PRESSURE COMPENSATING SYSTEM AND A HIGH-PRESSURE ELECTROLYSER SYSTEM COMPRISING THE SAME

Abstract

A pressure compensating system (1) for a dual fluid flow system, wherein the pressure compensating system (1) comprises: a fluid pipe (3) having a first fluid pipe portion (3a) and a second fluid pipe portion (3b), wherein the first fluid pipe portion (3a) has a first fluid inlet (5a) and a first fluid outlet (7a) for a first fluid flow (O.sub.2), wherein the second fluid pipe portion (3b) has a second fluid inlet (5b) and a second fluid outlet (7b) for a second fluid flow (H.sub.2) separate from the first fluid flow (O.sub.2), and a pressure compensator (11) arranged in the fluid pipe (3), separating the first fluid pipe portion (3a) and the second fluid pipe portion (3b), wherein the pressure compensator (11) is configured to move in the fluid pipe (3) between the first fluid outlet (7a) and the second fluid outlet (7b) to thereby at least partially obstruct one of the first fluid outlet (7a) and the second fluid outlet (7b) in response to a pressure differences between the first fluid pipe portion (3a) and the second fluid pipe portion (3b) to provide pressure compensation between the first fluid pipe portion (3a) and the second fluid pipe portion (3b).

Claims

1. A pressure compensating system (1) for a dual fluid flow system, wherein the pressure compensating system (1) comprises: a fluid pipe (3) having a first fluid pipe portion (3a) and a second fluid pipe portion (3b), wherein the first fluid pipe portion (3a) has a first fluid inlet (5a) and a first fluid outlet (7a) for a first fluid flow (O.sub.2), wherein the second fluid pipe portion (3b) has a second fluid inlet (5b) and a second fluid outlet (7b) for a second fluid flow (H.sub.2) separate from the first fluid flow (O.sub.2), and a pressure compensator (11) arranged in the fluid pipe (3), separating the first fluid pipe portion (3a) and the second fluid pipe portion (3b), wherein the pressure compensator (11) is configured to move in the fluid pipe (3) between the first fluid outlet (7a) and the second fluid outlet (7b) to thereby at least partially obstruct one of the first fluid outlet (7a) and the second fluid outlet (7b) in response to a pressure differences between the first fluid pipe portion (3a) and the second fluid pipe portion (3b) to provide pressure compensation between the first fluid pipe portion (3a) and the second fluid pipe portion (3b).

2. The pressure compensating system (1) as claimed in claim 1, wherein in a direction from the first fluid inlet (5a) to the second fluid inlet (5b) along the fluid pipe (3), the first fluid outlet (7a) is arranged after the first fluid inlet (5a) followed by the second fluid outlet (7b) followed by the second fluid inlet (5b).

3. The pressure compensating system (1) as claimed in claim 1 or 2, wherein the pressure compensator comprises (11) an incompressible fluid.

4. The pressure compensating system (1) as claimed in claim 3, wherein the incompressible fluid is a liquid.

5. The pressure compensating system (1) as claimed in claim 3 or 4, wherein the pressure compensator (n) comprises a first plunger (11a) and a second plunger (11b), wherein the incompressible fluid is provided between the first plunger (11a) and the second plunger (11b) which act to seal the incompressible fluid therebetween.

6. The pressure compensating system (1) as claimed in claim 5, wherein the first plunger (11a) is configured to be in fluid communication with the first fluid inlet (5a) and the second plunger (11b) is configured to be in fluid communication with the second fluid inlet (5b).

7. The pressure compensating system (1) as claimed in any of the preceding claims, wherein the first fluid pipe portion (3a) and the second fluid pipe portion (3b) are connected via a bend (9) and the pressure compensator (11) is located in the bend (9).

8. The pressure compensating system (1) as claimed in any of the preceding claims, wherein the first fluid pipe portion (3a) is provided with a first release valve (21a) configured to discharge fluid from the first fluid pipe portion (3a) in an initial state of the dual fluid flow system, and the second fluid pipe portion (3b) is provided with a second release valve (21b) configured to discharge fluid from the second fluid pipe portion (3b) in an initial state of the dual fluid flow system.

9. The pressure compensating system (1) as claimed in claim 8, wherein the first release valve (21a) and of the second release valve (21b) are configured to be controlled by external control.

10. The pressure compensating system (1) as claimed in claim 8 or 9, wherein the first release valve (21a) is a first solenoid valve and the second discharge vale (21b) is a second solenoid valve.

11. The pressure compensating system (1) as claimed in any of the preceding claims, wherein the first fluid outlet (7a) has a first axial section (13) with a tapering cross-section in the direction of the first fluid flow (O.sub.2) and a second axial section (15) downstream of the first axial section (13) with an increasing cross-section, and the second fluid outlet (7b) has a third axial section (17) with a tapering cross-section in the direction of the second fluid flow (H.sub.2) and a fourth axial section (19) downstream of the third axial section (17) with an increasing cross-section.

12. The pressure compensating system (1) as claimed in claim 11, wherein the first axial section (13) and the second axial section (15) form a converging-diverging nozzle structure which has a nozzle throat or waist having a diameter in the range of microns.

13. The pressure compensating system (1) as claimed in claim 11 or 12, wherein the third axial section (17) and the fourth axial section (19) form a converging-diverging nozzle structure which has a nozzle throat or waist having a diameter in the range of microns.

14. The pressure compensating system (1) as claimed in any of the preceding claims, comprising a first membrane assembly which includes PTFE membranes and a microporous filter membrane configured filter moisture and prevent flooding of the first fluid pipe portion (3a).

15. The pressure compensating system (1) as claimed in any of the preceding claims, comprising a second membrane assembly which includes PTFE membranes and a microporous filter membrane configured filter moisture and prevent flooding of the second fluid pipe portion (3b).

16. A high-pressure electrolyser system (23) comprising: an electrolyser stack (25) provided with an oxygen gas outlet (36a) and a hydrogen gas outlet (36b) and a water inlet (31) for filling the electrolyser stack (25) with water, a water inlet valve (33) configured to provide a one-way valve functionality of the water inlet (31), and a pressure compensating system (1) according to any of claims 1-15, wherein the first fluid inlet (5a) is connected to the oxygen gas outlet (36a) and the second fluid inlet (5b) is connected to the hydrogen gas outlet (36b).

17. The high-pressure electrolyser system (23) as claimed in claim 16, wherein the electrolyser stack (25) comprises a plurality of electrode plates (27a, 27b), each electrode plate (27a, 27b) having an inner metal frame (43) provided with electrode elements (46), and an outer heat conducting polymer frame (45) holding the inner metal frame (43).

18. The high-pressure electrolyser system (23) as claimed in claim 16 or 17, wherein the electrolyser stack (25) comprises a plurality of electrode plates (27a, 27b), each electrode plate (27a, 27b) comprising an outer frame (45) and electrode elements (46) extending in a space (47) between opposite sides of the outer frame (46), each electrode plate (27a, 27b) having a hydrogen channel (53) and an oxygen channel (51) extending through the outer frame (45), and a first outlet channel (59) and a second outlet channel (61) connecting the space (47) and one of the hydrogen channel (53) and the oxygen channel (51), wherein each first outlet channel (59) has a tapering shape in a direction from the space (47) to the hydrogen channel (53) or oxygen channel (51) and each second outlet channel (61) has a tapering shape in a direction from the hydrogen channel (53) or oxygen channel (51) to the space (47).

19. The high-pressure electrolyser system (23) as claimed in any of claims 16-18, comprising: a first pump and a second pump, a first sensor and a second sensor, and a pump control system configured to be connected to the first sensor and the second sensor, wherein the first sensor is configured to detect a water level in the electrolyser stack (25), wherein based on the water level the pump control system is configured to activate the first pump to pump more water into the electrolyser stack (25), wherein the second sensor is configured to detect a temperature in the electrolyser stack (25), wherein in case the temperature is above a threshold value, the pump control system is configured to activate the second pump to pump water from the electrolyser stack (25).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Examples of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

[0056] FIG. 1 schematically shows a top view of an example of a pressure compensating system;

[0057] FIG. 2 schematically shows an example of a high-pressure electrolyser system;

[0058] FIG. 3 schematically shows a front view of an electrode plate;

[0059] FIG. 4a shows a first section of the electrode plate in FIG. 3; and

[0060] FIG. 4b shows a second section of the electrode plate in FIG. 3 parallel with the section shown in FIG. 4a.

DETAILED DESCRIPTION

[0061] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0062] FIG. 1 shows an example of a pressure compensating system 1 for a dual fluid flow system. In the following, the dual fluid flow system will be exemplified by a high-pressure electrolyser system. It is to be noted that the pressure compensating system 1 could be used with any dual fluid flow system for which the two fluid flows may be have different pressure, for equalising the pressure between the two fluid flows.

[0063] The pressure compensating system 1 comprises a fluid pipe 3. The fluid pipe 3 has a first fluid pipe portion 3a and a second fluid pipe portion 3b. The first fluid pipe portion 3a has a first fluid inlet and a first fluid outlet 7a for a first fluid flow. The second fluid pipe portion 3b has a second fluid inlet 5b and a second fluid outlet 7b for a second fluid flow. In the following, the first fluid flow will be exemplified by oxygen gas flow O.sub.2 and the second fluid flow will be exemplified by hydrogen gas flow H.sub.2.

[0064] In a direction from the first fluid inlet 5a to the second fluid inlet 5b along the fluid pipe 3, the first fluid inlet 5a is followed by the first fluid outlet 7a. The first fluid outlet 7a is followed by the second fluid outlet 7b. The second fluid outlet 7b is followed by the second fluid outlet 5b.

[0065] The exemplified fluid pipe 3 has a bend 9 between the first fluid outlet 7a and the second fluid outlet 7b. According to the present example, the bend 9 provides a 180 degree turn of the fluid pipe 3. The first fluid pipe portion 3a and the second fluid pipe portion 3b are hence arranged in parallel.

[0066] The pressure compensating system 1 furthermore comprises a pressure compensator 11. The pressure compensator 11 is contained in the fluid pipe 3.

[0067] The pressure compensator 11 may be arranged in the bend 9. The pressure compensator 11 separates the first fluid pipe portion 3a and the second fluid pipe portion 3b.

[0068] The pressure compensator 11 comprises a first plunger 11a, a second plunger 11b and an incompressible fluid 11c arranged between the first plunger 11a and the second plunger 11b. The first plunger 11a hence forms a first end of the pressure compensator 11 and the second plunger 11b forms a second end of the pressure compensator 11. The first plunger 11a and the second plunger act as seals to maintain the incompressible fluid 11c therebetween.

[0069] The incompressible fluid 11c may preferably be an incompressible liquid. The first fluid outlet 7a is provided with a Venturi tube-like section forming a first nozzle. Hereto, the first fluid outlet 7a has a first axial section 13 with a tapering cross-section in the direction of the first fluid flow O.sub.2 and a second axial section 15 downstream of the first axial section 13 with an increasing cross-section.

[0070] The second fluid outlet 7b is provided with a Venturi tube-like section forming a second nozzle. Hereto, the second fluid outlet 7b has a third axial section 17 with a tapering cross-section in the direction of the second fluid flow H.sub.2 and a fourth axial section 19 downstream of the third axial section 17 with an increasing cross-section.

[0071] The first axial section 13 and the second axial section 15, and the third axial section 17 and the fourth axial section 19 hence form converging-diverging nozzles. These nozzles may be specially calibrated to achieve a desired output pressure. For example, if it would be desired to output a pressure of 300 bar and a hydrogen gas flow of 1 Nm.sup.3 per hour, assuming that the high-pressure electrolyser system can produce 1 Nm.sup.3 hydrogen gas per hour, the nozzles can be calibrated so that 1 Nm.sup.3/h hydrogen gas can only escape through the nozzle when the pressure reaches 300 bar in the electrolyser stack. The converging-diverging nozzle diameter, in particular the nozzle throat section or waist, which has the smallest diameter of the converging-diverging nozzle structure, will be different for the first fluid outlet 7a which discharges the oxygen gas, due to less oxygen than hydrogen and due to the different molecular weight of oxygen and hydrogen. The nozzle throat section or waist of the first fluid outlet 7a has a smaller diameter than the diameter of the nozzle throat section or waist of the second fluid outlet 7b. The smaller the diameter, the less gas can escape through the fluid outlet 7a, 7b under a certain pressure. The size of the diameter can be calculated in the design process required to flow a specific amount of gas under a certain pressure. In this manner, the output pressure capability can be changed by changing the converging-diverging nozzles. The throat diameters may according to one example be in the range of microns. Such diameters sizes can be made using for example laser micromachining. The first axial section 13 may be equal in length or longer than the second axial section 15. The third axial section 17 may be equal in length or longer than the fourth axial section 19. To this end, the converging portion of each of the first nozzle and the second nozzle may be longer than the corresponding diverging portion. The cross-sectional area of the converging portion may be reduced gradually until it reaches the throat diameter designed to achieve better compression, the calibrated flow rate per hour in a specific pressure, less flow-turbulence, reduced heat and increased nozzle life time. The outside surface of the first nozzle and the second nozzle may be provided with heat sink fins for natural convection of heat.

[0072] The exemplified pressure compensating system 1 furthermore comprises a first release valve 21a and a second release valve 21b. The first release valve 21a is configured to discharge fluid from the first fluid pipe portion 3a. The second release valve 21b is configured to discharge fluid from the second fluid pipe portion 3b. The first release valve 21a may for example be a solenoid valve. The second release valve 21b may for example be a solenoid valve.

[0073] The first release valve 21a may for example be arranged in a vertically upper or top region of the first fluid pipe portion 3a. Thereby, the evacuation of air may be facilitated because air is lighter than oxygen gas. Oxygen gas molecules may sink towards the bottom of the first fluid pipe portion 3a while air may rise upwards to the first release valve 21a.

[0074] The second release valve 21b may for example be arranged in a vertically lower or bottom region of the second fluid pipe portion 3b.

[0075] The first release valve 21a and the second release valve 21b may be configured to be controlled to open in an initial state of the pressure compensating system to discharge any gas contained in the fluid pipe 3 other than oxygen gas and hydrogen gas from the first fluid pipe portion 3a and the second fluid pipe portion 3b, respectively. When the fluid pipe 3 has been evacuated from such gas, the first release valve 21a and the second release valve 21b may be closed.

[0076] Thus, in operation, initially the first release valve 21a and the second release valve 21b are opened to evacuate any gas present in the fluid pipe 3 other than oxygen gas and hydrogen gas. When the gas has been evacuated, the release valves 21a and 21b are closed. In the even that the pressure compensating system 1 is used with an electrolyser oxygen gas O.sub.2 will flow into the first fluid pipe portion 3a and hydrogen gas H.sub.2 will flow into the second fluid pipe portion 3b. Eventually the pressure in the second fluid pipe portion 3b increases compared to the pressure in the first fluid pipe portion 3a due to the asymmetric oxygen gas and hydrogen gas production in electrolysis. This pressure difference causes the pressure compensator 11 to move inside the fluid pipe 3 towards the first fluid outlet 7a due to gas pressure on the second plunger 11b. The displacement of the pressure compensator 11 results in that the first plunger 11a partially obstructs or obstructs the first fluid outlet 7a, reducing the available cross-section for the oxygen gas O.sub.2 to escape through the first fluid outlet 7a. In this manner, the pressure will increase in the first fluid pipe portion 3a, eventually resulting in a pressure equalisation in the first fluid pipe portion 3a and the second fluid pipe portion 3b. During electrolysis the pressure compensator n will be displaced in either direction based on a current differential pressure in the first fluid pipe portion 3a and the second fluid pipe portion 3b. The pressure compensator 11 hence provides pressure equalisation by self-adjustment due to pressure-generated displacement in the fluid pipe 3.

[0077] Due to the Venturi tube-like design in the first fluid outlet 7a and the second fluid outlet 7b, there will be less heat generation, less turbulence, and less pressure drop as the first fluid and the second fluid exit the pressure compensating system 1.

[0078] FIG. 2 shows an example of a high-pressure electrolyser system 23. The high-pressure electrolyser system 23 comprises an electrolyser stack 25 and the pressure compensating system 1. The electrolyser stack 25 includes a plurality of electrode plates 27a and 27b. The electrode plates 27a and 27b are arranged in a stacked configuration one after the other. Each adjacent pair of electrode plates forms an electrolytic cell. Each electrode plate 27a and 27b is operated either as a cathode or as an anode, with each electrolytic cell having a cathode and an anode. Each electrode plate 27a and 27b has a frame structure comprising an inner frame and an outer frame, whereby a space is formed inside the inner frame. When the electrode plates 27a and 27b are stacked an electrolysis chamber configured to be filled with water is formed by the adjacent spaces.

[0079] The electrolyser stack 25 furthermore comprises a plurality of membranes. Each pair of adjacent electrode plates 27a and 27b is separated by a membrane so that each cathode forms a hydrogen chamber and each anode forms an oxygen chamber. The oxygen chambers and hydrogen chambers together form the electrolysis chamber. The membranes are configured to prevent hydrogen gas and oxygen gas to move between the electrode plates 27a and 27b in the electrolysis chamber.

[0080] The electrolyser stack 25 comprises a first end plate 29a forming a first end of the electrolyser stack 25 and a second end plate 29b forming a second end of the electrolyser stack 25. The electrode plates 27a and 27b are arranged between the first end plate 29a and the second end plate 29b.

[0081] The first end plate 29a is provided with two water inlets 31 configured to enable water to flow into the electrolysis chamber. The high-pressure electrolyser system 23 furthermore comprises two water inlet valves 33, one for each water inlet 31, configured to provide a check-valve functionality of a respective water inlet 31, a pump P and a pump controller 35.

[0082] The pump P is configured to pump water into the electrolyser stack 25 via the water inlets 31. The pump controller 35 is configured to control the pump P. For example, the pump controller 35 may be configured to operate the pump P only occasionally, such as once every hour. The pump controller 35 may hence use a timer function. The pump P may thus top up the water level in the electrolyser stack 25, which may hence be completely filled with water e.g. once every hour. Alternatively, other time frames may be used to operate the pump P by means of the pump controller 35. By operating the pump P only occasionally, energy may be saved while operating the high-pressure electrolyser system 23. Alternatively, the high-pressure electrolyser system may include one or more sensors to detect the water level in the electrolyser stack, wherein the pump controller 35 may be configured to control the pump based on the water level detected by the one or more sensors. As yet another alternative, the pump P could run at all times.

[0083] According to one example, the high-pressure electrolyser system 23 may comprise two pumps and two sensors. A first pump of the two pumps may be a high pressure pump and a second pump of the two pumps may be a low pressure pump. A first sensor may be configured to detect a water level in the electrolyser stack 25, and based on the water level, cause the pump controller 35 to activate the first pump to pump more water into the electrolyser stack. A second sensor may be configured to detect a temperature inside the electrolyser stack 25, and based on the temperature causes the pump controller 35, or alternatively another pump controller configured to control the second pump, to activate the second pump. The pump controller 35 or the other pump controller may be configured to activate the second pump for example if the temperature reaches a threshold value, for example 35 degrees or 40 degrees. The high-pressure electrolyser system may in general comprise a pump control system configured to control the first pump and the second pump. The pump control system may comprise the pump controller 35, which may be configured to control the first pump and according to one example also the second pump. The pump control system may according to one example comprise a dedicated controller configured to control the second pump. In this case, the pump control system would comprise two pump controllers, each configured to control a respective one of the first pump and the second pump. The second pump pumps water from the electrolyser stack when operated. The second pump hence functions as a cooling pump, enabling the high-pressure electrolyser system 23 to retain the same temperature to avoid overheating or self ignition of materials.

[0084] The electrolyser stack 25 further comprise an oxygen gas outlet 36a connected to the first fluid inlet 5a of the pressure compensating system 1 and a hydrogen gas outlet 36b connected to the second fluid inlet 5b of the pressure compensating system 1.

[0085] The high-pressure electrolyser system 23 may furthermore comprise gas outlet valves 37, which may be check-valves. The gas outlet valves 37 may be configured to allow a certain restricted gas flow of hydrogen gas and oxygen gas to flow out from the electrolyser stack 25 via the oxygen gas outlet 36a and the hydrogen gas outlet 36b to the pressure compensating system 1.

[0086] The first fluid outlet 7a of the pressure compensating system 1 may be connected to an oxygen gas pressure vessel 39 for storing the compressed oxygen gas and the second fluid outlet 7b may be connected to a hydrogen gas pressure vessel 41 for storing the compressed hydrogen gas.

[0087] FIG. 3 depicts an example of an electrode plate 27a or 27b. The exemplified electrode plate has an inner frame 43 and an outer frame 45. The inner frame 43 is preferably made of a metal with good electrical conducting properties, for example copper or aluminium. The inner frame 43 may hence be an inner metal frame. The outer frame 45 may be made of a heat conducting polymer. The outer frame 45 may hence be an outer heat conducting polymer frame. The outer frame 45 holds the inner frame 43. The outer frame 45 may for example be made by means of injection moulding. To this end, during manufacturing the inner frame may be placed inside an injection mould, wherein a heat conducting polymer is injected into the frame to form the outer frame 45.

[0088] The electrode plate 27a, 27b furthermore comprises electrode elements 46 extending between opposite sides of the inner frame 43 and hence also of the outer frame 45. The inner frame 43 delimits a space 47 in the region where the electrode elements 46 extend. This space 47 is an oxygen chamber in case the electrode plate is operated as an anode and a hydrogen chamber in case the electrode plate is operated as a cathode. The electrode plate 27a, 27b has a terminal 49 which is connected to the electrode elements 46 via the inner frame 43 and which is configured to be connected to a power supply.

[0089] The outer frame 45 is provided with an oxygen channel 51 and a hydrogen channel 53. Only one of these two channels 51 and 53 is configured to be in fluid communication with the space 47. In case the electrode plate is operated as an anode only the oxygen channel 51 is in fluid communication with the space 47 and in case the electrode plate is operated as a cathode only the hydrogen channel 53 is in fluid communication with the space 47. Since the electrode plates 27a and 27b are stacked alternatingly with a membrane covering the space 47 between them, every other electrode plate, i.e. every anode, will contribute to the oxygen gas stream in the oxygen channel 51 and every other plate, i.e. every cathode, will contribute to the hydrogen gas stream in the hydrogen channel 53.

[0090] In addition to the membranes, the electrolyser stack 25 may comprise a plurality of electrically insulating gaskets, each being sandwiched between two adjacent electrode plates 27a and 27b to provide electrical insulation and sealing between the electrode plates 27a and 27b.

[0091] Each electrode plate 27a, 27b may also comprise two water channels 55 and 57. A first water channel 55 of the two water channels may be connected to one of the water inlets 31 and a second water channel 57 of the two water channels may be connected to the other one of the water inlets 31. For an electrode plate 27a acting as anode the first water channel 55 is in fluid communication with the space 47, by means of a channel extending from the first water channel 55 to the space 47, while the second water channel 57 is not. For an electrode platen 27b acting as cathode the second water channel 57 is in fluid communication with the space 47 by means of a channel extending from the second water channel 57 to the space 47, while the first water channel 55 is not. This means that the anodes have their own water supply and the cathodes have their own water supply. This reduces the risk of cross-contamination between oxygen chambers and hydrogen chambers.

[0092] The first water channel 55 has a central channel portion and two oppositely arranged lateral fins 55a which are narrower compared to the central channel portion. The second water channel 57 has a central channel portion and two oppositely arranged lateral fins 57a which are narrower compared to the central channel portion. This provides the effect that the same or essentially the same water pressure can be provided along the length of the first water channel 55 and the second water channel 57 as they extend along the electrolyser stack 25. The oxygen channel 51 and the hydrogen channel 53 may according to one variation also have this configuration.

[0093] FIG. 4a shows a close-up view of a section through an electrode plate 27a acting as anode. The inner frame 43 and the outer frame 45 are provided with a first outlet channel 59 extending from the space 47 to the oxygen channel 51. The first outlet channel 59 has a Venturi tube-like design and is tapering in a direction from the space 47 to the oxygen channel 51. The hydrogen channel 53 of this electrode plate 27a has no opening to the space 47.

[0094] For an electrode plate 27b acting as a cathode the inner frame 43 and the outer frame 45 are provided with a first outlet channel extending from the space 47 to the hydrogen channel 53. The first outlet channel has a Venturi tube-like design and is tapering in a direction from the space 47 to the hydrogen channel 53. The oxygen channel 51 of this electrode plate 27b has no opening to the space 47.

[0095] FIG. 4b shows a close-up view of a section through the electrode plate 27a in parallel with the section shown in FIG. 4a but further downstream in the thickness direction of the electrode plate 27a. The inner frame 43 and the outer frame 45 are provided with a second outlet channel 61 extending from the space 47 to the oxygen channel 51. The second outlet channel 61 has a Venturi tube-like design and is tapering in a direction from the oxygen channel 51 to the space 47.

[0096] For an electrode plate 27b acting as a cathode the inner frame 43 and the outer frame 45 are provided with a second outlet channel extending from the space 47 to the hydrogen channel 53. The first outlet channel has a Venturi tube-like design and is tapering in a direction from the hydrogen channel 53 to the space 47.

[0097] The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.