WO2005036684A2

WO2005036684A2 - Power electronics for fuel cell power system

Info

Publication number: WO2005036684A2
Application number: PCT/US2004/033407
Authority: WO
Inventors: Jonathan R. Leehey; Lei Xia
Original assignee: Nuvera Fuel Cells, Inc.
Priority date: 2003-10-10
Filing date: 2004-10-08
Publication date: 2005-04-21
Also published as: WO2005036684A3

Abstract

This invention relates to a power conditioning system for connecting a fuel cell power module (FCPM) to an AC power grid. The power conditioning system comprises a one-stage bi-directional DC/AC inverter and an unregulated DC bus between the inverter and the FCPM. During fuel cell stack start up, FCPM parasitics such as pumps, blowers, compressors, etc. may be powered by the grid via the bidirectional inverter. After the FCPM is at its functional power level, it directly provides the power source for the fuel cell power module parasitics. The system is more efficient than prior arrangements.

Description

POWER ELECTRONICS FOR FUEL CELL POWER SYSTEM

RELATED APPLICATION This application claims the benefit of U.S. Provisional Application No. 60/510,321, filed October 10, 2003, the entire teachings of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Fuel cells are well-known electrochemical devices that produce direct current electrical power as a result of the chemical reaction of hydrogen and air across a selective membrane in the fuel cell. A fuel cell produces a low voltage, usually about 1 volt or less, at relatively high current. To produce a higher voltage output, several fuel cells are often connected in series, forming a fuel cell "stack." The voltage produced by a fuel cell may vary with the operating conditions and the load. For example, the voltage across the stack decreases as the power drawn from the stack increases, at constant fuel flow rate. If the fuel flow to a fuel cell stack is decreased, the cell stack will produce less power, which likewise will reduce stack voltage at constant current draw. The native current output of a fuel cell or stack is direct current. A fuel cell power module, as the term is used in this application, comprises a fuel cell stack or stacks, and supporting infrastructure (e.g., hydrogen supply, provision of air and cooling, controls, etc). Ancillary equipment is provided to allow interfacing of the fuel cell power module (FCPM) with an electric grid. The ancillary equipment has to solve two problems. The first problem is that most grid connection applications require alternating current electrical power (AC). In such a case, an inverter may be used to convert DC power to AC power. Inverters are known devices. It is the function of an inverter to accept, and if required manage and condition, DC electrical power supplied by the FCPM, and convert it to AC electrical power. The AC power is then distributed to a utility grid or directly to an AC load. Inverters are described in more detail below. A second problem is that the voltage produced by the stack will vary as a function of current draw. This makes the interfacing of the DC source and the grid significantly more complex. Standard practice in the fuel cell industry, as described in more detail below, has involved connecting a fuel cell stack to a direct current electrical power bus, and then via a DC/DC converter to another DC bus, thereby providing a regulated DC voltage output, while either compensating for fuel cell voltage fluctuations, or regulating the fuel cell output voltage. (See, for example, US 6,428,918, and US 5,714,874). These references do not directly concern connection to an AC grid. Connection of a fuel cell to an AC grid requires conversion of DC to AC, via a DC/ AC inverter. The simplest approach is to take regulated DC power from a DC/DC converter, as described above, and feed it to an inverter. Such an approach is illustrated in, for example, an article from Electronic Design News (EDN), July 6, 2000, on pages 48, 50 and 52. Such systems can be made from commercially available converters and inverters, but have certain inefficiencies. In particular, they cannot readily accept power from the grid at fuel cell startup, because DC/DC converters are typically not bi-directional. It is possible to build a "reversible" DC/DC converter, which effectively amounts to two DC/DC converters sharing common connections, one operating in the opposite direction to the other, but it is expensive. Moreover, efficiency is decreased, and cost increased, by having two stages of conversion. Hence, in current practice DC and AC power for startup operation of the fuel cell are typically supplied from the grid-connected side of the inverter through a separate DC/AC converter or power supply. However, this is not efficient in normal operation. We describe herein an innovative way of providing a bidirectional power connection between a fuel cell stack and its parasitics, and an electric grid, which significantly reduces cost and volume, and which may be more efficient in operation.

SUMMARY OF THE INVENTION In one aspect, the invention provides a method for providing a conditioned power flow between the electrical power output of a fuel cell power module and an alternating current electric grid, the method comprising selecting parasitic loads for the fuel cell power module that can be supplied by unregulated DC power, connecting such parasitic loads to an unregulated DC bus that is connected to said power module, and connecting said unregulated DC bus to a grid via a bidirectional inverter. The advantages of this configuration include the ability to supply AC loads with a minimal amount of conversion, during steady operation many DC loads can be supplied with no power conversion, and that by minimizing the conversion of power from DC to AC, or between voltages, the efficiency of the system is further increased. In another aspect, the invention comprises a system for connecting a fuel cell power module to an AC electric grid, the system comprising a fuel cell power module, a connection between the fuel cell power module and a bidirectional inverter; and a connection between the inverter and an AC electric grid. By use of a bidirectional inverter, the grid maybe used to generate DC voltage at startup, and also to power AC loads directly. In particular, DC-powered parasitic loads of the fuel cell power module are supplied by electricity at a point between the fuel cell power module and the inverter. The inverter may further comprise, or be accompanied by, an AC/ AC voltage transformer to match grid AC voltage requirements to fuel cell power system needs and outputs, and further to provide isolation where required. Moreover, the DC-powered parasitic loads are powered from the fuel cell power module during normal operation, and generally without requiring DC/DC voltage conversion. In another aspect, the invention describes a connection between a fuel cell power module and a utility grid in which there is no DC/DC converter between the outlet of the fuel cell power module and the connection to the grid. Instead, a bidirectional inverter serves as the principal power conversion system in line between a fuel cell power module and an AC grid. This minimizes equipment needs and improves efficiency. In any of these aspects, the system preferably comprises a system controller, which can regulate the operation of at least the fuel cell power module and the inverter. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Figure. 1 is a schematic illustration of a prior art system for connecting a fuel cell power module (FCPM) to an electric grid; Figure 2 is a schematic illustration of a power-converting system of the invention; and Figure 3 illustrates an alternative embodiment of the system of Figure 2.

DETAILED DESCRIPTION OF THE INVENTION A description of preferred embodiments of the invention follows. In this application, the term "hydrogen" is used to include both hydrogen gas

(H₂) and other hydrogen-containing gasses suitable for use in a fuel cell. Likewise, "air" includes any oxygen-containing gas suitable for use fuel in a fuel cell. A "grid" denotes an interconnected network for carrying AC electric power, as typified by the grids of electric utilities. However, a grid maybe more localized, such as within a neighborhood, or within a factory complex. An inverter is a device that converts direct current (DC) electrical power to alternating current (AC) electrical power. ("AC" and "DC" have their conventional electrical power meanings throughout this application). There are several types of inverter available, and the term "inverter" is used in the literature to describe devices of varying characteristics. As used herein, the term "bidirectional inverter" refers to a device that performs DC to AC conversion, without regulating of DC voltage levels, unless otherwise stated. Such a device does not contain a DC/DC voltage converter, and only contains an AC/AC transformer if so noted. Such an inverter is capable of "bidirectional" operation, i.e., not only does it supply an AC output when fed a DC input, but it can also supply DC voltage when supplied by an AC source, such as a grid. Such an inverter is also by definition a "single stage" inverter, i.e., there is only one stage of DC/ AC inversion or conversion in the device. In contrast, many standard, "off the shelf inverter systems comprise both an inverter and a DC/DC converter. The DC/DC converter section supplies a "regulated" level of DC voltage (i.e., constant output voltage over a range of input voltage) to the inverter section. Such devices often comprise two stages of DC/AC inversion, and are not bidirectional. This invention relates to an improved method of power conditioning for a fuel cell power module (FCPM) operatively connected to an electric grid. To illustrate the improvement, a schematic diagram of a prior art system for connecting a FCPM with a grid is shown in Figure 1. The system comprises a FCPM 9, connected by a DC line 10 to a switch or contactor 11 between the FCPM and a second DC line 12, which leads to a DC/DC converter 15. The DC/DC converter 15 produces a DC output that is regulated to provide DC current at a regulated voltage, via DC line 16, to an inverter 17. This type of inverter 17 is not bi-directional when connected to the DC/DC converter 15. The combination of a DC/DC converter 15 and inverter 17, as a pair of devices or as a single integrated device, serves only to supply AC power through an AC line 19. AC power is supplied to the grid 22 through a second contactor 21 connected via line 20 to the grid 22. Also attached to the grid section via an AC connection 23 are AC loads 26. These are those parasitic loads of the FCPM that require AC current. They are illustrated as being connected to the AC line 20, but can have a more complex connection (not illustrated) that allows the AC loads 26 to be supplied from the grid when the FCPM is not operating, and directly from the inverter when it is. There is optionally an AC/AC transformer (not illustrated) between the grid and the AC loads 26. One of the AC loads is a power supply (AC/DC converter) 24 that supports power-consuming DC equipment ("DC parasitics") in the FCPM. The power supply 24 supports DC-using parasitics via DC line 25, schematically described here as high power parasitics 27 and lower power, voltage-regulated parasitics 28. A battery and charger, if provided (not illustrated), could be connected to regulated DC line 16 or to unregulated DC line 12. The advantage of the configuration of Figure 1 is primarily in the ease of acquisition of the required parts, which is important for prototyping and small-scale production. For example, DC/DC converter 15 and inverter 17 can be obtained as an integrated single unit from stock. The disadvantage of the configuration of Figure 1 is inefficiency. The imposition of both a DC/DC converter and an inverter between the FCPM and the grid carries an efficiency penalty of several percent. Moreover, the need to supply the DC parasitics through a further set of converters, downstream of the inverter, adds further inefficiencies to the system. It should be noted that the DC parasitics, which include pumps and compressors, typically consume a significant fraction of the FCPS gross output, for example up to about 25%. Hence, the loss through extra conversion devices to obtain DC current for these devices is significant in terms of system efficiency. A schematic of a power-converting system of the invention is shown in Figure 2. This system offers improved efficiency, and is less complex than the system shown in Fig. 1. Numbering of parts identical to those in Fig. 1 is the same. The FCPM 9 is connected by a first line 10 and first contactor 11 to a second line 12 that serves as an unregulated DC bus. The fuel cell parasitics, represented by 27 and 28 as above, are connected via line 25 to line 14 and line 12. The devices requiring a regulated voltage are fed through a DC/DC converter 28, and devices 27 that do not require regulated voltage can be fed directly from line 14 via line 25 without intermediate conversion. In contrast to line 16 of Fig. 1, lines 12, 14 and 25 of Fig. 2 have a fluctuating voltage. Line 14 connects to a single-stage, bidirectional inverter 18, which may further comprise, or be connected to, an optional but preferred associated AC/AC transformer 13, which supplies power to (or accepts power from) the grid 22 via AC line 19, second contactor 21 and AC line 20. The AC-powered loads 26 associated with the FCPM are connected to line 20 as shown, or less preferably to line 19, or via a switchable arrangement (not illustrated) as described for Fig. 1. This mode of supplying AC loads is equivalent between this embodiment and the prior art. This embodiment differs from the prior art in that the DC loads are not supplied from AC line 20 or 23, but are powered directly from the fuel cell power module during normal operation. In addition, during startup, the DC parasitics of the FCPM can be supplied by the grid 22 via the bidirectional inverter 18. The pumps and blowers supplied via line 27, which are typically the largest parasitics, can be- driven by the inverter's DC output into line 14 with no additional power conversion. In the improved power conversion system of Figure 2, no DC/DC converter is required for the supply of energy to the grid. A smaller converter 28 is used for specific parasitic loads to provide a regulated DC voltage. The bi-directional single stage inverter 18 is an inherently reversible device, as is optional transformer 13, allowing energy to flow from the utility grid to the FCPM, or from the FCPM to the grid. This is made possible in this arrangement, in contrast to the prior art system of Fig. 1, because there is no DC/DC converter in the main power supply line between the FCPM and the grid. The absence of a DC/DC converter in the main power supply line between the FCPM and grid means that this connection can be made bidirectional (i.e. power can flow from the FCPM to the grid, and vice versa), and also improves the overall efficiency of power flow between these components. To the extent any DC/DC converter(s) are used, they are located on a side branch, and not on the main power bus between the FCPM and the grid. This simplification allows the connection of the fuel cell power module DC parasitics 27 and 28 to a bus 25 connected directly to the fuel cell power module via contactor 11. During startup operation, the parasitics are powered from the grid 22 via the inverter 18. The grid AC is first shifted to the proper voltage by the transformer 13, if required, and then is converted to DC by the inverter 18. The contactor 11, leading to the FCPM, is typically open in this mode, isolating the fuel cell. After the fuel cell is operating, contactor 11 is closed, and the parasitics are powered directly by the fuel cell via lines 12 and 25, using power that has not been converted wherever unregulated power can be used. With appropriate controllers, the major parasitics, such as pumps, blowers, and/or compressors, can be powered in this manner. This amounts to about a 15% increase in efficiency in these uses, since the losses that would have been incurred in the DC/DC converter, the inverter, and the power supply of Fig 1 (about 5% each) are avoided. Moreover, the power flows from the FCPM 9 to the grid 22 with only one stage 18 of power conversion, which can increase the efficiency of AC production by about 5%. Other system components are advantageously connected to the unregulated DC line 14 of Figure 2. Some of these are illustrated in Figure 3. A battery 33 and its charger 32, a DC/DC converter, may be connected via line 31 to the DC bus 26 to serve as an alternative source of electricity for the parasitics, for example during startup. The battery 33 may also serve as a back up source of electricity in case of grid unavailability, and may provide supplemental electrical power in addition to the power provided by the utility grid, or act as a buffer during transients. If the bus is required to power an application-specific device requiring regulated DC power, this can be supplied via a line 34 from the unregulated DC bus 14 to a dedicated converter 35 that supplies application-specific loads 36, which could be for example the power for a telephone substation, as illustrated here. Figure 3 also shows schematically some possible control elements of the system. These include a serial or other system bus 38, which connects via local controllers (e.g. 39, 40, 41, 42, 50) to key system components, and also to a system controller 51, typically a microprocessor or similar device to regulate the system. The system bus is preferably a robust high-speed bidirectional cornmunication network, such as a CAN (Control Area Network) type of network. Alternatives to CAN include Ethernet, RS232 and other bus systems. Possible connections from the bus 38 include connector 39 to the FCPM, connector 40 to the DC/DC converter (battery charger) 32, connector 41 to the power supply 35 for application-specific load, and connector 42 to the inverter 18. Also illustrated in Fig. 3 are further connection and control options, including a 24 VDC auxiliary voltage supply 43 from the inverter 18 to the fuel cell power module 9; a control line 44 for enablement of the inverter 18 by the FCPM 9; and control lines 45 and 46 to control the contactors 11 and 21. Line 47 allows the inverter to sense the current in line 12. In a fully integrated system, these functions will also be connected to the controller 51, either directly or via the bus 38. The use of a system controller and system network allows the activity of all system components to be monitored and controlled in a sophisticated way with minimal wiring. While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A method for providing a conditioned power flow between an electrical power output of a fuel cell power module and an alternating current electric grid, the method comprising: selecting parasitic loads for the fuel cell power module that can be supplied by unregulated direct current (DC) power; connecting the parasitic loads to an unregulated DC bus that is connected to the power module; and connecting said unregulated DC bus to the grid via a bidirectional inverter.

2. The method of Claim 1 wherein one or more DC parasitic loads of the power ■ module that require regulated DC power are connected to a DC/DC voltage converter that is supplied by the unregulated DC bus.

3. The method of Claim 2 wherein at least one of a battery and an application- specific function requiring regulated DC power is connected to a DC/DC voltage converter that is supplied by the unregulated DC bus.

4. The method of Claim 1 wherein one or more AC parasitic loads of the power module are powered directly from the grid, or from the connection of the grid to the bidirectional inverter.

5. The method of Claim 1 wherein the bidirectional inverter is connected to the power module via a path that does not include a DC/DC converter.

6. The method of Claim 1 wherein a transformer is associated with the bidirectional inverter.

7. The method Claim 1 , wherein during startup of the fuel cell power module, the DC-powered parasitic loads are powered from the grid via the bidirectional inverter.

8. The method of Claim 1 , wherein a system controller regulates the operation of at least the fuel cell power module and the bidirectional inverter.

9. The method of Claim 8 wherein the system controller is connected to the power module and to the bidirectional inverter via a local area network.

10. The method of Claim 8 wherein the system controller controls the opening and closing of contactors connecting the bidirectional inverter to the fuel cell power module and the grid.

11. A system for connecting a fuel cell power module to an alternating current (AC) electric grid, the system comprising: a fuel cell power module; a bidirectional inverter connected to the fuel cell by a first connection; and a second connection for connecting the bidirectional inverter to an AC electric grid.

12. The system of Claim 11, wherein one or more DC-powered parasitic loads of the fuel cell power module are powered from the first connection between the fuel cell power module and the bidirectional inverter.

13. The system of Claim 11 , wherein the bidirectional inverter further comprises an AC/ AC voltage transformer.

14. The system of Claim 11, further comprising one or more DC-powered parasitic loads of the fuel cell power module that are powered from the grid via the inverter during startup of the fuel cell power module.

15. The system Claims 11, further comprising one or more DC-powered parasitic loads of the fuel cell power module that are powered from the fuel cell power module during normal operation of the fuel cell power module.

16. The system of Claim 11, further comprising one or more AC-powered fuel cell power system parasitic loads that are powered from the second connection between the inverter and the grid.

17. The system Claim 11, further comprising one or more regulated DC loads that are powered from the first connection between the inverter and the fuel cell power module via a DC/DC converter.

18. The system Claim 11, wherein there is no DC/DC converter between the outlet of the fuel cell power module and the connection to the grid.

19. The system Claim 11, further comprising a system controller, wherein the system controller can regulate the operation of at least the fuel cell power module and the bidirectional inverter.

20 The use of a bidirectional inverter as the principal power conversion system in a conducting path between a fuel cell power module and an AC grid.

21. The use of Claim 20, wherein there is no DC/DC converter in the conducting patch between the grid and the power module.

22. The use of Claim 20, further comprising an AC/AC transformer between the inverter and the grid.