GB2522865A

GB2522865A - Fuel cell-based auxiliary power unit

Info

Publication number: GB2522865A
Application number: GB1402002.8A
Authority: GB
Inventors: David Graham Etteridge; Michael David Bailey; Colin John Halsey
Original assignee: GE Aviation Systems Ltd
Current assignee: GE Aviation Systems Ltd
Priority date: 2014-02-06
Filing date: 2014-02-06
Publication date: 2015-08-12
Anticipated expiration: 2034-02-06
Also published as: GB2522865B; GB201402002D0

Abstract

An auxiliary power unit (APU) (10), e.g. for an aircraft, providing electrical power to at least one load (30, 31, 38; 124) includes: an array (14; 214) of fuel cells comprising a plurality of sets (210, 212, 226) of series-connected fuel cells (216, 218, 220); at least one switch (222, 224) having a first state condition corresponding to an electrical coupling of at least two of the sets in parallel and a second state condition corresponding to an electrical uncoupling of the at least two of the sets in parallel; and a controller (12) operably coupled to the at least one switch and controlling the change of state of the switch between the first state condition and the second state condition. At least two of the sets are interconnected in parallel when the switch is in the first state condition to increase the current provided by the APU.

Description

FUEL CELL-BASED AUXILIARY POWER UNIT

BACKGROUND OF THE INVENTION

Contemporary aircraft may include an auxiliary power unit (APU) in addition to main propulsion engines, which are tvpicafly turbine engines. The APU may provide electrical S and pneumatic power to the aircraft including the provision of pressurized bleed air for main engine starting and the aircraft's environmental control system (ECS). The APU may deliver power to onboard aircraft systems while the aircraft is on the ground or in flight. Typically, APUs use gas turbine engines that burn jet fuel to turn a turbine that drives an electric generator. Alternatively, fuel cell APUs chemically convert fuel into I 0 electrical energy.

BRIEF DES CRIPTTON OF THE TNVENTTON

One aspect of the invention relates to an auxiliary power unit (APU) providing electrical power to at least one load, The APU comprises: an array of fuel cells comprising a plurality of sets of series-connected fuel cells; at least one switch having a first state condition corresponding to an electrical coupling of at least two of the sets in paraflel and a second state condition corresponding to an electrical uncoupling of the at least two of the sets in parallel; arid a controller operably coupled to the at least one switch and controlling the change of state of the switch between the first state condition and the second state condition. The at least two of the sets are interconnected in parallel when the switch is in the first state condition to increase the current provided by the APU.

BRIEF DESCRIPTION OF TIlE DRAWINGS

In the drawings: FIG, 1 is a block diagram of a fuel cell APU according to an embodiment of the present invention, FIG. 2 is a block diagram of the fuel cell stack of the fuel cell APU in FIG. 1.

FIG. 3 is a block diagram of a hydrogen supply system illustrating typical components for use with the fuel cell APIJ in FIG. 1.

FIG. 4 is a block diagram of the electrical power network configuration of the fuel cell array of the fuel cell APU in FIG. I. S FIG. S is a block diagram demonstrating the operation of the fuel cell APU with a bifurcated power output from the fuel cell array according to an embodiment of the present invention.

DETAILED DES CRIPTION OF THE INVENTION

Referring now to FIG. 1, an APIJ 10 with a fuel cell array 14 as an energy source may ID output the electrical power necessary for powering the aircraft electrical supply 38 and the compressor motor 24 that provides the output airflow 30, 31 for systems including the environmental conditioning system (ECS) 28 and the engine air-start system. To generate electricity, the APU 10 may include a fuel cell stack or array 14 operably coupled to a cooling system 16, a hydrogen supply 18 and an air supply system 20.

Electrical power output from the fuel cell array 14 may be coupled to the pneumatic power system 23, the electric power converter system 25 or both.

The pneumatic power system 23 may include a DC conditioner 22, a compressor motor 24, a compressor 26 and the ECS 28. The compressor 26 may selectively provide the output airflow 31 for the main engine start, the output airflow 30 for the cabin via the ECS 28 or both, The electric power converter system 25 may include an energy storage system 32, a DC conditioner 34 and a DC-to-AC inverter 36. The DC-to-AC inverter 36 may provide an electrical power output 38 used for running electrical systems on the aircraft. The electrical power output 38 may be a single or three-phase system and may preferably be configured to output 115 VAC at 400 Hz, though other power outputs including, for example, 28 VDC may be implemented, By communicably coupling to other elements of the APU 1 0, a controller 1 2 may direct the operation of the APTJ 10 and monitor the overall system and its individual components. Additionally, the controller 12 may receive additional instructions and report system faults to the flight deck controls via the aircraft data-bus 40 or be in communication with the bus power control unit (BPCU) to enable start up in an emergency situation. As described below, the controller 12 may be in electronic communication with such APU elements including, but not limited to the fuel cell array 14, the cooling system 16, the hydrogen supply 18, the air supply system 20, the compressor motor 24, the compressor 26, and one or more elements of the electric power converter system 25 such as the DC-to-AC inverter 36.

For example, the controller 12 may direct the hydrogen supply 18 to selectively open and close the hydrogen tank valves, sense the temperature or pressure of the tanks and report system faults along the aircraft data-bus 40. The controller 12 may direct the cooling system 16 to start, stop or otherwise control the speed of the cooling pumps and sense and report the coolant temperature and level. The controller 12 may relay start or stop commands to controlling elements of the fuel cell array 1 4 to generate a desired output power level as well as sense the actual power level output from the fuel cell array 14.

The controller 1 2 may direct the air supply system 20 to start, stop or otherwise control the air supply motor, The controller I 2 may direct the compressor motor 24 to start or stop or otherwise modulate the loading as determined by the current draw allowed (CDA) from the fuel cell array 14. The controller I 2 may direct one or more elements of the electric power converter system 25 to start or stop or otherwise modulate the loading as determined by the current draw allowed (CDA) from the fuel cell array 14. For example, the controller 12 may selectively switch an output contactor of the DC-to-AC inverter 36 to energize or dc-energize the electrical output 38.

Referring now to FIG. 2, a block diagram showing the inputs and outputs of the fuel cell array 14 is shown. As shown, the fuel cell array 14 is a proton exchange membrane (PEM) fuel cell. The PEM fuel cell 14 provides DC electrical power through the reaction of hydrogen and oxygen. Along the anode 118, the fuel cell array 114 contacts hydrogen that is provided from the hydrogen supply 18 as the fuel. Inside the fuel cell array 14, the hydrogen splits into ions and electrons. Any excess hydrogen I 19 is vented from the fuel cell array 14 and dispersed overboard. The ions pass through a semipermeable membrane generally made from ionomers to the cathode 120. The electrons travel through the DC load 124, returning to the cathode 120 where they combine with the hydrogen ions and oxygen supplied by the air supply system 20 to form water. A throttling valve 122 on the output of each fuel cell controls the air flow for each fuel cell.

The water discharges from the fuel cell array 14 along with the now oxygen-depleted air along the cathode 121. The water may be discharged overboard, used in the potable water tank on board the aircraft, used for lavatory frmnctions or for humidifying the air conditioning system.

Tn addition to the water and the oxygen-depleted air, the electro-chemical reaction may generate heat, For example, if the electro-chemical reaction is 50% efficient, then evefl' 1 kW of electrical energy produced, generates 1 kW of heat. Consequently, coolant from the coolant system 16 may be input 116 to the fuel cell array 14 to dissipate the heat and maintain the fuel cell array 14 at a desired temperature for optimum efficiency and lifespan. The coolant then may be output 117 from the fuel cell array 14 and cycle back through the cooling system 16.

While a proton exchange membrane (PEM) fuel cell is contemplated, it is understood that other types of fuel cells, particularly those with a low working temperature (ic. less than °C), may be implemented, A non-limiting list of fuel cell types that may be implemented in the fuel cell APU 14 includes magnesium-air, metal hydride, direct methanol, direct ethanol, phosphoric acid, microbial, direct fornic acid, electro-galvanic, zinc-air, enzymatic biofuel, regenerative, direct borohydride, protonic ceramic, direct carbon, and alkaline fuel cells.

Referring now to FIG. 3, a schematic diagram of the hydrogen supply system IS and its connection to the fuel cell array 14 is shown. The hydrogen that provides the fuel to the fuel cell array 14 may be stored on board the aircraft in at least one pressurized storage tank I 50. Types of storage tanks that may be used alone or in combination to store hydrogen may include high pressure cylindrical or conformal gaseous tanks, liquid hydrogen storage tanks and hydride storage vessels. Alternatively, by producing hydrogen from a fuel such as diesel or avtur through a process of reforming of hydrocarbons, the need for a large on-board storage tank of hydrogen may be mitigated.

The hydrogen supply system 18 may include an integrated tank valve 152 that may further include a temperature and pressure transducer 154, an over temperature relief valve 156, an automatic shutoff valve 158, a manual shutoff valve 160 and a tank fill line 162 with a check valve 164. The temperature and pressure transducer 154 may provide information about the contents of the storage tank 150 for use by an external refueling device, the APU controller 12 or other aircraft health monitoring systems. Should the 1 5 contents of the storage tank 1 50 exceed a predetermined temperature, the over temperature relief valve 156 may vent the tank pressure via a relief vent line to prevent a rupture of the storage tank ISO, In response to commands from external controls such as the APU controller 12 or an automatic shutoff system, the automatic shutoff valve 158 may selectively open or close to turn the hydrogen supply system on or off and thereby enable or disable the supply of hydrogen to the fuel cell array 14. Due to its high reliability and fast switching characteristics, a solenoid valve, that is, an electromechanically operated valve where electric current actuates a solenoid, may preferably embody the automatic shutoff valve 158, though other types of valves are contemplated. The manual shutoff valve 160 may allow for maintenance activities or prevent a storage tank 150 from supplying fuel to the fuel cell array 14. When closed, the manual shutoff valve 160 may override the automatic shutoff valve 158. To allow for the filling of the storage tank 150, the tank fill line 162 may include a check valve 164 to prevent hydrogen from leaking from the hydrogen supply system 1 8 during the commencement or conclusion of a refueling operation.

Coupled to the output of the integrated tank valve 152 opposite from the storage tank 150, an excess flow-rate valve 166 may close and shut off the hydrogen supply 18 from the storage tank ISO. Tn the event of a large leak such as may be created by a ruptured pipe in the hydrogen supply system I 8, the excess flow-rate valve 166 may self-actuate to S close automatically when the hydrogen gas flow exceeds a predetermined rate. Fuel moving through the excess flow-rate valve 166 at a pressure determined by that of the storage tank 150 may then pass through a pressure regulator 168 that drops the pressure from that in the storage tank 150 to a pressure appropriate for delivery of the hydrogen to the fuel cell array 14. Downstream of the pressure regulator 168 is a pressure switch 170 for monitoring the reduced pressure line. Should the pressure in the reduced pressure line rise a predetermined level above the pressure setting of the regulator, the controller 12 or a system safety controller may signal the automatic shutoff valve 158 to close. An overpressure relief valve 172 allows for the venting of fuel at an excess pressure in the reduced pressure line thereby preventing pipe or component rupture in the event that the pressure regulator 168 and automatic shutoff valve 158 fail. A three-way valve 174 placed between the overpressure valve 172 and the fuel cell array 14 may allow for the defueling of the hydrogen supply system 18 or connection to a low pressure source for direct supply to the fuel cell for on-aircraft fault finding or maintenance, Referring now to FIG. 4, a block diagram demonstrating the configuration of the electrical power network of a fuel cell array 214 is shown, Individual fuel cells, such as PEM fuel cells, under electrical loading conditions are typically associated with low voltages (e.g. 0.5 to 0.7 \TDC) though the actual output voltage per fuel cell may vary based upon the particular chemistry of the fuel cell. Therefore, a fuel cell array 214 may include an array of fuel cells arranged in sets of series-connected fuel cells. To control the output current and voltage of the fuel cell array, the plurality of electrically interconnected fuel cells may be linked in a series-parallel configuration. For example, to raise the output voltage level, a set 210 of fuel cells 216, 218, 220 may be connected in series, A number of sets 210, 212, 226 oF fuel cells may be connected in a parallel configuration to increase the current flow while maintaining the raised output voltage level. Consequently, the network of parallel-connected sets 210, 212, 226 of series-connected fuel cells 216, 218, 220 may deliver a desired power output without raising the output voltage to undesirable levels.

The number of fuel cells in a series-connected set and the number of sets implemented in the APU may be selected and adjusted based on the systems the APU will power. That is, the voltage and current requirements of the electrical and pneumatic power systems along with other system considerations such as redundancy requirements of the system determine the number and configuration of the fuel cells in the fuel cell array.

Each set 210, 212, 226 of series-connected fUel cells may be coupled in series to a switch I 0 222, 224 that may be set to an opened or closed state condition, Tn an opened state condition, the switch 222, 224 selectively decouples the respective set 210, 212 of series-connected fuel cells from the electrical output of the DC load 124. In a closed state, the switch 222, 224 selectively couples the respective set 210, 212 of series-connected fuel cells to the electrical output of the DC load 124. The switches 222, 224 may be any suitable power switching element or power controller such as a gate turn-off thyristor, an insulated-gate bipolar transistor, a metal oxide semiconductor field-effect transistor a silicon-controlled rectifier or a contactor, While shown in FIG. 4 as coupling sets of series-connected fUel cells in parallel, other switch configurations may be contemplated. For example, switches may be provided in between individual fuel cells of a set of series-connected fuel cells. Selectively controlling the state of the series-connected switches may control the output voltage level from each set of series-connected fuel cells, With such a configuration, the fuel cells in the fuel cell array can be selected by the controller to control the number of series-connected fuel cells in each set or array and the number of sets connected in parallel to control the voltage and current, respectively, outputted by the fuel cell array. It is also possible that the controller can subdivide the fuel cell array into distinct groupings of sets of series-connected fuel cells, including varying the number of fuel cells in each set and the number of each sets in each group. Tn this way, a single fuel cell array can be used to create different power supplies or sources, which can be coupled to loads with differing power requirements. Tn this sense, the subdividing creates virtual power supplies within the fuel cell array, with the output of the virtual power supplies being controlled by the controller.

Referring now to FIG. 5, a block diagram demonstrating the operation of the fuel cell APU as previously described, which has been subdivided into two power supplies to provide a bifurcated power output from the fuel cell array 314 is shown. A battery 312 is connected to the air supply system 20, the cooling system 16 and the fuel cell array 314.

The power output of the fuel cell array 314 is connected to deliver the electrical output 38 of the electrical supply system through the energy storage device 32, the DC conditioner 34 and the AC inverter 36. The power output of the fuel cell array 314 is also connected to deliver the air output 31 for the pneumatic systems including the ECS 28 through the DC conditioner 22 connected to the compressor 26 via the compressor motor controller 1 5 24A and compressor motor 24B.

Upon start-up of the fuel cell APU, the battery 312 powers a blower in the air supply system 20 to provide air for a set 328 of series-connected fuel cells 316, 318 to initialize the chemical fuel conversion process described above in FIG. 2. The set 328 of series-connected fuel cells 316, 318 may then generate sufficient power to fully drive the air supply system 20 to provide air for all the fuel cells in the fuel cell array 314 and power for the cooling system 16. Air for all of the fuel cells may feed from the air supply system to the fuel cell array 314. For example, via a common plenum, the air supply system may feed air to all of the fuel cells in the fuel cell array 314 simultaneously and individual flow control for each fuel cell is achieved by the use of a throttling valve on the output of each fuel cell as described above in FIG. 2.

As shown in FIG. 5, the fuel cell array 314 may be divided to provide separate DC output for the electrical supply and the pneumatic supply. The DC output from the set 328 or sets of series-connected fuel cells for the electrical supply fuel cells may be connected to an energy storage unit 32 such as a battery, capacitor or other power storage device to enable the electrical supply system to meet rapid load changes. The power output of the energy storage unit 32 may then couple to the DC conditioner 34 that conditions the DC power for use on a DC aircraft electrical system. Tn other words, the DC conditioner 34 may convert and regulate the voltage level of the input power from the fuel cell array 314 to a voltage level consistent with that required for the aircraft electrical system. For example, the DC conditioner 34 may convert an input voltage to 115 V for use in a 115 VAC, 400 Hz system, 270 V for a 270 VDC system or 28 V for a 28 DC system. If the electrical output 38 is an AC electrical system, a DC-AC inverter 36 may be provided after the DC conditioner 34 to convert the power from a DC input to an AC output.

The DC output of the set or sets 330, 332 of series-connected fuel cells provided to supply power for the pneumatic system may be connected to the DC conditioner 22. The DC conditioner may provide a stable power supply to the compressor motor controller 24A. The compressor motor controller 24A may then drive the DC compressor motor 1 5 24B. The compressor motor 24B connects by a shaft to the compressor 26. The compressor 26 generates compressed air in a volume and pressure required for the air outputs such as the ECS 28 and the air output 31 required for engine starting By splitting the DC power output from the fuel cell array 314 and controlling via controller 12 which sets of fuel cells are actively generating power, fuel and energy may be conserved. For example, should an aircraft operator only require aircraft electrical power, then only the set 328 or sets of fuel cells for the operation of the electrical supply will operate. Should the ECS 28 be required, the remaining sets 330, 332 of fuel cells operate to drive the electric compressor 26 and deliver air to the ECS 28.

Beneficial effects of the embodiments disclosed include operational and environmental advantages over conventional gas turbine APUs. In contrast to a conventional gas turbine APU that includes high speed rotating parts, a fuel cell APU includes a minimal number of moving parts, including the fuel cell array, which contains no moving parts. Desirable operating characteristics of a fuel cell APU include a low operating temperature (typically ranging from 50 to 180°C), a low audible noise level and no appreciable pollution beyond the water, heat and depleted oxygen air outputs.

Because fuel cells may run indefinitely provided clean fuel and air are available, unlike batteries whose performance degrades over time, the associated maintenance costs are S relatively low, Fuel cell APUs may have a lower maintenance burden than that of conventional gas turbine APUs. Fuel cell APUs may approach 20,000 operating hours whereas conventional gas turbine APUs average 12,000 hours between overhauls. Fuel cell APUs may also act as an emergency power system, replacing the Ram Air Turbine (RAT) because the fuel source is independent from that of the main engines.

ID Finally, the configuration and control of the sets of fuel cells for the fuel cell army enables a scalable technology. A fuel cell APU designed as described above may be quickly adapted for use between different aircraft types with different power requirements and modes of operation. For example, for any of the sets of series-connected fuel cells, the number of fuel cells in series can be controlled by switching the fuel cells in/out of series by changing the state of a corresponding switch, which can be called "series switches" for shorthand reference. The number of fuel cells in series can be used to control the voltage provided by the set of series-connected fuel cells. In a similar way, the number of sets of series-connected fuels cells coupled in parallel can be controlled by changing the state of a corresponding switch, which can be called "parallel switches" for shorthand reference. The number of parallel-connected sets of series-connected fuel cells can be selected to control the voltage of the fuel cell array. Thus, by controlling the series switches to control number of series-connected fuels cells in each set of the array and the parallel switches to control the number of parallel connected sets, it is possible to control both the voltage and current of the fuel cell array merely by controlling the state condition of the corresponding series/parallel switches.

Another technical benefit is the ability to sub-divide the fuel cell array as desired to provide different power sources for different loads, with each power source having a l0 predetermined voltage and current, which can be selected for the expected load. One example of such a sub-divided fuel cell array is the bifurcated fuel cell array of Fig. 5.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making S and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structura' elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. I 1

Claims

CLAIMS: An auxiliary power unit (APU) providing electrical power to at least one load comprising: an array of fuel cells comprising a plurality of sets of series-connected fuel cells; at least one first switch having a first state condition corresponding to an electrical coupling of at least two of the sets in parallel and a second state condition corresponding to an electrical uncoupling of the at least two of the sets in parallel; and a controller operably coupled to the at least one first switch and controlling the change of state of the first switch between the first state condition and the second state condition; wherein the at least two of the sets are interconnected in parallel when the switch is in the first state condition to increase the current provided by the APU.
2. The APU of claim 1, wherein a first grouping of sets of series-connected fuel cells provides electrical power to a first load and a second grouping of sets of series-connected fuel cells provides electrical power to a second load.
3. The APU of either of claim 1 or 2, wherein an output of the APU is at least one of: 400 Hz, 115 VAC, 28 VDC, 270 VDC, an environmental control system (ECS) or an air start system.

4, The APU of any preceding claim, wherein the at least one first switch is a gate turn-off thyristor, an insulated-gate bipolar transistor, a metal oxide semiconductor field-effect transistor, a silicon-controlled rectifier or a contactor, 5. The APU of any preceding claim, wherein the fuel cells are proton exchange membrane (PEM) fuel cells.6. The APU of any preceding claim, wherein the fuel cells have a working temperature below 200 °C.7, The APU of any of claims I to 4 and 6, wherein the fuel cells are magnesium-air, metal hydride, direct methanol, direct ethanol, phosphoric acid, microbial, direct formic acid, electro-galvanic, zinc-air, enzymatic biofuel, regenerative, direct borohydride, protonic ceramic, direct carbon, or alkaline fliel cells.8. The APU of any preceding claim, wherein the controller controls the change of state of the at least one first switch to control the current from the APU.9. The APU of any preceding claim, wherein the sets of series-connected fuel cells further comprise at least one second switch having a first state condition corresponding to an electrical coupling of at least two of the fuel cells in series and a second state condition corresponding to an electrical uncoupling of the at least two of the fuel cells wherein the at least two of the fuel cells are interconnected in series when the switch is in the first state condition to increase the voltage provided by the APU.10. The APU of claim 9, further comprising a plurality of first switches and a plurality of second switches, and the controller is operably coupled to and controls the state of the first and second switches to control the number of fuel cells connected in series for the sets of in-series fuel cells and the number of sets connected in parallel.11. The AYU of claim 10, wherein the controller operably controls the state of the switches to effect a subdividing of the array into multiple power sources.