Patent Description:
As will be appreciated, a fuel cell based power generation system includes a stack of fuel cells, auxiliary subunits, and associated circuitry. The auxiliary subunits typically include a fuel blower, an air blower, an air ventilator, associated motors, and the like. Initially, the auxiliary subunits are powered using an external power source, which in turn aids in heating the stack of fuel cells. Typically, the stack of fuel cells is heated for a time-period in a range from about <NUM> hours to about <NUM> hours until the stack of fuel cells attains a determined temperature at which the stack of fuel cells starts generating electric power. The longer the time required for heating up the stack of fuel cells, the more power is consumed from the external power source for powering of the auxiliary subunits. After the stack of fuel cells starts generating electric power, the electric power required for powering the auxiliary subunits is provided from the heated stack of fuel cells.

Different systems and techniques for providing power to the auxiliary subunits have been proposed. Conventional systems used for providing power to the auxiliary subunits call for use of power converters such as direct current (DC)-DC converters, buck boost converters, and the like. Generally, the auxiliary subunits are coupled to a DC bus and the stack of fuel cells is coupled to the DC bus via power converters.

Further, certain other conventional systems for providing power to the auxiliary subunits entail coupling the auxiliary subunits to both the DC bus and an alternating current (AC) bus. In some scenarios, such systems use three stage power conversion units (DC-AC-DC-AC). Use of the DC-DC converters, the buck boost converters, and/or the three stage power conversion units for powering the auxiliary subunits adds to cost, footprint, and complexity of the power generation systems and reduces efficiency.

<CIT> discloses an apparatus for heating a fuel cell stack in a cold start mode, comprising a fuel cell stack, a power converter, and a controller, wherein the power converter may include a power switch and resistive heating element that is thermally coupled to the fuel cell stack; the controller is configured to activate the power converter, if a temperature is below a predetermined temperature value, to draw current from the fuel cell stack to cause the fuel cell stack to generate heat; and heat from the power converter is also applied to the fuel cell stack. <CIT> discloses a fuel cell stabilisation system for stabilising the supply of electrical power from a fuel cell stack to a DC bus, including: a DC power supply; and a control configured to enable the DC power supply to supply electrical power to the DC bus. <CIT> discloses a system including a power module comprising at least one fuel cell generator for powering a load, and a bypass mechanism having a first, normally-open fast-acting switch that closes in <NUM>-<NUM> msec, and a second, normally-open switch in parallel with the first switch, the bypass mechanism being electrically connected between the load and a second power source, such as a grid source, where the first switch is configured to close in response to a fault event such that when the first switch is closed power to the load is provided from the second power source through the first switch, and the second switch closes after a predetermined time such that power to the load from the second source is provided through the second switch.

In accordance with aspects of the present specification, a fuel cell based power generation system is presented in accordance with claim <NUM>. The fuel cell based power generation system includes a fuel cell assembly configured to generate a DC power. Further, the fuel cell based power generation system includes at least one assembly switching element configured to operatively couple the fuel cell assembly to a first direct current (DC) bus. Furthermore, the fuel cell based power generation system includes at least one converter coupled between the first DC bus and an electrical grid. Moreover, the fuel cell based power generation system includes a plurality of auxiliary loads operatively coupled to the first DC bus at a location between the at least one assembly switching element and the at least one converter, where at least one of the plurality of auxiliary loads is configured to receive power from the fuel cell assembly via the at least one assembly switching element and a controller operatively coupled to the at least one converter, where the controller is configured to allow a voltage of the first DC bus to fluctuate within a range of voltage values.

In accordance with another aspect of the present specification, a method for operating a fuel cell based power generation system, where the fuel cell based power generation system includes a fuel cell assembly, at least one assembly switching element and a plurality of auxiliary loads operatively coupled to a first direct current (DC) bus is presented. The method includes heating the fuel cell assembly in a plurality of stages, where the plurality of stages includes at least a first stage and a second stage. The heating the fuel cell assembly includes during the first stage of the plurality of stages, powering at least one first auxiliary load corresponding to a portion or portions of the fuel cell assembly to heat the portion or portions of the fuel cell assembly. Further, heating the fuel cell assembly includes generating, using the portion or portions of the fuel cell assembly, a first DC power. Additionally, heating the fuel cell assembly includes during the second stage of the plurality of stages, powering at least one second auxiliary load corresponding to another portion or portions of the fuel cell assembly to heat the another portion or portions of the fuel cell assembly based at least in part on the first DC power to generate a second DC power.

Various features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms "first," "second," and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms "a" and "an" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term "or" is meant to be inclusive and mean one, some, or all of the listed items. The use of "including," "comprising" or "having" and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms "connected" and "coupled" are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. The term "operatively coupled," as used herein, refers to direct and indirect coupling. Furthermore, the terms "circuit" and "circuitry" and "controller" may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together to provide the described function.

As will be described in detail hereinafter, various embodiments of a fuel cell based power generation system and a method for operating the fuel cell based power generation system are presented. The exemplary fuel cell based power generation system may be employed in distributed power generation applications, micro-power generation, small-scale energy generation, and/or larger-scale applications, such as power plants or power stations. Also, the exemplary fuel cell based power generation system may be used in the automotive industry. The fuel cell based power generation system employs at least one switching element for coupling a fuel cell assembly to a direct current (DC) bus. In one specific embodiment, the switching element is devoid of controllable semiconductor switches, thereby reducing operational complexity and cost of the fuel cell based power generation system. Further, the DC bus is coupled to an electrical grid via a bidirectional converter.

Furthermore, the fuel cell assembly includes a plurality of fuel cell units coupled to one another. The exemplary method for operating the fuel cell based power generation system involves heating the fuel cell assembly in a plurality of stages. In one example, the fuel cell based power generation system involves heating the fuel cell assembly in two stages. In particular, auxiliary loads corresponding to the fuel cell assembly are powered in two stages. Accordingly, a start-up operation of the fuel cell based power generation system includes at least a first stage and a second stage.

Moreover, the powered auxiliary load aids in heating the fuel assembly. Initially, the auxiliary loads in the fuel cell based power generation system are powered by an external power source. Further, at the first stage of the start-up operation of the fuel cell based power generation system, a first set of fuel cell units in the fuel cell assembly is heated based on the power provided to auxiliary loads corresponding to the first set of fuel cell units from the external power source. The first set of fuel cell units starts generating power upon attaining a determined temperature value.

During the second stage of the start-up operation of the fuel cell based power generation system, in addition to the power drawn from the external power source, the power generated by the first set of fuel cell units aids in powering all the auxiliary loads corresponding to the fuel cell assembly. Consequently, a second set of the remaining fuel cell units is also heated. In one embodiment, once the power generated by the first set of fuel cell units is sufficient to power all the auxiliary loads, drawing power from the external power source to power the auxiliary loads may be ceased. Hence, use of exemplary method aids in reducing power consumed by the auxiliary loads from the external power source for heating the fuel cell assembly when compared to conventional methods of heating a fuel cell assembly where all the fuel cell units of the fuel cell assembly are heated together.

Turning now to the drawings, <FIG> is a diagrammatical representation <NUM> of an exemplary fuel cell based power generation system, according to aspects of the present specification. In particular, <FIG> depicts a fuel cell based power generation system <NUM> including a fuel cell assembly <NUM>, at least one assembly switching element <NUM>, a first DC bus <NUM>, auxiliary loads <NUM>, a converter <NUM>, and a controller <NUM>. As used herein, the term "controller" refers to integrated circuits (ICs), a computer, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit (ASIC), application-specific processors, digital signal processors (DSPs), field programmable gate arrays (FPGAs), and/or any other programmable circuits.

The fuel cell based power generation system <NUM> is coupled to an electrical grid <NUM>. In one embodiment, the electrical grid <NUM> may function as an external power source. In addition, the fuel cell based power generation system <NUM> may be coupled to other external power sources such as a battery, an uninterruptible power supply, a solar power generation system, a wind based power generation system, a thermoelectric device, another fuel cell based power generation system, or the like.

In one embodiment, the first DC bus <NUM> is a main DC bus. The fuel cell assembly <NUM> is coupled to the first DC bus <NUM> via the assembly switching element <NUM>. In accordance with aspects of the present specification, the assembly switching element <NUM> includes a diode coupled in series with a switch. The switch may include a fuse, a relay, a manually operated switch, mechanically operated switch, an electromechanically operated switch, a magnetically operated switch, or the like. Further, in one embodiment, the diode is a blocking diode. In one embodiment, when a non-controllable switch is used, cost and complexity of the fuel cell based power generation system <NUM> may be reduced.

Further, the first DC bus <NUM> is coupled to the converter <NUM>. The converter <NUM> is further coupled to the electrical grid <NUM> via a connector <NUM>. The connector <NUM> may include a disconnect switch, for example, employed to couple or de-couple the electrical grid <NUM> from the fuel cell based power generation system <NUM> based on operating conditions. In one embodiment, the converter <NUM> is a bidirectional inverter. The converter <NUM> is configured to transfer electrical power between the electrical grid <NUM> and at least one of the auxiliary loads <NUM> and the fuel cell assembly <NUM>. The electrical power may be at least one of a DC power and an AC power.

Moreover, the fuel cell assembly <NUM> includes a plurality of fuel cell units (shown in <FIG>), where the fuel cell units are operatively coupled to one another. Further, each fuel cell unit may include a plurality of fuel cell subunits (shown in <FIG>) coupled to one another. Also, each fuel cell subunit may include a stack of fuel cells (not shown) coupled to one another. Moreover, each of the fuel cells includes a cathode, an anode, and an electrolyte.

The auxiliary loads <NUM> are operatively coupled to the first DC bus <NUM>. Particularly, the auxiliary loads <NUM> are coupled to the first DC bus <NUM> at a location between the assembly switching element <NUM> and the converter <NUM>. In one embodiment, the auxiliary loads <NUM> are directly coupled to the first DC bus <NUM>. In another embodiment, the auxiliary loads are indirectly coupled to the first DC bus <NUM> (as shown below in the embodiment of <FIG>, for example). During the startup operation of the fuel cell based power generation system <NUM>, the auxiliary loads <NUM> are powered using power drawn from the electrical grid <NUM> via the converter <NUM>. In accordance with exemplary aspects of the present specification, the fuel cell assembly <NUM>, once at least partially available, is also configured to provide power to the auxiliary loads <NUM>. In particular, the power generated by the fuel cell assembly <NUM> is conveyed to the first DC bus <NUM> via the assembly switching element <NUM>. Subsequently, the power generated by the fuel cell assembly <NUM> is conveyed from the first DC bus <NUM> to the auxiliary loads <NUM>. The auxiliary loads <NUM> include a fuel blower, an air blower, an air ventilator, and the like.

In one embodiment, each fuel cell unit may have a corresponding auxiliary load <NUM>. In certain embodiments, each fuel cell subunit may also have a corresponding auxiliary load <NUM>. It may be noted that typically in fuel cell based power generating systems, the voltage of the DC bus is maintained at a single value. However, maintaining the voltage of the DC bus at the single value does not allow use of the DC bus to cater to different voltage requirements of all the auxiliary loads that are operatively coupled to the DC bus. Shortcomings of the presently existing systems are circumvented by the fuel cell based power generation system <NUM>.

To that end, the controller <NUM> is configured to allow a voltage of the first DC bus <NUM> to fluctuate within a range of voltage values. In one example, the controller <NUM> is configured to control the operation of the converter <NUM> to allow the voltage of the first DC bus <NUM> to fluctuate within the range of voltage values. In certain embodiments, the range of voltage values may include a lower limit and an upper limit of the voltage values. In one example, if a lower voltage limit of the fuel cell assembly <NUM> is greater than a lower voltage limit of the auxiliary loads <NUM>, the lower limit of the range of voltage values of the first DC bus <NUM> is equivalent to the lower voltage limit of the fuel cell assembly <NUM>. In another example, if the lower voltage limit of the auxiliary loads <NUM> is greater than the lower voltage limit of the fuel cell assembly <NUM>, the lower limit of the range of voltage values of the first DC bus <NUM> is equivalent to the lower voltage limit of auxiliary loads <NUM>.

It may be noted that if the voltage across the fuel cell assembly <NUM> is lower than the lower voltage limit of the fuel cell assembly <NUM>, the anode of the fuel cell assembly <NUM> may be oxidized, which in turn may adversely impact the fuel cell assembly <NUM>. Additionally, if the voltage provided to the auxiliary loads <NUM> is lower than the lower voltage limit of the auxiliary loads <NUM>, the operation of the auxiliary loads <NUM> is impacted.

Additionally, the upper limit of the range of voltage values is determined based on the maximum value of voltage that the auxiliary loads <NUM> can withstand. It may be noted that if the voltage provided to the auxiliary loads <NUM> exceeds the maximum value of voltage that the auxiliary loads <NUM> can withstand, the auxiliary loads <NUM> may break down. In one example, the range of voltage values of the first DC bus <NUM> may be in a range from about <NUM> volts to about <NUM> volts. In another example, the range of voltage values of the first DC bus <NUM> may be in a range from about <NUM> volts to about <NUM> volts. Maintaining voltage of the first DC bus <NUM> in the range of voltage values aids in catering to the different requirements of all the auxiliary loads <NUM> that are coupled to the first DC bus <NUM>.

Furthermore, in accordance with aspects of the present specification, the fuel cell assembly <NUM> having the plurality of fuel cell units is heated in two stages. In one embodiment, auxiliary loads <NUM> are configured to heat the fuel cell assembly <NUM> in a plurality of stages, such as the first stage and the second stage. In an even more specific embodiment, the auxiliary loads <NUM> corresponding to the plurality of fuel cell units are powered in two stages.

During the first stage of start-up operation of the fuel cell based power generation system <NUM>, auxiliary loads corresponding to the first set of fuel cell units of the fuel cell assembly <NUM> are powered using power provided by the electrical grid <NUM>. Powering of the auxiliary loads of the first set of fuel cell units aids in heating the first set of fuel cell units. Subsequently, the first set of fuel cell units generates power. Further, the power generated by the first set of fuel cell units is configured to power corresponding auxiliary loads.

Moreover, during the second stage of the start-up operation of the fuel cell based power generation system <NUM>, auxiliary loads corresponding to a second set of fuel cell units of the fuel cell assembly <NUM> are powered using the power generated by the first set of fuel cell units. Consequent to the powering of the auxiliary loads corresponding to the second set of fuel cell units, the second set of fuel cell units is heated. Once the second set of fuel cell units is heated, the second set of fuel cell units starts generating power.

Subsequently, a portion of power generated by the first and second set of fuel cell units is provided to power the auxiliary loads corresponding to the first and second set of fuel cell units. The remaining portion of the power generated by the first and second set of fuel cell units is supplied to the electrical grid <NUM> via the converter <NUM>. Since the power generated by the plurality of fuel cell units is employed to partially power the auxiliary loads during the start-up operation, the power consumed by the auxiliary loads from the external power source for heating the fuel cell assembly is considerably reduced. In one example, the plurality of fuel cell subunits may also be heated in two stages. The method of operating a fuel cell assembly will be described in greater detail with respect to subsequent figures.

Although the example of <FIG> depicts the fuel cell assembly <NUM> as being operatively coupled to the first DC bus <NUM> via a single assembly switching element <NUM>, use of any number of assembly switching elements <NUM> is anticipated. Examples of the architecture and method of operation of the fuel cell based power generation system will be described in greater detail with respect to subsequent figures.

Referring now to <FIG>, a diagrammatical representation <NUM> of one embodiment of the fuel cell based power generation system <NUM> of <FIG>, according to aspects of the present specification, is presented. In particular, <FIG> represents a detailed architecture of a <NUM> MW fuel cell based power generation system <NUM>.

The fuel cell based power generation system <NUM> includes a fuel cell assembly <NUM>. The fuel cell assembly <NUM> is operatively coupled to a first DC bus <NUM>, such as the first DC bus <NUM> of <FIG>. The first DC bus <NUM> includes a positive DC line <NUM>, a neutral line <NUM>, and a negative DC line <NUM>.

In the example of <FIG>, the fuel cell assembly <NUM> includes four fuel cell units <NUM>, <NUM>, <NUM>, <NUM>. For ease of explanation, the fuel cell units <NUM>, <NUM>, <NUM>, and <NUM> may be referred to as first, second, third, and fourth fuel cell units. In one embodiment, each of the fuel cell units <NUM>, <NUM>, <NUM>, <NUM> is configured to output a power of about <NUM> kW.

The first fuel cell unit <NUM> is operatively coupled to the first DC bus <NUM> via an assembly switching element <NUM>. In particular, a positive terminal of the assembly switching element <NUM> is coupled to the positive DC line <NUM> and a negative terminal of the assembly switching element <NUM> is coupled to a common terminal <NUM>. Similarly, the second, third, and fourth fuel cell units <NUM>, <NUM>, <NUM> are coupled to the first DC bus <NUM> via respective assembly switching elements <NUM>, <NUM>, <NUM>. In particular, a positive terminal of the assembly switching element <NUM> is coupled to the positive DC line <NUM> and a negative terminal of the assembly switching element <NUM> is coupled to the common terminal <NUM>. In a similar manner, negative terminals of the assembly switching elements <NUM>, <NUM> are coupled to the negative DC line <NUM> and positive terminals of the assembly switching elements <NUM>, <NUM> are coupled to the common terminal <NUM>. The common terminal <NUM> is coupled to the neutral line/ground line <NUM>.

Each of the assembly switching elements <NUM>, <NUM>, <NUM>, <NUM> includes a switch and optionally a diode in series with the switch (with an example of a switch and diode being shown in <FIG>). In one embodiment, the switch is a non-controllable switch such as a relay. In another embodiment, the switch is a manually operated switch. As discussed above with respect to <FIG>, use of other types of switches is also envisioned.

Furthermore, the fuel cell based power generation system <NUM> is operatively coupled to an external power source. In one embodiment, the external power source is an electrical grid <NUM>. The fuel cell based power generation system <NUM> also includes a converter <NUM>, such as the converter <NUM> of <FIG>. The electrical grid <NUM> is coupled to the first DC bus <NUM> via the converter <NUM>. In one embodiment, the converter <NUM> is a bidirectional inverter.

Additionally, the fuel cell based power generation system <NUM> includes auxiliary loads <NUM> that are operatively coupled to the first DC bus <NUM> via a variable frequency drive <NUM>. The auxiliary loads <NUM> operate in conjunction with the fuel cell assembly <NUM>. The auxiliary loads <NUM> may include air blower, fuel blower, air ventilators, and the like. The air blower, fuel blower, and air ventilators have associated motors. The variable frequency drive <NUM> is configured to control speed and torque of the motors associated with auxiliary loads <NUM> by varying a corresponding motor input frequency and/or voltage.

In the example of <FIG>, the auxiliary loads <NUM> include a first auxiliary load <NUM>, a second auxiliary load <NUM>, a third auxiliary load <NUM>, and a fourth auxiliary load <NUM>. The first auxiliary load <NUM> corresponds to the first fuel cell unit <NUM>, the second auxiliary load <NUM> corresponds to the second fuel cell unit <NUM>, the third auxiliary load <NUM> corresponds to the third fuel cell unit <NUM>, and the fourth auxiliary load <NUM> corresponds to the fourth fuel cell unit <NUM>. In one embodiment, the first, second, third, and fourth auxiliary loads <NUM>, <NUM>, <NUM>, <NUM> may be shared among the first, second, third, and fourth fuel cell units <NUM>, <NUM>, <NUM>, <NUM>.

As noted hereinabove, the fuel cell assembly <NUM> includes fuel cell units <NUM>, <NUM>, <NUM>, and <NUM>. Each of the fuel cell units <NUM>, <NUM>, <NUM>, and <NUM> may include a plurality of fuel cell subunits (shown in <FIG>). As noted hereinabove, the fuel cell assembly <NUM> may operate in conjunction with the corresponding auxiliary loads <NUM>. In addition, the fuel cell assembly <NUM> operates in conjunction with components (not shown in <FIG>) such as a natural gas pipeline, a burner, catalytic partial oxidation (CPOx) reformer, a heater, or the like.

In accordance with aspects of the present specification, a start-up operation of the fuel cell based power generation system <NUM> entails heating the fuel cell assembly <NUM> in a plurality of stages. In certain embodiments, the plurality of stages may include a first stage and a second stage of heating the fuel cell assembly <NUM>.

During the first stage, at least one first auxiliary load corresponding to a portion or portions of the fuel cell assembly <NUM> is powered to heat the portion or portions of the fuel cell assembly <NUM>. The portion or portions of fuel cell assembly <NUM> that have been heated may then be used to generate a first DC power. During the second stage, at least one second auxiliary load corresponding to another portion or other portions of the fuel cell assembly <NUM> is powered to heat the another portion or other portions of the fuel cell assembly <NUM> based at least in part on the first DC power to generate a second DC power. In one embodiment, the portion or portions of the fuel cell assembly <NUM> heated during the first stage include one or more of the plurality of fuel cell subunits, however, in another embodiment, the portion or portions of the fuel cell assembly heated during the first stage include one or more of the plurality of fuel cell units. The examples of <FIG> and <FIG> are described on a fuel cell unit basis and the example of <FIG> is described on a fuel cell subunit basis. Moreover, the portion or portions of the fuel cell assembly heated in the first stage may be either entire or partial fuel cell units.

In one example, the fuel cell assembly <NUM> may be heated in two stages. It may be noted that, in one embodiment, the number of fuel cells in the first fuel cell unit <NUM> is lower than the number of fuel cells in the combination of the other fuel cell units <NUM>, <NUM>, <NUM>. In such an embodiment, the time required for heating the first fuel cell unit <NUM> is substantially lower than the time required for heating the combination of the other fuel cell units <NUM>, <NUM>, <NUM>. Consequently, the first fuel cell unit <NUM> starts generating power within a shorter span of time, when compared to time required for generation of power by a combination of other fuel cell units <NUM>, <NUM>, <NUM>. In one embodiment, the first fuel cell unit <NUM> is heated during a first stage of the start-up operation of the system <NUM> and the other fuel cell units <NUM>, <NUM>, <NUM> are heated during a second stage of the start-up operation of the system <NUM>. As a result, the fuel cell assembly <NUM> starts generating power in a shorter time when compared to a scenario where a combination of the other fuel cell units <NUM>, <NUM>, <NUM> are heated during the first stage. Once the first fuel cell unit <NUM> starts generating power, the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM> are powered based at least in part on the power generated by the first fuel cell unit <NUM>, thereby reducing import of power from the electrical grid <NUM>.

In particular, during the first stage of the start-up operation of the fuel cell based power generation system <NUM>, the first auxiliary load <NUM> corresponding to the first fuel cell unit <NUM> is powered using the electrical grid <NUM>. In one embodiment, power from the electrical grid <NUM> is provided to the first auxiliary load <NUM> via the converter <NUM>. In one example, the first auxiliary load <NUM> is an air blower. Accordingly, in this example, the amount of air blown from the air blower <NUM> is based on the power provided to the air blower <NUM> from the electrical grid <NUM>. Further, the air from the air blower <NUM> is combined with fuel, such as natural gas provided from the natural gas pipeline and provided to the first fuel cell unit <NUM>. At least one of the burner, the reformer, and the heater may be employed to heat the fuel before providing the fuel to the first fuel cell unit <NUM>. Providing the fuel along with air from the air blower to the first fuel cell unit <NUM> aids in heating the first fuel cell unit <NUM>.

Once the first fuel cell unit <NUM> is heated to attain a determined temperature value, the first fuel cell unit <NUM> begins to generate power. In this example, the power generated by the first fuel cell unit <NUM> may be referred to as a first DC power. Further, the power later to be generated by the other fuel cell units <NUM>, <NUM>, and <NUM> may be referred to as a second DC power. Finally, the power provided to the first auxiliary load <NUM> from the electrical grid <NUM> is referred to as a third DC power. In one example, the determined temperature value may be about <NUM>° C. The first DC power is provided to the first auxiliary load <NUM> via the first DC bus <NUM> and the variable frequency drive. In one example, the first DC power is provided to the first auxiliary load <NUM> via the first DC bus <NUM> and the variable frequency drive when the assembly switching element <NUM> is in a closed state. It may be noted that, in one embodiment, the assembly switching element <NUM> is the closed state when a voltage at the first fuel cell unit is greater than voltage at the first DC bus <NUM>. In another embodiment, the assembly switching element <NUM> is opened when a voltage at the first fuel cell unit <NUM> is lower than the voltage at the first DC bus <NUM>.

Further, if the first DC power is sufficient to power the first auxiliary load <NUM>, then the power being supplied to the first auxiliary load <NUM> from the electrical grid <NUM> may be terminated and the first DC power may be used to power the first auxiliary load <NUM>.

Subsequently, at the second stage of start-up operation of the system <NUM>, the auxiliary loads <NUM>, <NUM>, <NUM> are powered. In one embodiment, the auxiliary loads <NUM>, <NUM>, <NUM> are powered using the power provided from the electrical grid <NUM>. In certain situations, the first DC power may be sufficient for powering additional auxiliary loads. Accordingly, in one example, additional auxiliary loads, such as the auxiliary load <NUM> may also be powered by the first DC power. Furthermore, in this example, the electrical grid <NUM> may continue to supply power to any remaining auxiliary loads, such as auxiliary loads <NUM>, <NUM>.

Also, if the first DC power provided to the first DC bus <NUM> is sufficient to power all the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM>, the converter <NUM> is configured to stop the import of power from the electrical grid <NUM> for powering the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM>. In this scenario, the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM> may be powered by the first DC power generated by the first fuel cell unit <NUM>.

Supply of power to the second, third, and fourth auxiliary loads <NUM>, <NUM>, <NUM>, results in the second, third, and fourth fuel cell units <NUM>, <NUM>, <NUM> being heated. Once the second, third, and fourth fuel cell units <NUM>, <NUM>, <NUM> attain a determined temperature value, the second, third, and fourth fuel cell units <NUM>, <NUM>, <NUM>, respectively, generate power. This power generated by the second, third, and fourth fuel cell units <NUM>, <NUM>, <NUM> is provided to the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM> via the first DC bus <NUM>. As noted hereinabove, the power generated by the second, third, and fourth fuel cell units <NUM>, <NUM>, <NUM> is referred to as a second DC power.

If a combination of the first and second DC powers provided to the first DC bus <NUM> exceeds the power required by the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM>, the converter <NUM> is configured to direct any excess power to the electrical grid <NUM>. In one example, if the power supplied to the first DC bus <NUM> is higher than a threshold value of power, the converter <NUM> is configured to direct the excess power to the electrical grid <NUM>. In one example, the threshold value of power of the first DC bus <NUM> may be summation of the power requirement of all the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM>. The phrase 'excess power," as used herein, is representative of a difference between the power generated by the fuel cell units <NUM>, <NUM>, <NUM>, <NUM> and the threshold value. In another example, if the power provided to the first DC bus <NUM> by the fuel cell units <NUM>, <NUM>, <NUM>, <NUM> is lower than the threshold value, the converter <NUM> is configured to import power from the electrical grid <NUM>. Consequently, the converter <NUM> is configured to "regulate" transfer of power between the electrical grid <NUM> and the first DC bus <NUM> thereby aiding in maintaining the voltage of the first DC bus <NUM> within the above-discussed range of voltage values.

As noted hereinabove, the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM> are powered using a combination of the power generated by the first fuel cell unit <NUM> and the power provided by the electrical grid <NUM> instead of entirely relying on the electrical grid <NUM> for powering the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM>. In certain other scenarios, the auxiliary loads <NUM>, <NUM>, ,<NUM>, <NUM> may be entirely powered by the power generated by the first fuel cell unit <NUM> without importing any power from the electrical grid <NUM>.

Therefore, the amount of power consumed from the electrical grid <NUM> to power the auxiliary loads <NUM>, <NUM>, <NUM>, <NUM> of the fuel cell based power generation system <NUM> is substantially lower when compared to the power consumed by a conventional fuel cell based power generation system. By way of example, a conventional <NUM> MW fuel cell based power generation system uses about <NUM> kWh to heat the fuel cell assembly. However, by implementing the <NUM> MW fuel cell based power generation system <NUM> as described hereinabove, the power required for heating the fuel cell assembly <NUM> is about <NUM> kWh, thereby resulting in a reduction in the power consumption of about <NUM> kWh. In one example, the amount of power drawn from the electrical grid <NUM> to power the auxiliary loads <NUM> of the fuel cell assembly <NUM> is reduced from <NUM> kWh to <NUM> kWh.

Although the example of <FIG> represents the auxiliary loads <NUM> as including the first, second, third, and fourth auxiliary loads <NUM>, <NUM>, <NUM>, <NUM>, different arrangements of the auxiliary loads are envisaged. Further, although the example of <FIG> represents use of only a single variable frequency drive to couple the first, second, third and fourth auxiliary loads to the first DC bus, use of different number of variable frequency drives is envisaged. Also, different techniques of coupling of the first, second, third, and fourth auxiliary loads to the first DC bus may be anticipated.

<FIG> is a diagrammatical representation <NUM> of one embodiment of a fuel cell unit for use in the fuel cell based power generation system <NUM> of <FIG>, according to aspects of the present specification. In particular, <FIG> represents one fuel cell unit such as the fuel cell units <NUM>, <NUM>, <NUM>, <NUM>.

The fuel cell unit <NUM> includes a plurality of fuel cell subunits <NUM> and a plurality of subunit switching elements <NUM>. One fuel cell subunit <NUM> is coupled to another fuel cell subunit <NUM> via a corresponding subunit switching element <NUM>. Further, positive terminals <NUM> of the subunit switching elements <NUM> are coupled together to form a positive DC line <NUM>. Similarly, negative terminals <NUM> of the subunit switching elements <NUM> are coupled together to form a negative DC line <NUM>. A combination of the positive DC line <NUM> and the negative DC line <NUM> forms a subunit DC bus <NUM>. The subunit DC bus <NUM> may be coupled to a first DC bus, such as the first DC bus <NUM> of <FIG>. Further, the first DC bus is coupled to an electrical grid. Therefore, the subunit DC bus <NUM> is coupled to the electrical grid via the first DC bus.

Each subunit switching element <NUM> typically includes a switch <NUM> and a blocking diode <NUM>. The switches <NUM> may include the same types of switches discussed above with respect to the fuel cell assembly and units. The subunit switching element <NUM> aids in de-coupling/coupling the fuel cell subunit <NUM> from the subunit DC bus <NUM> and thus, from the first DC bus <NUM>. The de-coupling of the fuel cell subunit <NUM> from the subunit DC bus <NUM> aids in preventing backflow of current to the fuel cell subunit <NUM> from the electrical grid.

Further, each fuel cell subunit <NUM> includes a plurality of fuel cells. Reference numeral <NUM> depicts a first set of fuel cell subunits (which may include one fuel cell subunit as shown or more fuel cell subunits) and reference numeral <NUM> depicts a second set of fuel cell subunits. In the current specification, the first set of fuel cell subunits <NUM> may also be referred to as one or more fuel cell subunits of the plurality of fuel cell subunits and the second set of fuel cell subunits <NUM> may also be referred to as other fuel cell subunits of the plurality of fuel cell subunits.

As noted hereinabove, a fuel, such as natural gas, is provided to the fuel cells. In one embodiment, the first set of fuel cell subunits <NUM> and the second set of fuel cell subunits <NUM> are configured to receive fresh fuel. The fresh fuel may be provided to the first set of fuel cell subunits <NUM> and the second set of fuel cell subunits <NUM> via independent fuel channels. In one embodiment, the first set of fuel cell subunits <NUM> is heated in a first stage and subsequently, the second set of fuel cell subunits <NUM> is heated in a second stage. In this embodiment, during the first stage, fresh fuel is provided to the first set of fuel cell subunits <NUM> and the second set of fuel cell subunits <NUM>. Subsequently, during the second stage, supply of the fresh fuel is continued to the second set of fuel cell subunits <NUM>, while the supply of fresh fuel to the first set of fuel cell subunits <NUM> is discontinued. In this scenario, an exhaust gas of the second set of fuel cell subunits <NUM> is provided to the first set of fuel cell subunits <NUM>. In this embodiment, the first set of fuel cell subunits <NUM> may be coupled in series with the second set of fuel cell subunits <NUM> and hence, the first and second sets of fuel cell subunits <NUM>, <NUM> are in fluid communication. Specifically, during the second stage, the first and second sets of fuel cell subunits <NUM>, <NUM> are in fluid communication, thereby aiding in supply of exhaust gas from the second set of fuel cell subunits <NUM> to the first set of fuel cell subunits <NUM>. The exhaust gas of the first set of fuel cell subunits <NUM> includes unreacted fuel, which may include methane, carbon monoxide, carbon dioxide, hydrogen, and steam.

Moreover, it may be noted that the number of fuel cells in the first set of fuel cell subunits <NUM> may be substantially lower than the number of fuel cells in the second set of fuel cell subunits <NUM>. In one example, the number of fuel cells in the first set of fuel cell subunits <NUM> is <NUM> and the number of fuel cells in the second set of fuel cell subunits <NUM> is <NUM>.

Also, each fuel cell unit <NUM> is associated with auxiliary loads (not shown in <FIG>) to generate power. In one embodiment, the auxiliary loads may be shared among the first and second set of fuel cell subunits <NUM>, <NUM>. In another embodiment, each of the first and second set of fuel cell subunits <NUM>, <NUM> may have a corresponding set of auxiliary loads.

During a start-up operation of the fuel cell unit <NUM>, at the first stage, power from an external power source is provided to auxiliary loads corresponding to the first set of fuel cell subunits <NUM>. In one example, the external power source may be an electrical grid, such as the electrical grid <NUM> of <FIG>. The first set of fuel cell subunits <NUM> is heated based on the power provided from the electrical grid to the corresponding auxiliary loads. Once the first set of fuel cell subunits <NUM> attains a determined temperature value, the first set of fuel cell subunits <NUM> generates a corresponding DC power. The DC power generated by the first set of fuel cell subunits <NUM> is referred to as a fourth DC power.

In one embodiment, the fourth DC power is conveyed from the first set of fuel cell subunits to the auxiliary loads of the fuel cell unit <NUM> via the subunit DC bus <NUM>. It may be noted that the fourth DC power is provided to the auxiliary loads in addition to the power provided from the electrical grid <NUM>. The supply of power from the electrical grid <NUM> to the auxiliary loads is reduced in light of supply of the fourth DC power from the fuel cell unit <NUM>. Thus, less power is imported from the electrical grid <NUM>. At a certain point in time, if the fourth DC power is sufficient for powering all the auxiliary loads of the fuel cell unit <NUM>, no power may be drawn from the external power source.

During the second stage of start-up operation of the fuel cell unit <NUM>, the second set of fuel cell subunits <NUM> may be heated. Consequently, the second set of fuel cell subunits <NUM> may generate a corresponding DC power. The power generated by the second set of fuel cell subunits <NUM> may be referred to as a fifth DC power. The fourth and fifth DC powers are provided to the first DC bus via the subunit DC bus <NUM>. In one example, the fourth power is a first portion of at least one of the first DC power and the second DC power. In another example, the fifth power is a second portion of at least one of the first DC power and the second DC power. It may be noted that a number of fuel cells heated during the first stage is lesser than a number of the fuel cells heated during the second stage.

According to aspects of the present specification, heating the first and second set of fuel cell subunits <NUM>, <NUM> at different stages aids in reducing the power consumed by auxiliary loads of the fuel cell unit <NUM> from the electrical grid <NUM>.

Although one set of fuel cell subunits has been shown in <FIG> which in one embodiment may be used to start one fuel cell unit that may be used in turn to start other fuel cell units, as discussed above with respect to <FIG>, in another embodiment, more than one fuel cell unit may have one or more fuel cell subunits that are started simultaneously. In such an embodiment, at least two fuel cell subunits may be heated simultaneously, where at least one of the at least two fuel cell subunits may be situated in a different fuel cell unit than other fuel cell subunit of the at least two fuel cell subunits.

<FIG> is a diagrammatical representation <NUM> of another embodiment of the fuel cell based power generation system <NUM> of <FIG>, according to aspects of the present specification. In particular, <FIG> represents a fuel cell based power generation system <NUM> including a plurality of fuel cell units <NUM>, a plurality of assembly switching elements <NUM>, a first DC bus <NUM>, and a converter <NUM>. The fuel cell based power generation system <NUM> is coupled to an external power source, such an electrical grid <NUM>. In another embodiment, the external power source may be a battery, an uninterruptible power supply (UPS), a solar based power source, a wind turbine based power source, a thermoelectric device, and another fuel cell based power generation system. In one embodiment, the converter <NUM> is a bidirectional inverter. In the example of <FIG>, the first DC bus <NUM> is a main DC bus and is operatively coupled to the electrical grid <NUM> via the converter <NUM>.

One fuel cell unit <NUM> is operatively coupled to another fuel cell unit <NUM> via a corresponding assembly switching element <NUM>. Additionally, each fuel cell unit <NUM> is operatively coupled to the first DC bus <NUM> via the corresponding assembly switching element <NUM>. Further, each fuel cell unit <NUM> includes a plurality of fuel cell subunits.

Moreover, the fuel cell based power generation system <NUM> includes a second DC bus <NUM> and an auxiliary load <NUM>. The second DC bus <NUM> is an auxiliary DC bus and is operatively coupled to the first DC bus <NUM> via an auxiliary switching element <NUM>. In one embodiment, the second DC bus is coupled between the auxiliary loads <NUM> and the first DC bus <NUM> via the auxiliary switching element <NUM>. In one example, the auxiliary switching element <NUM> includes a switch <NUM> and a blocking diode <NUM>. In one example, the switch <NUM> is a relay. The blocking diode <NUM> ensures that power flows in a single direction from the first DC bus <NUM> to the second DC bus <NUM>. It may be noted that in the illustrated embodiment of <FIG>, the voltage of the first DC bus <NUM> is higher than the voltage of the second DC bus <NUM>. In one embodiment, the voltage of the second DC bus <NUM> is allowed to fluctuate within a range of voltage values. In one example, the range of voltage values may be in a range from about <NUM> volts to about <NUM> volts.

Also, the fuel cell based power generation system <NUM> depicted in <FIG> for purposes of example includes an additional DC power source <NUM>. The additional DC power source <NUM> is coupled to the second DC bus <NUM>. In one embodiment, the additional DC power source <NUM> is a battery.

Further, the auxiliary load <NUM> is coupled to the second DC bus <NUM>. In this embodiment, the auxiliary load <NUM> is indirectly coupled to the first DC bus <NUM>. In one embodiment, during the start-up operation of the fuel cell based power generation system <NUM>, the additional DC power source <NUM> may be used to supply power to at least some of the auxiliary loads <NUM> of at least one of the fuel cell units of the plurality of fuel cell units. Use of the additional DC power source <NUM> in the fuel cell based power generation system <NUM> enables black start of the auxiliary loads <NUM>. The phrase 'black start of the auxiliary loads,' as used herein, refers to initial powering of the auxiliary loads even if the electrical grid <NUM> is de-coupled from the first DC bus <NUM>. Accordingly, implementing the system <NUM> as described with reference to <FIG> aids in ensuring an uninterrupted supply of power to the auxiliary load <NUM> even in the absence of supply of power from the electrical grid <NUM>.

<FIG> is a flow chart <NUM> representing a method for operating the fuel cell based power generation system <NUM> of <FIG>, according to aspects of the present specification. The method of <FIG> is described with respect to the components of <FIG> and <FIG>. In accordance with aspects of the present specification, the method <NUM> for operating the fuel cell based power generation system <NUM> includes heating the fuel cell assembly <NUM> in a plurality of stages, as indicated by step <NUM>. In one example, the plurality of stages includes at least a first stage and a second stage.

In certain embodiments, step <NUM> of heating the fuel cell assembly <NUM> in the plurality of stages includes sub-steps <NUM>, <NUM>, and <NUM>. In particular, at sub-step <NUM>, during the first stage of the plurality of stages, at least one first auxiliary load corresponding to at least one fuel cell unit of the plurality of fuel cell units is powered to heat the at least one fuel cell unit. Furthermore, the at least one fuel cell unit attains a determined temperature value consequent to the heating of the at least one fuel cell unit. By way of example, the auxiliary load <NUM> is provided power from the electrical grid <NUM> via the converter <NUM>. Consequently, the first fuel cell unit <NUM> is heated. As noted hereinabove with respect to <FIG>, the first auxiliary load <NUM> corresponds to the first fuel cell unit <NUM>. In particular, the first auxiliary load <NUM> aids in heating the first fuel cell unit <NUM>.

Moreover, at sub-step <NUM>, a first DC power is generated using the heated at least one fuel cell unit. For example, the first fuel cell unit <NUM> heated at sub-step <NUM> generates a first DC power once the first fuel cell unit <NUM> attains the determined temperature value. In one example, the determined temperature value is <NUM>° C. This first DC power is provided to the first DC bus <NUM>. If the value of the first DC power generated by the first fuel cell unit <NUM> is sufficient to meet the power requirement of the first auxiliary load <NUM>, the first auxiliary load <NUM> is provided power from the first fuel cell unit <NUM> instead of from the electrical grid <NUM>.

Furthermore, at sub-step <NUM>, during the second stage of the plurality of stages, at least one second auxiliary load corresponding to other fuel cell units of the plurality of fuel cell units is powered to heat the other fuel cell units based at least on the first DC power to generate a second DC power. In one embodiment, the first DC power generated by the first fuel cell unit <NUM> along with the power from the electrical grid <NUM> is provided to the auxiliary loads <NUM>, <NUM>, <NUM> of the other fuel cell units <NUM>, <NUM>, <NUM>. Consequently, the other fuel cell units <NUM>, <NUM>, <NUM> are heated.

In certain situations, if the value of the first DC power generated by the first fuel cell unit <NUM> is sufficient to power the auxiliary loads <NUM>, <NUM>, <NUM> of the other fuel cell units <NUM>, <NUM>, <NUM>, the converter <NUM> is configured to stop import of power from the electrical grid <NUM> for powering the auxiliary loads <NUM>, <NUM>, <NUM>. In this scenario, the other fuel cell units <NUM>, <NUM>, <NUM> are heated based only on the generated first DC power. The other fuel cell units <NUM>, <NUM>, <NUM> generate the second DC power based on the heating of the other fuel cell units <NUM>, <NUM>, <NUM>. The second DC power is also provided to the first DC bus <NUM>.

In one embodiment, a portion of the first DC power and the second DC power is used to power the auxiliary loads <NUM>, <NUM>, <NUM>. Further, the remaining portion of the first DC power and the second DC power is provided to the electrical grid <NUM> via the converter <NUM>. In particular, the controller <NUM> of <FIG> is employed to regulate the converter <NUM> such that if the first and second DC powers supplied to the first DC bus <NUM> from the fuel cell assembly <NUM> exceeds a threshold value, excess power is provided to the electrical grid <NUM> via the converter <NUM>. Supplying the excess power to the electrical grid <NUM> by the converter <NUM> aids in allowing the voltage of the first DC bus <NUM> to fluctuate within a range of voltage values. In particular, the voltage of the first DC bus <NUM> may vary between a lower limit of voltage and an upper limit of voltage. In one embodiment, the lower limit and the upper limit of voltage may be determined using the controller <NUM>. Using the exemplary method for operating the fuel cell based power generation system, the power consumed by auxiliary loads from the external power source for heating up the fuel cell assembly is reduced.

Furthermore, the foregoing examples, demonstrations, and process steps such as those that may be performed by the system may be implemented by suitable code on a processor-based system, such as a general-purpose or special-purpose computer. It should also be noted that different implementations of the present technique may perform some or all of the steps described herein in different orders or substantially concurrently, that is, in parallel. Furthermore, the functions may be implemented in a variety of programming languages, including but not limited to C++ or Java. Such code may be stored or adapted for storage on one or more tangible, machine readable media, such as on data repository chips, local or remote hard disks, optical disks (that is, CDs or DVDs), memory or other media, which may be accessed by a processor-based system to execute the stored code. Note that the tangible media may comprise paper or another suitable medium upon which the instructions are printed. For instance, the instructions may be electronically captured via optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in the data repository or memory.

Various embodiments of a fuel cell based power generation system and a method of operating the fuel cell based power generation system are presented. The systems and methods presented herein aid in heating the fuel cells of the fuel assembly in two stages. Use of the exemplary fuel cell based power generation system aids in heating a larger set of fuel cells using power provided by a smaller set of fuel cells and the external power source, instead of importing power entirely from the external power source. Accordingly, use of the exemplary fuel cell based power generation system aids in lowering the power consumed from the external power source. Moreover, the fuel cell based power generation system uses switching elements which in turn include switches.

Claim 1:
A fuel cell based power generation system (<NUM>), comprising:
a fuel cell assembly (<NUM>) configured to generate a DC power, the fuel cell assembly (<NUM>) comprising a plurality of fuel cell units (<NUM>, <NUM>, <NUM>, <NUM>) operatively coupled to one another;
at least one assembly switching element (<NUM>) configured to operatively couple the fuel cell assembly (<NUM>) to a first direct current (DC) bus (<NUM>);
at least one converter (<NUM>) coupled between the first DC bus and an electrical grid (<NUM>);
a plurality of auxiliary loads (<NUM>) operatively coupled to the first DC bus at a location between the at least one assembly switching element and the at least one converter, wherein each fuel cell unit (<NUM>, <NUM>, <NUM>, <NUM>) has a corresponding auxiliary load (<NUM>), wherein the auxiliary loads are configured to heat the fuel cell assembly in a plurality of stages, and wherein at least one of the plurality of auxiliary loads is configured to receive power from the fuel cell assembly via the at least one assembly switching element; and
a controller (<NUM>) operatively coupled to the at least one converter (<NUM>), wherein the controller is configured to allow a voltage of the first DC bus to fluctuate within a range of voltage values.