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Hydrogen storage tanks for vehicles: Recent progress and current status
Current Opinion in Solid State and Materials Science 15 (2011) 39–43
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Hydrogen storage tanks for vehicles: Recent progress and current status Scott W. Jorgensen General Motors Research and Development, CML 480-106-160, 30500 Mound Road, Warren, MI 48090, USA
a r t i c l e
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Article history: Received 26 July 2010 Accepted 26 September 2010 Available online 8 October 2010 Keywords: Hydrogen storage Engineering Automotive storage systems
a b s t r a c t Hydrogen storage is an important enabler for fuel cell vehicles. This brief summary provides an overview of the state of the art in the engineering of hydrogen storage tanks over a wide range of technologies as reported in the open literature. Significant progress has been made in hydrogen storage. In many of the alternate storage techniques full scale experimental systems have been built and tested. In some cases these systems can supply hydrogen at required rates under most conditions, but further refinement is needed. At present, compressed gas cylinders and, to a lesser extent, cryogenic tanks remain the storage systems closest to commercialization in vehicles. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction As scientists and engineers search for environmentally sustainable ways to maintain a vibrant economy, increasing research effort has been focused on transportation methods that generate vanishingly small amounts of emissions. Barring a startling breakthrough in battery technology that enables a cost effective battery electric vehicle, nearly all transportation will continue to depend on some sort of powerplant for long range travel. The value of batteries to increase energy efficiency has been demonstrated in the current generation of hybrids and will be taken another step forward in the coming months with plugged in vehicles such as the Chevy Volt. Yet even these clean and efficient vehicles will still generate some distributed emissions. An alternative is another inherently electric vehicle, the fuel cell vehicle (FCV) which generates no distributed emissions in converting chemical energy to electrical energy. FCVs can theoretically be operated on a variety of fuels but they are cleanest and most efficient operating on hydrogen. Were it not for the low density and highly non-ideal behavior of hydrogen, compressed gas (CH2) storage would be the unquestioned solution given its simplicity of design and control. However, hydrogen is an elusive molecule, it is highly diffusive, buoyant, and relative to other gases it is unusually expensive to compress or liquefy. Hydrogen has been widely stored in gas form for over a century and in liquid form since the 1940s. These early attempts at hydrogen storage have some parallels to our current situation, joints had to be especially tight to avoid loss, materials had to withstand the embrittlement possible with hydrogen, and for application in rockets, mass and volume were important considerations. Today’s vehicular applications have all these concerns plus cost, durability E-mail address: [email protected] 1359-0286/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2010.09.004
and consumer acceptance requirements that make the transportation market challenging for any new fuel/powertrain system. Eager to enable clean hydrogen technology, governments around the world have taken a variety of steps to advance the technological readiness of the FCV. Most major automobile makers have undertaken their own technology programs, often leveraging government work, while assiduously pursuing the economic and performance aspects of these vehicles. Many governments have offered generous support, some governments have offered tax incentives, but perhaps the most influential contribution has been the development of goals [1–3]. Of course given the differing driving speeds and distances of concern to the people of the EU, Japan, the United States, and other nations, it is natural that there is some variation in the hydrogen storage targets established in different parts of the world. Yet the common fundamentals of storing hydrogen and the requirements of advanced proton exchange membrane (PEM) fuel cells have led to a fairly homogenous set of storage goals worldwide. The first formal goals are well represented by the United States Department of Energy (DOE) goals of 2003 [2]. This set of goals was based on the high business risk of introducing a new fuel source, a new powerplant – the fuel cell, and a new power delivery system in the form of power electrics and traction motors. In such an environment goals tend to be set high because of the high risk of meeting all goals at the same time. Initially the long term goals (goals that would satisfy all sorts of customers over the full range of vehicles types currently sold) tended toward 10% storage by mass, liquid hydrogen density, fill times in 3–5 min, and costs comparable to gasoline vehicle systems. These were acknowledged as difficult goals to reach but they did generate a great diversification in the areas of hydrogen storage research. In time, areas of the globe that tend to favor smaller vehicles and shorter driving distances began to advocate less aggressive goals. As progress was made in
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the fuel cell and other subsystems some of the demands were adjusted until today the DOE goals for 2015 are 5.5% by mass and 40 g/L with 5 min fill times; roughly half the 2015 goals from 7 or 8 years ago [4]. None the less, meeting all the current targets of any government, or more importantly the demands of customers, is still a formidable challenge. 2. Hydrogen storage system progress The methods of hydrogen storage currently pursued fall into a few general categories, each with their unique challenges. For systems where the hydrogen carrier is regenerated off the vehicle the major challenges are handling the hydrogen carrier, efficient recycle of the carrier, and often, though not always, the management of heat generated during hydrogen generation. Systems where hydrogen is filled directly into the vehicle are frequently further divided into physical containment, ‘complex’ hydrides, metal hydrides, and adsorption systems. Physical systems are the most advanced and not surprisingly most focused on cost reduction, though lowering pressure or increasing the cryogenic operation temperature are also focus areas. Complex hydrides are subject to the need to have a favorable equilibrium at operating temperatures, so the entropy change from solid to moderate pressure gas has forced the enthalpy to exceed 30 kJ/mol, thus heat transfer and kinetics in a 3 min fill and the hydrogen release temperature are both major issues [5]. Metal hydrides have only recently reached cycleable hydrogen content near 3%, so capacity, the mass of the system, and the strength of the tank required are concerns. Adsorption systems are typically low density and low enthalpy so system volume and operating temperature (or pressure) are the focusing issues. Clearly the challenges and options for the total storage system are dictated by the storage media being used, and improvements in the areas noted will have significant impact on tank efficiency and cost. 2.1. Physical containment Most vehicle makers have favored compressed hydrogen tanks for initial launches due to the technical readiness of these tanks and the cost projections for both the vehicle and the infrastructure. Type IV (all composite) or in some areas type III tanks (metal lined composite) would be used. Significant progress has been reported in the actual performance of these tanks in demonstration fleets, and in the standards that govern these tanks. Fill rates similar to conventional vehicles can be achieved with pre-cooling of the hydrogen and communication during the fill [6]. In real world use by a variety of drivers the improvement in fill, and thus the range available to the driver, has been marked. In the DOE demo program, the average communication fill is nearly 25% faster and achieves a better range at the same time [7–9 and associated publications]. Cost is the greatest single barrier to wide scale use of CH2 tanks. Carbon fiber is well known as a cost driver for these tanks but assembly cost is also large at present, and there is room for cost reduction in the binder as well. These are all areas of active research and all areas where added research funding could help accelerate hydrogen fueled vehicles to cost competitiveness. Two potential barriers to use of CH2 tanks are public acceptance and infrastructure development. In Asia there is significant concern that 70 MPa systems will not gain acceptance as easily as lower pressure systems, energy providers have also endorsed a medium pressure infrastructure. It is not yet clear what storage pressure will prove most viable overall. An option is to store the high pressure gas in substructures such as capillaries or microspheres. These systems have seen a recent resurgence of experimentation [10,11]. The advantage is the possibility to use a small number of high
pressure filling plants for the storage material, with distribution of the filled spheres or capillary arrays to fuel stations, and presumptively return of the exhausted spheres in the same trucks. The very small dimensions of these structures allow for a thin glass envelope and potentially high specific mass. The wall of the tank can be a low pressure structure that is light, inexpensive and in some cases conformable to the underbody of the vehicle. At present the energy needed to release the hydrogen from microspheres or other glass structures, their lower volumetric efficiency, and the long term durability of the structures have prevented commercial consideration in vehicles. Another alternative CH2 technology actively researched in recent years is the use of internal skeletons. This concept uses a sophisticated design of struts or web elements in tension to resist the force of compressed gas [12]. Sealing of edges, practical joining of the skeleton to the shell, and long term durability in the automotive environment remain research questions of interest before these innovations move past the lab and simulation stage. In 2006 researchers reported marked improvement in liquid hydrogen (LH2) storage with the announcement of the best specific mass of any automotive hydrogen storage system to date, 15% [13]. While there are varying ideas on what ancillary materials such as straps and fasteners should be included in the calculation of the specific mass of the storage system, this is still a very impressive value for the tank and valve box, even though this value does exclude other parts required for use on a vehicle. There is some concern about maintaining low heat transfer into such a tank over the life of a vehicle that will experience a wide range thermal and vibrational fatigue conditions. From an energy efficiency perspective, the energy consumed in hydrogen liquefaction is a problem that has reduced interest in liquid LH2 systems in North America – though there has been reconsideration of the so called well to tank efficiency, which includes both production and delivery. In Europe there has been significant interest in LH2 over the years [14,15]. Loss of hydrogen due to boil off has been a problem, though recent work indicates that for the most recent tank designs venting occurs only in the case of infrequent driving [13]. None the less, boil off is an area that requires improvement before LH2 systems are likely to find wide acceptance [14]. An alternate design that greatly reduces this problem is the cryo-compressed tank, a combined approach using high pressure at cryogenic temperatures. Hydrogen is highly compressible, and in the supercritical region, as a dense, low-temperature, high-pressure fluid it can readily exceed the normal density of liquid hydrogen. Recent work shows such tanks approach the levels of gravimetric and volumetric capacity required [16], so cost is the main impediment. The DOE cites this method as attaining at least 5.5%mass and 42 g/L. The method is flexible in fueling source, using liquid, gas or cooled gas. The major unknowns in addition to the true long term cost are durability and public acceptance of the lower efficiency of producing LH2 relative to compressing hydrogen. 2.2. Storage in hydrides The desire to improve upon the storage capacity of pure physical containment systems has led to a great interest in storage in or on materials. It would be indeed remarkable if any of these new technologies, which 10–15 years ago were unproven ideas for storage media, could be ready today for vehicle integration. While substantial progress has been made in finding new complex hydrides with high capacity, none are easily available and ready for application, so the well studied NaAlH4 remained the medium of choice for early tank programs. It is recognized that the capacity of NaAlH4 precludes it from actual application; it is a surrogate for the high content, lower enthalpy material that many hope to discover.
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Three significant programs have been publicly reported, United Technologies Research Center (UT), Sandia National Labs/General Motors (SNL/GM), and the Hamburg University of Technology working with the GKSS research center under the EU’s STORHY program [17–19]. Of these the STORHY system has so far only been tested for absorption, but it is similar to the more completely studied systems; the design was selected from an exhaustive set of possible design options [19]. UT built a scale system and performed calculations on full tank designs. Their design was motivated largely by maximizing storage capacity with less emphasis on cost. The shell and tube design used by UT placed the storage medium in the shell and the oil coolant in the tubes [17]. This design uses a shell able to handle both the chemical environment and the pressure required for rapid filling; the tubes must also meet these requirements. Heat transfer is ensured by a suitable spacing of the coolant tubes. The SNL/GM system was guided by minimization of cost to a larger extent, though capacity was also a major driving force. That system also used a shell and tube storage system but selected the opposite arrangement of design elements, the coolant flows in the shell and the storage media is in the tubes. SNL/GM also selected the modular tank concept rather than a unitary tank [18]. In this design only the tubes must be able to handle the filling pressure; of course the tubes are significantly larger diameter than coolant tubes would be. Modular design achieves a semi-conformable tank which is of value in vehicles. The tank tested had much higher capacity than the others, storing 3 kg of hydrogen. Densification of alanate is accomplished in different ways in the two design types, with alanate inside the tubes a mechanical ram permitted densification to 1 g/cc, while packing the alanate in the shell was done using vibration to achieve 0.6– 0.75 g/cc in this more complex geometry. None of these tanks has demonstrated over 1% storage for a complete, working system, though they are still lab models and were deliberately overdesigned. UT designed a minimal mass prototype predicted to provide 2% overall storage based on a carbon fiber shell and minimized tubing, but only simulation data is available at present. The SNL/GM tank has undergone the most testing. Despite the slow discharge kinetics of the Na3AlH6 phase, the tank was able to follow very demanding automotive simulations. Fill time with this design was roughly 10 min and similar results are reported with the STORHY tank which also placed the alanate inside the tubes; this is longer than is customary with compressed tanks [18,19,7]. These results indicate solid storage tanks can meet automotive demand cycles but a better media is required to achieve the specific mass already possible using a CH2 system. While none of these studies offered long term cost estimates, the materials used suggest that at their current state of development complex hydride tanks are more expensive than compressed tanks, though optimization studies are required to truly compare costs. For several decades teams around the world have developed a number of fuel tanks based on a range of metal hydrides with moderate plateau pressures, such as the AB5 and AB2 families, among others [20]. Thermal conductivity, heat of reaction and activation energy have been found to be critical material properties in designing efficient systems. While volumetrically efficient they were frequently very inefficient gravimetrically. Recently work in this area has shifted to stationary power [21]. The automotive design focus for these conventional hydrides has switched to high pressure designs. This combination of a pressure tank with a solid hydride is aimed at achieving the advantages of both methods. By placing a metal hydride in a conventional pressure vessel, the volumetric capacity of the vessel can be increased substantially, though at the cost of lowering the specific mass of hydrogen. This design has been pursued at several locations, among them Toyota Motor Company and Purdue University. The Toyota designs which sparked interest in this concept [22,23 and references there-in]
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include a 7 kg hydrogen tank that has demonstrated 80% fill in 5 min without external cooling. This is possible using the thermal inertia of the tank, heat transfer elements inside the tank that vent heat via the radiator, and a storage media with an enthalpy of hydrogenation typically lower than 25 kJ/mol, which lowers the total amount of heat that must be vented. Because of the low enthalpy, these systems also provide sufficient hydrogen pressure to operate a fuel cell at temperatures well below 0 °C. Furthermore, the relatively low heat required to release hydrogen, combined with the significant pressurized void volume in the tank means these designs can respond quickly to hydrogen demand changes. The Purdue studies offer some light on the design sensitivities, especially the distribution of heat exchange elements relative to the thermal conductivity of the storage media [24,25]. The nondimensional conductance developed in these works is likely to assist in improving future tank designs in all storage systems using a solid media. A number of general purpose storage systems based on metal hydrides in pressurized containers have been studied with similar performance, confirming the capabilities of this general type of storage system [20]. While there are many benefits to combining a metal hydride with a pressure vessel, nonetheless the specific mass of hydrogen is compromised relative to a simple compressed hydrogen tank; a specific mass of approximately 2% is stored in a high pressure hydride tank compared to over 3.5% for a simple CH2 tank. If a high pressure hydride system were to actually advance past a CH2 tank, the hydride would need to release a specific mass of hydrogen higher than 4%. To date the highest total capacity reported is 3.6% [26] but as the capacity of transition metal hydrides has increased the hydrogen content at the beginning of the plateau has increased as well, causing the working capacity to remain near 2%. The mass increase for a high pressure hydride storage system relative to a simple CH2 tank will be exacerbated by mass compounding in the vehicle [27]. This important principle accounts for the added weight from the larger propulsion system and support systems required as the overall mass of the vehicle increases. Alternatively, the additional mass may be compensated by use of higher-cost, light-weight materials elsewhere in the vehicle. The cost of reducing the vehicle mass in conjunction with the cost of the chromium rich hydrides that have achieved the best technical results are barriers to implementation of these high pressure hydride systems. Magnesium hydride has also had a long association with vehicles. Recent papers [28 and references there-in] look at improvements in kinetics and heat transfer to improve function in the storage media. These works indicate that improving heat transfer via expanded natural graphite or similar materials has the same benefits mentioned in the discussion of complex hydride and metal hydride tanks: increasing the volume of storage media that can be served by a cooling surface and reducing fill time. Still, for fuel cell vehicles the very high heat of dehydrogenation and the high temperature of hydrogen release require 1/3 of the hydrogen stored in MgH2 to be consumed in the process of releasing the hydrogen. This is a major impediment to implementation in a fuel cell vehicle. Using magnesium as a storage medium seems more feasible in applications that generate abundant, high temperature, waste heat. 2.3. Adsorption storage methods A great number of publications have issued in recent years on new materials to adsorb hydrogen at low-temperature, but publications on full or scaled tanks are scarce [29]. These few published storage system results along with computational simulations indicate that the moderate heat of adsorption for hydrogen on surfaces may make it possible to successfully build tanks with no internal
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heat transfer elements and achieve a dormancy of multiple weeks [30,31]. Key to this achievement is the use of cold hydrogen that flows through the system serving as not only the hydrogen source but also as a coolant to carry off the heat produced by adsorption of that hydrogen. The materials used in adsorption systems are very high surface area powders with a low density. Accordingly, the density of hydrogen stored in the complete storage system is also low, especially including the additional volume occupied by the cryogenic tank system itself. To ameliorate this problem the storage media must be compressed to the extent practicable. Pellets, pucks and other structures of compressed storage media, often using a small amount of binder, are a convenient approach that may also reduce the difficulty and cost of assembly. As assembly cost can be an important fraction of total vehicle cost this is a meaningful consideration. Such reports as are available indicate that while compression of the storage media increases the density of the material, its specific hydrogen capacity is often reduced. Despite this capacity loss, reducing the total volume of the system by compressing the storage media also reduces the system mass and cost by reducing the size of the cryogenic pressure vessel required. Of course if the insulation could be removed entirely and the operating pressure were raised dramatically then the volumetric capacity of these tanks might approach or surpass the high pressure metal hydrides and potentially even the basic compressed gas cylinder. In recent years attention has turned to high pressure adsorption tanks [see for example 32]. In this approach it is important that the adsorbent has a positive excess-adsorption at the operation pressure, the heat transfer system must be minimal or unnecessary, and the cost of the adsorbent must be low. Temperature gradients of 60 K are predicted in this type of system when using a steel tank. A composite tank may reduce gradients but at the expense of higher overall tank temperature during filling, and a lower level of hydrogen stored. Until there is data on full scale tanks it will be unclear exactly what capacity is possible [14]. At present further study is warranted to establish the capacity and delivery characteristics so that the cost benefit analysis can be conducted. 2.4. Storage via reactive chemicals In the past, there has been significant interest in hydrolysis as a way to supply hydrogen due to its supposed simplicity, and also its rapid hydrogen production that might allow a fast response to transients in hydrogen demand. The Chrysler Natrium vehicle remains the definitive study of these materials in automotive use. That work along with a broad spectrum of other factors influenced the recent downturn in research on borohydride hydrolysis. Briefly, use of hydrolysis for hydrogen production on vehicles is limited by the high level of heat produced in the reaction, the high energy required to recycle the boron oxide and hydroxide byproducts, exceptional wear in moving parts contacting the borohydride slurry or solution, and the fact that when an acceptably low ratio of water to borohydride is used in the feed, then the byproducts can solidify at lower temperatures [33,34]. It does not seem likely that hydrolysis will replace the other options available for use on vehicles. Interesting progress has been made in other hydrogen carrier materials. Among the more publicized are organic liquid carriers with a lower heat of reaction and higher hydrogen capacity than the prototype benzene–cyclohexane system, and slurries or solutions carrying ammonia borane (NH3BH3) or alane (AlH3). Currently these sorts of systems have only been demonstrated in the lab [35–37]. The critical focus of research for these systems will be to regenerate the fuel efficiently in terms of both energy and conservation of the carrier material [38]. Based on Chrysler’s experience in testing the NaBO4 system, engineers will also need an
early focus on part wear and liquid/solid phase balance in all temperatures and conditions.
3. Discussion A common challenge to all systems is cost. This information is of course very confidential and so the only costs available are those provided by the inventors and a few third party investigations. Argonne National labs in conjunction with TIAX have conducted a cost analysis of most major types of hydrogen storage systems at volumes of 500,000 vehicles per year using a largely common set of assumptions [39–41]. While this analysis is not as detailed as the cost analysis done by OEMs and did not explore as many options to reduce cost – especially in the long term, this does provide a publicly available and self consistent view of early costs of the various systems. These works suggest that while the costs of all systems will drop markedly and the range of cost between types of storage system will decrease at high production volume, compressed gas and cryo-compressed tanks maintain a cost advantage relative to the technically viable alternative storage systems. Investigation of the sources of cost point to the price of the storage media as a major difference in many cases, but supporting equipment (heat transfer and required filtering equipment) is another important source of system cost. It is to be noted that while the hydrolysis systems look very attractive in these analysis this is because major costs occur off-board, and the systems were costed as if they would not need part replacement at frequent intervals. Another key to implementation is the understanding and refinement that comes from testing fleets of vehicles made with near production tooling. The fleets of vehicles operating on compressed hydrogen and liquid hydrogen give vehicle makers their best insight on how to improve product performance and reliability at lower cost. Reviewing the large public database available [7–9 and similar publications] it is possible to see the significant generation-to-generation improvement. All systems currently in the lab or in the scale-model stage will need to pass through this refining process to reach implementation. Given the importance of cost and dependability in the consumer purchase decision, much weight must be given to the results of cost analysis and performance testing. Based on the foregoing, it is clear why CH2 and LH2 storage systems are the current candidates for initial hydrogen powered vehicles: they can supply hydrogen at the required maximum rate, they can respond accurately to rapid changes in demanded hydrogen flow, the time required to fill the tank is comparable to the time required for gasoline or diesel fueled vehicles, and there is already an accumulation of data on demo vehicles using these technologies [see also 14] and references therein. While these tanks are expensive at the current, extremely low, production volumes, the raw materials and the amounts of those materials needed at least allow for the idea that prices could be sustainable at high volume. It is this aspect that defines the most important research for furthering storage of hydrogen on the initial fuel cell vehicles – cost reduction. This can take the form of novel methods to produce lower cost carbon fiber, ways to use fibers with a wider distribution of tensile strength, or better ways to assemble tanks to improve throughput and reduce capital costs. Another valuable research area is developing an understanding of hydrogen production, delivery, and storage systems to enable appropriate and effective codes and standards. The motivations for alternates to CH2 still hold, but until there is a technology development path for these methods to compete with CH2 in terms of cost, mass, volume, and performance in all conditions automobiles encounter, the alternate storage systems will stay in the lab. Research that is required to advance the
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alternative storage methods include development of light, inexpensive, durable materials for the storage tank structure, heat transfer systems that are highly effective for solid phase storage materials, and the ongoing search for improved storage materials. Reducing the heat load by reducing the enthalpy and especially improving the challenging solid state kinetics of complex hydrides is critical. Increasing capacity of all storage media is important as well. It is for these reasons that the majority of research on alternative storage systems remains focused on storage materials. Once a material is found that has the potential to perform as well as CH2, then the storage tank design must be optimized to mask any shortcomings of the storage material and take full advantage its strengths. The design knowledge from the alternative tanks described Sections 2.2–2.4 will be helpful, but it would be a mistake to assume no further engineering research is needed. The degree of refinement that has occurred in the less complex CH2 tank strongly indicates there will be many iterations of optimization on solidstate hydrogen storage tank design to further reduce cost and improve function. A few obvious areas of future engineering research are design for simpler assembly, designs to improve start time and performance in very cold weather, and improved integration into the vehicle. Of course even if the solid-state methods of storing and transporting hydrogen do not break into the automobile market they may still find valuable application in other areas with different requirements. A few examples of potential markets are stationary energy storage, off-road and specialty mobile applications including forklifts, mine vehicles and the like, back-up power, possibly even consumer goods. Indeed, hydride storage has already been used in one of the most environmentally and technically challenging applications known, spacecraft [42]. This is not by chance but rather because the storage medium was well suited to the needs of the project and its shortcomings were of secondary importance. In vehicles, those needed strengths are safety, performance in many environments for many years, and cost; the storage system that fills those requirements better than any other is likely to prevail in the long term. Acknowledgement The author wishes to thank Michael Herrmann, Mei Cai, and Mark Verbrugge for review of this paper. References [1] Strubel V. Publishable final activity report; 2008.. [2] Chalk SG, Miller JF. Key challenges and recent progress in batteries, fuel cells, and hydrogen storage for clean energy systems. J Power Sources 2006;159(1):73–80. [3] Conte M, Prosini PP, Passerini S. Overview of energy/hydrogen storage: stateof-the-art of the technologies and prospects for nanomaterials. Mater Sci Eng 2004;B108:2–8. [4] DOE targets for on-board hydrogen storage systems for light-duty vehicles; February 2009. . [5] Zhang J, Fisher TS, Ramachandran PV, Gore JP, Mudawar I. A review of heat transfer issues in hydrogen storage technologies. J Heat Trans 2005;127(12):1391–9. [6] Maus S, Hapke J, Ranong CN, Wüchner E, Friedlmeier G, Wenger D. Filling procedure for vehicles with compressed hydrogen tanks. Int J Hydrogen Energy 2008;33(17):4612–21. [7] Wipke K, Sprik S, Kurtz J, Ramsden T. Controlled hydrogen fleet and infrastructure demonstration and validation project: spring 2010; composite data products, final version. NREL report no. TP-560-48173; March 29, 2010. [8] A variety of analysis of performance of multiple generations of FCV. . [9] Wipke K, Sprik S, Kurtz J, Ramsden T, Garbak J. US fuel cell vehicle learning demonstration: status update and early second-generation vehicle results. World Electr Veh Assoc J 2009;3. [10] Shelby JE, Hall MM, Snyder MJ, Wachtel PB. A radically new method for hydrogen storage in hollow glass microspheres. DoE final report; 2009.
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System level analysis of hydrogen storage options. DoE annual merit review; 2008. [40] Ahluwalia RK, Hua TQ, Peng JK. Fuel cycle efficiencies of different automotive on-board hydrogen storage options. Int J Hydrogen Energy 2007;32(15):3592–602. [41] Ahluwalia RK, Hua TQ, Peng JK, Lasher S, McKenny K, Sinha J, et al. Technical assessment of cryo-compressed hydrogen storage tank systems for automotive applications. Int J Hydrogen Energy 2010;35(9):4171–84. [42] Pearson D, Bowman R, Prina M, Wilson P. The Planck sorption cooler: using metal hydrides to produce 20 K. J Alloy Compd 2007;446–447:718–22.
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Current Opinion in Solid State and Materials Science journal homepage: www.elsevier.com/locate/cossms
Hydrogen storage tanks for vehicles: Recent progress and current status Scott W. Jorgensen General Motors Research and Development, CML 480-106-160, 30500 Mound Road, Warren, MI 48090, USA
a r t i c l e
i n f o
Article history: Received 26 July 2010 Accepted 26 September 2010 Available online 8 October 2010 Keywords: Hydrogen storage Engineering Automotive storage systems
a b s t r a c t Hydrogen storage is an important enabler for fuel cell vehicles. This brief summary provides an overview of the state of the art in the engineering of hydrogen storage tanks over a wide range of technologies as reported in the open literature. Significant progress has been made in hydrogen storage. In many of the alternate storage techniques full scale experimental systems have been built and tested. In some cases these systems can supply hydrogen at required rates under most conditions, but further refinement is needed. At present, compressed gas cylinders and, to a lesser extent, cryogenic tanks remain the storage systems closest to commercialization in vehicles. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction As scientists and engineers search for environmentally sustainable ways to maintain a vibrant economy, increasing research effort has been focused on transportation methods that generate vanishingly small amounts of emissions. Barring a startling breakthrough in battery technology that enables a cost effective battery electric vehicle, nearly all transportation will continue to depend on some sort of powerplant for long range travel. The value of batteries to increase energy efficiency has been demonstrated in the current generation of hybrids and will be taken another step forward in the coming months with plugged in vehicles such as the Chevy Volt. Yet even these clean and efficient vehicles will still generate some distributed emissions. An alternative is another inherently electric vehicle, the fuel cell vehicle (FCV) which generates no distributed emissions in converting chemical energy to electrical energy. FCVs can theoretically be operated on a variety of fuels but they are cleanest and most efficient operating on hydrogen. Were it not for the low density and highly non-ideal behavior of hydrogen, compressed gas (CH2) storage would be the unquestioned solution given its simplicity of design and control. However, hydrogen is an elusive molecule, it is highly diffusive, buoyant, and relative to other gases it is unusually expensive to compress or liquefy. Hydrogen has been widely stored in gas form for over a century and in liquid form since the 1940s. These early attempts at hydrogen storage have some parallels to our current situation, joints had to be especially tight to avoid loss, materials had to withstand the embrittlement possible with hydrogen, and for application in rockets, mass and volume were important considerations. Today’s vehicular applications have all these concerns plus cost, durability E-mail address: [email protected] 1359-0286/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2010.09.004
and consumer acceptance requirements that make the transportation market challenging for any new fuel/powertrain system. Eager to enable clean hydrogen technology, governments around the world have taken a variety of steps to advance the technological readiness of the FCV. Most major automobile makers have undertaken their own technology programs, often leveraging government work, while assiduously pursuing the economic and performance aspects of these vehicles. Many governments have offered generous support, some governments have offered tax incentives, but perhaps the most influential contribution has been the development of goals [1–3]. Of course given the differing driving speeds and distances of concern to the people of the EU, Japan, the United States, and other nations, it is natural that there is some variation in the hydrogen storage targets established in different parts of the world. Yet the common fundamentals of storing hydrogen and the requirements of advanced proton exchange membrane (PEM) fuel cells have led to a fairly homogenous set of storage goals worldwide. The first formal goals are well represented by the United States Department of Energy (DOE) goals of 2003 [2]. This set of goals was based on the high business risk of introducing a new fuel source, a new powerplant – the fuel cell, and a new power delivery system in the form of power electrics and traction motors. In such an environment goals tend to be set high because of the high risk of meeting all goals at the same time. Initially the long term goals (goals that would satisfy all sorts of customers over the full range of vehicles types currently sold) tended toward 10% storage by mass, liquid hydrogen density, fill times in 3–5 min, and costs comparable to gasoline vehicle systems. These were acknowledged as difficult goals to reach but they did generate a great diversification in the areas of hydrogen storage research. In time, areas of the globe that tend to favor smaller vehicles and shorter driving distances began to advocate less aggressive goals. As progress was made in
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the fuel cell and other subsystems some of the demands were adjusted until today the DOE goals for 2015 are 5.5% by mass and 40 g/L with 5 min fill times; roughly half the 2015 goals from 7 or 8 years ago [4]. None the less, meeting all the current targets of any government, or more importantly the demands of customers, is still a formidable challenge. 2. Hydrogen storage system progress The methods of hydrogen storage currently pursued fall into a few general categories, each with their unique challenges. For systems where the hydrogen carrier is regenerated off the vehicle the major challenges are handling the hydrogen carrier, efficient recycle of the carrier, and often, though not always, the management of heat generated during hydrogen generation. Systems where hydrogen is filled directly into the vehicle are frequently further divided into physical containment, ‘complex’ hydrides, metal hydrides, and adsorption systems. Physical systems are the most advanced and not surprisingly most focused on cost reduction, though lowering pressure or increasing the cryogenic operation temperature are also focus areas. Complex hydrides are subject to the need to have a favorable equilibrium at operating temperatures, so the entropy change from solid to moderate pressure gas has forced the enthalpy to exceed 30 kJ/mol, thus heat transfer and kinetics in a 3 min fill and the hydrogen release temperature are both major issues [5]. Metal hydrides have only recently reached cycleable hydrogen content near 3%, so capacity, the mass of the system, and the strength of the tank required are concerns. Adsorption systems are typically low density and low enthalpy so system volume and operating temperature (or pressure) are the focusing issues. Clearly the challenges and options for the total storage system are dictated by the storage media being used, and improvements in the areas noted will have significant impact on tank efficiency and cost. 2.1. Physical containment Most vehicle makers have favored compressed hydrogen tanks for initial launches due to the technical readiness of these tanks and the cost projections for both the vehicle and the infrastructure. Type IV (all composite) or in some areas type III tanks (metal lined composite) would be used. Significant progress has been reported in the actual performance of these tanks in demonstration fleets, and in the standards that govern these tanks. Fill rates similar to conventional vehicles can be achieved with pre-cooling of the hydrogen and communication during the fill [6]. In real world use by a variety of drivers the improvement in fill, and thus the range available to the driver, has been marked. In the DOE demo program, the average communication fill is nearly 25% faster and achieves a better range at the same time [7–9 and associated publications]. Cost is the greatest single barrier to wide scale use of CH2 tanks. Carbon fiber is well known as a cost driver for these tanks but assembly cost is also large at present, and there is room for cost reduction in the binder as well. These are all areas of active research and all areas where added research funding could help accelerate hydrogen fueled vehicles to cost competitiveness. Two potential barriers to use of CH2 tanks are public acceptance and infrastructure development. In Asia there is significant concern that 70 MPa systems will not gain acceptance as easily as lower pressure systems, energy providers have also endorsed a medium pressure infrastructure. It is not yet clear what storage pressure will prove most viable overall. An option is to store the high pressure gas in substructures such as capillaries or microspheres. These systems have seen a recent resurgence of experimentation [10,11]. The advantage is the possibility to use a small number of high
pressure filling plants for the storage material, with distribution of the filled spheres or capillary arrays to fuel stations, and presumptively return of the exhausted spheres in the same trucks. The very small dimensions of these structures allow for a thin glass envelope and potentially high specific mass. The wall of the tank can be a low pressure structure that is light, inexpensive and in some cases conformable to the underbody of the vehicle. At present the energy needed to release the hydrogen from microspheres or other glass structures, their lower volumetric efficiency, and the long term durability of the structures have prevented commercial consideration in vehicles. Another alternative CH2 technology actively researched in recent years is the use of internal skeletons. This concept uses a sophisticated design of struts or web elements in tension to resist the force of compressed gas [12]. Sealing of edges, practical joining of the skeleton to the shell, and long term durability in the automotive environment remain research questions of interest before these innovations move past the lab and simulation stage. In 2006 researchers reported marked improvement in liquid hydrogen (LH2) storage with the announcement of the best specific mass of any automotive hydrogen storage system to date, 15% [13]. While there are varying ideas on what ancillary materials such as straps and fasteners should be included in the calculation of the specific mass of the storage system, this is still a very impressive value for the tank and valve box, even though this value does exclude other parts required for use on a vehicle. There is some concern about maintaining low heat transfer into such a tank over the life of a vehicle that will experience a wide range thermal and vibrational fatigue conditions. From an energy efficiency perspective, the energy consumed in hydrogen liquefaction is a problem that has reduced interest in liquid LH2 systems in North America – though there has been reconsideration of the so called well to tank efficiency, which includes both production and delivery. In Europe there has been significant interest in LH2 over the years [14,15]. Loss of hydrogen due to boil off has been a problem, though recent work indicates that for the most recent tank designs venting occurs only in the case of infrequent driving [13]. None the less, boil off is an area that requires improvement before LH2 systems are likely to find wide acceptance [14]. An alternate design that greatly reduces this problem is the cryo-compressed tank, a combined approach using high pressure at cryogenic temperatures. Hydrogen is highly compressible, and in the supercritical region, as a dense, low-temperature, high-pressure fluid it can readily exceed the normal density of liquid hydrogen. Recent work shows such tanks approach the levels of gravimetric and volumetric capacity required [16], so cost is the main impediment. The DOE cites this method as attaining at least 5.5%mass and 42 g/L. The method is flexible in fueling source, using liquid, gas or cooled gas. The major unknowns in addition to the true long term cost are durability and public acceptance of the lower efficiency of producing LH2 relative to compressing hydrogen. 2.2. Storage in hydrides The desire to improve upon the storage capacity of pure physical containment systems has led to a great interest in storage in or on materials. It would be indeed remarkable if any of these new technologies, which 10–15 years ago were unproven ideas for storage media, could be ready today for vehicle integration. While substantial progress has been made in finding new complex hydrides with high capacity, none are easily available and ready for application, so the well studied NaAlH4 remained the medium of choice for early tank programs. It is recognized that the capacity of NaAlH4 precludes it from actual application; it is a surrogate for the high content, lower enthalpy material that many hope to discover.
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Three significant programs have been publicly reported, United Technologies Research Center (UT), Sandia National Labs/General Motors (SNL/GM), and the Hamburg University of Technology working with the GKSS research center under the EU’s STORHY program [17–19]. Of these the STORHY system has so far only been tested for absorption, but it is similar to the more completely studied systems; the design was selected from an exhaustive set of possible design options [19]. UT built a scale system and performed calculations on full tank designs. Their design was motivated largely by maximizing storage capacity with less emphasis on cost. The shell and tube design used by UT placed the storage medium in the shell and the oil coolant in the tubes [17]. This design uses a shell able to handle both the chemical environment and the pressure required for rapid filling; the tubes must also meet these requirements. Heat transfer is ensured by a suitable spacing of the coolant tubes. The SNL/GM system was guided by minimization of cost to a larger extent, though capacity was also a major driving force. That system also used a shell and tube storage system but selected the opposite arrangement of design elements, the coolant flows in the shell and the storage media is in the tubes. SNL/GM also selected the modular tank concept rather than a unitary tank [18]. In this design only the tubes must be able to handle the filling pressure; of course the tubes are significantly larger diameter than coolant tubes would be. Modular design achieves a semi-conformable tank which is of value in vehicles. The tank tested had much higher capacity than the others, storing 3 kg of hydrogen. Densification of alanate is accomplished in different ways in the two design types, with alanate inside the tubes a mechanical ram permitted densification to 1 g/cc, while packing the alanate in the shell was done using vibration to achieve 0.6– 0.75 g/cc in this more complex geometry. None of these tanks has demonstrated over 1% storage for a complete, working system, though they are still lab models and were deliberately overdesigned. UT designed a minimal mass prototype predicted to provide 2% overall storage based on a carbon fiber shell and minimized tubing, but only simulation data is available at present. The SNL/GM tank has undergone the most testing. Despite the slow discharge kinetics of the Na3AlH6 phase, the tank was able to follow very demanding automotive simulations. Fill time with this design was roughly 10 min and similar results are reported with the STORHY tank which also placed the alanate inside the tubes; this is longer than is customary with compressed tanks [18,19,7]. These results indicate solid storage tanks can meet automotive demand cycles but a better media is required to achieve the specific mass already possible using a CH2 system. While none of these studies offered long term cost estimates, the materials used suggest that at their current state of development complex hydride tanks are more expensive than compressed tanks, though optimization studies are required to truly compare costs. For several decades teams around the world have developed a number of fuel tanks based on a range of metal hydrides with moderate plateau pressures, such as the AB5 and AB2 families, among others [20]. Thermal conductivity, heat of reaction and activation energy have been found to be critical material properties in designing efficient systems. While volumetrically efficient they were frequently very inefficient gravimetrically. Recently work in this area has shifted to stationary power [21]. The automotive design focus for these conventional hydrides has switched to high pressure designs. This combination of a pressure tank with a solid hydride is aimed at achieving the advantages of both methods. By placing a metal hydride in a conventional pressure vessel, the volumetric capacity of the vessel can be increased substantially, though at the cost of lowering the specific mass of hydrogen. This design has been pursued at several locations, among them Toyota Motor Company and Purdue University. The Toyota designs which sparked interest in this concept [22,23 and references there-in]
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include a 7 kg hydrogen tank that has demonstrated 80% fill in 5 min without external cooling. This is possible using the thermal inertia of the tank, heat transfer elements inside the tank that vent heat via the radiator, and a storage media with an enthalpy of hydrogenation typically lower than 25 kJ/mol, which lowers the total amount of heat that must be vented. Because of the low enthalpy, these systems also provide sufficient hydrogen pressure to operate a fuel cell at temperatures well below 0 °C. Furthermore, the relatively low heat required to release hydrogen, combined with the significant pressurized void volume in the tank means these designs can respond quickly to hydrogen demand changes. The Purdue studies offer some light on the design sensitivities, especially the distribution of heat exchange elements relative to the thermal conductivity of the storage media [24,25]. The nondimensional conductance developed in these works is likely to assist in improving future tank designs in all storage systems using a solid media. A number of general purpose storage systems based on metal hydrides in pressurized containers have been studied with similar performance, confirming the capabilities of this general type of storage system [20]. While there are many benefits to combining a metal hydride with a pressure vessel, nonetheless the specific mass of hydrogen is compromised relative to a simple compressed hydrogen tank; a specific mass of approximately 2% is stored in a high pressure hydride tank compared to over 3.5% for a simple CH2 tank. If a high pressure hydride system were to actually advance past a CH2 tank, the hydride would need to release a specific mass of hydrogen higher than 4%. To date the highest total capacity reported is 3.6% [26] but as the capacity of transition metal hydrides has increased the hydrogen content at the beginning of the plateau has increased as well, causing the working capacity to remain near 2%. The mass increase for a high pressure hydride storage system relative to a simple CH2 tank will be exacerbated by mass compounding in the vehicle [27]. This important principle accounts for the added weight from the larger propulsion system and support systems required as the overall mass of the vehicle increases. Alternatively, the additional mass may be compensated by use of higher-cost, light-weight materials elsewhere in the vehicle. The cost of reducing the vehicle mass in conjunction with the cost of the chromium rich hydrides that have achieved the best technical results are barriers to implementation of these high pressure hydride systems. Magnesium hydride has also had a long association with vehicles. Recent papers [28 and references there-in] look at improvements in kinetics and heat transfer to improve function in the storage media. These works indicate that improving heat transfer via expanded natural graphite or similar materials has the same benefits mentioned in the discussion of complex hydride and metal hydride tanks: increasing the volume of storage media that can be served by a cooling surface and reducing fill time. Still, for fuel cell vehicles the very high heat of dehydrogenation and the high temperature of hydrogen release require 1/3 of the hydrogen stored in MgH2 to be consumed in the process of releasing the hydrogen. This is a major impediment to implementation in a fuel cell vehicle. Using magnesium as a storage medium seems more feasible in applications that generate abundant, high temperature, waste heat. 2.3. Adsorption storage methods A great number of publications have issued in recent years on new materials to adsorb hydrogen at low-temperature, but publications on full or scaled tanks are scarce [29]. These few published storage system results along with computational simulations indicate that the moderate heat of adsorption for hydrogen on surfaces may make it possible to successfully build tanks with no internal
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heat transfer elements and achieve a dormancy of multiple weeks [30,31]. Key to this achievement is the use of cold hydrogen that flows through the system serving as not only the hydrogen source but also as a coolant to carry off the heat produced by adsorption of that hydrogen. The materials used in adsorption systems are very high surface area powders with a low density. Accordingly, the density of hydrogen stored in the complete storage system is also low, especially including the additional volume occupied by the cryogenic tank system itself. To ameliorate this problem the storage media must be compressed to the extent practicable. Pellets, pucks and other structures of compressed storage media, often using a small amount of binder, are a convenient approach that may also reduce the difficulty and cost of assembly. As assembly cost can be an important fraction of total vehicle cost this is a meaningful consideration. Such reports as are available indicate that while compression of the storage media increases the density of the material, its specific hydrogen capacity is often reduced. Despite this capacity loss, reducing the total volume of the system by compressing the storage media also reduces the system mass and cost by reducing the size of the cryogenic pressure vessel required. Of course if the insulation could be removed entirely and the operating pressure were raised dramatically then the volumetric capacity of these tanks might approach or surpass the high pressure metal hydrides and potentially even the basic compressed gas cylinder. In recent years attention has turned to high pressure adsorption tanks [see for example 32]. In this approach it is important that the adsorbent has a positive excess-adsorption at the operation pressure, the heat transfer system must be minimal or unnecessary, and the cost of the adsorbent must be low. Temperature gradients of 60 K are predicted in this type of system when using a steel tank. A composite tank may reduce gradients but at the expense of higher overall tank temperature during filling, and a lower level of hydrogen stored. Until there is data on full scale tanks it will be unclear exactly what capacity is possible [14]. At present further study is warranted to establish the capacity and delivery characteristics so that the cost benefit analysis can be conducted. 2.4. Storage via reactive chemicals In the past, there has been significant interest in hydrolysis as a way to supply hydrogen due to its supposed simplicity, and also its rapid hydrogen production that might allow a fast response to transients in hydrogen demand. The Chrysler Natrium vehicle remains the definitive study of these materials in automotive use. That work along with a broad spectrum of other factors influenced the recent downturn in research on borohydride hydrolysis. Briefly, use of hydrolysis for hydrogen production on vehicles is limited by the high level of heat produced in the reaction, the high energy required to recycle the boron oxide and hydroxide byproducts, exceptional wear in moving parts contacting the borohydride slurry or solution, and the fact that when an acceptably low ratio of water to borohydride is used in the feed, then the byproducts can solidify at lower temperatures [33,34]. It does not seem likely that hydrolysis will replace the other options available for use on vehicles. Interesting progress has been made in other hydrogen carrier materials. Among the more publicized are organic liquid carriers with a lower heat of reaction and higher hydrogen capacity than the prototype benzene–cyclohexane system, and slurries or solutions carrying ammonia borane (NH3BH3) or alane (AlH3). Currently these sorts of systems have only been demonstrated in the lab [35–37]. The critical focus of research for these systems will be to regenerate the fuel efficiently in terms of both energy and conservation of the carrier material [38]. Based on Chrysler’s experience in testing the NaBO4 system, engineers will also need an
early focus on part wear and liquid/solid phase balance in all temperatures and conditions.
3. Discussion A common challenge to all systems is cost. This information is of course very confidential and so the only costs available are those provided by the inventors and a few third party investigations. Argonne National labs in conjunction with TIAX have conducted a cost analysis of most major types of hydrogen storage systems at volumes of 500,000 vehicles per year using a largely common set of assumptions [39–41]. While this analysis is not as detailed as the cost analysis done by OEMs and did not explore as many options to reduce cost – especially in the long term, this does provide a publicly available and self consistent view of early costs of the various systems. These works suggest that while the costs of all systems will drop markedly and the range of cost between types of storage system will decrease at high production volume, compressed gas and cryo-compressed tanks maintain a cost advantage relative to the technically viable alternative storage systems. Investigation of the sources of cost point to the price of the storage media as a major difference in many cases, but supporting equipment (heat transfer and required filtering equipment) is another important source of system cost. It is to be noted that while the hydrolysis systems look very attractive in these analysis this is because major costs occur off-board, and the systems were costed as if they would not need part replacement at frequent intervals. Another key to implementation is the understanding and refinement that comes from testing fleets of vehicles made with near production tooling. The fleets of vehicles operating on compressed hydrogen and liquid hydrogen give vehicle makers their best insight on how to improve product performance and reliability at lower cost. Reviewing the large public database available [7–9 and similar publications] it is possible to see the significant generation-to-generation improvement. All systems currently in the lab or in the scale-model stage will need to pass through this refining process to reach implementation. Given the importance of cost and dependability in the consumer purchase decision, much weight must be given to the results of cost analysis and performance testing. Based on the foregoing, it is clear why CH2 and LH2 storage systems are the current candidates for initial hydrogen powered vehicles: they can supply hydrogen at the required maximum rate, they can respond accurately to rapid changes in demanded hydrogen flow, the time required to fill the tank is comparable to the time required for gasoline or diesel fueled vehicles, and there is already an accumulation of data on demo vehicles using these technologies [see also 14] and references therein. While these tanks are expensive at the current, extremely low, production volumes, the raw materials and the amounts of those materials needed at least allow for the idea that prices could be sustainable at high volume. It is this aspect that defines the most important research for furthering storage of hydrogen on the initial fuel cell vehicles – cost reduction. This can take the form of novel methods to produce lower cost carbon fiber, ways to use fibers with a wider distribution of tensile strength, or better ways to assemble tanks to improve throughput and reduce capital costs. Another valuable research area is developing an understanding of hydrogen production, delivery, and storage systems to enable appropriate and effective codes and standards. The motivations for alternates to CH2 still hold, but until there is a technology development path for these methods to compete with CH2 in terms of cost, mass, volume, and performance in all conditions automobiles encounter, the alternate storage systems will stay in the lab. Research that is required to advance the
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alternative storage methods include development of light, inexpensive, durable materials for the storage tank structure, heat transfer systems that are highly effective for solid phase storage materials, and the ongoing search for improved storage materials. Reducing the heat load by reducing the enthalpy and especially improving the challenging solid state kinetics of complex hydrides is critical. Increasing capacity of all storage media is important as well. It is for these reasons that the majority of research on alternative storage systems remains focused on storage materials. Once a material is found that has the potential to perform as well as CH2, then the storage tank design must be optimized to mask any shortcomings of the storage material and take full advantage its strengths. The design knowledge from the alternative tanks described Sections 2.2–2.4 will be helpful, but it would be a mistake to assume no further engineering research is needed. The degree of refinement that has occurred in the less complex CH2 tank strongly indicates there will be many iterations of optimization on solidstate hydrogen storage tank design to further reduce cost and improve function. A few obvious areas of future engineering research are design for simpler assembly, designs to improve start time and performance in very cold weather, and improved integration into the vehicle. Of course even if the solid-state methods of storing and transporting hydrogen do not break into the automobile market they may still find valuable application in other areas with different requirements. A few examples of potential markets are stationary energy storage, off-road and specialty mobile applications including forklifts, mine vehicles and the like, back-up power, possibly even consumer goods. Indeed, hydride storage has already been used in one of the most environmentally and technically challenging applications known, spacecraft [42]. This is not by chance but rather because the storage medium was well suited to the needs of the project and its shortcomings were of secondary importance. In vehicles, those needed strengths are safety, performance in many environments for many years, and cost; the storage system that fills those requirements better than any other is likely to prevail in the long term. Acknowledgement The author wishes to thank Michael Herrmann, Mei Cai, and Mark Verbrugge for review of this paper. References [1] Strubel V. Publishable final activity report; 2008.
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[11] Zhevago NK, Glebov VI. Hydrogen storage in capillary arrays. Energy Convers Manage 2007;48:1554–9. [12] Aceves SM, Berry GD, Weisberg AH, Espinosa-Loza F, Perfect SA. Advanced concepts for vehicular containment of compressed and cryogenic hydrogen. In: Proceeding of the 16th world hydrogen energy conference; 2006.