Hydrogen compression arrangement, system including the arrangement and method

Abstract

The compression arrangement comprises a hydrogen compressor and a return circuit having an inlet, which is fluidly coupled with the discharge side of the centrifugal compressor, and an outlet, which is fluidly coupled with the suction side of the centrifugal compressor. A head-loss control valve is positioned in the return circuit. The head-loss control valve is adapted to generate a controlled head loss in the return circuit when the compressor operates at a flowrate below the surge control line.

Description

HYDROGEN COMPRESSION ARRANGEMENT, SYSTEM INCLUDING THE ARRANGEMENT AND METHODDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure concerns hydrogen compression systems and methods.BACKGROUND ART

[0002] Many human activities are considered to cause climate changes. In particular, the production of energy through combustion of fossil fuels and the consequent generation of carbon dioxide is at the origin of increased concentration of greenhouse gas in the atmosphere with a consequent increase in global temperature.

[0003] The environmental impact of traditional energy production processes based on thermodynamic cycles fueled by fossil fuels, and the resulting climate changes induced thereby, triggered the search for alternative energy sources.

[0004] Hydrogen is widely used in several industrial chemical sectors, for instance to synthesize ammonia and derivatives thereof for agricultural purposes, or for oil refining operations, such as the desulfurization of fractions intended for fuel applications.

[0005] Recently, hydrogen has been taken into consideration as a possible alternative to fossil fuels, since the combustion thereof generates water vapor (steam) and does not produce carbon dioxide.

[0006] However, hydrogen does not exist in abundance in its natural state and shall therefore be produced starting from a raw material. Hydrogen can be produced by a variety of processes, depending upon the starting raw material used. Some of these processes are not without an environmental impact Among others, processes to produce hydrogen are particularly energy -intensive.

[0007] Some hydrogen production processes are based on natural gas or other hydrocarbons. The conversion of these raw materials into hydrogen requires energy, whichnowadays is derived mainly from fossil fuels. Specifically, production of hydrogen from natural gas is usually based on steam reforming (aka steam methane reforming), which involves reacting methane in the presence of water steam at high pressure and temperature in the presence of catalysts. The need to operate the process at high temperature (up to 1000°C) requires large amounts of thermal energy, usually generated by combustion of fossil fuels and consequent carbon dioxide generation.

[0008] Hydrogen produced by processes which release carbon dioxide in the environment is usually referred to as “gray hydrogen”.

[0009] In order to reduce the environmental impact of hydrogen production, carbon capture and storage (CCS) systems are usually used to capture carbon dioxide generated by the combustion of fossil fuels used in produce hydrogen reducing the amount of carbon dioxide released in the environment. Hydrogen produced with systems provided with carbon capture and storage is usually referred to as “blue hydrogen”. The use of CCS systems is not devoid of drawbacks, since carbon capture involves a reduction of the overall energy efficiency of the processes involved.

[0010] Hydrogen can further be generated by electrolysis of water, powered by electric current. Water is split in an electrolyzer into oxygen (O2), which returns to the atmosphere, and H2, If the electricity used to power the electrolyzer is generated 100% by renewable energy, such as photovoltaic panels, wind turbines, and the like, then the hydrogen produced is referred to as “green hydrogen”.

[0011] Green hydrogen has no environmental impact as far as carbon dioxide emissions is concerned, since the entire production process is carbon-free. However, processing green hydrogen, and in particular compressing the hydrogen for transportation or conversion purposes, may become challenging.

[0012] In fact, due to the nature of the renewable energy resources used to power the electrolyzer, green hydrogen production can be discontinuous and the flowrate may fluctuate and reduce drastically at certain times during the day. Centrifugal compressors used to compress hydrogen cannot be switched off when no hydrogen is available, as this can generate fatigue phenomena, which in turn can lead to failure of the compressor. In case of low flowrate of the hydrogen flow from the electrolyzer or whenproduction is idling, it is common practice to run the compressor at reduced speed to reduce the power absorbed by the driver, usually an electric motor. If needed, the antisurge control valve is opened, the suction valve and delivery valve are closed and the compressor is run at reduced speed until resumption of the hydrogen production.

[0013] This requires the use of a variable speed electric motor as a driver, which is expenses. Moreover, the rotary speed cannot be reduced below approximately 60% of the rated speed, to avoid crossing the critical speed of the compressor.

[0014] A novel system and method aimed at overcoming the above-mentioned drawbacks would be welcome in the art.SUMMARY

[0015] According to one aspect, disclosed herein is a hydrogen compression arrangement, which at least partly alleviates or solves the drawbacks mentioned above.

[0016] In embodiments disclosed herein the hydrogen compression arrangement comprises a hydrogen compressor with a suction side and a delivery side. The compression arrangement further includes a return circuit having an inlet fluidly coupled with the discharge side of the hydrogen compressor and an outlet fluidly coupled with the suction side of the hydrogen compressor. The compression arrangement can further include an anti-surge control valve positioned in anti-parallel to the compressor or to each compressor, if more than one compressor is foreseen. A head-loss control valve is positioned in the return circuit and adapted to respond to a drop of compressor flowrate below a threshold by generating a controlled head loss in the return circuit, such as to maintain the compressor in a stable operating condition at reduced flowrate, while recycling hydrogen through the return circuit from the delivery side to the suction side of the compressor.

[0017] If the compression arrangement comprises two compressors in parallel, a head-loss control valve can be provided in anti-parallel to each of said compressors. The two head-loss control valves can be controlled to react to a flowrate drop by partially open and recirculate a flowrate form the delivery side to the suction side of each compressor.

[0018] The head-loss control valve, or each head-loss control valve, can be adapted to respond to a drop of the compressor flowrate rate by reducing the flowrate at 50% or less, preferably at 30% or less, even more preferably at 5% or less of a design flowrate value.

[0019] According to another aspect, disclosed herein is a system for producing hydrogen. The system comprises a hydrogen production unit powered by a renewable energy source and a hydrogen compression arrangement as outlined above.

[0020] According to yet another aspect, disclosed herein is a method for operating a hydrogen compression arrangement comprising: a hydrogen compressor having a suction side and a delivery side; a return circuit having an inlet fluidly coupled with the discharge side of the hydrogen compressor and an outlet fluidly coupled with the suction side of the hydrogen compressor; an anti-surge control valve; and a head-loss control valve along the return circuit. According to embodiments disclosed herein, the method comprises the following steps: delivering a flow of hydrogen from a hydrogen source to the hydrogen compression arrangement; compressing the flow of hydrogen in the hydrogen compressor; and responsive to a hydrogen flowrate drop below a flowrate threshold, partially closing the head-loss control valve and recycling hydrogen from the delivery side of the hydrogen compressor to the suction side of the hydrogen compressor through the return circuit and the partially closed head-loss control valve; the partially closed head-loss control valve generating a concentrated head loss in the return circuit while recycling hydrogen through the return circuit, such that the compressor is maintained in a stable operating condition at the reduced flowrate.

[0021] Further features and embodiments of the compression arrangement, the system and the method according to the present disclosure are outlined below and set forth in the enclosed claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Reference is now made briefly to the accompanying drawings, in which:Fig.1 shows a system for producing hydrogen according to the present disclosure in a first embodiment of green hydrogen production system, including a compression arrangement;Fig.2 shows a pressure ratio vs. flowrate diagram of the hydrogen compression arrangement of the system of Fig.1;Fig.3 shows a diagram of power saving at reduced hydrogen flowrates;Fig.4 shows a system according to the present disclosure in a second embodiment;Fig.5 shows a system according to the present disclosure in a third embodiment;Fig.6 shows a system according to the present disclosure in a fourth embodiment; andFig.7 shows a system according to the present disclosure in a fifth embodiment.DETAILED DESCRIPTION

[0023] To overcome at least partly the drawbacks of hydrogen compression arrangements of the current art, a compression arrangement is disclosed herein, which includes novel features adapted to operate the hydrogen compressor at reduced flowrate and constant rotary speed, minimizing the power needed to run the compressor. In a green hydrogen production and compression plant, the novel compression unit allows a reduction of the plant costs, since a simple, constant-speed compressor driver can be used. The need to shut down the compressor completely, when hydrogen production becomes zero or too small, is also avoided. These advantages are achieved by providing a return circuit in anti-parallel with the compressor. A head-loss control valve is positioned in the return circuit. When the hydrogen flowrate is at or above a threshold value, defined by the point of intersection of the characteristic curve of the compressor and the anti-surge control line, the head-loss control valve is closed, the return circuit is inoperative and the compressor operates at a design rotary speed under stable conditions.

[0024] When the hydrogen flowrate from the hydrogen source drops below the minimum threshold, or becomes zero, the compressor continues to rotate at design speed, but part or the full hydrogen flow is recycled from the delivery side to the suction side of the compressor through the return circuit and through the head-loss control valve. The partially opened head-loss control valve generates a head-loss in the return circuit. The valve is opened to a percentage of its full aperture, such that the characteristic curve of the return circuit intersects the operating characteristic curve of the compressor in a point of stable operation. The flowrate is reduced at a minimum through partial closure of the head-control valve, such that the power absorbed by the motor driving the compressor is minimized. In some embodiments, the head-control valve can be closed such as to reach a total flowrate lower than 50%, preferably lower than 30%, even more preferably equal to or lower than 5% of the design flowrate value.

[0025] A stable operation of the hydrogen compressor at small or zero hydrogen flowrate from the source of green hydrogen is thus achieved, while the compressor is operated at constant speed, avoiding expensive variable speed motors, and the power consumption is minimizing while the need to stop the compressor is avoided.

[0026] A first embodiment of a hydrogen production system including a novel hydrogen compression arrangement according to the present disclosure is shown in Fig.1. The hydrogen production system 1 comprises a hydrogen source 3 and a hydrogen compression arrangement 5. The hydrogen source 3 is a source of green hydrogen. In some embodiments, the hydrogen source 3 includes one or more electrolyzers powered by electric power from an electric power distribution grid 7. In the schematic of Fig.1 a single electrolyzer 9 is shown, but it shall be understood that the hydrogen source may include a plurality of electrolyzers.

[0027] Electric power can be generated by one or more renewable energy resources globally shown at 10. Electric power can be generated by a photovoltaic field schematically shown at 11, by a wind farm schematically shown at 13, by solar concentrator power plants, by a hydroelectric power plant, by a tidal energy plant, or a combination thereof, for instance.

[0028] Hydrogen generated by the electrolyzer 3 can be at ambient pressure or at ahigher pressure, for instance if high-pressure electrolyzers are used. In general, hydrogen from the electrolyzer 9 needs to be further compressed. For this purpose, the system comprises the hydrogen compression arrangement 5. The hydrogen compression arrangement 5 includes a hydrogen compressor 17 driven into rotation by a source of mechanical power 18, typically an electric motor. In advantageous embodiments, the motor 18 is a constant-speed electric motor.

[0029] The hydrogen compressor 17 can be a multi-stage compressor. In some embodiments, the hydrogen compressor 17 is a multistage centrifugal compressor.

[0030] The compressor 17 comprises a suction side 17.1 fluidly coupled with the hydrogen source 3. The compressor 17 further comprises a delivery side 17.2, fluidly coupled with a pressurized hydrogen delivery duct 19. A suction valve 21 is arranged upstream of the suction side 17.1 and a delivery valve 23 is arranged downstream of the delivery side 17.2.

[0031] The hydrogen compression arrangement 5 further comprises a return circuit 25. The return circuit 25 includes an inlet 25.1, fluidly coupled with the discharge side 17.2 of the compressor 17, and an outlet 25.2, fluidly coupled with the suction side 17.1 of the compressor 17. When the return circuit 25 is open, hydrogen circulates in the return circuit 25 in anti -parallel to the compressor 17, from the discharge side 17.2 of the compressor 17 to the suction side 17.1 of the compressor 17.

[0032] The return circuit 25 can be used as an anti-surge line. An anti-surge control valve 27 can be positioned along the return circuit 25 and can be operated as known in the art. Specifically, the anti-surge control valve 27 can be a fast-opening valve, adapted to fully open in a short time, when the operating point of the compressor 17 approaches the surge control line, thus preventing surge phenomena from occurring.

[0033] In the embodiment of Fig.1 the return circuit 25 further comprises a head-loss control valve 29 positioned in the return circuit 25 in parallel to the anti-surge control valve 27. The head-loss control valve 29 is adapted to generate a controlled head loss in the return circuit 25 to stabilize the compressor operation at low or zero hydrogen flow, as will be described in more detail here below.

[0034] The hydrogen compression arrangement further comprises a cooler 31, whichin the embodiment of Fig. 1 is arranged along the return circuit 25, downstream of the head-loss control valve 29 and the anti-surge control valve 27, and upstream of the outlet 25.2 of the return circuit 25.

[0035] Fig. 2 shows a diagram of the characteristic curve of the compressor 17. The volumetric flow through the compressor is plotted on the horizontal axis and the compression ratio is plotted on the vertical axis. The operating curve of the compressor 17 under normal operating conditions is shown at Cl. SC2 is the surge control line. Stable operation of the compressor requires that the operating point remains on the right of the SC2. Flowrates below Fth at constant rotary speed of the compressor will cause the compressor to become unstable as the operating point will move along the increasing curve C2. Usually, the compressor is prevented from operating with flowrates on the left of the SC2 as an operating point on curve C2 becomes unstable.

[0036] The hydrogen flowrate available from the hydrogen source 3 depends on the availability of electric power on the electric power distribution grid 7. Since the electric power is generated by renewable resources, the hydrogen flowrate can drop below Fth or become zero, if reduced electric power or no electric power from the renewable resource is available.

[0037] To prevent the compressor operation to become unstable when the hydrogen production is idling, the common practice according to the current art is to reduce the rotary speed of the compressor and consequently to move on a different operating curve, which intersects the SC2 at a lower flowrate. This approach, however, requires the use of a variable speed motor for the compressor 17, which increases the cost of the system. Moreover, speed reduction under 60% of the rated speed should be avoided, to prevent the compressor from operating around the critical speed. A further drop of the hydrogen flowrate would therefore require stopping the compressor. Repeatedly shutting down and restarting the compressor causes fatigue stresses on the turbomachinery and shall be avoided.

[0038] To overcome the drawbacks of the current art, the hydrogen compression system 5 of the present disclosure uses the head-loss control valve 29, possibly in combination with either the suction valve 21, or the delivery valve 23 or both, to recycle a reduced hydrogen flowrate partly or fully through the return circuit 25 and to introducea controlled head loss in the return circuit 25 or in the main hydrogen circuit, such that the compressor continues to operate in a stable operating point, despite the reduction of the flowrate.

[0039] More specifically, when the green hydrogen production is idling and the hydrogen flowrate become zero, e.g., because no electric power or insufficient electric power is available, the suction valve 21 and the delivery valve 23 are closed, the antisurge control valve 27 remains closed, and the head-loss control valve 29 is partially opened, such that a reduced hydrogen flowrate is maintained in the return circuit 25. The head-loss control valve 29 introduces a concentrated pressure drop in the return circuit 25, such that the operating point of the compressor 17 is stable even if the operating point is on the left side of the surge control line SC2, i.e., the flowrate is lower than Fth. As mentioned, the flowrate of the partially closed head-loss control valve can be equal to or less than 50%, in some embodiments equal to or less than 30%, in other embodiments equal to or less than 5% of the design flowrate value.

[0040] In Fig.2 the curves C20, C50 and C90 are the characteristic curves of the return circuit 25 at variable opening values of the head-loss control valve 29. For instance, curve C90 shows the characteristic curve of the return circuit 25 with the headloss control valve 29 opened at 90%, C50 shows the characteristic curve of the return circuit 25 with the head-loss control valve 29 opened at 50% and curve C20 shows the characteristic curve with the head-loss control valve 29 opened at 20%.

[0041] As shown in Fig.2, the characteristic curves are steeply increasing curves. If the head-loss control valve 29 is open at 50%, the operating point P50 of the compressor 17 is defined by the intersection of the compressor characteristic curve C2 and the characteristic curve C50 of the return circuit 25, i.e., the external circuit in which the compressor 17 is inserted. The operating point P50 is a stable operating point for compressor 17, since a variation of the flowrate will be balanced by a variation of the head loss introduced in the return circuit 25 by the partially closed head-loss control valve 29. Specifically, if the flowrate through the compressor 17 drops, the external return circuit 25 will react with a reduction of the head loss, which counteracts the drop in flowrate and brings the compressor 17 back towards the operating point P50, which therefore becomes stable.

[0042] Due to the partial closure of head-loss control valve 29, therefore, the head loss added by the head-loss control valve 29 in the closed return circuit 25 stabilizes the operating point of the compressor 17 at reduced flowrate values without the need to modify the rotary speed of the compressor 17. While a plurality of stable operating points can be identified along the curve C2 by keeping the head-loss control valve 29 in a partially opened condition, in some embodiments, the head-loss control valve 29 is maintained at the most partialized (partially closed) condition at which the compressor operation is stable. The recirculating flowrate, and therefore the relevant power needed for recirculation, are thus minimized. In the exemplary embodiment shown in Fig.2, for instance, the operating point P20 is preferred over the operating point P50, since the compressor operates under stable conditions in both P20 and P50, but the power absorbed by the compressor is lower in point P20, due to the smaller flowrate processed by the compressor 17.

[0043] The benefit of this approach is twofold. On the one hand, the compressor 17 can operate at constant speed also in the area on the left of the SC2, even in conditions of zero flow of H2 from the hydrogen source 3. This avoids the need for a variable speed motor and for a variable frequency drive (VFD). A VFD is an expensive component and may be prone to wear, failure and needs maintenance, resulting in reduced availability of the system.

[0044] Secondly, by reducing the flowrate processed by the compressor 17, the power absorbed by the compressor 17 can be reduced. The electric power needed to drive the compressor under hydrogen idling conditions is, in fact, useless from a production point of view and is aimed only at avoiding shutdown of the compressor 17, as repeated shut-down and restart of the compressor in the long run may lead to fatigue failure of the turbomachine.

[0045] The power saving which can be achieved by reducing the flowrate under controlled head loss in the return circuit 25 is shown in Fig.3. A first curve A illustrates the power saving in percentage plotted on the vertical axis, as a function of the flowrate percentage, plotted on the horizontal axis. A second curve B illustrates the power saving as a function of speed reduction according to the current art approach, wherein the compressor speed is reduced to allow operation of the compressor at reduced flowrate.The 100% flowrate plotted on the right-most point of the horizontal axis in the diagram of Fig.3 is the minimum flowrate which the compressor 17 can process without entering the unstable operating range. In other words, the 100% flowrate in the diagram of Fig.3 is the flowrate Fth in Fig.2.

[0046] The diagram of Fig.3 shows that by operating the compressor at a flowrate of 8% with the head-loss control valve 29 in a partially closed condition will result in a power saving corresponding to a power saving achievable in a standard compression system by running the compressor a 50% of the rated speed. As mentioned, a speed reduction below 60% of the rated speed is normally avoided, to prevent turbomachinery problems due to vibrations when the fist critical speed is approached. Therefore, the system of the present invention achieves higher power savings if compared with the current art approach of variable compressor speed.

[0047] In conclusion, by rotating the compressor 17 at constant speed and compensating the absence of hydrogen flow from the hydrogen source 3 by recycling the flowrate through partialized head-loss control valve 29 allows the compression system to operate under stable conditions at very low flowrates allowing improved power saving with a simpler and less expensive constant speed motor 18 if compared with the current systems using speed reduction.

[0048] When a hydrogen flowrate is recycled through the return circuit, the cooler 31 removes heat generated by compression to prevent overheating of the recycled hydrogen.

[0049] Under some operating conditions, the hydrogen production may not be fully interrupted but the produced flowrate can be lower than the threshold Fth, at which the operation of the compressor 17 at nominal rotary speed becomes unstable. This situation can be addressed in different ways, which are all based on the idea of controlling the head loss of the external circuit to which the compressor 17 is connected, i.e., by modifying the characteristic curve of the circuit in which the compressor 17 is positioned, such that the operating point of the compressor 17 is defined by the intersection of the characteristic curve C2 of the compressor 17 and a steep characteristic curve of the external circuit.

[0050] In one embodiment, when the hydrogen flowrate drops under Fth, the suction valve 21 can be partially closed, the head-loss control valve 29 remains fully closed and the delivery valve 23 remains fully open. The head loss in the external circuit to which the compressor 17 is connected increases due to the concentrated head loss in suction valve 21 and the characteristic curve of the circuit becomes steeply increasing as a function of the flowrate. The operating point of compressor 17 becomes stable at low flowrate without the need for a speed reduction.

[0051] According to another embodiment, the suction valve 21 can remain fully open, the head-loss control valve 29 can remain fully closed and the head loss in the external circuit can be modulated by reducing the aperture of the delivery valve 21. The characteristic curve of the circuit becomes steeply increasing and the intersection thereof with curve C2 defines again a stable operating point at reduced flowrate without variation of the rotary speed of the compressor 17.

[0052] According to yet another embodiment, the suction valve 21 can remain fully open, while the head-loss control valve 29 and the delivery valve 21 are partially closed. The head loss downstream of the compressor 17 is increased and the characteristic curve of the circuit whereto the compressor 17 is connected becomes steep such that the operating point of the compressor on characteristic curve C2 becomes stable as noted above.

[0053] A combination of the above-described methods can be used.

[0054] The valves 21, 23, 29 can be controlled such as to modify the degree of par- tialization thereof as a function of the hydrogen flowrate from the hydrogen source 3.

[0055] With continuing reference to Figs. 1, 2 and 3, Fig.4 illustrates a schematic of a further embodiment of the system according to the present disclosure. The same reference numbers designate the same elements shown in Fig.l, which will not be described in detail again.

[0056] The main difference between the embodiment of Fig.l and the embodiment of Fig.4 is that in Fig.4 the return circuit 25 contains an anti-surge control valve 27 and a head-loss control valve 29, which are arranged in series rather than in parallel. Under normal operating conditions, the anti-surge control valve 27 is fully closed and thehead-loss control valve 29 is fully open, as the return circuit includes a portion of the delivery line extending from the delivery side of the compressor 17 and the valve 23, the head-loss control valve 29 being positioned in said portion of the delivery line.

[0057] When the compressor 17 approaches the surge control line SC2 for whatever reason, e.g., for a sudden drop of the flowrate, the anti-surge control valve 27 can be fully opened and the head-loss control valve 29 can be partially closed. The delivery valve 23 can be partially or fully closed. This avoids the onset of surging phenomena and modifies the characteristic curve of the circuit in which the hydrogen compressor 17 is placed, namely introduces a concentrated head loss at head-loss control valve 29.

[0058] When the hydrogen flowrate drops or stops altogether, e.g. because the hydrogen production is idling, the head-loss control valve 29 can be set to an intermediate partially opened condition, to introduce a concentrated head loss in the return circuit 25 as described in connection with Figs. 1 to 3, and the compressor 17 can continue operating at constant rotary speed and reduced flowrate. Differently from the embodiment of Figs. 1 to 3, since in Fig.4 the anti-surge control valve 27 and the head-loss control valve 29 are in series rather than in parallel, the anti-surge control valve 27 is maintained fully open.

[0059] As described above, under idling conditions or under low hydrogen flowrate from the hydrogen source, shut-down of the compressor is avoided, thus preventing occurrence of fatigue failure of the compressor, and the power needed to operate the compressor is substantially reduced (see Fig.3).

[0060] With continuing reference to Figs. 1, 2, 3 and 4, a further embodiment of the system according to the present disclosure is shown in Fig.5. The same reference numbers designate the same elements shown in Fig.l, which will not be described in detail again.

[0061] The main difference with respect to the embodiment of Fig.1, is that in Fig.5 the head-loss control valve 29 and the anti-surge control valve 27 are positioned along the return circuit 25 downstream of the cooler 31, such that the recycled hydrogen is cooled prior to entering the valve arrangement. In dotted line, an alternative position of the cooler 31 along the hydrogen delivery line 19 is shown

[0062] With continuing reference to Figs. 1 to 5, a yet further embodiment is shown in Fig.6. This embodiment differs from the embodiment of Fig.4 in that the cooler 31 is arranged upstream of the anti-surge control valve 27. An alternative position for the cooler 31 is shown in dotted lines, between the head-loss control valve 29 and the delivery valve 23.

[0063] In the embodiments of Figs. 4 and 6, where the anti-surge control valve 27 and the head-loss control valve 29 are positioned in series along the return circuit 25, the return circuit has an inlet 25.1 between the delivery side 17.2 of the compressor and the head-loss control valve 29. This means that, when hydrogen is partly or fully recycled, a circuit section between the delivery side 17.2 and the delivery valve 23 becomes part of the return circuit 25. Under normal operating conditions, when the head-loss control valve 29 is fully open and the anti-surge control valve 27 is fully closed, hydrogen flowing through the circuit section between the delivery side 17.2 of compressor 17 and the delivery valve 23 is fed to the delivery line 19 rather than recycled, and therefore the entire circuit section between the delivery side 17.2 of the compressor 17 and the delivery valve 23 forms part of the delivery line and does not operate as a return circuit.

[0064] In the embodiments described above, the operating condition of the compressor and of the relevant valve arrangement can be controlled by a control unit, not shown. The control unit can for instance detect the flowrate, the pressure ratio and the rotary speed of the compressor, among others. The control unit can be functionally coupled to the compressor, the anti-surge control valve 27 and the head-loss control valve 29. The control unit can be adapted to control the anti-surge control valve 27 and the head-loss control valve 29 to operate as described above, responsive to a surge condition, or to a flowrate drop at or below a flowrate threshold, set by the surge control line, for instance.

[0065] In other embodiments, the system disclosed herein may include more than one compressor. Fig.7 illustrates an embodiment with a compression arrangement including two compressors in series. The same reference numbers of Figs. 1-6 are used in Fig.7 to designate the same or equivalent parts or components, which will not be described in detail.

[0066] In the embodiment of Fig.7, the compression arrangement 5 includes a first compressor 17 with a suction side 17.1 and a delivery side 17.2. This latter is fluidly coupled with a suction side 18.1 of a second compressor 18. Reference number 18.2 indicates the delivery side of the second compressor 18.

[0067] Reference number 23 indicates a delivery valve of the first compressor 17, while reference 24 indicates a delivery valve of the second compressor 18. A first cooler 31 is located between the delivery side 17.2 of the first compressor 17 and the suction side 18.1 of the second compressor 18, while a second cooler 32 is located between the delivery side 18.2 of the second compressor 18 and the second delivery valve 24.

[0068] A first branch 25 or portion of a return circuit is arranged in anti-parallel with the first compressor 17. The first branch 25 of return circuit includes an inlet 25.1, fluidly coupled with the discharge side 17.2 of the first compressor 17, e.g. between the cooler 31 and the delivery valve 23, and an outlet 25.2, fluidly coupled with the suction side 17.1 of the first compressor 17, downstream of the suction valve 21. A head-loss control valve 29 is positioned between the inlet and the outlet of the branch 25 of the return circuit.

[0069] A second branch or portion 26 of the return circuit 26 is arranged in antiparallel with the second compressor 18. The second branch 26 of the return circuit includes an inlet 26.1, fluidly coupled with the discharge side 18.2 of the second compressor 18, between the cooler 32 and the delivery valve 24, and an outlet 26.2, fluidly coupled with the suction side 18.1 of the second compressor 18, downstream of the valve 23. A head-loss control valve 30 is positioned between the inlet and the outlet of the second return circuit 26.

[0070] In this embodiment a single anti-surge control valve is positioned along a common branch 28 of the return circuit, positioned in anti-parallel to the first compressor 17 and the second compressor 18. The common branch 28 of the return circuit an inlet which coincides with the inlet 26.1 of the second return circuit 26 and an outlet which coincides with the outlet 25.1 of the first return circuit 25.

[0071] As a matter of fact, in this embodiment, the return circuit comprises thereforethe branch 28 which is in anti-parallel to the serially arranged first compressor 17 and second compressor 18 and contains a single anti-surge control valve 27. The return circuit further includes a branch 25 in anti -parallel to the first compressor 17 and including the first head-loss control valve 29. Finally, the return circuit further includes the branch 26, in anti-parallel to the second compressor 18, and including the second head-loss control valve 29.

[0072] During normal operation, the anti-surge control valve 27 and both head-loss control valves 29, 30 are closed.

[0073] The anti-surge control valve 27 protects both compressors 17, 18 against surging and opens when needed if the operating point approaches the anti-surge line.

[0074] When the hydrogen flowrate drops below a threshold, or if no hydrogen is available from the hydrogen source 3, both head-loss control valves 29 and 30 can be partially opened to provide a return circuit for hydrogen recirculation at low flowrate under stable operating conditions through both compressors 17, 18 as described above in relation to the embodiments of Figs. 1 to 6. If no flowrate originates from the hydrogen source 3, the valves 21, 23 and 24 can be closed and the head-loss control valves 29, 30 can be partially closed, so that the two compressors continue operate at low flowrate and low power absorption.

[0075] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A hydrogen compression arrangement comprising: a hydrogen compressor comprising a suction side and a delivery side; a return circuit having an inlet fluidly coupled with the discharge side of the hydrogen compressor and an outlet fluidly coupled with the suction side of the hydrogen compressor; and an anti-surge control valve; characterized by a head-loss control valve positioned in the return circuit and adapted to respond to a drop of compressor flowrate below a threshold by generating a controlled head loss in the return circuit, such as to maintain the compressor in a stable operating condition at reduced flowrate, while recycling hydrogen through the return circuit from the delivery side to the suction side of the compressor.

2. The arrangement of claim 1 , wherein the anti-surge control valve and the head-loss control valve are arranged in parallel.

3. The arrangement of claim 2, wherein the anti-surge control valve is controlled such that when the compressor is maintained in stable operating condition at reduced flowrate, the anti-surge control valve is closed.

4. The arrangement of claim 1 , wherein the anti-surge control valve and the head-loss control valve are positioned in series.

5. The arrangement of claim 4, wherein the anti-surge control valve is controlled such that when the compressor is maintained in stable operating condition at reduced flowrate, the anti-surge control valve is fully open.

6. The arrangement of any one of the preceding claims, wherein the hydrogen compressor comprises a multistage compressor.

7. The arrangement of any one of the preceding claims, wherein the hydrogen compressor comprises a dynamic compressor.

8. The arrangement of any one of the preceding claims, wherein the hydrogen compressor comprises a multistage centrifugal compressor.

9. The arrangement of any one of the preceding claims, further comprising a suction valve upstream of the suction side of the hydrogen compressor and a delivery valve downstream of the delivery side of the hydrogen compressor.

10. The arrangement of claim 9, wherein the head-loss control valve is positioned between the delivery valve and the suction side of the compressor.

11. The arrangement of claim 9 or 10, wherein at least on of said suction valve and delivery valve is controlled to modulate a head loss in a circuit whereto the hydrogen compressor is fluidly coupled.

12. The arrangement of any one of the preceding claims, wherein the head-loss control valve is adapted to respond to the drop of the flowrate by reducing the flowrate at 50% or less, preferably at 30% or less, even more preferably at 5% or less of a design flowrate value.

13. The arrangement of any one of the preceding claims, wherein the hydrogen compressor comprises a first hydrogen compressor and a second hydrogen compressor in series; wherein the anti-surge valve is arranged in the return circuit between a delivery side of the second compressor and a suction side of the first compressor; wherein the head-loss control valve is positioned in the return circuit between the delivery side of the first compressor and the suction side of the first compressor; and wherein a second head loss control valve is positioned in the return circuit between a delivery side of the second compressor and a suction side of the second compressor.

14. A system for producing hydrogen, the system comprising: a hydrogen production unit powered by a renewable energy source; a hydrogen compression arrangement according to one or more of the preceding claims.

15. The system of claim 14 wherein the hydrogen production unit is an electrolysis production unit powered by electric energy generated by at least one renewable energy source.

16. A method for operating a hydrogen compression arrangement comprising: a hydrogen compressor having a suction side and a delivery side; a return circuit having an inlet fluidly coupled with the discharge side of the hydrogen compressor and an outlet fluidly coupled with the suction side of the hydrogen compressor; an anti-surge control valve; and a head-loss control valve along the return circuit; the method comprising the following steps: delivering a flow of hydrogen from a hydrogen source to the hydrogen compression arrangement; compressing the flow of hydrogen in the hydrogen compressor; and responsive to a hydrogen flowrate drop below a flowrate threshold, partially closing the head-loss control valve and recycling hydrogen from the delivery side of the hydrogen compressor to the suction side of the hydrogen compressor through the return circuit and the partially closed head-loss control valve; the partially closed head-loss control valve generating a concentrated head loss in the return circuit while recycling hydrogen through the return circuit, such that the compressor is maintained in a stable operating condition at the reduced flowrate.

17. The method of claim 16, wherein the anti-surge control valve and the head-loss control valve are positioned in parallel in the return circuit; and wherein the step of recycling hydrogen comprises the step of recycling hydrogen through the partially closed head-loss control valve while the anti-surge control valve is maintained in a close condition.

18. The method of claim 16, wherein the anti-surge control valve and the head-loss control valve are positioned in series in the return circuit; and wherein the step of recycling hydrogen comprises the step of recycling hydrogen through the partially closed head-loss control valve and through the anti-surge control valve, which is maintained in an open condition.

19. The method of claim 16, wherein the hydrogen compressor comprises a first hydrogen compressor and a second hydrogen compressor in series; wherein the anti-surge valve is arranged in the return circuit between a delivery side of the second compressor and a suction side of the first compressor; wherein the headloss control valve is positioned in the return circuit between the delivery side of the first compressor and the suction side of the first compressor; wherein a second head loss control valve is positioned in the return circuit between the delivery side of the second compressor and the suction side of the second compressor; and wherein the step of recycling hydrogen comprises the step of recycling hydrogen through the partially closed head-loss control valve and the partially closed second head-loss control valve, while the anti-surge control valve is maintained in a close condition.

20. The method of any one of claims 16 to 19, wherein the hydrogen compressor comprises a multi-stage compressor.

21. The method of any one of claims 16 to 20, further comprising the following steps responsive to the hydrogen flowrate drop: closing a suction valve upstream of the hydrogen compressor and a delivery valve downstream of the hydrogen compressor, wherein the inlet of the return circuit is positioned between the delivery side of the compressor and the delivery valve, and the outlet of the return circuit is positioned between the suction valve and the suction side of the compressor; and circulating hydrogen at reduced flowrate through the return circuit.

22. The method of claim 21, wherein the head-loss control valve is positioned between the delivery side of the compressor and the delivery valve downstream of the compressor.

23. The method of any one of claims 16 to 22, wherein the step of partially closing the head-loss control valve comprises reducing the flowrate of the headloss control valve at 50% or less, preferably at 30% or less, even more preferably at 5% or less of a design flowrate value.

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