Applications & Pathways

The Paris Climate Agreement seeks to keep global temperature increases under 2° Celsius ideally 1.5° Celsius. This goal necessitates significant emission reductions. By 2030 emissions are expected to range between 52 and 58 GtCO2e from their 2016 level of approximately 52 GtCO2e. This review paper explores a number of low and zero-carbon renewable fuels such as hydrogen green ammonia green methanol biomethane natural gas and synthetic methane (with natural gas and synthetic methane subject to CCUS both at processing and at final use) as alternative solutions for providing a way to rebalance transition paths in order to achieve the goals of the Paris Agreement while also reaping the benefits of other sustainability targets. The results show renewables will need to account for approximately 90% of total electricity generation by 2050 and approximately 25% of non-electric energy usage in buildings and industry. However low and zero-carbon renewable fuels currently only contributes about 15% to the global energy shares and it will take about 10% more capacity to reach the 2050 goal. The transportation industry will need to take important steps toward energy efficiency and fuel switching in order to achieve the 20% emission reduction. Therefore significant new commitments to efficient low-carbon alternatives will be necessary to make this enormous change. According to this paper investing in energy efficiency and lowcarbon alternative energy must rise by a factor of about five by 2050 in comparison to 2015 levels if the 1.5 °C target is to be realised.

Stringent sustainability goals are set for the next generation of aircraft. A promising novel airframe concept is the ultra-high aspect ratio Strut-Braced Wing (SBW) aircraft. Hydrogen-based concepts are active contenders for sustainable propulsion. The study compares a medium-range Liquid Hydrogen (LH2) to a kerosene-based SBW aircraft designed with the same top-level requirements. For both concepts overall design operating costs and emissions are evaluated using the tool SUAVE. Furthermore aerostructural optimizations are performed for the wing mass of SBW aircraft with and without wing-based fuel tanks. Results show that the main difference in the design point definition results from a higher zero-lift drag due to an extended fuselage housing the LH2 tanks with a small reduction in the required wing loading. Structural mass increases of the LH2 aircraft due to additional tanks and fuselage structure are mostly offset by fuel mass savings. While the fuel mass accounts for nearly 25% of the kerosene design’s Maximum Take-Off Mass (MTOM) this reduces to 10% for the LH2 design. The LH2 aircraft has 16% higher operating costs with emission levels reduced to 57–82% of the kerosene aircraft depending on the LH2 production method. For static loads the absence of fuel acting as bending moment relief in the wing results in an increase in wing structural mass. However the inclusion of roll rate requirements causes large wing mass increases for both concepts significantly outweighing dry wing penalties.

Hydrogen can manage intermittent Renewable Energy Sources (RES) especially in high-RES share systems. The energy transition calls for mature low cost low space solutions bringing the attention to unitized items such as the reversible Solid Oxide Cell (rSOC). This device made of a single unit can work as an electrolyzer and as fuel cell with high efficiency fuel flexibility and producing combined heat. The objective of this review is to identify and classify rSOC applications to the building sector as an effective solution and to show how much this technology is near to its commercialisation. Research & Development projects were analysed and discussed for a comprehensive overview. Conclusions show an increasing interest in the reversible technology although it is still at pre industrialisation stage with few real applications in the building sector of which the majority is reported commented and compared in this paper for the first time.

A viable green energy source for heavy industries and transportation is hydrogen. The internal combustion engine (ICE) when powered by hydrogen offers an economical and adaptable way to quickly decarbonize the transportation industry. In general two techniques are used to inject hydrogen into the ICE combustion chamber: port injection and direct injection. The present work examined direct injection technology highlighting the need to understand and manage hydrogen mixing within an ICE’s combustion chamber. Before combusting hydrogen it is critical to study its propagation and mixture behavior just immediately before burning. For this purpose the DI-CHG.2 direct injector model by BorgWarner was used. This injector operated at 35 barG and 20 barG as maximum and minimum upstream pressures respectively; a 5.8 g/s flow rate; and a maximum tip nozzle temperature of 250 ◦C. Experiments were performed using a high-pressure and hightemperature visualization vessel available at our facility. The combustion mixture prior to burning (spray) was visually controlled by the single-pass high-speed Schlieren technique. Images were used to study the spray penetration (S) and spray volume (V). Several parameters were considered to perform the experiments such as the injection pressure (Pinj) chamber temperature (Tch) and the injection energizing time (Tinj). With pressure ratio and injection time being the parameters commonly used in jet characterization the addition of temperature formed a more comprehensive group of parameters that should generally aid in the characterization of this type of gas jets as well as the understanding of the combined effect of the rate of injection on the overall outcome. It was observed that the increase in injection pressure (Pinj) increased the spray penetration depth and its calculated volume as well as the amount of mass injected inside the chamber according to the ROI results; furthermore it was also observed that with a pressure difference of 20 bar (the minimum required for the proper functioning of the injector used) cyclic variability increased. The variation in temperature inside the chamber had less of an impact on the spray shape and its penetration; instead it determined the velocity at which the spray reached its maximum length. In addition the injection energizing time had no effect on the spray penetration.

Unmanned aerial vehicles (UAVs) have become an integral part of modern life serving both civilian and military applications across various sectors. However existing power supply systems such as batteries often fail to provide stable long-duration flights limiting their applications. Previous studies have primarily focused on battery-based power which offers limited flight endurance due to lower energy densities and higher system mass. Proton exchange membrane (PEM) fuel cells present a promising alternative providing high power and efficiency without noise vibrations or greenhouse gas emissions. Due to hydrogen’s high specific energy which is substantially higher than that of combustion engines and battery-based alternatives UAV operational time can be significantly extended. This paper investigates the potential of PEM fuel cells as an alternative power source for electric propulsion in UAVs. This study introduces an adaptive fully functioning PEM fuel cell model developed using a reduced-order modeling approach and optimized for UAV applications. This research demonstrates that PEM fuel cells can effectively double the flight endurance of UAVs compared to traditional battery systems achieving energy densities of around 1700 Wh/kg versus 150–250 Wh/kg for batteries. Despite a slight increase in system mass fuel cells enable significantly longer UAV operations. The scope of this study encompasses the comparison of battery-based and fuel cell-based propulsion systems in terms of power mass and flight endurance. This paper identifies the limitations and optimal applications for fuel cells providing strong evidence for their use in UAVs where extended flight time and efficiency are critical.

This paper establishes a framework of boundary conditions for implementing hydrogen energy systems in ships identifying what is feasible within maritime constraints. To support a comprehensive understanding of hydrogen systems onboard vessels an extensive technical review of hydrogen storage and power systems is provided covering the entire power value chain. Key aspects include equipment arrangement integration of fuel cell powertrain and presentation of the complete storage system in compliance with regulations. Engineering considerations such as material selection and insulation equipment specifications (e.g. pressure relief valves and hydrogen purity) and system configurations are analysed. Key findings reveal that fuel cells must achieve operational lifespans exceeding 46000 h to be viable for maritime applications. Additionally reliance solely on volumetric energy density underestimates storage needs necessitating provisions for cofferdams ullage space tank heels and hydrogen conditioning areas. Regulatory gaps are identified including inadequate safety provisions and inappropriate material guidelines.

Fuel cell hybrid systems due to their combination of the high energy density of fuel cells and the rapid response capability of power batteries have become an important category of new energy vehicles. This paper discusses energy management strategies in hydrogen fuel cell vehicles. Firstly a detailed comparative analysis of existing PID control strategies and Adaptive Equivalent Consumption Minimization Strategies (A-ECMSs) is conducted. It was found that although A-ECMS can balance the energy utilization of the fuel cell and power battery well the power fluctuations of the fuel cell are significant leading to increased hydrogen consumption. Therefore this paper proposes an improved Adaptive Low-Pass Filter Equivalent Consumption Minimization Strategy (A-LPF-ECMS). By introducing low-pass filtering technology transient changes in fuel cell power are smoothed effectively reducing fuel consumption. Simulation results show that under the 6*FTP75 cycle the energy loss of A-LPF-ECMS is reduced by 10.89% (compared to the PID strategy) and the equivalent hydrogen consumption is reduced by 7.1%; under the 5*WLTC cycle energy loss is reduced by 5.58% and equivalent hydrogen consumption is reduced by 3.18%. The research results indicate that A-LPF-ECMS performs excellently in suppressing fuel cell power fluctuations under idling conditions significantly enhancing the operational efficiency of the fuel cell and showing high application value.

A promising energy carrier and storage solution for integrating renewable energies into the power grid currently being investigated is hydrogen produced via electrolysis. It already serves various purposes but it might also enable the development of hydrogen-based electricity storage systems made up of electrolyzers hydrogen storage systems and generators (fuel cells or engines). The adoption of hydrogen-based technologies is strictly linked to the electrification of end uses and to multicarrier energy grids. This study introduces a generic method to integrate and optimize the sizing and operation phases of hydrogen-based power systems using an energy hub optimization model which can manage and coordinate multiple energy carriers and equipment. Furthermore the uncertainty related to renewables and final demands was carefully assessed. A case study on an urban microgrid with high hydrogen demand for mobility demonstrates the method’s applicability showing how the multi-objective optimization of hydrogen-based power systems can reduce total costs primary energy demand and carbon equivalent emissions for both power grids and mobility down to  −145%. Furthermore the adoption of the uncertainty assessment can give additional benefits allowing a downsizing of the equipment.

Hydrogen-Integrated energy systems (HIESs) are pivotal in driving the transition to a low-carbon energy structure in China. This paper proposes a low-carbon economic scheduling strategy to improve the operational efficiency and reduce the carbon emissions of HIESs. The approach begins with the implementation of a stepwise carbon trading framework to limit the carbon output of the system. This is followed by the development of a joint operational model that combines hydrogen energy use and carbon capture. To improve the energy supply flexibility of HIESs modifications to the conventional combined heat and power (CHP) unit are made by incorporating a waste heat boiler and an organic Rankine cycle. This results in a flexible CHP response model capable of adjusting both electricity and heat outputs. Furthermore a comprehensive demand response model is designed to optimize the flexible capacities of electric and thermal loads thereby enhancing demand-side responsiveness. The integration of electric vehicles (EVs) into the system is analyzed with respect to their energy consumption patterns and dispatch capabilities which improves their potential for flexible scheduling and enables an optimized synergy between the demand-side flexibility and system operations. Finally a low-carbon economic scheduling model for the HIES is developed with the objective of minimizing system costs. The results show that the proposed scheduling method effectively enhances the economy low-carbon performance and flexibility of HIES operation while promoting clean energy consumption deep decarbonization of the system and the synergistic complementarity of flexible supply–demand resources. In the broader context of expanding clean energy and growing EV adoption this study demonstrates the potential of energy-saving emissionreduction systems and vehicle-to-grid (V2G) strategies to contribute to the sustainable and green development of the energy sector.

The dynamic response characteristics between the multiple energy flows of electricity-hydrogen-heat in the renewable energy DC off-grid hydrogen production system are highly coupled and nonlinear which leads to the complexity of its energy conversion and transmission law. This study proposes a model to describe the dynamic nonlinear energy conversion and transmission laws specific to such systems. The model develops a nonlinear admittance framework and a conversion characteristic matrix for multi-heterogeneous energy flow subsystems based on the operational characteristics of each subsystem within the DC off-grid hydrogen production system. Building upon this foundation an energy hub model for the hydrogen production system is established yielding the electrical thermal and hydrogen energy outputs along with their respective conversion efficiencies for each subsystem. By discretizing time the energy flow at each time node within the hydrogen production system is computed revealing the system’s dynamic energy transfer patterns. Experiments were conducted using measured wind speed and irradiance data from a specific location in eastern China. Results from selected typical days were analyzed and discussed revealing that subsystem characteristics exhibit nonlinear variation patterns. This highlights the limitations of traditional models in accurately capturing these dynamics. Finally a simulation platform incorporating practical control methods was constructed to validate the model’s accuracy. Validation results demonstrate that the model possesses high accuracy providing a solid theoretical foundation for further in-depth analysis of DC off-grid hydrogen production systems.