Frontiers in Energy最新文献

Japan aims to establish an international hydrogen supply chain by utilizing low-cost and abundantly available hydrogen sources and liquid hydrogen carriers to realize a future hydrogen economy that will enhance energy security and help achieve carbon neutrality. While hydrogen does not emit CO₂ when used as a fuel to generate energy, CO₂ emissions can be attributed to hydrogen due to the energy and other resources required at each stage of the hydrogen supply chain. Therefore, from a life cycle perspective, if hydrogen is to contribute to the world’s carbon neutrality goal, the entire hydrogen supply chain must be low-carbon. This paper explores the life cycle CO₂ emissions of international hydrogen supply chains envisaged by Japan. The target supply chains involve hydrogen produced from renewable electricity via electrolysis, as well as from fossil fuels with carbon capture and storage, sourced from resource-rich countries and imported to Japan using liquid hydrogen carriers such as liquid hydrogen, methylcyclohexane (MCH), and ammonia (NH₃). In addition, this paper addresses potential options for reducing life cycle CO₂ emissions to effectively establish a low-carbon hydrogen supply chain.

Direct air capture (DAC) is an emerging technology aimed at mitigating global warming. However, conventional DAC technologies and the subsequent utilization processes are complex and energy-intensive. An integrated system of direct air capture and utilization (IDACU) via in-situ catalytic conversion to fuels and chemicals is a promising approach, although it remains in the early stages of development. This review examines the current technical routes of IDACU, including solid-based dual-functional materials (DFMs) through thermo-catalysis, IDACU using liquid sorbents with thermo-catalysis, and non-thermal conversion methods. It covers the basic principles, reaction conditions, main products, material types, and the existing problems and challenges associated with these technical routes. Additionally, it discusses the recent advancements in solid-based DFMs for IDACU, with particular attention to the differences in material characteristics between carbon capture from flue gases (ICCU) and DAC. While IDACU technology holds significant promise, it still faces numerous challenges, especially in the design of advanced materials.

The curved bending regions of serpentine flow channels play a crucial role in mass transfer and the overall performance of the flow field in proton exchange membrane fuel cells (PEMFCs). This paper proposes a “2D Topology-Curvature Optimization” progressive design method to optimize the bend area structures, aiming to enhance PEMFC performance. Through numerical simulations, it compares the topology-curvature optimization model with both the algorithm-based optimization model and a validation model, and analyzes the mass transfer, heat transfer characteristics, and output performance of PEMFC under different flow fields. The results indicate that the optimized structures improve convection and diffusion within the flow field, effectively enhancing the transport and distribution of oxygen and water within the PEMFC. Performance improvements, ranked from highest to lowest, are TS-III > MD-G (Model-GA) > MD-P (Model-PSO) > TS-II > TS-I. Among the optimized models, TS-III (Topology Structure-III) exhibits the greatest increases in peak current density and peak power density, with improvement of 4.72% and 3.12%, respectively. When considering the relationship between performance improvement and pressure drop using the efficiency evaluation criterion (EEC), TS-II demonstrates the best overall performance.

Compressed CO₂ energy storage (CCES) system has received widespread attention due to its superior performance. This paper proposes a novel CCES concept based on gas-liquid phase change and cold-electricity cogeneration. Thermodynamic and exergoeconomic analyses are performed under simulation conditions, followed by an investigation of the impacts of various decision parameters on the proposed system. Next, a multi-objective optimization is conducted with the total energy efficiency and total product unit cost as the objective functions. Finally, brief comparisons are made between the proposed system and existing systems. The results indicate that the total energy efficiency of the proposed system reaches 79.21% under the given simulation conditions, outperforming the electrical efficiency of 61.27%. Additionally, the total product unit cost of the system is 25.61 $/GJ. A key component, T1, plays an important role due to its large exergy destruction rate (1.0591 MW) and total investment cost rate (154.85 $/h). Despite this, the exergoeconomic factors of T1 is only 41.08%, indicating that investing in T1 to improve the efficiency is practicable. The analysis shows that a lower CO₂ condensation temperature benefits the proposed system performance. While improving the isentropic efficiencies of the compressors and turbines enhances total energy efficiency, excessive isentropic efficiencies can lead to a significant increase in total product unit cost. Through multi-objective optimization, an optimal favorable operating condition is identified, yielding a compromise result with a total energy efficiency of 111.91% and a total product unit cost of 28.35 $/GJ. The proposed CCES system efficiently delivers both power and cooling energy, demonstrating clear superiorities over previous systems.

Proton exchange membrane (PEM) electrolyzer (EL) is regarded as a promising technology for hydrogen generation, offering load flexibility for electric grids (EGs), especially those with a high penetration of renewable energy (RE) sources. This paper proposes a PEM-focused economic dispatch strategy for EG integrated with wind-electrolysis systems. Existing strategies commonly assume a constant efficiency coefficient to model the EL, while the proposed strategy incorporates a bottom-up PEM EL model characterized by a part-load efficiency curve, which accurately represents the nonlinear hydrogen production performance, capturing efficiency variations at different loads. To model this, it first establishes a 0D electrochemical model to derive the polarization curve. Next, it accounts for the hydrogen and oxygen crossover phenomena, represented by the Faraday efficiency, to correct the stack efficiency curve. Finally, it includes the power consumption of ancillary equipment to obtain the nonlinear part-load system efficiency. This strategy is validated using the PJM-5 bus test system with coal-fired generators (CFGs) and is compared with a simple EL model using constant efficiency under three scenarios. The results show that the EL modeling method significantly influences both the dispatch outcome and the economic performance. Sensitivity analyses on coal and hydrogen prices indicate that, for this case study, the proposed strategy is economically advantageous when the coal price is below 121.6 $/tonne. Additionally, the difference in total annual operating cost between using the efficiency curve anda constant efficiency to model becomes apparent when the hydrogen price ranges from 2.9 to 5.4 $/kg.

The solid oxide fuel cell (SOFC) power system fueled by NH₃ is considered one of the most promising solutions for achieving ship decarbonization and carbon neutrality. This paper addresses the technical challenges faced by NH₃ fuel SOFC ship power system, including slow hydrogen (H₂) production, low efficiency, and limited space. It introduces an innovative a NH₃-integrated reactor for rapid H₂ production, establishes a safe and efficient all-electric SOFC all-electric propulsion system adaptable to various sailing conditions. The system is validated using a 2 kW prototype experimental rig. Results show that the SOFC system, designed for a target ship, has a rated power of 96 kW and an electrical efficiency of 60.13%, meeting the requirements for rated cruising conditions. Under identical catalytic scenarios, the designed reactor, with highly efficient heat transfer, measuring 1.1 m in length, can achieve complete NH₃ decomposition within 2.94 s, representing a 35% reduction in cracking time and a 42% decrease in required cabin space. During high-load voyage conditions, adjusting the circulation ratio (CR) and ammonia-oxygen ratio (A/O) improves system efficiency across a wide operational range. Among these adjustments, altering the A/O ratio proves to be the most efficient strategy. Under this configuration, the system achieves an efficiency of 55.02% at low load and 61.73% at high load, allowing operation across a power range of 20% to 110%. Experimental results indicate that the error for NH₃ cracking H₂ is less than 3% within the range of 570–700 °C, which is relevant to typical ship operation scenarios. At 656 °C, the NH₃ cracking H₂ rate reaches 100%. Under these conditions, the SOFC produces 2.045 kW of power with an efficiency of approximately 58.66%. The noise level detected is 58.6 dB, while the concentrations of CO₂, NO, and SO₂ in the flue gas approach zero. These findings support the transition of the shipping industry to green, clean systems, contributing significantly to future reductions in ocean carbon emissions.

Phase change energy storage technology has great potential for enhancing the efficient conversion and storage of energy. While triply periodic minimal surface (TPMS) structures have shown promise in improving heat transfer, research on their application in phase change heat transfer remains limited. This paper presents numerical simulations of composite phase change materials (PCMs) featuring TPMS skeletons, specifically gyroid, diamond, primitive, and I-graph and wrapped package-graph (I-WP) utilizing the lattice Boltzmann method (LBM). A comparative analysis of the effects of four TPMS skeletons on enhancing the phase change process reveals that the PCM containing the gyroid skeleton melts the fastest, with a complete melting time of 24.1% shorter than that of the PCM containing the I-WP skeleton. The PCM containing the gyroid skeleton is further simulated to explore the effects of the Rayleigh (Ra) number, Prandtl (Pr) number, and Stefan (Ste) number on the melting characteristics. Notably, the complete melting time is reduced by 60.44% when Ra is increased to 10⁶ compared to the case with Ra at 10⁴. Increasing the Pr number accelerates the migration of the mushy zone, resulting in fast melting. Conversely, the convective heat transfer effect from the heating surface decreases as the Ste number increases. The temperature differences caused by the local thermal non-equilibrium (LTNE) effect over time are significant and complex, with peaks becoming more pronounced nearer the heating surface. This study intends to provide theoretical support for the further development of TPMS skeletons in enhancing the phase change process.