The present invention relates to a method and a device for performing an ion-exchanging process. Examples of such processes are electro dialysis (ED), electro-deionisation (EDI), reverse electro dialysis (RED), and capacitive energy extraction based on Donnan Potential (CDP).

In conventional ion-exchange processes ion-exchange membranes are used. Adjacent membranes are separated by a spacer that is situated between two adjacent ion-exchange membranes, or in case of a spacerless system corrugated or profiled membranes can be used. These devices further comprise a number of cation and anion exchange membranes that are stacked in an alternating pattern between a cathode and an anode. Spacers maintain the distance between the adjacent membranes. However, spacers contribute to the pressure drop-off over the device. Also, spacers occupy part of the (effective) membrane surface.

Known methods and devices performing a reverse electro dialysis process can be used to generate energy when flows with different salinity mix. This power generation is possible due to the increase in entropy due to mixing flows with different salinities. Two of the possible existing systems to capture this energy are reverse electro dialysis (RED), and electro-deionisation and capacitive energy extraction based on Donnan potential (CDP), while electro dialysis is used for desalination or mineral recovery, for example.

Reverse electro dialysis relies on the potential difference over an ion exchange membrane with a concentrated salt solution on one side and a diluted salt solution on the other side of the membrane. Such method and device is disclosed in NL 1031148, for example. When the multiple anion exchange membranes (AEMs) and cation exchange membranes (CEMs) are stacked alternately, with concentrated and diluted salt solutions alternately in between the membranes, the voltage over each membrane is cumulated. At both ends of the RED stack, electrodes convert the ionic current into an electrical current (e.g. by using a reversible redox reaction), which can be used to power an electrical device.

In capacitive energy extraction based on Donnan potential, AEMs and CEMs are also stacked, but have a capacitive element (e.g. porous carbon) on one side of the membrane and one of the flow types (concentrated or diluted salt solution) on the other side of the membrane. The concentrated and diluted salt solutions succeed each other in time. When a concentrated salt solution is circulated in the compartments, ions are transported towards the capacitive element, due to the ion selective membranes. In a next stage, when a diluted salt solution is circulated in the compartments, the ions are transported back from the capacitive element towards the diluted salt solution. During both stages power is produced because a voltage is generated (due to the Donnan potential at the ion exchange membranes) and an electrical current is generated (due to the net transport of cationic or anionic species). The capacitive elements are conductive for electrons, to extract the produced power.

All these systems and processes are limited by the electrical resistance of the feed flow. In particular, the diluted feed flow causes a large electrical resistance. A thin compartment reduces the electrical resistance of this compartment and hence improves the power output significantly.

The present invention has as one of its objectives to provide a method that increases the overall performance of an ion-exchanging process using membranes.

This objective is achieved with the method for performing an ion-exchanging process according to the invention, the method comprising the steps of:

- providing a device comprising:

- a first electrode;

- a second electrode separated from the first electrode;

- a number of electrolyte compartments provided between the first and the second electrodes, wherein the electrolyte compartments are formed by a number of alternately provided cation exchange membranes and anions exchange membranes that are provided at a mutual distance, wherein the mutual distance is variable;

- providing flows having high and low osmotic concentrations; and

- changing the mutual distance in time and/or place.

In RED, ED and EDI the method according to the invention comprises a first compartment with a first electrode and a second compartment with a second electrode. In CDP the first and second electrode are provided in one compartment.

For example, in case of a method for performing a reverse electro dialysis process, the first electrode relates to an anode and the first compartment relates to an anode compartment. In such application the second electrode relates to a cathode and the second compartment relates to a cathode compartment. The anode and cathode compartments are filled with an anode fluidum and a cathode fluidum, respectively.

According to the present invention the wording fluidum comprises all media wherein molecules and/or ions with a low molecular rate may undergo diffusion movement, the media preferably including an aqueous medium or a gel on a water basis. The electrodes are

manufactured from suitable materials, such as carbon or other conductive material.

The first and second electrodes or electrode compartments are separated by a number, i.e. one or more, of cation exchange membranes and anion exchange membranes, which are placed alternately between the first and second electrodes. These membranes are known in the art. A cation exchange membrane essentially does not allow anions to pass and an anion exchange membrane essentially does not allow cations to pass. Between the cation exchange membranes and anion exchange membranes a number, i.e. one or more, of so-called electrolyte compartments are formed. In use, these electrolyte compartments are filled with low osmotic electrolyte solutions having low electrolyte

concentrations such as river water with a relatively low osmotic pressure or value, and/or high osmotic electrolyte solutions having electrolyte concentrations higher than the low osmotic electrolyte solutions, such as sea water with a relatively high osmotic pressure or value. The wording "electrolyte solutions" includes a solution of a number of positively and negatively ionised chemical species. It must be understood that high and low osmotic electrolyte solutions are relative terms and are to be considered relatively as the relative relationship of the electrolyte concentrations provides the driving force for the ion transport.

For example, for a reverse electro dialysis process an oxidative reaction takes place at the anode and a reductive reaction takes place at the cathode thereby generating energy.

The membranes in the device used in the method according to the invention are provided at a mutual distance. Often, this mutual distance is guarded by the use of spacers. According to the present invention the mutual distance of adjacent membranes varies over the length of the electrolyte compartment seen in the flow direction and/or varies in time. This change in mutual distance in time and/or over the length of the compartment seen in the flow direction can be applied gradually in a presently preferred embodiment, or alternatively be applied abrupt.

When refreshing and/or increasing the flow rate of an electrolyte through an electrolyte compartment the hydraulic friction or hydraulic resistance should be minimal to reduce the pressure difference between the inflow and outflow of the electrolyte. This requires a relatively large mutual distance. When performing the ion-exchange process with the device used in the method according to the present invention the ohmic resistance should be minimal. This requires a relatively small mutual distance. This is especially relevant for the electrolyte compartment filled with an electrolyte having a low osmotic concentration, for example the river water compartment, which has a relatively low conductivity. These sub-processes are performed alternately in time.

By varying the mutual distance, this mutual distance can be optimised in place over the distance of the electrolyte seen in the flow direction and/or optimised in time depending on the actual sub-process that takes place in the compartment when performing an ion-exchange process.

Depending on the actual ion-exchange process the device used in the method according to the invention is provided with a flow circuit for providing the required electrolyte at the desired electrolyte compartment. This may involve enabling switching the different flows, an electric circuit connecting the first and second electrodes, and/or moving means for moving the electrodes physically to another compartment, and/or other measures that are known to the skilled person when performing an ion-exchange process. The method according to the present invention enables an improved energy generation and/or improves desalination or mineral recovery.

In a preferred embodiment according to the present invention the method comprises adjusting means for adjusting the mutual distance.

By providing adjusting means the mutual distance between adjacent membranes can be selectively adjusted depending on the sub-process that takes place in the electrolyte compartment and/or the sub-process that is most relevant at a specific part of the electrolyte chamber, for example the entry or exit side.

In one of the presently preferred embodiments the adjusting means comprise a pump for providing a pressure difference in two adjacent compartments such that the mutual distance between the membranes adjust.

This embodiment makes use of the flexibility of the membranes that enable larger or smaller sized compartments depending on the pressure difference across each membrane. This requires that the compartments are not fully filled with a spacer or profile in case of profiled membranes. This can be achieved by providing the spacer or profile locally thinner than the sealing gaskets.

By manipulating the pressure in the electrolyte compartment the mutual distance between adjacent membranes can be adjusted in time. As mentioned earlier, this is especially relevant for the electrolyte compartment with low osmotic concentration, such as the river water compartment. By changing the pressure in this compartment the mutual distance between the membranes defining this electrolyte compartment can be adjusted. In a relatively thick compartment with a relatively large mutual distance the electrolyte experiences little hydraulic friction and, therefore, a low pressure difference between the inflow and outflow such that the fluid in this compartment can be refreshed quickly. Optionally, in the other compartment it is possible to even stop the flow during this period completely, thereby saving pumping power and/or providing this compartment with a relatively thick spacer such that the required pump energy for this compartment remains relatively low. By minimising the pressure in the river water compartment, or electrolyte with low osmotic concentration compartment, and/or increasing pressure in the other compartment the distance between the membranes is decreased for this compartment with low conductivity. This decreases the electrical resistance of the operation and the device. In fact, the power density that can be obtained by decreasing the mutual distance is significantly increased in case of RED or CDP. In case of ED and EDI lower energy consumption is achieved. It will be understood that a high power density remains as long as the concentration difference across the membranes remains high. Preferably, the river water or other electrolyte with low osmotic concentration is not refreshed or only to a limited extent, to save energy for example. As a result the concentration difference across the membrane decreases such that after a certain time period refreshing the electrolyte becomes necessary.

A further effect of varying the mutual distance and time is the reduction of (colloidal) fouling of the device used in the method, especially the reverse electro dialysis device for generating energy. This is achieved as fluctuating flow rates effectively remove fouling. In addition, moving the membranes will break up the diffusive boundary layer and creates some convection in the diffusion boundary layer such that the non-ohmic resistance, also referred to as concentration polarisation, is lower in such system.

In another presently preferred embodiment the adjusting means comprise extension means for extending the compartment.

By providing extension means the electrode compartments may move towards each other or move away from each other depending on the desired mutual distance between adjacent membranes. In this situation, the mutual distance between adjacent membranes is increased or decreased by changing the overall dimensions of the stack of membranes that is provided in the device used in the method according to the present invention. Alternatively or in combination therewith, the configuration of entry and exit sides of the compartments is changed by providing adjacent membranes at a varying angle relative to each other as seen in the flow direction of the fluid.

The extension means achieves similar effects as described above for the provision of a pressure difference. Optionally, both measures of providing a pressure difference and providing extension means are applied together to strengthen the effects of varying the mutual distance between adjacent membranes.

In a presently preferred embodiment, when using the pressure difference to adjust the mutual distance between adjacent membranes, when the mutual distance of the electrolyte compartment with low osmotic concentration is increased, the thickness of the adjacent electrolyte compartment with an electrolyte solution with high osmotic concentration is decreased. In this case the overall dimensions of the stack of membranes remain essentially constant. In other words, the increase of the mutual distance between the membranes in one compartment is achieved by reducing the mutual distance in the adjacent compartment. Alternatively, the mutual distance of one of the compartments, especially the compartment that is filled with an electrolyte solution having a high osmotic concentration, is kept constant, while the mutual distance in the other compartment, preferably having an electrolyte with a relatively low osmotic concentration, varies in time. Therefore, in this embodiment the dimensions of the stack of membranes that is used in the device used in the method according to the present invention vary in time.

The method according to the invention enables performing an ion-exchange process, including generating electricity by performing a reverse electro dialysis process or CDP, desalination or mineral recovery by performing an electro dialysis process or de-ionisation process. In a presently preferred embodiment the mutual distance is varied in a time interval between 0 and 1000 seconds (therefore excluding 0), preferably in a time interval between 0 and 100 seconds, more preferably in a time interval of 0.1-10 seconds, and most preferably about every second. Experimental results show the effect of varying the mutual distance and the effect of the time interval thereon.

The invention further relates to a device for performing an ion-exchange process, more particularly the method described earlier, the device comprising:

a first electrode;

a second electrode separated from the first electrode; and

a number of electrolyte compartments provided between the first and the second electrodes, wherein the electrolyte compartments are formed by a number of alternately provided cation exchange membranes and anions exchange membranes that are provided at a mutual distance, wherein the mutual distance is variable. The device according to the invention provides the same or similar effects and advantages as mentioned in relation to the method. For example, in one or more embodiments of the device according to the invention the change of the mutual distance can be performed over the length of the electrolyte compartment in the flow direction, providing adjusting means for adjusting the mutual distance, providing a pump to achieve a pressure difference in two adjacent compartments and/or providing extension means to adjust the mutual distance by extending the compartments.

In a further alternative preferred embodiment according to the present invention at least some of the edges of adjacent membranes are connected. Preferably, these connections are achieved by providing the membranes by folding two strips of membrane material. This folding can be performed resulting in a twisted paper chain type configuration (in Dutch: "muizentrap"). Preferably, the device comprises extension and rotation means for changing the mutual distance and/or angle between two adjacent membranes of this twisted paper type configuration. One of the advantages of this combined extension/translation and rotation movement is that all the membranes in this configuration are moved and will not stick to each other. This guarantees an optimal operation of the stack of membranes. In this embodiment, preferably the outer edges of two adjacent membranes are connected together, for example by gluing or melting. By having the remaining surface of these adjacent membranes flexible, the space between the adjacent membranes can be filled with more or less fluid depending on the sub-process that takes place in the device according to the present invention.

In a further alternative embodiment according to the present invention the mutual distance between adjacent membranes changes along the length of the electrolyte chamber. By changing the mutual distance between adjacent membranes along the length of the electrolyte chamber the actual mutual distance can be varied in place over the electrolyte compartment. Preferably, the mutual distance is relatively small near the inflow of fluid with low osmotic concentration thereby reducing the electric resistance at this part of the electrolyte compartment. As the electrolyte moves further in the electrolyte chamber this electric resistance slightly decreases such that the mutual distance can be increased. This can be achieved using a spacer and/or gasket which gradually change in thickness. Alternatively, or in addition thereto, profiled membranes can be used and equipped with profiles that show a gradual increase in depth/thickness of the flow channels of the profiled membranes. Preferably, the profiled membranes are provided with channels having a varying depth of the channels along the flow direction of the electrolyte chamber.

In another embodiment, or in addition to a previous embodiment, two adjacent membranes are put at an angle to achieve a changing mutual distance along the length of the electrolyte compartment. This provides the effect described earlier for the varying mutual distance along the length of the electrolyte compartment.

In a further preferred embodiment according to the present invention, the device further comprises a pressure exchanger.

To improve the energy efficiency a pressure exchanger can be provided thereby reducing pump losses. The exchange of the pressure of the out flowing fluid is used to increase the pressure of the in flowing fluid. This further improves the overall efficiency of the device according to the present invention.

Further advantages, features and details of the invention are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

- figure 1 shows a schematic overview of a device according to the invention for

performing a reverse electro dialysis process;

- figure 2 A-D shows a first embodiment of the device according to the invention when varying mutual membrane distance under the influence of pressure differences;

- figure 3 A-B shows the device of figure 2 with additional pressure exchangers;

- figure 4 shows a second embodiment of the device with inclined membranes;

- figure 5 shows a third embodiment of the device according to the present invention with profiled membranes having varying channel depths;

- figure 6 A-D shows a fourth embodiment of the device according to the present

invention with movable electrode compartments;

- figure 7 shows a fifth embodiment of the device according to the present invention with folded membranes that is especially relevant for CDP; and - figure 8 shows experimental results with the first embodiment of the device according to the present invention.

Figure 1 schematically shows an overview of a reversed electrodialysis process. As may be seen in figure 1, a number of cation change membranes (c) is placed between the anode (1) and the cathode (2). Between the anion exchange membranes (a) and the cation exchange membranes (c) electrolyte compartments are formed, wherein alternatingly seawater (s), with relatively high osmotic concentration, and river water (r), with low osmotic concentration, flows. Due to the concentration differences of ions in the sea water (s) and river water (r), the ions in the sea water (s) will be subjected to a driving force to move to the river water (r), to level the concentrations. For simplicity, in figure 1 only sodium and chlorine ions are presented as positive and negative ions.

As the anion exchange membranes (a) only allows anions to pass and the cation exchange membranes (c) only allow cations to pass, transport of anions and cations will proceed in opposite directions, The anions (CI ) will move in the direction of the anode (1), and the cations (Na ⁺ ) will move in the direction of the cathode (2). In order to maintain electric neutrality in the

compartments where the anode (1) is placed, an oxidation reaction takes place, and in the compartment where the cathode (2) is placed, a reduction reaction takes place. Hereby a flow of electrons is generated in the electric circuit (3) wherein the anode (1) and the cathode (2) are connected. In this electric circuit (3) electric work is performed by an electric apparatus (4), here symbolically presented by means of a bulb.

In figure 1 in dashes a dialytic cell, formed from a membrane couple of an anion exchange membrane (a) and a cation exchange membrane (c) and a mass of a solution having a high electrolyte concentration and a solution having a low electrolyte concentration (r). The number (N) of dialytic cells (here N=l) may be increased to increase the potential difference between the anode and the cathode.

Device 6 (figure 2A-2D) comprises a stack 8 of membranes. In the illustrated embodiment sea water 10, or another flow with relatively high osmotic concentration, is provided at inlet 12 of stack 8, and river water 14, or another flow with relatively low osmotic concentration, is provided at inlet 16 of stack 8. Outlet 18 (figure 2 A) is provided with valve 20, while outlet 22 is provided with switch 24 switching between a direct exit pipe and an exit pipe that is provided with valve 26.

Stack 8 is provided with anion exchange membranes 28 and cation exchange membranes 30 that are placed alternately. Between alternating membranes 28, 30 a sea water compartment 32 or river water compartment 34 is formed. In the illustrated embodiment membranes 28, 30 are of a flexible material such that when providing a surplus of pressure in sea water compartment 32 mutual distance dj of the adjacent membranes of this compartment increases, and mutual distance d ₂ between adjacent membranes of river water compartment 34 decreases. This pressure difference can be achieved by the use of at least one of the pumps 36, 38 provided for device 6.

In the illustrated embodiment the flow direction 40 of the electrolyte in the electrolyte chamber 32 with high osmotic concentration is parallel to the flow direction 42 of the fluid in the electrolyte chamber 34 with relatively low osmotic concentration. Alternatively, flow directions 44, 42 are in opposite directions, or provided at an angle, for example substantially perpendicular to each other.

In case there is a more or less constant pressure in the compartments 32 filled with seawater, for example by the use of pressure valve 20 after the outflow 8 of seawater, the pressure in the compartments filled with river water 34 is periodically higher and lower than the pressure in the seawater, regulated for example by the use of pressure valve 26 and a 3-way valve 24.

When the pressure in the river water compartments 34 is higher than that in the seawater compartments 32 the river water compartments 34 will be thick, or in other words, the mutual distance is increased (fig. 2 B). Due to the relatively thick river water compartments 34, the river water experiences little hydraulic friction (i.e. a low pressure difference between the inflow and outflow of river water) and the water in the river water compartments 34 can be refreshed quickly. In the seawater compartment 32, the flow can be stopped during this period (saving pumping power) or, alternatively, the entire seawater compartment may be equipped with a thicker spacer, such that the pump energy is always relatively low.

When the pressure in the river water compartments is lower than that in the seawater compartments the river water compartments are compressed due to the higher pressure in the seawater (figure 2 D). This can be achieved using 3-way valve 24 to guide river water from outlet 22 towards the exit (figure 2 C). As a result, the distance d ₂ between the membranes is smaller in the (weakly conductive) river water compartments 34, so that the electrical resistance of the cell as a whole decreases. As a result, at this stage, the power density that can be obtained from the RED cell is higher than when the river water compartments 34 would not be compressed. The period with a high power exists as long as the concentration difference across the membranes 28,30 remains high. When the pressure in the river water compartment is lower than that of the seawater compartment, the flow of river water can ultimately be stopped (fig. 2 C). Because the river water 14 is not or limited refreshed in this period, the concentration difference across the membrane decreases. Therefore, after a certain period of time, the pressure on the river water compartment needs to be increased so that the river water compartments can be refreshed easily with minimal hydraulic resistance, due to the large mutual membrane distance in this period. Varying the compartment thicknesses dl, d2 ensures that low pump energy is required and a high power is generated in case of RED, or minimal power is consumed in case of ED or EDI. Optionally, system 44 (figure 3 A) comprises pressure exchangers 46, 48 that can be used in addition to or instead of a pressure valve, thus allowing the pump losses (for producing a static high pressure) to reduce. At pressure exchanger 46, 48 the pressure of the out flowing water is used to increase the pressure of the in flowing water. This reduces hydraulic resistance in the system, because the static pressure controlled by the pressure exchangers is used to pressurize the inflow of feed water. For increased energy efficiency, the static pressure on the seawater compartments 32 can be reduced during the phase when there is pressure put on the river water compartments 34. This saves pump energy for the flow of seawater during this phase. Alternatively, the seawater flow can be stopped completely when the river water compartment is expanded and the seawater compartment is relatively thin (see system 50 in figure 3B).

System 52 provides an alternative embodiment (figure 4) wherein membranes 28, 30 are put at an angle a to each other. In the illustrated embodiment membranes 28, 30 are provided at an angle a relative to the average flow directions 40, 42. In the illustrated embodiment membranes 28, 30 have a relatively small distance d ₂ ' at the inlet 16, while having a relatively large mutual distance d ₂ " at exit 22. The distances in the sea water compartment 40 having mutual distance dj varies in the opposite direction such that along the length L of chambers 32, 34 dj + d ₂ is constant. It is noted that in the illustrated embodiment a linear relation is used between the distance di, d ₂ and the position along length L of electrolyte chambers 32, 34. It will be understood that other relations are also possible, for example parabolic and exponential.

In another embodiment 54 (figure 5) profiled membranes 56, 58 are alternately provided.

Profiled membranes 56, 58 comprise channels 60 that are separated with walls 62. In the illustrated embodiment channels 60 have a changing depth with the bottom of channels 60 being provided at an angle β with average flow direction 40, 42.

Both systems 52, 54 have a large distance d ₂ ' ' at the exit of the river water and a small distance d ₂ ' at the entrance of the river water. As the river water continues its way along length L of membranes 28, 30, 56, 58 ions are transported from the sea water to the river water such that the river water becomes more conductive along the length L and, therefore, distance d ₂ can be increased along length L in the flow direction. In both cases the configuration can be linear, exponential, parabolic etc. Also it will be understood that the sea water compartments 32 can be provided with, on average, a larger thickness or width thereby reducing the required pumping power for the fluid. Also, other types of fluid 10, 14 can be switched periodically and/or flow directions can be reversed.

In a fourth embodiment device 61 (figures 6 A-D, with figures 6 C-D cross-sections seen from below in device 61 as illustrated in figure 6 A) comprises a stack 62 with anion exchanging membranes 66 and cation exchanging membranes 68 that are placed alternately. Sealings 70 are provided to prevent leakage. In device 61 the other ends of adjacent membranes 66, 68 are connected by connection or sealing 72. At the outer ends of stack 62 first electrodes 74 and second electrodes 76 are provided with electrode rinse solution 78. At inlet 80 sea water flows into channel or manifold 81 and towards outlet 82. The sea water flows into electrolyte chamber 84. The other electrolyte chambers 86 are provided with river water from inlet 88 that is distributed through channel or manifold 90 and exits stack 62 at outlet 92. Guide 64 enables movement of electrode 76 in direction A and electrode 74 in direction B or in the opposite direction (figure 6B). In the illustrated embodiment membranes 66, 68 are provided with a circular surface. When membranes 66, 68 are closer together a large power density can be achieved as the electrical resistance is minimised in case of RED or CDP. In case of ED and EDI a lower energy consumption is achieved. When refreshing the fluid the distance between electrodes 74, 76 is increased thereby reducing the hydraulic friction.

Connection 72 is achieved by connecting anion exchange membrane 66 and cation exchange membrane 68 at the edges thereof by locally melting the membranes, pushing them locally together and cooling the membranes. Alternatively, the edges can be glued together.

Movement of electrodes 74, 76 in directions A, B can be performed using a motor and/or by using changes in the hydraulic pressure provided in the system.

A fifth embodiment 94 (figures 7A-C) comprises a housing or cylinder 96 with a current collector and capacitor 98. In the illustrated embodiment between the two current collectors and capacitors 98 a folded strip configuration (in Dutch: "muizentrap") 100 is provided. In the illustrated embodiment configuration 100 is provided from two strips 102 whereon are alternately provided anion exchange membrane material 104 and cation exchange membrane material 106 on the surface of strip 102 thereof.

When configuration 100 is stretched or extended with translation C and translation D a high power density can be obtained. In this case only one feed liquid or electrolyte type is required at the same time due to the capacitive material that is capable of storing ions. The electrolyte type is frequently switched in time such that ions move from and to the capacitive material and generate a potential difference over each ion selective membrane. Some of the potential differences can be used at the electrodes at the outer ends of configuration 100. Due to configuration 100 the membranes 104, 106, with in between supercapacitor 108 are moved equally for all compartments such that the distance between membrane 104 of one strip 102 and membrane 106 of another strip is equal for each repeating unit. This prevents the use of spacers or profiled membranes to prevent sticking together the membranes. Element 110 on strip 102 comprises a AEM 104 on a first side, a CEM 106 on the other side, and capacitor 108 between the AEM 104 and CEM 106.

Although the aforementioned devices are shown for reverse electrodialysis (RED), the same or similar embodiments can be applied to desalination in electrodialysis (ED), for example. In electrodialysis, a power supply is used to transport ions from one stream to the other stream, in order to obtain a desalinated stream. When the device would have a power supply, this system could function as a desalination device.

The required electrical power in ED depends on the ohmic resistance of the membranes and feed water, plus the Donnan potential (including concentration polarization) over each membrane. When the system of this invention is used, with variable mutual distance, instead of constant mutual membrane distance as in traditional ED systems, the ohmic resistance of the setup is low when the diluted compartment is compressed or at positions where the diluted compartment is smallest. This effect can be derived from the low ohmic resistance as proven for the RED setup. On the other hand, the variable mutual membrane distance is accompanied with a variable flow rate in the present invention. In electrodialysis, concentration polarization plays a major role, which would increase the required electrical power.

It will be understood that in relation to the illustrated embodiments other different dimensions and concentrations can be applied. For illustrative purposes some relevant values will be presented. The concentrated salt solution, for example NaCl or KC1 ranges from 0.25 to 6 M of soluble salt. The diluted salt solution preferably ranges from 0.001 to 4 M and is in any case lower than the concentrated salt solution. The thickness of individual compartments preferably ranges from 1 to 1000 μπι. Applied pressures range from 0.01 to 10 bar preferably, and more preferably between 0.1 and 2 bar. The capacitive element has a capacitance of preferably at least 100 F/m ² . The conductivity of the diluted solution is mostly below 10 mS/cm.

Experiment

A reverse electro dialysis stack with 5 cells according to device 6 illustrated in figure 2 is used in an experiment.

Each RED cell was composed of a cation exchange membrane (Fumatech FKS, 30 micrometer thick), an anion exchange membrane (Fumatech FAS, 30 micrometer thick), a seawater compartment and a river water compartment. The seawater compartment included a spacer of 300 micrometer thick and the river water compartment a composite spacer of 300 micrometer thick (near the inflow and outflow of the feed water) and a 100 micrometer thick at the center. Both compartments were sealed using a silicon gasket of 300 micrometer thick. The compartments are illustrated in figure 1. An additional membrane was used to separate the final compartment from the electrode compartment.

A Ti-mesh electrode, coated with Ru/Ir, with a dimension of 10 cm by 10 cm, was used. Hexacyanoferrate (Fe(CN) ₆ ^3~ / Fe(CN) ₆ ^3~ ) was circulated at 100 ml/min along the electrodes to facilitate the (reversible) redox reactions at the electrodes.

A seawater flow of 50 ml/min was applied, which corresponds to a velocity inside the compartments of 0.7 cm s and a residence time of 14 sec. A river water flow rate of 50 ml/min was applied only in combination with a larger pressure in this compartment than in the seawater compartments. During the stage when no pressure was given to the river water outlet, the river water flow was interrupted.

Experiments have been performed for several time intervals between varying mutual distances and several set pressures on the river water and seawater compartments. In all cases, the pressure on the seawater compartment was set constant, while the pressure on the river water compartment was set periodically higher and lower than that in the seawater compartment, as described earlier.

The obtained power was measured using a potentiostat (Ivium technologies) and applying amperometry at a constant voltage which is half of the open circuit voltage. The corresponding current density, which can be variable in time, is measured. The static pressure at the end of the feed water compartments was measured using an analog pressure gauge. The pressure drop over the feed water compartment was measured using a differential pressure meter.

The product of the voltage and current yields the obtained power. When the obtained power is divided by the membrane area, the power density is calculated. When using a variable mutual membrane distance over time, the resistance is variable over time, thus the current and thus the power density is variable in time. The power density of the described system is shown in figure 8, as function of the time, together with the power density when a fixed mutual membrane distance would be used. Such a fixed mutual membrane distance was imposed using the same stack, but then with a constant high or constant low pressure on the river water compartment. Results in figure 8 show the power density as function of time for a system with a variable intermembrane distance that was imposed by varying the pressure at the end of the river water compartment between 0 and 400 mbar at an interval of 10 seconds for both stages. In the illustrated experiments the pressure for the seawater compartment was 150 mbar and for the electrode rinse compartment 300 mbar. In the illustrated experiments the mutual distance for the river water compartment

(thickness) varied between 100 and 300 μπι. Measurements are compared with power density for cases where pressure at the river water outlet was continuous 0 mbar and continuous 400 mbar. In other experiments (not shown) different pressures were applied.

Figure 8 shows that the gross power density for an interval time of 1 second (figure 8, left- hand-side) is even higher than the power density when using a constant mutual membrane distance of 100 μπι (figure 8, upper dotted lines). The present system with variable mutual membrane distance may produce consequently a higher gross power density, due to a reduced non-ohmic resistance caused by the movement of the membranes. At an interval time of 10 seconds (figure 8, middle) or 30 seconds (figure 8, right-hand-side), the system with a variable mutual membrane distance shows two stages, as indicated with numbers 1 and 2 in figure 8 (middle). Stage 1 is measured when the river water compartment is compressed (i.e. the river water flow is stopped and the pressure is released). Stage 2 is measured when the river water compartment is expanded (i.e. when river water is supplied and pressure is high in the river water compartment).

The advantages of the present system are further improved when the power consumed for pumping is taken into account. The obtained gross power density minus the power consumed for pumping yields the net power density. The power consumed for pumping is derived from the product of the measured pressure drop between the inlet and outlet of the feed water and the flow rate. A pump efficiency of 100% is assumed. Table 1 shows that the case with a variable mutual membrane distance at an interval of 1 sec has the highest net power density. The systems with a variable mutual membrane distance at 10 or 30 second intervals have a slightly lower net power density than the conventional system with 100 μπι mutual membrane distance at a flow rate of 50 ml/min. However, at other flow rates, for example 100 ml/min, a variable mutual membrane distance at 10 seconds intervals obtains a higher net power density than a constant mutual membrane distance (see Table 2). Moreover, for lower pump efficiencies, which are the case in practical situations, the systems with variable mutual membrane distance benefit even more compared to the traditional systems with a constant mutual membrane distance.

Table 1: Obtained gross power, pumping power, and net power density, for systems with a constant mutual membrane distance and a variable mutual membrane distance, all at a feed water flow rate of 50 ml/min. A pump efficiency of 100% is assumed.

System (mutual membrane Average gross Average pumping Average net power distance) power density power density density

100 μπι 1.14 W/m ² 0.26 W/m ² 0.88 W/m ²

300 μπι 0.40 W/m ² 0.07 W/m ² 0.32 W/m ²

Variable, 1 sec interval 1.22 W/m ² 0.08 W/m ² 1.14 W/m ²

Variable, 10 sec interval 0.81 W/m ² 0.04 W/m ² 0.76 W/m ²

Variable, 30 sec interval 0.59 W/m ² 0.03 W/m ² 0.56 W/m ²

Table 2: Obtained gross power, pumping power, and net power density, for systems with a constant mutual membrane distance and a variable mutual membrane distance, all at a feed water flow rate of 100 ml/min. A pump efficiency of 100% is assumed.

System (mutual membrane Average gross Average pumping Average net power distance) power density power density density

100 μπι 1.24 W/m ² 0.54 W/m ² 0.71 W/m ²

300 μπι 0.41 W/m ² 0.16 W/m ² 0.25 W/m ²

Variable, 10 sec interval 0.84 W/m ² 0.09 W/m ² 0.75 W/m ²

The results of the test show the effect of varying the mutual distance, more specifically of varying the mutual distance in time, as compared to having a constant mutual distance under the same conditions and using the same materials.

The same trends are observed for other pressures, for example 500 mbar at the seawater compartment combined with 0 - 1500 mbar at the river water compartment, and 1000 mbar at the seawater compartment combined with 1500 mbar at the river water compartment. Additional experiments are done with other, more stiff, membrane types: Ralex CMH and Ralex AMH. These results show the same beneficial effect for a variable mutual membrane distance.

The present invention is by no means limited to the above described and preferred embodiments thereof. The rights sought are defined by the following claims within the scope of which many modifications can be envisaged.