Solar hydrogen by thermochemical water splitting cycles: design, modeling, and demonstration of a novel receiver/reactor for the high temperature decomposition of zno using concentrated sunlight

Solar fuels are emerging as a viable pathway to closing the gap between sustainable fuel production and consumption in the United States and the world. Hydrogen is among the list of attractive solar fuels, and when it is produced by concentrated sunlight and water it represents an elegant and benign energy harvesting cycle. As our energy-mix becomes more diverse, through increased adoption of intermittent renewables such as solar and wind, the value of energy storage will not only become more valuable, but completely necessary. Additionally, as our current fossil fuel basis for energy storage continues to deplete, research and development into alternative fuels will accelerate. Solar fuels produced by large-scale concentrated solar power will be able to uniquely address the demands of renewability, carbon neutrality, and industrial-scale as the emerging energy economy of the 21st century matures into a sustainable, long-term approach to energy harvesting, storage, and utilization. A novel solar-thermochemical reactor has been designed and constructed for the reduction of ZnO at high temperature using concentrated sunlight as the first step in a closed, two-step thermochemical cycle to produce hydrogen from water. Abbreviated as GRAFSTRR (Gravity-Fed Solar-Thermochemical Receiver/Reactor), the 10-20kW reactor is closed to the atmosphere, and features an inverted conical-shaped reaction surface along which ZnO powder descends continuously as a moving bed, undergoing a thermochemical reaction upon exposure to highly concentrated sunlight. The reactant feed is vibration-induced, metered, and gravity-driven. Beam-down, highly concentrated sunlight enters the reaction cavity through a small aperture, and Zn product gas is siphoned into a centrally-located exit stream via a stabilized vortex flow of inert gas originating from above the aperture plane. This thesis presents the design, modeling and successful demonstration of a novel solar fuels receiver/reactor. GRAFSTRR represents the unique combination of a beam-down, cavity-type receiver/reactor with the continuous and uniform delivery of reactants to a high fraction of cavity surface area, accomplished without the use of cavity rotation. The final design and operational concept was selected after an extensive review of existing technology, combined with numerous experimental and numerical investigations into concept feasibility. Reacting particle residence time was enhanced with a symmetric and inclined cavity geometry comprised of 15 interlocking alumina tile reaction surfaces. It was found that an adequate reaction cavity temperature (1200-2000°C) could be achieved with 10-20kW of concentrated solar radiation, and that ZnO powder could be accurately and consistently delivered in the controllable range of 0-2 g/s by a custom-designed hopper and metering spline powder feeder positioned above each tile. Further, with sufficient incorporation of ceramic insulation combined with areas of active water-cooling, the reactor can operate reliably for extended periods of time at high temperature (>6 hours). Lastly, it was shown by low temperature flow visualization, that a stable vortex, created by a series of tangential and radial jets located above the aperture, and attached to the centrally located product outlet can protect the solar window from contamination, and effectively drive products off the reaction surface and into the product outlet. In addition to heat transfer modeling developed to assist in the design of GRAFSTRR, a series of models were developed to investigate a single reacting ZnO particle subject to varying particle morphologies. It was found that a reacting particle composed of an agglomeration of smaller particles can have a significantly increased reactive surface area (>70%), leading to reduced residence time required for total particle decomposition. It was shown that a particle agglomerate of 165&mgr;m diameter, comprised of 2.5&mgr;m particles, requires as little as 1.5s to fully decompose inside the reaction chamber. A computational model developed to investigate Zn product removal in the high temperature environment showed that products could be effectively removed by a vortex flow, and that a stronger vortex, driven by the tangential jet flow rate, is more effective at vapor removal than radial gas flows. Lastly, on-sun experimentation was conducted in two phases using a high-flux solar simulator to demonstrate the high temperature operation of GRAFSTRR. In the first experimental campaign, reaction cavity temperatures exceeding 1100K were created with 3.5kW of concentrated solar power delivered through the aperture plane. The reactor and all system components performed as expected, and pre-sintering of ZnO was observed on the reaction surfaces. In the second experimental campaign, 7kW of concentrated solar power was delivered to GRAFSTRR to achieve a reaction cavity temperature above 1400K. A mixture of ZnO and beech charcoal, at a molar ratio of 1:1, was fed into the reaction cavity and peak production of Zn was measured at 0.135 mol/min. Product analysis showed high Zn-content ( >75%) in samples collected from inside the quench tube apparatus.