157 research outputs found

    Morphology Engineering of Porous Media for Enhanced Solar Fuel and Power Production

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    The favorable and adjustable transport properties of porous media make them suitable components in reactors used for solar energy conversion and storage processes. The directed engineering of the porous media's morphology can significantly improve the performance of these reactors. We used a multiscale approach to characterize the changes in performance of exemplary solar fuel processing and solar power production reactors incorporating porous media as multifunctional components. The method applied uses imaging-based direct numerical simulations and digital image processing in combination with volume averaging theory to characterize the transport in porous media. Two samples with varying morphology (fibrous vs. foam) and varying size range (mm vs. mu m scale), each with porosity between 0.46 and 0.84, were characterized. The obtained effective transport properties were used in continuum-scale models to quantify the performance of reactors incorporating multifunctional porous media for solar fuel processing by photoelectrochemical water splitting or power production by solar thermal processes

    Numerical Quantification of Coupling Effects for Radiation-Conduction Heat Transfer in Participating Macroporous Media: Investigation of a Model Geometry

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    Radiative-conductive heat transfer in porous media is usually investigated by decoupling the heat transfer modes and solving the volume-averaged continuum equations using effective transport properties. However, both modes are naturally coupled and coupling effect might significantly affect the results. We aim at providing quantitative understanding of the coupling effects occurring in a model geometry. This is an important first step towards improving the accuracy of heat transfer predictions in engineering applications. We developed a numerical method using a structured mesh and cell centered finite volumes and Monte Carlo ray tracing techniques in order to simulate the 3-dimensional and unsteady coupled radiative-conductive heat transfer in semitransparent macroporous media. We have optimized the numerical method with regards to memory and computational requirements leading to optimal performance and allowing to perform a parameter variation study for various steady state cases. We conducted a parameter study considering different optical and thermal material properties and boundary conditions in order to quantify the coupling effect between conduction and radiation, and to demonstrate its dependencies. In terms of thermal properties, it was found that the ratio of bulk thermal conductivities is governing the coupling effect. A distinct peak at a given conductivity ratio was found. The influence of optical properties is discussed in details. It was found that a significant coupling effect exists, reaching up to 15% of the total thermal heat flux. The verified modeling framework in conjunction with our non-dimensionalization offers a tool to investigate the importance of radiationconduction coupling in a quantitative manner. It is an important step towards understanding the detailed mechanisms of radiation and conduction coupling and provides engineering guidelines on the importance of these effects

    Design guidelines for concentrated photo-electrochemical water splitting devices based on energy and greenhouse gas yield ratios

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    Device and system design choices for solar energy conversion and storage approaches require holistic design guidelines which simultaneously respect and optimize technical, economic, sustainability, and operating time constraints. We developed a simulation platform which allows for the calculation of solar-to-hydrogen efficiency, hydrogen price, device manufacture and operation energy demand, and the component degradation and replacement time of photo-electrochemical water splitting devices. Utilizing this platform, we assessed 16 different design types representing all possible combinations of a system: (i) operating with or without irradiation concentration, (ii) utilizing high-performing and highcost or low-performing but low-cost photoabsorbers, (iii) utilizing high-performing and high-cost or low-performing but low-cost electrocatalysts, and (iv) operating with or without current concentration between the photoabsorber and the electrocatalyst. Our results show that device types exist with a global optimum (a Pareto point), simultaneously maximizing efficiency, while minimizing cost and the energy demand of manufacture and operation. In our examples, these happen to be the device types utilizing high irradiation concentration, as well as expensive photoabsorbers and electrocatalysts. These device types and designs were the most robust to degradation, exhibiting the smallest price sensitivity for increasing degradation rates. Other device types did not show a global optimum, but rather a set of partially optimized designs, i.e. a Pareto front, requiring a compromise and prioritization of either performance, cost, or manufacture and operation energy demand. In our examples, these happen to be the device types using low-cost photoabsorbers. The targeted utilization of irradiation and current concentration predicted that even device types utilizing expensive components can provide competitive solutions to photo-electrochemical water splitting. The quantification of the influence of component degradation on performance allows the suggestion of best practice for device operational time and component replacement. The framework and findings presented here provide holistic design guidelines for photo-electrochemical devices, and support the decision-making process for an integral and practical approach to competitive solar hydrogen production in the future

    Morphology Engineering of Porous Media for Enhanced Solar Fuel and Power Production

    Get PDF
    The favorable and adjustable transport properties of porous media make them suitable components in reactors used for solar energy conversion and storage processes. The directed engineering of the porous media's morphology can significantly improve the performance of these reactors. We used a multiscale approach to characterize the changes in performance of exemplary solar fuel processing and solar power production reactors incorporating porous media as multifunctional components. The method applied uses imaging-based direct numerical simulations and digital image processing in combination with volume averaging theory to characterize the transport in porous media. Two samples with varying morphology (fibrous vs. foam) and varying size range (mm vs. ÎĽm scale), each with porosity between 0.46 and 0.84, were characterized. The obtained effective transport properties were used in continuum-scale models to quantify the performance of reactors incorporating multifunctional porous media for solar fuel processing by photoelectrochemical water splitting or power production by solar thermal processe

    Techno-economic modeling and optimization of solar-driven high-temperature electrolysis systems

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    We present a techno-economic analysis of solar-driven hightemperature electrolysis systems used for the production of hydrogen and synthesis gas. We consider different strategies for the incorporation of solar energy, distinguished by the use of differing technologies to provide solar power and heat: i) thermal approaches (system 1) using concentrated solar technologies to provide heat and to generate electricity through thermodynamic cycles, ii) electrical approaches (system 2) using photovoltaic technologies to provide electricity and to generate heat through electrical heaters, and iii) hybrid approaches (system 3) utilizing concentrated solar technologies and photovoltaics to provide heat and electricity. We find that system 3 generates hydrogen at a high efficiency (ηSTF = 9.9%, slightly lower than the best performing system 1 with 10.6%) and at a low cost (Cfuel = $4.9/kg, lowest cost of all three systems) at reference conditions, providing evidence for the competitiveness of this hybrid approach for scaled solar hydrogen generation. Sensitivity analysis indicates an optimal working temperature for system 3 of 1350 K, which balances the increased thermal receiver losses with the reduced electrolysis cell potential when increasing the temperature. Lower working pressure always favors high system efficiency and low cost. The working current densities for thermoneutral voltage were determined for various temperature and pressure combinations, and trends for efficient and cost-effective thermoneutral operation were identified. The water conversion extent was optimized to avoid mass transport limitations in the electrodes while ensuring large fuel generation rates. For synthesis gas production, a H2/CO molar ratio of 2 can be achieved by tuning the inlet feeding molar ratio of CO2/H2O, temperature, and pressure. This study introduces a flexible simulation framework of solar-driven high-temperature electrolysis systems allowing for the assessment of competing solar integration approaches and for the guidance of the operational conditions maximizing efficiency and minimizing cost, providing pathways for scalable solar fuel processing

    Optical characterization of multi-scale morphologically complex heterogeneous media – Application to snow with soot impurities

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    A multi-scale methodology for the radiative transfer analysis of heterogeneous media composed of morphologically-complex components on two distinct scales is presented. The methodology incorporates the exact morphology at the various scales and utilizes volume-averaging approaches with the corresponding effective properties to couple the scales. At the continuum level, the volume-averaged coupled radiative transfer equations are solved utilizing i) effective radiative transport properties obtained by direct Monte Carlo simulations at the pore level, and ii) averaged bulk material properties obtained at particle level by Lorenz-Mie theory or discrete dipole approximation calculations. This model is applied to a soot-contaminated snow layer, and is experimentally validated with reflectance measurements of such layers. A quantitative and decoupled understanding of the morphological effect on the radiative transport is achieved, and a significant influence of the dual-scale morphology on the macroscopic optical behavior is observed. Our results show that with a small amount of soot particles, of the order of 1ppb in volume fraction, the reduction in reflectance of a snow layer with large ice grains can reach up to 77% (at a wavelength of 0.3 ÎĽm). Soot impurities modeled as compact agglomerates yield 2-3% lower reduction of the reflectance in a thick show layer compared to snow with soot impurities modeled as chain-like agglomerates. Soot impurities modeled as equivalent spherical particles underestimate the reflectance reduction by 2-8%. This study implies that the morphology of the heterogeneities in a media significantly affects the macroscopic optical behavior and, specifically for the soot-contaminated snow, indicates the non-negligible role of soot on the absorption behavior of snow layers. It can be equally used in technical applications for the assessment and optimization of optical performance in multi-scale media

    Modeling of concurrent CO2 and water splitting by practical photoelectrochemical devices

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    Concurrent solar generation of hydrogen and CO through photoelectrochemical (PEC) water and CO2 electrolysis, and the subsequent use of the product gas mixture in conventional Fischer-Tropsch processes, has the potential to provide a flexible pathway for direct solar generation of a variety of liquid fuels. In order for this approach to be practical, PEC devices must be designed to continuously and selectively provide awell-defined ratio of hydrogen to CO, independent of operating conditions.We develop a computational PEC device model providing insight into the dynamics and design requirements of such a device.We investigate a variety of combinations of catalysts (Ag, Cu, Ni, Pt, Co) and photoabsorbers (Si and Ga-based) under steady and transient solar irradiation conditions. Typical H2/CO ratios of 0.1 were observed for Ag-based electrodes, and ratios of 5 when using Cu-based electrodes. Variation in catalyst and photoabsorber properties provided guidance for the development of catalysts allowing for a H2/CO product ratio close to 2. Device design variations and the addition of Ni as a second cathode-side catalyst improved the generation of hydrogen, allowing H2/CO ratios to reach between 1.7 and 2.15. Transient simulations showed that product ratios vary significantly over the day and year, implying the use of storage or controlling measures or the addition of a water gas shift reactor. Our model provides insights and practical considerations for the design and implementation of a PEC device for the concurrent production of hydrogen and CO

    Optimizing Mesostructured Silver Catalysts for Selective Carbon Dioxide Conversion into Fuels

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    Mesostructured silver catalysts have shown a substantial rise in reaction selectivity for the conversion of CO2 into fuels with increasing thickness of ordered, inverse-opal films. The challenge lies in the optimization of the morphology to maximize the CO selectivity. We developed a 3D mass transport model utilizing the accurate electrode geometry to calculate local concentration distributions of CO2(aq), OH-, HCO3-, and CO32- by considering the buffer reactions in the electrolyte and modeling local catalytic surface reaction rates based on Butler-Volmer correlations. Validated with experimental data from the literature, the model predicted the suppression of the hydrogen evolution reaction with an inverse dependency on the hydroxide concentration and the promotion of the CO evolution reaction with a proportional dependency on the carbonate concentration. In order to increase the CO selectivity, we developed design guidelines that suggest high electrode roughness per film thickness, which translates to smaller pore size in practice. Further, the shallow pores of the electrode strongly reduced the overall CO selectivity as the mass transport to the bulk was non-limiting. We demonstrated that the introduction of an additional diffusion layer on top of the silver electrode can enhance the CO selectivity from as low as 39% to more than 90%

    Solar fuel processing efficiency for ceria redox cycling using alternative oxygen partial pressure reduction methods

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    Solar-driven non-stoichiometric thermochemical redox cycling of ceria for the conversion of solar energy into fuels shows promise in achieving high solar-to-fuel efficiency. This efficiency is significantly affected by the operating conditions, e.g. redox temperatures, reduction and oxidation pressures, solar irradiation concentration, or heat recovery effectiveness. We present a thermodynamic analysis of five redox cycle designs to investigate the effects of working conditions on the fuel production. We focused on the influence of approaches to reduce the partial pressure of oxygen in the reduction step, namely by mechanical approaches (sweep gassing or vacuum pumping), chemical approaches (chemical scavenger), and combinations thereof. The results indicated that the sweep gas schemes work more efficient at nonisothermal than isothermal conditions, and efficient gas phase heat recovery and sweep gas recycling was important to ensure efficient fuel processing. The vacuum pump scheme achieved best efficiencies at isothermal conditions, and at non-isothermal conditions heat recovery was less essential. The use of oxygen scavengers combined with sweep gas and vacuum pump schemes further increased the system efficiency. The present work can be used to predict the performance of solar-driven non-stoichiometric redox cycles and further offers quantifiable guidelines for system design and operation
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