Latent heat energy storage system is one of the promising solutions for efficient way of storing excess thermal energy during low consumption periods. One of the challenges for latent heat storage systems is the proper selection of the phase change materials (PCMs) for the targeted applications. As compared to organic PCMs, inorganic PCMs have some drawbacks, such as corrosion potential and phase separation; however, there are available techniques to overcome or minimize these drawbacks. On the other hand, inorganic PCMs are found to have higher thermal conductivity and storage capacity over organic PCMs. As a result inorganic PCMs have a great potential in thermal energy storage field, especially in medium to high temperature applications where organic PCMs are not a viable option. In this study, a detailed review of research outcomes and recent technological advancements in the field of inorganic phase change materials is presented while focusing on providing solutions to the associated disadvantages of this class of PCMs. Long term stability, thermal cycling performance, and heat transfer enhancements are also discussed in the context of this review.

A review on current status and challenges of inorganic phase change materials for thermal energy storage systems

Highly conductive composites made of phase change materials and graphite for thermal storage

The Advanced High-Temperature Reactor (AHTR) is a novel reactor design that utilizes the graphite-matrix high-temperature fuel of helium-cooled reactors, but provides cooling with a high-temperature fluoride salt. For applications at temperatures greater than 900 C the AHTR is also referred to as a Liquid-Salt-Cooled Very High-Temperature Reactor (LS-VHTR). This report provides an assessment of candidate salts proposed as the primary coolant for the AHTR based upon a review of physical properties, nuclear properties, and chemical factors. The physical properties most relevant for coolant service were reviewed. Key chemical factors that influence material compatibility were also analyzed for the purpose of screening salt candidates. Some simple screening factors related to the nuclear properties of salts were also developed. The moderating ratio and neutron-absorption cross-section were compiled for each salt. The short-lived activation products, long-lived transmutation activity, and reactivity coefficients associated with various salt candidates were estimated using a computational model. Table A presents a summary of the properties of the candidate coolant salts. Certain factors in this table, such as melting point, vapor pressure, and nuclear properties, can be viewed as stand-alone parameters for screening candidates. Heat-transfer properties are considered as a group in Sect. 3 in order to evaluate the combined effects of various factors. In the course of this review, it became apparent that the state of the properties database was strong in some areas and weak in others. A qualitative map of the state of the database and predictive capabilities is given in Table B. It is apparent that the property of thermal conductivity has the greatest uncertainty and is the most difficult to measure. The database, with respect to heat capacity, can be improved with modern instruments and modest effort. In general, ''lighter'' (low-Z) salts tend to exhibit better heat transfer and nuclear performance metrics. Lighter salts also tend to have more favorable (larger) moderating ratios, and thus should have a more favorable coolant-voiding behavior in-core. Heavy (high-Z) salts tend to have lower heat capacities and thermal conductivities and more significant activation and transmutation products. However, all of the salts are relatively good heat-transfer agents. A detailed discussion of each property and the combination of properties that served as a heat-transfer metric is presented in the body of this report. In addition to neutronic metrics, such as moderating ratio and neutron absorption, the activation properties of the salts were investigated (Table C). Again, lighter salts tend to have more favorable activation properties compared to salts with high atomic-number constituents. A simple model for estimating the reactivity coefficients associated with a reduction of salt content in the core (voiding or thermal expansion) was also developed, and the primary parameters were investigated. It appears that reasonable design flexibility exists to select a safe combination of fuel-element design and salt coolant for most of the candidate salts. Materials compatibility is an overriding consideration for high-temperature reactors; therefore the question was posed whether any one of the candidate salts was inherently, or significantly, more corrosive than another. This is a very complex subject, and it was not possible to exclude any fluoride salts based on the corrosion database. The corrosion database clearly indicates superior container alloys, but the effect of salt identity is masked by many factors which are likely more important (impurities, redox condition) in the testing evidence than salt identity. Despite this uncertainty, some reasonable preferences can be recommended, and these are indicated in the conclusions. The reasoning to support these conclusions is established in the body of this report.

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Assessment of Candidate Molten Salt Coolants for the Advanced High Temperature Reactor (AHTR)

The growing interest in energy applications of molten salts is justified by several of their properties. Their possibilities of usage as a coolant, heat transfer fluid or heat storage substrate, require thermo-hydrodynamic refined calculations. Many researchers are using simulation techniques, such as Computational Fluid Dynamics (CFD) for their projects or conceptual designs. The aim of this work is providing a review of basic properties (density, viscosity, thermal conductivity and heat capacity) of the most common and referred salt mixtures. After checking data, tabulated and graphical outputs are given in order to offer the most suitable available values to be used as input parameters for other calculations or simulations. The reviewed values show a general scattering in characterization, mainly in thermal properties. This disagreement suggests that, in several cases, new studies must be started (and even new measurement techniques should be developed) to obtain accurate values.

Molten salts database for energy applications

Material aspects of Solar Salt for sensible heat storage

Melting-crystallization and premelting properties of sodium nitrate-potassium nitrate. Enthalpies and heat capacities

The fusion properties (melting point, enthalpy and entropy of fusion) of the eutectic LiF-NaF-KF and the heat capacity of this system in the polycrystalline state and in the molten state have been investigated by the technique of high-temperature differential scanning calorimetry. The fusion properties and heat capacities may be summarized as follows: mp 462 + or - 2/sup 0/C; ..delta..H /SUB fus/ = 3.98 kcal mol/sup -1/; ..delta..S /SUB fus/ = 5.42 eu; C /SUB p/ (solid) = 12.8218 - (9.651 X 10/sup -3/)T + (1.513 X 10/sup -5/)T/sup 2/ and C /SUB p/ (melt) = 9.6360 + (10.487 X 10/sup -3/)T, both in cal mol/sup -1/K/sup -1/, respectively. The heat capacity temperature ranges are 57-450/sup 0/C (solid) and 475-590/sup 0/C (melt), and the accuracy limits for both the enthalpy and the heat capacities are + or -2.0%. For conversion to SI units, 1 cal = 4.184 J.

Fusion properties and heat capacities of the eutectic lithium fluoride-sodium fluoride-potassium fluoride melt

Graphs on the thermicity of reactions observed by the in-situ differential scanning calorimetry technique, fusion endotherms, and melting-crystallization behavior for potassium polysulfide electrolytes are presented.

Derek J. Rogers

Papers

Melting-crystallization and premelting properties of sodium nitrate-potassium nitrate. Enthalpies and heat capacities

Fusion properties and heat capacities of the eutectic lithium fluoride-sodium fluoride-potassium fluoride melt

Melting and pre-melting properties for a series of potassium polysulfides