WO2013112119A1

WO2013112119A1 - Fuel cell hybrid power system for large vehicles

Info

Publication number: WO2013112119A1
Application number: PCT/US2012/000039
Authority: WO
Inventors: Matthew P. Wilson; Sitaram Ramaswamy
Original assignee: Utc Power Corporation
Priority date: 2012-01-24
Filing date: 2012-01-24
Publication date: 2013-08-01

Abstract

A fuel cell hybrid power system (10) for a large vehicle, for example a bus, includes a fuel cell power plant (FCPP) (12); an electric energy storage system (EEES) (20) capable of being charged and discharged, and an electric traction system (ETS) (18) which is responsive to power from both the FCPP (12) and the (EEES) (20). The FCPP (12) and the EEES (20) are operatively connected at a common junction (26), and the ETS (18) is operatively connected to that common junction (26) via one or more DC/DC converters (16). An air supplier for the FCPP (12) is controlled by a controller (40) to regulate reactant air to, and concomitantly the output voltage of, the FCPP (12). The controller (40) acts to decrease the output voltage of the FCPP (12) during at least part of an interval when the vehicle is braking and the state of charge of the EEES (20) is increasing.

Description

Fuel Cell Hybrid Power System for Large Vehicle BACKGROUND

[0001 ] The disclosure relates generally to fuel cells, and more particularly to fuel cell hybrid power systems for vehicles. More particularly still, the disclosure relates to fuel cell hybrid power systems for large vehicles, as for example busses.

[0002] Fuel cell power plants (FCPP) are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus. In such power plants, one or typically a plurality, of fuel cells are arranged in a fuel cell stack, or cell stack assembly (CSA). Each cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. The anode and the cathode each have a respective catalyst surface, the catalyst typically being platinum (Pt). A reducing fluid, or fuel reactant, such as hydrogen is supplied to the anode electrode, and an oxidant reactant such as oxygen or air is supplied to the cathode electrode. In automotive applications, each cell typically uses a solid polymer proton exchange membrane (PEM) as the electrolyte, and the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce protons and electrons. The electrons are conducted to an external load circuit, such as one or more traction motors, and then returned to the cathode electrode, while the protons transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water.

[0003] Fuel cell power plants are increasingly finding application in hybrid automotive vehicles which combine the fuel cell(s) with some means of storing electrical energy to cooperatively supply electrical energy to electrical traction motors for driving/powering the vehicle. One typical example of such arrangement is disclosed in U. S. Patent 7,831,343 for Efficiency Optimized Hybrid Operation Strategy by Formanski et al, wherein the fuel cell is in cooperative combination with an electrical energy storage system (EESS) to supply electrical energy to power the electric traction system (ETS). The EESS may be a DC battery or ultra capacitor, or the like, which is conveniently capable of being charged/re-charged by the generator power available from the traction motor during regenerative braking.

[0004] Figure 1 depicts a simplified configuration of another existing fuel cell-based hybrid power system 1 10 for a vehicle 108, in which a fuel cell power plant (FCPP) 1 12 supplies electrical power, via a diode 1 14 and one or more DC/DC converters 1 16, to one or more drive motor/generators of an electrical traction system (ETS) 1 18. An electrical energy storage system (EESS) 120 comprised here of a storage battery is similarly connected to the electrical traction system 1 18 via a separate DC/DC converter 124. The DC/DC converters 1 16 and 124 are connected with the traction drive motor/generators of ETS 1 18 at a common junction 126. The battery of EESS 120 may supply power to the traction drive motor/generators of the ETS 1 18, and may also receive re-charging power from regenerative action of that/those

motor/generators during braking.

[0005] Typically, the existing arrangements for fuel cell power plants in hybrid vehicles have been directed to such application in conventional automotive vehicles having weights of several thousand kilograms or less( e. g., less than 3 or 4 tons), which have heretofore been powered by standard-size gasoline engines. Although still confronted with some issues of economic and efficient operation for extended periods, those issues are declining. However, the obstacles to commercially-viable application of fuel cells to much larger hybrid vehicles, such as busses, weighing perhaps as much as 17, 000 kilograms or more, are much greater. This is due in part to the requirement for a heavy duty vehicle drive train that requires a level of durability, reliability, and performance that makes it difficult to hybridize. The use of an energy storage system allows 10 to 20% improvement in fuel economy, but durability and reliability requires the system to be de-rated, or requires a tried and established technology, such as the use of a diesel engine as the principal source of power.

[0006] In order to be competitive with diesel engines as the principal source of power in large/heavy hybrid vehicles, fuel cell power plants should maximize vehicle drive train efficiency, while maintaining durability and reliability. Attainment of this objective correspondingly improves the life cycle value of the fuel cell power plant, thereby enhancing its competitiveness with diesel engines.

SUMMARY

[0007] A fuel cell hybrid power system for a large vehicle (e.g., greater than about 5,000 Kg, as for example a 17,000 Kg bus), comprises a fuel cell power plant (FCPP), an electric energy storage system (EESS) capable of being electrically charged and discharged, and an electric traction system (ETS) that is responsive to power from both the fuel cell power plant and the electric energy storage system. The FCPP and the EESS are operatively connected (electrically substantially in parallel) at a common junction, and the ETS is operatively connected to that common junction via one or more DC/DC converters. In a disclosed embodiment, the EESS may conveniently be an ultra capacitor. Further, in a disclosed embodiment, the voltage cycling of the FCPP during drive cycle operation has a rapid drop in voltage and a slow rise in voltage.

[0008] In a disclosed embodiment, an air supplier responsive to a control signal regulates reactant air supplied to, and concomitantly the output voltage of, the FCPP, and a controller provides the control signal to the air supplier to control

apportionment of power between the FCPP and the EESS.

[0009] In a disclosed embodiment, a voltage-based set point signal which is a function of the state of charge (SOC) of the EESS is connected as an input to the controller for regulating the control signal to the air supplier. That voltage-based control signal sets a fuel cell voltage which decreases during at least part, typically at least the initial half, of an interval during which the SOC of the EESS is increasing. A voltage at the common junction between the FCPP and the EESS is sensed and provided as a feedback control for the set point signal. This mode of controlling the fuel cell voltage during intervals when the EESS is recharging during braking serves to enhance the efficiency and durability of the FCPP by ensuring higher recovery of braking energy and suppression of fuel cell voltage.

[0010] In a disclosed embodiment, a current-based set point signal which is a function of the estimated reactant air flow demand is also connected, with the voltage-based set point signal, as an input to the controller for regulating the control signal to the air supplier. The controller includes selection logic responsive to conditions of the vehicle braking versus accelerating or remaining at constant velocity, for extending the voltage-based set point signal to said air supplier when the vehicle is braking, and alternatively extending the current-based set point signal to said air supplier when the vehicle is accelerating or at constant velocity.

[0011] In a disclosed embodiment, an electrical check valve, such as a power diode, is connected between the FCPP and the EESS to prevent reverse flow of current to the FCPP. The prevention of reverse current 'flow to the FCPP and the regulation of FCPP output voltage through regulation of the air supplier cooperatively serve to ensure higher recovery of braking energy and suppression of fuel cell voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Fig. 1 is a generalized block diagram of a fuel cell-based hybrid power system for a vehicle in accordance with the prior art;

[0013] Fig. 2 is a graph of vehicle operation in the representative

Business/ Arterial/Commuter (BAC) cycle, with emphasis on traction power and vehicle speed;

[0014] Fig. 3 is a schematic block diagram of a fuel cell-based hybrid power system for a large vehicle in accordance with a disclosed embodiment, including a fuel cell power plant (FCPP), an electric energy storage system (EESS) capable of being electrically charged and discharged, and an electric traction system (ETS)of a generalized fuel cell backup power system; and

[0015] Fig. 4 is a functional schematic block diagram of a portion of an included controller utilized in regulating the fuel cell-based hybrid power system of Fig. 3.

DETAILED DESCRD?TION

[0016] Following is described a fuel cell-based hybrid power system for a large vehicle in accordance with a disclosed embodiment. As used herein, the term "large vehicle" is intended to mean a land-based vehicle having a weight greater than about 5,000 Kg, typically in the range of 10,00,0 to 20, 000 Kg, with a disclosed example being associated with a 17,000 Kg bus.

[0017] Referring to Fig. 2, there is depicted a graph of power input to an electrical traction system (ETS) (shown upper in solid line) and of vehicle speed (shown lower in broken line ), for a 17,000 Kg bus, over a representative so-called BAC

(Business/ Arterial/Commuter) driving cycle having a duration of nearly 3,000 seconds (nearly 50 minutes). That depicted BAC cycle includes an initial mode ("Business") designated "1" during which there occur frequent stops and starts, and the speed is less than about 20 mph. A second mode (Arterial) is designated "2", and the speed is somewhat greater (35 mph) and the stops somewhat less frequent. A third mode (Commuter) is designated "3", and the speed is greater (50+ mph) and the stops much less frequent. Notable in each of the modes is the power delivered to the drive motors of the ETS during acceleration and constant velocity and, significantly, the power returned from the traction system during deceleration for use via regenerative braking. The relatively increased mass (weight) of the bus contributes greatly to the energy available during braking, especially from the higher speeds (1/2 mV²). Braking during the Business mode (1) may produce surges exceeding 100 kW; braking from the speeds of the Arterial mode (2) may produce power surges exceeding 275 kW; and braking from the 50+ mph speeds of the Commuter mode (3) may produce power surges exceeding 375 kW, with one such surge being depicted at the conclusion of the trace.

[0018] The well-known principal of regenerative braking is utilized to capture, as electrical energy, much of the energy otherwise lost to heat during braking. The kinetic energy converted to electrical energy during regenerative braking is utilized to charge, or re-charge, a suitable device in an electrical energy storage system (EESS). Correspondingly, that EESS is then able to supply a portion of the electrical energy required to power the traction motor(s) during periods of vehicle acceleration and/or maintenance of constant velocity.

[0019] Referring to Fig. 3, there is depicted a schematic block diagram of a fuel cell- based hybrid power system 10 for a large vehicle 8 in accordance with a disclosed embodiment. The example vehicle 8 may be a bus weighing 17,000 Kg, though other types of large vehicles weighing more than 5,000 Kg, and typically more than 10,000 Kg, may also employ an appropriately scaled fuel cell-based hybrid power system with corresponding benefit. Principal components of the fuel cell-based hybrid power system 10 are a fuel cell power plant (FCPP) 12, an electrical energy storage system (EESS) 20, and an electric traction system (ETS) 18.

[0020] The ETS 18 may include one or more power management electric motors (not shown) for driving the traction wheels of the bus 8 and, during braking, for supplying/returning electrical power to the EESS 20. The principal source of electrical power to drive the ETS 18 is the FCPP 12, which may conveniently include one or more PEM-type fuel cell assemblies and appropriate associated hardware, most of which is not shown separately in detail. For discussion of the disclosed

embodiment, there is depicted an air supplier, or blower, 22, for delivering an oxidant reactant such as air to the fuel cell assemblies of the FCPP 12. The air supplier 22 is conveniently shown as comprising a portion of the FCPP 12, and is capable of regulation to adjust the flow of air to the fuel cells. [0021] The FCPP 12 may conveniently be capable of providing about 450 V at idle, 350 V continuously, and 300 V at peak operating conditions. Similarly, it may provide a current of 40 A at idle, 470 A continuously, and 900 A at peak operating conditions, thus providing net power of about 15 kW at idle, 150 kW continuously, and 250 kW at peak operating conditions.

[0022] The EESS 20 is any of several suitable electrical energy storage systems capable of delivering a stored electrical charge to the ETS 18 and of correspondingly being charged/recharged by electrical energy by regenerative braking from the ETS 18 and/or by the FCPP12.In the disclosed embodiment, the EESS 20 is conveniently comprised of one or more ultra capacitors, separately numbered 21 , having the capability of accepting high currents on a repeated basis, as during braking events, while remaining reasonable in size and cost. The ultra capacitor(s) 21 are capable of 450 V when fully charged (100% SOC), and have a sink power capacity of 400kW (750A peak) and a source power capacity of 80 kW (<10 sec).

[0023] The EESS 20 and the FCPP 12 are connected substantially electrically in parallel at a common electrical junction 26, via power busses 30 and 32, respectively. This enables the voltages to be matched (i.e., the same) during phases of operation. A power diode 14 located in the power bus 32 intermediate the output of FCPP 12 and the junction 26 prevents reverse current on the FCPP during a braking event. The voltage appearing at the common electrical junction 26 is effectively connected to an input/output terminal at one end of a (bi-directional) DC/DC converter 16, which may be comprised of a suitable inverter and associated inductor(s), not separately shown. It will be appreciated that the converter 16 may in fact comprise one or more DC/DC converters. Correspondingly, the ETS 18 is electrically connected, via power bus 34, to an output/input terminal at the opposite end of the DC/DC converter 16.

Accordingly, the bi-directional delivery of electrical energy between the EESS 20 and FCPP 12 on the one hand and the ETS 18 on the other hand, is accomplished via the one or more DC/DC converters 16. This avoids the requirement of the separate DC/DC converter 124 depicted in Fig. 1.·

[0024] Various controls are associated with the various component portions of the fuel cell-based hybrid power system 10 and for the purpose of description of the disclosed embodiment, have been simply consolidated and depicted as a controller 40. It will be appreciated that controller 40 may take a variety of forms, may be centralized or distributed, and may represent digital, analog, or a combination of circuitry, and may be at least partly comprised of appropriately programmed software and/or firmware. For simplicity, only the functional aspects of controller 40 that are relevant to the disclosed embodiment are depicted and discussed.

[0025] The controller 40 is depicted as receiving signal inputs from and delivering control signals to, various portions of the fuel cell-based hybrid power system 10. The various signal transfer connections are shown for convenience as being hard wired signal leads, but it will be appreciated that other forms of transfer, e. g.

wireless, are intended to be included. Specifically, a signal lead 42 connects the voltage appearing at common electrical junction 26 to controller 40 for use by the controller. A state of charge (SOC) sensor 44 monitors the state of charge of the ultra capacitors 21 of the EESS 20 and conveys that value to controller 40 via signal lead 46. A signal lead 48 conveys an indication to the controller 40 of the output voltage and/or current of the FCPP 12. A signal lead 50 connecting the controller 40 and ETS 18 is depicted as being bi-directional for conveying appropriate drive status, e. g., accelerating or decelerating/braking, and/or control signals between them as may be required. The controller 40 delivers a control signal to the air supplier 22 via signal lead 52, and may receive an air flow rate status signal in return from the air supplier via signal lead 54.

[0026] Referring additionally to Fig. 4, which shows in greater functional detail the portion of the controller 40 of Fig. 3 that is relevant to the disclosed embodiment, there is functionally depicted the arrangement for regulating the reactant air flow to the FCPP 12, via the air supplier 22, which in turn regulates the voltage of the FCPP. The oxidant reactant supply is regulated via regulation of the air supplier 22, which in turn regulates, at least in significant part, the output voltage of the FCPP12. Normally such regulation of the air supplier 22 would be based only upon existing electrical current and an estimation of air flow required to the FCPP to meet that demand.

However, in the disclosed embodiment, provision is made for regulating the air flow of supplier 22 and thus the output voltage of the FCPP 12 as a function of the state of charge status of the EESS 20 during those intervals of braking/deceleration when energy is being restored to the ultra capacitor(s) 21 and there is little or no need for the FCPP to maintain a high output voltage. Indeed, it is beneficial to the efficiency and durability of the FCPP to quickly reduce the output voltage during those intervals, and to return that voltage to levels required during acceleration or even constant velocity only gradually. [0027] Accordingly, and referring particularly to Fig. 4, the air supplier control signal on lead 52, which in turn regulates FCPP 12 voltage, is a established part of the time as a function of estimated reactant flow required to support monitored electrical current on the FCPP 12, and another part of the time as a function of a voltage schedule correlated with the monitored state of charge of the EESS 20.

[0028] More particularly, function block 60 receives an input of existing current via signal lead 48 and provides an output signal 62 that is an estimation of required reactant (air) flow as a function of that current. That signal 62 is appropriately compared at comparator 64 with a feedback signal 54 of the actual air flow provided by the air supplier 22, and the resultant is extended through proportional/integral control 66 to provide a current-based candidate air supplier set point signal 68.

[0029] Similarly, function block 70, as a function of the state of charge of EESS 20 appearing as an input on signal lead 46, provides a preliminary voltage set point signal (Vset) 72. The correlation function between the SOC signal 46 and the resulting desired voltage set point is depicted graphically in block 70 as a schedule in which the maximum value of Vset is the maximum voltage to which the EESS 20 may be charged (VEEssmax) and exists nominally at a condition of maximum discharge of the EESS 20. The value of Vset declines gradually thereafter as the EESS 20 recharges and, at a point at which the EESS has recharged by about 50 % to 75% of a full charge, a constant value of Vset is maintained thereafter to completion of the charge. That signal 72 is appropriately compared at comparator 74 with a feedback signal 42 of the actual voltage of the junction 26, and the resultant is extended through proportional/integral control 76 to provide a voltage-based candidate air supplier set point signal 78. It will be appreciated and understood that it may also be possible to estimate the voltage at junction 26 based on FCPP 12 current and voltage, so long as the current from the fuel cell is greater than zero.

[0030] Both the current-based air supplier set point signal 68 and the voltage/state of charge-based air supplier set point signal 78 are extended as inputs to selection logic represented by block 80. The selection logic is responsive to a signal indicative of vehicle status with respect to accelerating, braking/decelerating, or substantially constant velocity, as might be provided by signal lead 50 or from elsewhere in the system, for selecting one or the other of the candidate air supplier set point signals 68 and 78 for connection via lead 52 to the air supplier 22. If the vehicle is accelerating or maintaining constant velocity, then the current-based set point signal 68 is selected. If, however, the vehicle is braking/decelerating, then the voltage/state of charge-based set point signal 78 is selected. In this way, during those intervals of mild to extreme deceleration from whatever pre-existing vehicle speed, it will be the voltage-based set point signal 78 that controls the air supplier 22. Accordingly, during those intervals, the air supplier 22 will be caused to relatively reduce the flow of air to the fuel cells of the FCPP 12, thereby relatively reducing the voltage at the output of the FCPP such that it is not, or substantially not, charging the EESS 20. Instead, electrical power provided by the regenerative braking serves to re-charge the EESS 20 during that interval, with little or no contribution from the FCPP 12. This mode of operation serves both to reduce the low/no-load output voltage of the FCPP 12, thereby mitigating fuel cell degradation or decay, and also to conserve and minimize electrical energy supply requirements on the FCPP 12 during an interval when it is not required by the ETS 18 and ample re-charge energy for the EESS 20 is available from the regenerative braking.

[0031] At such time as the vehicle 8 returns to an acceleration mode or one of constant speed, the full power capacity of the FCPP 12 and additionally that of the recharged EESS 20 are available for meeting the power requirement of the ETS 18. During vehicle acceleration, the control of air supplier 22 will determine the power mix between the EESS 20 and the FCPP 12. During drive cycle operation, the voltage cycling of the FCPP 12 has the inherent form of a rapid drop in voltage and a slow rise in voltage, which has been determined to minimize degrading forces on the fuel cells. This pattern of voltage cycling is due to the large vehicle 8 needing a lot of power during acceleration (from a stop or low speed) and the charging of the EESS 20 during a braking event.

[0032] Although the disclosure has been described and illustrated with respect to an exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the accompanying claims.

Claims

CLAIMS What is claimed is:

1. A large-vehicle fuel cell hybrid power system (10), comprising:

a fuel cell power plant (FCPP 12);

an electric energy storage system (EESS 20) capable of being electrically charged and discharged;

an electric traction system (ETS 18) that is responsive to power from both the fuel cell power plant and the electric energy storage system;

the fuel cell power plant (12) and the electric energy storage system (20) being operatively connected at a common junction (26); and

the electric traction system (18) being operatively connected (34) to said common junction (26) of said fuel cell power plant and said electric energy storage system via one or more DC/DC converters (16).

2. The large-vehicle fuel cell hybrid power system (10) , further including:

an air supplier (22) for the fuel cell power plant (12), the air supplier being responsive to a control signal (52) for regulating reactant air supplied to, and concomitantly the output voltage of, the fuel cell power plant; and

a controller (40) providing said control signal (52) to said air supplier to control apportionment of power between the fuel cell power plant and the electric energy storage system.

3. The large-vehicle fuel cell hybrid power system (10) of claim 2, wherein the controller (40) provides a voltage-based set point signal (78) which is a function (70) of the state of charge (SOC 44,46) of the electric energy storage system (20), said voltage-based set point signal (78) being connected (72,78,52) to the air supplier (22) to control the output voltage of the fuel cell power plant (12).

4. The large- vehicle fuel cell hybrid power system (10) of claim 3, wherein the set point signal (78) represents a fuel cell voltage (Vset) which decreases during at least part of an interval during which the state of charge of the electric energy storage system (20) is increasing.

5. The large-vehicle fuel cell hybrid power system (10) of claim 4, wherein the set point signal (78) represents a fuel cell voltage (Vset) which decreases during approximately the initial half of the interval during which the state of charge of the electric energy storage system (20) is increasing.

6. The large- vehicle fuel cell hybrid power system ( 10) of claim 4, wherein a voltage at the common junction (26) between the fuel cell power plant (12) and the electric energy storage system (20) is sensed and is provided (42) as feedback control for the set point signal (78).

7. The large-vehicle fuel cell hybrid power system (10) of claim 3 wherein the controller (40) provides a current-based set point signal (68) which is estimated reactant air flow demand as a function (60) of existing current 48, said current- based set point signal (68) being connected (62, 68 ,52) to the air supplier (22) to control the output voltage of the fuel cell power plant (12).

8. The large- vehicle fuel cell hybrid power system (10) of claim 7 wherein the controller (40) includes selection logic (80) responsive to signals (50) indicative of conditions of vehicle braking versus accelerating or remaining at constant velocity, for extending (52) the voltage-based set point signal (78) to said air supplier (22) for a condition of vehicle braking, and alternatively extending the current-based set point signal (68) to said air supplier for a condition of either vehicle acceleration or constant velocity.

9. The large-vehicle fuel cell hybrid power system (10) of claim 3, wherein an electrical check valve (14) is connected between the fuel cell power plant (12) and said junction (26) with the electric energy storage system (20) to prevent reverse flow of current to the fuel cell power plant.

10. The large-vehicle fuel cell hybrid power system (10) of claim 9 wherein the electrical check valve is a power diode (14).

11. The large-vehicle fuel cell hybrid power system (10) of claim 1 wherein the electric energy storage system (EESS 20) comprises one or more ultra capacitors (21).