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Sandia National Lab (Sandia), through the Concentrating Solar Power: Efficiently Leveraging Equilibrium Mechanisms for Engineering New Thermochemical Storage (CSP: ELEMENTS) funding program, is systematically designing, developing, characterizing, and demonstrating a robust and innovative storage cycle based on novel metal oxides with mixed ionic-electronic conductivity (MIEC). Thermal energy is stored as chemical potential in these materials through a reversible reduction-oxidation reaction; thermal energy from concentrated sunlight drives a highly endothermic reduction reaction that liberates lattice oxygen from the oxide to form O2 gas, leaving an energy-rich oxygen-depleted solid. When desired, the heat is recovered as the MIEC is re-oxidized in an exothermic reaction upon exposure to air.


The work for this project falls into three project elements: MIEC Materials Development, Reactor Design, and Systems Analysis and On-Sun Testing. The MIEC Materials Development element of the project utilizes chemical and analytical means of developing and characterizing new MIEC materials for utilization in Thermochemical Energy Storage (TCES). This process combines the significant experience of the project team and the unique set of tools and capabilities available to the team. The Reactor Design portion of the project is developing a Solar Receiver Reduction Reactor (SR3) and a Re-Oxidizing (ROx) Reactor. This development requires sophisticated computer modeling of multiphysics interactions including fluid and discrete particle flow, thermodynamics, kinetics, solar optics, heat and mass transfer, and structural effects. The models used to design reactor vessels are built and tested with the test data being used to improve and validate model performance. The final project element is Systems Analysis and On-Sun Testing in which high level systems and techno-economic analysis models are developed and utilized to evaluate system performance. The data from the smaller reactors tested at the bench/demo scales, coupled with optimizations from the systems and reactor analysis, are leveraged to design a reactor for a 100kWt on-sun test. The results from building the reactor will help inform modeling projections of performance and economics for full-scale systems utilizing this unique form of energy storage.


The project leverages the following innovations:

  • MIEC materials offer rapid reaction rates, deep reduction, no side-reactions, require no catalysts, and produce only oxygen
  • Directly illuminated particles eliminate heat transfer losses in receiver
  • Direct integration with air Brayton power cycle: compressor provides pressurized air to re-oxidizer that heats air and delivers to turbine
  • Gas/solid process has no need for expensive separators and produced oxygen is environmentally friendly and does not need to be stored

In the illustration below of the integrated solar thermochemical energy storage system being pursued by Sandia, the SR3, ROx, and associate hot and cold thermochemical energy storage tanks are denoted. On the right-hand side of the figure, the process flow is detailed indicating the proposed temperature changes to be realized as part of the energy storage cycle

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