A two-step cycle is considered for solar thermochemical energy storage based on particulate aluminum-doped calcium manganite reduction/oxidation reactions for direct integration into Air Brayton cycles. The two steps encompass 1) the storage of concentrated solar irradiation within endothermic reduction of aluminum-doped calcium manganite and 2) the delivery of heat to an Air-Brayton cycle via exothermic re-oxidation of oxygen-deficient aluminum-doped calcium magnanite. A 5 kW~th~ scale solar thermochemical reactor operating under vacuum was designed, modeled, and optimized to thermally reduce a continuous, gravity-driven flow of aluminum-doped calcium manganite particles. Granular flow behavior along inclined planes was used to develop thin, dense flows and control particle residence times. The granular flows were characterized in a tilt-flow rig, and particle image velocimetry was used to determine flow properties via frictional and velocity scaling relationships. Granular flow behavior of the aluminum-doped calcium manganite particles was compared to the flow behavior of Accucast ID50 ceramic particles, a state-of-the-art material used in particle heating receivers. The flow properties were integrated into a detailed heat and mass transfer model of the solar thermochemical reactor developed in ANSYS Fluent. The model was developed to predict reactor performance and investigate the influence of varying operating conditions. Reactor performance was characterized using the absorption efficiency of the delivered radiative input to a 40 mm aperture, the average outlet granular flow temperature, and the average reduction extent of the aluminum-doped calcium manganite particles.  A radiative input was delivered to the reactor from various lamp combinations of the Georgia Tech High-Flux Solar Simulator. The radiative input was modeled via Monte Carlo ray-tracing and coupled to the unstructured mesh within the ANSYS Fluent modeling domain. Radiative properties of the aluminum-doped calcium manganite particles and cavity materials were determined via a coupled monochromator with integrating sphere and Fourier transform infrared spectrometer with integrating sphere. Reactor operational parameters of particle mass flow rate, inclination angle, and heat input were varied to investigate reactor performance. A reactor design with 31° inclination angle, 230 g/min of particles, and 5.2 kW~th~ radiative input from the high-flux solar simulator was found to produce an outlet flow temperature of 1158 K, with stoichiometric deviations of 0.076 and a storage efficiency of 0.628 while avoiding particle overheating and promoting longer particle residence times. These results provided engineering guidance on reactor designs and operating conditions for achieving continuous, solar thermochemical energy storage with dense, granular flows of metal oxide particles. The ambient temperature characterization of the dense granular flow was important to reactor performance for different configurations and operating conditions.

 


Aluminum-Doped Calcium Manganite Particles for Solar Thermochemical Energy Storage: Reactor Design, Particle Characterization, and Heat and Mass Transfer Modeling

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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