Safety

Hydrogen is a key pillar in the global Net Zero strategy. Rapid scaling up of hydrogen production transport distribution and utilization is expected. This entails that hydrogen which is traditionally an industrial gas will come into proximity of populated urban areas and in some situations handled by the untrained public. To realize all their benefits hydrogen and its technologies must be safely developed and deployed. The specific properties of hydrogen involving wide flammability range low ignition energy and fast flame speed implies that any accidental release of hydrogen can be easily ignited. Comparing with conventional fuels combustion systems fueled by hydrogen are also more prone to flame instability and abnormal combustion. This paper aims to provide a comprehensive review about combustion research related to hydrogen safety. It starts with a brief introduction which includes some overview about risk analysis codes and standards. The core content covers ignition fire explosions and deflagration to detonation transition (DDT). Considering that DDT leads to detonation and that detonation may also be induced directly under special circumstances the subject of detonation is also included for completeness. The review covers laboratory medium and large-scale experiments as well as theoretical analysis and numerical simulation results. While highlights are provided at the end of each section the paper closes with some concluding remarks highlighting the achievements and key knowledge gaps.

With the world-wide decision to reduce carbon emissions through the Paris Agreement (2015) the demand for hydrogen-fuelled vehicles has been increasing. Although hydrogen is not a toxic gas it has a wide flammable range (4e75%) and can explode due to static electricity. Therefore studies on hydrogen safety are urgently required. In this study an explosion was induced by applying fire to the lower part of a fuel cell electric vehicle (FCEV). Out of three compressed hydrogen storage tanks installed in the vehicle two did not have hydrogen fuel and one was filled with compressed gaseous hydrogen of 700 bar and forcedly deactivated its temperature-activated pressure relief device. The side-on overpressure transducers were installed by distance in main directions to measure the side-on overpressure generated by the vehicle explosion. A 10 m-long protective barrier was installed on which reflected overpressure displacement and acceleration were measured to examine the effect of attenuation of explosion damage in the event of an accident. The vehicle exploded approximately 11 min after ignition generating a blast wave fireballs and fragments. The results of the experiment showed that the protective barrier could almost completely block explosive pressure smoke and scattering generated during an explosion. Through Probit function analysis the probabilities of an accident occurring were derived based on peak overpressure peak impulse and scattering. The results of this study can be used to develop standard operating procedures (SOPs) for firefighters as the base data for setting the initial operation location and deriving the safe separation distance.

In the frame of hydrogen-powered aircraft Airbus wants to understand all the H2 physics and explore every scenario in order to develop and manufacture safe products operated in a safe environment. Within the framework of a Large Eddy Simulation (LES) methodology for modeling turbulence a comparative numerical study of free under-expanded jet H2/AIR flame is conducted. The investigated geometry consists of straight nozzles with a millimetric diameter fed with pure H2 at upstream pressures ranging from 2 to 10 bar. Numerical results are compared with available experimental measurements such as; temperature signals using thermocouples. LES confirms its prediction capability in terms of shock jet structure and flame length. A particular attention is paid for capturing experimental unstable flame when upstream pressure decreases. Furthermore flame stabilization and flame anchoring are analyzed. Mechanisms of flame stabilization are highlighted for case 1 and stabilization criteria are tested. Finally an ignition map to reach flame stabilization is proposed for each case regarding the literature.

This study focuses on development of a CFD model able to simulate the experimentally observed critical nozzle diameter for hydrogen non-premixed flames. The critical diameter represents the minimum nozzle size through which a free jet flame will remain stable at all driving pressures. Hydrogen non-premixed flames will not blow-out at diameters equal to or greater than the critical diameter. Accurate simulation of this parameter is important for assessment of thermally activated pressure relief device (TPRD) performance during hydrogen blowdown from a storage tank. At TPRD diameters below the critical value there is potential for a hydrogen jet flame to blow-out as the storage tank vents potentially leading to hydrogen accumulation in an indoor release scenario. Previous experimental studies have indicated that the critical diameter for hydrogen is approximately 1 mm. In this study flame stability is considered across a range of diameters and overpressures from 0.1 mm to 2 mm and from 0.2 MPa to 20 MPa respectively. The impact of turbulent Schmidt number Sct which is the ratio of momentum diffusivity (kinematic viscosity) and mass diffusivity on the hydrogen concentration profile in the region near the nozzle exit and subsequent influence on critical diameter was investigated and discussed. For lower Sct values the enhanced mass mixing resulted in smaller predicted critical diameters. The use of value Sct=0.61 in the model demonstrated the best agreement with experimental values of the critical diameter. The model reproduced the critical diameter of 1 mm and then was applied to predict flame stability for under-expanded hydrogen jets.

National Nuclear Laboratory (NNL) is aiming to demonstrate through a research and development programme that nuclear enabled hydrogen can be used to support future clean energy systems. Demonstrating the safe operation of hydrogen facilities co-generating with a nuclear reactor will be key to enabling the deployment and success of nuclear enabled hydrogen technologies in the future. During the deployment continuity of supply will be paramount and possibly requires inter-seasonal storage. Co-generation is a means of using a source of energy in this case a nuclear reactor to efficiently produce power and thermal energy. Since a great deal of the heat energy is lost to the environment in a power plant making use of wasted energy for other useful output like the production of hydrogen and direct heating would be advantageous to plant economics and energy system flexibility. The civil nuclear industry is regulated around the world. This approach ensures that all the activities related to the production of power from nuclear and the hazards associated with ionising radiation are controlled in a manner which protects workers members of the public property and the environment. Nuclear safety assessments follow a rigorous process and are required as part of the Nuclear Site Licence. A fundamental requirement which is cited in the UK legislation is that the risks associated with all activities at the licensed site be reduced to As Low As Reasonably Practicable (ALARP). The principle places a requirement on duty holders to implement measures to reduce risk where doing so is considered reasonable and proportionate. The inclusion of risks for hazardous materials associated with the hydrogen production facilities need to be considered and this requires harmonisation of two different safety and regulatory governance regimes which have not previously interacted in this way. The safety demonstration for nuclear facilities is provided through the Safety Case.

A new dimensionless number (DN) is proposed in order to evaluate the performance of a high-pressure vessel composite structure. It shows that very few composite part is used at its maximum loading potential during bursting. Today for 70 MPa on-board type IV composite tanks DN values close to 20%. The suggested DN will be a useful indicator for an industrial application. By maximizing the DN at the design phase it is possible to minimize the mass of the composite structure of a CPV to reduce the manufacturing time and cost. To increase the DN as close as possible to 100% it is necessary to succeed in increasing the overall loading of the composite structure to have better oriented fibre. For this it seems necessary to find new processes which make it possible to better orient the fibre.

Climate change has prompted the international community to invest heavily in renewable energy sources in order to gradually replace fossil fuels. Whilst energy systems will be increasingly based on non-programmable renewable sources hydrogen is the main player when it comes to the role of energy reserve. This change has triggered a fast development of hydrogen production technologies with increasing use and installation of hydrogen generators (electrolyzers) in both the civil and industrial sector. The implementation of such investments requires the need for accurate design and verification of hydrogen systems with particular attention on fire safety. Due to its chemical-physical characteristics hydrogen is highly flammable and is often stored at very high-pressure levels. ISO 22734 and NFPA 2 are the main international standards which are currently available for the design of hydrogen generators and systems both of which include fire safety requirements. This paper analyses the main existing Regulations Codes and Standards (RCS) for hydrogen generators with the purpose of evaluating and comparing fire safety measures with focus on both active protection (detection systems extinguishing systems) and passive protection (safety distances separation walls). The scope of the paper is to identify safety measures which can be considered generally applicable and provide a reference for further fire safety regulations. The analysis carried out identifies potential gaps in RCS and suggests areas for potential future research.

Social risk is a comprehensive concept that considers not only internal/external physical risks but also risks (which are multiple varied and diverse) associated with social activity. It should be considered from diverse perspectives and requires a comprehensive evaluation framework that takes into account the synergistic impact of each element on others rather than evaluating each risk individually. Social risk assessment is an approach that is not limited to internal system risk from an engineering perspective but also considers the stakeholders development stage and societal readiness and resilience to change. This study aimed to introduce a social risk approach to assess the public safety of large-scale hydrogen systems. Guidelines for comprehensive social risk assessment were developed to conduct appropriate risk assessments for advanced science and technology activities with high uncertainties to predict major impacts on society before an accident occurs and to take measures to mitigate the damage and to ensure good governance are in place to facilitate emergency response and recovery in addition to preventive measures. In a case study this approach was applied to a hydrogen refueling station in Japan and risk-based multidisciplinary approaches were introduced. These approaches can be an effective supporting tool for social implementation with respect to large-scale hydrogen systems such as liquefied hydrogen storage tanks. The guidelines for social risk assessment of large-scale hydrogen systems are under the International Energy Agency Technology Collaboration Program Hydrogen Safety Task 43. This study presents potential case studies of social risk assessment for large-scale hydrogen systems for future.

The adoption of liquid hydrogen (LH2) as an energy carrier presents significant opportunities for distributing large quantities of hydrogen efficiently. However ensuring safety of LH2 transfer operations requires the evo lution of suitable technologies and regulatory framework. This study offers an extensive overview of technical considerations and safety aspects pertaining to liquid hydrogen installations and mobile applications. A signif icant lack of regulations specifically tailored for LH2 transfer operations is highlighted. Additionally experi mental findings and outcomes of the modelling activities carried out in previous research are presented shedding light on the combustion and ignition behaviour of liquid hydrogen during accident scenarios. The identification of research gaps and ongoing research projects underscores the importance of continued investigation and development of this critical area.

Natural ventilation is a well-known passive mitigation method to limit hydrogen build-up in confined spaces in case of accidental release [1-3]. In most cases a basic design of H2 infrastructure is adopted and vents installed for natural ventilation are adjusted according to safety targets and constraints of the considered structure. With the growing H2 mobility market the demand for H2 refueling infrastructure in our urban environment is on the rise. In order to meet both safety requirements and societal acceptance the design of such infrastructure is becoming more important. In this study a novel design concept is proposed for the hydrogen refueling station (HRS) by modifying physical structure while keeping safety consideration as the top priority of the concept. In this collaborative project between Air Liquide and the University of Delaware an extensive evaluation was performed on new structures of the processing container and dispenser of HRS by integrating safety protocols via passive means. Through a SWOT analysis combined with the most relevant approaches including analytical engineering models numerical simulations [4] and dedicated experimental trials an optimized design was obtained and its safety enhancement was fully evaluated. A small-scale processing container and an almost full-scale dispenser were built and tested to validate the design concepts by simulating accidental H2 release scenarios and assessing the associated consequences in terms of accumulation and potential flammable volumes formation. A conical dispenser and a V-shaped roof-top processing container which were easy to build and implement were designed and tested for this proof-of-concept study. This unique methodology from conception fundamental analysis investigation and validation through experimental design execution and evaluation is fully described in this study.