Patent Description:
A fuel cell is a device which converts chemical energy, such as energy stored in a hydrocarbon fuel, into electrical energy by way of an electrochemical reaction. Generally, a fuel cell includes an anode electrode and a cathode electrode separated by an electrolyte that serves to conduct electrically charged ions. High temperature fuel cells, such as molten carbonate fuel cells and solid oxide fuel cells, operate by passing a reactant fuel gas through the anode electrode, while oxidant gas (e.g., carbon dioxide and oxygen) is passed through the cathode electrode. In order to produce a desired power level, a number of individual fuel cells can be stacked in series. In operation, a fuel cell system can provide electrical power to a load, such as an electrical grid. If such a load is unexpectedly removed from the fuel cell system (e.g., the electric grid goes down), such removal can result in degradation of the fuel cell system in the form of thermal-mechanical stresses.

<CIT> which forms the basis for the preamble of claim <NUM>, describes methods and systems to control a fuel cell power plant system to provide load following capabilities and to improve power plant availability. One method is performed for controlling a fuel cell power plant system. The process includes controlling a variable load bank capable of absorbing output power from the fuel cell power plant such that the total load on the fuel cell power plant is constant, wherein the total load is a combination of an external load connected to the power plant and a load on the variable load bank. Further, the process may include maintaining a constant chemical reaction rate of the fuel cell power plant while maintaining a desired total load.

According to the present invention, there is provided a load leveling system as set out in Claim <NUM>.

A load leveling system described herein includes a fuel cell inverter, a direct current (DC) load bank, and a controller. The fuel cell inverter is configured to draw DC power generated by a fuel cell assembly. The DC load bank is connected to the fuel cell assembly in parallel with the fuel cell inverter. The controller is in communication with the fuel cell inverter and the DC load bank. The controller is configured to identify a reduction in a load being drawn by the fuel cell inverter. Responsive to the identification of the reduction of the load, the controller is also configured to divert the DC power generated by the fuel cell assembly from the fuel cell inverter to the DC load bank to prevent load cycling of the fuel cell assembly.

The foregoing is a summary of the disclosure and thus by necessity contains simplifications, generalizations, and omissions of detail. Consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, features, and advantages of the devices and/or processes described herein, as defined by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings.

The lifetime of a high temperature fuel cell system (e.g., a molten carbonate fuel cell system or a solid oxide fuel cell system) is adversely impacted by load cycling, which can be caused by a sudden reduction (or complete disappearance) of a load that is drawing power from the fuel cell system. Specifically, components of the fuel cell system experience increased thermal-mechanical stresses during such load cycling situations. Unplanned load cycling can occur due to unstable electric grid (i.e., load) conditions, which cause inadvertent tripping of the inverter(s) connected to the fuel cell system.

One way to help reduce the impact of load cycling due to a tripped inverter is to quickly reset the inverter such that most or all of the thermal-mechanical stresses on the fuel cell system are eliminated. Another way to help reduce the impact of load cycling in this situation is to improve inverter reliability and grid disturbance ride through capability of the overall system. However, such solutions are ineffective when the fuel cell inverter is down for a sustained period of time due to malfunction, a prolonged electric grid disturbance, etc. Described herein is a system that is designed to prevent load cycling (and the resultant stresses on the system) in the event that the fuel cell inverter(s) are tripped off and cannot be immediately reset due to unstable grid conditions, malfunction, other load failure, etc..

<FIG> is a block diagram of a fuel cell load leveling system <NUM> in accordance with an illustrative embodiment. The system <NUM>, which is connected to an electric grid <NUM>, includes a fuel cell assembly <NUM>, a fuel cell inverter <NUM>, an output transformer <NUM>, and an output breaker <NUM>. The system <NUM> also includes parasitic loads <NUM>, a controller <NUM>, and a direct current (DC) load bank <NUM>. In alternative embodiments, fewer, additional, and/or different components may be included in the fuel cell load leveling system <NUM>.

In an illustrative embodiment, the fuel cell assembly <NUM> is composed of one or more fuel cell columns, each of which may include one or more fuel cell stacks. In another illustrative embodiment, the fuel cells of fuel cell assembly <NUM> are molten carbonate fuel cells. In alternative embodiments, different types of fuel cells may be used. The fuel cell assembly <NUM> is used to generate direct current (DC) power that is received by the fuel cell inverter <NUM> via a bus line. The fuel cell inverter <NUM> can be a single inverter, or a plurality of inverters, depending on the implementation. Upon receipt of a DC demand signal generated by the controller <NUM>, the fuel cell inverter <NUM> inverts the DC power into alternating current (AC) power, which is fed to the output transformer <NUM>. The output transformer <NUM> steps the AC voltage received from the fuel cell inverter <NUM> up to a desired value that is compatible with the electric grid <NUM>. In alternative embodiments, the output transformer <NUM> may step down the voltage received from the fuel cell inverter <NUM>. The output breaker <NUM> can be used to disconnect the fuel cell load leveling system <NUM> from the electric grid <NUM> for maintenance, to operate the grid independently, etc..

As indicated in <FIG>, the parasitic loads <NUM> are connected to an output of the output transformer <NUM> via a bus line. As such, the parasitic loads <NUM> are able to receive the same stepped up AC voltage which is provided to the electric grid <NUM>. The parasitic loads <NUM> can include blowers, process heaters, water treatment units, heating, ventilating, and air conditioning (HVAC) systems, etc..

The DC load bank <NUM> is connected to an output of the fuel cell assembly <NUM> via the same bus line that connects the fuel cell assembly <NUM> to the fuel cell inverter <NUM> (i.e., the DC load bank <NUM> is connected to the fuel cell assembly <NUM> in parallel with the connection of the fuel cell assembly <NUM> to the fuel cell inverter <NUM>). In an alternative embodiment, different bus lines may be used to connect the fuel cell assembly <NUM> to the DC load bank <NUM> and to the fuel cell inverter <NUM>. As its name implies, the DC load bank <NUM> includes one or more direct current loads. These direct current loads are able to draw the DC power directly from fuel cell assembly <NUM> in the event that fuel cell inverter <NUM> is tripped off or malfunctions. As a result, load cycling and the associated thermal-mechanical stresses on the fuel cell assembly <NUM> are avoided.

The DC load bank <NUM> can include any DC load(s) known to those of skill in the art. For example, the DC load bank <NUM> can include actual loads that utilize the DC power generated by the fuel cell assembly <NUM> such as Variable Frequency motor drives, DC lighting, hydrogen electrolyzers, data center servers, etc. Alternatively, the DC load bank <NUM> can be configured to dissipate the DC power generated by the fuel cell assembly <NUM> such that the fuel cell assembly <NUM> is able to continue to operate until the fuel cell inverter <NUM> is once again operational and able to receive the DC power. In one implementation in which there is an available heat load, the DC load bank <NUM> may be a variable electric heater that is configured to recover the energy dissipated from the fuel cell assembly <NUM>.

In another embodiment, if there are critical loads in a backup application, the DC load bank <NUM> can also be used to maintain the fuel cell stacks at constant power while load following when power from the electric grid <NUM> is unavailable. Additionally, for multiple fuel cell systems with multiple inverters, a DC load bank can be implemented for each inverter to maintain the same capability of individualized stack current control. Alternatively, fuel cell systems having multiple inverters may utilize a common load bank that can be used for all of the fuel cell stacks if individualized stack current control is not required. Such a common load bank system is more cost effective than having an individual DC load bank associated with each stack of the fuel cell assembly.

The controller <NUM> can be a computerized controller that includes at least a processor, a memory, a transceiver, and an interface. In one embodiment, the memory of the controller <NUM> can include computer-readable instructions stored thereon. The computer-readable instructions can be executed by the processor to perform any of the operations described herein. The controller will receive a signal representative of the fuel cell stack output current from a sensor which will be compared to a desired current setpoint. The difference will be input to a proportional + integral (PI) control algorithm which will calculate a current demand signal which the controller will send to the DC load bank. The PI control algorithm will thus adjust the DC load as necessary to maintain the fuel cell stack current output at the desired setpoint. The transceiver allows the controller to communicate with other system components such as the parasitic loads <NUM>, the fuel cell inverter <NUM>, and the DC load bank <NUM>. The interface allows to user to interact with the controller <NUM> to enter commands, program the unit, view status and other information, etc..

As depicted in <FIG>, the controller <NUM> is in communication with the fuel cell inverter <NUM>, the parasitic loads <NUM>, and the DC load bank <NUM>. The controller <NUM> is configured to monitor and control the fuel cell inverter <NUM>. In the event that the inverter <NUM> is tripped off for any reason, the controller <NUM> ensures that the DC power from the fuel cell assembly <NUM> is instead received by the DC load bank <NUM>. In an illustrative embodiment, the controller <NUM> can also control the DC load bank <NUM> such that the amount of load present is equivalent (or substantially equivalent) to the amount of load that is no longer being drawn by the fuel cell inverter <NUM>. The controller <NUM> is also configured to monitor and control the parasitic loads <NUM>. In one embodiment, in the event of an inverter trip, a small inverter can be used to receive all or a portion of the DC power from the fuel cell assembly <NUM>, invert the DC power to AC power, and provide the AC power to the parasitic loads <NUM>. As a result, the parasitic loads <NUM> can continue to receive power in the event that the fuel cell inverter <NUM> is tripped off.

<FIG> is a flow diagram depicting operations for load leveling in a fuel cell system in accordance with an illustrative embodiment. In alternative embodiments, fewer, additional, and/or different operations may be performed Additionally, the use of a flow diagram is not meant to be limiting with respect to the order of operations performed. In an operation <NUM>, a fuel cell inverter is monitored. The fuel cell inverter can be the fuel cell inverter <NUM> discussed with reference to <FIG>, or any other fuel cell inverter or inverters, depending on the implementation. The monitoring, which is used to detect a partial or complete reduction in the load drawn by the inverter, can be performed by a controller such as the controller <NUM>.

In an operation <NUM>, the system identifies that the fuel cell inverter is drawing a reduced load. The identification may be made by a controller. The reduced load can be a partially reduced load or a zero load condition that results from electric grid disturbances, inverter malfunction, etc. In an operation <NUM>, the system diverts DC power generated by the fuel cell assembly from the fuel cell inverter to the DC load bank. Such diverting of the DC power is responsive to the identification of the reduced load being drawn by the inverter and is used to prevent load cycling of the fuel cell assembly. In an illustrative embodiment, the DC load bank can be connected to the fuel cell assembly in parallel with the fuel cell inverter. In at least some embodiments, the diverting may be performed at least in part by a controller such as the controller <NUM>.

In an operation <NUM>, a decision is made regarding whether the inverter is again operational. The decision, which can be made by the controller, can be based on the continued monitoring of the fuel cell inverter after diversion of DC power to the DC load bank. If it is determined in the operation <NUM> that the inverter is still not operational, DC power generated by the fuel cell assembly continues to be diverted to the fuel cell bank. If it is determined in the operation <NUM> that the inverter is once again operational, the DC power generated by the fuel cell assembly is diverted back to the inverter in an operation <NUM> such that normal operation can commence. Such diversion of the DC power back to the fuel cell inverter can also be performed by the controller. The system then continues to monitor the fuel cell inverter in the operation <NUM>.

Claim 1:
A load leveling system (<NUM>) comprising:
a fuel cell inverter (<NUM>) configured to receive direct current (DC) power generated by a fuel cell assembly (<NUM>);
a DC load bank (<NUM>), configured to be connected to the fuel cell assembly (<NUM>) in parallel with the fuel cell inverter (<NUM>); and
a controller (<NUM>) in communication with the fuel cell inverter (<NUM>) and the DC load bank (<NUM>), wherein the controller (<NUM>) is configured to:
identify a reduction in a load being drawn by the fuel cell inverter (<NUM>);
responsive to the identification of the reduction of the load, divert the DC power generated by the fuel cell assembly (<NUM>) from the fuel cell inverter (<NUM>) to the DC load bank (<NUM>) to prevent load cycling of the fuel cell assembly (<NUM>);
monitor a load condition of the fuel cell inverter (<NUM>) after diverting the DC power from the fuel cell inverter (<NUM>) to the DC load bank (<NUM>);
determine that the fuel cell inverter (<NUM>) is again operational based on the monitored load condition; and
based on the determination that the fuel cell inverter (<NUM>) is again operational, divert the DC power from the DC load bank (<NUM>) back to the fuel cell inverter (<NUM>).