The Breathing Cell: Cyclic Intermembrane Distance Variation in

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The breathing cell: cyclic intermembrane distance variation in reverse electrodialysis Jordi Moreno, Esther Slouwerhof, David A. Vermaas, Michel Saakes, and Kitty Nijmeijer Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02668 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016

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The breathing cell:

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cyclic intermembrane distance variation in reverse electrodialysis

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J. Moreno1,2, E. Slouwerhof1,2, D. A. Vermaas1, 2, M. Saakes1, K. Nijmeijer*, 2,3

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1

Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911MA Leeuwarden, The

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Netherlands

6 7 8

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Membrane Science & Technology, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands

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Current address: Membrane Materials and Processes, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

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*corresponding author: [email protected]

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Abstract

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The breathing cell is a new concept design that operates a reverse electrodialysis stack by varying

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in time the intermembrane distance. Reverse electrodialysis is used to harvest salinity gradient

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energy; a rather unknown renewable energy source from controlled mixing of river water and

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seawater. Traditionally, both river water and seawater compartments have a fixed

15

intermembrane distance. Especially the river water compartment thickness contributes to a large

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extent to the resistance of the stack due to its low conductivity. In our cyclic approach, two stages

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define the principle of the breathing concept; the initial stage, where both compartments

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(seawater and river water) have the same thickness and the compressed stage, where river water

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compartments are compressed by expanding the seawater compartments. This movement at a

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tunable frequency allows reducing stack resistance by decreasing the thickness of the river water

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compartment without increasing permanently the pumping losses. The breathing stacks clearly

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benefit from the lower resistance values and low pumping power required, obtaining high net

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power densities over a much broader flow rate range. The high frequency breathing stack (15 1 ACS Paragon Plus Environment

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cycles/min) shows a maximum net power density of 1.3 W/m2. Although the maximum gross and

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net power density ever registered (2.9 W/m2 and 1.5 W/m2, respectively) is achieved for a fixed

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120 µm intermembrane distance stack (without movement of the membranes), it is only obtained

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at a very narrow flow rate range due to the high pressure drops at small intermembrane distance.

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The breathing cell concept offers a unique feature, namely physical movement of the membranes,

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and thus the ability to adapt to the operational conditions and water quality.

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Introduction

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The world’s increasing demand for energy associated with CO2-emission and fossil fuel depletion

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has raised the interest in renewable and sustainable energy. Cleaner alternatives need to be

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found to reduce the emission of greenhouse gases1,2. Currently, solar-, wind-, biomass- and hydro-

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energy are well-known and used as sustainable energy sources. A rather new and unknown

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energy source is salinity gradient energy. Salinity gradient energy is a potentially large source for

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electricity generation. The Gibbs free energy of mixing is available when concentrated and diluted

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salt solutions mix. The potential power obtainable from mixing seawater and river water is large:

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approximately 2 TW when using the global discharge of rivers into the seas3,4. Reverse

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electrodialysis (RED) is used to harvest this energy. Other technologies to harvest salinity gradient

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power (SGP) are pressure retarded osmosis (PRO)5,6 and capacitive mixing (CAPMIX)7,8. A RED

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stack comprises an alternating series of cation exchange membranes (CEM) and anion exchange

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membranes (AEM). CEMs contain fixed negative charges allowing ideally only the passage of

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positively charged ions and AEMs contain fixed positive charges allowing ideally only the passage

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of negatively charged ions. The membranes are traditionally kept at a fixed distance using non-

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conductive net-spacers. The resulting potential difference over the membranes can be used to

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power an electrical device.

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Despite of the good prospects for this renewable energy source9, RED technology is not yet

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commercially attractive because of the relatively low gross power densities obtained until now; a

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net power density of 2 W/m2 is the target to be commercially attractive10–12. To achieve a high net

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power density, the internal resistance of the RED stack and the hydraulic friction of the feedwater

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should be as low as possible13.

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The internal resistance consists of the ohmic resistance and the non-ohmic resistance. The ohmic

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resistance is determined by three components: the resistance of the membranes, that of the net-

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spacers and that of the two feedwater compartments. The non-ohmic resistance includes the

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boundary layer resistance (also known as concentration polarization) and the effects due to the

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change in concentration of the bulk solution along the length of the channel. By reducing the

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intermembrane distance of the feedwater compartments the ohmic resistance can be reduced13.

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The dominant resistance is the river water resistance due to its low salt concentration and a

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reduced intermembrane distance immediately induces a lower ohmic resistance13. At the same

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time, the use of thin intermembrane distances increases the hydraulic friction and consequently

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increases the pumping power. This effect is even stronger when net-spacers are used to keep

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adjacent membranes at the same inter-mutual distance. Vermaas et al.13 achieved, when using

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0.507 M (30 g NaCl per kg water) as seawater and 0.017 M (1 g NaCl per kg water) as river water,

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a gross power density of 2.2 W/m2, but a net power density of only 1.2 W/m2 using a fixed

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intermembrane distance of 100 µm13. Kim et al.14 achieved a slighty higher gross power density of

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2.4 W/m2 but used much higher salt concentrations than the previous authors, i.e. 0.58 M as

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seawater and 0.017 M as river water, at the same fixed intermembrane distance (i.e. 100 µm).


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The use of net-spacers also increases the fouling behaviour of the stack when using natural feed

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waters15,16. The high pumping power required at low intermembrane distances narrows the flow

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rate range where the optimal net power output is achieved. Thin intermembrane distances ( 99% according to

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the manufacturer) and because of their much higher flexibility compared to that of thicker

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membranes. Also, these membranes have a low ionic resistance of 0.94 Ω·cm2 for the CEM and

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0.34 Ω·cm2 for the AEM measured in Cl- form in 0.5 M NaCl at 25 °C.

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Two titanium electrodes (mesh size 1.7 m2/m2) with 50 g/m2 galvanic platinum coating and with

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an active area of 10x10 cm2 were used as the anode and cathode (Magneto Special Anodes B.V.,

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The Netherlands). The electrode rinse solution used to facilitate the redox reactions at the

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electrodes was composed of 0.3 M K3Fe(CN)6 , K4Fe(CN)6 and 0.5 M NaCl in demineralized water.

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The electrode rinse solution was circulated at 3 cm3/s along the electrodes by using a peristaltic

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pump (Cole-Parmer, Masterflex L/S Digital drive USA). The electrode rinse solution compartment

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was created by placing a silicon rubber gasket with a thickness of 300 ± 5 µm. The compartment

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was kept at an over pressure of 30 ± 10 kPa with the help of a needle valve in order to avoid

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undesired bulging of the feed water compartments.


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To introduce the ‘breathing’ of the cell, an electronic one-way valve (Bürkert, Germany) was

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positioned at the outlet of the seawater compartments. This valve was automatically controlled

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by a Raspberry Pi (Raspberry Pi Foundation, United Kingdom), using an open source python code.

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The breathing was operated at two different on/off frequencies of 5 cycles/min and 15

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cycles/min.

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2. Feed water

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Artificial seawater and river water were used with a concentration of 0.507 M (30 g NaCl per kg

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water) and 0.017 M (1 g NaCl per kg water), respectively. These solutions were made with NaCl

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(technical grade, ESCO, The Netherlands) dissolved in demineralized water. The solutions were

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kept at 25 oC ± 0.5 oC with a heater (Tetratec HT300, Germany) and a pump. The artificial solutions

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were pumped continuously through the stack by using two peristaltic pumps (Cole-Parmer,

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Masterflex L/S Digital drive, USA). Measurements were performed in duplicate during 360

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seconds per flow rate at 0.8, 1.6, 2.5, 3.3, 4.1, 5 and 5.8 cm3/s in random order to avoid history

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effects. Flow velocities ranged from 0.69 cm/s to 5.55 cm/s for the 120 µm stack and 0.17 cm/s to

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1.38 cm/s for the 480 µm stack. The Reynolds number (Re) is calculated being defined as the

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Reynolds number for a wide empty channel (not corrected for the spacer porosity).

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=

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in which is the average flow velocity (m/s), Dh is the hydraulic diameter (m), is the density of

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water (kg/m3), is the (dynamic) viscosity of water (kg/(m·s)), Φ is the flow rate per feedwater

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compartment (m3/s) and b is the width of the feedwater compartment (m).

∙

∙

≈

∙

(eq.1) ∙



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3. Pressure drop measurements

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At the inlet and the outlet of the feed waters, pressure meters (Endress+Hauser Cerabar T,

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Germany) were placed to measure the pressure drop over the seawater and river water

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compartments. The pressure meters were connected to a data logger (Endress+Hauser Ecograph

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T, Germany) to obtain the data. The data logger measured the pressure every second. Before

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every experiment, air bubbles were removed from the tubes connected to the pressure meters, to

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ensure an accurate measurement. The pressure drop values used to calculate the consumed

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pumping energy are average pressure drop values measured during the full duration of the

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experiment (300 seconds).

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4. Electrochemical measurements

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A chrono-amperometric series was applied using a potentiostat (Ivium Technologies, The

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Netherlands). It involves an initial stage of 60 seconds without any current for the determination

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of the open circuit voltage (OCV), followed by a stage of 300 seconds measuring the current at a

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constant voltage equal to half of the OCV, i.e. at maximum current density. All experiments were

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done in duplicate. During the stage without current, electrochemical impedance spectroscopy

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(EIS) with an alternating sinusoidal signal of 5 KHz was applied to derive the ohmic resistance of

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the stack. One hundred analyses per second were performed and the real component of the

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impedance was used to extract the ohmic resistance. According to Długołęcki et al.

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resistance measured at high frequencies and alternating current represents the resistances of the

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membranes and the feed water compartments (i.e. ohmic resistance). Previous studies

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investigated the ohmic resistance by applying different current steps13,25, but this method cannot

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be applied in a breathing stack because the membranes move consequently changing the internal

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resistance.

24

, the


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5. Gross power density The gross power density can be calculated as follows: =

∙ (eq.2) !

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where P is the gross power density (W/m2), V is the voltage (V), I is the current (A) and Astack is the

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membrane stack area (0.2 m2).

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6. Pumping energy

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The power density required to pump the feed waters is determined from the measured pressure

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drop over the feed water compartments (hydraulic losses). The pumping power per membrane

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area (W/m2 membrane) can be calculated as 26,21:

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"#" =

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where Φ and Φ are the flow rates of river and seawater (m3/s) and ∆( and ∆( are the

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pressure drops (Pa) over the river and seawater compartments.

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∆"% ·% '∆" · (eq.3) !

7. Net power density

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The net power density, Pnet (W/m2), is calculated as the difference between the gross power

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density, Pgross (W/m2), and the power density consumed for pumping the feed waters, Ppump

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(W/m2):

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)*+ = − "#" (eq.4)

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Results

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1. Stack ohmic resistance

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In order to predict individual contributions to the stack ohmic resistance, the ohmic resistance per

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cell was calculated and compared with the experimentally measured values. In figure 3 the 12 ACS Paragon Plus Environment


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calculated ohmic resistance per cell for the two stages of the breathing stack configuration is

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presented. The resistance attributed to the membranes, the seawater compartment and the river

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water compartment is calculated using the experimental membrane specifications and the

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conductivity of the feed waters. To determine the contribution of the spacers (spacer shadow

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effect 20,24,27) in Figure 3, the total ohmic resistance was measured experimentally both for the 120

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µm and the 480 µm stacks and the difference between the calculated value and the measured

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value is attributed to the effect of the spacers13. To account for this effect in the breathing stack,

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the average value of this spacer effect of the 120 µm stack and that of the 480 µm stack is taken.

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The membrane resistance is considered independent of the thickness of the two feed water

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compartments, and consequently has the same resistance in both stages.

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2

Figure 3. Calculated area resistance per cell (in Ω⋅ cm ) originating from the spacers, the river water, the seawater and the membranes for a breathing cell at the initial stage and the compressed stage. Experimental data for membrane resistance and conductivity of the feed waters are used as input values.


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The thickness of the expanded seawater compartment (840 µm) originates from subtracting 120 µm (thickness of a totally compressed river water compartment) from the sum of the thickness of the two feed water compartments together at the initial stage (480 µm each = 960 µm).

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One of the most beneficial aspects of the breathing cell concept is the inter-membrane distance

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variation in time of the feed water compartments, especially for that of the river water

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compartment. The calculated values suggest a 47% lower ohmic resistance between the initial

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stage of the breathing cell (480 µm in both compartments) and the compressed stage (120 µm for

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the compressed river water compartment and 840 µm for the expanded seawater compartment).

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The thickness of the expanded seawater compartment (840 µm) originates from subtracting 120

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µm (thickness of a totally compressed river water compartment) from the sum of the thickness of

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the two feed water compartments together at the initial stage (480 µm each = 960 µm). The

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expansion of the sea water compartment results in an increase in resistance when the initial

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stage and the breathing stage are compared. However, this increase is minor compared to the

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decrease in resistance of the river water compartment upon compression. The dominance of the

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river water resistance, having a low electric conductivity due to the low salt concentration,

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provokes that any variation of the intermembrane distance in the river water compartment

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influences the overall stack performance 13.

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In order to validate and demonstrate experimentally the expansion of the seawater

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compartments and thus the compression of the river water compartments in time,

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electrochemical impedance spectroscopy measurements were conducted. In Figure 4 the

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experimentally measured ohmic resistance at time intervals of 0.01 s (in-phase component of

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impedance at 5 KHz) for different stack configurations is plotted against the time.



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Figure 4. Ohmic resistance per cell as a function of the time measured for every stack configuration. All 3 configurations were operated at the same flow rate per cell (5 cm /s). Dashed lines correspond to the calculated values of the initial stage (480 µm/480 µm) and the compressed stage (120 µm/840 µm) of the breathing cell. The straight lines at the bottom (120 µm) and at the top (480 µm) of the graph, represent the reference stacks, i.e, a stack with 120 µm spacers in both seawater and river water compartment and a stack with 480 µm spacers in both seawater and river water compartment, respectively (standard stack, no breathing). The time interval is 0.01 s for the impedance measurements at 5000Hz i.e. 100 analyses per second.

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The ohmic resistance of the reference stacks without the breathing movement (lower and upper

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line, 120 µm and 480 µm) remains constant during the measurement time, as expected because

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these stacks do not have any breathing movement. The 480 µm line lies above the theoretical 480

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µm line as calculated for the breathing stack. The reason for this is the spacer shadow effect. In

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the breathing configuration the spacer shadow effect is diminished as the inner part of the spacer

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is only 120 µm (only the inlet and outlet are 480 µm ) and thus the spacer does not completely

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touches the membrane area.


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The stacks with breathing movement (5 cycles/min and 15 cycles/min) present a strong cyclic

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variation of the ohmic resistance in time, demonstrating the movement of the membranes. The

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membrane movement of the breathing stack at a frequency of 5 cycles/min reduces the ohmic

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resistance by 30% in the compressed stage compared to the initial stage. At a frequency of 15

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cycles/min the reduction in ohmic resistance is only 15% in the compressed stage compared to

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the initial stage. Although one would expect that the resistances of the breathing stacks lay in

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between the resistance lines of the calculated values for the initial stage and that of the

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compressed stage, this is not the case, because the calculated values for the non breathing stacks

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include the spacer shadow effect. During expansion of the seawater compartments, free space is

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created in the seawater compartment (which has a spacer of only 480 µm thick, while the

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compartment expands to 840 µm), thus eliminating momentarily the spacer coverage of the

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membrane (spacer shadow effect), thus decreasing the ohmic resistance of the stack further more

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than the expected with the theoretical calculations20,28,24,29. This is probably the main reason why

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the ohmic resistance values of the breathing cell stacks (cyclic lines) are lower than the calculated

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values of the compressed stage (dashed line).

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Moreover, we expect that the river water compartment remains somewhat compressed after the

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compressed stage and does not return completely to the initial stage of 480 µm/480 µm stack,

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resulting in an overall reduction in ohmic resistance. This could be a consequence of keeping the

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electrode rinse solutions under a slight overpressure to avoid uncontrolled bulging of the

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membranes, leading to a slight compression of the whole stack16. This compression of the river

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water compartment also explains the difference in ohmic resistance values between the breathing

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stacks. The breathing stack with 15 cycles/min, remains more compressed than the 5 cycles/min

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breathing stack. Due to the high breathing frequency, the compartments do not return

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completely to their original position after each cycle. However, this relatively small effect is not 16 ACS Paragon Plus Environment


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visible in the pressure drop measurements for both compartments as a significant part of the

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pressure drop comes from the in- and outflows of the compartments.

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2. Gross power density

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Figure 5 shows the time-averaged gross power density calculated following equation 2 (standard

331

deviation is shown as error bar) and plotted against the flow rate for the different stack

332

configurations.

333 334 335

Figure 5. Gross power density as a function of the flow rate for the different stack configurations. Trend lines are added to guide the eye.

336 337

For all stack configurations the gross power density output increases when the flow rate per cell is

338

increased. At higher flow rates there is better re-supply of feedwater (i.e., keeping the maximum

339

concentration gradient along the compartment) and better mixing (decreased boundary layer

340

resistance)13,30. The breathing cell with a breathing frequency of 5 cycles/min shows a gross power

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density decay at higher flow rates. This is due to the closing of the valve in the seawater 17 ACS Paragon Plus Environment

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compartment that provokes internal leakage. Seawater is transferred to the river water

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compartment through internal leakage and thus reduces the salinity gradient. This is an undesired

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effect, but the consequence of the concept at certain specific conditions. The breathing stacks

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show gross power density values between those of the two stacks with a fixed intermembrane

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distance of 120 µm and 480 µm. A maximum value of 1.8 W/m2 for the gross power density was

347

achieved for the breathing stack with a breathing frequency of 15 cycles/min and a maximum

348

value of 1.6 W/m2 was achieved for the stack operating at a frequency of 5 cycles/min. The values

349

achieved for the stack operated at 15 cycles/min are higher than those obtained for the stack with

350

5 cycles/min. This again suggests that the river water compartments do not completely return to

351

their initial stage position after the compression stage, and the river water compartments remain

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slightly compressed. Therefore higher flow velocities inside the feed water compartments are

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achieved, although at the same flow rate. This results in a higher gross power density output. At

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the same time, the high flow velocities and the movement of the membranes of the breathing

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stacks disturb the diffusion boundary layer and improve the mixing of the feed waters near the

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membrane surface, especially in the river water compartment24. The highest gross power density

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value was achieved for the 120 µm fixed intermembrane distance stack. This stack, that also

358

benefits from the high flow velocities in both compartments, results in a gross power density of

359

2.9 W/m2, exceeding the previously reported value for gross power density of 2.2 W/m2,13 for the

360

same feed water concentrations and establishes a new maximum value for the gross power

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density in RED. The 480 µm fixed intermembrane distance stack achieved a maximum gross power

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density value of only 0.5 W/m2, especially due to the thick river water compartment resulting in a

363

high resistance. This value is comparable with a previous research 13.



364

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3. Pumping power

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In Figure 6Figure 62 the consumed pumping power density against the flow rate is presented.

366

Pumping power was calculated from experimentally measured values following equation 3

367

(standard deviation is shown as error bar). The pumping power increases with increasing flow rate

368

for all stack configurations. The pumping power is significantly pronounced when using the stack

369

with a fixed intermembrane distance of 120 µm, as expected. All stacks have an inlet net-spacer of

370

480 µm but the stack with an intermembrane distance of 120 µm has in both the seawater and

371

the river water compartment an inlet net-spacer of only 120 µm. Consequently, the pressure drop

372

and thus the pumping power in this stack configuration is much higher than in the other

373

configurations. As observed before13,31,32, the pressure drop is strongly dependent on the spacer

374

thickness.

375 376 377

Figure 62. Pumping power density plotted against the flow rate for every stack configuration. Trend lines are added to guide the eye.

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The major part of the energy consumed for pumping the feeds, and thus the pressure drop,

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predominantly comes from the friction at the inlet and outlets of the stacks. Consequently, as

381

both the breathing stacks and the stack with a fixed membrane distance of 480 µm have a much

382

wider inflow slit than the 120 µm stack, the three 480 µm stacks experience less friction and have

383

a much lower pressure drop than the narrower stack . Still the lowest pressure drop values are

384

achieved for the stack with a fixed intermembrane distance of 480 µm, due to the wide stack

385

compartments and consequently low friction inside the compartments. During a small fraction of

386

time, the breathing stacks, are in the compressed state, resulting in a temporary, short reduction

387

of the intermembrane distance and consequently a slight increase in pressure drop (which is

388

relatively minor compared to the pressure drop due to inflow effects). Since the pressure drop

389

values are averaged over one complete cycle, the average additional pressure drop due to

390

compression is also low. This demonstrates that the intermembrane distance variation in time

391

contributes to minimize the pumping power while keeping high gross power densities.

392

4. Net power density

393

In Figure 7Figure the net power density values, calculated as the difference between the gross

394

power density and the power consumed for pumping of the feed waters following equation 4

395

(standard deviation is shown as error bar), are shown as function of the flow rate for every stack

396

configuration.



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Figure 7. Net power density plotted against the flow rate for every stack configuration. Trend lines are added to guide the eye.

400 401

The breathing stacks clearly benefit from the low pumping power values obtaining high net power

402

densities over a broader flow rate range. The optimal flow rate giving the highest net power

403

density is between 2 cm3/s and 4 cm3/s in the breathing stack configurations. The high frequency

404

breathing stack with 15 cycles/min shows a maximum value of 1.3 W/m2 and the stack with a low

405

breathing frequency of 5 cycles/min has a maximum net power density of 1.1 W/m2.

406

The stack with a fixed intermembrane distance of 120 µm registers a maximum net power density

407

value of 1.5 W/m2. This is currently the highest net power density measured experimentally

408

known to the authors. To the best of our knowledge, the maximum net power density reported in

409

literature was 1.2 W/m2 using the same feed water concentrations13. At slighty higher flow rates,

410

the pumping power increases tremendously (Figure 6), directly resulting in a significantly lower

411

net power density as well. 21 ACS Paragon Plus Environment

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The stack with the wide compartments (a fixed membrane distance of 480 µm) shows the lowest

413

net power density due to the low gross power densities as a consequence of the dominance of

414

the high resistance of the river water compartment.

415 416

Outlook

417

The breathing cell concept offers a unique feature, namely physical movement of the membranes.

418

By compressing the river water compartment, the internal resistance of the stack decreases and

419

higher net power outputs can be achieved, despite the momentarily increase in pressure drop

420

along the compartment. This concept increases the robustness towards commercial applications

421

using real feed waters where stacks can be operated at a wide range of different flow rate

422

regimes if needed (e.g. adapting high or low flow rate or breathing frequency to the water

423

parameters and characteristics). However, new challenges concerning the (mechanical) lifetime

424

and integrity of the membranes have risen with this new stack concept and emphasizes the need

425

of more research about ion exchange membrane resistance towards continuous expansion and

426

stretching movements.

427

As the concentration of river water increases along the length of the stack, the effect of the

428

breathing will be more pronounced at the first stages of the stack than at the end of the stack,

429

where the salt concentration in the river water is significantly higher in real large-scale operation.

430

Earlier studies with sea and river water however, clearly showed the strong dominance of the low

431

conductivity of the river water on the power output in that case. As such for the seawater-river

432

water case, the breathing concept is an interesting option. When other feed waters with higher

433

conductivities are applied (e.g. seawater and water from the dead sea), this can be different, but

434

this needs to be considered for each specific case.



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Acknowledgements

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This work was performed in the cooperation framework of Wetsus, European Centre of Excellence

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for Sustainable Water Technology (www.wetsus.eu). Wetsus is co-funded by the Dutch Ministry of

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Economic Affairs and Ministry of Infrastructure and Environment, the Province of Fryslân, and the

441

Northern Netherlands Provinces. The authors like to thank the participants of the research theme

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“Blue Energy” for the fruitful discussions and their financial support.

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References

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Change, U. N.-F. convention on climate. Paris Agreement http://unfccc.int/files/meetings/paris_nov_2015/application/pdf/paris_agreement_english _.pdf (accessed Apr 12, 2016).

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The Breathing Cell: Cyclic Intermembrane Distance Variation in

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