Transcript File - Redox Flow Battery

Electrical equivalent modelling
Electrochemistry
Redox couples
Stack design
Elecrolyte flow circuit
optimization
Simulation
Electrode and membrane design
Electrical interfacing
Basic chemistry and
material science
Redox flow battery
Scale up, structural and
operation optimization of
flow geometries
Renewable energy
Distributed generation
Cost
Modlellng
optimization ,
and simualtion
Grid
Energy storage
System science
Electric vehicles
Trasients phenomena
04.03.2013
Aditya Poudyal
Electricity value chain
Fuel/Energy
Source
Coal
Nuclear
Generation
.....
Hydro
Transmission
Industry
Distribution
households
Office
buldings
Traditional way: Regulated utility, bundled functions.
Energy Storage
Renewables (Wind, Solar, )
Fuel/Energy
Source
Coal
Nuclear
Generation
.....
Unbundled services
Unbundled prices
New service strategies
Privatized services
Hydro
Transmission
Industry
Distribution
households
Distributed Generation
Office
buldings
Electrical energy storage along electricity
value chain
04.03.2013
Aditya Poudyal
Energy storage
04.03.2013
Aditya Poudyal
Candidates for grid storage (Electrochemical)
Liquid metal battery
Lithium Ion Battery
Lithium Ion Battery
Sodium sulfur
Sodium metal chloride
Nickel cadmium
Flow batteries
But they are not meeting the following challenges:
Un commonly high power
Long service lifetime and
Super low cost
04.03.2013
Aditya Poudyal
Energy Power
(€/kWh) (€/kW)
Lithium Ion Metal
compounds
oxides
containing Li
ions/Carbon
Lead acid
Nickel
cadmium
Sodium
sulfur
Lead
dioxide/lead
S/Na
Sodium
metal
chloride
Flow
batteries
Metal air
Vions/Vions
milliseconds
2.10
85-90%
(3%self
discharge/month,
4% electrical, 4%
heat)
milliseconds
1.3-1.35
5-15 100010000
30-50 secs-hrs 50-270
0.1-0.3% Developed Toxic remains
5-15 500-1000 0.1-0.3% Mature
Toxic remains
40-60 secs-hrs -
75%
2.1
(2% heating, 12%
chemical efficiency,
10% electrical)
85%
(2% heating, 9%
chemical, 4%
electrical)
150-240
(150-250)
65-75%
(3% electrolyte pumping,
10% electrical losses, 20%
electrochemical)
35,
16-33
(wh/liter)
Thermal needs
Room
temperature
Room
temperature
(50-80)
-
milliseconds
10-20 20002500
0.2-0.6% Developed Impact of cadmium in the
production step and
also for human health
(60-150)
1.4
Environmental
impact
Maturity
Self discharge
years cycles
85%
3.6-3.8 75-200 min-hrs 500(8% self
2000
(200discharge/month,
500)
3% electrical, 4%
electrochemical)
Nickel hydrate/ 50-90%
Sponge
Cadmium
Lifetime
Response time
Capital cost
Run time
(wh/kg)(wh/liter)
OCV (V)
Efficiency
Active materials
Comparison table for various storage systems
secs-hrs 210-250
125150
milliseconds 10-15
2500
20%
Developed
About 300
250300
<100usec
12000+ Small
Developed
Room
temperat
ure
600
150-3000
(50010000)
secs-10 125-150
hrs
5-10
100-300 Very small Developin Small
g
Vanadium
 Discovered in 1801 by a Spanish
Table: Stadard potential of vanadium couple s
minerologist Andres Manuel del Rio at in aqueous solution at 250O
 Named it after the Scandinavian
goddess of beauty Vanadis.
Vanadium
couples
Strong
acidic
solutions
Weak acidic
solutions
Neutral and
basic
solutions
 Rediscovered in 1830 by Swedish
chemist Nils Gabriel Sefstrom
V(V)-V(IV)
1.000
0.723
0.991
V(IV)-V(III)
0.337
0.481
0.542
V(III)-V(II)
-0.255
-0.082
-0.486
V(II)-V(0)
-1,13
-1.13
-0.820
 In 1867 isolated in nearly pure form
by Roose by reducing its chloride
with hydrogen.
 Steel grey metal which exists in
number of different oxidation states
i.e. -1, 0, +1, +2, +3, +4, and +5
04.03.2013
Aditya Poudyal
1. No problems of cross contamination.
2. High charge and voltage efficiency >> fast kinetics of the vanadium redox couples.
3. Low rate of gas evolution during charge rates associated with rapid charging cycles.
4. ”No memory effect” & ”Can be over charged and deeply discharged” without doing
permanent damage to the electrolyte and the cells.
Why all Vanadium?
5. Reusability of electrolyte >> Long cycle life 5. Fast response
6. Modularity
6. Safe operation
Challenges
1. Specific energy density
2. At high molar concentration precipitation occurs in th V5+ electrolyte at tempertaure
above 40oC and solid vanadium oxides in V2+ or V3+ solution below 10oC.
04.03.2013
Aditya Poudyal
Components of cell stack
OUT
End plate electrode
Bipolar electrode
Membrane
End plate electrode
Positive electrolyte
Negative electrolyte
IN
04.03.2013
Aditya Poudyal
ELECTROCHEMISTRY
Rxn occurs between electrolytes
No electrodeposition
Electrolytes are stored in external
tanks and circulated through the
stack.
Simultaeneous reaction occues at
the both side of electrolyte
Electrical balance is maintained by
proton migration across membranes.
Can be operated under the
temperature range of 10-(35)40oC.
Discharge: Electrons are removed
from Anolyte and trasnferred to the
Catholyte via external circuit.
04.03.2013
Aditya Poudyal
Vanadium concentrations during battery
operation
Salt
Charge
Discharge
Electrolyte
V2+
VSO4
↑
↓
Anolyte
V3+
0.5 V2 (SO4)3
↓
↑
Anolyte
V4+ or VO2+
VOSO4
↓
↑
Catholyte
V5+ or VO2+
0.5 (VO2) 2 SO4
↑
↓
Catholyte
04.03.2013
Aditya Poudyal
Electrolyte preparation
• Based on the the electrolysis of Vanadyl Sulphate.
• Catholyte is obtained from the electrolytic oxidation of VOSO4 solution and
anolyte from the elecrolytic reduction.
V3+ and V2+ (Reduction)
V5+ (Oxidation)
Negative compartment: Vanadyl Sulphate
Both are filled with the VOSO4 and electric
current is applied to the electrodes
Positive compartment: Sulphuric acid solution with a sulphate
concentration equivalent to the Vanadium concentration in
negative compartment.
VO
V
3
04.03.2013
2
 2H
e



e V
V
3
 H 2O
VO
2

 H 2 O  VO 2  2 H

e

2
Aditya Poudyal
Electrolyte stability




04.03.2013
Depends upon
 temperature,
 the vanadium concentration,
 the suplphric acid concentration and
 on the SOC.
At higher temperature
 Catholyte precipitaion at fully charged state.
 But not irreversible >> dissloves when discharging
Lower temperature
 V4+, V3+ and V2+ start to precipitate.
 Slows the rates of the reactions at the electrodes; operation at 0oC could result iin significantly
slower reaction rates.
Increasing the stability
 Use of inhibitors
 Dispersion: decrease the strength of attraction forces betn the particles
 Comlexing: forms new complexes with one of the ion involved in precipitation
 Threshold: inhibit the precipitation of certain compunds
 Use of heat treatment.
 Boil the electrolyte for few hours to remove the precipation process.
Aditya Poudyal
Electrical equivalent circuit
 R reaction and R resistive compise the
internal losses, reaction kinetics,
mass transport reisistance,
membrane resistacne, solution
resistance, electrode resistance and
bipolar resistance.
 Rfixedloss represent the parasitic
losses
 Ipump stands for the power
consumption by recirculation
pump, system controller, and
power loss from cell-stack-by pass.
 Celectrodes represnet the transie
component associated with the
electrode capacitance.
04.03.2013
Aditya Poudyal
VRB discharging and charging cycles:
 Charging take longer time than to
discharge it.
Ipump soars dramatically as the SOC
drops >> more catholyte and anolyte
are required to provide the same power
when the SOC lowers
Stack voltage is higher than the output
volatge when dscharging , stack volatge
is smaller when charging and it implies
internal losses.
Efficiency decreases by 5 % when SOC
is 0.2%.
04.03.2013
Aditya Poudyal
Trasients and response time
• Transients are essential
because of the importance
of the system ability to
respond to the fast change.
• Trasient behaviour is related
to the electrode
capacitance as well as
concentration depletion
close to electrodes.
Worst case transients were considered the operation is switched from -65A and then back.
Figure shows that it takes 0.045 seconds for battery voltag e to reach steady state
04.03.2013
Aditya Poudyal
Equilibrium Potential
The equilibrium voltage
corresponds to the sum of
equlibrium potential of each cell
in stack.
U
Equilibrium potential is given by
the Nernst equation and depends
upon vanadium species
concentration and the proton
concentrations.
 E  is standard potentials and
it is important parameter in nersnt
equation as it expresses the
reaction potentials at standard
conditions.
04.03.2013
stack
( t )  U eq ( t )  U loss ( t )
E  E 

RT
ln( Q )
nF
R is gas constant
T is temperature
F is Faraday constant
Aditya Poudyal
Standard potential
An ideal state where the battery
is at standard conditions:
Vanadium species at a
concentration of 1
All acticity coefficients
equal to 1 and temperature
250C.
Can be detemrined from
the thermodynamic
principle called the Gibbbs
free enthalpy, the
conservation of energy and
empirical parameter that
can be found in
electrochemical tables.
Standard Gibbs free enthalpy of reaction which
represents the change of free energy that accompanies
the formation of 1M of a substance from its component
elements at their standard states: 250C, 100kPa, and 1M
G
0
 H
r
 TS
o
r
Where the standard reaction enthalpy  H o r is the
difference of molar formation enthalpies between the
products and reagents
0
H r 

H
o
f , product


H
o
f , reagent
and the standard reaction of entropy  S 0 is the
r
differnce of molar formation entropies between the
products  S o f , product and the reagents  S o f , reagent
0
S r 
04.03.2013
o

S
o
f , product


S
o
f , reagent
Aditya Poudyal
Standard potentialInserting thermodynamical data the standard
The conservation of energy
relates the change in free energy
resulting from the transfer of n
moles of electrons to the
difference of potential E:
reaction enthalpy ∆H0r becomes :
H
0
r
 H
 G   nFE
E
 
G
nF
o
 
H
o
f ,V
2
o
f ,VO
 H
2
 H
o
f ,VO

o
f , H 2O
 2H
2
 H
o
f ,H
o
f , H 2O

= -155.6kJ/mol
Therefore standard potential can
e written as
o
 H
o
r
 TS
and similarly the standard reaction entropy
∆S0r
o
r
nF
S r
0
 S
 S
o
f ,V
2
o
f ,VO
 S
2
 S
o
f ,VO

2
o
f , H 2O
 S
 2S
o
f ,H
o
f , H 2O

”The standrad potentia is 1.23V
at 250C.”
=-121.7 J7mol.K
04.03.2013
Aditya Poudyal
Characteristic curve of the equilibrium
potential E for a single cell
04.03.2013
Aditya Poudyal
Electron exchange rate
Concentrat
involved
ion change are proportion al to the reaction
each time
the electrical
a redox reaction
occurs.
Therefore
rate and also it is known that
the concentrat ion change are proportion al to
current and the pace of the concentrat ion is set by the electrical

Q c  ne  e 
electron is
current flowing
through
cell.
 i ( t ) dt
where Q c is the charge, i the current, t the time, n e - the number
of electrons
and e the elementary
charge
--------------------------------------------------------------------------- the number
ne

1

eN
of electrons
 i (t ) d (t )
involved
for a given current is
where N A is the Avgardo
number
A
-----------------------------------------------------------------------------Therefore
.
N e  (t ) 
molar flow rate of electron
1
eN
i (t )
for single cell is
[mols/sec]
A
------------------------------------------------------------------------------and for the
.
N e  (t ) 
stack
N cell
eN
04.03.2013
containing
N cells is given by
i ( t ) [mol/s]
A
Aditya Poudyal
Proton concentrations
04.03.2013
Aditya Poudyal
Internal loss
• When current starts to flow
– Cell Voltage ≠ Nernst Voltage
• The losses are called overpotentials
– represents the energy required to force the redox
reaction to proceed at required rate
U loss ( t )   act ( t )   conc ( t )   ohm ( t )   ion ( t )
U loss ( t )  R eq , ch arg e / disch arg e i ( t )
Electrode phenomena and are associated with
 the energy required to initiate the charge transfer and
 Concentration difference between bulk solution and electrode surface
04.03.2013
Ohmic loss occurs in electrodes, the
bipolar plates and the collector plates.
Ionic loss occurs in electrolytes and
membranes
Aditya Poudyal
Efficiencies
Enegy released during discharge and energy
supplied during charge
•Defined for charge and discharge cycle for
constant currnet.
•Is meaure of ohmic and plarization losses
during the cycling.
•Can be maximized by contact, electrode,
electrolyte and membrane resistance
•By using an electrode material with good
electro-catalytic properties for the
reactions.
•Ratio of the charge withdrawn from the system
during the discharge to the charge supplied
•Can be caused by side reaction such as
oxygen and hydrogen evolution
•Cross mixing of electrolyte through
membrane due to ion transfer
•Unbalanced flowrates of the electroly
04.03.2013
Energy efficiency
 energy 
P
P
dich arg e
ch arg e
Coulombic
 coulombic 
( t ) dt
( t ) dt
efficiency
Q disch arg e

Q ch arg e
Voltage
efficiency
 energy 
U
U
dich arg e
ch arg e
( t ) dt
( t ) dt
i
i
disch arg e

ch arg e
( t ) dt
( t ) dt
 energy
 coulombic
Aditya Poudyal
Charge and discharge at costant currnet
Efficiencies at various currents.
The cycle starts at 2.5% SOC, and
charged upto 97.5% SOC and
again discharged to 2.5%
Cost breakdown
04.03.2013
Aditya Poudyal
04.03.2013
Aditya Poudyal
Thank you for the attention !!!
04.03.2013
Aditya Poudyal