Colloid and Interface Lab Department of Chemical Engineering HONGIK UNIVERSITY

Miok Ko†, Jennifer Moon ‡, and Won Ryoo†* †Department of Chemical Engineering, Hongik University, 72-1 Sangsu-Dong, Mapo-Ku, Seoul 121-791, Korea

& ‡Department of Chemical and Biological Engineering, 596 UCB, University of Colorado, Boulder, CO, 80309-0596, USA

Introduction

Conclusions

Experiments

Theory

• Increased oil prices and global warming calls for the development

of renewable energy.

• Entropy of mixing may be a novel source of energy.

• Reverse Electro Dialysis (RED) is a non-polluting technology

used to recover energy from the entropy of mixing.

• Currently, renewable energy occupies only 3% of total energy

production in Korea.

• Solar, wind, and hydro- power systems have been the focus thus far.

• Ion transport direction is controlled through use of anion and

cation exchange membranes.

• Brine and fresh water are brought into contact thru an alternating

series of anion exchange membranes (AEM) and cation exchange

membranes (CEM).

• The concentration difference between the solutions forces anions

and cations to migrate through the AEMs and CEMs.

• Space charging produces a potential difference that can be

recovered as electrical energy.

• Electricity is generated from the oxidation of Fe2+ and reduction

of Fe3+ at the anode and cathode, respectively.

Entropy of mixing effect

Selective ion transfer in AEM and CEM

• Explore the possibility of recovering energy from

the controlled mixing of salt and fresh water

• Develop and characterize RED stacked cells with

commercial ion exchange membranes

• Optimize the structure of the RED stacked cell to

maximize power output by varying channel thickness

and electrode material

Theory of electrical potential and current

V 1.05.000.1

3599.0ln

C/mol 96485

K 298 J/molK 314.892.0

]Na[

]Na[ln

rightL

leftH

oczF

RTV

• Nernst equation for electrical potential across a membrane (open-circuit voltage)

• Nernst-Planck equation for ionic current

dx

dV

RT

zF

dx

dDJ eff

]Na[]Na[dA

xFDi effsc

]Na[

Whr286.0

kJ 03.1

STHG

4

6

3

6 Fe(CN)eFe(CN)

Lumped-parameter Modeling

• Stacking resulted in linear increase in open circuit voltage. (~0.09 V/membrane)

• The short circuit current increased linearly with effective membrane area.

• High flowrates lead to increased short circuit current due to the reduction in concentration polarization

adjacent membrane surfaces.

• The maximum in short circuit current was limited by the ion transport in the ion exchange membraned

• The channel thickness was the most crucial factor in RED performance.

17 g NaCl/L

Fresh Water

1 L

Sea water

34 g NaCl/L

1L

Low entropy

High entropy

Energy consumed

for separation

Free energy

recovery

Renewable Energy Status (%) in S. Korea, 2011

Background

Objectives

AEM CEM

Principles of RED

Ion flux Short circuit current

• Boundary condition (BC)

» Uniform Concentration at Inlet:

• Model for convective diffusion

Reverse Electro Dialysis Equivalent Circuit Model (ECM)

• An ideal RED system can be described as an ECM.

• Concentration gradient is not uniform throughout

membrane area

• Curved streamline with a velocity field makes it

difficult to model

• Corner areas are not fully utilized

• Ion exchange membrane samples Channel design of previous study

• Uniform concentration gradient develops along flow direction

• Straight streamline enables simplified modeling

• Ions may migrate through the entire area of membrane

Improved channel design of this study

Schematic representation of RED

e-

e-

e-

02 LHL

z CCD

βdz

dCbV

02 LHH

z CCD

βdz

dCbV

thicknessmembrane

tcoefficiendiffusionD

fractionareaopenmeshβ

velocitylsuperficiaV

thicknessspacerb

z

2

where

• Local concentration

00 HZH CC

00 LZL CC

bV

Dz

LHLHH

zeCCCC

C

22

0000

bV

Dz

LHLHL

zeCCCC

C

22

0000

• Short circuit current

dACC

FDi LHsc

bV

DL

LHzsczeCCFwbVi

100

ntcoeffictiediffusionD

constantfaradayF

areaopeningmeshβ

thicknessspacerb

2

thicknessmembrane

widthmembranew

areamembraneA

where

Re Re

Rsp Rsp

Rf=R(AEM)+R(fresh water channel)

Rs=R(CEM)+R(sea water channel)

Rfp=R(parasitic resistance of fresh water channel)

Rsp=R(parasitic resistance of sea water channel)

Rssp=R(parasitic resistance of sea water slit)

Rsfp=R(parasitic resistance of fresh water slit)

Re=R(electrode)

Rs

Load resistance

Voltmeter

V

Ammeter A

RCEM Rs

Rfp

Rf Rf

R

Rfp

Rf Rs

fresh water

sea water

Anode Cathode

CEM CEM CEM CEM AEM AEM

AEM

Repeating unit

Rs Rf

Rfp

CEM AEM

Rsp

Rssp Rssp Rssp Rssp

Rsfp Rsfp Rsfp Rsfp

• Experimental conditions

0

0.5

1

1.5

2

2.5

3

0.0 1.0 2.0 3.0

Pm

ax (

W)

CL (g/L)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0

0.1

0.2

0.3

0.4

0.5

0.0 1.0 2.0 3.0

isc (A)

Voc

per

pair

cell

(V

)

CL (g/L)

Voc/units

isc

0 10 20 30 40 50 60 70 80

China

USA

Germany

Japan

Canada

India

UK

France

Brazil

Australia

Korea

RECAI Rank

Co

un

trie

s

Ranking of Countries Based on RE Capacity,

2013

(New & Renewable Energy Center, KEMCO, 2013)

(Adapted from Global Cleantech Center, 2014)

Fresh Water Concentration Simulation Results

Results & Discussion

Concentration (g/L) Diffusion coefficient * 106 (cm2/s) Membrane

thickness (mm)

Mean

velocity (Ṽ, cm/s)

Membrane dimension Channel

thickness (2b, mm)

Sea water (CH)

Fresh water (CL)

In-Water

(DW)

In-Membrane (DM)

Width

(W, cm)

Length (L, cm)

35 0.5 13.3 0.2~0.5 0.08 ~ 0.11 0.5 ~ 25 5 5 0.1 ~ 0.2

• RED cell design

• Press-cutter design for IEM and spacers

• RED Characterization apparatus

• Linear I-V characteristics,

thus maximum power

performance at the mid

point observed for stack

cells of 5 pair AEM/CEM.

• The maximum power

density measured as high

as 2.2 W/m2 for

synthesized membranes.

• The power performance

increased with an increase

in flowrate due to the

reduction in concentration

polarization.