METHOD OF STARTING UP FUEL CELL STACKS FROM FREEZING TEMPERATURES

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/908,374, filed March 27, 2007, where this provisional application is incorporated herein by reference in their entireties.

BACKGROUND

Field

The present invention relates to fuel cell stacks, and more specifically, to methods of starting up a fuel cell stack from subzero temperatures.

Description of the Related Art

Fuel cells, such as proton exchange membrane (PEM) fuel cells, are currently being developed for a variety of applications, such as automotive, backup power, and stationary applications. For polymer electrolyte membrane fuel cells, the preferred operating temperature range for PEM fuel cells is typically between 5O ⁰ C to 12O ⁰ C. Under normal conditions, the fuel cell stack can be started in a reasonable amount of time and quickly brought to the preferred operating temperature. However, at low temperatures, for example, at subzero temperatures, fuel cells typically take a longer time to start up and produce the required power output. Furthermore, the amount of water in the fuel cell stack before starting up, which varies for different shutdown conditions and may be unpredictable, will have an effect on the time to start up, particularly when starting up from temperatures below the freezing temperature of water.

Most current methods to start up fuel cell stacks from freezing conditions are based on heating up the fuel cell stack as quickly as possible. In some methods, heaters may be provided in the fuel cells and/or around the fuel cell stack to promote heating as the fuel cell stack is starting up, or may be used to prevent the fuel

cell stack from freezing even when it is not operating. In other methods, the fuel cell stack may be driven to as low a voltage as allowable (e.g., by drawing a high load without drawing useable power from the fuel cell stack) so that the heat generated from the fuel cell stack can be used to increase the temperature of the stack as quickly as possible.

One start up method is discussed in Japanese Patent Application No. 2005-71626. A determination is made of whether or not the measured cell temperature is at the freezing point or below. When the measured cell temperature is at the freezing point or below, a low temperature startup mode starts. When the cell voltage reaches 0.1 to 0.2 V, drawing of current from the fuel cell starts. When the cell voltage exceeds the specified voltage range, the drawn current is set at a maximum within the range allowed by the system (i.e., within a range not going below a given voltage). By doing this, the power generation reaction increases, so it is possible to promote self-heating of the fuel cell. A determination is then made of whether or not it is possible for refreezing of the generated water to occur. When it is determined that refreezing is not possible, the start up control ends and the process shifts to the normal operation mode. However, this method requires that self-heating continues until the temperature of the fuel cell exceeds the freezing temperature, which may lead to reduced fuel cell performance after start up due to excessive water generation. This performance loss may be recovered by performing drying procedures, such as the one described in U.S. Patent No. 6,709,777. However, such performance recovery methods typically require extra time to start up the fuel cell stack and/or additional fuel cell system components to carry out the methods, which is undesirable for commercial applications, in particular, automotive applications. Accordingly, there remains a need in the art to improve the starting up time of fuel cells and fuel cell stacks from temperatures below the freezing temperature of water. This invention addresses this problem and provides further related advantages.

BRIEF SUMMARY

In one embodiment, the invention relates to a method of commencing operation of an electrochemical fuel cell stack from temperatures below the freezing temperature of water, the method comprising the steps of: supplying reactants to the fuel cell stack; measuring at least one fuel cell characteristic representative of a fuel cell stack voltage; applying a load across the fuel cell stack so that the fuel cell stack voltage is maintained within a predetermined minimum fuel cell stack voltage range for a heating duration; determining a magnitude of the load upon expiry of the heating duration; and determining at least one of an operating parameter of the fuel cell stack and fuel cell system component based on the determined load.

In some embodiments, the heating duration expires when it reaches a predetermined heating duration. In other embodiments, the method further includes determining a load during the heating duration, wherein the heating duration expires if the determined load reaches a threshold load. The operating parameter(s) of the fuel cell stack and fuel cell system component may be further determined based on the heating duration, if the determined load reaches a threshold load.

In yet other embodiments, a fuel cell system is provided comprising a fuel cell stack and a control device, wherein the control device is configured to: determine a temperature of the fuel cell stack; apply a load across the fuel cell stack so that the fuel cell stack voltage is maintained within a predetermined minimum fuel cell stack voltage range for a heating duration, if the temperature of the fuel cell stack is below freezing temperatures; determine a magnitude of the load upon expiry of the heating duration; and determine at least one of an operating parameter of the fuel cell stack and a fuel cell system component based on the magnitude of the load. These and other aspects of the invention will be evident in view of the attached figures and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the figures are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn

to scale, and some of these elements are arbitrarily enlarged and positioned to improve figure legibility. Further, the particular shapes of the elements, as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the figures. Figure 1 shows a flow chart of a start up method according to one embodiment of the present invention.

Figure 2 shows a flow chart of a start up method according to another embodiment of the present invention.

DETAILED DESCRIPTION In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.

Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as "comprises" and "comprising" are to be construed in an open, inclusive sense, that is as "including but not limited to". Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

As mentioned before, most current methods for starting up a fuel cell stack from freezing conditions are based on heating up the fuel cell stack as quickly as possible, for example, by using heaters and/or driving the stack voltage as low as allowable while drawing little or no useable power from the fuel cell stack, to self-heat

the stack. (As used herein, "useable power" includes the power supplied to the external load as well as the power supplied to the fuel cell system components.) In situations where the fuel cell stack is subjected to low voltage operation for an extended period of time, damage to the fuel cell components may occur and/or hydrogen may be produced, which is a safety concern. As a result, in some applications, batteries may be used, alone or in combination with the fuel cell stack, to power the system auxiliary components required for start-up. These components may include various pumps and compressors in the fuel cell system, as well as heater(s) that may be in or around the fuel cell stack. However, at low temperatures, energy consumption from the battery is desirably kept as low as possible because the power available from the battery decreases with decreasing temperature. As a result, it is preferable to control the self- heating operation to minimize or prevent the aforementioned undesirable events from occurring while successfully starting up the fuel cell stack from freezing conditions within an acceptable start time. In this context, the U.S. Department of Energy targets a start up time of 30 seconds to 90% full power from -2O ⁰ C for automotive fuel cell systems by 2010.

Thus, the present invention is related to a method of starting up a fuel cell stack from temperatures below the freezing temperature of water and includes a self-heating phase and a start-up phase, (collectively, the "start-up time"). During the self-heating phase, a self-heating load is drawn for a threshold heating duration. At the end of the threshold heating duration, the magnitude of the load is determined and at least one operating parameter of the fuel cell stack and/or fuel cell system is selected based on the determined magnitude of the load. The start-up phase then commences and the operating parameter(s) is varied based on the selection. Figure 1 is a flow chart showing various steps according to one embodiment of the present method of commencing operation of an electrochemical fuel cell stack from temperatures below the freezing temperature of water. At step 102, a load request is made for starting up the fuel cell stack, which may be a signal requested by the user, and at step 104, the temperature of the fuel cell stack, T ₅ , is determined. If T _s is greater than freezing temperatures, the start-up proceeds according to a "normal" start-up protocol. Methods for starting up fuel cell stacks from temperatures greater

than freezing temperatures are well known and persons skilled in the art can readily select suitable start-up methods for a given fuel cell stack and/or system architecture.

However, if T ₅ is equal to or less than the freezing temperature of water, then a freeze-start protocol is employed. Reactants, such as a hydrogen-containing fuel and air, are supplied to the fuel cell stack at step 110. The temperatures, humidities, pressures, and flow rates of the reactants may be selected based on the temperature of the fuel cell stack or may be predetermined for all temperatures below freezing.

At step 112, the fuel cell stack voltage, V ₅ , is measured. Alternatively, other fuel cell characteristics representative of the stack voltage may be measured, such as the individual cell voltages or groups of cell voltages, and V _s determined therefrom.

At step 114, the self-heating phase begins. A load is applied across the fuel cell stack so that the measured V _s decreases from a first voltage (e.g., the open circuit voltage) to a predetermined allowable minimum fuel cell stack voltage, V _m j _n . V _m i _n may be determined based on a number of factors. In some cases, V _m j _n may be about OV to produce as much heat as possible. However, the desired operating voltage of the fuel cell system components required during the start-up may limit the value of V _m i _n . For example, the air compressor and/or the coolant pump may have desired minimum operating voltages that range from, for example, about 170V to about 240V. Therefore, in other embodiments, V _m j _n may be set to the minimum operating voltage of a fuel cell system component.

During the self-heating phase, V _s is maintained at about V _m j _n for a predetermined self-heating duration, Hp, so that heat may be produced for an extended period of time. In some embodiments, Hp may be selected based on the temperature of the stack and/or shutdown conditions. Alternatively, Hp may be determined based on the acceptable start-up time. In some examples, Hp may be 10 seconds, 30 seconds, or 60 seconds. One skilled in the art will appreciate that V _s may fluctuate slightly during the self-heating phase, for example, by as much as 20 mV/cell.

At the end of the self-heating phase at step 116, the magnitude of the load drawn, L _end , is determined. At step 118, at least one operating parameter of the fuel cell stack and/or fuel cell system component(s) is determined based on L _end - Suitable operating parameters of the fuel cell stack include a supply temperature, a

supply pressure, a supply flow rate, and a supply humidity of at least one of the fuel, oxidant, and coolant streams; and useable power drawn from the fuel cell stack;; as well as power supplied to any of the fuel cell system components, as described in the foregoing. Other examples of suitable operating parameters include a load ramp rate, a minimum stack voltage, and a coolant flow rate. For example, operation of the coolant pump may be commenced or the coolant flow rate may be increased or decreased, depending on Lend- In some embodiments, the operating parameter(s) may be determined by comparing L _end to a predetermined load. In other embodiments, a plurality of predetermined loads and operating parameters may be stored in a look-up table or other suitable media, and L _en a is compared to the predetermined loads to determine the corresponding operating parameter(s).

At step 120, the start-up phase is commenced and the operating parameter(s) is varied based on the determination at step 118. Typically, fuel cell operation during the start-up phase is different from "normal" start up protocols. For "normal" start-up protocols, the entire stack is above freezing temperatures. However, for the start-up methods of the present invention, the temperature of at least a portion of the stack may still be below freezing temperatures during at least a portion of start-up phase. Thus, fuel cell operation during the start-up phase may require operating conditions that are different from "normal" start-up protocols.

At step 122, the start-up phase is complete and normal operation proceeds. In some embodiments, the start-up phase is complete when the fuel cell stack reaches 50% full power. In other embodiments, the start-up phase is complete when the fuel cell stack reaches 80% full power or 90% full power. It is desirable to be able to select different operating conditions for at least a portion of the start-up, for example, for the start-up phase, because the shut down conditions of the fuel cell stack prior to freezing may not be the same in all instances. For example, in the event of an emergency shut down where the fuel cell stack is not subjected to a "normal" shut down procedure, the fuel cell stack may contain more water than usual. In another example, changes in ambient humidity during shutdown may affect the hydration state of the fuel cells. However, it has been discovered that the

magnitude of the load at the end of the self-heating phase, L _end , can be correlated to the required start-up time of the fuel cell stack from freezing conditions to 50% full power, regardless of the shut down conditions of the fuel cell stack (e.g., wet or dry). It is anticipated that L _end may be used to determine the desired operating conditions of the start-up phase to improve the overall start-up time of the fuel cell stack and/or indicate whether different operating conditions may be necessary to successfully start up the stack from freezing conditions. As a result, rigorous control of the shutting down conditions, such as performing specific purges, or specific knowledge of the condition of the fuel cell stack and/or system on shutdown or immediately prior to start-up will likely be unnecessary to successfully start up the stack from freezing conditions. Furthermore, by separating the start-up into two phases, the self-heating phase and the start-up phase, the duration of the self-heating operation, may be reduced in comparison to prior art start-up methods.

For example, if L _end indicates that the fuel cell stack will likely start up within an acceptable start-up time and can handle a greater demand for power, the power drawn from the stack may be increased at a faster rate during the start-up phase to improve the start-up time and/or consume less energy from the battery. However, if L _end indicates that the fuel cell stack will likely not start up within the acceptable startup time, useable power drawn from the stack may be increased at a slower rate or commenced at a later time in the start-up phase, and/or the reactants may be supplied at particular conditions during the start-up phase. For example, if excess water is suspected to be in the fuel cell stack, the reactants can be supplied at drier conditions and/or high pressures, which may be determined based on L _end . In other examples, operation of a heating device and/or recovery procedures, such as those described in U.S. Patent Nos. 6,329,089 and 6,472,090, may be triggered during and/or after the start-up phase.

In another embodiment of the present invention, the method may further include monitoring the self-heating time and determining the magnitude of the load drawn during the self-heating phase, as shown at step 214 of Figure 2. (As used herein and in the appended claims, "determining the magnitude of the load drawn during the self-heating phase" may include determining the magnitude of the load continuously or

intermittently.) At step 216, the self-heating phase is terminated when the monitored load reaches a threshold load, L ₁ -, and/or the monitored self-heating time reaches a threshold heating duration, Hx. L _T and H _T may be determined empirically or may be dependent on the fuel cell stack temperature and/or shut down conditions, as well as the acceptable start-up time. Persons skilled in the art will be able to select a suitable L _T and H _T for a given fuel cell stack design and/or fuel cell system architecture.

At step 218, operating parameter(s) of the stack and/or system determined based on the magnitude of the load at termination of the self-heating phase, L _end , and the length of the self-heating time. In this embodiment, the duration of the self-heating phase may be shortened in some situations because the self-heating phase terminates when the magnitude of the monitored load reaches L _T , without the self-heating time necessarily reaching H _T . AS a result, the self-heating phase may end earlier, thus commencing the start-up phase sooner, and thereby starting up the fuel cell stack more quickly. For example, if the magnitude of the load reaches L _T before the self-heating time reaches H _T , it gives an indication that the starting up conditions are favorable and the fuel cell stack will likely start up within the acceptable start-up time. The self-heating phase may then be terminated, even if the temperature of at least a portion of the stack may still be at freezing conditions, and the start-up phase is commenced. In any of the above embodiments, an energy device, such as a battery or supercapacitor bank, may be used alone or in combination with the fuel cell stack during the self-heating phase to heat up the stack faster. In one example, power from the energy device may be used to power fuel cell system auxiliary components during the self-heating phase, thus allowing V _m i _n to be less than the required minimum operating voltages of the balance of plant components. In another example, a heating device may be powered by the energy device to help heat up the stack during at least a portion of the self-heating phase.

Furthermore, in any of the above embodiments, a coolant may be circulated during at least a portion of the self-heating phase and/or start-up phase to evenly distribute the heat in the fuel cell and to prevent over-heating in localized regions of the fuel cells.

The method of the present invention may be implemented by a controller in the fuel cell system communicatively coupled to receive signals from various sensors, and/or to control the states of the reactants (pressure, flow rates, temperature), and the various pumps, compressors, and the like in the fuel cell system. For example, the controller may receive signals when the temperature of the fuel cell stack is below freezing temperatures, indicating that a self-heating operation is desired. The controller then sends command signals that cause a load to be applied for a heating duration, while maintaining the stack voltage at about the minimum stack voltage, so that heat is generated. The controller then receives signals indicative of the magnitude of the load upon expiry of the heating duration, and determines an operating parameter of the fuel cell stack and/or system, such as those discussed above, based on the magnitude of the load. The controller then sends command signals that specify at least one operating parameter of the fuel cell stack and/or system component for the start-up phase, and sends command signals that terminate the start-up phase when the useable power drawn from the fuel cell stack reaches a threshold, for example, 50%, 80%, or 90% full power. The controller may take a variety of forms such as microprocessors, microcontrollers, application-specific integrated circuits (ASIC), and/or digital signal processors (DSP), with or without associated memory structures such as read only memory (ROM) and/or random access memory (RAM). In some embodiments, the controller may be configured to store a predetermined load and self-heating duration, and/or a plurality of loads and self-heating durations in the form of a look-up table, for example.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.

While particular elements, embodiments, and applications of the present invention have been shown and described, it will be understood that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings.