An Evaluation of Containment Materials for High Temperature Metal Thermal Storage

Concentrated solar power (CSP) must decrease its levelized cost of electricity (LCOE) below the DOE SunShot program targets of 6 ¢/kWhe and improve its reliability to enable widespread adoption. Two features of CSP that will decrease LCOE and improve reliability are higher operating temperatures for the power cycle and thermal energy storage (TES). Thermaphase Energy and San Diego State University are developing the Liquid Metal Thermal Energy Storage System (LiMTESS), an innovative TES system based on phase change in Al-Si and Mg-Si alloys that stores thermal energy produced by gas-cooled solar receivers at temperatures above 800 C. Proper containment for Al-Si and Mg-Si alloys is critical for LiMTESS commercialization. Any containment vessel must be simultaneously compatible with the molten alloys and high-temperature oxidizing gases (e.g., air), facilitate heat transfer between the alloys and high-temperature oxidizing gases, and accommodate internal stresses associated with TES operation. A ceramic-metallic composite material (TCON) and select ceramics such as siliconized silicon carbide (SiSiC) and alumina initially showed promise in meeting these requirements. A series of thermal cycling tests were performed to check the integrity of the containment vessels. TCON produced macroscopic nodules during the thermal cycling that eliminated it from further consideration. On the other hand, SiSiC performed well when exposed to high-temperature, AlSi36, MgSi56, and air. To further evaluate SiSiC as a containment material, the research team conducted multiple thermal cycles with variable temperature profiles, duration of test, and gas environments. Before and after each thermal cycle, the team conducted a mass analysis and performed SEM and EDS analysis on prepared, treated samples. The results confirm SiSiC is a good candidate for a containment vessel. At this point the research team is evaluating Morcoset, a silicon carbide-based mortar, for creating an air-tight seal for SiSiC. The research team assessed the quality of the seal by using the mortar to seal MgSi56 and conducted thermal cycling tests to compare the mass loss of the system due to Mg vapor escaping the system to that of a controlled system with no alloys sealed. Results confirmed that Mg vapor did not exit the system. There is still more work to do, but preliminary results indicate the Morcoset + SiSiC system is a good containment system for AlSi36/MgSi56. In this paper the results of the long-duration thermal cycling tests as well as electron micrographs of the containment seals and phase change materials are presented.