WO2011050473A1

WO2011050473A1 - Method and system for desalinating saltwater while generating electricity

Info

Publication number: WO2011050473A1
Application number: PCT/CA2010/001718
Authority: WO
Inventors: Benjamin Stuart Sparrow; Joshua Aniket Zoshi; James Hing Bong Tang; Henry Kwan Keung Tsin; Nicholas Christian Roch
Original assignee: Saltworks Technologies Inc.
Priority date: 2009-10-30
Filing date: 2010-10-25
Publication date: 2011-05-05

Abstract

The present disclosure is directed at methods, systems and techniques for desalinating saltwater while generating electricity. A reverse electrodialysis (RED) stack is used to generate electricity, and the generated electricity is used to desalinate saltwater in an electrodialysis reversal (EDR) stack. As the RED stack relies on a concentration difference between two ionic fluids to generate electricity, a desalination plant that incorporates the RED and EDR stacks as described herein is referred to as a concentration difference energy plant. Brine discharge from a first desalination plant, such as a reverse osmosis plant, can be partially desalinated by the concentration difference energy plant, and the partially desalinated brine may optionally be returned to the first desalination plant for further desalination. This can result in several benefits. For example, the concentration difference energy plant can remove larger ionic species, which if not removed could cause scaling in the first desalination plant.

Description

METHOD AND SYSTEM FOR DESALINATING SALTWATER WHILE

GENERATING ELECTRICITY

TECHNICAL FIELD

The present disclosure is directed at a method and system for desalinating saltwater while generating electricity.

BACKGROUND

Over one quarter of Earth's population does not have adequate access to freshwater. Inadequate access to freshwater is detrimental, as it can lead to disease and malnutrition, limit agricultural development, and inhibit economic growth.

In contrast to freshwater, saltwater is readily available. Saltwater in the form of seawater constitutes about 97% of the water on Earth. Unless seawater is sufficiently desalinated, though, it is not only undrinkable but unsuitable for agriculture. "Desalination" refers to the process of removing anions and cations from saltwater. Seawater typically has a salt concentration of about 3.5% salt by mass; that is, about 35 grams of dissolved salt per liter of water. Another source of saltwater is salty, underground aquifer water, also known as "brackish water". The salt concentration of brackish water typically ranges from less than 1% to more than 18% salt by mass. In contrast, drinkable water typically has a salt concentration of, at most, about 0.04%.

Given the need for freshwater, and given the abundance of saltwater, there exists a continued need for methods and systems for producing freshwater by desalinating saltwater.

SUMMARY

According to a first aspect, there is provided a method for desalinating saltwater while generating electricity. The method includes generating electricity using a reverse electrodialysis (RED) stack, and utilizing the electricity to desalinate saltwater in an electrodialysis reversal (EDR) stack. The RED stack may have an output impedance that is matched to an input impedance of the EDR stack.

The RED stack may be one of a plurality of separate RED stacks electrically coupled to each other to generate the electricity, and the EDR stack may be one of a plurality of separate EDR stacks electrically coupled to each other to desalinate the saltwater. The method may also include determining a cost of electricity, and when the cost of electricity is below a low cost threshold, doing one or both of powering at least some of the separate EDR stacks using purchased electricity and converting at least some of the separate RED stacks into the EDR stacks to desalinate the saltwater. Additionally or alternatively, when the cost of electricity is above a high cost threshold, the method may include doing one or both of selling electricity generated using at least some of the RED stacks and converting at least some of the separate EDR stacks into the RED stacks to generate the electricity.

The method may also include periodically backflushing one or both of the RED and EDR stacks by reversing the direction of fluid flow through at least one of the stacks.

The method may also include determining one or more of water volume delivery need and power system need, and based on values for at least one of volume delivery need and power system need, accordingly adjusting at least one of rate at which the electricity is generated and at which the saltwater is desalinated.

The RED stack may be one of a plurality of RED stacks and the EDR stack may be one of a plurality of EDR stacks, and the method may also include utilizing a switching network configured to electrically couple any one or more of the RED stacks to any one or more of the EDR stacks. Any one or more of the RED stacks may have an output impedance that is matched to an input impedance of the any one or more EDR stacks.

A portion of the electricity generated using the RED stack may be transmitted to a power grid.

The RED stack may have an RED concentrate and an RED diluent flowing through it, and the method can also include evaporating water from at least some of the RED concentrate and the RED diluent exiting the RED stack, and harvesting salt that precipitates following evaporation. Substantially all the water may be evaporated from the at least some of the RED concentrate and the RED diluent. Additionally or alternatively, one or both of the RED concentrate and the RED diluent that is recycled may be heated utilizing waste heat from a process plant to facilitate evaporation to air. The EDR stack may also have an EDR diluent and product that is desalinated flowing therethrough, and the method may also include recycling the EDR diluent exiting the EDR stack into the RED diluent entering the RED stack. Additionally or alternatively, the method may also include mixing the EDR diluent exiting the EDR stack with a solution having a concentration less than an EDR diluent threshold to lower the concentration of the EDR diluent, and following mixing, recycling the EDR diluent exiting the EDR stack by using it as the EDR diluent entering the EDR stack. Additionally or alternatively, the method may also including mixing the RD diluent exiting the RED stack with a solution having a concentration less than an RED diluent threshold to lower the concentration of the RED diluent, and following mixing, recycling the RED diluent exiting the RED stack by using it as the RED diluent entering the RED stack.

The method may also include de-scaling the RED stack by reversing the polarity of the RED stack.

According to a second aspect, there is provided a system for desalinating saltwater while generating electricity. The system includes an RED stack configured to generate electricity, and an EDR stack electrically coupled to the RED stack such that the electricity generated by the RED stack is used to desalinate saltwater in the EDR stack.

The RED stack may have an output impedance that is matched to an input impedance of the EDR stack.

The RED stack may be one of a plurality of separate RED stack electrically coupled to each other to generate the electricity and the EDR stack may be one of a plurality of separate EDR stacks electrically coupled to each other to desalinate the saltwater. The system may also include a switching network electrically coupled between the plurality of RED stacks and the plurality of EDR stacks such that any one or more of the RED stacks can be electrically coupled to any one or more of the EDR stacks.

Any one or more of the RED stacks may have an output impedance that is matched to an input impedance of the any one or more EDR stacks.

The RED stack may have an RED concentrate and RED diluent flowing through it. The system may also include a salt harvesting device and a reconcentrator fluidly coupled to an outlet of the RED stack through which the RED concentrate and the RED diluent exit the RED stack and to the salt harvesting device. The reconcentrator may be configured to evaporate water from at least some of the RED concentrate and the RED diluent exiting the RED stack to facilitate harvesting of salt from the RED concentrate and RED diluent using the salt harvesting device. Substantially all the water may be evaporated from the at least some of the RED concentrate and the RED diluent.

The reconcentrator may be fluidly coupled between an outlet of the RED stack through which one or both of the RED concentrate and RED diluent exit the RED stack and an inlet of the RED stack through which the RED concentrate enters the RED stack such that one or both of the RED concentrate and RED diluent exiting the RED stack can be recycled for use as the RED concentrate that enters the RED stack.

The system may also include a process plant comprising a source of waste heat, and a heat exchanger fluidly coupled between the outlet of the RED stack and the reconcentrator configured to heat one or both of the RED concentrate and RED diluent to facilitate evaporation to air.

The RED stack may have an RED concentrate and an RED diluent flowing through it, and the EDR stack may have an EDR diluent and product that is desalinated flowing through it. The system may also include piping fluidly coupling an outlet of the EDR stack through which the EDR diluent exits the EDR stack to an inlet of the RED stack through which the RED diluent enters the RED stack such that the EDR diluent exiting the EDR stack can be recycled as the RED diluent entering the RED stack.

The system may also include piping fluidly coupling an outlet of the EDR stack through which the EDR diluent exits the EDR stack to an inlet of the EDR stack through the EDR diluent enters the EDR stack such that the EDR diluent exiting the EDR stack can be recycled as the EDR diluent entering the EDR stack; and an EDR diluent mixer disposed along the piping and fluidly coupled to a source of solution having a concentration less than an EDR diluent threshold. The EDR diluent mixer is configured to mix the solution and the EDR diluent being recycled to lower the concentration of the EDR diluent prior to the EDR diluent entering the EDR stack. Additionally or alternatively, the system may include piping fluidly coupling an outlet of the RED stack through which the RED diluent exits the RED stack to an inlet of the RED stack through which the RED diluent enters the RED stack such that the RED diluent exiting the RED stack can be recycled as the RED diluent entering the RED stack; and an RED diluent mixer fluidly coupled along the piping and to a source of solution having a concentration less than an RED diluent threshold. The RED diluent mixer can be configured to mix the solution and the RED diluent being recycled to lower the concentration of the RED diluent prior to the RED diluent entering the RED stack.

The RED stack can be de-scaled by reversing the polarity of the RED stack.

According to a third aspect, there is provided a system for desalinating saltwater while generating electricity. The system includes a first desalination plant for desalinating saltwater and having a brine discharge outlet; and a second desalination plant. The second desalination plant includes a RED stack configured to generate electricity, the RED stack having an RED concentrate and an RED diluent flowing therethrough; and an EDR stack having an EDR diluent and product flowing therethrough, the EDR stack electrically coupled to the RED stack such that the electricity generated by the RED stack is used to desalinate the product in the EDR stack. The brine discharge outlet is fluidly coupled to the EDR stack such that brine discharged from the first desalination plant is desalinated.

The brine discharge outlet may be fluidly coupled to the RED stack such that the brine discharged from the first desalination plant is used as the RED concentrate to generate the electricity. The second desalination plant may include a reconcentrator fluidly coupled between the brine discharge outlet and the RED stack. An outlet of the EDR stack may be fluidly coupled to the first desalination plant such that desalinated brine is output to the first desalination plant for further desalination. The RED stack may be de-scaled by reversing the polarity of the RED stack.

According to a fourth aspect, there is provided a method for desalinating saltwater while generating electricity. The method includes obtaining brine discharged from a first desalination plant, and utilizing the brine as product to be desalinated in a second desalination plant configured to desalinate saltwater according to a method including generating electricity using a RED stack, the RED stack having an RED concentrate and an RED diluent flowing therethrough; and utilizing the electricity to desalinate saltwater in an EDR stack having an EDR diluent and the product flowing therethrough, the EDR stack electrically coupled to the RED stack such that the electricity generated by the RED stack is used to desalinate the product in the EDR stack.

The brine discharged from the first desalination plant may be used as the RED concentrate. A portion of the brine may be evaporated to air to increase its concentration prior to using it as the RED concentrate. Desalinated brine may be returned from the second desalination plant to the first desalination plant for further desalination. The RED stack may be de-scaled by reversing the polarity of the RED stack.

According to a fifth aspect, there is provided a computer readable medium having encoded thereon statements and instructions to cause a controller to execute any of the foregoing methods.

Beneficially, the foregoing allows for electricity to be generated with the RED stacks while product is desalinated using the EDR stacks. Additionally, when brine discharge from a first desalination plant is partially desalinated by a concentration difference energy plant, a relatively high recovery ratio can be achieved and problems associated with scaling of conventional desalination plants can be mitigated by performing de-scaling on the concentration difference energy plant and by having the concentration difference energy plant remove the relatively large ionic species that are primarily responsible for scaling.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate one or more exemplary embodiments:

Figure 1 is a schematic view of a system for desalinating saltwater while generating electricity according to one embodiment in which a reverse electrodialysis (RED) stack is used to generate electricity to power an electrodialysis reversal (EDR) stack that desalinates the saltwater.

Figure 2 is a schematic view of a system for desalinating saltwater while generating electricity according to another embodiment in which the RED and EDR stacks form part of a plant.

Figure 3 is a schematic view of a system for desalinating saltwater while generating electricity according to another embodiment in which multiple RED and EDR stacks are electrically coupled using a switching network and in which the switching network is electrically coupled to a power grid.

Figure 4 is a schematic view of a system for desalinating saltwater while generating electricity according to another embodiment in which the RED and EDR stacks form part of a combined desalination plant that includes a first desalination plant that discharges brine, and in which the brine discharged from the first desalination plant is used to replenish RED concentrate used in the RED stack.

Figure 5 is a schematic view of a system for desalinating saltwater while generating electricity according to another embodiment in which the RED and EDR stacks form part of a plant having a salt harvesting device and a reconcentrator that are used in conjunction to substantially reduce the amount of liquid discharged from the plant.

Figure 6 is a schematic view of a system for desalinating saltwater while generating electricity according to another embodiment in which the RED and EDR stacks form part of a combined desalination plant that includes a first desalination plant that discharges brine, and in which the brine discharged from the first desalination plant is subsequently desalinated using the EDR stack, and is then subsequently returned to the first desalination plant for further desalination.

DETAILED DESCRIPTION

Two ionic solutions that differ only in the concentration of the solute dissolved therein have different amounts of chemical energy. This difference in chemical energy is hereinafter referred to as "concentration difference energy". For example, when equal volumes of solutions of saltwater and freshwater are placed in adjacent chambers and are separated from each other solely by a membrane that is water, but not ion, permeable, the concentration difference energy results in the two solutions seeking concentration equilibrium whereby water from the freshwater chamber flows into the saltwater chamber.

Similarly, when equal volumes of solutions of saltwater and freshwater are placed in adjacent chambers and are separated from each other solely by a membrane or salt bridge that is ion permeable, but less permeable to water, the concentration difference energy results in the two solutions seeking concentration equilibrium whereby ions flow from one solution to the other. The concentration difference energy therefore results in a voltage difference between the two chambers. A membrane that is ion permeable, but less permeable to water, is hereinafter referred to as an "ion exchange membrane". Ion exchange membranes include "cation exchange membranes" and "anion exchange membranes". Cation and anion exchange membranes are those membranes that allow only cations (positively charged ions) and anions (negatively charged ions) to pass therethrough, respectively. Exemplary cation exchange membranes include Neosepta CMX, CM-1; Ralex CMH-PES; Fumasep FKE, FKD; and Selemion CMV membranes. Exemplary anion exchange membranes include Neosepta AM-1, AFN, AMX; Ralex AMH-PES; Fumasep FAD; and Selemion DVS, APS membranes.

Concentration difference energy can be used to generate electricity in a reverse electrodialysis (RED) stack. In an RED stack, a series of chambers are separated by an alternating series of anion and cation exchange membranes. The chambers are typically tilled with one of two different ionic solutions: an RED concentrate and an RED diluent. The RED concentrate has a higher ionic concentration than the RED diluent. For any given adjacent pair of these chambers, one of the chambers in the pair contains the RED concentrate and the other of the chambers in the pair contains the RED diluent. The concentration difference energy that results is used to generate a voltage and current at electrodes that sandwich the chambers.

Electrodialysis reversal (EDR) stacks use a similar arrangement of chambers to desalinate saltwater. As with an RED stack, an EDR stack includes a series of chambers separated by an alternating series of anion and cation exchange membranes. Also as with an RED stack, the chambers are typically filled with one of two different ionic solutions: an EDR diluent and product feed, which is the saltwater to be desalinated. For any given pair of these chambers, one of the chambers in the pair contains the product feed and the other of the chambers in the pair contains the EDR diluent. A DC voltage can be applied across the EDR stack in order to drive cations and anions out of the chambers containing the product feed into the EDR diluent, thus desalinating the product feed.

The present disclosure is directed at a method and system that utilizes concentration difference energy to desalinate saltwater. To this end, an RED stack is used to generate electricity using concentration difference energy. The electricity is then used to drive an EDR stack to desalinate the saltwater. As discussed in more detail below, this method and system are beneficial for several reasons. For example, because RED and EDR stacks are similarly constructed, when economical to do so the RED stack may be converted into an EDR stack and used to desalinate saltwater, or the EDR stack may be converted into an RED stack and used to generate electricity that can be sold to an electrical utility.

Referring now to Figure 1, there is depicted an EDR stack 110 that is electrically coupled to an RED stack 150 via a conductor 126. At the ends of the RED stack 150 are an anode 103 and a cathode 104, each of which is adjacent to a different electrolyte chamber 156. Between the electrolyte chambers 156 is an alternating series of RED concentrate chambers 162 and RED diluent chambers 164. Separating adjacent RED concentrate and diluent chambers 162, 164, and separating the RED concentrate and diluent chambers 162, 164 from the electrolyte chambers 156, is either an anion exchange membrane 151 or a cation exchange membrane 153. In a similar fashion as the RED concentrate and diluent chambers 162, 164, the anion and cation exchange membranes 151, 153 are also arranged in an alternating series within the RED stack 150.

When in operation, RED concentrate is pumped through the RED concentrate chambers 162 via RED concentrate manifolds 152, RED diluent is pumped through the RED diluent chambers 164 via RED diluent manifolds 154, and an electrolyte is pumped through the electrolyte chambers 156 via electrolyte manifolds 108. The RED concentrate and the RED diluent are both ionic solutions, with the RED concentrate having a higher ionic concentration than the RED diluent; in an exemplary embodiment, both the RED concentrate and the RED diluent are saltwater. Because the RED concentrate and RED diluent are at different concentrations, a concentration gradient exists between the RED concentrate chambers 162 and the RED diluent chambers 164, which results in anions moving from the RED concentrate chambers 162 to the RED diluent chambers 164 through the anion exchange membranes 151, and cations moving from the RED concentrate chambers 162 to the RED diluent chambers 164 through the cation exchange membranes 153. Cations also move from the RED concentrate and diluent chambers 162, 164 adjacent to the electrolyte chambers 156 at the end of the RED stack 150 and into the electrolyte chambers 156 themselves. This movement of cations and anions throughout the RED stack 150 is referred to as an "ionic current", and in the electrolyte chambers 156 oxidation (at the anode 103) and reduction (at the cathode 104) reactions occur that convert this ionic current into an DC electrical current, which allows the RED stack 150 to be used as a voltage source. The voltage that the RED stack 150 generates is applied to the EDR stack 110.

In the exemplary embodiment of Figure 1, the structure of the EDR stack 110 is similar to that of the RED stack 150, although the manner in which the EDR stack 110 is used differs from that of the RED stack 150, and the nomenclature used to describe the EDR stack 110 consequently differs from that used to describe the RED stack 150. When in operation, product feed to be desalinated is pumped through EDR product chambers 112 via product feed manifolds 102, EDR diluent is pumped through EDR diluent chambers 114 via EDR diluent manifolds 105, and the electrolyte is pumped through the electrolyte chambers 156 via the electrolyte manifolds 108. As with the RED stack 150, at the ends of the EDR stack 110 are the anode 103 and the cathode 104.

When the voltage generated by the RED stack 150 is applied to the EDR stack 110, the electrical current applied to the electrodes of the EDR stack 110 is converted into ionic current in the EDR stack 1 10's electrolyte chambers 156. The electric field applied to the EDR stack 110 drives cations and anions out of the EDR product chambers 112 and through cation exchange membranes 153 and anion exchange membranes 151, respectively, and into adjacent EDR diluent chambers 114. A power supply 128 can be placed in the electric circuit if the voltage that the RED stack 150 generates is less than the threshold voltage required to desalinate the product feed. Alternatively, an electrical load (not depicted) can be placed in the electric circuit and operate using electricity generated by the RED stack 150.

In the exemplary embodiment of Figure 1, the same electrolyte can be used in all four of the electrolyte chambers 156. Alternatively, one type of electrolyte can be used in the RED stack 150, while another type of electrolyte can be used in the EDR stack 110. In the further alternative, different anolytes and catholytes can be used in any or all of the stacks 110, 150. For example, four different electrolytes (two different anolytes and two different catholytes) may be used in the embodiment of Figure 1. An exemplary list of electrolytes follows in Table 1 :

Table 1: Exemplary Electrolytes Electrolyte Half Cell Reaction Standard

Reduction Potential (V) i. Na₂S0₄ (aq) Anode 2H₂0<=± 0₂ (g) + 4H⁺ + 4e -1.23 Products: H₂ (g) & Cathode 4H₂0 + 4e <F± 2H₂ (g) + 40H -0.83

O2 (g) Net 4H₂0 2H₂ (g) + 0₂ (g) -2.06 ii. NaCl (aq) Anode 4Cl^' ¾=* 2Cl₂ + 4e^" -1.36 Products: Cl₂ (g) & Cathode 4H₂0 + 4e- 2H2 (g) + 40H- -0.83 NaOH (aq) Net 4H₂0 2H₂ (g) + 2C1₂ (g) + 40H- -2.16 iii. NaOH (aq) Anode 40FT ¾=t 2H₂0 + 0₂ (g) + 4e^" -0.40

Cathode 4H₂0 + 4e^" <=* 2H₂ (g) + 40H -0.83 Net 2H₂0 2H₂ (g) + 0₂ (g) -1.23 iv. HC1 (aq) Anode 2H₂0 0₂ (g) + 4H⁺ + 4e -1.23

Cathode 2H⁺ + 2e^" <=* H₂ (g) 0.00

Net 2H₂0 2H₂ (g) + 0₂ (g) -1.23 v. HC1 (aq) with gas Anode H₂ (g) 2H⁺ + 2e 0.00 diffusion anode Cathode 2H⁺ + 2e^_ 5=t H₂ (g) 0.00

Net 0.00 vi. Na₃Fe(CN)₆ or Anode Fe(CN)(T <=* Fe(CN)₆ ^3" + e - 0.36 K₃Fe(CN)₆ (aq) and Cathode Fe(CN)₆ ^3" + e Fe(CN)₆ ^4_ 0.36 Na₄Fe(CN)₆ or Net 0.00 K₄Fe(CN)₆ (aq) in

bulk of NaCl(aq)

vii. Na₂S₄0₆ (aq) and Anode S₂0₃ ^2" S₄0₆ ^2" + 2e^" - 0.08 Na₂S₂0₃ (aq) Cathode S₄0₆ ^2" + 2e ¾=t S₂0₃ ^2" 0.08

Net 0.00

Different electrolytes can be beneficial in different ways. For example, the electrolyte combination in row vi. of Table 1 is beneficial in that two forms of Fe(CN)₆ are stable and stay in solution, and in that the net electrode voltage for the overall redox reaction is zero. More energy can therefore be used for one or both of desalination and electricity generation. As another example, the electrolyte in row i. of Table 1 is beneficial in that hydrogen gas can be produced, albeit by utilizing energy that could otherwise be used for one or both of desalination and electricity generation.

Periodically, the stacks 110, 150 may be de-scaled by reversing the polarity of one or both of the stacks 110, 150. In order to reverse the polarity of the RED stack 150, the chambers that act as the RED concentrate chambers 162 in a forward polarity are used as the RED diluent chambers 164 in a reverse polarity, and the chambers that act as the RED diluent chambers 164 in the forward polarity are used as the RED concentrate chambers 162 in the reverse polarity. Similarly, in order to reverse the polarity of the EDR stack 110, the chambers that act as the EDR product chambers 112 in the forward polarity are used as the EDR diluent chambers 114 in the reverse polarity, and the chambers that act as the EDR diluent chambers 114 in the forward polarity are used as the EDR product chambers 112 in the reverse polarity. For both of the stacks 110, 150, the electrode that acts as the anode 103 in the forward polarity acts as the cathode 104 in the reverse polarity, and the electrode that acts as the cathode 104 in the forward polarity acts as the anode 103 in the reverse polarity. Reversible electrodes made of materials such as titanium coated with a noble metal, such as platinum, strontium, rubidium or a mixture of any of these noble metals, are used to facilitate reversibility.

While Figure 1 depicts stacks 110, 150 in which the anion and cation exchange membranes 151, 153 and the anode 103 and cathode 104 are linear, in an alternative embodiment the membranes 151, 153 and the anode 103 and cathode 104 may be curved such that the stacks 110, 150 take on a wound spiral shape. The stacks 110, 150 may also be manufactured in the form of other shapes (not depicted), so long as ionic current is able to flow through the stacks 1 10, 150.

Referring now to Figure 2, there is depicted a CDE plant 201 ("concentration difference energy plant" or "CDE plant" 201) that is able to desalinate saltwater while generating electricity by exploiting concentration difference energy using the RED stack 150. The CDE plant 201 includes a water source 202, which can be the ocean or a brackish water supply, for example. The water source 202 is fluidly coupled to a pre-treatment system 206 that treats the saltwater prior to desalination by removing debris, suspended solids, and organic matter that can foul or plug the equipment used in the CDE plant 201. The pre-treatment system 206 is fluidly coupled to and sends treated saltwater to an EDR product feed vessel 208, an RED diluent vessel 210, and an EDR diluent vessel 212, which respectively store the product feed, and RED diluent, and the EDR diluent for use in the stacks 110, 150 that form part of the CDE plant 201. The EDR diluent vessel 212 and product feed vessel 208 are fluidly coupled to the EDR stack 110, while the RED diluent vessel 210 is fluidly coupled to the RED stack 150. Also fluidly coupled to the RED stack 150 is an RED concentrate vessel 214, which stores the RED concentrate for circulation through the RED stack 150.

To desalinate the product feed, the product feed and EDR diluent are pumped through the EDR stack 1 10 and the RED concentrate and RED diluent are pumped through the RED stack 150. The product feed in the EDR product chambers 112 is maintained at a higher pressure than the EDR diluent in the EDR diluent chambers 114 such that if leakage occurs, the product flows into the EDR diluent chambers 1 14 and not vice-versa. Consequently, if leakage occurs the product is not contaminated. Similarly, the RED diluent in the RED diluent chambers 164 is maintained at a higher pressure than the RED concentrate in the RED concentrate chambers 162 such that if leakage occurs, the RED diluent flows into the RED concentrate chambers 162 and not vice- versa. This is beneficial in the embodiment of Figure 2, as some of the RED diluent may be routed to a reconcentrator 230 (as discussed in more detail below) and used to generate the RED concentrate after exiting the RED stack 150, whereas the RED concentrate exiting the RED stack 150 is not used to generate the RED diluent. Consequently, leakage from the RED diluent chambers 164 into the RED concentrate chambers 162 is less detrimental than any leakage that occurs in the other direction. As they flow through the stacks 1 10, 150, the concentrations of the EDR diluent and the RED diluent increase, while the concentrations of the RED concentrate and the product feed decrease. The product feed that is desalinated ("product") is routed from the EDR stack 110 to a post-treatment system 218 before being stored in a product storage vessel 220. The EDR diluent that exits the EDR stack 110 is routed to any or all of three destinations: back to the EDR diluent vessel 212 via a control valve 234; to the RED diluent vessel 210 via a control valve 236; and back to the water source 202 via a check valve 223. The proportion of the EDR diluent that is diverted to each of these three destinations can be determined by plant designers and operators in order to satisfy energy efficiency, product volume relative to intake water volume (recovery ratio), and discharge concentration requirements. The RED diluent that exits the RED stack 150 is routed to any or all of four destinations: to the reconcentrator 230 that is used to generate the RED concentrate via a control valve 238; to the EDR diluent vessel 212 via a control valve 240; back to the RED diluent vessel 210 via a control valve 237; and back to the water source 202 via a check valve 222. As with the EDR diluent, the proportion of the RED diluent sent to any of these destinations can be determined by plant designers and operators in order to satisfy energy efficiency, recovery ratio, and discharge concentration requirements. For example, when used EDR and RED diluent is recycled to the EDR and RED vessels 212, 210, energy efficiency of the CDE plant 201 decreases because the concentration differences within the stacks 1 10, 150 decrease, but recovery ratio increases since less water is taken from the water source 202. Plant designers and operators can balance the benefits offered by reduced saltwater intake and pre-treatment against reduced energy efficiency by adjusting the concentration within the EDR and RED vessels 212, 210. As another example, by diverting some of the RED diluent exiting the RED stack 150 to the reconcentrator 230, salt that the RED concentrate loses while passing through the RED stack 150 can be replaced; accumulation of precipitated salt in the RED diluent vessel 210 can, to a certain degree, be prevented; and the concentration of the RED diluent in the RED diluent vessel 210 can more easily be kept relatively low.

The RED concentrate exiting the RED stack 150 is recycled either directly back to the RED concentrate vessel 214, or back to the RED concentrate vessel 214 via the reconcentrator 230 and control valves 239, 243. The reconcentrator 230 evaporates water to air to maintain or increase the concentration of the RED concentrate. In so doing, the reconcentrator 230 also removes water from the used RED diluent that is diverted to the reconcentrator 230 via the control valve 238. Exemplary evaporative reconcentrators include evaporative ponds, evaporative spray ponds, natural draft evaporative towers, forced draft evaporative towers, and other suitable devices that can evaporate water from a solution to air. Optionally, the reconcentrator 230 can include an apparatus for capturing and condensing water vapor from the relatively humid air that results from evaporation. This apparatus can be, for example, a cool surface downwind of an evaporative spray pond, or a cool condensing surface internal to an evaporative tower that collects condensed water but that does not allow it to mix with the RED concentrate within the tower. The RED concentrate stored in the RED concentrate vessel 214 can exit the RED concentrate vessel 214 not only by being pumped through the RED stack 150, but by being sent to the reconcentrator 230 via a control valve 241 and by being periodically discharged through a discharge line via a control valve 231 to prevent salt from accumulating within the RED concentrate vessel 214. During typical operation, the control valves 241, 243 balance flow rates into and out of the RED concentrate vessel 214 in order to maintain the fluid volume and temperature of the reconcentrator 230 at levels set by the plant operator. The RED concentrate that is periodically discharged via the control valve 231 can be harvested for commercial use using known methods for extracting solid salt from concentrated brines.

In the CDE plant 201, the EDR diluent vessel 212 acts as an EDR diluent mixer. As mentioned above, the EDR diluent vessel 212 is fluidly coupled, via piping, to the pre-treatment system 206, to the EDR stack 1 10 to receive the EDR diluent that has been used in the EDR stack 1 10, and to the RED stack 150 to receive the RED diluent that has been used in the RED stack 150. Typically, the system operator will want to maintain the EDR diluent stored within the EDR diluent vessel 212 at a certain concentration ("EDR diluent threshold") that is above that of the saltwater obtained via the pre-treatment system 206, and below the concentrations of the used EDR and RED diluents. The system operator can accomplish this by controlling the proportion at which used EDR diluent, used RED diluent, and saltwater from the pre-treatment system 206 enter the EDR diluent vessel 212 such that the resulting mixture has a concentration that equals the EDR diluent threshold.

Similarly, as mentioned above, the RED diluent vessel 210 is fluidly coupled via piping to the pre-treatment system 206, to the EDR stack 1 10 to receive used EDR diluent, and to the RED stack 150 to receive used RED diluent. Typically, the system operator will want to maintain the RED diluent stored within the RED diluent vessel 210 at a certain concentration ("RED diluent threshold") that is above that of the saltwater obtained via the pre-treatment system 206, and far below the concentration of the RED concentrate so as to facilitate electricity generation. However, alternatively, the system operator may want to increase the recovery ratio of the system; i.e., the amount of product produced compared to the amount of saltwater drawn from the water source 202. If this is the goal, the RED diluent can be maintained at a concentration higher than that of the EDR diluent through recirculation of the RED diluent exiting the RED stack 150 back to the RED diluent vessel 210 through the control valve 237. This recirculation reduces the amount of make-up saltwater drawn from the pre-treatment system 206, which accordingly reduces the amount of saltwater drawn from the water source 202 and increases the recovery ratio. The system operator can control the concentration of the RED diluent by controlling the proportion at which used RED diluent, used EDR diluent, and saltwater from the pre-treatment system 206 enter the RED diluent vessel 210 such that the resulting mixture has a concentration that equals the RED diluent threshold.

While the CDE plant 201 of Figure 2 uses the vessels 210, 212 as mixers, in alternative embodiments (not depicted) the mixers may simply be a series of pipes carrying different fluids meeting at a common junction; the junction may include a mixing element that introduce "swirl" (i.e.: turbulent flow) to the flowing fluids such that the fluids mix downstream of the junction.

The CDE plant 201 is able to store low grade thermal energy, such as solar energy, in the form of concentrated saltwater; this stored solar energy is harnessed to generate electricity using the RED stack 150 which, in turn, is used to desalinate the product feed in the EDR stack 110. Alternative forms of low grade thermal energy include waste heat (e.g.: from a power plant); exemplary waste heat is between 30 to 80 °C. Many industries that require desalinated water tend to have low grade heat sources available. Alternatively, low grade heat may be extracted from solar thermal sources such as black bottomed ponds or solar water heaters. Beneficially, geographical areas that are dry and arid and consequently likely to have the most use for desalination technology are also those areas that tend to have less humid atmospheres, receive a great deal of solar radiation, and therefore be those areas in which water readily evaporates and for which the CDE plant 201 is well suited.

While the CDE plant 201 depicted in Figure 2 uses only one of the EDR stacks 110, in alternative embodiments (not depicted) more than one of the EDR stacks 1 10 can be used. For example, several of the EDR stacks 1 10 can be fluidly coupled in series, in which case the product feed is partially desalinated in each of the EDR stacks 110. Using several of the EDR stacks 110 in series to desalinate the product feed facilitates a higher reduction in the salt concentration of the product feed. Alternatively or additionally, several of the EDR stacks 1 10 may be fluidly coupled together in parallel, which facilitates desalination of a greater volume of the product feed. Also alternatively, the CDE plant 201 may be operated in batch mode, in which case discrete batches of product are produced by circulating all or a portion of the product that is output from the EDR stack 110 back to the product feed vessel 208 until the solution in the product feed vessel 208 has the desired salinity content. Referring now to Figure 3, there is shown a schematic view of a network 301 of multiple RED stacks 150a,b,c and EDR stacks 110a,b,c electrically coupled together such that any one or more of the RED stacks 150a,b,c can be electrically coupled to any one or more of the EDR stacks 1 10a,b,c via an electrical bus 303 and switches 302a-h. The network 301 may be incorporated into the CDE plant 201 in place of the single RED and EDR stacks 150,1 10 depicted in Figure 2. For example, two of the RED stacks 150a,b may power the three EDR stacks 110a,b,c by closing the switches 302a,b,c,d,e,f,h and by opening the switch 302g. This may be done, for example, to set the net impedance of the active RED stacks to be the complex conjugate of the active EDR stacks, or to isolate the RED stack 150c in order to perform maintenance on it. Beneficially, setting the impedance of the active RED stacks to be complex conjugate of the EDR stacks will result in a relatively high amount of power being transferred from the RED stacks to the EDR stacks.

At any given time, the RED stacks 150a,b,c and EDR stacks 110a,b,c may be electrically coupled to a power grid in the form of an AC power system 390 via the electrical bus 303 and one of a controllable inverter 360 and a controllable rectifier 370. The inverter 360 converts DC to AC, and the rectifier 370 converts AC to DC. In the event that the RED stacks 150a,b,c produce more electricity than is used by the EDR stacks 110a,b,c while desalinating, switches 302i,k can be closed and switches 302j,l can be opened, and excess electricity can be converted into AC by the inverter 360 and transmitted to the AC power system 390. Conversely, if the RED stacks 150a,b,c are not producing sufficient electricity to desalinate the product feed flowing through the EDR stacks 1 10a,b,c, the switches 302i,k can be opened and the switches 302j,l can be closed, and electricity can be purchased from the AC power system 390, rectified into DC by the rectifier 370, and used by the EDR stacks 110a,b,c. In an alternative embodiment (not shown), the RED stacks 150a,b,c and EDR stacks 110a,b,c may be electrically coupled to a DC power system. In such an alternative embodiment, the rectifier 370 and inverter 360 are not required.

Based on a variety of factors, a plant operator can use any or all of the RED stacks 150a,b,c and EDR stacks 110a,b,c to generate a relatively high amount of electricity for delivery and sale to the AC power system 390 for sale at the cost of the ability to desalinate saltwater. This can be accomplished by operating both the RED stacks 150a,b,c and the EDR stacks 110a,b,c as RED stacks. Alternatively, the plant operator can use any or all of the RED stacks 150a,b,c and EDR stacks 110a,b,c to desalinate a relatively high volume of product feed using electricity purchased from the AC power system 390. This can be accomplished by operating both the RED stacks 150a,b,c and the EDR stacks 110a,b,c as EDR stacks. The decision of whether to produce electricity for sale or to purchase electricity for desalination will be made based on several factors. Examples of these factors include: water volume delivery need (as delivery need increases, more electricity is utilized for desalination); power system needs (if the AC power system 390 requires more electricity, more electricity can be generated for delivery to the AC power system 390); a relative increase in demand for or the price of water relative to demand for or the price of power, which favors increasing the rate at which saltwater is desalinated by purchasing power and using the power for desalination; and a relative increase in demand for or the price of power relative to demand for or the price of water, which favors increasing the rate at which electricity is generated and sold to the AC power system 390. For example, if power system need is relatively low (e.g.: a cost of electricity is below a low cost threshold of $30/MWh) and water volume delivery need is relatively high (e.g.: such that water costs more than $l/m³), all of the switches 302a-h can be closed, the switches 302 i,k can be opened, the switches 302 j,l can be closed, and all of the stacks 150a,b,c and 110a,b,c can be operated as EDR stacks and used to desalinate product feed by using the electricity from the AC power system 190 that is rectified by the rectifier 370. Because of the structural similarity between an EDR stack and an RED stack, the RED stacks 150a,b,c can be operated as EDR stacks and used to desalinate product feed by replacing the RED concentrate and RED diluent with product feed and EDR diluent, and by applying a sufficient voltage across the stacks 150a,b,c. Analogously, if power system need is relatively high (e.g.: the cost of electricity is above a high cost threshold of, for example, $80/MWh) and water volume delivery need is relatively low, all of the switches 302a-h can be closed, the switches 302i,k can be closed, the switches 302j,l can be opened, and all of the stacks 150a,b,c and 1 10a,b,c can be operated as RED stacks and used to generate electricity, and this electricity can then be transmitted to the AC power system 190 via the inverter 360. Because of the structural similarly between an EDR stack and an RED stack, the EDR stacks 1 10a,b,c can be operated as RED stacks and used to generate electricity by replacing the product feed and EDR diluent with RED concentrate and RED diluent. Beneficially, operating in the manner as described above allows the plant operator to balance product production and electricity generation in accordance with market conditions. Also beneficially, as water can be stored more readily than electricity, by using the RED stacks 150,a,b,c and the EDR stacks 110a,b,c to produce product during periods of low cost electricity, electricity can effectively be stored as product.

The network 301 may be used to store electrical energy when electricity is relatively inexpensive ("off-peak times") and used to produce electrical energy when electricity is relatively expensive ("peak times"). For example, during the off-peak times RED concentrate production can be increased by diverting a portion of the RED diluent to the reconcentrator 230, and, conversely, during peak times electricity production can subsequently be increased using the RED stacks. Furthermore, process piping that enables all of the stacks 110a,b,c and 150a,b,c to be used to produce RED concentrate from one or both of the RED and EDR diluents can be added as a way to produce RED concentrate in place of or in addition to running the reconcentrator 230.

Referring now to Figure 4, there is depicted a combined desalination plant 401 that uses brine discharge from a first desalination plant 402 to replenish the RED concentrate used in a second desalination plant, the CDE plant 201, that incorporates the RED stack 150 and the EDR stack 110. The first desalination plant 402 may utilize technology such as reverse osmosis, multi-stage flash (MSF), multiple effect distillation (MED), electrodialysis, or vapor recompression. The first desalination plant 402 is fluidly coupled to the water source 202 via a flow control mechanism such as a control valve 405; the first desalination plant 402 obtains saltwater from the water source and produces freshwater that it outputs via a freshwater outlet 406 and the brine discharge, which it outputs via a brine discharge outlet 408. When the saltwater input to the first desalination plant 402 is about 3.5% salt by mass, the freshwater output from the first desalination plant 402 is typically about 0.04% salt by mass, and the brine discharge is typically between about 6 to 10% salt by mass, more particularly is between about 7 to 8% salt by mass, and more particularly still about 7.5% salt by mass. The brine discharge output by the first desalination plant 402 is diverted to any or all of three destinations: back to the water source 202 via a check valve 412; directly to the RED concentrate vessel 214 via control valve 424; and to the reconcentrator 230 via control valve 420. By diverting the brine discharge to either of the reconcentrator 230 and the RED concentrate vessel 214, the amount of evaporation that the reconcentrator 230 needs to perform in order to generate the RED concentrate can be reduced. Beneficially, this can make use of the brine discharge, which is a waste product that the first desalination plant 402 outputs, while also reducing the energy that the reconcentrator 230 uses.

Exemplary concentrations of the various saltwater solutions utilized in the combined desalination plant 401 follow:

• water source 202 and product feed vessel 208 = 3.5% salt by mass;

• product storage vessel 220 - 0.04% salt by mass;

• EDR diluent vessel 212 = 3.5 to 5% salt by mass (concentration increases with increased recirculation through the valve 234);

• RED diluent vessel 210 = 3.5 to 8% salt by mass (concentration increases with increased recirculation through the valve 237);

• RED concentrate vessel 214 = 14 to 22% salt by mass; and

• brine discharge from the first desalination plant 402 = 6% to 10% salt by mass.

When the concentration of the brine discharge from the first desalination plant 402 exceeds the RED diluent concentration, it will be beneficial to operate the combined desalination plant 401 by using the brine discharge such that one or both of the RED concentrate vessel 214 and the reconcentrator 230 are replenished with a solution having a higher concentration than the RED diluent. The brine discharge may also contain thermal energy, as occurs when the first desalination plant 402 is a thermal desalination plant such as an MSF or MED plant or if the brine discharge passes through a heat exchanger (not shown) that accepts heat from another energy source (e.g.: waste heat from a process plant) prior to entering the reconcentrator 230. Added thermal energy, regardless of whether it is from waste heat, the sun, or inherently present in the brine discharge by virtue of the manner in which the first desalination plant 402 produces freshwater, increases the temperature of the RED concentrate and therefore improves evaporation rates.

As a result of using the brine discharge from the first desalination plant 402, the saltwater discharged from the CDE plant 201 can be adjusted to have a higher or lower concentration than the brine discharge from the first desalination plant 402; this is achieved by adjusting the concentration of the EDR diluent by modulating the valves 234, 240, 236, 237 as described above. In the embodiment of Figure 4, the saltwater discharged from the CDE plant 201 has a concentration equal to the average, weighted by volume, of the concentration of the saltwater discharged via the check valves 222, 223; used RED diluent is discharged via the check valve 222, while used EDR diluent is discharged via the check valve 223. By outputting a lower concentration discharge than the first desalination plant 402, the combined desalination plant 401 mitigates concerns related to discharging relatively high concentration saltwater into the environment. Alternatively, the combined desalination plant 401 can be configured to output a higher concentration discharge than the brine discharged from the first desalination plant 402 alone, in which case overall brine volume is decreased. The combined desalination plant 401 can accomplish this by decreasing the amount of make-up saltwater channelled from the water source 202 to the pre-treatment system 206 through control valve 407, and correspondingly increasing the rate at which used RED and EDR diluent is recycled back the RED and EDR diluent vessels 210, 212, respectively, through the control valves 237, 234. Doing so increases the concentration of the RED and EDR diluents in the RED and EDR diluent vessels 210, 212 while reducing the amount of saltwater entering the plant 401 from the water source 202, which increases the salinity and decreases the volume of the saltwater discharged through the check valves 222, 223. This is particularly beneficial for inland desalination plants, as a lower volume of brine translates to lower costs for handling and otherwise disposing of the brine. In addition, locating the CDE plant 201 near the first desalination plant 402 facilitates cost reductions by allowing the two plants 201, 402 to share infrastructure such as post-treatment systems and water storage or distribution systems. Other exemplary shared infrastructure includes saltwater intakes, saltwater outlets, pre-treatment systems, post-treatment systems, power lines, water mains, control systems, and operations staff.

Referring now to Figure 5, there is shown a zero liquid discharge plant ("ZLD plant") 501 that can be used to produce precipitated salts, thereby effectively outputting no saltwater discharge. The exemplary ZLD plant 501 depicted in Figure 5 is substantially similar to the CDE plant 201 depicted in Figure 2, with certain exceptions. One exception is that the RED diluent vessel 210 does not receive water directly from the pre-treatment system 206. No water from the pre- treatment system 206 is directly added to the RED diluent vessel 210 because doing so could dilute the RED diluent such that liquid may have to be discharged from the plant 501 during operation, which is undesirable in a ZLD plant. Another exception is that the EDR diluent vessel 212 does not receive any of the used RED diluent. Adding used RED diluent to the EDR diluent vessel 212 would increase the concentration of the EDR diluent, thereby decreasing the concentration gradient between the EDR concentrate and EDR diluent and increasing the energy required to desalinate the product. Keeping the concentration of the RED diluent relatively high by not adding water to the RED diluent vessel 210 decreases the amount of electricity that the RED stack 150 generates; beneficially, however, keeping the concentration of the EDR diluent relatively low by not diverting used RED diluent to the EDR diluent vessel 208 decreases the amount of electricity required for desalination. In alternative embodiments of the ZLD plant 501 (not depicted), one or both of used RED diluent and water from the pre-treatment system 206 may be diverted to the EDR diluent vessel 208 and the RED diluent vessel 210, respectively. A third exception is that the discharge line and the control valve 231 that are used to discharge RED concentrate from the RED concentrate vessel 214 in the plant 201 are not present. Instead, as discussed in further detail below, a second reconcentrator 506 ("zero liquid discharge reconcentrator" or "ZLD reconcentrator") is fluidly coupled to the output of the reconcentrator 230 via a control valve 504. By controlling the degree to which the control valves 504, 243 are opened, the fluid leaving the reconcentrator 230 can be divided between the RED concentrate vessel 214 and the ZLD reconcentrator 506, as desired. A fourth exception, related to the ZLD reconcentrator 506, is that neither the used RED diluent nor the used EDR diluent are discharged directly back to the water source 202.

During normal operation of the ZLD plant 501, the RED stack 150 generates electricity and the EDR stack 110 desalinates the product feed, as described above. The concentration of the RED diluent stored in the RED diluent vessel 210 is maintained by mixing together used EDR diluent (which tends to lower the concentration of the RED diluent stored in the RED diluent vessel 210) and used RED diluent (which tends to raise the concentration of the RED diluent stored in the RED diluent vessel 210). The concentration of the EDR diluent stored in the EDR diluent vessel 212 is maintained by mixing together treated water from the water source 202 (which tends to lower the concentration of the EDR diluent stored in the EDR diluent vessel 212) and used EDR diluent (which tends to raise the concentration of the EDR diluent stored in the EDR diluent vessel 212). Used RED concentrate is routed to the reconcentrator 230 where the concentration of the used RED concentrate is increased through evaporation. From the reconcentrator 230, the used RED concentrate can be routed to the ZLD reconcentrator 506 where the concentration of the used RED concentrate is further increased, and then to a salt harvesting device in which substantially all the water can be evaporated from the RED concentrate and consequently from which can be harvested precipitated salts; an exemplary salt harvesting device is a sump 508. By increasing the concentration of the used RED concentrate in stages, mixing losses are reduced. Without the ZLD reconcentrator 506, operating the reconcentrator 230 such that the RED concentrate leaving the reconcentrator 230 has a salt concentration of above about 18% can be detrimental in that salt may precipitate within the CDE plant 201. In the embodiment of Figure 5, however, the reconcentrator 230 may increase the salt concentration of the RED concentrate to less than 18% and then transfer the RED concentrate to the ZLD reconcentrator 506, which increases the concentration of the concentrate to above 18%, and more particularly to concentrations higher than about 24% to achieve salt crystallization. Precipitated salt can then be removed from the sump 508. Beneficially, this lowers the risk that salt precipitation occurs anywhere in the CDE plant 201 aside from the ZLD reconcentrator 506 and the sump 508, and allows salt harvesting to occur in a controlled fashion.

In an alternative embodiment (not shown), either of the reconcentrator 230 and the ZLD reconcentrator 506 may output RED concentrate to an alternative type of salt harvesting device. For example, other exemplary salt harvesting devices are similar to evaporative reconcentrators - evaporative ponds, evaporative spray ponds, natural draft evaporative towers, and forced draft evaporative towers - with the ability to shut down the reconcentrator and collect the precipitated salts. A single one of the salt harvesting devices may be operated in batch mode in combination with sufficient RED concentrate storage volume, or many of the salt harvesting devices may be operated in batch mode at various concentrations approaching precipitation. Salt harvesting is beneficial in that harvested salt can be sold and not discharged into the environment, which may make obtaining regulatory approval for plant construction easier.

In an alternative embodiment (not shown), the ZLD plant 501 can alternatively produce a low volume, high concentration saltwater discharge of 12% salt by mass or greater, and this high concentration saltwater discharge can be discharged back to the water source 202 or otherwise stored or transported away for further treatment. For example, in an alternative embodiment it may be possible to discharge used RED and EDR diluent directly back to the water source 202.

The ZLD plant 501 can be used, for example, to desalinate underground saline aquifers. When ocean water is desalinated, the resulting brine is typically returned to the ocean. When an underground aquifer is desalinated, the brine may be sent back down to the saltwater aquifer; however, this increases the salt concentration of the aquifer, making further desalination from the same aquifer more difficult. The ZLD plant 501 allows saltwater to be desalinated without producing brine discharge. Being able to reduce the volume of saltwater in the aquifer without returning the brine to the aquifer also beneficially creates storage space in the aquifer into which carbon dioxide produced by an external process can be stored, for example.

Referring now to Figure 6, there is depicted another embodiment of the combined desalination plant 401 that includes the first desalination plant 602 fluidly coupled to the ZLD plant 501 such that brine discharged from the first desalination plant 602 is used as product feed in the CDE plant 201. In contrast to the embodiment of Figure 5, brine is not directly channelled from the first desalination plant 602 into the reconcentrator 230 of the ZLD plant 501. Instead, the ZLD plant 501 uses the brine discharged from the first desalination plant 602 instead of saltwater obtained from the water source 202. The ZLD plant 501 partially desalinates the brine in that it reduces the salt content of the brine, but not to levels suitable for potable, drinking water. The ZLD plant 501 and the first desalination plant 602 are fluidly coupled such that partially desalinated brine leaving the ZLD plant 501 is returned to the first desalination plant 602 for further desalination. Following this further desalination, the first desalination plant 602 outputs the product to the post-treatment system 218 and to the product vessel 220. In the context of Figure 6, "partially desalinated brine" refers to brine that has been desalinated, but not to the extent as ultimately desired by the plant operator. For example, if the combined desalination plan 401 is designed to output product in the form of potable drinking water at 0.04% salt by mass, then "partial desalination" refers to desalinating the brine discharged by the first desalination plant 602 to a level above 0.04%. In one embodiment, partial desalination may involve desalinating the brine discharged by the first desalination plant 401 to a concentration roughly equal to that of the water source 202, which is typically around 3.5% salt by mass. The combined desalination plant 401 of Figure 6 is beneficial in that it results in a higher recovery ratio than the first desalination plant 602 is capable of alone. In other words, by desalinating brine that would otherwise have to be discharged back to a concentration that the first desalination plant 602 can process, freshwater production of the first desalination plant 602 is increased while discharge volume of the combined desalination plant 401 is decreased. Decreasing brine volume is beneficial in that a lower volume of brine translates into lower costs associated with disposing or otherwise handling the brine, which is particularly beneficial for inland desalination plants.

Additionally, the effectiveness of desalination technologies such as RO, MED, and MSF is limited by scaling. However, the ZLD plant 501 can perform de-scaling by reversing ionic current, as discussed above. Furthermore, the ZLD plant 501 can remove the larger ionic species that are primarily responsible for scaling, such as Ca²⁺ or S0₄ ^2", efficiently, which is beneficial when partially desalinated brine is returned to the first desalination plant 602 for further desalination.

In an alternative embodiment (not depicted), the first desalination plant 602 may be similarly coupled to the CDE plant 201. That is, brine discharged from the first desalination plant 602 may be used as product feed and partially desalinated by the CDE plant 201 depicted in Figure 2, with the partially desalinated brine being returned to the first desalination plant 602 for additional desalination.

Any of the foregoing methods that are described as being used in conjunction with any of the foregoing stacks, systems, and plants can be stored on a computer readable medium for execution by a any suitable controller, such as a processor, microcontroller, programmable logic controller, field programmable gate array, or can be implemented in hardware using, for example, an application-specific integrated circuit. Exemplary computer readable media include disc-based media such as CD-ROMs and DVDs, magnetic media such as hard drives and other forms of magnetic disk storage, semiconductor based media such as flash media, random access memory, and read only memory. A control system (not shown), communicatively coupled to the components in the foregoing plants and systems, may access and execute the methods stored on the computer readable medium so as to automatically control the operation of any of the foregoing plants and systems.

For the sake of convenience, the exemplary embodiments above are described as various interconnected functional blocks or distinct modules. This is not necessary, however, and there may be cases where these functional blocks or modules are equivalently aggregated into a single stack, plant or operation with unclear boundaries. In any event, the functional blocks and software modules or features of the flexible interface can be implemented by themselves, or in combination.

While particular example embodiments have been described in the foregoing, it is to be understood that other embodiments are possible and are intended to be included herein. It will be clear to any person skilled in the art that modifications of and adjustments to the foregoing exemplary embodiments, not shown, are possible.

Claims

1. A method for desalinating saltwater while generating electricity, the method comprising:

(a) generating electricity using a reverse electrodialysis (RED) stack; and

(b) utilizing the electricity to desalinate saltwater in an electrodialysis reversal (EDR) stack.

2. A method as claimed in claim 1 wherein the RED stack has an output impedance that is matched to an input impedance of the EDR stack.

3. A method as claimed in claim 1 wherein the RED stack comprises one of a plurality of separate RED stacks electrically coupled to each other to generate the electricity and the EDR stack comprises one of a plurality of separate EDR stacks electrically coupled to each other to desalinate the saltwater, and further comprising:

(a) determining a cost of electricity; and

(b) when the cost of electricity is below a low cost threshold, doing one or both of powering at least some of the separate EDR stacks using purchased electricity and converting at least some of the separate RED stacks into the EDR stacks to desalinate the saltwater.

4. A method as claimed in claim 1 wherein the RED stack comprises one of a plurality of separate RED stacks electrically coupled to each other to generate the electricity and the EDR stack comprises one of a plurality of separate EDR stacks electrically coupled to each other to desalinate the saltwater, and further comprising:

(a) determining a cost of electricity; and

(b) when the cost of electricity is above a high cost threshold, doing one or both of selling electricity generated using at least some of the RED stacks and converting at least some of the separate EDR stacks into the RED stacks to generate the electricity.

A method as claimed in claim 1 wherein the RED stack comprises one of a plurality of RED stacks and the EDR stack comprises one of a plurality of EDR stacks, and further comprising utilizing a switching network configured to electrically couple any one or more of the RED stacks to any one or more of the EDR stacks.

A method as claimed in claim 5 wherein the any one or more of the RED stacks have an output impedance that is matched to an input impedance of the any one or more EDR stacks.

A method as claimed in claim 1 further comprising transmitting a portion of the electricity generated using the RED stack to a power grid.

A method as claimed in claim 1 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough, and further comprising:

(a) evaporating water from at least some of the RED concentrate and the RED diluent exiting the RED stack; and

(b) harvesting salt that precipitates following evaporation.

A method as claimed in claim 8 wherein substantially all the water is evaporated from the at least some of the RED concentrate and the RED diluent.

A method as claimed in claim 1 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough, and further comprising recycling one or both of the RED concentrate and the RED diluent exiting the RED stack for use as the RED concentrate that enters the RED stack.

A method as claimed in claim 10 further comprising heating one or both of the RED concentrate and the RED diluent that is recycled utilizing waste heat from a process plant to facilitate evaporation to air.

A method as claimed in claim 1 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough and the EDR stack has an EDR diluent and product that is desalinated flowing therethrough, and further comprising recycling the EDR diluent exiting the EDR stack as the RED diluent entering the RED stack.

13. A method as claimed in claim 1 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough and the EDR stack has an EDR diluent and product that is desalinated flowing therethrough, and further comprising:

(a) mixing the EDR diluent exiting the EDR stack with a solution having a concentration less than an EDR diluent threshold to lower the concentration of the EDR diluent; and

(b) following mixing, recycling the EDR diluent exiting the EDR stack by using it as the EDR diluent entering the EDR stack.

14. A method as claimed in claim 1 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough and the EDR stack has an EDR diluent and product that is desalinated flowing therethrough, and further comprising:

(a) mixing the RED diluent exiting the RED stack with a solution having a concentration less than an RED diluent threshold to lower the concentration of the RED diluent; and

(b) following mixing, recycling the RED diluent exiting the RED stack by using it as the RED diluent entering the RED stack.

15. A method as claimed in claim 1 further comprising de-scaling the RED stack by reversing the polarity of the RED stack.

16. A system for desalinating saltwater while generating electricity, the system comprising:

(a) a reverse electrodialysis (RED) stack configured to generate electricity; and

(b) an electrodialysis reversal (EDR) stack electrically coupled to the RED stack such that the electricity generated by the RED stack is used to desalinate saltwater in the EDR stack.

17. A system as claimed in claim 16 wherein the RED stack has an output impedance that is matched to an input impedance of the EDR stack.

18. A system as claimed in claim 16 wherein the RED stack comprises one of a plurality of separate RED stacks electrically coupled to each other to generate the electricity and the EDR stack comprises one of a plurality of separate EDR stacks electrically coupled to each other to desalinate the saltwater, and further comprising a switching network electrically coupled between the plurality of RED stacks and the plurality of EDR stacks such that any one or more of the RED stacks can be electrically coupled to any one or more of the EDR stacks.

19. A system as claimed in claim 18 wherein the any one or more of the RED stacks have an output impedance that is matched to an input impedance of the any one or more EDR stacks.

20. A system as claimed in claim 16 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough, and further comprising:

(a) a salt harvesting device; and

(b) a reconcentrator fluidly coupled to an outlet of the RED stack through which the RED concentrate and the RED diluent exit the RED stack and to the salt harvesting device, the reconcentrator configured to evaporate water from at least some of the RED concentrate and the RED diluent exiting the RED stack to facilitate harvesting of salt from the RED concentrate and RED diluent using the salt harvesting device.

21. A system as claimed in claim 20 wherein substantially all the water is evaporated from the at least some of the RED concentrate and the RED diluent.

22. A system as claimed in claim 16 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough, and further comprising a reconcentrator fluidly coupled between an outlet of the RED stack through which one or both of the RED concentrate and RED diluent exit the RED stack and an inlet of the RED stack through which the RED concentrate enters the RED stack such that one or both of the RED concentrate and RED diluent exiting the RED stack can be recycled for use as the RED concentrate that enters the RED stack.

23. A system as claimed in claim 22 further comprising:

(a) a process plant comprising a source of waste heat; and

(b) a heat exchanger fluidly coupled between the outlet of the RED stack and the reconcentrator configured to heat one or both of the RED concentrate and RED diluent to facilitate evaporation to air.

24. A system as claimed in claim 16 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough and the EDR stack has an EDR diluent and product that is desalinated flowing therethrough, and further comprising piping fluidly coupling an outlet of the EDR stack through which the EDR diluent exits the EDR stack to an inlet of the RED stack through which the RED diluent enters the RED stack such that the EDR diluent exiting the EDR stack can be recycled as the RED diluent entering the RED stack.

25. A system as claimed in claim 16 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough and the EDR stack has an EDR diluent and product that is desalinated flowing therethrough, and further comprising:

(a) piping fluidly coupling an outlet of the EDR stack through which the EDR diluent exits the EDR stack to an inlet of the EDR stack through the EDR diluent enters the EDR stack such that the EDR diluent exiting the EDR stack can be recycled as the EDR diluent entering the EDR stack; and

(b) an EDR diluent mixer disposed along the piping and fluidly coupled to a source of solution having a concentration less than an EDR diluent threshold, and wherein the EDR diluent mixer is configured to mix the solution and the EDR diluent being recycled to lower the concentration of the EDR diluent prior to the EDR diluent entering the EDR stack.

6. A system as claimed in claim 16 wherein the RED stack has an RED concentrate and an RED diluent flowing therethrough and the EDR stack has an EDR diluent and product that is desalinated flowing therethrough, and further comprising:

(a) piping fluidly coupling an outlet of the RED stack through which the RED diluent exits the RED stack to an inlet of the RED stack through which the RED diluent enters the RED stack such that the RED diluent exiting the RED stack can be recycled as the RED diluent entering the RED stack; and

(b) an RED diluent mixer fluidly coupled along the piping and to a source of solution having a concentration less than an RED diluent threshold, and wherein the RED diluent mixer is configured to mix the solution and the RED diluent being recycled to lower the concentration of the RED diluent prior to the RED diluent entering the RED stack.

27. A system as claimed in claim 16 wherein the RED stack is de-scaled by reversing the polarity of the RED stack.

28. A system for desalinating saltwater while generating electricity, the system comprising:

(a) a first desalination plant for desalinating saltwater and having a brine discharge outlet; and

(b) a second desalination plant comprising:

(i) a reverse electrodialysis (RED) stack configured to generate electricity, the RED stack having an RED concentrate and an RED diluent flowing therethrough; and

(ii) an electrodialysis reversal (EDR) stack having an EDR diluent and product flowing therethrough, the EDR stack electrically coupled to the RED stack such that the electricity generated by the RED stack is used to desalinate the product in the EDR stack; wherein the brine discharge outlet is fluidly coupled to the EDR stack such that brine discharged from the first desalination plant is desalinated.

29. A system as claimed in claim 28 wherein the brine discharge outlet is fluidly coupled to the RED stack such that the brine discharged from the first desalination plant is used as the RED concentrate to generate the electricity.

30. A system as claimed in claim 29 wherein the second desalination plant comprises a reconcentrator fluidly coupled between the brine discharge outlet and the RED stack.

31. A system as claimed in claim 28 wherein an outlet of the EDR stack is fluidly coupled to the first desalination plant such that desalinated brine it output to the first desalination plant for further desalination.

32. A system as claimed in claim 28 wherein the RED stack is de-scaled by reversing the polarity of the RED stack.

33. A method for desalinating saltwater while generating electricity, the method comprising:

(a) obtaining brine discharged from a first desalination plant; and

(b) utilizing the brine as product to be desalinated in a second desalination plant configured to desalinate saltwater according to a method comprising:

(i) generating electricity using a reverse electrodialysis (RED) stack, the RED stack having an RED concentrate and an RED diluent flowing therethrough; and

(ii) utilizing the electricity to desalinate saltwater in an electrodialysis reversal (EDR) stack having an EDR diluent and the product flowing therethrough, the EDR stack electrically coupled to the RED stack such that the electricity generated by the RED stack is used to desalinate the product in the EDR stack.

34. A method as claimed in claim 33 further comprising utilizing the brine discharged from the first desalination plant as the RED concentrate.

35. A method as claimed in claim 34 further comprising evaporating a portion of the brine to air to increase its concentration prior to using it as the RED concentrate.

36. A method as claimed in claim 33 further comprising returning desalinated brine from the second desalination plant to the first desalination plant for further desalination.

37. A method as claimed in claim 33 further comprising de-scaling the RED stack by reversing the polarity of the RED stack.

38. A computer readable medium having encoded thereon statements and instructions to cause a controller to execute a method as claimed in any one of claims 1 to 15 and 33 to 37.