Ejector, particularly for use with a blower for recirculating hydrogen in a proton exchange membrane fuel cell

Abstract

An ejector (100, 200, 300) includes a convergent portion (101, 201, 301); a cylindrical throat (102, 202, 302) downstream of the convergent portion and having a length Tl and a diameter Td; and a divergent portion (103, 203, 303) downstream of the throat and having a length Dl and a divergence angle Da between its internal surface and the flow axis (Fx). The primary nozzle has a minimum diameter Nd and an orifice (122, 222, 322) coaxial with the flow axis (Fx) and separated from an upstream end of the throat by a separation distance NTl, wherein: [6.1 ≥ (Td / Nd) ≥ 4.5], and [7.0 ≥ (Tl / Td) ≥ 3.5], and [1.1 ≥ (NTl / Td) ≥ 0.3], and [4.5° ≥ Da ≥ 2.0°], and [(Dl / Td) ≥ 10]. Preferably the secondary or suction inlet is coaxial with a primary flowpipe (123) defining the primary nozzle for a distance upstream of the nozzle orifice. The ejector (100, 200, 300) may be arranged in parallel with a downsized blower (521) to recirculate hydrogen at the anode (512) of a proton exchange membrane fuel cell power supply unit (500), wherein the ejector operates alone in a high load range (H) of the unit, and progressively replaces the operation of the blower (521) through a medium load range (M) which may be from about 25% to 44% of the maximum power output P of the unit.

Description

[0001] Description EJECTOR, PARTICULARLY FOR USE WITH A BLOWER FOR RECIRCULATING HYDROGEN IN A PROTON EXCHANGE MEMBRANE FUEL CELL Technical Field This disclosure relates to ejectors, and particularly (although not exclusively) to hydrogen recirculation arrangements in proton exchange membrane fuel cells in which an ejector operates in parallel with a blower. Background In this specification a PEMFC means a proton exchange membrane fuel cell, sometimes referred to as a polymer electrolyte membrane fuel cell. An ejector means a device for entraining a secondary fluid into a jet of primary fluid by suction generated by the motive power of the primary fluid. An ejector may be referred to by various other names including eductor, Venturi pump, and jet pump, all of which are considered herein as synonyms. Each functional unit of a PEMFC includes an anode and a cathode separated by an electrolytic membrane, which is to say, a membrane that includes or functions as an electrolyte to conduct ionic hydrogen while electrically insulating the anode from the cathode. Hydrogen gas is ionised at the anode to separate the electron from the proton. The hydrogen cation, consisting of a single proton, is much smaller than the neutral hydrogen atom and so is able to pass through the membrane to the cathode where it reacts with oxygen to form water. The electron cannot pass through the membrane and is conducted instead through a circuit between the anode and cathode to produce useful power. A typical fuel cell will include many such functional units arranged in a stack, with separate manifolds providing fluid communication with the anode and cathode sides of the membrane in all of the units. Catalysts are provided at the anode and cathode to promote the reactions. The gases are arranged to flow on the anode side of the membrane from a common, anode intake manifold to a common, anode exhaust manifold. Similarly, the gases are arranged to flow on the cathode side of the membrane from a common, cathode intake manifold to a common, cathode exhaust manifold. The cathode side of each unit is supplied with air via a the cathode intake manifold, while the exhaust fluids comprising oxygen depleted air and water that is formed at the cathode are extracted via the cathode exhaust manifold. In practical power generation applications, it is uneconomic to run a fuel cell on perfectly pure hydrogen, and so the hydrogen supplied to the anode may contain a variety of contaminants including inert gases and water. If the anode side of the membrane is arranged without an exhaust manifold (a so-called dead end anode) then these fluids can accumulate on the anode side of the membrane. In operation, water also migrates through the membrane at a rate that varies with the load on the fuel cell. The membrane is vulnerable (because thin and delicate) to static pressure imbalance between the anode and cathode sides, as well as to drying out and flooding due to the accumulation of liquid water. All of these conditions can impair the performance of the fuel cell. To avoid these problems, and to ensure an adequate hydrogen flow rate to the stack to respond to rapid changes in the power demand on the fuel cell, the hydrogen gas is recirculated continuously through the anode side of the stack, between the anode intake and exhaust manifolds. The recirculating flow is managed to maintain a target level of humidity at the anode while balancing the mass flow rate and static pressure across the anode and cathode sides of the membrane. A fraction of this recirculating flow is purged to remove the accumulated contaminants and excess water from the anode. The purged fraction may be combusted to drive a turbocompressor that supplies compressed air to the cathode intake manifold. Conventionally, the recirculating flow of hydrogen gas is driven by an electrically powered blower. Since the flowpaths through the stack are narrow, a lot of motive power is required, and so the blower represents a substantial parasitic load on the fuel cell, and makes a substantial contribution to the size and cost of the unit. By way of example, US2023387433 A1 discloses a proton exchange membrane power supply unit with a blower for recirculating hydrogen through the anode side of the stack. For more efficient operation, the hydrogen supply to the stack can be arranged to flow through the primary (motive power) flowpath of an ejector that replaces the blower to drive the circulating flow through its secondary (suction) intake. A number of studies have focused on optimising the design of the ejector for such applications. Generally, such studies have been based on models of fuel cells operating up to about 100kW maximum output power. The effectiveness of an ejector is represented by its entrainment ratio, defined as the ratio of the secondary (suction) mass flow rate to the primary (motive power) mass flow rate. The lower the entrainment ratio, the less secondary flow is entrained into the primary flow. As the mass flow rate through the ejector increases, so too does the back pressure, defined as the difference between the outlet boundary pressure (the static pressure at the outlet of the diffuser) and the suction inlet boundary pressure (the static pressure at the inlet to the secondary flowpath at the ejector.) The back pressure opposes the flow through the ejector, and so limits the entrainment ratio. At the lower end of the output power range of a fuel cell, the mass flow rate of hydrogen into the stack is too low to generate suction at the secondary inlet, and so the recirculating flow cannot be driven by an ejector. An ejector becomes more effective as the hydrogen flow rate increases, and so becomes a practical option towards the upper end of the range. However, the effectiveness of the ejector then diminishes as back pressure increases. It is difficult to design an ejector to operate effectively over more than a narrow range of mass flow rate through the primary flowpath. For these reasons it has been found challenging to design an ejector with a sufficiently high entrainment ratio to handle the hydrogen recirculation load in a power cell across much more than the upper quartile of the output power range of the cell. One approach to solving this problem is to use a combination of multiple ejectors which are optimised for operation over different parts of the output power range of the cell. However, this approach imposes additional control requirements for diverting the flow between the ejectors as the electrical load on the power cell fluctuates. By way of example, see: Chen, L.; Xu, K.; Yang, Z.; Yan, Z.; Dong, Z. Optimal Design and Operation of Dual-Ejector PEMFC Hydrogen Supply and Circulation System. Energies 2022, 15, 5427. (https: / / doi.org / 10.3390 / en15155427.) (Chen et al, 2022) Chen et al, 2022 discloses a PEMFC having dual ejectors intead of a blower for recirculating hydrogen at the anode. The ejectors are operated selectively over different parts of the range from 8% to 100% of the output power of the fuel cell. It is also known to operate an ejector in combination with a blower, as discussed in the following papers: He J, Choe SY, Hong C. Analysis and control of a hybrid fuel delivery system for a polymer electrolyte membrane fuel cell. J Power Sources 2008;185:973-984. (He at al, 2008) He, J.; Ahn, J.; Choe, S. Analysis and control of a fuel delivery system considering a two-phase anode model of the polymer electrolyte membrane fuel cell stack. J. Power Sources 2011, 196, 4655–4670. (https: / / doi.org / 10.1016 / j.jpowsour.2011.01.019) (He et al, 2011) He et al, 2008 models a fuel delivery control system for a PEMFC including a blower and an ejector, wherein the blower and the secondary flowpath of the ejector are arranged in parallel to recirculate hydrogen to the anode. Hydrogen gas can be supplied to the anode via a flow control valve that feeds the primary flowpath of the ejector, and via a low pressure regulator that bypasses the flow control valve and ejector. In low load mode, the flow control valve is closed, and hydrogen is supplied to the anode via the low pressure regulator and recirculated by the blower. In high load mode, the flow control valve is opened to supply additional hydrogen to the stack via the ejector, and the recirculating flow is driven by the ejector and the blower operating in parallel. He et al, 2011 models a fuel delivery control system for a PEMFC including a blower and an ejector, wherein the blower and the secondary flowpath of the ejector are arranged in parallel to recirculate hydrogen to the anode. Hydrogen gas can be supplied to the anode via a flow control valve that feeds the primary flowpath of the ejector, and via a low pressure regulator that bypasses the flow control valve and ejector. In low current mode, the flow control valve is closed, and hydrogen is supplied to the anode via the low pressure regulator and recirculated by the blower. In medium and high current modes, hydrogen is supplied to the anode via the flow control valve and ejector, and recirculated by the ejector and the blower, so that the power dissipated in the blower is reduced by the recirculation through the ejector. Summary of the Disclosure An ejector In accordance with the present disclosure there is provided an ejector including a first flowpath for carrying a mixed flow of primary and secondary fluids along a flow axis in a flow direction. The first flowpath is bounded by a first internal surface, which is a surface of revolution about the flow axis. When considered in the flow direction, the first flowpath includes: a convergent portion; a cylindrical throat downstream of the convergent portion and having a length Tl and a diameter Td; and a divergent portion downstream of the throat and having a length Dl and a divergence angle Da between the first internal surface and the flow axis. The ejector further includes: a secondary flowpath upstream of the convergent portion for conducting the secondary fluid to the convergent portion; and a primary flowpath including a nozzle portion, the nozzle portion having a minimum diameter Nd and terminating at a nozzle orifice to discharge the primary fluid as a jet from the nozzle orifice. The nozzle orifice is coaxial with the flow axis and upstream of the throat, and is separated from an upstream end of the throat by a separation distance NTl along the flow axis. In accordance with the disclosure: (Td / Nd) and and and 4.5° 2.0° and A power supply unit including the ejector In an embodiment, the disclosure further provides a power supply unit including: a proton exchange membrane fuel cell assembly having at least one anode and at least one cathode separated by at least one electrolytic membrane; an anode intake and an anode exhaust in fluid communication with the at least one anode; a cathode intake and a cathode exhaust in fluid communication with the at least one cathode; and a fluid handling system. The fluid handling system includes a cathode side subsystem, and an anode side subsystem. The cathode side subsystem is configured to supply an oxidant to the cathode intake, and to remove exhaust fluids from the cathode exhaust. The anode side subsystem includes a powered blower, and the ejector as described above. The blower and the ejector are arranged to define, respectively, first and second recirculation loops in parallel flow relation. The anode side subsystem is configured: to recirculate an anode fluid through the blower from the anode exhaust to the anode intake in the first recirculation loop; and to supply hydrogen, from a source of hydrogen external to the power supply unit, as the primary fluid to the primary flowpath of the ejector, and via the first flowpath of the ejector to the anode intake. The anode side subsystem is further configured, in the second recirculation loop: to recirculate the anode fluid from the anode exhaust as the secondary fluid to the secondary flowpath of the ejector, and to supply the mixed flow of primary and secondary fluids from the ejector to the anode intake. The unit is operable to produce a variable amount of power, from the proton exchange membrane fuel cell assembly, in an output power range from 0% to 100% of a maximum output power of the unit, the output power range including: a low load range above 0% of the maximum output power, a high load range up to 100% of the maximum output power, and a medium load range between the low load range and the high load range. The anode side subsystem is arranged to control the blower and the supply of hydrogen to the primary flowpath of the ejector: (a) to recirculate the anode fluid through the blower in the first recirculation loop throughout the low load range and the medium load range; and (b) to increase recirculation of the anode fluid through the ejector in the second recirculation loop, while reducing recirculation of the anode fluid through the blower in the first recirculation loop, as the output power of the unit increases in the medium load range; and (c) to recirculate the anode fluid through the ejector in the second recirculation loop, substantially without recirculation through the blower in the first recirculation loop, in the high load range. A method of producing power In a related aspect, the disclosure provides a method of producing power, including providing the power supply unit as described above, and further including steps S2a, S2b, and S2c, as follows. Step S2a: recirculating the anode fluid through the blower in the first recirculation loop throughout the low load range and the medium load range. Step S2b: increasing recirculation of the anode fluid through the ejector in the second recirculation loop, while reducing recirculation of the anode fluid through the blower in the first recirculation loop, as the output power of the unit increases in the medium load range. Step S2c: recirculating the anode fluid through the ejector in the second recirculation loop, substantially without recirculation through the blower in the first recirculation loop, in the high load range. Brief Description of the Drawings Further features and advantages will be appreciated from the various illustrative embodiments which will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying drawings, in which: Fig. 1 is a schematic drawing of a conventional PEMFC power supply unit with a full sized conventional blower for recirculating hydrogen at the anode, and having a maximum output power P of 0.5MW. Fig. 2 is a schematic drawing of the same power supply unit as shown in Fig. 1, except that the conventional blower is replaced by a first ejector in accordance with an embodiment of the disclosure, arranged in parallel with a downsized but otherwise conventional blower, and the control system is modified accordingly. This is referred to herein as the first power supply unit. Fig. 3 shows the internal surfaces of a second ejector, in longitudinal section along its flow axis. Fig. 4 shows the internal surfaces of a third ejector, in longitudinal section along its flow axis. Fig. 5 is an enlarged view of part of the nozzle portion of the second and third ejectors of Figs. 3 and 4, in longitudinal section along the flow axis. Fig. 6 shows the internal surfaces of the first ejector of Fig. 2, in longitudinal section along its flow axis. Fig. 7 is an enlarged view of part of the nozzle portion of the first ejector of Fig. 6, in longitudinal section along the flow axis. Fig. 8 shows the flow inside the second ejector of Fig. 3 as determined by CFD analysis. Fig. 9 shows the flow inside the first ejector of Fig. 6 as determined by CFD analysis. Fig. 10 shows the actual secondary (suction) flow rate Fs relative to the output power P of the power supply unit of Figs. 1 and 2, as determined by a three-dimensional CFD model, comparing the performance of the first ejector (Asub), the second ejector (Bsub), and the full sized blower (T). Fig. 11 shows the suction mass flow rate Fs through the hydrogen recirculation blower as a function of the power output P of the unit, as determined by the three-dimensional CFD model, comparing the full sized blower in the conventional unit of Fig. 1 (T) with the downsized blower in the first unit of Fig. 2 (D). Fig. 12 shows the suction mass flow rate Fs in the medium load range (M) as a function of the power output P of the unit, as determined by the three-dimensional CFD model, respectively: for the first ejector (A) and the downsized blower (D) in the first unit of Fig. 2, and for the full sized blower (T) in the conventional unit of Fig. 1. Fig. 13 shows the suction mass flow rate Fs as a function of the power output P of the unit, as determined by a two-dimensional axisymmetric CFD model, respectively: for the full size blower in the conventional unit of Fig. 1, represented by plot T, and for the third ejector 300 incorporated in the first unit of Fig. 2 in place of the first ejector. The third ejector 300 is represented by a group of 76 plots representing different parameter value combinations within the Basic Range shown in column A of Table 2. Of these, plot A represents the optimal parameter value combination shown in column B of Table 2, and plot A' represents a suboptimal combination as shown in column C of Table 2. Figs. 14, 15 and 16 show three alternative applications of the first, second or third ejector. Fig. 17 shows a method in accordance with an embodiment of the disclosure. Reference numerals and characters that appear in more than one of the figures indicate the same or corresponding elements in each of them. Fig. 1: the PEMFC power supply unit with full sized blower Fig. 1 is a schematic drawing of a conventional PEMFC power supply unit 2500. The conventional power supply unit 2500 includes a proton exchange membrane fuel cell assembly (PEMFC) 501 having at least one anode 510 and at least one cathode 550 separated by at least one electrolytic membrane 530. The PEMFC is formed as a stack of individual fuel cell units, each unit being a thin layered structure with the anode 510 and cathode 550 on opposite sides of the membrane 530. The anode and cathode are electrically conductive and are exposed to fluids, referred to herein as the anode fluid 40 which includes the gaseous hydrogen fuel 20 present at the anode 510, and the oxidant 50, e.g. air, and exhaust fluids 51 which are present at the cathode 550. The anode fluid 20 including hydrogen gas is supplied to the anode via an anode intake 511 and removed via an anode exhaust 512, both in fluid communication with the at least one anode 510. A cathode intake 551 and a cathode exhaust 552 are arranged in fluid communication with the at least one cathode 550. The unit 500 includes a fluid handling system including a cathode side subsystem 560, and a full sized blower 522. The cathode side subsystem 560 is configured to supply the oxidant 50 to the cathode intake 551, and to remove the exhaust fluids 51 from the cathode exhaust 552. In the illustrated example, the oxidant 50 is ambient air which may be drawn from outside the unit 2500 via an ambient air intake filter 561, and the exhaust fluids 51 include unreacted oxygen, nitrogen and water. In the conventional unit 2500 hydrogen recirculation at the anode is provided solely by a conventional blower 522, referred to herein as the full size blower, controlled by the control system 2502. The full sized blower 522 is powered by an electric motor (although it could be powered by any other prime mover), and is arranged to recirculate the anode fluid 40 through the blower 522 from the anode exhaust 512 to the anode intake 511. The anode fluid 40 includes hydrogen gas 20 (H2) supplied from an external source 22 to the anode intake 511, and other fluids, for example, nitrogen (N2) and water (H2O). The unit 2500 is operable to produce a variable amount of output power P from the proton exchange membrane fuel cell assembly 501, in an output power range from 0% to 100% of the maximum output power of the unit, which is 0.5MW in the illustrated example. The output power P is represented in the schematic illustration by the respective electrical terminals at the anode (+) and cathode (-) sides of the stack. A control system 2502 is arranged to control the operation of the full sized blower 522, the hydrogen supply to the anode intake 511 from the external fuel source 22, via a hydrogen supply control 503 (which may include for example a valve and / or a pressure regulator), and other functional elements of the unit 2500 so as to deliver the output power P responsive to the load on the unit. As indicated in Figs. 11 and 12, the output power range includes: a low load range L above 0% of the maximum output power P; a high load range H up to 100% of the maximum output power P; and a medium load range M between the low load range L and the high load range H. The PEMFC assembly or stack 501 is structured as well known in the art and so is not described in detail. The membrane 530 may be any medium permeable by hydrogen cations but not by electrons, particularly a polymer sheet material. Each anode 510 and cathode 550 may include a catalyst distributed on a conductive gas diffusion layer, with contoured plates defining flowpaths and conductors between the layers of the stack. The unit may include a coolant system (not shown) for providing a flow of coolant to coolant flowpaths also formed in the stack. The control system 2502 may include sensors (not shown) for sensing, for example, temperature, pressure, humidity, voltage, current, and / or other parameters at various points in the unit. The control system 2502 may include a processor and a memory including non-transient data executed on the processor to implement the control logic of the unit responsive to the external electrical load (demand) and other sensed parameters. Of course, the unit 2500 may include power management and transformation systems for converting the power output of the stack into useable line power from the unit, which are well known in the art and not further described. The cathode side subsystem 560 may include a purge line 567 to purge a fraction of the recirculated anode fluid 40. The purge line 567 may include a purge valve 568 operable by the control system 2502, 502 to control the flow, e.g. in a periodic manner, so as to remove contaminants from the anode 510 via the anode exhaust 512. The purged fraction of the anode fluid 40 will include hydrogen gas and so may be directed to a combustor 569 where it is combusted with the cathode exhaust fluids 51. The combustion product may be directed through a liquid separator 570 to separate and remove waste liquids 52 such as water or oil before flowing through the turbine 562 of a turbocompressor. The turbocompressor may be provided to manage and equalise pressure across the membrane 530 and to recover energy from the exhaust fluids 51 flowing from the cathode exhaust 552, and may include an electric motor or motor / generator 564 in addition to the compressor 563. The compressor 563 is driven by the electric motor 564 or by the flow of combustion product through the turbine 562 to compress ambient air 50 drawn in through the ambient air intake filter 561. The compressed air 50 may flow via an air conditioning unit 565 to a humidifier / dehumidifier 566. The humidifier / dehumidifier 566 may be cooled by the cooling system (not shown) and operable to manage the humidity of the air 50 flowing to the cathode intake 551, and may also help to remove moisture from the cathode exhaust fluids 51 upstream of the combustor 569. A bypass valve 571 may be operable to manage pressure differentials across the system. These and other features of the power supply unit 2500, 500 not described in detail may be arranged as known in the art, for example, as taught by US 2023387433 A1 or WO 2024086413 A1. Fig.2: the first PEMFC power supply unit including the first ejector and a downsized blower The first PEMFC power supply unit is the same as the conventional unit of Fig.1, also having a maximum power output of 0.5MW, except that the full sized blower 522 is replaced by an anode side subsystem 520 including a powered blower 521 and a first ejector 100, shown in more detail in Figs. 6 and 7, and the control system 502 is modified accordingly. The second ejector 200 or third ejector 300 may be used in place of the first ejector 100, as further discussed below. The blower 521 of the first unit 500 is the same as the full size blower 522 of the conventional unit 2500, except that it is operable to drive a maximum mass flow rate Fs that is less than the maximum mass flow rate of the full sized blower 522, and so is referred to herein as the downsized blower. As can be seen in the second line of Table 2, further explained below, the full size blower 522 in the conventional 0.5MW unit 2500 of Fig. 1 has a maximum mass flow rate of 93.34g / s at full load (P = 100%). In Figs. 12 and 13 plot D shows that the maximum mass flow rate of the downsized blower 521 is 30g / s at just under 25% load, after which point the first ejector 100 (plot A, Fig. 12) or third ejector (plot A, Fig. 13) supplies progressively more of the demanded suction power Fs of the unit, operating without assistance from the downsized blower 521 from not less than about 44% up to 100% of the output power P of the unit 500. Thus, the downsized blower 521 may be operable to recirculate the anode fluid 40 in the first recirculation loop 1521 up to a maximum mass flow rate of not more than one third of a maximum mass flow rate of the recirculation through the ejector 100 in the second recirculation loop 1100. The control system 502 of the first unit 500 is the same as the control system 2502 of the conventional unit 2500, except that it is modified accordingly to control the operation of the downsized blower 521 and the hydrogen supply 22 to the first ejector 100 as further discussed below. The downsized blower 521 and the first ejector 100 are arranged to define, respectively, first and second recirculation loops 1521, 1100 in parallel flow relation. The anode side subsystem 520 is configured to supply hydrogen 20, from the source 22 of hydrogen external to the power supply unit, as the primary fluid 20 to the primary flowpath of the first ejector 100, and via the first flowpath of the ejector 100 to the anode intake 511. The anode side subsystem 520 is further configured to recirculate the anode fluid 40 in the first recirculation loop 1521 and the second recirculation loop 1100. In the first recirculation loop 1521, the anode fluid 40 is recirculated through the downsized blower 521 from the anode exhaust 512 to the anode intake 511. In the second recirculation loop 1100, the anode fluid 40 is recirculated from the anode exhaust 512 as the secondary fluid 40 to the secondary flowpath 140 of the ejector 100, and the mixed flow 2040 of primary and secondary fluids 20, 40 is supplied from the ejector 100 to the anode intake 511. The CFD model The power supply units 500, 2500 and the first, second and third ejectors 100, 200, 300 were modelled using CFD (computational fluid dynamics) software, based on the boundary conditions indicated in Table 1, representative of the 0.5MW example as illustrated. After defining the boundary conditions and the key parameters and parameter relationships, the parameter values were generated and evaluated using an industry standard optimisation software (OptiSLang (RTM) from Ansys Inc of Canonsburg, PA, USA). Two CFD models were used: a two-dimensional axisymmetric CFD model, representing the third ejector 300 of Fig. 4; and a more computationally expensive three-dimensional CFD model, which was used to represent each of the first and second ejectors 100, 200. Each of the models was based on the boundary conditions as defined in Table 1, so representing the respective ejector 100, 200, 300 as incorporated into the second unit 20 of Fig. 2 in place of the first ejector 100 as illustrated. The two-dimensional axisymmetric model was less computationally intensive and so was used to generate a large number of data sets used for the optimisation analysis, including the 76 individual plots that were selected by the optimisation algorithm as shown in Fig. 13. The plots shown in Figs.10, 11 and 12 were generated by the three- dimensional model based respectively on the first and second ejectors 100, 200 as further discussed below. However, in each model, the ejector was characterised by the parameter values as listed in Table 2. The results obtained from the two-dimensional axisymmetric model of the third ejector 300 were found to closely, but not exactly, match the results obtained from the three-dimensional model of the first ejector 100, both of which, when modelled based on the same parameter value set, showed a better performance than the second ejector 200, due to the coaxial flow at the suction inlet as further discussed below. Thus, the two-dimensional axisymmetric model was a good predictor of the results obtained from the three-dimensional model. The difference (and close similarity) between the first and third ejectors as modelled respectively in the two- and three-dimensional CFD analysis can be appreciated by comparing plot A in Figs. 12 and 13, respectively. In Fig. 12, plot A represents the first ejector 100 having the optimal parameter value set in column B of Table 2. In Fig. 13, plot A represents the third ejector 300 also having the optimal parameter value set in column B of Table 2. It can be seen that the two plots are closely similar, but intersect plot T, representing the target value Fs of the full size blower (line 2 of Table 1) at different values of the power output P. In Fig. 12, plot A (first ejector 100) intersects T at P=44%. In Fig. 13, plot A (third ejector 300) intersects T at P=41.1%. It is believed that the apparent difference in performance reflects the difference between the two- and three-dimensional models rather than any significant difference in performance between the first and third ejectors 100, 300. In contrast, Fig.10 shows that the first and second ejectors 100, 200 exhibit a real difference in performance in the three-dimensional model, as further discussed below. Table 1 Power supply unit boundary conditions, constraints, and Target

[0002] Table 1 shows a full set of boundary conditions and constraints for the power supply unit 2500, 500 of Figs. 1 and 2, which was used as the basis of the CFD model to generate the plots and flow patterns as indicated in Figs. 8 - 13. In the first line of the table, the electrical load on the unit is represented by its output power P as a percentage of its maximum output power. The maximum output power of the unit is 0.5MW of electrical power, corresponding to P = 100%. The Target values for the suction (secondary) mass flow rate Fs in the second line of the table correspond to the actual mass flow rate Fs through the full size blower 522 in the modelled conventional unit 2500 of Fig. 1 in grams per second, as further discussed below. These values were used as target values for the optimisation algorithm and are indicated by the plot T in each of Figs. 10 - 13. The model assumes that the primary fluid 20 is hydrogen gas (H2), and the secondary (anode) fluid 40 is a mixture of hydrogen gas (H2), nitrogen gas (N2), and water (H2O). The model assumes that the primary fluid (hydrogen gas) 20 is supplied to the primary flowpath 120 of the ejector via the hydrogen supply control 503 at a maximum static pressure of less than 15bar-a (15 bar absolute). This pressure is determined by practical considerations in any given system, principally safety limitations, and can be controlled by the hydrogen supply control 503. In practice, the supply of hydrogen 20 will typically be at a pressure of at least 4 bar-a, preferably at least 6.4 bar-a, up to a maximum of 16 bar-a to 18 bar-a. A slightly improved entrainment ratio may be expected at 18 bar compared with the 15 bar pressure on which the model is based. The suction inlet boundary pressure is defined for the first and conventional units 500, 2500 at the anode exhaust 512, and as a boundary condition when modelling the first and second ejectors 100, 200 at the upstream axial end 141, 241 of the secondary flowpath. The ejector outlet discharge pressure is defined as a boundary condition when modelling the first and second ejectors 100, 200 at the downstream end 106, 206 of the divergent portion. Suction (secondary) mass flow rate Fs The full size blower 522 of the conventional unit 2500 of Fig. 1 is specified to deliver the Target value for the secondary (suction) mass flow rate Fs as shown in the second line of Table 1. This Target value is determined as known in the art to provide optimal efficiency by clearing water and contaminants from the anode 510 and maintaining a constant supply of hydrogen over its whole surface area. This allows the fuel cell 501 to operate normally by modulating the power output P as required to respond to rapid changes in the electrical load on the unit 2500. It can be seen that the full sized blower 522 is operable to recirculate hydrogen 20 through the anode 510 of the fuel cell 501 at up to a maximum mass flow rate of 93.34 grams per second at the maximum, 100% load (power output) P of the unit 2500. The Target value for Fs shown in the second line of Table 1 is represented by plot T in each of Figs. 10, 11, 12, and 13. In the first unit 500 of Fig. 2, the mass flow rate of hydrogen 20 from the supply 22 via the primary flowpath 120 of the ejector 100 to the stack 501 corresponds to the power output P of the stack 501; as power output P increases, mass flow rate increases. The actual value of the secondary (suction) flow rate Fs is governed by the primary (motive power) flow rate and the entrainment ratio, which in turn depends on the geometry of the first ejector 100. The suction or secondary mass flow rate Fs, as indicated in each of Figs. 10 - 13, represents the actual mass flow rate through the anode 510. Plot T in each of Figs. 10 - 13 represents the mass flow rate Fs driven by the full sized blower 522 in the conventional unit 2500 of Fig. 1. Plot A, Fig. 12, and plot Asub, Fig. 10 represent the mass flow rate Fs driven by the first ejector 100 in the first unit 500 of Fig. 2. Plot Bsub, Fig. 10 represents the mass flow rate Fs driven by the second ejector 200 in the first unit 500 of Fig. 2. Plots A and A', Fig. 13 represent the mass flow rate Fs driven by the third ejector 300 in the first unit 500 of Fig. 2. Plot D in Figs. 11 and 12 represents the mass flow rate Fs driven by the downsized blower 521 in the first unit 500 of Fig. 2. It will be understood that the total mass flow rate Fs in the first unit 500 of Fig. 2 corresponds to the mass flow rate driven by the downsized blower 521 acting on its own, in the low load range L; to the mass flow rate driven by the first, second or third ejector 100, 200, 300 in combination with the downsized blower 521 in the medium load range M; and to the mass flow rate driven by the first, second or third ejector 100, 200, 300 acting without assistance from the downsized blower 521 in the high load range H (as indicated by the arrow Fs in Figs. 3, 4 and 6.) Figs. 11 and 12: Low, Medium, and High load ranges Figs. 11 and 12 illustrate the operation of the first ejector 100 in parallel with the downsized blower 521 through the Low, Medium, and High load ranges L, M, H of the first power supply unit 500. Referring to Fig. 12, plot A represents the performance of the first ejector 100 at the optimal parameter value combination shown in column B in Table 2, as further explained below. At the optimal combination of parameter values represented by plot A, it can be seen that the first ejector 100 is able to make a useful contribution to the recirculating flow through the anode 510 over a surprisingly large proportion of the range of power output P of the unit 500, starting from about 25% of the power output P, and can supply the entire recirculation flow through the anode 510 without assistance from the downsized blower 521 in the high load range H from 44% to 100% of the maximum power output of the unit. Plot D shows the suction mass flow rate Fs through the downsized blower 521 over the same range of power output P, which declines progressively as the flow through the first ejector 100 increases. When the power output of the unit 500 rises to P=44% the power supply to the downsized blower 521 can be disconnected, leaving the first ejector 100 to supply the entire recirculating flow up to the maximum power output P=100%. The sub-optimal parameter value combinations represented by plots Asub and Bsub in Fig. 10 and by plot A' in Fig. 13 also indicate that the first, second or third ejector 100, 200, 300, having parameter values lying within the Basic Range in column A of Table 2, is able to supply the entire recirculating flow from a value at or below P=50% up to P=100%. By sub-optimal is meant that at this parameter value combination, the power output P at which the ejector 100 is able to supply the whole recirculating flow through the anode 510, marking the beginning of the high load range H, is higher than could be achieved by an optimal combination of parameter values, represented by plot A in Figs. 12 and 13, as further discussed below. By selecting the parameter values of the first, second or third ejector 100, 200, 300 within the Basic Range as indicated in column A of Table 2, further discussed below, the first, second or third ejector 100, 200, 300 can therefore be arranged to operate in parallel with the downsized blower 521 and to progressively supply the suction power Fs demanded by the unit 500 as the power output P increases, so that the power to the electric motor driving downsized blower 521 can be reduced progressively through the medium load range M and then switched off at the start of the high load range H. Accordingly, the anode side subsystem 520 (including the control system 502) is arranged to control the blower 521 and the supply of hydrogen 20 to the primary flowpath 120 of the ejector 100, 200, 300 in accordance with the control logic of the unit 500, which is as follows. Throughout the low load range L and the medium load range M, the anode fluid 40 is recirculated through the downsized blower 521 in the first recirculation loop 1521. As the output power P of the unit 500 increases in the medium load range M, recirculation of the anode fluid 40 through the ejector 100, 200, 300 in the second recirculation loop 1100 increases, while recirculation of the anode fluid 40 through the blower 521 in the first recirculation loop 1521 is reduced. The change in flow rate (and the change in power supplied to the downsized blower 521) may be progressive and smoothly (infinitely) variable through the medium range M as shown, although stepwise change is also possible. In the high load range H, the anode fluid 40 is recirculated through the ejector 100, 200, 300 in the second recirculation loop 1100, substantially without recirculation through the blower 521 in the first recirculation loop 1521. That is to say, the ejector 100, 200, 300 operates substantially without assistance from the downsized blower 521, although the downsized blower need not necessarily be disconnected from the power supply. In practice, the power supply to the electric motor or other prime mover of the downsized blower 521 can be managed in any way as appropriate to the equipment to obtain safe and efficient operation of the unit 500, and need not be disconnected at the beginning of the high load range H. For example, the downsized blower 521 could be left idling or running at a standby power level through all or part of the high load range H. A valve or other means (not shown) may also be arranged to prevent reverse flow through the downsized blower 521 during operation of the ejector 100, 200, 300. Preferably, as illustrated, the anode side subsystem 520 (including the control system 502) is arranged to control the downsized blower 521 and the supply of hydrogen 20 to the primary flowpath 120 of the ejector 100, 200, 300, to recirculate the anode fluid 40 through the ejector 100, 200, 300 in the second recirculation loop 1100, substantially without recirculation through the downsized blower 521 in the first recirculation loop 1521, throughout all of the high load range H up to 100% of the maximum output power P of the unit 500. Referring to Fig.11, plot T represents the suction mass flow rate Fs through the anode 510 of the conventional unit 2500 as shown in the second line of Table 1, while plot D represents the suction mass flow rate Fs through the downsized blower 521 in the first unit 500 of Fig. 2. It can be seen that the downsized blower 521 supplies the same suction mass flow rate Fs through the anode 510 as the full size blower 522 throughout the low load range L, up to a maximum suction mass flow rate Fs = 30g / s at just under 25% P. The suction mass flow rate Fs through the downsized blower 521 then starts to decline as the power output P increases through the medium range M. Accordingly, compared with the maximum suction mass flow rate Fs = 93.34g / s of the full size blower 522, the downsized blower 521 is designed to supply a maximum mass flow rate Fs of less than one third of the maximum mass flow rate Fs of the full size blower 522. Preferably, as shown, the medium load range M ends, and the high load range H begins, at an output power P of not more than 44% of the maximum output power of the unit 500. As shown in Figs 12 and 13 and further discussed below, by adopting the optimal parameter value combination (plot A, corresponding to column B in Table 2), the first or third ejector 100, 300 incorporated in the first unit 500 can reduce the point at which the medium load range M ends, and the high load range H begins, (which is to say, the point at which the downsized blower 521 can stop operating), to a surprisingly low value of P = 44% (as indicated by the three-dimensional model) or P = 41.1% (as indicated by the two-dimensional model.) The first ejector 100 Referring now to Figs. 6 and 7, the first ejector 100 as incorporated in the first unit 500 of Fig. 2 is shown in more detail. The ejector 100 includes a first flowpath 101, 102, 103 for carrying a mixed flow 2040 of primary and secondary fluids 20, 40 along a flow axis Fx in a flow direction F. The first flowpath 101, 102, 103 is bounded by a first internal surface 101', 102', 103', which is a surface of revolution about the flow axis Fx. When considered in the flow direction F, the first flowpath includes: a convergent portion 101; a cylindrical throat 102 downstream of the convergent portion 101 and having a length Tl and a diameter Td; and a divergent portion 103 (also referred to as the diffuser) downstream of the throat 102 and having a length Dl and a divergence angle or half cone angle Da between the first internal surface 103' and the flow axis Fx. The ejector 100 further includes a secondary flowpath 140 upstream of the convergent portion 101 for conducting the secondary fluid 40 to the convergent portion 101; and a primary flowpath 120. The primary flowpath 120 includes a nozzle portion 121, which has a minimum diameter Nd and terminates at a nozzle orifice 122 to discharge the primary fluid 20 as a jet 21 from the nozzle orifice 122. The nozzle orifice 122 is coaxial with the flow axis Fx and upstream of the throat 102, and is separated from the upstream end 104 of the throat 102 by a separation distance NTl along the flow axis Fx. In this specification, the term "cylindrical" refers to a right cylinder - which is to say, a circular cylinder, wherein the cylindrical boundary is defined by a circle projected along the perpendicular axis of the circle. As shown in Fig. 6, the throat 102 has an upstream end 104 at the downstream end of the convergent portion 101, and a downstream end 105 at the upstream end of the divergent portion 103. The upstream and downstream ends 104, 105 of the throat 102 can be identified at the angular discontinuity between the respectively, convergent and divergent first internal surface 101', 103' of the convergent and divergent portions 101, 103, and the first internal surface 102' of the throat 102. The first internal surface 103' of the divergent portion 103 may be frustoconical. The first internal surface 101' of the convergent portion 101 may be frustoconical. Although as illustrated in Fig. 2 the pipework includes a bend immediately downstream of the outlet 106 of the divergent portion, preferably the pipework connected to the downstream end 106, 206, 306 of the divergent portion of the ejector 100, 200, 300 is straight and axially aligned with the flow axis Fx for at length equal to a few times the diameter at the outlet 106, 206, 306. The straight portion may be cylindrical and equal in diameter to the diameter at the outlet 106, 206, 306. This was assumed to be the case in the CFD model and is recommended in any practical application to avoid performance losses. Optionally, a transition portion may be arranged immediately downstream of the outlet 106, 206, 306 of the divergent portion, in which the frustoconical first internal surface 103', 203', 303' of the divergent portion 103, 203, 303 blends smoothly into the internal surface of the downstream pipework. Although not illustrated, the generally frustoconical first internal surface 101', 201', 301' of the convergent portion 101, 201, 301 of the first, second or third ejector 100, 200, 300 may include a short, locally curved region that blends into the upstream end 104, 204, 304 of the cylindrical throat 102, 202, 302. This can help to minimize flow losses and avoid flow detachment from the first internal surface 102', 202', 302'. Preferably, the downstream end 105, 205, 305 of the throat 102, 202, 302 of the first, second or third ejector 100, 200, 300 marks an angular transition between the first internal surface 102', 202', 302' of the throat and the first internal surface 103', 203', 303' of the divergent portion 103, 203, 303. However, if alternatively there is a blended transition betweeen those first internal surfaces, then the downstream end 105, 205, 305 of the throat 102, 202, 302 is nevertheless taken to be the point at which the cylindrical first internal surface 102', 202', 302' of the throat 102, 202, 302 ends, and so any blended region will be included in the length Dl of the divergent portion 103, 203, 303. In accordance with the disclosure, the parameter values of the ejector 100, 200, 300 fall within the Basic Range as shown in column A of Table 2, further discussed below, which is to say: (Td / Nd) and and and 4.5° 2.0° and Note that the nozzle diameter Nd is selected to suit the maximum power output P of the unit as further discussed below, and so need not fall within the mentioned values which relate only to the example 0.5MW unit used for the CFD model. Most preferably the optimal parameter values are selected as listed in column B of Table 2, which is to say: (Td / Nd) = 5.4 and . . The effect of the remaining parameter value (Dl / Td) is further discussed below. This parameter value can be selected anywhere above the minimum value of (Dl / Td) Irrespective of the other selected parameter values, preferably (Dl / Irrespective of the parameter values selected within the Basic Range, preferably the performance of the ejector is improved by providing a coaxial suction inlet, as defined and further discussed below and as illustrated by the first ejector 100 of Figs. 6 and 7. A similar effect is provided by arranging two suction inlets symmetrically on either side of the flow axis Fx, as illustrated by the third ejector 300 of Fig. 4. Referring to Fig. 6, the convergent portion 101 defines a convergence angle Ca between the first internal surface 101' and the flow axis Fx. In order to minimise flow losses, preferably Ca = 5°. Referring particularly to Fig. 7, the primary flowpath 120 is bounded by a primary internal surface 120', which preferably is a surface of revolution about the flow axis Fx. As illustrated, when considered in the flow direction F, the nozzle portion 121 of the primary flowpath 120 may include a minimum diameter part 121'' that defines the minimum diameter Nd, a convergent part 121' upstream of the minimum diameter part 121'', and a divergent part 121''' downstream of the minimum diameter part 121'', wherein the divergent part 121''' extends from the downstream end of the minimum diameter part 121'' to the nozzle orifice 122. The divergent part 121''' defines a nozzle divergence angle Nad between the primary internal surface 120' and the flow axis Fx. For optimal performance, preferably Nad = 2.5°. Table 2 Ejector: key parameters and parameter values Table 2 lists the key parameters, which is to say, the most significant parameters that are found to determine the performance of the first, second and third ejectors 100, 200, 300, as shown in Figs. 3 - 7. The Basic Range in column A of Table 2 represents the parameter value combinations that provide a suction mass flow rate Fs through the first or third ejector 100, 300 in the first unit 500 of Fig. 2, at or above the benchmark or Target value of the full size blower 522 in the conventional unit 2500 of Fig. 1 as shown in the second line of Table 1, as calculated in the CFD model, throughout the range from 50% to 100% of the output power P of the unit. The optimisation algorithm identified a total of 76 parameter value combinations satisfying this condition, all represented in the graph of Fig. 13, including the optimal combination represented by plot A as listed in column B of Table 2. In Table 2, the figures given for the nozzle diameter Nd are only exemplary, corresponding to the flow rate to the stack 501 of the first unit 500 as illustrated, and assuming a hydrogen supply pressure in the range mentioned above. The maximum output power P of the illustrated first unit 500 is 0.5MW, the same as the conventional unit 2500, and for this power output, most preferably Nd = 4mm which is found to be the optimal value as shown in column B of Table 2. The first unit 500 can be scaled for any desired maximum output power P, whether larger or smaller than the 0.5MW illustrated example, in which case the nozzle diameter Nd and the downsized blower 521 will be proportionately larger or smaller to deliver the required flow to the stack 501. (A larger unit will require more flow and so a larger nozzle diameter Nd; a smaller unit will require less flow and so a smaller nozzle diameter Nd.) Irrespective of the size of the unit and the nozzle diameter Nd, the remaining parameter values of the first, second or third ejector 100, 200, 300 are selected to correspond to those shown in the Basic Range of column A in Table 2, and most preferably will be the optimal values as shown in column B in Table 2. Once the required value of Nd is defined to supply the required flow rate to the stack 501, the remaining parameter values including Td, Tl, Dl and NTl can be calculated based on the values in Table 2. It will be noted that no optimal value is given in column B for the parameter value (Dl / Td). This is because this parameter value can be selected freely above the minimum value of 10, more preferably 12, to suit the overall size and form factor of the unit 500, with progressively smaller effect on performance as the value increases. The parameter values of column D lie within the Basic Range as shown in column A, but are sub-optimal (which is to say, not optimal.) These parameter values are purely exemplary, and are shown because they were used for the CFD model that generated the plots shown in Fig. 10, further discussed below. By way of further example, column C gives another sub-optimal set of parameter values corresponding to plot A' as shown in Fig. 13. Fig. 13: parameter value selection The optimisation algorithm selected for parameter value combinations that met the design objective to maintain the actual secondary (suction) flow rate Fs at a value above the Target secondary (suction) flow rate Fs shown in the second line of Table 1, over the widest possible range of power output P, and up to the maximum power output P = 100% of the unit (corresponding to the maximum mass flow rate of 93.34 grams per second). Fig. 13 shows how the actual secondary (suction) mass flow rate Fs (here expressed in kg / s), as determined by the CFD model, varies with the load on the unit 500, 2500, represented by its output power P (as a percentage of the maximum output power of the unit), for different sets of key parameter values. Plot T represents the Target secondary (suction) mass flow rate Fs produced by the full size blower 522 in the conventional unit 2500 of Fig. 1, as shown in the second line of Table 1. Plot A represents the actual secondary (suction) mass flow rate Fs generated solely by the third ejector 300 having the optimal value combination identified by the optimisation algorithm, as shown in Column B of Table 2, wherein Nd = 4.0mm. Compared with the remaining plots, plot A remains above the target value T for the greatest part of the load range P. That is to say - at the optimal value combination, plot A, the third ejector 300 can operate without the downsized blower 521 to supply the Target suction mass flow rate Fs demanded by the unit 500 over the largest possible part of the load range P of the unit. For the other value combinations, represented by the remaining plots on the graph (75 in total), it is necessary to operate the downsized blower 521 up to relatively higher load points before the third ejector 300 is able to drive the required flow rate without assistance. Each of the remaining plots represents the actual secondary (suction) mass flow rate Fs generated solely by the third ejector 300 having one respective set of parameter values. For each plot, the set of parameter values comprises one discrete value for each of the six key geometric parameters of the third ejector 300, lying within the Basic Range as listed in column A of Table 2, wherein Nd lies in the range from 3.8mm to 4.2mm. The value sets were selected by the optimisation algorithm, taking the performance of the full sized blower 522 (plot T) as a target value, and represent the best identified performance characteristics. The Basic Range of column A in Table 2 represents, for each of the key parameters, the range of values contained within these value sets identified by the optimisation algorithm as shown in Fig. 13. In the first unit 500 of Fig. 2, the optimal ejector (which is to say, the first ejector 100 of Figs. 6 and 7 having the coaxial suction inlet, or the third ejector 300 of Figs. 4 and 5 having the symmetric suction inlets, each ejector embodying the optimal parameter values of Col. B in Table 1) generated sufficient suction power to supply the full recirculation mass flow demand of the fuel cell 501 over a range from 41.1% to 100% of the maximum power output of the unit 500, as indicated by the two dimensional model of Fig. 13, or from 44% to 100% P as indicated in the three dimensional model of Fig. 12. That is to say, where the unit 500 is equipped with the optimal ejector 100 or 300, the high load range H in which the ejector 100, 300 operates without assistance from the downsized blower 521 is from 41.1% or 44% to 100% of the maximum power output P of the unit 500. The modelled performance of the optimal ejector 100 or 300 is represented by plots A in Figs. 12 and 13. Table 3 Coefficient of importance Table 3 indicates the coefficient of importance, an indication of the relative contribution of each of the listed parameter values to the overall performance of the first, second or third ejector 100, 200, 300. In case it is desired to adjust one or more parameter values, e.g. to accommodate an external constraint, a new value combination can be selected by means of an optimisation algorithm. Optimisation algorithms per se are well known in the art. The coefficient of importance values shown in Table 3 can be used as known in the art to assist in tuning the selected algorithm to arrive more easily at the new optimal value combination. Note that it is possible to obtain a slightly better performance (so a slightly lower starting point for the high load range H) by elongating the divergent portion (diffuser) 103 to the optimal value at the upper end of the value range for the ratio of diffuser length to throat diameter (Dl / Td), as further discussed below. The preferred value combination took into account a practical constraint on this parameter so as to obtain a more compact ejector. The effect on efficiency is modest, as indicated by the coefficient of performance. Coaxial suction inlet Referring again to Figs.6 and 7, preferably, as illustrated by the first ejector 100, in addition to the parameter values which lie within the Basic Range as shown in column A of Table 2, the ejector in accordance with the disclosure further includes a coaxial suction inlet, which is defined as follows. The nozzle orifice 122 is located inside the convergent portion 101. The secondary flowpath (suction inlet) 140 is bounded by a second internal surface 140', which is a surface of revolution about the flow axis Fx. The second internal surface 140' may be cylindrical, as illustrated. The ejector 100 further includes a primary flow pipe 123 coaxial with the flow axis Fx, the primary flow pipe 123 extending within the secondary flowpath 140 and defining the primary flowpath 120. primary feed path 110 is arranged upstream of the primary flowpath 120, for conducting the primary fluid 20 to the primary flowpath 120. A secondary feed path 130 is arranged upstream of the secondary flowpath 140 to conduct the secondary fluid 40 to an upstream axial end 141 of the secondary flowpath 140. This concludes the definition of the coaxial suction inlet as referred to herein. When compared with the second ejector 200 of Figs.3 and 5, which does not have a coaxial suction inlet as defined above, it will be appreciated that the coaxial suction inlet of the first ejector 100 more effective aligns the jet 21 of primary fluid issuing from the nozzle orifice 122 with the length axis Fx of the first flowpath, as shown in Figs. 8 and 9, and this is found to provide a significant improvement in the entrainment ratio. Compared with the duplicated suction inlet of the third ejector 300, the coaxial suction inlet of the first ejector 100 requires only a single connection for the secondary flowpath 140 and so provides a more convenient arrangement for integration into the second unit 200 of Fig. 2 or other practical application. Referring again to Fig. 6, the secondary flowpath 140 has a length Sl and a diameter Sd. By way of comparison, Figs. 3 and 5 show the second ejector 200 which corresponds generally to the first ejector 100, having parameter values which lie within the Basic Range as shown in column A of Table 2, except that it does not have a coaxial suction inlet as defined herein. The second ejector 200 is of generally conventional design, except for special features (i) and (ii) identified below. Like the first ejector 100, the second ejector 200 includes a primary flowpath having a nozzle portion 221 with a nozzle orifice 222 and a minimum diameter Nd, and a first flowpath including a convergent portion 201 defined by its first internal surface 201', a cylindrical throat 202 defined by its first internal surface 202' and having an upstream end 204, and a divergent portion 203 defined by its first internal surface 203', all axially aligned along the flow axis Fx. The parameter values of the second ejector 200 are defined as for the first ejector 100 by reference to its corresponding features, as indicated in Figs. 3 and 5. The special features (i) and (ii) of the second ejector 200 are as follows. (i) When considered in longitudinal section, as shown, the secondary flowpath (suction inlet) 240 of the second ejector 200 is inclined at 45 degrees to the flow axis Fx to provide somewhat better performance relative to a typical prior art design in which the suction inlet would be perpendicular to the flow axis Fx. (ii) The parameter values of the second ejector 200 are the same as those of the first ejector 100, lying within the Basic Range of column A in Table 2. Figs. 4 and 5 show the third ejector 300 which corresponds generally to the first ejector 100, also having parameter values which lie within the Basic Range as shown in column A of Table 2, except that it has symmetrically arranged, dual suction inlets as shown, both oriented at the same oblique angle as the single suction inlet of the second ejector 200. Like the first ejector 100, the third ejector 300 includes a primary flowpath having a nozzle portion 321 with a nozzle orifice 322 and a minimum diameter Nd, and a first flowpath including a convergent portion 301 defined by its first internal surface 301', a cylindrical throat 302 defined by its first internal surface 302' and having an upstream end 304, and a divergent portion 303 defined by its first internal surface 303', all axially aligned along the flow axis Fx. The parameter values of the second ejector 300 are defined as for the first ejector 100 by reference to its corresponding features, as indicated in Figs. 4 and 5. Figs.8 - 10 illustrate the advantageous improvement in entrainment ratio (hence, suction mass flow rate Fs) obtained by virtue of the coaxial suction inlet of the first ejector 100 compared with the second ejector 200. shows the actual secondary (suction) flow rate Fs relative to the output power P of the PEMFC power supply unit 500, 2500, as determined by the CFD model, comparing the performance of the first and second ejectors 100, 200 when incorporated into the first unit 500 of Fig. 2, with that of the full sized blower 522 in the conventional unit 2500 of Fig.1. Plot Asub represents the performance of the first ejector 100 of Figs. 6 and 7 incorporated into the first power supply unit 500 as shown in Fig.2, having a nozzle diameter Nd = 4.0mm and sub-optimal parameter values as shown in Column D of Table 2. Plot Bsub represents the performance of the second ejector 200 of Figs. 3 and 5 when incorporated into the first power supply unit 500 of Fig. 2 in place of the first ejector 100, and having the same nozzle diameter Nd = 4.0mm and sub-optimal parameter values shown in Column D of Table 2. Plot T represents the performance of the full size blower 522 of the conventional power supply unit 2500 of Fig. 1, corresponding to the Target value of Fs in the second line of Table 1. The secondary (suction) mass flow rate Fs is shown in grams per second against the output power P of the unit 500, 2500 as a percentage of its maximum output power. It can be seen that the optimal (coaxial) suction inlet of the first ejector 100 raises the secondary (suction) flow Fs by about 10% when compared with the slightly enhanced suction inlet arrangement of the second ejector 200. Since the secondary (suction) flow Fs for the second ejector 200 (plot Bsub) lies above plot T (representing the benchmark performance of the full size blower 522) over the entire high load range from just under 50% to 100% of the maximum power output P of the unit, it can be appreciated that an ejector adapted with parameter values within the Basic Range as defined in column A of Table 2 can be used to replace the blower in a PEMFC power supply unit, even without the optimal coaxial suction inlet geometry. Figs. 8 and 9 Fig. 8 shows the gas velocity contours obtained from the three- dimensional CFD model of the second ejector 200 of Figs.3 and 5, illustrating how the primary nozzle jet 21 is deflected towards the suction flow inlet, degrading the suction performance of the ejector 200. Fig. 9 shows the gas velocity contours obtained from the three- dimensional CFD model of the first ejector 100 of Figs. 6 and 7, in which the primary nozzle jet 21 remains coaxial with the length axis Fx of the ejector 100. By adding the optimal coaxial suction inlet geometry as illustrated in Fig. 6, the target suction flow rate Fs can be supplied by the first or third ejector 100, 300 without assistance from the downsized blower 521 over a wider high load range from as little as 41.1% (plot A, Fig. 13) or 44% (plot A, Fig. 12) to 100% of the maximum power output P of the unit. The size of the blower can be reduced accordingly without degrading the performance of the unit. Diffuser length, final convergent portion The value (Dl / Td) has relatively small effect on performance, but must be at least 10, and preferably is at least 12. It can be increased indefinitely, and the higher the value, the better the performance. However, increasing (Dl / Td) increases the overall length of the ejector. The improvement in performance obtained by increasing (Dl / Td) to a value greater than 36 is very small, and the improvement obtained by increasing (Dl / Td) to a value greater than 40 is negligible. Preferably therefore, for a reasonably compact installation, (Dl / Td) At the upper end of this value range, the entrainment ratio is about 1% - 2% higher than at the lower end of the range. The upper end of the range drives the geometry of the ejector 100, 200, 300 to a relatively long overall length, and a relatively large diameter at the outlet end 106, 206, 306 of the divergent portion or diffuser 103, 203, 303. In the first ejector 100 as illustrated in Figs. 6 and 7, the divergent portion or diffuser 103 is truncated at a point selected to have a convenient diameter at the outlet end 106, resulting in a value of (Dl / Td) = 12.64 for the ratio of diffuser length to throat diameter. For any given value of Nd, the greater the value selected for (Dl / Td), the longer will be the divergent portion 103 and the larger the diameter of the outlet 106. Optionally, in order to reduce the diameter of the outlet 106, 206, 306 to a desired value for fluid connection to the system pipework, a final convergent portion (not shown) can be arranged downstream of the outlet 106, 206, 306 of the diffuser 103, 203, 303, coaxial with the central axis Fx of the ejector 100, 200, 300. This does not reduce the efficiency of the ejector 100, 200, or 300. Industrial Applicability By appropriately selecting the nozzle diameter Nd and determining the other key parameters as indicated in Table 2, the novel ejector 100, 200, 300 can be used to recirculate hydrogen at the anode of a PEMFC at any desired maximum output power. In this application, the advantageously high entrainment ratio of the novel ejector 100, 200, 300 over a surprisingly wide range of suction mass flow rate Fs, especially when configured with a coaxial suction inlet as illustrated by the first ejector 100, or a double suction inlet as illustrated by the third ejector 300, makes it possible for the ejector to drive the recirculating flow at the anode without assistance from the blower over the whole high load range from as little as 44% or even 41.1%, up to 100% of the maximum output power P of the power supply unit. This makes it possible to reduce the maximum capacity (mass flow rate) of the blower 521 to less than one third of what would be required for the same power supply unit without the ejector. The downsized blower 521 is substantially less expensive and more compact than the full size blower 522 of the conventional unit 2500 without the ejector, and the overall energy efficiency of the power supply unit 500 is improved. Fig. 17 illustrates a method of producing power by means of the novel ejector 100, 200, 300. Step S1 comprises providing a power supply unit 500 including the novel ejector 100, 200, 300 as earlier described. The unit 500 is then operated in accordance with Steps S2a, S2b, and S2c, which can be embodied in the logic of the control system 502, for example, as non-transient data stored in the memory and executed on the processor. The steps can be executed individually and / or simultaneously in any appropriate order, responsive to the fluctuating electrical load on the unit 500. Step S2a comprises recirculating the anode fluid 40 through the downsized blower 521 in the first recirculation loop 1521 throughout the low load range L and the medium load range M. Step S2b comprises increasing recirculation of the anode fluid 40 through the ejector 100, 200, 300 in the second recirculation loop 1100, while reducing recirculation of the anode fluid 40 through the downsized blower 521 in the first recirculation loop 1521, as the output power P of the unit 500 increases in the medium load range M. Step S2c comprises recirculating the anode fluid 40 through the ejector 100, 200, 300 in the second recirculation loop 1100, substantially without recirculation through the blower 521 in the first recirculation loop 1521, in the high load range H. Summary In summary, in embodiments, an ejector (100, 200, 300) includes a convergent portion (101, 201, 301); a cylindrical throat (102, 202, 302) downstream of the convergent portion and having a length Tl and a diameter Td; and a divergent portion (103, 203, 303) downstream of the throat and having a length Dl and a divergence angle Da between its internal surface and the flow axis (Fx). The primary nozzle has a minimum diameter Nd and an orifice (122, 222, 322) coaxial with the flow axis (Fx) and separated from an upstream end of the (Td / Nd) and [ 4.5° and [(Dl / Td) . Preferably the secondary or suction inlet is coaxial with a primary flowpipe (123) defining the primary nozzle for a distance upstream of the nozzle orifice. The ejector (100, 200, 300) may be arranged in parallel with a downsized blower (521) to recirculate hydrogen at the anode (512) of a proton exchange membrane fuel cell power supply unit (500), wherein the ejector operates alone in a high load range (H) of the unit, and progressively replaces the operation of the blower (521) through a medium load range (M) which may be from about 25% to 44% of the maximum power output P of the unit. Other potential applications for the novel ejector The novel ejector 100, 200, 300 has potential applications other than in fuel cells. Fig. 13 illustrates how the first, second or third ejector 100, 200, 300 in accordance with an embodiment of the disclosure may be employed to assist the compressor 601 in a compression cycle refrigeration system 600, making it possible to use a less powerful compressor 601. The ejector 100, 200, 300 is arranged to entrain vapour from the evaporator 602 at the suction inlet, with the high pressure liquid returning from the condenser 603 acting as the primary fluid 20 to produce motive power. The combined flow 2040 is directed through a separator 604, from which the gaseous component flows to the inlet of the compressor 601, and the liquid component flows through a throttle valve or valves 605 to the evaporator 602. Fig. 14 illustrates another potential application in which the first, second or third ejector 100, 200, 300 may be arranged to replace a conventional first stage compressor 702 to produce natural gas from low pressure wells 701. A fraction of the downstream produced flow from the conventional second stage compressor 703 (which is the only compressor required in the novel arrangement) is diverted to provide the primary flow 20 to the ejector 100, 200, 300. The low pressure gas is supplied as the secondary fluid 40 to the suction inlet, and the combined flow 2024 is directed to the inlet of the compressor 703. Fig. 15 illustrates another possible application in which the first, second or third ejector 100, 200, 300 is driven by the primary flow 20 from a high pressure gas well 801 to entrain the produced gas from low pressure gas wells 802 as the secondary fluid 40 via the suction inlet. A choke 803 may be arranged in parallel with the ejector. Many further applications and adaptations are possible within the scope of the claims. In the claims, reference numerals and characters are provided in parentheses, purely for ease of reference, and should not be construed as limiting features.

Claims

Claims 1. An ejector (100, 200, 300) including a first flowpath (101, 102, 103) for carrying a mixed flow (2040) of primary and secondary fluids (20, 40) along a flow axis (Fx) in a flow direction (F); the first flowpath (101, 102, 103) being bounded by a first internal surface (101', 102', 103'), the first internal surface (101', 102', 103') being a surface of revolution about the flow axis (Fx); the first flowpath (101, 102, 103) including, when considered in the flow direction (F): a convergent portion (101); a cylindrical throat (102) downstream of the convergent portion (101) and having a length Tl and a diameter Td; and a divergent portion (103) downstream of the throat (102) and having a length Dl and a divergence angle Da between the first internal surface (103') and the flow axis (Fx); the ejector (100) further including: a secondary flowpath (140) upstream of the convergent portion (101) for conducting the secondary fluid (40) to the convergent portion (101); and a primary flowpath (120) including a nozzle portion (121), the nozzle portion (121) having a minimum diameter Nd and terminating at a nozzle orifice (122) to discharge the primary fluid (20) as a jet (21) from the nozzle orifice (122); the nozzle orifice (122) being coaxial with the flow axis (Fx) and upstream of the throat (102), the nozzle orifice (122) being separated from an upstream end (104) of the throat (102) by a separation distance NTl along the flow axis (Fx); wherein: (Td / Nd) and6. An ejector (100) according to claim 1, wherein: the nozzle orifice (122) is located inside the convergent portion (101); andthe secondary flowpath (140) is bounded by a second internal surface (140'), the second internal surface (140') being a surface of revolution about the flow axis (Fx); and the ejector (100) further includes: a primary flow pipe (123) coaxial with the flow axis (Fx), the primary flow pipe (123) extending within the secondary flowpath (140) and defining the primary flowpath (120); a primary feed path (110) upstream of the primary flowpath (120), for conducting the primary fluid (20) to the primary flowpath (120); and a secondary feed path (130) arranged upstream of the secondary flowpath (140) to conduct the secondary fluid (40) to an upstream axial end (141) of the secondary flowpath (140).

7. An ejector (100) according to claim 6, wherein the secondary flowpath (140) has a length Sl and a diameter Sd, wherein Sl > Sd.

8. An ejector (100) according to claim 6, wherein the second internal surface (140') is cylindrical.

9. A power supply unit (500) including: a proton exchange membrane fuel cell assembly (501) having at least one anode (510) and at least one cathode (550) separated by at least one electrolytic membrane (530); an anode intake (511) and an anode exhaust (512) in fluid communication with the at least one anode (510); a cathode intake (551) and a cathode exhaust (552) in fluid communication with the at least one cathode (550); and a fluid handling system including: a cathode side subsystem (560), and an anode side subsystem (520); the cathode side subsystem (560) being configured:to supply an oxidant (50) to the cathode intake (551), and to remove exhaust fluids (51) from the cathode exhaust (552); the anode side subsystem (520) including: a powered blower (521), and an ejector (100, 200, 300) according to claim 1; the blower (521) and the ejector (100, 200, 300) being arranged to define, respectively, first and second recirculation loops (1521, 1100) in parallel flow relation, wherein the anode side subsystem (520) is configured: to recirculate an anode fluid (40) through the blower (521) from the anode exhaust (512) to the anode intake (511) in the first recirculation loop (1521); to supply hydrogen (20), from a source (22) of hydrogen external to the power supply unit (500), as the primary fluid to the primary flowpath (120) of the ejector (100), and via the first flowpath (101, 102, 103) of the ejector (100, 200, 300) to the anode intake (511); and, in the second recirculation loop (1100): to recirculate the anode fluid (40) from the anode exhaust (512) as the secondary fluid to the secondary flowpath (140) of the ejector (100, 200, 300), and to supply the mixed flow (2040) of primary and secondary fluids (20, 40) from the ejector (100, 200, 300) to the anode intake (511); wherein the unit (500) is operable to produce a variable amount of power (P), from the proton exchange membrane fuel cell assembly (501), in an output power range from 0% to 100% of a maximum output power of the unit (500), the output power range including: a low load range (L) above 0% of the maximum output power, a high load range (H) up to 100% of the maximum output power, and a medium load range (M) between the low load range (L) and the high load range (H);wherein the anode side subsystem (520) is arranged to control the blower (521) and the supply of hydrogen (20) to the primary flowpath (120) of the ejector (100, 200, 300): to recirculate the anode fluid (40) through the blower (521) in the first recirculation loop (1521) throughout the low load range (L) and the medium load range (M); and to increase recirculation of the anode fluid (40) through the ejector (100, 200, 300) in the second recirculation loop (1100), while reducing recirculation of the anode fluid (40) through the blower (521) in the first recirculation loop (1521), as the output power (P) of the unit (500) increases in the medium load range (M); and to recirculate the anode fluid (40) through the ejector (100, 200, 300) in the second recirculation loop (1100), substantially without recirculation through the blower (521) in the first recirculation loop (1521), in the high load range (H).

10. A power supply unit (500) according to claim 9, wherein the medium load range (M) ends, and the high load range (H) begins, at an output power (P) of not more than 44% of the maximum output power of the unit (500).

11. A power supply unit (500) according to claim 9, wherein the anode side subsystem (520) is arranged to control the blower (521) and the supply of hydrogen (20) to the primary flowpath (120) of the ejector (100, 200, 300), to recirculate the anode fluid (40) through the ejector (100, 200, 300) in the second recirculation loop (1100), substantially without recirculation through the blower (521) in the first recirculation loop (1521), throughout all of the high load range (H) up to 100% of the maximum output power (P) of the unit (500).

12. A power supply unit (500) according to claim 9, wherein the blower (521) is operable to recirculate the anode fluid (40) in the first recirculation loop (1521) up to a maximum mass flow rate of not more than onethird of a maximum mass flow rate of the recirculation through the ejector (100, 200, 300) in the second recirculation loop (1100).

13. A power supply unit (500) according to claim 9, wherein the maximum output power (P) of the unit (500) is 0.5MW, and Nd is 4mm.

14. A method of producing power (P), including: providing a power supply unit (500) according to claim 9 (S1); recirculating the anode fluid (40) through the blower (521) in the first recirculation loop (1521) throughout the low load range (L) and the medium load range (M) (S2a); increasing recirculation of the anode fluid (40) through the ejector (100, 200, 300) in the second recirculation loop (1100), while reducing recirculation of the anode fluid (40) through the blower (521) in the first recirculation loop (1521), as the output power (P) of the unit (500) increases in the medium load range (M) (S2b); and recirculating the anode fluid (40) through the ejector (100, 200, 300) in the second recirculation loop (1100), substantially without recirculation through the blower (521) in the first recirculation loop (1521), in the high load range (H) (S2c).

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