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Fraunhofer ISE
Design, characterisation and operation strategies of 1 KW all-vanadium redox flow battery
Martin Dennenmoser, Kolja Bromberger, Felix Owald, Karsten Korring, Tobias Schwind, Tom Smolinka, Matthias Vetter
Fraunhofer Institute for Solar Energy Systems ISE
The second International Flow Battery Forum,
Edinburgh - Scotland, 4th, 2011
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Introduction to redox flow batteries
Applications
Overview energy losses
Stack design
System description
Measurement results
Smart Redox flow Control
Summary
Agenda
V4+ V3+ V5+ V2+
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General layout of a RFB
Vanadium redox flow batteries:
Decoupling capacity from power
Modular design facilitates different applications
Fast response time
Efficiency >75 % possible
No irreversible cross-over of active mass over the membrane
Long calendar life, cycle stability (> 10.000)
Introduction to redox flow batteries
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General layout of a RFB
Vanadium redox flow batteries:
Decoupling capacity from power
Modular design facilitates different applications
Fast response time
Efficiency >75 % possible
No irreversible cross-over of active mass over the membrane
Long calendar life, cycle stability (> 10.000)
storage
Electrochemical converter
Introduction to redox flow batteries
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Fields of application:
Off-grid / mini-grid
kW/kWh range
Long term storage
Distribution network
MW/MWh range
Energy management
Industrial
Backup power
Load management
Introduction - application
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System overview
Components of a redox flow storage system
Stack
Pumps
Tanks
Valves
Pipes
Inverter
Controlling
Process technology
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System overview energy losses
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System overview energy losses
For an optimal process/energy efficiency inverter, pump control and stack design has to be optimized.
Energy efficiency of the stack is depending on:
Diffusion through the membrane
Internal shunt currents
Ohmic losses
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Hydraulically in parallel configuration causes
--> internal shunt currents
Stack design
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Hydraulically in parallel configuration causes
--> internal shunt currents
Stack design
Electrolyte is electrically conductive!
Internal shunt currents
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Hydraulically in parallel configuration causes
--> internal shunt currents
It is very important to minimize internal shunt currents
The internal shunt currents effects:
Coulombic efficiency
Self discharge
Operation strategies
Corrosion of the bipole plate
Stack design
Electrolyte is electrically conductive!
Internal shunt currents
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Modelling of internal shunt currents
Appropriate design of manifolds and in/outlet of the cells minimizes the internal shunt currents
But trade off between shunt currents and pressure drop has to be found
Stack design
Electrolyte is electrically conductive!
Internal shunt currents
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Stationary analysis of
Pressure drop
Fluid distribution
Parameter studies of
Porosity, permeability of porous electrodes
Velocity of electrolyte
Geometric design of manifolds
Fluid simulation of a half cell
CFD s imulation of an 700 cm half cell
Pre
ssu
re P
a
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Porosity: 95 %
Velocity: 19 ml/min to 76 ml/min per half cell
Stoichiometric flow rate 1 to 4
700 A/m2 current density
1.6 mol/l vanadium
2.0 mol/l sulfuric acid
Parameter study of electrolyte velocity
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Parameter study of electrolyte velocity
manifold inflow
manifold outflow
porous electrode
Velo
city
(m
/s)
Velocity: 19 ml/min
Stoichiometric flow rate
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Velocity: 38 ml/min
Parameter study of electrolyte velocity
Velo
city
(m
/s)
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Velocity: 57 ml/min
Parameter study of electrolyte velocity
Velo
city
(m
/s)
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Velocity: 76 ml/min
Parameter study of electrolyte velocity
Velo
city
(m
/s)
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Velocity: 76 ml/min
Line plots for velocity magnitude at electrode inflow and outflow
Comparison of fluid distribution at electrode inflow and outflow
Parameter study of electrolyte velocity
Velocity field at electrode inflow
Velocity field at electrode outflow
Velo
city
(m
/s)
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Electrode outflow
Velocity field is homogenous
Electrode inflow
Velocity field is not homogenous
depends strongly on velocity of electrolyte
Velocity field, electrode inflow and outflow
Optimized geometric design of manifolds to reach homogenous velocities at electrode inflow
Line graph: Velocity field, y component (m/s)
0,0
0,5
1,0
1,5
2,0
2,5
3,0
150 200 250100500
X 10-3
X 10-3
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
150 200 250100500
X
X
19ml/min
38ml/min
57ml/min
76ml/min
19ml/min
38ml/min
57ml/min
76ml/min
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Design of a 1 kW stack
18 cells
700 cm active area
Ca. 1.3 V nominal cell voltage
Max. 800 A/m
Max. charge power 1.6 kW
Flow through electrodes
Electrically in series and hydraulically in parallel
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Design of a 1 kW System
Measuring of
Pressure
Temperature
Single cell voltage
Electrolyte level in the tanks
Balancing of the electrolyte level
Valve control
Pump control
SoC determination over OCV measurement
1 kW redox flow system
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Voltage, current and power curves
cc-cv charging
cc discharging
600 A/m
Initialization plus 4 test cycles
Results power-voltage and current curve
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Influenced by
flow rate
cell design
different SoC in the stack
Homogenous single cell voltages are a sign for a good stack design
Results single cell voltage
1
1,1
1,2
1,3
1,4
1,5
1,6
1,7
0:00 1:12 2:24 3:36 4:48 6:00 7:12 8:24 9:36
duration [h:mm]
vo
ltag
e [
V]
cell 1 cell 3 cell 4 cell 8 cell 10 cell 12 average cell voltage
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Without pumps
Stack disconnected from electrolyte fluid cycle
100 % SoC after 2 h
Deinitialization begins after 15 h
Results single cell voltage self-discharge
0,8
0,9
1
1,1
1,2
1,3
1,4
1,5
1,6
1,7
1,8
0:00 4:48 9:36 14:24 19:12 24:00
duration [h:mm]
vo
ltag
e [
V]
cell 16 cell 12 cell 10 cell 8 cell 6 cell 4 cell 3 cell 2 cell 1
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Energy efficiency around 70 %
Coulombic efficiency > 90%
Voltage efficiency depends on the flow rate
Results with very low flow rate
Results - efficiencies
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System integration in the Messib project - Smart Redox Flow Control
Smart Redox Flow Control:
Battery management system
Interface between EMS, inverter and battery
Overview of the integration in the Solarhaus Freiburg
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Smart Redox Flow Control
Smart Redox Flow Control:
Control loops for devices of redox flow battery
Determination of set points (e.g. inverter, pumps)
Optimization of the process cycle
energy efficiency
Interface with energy management system
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Smart Redox Flow Control
Smart Redox flow Control
Pump control
Charge/
Discharge
controller
SOC forecast
SOC
Determination
Smart Redox Flow Control:
Control loops for devices of redox flow battery
Determination of set points (e.g. inverter, pumps)
Optimization of the process cycle
energy efficiency
Interface with energy management system
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Smart Redox Flow Control
AC-Grid VRB +
PeripheryInverter
Pu
mp
co
ntr
ol
Cu
rre
nt A
C S
et
Me
asu
rem
en
t
va
lue
s
Actual values
SOC-forecast
Power AC Demand
Smart Redox flow Control
Energy
Management
System
(EMS)
Pump control
Charge/
Discharge
controller
SOC forecast
SOC
Determination
Smart Redox Flow Control:
Control loops for devices of redox flow battery
Determination of set points (e.g. inverter, pumps)
Optimization of the process cycle
energy efficiency
Interface with energy management system
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Hardware implemen-
tation
Control algorithm
System validation
Parameter fitting
System modeling
Smart Redox Flow Control - development
Required steps for the development:
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Modelled components:
Inverter
Tanks
Stack
Pipes
Pumps
Reference cell
Smart redox flow control
Smart Redox Flow Control modelling in Modelica
System modelling in Modelica / Dymola
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Electrochemical characteristic:
Voltage according to depth of discharge and current density
Equivalent circuit
Results for discharge mode
Cel
l v
olt
age
[V]
Smart Redox Flow Control modelling in Modelica
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Electrochemical characteristic:
Dynamic of the stack
RC equivalent circuit
Important for control algorithm
Dynamic model of the voltage according to the SoC and the current density
Very fast response time!
Smart Redox Flow Control modelling in Modelica
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Energy loss model out of the inner resistance and coulombic efficiency model
The model shows the losses for charge and discharge according to SoC and the nominal power
SoC forecast
Optimized control strategies with several stacks
System layouts / design
Smart Redox Flow Control simulation in Modelica
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Stoichiometric flow rate depends on current and SOC
SOC difference between stack and tanks should not too large
Current density
600 A/m
discharging
Smart Redox Flow Control pump control
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Smart Redox Flow Control pump control
Stoichiometric flow rate depends on current and SOC
SOC difference between stack and tanks should not too large
Current density
600 A/m
discharging
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Stoichiometric flow rate depends on current and SOC
SOC difference between stack and tanks should not too large
Pump control is important for the energy efficiency of the VRFB
Smart Redox Flow Control pump control
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Stoichiometric flow rate depends on current and SOC
SOC difference between stack and tanks should not too large
Pump control is important for the energy efficiency of the VRFB
SOC Range also influences the energy efficiency
Smart Redox Flow Control SOC range variation
70
70,5
71
71,5
72
72,5
73
5-95 %
10-90 %
15-85 %
20-80 %
25-75 %
30-70 %
35-65 %
40-60 %
45-55 %
SOC Range
S
yste
m [
%]
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Summary
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May the flow be with you, and thank you for your kind attention!
Martin Dennenmoser
Fraunhofer ISE
Heidenhofstr. 2 / 79110 Freiburg / Germany
Ph: +49 761 4588 5682
www.ise.fraunhofer.de
Questions?
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