Fuel cell power system

Abstract

A fuel cell power system and a method of starting a fuel cell power system. The fuel cell power system includes a fuel cell system having one or more voltage outputs, one or more DC / DC converters each having an output and an input connectable to a voltage output of the fuel cell system, a DC voltage link connectable to the outputs of the one or more DC / DC converters, and a local load connectable to the DC voltage link. In the system, the local load is adapted to draw power from the fuel cell system to reduce the voltage of the fuel cell system.

Description

Technical Field

[0001] This invention relates to fuel cell power systems, and more specifically, to fuel cell power systems with a DC link. Background Technology

[0002] Fuel cells are increasingly being used to generate electricity for a variety of loads. In one application, the output voltage from the fuel cell system is fed to a DC link, and the voltage of the DC link is regulated to a certain range. The voltage of the DC link is typically further consumed by various loads using converters. The converters transform the voltage of the DC link for specific loads.

[0003] The DC link voltage is typically regulated by a boost DC / DC converter connected between the fuel cell output and the DC link. The fuel cell stack must be designed to operate below the rated system voltage, allowing the DC / DC converter to regulate the DC link voltage by increasing the fuel cell output voltage. The fuel cell output voltage is highest under open-circuit conditions and decreases with increasing load. Therefore, the DC / DC converter must be designed for both open-circuit voltage and full-load current, resulting in poor utilization of power conversion capacity. Consequently, the fuel cell output voltage decreases as the system load increases. The DC / DC converter is operated to maintain the DC link voltage within certain limits.

[0004] In large fuel cell systems with power ranges of several megawatts, fuel cell stacks are grouped together in specific fuel cell spaces. The output voltage is determined by stacking the fuel cells in series, specifically by stacking them on top of each other. Typically, there would be space to allow for more stacks in series, but voltage limitations prevent full utilization of the space height and require more floor space.

[0005] The above problem can be partially solved by using a buck-boost converter instead of a boost converter, but this requires more converter hardware or specific driver components. A boost converter can be configured with a conventional 6-pulse IGBT converter, but a buck-boost converter requires modified hardware. Summary of the Invention

[0006] The object of this invention is to provide a method and a system for implementing the method to solve the above-mentioned problems. This object is achieved by the method and system described below.

[0007] This invention is based on the concept of utilizing a local load during the startup phase of a fuel cell power system. This invention enables the connection of multiple fuel cell stacks in series and allows the fuel cells to operate at higher voltages during startup. When the fuel cell system starts up, the output voltage of the fuel cell system is initially directed to the local load. When the load increases to a level where the output voltage is below the rated system voltage, the fuel cell is connected to the main electrical system via a DC / DC converter. The DC / DC converter can be activated when the voltage drops below its rated voltage.

[0008] By using this invention, the number of power converters and cables in large fuel cell systems can be reduced. This invention will also enable more efficient packaging of fuel cell stacks and better utilization of available space. Attached Figure Description

[0009] In the following description, the invention will be given in more detail with reference to the accompanying drawings, in which:

[0010] Figure 1 A simplified structure of an embodiment of the present invention is shown;

[0011] Figure 2 and Figure 3 The polarization curves of the fuel cell stack are shown;

[0012] Figure 4 and Figure 5 It shows Figure 2 and Figure 3 The polarization curve with an operating range; and

[0013] Figure 6 An embodiment of the system of the present invention is shown. Detailed Implementation

[0014] Figure 1 A simplified circuit representing an embodiment of the present invention is shown. Figure 1 A fuel cell stack 11 is shown, with its output connected to a DC / DC converter 12. The converter is shown as including an inductor L, a diode D, a controllable switch S, and a capacitor C. Converter 12 is a boost converter, operable to increase the input voltage to a higher output voltage. The output of the DC / DC converter is connected to a main switch 13 to connect the converter's output voltage to a main load 15. The output of converter 12 also includes an auxiliary switch that can be used to connect the converter's output voltage to a local load.

[0015] By default, the main switch and auxiliary switch are operated such that either the main switch or the auxiliary switch is selectively turned on; that is, when one switch is controlled to be on, the other is simultaneously controlled to be in the off state. If an auxiliary load and its energy are required and used during operation, the auxiliary switch can be kept closed.

[0016] According to the present invention, a fuel cell power system includes a fuel cell system 11 having one or more voltage output terminals. Figure 1 In the example, the fuel cell system has a voltage output terminal. A fuel cell system refers to a system with multiple fuel cells connected in series. A fuel cell system may also include any auxiliary equipment that may be needed to operate the fuel cells in a manner that obtains electrical energy from them.

[0017] The fuel cell power system of the present invention further includes one or more DC-DC converters. Each DC-DC converter has an output terminal and an input terminal, and the input terminal can be connected to the voltage output terminal of the fuel cell system. The fuel cell system may include multiple voltage output terminals. The output terminals are formed by independent fuel cell stacks. Each DC-DC converter is connected to the voltage output terminal of the fuel cell system. Therefore, parallel DC-DC converters can be fed from parallel fuel cell stacks. Figure 1 In this embodiment, the DC / DC converter 12 is connected to the fuel cell stack 11, such that the voltage output terminal of the fuel cell system is connected to the input terminal of the DC / DC converter.

[0018] According to the present invention, the power system includes a DC voltage link that can be connected to the output of one or more DC / DC converters. Figure 1 In this implementation, DC voltage link 17 is shown connected to the output of the DC / DC converter. The capacitor C of converter 12 supports the voltage of the DC voltage link. In cases where the power system includes multiple DC / DC converters, the outputs of these converters are preferably connected to the same DC voltage link. In such a case, multiple converters feed power from individual fuel cell stacks to a common DC link.

[0019] Furthermore, the present invention includes a local load that can be connected to a DC voltage link. Figure 1 An embodiment is shown in which a local load can be connected to a DC voltage link using an auxiliary switch 14. This local load is adapted to draw power from the fuel cell system to reduce the voltage of the fuel cell system. In this invention, the local load can be used to load the fuel cell system to reduce its output voltage. When the current drawn from the fuel cell increases, the voltage at the fuel cell's output terminal decreases.

[0020] In the system of this invention, one or more DC / DC converters do not operate during the startup of the fuel cell system. It is well known that the output voltage of the fuel cell system is at its highest when there is zero current. That is, the open-circuit voltage of the fuel cell is the highest voltage obtainable from the fuel cell. When a load is applied to the fuel cell, i.e., when current is drawn from the fuel cell, the voltage drops considerably. When converter 12 is not operating during startup, the converter's switch S is not modulated. During startup... Figure 1 In a fuel cell system, a high voltage is generated at the input of the DC / DC converter. In known configurations, the fuel cell starts without load, reaches the open-circuit voltage, and once the open-circuit voltage is reached, a current path is formed through an additional switch. This operation in known configurations requires additional switching components. However, in this invention, current begins to flow through the inductor L and diode D and charges the capacitor C without causing the voltage to reach the open-circuit voltage value. As the voltage of the capacitor approaches the output voltage of the fuel cell stack, the current from the fuel cell stack decreases, increasing the output voltage of the fuel cell stack. Once the voltage of the capacitor approaches the rated voltage of the converter's switch S, an auxiliary load is connected to the system. The auxiliary load draws current from the fuel cell system, and therefore the output voltage of the fuel cell stack decreases. The auxiliary load can be a simple resistor sized such that when current from the fuel cell system flows through the resistor, the voltage of the capacitor, and therefore the voltage of the DC link, does not exceed the rated voltage of the system, and in particular, does not exceed the rated voltage of the controllable switch S.

[0021] When power from the fuel cell system is fed to a local load and the DC voltage is lower than the system's rated voltage, the DC / DC converter can be operated to increase the DC link voltage and control it to a desired level. Once the DC link voltage is regulated by one or more DC / DC converters, the main switch 15 and auxiliary switch 16 can be operated to connect the DC link voltage from the local load to the main load.

[0022] With this invention, the output voltage from a fuel cell system can be designed to be higher than the rated voltage of one or more converters connected to the fuel cell system.

[0023] Compared to conventional systems, more fuel cell stacks can be connected in series, resulting in a higher voltage. When the fuel cell system starts up, the fuel cell output voltage immediately increases to the open-circuit voltage (OCV), which can be higher than the rated system voltage. For example, the system voltage can be 1000VDC, while the fuel cell open-circuit voltage can be 1200VDC. At this point, the output voltage is connected to a local load capable of operating at a variable voltage up to the OCV level. This local load can be a simple resistor bank, a balancing component of the power plant, or some other load supplied directly or via a power converter.

[0024] When the local load begins to draw current from the fuel cell, the fuel cell's output voltage decreases according to the known polarization curve. Voltage and load current are correlated, such that under a given load, the output voltage has already decreased to a level below the rated system voltage. This load point is designed to be between 10% and 30% of the fuel cell system's maximum load. At this point, the fuel cell is connected to the main electrical system via a DC / DC converter, and the local load is disconnected.

[0025] Figure 2 Typical polarization curves are shown for a conventional system where the rated voltage 21 of the system is higher than the open-circuit voltage OCV of the fuel cell. Figure 2 In the diagram, the open-circuit voltage of the fuel cell stack is shown as approximately 950 volts. As can be seen, the output voltage of the fuel cell stack drops rapidly when current is drawn from the fuel cell. Typically, the minimum load for continuous operation of the fuel cell is approximately 20% to 30% of the rated load. At this point, the fuel cell output voltage has already dropped to approximately 80% of the open-circuit voltage. Furthermore, at full load, the output voltage is shown as approximately 65% ​​of the device's open-circuit voltage and rated voltage. As mentioned, in known designs, the electrical system has been designed to operate at a voltage equal to the open-circuit voltage.

[0026] Figure 3 The design achievable through this invention and its polarization curves are shown. The fuel cell stack has a higher polarization ratio than... Figure 2 The example shows an open-circuit voltage OCV that is higher than the open-circuit voltage OCV. However, the rated voltage of the system components 31, and especially the rated voltage of one or more converters, is different. Figure 2 The rated voltage is the same in the examples. Therefore, in Figure 3 In the example, the open-circuit voltage of the fuel cell stack is higher than the rated voltage of one or more converters connected to the output of the fuel cell stack. Since the power from the fuel cell stack is connected to a local load when the voltage from the fuel cell stack is higher than the rated voltage of the system, such a design is feasible by the present invention.

[0027] In designs achievable through this invention, the voltage under minimum load and Figure 2 The open-circuit voltages in the examples are at the same level. Furthermore, at full load, the fuel cell output voltage is approximately 80% of the rated voltage of one or more converters. The rated power of a fuel cell system can be increased with design specifications. Increasing the rated power using similar converters can significantly reduce losses in the system.

[0028] Figure 4 and Figure 5 It shows Figure 2 and Figure 3 The polarization curve. In Figure 4 and Figure 5 The additional vertical lines indicate the limits of normal operation, which are approximately 20% of the rated load. When using a fuel cell, the fuel cell load is maintained above 20% of the rated load. When the fuel cell load is below 20%, the fuel cell is in the start-up region. Using the fuel cell in this region is not feasible, and operation is primarily carried out at loads exceeding 20%.

[0029] Figure 4 The diagram also shows horizontal lines representing the system's rated voltage, and more particularly, the rated voltages of one or more DC / DC converters. From Figure 4 As can be seen, because the load of the fuel cell system affects the output voltage of the fuel cell, one or more DC / DC converters do not operate close to their rated voltage.

[0030] Figure 5 It also shows, as Figure 4 The corresponding horizontal and vertical lines are shown in the diagram. It can be seen that when the system is designed as permitted by this invention, the difference between the rated voltage and the operating voltage is much smaller. When operating close to the rated voltage, more power can be obtained from the system, and operation can be performed with reduced losses.

[0031] Figure 6 An embodiment of the system of the present invention is shown. In this embodiment, two fuel cell systems 51A and 51B are employed. Both fuel cell systems are shown as consisting of three fuel cell stacks 42, 43, and 44. Fuel cell systems 51A and 51B are parallel systems and have similar structures and components. Therefore, only one of these parallel systems, A, is considered here.

[0032] The voltage output of the fuel cell stack is connected to the DC / DC converter 41A. In this example, the boost DC / DC converter is formed by a three-phase inverter bridge with six controllable switching components and their anti-parallel diodes. A three-phase inverter bridge is chosen as an example here because they are typically available in a single module. However, if desired, the DC / DC converter used in this invention can also be formed from discrete components.

[0033] when Figure 6 When the fuel cell power system is started, the fuel cell begins to generate voltage. The fuel cell and its associated support equipment are started, and a current path is closed using a charging resistor CR1 (i.e., a DC charging resistor). As mentioned above, in known systems, the system voltage reaches the open-circuit voltage before the current path is closed. Once the charging resistor is closed, current begins to flow in the circuit. Since parallel current paths are similar, the operation is described here primarily with reference to a single fuel cell stack 42. The electrical parameters and power design of the charging resistor are related to the number of parallel DC / DC converters installed in the system.

[0034] Current flows from the fuel cell stack 42 through inductor HDCL to the DC / DC converter 41A. Figure 6 As can be seen, in a fuel cell system 51A, the positive output terminals of the fuel cell stack are connected together before the inductor HDCL. Current from the fuel cell flows through the inductor and the anti-parallel diode of the upper switch. For example, the anti-parallel diode of the switching component 45 is forward biased, and current can flow through the diode. The positive rail 47+ of the DC voltage link 47 is located in the current path after the diode. The current further flows through the capacitor bank 46 to the negative rail 47- of the DC voltage link. The capacitor bank is charged by this current, and the voltage across the capacitor bank is the voltage of the DC link. When a charging resistor is connected, the current further flows back to the negative terminal of the fuel cell through the charging resistor and the return line. The charging resistor is designed to keep the current value within an acceptable range so as not to trigger overcurrent protection.

[0035] The capacitor bank 46 and the DC link are charged toward the output voltage of the fuel cell stack, without operating the controllable switch of the DC / DC converter. As the DC link voltage approaches the output voltage of the fuel cell stack, the charging current decreases. This decrease in current also implies an increase in the fuel cell output voltage. When the DC link voltage reaches a specific preset value, a local load is connected, increasing the load on the fuel cell stack and reducing the output voltage from the fuel cell.

[0036] Figure 6A load resistor LR1 is included, which can be connected across the DC voltage link to create an additional load, causing the fuel cell stack's output voltage to drop below a desired limit. The load resistor LR1 acts as a local load and applies load to the fuel cell, allowing the DC / DC converter to start safely. After the load resistor has been in the circuit for a short time, the voltage has dropped to a safe level, and the DC / DC converter can start. Simultaneously, the charging resistor can be removed from the current path by opening the current path and closing another switch that forms a lower-ohmic current path. Furthermore, when the DC / DC converter is operated to control the DC link voltage, power can be fed to the main load 49, and the load resistor can be removed by opening the corresponding contactor or switch. Such a main load is not shown in the figures. However, it is clear that power from the fuel cell power system is fed to the consumer. In this example, the DC link extends to the main load via a contactor.

[0037] Figure 6 Another local load 48 is shown, which is formed by the converter and, in particular, by the inverter bridge. The output of the inverter bridge is shown connected to the resistor BR. The inverter bridge is connected to the DC voltage link and can be used to reduce the voltage of the fuel cell stack by feeding power to the resistor BR. Thus, instead of or in addition to the load resistor, the inverter bridge is also used to reduce the voltage of the fuel cell stack so that the DC / DC converter can be started.

[0038] According to the implementation, the rated voltage of the inverter bridge used as a local load is higher than the rated voltage of the DC / DC converter. When the rated voltage is higher than the open-circuit voltage of the fuel cell stack, the DC link voltage may initially be charged to a higher level. Then, when the inverter bridge is started, it draws power to allow the DC / DC converter to be safely started.

[0039] The advantage of using an inverter bridge as a local load is that it can be used to reduce the DC link voltage even when power is being fed to the main load 49. This can be advantageous in cases of sudden changes in system load. Furthermore, for example, when connecting a fuel cell power system to the main load, the inverter bridge as a local load can be used to control the DC link voltage level to match the main load voltage. Additionally, when increasing the number of fuel cells connected in parallel, the inverter bridge can be used to absorb voltage fluctuations caused by sudden changes and maintain the voltage at a desired level. The inverter bridge can also be controlled using a power reference, meaning a power value can be provided as a reference value to the inverter bridge. For example, when the power reference is 20% of the rated power, the system operates as described above. Figure 5 Within the described normal operating range.

[0040] exist Figure 6 In the example, the output of the inverter used as a local load is shown as a group of resistors. However, it is obvious, for example, that power can be fed to a motor or a grid-connected / off-grid converter instead of to resistors.

[0041] exist Figure 6 In the example, one or more DC / DC converters are formed by an inductor HDCL, an anti-parallel diode in the upper switching component, a lower switching component, and a capacitor. The resulting circuit corresponds to... Figure 1 The circuit is shown in the simplified structure. The upper switching component refers to the switching component connected to the positive rail 47+ of the DC link, and the corresponding lower switching component refers to the switching component connected to the negative rail 47-. The topology of the DC / DC converter is typical of a boost converter. Figure 6 Six boost converters are shown connected in parallel and feeding power to the same DC link.

[0042] It will be apparent to those skilled in the art that the inventive concept can be implemented in various ways as technology advances. The invention and its embodiments are not limited to the examples described above, but can be varied within the scope of the claims.

Claims

1. A fuel cell power system, comprising: A fuel cell system having one or more voltage output terminals; At least one DC / DC converter having an output terminal, an input terminal and a controllable switch, wherein the input terminal is connectable to the voltage output terminal of the fuel cell system; A DC voltage link, which can be connected to the output of the at least one DC / DC converter; as well as A local load, which can be connected to the DC voltage link, wherein, The local load is adapted to draw power from the fuel cell system to reduce the voltage of the fuel cell system. Wherein, the open-circuit voltage of the fuel cell system is higher than the rated voltage of the at least one DC / DC converter, and the fuel cell power system is adapted to: Before starting the at least one DC / DC converter, the fuel cell system is charged to the DC voltage link through the charging resistor and diode of the at least one DC / DC converter; Determine the voltage of the DC voltage link; When the voltage of the DC voltage link exceeds a set limit, the local load is connected to the DC voltage link to reduce the output voltage of the fuel cell system; and The operation of starting the at least one DC / DC converter by operating the controllable switch after the voltage of the DC voltage link drops to a safe level ensures safe startup of the at least one DC / DC converter.

2. The fuel cell power system according to claim 1, wherein, The local load is a resistor that can be connected between the positive and negative rails of the DC voltage link.

3. The fuel cell power system according to claim 1, wherein, The local load is a converter circuit, the input of which is connected between the positive and negative rails of the DC voltage link.

4. The fuel cell power system according to claim 1, wherein, The local load is a combination of a converter circuit and a load resistor connected between the positive and negative rails of the DC voltage link.

5. The fuel cell power system according to claim 3, wherein, The converter circuit is an inverter.

6. The fuel cell power system according to claim 5, wherein, The converter is adapted to feed power to the load.

7. The fuel cell power system according to claim 3, wherein, The rated voltage of the converter circuit is higher than the open-circuit voltage of the fuel cell system.

8. A method for starting a fuel cell power system according to claim 1, the method comprising: Before starting the at least one DC / DC converter, the fuel cell system is charged to the DC voltage link through the charging resistor and diode of the at least one DC / DC converter; Determine the voltage of the DC voltage link; When the voltage of the DC voltage link exceeds a set limit, a local load is connected to the DC voltage link to reduce the output voltage of the fuel cell system; as well as The operation of starting the at least one DC / DC converter by operating the controllable switch after the voltage of the DC voltage link drops to a safe level ensures safe startup of the at least one DC / DC converter.

9. The method according to claim 8, wherein, Connecting the local load to the DC voltage link includes: Connect the load resistor between the positive and negative rails of the DC voltage link.

10. The method according to claim 8, wherein, Connecting the local load to the DC voltage link includes: The converter circuit is operated, the input of which is connected to the positive and negative rails of the DC voltage link.

11. The method according to claim 10, wherein, Operating the converter circuit includes: Provide a power reference for the converter; The converter is operated to feed power to a load connected to the output of the converter according to the power reference.

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