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Abstract Solar energy is the most sustainable alternative to fossil fuels. The production of solar thermochemical fuels from water/carbon dioxide not only overcomes the intermittent nature of solar energy, but also allows for flexible transportation and distribution. In this paper, the challenges for solar thermochemical H2/CO production are reviewed. New perspectives and insights to overcome these challenges are presented. For two-step cycles, the main challenges are high temperatures, low conversions and the intensive oxygen removal work. Theoretically feasible temperature and pressure ranges are needed to develop reactant materials. The fundamental mechanism to reduce the temperature and the potential to improve the efficiency by minimizing the oxygen removal work need be revealed. Various material modification strategies and advanced reactors are proposed to improve the efficiency by reducing the temperature and enhancing heat transfer process. But the oxygen removal work required has not been minimized. For multi-step cycles, the main challenges are the separation of corrosive acid and insufficient reaction kinetics. For the separation of acids, many methods have been proposed. But these methods require extra energy and causes undesired side reactions or byproducts. The reaction kinetics have been enhanced by improving catalysts with noble materials or complex fabrication methods. Developing novel multi-step cycles using metal oxides, hydroxides and carbonates may be promising.

This review investigates the effect of different dopants on the oxygen evolution and carbon dioxide splitting abilities of the lanthanum manganites. Particular focus was placed on the lanthanide, alkaline earth metals, redox-active transition metal, and non-redox active Group 3 metals. The review suggests that a small ionic radius lanthanide on the A-site can increase the size discrepancy, leading to Mn-O6 octahedra tilting and more facile Mn-O bond breaking. Doping the A-site with a divalent alkaline earth element can increase the valance of the transition metal, leading to greater reduction capabilities. A transition metal with one electron in the eg orbital is the most effective for reduction while for oxidation, zero electrons in the high-energy eg orbitals is optimal. Finally, doping of the B-site with metals such as gallium or aluminium aids in sintering resistance and allows reactivity to remain constant over multiple cycles. Higher reduction temperatures and moderate re-oxidation temperatures also promote higher fuel yields as does the active reduction of the perovskite under hydrogen, although the total energy consumption implications of this are unknown. Far more is known about the mechanism of the reduction reaction than the oxidation reaction, therefore more research in this area is required.

We investigated, herein, the redox activity of partial substitution of the B-site in a series of lanthanum/strontium-manganese-based (LSM) perovskite oxide, La0.7Sr0.3Mn0.9X0.1O3 for solar two-step thermochemical fuel production using concentrated solar radiation as an energy source. We systematically investigated the effects of partial substitution in LaSrMnO3 in terms of their kinetics behavior, oxygen/CO productivity, thermal reduction/oxidation temperatures. Furthermore, repeatability was evaluated and compared among the samples prepared using the same procedure and studied using the same test method. We observed and evaluated the long-term thermal stability of the redox activity and valence variation of the constituting ionic species of the perovskite in the two-step thermochemical CO2 splitting. From the perspectives of superior activity and long-term repeatability, Ni-, Co-, and Mg-substituted LSM perovskites are promising for thermochemical two-step CO2/H2O splitting to produce synthetic gas.

Converting CO2 into fuels via solar-driven thermochemical cycles of metal oxides is promising to address global climate change and energy crisis challenges simultaneously. However, it suffers from low energy conversion efficiency (ηen) due to high sensible heat losses when swinging between reduction and oxidation cycles, and a single product of fuels can hardly meet multiple kinds of energy demands. Here, we propose an alternative way to upsurge energy conversion efficiency by integrating solar thermochemical CO2 splitting with a supercritical CO2 thermodynamic cycle. When gas phase heat recovery (εgg) is equal to 0.9, the highest energy conversion efficiency of 20.4% is obtained at the optimal cycle high pressure of 260 bar. In stark contrast, the highest energy conversion efficiency is only 9.8% for conventional solar thermochemical CO2 splitting without including a supercritical CO2 cycle. The superior performance is attributed to efficient harvesting of waste heat and synergy of CO2 splitting cycles with supercritical CO2 cycles. This work provides alternative routes for promoting the development and deployment of solar thermochemical CO2 splitting techniques.

Abstract In this study, authors present a new hybrid nuclear small modular reactor system assisted with wind energy for net zero emissions trigeneration system. Small modular reactors bring multiple advantages including (a) improved thermal efficiency, (b) better building efficiency due to modularity, and (c) less operation and maintenance costs compared to standard nuclear power generation. Furthermore, the greenhouse gas emissions from small modular reactors are lower than regular counterparts. This study hybridizes small modular reactors with wind turbines for producing three useful commodities, namely electricity, hydrogen, and hot water. A two‐step high‐temperature thermochemical cycle (based on hydrogen chloride gas) is used for hydrogen production, and its performance in terms of energy and exergy efficiencies is evaluated. Additionally, the exergy and energy analyses (by writing balance equations for each component of the system) are carried out to determine the thermodynamic feasibility of the proposed system. In order to observe the effects of various parameters such as the temperature of the thermochemical cycle steps, inlet gas turbine temperature, the pressure ratio of the gas turbine, actual wind speed, and current density on the system performance, a detailed parametric study is conducted. The results of this study show that the overall system can achieve an energy efficiency of about 57.5% and exergy efficiency of about 38.1%.

Non-abatable emissions are one of the decarbonization challenges that could be addressed with carbon-neutral fuels. One promising production pathway is the direct air capture (DAC) of carbon dioxide, followed by a solar thermochemical cycle and liquid fuel synthesis. In this study, we explore different combinations of these technologies to produce methanol from an economic perspective in order to determine the most efficient one. For this purpose, a model is built and simulated in Aspen Plus®, and a solar field is designed and sized with HFLCAL®. The inherent dynamics of solar irradiation were considered with the meteorological data from Meteonorm® at the chosen location (Riyadh, Saudi Arabia). Four different integration strategies are assessed by determining the minimum selling price of methanol for each technology combination. These values were compared against a baseline with no synergies between the DAC and the solar fuels production. The results show that the most economical methanol is produced with a central low-temperature DAC unit that consumes the low-quality waste heat of the downstream process. Additionally, it is determined with a sensitivity analysis that the optimal annual production of methanol is 11.8 kt/y for a solar field with a design thermal output of 280 MW.

This review explores the advances in the synthesis of ceria materials with specific morphologies or porous macro- and microstructures for the solar-driven production of carbon monoxide (CO) from carbon dioxide (CO2). As the demand for renewable energy and fuels continues to grow, there is a great deal of interest in solar thermochemical fuel production (STFP), with the use of concentrated solar light to power the splitting of carbon dioxide. This can be achieved in a two-step cycle, involving the reduction of CeO2 at high temperatures, followed by oxidation at lower temperatures with CO2, splitting it to produce CO, driven by concentrated solar radiation obtained with concentrating solar technologies (CST) to provide the high reaction temperatures of typically up to 1,500°C. Since cerium oxide was first explored as a solar-driven redox material in 2006, and to specifically split CO2 in 2010, there has been an increasing interest in this material. The solar-to-fuel conversion efficiency is influenced by the material composition itself, but also by the material morphology that mostly determines the available surface area for solid/gas reactions (the material oxidation mechanism is mainly governed by surface reaction). The diffusion length and specific surface area affect, respectively, the reduction and oxidation steps. They both depend on the reactive material morphology that also substantially affects the reaction kinetics and heat and mass transport in the material. Accordingly, the main relevant options for materials shaping are summarized. We explore the effects of microstructure and porosity, and the exploitation of designed structures such as fibers, 3-DOM (three-dimensionally ordered macroporous) materials, reticulated and replicated foams, and the new area of biomimetic/biomorphous porous ceria redox materials produced from natural and sustainable templates such as wood or cork, also known as ecoceramics.

The present study provides a thermodynamic analysis of solar thermochemical cycles for splitting of H2O or CO2. Such cycles, powered by concentrated solar energy, have the potential to produce fuels in a sustainable way. We extend a previous study on the thermodynamics of water splitting by also taking into account CO2 splitting and the influence of the solar absorption efficiency. Based on this purely thermodynamic approach, efficiency trends are discussed. The comprehensive and vivid representation in T-S diagrams provides researchers in this field with the required theoretical background to improve process development. Furthermore, results about the required entropy change in the used redox materials can be used as a guideline for material developers. The results show that CO2 splitting is advantageous at higher temperature levels, while water splitting is more feasible at lower temperature levels, as it benefits from a great entropy change during the splitting step.