Hydrogen plant

Abstract

A hydrogen plant (1) comprising - an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12); and - at least one turboexpander unit (20) connected to the oxygen outlet (12); wherein the at least one turboexpander unit (20) is connected to power a unit of the hydrogen producing plant (1) through a mechanical drive (30) directly connected to an output shaft of the turboexpander (20).

Description

[0001] HYDROGEN PLANT

[0002] The present invention relates to a hydrogen producing plant comprising at least one water electrolyser. More specifically the present invention relates to increasing the energy efficiency in such hydrogen plant by utilizing the oxygen bi-product from a hydrogen plant.

[0003] Background of the invention

[0004] In electrochemical electrolysis of water, electricity is used to split water into its two elemental components - hydrogen and oxygen. It is a well-known technology that has been used industrially on a small scale for well over a century, but with the rapidly expanding supply of cheap, clean electricity, the production of hydrogen from electrolysis is expected to grow from a capacity of a few megawatts into tens of gigawatts in the coming decade.

[0005] For example, modern high-pressure operation of the alkaline electrolysis modules is beneficial since it minimises water evaporation and lowers the cost of further pressurisation of the output H2-gas. Meanwhile, the O2-gas is currently depressurised and vented into the atmosphere due to its considered insignificant value.

[0006] It has been suggested in EP 31 40 435 B1 that the pressurised O2gas can perform useful work when passed through a turbine, which converts the potential energy stored in the gas into rotational (kinetic) energy, and which can be converted to electricity in a generator.

[0007] However, the generation of electricity comes with an unavoidable conversion loss which lowers the efficiency of the energy recovery.

[0008] It has also been suggested to combine hydrogen plants with wind power and / or solar power, in so-called “power to X” (PtX, P2X) facilities. For example, such facilities may be located offshore. Placing a hydrogen plant offshore provides plenty of water for the hydrogen production, but causes certain challenges, e.g. due to the hostile saline environment, remoteness, and lack of back-up systems, etc.

[0009] Summary of the invention

[0010] It is therefore an object of the invention to increase the variety of options.

[0011] In the broadest sense the objects of the invention are achieved by a hydrogen plant comprising

[0012] • an electrolysis unit having an oxygen outlet and a hydrogen outlet; and

[0013] • a turboexpander unit connected to the oxygen outlet; wherein the turboexpander unit is connected to power a unit of the hydrogen producing plant through a mechanical connection directly connected to an output shaft of the turboexpander unit.

[0014] However, in a first aspect of the invention, the objects of the invention are achieved by a hydrogen plant comprising

[0015] - an electrolysis unit having a hydrogen outlet and an oxygen outlet, and

[0016] - a turboexpander unit connected to the oxygen outlet, wherein the turboexpander unit is connected to power a vacuum unit of a distillation module, the distillation module connected to supply water to the electrolysis unit.

[0017] Thereby, clean or desalinated water for the electrolysis process in the electrolysis unit(s) of the hydrogen plant may be provided in ample amount, and surplus energy that would have otherwise been wasted is put to good use.

[0018] In an embodiment, the turboexpander unit is configured for powering a vacuum pump of the vacuum unit by a mechanical drive.

[0019] By directly utilizing the rotational work from turboexpander unit, rather than e.g. using a generator, battery and motor, considerable resources are saved, complexity reduced, and the energy is used in the most efficient way, with minimum losses. The hydrogen plant according to this first aspect of the invention is particularly useful in connection with offshore electrolysis applications, where clean water for the electrolysis units needs to be produced on site. By the hydrogen plant according to this aspect of the invention, an efficient vacuum distiller module could be installed together with the electrolysers (electrolysis units).

[0020] In a further embodiment of the first aspect of the invention, the vacuum unit comprises an inlet water pump, and the turboexpander unit connects to the inlet water pump via a mechanical drive, where the inlet water pump is configured to feed a pressurized inlet water stream into a vacuum unit pump connected to apply vacuum to the distillation module.

[0021] In a further embodiment, surplus heat from the electrolysis unit is used to heat water led into the distillation unit to accelerate the distillation process in the distillation module.

[0022] In a further embodiment, water to be treated in the distillation module is pre-heated by excess heat generated by the electrolysis unit by use of a heat exchanger.

[0023] In an embodiment thereof, a heat pump is inserted in a connection line between the electrolysis unit and the heat exchanger, to enhance the cooling of a fluid, such as produced gas, lye or cooling medium at the electrolysis unit, and so that heat energy transferred through the heat pump may be delivered at the heat exchanger at an increased temperature.

[0024] The objects of the invention may further be achieved in a second aspect of the invention by a hydrogen plant comprising:

[0025] - an electrolysis unit for producing hydrogen at a first pressure, and having a hydrogen outlet and an oxygen outlet;

[0026] - a turboexpander unit fluidly connected to the oxygen outlet; and

[0027] - a drying system for drying hydrogen received from the hydrogen outlet and comprising two zeolite beds, wherein hydrogen received from the hydrogen outlet can selectively be passed through one of the two zeolite beds for drying at the first pressure, while hydrogen at a second pressure lower than the first pressure can be passed through the other of the two zeolite beds to dry zeolite therein and regenerate the capacity for absorbing water of the zeolite, wherein wet hydrogen from the regenerating zeolite bed is passed through a cryo-condensor unit to dry the wet hydrogen received from the regenerating zeolite bed, wherein a compressor is fluidly connected to an outlet of the cryo-condensor unit in order to pressurize the hydrogen dried therein; and wherein the turboexpander unit is connected to power said compressor via a mechanical connection.

[0028] Thereby, in an energy-efficient way, hydrogen used for regenerating zeolite beds of a hydrogen drying system, may be dried.

[0029] By embodiments of the second aspect of the invention, which are described in the detailed part of the description, the hydrogen used for regenerating zeolite beds of a hydrogen drying system can be re-pressurized after drying, such that the hydrogen used is not wasted, but can instead re-enter the bulk hydrogen delivered from the hydrogen plant.

[0030] In a third aspect, the objects of the invention are achieved by a hydrogen plant comprising

[0031] - an electrolysis unit for producing hydrogen at a first pressure and having an hydrogen outlet and an oxygen outlet; and

[0032] - a turboexpander unit fluidly connected to the oxygen outlet;

[0033] - a compressor; wherein the turboexpander unit is connected to power the compressor; and wherein the compressor is fluidly connected to the hydrogen outlet of the electrolysis unit to increase pressure of the hydrogen therefrom.

[0034] Thereby, the pressurized oxygen produced as a bi-product in the electrolysis process may be utilized in an energy efficient way to further pressurize the produced hydrogen. This is useful in the transport of the hydrogen for example in a pipeline and / or when storing hydrogen underground in washed out salt domes or other storage facilities.

[0035] Further embodiments of the third aspect of the invention are described in the detailed description below.

[0036] In a fourth aspect, the objects of the invention are achieved by a hydrogen plant comprising

[0037] - an electrolysis unit having a hydrogen outlet and an oxygen outlet;

[0038] - a turboexpander unit fluidly connected to the oxygen outlet; and

[0039] - an oxygen tank fluidly connected to the oxygen outlet and to the at least one turboexpander unit, said oxygen tank being configured to collect and store oxygen.

[0040] Thereby, oxygen is made available for use for example in the above mentioned aspects, at all times, for example also when the hydrogen plant is shut down or in a stand-by mode.

[0041] In a fifth aspect, the objects of the invention are achieved by a hydrogen plant comprising

[0042] - an electrolysis unit having a hydrogen outlet and an oxygen outlet;

[0043] - a turboexpander unit fluidly connected to the oxygen outlet; and

[0044] - a hydrogen removal unit for combusting residual hydrogen in the oxygen from the hydrogen outlet in a catalytic process, wherein the hydrogen removal unit is fluidly connected between the oxygen outlet and the at least one turboexpander unit.

[0045] By using a residual content of hydrogen in the bi-product oxygen, an increased energy content in the oxygen gas downstream of the hydrogen removal unit will be provided to further energize the turboexpander unit. This increased energy content may be converted in a turboexpansion unit to provide increased rotation work in a rotation driven device via a mechanical drive. The rotation energy consuming device may be anyone of the devices described in connection with aspects one-four above. By this measure, residual hydrogen is also removed from the oxygen stream, which otherwise is almost always delivered to the atmosphere, and as hydrogen in itself also embodies a greenhouse gas potential, the measure ensures less pollution from the electrolyser plant, which is highly desirable.

[0046] Advantageous embodiments of the fifth aspect of the invention are described in the detailed part of the description below.

[0047] In a sixth aspect, the objects of the invention are achieved by a hydrogen plant comprising

[0048] - an electrolysis unit having a hydrogen outlet and an oxygen outlet;

[0049] - a first turboexpander unit connected to the oxygen outlet; and

[0050] - a second turboexpander unit connected to the oxygen outlet, wherein the first turboexpander unit is connected to power a vacuum unit of a distillation unit of the hydrogen plant according to any one of the embodiments of the first aspect of the invention, and wherein an inlet of the second turboexpander unit is further connected in series to an outlet of the first turboexpander unit such that oxygen that has passed through the first turboexpander unit is used to power the second turboexpander unit.

[0051] By this principle the oxygen byproduct of the electrolysis process in the hydrogen plant may be used to drive two or more of the processes described above in connection with aspects one - five.

[0052] Advantages embodiments of the sixth aspect of the invention are described in the detailed part of the description below.

[0053] It should be emphasized that the term "comprises / comprising / comprised of" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0054] Brief description of the drawings 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.

[0055] Fig. 1 illustrates a prior art part of hydrogen plant downstream of electrolysis unit or a plurality / group of electrolysis units, the part comprising a turboexpander unit that converts the energy from pressurised O2 gas to kinetic energy that drives a turbine;

[0056] Fig 2, in diagram form, shows a hydrogen plant according to a first aspect of the invention, where a turboexpander delivers energy for distillation / water purification unit;

[0057] Fig. 3, in diagram form, shows another embodiment of a hydrogen plant, where a turboexpander delivers energy for powering a distillation / water purification unit;

[0058] Fig. 4, in diagram form, shows a hydrogen plant according to a second aspect of the invention, where a turboexpander delivers energy for pressurizing used to regenerate a drying device for the hydrogen produced by the electrolysis unit(s);

[0059] Fig. 5, in diagram form, shows a hydrogen plant according to a third aspect of the invention, where a turboexpander delivers energy to pressurize hydrogen after production thereof in the electrolysis unit(s);

[0060] Fig. 6, in diagram form, shows a multi-step version of the pressurization method for hydrogen of Fig. 6;

[0061] Fig 7, in diagram form, illustrates use of a residual content of hydrogen in the oxygen to boost the energy that can be harvested from the oxygen produced in the hydrogen plant; Fig. 8, in diagram form, shows how multiple processes of hydrogen plant may be driven by using the bi-product oxygen to drive turbo expanding units arranged in parallel as well as in series, such that expanded oxygen may be used to drive a downstream turboexpander unit;

[0062] Fig. 9, in diagram form, shows an offshore plant, with an electrical battery and a pressurized oxygen tank;

[0063] Fig. 10, in diagram form, shows how hydrogen may be further compressed by way of the energy gleaned of the compressed oxygen;

[0064] Fig. 11 , in diagram form, shows use of energy gleaned of the compressed oxygen to pressurize hydrogen;

[0065] Fig.12, in diagram form, shows a situation where turboexpander units drive only electricity generators, and the electricity is consumed at electrically driven hydrogen compressors prior to the output of the hydrogen; and

[0066] Fig 13, in diagram form, shows the core process, where the oxygen stream is used to energize the turboexpander unit.

[0067] Detailed description of the embodiments

[0068] Fig. 1 illustrates a prior art part of a hydrogen plant, which part is downstream of electrolysis unit or plurality of electrolysis units, the part comprising a turboexpander unit that converts the energy from pressurised O2 gas to kinetic energy that drives a turbine. For safe operation of the electrolysis units, a regulator system is included for the gas to by-pass the turboexpander unit.

[0069] Fig. 1 shows - in diagram form - a part of a prior art hydrogen plant, the part formed downstream of a (not shown) electrolysis unit or plurality of electrolysis units. Fig. 1 does not show an entire hydrogen plant, but only a part thereof. A hydrogen plant may comprise one or more electrolysis units, also called stacks / electrolysis stacks or electrolysers. An electrolysis unit produces hydrogen and oxygen from water.

[0070] The electrolysis unit(s) 10 (seen in Fig. 2) may preferably be of the pressurized type, where the water for the electrolysis process is fed to the electrolysis units 10 under pressure so that the electrolysis process is conducted under pressure, such as 30 bar, or such as 60 bar or such as 90 bar or up to 110 bar, and where the resulting hydrogen (H2) is consequently delivered at the same pressure as the pressure of the electrolysis process. As a bi-product, oxygen (O2) is also delivered at the pressure of the electrolysis process.

[0071] High-pressure operation of the alkaline electrolysis modules, e.g. supplied from the applicant, is beneficial since it lowers the cost of pressurising the delivered H2-gas which is otherwise an arduous and costly task for the recipient of the gas.

[0072] The electrolysis unit or plurality of electrolysis units has a hydrogen (H2) outlet (not shown in Fig. 1) and an oxygen outlet 12. The oxygen outlet 12 is connected to an inlet 21 of a turboexpander unit 20 (turbine).

[0073] The hydrogen plant further comprises a turboexpander unit 20 that converts the energy from pressurised O2gas to kinetic energy that drives a generator 31.

[0074] Via a mechanical connection 30, such as a shaft, the turboexpander unit 20 may drive the generator 31 . The electrical energy thus extracted from the oxygen produced by the electrolysis process in the electrolysis unit or stack of electrolysis units may thus be used for various purposes. For safe operation of the electrolysis unit or stacks of electrolysis units, a regulator system including a valve 13 and a regulator 14 is included for enabling by-pass of the turboexpander unit 20. Depressurized oxygen may exit the turboexpander unit 20 via an outlet 22. The outlet 22 of the turboexpander unit 20 may connect to a stack cooling unit 25. Thus, the temperature decrease of oxygen depressurized in the turboexpander unit 20 may be used to regulate the temperature of the electrolysis unit or stacks of electrolysis units of the hydrogen plant.

[0075] Fig. 2 shows, a hydrogen plant 1 according to a first aspect of the invention. The hydrogen plant 1 comprises an electrolysis unit 10 or a plurality 100 of electrolysis units 10. The electrolysis unit 10 or plurality of electrolysis units has a hydrogen (H2) outlet 11 and an oxygen (O2) outlet 12.

[0076] Fig. 2 shows a hydrogen plant comprising 6 electrolysis units, also called stacks / electrolysis stacks or electrolysers. It will be appreciated that the inventive ideas described in the following will also work for fewer or more electrolysis units.

[0077] The hydrogen outlet 11 may connect downstream to a (not shown) hydrogen recipient, to a pipeline for transporting the hydrogen under pressure, or to other means of transporting or storing the hydrogen.

[0078] The oxygen outlet 12 is connected to an inlet 21 of a turboexpander unit 20 (turbine).

[0079] As is the case for the prior art system shown in Fig. 1 , for safe operation of the electrolysis unit 10 or plurality 100 of electrolysis units 10, a regulator system including a regulator 14 is included for the oxygen to by-pass the turboexpander unit 20. Depressurized oxygen may exit the turboexpander unit 20 via an outlet 22. The outlet 22 of the turboexpander unit 20 may in one embodiment connect to the surrounding environment. In another embodiment, similar to the situation in Fig. 1 , the outlet 22 of the turboexpander unit 20 may connect to a stack cooling unit (not shown in Fig. 2). Thereby, the temperature decrease of oxygen depressurized in the turboexpander unit 20 may be used to regulate the temperature of the electrolysis unit or plurality of electrolysis units of the hydrogen plant.

[0080] According to the first aspect of the invention, the turboexpander unit 20 is connected to power a vacuum unit 45 of a distillation module 40, where the distillation module 40 is connected to supply distilled water to the plurality 100 of electrolysis units 10. The vacuum unit 45 is configured for providing a vacuum to a vacuum device 41, such as an evaporator, of the distillation module 40, whereby water may be purified / cleaned, such as desalinated by the distillation module 40.

[0081] As shown in Fig. 2, water from the external environment may be passed into the vacuum device 41 via first water inlet 42. The water may be passed into the vacuum device 41 of the distillation module 40 by a first circulation pump 42. Clean (desalinated) water from the vacuum device 41 of the distillation module 40, may be led to the electrolysis unit 10 or plurality 100 of electrolysis units by a second circulation pump 43. These circulation pumps are independent from the vacuum unit 45.

[0082] The vacuum unit 45 providing vacuum to the vacuum device 41 for driving the distillation process may in principle (not shown) be powered by a pump with an electrical motor supplied for example with electricity by a generator 31 as shown in Fig. 1.

[0083] However, in an embodiment, the turboexpander unit 20 drives a pump of the vacuum unit 45 via a mechanical connection 30, such as a shaft, connecting the turboexpander unit 20 directly to the pump of the vacuum unit 45. In one such embodiment, and as illustrated in Fig. 2, such a pump is a vacuum pump 46. Examples of vacuum pumps are centrifugal or scroll pumps or ejector pumps.

[0084] In another embodiment, and as shown in Fig. 3, the turboexpander unit 20 may alternatively connect to an inlet water pump 47 directly via a mechanical drive 30, the inlet water pump 47 being configured to feed a pressurized inlet water stream into a vacuum pump 46, such as a vacuum ejector, connected to apply vacuum to the distillation module 40. In this case the vacuum pump is driven by the pressurized water flow from the inlet water pump 47.

[0085] Fig. 3 shows a hydrogen plant comprising 6 electrolysis units, also called stacks / electrolysis stacks or electrolysers. It will be appreciated that the inventive ideas described in the following will also work for fewer or more electrolysis units. In either of the above described situations, surplus heat from the electrolysis unit 10 may be used to heat water in the distillation module 30 in order to accelerate the distillation process in the distillation module 40. This is shown in Figs. 2 and 3.

[0086] In one, not shown embodiment, the surplus heat from the electrolysis unit 10 is used to heat water led into the distillation unit 30 in order to accelerate the distillation process in the distillation module 40.

[0087] In further embodiments, and as shown in both Fig. 2 and 3, the water to be treated in the distillation module 40 is pre-heated by excess heat generated by the electrolysis unit 10 by use of a heat exchanger 50. The heat exchanger is formed in a connection between the electrolysis unit 10 or plurality 100 of electrolysis units 10 and the distillation module 40.

[0088] In further embodiments (not shown), a heat pump is inserted a connection line between the electrolysis unit 10 or plurality 100 of electrolysis units 10, and the heat exchanger 50, in order to enhance the cooling of a fluid. The fluid may be a produced gas, lye or cooling medium at the electrolysis unit 10, so that heat energy transferred through the heat pump 48 may be delivered at the heat exchanger 50 at an increased temperature.

[0089] In further embodiments, water used for the distillation is also used to cool the electrolysers 10. This is a side effect of the connection between the distillation module 40 and the electrolysis unit 10 / plurality 100 of electrolysis units 10 via the heat exchanger 50.

[0090] The vacuum needed to perform vacuum distillation of water depends to a great extent on the specific design and operation mode of the distiller / distillation module 40. The vacuum would however not exceed what is considered a rough vacuum which can be achieved - in some embodiments - by a centrifugal and scroll pump or an ejector pump, such as exemplified above. Centrifugal and scroll pumps rely on a rotational element, and it would therefore be possible to directly couple a turboexpander 20 to one of these pumps, possible through some gearing. The exact energy consumption of such a pump again depends on the extent and volume of the vacuum. It is however claimed that vacuum distillation requires as little as 1.5-4.0 kWh / m3of produced fresh water. In the following scenario it will be assumed that this entire energy requirement is due to the vacuum pump alone. Additional heating of the water to accelerate the distillation is assumed to be supplied from waste heat from the electrolysis unit 10 itself, and the distillation process would therefore also help cooling the electrolysis unit 10. To examine if this scenario gives an energy surplus, the amount of oxygen produced from 1 m3of water will be calculated in the following.

[0091] 1 m3of water would correspond to 55509.3 moles of water, which would produce 27754.6 moles of O2. This corresponds to 888148.8 g of water or 629.45 Nm3of O2 (which is the amount of oxygen produced after 1 hour and 3 minutes of electrolysis at 100 % capacity). Nm is “Nm3is “normal cubic meter”. If this volume of oxygen gas was to be passed through the turboexpander, 20, 37.5 kWh of energy would be produced, meaning that 8 % of this energy is required to distil 1 m3of water (assuming 3 kWh / m3). There would thus be plenty of surplus power left to drive a heat exchanger 50 that would heat the saline water and cool the electrolysis units 10.

[0092] Now turning to Fig. 4, showing a hydrogen plant 1, according to a second aspect of the invention.

[0093] When producing hydrogen in an electrolysis unit 10, there is usually a residual water content in the produced hydrogen. Often this is not a problem. However, for some applications, it is desirable to remove water from the hydrogen gas, i.e. to dry the hydrogen gas, before transport and storage. An efficient way to dry the produced hydrogen is accomplished by leading the product gas through a vapor absorbing material, e.g. zeolite.

[0094] For this purpose, gas drying devices, such as zeolite beds 61 may be used.

[0095] However, during the drying process, the zeolite in the drying device become saturated with water and will therefore need regeneration. The regeneration entails removing the water content from the zeolite material, when the maximum capacity of the zeolite bed 61 has been reached.

[0096] This is done by passing low-pressure hydrogen through the zeolites while heating them. The hydrogen would subsequently need to be dried without the use of zeolites and re-pressurised in order not to waste hydrogen. Using only one drying device requires pausing the drying of the hydrogen, while the drying device is regenerated.

[0097] Therefore, regeneration is done by utilizing a dual drying system, where one dryer / drying device dries product gas (in this case hydrogen), while the other dryer / drying device is regenerated. Thus, a hydrogen gas drying system for a hydrogen plant preferably comprises two or more drying devices, such as zeolite beds 61 , so that drying the hydrogen gas may continue through one drying device, while the other drying device is being regenerated by removal of the water therefrom.

[0098] To regenerate a dryer / drying device, some of the dry product gas (hydrogen) is led back through the dryer at low pressure (for example 2 bar) and high temperature (for example 150°C), to desorb and carry away the water from the zeolite. The zeolite cannot contain much water at low pressure, and thus the water is desorbed by this process.

[0099] When the vessel is used for drying a product gas, the gas flows upwards. When the vessel is regenerating, gas flows downwards easing the water removal from the dryer.

[0100] Water containing warm hydrogen gas leaves the zeolites at 2 bar, where after it needs to be dried and pressurised back to system pressure (30 bar in this example).

[0101] As an approximation, 2 % of the produced and zeolite dried hydrogen will be depressurised and used to regenerate the one of the dryers, while the remaining 98 % is led to the gas recipient at system pressure. With a production rate of 1200 Nm3H2 / h (where Nm3 / h i.e. “Nm3 / Hour” is a standard unit used to measure gas flow rate), 24 Nm3H2 / h would thus be led through the wet zeolite bed in order to dry it. Subsequently, the wet, low-pressure H2-gas is dried in a cryo-condenser device, which is assumed to cool the gas to -30°C (cooled by the expanded O2 gas which has a temperature of -114°C). It would then require 2.2 kW to 3.27 kW for an isothermal and adiabatic compression respectively if an 80 % efficient compressor is assumed. This energy could be produced by expanding 55 Nm302 / h (corresponding to around 9 % of O2 production) from system pressure of 35 bar used for this calculation example.

[0102] Pressures up to 110 bar may well be realised, and higher pressure would only ensure release of more energy from the 02 gas stream.

[0103] The remaining oxygen would create additional 32.5 kW of energy (possibly from the same turboexpander which then could be coupled two different compressors). This energy could be used to compress all 1200 Nm3H2 / h from 35 bar to a pressure range within 64-68 bar for an isotropic and adiabatic compression respectively assuming no cooling of the hydrogen gas prior to compression.

[0104] According to the second aspect of the invention, a compressor 80 driven directly via a mechanical connection 30 to the turboexpander unit 20, which turboexpander unit 20 is driven by the oxygen side, performs the pressurisation. The expansion process of the oxygen produces cold enough gas to dry the zeolite-regenerating hydrogen gas though a cryogenic device. The design of this setup is illustrated in Fig. 4.

[0105] Turning now to Fig. 5, illustrating a hydrogen plant 1 according to a third aspect of the invention. In the production of hydrogen, both decrease of footprint of intermittent hydrogen storage vessels and / or ease of transport of hydrogen through gas-pipes, are items which benefit from increase in hydrogen output pressure.

[0106] Fig. 5 shows - in schematic form - a hydrogen plant 1 comprising a group / plurality 100 of hydrogen plants 10 connecting to a hydrogen pressurization system 2. The electrolysis unit(s) 10 may preferably be of the pressurized type, as described above. The at least one electrolysis unit 10 of the group / plurality 100 of electrolysis units 10 produced hydrogen at a first pressure, Pi , and has a common hydrogen outlet 11 and an oxygen outlet 12. The oxygen outlet 12 is as in the above-described aspects of the invention fluidly connected to a turboexpander unit 20. The turboexpander unit 20 is connected to power a compressor 120 of a hydrogen pressurization system 2. Thus, the turboexpander unit 20 is configured for powering compressor 120. The turboexpander unit 20 is connected to the compressor 120 by a mechanical drive 30 such as a shaft. Even if a gearbox is not indicated, such a device may be provided between the turboexpander and the compressor as is well known in the art to ensure the right rpm value of the compressor vis-a-vis the spinning speed of the turboexpander.

[0107] The compressor 120 is further fluidly connected to the hydrogen outlet 11 of the group / plurality 100 of electrolysis units 10. By leading hydrogen from the group / plurality 100 of electrolysis units 10 through the compressor 120, the output pressure of the produced hydrogen may be increased.

[0108] Thus, expansion of the oxygen gas stream drives at least one turboexpander of a turboexpander unit 20, which is coupled to a compressor 120 which further pressurises hydrogen received from the group / plurality 100 of electrolysis units 10.

[0109] When the oxygen from the oxygen outlet 12 of the group / plurality 100 of electrolysis units 10 is expanded through the turboexpander unit 20, the oxygen is cooled. The cold oxygen gas is in an embodiment of the third aspect of the invention used to cool the hydrogen gas prior to compression which would lower the energy consumption of the compression process.

[0110] This is shown in Fig. 5, where the connection 125 from the hydrogen outlet 11 to the compressor 120 is provided with a heat exchanger 130. The heat exchanger 130 is also connected to an outlet 22 of the turboexpander unit 20 via a fluid connection 135. The compressor 120 further connect to a compressed hydrogen outlet 129 via as fluid connection 126. The compressed hydrogen may be passed to a recipient such as a storage facility (tank(s) or e.g. a sub terrain salt dome), a subsea pipe system (if the hydrogen plant is placed offshore), or a land based pipe system via the compressed hydrogen outlet 129, or a combination of the mentioned.

[0111] The oxygen passed through the heat exchanger 130 may be vented to ambient through an oxygen vent 19 via a fluid connection136.

[0112] As an example, an electrolyser module (group 100 of 6 (six) electrolysis units 10), which at maximum capacity produces 1200 Nm3H2 / h, and consequently 600 Nm3O2 / h. The hydrogen gas is delivered to the hydrogen outlet 11 at system pressure (which may be 35 bar) and is cooled slightly through gas cleaning equipment (if present), such that both the hydrogen and oxygen gas would be approximately 50°C.

[0113] By passing 100 % of the oxygen through the turboexpander unit 20 and if the turboexpander unit 20 has an efficiency of 80 %, a power output of 35.7 kW can be expected. The coupling to the compressor 120 can be assumed to be lossless, such that 35.7 kW is available for H2-compression. Furthermore, the oxygen gas will be cooled to -114.9°C which could cool the hydrogen gas to 5°C in a passive heat exchanger 130 (depending on the type of heat exchanger 130 used). The compressor 120 approximately has a similar efficiency (to the turboexpander unit 120), and depending on the type of compression, the hydrogen pressure will be increased to 81 bar or 74 bar for an isothermal and adiabatic compression, respectively. If the hydrogen is not cooled prior to compression, the pressure may be increased to 72 bar or 67 bar for an isothermal and adiabatic compression, respectively.

[0114] In a further embodiment of the compression of the hydrogen gas may be obtained in a stepwise process. This is exemplified in Fig. 6.

[0115] In Fig 6, oxygen, from the oxygen outlet 12 of a (not shown) group 100 of electrolysis units 10, is passed through and expanded in a first turboexpander unit 20, where after the cooled and partly depressurized oxygen is passed through a heat exchanger 130 and further through a second turboexpander unit 20’, where the oxygen is further expanded. Thereafter, the oxygen may be vented to ambient through an oxygen vent 19.

[0116] The power generated by passing the oxygen through the second turboexpander unit 20’ is used in a first compression step, to compress hydrogen received from the hydrogen outlet 11 from the ( / not shown) group 100 of electrolysis units 10 and passed through a first compressor 120’. Power is transferred from the second turboexpander 20’ to the first compressor 120’ via a mechanical drive 30. The mechanical drive may be an axle, and may also comprise gearings if needed as is known in the art. When passing the hydrogen through the first compressor 120’, the hydrogen is compressed in the first step. Then the compressed hydrogen is passed through the heat exchanger 130, thereby exchanging heat with the cool oxygen passing through the other circuit of the heat exchanger 130. Thereby, the already partly compressed hydrogen is cooled before a second compression step, where the hydrogen is passed through a second compressor 120. The second compressor 120 is driven by the first turboexpander unit 20 via a mechanical drive 30. The mechanical drive 30 may be an axle and may as is known in the art comprise gearings.

[0117] Thereafter, the compressed hydrogen may be passed to a recipient such as a storage facility, (tank(s) or e.g. a sub terrain salt dome), a subsea pipe system (if the hydrogen plant is placed offshore), or a land based pipe system via a compressed hydrogen outlet 129.

[0118] Fig. 6 shows a two-step hydrogen pressurization process. It will be appreciated that the principle may in the context of the present hydrogen plant be expanded to three or more steps, similar to the steps described in connection with Fig. 6.

[0119] In a fourth aspect, the invention concerns a (not shown) hydrogen plant 1 having a means for storing energy from the high-pressure oxygen from the group 100 of electrolysis unit(s) 10.

[0120] The hydrogen plant 1 according to the fourth aspect of the invention comprises at least one electrolysis unit 10 or group 100 of electrolysis unit 10, having a common hydrogen outlet 11 and a common oxygen outlet 12. A turboexpander unit 20 is fluidly connected to the oxygen outlet 12, just as described in connection with the Figures 1-6 above.

[0121] In addition, the hydrogen plant 1 comprises an oxygen tank 172 seen in Fig. 9 which is fluidly connected to the oxygen outlet 12 of the group 100 of electrolysis units 10. Thereby, the oxygen tank may be filled with pressurized oxygen from the group 100 of electrolysis units 10 and saved for later use.

[0122] Further, the oxygen tank 172 is fluidly connected to the at least one turboexpander unit 20. Thereby, pressurized oxygen, stored in said oxygen tank may be used to extract energy for processes at the hydrogen plant 1 , as a supplement to the oxygen provided from production thereof in the group 100 of electrolysis units 10, or when production of oxygen in the group 100 of electrolysis units 10 is diminished or halted.

[0123] Such an oxygen tank 172 may be applied in connection with any of the aspects described above and below.

[0124] In the previously described aspects of the invention, energy, extracted from the pressurized oxygen produced in the group 100 of electrolysis units 10 along with the hydrogen, is used for various purposes in the hydrogen plant 1.

[0125] The energy output of the isentropic expansion process of the oxygen in the turboexpander 20 of the above describe aspects is theoretically directly proportional to the initial temperature of the oxygen to be expanded.

[0126] It would therefore be beneficial to increase the temperature of the oxygen gas originating from the oxygen outlet 12 of the group 100 of electrolysis units 10.

[0127] The oxygen outlet from 12 the group 100 of electrolysis units 10 contains some hydrogen due to gas crossover in each cell of the electrolysis units 10. According to the fifth aspect of the invention, this hydrogen content could be removed by reaction with oxygen to form water.

[0128] A similar system for removing residual oxygen from the hydrogen is often implemented on the H2 outlet. In that case such a system involves a so-called deoxo tank. In the presently described fifth aspect of the invention it is realized that a hydrogen removal unit 140 may be used to increase the oxygen temperature, and thereby the potential energy of the oxygen gas.

[0129] The reaction is catalytically activated in the hydrogen removal unit 140, which is similar to a de-oxo tank, and the exothermal reaction releases energy equal to the lower heating value of water formation (237.2 kJ / mol). 1 % (12 Nm3H2 / h) hydrogen in oxygen would release 34.82 kW of energy as heat, and with a flow of 600 Nm3O2 / h (and assuming that a total of 1.5 % of that volume is water vapour) this would result in a temperature increase of the gas from 50°C to approximately 208°C if no heat is released to the surroundings during the process. The energy output from the expansion process through the turboexpander unit 20 will therefore in theory increase by 48 %.

[0130] Therefore, according to the fifth aspect of the invention, and as illustrated in Fig. 7, a hydrogen plant 1 comprising an electrolysis unit 10 having a hydrogen outlet 11 and an oxygen outlet 12, and a turboexpander unit 20 fluidly connected to the oxygen outlet 12, may further comprises a hydrogen removal unit 140. The hydrogen removal unit 140 is configured for combusting residual hydrogen in the oxygen from the oxygen outlet 12 in a catalytic process. The hydrogen removal unit 140 is fluidly connected between the oxygen outlet 12 and the at least one turboexpander unit 20. Thereby, the increased energy content in the oxygen gas downstream of the hydrogen removal unit 140 is provided. As shown in Fig. 7 the turboexpander unit 20 may be connected to a rotation energy consuming device 150 via a mechanical drive 30. This mechanical drive 30 is preferably a shaft / axle. The rotation energy consuming device 150 may be anyone of the devices described in connection with aspects one-four above, i.e. a generator 31 , a vacuum pump 46, an inlet water pump 47, or a compressor 120. As mentioned, the oxygen from the electrolysis unit(s) 10 is often polluted with hydrogen. Typically, the hydrogen pollution in the oxygen is about 1%. This is often enough to increase the energy content sufficiently by combusting the hydrogen in the hydrogen removal unit 140. However, in case the hydrogen pollution is low or in situations where the energy content is desirably boosted, it may be beneficial to add a little hydrogen to the oxygen.

[0131] The addition of hydrogen to the oxygen needs to be maintained under a certain security level. A maximum content of hydrogen is 2% (2% hydrogen in pure oxygen). This leaves some headroom to the level of oxygen in hydrogen, where risks of explosions are at hand.

[0132] Therefore, in an embodiment of the fifth aspect the hydrogen plant 1 may further comprise a fluid connection 145 between the hydrogen outlet 11 and a hydrogen inlet 141 to the hydrogen removal unit 140. The fluid connection 145 comprises a valve 146 that may be opened to allow hydrogen to pass into the hydrogen removal unit 140, and to thereby increase the hydrogen content of the oxygen / hydrogen entering the hydrogen removal unit 140.

[0133] However, in a preferred embodiment thereof, the hydrogen plant 1 may further comprise a hydrogen collection tank 160. The hydrogen collection tank 160 is connected to the hydrogen outlet 11 to receive hydrogen from the electrolysis unit(s) 10. The hydrogen collection tank 160 is configured for collecting and storing hydrogen for use in the hydrogen removal unit 140 to increase the hydrogen content oxygen entering the hydrogen removal unit 140.

[0134] It consumes a lot of energy (electricity) to produce the hydrogen. Therefore, under normal circumstances, spending the produced hydrogen in this way would not provide any advantage. However, under exceptional circumstances, e.g. during startup and / or shutdown of the electrolysis unit(s) 10, hydrogen flows containing impurities may occur. By impurities is meant not only water or oxygen (the latter may be removed in the abovementioned so-called de-oxo tanks coupled to the hydrogen outlet), but also residues of purifying gas used to secure the plant against pockets of explosive mixtures of oxygen and hydrogen. The purifying gas is also called purge gas, and often Nitrogen (N2) is used. Nitrogen is harmless, and a small content may be acceptable in the outlet stream of hydrogen. But for some purposes even a low nitrogen level in the hydrogen is undesirable. Especially such a purge gas containing hydrogen, occurring often in the upstart and shutdown, could be collected in the above mentioned hydrogen collection tank 160 for use when beneficial in the hydrogen removal unit 140.

[0135] In a preferred embodiment of any of the previously described embodiments, the hydrogen removal unit 140 of the hydrogen plant 1 comprises a reaction tank holding granules (not shown) with surface parts comprising a catalytic component configured for catalyzing the reaction between oxygen and hydrogen for the formation of H2O. A catalytic component such as palladium is often used in the process of removing traces of oxygen from in the produced hydrogen gas, and in a similar process of removing traces of hydrogen from the oxygen gas, palladium or a similar catalytically active material may be used e.g. platinum.

[0136] In any of the above-mentioned embodiments, not shown temperature regulating means may further be provided to control the operating temperature in the hydrogen removal unit 140. In one embodiment hereof, the temperature regulating means comprises at least one burner (not show) arranged upstream of the hydrogen removal unit 140, which burner is configured for burning hydrogen in the stream of oxygen entering the hydrogen removal unit. Such a burner may produce steam to be entered into the hydrogen removal unit 140 and then into the turboexpander unit 20 with the increased energy content of the steam containing oxygen gas increasing the energy output from the turboexpander unit 20.

[0137] Alternatively, or additionally, the (not shown) temperature regulating means may comprise at least one water mist generating sprinkler, which at least one water mist generating sprinkler is arranged upstream of and / or inside the hydrogen removal unit 140. The water mist generating sprinkler is configured for providing a fine mist of water droplets in the hydrogen removal unit 140, so that the evaporation of the droplets into steam cools down the hydrogen removal unit. The advantage thereof is that the water vapour formed by heated and evaporated droplets occupies considerably more space than liquid water content added, thereby increasing the flow through the system and in particular into the turboexpander unit 20. Since the pressure is maintained approximately constant, the increased flow may be translated into the increased energy output in the turboexpander unit 20.

[0138] Injection of water may in principle be applied under normal operation if the temperature becomes too high in the granulate. However, water injection is particularly useful under addition of the extra hydrogen, where there is a risk of elevated temperature in the granulate tank. Using a hydrogen burner directly in the C^ flow, could replace cumbersome heat caps which are conventionally mounted on granulate tanks in order to ensure the operation temperature under starting up from cold.

[0139] Prior art system may use heaters in the O2 or H2 flow before the tank for removing impurities in a H2 or an O2 production flow. The sacrificial hydrogen spending in the O2 production flow or sacrificial O2 spending in a H2 production flow described here, may provide an alternative.

[0140] Cooling may often only be necessary, when hydrogen is added, e.g. at start-up in order to achieve operation temperature more quickly, or under stand-by, using O2 from tank, in order to harvest electricity.

[0141] A large oxygen tank allows for more energy storage, but by e.g. supplementing with a small H2 tank, the oxygen tank may be made considerably smaller, with the same energy storage potential, whereby cost may be reduced. However, further cooling may then be needed, and it is noted that the generated water also ensures that the energy in the hydrogen -oxygen combustion may be utilized in the turbine.

[0142] At any event, the water content in the oxygen stream will always be an issue, as it may lead to uncontrolled ice formation during a cooling / expansion process, which has the potential to block any of the devices in the oxygen treatment system. Liquid water removal by careful control of temperature and pressure is possible and may be combined with controlled ice formation and following ice melting and piping away of melted water where appropriate, as it is known from heat pumps used to extract heat from atmospheric air.

[0143] Above, various aspect of the invention are described one by one. It has however been realized that in a sixth aspect of the invention, some or all of the above described aspects may further be bundled in a beneficial way. The resulting hydrogen plant 1 is illustrated in diagram form in Fig. 8 and with reference, also, to the figures 2-7 as described above.

[0144] The hydrogen plant 1 according to the sixth aspect of the invention may comprise an electrolysis unit 10, as shown in Figs. 2-7, but not shown in Fig. 8, which electrolysis unit 10 or group 100 of electrolysis units 10 has a common hydrogen outlet 11 (not shown in Fig. 8) and a common oxygen outlet 12.

[0145] As shown in Fig. 8, the hydrogen plant 1 further comprises a first turboexpander unit 20, 20’ selectively connected to the oxygen outlet 12 of the electrolysis unit 10 or group 100 of electrolysis units 10. A first hydrogen supply valve 12’ is provided in a first fluid connection 26’ formed directly between the oxygen outlet 12 and the first turboexpander unit 20, 20’ to make the oxygen supply to the first turboexpander unit 20, 20’ selective.

[0146] The first turboexpander unit 20, 20’ is connected to power a vacuum unit 45 of a distillation unit 40 of a hydrogen plant 1 according to the any of the embodiments of the first aspect of the invention as described above in connection with Figs. 2 and 3. Preferably the first turboexpander unit 20, 20’ is connected to power a vacuum unit 45 via a mechanical drive 30, as indicated in Figs. 2, 3 and 8.

[0147] As also shown in Fig. 8, the hydrogen plant 1 further comprises second turboexpander unit 20, 20”. The second turboexpander unit 20, 20” is selectively connectable to the oxygen outlet 12. A second hydrogen supply valve 12” is provided in a second fluid connection 26” formed directly between the oxygen outlet 12 and the second turboexpander unit 20, 20” to make the oxygen supply to the second turboexpander unit 20, 20” selective. As is further shown in Fig. 8, an inlet 21 , 21” of the second turboexpander unit 20, 20” is fluidly connected in series to an outlet 22, 22’ of the first turboexpander unit 20, 20’. Thereby it is made possible that oxygen that has passed through the first turboexpander unit 20, 20’ is used to power the second turboexpander unit 20, 20”.

[0148] The oxygen that may be passed through the first turboexpander unit 20, 20’ comes from the oxygen outlet 12 of the electrolysis unit(s) 10 and therefore has a high pressure, similar to the process pressure in the electrolysis unit(s) 10, Pi, which may be 25-110 bar.

[0149] The oxygen passed from the first turboexpander unit 20, 20’ to the second turboexpander unit 20, 20” has already passed through the first turboexpander unit 20, 20’, where the oxygen has been expanded. Therefore, the pressure of the oxygen at the inlet 21 , 21 ” of the second turboexpander unit 20, 20” is P2, which is lower than Pi, an intermediate pressure.

[0150] In a preferred embodiment, of the hydrogen plant according to the sixth aspect of the invention, the second turboexpander unit 22, 22” is connected to power a compressor 80. This compressor 80 is in turn connected to a drying system 60 for drying hydrogen received from the hydrogen outlet 11 of a hydrogen plant 1 according to any one of the embodiments of the second aspect of the invention, and as described above in connection with Fig. 4.

[0151] In a further embodiment of either of the above mentioned embodiments, and as shown in Fig. 8, the hydrogen plant 1 further comprises a third turboexpander unit 20, 22’”. The third turboexpander unit 20, 22’” is selectively connectable to the oxygen outlet 12. A third hydrogen supply valve 12’” is provided in a third fluid connection 26’” formed directly between the oxygen outlet 12 and the third turboexpander unit 20, 20’” to make the oxygen supply to the third turboexpander unit 20, 20’” selective.

[0152] Further, and as indicated in Fig. 8, the third turboexpander unit 20, 22’” is connected to power a compressor 120 of a hydrogen pressurization system 2. The compressor 120 is fluidly connected to a hydrogen outlet 11 of the electrolysis unit 10 in order to increase pressure of the hydrogen, in a hydrogen plant according to any one of the embodiments of the third aspect of the invention as described above in connection with Figs. 5 and 6.

[0153] Further, an inlet 21, 2T” of the third turboexpander unit 20, 20”’ may be connected in series to an outlet 22, 22’ of the first turboexpander unit 20, 20’, such that oxygen that has passed through the first turboexpander unit 20, 20’ is used to power the third turboexpander unit 20, 20”.

[0154] Alternatively, and as also shown in Fig. 8, the inlet 21 , 21’” of the third turboexpander unit 20, 20” is also connected in series to an outlet 22, 22” of the second turboexpander unit 20, 20” such that oxygen that has passed through the second turboexpander unit 20, 20” may be used to power the third turboexpander unit 20, 20’”.

[0155] The hydrogen plant 1 according to any of the embodiments of the sixth aspect of the invention described above may further comprise an oxygen tank similar to the oxygen tank of the hydrogen plant 2 according to the fourth aspect of the invention described further above.

[0156] The oxygen tank may be filled with pressurized oxygen from the group 100 of electrolysis units 10 and saved for later use.

[0157] Further, because the oxygen tank is fluidly connected to the first turboexpander unit 20’, the second turboexpander unit 20” and / or the third turboexpander unit 20’”, the collected and stored pressurized oxygen, stored in said oxygen tank, may be used to extract energy for processes at the hydrogen plant 1 , either as a supplement to the oxygen during production thereof in the group 100 of electrolysis units 10, or when production of oxygen in the group 100 of electrolysis units 10 is diminished or halted.

[0158] Such an oxygen tank may be applied in connection with any of the embodiments of the sixth aspects of the invention as described above. The hydrogen plant 1 according to any of the embodiments of the sixth aspect of the invention described above may further comprise a hydrogen removal unit 140 of a hydrogen plant 1 according to any one of the embodiments of the fifth aspect of the invention, and as described above in connection with Fig. 7.

[0159] Further reference is now made to Fig. 8. When pressure in the oxygen delivery pipe 12”” connecting to the oxygen outlet 12 is low, only the third supply oxygen valve 12”’ is open, while the second oxygen supply valve 12” and the first oxygen supply valve 12’ remains closed.

[0160] If pressure in the oxygen delivery pipe 12”” increases, the second oxygen supply valve 12” may open and the third oxygen supply valve 12’” close, such that the process in the hydrogen drying system 60 and the hydrogen pressurization system 2 are connected in series, and both are active. In the hydrogen drying system 60, the second turboexpander 20, 20” delivers power W to regeneration process for the hydrogen drying system 60. The process is further designed to reuse hydrogen, which has been used in the zeolite beds for re-generating the zeolite dryers. This hydrogen is thus both water-rich and at low pressure such as at atmospheric pressure.

[0161] In regeneration process, the hydrogen stream is repressurized and cooled, such that water in the shape of condensed water is extracted, and the now dryer, and pressurized hydrogen is re-introduced into the hydrogen delivery line from the electrolyser prior to the zeolite drying phase.

[0162] If the pressure increases further in oxygen supply line 12””, all three processes, the distillation process 40; the regeneration process 60; and the hydrogen pressurization process 2 may be active at the same time.

[0163] Here, only first oxygen supply valve 12’ remains open, while the second oxygen supply valve 12” and the third oxygen supply valve 12’” are closed.

[0164] Each of processes 45, 60 and 120 are illustrated as driven by a single turboexpander wheel, however multiple steps of consecutive expansion steps may be employed in case machinery is available, and the thermodynamics thereby will allow more power, W, being transferred to the respective processes 45, 60 and 120.

[0165] A seventh aspect of the invention concerns a hydrogen plant 1 comprising an electrolysis unit 10 having a hydrogen outlet 11 and an oxygen outlet 12. A turboexpander unit 20 is connected to the oxygen outlet 12. The turboexpander unit is connected to power a unit of the hydrogen producing plant 1 through a mechanical drive 30 directly connected to an output shaft of the turboexpander 20. The unit may be any of the units described above in connection with aspects one through seven.

[0166] Some of the above aspects of the invention makes the invention particularly suitable to offshore use. In the following, further aspects of the invention which focuses on offshore use are:

[0167] A hydrogen plant 1 comprising at least one of:

[0168] 1) a battery adapted for buffering electric energy produced by an electric generator 31 driven by oxygen passed through a turboexpander unit 20, and

[0169] 2) a pressurized oxygen storage unit adapted to store oxygen at pressure levels up to the 90 bar, and further adapted to feed the pressurized oxygen to a turboexpander unit 20, to thereby store and release - when needed energy for use in some or more of the processes described above.

[0170] Further, at least one hydrogen storage tank 160 may be adapted to store hydrogen, a pipe connection 145 between this hydrogen storage tank 160 and a hydrogen removal unit 140 for removal of hydrogen and freeing the energy of hydrogen content in the oxygen supplied from the electrolysis unit(s) 19, which hydrogen removal unit 140 is arranged upstream of the turboexpander unit 20 for oxygen.

[0171] Further, at least one oxygen turboexpander unit 20 may be connected to an electric generator 31, and the power line from the electric generator 21 is connected to an electric motor which again is connected to drive a hydrogen compressor. The compressor is inserted in the output hydrogen gas stream between the hydrogen outlet from electrolysis units and a hydrogen gas distribution system. Any off-shore plant, see e.g., Fig. 9 has the challenge, that it is potentially an off-grid plant. In case of no wind and no solar power for providing electrical power to the offshore hydrogen plant 1 , the hydrogen plant 1 may well have no power of its own. This may be especially troublesome if the grid line connecting a singular hydrogen plant 1 with land based power grid is also cut off.

[0172] In order to overcome this obstacle, there is provided energy storages such as one or both of a battery 171 and an oxygen storage tank 172. The stored oxygen or electric power is used to remain in control of the hydrogen plant 1, also when no external power source is available. The energy in the oxygen from the oxygen storage tank 172 resides only in the pressure within the tank, and is released for use, such as in the turboexpander as described. If a hydrogen removal device 140 as disclosed with reference to Fig. 7 is part of the system, supplemental energy may be provided by means of a hydrogen tank 160 adapted for storing hydrogen. The hydrogen storage tank 160 is connected through a fluid connection / hydrogen gas pipe 145, to the hydrogen removal unit 140. An energy retrieving unit 180 for utilizing the energy stored in the pressurized oxygen is schematically indicated in Fig. 9, and shall comprise at least one turboexpander 20 and one or several of the above devices for utilizing the power of the pressurized oxygen, and for example additionally comprise a generator 31 for producing electricity, such that electric power shall be generated by the energy retrieving unit 180. The power from an electric power line 175 from the energy retrieving may be temporarily stored in a battery 171 as also indicated in Fig. 9.

[0173] It also so happens that the power up and / or power down times for electrolysers / electrolysis units 10 may be longer than power up and / or power down times for wind turbines and solar panels in changing weather. Thus, with use of weather forecasts, the stored energy may be used in the electrolysers during power down in order to extend this process, when external power is lost over a short time, and / or the stored energy may be used to prime the electrolysers / electrolysis units 10, 100, such that they are ready to absorb the energy from a wind turbine or a number of solar panels, in cases where wind or sun induced power is predicted to kick in with steep rise times. The effect of the system in Fig. 9 is that the power buffers (battery 171 and / or oxygen pressure tank 172) are used as explained above and are instrumental in ensuring control over the hydrogen plant 1 in cases where power from solar and wind is lost.

[0174] In Fig. 10, it is disclosed that the hydrogen is further compressed by way of the energy gleaned of the compressed oxygen, which the electrolyser / electrolysis unit 10 needs, must deliver out of the electrolyser / electrolysis unit 10 at the same pressure as the delivered hydrogen. Such further compression may provide a wider range for the hydrogen transport / storage and distribution system 190, such that the hydrogen may reach far off destinations without the aid of further pumps. This may also apply to land-based systems, which may feed into a hydrogen distribution system or storage systems, such as cavities in salt formations. Supplemental electricity generation and battery 171 is indicated in Fig. 10 as well as these further elements may prove very important in stand-alone instances for off-shore systems. In this system also an oxygen storage tank 172 and a hydrogen storage tank 160 are indicated, both intended to be coupled to the hydrogen removal unit 140, such that this system comprises both battery 171, compressed oxygen, and hydrogen as energy storages, intended for use in case all other power input to the system fail.

[0175] In Fig. 11 , the turboexpander unit 20 on the oxygen side is directly (via a shaft) connected to a turbo compressor 120 on the hydrogen side. This is a very simple solution, and the complexities of the system may be greatly reduced, which particularly at off-shore plants is an advantage. Also here, a hydrogen storage tank 170 is indicated, which is adapted to be coupled to a hydrogen removal device 140. With this system, a temporary boost of the power produced by the turbo compressor 120 may be achieved, as well as possible burns of hydrogen from nitrogen and hydrogen mixtures. Such mixtures may be produced during purge operations of the system, whereby hydrogen as well as oxygen residues are removed by flushing nitrogen through pipes and vessels. Fig. 12 discloses a situation where turboexpander units 20 drive only electricity generators 31 , and the electricity is consumed at electrically driven hydrogen compressors prior to the output of the hydrogen.

[0176] The Fig. 12 solution is not at all very smart energy wise (loss at turboexpander unit 20 to generator 31 , loss again from electric power in motor 173 to further compressed hydrogen 190), but may work all the same, especially as electricity is the pass par tout energy transferrer, and it’s use in virtually any application is state of the art. Also in this scenario, the use of de-oxy process for the oxygen, possibly in combination with hydrogen and oxygen storage tanks 176, 172 are optional addons. The go-to solution of electricity for energy transformation and storage allows such a system to be built mainly by standardized and commercially available pieces, which is of commercial value, and shall also lend increase up-times and fewer necessary service hours to the system, which are also benefits, which especially offshore plants shall enjoy, and which may make up for the increase in energy losses, which are bound to accompany the electrical components of the system such as electric generators 31 , motors 173 and batteries 171.

[0177] Fig. 13 discloses the core process, where the oxygen stream is used to energize the turboexpander unit 20 wheel, and mechanical power, W, is transferred via a rotating shaft of a mechanical drive 30, and used for any purpose in order to mitigate one or more challenges. Again, a hydrogen storage tank 160 and the hydrogen removal device 140 are disclosed which may be used whenever a boost of power is needed, such as in a start up process or when a long duration hot stand by situation may be desired and wind or solar power is not available in sufficient quantity.

[0178] In the following we will discuss the theoretical potential energy recovery from a turboexpander unit as described above.

[0179] The expansion process from a turboexpander unit can best be idealised as an isentropic expansion process, meaning that there is no heat loss (adiabatic), and the process is reversible (no entropy change). It is with these assumptions possible to calculate the theoretical energy output from the expansion process from basic thermodynamics. It is valid for an adiabatic expansion process that: where 1 / 1 / is the change in enthalpy (a measure of energy) per mass of expanded gas [kJ / kg] , Cpis the specific heat and Tj,f is the initial and final temperature of the expanded gas. The change in temperature is a consequence of the adiabatic expansion process where the gas cannot exchange heat with its surroundings, and thermodynamic relations between the pressure and temperature of such a process makes it possible to rewrite the above expression:

[0180] Here, y is the heat capacity ratio and is the initial and final expression. In reality, some heat will be lost to the surroundings, meaning that the actual final temperature, 7" / will be somewhat higher than theoretically calculated. Furthermore, some mechanical losses are expected to take place in the turbine. These considerations can be taken into account by multiplying W by an efficiency factor, . Finally, the output can be expressed as a power (measured in Watts) by multiplying W by the mass flow, m, of the oxygen outlet: itr

[0181] And the final temperature will likewise be expressed as:

[0182] Compression of the hydrogen gas is different. It is possible to compress the gas adiabatically, but this heats up the gas tremendously and requires a maximum amount of energy. A different (and somewhat opposite) type of compression is the reversible isothermal process. Here, the gas does not heat up, but is in equilibrium with its surroundings - unfortunately this process is only possible when the gas is compressed infinitely slow. In real situations, compression happens through a polytropic process which lies between the isothermal and adiabatic process. It is the same physics (and same equations) that expresses the energy consumption of the adiabatic compression, and the isothermal compression is described by: where R is the universal gas constant, and the other symbols have their usual meaning. With these expressions it would then be possible to estimate the amount energy that can be recovered from expansion of the pressurised oxygen and how well that energy can be utilised to compress hydrogen.

[0183] 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.

[0184] Items of the invention

[0185] Thus, as described above, the objects of the invention may be obtained by the below listed items.

[0186] A1. A hydrogen plant (1) comprising

[0187] - an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12), and

[0188] - a turboexpander unit (20) connected to the oxygen outlet (12), wherein the turboexpander unit (20) is connected to power a vacuum unit (45) of a distillation module (40), the distillation module (40) connected to supply water to the electrolysis unit (10).

[0189] A2. The hydrogen plant (1) according to item A1 , wherein the turboexpander unit (20) is configured for powering a vacuum pump (46) of the vacuum unit (45) by a mechanical drive (30).

[0190] A3. The hydrogen plant (1) according to item A1 , wherein the vacuum unit (45) comprises an inlet water pump (47), and wherein the turboexpander unit (20) connects to the inlet water pump (47) via a mechanical drive (30), the inlet water pump (47) being configured to feed pressurized inlet water stream into a vacuum unit pump (46) connected to apply vacuum to the distillation module (40).

[0191] A4. The hydrogen plant (1) according to any one of the items A1-A3, wherein surplus heat from the electrolysis unit (10) is used to heat water led into the distillation unit (30) in order to accelerate the distillation process in the distillation module (40).

[0192] A5. The hydrogen plant (1) according to any one of the items A1-A4, wherein water to be treated in the distillation module (40) is pre-heated by excess heat generated by the electrolysis unit (10) by use of a heat exchanger.

[0193] A6. The hydrogen plant (1) according to item A5, wherein a heat pump is inserted in a connection line between the electrolysis unit (10) and the heat exchanger, to enhance the cooling of a fluid, such as produced gas, lye or cooling medium at the electrolysis unit (10), and so that heat energy transferred through the heat pump may be delivered at the heat exchanger at an increased temperature.

[0194] B1. A hydrogen plant (1) comprising

[0195] - an electrolysis unit (10) for producing hydrogen at a first pressure (Pi), and having a hydrogen outlet (11) and an oxygen outlet (12);

[0196] - a turboexpander unit (20) fluidly connected to the oxygen outlet (12); and

[0197] - a drying system (60) for drying hydrogen received from the hydrogen outlet (11) and comprising two zeolite beds (61), wherein hydrogen received from the hydrogen outlet (11) can selectively be passed through one of the two zeolite beds (61) for drying at the first pressure (Pi) while hydrogen at a second pressure (P2) lower than the first pressure (Pi) can be passed through the other of the two zeolite beds (61) to dry zeolite therein and regenerate the capacity for absorbing water of the zeolite, wherein wet hydrogen from the regenerating zeolite bed (61) can be passed through a cryo-condensor unit (70) to dry the wet hydrogen received from the regenerating zeolite bed (61), wherein a compressor (80) is fluidly connected to an outlet (72) of the cryo- condensor unit (70) in order to pressurize the hydrogen dried therein; and wherein the turboexpander unit (20) is connected to power said compressor (80) via a mechanical connection (30).

[0198] B2. The hydrogen plant (1) according to item B1 , wherein the compressor (60) is configured to pressurize the gas to the first pressure (Pi).

[0199] B3. The hydrogen plant (1) according to item B1 or B2, wherein the first pressure (Pi) is 25-110 bar, such as 30 bar or 60 bar or between 90 and 100 bar.

[0200] B4. The hydrogen plant (1) according to any one of the items B1-B3, wherein the second pressure (P2) is 1-5 bars, such as 2 bar. B5. The hydrogen plant (1) according to any one of the items B1-B5, wherein the drying system comprises heating means for heating the hydrogen passed through a zeolite bed (61) during regeneration of the zeolite therein to a first temperature (Ti).

[0201] B6. The hydrogen plant (1) according to item B6, wherein the first temperature (Ti) is 100-200°C, such as 150°C.

[0202] B7. The hydrogen plant (1) according to any one of the items B1-B6, wherein the cryo-condenser unit (70) is configured to cool down the compressed hydrogen in a heat exchanger (55) which receives a cooling medium in the form of a stream of oxygen from the oxygen outlet (12) of the electrolysis unit (10) which has been decompressed by passing it through the turboexpander unit (20).

[0203] B8. The hydrogen plant (1) according to any one of the items B1-B7, wherein the hydrogen, at a second pressure (P2) lower than the first pressure (Pi), is passed through the other of the two zeolite beds (61) to dry zeolite therein and is passed via a fluid connection to the other of the two zeolite beds (61), in which fluid connection a de-pressurization unit (90) is arranged.

[0204] C1. A hydrogen plant (1) comprising

[0205] - an electrolysis unit (10) for producing hydrogen at a first pressure (Pi), and having an hydrogen outlet (11) and an oxygen outlet (12); and

[0206] - a turboexpander unit (20) fluidly connected to the oxygen outlet (12);

[0207] - a compressor (120); wherein the turboexpander unit (20) is connected to power the compressor (120); and wherein the compressor (120) is fluidly connected to the hydrogen outlet (11) of the electrolysis unit (10) to increase pressure of the hydrogen therefrom.

[0208] C2. The hydrogen plant (1) according to item C1 , wherein the turboexpander unit (20) is connected to power the compressor (120) via a mechanical drive (30).

[0209] C3. The hydrogen plant (1) according to item C1 or C2, wherein oxygen expanded through the turboexpander unit (20) is passed through a heat exchanger (130), and where hydrogen from the hydrogen outlet (11) is passed through the heat exchanger (130) whereby the expanded oxygen cools the hydrogen before it is compressed in the compressor (120).

[0210] C4. The hydrogen plant (1) according to item C1 or C2, wherein the hydrogen is cooled between compression steps by heat exchangers (130) placed between individual compression steps, such as between a first and a second compression step.

[0211] C5. The hydrogen plant (1) according to item C4, wherein the heat exchanger (130) receives expanded oxygen at low temperature as the cooling input, and cools down the flow of hydrogen.

[0212] D1. A hydrogen plant (1) comprising

[0213] - an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12);

[0214] - a turboexpander unit (20) fluidly connected to the oxygen outlet (12); and

[0215] - an oxygen tank fluidly connected to the oxygen outlet (12) and to the at least one turboexpander unit, said oxygen tank being configured to collect and store oxygen.

[0216] E1. A hydrogen plant (1) comprising

[0217] - an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12);

[0218] - a turboexpander unit (20) fluidly connected to the oxygen outlet (12); and

[0219] - a hydrogen removal unit (140) for combusting residual hydrogen in the oxygen from the hydrogen outlet (11) in a catalytic process, wherein the hydrogen removal unit (140) is fluidly connected between the oxygen outlet (12) and the at least one turboexpander unit (20).

[0220] E2. The hydrogen plant (1) according to item E1, further comprising a fluid connection (145) between the hydrogen outlet (11) and an inlet (141) to the hydrogen removal unit (140) to increase the hydrogen content of the oxygen / hydrogen entering the hydrogen removal unit (140). E3. The hydrogen plant (1) according to item E2, whereby the fluid connection (145) further comprises a hydrogen collection tank (160) connected to the hydrogen outlet (11) and configured for collecting hydrogen for use at the inlet (141) to the hydrogen removal unit (140) to increase the hydrogen content of the oxygen / hydrogen entering the hydrogen removal unit (140).

[0221] E4. The hydrogen plant (1) according to item E3, whereby the hydrogen removal unit (140) comprises a reaction tank holding granules with surface parts comprising a catalytic component adapted to catalyze the reaction between oxygen and hydrogen for the formation of H2O.

[0222] E5. The hydrogen plant (1) according to any one of the items E1-E4, wherein temperature regulating means are provided to ensure operating temperature in the hydrogen removal unit (140).

[0223] E6. The hydrogen plant (1) according to item E5, wherein the temperature regulating means comprises at least one burner arranged upstream from the hydrogen removal unit (140) for burning hydrogen in the stream of oxygen entering the hydrogen removal unit (140).

[0224] E7. The hydrogen plant (1) according to item E5 or E6, wherein the temperature regulating means comprise at least one water mist generating sprinkler upstream of and / or inside the hydrogen removal unit (140) and configured for providing a fine mist of water droplets in the hydrogen removal unit (140) so that the evaporation of the droplets into steam cools down the hydrogen removal unit (140).

[0225] F1. A hydrogen plant (1) comprising

[0226] - an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12);

[0227] - a first turboexpander unit (20, 20’) connected to the oxygen outlet (12); and

[0228] - a second turboexpander unit (20, 20”) connected to the oxygen outlet (12), wherein the first turboexpander unit (20, 20’) is connected to power a vacuum unit (45) of a distillation unit (40) of the hydrogen plant (1) according to any one of items A1-A6, and wherein an inlet (21, 21”) of the second turboexpander unit (20, 20”) is further connected in series to an outlet (22, 22’) of a first turboexpander unit (20, 20’) such that oxygen that has passed through the first turboexpander unit (20, 20’) is used to power the second turboexpander unit (20, 20”).

[0229] F2. The hydrogen plant (1) according to item F1 , wherein the second turboexpander unit (22, 22’) is connected to power a compressor (80) connected to a drying system (60) for drying hydrogen received from the hydrogen outlet (11) of a hydrogen plant (1) according to any one of the items B1-B9.

[0230] F3. The hydrogen plant (1) according to item F1 or F2, comprising a third turboexpander unit (20, 22’”) connected to the oxygen outlet (12), wherein the third turboexpander unit (20, 22’”) is connected to power a compressor (120) connected to a hydrogen outlet (11) of the electrolysis unit (10) to increase pressure of the hydrogen in a hydrogen plant according to any one of the items C1-C5, and wherein an inlet (21 , 21’”) of the third turboexpander unit (20, 20’”) is further connected in series to an outlet (22, 22’) of the first turboexpander unit (20, 20’) such that oxygen that has passed through the first turboexpander unit (20, 20’) is used to power the third turboexpander unit (20, 20”).

[0231] F4. The hydrogen plant (1) according to item F1 or F2, comprising a third turboexpander unit (20, 22’”) connected to the oxygen outlet (12), wherein the third turboexpander unit (20, 22’”) is connected to power a compressor (120) connected to a hydrogen outlet (11) of the electrolysis unit (10) to increase pressure of the hydrogen in a hydrogen plant according to any one of the items C1-C5, and wherein an inlet (21 , 21’”) of the third turboexpander unit (20, 20”) is further connected in series to an outlet (22, 22”) of the second turboexpander unit (20, 20”) such that oxygen that has passed through the second turboexpander unit (20, 20”) is used to power the third turboexpander unit (20, 20’”). F5. The hydrogen plant (1) according to any one of the items F1-F4 and further comprising an oxygen tank of a hydrogen plant (1) according to item D1. F6. The hydrogen plant (1) according to any one of the items F1-F5 and further comprising a hydrogen removal unit (140) of a hydrogen plant (1) according to items E1-E7.

[0232] G1 .A hydrogen plant (1) comprising - an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet

[0233] (12); and

[0234] - at least one turboexpander unit (20) connected to the oxygen outlet (12); wherein the at least one turboexpander unit (20) is connected to power a unit of the hydrogen producing plant (1) through a mechanical drive (30) directly connected to an output shaft of the turboexpander (20).

[0235] List of parts

[0236] 1 hydrogen plant / hydrogen producing plant

[0237] 2 hydrogen pressurization system

[0238] 10 electrolysis unit / electrolyser / water electrolyser

[0239] 11 hydrogen outlet from electrolysis unit

[0240] 12 oxygen outlet from electrolysis unit

[0241] 12’ oxygen supply valve to first turboexpander unit

[0242] 12” oxygen supply valve to second turboexpander unit

[0243] 12”’ oxygen supply valve to third turboexpander unit

[0244] 12”” oxygen supply pipe

[0245] 13 valve

[0246] 14 regulator for O2 (bypass of turboexpander)

[0247] 16 pressure regulator for H2

[0248] 18 outlet for after treated hydrogen

[0249] 19 oxygen vent

[0250] 20 turboexpander unit / turbine

[0251] 20’ first turboexpander unit

[0252] 20” second turboexpander unit

[0253] 20’” third turboexpander unit

[0254] 21 inlet to turboexpander unit

[0255] 22 outlet from turboexpander unit

[0256] 25 stack cooling unit

[0257] 26’ fluid connection from oxygen outlet to a first turboexpander unit

[0258] 26” fluid connection from oxygen outlet to a second turboexpander unit

[0259] 26’” fluid connection from oxygen outlet to a third turboexpander unit

[0260] 30 mechanical connection / shaft

[0261] 40 distillation module

[0262] 45 vacuum unit

[0263] 46 vacuum pump of vacuum unit

[0264] 47 inlet water pump of vacuum unit

[0265] 48 heat pump

[0266] 49 circulation pump

[0267] 50 heat exchanger

[0268] 60 drying system for drying hydrogen 61 zeolite beds of the drying system

[0269] 70 cryo-condensor unit, configured for drying wet hydrogen which has been used to regenerate zeolite material in zeolite bed of drying system

[0270] 71 inlet of cryo-condensor unit

[0271] 72 outlet of cryo-condensor unit

[0272] 75 heat exchanger of cryo-condensor unit

[0273] 80 compressor for re-pressurizing hydrogen used to regenerate zeolite

[0274] 90 de-pressurizing unit

[0275] 100 group of electrolysis units / plurality of electrolysis units

[0276] 120 compressor for pressurizing hydrogen

[0277] 125 fluid connection for hydrogen from the hydrogen outlet of the electrolysis unit to compressor

[0278] 126 fluid connection for hydrogen from compressor to a compressed hydrogen outlet

[0279] 129 compressed hydrogen outlet

[0280] 130 heat exchanger

[0281] 135 fluid connection for oxygen from the outlet of turboexpander unit to a heat exchanger

[0282] 136 fluid connection for oxygen from heat exchanger to vent to ambient

[0283] 140 hydrogen removal unit

[0284] 141 hydrogen inlet to the hydrogen removal unit

[0285] 145 fluid connection between the hydrogen outlet (from the hydrogen plant) and the hydrogen inlet to the hydrogen removal unit / hydrogen gas pipe

[0286] 150 rotation energy consuming device

[0287] 160 hydrogen tank

[0288] 171 battery

[0289] 172 oxygen tank

[0290] 173 electric motor

[0291] 174 Any electrically driven compressor adapted for H2 compression

[0292] 175 electric power cables / power line

[0293] 180 energy retrieving unit

[0294] 190 Subsea or land-based hydrogen transport and distribution tube system

[0295] 200 Hydrogen consumer, land or off- shore based

Claims

Claims1. A hydrogen plant (1) comprising- an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12), and- a turboexpander unit (20) connected to the oxygen outlet (12), wherein the turboexpander unit (20) is connected to power a vacuum unit (45) of a distillation module (40), the distillation module (40) connected to supply water to the electrolysis unit (10).

2. The hydrogen plant (1) according to claim 1 , wherein the turboexpander unit (20) is configured for powering a vacuum pump (46) of the vacuum unit (45) by a mechanical drive (30).

3. The hydrogen plant (1) according to claim 1 , wherein the vacuum unit (45) comprises an inlet water pump (47), and wherein the turboexpander unit (20) connects to the inlet water pump (47) via a mechanical drive (30), the inlet water pump (47) being configured to feed pressurized inlet water stream into a vacuum unit pump (46) connected to apply vacuum to the distillation module (40).

4. The hydrogen plant (1) according to any one of the claims 1-3, wherein surplus heat from the electrolysis unit (10) is used to heat water led into the distillation unit (30) in order to accelerate the distillation process in the distillation module (40).

5. The hydrogen plant (1) according to any one of the claims 1-4, wherein water to be treated in the distillation module (40) is pre-heated by excess heat generated by the electrolysis unit (10) by use of a heat exchanger.

6. The hydrogen plant (1) according to claim 5, wherein a heat pump is inserted in a connection line between the electrolysis unit (10) and the heat exchanger, to enhance the cooling of a fluid, such as produced gas, lye or cooling medium at the electrolysis unit (10), and so that heat energy transferred through the heat pump may be delivered at the heat exchanger at an increased temperature.

7. A hydrogen plant (1) comprising- an electrolysis unit (10) for producing hydrogen at a first pressure (Pi), and having an hydrogen outlet (11) and an oxygen outlet (12);- a turboexpander unit (20) fluidly connected to the oxygen outlet (12); and- a drying system (60) for drying hydrogen received from the hydrogen outlet(11) and comprising two zeolite beds (61), wherein hydrogen received from the hydrogen outlet (11) can selectively be passed through one of the two zeolite beds (61) for drying at the first pressure (Pi) while hydrogen at a second pressure (P2) lower than the first pressure (Pi) can be passed through the other of the two zeolite beds (61) to dry zeolite therein and regenerate the capacity for absorbing water of the zeolite, wherein wet hydrogen from the regenerating zeolite bed (61) can be passed through a cryo-condensor unit (70) to dry the wet hydrogen received from the regenerating zeolite bed (61), wherein a compressor (80) is fluidly connected to an outlet (72) of the cryo- condensor unit (70) in order to pressurize the hydrogen dried therein; and wherein the turboexpander unit (20) is connected to power said compressor (80) via a mechanical connection (30).

8. A hydrogen plant (1) comprising- an electrolysis unit (10) for producing hydrogen at a first pressure (Pi), and having an hydrogen outlet (11) and an oxygen outlet (12); and- a turboexpander unit (20) fluidly connected to the oxygen outlet (12);- a compressor (120); wherein the turboexpander unit (20) is connected to power the compressor (120); and wherein the compressor (120) is fluidly connected to the hydrogen outlet (11) of the electrolysis unit (10) to increase pressure of the hydrogen therefrom.

9. A hydrogen plant (1) comprising- an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet(12);- a turboexpander unit (20) fluidly connected to the oxygen outlet (12); andan oxygen tank fluidly connected to the oxygen outlet (12) and to the at least one turboexpander unit, said oxygen tank being configured to collect and store oxygen.

10. A hydrogen plant (1) comprising- an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12);- a turboexpander unit (20) fluidly connected to the oxygen outlet (12); and- a hydrogen removal unit (140) for combusting residual hydrogen in the oxygen from the hydrogen outlet (11) in a catalytic process, wherein the hydrogen removal unit (140) is fluidly connected between the oxygen outlet (12) and the at least one turboexpander unit (20).

11. A hydrogen plant (1) comprising- an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12);- a first turboexpander unit (20, 20’) connected to the oxygen outlet (12); and- a second turboexpander unit (20, 20”) connected to the oxygen outlet (12), wherein the first turboexpander unit (20, 20’) is connected to power a vacuum unit (45) of a distillation unit (40) of the hydrogen plant (1) according to any one of claims A1-A6, and wherein an inlet (21, 21”) of the second turboexpander unit (20, 20”) is further connected in series to an outlet (22, 22’) of a first turboexpander unit (20, 20’) such that oxygen that has passed through the first turboexpander unit (20, 20’) is used to power the second turboexpander unit (20, 20”).

12. A hydrogen plant (1) comprising- an electrolysis unit (10) having a hydrogen outlet (11) and an oxygen outlet (12); and- at least one turboexpander unit (20) connected to the oxygen outlet (12); wherein the at least one turboexpander unit (20) is connected to power a unit of the hydrogen producing plant (1) through a mechanical drive (30) directly connected to an output shaft of the turboexpander (20).

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