Para-H2 to ortho-H2 conversion in a full-scale automotive cryogenic pressurized hydrogen storage up to 345 bar

Hydrogen vehicles offer the potential to improve energy independence and lower emissions but suffer from reduced driving range. Cryogenic pressure vessel storage (also known as cryo-compressed storage) offers the advantage of higher densities than room temperature compressed although it has the disadvantage of cryogenic operating temperatures which results in boil-off when the temperature of the gas increases. In order to understand and optimize the time prior to boil-off, we have examined heat absorption from the transition between the two quantum states of the hydrogen molecule (para–ortho) in a full-scale (151 L internal volume) automotive cryogenic pressure vessel at pressures and temperatures up to 345 bar and 300 K, and densities between 14 and 67 g/L (2.1–10.1 kg H₂). The relative concentration of the two species was measured using rotational Raman scattering and verified by calorimetry. In fifteen experiments spanning a full year, we repeatedly filled the vessel with saturated LH₂ at near ambient pressure (2–3 bar), very low temperatures (20.3–25 K), varying densities, and very high para-H₂ fraction (99.7%). We subsequently monitored vessel pressure and temperature while performing periodic ortho-H₂ concentration measurements with rotational Raman scattering as the vessel warmed up and pressurized due to environmental heat entry. Experiments show that para–ortho H₂ conversion typically becomes active after 10–15 days of dormancy (“initiation” stage), when H₂ temperature reaches 70–80 K. Para–ortho H₂ conversion then approaches completion (equilibrium) in 25–30 days, when the vessel reaches 100–120 K at ∼50 g/L density. Warmer temperatures are necessary for conversion at lower densities, but the number of days remains unchanged. Vessel dormancy (time that the vessel can absorb heat from the environment before having to vent fuel to avoid exceeding vessel rating) increased between 3 and 7 days depending on hydrogen density, therefore indicating a potentially large benefit for reduced fuel venting in cryogenic pressurized hydrogen storage.     

Numerical analysis of accidental hydrogen releases from high pressure storage at low temperatures

Evaluations of the performance of simplified engineering and CFD models are important to improve risk assessment tools e.g. to predict accurately releases from various types of hydrogen storages. These tools have to predict releases from a wide range of storage pressures (up to 80 MPa) and temperatures (down to 20 K), e.g. cryogenic compressed gas storage covers pressures up to 35 MPa and temperatures between 33 K and 338 K. Accurate calculations of high pressure releases require real gas EOS. This paper compares a number of EOS to predict hydrogen properties typical in different storage types. The vessel dynamics are modeled using a simplified engineering and a CFD model to evaluate the performance of various EOS to predict vessel pressures, temperatures mass flow rates and jet flame lengths. It is shown that the chosen EOS and the chosen specific heat capacity correlation are important to model accurately hydrogen releases at low temperatures.     

Vehicular storage of hydrogen in insulated pressure vessels

This paper describes an alternative technology for storing hydrogen fuel onboard vehicles. Insulated pressure vessels are cryogenic capable vessels that can accept cryogenic liquid hydrogen, cryogenic compressed gas or compressed hydrogen gas at ambient temperature. Insulated pressure vessels offer advantages over conventional storage approaches. Insulated pressure vessels are more compact and require less carbon fiber than compressed hydrogen vessels. They have lower evaporative losses than liquid hydrogen tanks, and are lighter than metal hydrides.

The paper outlines the advantages of insulated pressure vessels and describes the experimental and analytical work conducted to verify that insulated pressure vessels can be safely used for vehicular hydrogen storage. Insulated pressure vessels have successfully completed a series of certification tests. A series of tests have been selected as a starting point toward developing a certification procedure. An insulated pressure vessel has been installed in a hydrogen fueled truck and tested over a six month period.     

Flame propagation regimes and critical conditions for flame acceleration and detonation transition for hydrogen-air mixtures at cryogenic temperatures

A series of more than 100 experiments with hydrogen-air mixtures have been performed at cryogenic temperatures from 90 to 130 K and ambient pressure. A wide range of hydrogen concentrations from 8 to 60%H2 in a shock tube of 5-m long and 54 mm id was tested. Flame propagation regimes were investigated for all hydrogen compositions at three different blockage ratios 0, 30% and 60% as a function of initial temperature. Piezoelectric pressure sensors and InGaAs photo-diodes have been applied to monitor the flame and shock propagation velocity of the combustion process. More than 150 experiments at ambient pressure and temperature were conducted as the reference data for cryogenic experiments. The critical expansion ratio σ1 for an effective flame acceleration to the speed of sound was experimentally found at cryogenic temperatures. The detonability criteria for smooth and obstructed channels were used to evaluate the detonation cell sizes at cryogenic temperatures as well. The main peculiarities of cryogenic combustion with respect to the safety assessment were that the maximum combustion pressure was several times higher and the run-up-distance to detonation was two times shorter compared to ambient temperature independent of lower chemical reactivity at cryogenic conditions.     

Safe,long range,inexpensive and rapidly refuelable hydrogen vehicles with cryogenic pressure vessels

Hydrogen storage is often cited as the greatest obstacle to achieving a hydrogen economy free of environmental pollution and dependence on foreign oil. A compact high-pressure cryogenic storage system has promising features to the storage challenge associated with hydrogen-powered vehicles. Cryogenic pressure vessels consist of an inner vessel designed for high pressure (350 bar) insulated with reflective sheets of metalized plastic and enclosed within an outer metallic vacuum jacket. When filled with pressurized liquid hydrogen, cryogenic pressure vessels become the most compact form of hydrogen storage available. A recent prototype is the only automotive hydrogen vessel meeting both Department of Energy's 2017 weight and volume targets. When installed onboard an experimental vehicle, a cryogenic pressure vessel demonstrated the longest driving distance with a single H₂ tank (1050 km). In a subsequent experiment, the vessel demonstrated unprecedented thermal endurance: 8 days parking with no evaporative losses, extending to a month if the vehicle is driven as little as 8 km per day. Calculations indicate that cryogenic vessels offer compelling safety advantages and the lowest total ownership cost of hydrogen storage technologies. Long-term (∼10 years) vacuum stability (necessary for high performance thermal insulation) is the key outstanding technical challenge. Testing continues to establish technical feasibility and safety.     

High-density automotive hydrogen storage with cryogenic capable pressure vessels

LLNL is developing cryogenic capable pressure vessels with thermal endurance 5–10 times greater than conventional liquid hydrogen (LH₂) tanks that can eliminate evaporative losses in routine usage of (L)H₂ automobiles. In a joint effort BMW is working on a proof of concept for a first automotive cryo-compressed hydrogen storage system that can fulfill automotive requirements on system performance, life cycle, safety and cost. Cryogenic pressure vessels can be fueled with ambient temperature compressed gaseous hydrogen (CGH₂), LH₂ or cryogenic hydrogen at elevated supercritical pressure (cryo-compressed hydrogen, CcH₂). When filled with LH₂ or CcH₂, these vessels contain 2–3 times more fuel than conventional ambient temperature compressed H₂ vessels. LLNL has demonstrated fueling with LH₂ onboard two vehicles. The generation 2 vessel, installed onboard an H₂-powered Toyota Prius and fueled with LH₂ demonstrated the longest unrefueled driving distance and the longest cryogenic H₂ hold time without evaporative losses. A third generation vessel will be installed, reducing weight and volume by minimizing insulation thickness while still providing acceptable thermal endurance. Based on its long experience with cryogenic hydrogen storage, BMW has developed its cryo-compressed hydrogen storage concept, which is now undergoing a thorough system and component validation to prove compliance with automotive requirements before it can be demonstrated in a BMW test vehicle.     

Vacuum generation by hydrogen permeation to atmosphere through austenitic and nickel-base-alloy vessel walls at temperatures from 573 K to 773 K

The permeation of hydrogen through metals is of great concern in hydrogen containment systems. In this study, hydrogen contained in seamless coiled tube vessels made of SUS 316L and Inconel 625 permeated the vessel walls at temperatures from 573 K to 773 K, and the decreasing interior pressure of the vessels was monitored for an extended period to characterize the behavior of the pressure change. It was found that the pressure became lower than the surrounding atmospheric pressure, and the vessels reached a vacuum. Hydrogen permeabilities through SUS 316L and Inconel 625 were determined from the pressure drop measurements. In order to ensure the reliability of the measurements, the permeabilities were also determined with a gas chromatograph that measured the concentration of hydrogen completely permeating the vessel wall. The permeabilities obtained with the two methods were in good agreement with each other. The pressure drop behavior was compared to, and found to be consistent with, theoretical calculations performed using the obtained permeabilities based on Fick’s law of diffusion.     

Thermodynamic analysis of the para-to-ortho hydrogen conversion in cryo-compressed hydrogen vessels for automotive applications

Cryo-compressed hydrogen storage has potential applications in fuel-cell vehicles due to its large storing density and thermal endurance. The dormancy of storage can be extended when considering the endothermic conversion of para-to-ortho hydrogen. In present study, a thermodynamic model is established to analyze the effect of the conversion in a cryogenic pressure vessel. The influence of the parameters such as the filling density, initial temperature and initial ortho hydrogen fraction is studied. It is demonstrated that different “transition pressures” for the vessels exist for different filling densities. The conversion can carry out sufficiently and the dormancy can be extended significantly when the designed release pressure of the vessel matches with the transition pressure. The heat of absorption increases with the initial o-H₂ fraction, whereas the peak of conversion rate occurs earlier for the vessel with a large initial o-H₂ fraction. The dormancy can be extended by 163% for the vessel with filling density of 70 kg/m³. The investigations on the effect of the para-to-ortho hydrogen conversion can provide useful guideline for the design of cryo-compressed hydrogen vessels.     

The storage performance of automotive cryo-compressed hydrogen vessels

Cryo-compressed hydrogen storage promises to deliver the highest system storage density leading to practical vehicles with range comparable to today's gasoline vehicles and fundamental cost and safety advantages. However, cryogenic vessels are complex systems, continuously drifting in thermodynamic space depending on use patterns, insulation performance, vessel characteristics, liquid hydrogen pump performance, and para-H₂ to ortho-H₂ conversion. In this paper, cryogenic vessel fill density results from a previous publication are extended to calculate system storage performance, including volumetric (gH₂/L), gravimetric (H₂ weight fraction), and vent losses over a broad range of conditions. The results confirm previous experiments and models indicating that cryogenic pressure vessels have maximum system density of all available storage technologies while avoiding vent losses in all but the most extreme situations. Design pressures in the range 250–350 bar seem most advantageous due to high system density and low weight and cost, although determining an optimum pressure demands a complete economic and functional analysis. Future insulation, vessel, and liquid hydrogen pump improvements are finally analyzed that, while not experimentally demonstrated to date, show promise of being feasible in the future as their level of technical maturity increases, leading to maximum H₂ storage performance for cryo-compressed storage. If proven feasible and incorporated into future cryogenic vessels, these improvements will enable 50 + gH₂/L system density at 10+% H₂ weight fraction.     

A rapid compression machine with crevice containment

A rapid compression machine (RCM) incorporating ‘crevice containment’ is designed and fabricated. ‘Crevice containment’ maintains the advantage of suppression of piston-motion induced roll-up vortex while avoiding undesirable multi-dimensional effects of crevice. The geometry of the combustion chamber is optimized with computational fluid dynamic simulations. The designed RCM is demonstrated to provide highly reproducible experimental data at compressed gas pressures up to 100 bar. Pressure traces also reveal that ‘crevice containment’ leads to significant reduction in the post-compression pressure drop. Further, the importance of ensuring instrumentation calibration and avoiding thermal shock of pressure sensor is highlighted to avoid systematic errors in measurements. High fidelity experiments are conducted for autoignition of hydrogen at compressed pressure of 50 bar. The experimental data is properly modeled by the kinetic mechanism from O’Conaire et al. [M. O’Conaire, H.J. Curran, J.M. Simmie, W.J. Pitz, C.K. Westbrook, Int. J. Chem. Kinet. 36 (11) (2004) 603–622] and discrepancy is noted from a recent mechanism [Z. Hong, D.F. Davidson, R.K. Hanson, Combust. Flame 158 (2011) 633–644].     

The design and optimization of a cryogenic compressed hydrogen refueling process

Cryo-compressed hydrogen storage has excellent volume and mass hydrogen storage density, which is the most likely way to meet the storage requirements proposed by United States Department of Energy（DOE）. This paper contributes to propose and analyze a new cryogenic compressed hydrogen refueling station. The new type of low temperature and high-pressure hydrogenation station system can effectively reduce the problems such as too high liquefaction work when using liquid hydrogen as the gas source, the need to heat and regenerate to release hydrogen, and the damage of thermal stress on the storage tank during the filling process, so as to reduce the release of hydrogen and ensure the non-destructive filling of hydrogen. This paper focuses on the study of precooling process in filling. By establishing a heat transfer model, the dynamic trend of tank temperature with time in the precooling process of low-temperature and high-pressure hydrogen storage tank under constant pressure is studied. Two analysis methods are used to provide theoretical basis for the selection of inlet diameter of hydrogen storage tank. Through comparative analysis of the advantages and disadvantages of the two analysis methods, it is concluded that the analysis method of constant mass flow is more suitable for the selection in practical applications. According to it, the recommended diameter of the storage tank at the initial temperature of 300 K, 200 K and 100 K is selected, which are all 15 mm. It is further proved that the calculation method can meet the different storage tank states of hydrogen fuel cell vehicles when selecting the pipe diameter.     

The potential for avoiding hydrogen release from cryogenic pressure vessels after vacuum insulation failure

This paper presents an analysis of vacuum insulation failure in an automotive cryogenic pressure vessel (also known as cryo-compressed vessel) storing hydrogen. Vacuum insulation failure increases heat transfer into cryogenic vessels by about a factor of 100, potentially leading to rapid pressurization and venting of the cryogen to avoid exceeding maximum allowable working pressure (MAWP). Hydrogen release to the environment may be dangerous, especially if the vehicle is located in a closed space (e.g. a garage or tunnel) at the moment of insulation failure. We therefore consider utilization of the hydrogen in the vehicle fuel cell and dissipation of the electricity by operating vehicle accessories or electric resistances as an alternative to releasing hydrogen to the environment. We consider two strategies: initiating hydrogen extraction immediately after vacuum insulation failure or waiting until maximum operating pressure is reached before extraction. The results indicate that cryogenic pressure vessels have thermodynamic advantages that enable slowing down hydrogen release to moderate levels that can be consumed in the fuel cell and dissipated in vehicle accessories supplemented by electric resistances, even in the worst case when the insulation fails at the moment when the vessel stores hydrogen near its maximum density (70 g/L at 300 bar). The two proposed strategies are therefore feasible, and the best alternative can be chosen based on economic and/or implementation constraints.     

Pressurized hydrogen from charged liquid organic hydrogen carrier systems by electrochemical hydrogen compression

We demonstrate that the combination of hydrogen release from a Liquid Organic Hydrogen Carrier (LOHC) system with electrochemical hydrogen compression (EHC) provides three decisive advantages over the state-of-the-art hydrogen provision from such storage system: a) The EHC device produces reduced hydrogen pressure on its suction side connected to the LOHC dehydrogenation unit, thus shifting the thermodynamic equilibrium towards dehydrogenation and accelerating the hydrogen release; b) the EHC device compresses the hydrogen released from the carrier system thus producing high value compressed hydrogen; c) the EHC process is selective for proton transport and thus the process purifies hydrogen from impurities, such as traces of methane. We demonstrate this combination for the production of compressed hydrogen (absolute pressure of 6 bar) from perhydro dibenzyltoluene at dehydrogenation temperatures down to 240 °C in a quality suitable for fuel cell operation, e.g. in a fuel cell vehicle. The presented technology may be highly attractive for providing compressed hydrogen at future hydrogen filling stations that receive and store hydrogen in a LOHC-bound manner.     

Cryo-adsorptive hydrogen storage on activated carbon. I: Thermodynamic analysis of adsorption vessels and comparison with liquid and compressed gas hydrogen storage

This paper presents a thermodynamic analysis of cryo-adsorption vessels for hydrogen storage. The analysis is carried out with an unsteady lumped model and gives a global assessment of the behavior of the storage system during operation (discharge), dormancy and filling. The adsorbent used is superactivated carbon AX-21™. Cryogenic hydrogen storage, either by compression or adsorption, takes advantage of the effect of temperature on the storage density. In order to store 4.1 kg H₂ in 100 L, a pressure of 750 bar at 298 K is necessary, but only 150 bar at 77 K. The pressure is further reduced to 60 bar if the container is filled with pellets of activated carbon [7]. However, adsorption vessels are submitted to intrinsic thermal effects which considerably influence their dynamic behavior and due to which thermal management is required for smooth operation. In this analysis, among energy balances for filling and discharge processes, the influence of the intrinsic thermal effects during vessel operation is presented. Hydrogen losses during normal operation as well as during long periods of inactivity are also considered. The results are compared to those obtained in low-pressure and high-pressure insulated LH₂ and CH₂ tanks.     

Effect of heat transfer through the release pipe on simulations of cryogenic hydrogen jet fires and hazard distances

Jet flames originated by cryo-compressed ignited hydrogen releases can cause life-threatening conditions in their surroundings. Validated models are needed to accurately predict thermal hazards from a jet fire. Numerical simulations of cryogenic hydrogen flow in the release pipe are performed to assess the effect of heat transfer through the pipe walls on jet parameters. Notional nozzle exit diameter is calculated based on the simulated real nozzle parameters and used in CFD simulations as a boundary condition to model jet fires. The CFD model was previously validated against experiments with vertical cryogenic hydrogen jet fires with release pressures up to 0.5 MPa (abs), release diameter 1.25 mm and temperatures as low as 50 K. This study validates the CFD model in a wider domain of experimental release conditions - horizontal cryogenic jets at exhaust pipe temperature 80 K, pressure up to 2 MPa ab and release diameters up to 4 mm. Simulation results are compared against such experimentally measured parameters as hydrogen mass flow rate, flame length and radiative heat flux at different locations from the jet fire. The CFD model reproduces experiments with reasonable for engineering applications accuracy. Jet fire hazard distances established using three different criteria - temperature, thermal radiation and thermal dose - are compared and discussed based on CFD simulation results.     

The isentropic expansion energy of compressed and cryogenic hydrogen

Pressure is often perceived as the single most important parameter when considering the safety of a storage system, for example when calculating the pneumatic energy that could be released in the event of a sudden accidental failure (or burst energy). In this paper, we investigate the role of temperature as another degree of freedom for minimizing the burst energy. Results are first presented for ideal gases, for which the relationship between burst energy as a function of initial and final volumes, temperature and pressures can be expressed analytically. Similar analysis is then derived for the specific case of H₂ using real gas equations of state. Assuming the expansion is isentropic, which holds for an adiabatic and sudden release as in a burst, it is shown that the energy released during a sudden burst is a weak function of pressure, revealing that the effect of increasing pressure is negligible beyond a certain value (∼100 bar); whereas the burst energy is a linear function of temperature. This suggests that temperature controls the burst energy in a much greater way. This analysis is carried out in the frame of onboard H₂ storage systems, for which it is shown that the use of cryogenic temperature for hydrogen vehicles, where risks of collision and impact on the surroundings are high, appears as a safety feature since burst energy is up to 18 times less than room temperature, high pressure storage.     

Modeling and simulation of a composite high-pressure vessel made of sustainable and renewable alternative fibers

To substitute the standard carbon fiber reinforced epoxy composite, sustainable and renewable alternative fibers are investigated for the use of high-pressure vessel through a finite element model. The standard T700S carbon fiber pressure vessel exhibits a minimum burst pressure of 1483 bar on the first layer oriented at 90°. The burst occurs in the central part showing a safe burst and the radial deformation reaches 1.12 mm. Several alternative fibers (basalt, E-glass, flax and recycled T700S carbon) are compared to the T700S carbon fiber. It results of lower burst pressures and none of the alternative composites caters for the minimum pressure threshold of 1400 bar. According to the storage pressure and in respect of the mechanical requirements, hybrid vessels integrating alternative and T700S carbon fibers are proposed to improve physical, environmental and economic performances. From an economic point of view, the optimal vessels are the E-glass/T700S carbon hybrid vessel and E-glass vessel for 700 and 350 bar, respectively. Regarding the environmental impact, the most suitable fibers are basalt/T700S carbon for a 700-bar storage and E-glass for 350 bar. Concerning the vessel mass, T700S carbon composite stays obviously the best candidate for a 700-bar storage but at 350 bar T700S carbon/flax fibers composite appears to be more efficient.     

Parametric study of a laser ignited hydrogen–air mixture in a constant volume combustion chamber

Experiments were carried out in a constant volume combustion chamber (CVCC) to investigate flame kernel development and flame speed of hydrogen–air mixtures having different fuel–air ratios. A Q-switched Nd: YAG laser with 1064 nm wavelength and pulse duration of 6–9 ns was used for ignition by generating laser induced plasma inside the CVCC. In this study, laser induced ignition of hydrogen–air mixtures was investigated using different initial chamber filling pressures (P = 2.5 bar–10 bar) at different initial temperatures (373 K–523 K). A variable optical setup with converging lenses having different focal lengths (f = 100–250 mm) were used to position the plasma at various locations inside the CVCC. A high speed camera recorded the flame kernel development and a piezoelectric pressure transducer recorded the pressure–time history for all the experiments. The main objective of this study was to determine the dependence of combustion properties of laser ignited hydrogen–air mixtures on lasers, optical configurations and initial conditions prevailing in the CVCC.     

Dynamics of cryogenic hydrogen storage in insulated pressure vessels for automotive applications

A dynamic model is used to characterize cryogenic H₂ storage in an insulated pressure vessel that can flexibly hold liquid H₂ and compressed H₂ at 350 bar. A double-flow refueling device is needed to ensure that the tank can be consistently refueled to its theoretical capacity regardless of the initial conditions. Liquid H₂ charged into the tank is stored as supercritical fluid if the initial tank temperature is >120 K and as a subcooled liquid if it is <100 K. An in-tank heater is needed to maintain the tank pressure above the minimum delivery pressure. Even if H₂ is stored as a supercritical fluid, liquid H₂ will form as H₂ is withdrawn and will further transform to a two-phase mixture and ultimately to a superheated gas. The recoverable fraction of the total stored inventory depends on the minimum H₂ delivery pressure and the power rating of the heater. The dormancy of cryogenic H₂ is a function of the maximum allowable pressure and the pressure of stored H₂; the evaporative losses cannot deplete H₂ from the tank beyond 64% of the theoretical storage capacity.     

Experimental analyses of the spatial varying temperature development of type-IV hydrogen pressure vessels in cyclic tests considering different length to diameter ratios

Establishing hydrogen as a reliable energy carrier is closely linked to the performance and safety level of the storage systems. During operation, the storage systems such as composite over-wrapped pressure vessels (COPVs) are exposed to complex physical, mechanical and thermal loads. Since the mechanical and physical properties of the used materials are strongly temperature-dependent, thermal influences must be taken into account for the vessel design. The effect of the vessel geometry, in particular the length-to-diameter ratio, as well as filling conditions on the temperature distribution within the fluid is analysed through the examination of cyclic tests in accordance with ANSI/CSA HGV2 and UN GTR N0.13/ECE R134. The gas temperature development during the cyclic tests is determined using a measuring device that allows a spatially distributed temperature measurement at eight vertical and horizontal positions. Two sizes of vessels are investigated characterised by the same inner and outer diameter but different length. It is observed that the length-to-diameter ratio substantially influences the temperature distribution within the fluid for room as well as elevated ambient temperatures at comparable filling conditions. Furthermore, the mass flow of the gas influences the temperature distribution within the fluid and shows an increased spatial and thermal inhomogeneity at higher gas mass flow. In addition, it can be observed that the temperature increase (ΔT) during filling depends significantly on the vessel temperature distribution. In the transient case of filling directly after emptying the vessel, a temperature increase of 36 K compared to the initially homogeneous vessel temperature has been found. Moreover, gas temperature differences of up to 97 K between the end of filling and the end of emptying can be observed, which is a significant thermal load for the vessels. Thus, the results presented here provide a broad data basis as input and boundary conditions for numerical fluid dynamic and structural analyses of pressure vessels made of carbon-fibre-reinforced plastics (CFRP).