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.     

Modeling of sudden hydrogen expansion from cryogenic pressure vessel failure

We have modeled 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 favorable conditions to attempt fail-safe cryogenic vessel designs that do not harm people or property even after catastrophic failure of the inner pressure vessel.     

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.     

Thermal design analysis of a 1 L cryogenic liquid hydrogen tank for an unmanned aerial vehicle

Thermal design analysis of a 1-L cryogenic liquid hydrogen storage tank without vacuum insulation for a small unmanned aerial vehicle was carried out in the present study. To prevent excess boil-off of cryogenic liquid hydrogen, the storage tank consisted of a 1-L inner vessel, an outer vessel, insulation layers and a vapor-cooled shield. For a cryogenic storage tank considered in this study, the appropriate heat inleak was allowed to supply the boil-off gas hydrogen to a proton electrolyte membrane fuel cell as fuel. In an effort to accommodate the hydrogen mass flow rate required by the fuel cell and to minimize the storage tank volume, a thermal analysis for various insulation materials was implemented here and their insulation performances were compared. The present thermal analysis showed that the Aerogel thermal insulations provided outstanding performance at the non-vacuum atmospheric pressure condition. With the Aerogel insulation, the tank volume for storing 1-L liquid hydrogen at 20 K could be designed within a storage tank volume of 7.2 L. In addition, it was noted that the exhaust temperature of boil-off hydrogen gas was mainly affected by the location of a vapor-cooled shield as well as thermal conductivity of insulation materials.     

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.     

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.     

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.     

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.     

Operating experience with a liquid-hydrogen fueled buick and refueling system

An investigation of liquid-hydrogen storage and refueling systems for vehicular applications was made in a recently completed project. The vehicle used in the project was a 1979 Buick Century sedan with a 3.8.1, displacement turbocharged V6 engine and an automatic transmission. The vehicle had a fuel economy for driving in the high altitude Los Alamos area that was equivalent to 2.4 km/l of liquid hydrogen or 8.9 km/l of gasoline on an equivalent energy basis. About 22% less energy was required using hydrogen rather than gasoline to go a given distance based on the Environmental Protection Agency estimate of 7.2 km/l of gasoline for this vehicle. At the end of the project the engine had been operated for 138 h and the car driven 3633 km during the 17 months that the vehicle was operated on hydrogen. Two types of onboard liquid-hydrogen storage tanks were tested in the vehicle; the first was an aluminium Dewar with a liquid-hydrogen capacity of 1101.; the second was a Dewar with an aluminium outer vessel, two copper vapor-cooled thermal radiation shields, and a stainless steel inner vessel with a liquid-hydrogen capacity of 1551. The Buick had an unrefueled range of about 274 km with the first liquid-hydrogen tank and about 362 km with the second. The Buick was fueled at least 65 times involving a minimum of 8.1 kl of liquid hydrogen using various liquid-hydrogen storage Dewars at Los Alamos and a semiautomatic refueling station. A refueling time of 9 min was achieved, and liquid-hydrogen losses during refueling were measured. The project has demonstrated that liquid-hydrogen storage onboard a vehicle, and its refueling, can be accomplished over an extended period without any major difficulties; nevertheless, appropriate testing is still needed to quantitatively address the question of safety for liquid-hydrogen storage onboard a vehicle.     

A comparative analysis of the cryo-compression and cryo-adsorption hydrogen storage methods

While conventional low-pressure LH₂ dewars have existed for decades, advanced methods of cryogenic hydrogen storage have recently been developed. These advanced methods are cryo-compression and cryo-adsorption hydrogen storage, which operate best in the temperature range 30–100 K. We present a comparative analysis of both approaches for cryogenic hydrogen storage, examining how pressure and/or sorbent materials are used to effectively increase onboard H₂ density and dormancy. We start by reviewing some basic aspects of LH₂ properties and conventional means of storing it. From there we describe the cryo-compression and cryo-adsorption hydrogen storage methods, and then explore the relationship between them, clarifying the materials science and physics of the two approaches in trying to solve the same hydrogen storage task (∼5–8 kg H₂, typical of light duty vehicles). Assuming that the balance of plant and the available volume for the storage system in the vehicle are identical for both approaches, the comparison focuses on how the respective storage capacities, vessel weight and dormancy vary as a function of temperature, pressure and type of cryo-adsorption material (especially, powder MOF-5 and MIL-101). By performing a comparative analysis, we clarify the science of each approach individually, identify the regimes where the attributes of each can be maximized, elucidate the properties of these systems during refueling, and probe the possible benefits of a combined “hybrid” system with both cryo-adsorption and cryo-compression phenomena operating at the same time. In addition the relationships found between onboard H₂ capacity, pressure vessel and/or sorbent mass and dormancy as a function of rated pressure, type of sorbent material and fueling conditions are useful as general designing guidelines in future engineering efforts using these two hydrogen storage approaches.     

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.     

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).     

Cold hydrogen delivery in glass fiber composite pressure vessels: Analysis,manufacture and testing

This paper describes Lawrence Livermore National Laboratory (LLNL) and Spencer Composites Corporation (SCC) efforts in demonstrating an innovative approach to hydrogen delivery. This approach minimizes hydrogen delivery cost through utilization of glass fiber pressure vessels at 200 K and 70 MPa to produce a synergistic combination of container characteristics and properties of hydrogen gas: (1) hydrogen cooled to 200 K is ∼35% more compact for a small increase in theoretical storage energy (exergy); and (2) these cold temperatures (200 K) strengthen glass fibers by as much as 50%, expanding trailer capacity without the use of much more costly carbon fiber composite vessels.     

Properties improvement of composite layer of cryo-compressed hydrogen storage vessel by polyethylene glycol modified epoxy resin

The composite layer of cryo-compressed hydrogen (CcH₂) storage vessel is subjected to combined action of cryogenic temperate (CT) and high pressure, resulting in the failure of matrix. Herein, the transverse mechanical properties of composites were proved that plays a dominant role in the vessel through theoretical analysis, and epoxy resin was modified by polyethylene glycol 600 (PEG 600) to obtain high strength and toughness at CT. Subsequently, the composites prepared from modified resins were applied to the design of the vessel. The results demonstrate that the modified epoxy resin maintains high strength of 73.38 MPa while reducing elastic modulus by 45.31%. The 3 wt% PEG 600 modifier reduces winding plies by 32.65% and increases the utilization rate of carbon fiber strength by 50.69%. Conventional mechanical properties of carbon fibers can be employed in CcH₂ storage vessel design, with only a 6.06% increase in winding plies while preserving higher safety.     

Vehicle refueling with liquid hydrogen thermal compression

We have modeled an approach for dispensing pressurized hydrogen to 350 and/or 700 bar vehicle vessels. Instead of relying on compressors, this concept stores liquid hydrogen in cryogenic pressure vessels where pressurization occurs through heat transfer, reducing the station energy footprint from 12 kW h/kgH₂ of energy from the US grid mix to 1.5–2 kW h/kgH₂ of heating. This thermal compression station presents capital cost and reliability advantages by avoiding the expense and maintenance of high-pressure hydrogen compressors, at the detriment of some evaporative losses. The total installed capital cost for a 475 kg/day thermal compression hydrogen refueling station is estimated at about $611,500, an almost 60% cost reduction over today's refueling station cost. The cost for 700 bar dispensing is $5.23/kg H₂ for a conventional station vs. $5.45/kg H₂ for a thermal compression station. If there is a demand for 350 bar H₂ in addition to 700 bar dispensing, the cost of dispensing from a thermal compression station drops to $4.81/kg H₂, which is similar to the cost of a conventional station that dispenses 350 bar H₂ only. Thermal compression also offers capacity flexibility (wide range of pressure, temperature, and station demand) that makes it appealing for early market applications.     

Modeling of adsorption storage of hydrogen on activated carbons

The storage of hydrogen on board vehicles is one of the most critical issues for the transition towards an hydrogen-based transportation system. An electric vehicle powered by a typical gasoline tank will require 3.1 kg of hydrogen (H₂) to achieve a range of 500 km. Compared to a typical gasoline tank, this would correspond to a hydrogen density of 65 kg/m³ (including the storage system) and 6.5 wt%. Presently, only liquid hydrogen (LH₂) systems with a density of 51 kg/m³ and 14 wt% is close to this target. However, LH₂ is costly and requires more complex refueling systems. The physical adsorption of hydrogen on activated carbon can reduce the pressure required to store compressed gases. Though an efficient adsorption-based storage system for vehicular use of natural gas can be achieved at room temperature, the application of this technology to hydrogen using activated carbon as the adsorbent requires its operation at cryogenic temperature. We present the results of a parametric and comparative study of adsorption and compressed gas storage of hydrogen as a function of temperature, pressure and adsorbent properties. In particular, the isothermal hydrogen storage and net storage densities for passive and active storage systems operating at 77, 150 and 293 K are compared and discussed.     

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.     

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.     

The fill density of automotive cryo-compressed hydrogen vessels

Cryo-compressed hydrogen storage promises to deliver highest system storage density leading to 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. This paper shows a comprehensive evaluation of all factors affecting cryogenic vessel fill density, in an effort to evaluate system performance vs. operational parameters over a broad range of conditions. The results confirm previous experiments and models indicating that cryogenic vessels have maximum fill density of all available storage technologies, and fill density is most sensitive to daily driving distance and insulation performance. It is finally predicted that para-H₂ to ortho-H₂ conversion will affect most automobiles, increasing fill density by up to 5.3%. In a future world dominated by cryogenic H₂ fueled vehicles, para-H₂ to ortho-H₂ conversion inside the vessel will be the closest contact an average person will have with quantum mechanics outside of consumer electronics.