The document summarizes research on recovering energy from the entropy of mixing salt and fresh water using reverse electrodialysis (RED). Key points: 1) RED uses a series of alternating anion and cation exchange membranes to generate electricity from the concentration difference as ions migrate through the membranes. 2) Experiments were conducted to optimize RED cell design parameters like channel thickness and electrode material to maximize power output. 3) Results showed that power increased linearly with more membranes and effective area, and higher flowrates reduced concentration polarization for higher current. Channel thickness was the most important design factor.

1. 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
ExperimentsTheory
• 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
V1.0
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K298J/molK314.892.0
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rightL
leftH
oc
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RT
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• Nernst equation for electrical potential across a membrane (open-circuit voltage)
• Nernst-Planck equation for ionic current

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

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6
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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  LH
L
z CC
D
β
dz
dC
bV

  02  LH
H
z CC
D
β
dz
dC
bV
 thicknessmembrane
tcoefficiendiffusionD
fractionareaopenmeshβ
velocitylsuperficiaV
thicknessspacerb
z






2
where
• Local concentration
00 HZH CC 
00 LZL CC 
bV
Dz
LHLH
H
z
e
CCCC
C 






 





 

22
0000
bV
Dz
LHLH
L
z
e
CCCC
C 






 





 

22
0000
• Short circuit current
dA
CC
FDi LH
sc  




 



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




bV
DL
LHzsc
z
eCCFwbVi 

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
AmmeterA
RCEMRs
Rfp
Rf Rf
R
Rfp
RfRs
fresh water
sea water
Anode Cathode
CEM CEM CEM CEMAEM 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
Pmax(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)
Vocperpaircell(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
Countries
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.