Controlling electrical power of a fuel cell of a vehicle

Abstract

A method of controlling electrical power of a fuel cell (204-Fig.2) of a vehicle comprises: receiving a power demand 602-610; obtaining an efficiency parameter 614, indicative of an efficiency at whic

Description

TECHNICAL FIELD The present disclosure relates to controlling electrical power of a fuel cell of a vehicle. Aspects of the invention relate to a control system, to a vehicle, to a method, and to computer readable instructions. BACKGROUND A fuel cell converts chemical energy of a fuel (typically hydrogen) and oxygen to electrical energy. In a fuel cell powered vehicle, the generated electrical energy is stored by an electrical energy storage means (e.g., traction battery), and consumed by various consumers including an electric motor. The electric motor is configured to produce torque (drive torque) fordriving the vehicle. If a torque increase is requested via an accelerator pedal or other requestor, the fuel cell’s electrical power output may be increased in response to ensure that sufficient electrical power is available. If a torque decrease is requested, the fuel cell’s electrical power output may be decreased in response. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a vehicle, a method, and computer readable instructions as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for controlling electrical power of a fuel cell of a vehicle, the control system comprising one or more processors collectively configured to: receive a power demand; obtain an efficiency parameter indicative of an efficiency at which the fuel cell is capable of generating the power demand; determine an arbitrated electrical power request in dependence on at least the efficiency parameter and the power demand, wherein the arbitrated electrical power request comprises one of a peak efficiency power, or the power demand, selected in dependence on a value of the efficiency parameter, and wherein the peak efficiency power is approximately equal to electrical power that the fuel cell is configured to produce at peak efficiency; and output a control signal requesting the fuel cell to generate electrical power in dependence on the arbitrated electrical power request. An advantage is that fuel efficiency is improved because the fuel cell is selectively operated at its peak efficiency. A fuel cell has a power curve, such that it is more efficient to run at certain power levels such as 5-15kW. An electrical energy storage means (electrical energy storage device) of the vehicle may be configured to supply or store a difference between the arbitrated electrical power request and the power demand. Optionally, the electrical energy storage means is a traction battery or the like. Therefore, for example, the electrical energy storage means may be configured to supply increased electrical power in dependence on the 1 power demand increasing, while the fuel cell may be controlled to keep outputting its peak efficiency power despite the power demand increasing. A further advantage is therefore that the power demand can be satisfied by discharging or charging the electrical energy storage means, while the fuel cell is kept at its peak efficiency power. Therefore, the required amount of power is generated in a more fuel efficient manner. Overall efficiency is improved because charging or discharging the electrical energy storage means is more efficient than operating the fuel cell at less efficient power levels. The control system comprises one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to: receive the power demand; obtain the efficiency parameter; determine the arbitrated electrical power request; and output the control signal. Optionally, determining the arbitrated electrical power request comprises selecting one of the peak efficiency power and the power demand, in dependence on a value of the efficiency parameter. Optionally, the determination of the arbitrated electrical power request comprises selecting the peak efficiency power in dependence on the efficiency parameter indicating a falling efficiency of the fuel cell. Optionally, the arbitrated electrical power request comprises the peak efficiency power when the efficiency parameter indicates a falling efficiency of the fuel cell. Optionally, the determination of the arbitrated electrical power request comprises selecting the peak efficiency power in dependence on the efficiency parameter indicating an above-threshold inefficiency of the fuel cell. An advantage is that fuel efficiency is improved because the fuel cell’s power output is kept close to its peak efficiency. Therefore, the fuel cell is prevented from outputting significantly higher or lower powers at which the efficiency would be too low. The fuel cell’s output may be fixed at a constant peak efficiency power despite variation of the power demand. Optionally, the determination of the arbitrated electrical power request comprises selecting the power demand in dependence on the efficiency parameter indicating an efficiency of the fuel cell being proximal to the peak efficiency power. Optionally, the arbitrated electrical power request comprises the power demand when the efficiency parameter indicates an efficiency of the fuel cell equal to or proximal to the peak efficiency power. Optionally, the determination of the arbitrated electrical power request comprises selecting the power demand in dependence on the efficiency parameter indicating a below-threshold inefficiency of the fuel cell. Optionally, the inefficiency threshold defines at least in part a peak efficiency range in which the fuel cell is allowed to match the power demand. An advantage is that the fuel cell is allowed to track the power demand as long as its output is close to the peak efficiency power, during which time the fuel cell is efficient and the electrical energy storage means does not need to be discharged or charged to track the power demand. Therefore, the fuel cell is allowed to vary its power in a fuel efficient range, that does not rely on battery charging or battery assistance to satisfy the power demand. Optionally, the arbitrated electrical power request comprises the peak efficiency power when the power demand increases while the power demand is equal to or greater than the peak efficiency power. Optionally, the determination of the arbitrated electrical power request comprises selecting the peak efficiency power in dependence on the power demand increasing while the power demand is equal to or greater than the peak efficiency power. Optionally, the determination of the arbitrated electrical power request comprises selecting the peak efficiency power in dependence on the power demand indicating an above-threshold power demand (power threshold). An advantage is that fuel efficiency is improved because the fuel cell is prevented from outputting high power levels, because high power levels are less efficient. Optionally, the determination of the arbitrated electrical power request comprises selecting the power demand in dependence on the power demand being within a peak efficiency range, and comprises selecting the peak efficiency power in dependence on the power demand being outside the peak efficiency range. Optionally, the peak efficiency range is associated with the inefficiency threshold and the power threshold defined above. Optionally, the control system is configured to: receive an energy parameter dependent on stored electrical energy in the electrical energy storage means; and determine whether the energy parameter satisfies a condition associated with the stored electrical energy in the electrical energy storage means being less than a target, wherein the arbitrated electrical power request is configured to, in dependence on the energy parameter satisfying the condition, control the fuel cell to generate the power demand and an additional charging power to charge the electrical energy storage means. Optionally, the energy parameter is indicative of a state of charge of the electrical energy storage means. An advantage is that the fuel cell takes over satisfying the power demand if the energy parameter of the electrical energy storage means is less than the target, and also simultaneously charges the electrical energy storage means due to the additional charging power. The energy parameter may be dependent on a state of charge of the electrical energy storage means. Optionally, determining the arbitrated electrical power request comprises: determining whether the condition associated with the stored electrical energy in the electrical energy storage means being less than the target is satisfied; in dependence on satisfaction of the condition, selecting a power comprising the sum of the power demand and the additional charging power; and in dependence on non-satisfaction of the condition, selecting one of the peak efficiency power and the power demand, in dependence on a value of the efficiency parameter. Optionally, determining the arbitrated electrical power request comprises determining whether the power demand is greater than an efficiency power capability, wherein the efficiency power capability is dependent on a combination of battery power output (e.g., power output capability of the electrical energy storage means), and fuel cell power output at the peak efficiency power, and wherein the arbitrated electrical power request is increased from the peak efficiency power to satisfy the power demand when the power demand is greater than the efficiency power capability. An advantage is that when the power demand reaches particularly high values, the fuel cell’s power output is increased from the peak efficiency power if the electrical energy storage means has insufficient power output capability to supply the difference between the peak efficiency power and the power demand. The amount by which the fuel cell’s power output is increased above the peak efficiency power is sufficient to allow the sum of the increased fuel cell power output and the battery power at its power output capability to track the power demand. Optionally, the determination of the arbitrated electrical power request comprises determining a difference (efficiency difference) between the efficiency parameter and a peak-efficiency parameter, and determining whether the efficiency difference is greater than an inefficiency threshold. The inefficiency threshold may be as described above. Optionally, the efficiency parameter indicates an efficiency at which the fuel cell is capable of generating the power demand. Optionally, the peak-efficiency parameter indicates a peak fuel cell efficiency. Optionally, the peak-efficiency parameter indicates an efficiency at which the fuel cell is capable of generating the peak efficiency power. Optionally, the inefficiency threshold is associated with a power value less than the peak efficiency power. Optionally, the inefficiency threshold comprises hysteresis such that the inefficiency threshold is dependent on whether the efficiency difference is rising or falling. An advantage is improved control performance. Optionally, the determination of the arbitrated electrical power request is further dependent on a value of the power demand relative to a power threshold. Optionally, the power threshold comprises hysteresis such that the power threshold is dependent on whether the power demand is rising or falling. Optionally, the arbitrated electrical power request comprises the peak efficiency power in dependence on the efficiency difference being greater than the inefficiency threshold and the power demand being less than the power threshold. Optionally, the determination of the arbitrated electrical power request is configured to select the peak efficiency power in dependence on the efficiency difference being greater than the inefficiency threshold and the power demand being less than the power threshold. Optionally, the arbitrated electrical power request comprises the peak efficiency power in dependence on the difference between the efficiency parameter and the peak-efficiency parameter being greater than the inefficiency threshold and the power demand being greater than the power threshold. Optionally, the determination of the arbitrated electrical power request is configured to select the peak efficiency power in dependence on the difference between the efficiency parameter and the peak-efficiency parameter being greater than the inefficiency threshold and the power demand being greater than the power threshold. Optionally, the arbitrated electrical power request comprises the peak efficiency power in dependence on the difference between the efficiency parameter and the peak-efficiency parameter being less than the inefficiency threshold and the power demand being greater than the power threshold. Optionally, the determination of the arbitrated electrical power request is configured to select the peak efficiency power in dependence on the difference between the efficiency parameter and the peak-efficiency parameter being less than the inefficiency threshold and the power demand being greater than the power threshold. Optionally, the arbitrated electrical power request comprises the power demand in dependence on the difference between the efficiency parameter and the peak-efficiency parameter being less than the inefficiency threshold and the power demand being less than the power threshold. Optionally, the determination of the arbitrated electrical power request is configured to select the power demand in dependence on the difference between the efficiency parameter and the peak-efficiency parameter being less than the inefficiency threshold and the power demand being less than the power threshold. An advantage of the two thresholds is allowing for the separate calibration and tuning of thresholds for low and high power demands. Optionally, the power demand comprises a current power demand or a predicted power demand. According to another aspect of the present invention there is provided a vehicle comprising the control system and the fuel cell. Optionally, the vehicle further comprises the electrical energy storage means. According to a further aspect of the present invention there is provided a method of controlling electrical power of a fuel cell of a vehicle, the method comprising: receiving a power demand; obtaining an efficiency parameter indicative of an efficiency at which the fuel cell is capable of generating the power demand; determining an arbitrated electrical power request in dependence on at least the efficiency parameter and the power demand, wherein the arbitrated electrical power request comprises one of a peak efficiency power, or the power demand, selected in dependence on a value of the efficiency parameter, and wherein the peak efficiency power is approximately equal to electrical power that the fuel cell is configured to produce at peak efficiency; and outputting a control signal requesting the fuel cell to generate electrical power in dependence on the arbitrated electrical power request. According to a further aspect of the present invention there is provided a control system for controlling electrical power of a fuel cell of a vehicle, the control system comprising one or more processors collectively configured to: receive a power demand; determine an arbitrated electrical power request in dependence on at least the power demand, wherein the arbitrated electrical power request is configured to request a power output of the fuel cell that inhibits variation of fuel cell power from a peak efficiency power of the fuel cell; and output a control signal requesting the fuel cell to generate electrical power in dependence on the arbitrated electrical power request. According to a further aspect of the invention there is provided computer readable instructions which, when executed by a computer, are arranged to perform any one or more of the methods described herein. According to a further aspect of the invention there is provided a non-transitory computer readable medium comprising computer readable instructions that, when executed by one or more electronic processors, causes the one or more electronic processors to carry out any one or more of the methods described herein. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination that falls within the scope of the appended claims. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination that falls within the scope of the appended claims, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 illustrates a perspective view illustrating an example of a vehicle; FIG. 2 illustrates a schematic view illustrating an example of a fuel cell powertrain; FIG. 3 illustrates a graph illustrating an example of a fuel cell efficiency curve; FIG. 4 illustrates a schematic view illustrating an example of a control system; FIG. 5 illustrates a schematic view illustrating an example of a non-transitory computer-readable storage medium; FIG. 6 illustrates a flowchart illustrating an example of a method; FIGS. 7A-7E illustrate graphs of example values of power demand for the fuel cell, fuel cell efficiency, fuel cell efficiency difference, an arbitrated electrical power request for the fuel cell, and battery power; FIG. 8 illustrates a flowchart illustrating an example of a method related to stored electrical energy; and FIG. 9 illustrates a flowchart illustrating an example of a method related to efficiency power capability. DETAILED DESCRIPTION A vehicle 1 in accordance with an embodiment of the present invention is described herein with reference to the accompanying FIG. 1. In some, but not necessarily all examples, the vehicle 1 is a passenger vehicle, also referred to as a passenger car or as an automobile. In other examples, embodiments of the invention can be implemented for other applications, such as commercial vehicles. FIG. 2 illustrates a powertrain 200 of the vehicle 1. The powertrain 200 comprises a fuel cell 204, and an electrical energy storage means in the form of a traction battery 206. In a non-limiting example, the fuel cell 204 is a hydrogen fuel cell. The fuel cell 204 is connected to a fuel tank 202 storing a chemical fuel such as gaseous hydrogen. The fuel cell 204 is configured to convert the chemical energy of the fuel into electrical energy through an electrochemical reaction. The fuel cell 204 is also connected to an electrical bus 201 of the vehicle 1. The fuel cell 204 is configured to generate electrical energy and output the electrical energy to the electrical bus 201. The traction battery 206 is also connected to the electrical bus 201. When discharging, the traction battery 206 is configured to output electrical energy to the electrical bus 201. When charging, the traction battery 206 is configured to receive and store electrical energy from the electrical bus 201. One or more electrical inverters 208 are also connected to the electrical bus 201. Each inverter 208 is connected to a corresponding electric machine 210, such as an electric motor. The electric machine 210 is mechanically connected to one or more wheels of the vehicle 1 (not shown in FIG. 2). When positive torque is requested, the electric machine 210 and inverter 208 are collectively configured to convert the electrical energy on the electrical bus 201 into mechanical energy of the vehicle 1. When negative torque is requested for charging the traction battery 206, the opposite energy transformation takes place. FIG. 3 is a graph schematically illustrating a curve of efficiency E of the fuel cell 204 compared to an electrical power output P of the fuel cell 204. The line is not based on any particular model of fuel cell. The efficiency curve illustrates a peak efficiency Emax, and falling efficiency to either side of the peak efficiency Emax. The peak efficiency Emax corresponds to a peak efficiency power P_Emax, meaning the fuel cell power P at which the fuel cell 204 is operating at its peak efficiency Emax. FIG. 3 shows that the peak efficiency power P_Emax is quite low relative to a peak power of the fuel cell 204. For example, the peak efficiency power P_Emax may be from the range 5% to 15% of the rated peak power 7 of the fuel cell 204. For an 80kW fuel cell, the peak efficiency power P_Emax may be in the order of 4kW to 12kWdepending on the implementation. With reference to FIG. 4, there is illustrated a control system 400 for a vehicle 1. The control system 400 comprises one or more controllers 401. The control system 400 is configured to receive power demand data directly or indirectly from a power requestor 414 such as an accelerator pedal sensing system, or an automated driving control system, and determine an arbitrated electrical power request. The control system 400 may then output a control signal to control the fuel cell 204. The control system 400 as illustrated in FIG. 4 comprises one controller 401, although it will be appreciated that this is merely illustrative. The controller 401 comprises processing means 404 and memory means 406. The processing means 404 may be one or more electronic processing device 404 which operably execute computer-readable instructions. The memory means 406 may be one or more memory device 406. The memory means 406 is electrically coupled to the processing means 404. The memory means 406 is configured to store instructions, and the processing means 404 is configured to access the memory means 406 and execute the instructions stored thereon. The controller 401 comprises an input means 410 and an output means 412. The input means 410 may comprise an electrical input 410 of the controller 401. The output means 412 may comprise an electrical output 412 of the controller 401. The controller 401 may have an interface 402 comprising an electrical input / output I / O 410, 412, or an electrical input 410, or an electrical output 412, for receiving information and interacting with external components. The input 410 is arranged to receive a power demand signal from a power requestor 414. The power demand signal is an electrical signal which is indicative of a requested electrical instant power available on the electrical bus 201. The output 412 is arranged to output a fuel cell control signal, indicative of requested fuel cell power for controlling a power level of the fuel cell 204. FIG. 5 illustrates a non-transitory computer-readable storage medium 500 comprising the instructions (computer software). FIG. 6 illustrates a method 600 according to an embodiment of the invention. The method 600 is a method of controlling electrical power of the fuel cell 204 of the vehicle 1. The method 600 may be performed by the control system 400 illustrated in FIG. 4. In particular, the memory 406 may comprise computer-readable instructions 408 which, when executed by the processor 404, perform the method 600. In summary, the method 600 comprises: at blocks 602 to 610, receiving a power demand P_fc; at block 614, obtaining an efficiency parameter E indicative of an efficiency at which the fuel cell 204 is capable of generating the power demand P_fc; at block 624, determining an arbitrated electrical power request P_arb_fc in dependence on at least the efficiency parameter E and the power demand P_fc, wherein the arbitrated electrical power request P_arb_fc comprises one of a peak efficiency power P_Emax, or the power demand P_fc, selected in dependence on a value of the efficiency parameter E, and wherein the peak efficiency power P_Emax is approximately equal to electrical power that the fuel cell 204 is configured to produce at peak efficiency Emax; and at block 626, outputting a control signal requesting the fuel cell 204 to generate electrical power in dependence on the arbitrated electrical power request P_arb_fc. FIG. 6 also illustrates further blocks demonstrating optional implementation features of the method 600. Blocks 602-610 relate to input data. Block 602 is a data source block indicating a current power demand P_fc_cur of the fuel cell 204. Block 606 indicates a predicted power demand P_fc_pred of the fuel cell 204. Block 610 is an arbitration block determining a maximum of the current power demand P_fc_cur and the predicted power demand P_fc_pred. A power demand P_fc refers to an electrical power demand, indicating an electrical power that is requested on the electrical bus 201. A power demand P_fc may be determined by a controller 401 inside or outside the control system 400. A power demand P_fc may be determined in dependence on a tractive demand power, and in dependence on an auxiliary system demand power. The tractive demand power relates to a value of electrical bus power which is demanded for satisfying a propulsive torque request. The propulsive torque request may be determined in dependence on accelerator pedal depression. If the vehicle 1 is under automated control, the propulsive torque request may be determined at least partially automatically in dependence on machine vision sensing and an automated driving algorithm. The propulsive torque request may be converted from torque units to electrical power units. An auxiliary system demand power relates to a value of electrical bus power which is demanded for satisfying a power requirement of a system which is auxiliary to the electric machine 210, such as a heating, ventilation and cooling (HVAC) system. The power demand P_fc may therefore be dependent on the sum of the tractive demand power and the auxiliary system demand power. The current power demand P_fc_cur may be determined reactively in response to the current tractive demand power and current auxiliary system demand power. The predicted power demand P_fc_pred may be determined predictively, wherein at least one of the tractive demand power or the auxiliary system demand power is determined predictively. As an example, determining the predicted power demand P_fc_pred may start with receiving electronic horizon data at block 604. The electronic horizon data is based on a predictive model of a road ahead. The predictive model is updated in dependence on various data sources such as location data, digital map data, route data, etc. Block 606 comprises determining the predicted power demand P_fc_pred for a route segment ahead of the vehicle 1. Block 606 outputs the predicted power demand P_fc_pred to arbitration block 610. Arbitration block 610 receives the current power demand P_fc_cur and predicted power demand P_fc_pred, determines the maximum of the two, and outputs the maximum as a power demand P_fc to the next block 614. FIG. 7A is an example graph illustrating the power demand P_fc output by block 610, over time ‘t’. The power demand P_fc increases linearly from a first nonzero positive value, such as 1% of maximum fuel cell power, to a second nonzero positive value, such as 100% of maximum fuel cell power. If predictive functionality is omitted, blocks 604-610 may be omitted such that the current power demand P_fc_cur is received by block 614. The subsequent blocks 614-624 define an arbitration process for determining an arbitrated electrical power request P_arb_fc for the fuel cell 204, in dependence on the power demand P_fc output by block 610. Block 614 obtains an efficiency parameter E indicative of an efficiency at which the fuel cell 204 is capable of generating the power demand P_fc received from block 610. For example, block 614 may lookup the efficiency parameter E in dependence on the value of the power demand P_fc. This may comprise looking up the efficiency parameter E using a lookup table which relates values of the power demand P_fc to values of the efficiency parameter E. Alternatively, a formula may be used. FIG. 7B is an example graph illustrating the value of the efficiency parameter E corresponding to the power demand P_fc of FIG. 7A. FIG. 7B illustrates the efficiency parameter E rising with the power demand P_fc at a steep gradient, to a value corresponding to the peak efficiency Emax at which the peak efficiency power P_Emax is produced. The efficiency parameter E then reduces, at a lower-magnitude gradient, as the power demand P_fc further increases. Block 616 receives the efficiency parameter E, and determines an efficiency difference AE between the efficiency parameter E and a peak-efficiency parameter Emax. The peak-efficiency parameter Emax is the peak efficiency referred to earlier, indicating the peak fuel cell efficiency. In some examples, the peak-efficiency parameter Emax is a maximum-efficiency parameter, the peak fuel cell efficiency is a maximum fuel cell efficiency, and the peak efficiency power P_Emax is a maximum efficiency power. FIG. 7C is an example graph illustrating the value of the efficiency difference AE corresponding to the efficiency parameter E of FIG. 7B. FIG. 7C illustrates the efficiency difference curve having an inverted shape relative to the curve of FIG. 7B. The efficiency difference AE decreases to a minimum value of zero which occurs when the efficiency parameter E is equal to the peak-efficiency parameter Emax, such that there is zero difference. This represents the situation in which the power demand P_fc is equal to the peak efficiency power P_Emax. The efficiency difference AE then increases again as power demand P_fc further increases, due to the efficiency E falling from its peak. Decision block 618 receives the efficiency difference AE from block 616, and determines whetherthe efficiency difference AE is greater than an inefficiency threshold AE_thrs. This threshold is schematically illustrated in FIG. 7C. When the inefficiency threshold AE_thrs is exceeded, this indicates that the efficiency of the fuel cell 204 is relatively low compared to the peak-efficiency parameter Emax. If the inefficiency threshold AE_thrs is exceeded, the method 600 proceeds to decision block 620. If not, the method 600 proceeds to decision block 622. Since efficiency decreases sharply when the power demand P_fc decreases towards zero from a value below the peak efficiency power P_Emax, the inefficiency threshold AE_thrs is associated with a power value less than the peak efficiency power P_Emax. The power value may be a few kW. Whether the inefficiency threshold AE_thrs is exceeded again at high power demand P_fc depends on the value of the inefficiency threshold AE_thrs, as well as the rate at which efficiency E falls as power demand P_fc increases. FIGS. 7A-7C do not show high power demand P_fc causing the efficiency difference AE to exceed the inefficiency threshold AE_thrs, although it could do in other examples. The inefficiency threshold AE_thrs may be a value selected from the range 1% to 10% or 2% to 5% of the peak-efficiency parameter Emax. Although not shown in the graphs, the inefficiency threshold AE_thrs may comprise hysteresis such that the inefficiency threshold AE_thrs is dependent on whether the efficiency difference AE rises or falls past the efficiency threshold AE_thrs. An advantage is improved control performance. Decision block 620 determines whetherthe power demand P_fc received from block 610 is less than a power threshold P_thrs. FIG. 7A illustrates the power threshold P_thrs, which is equal to or greater than the peak efficiency power P_Emax. If greater, the power threshold P_thrs may be a value within 10 kW or within 5 kW of the peak efficiency power P_Emax. If the power difference is less than the power threshold P_thrs, the decision block 620 selects the peak efficiency power P_Emax and requests it from the next block 624. If the power difference is instead greater than the power threshold P_thrs, the decision block 620 instead proceeds to a further decision block 622. In summary, decision blocks 618 and 620 request the peak efficiency power P_Emax when the efficiency parameter E is low and the power demand P_fc is low. In FIGS. 7A-7E this is represented to the left side of the graphs, when the efficiency difference AE is greater than the inefficiency threshold AE_thrs. FIG. 7D illustrates the arbitrated electrical power request P_arb_fc being a fixed value equal to the peak efficiency power P_Emax when the efficiency difference AE is greater than the inefficiency threshold AE_thrs, while the requested power demand P_fc is lower than the peak efficiency power P_Emax in this region. As a consequence of requesting the peak efficiency power P_Emax, the fuel cell 204 produces, at its maximum efficiency, slightly too much electrical power compared to what was originally requested. This is represented by the solid line being higher than the dashed line in FIG. 7D. The excess electrical power is used to efficiently charge the traction battery 206 as shown in the negative power region of the battery power graph of FIG. 7E. This is preferable compared to the fuel cell 204 inefficiently outputting the power demand P_fc. If the traction battery 206 is unable to store the excess electrical energy, appropriate control logic may be implemented to ignore / override the peak efficiency power P_Emax and request the power demand P_fc instead, or something closer to the power demand P_fc. Although not shown in the graphs, the power threshold P_thrs may comprise hysteresis such that the power threshold P_thrs is dependent on whether the power demand P_fc rises or falls past the power threshold P_thrs. In some examples, hysteresis is added to the peak efficiency power P_Emax for rising power demand, and / or subtracted from the peak efficiency power P_Emax for falling power demand. Decision block 622 determines whether the power demand P_fc received from block 610 is greater than the power threshold P_thrs. The power threshold P_thrs may be the same threshold as that of decision block 620. If the power difference is greater than the power threshold P_thrs, the decision block 622 selects the peak efficiency power P_Emax and sends this as a request to the next block 624. If the power difference is less than the power threshold P_thrs, the decision block 622 selects the original power demand P_fc and sends this as a request to the next block 624. Therefore, in response to decision block 618 determining that the efficiency difference AE is not greater than the inefficiency threshold AE_thrs, and decision block 622 determining that the power demand P_fc is less than the power threshold P_thrs, the power demand P_fc is requested. In FIGS. 7A-7E this is represented in the narrow region between the efficiency threshold AE_thrs and power threshold P_thrs, and proximal to the peak efficiency power P_Emax. FIG. 7D shows the arbitrated electrical power request P_arb_fc (solid line) tracking the power demand P_fc (dashed line). Since the fuel cell 204 is operating very efficiently in this region, there is a negligible efficiency penalty arising from tracking the power demand P_fc, due to the deviation from the peak efficiency power P_Emax being so small. As the power demand P_fc continues to increase, the power threshold P_thrs will at some point be exceeded. This switches the decision block 622 to requesting the peak efficiency power P_Emax. In FIGS. 7A-7E this is represented to the right side of the graphs, when the power demand P_fc is greater than the power threshold P_thrs. FIG. 7D illustrates the arbitrated electrical power request P_arb_fc being a fixed value equal to the peak efficiency power P_Emax. As a consequence, the fuel cell 204 produces, at its maximum efficiency, insufficient electrical power compared to what is requested. As shown in FIG. 7E, the battery power P_bat of the traction battery 206 provides an assistance role (akin to torque fill). The traction battery 206 discharges to supply the remaining electrical power required to satisfy the power demand P_fc. This is preferable compared to the fuel cell 204 inefficiently outputting the power demand P_fc. If the battery power P_bat of the traction battery 206 is unable to supply the required extra electrical energy, appropriate control logic may be implemented to ignore / override the peak efficiency power P_Emax and request the power demand P_fc instead, or something closer to the power demand P_fc. Consider also the situation in which decision block 618 determines that the efficiency difference AE is greater than the inefficiency threshold AE_thrs, and decision block 620 determines that the power demand P_fc is not less than the power threshold P_thrs. In this situation, decision block 620 is connected to decision block 622 which determines that the power demand P_fc is greater than the power threshold P_thrs, and therefore requests the peak efficiency power P_Emax. This situation is not shown in FIGS. 7A-7E, but could arise if the inefficiency difference AE again rises above the inefficiency threshold AE_thrs at high power levels. If this situation arises, the peak efficiency power P_Emax will still be requested, and the traction battery 206 will still supply the remaining required electrical power. The right-hand-most side of FIG. 7D illustrates another scenario which is described later in relation to FIG. 9. FIG. 7D shows that the arbitrated electrical power request P_arb_fc stays either at or close to the peak efficiency power P_Emax of the fuel cell 204 over a wide range of power demands P_fc of the fuel cell 204. FIG. 7E shows the battery power P_bat of the traction battery 206, illustrating that whenever there is a difference between the power demand P_fc and the arbitrated electrical power request P_arb_fc, the traction battery 206 will supply or store the difference between the two. Therefore, the power of the traction battery 206 of FIG. 7E corresponds to the difference between the solid and dashed lines in FIG. 7D. Although not shown in the graphs, the power threshold P_thrs of decision block 622 may comprise hysteresis such that the power threshold P_thrs is dependent on whether the power demand P_fc rises or falls past the power threshold P_thrs. The decision blocks 618, 620, 622 define a range of thresholds, which allow the power demand P_fc for low, medium, and high powers to be tuned / calibrated separately. This allows optimisation of control performance. In other implementations, it is instead possible to have fewer thresholds and decision blocks. Block 624 illustrates an arbitration block connected to the outputs of the decision blocks 620 and 622. Block 624 may for example saturate the received power demand P_fc or peak efficiency power P_Emax in dependence on predetermined maximum and / or minimum allowable fuel cell power limits. These limits (or limit) may vary in operation. In other examples, block 624 may be omitted. The signal 626 in FIG. 6 indicates the arbitrated electrical power request P_arb_fc output by the method 600. In many circumstances it may be based on and / or equal to eitherthe power demand P_fcorthe peak efficiency power P_Emax, based on the earlier operations. The control system 400 outputs to the inverter(s) 208 a control signal requesting the fuel cell 204 to generate electrical power in dependence on the arbitrated electrical power request P_arb_fc. If no further signal processing is required, the signal 624 could be the final output signal sent to the inverter(s) 208. FIG. 8 illustrates a method 800 which may be performed in addition to the method 600 of FIG. 6. The method 800 is used to override the method 600 when the traction battery 206 has a below-target stored electrical energy, referred to as state of charge. Block 802 comprises receiving an energy parameter dependent on stored electrical energy in the traction battery 206. Decision block 804 comprises determining whether the energy parameter satisfies a condition associated with the stored electrical energy in the electrical energy storage means being less than a target. For example, the energy parameter may indicate a state of charge error indicating how far the state of charge is below a setpoint target, and decision block 804 may determine that the error is greater than a threshold. Alternatively, or additionally, the energy parameter may indicate the state of charge, and decision block 804 may determine that the state of charge is below a threshold. If the condition is satisfied, the method 800 proceeds to block 806. Block 806 comprises setting the arbitrated electrical power request P_arb_fc so that the arbitrated electrical power request P_arb_fc is configured to control the fuel cell 204 to generate the power demand P_fc and an additional charging power to charge the electrical energy storage means. Optionally, the energy parameter is indicative of a state of charge of the electrical energy storage means. When the traction battery 206 is supplying electrical power, the state of charge may drop too far, and the condition 804 may be triggered so that the arbitrated electrical power request P_arb_fc increases from the peak efficiency power P_Emax to a much higher value. The new higher value corresponds to the sum of the original power demand P_fc, plus an additional charging power. Therefore, the power demand P_fc is satisfied and the traction battery 206 is charged, at the same time. The arbitrated electrical power request P_arb_fc may be greater than the power demand P_fc by a positive offset amount, the offset amount setting the additional charging power. FIG. 9 illustrates a method 900 which may be performed in addition to the method 600 and / or 800. The method 900 is used to override the method 600 when the power demand P_fc is too high. Block 902 comprises determining an efficiency power capability P_Ecap, comprising the sum of the peak efficiency power P_Emax and a power output capability P_bat_cap of the traction battery 206. This represents the maximum amount of power that can be output while the fuel cell’s power is inhibited to the peak efficiency power P_Emax. Any additional power demand P_fc would exceed the traction battery’s capabilities, and would require the fuel cell power to be increased if the power demand P_fc is to be satisfied. The power output capability P_bat_cap of the traction battery 206 may be received from a battery controller of the traction battery 206. The power output capability P_bat_cap may comprise an electrical power limit, for example. This limit may vary with factors such as monitored state of charge, monitored state of health, temperature, etc. Block 904 comprises determining whether the power demand P_fc is greater than the efficiency power capability P_Ecap. If so, the method 900 proceeds to block 906 which comprises increasing the arbitrated electrical power request P_arb_fc to an increased level such that the combined power output satisfies the power demand P_fc. This is represented in the right-hand-most side of FIGS. 7A-7E, when the power demand P_fc is highest. FIG. 7E shows the battery power P_bat reaching the power output capability P_bat_cap of the traction battery 206. FIG. 7D shows the arbitrated electrical power request P_arb_fc of the fuel cell 204 increasing from the peak efficiency power P_Emax, to supply the difference between the power demand P_fc and the power output capability P_bat_cap of the traction battery 206. Therefore, the sum of the two powers satisfies the power demand P_fc. It is to be understood that the or each controller 401 can comprise a control unit or computational device having one or more electronic processors (e.g., a microprocessor, a microcontroller, an application specific integrated circuit (ASIC), etc.), and may comprise a single control unit or computational device, or alternatively different functions of the or each controller401 may be embodied in, or hosted in, different control units or computational devices. As used herein, the term “controller,” “control unit,” or “computational device” will be understood to include a single controller, control unit, or computational device, and a plurality of controllers, control units, or computational devices collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause the controller 401 to implement the control techniques described herein (including some or all of the functionality required for the method(s) described herein). The set of instructions 408 could be embedded in said one or more electronic processors 404 of the controller 401; or alternatively, the set of instructions 408 could be provided as software to be executed in the controller 401. A first controller or control unit may be implemented in software run on one or more processors. One or more other controllers or control units may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller or control unit. Other arrangements are also useful. The, or each, electronic processor 404 may comprise any suitable electronic processor (e.g., a microprocessor, a microcontroller, an ASIC, etc.) that is configured to execute electronic instructions 408. The, or each, electronic memory device 406 may comprise any suitable memory device and may store a variety of data, information, threshold value(s), lookup tables or other data structures, and / or instructions therein or thereon. In an embodiment, the memory device 406 has information and instructions for software, firmware, programs, algorithms, scripts, applications, etc. stored therein or thereon that may govern all or part of the methodology described herein. The processor, or each, electronic processor 404 may access the memory device 406 and execute and / or use that orthose instructions and information to carry out or perform some or all of the functionality and methodology described herein. The at least one memory device 406 may comprise a computer-readable storage medium (e.g. a non-transitory or non-transient storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors / computational devices. Examples of the form include, without limitation: a magnetic storage medium (e.g. floppy diskette); optical storage medium (e.g. CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g. EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information / instructions. It will be appreciated that embodiments of the present invention can be realised in any suitable form of hardware, software or a combination of hardware and software. For example, it is contemplated that the present invention is not limited to being implemented by way of programmable processing devices, and that at least some of, and in some embodiments all of, the functionality and or method steps of the present invention may equally be implemented by way of non-programmable hardware, such as by way of non-programmable ASIC, Boolean logic circuitry, etc. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. In other implementations, it is possible that block 618 could compare a power difference with a power threshold P_thrs, and blocks 620, 622 could compare an efficiency difference AE with an inefficiency threshold AE_thrs. It is also possible for the method 600 to be based only on efficiency such that blocks 620, 622 are omitted, or only based on power demand P_fc (which itself is an approximate indicator of efficiency) such that blocks 614, 616, 618 are omitted. The blocks illustrated in FIGS. 6, 8, and 9 may represent steps in a method and / or sections of code in the computer program 408. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Claims

1. A control system for controlling electrical power of a fuel cell of a vehicle, the control system comprising one or more processors collectively configured to:receive a power demand P_fc;obtain an efficiency parameter E indicative of an efficiency at which the fuel cell is capable of generating the power demand P_fc;determine an arbitrated electrical power request P_arb_fc in dependence on at least the efficiency parameter E and the power demand P_fc, wherein the arbitrated electrical power request P_arb_fc comprises one of a peak efficiency power P_Emax, or the power demand P_fc, selected in dependence on a value of the efficiency parameter E, and wherein the peak efficiency power P_Emax is approximately equal to electrical power that the fuel cell is configured to produce at peak efficiency Emax; andoutput a control signal requesting the fuel cell to generate electrical power in dependence on the arbitrated electrical power request P_arb_fc.

2. The control system of claim 1, wherein the arbitrated electrical power request P_arb_fc comprises the peak efficiency power P_Emax when the efficiency parameter E indicates a falling efficiency of the fuel cell.

3. The control system of claim 1 or 2, wherein the arbitrated electrical power request P_arb_fc comprises the power demand P_fc when the efficiency parameter E indicates an efficiency of the fuel cell equal to or proximal to the peak efficiency power P_Emax.

4. The control system of claim 1, 2, or 3, wherein the arbitrated electrical power request P_arb_fc comprises the peak efficiency power P_Emax when the power demand P_fc increases while the power demand P_fc is equal to or greater than the peak efficiency power P_Emax.

5. The control system of any preceding claim, wherein an electrical energy storage means of the vehicle is configured to supply or store a difference between the arbitrated electrical power request P_arb_fc and the power demand P_fc.

6. The control system of claim 5, configured to:receive an energy parameter dependent on stored electrical energy in the electrical energy storage means; anddetermine whether the energy parameter satisfies a condition associated with the stored electrical energy in the electrical energy storage means being less than a target,wherein the arbitrated electrical power request P_arb_fc is configured to, in dependence on the energy parameter satisfying the condition, control the fuel cell to generate the power demand P_fc and an additional charging power to charge the electrical energy storage means.

7. The control system of any preceding claim, wherein determining the arbitrated electrical power request P_arb_fc comprises determining whether the power demand P_fc is greater than an efficiency power capability P_Ecap, wherein the efficiency power capability P_Ecap is dependent on a combination of battery power 17output, and fuel cell power output at the peak efficiency power P_Emax, and wherein the arbitrated electrical power request P_arb_fc is increased from the peak efficiency power P_Emax to satisfy the power demand P_fc when the power demand P_fc is greater than the efficiency power capability P_Ecap.

8. The control system of any preceding claim, wherein the determination of the arbitrated electrical power request P_arb_fc comprises determining a difference AE between the efficiency parameter E and a peakefficiency parameter Emax, and determining whetherthe difference AE isgreaterthan an inefficiency threshold AE_thrs.

9. The control system of claim 8, wherein the inefficiency threshold AE_thrs comprises hysteresis such that the inefficiency threshold AE_thrs is dependent on whetherthe difference AE is rising or falling.

10. The control system of claim 8 or 9, wherein the determination of the arbitrated electrical power request P_arb_fc is further dependent on a value of the power demand P_fc relative to a power threshold P_thrs.

11. The control system of claim 10, wherein the power threshold P_thrs comprises hysteresis such that the power threshold is dependent on whetherthe difference between the power demand P_fc and the power threshold is rising or falling.

12. The control system of claim 10 or 11, wherein the arbitrated electrical power request P_arb_fc comprises the peak efficiency power P_Emax in dependence on the difference AE between the efficiency parameter E and the peak-efficiency parameter Emax being greater than the inefficiency threshold AE_thrs, and the power demand P_fc being less than the power threshold P_thrs.

13. The control system of claim 10, 11, or 12, wherein the arbitrated electrical power request P_arb_fc comprises the peak efficiency power P_Emax in dependence on the power demand P_fc being greater than the power threshold P_thrs.

14. The control system of any one of claims 10 to 13, wherein the arbitrated electrical power request P_arb_fc comprises the power demand P_fc in dependence on the difference AE between the efficiency parameter E and the peak-efficiency parameter Emax being less than the inefficiency threshold AE_thrs and the power demand P_fc being less than the power threshold P_thrs.

15. A vehicle 1 comprising the control system and the fuel cell of any one of the preceding claims.

16. A method of controlling electrical power of a fuel cell of a vehicle 1, the method comprising:receiving a power demand P_fc;obtaining an efficiency parameter E indicative of an efficiency at which the fuel cell is capable of generating the power demand P_fc;determining an arbitrated electrical power request P_arb_fc in dependence on at least the efficiency parameter E and the power demand P_fc, wherein the arbitrated electrical power request P_arb_fc comprises 18one of a peak efficiency power P_Emax, or the power demand P_fc, selected in dependence on a value of the efficiency parameter E, and wherein the peak efficiency power P_Emax is approximately equal to electrical power that the fuel cell is configured to produce at peak efficiency Emax; andoutputting a control signal requesting the fuel cell to generate electrical power in dependence on the 5 arbitrated electrical power request P_arb_fc.

17. Computer readable instructions which, when executed by a computer, are arranged to perform a method 600 according to claim 16.10

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