WO2014091199A1

WO2014091199A1 - Electricity generation using a reverse electrodialysis cell with means agains fouling

Info

Publication number: WO2014091199A1
Application number: PCT/GB2013/053133
Authority: WO
Inventors: Willem VAN BAAK
Original assignee: Fujifilm Manufacturing Europe Bv; Fujifilm Imaging Colorants Limited
Priority date: 2012-12-12
Filing date: 2013-11-27
Publication date: 2014-06-19
Also published as: GB201222348D0

Abstract

Amethod for generating electricity comprising the steps: (A) feeding a concentrated ionic solution through a first pathway in a reverse electrodialysis unit comprising a membrane stack having electrodes and alternating cation and anion exchange membranes; and (B) feeding a dilute ionic solution through a second pathway in said reverse electrodialysis unit;and (C)regenerating the concentrated and dilute ionic solutions from solution(s) exiting from the reverse electrodialysis unit and recycling said regenerated concentrated and dilute ionic solutions back through said reverse electrodialysis unit; wherein: (i)as the concentrated and dilute ionic solutions pass through their respective pathways, solute migrates from the concentrated ionic solution through the membranes to the dilute ionic solution, thereby generating electricity; (ii)the method comprises the further step (D) of alternating which of the first and second pathways carries the said concentrated and dilute ionic solutions, and/or bringing at least one of said solutions into contact with a biocide and/or irradiating at least one of said solutions with ultraviolet light.

Description

ELECTRICITY GENERATION USING A REVERSE ELECTRODIALYSIS CELL

WITH MEANS AGAIN S FOULING

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for generating electricity using reverse electrodialysis.

Reverse electrodialysis ("RED") is a known technique for generating electricity from the free energy of mixing of two ionic solutions. This technique typically uses a reverse electrodialysis unit comprising a membrane stack having alternating cation and anion exchange membranes, electrodes (typically at each end of the stack), a first pathway through the reverse electrodialysis unit for a concentrated ionic solution, and a second pathway through the reverse electrodialysis unit for a dilute ionic solution. When the concentrated ionic solution is fed through the first pathway, and the dilute ionic solution is fed through the second pathway, solute from the concentrated solution in the first pathway migrates through the membranes to the dilute solution in the second pathway, this being accompanied by the generation of an output electricity across the electrodes at the ends of the stack. The voltage generated by the concentration difference across each pair of membranes is low, but this voltage is multiplied by increasing the number of alternating cation and anion exchange membranes separating the two solutions in the membrane stack. RED is of interest for the production of electricity in an environmentally-friendly way. The ionic solutions exiting RED units are usually sent to waste, e.g. by releasing them into rivers or the sea.

In an age where gas and electricity prices are continually rising, there is a desire to maximise the efficiency of electricity generation. The present invention seeks to provide a method and apparatus for increasing the average electricity output of an RED unit per unit time. In one embodiment, the present invention may also use low grade or waste heat to further improve the power output of an RED unit.

According to a first aspect of the present invention, there is provided a method for generating electricity comprising the steps:

(A) feeding a concentrated ionic solution through a first pathway in a reverse electrodialysis unit comprising a membrane stack having electrodes and alternating cation and anion exchange membranes; and

(B) feeding a dilute ionic solution through a second pathway in said reverse electrodialysis unit; and

(C) regenerating the concentrated and dilute ionic solutions from solution(s) exiting from the reverse electrodialysis unit and recycling said regenerated concentrated and dilute ionic solutions back through said reverse electrodialysis unit;

wherein: (i) solute from the concentrated ionic solution migrates through the membranes to the dilute ionic solution, thereby generating electricity; and

(ii) the method comprises the further step (D) of alternating which of the first and second pathways carries the said concentrated and dilute ionic solutions, and/or bringing at least one of said solutions into contact with a biocide, and/or irradiating at least one of said solutions with ultraviolet light. The present method has a number of advantages over conventional RED systems which send the output of the RED unit to waste. For example, the regeneration and recycling of ionic solutions which occurs in step (C) means that the method can be performed in locations where there are not vast, renewable sources of salty and fresh water close to hand e.g. it is not limited to coastal areas which have ready supplies of sea and fresh/brackish water. In addition, the method obviates the problems of continuously transporting ionic solutions the RED unit and of disposing of spent brine. The present invention may also increase the average electricity output of the RED unit per unit time, for example by increasing the reliability and reducing the maintenance time and costs as a result of feature (ii) above.

The three options referred to in feature (ii) of (a) alternating which of the first and second pathways carries the said concentrated and dilute ionic solutions; (b) bringing at least one of the ionic solutions into contact with a biocide; and (c) irradiating at least one of the ionic solutions with ultraviolet ("UV") light; may each be performed alone or in combination with one or more of the other two options. For example, one may perform option (a) without (b) or (c); (b) without (a) or (c); (c) without (a) or (b); (a) and (b) without (c); (a) and (c) without (b); (b) and (c) without (a); or one may perform all three options of (a), (b) and (c).

By alternating which of the first and second pathways carries the said concentrated and dilute ionic solutions, one may increase the average electricity output of the RED unit per unit time by reducing scale formation, salt build-up and biofilm formation in the RED unit. For example, when the concentrated ionic solution is fed through the first pathway for an extended period of time, some of the solute dissolved therein may come out of solution and begin to restrict the flow of the concentrated ionic solution through the membranes and/or through the pathway. By periodically feeding the dilute ionic solution through the first pathway, one may use the dilute solution to dissolve any solute or scale which has come out of the concentrated solution. Also by periodically feeding the concentrated ionic solution through the second pathway, the concentrated solution may kill solute- sensitive bacteria and reduce biofilm formation (due to its high ionic content) in the second pathway. The reduction in scale formation, salt build-up and biofilm formation increases the overall efficiency of the RED unit and reduces the frequency with which the RED unit must be switched-off and cleaned. One may alternate which of the first and second pathways carries the said concentrated and dilute ionic solutions in a number of ways and this alternation may be repeated as many times or as often as desired. For example, one may periodically redirect the concentrated ionic solution such that it passes through the pathway which was previously occupied by the dilute ionic solution and redirect the dilute ionic solution such that it passes through the pathway which was previously occupied by the concentrated ionic solution. This redirection may be achieved by, for example, using a flow valve which alternates the flow of concentrated and dilute ionic solutions through the first and second pathways.

Preferably the method is performed such that the pathways through which the concentrated ionic solution and the dilute ionic solution pass are repeatedly alternated. The frequency of alternation may be varied between wide limits, depending on, for example, the rate at which the membranes foul. In RED units where salt or scale build-up or fouling occurs slowly, the frequency of alternation will generally be chosen to be less than for RED units where the membranes scale or foul more quickly. Typically the frequency of alternation is, on average, from 6 times per hour to once per month, especially once per hour to once per week. The frequency of alternation may be high e.g. on average once every 10 to 60 minutes, or it may be low, e.g. on average once per week to once per month. Also an intermediate frequency of alternation may be used, e.g. of once per hour to once per week as indicated above. In this way membrane fouling can be reduced.

The average electricity output of RED units per unit time may also be enhanced by bringing one or more of solutions into contact with the biocide and/or by irradiation with UV light, e.g. the concentrated ionic solution, the dilute ionic solution and/or one or both of the solutions exiting the RED unit. Preferably, however, the solution which is brought into contact with the biocide and/or irradiated by UV light is the concentrated ionic solution and/or the dilute ionic solution.

The biocide and/or UV irradiation may kill bacteria and reduce biofilm formation, increasing the efficiency of the RED unit and reducing the frequency with which the RED unit must be switched-off and cleaned.

In typical RED methods which lack the regeneration and recycling step (C), the output of the unit is sent to waste. Therefore the user is unlikely to even contemplate adding a biocide to the ionic solutions because the biocide would also be sent to waste, thereby significantly increasing the cost of performing RED and because of the polluting effect on the environment that would be the consequence of using biocides. In contrast, the present inventors have conceived the idea of using regeneration and recycling step (C) in combination with a biocide, allowing the biocide to be re-used in subsequent cycles. As a result the cost of bringing at least one of the solutions into contact with a biocide is greatly reduced by the present method.

The solution(s) may be brought into contact with a biocide by any suitable method, for example one may add a biocide to one or both of the ionic solutions or one may pass one or both of the ionic solutions through a bed of solid biocide chemicals.

The preferred approach is to add a biocide to one or both of the ionic solutions. The biocide may be added to the solution(s) periodically, for example on an hourly, daily or weekly basis. If desired, the timing and/or amount of biocide added may be done in response to a measured biocide concentration within the relevant ionic solution(s).

Use of a biocide has an advantage over irradiating one or both of the ionic solutions with UV light in that it uses little or no electricity.

Although the present method essentially re-uses biocide, it is still preferred to use an inexpensive biocide.

The identity of the biocide is not particularly limited and includes, for example, chlorine, bromine, chloramines, biguanide salts, peroxy compounds (e.g. H2O2), ozone, quaternary ammonium compounds, silver ions, silver nanoparticles and combinations comprising two or more thereof.

Peroxy compounds (e.g. hydrogen peroxide) are preferably used in conjunction with a stabiliser (e.g. a phosphate).

The quaternary ammonium compounds include polyquaternary ammonium compounds and monomeric quaternary ammonium compounds.

The chlorine and bromine are preferably provided by adding a hypochlorite, hypobromite and/or halogenated hydantoin to the relevant solution, e.g. in the form of a sodium, lithium, ammonium, potassium or calcium salt.

Examples of biocides include sodium hypochlorite, potassium hypochlorite, ammonium hypochlorite, calcium hypochlorite, lithium hypochlorite, activated sodium bromide, chloramine, sodium dichloroisocyanurate, trichloroisocyanurate, potassium peroxymonopersulphate, hydrogen peroxide, silver dihydrogen citrate, polyaminopropyl biguanide. Examples of halogenated hydantoins include dichlorodimethylhydantoin, bromochlorodimethylhydantoin and dibromodimethylhydantoin.

A variety of polyquaternary ammonium compounds may be used in the various aspects of the present invention, including poly(hexamethylammonium) chloride, poly[oxyethylene-(dimethylimino) ethylene-(dimethylimino) ethylene dichloride] and dodecamethylenedimethylimino chloride. The aforementioned biocides are commercially available.

For cost reasons, the preferred biocide comprises a hypochlorite, especially sodium or calcium hypochlorite. The amount of biocide included in the ionic solution can be varied between quite wide limits and depends to some extent on what other steps (if any) are also been taken to reduce biofouling, the biological load of the ionic solutions and the frequency with which the biocide is added. Typically, however, the ionic solutions preferably contain 5 to 500 parts per million by weight (ppm) of biocide, preferably 10 to 250 ppm, especially 20 to 100 ppm, relative to the weight of ionic solution.

To reduce the risk of developing resistance to one type of biocide one may change the type of biocide from time to time.

The average electricity output of RED unit per unit time may also be enhanced by irradiating one or more of the solutions with UV light. In RED methods lacking the regeneration and recycling step (C), irradiation of the solutions with UV light would be expensive because one would need to continuously irradiate the ionic solutions entering the RED unit. In contrast, the present method, which includes regeneration and recycling step (C), may periodically (or discontinuously) irradiate one or more of the solutions with UV light. This periodic irradiation is possible because the present method may re-use solutions which have already been sterilised by UV light in a previous cycle and therefore still have a low bacterial content. As a result, in the present method it is not necessary to continuously irradiate the ionic solutions before they enter the RED unit. Instead one may periodically irradiate one or both of the ionic solutions (or the solutions exiting the RED unit), thereby greatly reducing the energy cost compared to continuous irradiation.

The UV light may be UV-A (e.g. 400 - 315 nm), UV-B (e.g. >315 - 280 nm) and/or UV-C (e.g. >280 - 100 nm) light, however it is preferred that at least one of the solutions is irradiated using a lamp which emits UV-C light, e.g. a lamp which emits UV light at least 10% (more preferably at least 20%, especially at least 30%) of which is UV-C light. This is because the short wavelength of UV-C light is particularly effective for killing microorganisms.

Lamps which may be used to irradiate the solution(s) include amalgam UV lamps, low pressure UV lamps and medium pressure UV lamps.

Low-pressure UV lamps typically offer high efficiencies (approximately 35% UV-C output) at a low density (e.g. about 1 W/cm³).

Amalgam UV lamps are essentially a higher-power version of low-pressure lamps. Their efficiency is slightly lower than that of traditional low-pressure lamps (approximately 33% UV-C output) and the power density is typically higher (e.g. about 2 to 3 W/cm³).

Medium-pressure UV lamps often have a broad and pronounced peak-line spectrum and a high radiation output but lower UV-C efficiency of 10% or less. Typically the power density is 30 W/cm³ or greater. Depending on the quartz glass used for the lamp body, low-pressure and amalgam UV lamps usually emit light at 254 nm (and 185 nm which can generate ozone).

The regeneration which occurs in step (C) may be done by a number of techniques, with thermal regeneration being preferred.

Since step (C) can take place at relatively low temperature differences, the method is particularly useful where low temperature heat sources are available, such as solar energy, geothermal energy or waste heat.

Preferably the method is performed without mixing of the (raw) solutions exiting the reverse electrodialysis unit. For example, one may pass one or both of the solutions exiting the reverse electrodialysis unit into regeneration unit(s) which heat the solutions(s), thereby evaporating liquid therefrom and creating an ion-free liquid (i.e. a liquid having a very low solute content) and a concentrate. The ion- free liquid and concentrate may then be mixed with streams exiting the reverse electrodialysis unit to change their solute content, thereby regenerating the concentrated ionic solution and the dilute ionic solution.

Thermal regeneration preferably comprises:

a) evaporation of liquid (e.g. water) from one or both of the solution(s) exiting the RED unit and condensation of the evaporated liquid, thereby providing an ion-free liquid and a concentrate;

b) mixing of the ion-free liquid with one or both of the solution(s) exiting the RED unit, thereby regenerating the dilute ionic solution; and

c) mixing of the concentrate with one or both of the solution(s) exiting the RED unit, thereby regenerating the concentrated ionic solution.

For efficiency reasons, in step b) it is preferred that ion-free liquid is mixed with the solution exiting the RED unit which has the lowest solute content (i.e. the solution derived from the dilute ionic solution). Similarly, in step c) it is preferred that the concentrate is mixed with the solution exiting the RED unit which has the highest solute content (i.e. the solution derived from the concentrated ionic solution).

Typically the electrodes are located at each end of the stack.

The recycling/recirculating feature of the present method opens up the possibility of further additives being present in the concentrated solution and/or dilute solution that previously would have been impractical on cost grounds. For example, the concentrated solution and/or dilute solution may optionally comprise one or more surfactant, dispersant, pH buffer, anti-foaming agent and/or anti- corrosive agent, e.g. in an amount of 0.01 to 5wt%, especially 0.1 to 3wt%. Inclusion of the surfactant and/or dispersant can help the breaking down and removal of fouling materials from the membranes and also assist flow of the solutions through the RED unit. An anti-foaming agent will prevent foaming, e.g. when a solution contains a surfactant. Anti-corrosive agents are particularly useful when any of the solutions come into contact with corrodible materials, e.g. metal parts, pipes, anodes, cathodes and casings etc.. pH buffers can reduce corrosion and biofilm build-up.

Suitable surfactants include ionic surfactants, non-ionic surfactants and mixtures thereof.

Suitable ionic surfactants include anionic and cationic surfactants. Examples of suitable anionic surfactants are: alkyl benzene sulphonates (e.g. 25 sodium dodecylbenzene sulphonate); alkyl sulphates; alkyl ether sulphates; sulphosuccinates; phosphate esters; fatty acid carboxylates, including alkyl carboxylates; and alkyl or aryl alkoxylated carboxylates, which include, for example, alkyl ethoxylated carboxylates, alkyl propoxylated carboxylates and alkyl ethoxylated/propoxylated carboxylates.

Examples of suitable cationic surfactants are: quaternary ammonium salts; benzalkonium chloride; ethoxylated amines.

Examples of non-ionic surfactants are: alkyl ethoxylates; alkyl propoxylates; alkyl aryl ethoxylates; alkyl aryl propoxylates; and ethylene oxide/propylene oxide copolymers.

Suitable dispersants include Solsperse^® 32000, optionally together with the synergist Solsperse^® 5000, (available from Lubrizol) and BYK-168 (available from BYK Chemie).

Suitable anti-foaming agents include silicone anti-foaming agents, e.g.

Silicone Antifoam Emulsion SE57 (Wacker), TRITON^® CF32 (Rohm & Haas),

AKYPO^® LF (ethercarboxylic acid Chem Y), Agitan 190 (Mtinzing Chemie), TEGO^® Foamese 825 (modified polysiloxane, TEGO Chemie Service GmbH,

Germany). Silicone-based antifoaming agents are preferred. They are either dispersible or soluble in water.

Suitable anticorrosive agents include phosphonic acids and their salts like hydroxyethyl phosphonic acid and its salts, amino trismethylene phosphonic acid and its salts, and diethylentriaminpentamethylene phosphonic acid and its salts; phosphates, e.g. trisodium phosphate; and borates, e.g. borax.

Optionally the solutions may also contain a pH buffering agent, e.g. tris(hydroxymethyl)-aminomethane (TRIS), hydrogen phosphates, glycine, 3-

(cyclohexylamino)-propane sulfonic acid (CAPS), hydrogen carbonates, borates including borax, 2-amino-2-methyl-1 -propanol (AMP), 3-(cyclohexylamino)-2- hydroxy-1 -propane-sulfonic acid (CAPSO) or 2-(N-cyclohexylamino)ethan-sulfonic acid (CHES).

The invention is herein described, somewhat diagrammatically, and by way of example only, with reference to the accompanying drawing, wherein:

FIG. 1 illustrates one embodiment of the present invention. Referring to FIG. 1 , the concentrated ionic solution is fed into RED unit 202 through inlet 204 and exits, with a somewhat lower ionic concentration, via outlet 208; whereas the dilute ionic solution is fed into unit 202 via inlet 206 and exits, with a somewhat higher ionic concentration, via outlet 210. The exiting solution 210 enters the regeneration unit 203 comprising a distillation unit receiving heat from a heat source and optionally a condensation unit for condensing the distillate. The distillation unit separates the solution derived from outlet 210 into an ion-free liquid and a concentrate. The ion-free liquid is mixed with another portion of liquid derived from outlet 210, thereby regenerating the dilute ionic solution which exits the regeneration unit via outlet 21 1 and is then pumped back to inlet 206 via pump 234. Meanwhile the concentrate created in regeneration unit 203 exits via outlet 209 and is mixed with the liquid derived from outlet 208, thereby regenerating the concentrated ionic solution which is pumped back to inlet 204 via pump 230. In FIG. 1 the power output of the method may be enhanced by making the concentrated ionic solution hotter than the dilute ionic solution by a means for heating the concentrated ionic solution (not shown).

The electrical energy generated across the electrodes in the RED unit 202 is outputted at 216.

The pathways through which the concentrated ionic solution and the dilute ionic solution pass may be alternated by use of one or more valves (not shown) which directs the flow of the solutions.

Because of the high volumetric latent heat of vaporization of water, the use of water as sole solvent in the ionic solutions requires a large amount of heat to raise its temperature, thereby decreasing the thermal efficiency of the method. For this reason, one the ionic solutions optionally comprise an organic solvent having a lower volumetric latent heat of vaporization than water (either instead of or in addition to water), especially a water-miscible organic solvent, particularly those having a boiling point below 100°C, e.g. methyl and/or ethyl alcohol. Examples of the solute that could be used with such an organic solvent include ferric chloride and/or sodium nitrate.

Many industrial activities generate low grade, unused heat. For example, gas and coal fired power stations generate not only electricity but also a large amount of heat which is often wasted because it simply warms the surrounding environment. Low grade heat is often available from natural sources, e.g. from solar and geothermal sources. The present method may put such low grade or waste heat to good use by using it to heat at least one of the ionic solutions.

The concentrated ionic solution and the dilute ionic solution may be the same temperature or different temperatures as they enter the RED unit.

In one embodiment, both the concentrated ionic solution and the dilute ionic solution are hotter than the immediate vicinity of the RED unit as they enter the RED unit. The elevated temperature for both ionic solutions can be useful to remove scale build-up and/or to heat sterilise the RED unit, thereby increasing the power output of the RED unit per unit time by reducing the 'down time' required for cleaning the unit. The elevated temperatures can also kill or slow down the reproduction of unwanted organisms in the RED unit, e.g. bacteria.

The optional temperature difference between the ionic solutions may be achieved by, for example, cooling one of the ionic solutions and not cooling the other ionic solution (or by cooling the other ionic solution to a lesser extent). However, as one of the objectives of the invention is to make use of low grade heat and waste heat, in a preferred embodiment the method comprises the step of heating one of the ionic solutions (especially the concentrated ionic solution) and not heating the other ionic solution (especially the dilute ionic solution), or the step of heating both of the ionic solutions such that one of the ionic solutions (especially the concentrated ionic solution) is hotter than the other ionic solution (especially the dilute ionic solution).

The temperature of the concentrated ionic solution as it enters the reverse electrodialysis unit is preferably 20 to 200°C, more preferably 30 to 150°C, especially 50 to 120°C.

In one embodiment the temperature of the dilute ionic solution as it enters the reverse electrodialysis unit is preferably 0 to 40°C, more preferably 1 to 35°C, especially 5 to 30°C.

As the ionic solutions enter the reverse electrodialysis unit, it is preferred that the concentrated ionic solution is 5 to 200°C hotter than the dilute ionic solution, more preferably 10 to 149°C, especially 1 1 to 100°C, more especially 12 to 75°C hotter than the dilute ionic solution.

When both the dilute ionic solution and the concentrated ionic solution are hot as they enter the reverse electrodialysis unit, it is preferred for the dilute ionic solution and the concentrated ionic solution to each independently have a temperature of 30 to 150°C, more preferably 40 to 120°C, especially 45 to 80°C as they enter the electrodialysis unit.

The heat source which may be used to obtain the above temperatures is not particularly limited, but to achieve most benefit from the invention one will typically use waste or low grade heat, for example solar energy, geothermal energy and/or waste heat, e.g. from a power plant, cooling tower or manufacturing process. The same heat source can also be used in regeneration step (C) to evaporate liquid (e.g. water) from the solution(s) exiting the RED unit or from a mixture comprising one or both of such solutions.

The ionic solutions typically comprise a solute (in this specification 'solute' and 'salt' are used interchangeably and the word 'salt' is not limited to sodium chloride), water and optionally a water-miscible organic solvent. The solute typically comprises one or more salts. Preferred salts have a high solubility in the liquid medium in which they are dissolved. The salt(s) preferably comprise a cation selected from sodium, potassium, lithium, caesium, rubidium and/or ammonium, i.e. the salt comprises a sodium, potassium, lithium, caesium, rubidium and/or ammonium salt. The salt preferably comprises an anion selected from azide, bromide, carbonate, chloride, chlorate, fluoride, iodide, hydrogen carbonate, nitrate, nitrite, selenide, perchlorate, thiocyanate, thiosulphate, acetate and/or formate. More preferably the salt comprises an anion selected from azide, bromide, carbonate, chlorate, fluoride, iodide, nitrate, nitrite, perchlorate, thiocyanate, thiosulphate, acetate and/or formate.

As examples of sodium salts there may be mentioned sodium formate, acetate, azide, chloride, bromide, iodide, carbonate, hydrogen carbonate, chlorate, perchlorate, nitrate, nitrite, thiocyanate, thiosulphate and mixtures comprising two or more thereof.

As examples of potassium salts there may be mentioned potassium formate, acetate, fluoride, chloride, bromide, iodide, azide, carbonate, hydrogen carbonate, perchlorate, nitrate, nitrite, phosphate, thiocyanate, thiosulphate and mixtures comprising two or more thereof.

As examples of lithium salts there may be mentioned lithium formate, acetate, chloride, bromide, iodide, azide, bromate, chlorate, perchlorate, nitrate, nitrite, selenide, thiocyanate and mixtures comprising two or more thereof.

As examples of caesium salts there may be mentioned caesium formate, acetate, fluoride, chloride, bromide, iodide, azide, carbonate, hydrogen carbonate, perchlorate, nitrate, nitrite, thiocyanate and mixtures comprising two or more thereof.

As examples of rubidium salts there may be mentioned rubidium formate, acetate, fluoride, chloride, bromide, iodide, carbonate, hydrogen carbonate, nitrate, nitrite, thiocyanate and mixtures comprising two or more thereof.

As examples of ammonium salts there may be mentioned ammonium formate, acetate, fluoride, chloride, bromide, iodide, carbonate, hydrogen carbonate, chlorate, sulphate, hydrogen sulphate, nitrate, nitrite, thiocyanate, thiosulphate and mixtures comprising two or more thereof.

The ionic solutions comprise one or more than one salt (as a solute), e.g. two or more of the foregoing salts. In a preferred embodiment one or both of the ionic solutions comprise water and at least 0.02 mol/l of a solute other than sodium chloride.

The salt preferably comprises a cation which has an (unhydrated) ionic radius larger than 0.1 nm, more preferably larger than 0.13 nm. Suitable cations include sodium (ionic radius 0.10 nm) potassium (ionic radius 0.14 nm), rubidium (ionic radius 0.15 nm), caesium (ionic radius 0.17 nm) and ammonium (ionic radius 0.15 nm).

The salt(s) preferably comprise(s) an anion which has an (unhydrated) ionic radius larger 0.15 nm, more preferably larger than 0.18 nm. Suitable anions include chloride (ionic radius 0.18 nm), bromide (ionic radius 0.20 nm), iodide (ionic radius 0.22 nm), nitrate (ionic radius 0.19 nm), nitrite (ionic radius 0.19 nm), thiocyanate, hydrogen carbonate, formate and acetate.

The salt(s) used in the concentrated ionic solution may be identical to or different from the salt(s) used in the diluted ionic solution second stream, although typically the salts present in both solutions will become the same as the method continues due to the potential mixing of the outlet streams.

The ionic solutions preferably comprise sodium nitrate, sodium nitrite, lithium nitrate, lithium nitrite, lithium acetate, potassium formate, potassium acetate, potassium nitrate, potassium nitrite, potassium thiocyanate, ammonium formate, ammonium acetate, ammonium nitrate and/or ammonium thiocyanate.

When the ionic solutions comprise more than one salt, the number of moles/litre (mol/l) of solute is the total for all of the salts present. For example, if 1 litre of an ionic solution contains 0.1 mol of salt A and 0.15 mol of salt B then the ionic solution contains (0.1 + 0.15 mol/l) solute = 0.25 mol/l solute.

Preferably the ionic solutions comprise a salt which has solubility in water at

20°C (S20C) of at least 4mol/Kg (i.e. 4 moles of salt per Kg of water), more preferably at least 5mol/Kg, especially at least 8mol/Kg, more especially at least 10mol/Kg.

The values of S20C and S6oc (solubility in water at 60°C) for various salts are shown in Table 1 below:

Table 1

Salt/Solute Formula S₂oc S60C

(mol/kg water) (mol/kg water)

Sodium acetate NaC₂H₃0₂ 5.7 17.0

Sodium bromide NaBr 8.8 1 1 .5

Sodium chlorate NaCIOs 9.0 12.9

Sodium chloride NaCI 6.1 6.3

Sodium formate NaHC0₂ 1 1 .9 17.9

Sodium hydrogen carbonate NaHC0₃ 1 .1 1 .9

Sodium iodide Nal 1 1 .9 17.1

Sodium nitrate NaN0₃ 10.3 14.4

Sodium nitrite NaN0₂ 1 1 .7 16.1

Sodium perchlorate NaCI0₄ 16.4 23.5

Sodium thiosulphate Na₂S20₃ 4.6 5.3

Potassium acetate KC₂Hs0₂ 26.1 35.7 Salt/Solute Formula S20C S60C

(mol/kg water) (mol/kg water)

Potassium azide KN₃ 5.6

Potassium bromide KBr 5.5 7.2

Potassium carbonate K2CO3 1 1 .2 12.8

Potassium chloride KCI 4.6 6.1

Potassium formate KHCO2 40.1 56.1

Potassium fluoride KF 16.4 24.5

Potassium hydrogen carbonate KHCO3 3.4 6.6

Potassium iodide Kl 8.7 10.6

Potassium nitrate KNO3 4.6 10.2

Potassium nitrite KN0₂ 36.0 40.9

Potassium thiocyanate KSCN 23.0 38.4

Potassium thiosulphate K2S2O3 8.2 12.5

Ammonium acetate NH4C2H3O2 18.6 40.4

Ammonium bromide NH₄Br 7.8 1 1 .0

Ammonium chloride NH₄CI 7.0 10.3

Ammonium formate NH₄HC0₂ 22.7 49.4

Ammonium hydrogen carbonate NH₄HC0₃ 2.7 7.5

Ammonium iodide NH₄I 1 1 .9 14.4

Ammonium nitrate NH₄N0₃ 24.0 52.6

Ammonium thiocyanate NH₄SCN 22.3 45.5

Ammonium thiosulphate (NH₄)₂S₂0₃ 1 1 .7

Caesium acetate CsC₂H₃0₂ 52.6

Caesium azide CsN₃ 17.5

Caesium bromide CsBr 5.1

Caesium chloride CsCI 1 1 .1 13.6

Caesium fluoride CsF 21 .2

Caesium formate CsHC0₂ 25.3

Caesium iodide Csl 2.9 5.8

Caesium nitrate CsNOs 1 .2 4.3

Rubidium bromide RbBr 6.5 9.5

Rubidium fluoride RbF 28.7

Rubidium formate RbHC0₂ 42.5 69.0

Rubidium hydrogen carbonate RbHC0₃ 7.5

Rubidium iodide Rbl 6.8

Rubidium nitrate RbN03 3.6 13.6

Lithium acetate UC2H3O2 6.2

Lithium azide LiN₃ 13.7 17.7

Lithium bromide LiBr 18.4 25.6

Lithium chloride LiCI 19.6 23.2

Lithium chlorate L1CIO3 41 .1 85.9

Lithium formate L1HCO2 7.6 12.4

Lithium iodide Lil 12.3 15.1 Salt/Solute Formula S20C S60C

(mol/kg water) (mol/kg water)

Lithium nitrate LiNOs 10.2 25.4

Lithium nitrite LiN0₂ 18.3 33.4

Lithium thiocyanate LiSCN 17.5

The total concentration of solute (i.e. salts) in the concentrated ionic solution as it enters the reverse electrodialysis unit is preferably 0.6 to 100mol/l, more preferably 1.5 to 80mol/l, especially 3 to 75mol/l.

The total concentration of solute (i.e. salts) in the dilute ionic solution as it enters the reverse electrodialysis unit is preferably 0.03 to 4mol/l, more preferably 0.05 to 3mol/l, especially 0.06 to 2.5mol/l, more especially 0.08 to 2mol/l.

In a preferred embodiment the concentrated ionic solution and the dilute ionic solution each independently comprise sodium nitrate, sodium nitrite, lithium nitrate, lithium nitrite, lithium acetate, potassium formate, potassium acetate, potassium nitrite, potassium thiocyanate, ammonium formate, ammonium acetate, ammonium nitrate and/or ammonium thiocyanate. Preferably the ionic solutions comprise at least 0.02 mol/l of solute. Preferably the identity of the solute present in the concentrated ionic solution and the dilute ionic solution are substantially the same.

The total concentration of salt(s) in the concentrated ionic solution as it enters the reverse electrodialysis unit is higher than the total concentration of salt(s) in the dilute ionic solution as it enters the reverse electrodialysis unit.

Preferably the method is performed such that the following equation is satisfied:

S1/S2 > Y

wherein:

S1 is the mol/L concentration of solute (e.g. salt) in the concentrated ionic solution as it enters the reverse electrodialysis unit;

S2 is the mol/L concentration of solute (e.g. salt) in the dilute ionic solution as it enters the reverse electrodialysis unit; and

Y is at least 30.

Preferably Y is at least 40, more preferably at least 50, especially at least 60, more especially at least 80, 100 or 120. Preferably Y is less than 1000, more preferably less than 200, especially less than 150.

In a preferred embodiment, Y is at least 40 (e.g. 40 to 200), S20C is at least 4mol/kg of water.

In another preferred embodiment, Y is at least 40 (e.g. 40 to 200), S20C is at least 8mol/kg of water. In another preferred embodiment, Y is 50 to 150, S20C is at least 5mol/kg of water.

In another preferred embodiment, Y is 50 to 150, S20C is at least 10mol/kg of water.

In another preferred embodiment, Y is at least 40 (e.g. 40 to 200), the concentrated ionic solution and the dilute ionic solution each independently comprise sodium bromide, sodium chlorate, sodium iodide, sodium nitrate, sodium nitrite, sodium formate, sodium acetate, sodium perchlorate, potassium fluoride, potassium iodide, potassium formate, potassium acetate, potassium carbonate, potassium nitrite, potassium thiocyanate, lithium nitrate, lithium nitrite, lithium acetate, ammonium formate, ammonium acetate, ammonium chloride, ammonium bromide, ammonium iodide, ammonium nitrate, ammonium thiosulphate and/or ammonium thiocyanate and, as the ionic solutions enter the reverse electrodialysis unit, the concentrated ionic solution is 5 to 200°C hotter than the dilute ionic solution.

In another preferred embodiment, Y is at least 40 (e.g. 40 to 200) and the concentrated ionic solution and the dilute ionic solution each independently comprise potassium formate, potassium acetate, potassium nitrate, potassium nitrite, potassium thiocyanate, ammonium formate, ammonium acetate, ammonium nitrate and/or ammonium thiocyanate.

In another preferred embodiment, Y is 50 to 150 and the concentrated ionic solution and the dilute ionic solution each independently comprise potassium formate, potassium acetate, potassium nitrate, potassium nitrite, potassium thiocyanate, ammonium formate, ammonium acetate, ammonium nitrate and/or ammonium thiocyanate.

In another preferred embodiment, Y is 40 to 200, S20C is at least 4mol/kg of water and, as the ionic solutions enter the reverse electrodialysis unit, the concentrated ionic solution is 5 to 200°C hotter than the dilute ionic solution.

In another preferred embodiment, Y is 50 to 150, S20C is at least 5mol/kg of water and, as the ionic solutions enter the reverse electrodialysis unit, the concentrated ionic solution is 10 to 149°C hotter than the dilute ionic solution.

In another preferred embodiment, Y is 50 to 150, S20C is at least 10mol/kg of water and, as the ionic solutions enter the reverse electrodialysis unit, the concentrated ionic solution is 1 1 to 100°C hotter than the dilute ionic solution.

In another preferred embodiment, Y is 40 to 200, the concentrated ionic solution and the dilute ionic solution each independently comprise potassium formate, potassium acetate, potassium nitrate, potassium thiocyanate, ammonium formate, ammonium acetate, ammonium nitrate and/or ammonium thiocyanate and, as the ionic solutions enter the reverse electrodialysis unit, the concentrated ionic solution is 5 to 200°C hotter than the dilute ionic solution. In another preferred embodiment, Y is 50 to 150, the concentrated ionic solution and the dilute ionic solution each independently comprise potassium formate, potassium acetate, potassium nitrate, potassium thiocyanate, ammonium formate, ammonium acetate, ammonium nitrate and/or ammonium thiocyanate and, as the ionic solutions enter the reverse electrodialysis unit, the concentrated ionic solution is 12 to 75°C hotter than the dilute ionic solution.

In another preferred embodiment, Y is at least 40 (e.g. 40 to 200), the concentrated ionic solution and the dilute ionic solution each independently comprise sodium bromide, sodium chlorate, sodium iodide, sodium nitrate, sodium nitrite, sodium formate, sodium acetate, sodium perchlorate, potassium fluoride, potassium iodide, potassium formate, potassium acetate, potassium carbonate, potassium nitrite, potassium thiocyanate, lithium nitrate, lithium nitrite, lithium acetate, ammonium formate, ammonium acetate, ammonium chloride, ammonium bromide, ammonium iodide, ammonium nitrate, ammonium thiosulfate and/or ammonium thiocyanate and, as the ionic solutions enter the reverse electrodialysis unit, the dilute ionic solution is 5 to 200°C hotter than the concentrated ionic solution.

In another preferred embodiment, Y is at least 40 (e.g. 40 to 200), the concentrated ionic solution and the dilute ionic solution each independently comprise sodium bromide, sodium chlorate, sodium iodide, sodium nitrate, sodium nitrite, sodium formate, sodium acetate, sodium perchlorate, potassium fluoride, potassium iodide, potassium formate, potassium acetate, potassium carbonate, potassium nitrite, potassium thiocyanate, lithium nitrate, lithium nitrite, lithium acetate, ammonium formate, ammonium acetate, ammonium chloride, ammonium bromide, ammonium iodide, ammonium nitrate, ammonium thiosulfate and/or ammonium thiocyanate and, as the ionic solutions enter the reverse electrodialysis unit, the dilute ionic solution and the concentrated ionic solution have about the same temperature, i.e. differ less than 5°C in temperature.

Preferably at least one, more preferably at least half, especially all of the membranes have an uneven surface profile comprising peaks and valleys. The average height difference between the peaks and valleys is preferably at least 50pm, more preferably at least 75pm.

When the membranes do not have an uneven surface comprising peaks and valleys it is preferred that the stack further comprises spacers which keep the cationic and anionic membranes apart, thereby creating a pathway between said membranes. Preferably the spacers are ion-conducting and keep the cationic and anionic create pathways of average width 50 to 400 pm, more preferably 100 to 250 pm. Preferably at least one, more preferably at least half, especially all of the membranes have an electrical resistance below 1 .5 ohm. cm², more preferably below 1 .0 ohm. cm².

Preferably at least one, more preferably at least half, especially all of the membranes comprise a porous support and an ionically-charged polymeric layer present on the support. Such composite membranes are available from FUJIFILM Manufacturing Europe b.v..

Preferably at least one, more preferably at least half, especially all of the membranes have a thickness, including the support (when present) of less than 250pm, more preferably between 10 and 200pm, most preferably between 20 and 150pm.

Preferably at least one, more preferably at least half, especially all of the membranes have an ion exchange capacity of at least 0.1 meq/g, more preferably of at least 0.3meq/g, especially more than 0.6meq/g, more especially more than 1 .0meq/g, based on the total dry weight of the membrane and any porous support and any porous strengthening material which remains in contact with the resultant membrane. Ion exchange capacity may be measured by titration as described by Djugolecki et al, J. of Membrane Science, 319 (2008) on page 217.

Preferably at least one, more preferably at least half, especially all of the membranes have a permselectivity for small cations (e.g. Na⁺ or CI^"), measured at a concentration of 0.05 and 0.5 M in both compartments respectively, of above 90%, more preferably above 95%. The permselectivity may be measured by the method described in WO2013/14420.

Preferably at least one, more preferably at least half, especially all of the membranes exhibit a swelling in water of less than 100%, more preferably less than 75%, most preferably less than 60%. The degree of swelling can be controlled by appropriate selection of the components used to prepare the membrane, e.g. the amount of crosslinker and/or the amount of non-curable compounds, and further by the properties of the porous support (when present).

Electrical resistance, permselectivity and % swelling in water may be measured by the methods described by Djugolecki et al, J. of Membrane Science, 319 (2008) on pages 217-218.

Typically the membranes are substantially non-porous e.g. the pores are smaller than the detection limit of a standard Scanning Electron Microscope (SEM). Thus using a Jeol JSM-6335F Field Emission SEM (applying an accelerating voltage of 2kV, working distance 4 mm, aperture 4, sample coated with Pt with a thickness of 1 .5nm, magnification 100,000x, 3° tilted view) the average pore size is generally smaller than 5nm, preferably smaller than 2 nm.

The membranes preferably have low water permeability so that ions may pass through the membranes and water molecules do not pass through the membranes. Preferably at least one, more preferably at least half, especially all of the membranes have a water permeability lower than 1 .10^"7 m³/m².s.kPa, more preferably lower than 1 .10^"8 m³/m².s.kPa, most preferably lower than 5.10^"9 m³/m².s.kPa, especially lower than 1 .10^"9 m³/m².s.kPa.

The porous supports which may be included in the membranes include woven and non-woven synthetic fabrics, e.g. polyethylene, polypropylene, polyacrylonitrile, polyvinyl chloride, polyester, polyamide, and copolymers thereof; and porous membranes based on, for example, polysulfone, polyethersulfone, polyphenylenesulfone, polyphenylenesulfide, polyimide, polyetherimide, polyamide, polyamideimide, polyacrylonitrile, polycarbonate, polyacrylate, cellulose acetate, polypropylene, poly(4-methyl 1 -pentene), polyinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, polychlorotrifluoroethylene, and copolymers thereof.

Various porous supports are available commercially, e.g. from Freudenberg Filtration Technologies (Novatexx™ materials) and Sefar AG.

Preferred cation exchange membranes comprise sulpho, carboxyl and/or phosphato groups, especially sulpho and/or carboxyl groups.

Preferred anion exchange membranes comprise quaternary ammonium groups.

Preferably at least one, more preferably at least half, especially all of the cation exchange membranes comprise an ionically-charged polymeric layer obtained from a curable composition comprising:

(i) 2.5 to 70 wt% crosslinker comprising at least two acrylamide groups;

(ii) 20 to 65 wt% curable ionic compound comprising one ethylenically unsaturated group and at least one anionic group;

(iii) 5 to 45 wt% solvent; and

(iv) 0 to 10 wt% of free radical initiator.

The crosslinker is preferably of the Formula (1 ):

Formula (1 ) wherein:

Ri and R₂ are each independently H or methyl;

R₃ and R₄ are each independently H, alkyl, R₃ and R₄ together with the N

groups to which they are attached and Y form an optionally substituted 6- or 7-membered ring; and Y is an optionally substituted and optionally interrupted alkylene group.

When R₃ or R₄ is alkyl it is preferably Ci_-4-alkyl.

When R₃ and R₄ together with the N groups to which they are attached and Y form an optionally substituted 6- or 7-membered ring they preferably form a piperazine, homopiperazine or triazine ring.

The optional interruptions which may be present in Y are preferably ether or, more preferably, amino groups. Y is preferably of the formula -(C_nH_2n)- wherein n is 1 , 2 or 3.

As examples of suitable crosslinkers there may be mentioned Ν,Ν'- methylene bis(meth) acrylamide, N,N'-ethylenebis(meth)acrylamide, Ν,Ν'- propylenebis(meth)acrylamide, N,N'-butylenebis(meth)acrylamide, N,N'-( ,2- dihydroxyethylene) bis-(meth)acrylamide, 1 ,4-diacryoyl piperazine, 1 ,4- bis(acryloyl)homopiperazine, triacryloyl-tris(2-aminoethyl)amine, triacroyl diethylene triamine, tetra acryloyl triethylene tetramine, 1 ,3,5-triacryloylhexahydro- 1 ,3,5-triazine and/or 1 ,3,5-trimethacryloylhexahydro-1 ,3,5-triazine. The term '(meth)' is an abbreviation meaning that the 'meth' is optional, e.g. N,N'-methylene bis(meth) acrylamide is an abbreviation for Ν,Ν'-methylene bis acrylamide and Ν,Ν'-methylene bis methacrylamide.

Preferred curable ionic compounds comprise an acidic group, for example a sulpho, carboxy and/or phosphato group. Examples of curable ionic compounds include acrylic acid, beta carboxy ethyl acrylate, maleic acid, maleic acid anhydride, vinyl sulphonic acid, phosphonomethylated acrylamide, (2- carboxyethyl)acrylamide and 2-(meth)acrylamido-2-methylpropanesulfonic acid.

The solvent is preferable water or a mixture comprising water and an organic solvent, especially a water-miscible organic solvent.

When the solvent comprises water and an organic solvent the weight ratio of watenorganic solvent is preferably higher than 2:3, more preferably between 10: 1 and 1 : 1 , more preferably between 10: 1 and 1 :2, especially between 4: 1 and 1 : 1 , and more especially between 3: 1 and 2: 1 .

The organic solvent is optionally a single organic solvent or a combination of two or more organic solvents. Preferred organic solvents include Ci_-4 -alcohols (e.g. methanol, ethanol and propan-2-ol, diols (e.g. ethylene glycol and propylene glycol), triols (e.g. glycerol), carbonates (e.g. ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, di-t-butyl dicarbonate and glycerin carbonate), dimethyl formamide, acetone, N-methyl-2-pyrrolidinone and mixtures comprising two or more thereof. A particularly preferred organic solvent is propan-2-ol.

Preferably the composition comprises 0.01 to 10 wt%, more preferably 0.05 to 5 wt%, especially 0.1 to 2 wt% free radical initiator. The preferred free radical initiator is a photoinitiator. Especially preferred photoinitiators include alpha- hydroxyalkylphenones, e.g. 2-hydroxy-2-methyl-1 -phenyl propan-1 -one and 2- hydroxy-2-methyl-1 -(4-tert-butyl-) phenylpropan-1 -one, and acylphosphine oxides, e.g. 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, and bis(2,4,6- trimethylbenzoyl)-phenylphosphine oxide.

Preferably at least one, more preferably at least half, especially all of the anion exchange membranes comprise an ionically-charged polymeric layer obtained from a curable composition comprising:

(i) 2.5 to 70 wt% crosslinker comprising at least two acrylamide groups;

(ii) 12 to 65 wt% curable ionic compound comprising one ethylenically unsaturated group and at least one cationic group;

(iii) 10 to 70 wt% solvent; and

(iv) 0 to 10 wt% of free radical initiator.

The preferred crosslinker, solvent and radical initiator used to prepare the anion exchange membrane are as described above in relation to the cation exchange membrane.

Preferred curable ionic compounds comprise a quaternary ammonium group. Examples of such compounds include (3-acrylamidopropyl) trimethylammonium chloride, 3-methacrylamidopropyl trimethyl ammonium chloride, (ar-vinylbenzyl) trimethylammonium chloride, (2- (methacryloyloxy)ethyl)trimethylammonium chloride, [3-(methacryloylamino)propyl] trimethyl ammonium chloride, (2-acrylamido-2-methylpropyl) trimethylammonium chloride, 3-acrylamido-3-methylbutyl trimethyl ammonium chloride, acryloylamino- 2-hydroxypropyl trimethyl ammonium chloride, N-(2-aminoethyl)acrylamide trimethyl ammonium chloride and mixtures comprising two or more thereof.

Preferably the flow velocity of the ionic solutions through the pathways is less than 4 cm/s, more preferably less than 2cm/s, especially less than 1 .5cm/s; the velocity may even be as low as 0.2, 0.4 or 0.6 cm/s. These preferences arise because higher flow velocities require more energy to pump the solutions, thereby reducing the efficiency of electricity generation. The flow velocity of the dilute ionic solution may be lower than, higher than or the same as the flow velocity of the concentrated solution.

According to a second aspect of the present invention there is provided a device for generating electricity comprising:

(A) a reverse electrodialysis unit comprising a membrane stack having electrodes, alternating cation and anion exchange membranes, a first pathway through the reverse electrodialysis unit for a concentrated ionic solution, and a second pathway through the reverse electrodialysis unit for a dilute ionic solution;

(B) a means for feeding a concentrated ionic solution into said first pathway; (C) a means for feeding a dilute ionic solution into said second pathway, whereby solute from the concentrated solution in the first pathway migrates through the membranes to the dilute solution in the second pathway, thereby generating electricity;

(D) a means for alternating which of the first and second pathways carries the said concentrated and dilute ionic solutions, and/or a means for bringing at least one of said solutions into contact with a biocide and/or a means for irradiating at least one of said solutions with ultraviolet light;

(E) a means for regenerating the concentrated and dilute ionic solutions from solutions exiting from the reverse electrodialysis unit; and

(F) a recycling means for recycling said regenerated concentrated and dilute solutions back to said first and second pathways in the reverse electrodialysis unit.

Preferably the device further comprises a concentrated ionic solution and a dilute ionic solution, preferably such solutions being as hereinbefore described in relation to the first aspect of the present invention.

The means for feeding the ionic solutions into the pathways are typically inlets, e.g. pipes. The pathways through the electrodialysis unit typically comprise a cationic exchange membrane wall and an anionic exchange membrane wall. As ionic solutions pass along the pathways, ions migrate through the membrane walls from the concentrated ionic solution to the dilute ionic solution in order to bring the solute concentrations of the two ionic solutions closer together, thereby generating electricity. As a consequence, during the method the solute concentration of the dilute ionic solution increases and the solute concentration of the concentrated ionic solution decreases.

The means for bringing at least one of said solutions into contact with a biocide is preferably a dosing system, e.g. a dosing system which adds biocide to the concentrated solution and/or the dilute solution.

The means for irradiating at least one of said solutions is preferably a UV lamp, preferably a lamp as hereinbefore described in relation to the method of the present invention.

The means for alternating which of the first and second pathways carry the said concentrated ionic solution and the dilute ionic solution is typically a valve. Such a valve can be used to switch the flows of concentrated ionic solution and the dilute ionic solution from one pathway to another.

Optionally the device further comprises a means for heating one or both of the ionic solutions. This means is not particularly limited. However to take full advantage of the present invention, one will preferably use an inexpensive means, for example a heating element which is heated by waste or unwanted heat, e.g. solar power, heat from a geothermal source or waste heat from a power station or cooling tower.

In one embodiment the heating element is heated by thermal energy obtained from a photovoltaic ("PV") power generation system. Since increased temperature has a deleterious effect on many PV power generation systems and requires cooling of solar cells, coupling the use of unwanted heat from such a system to drive efficient production of electricity creates a more efficient system. Also, many large solar concentrated thermal power plants (CSP) that are operating at high temperature such as steam turbine-generators or Stirling engine generators, produce unwanted, waste heat at 40 to 90°C, which can be effectively and efficiently used to generate power by the present invention.

Any waste heat from industries (e.g. from power plants) or geothermal energy could also be utilized in the present method and device.

By utilizing waste energy (i.e., heat) produced through the operation of solar thermal or (concentrated) photovoltaic cell electricity-producing systems to increase the temperature of the concentrated ionic stream, some of the efficiency typically lost through the operation of solar thermal or photovoltaic cell electricity- producing systems can be recovered.

The means for regenerating the concentrated and dilute ionic solutions from solutions exiting from the reverse electrodialysis unit preferably comprises a heating element for evaporating liquid and optionally a cooling element for condensing evaporated liquid. The liquid typically is or comprises one or both of the solutions exiting the reverse electrodialysis unit. The cooling element, when present, optionally comprises recirculating water which cools vapour obtained from evaporating the aforementioned liquid.

The means for regenerating the concentrated and dilute ionic solutions from solutions exiting from the reverse electrodialysis unit preferably comprises a distillation unit which is capable of:

(i) evaporating liquid (e.g. water) from one or both of the solution(s) exiting the RED unit and condensing the evaporated liquid to give an ion-free liquid and a concentrate;

(ii) mixing of the ion-free liquid with one or both of the solution(s) exiting the RED unit, thereby regenerating the dilute ionic solution; and

(iii) mixing of the concentrate with one or both of the solution(s) exiting the RED unit, thereby regenerating the concentrated ionic solution.

If desired the reverse electro-dialysis device may comprise an integral heat exchange element. Likewise, a second heat transfer fluid may be cooled by heat exchange with ambient air or another cooler environment such as the subsurface soil. Non-limiting Examples of how the invention may be put into effect are described below.

EXAMPLE 1

A reverse electrodialysis unit may be modified to include a valve which periodically alternates which of the pathways the concentrated and dilute ionic streams pass through. Cation and anion exchange membranes comprising a porous support and an ionically charged polymeric layer may be obtained from FUJIFILM Manufacturing Europe b.v.. One may use titanium mesh electrodes, coated with Ru-lr mixed metal oxides (MMO) with dimensions of 12.5 cm x 8.0 cm (De Nora Tech, Inc., USA).

Dilute (0.1 mol/l, 20°C) and concentrated (5mol/l, 50°C) aqueous ammonium acetate solutions may be recycled through the device illustrated in FIG. 1 , using waste heat from a power station to power the regeneration unit 203. Periodically the pathways through which concentrated and dilute ionic streams pass through the RED unit are alternated.

Example 2

Example 1 may be repeated except that instead of alternating which of the pathways the concentrated and dilute ionic streams pass through, the concentrated and dilute ionic streams contain 50-60 ppm dissolved chlorine gas (obtained by adding sodium hypochlorite to the solution).

Example 3

Example 1 may be repeated except that instead of alternating which of the pathways the concentrated and dilute ionic streams pass through, the RED unit is fitted with a UV-C Miracle Wand lamp obtained from MBR Imports UK Ltd. This UV-C lamp is used to periodically irradiate the concentrated and dilute ionic streams.

Claims

1 . A method for generating electricity comprising the steps:

wherein:

(i) solute from the concentrated ionic solution migrates through the membranes to the dilute ionic solution;

(ii) the method comprises the further step (D) of alternating which of the first and second pathways carries the said concentrated and dilute ionic solutions, and/or bringing at least one of said solutions into contact with a biocide and/or irradiating at least one of said solutions with ultraviolet light.

2. The method according to claim 1 which comprises the step (D) of alternating which of the first and second pathways carries the said concentrated and dilute ionic solutions.

3. The method according to claim 1 or 2 which comprises the step of bringing at least one of said solutions into contact with a biocide.

4. The method according to claim 3 wherein the biocide is selected from chlorine, bromine, biguanide salts, peroxy compounds, ozone, quaternary ammonium compounds, silver ions, silver nanoparticles and combinations comprising two or more thereof.

5. The method according to any one of the preceding claims which comprises the step of irradiating at least one of said solutions with ultraviolet light.

6. The method according to any one of the preceding claims which comprises the step of irradiating at least one of said solutions with UV-C light.

7. The method according to any one of the preceding claims wherein the concentrated ionic solution and the dilute ionic solution differ in temperature by 5°C or more as they enter the RED unit.

8. The method according to any one of the preceding claims wherein the dilute ionic solution and the concentrated ionic solution each independently have a temperature of 30 to 150°C as they enter the electrodialysis unit.

9. The method according to any one of the preceding claims wherein the ionic solutions comprise a salt and water, wherein the salt comprises a cation selected from sodium, potassium, lithium, caesium, rubidium and/or ammonium and an anion selected from azide, bromide, carbonate, chloride, chlorate, fluoride, iodide, hydrogen carbonate, nitrate, nitrite, perchlorate, selenide, thiocyanate, thiosulphate, acetate and/or formate.

10. The method according to any one of the preceding claims wherein the ionic solutions comprise a salt and water, wherein the salt comprises sodium bromide, sodium chlorate, sodium iodide, sodium nitrate, sodium nitrite, sodium formate, sodium perchlorate, potassium carbonate, potassium formate, potassium acetate, potassium fluoride, potassium iodide, potassium nitrite, potassium nitrate, potassium thiosulfate, potassium thiocyanate, lithium nitrate, lithium nitrite, lithium acetate, ammonium formate, ammonium acetate, ammonium iodide, ammonium nitrate, ammonium thiosulfate and/or ammonium thiocyanate.

1 1 . The method according to any one of the preceding claims which is performed such that the following equation is satisfied:

S1/S2 > Y

wherein:

S1 is the mol/l concentration of solute in the concentrated ionic solution as it enters the reverse electrodialysis unit;

S2 is the mol/l concentration of solute in the dilute ionic solution as it enters the reverse electrodialysis unit; and

Y is at least 30.

12. The method according to any one of the preceding claims wherein the concentration of the dilute ionic solution is 0.06 to 2.5mol/l as it enters the reverse electrodialysis unit.

13. The method according to any one of the preceding claims wherein the concentrated solution and/or dilute solution comprise one or more surfactant, dispersant, pH buffer, anti-foam ing agent and/or anti-corrosive agent.

14. The method according to any one of the preceding claims wherein the flow velocity of the ionic solutions through the pathways is less than 4 cm/s.

15. The method according to any one of the preceding claims wherein at least one of the concentrated and dilute ionic solutions comprise water and at least 0.02 mol/l of a solute other than sodium chloride.

16. The method according to any one of the preceding claims wherein at least half of the membranes have a thickness, including the support (when present) of less than 250pm.

17. The method according to any one of the preceding claims wherein at least half of the cation exchange membranes comprise an ionically-charged polymeric layer obtained from a curable composition comprising:

(i) 2.5 to 70 wt% crosslinker comprising at least two acrylamide groups;

(iii) 5 to 45 wt% solvent; and

(iv) 0 to 10 wt% of free radical initiator.

18. The method according to any one of the preceding claims wherein at least half of the anion exchange membranes comprise an ionically-charged polymeric layer obtained from a curable composition comprising:

(i) 2.5 to 70 wt% crosslinker comprising at least two acrylamide groups;

(iii) 10 to 70 wt% solvent; and

(iv) 0 to 10 wt% of free radical initiator.

19. A device for generating electricity comprising:

(B) means for feeding a concentrated ionic solution into said first pathway; (C) a means for feeding a dilute ionic solution into said second pathway, whereby solute from the concentrated solution in the first pathway migrates through the membranes to the dilute solution in the second pathway, thereby generating electricity;

(E) a means for regenerating concentrated and dilute ionic solutions from the solutions exiting from the reverse electrodialysis unit; and

(F) a recycling means for recycling said regenerated concentrated and dilute solutions back through the reverse electrodialysis unit.

20. A device according to claim 19 which further comprises a concentrated ionic solution and a dilute ionic solution.

21 . A device according to claim 20 wherein the ionic solutions comprise water and a salt which has solubility in water at 20°C of at least 8mol/kg of water.

22. A device according to claim 20 wherein the ionic solutions comprise a salt and water, wherein the salt comprises a cation selected from sodium, potassium, lithium, caesium, rubidium and/or ammonium and an anion selected from azide, bromide, , carbonate, chloride, chlorate, fluoride, iodide, hydrogen carbonate, nitrate, nitrite, perchlorate, selenide, thiocyanate, thiosulfate, acetate and/or formate.

23. A device according to claim 20 wherein the ionic solutions comprise a salt and water, wherein the salt comprises sodium bromide, sodium chlorate, sodium iodide, sodium nitrate, sodium nitrite, sodium formate, sodium perchlorate, potassium carbonate, potassium formate, potassium acetate, potassium fluoride, potassium iodide, potassium nitrite, potassium nitrate, potassium thiosulfate, potassium thiocyanate, lithium nitrate, lithium nitrite, lithium acetate, ammonium formate, ammonium acetate, ammonium iodide, ammonium nitrate, ammonium thiosulfate and/or ammonium thiocyanate.

24. A device according to any one of claims 20 to 23 wherein the concentrated ionic solution is present in the means for introducing the concentrated ionic solution into said first pathway and the dilute ionic solution is present in the means for introducing the dilute ionic solution into said second pathway and the following equation is satisfied: S1/S2 > Y

wherein:

S1 is the mol/l concentration of solute in the said concentrated ionic solution;

S2 is the mol/l concentration of solute in the said dilute ionic solution; and

Y is at least 30.

25. A device according to any one of claims 20 to 24 which comprises a means for heating one of the ionic solutions to a temperature higher than the other ionic solution and/or for irradiating at least one of the ionic solutions with ultraviolet light.

26. Use of the device according to any of claims 19 to 25 for the generation of electrical energy.