Patent Publication Number: US-2021175524-A1

Title: Fuel cell with protection from pressure imbalance

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of International Application No. PCT/M2019/057018, filed Aug. 20, 2019, which claims priority to and the benefit of U.S. Provisional Patent Application No. 62/720,400, filed Aug. 21, 2018, and U.S. Provisional Patent Application No. 62/784,210, filed Dec. 21, 2018. The entire disclosures of International Application No. PCT/IB2019/057018, U.S. Provisional Patent Application No. 62/720,400, and U.S. Provisional Patent Application No. 62/784,210 are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present application relates generally to the field of fuel cell systems, and more specifically, to systems with protection from pressure imbalance between the anode and cathode of the fuel cell. 
     In conventional fuel cell systems, when an anode exhaust processing system is included, an anode blower is generally incorporated to provide pressure balance from the anode to the cathode of the fuel cell. Pressure balance means the anode pressure is nearly the same as the cathode pressure, to within a few inches water column difference. During upsets in the operation of the system, for example when the fuel cell power output rapidly increases or decreases, a pressure imbalance results from an instantaneous reduction or increase in the volumetric flow rate of the fuel cell anode exhaust. Moreover, the anode blower generally takes several seconds to reduce speed or increase speed to compensate for this increase or reduction in the anode exhaust flow rate. During this delay, the flow rate of anode exhaust supplied to the anode blower is either insufficient (in the power reduction case) or too great (in the power increase case), relative to the flow rate being drawn into the anode blower. The sudden difference in flow rates results in a decrease or increase in the anode pressure relative to pressure in the cathode (i.e., anode under-pressurization or anode over-pressurization). The anode under-pressurization or over-pressurization may be severe enough to cause damage to the fuel cell, typically by damaging the fuel cell manifold and/or the fuel cell manifold seals. 
     In some fuel cell manifold designs, anode under-pressurization greater than (i.e., more negative than) −7 inches of water-column pressure (iwc), measured as the difference in pressure between the anode and the cathode, is considered potentially damaging to the fuel cell. Under-pressurization greater than −10 iwc is considered likely to cause fuel cell damage. Anode under-pressurization greater than −15 iwc is considered very likely to cause fuel cell damage. Fuel cell damage may be limited to damage of the fuel cell manifolds and the manifold seals. In the case of anode under-pressurization, more severe damage may result from the manifold collapsing, causing mechanical damage to additional components of the fuel cell (e.g., the internal fuel delivery system) or may damage the cells in other ways (e.g., electrical short to the cells). Repairing damage due to under-pressurization may be very costly, sometimes exceeding the value of the fuel cell itself. 
     Similar to under-pressurization, anode over-pressurization may damage the fuel cell system. Over-pressurization may occur when the fuel cell power output suddenly increases, causing a corresponding increase in the anode exhaust volumetric flow rate that outpaces the compensating speed increase in the anode blower. Other causes of anode over-pressurization are blower failure or failure of a valve in the anode exhaust line while the fuel cell is in steady operation. Additionally, anode over-pressurization may occur while the fuel cell power plant is shut down and the fuel cell anode is blocked in for controlled purge through a vent line if there are erroneously high flow rates into the fuel cell anode (e.g., excessively high nitrogen purge). 
     Anode over-pressurization greater than (more positive than)+7 iwc is considered potentially damaging to the fuel cell manifold gasket. Over-pressurization greater than +10 iwc is considered likely to cause damage, and over-pressurization greater than +15 iwc is considered very likely to cause damage. Damage from over-pressurization typically includes blowing out the manifold gasket material. 
     It may be advantageous to limit or completely avoid anode under-pressurization in the fuel cell system with a recycle line around the anode blower and completely avoid anode over-pressurization by venting the anode exhaust through a back pressure control system, such as a level-adjusting water seal or a differential pressure relief valve. By preventing or limiting both anode under-pressurization and anode over-pressurization, fuel cell damage and subsequent costly repairs may be avoided. 
     SUMMARY 
     One embodiment relates to a fuel cell system, including a fuel cell. The fuel cell includes an anode having an anode inlet configured to receive anode feed gas, and an anode outlet configured to output anode exhaust. The fuel cell further includes a cathode having a cathode inlet and a cathode outlet. The fuel cell system further includes an anode blower configured to receive the anode exhaust and output a higher-pressure anode exhaust. The fuel cell system further includes a blower recycle line configured to receive a portion of the higher-pressure anode exhaust downstream from the anode blower and to output the portion of the higher-pressure anode exhaust upstream from the anode blower, preferably upstream of the anode exhaust processing equipment. The fuel cell system further includes a first valve disposed in the blower recycle line, the first valve configured to open when the anode of the fuel cell is under-pressurized, thereby protecting the fuel cell system against anode over- and under-pressurization. 
     One embodiment relates to a fuel cell system, including a fuel cell. The fuel cell includes an anode having an anode inlet configured to receive anode feed gas, and an anode outlet configured to output anode exhaust. The fuel cell further includes a cathode having a cathode inlet and a cathode outlet. The fuel cell system further includes an anode blower configured to receive the anode exhaust and output a higher-pressure anode exhaust. The fuel cell system further includes a blower recycle line configured to receive a portion of the higher-pressure anode exhaust downstream from the anode blower and to output the portion of the higher-pressure anode exhaust upstream from the anode blower, preferably upstream of the anode exhaust processing equipment. The fuel cell system further includes a control system and a first valve disposed in the blower recycle line, the first valve configured to open when the first valve receives a signal from the control system to avoid anode under-pressurization. 
     One embodiment relates to a fuel cell system, including a fuel cell. The fuel cell includes an anode having an anode inlet configured to receive anode feed gas, and an anode outlet configured to output anode exhaust. The fuel cell further includes a cathode having a cathode inlet and a cathode outlet. The fuel cell system further includes an anode blower configured to receive the anode exhaust and output a higher-pressure anode exhaust. The fuel cell system further includes a blower recycle line configured to receive a portion of the higher-pressure anode exhaust downstream from the anode blower and to output the portion of the higher-pressure anode exhaust upstream from the anode blower. The fuel cell system further includes a first valve disposed in the blower recycle line, the first valve configured to open when the anode of the fuel cell is under-pressurized, thereby protecting the fuel cell against anode under-pressurization. The fuel cell system may further include a control system configured to send a signal to the first valve, causing it to open when the fuel cell anode is under-pressurized. The system further includes a level-adjusting water seal which limits the fuel cell anode pressure by having its water level adjusted by the cathode inlet pressure, thereby protecting the fuel cell system from anode over-pressurization by venting excess anode exhaust gas to atmosphere while maintaining the desired anode pressure during upsets when the blower cannot maintain the desired anode-to-cathode pressure differential. 
     Another embodiment relates to a fuel cell system, including a fuel cell. The fuel cell includes an anode having an anode inlet configured to receive anode feed gas, and an anode outlet configured to output anode exhaust. The fuel cell further includes a cathode having a cathode inlet and a cathode outlet. The fuel cell system further includes an anode blower configured to receive the anode exhaust and output a higher-pressure anode exhaust. The fuel cell system further includes a blower recycle line configured to receive a portion of the higher-pressure anode exhaust downstream from the anode blower and to output the portion of the higher-pressure anode exhaust upstream from the anode blower. The fuel cell system further includes a first valve disposed in the blower recycle line, the first valve configured to open when the anode of the fuel cell is under-pressurized, thereby protecting the fuel cell against anode under-pressurization. The fuel cell system may further include a control system configured to send a signal to the first valve, causing it to open when the fuel cell anode is under-pressurized. The system further includes a differential pressure regulator to maintain the anode outlet pressure relative to the cathode inlet pressure by venting excess anode exhaust gas to atmosphere while maintaining the desired anode pressure during upsets when the blower cannot maintain the desired anode-to-cathode pressure differential, thereby protecting the fuel cell against anode over-pressurization. 
     Another embodiment relates to a method of controlling pressure in a fuel cell system, including receiving anode feed gas at an anode inlet, receiving cathode feed gas at a cathode inlet, and outputting anode exhaust from an anode outlet. The method further includes measuring a first pressure at one of the anode inlet or the anode outlet, measuring a second pressure at the cathode inlet, and determining a first pressure differential between the first pressure and the second pressure. The method further includes receiving the anode exhaust at a blower inlet and controlling a first pressure differential by means of a blower speed controller. The method further includes outputting the anode exhaust from a blower outlet at a higher pressure than at the anode blower inlet, and receiving at least a portion of the higher-pressure anode exhaust at a blower recycle line configured to output the higher-pressure anode exhaust upstream from the blower inlet. 
     Another aspect of the fuel cell system relates to an anode exhaust line configured to receive the anode exhaust from the anode, a vessel partially filled with water, a water seal downpipe extending away from the anode exhaust line and with at least a portion extending generally downward into the vessel, such that a water seal downpipe outlet is disposed in the water, and a vent defined in the vessel above a waterline, the vent configured to output anode exhaust. 
     Another aspect of the fuel cell system relates to a water level in the vessel being defined relative to a vertical position of the water seal downpipe outlet. The water level provides a water seal pressure at the water seal downpipe outlet, such that anode exhaust is output through the vent when the first pressure differential is greater than the water seal pressure. 
     Another aspect of the fuel cell system relates to an anode exhaust line configured to receive the anode exhaust from the anode, a pressure relief line extending from the anode exhaust line, and a pressure relief valve disposed on the pressure relief line and configured to vent anode exhaust. 
     Another aspect of the fuel cell system relates to a heat sink disposed about the pressure relief line upstream from the pressure relief valve, the heat sink configured to absorb heat from the anode exhaust. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a fuel cell system with the anode-to-cathode pressure differential controlled by the speed of an anode booster blower, without an anode under-pressure protection system or anode over-pressure protection system. 
         FIG. 2  is a schematic view of a fuel cell system like that shown in  FIG. 1 , with pressure protection systems added to avoid both anode under-pressurization and anode over-pressurization, according to the exemplary embodiments. 
         FIG. 3  is a schematic view of one exemplary embodiment of a back pressure control system, including a level-adjusting water seal apparatus for a fuel cell system, configured to receive anode exhaust gas and vent it to atmosphere during upsets in operation of a fuel cell system to avoid anode over-pressurization, according to an exemplary embodiment. 
         FIG. 4  is a schematic of another embodiment of a back pressure control system, including a differential pressure regulator valve apparatus configured to receive anode exhaust gas and vent it to atmosphere during upsets in operation of a fuel cell system to avoid anode over-pressurization, according to another exemplary embodiment. 
         FIG. 5  is a schematic of a variation of the differential pressure regulator valve apparatus shown in  FIG. 4 , configured to receive anode exhaust gas and vent it to atmosphere during upsets in operation of a fuel cell system to avoid anode over-pressurization, according to a further exemplary embodiment. 
         FIG. 6  is a schematic view of a fuel cell system with a pressure surge protection system, according to an exemplary embodiment. 
         FIG. 7  is a schematic view of a fuel cell system with a pressure surge protection system, according to another exemplary embodiment. 
         FIG. 8  is a plot showing a pressure surge in a fuel cell system without protection from a pressure surge protection system. 
         FIG. 9  is a plot showing pressure surge in a fuel cell system with protection from a pressure surge protection system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the FIGURES generally, a fuel cell system is shown according to various exemplary embodiments. The fuel cell system includes a fuel cell and an anode exhaust processing system for processing or converting anode exhaust output by the fuel cell for use in other parts of a fuel cell system or for export. The fuel cell system includes an anode blower configured to maintain the fuel cell anode pressure to be very similar to the fuel cell cathode pressure by means of an anode blower speed controller. The anode blower speed controller measures the difference between the anode pressure and the cathode pressure and adjusts the blower speed to maintain the desired pressure differential. A variety of events may cause the pressure differential to be out of specification despite this pressure control, such as a sudden decrease or increase in fuel cell power output, anode blower failure, or upset in the anode exhaust processing equipment, which may result in damage to the fuel cell. Accordingly, the fuel cell system may include fuel cell pressure protection systems configured to minimize or eliminate pressure imbalances between the fuel cell anode and the fuel cell cathode, as will be described in more detail below. 
     The fuel cell system includes an anode blower (e.g., booster blower, compressor, etc.), which is configured to provide pressure balance in the fuel cell between the anode and the cathode. Specifically, the anode blower offsets the added pressure drop of the anode exhaust processing system and/or pushes the processed anode exhaust to a higher-pressure region for further processing or use in a higher-pressure fuel cell. For example, in a high-efficiency fuel cell system (e.g., a fuel cell system with a Direct Fuel Cell (“DFC”)), the anode exhaust from a first (e.g., topping) fuel cell is cooled to condense and remove a significant amount of water from the anode exhaust stream. Then the processed anode exhaust is fed to an anode blower to force the stream into a second (e.g., bottoming) fuel cell, operating at a higher pressure than the first fuel cell. An anode blower may also be required in a fuel cell system for carbon capture (e.g., harvesting carbon dioxide from anode exhaust) or hydrogen capture (e.g., harvesting hydrogen from anode exhaust), to overcome the pressure drop from anode exhaust gas processing specific to the system, and to send the anode exhaust to higher-pressure sub-systems. 
     Referring now to  FIG. 1 , a fuel cell system  100  is shown in a baseline configuration, with no specific embodiments, with the pressure at the fuel cell anode inlet  108  or fuel cell anode exit  110  controlled to be similar to the pressure at the cathode inlet  112  by anode blower  124 . The fuel cell system  100  includes a fuel cell  102  having an anode  104  and a cathode  106 . The anode  104  includes an anode inlet  108 , configured to receive anode feed gas, and an anode outlet  110 , configured to output anode exhaust. An anode inlet pressure P 1  may be defined as the pressure of the anode feed gas at the anode inlet  108  (e.g., at an anode inlet manifold coupled to the anode inlet  108  for receiving the anode feed gas). An anode outlet pressure P 2  may be defined as the pressure of the anode exhaust at the anode outlet  110  (e.g., at an anode outlet manifold coupled to the anode outlet  110  for receiving the anode exhaust). 
     The cathode  106  includes a cathode inlet  112 , configured to receive cathode feed gas, and a cathode outlet  114 , configured to output cathode exhaust. A cathode inlet pressure P 3  may be defined as the pressure of the cathode feed gas at the cathode inlet  112  (e.g., at a cathode inlet manifold coupled to the cathode inlet  112  for receiving the cathode feed gas). The cathode inlet pressure P 3  may also be defined as the pressure of a vessel housing the fuel cell  102 , for example, when the fuel cell  102  is sealed within the vessel and cathode feed gas is supplied to the vessel for introduction to the cathode  106 . In this configuration, the vessel may operate as the cathode manifold. 
     A first pressure differential transmitter  116  measures a first pressure differential P 4  between the anode inlet pressure P 1  and the cathode inlet pressure P 3 . Similarly, a second pressure differential transmitter  118  measures a second pressure differential P 5  between the anode outlet pressure P 2  and the cathode inlet pressure P 3 . The first and/or second pressure differential transmitters  116 ,  118  communicate either wired or wirelessly with a control system  120  (e.g., a computer), which receives the measurements of the first and/or second pressure differentials P 4 , P 5 , and controls various aspects of the fuel cell system  100  to control the pressure differential between the anode  104  and the cathode  106 . Specifically, the baseline fuel cell system with no embodiments for pressure protection controls the anode pressure to be similar to the cathode pressure by modulation of the blower speed in response to the measured differential pressure at  116  or  118 . For example, if the anode pressure is low compared to the cathode pressure, then differential pressure P 4  or P 5  will be lower than desired, and the anode blower speed controller  130  will be commanded by control system  120  to reduce speed. Conversely, if the anode pressure is high compared to the cathode pressure then differential pressure P 4  or P 5  will be greater than desired and the anode blower speed controller  130  is commanded by control system  120  to increase speed. While the present application describes controlling the fuel cell system  100  based on the first pressure differential P 4 , it should be understood that the same structure and methods may be applied to control the second pressure differential P 5 . For example, various fuel cell systems may measure and control portions of the fuel cell system  100  based on one or both of the first pressure differential P 4  and/or the second pressure differential P 5 . 
     After anode exhaust is output from the anode  104 , it is fed through the anode outlet manifold  110  to an anode exhaust line  121  (e.g., conduit) to a processing system  122  configured to process the anode exhaust by reacting or isolating certain components (e.g., byproducts) in the anode exhaust. For example, the processing system  122  may react carbon monoxide and water vapor in the anode exhaust to form additional hydrogen, and the processing system  122  may isolate and separately output at least one of water, carbon dioxide, or hydrogen from the anode exhaust, while at the same time cooling the anode exhaust. The processing system  122  then outputs a processed anode exhaust (i.e., a processed stream). 
     The fuel cell system  100  further includes an anode blower  124  having a blower inlet  126  and a blower outlet  128 . The anode blower  124  is configured to receive the processed stream at the blower inlet  126 . The anode blower  124  compresses the processed stream, increasing the pressure of the processed stream output from the blower outlet  128  (i.e., higher-pressure anode exhaust). A blower inlet pressure P 6  may be defined as the pressure of the processed stream at the blower inlet  126  and a blower outlet pressure P 7  may be defined as the pressure of the processed stream at the blower outlet  128 . After passing the processed stream through the anode blower  124  while it is operating, the blower outlet pressure P 7  is higher than the blower inlet pressure P 6 . The processed stream is then output from the fuel cell system  100  for further processing, collection, or export from the fuel cell system. 
     A speed controller  130  controls the speed of the anode blower  124  and is connected to the control system  120 , such that the anode blower  124  may be controlled based on measurements taken at the first and/or second pressure differential transmitters  116 ,  118 . 
     Back Pressure Control System 
     Referring now to  FIG. 2 , according to another embodiment, the fuel cell system  200  includes at least one anode under-pressure protection system  234  and at least one anode over-pressure protection system (or back pressure control system)  236 . The anode under-pressure protection system  234  is configured to prevent excessive anode under-pressure surges in the fuel cell  202 , for example as may be caused by sudden decreases in the anode outlet volumetric flow rate that are not immediately met by changes in speed of the anode blower  224 . 
     Referring still to  FIG. 2 , the anode under-pressure protection system  234  includes a blower recycle line  246 . The blower recycle line  246  fluidly connects a portion of the fuel cell system  200  downstream from the blower outlet  228  to a portion of the fuel cell system  200  upstream from the blower inlet  226  (e.g., between the fuel cell anode outlet  210  and the blower inlet  226 ). The blower recycle line  246  is configured to pass at least a portion of the higher-pressure processed stream from the blower outlet  228  back to the lower-pressure stream at the blower inlet  226 , thereby increasing the pressure at the anode outlet manifold  210  to reduce or eliminate anode under-pressurization. While  FIG. 2  shows the blower recycle line  246  connects to a portion of the fuel cell system  200  between the anode outlet  210  and the processing system  222  in the fuel cell anode exhaust line  221 , according to other exemplary embodiments, the blower recycle line  246  may connect to a portion of the fuel cell system  200  downstream from the processing system  222  or any intermediate point within the processing system  222 . 
     The blower recycle line  246  includes a first valve  248  and a second valve  250  connected along the blower recycle line  246  in series. The first valve  248  is an automated valve connected to the control system  220 , and remains in a closed position until it receives a command to open. When the fuel cell system  200  has anode under-pressurization, at least one of the first or second pressure differential transmitters  216 ,  218  signal to the control system  220  that the fuel cell system  200  is in an anode under-pressurization condition. The control system  220  then sends a signal to the first valve  248  to open, at which point the first valve  248  moves from a closed position to an opened position. According to an exemplary embodiment, the first valve  248  may be configured to open within approximately 200 milliseconds from receiving the signal from the control system  220 . According to another exemplary embodiment, the first valve  248  may be configured to open within approximately 200 milliseconds from the first detection of an anode under-pressurization condition in the fuel cell system  200  has occurred. For example, the valve  248  may open upon sensing at least one of the first or second pressure differentials P 4 , P 5  is approximately −2 iwc. When the first valve  248  is opened, the higher-pressure anode exhaust passes from the blower outlet  228 , through the blower recycle line  246 , to the fuel cell anode exhaust line  221 . The first valve  248  may also be configured to open upon the fuel cell system  200  sensing a loss of electrical load in the fuel cell  202  or loss of power to the first valve  248 . The first valve  248  may also be configured to open upon a signal from the control system to command the fuel cell to drop all electrical load or a certain percentage of electrical load. 
     The second valve  250  is a pressure control valve, which is configured to restrict the flow of the processed stream passing through the blower recycle line  246  and received at the anode exhaust line  221  such that the impact of opening the first valve  248  on the anode under-pressurization can be controlled by presetting the opening of the second valve  250  according to the fuel cell system  200  operating condition (e.g., at a pre-determined opening). For example, when the first valve  248  opens, the higher-pressure processed stream is recycled through the blower recycle line  246  to a position upstream from the blower  224 , which may lead to over-pressurization (e.g., backpressure) on the fuel cell  202 . The second valve  250  controls the volume flow rate in the blower recycle line  246  in order to limit over-pressurization on the fuel cell  202  upon opening of the first valve  248 . According to an exemplary embodiment, the second valve  250  may be a pre-set manual hand valve, an orifice, an automated valve configured to change its position (e.g., percentage opened or closed) based on the power output of the fuel cell  202 , or other suitable valves. Although  FIG. 2  shows the second valve  250  disposed downstream from the first valve  248 , according to another exemplary embodiment, the second valve  250  may be disposed upstream from the first valve  248 . 
     A third pressure differential transmitter  232  measures a third pressure differential P 8  between the blower outlet pressure P 7  and the blower inlet pressure P 6 . Like the first and second pressure differential transmitters ( 216 ,  218 ) the third pressure differential transmitter  232  communicates either wired or wirelessly with the control system  220 . When the first valve  248  is opened and the higher-pressure processed stream output from the blower outlet  228  is recycled back to the lower-pressure anode exhaust line  221 , the anode blower  224  may no longer be able to effectively control the first and/or second pressure differentials P 4 , P 5  between the anode  204  and the cathode  206 . For this reason, in some configurations, when the first valve  248  is opened the blower speed may only be modulated down (i.e., decelerated) or stopped, and may not be modulated up (i.e., accelerated). This deceleration of the anode blower  224  may be measured by measuring a drop in the third pressure differential P 8 . The magnitude of P 8  may be used to determine when it is safe to close the first valve  248 , i.e., the magnitude of P 8  determines the impact of the closure of first valve  248 . 
     After opening the first valve  248  to avoid anode under-pressurization, the first valve  248  must be closed again so that the anode blower  224  may resume normal control of the first and/or second pressure differentials P 4 , P 5 . Re-closure of the first valve  248  may occur while the fuel cell  202  is still under load, or after the fuel cell  202  has shed all of its load and is in an idle state ready to resume loaded operation. The control system  220  may close the first valve  248  when various requirements are met. A first requirement may include keeping the first valve  248  open for a pre-determined minimum amount of time (e.g., approximately 2 seconds), long enough to ensure that the event that caused the first valve  248  to open has completed. A second requirement may include the third pressure differential P 8  being below a pre-determined threshold (e.g., approximately 15 iwc), such that the fuel cell anode  204  does not become under-pressurized upon re-closure of the first valve  248 . A third requirement may include the first pressure differential P 4  being above a pre-determined threshold (e.g., greater than −1 iwc, greater than 0 iwc, or greater than +1 iwc, etc.). After the requirements are met and the first valve  248  is closed, the control system  220  may modulate the anode blower speed, to either accelerate the anode blower  224  to reduce the first pressure differential P 4 , or decelerate the anode blower  224  to increase the first pressure differential P 4 . Therefore, while the first valve  248  is open, the blower speed may only be decreased by the speed controller, and while closed, the anode blower is under normal controls and the speed may either be increased or decreased. While the above discussion defines pre-determined thresholds relative to the first pressure differential P 4 , according to other exemplary embodiments, the pre-determined thresholds may be taken relative to the second pressure differential P 5 . 
     While  FIGS. 1 and 2  show the processing system ( 122 ,  222 ) disposed between the fuel cell ( 102 ,  202 ) and the anode blower ( 124 ,  224 ), according to other exemplary embodiments, the processing system ( 122 ,  222 ) may be disposed downstream from the anode blower ( 124 ,  224 ), such that the anode blower ( 124 ,  224 ) receives anode exhaust directly from the anode ( 104 ,  204 ) rather than processed anode exhaust. 
     Referring again to  FIG. 2 , the fuel cell system  200  includes, in addition to the anode under-pressure protection system  234 , at least one back pressure control system (i.e., anode over-pressure protection system)  236 . The back pressure control system  236  is connected to the cathode inlet at the cathode in pressure (P 3 ) through cathode gas line  309  and receives anode exhaust gas at anode out pressure (P 2 ) through the anode exhaust line  121 . The back pressure control system  236  is configured to prevent excessive anode over-pressure in the fuel cell  202 , for example as may be caused by sudden increases in the anode outlet volumetric flow rate that are not immediately met by changes in speed of the anode blower  224 . 
     Referring now to  FIG. 3 , an exemplary embodiment of a back pressure control system for a fuel cell system includes a level-adjusting water seal apparatus (i.e., water seal)  300 , which limits a first system pressure (e.g., a fuel cell anode outlet pressure, P andode ) and has its water level adjusted by a second system pressure (e.g., a fuel cell cathode inlet pressure, P cathode ) having a known relationship (P anode −P cathode =P diff ) with the first system pressure. P anode  and P cathode  may be equal to the anode out pressure P 2  and cathode in pressure P 3 , respectively. One advantage of such a configuration is that the water seal apparatus  300  may provide a fast-acting passive control of the water level in the water seal to provide over-pressure protection at all times and across all operating modes of the fuel cell system, including in normal operation and shut down modes, as well as during operational transients, such as large changes in load or rapid shut down, when the system pressures are also changing, sometimes rapidly. 
     Referring still to  FIG. 3 , the water seal  300  includes a water seal tank  302  containing water  304  and a water seal downpipe  306 . Water seal downpipe  306  has a water seal downpipe inlet  308 , connected to the anode exhaust line (e.g.,  221  of  FIG. 2 ) and therefore sensing a first system pressure (e.g., the fuel cell anode outlet pressure, P 2 ), and a water seal downpipe outlet  310  submerged under the water  304  in the water seal tank  302 . The water seal tank  302  vents to atmosphere through an atmospheric vent  312 . Therefore, the pressure over the water seal tank  302  is atmospheric pressure. 
     The water seal tank  302  is fluidly connected by a water line  320  to a reservoir tank  322 , which is connected to a second system pressure (e.g., the fuel cell cathode pressure P 3 ) by line  309 , which connects to the cathode inlet line  212  of  FIG. 2 . The level-adjusting water seal  300  operates with a fixed volume of water interspersed between the reservoir tank  322  and the water seal tank  302 , and the water is pushed back and forth to rapidly adjust the water level  314  in the water seal tank  302  to always maintain the proper over-pressure burst protection of the anode exhaust line  221 . As the second system pressure (e.g., P cathode ) increases or decreases, the water seal level  314  adjusts because the reservoir water  324  is forced to and from the water seal tank  302  from the reservoir tank  322  through the water line  320 . 
     The reservoir tank  322  is larger in diameter, and therefore has a greater cross-sectional area, than the water seal tank  302  (less the cross-sectional area of the water seal downpipe  306 ), such that changes in second system pressure primarily affect the water level in the water seal tank  302 . The ratio of the fluid cross-sectional areas is selected to predictably alter the water seal level  314  as the cathode pressure changes. As cathode pressure increases, the water seal level  314  increases as much as the reservoir tank level  326  decreases, multiplied by the ratio of the cross-sectional areas. For example, if the reservoir tank  322  cross-sectional area is 5 times that of water seal tank  302  (less water seal downpipe  306 ), then the water seal level  314  will increase 5 inches for every 1-inch decrease in reservoir tank level  326 . Because the water seal tank  302  is vented to atmosphere, the water height difference between one tank and the other is always equal to the cathode pressure, as sensed through line  309 , which is connected to the fuel cell cathode inlet line  212  ( FIG. 2 ). Although the ratio of cross-sectional areas need not be limited to any particular range, it is often advantageous to have the ratio of cross-sectional areas between the reservoir  322  and the water seal tank  302  (less water seal downpipe  306 ) equal to about 3 to 6, to control the impact of water level changes in the water seal  314  at various operating conditions. Table 1 shows how the water seal  300  maintains a protective level of water in the water seal tank  302  across the full range of fuel cell operations. In this respect, the water seal  300  acts as a differential pressure vent. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Water and Protection Levels in Level-Adjusting Water Seal When 
               
               
                 Reservoir Tank/WS Tank Cross-Sectional Area Ratio Equals 3.21 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Measured 
                   
                   
                   
               
               
                   
                 Cathode Inlet 
                 Anode Exhaust 
                 Pressue at FIG. 
               
               
                   
                 Pressure @ FIG. 
                 Manifold Pressure 
                 2 Line 121 
                 WS Tank Level 
                 Overburst 
                 Reservoir Tank 
               
               
                 Case 
                 2 line 112 (iwc) 
                 Set Point (iwc) 
                 (iwc) 
                 (iwc) 
                 Protect (iwc) 
                 Level (iwc) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Shut Down 
                 0 
                 0 
                 0 
                 6.5 
                 6.5 
                 6.5 
               
               
                 HSBY 
                 21.2 
                 20.7 
                 20.6 
                 22.7 
                 2.1 
                 1.5 
               
               
                 Set Case 
               
               
                 Half Load 
                 15.4 
                 14.9 
                 13.7 
                 18.3 
                 4.6 
                 2.9 
               
               
                 Full Load 
                 27.6 
                 27.1 
                 21.7 
                 27.6 
                 5.9 
                 0.0 
               
               
                   
               
            
           
         
       
     
     Referring still to  FIG. 3 , the water seal  300  may include additional optional features to maintain the volume of water within the water seal  300  within a desired range. The water seal  300  operates with a fixed volume of water which can be rapidly transferred between the reservoir tank  322  and the water seal tank  302 . However, water may accumulate in the water seal  300  due to connection to the anode gas through water seal downpipe  306 , which has high dew point, causing water to gradually condense in the water seal tank  302 . Accordingly, water may occasionally be removed from the water seal tank  302  by means of a level control sub-assembly  332 . Level control sub-assembly  332  may include a level transmitter  336  and level control pump  338  controlled by a level controller  333 . The level control pump  338  is fluidly connected to the water seal tank  302  through a water pipe  334 . Under all operating conditions, the level transmitter  336  receives a set point, based on the cathode inlet pressure P cathode  (P 3 ), turning on the level control pump  338  as necessary to prevent the buildup of excess water in the water seal tank  302 . The water level control sub-assembly  332  will thereby occasionally remove water that accumulates from condensation of humidity in the process gases, or otherwise reduce the water seal level  314  as necessary to maintain a desired operating condition. 
     As shown in  FIG. 3 , the water seal  300  may include additional optional features. For example, a disentrainment zone  328  may be interposed between the top  330  of the water seal tank  302  and the atmospheric vent  312 . The disentrainment zone  328  has a larger cross-sectional area than the water seal tank  302  to permit venting of high anode exhaust flow rates without entraining water. The disentrainment zone  328  thereby allows the water seal apparatus  300  to protect the fuel cell system over a wider range of operational upsets. 
     Referring now to  FIG. 4 , another exemplary embodiment of a back pressure control system for a fuel cell system includes a back pressure control valve or differential pressure regulator apparatus  400  that can perform the same function as the water seal  300  ( FIG. 3 ). Differential pressure regulator apparatus  400  includes a control valve  402  and an actuator  404  to maintain the anode-to-cathode differential pressure. In this exemplary embodiment, the differential pressure regulator apparatus  400  is not in series with the blower  224  ( FIG. 2 ). Instead, it vents to the atmosphere through a valve  402 . Because the flow rates change very quickly during upsets, the valve  402  must be fast-acting (e.g., preferably operating on the order of 200 milliseconds) to protect the fuel cell. During an emergency shutdown, which may result from a high anode-to-cathode pressure differential, the valve  402  opens, venting anode exhaust gas to the atmosphere through a vent line  410 . Because the cathode inlet pressure and the anode pressure are very similar during normal operation, there is potential for valve  402  to leak to the atmosphere, depending on the valve type. To prevent this potential leakage, during normal operation, a fast-acting valve  408  may be added to prevent anode exhaust gas from venting to the atmosphere. Valve  408  would open quickly during any shutdowns, including a shutdown caused by high anode pressure. 
     Referring still to  FIG. 4 , control valve  402  and actuator  404  maintain the anode outlet pressure approximately equal to the cathode inlet pressure during upsets. During normal operation, the cathode inlet  212  ( FIG. 2 ) is connected to the cathode sensing line  309 , which is connected to a first zone  405  of the valve actuator  404 . Meanwhile, anode outlet  210  ( FIG. 2 ) is connected to the anode sensing line  416 , which is connected to a second zone  406  of the actuator  404 . When P 3  (P cathode ) increases, it exerts pressure on a first side of the diaphragm (not shown) within the actuator  404 , thereby closing the valve  402  to reduce anode exhaust gas flow and increase P 2  (P anode ). When P 3  (P cathode ) decreases, the cathode inlet gas exerts less pressure on the first side of the diaphragm (not shown) within the actuator  404 , thereby partially or completely opening the control valve  402  to increase the anode exhaust gas flow and decrease P 2  (P anode ). Similarly, when P 2  (P anode ) increases, anode exhaust gas exerts greater pressure on the second side of the diaphragm within the actuator  404 , partially or completely opening the control valve  402  and reducing P 2  (P anode ). 
     As shown in  FIG. 4 , the differential pressure regulator apparatus  400  may include additional optional features. For example, a flow switch  418  may be included downstream from the control valve  402  to notify the operator if the valve  408  fails. The flow switch  418  is preferably located at the high point in the line, close to the anode exhaust line  221 . Preferably, the flow switch  418  is downstream from the control valve  402  to allow easier maintenance on the valve  402 . 
     Referring now to  FIG. 5 , because of the low differential pressure desired, there may be the possibility of a leak through the valve  502  during normal operation, especially when a passive control system is used as shown in  FIGS. 4 and 5 . In this case, a fast-acting valve  508  may optionally be included in the anode pressure line  516  to the back pressure regulator valve  502  with a small vent line or pressure equalization line  520  and open immediately whenever the unit shuts down. This maintains a high pressure differential on the regulator valve  502  during normal operation to prevent leakage. 
     Referring still to  FIG. 5 , another exemplary embodiment of the back pressure control system is differential pressure regulator apparatus  500 , where the differential pressure regulator apparatus  500  is not in series with a blower  224  ( FIG. 2 ). In this exemplary embodiment, the valve  508  may be located on the anode sensing line  516  between the anode outlet  210  ( FIG. 2 ) and the second zone  506  of the valve actuator  504 . A small pressure equalization line  520  from the second zone  506  of the valve actuator  504  is routed downstream of the valve  502 . Additionally, the anode sensing line  516  should be sufficiently large relative to the pressure equalization line  520  that anode exhaust gas flow through the pressure equalization line  520  is insufficient to affect the pressure on the actuator  504 . 
     During normal operation, the valve  508  is closed so that the pressure in the second zone  506  of the regulator actuator  504  is zero and the valve  502  is closed due to the pressure of the cathode inlet gas in the first zone  505  of the actuator  504 . During an emergency shutdown, the valve  508  opens to the anode sensing line  516  so the pressure exerted within the second zone  506  of the actuator  504  is equal to the anode out pressure, P 2 . The anode-to-cathode pressure differential is thereby maintained at the desired value. In the event of anode over-pressurization, the anode exhaust gas is vented to the atmosphere through vent line  510 . 
     Referring still to  FIG. 5 , when valve  508  is open (e.g., during an upset), control valve  502  and actuator  504  maintain the anode outlet pressure approximately equal to the cathode inlet pressure. The cathode inlet  212  ( FIG. 2 ) is connected to the cathode sensing line  309 , which is connected to a first zone  505  of the valve actuator  504 . Meanwhile, anode outlet  210  ( FIG. 2 ) is connected to the anode sensing line  516 , which is connected to a second zone  506  of the actuator  504 . When P 3  (P cathode ) increases, it exerts pressure on a first side of the diaphragm (not shown) within the actuator  504 , thereby closing the valve  502  to reduce anode exhaust gas flow and increase P 2  (P anode ). When P 3  (P cathode ) decreases, the cathode inlet gas exerts less pressure on the first side of the diaphragm (not shown) within the actuator  504 , thereby partially or completely opening the control valve  502  to increase the anode exhaust gas flow and decrease P 2  (P anode ). Similarly, when P 2  (P anode ) increases, anode exhaust gas exerts greater pressure on the second side of the diaphragm within the actuator  504 , partially or completely opening the control valve  502  and reducing P 2  (P anode ). 
     Referring still to  FIG. 5 , additional optional features may be included in the differential pressure regulator apparatus  500 . A small solenoid valve (not shown) may be placed in the pressure equalization line  520  and configured to automatically close during an ESD. A flow switch  518  may also be included downstream from the control valve  502  to notify the operator if the valve  508  fails. The flow switch  518  is preferably located at the high point in the line, close to the anode exhaust line  221 . Preferably, the flow switch  518  is downstream from the control valve  502  to allow easier maintenance on the valve  502 . 
     Although  FIGS. 4 and 5  show passive back pressure regulator valves  402 ,  502  where the valve actuator  404 ,  504  is controlled directly by pressure connections to the cathode inlet and anode outlet, in other embodiments (not shown), the pressure controls may pass through an electronic control system, provided that the electronic control system does not delay the fast action of the valve  402 ,  502  required during a shutdown. 
     Pressure Surge Protection System 
     Referring now to  FIG. 6 , in another embodiment, the fuel cell system  600  includes a pressure surge protection system  634 . The pressure surge protection system  634  is configured to prevent excessive pressure surges in the fuel cell  602  caused by sudden changes in the anode outlet volume flow rate that are not immediately met by changes in speed of the anode blower  624 . The pressure surge protection system  634  includes a water seal  636  having a vessel  638  (e.g., reservoir) partially filled with water or other suitable liquid. A water seal downpipe  640  (e.g., pipe, line, conduit, etc.) has a starting point that is fluidly connected to the anode exhaust line  621  and defines a water seal downpipe outlet  642  at an opposing end from the anode exhaust line  621 . The water seal downpipe  640  extends generally upward from the anode exhaust line  621  and then generally downward into the vessel  638 , such that a water seal downpipe outlet  642  is disposed in the water, forming an air lock with a water seal. In this configuration, anode exhaust may vent to the atmosphere or other location through a vent  644  from the anode exhaust line  621 , through the water in the vessel  638 . The vent  644  may be defined in an upper portion of the vessel  638  and is configured to output anode exhaust from the fuel cell system  600 . For example, the vent  638  may output the anode exhaust to the atmosphere or may be fluidly connected to another system configured to capture and store the vented anode exhaust. The vent  644  may also be configured to allow air above the water to maintain an atmospheric pressure, such that the vessel  638  does not increase in pressure as anode exhaust is received therein. 
     The vessel  638  may be filled with water to a desired water level, which is measured vertically from the water seal downpipe outlet  642 . Water may be added to or drained from the vessel  638  to control a water seal pressure P 9  measured at the water seal downpipe outlet  642 . The water seal pressure P 9  varies directly with the water level in the vessel  638 . For example, when the water level is 2 inches above the water seal downpipe outlet  642 , the water seal pressure P 9  is 2 iwc. Anode exhaust will vent from the anode exhaust line  621  when the anode outlet pressure P 2  exceeds the water seal pressure P 9 . In this configuration, the water seal  636  allows for venting anode exhaust in the over-pressurization condition, but prevents mixture of outside gas with the anode exhaust in the anode exhaust line  621 . According to an exemplary embodiment, the water level may be set such that over-pressurization is limited to 6 iwc in the anode  604 . According to other exemplary embodiments, the water level may be set such that over-pressurization is limited to 10 or 15 iwc. 
     Referring still to  FIG. 6 , the pressure surge protection system  634  includes a blower recycle line  646 . The blower recycle line  646  fluidly connects a portion of the fuel cell system  600  downstream from the blower outlet  628  to a portion of the fuel cell system  600  upstream from the blower inlet  626  (e.g., between the processing system  622  and the blower inlet  626 ). The blower recycle line  646  is configured to pass at least a portion of the higher-pressure processed stream from the blower outlet  628  back to the lower-pressure stream at the blower inlet  626 , thereby increasing the pressure at the anode outlet manifold  610  to reduce or eliminate anode under-pressurization. 
     The blower recycle line  646  includes a first valve  648  and a second valve  650  connected along the blower recycle line  646  in series. The first valve  648  is an automated valve connected to the control system  620 , and remains in a closed position until it receives a command to open. When the fuel cell system  600  has anode under-pressurization, at least one of the first or second pressure differential transmitters  616 ,  618  signal to the control system  620  that the fuel cell system  600  is in an anode under-pressurization condition. The control system  620  then sends a signal to the first valve  648  to open at which point the first valve  648  moves from a closed position to an opened position. According to an exemplary embodiment, the first valve  648  may be configured to open within approximately 200 milliseconds from receiving the signal from the control system  620 . According to another exemplary embodiment, the first valve  648  may be configured to open within approximately 200 milliseconds from the first detection of an anode under-pressurization condition in the fuel cell system  600  has occurred. For example, the valve  648  may open upon sensing at least one of the first or second pressure differentials P 4 , P 5  is approximately −2 iwc. When the first valve  648  is opened, the higher-pressure anode exhaust passes from the blower outlet  628 , through the blower recycle line  646 , to the blower inlet  626 . The first valve  648  may also be configured to open upon the fuel cell system  600  sensing a loss of electrical load in the fuel cell  602  or loss of power to the first valve  648 . The first valve  648  may also be configured to open upon a signal from the control system to command the fuel cell to drop all electrical load or a certain percentage of electrical load. 
     The second valve  650  is a pressure control valve, which is configured to restrict the flow of the processed stream passing through the blower recycle line  646  and received at the blower inlet  626  such that the impact of opening the first valve  648  on the anode under-pressurization can be controlled by presetting the opening of the second valve  650  (e.g., at a pre-determined pressure). For example, when the first valve  648  opens, the higher-pressure processed stream is recycled through the blower recycle line  646  to a position upstream from the blower  624 , which may lead to over-pressurization (e.g., backpressure) on the fuel cell  602  and therefore excessive venting of anode exhaust gas through the water seal vent  644 . The second valve  650  controls the volume flow rate in the blower recycle line  646  in order to limit over-pressurization on the fuel cell  602  upon opening of the first valve  648 . It should be noted that even if the second valve  650  is in a fully opened position and minor over-pressurization of the fuel cell  602  occurs, the presence of the water seal  636  limits the amount of over-pressurization on the fuel cell  602  to below a threshold that may cause damage. According to an exemplary embodiment, the second valve  650  may be a pre-set manual hand valve, an orifice, an automated valve configured to change its position (e.g., percentage opened or closed) based on the power output of the fuel cell  602 , or other suitable valves. While  FIG. 6  shows the second valve  650  disposed downstream from the first valve  648 , according to another exemplary embodiment, the second valve  650  may be disposed upstream from the first valve  648 . 
     When the first valve  648  is opened and the higher-pressure processed stream output from the blower outlet  628  is recycled back to the lower-pressure blower inlet  626 , the anode blower  624  may no longer be able to effectively control the first and/or second pressure differentials P 4 , P 5  between the anode  604  and the cathode  606 . For this reason when the first valve  648  is opened, the anode blower  624  is then signaled by the speed controller  630  to modulate speed to control the first and/or second pressure differentials P 4 , P 5 . In some configurations, when the first valve  648  is opened, the blower speed may only be modulated down (i.e., decelerated) or stopped, and may not be modulated up (i.e., accelerated). This deceleration of the anode blower  624  may be measured by measuring a drop in the third pressure differential P 8 . In this configuration, according to some embodiments, as the anode blower  624  slows down, the load (e.g., pressure) on the fuel cell  602  is reduced until there is no longer a load present and/or the anode blower  624  is stationary. In this condition, the fuel cell  602  is in a “hot standby” condition, such that it is configured to generate electricity as soon as the anode blower  624  begins to accelerate. 
     After opening the first valve  648  to avoid anode under-pressurization, the first valve  648  must be closed again so that the anode blower  624  may resume normal control of the first and/or second pressure differentials P 4 , P 5 . Re-closure of the first valve  648  may occur while the fuel cell  602  is still under load, or after the fuel cell  602  has shed all of its load and is in an idle state ready to resume loaded operation. The control system  620  may close the first valve  648  when various requirements are met. A first requirement may include keeping the first valve  648  open for a pre-determined minimum amount of time (e.g., approximately 2 seconds), long enough to ensure that the event that caused the first valve  648  to open has completed. A second requirement may include the third pressure differential P 8  being below a pre-determined threshold (e.g., approximately 15 iwc), such that the fuel cell anode  604  does not become under-pressurized upon re-closure of the first valve  648 . A third requirement may include the first pressure differential P 4  being above a pre-determined threshold (e.g., greater than −1 iwc, greater than 0 iwc, or greater than +1 iwc, etc.). After the requirements are met and the first valve  648  is closed, the control system  620  may modulate the anode blower speed, to either accelerate the anode blower  624  to reduce the first pressure differential P 4 , or decelerate the anode blower  624  to increase the first pressure differential P 4 . Therefore, while the first valve  648  is open, the blower speed may only be decreased by the speed controller, and while closed the anode blower is under normal controls and the speed may either be increased or decreased. While the above discussion defines pre-determined thresholds relative to the first pressure differential P 4 , according to other exemplary embodiments, the pre-determined thresholds may be taken relative to the second pressure differential P 5 . For example, where the fuel cell  602  has a low pressure drop configuration, the first pressure differential P 4  may be substantially similar to the second pressure differential P 5 . 
     Referring now to  FIG. 7 , the fuel cell system  700  is shown with a pressure surge protection system  734  according to another exemplary embodiment. In place of or in addition to the water seal (e.g.,  636 ,  FIG. 6 ), the pressure surge protection system  734  includes a pressure relief valve  752  extending from the anode exhaust line  721  through a pressure relief line  754 . The pressure relief valve  752  is generally in a closed position and is set to open at a pre-determined pressure, such that when the anode exhaust reaches the pre-determined pressure, the anode exhaust is momentarily output from the fuel cell system  700  substantially the same way as in the water seal  636  ( FIG. 6 ). The pre-determined pressure may be selected to limit damage to the fuel cell  702  due to anode over-pressurization of the fuel cell  702 . 
     Because the pressure relief valve  752  may be sensitive to short durations of high-temperature anode exhaust passing therethrough, heat in the pressure relief valve  752  may be controlled with a heat sink  756  coupled to the pressure relief line  754  upstream from the pressure relief valve  752 . The heat sink  756  includes a column packed with metal or other heat absorbing material disposed on the pressure relief line  754 , such that heat may be transferred from the anode exhaust to the heat sink  756 . The heat sink  756  may be configured (e.g., sized) to absorb enough heat from the anode exhaust, such that the anode exhaust may pass through the pressure relief valve  752  for several seconds (e.g., until over-pressurization of the fuel cell  702  is resolved), without damaging the pressure relief valve  752 . The heat sink  756  then transfers heat to the environment until it reaches equilibrium (e.g., ambient temperature). 
     A temperature sensor  758  may be disposed on the pressure relief line  754  between the heat sink  756  and the pressure relief valve  752 . The temperature sensor  758  measures the temperature of the anode exhaust gas received at the pressure relief valve  752  and transmits the temperature to the control system  720 . If the temperature measured at the temperature sensor  758  is too high (e.g., above a threshold temperature) for a prolonged period of time, such that the anode exhaust is likely to damage the pressure relief valve  752 , the control system  720  may shut down the fuel cell  702  in order to reduce or stop the flow of anode exhaust through the pressure relief valve  752 . 
     While  FIGS. 6 and 7  show the blower recycle line ( 646 ,  746 ) connects to a portion of the fuel cell system ( 600 ,  700 ) between the processing system ( 622 ,  722 ) and the blower inlet ( 626 ,  726 ), according to other exemplary embodiments, the blower recycle line ( 646 ,  746 ) may connect to a portion of the fuel cell system ( 600 ,  700 ) upstream from the processing system ( 622 ,  722 ) or any intermediate point within the processing system ( 622 ,  722 ). 
     While  FIGS. 6 and 7  show the processing system ( 622 ,  722 ) disposed between the fuel cell ( 602 ,  702 ) and the anode blower ( 624 ,  724 ), according to other exemplary embodiments, the processing system ( 622 ,  722 ) may be disposed downstream from the anode blower ( 624 ,  724 ), such that the anode blower ( 624 ,  724 ) receives anode exhaust directly from the anode ( 604 ,  704 ), rather than processed anode exhaust. According to other exemplary embodiments, the water seal downpipe  640  ( FIG. 6 ) and/or the pressure relief line  754  ( FIG. 7 ) may be disposed either upstream or downstream from the processing system ( 622 ,  722 ), provided that the water seal downpipe  640  and/or the pressure relief line  754  is upstream from the anode blower ( 624 ,  724 ). In this configuration the fuel cell ( 602 ,  702 ) is protected from anode over-pressurization. 
     Referring now to  FIG. 8 , a plot of test results of the first pressure differential P 4  and percentage of fuel cell power is shown for a fuel cell system without the pressure surge protection system  634 . As provided in  FIG. 8 , when the fuel cell  602  suddenly drops from approximately 72% of full load capacity to 0% (e.g., the fuel cell instantaneously sheds its load), the first pressure differential P 4  drops by approximately 17 iwc within 3 seconds, and stays below −15 iwc for approximately 6 seconds before beginning to recover. It should be noted that  FIG. 8  shows a sustained first pressure differential P 4  of approximately −16.3 iwc. However, during testing the first pressure differential transmitter  616  could only measure as low as −16.3 iwc.  FIG. 8  further shows an estimated curve with peak under-pressurization of approximately −25 iwc, 5 seconds after fuel cell shutdown, based on other operating parameters measured during the test. Based on the results from this test, a sudden reduction from 100% of full load capacity to 0% is expected to change the first pressure differential P 4  by approximately −35 iwc, which would very likely cause damage to the fuel cell  602 . 
     Referring now to  FIG. 9 , a plot of test results of the first pressure differential P 4  and percentage of fuel cell power is shown for a fuel cell system with the pressure surge protection system  634  having a configuration as shown in  FIG. 6 . As provided in  FIG. 9 , when the fuel cell  602  suddenly drops from 100% of full load capacity to 0%, the first pressure differential P 4  drops to −5 iwc by 2 seconds after the drop in the load. As the first pressure differential P 4  drops, the controller  620  senses the change in pressure differential and the first valve  648  is opened within approximately 200 milliseconds. The first pressure differential P 4  then suddenly rises again. After approximately 8 seconds following the drop in the load on the fuel cell  602 , the water seal  636  limits the increase of the first pressure differential P 4  to approximately 5 iwc. (While this test was performed with a water seal  636 , similar results are expected with the pressure relief valve  752  (e.g., as configured as shown in  FIG. 7 ).)  FIG. 9  then shows the first pressure differential P 4  dropping to approximately −8 iwc before gradually returning back to approximately 1 iwc, as the first valve  648  is closed and the anode blower  624  resumes control of the first and/or second pressure differential P 4 , P 5  as controlled by the speed controller  630 . Unlike the system shown in  FIG. 8 , while  FIG. 9  still has pressure surges, the first pressure differential P 4  is substantially limited in magnitude to within a range (e.g., between approximately −8 and +5 iwc). This range is within the range unlikely to cause damage to the fuel cell  602 , whereas the fuel cell system without the pressure surge protection system  634  does not fall within such range. It should be recognized that  FIG. 9  shows a complete loss of load from the fuel cell  602 , which represents the most severe pressure surge condition. Where the fuel cell  602  has a sudden reduction in load, but not all the way to 0%, the magnitude of the pressure surge is expected to be less than shown in  FIG. 9 . 
     As utilized herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of this disclosure as recited in the appended claims. 
     It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples). 
     The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. 
     References herein to the position of elements (e.g., “top,” “bottom,” “above,” “below,” etc.) are merely used to describe the orientation of various elements in the FIGURES. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure. 
     It is to be understood that although the present invention has been described with regard to preferred embodiments thereof, various other embodiments and variants may occur to those skilled in the art, which are within the scope and spirit of the invention, and such other embodiments and variants are intended to be covered by corresponding claims. Those skilled in the art will readily appreciate that many modifications are possible (e.g., variations in dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. For example, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present disclosure.