GB2194669A - Improvements in electric batteries - Google Patents

Abstract

A process, apparatus and system for reverse electrodialysis utilizing a supply of salt water and a supply of fresh water by passing the liquids through many cells of a battery comprising the improvements (independently or in any combination), (1) The salt water is fed to a battery through a distributor such that electric current is not free to pass along a salt water conduit between a first group of cells and a second group of cells: (2) One or more batteries operate as a heat exchanger to warm up the salt water and fresh water to a temperature above ambient temperature: (3) Salt water for reverse electrodialysis is pumped through one or more batteries by means of tidal power, and fresh water is pumped through one or more batteries by means of hydrostatic head available from a river. p

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 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 liquid 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. 2909) 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 devides 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 applicable 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 comparment containing either salt water or fresh water; the compartment is bounded on at least one side by semipermeable membrane; the compartment has a duct for supplying the water and duct for withdrawing the water; usually a cell has two membrances 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 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.

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 group being electrically isolated 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 ) wherein one of said first and second sets is supplied with liquid at higher than ambient temperature and warms liquid in the other set by heat exchange within said battery.

Preferably salt water is supplied at ambient temperature and fresh water at above ambient temperature from a waste water source.

Preferably there are a plurality of batteries connected in series for the supply of salt water and fresh water and wherein steam or hot water is injected into at least part of the fresh water connexion between at least one pair of batteries.

According to a third 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.

According to a fourth aspect of the invention there is provided a system for the prodution of electric power by reverse- electrodialysis including plant comprising one or more batteries of cells separated by ion exchange membranes, means for supplying alternate cells with water containing sea water as electrolyte and water containing little or no electrolyte ("fresh water") and suitable electrical connexion means, the plant being installed at or near a coast at a level intermediate between high tide level and low tide level and being supplied with sea water at lower tide periods via a feed reservoir filled during periods of high tide, sea water waste being released to a sink reservoir at a level suitable for drainage to sea at periods of low tide.

Embodiments of apparatus and preferred forms of the methods and systems of the invention are hereafter described with refer ence to the accompanying drawings, in which Figure 1 is a diagrammtic 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-section of the mehcanism of Fig. 3a.

Figure 4 is a circuit diagram illustrating a salt water distribution scheme.

Figure 5 is a diagram showing batteries ar ranged for internal heat exchange, Figure 6 is a diagrammatic plan in side view demonstrating a cycle of operations, Figure 7a is a plan view of a second distributor mechanism.

Figure 7b is a cross-section of the mechanism of Fig. 7a, Figure 8a is a plan view of a third distributor mechanism, and Figure 8b is a side view of the mechanism of Fig. 8a.

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 with 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 many cells 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 fell 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 insulator.

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. 4.

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 later, it is proposed to used 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. 7a and 7b.

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. 7a and 7b) 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. 4.

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, 39, 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. 8a and 8b.

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.

4.

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

polypropylene.

An industrial facility for producing about 500 kW of electricity requires flowrates of salt water and fresh water in the order of 1000 kg per second. A facility of this size has many batteries in parallel and in series.

These flowrates through cells of the batteries produce a fall in hydrostatic pressure, known to engineers as "pressure drop". A means of overcoming the problem of pressure drop is given in one aspect of my invention by using tides, as described later in this specification.

I have found that there is a considerable benefit in operating a battery at a temperature above ambient temperature, for example a battery of five cells was operated in which fresh water was passed into cells 1, 3 and 5, and salt water was passed into cells 2 and 4; the salt water contained 290 grams of sodium chloride per litre. When the liquids were at 10 C and were passed through the battery, the voltage generated was 0.096 V, but when the temperature was raised to 40 C the voltage rose to 0.19 V and the current rose correspondingly.

In another experiment a battery consisted of seven cells and fresh water was passed through cells 2, 4 and 6. Salt water was passed through cells 1, 3, 5 and 7; the concentration of salt (sodium chloride) was 35 grams per litre. The experiment was repeated at a number of different temperatures and the voltage between the electrodes was measured on each occasion. The results are given in the table.

Average temperature of Volt the liquids withdrawn from the Battery, C 18 0.43 21 0.46 30 0.50 35 0.54 44 0.56 48 0.60 These experimental results show that the voltage generated by the battery is increased by a substantial percentage as the temperature is raised above 18 C, and this increase in voltage would be a great benefit to an operator of an industrial facility.

I have considered a facility for producing about 500 kW of electricity (as mentioned above); the power output could be increased by about 200 kW if the liquid were fed into the batteries at 35 C instead of 10 C. However the extra heat required to heat both liquids from 10 C to 35 C (each at 100 kg per second) is about 209000 kW. Hence operating the batteries at an elevated tempera ture has the disadvantage that the heat required is over two orders of magnitude greater than the electric power output.

It is known that coal-fired electric power stations, nuclear power stations, oil refineries and chemical works usually generate as a waste product hot water or low pressure steam, which are almost useless for generating power. These sources of heat are very suitable for heating the aqueous liquids for the batteries in the present invention. However a simple conventional heat exchanger between the waste hot water and salt water to the batteries -would not be very practical or effective for a number of reasons: (1) The heat exchanger would be very large.

(2) A larger heat exchanger would introduce additional pressure drop and therefore more power would be required for pumping the liquids through the heat exchanger.

(3) The waste hot water available from a typical power station or oil refinery is an order of magnitude lower than the flowrates indicated in the present battery system.

The present invention in its second aspect overcomes these problems.

I have found that a battery can be designed to act like a heat exchanger, and therefore the facility can be put together in such a way that the batteries do double duty in producing electric power and in warming up the liquids.

In this aspect at least two batteries are used which are fed by salt water and fresh water and (1) At least one of the batteries operates at a temperature above ambient temperature.

(2) A supply of steam or hot water is injected into some or all of the fresh water streams between two or more batteries.

(3) Heat exchange takes place between fresh water streams and salt water streams in the batteries.

(In a battery is is appropriate to insert spacers between the membranes in order to make the liquids take a tortuous path through the cells; this arrangement adds to the effectiveness of heat transfer. This application of spacers is well known to chemical engineers and needs no further description herein).

An embodiment is described with reference to Fig. 5. The facility has a supply of steam or hot water, 51, a supply of fresh water, 61, and a supply of salt water, 71. There are three batteries (or sets of batteries) 58, 59 and 60. 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 order to distinguish them from the more concentrated salt solutions.

The fresh water streams 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 this invention the batteries (or sets of batteries) 58 and 60, operate over a temperature range, but their average temperature is each above ambient temperature. The temperature of battery (or sets of batteries) 59, is substantially above ambient temperature.

It will be appreciated by those skilled in the arts that this invention is a very economical way of utilizing the waste heat from power stations, oil refineries, chemical works and the like.

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 cells 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 5 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 those 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 expected to be 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.

It will be- appreciated by engineers that power is required to pump salt water and fresh water through the batteries, and Jagur Grodzinski and Kramer have shown that the power for the pumps is an appreciable fraction of the power obtainable from the batteries. Therefore pumping the liquids detracts from the economic usefulness of the batteries.

In the fourth aspect of the invention it is proposed to overcome this problem by using water power from a river supply and tidal power from the sea. In each case this power comes from the available hydrostatic height of water.

Batteries may be installed at a level between high tide level and low tide level at a coastal site; there may be two lagoons (or reservoirs) and means for passing salt water from a relatively high tide level through the batteries to a relatively low tide level, and means for passing fresh water from a relatively high level through the batteries to relatively low tide level.

The system is described with reference to Fig. 6, which is intended to show the cycle of operations as the sea water goes from high tide through to low tide and back to high tide.

There are two lagoons (or reservoirs) 81 and 82, there is a conduit, 91, for filling lagoon, 81, with salt water at high tide, and there is a conduit, 92, for emptying lagoon, 82, at low tide. The group of one or more batteries, 83, is positioned approximately midway between the high tide level and the low tide level.

There is a conduit, 93, supplying salt water from lagoon, 81, to the battery group, and a conduit, 94, taking salt water from the battery group to lagoon, 82. There is a conduit, 95, supplying fresh water to the battery group; typically the supply of fresh water comes from a point in a river upstream of the lagoons.

There is a conduit, 96, discharging fresh water from the battery group to lagoon, 82.

There is a conduit, 97, taking salt water from the sea to the battery group. There are valves and other instruments (not shown) for controlling the flow of liquids; control valves and the like are well understood by engineers and require no further description herein.

The process runs a continual cycle from high tide to low tide and back to high tide again.

(1) At high tide salt water flows from the sea along conduit, 97, to the battery group and thence along conduit, 94, to lagoon, 82, which has been previously emptied at low tide. Also at high tide, lagoon, 81, is filled to the highest possible level with salt water via conduit, 91.

(2) When the tide tide falls to about 70% of high tide, the supply of salt water by conduit, 97; is discontinued and salt water is supplied to the battery group from lagoon, 81, via conduit, 93.

(3) At low tide the supply of salt water continues from lagoon, 81, and effluent from the battery group continues to lagoon, 82, but lagoon, 82, is discharged to as low a level as possible to the sea via conduit, 92.

(4) As the tide rises the flow of salt water continues from lagoon, 81, through the battery group to lagoon, 82.

(5) Finally, when the tide rises to 70% of full tide, the salt water is supplied directly to the battery group via conduit, 97, and lagoon, 81, is refilled with salt water via conduit, 91.

(6) During all the operations (1) to (5) fresh water is supplied to the battery group via conduit, 95, and fresh water is discharged via conduit, 96, to lagoon, 82.

In Fig. 6 for those sections denoted by "mid-tide" and "low tide" some of the conduits are omitted; this omission is partly for simplicity and partly to show that those conduits are not used at those periods of the cycle.

My invention is most suitably applied near a river estuary where there is a plentiful supply of salt water and fresh water; there should desirably be available a supply of hot water or steam. My invention is particulary applicable in Great Britain because many estuaries in Great Britain are the sites for power stations or chemical works; examples are Teeside, Severnside and the Mersey estuary.

My invention produces electricity as direct current. This is particularly useful at Teeside and Mersey, because the chemical works at these sites use D.C. for electro-chemical processes.

The batteries in my invention produce some hydrogen at the positive electrode. This hydrogen can be collected as a by-product.

Claims (17)

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 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 plurality 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 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") wherein one of said first and second sets is supplied with liquid at higher than ambient temperature and warms liquid in the other set by heat exchange with a said battery.

9. A method according to claim 8 in which salt is supplied at ambient temperature and fresh water at above ambient temperature from a waste water source.

10. A method according to claim 8 or claim 9 in which there are a plurality of batteries connected in series for the supply of salt water and fresh water and wherein steam or hot water is injected into at least part of the fresh water connexion between at least one pair of batteries.

11. A method of reverse electrodialysis utilizing one or more batteries of cells separated by ion exchange membranes, in which a first set of alternative 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") 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.

12. A method according to claim 11 including the steps of any one of claims 8 to 10.

13. A method according to any of claims 8 to 12 carried out utilizing apparatus according to any one of claims 1 to 7.

14. A system for the production of electric power by reverse electrodialysis including plant comprising one or more batteries of cells separated by ion exchange membranes, means for supplying alternate cells with water containing sea water as electrolyte and water containing little or no electrolyte ("fresh water'') and suitable electrical connexion means.

the plant being installed at or near a coast at a level intermediate between high tide level and low tide level and being supplied with sea water at lower tide periods via a feed reservoir filled during periods of high tide, sea water waste being released to a sink reservoir at a level suitable for drainage to sea at periods of low tide.

15. A system according to claim 14 wherein the plant includes apparatus according to any of claims 1 to 7.

16. A system according to claim 14 or claim 15 arranged for operation by a method according to any of claims 8 to 12.

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