United States

The development and promulgation of codes and standards are essential to establish a market-receptive environment for commercial hydrogen-based products and systems. The focus of the U.S. Department of Energy (DOE) is to conduct the research and development (R&D) needed to strengthen the scientific basis for technical requirements incorporated in national and international standards codes and regulations. In the U.S. the DOE and its industry partners have formed a Codes and Standards Tech Team (CSTT) to help guide the R&D. The CSTT has adopted an R&D Roadmap to achieve a substantial and verified database of the properties and behaviour of hydrogen and the performance characteristics of emerging hydrogen technology applications sufficient to enable the development of effective codes and standards for these applications. However to develop a more structured approach to the R&D described above the CSTT conducted a workshop on Risk Assessment for Hydrogen Codes and Standards in March 2005. The purpose of the workshop was to attain a consensus among invited experts on the protocols and data needed to address the development of risk-informed standards codes and regulations for hydrogen used as an energy carrier by consumers. Participants at the workshop identified and assessed requirements methodologies and applicability of risk assessment (RA) tools to develop a framework to conduct RA activities to address for example hydrogen fuel distribution delivery on-site storage and dispensing and hydrogen vehicle servicing and parking. The CSTT was particularly interested in obtaining the advice of RA experts and representatives of standards and model code developing organizations and industry on how data generated by R&D can be turned into information that is suitable for hydrogen codes and standards development. The paper reports on the results of the workshop and the RA activities that the DOE’s program on hydrogen safety codes and standards will undertake. These RA activities will help structure a comprehensive R&D effort that the DOE and its industry partners are undertaking to obtain the data and conduct the analysis and testing needed to establish a scientific and technical basis for hydrogen standards codes and regulations.

A combined experimental and modeling program is being carried out at Sandia National Laboratories to characterize and predict the behavior of unintended hydrogen releases. In the case where the hydrogen leak remains unignited knowledge of the concentration field and flammability envelope is an issue of importance in determining consequence distances for the safe use of hydrogen. In the case where a high-pressure leak of hydrogen is ignited a classic turbulent jet flame forms. Knowledge of the flame length and thermal radiation heat flux distribution is important to safety. Depending on the effective diameter of the leak and the tank source pressure free jet flames can be extensive in length and pose significant radiation and impingement hazard resulting in consequence distances that are unacceptably large. One possible mitigation strategy to potentially reduce the exposure to jet flames is to incorporate barriers around hydrogen storage equipment. The reasoning is that walls will reduce the extent of unacceptable consequences due to jet releases resulting from accidents involving high-pressure equipment. While reducing the jet extent the walls may introduce other hazards if not configured properly. The goal of this work is to provide guidance on configuration and placement of these walls to minimize overall hazards using a quantitative risk assessment approach. Detailed Navier-Stokes calculations of jet flames and unignited jets are used to understand how hydrogen leaks and jet-flames interact with barriers. The effort is complemented by an experimental program that considers the interaction of jet flames and unignited jets with barriers.

A simple approximate method for evaluation of blast effects and safety distances for unconfined hydrogen explosions is presented. The method includes models for flame speeds hydrogen distribution blast parameters and blast damage criteria. An example of the application of this methodology for hydrogen releases in three hypothetical obstructed areas with different levels of congestion is presented. The severity of the blast effect of unconfined hydrogen explosions is shown to depend strongly on the level of congestion for relatively small releases. Extremely large releases of hydrogen are predicted to be less sensitive to the congestion level.

A total of 12 ventilation experiments with various combinations of hydrogen release rates and ventilation speeds were performed in order to study how ventilation speed and release rate effect the hydrogen concentration in a closed system. The experiential facility was constructed out of steel plates and beams in the shape of a rectangular enclosure. The volume of the test facility was about 60m3. The front face of the enclosure was covered by a plastic film in order to allow visible and infrared cameras to capture images of the flame. The inlet and outlet vents were located on the lower front face and the upper backside panel respectively. Hydrogen gas was released toward the ceiling from the center of the floor. The hydrogen gas was released at constant rate in each test. The hydrogen release rate ranged from 0.002 m3/s to 0.02 m3/s. Ventilation speeds were 0.1 0.2 and 0.4 m3/s respectively. Ignition was attempted at the end of the hydrogen release by using multiple continuous spark ignition modules on the ceiling and next to the release point. Time evolution of hydrogen concentration was measured using evacuated sample bottles. Overpressure and impulse inside and outside the facility were also measured. The mixture was ignited by a spark ignition module mounted on the ceiling in eight of eleven tests. In the other three tests the mixture was ignited by spark ignition modules mounted next to the nozzle. Overpressures generated by the hydrogen deflagration in most of these tests were low and represented a small risk to people or property. The primary risk associated with the hydrogen deflagrations studied in these tests was from the fire. The maximum concentration is proportional to the ratio of the hydrogen release rate to the ventilation speed within the range of parameters tested. Therefore a required ventilation speed can be estimated from the assumed hydrogen leak rate within the experimental conditions described in this paper.

We have modelled sudden hydrogen expansion from a cryogenic pressure vessel. This model considers real gas equations of state single and two-phase flow and the specific “vessel within vessel” geometry of cryogenic vessels. The model can solve sudden hydrogen expansion for initial pressures up to 1210 bar and for initial temperatures ranging from 27 to 400 K. For practical reasons our study focuses on hydrogen release from 345 bar with temperatures between 62 K and 300 K. The pressure vessel internal volume is 151 L. The results indicate that cryogenic pressure vessels may offer a safety advantage with respect to compressed hydrogen vessels because i) the vacuum jacket protects the pressure vessel from environmental damage ii) hydrogen when released discharges first into an intermediate chamber before reaching the outside environment and iii) working temperature is typically much lower and thus the hydrogen has less energy. Results indicate that key expansion parameters such as pressure rate of energy release and thrust are all considerably lower for a cryogenic vessel within vessel geometry as compared to ambient temperature compressed gas vessels. Future work will focus on taking advantage of these favourable conditions to attempt fail-safe cryogenic vessel designs that do not harm people or property even after catastrophic failure of the inner pressure vessel.

Solid state materials such as metal and chemical hydrides have been proposed and developed for high energy density automotive hydrogen storage applications. As these materials are implemented into hydrogen storage systems developers must understand their behavior during accident scenarios or contaminated refueling events. An ability to predict thermal and chemical processes during contamination allows for the design of safe and effective hydrogen storage systems along with the development of appropriate codes and standards. A model for the transport of gases within an arbitrary-geometry reactive metal hydride bed (alane -AlH3) is presented in this paper. We have coupled appropriate Knudsen-regime permeability models for flow through packed beds with the fundamental heat transfer and chemical kinetic processes occurring at the particle level. Using experimental measurement to determine and validate model parameters we have developed a robust numerical model that can be utilized to predict processes in arbitrary scaled-up geometries during scenarios such as breach-in-tank or contaminated refueling. Results are presented that indicate the progression of a reaction front through a compacted alane bed as a result of a leaky fitting. The rate of this progression can be limited by; 1) restricting the flow of reactants into the bed through densification and 2) maximizing the rate of heat removal from the bed.

The properties of hydrogen compared to conventional fuels such as gasoline and diesel are substantially different requiring adaptations to the design and layout of test cells for hydrogen fuelled engines and vehicles. A comparison of hydrogen fuel properties versus conventional fuels in this paper provides identification of requirements that need to be adapted to design a safe test cell. Design examples of actual test cells are provided to showcase the differences in overall layout and ventilation safety features fuel supply and metering and emissions measurements. Details include requirements for ventilation patterns the necessity for engine fume hoods as well as hydrogen specific intake and exhaust design. The unique properties of hydrogen in particular the wide flammability limits and nonvisible flames also require additional safety features such as hydrogen sensors and flame cameras. A properly designed and implemented fuel supply system adds to the safety of the test cell by minimizing the amount of hydrogen that can be released. Apart from this the properties of hydrogen also require different fuel consumption measurement systems pressure levels of the fuel supply system additional ventilation lines strategically placed safety solenoids combined with appropriate operational procedures. The emissions measurement for hydrogen application has to be expanded to include the amount of unburned hydrogen in the exhaust as a measurement of completeness of combustion. This measurement can also be used as a safety feature to avoid creation of ignitable hydrogen-air mixtures in the engine exhaust. The considerations provided in this paper lead to the conclusion that hydrogen IC engines can be safely tested however properly designed test cell and safety features have to be included to mitigate the additional hazards related to the change in fuel characteristics.

For large-scale distribution and use of energy carriers classified as hazardous material in many countries as a method to assist land use planning to grant licenses to design a safe installation and to operate it safely some form of risk analysis and assessment is applied. Despite many years of experience the methods have still their weaknesses even the most elaborated ones as e.g. shown by the large spread in results when different teams perform an analysis on a same plant as was done in EU projects. Because a fuel as hydrogen with its different properties will come new in the daily use of many people incidents may happen and risks will be discussed. HySafe and other groups take good preparatory action in this respect and work in the right direction as appears from various documents produced. However already a superficial examination of the results so far tells that further cooperative work is indispensable. To avoid criticism skepticism and frustration not only the positive findings should be described and general features of the methods but the community has also to give strong guidance with regard to the uncertainties. Scenario development appears to be very dependent on insight and experience of an individual analyst leak and ignition probability may vary over a wide range of values Computational Fluid Dynamics or CFD models may lead to very different result. The Standard Benchmark Exercise Problems SBEPs are a good start but shall produce guidelines or recommendations for CFD use or even perhaps certification of models. Where feasible narrowing of possible details of scenarios to the more probable ones taking into account historical incident data and schematizing in bowties more explicit use of confidence intervals on e.g. failure rates and ignition probability estimates will help. Further knowledge gaps should be defined.

Hydrogen is recognized as the “future fuel” and the most promising alternative of fossil fuels due to its remarkable properties including exceptionally high energy content per unit mass (142 MJ/kg) low mass density and massive environmental and economical upsides. A wide spectrum of methods in H₂ production especially carbon-free approaches H2purification and H2storage have been investigated to bring this energy source closer to the technological deployment. Hydrogen hydrates are among the most intriguing material paradigms for H₂storage due to their appealing properties such as low energy consumption for charge and discharge safety cost-effectiveness and favorable environmental features. Here we comprehensively discuss the progress in understanding of hydrogen clathrate hydrates with an emphasis on charging/discharging rate of H₂ (i.e. hydrate formation and dissociation rates) and the storage capacity. A thorough understanding on phase equilibrium of the hydrates and its variation through different materials is provided. The path toward ambient temperature and pressure hydrogen batteries with high storage capacity is elucidated. We suggest that the charging rate of H₂ in this storage medium and long cyclic performance are more immediate challenges than storage capacity for technological translation of this storage medium. This review and provided outlook establish a groundwork for further innovation on hydrogen hydrate systems for promising future of hydrogen fuel.

Automotive proton-exchange membrane fuel cells (PEMFCs) have finally reached a state of technological readiness where several major automotive companies are commercially leasing and selling fuel cell electric vehicles including Toyota Honda and Hyundai. These now claim vehicle speed and acceleration refueling time driving range and durability that rival conventional internal combustion engines and in most cases outperform battery electric vehicles. The residual challenges and areas of improvement which remain for PEMFCs are performance at high current density durability and cost. These are expected to be resolved over the coming decade while hydrogen infrastructure needs to become widely available. Here we briefly discuss the status of automotive PEMFCs misconceptions about the barriers that platinum usage creates and the remaining hurdles for the technology to become broadly accepted and implemented.