Abstract:
A delivery system for the delivery of a gaseous fuel such as for example hydrogen, to a consumer, in particular to a fuel cell stack, wherein the fuel comes from a source with a higher pressure level, the delivery system comprising at least one mechanical pressure regulating valve which has a valve member biased by means of a spring and which is designed to regulate the fuel from the higher pressure level down to a lower pressure level which serves as the feed pressure for a control valve, wherein the control valve is connectable via a feed line to a consumer and determines the mass flow of the fuel coming from the source which is supplied to the consumer and wherein changes of the pressure prevailing at the consumer side of the control valve arise with power changes of the consumer, is characterized in that the said pressure prevailing at the consumer side of the control valve, or a value proportional to the latter or corresponding to it, can be supplied via a control line to the mechanical pressure regulating valve to supplement the spring bias acting on the valve member of the mechanical pressure regulating valve. In addition, a fuel cell system and a method for delivering a gaseous fuel are claimed.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a delivery system for the delivery of a gaseous fuel such as for example hydrogen, to a consumer, in particular to a fuel cell stack, wherein the fuel comes from a source with a higher pressure level, the delivery system comprising at least one mechanical pressure regulating valve which has a valve member biased by a spring and which is designed to regulate the fuel from the higher pressure level down to a lower pressure level which serves as the feed pressure for a control valve, with the control valve being connectable via a feed line to a consumer and determining the mass flow of the fuel coming from the source which is supplied to the consumer and with changes of the pressure prevailing at the consumer side of the load valve arising with power changes of the consumer. The invention furthermore relates to a fuel cell system and also to a method of delivering a gaseous fuel.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the development of alternative drive concepts for automotive applications electrical propulsion in conjunction with a low temperature fuel cell system as an electrochemical energy converter has particularly increased in significance. The choice of the fuel has in this respect a substantial influence on the complexity of the fuel cell system. When using organic fuels such as for example methanol or gasoline, the system complexity increases because of the required reformation of fuel to form hydrogen-rich gas. The use of pure hydrogen results in a substantial simplification of the system.  
           [0003]    In the following hydrogen will principally be referred to as the gaseous fuel, the invention is however not restricted to the use of hydrogen as the gaseous fuel but rather relates in principle to the supply of any desired gaseous fuels to any desired consumers which have a changing requirement for fuel in operation.  
           [0004]    For the more detailed explanation of the invention a fuel cell system will be considered which is supplied with compressed hydrogen from a hydrogen tank. The hydrogen gas is supplied in dependence on the load to the fuel cell stack at the anode side. Oxygen or impure oxygen (air) is supplied to the cathode side of the fuel cell stack in dependence on the load. The desired load value results essentially from the performance demanded of the overall vehicle as selected by the driver (acceleration, braking etc.) and also on the power requirement of the electrical loads which are involved in the fuel cell system.  
           [0005]    With a fuel cell system of this kind it is necessary to restrict the pressure of the hydrogen gas from a high pressure level in the tank to a lower pressure level for the fuel cell system at the outlet of the tank. The low pressure level is not a single absolute pressure point but rather a range of pressures at low level.  
           [0006]    The known delivery systems for delivering a gaseous fuel have been designed hitherto so that at least one mechanically operating pressure regulating valve and also a control valve are provided connected in series between the source of the gaseous fuel with a higher pressure level and the fuel cell system. The task of the mechanically operating pressure valve has hitherto been to produce a constant feed pressure and the control valve is controlled in accordance with the load demands in order to control the mass flow of fuel supplied to the fuel cell stack.  
           [0007]    The control valve had to be designed to handle the whole pressure range between the constant feed pressure of the mechanical pressure regulating valve and the smallest pressure prevailing at the fuel cell side of the valve. This leads to relatively high demands being placed on the quality of the control valve and its ability to respond.  
         SUMMARY OF THE INVENTION  
         [0008]    The principal object of the present invention is to design a delivery system of the initially named kind and a fuel cell system and a method of delivering a gaseous fuel so that the demands made on the control valve are smaller, whereby favourably priced control valves can be used and a high quality control of the delivered fuel mass flow can nevertheless be achieved.  
           [0009]    In order to satisfy this object there is provided an apparatus for the delivery of a gaseous fuel, such as hydrogen, to a consumer, wherein said fuel comes from a source having a higher pressure level (P3), said apparatus comprising at least one mechanical pressure regulating valve having a valve member and a spring for exerting a spring bias on said valve member, said pressure regulating valve being adapted to regulate the fuel from said higher pressure level (P3) down to a lower pressure level (P2), a control valve having an inlet adapted to receive fuel at said lower pressure (P2) and an outlet, a feed line adapted to connect said outlet of said control valve to a said consumer, said control valve being adapted to determine a fuel mass flow supplied from said source to a said consumer, there being a reduced pressure (P1) at said outlet of said control valve said reduced pressure (P1) changing with power changes of a said consumer and a control line for communicating one of said reduced pressure (P 1 ) prevailing at said outlet of said control valve, a value proportional to said reduced pressure and a value corresponding to said reduced pressure to said mechanical pressure regulating valve to supplement said spring bias acting on the valve member of the mechanical pressure regulating valve.  
           [0010]    Also in order to satisfy this object there is further more provided a fuel cell system comprising fuel cell stack consisting of a plurality of fuel cells and having an anode side with an anode inlet and an anode outlet and also a cathode side with a cathode inlet and a cathode outlet, a source of one of oxygen or air at said cathode side for the delivery of one of oxygen and air via said cathode inlet to said cathode side of said fuel cell stack, a source of a gaseous fuel for said anode side of said fuel cell stack, said source being adapted to deliver fuel at a higher pressure level, a feed-line which leads from said source of gaseous fuel to said anode inlet, at least one mechanical pressure regulating valve provided in said feed-line, said mechanical pressure regulating valve having a valve member and a spring for exerting a spring bias on said valve member and being adapted to regulate the fuel from said higher pressure level down to a lower pressure level, a control valve arranged downstream of said mechanical pressure regulating valve and adapted to determine a mass flow of fuel supplied to the anode side of said fuel cell stack from said source of gaseous fuel, an anode outlet valve which is connected to said anode outlet of the fuel cell stack, a control line which leads, from one of a point between the control valve and the anode exhaust gas valve at said anode side of said fuel cell stack and a point at said cathode side of said fuel cell stack downstream of said source of one of oxygen and air, to said mechanical pressure regulating valve and being connected to it to assist said spring bias.  
           [0011]    Furthermore there is provided a method for delivering a gaseous fuel to a consumer said fuel coming from a source with a higher pressure level, there being a mechanical pressure regulating valve having a valve member and a spring for exerting a spring bias on said valve member, said mechanical pressure regulating valve being adapted to regulate said fuel from said higher pressure level down to a lower pressure level and a control valve for receiving fuel at said lower pressure as a feed pressure and being adapted to feed fuel at another pressure via a feed-line to said consumer, said control valve determining a mass flow of fuel supplied to said consumer and with changes of said another pressure prevailing at the consumer side of the control valve arising with power changes of the consumer, the method comprising communicating one of said another pressure prevailing at a consumer side of the control valve, a value proportional to said another pressure and a value corresponding to said another pressure to said mechanical pressure regulating valve to supplement said spring bias acting on said valve member of said mechanical pressure regulating valve.  
           [0012]    Through the measure taken in accordance with the invention the pressure at the outlet of the mechanical pressure regulating valve is changed in accordance with the pressure changes downstream of the control valve in the same sense, whereby the pressure difference between the inlet side of the control valve and the outlet side of the control valve becomes smaller. In this way, the demands placed on the control valve, which is disposed between the mechanical regulating valve and the consumer or the fuel cell stack and which sets the required quantity of the gaseous fuel in dependence on the load, are reduced.  
           [0013]    It is not absolutely essential for the control line to return the actual pressure at the outlet side of the control valve to the mechanical pressure regulating valve but rather it is sufficient to apply a pressure to the mechanical pressure regulating valve as a control pressure which behaves in accordance with the pressure level at the outlet of the control valve, i.e. changes in the same sense as the pressure level of the outlet of the control valve, preferably proportional to this pressure. The required adaptation of the outlet pressure of the mechanical pressure regulating valve to the outlet pressure at the outlet side of the control valve can be achieved through the specific design of the area which is effective with the additional pressure dependent biasing of the valve member in the mechanical pressure regulating valve. Accordingly, the control line can branch off from any desired point at the anode side of the fuel cell stack, for example at the outlet of the control valve itself, at the line between the control valve and the anode inlet of the fuel cell stack, from a distribution passage of the fuel cell stack at the anode side or from an outlet line which leads from the anode outlet of the fuel cell stack to a discharge valve, or indeed from a point in a return flow path between the anode outlet and the anode inlet of the fuel cell stack, if such a return flow path is provided.  
           [0014]    Since the oxygen supply to the cathode side of the fuel cell system is likewise controlled in load dependent manner the control line can also be branched off from a point at the cathode side of the fuel cell stack downstream of the air compressor which is normally provided and which acts as the oxygen source for the fuel cell stack, providing the branch off takes place at a position where the pressure changes take place in phase with the pressure changes at the anode side of the fuel cell stack. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention will now be explained in more detail with reference to embodiments and to the single drawing which shows a fuel cell system designed in accordance with the invention in schematic form. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    The reference numeral  10  designates the fuel cell system which includes a consumer in the form of a fuel cell stack  12 . The fuel cell stack  12  consists of a plurality of individual fuel cells which are schematically designated by  14 . The fuel cell stack  12  has an anode side  16  with an anode inlet  18  and an anode outlet  20  and also a cathode side  22  with a cathode inlet  24  and a cathode outlet  26 .  
         [0017]    In manner known per se each individual fuel cell  14  has an anode, a cathode and between them a membrane (not shown) with each of the so-called MEAs (Membrane Electrode Assemblies) consisting of an anode and cathode in the membrane between two so-called bipolar plates (likewise not shown). Flow passages for oxygen or atmospheric oxygen are provided between the one bipolar plate and the cathode while flow passages which serve for the supply of hydrogen to the anode are likewise provided between the other bipolar plate and the anode.  
         [0018]    The flow passages at the anode side of the fuel cells are connected together so that all fuel cells can be simultaneously supplied with fuel via the anode inlet  18 , and so that excess hydrogen and also other exhaust gases of the fuel cells, such as for example water in vapour form and nitrogen which comes from the atmospheric oxygen delivered to the cathode side, can be led out of the fuel cell stack at the anode outlet  20  via the line  55 . The flow through the anodes of the connected together fuel cells it is indicated schematically in the Figure via the line  28 . In similar manner the flow passages at the cathode side of the fuel cells are connected together to form a flow path  30  from the cathode inlet  24  to the cathode outlet in the fuel cell stack  12 , with the exhaust gases arising at the cathode side  22  being able to be discharged into the atmosphere via the line  25 . The bipolar plates of individual fuel cells  14  are connected in series or in parallel to one another. In operation a voltage arises at the two output terminals  32  and  34 . This voltage is available as a power source for non-illustrated devices, for example for the propulsion of the motor vehicle in which the fuel cell system is incorporated, and also for the driving of other units, which are necessary for the operation of the fuel cell system.  
         [0019]    The design of fuel cell stacks, or of the fuel cells contained therein is well known from diverse documents so that it is not necessary to go into more detail here on the specific design of the fuel cell stack.  
         [0020]    It is important that a gaseous fuel has to be supplied to the anode side  16  of the fuel cell stack  12 . When using hydrogen, the hydrogen can be taken from a source in the form of a hydrogen tank  36 . More specifically the hydrogen comes from the hydrogen tank  36  via a mechanical pressure regulating valve  38  and also via a solenoid actuated switch-off valve  40  and a manually actuatable cut-off valve  42  to a control valve  44  which supplies the fresh hydrogen via a line  46  to the anode inlet  18  of the fuel cell stack  12 .  
         [0021]    In operation the control valve  44  is controlled, with the valves  40  and  42  being open, in accordance with the power required by the driver of the motor vehicle via a control  48  in order to feed the required mass flow of fresh hydrogen into the anode side of the fuel cell stack  12 .  
         [0022]    At the same time as the load dependent control of the control valve  44  by the control  48 , the control  48  controls an electric motor  50  which drives a compressor  52  and feeds atmospheric oxygen via a line  54  and the cathode inlet  24  into the cathode side  22  of the fuel cell stack  12 .  
         [0023]    In the fuel cell stack  12  protons, which are delivered by the hydrogen that is supplied, migrate from the anode side  16  of the individual fuel cells through the membranes to the cathode side  22  and react with the atmospheric oxygen that is supplied at catalysts which are provided there in order to form water. This reaction leads to electrical potentials arising at the bipolar plates which serve in summed form for the power which can be taken off at the terminals  32  and  34 .  
         [0024]    During the electrochemical reaction in the individual fuel cells nitrogen molecules diffuse from the cathode side to the anode side and leave the anode side  16  via the anode outlet  20  together with the unconsumed hydrogen and water vapour. These anode exhaust gases can be discharged continuously via an anode exhaust valve  56 . They are then normally supplied to a catalytic burner (not shown) for heat recovery via a line  57  and are reacted there with atmospheric oxygen for the production of heat. The exhaust gases present after the burner consisting of nitrogen and water vapour can be discharged without reservation to the atmosphere. The anode exhaust valve  56  can however also be discontinuously opened in order to discharge exhaust gases from the fuel cell stack  12  from time to time, for example, when the nitrogen concentration at the anode side of the fuel cell stack  12  has risen to a level at which the efficient operation of the fuel cell stack would suffer. It is however also known to supply the anode exhaust gases to the cathode side  22  of the fuel cell stack  12  so that the hydrogen component at the cathode side reacts directly with oxygen formed water and can be dispensed off in this manner. The present invention can also be used with such a system.  
         [0025]    The possibility however also exists of providing a return line between the anode outlet  20  and the anode inlet  18  and indeed with the pump  60  which ensures that the return gases have a pressure level matched to that at the anode inlet  18  in order to maintain the flow. When using such a return flow a portion of the anode exhaust gases can be discharged either continuously or discontinuously via the anode exhaust valve  56  and the line  57 .  
         [0026]    In the drawing the hydrogen tank  36 , the mechanical pressure regulating valve  38 , the solenoid actuated switch-off valve  40  and also the manually actuatable cut-off valve  52  are shown in a frame  62 . Since this part is frequently supplied by specialist suppliers it is known per se.  
         [0027]    The mechanical pressure regulating valve  38  serves to regulate the higher pressure level P 3  in the hydrogen tank  36 , which can for example lie at 350 bar, down to a lower pressure level P 2  which can for example lie only somewhat above 1 bar.  
         [0028]    With a mechanical pressure regulating valve of this kind a spring  66  which exerts a force on a piston is pre-stressed via an adjusting screw  64 . The piston then for example drives a valve member which cooperates with a counter member or valve seat (not shown) in the mechanical pressure regulating valve, which exerts the actual regulating function. Such mechanical pressure regulating valves normally have a connection to the atmosphere which serves as a reference pressure.  
         [0029]    In some systems  62 , such as can be obtained from a supplier, a second mechanical pressure regulating valve is provided at the tank side of the pressure regulating valve in series with the mechanical pressure regulating valve  38  shown here, since it is difficult to realize the reduction of the pressure P3 from about 350 bar to somewhat over 1 bar with one mechanical pressure regulating valve. When a second mechanical pressure regulating valve is provided it is known to direct a control line from the outlet side of the mechanical pressure regulating valve  38  to the reference input of the second pressure regulating valve, as will be later explained in more detail.  
         [0030]    In accordance with the present invention a control line  70  leads from the outlet side of the control valve  44  to the reference pressure input of the pressure regulating valve  38  so that, when the pressure drops at the fuel cell side of the control valve  44  the reference pressure at the mechanical pressure regulating valve  38  likewise falls. Since the force from the reference inlet acts in the same direction as the spring this leads to a situation in which the outlet pressure P 2  of the mechanical pressure regulating valve  38  and thus also the pressure at the inlet side of the control valve  44  likewise falls, whereby the pressure difference at the control valve  44  between its inlet and outlet sides is smaller. In this way the range of pressure difference which has to be controlled by the control valve  44  is always kept small, which reduces the requirements placed on the control valve  44 . This does not signify that the pressure difference itself must be kept small but rather that the fluctuations of the pressure difference must always be kept small.  
         [0031]    As stated the spring  66  which exerts a force on the piston is pre-stressed by the adjusting screw  64 . The pressure P 1  in the control or reference line  70  acts in the pressure regulating valve  38  in the same direction as the spring  66  with a force equal to P1×A where A is the effective area for the applied control pressure in the pressure regulating valve. The valve outlet pressure P2 acts opposite to the two forces (spring, pressure of the control line). Depending on the pressure a piston which represents the valve member or actuates it moves into an open or closed position.  
         [0032]    In the operating mode with the electromagnetically actuatable switch-off valve  40  open and the cut-off valve  42  open the anodes of the fuel cells require hydrogen gas depending on the load requirement. This hydrogen gas is set by the control valve  44 .  
         [0033]    The connection of the control line  70  from the anode side  16  to the pressure regulating valve  38  has the advantage, that the pressure at the tank outlet P2 is not kept constant relative to the atmospheric pressure, but rather varies in accordance with the anode pressure P1. This signifies that the pressure difference (P2−P1) varies significantly less across the control valve  44  during the operation of the fuel cells than in the previous solution. As a result of the smaller variation in the pressure difference P2−P1 over the valve  44  the demands on the operating range of this valve drop which leads to cost reduction. Moreover, when using a two stage tank regulator, i.e. with two pressure regulating valves connected in series a more stable regulating behaviour can also be expected. As stated the control line  70  can be connected to any desired point between the valve  56  and the valve  55 , however the point directly at the outlet of the valve  44  is preferred.  
         [0034]    As an alternative the control line  70  of the pressure regulating valve  38  can also be connected to any desired point at the cathode side of the fuel cell system. The pre-condition for this is, that the pressure level of the cathode changes in phase with the anode, i.e. the pressure difference between the cathode and the anode remains constant apart from sensor error.  
         [0035]    Various possibilities exist for connecting the control line  70  to the fuel cell system and are indicated by stub lines which are designated with the reference numerals  70 A,  70 B,  70 C,  70 D,  70 E and  70 F. The stub line  70 A lies directly at the anode inlet  18  of the fuel cell stack  12  whereas the stub line  70 B lies directly in front of the anode exhaust valve  56 . The stub line  70 C is connected to the anode outlet side of the fuel cell stack  12  at the return line  65  while the stub line  70 D is provided at the return line  58  after the pump  60 . The stub line  70 F is provided at the cathode inlet side of the fuel cell system  12  while the stub line  70 G is provided at the cathode outlet side of the fuel cell stack and for example branches off from the outlet line  25 .  
         [0036]    In the known system with two pressure regulating valves connected in series the two pressure regulating valves communicate with one another via a control or reference line in order to achieve a dynamic pressure regulating behaviour of the pressure P 2  with varying hydrogen take-off via the line  72 . In this connection the two pressure regulating valves operate as follows:  
         [0037]    The second pressure regulating valve, i.e. the pressure regulating valve which is arranged closest to the fuel cell stack has, as in the pressure regulating valve  38  used in accordance with the invention, a control reference line which is however operated atmospherically. That is to say, that the second pressure stage cannot receive any information from the anode but rather utilizes the atmosphere as a reference measurement point and thus always regulates to a pressure difference relative to the atmosphere which is set by the spring bias. The consequence is that when using the known system with varying anode pressure the pressure difference across the control valve  44  varies and thus the working range of the control valve  44  is unnecessarily increased. The design of the invention avoids this disadvantage as discussed above.  
         [0038]    The design of such mechanical pressure regulating valves  38  is known per se. One example of such a pressure regulating valve is known from the European patent specification EP 0 585 810 B1. There the reference connection  64  is connected either to the atmosphere or a higher pressure can be applied from a pump, if the installation of the pressure regulating valve makes this necessary. However in EP 0 585 510 B1 the reference connection is provided on the wrong side of the membrane for the purpose of the present invention. The valve could however be converted for the purpose of the present invention by arranging a depression capsule to which the control pressure P1 is supplied at the other side of the valve member from the spring and acts via a pin guided coaxially to the valve seat on the valve member coupled to the membrane and biased by the spring, both in the tension direction and also in the pressure direction.