Electrolyser System

Abstract

An electrolyser system comprises a water electrolyser having first and second electrode compartments, and a vessel having first and second chambers, the first compartment connected to the first chamber and the second compartment connected to the second chamber, via valved ports, wherein the first chamber also comprises a valved outlet and the second chamber also comprises a valved inlet, and wherein the system includes pressure sensing means to open or close the valves.

Description

CROSS-REFERENCE TO A RELATED APPLICATION

This application claims priority to Great Britain Application No. 0714140.1, filed Jul. 19, 2007, which is incorporated herein in its entirety.

FIELD OF THE INVENTION

This invention relates to an electrolyser system for militating against osmotic drag.

BACKGROUND OF THE INVENTION

An ionic exchange membrane electrolyser consists of two catalytic electrodes separated by an ionically conductive solid polymer electrolyte. Cationic and Anionic material may be used to form the solid polymer electrolyser membrane. In both cases if the electrolyser operates in a system which is not open to the surrounding environment (closed system) it will pressurise at a rate dependent upon the volume of the head space within the separation towers (the vessels that either supply water to, or collect gas and water from, the electrolyser), system temperature and the gas production rate.

Electro-osmotic drag occurs in all solid polymer electrolyte membrane electrochemical cells. It is a process by which water is transported through the membrane in the direction of ion transport (i.e. for cationic exchange (CE) systems, from the oxygen to the hydrogen side). The degree of drag is directly related to the operational current of the electrolyser, the temperature of the system and the chemistry of the membrane. Therefore, for CE closed electrolyser systems this will cause the oxygen side of the electrolyser to become devoid of water and to pressurise more slowly than predicted, simultaneously the hydrogen side will have an increased volume of water and pressurise at a greater than predicted rate.

In order to combat the effects of osmotic drag the water needs to be removed from the hydrogen separation vessel and water needs to be added into the oxygen separation vessel and the differential pressure between the separation towers needs to be equalised in order to reduce the differential pressure across the membrane. Due to the risk of explosion, it is not possible to transfer water from the hydrogen tower to oxygen tower directly, since dissolved hydrogen gas within the transferred water may come out of solution, creating an explosive mixture of hydrogen and oxygen gas within the restricted space of the pressurised oxygen separation vessels; therefore fresh water needs to be used.

The reverse is true for an anionic exchange (AE) system; the drag will carry water from the hydrogen side to the oxygen side, resulting in a decreased rate of pressurisation in the hydrogen vessel and an increased rate of pressurisation in the oxygen vessel. This can be combated by removing water from the oxygen vessel and introducing fresh water to the hydrogen vessel.

SUMMARY OF THE INVENTION

The present invention is based on the realisation that it is possible to utilise the different pressures within a closed system of an electrolyser to drive a “pump”. This “pump” can inject fresh water from a low pressure source into the highly pressurised vessel minimising the differential pressure across the electrolyser membrane and resulting in improved water management.

According to a first aspect, the present invention is an electrolyser system comprising a water electrolyser having first and second electrode compartments, and a vessel having first and second chambers, the first compartment connected to the first chamber and the second compartment connect to the second chamber, via valved ports, wherein the first chamber also comprises a valved outlet and the second chamber also comprises a valved inlet, and wherein the system includes pressure sensing means to open or close the valves.

According to a second aspect, the present invention is a method for militating against osmotic drag in an electrolyser having first and second electrode compartments and that produces hydrogen and oxygen by electrolysis of water, which comprises removing water from the first electrode compartment into the first chamber of a vessel, and supplying water from the second chamber of the vessel into the second electrode compartment, wherein the flow of water is controlled by the relative pressures in the first and second electrode compartments.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 illustrates an electrolyser system of the invention and also illustrates a vessel for operation with a CE system, in which the Ph>Po; the reverse is true for AE systems.

DESCRIPTION OF PREFERRED EMBODIMENTS

The electrolyser system comprises a water electrolyser (which produces hydrogen and oxygen) having first and second electrode compartments and a vessel having first and second chambers. The first chamber is connected to the first electrode compartment via a valved port, and the second chamber is connected the second electrode compartment via a valved port. In one embodiment, the first electrode compartment is the cathode and the second electrode compartment is the anode. In another embodiment, the first electrode compartment is the anode and the second electrode compartment is the cathode.

The, or each, valved port may comprise a separation tower. A “separation tower” or “separation vessel”, as described herein, is a container suitable for positioning between the water electrolyser and the vessel, in an electrolyser system of the invention. It may be connected to the vessel and the electrolyser, via valved ports. Alternatively, it may be integral with the electrolyser.

When the first electrode compartment is the cathode and the second electrode compartment is the anode, the tower separating the first compartment and the first chamber will contain H₂/water mix, and the tower separating the second compartment and the second chamber will contain an O₂/water mix. When the first electrode compartment is the anode and the second electrode compartment is the cathode, the tower separating the first electrode compartment and the first chamber will contain an O₂/water mix, and the tower separating the second electrode compartment and the second chamber will contain a H₂/water mix.

The separation tower may be a pespex cylindrical tower. It may have a valved port near the bottom of the tower to supply water to the electrolyser, and a valved port near the top of the tower to accept water/gas from the electrolyser. The separation tower allows the waters to be re-circulated and degassed after passing through the electrolyser. As the gas builds up, it may be transferred to a storage container.

The pressure sensing means may sense pressure in the first and second chambers of the vessels and/or in the first and second electrode compartments. The pressure sensing means may either directly or indirectly cause the opening and closing of the valves.

Preferably the pressure sensing means is a moveable member separating the first and the second chambers of the vessel, so that the contents of the two chambers are isolated from each other. The moveable member preferably moves between a first and second position. When it reaches the first position, it causes the valves of the ports to open and the valves of the inlet and the outlet to close. When it reaches the second position, it causes the valves of the ports to close and the valves of the inlet and outlet to open.

In one embodiment, the member opens or closes the valves by activating a switch at either the first or second position. The switches may be micro-switches, which are placed either inside the chambers, or externally. For external switching the moveable member may have an arm which projects out of the chamber, which can then activate external micro-switches.

Mechanical actuators may be used to open and close the valves. These are driven/commanded by micro-switches, which can either be intelligently coupled to pressure sensors within the system, or connected to an external arm, which is connected to the piston. As the piston moves, so too does the arm, which makes/breaks electrical connections, and commands the valves to open or close.

The invention will now be described, by way of example only, with reference to the accompanying drawing.

FIG. 1 illustrates an electrolyser system of the invention. It shows a vessel suitable for operation with a cationic exchange (CE) electrolyser (not shown).

The vessel (FIG. 1) comprises:

• • four valves: Vh is connected to a hydrogen separation vessel (not shown) of an electrolyser; Vo is connected to an oxygen separation vessel (not shown) of an electrolyser; Vd is connected to a drain; and Vf is connected to a fresh water supply.
  • a sliding section defining Chamber A and Chamber B, within a high pressure cylinder capable of moving between positions A and B, and containing four ports (each connected to an appropriate valve).
  • switches at positions A and B to activate the opening and closing of the valves.

At the start of the electrolyser operation, Vf and Vd are closed, and Vh and Vo are open. The pressure in chamber section A is therefore equal to the pressure in the hydrogen separation vessel (Ph) and the pressure in chamber section B is equal to the pressure in the oxygen separation vessel (Po). Before operation, Ph is equal to Po; during operation, Pf is higher than Pd.

During operation the electrolyser will produce hydrogen and oxygen in a 2:1 ratio, if the gas towers are of equal size the system will pressurise so that the pressure in the hydrogen vessel (Ph) becomes greater (approximately twice) than the pressure in the oxygen vessel (Po). The water levels will change so that the water level in the hydrogen vessel (Wh) increases (due to osmotic drag influx) and the water level in the oxygen vessel (Wo) will decrease (due to losses from osmotic drag and the utilisation of water for electrolysis), further increasing the pressure differential between Ph and Po. Since valves Vh and Vo are open the pressures in section A and section B will correspond to the pressures in the respective vessels.

As Ph increases at a faster rate than Po, the sliding section in the chamber will be pushed from position A to position B in an attempt to equalise the pressure in each of the chambers. Once the sliding section reaches position B, a micro-switch closes valves Vo and Vh, and opens Vd and Vf. When Vd opens to the atmosphere, the hydrogen water will drain out of the chamber, as the pressure in chamber A is greater than atmospheric pressure; the pressure in section A will therefore decrease. Fresh water will be pushed into section B through the open valve, from the water source (which has a pressure above atmospheric), causing the sliding mechanism to move from position B to position A. A mains water supply is at adequate pressure to cause the sliding mechanism to move from B to A. This reduces the need for a high pressure water pump. This means both reduced capital, operational and maintenance costs. Once at position A, a micro-switch will close valves Vd and Vf and open valves Vh and Vo, again allowing the sliding mechanism to move towards position B to negate the pressure build-up. This can be used as a continuous system.

FIG. 1 illustrates a vessel for operation with a CE system, in which the Ph>Po; the reverse is true for AE systems. AE systems require water to be removed from the O₂ vessel and introduced into the H₂ vessel; this requires Po>Ph. Simple modifications to the vessel design will allow the cassette to operate with an AE system. Specifically, the H₂ vessel must have a volume of more than twice the volume of the O₂ vessel.

Claims

1. An electrolyser system comprising a water electrolyser having first and second electrode compartments, and a vessel having first and second chambers, the first compartment connected to the first chamber and the second compartment connected to the second chamber, via valved ports, wherein the first chamber also comprises a valved outlet and the second chamber also comprises a valved inlet, and wherein the system includes pressure sensing means to open or close the valves.

2. The system according to claim 1, wherein the, or each, valved port comprises a separation tower.

3. The system according to claim 1, wherein the pressure sensing means comprises a member separating the first and second chambers, moveable between first and second positions, whereby the member causes the ports to open and the inlet and outlet to close when it reaches the first position, and causes the ports to close and the inlet and outlet to open when it reaches the second position.

4. The system according to claim 3, wherein the member causes the opening or closing of the ports, the inlet and the outlet, by activating switches at the first and second positions.

5. The system according to claim 1, wherein the inlet is connected to a water supply.

6. The system according to claim 1, wherein the first chamber is connected to the cathode compartment of the electrolyser, and the second chamber is connected to the anode compartment.

7. The system according to claim 1, wherein the first chamber is connected to the anode compartment of the electrolyser, and the second chamber is connected to the cathode compartment.

8. A method for militating against osmotic drag in an electrolyser having first and second electrode compartments and that produces hydrogen and oxygen by electrolysis of water, which comprises removing water from the first electrode compartment into the first chamber of a vessel, and supplying water from the second chamber of the vessel into the second electrode compartment, wherein the flow of water is controlled by the relative pressures in the first and second electrode compartments.

9. The method according to claim 8, wherein the first electrode compartment is the cathode and the second electrode compartment is the anode.

10. The method according to claim 8, wherein the first electrode compartment is the anode and the second electrode compartment is the cathode.