We describe and analyze the efficiency of a new solar-thermochemical reactor concept, which employs a moving packed bed of reactive particles produce of H2 or CO from solar energy and H2O or CO2. The packed bed reactor incorporates several features essential to achieving high efficiency: spatial separation of pressures, temperature, and reaction products in the reactor; solid–solid sensible heat recovery between reaction steps; continuous on-sun operation; and direct solar illumination of the working material. Our efficiency analysis includes material thermodynamics and a detailed accounting of energy losses, and demonstrates that vacuum pumping, made possible by the innovative pressure separation approach in our reactor, has a decisive efficiency advantage over inert gas sweeping. We show that in a fully developed system, using CeO2 as a reactive material, the conversion efficiency of solar energy into H2 and CO at the design point can exceed 30%. The reactor operational flexibility makes it suitable for a wide range of operating conditions, allowing for high efficiency on an annual average basis. The mixture of H2 and CO, known as synthesis gas, is not only usable as a fuel but is also a universal starting point for the production of synthetic fuels compatible with the existing energy infrastructure. This would make it possible to replace petroleum derivatives used in transportation in the U.S., by using less than 0.7% of the U.S. land area, a roughly two orders of magnitude improvement over mature biofuel approaches. In addition, the packed bed reactor design is flexible and can be adapted to new, better performing reactive materials.

High-resolution neutron radiography was used to image an operating proton exchange membrane fuel cell in situ. The crosssectional liquid water profile of the cell was quantified as a function of cell temperature, current density, and anode and cathode gas feed flow rates. Detailed information was obtained on the cross-sectional water content in the membrane electrode assembly and the gas flow channels. At low current densities, liquid water tended to remain on the cathode side of the cell. Significant liquid water in the anode gas flow channel was observed when the heat and water production of the cell were moderate, where both water diffusion from the cathode and thermal gradients play a significant role in determining the water balance of the cell. Within the membrane electrode assembly itself, the cathode side was moderately more hydrated than the anode side of the assembly from 0.1 to 1.25 A cm −2 . The total liquid water content of the membrane electrode assembly was fairly stable between current densities of 0.25 and 1.25 A cm −2 , even though the water in the gas flow channels changed drastically over this current density range. At 60°C, the water content in the center of the gas diffusion layer was depleted compared to the membrane or gas flow channel interfaces. This phenomenon was not observed at 80°C where evaporative water removal is prevalent.

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Neutron imaging experiments were carried out to measure the water content of an operating proton exchange membrane fuel cell ͑PEMFC͒ under varying conditions of current density and temperature. It was found that the water content of the PEMFC is strongly coupled to the current density and temperature of the cell. These measurements indicate that changes in water content lag changes in current density by at least 100 s, both when the current density was increased and decreased. Less liquid water was measured in the cells when operating at 80°C than at 40°C. At 60°C cell temperature, a peak in water content was observed around 650 mA/cm 2 and the water content was found to decrease with increasing current density. This is explained in the context of cell heating by performing a simple thermal analysis of an operating PEMFC so as to yield quantitative information on the waste heat and its effects on the liquid water contained in the cell.Understanding liquid water content and its distribution within an operating proton exchange membrane fuel cell ͑PEMFC͒ is critical to designing high-performance systems and formulating rational models for simulating PEMFC behavior. The generation, transport, and removal of liquid water are key phenomena that occur in a PEMFC. Effective water transport through and removal from the membrane electrode assembly ͑MEA͒ is crucial to achieving high current density and maintaining PEMFC performance. In the design and optimization of PEMFCs, it is important to be able to quantify the water content in an operating cell in order to gain insight into the dominant phenomena or processes that influence liquid water transport and removal. This work is concerned with the measurement of liquid water in an operating PEMFC under various temperatures, relative humidities, and current densities. Neutron imaging, or radiography, is a useful tool for gaining qualitative and quantitative insight into liquid water content and distribution in near real-time ͑temporal resolution ϳ1 s͒.Both Tuber et al. 1 and Yang et al. 2 used optical methods for imaging water in PEMFCs under a range of operating conditing. In order to use optical techniques, a transparent fuel cell must be fabricated. Optical imaging is capable of high spatial and temporal resolutions for the elucidation of dynamic processes, but optical techniques suffer from fogging of the transparent window under humidified conditions and it is more difficult ͑though possible͒ to obtain quantitative information. Furthermore, optical investigations are limited to studying liquid water in the gas flow channels because that is the only visible water in the fuel cell; liquid water inside the gas diffusion layers ͑GDLs͒ cannot be imaged using optical techniques. Tuber et al. 1 were able to correlate the appearance of water in the cell with a drop in current density, although they did not quantify the liquid water in the cell. Yang et al. 2 focused on the appearance and dynamics of liquid water droplet formation and breakup in the gas flow channels. Their work ...

Thermochemical cycles are a type of heat engine that utilize high-temperature heat to produce chemical work. Like their mechanical work producing counterparts, their efficiency depends on the operating temperature and on the irreversibility of their internal processes. With this in mind, we have invented innovative design concepts for two-step solar-driven thermochemical heat engines based on iron oxide and iron oxide mixed with other metal oxide (ferrites) working materials. The design concepts utilize two sets of moving beds of ferrite reactant materials in close proximity and moving in opposite directions to overcome a major impediment to achieving high efficiency—thermal recuperation between solids in efficient countercurrent arrangements. They also provide an inherent separation of the product hydrogen and oxygen and are an excellent match with a high-concentration solar flux. However, they also impose unique requirements on the ferrite reactants and materials of construction as well as an understanding of the chemical and cycle thermodynamics. In this paper, the counter-rotating-ring receiver∕reactor∕recuperator solar thermochemical heat engine concept is introduced, and its basic operating principles are described. Preliminary thermal efficiency estimates are presented and discussed. Our results and development approach are also outlined.