Onboard Storage Alternatives for Hydrogen Vehicles - Energy & Fuels

Energy & Fuels 1998, 12, 49-55

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Onboard Storage Alternatives for Hydrogen Vehicles† Gene D. Berry‡ and Salvador M. Aceves* Lawrence Livermore National Laboratory, 7000 East Ave., L-640, Livermore, California 94551 Received June 23, 1997. Revised Manuscript Received September 30, 1997X

Three viable technologies for storing hydrogen fuel on cars are currently available: compressed gas, metal hydride adsorption, and cryogenic liquid. However, each of these has significant disadvantages: volume, weight, boiling losses, or energy to compress or liquefy the hydrogen. Two alternative approaches are analyzed in this paper: pressure vessels with cryogenic capability and a combination of a metal hydride and liquid hydrogen storage. These alternatives are compared to baseline compressed hydrogen and liquid hydrogen (LH2) storage in terms of volume, vehicle range, dormancy, energy required for fuel processing, and cost. The results indicate that the alternative methods can result in a reduced volume, if packaging is a constraint; or in an extended range, if this is a desirable feature. Cryogenic pressure vessels, with one-fifth the insulation of LH2 systems, have comparable or better dormancy than LH2 systems. Energy requirements and cost appear favorable for the alternative systems.

Introduction Development and use of hydrogen-fueled automobiles and light trucks would yield important strategic, environmental, and economic benefits to the nation by reducing imported petroleum use, urban air pollution, and greenhouse gas emissions. Ultimately, hydrogen technologies could revolutionize energy and transportation markets worth more than $1 trillion annually in the next 20-25 years. Recent studies have indicated that hydrogen fuel costs are reasonable (less than $0.04/ km in 34 km/L gasoline equivalent vehicles; $0.06/mile in 80 mpg) using off-peak electricity or natural gas to either produce hydrogen on site at fueling stations, or delivering liquid hydrogen (LH2) by truck from larger, centralized plants.1-3 A more difficult issue facing the transition to hydrogen fuel is onboard storage for hydrogen vehicles. The onboard storage technology employed for hydrogen vehicles may dramatically influence the vehicle’s cost, range, performance, and fuel economy, as well as shape the scale, investment requirements, energy use, and potential emissions of a hydrogen refueling infrastructure. * Corresponding author. Phone: (510) 422 0864. Fax (510) 423 0618. E-mail: [email protected]. † Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. ‡ E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Ogden, J. M.; Dennis, E.; Steinbugler, M.; Strohbehn, J. Hydrogen Energy Systems Studies; Final Report to NREL for Contract No. XR-11265-2; Center for Energy and Environmental Studies, Princeton University, Princeton, NJ, 1995. (2) Berry, G. D. Hydrogen as a Transportation Fuel: Costs and Benefits; Lawrence Livermore National Laboratory Report UCRL-ID123465, 1996. (3) Thomas, C. E. Integrated Analysis of Transportation Demand Pathway Options for Hydrogen Production, Storage, and Distribution; Directed Technologies Inc. Arlington, VA. Draft Final Report under Modification 3 to Subcontract No. ACF-4-14266-01 for the National Renewable Energy Laboratory, 1997.

Hydrogen vehicles achieving 17-34 km/L (40-80 mpg) fuel economy need to store at least 5 kg of fuel onboard for adequate driving range (320-640 km; 200400 miles). Hydrogen could be stored compactly as a cryogenic liquid, or in hydride powders based on Mg, Fe, Ti, La, Ni, etc., which adsorb hydrogen and release it when warmed. LH2 storage suffers from potential hydrogen losses due to evaporation; and hydride-based approaches suffer from weight and cost concerns. Ambient high-pressure hydrogen storage has recently become more practical with improvements in composite materials, and fuel-efficient vehicle designs. Systems storing high-pressure (34.4 MPa; 5000 psia) hydrogen gas in composite vessels on fuel cell vehicles can weigh less than the equivalent-range full gasoline tank on an internal combustion engine vehicle. Composite pressure vessel costs depend principally on carbon fiber costs. Low-volume production cost projections are approximately $2000 for 5 kg of H2 storage. Mass production may lower costs to about $600.4 The crucial issue facing ambient (i.e., room temperature) high-pressure storage (i.e., 34.4 MPa) may be volume.5 More than 227 L (60 gal) are required to store 320-640 km (200-400 miles) worth of hydrogen fuel (5 kg) on a vehicle, raising packaging issues for lightduty vehicles. The volume can be reduced by shortening the driving range. Even in the United States, where driving distances are relatively large, it is estimated that 99% of trips and 85% of kilometers driven in (4) James, B. D.; Lomax, F. D.; Baum, G. N.; Thomas, C. E.; Kuhn, I. F. Jr. Comparison of Onboard Hydrogen Storage for Fuel Cell Vehicles; Directed Technologies Inc. Arlington, VA. Final Report under Subcontract 47-2-R31148 for Ford Motor Company under Prime Contract DE-ACO2-94CE50389 “Direct Hydrogen-Fueled Proton Exchange Membrane (PEM) Fuel Cell System for Transportation Applications” to the U.S. Department of Energy, 1996. (5) Pentastar Electronics, Inc. Direct-Hydrogen-Fueled ProtonExchange-Membrane Fuel Cell System for Transportation Application. Conceptual design report prepared for the U.S. Department of Energy Office of Transportation Technologies (Contract no. DE-AC0294CE50390), Huntsville, AL, March 1997.

S0887-0624(97)00094-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998

50 Energy & Fuels, Vol. 12, No. 1, 1998

Berry and Aceves

Table 1. Vessels for Vehicles with Same Range (640 km; 400 miles) as the 34.4 MPa (5000 psia) Compressed Hydrogen Vessel vessel parameter external volume, L internal volume, L volumetric eff, % max pressure, MPa mass of LH2, kg mass H2 in hydride, kg mass, compressed H2, kg total H2 mass, kg kg H2/m3 insulation thickness, cm total vessel weight, kg vehicle empty weight, kg vehicle fuel economy, km/L vehicle range with compressed H2, km max vehicle range, km

34.4 MPa (5000 psia) H2

cryogenic press. vessel 34.4 MPa

LH2 tank

237 216 90.9 34.4

126 81.2 64.6 34.4 5.17

135 78.5 58.6 0.69 5

195 49.3 77.7 0.69 3.14 2.5

5 5 21.1

1.88 5.17 41.1 1 66 1146 33.4 238 640

5 37.1 5 31.3 1091 34.1

5.64 28.9 5 213 1383 30.2 284 640

37.0 1100 34.0 640 640

passenger vehicles are for distances less than 120 km (75 miles) one way.6 Initially, however, hydrogen refueling sites may be limited. Consumers may demand cars with extended driving range. This study examines two alternative onboard hydrogen storage approaches in light of the issues existing with the currently available technologies. The first alternative is to use a cryogenic-capable pressure vessel. These vessels can dramatically improve upon the range and/or volume of compressed gas storage. Ideally, these systems could refuel with LH2 for occasional long distance trips, while using ambient high-pressure hydrogen (20.6-34.4 MPa; 3000-5000 psia) for the vast majority of driving needs. The second alternative is a hybrid system using both metal hydride storage and conventional LH2 tanks. A relatively small capacity hydride (storing about 2.5 kg of H2) is sufficient for a 288 km (180 mile) range, and can extend dormancy and allow ambient refueling, perhaps even at home. This study analyzes and compares these alternatives to two baseline technologies: ambient 34.4 MPa (5000 psia) storage and low-pressure LH2 storage. The comparison is done in terms of volume, range, venting losses, energy requirements, and cost. Later work will address cost with more detail and potential implications for a hydrogen refueling infrastructure. Methodology Modeling in this paper uses optimistic storage parameters for characterizing the two baseline storage technologies (compressed gas and liquid hydrogen), and conservative assumptions when analyzing alternative storage systems, to account for their relative immaturity. Data for baseline systems are drawn from a study by James et al.,4 while data for cryogenic composite pressure vessels are adapted from a recent study of ambient compressed natural gas storage.7 The conceptual designs for the different storage systems are added to a previously constructed hydrogen vehicle simulation.8 The vehicle fuel economy ranges (6) National Highway Traffic Safety Administration. Personal Travel in the United States Volume 1-2. 1983-1984 Nationwide Personal Transportation Study. August 1986. Table E-22, p E-23. (7) Institute of Gas Technology. Compressed Natural Gas Storage Optimization for Natural Gas Vehicles; Gas Research Institute Report GRI-96/0364, Des Plaines, IL 60018-1804, 1996. (8) Aceves, S. M.; Smith, J. R. A Hybrid Vehicle Evaluation Code and Its Application to Vehicle Design; SAE paper 950491, 1995.

640

LH2 tank and hydride

Figure 1. Conceptual design for a hydrogen hybrid electric vehicle achieving 34 km/L (80 mpg) equivalent fuel economy. Several onboard fuel storage options are shown.

from 30 to 34 km/L (70-80 mpg) gasoline-equivalent, representing a fuel cell electric vehicle or hybrid electric vehicle using an optimized hydrogen engine. PNGV class vehicle parameters for glider mass and drag are used as a basis (see Figure 1). The baseline compressed hydrogen storage (34.4 MPa, 5000 psia) has a volume of 237 L (63 gal) for storing 5 kg of hydrogen, and the resulting vehicle range is 640 km (400 miles). Alternative systems are designed to achieve a range or a total volume equal to this baseline system. Fuel consumption, venting, refueling, and onboard storage system pressures, temperatures, etc. are simulated under a variety of conditions to explore the tradeoffs, advantages, and disadvantages of each technology. The mass of onboard fuel storage systems ranges from 30 to 200 kg. To ensure a balanced comparison of storage systems, the vehicle analysis includes a 30% structural mass compounding factor (i.e., for every kg of incremental mass added to a vehicle design, an additional 0.3 kg is added to account for strengthening, etc. necessary for the additional mass), and it takes into account the additional power train mass required to maintain a fixed acceleration capability, and the additional fuel storage capacity to maintain constant vehicle range. Technical sketches of each storage system analyzed are given below. The storage system parameters are given in Tables1 and 2. Ambient Temperature 34.4 MPa Storage. James et al.4 estimate that an external volume of 323 L and 50-66 kg (depending upon the strength of carbon fibers used) are required to store 6.8 kg of H2 at 5000 psia in an advanced metallized polymer liner vessel wound with

Onboard Storage Alternatives for Hydrogen Vehicles

Energy & Fuels, Vol. 12, No. 1, 1998 51

Table 2. Vessels with Same External Volume as 34.4 MPa (5000 psia) Baseline Compressed Hydrogen Vessel vessel parameter external volume, L internal volume, L volumetric eff, % max pressure, MPa mass of LH2, kg mass H2 in hydride, kg mass, compressed H2, kg total H2 mass, kg kg H2/m3 insulation thickness, cm total vessel weight, kg vehicle empty weight, kg vehicle fuel economy, km/L vehicle range with compressed H2, km max vehicle range, km

34.4 MPa (5000 psia) H2 237 216 90.9 34.4 5 5 21.1 37.0 1100 34.0 640 640

LH2 tank

LH2 tank and hydride

237 153 64.6 0.69 9.75

237 78.5 58.6 0.69 5 2.5

9.75 41.1 5 61.1 1139 33.5

7.5 31.6 5 223 1398 30.0 285 853

1235

cryogenic press. vessel 20.6 MPa

34.4 MPa

237 178 75.0 20.6 11.4

237 161 67.8 34.4 10.3

2.57 11.4 47.9 1 104 1207 32.6 317 1401

3.73 10.3 43.2 1 131 1250 32.0 452 1244

carbon fiber (with a safety factor of 2.25). Scaling this (linearly, which likely understates the volume required, and is therefore conservative) to the 5 kg of H2 storage considered in this paper, results in an external volume of 237 L (62.7 gal) and a mass of 37 kg (including the H2 fuel). These characteristics represent the baseline storage system for comparison to other systems. Liquid Hydrogen Storage. James et al.4 estimate that an aluminum-walled LH2 tank containing 6.8 kg of H2 weighs 42.5 kg with an external volume of 187 L, using 5 cm of non-load-bearing multilayer vacuum insulation to achieve a boil-off rate of 1.8%/day with a heat leak of 0.64 W, 0.29 W of which is parasitic (i.e., not through the insulation). For comparison purposes we postulate a spherical LH2 tank, with a 0.29 W parasitic heat leak in order to represent the greatest thermal performance and smallest volume possible with LH2 storage. An LH2 tank sized for an external volume of 237 L (equal to the baseline ambient 34.4 MPa pressure vessel) would store 9.75 kg of LH2. Conceptual designs sized to 5 kg of LH2 and 3.14 kg of LH2 are examined as well (alone and/or supplemented by hydride buffer storage). Cryogenic Compressed Storage. Cryogenic conceptual designs use GRI pressure vessel data7 for carbon-composite pressure vessels with 2.5 mm thick aluminum liners, rated to 20.6 MPa (3000 psia). The values reported in ref 7 for the pressure vessels are scaled to higher pressures in some cases, assuming a linear relationship between carbon fiber requirements and pressure. Designs adapted from GRI data are heavier and less volume efficient (85% vs 90%) than the ambient 34.4 MPa (5000 psia) baseline vessel described earlier. This assumption should account for potential additional material requirements to meet the mechanical demands of cryogenic cycling. Preliminary cryogenic temperature cycling tests of similar carbon/ aluminum composite pressure vessels have shown no strength degradation.9 The pressure vessels are insulated with 1 cm of multilayer insulation (5 times less than the LH2 tanks). Three cryogenic pressure vessel conceptual designs are considered. They are sized to achieve either the same driving range (640 km), or

external volume (237 L), as the baseline ambient 34.4 MPa storage system.

(9) Structural Composites Industries. Performance of Lightweight Kevlar/Aluminum and Carbon/Aluminum Composite Pressure Vessels at Cryogenic Temperatures. SCI special report no. 86712, prepared by Vicki Lynn, Pomona, CA, May 1986.

(10) Peschka, W. Liquid Hydrogen: Fuel of the Future; SpringerVerlag: New York, 1992. (11) VanWylen, G. J.; Sonntag, R. E. Fundamentals of Classical Thermodynamics; John Wiley and Sons: New York, 1978.

(1) 34.4 MPa, 5.17 kg of LH2 capacity, 640 km range (2) 34.4 MPa, 10.26 kg of LH2 capacity, 237 L external volume (3) 20.6 MPa, 11.37 kg of LH2 capacity, 237 L external volume It is presumed that LH2 refueling will be possible at high pressure, to accommodate warm tanks which are partially filled with high-pressure hydrogen. Highpressure LH2 (25 MPa) pumps have been successfully demonstrated.10 Liquid Hydrogen Tanks Supplemented by Metal Hydrides. Two combination LH2/metal hydride storage systems are simulated to explore potential evaporation losses of the liquid hydrogen tank; one of these combinations has the same range (640 km), and the other has the same system volume (237 L) as the baseline 34.4 MPa ambient temperature pressure vessel. LH2 tanks are again modeled as idealized spheres, using the same methodology as for LH2-only systems. Hydrides are modeled as Fe-Ti-based alloys, assumed to achieve 1.3% H2 system weight, and a system density of 23.3 kg H2/m3 (scaled from 3 to 5 kg of H2 capacity systems).4 For dormancy calculations, the hydrides are treated as initially empty. For range calculations the hydrides are treated as initially full. The two combinations being analyzed are

(1) Fe-Ti hydride 2.5 kg of H2 capacity, 3.14 kg of LH2 capacity tank, 640 km total range (2) Fe-Ti hydride 2.5 kg of H2 capacity, 5.0 kg of LH2 capacity tank, 237 L total volume Thermal Analysis of Cryogenic Vessels Thermal analysis of the cryogenic vessels uses an expression for the first law of thermodynamics,11 taking into account the internal energy of the vessel materials


Figure 2. External storage system volume required to achieve 640 km (400 mile) range in a PNGV class hydrogen vehicle, using the storage systems in Table 1 and the vehicle parameters in Figure 1.

and fuel stored, and the enthalpy of the hydrogen flowing into and out of the vessel, as well as the heat transfer from the environment into the vessel. Heat transfer rate into the vessel has two components: heat transfer through the insulation, and parasitic heat transfer. Heat transfer through the insulation is calculated assuming a conductivity of 0.000 05 W/(m K) for vacuum multilayer insulation.4 Parasitic heat transfer (0.29 W, as mentioned earlier) is assumed to be constant throughout the process. The main assumptions in the thermal analysis are as follows: conditions are uniform inside the vessel; kinetic and potential energies are neglected; gaseous hydrogen is always extracted from the vessel; and hydrogen properties are equal to those for parahydrogen, as given by McCarty.12 No conversion between the para and ortho phases of hydrogen is considered in the analysis. This last assumption yields conservative results, because conversion from parahydrogen to orthohydrogen is endothermic. Any conversion would therefore tend to keep the vessel cold, reducing venting losses. A key point is that cryogenic pressure vessels are expected to have excellent dormancy due to (1) higher pressure capacity, extending the time before boiloff hydrogen is released, and (2) self-cooling of the stored hydrogen due to the flow work11 done as hydrogen is exhausted. Results Comparison of System Volume for Equal Range. Figure 2 shows the volume required to achieve a 640 km range in a PNGV class hydrogen vehicle, for vessels listed in Table 1. The baseline ambient 34.4 MPa (5000 psia) pressure vessel is rather large, requiring 237 L (63 gal). Storing this hydrogen as a cryogenic liquid in a conventional LH2 tank reduces system volume to 135 L, but the cryogenic pressure vessel is even more compact (126 L), because of reduced insulation requirements. The fourth option, a small LH2 tank combined with a metal hydride, sacrifices volume efficiency to extend dormancy and provide ambient range. Even with the fuel economy penalty for the heavier hydride tank, the LH2/hydride combination tank achieves the range of the baseline 34.4 MPa ambient pressure vessel (12) McCarty, R. D. Hydrogen: Its Technology and Implications, Hydrogen Properties, Volume III; CRC Press: Cleveland, OH, 1975.

Berry and Aceves

Figure 3. Driving range for a PNGV class vehicle for hydrogen storage systems with 237 L external volumes, for the storage systems detailed in Table 2. Cryogenic ranges are given where applicable, indicating the range available when refueled with LH2 (with 10% ullage space).

for only 80% of the baseline volume. In addition, the combination LH2/hydride allows this volume to be subdivided into two roughly equal volumes, possibly easing automotive body design constraints and packaging concerns. While the two alternative systems are sized to 640 km LH2 ranges, they also offer shorter, but significant (238-284 km) ranges, when fueled with ambient temperature hydrogen. Comparison of Vehicle Range for Equal Volume. Figure 3 shows driving range (both ambient and cryogenic where applicable) in a PNGV class hydrogen vehicle for five hydrogen storage technology options (from Table 2) with an equal external volume (237 L). It can be seen that cryogenic-capable hydrogen storage systems fueled with LH2 offer about twice the 640 km range of an ambient 34.4 MPa vessel. A cryogenic 34.4 MPa vessel of equal external volume achieves a cryogenic range of 1244 km and an ambient range of 452 km. Lowering the pressure from 34.4 to 20.6 MPa decreases ambient range from 452 to 317 km, but also reduces carbon fiber requirements, lowering costs and increasing internal volume to provide a cryogenic range of 1401 km. The thicker insulation requirements of lowpressure LH2 tanks reduce the internal volume available relative to cryogenic pressure vessels, so range for the LH2 tank is 1235 km. Supplementing a LH2 tank with an Fe-Ti hydride bed can provide ambient range (285 km), but the necessarily smaller LH2 tank reduces the total system range (LH2 tank and hydride combined) to 853 km. In summary, the cryogenic designs provide 35-70% of the range of the baseline 34.4 MPa technology on ambient-temperature hydrogen and 130-220% of the baseline range using LH2. Evaporation Losses of Cryogenic Hydrogen Systems. Figures 4-9 show the results of simulations calculating hydrogen fuel vented using each cryogenic hydrogen storage system. Three scenarios are chosen to explore the dormancy characteristics of liquid hydrogen tanks with and without hydrides and cryogenic pressure vessels. All of the storage systems are assumed to have been cooled to 20 K when fueled with LH2. Case 1. Vehicle is refueled to its maximum cryogenic range (with LH2) and parked indefinitely. This is the worst-case scenario that results in maximum venting losses. Figure 4 compares systems sized to achieve 640


Figure 4. Hydrogen fuel remaining in storage system as a function of time under a worst case scenario (vehicle is parked immediately after refueling) for the storage systems with fixed range (in Table 1). The hydride is assumed to be initially empty for the combination LH2/hydride systems.

Figure 5. Hydrogen fuel remaining in storage system as a function of time under a worst case scenario: vehicle is parked immediately after refueling, for the storage systems with fixed volume (in Table 2). The hydride is assumed to be initially empty for the combination LH2/hydride systems.


Figure 7. Hydrogen fuel remaining in storage system as a function of time under a “realistic” worst case scenario (vehicle is driven 120 km and then parked indefinitely) for the storage systems with fixed volume (in Table 2). The hydride is assumed to be initially empty for the combination LH2/hydride systems.

Figure 8. Hydrogen fuel remaining in storage system as a function of time, when the storage systems are partially fueled, to provide a 640 km (400 mile) range, for the storage systems with fixed volume (in Table 2). The hydride is assumed to be initially empty for the combination LH2/hydride systems.

Figure 6. Hydrogen fuel remaining in storage system as a function of time under a “realistic” worst case scenario (vehicle is driven 120 km and then parked indefinitely) for the storage systems with fixed range (in Table 1). The hydride is assumed to be initially empty for the combination LH2/hydride systems.

Figure 9. Cumulative losses (from a full tank of cryogenic hydrogen fuel) as a function of average daily driving distance for vehicles using the storage systems given in Tables 1 and 2. All storage systems are assumed to be cooled to 20 K upon refueling.

km ranges (listed in Table 1). The LH2 tank starts losing hydrogen after only 1 week of dormancy, and loses almost all of the hydrogen after 45 days. The cryogenic pressure vessel has 2 weeks of dormancy, but losses occur slowly, with over 60% of the hydrogen remaining after 60 days. The combination LH2/hydride has no losses for 3 weeks.

Figure 5 shows how bigger vessels (listed in Table 2) behave under the same scenario. All vessels in Figure 5 have a 237 L external volume. The conventional LH2 tank reaches maximum pressure after being parked 7-8 days and begins to vent hydrogen, losing nearly all of its hydrogen after 60 days. A 20.6 MPa cryogenic pressure vessel, under the same scenario, begins venting


hydrogen slightly later, after 9-10 days, but after being parked 60 days has nearly half of its fuel remaining (enough to achieve a 640 km range). A 34.4 MPa pressure vessel remains dormant more than twice as long as the LH2 tank (17 days), still retaining 70% of its hydrogen fuel after 2 months. If the LH2 tank is equipped with a 2.5 kg of H2 buffer hydride, then dormancy can be extended an additional 2 weeks and boiloff losses comparable to a 34.4 MPa pressure vessel can be achieved for times less than 40 days. Dormancy is only weakly dependent on size (i.e., surface to volume ratios) for cryogenic pressure vessels. Case 2. Vehicle is refueled to its maximum cryogenic range (with LH2), driven 120 km, and then parked indefinitely. This case is considered a “realistic” worstcase scenario, and the results are shown in Figures 6 and 7. Figure 6 shows the constant-range systems (Table 1), and Figure 7 shows the constant-volume systems (Table 2). The 120 km trip requires between 10 and 30% of the fuel stored in the different storage systems. In the case of LH2 systems, this small amount of driving can extend dormancy times to 9-14 days depending on tank size. For 34.4 MPa cryogenic pressure vessels, dormancy is extended to 3 weeks for both large and small tanks. Figures 6 and 7 also show that for a “realistic” worst-case scenario, adding a hydride bed to the LH2 tanks reduces range, but virtually eliminates dormancy concerns. Case 3. Vehicle is refueled (with LH2) to achieve a 640 km range and then parked indefinitely. Figure 8 compares the dormancy of cryogenic pressure vessels and LH2 tanks on an equal range basis. In this case the fuel systems (of equal external volume, listed in Table 2) are refueled with only enough liquid hydrogen to achieve a 640 km range. Then they are parked indefinitely. Cryogenic pressure vessels have a much greater dormancy than conventional LH2 tanks on an equal range basis, even though only a fifth of the insulation thickness is used. The 34.4 MPa cryogenic vessel does not vent hydrogen even after 60 days when filled to achieve a 640 km range. The 20.6 MPa vessel begins to vent after 40 days, and then only at a very slow rate (2 g/h). The preceding dormancy scenarios have established that, under worst-case scenarios, all of the cryogenic hydrogen storage systems considered can remain idle for more than 5 days without venting hydrogen. Moderate driving before parking extends the dormancy to over a week. A clearer and probably more relevant comparison of hydrogen onboard storage systems is shown in Figure 9. Cumulative fuel losses (out of one complete refueling) due to venting are calculated for each storage system as a function of average daily driving distance. LH2 tanks begin to lose fuel when driven less than 25-30 km daily. The 20.6 MPa cryogenic vessel vents no hydrogen when driven more than 16 km/day. Increasing the maximum vessel pressure to 34.4 MPa reduces the no-loss driving requirement to only 5-7 km/day. Adding hydride storage to LH2 tanks reduces losses to similar levels. Given that the U.S. passenger vehicles average 48 km (30 miles) per day, it can be reasonably expected that venting of hydrogen fuel would be negligible for all but the lowpressure LH2 systems, especially given that vehicles

Berry and Aceves

driven less than 16 km/day could use ambient hydrogen refueling. LH2-only vehicles, however, when driven 1015 miles per day could suffer 5-25% fuel economy loss. This is similar to the fuel economy loss incurred by heavy hydride systems. Energy Efficiency of Hydrogen Storage Options. An important element of the rationale for hydrogen (and other alternative fuels) is the reduction of greenhouse gases and other emissions. Particularly in the case of greenhouse gases, the energy required to produce, store, and use hydrogen is a good indicator of potential emissions. Furthermore, in the case of hydrogen, storage can require relatively large amounts of energy, directly, or indirectly (e.g., through additional storage system mass). It can be useful to express the energy required to store hydrogen as a fraction of the lower heating value, or LHV (33.32 kWh/kg of H2). Production of LH2 storage is the most energy intensive, requiring about 12 kWh of electricity per kilogram of hydrogen or about 35% of the energy content (LHV) of the hydrogen for liquefaction.13 Electricity requirements for compressed hydrogen vary with pressure, number of stages, etc. but are much lower (about 2.75 kWh/kg of H2).1 Low-pressure refueling of hydrides can eliminate the need for compression, requiring only waste heat to release the hydrogen, assuming a high enough engine or fuel cell exhaust temperature. Also important, however, is the effect upon fuel economy of hydrogen storage systems. Relative to the baseline 34.4 MPa (5000 psia) and conventional LH2 storage systems, the alternative storage systems are heavier, resulting in vehicle fuel economies as low as 30 km/L (70.8 mpg) (compared to 34 km/L for the baseline), after vehicle mass compounding. Hydrides, which may eliminate the need for compression electricity in the storage cycle, suffer an offsetting fuel economy penalty of up to 12%. LH2 systems driven less than 15 miles daily vent hydrogen and may suffer similar fuel economy losses (see Figure 9). Cryogenic pressure vessels, on the other hand, can drastically reduce or eliminate venting losses, and only require LH2 for very long trips (accounting for 15% of vehicle kilometers traveled in United States). When refueled with ambient high-pressure H2 at 20.6-34.4 MPa, compression energy requirements are equal or lower than 34.4 MPa ambient vessels. The fuel economy penalty due to extra mass of the cryogenic pressure vessels considered in this analysis is much lower than hydride systemssonly 1.5% on an equal range basis (see Table 1). In summary, ambient 34.4 MPa hydrogen offers much lower energy requirements than LH2 and significantly higher fuel economy than hydride/LH2 systems. Cryogenic pressure vessels can approach the energy requirements of ambient temperature compressed H2 storage, if they provide sufficient ambient range. The designs used in this analysis achieve 238-452 km (150-283 miles) ambient ranges. Storage System Capital Cost. Cryogenic pressure vessels could cost significantly less than conventional LH2 and ambient high-pressure storage because of dramatically reduced insulation and carbon fiber requirements relative to each technology. (13) Bracha, M.; Lorenz, G.; Patzelt, A.; Wanner; M. Large-Scale Hydrogen Liquefaction in Germany; Int. J. Hydrogen Energy 1994, 19, 53-59.


For example, the ambient 34.4 MPa pressure vessel (5 kg of H2) used as a baseline for comparison here is projected to cost $100-200/kg of H2 capacity in highvolume mass production (i.e., when carbon fiber is inexpensive). Of this cost, carbon fiber accounts for 4050%. A LH2 tank of equal capacity is projected to cost $70-150/kg H2 capacity of which cryogenic insulation accounts for 35-40%.4 Cryogenic pressure vessels require less than a fifth the insulation of the LH2 tank and store over twice the hydrogen of the baseline 34.4 MPa tank in this example (see Table 2). Depending upon ambient range requirements, the cryogenic vessel may operate at lower pressure, reducing carbon fiber requirements further. A conservative upper-bound cost estimate for cryogeniccapable pressure vessels can be calculated by combining the capital costs of conventional LH2 and ambient highpressure tank systems, scaled by the hydrogen density of each system relative to the LH2 storage density of the cryogenic pressure vessel. This calculation, based on the values listed above, results in projected cost of $114-240/kg H2, or $570-1200 for a 5 kg H2 tank. This is without any credit taken for lower parts count, reduced insulation, or carbon fiber requirements. Approximately 60% of the total cost would be associated with the LH2 equipment and 40% with the high pressure requirements of the cryogenic pressure vessel. Using the materials cost fractions cited above for cryogenic insulation in LH2 tanks, up to a 20% cost reduction might be expected to accrue from the reduced insulation needs of cryogenic pressure vessels. This results in an expected cost of $90-190/kg of LH2 capacity or $450-950 for a 640 km range 34.4 MPa cryogenic pressure vessel. This is slightly lower than the ambient vessel cost estimates from which it is derived. Carbon fiber costs in this estimate still account for $20-40/kg H2 capacity. If fiber costs scale directly with pressure then reducing the cryogenic pressure vessel to 20.6 MPa would result in a projected cost of $80-175/kg of H2. Reducing the pressure also reduces ambient range, however, so 20.6 MPa cryogenic vessels would probably be best suited to larger capacity tanks. These cost projections roughly translate to an $8001800 cost for the 1401 km range cryogenic pressure vessel used in the simulations. For reference, the baseline ambient 34.4 MPa pressure vessel providing only 640 km of range is estimated to cost $500-1000. In summary, cryogenic pressure vessels appear to offer comparable or perhaps lower cost than baseline technologies. Refined cost projections may change this picture somewhat, but the principal fact is that cryogenic pressure vessels store hydrogen at high density and pressure, reducing insulation and other materials related costs per kg of hydrogen stored. If materials


costs dominate in mature production of a technology, then their ultimate costs will likely be lower than other options. This will also be true if carbon fiber and vacuum insulation remain relatively costly, as is the case today. Conclusions This study has shown the significant potential volume, range, and energy efficiency advantages of using cryogenic-capable pressure vessels for hydrogen storage. They offer twice the range or half the volume of ambient high-pressure storage, without necessarily incurring the full energy penalty of LH2 storage, and with greatly enhanced dormancy. They also avoid the fuel economy penalties and mass compounding of combination LH2/ hydride systems with similar dormancy and ambient range. Cryogenic-capable pressure vessels also facilitate the use of small-scale on-site hydrogen production technologies which circumvent the energy penalties of liquefaction. Cryogenic pressure vessels also appear able to achieve comparable and perhaps lower costs than conventional storage through reduced insulation and carbon fiber requirements. Costs are likely to be sensitive to the balance between the pressure to meet ambient range requirements and the volume required to meet longrange driving requirements as well as assembly and ancillary costs. Future Work A refueling infrastructure for hydrogen vehicles using cryogenic pressure vessels would have to provide both LH2 and ambient high-pressure hydrogen. The relative amounts of these requirements will do a lot to shape the energy use and potential fuel cycle emissions associated with hydrogen vehicles, and hence the benefits they offer over today’s gasoline vehicles or other alternatives. In the near future, driving pattern data and information from electric vehicle market studies will be used to better understand ambient range requirements, and the impact of this ambient range on the energy, fuel cycle emissions, and costs of hydrogen vehicles using cryogenic pressure vessels. Knowledge of ambient range will also help to refine cost estimates and direct technical validation efforts on cryogenic pressure vessels. Acknowledgment. The authors acknowledge the encouragement and assistance of Glenn Rambach and Ray Smith in proposing and analyzing cryogenic pressure vessels for storing hydrogen on vehicles. EF9700947

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