Abstract:
A water atomization apparatus for a fuel cell system, comprising a plurality of fuel cells connected together to form a fuel cell stack and each having an anode, a cathode and a membrane, with the fuel cell stack having an anode side with an inlet for a fuel and an outlet for non-consumed fuel and exhaust gases which arise at the anode side, a cathode side with an inlet for a gaseous oxidation agent such as air and an outlet for exhaust gases arising at the cathode side, and a compressor connected upstream of the cathode side inlet, is characterized in that the water atomizing apparatus comprises a supply tank for deionized water, a pressure pump which is connected to the supply tank, a reservoir which is fed by the pressure pump and contains deionized water under pressure in operation, a pressure regulating valve having an inlet connected to the reservoir and determining the operating pressure which prevails in the reservoir and also at least one controllable injection valve which injects atomized water into the cathode side and or into the anode side of the fuel cell stack.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a water atomization apparatus for a fuel cell system and also to a method for the humidification of a gas flow supplied to a fuel cell system.  
         BACKGROUND OF THE INVENTION  
         [0002]    As is generally known, a fuel cell system frequently comprises a plurality of fuel cells assembled together -to form a fuel cell stack which each have an anode, a cathode and a membrane. The anodes of the individual fuel cells are electrically connected together so that one speaks of the anode side of the fuel cell stack. In the same manner the cathodes of the individual fuel cells are electrically connected together and one speaks here of the cathode side of the fuel cell stack. At the anode side the fuel cell stack has an inlet for a fuel and an outlet for a non-consumed fuel and also for exhaust gases which arise at the anode side. At the cathode side the fuel cell stack likewise has an inlet, either for a gaseous oxidizing agent such as air and an. outlet for exhaust gases which arise at the cathode side, with a compressor normally being connected: upstream of the cathode side inlet of,the fuel cell stack.  
           [0003]    It is generally known that the membranes of the individual fuel cells must be kept moist in operation in order, on the one hand, to protect them from damage and, on the other hand, to achieve a high degree of efficiency.  
           [0004]    In the operation of a PEM fuel cell (PEM=proton exchange membrane) protons which originate from the hydrogen component of the fuel supplied to the anode side migrate through the humidified membranes and react at the cathode side with the oxidizing agent, which is normally supplied in the form of atmospheric oxygen, and thereby form water with the simultaneous generation of electrical power. Thus water always arises at the cathode side. In the operation of the fuel cell stack a part of this water diffuses through the membranes to the anode side of the fuel cell stack so that both sides of the fuel cell stack are humidified. Since the water which is produced is frequently present in excess it is removed from the anode side and the cathode side and then collected and/or drained off.  
           [0005]    Despite this water which is produced in operation, the problem nevertheless remains that the gaseous oxidizing agent supplied to the cathode side and also the fuel supplied to the anode side have to be adequately humidified so that in all operating states of the fuel cell system, for example during startup and with dry environmental conditions, i.e. dry air, the membranes are always kept moist and an adequate humidity is present both at the anode side and also at the cathode side.  
           [0006]    The active humidification of the gas flows that are supplied is frequently restricted to the cathode side of the fuel cell system, because this is generally sufficient in order to humidify the membranes and, as already stated above, a diffusion through the membranes to the anode side of a fuel cell system in any event takes place. Up to now relatively complicated devices are known for the humidification of the gas flows which are operated with fully demineralized, i.e. deionized water which originates from the operation of the fuel cell system. Since such humidifying apparatuses are not only complicated but also take up a relatively large amount of space another system has been tested at the applicants premises in which two nozzles are supplied with deionized water from a pressure pump and the nozzles spray water into the induced air stream. It has, however, been shown that droplet formation can occur, in particular on switching on the pressure pump, which can lead to problems. When switching on the pressure pump a sudden pressure loading of the nozzles arises which leads to the droplet formation, with the droplets being able to migrate under some circumstances up to and into the fuel cell stack. Such droplets can have a negative effect on the degree of efficiency of a fuel cell system because they for example block fine flow passages in the area of the cathode. Moreover, it can transpire that a water droplet lands on a temperature sensor provided in the fuel cell system or in the fuel cell stack and cools it down to such an extent that the outlet signal of the temperature sensor simulates a temperature reduction of the fuel cell stack and leads, via the control of the fuel cell system, to an unnecessary and undesired switching off of the fuel cell system.  
         SUMMARY OF THE INVENTION  
         [0007]    In order to satisfy this object there is provided in accordance with the invention, a water atomization apparatus for a fuel cell system, the apparatus comprising: a supply tank for deionized water, a pressure pump connected to said supply tank, a reservoir which is fed by said pressure pump and contains deionized water under pressure in operation, a pressure regulating valve having an inlet connected to said reservoir and determining an operating pressure which prevails in said reservoir and at least one controllable injection valve connected to said reservoir for the delivery of atomized water.  
           [0008]    An apparatus of this kind ensures, in operation, that deionized water which stands under pressure is always contained in the reservoir. Thus, an at least substantially constant pressure always exists at the, inlet side of the injection valve on injecting water through the controllable injection valve and this forms a first precondition for a fine atomization of the water. This fine atomization of the water is also further improved by the use of a controllable injection valve in contrast to a simple nozzle.  
           [0009]    A water atomization apparatus of this kind can not only be used for the fuel cell stack but also in other units which have a role to play in a fuel cell system, such as for example in a reformation unit which is used for the conversion of a hydrocarbon into a hydrogen rich synthesized gas or in another fuel processing system.  
           [0010]    When the water atomization apparatus in accordance with the invention is used with a fuel cell stack, the controllable injection valve connected to the reservoir can inject the atomized water into the cathode side or into the anode side of the fuel cell stack. When using two or more injection valves the possibility also exists of injecting atomized water both into the cathode side and also into the anode side of the fuel cell stack. It is particularly favorable that the required humidification of the gases can take place both at the cathode side and also at the anode side with one water atomization apparatus.  
           [0011]    It is particularly advantageous for the reservoir to be formed as an injection gallery, which can, for example, be arranged in tube form alongside the fuel cell stack. This tube form represents a favorably priced but nevertheless compact arrangement. As many connections as desired can be provided along the injection gallery so that a corresponding member of injection valves can be connected to the injection gallery.  
           [0012]    Particularly preferred is a water atomization apparatus in which first and second injection valves are provided, the first injection valve being active at a lower power yield of the fuel cell system and the second injection valve being active, additionally or alternatively to the first injection valve, at a higher power yield.  
           [0013]    In this manner a situation can be achieved in which only one kind of injection valve is provided which is ideally designed for the adjustable range of delivery and the increased requirement for water at a higher power yield of the fuel cell stack is satisfied in that the second injection valve is taken into operation in addition to the first injection valve. Naturally three or more injection valves can also be provided when it is necessary to atomize larger quantities of water per unit of time or to cover fuel cell systems of different sizes with one water atomization apparatus. In this way a type of basic module for the water atomization apparatus arises which can be used with fuel cell systems of different sizes, i.e. with different power yields. The adaptation of the basic module to the different fuel cell systems then only resides in controlling the pressure pump in accordance with the respective fuel cell system, i.e. appropriately selecting its power and providing a different number of injection valves. For this purpose one and the same basic module and one model of an injection valve or only a few different models of injection valves can be used for a large number of purposes, which leads to cost savings in manufacturing and in storage.  
           [0014]    In a particularly preferred embodiment a control is provided to which the or each injection valve is connected, with the control being designed in order to control the opening and/or closing and/or the degree of opening of the or each injection valve. It is most favorable when the control is designed to control the or each injection valve by means of a PWM signal (pulse width modulated signal) in order to hereby preset a metered injection quantity of water from the respective injection valve. In other words the injection valve is opened by the control. A certain quantity of water is atomized and the valve is subsequently closed again, with the duration of opening of the valve being determined by the PWM signal. The frequency, with which the valve is repeatedly opened, is determined by the control, so that the quantity of water dispensed is determined by this frequency and by the respective duration of opening.  
           [0015]    The injection, valve itself can in principle be formed in accordance with a petrol injection valve of a motor vehicle. In this connection it is only important that all the parts which come into the contact with the deionized water are formed as parts resistant to deionized water.  
           [0016]    Although it would in principle be possible to design a control so that the controlling of the injection valves is effected in accordance with the respective operating state of the fuel cell system, it is more favorable to provide a humidity sensor which is associated with the cathode side of the fuel cell stack and connected to the control. In this case the humidity sensor delivers a precise information to the control on the degree of humidity that is present and enables a simplified characteristic field map control for the controlling of the injection valves in order to achieve the desired humidity level.  
           [0017]    The humidity sensor is preferably associated with the cathode side outlet and/or arranged after the latter.  
           [0018]    When the water atomization apparatus of the invention is used for the humidification of the anode side of the fuel cell stack then a humidity sensor is associated with the anode side of a fuel cell stack and connected to the control. In this case the humidity sensor is also preferably associated with the anode side outlet and/or arranged after it.  
           [0019]    Further particularly preferred designs of the water atomization apparatus and of the components used therein can be found in the subordinate claims.  
           [0020]    The method in accordance with the invention for the humidification of a gas flow supplied to a fuel cell system comprises the steps of: taking deionized water from a water supply tank and forwarding it by means of a pressure pump into a reservoir for storing said deionized water under pressure, determining one of a preset pressure level and a predetermined pressure range in said reservoir by means of a valve and injecting atomized water into said gas flow via at least one controllable injection valve connected to said reservoir.  
           [0021]    Particularly preferred variants of the method of the invention can also be found in the subordinate claims.  
           [0022]    The invention brings the advantages that one can better atomize deionized water. There is no droplet formation so that water droplets cannot be blown by the high air pressure of the compressor into the stack or land on the temperature sensor. The control time for the valves can be accurately preset and can be much better controlled than the time in which a nozzle is acted on by a pressure pump which is switched on and off. Thus, by means of the invention, the quantity of water injected can be substantially better controlled than is the case in the prior art.  
           [0023]    The system is also better projected by the invention against undesired shutting down of the system, which was previously caused by droplet formation on the introduction of deionized water.  
           [0024]    The invention will now be explained in the following in more detail with reference to embodiments and to the accompanying drawing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The single figure of the drawing shows, in a highly schematic form and not true to scale, a PEM fuel cell system  10  which consists of a plurality of fuel cells  14  connected together to form a fuel cells tack  12 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    The fuel cells  14  each have, in manner known per se—as indicated by the enlarged representation in the circle—an anode  16 , a cathode  18  and a membrane  20 . For each fuel cell  14  the arrangement of the anode  16 , the cathode  18  and the membrane  20 , which forms the so-called MEA (Membrane Electrode Assembly) is arranged between two so-called bipolar plates  22  and each bipolar plate is arranged between two adjacent membrane electrode assemblies, that is apart from the end plates of the stack. At one side each such bipolar plate  22  forms, together with the anode  16  of the one fuel cell, a flow field for a fuel. At the other side each said bipolar plate forms, together with the cathode  18  of the adjacent fuel cell, a flow field for the gaseous oxidizing agent. The flow fields are frequently formed by fine channels which are worked or machined into the bipolar plate. The bipolar plates are each frequently assembled from two plates which lie surface to surface against one another, with cooling passages for a gaseous or liquid coolant being provided between the two plates. The design of fuel cells is well known per se and will not be described further here, because this specific design of the fuel cell is not of importance for the present invention.  
         [0027]    It suffices to say that the fuel cells are so arranged in the fuel cell stack that the anodes are connected together and thus form an anode side  26  of the fuel cell stack. The cathodes of the individual fuel cells are likewise connected together and form a cathode side of the fuel cell stack.  
         [0028]    The anode side  26  of the fuel cell stack has an inlet  30  for a fuel and an outlet  32  for non-consumed fuel as well as exhaust gases which arise at the anode side. In similar manner the cathode side  28  of the fuel cell stack  12  has an inlet  34  for a gaseous oxidizing agent such as air and an outlet  36  for exhaust gases arising at the cathode side. A compressor  38  is connected in front of the cathode side inlet  34  and is driven by an electric motor  40 . The compressor sucks in air via a line  42  and an air filter  44  and feeds it into the cathode side inlet  34  of the cathode side  28  of the fuel cell stack  12 . The air arriving at the inlet  34  is distributed through the internal flow passages of the fuel cell stack  12  into the anode side flow fields of the individual fuel cells  14 , with a part of the oxygen contained in the sucked in air reacting catalytically with protons coming from the anode side  26  at the cathode side and thereby producing water and: generating electrical power which can be tapped off at the terminals  46  and can for example be used for the electrical propulsion of a motor vehicle which has the fuel cell system as a source of propulsion.  
         [0029]    The cathode side exhaust gases, which principally consist of nitrogen, which is a component of the induced air and is not consumed in the fuel cell stack  12 , of water vapor and fine droplets of water which arise by the reaction of oxygen with protons coming from the anode side and also of non-consumed oxygen, pass via a humidity sensor  48  and also a water separator device  50  to a valve  52  which, on the one hand, determines the operating pressure prevailing at the cathode side  28  and, on the other hand, discharges the cathode side exhaust gases, less the water component which is separated out at the separating device  50 , into the environment via the line  54 . In this connection it is rioted that the so discharged nitrogen, the water and the residual oxygen do not cause any pollution of the environment because they represent natural components of the environment.  
         [0030]    The adjustment of the valve  52  is effected by a control  54  which is connected via a line  56  to the valve  52 . For the line  56  only the respective connections to the control  54  and to the valve  52  are indicated in order to not unnecessarily complicate the drawing by this line and by the many lines which must otherwise be drawn in.  
         [0031]    The humidity sensor  48  is also connected via a line  58  to the control  54 . At the outlet of the water separating device  50  there are located two valves  60  and  62  respectively, with the valve  60  being connected via the line  64  to the control  54 . The valve  60  can be opened by the control  54  in order to feed water via the line  66  into the supply tank  68 . The supply tank  68  has filling level indicator  70  which is connected via a line  72  to the control  54 . Thus the level indicator  70  indicates to the control at what points in time the valve  60  has to be opened in order to refill the tank  68  and closed again when the supply tank  68  is filled up.  
         [0032]    The valve  62  is normally closed and is in any event closed during the filling of the supply tank  68 . It can, however, be opened in order to drain excess water from the water separating device  50 . The valve  62  is likewise connected to the control  54  and indeed via a line  74 .  
         [0033]    The drive motor  40  for the compressor  38  is also connected to the control  54  and indeed via a line  76 .  
         [0034]    At the anode side  26  of the fuel cell stack  12  there is located a supply tank  80  which in this example contains hydrogen, with the hydrogen from the supply tank  80  being able to be supplied via the valve  82  to the anode side inlet  30  of the fuel cell stack  12 . The valve  82  which regulates the quantity of newly supplied fuel is connected to the control  54  via the line  84 .  
         [0035]    The hydrogen supplied flows through the flow fields provided at the anode side, with a part of the hydrogen being converted at the catalyst provided at the anode side into protons which migrate through the membranes  20  of the fuel cells  12  and react at the cathode side in the already described way and manner with the supplied atmospheric oxygen. The non-consumed fuel, here in the form of hydrogen, then leaves the anode side  26  of the fuel cell stack  12  via the outlet  32  together with water vapor and nitrogen which has diffused through the membranes of the fuel cells from the cathode side  28  to the anode side  26 . The gases emerging from the outlet  32  flow through the line  85  via a further humidity sensor  86  and are then supplied again by a pump  90  to the anode side inlet  30 . The humidity sensor  86  is connected via a line  92  to the control  54 . In this manner the control  54  receives information concerning the prevailing relative humidity at the anode side  26  of the fuel cell stack  12 .  
         [0036]    Since the increasing accumulation of nitrogen at the anode side  26  of the fuel cell stack in operation leads to the current generation being impaired, a part of the gases flowing at the anode side is continuously or discontinuously discharged via the valve  88 , with the valve  88  being controlled for this purpose via the line  94  by the control  54 . In manner known per se the exhaust gases discharged via the valve  88  are supplied to a catalytic burner device which removes the remainder of the hydrogen content by combustion, so that the cleaned exhaust gases can be discharged without reservation into the environment, because they only consist of natural components of the environmental air.  
         [0037]    The reference numeral  100  points to a cooling circuit having a pump  102  which pumps a coolant liquid through a radiator  104  and into the cooling passages provided in the bipolar plates in order to keep the fuel cell stack  12  in a desired temperature range.  
         [0038]    The fully demineralized, i.e. deionized water present in the water supply tank  68  is sucked in by a pump  110  via a line  112  and is pumped via a further pressure line  114  into a reservoir in the form of an elongate injection gallery  116 . A pressure above atmospheric pressure builds up in the injection gallery  116  which can lie between 0.2 bar and 10 bar and normally lies between 1 and 3 bar. The maximum pressure is restricted by a pressure regulator  118  in the sense that when the pressure present in the reservoir  116  thus reaches the maximum set pressure the pressure regulator  118  discharges or feeds a part of the water stored in the gallery  116  back into the water supply tank  68  via the return line  120 .  
         [0039]    The pressure regulating valve  118  can be a purely mechanically acting pressure regulating valve, can however also be an electronically controlled pressure regulating valve which is connected via a control line  122  to the control  54 . Thus the maximum pressure level in the injection gallery  116  can be determined by the control  54  via the control line  122 .  
         [0040]    The reference numeral  124  points to a pressure sensor which can be connected to the injection gallery  116  to determine the pressure prevailing there. The pressure sensor  124  is connected via a line  126  to the control  54  and delivers an actual value for the pressure prevailing in the injection gallery  116  to the control  54 , which can take this into account when controlling the pressure regulating valve  118  via the line  122 . The pressure sensor  124  is not essential when using a purely mechanically acting pressure regulating valve, can however nevertheless be useful in order to give the control  54  information concerning the operating state of the water atomization device.  
         [0041]    The reference numeral  128  points to a bleed valve which can be actuated manually in order to bleed air from the injection gallery  116  at the pressure side on taking the system to operation. The bleed valve  128  can however also be a mechanical self bleeding valve. The valve  128  could however also be an electronically controlled valve which is controlled from time to time in order to bleed the injection gallery  116  at intervals, when it turns out that air repeatedly collects in the injection gallery  116 .  
         [0042]    The reference numeral  130  points to an optionally provided accumulator with a gas cushion  132  which stands under pressure and which is separated from the liquid contained in the gallery  116  by a membrane  136 . This accumulator  130  can be used in order to suppress pressure peaks or fluctuations in the over pressure in the injection gallery  116 , if it turns out that pressure peaks or fluctuations in pressure are problematic.  
         [0043]    In this example three injection valves  140 ,  142  and  144  are connected to the injection gallery  116  and indeed via respective pressure lines  146 ,  148  and  150 . The first injection valve  140 , which can be designed in accordance with a petrol injection valve known per se for a normal motorcar, serves, on being energized by the line  152 , to inject water in finely atomized form into the inlet  39  of the compressor  38 . The control line  152  is connected to the control  54  and injection valve  140  receives so-called PWM signals (pulse width modulatable voltage signals) from the control  54  which cause the injection valve  140  to open, to atomize water and to close again, with the total quantity of water injected depending on the frequency of the opening and closing processes and also on the duration of each injection process. The second injection valve  142  likewise serves to inject water into the compressor inlet, is however located for space reasons at a different position at the compressor  38 .  
         [0044]    The injection valve  142  is provided with its own control line  154  which is connected to the control  54 .  
         [0045]    The advantage of using two injection valves  141 ,  142  at the cathode side of the fuel cell stack  12  lies in the fact that for a low power yield only one injection valve, for example  140 , must be controlled in order to take care of the required humidification of the inflowing air. If, in contrast, a larger quantity of water is required for the humidification of the inflowing air for a higher power yield, then further atomized water can additionally be injected via the valve  142  into the inflowing air.  
         [0046]    The third injection valve  144  is provided at the anode side  26  and is controlled via the control line  156  from the control  54 . The third injection valve  144  serves to adequately humidify the fuel supplied to the anode side  26  of the fuel cell stack when the humidity determined by the humidity sensor  68  is not sufficient. Since the control  54  obtains information both at the cathode side  28  and also at the anode side concerning the humidity prevailing there from the respective humidity sensor  48 ,  86 , the control  54  can straightforwardly determine the metered quantity of atomized water which is to be injected into both sides of the fuel cell stack and can control the injection valves  140 ,  142  and  144  accordingly.  
         [0047]    In the water supply tank fully demineralized (deionized water) is present because such water can be taken from the cathode side exhaust gases. Fully demineralized water is to be preferred, because one can ensure in this manner that no salt residues deposit in the system and impair the action of the system. The use of deionized water involves the danger of leaching out chemical components from various components. Accordingly all components of this system which come into contact with the deionized water must be resistant against deionized water. Favorable in this connection is the manufacture of the individual components, which come into contact with deionized water, of stainless steel or of other materials which are coated with Teflon.  
         [0048]    In operation the pressure in the injection gallery  116  is increased by the electric motor  110 . The pressure level in the injection gallery  116  is variably adjustable via the pressure regulator  118  which is inserted into the return flow line  120  of the injection gallery  116 . The pressure regulator  118  ensures that a predetermined pressure is present in the system, i.e. in the injection gallery  116 . If the pressure in the system sinks, for example because water is taken from the injection gallery  116 , the electric pump  110  is activated in order to reestablish the pressure. The pressure regulating valve  118  can be a purely mechanical valve, but also an electrically adjustable pressure regulating valve. In an electrically controlled pressure valve one can, with the aid of the control apparatus  54 , set the pressure via the voltage outlet  122 . This system additionally requires the pressure sensor  124  as a feedback concerning the actual pressure in the system.  
         [0049]    With a mechanical pressure regulating valve the pressure is preset to the desired value. It is possible to bleed the system through a manually operable bleeding valve  128 . One can also use a mechanical self bleeding valve for this purpose in order to carry out the bleeding.  
         [0050]    The pressure of the injection nozzle (injection valve), present as a result of the pressure in the injection gallery, ensures that on controlling the valve or the valves the atomization (spray) is significantly finer than is the case with the present system.  
         [0051]    With a high air pressure at the cathode side the air is preheated and thus becomes drier. Depending on the temperature, deionized water is injected into the compressor under the control of a characteristic field map, which has to be established. At very high temperatures it can be necessary to use a second injection valve. The injection system corresponding to the invention can be enlarged as desired to a plurality of injection nozzles and can thus be easily adapted to different sizes of the fuel cell stack, i.e., with large fuel cell stacks  12 , a plurality of injection nozzles of the same design can be used both at the cathode side and also at the anode side in order to cover the requirement for atomized water. It is possible to meter the quantity injected very accurately through the control by means of a PWM signal (pulse width modulatable voltage signal).  
         [0052]    Although the present water atomization apparatus has been described in conjunction with PEM fuel cells it can be used with all types of fuel cells where the humidification of operating gases is necessary.