WO2018229505A1 - Electricity generation though reverse electrodialysis - Google Patents

Electricity generation though reverse electrodialysis Download PDF

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Publication number
WO2018229505A1
WO2018229505A1 PCT/GB2018/051660 GB2018051660W WO2018229505A1 WO 2018229505 A1 WO2018229505 A1 WO 2018229505A1 GB 2018051660 W GB2018051660 W GB 2018051660W WO 2018229505 A1 WO2018229505 A1 WO 2018229505A1
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ionic solution
concentrated
dilute
mol
reverse electrodialysis
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PCT/GB2018/051660
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French (fr)
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WO2018229505A9 (en
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Willem VAN BAAK
Arash EMDADI
Marina MICARI
Maurizio BEVACQUA
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Fujifilm Manufacturing Europe Bv
Fujifilm Imaging Colorants Limited
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Publication of WO2018229505A1 publication Critical patent/WO2018229505A1/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/42—Electrodialysis; Electro-osmosis ; Electro-ultrafiltration; Membrane capacitive deionization
    • B01D61/422—Electrodialysis
    • B01D61/423—Electrodialysis comprising multiple electrodialysis steps
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/22—Fuel cells in which the fuel is based on materials comprising carbon or oxygen or hydrogen and other elements; Fuel cells in which the fuel is based on materials comprising only elements other than carbon, oxygen or hydrogen
    • H01M8/227—Dialytic cells or batteries; Reverse electrodialysis cells or batteries
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/42—Electrodialysis; Electro-osmosis ; Electro-ultrafiltration; Membrane capacitive deionization
    • B01D61/422—Electrodialysis
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
    • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
    • C02F1/46104—Devices therefor; Their operating or servicing
    • C02F1/46176—Galvanic cells
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
    • C02F1/469—Treatment of water, waste water, or sewage by electrochemical methods by electrochemical separation, e.g. by electro-osmosis, electrodialysis, electrophoresis
    • C02F1/4693—Treatment of water, waste water, or sewage by electrochemical methods by electrochemical separation, e.g. by electro-osmosis, electrodialysis, electrophoresis electrodialysis
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0002—Aqueous electrolytes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • This invention relates to a method and apparatus for generating electricity using reverse electrodialysis ("RED").
  • electricity may be generated from the free energy of mixing of two ionic solutions by reverse electrodialysis.
  • This technique uses a reverse electrodialysis unit comprising a membrane stack having alternating cation and anion exchange membranes, an electrode 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.
  • the concentrated ionic solution is introduced into the first pathway
  • the dilute ionic solution is introduced into the second pathway
  • solute from the concentrated solution in the first pathway passes 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 to separate the two solutions in the membrane stack.
  • WO2014080188 suggests the use in RED of a dilute ionic solution having solute content of at least 0.03mol/l as it enters the RED unit.
  • An object of the present invention is to provide a method and apparatus for improving the amount of energy which may be generated by RED.
  • a method for generating electricity comprising the steps:
  • the concentrated ionic solution and the dilute ionic solution each comprise a main salt; and ( ⁇ ) the main salt present in each of the concentrated ionic solution and the dilute ionic solution as they enter the reverse electrodialysis unit is such that Equation 1 is satisfied: ACc/ACd is at least 1 .5
  • ACc is the activity coefficient of the main salt present in the concentrated ionic solution when measured at a concentration of 6 mol/kg water at 25°C; and ACd is the activity coefficient of the main salt present in the dilute ionic solution when measured at a concentration of 0.5 mol/kg water at 25°C.
  • the main salt present in an ionic solution is the salt which is present in that solution in highest number of moles per kg solvent.
  • the ionic solution contains two or more salts in identical mol/kg amounts (calculated as mol/kg solvent) and all other salt(s) (if any) are present in lower amounts then the main salts are the salts having the highest identical mol/kg amounts, i.e. Equation 1 applies equally to each of the main salts having the identical, highest amounts.
  • the activity coefficient of the main salt when measured at a concentration of 6 mol/kg water at 25°C is preferably at least 1 .0, more preferably at least 1 .2.
  • a high activity coefficient for the salt in the concentrated ionic solution is desired because it correlates with a high power density.
  • the ratio of ACc/ACd is preferably at least 1 .7, e.g. from 1 .8 to 10.
  • the activity coefficient of a salt may be determined by, for example, the methods described by Bromley, by Hamer and Wu or by Staples. The method of Bromley is preferred, as described in detail in AIChE Journal (Vol. 19, No. 2, page 313-320).
  • the activity coefficient (AC) for monovalent salts can be determined using Equation 2 below.
  • Log AC -0.51 1 x M 1 2 /(1 + M 1 2 ) + (0.06 + 0.6B) x M/(1 + 1.5M) 2 + B x M
  • Equation 2 wherein M is the molar concentration of the salt (in mol/kg solvent) and B is a value determined experimentally as listed by Bromley in AIChE Journal (Vol. 19, No. 2, page 313-320).
  • the total concentration of solute in the dilute ionic solution as it enters the reverse electrodialysis unit is at least 0.03mol/kg solvent.
  • the preferred concentrations mentioned herein refer to the concentrations when the process is being performed in the steady state (at start-up the concentrations may be slightly different due to mixing with liquid already in the RED device).
  • the method preferably further comprises the step (C) of regenerating the concentrated and dilute ionic solutions from the solutions exiting from the reverse electrodialysis unit and recycling said regenerated concentrated and dilute solutions back through said respective first and second pathways in the reverse electrodialysis unit.
  • the method may be operated as a continuous process wherein the steady state is the stable process condition after the start-up phase.
  • the process is executed at a high temperature, e.g. at 40°C or higher.
  • a high temperature e.g. at 40°C or higher.
  • the amount of generated electricity is higher than when the process is executed at room temperature or lower.
  • the method is not restricted to locations where there are large quantities of suitable mixing pairs in close proximity, i.e. concentrated and dilute ionic solutions. On the contrary, the present process can be used much more widely. In addition, the above-described method obviates the problems of transporting mixing pairs to the RED unit, and of disposing of spent brine.
  • An additional advantage of regenerating the concentrated and dilute ionic solutions is that fouling by contaminants from the feed streams can be prevented.
  • the ionic solutions are not obtained directly from natural sources, e.g. from a river water, ground water or sea water.
  • natural sources e.g. from a river water, ground water or sea water.
  • solute(s) is not limited to what is available from external sources but can be optimized to obtain a higher electrical energy production.
  • the heat source which may optionally be used to heat the concentrated and dilute ionic solutions is not particularly limited and it includes, for example, solar energy and unwanted and low grade heat, e.g. waste heat from power plants or the heat generated in industrial cooling towers or manufacturing processes.
  • the same heat source can also be used in optional 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.
  • step (C) can take place at relatively low temperature differences, the process is particularly useful where low temperature heat sources are available, such as solar energy, geothermal energy, and waste heat.
  • the regeneration which occurs in optional step (C) may be done by a number of techniques, with thermal regeneration being preferred.
  • the process is performed without mixing of the (raw) streams exiting the reverse electrodialysis unit.
  • one may pass (parts of) one or both of the streams exiting the reverse electrodialysis unit into regeneration unit(s) which (further) heat the stream(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 step preferably comprises:
  • liquid e.g. water
  • step iii) 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).
  • step iii) 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).
  • An example of a suitable thermal regeneration system is membrane distillation (MD).
  • the electrodes are located at each end of the stack.
  • both the concentrated and the dilute ionic solution are 'hot', e.g. each having a temperature of at least 40°C, e.g. at least at 50°C, at least at 60°C or at least at 80°C and optionally up to 100°C.
  • the power output of the RED device is increased.
  • solubility of solute(s) and the ion transport through the membranes are higher for hot solutions than for cold solutions.
  • one or both of the solutions are at a high temperature, e.g. higher than 40°C.
  • the solutions likely will reach thermal equilibrium rapidly.
  • activity coefficients generally tend to be somewhat lower at higher temperatures, the diffusivity of salts increases significantly at higher temperatures and this can often more than compensate for the reduction in activity coefficient.
  • the ionic solutions may 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.
  • the ionic solutions optionally comprise one or more than one salt.
  • Preferred salts have a high solubility in the liquid medium in which they are dissolved.
  • the terms "solute” and “salt” are often used interchangeably.
  • the ionic solutions typically comprise solute (i.e. one or more salts), water and optionally a water-miscible organic solvent.
  • the salt(s) present in the concentrated ionic solution may be the same as the salts used in the dilute ionic solution or they may be different to the salts used in the dilute ionic solution.
  • the dilute ionic solution may comprise a salt that cannot pass through the membranes to the first pathway.
  • the salt that cannot pass through the membranes to the first pathway has the function of lowering the electrical resistance of the dilute ionic solution.
  • the main salt present in the concentrated ionic solution is the same as the main salt present in the dilute ionic solution.
  • Table 1 shows the activity coefficient of various salts at different ionic concentrations (at 6 mol/kg water and at 0.5 mol/kg water) at 25°C and the solubility in water at 20°C (S 20 c) and at 80°C (S 80 c):
  • the salts listed in Table 1 which have an ACc/ACd ratio of at least 1 .5 are suitable for use as the main salt in both the concentrated ionic solution and the dilute ionic solution of the same RED device due to the significant difference in activity coefficients at 6 mol/kg vs. 0.5 mol/kg.
  • the main salt present in the concentrated ionic solution has a solubility in water (Ssoc) of at least 8 mol/kg water, more preferably at least 10 mol/kg water, especially at least 15 mol/kg water, more especially at least 20 mol/kg water, when measured at 80°C.
  • the concentrated ionic solution comprises (in the steady state) at least 4 mol/kg solvent (more preferably at least 5 mol/kg solvent (e.g. 5 to 40 mol/kg solvent) especially at least 6 mol/kg solvent (e.g. 6 to 20 mol/kg solvent) or more especially at least 8 mol/kg solvent (e.g. 8 to 15 mol/kg solvent)) of the main salt as it enters the reverse electrodialysis unit.
  • the total concentration of salts present in the concentrated ionic solution (in the steady state) as it enters the reverse electrodialysis unit is preferably at least 4 mol/kg solvent (more preferably at least 5 mol/kg solvent (e.g. 5 to 40 mol/kg solvent) especially at least 6 mol/kg solvent (e.g. 6 to 20 mol/kg solvent) or more especially at least 8 mol/kg solvent (e.g. 8 to 15 mol/kg solvent)).
  • the main salt present in the dilute ionic solution is identical to the main salt present in the concentrated ionic solution.
  • Preferred main salts include the potassium, caesium, rubidium, ammonium and tetramethylammonium salts of formic acid, acetic acid and fluoride anion (except potassium fluoride), the sodium salt of acetic acid, sodium bromide, sodium iodide, sodium thiocyanate and all lithium salts except lithium fluoride. Less preferred is lithium acetate.
  • the main salt present in the concentrated ionic solution is preferably selected from ammonium acetate, tetramethylammonium fluoride, caesium acetate, caesium fluoride, caesium formate, lithium acetate, lithium bromide, lithium chloride, lithium chlorate, lithium iodide, lithium nitrate, lithium perchlorate, lithium thiocyanate, potassium acetate, potassium formate, sodium acetate, sodium bromide, sodium iodide, sodium thiocyanate, rubidium acetate, rubidium formate and rubidium fluoride, all of which satisfy Equation 1 above as can be seen from Table 1 above.
  • the main salt(s) present in the concentrated ionic solution and in the dilute ionic solution are monovalent. This preference arises because in the concentrated ionic solution monovalent ions generate more power than multivalent ions.
  • the dilute ionic solution comprises at least
  • the total concentration of salts present in the dilute ionic solution as it enters the reverse electrodialysis unit is (in the steady state) preferably at least 0.03 mol/kg solvent.
  • the initial concentration of the main salt in the dilute ionic solution may be lower and even close to zero since in the RED device solute will diffuse from the concentrated ionic solution to the dilute ionic solution rather rapidly.
  • the steady state it is not practical to maintain a very low concentration in the dilute ionic solution because this requires more regeneration effort than necessary so an optimum will be selected taking the degree of regeneration and magnitude of concentration difference between dilute and concentrated ionic solutions into account.
  • a very low concentration of ions in the dilute ionic solution i.e. less than 0.03 mol/kg solvent) may increase the electrical resistance of the stack resistance which is not desirable.
  • the concentration of the solute in the concentrated ionic solution gradually decreases due to diffusion of ions from the concentrated ionic solution to the dilute ionic solution.
  • concentration of solute in the dilute ionic solution typically increases as it passes through the RED device, e.g. to 0.5 mol/kg solvent, or to 1.0 mol/kg solvent or to an even higher concentration.
  • the ionic solutions optionally comprise one or more than one salt (as the solute), e.g. two or more salts.
  • the ionic solutions comprise water and essentially one salt. All other salts are then present in only trace amounts or are absent.
  • Preferred salts comprise a cation which has an (unhydrated) ionic radius larger than 0.09nm.
  • 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).
  • Preferred salts comprise an anion which has an (unhydrated) ionic radius larger 0.12 nm.
  • Suitable anions include fluoride (ionic radius 0.13 nm), chloride (ionic radius 0.18 nm), bromide (ionic radius 0.20 nm), iodide (ionic radius 0.22 nm), nitrate (ionic radius 0.20 nm), chlorate, perchlorate (ionic radius 0.24 nm), thiocyanate (ionic radius 0.21 nm), formate (ionic radius 0.20 nm) and acetate (ionic radius 0.23 nm).
  • the number of moles solute/kg solvent is the total for all of the salts present.
  • the solvent preferably comprises water and optionally a water miscible (organic) solvent.
  • the total concentration of solute (i.e. all salts) in the concentrated ionic solution as it enters the reverse electrodialysis unit is in the steady state preferably 1 .5 to 80mol/kg solvent, especially 3 to 75mol/kg solvent.
  • the total concentration of solute (i.e. all salts) in the dilute ionic solution as it enters the reverse electrodialysis unit is in the steady state preferably 0.03 to 1 mol/kg solvent, especially 0.05 to 0.8mol/kg solvent, e.g. 0.05 to 0.3mol/kg solvent.
  • the identity of the solute present in the concentrated ionic solution and the dilute ionic solution are substantially the same.
  • solute e.g. salt
  • solute e.g. salt
  • Y is at least 30.
  • Y is at least 40, more preferably at least 50, especially at least
  • Y is less than 1000, more preferably less than 200, especially less than 150.
  • the temperature of the concentrated ionic solution and/or of the dilute ionic solution as it enters the reverse electrodialysis unit is preferably 40 to 100°C, more preferably 50 to 100°C, especially 60 to 95°C, more especially 70 to 90°C, e.g. 75°C, 80°C or 85°C.
  • 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.
  • the stack further comprises spacers which keep the cationic and anionic membranes apart, thereby creating a pathway between said membranes.
  • the spacers are ion-conducting and keep the cationic and anionic membranes apart creating pathways of average width 50 to 400 pm, more preferably 100 to 250 ⁇
  • the membranes Preferably at least one, more preferably at least half, especially all of the membranes have an electrical resistance below 1 .5 ohm. cm 2 , more preferably below 1 .0 ohm. cm 2 .
  • the membranes are stable at high temperatures (e.g. the permselectivity of the membranes does not deteriorate in time, especially at high concentrations).
  • the permselectivity reduces by less than 5% when the membranes are used continuously at a temperature of 80°C for a period of 16 hours.
  • the membranes are optimized for the concentration and salt(s) which are present in the streams which come into contact with the membrane.
  • Preferred membranes have a low electrical resistance (e.g.
  • a high permselectivity e.g. a permselectivity of more than 80%, more preferably more than 90%
  • the flow velocity of the ionic solutions through the pathways is less than 4 cm/s, more preferably less than 2 cm/s, especially less than 1 .5 cm/s; the velocity may even be as low as 0.2, 0.4 or 0.6 cm/s.
  • 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.
  • 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;
  • the concentrated ionic solution and the dilute ionic solution each comprises a main salt; and ( ⁇ ) the main salt present in each of the concentrated ionic solution and the dilute ionic solution as they enter the reverse electrodialysis unit is such that following Equation 1 is satisfied: ACc/ACd is at least 1 .5
  • ACc is the activity coefficient of the main salt present in the concentrated ionic solution when measured at a concentration of 6 mol/kg water at 25°C; and ACd is the activity coefficient of the main salt present in the dilute ionic solution when measured at a concentration of 0.5 mol/kg water at 25°C.
  • the means for introducing 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.
  • ions migrate through the membrane walls from the concentrated ionic solution to the dilute ionic solution in order to bring the concentrations of the two ionic solutions closer together, thereby generating electricity.
  • solute concentration of the dilute ionic solution increases and the solute concentration of the concentrated ionic solution decreases.
  • Fig. 1 shows a reverse electrodialysis unit which may be used to perform the method of the present invention
  • Fig. 2 shows the power densities achieved using various salts at an S1/S2 ratio of 12 when the ionic solutions were at a temperature of at 80°C.
  • Fig. 3 shows the power densities achieved using various salts at an S1/S2 ratio of 12 when the ionic solutions were at a temperature of at 20°C.
  • Fig. 4 shows the power densities achieved using various salts at an S1/S2 ratio of 1 10 when the ionic solutions were at a temperature of at 80°C.
  • the reverse electrodialysis unit shown in Fig. 1 comprises a membrane stack (1 ) comprising a first pathway for a concentrated ionic solution, a second pathway for a dilute ionic solution, electrodes and alternating cation and anion exchange membranes (not shown).
  • Dilute ionic solution (14) and concentrated ionic solution (15) are brought to the desired temperature in water bath (2) with stirring using mixers (3) and (4) respectively.
  • the dilute ionic solution (14) and the concentrated ionic solution (15) are then pumped into the stack (1 ) using pumps (6) and (7).
  • the reverse electrodialysis unit further comprises multimeter (9) for measuring the current and multimeter (10) for measuring the potential and external load (5) provides a varying resistance. Dilute ionic solution which has exited the stack passes to vessel (12) and concentrated ionic solution which has exited the stack passes to vessel (13).
  • the means for heating the concentrated ionic solution and/or the dilute ionic solution 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.
  • 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.
  • 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.
  • PV photovoltaic
  • CSP solar concentrated thermal power plants
  • waste heat from industries e.g. from power plants
  • geothermal energy could also be utilized in the present process and device.
  • the means for regenerating the concentrated and dilute ionic 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:
  • evaporating liquid e.g. water
  • condensing the evaporated liquid to give an ion-free liquid and a concentrate
  • the reverse electro-dialysis device may comprise an integral heat exchange element.
  • a second heat transfer fluid may be cooled by heat exchange with ambient air or another cooler environment such as the subsurface soil.
  • the concentrated and dilute ionic solutions are regenerated using an electrodialysis (ED) device, for example by a process comprising separating at least a part of the dilute ionic solution and/or the concentrated ionic solution both exiting the reverse electrodialysis into a dilute and concentrate stream.
  • the ED device uses an electrical field to force ions to flow from a dilute ionic solution to a more concentrated.
  • the RED unit and the ED device each comprise membrane stacks and the membrane stacks present in the RED unit are different from the membrane stacks present in the ED device.
  • excess electricity is used, e.g. during periods that the production of electrical energy exceeds the demand. During periods that the demand exceeds the production capacity the RED unit can be activated to produce additional electrical energy.
  • excess energy may be stored and used to assist the regeneration step at a different point in time. The use of excess electricity in this way makes the method particularly flexible.
  • the reverse electro-dialysis device in combination with storage tanks may be used to store energy for use in periods when there is high demand for electrical energy.
  • a third aspect of the present invention there is provided the use of the device according to the second aspect of the present invention for the generation and/or storage of electrical energy.
  • a RED unit comprised a CS100 membrane stack obtained from Deukum, Germany, having an anode, a cathode, a first pathway for a concentrated ionic solution, a second pathway for a dilute ionic solution.
  • the stack comprised 10 cell pairs (10 for each pathway with the first and second pathways in alternating order), each cell pair comprising an anion exchange membrane (AEM) and a cation exchange membrane (CEM) which together define a pathway and together with an adjacent cell pair a second pathway.
  • AEM anion exchange membrane
  • CEM cation exchange membrane
  • the effective cross-sectional area of the AEMs and CEMs was 10x10cm to give effective membrane surface area of 0.01 m 2 .
  • the membrane stack further comprised two electrolyte compartments, one at each side of the membrane stack.
  • the electrolyte compartments each comprised two CEMs to reduce leakage of electrolyte out of the electrolyte compartments.
  • the 10 AEMs and 13 CEMs were commercially available Type 10 membranes provided by Fujifilm, The Netherlands.
  • Each cell comprised a spacer of thickness 310 pm having a silicon gasket and a PE netting, and was obtained from Deukum, Germany.
  • the RED unit comprising the membrane stack is illustrated in Fig. 1 .
  • Concentrated ionic solutions were prepared comprising 6 mol of each of the salts indicated in Table 2 below per kg of water.
  • Dilute ionic solutions were prepared comprising 0.5 mol of each of the salts indicated in Table 2 below per kg of water.
  • the solubility of each salt at 20°C and 80°C is shown in Table 2 below:
  • the dilute ionic solution was pumped into the stack at a rate of 203 rpm and the concentrated ionic solution was pumped into the stack at a rate of 373 rpm corresponding to a flow velocity of 2.0 cm/s and 3.5 cm/s respectively.
  • An electrolyte solution (0.1 M potassium hexacyanoferrate (II), 0.1 M potassium hexacyanoferrate (III) and 0.25M NaCI) was circulated through the electrolyte compartments at each end of the stack using a third MasterFlex peristaltic pump at a rate of 345 ml/min.
  • An external load (Codix from Kubler, Germany) was applied at a range of resistance values (from 0.5 to 19 ohm).
  • the stack resistance and Power Density (PD) were determined according to the method as described in Veerman et al, Journal of Membrane Science, 327, (2009), 136-144. The data were not corrected for the so-called blank resistance of the membrane stack.
  • the membranes were equilibrated by filling the membrane stack with concentrated ionic solution and applying a current of 0.1 A for one minute using a PGSTAT 204 galvanostat from Autolab, The Netherlands.
  • OCV Open Circuit Voltage
  • Fig. 2 The PD of the salts listed in Table 2 above at 80°C versus applied voltage potential are shown in Fig. 2 and in Fig. 3 are shown the PD of NaNO 3 , NaCI and LiBr at 20°C.
  • Fig. 2 and Fig. 3 the salts having an ACc/ACd falling outside of claim 1 produced a lower PD than the salts falling within claim 1 .
  • the PD achieved using ionic solutions at 80°C was much higher than the PD achieved using ionic solutions at 20°C.
  • the membrane stack comprised 40 cell pairs
  • the dilute ionic solution had a concentration of 0.05 mol/kg water
  • the concentrated ionic solution had a concentration of 5.5 mol/kg water; and • the spacer for the second pathway (for the dilute ionic solution) had a thickness of 150pm (from Deukum, Germany) to reduce the resistance of the second pathway.
  • the spacer in the first pathway was not changed.

Abstract

A method for generating electricity comprising the steps: (A) passing 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) passing a dilute ionic solution through a second pathway in said reverse electrodialysis unit, whereby solute from the concentrated solution in the first pathway passes through the membranes to the dilute solution in the second pathway, thereby generating electricity; wherein: (i) the concentrated ionic solution and the dilute ionic solution each comprise a main salt; and (ii) the main salt present in each of the concentrated ionic solution and the dilute ionic solution as they enter the reverse electrodialysis unit is such that Equation 1 is satisfied: ACc/ACd is at least 1.5 (Equation 1) wherein ACc and ACd are the activity coefficients of the main salts present in the concentrated and dilute ionic solutions when measured at a concentration of 6 mol/kg water at 25°C and at a concentration of 0.5 mol/kg water at 25°C respectively.

Description

ELECTRICITY GENERATION THOUGH REVERSE ELECTRODIALYSIS
This invention relates to a method and apparatus for generating electricity using reverse electrodialysis ("RED").
As is known, electricity may be generated from the free energy of mixing of two ionic solutions by reverse electrodialysis. This technique uses a reverse electrodialysis unit comprising a membrane stack having alternating cation and anion exchange membranes, an electrode 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 introduced into the first pathway, and the dilute ionic solution is introduced into the second pathway, solute from the concentrated solution in the first pathway passes 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 to separate the two solutions in the membrane stack.
Several documents describe the use of RED at coastal areas where seawater and river water provide the concentrated and dilute ionic streams respectively.
However the concentration difference between seawater and river water is rather small and this small difference limits the power density which can be produced from them to about 1 W/m2 of membrane surface.
WO2014080188 suggests the use in RED of a dilute ionic solution having solute content of at least 0.03mol/l as it enters the RED unit.
An object of the present invention is to provide a method and apparatus for improving the amount of energy which may be generated by RED.
According to a first aspect of the present invention, there is provided a method for generating electricity comprising the steps:
(A) passing 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) passing a dilute ionic solution through a second pathway in said reverse electrodialysis unit, whereby solute from the concentrated ionic solution in the first pathway passes through the membranes to the dilute ionic solution in the second pathway, thereby generating electricity;
wherein:
(i) the concentrated ionic solution and the dilute ionic solution each comprise a main salt; and (ϋ) the main salt present in each of the concentrated ionic solution and the dilute ionic solution as they enter the reverse electrodialysis unit is such that Equation 1 is satisfied: ACc/ACd is at least 1 .5
(Equation 1 )
wherein:
ACc is the activity coefficient of the main salt present in the concentrated ionic solution when measured at a concentration of 6 mol/kg water at 25°C; and ACd is the activity coefficient of the main salt present in the dilute ionic solution when measured at a concentration of 0.5 mol/kg water at 25°C.
The main salt present in an ionic solution is the salt which is present in that solution in highest number of moles per kg solvent. Where the ionic solution contains two or more salts in identical mol/kg amounts (calculated as mol/kg solvent) and all other salt(s) (if any) are present in lower amounts then the main salts are the salts having the highest identical mol/kg amounts, i.e. Equation 1 applies equally to each of the main salts having the identical, highest amounts.
The activity coefficient of the main salt when measured at a concentration of 6 mol/kg water at 25°C is preferably at least 1 .0, more preferably at least 1 .2. A high activity coefficient for the salt in the concentrated ionic solution is desired because it correlates with a high power density.
The ratio of ACc/ACd is preferably at least 1 .7, e.g. from 1 .8 to 10.
The activity coefficient of a salt may be determined by, for example, the methods described by Bromley, by Hamer and Wu or by Staples. The method of Bromley is preferred, as described in detail in AIChE Journal (Vol. 19, No. 2, page 313-320).
The activity coefficient (AC) for monovalent salts can be determined using Equation 2 below. Log AC = -0.51 1 x M1 2/(1 + M1 2) + (0.06 + 0.6B) x M/(1 + 1.5M)2 + B x M
(Equation 2) wherein M is the molar concentration of the salt (in mol/kg solvent) and B is a value determined experimentally as listed by Bromley in AIChE Journal (Vol. 19, No. 2, page 313-320).
Preferably the total concentration of solute in the dilute ionic solution as it enters the reverse electrodialysis unit (when measured in the steady state) is at least 0.03mol/kg solvent.
The preferred concentrations mentioned herein refer to the concentrations when the process is being performed in the steady state (at start-up the concentrations may be slightly different due to mixing with liquid already in the RED device).
The method preferably further comprises the step (C) of regenerating the concentrated and dilute ionic solutions from the solutions exiting from the reverse electrodialysis unit and recycling said regenerated concentrated and dilute solutions back through said respective first and second pathways in the reverse electrodialysis unit. In this way, the method may be operated as a continuous process wherein the steady state is the stable process condition after the start-up phase.
In a preferred embodiment, the process is executed at a high temperature, e.g. at 40°C or higher. At a high temperature the amount of generated electricity is higher than when the process is executed at room temperature or lower.
The method is not restricted to locations where there are large quantities of suitable mixing pairs in close proximity, i.e. concentrated and dilute ionic solutions. On the contrary, the present process can be used much more widely. In addition, the above-described method obviates the problems of transporting mixing pairs to the RED unit, and of disposing of spent brine.
An additional advantage of regenerating the concentrated and dilute ionic solutions is that fouling by contaminants from the feed streams can be prevented.
Preferably the ionic solutions are not obtained directly from natural sources, e.g. from a river water, ground water or sea water. Further due to the closed character of the system the choice of solute(s) is not limited to what is available from external sources but can be optimized to obtain a higher electrical energy production.
The heat source which may optionally be used to heat the concentrated and dilute ionic solutions is not particularly limited and it includes, for example, solar energy and unwanted and low grade heat, e.g. waste heat from power plants or the heat generated in industrial cooling towers or manufacturing processes. The same heat source can also be used in optional 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.
Since step (C) can take place at relatively low temperature differences, the process is particularly useful where low temperature heat sources are available, such as solar energy, geothermal energy, and waste heat.
The regeneration which occurs in optional step (C) may be done by a number of techniques, with thermal regeneration being preferred.
Preferably the process is performed without mixing of the (raw) streams exiting the reverse electrodialysis unit. For example, one may pass (parts of) one or both of the streams exiting the reverse electrodialysis unit into regeneration unit(s) which (further) heat the stream(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 step preferably comprises:
i) evaporation of liquid (e.g. water) from one or both of the solution(s) exiting the RED unit and condensation of 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.
For efficiency reasons, in step ii) 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 iii) 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). An example of a suitable thermal regeneration system is membrane distillation (MD).
Typically the electrodes are located at each end of the stack.
In a preferred embodiment, both the concentrated and the dilute ionic solution are 'hot', e.g. each having a temperature of at least 40°C, e.g. at least at 50°C, at least at 60°C or at least at 80°C and optionally up to 100°C. At such higher temperatures the power output of the RED device is increased. Also generally the solubility of solute(s) and the ion transport through the membranes are higher for hot solutions than for cold solutions. Especially when heat is applied for the regeneration step it is economically beneficial to perform the method such that one or both of the solutions are at a high temperature, e.g. higher than 40°C. Within the RED stack the solutions likely will reach thermal equilibrium rapidly. Although activity coefficients generally tend to be somewhat lower at higher temperatures, the diffusivity of salts increases significantly at higher temperatures and this can often more than compensate for the reduction in activity coefficient.
Because of the high volumetric latent heat of vaporization of water, the use of water increases the heat required for producing electricity, thereby decreasing the thermal efficiency of the process. For this reason, it may be desirable for the ionic solutions to 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. The ionic solutions optionally comprise one or more than one salt. Preferred salts have a high solubility in the liquid medium in which they are dissolved. In this specification, the terms "solute" and "salt" are often used interchangeably.
The ionic solutions typically comprise solute (i.e. one or more salts), water and optionally a water-miscible organic solvent.
The salt(s) present in the concentrated ionic solution may be the same as the salts used in the dilute ionic solution or they may be different to the salts used in the dilute ionic solution. For instance, the dilute ionic solution may comprise a salt that cannot pass through the membranes to the first pathway. Typically the salt that cannot pass through the membranes to the first pathway has the function of lowering the electrical resistance of the dilute ionic solution. Preferably the main salt present in the concentrated ionic solution is the same as the main salt present in the dilute ionic solution.
Table 1 below shows the activity coefficient of various salts at different ionic concentrations (at 6 mol/kg water and at 0.5 mol/kg water) at 25°C and the solubility in water at 20°C (S20c) and at 80°C (S80c):
Table 1
Salt S2oc Ssoc ACc ACd Ratio
(mol/kg (mol/kg ACc/ACd water) water)
Ammonium acetate 18.6 69.1 1.27 0.70 1.81
Ammonium bromide 7.8 12.8 0.40 0.62 0.65
Ammonium chloride 7.0 12.3 0.58 0.65 0.89
Ammonium fluoride 22.3 0.48 0.63 0.76
Ammonium formate 22.7 81.8 0.81 0.67 1.21
Ammonium iodide 11.9 15.8 0.59 0.65 0.91
Ammonium nitrate 24.0 72.5 0.27 0.60 0.45
Ammonium perchlorate 1.8 4.1 0.18 0.58 0.31
Ammonium thiocyanate 22.3 0.63 0.65 0.97
Tetramethyl ammonium fluoride 6.9 7.17 0.79 9.08
Caesium acetate 52.6 70.1 2.56 0.75 3.41
Caesium bromide 5.1 0.41 0.62 0.66
Caesium chloride 11.1 14.8 0.45 0.63 0.71
Caesium fluoride 21.2 1.54 0.71 2.17
Caesium formate 25.3 1.70 0.72 2.36
Caesium iodide 2.9 7.3 0.34 0.61 0.56 Salt S20C S80C ACc ACd Ratio
(mol/kg (mol/kg ACc/ACd water) water)
Caesium nitrate 1.2 6.9 0.09 0.53 0.17
Lithium acetate 6.2 1.19 0.69 1.72
Lithium bromide 18.4 28.2 3.65 0.78 4.68
Lithium chloride 19.7 26.4 2.60 0.75 3.47
Lithium chlorate 41.2 3.24 0.77 4.21
Lithium formate 7.6 17.8 0.81 0.67 1.21
Lithium iodide 12.3 32.5 5.45 0.81 6.73
Lithium nitrate 10.2 1.61 0.71 2.27
Lithium perchlorate 5.3 12.0 4.66 0.79 5.90
Lithium thiocyanate 17.5 4.17 0.79 5.28
Potassium acetate 26.1 38.8 2.28 0.74 3.08
Potassium bromide 5.5 8.0 0.66 0.65 1.02
Potassium chlorate 0.6 3.1 0.16 0.57 0.28
Potassium chloride 4.6 6.9 0.61 0.65 0.94
Potassium formate 40.1 68.9 1.37 0.70 1.96
Potassium fluoride 16.3 25.8 0.96 0.68 1.41
Potassium iodide 8.7 11.6 0.79 0.67 1.18
Potassium nitrate 3.3 16.5 0.13 0.56 0.23
Potassium thiocyanate 23.0 50.6 0.53 0.64 0.83
Sodium acetate 5.7 18.7 1.88 0.73 2.58
Sodium bromide 8.8 11.7 1.24 0.70 1.77
Sodium chlorate 9.0 15.7 0.52 0.64 0.81
Sodium chloride 6.2 6.5 0.98 0.68 1.44
Sodium fluoride 1.0 1.2 0.46 0.63 0.73
Sodium formate 11.9 20.3 0.90 0.67 1.34
Sodium iodide 11.9 19.7 1.74 0.72 2.42
Sodium nitrate 10.3 17.4 0.37 0.62 0.60
Sodium perchlorate 16.4 25.0 0.69 0.66 1.05
Sodium thiocyanate 17.1 24.9 1.25 0.70 1.79
Rubidium acetate 2.45 0.75 3.27
Rubidium bromide 6.5 0.51 0.64 0.80
Rubidium chloride 7.5 10.5 0.54 0.64 0.84
Rubidium fluoride 12.5 1.08 0.69 1.57
Rubidium formate 42.5 1.49 0.71 2.10
Rubidium iodide 6.8 0.51 0.64 0.80 Salt S20C Ssoc ACc ACd Ratio
(mol/kg (mol/kg ACc/ACd water) water)
Rubidium nitrate 3.6 21.0 0.13 0.56 0.23
The salts listed in Table 1 which have an ACc/ACd ratio of at least 1 .5 are suitable for use as the main salt in both the concentrated ionic solution and the dilute ionic solution of the same RED device due to the significant difference in activity coefficients at 6 mol/kg vs. 0.5 mol/kg.
Preferably the main salt present in the concentrated ionic solution has a solubility in water (Ssoc) of at least 8 mol/kg water, more preferably at least 10 mol/kg water, especially at least 15 mol/kg water, more especially at least 20 mol/kg water, when measured at 80°C.
Preferably the concentrated ionic solution comprises (in the steady state) at least 4 mol/kg solvent (more preferably at least 5 mol/kg solvent (e.g. 5 to 40 mol/kg solvent) especially at least 6 mol/kg solvent (e.g. 6 to 20 mol/kg solvent) or more especially at least 8 mol/kg solvent (e.g. 8 to 15 mol/kg solvent)) of the main salt as it enters the reverse electrodialysis unit. The total concentration of salts present in the concentrated ionic solution (in the steady state) as it enters the reverse electrodialysis unit is preferably at least 4 mol/kg solvent (more preferably at least 5 mol/kg solvent (e.g. 5 to 40 mol/kg solvent) especially at least 6 mol/kg solvent (e.g. 6 to 20 mol/kg solvent) or more especially at least 8 mol/kg solvent (e.g. 8 to 15 mol/kg solvent)).
In order to facilitate regeneration of the dilute and concentrated ionic solutions, it is preferred that the main salt present in the dilute ionic solution is identical to the main salt present in the concentrated ionic solution.
Preferred main salts include the potassium, caesium, rubidium, ammonium and tetramethylammonium salts of formic acid, acetic acid and fluoride anion (except potassium fluoride), the sodium salt of acetic acid, sodium bromide, sodium iodide, sodium thiocyanate and all lithium salts except lithium fluoride. Less preferred is lithium acetate.
The main salt present in the concentrated ionic solution is preferably selected from ammonium acetate, tetramethylammonium fluoride, caesium acetate, caesium fluoride, caesium formate, lithium acetate, lithium bromide, lithium chloride, lithium chlorate, lithium iodide, lithium nitrate, lithium perchlorate, lithium thiocyanate, potassium acetate, potassium formate, sodium acetate, sodium bromide, sodium iodide, sodium thiocyanate, rubidium acetate, rubidium formate and rubidium fluoride, all of which satisfy Equation 1 above as can be seen from Table 1 above. Preferably the main salt(s) present in the concentrated ionic solution and in the dilute ionic solution are monovalent. This preference arises because in the concentrated ionic solution monovalent ions generate more power than multivalent ions.
Preferably (in the steady state) the dilute ionic solution comprises at least
0.03 mol/kg solvent of the main salt as it enters the reverse electrodialysis unit. The total concentration of salts present in the dilute ionic solution as it enters the reverse electrodialysis unit is (in the steady state) preferably at least 0.03 mol/kg solvent. At start-up of the process the initial concentration of the main salt in the dilute ionic solution may be lower and even close to zero since in the RED device solute will diffuse from the concentrated ionic solution to the dilute ionic solution rather rapidly. However in the steady state it is not practical to maintain a very low concentration in the dilute ionic solution because this requires more regeneration effort than necessary so an optimum will be selected taking the degree of regeneration and magnitude of concentration difference between dilute and concentrated ionic solutions into account. Furthermore a very low concentration of ions in the dilute ionic solution (i.e. less than 0.03 mol/kg solvent) may increase the electrical resistance of the stack resistance which is not desirable.
In the RED device the concentration of the solute in the concentrated ionic solution gradually decreases due to diffusion of ions from the concentrated ionic solution to the dilute ionic solution. Thus the concentration of solute in the dilute ionic solution typically increases as it passes through the RED device, e.g. to 0.5 mol/kg solvent, or to 1.0 mol/kg solvent or to an even higher concentration.
The ionic solutions optionally comprise one or more than one salt (as the solute), e.g. two or more salts. In a preferred embodiment the ionic solutions comprise water and essentially one salt. All other salts are then present in only trace amounts or are absent.
Preferred salts comprise a cation which has an (unhydrated) ionic radius larger than 0.09nm. 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).
Preferred salts comprise an anion which has an (unhydrated) ionic radius larger 0.12 nm. Suitable anions include fluoride (ionic radius 0.13 nm), chloride (ionic radius 0.18 nm), bromide (ionic radius 0.20 nm), iodide (ionic radius 0.22 nm), nitrate (ionic radius 0.20 nm), chlorate, perchlorate (ionic radius 0.24 nm), thiocyanate (ionic radius 0.21 nm), formate (ionic radius 0.20 nm) and acetate (ionic radius 0.23 nm).
When the ionic solutions comprise more than one salt, the number of moles solute/kg solvent is the total for all of the salts present. The solvent preferably comprises water and optionally a water miscible (organic) solvent. The total concentration of solute (i.e. all salts) in the concentrated ionic solution as it enters the reverse electrodialysis unit is in the steady state preferably 1 .5 to 80mol/kg solvent, especially 3 to 75mol/kg solvent.
The total concentration of solute (i.e. all salts) in the dilute ionic solution as it enters the reverse electrodialysis unit is in the steady state preferably 0.03 to 1 mol/kg solvent, especially 0.05 to 0.8mol/kg solvent, e.g. 0.05 to 0.3mol/kg solvent.
Preferably the identity of the solute present in the concentrated ionic solution and the dilute ionic solution are substantially the same.
Preferably the method is performed such that the following equation is satisfied:
S1/S2 > Y
wherein:
51 is the concentration of solute (e.g. salt) in mol/kg solvent in the concentrated ionic solution as it enters the reverse electrodialysis unit in the steady state;
52 is the concentration of solute (e.g. salt) in mol/kg solvent in the dilute ionic solution as it enters the reverse electrodialysis unit in the steady state; 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.
The temperature of the concentrated ionic solution and/or of the dilute ionic solution as it enters the reverse electrodialysis unit is preferably 40 to 100°C, more preferably 50 to 100°C, especially 60 to 95°C, more especially 70 to 90°C, e.g. 75°C, 80°C or 85°C.
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 membranes apart creating pathways of average width 50 to 400 pm, more preferably 100 to 250 μητ
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 the membranes are stable at high temperatures (e.g. the permselectivity of the membranes does not deteriorate in time, especially at high concentrations). Preferably the permselectivity reduces by less than 5% when the membranes are used continuously at a temperature of 80°C for a period of 16 hours. Preferably the membranes are optimized for the concentration and salt(s) which are present in the streams which come into contact with the membrane. Preferred membranes have a low electrical resistance (e.g. a resistance of less than 3 ohm. cm, more preferably less than 2 ohm. cm) and a high permselectivity (e.g. a permselectivity of more than 80%, more preferably more than 90%), e.g. under the conditions at which the RED unit is used.
Preferably the flow velocity of the ionic solutions through the pathways is less than 4 cm/s, more preferably less than 2 cm/s, especially less than 1 .5 cm/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 introducing a concentrated ionic solution into said first pathway;
(C) a means for introducing a dilute ionic solution into said second pathway,
(D) optionally a means for heating the concentrated ionic solution and/or the dilute ionic solution;
(E) a means for regenerating the concentrated and dilute ionic solutions exiting from the reverse electrodialysis unit;
(F) a recycling means for recycling said regenerated concentrated and dilute solutions back to said first and second pathways in the reverse electrodialysis unit;
(G) a concentrated ionic solution present in the first pathway; and
(H) a dilute ionic solution present in the second pathway;
wherein:
(i) the concentrated ionic solution and the dilute ionic solution each comprises a main salt; and (ϋ) the main salt present in each of the concentrated ionic solution and the dilute ionic solution as they enter the reverse electrodialysis unit is such that following Equation 1 is satisfied: ACc/ACd is at least 1 .5
(Equation 1 )
wherein:
ACc is the activity coefficient of the main salt present in the concentrated ionic solution when measured at a concentration of 6 mol/kg water at 25°C; and ACd is the activity coefficient of the main salt present in the dilute ionic solution when measured at a concentration of 0.5 mol/kg water at 25°C.
The means for introducing 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 concentrations of the two ionic solutions closer together, thereby generating electricity. As a consequence, during the process the solute concentration of the dilute ionic solution increases and the solute concentration of the concentrated ionic solution decreases.
Fig. 1 shows a reverse electrodialysis unit which may be used to perform the method of the present invention
Fig. 2 shows the power densities achieved using various salts at an S1/S2 ratio of 12 when the ionic solutions were at a temperature of at 80°C.
Fig. 3 shows the power densities achieved using various salts at an S1/S2 ratio of 12 when the ionic solutions were at a temperature of at 20°C.
Fig. 4 shows the power densities achieved using various salts at an S1/S2 ratio of 1 10 when the ionic solutions were at a temperature of at 80°C.
The reverse electrodialysis unit shown in Fig. 1 comprises a membrane stack (1 ) comprising a first pathway for a concentrated ionic solution, a second pathway for a dilute ionic solution, electrodes and alternating cation and anion exchange membranes (not shown). Dilute ionic solution (14) and concentrated ionic solution (15) are brought to the desired temperature in water bath (2) with stirring using mixers (3) and (4) respectively. The dilute ionic solution (14) and the concentrated ionic solution (15) are then pumped into the stack (1 ) using pumps (6) and (7). Inside the stack (1 ), the concentrated ionic solution passes through the first pathway and the dilute ionic solution passes through the second pathway, whereby solute from the concentrated ionic solution in the first pathway passes through membranes to the dilute ionic solution in the second pathway, thereby generating electricity. During the method, electrolyte stored in vessels (1 1 ) is pumped into electrode compartments of the membrane stack (1 ) using pump (8). The reverse electrodialysis unit further comprises multimeter (9) for measuring the current and multimeter (10) for measuring the potential and external load (5) provides a varying resistance. Dilute ionic solution which has exited the stack passes to vessel (12) and concentrated ionic solution which has exited the stack passes to vessel (13).
The means for heating the concentrated ionic solution and/or the dilute ionic solution (e.g. to a temperature higher than the temperature of the environment) 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 process and device.
The means for regenerating the concentrated and dilute ionic 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.
In an alternative embodiment, the concentrated and dilute ionic solutions are regenerated using an electrodialysis (ED) device, for example by a process comprising separating at least a part of the dilute ionic solution and/or the concentrated ionic solution both exiting the reverse electrodialysis into a dilute and concentrate stream. Optionally the ED device uses an electrical field to force ions to flow from a dilute ionic solution to a more concentrated. Preferably the RED unit and the ED device each comprise membrane stacks and the membrane stacks present in the RED unit are different from the membrane stacks present in the ED device. In this embodiment it is preferred that for the regeneration step excess electricity is used, e.g. during periods that the production of electrical energy exceeds the demand. During periods that the demand exceeds the production capacity the RED unit can be activated to produce additional electrical energy. Alternatively, excess energy may be stored and used to assist the regeneration step at a different point in time. The use of excess electricity in this way makes the method particularly flexible.
The reverse electro-dialysis device in combination with storage tanks may be used to store energy for use in periods when there is high demand for electrical energy.
According to a third aspect of the present invention there is provided the use of the device according to the second aspect of the present invention for the generation and/or storage of electrical energy.
Examples
(i) The RED Unit
In the following Examples and Comparative Examples electricity was generated using a RED unit. This unit comprised a CS100 membrane stack obtained from Deukum, Germany, having an anode, a cathode, a first pathway for a concentrated ionic solution, a second pathway for a dilute ionic solution. The stack comprised 10 cell pairs (10 for each pathway with the first and second pathways in alternating order), each cell pair comprising an anion exchange membrane (AEM) and a cation exchange membrane (CEM) which together define a pathway and together with an adjacent cell pair a second pathway. The effective cross-sectional area of the AEMs and CEMs was 10x10cm to give effective membrane surface area of 0.01 m2. The membrane stack further comprised two electrolyte compartments, one at each side of the membrane stack. The electrolyte compartments each comprised two CEMs to reduce leakage of electrolyte out of the electrolyte compartments. The 10 AEMs and 13 CEMs were commercially available Type 10 membranes provided by Fujifilm, The Netherlands. Each cell comprised a spacer of thickness 310 pm having a silicon gasket and a PE netting, and was obtained from Deukum, Germany. The RED unit comprising the membrane stack is illustrated in Fig. 1 .
(ii) Preparation of Concentrated and Dilute Ionic Solutions
Concentrated ionic solutions were prepared comprising 6 mol of each of the salts indicated in Table 2 below per kg of water. Dilute ionic solutions were prepared comprising 0.5 mol of each of the salts indicated in Table 2 below per kg of water. The solubility of each salt at 20°C and 80°C is shown in Table 2 below:
Table 2. Solubilities of salts used in the experiments
Figure imgf000016_0001
is potassium acetate)
The recipes for the salt solutions were as follows:
Table 3. 0.5m salt solution recipes
Salt MW Amount of salt (g) Amount of water (kg)
NaCI 58.44 1 16.88 4.0
NaBr 102.89 205.79 4.0
NaN03 84.99 169.99 4.0
LiBr 86.85 173.69 4.0
KAc 98.14 196.28 4.0 Table 4. 6m salt solution recipes
Figure imgf000017_0001
The salts used were as follows:
NaN03, 98% from Alfa Aesar, Germany.
NaBr, 97% from Alfa Aesar, Germany.
LiBr, 99% from Sigma Aldrich.
KAc, 99% from Sigma Aldrich.
NaCI, 99.5% from Sigma Aldrich.
The ACc and ACd are shown in Table 5 below along with the calculated value of ACc/ACd:
Table 5. Activity coefficients of salts used in the experiments
Figure imgf000017_0002
(iii) Generation of Electricity
Each of the salts listed in Table 2 above was evaluated as follows:
Four litres of each of the dilute ionic solution (0.5 mol/kg of water) containing the salt under evaluation and the concentrated ionic solution (6 mol/kg of water) containing the same salt, as described in section (ii) above were poured into separate 10 L stainless steel vessels placed in a water bath set at 85°C. The solutions in each vessel were mixed continuously using IKA Eurostar 20 mixers at a rate of 50 rpm. The solutions were pumped in co-current mode into the membrane stack described in Section (i) above using MasterFlex peristaltic pumps obtained from Cole-Parmer, USA. The dilute ionic solution was pumped into the stack at a rate of 203 rpm and the concentrated ionic solution was pumped into the stack at a rate of 373 rpm corresponding to a flow velocity of 2.0 cm/s and 3.5 cm/s respectively. An electrolyte solution (0.1 M potassium hexacyanoferrate (II), 0.1 M potassium hexacyanoferrate (III) and 0.25M NaCI) was circulated through the electrolyte compartments at each end of the stack using a third MasterFlex peristaltic pump at a rate of 345 ml/min.
Two multimeters (Model 289 from Fluke, USA) were used to measure current and potential during the experiments, one connected in series with the membrane stack for current measurements and one in parallel to the membrane stack for potential measurements.
An external load (Codix from Kubler, Germany) was applied at a range of resistance values (from 0.5 to 19 ohm).
From the measurements, the stack resistance and Power Density (PD) were determined according to the method as described in Veerman et al, Journal of Membrane Science, 327, (2009), 136-144. The data were not corrected for the so-called blank resistance of the membrane stack.
At the start of each experiment the membranes were equilibrated by filling the membrane stack with concentrated ionic solution and applying a current of 0.1 A for one minute using a PGSTAT 204 galvanostat from Autolab, The Netherlands. At the start of the experiments the Open Circuit Voltage (OCV) of the membrane stack was measured without external load. At the end of the experiments the OCV of the membrane stack was recorded again and was found to be comparable to the value measured at the start of the experiments.
The activity coefficients of the salt present in the dilute and concentrated ionic solutions were calculated as indicated in the description.
(iv) Results
The PD of the salts listed in Table 2 above at 80°C versus applied voltage potential are shown in Fig. 2 and in Fig. 3 are shown the PD of NaNO3, NaCI and LiBr at 20°C. As can be seen from Fig. 2 and Fig. 3, the salts having an ACc/ACd falling outside of claim 1 produced a lower PD than the salts falling within claim 1 . Furthermore, the PD achieved using ionic solutions at 80°C was much higher than the PD achieved using ionic solutions at 20°C.
The experiments described above in sections (i) to (iii) were repeated for NaCI and KAc except for the following changes:
· the membrane stack comprised 40 cell pairs;
• the membranes were Type 12 membranes from Fujifilm;
• the dilute ionic solution had a concentration of 0.05 mol/kg water;
• the concentrated ionic solution had a concentration of 5.5 mol/kg water; and • the spacer for the second pathway (for the dilute ionic solution) had a thickness of 150pm (from Deukum, Germany) to reduce the resistance of the second pathway. The spacer in the first pathway was not changed.
The PD of these salts at 80°C versus applied voltage potential is shown in
Fig. 4. Results
The results of all experiments are shown in Table 6 below: Table 6.
Figure imgf000019_0001
In Ex5 using KAc at 80°C a very high max PD of 28 W/m2.cell pair was obtained.

Claims

1 . A method for generating electricity comprising the steps:
(A) passing 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) passing a dilute ionic solution through a second pathway in said reverse electrodialysis unit, whereby solute from the concentrated ionic solution in the first pathway passes through the membranes to the dilute ionic solution in the second pathway, thereby generating electricity;
wherein:
(i) the concentrated ionic solution and the dilute ionic solution each comprise a main salt; and
(ii) the main salt present in each of the concentrated ionic solution and the dilute ionic solution as they enter the reverse electrodialysis unit is such that
Equation 1 is satisfied:
ACc/ACd is at least 1 .5
(Equation 1 )
wherein:
ACc is the activity coefficient of the main salt present in the concentrated ionic solution when measured at a concentration of 6 mol/kg water at 25°C; and
ACd is the activity coefficient of the main salt present in the dilute ionic solution when measured at a concentration of 0.5 mol/kg water at 25°C.
2. The method according to claim 1 wherein the main salt present in the concentrated ionic solution has a solubility in water of at least 8 mol/kg water when measured at 80°C.
3. The method according to claim 1 or 2 wherein the activity coefficient of the main salt when measured at a concentration of 6 mol/kg water at 25°C is at least 1 .0.
4. The method according to any one of the preceding claims wherein the concentrated ionic solution comprises at least 4 mol/kg solvent of the said main salt as it enters the reverse electrodialysis unit.
5. The method according to any one of the preceding claims wherein the dilute ionic solution comprises at least 0.03 mol/kg solvent of the said main salt as it enters the reverse electrodialysis unit.
6. The method according to any one of the preceding claims wherein the total concentration of salts present in the concentrated ionic solution as it enters the reverse electrodialysis unit is at least 4 mol/kg solvent.
7. The method according to any one of the preceding claims wherein the total concentration of salts present in the dilute ionic solution as it enters the reverse electrodialysis unit is at least 0.03 mol/kg solvent
8. The method according to any one of the preceding claims wherein the main salt present in the dilute ionic solution is identical to the main salt present in the concentrated ionic solution.
9. The method according to any one of claims 1 to 8 wherein the main salt present in the concentrated ionic solution is selected from ammonium acetate, tetramethylammonium fluoride, caesium acetate, caesium fluoride, caesium formate, lithium acetate, lithium bromide, lithium chloride, lithium chlorate, lithium iodide, lithium nitrate, lithium perchlorate, lithium thiocyanate, potassium acetate, potassium formate, sodium acetate, sodium bromide, sodium iodide, sodium thiocyanate, rubidium acetate, rubidium formate and rubidium fluoride.
10. The method according to any one of the preceding claims wherein the cations of the main salt(s) present in the concentrated ionic solution and in the dilute ionic solution each have an ionic radius of at least 0.09 nm and the anions of the main salt(s) each have an ionic radius of at least 0.12 nm.
1 1 . The method according to any one of the preceding claims wherein the main salt(s) present in the concentrated ionic solution and in the dilute ionic solution are monovalent.
12. The method according to any one of the preceding claims wherein both the concentrated and the dilute ionic solution each have a temperature of at least
40°C.
13. The method according to any one of the preceding claims which further comprises the step (C) of regenerating the concentrated and dilute ionic solutions from the solutions exiting from the reverse electrodialysis unit and recycling said regenerated concentrated and dilute solutions back through said respective first and second pathways in the reverse electrodialysis unit.
14. The method according to any one of the preceding claims which is performed such that the following equation is satisfied:
S1/S2 > Y
wherein:
51 is the concentration of solute in mol/kg solvent in the concentrated ionic solution as it enters the reverse electrodialysis unit;
52 is the concentration of solute in mol/kg solvent in the dilute ionic solution as it enters the reverse electrodialysis unit; and
Y is at least 30.
15. The method according to any one of the preceding claims wherein the flow rate of the ionic solutions through the pathways is less than 4 cm/s.
16. 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) means for introducing a concentrated ionic solution into said first pathway;
(C) a means for introducing a dilute ionic solution into said second pathway,
(D) optionally a means for heating the concentrated ionic solution and/or the dilute ionic solution;
(E) a means for regenerating concentrated and dilute ionic solutions exiting from the reverse electrodialysis unit;
(F) a recycling means for recycling said regenerated concentrated and dilute solutions back to said first and second pathways in the reverse electrodialysis unit;
(G) a concentrated ionic solution present in the first pathway; and
(H) a dilute ionic solution present in the second pathway;
wherein:
(i) the concentrated ionic solution and the dilute ionic solution each comprises a main salt; and (ii) the main salt present in each of the concentrated ionic solution and the dilute ionic solution as they enter the reverse electrodialysis unit is such that following Equation 1 is satisfied: ACc/ACd is at least 1 .5
(Equation 1 )
wherein:
ACc is the activity coefficient of the main salt present in the concentrated ionic solution when measured at a concentration of 6 mol/kg water at 25°C; and ACd is the activity coefficient of the main salt present in the dilute ionic solution when measured at a concentration of 0.5 mol/kg water at 25°C.
17. The device according to claim 16 wherein the main salt present in the concentrated ionic solution has a solubility in water of at least 8 mol/kg water when measured at 80°C.
18. The device according to claim 16 or 17 wherein the activity coefficient of the main salt when measured at a concentration of 6 mol/kg water at 25°C is at least 1 .0.
19. The device according to any one of claims 16 to 18 wherein the concentrated ionic solution comprises at least 4 mol/kg solvent of the said main salt as it enters the reverse electrodialysis unit.
20. The device according to any one of claims 16 to 19 wherein the dilute ionic solution comprises at least 0.03 mol/kg solvent of the said main salt as it enters the reverse electrodialysis unit.
21 . The device according to any one of claims 16 to 20 wherein the total concentration of salts present in the concentrated ionic solution as it enters the reverse electrodialysis unit is at least 4 mol/kg solvent.
22. The device according to any one of claims 16 to 21 wherein the total concentration of salts present in the dilute ionic solution as it enters the reverse electrodialysis unit is at least 0.03 mol/kg solvent.
23. The device according to any one of claims 16 to 22 wherein the main salt present in the dilute ionic solution is identical to the main salt present in the concentrated ionic solution.
24. The device according to any one of claims 16 to 23 wherein the main salt present in the concentrated ionic solution is selected from ammonium acetate, tetramethylammonium fluoride, caesium acetate, caesium fluoride, caesium formate, lithium acetate, lithium bromide, lithium chloride, lithium chlorate, lithium iodide, lithium nitrate, lithium perchlorate, lithium thiocyanate, potassium acetate, potassium formate, sodium acetate, sodium bromide, sodium iodide, sodium thiocyanate, rubidium acetate, rubidium formate and rubidium fluoride.
25. The device according to any one of claims 16 to 24 wherein the cation of the main salt(s) present in the concentrated ionic solution and in the dilute ionic solution each have an ionic radius of at least 0.09 nm and the anions of the main salt(s) each have an ionic radius of at least 0.12 nm.
26. The device according to any one of claims 16 to 25 wherein the main salt(s) present in the concentrated ionic solution and in the dilute ionic solution are monovalent.
27. The device according to any one of claims 16 to 26 which further comprises (I) a means for regenerating the concentrated and dilute ionic solutions exiting from the reverse electrodialysis unit and recycling said regenerated concentrated and dilute solutions back through said respective first and second pathways in the reverse electrodialysis unit.
28. The device according to any one of claims 16 to 27 wherein the solutions are such that the following equation is satisfied:
S1/S2 > Y
wherein:
51 is the concentration of solute in mol/kg solvent in the concentrated ionic solution as it enters the reverse electrodialysis unit;
52 is the concentration of solute in mol/kg solvent in the dilute ionic solution as it enters the reverse electrodialysis unit; and
Y is at least 30.
29. Use of the device according to any of claims 16 to 28 for the generati and/or storage of electrical energy.
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Cited By (3)

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US11502323B1 (en) 2022-05-09 2022-11-15 Rahul S Nana Reverse electrodialysis cell and methods of use thereof
US11502322B1 (en) 2022-05-09 2022-11-15 Rahul S Nana Reverse electrodialysis cell with heat pump
US11855324B1 (en) 2022-11-15 2023-12-26 Rahul S. Nana Reverse electrodialysis or pressure-retarded osmosis cell with heat pump

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US20140255813A1 (en) * 2013-03-11 2014-09-11 BlueCell Energy, LLC Energy generation and storage using electro-separation methods and devices

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US20140255813A1 (en) * 2013-03-11 2014-09-11 BlueCell Energy, LLC Energy generation and storage using electro-separation methods and devices

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Publication number Priority date Publication date Assignee Title
US11502323B1 (en) 2022-05-09 2022-11-15 Rahul S Nana Reverse electrodialysis cell and methods of use thereof
US11502322B1 (en) 2022-05-09 2022-11-15 Rahul S Nana Reverse electrodialysis cell with heat pump
US11563229B1 (en) 2022-05-09 2023-01-24 Rahul S Nana Reverse electrodialysis cell with heat pump
US11611099B1 (en) 2022-05-09 2023-03-21 Rahul S Nana Reverse electrodialysis cell and methods of use thereof
US11699803B1 (en) 2022-05-09 2023-07-11 Rahul S Nana Reverse electrodialysis cell with heat pump
US11855324B1 (en) 2022-11-15 2023-12-26 Rahul S. Nana Reverse electrodialysis or pressure-retarded osmosis cell with heat pump

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