WO2008073119A1

WO2008073119A1 - Maintaining performance of a fuel cell power supply

Info

Publication number: WO2008073119A1
Application number: PCT/US2006/061840
Authority: WO
Inventors: Eric R. Strayer; Praveen Narasimhamurthy; Michael S. Billups
Original assignee: Utc Power Corporation
Priority date: 2006-12-11
Filing date: 2006-12-11
Publication date: 2008-06-19

Abstract

An exemplary method of monitoring the fuel cell includes operating the fuel cell in an ohmic loss region of performance of the fuel cell. At least one characteristic of the fuel cell performance operating in the ohmic loss region is determined. An exemplary power supply unit includes a fuel cell and a controller configured to cause the fuel cell to operate in an ohmic loss region of the fuel cell performance in the absence of the fuel cell providing power to a load. The controller is configured to determine at least one characteristic of the fuel cell performance operating in the ohmic loss region.

Description

MATNTATNTNG PERFORMANCE OF A FUEL CELL POWER

SUPPLY

BACKGROUND

[0001] Back-up power supply systems are well known and in widespread use. Typical arrangements include a backup power supply that provides power to a load (e.g., power to a building or complex) whenever a main power supply grid fails to deliver sufficient power. Different types of backup power supplies are known. Some include a fuel cell power plant that generates electricity in a known manner to be the source of backup power.

[0002] Back-up power supplies have to be reliable and available at all times to serve their intended purpose. When the backup power supply relics upon a fuel cell, there are several issues that must be addressed. For example, it is necessary to ensure that the fuel cell power plant is ready to operate at a needed level at any time. When a backup power supply remains dormant for an extended period, this can be challenging. Typical fuel cell power plant arrangements are subject to potential performance loss over time. Some forms of performance loss are even more of an issue when the fuel cell power plant remains dormant for extended periods while the main power supply grid is fully functional.

[0003] For example, ohmic loss can develop in a fuel cell power plant that degrades the performance of the assembly. Ohmic loss typically results in a loss of voltage output at a given current level. Detecting ohmic loss can be challenging because fuel cell assemblies typically operate at a level that corresponds to rated power of the fuel cell power plant that is generally outside the ohmic range. Performance loss in this region tends to be related to mass transfer issues and does not provide a good indication of any problems stemming from other issues.

[ooo4] There is a need for reliably detecting ohmic loss in a fuel cell. SUMMARY

[0005] An exemplary method of monitoring a fuel cell includes operating the fuel cell at a level corresponding to an IR region of the fuel cell performance when the fuel cell would otherwise be dormant because it is not needed as a main source of power. This allows for detecting when there has been ohmic loss.

[0006] In one example the fuel cell is part of a fuel cell power plant that is a backup source of power for situations when a main power grid does not provide adequate power to a load. Such an example includes operating the fuel cell in the IR region when the main power grid does provide adequate power. One example includes operating the fuel cell at a voltage that exceeds a voltage of the main power grid. In one example, the fuel provides part of the power to the load and the main power grid provides a remainder of the power to the load.

[0007] In one example, operating the fuel cell in the IR region includes generating power for charging an energy storage system such as start up batteries associated with the fuel cell.

[0008] In one example, when the fuel cell performance in the IR region is outside of a desired range, the method includes providing an indication that corrective action is needed. Examples of corrective action include further diagnostic procedures, a conditioning procedure or a maintenance procedure.

[0009] The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

BRIEF DESCRIPTION

[00010] Figure 1 schematically illustrates selected portions of an example power supply system that is useful with an embodiment of this invention.

[00011] Figure 2 is a flowchart diagram summarizing one example approach useful with an embodiment of this invention. [oooi2] Figure 3 graphically illustrates a performance of an example fuel cell operated according to an embodiment of this invention. DETAILED DESCRIPTION

[oooi3] Disclosed example embodiments allow for determining when there is ohmic loss that may adversely affect the performance of a fuel cell for situations like providing backup power to a load, for example. The example techniques provide the ability to detect ohmic loss so that corrective action can be taken to avoid having inadequate power delivered from a backup power supply unit including a fuel cell, for example.

[00014] Figure 1 schematically shows selected portions of a powering arrangement 20 for supplying electrical power to a load 22. A main power supply grid 24 provides electrical power to the load 22. The load 22 in one example represents the electrical power needs in a building. A backup power supply unit 26 provides electrical power to the load 22 whenever the main power supply grid 24 is unable to supply adequate power to the load 22. The backup power supply unit 26 may be used when there is a total failure of the main power supply grid 24 or when there is compromised performance of the grid 24 so that the load 22 receives adequate power.

[oooi5] The example backup power supply unit 26 includes a fuel cell 30 that generates electricity. The fuel cell 30 may comprise a single cathode and electrode arrangement or a plurality of them in a cell stack assembly. Various fuel cell types may be used. One example includes a PEM fuel cell. A controller 32 is configured to control and monitor operation of the backup power supply unit 26 and the fuel cell 30.

[00016] There will be times when the backup power supply unit 26 is not needed because the main power supply grid 24 is operating as intended. During such times it is possible for the fuel cell 30 to experience a drop in performance because of ohmic loss. Inactive fuel cells experience ohmic loss for known reasons. The illustrated example controller 32 is configured to operate the backup power supply unit 26 in a manner that allows for detecting and addressing any ohmic loss that may develop.

[oooi7] Figure 2 includes a flow chart diagram 40 that summarizes one example approach. At 42 a determination is made that the backup power supply unit is not in use to provide power to the load 22. During such a condition, the controller causes the backup power unit 26 to operate at 44 such that the fuel cell 30 operates in an ohmic loss region or IR region of performance for the fuel cell 30. [00018] Figure 3 includes a plot 50 of an example fuel cell performance. Such a plot may exhibit the performance of a single fuel cell or an entire cell stack assembly. This invention is equally applicable to either situation. In this example, the fuel cell performance is characterized by three distinct regions. A first region is shown at 52. This region corresponds to the activation polarization region of fuel cell operation. A second region, which is shown at 54, is the IR region or the ohmic loss region. The example controller 32 operates the example fuel cell 30 in this region for determining whether there has been any ohmic loss that will potentially affect the performance of the backup power supply unit 26. A third region is shown at 56, which is called the concentration polarization region. This region 56 is outside of the normal operating range 58 of the fuel cell 30.

[oooi9] During periods of inactivity of the backup power supply unit 26 when it is not needed for supplying power to the load 22, the controller 32 causes operation of the fuel cell 30 in the IR region 54. In one example, this is accomplished by using the fuel cell 30 to charge an energy storage system (not illustrated) such as start up batteries used for the backup power supply unit 26. Tn another example, the controller 32 causes the fuel cell 30 to provide power to the load 22 even when the main power supply grid 24 is providing power to the load 22. The controller 32 in one such example maintains an output voltage of the backup power supply unit 26 above that of the main power supply grid 24. In another example, the controller 32 uses a combination of these techniques.

[00020] The controller 32 is programmed with information about which voltage and current values on a VI curve 60 are within the IR region 54 for the fuel cell 30. The controller 32 selects an appropriate operating condition to ensure that the fuel cell 30 is operating in the IR region 54. During such operation, the controller determines whether the actual fuel cell performance is within a desired range of an expected or ideal performance. This is shown at 64 in Figure 2.

[00021] As can be appreciated from Figure 3, the VI curve 60 in the IR region 54 has a generally linear slope showing the expected or ideal relationship between the voltage and current values. When there is ohmic loss, the slope of the curve in the IR region changes as shown at 62, for example. At a given current value, there is a difference ΔV between the expected or ideal voltage from the curve 60 and the actual voltage on the curve 62. The controller 32 in one example is programmed to determine whether such a difference is outside of a predetermined acceptable range. When the difference ΔV is within an acceptable range, the controller continues monitoring the operation and performance until an unacceptable condition occurs.

[00022] In one example, the acceptable range for ΔV is based on when cell stack output falls below an input cutoff voltage of mating power electronics such as a DC/DC converter or an AC/DC converter. The exact range will vary based on a particular power plant configuration.

[00023] In another example, the controller determines whether the actual voltage value on the curve 62 for a given current value is within an acceptable range of the corresponding expected or ideal voltage from the curve 60. Those skilled in the art who have the benefit of this description will be able to determine what is an acceptable range for their particular situation.

[00024] When there is an undesirable deviation in performance within the IR region 54, the controller 32 provides an indication that some corrective action should be taken at 66. Examples of corrective action include performing further diagnosis of the fuel cell 30, performing a conditioning procedure to enhance the performance of the fuel cell 30 and performing a maintenance procedure to repair or replace the fuel cell 30 or an appropriate portion of the backup power supply unit 26.

[00025] One example conditioning procedure includes starting the fuel cell and running the stack to power the stack's own parasitic loads or application loads to regenerate water. This technique allows for rchydrating the stack. Another example conditioning procedure includes circulating water into the cell stack by initiating a coolant pump without running the fuel cell. Transport through the plate of the stack will rehydrate the membranes. Additionally, contaminants can be cleansed from the system using an inline demineralizer.

[00026] An example diagnostic procedure includes starting the fuel cell and running the stack to power the stack's own parasitic loads or applications loads. The diagnosis includes monitoring the stack voltage versus the input voltage.

[00027] An example maintenance procedure includes periodic operation of a coolant pump for hydrating the stack by circulating water into the stack.

[00028] There are different types of performance losses that may be detected by operating the fuel cell stack in the ohmic loss region. One type of loss is referred to as an activity loss and is independent of current density. An activity loss is characterized by an approximately constant loss (in mV) at all current levels. When there is such an activity loss, one example includes exposing the anodes of the cell stack to air at elevated temperatures in an attempt to oxidize contaminants on the anodes. Alternatively, reducing the cathodes to remove oxide build-up on the cathodes can be used.

[00029] Ohmic losses are proportional to current density. An ohmic loss is characterized by an approximately constant mV/Amps loss slope. When an ohmic loss is detected, circulating coolant water through the stack and a demineralizer bed can be used to assist in removing ionic contaminants from the stack. In some examples, replacing the demineralized bed will be useful for addressing ohmic losses. [00030] Another type of loss is the mass-transport loss that has an approximately exponential relationship with current density. Reducing the cathodes to remove oxide build-up on the cathodes is one technique for addressing a mass- transport loss. Another technique includes running the cell stack with elevated coolant vacuum pressure to help remove excess water from the electrodes, gas diffusion layers or both.

[00031] Determining the type of loss is accomplished in one example by determining the fuel cell stack performance at a plurality of points in the ohmic loss region. Determining how the loss changes with increasing current density allows for determining the type of loss. Once the type of loss has been identified, an appropriate one of the techniques mentioned above may be implemented depending on the needs of a particular situation.

[00032] Given this description, those skilled in the art will realize how often to use the example monitoring techniques for obtaining fuel cell performance information adequate for their particular needs.

[00033] There are several advantages associated with the disclosed examples. One is that the backup power supply unit 26 is more fully autonomous in that it is able to automatically diagnose a potential problem with performance over time. Another advantage is that the backup power supply unit 26 is more reliably available because potential performance degradation can be detected and addressed before it would interfere with actual use for powering the load 22.

[00034] The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this invention. The scope of legal protection given to this invention can only be determined by studying the following claims.

Claims

We claim: 1. A method of monitoring a fuel cell, comprising: operating the fuel cell in an ohmic loss region of performance of the fuel cell; and determining at least one characteristic of the fuel cell performance operating in the ohmic loss region.

2. The method of claim 1 , comprising determining whether the at least one determined characteristic indicates ohmic loss.

3. The method of claim 1, comprising operating the fuel cell in the ohmic loss region in the absence of the fuel cell providing power to an external load.

4. The method of claim 3, comprising providing power to the external load from the fuel cell; and maintaining a voltage level of the fuel cell above a voltage level of another source of power simultaneously providing power to the load.

5. The method of claim 1 , comprising determining whether an actual voltage level of the fuel cell operating in the ohmic loss region for a corresponding current level is within an acceptable range of an expected voltage level.

6. The method of claim 1, comprising determining a difference between an actual voltage level of the fuel cell operating in the ohmic loss region and an expected voltage level for a corresponding current level; and determining whether the determined difference is within an acceptable range.

7. The method of claim 1 , comprising operating the fuel cell in the ohmic loss region to charge an energy storage system.

8. The method of claim 1, comprising determining that actual fuel cell performance from operating in the ohmic loss region is indicative of ohmic loss; and providing an indication that corrective action is desired.

9. The method of claim 9, comprising performing at least one of a diagnostic procedure, a conditioning procedure or a maintenance procedure responsive to the provided indication.

10. A power supply unit, comprising: a fuel cell; and a controller configured to cause the fuel cell to operate in an ohmic loss region of the fuel cell performance in the absence of the fuel cell providing power to a load, the controller being configured to determine at least one characteristic of the fuel cell performance operating in the ohmic loss region.

11. The power supply unit of claim 10, wherein the controller is configured to determine whether the at least one determined characteristic indicates ohmic loss.

12. The power supply unit of claim 10, wherein the controller is configured to cause the fuel cell to provide power to the load and maintain a voltage level of the fuel cell above a voltage level of another source of power simultaneously providing power to the load.

13. The power supply unit of claim 10, wherein the controller is configured to determine whether an actual voltage level of the fuel cell operating in the ohmic loss region for a corresponding current level is within an acceptable range of an expected voltage level.

14. The power supply unit of claim 10, wherein the controller is configured to determine a difference between an actual voltage level of the fuel cell operating in the ohmic loss region for a corresponding current level and an expected voltage level and determine whether the determined difference is within an acceptable range.

15. The power supply unit of claim 10, wherein the controller is configured to operate the fuel cell to charge an energy storage system.

16. The power supply unit of claim 10, wherein the controller is configured to determine that actual fuel cell performance from operating in the ohmic loss region corresponds to ohmic loss and to provide an indication that corrective action is desired.