Fuel cell system with load management

A fuel cell system having partial and/or total redundancy of at least one operational component, such as a redundancy of fuel cell stacks and/or fuel processors. In some embodiments, the fuel cell system includes a plurality of fuel cell stacks adapted to deliver the same maximum rated power output as a comparative fuel cell system having only a single fuel cell stack. In some embodiments, the fuel cell system includes a plurality of fuel cell stacks adapted to deliver more than the maximum rated power output of the comparative fuel cell system. In some embodiments, the fuel cell system includes a plurality of fuel cell stacks having at least n+1 (or total) redundancy compared to a fuel cell system having only a single fuel cell stack. In some embodiments, the fuel cell system includes a control system and/or structure adapted to limit the applied load to the system.

FIELD OF THE INVENTION

The present invention relates generally to energy-production systems, and more particularly to fuel cell systems that include a plurality of fuel cell stacks.

BACKGROUND OF THE INVENTION

Fuel cell systems include a fuel processor and a fuel cell stack. The fuel cell stack produces an electric current from the product stream of the fuel processor. For example, the fuel processor may produce hydrogen gas or hydrogen-rich gas from common feed stocks, such as water, a carbon-containing feedstock, or both. The fuel cell stack produces an electric current from the hydrogen gas.

An example of a conventional fuel cell system is shown inFIG. 1and indicated generally at10. System10includes a fuel processing assembly11and a fuel cell stack22. Fuel processing assembly11includes a suitable fuel processor12and a feed stream delivery system17, which delivers a feed stream16to the fuel processor. Fuel processor12is adapted to produce a product hydrogen stream14containing hydrogen gas from feed stream16, which contains the feedstock for the fuel processor.

The composition and number of individual streams forming feed stream16will tend to vary depending on the mechanism by which fuel processor12is adapted to produce product hydrogen stream14. For example, if fuel processor12produces stream14by steam or autothermal reforming, feed stream16contains a carbon-containing feedstock18and water20. If fuel processor12produces stream14by pyrolysis or catalytic partial oxidation of a carbon-containing feedstock, feed stream16contains a carbon-containing feedstock and does not include water. If fuel processor12produces stream14by electrolysis, feed stream16contains water and does not contain a carbon-containing feedstock. Examples of carbon-containing feedstocks include alcohols and hydrocarbons. When the feed stream contains water and a carbon-containing feedstock that is soluble with water, the feed stream may be a single stream, such as shown inFIG. 1. When the carbon-containing feedstock is not miscible in water, the water and carbon-containing feedstock are delivered in separate feed streams, such as shown inFIG. 2.

Fuel cell stack22is adapted to produce an electric current from the portion of product hydrogen stream14delivered thereto. Fuel cell stack22includes a plurality of fuel cells24integrated together between common end plates23, which contain fluid delivery/removal conduits (not shown). Examples of conventional fuel cells include proton exchange membrane (PEM) fuel cells and alkaline fuel cells. Fuel cell stack22may receive all of product hydrogen stream14. Some or all of stream14may additionally, or alternatively, be delivered, via a suitable conduit, for use in another hydrogen-consuming process, burned for fuel or heat, or stored for later use.

Fuel cell stack22receives at least a substantial portion of product hydrogen stream14and produces an electric current26therefrom. This current can be used to provide electrical power to an associated energy-consuming device28, such as a vehicle or a house or other residential or commercial dwelling.

InFIG. 3, an illustrative example of a fuel cell stack is shown. Stack22(and the individual fuel cells24contained therein) includes an anode region32and a cathode region34, which are separated by an electrolytic membrane or barrier36through which hydrogen ions may pass. The regions respectively include anode and cathode electrodes38and40. The anode region32of the fuel cell stack receives at least a portion of product hydrogen stream14. Anode region32is periodically purged, and releases a purge stream48, which may contain hydrogen gas. Alternatively, hydrogen gas may be continuously vented from the anode region of the fuel cell stack and re-circulated. The purge streams may be vented to the atmosphere, combusted, used for heating, fuel or as a feedstock for the fuel processing assembly. The purge streams from the fuel cell stacks may be integrated into a suitable collection assembly through which the combined purge stream may be used for fuel, feedstock, heating, or otherwise harvested, utilized or stored.

Cathode region34receives an air stream42, and releases a cathode air exhaust stream44that is partially or substantially depleted in oxygen. Air stream42may be delivered by an air delivery system46, which is schematically illustrated inFIG. 3and may take any suitable form, such as a fan, blower or the like. Electrons liberated from the hydrogen gas cannot pass through barrier36, and instead must pass through an external circuit49, thereby producing electric current26that may be used to meet the load applied by device28. Current26may also be used to power the operation of the fuel cell system. The power requirements of the fuel cell system are collectively referred to as the balance of plant requirements of the fuel cell system.

Because fuel cell system10relies upon a single fuel cell stack and a single fuel processor, it suffers from some limitations due to its reliance on those components. For example, if stack22requires maintenance, is damaged or otherwise needs to be removed from service, system10is unable to provide power to device28, other than previously stored power, if any. Similarly, if fuel processor12requires maintenance, is damaged or otherwise needs to be removed from service, system10is unable to provide feedstock, such as product hydrogen stream14, to fuel cell stack22, other than previously stored feedstock, if any.

SUMMARY OF THE INVENTION

The present invention is directed to a fuel cell system having a redundancy of at least one operational component, such as a redundancy of fuel cell stacks and/or a redundancy of fuel processors. In some embodiments, the fuel cell system may include a plurality of fuel cell stacks adapted to provide partial and/or total redundancy. In some embodiments, the fuel cell system includes a plurality of fuel cell stacks adapted to deliver the same maximum rated power output of a comparative fuel cell system having only a single fuel cell stack, thereby providing partial redundancy. In some embodiments, the fuel cell system includes a plurality of fuel cell stacks adapted to deliver more than the maximum rated power output of a comparative fuel cell system having only a single fuel cell stack. In some embodiments, the fuel cell system includes a plurality of fuel cell stacks having at least n+1 (or total) redundancy compared to a fuel cell system having only a single fuel cell stack. In some embodiments, the fuel cell system includes a control system. In some embodiments, the fuel cell system may include a plurality of fuel processors to provide partial or total redundancy.

DETAILED DESCRIPTION AND BEST MODE OF THE INVENTION

A fuel cell system constructed according to the present invention is shown inFIG. 4and generally indicated at60. System60includes a fuel processing assembly62, which includes a fuel processor64that is adapted to produce a product hydrogen stream66from a feedstock delivered via feed stream68. It should also be understood that the components of system60have been schematically illustrated and that the fuel cell system may include additional components other than those specifically illustrated in the figures, such as feed pumps, air delivery systems, heat exchangers, heating assemblies and the like, such as disclosed in the incorporated references.

Fuel processor64may produce product hydrogen stream66via any suitable mechanism. Examples of suitable mechanisms include steam reforming and autothermal reforming, in which reforming catalysts are used to produce hydrogen gas from a feed stream containing a carbon-containing feedstock and water. Other suitable mechanisms for producing hydrogen gas include pyrolysis and catalytic partial oxidation of a carbon-containing feedstock, in which case the feed stream does not contain water. Still another suitable mechanism for producing hydrogen gas is electrolysis, in which case the feedstock is water. InFIG. 4, feed stream68is shown being delivered as two separate streams from respective feed stock delivery systems70. It is within the scope of the invention that the feed stream may be a single stream or may be more than two streams. Similarly, the feed stock delivery system may take any suitable form, such as a pump connected to a supply of feedstock, a valve assembly associated with a pressurized stream of feedstock, etc.

For purposes of illustration, the following discussion will describe fuel processor64as a steam reformer adapted to receive a feed stream68containing a carbon-containing feedstock72and water74. However, it is within the scope of the invention that the fuel processor64may take other forms, as discussed above. Examples of suitable carbon-containing feedstocks72include at least one hydrocarbon or alcohol. Examples of suitable hydrocarbons include methane, propane, natural gas, diesel, kerosene, gasoline and the like. Examples of suitable alcohols include methanol, ethanol, and polyols, such as ethylene glycol and propylene glycol. When the carbon-containing feedstock is soluble in water, the carbon-containing feedstock and water may be, but are not necessarily, delivered in a single feed stream68, such as shown inFIG. 5. When the carbon-containing feedstock is not soluble in water, separate feed streams68are used, such as shown inFIG. 4.

Examples of suitable steam reformers are disclosed in U.S. patent application Ser. No. 09/291,447, which is entitled “Fuel Processing System,” was filed on Apr. 13, 1999, and the disclosure of which is hereby incorporated by reference. Examples of other components of fuel processing assembly62are also disclosed in U.S. patent application Ser. No. 09/190,917, which is entitled “Integrated Fuel Cell System,” was filed on Nov. 12, 1998, and the disclosure of which is hereby incorporated by reference.

As shown inFIG. 4, fuel processing assembly62is adapted to deliver at least a portion of product hydrogen stream66to a plurality of fuel cell stacks76. Collectively, the plurality of fuel cell stacks76may be referred to as a stack assembly77. Stacks76produce an electric current78from the portion of product hydrogen stream66delivered thereto, and this current may be used to satisfy the energy demands, or applied load, of an energy-consuming device80. Illustrative examples of devices80include, but should not be limited to, a motor vehicle, recreational vehicle, boat, tools, lights or lighting assemblies, appliances (such as household or other appliances), one or more residential dwellings, such as a household, apartment, townhouse, condominium, etc.), commercial buildings, microwave relay stations, signaling or communication equipment, etc. It should be understood that device80is schematically illustrated inFIG. 4and is meant to represent one or more devices or collection of devices that are adapted to draw electric current from the fuel cell system. To further illustrate this point, device80is shown inFIG. 5as including a pair of devices801and802. Each device80has a plurality of operational states that includes at least a first operational state, in which the device is applying at least a portion of the applied load on fuel cell stack assembly77, and a second operational state, in which the device is not applying a load on stack assembly77.

In the illustrative example shown inFIG. 4, four stacks are shown and are generally indicated at761-764. It is important to distinguish that system60includes a plurality of fuel cell stacks76, and not merely a single fuel cell stack containing a plurality of fuel cells. As shown, each fuel cell stack contains one or more fuel cells82(and typically contains a plurality of fuel cells) connected between common end plates84and having common fluid conduits. Examples of suitable fuel cells are proton-exchange-membrane (PEM) fuel cells and alkaline fuel cells, however, any other suitable fuel cell may be used. Similarly, the stacks and cells may be similar in construction to the stack shown inFIG. 3. The fuel cells82in each stack function as a unit to produce electric power from the feedstock delivered to the stack, such as from the portion of product hydrogen stream66that is delivered to the stack. Each stack has a plurality of operational states that include at least a first operational state, in which the stack is receiving at least a portion of product hydrogen stream66and producing an electric current therefrom, and a second operational state, in which the fuel cell stack is not producing an electric current (and typically not receiving a portion of stream66).

Unlike a single fuel cell stack, each stack in assembly77may operate independent of the other stacks. By this it is meant that if one of the stacks fails or is otherwise removed from operation, such as for maintenance or repair, the other stacks may continue to operate and thereby produce current78to satisfy at least a portion of the applied load from device80. Although the total rated power output of the stack assembly will not be available when at least one of the stacks is off-line or otherwise not producing an electric current, the stack assembly will still be able to produce a portion of its rated power output as long as at least one of its stacks is operating. In other words, stack assembly77provides an alternative to having either a single functioning stack, in which the maximum rated power output is available to supply the applied load of device80, and no functioning stack, in which no power output is available, other than from previously stored power, if any.

To further expand upon the utility of stack assembly77, it may be helpful to define some terms that are used herein and to provide some illustrative applications of stack assembly77and system60. As used herein, “maximum rated power output” refers to the power output that a fuel cell stack76is designed, or configured, to produce. For example, manufacturers of fuel cell stacks, such as Energy Partners, Plug Power, Nuvera, H-Power, Ballard Power, International Fuel Cells, Teledyne and others, rate their stacks with a maximum power output within which the stacks are designed to safely operate. Similarly, the term “total rated power output” refers to the combined maximum rated power output of a plurality of fuel cell stacks in stack assembly77. As used herein, “maximum desired power output” refers to the maximum power output a fuel cell stack or stack assembly needs to provide to satisfy the applied load from device80. As used herein, the term “intermediate power output” and “intermediate rated power output” refer to the output of a fuel cell stack assembly that is less than its total rated power output. For example, if a stack assembly includes three stacks and one of the stacks is offline, the stack assembly will be able to provide an intermediate power output, namely the sum of the maximum rated power outputs of the remaining two stacks. This output may also be referred to as the available rated power output of the stack assembly, which will change depending upon the number of available stacks at a particular time.

A particular device80may apply a fixed or a variable load to fuel cell system60(and fuel cell stack assembly77). The magnitude and variability of the applied load will tend to vary with the particular type and construction of device and application within which the device is used. For example, a particular device80may normally apply loads to stack assembly77within the range of 300 W and 15 kW. Stack assembly77, in turn, may be designed to provide up to 20 kW of power so that the stack assembly may satisfy this load, supply the balance-of-plant requirements of system60, and additionally or alternatively provide a buffer in case the applied load from device80on some occasion exceeds its normal range of values. In such a configuration, the total rated power output of the stack assembly is 20 kW, and the maximum desired power output of stack assembly77is 15 kW. The maximum rated power output of the individual stacks76in stack assembly77may vary, such as depending upon the number of stacks and the degree of desired redundancy, as discussed in more detail herein.

For purposes of illustration, the following ranges of operating power requirements of exemplary devices80are presented. Automobiles generally require 5-75 kW of power, with 5 kW representing cruising on a level surface and 75 kW representing hard acceleration. A backup power supply for a household generally requires power in the range of 300 W to 13-15 kW. A recreational vehicle, mobile home or the like typically has a power requirement in the range of 300 W to 7-10 kW, excluding motive power requirements. By this it is meant that this range of power represents the electrical power needed to run the heating, air conditioning and appliances of the recreational vehicle, but not the power to drive the vehicle. Seacraft, such as sailboats, tend to have power requirements in the range of 100-300 W to 2-5 kW. Some devices80, such as uninterruptable power supplies (UPSs) may be designed for a particular power requirement that depends upon the specific environment in which the power supply is used. For example, a UPS that is configured to provide power to a personal computer may only require 300 W of power. However, a UPS designed to provide power to communications equipment, signaling equipment, a laboratory, a network of computers, or the like may have a much higher power requirement.

It should be understood that the above ranges are illustrative examples and that similar devices80may operate outside of the identified ranges. Similarly, it should be understood that the above ranges are referred to as operating power requirements because each of the devices may be disconnected or shut down, in which case the device will have no power requirement.

It is within the scope of the present invention that the number of fuel cell stacks76in stack assembly77may vary from two to several dozen or more stacks. Because system60includes a plurality of independent stacks76, each stack may be smaller and may have a maximum rated power output that is less than would otherwise be required if only a single stack was used, such as stack22in system10. When smaller stacks are used, they will typically be less expensive than a single larger stack. This decrease in individual stack cost is somewhat buffered by the increase in additional controls and fluid conduits required for the additional stack. As discussed in more detail herein, each of the stacks may alternatively be equal in rated power output to the corresponding stack22in system10.

As a comparative example, consider a fuel cell system10designed to provide 3 kW of backup power (such as emergency or standby power applications) to a household. Continuing this example, the system may provide power to satisfy the balance-of-plant requirements of the fuel cell system (namely, the power required by the components of system60). The balance-of-plant requirements and losses in power electronics typically range from a few hundred Watts to approximately 1 kW. In such a system, the maximum desired power output is 3 kW, and the maximum rated power output of stack22may be 4 kW. System10therefore, is able to provide the maximum desired power output of the household, as well as to provide the system's balance-of-plant requirements. However, if stack22needs to be removed from operation, such as if the stack fails, operates beyond acceptable operating parameters, needs to be upgraded, is contaminated, or needs to be otherwise serviced, inspected, or repaired, system10cannot provide power to the household until the stack is back in service. During this time, the household is without its source of backup power. As a reminder, this is an illustrative example and the values in a particular system may vary. For example, if the balance-of-plant requirements of a particular system exceed 1 kW, then a fuel cell stack22should be selected that has a higher maximum rated power output.

If fuel cell system60is used instead of system10, the system is able to provide at least an intermediate power output, even if one of the stacks76in stack assembly77fails or is otherwise offline, shut down or otherwise removed from service. For example, if stacks761-764each have a maximum rated power output of 1 kW, the stack assembly will have a total rated power output that equals the rated power output of system10. Should one of the stacks be offline, stack assembly77(and system60) will still be able to provide an intermediate power output of 3 kW. In such a situation, the maximum desired power output of the household or other device80may not be able to be met, but at least a portion, and in some cases a substantial portion of this maximum rated power output may be met. Accordingly, stack assembly77may be described as having a first operational state, in which all of stacks76are producing an electric current, a second operational state, in which none of stacks76are producing an electric current, and a third operational state, in which at least one of stacks76is producing an electric current and at least one of stacks76is not. Furthermore, it should be remembered that many devices80apply a load that corresponds to less than their maximum desired power output during a majority, if not a substantial portion (80% or more) of their operating time. In such a situation, the stack assembly will be able to meet the applied load of the household even though it is not able to provide the maximum desired power output.

From the above example, the utility is demonstrated of a stack assembly77comprised of individual stacks76that collectively have a total rated power output that equals the maximum desired power output of device80. Such a system60may be described as having a fuel cell stack assembly, or a plurality of fuel cell stacks, that individually have rated power outputs that are less than the maximum desired power output from the fuel cell system, but collectively satisfy the maximum desired power output of the system. Such a system may also be described as having intermediate or partial redundancy, in that the system may provide an intermediate power output even if one or more of the individual stacks fails (so long as there is at least one operating stack).

As discussed above, the number of fuel cell stacks76in stack assembly77may vary, but will always include at least two fuel cell stacks. For example, the stack assembly described in the context of the above example may alternatively include eight stacks76with rated power outputs of 500 W, two stacks76with rated power outputs of 2 kW, three stacks76with rated power outputs of 1.33 kW, etc.

In some embodiments of the invention, it may be desirable for the power rating of each individual stack76to be sufficient to provide at least one “extra” stack that will enable the system to still achieve the maximum rated power output even if one or more stacks fails or is offline, such as for servicing, maintenance or repair. For example, assuming a 4 kW system is desired, having five stacks rated at 1 kW enables the maximum desired power output to still be achieved even if one of the stacks needs to be taken offline or fails. In this configuration, system60may be described as having n+1 redundancy, in that it may still provide the maximum desired power output even if one stack is not producing power. It is within the scope of the invention that any desired level of redundancy may be provided, such as n+2 redundancy, n+3 redundancy, etc. When the fuel cell stack assembly includes at least n+1 redundancy, it may be referred to as having total redundancy, in that the stack assembly (and corresponding fuel cell system) may still provide the maximum desired power output even if one (or more, depending on the level of redundancy) individual stacks fail or are otherwise offline.

It should be understood that the increased system reliability provided by having additional stacks should be weighed against the expense of these additional stacks, such as the upfront costs, operating expenses, system demands, etc. Therefore, there is not a best configuration for all users and all purposes. Instead, a particular system may be selected depending upon such factors as the acceptable cost for the system, the desired level of intermediate redundancy and the desired level of total redundancy. For many applications, n+1 redundancy will be desirable. Of course, if a single fuel cell stack has a sufficiently high reliability, no redundancy may be required. However, it is often difficult to predict the actual reliability of a particular stack, especially when the stack may fail due to a failure of a system component up- or down-stream of the stack. In many applications, some degree of redundancy may be required as a safeguard against the consequences of having a fuel cell system that cannot produce any current.

Because system60includes a plurality of fuel cell stacks76instead of the single stack22shown inFIGS. 1 and 2, the stacks may be brought online incrementally as needed to meet the applied load by device80(and/or the balance-of-plant requirements of the fuel cell system, which may be greater during startup of the system). Thus, for loads less than maximum, only that number of fuel cells necessary to meet the load demand are brought online and made operational. For demanding applications such as residential applications where extremely long lifetimes are required of the stacks, and where loads are cycling daily between maximum rated power output and minimum power output, only operating those fuel cell stacks needed to meet the load demand will result in reduced operating hours on the fuel cell stacks and longer lifetime. Thus, instead of having a single stack that is always online when the system is in use, system60may conserve the operative life of the individual fuel cell stacks by only utilizing the number of stacks necessary to meet the load applied by device80. The operational state of the individual stacks may be manually selected, may be automatically controlled responsive to the magnitude of the applied load (from device80and system60), or may be controlled by a control system, as discussed in more detail herein.

The fuel cell stacks may be electrically connected in series, parallel or a combination of series and parallel to meet the output voltage requirements of system60. For example, the four 1 kW stacks discussed in the above illustration each may yield 12 VDC under load. These stacks may be electrically connected in series to yield an output of 48 VDC to the power electronics. It should be understood that these values are merely meant to provide illustrative examples, and that the voltage of the current produced by stack assembly77varies with the applied load. Preferably the fuel cell stacks are electrically isolated from each other to facilitate maintenance, service, replacement, etc. of one of more fuel cell stacks while the remaining stacks continue to supply electric power to device80.

InFIG. 5, an embodiment of system60is shown that includes a power management module81through which electric power from the plurality of fuel cell stacks is delivered to device80. As shown, power (or current)78from stack assembly77passes through module81and then is subsequently delivered to device80as indicated at83. When device80requires AC power, module81will include an inverter for converting the DC power from the fuel cell stacks to AC power. An example of a power management module including an inverter85is schematically illustrated inFIG. 6. Module81may additionally or alternatively include a battery assembly86containing one or more batteries88and associated chargers90, which are adapted to store excess power, as well a switching assembly92that is adapted to selectively deliver power from stack assembly77to either device80or to the battery assembly. Module81may additionally, or alternatively, include at least one DC-DC converter93, such as at least one boost DC-DC converter that increases the voltage of current78or at least one buck DC-DC converter that decreases the voltage of current78. Converter93receives the unregulated DC stream from stack assembly77, the voltage of which is variable with the applied load, and regulates the voltage of the stream to a selected value. The selected value may be more or less than the unregulated voltage and it may also vary depending upon whether the output stream of the converter is going to battery assembly86or device80. Module81may contain a DC-DC converter for each stack76, or alternatively, each fuel cell stack may be electrically connected to, or include, a dedicated DC-DC converter93, such as schematically illustrated inFIG. 7with dashed lines. As shown, the DC-DC converters may be integrated with fuel cell stacks76, with contactors100, or they may be discrete units downstream from the fuel cell stacks. The regulated DC output from the dedicated DC-DC converters may be connected in parallel or series. It should be understood that module81may include components other than those discussed herein, and that not all of the above components are required in every embodiment of a power management module.

InFIG. 7, a further embodiment of system60is shown and includes a delivery manifold assembly94that receives at least a portion of product hydrogen stream and distributes the stream to the fuel cell stacks forming stack assembly77. As shown, assembly94receives product hydrogen stream66and distributes hydrogen streams96to stacks76. Preferably, the manifold assembly is adapted to only deliver hydrogen gas to the operating stacks in stack assembly77. To further illustrate that the number of fuel cell stacks in stack assembly77may vary, stack assembly77is shown inFIG. 7with only a pair of fuel cell stacks76. As discussed, there must be at least two stacks to provide some level of intermediate and/or total redundancy.

Fuel cell system60may include a valve assembly98adapted to regulate, or selectively interrupt, the flow of hydrogen gas from manifold assembly94to selected ones of the fuel cell stacks in stack assembly77. The valve assembly may additionally be adapted to regulate or selectively interrupt the flow of hydrogen gas to the entire stack assembly. Valve assembly98may include any suitable structure for selectively enabling or interrupting the flow of hydrogen gas to stack assembly77and/or stacks76. Examples of suitable devices include flow regulators, valves, switches, switch assemblies, solenoids, and the like. InFIG. 7, valve assembly98is shown integrated within manifold94. It is within the scope of the present invention, however, that valve assembly98may be located external the delivery manifold assembly, although in some embodiments it may still be in direct or indirect cooperative communication therewith.

Fuel cell system60may also include contactors or other suitable devices100that may be actuated to electrically isolate one or more of the fuel cell stacks76in assembly77from the applied load. The contactors may be actuated either manually, such as to remove a stack for servicing, automatically, such as upon exceeding certain operating parameters or load conditions, and/or by a control system. For example, a contactor may be actuated to remove a particular stack from service if the stack is operating at too high of temperature, if the potential in the stack is too low, if the stack has been contaminated, such as by exposure to carbon monoxide, if the stack needs to be serviced or inspected, or if the stack is not needed to meet the applied load.

Stack assembly77may receive feeds other than hydrogen gas. For example, fuel cell stacks76may each receive an air stream102from an air delivery system104. As discussed above with respect toFIG. 3, the air streams are delivered to the cathode regions of the stacks. The air streams may be distributed to the stacks by delivery manifold assembly94, such as shown inFIG. 8. In the illustrated embodiment, air delivery system104, which may take any suitable form, delivers an air stream106to manifold assembly94, which in turn distributes air streams102to stacks76. The embodiment of fuel cell system60shown inFIG. 8also illustrates switch assemblies98located external manifold assembly94. Furthermore, stack assembly77is shown including individual fuel cell stacks761,762through76n, to illustrate that any selected number of stacks may be used.

Stacks76may also receive a cooling fluid stream108that regulates the operating temperature of the stacks. An example of a cooling fluid supply, or delivery system is schematically illustrated at110inFIG. 8and may take any suitable form. System110delivers a stream112of cooling fluid to the manifold assembly, which in turn delivers streams108to the individual stacks. Examples of suitable cooling fluids, include, but are not limited to, air, water, glycols, and water-glycol mixtures. The cooling fluid streams may form a cooling fluid loop, or the streams may be vented, exhausted, or otherwise used or disposed of after being used to cool stacks76. It should be understood that the cooling fluid is not introduced directly into the anode or cathode regions of the stacks. Instead, it may flow through a jacket that surrounds a fuel cell stack, between fuel cells82forming the stack, and/or through conduits extending through the anode and/or cathode regions.

Similar to hydrogen streams96, it is preferable that air streams102are only delivered to the stacks in stack assembly77that are operating to produce current stream78. For example, delivering an air stream to a PEM fuel cell that is not being used to produce an electric current may dry out the electrolytic membrane used in the stack's cells. Cooling fluid streams108may be delivered to only the operating stacks or may be delivered to all of the stacks in stack assembly77at all times. For example, it may be less demanding or require less resources to maintain a continuous flow of cooling fluid to all of the stacks than to regulate and selectively interrupt the flow of cooling fluid.

While a single delivery manifold assembly94is shown inFIG. 7, fuel cell system60may include separate assemblies for each of the feeds to stack assembly77. An exemplary embodiment of such a fuel cell system60is shown inFIG. 9, in which hydrogen gas is distributed by manifold assembly94, air is distributed by manifold assembly94′ and cooling fluid is distributed by manifold assembly94″. It is further within the scope of the invention that the individual stacks may receive any or all of these streams directly from the above-described supplies or sources without requiring a manifold assembly, and that each of the stacks may receive one or more of these streams from an independent supply or delivery system.

When streams96,102and/or108are delivered to the individual stacks76in stack assembly77via a delivery manifold assembly, it is preferable that the streams are delivered in parallel, rather than in series. This configuration enables all of the stacks to receive the respective streams at essentially the same composition, temperature, etc.

InFIG. 10, an embodiment of fuel cell system60is shown in which the system includes a control system120with a controller122that is adapted to manage the operation of system60. As shown, controller122communicates with various components of the fuel cell system via communication links124. Links124may be any suitable form of mechanical, wired or wireless communication between the controller and the corresponding portions of the fuel cell system. The communication links may enable one- or two-way communication. Two-way communication links enable the controller to receive inputs from and send control signals to various components of the fuel cell system. Examples of suitable inputs include one or more current operating conditions, such as temperature, pressure, flow rate, composition, state of actuation, load, etc. These inputs may be received from the corresponding component directly, or from sensor assemblies126associated with the selected components.

Illustrative communication links124and sensors126are shown inFIG. 10, however, it should be understood that it is within the scope of the present invention that control system120may not include all of the links and sensors shown inFIG. 10in all embodiments. Similarly, the control system may also include additional sensors and links, such as in communication with fuel processing assembly62(and its fuel processor(s)64) and/or device80.

Control system120may be used to selectively isolate a stack from the applied load by sending a control signal to the corresponding contactor100. For example, a stack may be isolated from the applied load if the stack is determined, such as from communication from sensor assembly126, other sensors or detectors, manual observation, or the like, to be operating outside of acceptable operating parameters.

In embodiments of the fuel cell system in which each fuel cell stack76includes its own DC-DC converter, each DC-DC converter may be adapted to automatically isolate the corresponding stack if the stack is delivering substandard performance in response to the applied load. For example, if a particular DC-DC converter does not receive current78having a voltage that exceeds a selected minimum voltage, then the DC-DC converter automatically isolates the stack from the applied load, such as by actuating contactor100or a suitable contactor or other switch associated with the DC-DC converter.

Control system120may additionally or alternatively be used to selectively adjust or interrupt the flow of hydrogen gas, air and/or cooling fluid to one or more of the stacks forming assembly77. For example, the flow of hydrogen and air, and optionally cooling fluid to a particular stack may be interrupted so that the stack does not produce electric current. Typically, the corresponding contactor100will also be actuated to isolate the stack from feeds and from the applied load. Control system120may also isolate one or more of the stacks if a contaminant in the hydrogen gas stream is detected, such as to prevent the contaminated hydrogen gas from being delivered to the stack.

Again, returning to the illustrative example of a residential fuel cell system rated at 4 kW gross electric or 3 kW net electric, when the load demand falls to a value substantially less than 3 kW the fuel cell controller may send a signal to turn off and isolate one or more of stacks76. Especially during those periods when power consumption is at a minimum in a normal residential home, such as late night and mid-day, up to three of the four fuel cell stacks may be signaled to turn off and be electrically isolated to reduce the net power output to less than 1 kW, sufficient only to meet the minimum load demands of the residence during periods of minimal power requirements. In this example, if the periods of minimum power consumption in the house last 12 hours each day, and if only one 1-kW stack is required to be online to meet the minimum loads, then taking 3 of the 4 fuel cell stacks offline will effectively increase the lifetime of the stacks by 60%. (In a four-day period each fuel cell stack will operate for one full day and three half days, or 60 hours of operation for every 96-hour period.)

It should be understood that the increase in lifetime is proportional to the percentage of total system operating time that a particular stack is offline. Controller122may be adapted to select the stack to be removed from service according to a predetermined sequence, or alternatively, the stack may be randomly selected or it may be rotated. A predetermined sequence maximizes the operating hours of a particular stack, while minimizing the life of the others. In this situation, the maximized stack is going to fail much sooner than the rest, but only that particular cell will need replacement. If in a rotational sequence, in which the particular stack that remains online is rotated sequentially between the stacks, such as on an hourly, daily, weekly, or monthly basis, the overall operating time of the stacks will be approximately the same, meaning that all of the stacks will tend to need replacement at approximately the same time, however, this time will be considerably longer than the time required to replace the single maximized stack in the predetermined sequence configuration.

Control system120may include a user interface130in communication with the controller. User interface130enables a user to monitor and/or interact with the operation of the controller. An illustrative example of a user interface130is shown inFIG. 12. As shown, interface130includes a display region132with a screen134or other suitable display mechanism in which information is presented to the user. For example, display region132may display the current values measured by one or more of sensor assemblies126, the threshold and actual operating parameters of system60or device80, the applied load to the stack assembly and individual stacks therein, the potential and other operating parameters of the stacks, etc. Previously measured values may also be displayed. Other information regarding the operation and performance of the fuel processing system may also be displayed in region132.

User interface130may also include a user input device136through which a user communicates with the controller. For example, input device136may enable a user to input commands to change the operating state of the fuel cell system, to change one or more of the stored threshold values and/or operating parameters of the system, and/or to request information from the controller about the previous or current operating parameters of the system. Input device136may include any suitable device for receiving user inputs, including rotary dials and switches, pushbuttons, keypads, keyboards, a mouse, touch screens, etc. Also shown inFIG. 8is a user-signaling device138that alerts a user when an acceptable threshold level has been exceeded and the fuel cell stack has been isolated. Device138may include an alarm, lights, or any other suitable mechanism or mechanisms for alerting users.

It should be understood that it is within the scope of the present invention that the fuel cell system may include a controller without a user interface, and that it is not required for the user interface to include all of the elements described herein. The elements described above have been schematically illustrated inFIG. 12collectively, however, it is within the scope of the present invention that they may be implemented separately. For example, the user interface may include multiple display regions, each adapted to display one or more of the types of user information described above. Similarly, a single user input device may be used, and the input device may include a display that prompts the user to enter requested values or enables the user to toggle between input screens.

In applications where system60includes a power management module81with an inverter85, these load managing controls by system120also allow the electronics to be designed for lower magnitude peak power, with resultant cost savings. Such load managing controls may be particularly effective when device80operates at an intermediate power output during a majority or a significant portion of the time, with the maximum desired power output only being required a small percentage of the time. An example of such a device80is a household, which may apply loads in the range of a few hundred Watts to 13-15 kW to system60. However, other than during peak periods, such as one- or two-hour periods in the mornings and evenings, the household typically applies a load that is much less than its maximum desired power output.

Continuing with the illustrative example of a residential fuel cell system, load managing controls can effectively be used with a fuel cell system to reduce (manage) the magnitude of peaking loads in the household. This may be accomplished using switching modules140that major household appliances (dryer, dishwasher, hairdryer, microwave oven, coffee maker, etc.) are plugged into. The switching modules (collectively referred to as a switching module assembly) communicate with each other, controller122, or both, and are capable of recognizing loads (appliances) of higher priority and signaling lower priority appliances to switch off. For example, a high priority appliance such as a microwave oven may signal a low priority appliance (such as a dishwasher or clothes dryer) to turn off so that the microwave oven may be used without significantly increasing the total household load demand. The signal originates through the switching module, which may be built into or otherwise integrated with the appliance, or it may be a separate module that the appliance is plugged into. Switching modules140may communicate with other switching modules by radio or by electrical signals sent through the existing household wiring. Frequency scrambled communication through existing wires is particularly effective, although any suitable communication link may be used. Additionally, or alternatively, the switching modules may communicate with controller122, which in turn directs the selective on/off configuration (or operational state) of the appliances. The priority, or hierarchy of the devices or associated modules may be established by any suitable mechanism, such as by being predetermined by the individual switching modules (such as by having high priority modules, low priority modules, medium priority modules, etc.), or stored by the control system or switching module assembly.

An illustrative example of a device80having load limiting controls, which are collectively indicated at142is shown inFIG. 13. As shown, device80includes devices801-804, each of which includes or communicates with a switching module140. The modules are shown being in communication with each other via communication links144, and additionally or alternatively, in communication with controller122with communication link124.

Managing the peak electrical loads of a household can result in the peak load demand being decreased. For example, in the continuing example used herein a household may have a maximum desired power output of 10-15 kW. This maximum desired power output may be reduced by 25%, 50% or more through the use of load limiting controls. For example, the maximum desired power output may be reduced to a range of 4-8 kW. As a result, the fuel cell system can effectively use fuel cell stacks with lower net electrical power output and the power electronics can be substantially downsized. The cost savings may be significant.

It should be understood that the invented fuel cell system, including the control system and load limiting controls, may be applied to energy-consuming devices80other than the residential household described above. Examples of other suitable devices include commercial buildings, vehicles, microwave relay stations, lights, appliances, tools, communication equipment, signaling devices and other devices80described herein.

As discussed previously, fuel processor64is any suitable device that produces hydrogen gas. Preferably, the fuel processor is adapted to produce substantially pure hydrogen gas, and even more preferably, the fuel processor is adapted to produce pure hydrogen gas. For the purposes of the present invention, substantially pure hydrogen gas is greater than 90% pure, preferably greater than 95% pure, more preferably greater than 99% pure, and even more preferably greater than 99.5% pure. Suitable fuel processors are disclosed in U.S. Pat. Nos. 5,997,594 and 5,861,137, pending U.S. patent application Ser. No. 09/291,447, which was filed on Apr. 13, 1999, and is entitled “Fuel Processing System,” and U.S. Provisional Patent Application Ser. No. 60/188,993, which was filed on Mar. 13, 2000 and is entitled “Fuel Processor,” each of which is incorporated by reference in its entirety for all purposes.

An example of a suitable fuel processor64is a steam reformer. An example of a suitable steam reformer is shown inFIG. 14and indicated generally at150. Reformer150includes a reforming, or hydrogen-producing, region152that includes a steam reforming catalyst154. Alternatively, reformer150may be an autothermal reformer that includes an autothermal reforming catalyst. In reforming region152, a reformate stream156is produced from the water and carbon-containing feedstock forming feed stream68. The reformate stream typically contains hydrogen gas and impurities, and therefore is delivered to a separation region, or purification region,158, where the hydrogen gas is purified. In separation region158, the hydrogen-containing stream is separated into one or more byproduct streams, which are collectively illustrated at160, and a hydrogen-rich stream162by any suitable pressure-driven separation process. InFIG. 14, hydrogen-rich stream162is shown forming product hydrogen stream66.

An example of a suitable structure for use in separation region158is a membrane module164, which contains one or more hydrogen permeable metal membranes166. Examples of suitable membrane modules formed from a plurality of hydrogen-selective metal membranes are disclosed in U.S. patent application Ser. No. 09/291,447, which was filed on Apr. 13, 1999, is entitled “Fuel Processing System,” and the complete disclosure of which is hereby incorporated by reference in its entirety for all purposes. In that application, a plurality of generally planar membranes are assembled together into a membrane module having flow channels through which an impure gas stream is delivered to the membranes, a purified gas stream is harvested from the membranes and a byproduct stream is removed from the membranes. Gaskets, such as flexible graphite gaskets, are used to achieve seals around the feed and permeate flow channels. Also disclosed in the above-identified application are tubular hydrogen-selective membranes, which also may be used. Other suitable membranes and membrane modules are disclosed in U.S. patent application Ser. No. 09/618,866, which was filed on Jul. 19, 2000 and is entitled “Hydrogen-Permeable Metal Membrane and Method for Producing the Same,” the complete disclosure of which is hereby incorporated by reference in its entirety for all purposes. Other suitable fuel processors are also disclosed in the incorporated patent applications.

The thin, planar, hydrogen-permeable membranes are preferably composed of palladium alloys, most especially palladium with 35 wt % to 45 wt % copper. These membranes, which also may be referred to as hydrogen-selective membranes, are typically formed from a thin foil that is approximately 0.001 inches thick. It is within the scope of the present invention, however, that the membranes may be formed from hydrogen-selective metals and metal alloys other than those discussed above, hydrogen-permeable and selective ceramics, or carbon compositions. The membranes may have thicknesses that are larger or smaller than discussed above. For example, the membrane may be made thinner, such as by rolling, sputtering or etching with a commensurate increase in hydrogen flux. The hydrogen-permeable membranes may be arranged in any suitable configuration, such as arranged in pairs around a common permeate channel as is disclosed in the incorporated patent applications. The hydrogen permeable membrane or membranes may take other configurations as well, such as tubular configurations, which are disclosed in the incorporated patents.

Another example of a suitable pressure-separation process for use in separation region158is pressure swing adsorption (PSA). In a pressure swing adsorption (PSA) process, gaseous impurities are removed from a stream containing hydrogen gas. PSA is based on the principle that certain gases, under the proper conditions of temperature and pressure, will be adsorbed onto an adsorbent material more strongly than other gases. Typically, it is the impurities that are adsorbed and thus removed from reformate stream156. The success of to using PSA for hydrogen purification is due to the relatively strong adsorption of common impurity gases (such as CO, CO2, hydrocarbons including CH4, and N2) on the adsorbent material. Hydrogen adsorbs only very weakly and so hydrogen passes through the adsorbent bed while the impurities are retained on the adsorbent. Impurity gases such as NH3, H2S, and H2O adsorb very strongly on the adsorbent material and are therefore removed from stream156along with other impurities. If the adsorbent material is going to be regenerated and these impurities are present in stream156, separation region158preferably includes a suitable device that is adapted to remove these impurities prior to delivery of stream156to the adsorbent material because it is more difficult to desorb these impurities.

Adsorption of impurity gases occurs at elevated pressure. When the pressure is reduced, the impurities are desorbed from the adsorbent material, thus regenerating the adsorbent material. Typically, PSA is a cyclic process and requires at least two beds for continuous (as opposed to batch) operation. Examples of suitable adsorbent materials that may be used in adsorbent beds are activated carbon and zeolites, especially 5 Å (5 angstrom) zeolites. The adsorbent material is commonly in the form of pellets and it is placed in a cylindrical pressure vessel utilizing a conventional packed-bed configuration. It should be understood, however, that other suitable adsorbent material compositions, forms and configurations may be used.

Reformer150may, but does not necessarily, further include a polishing region168, such as shown inFIG. 15. Polishing region168receives hydrogen-rich stream162from separation region158and further purifies the stream by reducing the concentration of, or removing, selected compositions therein. For example, when stream162is intended for use in a fuel cell stack assembly, such as assembly77, compositions that may damage the fuel cell stack, such as carbon monoxide and carbon dioxide, may be removed from the hydrogen-rich stream. The concentration of carbon monoxide should be less than 10 ppm (parts per million) to prevent the control system from isolating the fuel cell stack. Preferably, the system limits the concentration of carbon monoxide to less than 5 ppm, and even more preferably, to less than 1 ppm. The concentration of carbon dioxide may be greater than that of carbon monoxide. For example, concentrations of less than 25% carbon dioxide may be acceptable. Preferably, the concentration is less than 10%, even more preferably, less than 1%. Especially preferred concentrations are less than 50 ppm. It should be understood that the acceptable minimum concentrations presented herein are illustrative examples, and that concentrations other than those presented herein may be used and are within the scope of the present invention. For example, particular users or manufacturers may require minimum or maximum concentration levels or ranges that are different than those identified herein.

Region168includes any suitable structure for removing or reducing the concentration of the selected compositions in stream162. For example, when the product stream is intended for use in a PEM fuel cell stack or other device that will be damaged if the stream contains more than determined concentrations of carbon monoxide or carbon dioxide, it may be desirable to include at least one methanation catalyst bed170. Bed170converts carbon monoxide and carbon dioxide into methane and water, both of which will not damage a PEM fuel cell stack. Polishing region168may also include another hydrogen-producing device172, such as another reforming catalyst bed, to convert any unreacted feedstock into hydrogen gas. In such an embodiment, it is preferable that the second reforming catalyst bed is upstream from the methanation catalyst bed so as not to reintroduce carbon dioxide or carbon monoxide downstream of the methanation catalyst bed.

Steam reformers typically operate at temperatures in the range of 200° C. and 700° C., and at pressures in the range of 50 psi and 1000 psi, although temperatures outside of this range are within the scope of the invention, such as depending upon the particular type and configuration of fuel processor being used. Any suitable heating mechanism or device may be used to provide this heat, such as a heater, burner, combustion catalyst, or the like. The heating assembly may be external the fuel processor or may form a combustion chamber that forms part of the fuel processor. The fuel for the heating assembly may be provided by the fuel processing system, or fuel cell system, by an external source, or both.

InFIGS. 14 and 15, reformer150is shown including a shell151in which the above-described components are contained. Shell151, which also may be referred to as a housing, enables the fuel processor, such as reformer150, to be moved as a unit. It also protects the components of the fuel processor from damage by providing a protective enclosure and reduces the heating demand of the fuel processor because the components of the fuel processor may be heated as a unit. Shell151may, but does not necessarily, include insulating material153, such as a solid insulating material, blanket insulating material, or an air-filled cavity. It is within the scope of the invention, however, that the reformer may be formed without a housing or shell. When reformer150includes insulating material153, the insulating material may be internal the shell, external the shell, or both. When the insulating material is external a shell containing the above-described reforming, separation and/or polishing regions, the fuel processor may further include an outer cover or jacket external the insulation.

It is further within the scope of the invention that one or more of the components may either extend beyond the shell or be located external at least shell151. For example, and as schematically illustrated inFIG. 15, polishing region168may be external shell151and/or a portion of reforming region152may extend beyond the shell. Other examples of fuel processors demonstrating these configurations are illustrated in the incorporated references and discussed in more detail herein.

As discussed previously, a fuel cell system according to the present invention may additionally or alternatively include partial or total redundancy regarding its fuel processors. An example of such a system is shown inFIG. 16and generally indicated at180. As shown, system180includes a fuel processing assembly62that includes a plurality of fuel processors64. To illustrate that the fuel cell system includes at least two fuel processors, and may include more than two fuel processors, the fuel processors are indicated at641to64n. It should be understood that “n” may be as low as 2 and may be any selected number from two to a dozen or more. Similarly, although system180is shown also having a redundancy of fuel cell stacks76, system180may be implemented with only a single stack76.

INDUSTRIAL APPLICABILITY

The present invention is applicable to energy-producing systems, and more particularly to fuel processing and fuel cell systems.