Transmission, Distribution & Storage

A new protocol is presented to directly characterise the toughness of microstructural regions present within the weld heat-affected zone (HAZ) the most vulnerable location governing the structural integrity of hydrogen transport pipelines. Heat treatments are tailored to obtain bulk specimens that replicate predominantly ferriticbainitic bainitic and martensitic microstructures present in the HAZ. These are applied to a range of pipeline steels to investigate the role of manufacturing era (vintage versus modern) chemical composition and grade. The heat treatments successfully reproduce the hardness levels and microstructures observed in the HAZ of existing natural gas pipelines. Subsequently fracture experiments are conducted in air and pure H2 at 100 bar revealing a reduced fracture resistance and higher hydrogen embrittlement susceptibility of the HAZ microstructures with initiation toughness values as low as 32 MPa√ m. The findings emphasise the need to adequately consider the influence of microstructure and hard brittle zones within the HAZ.

The rock wettability is one of the most critical parameters that influences rock storage potential trapping and H2 withdrawal rate during Underground hydrogen storage (UHS). However the existing review articles on wettability of H2-brine-rock systems do not provide detailed information on complexities introduced by reservoir wettability influencing parameters such as high pressure temperature salinity conditions micro-biotic effects cushion gases and organic acids relevant to subsurface environments. Therefore a comprehensive review of existing research on various parameters influencing rock wettability during UHS and residual trapping of H2 was conducted in this study. Literature that provides insight into molecular-level interaction through machine learning and molecular dynamic (MD) simulations and role of surface-active chemicals such as nanoparticles surfactants and wastewater chemicals were also reviewed. The review suggested that UHS could be feasible in clean geo-storage formations but the presence of rock surface contaminants at higher storage depth and microbial effects should be accounted for to prevent over-estimation of the rock storage potentials. The H2 wettability of storage/caprocks and associated risks of UHS projects could be higher in rocks with high proportion of carbonate minerals organic-rich shale and basalt with high plagioclase minerals content. However treatment of rock surfaces with nanofluids surfactants methylene blue and methyl orange has proven to alter the rock wettability from H2-wet towards water-wet. Research results on effect of rock wettability on residually trapped hydrogen and snap-off effects during UHS are contradictory thus further studies would be required in this area. The review generally concludes that rock wettability plays prominent role on H2 storage due to the frequency and cyclic loading of UHS hence it is vital to evaluate the effects of all possible wettability influencing parameters for successful designs and implementation of UHS projects.

Hydrogen handling equipment suffers from interaction with their operating environment which degrades the mechanical properties and compromises component integrity. Hydrogen-assisted cracking is responsible for several industrial failures with potentially severe consequences. A thorough failure analysis can determine the failure mechanism locate its origin and identify possible root causes to avoid similar events in the future. Postmortem fractographic analysis can classify the fracture mode and determine whether the hydrogen-metal interaction contributed to the component’s breakdown. Experts in fracture classification identify characteristic marks and textural features by visual inspection to determine the failure mechanism. Although widely adopted this process is time-consuming and influenced by subjective judgment and individual expertise. This study aims to automate fractographic analysis through advanced computer vision techniques. Different materials were tested in hydrogen atmospheres and inert environments and their fracture surfaces were analyzed by scanning electron microscopy to create an extensive image dataset. A pre-trained Convolutional Neural Network was finetuned to accurately classify brittle and ductile fractures. In addition Grad-CAM interpretability method was adopted to identify the image regions most influential in the model’s prediction and compare the saliency maps with expert annotations. This approach offered a reliable data-driven alternative to conventional fractographic analysis.

The effect of tempering temperature and tempering time on the density of hydrogen traps hydrogen diffusivity and microhardness in a vanadium-modified AISI 4140 martensitic steel was determined. Tempering parameters were selected to activate the second third and fourth tempering stages. These conditions were intended to promote specific microstructural transformations. Permeability tests were performed using the electrochemical method developed by Devanathan and Stachurski and microhardness was measured before and after these tests. It was observed that hydrogen diffusivity is inversely proportional to microhardness while the density of hydrogen traps is directly proportional to microhardness. The lowest hydrogen diffusivity the highest trap density and the highest microhardness were obtained in the as-quenched condition and the tempering at 286 ◦C for 0.25 h. In contrast tempering at a temperature corresponding to the fourth tempering stage increases hydrogen diffusivity and decreases the density of hydrogen traps and microhardness. However as the tempering time or temperature increases the opposite occurs which is attributed to the formation of alloy carbides. Finally hydrogen has a softening effect for tempering temperatures corresponding to the fourth tempering stage tempering times of 0.25 h and in the as-quenched condition. However with increasing tempering time hydrogen has a hardening effect.

Underground hydrogen storage (UHS) represents a large-scale energy storage system aiming to ensure a consistent supply by storing hydrogen generated from surplus energy. In the practice of UHS cushion gas is typically injected into the formation to maintain reservoir pressure for efficient hydrogen withdrawal. This paper reviews the impact of cushion gas on the performance of UHS from both experimental and numerical simulation perspectives. The thermophysical (e.g. density viscosity compressibility and solubility) and petrophysical (interfacial tension wettability and relative permeability) properties as well as the mixing and diffusion behavior of different cushion gases were compared. The corresponding impact of different cushion gases on plume migration and trapping potential is then discussed. Furthermore this review critically analyzes and explains the impact of various factors on the performance of UHS including the type of cushion gas the composition of cushion gas mixtures the volume of injected cushion gas and the effects of bio-methanation processes. The corresponding analysis specifically focuses on key performance indicators including H2 recovery factor formation pressure brine production and H2 outflow purity. Thus this review provides a comprehensive analysis of the role of cushion gas in UHS offering insight into the effective management and optimization of cushion gas injection in field-scale UHS operations.

As a key zero-carbon energy carrier the accurate measurement of liquid hydrogen flow in its industrial chain is crucial. However the ultra-low temperature ultra-low density and other properties of liquid hydrogen can introduce calibration errors. To enhance the measurement accuracy and reliability of liquid hydrogen flow this study investigates the heat and mass transfer within a 1 m3 non-vented storage tank during the calibration process of a liquid hydrogen flow standard device that integrates combined dynamic and static gravimetric methods. The vertical tank configuration was selected to minimize the vapor–liquid interface area thereby suppressing boil-off gas generation and enhancing pressure stability which is critical for measurement accuracy. Building upon research on cryogenic flow standard devices as well as tank experiments and simulations this study employs computational fluid dynamics (CFD) with Fluent 2024 software to numerically simulate liquid hydrogen flow within a non-vented tank. The thermophysical properties of hydrogen crucial for the accuracy of the phase-change simulation were implemented using high-fidelity real-fluid data from the NIST Standard Reference Database as the ideal gas law is invalid under the cryogenic conditions studied. Specifically the Lee model was enhanced via User-Defined Functions (UDFs) to accurately simulate the key phasechange processes involving coupled flash evaporation and condensation during liquid hydrogen refueling. The simulation results demonstrated good agreement with NASA experimental data. This study systematically examined the effects of key parameters including inlet flow conditions and inlet liquid temperature on the flow characteristics of liquid hydrogen entering the tank and the subsequent heat and mass transfer behavior within the tank. The results indicated that an increase in mass flow rate elevates tank pressure and reduces filling time. Conversely a decrease in the inlet liquid hydrogen temperature significantly intensifies heat and mass transfer during the initial refueling stage. These findings provide important theoretical support for a deeper understanding of the complex physical mechanisms of liquid hydrogen flow calibration in non-vented tanks and for optimizing calibration accuracy.

The development of efficient and sustainable hydrogen storage materials is a key challenge for realizing hydrogen as a clean and flexible energy carrier. Among various options metal hydrides offer high volumetric storage density and operational safety yet their application is limited by thermodynamic kinetic and compositional constraints. In this work we investigate the potential of machine learning (ML) to predict key thermodynamic properties—equilibrium plateau pressure enthalpy and entropy of hydride formation—based solely on alloy composition using Magpie-generated descriptors. We significantly expand an existing experimental dataset from ~400 to 806 entries and assess the impact of dataset size and data augmentation using the PADRE algorithm on model performance. Models including Support Vector Machines and Gradient Boosted Random Forests were trained and optimized via grid search and cross-validation. Results show a marked improvement in predictive accuracy with increased dataset size while data augmentation benefits are limited to smaller datasets and do not improve accuracy in underrepresented pressure regimes. Furthermore clustering and cross-validation analyses highlight the limited generalizability of models across different material classes though high accuracy is achieved when training and testing within a single hydride family (e.g. AB2). The study demonstrates the viability and limitations of ML for accelerating hydride discovery emphasizing the importance of dataset diversity and representation for robust property prediction.

A thermodynamic modeling framework is introduced to describe hydrogen refueling station configurations and capture detailed thermal dynamics in vehicle tanks with large aspect ratios. With an aspect ratio larger than three axial discretization of temperature allows to recover accurate temperature profiles and show that the gas and liner temperature are always highest towards the rear of the tanks. The framework is validated against experimental data and used to investigate the internal heat transfer dynamics. As aspect ratio grows larger the amount of cooling received by the rear region decreases as the incoming turbulent jet does not reach the latter. The current thermal management strategy of pre-cooling is therefore limited showing marginal improvements even with a cooling temperature of -50 ◦C. Potential solutions are to increase the filling duration time or to carefully design the tank with higher thermal diffusivity and adequate external means of cooling.

This project has investigated equipment used to carry out purging of gas networks with a view to providing tooling for commissioning SGN’s H100 Fife project.

It has built on work including the HyPurge NIA2_SGN0008 and Lot 1 of the Hydrogen Skills and Standards for Heat projects. The project has further advanced the body of purge theory founded and developed in those previous projects. The HyPurge project showed that direct purging was feasible with hydrogen; this project has investigated some of the hazards presented and recommended tooling to mitigate those hazards.

Flame arrestors are specified for certain network operations involving Natural Gas. It is recommended that the current procedures regarding flame arrestors are kept for hydrogen and a range of flame arrestors suited to hydrogen use has been identified.

Purge tables specify minimum speeds for purging related to pipe diameter. These minimum purge speeds are used to suppress the buoyancy driven effect of a less dense gas to preferentially flow over a denser gas. The lower buoyancy of hydrogen suggests an increase in purge speed of 1.7x those recommended for methane. This increase is not required in smaller diameters (100 mm and below) where it has been found that diffusion effects dominate purge performance resulting in greater flexibility for purging. Therefore purge tables have been produced giving recommended minimum purge speeds for methane and hydrogen according to the PE pipe diameters proposed in the H100 Fife project.

A purge stack with additional features to assist with hydrogen purging has been developed in this project. The features include a restriction at the end of the stack to mitigate burn-back in the event of a vent ignition. Specific restriction sizes are linked to the diameter of network pipes being purged and each individual restriction is tailored to achieve the correct purge speed for the given network pipe diameter. A pressure gauge on the stack indicates sufficient back pressure showing the correct purge flow is being achieved. The stack also includes a hydrogen wHystle (developed by Steer independently) to provide feedback on purge progress in real time.

A review of non-sparking tool requirements has been carried out. Purge operations are such that it is unlikely that non-sparking tools will provide a significant reduction in hazard. The conclusions from this are that the current recommendations from SGN’s mainlay procedures on non-sparking tools and ignition prevention will be suitable for hydrogen use.

A preliminary investigation into the consequences of in-pipe ignitions has been carried out. The investigation has shown that the overpressures generated are affected by several different factors. The proportion of the pipe that contains the flammable mixture affects the ability of the system to absorb the overpressure through non-flammable gas buffer zones. Once detonable zones increase in size then the absolute length of the detonable zone in relation to pipe diameter becomes a dominant factor. The most significant hazard to be prevented is an in-pipe detonation therefore the volume of detonable mixture is an important factor that may result in a limit to the permitted length for direct purging in a given pipe diameter.

The hazards presented during purging have been investigated and three specific hazards have been studied. These are ignition of the vent in-pipe ignition and burn back from a vent ignition into the pipe. Although none of these events are likely to occur ignition of the vent is the most likely and the consequence of this is similar with hydrogen and methane. In-pipe ignition is the event with the greatest consequence and although very unlikely this should be avoided.

Proposed further work includes: data mining from the body of purge studies to date identification of the growth of flammable and detonable zones vs. purge length a study into static electricity generation and consequence testing on ignitions in a variety of 90 mm and 125 mm PE pipes of different lengths.

This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.

Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz

There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from supplying Natural Gas to Hydrogen in multi-occupancy buildings (MOBs). Previous work undertaken during project ‘MOBs Work Pack 2 Asset Information Review’ identified the following gap in technical evidence relating to network pipeline capacity:

♦ The adequacy of the diameter of existing risers and laterals to supply the energy required with Hydrogen need to be investigated.

♦ The effects of an increased flow rate velocity or increased pressure (pipe integrity) should it be required to meet the demand without increasing the diameter of risers and laterals. This would need to consider the effect of altitude on Hydrogen riser systems the pressure drops from existing fittings and additional safety devices installed (e.g. excess flow valves) and the minimum pressure required to ensure safe operation of Hydrogen appliances.

SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBs to Hydrogen. The program is formed through 4 main work packs broken down into 2 main stage gated programmes (Part A = WP1 2 & Part B = WP3 4). This report is part of Work Pack 3 and summarises Task 3. The objective of Task 3 is to address evidence by examining the effects of increased volumetric flowrate velocity and/or increased pressure (pipe integrity) using the OLGA (V2021.2) pipeline simulator.

An earlier report described the survey of eighteen multi-occupancy buildings of various heights ages and construction methods. Of the eighteen multi-occupancy buildings eight were selected for analysis of capacity. A further two buildings representative of standard riser and lateral design were modelled with the data taken from the SGN management procedure SGN/PM/RL/1.

This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.

Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.