©sja 2007 1 søren juhl andreasen and søren knudsen kær aalborg university institute of energy...
TRANSCRIPT
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©SJA 2007
Søren Juhl Andreasen and Søren Knudsen Kær
Aalborg UniversityInstitute of Energy Technology
Dynamic Model of High Temperature PEM Fuel Cell Stack Temperature
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Presentation outline
∙ HTPEM features∙ Experimental fuel cell system setup∙ Previous work
▫ Stack temperature profile identification
∙ Governing equations▫ Energy balance▫ Fuel cell power input▫ Convection▫ Conduction
∙ Model definitions∙ Model assumptions∙ HTPEM FC stack temperature control
▫ Current feedforward▫ PI controller
∙ Model validation▫ Heating▫ Operation▫ Pulsating air flow
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HTPEM PBI(H3PO4)-membrane features
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 100oC) to avoid water condensation.▫ High demands for materials at these elevated temperatures
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Performance of HTPEM fuel cell
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HTPEM FC System- pure hydrogen
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Previous work – Initial experimental results
Stack temperature profile identification
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Fuel cell stack energy balance
Energy balance :
Fuel cell heat input :
External heat input :
Forced Convection :
Heat Conduction :
Natural Convection :
PWMtotalexternalin DPQ ,
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Manifold and gas channel temperature
T
xmanifold
endmiddle
front
Tmanifold,in,front
Tmanifold,in,middle
Tmanifold,in,end
T
xchannel
Tmanifold,in,front + Tmanifold,in,middleTchannel,in,front =
2
Texit,channel,middleTexit,channel,front Texit,channel,end
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Model assumptions
∙ 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.
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HTPEM FC stack temperature control
SystemController
TmeasuredTreference
Ublower
Ireference
I->mAir
FC air flow – PI controller with Current feedforward
Stack temperature 160-180 oC, what Tmeasured should be used?
+
+-
+
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Typical stack temperature control case
Middle temperature controlled End temperature controlled
Initial heating followed by 20 A load step.
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Model validation - Electrical heating
Experiment :
400 W heating
Simulation :
350 W heating
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Model validation – Constant current
Experiment :
20 A load
Simulation :
1500 W heating,
20 A load
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Model validation – Pulsating air flow
Operation :
small current ramp,
20 A load
air flow pulsing
no controls
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Example of experimental data
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Conclusions and future work
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
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Thank you for your attention!