Method for controlled release of hydrogen and oxygen from at least one alkaline electrolyser stack and electrolyser and control module

Abstract

A method for controlled release of hydrogen and oxygen from at least one alkaline electrolyser stack is provided having at least one gas separation diaphragm (19) whereby oxygen gas and anolyte from a first side of the diaphragm (19) are introduced into an oxygen separation tank (2) provoking an oxygen gas pressure, PO and an anolyte fluid level, Lan in the oxygen separation tank (2), and whereby hydrogen gas and catholyte from a second side of the diaphragm (19) are introduced into a hydrogen separation tank (3) provoking a hydrogen gas pressure, PH and a catholyte fluid level Lca in the hydrogen separation tank (3), whereby the fluid level differences, dL = Lca – Lan pressure differences, dPdP and pressures, PO, PH are determined, and i. the deviation between a desired fluid level difference dL des and actual measured fluid level difference dL is used to calculate a new desired pressure difference dP des, a. a pressure control module controls the vent off of hydrogen and oxygen from the respective separation tanks in order to thereby introduce and maintain the calculated desired pressure difference dP des whereby ii. a new actual fluid level difference dL is obtained and measured and used in a control loop involving point i. in order to minimise the deviation between a desired fluid level difference dL des and the just measured actual fluid level difference dL.

Description

[0001] METHOD FOR CONTROLLED RELEASE OF HYDROGEN AND OXYGEN FROM AT LEAST ONE ALKALINE ELECTROLYSER STACK AND ELECTROLYSER AND CONTROL MODULE

[0002] The present invention relates to a method for controlled release of hydrogen and oxygen from at least one alkaline electrolyser stack. Further, the invention relates to an electrolyser and control module.

[0003] Background of the invention

[0004] Electrolysers for electrolysation of water such as pressurized alkali electrolysers deliver hydrogen and oxygen from each their side of a diaphragm in each cell in a cell stack. It remains a delicate task to ensure that gas pressures in separation tanks for the produced hydrogen and oxygen is continually balanced correctly, such that the diaphragm or diaphragms are not unduly pressurized at one side thereof. Such one-sided pressurizations above a predetermined level, may lead to unwanted flows of electrolyte from a cathode side to an anode side of the diaphragm or vice versa, which again will result in less pure gasses being released at the two sides of the diaphragm. Also, the diaphragm or adjacent electrode may suffer damages due to such pressure imbalance.

[0005] It has been suggested to install a cross pressure balance system whereby pressure in one of the separation tanks is guided in accordance with the pressure in the other separation tank. This construction is basically fine, however in case fluid levels in the two tanks due to an unplanned event become biased, it will be difficult to regulate fluid levels back to the correct level, and even if the pressures above the fluid in each separation tank are balanced correctly, the fluid level difference may lead to undesired cross diaphragm flows. It is further a challenge to balance the pressures in alkaline pressurized electrolysers with pressures above 30 bar, where pressure differences in the millibar ranges between pressures at the two sides of the separation diaphragm may cause the diaphragm to move and either become damaged itself or cause damage to an adjacent electrode. Summary of the invention

[0006] The objects of the invention are achieved by a new method for controlled release of hydrogen and oxygen from at least one alkaline electrolyser stack having at least one gas separation diaphragm whereby, a. oxygen gas and anolyte from a first side of the diaphragm are introduced into an oxygen separation tank provoking an oxygen separation tank gas pressure, PO, and an anolyte fluid level, Lan, in the oxygen separation tank, and whereby b. hydrogen gas and catholyte from a second side of the diaphragm are introduced into a hydrogen separation tank provoking a hydrogen separation tank gas pressure, PH, and a catholyte fluid level, Lea, in the hydrogen separation tank.

[0007] According to the method, i. the fluid level differences dL=(Lca - Lan), pressure differences between the pressures in the two tanks, dP and separation tank pressures, PO and PH, are determined, and the deviation between a desired fluid level difference dL des and actual measured fluid level difference dL are used to calculate a new desired pressure difference dP des and ii. a pressure control module controls the vent off of hydrogen and oxygen from the respective separation tanks in order to thereby introduce and maintain the calculated desired pressure difference dP des whereby, a new actual fluid level difference dL is obtained and measured and used in a control loop involving point i. and ii in order to minimise the deviation between a desired fluid level difference dL des and the just measured actual fluid level difference dL.

[0008] The i. and ii. points designate a fluid level difference outer control loop, whereby an actual measured level difference, dL, is compared to a desired level deference, dL des, and whereby the control loop works to maintain a smallest possible deviation between measured and desired fluid level difference in the oxygen and hydrogen separation tanks. Usually, the desired fluid level difference is zero. This outer control loop seeks to ensure the desired fluid level difference by means of the pressures above the fluids in the two tanks. This is possible, as any lasting substantial pressure difference between the pressure in the anolyte tank and the pressure in the catholyte tank will provoke a flow of electrolyte across the diaphragm in the cells and thus lead to a shift in level difference. In order to take the time constants associated with the cross-diaphragm flow into account, the outer control loop comprises a filter or calculation unit adapted to ensure a stable and precise regulation towards the desired setpoint, also whenever a new setpoint is introduced. This allows for a controlled movement from an undesired fluid level difference and back to a desired fluid level difference, which again ensures that impurities in the produced gasses on the two sides of the membranes will remain within predetermined limits.

[0009] The venting off of hydrogen and oxygen from the respective tanks is preferably undertaken in an inner control loop, which seeks to introduce and maintain the desired pressure difference. As mentioned, this desired pressure difference is the pressure difference, which is calculated in order to provide the desired fluid level difference such as a zero fluid level difference or a step on the way towards a zero fluid level difference. This venting off of oxygen and hydrogen takes place under the guidance of the inner control loop, which compares actual measured pressure differences with the provided setpoint for the pressure difference between the hydrogen and oxygen separation tanks. Time constants, such as reaction times between the provision of a regulation input to a particular valve member and actual measured change in the pressure differences between pressures in the two tanks are handled in the inner control loop by a calculation or filter unit which calculate the right change in the setting of a particular valve, to change a flow of gas therethrough and thereby close or minimise a deviation between a measure pressure difference and the setpoint pressure difference.

[0010] In an embodiment of the invention, gas is delivered from either the hydrogen separation tank or the oxygen separation tank to a recipient by firstly leading the gas through a controllable pressure reduction valve and further through a controllable back pressure valve whereby the back pressure valve setpoint is guided by the deviation between a chosen desired pressure loss over the pressure reduction valve and the actual size of the pressure loss over the pressure reduction valve.

[0011] The setpoint of the pressure reduction valve, is guided by the deviation between the calculated desired pressure differences between the pressures in each of the two separation tanks and a measured actual pressure difference between the pressures in the two separation tanks.

[0012] The recipient can be a pump and a storage facility, or it can be the atmosphere. The back pressure valve setpoint when it receives gas from the pressure reduction valve is guided by the pressure module whereby a control loop is provided such as to maintain a predefined pressure loss over the pressure reduction valve. Thus, whenever sufficient gas is produced in the electrolyser stack, a predefined flow through the pressure reduction valve is maintained. The setpoint for the controllable pressure reduction valve is chosen through a control loop of its own, which is adapted to constantly minimise the deviation between the tank pressure differences and a desired setpoint for the tank pressure differences by moving the setting of the pressure reduction valve.

[0013] In both control loops of this embodiment, a calculation unit or digital filter is provided to ensure that an agile but stable pressure regulation regime is maintained which takes the dynamic nature of the system into account.

[0014] The pressure reduction valve and back pressure valve are provided with or without intermediate gas handling elements between them. Gas handling equipment could comprise scrubbers and / or dryers adapted to ensure that the gas delivered has the desired quality whether it be hydrogen or oxygen.

[0015] It is noticed that the setpoint for the backpressure valve is defined by the electric current running through the stack. When a given electric current is supplied to the stack, a predefined stream of hydrogen and oxygen is ideally generated at the two sides of the diaphragm or multitude of diaphragms, and whenever one part of oxygen is produced, two parts of hydrogen will be generated. However, time lapses between electric charge supply at the electrodes and the rise of pressures in the tanks, and unideal effects leading to minute amounts of hydrogen in the oxygen gas and vice-versa and the influence of other conditions such as temperature in the stack, make it less predictable when a given gas production rate is achieved. The pressure module is constructed with the view to tackle this problem such that desired pressures and desired fluid levels are maintained during both full load and production ramp-up or down and the produced gasses are supplied to the recipients at predefined pressures. It is noticed that the gas may be piped directly from the pressure reduction valve to the back pressure valve, or alternatively, one, two or more steps of cleaning and / or drying may be provided between the pressure reduction valve and the back pressure valve.

[0016] In an embodiment, an absolute system pressure is maintained in that the pressure control module is adapted to vent off oxygen from the oxygen separation tank through a controllable back pressure valve to thereby maintain an absolute system pressure. Alternatively, an absolute system maximum pressure is maintained in that the pressure control module vents of hydrogen from the hydrogen separation tank through a controllable back pressure valve to thereby maintain an absolute system pressure.

[0017] As mentioned, oxygen is produced at a lower rate than hydrogen, and thus hydrogen shall be vented off during a ramp-up process of the stack in order to ensure that the desired balance of pressures in the two separation tanks is maintained. However, at some point a desired system pressure is reached in the oxygen separation tank or hydrogen separation tank, and here the gas in the other tank is to be vented off in a controlled manner, to maintain this pressure during production. A control loop for the system pressure is provided as part of the pressure control module, whereby a setpoint for the system pressure is maintained. Thus, a deviation from the chosen setpoint and a measured system pressure is used as basis for calculation of a regulation action, which is then fed to a controllable back pressure valve, in order to change the flow of gas therethrough. Whenever the gas is vented off, it will also influence the pressure difference between the pressure in the other separation tank and this will provoke a balancing action through the mentioned pressure balance and control loop. In an embodiment, hydrogen shall be vented off or retained to maintain the desired pressure difference (usually zero pressure difference between pressures in the two separation tanks), while oxygen is vented off to maintain the desired system pressure. In another embodiment, oxygen shall be vented off or retained to maintain the desired pressure difference (usually zero pressure difference between pressures in the two separation tanks), while hydrogen is vented off to maintain the desired system pressure. The recipient of the oxygen may be the atmosphere, or it may be a pump and storage facility in case the oxygen is to be used in a further process, such as in a land-based fish farming facility or industry or health service.

[0018] In an embodiment of the invention, it is further ensured that between the controllable pressure reduction valve and the controllable back pressure valve, firstly oxygen residues are removed and secondly water vapor is removed from the hydrogen gas stream.

[0019] It is noticed that the removal of the possible oxygen residues from the hydrogen gas stream is performed by allowing a controlled oxidation of hydrogen in the gas stream thereby consuming a predefined percentage of available oxygen for the formation of water, possibly by the interaction of a suitable catalyst. Water and heat are generated in this process and the resulting oxygen depleted hydrogen gas stream will have a higher temperature and increased water content. The oxygen depleted hydrogen gas stream is now piped through a water removal apparatus. One way of removing the water is by cooling the hydrogen below the condensation point for water vapor at the particular pressure and then drain the liquid water away. This process leaves the hydrogen gas stream clean from oxygen and water as desired, and thereby ready for the recipient. The provision of the hydrogen cleaning and drying between the two valves: the controllable pressure reduction valve and the controllable back pressure valves ensure that the venting off of hydrogen during production remains well-managed and precise.

[0020] In an embodiment, the setting of the controllable back pressure valve is chosen according to electrical operational parameters of at least one alkaline electrolyser stack. The electrical current feed to the stack is the most decisive parameter for the production of hydrogen and oxygen and thus also indicative for the projected delivery from the hydrogen and from the oxygen separation tanks. Thus, this parameter is the best predictor for the correct setting of the back pressure valve.

[0021] The invention also comprises an electrolyser and control module. The electrolyser comprises at least one cell stack having at least one gas separation diaphragm whereby pipes are arranged to direct oxygen gas and alkaline anolyte from a first side of the diaphragm within the stack and into an oxygen separation tank, and whereby pipes are arranged to direct hydrogen gas and catholyte from a second side of the diaphragm and into a hydrogen separation tank. Further, fluid level sensors are provided for the two separation tanks, and a difference pressure gauge is provided and arranged to provide a difference pressure signal, and additionally pressure gauges responsive to the absolute pressures in the two tanks are provided.

[0022] According to this invention the pressure control module comprises a. an outer calculation and control loop having a calculation unit which is adapted to minimise a deviation between a desired fluid level difference dL des and the measured fluid level difference dL whereby the outer control loop calculation unit is further adapted to calculate a desired pressure difference dP des between the pressures in the two separation tanks, and b. an inner calculation and control loop i. adapted to receive a desired pressure difference dP des calculated by the outer control loop, and whereby the inner calculation and control loop is adapted to regulate controllable valves in order to vent off hydrogen and / or oxygen from the separation tanks to a recipient in order to minimise the deviation between the desired calculated pressure difference dP des and the measured pressure difference dP to thereby ii. maintain the desired fluid level difference dL des between fluid levels in the oxygen and hydrogen separation tanks. The inner and the outer calculation and control loops may comprise well-known programmable PID regulator units, to which a setpoint for a process and current output value delivered by the process is provided, and which based on a deviation between these two, predicts the best possible change of the setting of a regulated device which determines the output of the process, in order to constantly minimise the deviation between the setpoint and the output value. The outer calculation and control loop attempts to keep the fluid levels in the two tanks balanced, usually such as by maintaining the same fluid level in the tanks. The outer control loop delivers a calculated new desired fluid level difference based on the deviation between the measured fluid level differences and the current setpoint for fluid level.

[0023] The calculated new desired fluid level difference is translated into desired pressure difference between the pressures in the oxygen and hydrogen separation tanks. This is doable as the pressure levels in the two tanks will translate into pressure level differences between the two sides of the diaphragms in the stack, and ultimately such a pressure level difference between a first and second side of the diaphragms will lead to a cross diaphragm flow. Thus, the pressure level difference is used as a means to keep the fluid levels within the two tanks at a desired point.

[0024] The pressure difference between pressures in the tanks is regulated in a control loop of its own, the so-called inner control loop. Based on the calculated desired pressure differences communicated from the outer control loop and the actual pressure difference between pressures in the two tanks, a regulation output is provided such as to change the setting of a particular valve. The changed setting will result in a new measurable pressure difference and the loop is concluded when this new pressure difference is gauged and fed to the calculation and control device in the loop.

[0025] It is mentioned that the gasses in the pipes and vessels, as well as valve members react swiftly to changes of the setting of valve members and this dynamic of the system is accommodated in the preferred settings of the parameters in the inner control loop. Similarly, cross diaphragm flows are usually kept slow, and thus a different set of calculation parameters will be used in the outer control loop. This does not prevent a faster cross diaphragm flow, in case this is desired, and it requires only a different set of filter or calculation parameters for the two control loops. With this control module, it is easy to ensure that pressure differences between the pressures in the two separation tanks remain within predefined constraints.

[0026] In an embodiment, vent pipes are arranged between the separation tanks and a hydrogen recipient or an oxygen recipient respectively, and in one of these pipes, a controllable pressure reduction valve is provided and further, in the one vent pipe between the mentioned pressure reduction valve and the gas recipient, a controllable back pressure valve is provided, whereby the inner control and calculation loop of the control module is adapted to a. guide the setting of the mentioned back pressure valve in order to realise a chosen desired pressure loss over the pressure reduction valve and further adapted to b. guide the setting of the pressure reduction valve to minimise a deviation between: i. the calculated desired pressure difference between the pressures in each of the two separation tanks, and ii. a measured actual pressure difference between the pressures in the two separation tanks.

[0027] A control loop for guiding the setting of the pressure reduction valve is thus provided in order for a calculated desired pressure difference between the pressures in the tanks to be maintained. A further control loop is provided, which guides the back pressure valve in order to realize a chosen desired pressure loss over the pressure reduction valve. The setpoint of the back pressure valve is a system parameter, which depends on the expected flow of gas to the recipient. Thus, the two valves: the back pressure valve and the pressure reduction valve in the gas vent pipe and the control loops shall ensure, that the pressure in the separation tanks remains balanced, even when gas production is not entirely according to predictions. A gas separation tank pressure meter and a gas back pressure meter are provided in order to gauge the pressure loss over the pressure reduction valve. In an embodiment of the invention, a back pressure valve is provided in a vent pipe from one of the separation tank vent pipes which is not provided with the mentioned control loop and further control loop. The control module also comprises a control loop adapted to minimize the deviation between a predetermined desired absolute pressure in the gas in the separation tank vent pipe without the two control loops, and the actual measured pressure in this tank.

[0028] The oxygen separation tank receives less produced gas than the hydrogen separation tank. Thus, during ramp-up, release of oxygen will not take place before a system pressure maximum value under normal processing conditions is reached. Some time prior to this pressure being registered in the oxygen separation tank, hydrogen needs must be vented off, such as to maintain the balance between the pressures in the two separation tanks.

[0029] In an embodiment of the invention, in the hydrogen gas stream between the pressure reduction valve and the back pressure valve, an oxygen removal device is provided, and in the hydrogen gas stream between the oxygen removal device and the back pressure valve, a water vapor removal device is provided.

[0030] Thereby the hydrogen gas reaches the recipient through the hydrogen vent pipe but encounters an oxygen removal device and a dryer on the way to the recipient. The oxygen removal device will add heat and a bit of water to the hydrogen stream, and thus it is customary to have the oxygen removal device in the gas line prior to the dryer. It is mentioned that the back pressure meter and the hydrogen separation tank pressure meter are arranged with both pressure reduction valve, oxygen removal device, and dryer therebetween. At any given flow, the oxygen removal device and the dryer shall provide a constant pressure loss and thus their presence shall not be a disturbance to the control loop guiding the setting of the pressure reduction valve.

[0031] In a further embodiment, the controllable back pressure valve has a regulator, whereby this regulator is adapted to choose a setpoint for the back pressure valve according to a signal indicative of a hydrogen or an oxygen production rate such that in an embodiment where the controllable back pressure valve is part of the oxygen vent pipe, the oxygen production rate signal is supplied to the regulator and in an embodiment where the controllable back pressure valve is part of the hydrogen vent pipe, the hydrogen production rate signal is supplied to the regulator.

[0032] The regulator is part of the control loop for the back pressure valve, which shall thus receive the setpoint input, which is calculated according to the current supplied to the stack and is indicative of a desired pressure loss over the pressure reduction valve which will be realised when a predetermined flow of hydrogen flows from the hydrogen separation tank to the recipient. The two controllable valves: the back pressure valve and the pressure reduction valve together with the controllable back pressure valve and their respective control loops thus allow for a fine-tuned control of the difference between the pressure in the two separation tanks.

[0033] Various exemplifying and non-limiting embodiments both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and nonlimiting embodiments when read in conjunction with the accompanying drawings.

[0034] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.

[0035] Brief description of the drawings

[0036] In the following, the invention will be described in greater detail with reference to embodiments shown by the enclosed figures. It should be emphasized that the embodiments shown are used for example purposes only and should not be used to limit the scope of the invention.

[0037] Fig. 1 is an overall view of a diagram of the hydrolyser and control system, Fig. 1A shows an overall view of the major components of a system with 6 stacks, not showing valves and sensors,

[0038] Fig. 2 shows a simplified version of central parts of the hydrolyser,

[0039] Fig. 3 is a route diagram of the outer and inner control loops,

[0040] Fig. 4 shows an enlarged section of the diagram in Fig. 1, and

[0041] Fig. 5 is an enlarged section of the diagram in Fig. 1.

[0042] Detailed description of the embodiments

[0043] Throughout the description and claims the following names are used:

[0044] Lea measured fluid level in the hydrogen separation tank

[0045] Lan measured fluid level in the oxygen separation tank dL measured fluid level difference dL des desired fluid level difference dl set setpoint fluid level difference

[0046] PH measured pressure in the hydrogen separation tank

[0047] PO measured pressure in the oxygen tank dP measured pressure difference between pressure in the oxygen tank and pressure in the hydrogen tank dP des desired pressure difference between pressures in the two separation tanks dP set setpoint for pressure difference.

[0048] In Fig. 1, an example of a cell stack 1 is shown and the plurality of individual cells are indicated by the vertical lines between a pair of end flanges or current injection electrodes. The electrical power supply 4 is schematically indicated and comprises power lines, which enter the stack at endpoints thereof. The power supply 4 provides a DC current to the stack at a predetermined voltage. In the example of Figs. 1, 3, 4 and 5, the hydrogen is vented off through a pressure reduction valve and a controllable second back pressure valve 11, where the oxygen is vented off through a controllable first back pressure valve 10 only. However, as indicated above, oxygen may be piped through a pressure reduction valve and back pressure valve with the described control loops, taking into account the level differences and actual pressure differences in the two separation tanks, and hydrogen vented off through a simple controllable back pressure valve.

[0049] In Fig. 2, an even more schematic representation of a stack is provided whereby one cell 1 is shown which represents the entire cell stack 1. The cell 1 comprises a diaphragm 19, and at one side of the diaphragm a cathode shall be provided (not shown) and at the other side an anode shall reside (not shown). When an electrical potential is established between anode and cathode, the cathode shall release hydrogen and the anode will release oxygen, provided that these electrodes are immersed in a catholyte and anolyte respectively and charged with an electrical DC potential difference. With reference to Figs. 1, 1A and 2, the basics of the system is described here. An anolyte return pipe 7 is indicated and is adapted to return anolyte to the cell 1 or cell stack / s 1 from an oxygen separation tank 2, and also an oxygen and anolyte delivery pipe 5 is shown, which is adapted to pipe a mixture of anolyte and oxygen into the oxygen separation tank 2. An oxygen vent pipe 23 is shown which shall lead oxygen out of the oxygen tank 2 and on to an oxygen recipient 22 (see Fig. 1), such as the atmosphere. Similarly, a catholyte return pipe 8 is shown, which returns catholyte from the hydrogen separation tank 3 into the cell 1 (or cell stack / s 1), as well as a hydrogen and catholyte delivery pipe 6 leading from the cell 1 to the hydrogen separation tank 3. A hydrogen vent pipe 24 is disclosed, which shall feed hydrogen from the hydrogen separation tank 3 towards a hydrogen recipient 21 (shown schematically in Figs. 1 and 1A).

[0050] Fluid level meters 12, 13 are connected to the two separation tanks 3, 2 shown in Fig. 1, and the fluid level meters 12, 13 are adapted to provide an electric signal indicative of the fluid level in each tank, Lea for hydrogen separation tank fluid level, Lan for the fluid level in the oxygen separation tank 2. The level signals Lea and Lan are subtracted from each other, giving rise to a fluid level difference signal dL. In an outer control loop shown in Fig. 3, the thus measured level difference dL is compared to a setpoint dl set for the level difference at the outer loop comparator 32, and the resulting error or deviation signal undergoes calculation in the outer loop calculation unit 33 to determine a regulation value or desired fluid level difference dL des. The filter or calculation unit 33 accommodates the dynamics of the fluid level difference changes and limitations which guide such changes. This calculated desired fluid level difference, dL des feeds into an inner regulation loop 27.

[0051] As seen in Fig. 1 and Fig. 2, the two separation tanks 2, 3 in the example are provided in the same height above the stack 1 , and are thus also level with each other, and the tanks furthermore have the same shape. However, it is possible to use differently shaped tanks, as well as tanks 2, 3 arranged in each their height. In such cases, the Lea and Lan measures need each their normalisation curve or table, such that when normalised, a zero-level difference will correspond to a zero pressure difference between pressures at a first and a second side of the diaphragm.

[0052] The inner control loop 27 is adapted to control the pressure difference between the pressure PH and PO in the two separation tanks. The loop 27 accepts the calculated desired level difference signal dL des and treats it as a setpoint level for the pressure difference dP between the pressures in the two separation tanks in the gasses above the fluid. A simple calculation (not indicated in Fig. 3) converts the level difference signal dL des into a pressure difference dP set, considering the density of the electrolyte in the two tanks and the height between fluid level in the respective separation tank and height between the diaphragms and the fluid level. The thus reached pressure difference setpoint dP set and the measured pressure difference dP are compared at the inner loop comparator 34. The comparator calculates the deviation, and the deviation signal is forwarded to the inner loop calculation or filter unit 35. A pressure reduction valve control signal 28 (also indicated in Fig. 4) is calculated at the inner calculation unit 35 and is forwarded to the control valve 17. The resulting output of the regulation action at control valve 17 is a new pressure difference dP which will be determined at the pressure difference meter 16. By increasing or decreasing the flow through the reduction valve 17, the pressure in the hydrogen separation tank will either rise or fall and thereby a better approximation to the desired pressure difference value dP des may be reached. By the change in actual difference pressure, the fluid level differences will gradually change, especially if a pressure difference between the pressures in the tanks is noticeable. This action is indicated at box 36 of the outer loop which represents cross diaphragm flows within the stack 1.

[0053] In Fig. 4 and Fig. 1 the pressure reduction valve control signal 28 is indicated, and as disclosed, there are two pressure reduction valves 17. However, one valve or two or more valves may be provided in parallel at this place to better accommodate the range of flows through chosen valves 17.

[0054] The pressure reduction valve 17 is considered a fine-tuning option to ensure a desired precision and dynamic of the regulation of the pressure difference between hydrogen and oxygen separation tanks 2, 3. A further valve, the so-called controllable second back pressure valve 11 is also provided and is controlled automatically through a control loop of its own. This is better seen in Fig. 5 and in Fig. 1. The controllable second back pressure valve 11 is guided by the deviation between a setpoint signal 29 and a measured signal 30. The measured signal 30 constitutes the difference between the pressure signal from the second back pressure meter 25 right upstream of the second back pressure valve 11 and the pressure signal from the hydrogen separation tank pressure meter 14 arranged upstream of the second pressure reduction valve 17. This difference is indicative of the flow over the hydrogen pressure reduction valve 17. The setpoint signal 29 is commensurate with the projected flow of hydrogen generated at a given current supply to the stack 1.

[0055] As indicated in Fig. 1 , the oxygen is vented off from the oxygen separation tank 2 whenever an absolute oxygen pressure measured at the oxygen separation tank pressure meter 15 reaches a predetermined value. To ensure controlled venting off of oxygen, the control module 20 further comprises a control loop. The loop receives a setpoint system pressure signal 9, which is commensurate with a predetermined maximum system pressure under usual production conditions. Further, a measured signal from the oxygen separation tank pressure meter 15 is provided and based on the setpoint system pressure signal and the oxygen separation tank pressure meter signal, the control loop is adapted to generate a deviation signal. Based on the deviation signal, the calculation and control unit generates a setting for the controllable first back pressure valve 10 to ensure that the actual pressure obtained from the separation tank pressure meter 15 remains as close as possible to the setpoint system pressure signal 9. Optionally, the signal from the first back pressure meter 18 is also or alternatively taken into account in the controllable first back pressure valve control loop.

[0056] As seen in Fig. 1 , a so named deoxo and dryer unit 31 is provided between the pressure reduction valve 17 and the controllable second back pressure valve 11. This unit is comprised of two sub-units, whereby firstly any oxygen is removed from the hydrogen by simply allowing hydrogen to burn and consume the oxygen and produce small amounts of water vapor and some heat. This is done by feeding the gas stream through a reactor holding a catalyst, which is adapted to promote this combustion, which due to the low amounts of oxygen in the hydrogen gas will not proceed spontaneously outside of the reactor. The produced water is removed from the gas stream along with water already in the stream as water vapor. This may be done by chilling the gas stream down to the temperature, in which the dew or condensation point of the water vapor will ensure a sufficiently dry hydrogen gas stream. Alternatively, or in combination with chilling, water absorption materials may be employed.

[0057] It is mentioned that the pressure difference meter 16 and the two pressure gauges 14, 15 representing oxygen separation tank pressure and hydrogen separation tank pressure do not represent a real redundancy as the pressure gauges 14, 15 are arranged some distance downstream from the respective tanks 2, 3 in respective oxygen and hydrogen vent pipes 23, 24. The pressure difference meter 16 is an agile measuring instrument, which will provide a better and swifter real pressure difference value representing the pressure difference between the tanks 2, 3, and thus provide a safer and more precise input to the control loop than would the calculated difference between the signals from the pressure gauges 14,15. It is to be noted that the figures and the above description have shown the example embodiments in a simple and schematic manner. Many of the specific mechanical details have not been shown since the person skilled in the art should be familiar with these details and they would just unnecessarily complicate this description.

[0058] List of parts

[0059] 1 cell stack

[0060] 2 oxygen separation tank

[0061] 3 hydrogen separation tank

[0062] 4 electrical power supply

[0063] 5 oxygen and anolyte delivery pipe

[0064] 6 hydrogen and catholyte delivery pipe

[0065] 7 anolyte return pipe

[0066] 8 catholyte return pipe

[0067] 9 setpoint system pressure signal

[0068] 10 (controllable) first back pressure valve

[0069] 11 (controllable) second back pressure valve

[0070] 12 anolyte fluid level meter

[0071] 13 catholyte fluid level meter

[0072] 14 hydrogen separation tank pressure meter

[0073] 15 oxygen separation tank pressure meter

[0074] 16 pressure difference meter

[0075] 17 (controllable) hydrogen pressure reduction valve

[0076] 18 first back pressure meter

[0077] 19 gas separation diaphragm

[0078] 20 control module

[0079] 21 hydrogen recipient

[0080] 22 oxygen recipient

[0081] 23 oxygen vent pipe

[0082] 24 hydrogen vent pipe

[0083] 25 second back pressure meter

[0084] 26 outer control loop

[0085] 27 inner control loop

[0086] 28 pressure reduction valve control signal

[0087] 29 controllable back pressure valve setpoint signal

[0088] 30 controllable back pressure valve measured signal

[0089] 31 deoxo and dryer

[0090] 32 outer loop comparator 33 outer loop calculation unit

[0091] 34 inner loop comparator

[0092] 35 inner loop calculation unit

Claims

Claims1. Method for controlled release of hydrogen and oxygen from at least one alkaline electrolyser stack (1) having at least one gas separation diaphragm (19) whereby a. oxygen gas and anolyte from a first side of the diaphragm (19) are introduced into an oxygen separation tank (2) provoking an oxygen separation tank gas pressure, PO and an anolyte fluid level, Lan in the oxygen separation tank (2), and whereby b. hydrogen gas and catholyte from a second side of the diaphragm (19) are introduced into a hydrogen separation tank (3) provoking a hydrogen separation tank gas pressure, PH and a catholyte fluid level Lea in the hydrogen separation tank (3), whereby i. the fluid level differences, dL = (Lea - Lan), pressure differences, dP and pressures, PO and PH are determined, and the deviation between a desired fluid level difference dL des and actual measured fluid level difference dL are used to calculate a new desired pressure difference dP des and ii. a pressure control module controls the vent off of hydrogen and oxygen from the respective separation tanks (2,3) in order to thereby introduce and maintain the calculated desired pressure difference dP des whereby, a new actual fluid level difference dL is obtained and measured and used in a control loop involving point i. and ii. in order to minimise the deviation between a desired fluid level difference dL des and the just measured actual fluid level difference dL.

2. The method according to claim 1 , characterised in that one of the two hydrogen gas or oxygen gas is delivered from the respective separation tank (3, 2) to its recipient (21 , 22) by firstly leading the gas through a controllable second pressure reduction valve (17) and further through a controllable second back pressure valve (11) and towards a recipient (21 , 22), whereby a setpoint of the back pressure valve (11) is guided by the deviation between a chosen desired pressure loss over the pressure reduction valve (17) and a measured pressure loss over the pressure reduction valve (17) and whereby the setpoint of the pressure reduction valve (17) is guided by a deviation between the calculated desired pressure differences dP desand a measured actual pressure difference dP between the pressures in the two separation tanks (2, 3).

3. The method according to claim 2, characterised in that an absolute system maximum pressure is maintained in that the pressure control module (20) vents off oxygen from the oxygen separation tank (2) through a controllable first back pressure valve (10), or vents off hydrogen from the hydrogen separation tank (3) through a controllable first back pressure valve (10) to thereby maintain an absolute system pressure.

4. The method according to claim 3, characterised in that between the controllable pressure reduction valve (17) and a controllable second back pressure valve (11) firstly oxygen residues are removed and secondly water vapor is removed from the hydrogen gas stream when hydrogen is vented off through pressure reduction valve (17).

5. The method according to claim 4, characterised in that the setting of the controllable second back pressure valve (11) is chosen according to electrical operational parameters of at least one alkaline electrolyser stack (1).

6. Electrolyser and control module (20), whereby the electrolyser comprises at least one cell stack (1) having at least one gas separation diaphragm (19) whereby pipes (5) are arranged to direct oxygen gas and alkaline anolyte from a first side of the diaphragm (19) within the stack and into an oxygen separation tank (2), and whereby pipes (6) are arranged to direct hydrogen gas and catholyte from a second side of the diaphragm (19) and into a hydrogen separation tank (3), and whereby fluid level sensors (12, 13) are provided for the two separation tanks (2, 3), and a difference pressure gauge (16) is provided and arranged to provide a difference pressure signal dP, and also pressure gauges (14, 15) responsive to the pressures in the two tanks (2, 3) are provided, characterised in that the pressure control module (20) comprises a. an outer calculation and control loop having a calculation unit which is adapted to minimise a deviation between a desired fluid level difference dL des and the measured fluid level difference dL whereby the outer controlloop calculation unit is further adapted to calculate a desired pressure difference between the pressures in the two separation tanks, and b. an inner calculation and control loop i. adapted to receive a desired pressure difference calculated by the outer control loop, and whereby the inner control loop and calculation unit is adapted to regulate controllable valves (10, 11 , 17) in order to vent off hydrogen and / or oxygen from the separation tank or tanks to a recipient in order to minimise the deviation between the desired calculated pressure difference dP des and the measured pressure difference dP to thereby ii. maintain the desired fluid level difference dL des between fluid levels in the oxygen and hydrogen separation tanks (2, 3).

7. The electrolyser and control module (20) according to claim 6, characterised in that, in one of vent pipes (24, 23) arranged between a separation tank (3, 2) and a recipient (21 , 22), a controllable pressure reduction valve (17) is provided, and that in the vent pipe (23, 24) between the controllable pressure reduction valve (17) and the gas recipient (21, 22) a controllable second back pressure valve (11) is provided, whereby inner control and calculation loop of the control module (20) is adapted to a. guide the setting of the back pressure valve (11) in order to realise a chosen desired pressure loss over the controllable pressure reduction valve (17) and further adapted to b. guide the setting of the pressure reduction valve (17) to minimise a deviation between: i. the calculated desired pressure difference dP des between the pressures in each of the two separation tanks (2, 3), and ii. a measured actual pressure difference dP between the pressures in the two separation tanks (2, 3).

8. The electrolyser and control module (20) according to claim 7, characterised in that, in the vent pipe (23, 24) without the pressure reduction valve (17), a controllable first back pressure valve (10) is provided between the recipient (22, 21) and the corresponding separation tank (2, 3) whereby the control module furthercomprises a control loop adapted to minimize the deviation between a predetermined desired absolute pressure in the respective separation tank and the actual measured pressure (PO, PH) in that tank.

9. The electrolyser and control module (20) according to any one of the claims 6-8, characterised in that, in a hydrogen gas stream between the pressure reduction valve (17) and the controllable second back pressure valve (11) i. an oxygen removal device (31) is provided, and that in the hydrogen gas stream between the oxygen removal device and the controllable second back pressure valve (11), ii. a water vapor removal device (31) is provided.

10. The electrolyser and control module (20) according to any one of the claims 7-9, characterised in that, the controllable second back pressure valve (11) has a regulator, whereby this regulator is adapted to choose a setpoint for the controllable second back pressure valve (11) according to a signal indicative of a hydrogen production rate when the hydrogen vent pipe is connected to the controllable second back pressure valve (11) and whereby the regulator is adapted to choose a setpoint to the controllable second back pressure valve (11) according to a signal indicative of an oxygen production rate in the electrolyser when the oxygen vent pipe (23) is connected to the controllable second back pressure valve (11).

Images