GB2197116A - Reverse electrodialysis - Google Patents

Abstract

Apparatus for reverse electrodialysis including one or more batteries of cells separated by ion exchange membranes, means for supplying alternate cells with water containing electrolyte ("salt water") and water containing little or no electrolyte ("fresh water") and suitable electrical connexion means, wherein means are provided for electrically isolating a part of the supply means which serves at least a first salt water cell or group of such cells from a part of the supply means which serves at least a second salt water cell or group of such cells to prevent or reduce loss of electric power through the supply means. Figs. 3a and 3b show a distributor in which salt water is supplied to central conduit 32, and via conduit 37, to one of conduits 33. The disc 34 rotates so that each conduit 33 receives a supply of salt water in turn, but there is never any electrical contact between any of the conduits 33. Fig. 6 shows a way in which the distributor can be applied to a battery. Alternative distributors are shown in Figs. 4a/4b and 5a/5b respectively. <IMAGE>

Description

SPECIFICATION Improvements in electric batteries The present invention relates particularly but not exclusively to a device for obtaining electrical energy from a supply of salt water and a supply of water which contains little or no salt.

It is known that "Free Energy" of mixing is available from a system in which there is a supply of relatively concentrated salt solution and a supply of liqui#d containing no salt or very little salt. Such systems have been described by R. E. Pattle (Brit. Pat. 731729, and in Nature, Vol. 174, p. 660, 1954, and in Chemical Process Engineering, Vol. 36, p.

351, 1955), by J. N. Weinstein and F. B.

Leitz (in 1976, Science, Vol. 191, p. 557), by A. T. Emren and S. B. Bergstrom (in 1977, Proceedings of the International Conf. on Alternative Energy Sources, Florida, p. 2809) and by J. Jagur-Grodzinski and R. Kramer (in 1986, Industrial and Engineering Chemistry, Process Design Development, Vol. 25, p.

443). None of these studies has led to the manufacture of a practical facility on an industrial scale. All of the devices described in the above papers have practical problems. These problems will be referred to again later in this specification. The invention to be described in this specification overcomes these problems.

Certain of the improvements to be described are also appliable to the desalination of salt water by electrodialysis.

Definitions and Explanations.

The term, "Salt Water", in this specification means an aqueous solution of any electrolyte: typically the solution is sea water, which contains about 32 kg of salts per cubic metre.

In this specification "Fresh Water" refers to water with very little dissolved salts compared with sea water, e.g. fresh water contains less than 3 kg of salts per cubic metre.

A "Cell" is a compartment containing either salt water or freshwater; the compartment is bounded on at least one side by a semipermeable membrane; the compartment has a duct for supplying the water and a duct for withdrawing the water; usually a cell has two membranes on opposite walls, one is an anion exchange membrane and the other is a cation exchange membrane.

A "Battery" is a device with a number of cells in series; typically the cells are assembled such that the membranes are alternately anion and cation exchange membranes, and the cells contain alternately salt water and fresh water; at the two ends of the series of cells there are placed electrodes which are in contact with the water in the respective end cell.

A battery as described above can be utilized to produce electric power; this process is termed "Reverse Electrodialysis".

PTFE is an abbreviation for poly-tetra-fluoroethylene, which is a solid plastic and which is an electrical insulator and which has the property of a low coefficient of friction; in the present specification "PTFE" is meant to include any plastic which has properties similar to poly-tetra-fluoro-ethylene.

According to one aspect of the invention there is provided an apparatus for reverse electrodialysis including one or more batteries of cells separated by ion exchange membranes, means for supplying alternate cells with water containing electrolyte ("salt water") and water containing little or no electrolyte ("fresh water") and suitable electrical connexion means, wherein means are provided for electrically insolating a part of the supply -means which serves at least a first salt water cell or group of such cells from a part of the supply means which serves at least a second salt water cell or group of such cells to prevent or reduce loss of electric power through said supply means.

Preferably there are included a plurality of salt water supply parts each serving a group of e.g. 10 to 100 (preferably 20 or less) salt water cells each groupbeing electrically iso.lated from the other groups.

Preferably said isolating means includes a mechanism for diverting salt water from a supply sequentially to at least two supply parts.

Preferably each said supply part includes a header of sufficient capacity to maintain a constant flow to a said group of cells.

One form of mechanism may comprise a rotary valve having a plurality of ports communicating successively and without overlap with a common transfer duct.

Another form of mechanism may comprise a plurality of juxtaposed headers and a transfer member which shifts the supply of salt water from one header to another.

Large scale apparatus may include different types of mechanism.

According to a second aspect the invention provides a method of reverse electrodialysis utilizing one or more batteries of cells separated by ion exchange membranes, in which a first set of alternate cells is supplied with water containing electrolyte ("salt water") and a second set of alternate cells is supplied with water containing little or no electrolyte ("fresh water") and including the step of electrically isolating a part of the supply means which serves at least a first salt water cell or group of such cells from a part of the supply means which serves at least a second salt water cell or group of such cells to prevent or reduce loss of electric power through said supply means.

This specification may be considered in conjunction with my two co-pending patent applications.

Embodiments of apparatus and preferred forms of the methods and systems of the invention are hereafter described with reference to the accompanying drawings, in which Figure 1 is a diagrammatic cross-section of a battery arranged for reverse electrodialysis, Figure 2 is a plan view of the battery of Fig.

1, Figure 3a is a plan view of a first distributor mechanism, Figure 3b is a cross-sction of the mechanism of Fig. 3a, Figure 4a is a plan view of a second distributor mechanism, Figure 4b is a cross-section of the mechanism of Fig. 4a, Figure 5a is a plan view of a third distributor mechanism, Figure 5b is a side view of the mechanism of Fig. 5a, Figure 6 is a circuit diagram illustrating a salt water distribution scheme, and Figure 7 is a diagram showing batteries arranged for internal heat exchange (which relates to one of my co-pending patent applica tions).

An experimental battery is described with reference to Figs. 1 and 2.

A multiplicity of rubber sheets, 11, each 8mm thick, have a rectangular hole cut into them. Between each of the rubber sheets there are placed alternately anion exchange membranes, 14, and cation exchange membranes, 15. The membranes are placed across the rectangular holes (mentioned above) such that each rectangular space forms a cell; alternate cells are allocated for salt water (101 to 191) and fresh water (200 to 291). (In the subsequent description, reference to cell 101 implies all the cells in the "one hundred" series, and reference to cell 201 implies all the cells in the "two hundred" series).

The whole group of cells is bounded by two wooden blocks, 16 and 17, and the whole assembly is held firmly together by bolts, 18, and nuts, 19. Electrodes, 20 and 21, are located next to the wooden blocks in the end cells.

In the top of cell, 101, there is located a tube, 22, for the supply of salt water, and there is a tube, 23, for the withdrawal of salt water. In the top of cell, 201, there is a tube, 24, for the supply of fresh water, and the tube, 25, for the withdrawal of fresh water. It should be noted in Fig. 2 that the supply tubes, 22 and 24, are on opposite positions of the top face of the battery. The significance of this is that the salt water and fresh water pass through their respective cells counter-currently.

The tubes, 22, 23, 24, and 25, have an internal diameter of about 2 mm.

In Figs. 1 and 2 the negative electrode is item 20, and the positive electrode is 21, but if the salt water and fresh water are interchanged, then electrode 20 becomes positive and electrode 21 becomes negative.

Emren and Bergstrom did a theoretical study of a system of batteries for the production of electricity from sea water and fresh water.

Each battery was designed to produce 20 kW. However, their design would not have worked efficiently in practice because they had no electrical insulation between the salt water supplies to the multiplicity of cells. Since salt solutions are conductors of electricity there would be a loss of voltage across each of their batteries. They had a common feed pipe for salt water to a multiplicity of cells. The same inefficient feature is shown in the diagram in the paper by Weinstein and Leitz, and in the diagram in the British Patent specification, 731729, by Pattle. Pattle states that the fraction of Free Energy converted to electrical energy in his apparatus was low; in one case it was 0.56%.

Using a battery like that shown in Figs. 1 and 2, fresh water was supplied to the cells in the "one hundred" series, and sea water to the cells in the "two hundred" series. The sea water supply to each cell was kept separate from every other cell, (i.e. the salt water supply). The average voltage across one pair of cells was in the order of 0.05 Volt. (A "pair of cells" means a salt water cell and a fresh water cell next to each other). When the system was changed so that there was a common salt water supply to two cells which were one space apart (e.g. cells 200 and 201 in Fig. 1) the loss in voltage was not measurable.

In another experiment, and battery of manycells gave a voltage of 1.95, but when there was caused to be a common salt water supply to the two end cells (i.e. cells which were not one space apart, but many spaces apart) the voltage feell to 1.50. This result indicates that electrical current had passed along the salt water connexion and useful power was lost.

In an industrial application, a typical battery would have at least 200 cells in series in order to generate a useful voltage of 5 Volts or more; in this case if there were a common salt water supply to (say) cells 101 and 191 in Fig. 1, an electrical current would pass along the salt solution and useful power would be lost.

In the first aspect of the invention there is provided a means for electrical insulation of a first cell or a first group of cells from a second group of cells such that electrical current is not free to pass along a salt water conduit between the first cell or group of cells and the second group of cells.

This may be achieved, for example, by a mechanism which provides sequentially salt water to a first conduit leading to a first group of cells, and which at the same time withholds salt water from a second conduit leading to a second group of cells, whereafter the said mechanism sequentially provides salt water to the second conduit while withholding salt water from the first conduit.

An example of such a mechanism is a distributor which comprises a stationary member and a rotating member, the stationary member at its centre is connected to a conduit for the supply of salt water, and at a radius from the said centre there are connected a number of pipes to the said stationary member, and the rotating member makes a leak-tight contact with the stationary member, and the rotating member includes a conduit which allows a flow of salt water from the said central connexion to one of the pipes at the said radius; the stationary member and the rotating member are made of PTFE or similar electrical insuiator.

A distributor in this form is described with reference to Figs. 3a and 3b. A thick circular disc, 31, has fixed into it a central conduit, 32, and the number of conduits, 33, (eight of them in the Figure) are fixed on a constant radius from the central conduit. A second disc, 34, is placed against the first disc, 31, and the two discs have mating surfaces. The disc, 34, has the means for rotation (not shown), such means could be a belt drive or a cog-wheel attached to an electric motor. The disc, 34, has a central port, 35, and a port, 36, at a distance from the first port equal to the constant radius stated above. Ports 35 and 36 are connected by a conduit, 37.

The distributor operates with a supply of salt water to the central conduit, 32; the salt water passes through the conduit, 37, to one of the conduits, 33. The disc, 34, rotates such that each conduit, 33, receives a supply of salt water in turn, but there is never any electrical contact between any of the conduits, 33. There are means provided such as bearings (not shown) for holding the two discs together such that leaks from one conduit to another are negligible. PTFE is a very suitable material of construction for the distributor because PTFE is an electrical insulator, and it has a low coefficient of friction.

One way in which the distributor can be applied to a battery is demonstrated in Fig. 6.

Salt water is supplied along conduit, 32, to a distributor, 73, and thence there are eight conduits, 331 to 338. The battery, 58, has (in this example) 160 cells receiving salt water; these cells are numbered in sequence 101 to 260 such that conduit 331 feeds cells 101 to 120, (in parallel), conduit, 332, feeds cells, 121 to 140, and so on (333 feeds 141 to 160; 334 feeds 161 to 180; 335 feeds 181 to 200; 336 feeds 201 to 220; 337 feeds 221 to 240; 338 feeds 241 to 260). Each of the conduits, 331 to 338, has attached to it a head tank, 41, (only one shown). The purpose of the head tank is to maintain a continuous flow of liquid at approximately steady pressure through the cells, and to damp out surges in pressure of the salt water introduced by the distributor to the battery.For example, when the distributor switches to conduit, 331, there is a high flow generated in that conduit but part of the salt water is diverted into the head tank, 41. Then when the distributor is delivering salt water to conduits, 332 to 338, salt water flows out of the head tank into the cells, 101 to 120.There are also head tanks (not shown) on conduits, 332 to 338. As will be described in one of my co-pending patent applications, it is proposed to use tidal power to pump the salt water into the cells, and therefore the head of water will not be more than 5 metres; therefore the head tanks do not need to be more than 5 metres high.

Another form of distributor mechanism is described with reference to Figs. 4a and 4b.

Vessel, 42, is divided into a number of vertical sectors like prisms by means of vertical partitions, 43. In the present example there are eight such prismatic sectors. Each of the sectors has at the bottom a withdrawal conduit, 331 to 338. In the centre of the vessel there is a fixed vertical feed conduit, 32, and at the top of this feed conduit there is a moveable rotating arm, 37.

The present example of a distributor operates by having salt water forced up the feed conduit and out through the rotor arm, 37, into one of the sectors. The rotor arm has a lateral curve (as shown in Figs. 4a and 4b) such that the flow of salt water causes the rotor to rotate continually. In this way the salt water is distributed to each of the sectors in turn. The salt water then flows out through conduits, 331 to 338, to separate groups of cells in a battery. The vessel, 42, is erected at such a height that it operates like a multiple head tank to the battery system as shown in Fig. 6.

The vessel and its partitions are made of a material which is an electrical insulator, e.g.

polypropylene. In this way there is no electrical contact between the salt water supplies in conduits, 331 to 338, even although salt water is an electrical conductor.

The rotor, 37, includes an internal sleeve, 38, which provides the means for locating the rotor in the fixed conduit, and the rotor has a flange, 339, which rests on a flange, 40, at the top of the fixed conduit. A disc, 43, holds the rotor loosely in position. A small leakage of salt water past these flanges does not detract from the effectiveness of the distributor.

The rotor and the flange, 40, could suitably be made of PTFE or nylon such that the rotor moves with minimum friction.

In a modification of the above mechanism the salt water may be allowed to pass downwardly into the rotor arm 37.

Another form of distributor mechanism is described with reference to Figs. 5a and 5b.

A rectangular vessel, 442, is divided into a number of compartments by means of vertical partitions 443 and 444. There are eight such compartments in the present example and they are open at the top. Each of the compartments has at the bottom a withdrawal conduit, 431 to 438.

Above vessel, 442, there is a long trough, 446, which has a longitudinal divider, 447, such that there are effectively two compartments to the trough. The trough can, to a limited degree, pivot about the base, 448, such that the trough can take up position A or position B.

Above the trough there is a feed conduit, 449. The distributor operates by salt water flowing along the feed conduit into one of the trough compartments; when this fills up the weight of the salt water makes the trough pivot over, such that a load of salt water falls from the trough compartment into a row of vessel compartments below. In this orientation the alternative trough compartment is filled with salt water from the feed conduit, until the trough pivots over again and a second row of vessel compartments are supplied with salt water.

The salt water flows out through conduits, 431 to 438, to separate groups of cells in a battery. The vessel, 442, is erected at such a height that it operates like a multiple head tank to the battery system as shown in Fig.

6.

The vessel and its partitions are made of a material which is an electrical insulator, e.g.

polypropylene.

Fig. 7 shows a flowsheet for a number of batteries, 58, 59 and 60, in series. This embodiment is described in detail in one of my co-pending patent applications.The facility has a supply of steam or hot water, 51, a supply of fresh water, 61, and a supply of salt water, 71. Fresh water passes along conduit, 62, and is divided into a number of streams, 63, which enter battery, 60. In battery, 60, the fresh water warms up by means of hot salt water, 78 (to be described). As electric current is generated in battery, 60, so the fresh water acquires some salt content, but for the purposes of this description these streams will be termed "fresh water" in oder to distinguish them from the more concentrated salt solutions.

The fresh water steams leave battery, 60, via conduits, 64, and they are mixed with hot water (or steam) 52, and the resulting hot water streams, 65, enter battery 59, which operates at a temperature (typically 35 C) which is above ambient temperature. The hot fresh water leaves battery, 59, via conduits, 66, and enters battery, 58, where the hot fresh water gives up its heat by warming salt water streams, 74, (to be described). The fresh water is rejected from the facility via conduits, 68, at approximately ambient temperature.

Salt water passes along conduit, 72, and is divided into a number of streams, 74, by means of the distributor, 73, (already described). The multiplicity of salt water streams, 74, are warmed up in battery, 58, as already described. The salt water streams, 76, leaving battery, 58, are (typically) at about 34 C. The salt water streams, 76, enter battery, 59, and leave via streams, 78, which then warm up the fresh water in battery, 60, as already described. In this way the salt water streams, 78, give up all their heat and are rejected from the facility via conduits, 79, at approximately ambient temperature.

In the prior art described by Pattle (Brit. Pat.

731729) and by Jagur-Grodzinski and Kramer, the salt water from the multiplicity of cells is combined into a single exit stream, and the fresh water from the multiplicity of streams is combined into another single exit stream. It should be noted that this "fresh water" contains some salt when it leaves the cells, and therefore this discharged water is also an electrical conductor.

The combining of these conducting streams is another source of lost electric power. In my invention this loss of power is eliminated by keeping the discharged streams separate and maintaining electrical isolation of groups of cells. This feature is exemplified in Fig. 7 which shows the concept that discharged streams are not mixed. The distributor, 73, divides the salt water into a number of streams, 74, which enter the battery, 58; the salt water streams leaving battery, 58, are not mixed but maintain their separate identity as streams 76, and when these streams leave battery, 59, they maintain their separate identity as streams 78, and when these streams leave battery, 60, they maintain their separate identity as streams 79.

In one embodiment of the present invention the streams, 79, are discharged into a lagoon.

Short-circuiting of electric current through the lagoon is negligible if the discharge conduits are not adjacent to each other. Alternatively some form of isolating means can be installed on these discharge conduits, for example the conduits could discharge into separate pans of a water wheel.

In the same way the fresh water streams, 64, leaving battery, 60, maintain their separate identity, and again streams, 66, maintain their separate identity leaving battery, 59, and again streams, 68, maintain their separate identity exit battery 58.

The batteries in my invention produce at the positive electrode some hydrogen which can.

be collected as by-product.

Claims (10)

1. Apparatus for reverse electrodialysis including one or more batteries of cells separated by ion exchange membranes, means for supplying alternate cells with water containing electrolyte ("salt water") and water containing little or no electrolyte ("fresh water") and suitable electrical connexion means, wherein means are provided for electrically isolating a part of the supply means which serves at least a first salt water cell or group of such cells from a part of the supply means which serves at least a second salt water cell or group of such cells to prevent or reduce loss of electric power through said supply means.

2. Apparatus according to claim 1 including a plurality of salt water supply parts each serving a group of 100 or less salt water cells each group being electrically isolated from other groups.

3. Apparatus according to claim 1 including a plurality of salt water supply parts each serving a group of 20 or less salt water cells each group being electrically isolated from other groups.

4. Apparatus according to any of the preceding claims wherein said isolating means includes a mechanism for diverting salt water from a supply sequentially to at least two supply parts.

5. Apparatus according to claim 4 wherein each said supply part includes a header of sufficient capacity to maintain a constant supply to a said group of cells.

6. Apparatus according to Claim 4 or Claim 5 wherein said mechanism comprises a rotary valve having a plurality of ports communicating successively and without overlap with a common transfer duct.

7. Apparatus according to Claim 5 or Claim 6 wherein #said mechanism comprises a piurality of juxtaposed headers and a transfer member which shifts the supply of salt water from one header to another.

8. A method of reverse electrodialysis utilizing one or more batteries of cells separated by ion exchange membranes, in which a first set of alternatecells is supplied with water containing elecrrolyte ("salt water") and a second set of alternate cells is supplied with water containing little or no electrolyte ("fresh water") including means which serves at least a first salt water cell or group of such cells from a part of the supply means which serves at least a second salt water cell or group of such cells to prevent or reduce loss of electric power through said supply means.

9. A system for the production of electricity by reverse electrodialysis substantially as described herein with reference to the accompanying drawings.

10. A method for the production of hydrogen by reverse electrodialysis substantially as described herein with reference to the accompanying drawings.