©SJA 2007 1 Søren Juhl Andreasen and Søren Knudsen Kær Aalborg University Institute of Energy...

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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 – Pulsating air flow

Operation :

small current ramp,

20 A load

air flow pulsing

no controls

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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

©SJA 2007 1 Søren Juhl Andreasen and Søren Knudsen Kær Aalborg University Institute of Energy...

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