1
Dynamic Model of High Temperature PEM Fuel Cell Stack Temperature
Søren Juhl Andreasen and Søren Knudsen Kær
Aalborg University
Institute of Energy Technology
©SJA 2007
Presentation outline
2
∙
∙
∙
HTPEM features
Experimental fuel cell system setup
Previous work
▫
∙
Governing equations
▫
▫
▫
▫
∙
∙
∙
Current feedforward
PI controller
Model validation
▫
▫
▫
©SJA 2007
Energy balance
Fuel cell power input
Convection
Conduction
Model definitions
Model assumptions
HTPEM FC stack temperature control
▫
▫
∙
Stack temperature profile identification
Heating
Operation
Pulsating air flow
HTPEM PBI(H3PO4)-membrane features
3 Operating
▫
▫
▫
▫
conditions
FC operating conditions 120-200oC, preferred range (160-180oC)
Allowable CO content 1-3% (10000-30000 ppm)
No humidification of anode- and cathode flows
Fast response to load changes due to high temperatures (even with CO)
Advantages
▫
▫
▫
▫
▫
▫
No humidification of cathode or anode => Very simple FC system and stack design
No liquid water should be present in FC membranes => Simple stack design
Large CO-tolerance (1-3%), LTPEM is typically 10-100ppm
Possible system integration with simple reformer, due to high CO tolerance
Storing hydrogen as a liquid hydrocarbon => methanol, ethanol etc.
Avoiding and extra cooling circuit, by using extra cathode air
Disadvantages (Challenges)
▫
▫
▫
Lower cell voltage = Lower efficiency (not as low as DMFC though)
Start-up time is often long because of high operating temperatures (min 100 oC) to avoid water condensation.
High demands for materials at these elevated temperatures
©SJA 2007
Performance of HTPEM fuel cell
4
©SJA 2007
©SJA 2007
HTPEM FC System- pure hydrogen
5
©SJA 2007
Previous work – Initial experimental results
6
©SJA 2007
Stack temperature profile identification
Fuel cell stack energy balance
7
Energy balance :
Fuel cell heat input :
External heat input :
Heat Conduction :
Forced Convection :
Natural Convection :
©SJA 2007
Q in,external  Ptotal  DPWM
Manifold and gas channel temperature
8
Tchannel,in,front =
Tmanifold,in,front + Tmanifold,in,middle
2
Tmanifold,in,end
Tmanifold,in,middle
T
Tmanifold,in,front
T
xmanifold
middle
end
xchannel
front
Texit,channel,front
©SJA 2007
Texit,channel,middle
Texit,channel,end
Model assumptions
9
∙
∙
∙
∙
∙
∙
∙
∙
∙
∙
©SJA 2007
Quasi-steady-state : Constant surface temperature.
Fuel cell stack modelled as three lumps.
Constant Urev of 1.2V.
Fuel cell heat generation calculated at steady-state.
No axial and in-plane heat conduction between lumps.
Additional heating in inlet plate and BPP junction modelled as small
constant gain.
Heat transfer in the MEA is neglected.
Hydrogen heating and cooling effects neglected.
Constant air mass flow in channels, consumption subtracted.
Small natural convection term added.
HTPEM FC stack temperature control
10
FC air flow – PI controller with Current feedforward
Ireference
I->mAir
Treference
Ublower
+
Controller
+
+
Tmeasured
System
Stack temperature 160-180 oC, what Tmeasured should be used?
©SJA 2007
Typical stack temperature control case
11
Initial heating followed by 20 A load step.
Middle temperature controlled
©SJA 2007
End temperature controlled
Model validation - Electrical heating
12
Experiment :
400 W heating
Simulation :
350 W heating
©SJA 2007
Model validation – Constant current
13
Experiment :
20 A load
Simulation :
1500 W heating,
20 A load
©SJA 2007
Model validation – Pulsating air flow
14
Operation :
small current ramp,
20 A load
air flow pulsing
no controls
©SJA 2007
Example of experimental data
15
©SJA 2007
Conclusions and future work
16
Conclusions
∙ Developed model can with good agreement predict fuel cell stack temperature
dynamics.
∙ Developed model can within acceptable ranges predict the steady-state values
of the fuel cell stack temperatures.
∙ The modelled exhaust temperature must be improved for use as a direct
control feedback.
∙ Minimization of measured temperatures should be examnied using model
based control.
Furture Work
∙ Manifold and channel temperature dynamics
∙ Air flow subtraction along the channel
∙ Discrete (at cell level) model
∙ Model validation on 1 kW HTPEM stack with other geometry
©SJA 2007
Thank you for your attention!
17
©SJA 2007