Search

Considerable effort has been devoted to the characterization of thermal properties of the different types of materials that can be used as thermal energy storage (TES) media, but scarce literature exists concerning the materials to manufacture the tanks that can be used to contain these storage media. One of the main concerns when selecting the most suitable material for these tanks is its resistance to corrosion due to molten salts that constitute the TES system. Dynamic gravimetric analysis is a newly proposed method for the study of corrosion on metals, which optimizes the standard procedure described by ASTM G1–03. The new technique avoids the direct handling of samples, so more accurate values can be obtained. In this work, the resistance to corrosion of AISI 316 stainless steel samples in contact with commercial grade molten salts of the Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 system, at 600 °C for different exposure times, has been determined by using this new methodology. The results show that the initial corrosion rate is lower at higher amounts of lithium carbonate present in the molten salts mixture. [ABSTRACT FROM AUTHOR]

The eutectic Na 2 SO 4 -NaCl salt was investigated as a novel high temperature phase change material for solar thermal energy storage due to its low cost and melting point (626.0 °C) interested. The measured melting point and heat of fusion of the eutectic salt are 625.3 °C and 263.3 J/g, which are in agreement with their theoretical values. The eutectic salt shows good thermal stability after 100 melting-freezing thermal cycles in the range of 550–680 °C in air. Three types of the eutectic Na 2 SO 4 -NaCl-ceramic composites have been designed and evaluated to address the salt’ corrosion issues with containment materials. Results show that eutectic Na 2 SO 4 -NaCl/both α-alumina and mullite composites with a mass ratio of salt to ceramics of 1:1 (wt%) have good thermal stability without cracking as well as with weight losses of 0.50% and 0.74% and the decrease of latent heat by 1% and 2% respectively after 20 thermal cycles, where no phase separation and chemical reaction occur. Their measured melting temperature and latent heat are consistent with their theoretical values. Addition of α-alumina and mullite to the eutectic Na 2 SO 4 -NaCl salt can reduce the eutectic Na 2 SO 4 -NaCl salt’ supercooling. In contrast, the sample of the Na 2 SO 4 -NaCl/diatomite composite cracked after the cycles. [ABSTRACT FROM AUTHOR]

The use of paraffin, salts and salt hydrates as phase change materials (PCMs) have been researched extensively and used in a number of commercial applications. However, metals and metal alloys, which possess a high storage density on a volume basis as well as a substantially higher thermal conductivity, has received much less attention. This paper discusses the considerations for the use of metal and metal alloys as phase change materials for high temperature thermal storage applications, as well as summarises the literature on the limited research in this area. Although some pure metals and metal alloys present interesting thermal properties to be used as PCMs in thermal storage systems, there is a lack of understanding on the implications of the metallurgical aspects related to the melting and solidification of these materials under thermal cycling at high temperatures. The main issues to be considered include vapour pressure, undercooling, corrosion, segregation, changes in composition and microstructure, changes in thermal properties and undesired reactions. Further research is needed before these materials can be used as PCMs in thermal energy storage systems in industry. [ABSTRACT FROM AUTHOR]

Thermal energy storage (TES) is a critical component in a concentrated solar power (CSP) plant since it is able to provide dispatchability and increase the capacity factor of the plant. Recently the Brayton power cycle using supercritical carbon dioxide (s-CO 2 ) has attracted considerable attention as it allows a higher thermal to electric power conversion efficiency compared to the conventional Rankine cycle using subcritical steam. However, no commercial TES has yet been developed for integration with a s-CO 2 based plant. One reason is the lack of a suitable storage material. This work explores the use of a eutectic NaCl-Na 2 CO 3 salt as a reliable high temperature phase change material (PCM). The PCM has been thermally cycled up to 1000 times. Its thermo-physical properties have been measured before and after it has been subjected to the thermal cycling and its corrosion behavior has been investigated. This eutectic salt shows good thermal stability without degradation after cycling 1000 times between 600 and 650 °C. The corrosion rate on stainless steel 316 (SS316) increases linearly up to 350 cycles, and thereafter it stabilizes at 70 mg/cm 2 . [ABSTRACT FROM AUTHOR]

The mechanisms leading to corrosion in stainless steel containments for thermal energy storage through phase change materials, such as carbonates and chlorides, are crucial for understanding the degradation of these steel alloys. A comprehensive study of this area will allow for down-selection of materials suitable for solar thermal energy storage (TES) operation at an elevated temperature range. Samples of stainless steel (SS) 316L were subjected to a cyclic heat environment of 550–750 °C in air and argon headspace environments, in the presence of a carbonate salt phase change material (PCM). A series of complementary microscopy, spectroscopy and diffractometry analytical techniques were applied to the corroded SS316L. Corrosion rate, interface formation and chemical products with respect to thermal cycling are presented with associated degradation mechanism explained and comparisons are made among different gas environments and varied immersion conditions. In the PCM under ambient air conditions, steel surfaces were mainly corroded by the penetration of oxidants from air, such as H 2 O or O 2 , along grain boundaries. SS316L samples demonstrated more severe degradation when exposed to air with a small fraction of PCM vapour present compared to when they were exposed to liquid or solid PCM. Cycling in Ar resulted in less corrosion in samples as opposed to when they were exposed to an air environment. In an Ar environment corrosion is driven via the formation of chromite, while the SS316L showed a less degradation when exposed to Ar with a small fraction of PCM vapour compared to when it was immersed in PCM. The mechanisms for degradation in air and in Ar are common in that the corrosion products of Cr and Ni dissolve in the PCM thus removing any layer that could protect against corrosion. The present study provides insight into corrosion of stainless-steel when exposed to carbonate salts, in air and inert gas environments, and contributes to down-selection of materials for solar thermal energy storage. • SS316L compatibility test for PCM based solar thermal storage at high temperatures. • Determine the migration of elements including Na form the PCM into SS316L. • Identifying the depletion of steel components dissolving into the liquid PCM. • Corrosion mechanism of steels under inert gas in the presence of carbonates. [ABSTRACT FROM AUTHOR]

In this work, the eutectic Na 2 CO 3 –NaCl molten salt was investigated as a new high temperature phase change material for solar thermal energy storage. The composition of the eutectic binary salt was determined with the aid of FactSage software and its thermophysical properties were investigated using a Simultaneous Thermal Analyzer (STA) and X-Ray Diffraction (XRD). Inductively coupled plasma analysis has shown that the composition of the as-prepared sample is consistent with the nominal one. The STA results exhibit that the melting point of the eutectic salt is 637.0 °C and its heat of fusion is 283.3 J/g whereas its specific heat is a function of temperature, which all are in agreement with the theoretical values determined by the FactSage software. The thermal stability analysis indicates that the eutectic molten salt has good thermal stability without weight loss in a CO 2 environment at temperatures below 700 °C, compared with 0.51% weight loss in a N 2 atmosphere. The weight loss observed in the latter, is most likely to be due to the salt׳s decomposition at high temperature. The thermophysical properties of the salt such as melting temperature, latent heat of fusion and solidification, varied marginally after 50 and 100 thermal cycle tests. This demonstrates that the eutectic Na 2 CO 3 –NaCl salt is a promising high temperature phase change material when used in a CO 2 environment or encapsulation. [ABSTRACT FROM AUTHOR]

Analysis of stainless steel 316 as a containment material in the presence of a phase change material (PCM) cycled at high temperature was carried out through a combination of X-ray photoelectron spectroscopy and X-ray diffraction methods. In this work, stainless steel tokens were half-immersed in a chloride carbonate-based PCM, which were then thermally cycled in an air or a nitrogen environment up to 500 times, allowing the PCM to repeatedly transform between solid and liquid phases. Spectroscopy and diffraction methods were applied on the tokens, as well as the cycled PCM, to investigate the extent and nature of corrosion in such steel alloys. With varying sputtering conditions, the oxidation state at different depths of token surfaces was quantified. From the outermost corroded layer through to the bulk, this study shows a gradual change in distribution in both Cr and oxidation of Fe, while Cr was specifically found to have depleted and migrated into the PCM. The oxidation and depletion have been found to increase with increasing exposure time to PCM while sigma-phase structure embrittled in the corrosion layer. [Display omitted] • Chemical degradation of thermal cycled SS 316 with PCM was analysed. • X-ray photoelectron spectroscopy methods were applied to analyse corrosion layers. • Chloride carbonate based PCM in air and nitrogen environment was studied. • With varying sputtering conditions, oxidation state of steel was quantified. [ABSTRACT FROM AUTHOR]

The formation and mechanism that drive corrosion in stainless steel as a containment material in the presence of phase change materials is of importance in solar thermal energy storage systems. In our work, half-immersed stainless steel 316 tokens in a carbonate-based phase change material (PCM 638) have been investigated. The samples were thermally cycled in air and Nitrogen environment up to 500 times within a high-temperature range, allowing the PCM to transform between solid and liquid states repeatedly. As a consequence of thermal cycling, severe thickness loss on steel token was observed, indicating a degree of oxidation occurring, which depleted the uncorroded steel. Cross-sectional microstructural analysis was carried out to determine the elemental distribution and structural morphology along the corrosion layers. This study shows that thermal cycling of SS 316 in PCM results in active Ni and Cr migration to the surface, leading to a significant depletion of Cr from steel moving into the PCM. Cr and Fe on the surface are found to oxidise with varying degrees, with respect to the exposure time. The depletion of Cr was found to be higher in SS 316 immersed in PCM, while also increasing with exposure time. [Display omitted] • Degradation of stainless steel as a containment material at high temperature is studied. • Corrosion of steel against PCM at accelerated thermal cycling is explained. • The degree of oxidation in corrosion layers due to reactions with PCM is explained. • Structural morphology along with active Ni and Cr migration of steel is found. • Relationship between elemental depletion and exposure time is explained. [ABSTRACT FROM AUTHOR]