High-temperature stability of ternary nitrate molten salts for solar thermal energy systems

Long-Term Evaluation of a Ternary Mixture of Molten Salts in Solar Thermal Storage Systems: Impact on Thermophysical Properties and Corrosion

Solar thermal plants typically undergo trough operational cycles spanning between 20 and 25 years, highlighting the critical need for accurate assessments of long-term component evolution. Among these components, the heat storage media (molten salt) is crucial in plant design, as it significantly influences both the thermophysical properties of the working fluid and the corrosion of the steel components in thermal storage systems. Our research focused on evaluating the long-term effects of operating a low-melting-point ternary mixture consisting of 30 wt% LiNO3, 57 wt% KNO3, and 13 wt% NaNO3. The ternary mixture exhibited a melting point of 129 °C and thermal stability above 550 °C. Over 15,000 h, the heat capacity decreased from 1.794 to 1.409 J/g °C. Additionally, saline components such as CaCO3 and MgCO3, as well as lithium oxides (LiO and LiO2), were detected due to the separation of the ternary mixture. A 30,000 h exposure resulted in the formation of Fe2O3 and the presence of Cl, indicating prolonged interaction with the marine environment. This investigation highlights the necessity of analyzing properties under actual operating conditions to accurately predict the lifespan and select the appropriate materials for molten salt-based thermal storage systems.

Influence of atmosphere and austenitic stainless steel on the solar salt corrosivity

The utilization of the Solar Salt (60 wt%NaNO3/40 wt%KNO3) mixture as a Thermal Energy Storage (TES) medium is gaining importance due to its scalability and cost-effectiveness. However, the corrosion of metallic components presents a significant challenge. This study explores the intricate interplay between salt chemistry and its corrosivity, particularly at elevated temperatures exceeding the state-of-the-art bulk temperature 565 °C. The study manipulates salt decomposition by adjusting the oxygen partial pressure in the purge gas over Solar Salt and investigates the evolution of salt chemistry with and without the presence of steel. It analyzes the corrosion behavior of two types of stainless steel, AISI 316L and AISI 310, under different gas purging atmospheres. Furthermore, it employs a gold particle tracing technique to identify and monitor the formation and growth of the corrosion layer on the steel surface. The results reveal that nitrogen gas purging significantly enhances salt decomposition and its corrosivity over time. The presence of steel also influences salt decomposition depending on the purged gas atmosphere. In a nitrogen atmosphere, the presence of steel can increase the nitrite levels, while an air atmosphere results in an elevated concentration of oxide ions. In air, the AISI 310 alloy shows slightly better performance than AISI 316L. Both alloys experience substantial mass loss in the nitrogen-purged atmosphere. Interestingly, the presence of gold particles within the middle of the corrosion layer in the air purged atmosphere visually illustrates a counter diffusion involving various cations and anions across the corrosion layer.

Molecular Dynamics Simulations of Nitrate/MgO Interfaces and Understanding Metastability of Thermochemical Materials

We explore the role of molten nitrate interfaces on MgO surface treatment for improving the reversibility of thermochemical energy storage via sorption and desorption of water or CO₂. Our molecular dynamics simulations focus on melts of LiNO₃, NaNO₃, KNO₃, and the triple eutectic mixture Li_0.38Na_0.18K_0.44NO₃ on the surface of MgO to provide atomic scale details of adsorbed layers and to rationalize interface energies. On this basis, a thermodynamic model is elaborated to characterize the effect of nitrate melts on the dehydration of Mg(OH)₂ and to quantitatively explain the difference in dehydration temperatures of intact and LiNO₃-doped Mg(OH)₂.

Assessment and Perspectives of Heat Transfer Fluids for CSP Applications

Different fluid compositions have been considered as heat transfer fluids (HTF) for concentrating solar power (CSP) applications. In linear focusing CSP systems synthetic oils are prevalently employed; more recently, the use of molten salt mixtures in linear focusing CSP systems has been proposed too. This paper presents a comparative assessment of thermal oils and five four nitrate/nitrite mixtures, among the ones mostly employed or proposed so far for CSP applications. The typical medium-size CSP plant (50 MWe) operating with synthetic oil as HTF and the “solar salt” as TES was considered as a benchmark. In the first part of the paper, physical properties and operation ranges of different HTFs are reviewed; corrosion and environmental issues are highlighted too. Besides an extensive review of HTFs based on data available from the open literature, the authors report their own obtained experimental data needed to thoroughly compare different solutions. In the second part of the paper, the impact of the different HTF options on the design and operation of CSP plants are analyzed from techno-economic perspectives.

Molten Salts for Sensible Thermal Energy Storage: A Review and an Energy Performance Analysis

A comprehensive review of different thermal energy storage materials for concentrated solar power has been conducted. Fifteen candidates were selected due to their nature, thermophysical properties, and economic impact. Three key energy performance indicators were defined in order to evaluate the performance of the different molten salts, using Solar Salt as a reference for low and high temperatures. The analysis provided evidence that nitrate-based materials are the best choice for the former and chloride-based materials are best for the latter instead of fluoride and carbonate-based candidates, mainly due to their low cost.

Mechanisms of absorption and desorption of CO2 by molten NaNO3-promoted MgO

We present details of the mechanism of absorption and desorption of carbon dioxide by molten NaNO₃-promoted MgO and their implications for the applications of alkali nitrate-promoted MgO absorbents with many repeated absorption and desorption cycles.