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B+S / Session B4: News from the Science Sector

TECHNOLOGICAL CHALLENGES AND PROGRESS IN THE DEVELOPMENT OF CELL STACKS FOR REDOX FLOW BATTERIES

Kolja Bromberger, Johannes Kaunert, Christian Reinke, Malte Schlüter, Tom Smolinka

Fraunhofer-Institut für Solare Energiesysteme ISE

World of Energy Solutions Conf.Stuttgart, September 30, 2013

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Agenda

Introduction to redox flow batteries

Material screening

Overview required parameter

Test bench for material screening

Performance data

Design of a 5 kW stack

General features

Test bench for testing

Performance data

Summary and outlook

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There Are Many Options to Store Eletrical Energy!

Different principles:

Electrochemical

Chemical

Mechanical

Electro-magnetic

There is not the only one and universal storage type!

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RFB in general:

Decoupling capacity from power

Modular design facilitate different applications

Fast response time (μs – ms)

VRFB in particular:

high efficiency (>75 % possible)

no irreversible cross-over of active mass over the membrane

Long calendar life

excellent cycle ability (> 10.000)

No self discharge in the tanks

Why Vanadium Redox Flow Batteries?

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V4+ V3+V5+ V2+

All-Vanadium Redox Flow Battery (VRFB)

Material characterisation

Electrochemical modelling

CFD simulations of cells and stacks

Cell and stack development

Characterisation and optimisation of the complete system

System modelling and simulation

Model-based development of controls and implementation in embedded systems

Lifetime cost analysis

System integration, e.g. in hybrid PV systems

Redox Flow Batteries: Portfolio at Fraunhofer ISE

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Major Challenges in Cell and Stack Design

High coulombic efficiency

Assembly must be leak-free

Low vanadium ions cross-over (membrane)

Minimising shunt currents (bypass)

High energy efficiency

Low overpotential and low ohmic losses

Low electrical contact resistances

Uniform flow of the electrode at cell and stack level

Balanced single cell voltage (no H2, O2 formation)

Low pressure loss to minimise pumping energy

Simple design for frame and sealing

Easy and fast assembly/disassembly -> Quick material changes

Simple fabrication, suitable for injection molding

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Main components forcell/stack design:

Membrane

Electrode

Electrolyte

(Bipolar plate)

Material Screnning and Parameter ExtractionWhat kind of parameter do we need?

Symbol Unit Description (electrode)

kin

eti

cs k0 m/s reaction rate constant

AR m²/m³ specific reaction surface

α - transfer coefficientele

ctri

c ρx Ω m through plane resistance

ρy,z Ω m² in plane resistance

oth

er K m² permeability

E N/m² Young's or elastic modulus

Symbol Unit Description (membrane)

σion. S/m ionic conductivity

S % selectivity

g cm-2 h-1 mass transfer

τSD % cm-2 h-1 self-discharge

chemical stability

mechanical stability

Symbol Unit Description (electrolyte)

σion. S/m ionic conductivity

ρQ Ah l-1 charge density

ω Wh l-1 energy density

η Pa s viscosity

ρ kg/m3 density

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Two test cells in parallel

Power electronics:

Current: 2x 0 – 30 A

Voltage: 2x 0 – 10 V

Flow rate: 4x 100 ml/min

Measured value for each cell:

Voltage, current

Impedance spectroscopy

2 x reference cells (OCV)

4 x pressure, 6 x temperature

2 x flow rate, 2 x tank level

LabView controlled

Material Screening and Parameter ExtractionTest set-up for high throughput under defined conditions

Cell 1 Cell 2

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Material Screnning and Parameter ExtractionDevelopment of suitable test procedures is essential

Defined cc cycling

charge: 1,7 V

discharge: 0,9 V

initialisation @ 400 A/m²

Variation of current densityCell 1 Cell 2

Variation of different parameters

electrode compression (25 - 58%)

temperature (25 °C)

mass flow (100 ml)

current density (40-60-80 mA/cm²)

Resulting parameter

CE/VE/EE

ASR

ohmic part

non-ohmic part

pressure loss

mass transfer

reaction turnover

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Material Screening and Parameter ExtractionEvaluation of different electrodes

40 cm2 test cell

Operation parameters:

Electrode compression (25%)

Temperature (25 °C)

Mass flow (100 ml)

Current density (60 mA/cm²)

Membrane M-2

Electrodes provided by different commercial suppliers

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Material Screnning and Parameter ExtractionEvaluation of different membranes

40 cm2 test cell

Operation parameters:

Electrode compression (25%)

Temperature (25 °C)

Mass flow (100 ml)

Current density (60 mA/cm²)

Electrode E-2

Membranes provided by different commercial suppliers

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Filter press configuration

Cells are connected electrically in series, hydraulically in parallel

Flow through electrode

Bipolar plates are bonded into two flow frames

Felts, O-rings, membranes are placed between the frames

Design of 5kW StackGeneral features (1)

One cell

FeltBipolar plate

Flow frame(Anolyte)

Flow frame(Catholyte)

Membrane

O-ring sealing

This work was partly funded by the German BMBF.

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Design of 5kW StackGeneral features (2)

Intermediate segment

Reducing shunt currents

Felt compression of 25%

Reducing ohmic losses

Single cell voltage

Frame design suitable for injectionmolding

Optimized use of materials

Weight with end plates: ca. 240kg

Dimensions :660 x 660 x 675mm:(width x height x depth)

This work was partly funded by the German BMBF.

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Design of a 5kW StackElectrical Specifications

Data peak / nom.

cell voltage [V] 1.2 / 1.1

current density [mA/cm2] 60 / 40

cell area [cm2] 2016

no. cells [-] 40

stack voltage [V] 48 / 44

stack current [A] 120 / 80

stack power [kW] 5.7 / 3.5

This work was partly funded by the German BMBF.

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Power electronic

Current: 0 – 600 A

Voltage: 0 – 60 V

Flow rate: 3 – 60 l/min

Main measuring instruments

Stack voltage, current

60 single cell voltage channels

2 x reference cells (OCV)

4 x pressure, 6 x temperature

2 x flow rate, 2 x tank level

Max. tank volume: 2 x 200 l

LabView controlled

Different modes: cc,cv,cp

Performance Data of the 5kW StackTest rig for stack measurements

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Performance Data of the 5kW StackTypical charge and discharge performance

This work was partly funded by the German BMBF.

20-cell block

Electrodes and membrane as received

Complete cycles:

cc-cv charging

cc discharging

SOC: 15 - 85 %

Different current densities

Constant flow

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Performance Data of the 5kW StackTypical charge and discharge performance

This work was partly funded by the German BMBF.

20-cell block

Electrodes and membrane as received

Complete cycles:

cc-cv charging

cc discharging

SOC: 15 - 85 %

Different current densities

Constant flow

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Performance Data of the 5kW StackDifferent efficiencies

This work was partly funded by the German BMBF.

Calculation of different stack efficiencies:

CC - Coulombic Eff.

VE - Voltage Eff.

EE - Energy Eff.

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Summary and Outlook

VRFB can be tailored to many applications, but has to be competitive against LA -Li-Ion - NaS - (H2 systems)

Stack design is always a trade-off between:application available materials production volume

Parameter extraction for selected materials is essential for stack design

Appropriate material selection allows efficiencies EE > 85% on cell level

A 5 kW stack platform has been development at Fraunhofer ISE

Cell area of some 2,000 cm² enables power range of 2 - 10 kW

Intermediate flow segment for reducing shunt currents

5 kW VRFB system was built up and integrated in a PV battery system

Outlook:

Long term investigation of 5 kW system

Transferring excellent cell results on stack level

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Thanks a lot for your kind attention!

Fraunhofer-Institut für Solare Energiesysteme ISE

Dr. Tom Smolinka

tom.smolinka@ise.fraunhofer.de

www.ise.fraunhofer.de