Comparison of Energy Efficiency and Power Density in Pressure

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Comparison of Energy Efficiency and Power Density in Pressure Retarded Osmosis and Reverse Electrodialysis Ngai Yin Yip, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5029316 • Publication Date (Web): 26 Aug 2014 Downloaded from http://pubs.acs.org on September 1, 2014

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Environmental Science & Technology

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Comparison of Energy Efficiency and Power Density in Pressure Retarded Osmosis and Reverse Electrodialysis

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Revised: August 20, 2014

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Ngai Yin Yip and Menachem Elimelech*

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Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286, USA

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*

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Corresponding author: Email: [email protected], Phone: +1 203 432 2789.

ACS Paragon Plus Environment


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ABSTRACT

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Pressure retarded osmosis (PRO) and reveres electrodialysis (RED) are emerging membrane-

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based technologies that can convert chemical energy in salinity gradients to useful work. The

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two processes have intrinsically different working principles: controlled mixing in PRO is

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achieved by water permeation across salt-rejecting membranes, whereas RED is driven by ion

15

flux across charged membranes. This study compares the energy efficiency and power density

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performance of PRO and RED with simulated technologically-available membranes for natural,

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anthropogenic, and engineered salinity gradients (seawater–river water, desalination brine–

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wastewater, and synthetic hypersaline solutions, respectively). The analysis shows that PRO can

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achieve both greater efficiencies (54–56%) and higher power densities (2.4–38 W/m2) than RED

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(18–38% and 0.77–1.2 W/m2). The superior efficiency is attributed to the ability of PRO

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membranes to more effectively utilize the salinity difference to drive water permeation and better

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suppress the detrimental leakage of salts. On the other hand, the low conductivity of currently

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available ion exchange membranes impedes RED ion flux and, thus, constrains the power

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density. Both technologies exhibit a tradeoff between efficiency and power density: employing

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more permeable but less selective membranes can enhance the power density, but undesired

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entropy production due to uncontrolled mixing increases and some efficiency is sacrificed.

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When the concentration difference is increased (i.e., natural→anthropogenic→engineered

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salinity gradients), PRO osmotic pressure difference rises proportionally but not so for RED

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Nernst potential, which has logarithmic dependence on the solution concentration. Because of

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this inherently different characteristic, RED is unable to take advantage of larger salinity

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gradients, whereas PRO power density is considerably enhanced. Additionally, high solution

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concentrations suppress the Donnan exclusion effect of the charged RED membranes, severely

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reducing the permselectivity and diminishing the energy conversion efficiency. This study

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indicates that PRO is more suitable to extract energy from a range of salinity gradients, while

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significant advancements in ion exchange membranes are likely necessary for RED to be

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competitive with PRO.

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INTRODUCTION

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The Gibbs free energy from mixing two solutions of different concentration can be harnessed for

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useful work.1, 2 The salinity gradient can be from various sources,2 such as, the mixing of fresh

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river water with salty seawater which occurs naturally as part of the hydrological cycle. A recent

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study showed that the ~37,300 km3 annual global river discharge represents a substantial source

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of clean and renewable energy that can potentially generate electricity for over half a billion

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people.3 Alternatively, anthropogenic waste streams can be utilized; e.g., concentrated brine

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from desalination plants can be paired with wastewater effluent from treatment facilities and the

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power generated can partially offset the desalination energy cost.4,

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approximately one-third of the energy consumed as thermal losses and the worldwide ~9,400

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TWh/y of mostly low-temperature rejected heat can be recaptured for useful work production.6

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Closed systems that hybridize energy production technologies with thermal separation methods

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can access this industrial waste heat, and also low-temperature geothermal energy, using

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engineered salinity gradients.2 Useful work is generated from the controlled mixing of synthetic

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hypersaline solutions in an energy production stage, while a solution regeneration stage

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thermally separates the mixture to reconstitute the salinity gradient, essentially converting

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thermal energy to electricity.

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Industries discharge

Several approaches have been proposed to harness salinity energy, including pressure

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retarded osmosis (PRO),7,

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osmotically-induced nanofluidic electric currents,13 and hydrogels.14 Among the technologies,

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membrane-based PRO and RED have been demonstrated at pilot-scale and are considerably

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more advanced.8,

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across a salt-rejecting semipermeable membrane into a more concentrated “draw” solution. The

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expanding volume of the draw solution is depressurized through a hydroturbine to produce

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useful work. RED, on the other hand, is driven by the Nernst potential — another manifestation

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of the chemical potential difference. The technology employs ion exchange membrane pairs to

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selectively allow counter-ion permeation and the net ion flux is converted to an electric current

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for power generation.

15

8

reverse electrodialysis (RED),9,

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capacitive mixing,11,

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PRO utilizes the osmotic pressure difference to drive water permeation

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PRO and RED have fundamentally different working principles as well as operating

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constraints and, thus, are anticipated to have different performance in salinity energy extraction.

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Furthermore, the permeability and selectivity of the polymeric membranes used in PRO and the

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ion exchange membranes employed in RED are not at equivalent technological levels.16,

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Therefore, it would be instructive to quantitatively analyze the strengths and weaknesses

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characteristic to the technologies. Evaluation of PRO and RED potential performance with

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current state-of-the-art membranes can shed light on the viability of harnessing energy from

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various salinity gradient sources, and reveal vital insights that can inform future development of

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the technologies. Numerous studies have examined the energy conversion performance of the

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two processes separately.3, 18-22 The few that have pitted PRO and RED in direct comparison

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examined either power density or efficiency,23, 24 but no study encompassed both metrics.

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This study aims to identify the comparative advantages of PRO and RED and examine their

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practical feasibility for salinity energy extraction.

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available high performance PRO and RED membranes are employed in the analysis, and tradeoff

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relations governing the membrane parameters are incorporated into the evaluation. The energy

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efficiency and power density attainable with the simulated membranes are simultaneously

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assessed for three categories of salinity gradients: natural (seawater–river water), anthropogenic

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(desalination brine–wastewater effluent), and engineered (synthetic hypersaline solutions). The

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prospects and limitations intrinsic to PRO and RED are highlighted, and the working principles

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and primary membrane parameters affecting performance are identified and discussed. The

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analytical insights of this study can serve to guide membrane and process development for the

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advancement of PRO and RED energy production from salinity gradients.

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ENERGY PRODUCTION FROM SALINITY GRADIENTS

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Both PRO and RED convert the chemical energy stored in salinity gradients to useful work by

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the controlled mixing of two solutions of different concentrations.

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technologies have different working principles, operating considerations, and membrane

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properties and, thus, are expected to produce distinct power generation performance. In this

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section, the two technologies are briefly introduced and the governing transport equations are

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presented. Detailed description of the processes can be found in previous studies.3, 8, 10, 25 Here,

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the fundamental differences between PRO and RED are emphasized, and the different salinity

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gradients used in this analysis are discussed.

3

Properties simulating technologically-


However, the two

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Pressure Retarded Osmosis. Figure 1A shows the schematic of a PRO process, where

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a semipermeable membrane separates a low concentration (LC) solution and a pressurized high

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concentration (HC) solution. Because of the difference in salt concentration, an osmotic pressure

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difference, ∆πm, develops across the membrane that drives water permeation from the LC

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solution into the more concentrated HC solution. The expanding volume of the pressurized HC

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solution powers a hydroturbine to produce useful work.3 As the semipermeable membrane is not

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perfectly selective, some salt diffuses from the saltier side to the more dilute LC solution (Figure

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1). The leakage of salts across the membrane represents an uncontrolled mixing that undesirably

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lowers the extractable energy in PRO.

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FIGURE 1

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An actual PRO system would consist of membrane modules in continuous flow operation.8, 26

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Although Figure 1A seemingly depicts a batch process, the model is equivalent to a module

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operating in co-current configuration by assuming ideal plug-flow (i.e., the solution

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concentrations while advancing along the axial length of the module correspond to the conditions

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in the batch process as controlled mixing progresses). The governing equation for water flux in

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PRO, Jw, is27

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   −Jw   JwS   π HC exp    − π LC exp    k  D     J w = A ( ∆π m − ∆P ) = A  − ∆P  1 + B  exp  J w S  − exp  − J w         J w    D   k 

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where πHC and πLC are the osmotic pressures of the HC and LC solutions, respectively, D is the

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diffusion coefficient of the salt in the LC solution, and ∆P is the hydraulic pressure applied to the

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HC solution. The osmotic pressure is determined by the salt concentration, c, and can be

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approximated using the van’t Hoff equation:3

(1)

117

π ≈ ν RgTc

118

for relatively dilute solutions (90 bar (1,300 psi) while

521

retaining their structural and transport properties.

522

reported in literature is 48 bar (700 psi) on a coupon-sized membrane in a laboratory testing

523

cell.35 The large pressurization necessary for high salinity operation is likely to detrimentally

18

Presently, the highest ∆P demonstration



524

alter the membrane properties and, therefore, improving the membrane mechanical robustness,

525

together with the apt design of spacer support and membrane module, will be critical.35, 42

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The comparison presented in this study centers on membrane-level performance. Actual

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power generation installations would further comprise engineering components that are different

528

for PRO and RED. For example, PRO requires pumps, pressure exchangers, and hydroturbines

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to convert mechanical expansion of the HC draw solution to electrical energy. RED employs a

530

reversible redox couple at the end electrodes to directly convert salinity energy to electricity

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without a mechanical intermediate, and requires pumping energy to circulate the solutions

532

through narrow stack channels. Additionally, foulants present in natural and anthropogenic input

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streams (e.g., river water and wastewater effluent) can detrimentally lower PRO and RED

534

productivity.54, 55 Further cost-efficiency analysis of the system-level components, taking into

535

account fouling impacts and pretreatment, are necessary to more accurately assess the practical

536

potential of power generation from salinity gradients.

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ASSOCIATED CONTENT

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Supporting Information

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Details of the molar concentration approximations for PRO and RED driving force; discussion

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on the practical operation of PRO module and RED stack; activity coefficient of water and salt

541

(NaCl) as a function of mole fraction (Figure S1); osmotic pressure difference and Nernst

542

potential, with and without simplifying assumptions, as a function of the molar concentration

543

(Figure S2); representative plot of power density and efficiency for a range of constant applied

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hydraulic pressure in PRO and constant external load resistance in RED (Figure S3). This

545

material is available free of charge via the Internet at http://pubs.acs.org.

546 547

ACKNOWLEDGMENTS

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We acknowledge the support received from the National Science Foundation under Award Number

549

CBET 1232619 and from the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department

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of Energy, via Grant DE-AR0000306. We also acknowledge the Graduate Fellowship (to Ngai Yin Yip)

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made by the Environment and Water Industrial Development Council of Singapore.

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REFERENCES

553 554

1. Pattle, R. E., Production of Electric Power by mixing Fresh and Salt Water in the Hydroelectric Pile. Nature 1954, 174, (4431), 660-660.

555 556

2. Logan, B. E.; Elimelech, M., Membrane-based processes for sustainable power generation using water. Nature 2012, 488, (7411), 313-319.

557 558 559

3. Yip, N. Y.; Elimelech, M., Thermodynamic and Energy Efficiency Analysis of Power Generation from Natural Salinity Gradients by Pressure Retarded Osmosis. Environ Sci Technol 2012, 46, (9), 5230-5239.

560 561 562

4. Han, G.; Zhang, S.; Li, X.; Chung, T.-S., High performance thin film composite pressure retarded osmosis (PRO) membranes for renewable salinity-gradient energy generation. J Membrane Sci 2013, 440, 108-121.

563 564 565

5. Achilli, A.; Prante, J. L.; Hancock, N. T.; Maxwell, E. B.; Childress, A. E., Experimental Results from RO-PRO: A Next Generation System for Low-Energy Desalination. Environ Sci Technol 2014, 48, (11), 6437-6443.

566 567 568

6. Lin, S.; Yip, N. Y.; Cath, T. Y.; Osuji, C. O.; Elimelech, M., Hybrid Pressure Retarded Osmosis–Membrane Distillation System for Power Generation from Low-Grade Heat: Thermodynamic Analysis and Energy Efficiency. Environ Sci Technol 2014, 48, (9), 5306-5313.

569

7.

570 571

8. Achilli, A.; Childress, A. E., Pressure retarded osmosis: From the vision of Sidney Loeb to the first prototype installation - Review. Desalination 2010, 261, (3), 205-211.

572 573

9. Weinstein, J. N.; Leitz, F. B., Electric-Power from Difference in Salinity - Dialytic Battery. Science 1976, 191, (4227), 557-559.

574 575 576

10. Post, J. W.; Hamelers, H. V. M.; Buisman, C. J. N., Energy recovery from controlled mixing salt and fresh water with a reverse electrodialysis system. Environ Sci Technol 2008, 42, (15), 5785-5790.

577 578

11. Brogioli, D., Extracting Renewable Energy from a Salinity Difference Using a Capacitor. Phys Rev Lett 2009, 103, (5).

579 580

12. La Mantia, F.; Pasta, M.; Deshazer, H. D.; Logan, B. E.; Cui, Y., Batteries for Efficient Energy Extraction from a Water Salinity Difference. Nano Lett 2011, 11, (4), 1810-1813.

581 582 583

13. Siria, A.; Poncharal, P.; Biance, A. L.; Fulcrand, R.; Blase, X.; Purcell, S. T.; Bocquet, L., Giant osmotic energy conversion measured in a single transmembrane boron nitride nanotube. Nature 2013, 494, (7438), 455-458.

Loeb, S., Osmotic Power-Plants. Science 1975, 189, (4203), 654-655.

20



Page 22 of 32

584 585 586

14. Zhu, X.; Yang, W.; Hatzell, M. C.; Logan, B. E., Energy recovery from solutions with different salinities based on swelling and shrinking of hydrogels. Environ Sci Technol 2014, 48, (12), 7157-63.

587 588

15. News Release: Progress Blue Energy generation by RED. http://www.fujifilmmembranes.com/item/progress-blue-energy-generation-by-red (June 05),

589 590 591

16. Yip, N. Y.; Elimelech, M., Performance Limiting Effects in Power Generation from Salinity Gradients by Pressure Retarded Osmosis. Environ Sci Technol 2011, 45, (23), 1027310282.

592 593 594

17. Veerman, J.; de Jong, R. M.; Saakes, M.; Metz, S. J.; Harmsen, G. J., Reverse electrodialysis: Comparison of six commercial membrane pairs on the thermodynamic efficiency and power density. J Membrane Sci 2009, 343, (1–2), 7-15.

595 596

18. Thorsen, T.; Holt, T., The potential for power production from salinity gradients by pressure retarded osmosis. J Membrane Sci 2009, 335, (1–2), 103-110.

597 598

19. Lin, S.; Straub, A. P.; Elimelech, M., Thermodynamic limits of extractable energy by pressure retarded osmosis. Energy & Environmental Science 2014, 7, (8), 2706.

599 600 601

20. Długołȩcki, P.; Gambier, A.; Nijmeijer, K.; Wessling, M., Practical Potential of Reverse Electrodialysis As Process for Sustainable Energy Generation. Environ Sci Technol 2009, 43, (17), 6888-6894.

602 603

21. Vermaas, D. A.; Guler, E.; Saakes, M.; Nijmeijer, K., Theoretical power density from salinity gradients using reverse electrodialysis. Energy Procedia 2012, 20, 170-184.

604 605

22. Daniilidis, A.; Herber, R.; Vermaas, D. A., Upscale potential and financial feasibility of a reverse electrodialysis power plant. Applied Energy 2014, 119, 257-265.

606 607 608

23. Feinberg, B. J.; Ramon, G. Z.; Hoek, E. M. V., Thermodynamic Analysis of Osmotic Energy Recovery at a Reverse Osmosis Desalination Plant. Environ Sci Technol 2013, 47, (6), 2982-2989.

609 610 611

24. Post, J. W.; Veerman, J.; Hamelers, H. V. M.; Euverink, G. J. W.; Metz, S. J.; Nymeijer, K.; Buisman, C. J. N., Salinity-gradient power: Evaluation of pressure-retarded osmosis and reverse electrodialysis. J Membrane Sci 2007, 288, (1–2), 218-230.

612 613 614

25. Yip, N. Y.; Vermaas, D. A.; Nijmeijer, K.; Elimelech, M., Thermodynamic, Energy Efficiency, and Power Density Analysis of Reverse Electrodialysis Power Generation with Natural Salinity Gradients. Environ Sci Technol 2014, 48, (9), 4925-4936.

615 616

26. Cath, T. Y.; Childress, A. E.; Elimelech, M., Forward osmosis: Principles, applications, and recent developments. J Membrane Sci 2006, 281, (1–2), 70-87.

21


Page 23 of 32


617 618 619

27. Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffrnan, J. D.; Hoover, L. A.; Kim, Y. C.; Elimelech, M., Thin-Film Composite Pressure Retarded Osmosis Membranes for Sustainable Power Generation from Salinity Gradients. Environ Sci Technol 2011, 45, (10), 4360-4369.

620 621

28. Robinson, R. A.; Stokes, R. H., Electrolyte solutions. 2nd rev. ed.; Dover Publications: Mineola, NY, 2002; p xv, 571 p.

622 623

29. Veerman, J.; Saakes, M.; Metz, S. J.; Harmsen, G. J., Reverse electrodialysis: evaluation of suitable electrode systems. J Appl Electrochem 2010, 40, (8), 1461-1474.

624 625 626

30. Veerman, J.; Saakes, M.; Metz, S. J.; Harmsen, G. J., Reverse electrodialysis: Performance of a stack with 50 cells on the mixing of sea and river water. J Membrane Sci 2009, 327, (1–2), 136-144.

627 628

31. Vermaas, D. A.; Saakes, M.; Nijmeijer, K., Enhanced mixing in the diffusive boundary layer for energy generation in reverse electrodialysis. J Membrane Sci 2014, 453, 312-319.

629 630

32. Pitzer, K. S.; Peiper, J. C.; Busey, R. H., Thermodynamic Properties of Aqueous Sodium Chloride Solutions. Journal of Physical and Chemical Reference Data 1984, 13, (1), 1-102.

631 632

33. Lee, K. L.; Baker, R. W.; Lonsdale, H. K., Membranes for power generation by pressureretarded osmosis. J Membrane Sci 1981, 8, (2), 141-171.

633 634

34. Bui, N.-N.; McCutcheon, J. R., Nanofiber Supported Thin-Film Composite Membrane for Pressure-Retarded Osmosis. Environ Sci Technol 2014, 48, (7), 4129-4136.

635 636 637

35. Straub, A. P.; Yip, N. Y.; Elimelech, M., Raising the Bar: Increased Hydraulic Pressure Allows Unprecedented High Power Densities in Pressure-Retarded Osmosis. Environmental Science & Technology Letters 2013, 1, (1), 55-59.

638 639 640

36. Geise, G. M.; Park, H. B.; Sagle, A. C.; Freeman, B. D.; McGrath, J. E., Water permeability and water/salt selectivity tradeoff in polymers for desalination. J Membrane Sci 2011, 369, (1–2), 130-138.

641 642

37. Elimelech, M.; Phillip, W. A., The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 2011, 333, (6043), 712-717.

643 644

38. Fritzmann, C.; Löwenberg, J.; Wintgens, T.; Melin, T., State-of-the-art of reverse osmosis desalination. Desalination 2007, 216, (1–3), 1-76.

645 646 647

39. Arena, J. T.; McCloskey, B.; Freeman, B. D.; McCutcheon, J. R., Surface modification of thin film composite membrane support layers with polydopamine: Enabling use of reverse osmosis membranes in pressure retarded osmosis. J Membrane Sci 2011, 375, (1–2), 55-62.

648 649 650

40. Chou, S.; Wang, R.; Shi, L.; She, Q.; Tang, C.; Fane, A. G., Thin-film composite hollow fiber membranes for pressure retarded osmosis (PRO) process with high power density. J Membrane Sci 2012, 389, 25-33.

22



Page 24 of 32

651 652 653

41. Hoover, L. A.; Schiffman, J. D.; Elimelech, M., Nanofibers in thin-film composite membrane support layers: Enabling expanded application of forward and pressure retarded osmosis. Desalination 2013, 308, 73-81.

654 655 656

42. She, Q.; Jin, X.; Tang, C. Y., Osmotic power production from salinity gradient resource by pressure retarded osmosis: Effects of operating conditions and reverse solute diffusion. J Membrane Sci 2012, 401–402, 262-273.

657 658 659

43. Geise, G. M.; Hickner, M. A.; Logan, B. E., Ionic Resistance and Permselectivity Tradeoffs in Anion Exchange Membranes. ACS Applied Materials & Interfaces 2013, 5, (20), 10294-10301.

660 661 662

44. Guler, E.; Zhang, Y.; Saakes, M.; Nijmeijer, K., Tailor-Made Anion-Exchange Membranes for Salinity Gradient Power Generation Using Reverse Electrodialysis. ChemSusChem 2012, 5, (11), 2262-2270.

663 664 665

45. Długołęcki, P.; Nymeijer, K.; Metz, S.; Wessling, M., Current status of ion exchange membranes for power generation from salinity gradients. J Membrane Sci 2008, 319, (1–2), 214222.

666 667

46. Güler, E.; Elizen, R.; Vermaas, D. A.; Saakes, M.; Nijmeijer, K., Performancedetermining membrane properties in reverse electrodialysis. J Membrane Sci 2013, 446, 266-276.

668 669

47. Baker, R. W., Membrane technology and applications. 3rd ed.; John Wiley & Sons: Chichester, West Sussex ; Hoboken, 2012; p xiv, 575 p.

670 671

48. Mulder, M., Basic principles of membrane technology. 2nd ed.; Kluwer Academic: Dordrecht ; Boston, 1996; p 564 p.

672 673 674

49. Daniilidis, A.; Vermaas, D. A.; Herber, R.; Nijmeijer, K., Experimentally obtainable energy from mixing river water, seawater or brines with reverse electrodialysis. Renewable Energy 2014, 64, 123-131.

675 676

50. Vermaas, D. A.; Saakes, M.; Nijmeijer, K., Doubled Power Density from Salinity Gradients at Reduced Intermembrane Distance. Environ Sci Technol 2011, 45, (16), 7089-7095.

677 678

51. Skilhagen, S. E., Osmotic power — a new, renewable energy source. Desalin Water Treat 2010, 15, (1-3), 271-278.

679 680 681

52. Post, J. W.; Goeting, C. H.; Valk, J.; Goinga, S.; Veerman, J.; Hamelers, H. V. M.; Hack, P. J. F. M., Towards implementation of reverse electrodialysis for power generation from salinity gradients. Desalin Water Treat 2010, 16, (1-3), 182-193.

682 683 684

53. Vermaas, D. A.; Veerman, J.; Yip, N. Y.; Elimelech, M.; Saakes, M.; Nijmeijer, K., High Efficiency in Energy Generation from Salinity Gradients with Reverse Electrodialysis. Acs Sustain Chem Eng 2013, 1, (10), 1295-1302.

23


Page 25 of 32


685 686 687

54. Yip, N. Y.; Elimelech, M., Influence of Natural Organic Matter Fouling and Osmotic Backwash on Pressure Retarded Osmosis Energy Production from Natural Salinity Gradients. Environ Sci Technol 2013, 47, (21), 12607-12616.

688 689

55. Vermaas, D. A.; Kunteng, D.; Saakes, M.; Nijmeijer, K., Fouling in reverse electrodialysis under natural conditions. Water Research 2013, 47, (3), 1289-1298.

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Page 26 of 32

690

Table 1. Properties of current high performance membranes (thin-film composite polyamide

691

membranes for PRO and ion exchange membranes for RED) analyzed in this study.

Selectivity I

Selectivity II

PRO Membranes a

1.5

3.0

a

Salt (NaCl) permeability, B (10−8 m/s)

1.25

10

b

500

500

Water permeability, A (L m−2h−1bar−1)

Structural parameter, S (µm)

RED Membranes c

Area specific resistance, ASR (Ωcm2)

3.0

1.5

c,d

0.95 / 0.90 / 0.80

0.90 / 0.80 / 0.60

d

3.5 / 11 / 26

19 / 44 / 94

Permselectivity, α (–)

Molar water permeation ratio, θ (–) 692

a

693

relationship of polyamide thin-film composite membranes.16

694

b

695

withstanding high hydraulic pressures.35

696

c

697

performance ion exchange membranes constrained by conductivity–permselectivity tradeoff.43

698

d

699

/ 1.2 / 4.0 M NaCl) of the surrounding solutions, while the molar water permeation ratio detrimentally

700

rises.49

Water and salt permeability of the PRO membranes are based on the permeability–selectivity tradeoff

The structural parameter is selected to simulate a membrane with woven fabric support capable of

Area specific resistance and permselectivity values are chosen to simulate technologically-available high

Permselectivity of the IEMs is assumed to deleteriously decrease with increasing salt concentration (0.6

25


Page 27 of 32


701

702

Figure 1. Schematic of (A) pressure retarded osmosis (PRO) and (B) reverse electrodialysis

703

(RED). In PRO, the salinity gradient produces an osmotic driving force for water flux across the

704

semipermeable membrane, and the increasing volume of the pressurized high concentration (HC)

705

solution powers a hydroturbine to produce useful work. Some salt leaks from the HC solution to

706

the low concentration (LC) solution as the membrane is not perfectly selective. In RED, the

707

concentration difference across the ion exchange membranes produces a Nernst potential and the

708

membranes selectively allow the transport of counter ions. The ion flux is converted to an

709

electric current, I, with a redox couple circulating between the end electrodes and useful work is

710

produced by the external load of resistance RL. Water diffuses to the HC solution, while some

711

co-ions leak across to the LC solution as the membranes are not perfectly selective. Only one

712

RED membrane pair is shown to illustrate the process.

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Osmotic Pressure Difference, ∆π (bar)

40

A 30

PRO Frictional Losses Uncontrolled Mixing Losses

20 ∆P 10

Unutilized Energy Useful Work, W

0 0.0

0.2

0.4

0.6

0.8

1.0

Electromotive Force, ξemf (mV)

0 Fraction of LC Solution Permeated, ∆V/VLC

400

B 300

Resistive Losses

ξL 200

Useful Work, W Uncontrolled Mixing Losses

100 0 0.0

713

RED

0.1

0.2

0.3

0.4

Unutilized Energy

0.5

Fraction of HC Salt Permeated, ∆ns/n0s,HC

714

Figure 2. Representative plots of useful work, W (black patterned areas), frictional/resistive

715

losses (blue patterned areas), uncontrolled mixing losses (red patterned areas), and unutilized

716

energy (green patterned areas) for A) PRO and B) RED. The vertical axes are the driving force

717

for water and ion flux (osmotic pressure difference, ∆π, and electromotive force, ξemf, for PRO

718

and RED, respectively), while the horizontal axes denote progress of the energy production

719

0 process (fraction of water permeated from the low concentration (LC) solution, ∆ V VLC , and

720

0 fraction of salt permeated from the high concentration (HC) solution, ∆ns ns,HC , respectively).

721

For both processes, the HC solution is 0.6 M NaCl and the LC solution is 1.5 mM NaCl to

722

simulate seawater–river water salinity gradient.

723

column “Selectivity I” of Table 1. To maximize η, ∆P in PRO is 14.2 bar (about half of initial

724

∆π) while RLAm = 9.15 Ωcm2 in RED to maximize PD.

27

Membrane properties are presented in the


Page 29 of 32


100

PRO RED PRO RED PRO RED

80

0.6 M1.5 mM

56.1%

1.2 M10 mM

18.1%

0

33.1%

20

53.9%

40

37.8%

60 53.9%

Percent of ∆Gmix (%)

Unutilized Energy Uncontrolled Mixing Losses Frictional/Resistive Losses Useful Work

4.0 M17 mM

(seawater(brine(engineered river water) wastewater) solutions)

HC-LC Solution NaCl Concentration

725

726

Figure 3. Efficiency of work extraction (grey columns), percent of energy expended to drive

727

water or counter ion flux (blue patterned columns), percent of energy lost to uncontrolled mixing

728

(red patterned columns), and portion of unutilized energy (green patterned columns) for PRO and

729

RED. The HC–LC solution concentrations are 0.6 M–1.5 mM, 1.2 M–10 mM, and 4.0 M–17

730

mM NaCl to simulate seawater–river water, seawater desalination plant brine–wastewater, and

731

engineered solutions, respectively. The PRO and RED membrane properties are presented in the

732

column “Selectivity I” of Table 1. ∆P in PRO is 14.2, 28.6, and 96.7 bar and RED RLAm is 9.15,

733

7.63, and 6.82 Ωcm2 for the 0.6 M–1.5 mM, 1.2 M–10 mM, and 4.0 M–17 mM HC-LC solutions,

734

respectively.

28



Power Density

Page 30 of 32

Efficiency

PRO RED

8

A

Seawater-River Water 0.6 M-1.5 mM

53.9%

6

44.1%

4

3.7

37.8%

20 1.1

0.77

53.9%

8.9

-II ED

R

R

20

-II

I

ED R

R

ED -

O -II

PR

PR

O -I

0

Engineered Solutions 4.0 M-17 mM 56.1%

80

C

47.8% 38.0 40.2

40 18.1%

20 3 2 1 0

60

20 1.8

1.2

5.5%

Efficiency, η (%)

ED

-II

-I R ED

PR

PR O

O -I I

0 -I

Power Density, PD (W/m2)

22.6% 0.86 0.86

60 40

40

33.1%

6.9

4 2 1 0

80

60

R

8

80

45.7%

12

100

735

Brine-Wastewater 1.2 M-10 mM

B

Efficiency, η (%)


24

ED -I

0 PR O -I PR O -II

0

16

60

31.1% 40

2.4

2

20

80

Efficiency, η (%)


10

736

Figure 4.

Membrane power density (columns, left vertical axis) and efficiency of work

737

extraction (symbols, right vertical axis) for PRO and RED (blue and red data representations,

738

respectively) at 0.6 M, 1.2 M, and 4.0 M NaCl high concentration (HC) solution concentrations.

739

Two sets of membranes with imperfect selectivity are examined: membrane–I has moderate

740

selectivity imperfections (unshaded columns and symbols), while membrane–II has more severe

741

imperfection in selectivity (shaded columns and symbols).

742

membrane properties can be found in Table 1. For Selectivity II, ∆P in PRO is 13.1, 26.3, and

743

88.9 bar and RED RLAm is 6.05, 6.25, and 4.19 Ωcm2 for the 0.6 M–1.5 mM, 1.2 M–10 mM, and

744

4.0 M–17 mM HC-LC solutions, respectively, while ∆P and RLAm for Selectivity I can be found

745

in the caption of Figure 3. 29


Details of the PRO and RED

Page 31 of 32


HC-LC Solution NaCl Concentration 4.0 M-17 mM (engineered solutions) 1.2 M-10 mM (brine-wastewater) 0.6 M-1.5 mM (seawater-river water)

Osmotic Pressure Difference, ∆π (bar)

250 200

A

PRO

150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Electromotive Force, ξemf (mV)

Fraction of LC Solution 0 Permeated, ∆V/VLC (-)

746

400

B

300

RED

350 300

200

250 200 0.0

2.0x10-3 4.0x10-3

100 0 0.0

0.4

0.8

1.2

1.6

2.0

Salt Permeated per HC Solution 0 Volume, ∆ns/VHC (mols/L)

747

Figure 5. (A) Osmotic pressure difference, ∆π (driving force for water flux in PRO), as a

748

0 function of the fraction of water permeated from the low concentration (LC) solution, ∆V VLC .

749

(B) Electromotive force, ξemf (driving force for ion flux in RED), as a function of the moles of

750

0 salt permeated per unit volume of the high concentration (HC) solution, ∆ns VHC . Inset in B)

751

shows the initial ξemf of the RED cell. Blue solid, green dashed, and red dotted lines represent

752

natural, anthropogenic, and engineered salinity gradients, respectively. Perfectly selective PRO

753

and RED membranes are assumed.

30



754

TOC ART

755

756

31


Page 32 of 32

Comparison of Energy Efficiency and Power Density in Pressure

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