Apparatus for high efficiency operation of fuel cell systems and method of manufacturing same

A drive circuit comprising a DC bus configured to supply power to a load, a first fuel cell coupled to the DC bus and configured to provide a first power output to the DC bus, and a second fuel cell coupled to the DC bus and configured to provide a second power output to the DC bus supplemental to the first fuel cell. The drive circuit further includes an energy storage device coupled to the DC bus and configured to receive energy from the DC bus when a combined output of the first and second fuel cells is greater than a power demand from a load, and provide energy to the DC bus when the combined output of the first and second fuel cells is less than the power demand from the load.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to systems that derive their power from fuel cells, and, more specifically, to an apparatus and method for improving the service life and efficiency of such systems.

Fuel cell technology has been incorporated in vehicles ranging from automobiles and buses to forklift trucks. While vehicles using fuel cell propulsion systems may produce low to near-zero emissions, incorporating fuel cell systems into such vehicles typically increases a cost of the vehicle (both in initial cost as well as in operating costs due to a relatively short service life of the fuel cell system) and reduces the range the vehicles may travel. Accordingly, acceptance of fuel cell technology vehicles has generally been limited in the marketplace.

Typically, fuel cell propulsion systems are sized to meet the peak transient requirements for system operation. In a fuel cell vehicle, peak transients generally occur over periods of steep acceleration, during which the system draws significantly more power from the fuel cell than during periods where the vehicle moves at constant speed. Sizing fuel cells to meet peak power requirements during periods of steep acceleration may result in vehicles which have fuel cells that are significantly larger than desired for the majority of driving situations.

Developing fuel cell vehicles with single fuel cells designed to meet the maximum power demand requirements, typically results in fuel cells that are expensive, heavy, and that have a short service life. Because the service lifetime of a fuel cell generally decreases as the total number of transients experienced by the fuel cell increases, having a single large cell may result in frequent replacement of one of the most expensive components in the vehicle. Because the cost of replacing a fuel cell can be a large percentage of the vehicle's total operating costs, decreasing the size of fuel cells and increasing the service life of the fuel cell are two factors in reducing the overall cost of operation of fuel cell vehicles.

It would therefore be desirable to have a fuel cell propulsion system that reduces the number of transients experienced by the fuel cell. It would also be desirable to have a propulsion system in which the size and cost of the fuel cell can be reduced from levels typical for current propulsion systems while offering performance comparable to systems having larger fuel cells.

BRIEF DESCRIPTION OF THE INVENTION

According to an aspect of the invention, a drive circuit comprising a DC bus configured to supply power to a load, a first fuel cell coupled to the DC bus and configured to provide a first power output to the DC bus, and a second fuel cell coupled to the DC bus and configured to provide a second power output to the DC bus supplemental to the first fuel cell. The drive circuit further includes an energy storage device coupled to the DC bus and configured to receive energy from the DC bus when a combined output of the first and second fuel cells is greater than a power demand from the load, and provide energy to the DC bus when the combined output of the first and second fuel cells is less than the power demand from the load.

In accordance with another aspect of the invention, a method of manufacturing that includes configuring a DC link to provide electrical power to a traction motor, the traction motor having a loading on the DC link, coupling a first fuel cell and a second fuel cell to the DC link, each fuel cell configured to output electrical power to the DC link, and coupling a first energy storage device to the DC link, the first energy storage device configured to receive energy from the DC link when a combined power output of the first and second fuel cells is greater than a power demand from a loading and configured to provide energy to the DC link when the combined output of the first and second fuel cells is less than the power demand from the loading.

In accordance with another aspect of the invention, a fuel cell propulsion system including a first fuel cell configured to output power to a vehicle traction motor load, a second fuel cell configured to output power to the vehicle traction motor load, and an energy storage device configured to output power to the vehicle traction motor load. The system further includes a controller configured to regulate energy to and from the energy storage device such that the fuel cells provide energy to the energy storage device when combined power output from the fuel cells exceeds a power demand of the vehicle traction motor, and the energy storage device provides power to vehicle traction motor when combined power output from the fuel cells is less than the power demand of the vehicle traction motor.

DETAILED DESCRIPTION

The invention includes embodiments that relate to hybrid and electric vehicles. The invention also includes embodiments that relate to an auxiliary drive apparatus and to methods for manufacturing auxiliary drive systems.

According to an embodiment of the invention, a hybrid vehicle propulsion system100is illustrated inFIG. 1. Electrical power is provided to a DC bus or DC link102via a traction motor drive circuit104. In this embodiment, a traction motor106is coupled to a voltage inverter108, which is coupled to a dynamic retarder110via DC bus102. A high-side energy storage device112is coupled to the retarder110via DC bus102as well. Traction motor drive circuit104includes one or more DC-to-DC voltage converters114that are coupled to fuel cells116,118, a low-side energy storage device120, and auxiliary loads122.

In embodiments of the invention, low side energy storage device120may be a battery, an ultracapacitor, a flywheel, or the like. In embodiments of the invention, voltage converters114may be a buck regulator, a buck converter, a boost regulator, a boost converter, or a bi-directional buck/boost converter. Fuel cells116,118are coupled to fuel cell system controls124. A controller126is coupled to fuel cell system controls124, voltage converters114, voltage inverter108, dynamic retarder110, and motor106.

In operation, voltage inverter108receives a DC power signal provided by traction motor drive circuit104to DC bus102and converts the DC signal into an AC power signal suitable to drive traction motor106, which may be configured to propel a hybrid vehicle (not shown). Traction motor drive circuit104generates a DC power signal via fuel cells116,118and low-side energy storage device120. The DC power signal is output to DC bus102via the one or more voltage converters114. Dynamic retarder110is used during the recapture of electrical energy from traction motor106during braking. High-side energy storage device112is configured to supply electrical power to inverter108in one operating mode, during vehicle acceleration for example. In another operating mode, during regenerative braking for example, high-side energy storage device112may supply electrical power via the voltage converters114to low-side energy storage device120, or to auxiliary loads122.

Controller126regulates the output of both voltage inverter108and DC-to-DC voltage converters114. Through its control of the output voltage of each of the voltage converters114, controller126determines what proportion of the electrical energy driving traction motor106comes from each of the fuel cells116,118and from low-side energy storage device120. Controller126also regulates the operation of fuel cell control system124, which is configured to implement on/off sequencing of fuels cells116,118to extend the service life thereof while providing optimal efficiency independent of the instantaneous power demands of traction motor106. While fuel cell control system124is depicted as a single element inFIG. 1, in an embodiment of the invention, each fuel cell116,118has its own fuel cell controller, as discussed below and shown inFIG. 3. Furthermore, although only two fuel cells116,118are illustrated, embodiments of the invention illustrated herein are not limited to two, but may include more than two fuel cells coupled to voltage converters114and controlled by fuel cell system control124.

FIG. 2illustrates a hybrid vehicle propulsion system200according to an embodiment of the invention. System200, in this embodiment, is a “plug-in” version configured to receive power from a utility grid230. And, although illustrated with respect toFIG. 2, it is to be understood that the hybrid systems disclosed herein may all be configured to receive power from a utility grid, such as illustrated with respect toFIG. 2.

According to this embodiment of the invention, hybrid vehicle propulsion system200includes a traction motor drive circuit204, which provides electrical power to a DC bus or DC link202. In this embodiment, a traction motor206is coupled to a voltage inverter208. A dynamic retarder210is also coupled to inverter208via DC bus202. A high-side energy storage device212, which may be one of a battery and an ultracapacitor, is coupled to dynamic retarder210via DC bus202as well. Traction motor drive circuit204comprises one or more bi-directional boost converters214that are coupled to fuel cells216,217,218,219, and to low-side energy storage devices222,223. The fuel cells216,217,218,219are coupled to the boost converters214via a plurality7of coupling devices221, each of which may be a diode, a contactor, a semiconductor switch, or the like.

In embodiments of the invention, low-side energy storage device222may be one of a battery, an ultracapacitor, and a flywheel. Fuel cells216,217,218, and219are coupled to fuel cell system controls224. A controller226is coupled to fuel cell system controls224, boost converters214, and inverter208. An AC-to-DC converter228is coupled between high-side energy storage device212and three-phase utility grid230.

In operation, an AC signal from grid230is converted by an AC-to-DC converter228into a DC signal, the energy from which can be stored in high-side energy storage device212, low-side energy storage device222, low-side energy storage device223, or a combination thereof. In one embodiment, each of the plurality of fuel cells216-219is coupled to a distinct bi-directional buck/boost converter of bi-directional buck/boost converters214, forming pairs thereof. In this embodiment, the plurality of fuel cells216-219may be regenerative or non-regenerative. In an embodiment where fuel cells216-219are regenerative fuel cells, bi-directional buck/boost converters214permit recharging of fuel cells216-219during regenerative braking. The plurality of coupling devices221in system200may include one coupling device for each fuel cell/bi-directional buck/boost converter pair. When the plurality of coupling devices221is a contactor or semiconductor switch, controller226can fully isolate a respective fuel cell216-219from the remainder of circuit200. Electrical energy is thus supplied to traction motor206via bi-directional buck/boost converters214, which are also configured to deliver electrical energy from traction motor206during regenerative braking to the low-side energy storage devices222,223.

In an embodiment of the invention, system200is employed in a multiple fuel cell vehicle. In such an embodiment, controller226is configured to operate fuel cells, such as fuel cells216,217, at a relatively non-varying power output in response to power demands from traction motor206. This relatively stable output is maintained independent of the transient or varying power demands from traction motor206which may be due to different modes of vehicle operation. However, when there are no power demands on the fuel cells216,217, such as, for example, at a stop light, controller226may instruct fuel cells216-219to supply no power until the user accelerates the vehicle. In such a case, the power output from the fuel cells could be reduced. In order for fuel cells216,217to maintain a non-varying output independent of the varying power demands from traction motor206, the output level of fuel cells216,217should be at or below the minimum power level used during vehicle operation.

During periods of acceleration or when climbing a steep hill, traction motor206may demand power in excess of that being supplied by fuel cells216,217. Such sharp increases or variances in power demand may be referred to as transients and can reduce service life of the fuel cell. Because fuel cells216,217maintain relatively non-varying power outputs, power demands in excess of that supplied by fuel cells216,217, including transient or varying power demands, are met by additional fuel cells, for example218,219, along with low-side energy storage devices222,223and high-side energy storage device212. In this manner, fuel cell service lifetimes are extended because fuel cells216,217are not exposed to transient demands. And while fuel cells218,219supply the supplemental power needed during transient demands, these cells may not have to supply energy when the vehicle operates in a low-demand mode, such as cruising at constant speed or driving at low-speed.

Operating the fuel cells in this manner may also be more economical in that the fuel cells can be smaller than would be possible in a vehicle powered only by a single fuel cell. For example, a single fuel cell vehicle may, at times of peak demand, use 150 kW of power. In this case, the single fuel cell would have to be capable of supplying the 150 kW. As such, the fuel cell could be large and costly to operate and replace and would be exposed to transient power demands, thus limiting the service lifetime of the fuel cell. However, a fuel cell propulsion system according to an embodiment of the invention may include a fuel cell of 40 kW providing a relatively stable or non-varying power output adequate for low-power-demand operating modes. Controller226is configured to meet transient power demands using one or more energy storage devices, such as storage devices212,222,223, and using one or more fuel cells, such as fuel cells218,219to supply supplemental power. Controller226could alternate between the two supplemental fuel cells218,219in responding to transient power demands, thus extending the service life of each cell. Cost savings may be realized through both a longer service life for fuel cells and through the use of smaller, less costly fuel cells.

FIGS. 3,6, and7illustrate traction motor drive circuits300,600,700, respectively, according to embodiments of the invention. Thus, circuits300,600,700may be applicable to the hybrid vehicle propulsion system100illustrated inFIG. 1or to the “plug-in” version of a hybrid vehicle propulsion system200illustrated inFIG. 2.

An embodiment of a traction motor drive circuit300is illustrated inFIG. 3and includes two fuel cells302,304, each of which is coupled to a respective DC-to-DC voltage converter306,308. Each of DC-to-DC voltage converters306,308may be one of a uni-directional boost converter and a bi-directional buck/boost converter. Uni-directional boost converters would not supply recharging voltage to fuel cells302,304when, for example, fuel cells302,304are non-regenerative-type cells. In this case, using uni-directional boost converters may provide a cost advantage over bi-directional voltage converters.

DC-to-DC voltage converters306,308are coupled to DC bus/DC link310. Each fuel cell302,304is coupled to a respective fuel cell controller312,314. An energy storage device316, which may be one of a battery, an ultracapacitor, and a flywheel, is coupled to two bi-directional buck/boost converters318,320, both of which are coupled to DC bus310. Auxiliary loads317may also be electrically connected to energy storage device316, and receive power from energy storage device316or from DC link through one or more bi-directional buck/boost converters318,320Though only two fuel cells302,304, two fuel cell controllers312,314, and two DC-to-DC voltage converters306,308are illustrated, one skilled in the art will recognize that embodiments of system300may include more than two fuel cells, fuel controllers and voltage converters. Accordingly, one skilled in the art will recognize that embodiments of system300may include more than two bi-directional buck/boost converters318,320, with each of the more than two bi-directional buck/boost converters coupled to DC bus112and energy storage device316.

Power output of fuel cells302,304is controlled by fuel cell controllers312,314, respectively. Together, the individual fuel cell controllers constitute a fuel cell control system, such as fuel cell control system224shown inFIG. 2. In operation, each of the DC-to-DC voltage converters306,308boosts the voltages from fuel cells302,304when commanded by controllers312,314and supplies the boosted voltage to DC bus/link310. Similarly, when commanded to do so by controller226, bi-directional buck/boost converters318,320boosts the voltage from energy storage device316and supplies the boosted voltage to DC bus310.

The level to which the output voltages of fuel cell302,304are boosted, or stepped up, depends on the manner in which DC-to-DC voltage converters306,308are regulated by controller226. Similarly, the level to which the output voltage of energy storage device316is boosted depends on the manner in which bi-directional buck/boost converters318,320are regulated by controller226.

FIG. 4illustrates an exemplary boost converter400usable in embodiments of boost converters herein. Boost converter400has a transistor or switch402used to control the output voltage of the device400. In an embodiment of the invention, a controller such as controller226(shown inFIG. 2) opens and closes switch402using pulse-width modulation (PWM) to generate the desired output voltage. Pulse-width modulation of a power supply, such as fuel cell302(shown inFIG. 3) and DC-to-DC voltage converter306(shown inFIG. 3), involves modulation of the power supply duty cycle. The resulting output is a series of square waves. By controlling the timing of the square waves, the power supply output signal can be made to simulate a range of DC voltage values.

FIG. 5shows an exemplary bi-directional buck/boost converter500usable in embodiments of bi-directional buck/boost converters herein. Bi-directional buck/boost converter500has two transistor or switches502,504used to control the output voltage of the device. In an embodiment of the invention, controller226(shown inFIG. 2) operates the transistors502,504using PWM to generate the desired output voltage. Power can flow in both directions through converter500. However, the output voltage can be boosted only in one direction, that is, when power is output at a first terminal506. For power flowing in the other direction and output to a second terminal508, bi-directional converter500acts as a buck, or step-down, converter.

FIG. 6illustrates an embodiment of a traction motor drive circuit600in which a plurality of fuel cells602is coupled to a coupling device604. In an alternate embodiment of invention, circuit600includes one coupling device for each of the plurality of fuel cells602. The combined output of the plurality of fuel cells602is coupled through coupling device604to a plurality of bi-directional buck/boost converters606. In an alternate embodiment of the invention, the combined output of the plurality of fuel cells602is also electrically coupled through coupling device604to auxiliary loads611(shown in phantom). Auxiliary loads611can be powered directly from fuel cells602or via bi-directional buck/boost converters606using power from DC Link608. Fuel cells602are also coupled to fuel cell control system607, which, in an embodiment of the invention, includes a separate fuel cell controller (not shown) for each of the plurality of fuel cells602. Each of the plurality of bi-directional buck/boost converters606is coupled to DC link or bus608. A first energy storage device610, which may be a battery or an ultracapacitor, is coupled to a first bi-directional buck/boost converter612. A second energy storage device614, which may be a battery, an ultracapacitor, or a flywheel, is coupled to a second bi-directional buck/boost converter616. First and second bi-directional buck/boost converters612,616are coupled to DC bus608. One skilled in the art will recognize that circuit600is not limited to two energy storage devices but may include a plurality of energy storage devices coupled to one or more bi-directional buck/boost converters.

The use of bi-directional buck/boost converters606allows for recharging of regenerative-type fuel cells602during regenerative braking. Similarly, bi-directional buck/boost converters612,616allow for recharging of energy storage devices610,614during regenerative braking. Having multiple energy storage devices610,614may increase the amount of electrical energy available to devices powered from DC bus608. However, the amount of electrical energy supplied to DC bus608via energy storage devices610,614is determined by the manner in which the voltage output of each of bi-directional buck/boost converters612,616is regulated.

Bi-directional buck/boost converters606step up the voltage output from the plurality of fuel cells602and supply the stepped up voltage to DC bus608. Similarly, bi-directional buck/boost converters612,616step up the voltages from energy storage devices610,614and supplies the stepped up voltages to DC bus/DC link608. The amount of electrical energy output by fuel cells602is determined by the manner in which fuel cell output is regulated by fuel cell control system607. During low-speed driving or cruising at constant speed, fuel cell control system607may be commanded to operate a subset of the plurality of fuel cells602to output power at a non-varying rate to propel the vehicle. During periods of acceleration, when more power is needed, circuit600may meet the transient power demands by drawing supplemental power from energy storage devices610,614and from fuel cells602.

FIG. 7illustrates a traction motor drive circuit700according to an embodiment of the invention. Traction motor drive circuit700includes a plurality of fuel cells702whose combined output is coupled to a boost converter704coupled to DC link706. In an alternate embodiment of circuit700, each of the plurality of fuel cells702is coupled to a coupling device708(shown in phantom), which may be a semiconductor switch, a diode, a contactor, or the like. A first energy storage device710is coupled to a first bi-directional buck/boost converter712, and a second energy storage device714is coupled to a second bi-directional buck/boost converter716. Each of the first and second energy storage devices710,714may be a battery, an ultracapacitor, or a flywheel. Each of the first and second bi-directional buck/boost converters712,716is coupled to DC link706.

In operation, the output voltage of the plurality of fuel cells702is stepped up by boost converter704and output to DC link706. Similarly, the output voltages of storage devices710,714are boosted by converters712,716, respectively, and supplied to DC bus706. In an alternate embodiment, a contactor- or switch-type coupling device708may be operated to isolate the plurality of fuel cells702from the remainder of circuit700. Alternate embodiments may also include a coupling device718(shown in phantom) coupled between the outputs of first energy device710and second energy device714. Coupling device718could be used to recharge one energy storage device by a second energy storage device. For example, if first energy storage device710is a battery and second energy storage device714is an ultracapacitor, the battery710could supply the ultracapacitor714with electrical energy via coupling device718.

FIG. 8is a graphic illustration of a power output plot800of an exemplary hybrid vehicle propulsion system having two fuel cells, a battery, and a load such as an electric motor according to embodiments of the invention. A power demand curve802illustrates an exemplary power demand of the electric motor. A plurality of curves804and811respectively illustrate power provided by first and second fuel cells. An energy storage device power curve812illustrates power output by an energy storage device such as, for example, a battery.

The first fuel cell is operated to experience minimum transients, and the second fuel cell is operated to experience transients when limits of the energy storage device have been reached according to embodiments of the invention. As illustrated, first fuel cell power804is stable or non-varying despite the transiently varying power demand802. Second fuel cell power811is stable during periods when the electric motor power demand802is less than the combined power outputs of first fuel cell804, second fuel cell811, and battery power812such as during period809. During period809, the power demand802is variable, and at times exceeds the total output of the first and second fuel cells804,811, and at other times is less than the total output of the first and second fuel cells804,811. When the power demand is less than the combined power output of the first and second fuel cells804,811, output812from the battery is not needed, and the excess fuel cell energy charges the battery. As illustrated inFIG. 8, when battery power812drops below a zero axis814, the battery may absorb excess power from the first and second fuel cells.

At times during operation, power demand802may experience spikes806,808that exceed the stable output of both fuel cells802,811and battery812. For example, battery power output812may be limited by the physical properties of the battery as illustrated by plateaus803,816. Under such occurrences of spikes806,808, when battery power812reaches maximum battery output803,816, power output of the second fuel cell transiently increases, respectively, at820and822to meet the additional power demand.

Thus, because the battery initially responds to transient power demands exceeding the stable power outputs of the two fuel cells, the second fuel cell responds to transient demands that exceed the combined stable power outputs of the two fuel cells and the power output of the battery. Reducing exposure of the second fuel cell to transient power demands when limits of the battery have been reached increases the service life of the second fuel cell. Further, because the first fuel cell does not respond to power demand transients such as at spikes806,808, the first fuel cell has a longer service life and, further, does not have to be sized to meet power demands greater than the stable output804.

FIG. 9is a graphic illustration900of power output of an exemplary hybrid vehicle propulsion system having two fuel cells, a battery and a load, such as an electric motor, according to embodiments of the invention. That illustrated inFIG. 9is similar to that illustrated inFIG. 8. However, in this embodiment the maximum power output of the battery is halved and the second fuel cell is caused to experience greater transient fluctuations under the same assumed loading conditions.

Referring now toFIG. 9, a power output plot900is illustrated according to an embodiment of the invention. Power output plot900shows a power demand curve902similar to power demand curve802shown inFIG. 8. A power output curve904of a first fuel cell shows stable or non-varying power output despite varying power demands of the electric motor902. A power output curve910is plotted of a second fuel cell having half the power output of the second fuel cell used for output curve811illustrated inFIG. 8. An energy storage device or battery power curve from912illustrates power output by, for example, a battery having half the power output of the battery used for output curve802illustrated inFIG. 8. Curve910shows a non-zero base power output911that is stable while the electric motor power demand from curve902is less than the stable power output of the first fuel cell904. Curve910shows another non-zero base power output913that is stable while the electric motor power demand from curve902is greater than the stable power output of the first fuel cell904and while the electric motor power demand from curve902is less than a maximum battery power output as illustrated by plateaus916,918of curve912.

Because the battery, in this embodiment, has half the power output of that used inFIG. 8, the second fuel cell is operated to respond to more transient demands than under the same load conditions as those illustrated inFIG. 8such that the first fuel cell experiences minimum transients. When the electric motor power demand902is less than the combined power output of the first and second fuel cells904,910, the excess fuel cell energy charges the battery as illustrated when battery power output line912drops below the zero axis914on graph900, which indicates that the battery is absorbing excess power from the first and second fuel cells. The battery power output912adds supplemental power to the first and second fuel cell power outputs904,910when the power demand902exceeds the power output904of the first fuel cell and the base power output913of the second fuel cell. When the battery output reaches its maximum shown at916,918, the power output of the second fuel cell increases at920,922to meet the power demanded at906,908. Because the battery initially responds to transient demands exceeding the stable power outputs of the two fuel cells, the responses of the second fuel cell to transient power demands are reduced. However, because the battery power output maximum is less than that illustrated inFIG. 8, the transient response requirements of the second fuel cell are increased.

Thus, between the two illustrations ofFIGS. 8 and 9, one skilled in the art will recognize that a tradeoff may be made between battery capacity and transient requirements of the second fuel cell. A large battery capacity may reduce or eliminate the transient requirements of the fuel cell under designated operating conditions. However, it does so at the expense of a larger and more expensive battery. Conversely, a smaller battery capacity may result in increased transient requirements of the second fuel cell, thus negatively impacting its life. As such, the up-front cost of a large battery may be used to offset the long-term life costs of a fuel cell, and vice versa, according to embodiments of the invention.

According to one embodiment of the invention, a drive circuit comprising a DC bus configured to supply power to a load, a first fuel cell coupled to the DC bus and configured to provide a first power output to the DC bus, and a second fuel cell coupled to the DC bus and configured to provide a second power output to the DC bus supplemental to the first fuel cell. The drive circuit further includes an energy storage device coupled to the DC bus and configured to receive energy from the DC bus when a combined output of the first and second fuel cells is greater than a power demand from the load, and provide energy to the DC bus when the combined output of the first and second fuel cells is less than the power demand from the load.

In accordance with another embodiment of the invention, a method of manufacturing that includes configuring a DC link to provide electrical power to a traction motor, the traction motor having a loading on the DC link, coupling a first fuel cell and a second fuel cell to the DC link, each fuel cell configured to output electrical power to the DC link, and coupling a first energy storage device to the DC link, the first energy storage device configured to receive energy from the DC link when a combined power output of the first and second fuel cells is greater than a power demand from a loading and configured to provide energy to the DC link when the combined output of the first and second fuel cells is less than the power demand from the loading.

In accordance with yet another embodiment of the invention, a fuel cell propulsion system including a first fuel cell configured to output power to a vehicle traction motor load, a second fuel cell configured to output power to the vehicle traction motor load, and an energy storage device configured to output power to the vehicle traction motor load. The system further includes a controller configured to regulate energy to and from the energy storage device such that the fuel cells provide energy to the energy storage device when combined power output from the fuel cells exceeds a power demand of the vehicle traction motor, and the energy storage device provides power to vehicle traction motor when combined power output from the fuel cells is less than the power demand of the vehicle traction motor.