GB2195818A - Reverse electrodialysis - Google Patents

Abstract

In a method of reverse on electrodialysis utilizing one or more batteries 58, 59, 60 of cells separated by ion exchange membranes (14, 15, Fig. 1) in which a first set (101 to 191) of alternate cells is supplied with water containing electrolyte ("salt water") and a second set (200 to 291) of alternate cells is supplied with water containing little or no electrolyte ("fresh water"), one of the first and second sets is supplied with liquid at higher than ambient temperature and warms liquid in the other set by heat exchange within a battery. The liquid supplied at higher then ambient temperature may be the fresh water supply 64 which is heated by hot water or steam 51 from a waste water source e.g. a power station, oil refinery or chemical works. The invention is best applied near a river estuary where there is a plentiful supply of salt water and fresh water. By operating the battery at higher than ambient temperature the voltage of the batter is increased. <IMAGE>

Description

SPECIFICATION Electric batteries I, Bernard Ramsay Bligh, a British subject, of 4, Saint James's Avenue, Hampton Hill, Middlesex, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- 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 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 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 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 g of salts per cubic metre.

In this specification "Fresh Water" refer 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 fresh water; 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".

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

According to one 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 a 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.

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 3 is a diagram showing batteries arranged for internal heat exchange.

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 the 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.

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.

I 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 experimenta! results show that the voltage generated by the battery is increased by a substantial percentage as the temperature is raised abpove 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; the power output could be increased by about 200 kW if the liquids 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 1000 kg per second) is about 209000 kW. Hence operating the batteries at an elevated temperature 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 it 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. 3. 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, (described in my co-pending patent). 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.

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 particularly 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.

Claims (4)

1. A method for 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 temperatures and warms liquid in the other set by heat exchange within a said battery.

2. A method according to claim 1 in which salt water is suplied at ambient temperature and fresh water is at above ambient temperature from a waste water source.

3. A method according to claim 1 or claim 2 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.

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