Patent Publication Number: US-2013252117-A1

Title: Apparatus and method for humidified fluid stream delivery to fuel cell stack

Description:
TECHNICAL FIELD 
     Embodiments disclosed herein generally relate to an apparatus and method for humidified fluid stream delivery to a fuel cell stack. 
     BACKGROUND 
     It is generally known that a number of fuel cells are joined together to form a fuel cell stack. Such a stack generally provides electrical current in response to electrochemically converting hydrogen and oxygen into water and energy. The electrical current is used to provide power for various electrical devices in the vehicle or in other suitable mechanisms. 
     An inherent deficiency of a fuel cell membrane is that the membrane requires humidification to operate properly. Due to such a condition, an additional subsystem is needed to adequately humidify the membrane. During operation of the fuel cell in an automotive environment, the fuel cell operates at lower powers (i.e., current densities), leading to increased humidification demand since not enough product water is being generated. 
     Conventional systems deliver water in the air and hydrogen streams that are fed to the fuel cell stack to ensure that such membranes are kept moist. While it may be necessary to ensure that membranes are kept moist, one consideration should account for not providing too much water in the air and hydrogen streams since such excess water may clog membranes in the fuel cell and lead to inefficient operation of the fuel cell stack. 
     One example of humidifying an air stream in connection with a fuel cell is set forth below. 
     Japanese Patent Publication No. JP20100198743 to Toshikatsu et al. discloses a fuel cell system that includes a fuel cell in which oxidant gas and fuel gas are supplied. The fuel cell generates power by electro-chemical reaction of these oxidant gas and fuel gas. The fuel cell system further includes a humidifier that transfers moisture contained in the oxidant gas discharged from the fuel cell to the oxidant gas to be supplied to the fuel cell and a compressor that compresses the oxidant gas humidified by the humidifier and sends to the fuel cell. The fuel cell system further includes condensing means that condenses power generation produced water discharged from the fuel cell and stores it. The condensed water stored by the condensing means is supplied to the spacing between the humidifier and the compressor upstream of the fuel cell. 
     SUMMARY 
     An apparatus for providing a humidified cathode fluid stream to a fuel cell stack is disclosed. The apparatus comprising a first humidifier including membranes and a compressor. The first humidifier is configured to receive a cathode fluid stream and to humidify the cathode fluid stream with water from a recirculated fluid stream to provide a first humidified cathode stream. The compressor is configured to receive the first humidified cathode stream and to provide a first pressurized humidified cathode stream. The compressor is further configured to generate a pressure differential across the first humidifier such that the membranes are humidified with the water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the present disclosure are pointed out with particularity in the appended claims. However, other features of the various embodiments will become more apparent and will be best understood by referring to the following detailed description in conjunction with the accompany drawings in which: 
         FIG. 1  depicts an apparatus for humidifying a fluid stream that is passed to a fuel cell stack in accordance to one embodiment; 
         FIG. 2  depicts the manner in which more water may be driven across membranes of a first humidifier in accordance to one embodiment; and 
         FIG. 3  depicts an apparatus for humidifying a fluid stream that is passed to the fuel cell stack in accordance to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present disclosures are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present disclosure. 
     Fuel cell technology has been well acknowledged to offer the promise of generating clean and efficient power for stationary, portable and transportation applications. The proton exchange membrane fuel cell (PEMFC) is attractive for vehicle applications, relative to other fuel cell designs, due to its higher power density and lower temperature of operation. Conventional PEMFCs separate the electrochemical reactions that generate electricity, as well as the gas reactants, with the proton exchange membrane (PEM) itself. While PEMs allow the transport of protons from an anode to a cathode, PEM fuel cells generally need both moisture content in the proton exchange membrane and sufficient moisture in the anode fuel stream to provide water to the protons to enable the transport. Humidified fuel and air streams are commonly used to provide the needed moisture to assure the function. One method of humidifying the reagent streams is by utilizing a gas to gas (“G2G”) humidifier. There are a number of G2G humidifying devices and a large number of such G2G humidifiers comprise numerous layers (or membranes) or numerous thin tubes in order to obtain adequate surface to produce the desired humidifying effect. 
     Although G2G humidifiers have been proven in fuel cell applications as a simple, robust and reliable way to humidify air stream, their size (and efficiency) represent an ongoing consideration. For example, a G2G humidifier for a 90 kW system may be as large as 42 liters in volume. Additionally, G2G humidifiers generally include membranes and an intercooler may be needed in order to avoid melting of the membranes in the G2G humidifier. If G2G humidifer membranes can withstand higher temperatures, then the intercooler may be down sized or simply removed. 
     Various fuel cell systems implementations as disclosed herein contemplate positioning the G2G humidifier before a compressor. In this implementation, an air stream is provided to the compressor, which then increases the pressure of the air stream for delivery to the fuel cell stack. By positioning the G2G humidifier before the compressor while delivering the air stream to the fuel cell stack, it is possible to take advantage of the lower pressure on a dry side of the G2G humidifier to drive more water across membranes (e.g., in the G2G humidifier) to humidify the incoming air stream that is delivered to the a fuel cell stack. For example, a higher pressure differential across the membrane of the G2G humidifier enables more water (i.e., more humidity) to pass through the membranes thereby causing the air stream to achieve optimal humidity levels. Such a condition may enable the size/area of the G2G humidifier to be reduced. A reduction in size/area of the G2G humidifier may reduce cost/complexity. 
     In addition, by humidifying the air stream via the G2G humidifier that is positioned prior to the compressor, the output air stream from the compressor may be cooler thus allowing for a reduction in size of an intercooler of simply eliminating the need for the intercooler. This condition may obviate the need for a full size intercooler in the fuel cell system, which further reduces the cost/complexity of the fuel cell system. Generally, intercoolers are needed to cool the incoming air stream to prevent membranes within the G2G humidifier from melting. It is recognized that the compressor may output the air stream at a higher temperature. However, by positioning the G2G humidifier prior to the compressor, more energy is needed in order to heat the water in the air stream therefore causing the overall mixture of water and air to exhibit a lower temperature. Such a condition may be attributed to the G2G humidifier lowering the temperature of dry gas (i.e., supply air) entering into the G2G humidifier due to condensation, evaporation, and other factors. While the temperature of the dry gas may increase, such a temperature increase may not rise to the level as that expected by a pure “temperature exchange.” As such, the temperature may be lower than usual and it may be necessary to expend more energy to heat the water in the air stream. 
     In yet another implementation as disclosed herein, multiple G2G humidifiers may be provided such that water discharged from a stack outlet of the fuel cell stack is provided to the G2G humidifiers to humidify the air stream both pre and post compressor streams. A small intercooler may be needed depending on the types of material(s) that are used for the membranes of the G2G humidifier(s) for the various implementations disclosed herein. Similar to the implementation noted above, this implementation also takes advantage of the higher-pressure differential across the membranes of the G2G humidifier thereby causing more water to pass through the membranes, which results in an increase in humidity levels within the air stream. 
       FIG. 1  depicts an apparatus  10  for humidifying a fluid stream that is passed to a fuel cell stack  12  in accordance to one embodiment. The fuel cell stack  12  is configured to generate electrical current for powering one or more various devices (not shown) in a vehicle (or other apparatus) in response to electrochemically converting hydrogen and oxygen into water and energy. The electrical current is used to provide power for various electrical devices in the vehicle (or other apparatus). It is recognized that the apparatus  10  and the fuel cell stack  12  may be implemented in any application in which it is desired to generate electrical current through the use of electrochemically converting hydrogen and oxygen. 
     A tank (or supply)  14  provides a supply fuel stream (or an anode stream) in the form of hydrogen. The supply fuel stream comprises compressed hydrogen. While compressed hydrogen may be used in the apparatus  10 , any hydrogen fuel source may be implemented in the apparatus  10 . For example, liquid hydrogen, hydrogen stored in various chemicals such as sodium borohydride or alanates, or hydrogen stored in metal hydrids may be used instead of compressed gas. 
     A tank valve  16  controls the flow of the supply hydrogen. A pressure regulator  18  regulates the flow of the supply hydrogen to the fuel cell stack  12 . A humidifier  20  may be optionally provided to add water into the input fuel stream for generating a humidified input fuel stream. Water vapor in the humidified input fuel stream may be needed to ensure that membranes in the fuel cell stack  12  remain humidified to provide for optimal operation of the fuel cell stack  12 . It is recognized that a recirculated hydrogen stream may be provided from an outlet of the fuel cell stack  12  in lieu of, or in combination with the humidifier  20  to humidify the input fuel stream to generate the humidified input fuel stream. For example, the recirculated hydrogen (or anode) stream may be delivered to the input fuel stream to humidify the same for providing the humidified fuel stream. 
     A first fluid stream (or cathode stream) which comprises dry air is fed to a first humidifier  22 . A compressor  28  and a second humidifier  34  are in fluid communication with the first humidifier  22  and the fuel cell stack  12 . The first humidifier  22  and the second humidifier  34  may each be implemented as a G2G humidifier or other suitable device. An example of a G2G humidifier is set forth in U.S. Pat. No. 8,003,265, entitled “Gas Conditioning Device and Method” filed on May 11, 2006 to Schank et al., which is hereby incorporated by reference in its entirety. 
     The first humidifier  22  includes a first inlet  24  (or dry gas inlet) for receiving the dry air. The first humidifier  22  adds water to the cathode stream to humidify the same. The first humidifier  22  includes a second inlet  30  (or wet gas inlet) for receiving water that is recirculated, or recirculate water  51  (e.g., product water) from the fuel cell stack  12  and passed through the second humidifier  34  (and/or passed through the first humidifier  22 ) in response to electrochemically converting the air and the hydrogen (e.g., generating electrical power). Water in the cathode stream may be needed to ensure that membranes (not shown) in the fuel cell stack  12  remain moist to provide for optimal operation of the fuel cell stack  12 . 
     A first outlet  26  of the first humidifier  22  provides a humidified cathode stream  50 . The compressor  28  receives the humidified cathode stream  50  and increases the pressure of the same to provide a first pressurized humidified cathode stream  52 . The first humidifier  22  and the second humidifier  34  each generally include a plurality of membranes  23 . Such membranes  23  may be formed of GORE-TEX®, Nafion® or other suitable materials. The membranes  23  generally define dry air channels and at least one wet channel. Water that is provided by the fuel cell stack  12  and the second humidifier  34  flows into the wet gas inlet  30  of the first humidifier  22 . Air that enters into the first humidifier  22  is humidified as the water passes from the wet channel and into the dry air channels thereof. By positioning the compressor  28  after the first humidifier  22 , it is recognized that more water can be driven across the membranes  23  of the first humidifier  22 . Such a condition may reduce the overall size of the membranes  23  and consequently the size of the first humidifier  22  thereby reducing cost. 
       FIG. 2  generally depicts the manner in which more water may be driven across the membranes  23  of the first humidifier  22 . As shown, in moments in which a lower partial pressure (or low partial pressure) is exhibited (see  80 ) (e.g., prior to the compressor  28 ), more water is driven across the membranes  23  of the first humidifier  22  because of a higher pressure differential, which enables the air to absorb more water (see  82  which is indicative of more water being driven through the membranes of the first humidifier  22 ). As shown at  84 , when an increase in partial pressure (or increased partial pressure) is exhibited, less water is driven through the membranes  23  of the first humidifier  22  (see  86  which is indicative of less water being driven through the membranes  23  of the first humidifier  22 ). The compressor  28  creates the high pressure differential thereby allowing more water to pass to the membranes  23 . The high pressure differential is generated because the partial pressure on the wet side of the first humidifier  22  (e.g., at the first outlet  26 ) is high and the partial pressure on the dry side of the first humidifier  22  (e.g., at the first inlet  24 ) is low. 
     Referring back to  FIG. 1 , the compressor  28  pressurizes the humidified cathode stream  50  and delivers the first pressurized humidified cathode stream  52  to the second humidifier  34 . The second humidifier  34  includes a gas inlet  36  and a wet gas inlet  38 . The fuel cell stack  12  provides water (or the recirculated water  51 ) to the wet gas inlet  38 . The second humidifier  34  receives the first pressurized humidified cathode stream  52  to add more water into the same. The second humidifier  34  adds more water into the first pressurized humidified cathode stream  52  to provide a final pressurized humidified cathode stream  54  to the fuel cell stack  12 . 
     The first pressurized humidified cathode stream  52  is generally at a higher temperature than the temperature of the humidified cathode stream  50  after being pressurized by the compressor  28 . Because the first pressurized humidified cathode stream  52  is at a higher temperature than that of the humidified cathode stream  50 , the first pressurized humidified cathode stream  52  is capable of storing more water. To take advantage of such a condition, the second humidifier  34  is provided to add more water into the first pressurized humidified cathode stream  52  to provide the final pressurized humidified cathode stream  54 . This is done prior to ensure that the membranes of the fuel cell stack  12  are kept humidified. It is recognized that the size of the first humidifier  22  and the second humidifier  34  may be similar to or different from one another. The second humidifier  34  provides the final pressurized humidified cathode stream  54  that is delivered to the fuel cell stack  12 . 
     An exhaust valve  40  is fluidly coupled to an outlet  39  of the first humidifier  22 . The outlet  39  delivers water (or the recirculated water  51 ) from the first humidifier  22  to the valve  40 . The valve  40  may be controlled by a controller (not shown) to regulate pressure (or flow) on the cathode side of the fuel cell stack  12 . Various humidity sensors (not shown) may be positioned along the cathode side to monitor the humidity of the cathode stream as it passes through the first humidifier  22  and the second humidifier  34 . Depending on the design, the compressor  28  controls the pressure of the cathode fluid stream while the valve  40  controls the flow of the cathode fluid stream in the apparatus  10  (e.g., in the case of a centrifugal compressor). Or, alternatively, the compressor (if a positive displacement device) controls the flow on the cathode, while the valve  40  adjust the pressure of the cathode fluid. 
       FIG. 3  depicts an apparatus  70  for humidifying a fluid stream that is passed to the fuel cell stack  12  in accordance to another embodiment. Operation of the manner in which the humidified input fuel stream is provided to the fuel cell stack  12  is similar to that described above in connection with  FIG. 1 . Operation of the manner in which the final pressurized humidified cathode stream  54  is provided to the fuel cell stack  12  is different than that described in connection with  FIG. 1 . For example, the apparatus  70  utilizes a single G2G humidifier arrangement (or the first humidifier  22 ) for providing the final pressurized humidified cathode stream  54  to the fuel cell stack  12  as opposed to a multi-G2G humidifier arrangement. 
     Dry air is delivered to the first humidifier  22  to the dry air inlet  24  and the recirculated water  51  is delivered to the first humidifier  22  at the wet gas inlet  30  from the fuel cell stack  12 . The first humidifier  22  adds water to the dry air in a similar manner to that described above in connection with  FIG. 1 . For example, the dry air that enters into the first humidifier  22  is humidified as the water passes from the wet channel and into the dry air channels (e.g., where the dry air flows) as defined by the membranes  23  of the first humidifier  22 . The compressor  28  receives the humidified cathode stream  50  from the first outlet  26  of the first humidifier  22  and provides the first pressurized humidified cathode stream  52 . The first humidifier  22  provides more water into the humidified cathode stream  50  due to the high pressure differential that is present between the pressure of the dry air as it enters into the first inlet  24  of the first humidifier  22  and the pressure of the wet air as it exits from the first outlet  26  of the first humidifier  22 . As noted above, in connection with  FIG. 2 , the high pressure differential enables more water to flow through the membranes  23  of the first humidifier  22  and into the dry air thereby providing for a decrease in the size of the first humidifier  22 . The positioning of the first humidifier  22  prior to the compressor  28 , enables the compressor  28  to generate the high pressure differential within the first humidifier  22  to adequately humidify the membranes  23  therein. 
     An outlet of the compressor  28  provides the first pressurized humidified cathode stream  52  where it is delivered to back to an inlet  42  of the first humidifier  22 . The compressor  28  provides the first pressurized humidified cathode stream  52  back to the first humidifier  22  at a temperature that is greater than the temperature of the humidified cathode stream  50  that is received from the first outlet  26  of the first humidifier  22 . Since the first pressurized humidified cathode stream  52  is hotter after it is passed through the compressor  28 , such a condition enables the first pressurized humidified cathode stream  52  to receive more water. To add more water into the first pressurized humidified cathode stream  52 , the compressor  28  delivers the same to back to the first humidifier  22  so that more water can be added to the first pressurized humidified cathode stream  52  to generate the final pressurized humidified cathode stream  54 . An outlet  44  of the first humidifier  22  delivers the final pressurized humidified cathode stream  54  to the fuel cell stack  12  to generate the electrical power. The exhaust valve  40  is fluidly coupled to the outlet  39  of the first humidifier  22  where it controls the flow of the re-circulated water either into an exhaust (e.g., out of the apparatus  10 ) or back into the first humidifier  22 . Various humidity sensors (not shown) may be positioned along the cathode path to monitor the humidity of the cathode stream as it passes through the first humidifier  22  and onto the fuel cell stack  12 . 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.