Hydrogen and Fuel Cells in Submarinesdownload.xuebalib.com/4orihNcQwP5k.pdf · PEM Fuel Cells for Submarines 42.3.1 Introduction The principal design and operation of PEM fuel cells

42Hydrogen and Fuel Cells in SubmarinesStefan Krummrich and Albert Hammerschmidt

42.1Background

Non-nuclear submarines are generally based on a diesel-electric power supplysystem. A diesel generator in combination with lead acid batteries supplies thesubmarine’s network system. The battery is charged during surfaced or snorkeloperation of the submarine, because air is required for operation of the dieselengines. In submerged operation traditionally the entire power is taken from thelead acid batteries.

The limited capacity of the lead acid batteries results in a relatively short sub-merged operating period of several days, until resurfacing is required again torecharge the batteries. During this period of snorkeling the submarine is easilydetectable – this is the main reason for the efforts that have been spent in recentdecades to develop air-independent propulsion (AIP) systems for submarines.

The major requirements for such AIP systems are:

� high energy density (plant including storage of reactants);� high efficiency (→ low heat transfer to sea water);� low noise level;� low magnetic signature;� small size;� low weight;� low effort for maintenance/no extra crew.Many different technologies have been considered as AIP systems, but today

fuel cell technology is by far the most successful technology in this area.

991

Hydrogen Science and Engineering: Materials, Processes, Systems and Technology, First Edition.Edited by Detlef Stolten and Bernd Emonts. 2016 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2016 by Wiley-VCH Verlag GmbH & Co. KGaA.

42.1.1

When it All Began . . .

Fuel cells have been under development by ThyssenKrupp Marine Systems formore than 20 years. At the beginning of the 1980s a land based test site wasrealized with a fuel cell system and a lead acid battery, comprising all supplysystems for the reactants hydrogen and oxygen. The plant was built up withalkaline fuel cells, because polymer electrolyte membrane (PEM) FC technologywas under development at Siemens, but not yet available at that time. Afterextensive testing of the plant, the system was installed onto the submarine U1, aClass 205 submarine that was still in operation with the German Navy. In 1988,first sea tests were performed. Thanks to the successful results of the testing, theHDW Class 212A submarine was designed from scratch based on fuel cell tech-nology. The development of the PEM-fuel cells at Siemens was intensified. Theresult was the first PEM FC Module in 1994 with 30–40 kW power output,showing excellent operational performance.

Furthermore, system developments for the supply systems were performed torealize a safe and reliable system operation on board a submarine. Taking intoconsideration the very special requirements like operation in a closed atmo-sphere, shock loads, very strong acoustic requirements, and the need for a non-magnetic design, these tasks were very challenging. The type of hydrogen storagewas also carefully evaluated, with the result that metal hydride storage was thebest choice for submarines. The only problem regarding this choice was thatmetal hydride storage cylinders were available in finger-sized bottles for labora-tories and so on, but not as the very large storage tanks needed to fulfill thestorage capacity requirements for submarine applications. Therefore, thesehydrogen storage cylinders had to be developed. This development was per-formed at ThyssenKrupp Marine Systems, from laboratory via the type approvaltesting by the German authorities up to the implementation of a reliable andquality optimized manufacturing process.

42.2The HDW Fuel Cell AIP System

Figure 42.1 shows the system installed onboard the HDW Class 212A and 214submarines.

The HDW fuel cell systems consists of the reactant storage of hydrogen andoxygen, the fuel cell modules developed by Siemens, a distillate cooling watersystem, and a system for handling the product water and residual gases.

The fuel cells are operated on the pure reactants hydrogen and oxygen. Thereaction water is stored onboard to realize a closed system – no substances haveto be sent overboard, no weight loss occurs by the consumption of the reactants,therefore no weight compensation is needed. A DC/DC converter regulates theoutput voltage of the system to fit with the actual boats network voltage.

992 42 Hydrogen and Fuel Cells in Submarines

42.3PEM Fuel Cells for Submarines

42.3.1

Introduction

The principal design and operation of PEM fuel cells are described in depth else-where in the present book.

PEM fuel cells convert chemical energy which is bound in hydrogen and oxy-gen via an electrochemical process into electrical energy. They are, in principle,well suited for use in a submarine environment: the energy conversion happensat a relatively low temperature (70–80 °C), the conversion is very effective (highelectrical efficiency), and as an electrochemical process is – compared to combus-tions processes – silent. All these properties are favorable for the tactical needs ofa modern submarine as described in the requirements above: the relatively lowtemperature and the highly efficient energy conversion are advantageous for lowlevel heat dissipation around the submarine, keeping the thermal signature low.

Beyond these intrinsic properties of PEM fuel cells several additional and spe-cific requirements should be fulfilled when using them as energy source in theunderwater application. In addition to the major requirements for AIP systems(Section 42.1) the fuel cell modules should be characterized by:

� low volume of off-gases (residual gas hydrogen and oxygen) from fuel celloperation;

Figure 42.1 Overview of AIP system for submarines.

42.3 PEM Fuel Cells for Submarines 993

� high power density of the fuel cell module with a high degree of integrationprocess equipment;� low electrical stray field;� shock resistance;� compliance with submarine safety requirements.

42.3.2

The Oxygen/Hydrogen Cell Design

42.3.2.1 Constructive Features/Cell Design of Siemens PEM Fuel CellThe basic constructional features of PEM fuel cells (Figure 42.2) are the protonconducting membrane (mostly based on a perfluorinated polymer, e.g., Nafion),two catalyst layers (on the anode and cathode sides), a gas diffusion layer onboth sides, and the bipolar or cooling plates. To achieve the power densitymetal-based bipolar plates are used that allow simple realization of a water-cooled and “thin” design.

The corrosive environment, which consists in the cathode compartment ofpure oxygen and hot, deionized water, requires a high stability of the appliedmaterials. Stainless, nickel-rich steel fulfils these stability requirements easily.This material can be embossed and welded to produce the structures for uni-form coolant flow, gas supply, and residual gas product water removal.

Figure 42.2 Schematic set-up of a PEM fuel cell [1].

994 42 Hydrogen and Fuel Cells in Submarines

Figure 42.3 shows bipolar plates as used in the fuel cell module BZM 34(design E4) and BZM 120 (design D4). The surface is gold-plated to ensure suffi-cient electrical contact between the metal and the carbon-paper based diffusionlayer. The black gasket material, a fluoro-elastomer around the edge of thebipolar plate, is provided with axial gas channels conducting the media (coolant,reactant, product water) to and from the cells where they are consumed orreleased. The membrane electrode assembly (MEA), a five-layered componentconsisting of the membrane, catalyst, and gas diffusion layer, is placed inbetween two of these cooling units. An appropriate treatment of the diffusionlayer is necessary to make it hydrophobic and to squeeze the product water outof the porous structure.

42.3.2.2 Results from Fuel Cell OperationFigure 42.4 shows the significant difference of fuel cell operation between hydro-gen/oxygen (blue) and hydrogen/air (green). The membrane basis for theseinvestigations is DuPont Nafion 115/117, respectively. In air operation a strongreduction of the cell voltage at a given current density can be observed even withthe same membrane material.

The reason for this is the lower partial pressure of oxygen and transport phe-nomena at the reactive interface. But there is also a significant difference in pureoxygen operation between the thinner Nafion 115 (nominal thickness @ drystate 125 μm) and the thicker Nafion 117 (175 μm) – in respect to power bynearly the factor of 1.5. The reason for this is the higher ohmic resistance of thethicker membrane.

42.3.3

Constructive Feature of Fuel Cell Module for Submarine Use

42.3.3.1 PreconditionsFuel cells used in submarines are operated in a frequently closed environmentwith a confined volume and defined gas pressure. Both requirements must be

Figure 42.3 Metal-based bipolar plate, Design E4 (a) and Design D4 (b).

42.3 PEM Fuel Cells for Submarines 995

considered in handling the off-gases. To avoid complicated or energy consumingresidual gas treatment like compression and/or bringing it outboard it is impor-tant to know how to reduce the amount of residual gas as far as possible. A firststep is to limit the impurities in the reactants: the quality of the oxygen used inthe fuel cells (provided as liquid oxygen) is 99.5% purity with impurities likenitrogen or argon. Medical oxygen normally meets these requirements. Hydro-gen is even much purer (99.999%) since it is stored in metal hydrides. Thesemetal alloys act as purifiers: gases like CO2, O2, or water vapor are tightlyadsorbed within the lattice structure of the metal alloys, reducing the storagecapacity with time.

42.3.3.2 Cascaded Fuel Cell Stacks [2]Figure 42.5 shows the construction principles of a cascaded fuel cells stack. Toavoid unacceptably large hydrogen/oxygen quantities as residual gases a Siemensfuel cell module consists of several internal gas loops on the oxygen and on thehydrogen side, which physically do not need to be at the same location on bothsides.

The dry reactants, which are released from the gas supply at pressuresbetween 2 and 4 bar, pass through the membrane humidifier and the sections orcascades of fuel cells. Each section/cascade consists of a certain number of cells,which decreases from inlet to exit. The number of cells is adjusted correspond-ing to the depletion of reactants due to the electrochemical reaction within thecells in order to minimize the volume of the residual gas. After each loop the

Figure 42.4 Voltage–current characteristic of a PEM fuel cell at different operation conditions.

996 42 Hydrogen and Fuel Cells in Submarines

reactants are separated from product water and conducted to the next cascade.The product or product water is collected separately. The last cascades of theanode and cathode side are the purging cells which define the volume and thequality (i.e., composition of reactant and inert gases) of the residual gas.

Figure 42.6 shows a typical purging behavior of a hydrogen purging cell.Whereas all fuel cells except the purging cells are operated with a continuousflow of reactants the purging cells are operated in the dead-ended mode. Due tothe enrichment of inert gases and/or water and the operational current, the

Figure 42.5 Schematic set-up of a cascaded fuel cell module with humidifier and purgingcells.

Figure 42.6 Typical behavior of a series of four hydrogen purging cells operating under fullload.

42.3 PEM Fuel Cells for Submarines 997

voltage of the purging cells group shows a time-dependent behavior starting at ahigh voltage level with decay with time.

The trigger of the purging operation must be adjusted depending on accept-able purging volume or voltage levels [3]. The principle of the purging cellsallows us to dispose the residual gases from both sides individually. Hydrogencan be brought into a hydrogen-rich environment, for example, a battery com-partment, and is combusted there on recombination catalysts, whereas oxygencan be released into the boat atmosphere provided the amount of oxygen can bebreathed away by the crew.

42.3.3.3 Pressure Cushion for Uniform Current Distribution [4]The fuel cell modules are operated at current densities up to 1000 mA cm�2; thestacks may consist of up to 200 single cells and more. It is important to guaran-tee a uniform contact pressure between the cooling units or bipolar plates andthe membrane electrode units. Uneven current distribution has an impact onthe potential distribution on the bipolar plate. As a result, one MEA may notoperate homogenously without uniform water production and draining orinhomogeneous production of reaction heat. This may influence life time by par-tially drying out the membrane or inducing corrosion.

Fuel cell stacks with a rather high number of large area cells cannot simply bekept together by tie rods if extra boundary conditions must be considered likeuniform pressure and sufficient gas tightness.

Using a pressure inducing element inside the cell stack has been considered tobe the best solution for the Siemens fuel cell stacks (Figure 42.7).

The pressure cushion consists of an inflatable element that is filled with a flex-ible, electrically conductive material to ensure good electrical conductivity

Figure 42.7 Fuel cell stack with integrated pressure cushions for uniform pressuredistribution.

998 42 Hydrogen and Fuel Cells in Submarines

through the cushion. They ensure, together with the rigid tie rods and the stiffend plates, the necessary forces onto the fuel cell elements for good electricalconductivity. Figure 42.8 shows the quality of uniform pressing achieved by thepressure cushions as measured with a pressure sensitive paper material.

42.3.3.4 Fuel Cell ModuleA schematic set-up of a Siemens PEM fuel cell stack is shown in Figure 42.9.The essential components of the stack are the hydrogen/oxygen humidifier, thecascaded cells stack within the pole plates, and the water separators to removethe product water from the gas stream, which are placed at the end plate.

The fuel cell module is supplied with dry reactants delivered from the gas stor-age (see below); the reactants are internally humidified and directly conductedinto the fuel cell stack. The reaction water is employed to humidify the reactantsand part of the reaction enthalpy produced during fuel cell operation is used asevaporation heat inside the humidifier.

Figure 42.8 Measurement of uniform pressure application inside a fuel cell (design D4).

Figure 42.9 Schematic set-up of a cascaded Siemens PEM fuel cell stack with integratedhumidifier, the cascaded stack and water separator.

42.3 PEM Fuel Cells for Submarines 999

The advantage of the design is the simple use of dry reactants due to internalhumidification.

The fuel cell module consists of the stack described above and, in addition, pro-cess equipment to handle the media like gases and cooling water (Figure 42.10).

Most of this process-related equipment is arranged in front of the fuel cellstack. It consists of valves and sensors for pressure, temperature, current, volt-age, and flow measurements. The pipes and tube made of insulating material areused to conduct the media internally. The front plate of the module equippedwith the electrical power connectors and a connecting block as interface formedia supply is tightly connected to a container or pressure vessel. Due to itsgas tightness the container is an active part of the safety system of the fuel cellplant. For this the pressure container is pressurized by nitrogen such that in caseof an internal leak the critical media like hydrogen or oxygen are forced toremain inside the fuel cells stack. In case of a leakage no gas can leave the stackbut nitrogen may enter and induces a pressure increase or voltage drop thatshuts off the control system.

These constructional principles are valid for both BZM 34 and BZM 120 fuelcell modules (Figure 42.11). The essential difference between both modules isthe cell area (see Figure 42.3, design E4 versus D4), the size of the active area,and the current density. The reason for the higher current density in BZM 120is the use of Nafion 115 instead Nafion 117 in BZM 34 (see also Figure 42.4).The technical data of both modules are listed in Table 42.1.

42.3.3.5 Results from Fuel Cell Module OperationFuel cells modules can be easily operated when they are supplied by sufficientreactants, service gas, cooling water, and a load bench, provided they are con-nected to an appropriate control unit including the appropriate program. Sincethis is very product and process specific it will not reported here.

Figure 42.10 Siemens PEM fuel cell Module BZM 120 without (a) and with pressurecontainer (b).

1000 42 Hydrogen and Fuel Cells in Submarines

Figure 42.12 presents a snapshot of the performance data of a BZM 120 oper-ating onboard a submarine – mainly load steps between open circuit voltage andnominal load.

42.3.3.6 Safety Features of Submarine Fuel Cell ModulesThe safe operation of PEM fuel cells onboard submarines is especially importantdue to the closed environment of the installation and the reactants hydrogen andoxygen, which are close together.

The fuel cell modules themselves are intrinsically safe. A basis of the safetyphilosophy is the pressure vessel concept discussed above. Another aspect is theprocess safety, provided by continuous measurement of physical data like tem-peratures, reactant pressure, flow, module current and voltage, and pairs of sin-gle cell voltages. These data are continuously compared with target and limitvalues and in case of a deviation appropriate measures are taken like the displayof a warning signal, alarm, or automatic shut-off. This requires a fast PLC (pro-grammable logic controller), which controls the automatic shut-down process,

Figure 42.11 Inside of BZM 34 (a) and BZM 120 (b).

Table 42.1 Technical data of BZM 34 and BZM 120 [5].

BZM 34 BZM 120

Rated power (kW) 30–40 120

Number of cells ∼70 320Rated current (A) 560

Rated voltage (V) >50 215

Working temperature (°C) 70–80 70–80

Size (cm3) 47× 47× 143 176× 53× 50

Weight (incl. pressure vessel) (kg) 650 900

Efficiency at full load (%) >60 >55

Efficiency at 20% load (%) >70 >65

42.3 PEM Fuel Cells for Submarines 1001

consisting of the interruption of the media supply and the removal of reactantsinside the fuel cells.

42.4Hydrogen Storage

The hydrogen storage system for submarines is based on metal hydride storagecylinders, which have been developed by and are produced by ThyssenKruppMarine Systems. This type of hydrogen storage ideally fulfills the specificrequirements of this niche application. Compared to compressed hydrogen, themetal hydride storage offers much higher volumetric storage densities. Theusage of liquid hydrogen was also considered, but because a cryogenic hydrogentank has significant boil-off losses, and since the quantity of hydrogen requiredwould result in a tank system with enormous influence on the entire submarinedesign, the metal hydride storage was chosen.

Today’s metal hydride storage cylinders offer the highest safety combined withexcellent operational features for the submarine. Waste heat from the fuel cellsis used for discharging the tanks. As consequence, less waste heat is dissipatedinto the surrounding sea. During recharging of the metal hydride the cylindersshould be cooled to enable a fast filling procedure.

The high weight of metal hydride compared to other storage technologies isno problem for the submarine application. Taking into consideration that non-nuclear submarines need lead in the keel area to achieve weight balance (princi-ple of Archimedes), it is relatively easy to understand that the hydrogen storagecylinders can be installed by simply taking less or even no lead. Nevertheless, the

Figure 42.12 Snapshot of the performance of a BZM 120 operated under quick load changes(red: current, blue: voltage, x-axis: time in minutes).

1002 42 Hydrogen and Fuel Cells in Submarines

storage cylinders are installed outside the pressure hull, and therefore have tomeet very harsh requirements like diving pressure, salt water environment, andmaximum shock loads.

The storage cylinders are currently produced in several cassettes filled withmetal hydride. These cassettes are then put into a pressure vessel. One cylinder isapproximately 5 m long and has a diameter of approximately 500 mm (Fig-ure 42.13). After the production procedure the storage cylinders have to be acti-vated by several charging and discharging cycles. This is followed by theinstallation of two half shells made of glass fiber reinforced plastic around thecylinders. These shells are required for heating of the storage cylinders duringoperation with seawater that has been warmed up by the distillate cooling system.

Taking into consideration the usage of metal hydride, their behavior has to beunderstood in detail. Customers in most cases ask: How much hydrogen can bestored reversibly in the cylinders?

Figure 42.14 shows the influence of temperature and filling level on the pres-sure of the storage cylinders. It becomes obvious that the storage capacity is

Figure 42.13 Metal hydride cylinder for submarines.

Figure 42.14 CPI (concentration–pressure isotherm) characteristics of metal hydride.

42.4 Hydrogen Storage 1003

increased in cold conditions, provided that the pressure is the limiting factor forthe hydride storage cylinder. Therefore, the storage capacity of the hydride stor-age system onboard submarines is directly influenced by the specified seawatertemperature – the higher the temperature, the lower the storage capacity.

For refueling of the metal hydride ThyssenKrupp Marine Systems has devel-oped a specific reactant filling station. This filling station controls the pressureand flow of hydrogen into the storage cylinders to achieve optimum conditionsfor maximum filling in a relatively short time. Nevertheless, as during the fillingprocedure heat has to be withdrawn from the storage cylinders, the procedure isa time-consuming action.

The number of metal hydride cylinders differs from submarine to submarine,and is related to the customer-specific requirements regarding the amount ofAIP energy. To date, not even one single failure has occurred during operationin or at one of the metal hydride storage cylinders, with more than 500 of themproduced so far.

42.5The Usage of Pure Oxygen

Many challenges during the fuel cell development are caused by the usage ofpure oxygen inside the fuel cells. The details are discussed above.

Storage of the required amount of oxygen inside the submarine is realizedwith cryogenic liquid oxygen tanks. In the HDW Class 212A submarines twoliquid oxygen tanks are located outside the pressure hull (Figure 42.15), while in

Figure 42.15 HDW Class 212A submarine.

1004 42 Hydrogen and Fuel Cells in Submarines

HDW Class 214 boats (Figure 42.16) one large tank is located inside, hangingdirectly underneath the elastic platform deck, on which the HDW fuel cell sys-tem is located.

42.6System Technology – Differences Between HDW Class 212A and Class 214 Submarines

The first submarines that have been developed from scratch based on fuel celltechnology are the HDW Class 212A submarines for the German and ItalianNavies. In these submarines the HDW fuel cell system is implemented electri-cally without a DC/DC-converter. This results in very harsh operating condi-tions for the fuel cells, because all dynamic load changes inside the ship’selectrical network system directly affect the fuel cell modules. Thanks to theunique development of cascaded fuel cells, these harsh requirements can be ful-filled by the Siemens fuel cells.

Furthermore, the HDW Class 212A submarines have an entirely redundantsystem design – a failure of one single component will not lead to system degra-dation. Therefore, nine FC modules are installed, with only eight in operation, inorder to have a spare module in case of a module-internal failure. On a systemlevel, the HDW fuel cell system is the only alternative to the diesel engine,because only one single diesel generator is installed.

The HDW Class 214 submarines are equipped with the improved Siemens FCmodules FCM120. Furthermore, a DC/DC converter is installed to adapt the FCoutput voltage to the ships network voltage. The system that has been developed

Figure 42.16 HDW Class 214 submarine.

42.6 System Technology – Differences Between HDW Class 212A and Class 214 Submarines 1005

for HDW class 214 can therefore also be integrated in other submarines, like forexample in the HDW Class 209 submarine (Portugal and Greece) or the HDWDolphin AIP Class submarine.

Both on HDW Class 212A and Class 214 submarines, the fuel cell control sys-tem realizes a fully automated operation of the fuel cell system. This means thatno additional crew member is required onboard for the fuel cell systemoperation.

42.7Safety Concept

The integration of a HDW fuel cell system working with pure hydrogen andoxygen inside the submarine’s atmosphere is only possible when taking into con-sideration the technical safety at a very early design stage. Generally, any possi-bility of a hydrogen–oxygen mixture must be avoided. Furthermore, anyuncontrolled outflow of one of these reactants into the submarine’s atmospheremust be prohibited. These design criteria led to a system without any safetyvalves, which would usually be utilized in on-shore applications. The basicdesign principle is to ensure a double safety barrier between hydrogen/oxygenand the submarine’s atmosphere. All actuators and sensors are designed accord-ing to the double-barrier principle. The fuel cell modules are encapsulated in apressure container filled with nitrogen at a higher pressure than the correspond-ing hydrogen or oxygen pressures.

42.8Developments for the Future – Methanol Reformer for Submarines

Although the existing HDW fuel cell system offers many advantages, for sub-marines larger than approximately 2000 t displacement with high AIP require-ments a different solution is preferable. The system based on metal hydridestorage cylinders is relatively heavy, resulting in the amount of hydrogen storedon board being limited by the capability of the submarine to carry them, havingin mind the principle of Archimedes.

Generally, liquid fuels have high volumetric and gravimetric energy contentand are easy to handle. These advantages in combination with fuel cell perform-ance motivated ThyssenKrupp Marine Systems to initiate the development of areformer system for onboard hydrogen production.

42.8.1

System Configuration

At the beginning of the development the requirements for the reformers weredefined. A major requirement was operation based on the existing and proven

1006 42 Hydrogen and Fuel Cells in Submarines

Siemens fuel cells. Furthermore, the exhaust gas (CO2) pressure should be high,to enable the discharge of exhaust gas into the surrounding seawater without theneed for an additional exhaust gas compressor. Of major importance were theoverall system efficiency and the reliability and availability of the system.

Based on these requirements, the choice was a methanol–steam reformer sys-tem operated at elevated pressure. Hydrogen purification is performed with amembrane purification unit. The required thermal energy is produced in a high-pressure oxygen burner. Figure 42.17 shows an overview of the process.

The methanol is mixed with water, evaporated and fed to the steam reformer.The reforming reactor is heated by a boiling water cycle operating between 60and 100 bar. The methanol–water mixture is converted into a hydrogen-rich gasmixture at a temperature between 250 and 300 °C. This reformate gas is furtherprocessed in a gas purification unit based on hydrogen-permeable membranes.The major fraction of hydrogen passes through the membrane and can be feddirectly to the Siemens fuel cell. The rest of the reformate gas is burned withpure oxygen in the burner, under the addition of methanol, to provide therequired heat for the reforming process. The only product gas from the reformeris CO2; the H2O in the exhaust gas is condensed and reused internally.

The methanol reformer itself will be operated in an enclosure with severalsafety features, to protect the crew from any harmful gases and liquids.

42.8.2

Challenges of the Methanol Reformer Development

The methanol reformer development was started based on the procurement of afunctional demonstrator. This plant was operated with the process conditions thatwere foreseen for the submarine plant. The process itself worked well, but manycomponents showed weaknesses regarding the special onboard requirements.

Nevertheless, after showing that the methanol reforming process works wellunder the special process conditions that had to be chosen, a major effort has tobe made to achieve a submarine-suitable layout of the system.

Figure 42.17 Overview of methanol reformer system.

42.8 Developments for the Future – Methanol Reformer for Submarines 1007

42.8.3

Hydrogen Purification Membranes

One major milestone was a successful shock test of the palladium membranesused for the separation of hydrogen from the reformate gas stream. This shocktest was performed with membranes that were saturated with hydrogen, theoperational differential pressure was applied, and the operating temperature wasapplied to the membrane. This kind of shock testing is a specific requirement forsystems and components that are foreseen to be integrated onboard asubmarine.

Because the existing Siemens FC modules are foreseen to be coupled with thereformer, the purity requirements are extremely high. Therefore, any leakagefrom the reformate (feed) to the hydrogen (permeate) must be avoided. Theseunique requirements led to the choice of a thick film membrane. This kind ofmembrane shows highest stability, although the costs are comparatively high.No membrane based on a support structure, for example, sintered metal with athin palladium film, showed the required stability for operating cycles (tempera-ture as well as load changes), lifetime, and the special submarine requirements.

42.8.4High Pressure Catalytic Oxidation

The burner inside the methanol reformer is designed as a multi-stage catalyticoxidation device, because the catalytic oxidation does not require an ignitionsystem or a flame monitoring system. These two mentioned systems are notavailable in a submarine-suitable design. Furthermore, the oxygen control of amulti-stage burner is beneficial compared to a flame burner, and shows onlyminor deviation in the oxygen concentration in the exhaust gas.

The burner is a patented unique design, operating at each stage at sub-stoichi-ometric conditions. The temperature in each stage should be limited by onlypartial oxidation. Even if this technology seems to be unusual and complicatedrelated to the control system of the burner, the control can be performed bysimple temperature sensors and an oxygen sensor. The catalytic oxidation deviceshows best exhaust gas quality and very high modulation ability.

42.8.5Integration on Board a Submarine

The conceptual integration of a reformer system on board requires the consider-ation of several special requirements derived from the special environment. Tomeet all requirements for onboard operation, a special safety concept has beenworked out. This safety concept led to the solution of an encapsulated reformersystem. A view of the encapsulated reformer is shown in Figure 42.18. Inside theencapsulation, a special ventilation system is installed for cooling purposes.Furthermore, the encapsulated system is continuously monitored for leakages

1008 42 Hydrogen and Fuel Cells in Submarines

(CO, H2, CH3OH). In the unlikely event of the detection of any of these sub-stances, a special air purification device (catalytic oxidation) is operated to con-vert these substances into harmless gases. The escape of these substances intothe submarine’s atmosphere is effectively avoided.

The methanol reformer system itself is installed on an elastic platform insidethe submarine. The LOX (liquid oxygen) tank is located below the platform.Generally, the size of the LOX tank is the dominant factor for the submarinedesign. Because methanol is a liquid energy carrier, it can be stored on board intanks that are part of the ship’s structure.

The dynamic behavior of fuel cells is much faster than the dynamics of thehydrogen production inside the reformer. As submarines always have a verylarge battery installed, the dynamic requirements can be met by the entireenergy system, consisting of fuel cell system (+ reformer) and battery. To controlthe load of the fuel cell system in relation to the reformer dynamics, the DC/DC-converter controls the load of the fuel cell in accordance with the actual hydro-gen output of the reformer.

42.9Conclusion

The usage of fuel cells onboard submarines is one of the most successful appli-cations for fuel cells in the world. The HDW fuel cell system based on metalhydride storage of hydrogen fits best the unique requirements of a submarine. Incombination with the Siemens fuel cells with their cascaded design the system isabsolutely quiet and highly efficient. Today, submarine fuel cell technology canbe considered as a mature technology that enables navies to operate submergedsubmarines for several weeks.

The reformer as an additional option offers further increased underwaterendurance. All advantages and challenges derived from the usage of fuel cells,

Figure 42.18 Methanol reformer system including (a) and excluding (b) encapsulation.

42.9 Conclusion 1009

metal hydride storage, and pure oxygen have been addressed – resulting in aproven system for every day submarine use.

The experience gained in the entire submarine fuel cell development thatbegan in the 1980s shall contribute to the development and market penetrationof fuel cell systems also in other, for example, civilian applications. In addition,the success story of submarine fuel cells shows that customer benefit is ofutmost importance during the implementation of a new technology.

References

1 Hammerschmidt, A. (2007) Fuel cellpropulsion of submarines. Naval Forces,special issue, 28, 132.

2 Mehltretter, I. (2010) EP 2122737.3 Strasser, K. (1997) EP 0596366 A1.4 Hartnack, H., Lersch, J., and Mattejat, A.

(2010) EP 1627445.

5 Hammerschmidt, A. and Scholz, D. (2011)Electrical platform and fuel cell based AIPsystems for submarines. Presented atSubmarine Institute of Australia, Inc.Technology Conference, Adelaide,Australia, 8–10 November 2011.

1010 42 Hydrogen and Fuel Cells in Submarines

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42. Hydrogen and Fuel Cells in Submarines42.1 Background42.1.1 When it All Began . . .

42.2 The HDW Fuel Cell AIP System42.3 PEM Fuel Cells for Submarines42.3.1 Introduction42.3.2 The Oxygen/Hydrogen Cell Design42.3.2.1 Constructive Features/Cell Design of Siemens PEM Fuel Cell42.3.2.2 Results from Fuel Cell Operation

42.3.3 Constructive Feature of Fuel Cell Module for Submarine Use42.3.3.1 Preconditions42.3.3.2 Cascaded Fuel Cell Stacks [2]42.3.3.3 Pressure Cushion for Uniform Current Distribution [4]42.3.3.4 Fuel Cell Module42.3.3.5 Results from Fuel Cell Module Operation42.3.3.6 Safety Features of Submarine Fuel Cell Modules

42.4 Hydrogen Storage42.5 The Usage of Pure Oxygen42.6 System Technology - Differences Between HDW Class 212A and Class 214 Submarines42.7 Safety Concept42.8 Developments for the Future - Methanol Reformer for Submarines42.8.1 System Configuration42.8.2 Challenges of the Methanol Reformer Development42.8.3 Hydrogen Purification Membranes42.8.4 High Pressure Catalytic Oxidation42.8.5 Integration on Board a Submarine

42.9 ConclusionReferences

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