PBN Crucibles Empower High-Temperature Corrosion Research in Solar Thermal Energy Storage
CSP introduction
As the world turns towards cleaner and renewable energy sources, solar power stands out for its potential and sustainability. Concentrated Solar Power (CSP) systems are particularly noteworthy as they harness sunlight through a field of heliostats and convert it into electricity using a combination of thermal transfer systems and turbines. These systems rely heavily on efficient thermal energy storage, which necessitates the use of stable heat transfer fluids (HTFs) at high temperatures, often exceeding 700°C. Research has suggested the use of molten salts, such as LiF-NaF-KF (FLiNaK), LiF-BeF2 (FLiBe), MgCl2-KCl, or LiCl-NaCl-KCl, as potential HTFs capable of operating under these extreme conditions.
However, the interaction of these HTFs with the alloys used in receivers and heat exchangers at high temperatures can lead to corrosion, which reduces heat transfer efficiency and system longevity. Minimizing high-temperature corrosion is therefore critical for enhancing system lifetime, heat transfer efficiency, and the overall economics of high-temperature CSP systems.
Research about corrosion mechanism
In a publication titled "Dimensionless Analysis for Predicting Corrosion of Fe-Ni-Cr Alloys in Molten Salt Environments for Concentrated Solar Power Systems," found in the journal Corrosion, researchers Cho Hyun-Seok and Van Zee John from the Department of Chemical and Biological Engineering at the University of Alabama, along with their team, developed a corrosion model that incorporates fluid flow and selective oxidation of chromium. Chromium selective oxidation is observed as the primary corrosion mechanism in high-temperature alloys when exposed to molten salts. Their model, which integrates reaction mechanisms with Computational Fluid Dynamics (CFD), predicts the distribution of temperature and velocity within the molten salts, thereby forecasting the corrosion rate and potential.
For their experiments, the researchers used pyrolytic boron nitride (PBN) crucibles, which required cleaning and degreasing prior to their use. The PBN crucibles are chosen for their exceptional properties under the extreme conditions of the experiments. PBN exhibits high thermal stability, capable of withstanding temperatures well beyond the operational range of CSP systems, ensuring it does not react with the molten salts.
Moreover, PBN has a low wettability with molten salts, which prevents the formation of a film on the crucible's surface, minimizing contamination and interaction with the environment. Its chemical inertness also prevents reactions that could skew the results of the corrosion experiments.
By utilizing PBN crucibles within an inert atmosphere, the team was able to mix salts at the required concentrations, prepare eutectic mixtures, and load them into the crucibles without the risk of introducing impurities or experiencing unwanted chemical reactions. These PBN crucibles were then placed within stainless steel containers, which were subsequently positioned in an Inconel reactor equipped with an argon gas shroud and stirring mechanism for the salt medium. The entire setup was isolated from the surrounding laboratory environment within a Carbolite tube furnace, controlled for internal gas flow.
The choice of PBN crucibles in these high-temperature corrosion experiments highlights the importance of selecting the right materials to ensure the integrity and accuracy of the results, which is crucial for advancing the field of solar thermal energy storage and improving the efficiency and durability of CSP systems.