Variable anode flow rate for fuel cell vehicle start-up

A fuel cell system is disclosed with a fuel cell stack having a plurality of fuel cells, the fuel cell stack including an anode supply manifold and an anode exhaust manifold, a sensor for measuring at least one of an environmental condition affecting the fuel cell stack and a characteristic of the fuel cell stack, wherein the sensor generates a sensor signal representing the measurement of the sensor; and a processor for receiving the sensor signal, analyzing the sensor signal, and controlling a flow rate of a fluid flowing into the anode supply manifold based upon the analysis of the sensor signal.

FIELD OF THE INVENTION

The invention relates to fuel cell systems. More particularly, the invention is directed to a fuel cell system and a method for facilitating a variable anode flow rate during a start-up of the fuel cell system.

BACKGROUND OF THE INVENTION

A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.

A typical fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrode-assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of generating a quantity of electricity sufficient to power a vehicle.

During periods of non-operation, a quantity of air accumulates in the anodes of the fuel cell stack. Upon start-up of the fuel cell stack, hydrogen is supplied to the anodes. The hydrogen contacts the air and creates a “hydrogen-air front” that passes over the anodes. The hydrogen-air front is known to degrade fuel cell performance. In particular, the presence of both hydrogen and air on the anode results in a localized short between a portion of the anode that sees hydrogen and a portion of the anode that sees air. The localized short causes a reversal of current flow and increases the cathode interfacial potential, resulting in a rapid corrosion of the fuel cell carbon substrates and catalyst supports. The rate of carbon corrosion has been found to be proportional to a time that the hydrogen-air front exists and a magnitude of the localized voltage at the hydrogen-air front.

It is known in the art to rapidly purge the anodes of the accumulated air with hydrogen and minimize the time that the hydrogen-air front exists on the anodes. The purge is often designed to substantially and evenly fill the anode inlet header with hydrogen without exhausting an excess of hydrogen from the fuel cell system. An illustrative purge method is disclosed in applicant's co-pending U.S. application Ser. No. 11/762,845, incorporated herein by reference in its entirety. Typically, a time required to purge the anodes is calculated in advance, and based on the volume of the fuel cell stack and the flow rate of the hydrogen. However, the quantity of air that has accumulated on the anodes varies with different shut-down periods and conditions. Additionally, variations in pressure, pressure measurements, flow rates, flow control and composition of the gases on the anodes after shut-down periods may vary widely. Therefore, the time required to push the accumulated air from the anodes, as well as the volume and flow rate of hydrogen for purging the anodes, is generally not optimized. As the optimal end point of the purge is often difficult to predict, systems known in the art have been unable to purge the anodes with hydrogen without exhausting an undesirable quantity of hydrogen to the atmosphere.

Additionally, known systems have also employed a dead-short circuit during start-up of the fuel cell stack. In dead-short systems, a circuit with a shorting resistor, for example, is used to minimize the localized voltage during start-up of the fuel cell stack. Resistance to carbon corrosion during start-up of the fuel cell stack is thereby optimized. In order for the dead-short system to work properly, however, each fuel cell in the fuel cell stack must have substantially equal quantities of hydrogen for the duration of the dead-short. A fuel cell that is deficient in hydrogen may experience undesirable, localized “hot-spots” if subjected to the dead-short.

There is a continuing need for a fuel cell system and a method that provide a rapid and reliable start-up. It would be desirable to develop a fuel cell system and a method for facilitating a variable anode flow rate during a start-up of the fuel cell system, wherein the fuel cell system and the method minimize an anode fill time, while also minimizing degradation of the fuel cell system due to a start-up procedure.

SUMMARY OF THE INVENTION

Concordant and consistent with the present invention, a fuel cell system and a method for facilitating a variable anode flow rate during a start-up of the fuel cell system, wherein the fuel cell system and the method minimize an anode fill time, while also minimizing degradation of the fuel cell system due to a start-up procedure, has surprisingly been discovered.

In one embodiment, a fuel cell system comprises: a fuel cell stack having a plurality of fuel cells, the fuel cell stack including an anode supply manifold and an anode exhaust manifold, a sensor for measuring at least one of an environmental condition affecting the fuel cell stack and a characteristic of the fuel cell stack, wherein the sensor generates a sensor signal representing a measurement of the sensor; and a processor for receiving the sensor signal, analyzing the sensor signal, and controlling a flow rate of a fluid flowing into the anode supply manifold based upon the analyzed sensor signal.

In another embodiment, a fuel cell system comprises: a fuel cell stack having a plurality of fuel cells, the fuel cell stack including an anode supply manifold and an anode exhaust manifold, a sensor for measuring at least one of a volume of fluid injected into the anode supply manifold, a voltage across at least one fuel cell, a current supplied from the fuel cell stack, a shut-down time period, and an environmental factor, wherein the sensor generates a sensor signal representing a measurement of the sensor; and a processor for receiving the sensor signal, analyzing the sensor signal, and controlling a flow rate of a fluid flowing into the anode supply manifold based upon the analyzed sensor signal.

The invention also provides methods for controlling a flow rate of a fluid in a fuel cell system.

One method comprises the steps of: providing a fuel cell stack having a plurality of fuel cells, the fuel cell stack including an anode supply manifold and an anode exhaust manifold, measuring at least one of an environmental condition affecting the fuel cell stack and a characteristic of the fuel cell stack; and controlling a flow rate of a fluid flowing into the anode supply manifold based upon an analysis of at least one of the environmental condition measured and the characteristic of the fuel cell stack measured.

FIG. 1illustrates a PEM fuel cell stack10according to the prior art. For simplicity, only a two-cell stack (i.e. one bipolar plate) is illustrated and described inFIG. 1, it being understood that a typical fuel cell stack will have many more such cells and bipolar plates. The fuel cell stack10includes a pair of membrane electrode assemblies (MEAs)12,14separated by an electrically conductive bipolar plate16. The MEAs12,14and the bipolar plate16are stacked between a pair of clamping plates18,20and a pair of unipolar end plates22,24. The clamping plates18,20are electrically insulated from the end plates22,24by a gasket or a dielectric coating (not shown). A working face26,28of each of the unipolar end plates22,24, as well as the working faces30,32of the bipolar plate16, include a plurality of grooves or channels34,36,38,40adapted to facilitate the flow of a fuel such as hydrogen and an oxidant such as oxygen therethrough. Nonconductive gaskets42,44,46,48provide seals and an electrical insulation between the components of the fuel cell stack10. Gas-permeable diffusion media50,52,54,56such as carbon or graphite diffusion papers substantially abut each of an anode face and a cathode face of the MEAs12,14. The end plates22,24are disposed adjacent the diffusion media50,56respectively. The bipolar plate16is disposed adjacent to the diffusion media52on the anode face of the MEA12and adjacent the diffusion media54on the cathode face of the MEA14.

As shown, each of the MEAs12,14, the bipolar plate16, the end plates22,24, and the gaskets42,44,46,48include a cathode supply aperture58, a cathode exhaust aperture60, a coolant supply aperture62, a coolant exhaust aperture64, an anode supply aperture66, and an anode exhaust aperture68. A cathode supply is formed by the alignment of adjacent cathode supply apertures58formed in the MEAs12,14, the bipolar plate16, the end plates22,24, and the gaskets42,44,46,48. A cathode exhaust manifold is formed by the alignment of adjacent cathode exhaust apertures60formed in the MEAs12,14, the bipolar plate16, the end plates22,24, and the gaskets42,44,46,48. A coolant supply manifold is formed by the alignment of adjacent coolant supply apertures62formed in the MEAs12,14, the bipolar plate16, the end plates22,24, and the gaskets42,44,46,48. A coolant exhaust manifold is formed by the alignment of adjacent coolant exhaust apertures64formed in the MEAs12,14, the bipolar plate16, the end plates22,24, and the gaskets42,44,46,48. An anode supply manifold is formed by the alignment of adjacent anode supply apertures66formed in the MEAs12,14, the bipolar plate16, the end plates22,24, and the gaskets42,44,46,48. An anode exhaust manifold is formed by the alignment of adjacent anode exhaust apertures68formed in the MEAs12,14, the bipolar plate16, the end plates22,24, and the gaskets42,44,46,48.

A hydrogen gas is supplied to the fuel cell stack10through the anode supply manifold via an anode inlet conduit70. An oxidant gas is supplied to the fuel cell stack10through the cathode supply manifold of the fuel cell stack10via a cathode inlet conduit72. An anode outlet conduit74and a cathode outlet conduit76are provided for the anode exhaust manifold and the cathode exhaust manifold, respectively. A coolant inlet conduit78and a coolant outlet conduit80are in fluid communication with the coolant supply manifold and the coolant exhaust manifold to provide a flow of a liquid coolant there through. It is understood that the configurations of the various inlets70,72,78and outlets74,76,80inFIG. 1are for the purpose of illustration, and other configurations may be chosen as desired.

FIG. 2shows an anode side of a fuel cell system100according to an embodiment of the invention. The fuel cell system100includes a fuel cell stack110having a plurality of fuel cells112. Each of the fuel cells112has an anode (not shown) and a cathode (not shown) with an electrolyte membrane (not shown) disposed therebetween. The fuel cell stack110further includes a first end114and a second end116. As described herein, the first end114is known as the “dry end” and the second end116is known as the “wet end.”

In the embodiment shown, the fuel cell system100includes an anode supply manifold118, an anode exhaust manifold120, a plurality of sensors122,124,126, a resistive device128, and a processor130. It is understood that additional components and systems may be included in the fuel cell system100such as a recycle sub-system, for example.

The anode supply manifold118is in communication with the anodes of the fuel cells112and provides fluid communication between a source of hydrogen132and the fuel cells112. It is understood that other fluid sources may be used such as nitrogen and air, for example. As shown, the anode supply manifold118receives a flow of gaseous hydrogen through an anode inlet conduit134from the source of hydrogen132. The anode inlet conduit134defines a volume between the source of hydrogen132and the anode supply manifold118. It is understood that the anode inlet conduit134may have any desired cross-sectional area and may further include a chamber, for example. As illustrated, the fuel cell system100includes a first valve136, also known as a purge valve, in fluid communication with the anode supply manifold118. The first valve136is disposed at the first end114of the fuel cell stack110, spaced from the anode inlet conduit134. The first valve136includes an inlet138for receiving a fluid flow and an outlet140for exhausting a fluid when the first valve136is in an open position.

The anode exhaust manifold120of the fuel cell system100provides fluid communication between the anodes of the plurality of fuel cells112and an exhaust system142. The anode exhaust manifold120receives the fluid flowing through the anodes of the fuel cells112. The fluid caused to flow through the anodes may be gaseous hydrogen, air, or water. A second valve144is in fluid communication with the anode exhaust manifold120and is disposed at the second end116of the fuel cell stack110. The second valve144facilitates purging or flushing of a fluid from the anode exhaust manifold120. It is understood that the second valve144may be disposed at the first end114of the fuel cell stack110, if desired. It is further understood that the second valve144may flush fluid to a cathode inlet (not shown), for example. Specifically, the second valve144includes an inlet146for receiving a fluid flow and an outlet148for exhausting a fluid when the second valve144is in an open position.

The sensors122,124,126provide a means to measure characteristics of the fuel cell system100and a surrounding environment. Specifically, at least one of the sensors122,124,126is adapted to measure a voltage across at least one of the fuel cells112. Another one of the sensors122,124,126is adapted to measure a characteristic of the fluid flowing into the anode supply manifold118such as a cumulative volume of the fluid, for example. Another one of the sensors122,124,126is adapted to measure an environmental characteristic affecting the fuel cell stack110. As a non-limiting example, the environmental characteristic is one of a temperature, a time period, a composition of a fluid flowing through the anode side of the fuel cell stack110, an age of the fuel cell stack110, and a pressure level at various points in the fuel cell system100. It is understood that other characteristics and system parameters may be measured such as a current flowing through the resistive (ohmic) device128, for example. It is further understood that each of the sensors122,124,126is adapted to transmit a sensor signal to the processor130, wherein the sensor signal represents the measurement data of an associated one of the sensors122,124,126.

The resistive device128is in electrical communication with the fuel cell stack110. The resistive device128may be adjustable to place a desired resistive load on the fuel cell stack110. However, a skilled artisan should understand that other suitable resistive loads may be used as desired. In an illustrative embodiment, the resistive device128is adapted to place a resistive load on the fuel cell stack110during startup, thereby limiting cell potential and militating against fuel cell degradation induced by carbon corrosion. As a non-limiting example, the resistive device128is coupled to a plurality of terminals (not shown) of the fuel cell stack110, the resistive device128adapted to short the fuel cell stack110as desired.

The processor130illustrated is in communication with the sensors122,124,126, the source of hydrogen132, the first valve136, and the second valve144. As such, the processor130is adapted to receive each of the sensor signals transmitted from the sensors122,124,126, analyze the sensor signals, and control a flow rate of a fluid flowing into the anode supply manifold118in response to the analysis of the sensor signals. It is understood that the processor130may control a flow rate of a fluid flowing into the anode supply manifold118by controlling an open/closed position of the valves136,144. It is further understood that the processor130may directly control a flow of fluid from the source of hydrogen132by regulating an injector, for example.

As shown, the processor130analyzes and evaluates the sensor signals based upon an instruction set150. The instruction set150, which may be embodied within any computer readable medium, includes algorithms, formulas, and processor executable instructions for configuring the processor130to perform a variety of tasks. It is understood that the processor130may execute a variety functions such as controlling the functions of the sensors122,124,126.

In certain embodiments, the processor130may include a storage device152. The storage device152may be a single storage device or may be multiple storage devices. Furthermore, the storage device152may be a solid state storage system, a magnetic storage system, an optical storage system or any other suitable storage system or device. It is understood that the storage device152is adapted to store the instruction set150. Other data and information may be stored in the storage device152, as desired.

The processor130may further include a programmable component154. It is understood that the programmable component154may be in communication with any other component of the fuel cell system100such as the sensors122,124,126, for example. In certain embodiments, the programmable component154is adapted to manage and control processing functions of the processor130. Specifically, the programmable component154is adapted to control the analysis of the sensor signals. It is understood that the programmable component154may be adapted to store data and information on the storage device152, and retrieve data and information from the storage device152.

In use, gaseous hydrogen is supplied to the anode supply manifold118, thereby causing hydrogen to flow through the active areas of the anode portions of each of the fuel cells112, as shown inFIG. 3. As hydrogen flows through the active areas of the fuel cells112, air is purged from the fuel cells112and the anode exhaust manifold120through the second valve144(referred to as an anode fill or stack flush procedure).

In certain embodiments, the stack flush procedure may be executed following a header purge procedure. During the header purge procedure, the first valve136and the second valve144are closed and gaseous hydrogen is caused to flow from the hydrogen source132and into the anode inlet conduit134with no substantial amount of gaseous hydrogen flowing into the anode supply manifold118. Because the valves136,144are closed, a flow of hydrogen into the anode inlet conduit134causes the contents of the anode supply manifold118, typically air, to flow into the active area of the fuel cell stack110. During a pressure build step, the fluid pressure within the fuel cell stack110is increased by continuing to introduce hydrogen into the anode inlet conduit134. Once a desired pressure in the fuel cell stack110is reached, the first valve136is opened and the gaseous hydrogen flows into and through the anode supply manifold118. Since the active areas of the fuel cells112in the fuels cell stack110are pressurized, the gaseous hydrogen is caused to flow through the anode supply manifold118and to the first valve136, but is not permitted to flow into the active areas of the fuel cells112. Once the gaseous hydrogen has substantially filled the anode supply manifold118, the first valve136is closed. Next, the second valve144is opened, and the gaseous hydrogen is continuously supplied to the anode supply manifold118, thereby causing hydrogen to flow through the active areas of the anode portions of each of the fuel cells112.

It is understood that the flow rate of hydrogen through the active areas of the fuel cells112may be maximized in order to minimize the time any hydrogen-air fronts are present in the active area of the fuel cell stack110and to minimize start-up time. Specifically, favorable results have been achieved where a hydrogen flow rate is maximized and a voltage across the fuel cells112is minimized. As a non-limiting example, to achieve a low initial voltage across the fuel cells112for a standard start-up procedure, hydrogen injection into the anode supply manifold118is scheduled according to at least one of the following: a total volume of hydrogen injected into the anode subsystem (analogous to the location of the hydrogen/air front); a measured cell voltage, an elapsed time, and environmental factors.

Specifically, each of the sensors122,124,126, measure characteristics and levels at various positions in the fuel cell system100. Each of the sensors122,124,126, transmits the sensor signal representing the measured data and information to the processor130. Once received, the processor130analyzes the data and information represented by each of the sensor signals and controls the flow rate of the hydrogen gas flowing into the anode inlet conduit134. It is understood that the processor130may directly control the flow rate of the hydrogen gas by regulating an injector or supply control device. It is further understood that the processor130may control the flow rate of the hydrogen gas by regulating the open/close position of the valves136,144.

As a non-limiting example, as hydrogen gas flows into the anode inlet conduit134, the total volume of the hydrogen gas injected is measured by at least one of the sensors122,124,126and the processor130estimates the location of a hydrogen/air front based upon the measured volume. Accordingly, the injection flow rate is modified as the hydrogen/air front reaches a pre-determined location (e.g. an end of the fuel cells112).

As a further example, as hydrogen gas flows into the active areas of the fuel cells112, a range of cell voltages (between a minimum cell voltage and a maximum cell voltage) increases until the fuel cells112have excess hydrogen. Thus, at least one of the sensors122,124,126measures a voltage across at least one of the fuel cells112and the processor130regulates the hydrogen injection flow rate in response to the measured voltage. In certain embodiments, the hydrogen injection flow rate remains constant until a voltage peak is detected. Thereafter, the flow rate is modified.

As yet another example, as hydrogen flows into the active areas of the fuel cells112, an electric current supplied from the fuel cell stack110to the resistive load represents a function of an oxidation state of the anode and cathode electrodes. Because the anode is being filled with hydrogen, the anode current-carrying capability is increasing throughout the anode fill. Due to a lack of fresh air supplied to the cathode, the ability of the cathode to produce current is decreasing throughout the start up. When corrected for concentration cell current, the peak of the stack current measured during the start up will signify the end of anode starvation and the beginning of cathode starvation. The hydrogen injection flow rate remains constant until the current peak is detected, where it is then modified.

FIG. 4illustrates an anode side of a fuel cell system100′ according to another embodiment of the present invention similar to the fuel cell system100ofFIG. 2, except as described below. Structure repeated from the description ofFIG. 2includes the same reference numeral. Variations of structure shown inFIG. 2include the same reference numeral and a prime (′) symbol. As shown inFIG. 4, the fuel cell system100further includes a fuel injector156, a jet pump158, and a recycle loop160. It is understood that additional components and systems may be included in the fuel cell system100′, as desired.

The fuel injector156and the jet pump158are disposed between the source of hydrogen132and the anode inlet conduit134′. The injector156and the jet pump158provide control over the flow of hydrogen into the anode inlet conduit134′. It is understood that additional components may be in communication with the source of hydrogen132and the anode inlet conduit134′ such as a pressure regulator and recirculation pump, for example. Other components or systems may be in communication with the anode inlet conduit134′, as desired.

The anode exhaust manifold120of the fuel cell system100′ provides fluid communication between the anodes of the fuel cells112, and at least one of the exhaust system142and the recycle loop160. The anode exhaust manifold120receives a fluid flowing through the anodes of the fuel cells112. As a non-limiting example, the fluid caused to flow through the anodes may be gaseous hydrogen, air, or water. Other fluids may be used, as desired. In the embodiment shown, the fuel cell system100′ includes an anode exhaust conduit162in fluid communication with the anode exhaust manifold120. The anode exhaust conduit162defines a volume between the anode exhaust manifold120and at least one of the exhaust system142and the recycle loop160. It is understood that the anode exhaust conduit162may have any desired cross-sectional area and may further include a chamber, for example.

The recycle loop160provides fluid communication between the anode exhaust manifold120and the anode supply manifold118. In certain embodiments, the recycle loop160includes at least a portion of the anode inlet conduit134′, at least a portion of the anode exhaust conduit162, a recycle conduit164, and a water separator166. However, it is understood that other components may be included, as desired.

The recycle conduit164defines a volume between the anode inlet conduit134′ and the anode exhaust conduit162. It is understood that the recycle conduit164may have any desired cross-sectional area and may further include a chamber, for example. In the embodiment shown, the recycle conduit164is in fluid communication with the water separator166and the jet pump158. It is understood that the recycle conduit164may be in direct communication with at least one of the anode inlet conduit134′, the anode supply manifold118, the anode exhaust manifold120, and the anode exhaust conduit162. It is further understood that other components or systems may be disposed between the recycle conduit164and at least one of the anode inlet conduit134′, the anode supply manifold118, the anode exhaust manifold120, and the anode exhaust conduit162, as desired.

The water separator166is disposed between the anode exhaust conduit162and the recycle conduit164and is adapted to remove excess humidification or product water from the fluid received from the anode exhaust conduit162. Accordingly, the water separator166is in fluid communication with the second valve144. The second valve144drains or bleeds the product water collected in the water separator144. As shown, the second valve144is in further fluid communication with the exhaust system142and is adapted to drain the product water into the exhaust system142. However, it is understood that the second valve144may drain the product water into any system or device, as desired. It is further understood that additional valves and control devices may be included, as desired.

In use, gaseous hydrogen is supplied to the anode supply manifold118, thereby causing hydrogen to flow through the active areas of the anode portions of each of the fuel cells112. As hydrogen flows through the active areas of the fuel cells112, air is purged from the fuel cells112and the anode exhaust manifold120through the second valve144(referred to as an anode fill or stack flush procedure). It is understood that the flow rate of hydrogen through the active areas of the fuel cells may be maximized in order to minimize the time any hydrogen-air fronts are present in the active area of the fuel cell stack110. Specifically, favorable results have been achieved where a hydrogen flow rate is maximized and a voltage across the fuel cells112is maintained as low as possible.

As a non-limiting example, to achieve a low initial voltage across the fuel cells112for a standard start-up procedure, hydrogen injection into the anode supply manifold118is scheduled according to at least one of the following: a total volume of hydrogen injected into the anode subsystem (analogous to the location of the hydrogen/air front); a measured cell voltage, an elapsed time, and environmental factors.

Specifically, each of the sensors122,124,126, measure characteristics and levels at various positions in the fuel cell system100. Each of the sensors122,124,126, transmits the sensor signal representing the measured data and information to the processor130. Once received, the processor130analyzes the data and information represented by each of the sensor signals and controls the flow rate of the hydrogen gas flowing into the anode inlet conduit134. It is understood that the processor130may directly control the flow rate of the hydrogen gas by regulating the fuel injector156. It is further understood that the processor130may control the flow rate of the hydrogen gas by regulating the open/close position of the valves136,144.

Accordingly, the fuel cell system100,100′ and method for variable anode flow rate during a start-up procedure of the fuel cell system provide a basis to tune and modify the fuel cell start-up to maximize efficiency, durability, and reliability of the fuel cell system100,100′ from beginning-of-life to end-of-life. Specifically, the fuel cell system100,100′ and method minimize an anode fill time, while also minimizing degradation of the fuel cell system due to a start-up procedure

From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, make various changes and modifications to the invention to adapt it to various usages and conditions.