Electrochemical fuel conversion system

Abstract

A method of operating an electrochemical fuel conversion system which comprises a plurality of cells arranged in groups and a plurality of switches which are for selectively engaging and disengaging r

Description

The present invention relates to an electrochemical fuel conversion system that is adapted for removing the need for AC / DC conversion. An electrochemical fuel conversion system is a system that comprises an electrochemical fuel conversion device, such as a fuel cell or electrolyser cell. A traditional electrochemical fuel cell system operating as a fuel cell system in its power delivery mode typically provides an electrical output across its cells’ electrolytes as a DC (direct current) power output. However, most commercial, industrial or domestic power supplies need to be an AC (alternating current) supply. Traditionally, therefore, a fuel cell system will be connected to an inverter to convert the DC output into an AC signal so that it can be used as or connected to a mains circuit. A traditional electrochemical fuel cell system when instead operating as an electrolyser system, needs a DC power supply to be supplied to it, whereupon fuel (e.g. carbon dioxide or water, amongst other possible inputs) that is fed to the electrolyser system can be separated into its component parts (i.e. oxygen and carbon monoxide or oxygen and hydrogen) across the electrolytes. Therefore, as most networked power supplies - i.e. mains power, are supplied as an AC power supply, the electrolyser system will need to be connected to the mains supply via an AC / DC converter to convert the AC mains supply into an DC signal for delivery to the electrolyser. The use of AC / DC conversion, however, is costly at an industrial level since the power electronics for industrial scale AC / DC conversion is expensive - often at least 3-5% of the overall system cost. Non-perfect AC / DC conversion efficiencies can also lead to lower overall fuel to electrical power efficiencies in the system as a whole, which is also disadvantageous. These losses are even more significant when a system is run at less than full load. The present invention seeks to avoid the need for costly AC / DC power electronics in a fuel cell system, be that one operating in a power delivery mode or in a reverse (electrolyser) mode. Similarly, it is desirable to have a back-up should the AC / DC power electronics fail. It will be appreciated that some fuel cells can operate both in a power delivery mode and a reverse mode (i.e. an electrolyser mode, which may also be referred to as a regenerative mode), but others will only be able to function in the first of these modes. Similarly, some electrolysers can operate both in an electrolyser mode and a reverse mode (power delivery mode). As such, the terms fuel cell and electrolyser can be interchangeable to a certain extent. However, again there will be electrolysers that can only function in an electrolyser mode. The present invention has a similar application for any of these fuel cells or electrolysers, and in particular the systems that incorporate many such cells within their fuel cell / electrolyser cell system - i.e. within an electrochemical fuel conversion system, which may operate in an electrolysis mode or a power delivery mode. According to the present invention there is provided a method of operating an electrochemical fuel conversion system, the electrochemical fuel conversion system comprising a plurality of cells arranged in groups and a plurality of switches; wherein the switches are for selectively engaging and disengaging respective ones of the cells or groups with a circuit; the method comprising repetitively and sequentially increasing and then decreasing the number of cells or groups in engagement with the circuit, in most cases, in an oscillatory pattern. The cells may be fuel cells or electrolyser cells, and may be reversible (also referred to as regenerative) and so the electrochemical fuel conversion system may be a (reversible) fuel cell system or (reversible) electrolyser cell system. The circuit is typically an electrical circuit. In some embodiments the cells are either fuel cells or electrolyser cells. These terms may be synonymous as some fuel cells and some electrolyser cells can be used in either or both a power generation mode or in an electrolysis mode (in which they split a compound into constituent or simpler parts, i.e. water into hydrogen and oxygen, carbon dioxide into carbon monoxide and oxygen and nitrogen dioxide into nitrogen monoxide and oxygen). The cells or groups may be configured to be selectively connected in series with one or more other of the cells or groups within the circuit. In some embodiments, the number of cells or groups connected in series within the circuit increases and decreases with the oscillatory pattern. In some embodiments, one or more of the cells or groups are configured to be selectively connected in parallel with one or more other cell or group within the circuit. By connecting in parallel, a current output can be increased. This is beneficially combined with connecting various cells or groups in series, which increase the voltage output. In some embodiments, the number of cells or groups connected in parallel within the circuit increases and decreases with the oscillatory pattern. However, in usual embodiments it is the number of cells or groups connected in series within the circuit that increases and decreases with the oscillatory pattern so that the voltage and current output (of the circuit or cells, depending on mode) oscillates as an alternating voltage and current (it will be understood that alternating or oscillating current encompasses an alternating or oscillating voltage). In some embodiments, the groups comprise first and second sub groups, with the second sub group being selectively engaged with the circuit with an opposite polarity to the first sub group. In some embodiments this is achieved using a circuit that utilises one or more diodes and / or one or more switch to control or set the polarity. In some embodiments the circuit is configured so that current to or from the cells of the wrong polarity is blocked, for example using one or more diodes. In some embodiments, the electrochemical fuel conversion system is operating as an electrolyser system in an electrolysis mode (also referred to as a power consumption mode or a regenerative mode), and an AC power supply is supplied to the circuit. In some embodiments, the AC power supply provides an alternating current at a frequency that either matches or is half of a frequency of the oscillatory pattern. In some embodiments the half frequency is achieved through a rectified alternating current. In some embodiments, the frequency of the oscillatory pattern could vary cell to cell or group to group (i.e., different cells / groups have different duty cycles), and thus might not be constant multiple of the current’s frequency. In some embodiments, the switches are configured to engage a first number of cells or groups in series with the circuit while a magnitude of the input voltage is below a first threshold value and to engage a second number of cells or groups in series with the circuit when the magnitude of the voltage exceeds the first threshold value, the second number being larger than the first number. In some embodiments, the switches are configured to engage a third number of cells or groups in series with the circuit when the input voltage exceeds a second threshold value that is larger than the first threshold value, the third number being larger than the first and second numbers. In some embodiments, the switches are instead configured to engage a first number of cells or groups in series with the circuit while a magnitude of the current is below a first threshold value and to engage a second number of cells or groups in series with the circuit when the magnitude of the current exceeds the first threshold value, the second number being larger than the first number. Furthermore, the switches may be configured to engage a third number of cells or groups in series with the circuit when the current exceeds a second threshold value that is larger than the first threshold value, the third number being larger than the first and second numbers. In such cases the switches may be configured to provide a sequence of switched states comprising consecutively the first, the second and the third number of cells or groups, and subsequently, consecutively, the third, the second and the first number of cells or groups. The switches may be configured to engage a fourth, fifth or sixth, etc., number of cells or groups in series with the circuit when the voltage (or current) exceeds a third, fourth, fifth, etc., threshold value that is each larger than the previous threshold value, the fourth, fifth, sixth, etc. number being larger than the preceding numbers. In some embodiments there may be 8, 10 or 12 or more cells or groups. For example, if 24 stacks are provided, 12 groups could be easily adopted by arranging stacks in pairs within a given group. Other configurations are also possible, using singles, pairs or larger groups of stacks in each group. Typically the first number is one, the second number is two, the third number is three, the fourth number is four, the fifth number is five, the sixth number is six, etc., etc.. By increasing the number of cells or groups that are connected in series, for an electrolyser operating in its regenerative mode, the alternating current’s oscillations can be accommodated in the groups without any group being supplied power in excess of that group’s operational limits. As the AC supply has a generally sinusoidal current curve, within each wavelength the thresholds are crossed four times (twice with a positive current and twice with a negative current). In some embodiments, the fuel cell groups may comprise first and second sub groups, with the second sub group being selectively engaged with the circuit with an opposite polarity to the first sub group, such that both the positive current within the sinusoidal current curve and the negative current within the sinusoidal current curve operate one or more of the fuel cell groups. For example, the first sub group may connect to the circuit for the positive current and the second sub group may connect to circuit for the negative current. This then fully utilises the current cycle of the AC supply. In some embodiments, the electrochemical fuel conversion system is operating in a power delivery mode, and an oscillating output current (likewise voltage) is generated through the circuit by the electrochemical fuel conversion system. In some embodiments, the oscillating output current has a frequency and the frequency of the oscillating output current either matches or is half of a frequency of the oscillatory pattern. In some embodiments, the switches are configured to switch between engaging a first number of cells or groups in series with the circuit to produce a first output current with a first magnitude within a first range of magnitudes, and engaging a second number of cells or groups in series with the circuit to produce a second output current with a second magnitude within a second range of magnitudes, with the second magnitude being higher than the first magnitude. In some embodiments, the switches are configured to additionally provide a switched state with a third number of cells or groups engaged in series with the circuit to produce a third output current with a third magnitude within a third range of magnitudes, the third magnitude being higher than the first and second magnitudes, the switching providing a sequence of switched states comprising consecutively the first, the second and the third number of cells or groups, and subsequently, consecutively, the third, the second and the first number of cells or groups. Typically this sequence repeats continuously, the system thus having an oscillating output current, as the configurations with the first, the second and the third number of cells or groups each output a different current. The switches may be configured to additionally provide switched states in which they engage a fourth, fifth or sixth, etc., number of cells or groups in series with the circuit, each having a different output current. The fourth number of cells or groups provides a current with a fourth magnitude that is higher than the third magnitude, the fifth number of cells or groups provides a current with a fifth magnitude that is higher than the fourth magnitude, the sixth number of cells or groups provides a current with a sixth magnitude that is higher than the fifth magnitude; etc. etc.. These increasing numbers of cells or groups can be implemented also in the sequence - for example there being a sequence of switched states comprising consecutively the first, the second, the third, the fourth, the fifth, and then the sixth number of cells or groups, and subsequently, consecutively, the sixth, the fifth, the fourth, the third, the second and then the first number of cells or groups. This sequence can similarly repeat continuously, the system thus having the oscillating output current, but with a smoother curve. As with the electrolyser mode, as discussed above, larger numbers of groups are also possible in the power delivery mode. Due to the intended frequency of the output current - matching, for example, the 50-60 Hz frequency of mains power, the switches need to switch quickly between these switched states. This imposes a practical limit on the number of switched states that can be implemented during each oscillatory cycle. However, that practical limit will often be rather larger than the number of groups available. For example, typical arrays of groups of cells (or stacks) may contain 10 to 50 such groups. In some cases there may be 10 groups, and the first through fifth number of groups may be multiples of one (i.e, 1, 2, 3...10) or two (i.e., 2, 4,... 10). In some cases there may be 12 groups, and the first through sixth number of groups may be multiples of one (i.e, 1, 2, 3...12, therefore twelve switched states), two (i.e., 2, 4,...12, therefore six switched states), three (i.e., 3, 6...12, therefore four switched states) etc. In some cases there may be 24 groups, and the first through twelfth number of groups may be multiples of two (i.e., 2, 4,...24, therefore twelve switched states), three (i.e., 3, 6...24, therefore eight switched states) etc. Typically the first number is one, the second number is two, the third number is three, the fourth number is four, the fifth number is five, the sixth number is six, etc., etc.. As the output current is intended to match a mains supply, the fuel cell groups may comprise first and second sub groups, with the second sub group being selectively engaged with the circuit with an opposite polarity to the first sub group, as previously discussed. With this configuration, the sequence can produce both positive and negative currents and thus an AC output. For example, the sequence may comprise engaging, sequentially and consecutively, the first, the second and the third number of fuel cell groups from the first sub group, and subsequently, consecutively, the third, the second and the first number of fuel cell groups from the first sub group, followed by, sequentially and consecutively, the first, the second and the third number of fuel cell groups from the second sub group, and subsequently, consecutively, the third, the second and the first number of fuel cell groups from the second sub group. This in turn then produces both a positive current “curve” and a negative current “curve”, in a repeatable sequence, thus providing an AC supply. In some embodiments, the method comprises modifying a duty cycle of a cell or group based on health characteristics of the cell or group. In some embodiments, the method comprises modifying a pulse-width modulation frequency based on health characteristics of a cell or group. In some embodiments, the method comprises locating the cells or groups within a vessel, and arranging the switches external to the vessel. In some embodiments, the groups are maintained at a higher temperature than the switches. In some embodiments, the groups are maintained in a higher pressure environment than the switches. For example, the vessel may be a pressure vessel. In some embodiments, the cells comprise solid oxide cells (i.e., solid oxide fuel cells or solid oxide electrochemical cells, either / both of which may be reversible). In some embodiments, the cells are metal supported solid oxide cells. In some embodiments the metal is steel. With a metal, e.g. steel, support, there can be more thermal inertia, which makes the cells more resilient to current cycles. In some embodiments, the cells are intermediate temperature cells - operating at a temperature between 400 and 700 degrees C. At such temperatures, the cells suffer less from thermal losses than higher temperature cells and so are more robust to any temperature variation (which is of a lesser amplitude than for higher temperature cells). In some embodiments, the groups are configured into at least three phase-targeting groups, each one of the three phase targeting groups being assigned to a phase of a 3-phase circuit. With this configuration, a three phase implementation is possible. A similar, but differently divided sub-categorisation may be suitable also for other polyphasic systems - for example, four phase-targeting groups for a four phase system, etc. In some embodiments, each group comprises one or more stacks of cells or a part of a stack of cells. In some embodiments, the electrical connection of the fuel cell system is oscillatory about a non-zero value. This may be a voltage and / or a current. In some embodiments, the electrical connection of an electrochemical fuel conversion system is a time varying supply and the selective switching of groups is used to vary the supplied power load or react to the varying power input. In some embodiments this can be utilised to allow for a varying total power output received from a solar array or a solar farm. Similarly, other forms of electricity generation can have variable outputs, particularly wind power and tidal power. In some embodiments, an electrical power output or electrical power input of the electrochemical fuel conversion system is maintained at a substantially constant level during an operational period of time and the system has a control to selectively isolate or connect, or to reverse the polarity of, the individual groups, or collections of said groups, or cells within said groups, to improve an operational lifetime or performance of one or more of the cells or groups. For example, in the case of there being three groups, each with one or more cells or stacks, and one group is operating at a lower performance than the others, it can be used less when a demand is lower, or it can be selected for operations when load balancing or load sharing is desired. In another example, it is possible to predict a lifetime of a cell, group or stack, and it is possible to use cells, groups, or stacks with a longer operational lifetime remaining more than those with a shorter operational lifetime remaining, thus prolonging the overall operational lifetime of the system before a maintenance shutdown is needed. The substantially constant level may refer to an AC power. In some embodiments, damaged or otherwise non-functional cells or groups are switched out of use. As a result they will no longer participate in the operational sequence (oscillatory or otherwise), thus also prolonging the overall operational lifetime of the system before a maintenance shutdown is needed. The present invention also concerns an electrochemical fuel conversion system comprising a plurality of cells arranged in groups and a plurality of switches, wherein the switches are configured for selectively engaging and disengaging respective ones of the cells or groups with a circuit - for example an electrical or fluidic circuit, and a control circuit configured to repetitively and sequentially increase and then decrease a number of cells or groups in engagement with the circuit in an oscillatory pattern. The circuit is typically an electrical circuit. In some embodiments the cells are either fuel cells or electrolyser cells. These terms may be synonymous as some fuel cells and some electrolyser cells can be used in either or both a power generation mode or in a electrolysis (or power consumption) mode (in which they split a compound into constituent or simpler parts, i.e. water into hydrogen and oxygen, carbon dioxide into carbon monoxide and oxygen and nitrogen dioxide into nitrogen monoxide and oxygen. The system may be configured to carry out a method as defined above. The present invention also concerns a controller configured to perform the methods discussed herein. The present invention also concerns a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method as discussed above. In particular, when the program is executed by a computer, cause the computer to repetitively and sequentially increasing and then decreasing a number of cells or groups in engagement with the circuit in an oscillatory pattern. The program may be executed in (communication with) an electrochemical fuel conversion system, the electrochemical fuel conversion system comprising a plurality of cells arranged in groups and a plurality of switches, wherein the switches are for selectively engaging and disengaging respective ones of the cells or groups with a circuit. The present invention also concerns a non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform (or cause the processor to perform) the steps of the method as discussed above. In particular, when executed by a processor, perform (or cause the processor to perform): repetitively and sequentially increasing and then decreasing a number of cells or groups in engagement with the circuit in an oscillatory pattern. The instructions may be executed in (communication with) an electrochemical fuel conversion system, the electrochemical fuel conversion system comprising a plurality of cells arranged in groups and a plurality of switches, wherein the switches are for selectively engaging and disengaging respective ones of the cells or groups with a circuit. The present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically shows a typical cell from an electrochemical fuel conversion device or system, which may be within a stack, or within a group of cells; Figure 2 shows a schematic circuit representing switches for controlling how many cells or groups of cells (or stacks) are connected in series within the circuit; Figure 3 shows a logic table for the circuit of Figure 2 for use by a control system; Figures 4 and 5 show two options for groups to connect to the circuit for particular parts of the electrical power wave form, between which the control system may choose to use, or to switch or alternate between; Figure 6 schematically illustrates a simplified diode arrangement for managing reverse polarity cells or groups of cells; Figure 7 shows three arrangements similar to that of claim 6, each connected to or as a different phase of a three phase power supply; Figure 8 shows a possible configuration for each cell arrangement within the arrangements of Figures 6 and 7; Figure 9 schematically shows a modified version of Figure 6, with a plurality of groups of cells held in series for each polarity to increase the overall voltage capacity; and Figure 10 shows a further example of a typical stepped response curve achievable with the present invention compared against a true sinewave, showing both positive and negative output voltages. Referring first to Figure 1 there is schematically shown a typical cell - an electrochemical fuel conversion device, for example a fuel cell or an electrolyser cell, multiples of which may be stacked in one or more stacks within an electrochemical fuel conversion system, forming groups of cells. Hereinafter, for simplicity, this will be described as an electrolyser system comprising electrolyser cells. Referring first to Figure 1, the basic structure and operation of a typical electrolyser cell for use in a regenerative (also referred to as electrolysis) mode within an electrolyser system is shown. This electrolyser cell 11 will be within an electrolyser of an electrolyser system. It should be noted that other ancillary components related to the electrolyser cell 11 would also be included in an electrolyser system. These usually include heat exchangers, heaters, valves and sensors. It will be understood by a skilled person that a similar or identical structure for a cell is also used in a fuel cell that operates in a power delivery mode. However, it effectively operates in reverse to the electrolyser operating in the regenerative (electrolysis) mode. The electrolyser cell 11 comprises an anode (or oxygen electrode) 33, a cathode (or fuel electrode) 34 and an electrolyte 35. Such a structure for an electrolyser cell 11 is well known in the art. In this example, the electrolysis of water will be discussed, whereby the “fuel” for the electrolyser system is water, although other fluids and gases can similarly be electrolysed, i.e. to split them into their component parts, such as oxygen and hydrogen for water or oxygen and carbon monoxide for carbon dioxide. Water - here in the form of steam 43 from a water source - which may be a steam source if less internal heating of the water is desired in the electrolyser system 20 - is passed over the cathode 34 via inlet 41 and hot air (or N2 or O2) 42 can be passed over the anode 33 via inlet 40 as a sweep gas. To power the electrolyser cell, an electric current is applied across the electrolyser cell 11 via electric terminals / connections 36, 37 at the anode and cathode sides of the electrolyser cell 11. These terminals may be positioned adjacent to one-another on one side or end of a stack of cells 11, for example by having stacked cells in parallel and extending one terminal to the other end of the stack 12 using a bus bar, as known in the art. Via the terminals 36, 37, a voltage can be applied across the stack 12 - and thus a current is passed through the cells 11. The application of electrical power through an electrolyser 10 in this manner is well known in the art. As a consequence of the electrical current, an electrolytic reaction occurs across the electrolyte 35, with oxygen ions passing across the electrolyte 35 from the cathode 34 to the anode 33. Steam breaks down into hydrogen on the cathode side of the electrolyser cell 11 and oxygen is produced at the anode side. The oxygen can be extracted from the cell via an air flow provided by the sweep gas flow provided by the hot air 42, thus venting it out of an off-gas outlet 38 on the anode side of the electrolyser cell 11. That output is thus generally an oxygen enriched air flow (with the produced oxygen enriching the sweep gas flow, if present). The hydrogen can instead be extracted and vented out of another off-gas outlet 39 on the cathode side of the electrolyser cell 11. This hydrogen typically will be mixed with any remaining steam from the initial supply, as the splitting of the input steam into oxygen and hydrogen is usually only in respect of a proportion of the supplied steam. The hydrogen is thus vented as a ‘wet’ hydrogen off-gas on the cathode side. Thus, the steam exiting the cathode side is hydrogen enriched, and the sweep gas exiting the anode side is oxygen enriched. Such operational characteristics of electrolyser cells, including SOECs, are well known in the art. If this electrolyser cell (the electrochemical fuel conversion device) is instead operating in a power delivery mode, instead of supplying (as an example) steam at an inlet and electricity at terminals thereof for powering the cell, and then producing oxygen and hydrogen, the fuel is instead usually supplied as a hydrocarbon gas, a hydrogen enriched gas or hydrogen and an oxidant (e.g., oxygen or air) is supplied at the inlet 40, and during operation of the cell an electrochemical reaction occurs across the electrolyser to instead output an electrical current from the terminals, along with steam and possibly carbon dioxide and / or carbon monoxide at the outlets, dependent upon the choice of fuel. Such operational characteristics of an electrochemical fuel conversion device (fuel cells and electrolysers) are well known in the art. Referring next to Figure 2 there is shown a schematic circuit 18 representing a simplified electrochemical fuel conversion system 14 configured to output electricity for use by a load 24. Within the circuit are a plurality of switches 22 (A to J) for controlling how many groups of cells (or cells or stacks) are connected in series within the circuit 18. In this example and the examples that follow, the cells are operating in a power delivery mode whereby there is the load to which the generated voltage and current is delivered. The present invention operates is the same manner for a regenerative mode of the cells, albeit with power instead being delivered to the cells (or stacks of cells or groups of cells within one or more stacks) from a power source, rather than being output by those cells to a load 24. In this electrochemical fuel conversion system 14, the switches 22 are selectively opened and closed as per the logic table shown in Figure 3 by a control system. In this example, this will selectively connect in series one or more of the groups 16 without ever connecting them in parallel. For this purpose, each group has a switch at each end, and each group has a switchable circuit in series to each of the other groups. There are three groups in this example whereby there are nine switches in total. Following the logic table of Figure 3, a first condition in which only group 1 is connected to the load, all switches are open (non-connected in circuit, and logic 0 in the table) except for switches 22A and 22D - the two switches at each end of group 1, and thus the switches to connect the other groups in series are all open. However, to connect only group 2, instead only switches 22B and 22E are closed (logic 1 in Figure 3). They are the switches at the ends of group 2. All the other switches are open (logic 0). When instead connecting only group 3, just switches 22C and 22F are closed (logic 1) and all the others are open (logic 0). However, to connect groups 1 and 2 and 3 in series within the circuit, switch 22A needs to be closed, but switch 22D needs to be open, and switches 22G (between groups 1 and 2), 22H (between groups 2 and 3) and 22F need to be closed to bring groups 2 and 3 in series with group 1 within the circuit. In such a case, switch 22G may connect a negative terminal of group 1 to a positive terminal of group 2, likewise switch 22H may connect a negative terminal of group 2 to a positive terminal of group 3, in order to connect the groups in series. All the other switches are open. The logic table in Figure 3 shows seven different configurations for the groups, with each permutation of a series connection being shown. Parallel configurations are excluded, but may be utilised in a larger, more complex circuit. In Figure 3, “12s” indicates group 1 and 2 in series, for instance. Referring next to Figures 4 and 5, two potential sequences for the switching are shown. These represent a sequence of options for the groups 16 to connect to the circuit 18 for mirroring or representing particular parts of a desired alternating electrical current power supply, in which the voltage (like the current) varies over time. These are represented in graph form, with a true sinewave being represented to show a target general form, and a linear sawtooth wave form representing a simplified form of the achievable power output. In Figure 4, there is represented in the sawtooth wave form, an initial condition where none of the groups are connected, followed by a second condition in which just group 1 is connected. There is then a third condition in which groups 1 and 2 are connected and a fourth condition in which all three - groups 1,2 and 3 - are connected. There is then a fifth condition in which groups 2 and 3 are connected, a sixth condition in which just group 3 is connected and then finally a seventh condition where none of the groups are connected. Figure 5 simply shows a configuration in which the groups are switched in in a different order. In both Figure 4 and 5, it is intended that each group is switched into circuit for the same amount of time as the others, whereby a uniform operational lifetime will be experienced by all the groups. As each group has a voltage output approximating a nominal voltage output, when just one group is connected 1x that nominal voltage output is passed to the load (it will be the voltage of that group). Then, when two are connected, 2x that nominal voltage output is passed to the load. This is represented in the graph. The actual voltage will be the combined voltage of the two groups that are connected in series. Then when three are connected, 3x the nominal voltage is output. Again the actual voltage will be the combined voltage of the three groups that are connected in series. Then as this then steps down to two groups and then one group being connected in series, the output voltages drops again to 2x and then 1x the nominal voltage. Again the actual voltage will be the combined voltage of the groups that are connected in series at any point in time. By sequentially repeating this, an oscillating voltage output is generated, and by doing this at a frequency that matches the target alternating current power supply, an approximation of that alternating current power supply can be achieved. The more steps present in this sequence, the closer it becomes possible to approximate the target sinewave. It is to be appreciated, however, that an AC power supply oscillates at a frequency that is usually between 50 and 60 Hz. The switching of the switches 22 thus need to be able to match that. There is thus a practical limit as to how many groups 16 can be used within the circuit 18, whereby a true sinewave likely cannot be achieved with this process until voltage and current smoothing circuitry is added (for example, including one or more inductors and / or capacitors). It is within the scope of the present invention to incorporate smoothing circuitry alongside the core components described herein. Furthermore, whereas the above example only considers a positive voltage output, it will be appreciated that a true sinewave power supply oscillates between positive and negative currents and voltages. In order to accommodate this within the present invention, it is possible to provide further switches to swap the relative polarity of the groups of cells and the load. Alternatively, it is possible to incorporate within the circuit 18 two or more switchable sub-groups or arrays 26, 28 of groups 16 of cells 11, with opposing polarities. An example of this is shown in Figure 6, which schematically illustrates a simplified diode arrangement for managing the reversed polarities of the different sub groups 26, 28, each sub group comprising multiple groups 16 of cells 11 that can be selectively switched into series as discussed above. With this arrangement, in a first array 26, the groups 16 can be switched in and out of series with the circuit 18 in the manner previously described. This can produce the voltage response indicated by Figures 4 and 5. Then, however, the same occurs to the array 28 with the reversed polarity. That then instead produces the response with effectively a negative voltage in the eyes of the load 24. Switching then back to the first array 26, a positive voltage is provided, and this sequence then continues. As a result an alternating current output can be approximated, both with positive and negative voltages and currents. Although it is appreciated that it is possible to switch out of the circuit all the groups of cells that are not required to participate at any point in time, whereby the positive and negative currents can be oscillating, in practice there will be some overlap between the outputs from individual groups of cells. To prevent the cells encountering reverse polarities, Figure 6 shows that each array is fitted with an appropriate diode 30, 32. In the case of a regenerative mode electrochemical fuel conversion system, diodes may instead provide rectification of the incoming AC power, so that the received voltages are always appropriate for the arrays, although switching between positive and negative arrays is also possible instead, much like as described above for the power delivery mode. It is also to be appreciated that although reversing the voltage is typically not desired, in some forms of electrochemical fuel conversion system, there are benefits from running, selectively, in both a power delivery mode and a regenerative mode as in certain circumstances, and at carefully controlled voltages, this can reverse damage to or remove undesired chemical accumulations from the electrodes and electrolyte. For example, in an electrolyser cell with a target operational voltage of about 1.28V, at input voltages above 0.9v the cell will electrolyse fuel (e.g. water) to produce the oxygen and hydrogen, whereas if the voltage is allowed to be reduced to a number a little below that voltage, it can start to regenerate the cell, operating in fuel cell mode, improving its operational lifetime. Some reverse voltage flow can thus be utilised to make use of this benefit. Referring next to Figure 7 there is shown three arrangements similar to that of Figure 6, each forming a circuit 18A, 18B, 18C that is connected to, or as, a different phase of a three phase power supply. Each arrangement forms one of three phase targeting groups 44, 46, 48. A first phase targeting group 44 can be connected to a first phase, a second phase targeting group 46 can be connected to a second phase and a third phase targeting group 48 can be connected to a third phase. Each phase targeting group has similar first and second arrays 26, 28, and diodes 30, 32, to that of Figure 6. Furthermore, each phase targeting group can be operated at the same frequency of oscillation between positive and negative voltages, but their waveforms are typically going to be shifted through 120 degrees so as to match a commercial three phase power supply. Referring next to Figure 8 there is shown a possible configuration for each group 16 of cells 11 within the circuits of Figures 6 and 7. This configuration is effectively the same as in Figure 2, but with terminals 50 in place of the load 24 so that the circuits can be connected as appropriate into the positions of the first array 26 and the second array 28 using the correct polarity. Referring next to Figure 9, there is schematically shown a modified version of Figure 6, with a plurality of groups of cells held in series for each polarity to increase the overall voltage capacity. As a target output or input voltage is 240V for mains electricity, it will be normal for a large number of cells to be operated in series within any group, but in some instances the cells in series, which may form stacks, need additionally to be held in series to achieve the desired 240v as commonly used in the United Kingdom and Europe, amongst other countries (or 110 V as used in the US and other countries). In a similar way a plurality of groups of cells held in parallel, with plural groups for each polarity may be used to increase the overall current capacity. It is to be appreciated that in an industrial plant in which these electrochemical fuel conversion systems are likely to be adopted, it is likely that the cells will be configured in stacks of between 100 and 400 cells, and groups of cells may be in the form of groups of stacks. For example, 10 to 30 stacks may form a typical array, albeit with numerous stacks being configured as sets that are collectively and selectively connected in series. Finally, reference is made to Figure 10, which shows a typical stepped voltage output curve that may be achievable with the present invention, compared against a true sinewave. As can be seen this example utilised arrays with normal and reversed polarities, whereby it shows both positive and negative output voltages. Furthermore, it shows a maximum of four rather than three connected groups within the series connection, which allows the curve to more closely approximate the sinewave, albeit still with a stepped output. Other versions with five, six, eight, ten or twelve steps can also be provided, or more if the switching speed is achievable. With microprocessor control, such rapid switching is feasible. The present invention has therefore been described above, purely by way of example, with reference to the accompanying drawings. Modifications in detail may be made to the invention within the scope of the claims as appended hereto.

Claims

1. A method of operating an electrochemical fuel conversion system, the electrochemical fuel conversion system comprising a plurality of cells arranged in groups and a plurality of switches;wherein the switches are for selectively engaging and disengaging respective ones of the cells or groups with a circuit;the method comprising repetitively and sequentially increasing and then decreasing a number of cells or groups in engagement with the circuit in an oscillatory pattern.

2. The method of claim 1, wherein the cells or groups are configured to be selectively electrically connected in series with one or more other of the cells or groups within the circuit.

3. The method of claim 2, wherein the number of cells or groups connected in series within the circuit increases and decreases with the oscillatory pattern.

4. The method of any one of claims 1 to 3, wherein the groups comprise first and second sub groups, with the second sub group being selectively engaged with the circuit with an opposite polarity to the first sub group.

5. The method of any one of claims 1 to 4, wherein the electrochemical fuel conversion system is operating as an electrolyser system in an electrolysis mode, and an AC power supply is supplied to the circuit.

6. The method of claim 5, wherein the AC power supply provides an alternating current at a frequency that either matches or is half of a frequency of the oscillatory pattern.

7. The method of claim 5 or claim 6, wherein the switches are configured to engage a first number of cells or groups in series with the circuit while a magnitude of the current is below a first threshold value and to engage a second number of cells or groups in series with the circuit when the magnitude of the current exceeds the first threshold value, the second number being larger than the first number.

8. The method of claim 7, wherein the switches are configured to engage a third number of cells or groups in series with the circuit when the current exceeds a second threshold value that is larger than the first threshold value, the third number being larger than the first and second numbers.

9. The method of any one of claims 1 to 4, wherein the electrochemical fuel conversion system is operating in a power delivery mode, and an oscillating output current is generated through the circuit by the electrochemical fuel conversion system.

10. The method of claim 9, wherein the oscillating output current has a frequency and the frequency of the oscillating output current either matches or is half of a frequency of the oscillatory pattern.

11. The method of claim 9 or claim 10, wherein the switches are configured to switch between engaging a first number of cells or groups in series with the circuit to produce a first output current with a first magnitude within a first range of magnitudes, and engaging a second number of cells or groups in series with the circuit to produce a second output current with a second magnitude within a second range of magnitudes, with the second magnitude being higher than the first magnitude.

12. The method of claim 11, wherein the switches are configured to additionally provide a switched state with a third number of cells or groups engaged in series with the circuit to produce a third output current with a third magnitude within a third range of magnitudes, the third magnitude being higher than the first and second magnitudes, the switching providing a sequence of switched states comprising consecutively the first, the second and the third number of cells or groups, and subsequently, consecutively, the third, the second and the first number of cells or groups.

13. The method of any one of the preceding claims, comprising modifying a duty cycle of a cell or group based on health characteristics of the cell or group.

14. The method of any one of the preceding claims, comprising modifying a pulse-width modulation frequency based on health characteristics of a cell or group.

15. The method of any one of the preceding claims, comprising locating the cells or groups within a vessel, and arranging the switches external to the vessel.

16. The method of any one of the preceding claims, wherein the cells comprise solid oxide cells.

17. The method of any one of the preceding claims, wherein the groups are configured into at least three phase-targeting groups, each one of the three phase targeting groups being assigned to a phase of a 3-phase electrical circuit.

18. The method of any one of the preceding claims, wherein each group comprises one or more stacks of cells or a part of a stack of cells.

19. The method of any one of the preceding claims wherein the electrical input / output of the fuel cell system is oscillatory about a non-zero value.

20. The method of any one of the preceding claims wherein the electrical connection of a electrochemical fuel conversion system is a time varying supply and the selective switching of groups is used to vary the supplied power load or react to the varying power input.

21. The method of any one of the preceding claims wherein an electrical power output or electrical power input of the electrochemical fuel conversion system is maintained at a substantially constant level during an operational period of time and the system has a control to selectively isolate or connect, or to reverse the polarity of, the individual groups, or collections of said groups, or cells within said groups, to improve an operational lifetime or performance of one or more of the cells or groups.

22. The method of any one of the preceding claims wherein damaged or otherwise nonfunctional cells or groups are switched out of use.

23. An electrochemical fuel conversion system comprising a plurality of cells arranged in groups and a plurality of switches, wherein the switches are configured for selectively engaging and disengaging respective ones of the cells or groups with a circuit, and a control circuit configured to repetitively and sequentially increase and then decrease a number of cells or groups in engagement with the circuit in an oscillatory pattern.

24. The electrochemical fuel conversion system of claim 23, wherein the cells are either fuel cells or electrolyser cells, the system configured to carry out a method according to any one of claims 1 to 22.

25. A controller configured to perform the method of any one of claims 1 to 22 or an electrochemical system comprising a controller configured to perform the method of any one of claims 1 to 22.10 09 25AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:-CLAIMS:

1. A method of operating an electrochemical fuel conversion system as an electrolyser system in an electrolysis mode, the electrochemical fuel conversion system comprising a plurality of cells arranged in groups and a plurality of switches;wherein the switches are for selectively engaging and disengaging respective ones of the cells or groups with a circuit;the method comprising supplying an AC power to the circuit and repetitively and sequentially increasing and then decreasing a number of cells or groups in engagement with the circuit in an oscillatory pattern.

2. The method of claim 1, wherein the cells or groups are configured to be selectively electrically connected in series with one or more other of the cells or groups within the circuit.

3. The method of claim 2, wherein the number of cells or groups connected in series within the circuit increases and decreases with the oscillatory pattern.

4. The method of any one of claims 1 to 3, wherein the groups comprise first and second sub groups, with the second sub group being selectively engaged with the circuit with an opposite polarity to the first sub group.

5. The method of any one of the preceding claims, wherein the AC power supply provides an alternating current at a frequency that either matches or is half of a frequency of the oscillatory pattern.

6. The method of any one of the preceding claims, wherein the switches are configured to engage a first number of cells or groups in series with the circuit while a magnitude of the current is below a first threshold value and to engage a second number of cells or groups in series with the circuit when the magnitude of the current exceeds the first threshold value, the second number being larger than the first number.

7. The method of claim 6, wherein the switches are configured to engage a third number of cells or groups in series with the circuit when the current exceeds a second threshold value that is larger than the first threshold value, the third number being larger than the first and second numbers.10 09 258. The method of any one of the preceding claims, comprising modifying a duty cycle of a cell or group based on health characteristics of the cell or group.

9. The method of any one of the preceding claims, comprising modifying a pulse-width modulation frequency based on health characteristics of a cell or group.

10. The method of any one of the preceding claims, comprising locating the cells or groups within a vessel, and arranging the switches external to the vessel.

11. The method of any one of the preceding claims, wherein the cells comprise solid oxide cells.

12. The method of any one of the preceding claims, wherein the groups are configured into at least three phase-targeting groups, each one of the three phase targeting groups being assigned to a phase of a 3-phase electrical circuit.

13. The method of any one of the preceding claims, wherein each group comprises one or more stacks of cells or a part of a stack of cells.

14. The method of any one of the preceding claims wherein the electrical input of the electrochemical fuel conversion system is oscillatory about a non-zero value.

15. The method of any one of the preceding claims wherein the electrical connection of a electrochemical fuel conversion system is a time varying supply and the selective switching of groups is used to react to the varying power input.

16. The method of any one of the preceding claims wherein an electrical power input of the electrochemical fuel conversion system is maintained at a substantially constant level during an operational period of time and the system has a control to selectively isolate or connect, or to reverse the polarity of, the individual groups, or collections of said groups, or cells within said groups, to improve an operational lifetime or performance of one or more of the cells or groups.

17. The method of any one of the preceding claims wherein damaged or otherwise nonfunctional cells or groups are switched out of use.

18. An electrochemical fuel conversion system configured to operate in an electrolysis mode, the electrochemical fuel conversion system comprising a plurality of cells arranged ingroups and a plurality of switches, wherein the switches are configured for selectively engaging and disengaging respective ones of the cells or groups with a circuit, and a control circuit configured to repetitively and sequentially increase and then decrease a number of cells or groups in engagement with the circuit in an oscillatory pattern and an AC power supply is configured to supply AC power to the circuit.

19. The electrochemical fuel conversion system of claim 18, wherein the cells are electrolyser cells, the system configured to carry out a method according to any one of claims 1 to 17.

20. A controller configured to perform the method of any one of claims 1 to 17 or an electrochemical system comprising a controller configured to perform the method of any one of claims 1 to 17.

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