Publications

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A two-step cycle was considered for solar thermochemical energy storage based on aluminum-doped calcium manganite reduction/oxidation reactions for direct integration into Air Brayton cycles. The two steps encompassed (1) the storage of concentrated solar direct irradiation via the thermal reduction of aluminum-doped calcium manganite and (2) the delivery of heat to an Air-Brayton cycle via re-oxidation of oxygen-deficient aluminum-doped calcium manganite. The re-oxidized aluminum-doped calcium manganite was fed back to the first step to complete the cycle. A 5 kWth solar thermochemical reactor operating under vacuum was fabricated and tested to examine the first cycle reduction step. Reactor operating conditions and high-flux solar simulator control were tuned for continuous reactor operation with particle temperatures >1073 K. Continuous operation was achieved using intermittent, dense granular flows. A maximum absorption efficiency of 64.7% was demonstrated, accounting for both sensible and chemical heat storage.

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A coupled electrochemical/thermochemical cycle was investigated to produce hydrogen from renewable resources. Like a conventional thermochemical cycle, this cycle leverages chemical energy stored in a thermochemical working material that is reduced thermally by solar energy. However, in this concept, the stored chemical energy only needs to be partially, but not fully, capable of splitting steam to produce hydrogen. To complete the process, a proton-conducting membrane is driven to separate hydrogen as it is produced, thus shifting the thermodynamics toward further hydrogen production. This novel coupled-cycle concept provides several benefits. First, the required oxidation enthalpy of the reversible thermochemical material is reduced, enabling the process to occur at lower temperatures. Second, removing the requirement for spontaneous steam-splitting widens the scope of materials compositions, allowing for less expensive/more abundant elements to be used. Lastly, thermodynamics calculations suggest that this concept can potentially reach higher efficiencies than photovoltaic-to-electrolysis hydrogen production methods. This Exploratory Express LDRD involved assessing the practical feasibility of the proposed coupled cycle. A test stand was designed and constructed and proton-conducting membranes were synthesized. While the full proof of concept was not achieved, the individual components of the experiment were validated and new capabilities that can be leveraged by a variety of programs were developed.

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A novel concept for coupling a thermochemical cycle with an electrochemical separation device for the generation of hydrogen from steam is reported and a thermodynamic analysis of the system is presented. In a conventional thermochemical cycle, an oxygen carrier material is thermally reduced, cooled, and then reoxidized in steam thereby generating hydrogen. However, this process often requires high temperatures (>1700 K) and/or low oxygen partial pressures (<0.001 atm) in order to meet thermodynamic requirements. Such extreme conditions can adversely affect the stability of the reactive oxides, reactor materials, and system efficiency. In our proposed technology, we seek to decrease the required reduction temperature by several hundred degrees Kelvin by relaxing the requirement for spontaneous oxidation reaction at atmospheric pressure. This is accomplished by incorporating a proton-conducting membrane (PCM) to separate hydrogen produced at equilibrium concentrations from reactant steam. We also suggest the use of mixed ionic-electronic conducting (MIEC) oxygen carrier materials that reduce through a continuum of oxidation states at lower temperatures (∼1200 °C). This concept allows the generation of a high-quality hydrogen stream while avoiding the challenging high temperatures/low partial pressures required in conventional water-splitting reaction schemes.

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