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  1. Abstract

    Bio-inspired flying robots (BIFRs) which fly by flapping their wings experience continuously oscillating aerodynamic forces. These oscillations in the driving force cause vibrations in the motion of the body around the mean trajectory. In other words, a hovering BIFR does not remain fixed in space; instead, it undergoes oscillatory motion in almost all directions around the stationary point. These oscillations affect the aerodynamic performance of the flier. Assessing the effect of these oscillations, particularly on thrust generation in two-winged and four-winged BIFRs, is the main objective of this work. To achieve such a goal, two experimental setups were considered to measure the average thrust for the two BIFRs. The average thrust is measured over the flapping cycle of the BIFRs. In the first experimental setup, the BIFR is installed at the end of a pendulum rod, in place of the pendulum mass. While flapping, the model creates a thrust force that raises the model along the circular trajectory of the pendulum mass to a certain angular position, which is an equilibrium point and is also stable. Measuring the weight of the BIFR and the equilibrium angle it obtains, it is straightforward to estimate the average thrust, by moment balance about the pendulum hinge. This pendulum setup allows the BIFR model to freely oscillate back and forth along the circular trajectory about the equilibrium position. As such, the estimated average thrust includes the effects of these self-induced vibrations. In contrast, we use another setup with a load cell to measure thrust where the model is completely fixed. The thrust measurement revealed that the load cell or the fixed test leads to a higher thrust than the pendulum or the oscillatory test for the two-winged model, showing the opposite behavior for the four-winged model. That is, self-induced vibrations have different effects on the two BIFR models. We felt that this observation is worth further investigation. It is important to mention that aerodynamic mechanisms for thrust generation in the two and four-winged models are different. A two-winged BIFR generates thrust through traditional flapping mechanisms whereas a four-winged model enjoys a clapping effect, which results from wing-wing interaction. In the present work, we use a motion capture system, aerodynamic modeling, and flow visualization to study the underlying physics of the observed different behaviors of the two flapping models. The study revealed that the interaction of the vortices with the flapping wing robots may play a role in the observed aerodynamic behavior of the two BIFRs.

     
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  2. Long duration energy storage (LDES) is economically attractive to accelerate widespread renewable energy deployment. But none of the existing energy storage technologies can meet LDES cost requirements. The newly emerged solid oxide iron air battery (SOIAB) with energy-dense solid Fe as an energy storage material is a competitive LDES-suitable technology compared to conventional counterparts. However, the performance of SOIAB is critically limited by the kinetics of Fe3O4reduction (equivalent to charging process) and the understanding of this kinetic bottleneck is significantly lacking in the literature. Here, we report a systematic kinetic study of Fe3O4-to-Fe reduction in H2/H2O environment, particularly the effect of catalyst (iridium) and supporting oxides (ZrO2and BaZr0.4Ce0.4Y0.1Yb0.1O3). With in situ created Fe3O4, the degree of reduction is measured by the change of H2O and H2concentrations in the effluent using a mass spectrometer, from which the kinetic rate constant is extracted as a function of inlet H2concentration and temperature. We find that kinetics can be nicely described by Johson-Mehl-Avrami (JMA) model. We also discuss the stepwise reduction mechanisms and activation energy for the reduction process.

     
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  3. To cut CO2emissions, we propose to directly convert shale gas into value-added products with a new H2/O2co-transport membrane (HOTM) reactor. A Multiphysics model has been built to simulate the membrane and the catalytic bed with parameters obtained from experimental validation. The model was used to compare C2 yield and CH4conversion rate between the membrane reactor and the state-of-the-art fixed-bed reactor with the same dimensions and operating conditions. The results indicate that (1) the membrane reactor is more efficient in consuming CH4for a given amount of fed O2. (2) The C2 selectivity of the membrane reactor is higher due to the gradual addition of O2into the reactor. (3) The current proposed membrane reactor can have a decent proton molar flux density but most of the proton molar flux will contribute to producing H2O on the feed side under the current operating conditions. The paper for the first-time projects the performance of the membrane reactor for combined H2O/H2removal and C2 production. It could be used as important guidance for experimentalists to design next generation natural gas conversion reactors.

     
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  4. Long duration electricity storage (LDES) with 10+ hour cycle duration is an economically competitive option to accelerate the penetration of renewable energy into the utility market. Unfortunately, none of the available energy storage technologies can meet the LDES’ requirements for duration and cost. We here report a focused kinetic study on Fe-oxide reduction process, which is a key step for solid oxide iron-air battery; the latter has been recently demonstrated as a LDES compatible battery. The study clearly shows that Ir is an excellent catalyst to boost the sluggish Fe-oxide reduction kinetics. 
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  5. Long duration electricity storage (LDES) with 10+ hour cycle duration is an economically competitive strategy to accelerate the penetration of renewable energy into the utility market. Unfortunately, none of the available energy storage technologies can meet the LDES requirements in terms of duration and cost. The newly emerged solid-oxide iron–air batteries (SOIABs) with energy-dense solid iron as an energy storage material have inherent advantages for LDES applications. Herein, we report for the first time the LDES capability of SOIABs even at a laboratory scale. We show that SOIABs with an Ir-catalyzed Fe-bed can achieve excellent energy density (625 W h kg −1 ), long cycle duration (12.5 h) and high round-trip efficiency (∼90%) under LDES-related working conditions. Given the excellent low-rate performance and the use of earth-abundant, low-cost Fe as an energy storage material, we conclude that the SOIAB is a well-suited battery technology for LDES applications. 
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