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Requirements for advanced mobile storage systems
ht. J. Hydrogen Energy, Vol. 23, No. 9, pp. 803-814, 1998
Q 1998Published by Elsevier ScienceLtd on behalf of the International Association for Hydrogen Energy All rights reserved. Printed in Great Britain PII: SO360-31!N(97)00120-1 03s3199198 $19.00+0.00
Pergamon
REQUIREMENTS
FOR ADVANCED
MOBILE STORAGE SYSTEMS
R. EWALD Messer Griesheim GmbH Industriegase, D-47805 Krefeld, Germany
Abstract-Today, it is generally considered as a fact that hydrogen will play a decisive role in a future energy system, when fossil fuels have become scarceand thus expensive and/or unsuitable becauseof ecological reasons. Moreover, it is likely that hydrogen will be applied first and mainly for transportation at least until an efficient storage of electricity on-board of vehicles is available. Based on the experience of the Industrial Gases Industry and of chemical process engineering, on space technology and new developments, different methods of hydrogen storage have been proposed and partly investigated in the past. An important result is that the most favorable type of fuel tank system depends on the kind of vehicle and the mode of its operation. The paper gives a review of on-board storage methods with examples of applications. Requirements for storage systemswith respect to the various means of transportation are discussed, emphasizing ground transport by car and bus and mentioning aviation. Further development is necessarywhich has to be undertaken now as long as we are not under constraint regarding economy, safety and common acceptance.It is shown that the development of hydrogen technology can benefit the introduction of natural gas powered vehicles, in particular if it is stored as cryogenic liquid. 0 1998 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy
1. INTRODUCTION About 100 years ago the storage of hydrogen at a reduced volume in gas tight high pressure steel cylinders at 150 bar was developed. Long-term storage of hydrogen and transport to the point of use is possible since. Over severa decades the handling of hydrogen has been a domain of the chemical industry and in particular of the industrial gases companies. Together with the cylinder manufacturers, they developed the 200 bar steel cylinder which is generally used for transport of gaseous hydrogen still today. Road trailers with this type of pressure vessel (Fig. 1) are able to carry more than 4000 standard m3 of hydrogen. The start of a hydrogen powered Centaur type rocket in 1963 can be considered as the beginning of a specific liquid hydrogen technology. Cryogenic storage and transport equipment has been developed and is now the backbone of a safe and reliable system for distributing liquid hydrogen on an industrial level. Thus, the hydrogen technology was already well advanced when solutions for an on-board storage for hydrogen powered vehicles were searched for, in particular after the perceptions of the Club of Rome and the first petrol crisis initiated the idea of a hydrogen dominated energy system. 803
2. METHODS AVAILABLE FOR ON-BOARD STORAGE In the following the characteristic properties of the various methods (Fig. 2) will be discussedwith respect to on-board storage of hydrogen: Pressure vessel at ambient temperature
Storing the gas in normal 200 bar steel cylinders offers a considerable reduction of storage volume (by a factor of 180 compared to atmospheric pressure), but the packaging in steel results in a dead weight a 100 times the mass of the hydrogen. Composite pressure vessels, with a metallic liner and a
wrapping of fibers embedded in resin [l], constitute a considerable progress towards lightweight design with the worthwhile option of higher pressure up to 300 bar. Much higher pressures are not desirable because of the
increasing technical effort for infrastructure and high pressure accessoriesas well as due to the “filling factor” of hydrogen decreasingprogressively with rising pressure (difference of real gas from ideal gas law). Composite pressure vessels are used at an industrial
scale, e.g. for H, transport trailers. The 50% gain in
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Fig. 1. Trailer for transport of 4000 standard m3 of hydrogen at 200 bar.
@# cryo-compressed
5-u OS
1
2
BOKI
5
10
Grovlmelrlc Density
20
[ %H2 I
Fig. 2. Efficiency of hydrogen storage systems.
trailer transport capacity justifies the higher investment costs only for special transportation tasks.
refrigerant, e.g. liquid nitrogen, and an efficient thermal insulation are necessary.
Pressure vessel at low temperature
Pressure vessel combined with low temperature adsorption (cry0 adsorption)
By cooling the compressed gas to e.g. 80 K (liquid nitrogen temperature) an increase of storage density by factor 3.5 can be obtained. The method requires materials compatible with low temperatures and still needs research, especially for compound materials. Compared to conventionally pressurized hydrogen, various process equipment for cooling down and warming up as well as
Within a certain pressure range, a further increase in storage density is possible by filling the pressure vessel with an adequate adsorbent [2]. The gain of density dependsmainly on the type of adsorbent, on the pressure level, on the degree of desorption of hydrogen during discharging, and on the influence of poisoning of the
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adsorbent by impurities. As in the precedent case,special process equipment and energy sources have to be provided for filling and withdrawal. Liquid hydrogen
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Methanol and MCH being liquid at ambient temperature are easily stored on-board like gasoline. Methane or natural gas, on the other hand, needsstorage methods similar to those described for hydrogen. For the use of hydrogen in the engine (or fuel cell) the necessaryreforming has to take place on board the vehicle. In the caseof an internal combustion engine or certain fuel cells, the direct use of the organic hydride as a fuel would be the better solution, particularly in the caseof the hydrogen-rich methane.
The high density of more than 800 times the density of the gas at atmospheric conditions makesliquid hydrogen extremely attractive for on-board storage. There is no need for high pressurization. So in spite of the necessary thermal insulation the containing structure is lightweight Microspheres in comparison to other storage methods. In addition, the vessel is not subject to extreme pressure and/or temThe storage of hydrogen in hollow glass sphereswith perature cycles for filling and emptying. Due to the high a diameter of SO-100NM is still in a laboratory stage costs for liquefaction and the special handling require- and is mentioned for completenessonly. ments liquid hydrogen as an alternative fuel is not without competition. 3. REQUIREMENTS AND SOLUTIONS FOR DIFFERENT TYPES OF VEHICLES Sluxh hydrogen A mixture of about 50% solid and 50% liquid hydrogen at the triple point temperature (13.8 K) and the correspondent vapor pressure (0.07 bar) is called “slush hydrogen” [3]. Its higher density (15% more than liquid hydrogen) and higher refrigeration capacity (18% more) as well as its flow behavior similar to the liquid phase ha\e been considered as an advantage and investigated for space flight in the 1960sand for the planned supersonic spaceshuttle carrier in the 1980s[4]. The idea was given up becauseof, among others, the high production costs and the difficult handling causedby the fact that the vapor pressure is lower than the atmospheric pressure. Inorganic solid (= metal) hydrides
Suitable alloys on an iron-titanium basis are capable to bind hydrogen and thus to condense it to an amount of approximately twice the density of the compressedgas, whereas the mass stored is, with l-2% of the system mass, in the samerange. Since the formation of the hydride is an exothermic reaction, the storagemasshas to be cooled during loading and heated for discharging. That means additional processequipment which has to be added to the relatively expensive and heavy absorption material. On the other hand, the need of heat for discharging supports safeoperation. Even in case of a tank leakage, without simultaneous heating the quantity of hydrogen liberated to atmosphere is restricted. Impurities in the hydrogen to be stored may causea passivation of the hydrating material, thus reducing the storage capacity. Organic liquid hydrides
Organic compounds which can releasehydrogen by a chemical reforming reaction may be considered as hydrides, too, e.g. l l l
Methanol Methane Methylcyclohexane (MCH)
Requiremen tslgenerai
The final specification of one certain on-board storage system is the result of an optimization of the different requirements, the importance of each requirement depending on the intended applications as there are: l l l l l
Ground vehicles (passengercars, buses,. ) Aircraft Water vehicles (small ships, submarines,. ) Rail vehicles Spaceflight vehicles
The optimization is also a function of the energy transforming device chosen, e.g. a Internal combustion (IC) engine l Gasturbine l Fuel cell combined with electric drive l Rocket engine The most important requirements to be fulfilled by a storage system are discussedin the following. The storage capacity, which results from the type, the size, the drlvmg cycle, and the range of the vehicle, determines largely the type of storage system to be chosen. As a general rule low mass (weight) is important if acceleration plays a preponderant role, which is obviously the case for space and air flight, but also for the stop-and-go operation of passengercars and city buses. It is much lessimportant for trains and ships. SmaD storage volume is nearly as important as low mass.The usable spaceon a vehicle of given size is needed e.g. for passengersand should not be reduced considerably. Making the vehicle larger is often not possible (e.g. regulation existing for the maximum size of a city bus) or not convenient because of the negative influence on the overall weight and on the aerodynamic drag, both increasing fuel consumption. The compatibility of the tank geometry with the structure of the vehicle-is a bigger problem for the on-board storage of hydrogen than it is for conventional fuels.
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The shape of a gasoline tank for a modern passenger car is made of plastic material and fills unused cavities of the car body, in order to leave a maximum spacefor the passengersor other payload. A similar principle is applied for airplanes where the kerosenetank is mainly placed in the wings. A storage vessel for hydrogen, on the other hand, should have a surface-to-volume ratio as low as possible. This leads to weight reduction for high pressure storage and good thermal insulation for liquid hydrogen storage. In both cases the adopted solution is generally a cylindrical shape.For a hydride storage system,similar considerations also lead to a compact tank. The stringent requirements concerning the shape of the tanks have consequencesfor the design of hydrogen driven vehicles. Gasoline or diesel fueled cars or buses modified to be operated with hydrogen will generally not present an optimum solution. Successfulvehicles will have to be “tailored” for hydrogen. The lifetime of the storage system mainly has to coincide with the lifetime of the whole vehicle. That means a very short lifetime for a carrier rocket which brings a satellite into orbit within a few minutes and will not be used any more afterwards. For an airplane or a submarine, on the other hand, a lifetime of 20 years or more is required. For that reason the design of tanks and equipment cannot be the same for both cases; technological solutions and experience can be transferred only partly from one to the other. The required degreeof reliability of an on-board storage systemis linked to the nature and the purposes of the vehicle. It is evident that reliability plays a more important role for the use in space or air flight than for terrestrial use. Reliability is a crucial factor for the economical and safe operation of a vehicle, and hence decisive for the public acceptance of a future hydrogen fueled transport system. Regardless of the type of hydrogen driven vehicle, safety in operation is the basic precondition for an introduction to public use. This is particularly true for passenger transport devices. Experts know that the hazard in handling hydrogen is generally overestimated [S]. As the tank is indeed the central part of the storage system, the importance of the accessories is often overlooked. It is true that, in principle, all elements necessary for the on-board fuel management are available and in industrial use, e.g. high pressure and cryogenic valves, regulators, gauges for temperature, pressure and liquid level, pumps, metering devices, heat exchangers, couplings. But it is a matter of fact that all this equipment has to be adapted to the special conditions of a moving device which is handled (or at least used) by technical laymen. That means, besides fulfilling the above-mentioned requirements, the different accessoriesmust be simple but efficient in use and their number should be as reduced as possible, so that an easy handling in normal operation, maintenance, and repair is guaranteed. Optimizing a hydrogen storage systemby observing all discussedrequirements is a challenging engineering task. Which storage method is chosen for a defined application
depends on the total costs of the system, where both capital and operational costs together have to be considered. Research and development as well as field tests have to be carried out in order to get the necessary information on the system costs. Ground vehicles: aspects, examples
Numerous tests on hydrogen driven cars have been carried out in different countries over the last 20 years. In the beginning, the aspect of optimum fuel storage was secondary, and normal high pressure cylinders served for hydrogen reservoir. Later on, the metallic hydrides and liquid hydrogen played an increasing role. On the hydride side, the fleet of various types of vehicles (individual cars, taxis, vans) tested over around 800,000 km by Daimler-Benz in Berlin in the 1980sis well known 16, 71.
The advantage of liquid hydrogen for on-board storage has been recognized very early [8]. A Ford F250 pick-up truck modified in the U.S. in 1971 is reported to be the first liquid hydrogen fueled car [9], the technology was pushed forward by the Los Alamos Scientific Laboratory (since 1973) the Musashi Institute of Technology in Japan (since 1975) and the German Aerospace Research Association (DLR) (since 1978). With these works the use of liquid hydrogen appeared increasingly attractive. In the middle of the 1980s the German automobile manufacturer BMW decided to develop a liquid hydrogen powered car. One of the main tasks was the development and production of light weight, low volume, and highly insulated on-board tanks with official approval for operation on public roads. The co-operation with partners in the cryogenic and gasesindustry resulted in a stepwise development of a 120 1tank from a first design (Fig. 3), completely filling the car trunk, to an advanced tank which is installed behind the rear seat and leaves enough spacefor payload (Fig. 4). Over 10 years of development [lo] have improved the tank performance (Fig. 5) by factor 2 for insulation efficiency and mass and by factor 3 for volume and production costs. A schemeof one of the most recent tank designsis given by Fig. 6. Details on design, equipment, and operational aspectsare described in [ 111. The volume and hence the weight of the internal combustion engine for the same power output can be decreased by replacing the external mixture formation (applied e.g. by BMW) by direct high pressure injection of hydrogen (internal mixture formation). This principle is pursued by the Musashi Institute where appropriate high pressurehydrogen pumps (Fig. 2) and injection nozzles are developed and tested in passengercars [ 12, 131. Of course, high pressure injection can also be effectuated using high pressure storage of hydrogen. But that would result in higher on-board massas discussedabove, with a large part of the stored hydrogen (up to the injection pressure) not being usable. During the last years, the application of hydrogen for public transport has been investigated, especially in the
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STORAGE
Fig. 3. LH, fueled experimental car, 1984 (Source: DLR/Messer
. .
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Griesheim).
Fig. 4. \ fiew into the tlrunk of a LH, fueled e& riment, al car, 1995 (Source: BMW/Messer
framework of the Euro-Quebec Hydro Hydrogen Pilot Project (EQHHPP). Examples are: l
l
l
The world’s first city bus (Fig. 7) with an internal combustion engine and equipped with a liquid hydrogen storage systemhas been presented to the public in 1994 by Hydrogen Systems,Belgium, and Messer Griesheim, Germany [14, 151. An experimental city bus with an internal combustion engine has been realized in 1995 by MAN, Germany. The 3 x 200 1liquid hydrogen tanks delivered by Linde, Germany, are oval shaped and located under the bus floor [16]. Another experimental city bus with a PEM fuel cell as
Grie
an energy transformer is being built by Ansaldo, Italy [17]. 600 1liquid hydrogen will be stored in three superinsulated cylindrical vessels,manufactured by Messer Griesheim, and situated on the roof for reason of available spaceand safety. The experimental city bus of the Swiss-German“Hypasse” project [ 181is intended to be operated with a high pressure hydrogen fueled IC engine. Internal mixture formation is realized using 300 bar aramide fiber reinforced vessels with aluminum liner. In the United States, Ballard Power Systems have recently realized the second city bus equipped with a PEM fuel cell. The operational range is 400 km. Hydro-
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Slorage
Factor
tMJke1 Evaporellon
Rele
Fig. 5. Progress of efficiency of prototype LH2 fuel tanks.
Fig. 6. Modern on-board LH, fuel tank (schematic).
gen is stored in 150 bar fiberglass-wound aluminum cylinders installed on the roof. Most of the comparisons of on-board hydrogen storage systemswhich have been published in the past [9, 19, 201,concluded that liquid storage provides an advantage over pressurized hydrogen or hydrides, particularly if a higher operational range (2OS500 km) has to be taken into account. A recent study of the Musashi Institute (Fig. 8) confirms that a passengercar with a reasonable range i.e. with reasonable tank content has to be liquid fueled. Hydride and pressurized hydrogen at 150 bar show the additional inconvenience, besides the lower range, that their storage systemwould occupy nearly the whole payload available [21].
However, this may not be the final word on the subject becauseof the progress in technology, e.g. by upcoming reliable, highly efficient fuel cells (instead of internal combustion engines) and lightweight composite pressurevessels.This is confirmed by a study of Los Alamos Scientific Laboratories [22]. It shows that the reduction in hydrogen consumption due to fuel cell electric drive instead of IC engine results in a much smaller difference between the three modes of storageconsidered. For a given drive cycle and range, hydride storage (massfraction 1.5%) requires 18% more fuel than liquid hydrogen storage in conventional superinsulated vessels. Under the same conditions high pressurestorage in 330 bar composite vessels (mass fraction 4%, safety factor 2.5) results in only 4%
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Fig. 7. First LH, fueledexperimentalcity buswith internal combustionengine,1994.
0
Driving Range L (km) 300 100 200
I
400
A W=Weight of Fuel and Fuel Tank A W=lOO kg-LHz
Hydrogen Fueled Vehicles
,A~=S&kg-High
1 Pressure Cylinder
A W=300 kg-Metal Hydrides Electric Car
A W&SO0 kg-Battery Weight of Vehicle: 1500 kg without A W
Fig. 8. H2fueledvehicles:driving rangeandstorageweightfor differentstoragemethods(Source:MusashiInstitute of Technology).
higher consumption. Of course, more advancedcryogenic on-board tanks as those developed in Europe would probably have shown a greater advantage for liquid storage. The samestudy [22] points out that for a 500 km range the storage system volume for compressedgas is nearly three times that for liquid; but on the other hand it emphasizes that the compressed gas may be stored in multiple tanks at different locations in the vehicle. It seemslikely that the discussedrelationships between the storage modes are not only valid for cars of different sizes but also for buses. The question, which hydrogen storage system is the best for ground vehicles will be answeredby further development of the storage elements. The potential for R & D in the field is by far not exhausted.
Future research and development for ground vehicles
Independent of the kind of storage, an essential task is to demonstrate that hydrogen fueled vehicles can be operated as safe as conventional ones. Further detailed tests [23] have to be carefully carried out for each realistic operation failure and the caseof accident. For liquidstorage nearly all components merit an effort for further improvement, i.e. adaptation to the special requirements for on-board service. The following tasks are mentioned by way of example: a A new or modified tank insulation should allow low production costs as well as tank shapes other than cylindrical in order to bring the tank geometry in line with the spaceavailable on the vehicle. To reduce outer tank volumes the insulation interspace should be
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reduced to a minimum. In parallel, the requirements of insulation efficiency should be limited to what is really necessaryby providing an appropriate process design, e.g. boil-off management. Today’s performance of 1% daily evaporation rate might be exaggerated in some cases. l Plastic or composite materials with negligible permeation for hydrogen should be qualified as a structural tank material in order to reduce massand costs. l Simple tank design will contribute to low production costs, too. l If, even for mass production, an acceptable cost level cannot be achieved, the tanks should be conceived for being reused after the end of the vehicle lifetime. l Increasing the reliability of high pressure pumps and injectors should allow a larger application of internal mixture formation. Pumping principles other than mechanical should be considered. l The refrigeration capacity of liquid hydrogen should be used for cold injection and/or refrigeration of the combustion air. l In the case of external mixture formation an on-board low pressurefuel pump would avoid pressurizing of the tank and thus permit a simplified tank design. l Reliable, accurate, and low cost level gaugesand hydrogen gas detectors have to be developed. a The no-loss refueling requires an integrated process design comprising fuel station and on-board storage system. In that context reliability and accuracy of hydrogen metering devices have to be investigated to the end of correct invoicing to future customers. The potential of the composite vessel technology has not yet come to its end. Development of lighter and cheaper vesselshas to be pursued in the field of l
l l
l
wrapping material: high resistance and low costs of fibers and resins, liner: plastic instead of metallic, reliability to be increased: in order to reduce the safety factor for wrap thickness and to extend the periodic inspection intervals, low temperature qualification of the composite materials for the caseof cryogenic storage.
The cryoacisorption storage method would require much more research in order to find a high capacity, as well as light and cheap adsorption material. Metallic hydride storage has been investigated thoroughly in the past without considerably improving the low massratio. A big step advancing this technology does not seemto be in view. Organic hydrides merit R & D attention in order to investigate whether dehydrogenation necessaryto feed a fuel cell in an on-board reformer device can be realized without occupying volume and restricting payload to an unacceptable extent. Airplanes: aspects, examples, R & D need
Compared to kerosene, hydrogen contains 2.8 times more energy per mass unit. This makes hydrogen very
attractive as fuel for airplanes. Due to its low density it has to be stored in liquid state. The first experimental plane to fly with liquid hydrogen as propellant was a B57 Canberra in 1956/57. Later on, activities in this field were limited to hydrogen engines. This work was done by Pratt & Whitney, 195658. In the 1970sLockheed studied the feasibility of operating liquid hydrogen powered air freighters between the USA and the Middle East [9]. In the 1980s a supersonic intercontinental airplane which was also to be used as a carrier for spaceshuttles has been studied in several countries. The air-breathing engineswere planned to burn hydrogen. The refrigerating capacity of liquid hydrogen was proposed to be used for cooling the fuselage which was intended to be partly identical with the outer shell of the cryogenic tank. In 1988 a demonstration flight of a modified TU 154 with one of three engines running on hydrogen took place in the former Soviet Union. The liquid hydrogen reservoir was a cryogenic tank situated in the cabin. Within the German-Russian Project “Cryoplane”, the use of liquid hydrogen as a fuel for a modified Airbus 320, has been investigated since 1990 [24]. The requirements for the storage of liquid hydrogen on-board a plane differs essentially from those of ground vehicle tank systems. The most important shall be mentioned: l
l
l
l
The low mass design of the tank structure has high priority. A highly efficient insulation has to assurethat no hydrogen blow-off occurs during 12 hours of stand-still at night. Lifetime and reliability of the tank have to meet 120,000 pressure cycles. Liquid hydrogen pumps with high massBow and pressure head are compulsory to feed the engine.
In the caseof a modified Airbus 3 10with a flight range of 5000 km, 15,600 kg, i.e. more than 200 liquid m3 of hydrogen i.e. four times the volume of the energetic equivalent of kerosene have to be stored on board. A “fuselage-top” tank configuration (Fig. 9) has beenfound to be most suitable. For the realization of a first demonstrator the modification of a shorter range plane, a DO 328 with two hydrogen tanks attached to the wings, is planned (Fig. 10). For the whole project, important R & D activities are necessary,especially with regard to: l l
l l
Tank material (aluminum, aluminum composites, . . ) Tank insulation: material (MLI, foam, mineral fiber, microspheres, . . . ) and configuration (inside or outside, the hydrogen containing shell) Pumps and valves (bearings, seals,. . . ) The possible use of the refrigerating capacity of the liquid hydrogen for obtaining a laminar air flow on the fuselage would probably affect the concept of the storage system.
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Fig. 9. Cryoplane project: LH, tank configuration for modified Airbus 310 (Source: DASA)
Fig. 10. Cryoplane project: LH, tank configuration for modified DO 328 (Source: DASA).
Watercraft: aspects and examples
The subject of hydrogen driven ships did not find much attention in the past. Two examples of hydrogen powered boats shall be mentioned here. In the frame of the EQHHP project of a smallpassenger boat on the Lago Maggiore, Italy, (Fig. 11) will be equipped with a PEM fuel cell electric drive that is supplied with hydrogen. The 3 x 200 1 liquid tank groups will be installed on the rear roof. The overall concept is elaborated by Ansaldo and similar to that of the city bus described above. The fuel cell is manufactured by DeNora, the cryogenic storage system by Messer Grie-
sheim. Four submarines which have been ordered by the German navy are under construction at Howaldtswerke Deutsche Werft in Kiel. This new submarine class U2 12
is powered with a PEM fuel cell made by Siemenswhich runs on hydrogen and oxygen. The hydrogen is stored in long cylinders filled with an iron-titanium hydride. They are attached to the inner hull (Fig. 12) in the lower part of the spacebetween inner and outer hull in the rear of the boat. The relatively large mass of the hydrogen storage system is not relevant in the caseof a submarine. On the contrary, the massof the systemprovides a better stability of the vessel so that the advantages of a hydride storage discussedabove can fully be used [25]. 4. CONCLUSION Numerous test vehicles examined during the last ten years were to demonstrate the feasibility of using hydrogen as a propellant. A result is that the type of hydrogen
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Fig. 11.Passenger boat to be LH2 fueled.
Fig. 12.Hz fueledsubmarine:Configurationof hydride tanksat the hull (Source:HDW).
storage is a decisive factor. Three storage methods can be handled today, others are still in an early development stage. The choice between pressurized gas, cryogenic liquid and hydrides depends on the parameters given by the intended application of the vehicle and, what is often neglected, on the availability and the price of the fuel. For aviation only liquid hydrogen is possible. For road traffic liquid is advantageous, too, but further work in researchand development may change the situation. Parallel R & D efforts for aviation and road traffic will push forward the liquid storage technology. The realization of prototypes and demonstrators showed that-contrary to opinion sometimes expressed-a multitude of technological problems have still to be solved before introducing the hydrogen technology for the use of everyone under reliable and economic con-
ditions. This R & D has to be done now, not in a far future. becauseof at least two reasons: l
l
The product cycles in the automobile and particularly in the aircraft industry are long. The use of hydrogen for applications being already economical soon will be favored.
An example for such an application is the upcoming use of natural gas for vehicle propulsion. City bus fleets are already running on both, compressedgaseous(CNG) and liquid natural gas (LNG). The technology for storing the fuels on board which is similar to that of hydrogen is being applied to individual passengercars [26, 271.Fig. 13 shows a passengercar equipped with an LNG drive which has been in front of a mobile refueling station.
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Fig. 13. LNG fueled car with mobile refueling station.
Another example concerns the aviation field: Tupolew in Russia intends to develop a LNG powered plane. Finally, it can be stated that it is necessaryand worthwhile to advance the on-board storagetechnology. Only a mature and available technology will find its application. Ac&owledgemenf-The author expresses his gratitude to F. J. Edeskuty and S. Furuhama for having contributed valuable ideas to this paper.
REFERENCES 1. Dtiren, C., von Hagen, I., Junker, G., Kulgemeyer, A. and Rasche, C., Safety Aspects of Composite Pressure Vessels i’or the Transportation of Hydrogen on Trailers and for CNG Tanks in Vehicles, 3R International, Heft 6/1994, Vulkan-Verlag. 2. Hynek, S., Fuller, W., Bentley, J. and McCullough, J., Hydrogen Storage by Carbon Sorption, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994, p. 985, Int. Ass. Hydr. En. 3. Ewald, R., ProprittCs, utilisation et production de l’hydrogine et du deut&um sblides, Note CEA N963, 1968, Centre d’ktudes NuclCaires de Grenoble. 4. Dewitt, R. L., Hardy, T. L., Whalen, M. V. and Richter, G. P., Slush Hydrogen (SLH3 Technology Development for Application to the National Aerospace Plane (NASP), Advances in Cryogenic Engineering, Vol. 35, 1990, p. 1741, Plenum Press, N.Y. 5. BaumgBrtner, K. and Ewald, R., Sicherheitstechnik bei der Handhabune. von Wasserstoff, Chemie-Inu.-Technik 4.1987. 6. Feucht, K.,-Hiilzel, D. and. Hurich, *., Perspective of Mobile Hydrogen on Trailers and for CNC Tanks in Vehicles, Hydrogen Energy Progress VII, Proc. VIIth WHEC, 1988, p. 1963, Pergamon Press, N.Y. 7. Alternative Energy Sources for Road Transport: Hydrogen Drive Test, 1990, Verlag TtiV Rheinland, K6ln.
8. Williams, L. O., Hydrogen Powered Automobiles must use Liquid, Cryogenics, Dec. 1973. of the Future, 9. Peschka, W., Liquid Hydrogen-Fuel Springer, Wien/New York, 1992. 10. Ewald, R. and Kesten, M., Cryogenic Equipment of Liquid Hydrogen Powered Automobiles, Advances in Cryogenic Engineering No. 35, 1989, Plenum Press, N.Y. 11. Michel, F., Fieseler, H., Meyer, G. and TheiBen, F., OnBoard Equipment for Liquid Hydrogen Vehicles, Hydrogen Enerqv Prowess XI. Proc. XIth WHEC. 1996. Int. Ass. Hyd;.‘En. .. 12. Yamane, K., Nakamura, S., Nosset, T. and Furuhama, S., A Study on a Liquid Hydrogen Pump with Self-ClearanceAdjustment Structure, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994, p, 1919, Int. Ass. Hydr. En. 13. Furuhama, S., Hydrogen Engine Systems for Land Vehicles, ht. J. Hydrogen Energy, Voi. 14, No. 12, 1989, p. 907. 14. Vandenborre, H. and Sierens. R.. Greenbus. A Hvdroeen Fuelled City Bus, Hydrogen Eneriy Progress X, Pioc. %th WHEC, 1994, p. 1959, Int. Ass. Hydr. En. als Beispiel fiir ver15. Wurster, R., H,-Stadtbusprojekte schiedene ELI-Proiekte. YDI-Berichte. Nr. 1201. 1995. D. * 187, VDI-Verlag, i)iis&ldorf. 16. Riidiger, H., Seifers, S., Holzer, H. and Wolf, J., Liquid Hydrogen Storage System for Urban Bus, Hydrogen Energy Progress X. Proc. Xth WHEC, 1994, p. 967, Int. Ass. Hydr. En. 17. Marcenaro, B. G., EQHHPP FC Bus: Status of the Project and Presentation of the First Experimental Results, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994, p. 1447, Int. Ass. Hydr. En. Powered Automobiles using 18. Zieger, J., Hypasse-Hydrogen Seasonal and Weekly- S&&s of Electricity, H.vdrogen Enerqy Prowess X, Proc. Xth WHEC. 1994. D. 1367. lnt. Ass. Hydr. En. 19. Reister, D. and Strobl, W., Current Development and Outlook for the Hydrogen-Fuelled Car, Hydrogen Energy Proyrrss IX. Proc. IXth WHEC, 1992, p. 1201, Int. Ass. Hydr. En. Automobiles: On20. Ewald, R., Liquid Hydrogen-Fuelled
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Board and Stationary Cryogenic Installations, Cryogenics, Pro. Int. Cryog. Eng. Conf. XIII, 1990, p. 38, Cryogenics 1990,Vol. 30, Sept. Suppl., Int. Ass. Hydr. Eng. 2 1. Furuhama, S., Private Communication. 22. Daugherty, M. A., Prenger, F. C., Daucy, D. E., Hill, D. D. and Edeskuty, F. J., A Comparison of Hydrogen Vehicle Storage Options using the EPA Urban Driving Cycle Schedule, Advances in Cryogenic Engineering, Vol. 41, 1995,Plenum Press,N.Y. 23. Pehr, K., Aspects of Safety and Acceptance of LH, Tank Systems in Passenger Cars, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994,p. 1399,Int. Ass. Hydr. En.
24. Pohl, H. W., Hydrogen in Future Civil Aviation, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994, p. 1969, Int. Ass Hydr. En. 25. HDW-Wasserstoff-Energietechnologie 9/94, Information Brochure of Howaldtswerke Deutsche Werft, Kiel (communicated by K. Knaack). 26. Kesten, M., Equipment for the On-Board Gas Storage and the Refuelling of LNG Vehicles, Co& Proc. of Methamotion 1995(London), published by EUROPOINT, Harderwijk. 27. Strobl, W. and Heck, E., Wasserstoffantrieb und miigliche Zwischenschritte, VDI-Berichte, Nr. 1201, 1995, p. 173, VDI-Verlag Dusseldorf.
Q 1998Published by Elsevier ScienceLtd on behalf of the International Association for Hydrogen Energy All rights reserved. Printed in Great Britain PII: SO360-31!N(97)00120-1 03s3199198 $19.00+0.00
Pergamon
REQUIREMENTS
FOR ADVANCED
MOBILE STORAGE SYSTEMS
R. EWALD Messer Griesheim GmbH Industriegase, D-47805 Krefeld, Germany
Abstract-Today, it is generally considered as a fact that hydrogen will play a decisive role in a future energy system, when fossil fuels have become scarceand thus expensive and/or unsuitable becauseof ecological reasons. Moreover, it is likely that hydrogen will be applied first and mainly for transportation at least until an efficient storage of electricity on-board of vehicles is available. Based on the experience of the Industrial Gases Industry and of chemical process engineering, on space technology and new developments, different methods of hydrogen storage have been proposed and partly investigated in the past. An important result is that the most favorable type of fuel tank system depends on the kind of vehicle and the mode of its operation. The paper gives a review of on-board storage methods with examples of applications. Requirements for storage systemswith respect to the various means of transportation are discussed, emphasizing ground transport by car and bus and mentioning aviation. Further development is necessarywhich has to be undertaken now as long as we are not under constraint regarding economy, safety and common acceptance.It is shown that the development of hydrogen technology can benefit the introduction of natural gas powered vehicles, in particular if it is stored as cryogenic liquid. 0 1998 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy
1. INTRODUCTION About 100 years ago the storage of hydrogen at a reduced volume in gas tight high pressure steel cylinders at 150 bar was developed. Long-term storage of hydrogen and transport to the point of use is possible since. Over severa decades the handling of hydrogen has been a domain of the chemical industry and in particular of the industrial gases companies. Together with the cylinder manufacturers, they developed the 200 bar steel cylinder which is generally used for transport of gaseous hydrogen still today. Road trailers with this type of pressure vessel (Fig. 1) are able to carry more than 4000 standard m3 of hydrogen. The start of a hydrogen powered Centaur type rocket in 1963 can be considered as the beginning of a specific liquid hydrogen technology. Cryogenic storage and transport equipment has been developed and is now the backbone of a safe and reliable system for distributing liquid hydrogen on an industrial level. Thus, the hydrogen technology was already well advanced when solutions for an on-board storage for hydrogen powered vehicles were searched for, in particular after the perceptions of the Club of Rome and the first petrol crisis initiated the idea of a hydrogen dominated energy system. 803
2. METHODS AVAILABLE FOR ON-BOARD STORAGE In the following the characteristic properties of the various methods (Fig. 2) will be discussedwith respect to on-board storage of hydrogen: Pressure vessel at ambient temperature
Storing the gas in normal 200 bar steel cylinders offers a considerable reduction of storage volume (by a factor of 180 compared to atmospheric pressure), but the packaging in steel results in a dead weight a 100 times the mass of the hydrogen. Composite pressure vessels, with a metallic liner and a
wrapping of fibers embedded in resin [l], constitute a considerable progress towards lightweight design with the worthwhile option of higher pressure up to 300 bar. Much higher pressures are not desirable because of the
increasing technical effort for infrastructure and high pressure accessoriesas well as due to the “filling factor” of hydrogen decreasingprogressively with rising pressure (difference of real gas from ideal gas law). Composite pressure vessels are used at an industrial
scale, e.g. for H, transport trailers. The 50% gain in
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Fig. 1. Trailer for transport of 4000 standard m3 of hydrogen at 200 bar.
@# cryo-compressed
5-u OS
1
2
BOKI
5
10
Grovlmelrlc Density
20
[ %H2 I
Fig. 2. Efficiency of hydrogen storage systems.
trailer transport capacity justifies the higher investment costs only for special transportation tasks.
refrigerant, e.g. liquid nitrogen, and an efficient thermal insulation are necessary.
Pressure vessel at low temperature
Pressure vessel combined with low temperature adsorption (cry0 adsorption)
By cooling the compressed gas to e.g. 80 K (liquid nitrogen temperature) an increase of storage density by factor 3.5 can be obtained. The method requires materials compatible with low temperatures and still needs research, especially for compound materials. Compared to conventionally pressurized hydrogen, various process equipment for cooling down and warming up as well as
Within a certain pressure range, a further increase in storage density is possible by filling the pressure vessel with an adequate adsorbent [2]. The gain of density dependsmainly on the type of adsorbent, on the pressure level, on the degree of desorption of hydrogen during discharging, and on the influence of poisoning of the
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adsorbent by impurities. As in the precedent case,special process equipment and energy sources have to be provided for filling and withdrawal. Liquid hydrogen
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Methanol and MCH being liquid at ambient temperature are easily stored on-board like gasoline. Methane or natural gas, on the other hand, needsstorage methods similar to those described for hydrogen. For the use of hydrogen in the engine (or fuel cell) the necessaryreforming has to take place on board the vehicle. In the caseof an internal combustion engine or certain fuel cells, the direct use of the organic hydride as a fuel would be the better solution, particularly in the caseof the hydrogen-rich methane.
The high density of more than 800 times the density of the gas at atmospheric conditions makesliquid hydrogen extremely attractive for on-board storage. There is no need for high pressurization. So in spite of the necessary thermal insulation the containing structure is lightweight Microspheres in comparison to other storage methods. In addition, the vessel is not subject to extreme pressure and/or temThe storage of hydrogen in hollow glass sphereswith perature cycles for filling and emptying. Due to the high a diameter of SO-100NM is still in a laboratory stage costs for liquefaction and the special handling require- and is mentioned for completenessonly. ments liquid hydrogen as an alternative fuel is not without competition. 3. REQUIREMENTS AND SOLUTIONS FOR DIFFERENT TYPES OF VEHICLES Sluxh hydrogen A mixture of about 50% solid and 50% liquid hydrogen at the triple point temperature (13.8 K) and the correspondent vapor pressure (0.07 bar) is called “slush hydrogen” [3]. Its higher density (15% more than liquid hydrogen) and higher refrigeration capacity (18% more) as well as its flow behavior similar to the liquid phase ha\e been considered as an advantage and investigated for space flight in the 1960sand for the planned supersonic spaceshuttle carrier in the 1980s[4]. The idea was given up becauseof, among others, the high production costs and the difficult handling causedby the fact that the vapor pressure is lower than the atmospheric pressure. Inorganic solid (= metal) hydrides
Suitable alloys on an iron-titanium basis are capable to bind hydrogen and thus to condense it to an amount of approximately twice the density of the compressedgas, whereas the mass stored is, with l-2% of the system mass, in the samerange. Since the formation of the hydride is an exothermic reaction, the storagemasshas to be cooled during loading and heated for discharging. That means additional processequipment which has to be added to the relatively expensive and heavy absorption material. On the other hand, the need of heat for discharging supports safeoperation. Even in case of a tank leakage, without simultaneous heating the quantity of hydrogen liberated to atmosphere is restricted. Impurities in the hydrogen to be stored may causea passivation of the hydrating material, thus reducing the storage capacity. Organic liquid hydrides
Organic compounds which can releasehydrogen by a chemical reforming reaction may be considered as hydrides, too, e.g. l l l
Methanol Methane Methylcyclohexane (MCH)
Requiremen tslgenerai
The final specification of one certain on-board storage system is the result of an optimization of the different requirements, the importance of each requirement depending on the intended applications as there are: l l l l l
Ground vehicles (passengercars, buses,. ) Aircraft Water vehicles (small ships, submarines,. ) Rail vehicles Spaceflight vehicles
The optimization is also a function of the energy transforming device chosen, e.g. a Internal combustion (IC) engine l Gasturbine l Fuel cell combined with electric drive l Rocket engine The most important requirements to be fulfilled by a storage system are discussedin the following. The storage capacity, which results from the type, the size, the drlvmg cycle, and the range of the vehicle, determines largely the type of storage system to be chosen. As a general rule low mass (weight) is important if acceleration plays a preponderant role, which is obviously the case for space and air flight, but also for the stop-and-go operation of passengercars and city buses. It is much lessimportant for trains and ships. SmaD storage volume is nearly as important as low mass.The usable spaceon a vehicle of given size is needed e.g. for passengersand should not be reduced considerably. Making the vehicle larger is often not possible (e.g. regulation existing for the maximum size of a city bus) or not convenient because of the negative influence on the overall weight and on the aerodynamic drag, both increasing fuel consumption. The compatibility of the tank geometry with the structure of the vehicle-is a bigger problem for the on-board storage of hydrogen than it is for conventional fuels.
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The shape of a gasoline tank for a modern passenger car is made of plastic material and fills unused cavities of the car body, in order to leave a maximum spacefor the passengersor other payload. A similar principle is applied for airplanes where the kerosenetank is mainly placed in the wings. A storage vessel for hydrogen, on the other hand, should have a surface-to-volume ratio as low as possible. This leads to weight reduction for high pressure storage and good thermal insulation for liquid hydrogen storage. In both cases the adopted solution is generally a cylindrical shape.For a hydride storage system,similar considerations also lead to a compact tank. The stringent requirements concerning the shape of the tanks have consequencesfor the design of hydrogen driven vehicles. Gasoline or diesel fueled cars or buses modified to be operated with hydrogen will generally not present an optimum solution. Successfulvehicles will have to be “tailored” for hydrogen. The lifetime of the storage system mainly has to coincide with the lifetime of the whole vehicle. That means a very short lifetime for a carrier rocket which brings a satellite into orbit within a few minutes and will not be used any more afterwards. For an airplane or a submarine, on the other hand, a lifetime of 20 years or more is required. For that reason the design of tanks and equipment cannot be the same for both cases; technological solutions and experience can be transferred only partly from one to the other. The required degreeof reliability of an on-board storage systemis linked to the nature and the purposes of the vehicle. It is evident that reliability plays a more important role for the use in space or air flight than for terrestrial use. Reliability is a crucial factor for the economical and safe operation of a vehicle, and hence decisive for the public acceptance of a future hydrogen fueled transport system. Regardless of the type of hydrogen driven vehicle, safety in operation is the basic precondition for an introduction to public use. This is particularly true for passenger transport devices. Experts know that the hazard in handling hydrogen is generally overestimated [S]. As the tank is indeed the central part of the storage system, the importance of the accessories is often overlooked. It is true that, in principle, all elements necessary for the on-board fuel management are available and in industrial use, e.g. high pressure and cryogenic valves, regulators, gauges for temperature, pressure and liquid level, pumps, metering devices, heat exchangers, couplings. But it is a matter of fact that all this equipment has to be adapted to the special conditions of a moving device which is handled (or at least used) by technical laymen. That means, besides fulfilling the above-mentioned requirements, the different accessoriesmust be simple but efficient in use and their number should be as reduced as possible, so that an easy handling in normal operation, maintenance, and repair is guaranteed. Optimizing a hydrogen storage systemby observing all discussedrequirements is a challenging engineering task. Which storage method is chosen for a defined application
depends on the total costs of the system, where both capital and operational costs together have to be considered. Research and development as well as field tests have to be carried out in order to get the necessary information on the system costs. Ground vehicles: aspects, examples
Numerous tests on hydrogen driven cars have been carried out in different countries over the last 20 years. In the beginning, the aspect of optimum fuel storage was secondary, and normal high pressure cylinders served for hydrogen reservoir. Later on, the metallic hydrides and liquid hydrogen played an increasing role. On the hydride side, the fleet of various types of vehicles (individual cars, taxis, vans) tested over around 800,000 km by Daimler-Benz in Berlin in the 1980sis well known 16, 71.
The advantage of liquid hydrogen for on-board storage has been recognized very early [8]. A Ford F250 pick-up truck modified in the U.S. in 1971 is reported to be the first liquid hydrogen fueled car [9], the technology was pushed forward by the Los Alamos Scientific Laboratory (since 1973) the Musashi Institute of Technology in Japan (since 1975) and the German Aerospace Research Association (DLR) (since 1978). With these works the use of liquid hydrogen appeared increasingly attractive. In the middle of the 1980s the German automobile manufacturer BMW decided to develop a liquid hydrogen powered car. One of the main tasks was the development and production of light weight, low volume, and highly insulated on-board tanks with official approval for operation on public roads. The co-operation with partners in the cryogenic and gasesindustry resulted in a stepwise development of a 120 1tank from a first design (Fig. 3), completely filling the car trunk, to an advanced tank which is installed behind the rear seat and leaves enough spacefor payload (Fig. 4). Over 10 years of development [lo] have improved the tank performance (Fig. 5) by factor 2 for insulation efficiency and mass and by factor 3 for volume and production costs. A schemeof one of the most recent tank designsis given by Fig. 6. Details on design, equipment, and operational aspectsare described in [ 111. The volume and hence the weight of the internal combustion engine for the same power output can be decreased by replacing the external mixture formation (applied e.g. by BMW) by direct high pressure injection of hydrogen (internal mixture formation). This principle is pursued by the Musashi Institute where appropriate high pressurehydrogen pumps (Fig. 2) and injection nozzles are developed and tested in passengercars [ 12, 131. Of course, high pressure injection can also be effectuated using high pressure storage of hydrogen. But that would result in higher on-board massas discussedabove, with a large part of the stored hydrogen (up to the injection pressure) not being usable. During the last years, the application of hydrogen for public transport has been investigated, especially in the
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STORAGE
Fig. 3. LH, fueled experimental car, 1984 (Source: DLR/Messer
. .
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Griesheim).
Fig. 4. \ fiew into the tlrunk of a LH, fueled e& riment, al car, 1995 (Source: BMW/Messer
framework of the Euro-Quebec Hydro Hydrogen Pilot Project (EQHHPP). Examples are: l
l
l
The world’s first city bus (Fig. 7) with an internal combustion engine and equipped with a liquid hydrogen storage systemhas been presented to the public in 1994 by Hydrogen Systems,Belgium, and Messer Griesheim, Germany [14, 151. An experimental city bus with an internal combustion engine has been realized in 1995 by MAN, Germany. The 3 x 200 1liquid hydrogen tanks delivered by Linde, Germany, are oval shaped and located under the bus floor [16]. Another experimental city bus with a PEM fuel cell as
Grie
an energy transformer is being built by Ansaldo, Italy [17]. 600 1liquid hydrogen will be stored in three superinsulated cylindrical vessels,manufactured by Messer Griesheim, and situated on the roof for reason of available spaceand safety. The experimental city bus of the Swiss-German“Hypasse” project [ 181is intended to be operated with a high pressure hydrogen fueled IC engine. Internal mixture formation is realized using 300 bar aramide fiber reinforced vessels with aluminum liner. In the United States, Ballard Power Systems have recently realized the second city bus equipped with a PEM fuel cell. The operational range is 400 km. Hydro-
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Slorage
Factor
tMJke1 Evaporellon
Rele
Fig. 5. Progress of efficiency of prototype LH2 fuel tanks.
Fig. 6. Modern on-board LH, fuel tank (schematic).
gen is stored in 150 bar fiberglass-wound aluminum cylinders installed on the roof. Most of the comparisons of on-board hydrogen storage systemswhich have been published in the past [9, 19, 201,concluded that liquid storage provides an advantage over pressurized hydrogen or hydrides, particularly if a higher operational range (2OS500 km) has to be taken into account. A recent study of the Musashi Institute (Fig. 8) confirms that a passengercar with a reasonable range i.e. with reasonable tank content has to be liquid fueled. Hydride and pressurized hydrogen at 150 bar show the additional inconvenience, besides the lower range, that their storage systemwould occupy nearly the whole payload available [21].
However, this may not be the final word on the subject becauseof the progress in technology, e.g. by upcoming reliable, highly efficient fuel cells (instead of internal combustion engines) and lightweight composite pressurevessels.This is confirmed by a study of Los Alamos Scientific Laboratories [22]. It shows that the reduction in hydrogen consumption due to fuel cell electric drive instead of IC engine results in a much smaller difference between the three modes of storageconsidered. For a given drive cycle and range, hydride storage (massfraction 1.5%) requires 18% more fuel than liquid hydrogen storage in conventional superinsulated vessels. Under the same conditions high pressurestorage in 330 bar composite vessels (mass fraction 4%, safety factor 2.5) results in only 4%
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ADVANCED MOBILE STORAGESYSTEMS
Fig. 7. First LH, fueledexperimentalcity buswith internal combustionengine,1994.
0
Driving Range L (km) 300 100 200
I
400
A W=Weight of Fuel and Fuel Tank A W=lOO kg-LHz
Hydrogen Fueled Vehicles
,A~=S&kg-High
1 Pressure Cylinder
A W=300 kg-Metal Hydrides Electric Car
A W&SO0 kg-Battery Weight of Vehicle: 1500 kg without A W
Fig. 8. H2fueledvehicles:driving rangeandstorageweightfor differentstoragemethods(Source:MusashiInstitute of Technology).
higher consumption. Of course, more advancedcryogenic on-board tanks as those developed in Europe would probably have shown a greater advantage for liquid storage. The samestudy [22] points out that for a 500 km range the storage system volume for compressedgas is nearly three times that for liquid; but on the other hand it emphasizes that the compressed gas may be stored in multiple tanks at different locations in the vehicle. It seemslikely that the discussedrelationships between the storage modes are not only valid for cars of different sizes but also for buses. The question, which hydrogen storage system is the best for ground vehicles will be answeredby further development of the storage elements. The potential for R & D in the field is by far not exhausted.
Future research and development for ground vehicles
Independent of the kind of storage, an essential task is to demonstrate that hydrogen fueled vehicles can be operated as safe as conventional ones. Further detailed tests [23] have to be carefully carried out for each realistic operation failure and the caseof accident. For liquidstorage nearly all components merit an effort for further improvement, i.e. adaptation to the special requirements for on-board service. The following tasks are mentioned by way of example: a A new or modified tank insulation should allow low production costs as well as tank shapes other than cylindrical in order to bring the tank geometry in line with the spaceavailable on the vehicle. To reduce outer tank volumes the insulation interspace should be
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reduced to a minimum. In parallel, the requirements of insulation efficiency should be limited to what is really necessaryby providing an appropriate process design, e.g. boil-off management. Today’s performance of 1% daily evaporation rate might be exaggerated in some cases. l Plastic or composite materials with negligible permeation for hydrogen should be qualified as a structural tank material in order to reduce massand costs. l Simple tank design will contribute to low production costs, too. l If, even for mass production, an acceptable cost level cannot be achieved, the tanks should be conceived for being reused after the end of the vehicle lifetime. l Increasing the reliability of high pressure pumps and injectors should allow a larger application of internal mixture formation. Pumping principles other than mechanical should be considered. l The refrigeration capacity of liquid hydrogen should be used for cold injection and/or refrigeration of the combustion air. l In the case of external mixture formation an on-board low pressurefuel pump would avoid pressurizing of the tank and thus permit a simplified tank design. l Reliable, accurate, and low cost level gaugesand hydrogen gas detectors have to be developed. a The no-loss refueling requires an integrated process design comprising fuel station and on-board storage system. In that context reliability and accuracy of hydrogen metering devices have to be investigated to the end of correct invoicing to future customers. The potential of the composite vessel technology has not yet come to its end. Development of lighter and cheaper vesselshas to be pursued in the field of l
l l
l
wrapping material: high resistance and low costs of fibers and resins, liner: plastic instead of metallic, reliability to be increased: in order to reduce the safety factor for wrap thickness and to extend the periodic inspection intervals, low temperature qualification of the composite materials for the caseof cryogenic storage.
The cryoacisorption storage method would require much more research in order to find a high capacity, as well as light and cheap adsorption material. Metallic hydride storage has been investigated thoroughly in the past without considerably improving the low massratio. A big step advancing this technology does not seemto be in view. Organic hydrides merit R & D attention in order to investigate whether dehydrogenation necessaryto feed a fuel cell in an on-board reformer device can be realized without occupying volume and restricting payload to an unacceptable extent. Airplanes: aspects, examples, R & D need
Compared to kerosene, hydrogen contains 2.8 times more energy per mass unit. This makes hydrogen very
attractive as fuel for airplanes. Due to its low density it has to be stored in liquid state. The first experimental plane to fly with liquid hydrogen as propellant was a B57 Canberra in 1956/57. Later on, activities in this field were limited to hydrogen engines. This work was done by Pratt & Whitney, 195658. In the 1970sLockheed studied the feasibility of operating liquid hydrogen powered air freighters between the USA and the Middle East [9]. In the 1980s a supersonic intercontinental airplane which was also to be used as a carrier for spaceshuttles has been studied in several countries. The air-breathing engineswere planned to burn hydrogen. The refrigerating capacity of liquid hydrogen was proposed to be used for cooling the fuselage which was intended to be partly identical with the outer shell of the cryogenic tank. In 1988 a demonstration flight of a modified TU 154 with one of three engines running on hydrogen took place in the former Soviet Union. The liquid hydrogen reservoir was a cryogenic tank situated in the cabin. Within the German-Russian Project “Cryoplane”, the use of liquid hydrogen as a fuel for a modified Airbus 320, has been investigated since 1990 [24]. The requirements for the storage of liquid hydrogen on-board a plane differs essentially from those of ground vehicle tank systems. The most important shall be mentioned: l
l
l
l
The low mass design of the tank structure has high priority. A highly efficient insulation has to assurethat no hydrogen blow-off occurs during 12 hours of stand-still at night. Lifetime and reliability of the tank have to meet 120,000 pressure cycles. Liquid hydrogen pumps with high massBow and pressure head are compulsory to feed the engine.
In the caseof a modified Airbus 3 10with a flight range of 5000 km, 15,600 kg, i.e. more than 200 liquid m3 of hydrogen i.e. four times the volume of the energetic equivalent of kerosene have to be stored on board. A “fuselage-top” tank configuration (Fig. 9) has beenfound to be most suitable. For the realization of a first demonstrator the modification of a shorter range plane, a DO 328 with two hydrogen tanks attached to the wings, is planned (Fig. 10). For the whole project, important R & D activities are necessary,especially with regard to: l l
l l
Tank material (aluminum, aluminum composites, . . ) Tank insulation: material (MLI, foam, mineral fiber, microspheres, . . . ) and configuration (inside or outside, the hydrogen containing shell) Pumps and valves (bearings, seals,. . . ) The possible use of the refrigerating capacity of the liquid hydrogen for obtaining a laminar air flow on the fuselage would probably affect the concept of the storage system.
ADVANCED MOBILE STORAGE SYSTEMS
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Fig. 9. Cryoplane project: LH, tank configuration for modified Airbus 310 (Source: DASA)
Fig. 10. Cryoplane project: LH, tank configuration for modified DO 328 (Source: DASA).
Watercraft: aspects and examples
The subject of hydrogen driven ships did not find much attention in the past. Two examples of hydrogen powered boats shall be mentioned here. In the frame of the EQHHP project of a smallpassenger boat on the Lago Maggiore, Italy, (Fig. 11) will be equipped with a PEM fuel cell electric drive that is supplied with hydrogen. The 3 x 200 1 liquid tank groups will be installed on the rear roof. The overall concept is elaborated by Ansaldo and similar to that of the city bus described above. The fuel cell is manufactured by DeNora, the cryogenic storage system by Messer Grie-
sheim. Four submarines which have been ordered by the German navy are under construction at Howaldtswerke Deutsche Werft in Kiel. This new submarine class U2 12
is powered with a PEM fuel cell made by Siemenswhich runs on hydrogen and oxygen. The hydrogen is stored in long cylinders filled with an iron-titanium hydride. They are attached to the inner hull (Fig. 12) in the lower part of the spacebetween inner and outer hull in the rear of the boat. The relatively large mass of the hydrogen storage system is not relevant in the caseof a submarine. On the contrary, the massof the systemprovides a better stability of the vessel so that the advantages of a hydride storage discussedabove can fully be used [25]. 4. CONCLUSION Numerous test vehicles examined during the last ten years were to demonstrate the feasibility of using hydrogen as a propellant. A result is that the type of hydrogen
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Fig. 11.Passenger boat to be LH2 fueled.
Fig. 12.Hz fueledsubmarine:Configurationof hydride tanksat the hull (Source:HDW).
storage is a decisive factor. Three storage methods can be handled today, others are still in an early development stage. The choice between pressurized gas, cryogenic liquid and hydrides depends on the parameters given by the intended application of the vehicle and, what is often neglected, on the availability and the price of the fuel. For aviation only liquid hydrogen is possible. For road traffic liquid is advantageous, too, but further work in researchand development may change the situation. Parallel R & D efforts for aviation and road traffic will push forward the liquid storage technology. The realization of prototypes and demonstrators showed that-contrary to opinion sometimes expressed-a multitude of technological problems have still to be solved before introducing the hydrogen technology for the use of everyone under reliable and economic con-
ditions. This R & D has to be done now, not in a far future. becauseof at least two reasons: l
l
The product cycles in the automobile and particularly in the aircraft industry are long. The use of hydrogen for applications being already economical soon will be favored.
An example for such an application is the upcoming use of natural gas for vehicle propulsion. City bus fleets are already running on both, compressedgaseous(CNG) and liquid natural gas (LNG). The technology for storing the fuels on board which is similar to that of hydrogen is being applied to individual passengercars [26, 271.Fig. 13 shows a passengercar equipped with an LNG drive which has been in front of a mobile refueling station.
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Fig. 13. LNG fueled car with mobile refueling station.
Another example concerns the aviation field: Tupolew in Russia intends to develop a LNG powered plane. Finally, it can be stated that it is necessaryand worthwhile to advance the on-board storagetechnology. Only a mature and available technology will find its application. Ac&owledgemenf-The author expresses his gratitude to F. J. Edeskuty and S. Furuhama for having contributed valuable ideas to this paper.
REFERENCES 1. Dtiren, C., von Hagen, I., Junker, G., Kulgemeyer, A. and Rasche, C., Safety Aspects of Composite Pressure Vessels i’or the Transportation of Hydrogen on Trailers and for CNG Tanks in Vehicles, 3R International, Heft 6/1994, Vulkan-Verlag. 2. Hynek, S., Fuller, W., Bentley, J. and McCullough, J., Hydrogen Storage by Carbon Sorption, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994, p. 985, Int. Ass. Hydr. En. 3. Ewald, R., ProprittCs, utilisation et production de l’hydrogine et du deut&um sblides, Note CEA N963, 1968, Centre d’ktudes NuclCaires de Grenoble. 4. Dewitt, R. L., Hardy, T. L., Whalen, M. V. and Richter, G. P., Slush Hydrogen (SLH3 Technology Development for Application to the National Aerospace Plane (NASP), Advances in Cryogenic Engineering, Vol. 35, 1990, p. 1741, Plenum Press, N.Y. 5. BaumgBrtner, K. and Ewald, R., Sicherheitstechnik bei der Handhabune. von Wasserstoff, Chemie-Inu.-Technik 4.1987. 6. Feucht, K.,-Hiilzel, D. and. Hurich, *., Perspective of Mobile Hydrogen on Trailers and for CNC Tanks in Vehicles, Hydrogen Energy Progress VII, Proc. VIIth WHEC, 1988, p. 1963, Pergamon Press, N.Y. 7. Alternative Energy Sources for Road Transport: Hydrogen Drive Test, 1990, Verlag TtiV Rheinland, K6ln.
8. Williams, L. O., Hydrogen Powered Automobiles must use Liquid, Cryogenics, Dec. 1973. of the Future, 9. Peschka, W., Liquid Hydrogen-Fuel Springer, Wien/New York, 1992. 10. Ewald, R. and Kesten, M., Cryogenic Equipment of Liquid Hydrogen Powered Automobiles, Advances in Cryogenic Engineering No. 35, 1989, Plenum Press, N.Y. 11. Michel, F., Fieseler, H., Meyer, G. and TheiBen, F., OnBoard Equipment for Liquid Hydrogen Vehicles, Hydrogen Enerqv Prowess XI. Proc. XIth WHEC. 1996. Int. Ass. Hyd;.‘En. .. 12. Yamane, K., Nakamura, S., Nosset, T. and Furuhama, S., A Study on a Liquid Hydrogen Pump with Self-ClearanceAdjustment Structure, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994, p, 1919, Int. Ass. Hydr. En. 13. Furuhama, S., Hydrogen Engine Systems for Land Vehicles, ht. J. Hydrogen Energy, Voi. 14, No. 12, 1989, p. 907. 14. Vandenborre, H. and Sierens. R.. Greenbus. A Hvdroeen Fuelled City Bus, Hydrogen Eneriy Progress X, Pioc. %th WHEC, 1994, p. 1959, Int. Ass. Hydr. En. als Beispiel fiir ver15. Wurster, R., H,-Stadtbusprojekte schiedene ELI-Proiekte. YDI-Berichte. Nr. 1201. 1995. D. * 187, VDI-Verlag, i)iis&ldorf. 16. Riidiger, H., Seifers, S., Holzer, H. and Wolf, J., Liquid Hydrogen Storage System for Urban Bus, Hydrogen Energy Progress X. Proc. Xth WHEC, 1994, p. 967, Int. Ass. Hydr. En. 17. Marcenaro, B. G., EQHHPP FC Bus: Status of the Project and Presentation of the First Experimental Results, Hydrogen Energy Progress X, Proc. Xth WHEC, 1994, p. 1447, Int. Ass. Hydr. En. Powered Automobiles using 18. Zieger, J., Hypasse-Hydrogen Seasonal and Weekly- S&&s of Electricity, H.vdrogen Enerqy Prowess X, Proc. Xth WHEC. 1994. D. 1367. lnt. Ass. Hydr. En. 19. Reister, D. and Strobl, W., Current Development and Outlook for the Hydrogen-Fuelled Car, Hydrogen Energy Proyrrss IX. Proc. IXth WHEC, 1992, p. 1201, Int. Ass. Hydr. En. Automobiles: On20. Ewald, R., Liquid Hydrogen-Fuelled
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