Method and device for operating fuel cells with artificial air

Abstract

The invention relates to a fuel cell system (1) suitable for operation with a cathode operating gas containing oxygen and inert gas and an anode operating gas containing hydrogen and inert gas; an appliance system operated by means of the fuel cell system (1); and a method for operating the fuel cell system (1). In the method according to the invention, the single components of the operating gases are stored separately, and mixed to the required portions during operation of the fuel cell system, thereby constantly recirculating the inert portion of the operating gases. During operation of the fuel cell system, gases are neither taken in from the environment nor released into the environment nor are fuel cell exhaust gases stored in the fuel cell system or the appliance system. In an alternative variation, only the anode operating gas is mixed and recirculated, while the cathode operating gas and the cathode exhaust gas are taken from the environment and released into the environment, respectively.

Claims

1. A fuel cell system suitable for operation with a cathode operating gas containing oxygen and inert gas and an anode operating gas containing hydrogen and inert gas, comprising: a fuel cell arrangement having at least one fuel cell, wherein the fuel cell comprises a cathode having a cathode flow region and an anode having an anode flow region; an oxygen source, a hydrogen source and an inert gas source; a cathode gas circuit comprising a cathode operating gas flow path for feeding the cathode operating gas into the cathode flow region of the cathode, the cathode flow region, a cathode exhaust gas flow path for receiving cathode exhaust gas from the cathode flow region and for recirculating the cathode exhaust gas into the cathode operating gas flow path, and a transition point where the cathode exhaust gas flow path transitions into the cathode operating gas flow path; an anode gas circuit comprising an anode operating gas flow path for feeding the anode operating gas into the anode flow region of the anode, the anode flow region, an anode exhaust gas flow path for receiving anode exhaust gas from the anode flow region and for recirculating the anode exhaust gas into the anode operating gas flow path, and a transition point where the anode exhaust gas flow path transitions into the anode operating gas flow path; means for separating liquid water from the cathode exhaust gas in the cathode exhaust gas flow path and means for separating liquid water from the anode exhaust gas in the anode exhaust gas flow path; means for generating a flow in the cathode gas circuit and means for generating a flow in the anode gas circuit, wherein one or more of the means for generating a flow in the cathode gas circuit and the means for generating a flow in the anode gas circuit are a pump or a jet nozzle; a pressure sensor in the cathode gas circuit, a pressure sensor in the anode gas circuit, a temperature sensor in the cathode gas circuit and optionally a temperature sensor in the anode gas circuit for determining an actual amount of a gas in the cathode gas circuit and for determining an actual amount of gas in the anode gas circuit; an inert gas flow path leading from the inert gas source to the transition point in the cathode gas circuit or to a point upstream of the transition point of the cathode gas circuit, and an inert gas flow path leading from the inert gas source to the transition point in the anode gas circuit or to a point upstream of the transition point of the anode gas circuit; an oxygen flow path leading from the oxygen source to the transition point in the cathode gas circuit and a hydrogen flow path leading from the hydrogen source to the transition point in the anode gas circuit; means for supplying inert gas to the cathode gas circuit and to the anode gas circuit until a nominal amount of inert gas is reached in the cathode gas circuit and the anode gas circuit; means for supplying oxygen to the cathode gas circuit until a nominal amount of oxygen is reached in the cathode gas circuit; means for supplying hydrogen to the anode gas circuit until a nominal amount of hydrogen is reached in the anode gas circuit; means for generating a vacuum in the cathode gas circuit; and means for generating a vacuum in the anode gas circuit; wherein said cathode gas circuit and said anode gas circuit are configured to be under atmospheric pressure before initiation of operation of said fuel cell arrangement, and wherein said means for generating a vacuum in the cathode gas circuit and said means for generating a vacuum in the anode gas circuit are configured to be used before said initiation of operation of said fuel cell arrangement; wherein said means for generating a vacuum in the cathode gas circuit and said means for generating a vacuum in the anode gas circuit are further configured to allow a reduction of inert gas partial pressure in said cathode gas circuit and said anode gas circuit, respectively, before said initiation of operation of the fuel cell arrangement, when said fuel cell system is configured to operate at said inert gas partial pressure below said atmospheric pressure; and, wherein the fuel cell system is configured to not receive matter from an environment during operation of the at least one fuel cell, and not release matter into the environment, and not store fuel cell exhaust gas, and store water in liquid form.

2. The fuel cell system according to claim 1, further comprising means for feeding either oxygen from the oxygen flow path or inert gas from the inert gas flow path into the cathode gas circuit at the transition point of the cathode gas circuit and/or means for feeding either hydrogen from the hydrogen flow path or inert gas from the inert gas flow path into the anode gas circuit at the transition point of the anode gas circuit.

3. The fuel cell system according to claim 1, in which one or more of the means for supplying inert gas to the cathode gas circuit and to the anode gas circuit is a pressure reducer in the inert gas flow path, the means for supplying oxygen to the cathode gas circuit is a pressure reducer in the oxygen flow path, the means for supplying hydrogen to the anode gas circuit is a pressure reducer in the hydrogen flow path.

4. The fuel cell system according to claim 1, further comprising at least one container for storing said water in liquid form, the container being in fluid connection with the means for separating liquid water from the cathode exhaust gas and/or with the means for separating liquid water from the anode exhaust gas, via a water pump.

5. The fuel cell system according to claim 1, further comprising means for discharging gas from the cathode exhaust gas flow path and/or means for discharging gas from the anode exhaust gas flow path.

6. The fuel cell system according to claim 1, wherein said fuel cell system is coupled with a manned or unmanned submarine vehicle.

7. A fuel cell system suitable for operation with air as a cathode operating gas and an anode operating gas containing hydrogen and nitrogen, comprising: a fuel cell arrangement having at least one fuel cell, wherein the fuel cell comprises a cathode having a cathode flow region and an anode having an anode flow region; an oxygen source, a hydrogen source, and a nitrogen source; a cathode gas flow path comprising a cathode operating gas flow path for feeding the cathode operating gas into the cathode flow region of the cathode, the cathode flow region, and a cathode exhaust gas flow path for receiving cathode exhaust gas from the cathode flow region; an anode gas circuit comprising an anode operating gas flow path for feeding the anode operating gas into the anode flow region of the anode, the anode flow region, an anode exhaust gas flow path for receiving anode exhaust gas from the anode flow region and for recirculating the anode exhaust gas into the anode operating gas flow path, and a transition point where the anode exhaust gas flow path transitions into the anode operating gas flow path; means for separating liquid water from the anode exhaust gas in the anode exhaust gas flow path; means for generating a flow in the anode gas circuit, wherein the means for generating a flow in the anode gas circuit is a pump or a jet nozzle; a pressure sensor in the cathode gas flow path, a pressure sensor in the anode gas circuit, a temperature sensor in the cathode gas flow path and optionally a temperature sensor in the anode gas circuit for determining an actual amount of a gas in the cathode gas flow path and for determining an actual amount of a gas in the anode gas circuit; a nitrogen flow path from the nitrogen source to the transition point in the anode gas circuit or to a point upstream of the transition point in the anode gas circuit; a hydrogen flow path leading from the hydrogen source to the transition point in anode gas circuit; means for supplying nitrogen to the anode gas circuit until a nominal amount of nitrogen is reached in the anode gas circuit; means for supplying hydrogen to the anode gas circuit until a nominal amount of hydrogen is reached in the anode gas circuit; a valve in the cathode exhaust gas flow path; and means for generating a vacuum in the anode gas circuit; wherein said anode gas circuit is configured to be under atmospheric pressure before initiation of operation of said fuel cell arrangement, and wherein said means for generating a vacuum in the anode gas circuit is configured to be used before said initiation of operation of said fuel cell arrangement; wherein said means for generating a vacuum in the anode gas circuit is further configured to allow a reduction of inert gas partial pressure in said anode gas circuit, before said initiation of operation of the fuel cell arrangement, when said fuel cell system is configured to operate at said inert gas partial pressure below said atmospheric pressure; and, wherein the fuel cell system is configured to not receive anode operating gas from an environment during operation of the at least one fuel cell, and not release anode exhaust gas into the environment, and not store anode exhaust gas, and store water in liquid form.

8. The fuel cell system comprising a fuel cell system according to claim 7, wherein said fuel cell system is coupled with a manned or unmanned submarine vehicle.

9. A method for operating a fuel cell system with a cathode operating gas containing oxygen and inert gas and an anode operating gas containing hydrogen and inert gas, the fuel cell system comprising: a fuel cell arrangement having at least one fuel cell, wherein the fuel cell comprises a cathode having a cathode flow region and an anode having an anode flow region; a cathode gas circuit comprising a cathode operating gas flow path, the cathode flow region, a cathode exhaust gas flow path, and a transition point where the cathode exhaust gas flow path transitions into the cathode operating gas flow path; and an anode gas circuit comprising an anode operating gas flow path, the anode flow region, the anode exhaust gas flow path, and a transition point where the anode exhaust gas flow path transitions into the anode operating gas flow path, the method comprising the following steps: generating a vacuum in the anode gas circuit to allow a reduction of inert gas partial pressure in said anode gas circuit, before initiating operation of the fuel cell arrangement, when said fuel cell system operates at said inert gas partial pressure below said atmospheric pressure; and, configuring said anode gas circuit to be under atmospheric pressure before initiating operation of said fuel cell arrangement; feeding a cathode operating gas containing oxygen and inert gas into the cathode flow region of the fuel cell and feeding an anode operating gas containing hydrogen and inert gas into the anode flow region of the fuel cell, wherein an oxygen concentration in the cathode operating gas has a predetermined nominal value and a hydrogen concentration in the anode operating gas has a predetermined nominal value; having oxygen and hydrogen react in the fuel cell, thereby generating electrical energy, a cathode exhaust gas containing inert gas and water, and an anode exhaust gas containing inert gas and water; separating liquid water from the cathode exhaust gas and from the anode exhaust gas, thereby generating a cathode exhaust gas free of liquid water and an anode exhaust gas free of liquid water; feeding the entire cathode exhaust gas free of liquid water into the cathode operating gas flow path and feeding the entire anode exhaust gas free of liquid water into the anode operating gas flow path; determining an actual value of the oxygen concentration of the gas in the cathode operating gas flow path and determining an actual value of the hydrogen concentration of the gas in the anode operating gas flow path; feeding oxygen into the cathode operating gas flow path until the predetermined nominal value of the oxygen concentration of the cathode operating gas is reached, and feeding hydrogen into the anode operating gas flow path until the predetermined nominal value of the hydrogen concentration of the anode operating gas is reached; and maintaining a gas flow rate in the cathode gas circuit and the anode gas circuit while: not receiving anode operating gas from an environment during operation of the fuel cell arrangement, and not releasing anode exhaust gas into the environment, and not storing anode exhaust gas, and storing water in liquid form.

10. The method according to claim 9, in which, before taking the fuel cell arrangement into operation, the following steps are performed: filling the cathode gas circuit and the anode gas circuit by evacuation and/or feeding in inert gas, each substantially simultaneously, with such an amount of inert gas that when feeding oxygen into the cathode operating gas flow path under operating conditions of the fuel cell system, a cathode operating gas with the predetermined nominal value of the oxygen concentration is formed, and when feeding hydrogen into the anode operating gas flow path under the operating conditions of the fuel cell system, an anode operating gas with the predetermined nominal value of the hydrogen concentration is formed; setting the operating conditions of the fuel cell system; feeding oxygen into the cathode operating gas flow path until the predetermined nominal value of the oxygen concentration of the cathode operating gas is reached, and substantially simultaneously; feeding hydrogen into the anode operating gas flow path until the predetermined nominal value of the hydrogen concentration of the anode operating gas is reached.

11. The method according to claim 9, in which the oxygen concentration in the cathode operating gas flow path and the hydrogen concentration in the anode operating gas flow path are determined regularly or continuously.

12. The method according to claim 9, in which nitrogen is used as inert gas.

13. The method according to claim 9, wherein the method further comprises coupling said fuel cell system with a manned or unmanned submarine vehicle.

14. A method for operating a fuel cell system with air as a cathode operating gas and an anode operating gas containing hydrogen and nitrogen, wherein the fuel cell system comprises: a fuel cell arrangement having at least one fuel cell, wherein the at least one fuel cell comprises a cathode having a cathode flow region and an anode having an anode flow region; a cathode gas flow path comprising a cathode operating gas flow path, the cathode flow region, and a cathode exhaust gas flow path; and an anode gas circuit comprising an anode operating gas flow path, the anode flow region, an anode exhaust gas flow path, and a transition point where the anode exhaust gas flow path transitions into the anode operating gas flow path; the method comprising the following steps: generating a vacuum in the anode gas circuit to allow a reduction of inert gas partial pressure in said anode gas circuit, before initiating operation of the fuel cell arrangement, when said fuel cell system operates at said inert gas partial pressure below said atmospheric pressure; and, configuring said anode gas circuit to be under atmospheric pressure before said initiating operation of said fuel cell arrangement; feeding air as cathode operating gas into the cathode flow region of the fuel cell, wherein the air has an oxygen concentration, and feeding an anode operating gas containing hydrogen and nitrogen into the anode flow region of the fuel cell, wherein a hydrogen concentration in the anode operating gas has a predetermined nominal value corresponding to the oxygen concentration in the air; having oxygen and hydrogen react in the fuel cell, thereby generating electrical energy, a cathode exhaust gas containing nitrogen and water, and an anode exhaust gas containing nitrogen and water; separating liquid water from the anode exhaust gas, thereby generating an anode exhaust gas free of liquid water; feeding the entire anode exhaust gas free of liquid water into the anode operating gas flow path; determining an actual value of the hydrogen concentration of the gas in the anode operating gas flow path; optionally determining the oxygen concentration of the air in the cathode operating gas flow path; feeding hydrogen into the anode operating gas flow path until the predetermined nominal value of the hydrogen concentration of the anode operating gas is reached; maintaining a gas flow rate in the anode gas circuit and the cathode gas flow path; and discharging the cathode exhaust gas from the cathode exhaust gas flow path while: not receiving anode operating gas from an environment during operation of the fuel cell arrangement, and not releasing anode exhaust gas into the environment, and not storing anode exhaust gas, and storing water in liquid form.

15. The method according to claim 14, in which, before taking the fuel cell arrangement into operation, the following steps are performed: feeding air into the cathode gas flow path and substantially simultaneously filling the anode gas circuit by evacuation and/or feeding in nitrogen, with such an amount of nitrogen that when feeding hydrogen into the anode operating gas flow path under operating conditions of the fuel cell system, an anode operating gas with the predetermined nominal value of the oxygen concentration is generated, said nominal value corresponding to the oxygen concentration in the air; setting the operating conditions of the fuel cell system; feeding hydrogen into the anode operating gas flow path until the predetermined nominal value of the hydrogen concentration of the anode operating gas is reached.

16. The method according to claim 14, in which the hydrogen concentration in the anode operating gas flow path and, optionally, the oxygen concentration in the cathode operating gas flow path are determined regularly or continuously.

17. The method according to claim 14, wherein the method further comprises coupling said fuel cell system with a manned or unmanned submarine vehicle.

Description

(1) In the following, the invention will be further illustrated by means of drawings. It is noted that the drawings are neither drawn to scale nor proportional. Furthermore, only the features essential for understanding the present invention are shown. It is understood that additional features may be present and that not all features shown are essential for the functioning of the present invention. In the figures:

(2) FIG. 1 shows a schematic illustration of an embodiment of a fuel cell system according to the invention,

(3) FIG. 2 shows a schematic illustration of an alternative embodiment of a fuel cell system according to the invention, and

(4) FIG. 3 shows a schematic illustration of another alternative embodiment of a fuel cell system according to the invention.

(5) FIG. 1 shows a schematic illustration of an embodiment of a fuel cell system 1 according to the invention. The fuel cell system 1 comprises a fuel cell arrangement 2 consisting, in the illustrated embodiment, of a single fuel cell 3. In reality, a fuel cell arrangement comprises a plurality of fuel cells, typically several fuel cell stacks, each having a plurality of fuel cells. The fuel cells are of an actually conventional construction, for example polymer electrolyte membrane fuel cells having a cathode 10 and an anode 20, which are each supplied with operating gas over as much of their area as possible. The operating gas typically flows in flowing fields, which are in FIG. 1 schematically illustrated as the cathode flow region 13 and the anode flow region 23. For cooling, the illustrated fuel cell 3 comprises a cooling plate 8.

(6) The fuel cell system is operated with artificial air, i.e. with a mixture of oxygen and nitrogen, the oxygen content of the artificial air preferably being 20 to 50 volume percent, especially preferably 40 to 50 volume percent. During operation of the fuel cell system 1, the artificial air is continuously generated from the components oxygen and nitrogen and supplied to the fuel cell arrangement 2. The operating gas on the anode side is a mixture of hydrogen and nitrogen, which is also continuously generated from the components hydrogen and nitrogen and supplied to the fuel cell arrangement 2 during operation of the fuel cell system 1. The hydrogen concentration in the anode operating gas equals the oxygen concentration in the cathode gas circuit.

(7) The reaction gases oxygen and hydrogen as well as the inert gas nitrogen are provided in suitable reservoirs, in the illustrated embodiment a compressed-oxygen cylinder 30, a compressed-hydrogen cylinder 40 and a compressed-nitrogen cylinder 50. The nitrogen reservoir may be much smaller than the reaction gas reservoirs, because no nitrogen is consumed during the fuel cell reaction, since during the entire fuel cell operation, the same amount of nitrogen is circulated.

(8) The size of the reaction gas reservoirs depends on the scheduled fuel cell operation time. The reservoirs are of course not limited to compressed gas cylinders.

(9) An essential aspect of the present invention is the provision of the fuel cell system with a closed cathode gas circuit 11, into which either nitrogen or oxygen is fed in, and with a closed anode gas circuit 21, into which either hydrogen or nitrogen is fed in. The cathode gas circuit 11 is composed of a cathode operating gas flow path 12, which transitions into the cathode flow region 13 at the fuel cell gas inlet, which, in turn, transitions into a cathode exhaust gas flow path 14 at the fuel cell gas outlet. The cathode exhaust gas flow path 14, in turn, opens at a transition point 15 into the cathode operating gas flow path 12. The anode gas circuit 21 is composed of an anode operating gas flow path 22, which transitions into the anode flow region 23 at the fuel cell gas inlet, which in turn transitions into an anode exhaust gas flow path 24 at the fuel cell gas outlet. The anode exhaust gas flow path 24 opens into the anode operating gas flow path 22 at a transition point 25. The fuel cell system of the present invention is thus adapted for fully recirculating the fuel cell exhaust gases and not releasing any exhaust gas into the environment. The flow paths are hose lines or pipes.

(10) The cathode flow region 13 and the anode flow region 23 are commonly fanned out, i.e. there are gas distributors at the fuel cell gas inlet, which distribute the cathode-operating gas and the anode-operating gas as evenly as possible over the entire fuel cell arrangement 2, and there are collectors at the fuel cell gas outlet, which collect the cathode exhaust gas and the anode exhaust gas and feed them into the cathode exhaust gas flow path 14 and the anode exhaust gas flow path 24, respectively.

(11) A pressure sensor 18 in the cathode operating gas flow path 12 and a temperature sensor 19 with the cathode exhaust gas flow path 14 serve to determine the gas pressure and the gas temperature in the cathode gas circuit 11.

(12) A pressure sensor 28 in the anode operating gas flow path 22 and a temperature sensor 29 in the anode exhaust gas flow path 24 serve to determine the pressure and the temperature of the gas in the anode gas circuit 21. However, it is also sufficient to only provide one of the temperature sensors 19, 29, preferably the temperature sensor 19 in the cathode gas circuit, because the gas temperatures in the anode gas circuit and the cathode gas circuit are approximately the same both during the filling procedure and during operation of the fuel cell system. Furthermore, the sensors may be located at an arbitrary location in the cathode gas circuit 11 and in the anode gas circuit 21. The system's electronics can calculate the amount of the gas in the cathode gas circuit 11 and in the anode gas circuit 21 from the measured pressure and the measured temperature.

(13) In the illustrated embodiment, oxygen is fed from the compressed gas cylinder 30 via an oxygen flow path 31 (oxygen line 31), in which a pressure reducer 33 is located, to a valve 32 to feed it into the cathode gas circuit 11. Hydrogen is guided from the compressed gas cylinder 40 via a hydrogen flow path (hydrogen line) 41, in which a pressure reducer 43 is located, to a valve 42 for feeding it into the anode gas circuit 21. Nitrogen is guided from a compressed gas cylinder 50 via an inert gas flow path 51, 52 to the valve 32 to feed it into the cathode gas circuit 11 and similarly via an inert gas flow path 51, 54 to the valve 42 to feed it into the anode gas circuit 21. In the partial section 51 of the inert gas flow path, a pressure reducer 53 and an optional stop valve 55 are provided, the stop valve 55 making it possible to reliably prevent nitrogen from flowing into the cathode gas circuit 11 and the anode gas circuit 21 at the wrong point in time. By providing the pressure reducer 53 in the partial section 51 of the inert gas flow path, it is ensured that the same inert gas partial pressure is set in the cathode gas circuit 11 and in the anode gas circuit 21.

(14) In the illustrated embodiment, nitrogen and oxygen are, via a common means 32, which allows either oxygen or nitrogen to be fed in, such as a valve that can be switched between supplying oxygen and supplying nitrogen, fed into the cathode gas circuit 11 at a feed-in point (transition point) 15. Nitrogen and hydrogen are analogously, via a common valve 42, which can be switched between supplying hydrogen and supplying nitrogen, fed into the anode gas circuit 21 at a feed-In point (transition point) 25, Suitable valves 32 and 42 are for example 3-2-way magnetic valves, in general, magnetic valves are preferably used for all valves.

(15) As an alternative, it is also possible to separately supply oxygen and nitrogen to the cathode gas circuit 11 and/or hydrogen and nitrogen separately to the anode gas circuit 21. The point where oxygen is fed in defines the transition point 15 and the point where hydrogen is fed in defines the transition point 25. Nitrogen can principally be fed in at an arbitrary location of the cathode gas circuit 11 and the anode gas circuit 21, respectively, of course outside the fuel cells themselves. If the supplies are separated, it is preferable to provide a stop valve in the inert gas flow path, the oxygen flow path and the hydrogen flow path in order to prevent nitrogen and oxygen from being simultaneously fed into the cathode gas circuit and nitrogen and hydrogen from being simultaneously fed into the anode gas circuit.

(16) The fuel cell arrangement 2 is, on the cathode side, operated with artificial air, for example having an oxygen portion of 50 volume percent, and on the anode side, with a hydrogen/nitrogen mixture. If the oxygen portion is 50 volume percent, the hydrogen portion is also 50 volume percent. Before starting continuous fuel cell operation and drawing energy, the anode gas circuit 21 and the cathode gas circuit 11 are filled with the desired operating gases. This procedure is explained by means of concrete exemplary numbers in the following.

(17) Formula

(18) $p.Math.V=\frac{m}{M}.Math.R.Math.T$
(p=pressure; V=volume, m=mass; M=molar mass; R=gas constant; T=temperature), which can be applied with very good approximation, implies that for setting the desired reaction gas concentrations (nominal concentrations), pressure and temperature, as well as mass and molar mass of the involved gases and the volume to be filled, are of importance.

(19) For an exemplary volume of the cathode gas circuit of V.sub.g=0.0035 m.sup.3, a desired reaction pressure (nominal pressure) of the cathode operating gas of p.sub.g=4451 hPa absolute (445100 kg.Math.m.sup.1.Math.s.sup.2 absolute), a temperature of the gas in the cathode gas circuit of T.sub.g=327 K (54 C.) and a desired oxygen concentration of 50 Vol% (.sub.O2=0.5), for a molar mass of oxygen M.sub.O2=15.9994 g.Math.mol.sup.1, a molar mass of nitrogen M.sub.N2=14.0067 g.Math.mol.sup.1 and the gas constant R=8.312 J.Math.mol.sup.1.Math.K.sup.1, for the entire mass of the gas m.sub.g=m.sub.O2+m.sub.N2 in the cathode gas circuit 11 at a stable point of time before the continuous operation of the fuel cells, i.e. before starting to draw current without a gaseous or liquid water portion, this yields:

(20) $m_g=\left[M_{O2}.Math.{}_{O2}+M_{N2}\left(1-{}_{O2}\right)\right].Math.\frac{p_g.Math.V_g}{R.Math.T_g}$

(21) Applying the above numerical values yields 11 m.sub.g=9.170 g for the required overall mass of the gas in the cathode gas circuit. When taking the ratio of the molar masses of oxygen and nitrogen M.sub.O2/M.sub.N2=15.9994:14.0067 into consideration, the mass of oxygen will be m.sub.O2=4.585 g and the mass of nitrogen will be m.sub.N2=4.281 g.

(22) If nitrogen is filled in at a temperature of 23 C. (298K), a nitrogen partial pressure p.sub.N2 must be set, which results from:

(23) $p_{N2}=\frac{m_{N2}.Math.R.Math.296K}{M_{N2}.Math.V_g}=2149\mathrm{hPa}\left(\mathrm{absolute}\right).$

(24) This partial pressure is set in the cathode gas circuit 11 and in the anode gas circuit 21 when taking the fuel cell system into operation.

(25) The above calculation, however, does not consider the fact that during the fuel cell reaction, water is generated as a reaction product, a certain proportion of which is taken along in gaseous form in the cathode gas circuit and the anode gas circuit. The gaseous water replaces part of the inert gas such that when taking the fuel cell system into operation, correspondingly less inert gas must be fed into the cathode gas circuit 11 and into the anode gas circuit 21. The required amount of inert gas when considering the generated reaction water, can be calculated according to the Wagner equation

(26) $p_{\mathrm{sat}}-p_c.Math.\exp \left\{\frac{T_c}{T_g}.Math.\left[A.Math.\left(1-\frac{T_g}{T_c}\right)+B.Math.{\left(1-\frac{T_g}{T_c}\right)}^{15}+C.Math.{\left(1-\frac{T_g}{T_c}\right)}^3+D.Math.{\left(1-\frac{T_g}{T_c}\right)}^6\right]\right\}$

(27) p.sub.sat refers to the saturation pressure, p.sub.c to the critical gas pressure and T.sub.c to the critical water temperature. p.sub.c is 220600 hPa and T.sub.c is 641.1 K. T.sub.g refers to the temperature of the gas in the cathode gas circuit and the anode gas circuit, respectively, and A, B, C, D are Wagner coefficients (A=7.71374, B=1.31467, C=2.51444, D=1.72542). With respect to the Wagner equation and the values cited above, reference is made to the VDI Wrmeatlas 10.sup.th edition, Springer-Verlag Berlin, Heidelberg 2006.

(28) Applying the parameters yields .sub.H2O=p.sub.sat/p.sub.g=0.249 for the concentration .sub.H2O of gaseous water in the cathode gas circuit and the anode gas circuit.

(29) p.sub.g refers to the nominal pressure of the cathode operating gas and the anode operating gas, respectively (4451 hPa absolute).

(30) The overall mass of the gas m.sub.g=m.sub.O2+m.sub.N2+m.sub.H2O in the cathode gas circuit 11 at a stable point in time during operation of the fuel cell thus yields

(31) $m_g=\left[M_{O2}.Math.{}_{O2}+M_{N2}.Math.\left(1-{}_{O2}-{}_{H2O}\right)+M_{H2O}.Math.{}_{H2O}\right].Math.\frac{p_g.Math.V_g}{R.Math.T_g}$

(32) V.sub.g, T.sub.g and .sub.O2 are to be specified as stated above for the calculation without a gaseous water portion.

(33) Applying the parameters yields m.sub.O2=4.585 g for the mass of oxygen, m.sub.N2=2.018 g for the mass of nitrogen and M.sub.H2O=2.567 g for the mass of gaseous water. The overall mass m.sub.g of the gas is 9.170 g.

(34) If the temperature T.sub.0 is 296K when filling the anode gas circuit and the cathode gas circuit with nitrogen when taking the fuel cell system into operation, a nitrogen pressure needs to be set that results from

(35) $p_{N2}=\frac{m_{N2}.Math.R.Math.T_0}{M_{N2}.Math.V_g}$

(36) This yields a nitrogen pressure p.sub.N2 of 1013 hPa absolute.

(37) When taking the fuel cell system 1 into operation and before taking the fuel cell arrangement 2 into operation, a nitrogen partial pressure of 1013 hPa is substantially simultaneously set in the cathode gas circuit and the anode gas circuit. The nitrogen partial pressure of 1013 hPa in the cathode gas circuit 11 is set by opening the valve 55 in the inert gas flow path 51, thus having nitrogen flow to the 3-2-way valve 32, which can be switched between supplying oxygen and supplying nitrogen. The valve 32 is switched to nitrogen supply such that nitrogen flows into the cathode gas circuit 11 through a flow path 34 at the transition point 15. The nitrogen pressure is measured by means of the pressure sensor 18, and the pressure reducer 53 in the inert gas flow path 51 compares the measured pressure with the nominal value of 1013 hPa and lets nitrogen flow in until a nitrogen pressure of 1013 hPa is reached (the pressures each refer to absolute pressures).

(38) The anode gas circuit 21 is filled with nitrogen substantially simultaneously with filling the cathode gas circuit 11. Filling the anode gas circuit substantially simultaneously with the same nitrogen pressure as exists in the cathode gas circuit is necessary for preventing nitrogen from migrating due to partial pressure compensation. For filling the anode gas circuit 21 with nitrogen, the 3-2-way valve 42, which can be switched between supplying hydrogen and nitrogen, is switched to nitrogen supply such that nitrogen flows through a nitrogen flow path 44 to the transition point 25 and into the anode gas circuit 21. The nitrogen pressure in the anode gas circuit 21 is measured by means of the pressure sensor 28. The pressure reducer 55 compares the measured pressure with the nominal pressure of 1013 hPa to be set and lets nitrogen flow in until this pressure is reached.

(39) Subsequently, the operating gas mixtures are produced. To this end, the valve 32 is switched to oxygen supply and the valve 42 is switched to hydrogen supply. Since in the embodiment, the cathode operating gas has an oxygen portion of 50 volume percent, the oxygen partial pressure p.sub.O2 to be set equals the nitrogen partial pressure p.sub.N2 without considering the reaction water, ergo 2149 hPa. For the overall operating gas pressure p.sub.g at the filling temperature of 23 C., a pressure of 4156 hPa thus needs to be set. This pressure is set in the cathode gas circuit 11 analogously to the nitrogen partial pressure, i.e. the pressure p.sub.g is measured by means of the pressure sensor 18, and a pressure reducer 33 compares the measured pressure with the required nominal value. As long as the measured pressure is smaller than the required nominal value of 4156 hPa, the pressure reducer valve is opened far enough to have sufficient oxygen flow into the cathode gas circuit in order to reach the required nominal value. As soon as the pressure measured by the pressure sensor has reached the required nominal value, the valve of the pressure reducer 33 closes. Simultaneously, in the anode gas circuit 21, a gas pressure p.sub.g=p.sub.H2+p.sub.N2 of 4156 hPa is also set by measuring the pressure p.sub.g in the anode gas circuit 21 by means of the pressure sensor 28 and then comparing the measured pressure of the pressure reducer 43 to the setpoint. The valve of the pressure reducer 43 is opened to let hydrogen flow into the anode gas circuit 21 until reaching the setpoint. The pressure reducer valve is then closed. The valves 32 and 42 keep theft position, i.e. they remain set to oxygen flow and hydrogen flow, respectively. The fuel cell system 1 is now ready to take the fuel cell arrangement 2 into operation. The pressures each refer to absolute pressures.

(40) The above example was chosen such that the nitrogen partial pressure to be set approximately corresponds to the atmospheric pressure, such that the suitable nitrogen partial pressure is set by simply flushing the cathode gas circuit and the anode gas circuit with nitrogen. Under operating conditions, however, this yields operating gas pressures above the preferred range of 300 to 1000 hPa (positive pressure) according to the present invention. For setting operating gas pressures in the preferred range, nitrogen partial pressures (absolute pressures) need to be set such as to be smaller than the atmospheric pressure, i.e. the cathode gas circuit and the anode gas circuit must be evacuated before setting the desired inert gas pressures. To this end, the cathode gas circuit and the anode gas circuit each preferably have a means for generating a vacuum, such as a vacuum pump (not shown in the figures), provided therein. Small, light pumps with low throughput are sufficient, because there is no need to generate a high vacuum. It is sufficient to be able to generate the nitrogen partial pressure to be set (for example approximately 200 to 800 hPa) or a pressure slightly below the nitrogen partial pressure to be set, such that the desired nitrogen partial pressure (nominal nitrogen partial pressure) can be set by supplying nitrogen as described above.

(41) Before taking the fuel cell arrangement 2 into operation and preferably already while filling the cathode gas circuit 11 and the anode gas circuit 21, a recirculating flow is generated both in the cathode gas circuit and the anode gas circuit in order to achieve a proper gas distribution and mixing of inert gas and reaction gas, for example by means of a recirculation pump 17 in the cathode exhaust gas flow path 14 and by means of a recirculation pump 27 in the anode exhaust gas flow path 24. As an alternative, one or both pumps may be replaced by a jet nozzle. It is important to maintain a flow rate in order to ensure that fresh operating gases are constantly transported into the fuel cells and consumed gases and water formed during the fuel cell reaction are transported out of the fuel cells.

(42) The water formed during the fuel cell reaction needs to be removed from the fuel cell exhaust gas, because it would otherwise continue to enrich in the cathode gas circuit and the anode gas circuit and eventually flood the fuel cells. Therefore, a water separator 16 is provided in the cathode exhaust gas flow path 14 and a water separator 26 is provided in the anode exhaust gas flow path 24. In the water separators 16, 26, the liquid water is separated from the gas flow and collected, while gaseous water remains in the cathode exhaust gas and the anode exhaust gas. After separating the liquid water, the entire exhaust gas is fed into the cathode operating gas flow path 12 and the entire anode exhaust gas is fed into the anode operating gas flow path 22. Since the fuel cell exhaust gases are fed into the operating gas flow paths during the operation of the fuel cell arrangement 2, the operating gases become depleted of the reaction gases oxygen and hydrogen, such that the pressure measured by the pressure sensors 18 and 28 is lower than the nominal pressure at the respective gas temperature measured by means of the temperature sensors 19 and/or 29 in the cathode exhaust gas flow path 14 and/or the anode exhaust gas flow path 24. According to the invention, the pressure in the cathode gas circuit 11 and the anode gas circuit 21 is, however, kept constant during the operation of the fuel cell arrangement 2. To this end, a means for supplying oxygen to the cathode gas circuit 11 and a means for supplying hydrogen to the anode gas circuit 21 is provided such that the supplied amounts of oxygen and hydrogen can be regulated. In the illustrated embodiment, a pressure reducer 33 and a pressure reducer 43 are used. The pressure in the cathode gas circuit 11 and anode gas circuit 21 is kept constant by having the valve of the pressure reducer 33 and the valve of the pressure reducer 43 open sufficiently far for having oxygen and hydrogen continuously flow into the cathode gas circuit 11 and the anode gas circuit 21, respectively, in order to supplement the consumed oxygen and the consumed hydrogen, respectively.

(43) As alternative means for appropriately supplying oxygen, hydrogen and nitrogen, mass flow regulators may be used.

(44) In the illustrated embodiment, the water separators 16 and 26 are each provided with a ievel switch 67 and 68, respectively, and with a water drain valve 64 and 65, respectively. The level switches 67, 68 monitor the fill level of the water separators 16, 26 and ensure that a predetermined filling level is not exceeded. As soon as the water level in the water separator 16, 26 has risen sufficiently far for wetting the level switches, the water drain valves 64, 65 are opened and water is drained. The drain time is chosen such that some water remains in the water separators 16, 26 to prevent cathode exhaust gas and anode exhaust gas from flowing out. Suitable drain times range between 1 and 3 seconds. The drained water flows through pipes 63, 63 into a water collection tank 60, supported by a water pump 61, which is operated each time one of the water drain valves 64, 65, or both, are opened.

(45) The illustrated embodiment comprises a pressure switch 4 in the cathode operating gas flow path 12 and a pressure switch 6 in the anode operating gas flow path 22. These pressure switches monitor the pressure of the operating gases and switch the entire system into a safe mode by means of a safety circuit if a predetermined maximum pressure of the cathode operating gas and the anode operating gas, respectively, is exceeded, as described above.

(46) In the nitrogen flow paths and 52 and 54, non-return valves 56, 57 are provided. The non-return valve 56 prevents a return flow of the cathode operating gas if the valve 32 is erroneously switched to nitrogen flow during operation of the fuel cell arrangement 2 and the non-return valve 57 prevents a return flow of the anode operating gas if the valve 42 is erroneously switched to nitrogen flow during operation of the fuel cell arrangement 2.

(47) Another embodiment of a closed fuel cell system 1 according to the invention is schematically illustrated in FIG. 2. The fuel cell system according to the embodiment illustrated in FIG. 2 is, with respect to most of the components, identical to the fuel cell system illustrated in FIG. 1. The same reference numbers refer to the same or to corresponding components. The fuel cell system illustrated in FIG. 2 comprises only one temperature sensor 19 in the cathode gas circuit 11. A connectable bleeding resistor 9 provides for the generation of fuel cell power and thus for the consumption of reaction gases after switching off the fuel cell system. Furthermore, in the embodiment illustrated in FIG. 2, as means for generating a flow in the cathode gas circuit 11 and the anode gas circuit 21, venturi nozzles 17 and 27 are provided at the transition point 15 of the cathode gas circuit and the transition point 25 of the anode gas circuit. By having gas flow from the lines 34 and 44, respectively, into the venturi nozzles, the exhaust gas from the lines 14 and 24 is sucked in and fed into the cathode operating gas flow path 12 and the anode operating gas flow path 22, respectively.

(48) Furthermore, the fuel cell system illustrated in FIG. 2 comprises a valve 5 for discharging gas from the cathode gas circuit 11 and a valve 7 for discharging gas from the anode gas circuit 21. After switching off the fuel cell system or at least before retaking the fuel cell system into operation, the remaining gases in the system and the remaining water in the system should be drained. This may, for example, be carried out by the valve 5 in the cathode exhaust gas flow path 14 and the valve 7 in the anode exhaust gas flow path 24, as well as a water drain valve 66. The gases and the water are released into the environment of the fuel cell system or, in a fuel cell system built into an appliance system, into the environment of the appliance system the fuel cell system is built into, i.e. into the atmosphere, however only after the mission to be performed by the appliance system is completed. However, during an ongoing mission, the appliance system represents a completely closed system, which is, in particular, of importance in cases of vehicles such as submarine vehicles. The gases may, however, after completing the mission to be performed by the appliance system, also be discharged from the gas circuits in a different way than by means of valves 5, 7, for example together with the water collected in the water separators 16, 26 through the outlets thereof.

(49) Another embodiment of a fuel cell system 1 according to the invention is schematically illustrated in FIG. 3. The embodiment illustrated in FIG. 3 is a semi-closed system, i.e. the system is only closed on the anode side, while on the cathode side, air can be sucked in from the environment and the oxygen-depleted air can be re-released into the environment after the fuel cell reaction. The fuel cell system according to the embodiment illustrated in FIG. 3 is, on the anode side, identical to the fuel cell system illustrated in FIG. 2. The same reference numbers refer to the same or to corresponding components.

(50) The fuel cell system 1 according to FIG. 3 comprises a cathode gas flow path 11, which comprises a cathode operating gas flow path 12, a cathode flow region 13 and a cathode exhaust gas flow path 14. The cathode operating gas flow path 12 and the cathode exhaust gas flow path 14 are fluidly separated from each other. Air, preferably natural ambient air, is fed in as cathode operating gas into the cathode gas flow path 30 through an air source 11. A preferred air source is a blower with a performance that ensures a sufficient flow rate of the cathode operating gas in the cathode gas flow path 11.

(51) The cathode gas flow path 11 has a sensor 35 for detecting the oxygen concentration and the nitrogen portion, respectively, in the supplied cathode operating gas, a pressure sensor 18, a temperature sensor 19 and a pressure switch 4 provided therein. The sensors 35, 18 and 19 and the pressure switch 4 are optional components. The stop valve 32 illustrated in FIG. 3, which allows for separating the cathode gas flow path 11 from the air source 30, is also optional. The air source 30 and the valve 32 are connected by means of an air flow path 31.

(52) Air supplied by the air source 30 flows into the cathode operating gas flow path 12, flows through the cathode flow region 13 and ultimately exits the fuel cell arrangement as oxygen-depleted cathode exhaust gas through the cathode exhaust gas flow path 14. The cathode exhaust gas flow path 14 releases the cathode exhaust gas into the environment. A means for providing a certain resistance to the exiting cathode exhaust gas and at the same time preventing a potential flow of gas in the counter direction, such as a spring-biased non-return valve or a throttle valve, is provided in the cathode exhaust gas flow path 14. The means 5 ensures the maintenance of the desired cathode operating gas pressure during the operation of the fuel cell system 1.

(53) Before taking the fuel cell system according to FIG. 3 into operation, ambient air is first taken into the cathode operating gas flow path 12 by means of the air source 30 and, simultaneously, nitrogen is taken into the anode operating gas flow path 22 from the nitrogen source 50 (if applicable, after first evacuating the anode gas circuit 21), thereby setting a nitrogen partial pressure corresponding to the nitrogen partial pressure of the air in the cathode operating gas flow path 12. Setting the required nitrogen partial pressure in the anode gas circuit 21 is carried out in the same manner as described above for the closed systems. Subsequently, hydrogen is fed into the anode operating gas flow path 22 from the hydrogen source 40 until the same pressure exists in the anode gas circuit 21 and the cathode gas flow path 11. On the anode side, the procedure is the same as described above for the closed systems. Of course, it must be taken into consideration here that, during the operation of the fuel cell system, the temperature changes and product water is formed. On the anode side, the product water needs to be separated from the anode exhaust gas and collected in a collection container, as described above. The separation of product water from the cathode exhaust gas is optional. Alternatively, the product water may also be released into the environment together with the cathode exhaust gas.

(54) A closed fuel cell system may, with slight modifications, also be operated as a system closed on the anode side or a system closed on the cathode side. If, for example, the system illustrated in FIG. 2 that is closed both on the anode side and on the cathode side is to be operated as a system that is closed on the anode side but open on the cathode side, a possibility for separating the cathode operating gas flow path 12 from the cathode exhaust gas flow path 14 must be provided between the cathode operating gas flow path 12 and the cathode exhaust gas flow path 14. i.e. between the water separator 16 and the venturi nozzle 17. This may, for example, be carried out by means of a simple stop valve, such as valves 5 or 55. The valve 5 for draining gas from the cathode gas circuit 11 may be replaced by means 5 from FIG. 3, or such means 5 may be additionally provided in the cathode exhaust gas discharge path. By means of a junction in the oxygen flow path 31 between the pressure reducer 33 and the 3-2-way valve 32, the oxygen source 30 may be decoupled and replaced by an air source 30. The fuel cell system illustrated in FIG. 2 is then ready for operation as a system that is only closed on the anode side. Analogously, a system that is closed on the cathode side, but open on the anode side, may be achieved by modification on the anode side.