Hybrid fuel cell system for load following and backup in a microgrid, and its operating method.

Abstract

To provide direct current (DC) power sources, such as fuel cell systems with different types of fuel cells for managing variable load demand.SOLUTION: A method for operating a fuel cell system comprises: drawing a base-level DC electric energy from a first fuel cell 302 of a first type to a combined DC bus 312a; measuring a DC voltage at the combined DC bus 312a; determining whether the DC voltage at the combined DC bus 312a falls short of a DC voltage threshold; and drawing a variable DC electric energy from a second fuel cell 304 of a second type different from the first type in response to determining that the DC voltage at the combined DC bus 312a falls short of the DC voltage threshold.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a direct current (DC) power source such as a fuel cell system including different types of fuel cells for managing variable load demands.

Background Art

[0002] Electrical energy generator systems can support variable loads through various configurations. These electrical energy generator systems are expected to continuously vary their electrical energy output to maintain power quality for variable loads. Known energy generator systems rely on non-power generation type electrical energy storage systems such as batteries, combustion type energy generators such as diesel generators, and / or external electrical energy sources such as electrical utility grids to provide the electrical energy output for the variable portion of the load.

Brief Description of the Drawings

[0003] [Figure 1] FIG. 1 is a perspective view of a fuel cell system according to various embodiments suitable for implementing various embodiments. [Figure 2] FIG. 2 is a schematic side cross-sectional view of a hot box according to various embodiments suitable for implementing various embodiments. [Figure 3] FIG. 3 is a block diagram of components of a hybrid fuel cell system suitable for implementing various embodiments. [Figure 4A] FIG. 4A is a block diagram of components of a hybrid fuel cell system using hydrogen fuel suitable for implementing various embodiments. [Figure 4B] FIG. 4B is a block diagram of components of a hybrid fuel cell system using hydrogen fuel suitable for implementing various embodiments. [Figure 5A] FIG. 5A is a block diagram of components of a hybrid fuel cell system using hydrocarbon gas fuel suitable for implementing various embodiments. [Figure 5B]Figure 5B is a block diagram of the components of a hybrid fuel cell system using hydrocarbon gas fuel, suitable for implementing various embodiments. [Figure 5C] Figure 5C is a block diagram of the components of a hybrid fuel cell system using hydrocarbon gas fuel, suitable for implementing various embodiments. [Figure 5D] Figure 5D is a block diagram of the components of a hybrid fuel cell system using hydrocarbon gas fuel, suitable for implementing various embodiments. [Figure 5E] Figure 5E is a block diagram of the components of a hybrid fuel cell system using hydrocarbon gas fuel, suitable for implementing various embodiments. [Figure 5F] Figure 5F is a block diagram of the components of a hybrid fuel cell system using hydrocarbon gas fuel, suitable for implementing various embodiments. [Figure 5G] Figure 5G is a block diagram of the components of a hybrid fuel cell system using hydrocarbon gas fuel, suitable for implementing various embodiments. [Figure 6] Figure 6 is an example plot of load demand against time during load following for a grid-independent hybrid fuel cell system. [Figure 7] Figure 7 is an illustrative plot of load demand against time during load following for a microgrid-backed hybrid fuel cell system. [Figure 8] Figure 8 is an illustrative plot of load demand against time during load following for a microgrid-backed hybrid fuel cell system. [Figure 9] Figure 9 is a process flowchart illustrating load following and backup methods for hybrid fuel cell systems according to various embodiments. [Figure 10] Figure 10 is a process flowchart illustrating load following and backup methods for hybrid fuel cell systems according to various embodiments. [Overview of the project]

[0004] According to one embodiment, a method for operating a fuel cell system includes drawing base-level DC electrical energy from a first fuel cell of a first type to a coupled DC bus, measuring the DC voltage in the coupled DC bus, determining whether the DC voltage in the coupled DC bus is below a DC voltage threshold, and, in response to determining that the DC voltage in the coupled DC bus is below a DC voltage threshold, drawing variable DC electrical energy from a second fuel cell of a second type, different from the first type.

[0005] According to another embodiment, the fuel cell system comprises a first fuel cell of a first type, a second fuel cell of a second type different from the first type, a first DC / DC converter electrically connected to the first fuel cell via a first DC (direct current) bus, a second DC / DC converter electrically connected to the second fuel cell via a second DC bus, and a DC / AC (alternating current) inverter electrically connected in parallel to the first DC / DC converter and the second DC / DC converter via a coupled DC bus. [Modes for carrying out the invention]

[0006] Various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings to refer to the same or similar parts. References to specific examples and embodiments are for illustrative purposes only and are not intended to limit the scope of the claims.

[0007] As used herein, the terms “storage system” and “energy storage system” refer to any form of energy storage that can be used interchangeably and can be converted into electricity, such as an electrical storage unit, a mechanical storage unit, an electromechanical storage unit, an electrochemical storage unit, or a thermal storage unit. Examples include batteries, capacitors, supercapacitors, flywheels, liquid reservoirs, gas reservoirs, and the like. In some embodiments, the energy storage system may comprise any combination of components configured to control the electrical energy output of the energy storage system, such as an electrical connector and / or an electrical energy regulator, in response to signals from a controller and / or an electrical energy bus.

[0008] As used herein, the terms “electrical energy” and “electrical energy output” refer to quantities of voltage, current, or power. The examples given herein with respect to voltage do not limit the claims and the scope of this specification to such types of electrical energy and electrical energy output.

[0009] Various embodiments include electrical circuits, electrical components, and methods for load following and backup of a hybrid fuel cell system. A hybrid fuel cell system using multiple types of fuel cells having different characteristics suitable for different situations can be configured to control different fuel cells (e.g., a first fuel cell stack including a first type of fuel cell such as a solid oxide fuel cell, and a second fuel cell stack including a second type of fuel cell such as a proton exchange membrane fuel cell) in different ways. For example, a hybrid fuel cell system can be configured to control a first fuel cell (e.g., a first type of fuel cell stack) for a consistent electrical energy output, and a second fuel cell (e.g., a second type of fuel cell stack) for electrical energy output when load demand exceeds the consistent electrical energy output of the first fuel cell system. A hybrid fuel cell system can be configured to control a second fuel cell to follow variable load demand that exceeds the consistent electrical energy output of the first fuel cell.

[0010] Electrical energy generator systems can support variable loads through various configurations. These systems are expected to continuously vary their electrical energy output to maintain power quality for variable loads. Some known energy generator systems rely on non-generating electrical energy storage systems, such as batteries, to provide electrical energy output for the variable portion of the load. For example, an electrical energy generator system configured as a grid-isolated or parallelized microgrid is configured to support the variable portion of the load by using a non-generating electrical energy storage system. The non-generating electrical energy storage system outputs electrical energy in accordance with the variable portion of the load. Such electrical energy generator systems incorporating non-generating electrical energy storage systems have several problems, including the complexity of field design for optimizing the size of the electrical energy generator and the non-generating electrical energy storage unit, a lack of flexibility to intermittent overloads unless there is sufficient capacity margin, and poor round-trip efficiency of power from the non-generating electrical energy storage unit.

[0011] Other known electrical energy generator systems rely on combustion-type energy generators, such as diesel generators, to provide electrical energy output for the variable portion of the load. For example, an electrical energy generator system configured as a grid-isolated or parallelized microgrid is configured to support the variable portion of the load by using a combustion-type energy generator. The combustion-type energy generator outputs electrical energy in accordance with the variable portion of the load. Such electrical energy generator systems incorporating a combustion-type energy generator have problems with emissions generated by the combustion of fuel by the combustion-type energy generator.

[0012] Other known electrical energy generator systems rely on external electrical energy sources, such as the electric utility grid, to provide electrical energy output for the variable portion of the load. For example, an electrical energy generator system can be configured as a grid-parallelized microgrid that relies on the electric utility grid to support the variable portion of the load. However, in the event of an electric utility grid failure or emergency grid isolation event, the electrical energy generator system must rely on a non-generating electrical energy storage system or a combustion-type energy generator to support the variable portion of the load. During prolonged grid failures or isolation (e.g., several days to several weeks, or power outage events caused by wildfires or other weather events such as hurricanes), an electrical energy generator system with a non-generating electrical energy storage system is not economical from the standpoint of equipment and real estate costs. During prolonged grid failures or isolation, an electrical energy generator system with a combustion-type energy generator faces problems due to increased emissions from the combustion of fuel by the combustion-type energy generator over the extended duration.

[0013] Embodiments herein address the aforementioned problems associated with known electrical energy generator systems that use non-generating electrical energy storage systems and / or combustion-type energy generators to support variable portions of the load by implementing a hybrid fuel cell system using multiple types of fuel cells having different characteristics suited to different situations. The system can be configured to control different fuel cells in different ways. The inventors have found that solid oxide fuel cells (SOFCs) offer high efficiency and long life at constant power output and are therefore well-suited for base load applications in microgrids. In contrast, proton exchange membrane (PEM) fuel cells, due to their lower operating temperature, start up faster than SOFCs, have better load-following capabilities, and are well-suited for backup power and fluctuating loads in island mode microgrids. The hybrid fuel cell system can be configured to control SOFCs (e.g., SOFC stacks) for consistent electrical energy output and to control PEM fuel cells (e.g., PEM fuel cell stacks) for electrical energy output to meet load demands exceeding the consistent electrical energy output of SOFCs. Hybrid fuel cell systems can be configured to control PEM fuel cells to follow variable load demands that exceed the consistent electrical energy output of SOFCs. Hybrid fuel cell systems using both SOFCs and PEM fuel cells can utilize SOFCs to support the base portion of the load and PEM fuel cells to support the variable portion of the load, and can provide superior performance to known electrical energy generator systems that use non-generating electrical energy storage systems and / or combustion-type energy generators to support the variable portion of the load. Similarly, hybrid fuel cell systems that use SOFCs to support the base portion of the load and PEM fuel cells to support the variable portion of the load can provide superior performance to fuel cell systems that use SOFCs and PEM fuel cells separately.

[0014] The hybrid fuel cell system can be controlled such that the SOFC can provide a consistent electrical energy output, and the PEM fuel cell can provide an electrical energy output that follows variable load demands exceeding the consistent electrical energy output of the SOFC, enabling the hybrid fuel cell system to support variable loads in various grid-independent, grid-fault, or grid-isolation situations. For example, the hybrid fuel cell system can be controlled to support variable loads in grid-independent microgrid applications to enable variable load following and / or emergency load support. In another example, the hybrid fuel cell system can be controlled to support variable loads in grid-connected microgrid applications to enable peak shaving, grid-fault backup, and / or emergency load support.

[0015] The control of the hybrid fuel cell system can be based on a direct current (DC) voltage or a DC voltage in a combined DC bus configured to transmit DC electrical energy from both the SOFC and the PEM fuel cell. The combined DC bus can transmit DC electrical energy to a load. An AC (alternating current) / DC inverter can receive DC electrical energy from the combined DC bus, convert the DC electrical energy to AC electrical energy, and transmit the AC electrical energy to the load. The AC / DC inverter can be controlled to draw DC electrical energy from the PEM fuel cell in response to determining that the DC voltage in the combined DC bus is below a threshold. The AC / DC inverter can be controlled to increase or decrease the DC electrical energy drawn from the PEM fuel cell in response to determining whether the DC voltage in the combined DC bus meets a variable portion of the load demand of a load exceeding the threshold. The AC / DC inverter can be controlled to stop drawing DC electrical energy from the PEM fuel cell in response to determining that the DC voltage in the combined DC bus does not exceed the threshold.

[0016] FIG. 1 shows an example of an electrical energy generator including a modular fuel cell system fully described by U.S. Patent No. 8,440,362. This U.S. Patent is incorporated herein by reference with respect to the description of the modular fuel cell system. The modular system can include the modules and components described above and in U.S. Patent No. 9,190,693. This U.S. Patent is incorporated herein by reference with respect to the description of the modular fuel cell system. The modular design of the fuel cell system housing 10 provides for flexible system installation and operation.

[0017] The modular fuel cell system housing 10 includes a plurality of power module housings 12 (which house fuel cell power module components), one or more fuel input (i.e., fuel processing) module housings 16, and one or more power conditioning (i.e., electrical output) module housings 18. For example, the system housing can include any desired number of modules, such as 2 to 30 power modules, for example 6 to 12 power modules. FIG. 1 shows a system housing 10 that includes six power modules (six modules stacked in a single row) on a common base 20, one fuel processing module, and one power conditioning module. Each module can include its own cabinet or housing. Alternatively, the power conditioning module and the fuel processing module can be combined into a single input / output module located within one cabinet or housing 14. For simplicity, each housing 12, 14, 16, 18 is hereinafter referred to as a "module".

[0018] Although a single row of power modules 12 is shown, the system may include two or more rows of modules 12. For example, the system may include two rows of power modules stacked back-to-back.

[0019] Each power module 12 is configured to house one or more hot boxes 13. Each hot box houses one or more stacks or columns (not shown for clarity) of fuel cells, such as one or more stacks or columns of solid oxide fuel cells having a ceramic oxide electrolyte separated by a conductive interconnect plate. Other types of fuel cells, such as PEM, molten carbonate, and phosphoric acid, may also be used.

[0020] The modular fuel cell system housing 10 also includes one or more input or fuel processing modules 16. Each module 16 comprises a cabinet for housing components used for fuel pretreatment, such as a desulfurization bed. The fuel processing modules 16 can be designed to process different types of fuel. For example, a diesel fuel processing module, a natural gas fuel processing module, and an ethanol fuel processing module can be provided in the same or separate cabinet. Different bed compositions tailored to specific fuels can be provided within each module. The processing module(s) 16 can process at least one of the fuels selected from natural gas, compressed natural gas, methane, propane, liquefied petroleum gas, gasoline, diesel, household heating oil, kerosene, JP-5, JP-8, aviation fuel, hydrogen, ammonia, ethanol, methanol, synthesis gas, biogas, biodiesel, and other suitable hydrocarbons or hydrogen-containing fuels supplied from a pipeline. A reformer 17 may be placed within the fuel processing module 16 if desired. Alternatively, if it is desirable to thermally integrate the reformer 17 with the fuel cell stack(s), a separate reformer 17 can be placed in each hot box 13 within each power module 12. Furthermore, if an internally reformed fuel cell is used, the external reformer 17 can be completely omitted.

[0021] The modular fuel cell system enclosure 10 also includes one or more power adjustment modules 18. The power adjustment module 18 comprises components for converting DC power generated by the fuel cell stack to AC power, electrical connectors for AC power output to the grid, circuits for managing electrical transients, and a cabinet housing a system controller (e.g., a computer or a dedicated control logic device or circuit). The power adjustment module 18 can be designed to convert DC power from the fuel cell modules to different AC voltages and frequencies. Designs can be provided for 208V, 60Hz; 480V, 60Hz; 415V, 50Hz and other common voltages and frequencies.

[0022] The fuel processing module 16 and the power adjustment module 18 may be housed in a single input / output cabinet 14. If a single input / output cabinet 14 is provided, the modules 16 and 18 can be arranged vertically within the cabinet 14 (for example, with the components of the power adjustment module 18 above the desulfurization canister / floor of the fuel processing module 16) or side by side.

[0023] As shown in the exemplary embodiment of Figure 1, one input / output cabinet 14 is provided for one row of six power modules 12, and the six power modules 12 are arranged in a linear, side-by-side manner on one side of the input / output cabinet 14. The row of modules can be positioned, for example, adjacent to a building to which the system provides power (e.g., so that the back of the module cabinets faces the wall of the building). Although one row of power modules 12 is shown, the system may comprise two or more rows of modules 12. For example, as described above, the system may comprise two rows of power modules stacked back-to-back.

[0024] Each of the power module 12 and the input / output module 14 is provided with a door 30 (e.g., a hatch, access panel, etc.) that allows access to the internal components of the module (e.g., for maintenance, repair, replacement, etc.). According to one embodiment, the modules 12 and 14 are arranged in a linear arrangement with the door 30 on only one side of each cabinet, allowing consecutive rows of the system to be installed end-to-end. In this way, the size and capacity of the fuel cell housing 10 can be adjusted by additional modules 12 or 14 and base 20, while minimizing the need to rearrange the existing modules 12 and 14 and base 20. If desired, the door 30 to the module 14 may be located on the side of the cabinet rather than the front.

[0025] Figure 2 shows a plan view of a fuel cell system hotbox 13 comprising a fuel cell stack or column 40. The hotbox 13 is shown as comprising a fuel cell stack or column 40, however, the hotbox 13 may comprise two or more stacks or columns 40. The stacks or columns 40 may contain fuel cells 45 that are stacked on top of each other and electrically connected, with interconnects 50 located between the fuel cells 45. The first and last fuel cells 45 in the stack or column are positioned between their respective end plates 60 and interconnects 50. The end plates 60 are electrically connected to the electrical output of the fuel cell stack or column 40. The hotbox 13 may comprise other components such as fuel conduits, air conduits, seals, and electrical contacts, and can be incorporated into a fuel cell system that includes balance of plant components. The fuel cell 45 may be a solid oxide fuel cell comprising a ceramic electrolyte such as yttria-stabilized zirconia (YSZ) or scandia-stabilized zirconia (SSZ), an anode electrode such as nickel-YSZ, Ni-SSZ, or nickel-samaria-doped ceria (SDC) cermet, and a cathode electrode such as lanthanum strontium manganite (LSM). The interconnect 50 and / or end plate 60 may include any suitable gas-impermeable conductive material, such as a chromium-iron alloy, for example, an alloy containing 4 wt% to 6 wt% iron and the remainder chromium. The interconnect 50 electrically connects adjacent fuel cells 45 and provides channels for fuel and air to reach the fuel cells 45.

[0026] Figure 3 shows a hybrid fuel cell system 300 suitable for carrying out various embodiments. Referring to Figures 1 to 3, the hybrid fuel cell system 300 may comprise an SOFC stack 302 and a PEM fuel cell stack (referred to herein as “PEM stack” for brevity) 304 configured to generate DC electrical energy to support a load (not shown). The SOFC stack 302 may be located in one or more power modules 12 shown in Figure 1, while the PEM stack 304 may be located in one or more other power modules 12 shown in Figure 1. Alternatively, the SOFC stack 302 and PEM stack 304 may be located in different power modules 12 located in different rows and / or on different bases. Alternatively, the SOFC stack 302 and PEM stack 304 may be located in the same power module 12. It should be understood that the system 300 may comprise multiple SOFC stacks 302 and / or multiple PEM stacks 304.

[0027] The hybrid fuel cell system 300 can be equipped with various power adjustment elements such as any number and combination of DC / DC converters 308a, 308b and at least one DC / AC inverter 310. The SOFC stack 302 can be electrically connected to the DC / DC converter 308a via a DC bus 312a. The PEM stack 304 can be electrically connected to the DC / DC converter 308b via a DC bus 312b. The DC buses 312a and 312b can be configured to transmit DC electrical energy from the SOFC stack 302 to the DC / DC converter 308a and from the PEM stack 304 to the DC / DC converter 308b. The DC / DC converters 308a and 308b can be electrically connected to the DC / AC inverter 310 via a coupled DC bus 314. The coupled DC bus 314 can be configured to transmit DC electrical energy from the DC / DC converters 308a and 308b to the DC / AC inverter 310.

[0028] The hybrid fuel cell system 300 may optionally include an electrical energy storage system 306. For example, the electrical energy storage system 306 may include an electrical storage unit, a mechanical storage unit, an electromechanical storage unit, an electrochemical storage unit, a thermal storage unit, etc. Examples include a battery, a capacitor, a supercapacitor, a flywheel, a liquid reservoir, a gas reservoir, etc. The hybrid fuel cell system 300 may include various additional optional power adjustment elements, such as any number and combination of DC / DC converters 308c. The electrical energy storage system 306 can be electrically connected to the DC / DC converters 308c via a DC bus 312c. The DC bus 312c can be configured to transmit DC electrical energy from the electrical energy storage system 306 to the DC / DC converters 308c. The DC / DC converters 308c can be electrically connected to a DC / AC inverter 310 via a coupled DC bus 314. The coupled DC bus 314 can be configured to transmit DC electrical energy from the DC / DC converter 308c to the DC / AC inverter 310.

[0029] The hybrid fuel cell system 300 may include any number and combination of controllers 320 (e.g., a central processing unit (CPU), a microcontroller, a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), or any other software-programmable processor) that are communicated to (e.g., via wired and / or wireless data connections) the SOFC stack 302, the PEM stack 304, the electrical energy storage system 306, the DC / DC converters 308a-308c, the DC / AC inverter 310, the DC buses 312a-312c, and / or the coupled DC bus 314. For example, one or more controllers 320 may be components of the hybrid fuel cell system 300 that are communicated to and external to the SOFC stack 302, the PEM stack 304, the electrical energy storage system 306, the DC / DC converters 308a-308c, and / or the DC / AC inverter 310. As another example, one or more controllers 320 can be a component of a hybrid fuel cell system 300, communicating with and integrated with DC / DC converters 308a-308c and / or DC / AC inverters 310. One or more controllers 320 can be configured to provide control signals to the SOFC stack 302, PEM stack 304, electrical energy storage system 306, DC / DC converters 308a-308c, and / or DC / AC inverters 310, and / or to directly control their functions. One or more controllers 320 can be configured to receive signals from the SOFC stack 302, PEM stack 304, electrical energy storage system 306, DC / DC converters 308a-308c, and / or DC / AC inverters 310 that are configured to indicate DC voltages in DC buses 312a-312c and / or coupled DC bus 314.One or more controllers 320 can be configured to directly measure the DC voltage in the DC buses 312a-312c and / or the coupled DC bus 314 in the SOFC stack 302, PEM stack 304, electrical energy storage system 306, DC / DC converters 308a-308c, DC / AC inverter 310, DC buses 312a-312c, and / or coupled DC bus 314.

[0030] The SOFC stack 302 can be configured to provide DC electrical energy to the DC / DC converter 308a via the DC bus 312a. The SOFC stack 302 is sized and can be configured to provide base-level stack DC electrical energy, which may be determined by the base-level load demand of the load in which the hybrid fuel cell system 300 is deployed. The base-level stack DC electrical energy can be configured as the amount of DC voltage required to support the base-level load demand. Optionally, the amount of DC voltage output by the SOFC stack 302 can be controlled to a constant amount of DC voltage, such as the base-level stack DC electrical energy. The amount of DC voltage output by the SOFC stack 302 can be controlled by the controller 320.

[0031] The DC / DC converter 308a can be configured to provide DC electrical energy (e.g., DC power) to the DC / AC inverter 310 via the coupled DC bus 314. The DC / DC converter 308a can also be configured to provide base-level regulated DC electrical energy, which may be determined by the base-level load demand of the load in which the hybrid fuel cell system 300 is deployed. Base-level regulated DC electrical energy can be configured as the amount of DC voltage required to support the base-level load demand. The amount of DC voltage output by the DC / DC converter 308a can be controlled to be a constant amount of DC voltage, such as base-level regulated DC electrical energy. The amount of DC voltage output by the DC / DC converter 308a can be controlled by the controller 320.

[0032] The PEM stack 304 can be configured to provide DC electrical energy (e.g., DC power) to the DC / DC converter 308b via the DC bus 312b. The PEM stack 304 is sized and can be configured to provide variable level stack DC electrical energy, which can be determined by the variable level load demand of the load in which the hybrid fuel cell system 300 is deployed. The variable level load demand can be a certain level of load demand added to the base level load demand of the load in which the hybrid fuel cell system 300 is deployed. The variable level stack DC electrical energy can be configured as the amount of DC voltage required to support the variable level load demand. The amount of DC voltage output by the PEM stack 304 can be controlled as a variable amount of DC voltage, such as the variable level stack DC electrical energy. The amount of DC voltage output by the PEM stack 304 can be controlled by the controller 320.

[0033] The DC / DC converter 308b can be configured to provide DC electrical energy to the DC / AC inverter 310 via the coupled DC bus 314. The DC / DC converter 308b can also be configured to provide variable-level regulated DC electrical energy, which may be determined by the variable-level load demand of the load in which the hybrid fuel cell system 300 is deployed. The variable-level regulated DC electrical energy may be configured as the amount of DC voltage required to support the variable-level load demand. The amount of DC voltage output by the DC / DC converter 308b can be controlled as a variable amount of DC voltage, such as the variable-level regulated DC electrical energy. The amount of DC voltage output by the DC / DC converter 308b can be controlled by the controller 320.

[0034] The DC / AC inverter 310 can be configured to provide AC electrical energy (e.g., AC power) to a load. The DC / AC inverter 310 can be configured to provide AC electrical energy to support the load demand of the load to which the hybrid fuel cell system 300 is deployed. The load demand can be any level of load demand, including but not limited to base-level load demand and variable-level load demand. The DC / AC inverter 310 can draw DC electrical energy from the coupled DC bus 314 in relation to the load demand, convert the DC electrical energy to AC electrical energy, and provide AC electrical energy to the load, which can be electrically connected to the hybrid fuel cell system 300 by the AC bus. The DC / AC inverter 310 can increase or decrease the DC electrical energy drawn from the coupled DC bus 314 in response to corresponding increases or decreases in the load demand. The load demand can be configured as a combination of base-level load demand and variable-level load demand. For example, in response to an increase in load demand due to an increase in variable-level load demand, the DC / AC inverter 310 can increase the DC electrical energy drawn from the coupled DC bus 314. In response to the decrease in load demand due to the reduction in variable level load demand, the DC / AC inverter 310 can reduce the amount of DC electrical energy it draws from the coupled DC bus 314. The amount of DC electrical energy drawn by the DC / AC inverter 310 can be controlled by the controller 320.

[0035] The controller 320 can be configured to measure the DC voltage in the coupled DC bus 314. For example, the controller 320 can receive signals from DC / DC converters 308a, 308b and / or DC / AC inverters 310, which are configured to show the controller 320 the DC voltage in the coupled DC bus 314. In another example, the controller 320 can directly measure the DC voltage in the coupled DC bus 314. The controller 320 can compare the DC voltage in the coupled DC bus 314 to a DC voltage threshold. For example, the DC voltage threshold can be a predetermined DC voltage target. If the DC voltage in the coupled DC bus 314 is below the DC voltage threshold, the controller 320 can determine that the DC voltage in the coupled DC bus 314 is insufficient. A DC voltage deficiency in the coupled DC bus 314 may occur because the load on which the hybrid fuel cell system 300 is deployed requires more AC electrical energy than can be provided by the base-level stack DC electrical energy from the SOFC stack 302 via the DC / DC converter 308a. Therefore, variable level load demand may be included as part of the load demand. For example, an increase in variable level load demand may result in an increase in the DC electrical energy drawn by the DC / AC inverter 310 and a decrease in the DC voltage in the coupled DC bus 314. The controller 320 can increase the DC voltage in the coupled DC bus 314 to at least achieve a DC voltage threshold. The increase in voltage in the coupled DC bus 314 may, in fact, be sufficient to support the load demand. The controller 320 can signal and / or directly control the DC / DC converter 308b to initiate the drawing of DC electrical energy from the PEM stack 304 via the DC bus 312b. The controller 320 can signal and / or directly control the DC / DC converter 308b to draw a certain amount of DC electrical energy so that the DC voltage in the coupled DC bus 314 is sufficient to at least achieve a DC voltage threshold.The amount of DC electrical energy drawn from the PEM stack 304 by the DC / DC converter 308b may be sufficient to compensate for any DC voltage deficiency in the coupled DC bus 314 and at least achieve the DC voltage threshold. Thus, in practice, the amount of DC electrical energy drawn from the PEM stack 304 by the DC / DC converter 308b may be sufficient to support the variable-level load demand portion of the load demand. When the PEM stack 304 is off, it can be restarted in response to a signal from the controller 320 and / or in response to power being drawn by the DC / DC converter 308b. Because the PEM stack 304 can be restarted more quickly than the SOFC stack 302, the PEM stack 304 can be used to quickly provide the additional DC electrical energy (i.e., power) needed to meet the DC voltage threshold, i.e., the load demand.

[0036] If the DC voltage in the coupled DC bus 314 falls below the DC voltage threshold while the DC / DC converter 308b is drawing DC electrical energy from the PEM stack 304, the controller 320 can determine that the DC voltage in the coupled DC bus 314 is insufficient. A DC voltage deficiency in the coupled DC bus 314 may occur because the load is requesting an amount of AC electrical energy exceeding what is provided by the base-level stack DC electrical energy from the SOFC stack 302 via the DC / DC converter 308a and the variable-level stack DC electrical energy from the PEM stack 304 via the DC / DC converter 308b. The controller 320 can be configured to increase the DC voltage in the coupled DC bus 314 to at least achieve the DC voltage threshold. The increase in voltage in the coupled DC bus 314 may actually be sufficient to support the increase in load demand, which may include an increase in the variable-level load demand portion of the load demand. For example, an increase in variable-level load demand may result in an increase in the DC electrical energy drawn by the DC / AC inverter 310 and a decrease in the DC voltage in the coupled DC bus 314. The controller 320 can signal and / or directly control the DC / DC converter 308b to increase the amount of DC electrical energy drawn from the PEM stack 304 via the DC bus 312b. The controller 320 can signal and / or directly control the DC / DC converter 308b to draw a certain amount of DC electrical energy so that the DC voltage in the coupled DC bus 314 is sufficient to achieve at least a DC voltage threshold. The amount of DC electrical energy drawn from the PEM stack 304 by the DC / DC converter 308b may, in fact, be sufficient to support the variable-level load demand portion of the load demand.

[0037] Conversely, if the DC voltage in the coupled DC bus 314 exceeds a DC voltage threshold while the DC / DC converter 308b is drawing DC electrical energy from the PEM stack 304, the controller 320 can determine that the DC voltage in the coupled DC bus 314 is excessive. An excess of DC voltage in the coupled DC bus 314 may occur when the load requests an amount of AC electrical energy less than what is provided by the base-level stack DC electrical energy from the SOFC stack 302 via the DC / DC converter 308a and the variable-level stack DC electrical energy from the PEM stack 304 via the DC / DC converter 308b. The controller 320 can be configured to reduce the DC voltage in the coupled DC bus 314 to achieve the DC voltage threshold. The reduction in voltage in the coupled DC bus 314 may actually be sufficient to support a reduction in load demand, which may include a reduction in the variable-level load demand portion of the load demand. For example, a decrease in variable-level load demand may result in a decrease in the DC electrical energy drawn by the DC / AC inverter 310 and an increase in the DC voltage in the coupled DC bus 314. The controller 320 can signal and / or directly control the DC / DC converter 308b to reduce and / or stop the DC electrical energy drawn from the PEM stack 304 via the DC bus 312b. The controller 320 can signal and / or directly control the DC / DC converter 308b to draw or stop drawing a certain amount of DC electrical energy so that the DC voltage in the coupled DC bus 314 is sufficient to achieve a DC voltage threshold. The amount of DC electrical energy drawn from the PEM stack 304 by the DC / DC converter 308b may, in fact, be sufficient to support the variable-level load demand portion of the load demand.The variable level load demand may decrease to a level where the load demand is approximately equal to or less than the base level load demand, and as a result, the amount of DC electrical energy drawn from the coupled DC bus 314 by the DC / AC inverter 310 may decrease, such that the DC voltage in the coupled DC bus 314 may be approximately equal to or greater than the DC voltage threshold. In such a case, the controller 320 can signal the DC / DC converter 308b and / or directly control the DC / DC converter 308b to stop drawing DC electrical energy from the PEM stack 304 via the DC bus 312b. In such a case, the load demand can actually be supported by the base level stack DC electrical energy from the SOFC stack 302 without the variable level stack DC electrical energy from the PEM stack 304.

[0038] An optional electrical energy storage system 306 can be configured to provide DC electrical energy to an optional DC / DC converter 308c via an optional DC bus 312c. The optional electrical energy storage system 306 is sized and can be configured to provide storage system DC electrical energy, which can be configured to modify the quantity and / or quality of AC electrical energy supplied to the load into which the hybrid fuel cell system 300 is deployed. The storage system DC electrical energy can be configured to provide the amount of DC voltage necessary to modify the quantity and / or quality of AC electrical energy. For example, if the combined electrical energy output by the SOFC stack 302 and the PEM stack 304 cannot reach a DC voltage threshold, additional electrical energy (i.e., power) is supplied from the storage system 306 to the inverter 310. Alternatively, if the SOFC stack 302 and / or the PEM stack 304 are taken offline (e.g., for maintenance or due to fuel shortage), electrical energy is supplied from the storage system 306 to the inverter 310. The amount of DC voltage output by the electrical energy storage system 306 can be controlled as a certain amount of DC voltage, such as the DC electrical energy of the storage system. The amount of DC voltage output by the electrical energy storage system 306 can be controlled by the controller 320.

[0039] The DC / DC converter 308c can be configured to supply DC electrical energy to the DC / AC inverter 310 via the coupled DC bus 314. The DC / DC converter 308c can also be configured to supply storage system-adjusted DC electrical energy. The storage system-adjusted DC electrical energy can be configured as a DC voltage of the amount necessary to modify the quantity and / or quality of AC electrical energy. The amount of DC voltage output by the DC / DC converter 308c can be controlled as a certain amount of DC voltage, such as storage system-adjusted DC electrical energy. The amount of DC voltage output by the DC / DC converter 308c can be controlled by the controller 320.

[0040] Figures 4A and 4B show hybrid fuel cell systems 400a and 400b using hydrogen fuel suitable for carrying out various embodiments. Referring to Figures 1 to 4B, the hybrid fuel cell systems 400a and 400b can comprise any number and combination of components of the hybrid fuel cell system 300, including an SOFC stack 302, a PEM stack 304, and various electrical energy adjustment elements such as any number and combination of DC / DC converters 308a, 308b and a DC / AC inverter 310, as described with reference to Figure 3. The SOFC stack 302 can be electrically connected to the DC / DC converter 308a via a DC bus 312a. The PEM stack 304 can be electrically connected to the DC / DC converter 308b via a DC bus 312b. The DC / DC converters 308a and 308b can be electrically connected to the DC / AC inverter 310 via a coupled DC bus 314. The hybrid fuel cell systems 400a and 400b may optionally include an electrical energy storage system 306 and various optional electrical energy adjustment elements such as any number and combination of DC / DC converters 308c. The electrical energy storage system 306 can be electrically connected to the DC / DC converters 308c via a DC bus 312c. The DC / DC converters 308c can be electrically connected to a DC / AC inverter 310 via a coupled DC bus 314. The hybrid fuel cell systems 400a and 400b may also include any number and combination of controllers 320 that are communicated to the SOFC stack 302, PEM stack 304, electrical energy storage system 306, DC / DC converters 308a to 308c, DC / AC inverter 310, DC buses 312a to 312c, and / or coupled DC bus 314.

[0041] The hybrid fuel cell systems 400a and 400b can be fluid-connected to one or more hydrogen fuel sources. Specifically, at least one of the SOFC stack 302 and / or PEM stack 304 of the hybrid fuel cell systems 400a and 400b can be fluid-connected to one or more hydrogen fuel sources. For example, the SOFC stack 302 and PEM stack 304 can be fluid-connected to an external hydrogen fuel source 322 such as a hydrogen fuel storage tank (e.g., a hydrogen gas storage tank). The hydrogen fuel source may include hydrogen tanks (e.g., cylinders) that are filled at a hydrogen pumping station and then delivered to a power plant site, and / or filled at a power plant site by a hydrogen tanker truck. The SOFC stack 302 and PEM stack 304 can receive hydrogen from the hydrogen fuel source 322 and use that hydrogen to generate base-level stack DC electrical energy and variable-level stack DC electrical energy. The SOFC stack 302 and PEM stack 304 can receive or draw hydrogen from a fluid-connectable hydrogen fuel source at a constant and / or variable rate. For example, the SOFC stack 302 can receive hydrogen at a fixed rate from a fluid-connectable hydrogen fuel source, while the PEM stack 304 can receive hydrogen at a variable rate from a fluid-connectable hydrogen fuel source.

[0042] In one embodiment of the hybrid fuel cell system 400a shown in Figure 4A, the SOFC stack 302 and the PEM stack 304 can be separately fluidly connected to an external hydrogen fuel source 322 via their respective hydrogen fuel conduits (e.g., pipes, etc.) 324a and 324b.

[0043] In another embodiment shown in Figure 4B relating to the hybrid fuel cell system 400b, the SOFC stack 302 can be fluidly connected to an external hydrogen fuel source 322 via a hydrogen fuel conduit 324. The PEM stack 304, on the other hand, is not directly connected to the external hydrogen fuel source 322. Instead, the fuel inlet of the PEM stack 304 is fluidly connected to the anode exhaust of the SOFC stack 302 via an exhaust conduit 326, so that the fuel inlet of the PEM stack 304 receives hydrogen from the anode exhaust of the SOFC stack. The SOFC stack 302 receives hydrogen as a fuel inlet flow at the anode electrode and air at the cathode electrode, and can output a mixture of water and unused hydrogen as an anode exhaust flow from the anode electrode. The anode exhaust flow is then supplied as fuel to the fuel inlet of the PEM stack 304 via the exhaust conduit 326.

[0044] Figures 5A to 5G show hybrid fuel cell systems 500a to 500g using hydrocarbon fuels (e.g., natural gas, pure methane, pentane, biogas, etc.) suitable for carrying out various embodiments. Referring to Figures 1 to 5G, the hybrid fuel cell systems 500a to 500g may comprise any number and combination of components of the hybrid fuel cell system 300, including an SOFC stack 302, a PEM stack 304, and various electrical energy adjustment elements such as any number and combination of DC / DC converters 308a, 308b and DC / AC inverters 310, as described with reference to Figure 3. The SOFC stack 302 can be electrically connected to the DC / DC converter 308a via a DC bus 312a. The PEM stack 304 can be electrically connected to the DC / DC converter 308b via a DC bus 312b. The DC / DC converters 308a and 308b can be electrically connected to the DC / AC inverter 310 via a coupled DC bus 314. The hybrid fuel cell systems 500a to 500g may optionally include an electrical energy storage system 306 and various optional electrical energy adjustment elements such as any number and combination of DC / DC converters 308c. The electrical energy storage system 306 can be electrically connected to the DC / DC converters 308c via a DC bus 312c. The DC / DC converters 308c can be electrically connected to a DC / AC inverter 310 via a coupled DC bus 314. The hybrid fuel cell systems 500a to 500g may include any number and combination of controllers 320 that are communicated to the SOFC stack 302, PEM stack 304, electrical energy storage system 306, DC / DC converters 308a to 308c, DC / AC inverter 310, DC buses 312a to 312c, and / or coupled DC bus 314.

[0045] The hybrid fuel cell systems 500a to 500g can be fluidly connected to one or more hydrocarbon fuel sources. Specifically, the SOFC stack 302 of the hybrid fuel cell systems 500a to 500g can be fluidly connected to one or more external hydrocarbon fuel sources 522 via hydrocarbon fuel conduits 524, while the PEM stack is fluidly connected to the anode exhaust of the SOFC stack 302 and / or to an external hydrogen fuel source 322. The hydrocarbon fuel source 522 may include gas pipelines such as city natural gas lines and / or hydrocarbon fuel storage tanks such as gas or liquid fuel tanks. The hydrocarbon fuel source 522 can supply gas or liquid fuel such as natural gas, methane, biogas, or propane to the fuel inlet of the SOFC stack 302 via the hydrocarbon fuel conduits 524.

[0046] In the examples shown in Figures 5A to 5C, the fuel inlet PEM stack 304 is fluidly connected to the anode exhaust of the SOFC stack 302. The SOFC anode exhaust may contain a mixture of CO, CO2, H2, and H2O (and unused hydrocarbon fuel). A hydrogen pump, such as an electrochemical hydrogen pump (not shown), can be used to separate hydrogen (and optionally water vapor) from the SOFC anode exhaust and supply it to the PEM stack 304. Based on the utilization rate of hydrogen in the SOFC anode exhaust, approximately 30% to 40% of the SOFC stack rating can be achieved in the PEM stack 304.

[0047] In the example shown in Figure 5A, the PEM stack 304 can receive hydrogen from the SOFC anode exhaust of the SOFC stack 302 via the exhaust conduit 326 and the hydrogen buffer tank 502 located on the exhaust conduit 326. The hydrogen can be temporarily stored in the hydrogen buffer tank 502 and then supplied to the PEM stack 304 during the operation of the PEM stack. Water vapor (i.e., moisture) can be removed from the hydrogen stored in the hydrogen buffer tank 502.

[0048] In the example shown in Figure 5B, the PEM stack 304 can receive hydrogen directly from the SOFC stack 302 via the exhaust conduit 326 without using the hydrogen buffer tank 502, from the SOFC anode exhaust. The hydrogen pump can supply both hydrogen and steam (i.e., humidified hydrogen) from the anode exhaust of the SOFC stack 302 to the fuel inlet of the PEM stack 304.

[0049] In the example shown in Figure 5C, the PEM stack 304 can receive hydrogen from the SOFC anode exhaust of the SOFC stack 302 via a first exhaust conduit 326a, which includes a hydrogen buffer tank 502, and directly from the SOFC stack 302 via a second exhaust conduit 326b. In this case, the hydrogen buffer tank 502 can supply additional hydrogen fuel required by the PEM stack 304 if the second exhaust conduit 326b is unable to supply sufficient hydrogen fuel required by the PEM stack while the second anode exhaust conduit 326b is supplying humidified hydrogen fuel to the PEM stack 304.

[0050] In the examples shown in Figures 5D to 5G, the PEM stack 304 is fluidly connected to an external hydrogen fuel source 322. In the example shown in Figure 5D, the PEM stack 304 receives hydrogen from the hydrogen storage tank 322 via the hydrogen fuel conduit 324b. In this example, the PEM stack 304 is not fluidly connected to the anode exhaust of the SOFC stack 302. In the example shown in Figure 5E, the PEM stack 304 can receive hydrogen from the SOFC anode exhaust of the SOFC stack 302 directly from the SOFC stack 302 via the anode exhaust conduit 326 and from the hydrogen storage tank 322 via the hydrogen fuel conduit 324b. In the example shown in Figure 5F, the PEM stack 304 can receive hydrogen from the SOFC anode exhaust of the SOFC stack 302 via the hydrogen buffer tank 502 and conduit 326, and from the hydrogen storage tank 322 via conduit 324b. In the example shown in Figure 5G, the PEM stack 304 can receive hydrogen from the SOFC anode exhaust of the SOFC stack 302 via the hydrogen buffer tank 502 and conduit 326a, from the SOFC anode exhaust directly from the SOFC stack 302 via conduit 326b, and from the hydrogen storage tank 322 via conduit 324b.

[0051] Figure 6 shows an exemplary plot 600 of load demand over time during load following for a grid-independent hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G). Referring to Figures 1-6, the hybrid fuel cell system can generate enough DC electrical energy to achieve a DC voltage threshold at least on the DC bus (e.g., coupled DC bus 314 in Figures 3-5G). The DC voltage threshold can be a predetermined DC voltage target. For example, the DC voltage threshold can be a DC voltage sufficient to support the load demand of the load on which the hybrid fuel cell system is deployed. The load demand may include a base-level load demand 602 portion, and the DC electrical energy may include base-level stack DC electrical energy generated by the SOFC stack 302 and base-level regulated DC electrical energy output by the DC / DC converter 308a, which may be sufficient to support the base-level load demand 602. The load demand may include the variable-level load demand 604 portion, and the DC electrical energy may include the variable-level stack DC electrical energy generated by the PEM stack 304 and the variable-level adjusted DC electrical energy output by the DC / DC converter 308b, which may be sufficient to support the variable-level load demand 604. The base-level load demand 602 may be part of the load demand that is insufficient or equal to the load demand threshold 606. The variable-level load demand 604 may be part of the load demand that exceeds the load demand threshold 606.

[0052] The SOFC stack 302 can generate base-level stack DC electrical energy of a constant output, and the DC / DC converter 308a can generate a base-level regulated DC electrical energy output of a constant output that may be sufficient to achieve at least a DC voltage threshold. In fact, the base-level regulated DC electrical energy may be sufficient to support the base-level load demand 602. The PEM stack 304 can generate variable-level stack DC electrical energy of a certain output, and the DC / DC converter 308b can generate a variable-level regulated DC electrical energy output of a certain output that may be sufficient to achieve at least a DC voltage threshold while the variable-level load demand 604 is present. In fact, the variable-level regulated DC electrical energy may be sufficient to support the variable-level load demand 604. The variable-level stack DC electrical energy and / or variable-level regulated DC electrical energy vary over time in response to variations in the variable-level load demand 604 over time, so as to achieve at least a DC voltage threshold.

[0053] The DC / AC inverter 310 can output AC power converted inversely from base-level adjusted DC power to support load demand. The DC / AC inverter 310 can also output base-level adjusted DC power and AC power converted inversely from variable-level adjusted DC power to support load demand if the load demand includes a variable-level load demand 604 portion that results in load demand exceeding the load demand threshold 606. In some examples, the DC / AC inverter 310 can further output base-level adjusted DC power, variable-level adjusted DC power, and AC power converted inversely from stored system DC electrical energy generated by the electrical energy storage system 306 to support load demand if the load demand includes a variable-level load demand 604 portion that results in load demand exceeding the load demand threshold 606.

[0054] Figure 7 shows an exemplary plot of load demand over time during load following for a microgrid-backed hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G). Referring to Figures 1-7, the hybrid fuel cell system can generate enough DC electrical energy to achieve at least a DC voltage threshold on the DC bus (e.g., coupled DC bus 314 in Figures 3-5G). The DC voltage threshold can be a predetermined DC voltage target. For example, the DC voltage threshold can be a DC voltage sufficient to support the load demand of the load on which the hybrid fuel cell system is deployed. The load demand may include a base-level load demand portion 602, and the DC electrical energy may include base-level stack DC electrical energy generated by the SOFC stack 302 and base-level regulated DC electrical energy output by the DC / DC converter 308a, which may be sufficient to achieve at least the DC voltage threshold. In fact, the base-level regulated DC electrical energy may be sufficient to support the base-level load demand 602, as described herein with reference to Figure 6. The load demand may include a variable-level load demand 704 portion, and the electric utility grid may provide sufficient AC electrical energy to support the variable-level load demand 704. The variable-level load demand 704 may be a portion of the load demand that exceeds the load demand threshold 606.

[0055] However, in some cases, the electrical utility grid may not be able to provide sufficient AC electrical energy to support the variable-level load demand 704. For example, during the period between the first time 708 and the second time 710, the electrical utility grid may not be able to provide sufficient AC electrical energy to support a portion of the variable-level load demand 704. The portion of the variable-level load demand not provided by the grid is shown as element 712 in Figure 7. In this case, the PEM stack 304 is started and used as a backup power source, and the DC electrical energy may include the variable-level stack DC electrical energy generated by the PEM stack 304 and the variable-level regulated DC electrical energy output by the DC / DC converter 308b, which may be sufficient to achieve at least the DC voltage threshold while the variable-level load demand portion 712 is present. In fact, the variable-level regulated DC electrical energy may be sufficient to support the variable-level load demand portion 712. The variable-level stack DC electrical energy and / or variable-level regulated DC electrical energy fluctuate over the same period of time in response to the fluctuations in the variable-level load demand portion 712 over time, so as to achieve at least the DC voltage threshold. For example, the above time can be the period between the first time 708 and the second time 710, during which the electric utility grid may not be able to provide sufficient AC electrical energy to support the variable level load demand portion 712.

[0056] The DC / AC inverter 310 can output AC power converted inversely from base-level adjusted DC power to support load demand. The DC / AC inverter 310 can also output base-level adjusted DC power and AC power converted inversely from variable-level adjusted DC power to support load demand if the load demand includes a variable-level load demand portion 712 that results in load demand exceeding the load demand threshold 606. In some examples, the DC / AC inverter 310 can further output base-level adjusted DC power, variable-level adjusted DC power, and AC power converted inversely from stored system DC electrical energy generated by the electrical energy storage system 306 to support load demand if the load demand includes a variable-level load demand portion 712 that results in load demand exceeding the load demand threshold 606.

[0057] Figure 8 shows an exemplary plot of load demand over time during load following for a microgrid-backed hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G). Referring to Figures 1-8, the hybrid fuel cell system can generate enough DC electrical energy to achieve at least a DC voltage threshold on the DC bus (e.g., coupled DC bus 314 in Figures 3-5G). The DC voltage threshold can be a predetermined DC voltage target. For example, the DC voltage threshold can be a DC voltage sufficient to support the load demand of the load on which the hybrid fuel cell system is deployed. The load demand may include a base-level load demand portion 602, and the DC electrical energy may include base-level stack DC electrical energy generated by the SOFC stack 302 and base-level regulated DC electrical energy output by the DC / DC converter 308a, which may be sufficient to achieve at least a DC voltage threshold. In fact, the base-level regulated DC electrical energy may be sufficient to support the base-level load demand 602, as described herein with reference to Figure 6. The load demand may include the variable-level load demand 804 portion, and the DC electrical energy may include the variable-level stack DC electrical energy generated by the PEM stack 304 and the variable-level regulated DC electrical energy output by the DC / DC converter 308b, which may be sufficient to achieve at least the DC voltage threshold while the variable-level load demand 804 is present. In fact, the variable-level regulated DC electrical energy may be sufficient to support the variable-level load demand 804. The variable-level load demand 804 may be part of the load demand that exceeds the load demand threshold 606.

[0058] In some cases, due to scheduled or event-based emergency system activations, the load demand may increase by an amount equal to the variable-level load demand 804. Scheduled emergency system activations may be performed, for example, for maintenance and testing. Event-based emergency system activations may be performed, for example, based on emergency criteria, such as the activation of a fire suppression system due to fire detection, or the activation of a backup power system due to power loss caused by a weather event. The PEM stack 304 can generate variable-level stack DC electrical energy of a certain output, and the DC / DC converter 308b can generate a variable-level regulated DC electrical energy output of a certain output that may be sufficient to achieve at least a DC voltage threshold while the variable-level load demand 804 is present. In fact, the variable-level regulated DC electrical energy may be sufficient to support the variable-level load demand 804. The variable-level stack DC electrical energy and / or variable-level regulated DC electrical energy fluctuate over time in response to fluctuations in the variable-level load demand 804 over time, so as to achieve at least a DC voltage threshold. For example, the variable level load demand 804 may ramp up quickly to power an emergency system or ramp down to shut down an emergency system. For example, the variable level load demand 804 may be variable or constant between ramping up and ramping down for the operation of the emergency system. The amount of time between ramping up and ramping down for the operation of the emergency system may vary from a few seconds to several weeks for different circumstances. The activation of the emergency system may induce the load demand to exceed the load demand threshold 606 by the amount of the variable level load demand 804, affecting the voltage on the DC bus and generating an output of variable level stack DC electrical energy in the PEM stack 304 that may be sufficient to support achieving at least the DC voltage threshold.

[0059] The DC / AC inverter 310 can output AC power converted inversely from base-level adjusted DC power to support load demand. The DC / AC inverter 310 can also output base-level adjusted DC power and AC power converted inversely from variable-level adjusted DC power to support load demand if the load demand includes variable-level load demand 804 that results in load demand exceeding the load demand threshold 606. In some examples, the DC / AC inverter 310 can further output base-level adjusted DC power, variable-level adjusted DC power, and AC power converted inversely from stored system DC electrical energy generated by the electrical energy storage system 306 to support load demand if the load demand includes variable-level load demand 804 that results in load demand exceeding the load demand threshold 606.

[0060] Figure 9 is a process flowchart of load-following and backup methods for hybrid fuel cell systems according to various embodiments. Referring to Figures 1 to 9, Method 900 can be implemented using one or more controllers 320 configured to receive signals from any number or combination of SOFC stacks 302, PEM stacks 304, electrical energy storage systems 306, DC / DC converters 308a to 308c, DC / AC inverters 310, DC buses 312a to 312c, and / or coupled DC buses 314. Method 900 can be implemented using one or more controllers 320 configured to transmit control signals to any number and combination of SOFC stacks 302, PEM stacks 304, electrical energy storage systems 306, DC / DC converters 308a to 308c, and / or DC / AC inverters 310. Hardware implementing Method 900 is referred to herein as “control device” to encompass alternative configurations. Blocks 902–916 of any number and combination may be performed periodically, repeatedly, or sequentially, and / or simultaneously with any other blocks 902–916.

[0061] In block 902, the control device can extract DC electrical energy from the first fuel cell (e.g., SOFC stack 302 in Figures 3 to 5G) of the fuel hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a to 500g in Figures 3 to 5G). The control device can provide a signal configured to instruct the first fuel cell and / or the first DC / DC converter (e.g., DC / DC converter 308a in Figures 3 to 5G) to output and / or extract a certain amount (e.g., a setpoint) of base-level stack DC electrical energy. The control device can directly control the function of the first fuel cell and / or the first DC / DC converter to cause it to output and / or extract base-level stack DC electrical energy. The base-level stack DC electrical energy can be predetermined, for example, based on the base-level load demand of the load in which the hybrid fuel cell system is deployed.

[0062] In block 904, the control unit can supply DC electrical energy (i.e., power) to a coupled DC bus (e.g., coupled DC bus 314 in Figures 3 to 5G). The control unit can provide a signal configured to instruct a first DC / DC converter (e.g., DC / DC converter 308a in Figures 3 to 5G) to output a certain amount (e.g., a setpoint) of base-level adjusted DC electrical energy to the coupled DC bus. The control unit can directly control the function of the first DC / DC converter to cause it to output base-level adjusted DC electrical energy. The base-level adjusted DC electrical energy can be predetermined, for example, based on the base-level load demand of the load to which the hybrid fuel cell systems (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a to 500g in Figures 3 to 5G) are deployed.

[0063] In block 906, the control unit can measure the DC voltage of a coupled DC bus (e.g., coupled DC bus 314 in Figures 3 to 5G). The control unit can receive signals from a DC / DC converter (e.g., DC / DC converters 308a to 308c in Figures 3 to 5G) and / or a DC / AC inverter (e.g., DC / AC inverter 310 in Figures 3 to 5G) that represent the DC voltage on the coupled DC bus, as shown in the control unit. The control unit can be configured in the DC / DC converter, DC / AC inverter, and / or coupled DC bus to directly measure the DC voltage on the coupled DC bus.

[0064] In decision block 908, the control device can determine whether the DC voltage is below the DC voltage threshold. The DC voltage threshold can represent the DC voltage threshold of a coupled DC bus (e.g., coupled DC bus 314 in Figures 3 to 5G). The DC voltage threshold can be a predetermined DC voltage target. For example, the DC voltage threshold can be based on a voltage sufficient for the load on which the hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a to 500g in Figures 3 to 5G) is deployed. The control device can compare the DC voltage measured in block 906 with the DC voltage threshold. In response to the determination that the DC voltage is not below the DC voltage threshold (i.e., decision block 908 = "No"), the control device can measure the DC voltage of the coupled DC bus in block 906.

[0065] In response to the determination that the DC voltage falls below the DC voltage threshold (i.e., determination block 908 = "Yes"), the control unit may, in block 910, draw DC electrical energy from the second fuel cell (e.g., the PEM stack 304 in Figures 3-5G). The management of the DC electrical energy drawn from the second fuel cell is further described with respect to method 1000 with reference to Figure 10. The control unit may, in block 906, repeat the measurement of the DC voltage of the coupled DC bus.

[0066] In the alternative embodiment described above with reference to Figure 7, the control unit may also determine whether utility grid power is unavailable before drawing DC power from the second fuel cell. In the alternative embodiment described above with reference to Figure 8, the control unit may also determine whether an emergency (e.g., grid failure or maintenance request) has occurred before drawing DC power from the second fuel cell.

[0067] In block 912, the control unit can draw DC electrical energy from a coupled DC bus (e.g., coupled DC bus 314 in Figures 3 to 5G). The control unit can provide a signal configured to instruct a DC / AC inverter (e.g., DC / AC inverter 310 in Figures 3 to 5G) to draw a certain amount (e.g., a setpoint) of DC electrical energy from the coupled DC bus. The control unit can directly control the function of the DC / AC inverter to draw DC electrical energy. The amount of DC electrical energy can be based on the base-level load demand and / or variable-level load demand of the load in which the hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3 to 5G) is deployed.

[0068] In block 914, the control unit can convert DC electrical energy to AC electrical energy. The control unit can provide a DC / AC inverter (e.g., DC / AC inverter 310 in Figures 3 to 5G) with a signal configured to indicate parameters for converting DC electrical energy to AC electrical energy. The control unit can directly control the function of the DC / AC inverter to convert DC electrical energy to AC electrical energy. For example, the control unit can indicate or control characteristics of the AC electrical energy such as voltage, current, and power. The characteristics of the AC electrical energy can be based on the load to which the hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a to 500g in Figures 3 to 5G) is deployed, including base-level load demand and / or variable-level load demand. In block 916, the control unit can output AC electrical energy. AC electrical energy can be output to a hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G) and to an AC bus that can be electrically connected to the load in which the hybrid fuel cell system is deployed.

[0069] Figure 10 is a process flowchart illustrating load following and backup methods for hybrid fuel cell systems according to various embodiments.

[0070] Block 1002 in Figure 10 corresponds to block 910 in Figure 9, where the control device can extract DC electrical energy from a second fuel cell (e.g., PEM stack 304 in Figures 3-5G) of a fuel hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G). The control device can provide a signal configured to instruct the second fuel cell and / or a second DC / DC converter (e.g., DC / DC converter 308b in Figures 3-5G) to output and / or extract a certain amount (e.g., a setpoint) of variable level stack DC electrical energy. The control device can directly control the function of the second fuel cell and / or the second DC / DC converter to cause it to output and / or extract variable level stack DC electrical energy. The variable level stack DC electrical energy can be determined based on a comparison of the DC voltage measured in block 906 with a DC voltage threshold. For example, the variable level stack DC electrical energy can be determined as the difference between the DC voltage measured in block 906 and the DC voltage threshold.

[0071] In decision block 1004, the control device can determine whether the DC voltage is below the DC voltage threshold. The DC voltage threshold can represent the DC voltage threshold of a coupled DC bus (e.g., coupled DC bus 314 in Figures 3-5G). The DC voltage threshold can be a predetermined DC voltage target. For example, the DC voltage threshold can be based on a voltage sufficient for the load on which the hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G) is deployed. The control device can compare the DC voltage measured in block 906 of method 900, as described with reference to Figure 9, with the DC voltage threshold. Block 906 can be performed periodically, repeatedly, or continuously, and / or simultaneously with any number and combination of blocks 1002-1014, so that the control device can utilize the updated DC voltage measurement in decision block 1004.

[0072] In response to the determination that the DC voltage is below the DC voltage threshold (i.e., determination block 1004 = "Yes"), the control unit may, in block 1014, increase the DC electrical energy drawn from the second fuel cell (e.g., PEM stack 304 in Figures 3-5G) of the fuel hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G). The control unit may provide the second fuel cell and / or the second DC / DC converter (e.g., DC / DC converter 308b in Figures 3-5G) with a signal configured to indicate an amount (e.g., a setpoint) to increase the variable level stack DC electrical energy to output and / or draw. The control unit may directly control the function of the second fuel cell and / or the second DC / DC converter to cause it to output and / or draw the increased variable level stack DC electrical energy. The increase in the variable level stack DC electrical energy can be determined, for example, based on a comparison between the DC voltage measured in block 906 and the DC voltage threshold. For example, the variable level stack DC electrical energy can be determined as the difference between the DC voltage measured in block 906 and the DC voltage threshold. The increase in the variable level stack DC electrical energy can be predetermined by a preset amount, etc., which allows for repeated increases so that the DC voltage does not fall below the DC voltage threshold. The control device can repeatedly make a determination in the determination block 1004 as to whether or not the DC voltage falls below the DC voltage threshold.

[0073] In response to the determination that the DC voltage is not below the DC voltage threshold (i.e., decision block 1004 = "No"), the control unit can determine in decision block 1006 whether the DC voltage exceeds the DC voltage threshold. The control unit can compare the DC voltage measured in block 906 of method 900, as described with reference to Figure 9, with respect to the DC voltage threshold. Block 906 can be performed periodically, repeatedly, or continuously, and / or simultaneously with any number and combination of blocks 1002-1014, so that the control unit can utilize updated DC voltage measurements in decision block 1006. In response to the determination that the DC voltage does not exceed the DC voltage threshold (i.e., decision block 1006 = "No"), the control unit can repeat the determination in decision block 1004 whether the DC voltage is below the DC voltage threshold.

[0074] In response to the determination that the DC voltage exceeds the DC voltage threshold (i.e., determination block 1006 = "Yes"), the control unit may, in block 1008, reduce the DC electrical energy drawn from the second fuel cell (e.g., PEM stack 304 in Figures 3-5G) of the fuel hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G). The control unit may provide the second fuel cell and / or the second DC / DC converter (e.g., DC / DC converter 308b in Figures 3-5G) with a signal configured to indicate the amount (e.g., a setpoint) to reduce the variable level stack DC electrical energy to output and / or draw. The control unit may directly control the function of the second fuel cell and / or the second DC / DC converter to cause it to output and / or draw the reduced variable level stack DC electrical energy. The decrease in the DC electrical energy of the variable level stack can be determined, for example, based on a comparison between the DC voltage measured in block 906 and the DC voltage threshold. For example, the decrease in the DC electrical energy of the variable level stack can be determined as the difference between the DC voltage measured in block 906 and the DC voltage threshold. The decrease in the DC electrical energy of the variable level stack can be predetermined by a preset amount, etc., which allows for repeated reductions so that the DC voltage does not exceed the DC voltage threshold.

[0075] In decision block 1010, the control device can determine whether the DC voltage is equal to the DC voltage threshold. The control device can compare the DC voltage measured in block 906 of method 900, as described with reference to Figure 9, with respect to the DC voltage threshold. In response to the determination that the DC voltage is not equal to the DC voltage threshold (for example, decision block 1010 = "No"), the control device can repeat the determination in decision block 1004 to determine whether the DC voltage is insufficient to the DC voltage threshold.

[0076] In response to the determination that the DC voltage is equal to the DC voltage threshold (e.g., determination block 1010 = "Yes"), the control unit may, in block 1012, stop drawing DC electrical energy from the second fuel cell (e.g., PEM stack 304 in Figures 3-5G) of the fuel hybrid fuel cell system (e.g., hybrid fuel cell systems 300, 400a, 400b, 500a-500g in Figures 3-5G). The control unit may provide a signal configured to instruct the second fuel cell and / or the second DC / DC converter (e.g., DC / DC converter 308b in Figures 3-5G) to stop outputting and / or drawing variable level stack DC electrical energy. The control unit can also directly control the function of the second fuel cell and / or the second DC / DC converter to stop outputting and / or drawing variable level stack DC electrical energy.

[0077] The above-described descriptions of the embodiments of the disclosure are provided to enable those skilled in the art to practice or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may apply to other embodiments without departing from the scope of the invention. Accordingly, the invention is not intended to be limited to the embodiments shown herein, but rather to be given the broadest scope consistent with the principles and novel features disclosed herein.

[0078] The above-described method and diagrams are provided merely as illustrative examples and are not intended to require or imply that the steps of the various embodiments must be performed in the order presented. As those skilled in the art will understand, the order of the steps in the above embodiments can be performed in any order. Furthermore, terms such as “thereafter,” “then,” and “next” are not intended to limit the order of the steps. These terms are used only to guide the reader throughout the description of the method.

[0079] One or more figures are used to illustrate exemplary embodiments. The use of the figures is not intended to limit the order of operations performed. The above description of exemplary embodiments is presented for illustrative and explanatory purposes only. It is not intended to be exhaustive or limiting with respect to the exact form disclosed, and modifications and variations may be possible in light of the above teachings or obtained from the practice of the disclosed embodiments. The scope of the invention is intended to be defined by the claims and equivalents attached herein.

[0080] Control devices and control elements including the controller 320 described herein can be implemented using a computing device (such as a computer) which includes a programmable processor, memory, and other components in which instructions for performing a specific function are programmed, or can be implemented within a processor designed to perform a specific function. The processor can be any programmable microprocessor, microcomputer, or one or more multiprocessor chips that can be configured to perform a variety of functions, including the functions of the various embodiments described herein, by software instructions (applications). In some computing devices, multiple processors may be provided. Typically, software applications can be stored in internal memory before being accessed and loaded into the processor. In some computing devices, the processor may have sufficient internal memory to store application software instructions.

[0081] Various exemplary logic blocks, modules, circuits, and algorithmic steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or a combination of both. To clearly illustrate this hardware and software compatibility, various exemplary components, blocks, modules, circuits, and steps have been outlined above in terms of their functionality. Whether such functionality is implemented as hardware or software depends on the specific application and the design constraints imposed on the overall system. Those skilled in the art can implement the described functionality in various ways for specific applications, but such decisions should not be construed as resulting in a departure from the scope of the invention.

[0082] Hardware used to implement the various exemplary logic, logic blocks, modules, and circuits described in relation to the embodiments disclosed herein may be implemented or run using a control device that may be or may include a general-purpose processor, digital signal processor (DSP), application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware component, or any combination thereof, designed to perform the functions described herein. The general-purpose processor may be a microprocessor, but in alternative forms, the processor may be any conventional processor, controller, microcontroller, or state machine. The processor may also be implemented as a combination of computing devices, for example, a DSP and a microprocessor, multiple microprocessors, one or more microprocessors working in conjunction with a DSP core, or any other combination of such configurations. Alternatively, some blocks or methods may be implemented by circuit sections specific to a given function.

[0083] The above-described embodiments of the disclosure are provided to enable a person skilled in the art to implement or use any of the embodiments described. Various modifications to these embodiments will be readily apparent to a person skilled in the art, and the general principles defined herein may apply to other embodiments without departing from the scope of this disclosure. Accordingly, the claims are not intended to be limited to the embodiments shown herein, but are given the broadest scope consistent with the language of the claims and the principles and novel features disclosed herein.

Claims

1. A first fuel cell of a first electrolyte type, A second fuel cell having a second electrolyte type different from the first electrolyte type, A first DC / DC converter electrically connected to the first fuel cell via a first DC bus, A second DC / DC converter electrically connected to the second fuel cell via a second DC bus, A DC / AC inverter is electrically connected in parallel to the first DC / DC converter and the second DC / DC converter via a coupled DC bus, Electrical energy storage systems, A third DC / DC converter is electrically connected to the aforementioned electrical energy storage system and configured to output DC electrical energy from the electrical energy storage system to the DC / AC inverter, Equipped with a controller, The DC / AC inverter is electrically connected in parallel to the first DC / DC converter, the second DC / DC converter, and the third DC / DC converter via the coupled DC bus. The aforementioned controller, The second fuel cell is used to extract DC electrical energy, and the first fuel cell is used to extract base-level DC electrical energy. To determine whether the DC voltage in the coupled DC bus exceeds the DC voltage threshold, In response to determining that the DC voltage in the coupled DC bus exceeds the DC voltage threshold, the extraction of DC electrical energy from the second fuel cell is reduced to a level where the DC voltage in the coupled DC bus is less than or equal to the DC voltage threshold, and the extraction of DC electrical energy from the second fuel cell is continued at that level, while base-level DC electrical energy is extracted from the first fuel cell. A controller has executable instructions that perform an operation including, Fuel cell system.

2. The first fuel cell is a solid oxide fuel cell located within a solid oxide fuel cell stack, The fuel cell system according to claim 1, wherein the second fuel cell is a proton exchange membrane fuel cell located within a proton exchange membrane fuel cell stack.

3. The solid oxide fuel cell stack is fluidly connected to a hydrocarbon fuel source. The fuel cell system according to claim 2, wherein the proton exchange membrane fuel cell stack is fluidly connected to at least one of a hydrogen fuel source or the anode exhaust of the solid oxide fuel cell stack.

4. The fuel cell system according to claim 3, wherein the proton exchange membrane fuel cell stack is fluidly connected to the anode exhaust of the solid oxide fuel cell stack.

5. The fuel cell system according to claim 4, wherein the proton exchange membrane fuel cell stack is directly fluid-connected to the anode exhaust of the solid oxide fuel cell stack through an anode exhaust conduit, or is indirectly fluid-connected to the anode exhaust of the solid oxide fuel cell stack through a hydrogen buffer tank.

6. The fuel cell system according to claim 2, wherein a solid oxide fuel cell stack and a proton exchange membrane fuel cell stack are fluidly connected to a hydrogen fuel source.

7. The aforementioned controller, To measure the DC voltage in the coupled DC bus, To determine whether the DC voltage in the coupled DC bus is insufficient to reach the DC voltage threshold, It is configured to have a controller executable instruction that performs the operation, Extracting DC electrical energy from the second fuel cell is done in response to the determination that the DC voltage in the coupled DC bus is insufficient to the DC voltage threshold, A fuel cell system according to claim 1, including the above.

8. Measuring the DC voltage in the coupled DC bus includes measuring the DC voltage of the base-level DC electrical energy from the first fuel cell in the coupled DC bus. The fuel cell system according to claim 7, wherein, in response to the determination that the DC voltage in the coupled DC bus is insufficient to the DC voltage threshold, drawing the DC electrical energy from the second fuel cell includes initiating the drawing of a variable level of DC electrical energy from the second fuel cell.

9. Measuring the DC voltage in the coupled DC bus includes measuring the DC voltage in the coupled DC bus of the base-level DC electrical energy from the first fuel cell and the variable-level DC electrical energy from the second fuel cell. The fuel cell system according to claim 7, wherein, in response to the determination that the DC voltage in the coupled DC bus is insufficient to the DC voltage threshold, drawing the DC electrical energy from the second fuel cell includes increasing the variable level DC electrical energy drawn from the second fuel cell.

10. The aforementioned controller, To determine whether utility grid power is available to meet load demand, or whether an emergency situation has occurred, If the aforementioned utility grid power is unavailable to meet the load demand, or if the aforementioned emergency condition occurs, the second fuel cell is started and DC electrical energy is drawn from the second fuel cell. The fuel cell system according to claim 7, further comprising a controller executable command that performs an operation further including the following:

11. Extracting the DC electrical energy from the second fuel cell includes extracting a variable level of DC electrical energy from the second fuel cell, In response to the determination that the DC voltage in the coupled DC bus exceeds the DC voltage threshold, the extraction of DC electrical energy from the second fuel cell is reduced to the level at which the DC voltage in the coupled DC bus falls short of or equals the DC voltage threshold, and continuing to extract DC electrical energy from the second fuel cell at that level is equivalent to reducing the variable level of DC electrical energy extracted from the second fuel cell in response to the determination that the DC voltage in the coupled DC bus exceeds the DC voltage threshold. A fuel cell system according to claim 1, including the following:

12. The aforementioned controller, To determine whether the DC voltage in the coupled DC bus is equal to the DC voltage threshold, In response to the determination that the DC voltage in the coupled DC bus is equal to the DC voltage threshold, the extraction of the variable level DC electrical energy from the second fuel cell is stopped. The fuel cell system according to claim 11, further comprising a controller executable command that performs an operation further including the following:

13. A method for operating a fuel cell system, The first fuel cell of the first electrolyte type extracts base-level DC electrical energy to a coupled DC bus via a first DC / DC converter, To measure the DC voltage in the coupled DC bus, To determine whether the DC voltage in the coupled DC bus is insufficient to reach the DC voltage threshold, In response to determining that the DC voltage in the coupled DC bus is insufficient to reach the DC voltage threshold, variable-level DC electrical energy is extracted from a second fuel cell of a second electrolyte type, which is different from the first electrolyte type, via a second DC / DC converter. To determine whether the DC voltage in the coupled DC bus exceeds the DC voltage threshold, In response to determining that the DC voltage in the coupled DC bus exceeds the DC voltage threshold, the variable-level DC electrical energy drawn from the second fuel cell via the second DC / DC converter is reduced to a level where the DC voltage in the coupled DC bus is less than or equal to the DC voltage threshold, and the drawing of DC electrical energy from the second fuel cell is continued at that level, while base-level DC electrical energy is drawn from the first fuel cell. In response to determining that the DC voltage in the coupled DC bus is insufficient to meet the DC voltage threshold, DC electrical energy is drawn from the electrical energy storage system via a third DC / DC converter. The coupled DC energy from the coupled DC bus is supplied to the DC / AC inverters that are electrically connected in parallel to the first DC / DC converter, the second DC / DC converter, and the third DC / DC converter via the coupled DC bus, Methods that include...

14. The first fuel cell is a solid oxide fuel cell located within a solid oxide fuel cell stack, The method according to claim 13, wherein the second fuel cell is a proton exchange membrane fuel cell located within a proton exchange membrane fuel cell stack.

15. Measuring the DC voltage in the coupled DC bus includes measuring the DC voltage of the base-level DC electrical energy from the first fuel cell in the coupled DC bus. The method according to claim 14, wherein, in response to the determination that the DC voltage in the coupled DC bus is insufficient to the DC voltage threshold, drawing the variable level DC electrical energy from the second fuel cell includes initiating the drawing of the variable level DC electrical energy from the second fuel cell.

16. Measuring the DC voltage in the coupled DC bus includes measuring the DC voltage in the coupled DC bus of the base-level DC electrical energy from the first fuel cell and the variable-level DC electrical energy from the second fuel cell. The method according to claim 14, wherein, in response to the determination that the DC voltage in the coupled DC bus is insufficient to the DC voltage threshold, drawing the variable level DC electrical energy from the second fuel cell includes increasing the variable level DC electrical energy drawn from the second fuel cell.

17. To determine whether the DC voltage in the coupled DC bus is equal to the DC voltage threshold, In response to the determination that the DC voltage in the coupled DC bus is equal to the DC voltage threshold, the extraction of the variable level DC electrical energy from the second fuel cell is stopped. The method according to claim 14, further comprising:

18. To determine whether utility grid power is available to meet load demand, or whether an emergency situation has occurred, If the utility grid power is unavailable to meet the load demand, or if the emergency condition occurs, the second fuel cell is started to draw the variable-level DC electrical energy from the second fuel cell. The method according to claim 14, further comprising:

Images