Current control for parallel fuel cell stacks

A fuel cell system includes fuel cell stacks electrically connected in parallel and supplying a gross current to a load. A controller determines the gross load current, and produces a desired current through the load by adjusting, based on the gross load current, at least one parameter affecting at least one of the inputs to and outputs from the system. This system allows a stack design and its voltage output to be kept constant while stacks are added for increased power.

FIELD OF THE INVENTION

The present invention relates to fuel cells and, more particularly, to controlling current shared among a plurality of fuel cells.

BACKGROUND OF THE INVENTION

Fuel cells produce electricity through electrochemical reaction and have been used as power sources in many applications. Fuel cells can offer significant benefits over other sources of electrical energy, such as improved efficiency, reliability, durability, cost and environmental benefits. Fuel cells may eventually be used in automobiles and trucks. Fuel cells may also power homes and businesses.

There are several different types of fuel cells, each having advantages that may make them particularly suited to given applications. One type is the proton exchange membrane (PEM) fuel cell, which has a membrane sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and air or oxygen (O2) is supplied to the cathode.

In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). Because the membrane is proton conductive, the protons are transported through the membrane. The electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+) and electrons (e−) are taken up to form water (H2O).

In fuel cell applications, a plurality of fuel cells are combined in series to form one or more stacks. Power demands of different fuel cell applications vary. Changing the active area of the fuel cell in a stack can scale power. This approach, however, requires fuel cells to be redesigned and/or retooled for the different power levels, which is a costly approach.

Other ways of scaling the power output of fuel cell stacks include varying the number of cells in a stack and/or connecting stacks in series. When changing stack output voltages, it is usually necessary to redesign supporting components such as compressors. When stacks are connected in series to allow power scaling, different output voltages are produced, which complicates the design of accessory loads and electrical interfaces.

SUMMARY OF THE INVENTION

A fuel cell system according to one embodiment of the present invention includes a plurality of fuel cell stacks connected in parallel and supplying a gross current to a load. The system has a plurality of inputs to and a plurality of outputs from the stacks. A controller produces a desired current through the load by adjusting, based on the gross load current, at least one parameter affecting at least one of the inputs and outputs.

In another embodiment, a fuel cell system includes a plurality of fuel cell stacks electrically connected in parallel to supply a load. Each stack includes a plurality of inputs and outputs affected by a plurality of parameters. A controller determines a current from one of the stacks to the load, and, based on the determined current, adjusts at least one of the parameters to produce a desired current through the load.

An embodiment of a method for controlling power to a load supplied by a plurality of fuel cells includes combining the fuel cells to provide a plurality of fuel cell stacks. The method further includes connecting the stacks in parallel, and controlling at least one of an input to and an output from a given stack to provide a desired current through the given stack.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now toFIG. 1, the present invention will be described in conjunction with a fuel cell10that includes a membrane electrode assembly (MEA)12. Skilled artisans will appreciate that other types of fuel cells are contemplated and may be employed without departing from the invention. Preferably, the MEA12is a proton exchange membrane (PEM). The MEA12includes a membrane14, a cathode16, and an anode18. The membrane14is sandwiched between the cathode16and the anode18.

A cathode diffusion medium20is layered adjacent to the cathode16opposite the membrane14. An anode diffusion medium24is layered adjacent to the anode18opposite the membrane14. The fuel cell assembly10further includes a cathode flow channel26and anode flow channel28. The cathode flow channel26receives and directs air or oxygen (O2) from a source to the cathode diffusion medium20. The anode flow channel28receives and directs hydrogen (H2) from a source to the anode diffusion medium24.

In the fuel cell assembly10, the membrane14is a cation permeable, proton conductive membrane having H+ions as the mobile ion. The fuel gas is hydrogen (H2) and the oxidant is oxygen or air (O2). The overall cell reaction is the oxidation of hydrogen to water and the respective reactions at the anode18and the cathode16are as follows:
H2=2H++2e−
0.5 O2+2H++2e−=H2O

Since hydrogen is used as the fuel gas, the product of the overall cell reaction is water. Typically, the water that is produced is rejected at the cathode16, which is a porous electrode including an electrocatalyst layer on the oxygen side. The water may be collected as it is formed and carried away from the MEA12of the fuel cell assembly10in any conventional manner.

The cell reaction produces a proton exchange in a direction from the anode diffusion medium24towards the cathode diffusion medium20. In this manner, the fuel cell assembly10produces electricity. An electrical load30is electrically connected across a first plate32and a second plate34of the MEA12to receive the electricity. The plates32and/or34are bipolar plates if a fuel cell is located adjacent to the respective plate32or34or end plates if a fuel cell is not adjacent thereto.

Referring now toFIG. 2, a fuel cell system according to one embodiment is indicated generally by reference number100. The system100includes a plurality of fuel cell stacks104electrically connected in parallel and providing a gross load current to a primary load108and an ancillary load, e.g., a cathode compressor motor assembly116. Two fuel cell stacks104aand104bare included in the embodiment shown inFIG. 2, although more than two stacks are contemplated in other embodiments. Generally, electrically connecting fuel cell stacks104in parallel fixes an output voltage range. Power-producing capacity can be increased by adding parallel stack(s)104to the system100. When a plurality of stacks104are operated in parallel, a resulting parallel bus voltage generated from the parallel configuration is matched by all the stacks104in the parallel configuration, in accordance with Kirchhoff's voltage law.

When the fuel cell stacks104are connected in parallel, current produced by a given stack104may differ significantly from other connected stacks104. Such difference may be due in part to variability in stack material, build process, operating flows and other normal variations. In order to minimize stack degradation, maximize power output, and maximize efficiency, it is desirable to independently control the currents produced by each stack104at or close to a desired set point to achieve operating conditions appropriate for each stack104. Although current controls might be provided using power conversion electronics, such controls could add considerable cost, mass and volume to the system100.

Therefore, in the present embodiment and as further described below, a system controller112is configured to produce a desired current across the loads108and116by adjusting, based on the gross load current, at least one parameter affecting at least one input to and/or output from at least one of the stacks104. Such parameters include but are not limited to pressure, humidity, stoichiometry, nitrogen dilution and temperature. Thus current generated by a stack104can be controlled and varied, for example, by controlling the anode and/or cathode gas streams for that stack, as further described below. The system100can control stack currents independently of one another and to different levels, for example, based on different stack current set points in proportion to the active areas of the stacks104.

In the fuel cell system100, cathode reactions are processed in the following manner. A cathode compressor116is powered by the stacks104and controlled by the system controller112via a DC/AC converter120. Oxidant is drawn from atmospheric pressure through the cathode compressor116. The oxidant is discharged at positive pressure from the cathode compressor116into a common cathode inlet manifold124. The oxidant is forced into cathode distribution inlets128and132. Mass flow of the oxidant is controlled via cathode inlet control valves136and140. The regulated oxidant mass flows are forced into cathode inlet streams144and146respectively for the fuel cell stacks104aand104b. Upon completing a proton exchange reaction, waste oxidant gases are exhausted via cathode exhaust distribution outlets150and152, in which are respectively positioned back-pressure control valves198aand198b. The oxidant waste gases are combined in a common cathode exhaust stream156.

Anode reactions are processed in the following manner. Pressurized hydrogen enters an anode common inlet manifold160and is forced into anode distribution inlets162and164. Mass flow of the respective hydrogen streams is controlled via anode inlet control valves166and168. Regulated hydrogen mass flows are forced into anode inlet streams170and172for the fuel cell stacks104aand104brespectively. Upon completing a proton exchange reaction, waste anode gases are exhausted via anode exhaust distribution outlets174and176. Anode waste gases are combined in a common anode exhaust stream178.

The stacks104generate electricity in the following manner. Electrons generated by proton exchange reactions within each fuel cell stack104are collected at negative electrical terminals182aand182b. Current is conducted via a system negative voltage bus184through optional diodes186to serve the cathode compressor motor assembly116and the primary system load108. Return currents are conducted via a positive voltage bus188. The fuel cell stacks104aand104bare electrically connected in parallel. Thus voltages across each of the stacks are equal. Electrons are produced at a rate at least partly determined by cathode and anode gas mass flow rates through the cathode and anode inlet streams144and170for fuel cell stack104a, and streams146and172for fuel cell stack104b.

Currents generated by the fuel cell stacks104aand104bare measured respectively by current sensors190aand190b, which are in electrical communication with the controller112. The controller112monitors the current sensors190and uses current information for each stack104to determine a total (i.e., gross) load current generated by the stacks104aand104b. Alternatively or additionally, the controller112uses oxygen sensors194and196to determine the gross current generated by the stacks104aand104b. For example, the sensor194amonitors oxygen content of the cathode inlet stream144to the stack104a. The sensor196amonitors oxygen content of the cathode exhaust distribution outlet150. The controller112uses information from the sensors194aand196ato determine oxygen consumption across the stack104a. The controller112uses the oxygen consumption across the stack104ato determine current generated by the stack104a. Thus, although both current sensors and oxygen sensors are illustrated inFIG. 2, embodiments are contemplated in which oxygen sensors are not included, and other embodiments are contemplated in which current sensors are not included. Although a controller may calculate (rather than sense) current values from oxygen sensor information, oxygen sensors may be less expensive than current sensors.

Stack temperatures and relative humidity can be controlled via on or more thermal subsystems114in communication with the controller112. For simplicity, only one thermal subsystem114is shown inFIG. 2, and only one stack104is shown connected to the thermal subsystem114. However, each stack104preferably would be configured with a thermal subsystem. It also is contemplated that a single thermal subsystem114could provide cooling for all of the stacks104. The controller112can adjust stack temperatures, for example, by controlling the circulation of coolant through a coolant loop118in a stack104. The thermal subsystem114is configured to communicate information to the controller112pertaining, for example, to stack temperature, radiator temperature, bypass valve position, coolant pump motor speed, and radiator fan speed. The controller112can use such information, for example, to control the pumping and flow of coolant through the thermal subsystem114and the stacks104.

The controller112uses the gross load current to set a gross cathode stream mass flow rate through the compressor116. The controller112generates a total cathode and anode mass flow control set point to obtain a desired total current generation for serving the primary system load108and the cathode compressor motor assembly116. One or more parameters affecting a particular stack104can be adjusted, for example, according to applicable parameter relationships, to control current produced by the particular stack104.

An embodiment of a method performed by the controller112for adjusting one or more parameters affecting the stacks104is indicated generally inFIG. 3by reference number200. The controller112starts at step202and sets a stack index M equal to 1 at step204. At step208the controller112measures and/or estimates current at each of N parallel-connected, operating fuel cell stacks104. If at step212the controller determines that current through a fuel cell stack corresponding to index value M (“the Mth stack”) exceeds an upper threshold current value Ihigh, the controller at step216adjusts at least one parameter affecting the Mth stack and checks the Mth stack current again at step212. If at step220current through the Mth stack is less than Ihighbut not greater than a lower threshold current value Ilow, the controller at step224adjusts at least one parameter affecting the Mth stack and checks the stack current again at step220.

If current through the Mth stack falls between Ihighand Ilowand M does not equal N at step228, then the controller112increments the index M, e.g., by one, and control returns to step208. At step208the controller112measures and/or estimates current from the N stacks104and proceeds to step212to check current through a stack corresponding to index value M+1. If M equals N at step228, control returns to step204, M is reset to 1, and so on.

One or more parameters affecting the stack(s)104can be adjusted in a plurality of ways. For example, a proportion of oxidant to hydrogen to a particular stack104can be adjusted to drive the load current for that stack to a desired value. Thus, using the anode and cathode mass flow control valves168and136, the system controller112can adjust stack104astoichiometry and/or gas pressure(s) to set and control an amount of current generated from fuel cell stack104a. Using the anode and cathode mass flow control valves166and140, the system controller112can set and control an amount of current generated from fuel cell stack104b. Relative humidity in a stream is affected, for example, by pressure. Accordingly, relative humidity of a cathode stream of stack(s)104can be adjusted in the system100via back-pressure control valves198aand/or198b.

Embodiments are contemplated wherein stack104anode exhaust streams174and/or176are re-circulated through the system100, possibly resulting at times in nitrogen accumulation in stack cells10. The controller112can detect the foregoing condition, known as nitrogen dilution, by monitoring voltages across the stacks104. Where the controller112determines that stack voltages have dropped due to nitrogen dilution, the exhaust streams174and/or176are vented and fresh hydrogen input is introduced at the anode common inlet manifold160.

The diodes186are optional for protecting stacks104against reverse bus currents. A given stack104can be electrically isolated from the system100by opening a contactor192aor192bconnected between the given stack terminal182and the bus184. Thus a selected stack can be switched in and/or out of the fuel cell system100as may be desired, while other stack(s)104may be kept in operation. In an embodiment having more than two stacks, more than one stack104may be de-energized at any one time. Voltage may vary with current, for example, according to a polarization curve which is a function of controlled parameters, design and fuel cell behavior.

In the present embodiment, an amount of power being generated by the system100can be changed by adding stacks104to the system100and/or taking stacks out of the system100. Where the stacks104are of a standard size, e.g., having equal numbers of fuel cells and common or differing active areas, the power can be changed by discrete amounts related to the stack size. The power produced from each of the stacks104of the system100can be tuned, i.e., balanced, within a fixed bus voltage range, by control of parameters as previously described.

The foregoing system and related methods make it possible to keep a stack design and its voltage output constant while adding one or more stacks to produce higher amounts of current. Where stacks are combined to form fuel cell power modules for a particular application, paralleling of modules is beneficial because the application voltage remains the same as the power level increases. Embodiments of the present invention allow current in each stack to be controlled. Such control can be adapted for degradation of an individual stack over its life. A comparatively less stable stack can be allowed to operate at a comparatively low operating point. In the event of loss of an individual stack, although total output power may be reduced, the foregoing system can remain in operation. Thus reliability, flexibility and efficiency of a fuel cell system are increased where such system is configured in accordance with the foregoing embodiments.

Power levels can be scaled in the foregoing system without having to change the size of an active cell area. Common voltages can be provided for accessory components, for example, for a compressor and/or an isolation detection system. Parallel electrical operation of fuel cell stacks and modules is achieved without costly power electronics.