Method and apparatus for controlling a combined heat and...

Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation

Reexamination Certificate

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Details

C429S006000, C429S006000, C429S010000

Reexamination Certificate

active

06740437

ABSTRACT:

BACKGROUND
The invention generally relates to a combined heat and power fuel cell system and associated methods of operation.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H
2
→2H
+
+2e

at the anode of the cell, and
O
2
+4H
+
+4e

→2H
2
O at the cathode of the cell.
A typical fuel cell has a terminal voltage of up to about one volt DC. For purposes of producing much larger voltages, multiple fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow field plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow field channels and orifices to, as examples, route the reactants and products through the fuel cell stack. A PEM is sandwiched between each anode and cathode flow field plate. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow field channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair of catalyst layers are often referred to as a membrane electrode assembly (MEA). An MEA sandwiched by adjacent GDL layers is often referred to as a membrane electrode unit (MEU).
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. The amount of a reactant supplied may be referred to in terms of “stoich”. For example, for a given electrical load on a fuel cell, one stoich of hydrogen and one stoich of air would refer to the minimum amount of each reactant theoretically required to produce enough electrons to satisfy the load (assuming all of the reactants will react). However, in some cases, not all of the hydrogen or air supplied will actually react, so that it may be necessary to provide excess fuel and air stoichiometry so that the amount actually reacted will be appropriate to satisfy a given power demand.
Hydrogen that is not reacted in the fuel cell may be vented to the atmosphere with the fuel cell exhaust, and in some cases may be oxidized before it is vented. Such exhaust may also contain small amounts of hydrocarbons that “slip” through the fuel processor without being reacted. Substantial heat may be generated as these exhaust components are oxidized, for example by mixing them with air and passing them through a platinum-coated ceramic monolith similar to an automotive catalytic converter.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the load may not be constant, but rather the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time.
There is a continuing need for systems and algorithms to achieve objectives including the foregoing in a robust and cost effective manner.
SUMMARY
The invention provides a combined heat and power fuel cell system and associated methods of operation. Such systems are commonly referred to as cogeneration systems. In general, the system and methods of the invention relate to operation of a fuel cell system among various modes and configurations to balance heat and power demand signals. The fuel cell system is coupled to both a power sink and a heat sink. A controller is adapted to coordinate response to data signals from the power sink and the heat sink. As examples, such data signals from the heat sink may include a temperature indication or a heat demand signal (such as from a thermostat), and such data signals from the power sink may include a voltage or current measurement, an electrical power demand signal, or an electrical load.
In one aspect, a fuel cell system is provided that includes a fuel cell, a fuel supply, an oxidant supply, a power demand sensor, a heat demand sensor, and a controller. The fuel cell is adapted to receive a fuel flow from the fuel supply, and an oxidant flow from the oxidant supply. The controller is connected to each of the fuel supply, oxidant supply, power demand sensor, and heat demand sensor. The controller is further adapted to receive a power demand signal from the power demand sensor and a heat demand signal from the heat demand sensor.
In a first state, the controller is configured to reduce at least one of the fuel flow and oxidant flow when there is no heat demand signal and no power demand signal. In a second state, the controller is configured to increase at least one of the fuel flow and oxidant flow when there is no heat demand signal and there is a power demand signal. In a third state, the controller is configured to increase at least one of the fuel flow and oxidant flow when there is no power demand signal and there is a heat demand signal. In a fourth state, the controller is configured to increase at least one of the fuel flow and oxidant flow when there is a power demand and a heat demand signal.
In some embodiments, the power demand sensor is a fuel cell voltage sensor that produces a power demand signal when a voltage of the fuel cell falls below a predetermined level. The power demand sensor can also be a fuel cell current sensor that produces a power demand signal when an output current of the fuel cell exceeds a predetermined level. The power demand sensor can also include a fuel cell output current sensor an electrical load sensor, wherein the power demand sensor produces a power demand signal when an electrical load on the fuel cell exceeds an output current of the fuel cell. It will be appreciated that the electrical load on the fuel cell can include a parasitic system electrical load and an application electrical load. For example, the parasitic load can refer to internal components such as pumps and blowers that are powered by the fuel cell. The application load can

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