Gasoline fuel cell power system transient control

A reformer fuel feed stream in fuel cell power system is catalytically reacted into a reformate stream for a fuel cell stack by adjusting fuel supply throughput according to a predefined throughput ramp when a time derivative and the current demand for the fuel cell deviates from zero by more than an acceleration threshold value. The fuel cell power system is of special value when deployed on a vehicle where the load command is derived from the accelerator pedal of the vehicle.

FIELD OF THE INVENTION

The present invention relates to fuel cell power system operation, especially to load transient operation of a fuel cell power system having a reformer that converts hydrocarbon to a hydrogen-containing feed for a fuel cell stack.

BACKGROUND OF THE INVENTION

Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system.

As fuel cell power systems are deployed into application having transient power demands such as motor vehicles, fuel cell power system response becomes an issue of concern. In this regard, some components in many fuel cell power systems are designed for operation in a relatively steady state dynamic context where load transients are best accommodated over a relatively long period of time. Vehicles, however, require fairly rapid load change response by the fuel cell power system. In addition to prompt response for the vehicle, the power system ideally maintains nominal voltage output levels during load transients and also effectively handles thermal and/or stoichiometric transients such as carbon monoxide spikes and hydrogen starvation that accompany the load transients.

When a fuel cell power system processes a hydrocarbon by steam reformation and/or partial oxidation to feed high hydrogen content reformate to a fuel cell stack, responsiveness and long-term robustness are needed in both the fuel cell stack and in the reforming process. One problem in this regard occurs when a dramatic upward demand transient on a fuel cell depresses stack output voltage as insufficient hydrogen flows to the fuel cell stack to sustain the voltage during the transient. This condition occurs if the hydrocarbon reforming rate does not accelerate to essentially match acceleration in demand. Another problem is that unacceptably low cell output voltage during the load transient can result from carbon monoxide “spikes” in reformate gas if water vaporization rate change lags the acceleration in load. Reactor durability is also adversely affected as the fuel vaporization rate change lags the acceleration in load and commensurate temperature “spikes” damage the reforming catalyst.

While one solution to the above problems is to delay the response of the vehicle to a change in load command so that essentially steady-state conditions are sustained in the power system, such a solution is unacceptable for drivers conditioned to expect the responsiveness provided by an internal combustion engine. Such a solution is also potentially dangerous for a vehicle operating in a transportation infrastructure built for immediate responsiveness.

What is needed is a fuel cell power system that responds smoothly and comprehensively to load transients. The present invention provides a solution to this need.

SUMMARY OF THE INVENTION

The invention provides for catalytically reacting at least one reformer feed stream into a fuel supply stream for a fuel cell by adjusting throughput of each feed stream according to a predefined throughput ramp when the rate of change in a load command measurement for the fuel cell deviates from zero by more than a threshold value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In overview, the preferred embodiment of the present invention uses a feed forward control approach to minimize the effect of water and fuel vaporizer lag once a load transient has been initiated. More specifically, the rate of change in the load command (the time derivative of the current demand) triggers a pre-determined step change adjustment to begin a pre-determined adjustment of fuel and water flows to the primary reactor of the reformer (e.g. an auto-thermal reactor also designated as the ATR). In this regard, the anode stoichiometry is promptly modified according to a predefined throughput step when a sudden change in the load command is detected. The step changes in feed settings adjust the steam to carbon (S/C) ratio and oxygen to carbon (O/C) ratio in conjunction with the general adjustments to the feed settings of the power system. In other words, the present invention provides means for varying the operational state of the fuel processor system to account for large deviations from a steady-state or quasi-steady-state operation of the system.

In a fuel reformer based fuel cell system (where a fuel such as gasoline or methanol is reformed to a hydrogen-containing feed stream for the fuel cell stack), liquid fuel and water flowing to the fuel reforming system must be vaporized prior to entry into the primary reactor (ATR). When increased electrical output is required from the fuel cell stack, commensurately greater fuel and water must be vaporized in order to provide the increased electrical output. During an up-transient, when the electrical demand increases rapidly, a time lag affects achieving the necessary higher flow rates of fuel and water vapor. Even if fuel and water are delivered to the power system with appropriate increase in mass flow, sizing of the vaporizers within the system usually establishes the basis for lag to the system as a whole. Undesirable instantaneous O/C ratio increase and S/C ratio decrease occur since there is relatively little time lag respective to air flow into the ATR during an up-transient. Unacceptable stack output voltage decline (as a result of insufficient hydrogen flow and carbon monoxide “spikes” in the reformate gas) and reactor catalyst degradation (from temperature “spikes”) occur as the O/C and S/C ratio metrics within the ATR deviate, during load command transients, from mismatches in the time constants of their associated feeds.

The carbon monoxide problem is mitigated in the present invention by water enrichment whereby extra water and energy is delivered into the water vaporizer (increasing the S/C ratio) at the beginning of the up-transient. The water enrichment triggers when the time derivative of requested load (i.e. current demand) exceeds a threshold value. The higher S/C ratio continues to be delivered throughout the ramp-up in load command, until the rate of change in desired load falls below an appropriate threshold trigger value. When the desired load shifts below the threshold, the S/C ratio transitions back to the steady state operations value.

The O/C ratio controls operating temperature in the ATR. An increase in the O/C ratio commensurately increases ATR temperature and, conversely, a decrease in the O/C ratio decreases ATR temperature. The temperature at the inlet of the ATR is normally used as feedback to determine how to change the O/C ratio to obtain the desired operating temperature (approximately 750 degrees C.). This control approach works well in steady-state operation, but responds too slowly in up-transients, resulting in the aforementioned temperature spike. This problem is mitigated in the present invention by issuing an O/C correction based on the time derivative of the current demand. Based on the magnitude of the derivative, the O/C ratio is immediately decreased by a calibrated amount at the start of an up-transient. After this initial correction, the O/C ratio is continuously adjusted based on real time feedback of ATR temperature. Conversely, an ATR temperature decline can be avoided by increasing the O/C ratio by a calibrated amount at the start of a downward transient.

The present invention is further understood with reference to a generic fuel cell power system. Therefore, before further describing the invention, a general overview of the fuel cell power system of the invention is provided. In the system, a hydrocarbon fuel is processed in a fuel processor, for example, by reformation and partial oxidation processes, to produce a reformate gas which has a relatively high hydrogen content on a volume or molar basis. Therefore, reference is made to hydrogen-containing as having relatively high hydrogen content. The present invention is hereafter described in the context of a fuel cell fueled by an H2-containing reformate regardless of the method by which such reformate is made. It is to be understood that the principles embodied herein are applicable to fuel cells fueled by H2obtained from any source, including reformable hydrocarbon and hydrogen-containing fuels such as methanol, ethanol, gasoline, alkaline, or other aliphatic or aromatic hydrocarbons.

As shown inFIG. 1, fuel cell power system100includes a fuel processor112for catalytically reacting a reformable hydrocarbon fuel stream114, and water stream116in the form of steam from a water vaporizer178and water reservoir146. Fuel stream114is regulated by control valve172and water stream116is regulated by control valve176. In some fuel processors, air is also used in a combination partial oxidation/steam reforming reaction. In this case, fuel processor112also receives an air stream118. Air stream118is regulated by control valve174. Fuel processor112contains one or more reactors wherein reformable hydrocarbon fuel in stream114undergoes dissociation in the presence of steam in stream116and air in stream118to produce hydrogen-containing reformate exhausted from fuel processor112in reformate stream120. Fuel processor112typically also includes one or more downstream reactors, such as water-gas shift (WGS) and/or preferential oxidizer (PrOx) reactors for reducing the level of carbon monoxide in reformate stream120to acceptable levels, for example, below 20 ppm. H2-containing reformate120is fed to the anode chamber of fuel cell stack system122. As should also be apparent, an oxidant having greater than about 25 weight percent oxygen is, in some embodiments, fed or provided in stream124in the place of air.

As used herein, “water” means water that, in compositional nature, is useful for operation of a fuel cell power system. While certain particulates are acceptable in generally available water, they might cause plugging in addition to plugging caused by particulates in the oxidant gas. Therefore, as should be apparent, the water used must be appropriately cleaned before being introduced into the fuel cell power system.

The hydrogen of reformate stream120and the oxygen of oxidant stream124react in fuel cell stack system122to produce electricity. Anode exhaust (or effluent)126from the anode side of fuel cell stack system122contains some unreacted hydrogen. Cathode exhaust (or effluent)128from the cathode side of fuel cell stack system122may contain some unreacted oxygen. These unreacted gases represent additional energy for recovery in combustor130, in the form of thermal energy, for various heat requirements within power system100, such as heating of vaporizer178to vaporize water and also for heating and vaporizing fuel114. A hydrocarbon fuel132and/or anode effluent126are combusted, catalytically or thermally, in combustor130with oxygen provided to combustor130either from air in stream134or from cathode effluent stream128, depending on power system100operating conditions. In one embodiment, fuel132and fuel114are provided from the same fuel source. Combustor130discharges exhaust stream154to the environment, and the heat generated thereby is used in vaporizer178and fuel processor112as needed.

In one embodiment, energy store186buffers electricity184from fuel cell122to a consuming system of the generated electrical power. Energy store186may include a battery in one embodiment or, in an alternative embodiment, an ultra-capacitor.

Control module164controls control valves172,174, and176, and also energy input regulator180in response to one or more control signals including a temperature signal from temperature indicator170, a current demand signal from an accelerator pedal188operated by a human driver in a vehicle embodiment, and a current signal from current sensor182(or electrical power sensor182) associated with the fuel cell122. In one embodiment, energy input regulator180is essentially a control valve controlling fuel132to combustor (heater)130. In an alternative embodiment, energy input regulator180is a local controller having affiliated control elements and sensors for specifically controlling combustor130. In this latter embodiment, control module164inputs a set point signal for the level of energy that should be output from combustor130for input to vaporizer178. Process control166(also denoted as “software” and/or “executable logic” and/or an “executable program” as a data schema holding data and/or formulae information and/or program execution instructions) is provided in control module164for controlling operation of power system100. In one embodiment, computer164and process control166are provided as an ASIC (application-specific integrated circuit).

Fuel cell power system100may be stationary or may be an auxiliary power system in a vehicle. In a preferred embodiment, however, fuel cell power system100powers a vehicle such as a passenger car, truck, or van.FIG. 2andFIG. 3present a vehicle220to illustrate components of such a vehicular system100in electrochemical propulsion system210in vehicle220. Electrochemical propulsion system210is positioned in front compartment266of vehicle220and supported on frame rails290. Drive system248transmits mechanical power from electric drive motor250to provide traction power for vehicle220.

Fuel cell122generates electricity from individual bipolar fuel cell plates238to at least one electric drive motor250operatively connected to front vehicle wheels252via front axle256. In an alternative embodiment, motor250drives rear vehicle wheels254via rear axle258. Voltage converter272adjusts voltage in generated electricity for use in auxiliary vehicle components. Fuel114(such as, without limitation, gasoline, methanol, or diesel fuel) is stored in fuel tank222in rear underbody compartment224.

Thermal management system260includes heat exchanger262and adjacent cooling fan264positioned to dispute heat generated in this propulsion system210with cool incoming air at the forward end of front compartment266. Hydrogen-containing product of (optional) carbon monoxide reduction reactor232(such as water-gas shift (WGS) and/or preferential oxidizer (PrOx) reactors) used to reduce the level of carbon monoxide in reformate stream120to acceptable levels as previously referenced is delivered to (optional) cooler234and thence to fuel cell122. In an embodiment where cooler234is not needed, fuel cell122is downstream of reactor232.

Air generator240includes closely coupled air compressor242and optional cathode humidifier244to humidify fuel cell112oxidant. If used, humidifier244receives deionized water from water reservoir146. Air cooler247may be included as part of air generator240. Inlet air is provided through filter274as mounted to the inlet of compressor242. Exhaust154from combustor130is delivered to expander276, which powers compressor242.

Turning now to further detail in process control166,FIG. 4presents an overview for adjusting throughput of valves172,174,176and regulator180according to a predefined throughput ramp when the time derivative of the current demand deviates from zero by more than a threshold value to initiate a transient control mode. Once in this mode, step change adjustments are made in flows of fuel and water as well as the steam to carbon (S/C) and oxygen to carbon (O/C) ratios for the primary reactor (ATR) of the reformer112to adjust these flows in a predefined manner to account for this transient demand. In this regard, the throughput of one or more of valves172,174,176and regulator180may be promptly modified according to a predefined throughput ramp to provide more feed promptly to the reactor and so that the anode stoichiometry requirement is also promptly modified according to the predefined throughput ramp when the transient control mode is initiated.

As should be appreciated, the present invention contemplates initiation of the transient control mode when the rate of change in the current demand in either a positive direction (acceleration) or a negative direction (deceleration). Thus, a negative rate of change similarly adjusts throughput of control valves172,174,176and regulator180according to a second predefined throughput ramp when the rate of change of the current demand deviates from zero by more than a second threshold value.

Process control166proceeds from Start404to Measure Load Command408, which measures the position of accelerator pedal188. This measurement is stored in database420in the Store Load Command412operation. In the next operation, Compute Derivative416, a determination of the rate of change or time derivative of the recent measurements taken in step408is computed by using recent data from database420to yield a rate of change in the current demand (i.e., the current demand derivative or CDD). In this regard, the current demand derivative may be negative or positive in value. As previously noted, the current demand derivative may be computed as a time derivative of the position of accelerator pedal188, or it may be an estimated derivative or other rate of change indicator based on change of the current demand over a period of time. In one embodiment, a differential operational amplifier generates a signal representing the current demand derivative, Operation408measures input from the operational amplifier and the program immediately proceeds to Operation424. In another embodiment, a determination of the current demand derivative is taken from current (power measurement) sensor182as it measures electrical power delivered from fuel cell122.

After determination of the current demand derivative to fuel cell122, process control166proceeds to Decision424to ascertain if an offset step to adjust throughput of control valves172,174,176and regulator180is already active. If the transient control offset is active (i.e., YES), then process control166proceeds to determine if the current demand derivate exceeds a threshold value TV1at decision block428. Based on the outcome of decision block428, process control166will proceed to a stoichiometry (S/C and O/C) transient offset calculation at block430when the threshold value is exceeded or to deactivate the transient control offset at block436when the threshold value TV1is not exceeded428.

If the transient control offset is not active (i.e., NO), process control166proceeds to Decision432to determine if the current demand derivative exceeds a threshold value TV2. Based on the outcome of decision block432, process control166will proceed to the stoichiometry (S/C and O/C) transient offset calculation at block430when the threshold value is exceeded or to deactivate the transient control offset at block436when the threshold value TV1is not exceeded.

After accounting for the transient control offset, process control166operates to adjust the control valves172,174,176and regulator180in accordance with the computed control signals at blocks440,444,448,452respectively. One skilled in the art will recognize that the threshold values TV1or TV2for the current demand derivative may differ depending on the sign of the current demand derivative (i.e. a positive or negative derivative indicating an increasing or decreasing rate of change). In an alternative embodiment, a comparison of the absolute value of the current demand derivative my be used for a single threshold value.

The stoichiometry (S/C and O/C) transient offset calculation at block430is further detailed in flow chart500ofFIG. 5. Entry to Start502is from either Decision428or Decision432. Process control166proceeds from Start502to Block504where the control parameters of the current demand derivative are retrieved. These control parameters represent the state (activated, deactivated), direction (positive, negative) and the magnitude of the S/C and O/C transient offset.

Next, process control166retrieves the system parameters at block506to evaluate the need to modify the control parameters including the state, direction or magnitude of the offset. In this regard, various system parameters are compared to predetermined threshold values to determine if subsequent action should be taken. System parameters which represent the operating condition of system could include the power demand, the power output, the operating temperature of various components in the system, the hydrocarbon content of the effluent, amount others. These system parameters may be used independently or in any combination to arrive at suitable assessment of the operating condition of the system.

Based on this data, a revised S/C and O/C transient offset is calculated at block508which are used to adjust the control parameters at block510. These modified control parameters include state, direction and magnitude of the transient offset which are subsequently used to compute control signals for the system feed valves172,174,176and regulator180.

In modifying the control parameters, set points for valves172,174,176and regulator180may be adjusted by the process control166to effectively implement feed forward control in set point values. In this regard, set points for valves172,174,176and regulator180are modified according to respective predefined throughput steps with the step rates set to be sufficient for providing acceptable output voltage, carbon monoxide level in the effluent feed stream to the fuel cell, hydrocarbon concentration in the effluent feed stream to the fuel cell, and the like during the transient demand event. When the offset state is inactive (i.e., OFF), the fuel cell power system100is controlled according to a conventional feedback methodology appropriate for steady state operation. Process control166then returns from block512to control point438inFIG. 4at which set point adjustments are made to position the HC feed valve172, air feed valve174, H2O valve176and regulator180in accordance with the process control166.

Benefits from the feed forward control approach initiated upon identification of a load transient are further appreciated from comparative consideration ofFIG. 6andFIG. 7.FIG. 6presents plotted data600for a fuel cell power system100under traditional feedback control during a relatively slow up-transient respective to a moderate rate of change in the current demand on the fuel cell stack122. In this regard, plotted data600shows evidence of the three problems (stack voltage decline, carbon monoxide “spiking”, and temperature “spiking”) described previously.

Data600includes a time plot of current demand604(in amperage, on a scale of 0 to 1000 amps), reactor (ATR) temperature608(in degrees C., on a scale of 0 to 1000 degrees C.), average cell voltage612(in millivolts, on a scale of 0 to 1000 millivolts), minimum cell voltage616(the lowest voltage of any fuel cell in fuel cell stack122in millivolts, on a scale of 0 to 1000 millivolts), and stack carbon monoxide620(in parts per million, on a scale of 0 to 1000 ppm). Note the dramatic drop in average cell voltage612and minimum cell voltage616. Note also the large increase in carbon monoxide (to greater than 1000 ppm) entering the fuel cell stack. There is also a significant rise in ATR temperature608, approaching 900 degrees C.

FIG. 7presents plotted data700for the same fuel cell power system as forFIG. 6under feed forward control as described herein during essentially similar rate of change in the current demand as inFIG. 6. In this regard, plotted data700shows evidence of resolution of the three problems (stack voltage decline, carbon monoxide “spiking”, and temperature “spiking”) described previously respective to the fuel cell power system transient ofFIG. 6.

Plotted data700includes a time plot of current demand704, reactor (ATR) temperature708, average cell voltage712, minimum cell voltage716, and stack carbon monoxide720(all comparably scaled to the scale of data600inFIG. 6). Note the greatly reduced carbon monoxide concentration720entering the stack compared to carbon monoxide concentration620of data600, and the improved average cell voltage (voltage712as compared to voltage612, especially at about 40 seconds) and minimum cell voltages (voltage716as compared to voltage616, especially at about 40 seconds where the decline in616spikes downward to below 300 mV from above 600 mV and the decline in716has a more limited down spike to the 400 mV from about 600 mV). Additionally, the peak ATR temperature708is just over 800 degrees C. as compared to the peak ATR temperature608of 900 degrees C. in data600ofFIG. 6.

In a comparative summary between the feed forward approach of the preferred embodiments (during a transient) against a feedback approach, the feed forward approach provides for reduced levels of carbon monoxide to the fuel cell stack and accompanying higher cell voltage and stack efficiency. A potential further exists for reduced CO loading of precious metal catalyst with the fuel cell since carbon monoxide excursions are minimized. Reduced temperature excursions in the auto-thermal reactor provides efficacy respective to catalyst and reactor vessel durability. Significantly improved transient load response has also been demonstrated. System up-transient response also improved from approximately 160 seconds to 45 seconds after implementing the control changes according to the preferred embodiments.

Sizing of fuel cell power system100components and definition of predetermined ramp rates for handling up-transient response of reformer output during an increase in the current demand are iteratively interdependent. This interrelationship between component capacity and dynamic responsiveness is inherent in any engine design. In this regard, predetermined ramp rates are defined in existing embodiments to enable desired output conditions to be achieved. In new systems, desired responsiveness according to a predefined ramp rate for the current demand derivation is a criterion for design so that the fuel cell power system is engineered to provide the desired response.