Advanced components in next-generation concentrating solar power (CSP) applications will require advanced heat-transfer fluids and thermal-storage materials that work from about 550°C to at least 720°C, for integration with advanced power-conversion systems. To reach the cost target, less-expensive salts such as molten chlorides have been identified as high-temperature fluid candidates.High-strength alloys need to be identified and their mechanical and chemical degradation must be minimized to be used in CSP applications. Approaches for corrosion mitigation need to be investigated and optimized to drive down corrosion rates to acceptable levels-in the order of tens of micrometers per year-for achieving a long system lifetime of at least 30 years.Surface passivationis agood corrosion mitigation approach because the alloy could then be exposed to both the liquid and the vapor phases of the salt mixture. In this investigation,we pre-oxidized the alumina-forming alloys Inconel 702, Haynes 224, and Kanthal APMT at different temperatures, dwelling times, and atmospheres to produce the passivation by forming protective oxides at the surface. The pretreated alloys were latercorrodedin molten MgCl 2-64.41 wt% KClat 700°C in a flowing Ar atmosphere. We performed electrochemical techniques such as open-circuit potential followed by a potentiodynamic polarization sweep and conventional long-term weight-change tests to down-select the best-performing alloy and pre-oxidation conditions. The best corrosion results were obtained for In702 pre-oxidized in zero air at 1050°C for 4 h. Metallographic characterization of the pre-oxidized alloys and of the corroded surfaces showed that the formation of dense and uniform alumina scale during the pre-oxidation appears to protect the alloy from attack by molten chloride.
a b s t r a c tLithium, mainly used in electrical energy storage, has also been studied in thermal energy storage. It is recognized as a "critical material" and is produced from minerals and from brines. Chile is one of the biggest producers, here from brine and with lower costs than in other countries. With sensible heat storage, in solar power plants lithium is seen as a way to improve the properties of molten salts used today. The low melting point in these ternary salts with lithium, represent a considerable reduction in the maintenance and operational costs associated with current solar technology, demonstrating that the fluids showed, are potential candidates for thermal energy storage (TES) in concentrated solar plants (CSP) plants. Many materials have been studied and proposed to be used as phase change materials (PCM). Between the multiple materials studied to be used in PCM, lithium materials and mixtures are listed as potential PCM for building applications and for high temperature applications. In thermochemical energy storage, lithium compounds have been used mainly in chemical heat pumps, following their use in absorption cooling.
Physical characterization and thermal properties of bischofite, a by-product from the nonmetallic industry, were determined and compared with those to MgCl 2 •6H 2 O with the idea of using it as phase change material in thermal energy storage applications. The melting point and heat of fusion were measured in the temperature range from 30°C to 150°C, where T fus and ΔH fus were 100°C and 115 kJ/kg for bischofite, and 114.5°C and 135 kJ/kg for MgCl 2 •6H 2 O. The solid heat capacity was determined from 25°C to 60°C, being 2.1 kJ/(kg•K) at 60°C for both samples. The measurements of the liquid heat capacity of bischofite were done from 105°C to 113°C and the Cp showed linear increase from 5.61 kJ/(kg•K) to 9.01 kJ/(kg•K). The thermal stability test (30 heating/cooling cycles) of bischofite and MgCl 2 •6H 2 O shows subcooling of about 37K and 29K, respectively. The solid and liquid densities were determined using the pycnometrically method; for bischofite, ρ solid decrease from 1686 (at 30°C) to 1513 kg/m 3 (at 50°C) and ρ liq was 1481 kg/m 3 (at 115°C). Based on the thermophysical properties evaluated, the energy storage density was evaluated for both materials, being 170 J/cm 3 for bischofite and 192 J/cm 3 for MgCl 2 •6H 2 O. This study established that bishofite is a promissory PCM with similar thermophysical characteristics to magnesium chloride hydrate, but with a lower cost. Nevertheless, further work is needed to overcome the two main problems found, subcooling and segregation of the material.
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