System and method for operating a fuel cell system

Abstract

The present application relates to a system and a method for operating a fuel cell system, and more particularly, to a system and a method for controlling operation of a fuel cell system, the fuel cell system comprising: a fuel cell unit comprising a fuel cell stack comprising a cathode and an anode; and a cathode recirculation channel configured to divert the cathode exhaust gas stream to the cathode inlet line. A control system is configured to, in response to a value of a power output requested from the fuel cell system being below a first threshold power level, control a target coolant inlet temperature of coolant at a coolant inlet of the fuel cell stack and control an air pressure at the cathode. In response to the value of the power output being below at least one second threshold power level, in addition, the oxygen partial pressure in the air stream may be reduced by controlling the volumetric flow rate of the cathode exhaust stream directed to the cathode inlet line.

Description

Technical Field

[0001] The present disclosure generally relates to operating a fuel cell system when a requested power output from the fuel cell system is reduced.

[0002] The present disclosure can be applied to heavy-duty vehicles such as trucks, buses, and construction equipment, as well as to generator sets. Background Art

[0003] A fuel cell is an electrochemical device that includes an electrolyte sandwiched between two electrodes such as an anode and a cathode. A solid polymer electrolyte fuel cell employing a proton exchange membrane (PEM) generates electrical power or electric energy via an electrochemical reaction between a fuel such as hydrogen received at the anode or anode side and an oxidant such as oxygen or air received at the cathode or cathode side. PEM fuel cells are considered well-suited for vehicle applications as well as stationary applications employing fuel cell systems. Typically, two or more fuel cells are arranged together to form a fuel cell stack in order to provide a higher output voltage. One or more fuel cell stacks can form a fuel cell unit, and a fuel cell system can include one or more fuel cell units.

[0004] In a fuel cell, metals such as palladium and platinum are used as catalysts to facilitate the electrochemical reaction between hydrogen and an oxidizing gas, where the reaction occurs across a membrane or electrolyte. A support structure made of a catalyst carrier material such as carbon is configured to support the membrane, and the metal catalyst is typically used on the catalyst carrier material. The conditions of the catalyst and the carrier material define the durability, lifetime, and performance of the fuel cell system. The catalyst needs to operate under certain conditions to produce an effective reaction rate. Thus, an excessive increase in the single cell voltage, which can be defined as the cell voltage or potential of each fuel cell of each fuel cell stack in a fuel cell system, can lead to degradation of the catalyst and the carrier material. Therefore, during operation of a fuel cell system, it is desirable to maintain the single cell voltage below a certain voltage level.

[0005] In fuel cell systems with higher power, which are attracting increasing attention in the automotive and other fields, a problem occurs at low loads. Specifically, at higher power of the fuel cell system, the single cell potential can reach a safety threshold. Thus, an excessive amount of power may be generated, which is wasted, thereby reducing the efficiency of the fuel cell system. In addition, as the power of the fuel cell system decreases, the single cell potential increases to a value that negatively affects the conditions of the catalyst and the carbon carrier material. Therefore, it can be challenging to control the fuel cell system at lower loads in a manner that takes into account the potential degradation of the catalyst and the carrier material.

[0006] Various methods have been proposed for reducing the single cell potential in the case of a reduced power demand from a fuel cell system when the current density in the fuel cell system decreases. However, there is still a need for improved methods for controlling the operation of a fuel cell system in the case of a reduced power demand on the fuel cell system. SUMMARY OF THE INVENTION

[0007] Aspects of the present disclosure relate to controlling a fuel cell system at low load in a manner that limits the single cell voltage or potential to a value below 0.8 V when the power demand on the fuel cell system decreases. This allows for avoiding degradation of the catalyst and carrier materials. The fuel cell system can be deployed in a fuel cell vehicle or a stationary application.

[0008] In one aspect, a fuel cell system is provided that includes a fuel cell unit that includes a fuel cell stack that includes a cathode and an anode. The cathode is configured to receive an air stream via a cathode inlet line and provide a cathode exhaust stream via a cathode outlet line. The fuel cell system further includes a cathode recirculation channel, a coolant system, and a control system. The cathode recirculation channel and the coolant system can be part of the fuel cell unit. The cathode recirculation channel fluidly connects the cathode outlet line to the cathode inlet line, thereby redirecting the cathode exhaust stream to the cathode inlet line such that the cathode exhaust stream is mixed with the air stream received by the cathode inlet line. The coolant system is configured to circulate coolant through the fuel cell stack. The coolant system includes a coolant inlet line and a coolant outlet line. The coolant inlet line is configured to direct the coolant to the fuel cell stack. The coolant outlet line is configured to direct the coolant away from the fuel cell stack after the coolant has passed through the fuel cell stack. The control system includes processing circuitry that is configured to: obtain a value of a power output requested from the fuel cell system; and in response to the value of the power output being below a first threshold power level, monitor the voltage of the fuel cell stack; and control a target coolant inlet temperature of the coolant at the coolant inlet, thereby maintaining the voltage of the fuel cell stack below 0.8 V.

[0009] The voltage of the fuel cell stack can be the single cell voltage determined for the fuel cell stack. In some examples, the single cell voltage can be determined as the measured voltage of the fuel cell stack divided by the number of fuel cells in the fuel cell stack. Thus, the single cell voltage can be determined as the average single cell voltage, and this single cell voltage represents the voltage of each fuel cell in each fuel cell stack. In some examples, the single cell voltage can be determined based on the measured voltage of the entire fuel cell unit, in which case the measured voltage is divided by the number of stacks and the number of cells in each stack. In some examples, the voltage of each fuel cell is measured, and the average of these measured voltages can be regarded as the single cell voltage of the fuel cell stack.

[0010] The processing circuit of the control system can be configured to control the target coolant inlet temperature by increasing the target coolant inlet temperature to a temperature threshold level.

[0011] Technical advantages include controlling the fuel cell system in a manner that allows reducing the power demand or load on the fuel cell system as needed, while avoiding the risk of the individual cell potential increasing to a value higher than the safety threshold of 0.8V. By increasing the target coolant inlet temperature to increase the temperature of the fuel cell stack, the efficiency of the fuel cell system is intentionally reduced. Thus, the fuel cell stack may need to reduce the oxygen partial pressure in the air stream fed to the cathode less in order to operate without the single cell voltage increasing to a value higher than 0.8V. Therefore, the system of the present disclosure provides improved control of the fuel cell system at low power demand in a manner that avoids causing degradation of the catalyst and carrier materials of the fuel cell stack.

[0012] The processing circuit of the control system can be configured to control the air pressure at the cathode of the fuel cell stack when controlling and / or adjusting the target coolant inlet temperature. In some examples, as the target coolant inlet temperature increases, in order to prevent membrane dehydration, the air pressure at the cathode also increases so that the cathode inlet air relative humidity (RH) can be maintained at an acceptable level.

[0013] In some examples, the temperature threshold level is 80°C. In some examples, the temperature threshold level can be 81°C or 82°C or 83°C or 84°C.

[0014] In some examples, the processing circuit of the control system can be configured to, in response to the value of the power output being lower than at least one second threshold power level that is smaller than the first threshold power level, when the target coolant inlet temperature is at the temperature threshold, control the cathode recirculation channel to direct a portion of the cathode exhaust gas stream to mix with the air stream, thereby reducing the oxygen partial pressure of the air stream.

[0015] In some examples, the processing circuitry of the control system is configured to control at least one flow control valve fluidly connected to the cathode recirculation passage and a cathode recirculation pump fluidly connected to the cathode recirculation passage and configured to operate to drive the cathode exhaust gas flow through the cathode recirculation passage based on the value of the power output.

[0016] In some examples, the fuel cell unit includes one fuel cell unit. Accordingly, the fuel cell system includes a single fuel cell unit.

[0017] In some examples, the value of the obtained power output is a predicted value of the power output of the fuel cell system.

[0018] In one aspect, a fuel cell vehicle is provided that includes a fuel cell system according to an example of the present disclosure.

[0019] In one aspect, a method for operating a fuel cell system is provided, the fuel cell system including a fuel cell unit that includes a fuel cell stack that includes a cathode and an anode, the cathode being configured to receive an air flow via a cathode inlet line and provide a cathode exhaust gas flow via a cathode outlet line. The method includes: obtaining a value of a power output requested from the fuel cell system; monitoring a voltage of the fuel cell stack in response to the value of the power output being below a first threshold power level; and controlling a target coolant inlet temperature of coolant at an inlet of the coolant of the fuel cell stack, thereby maintaining the voltage of the fuel cell stack below 0.8V. The method includes: controlling the target coolant inlet temperature in response to the value of the power output being below at least one second threshold power level that is less than the first threshold power level; and controlling an oxygen partial pressure of the air flow, wherein the oxygen partial pressure is controlled by controlling a volumetric flow rate of the cathode exhaust gas flow that is directed from the cathode outlet line to the cathode inlet line to be mixed with the air flow.

[0020] The technical benefits of the method include benefits similar to those described above in connection with the fuel cell system. Accordingly, the method of the present disclosure provides improved control of the fuel cell system at low power demands in a manner that avoids causing degradation of the catalyst and carrier materials of the fuel cell stack of the fuel cell system.

[0021] A lower value of the power output requires a lower oxygen partial pressure of the air flow.

[0022] In some examples, the method includes controlling the target coolant inlet temperature by increasing the target coolant inlet temperature to a temperature threshold level. In some examples, the temperature threshold level is 80°C.

[0023] In some examples, the method includes controlling the air pressure at the cathode of a fuel cell stack while controlling a target coolant inlet temperature.

[0024] In some examples, controlling the oxygen partial pressure of the air stream includes controlling at least one flow control valve and a cathode recirculation pump fluidly connected to a cathode recirculation passage that fluidly connects the cathode outlet line to the cathode inlet line, thereby redirecting the cathode exhaust stream to the cathode inlet line such that the cathode exhaust stream is mixed with the air stream, wherein the at least one flow control valve and the cathode recirculation pump are controlled based on the value of the power output. For example, the at least one flow control valve may include: a first flow control valve positioned at a junction between the coolant outlet line and the cathode recirculation passage; and a second flow control valve positioned at a junction between the cathode recirculation passage and the coolant inlet line. The valves may be activated or actuated to at least partially open to allow a portion of the cathode exhaust stream in the coolant outlet line to circulate through the cathode recirculation passage and reach the cathode inlet line, whereby the portion of the cathode exhaust stream is mixed with the air stream supplied to the cathode inlet.

[0025] In some examples, the value of the power output obtained is a predicted value of the power output of the fuel cell system.

[0026] In one aspect, a control system for controlling a fuel cell system of a fuel cell vehicle is provided. The control system includes a processing circuit configured to perform a method according to an example of the present disclosure.

[0027] In one aspect, according to an example of the present disclosure, a fuel cell vehicle is provided that includes a control system and / or communicates with the control system.

[0028] In one aspect, a computer program product including computer-executable instructions is provided that, when executed by a processing circuit, causes the processing circuit to perform a method according to any aspect and example of the present disclosure.

[0029] In one aspect, a computer-readable storage medium having stored thereon a computer program product is provided, the computer program product including computer-executable instructions that, when executed by a processing circuit, cause the processing circuit to perform a method according to any aspect and example of the present disclosure.

[0030] Additional features and advantages are disclosed in the following description, claims, and drawings. Additionally, those skilled in the art will appreciate additional advantages upon a review of the disclosure or through practice of the disclosure as described herein. Also disclosed herein are control units, computer program products, and computer-readable media associated with the technical effects and corresponding advantages discussed above. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Aspects of the present disclosure, which are cited as examples, will be described in more detail below with reference to the drawings.

[0032] Figure 1 A side view of an example of a vehicle including a fuel cell system in which a method according to an example of the present disclosure can be implemented is illustrated.

[0033] Figure 2 is illustrative of according to one example Figure 1 block diagram of the fuel cell system and control system of a vehicle.

[0034] Figure 3 is a diagram illustrating an example of a fuel cell system in which a method according to an example of the present disclosure can be implemented.

[0035] Figure 4 is a diagram illustrating another example of a fuel cell system in which a method according to an example of the present disclosure can be implemented.

[0036] Figure 5 is a diagram illustrating another example of a fuel cell system in which a method according to an example of the present disclosure can be implemented.

[0037] Figure 6A is a flowchart illustrating a method for operating a fuel cell system according to one example.

[0038] Figure 6B is a flowchart illustrating a method for operating a fuel cell system according to one example.

[0039] Figure 7A and Figure 7B is a schematic block diagram illustrating an example of a control system of a fuel cell system according to one example. DETAILED DESCRIPTION

[0040] A fuel cell system capable of generating high power or higher power may include a single fuel cell unit that includes one or more fuel cell stacks. In various applications (e.g., in fuel cell vehicles), a single fuel cell unit may be desirable for several reasons, including, for example, space-saving considerations, more straightforward maintenance, and simplified control strategies. At the same time, in such fuel cell systems with improved efficiency, at low loads, the single cell voltage can reach a safety threshold at a relatively high power level, e.g., up to 50 kilowatts (kW) in some cases. Thus, in some cases, once the fuel cell system enters an idle mode or state at low load (i.e., low current density), the single cell voltage may quickly reach the safety threshold. This makes it challenging to operate the fuel cell system correctly at low load without the risk of damaging the catalyst and support materials of the fuel cell stack. Additionally, a relatively high idle power is wasted, which reduces the efficiency of the fuel cell system.

[0041] Therefore, to control catalyst degradation and thereby extend the life of the fuel cell system, it is desirable to limit the maximum value that the single cell voltage can reach at low current density of the fuel cell system. When the single cell voltage (which may also be interchangeably referred to as single cell potential) increases to a value of 0.8 V or above, platinum dissolution begins, thereby reducing the electrochemically active surface area (ECSA), which in turn leads to a reduction in the efficiency and life of the fuel cell system. Thus, if the maximum value allowed for the single cell voltage is set to 0.8 V or just below 0.8 (e.g., 0.79 V), the risk of catalyst and support material degradation can be reduced or avoided.

[0042] Solutions involving stop / start strategies have been proposed such that the fuel cell system can be shut down when the single cell voltage reaches 0.8 V. For example, when the power required by the fuel cell system drops below a certain value (e.g., 50 kW in some scenarios), the fuel cell system may be shut down. The drawback of such methods is that the frequent stop and start of the fuel cell system induces degradation mechanisms in the catalyst support material, such as carbon corrosion. This results in the reduction of platinum and thus the reduction of ECSA, which leads to the structural collapse of the catalyst and thus increases the mass transfer loss. Therefore, the stop / start strategy may have a negative impact on the efficiency and life of the fuel cell system.

[0043] The methods and systems according to the present disclosure employ a control strategy that involves using a cathode recirculation loop or passage that recirculates a cathode exhaust gas stream back into the inlet of the cathode in order to reduce the oxygen partial pressure of the air stream fed to the cathode inlet. The control strategy also uses active control of a target inlet coolant temperature of the coolant stream in the coolant system, where the target inlet coolant temperature is controlled as a function of the power required from the fuel cell system by an application such as a vehicle. The actual inlet and outlet coolant temperatures are monitored. A controller or control system, which is configured to control the operation of the fuel cell system, is configured and adapted to control the target inlet coolant temperature and regulate the volumetric flow rate of the cathode exhaust gas stream that is diverted or recirculated into the air stream to adjust the oxygen partial pressure of the air stream. Additionally, the cathode pressure level is controlled. The control system performs control of the target inlet coolant temperature and the volumetric flow rate of the recirculated cathode exhaust gas stream while monitoring the single cell voltage of the fuel cell system, thereby controlling the single cell voltage not to exceed a safety threshold (e.g., 0.8 V).

[0044] Accordingly, in a situation where the power demand from the fuel cell system decreases such that the fuel cell system operates at low load or interchangeably at low current density, the controller may start monitoring the voltage of each fuel cell stack of the fuel cell system to estimate the single cell voltage. Once the power demand drops below a certain threshold power level, the controller may control the coolant system to increase the target inlet coolant temperature. If the power demand decreases, the controller may both control the target inlet coolant temperature to increase and control the cathode recirculation passage to direct a portion of the cathode exhaust gas stream to mix with the air stream, thereby reducing the oxygen partial pressure of the air stream. The volumetric flow rate of the cathode exhaust gas stream in the cathode recirculation passage may be controlled by controlling, for example, a recirculation pump and one or more flow rate valves in the cathode recirculation passage.

[0045] Although a method where only the cathode exhaust gas stream is diverted to mix with, for example, an air stream fed to the cathode from the outside may potentially lead to oxygen starvation in the fuel cell stack due to a sharp decrease in the oxygen partial pressure, the combination of this technique with an increase in the target inlet coolant temperature alters the efficiency of the fuel cell stack. Accordingly, due to the decrease in the efficiency of the fuel cell stack, a smaller decrease in the oxygen partial pressure in the air stream is required to achieve the lower power required by the fuel cell system. In this way, lower power can be achieved without the risk of the single cell potential increasing above the safety threshold of 0.8 V. Thus, the systems and methods of the present disclosure advantageously provide improved control of the fuel cell system under low power demand in a manner that avoids causing degradation of the catalyst and carrier materials.

[0046] In some examples, the processing circuitry of the control system may be configured to control the air pressure at the cathode of the fuel cell stack as the value of the target coolant inlet temperature is adjusted. For example, the air pressure at the cathode, or the cathode air pressure, may increase as the target coolant inlet temperature increases.

[0047] Figure 1 A side view of a vehicle 10 according to an example of the present disclosure is depicted. The vehicle 10 is shown as a truck, such as a heavy-duty truck for towing one or more trailers (not shown). The vehicle 10 may be a fuel cell electric vehicle (FCEV) or a hybrid vehicle. It should be understood that the present disclosure is not limited to any other particular type of vehicle, but may be used for any other type of vehicle, such as buses, construction equipment, e.g., wheel loaders or excavators, passenger vehicles, aircraft, and marine vessels. The present disclosure also applies to other applications not related to vehicles, including stationary applications.

[0048] As Figure 1 schematically shown, the vehicle 10 includes a fuel cell system 20, which includes a fuel cell unit 21, and the fuel cell unit includes one or more fuel cell stacks 22. The fuel cell system 20 can be used to power one or more electric drive motors (not shown) that generate propulsion force for the vehicle 10. The fuel cell system 20 can additionally or alternatively be used to power other power-consuming devices of the vehicle 10 (not shown) (such as an electric motor for a crane, an electric motor for a refrigerator system, an electric motor for an air conditioning system, or any other power-consuming function of the vehicle 10). Thus, the fuel cell system 20 can additionally or alternatively be used to power a power take-off (PTO) device, which is a device that transfers the power of an electric motor to another piece of equipment. The vehicle 10 may include one or more PTO devices or be coupled to or associated with the one or more PTO devices.

[0049] The fuel cell unit 21 of the fuel cell system 20 includes one or more fuel cell stacks (shown as fuel cell stacks 22 in Figure 1 , and each fuel cell stack includes two or more fuel cells. In the examples herein, the fuel cell unit 21 may include a plurality of fuel cell stacks 22, and any reference to a fuel cell stack 22 applies to two or more fuel cell stacks. The fuel cell unit 21 is arranged to provide the necessary supplies of hydrogen fuel (H2), an oxidant (such as air), cooling, heating, etc. to the fuel cells. The fuel cell system 20 may include Figure 1Various components not shown in the figure. In some examples, the fuel cell system 20 may include more than one fuel cell unit 21, and each fuel cell unit may include its own controller system, which may be communicatively connected to a controller or control unit or control system for the entire fuel cell system 20.

[0050] The fuel cell system 20 may include a single fuel cell system, two fuel cell systems, or more than two fuel cell systems, such as three or more fuel cell systems. Additionally, when several fuel cell units or systems are provided, the fuel cell systems may be controlled independently or jointly. When controllable independently, each fuel cell system may be controlled to an on state or an off state regardless of the state of other fuel cell systems. When two or more of the fuel cell systems are controllable jointly, these fuel cell systems may be jointly controlled to an on state or an off state, i.e., all fuel cell systems are jointly controlled to the same state. Two fuel cell systems may be controlled dependently on each other in some cases such that one of the fuel cell systems is controlled to an on state or an off state based on the state of the other fuel cell system.

[0051] According to an example of the present disclosure, the vehicle 10 further includes a control device or control system 30. The control system 30 is configured to control the operation of the fuel cell system 20. In some examples, the control system 30 may be implemented as a combination of one or more on-vehicle and non-on-vehicle control systems. The control system 30 is configured to control the fuel cell system 20 and one or more fuel cell units 21 by sending control signals and by receiving status information related to the fuel cell system 20. The control system 30 is configured to receive information from various sensors, which include a temperature sensor, a humidity sensor, a voltage sensor, and one or more sensors among other sensors included in the fuel cell system 20 and / or the vehicle 10 or associated with the fuel cell system and / or the vehicle. For example, one or more temperature sensors may be positioned such that they can obtain measurements of the temperature of the coolant system and other components of the fuel cell system 20. One or more voltage sensors may be included in the fuel cell unit to monitor the voltage of the fuel cell stack. Various sensors may obtain measurements regarding the operation of the components of the fuel cell system 20. For example, one or more temperature sensors may measure the inlet coolant temperature and the outlet coolant temperature of the coolant system of the fuel cell unit 21.

[0052] The control system 30 may also be communicatively coupled to an internal database, an external database, or a combination thereof to receive data related to a route that the vehicle has traveled or will travel. The data related to the route may include a starting point, an ending point, and the path that the vehicle 10 travels between the starting point and the ending point. The control system 30 may store and / or have access to data related to the route, which includes terrain information, speed limits, obstacles, etc. The control system 30 may also receive real-time or near real-time information about the route, which includes traffic congestion, accident reports, road closures, construction, etc. The control system 30 may also store and / or have access to historical data related to the driver's driving pattern, historical data about vehicle operation (e.g., the locations where the vehicle has traveled, the frequency and locations of stops), historical data about environmental conditions at the locations where the vehicle has traveled, etc. The control system 30 may also receive data from a weather service, which may include data about predicted weather conditions and other types of data. The control system 30 may have data about the vehicle's current location and the predicted location where the vehicle is expected to travel along the route.

[0053] The control system 30 is an electronic control unit and may include processing circuitry that is adapted to execute a computer program (such as computer-executable instructions) to perform methods in accordance with aspects of the present disclosure. The control system 30 may include hardware and / or software for performing methods in accordance with aspects of the present disclosure. The control system 30 may be represented as a computer. The control system 30 may be constituted by one or more separate sub-units, and the control system 30 may communicate by using wired and / or wireless communication technologies.

[0054] As Figure 1 shown, the vehicle 10 includes an energy storage system (ESS) 23, such as one or more batteries and / or one or more supercapacitors for storing electrical energy, which includes excess electrical energy generated by the fuel cell system 20. The ESS 23 may store energy regenerated during braking (such as regenerative braking), and / or the ESS may be configured to be charged by a charger (such as from an external power grid). The ESS 23 is configured to assist the fuel cell system 20 in supplying energy to the drive motor to meet the power / energy requirements of the vehicle 10. The ESS 23 may be configured to provide additional power in cases where the fuel cell system 20 cannot provide all of the required power, or where the fuel cell system 20 is not suitable for providing all of the required power. The ESS 23 may provide electrical energy storage for electrical energy generated by the fuel cell system at low loads, assist the fuel cell system 20 in generating power at higher loads, or in some cases may be used as the primary energy supply device. The fuel cell system 20 and the ESS 23 may supply power to one or more auxiliary systems of the vehicle 10.

[0055] The vehicle 10 may also include Figure 1Various other components not shown.

[0056] Although the present disclosure is described with respect to a vehicle such as a truck, aspects of the present disclosure are not limited to that particular vehicle, but may also be used in other vehicles such as passenger cars, off-road vehicles, airplanes, and ships. The present disclosure may also be applied to ships and stationary applications such as grid-connected auxiliary generators or generators independent of the grid.

[0057] Figure 2 An example of a fuel cell system 20 that may be deployed in a fuel cell vehicle such as vehicle 10 is further illustrated. As shown, the fuel cell system 20 includes a fuel cell unit 21, which in turn includes a fuel cell stack 22. The fuel cell unit 21 may include more than one stack, and the fuel cell stack 22 is shown as an example. The operation of the fuel cell system 20 is controlled by a control system 30 also shown in Figure 2 as well.

[0058] The fuel cell stack 22 includes an anode side or anode 24, a cathode side or cathode 26, and an electrolyte 28 (such as, for example, a proton exchange membrane (PEM)) sandwiched between the anode 24 and the cathode 26. The fuel cell stack 22 is configured to generate electricity through an electrochemical reaction between hydrogen and an oxidant.

[0059] The anode 24 receives a fuel such as hydrogen, which may be supplied from a hydrogen storage device 33 (e.g., a refillable and / or replaceable hydrogen container or tank). Any other suitable hydrogen source may be used to supply hydrogen to the anode 24. The anode 24 is configured to receive hydrogen via an anode inlet line 34, and an anode outlet line 36 may be configured to receive the operating by-products of the anode 24 and direct the operating by-products away from the anode 24.

[0060] The cathode 26 receives, at a cathode inlet 40, air or an air stream from the surrounding environment, for example, via a cathode inlet line 38. The air may be filtered and pressurized by an air compressor 39. In some examples, the air compressor 39 may be an electric turbocharger (ETC) including a turbine and a compressor. Due to the compression of the air compressor 39, the air stream may be heated. The air compressor 39 may be fluidly coupled to a charge air cooler (CAC) ( Figure 2 not shown), which is configured to cool the air stream in the cathode inlet line 38 supplied to the cathode 26. The air stream may also be humidified by a humidifier (not shown).

[0061] The cathode 26 is configured to discharge a cathode exhaust gas stream flowing through the cathode outlet line 44 from the cathode outlet 42. The cathode exhaust gas stream may include liquid water and steam generated as by-products of the electrochemical reaction in the fuel cell stack 22. The cathode exhaust gas stream may be carried via the cathode outlet line 44 to the exterior (e.g., outside the vehicle 10), as shown by arrow 45. In some embodiments, the liquid water and steam may be at least partially treated and reused, for example, the liquid water may be extracted from the cathode exhaust gas stream.

[0062] In the example herein, the cathode outlet line 44 is fluidly coupled to a cathode recirculation loop or passage 46 that is configured to fluidly connect or couple the cathode outlet 42 to the cathode inlet 40 to divert a cathode exhaust gas stream (e.g., a portion of the cathode exhaust gas stream) to the cathode inlet 40 such that the cathode exhaust gas stream (e.g., a portion thereof) is mixed with the air stream received by the cathode 26 at the cathode inlet 40. The cathode recirculation loop or passage 46 may also be referred to as an exhaust gas recirculation (EGR) loop or passage. The configuration of the cathode recirculation passage (such as, for example, the cathode recirculation passage 46) according to aspects of the present disclosure may depend on the overall layout of the fuel cell system, the components used, the power level requirements, the application (such as, for example, a vehicle, a generator set, a ship, etc.).

[0063] The volumetric flow rate of the cathode exhaust gas stream in the cathode recirculation passage 46 can be controlled according to the power demand from the fuel cell system, as discussed in more detail below. The volumetric flow rate of the cathode exhaust gas stream in the cathode recirculation passage 46 can be controlled via at least one flow control valve. Thus, as Figure 2 shown, a first flow control valve 48 can be positioned to allow a portion of the cathode exhaust gas stream to be diverted from the cathode outlet line 44 to the cathode recirculation passage 46. The first flow control valve 48 is positioned at the junction between the coolant outlet line 44 and the cathode recirculation passage 46. The first flow control valve 48 can be, for example, a three-way proportional valve, but other types of valves can also be used. The opening of the first flow control valve 48 can be controlled to control the volumetric flow rate of the cathode exhaust gas in the cathode recirculation passage 46. The cathode recirculation passage 46 may include a cathode recirculation pump 50 that is configured to pump the cathode exhaust gas stream through the cathode recirculation passage 46.

[0064] A second flow control valve 52 is positioned to also control the volumetric flow rate of the cathode exhaust gas stream flowing from the cathode recirculation passage 46 into the cathode inlet line 38 such that the partial pressure of oxygen in the air stream supplied to the cathode 26 is reduced. As Figure 2As shown, the second flow control valve 52 is positioned at the junction between the cathode recirculation passage 46 and the coolant inlet line 38. The second flow control valve 52 can be, for example, a two-way proportional valve, but other types of valves can also be used. The opening degree of the second flow control valve 52 can be controlled to control the volumetric flow rate of the cathode exhaust gas diverted from the cathode recirculation passage 46 to the cathode inlet line 38.

[0065] In some examples, one or both of the cathode outlet line 44 and the cathode recirculation passage 46 can include a water separator or another similar component configured to extract water from the cathode exhaust gas stream. The presence of one or more water separators is optional and can depend on the amount of liquid water present in the cathode exhaust gas. In some examples, one or more water separators can be present and can be selectively activated, for example, when it is desired to reduce the amount of liquid water in the cathode exhaust gas stream. It may be desirable to provide a drier cathode exhaust gas to the air stream supplied to the cathode at the cathode inlet 40.

[0066] In addition, in some examples, the cathode air pressure (also referred to as the air pressure in the cathode) can be controlled to prevent drying of the membrane 28 of the fuel cell stack 22. For example, the air pressure control valve 54 can be fluidly coupled to the cathode inlet line 38, and the valve 54 can be controlled to regulate the air pressure in the cathode. In some embodiments, additionally or alternatively, the air compressor 39 (such as, for example, an ETC) can be controlled to regulate the air pressure in the cathode.

[0067] As Figure 2 As shown, the fuel cell unit 21 includes a coolant system or coolant circuit 56 that is configured to circulate coolant through the fuel cell stack 22. The coolant system 56 circulates the coolant such that the coolant flows into the coolant inlet 60 of the fuel cell stack 22 via the coolant inlet line 58 and out of the coolant outlet 64 of the fuel cell stack 22 via the coolant outlet line 66. The coolant system 56 can include components such as, for example, a radiator, a coolant tank for storing coolant, a pump for circulating the coolant, and a heater. The coolant passes through and circulates in the fuel cell stack 22, and the coolant is output from the fuel cell stack via the coolant outlet 64, which is fluidly coupled to the coolant outlet line 66 that is configured to direct the coolant away from the fuel cell stack 22.

[0068] The coolant system 56 (e.g., circuit) is configured to regulate the temperature of the fuel cell stack 22, including dissipating heat generated due to the operation of the fuel cell stack 22. The temperature of the coolant at the coolant inlet 60 and the coolant outlet 64 can be controlled according to, for example, the load on the fuel cell system 20 and other factors. Figure 2Schematically shown, the coolant system 56 may include a coolant inlet temperature sensor 62 and a coolant outlet temperature sensor 68, which are configured to monitor the temperatures of the coolant at the coolant inlet 60 and the coolant outlet 64, respectively. More than two temperature sensors may be deployed in various specific embodiments.

[0069] In an example according to the present disclosure, the target coolant inlet temperature of the coolant at the coolant inlet 60 can be controlled. In some examples, the target coolant inlet temperature may be coupled to controlling the cathode pressure level, thereby maintaining the voltage of the fuel cell stack 22 below 0.8V. The target coolant inlet temperature can be defined as a set point, that is, the value of the coolant temperature set to be achieved at the coolant inlet 60. The actual temperature of the coolant at the coolant inlet 60 can be measured, for example, using the coolant inlet temperature sensor 62.

[0070] The operation of the fuel cell unit 21 is controlled by the control system 30. In the example herein, as Figure 2 shown, the control system 30 includes a processing circuit 32, which is configured to execute computer-executable instructions that can perform the method according to the example of the present disclosure when executed by the processing circuit 32. The control system 30 further includes a memory 31, which is configured to store computer-executable instructions and various data. In some specific embodiments, the computer-executable instructions may be stored at least partially remotely, for example, in a remote memory. The memory 31 can obtain data from various sources, including from one or more remote storage devices.

[0071] The processing circuit 32 of the control system 30 can be configured to obtain the value of the power output requested from the fuel cell system 20. The power output requested from the fuel cell system 20 is defined by the load applied to the fuel cell system 20. The processing circuit 32 is also configured to monitor the voltage of the fuel cell stack 22 in response to the value of the power output being lower than a first threshold power level. For example, in Figure 2 the voltage sensor 55 schematically shown can be configured to monitor the voltage of the fuel cell stack 22. One or more suitable voltage sensors can be used. The single-cell voltage of the fuel cell stack 22 can be determined as, for example, the total voltage of the fuel cell stack 22 measured by the voltage sensor 55 divided by the number of fuel cells in the fuel cell stack 22. The processing circuit 32 is also configured to control the target coolant inlet temperature of the coolant at the coolant inlet 60 and control the cathode pressure level to avoid degradation of the membrane 28, thereby maintaining the voltage of the fuel cell stack 22 below 0.8V (which is a value: above which the catalyst and carrier materials may be damaged).

[0072] In the examples herein, the processing circuitry 32 of the control system 30 may be configured to control a target coolant inlet temperature by increasing the target coolant inlet temperature to a temperature threshold level. The processing circuitry 32 may be configured to, in response to a value of the power output being lower than at least one second threshold power level that is less than a first threshold power level, when the target coolant inlet temperature is at the temperature threshold, control the coolant system 56 to direct a portion of the cathode exhaust gas stream to mix with the air stream, thereby reducing the oxygen partial pressure of the air stream. The second threshold power level may include more than one threshold power level. The value of the power output may be compared with one or more of the second threshold power levels, and the amount of the cathode exhaust gas stream directed to mix with the air stream in the cathode inlet line via the cathode recirculation channel may increase with the value of the power output. Thus, as the value of the power output requested from the fuel cell system 20 decreases, the oxygen partial pressure of the air stream may decrease. In other words, a lower current density in the fuel cell system 20 may require a lower oxygen partial pressure of the air stream. This, in combination with the increase in the target coolant inlet temperature, allows the fuel cell system 20 to operate at lower loads while maintaining the voltage of the fuel cell stack below 0.8V. A lower value of the power output requires a lower oxygen partial pressure of the air stream.

[0073] In some examples, the processing circuitry 32 of the control system 30 is configured to control at least one flow control valve (e.g., the first flow control valve 48 and the second flow control valve 52) fluidly coupled to the cathode recirculation channel, and a cathode recirculation pump 50 fluidly coupled to the cathode recirculation channel 46 and configured to operate to drive the cathode exhaust gas stream through the cathode recirculation channel 46, based on the value of the power output.

[0074] In some examples, the fuel cell unit 21 includes one fuel cell unit. Thus, the fuel cell system 20 may include a single fuel cell unit.

[0075] It should be understood that Figure 2 the configuration of the fuel cell system 20 shown in Figure 2 is exemplary. Additionally, the fuel cell system 20 may include

[0076] It should also be understood that the cathode recirculation channel can be fluidly connected to the cathode inlet line at different positions along the cathode inlet line, which is configured to deliver an air stream from an air source (usually the ambient environment) to the inlet of the cathode. Figure 3 、 Figure 4 and Figure 5 illustrates an example of a fuel cell system according to aspects of the present disclosure, in which the cathode recirculation channel is fluidly coupled to the cathode inlet line at different positions along the cathode inlet line.

[0077] Figure 3 illustrates a fuel cell unit in which the cathode recirculation channel is connected to the cathode inlet after or downstream of the humidifier. Figure 3 Illustrates a fuel cell system 200 including a fuel cell unit 221, the fuel cell unit including one or more fuel cell stacks, where the fuel cell stack 222 is shown as a representative. Figure 3 The fuel cell system 200 of Figure 2 is generally similar to the fuel cell system 20 of Figure 3 so the components of the fuel cell system 200 of

[0078] The coolant inlet line 258 is configured to direct a coolant stream or coolant to the fuel cell stack 222 such that the coolant passes through the fuel cell stack 222, and the coolant outlet line 266 receives the coolant after the coolant has passed through the fuel cell stack 222.

[0079] The cathode receives, at the cathode inlet 240, an air or air stream, for example from the ambient environment, via the cathode inlet line 238. The air can be filtered and pressurized by an air compressor 239. The air compressor 239 can be fluidly coupled to a charge air cooler (CAC) 241, which is positioned downstream of the air compressor 239, i.e., farther from the air inlet than the air compressor 239. In Figure 3 the example of

[0080] the cathode of the fuel cell stack 222 is configured to discharge a cathode exhaust stream at the cathode outlet 242 such that the cathode exhaust stream flows through the cathode outlet line 244. As Figure 3As shown, the cathode outlet line 244 is fluidly coupled to a cathode recirculation loop or passage 246 that fluidly connects the cathode outlet 242 to the cathode inlet 240 to divert the cathode exhaust gas stream to the cathode inlet 240 such that the cathode exhaust gas stream is mixed with the air stream received by the cathode at the cathode inlet 240. The cathode recirculation loop or passage 246 may also be referred to as an exhaust gas recirculation (EGR) loop or passage.

[0081] The volumetric flow rate of the cathode exhaust gas stream in the cathode recirculation passage 246 may be controlled via at least one flow control valve. Thus, as Figure 3 shown, a flow control valve 248 may be positioned to allow a portion of the cathode exhaust gas stream to divert from the cathode outlet line 244 to the cathode recirculation passage 246. The opening of valve 248 may be controlled to control the volumetric flow rate of the cathode exhaust gas in the cathode recirculation passage 246. The cathode recirculation passage 246 may include a cathode recirculation pump 250 that is configured to pump the cathode exhaust gas stream through the cathode recirculation passage 46. A flow control valve 252 is positioned to further control the volumetric flow rate of the cathode exhaust gas stream flowing from the cathode recirculation passage 246 into the cathode inlet line 238 such that the partial pressure of oxygen in the air stream supplied to the cathode 226 is reduced. As Figure 3 shown, in the fuel cell system 200, the cathode recirculation passage 246 is fluidly coupled to the cathode inlet line 238 after or in other words downstream of the humidifier 243.

[0082] Figure 4 Illustrated is a fuel cell system 300 that includes a fuel cell stack 321 that includes at least one fuel cell stack 322. The configuration of the fuel cell system 300 is similar to Figure 3 that of the fuel cell system 200 and only some components of Figure 4 the fuel cell system 300 are shown. The anode of the fuel cell stack 322 is configured to receive hydrogen via an anode inlet line 334 and discharge by-products of anode operation via an anode outlet line 336. A coolant inlet line 358 is configured to direct a coolant stream or coolant to the fuel cell stack 322 such that the coolant passes through the fuel cell stack 322 and a coolant outlet line 366 receives the coolant after the coolant has passed through the fuel cell stack 322. Figure 4 Is shown as an example of the location where the cathode recirculation passage is fluidly coupled to the cathode inlet line. Figure 4The cathode inlet line 338 of the fuel cell system 300 does not include a humidifier. The cathode recirculation passage 346 (fluidically coupled to the cathode outlet line 344, which is configured to carry the cathode exhaust gas flow) is configured to be fluidly coupled to the cathode inlet line 338 between the air compressor 339 and the CAC 341, that is, downstream of the air compressor 339 and upstream of the CAC 341.

[0083] Figure 5 The fuel cell system 400 includes a fuel cell stack 421, which includes at least one fuel cell stack 422. The configuration of the fuel cell system 400 is similar to Figure 3 The configuration of the fuel cell system 200 is shown. Figure 5 The anodes of the fuel cell stack 422 are configured to receive hydrogen gas via an anode inlet line 434 and to exhaust byproducts of anode operation via an anode outlet line 436. The coolant inlet line 458 is configured to direct a coolant flow or coolant to the fuel cell stack 422 so that the coolant passes through the fuel cell stack 422, and the coolant outlet line 466 receives the coolant after the coolant has passed through the fuel cell stack 422. Figure 5 Shown is an example of where the cathode recirculation channel is fluidly coupled to the cathode inlet line. Figure 5 The cathode inlet line 438 of the fuel cell system 400 does not include a humidifier. The cathode recirculation passage 446 (fluidically coupled to the cathode outlet line 444, which is configured to carry the cathode exhaust gas flow) is configured to be fluidly coupled to the cathode inlet line 438 upstream of the air compressor 439 and the CAC 441 positioned downstream of the air compressor 439.

[0084] Various other configurations of fuel cell systems and cathode recirculation passages may be coupled to the coolant inlet line at appropriate locations along the coolant inlet line. Figure 3 , Figure 4 and Figure 5 The fuel cell system shown in can be configured to be controlled according to the examples of the present disclosure. It should be noted that Figure 2 The fuel cell system 20 can be modified to be similar to Figure 3 , Figure 4 and Figure 5 The invention may be configured with a fuel cell system of any one of the foregoing, or configured in another manner.

[0085] In an example according to aspects of the present disclosure, the temperature of the fuel cell stack can be increased, for example, by increasing the target coolant inlet temperature. Additionally, in some cases, the oxygen partial pressure in the air stream supplied to the input of the cathode can be reduced, for example, via a cathode recirculation loop or passage (e.g., EGR). This allows for a reduction in power without having to significantly reduce the oxygen mass fraction in the air stream, thereby reducing the risk of oxygen starvation that can occur when the oxygen partial pressure in the air stream drops sharply. Depending on how low the power request from the fuel cell system is (e.g., how low the idle power required is), the need to reduce the oxygen partial pressure decreases with higher idle power. And as the temperature of the fuel cell stack increases, the operation of the cathode recirculation loop or passage helps to control the membrane humidity. In other words, using both the reduction in the oxygen partial pressure in the air stream and the increase in the fuel cell stack temperature allows for improved management of the fuel cell system under low power demand.

[0086] The operation of a fuel cell system according to aspects of the present disclosure is illustrated in the following example. Thus, by way of example only, the value of the power output requested from the fuel cell system can be higher than 60 W. The single cell potential needs to be limited to 0.8 V. As long as the value of the power output remains higher than or equal to 60 kW, the target coolant inlet temperature can be maintained at the normal coolant inlet set point, for example, such that the temperature of the stack is also normally controlled and the cathode recirculation passage is not activated, i.e., the cathode exhaust gas stream is not supplied to mix with the air stream fed to the cathode inlet. For a NAFION-type membrane, the normal coolant inlet set point can be defined as, for example, 60 °C. Another temperature value can represent the normal coolant inlet set point.

[0087] Continuing with this example, if the power request of the fuel cell system drops below 60 kW, under normal operating conditions and as the current density decreases, the voltage of the fuel cell stack will increase, and thus the single cell potential will rise above 0.8 V.

[0088] According to aspects of the present invention, as the power requested from the fuel cell system (e.g., the power requested by a vehicle in which the fuel cell system is deployed) drops below 60 kW, the fuel cell control unit (FCCU) (such as, for example, controller 30 ( Figure 1 and Figure 2 shown in)) continuously evaluates the fuel cell stack voltage to estimate the single cell voltage. The FCCU controls the fuel cell stack temperature and the volumetric flow rate of the cathode exhaust gas stream in the cathode recirculation passage based on the power requested from the fuel cell system.

[0089] When the power requested from the fuel cell system (also referred to as net power) needs to be reduced by a relatively high value (e.g., from 60 kW to 20 kW), the requested volumetric flow rate of the cathode exhaust gas stream from the cathode recirculation channel may be relatively high because a significant reduction in the oxygen partial pressure in the cathode inlet air stream is required. The vehicle needs the fuel cell system to provide power called net power. As an example, Table 1 below illustrates the relationship of the oxygen mass fraction required to achieve a certain fuel cell power and a certain single cell voltage as a function of the target coolant inlet temperature (also referred to as the coolant inlet set point).

[0090]

[0091] However, the required significant reduction in the oxygen partial pressure significantly increases the risk of oxygen starvation in the fuel cell stack. As a result, while the fuel cell power output may be unstable, an undesired degradation mechanism may be triggered. Therefore, in some cases, using only the cathode recirculation channel may not be a suitable solution.

[0092] The fuel cell stack temperature has a function of changing the fuel cell stack efficiency in such a way that, for example, as the target coolant inlet temperature increases, the fuel cell stack temperature increases, the fuel cell efficiency decreases, and thus not such a high reduction in the oxygen partial pressure is required to achieve the lower power required by the fuel cell system, as shown in the example of Table 1. In an example according to the present disclosure, the cathode recirculation channel is thus used to both reduce the oxygen partial pressure and increase the humidity of the incoming air (i.e., the air stream fed to the cathode) to help compensate for the reduction in membrane / cell / stack humidity when the fuel cell stack temperature increases.

[0093] In some examples, such as referring to the example values shown in Table 1, the control system may implement the following control strategy. When the power consuming device (e.g., a fuel cell vehicle) requests a lower power from the fuel cell system (e.g., below 70 kW, or below 65 kW, or in some examples below 60 kW), if the fuel cell membrane cannot operate properly at a higher temperature, the target coolant inlet temperature may be increased to a certain value, such as 80 °C for example. As the target coolant inlet temperature increases, the single cell voltage is monitored. The fuel cell stack temperature is also monitored, for example as the temperature of the coolant at the coolant inlet of the fuel cell stack and the temperature of the coolant at the coolant outlet of the fuel cell stack. If the temperature at the coolant inlet is already at 80 °C and the need to reduce the power output of the fuel cell system has not been met, then, as an example, one or more flow control valves and recirculation pumps in the cathode recirculation channel according to an example of the present disclosure may be actuated or operated to divert at least a portion of the cathode exhaust gas stream into the air stream fed to the cathode inlet of the fuel cell stack.

[0094] Figure 6AAn example of a process or method for operating a fuel cell system including a fuel cell unit is illustrated. The fuel cell unit includes a fuel cell stack, and the fuel cell stack includes a cathode and an anode. The cathode is configured to receive an air stream via a cathode inlet line and provide a cathode exhaust stream via a cathode outlet line. The air stream is received at a cathode inlet that is in fluid communication with the cathode inlet line, and the cathode exhaust stream is discharged from a cathode outlet that is in fluid communication with the cathode outlet line. The fuel cell unit includes a cathode recirculation passage that fluidly connects the cathode outlet line to the cathode inlet line, thereby diverting a cathode exhaust stream (e.g., a portion of the cathode exhaust stream) to the cathode inlet line such that the cathode exhaust stream (e.g., the portion of the cathode exhaust stream) is mixed with the air stream received by the cathode inlet line. The amount of the cathode exhaust stream diverted to the cathode inlet line via the cathode recirculation passage depends on a desired reduction in the oxygen partial pressure of the air stream delivered from an air source (e.g., ambient air) to the cathode inlet via the cathode inlet line.

[0095] The fuel cell unit further includes a coolant system that is configured to circulate coolant through the fuel cell stack. The coolant system includes a coolant inlet line and a coolant outlet line. The coolant inlet line is configured to direct coolant to the fuel cell stack, and the coolant outlet line is configured to direct coolant away from the fuel cell stack after the coolant has passed through the fuel cell stack.

[0096] The fuel cell system can be Figure 2 the fuel cell system 20, or the fuel cell system 300 ( Figure 3 ), or the fuel cell system 400 ( Figure 4 ), or a fuel cell system having another configuration and configured to operate in accordance with an example of the present disclosure. The cathode includes a cathode inlet configured to receive an air stream and a cathode outlet configured to provide a cathode exhaust stream. For example, Figure 2 the fuel cell system 20 includes an anode 24 and a cathode 26. The method 600 is described below for illustrative purposes only in connection with Figure 2 and it should be understood that another fuel cell system can be used.

[0097] In addition, it should be noted that the processes shown in connection with Figure 6A the process or method 600 are shown under the assumption that as the load on the fuel cell system decreases (e.g., as the vehicle decelerates or stops), the ESS of the fuel cell system may not be able to accept excess power or it may only accept limited power.

[0098] The method 600 can be performed by a controller or control system such as Figure 1 and Figure 2The control system 30 shown in FIG. is implemented. The processing circuitry 32 of the control system 30 may execute computer-executable instructions, such as those stored in the memory of the control system 30, which, when executed, cause the processing circuitry to perform method 600.

[0099] Process 600 may start at any suitable time. For example, the process may be performed as the vehicle including the fuel cell system 20 is traveling along a route and the load on the fuel cell system 20 changes. When the vehicle 10 is traveling downhill, when the vehicle decelerates due to, for example, traffic conditions, and / or in various other situations, the power demand from the fuel cell system 20 may decrease. The vehicle may travel along the route (e.g., as part of a mission or task) that may include traveling from a starting point to an end point. The vehicle may be part of a fleet. Additionally, even though process 600 is described herein by way of example as being implemented in a fuel cell system of a vehicle, it should be understood that process 600 may be implemented in a stationary application.

[0100] At Figure 6A In block 602 shown in FIG., process 600 includes obtaining a value of the power output requested from the fuel cell system. The value of the power output may be the current value of the power output currently requested from the fuel cell system as the fuel cell system operates. The value of the power output is defined by the load on the fuel cell system, which varies according to the state of the application (e.g., the vehicle including the fuel cell system). As the vehicle is traveling and requesting power from the fuel cell system, the value of the power output or the requested power output may be repeatedly obtained.

[0101] In some examples, the value of the power output of the fuel cell system may be a predicted value of the power output of the fuel cell system. For example, for a fuel cell vehicle traveling along a route, the control system 30 may predict that the vehicle is approaching a downhill section of the route or road where the vehicle will travel downhill. Based on the steepness of the hill and other factors (e.g., the weight of the vehicle, which may also carry a load), a decrease in the power demand from the fuel cell system may be predicted or estimated or determined.

[0102] A decrease in the power output required by the fuel cell system may be detected and / or predicted in various other situations and based on various factors. For example, the vehicle may decelerate, e.g., based on driver input and / or automatically. Additionally, as an example, when it is detected that the speed limit in the route traveled by the fuel cell vehicle is about to decrease, a decrease in the value of the power output requested from the fuel cell system may be predicted.

[0103] At decision block 604, process 600 includes determining whether a value of power output is below a first threshold power level. This determination includes comparing the value of power output with the first threshold power level. The first threshold power level can be, for example, about 60 kW. In some examples, the first threshold power level can vary in a range from about 60 kW to about 75 kW, or from about 60 kW to about 70 kW.

[0104] In response to determining that the value of power output is not below the first threshold power level (i.e., is above or at the first threshold power level), process 600 can return to block 602, as Figure 6A shown, where one or more subsequent values of power output requested from the fuel cell system are obtained.

[0105] At block 606, in response to determining that the value of power output is below the first threshold power level, the control system can monitor the voltage of the fuel cell stack of the fuel cell unit of the fuel cell system. In some examples, a power output with a value below the first threshold power level can be referred to as idle power. When the power output value is below the first threshold power level, the fuel cell system can be considered to be in an idle mode or state. In other words, in some examples, when the value of power output is below the first threshold power level, the fuel cell system operates in an idle mode or state.

[0106] When the load on the fuel cell system is reduced to a point where the single cell voltage can rise to a value that poses a risk of damage to the catalyst and / or carbon support material, the value of the power output requested from the fuel cell system, which can be the current value or a predicted value, can be below the first threshold power level. When the value of power output is below the first threshold power level, the control system 30 can begin to monitor the voltage of the fuel cell stack. For example, a voltage sensor such as Figure 2 voltage sensor 55 as shown can be used to obtain a measurement of the voltage of the fuel cell stack 22, and the single cell voltage can be determined based on the obtained measurement.

[0107] In some examples, the single cell voltage of the fuel cell stack 22 can be determined as the measured voltage of the fuel cell stack 22 divided by the number of fuel cells in the fuel cell stack 22. Thus, the single cell voltage can be determined as an average single cell voltage, and this single cell voltage represents the voltage of each fuel cell in each fuel cell stack. In some examples, the single cell voltage can be determined based on the measured voltage of the entire fuel cell unit, in which case the measured voltage is divided by the number of stacks and the number of cells in each stack. In some examples, the voltage of each fuel cell is measured, and the average of these measured voltages can be considered as the single cell voltage of the fuel cell stack. Regardless of the manner in which the single cell voltage of the fuel cell stack is measured, the fuel cell system is controlled such that the single cell voltage does not exceed 0.8 V.

[0108] In some examples, the value of the power output can be below the first threshold power level but above the second threshold power level, which is lower than the first threshold power level. Thus, to allow the load on the fuel cell system to be reduced to a desired lower value, it may be sufficient to reduce the target coolant inlet temperature, as discussed further below. If the value of the requested power output is further reduced (e.g., reduced to a value below the second threshold power level), then it will be necessary to reduce the oxygen partial pressure in the air stream fed to the cathode, as will also be discussed below.

[0109] In some examples, the value of the power output can be low enough such that it is below the first threshold power level and also below the second threshold power level, which is lower than the first threshold power level. For example, the vehicle may suddenly decelerate and the value of the power output may rapidly decrease to a value below the second threshold power level. The fuel cell system can enter an idle mode or state.

[0110] At block 608, process 600 includes adjusting or controlling the target coolant inlet temperature, thereby maintaining the voltage of the fuel cell stack below 0.8 V. The target coolant inlet temperature can be adjusted or controlled based on the value of the power output by increasing the target coolant inlet temperature to a temperature threshold level. In some examples, the temperature threshold level is 80 °C. In some examples, the temperature threshold level is 60 °C. In some examples, the temperature threshold level is 70 °C. In some examples, the temperature threshold level is in the range between 60 °C and 80 °C. As the target coolant inlet temperature is controlled, the voltage of the fuel cell stack is monitored.

[0111] The target coolant inlet temperature is increased, thereby increasing the temperature of the fuel cell stack of the fuel cell system. As the target coolant inlet temperature increases, the increase in the fuel cell stack temperature affects the fuel cell stack efficiency, and the fuel cell stack efficiency may decrease as the fuel cell stack temperature increases. The efficiency of the fuel cell system is intentionally reduced.

[0112] As the fuel cell stack becomes less efficient, the fuel cell stack will need to reduce the oxygen partial pressure in the air stream at the input to the cathode less in order to achieve a lower value of the power requested from the fuel cell system. This is an advantage because the risk of oxygen starvation in the fuel cell stack is reduced or eliminated in this way. As another advantage, the control of the net power of the fuel cell system is faster and more stable. In addition, the control of the EGR mass flow rate and the single cell voltage is improved.

[0113] At block 610, process 600 includes controlling the air pressure in the cathode of the fuel cell stack of the fuel cell unit, where the air pressure in the cathode is also referred to as the cathode air pressure. In some cases, in response to a decrease in the power output value of the fuel cell system, the control system may control the fuel cell system to increase the cathode air pressure. The cathode air pressure can be controlled to prevent the membrane of the fuel cell from drying out. When increasing the target coolant inlet temperature, at block 608, the likelihood of the membrane of the fuel cell stack drying out increases. To avoid this, that is, to be able to raise the target coolant inlet temperature to above a specific level, the air pressure in the cathode of the fuel cell stack must also be increased. This prevents the membrane from drying out. This can be achieved by active control (e.g., an electric turbocharger (ETC) of an air compressor) to increase the air pressure on the cathode side.

[0114] Accordingly, in some examples, the control system obtains or determines a power request from the fuel cell system, determines a target coolant inlet temperature, and determines whether additional boosting is also required, that is, whether to control the air compressor to increase the air pressure. It should be understood that the processes at blocks 608 and 610 can be performed simultaneously or substantially simultaneously, that is, with some overlap in time, and blocks 608 and 610 are illustrated separately only for illustrative purposes.

[0115] At decision block 612, process 600 includes determining whether the value of the power output requested from the fuel cell system is less than at least one second threshold power level that is less than a first threshold power level. This determination includes comparing the value of the power output with at least one second threshold power level, which may include more than one second threshold power level, such that the value of the power output can be compared with more than one value of the second threshold power level.

[0116] In some examples, the value of the power output requested from the fuel cell system may first decrease to below the first threshold power level but above the second threshold power level, and then further decrease to the second threshold power level, which can be one or more power levels. As an example, the value of the power output may first decrease to below the first threshold power level (which can be 60 KW or a value greater than 60 kW), then decrease to below 40 KW, and then decrease to below 30 KW, where the second threshold power level includes two second threshold power levels (40 kW and 30 kW). The second threshold power level can have any other value below the first threshold power level. For example, the second threshold power level may include 40 kW, or 30 kW, or 20 kW, or another suitable value.

[0117] It should be noted that the specific value or range of values of the first threshold power level and one or more values of the second threshold power level may depend on the characteristics of the fuel cell unit and the entire fuel cell system that may include one or more fuel cell units. In some examples according to aspects of the present disclosure, the fuel cell system includes one (i.e., a single) high-efficiency or higher-efficiency fuel cell unit. Compared with a fuel cell system having a lower efficiency, a fuel cell system having a higher efficiency may have a higher first threshold power value.

[0118] In response to determining that the value of the power output is not less than the second threshold power level (i.e., greater than or at the second threshold power level), process 600 may return to optional block 616, as Figure 6A shown, where it is determined whether the value of the power output is further decreased, as discussed in more detail below.

[0119] At block 614, in response to the value of the power output being lower than at least one second threshold power level that is smaller than the first threshold power level, process 600 includes controlling the target coolant inlet temperature and controlling the oxygen partial pressure of the air flow, where the oxygen partial pressure is controlled by controlling the volumetric flow rate of the cathode exhaust gas stream that is directed from the cathode outlet line to the cathode inlet line to mix with the air flow. Thus, the target coolant inlet temperature is controlled, and the oxygen partial pressure in the air flow is adjusted or controlled. Thus, in response to the value of the power output being lower than at least one second threshold power level that is smaller than the first threshold power level, when the target coolant inlet temperature is at the temperature threshold, the control system may control the cathode recirculation channel to direct a portion of the cathode exhaust gas stream to mix with the air flow, thereby reducing the oxygen partial pressure of the air flow. In some examples, the oxygen partial pressure may be reduced according to the desired decrease in the power demand of the fuel cell system (e.g., according to the values shown in Table 1 herein), but it should be understood that the values in Table 1 are shown only as non-limiting examples. Other values may be used depending on the details of the fuel cell system and other factors.

[0120] The control of the oxygen partial pressure of the air oxygen flow may include controlling at least one flow control valve and a cathode recirculation pump fluidly coupled to the cathode recirculation channel that fluidly connects the cathode outlet line to the cathode inlet line, thereby diverting a portion of the cathode exhaust gas stream to the cathode inlet line such that the cathode exhaust gas stream mixes with the air oxygen flow. The control system may control the at least one flow control valve and the cathode recirculation pump according to the value of the power output. The at least one flow control valve and the cathode recirculation pump may be activated and / or actuated to allow a portion of the cathode exhaust gas stream in the cathode outlet line to flow through the cathode recirculation channel to the cathode inlet line. For example, referring to Figure 2, the first flow control valve 48, the second flow control valve 52, and the cathode recirculation pump 50 can be controlled to allow a portion of the cathode exhaust gas flow in the cathode outlet line 44 to be directed through the cathode recirculation passage 46 to the cathode inlet line 38.

[0121] In the examples herein, depending on how low the idle power of the fuel cell system is, the need to control the cathode recirculation passage to reduce the oxygen partial pressure decreases with higher idle power. At the same time, as the fuel cell stack temperature increases, the cathode recirculation passage helps control the humidity of the membrane.

[0122] At block 616, the control system can optionally determine whether the value of the power output requested from the fuel cell system further decreases. In response to determining that the value of the power output continues to decrease, the power output can be compared with one or more successively decreasing second threshold power levels among at least one second threshold power level. Thus, as Figure 6A shown, if it is determined that the value of the power output requested from the fuel cell system further decreases, process 600 can return to block 612, where the value of the power output is compared with another second threshold power level (such as a second threshold power level having a lower value than the second threshold power level previously compared with the value of the power output). According to an example of the present disclosure, as the value of the power output continues to decrease, the fuel cell system is controlled to accommodate the reduced load on the fuel cell system.

[0123] In response to determining that the value of the power output does not further decrease, process 600 can end, as Figure 6A shown. However, it should be understood that as the fuel cell system is operating, process 600 can be continuously executed. For example, as the load on the fuel cell system increases, process 600 can return to Figure 6A block 602 schematically shown, where one or more values of the power output requested from the fuel cell system are obtained or acquired.

[0124] Figure 6B Illustrates an example of process 600a that can be executed as the value of the power output requested from the fuel cell system increases. For example, the load on the fuel cell system may increase, and the fuel cell system may exit the idle state or mode. Process 600a can be executed by the control system 30. As the fuel cell system is operating (e.g., as the vehicle is traveling along a route and the load on the fuel cell system changes), the control system 30 can execute processing according to Figure 6A process 600 as the load decreases and execute processing according to Figure 6B process 600a as the load increases.

[0125] In Figure 6BAt block 618, a value of the power output requested from the fuel cell system can be obtained or acquired. As the fuel cell system is operating, the value of the power output requested from the fuel cell system can be continuously acquired.

[0126] At decision block 620, it can be determined whether the value of the power output is higher than at least one second threshold power level. In some examples, when the at least one second threshold power level includes more than one second threshold power level, the value of the power output can be compared with the second threshold power level having the lowest value. For example, if the second threshold power levels include threshold power levels having values of 40 kW, 30 kW, and 20 kW, respectively, the current value of the power output can be compared with the lowest value (i.e., 20 kW).

[0127] If the value of the power output is not higher than at least one second threshold power level (i.e., if the value of the power output is not higher than any of the second threshold power levels used), process 600a can return to block 618, where a further value of the power output can be obtained. In such cases, the control system can continue to control the fuel cell system according to Figure 6A process 600.

[0128] At decision block 622, in response to determining that the value of the power output is higher than at least one second threshold power level (indicating an increase in the load), the control system can determine whether the oxygen partial pressure in the air stream has decreased. In other words, it is determined whether the cathode recirculation loop or passage (e.g., is currently being used) has been used, and when the load on the fuel cell system decreases, the oxygen partial pressure in the air stream decreases. As discussed above, one or more flow control valves and a cathode recirculation pump fluidly coupled to the cathode recirculation loop or passage can be activated and / or actuated to allow a portion of the cathode exhaust gas stream to be directed to the cathode inlet line.

[0129] At block 624, in response to determining that the oxygen partial pressure in the air stream has decreased, the oxygen partial pressure in the air stream can be increased. For example, the flow control valve and the cathode recirculation pump can be controlled to reduce the volumetric flow rate of the cathode exhaust gas stream directed from the cathode recirculation pump to the cathode inlet line, thereby increasing the oxygen partial pressure in the air stream in the cathode inlet line.

[0130] As the value of the power output requested from the fuel cell system continues to increase, the oxygen partial pressure in the air stream can increase accordingly. At a certain point (e.g., when the value of the power output rises above the second threshold power level having the highest value among the at least one second threshold power levels), at least one flow control valve and the cathode recirculation pump can be controlled to completely block the cathode exhaust gas stream from entering the cathode recirculation passage.

[0131] At block 626, the control system may control a fuel cell system (e.g., a coolant system) to reduce a target coolant inlet temperature. The target coolant inlet temperature is reduced such that the efficiency of the fuel cell system is increased because it may not be necessary to intentionally reduce its efficiency as the load increases.

[0132] As Figure 6B shown, at block 626, the control system may also control the coolant system to reduce the target coolant inlet temperature in combination with controlling the air pressure at the cathode in response to determining that the oxygen partial pressure in the air stream has not decreased at block 622 (which may occur when only the target coolant inlet temperature is controlled). In other words, if the value of the power output does not drop low enough to activate the cathode recirculation channel as the power output decreases, only the target coolant inlet temperature decreases as the load on the fuel cell system increases. The air pressure at the cathode may be reduced.

[0133] In an example according to aspects of the present disclosure, as the value of the power output requested from the fuel cell system continues to increase, the target coolant inlet temperature may be reduced to a value considered to be a normal target coolant inlet temperature. Thus, as the value of the requested power output may increase above a first threshold power level, the target coolant inlet temperature may be controlled at a level or within a range considered to be a normal target coolant inlet temperature.

[0134] If the load on the fuel cell system continues to increase or varies within a certain range without dropping below the first threshold power level, then after the process at block 626 of Figure 6B the process at block 618 of Figure 6B may be performed. If the load on the fuel cell system decreases, then after the process at block 626 of Figure 6B the process at block 602 of Figure 6A may be performed. In either of these cases, the value of the power output is obtained or acquired and then may be compared with one or more threshold power levels as described herein.

[0135] As described above, the method according to an example of the present disclosure may be implemented in a power generation unit. In some examples, the method according to an example of the present disclosure may be implemented in a stationary system including a fuel cell system.

[0136] To perform the method steps described herein, the control system 30 may be configured to perform the processes described in connection with Figure 6A and Figure 6B and / or any other example according to the present disclosure. For example, the control system 30 may include as Figure 7A and Figure 7BThe arrangement shown. The control system 30 can be located at any suitable location in the vehicle 10 or in another system that implements the methods according to aspects of the present disclosure. In some examples, the control system 30 can be a remote control system or some of its functions can be performed in a remote control system.

[0137] As Figure 7A shown, the control system 30 includes a processing circuit 32, a memory 31, and an input and output interface 700 that is configured to communicate with any necessary components and / or entities of the examples herein. The input and output interface 700 can include wireless and / or wired receivers and wireless and / or wired transmitters. In some examples, the input and output interface 700 can include wireless and / or wired transceivers. The control system 30 can use the input and output interface 700 to control and communicate with various sensors, actuators, subsystems, and / or interfaces of the fuel cell system and the vehicle 10 by using any one or more of a controller area network (CAN) bus, an Ethernet cable, Wi-Fi, Bluetooth, and / or other network interfaces.

[0138] The methods described herein can be implemented using a processing circuit (e.g., one or more processors such as the processing circuit 32 of the control system 30) together with computer program code stored in a computer-readable storage medium for performing the functions and actions of the examples herein.

[0139] The memory 31 can include one or more memory units. The memory 31 includes computer-executable instructions that can be executed by the processing circuit 32 of the control system 30. The memory 31 is configured to store, for example, information, data, etc., and computer-executable instructions to perform the methods according to the examples herein when executed by the processing circuit 32. The control system 30 can additionally obtain information from an external memory.

[0140] The methods according to aspects of the present disclosure can be implemented by, for example, a computer program product 710 or a computer program that includes computer-executable instructions (i.e., a software code portion) that, when executed on at least one processor (e.g., the processing circuit 32), cause the at least one processor to perform the actions described herein, as performed by the control system 30.

[0141] In some examples, the computer program product 710 is stored on a computer-readable storage medium 720. The computer-readable storage medium 720 can be, for example, a magnetic disk, a universal serial bus (USB) stick, or a similar device. The computer-readable storage medium 720 on which the computer program product is stored can include computer-executable instructions that, when executed by the processing circuit 32, cause the processing circuit 32 to perform the actions of the method according to the examples of the present disclosure described herein, as performed by the control system 30.

[0142] As Figure 7B shown, the control system 30 can include an obtaining unit 702. The control unit system 30, the processing circuit 32, and / or the obtaining unit 702 are configured to obtain a value of the power output requested from the fuel cell system, the value of the power output indicating the power demand from the fuel cell system during the operation of a fuel cell vehicle or another power-consuming device in which the fuel cell system is deployed. The control unit system 30, the processing circuit 32, and / or the obtaining unit 702 can be configured to determine the value of the power output of the fuel cell system. In some examples, the obtained value of the power output can be a predicted value of the power output of the fuel cell system. The value of the power output can be obtained as part of the processing shown at block 602 of Figure 6A and / or as part of the processing shown at block 618 of Figure 6B .

[0143] In some examples, the value of the power output can be a predicted value of the power output desired to be requested from the fuel cell system. The value of the power output can be predicted using, for example, characteristics of the route on which a vehicle including the fuel cell system is traveling and other relevant characteristics, such as the current location of the vehicle, current and upcoming traffic conditions along the route, actual and / or predicted weather, and other characteristics. For example, the control system can predict that the vehicle is approaching a portion of the route where the power demand on the fuel cell system will decrease, for example, the vehicle will be traveling downhill, or the vehicle will stop for a certain duration, or another condition will cause the vehicle to operate such that the power output requested from the fuel cell system will decrease.

[0144] In some examples, the control system 30 and / or the processing circuit 32 can include a prediction unit (not shown separately herein) that is configured to predict or estimate the value of the power output based on characteristics of the route on which the vehicle is traveling and various other characteristics.

[0145] In some examples, the fuel cell system can be included in a stationary application and data such as, for example, the previous operating history of the stationary application and / or other factors can be used to predict the value of the power output to be requested from the fuel cell system of the stationary application.

[0146] Similarly, asFigure 7B As shown, the control system 30 may include a determination unit 704. The control system 30, the processing circuit 32, and / or the determination unit 704 are configured to determine whether the value of the power output is below a first threshold power level. The value of the power output may be determined or predicted. The first threshold power level may be, for example, about 60 kW. In some examples, the first threshold power level may vary within a range from about 60 kW to about 75 kW, or from about 60 kW to about 70 kW. In some examples, the first threshold power level may be a value within a value range.

[0147] The control system 30 may include a monitoring unit 706. The control system 30, the processing circuit 32, and / or the monitoring unit 706 are configured to monitor the voltage of the fuel cell stack of the fuel cell unit of the fuel cell system in response to the value of the power output being below the first threshold power level. Once it is determined that the load on the fuel cell system has decreased such that the value of the power output is below the first threshold power level, the control system 30, the processing circuit 32, and / or the monitoring unit 706 may begin to monitor the voltage of the fuel cell stack. The control system 30, the processing circuit 32, and / or the monitoring unit 706 may employ one or more voltage sensors associated with the fuel cell stack and configured to obtain voltage measurement results of the fuel cell stack. In the examples herein, the control system 30 is configured to control the operation of the fuel cell system such that the voltage of the fuel cell stack (also referred to as the single cell voltage) remains below 0.8 V, which may be determined based on the voltage sensor measurement results. As discussed herein, if the single cell voltage is equal to or exceeds 0.8 V, the catalyst and carrier materials of the fuel cell stack may begin to degrade, which reduces the durability and overall life of the fuel cell system.

[0148] The control system 30 may include a control unit 708. The control system 30, the processing circuit 32, and / or the control unit 708 are configured to control the target coolant inlet temperature of the coolant at the coolant inlet of the fuel cell stack in response to the value of the power output being below the first threshold power level and as the voltage of the fuel cell stack is monitored, thereby maintaining the voltage of the fuel cell stack below 0.8 V. The control of the target coolant inlet temperature is performed by increasing the target coolant inlet temperature to a temperature threshold level. In some examples, the temperature threshold level is 80 °C. In some examples, the temperature threshold level includes other values.

[0149] The control system 30, the processing circuit 32, and / or the monitoring unit 706 may be configured to monitor temperature sensors that may be positioned to obtain temperature measurement results of the coolant, thereby controlling and / or monitoring the temperature of the fuel cell stack. For example, as Figure 2As shown, the coolant system 56 may include a coolant inlet temperature sensor 62 and a coolant outlet temperature sensor 68, which are configured to monitor the temperature of the coolant at the coolant inlet 60 and the coolant outlet 64, respectively. The control system 30, the processing circuit 32, and / or the monitoring unit 706 may be configured to obtain temperature measurements obtained by the coolant inlet temperature sensor 62 and the outlet temperature sensor 68, as well as various other sensors that may be deployed in the fuel cell system.

[0150] In some examples, the control system 30, the processing circuit 32, and / or the control unit 708 may be configured to control the air pressure at the cathode of the fuel cell stack as the value of the target coolant inlet temperature is controlled. For example, the air pressure at the cathode may increase as the target coolant inlet temperature increases. The air pressure at the cathode may be controlled by controlling an air compressor that is configured to pressurize an air stream, for example, delivered from the ambient environment to the cathode inlet. Controlling (e.g., increasing the air pressure at the cathode inlet) may prevent the membrane of the fuel cell stack from drying out as the target coolant inlet temperature increases.

[0151] The control system 30, the processing circuit 32, and / or the determination unit 704 may be configured to determine whether the value of the power output is lower than at least one second threshold power level that is smaller than a first threshold power level. The at least one second threshold power level may include a plurality of threshold power levels such that the value of the power output may be compared with more than one second threshold power level, where the second threshold power levels may have continuously decreasing values. By way of non-limiting example only, the second threshold power levels may include four threshold power levels having values of 50 kW, 40 kW, 30 kW, and 20 kW, respectively. Also by way of non-limiting example, when the at least one second threshold power level includes a value less than 60 kW, the first threshold power level may include a value of 60 kW or greater than 60 kW.

[0152] In an example in accordance with the present disclosure, when the power demand of the fuel cell system (referred to as the value of the power output requested from the fuel cell system) decreases to a certain extent (e.g., drops to a value below the first threshold power level), the target coolant inlet temperature may increase to a certain temperature threshold level (e.g., 80 °C). The air pressure at the cathode may be controlled simultaneously. The power demand may be lower than the first threshold power level but higher than a second threshold power level that is smaller than the first threshold power level, such that increasing the target coolant inlet temperature and controlling, for example, increasing the air pressure at the cathode may be sufficient to allow the power demand to decrease as needed while maintaining the voltage of the fuel cell stack below 0.8 V.

[0153] As the power demand for the fuel cell system is further reduced (e.g., dropped below a second threshold power level), in addition to increasing the target coolant inlet temperature, it is also necessary to reduce the oxygen partial pressure in the cathode input air stream to continue to maintain the voltage of the fuel cell stack below 0.8V and allow for further reduction of the power demand.

[0154] Thus, in some examples, the control system 30, the processing circuit 32, and / or the control unit 708 may be configured to control the target coolant inlet temperature and also control the oxygen partial pressure of the air stream in response to the value of the power output being lower than at least one second threshold power level that is smaller than the first threshold power level, wherein the oxygen partial pressure is controlled by controlling the volumetric flow rate of the cathode exhaust gas stream that is directed from the cathode outlet line to the cathode inlet line to mix with the air stream. Thus, when the target coolant inlet temperature is at the temperature threshold, the control system 30, the processing circuit 32, and / or the control unit 708 may be configured to control the cathode recirculation passage to direct a portion of the cathode exhaust gas stream to mix with the air stream, thereby reducing the oxygen partial pressure of the air stream.

[0155] In some examples, the control system 30, the processing circuit 32, and / or the control unit 708 may be configured to control at least one flow control valve fluidly coupled to the cathode recirculation passage and a cathode recirculation pump fluidly coupled to the cathode recirculation passage according to the value of the power output, and the cathode recirculation pump is configured to operate to drive the cathode exhaust gas stream through the cathode recirculation passage. The air pressure at the cathode of the fuel cell stack is controlled because the target coolant inlet temperature is controlled, and the oxygen partial pressure in the air stream is also reduced by mixing the air stream (e.g., fresh air from the outside), wherein a portion of the cathode exhaust gas stream is diverted from the cathode output line to the cathode input line, and the cathode input line is configured to feed the air stream to the cathode.

[0156] In some examples, as discussed above, the at least one second threshold power level may include more than one threshold power level, and the value of the power output may be compared with more than one second threshold power level. The smaller the value of the second threshold power level, the greater the reduction in the oxygen partial pressure of the cathode inlet air stream that may be required. In other words, a lower value of the power output requested from the fuel cell system requires a lower oxygen partial pressure of the air stream.

[0157] In some examples, the control system 30, the processing circuitry 32, and / or the determination unit 704 may be configured to determine whether the value of the power output requested from the fuel cell system is further decreased. The control system 30, the processing circuitry 32, and / or the determination unit 704 may also be configured to compare the value of the power output with one or more successively decreasing second threshold power levels among at least one second threshold power level in response to determining that the value of the power output continues to decrease. The control system continues to control the operation of the fuel cell system based on the value of the power output. For example, referring to Table 1, the oxygen partial pressure may be adjusted according to the desired decrease in the power demand of the fuel cell system.

[0158] In some examples, the control system 30, the processing circuitry 32, and / or the determination unit 704 may be configured to determine whether the value of the power output is higher than at least one second threshold power level, such as as Figure 6B shown at block 620. This may occur when the load on the fuel cell system increases. For example, the fuel cell system may exit the idle mode.

[0159] In some examples, the control system 30, the processing circuitry 32, and / or the determination unit 704 may be configured to determine whether the oxygen partial pressure in the air stream has been decreased in response to determining that the value of the power output is higher than at least one second threshold power level, such as as Figure 6B shown at block 622.

[0160] In some examples, the control system 30, the processing circuitry 32, and / or the control unit 708 may be configured to control the cathode recirculation channel to increase the oxygen partial pressure in the air stream in response to determining that the oxygen partial pressure in the air stream has been decreased, such as as Figure 6B shown at block 624. In some examples, the control system 30, the processing circuitry 32, and / or the control unit 708 may also be configured to control the fuel cell system (e.g., the coolant system) to decrease the target coolant inlet temperature, such as referring to Figure 6B block 626.

[0161] Those skilled in the art should understand that the units in the above control system 30 may refer to a combination of analog and digital circuits, and / or one or more processors configured with software and / or firmware stored in the control system 30, and the software and / or firmware may execute the methods according to the embodiments of the present disclosure when executed by the corresponding one or more processors. One or more of these processors and other digital hardware may be included in a single application specific integrated circuit (ASIC), or several processors and various digital hardware may be distributed on several separate components, whether packaged separately or assembled into a system on a chip.

[0162] The operational steps described in any of the exemplary aspects of this document are described to provide examples and discussions. These steps may be performed by hardware components, embodied in machine-executable instructions to cause a processor to perform these steps, or performed by a combination of hardware and software. Although a specific order of method steps may be shown or described, the order of the steps may vary. Additionally, two or more steps may be performed simultaneously or partially simultaneously.

[0163] The terms used herein are for the purpose of describing particular aspects only and are not intended to limit the disclosure. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and / or" includes any and all combinations of the associated listed items. It should also be understood that the terms "comprises" and / or "comprising", when used herein, specify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or combinations thereof.

[0164] It will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, without departing from the scope of the disclosure, a first element may be termed a second element, and similarly, a second element may be termed a first element.

[0165] Relative terms such as "below" or "above", or "upper" or "lower", or "horizontal" or "vertical" may be used herein to describe the relationship of one element to another, as shown in the figures. It should be understood that these terms, as well as those discussed above, are intended to cover different device orientations in addition to the orientation depicted in the figures. It should be understood that when an element is referred to as "connected to" or "coupled to" another element, the element may be directly connected or coupled to the other element or there may be intervening elements. In contrast, when an element is referred to as "directly connected to" or "directly coupled to" another element, there are no intervening elements.

[0166] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It should also be understood that, unless clearly defined herein, the terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art, and should not be interpreted in an idealized or overly formal sense.

[0167] It should be understood that the present disclosure is not limited to the aspects described above and shown in the drawings; rather, those skilled in the art will recognize that many changes and modifications can be made within the scope of the present disclosure and the appended claims. In the drawings and the specification, various aspects have been disclosed for illustrative purposes only and not for purposes of limitation, and the scope of the inventive concept is set forth in the claims above.

Claims

1. A fuel cell system (20, 200, 300, 400), comprising: A fuel cell unit (21, 221, 321, 421) comprising a fuel cell stack (22, 222, 322, 422), the fuel cell stack comprising a cathode and an anode, the cathode being configured to receive an air flow via a cathode inlet line (38, 238, 338, 438) and to provide a cathode exhaust flow via a cathode outlet line (44); a cathode recirculation passage (46, 246, 346, 446) fluidly connecting the cathode outlet line to the cathode inlet line, thereby diverting the cathode exhaust flow to the cathode inlet line so that the cathode exhaust flow mixes with the air flow received by the cathode inlet line; a coolant system (56) configured to circulate a coolant through the fuel cell stack (22, 222, 322, 422), the coolant system comprising a coolant inlet line (58, 258, 358, 458) configured to direct the coolant to the fuel cell stack and a coolant outlet line (66, 266, 366, 466) configured to direct the coolant away from the fuel cell stack after the coolant has passed through the fuel cell stack (22, 222, 322, 422); A control system (30), the control system comprising a processing circuit (32), the processing circuit being configured to: obtaining a value of power output requested from the fuel cell system; and In response to said value of said power output being below a first threshold power level, monitoring the voltage of the fuel cell stack; as well as A target coolant inlet temperature of the coolant at a coolant inlet is controlled, thereby maintaining the voltage of the fuel cell stack below 0.8V. 2 . The fuel cell system of claim 1 , wherein the processing circuitry of the control system is configured to control the target coolant inlet temperature by increasing the target coolant inlet temperature to a temperature threshold level.

3. A fuel cell system as described in claim 1 or 2, wherein the processing circuit of the control system is configured to control the air pressure at the cathode of the fuel cell stack while controlling the target coolant inlet temperature.

4. The fuel cell system of claim 2 or 3, wherein the temperature threshold level is 80°C.

5. A fuel cell system as described in any one of claims 1 to 4, wherein the processing circuit of the control system is configured to control the cathode recirculation channel to direct a portion of the cathode exhaust gas flow to mix with the air flow, thereby reducing the oxygen partial pressure of the air flow, in response to the value of the power output being lower than at least one second threshold power level that is smaller than the first threshold power level, when the target coolant inlet temperature is at the temperature threshold.

6. A fuel cell system as described in claim 5, wherein the processing circuit of the control system is configured to control at least one flow control valve (48, 52, 248, 252) fluidly connected to the cathode recirculation channel and a cathode recirculation pump (50, 250) fluidly connected to the cathode recirculation channel and configured to operate to drive the cathode exhaust gas flow through the cathode recirculation channel according to the value of the power output.

7. The fuel cell system of any one of claims 1 to 6, wherein the fuel cell unit comprises one fuel cell unit.

8. The fuel cell system according to any one of claims 1 to 7, wherein the value of the power output obtained is a predicted value of the power output of the fuel cell system.

9. A fuel cell vehicle (10), comprising the fuel cell system according to claim 1.

10. A method (600) for operating a fuel cell system, the fuel cell system comprising a fuel cell unit, the fuel cell unit comprising a fuel cell stack, the fuel cell stack comprising a cathode and an anode, the cathode being configured to receive an air flow via a cathode inlet line and to provide a cathode exhaust flow via a cathode outlet line, the method comprising: obtaining (602) a value of a power output requested from the fuel cell system; In response to (604) said value of said power output being below a first threshold power level, monitoring (606) a voltage of the fuel cell stack, and controlling (608) a target coolant inlet temperature of coolant at a coolant inlet of the fuel cell stack, thereby maintaining the voltage of the fuel cell stack below 0.8V; and In response to (612) the value of the power output being below at least one second threshold power level that is less than the first threshold power level, controlling (608) the target coolant inlet temperature and controlling (614) the oxygen partial pressure of the air flow, wherein the oxygen partial pressure is controlled by controlling a volumetric flow rate of the cathode exhaust gas flow directed from the cathode outlet line to the cathode inlet line to mix with the air flow. 11 . The method of claim 10 , comprising controlling the target coolant inlet temperature by increasing the target coolant inlet temperature to a temperature threshold level.

12. The method of claim 11, wherein the temperature threshold level is 80°C.

13. A method as claimed in any one of claims 10 to 12, wherein lower values ​​of the power output require lower oxygen partial pressures of the air stream.

14. The method of any one of claims 10 to 13, comprising controlling (610) air pressure at the cathode of the fuel cell stack while controlling the target coolant inlet temperature.

15. The method of any one of claims 10 to 14, wherein controlling the oxygen partial pressure of the air flow comprises controlling at least one flow control valve and a cathode recirculation pump fluidly connected to a cathode recirculation channel, the cathode recirculation channel fluidly connecting the cathode outlet pipeline to the cathode inlet pipeline, thereby diverting the cathode exhaust gas flow to the cathode inlet pipeline so that the cathode exhaust gas flow is mixed with the air flow, wherein the at least one flow control valve and the cathode recirculation pump are controlled according to the value of the power output.

16. The method of any one of claims 10 to 15, wherein the value of the power output obtained is a predicted value of the power output of the fuel cell system.

17. A control system (30) for controlling a fuel cell system (20) of a fuel cell vehicle (10), the control system (30) comprising a processing circuit (32) configured to perform the method according to any one of claims 10 to 16.

18. A fuel cell vehicle (10) comprising the control system (30) according to claim 17.

19. A computer program product comprising computer executable instructions which, when executed by a processing circuit, cause the processing circuit to perform the method of any one of claims 10 to 16.

20. A computer readable storage medium having stored thereon a computer program product, the computer program product comprising computer executable instructions which, when executed by a processing circuit, cause the processing circuit to perform the method of any one of claims 10 to 16.

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