A water electrolyser system and method for producing compressed hydrogen

Abstract

The present invention relates to a water electrolyser system for production of compressed hydrogen, comprising a water electrolyser stack, a multiphase pump arranged downstream of the electrolyser stack and a hydrogen gas / liquid separator. The multiphase pump is arranged between the water electrolyser stack and the hydrogen gas / liquid separator. The present invention also relates to a method for production of compressed hydrogen in a water electrolyser system including: supplying deionized water or liquid electrolyte to a water electrolyser stack; producing hydrogen in a water electrolyser stack; compressing a mixture of produced hydrogen and entrained deionized water or liquid electrolyte in a multiphase pump; and separating the compressed mixture of produced hydrogen and entrained deionized water or liquid electrolyte in a hydrogen gas / liquid separator.

Description

A WATER ELECTROLYSER SYSTEM AND METHOD FOR PRODUCING COMPRESSED HYDROGENTechnical Field

[0001] The invention relates generally to hydrogen production and processing systems and more particularly to enhancement of the performance of an integrated hydrogen production and processing system that includes water electrolyser systems for hydrogen production and compression sub-systems.Background Art

[0002] Water electrolyser systems for hydrogen production include alkaline water electrolysis (AWE), proton exchange membrane electrolysis (PEM), solid oxide electrolysis (SOEC) and anion exchange membrane electrolysis (AEM). The water electrolyser cells of all technologies include an anode and a cathode, as well as a diaphragm, membrane, or electrolyte. AWE systems use liquid alkaline electrolyte solution and a diaphragm. AEM systems use a less alkaline liquid electrolyte and an anion exchange membrane. PEM systems do not contain liquid electrolytes, but use deionized water and a polymer-electrolyte membrane, also known as a proton exchange membrane (PEM).

[0003] The development of infrastructure for low-cost distribution, delivery, storage and use of hydrogen is reliant on cost and energy efficient hydrogen compression. Hydrogen is one of the most challenging gases to compress due to its low molecular weight and small size. There are various compressors that can be used and ultimately the choice of compression technology, associated costs and energy use will depend on where in the supply chain it is used.

[0004] Mechanical compressors are widely used and are designed based on the direct conversion of mechanical energy into compressed gas energy. There are several classifications, but most compressors used today for gaseous H2 compression are either positive displacement compressors or dynamic compressors.

[0005] Centrifugal compressors are examples of dynamic compressors that are most commonly used in applications that require high throughput and moderatecompression ratios. They compress the process gas using a rotating impeller with radial blades that imparts kinetic energy to the process gas by increasing its velocity. Unlike reciprocating compressors, the compression ratio largely depends on the molecular weight of the gas in the centrifugal compressor. Because of the low molecular weight of H2, the impeller speed must be increased, or additional compression stages must be added compared to compression of e.g. natural gas. Increasing current impeller tip speeds is very challenging due to material strength limitations and H2 embrittlement issues.

[0006] Reciprocating piston compressors are ideal for low to moderate flow and high-pressure applications. They are positive displacement machines that work via compression and displacement of gases. A single stage reciprocating compressor is designed using a piston and system where the piston is driven by a crankshaft, converting rotary motion into linear motion. The cylinder uses two automatic valves - one for gas suction and the other for gas discharge and the energy needed for compression is provided by either an electrical or thermal source.

[0007] Another option to produce hydrogen for use or storage at high pressure is to perform water electrolysis at the required pressure directly, generating both hydrogen and oxygen at high pressure. Alternatively, differential-pressure electrolysis may be employed to generate hydrogen at high pressure and oxygen at substantially atmospheric pressure. In a state-of-the-art PEM (proton-exchange membrane) electrolyser, a low-pressure pump provides liquid water at near-ambient pressure to the anode side of the electrolyser stack. When DC current is applied, the water is decomposed at the anode to oxygen, protons and electrons. The oxygen is separated from the excess circulating water in a low-pressure gas / water separator. All functions on the anode side are usually conducted at near-ambient pressure. The protons, along with some water, are electrochemically transported across the membrane to the cathode, where they react with the externally transported electrons to produce hydrogen at the required higher operating pressure. The hydrogen is then separated from the transported water in a high-pressure gas / water separator. The hydrogen may further be compressed in a single-phase compressor outside the integrated PEM electrolysis process after the separation of hydrogen and water. The separated water is reintroduced to the electrolysis process.

[0008] To date, high pressure water electrolysers have been fabricated that either generate both hydrogen and oxygen at pressures up to 200 bar or generate hydrogen at 200 bar and oxygen at near atmospheric pressure. For example, US8282811 B2 discloses a water electrolyser that operates at a differential pressure (H2>02) of 2500 psia using plastic materials as frames and proton-exchange membranes (PEMs) as solid-polymer electrolytes. Electrolysers operating totally or partially at these high pressures are however expensive, involve complex construction, and present safety hazards due to higher risk of hydrogen leaks. Such electrolysers providing outlet pressures above 40 bar have not been a commercial success. Most PEM electrolyser manufacturers today therefore offer electrolysers with a hydrogen output pressure between 15 and 40 bar. Even at these pressures there is a significant amount of hydrogen diffusing from the cathode compartment to the anode compartment causing the formation of a H2 / O2 gas mixture in the anode compartment. This hydrogen crossover phenomenon forces the use of thick membranes (> 50pm) to limit the amount of hydrogen that is mixed with the oxygen to prevent flammable or explosive gas mixtures to be formed. The use of thick membranes increases the cost and leads to higher ohmic resistances in the PEM electrolyser cells which increases the energy demand for the electrolyser process.

[0009] In a conventional electrolyser process, any additional hydrogen compression beyond the operating pressure of the water electrolyser stack takes place using a conventional single-phase compressor or wet gas compressors outside the electrolysis process, after the separation of liquid and hydrogen in the separator vessel. In addition to the compressor, the process can also include a buffer tank and / or a gas drier before the compressor.

[0010] The energy needed to compress hydrogen is relatively high compared to other gases such as natural gas. For an adiabatic compression of hydrogen from 1 to 200 bar, the energy need is ~10 MJ / kg, or about 10% of the energy content of the compressed hydrogen. The ideal isothermal compression would in comparison need only ~6 MJ / kg.

[0011] Due to the technical difficulties and the high energy demand for compression of hydrogen as described above, there exists a need in the art for improved systems and methods for compressing hydrogen in combination with production from electrolysers.

[0012] Multiphase pumps are used in many different industries, where it is necessary to convey or compress a multiphase process fluid which comprises a mixture of a plurality of phases, for example a liquid phase and a gaseous phase. A multiphase pump is essentially a hybrid of a pump and a compressor and thus, also referred to as a multiphase compressor. In the present disclosure, the term multiphase pump will be used. The ratio of the gaseous phase in a multiphase mixture is commonly measured by the dimensionless gas volume fraction (GVF) designating the volume ratio of the gas in the multiphase process fluid. A multiphase pump may be designed for conveying multiphase process fluids having a GVF from close to 0% to close to 100%, i.e. all process fluids from a pure liquid (GVF = 0%) to a pure gas (GVF = 100%).Summary of invention

[0013] According to a first aspect of the present invention, there is provided a water electrolyser system for hydrogen production, comprising an electrolyser stack, a multiphase pump arranged downstream of the electrolyser stack; and a hydrogen gas / liquid separator, wherein the multiphase pump is arranged between the water electrolyser stack and the hydrogen gas / liquid separator.

[0014] In one embodiment, the water electrolyser stack comprises a PEM water electrolyser stack comprising a plurality of proton exchange membrane (PEM) water electrolyser cells, each PEM water electrolyser cell comprises an anode compartment, a cathode compartment and a proton exchange membrane; the cathode compartment is configured to be supplied with deionized water through a stack cathode inlet, and the anode compartment is configured to be supplied with air through a stack anode inlet. In this embodiment, a control valve is arranged between the hydrogen gas / liquid separator and the stack cathode inlet to control supply of deionized water to the PEM water electrolyser stack. A humidifier is not required, but may be included to provide humidified air to the anode compartment.

[0015] In another embodiment, the PEM water electrolyser stack comprises a plurality of proton exchange membrane water electrolyser cells, each water electrolyser cell comprises an anode compartment, a cathode compartment; the anode compartment is configured to be supplied with deionized water through astack anode inlet; the cathode compartment comprises a cathode outlet where a mixture of produced hydrogen and entrained water exits. This embodiment further includes an oxygen gas / liquid separator separating a mixture of water and produced oxygen. In this embodiment, the at least one control valve is arranged between the hydrogen gas / liquid separator and the oxygen gas / liquid separator to control supply of deionized water to the PEM water electrolyser stack.

[0016] In another embodiment, the water electrolyser stack comprises an AEM water electrolyser stack or an AWE water electrolyser stack, and a pump supplying the AEM water electrolyser stack or the AWE water electrolyser stack with a liquid alkaline electrolyte.

[0017] According to a second aspect of the present invention, there is provided a method for production of hydrogen in a water electrolyser system including: supplying water or liquid electrolyte to a water electrolyser stack, producing hydrogen in the water electrolyser stack, compressing a mixture of produced hydrogen and entrained deionized water or liquid electrolyte in a multiphase pump; and separating the compressed mixture of produced hydrogen and entrained deionized water or liquid electrolyte in a hydrogen gas / liquid separator.

[0018] In one embodiment of the method, the outlet pressure from the multiphase pump is between 2 and 100, preferably 4 and 50 bar above the outlet pressure from the electrolyser stack. The gas volume fraction in the mixture of produced hydrogen and deionized water or liquid electrolyte may be within the range 5 to 95%. Deionized water or liquid electrolyte may be circulated from the hydrogen gas / liquid separator via a control valve to the water electrolyser stack.Figures

[0019] Figure 1 shows a schematic diagram of one embodiment of PEM water electrolyser system for hydrogen production.

[0020] Figure 2 shows another schematic diagram of one embodiment of PEM water electrolyser system for hydrogen production.

[0021] Figure 3 shows a schematic diagram of an anion exchange membrane or liquid alkaline water electrolyser system for hydrogen production.

[0022] Figure 4 shows an alternative configuration of an anion exchange membrane or liquid alkaline water electrolyser system for hydrogen production.

[0023] Figure 5 is a diagram showing the energy required to compress hydrogen from ambient conditions to different final pressures.Detailed description of the invention

[0024] The present invention relates to a use of a multiphase pump integrated in a water electrolysis process to compress a mixture of hydrogen and deionized water or liquid electrolyte and simultaneously circulate deionized water or liquid electrolyte in the process.

[0025] The water electrolysis process can be either a proton exchange membrane PEM, a liquid alkaline (AWE) or an anion exchange membrane (AEM) electrolysis process. The functionality of the multiphase pump in the different processes is similar, with the main difference being the circulated liquid which in a PEM electrolyser is ultrapure, deionized water, in a liquid alkaline water electrolyser the liquid electrolyte is an alkaline aqueous electrolyte, usually concentrated potassium hydroxide (25-30wt% KOH), and in an AEM electrolyser the liquid electrolyte is an alkaline aqueous electrolyte, usually a dilute KOH solution (0.01-1 M KOH). In the present detailed description of the invention, embodiments of PEM water electrolyser systems and an AEM or liquid alkaline water electrolyser systems will be described.

[0026] An integrated system is disclosed for the combined compression of hydrogen produced by an electrolyser stack as described above and circulation of deionized water or liquid electrolyte through the water electrolyser stack using a multiphase pump, enabling a cost effective and energy efficient compression of hydrogen produced in an electrolysis process.

[0027] In one embodiment of the invention, shown in Fig. 1 , the system comprises a hydrogen gas / liquid separator 10, a PEM water electrolyser stack 11 connected to the hydrogen gas / liquid separator 10 via a stack cathode inlet 14, a pressure control valve 13a positioned in the connection line between the hydrogen gas / liquid separator 10 and the PEM water electrolyser stack 11 , a multiphase pump 12 with a pump inlet 22 connected to a cathode outlet 15 of the PEM water electrolyser stack 11 and a pump outlet 23 connected to the hydrogen gas / liquid separator 10.

[0028] The stack cathode inlet 14 of the PEM water electrolyser stack may be supplied with deionized water via the deionized water supply line 21 and the hydrogen gas / liquid separator 10 via the pressure control valve 13a and a small amount of the supplied water is converted to oxygen in the anode compartment and hydrogen in the cathode compartment. Air is supplied through a stack anode inlet 16. A humidifier 19 is not required, but may be included to provide humidified air to the anode compartment. A mixture of humidified oxygen and air may be led from the stack anode outlet 17 to the humidifier 19. Generally, the hydrogen gas is produced at a pressure level between ambient pressure (1 bar) and 50 bar, depending on the PEM water electrolyser stack and system design. In an embodiment of this invention, the gas is produced at a pressure level between 1 and 10 bar. The remaining water acts as a coolant medium to remove excess heat formed in the electrochemical reaction and exits the PEM water electrolyser stack 11 together with the produced hydrogen as a two-phase flow. The gas volume fraction in this two-phase flow can vary between 2-90%, depending on the operating conditions of the PEM water electrolyser stack.

[0029] The two-phase flow consisting of hydrogen and deionized water enters the hydrogen multiphase pump 12 where it is compressed to a higher pressure than the pressure at the stack cathode outlet 15. The level of this pressure can vary between 1 and 100 bar above the outlet pressure from the electrolyser stack, depending on the operating pressure of the PEM water electrolyser stack and the desired design pressure of the hydrogen gas / liquid separator 10. In an embodiment of this invention, the outlet pressure from the multiphase pump 12 is between 4 and 50 bar above the outlet pressure from the electrolyser stack.

[0030] In another embodiment of the invention, shown in Fig 2, the system comprises a oxygen gas / liquid separator 20 and a hydrogen gas / liquid separator 10, a PEM water electrolyser stack 11 connected to the oxygen gas / liquid separator 20 via a stack anode inlet 16 and to a stack anode outlet 17, a multiphase pump 12 with pump inlet 22 connected to a stack cathode outlet 15 of the PEM water electrolyser stack 11 and a pump outlet 23 connected to the hydrogen gas / liquid separator 10 and the pressure control valve 13b positioned on the connection line between the hydrogen gas / liquid separator 10 and the oxygen gas / liquid separator 20.

[0031] The anode inlet 16 of the PEM water electrolyser stack 11 is supplied with deionized water from the oxygen gas / liquid separator 20 via pump 18a and a small amount of the supplied water is converted to oxygen in the anode compartment and hydrogen in the cathode compartment. Deionized water is supplied via supply line 21. Generally, the hydrogen gas is produced at a pressure level between ambient pressure (1 bar) and 50 bar, depending on the PEM water electrolyser stack and system design. Some water (between 2 and 5 water molecules per proton, depending on membrane material and operating conditions) is also transferred through the membrane separating the anode compartment and the cathode compartment by electroosmosis. This water exits the cathode compartment of the PEM water electrolyser stack 11 together with the produced hydrogen as a two- phase flow. The gas volume fraction in this two-phase flow can vary between 5-95%, depending on the operating conditions of the PEM electrolyser stack. The two-phase flow consisting of hydrogen and deionized water enters the multiphase pump 12 where it is compressed to a pressure between 1 and 100 bar above the outlet pressure from the PEM water electrolyser stack 11 . The level of this pressure depends on the operating pressure of the PEM water electrolyser stack 11 and the desired design pressure of the hydrogen gas / liquid separator 10. In an embodiment of this invention, the outlet pressure from the multiphase pump 12 is between 4 and 50 bar above the outlet pressure from the PEM water electrolyser stack 11 . The water entering the hydrogen gas / liquid separator 10 from the multiphase pump 12 is re-injected to the oxygen gas / liquid separator through the pressure control valve 13b via a fluid connection line 24. This fluid connection line can optionally also consist of a dedicated degassing compartment to remove dissolved hydrogen from the water before it enters the oxygen I iquid / gas separator 20.

[0032] In a third embodiment of the invention, shown in Fig 3, the system comprises a oxygen gas / liquid separator 20 and a hydrogen gas / liquid separator 10, an anion exchange membrane or liquid alkaline water electrolyser stack 11 connected to a liquid electrolyte pump 18b via a stack anode inlet 16 and a stack cathode inlet 14, the liquid electrolyte pump 18b connected to the oxygen gas / liquid separator 20 and the hydrogen gas / liquid separator 10 with a pressure control valve 13c on the connection line between the hydrogen / gas liquid separator 10 and the liquid electrolyte pump inlet, a multiphase pump 12 with a pump inlet 22 connected to astack cathode outlet 15 of the one or more AEM or liquid alkaline electrolyser stack(s) 11 and a compressor outlet 23 connected to the hydrogen gas / l iquid separator 10. Water supply line 21 supplies deionized water to the oxygen gas / liquid separator 20.

[0033] The liquid pump inlet is supplied with electrolyte from both the oxygen gas / liquid separator 20 and the hydrogen gas / liquid separator 10, and the stack anode and cathode inlets (16, 14) are supplied with liquid electrolyte from the pump 18b and a small amount of the water in the electrolyte is converted to oxygen in the anode compartment and hydrogen in the cathode compartment.

[0034] The oxygen and hydrogen gas are produced at a pressure level between ambient pressure (1 bar) and 50 bar, depending on the AEM or liquid alkaline water electrolyser stack and system design. Liquid electrolyte exits the anode compartment of the AEM or liquid alkaline water electrolyser stack 11 together with the produced oxygen as a two-phase flow and enters the oxygen gas / liquid separator 20.

[0035] Liquid electrolyte exits the cathode compartment of the AEM or liquid alkaline water electrolyser stack 11 together with the produced hydrogen as a two-phase flow. The gas volume fraction in this two-phase flow can vary between 5-95%, depending on the operating conditions of the AEM or liquid alkaline water electrolyser stack 11. The two-phase flow consisting of hydrogen and liquid electrolyte enters the multiphase pump 12 where it is compressed to a higher pressure. The level of this pressure can vary between 2 and 100 bar, depending on the operating pressure of the AEM or liquid alkaline water electrolyser stack 11 and the desired design pressure of the hydrogen gas / liquid separator 10. In a preferred embodiment of this invention, the outlet pressure from the multiphase pump 12 is between 4 and 50 bar.

[0036] The multiphase pump / compressor 12 used in the described embodiments, can be of any design and can consist of one or more compression stages, depending on the desired total compression ratio over the complete compressor. Twin-screw, helicoaxial progressing-cavity or centric reciprocating pumps are examples of multiphase pumps that can be used.

[0037] In a PEM electrolysis process, at least one pressure control valve (13a, 13b) controls supply of deionized water to at least one electrolyser stack. In a liquid alkaline or an anion exchange membrane (AEM) electrolysis process, at least onepressure control valve (13c) controls supply of liquid electrolyte (concentrated or diluted KOH) to at least one electrolyser stack.

[0038] In Fig. 4, an alternative configuration of the liquid alkaline or anion exchange membrane electrolysis process is shown. The cathode inlet 14 and the anode inlet 16 to the water electrolyser stack are separated, and a pump 18c circulates liquid electrolyte to the anode side of the AWE or AEM water electrolyser stack 11 . In addition, there is a balancing line 25 between the anode inlet 16 and the cathode inlet 14 which function is to equalize concentration differences in the electrolyte on the anode and cathode side. Other reference signs in this figure correspond to the reference signs in Fig. 3.

[0039] In all embodiments described above, but not shown in the figures, more than one water electrolyser stack may be used.

[0040] Advantages of the invention:

[0041] The use of a multiphase pump downstream of a PEM water electrolyser stack in the PEM water electrolyser process instead of a conventional single phase water pump upstream of the PEM water electrolyser stack provides several advantages over state of the art. For example, the compression of hydrogen in a hydrogen / water two phase mixture enables a close to isothermal compression of the gas due to the cooling effect of the water present: The high mass fraction of water in the two-phase mixture enables the water to adsorb the heat generated by the gas compression with only marginal temperature increase due to a much higher heat capacity.

[0042] In a conventional PEM electrolyser process, the hydrogen compression takes place outside the integrated PEM electrolysis process, after the separation of water and gas, and the compression process will be close to an adiabatic compression. As shown in Figure 5, the energy use in an isothermal compression process is significantly lower than in an adiabatic process, thus the use of a multiphase pump will reduce the overall energy consumption of the process.

[0043] Another advantage is that the multiphase pump replaces two other process equipment, namely the conventional single-phase liquid pump and the single-phase hydrogen gas compressor, reducing the overall complexity, cost and service requirements of the system.List of figure reference signs

Claims

Claims1 . A water electrolyser system for production of compressed hydrogen, comprising a water electrolyser stack; a multiphase pump arranged downstream of the water electrolyser stack; and a hydrogen gas / liquid separator; wherein the multiphase pump is arranged between the water electrolyser stack and the hydrogen gas / liquid separator.

2. The water electrolyser system of claim 1 , wherein the water electrolyser stack comprises a proton exchange membrane (PEM) water electrolyser stack, an anion exchange membrane (AEM) water electrolyser stack or a liquid alkaline water electrolyser stack (AWE).

3. The water electrolyser system of claim 1 or 2, wherein the water electrolyser stack comprises a PEM water electrolyser stack comprising a plurality of PEM water electrolyser cells, each PEM water electrolyser cell comprises an anode compartment, a cathode compartment and a proton exchange membrane; the cathode compartment is configured to be supplied with deionized water through a stack cathode inlet, and the anode compartment is configured to be supplied with air through a stack anode inlet.

4. The water electrolyser system of claim 3, wherein a control valve controlling supply of deionized water to the cathode compartment is arranged between the hydrogen gas / liquid separator and the stack cathode inlet.

5. The water electrolyser system of claims 1 or 2, wherein the water electrolyser stack comprises a PEM water electrolyser stack comprising a plurality of proton exchange membrane water electrolyser cells, each water electrolyser cell comprises an anode compartment and a cathode compartment;the anode compartment is configured to be supplied with deionized water through a stack anode inlet; and the cathode compartment comprises a cathode outlet where a mixture of produced hydrogen and entrained deionized water exits.

6. The water electrolyser system of claim 5, further comprising an oxygen gas / liquid separator separating a mixture of water and produced oxygen.

7. The water electrolyser system of claim 5 or 6, wherein a control valve for controlling supply of deionized water to the anode is arranged between the hydrogen gas / liquid separator and the oxygen gas / liquid separator.

8. The water electrolyser system of claim 1 or 2, wherein the water electrolyser stack comprises an AEM water electrolyser stack or an AWE water electrolyser stack, and a pump supplying the AEM water electrolyser stack or the AWE water electrolyser stack with a liquid alkaline electrolyte.

9. Method for production of compressed hydrogen in a water electrolyser system, including: supplying deionized water or liquid electrolyte to a water electrolyser stack; producing hydrogen in the water electrolyser stack; compressing a mixture of produced hydrogen and entrained deionized water or liquid electrolyte in a multiphase pump; and separating the compressed mixture of produced hydrogen and entrained deionized water or liquid electrolyte in a hydrogen gas / liquid separator.

10. The method of claim 9, wherein the water electrolyser stack comprises a proton exchange membrane (PEM) water electrolyser stack, an anion exchange membrane (AEM) water electrolyser stack or a liquid alkaline (AWE) water electrolyser stack.11 . The method of claim 9 or 10, wherein the outlet pressure from the multiphase pump is between 2 and 100 bar, preferably between 4 and 50 bar,above the outlet pressure from the water electrolyser stack.

12. The method of any of claims 9 to 11 , wherein the gas volume fraction in the mixture of produced hydrogen and deionized water or liquid electrolyte is within the range 5 to 95%.

13. The method of any of claims 9 to 12, further comprising circulating deionized water or liquid electrolyte from the hydrogen gas / liquid separator via a control valve to the water electrolyser stack.

Images