We present an advanced thermodynamic model for a water-splitting solar reactor system employing Zr-doped ceria as the redox material and inert sweep gas to obtain the desired oxygen partial pressures in the reduction chamber. Conservation of mass and species, conservation of energy, and the Gibbs’s criteria are employed to predict solar-to-fuel efficiencies. Efficiencies vary widely with operating conditionsreactor temperatures and pressuresin addition to material thermodynamic properties, making it difficult to compare the performance of proposed redox materials. We determine the maximum efficiencies theoretically achievable with selected redox materials by simultaneous multivariable optimization of all operational parameters within their meaningful ranges. For the baseline case of zero solid heat recovery and 75% gas heat recovery, the results demonstrate that a modest efficiency improvement can be achieved by doping ceria with 10% and 15% Zr as compared to pure ceria within a narrow reduction temperature range of 1700–1850 K. However, this efficiency benefit is achieved at the cost of low oxidation temperature operation, which may lower the realistic maximum efficiencies if the oxidation step is kinetically limited. Four different reactor flow configurations are considered, including a newly developed model for counter-current flow. It is found that a maximum solar-to-fuel efficiency of 7.8% for water splitting can be attained with state-of-the-art reactors operated at 1773 K, 95% gas heat recovery, and no solid heat recovery, based on our model assumptions. In terms of potential efficiency enhancement, peak efficiencies of 26.4% and 25.2% can be achieved for inert gas sweeping and vacuum pumping, respectively, at reduction temperature of 1900 K, 95% gas heat recovery, and 90% solid heat recovery. The model results provide insights that help guide reactor design and operation as well as potential redox material selection.

We present a thermodynamic model describing the operation of solar thermochemical reduction and oxidation chambers utilizing a non-stoichiometric metal oxide. The system under consideration is a generic reactor implementing an ideal counter-current flow (CF) configuration with prescribed inlet conditions of reactant flow rates and thermodynamic states. Conservation of species and mass as well as Gibbs’s criterion are used to determine the maximum and minimum limits of oxygen non-stoichiometry for reduction and oxidation, respectively, under a CF configuration. The methodology presented here is first used to analyze the previous models appearing in literature. It is found that existing efforts to model the CF configuration can violate Gibbs’s criterion. Motivated by this, a revised CF model is formulated and ensures the criterion is met thereby ensuring process spontaneity of the desired reaction for all conditions existing within the reaction chambers. The model identifies the highest reduction (oxidation) extent possible for a given inlet condition of the reduction (oxidation) chamber. This work offers an enhanced understanding of the CF flow configuration that will lead to more realistic estimates of the upper limit on solar-to-fuel efficiency for a reactor system.

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There are a number of technical and socioeconomic factors converging to position photovoltaic (PV) windows as a transformative technology for the energy landscape of the future: 1) Urban areas currently account for 67-76% of global final energy consumption. 2) 70% of the world's population will live in urban areas by 2050. 3) The overwhelming architectural trend is away from opaque building components and toward all-glass facades. 4) Photovoltaics are becoming extremely affordable, and the most expensive components in a conventional module are the glass and transparent metals-components that are already in highly-insulating glazing. 5) Buildings are increasingly built to easily integrate with photovoltaic energy generation using DC microgrids and on-site energy generation to balance high demand on the grid. Rational design of PV windows is of paramount importance to realize their impact. In this work, we provide an analysis on the theoretical performance of PV windows using a detailed balance model to understand the complex design space of power conversion efficiency, visible light transmittance, solar heat gain coefficient, and color. We find there are two distinct regimes for PV window absorber design. The first lowvisible light transmittance regime validates the most prevalent approach to semitransparent PV windows in which conventional absorber materials (Si, CIGS, CdTe, CZTS, perovskite, etc.) are thinned to allow visible light to pass through. In this thinned-absorber regime, an ideal bandgap of ~1.35 eV maximizes performance, which is consistent with the famous Shockley-Queisser limit. However, we identify a second, high visible light transmittance regime in which the ideal bandgap for maximum power conversion efficiency increases monotonically from 2 to 3 eV with increasing visible light transmittance. In this tuned-bandgap regime, the solar cell exhibits lower losses and tunable solar heat gain and color.

Urban centers across the globe are responsible for a significant fraction of energy consumption and CO2 emission. As urban centers continue to grow, the popularity of glass as cladding material in urban buildings is an alarming trend. Dynamic windows reduce heating and cooling loads in buildings by passive heating in cold seasons and mitigating solar heat gain in hot seasons. Here, reduced energy consumption in highly glazed buildings in a mesoscopic building energy model is demonstrated when thermochromic windows are employed. Savings are realized across eight disparate climate zones of the United States. The model is used to determine ideal critical transition temperatures of 20–27.5 °C for thermochromic windows based on metal halide perovskite materials. Ideal transition temperatures are realized experimentally in composite metal halide perovskite films composed of perovskite crystals and an adjacent reservoir phase. The transition temperature is controlled by cointercalating methanol, instead of water, with methylammonium iodide and tailoring the hydrogen‐bonding chemistry of the reservoir phase. Thermochromic windows based on metal halide perovskites represent a clear opportunity to mitigate the effects of energy‐hungry buildings.

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A thermodynamic model of an isothermal ceria-based membrane reactor system is developed for fuel production via solar-driven simultaneous reduction and oxidation reactions. Inert sweep gas is applied on the reduction side of the membrane. The model is based on conservation of mass, species, and energy along with the Gibbs criterion. The maximum thermodynamic solar-to-fuel efficiencies are determined by simultaneous multivariable optimization of operational parameters. The effects of gas heat recovery and reactor flow configurations are investigated. The results show that maximum efficiencies of 1.3% (3.2%) and 0.73% (2.0%) are attainable for water splitting (carbon dioxide splitting) under counter- and parallel-flow configurations, respectively, at an operating temperature of 1900 K and 95% gas heat recovery effectiveness. In addition, insights on potential efficiency improvement for the membrane reactor system are further suggested. The efficiencies reported are found to be much lower than those reported in literature. We demonstrate that the thermodynamic models reported elsewhere can violate the Gibbs criterion and, as a result, lead to unrealistically high efficiencies. The present work offers enhanced understanding of the counter-flow membrane reactor and provides more accurate upper efficiency limits for membrane reactor systems.