University of Pisa performed experimental tests in a 1m3 facility, which shape and dimensions resemble a gas cabinet, for the HySEA project, founded by the Fuel Cells and Hydrogen 2 Joint Undertaking with the aim to conduct pre-normative research on vented deflagrations in real-life enclosures and containers used for hydrogen energy applications, in order to generate experimental data of high quality. The test facility, named Small Scale Enclosure (SSE), had a vent area of 0,42m2 which location could be varied, namely on the top or in front of the facility, while different types of vent were investigated. Three different ignition location were investigated as well, and the range of Hydrogen concentration ranged between 10 and 18% vol. This paper is aimed to summarize the main characteristics of the experimental campaign as well as to present its results.

Fuel cell vehicles and some compressed natural gas vehicles are equipped with carbon fiber reinforced plastic (CFRP) composite cylinders. Each of the cylinders has a pressure relief device designed to detect heat and release the internal gas to prevent the cylinder from bursting in a vehicle fire accident. Yet in some accident situations, the fire may be extinguished before the pressure relief device is activated, leaving the high-pressure fuel gas inside the fire-damaged cylinder. To handle such a cylinder safely after an accident it is necessary that the cylinder keeps a sufficient post-fire strength against its internal gas pressure, but in most cases it is difficult to accurately determine cylinder strength at the accident site. One way of solving this problem is to predetermine the post-fire burst strengths of cylinders by experiments. In this study, automotive CFRP cylinders having no pressure relief device were exposed to a fire to the verge of bursting; then after the fire was extinguished the residual burst strengths and the overall physical state of the test cylinders were examined. The results indicated that the test cylinders all recorded a residual burst strength at least twice greater than their internal gas pressure.

For refueling on-board hydrogen tanks, table-based or formula based protocols are commonly used. These protocols are designed to achieve a tank filling close to 100% SOC (State of Charge) in s safe way: without surpassing temperature (-40°C to 85°C) and pressure limits (125% Nominal Working Pressure, NWP). The ambient temperature, the initial pressure and the volume category of the (compressed hydrogen storage system, CHSS are used as inputs to determine the final target pressure and the pressure ramp rate (which controls the filling duration). However, abnormal out-of-spec events (e.g. misinformation of storage system status and characteristics of the storage tanks) may occur and result in a refueling in which the safety boundaries are surpassed. In the present article, the possible out of specification (out-of-spec) events in a refueling station have been analyzed. The associated hazards when refueling on-board hydrogen tanks have been studied. Experimental results of out-of-spec event tests performed on a type 3 tank are presented. The results show that on the type 3 tank, the safety temperature limit of 85°C was only surpassed under a combination of events; e.g. an unnoticed stop of the cooling of the gas combined with a wrong input of ambient temperature at a very warm environment. On the other hand, under certain events (e.g. cooling the gas below the target temperature) and in particular under cold environmental conditions, the 100% SOC limit established in the fuelling protocols has been surpassed.

In the event of a fire, composite pressure vessels behave very differently from metallic ones: the material is degraded, potentially leading to a burst without significant pressure increase. Hence, such objects are, when necessary, protected from fire by using thermally-activated devices (TPRD), and standards require testing cylinder and TPRD together. The pre-normative research project FireComp aimed at understanding better the conditions which may lead to burst, through testing and simulation, and proposed an alternative way of assessing the fire performance of composite cylinders. This approach is currently used by Air Liquide for the safety of composite bundles carrying large amounts of hydrogen gas.

Type IV pressure vessels are commonly used for hydrogen on-board, stationary or bulk storages. During their lifetime, they can be submitted to mechanical impacts, creating damage within the composite structure, not necessarily correlated to what is visible from the outside. When an impact is suspected, or when a cylinder is periodically inspected, it is necessary to determine whether it can safely stay in service or not. The FCH JU project Hypactor aims at creating a large database of impacts, characterized by various non destructive testing (NDT) methods, in order to provide reliable pass-fail criteria for damaged cylinders. This paper presents some of the tests results, investigating short term (burst) and long term (cycling) performance of impacted cylinders, and the recommendations that can be made for impact testing and NDT criteria calibration.

The 3D model of conjugate heat transfer from a fire to compressed gas storage cylinder is described. The model predictions of temperature outside and inside the cylinder as well as pressure increase during a fire are compared against a fire test experiment. The simulation reproduced measured in test temperatures and pressures. The original failure criterion of the cylinder in a fire has been applied in the model. This allowed for the prediction of the cylinder catastrophic rupture time with acceptable engineering accuracy. The significance of 3D modelling is demonstrated, and recommendations to improve safety of high-pressure composite tanks are given.

The development of the temperature field in hydrogen tanks during the filling process has been investigated with Computational Fluid Dynamics (CFD). Measurements from experiments undertaken at the JRC GasTef facility have been used to develop and validate the CFD modelling approach. By means of the CFD calculations, the effect of injector direction on the temperature distribution has been analysed. It has been found that the dynamics of the temperature field, including the development of potentially detrimental phenomena like thermal stratification and temperature inhomogeneity e.g. hot spots, can be significantly affected by the injector orientation.

Previous studies have demonstrated that while blowdown pressure is reproduced well by both adiabatic and isothermal analytical models, the dynamics of temperature cannot be predicted well by either model. The reason for the last is heat transfer to cooling during expansion gas from the vessel wall. Moreover, when exposed to an external fire, the temperature inside the vessel increases, i.e. when a thermally activated pressure relief device (TPRD) is still closed, with subsequent pressure increase that may lead to a catastrophic rupture of the vessel. The choice of a TPRD exit orifice size and design strategy are challenges: to provide sufficient internal pressure drop in a fire when the orifice size is too small; to avoid flame blow off expected with the decrease of pressure during the blowdown; to decrease flame length of subsequent jet fire as much as possible by the decrease of the orifice size under condition of sufficient fire resistance provisions, to avoid pressure peaking phenomenon, etc. The adiabatic model of blowdown [1] was developed using the Abel-Nobel equation of state and the original theory of underexpanded jet [2]. According to experimental observations, e.g. [3], heat transfer plays a significant role during the blowdown. Thus, this study aims to modify the adiabatic blowdown model to include the heat transfer to non-ideal gas. The model accounts for a change of gas temperature inside the vessel due to two “competing” processes: the decrease of temperature due to gas expansion and the increase of temperature due to heat transfer from the surroundings, e.g. ambience or fire, through the vessel wall. This is taken into account in the system of equations of adiabatic blowdown model through the change of energy conservation equation that accounts for heat from outside. There is a need to know the convective heat transfer coefficient between the vessel wall and the surroundings and wall size and properties to define heat flux to the gas inside the vessel. The non-adiabatic model is validated against available experimental data. The model can be applied as a new engineering tool for the inherently safer design of hydrogen tank-TPRD system.

Existing regulations and standards for the approval of composite cylinders in hydrogen service are currently based on deterministic criteria (ISO 11119-3, UN GTR No. 13). This paper provides a systematic analysis of the load cycle properties resulting from these regulations and standards. Their characteristics are compared with the probabilistic approach of the BAM. Based on Monte-Carlo simulations the available design range of all concepts is compared. In addition, the probability of acceptance for potentially unsafe design types is determined.

With the simple structure and refueling process, the compressed hydrogen storage system is currently widely used. However, thermal effects during charging-discharging cycle may induce temperature change in storage tank, which has significant impact on the performance of hydrogen storage and the safety of hydrogen storage tank. In our previous works, the final hydrogen temperature, the hydrogen pre-cooling temperature and the final hydrogen mass during refueling process were expressed based on the analytical solutions of a single zone (hydrogen gas) lumped parameter thermodynamic model of high pressure compressed hydrogen storage systems. To address this issue, we once propose a single zone lumped parameter model to obtain the analytical solution of hydrogen temperature, and use the analytical solution to estimate the hydrogen temperature, but the effect of the tank wall is ignored. For better description of the heat transfer characteristics of the tank wall, a dual zone (hydrogen gas and tank wall) lumped parameter model will be considered for widely representation of the reference (experimental or simulated) data. Now, we extend the single zone model to the dual zone model which uses two different temperatures for gas zone and wall zone. The dual zone model contains two coupled differential equations. To solve them and obtain the solution, we use the method of decoupling the coupled differential equations and coupling the solutions of the decoupled differential equations. The steps of the method include: (1) Decoupling of coupled differential equations; (2) Solving decoupled differential equations; (3) Coupling of solutions of differential equations; (4) Solving coupled algebraic equations. Herein, three cases are taken into consideration: constant inflow/outflow temperature, variable inflow/outflow temperature and constant inflow temperature and variable outflow temperature. The corresponding approximate analytical solutions of hydrogen temperature and wall temperature can be obtained. With hydrogen temperature and hydrogen mass, the hydrogen pressure can be calculated with using the equation of state for ideal gas. Besides, the two coupled differential equations can also be solved numerically and the simulated solution can also be obtained. The SAE J2601 developed a standard fueling protocol based on the so-called MC Method to a formula based approach, as an extension of the early look-up table approach. This study will help to set up another formula based approach of refueling protocol for gaseous hydrogen vehicles.