PEM fuel cell stack experiments applying air exhaust recirculation ...

PEM fuel cell stack experiments
applying air exhaust recirculation
for air humidification
Sept., 23 th , 2009
Beom Jun Kim * , Sung-il Kim,
Soo-young Byun, Min-soo Kim
Dept. of Mechanical & Aerospace Engineering
Seoul National University, South Korea

Outline
1. Introduction
- Needs for Humidification in PEM Fuel Cell
- Kinds of Humidification Methods
- Basic Concept of Humidification using Exhaust Air
Recirculation
2. Theoretical Analysis
- Solving Basic Species Equations
- Results of Calculation
3. PEM Fuel Cell Stack Experiments
- Fuel Cell Operation with 20% Exhaust Air
Recirculation
- Effects of Oxygen Mole Fraction and Flow Increase
4. Conclusion
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

1. Introduction

Needs for Reactants Humidification in PEMFC
Not Enough
Hydration
Dry Out
1. Life Span, Durability
2. Performance, Ion
Conductivity
Membrane
Catalyst
GDL Layer
Gas
distribution
channel
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Kinds of Humidifier
Enthalpy Wheel
-The rotating
Cordierite drum for
Heat and Vapor
Exchanger
-Simple, Low
Parasitic Power
Loss
- Cost of
Maintenance
Internal Humidifier
- No requiring
additional devices
- Insufficient
Humidification
- Hard to direct
application
Dry gas
Humid gas
Membrane Humidifier
- Simple, No
Parasitic Power
Loss
- Expensive
- Bulky
- Difficult to Control
the Humidity
Water
Figure Bubbling
Heater
Steam
Figure Steam
injection
Figure Direct water injection
with inter-digitized flow
channels
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Basic Concept of Exhaust Air Recirculation
AIR INLET
AIR SUPPLY
AIR OUTLET
AIR
BLOWER
MIXER
AIR FILTER
PEMFC STACK
EXHAUST AIR
– It does not need any additional blower for the air recycle.
– Fresh air from the ambient and Exhaust air are mixed before the
blower, and supplied to the fuel cell stack.
– There is a trade-off between the humidity increase and the oxygen
concentration decrease according to the recirculation ratio.
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

2. Theoretical Analysis

Solving the Species Conservation Equations
AIR INLET
n&
+ n&
N _ Amb O _ Amb
2 2
n& O 2 _ Consumed
AIR OUTLET
AIR BLOWER
MIXER
AIR
SUPPLY
x ´ ( n& + n& + n&
)
O _ Ex N _ Ex H O _ Ex
2 2 2
PEMFC STACK
( 1 - x )( n& O _ Ex
+ n& N _ Ex
+ n&
H O _ Ex )
2 2 2
1)
n& + x´ n& = n& + n&
O _ Amb O _ Ex O _ Consumed O _ Ex
2 2 2 2
2) n& N 2
2 _ Amb
+ x´ n& N2 _ Ex
= n&
N2
_ Ex
0.79
3) n&
N2 _ Amb
= ´ n&
O2
_ Amb
0.21
1
4) n& O2 _ Consumed
= ( n& O2 _ Amb
+ x´
n&
O2
_ Ex)
SR
Psat , Ex
5) n& H 2O _ Ex
= ´ ( n& O2 _ Ex
+ n&
N2
_ Ex
)
P - P
Ex
sat , Ex
EXHAUST AIR
1 1
6) k ´ 2 ´ ´ n& O2 _ Consumed
= ´ n& O2 _ Amb
+ x ´ ( n& O2 _ Ex
+ n& N 2 _ Ex
+ n&
H 2O _ Ex
)
0.21 0.21
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Solving the Species Conservation Equations
• Results
7) SR
8) X
O
9) X
10)
2
2
_ IN
H O _
X
P
ì
w, Ex
2 ï æ P öü
w,
Ex ï
´ 4.76x + í9.52k + 3.76 - ý x -9.52k
PEx - P ç
w, Ex
PEx P ÷
ï è -
w,
Ex øï
=
î
þ
æ P
ö
w,
Ex
( x - 1)
´ 4.76x
+
ç
4.76
PEx
P
÷
è -
w,
Ex
ø
SR
=
9.52k
Pw , Ex
x ´ ´ é4.76 ´ {( 1- SR2)
x + SR}
-1ù
PEx
- P ë
û
w,
Ex
IN
=
9.52 k(1 - x)
O _ DA_
IN
2
X
O _ IN
SR(1 - x)
1-
X
P
H2O
_ IN
k x x é SR x SR
P - P ë
2
= =
{( ) }
w,
Ex
9.52 (1 - ) - ´ ´ 4.76´ 1- + -1
Ex
w,
Ex
ù
û
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Solving the Species Conservation Equations
• Results
SR
2.4
2.2
2
1.8
1.6
1.4
1.2
k=1
k=1.1
k=1.2
Oxygen mole fraction in dry air
0.25
0.2
0.15
0.1
0.05
k=1
k=1.1
k=1.2
1
0
0 0.1 0.2 0.3 0.4 0.5
Recirculation ratio, x
0 0.1 0.2 0.3 0.4 0.5
Recirculation ratio, x
– As inceasing recirculation ratio, concentration of oxygen is decreased.
– At 20% recirculation ratio, SR is 1.68. However as flow rate is increased, SR
is increased.
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Solving the Species Conservation Equations
• Results
60
80
Relative humidity @65℃ (%)
50
40
30
20
10
k=1
k=1.1
k=1.2
Relative humidity @65℃ (%)
70
60
50
40
30
20
10
0
60℃
65℃
70℃
75℃
0
0 0.1 0.2 0.3 0.4 0.5
Recirculation ratio, x
0.0 0.1 0.2 0.3 0.4 0.5
Recirculation ratio, x
– With 20% recirculation ratio, we can obtain relative humidity 29.58% at
65℃.
– The bigger dew point of exhaust air, The bigger effect of humidification
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

3. PEM Fuel Cell
Stack Experiments

Design of Experimental Setup
Inverter
P
Dewpoint
Meter
Ambient
Air
T
Air
Inlet
(+)
(-)
Filter
MFM
AC Blower
Air
Outlet
Control Valve
2
Orifice
Flow Meter
Orifice
Flow Meter
T
Air Exhaust
P
Dewpoint
Meter
Gas Analyzer
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Devices applied to the Experimental Setup
5 cells Fuel Cell Stack Control Valve (Needle Valve
+ Stepping Motor + Driver)
Dew Point Meter
Orifice Flow Meter Air Blower Mass Flow Controller
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Devices applied to the Experimental Setup
Fuel Cell Stack Setting
Electric Loader
Air Supply and Exhaust Lines
Fuel Cell Test Stand
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Experiments of 1 kW Fuel Cell Stack
with Air Recirculation Humidification
• Test Conditions
Pressure
[Fuel Cell Operation]
SR
[O2]
SR
[H2]
Temperature(℃)
[Cathode Inlet, Air]
Temperature(℃)
[Incoming Air]
Temperature(℃) Temperature(℃)
[Anode Inlet, H2] [Coolant Inlet]
Dewpoint (℃)
[Cathode Inlet]
Dewpoint(℃)
[Anode Inlet]
Bubbler 1.01325 bar 2 1.5 65 - 65 65 65 65
20% Recirc
ulation
1.01325 bar 1.68 1.5 65 60 65 65 38 65
• Recirculation ratio is controled by
the control valve
• Control the external air flow rate
with the normal flow rate
multiplied by the following
coefficient equation.
{( ) }
(11) 0.5´ 1- SR x + SR
2
Current Density Current External Air Flow Rate (SLPM)
Bubbler 20% Recirculation
(mA/cm 2 ) (A) H 2 AIR Air
0 0.00 0.000 0.000 0.00
40 10.00 0.523 1.663 1.28
80 20.00 1.045 3.327 2.56
160 40.00 2.091 6.653 5.12
320 80.00 4.182 13.307 10.25
480 120.00 6.272 19.960 15.37
640 160.00 8.363 26.613 20.49
800 200.00 10.454 33.267 25.62
1000 250.00 13.068 41.584 32.02
1200 300.00 15.681 49.900 38.42
1400 350.00 18.295 58.217 44.83
1600 400.00 20.908 66.534 51.23
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Experiments of 1 kW Fuel Cell Stack
with Air Recirculation Humidification
• Polarization Curves
Voltage (V)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Current density (A/cm 2 )
Cell 1, 65℃ 100%RH
Cell 2, 65℃ 100%RH
Cell 3, 65℃ 100%RH
Cell 4, 65℃ 100%RH
Cell 5, 65℃ 100%RH
Cell 1, 20% Recycle
Cell 2, 20% Recycle
Cell 3, 20% Recycle
Cell 4, 20% Recycle
Cell 5, 20% Recycle
- Differences in voltage at each single cell are increased as current
density is increased.
- At 20% recirculation and 1 A/cm 2 , average single cell voltage is
0.536 V.
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Experiments of 1 kW Fuel Cell Stack
with Air Recirculation Humidification
• Power Curves
250
200
65℃
100%RH
Power (W)
150
100
50
0
0 0.5 1 1.5 2
Current denstiy (A/cm 2 )
- Average single cell voltage is decreased and average single cell
power is decreased upto 23W at current densitiy 1 A/cm 2 , .
- Without increase in flow rate the concentration loss is big.
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Experiments of 1 kW Fuel Cell Stack
with Varying O 2 Mole Fraction and SR
• Polarization Curves
1.2
1
0.8
Normal air(O2 mole fraction 0.21)
O2 mole fraction 0.19
O2 mole fraction 0.17
10% flow rate increase, oxygen 0.19
10% flow rate increase, oxygen 0.17
Voltage (V)
0.6
0.4
0.2
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Current density (A/cm 2 )
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Experiments of 1 kW Fuel Cell Stack
with Varying O 2 Mole Fraction and SR
• Power Curves
200
180
160
140
Power (W)
120
100
80
60
40
20
0
Normal air(O2 mole fraction 0.21)
O2 mole fraction 0.19
O2 mole fraction 0.17
10% flow rate increase, oxygen 0.19
10% flow rate increase, oxygen 0.17
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Current density (cm/m 2 )
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Conclusion
• When k is 1.0 and recirculation ratio is 20%, we can obtain
relative humidity 29.58% at 65℃. It is not sufficient with
only exhaust air recirculation.
• By applying this method to the hybrid humidification
system with membrane humidifier, we can achieve the
good humidification system.
• This hybrid humidification system is easier to control,
cheaper and smaller than membrane humidifier.
SEOUL NATIONAL UNIVERSITY, SOUTH KOREA
Fuel Cell System Laboratory

Thank you !