Lined Molten Salt Tank Evaluation (LIMOSA) Phase 3 of Agreement 34209 CSP Gen3: Liquid-Phase Pathway to SunShot

The U.S. Department of Energy (DOE) established the Concentrating Solar Power Generation 3 (CSP Gen3) program to promote the development of advanced CSP systems capable of producing electricity at a levelized cost of energy (LCOE) less than $60/MWh, based on criteria published in the CSP Gen3 Roadmap and a subsequent funding opportunity announcement (Gen3 FOA). The National Laboratory of the Rockies (NLR) was selected to lead the "Liquid Pathway" project to meet these objectives. The Liquid Pathway proposed the use of low-cost molten chloride salts for energy storage, mated with an operationally flexible solar receiver that employs liquid-metal sodium for heat capture and transfer to the storage salt. The results of project Phases 1 and 2 were documented in a prior report on OSTI. A formal risk registry was developed and tracked during Phase 2. The risk registry captured project risks across over 17 component and subsystem categories. The most significant risks included the integrity of the tank liner, the performance of the salt valves, the impact of salt vapor on system components, pressure- and chemical-sensor performance, and risks associated with liquid-metal sodium use. The U.S. DOE did not select the Liquid Pathway for pilot-scale demonstration under the Gen3 CSP program. However, a Phase 3 project was authorized to evaluate the proposed internally lined molten salt tank, which became known as the LIMOSA project. This report documents the findings under Phase 3 construction and testing of a small-scale internally insulated molten salt tank. LIMOSA was designed to de-risk the feasibility of an internally insulated tank for salt containment. While a purely refractory internal liner was deemed unable to prevent salt wetting of the insulation, the LIMOSA project promoted the use of a graphite barrier between the liner hotface and underlying insulation layers to stop molten salt penetration. The LIMOSA team constructed a small-scale tank that included a graphite-foil barrier within the refractory liner. Despite encouraging bench-scale testing, the liner failed shortly after transferring 164 kg of molten chloride salt into the tank. Transfer was made at 550 degrees Celsius and the failure was detected while the tank was at 620 degrees Celsius, being heated on its way to the design point temperature of 720 degrees Celsius. Subsequent analysis of the LIMOSA tank liner showed the graphite adhesive in the liner had failed. Although the graphite foil itself appeared undamaged, the adhesive was cracked and brittle, leading to salt intrusion to the underlying insulated layers. Testing of three different graphite adhesives and two different graphite-foil sheet suppliers in molten chloride salt for 500 hours under controlled conditions showed the same behavior. One of the adhesives, Aremco 551 RN, performed better than the others, but it was still negatively affected impacted by the approximately 750 degrees Celsius salt. Nonetheless, the use of graphite remains a promising approach for molten chloride containment, and recommendations for future work include (1) switching to a high-temperature adhesive that is resistant to salt attack, (2) moderate vacuum purging to ensure no residual oxygen in the tank, and (3) introducing better cover gas mixing withing the tank during preheating. In other outcomes, two types of hotface mortar were tested, but as expected, the mortar joints between the hotface bricks were fully permeated by salt, highlighting the importance of an impermeable barrier. Prior to failure, the tank heat loss and shell temperature were consistent with the targeted performance for the design. The heat flux values ranged from about 200 to 300 W/m2 at the lid and between 110 and 200 W/m2 at the tank bottom, as the temperature of the salt zone was raised from 550 degrees Celsius to 620 degrees Celsius. The shell wall temperature remained below 60 degrees Celsius. However, the limited duration of the experiment and the inability to reach the target operating temperature of 720 degrees Celsius must be taken into consideration. The LIMOSA project confirmed the presence of salt in cover-gas exhaust tubing, indicating it will be critical to design containment and transfer systems for the presence of salt and possible precipitation thereof from the vapor phase. The deposited material included Na, K, Mg, and Cl, indicated all constituents of the ternary chloride entered the vapor phase; however, specific crystal form was not determined. Lastly, no salt was detected in the fiber blanket insulation of the tank lid, implying that the introduction of cover gas within the lid insulation and use of a thin metal barrier at the face of the insulation successfully prevented salt vapor diffusion into the insulation.