United Kingdom

This paper explores the current scope and direction of the emerging global governance of hydrogen within the broader context of the energy transition where technological innovation and institutional change intersect. Hydrogen as a critical yet complex energy vector requires coordinated governance efforts to navigate its development effectively. To this end we critically engage with key challenges facing the hydrogen sector and examine how institutional frameworks are addressing these issues. Departing from the broader scholarship on global energy governance we conceptually leverage the socio-technical transition and innovation system liter ature to understand the complexities underpinning the development of the global hydrogen economy. We identify three overarching issue areas pertaining to the nature and role of hydrogen in the global energy system: end-use sector development infrastructure and trade and environmental and socio-economic sustainability. Each of these areas presents distinct challenges to hydrogen’s global governance from stimulating supply and demand to managing geo-economic challenges and establishing comprehensive certification and standards. Through mapping multilateral institutions at the global and regional levels and their main objectives we offer insights into the emerging institutional architecture related to hydrogen and identify potential gaps in current governance. Our findings suggest that while newer hydrogen-specific institutions complement the broader agenda of the main established international organizations the overall global hydrogen structure remains a patchwork of diverse actors and frameworks each addressing hydrogen-related challenges to varying degrees. Our research contributes to a nuanced understanding of global governance in the hydrogen sector and advances scholarly discussions on how institutional and actor dynamics shape the emergence and development of new technologies.

The lack of clarity and uncertainty about hydrogen’s role demand applications and economics has been a barrier to the development of the hydrogen economy. In this paper an optimisation model for the integrated planning and operation of hydrogen and electricity systems is presented to identify the role of hydrogen technologies and linepack in decarbonising energy systems improving system flexibility and enhancing energy system security and resilience against extreme weather events. The studies are conducted on Great Britain’s (GB) 2050 net-zero electricity and gas transmission systems to analyse the hydrogen transport and capacity requirements within the existing infrastructure under different scenarios. This includes sensitivities on the level of flexibility high gas prices hydrogen production mixes enabled reversibility of electrolysers electricity generation cost and hydrogen storage facilities. In all sensitivity scenarios efficient hydrogen transport within the existing infrastructure is enabled by the optimal allocation of green and blue hydrogen sources distributed storage facilities and the intra-day flexibility provided by linepack. The findings highlight that increased renewable deployment transfers intermittency to the hydrogen network requiring greater linepack flexibility compared to the current paradigm (up to 83%). Furthermore the necessity of synergy between different gas and electricity systems components in providing flexibility security and resilience is quantified.

This work investigated hydrogen production from biomass feedstocks (i.e. glucose starch lignin and cellulose) using a 100 mL h-type proton exchange membrane electrolysis cell. Biomass electrolysis is a promising process for hydrogen production although low in technology readiness level but with a series of recognised advantages: (i) lower-temperature conditions (compared to thermochemical processes) (ii) minimal energy consumption and low-cost post-production (iii) potential to synthesise high-volume H2 and (iv) smaller carbon footprint compared to thermochemical processes. A Lewis acid (FeCl3 ) was employed as a charge carrier and redox medium to aid in the depolymerisation/oxidation of biomass components. A comprehensive analysis was conducted measuring the H2 and CO2 emission volume and performing electrochemical analysis (i.e. linear sweep voltammetry and chronoamperometry) to better understand the process. For the first time the influence of temperature on current density and H2 evolution was studied at temperatures ranging from ambient temperature (i.e. 19 ◦C) to 80 ◦C. The highest H2 volume was 12.1 mL which was produced by FeCl3 -mediated electrolysis of glucose at ambient temperature which was up to two times higher than starch lignin and cellulose at 1.20 V. Of the substrates examined glucose also showed a maximum power-to-H2 -yield ratio of 30.99 kWh/kg. The results showed that hydrogen can be produced from biomass feedstock at ambient temperature when a Lewis acid (FeCl3 ) is employed and with a higher yield rate and a lower electricity consumption compared to water electrolysis.

The Gulf Cooperation Council (GCC) comprising: Saudi Arabia United Arab Emirates Kuwait Qatar Oman and Bahrain is home to an abundant number of resources including natural gas and solar and wind energy (renewables). Because of this the region is favourably positioned to become a significant player in both blue and green hydrogen production and their export. Current dependence on fossil fuels and ambitious national targets for decarbonisation have led the region and world to research the feasibility of switching to a hydrogen economy. This literature review critically examines the current advantages and strategies adopted by the GCC to expedite the implementation of hydrogen supply chains as well as investigation into the methodologies employed in current research for the modelling and optimisation of hydrogen supply chains. Insight into these endeavours is critical for stakeholders to assess the inherent challenges and opportunities in establishing a sustainable hydrogen economy. Despite a substantial global effort establishing a solid hydrogen supply chain presently faces various obstacles including the costs of clean hydrogen production. Scaling-up storage and transport methods is an issue that affects all types of hydrogen including carbon-intensive (grey) hydrogen. However the current costs of green hydrogen production mostly via the process of electrolysis is a major obstacle hindering the widescale deployment of clean hydrogen. Research in this literature review found that compressed gas and cryogenic liquid options have the highest storage capacities for hydrogen of 39.2 and 70.9 kg/m3 respectively. Meanwhile for hydrogen transportation pipelines and cryogenic tankers are the most conventional and efficient options with an efficiency of over 99 %. Cryogenic ships to carry liquid hydrogen also show potential due to their large storage capacities of 10000 tonnes per shipment However costs per vessel are currently still very expensive ranging between $ 465 and $620 million.

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.

Hydrogen presents the UK with a substantial opportunity to drive economic growth and secure skilled jobs by leveraging our natural geological and geographical advantages robust supply chain and existing energy expertise. Hydrogen UK’s most recent Economic Impact Assessment estimates that the hydrogen sector in the UK could support approximately 30000 direct jobs and contribute more than £7 billion gross value added annually by 2030. On a global scale the hydrogen market is projected to be worth $2.5 trillion by 2050.

With international competition increasing the UK must act now to capitalise on this potential. These projections are supported by a recognition that hydrogen is one of the key solutions to decarbonising the UK economy complementing other low-carbon solutions such as electrification carbon capture biofuels and energy efficiency. Additionally hydrogen will play a vital role in enhancing the UK’s energy security by storing domestically produced energy to balance intermittent renewable sources like wind and solar. As a critical component of the clean energy transition hydrogen is indispensable to achieving net zero.

As it stands the UK is well placed to capitalise on the hydrogen opportunity and emerge as a global leader. We have made early strides in establishing a framework for hydrogen development with various pilot projects and strategic investments already underway. However the next five years will be critical for the sector as we move from strategy and planning to development and delivery. It is imperative to get the first lowcarbon production projects over the line and into construction as a matter of urgency and then deliver substantial infrastructure development regulatory clarity and sustained financial support to scale-up production and distribution. A new Government presents an opportunity for policymakers to solidify commitments and accelerate the deployment of hydrogen technology ensuring the UK remains competitive in the global race.

Our manifesto outlines policy recommendations for the new UK Government to take across production distribution and storage infrastructure end use applications trade and beyond which will support a thriving British industrial base that creates jobs and growth for British people. To achieve this the UK hydrogen industry calls on policymakers to speed up the deployment of hydrogen through the recommendations set out in this Manifesto.

This report can be found on Hydrogen UK's website.

Hydrogen is one of the key solutions to decarbonising the UK economy along with other carbon abatement solutions such as electrification CCUS biofuels and energy efficiency. It provides a low carbon alternative to fossil fuels that has many of the same desirable features such as burning with a high temperature flame without producing carbon emissions during combustion. Hydrogen will be particularly valuable in hard-to-decarbonise sectors that have few cost-effective alternatives including elements of industry heavy transport and dispatchable power generation. However it’s use could be much more widespread depending on how costs preferences and policy for different low carbon solutions develop. The Government’s Hydrogen Strategy estimates that based on analysis from the Climate Change Committee (CCC) in 2050 between 20% and 35% of the UK’s final energy demand could be met with low carbon hydrogen1 . While hydrogen provides a promising solution to reducing emissions current deployment of low carbon hydrogen is low with almost all hydrogen in the UK produced from unabated fossil fuels resulting in high emissions. In the UK hydrogen production must meet the Low Carbon Hydrogen Standard (LCHS) to access government support. This is currently set at 20g CO2 e/MJ(LHV) and will ensure that future deployment will deliver significant emissions reductions when switching from fossil fuels2. The period to 2030 will be a critical time for the UK to seize the economic opportunity presented by low carbon hydrogen sector. Internationally increasing attention has been placed on hydrogen as a solution to global emissions. In the USA the Inflation Reduction Act (IRA) has provided fixed rate tax credits of up to $3/kg (£2.4/kgII) for clean hydrogen production3. Closer to home the EU is targeting 10 million tonnes of domestic electrolytic production and an additional 10 million tonnes of electrolytic hydrogen imports by 20304. This will be achieved through a variety of policy levers including an auction for fixed price subsidy support for electrolytic production with a ceiling of €4.5/kg5 (£3.84/kgIII). In the UK Government have set an ambitious target of up to 10 GW of low carbon hydrogen production by 2030 with at least half of this from electrolytic sources6. This will be supported by the Hydrogen Production Business Model (HPBM) a two-way variable CfD which could potentially provide hydrogen for a price as low as the natural gas price7 . As global supply chains investment and skills are in international competition the UK must continue its ambitious hydrogen aspirations to ensure the decarbonisation and economic opportunity presented by low carbon hydrogen is captured. This study estimates the economic impact of the low carbon hydrogen sector in the UK by 2030. The impact is assessed by estimating the costs of hydrogen deployment and applying employment and GVA multipliers to these costs based on historic economic activity. These estimates are broken down by different forms of low carbon hydrogen production and end use as well as the enabling infrastructure required to connect production and demand namely hydrogen networks and storage. Both the employment and GVA are estimated for each of these value chain elements for every year between 2024 and 2030. Employment and economic growth from the hydrogen sector will be created across the UK with many benefits arising in regions that have faced historic underinvestment such as the industrial clusters and Scotland. Beyond the high-level economic benefits estimated in this study the hydrogen sector creates an opportunity for the hundreds of thousands oil and gas sector jobs in the UK to transition to a low carbon alternative.

This report can be found on Hydrogen UK's website.

This report explores how hydrogen could be taken up in the UK and how this in turn translates to each sector from both global and UK perspectives to understand the practical implications of global and UK targets and projections on hydrogen innovation opportunities:

♦ Assessing demand for hydrogen sets out the context and the approach taken in the assessment of global and UK sector hydrogen needs including the development of specific UK scenarios for hydrogen deployment and innovation across the energy system and supply chain.

♦ Key insights discusses the insights and an overview of the outputs from the implementation of the UK deployment scenarios in whole energy system modelling.

♦ Hydrogen production storage and distribution and demand explore these areas in more detail setting out the current state and potential trajectories for hydrogen in each sector both globally and in the UK up to 2050.

This report can also be downloaded free on the Hydrogen Innovation Initative website.

Future pathways for Great Britain’s energy system decarbonization have highlighted the importance of lowcarbon hydrogen as an energy carrier and demand flexibility support. However the potential application within various sectors (heating industry transport) and production capacity through different technologies (methane reformation with carbon capture biomass gasification electrolysis) is highly varying introducing substantial uncertainties for hydrogen infrastructure development. This study sets out infrastructure priorities and identifies locational flexibility for hydrogen supply and demand options. Advances on limitations of previous research are made by developing an open-source model of the hydrogen system of Great Britain based on three Net Zero scenarios set out by National Grid in their Future Energy Scenarios in high temporal and spatial resolution. The model comprehensively covers demand sectors and supply options in addition to extending the locational considerations of the Future Energy Scenarios. This study recommends prioritizing the establishment of green hydrogen hubs in the near-term aligning with demands for synthetic fuels production industry and power which can facilitate the subsequent roll out of up to 10GW of hydrogen production capacity by 2050. The analysis quantifies a high proportion of hydrogen supply and demand which can be located flexibly.

The current global energy environment is experiencing a substantial shift towards minimizing carbon emissions and enhancing sustainability due to persistent problems. Demand for sustainable end-to-end energy solutions has boosted green hydrogen as the solution to decarbonize the world. The current study has identified and evaluated 7 main criteria of 27 sub-criteria for enabling the hydrogen supply Chains for decarbonization using the Fuzzy DEMATEL technique. The results show that the most prominent enablers criteria under causal factors are: cluster-based approach for developing a green hub Cost and investment decisions Hydrogen trade policy and regulatory actions and Technology. The effect group factors include: Assessment of ecological concerns- Ecology effect Availability of Energy sources and Awareness and public outreach. This study offers insights to understand the dynamics of the hydrogen supply chains and its way ahead towards decarbonization and transition towards a low-carbon economy. This research helps various academic and industrial stakeholders to give pace to green hydrogen uptake as a vital decarbonization tool and act as a base for strategic and collaborative decisions for a resilient and responsible energy landscape.

Though being attractive on railway decarbonisation for regional lines excessive cost caused by immature hydrogen supply chain is one of the significant hurdles for promoting hydrogen traction to rolling stocks. Therefore we conduct bespoke research on the UK’s hydrogen supply chain for railway concentrating on hydrogen transportation. Firstly a map for the planned hydrogen production plants and potential hydrogen lines is developed with the location capacity and usage. A spatially explicit model for the hydrogen supply chain is then introduced which optimises the existing grid-based methodology on accuracy and applicability. Compressed hydrogen at three pressures and liquid hydrogen are considered as the mediums incorporating by road and rail transport. Furthermore three scenarios for hydrogen rail penetration are simulated respectively to discuss the levelised cost and the most suitable national transport network. The results show that the developed model with mix-integer linear programming (MILP) can well design the UK’s hydrogen distribution for railway traction. Moreover the hydrogen transport medium and vehicle should adjust to suit for different era where the penetration of hydrogen traction varies. The levelised cost of hydrogen (LCOH) decreases from 6.13 £/kg to 5.13 £/kg on average from the conservative scenario to the radical scenario. Applying different transport combinations according to the specific situation can satisfy the demand while reducing cost for multi-supplier and multitargeting hydrogen transport.

This paper presents a comprehensive safety assessment of hydrogen production using Alkaline Water Electrolysis (AWE). The study utilizes various risk assessment methodologies including Hazard Identification (HAZID) What-If analysis Fault Tree Analysis (FTA) Event Tree Analysis (ETA) and Bow Tie analysis to systematically identify and evaluate potential hazards associated with the AWE process. Key findings include the identification of critical hazards such as hydrogen leaks oxygen-related risks and maintenance challenges. The assessment emphasizes the importance of robust safety measures including preventive and mitigative strategies to manage these risks effectively. Consequence modeling highlights significant threat zones for thermal radiation and explosion risks underscoring the need for comprehensive safety protocols and emergency response plans. This work contributes valuable insights into hydrogen safety providing a framework for risk assessment and mitigation in hydrogen production facilities crucial for the safe and sustainable development of hydrogen infrastructure in the global energy transition.

This study explored the applications of liquid hydrogen (LH2 ) in aerospace projects followed by an investigation into the efficiency of ramjets scramjets and turbojets for hypersonic flight and the impact of grey blue and green hydrogen as an alternative to JP-7 and JP-8 (kerosene fuel). The advantage of LH2 as a propellant in the space sector has emerged from the relatively high energy density of hydrogen per unit volume enabling it to store more energy compared to conventional fuels. Hydrogen also has the potential to decarbonise space flight as combustion of LH2 fuel produces zero carbon emissions. However hydrogen is commonly found in hydrocarbons and water and thus it needs to be extracted from these molecular compounds before use. Only by considering the entire lifecycle of LH2 including the production phase can its sustainability be understood. The results of this study compared the predicted Life Cycle Assessment (LCA) emissions of the production of LH2 using grey blue and green hydrogen for 2030 with conventional fuel (JP-7 and JP-8) and revealed that the total carbon emissions over the lifecycle of LH2 were greater than kerosene-derived fuels.

To achieve the net zero carbon emissions goals by 2050 the transition to cleaner forms and carriers of energy should be accelerated without though jeopardizing the public safety. Although hydrogen has been deemed to play significant role in the energy transition for years now there are still concerns for its risks that hamper its widespread implementation in several applications like for instance automobile applications. Hydrogen-powered vehicles raise concerns about their safety especially inside confined spaces like tunnels and thus research on that topic has been intensified during the last years. In this context experiments have been conducted by UK HSE within the EU-funded project HyTunnel-CS to examine hydrogen dispersion and deflagration inside a scaled tunnel resulting from fuel cell car bus and train release.<br/>In this work that was also carried out within the HyTunnel-CS we present the Computational Fluid Dynamics (CFD) simulations of the HSE unignited experiments. Blowdown tests related to high-pressure hydrogen releases through Thermal Pressure Relief Device (TPRD) installed in car and in train were modeled using the ADREA-HF code. The scope of these simulations was two-fold: a) contribute to the design of the experiments (e.g. indicate sensor positioning ignition point etc.) and the interpretation of hydrogen behavior and b) validate the CFD code. For the former pre-test simulations preceded the experiments to provide design recommendations. When the experiments were conducted the measurements were used for the code validation. Overall the CFD results are in satisfactory agreement with the experiments. Finally simulations with different ventilation rates and with model vehicles inside the tunnel were conducted to examine their effect on mixture dispersion and tunnel safety.

The depletion and overuse of fossil fuels present formidable challenge to energy supply system and environment. The human society is in great need of clean renewable and sustainable energy which can guarantee the long-term utilization without leading to escalation of greenhouse effect. Hydrogen as an extraordinary secondary energy is capable of realizing the target of environmental protection and transferring the intermittent primary energy to the application terminal while its nature of low volumetric energy density and volatility need suitable storage method and proper carrier. In this context liquid organic hydrogen carrier (LOHC) among a series of storage methods such as compressed and liquefied hydrogen provokes a considerable amount of research interest since it is proven to be a suitable carrier for hydrogen with safety and stability. However the dehydrogenation of hydrogen-rich LOHC materials is an endothermic process and needs large energy consumption which hampers the scale up of the LOHC system. The heat issue is thus essential to be addressed for fulfilling the potential of LOHC. In this work several strategies of heat intensification and management for LOHC system including the microwave irradiation circulation of exhaust heat and direct LOHC fuel cell are summarized and analyzed to provide suggestions and directions for future research.

Present work investigates the potential of a long-range commercial blended wing body configuration powered by hydrogen combustion engines with future airframe and propulsion technologies. Future technologies include advanced materials load alleviation techniques boundary layer ingestion and ultra-high bypass ratio engines. The hydrogen combustion configuration was compared to the configuration powered by kerosene with respect to geometric properties performance characteristics energy demand equivalent CO2 emissions and Direct Operating Costs. In addition technology sensitivity studies were performed to assess the potential influence of each technology on the configuration. A multi-fidelity sizing methodology using low- and mid-fidelity methods for rapid configuration sizing was created to assess the configuration and perform robust analyses and multi-disciplinary optimizations. To assess potential uncertainties of the fidelity of aerodynamic analysis tools high-fidelity aerodynamic analysis and optimization framework MACHAero was used for additional verification. Comparison of hydrogen and kerosene blended wing body aircraft showed a potential reduction of equivalent CO2 emission by 15% and 81% for blue and green hydrogen compared to the kerosene blended wing body and by 44% and 88% with respect to a conventional B777-300ER aircraft. Advancements in future technologies also significantly affect the geometric layout of aircraft. Boundary layer ingestion and ultra-high bypass ratio engines demonstrated the highest potential for fuel reduction although both technologies conflict with each other. However operating costs of hydrogen aircraft could establish a significant problem if pessimistic and base hydrogen price scenarios are achieved for blue and green hydrogen respectively. Finally configurational problems featured by classical blended wing body aircraft are magnified for the hydrogen case due to the significant volume requirements to store hydrogen fuel.

Decarbonising and decontaminating remote regions in the world presents several challenges. Many of these regions feature isolation dispersed demand in large areas and a lack of economic resources that impede the development of robust and sustainable networks. Furthermore isolated systems in the developing world are mostly based on diesel generation for electricity and firewood and liquefied petroleum gas for heating as these options do not require a significant infrastructure cost. In this context we present a stochastic multi-energy multi-microgrid system planning model that integrates electricity heat and hydrogen networks in isolated systems. The model is stochastic to capture uncertainty in renewable generation outputs particularly hydro and wind and thus design a multi-energy system proved secured against such uncertainty. The model also features two distinct constraints to limit the emissions of CO2 (for decarbonisation) and particulate matter (for decontamination) and incorporates firewood as a heating source. Moreover given that the focus is on low-voltage networks we introduce a fully linear AC power flow equations set allowing the planning model to remain tractable. The model is applied to a real-world case study to design a multi-energy multi-microgrid system in an isolated region in Chilean Patagonia. In a case with a zero limit over direct CO2 emissions the total system’s cost increases by 34% with respect to an unconstrained case. In a case with a zero limit over particulate matter emissions the total system’s cost increases by 189%. Finally although an absolute zero limit over both particulate matter and direct CO2 emissions leads to a total system’s cost increase of 650% important benefits in terms of decarbonisation and decontamination can be achieved at marginal cost increments.

Green hydrogen is set to become the energy carrier of the future provided that production technologies such as electrolysis and solar water splitting can be scaled to global dimensions. Testing these hydrogen technologies on the MW scale requires the development of dedicated new test facilities for which there is no precedent. This perspective highlights the challenges to be met on the path to implementing a test facility for large-scale water electrolysis photoelectrochemical and photocatalytic water splitting and aims to serve as a much-needed blueprint for future test facilities based on the authors’ own experience in establishing the Hydrogen Lab Leuna. Key aspects to be considered are the electricity and utility requirements of the devices under testing the analysis of the produced H2 and O2 and the safety regulations for handling large quantities of H2 . Choosing the right location is crucial not only for meeting these device requirements but also for improving financial viability through supplying affordable electricity and providing a remunerated H2 sink to offset the testing costs. Due to their lower TRL and requirement for a light source large-scale photocatalysis and photoelectrochemistry testing are less developed and the requirements are currently less predictable.

With the aggravation of global environmental pollution problems and the need for energy restructuring hydrogen energy as a highly clean resource has gradually become a hot spot for research in countries around the world. Facing the requirement of distributed hydrogen in refueling the original station for hydrogen transportation and other usage this paper proposes a comprehensive energy system planning model for hydrogen refueling stations to obtain the necessary devices construction the devices’ capacity decisions and the optimal operation behaviors of each device. Comparing to traditional single hydrogen producing technics in the traditional planning model the proposed model in this paper integrates both water-electrolysis-based and methanol-based manufacturing technics. A two-level optimization model is designed for this comprehensive system. The result of the numerical study shows that the proposed model can achieve a better optimal solution for distributed hydrogen production. Also it considers the single producing situation when price of one primary resource is sufficient higher than the other.

Considerable progress has been made recently in the use of nanoporous materials for hydrogen storage. In this article the current status of the field and future challenges are discussed ranging from important open fundamental questions such as the density and volume of the adsorbed phase and its relationship to overall storage capacity to the development of new functional materials and complete storage system design. With regard to fundamentals the use of neutron scattering to study adsorbed H2 suitable adsorption isotherm equations and the accurate computational modelling and simulation of H2 adsorption are discussed. The new materials covered include flexible metal–organic frameworks core–shell materials and porous organic cage compounds. The article concludes with a discussion of the experimental investigation of real adsorptive hydrogen storage tanks the improvement in the thermal conductivity of storage beds and new storage system concepts and designs.

Hydrogen embrittlement in metals (HE) is a serious challenge for the use of high strength materials in engineering practice and a major barrier to the use of hydrogen for global decarbonization. Here we describe the factors and variables that determine HE susceptibility and provide an overview of the latest understanding of HE mechanisms. We discuss hydrogen uptake and how it can be managed. We summarize hydrogen trapping and the techniques used for its characterization. We also review literature that argues that hydrogen trapping can be used to decrease HE susceptibility. We discuss the future research that is required to advance the understanding of HE and hydrogen trapping and to develop HE-resistant alloys.

This paper offers a comprehensive analysis of the historical evolution and the current state of the aviation industry with a particular emphasis on the critical need for this sector to decarbonize. It delves into emerging propulsion technologies such as battery electric and hydrogen-based systems assessing their potential impact on sustainability within the aviation sector. Special attention is devoted to the global regulatory framework notably carbon offsetting and emission reduction scheme for international aviation which encapsulates initiatives such as lower carbon aviation fuels and sustainable aviation fuels. Examining the environmental challenges facing aviation the paper underscores the necessity for a balanced and comprehensive strategy that integrates various approaches to achieve sustainable solutions. By addressing both the historical context and contemporary advances the paper aims to provide a nuanced understanding of the complexities surrounding aviation's decarbonization journey acknowledging the industry's strides while recognizing the ongoing challenges in the pursuit of sustainability.

Hydrogen and ammonia are primary carbon-free fuels that have massive production potential. In regard to their flame properties these two fuels largely represent the two extremes among all fuels. The extremely fast flame speed of hydrogen can lead to an easy deflagration-to-detonation transition and cause detonation-type engine knock that limits the global equivalence ratio and consequently the engine power. The very low flame speed and reactivity of ammonia can lead to a low heat release rate and cause difficulty in ignition and ammonia slip. Adding ammonia into hydrogen can effectively modulate flame speed and hence the heat release rate which in turn mitigates engine knock and retains the zero-carbon nature of the system. However a key issue that remains unclear is the blending ratio of NH3 that provides the desired heat release rate emission level and engine power. In the present work a 3D computational combustion study is conducted to search for the optimal hydrogen/ammonia mixture that is knock-free and meanwhile allows sufficient power in a typical spark-ignition engine configuration. Parametric studies with varying global equivalence ratios and hydrogen/ammonia blends are conducted. The results show that with added ammonia engine knock can be avoided even under stoichiometric operating conditions. Due to the increased global equivalence ratio and added ammonia the energy content of trapped charge as well as work output per cycle is increased. About 90% of the work output of a pure gasoline engine under the same conditions can be reached by hydrogen/ammonia blends. The work shows great potential of blended fuel or hydrogen/ammonia dual fuel in high-speed SI engines.

Hydrogen is gaining prominence in global efforts to combat greenhouse gas emissions and climate change. While steam methane reforming remains the predominant method of hydrogen production alternative approaches such as water electrolysis and methane cracking are gaining attention. The bridging technology – methane cracking – has piqued scientific interest with its lower energy requirement (74.8 kJ/mol compared to steam methane reforming 206.278 kJ/mol) and valuable by-product of filamentous carbon. Nevertheless challenges including coke formation and catalyst deactivation persist. This review focuses on two main reactor types for catalytic methane decomposition – fixed-bed and fluidised bed. Fixed-bed reactors excel in experimental studies due to their operational simplicity and catalyst characterisation capabilities. In contrast fluidised-bed reactors are more suited for industrial applications where efforts are focused on optimising the temperature gas flow rate and particle characterisation. Furthermore investigations into various fluidised bed regimes aim to identify the most suitable for potential industrial deployment providing insights into the sustainable future of hydrogen production. While the bubbling regime shows promise for upscaling fluidised bed reactors experimental studies on turbulent fluidised-bed reactors especially in achieving high hydrogen yield from methane cracking are limited highlighting the technology’s current status not yet reaching commercialisation.

As an important energy source to achieve carbon neutrality green hydrogen has always faced the problems of high use cost and unsatisfactory environmental benefits due to its remote production areas. Therefore a liquid-gaseous cascade green hydrogen delivery scheme is proposed in this article. In this scheme green hydrogen is liquefied into high-density and low-pressure liquid hydrogen to enable the transport of large quantities of green hydrogen over long distances. After longdistance transport the liquid hydrogen is stored and then gasified at transfer stations and converted into high-pressure hydrogen for distribution to the nearby hydrogen facilities in cities. In addition this study conducted a detailed model evaluation of the scheme around the actual case of hydrogen energy demand in Chengdu City in China and compared it with conventional hydrogen delivery methods. The results show that the unit hydrogen cost of the liquid-gaseous cascade green hydrogen delivery scheme is only 51.58 CNY/kgH2 and the dynamic payback periods of long- and short-distance transportation stages are 13.61 years and 7.02 years respectively. In terms of carbon emissions this scheme only generates indirect carbon emissions of 2.98 kgCO2/kgH2 without using utility electricity. In sum both the economic and carbon emission analyses demonstrate the advantages of the liquidgaseous cascade green hydrogen delivery scheme. With further reductions in electricity prices and liquefication costs this scheme has the potential to provide an economically/environmentally superior solution for future large-scale green hydrogen applications.