studied thermal conductivity enhancements of PCMs with different CNFs (carbon nanofibers). A CNF net was used in Nomura’s study [124], while Warzoha [126] used a herringbone CNF. Both studies found a good level of enhancement. Similar to the discovery of Yin [97], Frusteri [122] also showed that a critical value exists for the mass fraction of CNFs. PCM embedded porous matrices. One practical measure to increase thermal conductivity is to use composite materials composed of molten salts, then strengthening the materials using some other material for use in a high-temperature phase change TES system. This approach can reasonably combine the phase change thermal storage capacity of molten salts with the excellent conductivity ability of the added materials. By experiment and simulation, Zhao et al. [82,83] proved that thermal conductivity could be considerably improved by using metal foams filled with sodium nitrate and an expanded graphite matrix. Acem, Tian and Zhang [84–86] also embedded PCMs in metal foams to enhance the thermal conductivity of the PCMs. Their experimental results showed that porous metal foam could effectively increase the heat transfer rate of the energy storage system. The experimental results by Allen and Xie [87,88] showed that the heat transfer performance could be increased 25.2% by embedding PCMs in a porous aluminum metal foam. The experimental results by Zhuo et al. [89–92] showed that the heat transfer rate could be improved by a factor of 28.1 for solid state PCMs and by a factor of 3.1 for liquid states

Besides CNF, CNTs (carbon nanotubes) are also a familiar carbon particle additive. The experimental results of Tao and others [119,127– 129] showed that CNTs could increase the thermal conductivity of PCMs up to 56.98%. Yu and Cui [116,130] compared the use of CNTs and CNFs on thermal conductivity reinforcement, and found that CNTs were more effective. Beyond carbon particles, Cu, nano-titania and nanomagnetite particles have also been studied for the thermal conductivity enhancement of PCMs by Kibria and others [109,131– 136]. The experimental results from these researchers have shown that nano-titania can apparently increase the heat capacity of PCMs, while nanomagnetite particles can increase their thermal conductivity and heat capacity simultaneously. Other potential thermal conductivity enhancement particles also exist, such as NaOH, KOH, silica particles,


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and found that the solidification time of the PCM in the container was reduced by 60%. Hu et al. [162] designed a new gravity heat pipe configuration to strengthen heat transfer of PCMs. Allen and many other researchers [163–167] achieved significant heat enhancement results over using any single method by combining different strengthening methods .

and so on. Luo [137] indicated that NaOH and KOH particles can effectively increase thermal conductivity of PCMs at high temperatures, and Shin et al. [138] used silica nanoparticles to increase the specific heat of alkali chloride salt eutectics in a liquid state. PCM microencapsulation. Encapsulation is also an effective method to improve the heat transfer performance of PCMs. The advantages of encapsulation are that the apparent effective surface area can be increased, corrosion of the PCM on the container is effectively suppressed and leakage can be avoided. Cui and other researchers [139–147] conducted a considerable number of experimental investigations on the encapsulation of different PCMs, finding that the technique greatly improved heat transfer performance of the PCMs. Garcia-Romero [141] and Bellan [147] conducted their heat transfer performance measurements at high temperatures. The experimental results of Özonur [148] and Regin [149] show that the heat transfer efficiency of any PCM may be lowered to some degree. Readers can refer to more detailed summarizations and discussions related to encapsulation found in review articles by Jacob [150], Liu [151] and Liu [152]. Jacob [150] also provided a detailed summarization of heat transfer enhancement studies on encapsulation at elevated temperatures.

Many different heat transfer strengthening methods have realized great success under experimental conditions. However, few of them have been studied in large-scale applications since molten salts remain the main PCM material, and many of strengthening methods are not applicable in practice due to the poor compatibility of molten salts with other materials. In future stages of investigation, materials with good compatibility (such as ceramics, etc.) should be the research focus. Moreover, this approach lacks basic theoretical guidance for the heat transfer enhancement study of PCMs, as experimental and simulation investigations are currently the only two methods used in these studies. Table 4 gives a comparison of above thermal conductivity enhancement methods. 3.2. Metal alloys Although possessing many advantages as a PCM, molten salts still have some limitations that need to be overcome, such as their low thermal conductivity and the liquid stratification problems previously mentioned. In comparison, the thermal conductivity of metal alloys is dozens to hundreds of times greater than that of molten salts [5]. Furthermore, metal alloys usually have large TES densities and perfect cycling stability, which contributes to their great application potential. For example, molten metals and eutectic alloys have been used as heat transfer fluids in nuclear power plants [168–171]. Among metal alloys, aluminum alloys are considered to have the highest potential for high-temperature TES applications due to their suitable phase change temperatures and relatively low causticity compared with many other alloys. The thermophysical properties of some aluminum alloys are summarized in Table 5. As early as the 1980s, American scholar Birchenall, French scientist Achard and the Russian research group of Cherneeva et al. [172–177] experimentally investigated some binary and multi-component aluminum alloys. Their studies showed that these aluminum alloys not only possessed suitable phase change temperatures (327–657 °C) and large latent heats, but also enjoyed large thermal conductivities and excellent thermal stability. Compared with high-temperature molten salt systems, the TES performance of systems using such materials would be significantly improved. However, correspondingly more stringent requirements for the container materials become necessary due to the strong liquid corrosion characteristics of aluminum alloys. Even so, a great deal of attention has been paid to aluminum alloys in recent years. Gasanaliev [178] measured the phase change temperatures and latent heats of a variety of binary and multi-component aluminum alloys containing Si, Cu, Mg and/or other elements. Kenisarin [17] and Liu [19] summarized the melting point, heat of fusion and corrosive Multiple PCM method. The temperature of the fluid on the high-temperature side of the exchanger will gradually decrease along the flow direction in a heat transfer process, with the temperature difference between the hot fluid and the PCM gradually decreasing. The multiple PCM method solves the above problem. In the different stages of the heat exchanger, a variety of PCMs with different phase change temperatures are arranged to adapt to the local temperature differences at the corresponding positions. Thus, each PCM can be optimized for each position in terms of heat transfer for the respective conditions. Gawlik, Hu and some other researchers [153–157] conducted numerous experiments on the multiple PCM method. Their results showed that multiple PCMs are more efficient than any individual PCM method, and the heat transfer rate can be increased up to 47.5%. By comparing the heat transfer performance of double PCMs and individual PCMs, a theoretical selection and matching equation used for double PCMs was proposed by Tao el al [158] based on entransy theory. Other methods. In addition to the above five main methods, there are other heat transfer enhancement methods related to PCMs that have been studied by scholars and which have produced encouraging results. Similar to the effect of using porous matrices, placing other structures in PCMs with high thermal conductivity can also strengthen heat transfer results. Velraj and Ettouney [159,160] showed that the heat transfer rate could be increased by 20% by placing some metal structures in the PCM. Abdollahzadeh [161] used a PCM container with a wavy surface instead of a conventional plane surface, Table 4 Comparison of different thermal conductivity enhancement methods. Methods



Fins and extended surfaces

Increasing heat transfer area.

PCM embedded porous matrices

Increasing heat transfer area, forming a thermal transfer network, and increasing thermal conductivity of PCM. Increasing thermal conductivity of PCM by the particles with high thermal conductivity. Increasing heat transfer area. Increasing average temperature difference.

Increasing total weight, increasing total cost, properties of PCM are not changed. Porous material is expensive, it can reduce total heat storage capacity and increase total weight probably. Sedimentation of highly conductive particles may appear, and these particles can hardly form a heat transfer network. Costly, and will reduce the mass per unit volume of PCM. Only adapt to the design conditions, and maybe not useful at variable working conditions.

Dispersion of highly conductive particles within the PCM PCM Microencapsulation Multiple PCM method


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Table 5 Thermophysical properties of some aluminum alloys. Composition (wt%)

Tm (°C)

ρ (kg/m3)

ΔHm (kJ/kg)

ΔHv MJ/m3

cp,s kJ/ (kg K)

34Mg 8Si 12Si 12.5Si 12.6Si 20Si 96Zn 33.2Cu Si/ Fea 34Cu/1.7Sb 13Cu/15Zn 5.25Si/ 27Cu 5Si/ 30Cu 13.2Si,/5Mg 34Mg/6.42Zn 35Mg/ 6Zn 22Cu/18Mg/6Zn 24.5Cu/12Mg/18Zn 26Cu/5Mg/20.5Zn 5.2Si/28Cu/2.2Mg

450 576 576 577 576 576 381 548 577 545 493.3–598 520 571 552 447–450 443 520 460–624 458–488.3 507


310 428.9 560 515 463.4 528.4 138 351 515 331 158.3 365.8 422 533.1 329–316 310 305 315.3 163.8 374


1.73 1.058 1.038 1.49 1.037 0.970


2700 2250

6630 3424 2600 4000 3420 2730 2393 2380 3140 3800 3860 4400


916 1200 1339 1324 538.8 1150

740 960 1197.3 632.3 1664

cp,l kJ/ (kg K)


λs W/ (m K)

λl W/ (m K)




160 180


[186] [180] [233] [186] [180] [180] [178] [186] [5] [178] [187] [180] [186] [180] [195] [186] [17] [187] [187] [178]


1.11 0.939


0.875 1.30 1.123 1.049 (100 ℃) 1.63 1.51

1.438 1.20 1.249 1.426 (478 ℃) 1.46 1.13

130 180


The mass percentage is unclear.

Huang et al. [180,181] conducted a number of experiments using Albased alloys and pointed out that Al-Si-Mg alloys have the best thermal storage capacity, Al-Si-Cu alloys have the longest life expectancy and Al-Si alloys have the best comprehensive performance. In addition, AlSi alloys with different amounts of Si were analyzed in various ways, and the variation in latent heat and specific heat with Si content,

properties of many different metal alloys used for high-temperature TES applications in their review article. Zou [179] conducted an investigation on the thermal storage performance and thermal cycle stability of Al-Si alloys, and showed that the latent heat of Al-13Si (mass percent, the same below) decreased by 10.5% after 720 thermal cycles while the phase change temperature was essentially constant.

Table 6 Various thermal physical property measuring experiments of PCMs using DSC and DTA methods. Techniques





NaNO3, KNO3 LiNO3 NaNO2, Ca(NO3)2 Li2CO3, K2CO3 NaCl, KCl, MgCl2, CaCl2 Al, Mg, Zn Si, Cu Ti, Ag Fe3O4, Al2O3 Montmorillonite, Vermiculite, Diatomite HDPE Paraffin Soy wax Graphite

[12,13,53,54,231,235,239,243] [44,54,231,239] [12,13] [44,119] [60,61] [11,44,52,187,196] [11,44,52,187] [137,166] [134,212] [230,236,237] [110,162] [110,120,126,130,134,212] [130] [110,139,164,230,231,234,235,237,240,241,243]

CF, CNF, CNT SWCNT, MWCNT, C60 GNP, xGNP Graphene EF, EC, EP NPG PEG, GO Erythritol Cement mortar, Concretes mortar RT100, RT40, RT20 Polyaniline Docosane, PMMA C6H6O2, C6H14O6 Capric acid, Myristic Acid, Stearic acid Halloysite nanotube n-hexadecane/n-octadecane Ag Al, Ca, Cu, Mg, P, Si, Zn NaNO3, KNO3, NaOH, KOH, ZnCl2, NaCl, KCl n-hexadecane, n-octadecane

[124,128,130] [119] [118,120,128] [119,231] [244] [234] [118,230] [124,137] [139,140,241] [64,162,236] [143] [145] [155] [143,237,240] [240] [166] [52,166] [52,173] [114]

STA-409PC, DSC 8000, DSC111, Q100 DSC200PC, Q100 STA-409PC – SDT Q600 Q10-V5.1-Build191, STA449C STA449C Q2000, DSC4000 Diamond DSC-131, Jade DSC8500, DSC204F1 DSC8500, 2920, Q1000, Diamond Q1000 DSC8500, DSC111, DSC131, DSC200F3, DSC8000, Q100, Q200, Jade, STA449C Q1000 – Q20, 2920 Q100 Q100 DSC200F3 Q20, Jade Q2000 Q200, Micro DS3, DSC200F3 DSC204F1 DSC200F3 DSC131 DSC822e DSC-131, STA449C, DSC200F3 STA449C DSC4000 Du Pont 900DTA, SEIKO 6200 Du Pont 900DTA TG SETSYS 7000


SEIKO 6200



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detection temperature of DSC only reaches 1000 °C, although this is usually sufficient to satisfy most PCM measurement requirements. Table 6 summarizes the DSC and DTA experiments on PCMs available in the literature. In DSC measurement, corrosion, leakage and oxidation and other factors which may affect the test results should be considered firstly in order to ensure the accuracy of the test. If a resoluble or volatile sample, for example, hydrous salt as a case, is measured, a closed crucible should be used to avoid the escape of the resolved vapor or water.

cycling time and heat preservation time were also presented. Zhang and Lorenzin [5,182] gave a detailed discussion on the reasons behind the corrosiveness of liquid metals at high temperatures. These studies found that time, temperature and melt components were the main factors affecting corrosion. Liu, Félix and Prashanth [183– 185] analyzed the thermal storage performance of Al-12Si alloy, determining that 42SrMo heat resistant steel is a suitable container material. Cheng et al. [186,187] successfully prepared 20 different aluminum multi-component alloys applicable to high-temperature phase change TES for CSP systems, and analyzed the effect of different element additions on melting point and latent heat. Solé, Kwon, Rashidi and other researchers [188–194,205–211] have recently carried out numerous studies on Al corrosion in heat exchangers. Sun et al. [195,196] tested the thermal storage properties of Al34Mg-6Zn, as well as its compatibility with stainless steel/carbon steel, and found that the melting point remained almost constant, although the thermal cycling time increased and the heat of fusion slightly decreased. This review shows that the current level of research on metal alloys is still far from adequate, even though numerous investigations have been completed on the thermophysical properties of various alloys. As the temperatures and phase states of metal alloys will change during endothermic and exothermic processes, the corresponding thermophysical properties will more or less change as well. This directly relates to the heat storage and heat transfer performance of the system, thereby affecting efficiency and cost. Unfortunately, research on changes in performance of the thermophysical properties of different aluminum alloys as a result of the phase change process is obviously insufficient. Take conductivity, for example. Many new alloys possess large heat storage capacities, but their thermal conductivity values are unavailable in the literature. Therefore, it is necessary to accumulate a more comprehensive dataset containing the thermophysical properties of metal alloys, and conduct an analysis on variations in thermal properties as a function of the phase change process. Undoubtedly, metal alloys have the advantage of a larger heat storage capacity and higher thermal conductivity. However, their poor performance in terms of liquid corrosion under high temperatures conditions results in poor compatibility with the container material, the biggest barrier when it comes to practical TES applications. Even with numerous studies done on the compatibility between alloys and container materials, it seems that the available literature still lacks of systemic and regularity. Therefore, these compatibility issues must be investigated further, followed by a systemic search for a reasonable encapsulation method.

3.3.2. Thermal conductivity measurement methods Thermal conductivity is a key thermophysical property of PCM materials. The characteristic represents the heat transfer ability of the medium. The main drawback of molten salts is their low thermal conductivity, which blocks the heat adsorption of the PCM in its solid state, which is why the main purpose of enhancement studies of molten salts, as stated in Section 3.1.2, is to elevate the thermal conductivity of the PCMs. Most of a material’s thermal conductivity data can only be accumulated through experimental methods. There exist many different thermal conductivity measurement techniques, classified into two categories: the steady method and the transient method [197,198]. The principle of the steady method is based on the one-dimensional steady state Fourier law. While simple in theory, the technique takes a long time to realize a stabilized condition in actual experiments. The guarded hot plate (GHP) technique is a typical steady method [199,200]. It is a standard method, and more suitable to measuring samples with a relatively low thermal conductivity. The transient methods are based on the unsteady heat conduction equation, which determines the thermal conductivity of a medium by detecting the transient temperature variation during heating of the sample. The transient method has become widely used for thermal conductivity determination as it is a quick measurement and provides highly accuracy results. Transient plane source (TPS), transient hot-wire (THW), transient hot-strip (THS), and laser flash analysis (LFA) are all very frequently used transient thermal conductivity measurement techniques. The TPS method is used to determine the thermal conductivity of a sample by detecting the change in temperature at a disc-type heating source embedded in the infinite medium [201–207]. The THW method detects the temperature rise of a long thin heater embedded in an infinite sample, and then uses the data to determine the thermal conductivity of the medium [208–217]. This method is widely used to determine the thermal conductivity of powdered and fluid samples. The principle of the THS method is very similar to the THW method, where a thin metal strip instead of a wire is used as the heating source [218– 221]. The LFA method was proposed by Parker [222] in 1961. Thermal diffusivity is measured directly by the LFA method, and thermal conductivity is derived. The merits of the LFA method include very short measurement time, a very wide temperature range for measurements, and the fact that very wide materials can be measured. Similar to the above three transient methods, the LFA method has achieved significant progress after decades of development and practice. The LFA method can not only measure the thermal diffusivity of metals, it also has been used to measure the diffusivity of liquid [223], molten masses [224,225], translucent materials [226], multi-layer composites [227,228] and thin film materials [229]. In general, the first three transient methods noted above are more suitable for measuring samples with relatively low thermal conductivities, while the laser flash method is more suitable for measuring samples with high thermal conductivity. Table 7 lists some thermal conductivity experiments on PCMs and their methods. In addition to the listed methods in Table 7, the thermal conductivity of composite materials can be approximately estimated through the known thermal conductivities of component materials. Thermal conductivity has also been observed directly by using time of temperature change to verify the strengthening effect in some strengthening studies [230,231].

3.3. Measurement methods of thermophysical properties 3.3.1. Melting point, latent heat and specific heat For PCM selection, thermophysical properties are the primary consideration, with melting temperature, phase change latent heat and specific heat the three main properties of PCMs. Thus, accurate and effective determination of the melting point temperature, latent heat and specific heat of a PCM is critical. At present, differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are the two most effective methods for simultaneously determining the melting point, latent heat and specific heat of a material. Of the two methods, DTA can only test the phase change temperature of the material, while DSC can test both the phase change temperature and the heat change in the phase change process. The exothermic and endothermic peaks on a DSC curve represent the heat emission and heat absorption, respectively. However, the exothermic and endothermic peaks on the DTA curve have no definite physical meaning. As a result, DSC is currently more popular for experimental measurements. However, it should be noted that the maximum detection temperature of DTA can be up to 1500–1700 °C, although with a relatively lower sensitivity and precision than DSC. On the other hand, the maximum 10

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Although with many methods in thermal conductivity determination, there is not an universal method which is suited to all of the materials. If it is expected to measure the thermal conductivity of the sample rapidly, the transient method should be used. The measuring time is very long for all of the steady methods, especially at elevated temperatures, it will need a long time to realize a stable surroundings. The TPS, THW, and THS methods are more adaptable to measure the samples with low thermal conductivities, and the LFA method is more adaptable to measure the samples with high thermal conductivities, and electric conductive materials. If metal alloys are measured, for example, the LFA method is recommended. If the thermal conductivity of PCMs at fusion or liquid station is measured, natural convection should be avoided in measurement process. In addition, corrosion, leakage, and oxidation should be avoided in thermal conductivity measurement process.

RT-1800 K Insulation materials, loose or powder nonmetal materials No more than 1100 K Homogeneous solid materials, heterogeneous materials and porous materials

0.005–2 W/(m K)

RT-3000 K Low conductivity materials

Thermal conductivity Measurement ranges Temperature range Materials

-100–3000 K Metal, conductor, semiconductor, building materials, film materials, multilayer materials and liquid materials

0.02–20 W/(m K) 0.005–500 W/(m K)

Simple in principle; Low cost; Simple in operation; Thermal conductivity is obtained directly. Advantage

Short test cycle; Small sample; Wide temperature range; Suitable for various kinds of materials. 0.02–2000 W/(m K)


[105,116,122,123,128,130,208– 221,240,243] Large measuring range; Small system error; Simple structure; Convenient in operation. [82–84,91,103,110,111,118,121,126,129,134,139,142,166,201– 207,233,242] Short testing time; High accuracy; Small sample is required; Simple in sample preparation. [55–58,74,110,116,117,122,141,191,214,215,238] Literatures

LFA THW/THS TPS Steady-state method Technique

Table 7 Common methods for measuring thermal conductivity.

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4. Conclusions Heat storage material exploitation and selection are very important for the development of high-temperature TES technologies used in CSP systems. Nevertheless, a systematic and integrated study of hightemperature PCMs and high-temperature thermal energy storage processes is still lacking. Based on the collation and analysis of relevant literature, this review evaluated current efforts and the prospects of future related research topics, drawing the following main conclusions. (1) The selection principles of PCMs must satisfy the multifaceted requirements of thermodynamics, kinetics, chemistry and economics. When selecting materials for practical applications, we should not only measure the merits of the properties of the materials themselves, but also comprehensively take into account various objective factors such as heat transfer conditions. In practical applications, it is difficult for materials to meet all of the conditions simultaneously. Therefore, after ensuring that a material at least meets the main criteria (suitable phase change temperature and large latent heat), techniques should be developed to compensate for any remaining shortcomings and deficiencies of the material. (2) PCMs and latent heat TES systems have been investigated for decades, but unfortunately, no high-temperature PCM technology has been commercialized. Instead, PCMs are applied in thermal storage systems by using their sensible heat in spite of their high freezing points and lower thermal storage density compared with latent heat storage. This phenomenon is due to many factors beyond the shortcomings of the heat storage medium itself, with complexity of the system another primary barrier to development. (3) The level of research on molten salt and salt compositions for hightemperature phase change TES systems remains insufficient, although the importance of heat transfer enhancement investigations have been recognized by researchers. In most completed studies, many of heat transfer strengthening methods have realized great success in experimental conditions. However, many of strengthening methods are not applicable in practical application due to the poor compatibility of molten salts with other materials. In future investigations, it is important that researchers pay more attention to materials with good compatibility. (4) The temperature dependent thermophysical properties of most PCMs are not provided by researchers in their published works. The thermal storage performance of metal alloys, especially aluminum alloys, seems very sound due to their high thermal conductivity, but the problem of high liquid causticity must still be resolved. Accordingly, compatibility between metal alloys and container materials, and effective encapsulation methods, are important directions for further research. (5) The thermophysical properties of PCMs are undoubtedly of primary importance for TES technology development. Although many scholars have developed and studied a variety of high11

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