Humidity Control of a Polymer Electrolyte Membrane Fuel Cell (PEMFC)

Transcription

1 Humidity Control of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) Kevin C. Lauzze Donald J. Chmielewski Illinois Institute of Technology

2 Outline Background and PEMFC Modeling Steady State Analysis Desired Operating Window Temperature Control Power Control Open-Loop/Closed-Loop Responses Open-Loop/Closed-Loop Responses Humidity Control Open-Loop/Closed-Loop Responses Illinois Institute of Technology

3 PEMFC system Focus of our model Illinois Institute of Technology 3

4 PEMFC Model Oxygen Balance: Water Balance: Gas Phase Energy Balance: Membrane Energy Balance: V V V c c c dt dt dc dt dc dt dt dt m O H air O F F in a F in a UA ( mc ) p C in a T in O C in air mem F C in H O F T ( T F C a air a O a air T UA ( C ) m H 1 O ) r H r p O H air Q ( mc (j) O heat p (j) ( T (j) ) m mem T air Q ) cool where : j Current Density (ma/cm r in H j Fa F O ( ) a in C T ) Illinois Institute of Technology 4

5 Rate of Water Production j E tm Where: V r H O ( j) F ja F Where the current density is defined by the Nernst Equation: C V i cell o cell RT m log( ~ Concentration of C C ~ Operating Voltage H H O C 1 O Species (0.8 V) ( RT i air ) 1 ) Illinois Institute of Technology 5

6 Heat Generation Rate Heat Generation = Reversible losses + Irreversible losses Q heat o o ja ( j) ( H353 G353) ( ja) F R Where: Resistance is R 1 tm A ~ Membrane Conductivity t m ~ Membrane Thickness Illinois Institute of Technology 6

7 Ohmic Resistance Electrical Conductivity,, Increases with Humidity RH x w P P (T) sat x w = 0.35 Illinois Institute of Technology 7

8 Mass Transfer Resistance B CO membrane ( B S 1 S K xo x ) ( ( O r HO j CO )) S CO j Decreasing K Flooding: K 0 as RH 100%, due to a liquid film formation Illinois Institute of Technology 8

9 PEMFC Operating Window Membrane too dry (Large Ohmic Resistance) 80% RH 100% Membrane Flooded (Large Mass Transfer Resistance) 60 C T RH 0 x w P P sat air (T) C Illinois Institute of Technology 9

10 Steady State Analysis (Current Density vs. Air Flow and Temperature) Illinois Institute of Technology 10

11 Steady State Analysis (Relative Humidity vs. Air Flow and Temperature) Illinois Institute of Technology 11

12 Steady State Recap (Current Density & RH vs Air Flow and Temperature) Flooded Membrane Dehydrated Membrane Air Flow (m 3 /s) Illinois Institute of Technology 1

13 Possible MV s and CV s Control Variables: Relative Humidity, Temperature, Current / Power Output Manipulated Variables: Cooling Rate, Air Flow Rate, Stack Voltage, Pressure Illinois Institute of Technology 13

14 Open Loop Temperature Response With Absolutely no Control the System is Unstable! This observation is supported by the Multiple Steady- State Results of Benziger (00) and Debenedetti (1983) Illinois Institute of Technology 14

15 Temperature Control-Loop (sp) T mem PI Q cool PEMFC Tmem Illinois Institute of Technology 15

16 Temperature Control Set Point Changes Illinois Institute of Technology 16

17 Temperature Control Set Point Changes Illinois Institute of Technology 17

18 Air Flow Step Tests Air Flow Power (sp) T mem PI Q cool PEMFC Tmem Illinois Institute of Technology 18

19 Air Flow Step Test Results Illinois Institute of Technology 19

20 Air Flow Step Tests (cont) RH Tgas (C) Tmem (C) Illinois Institute of Technology 0

21 Power Control Power sp PI Air Flow Power (sp) T mem PI Q cool PEMFC T mem Illinois Institute of Technology 1

22 Power Set Point Changes Illinois Institute of Technology

23 Power Set Point Changes (cont) RH Tgas(C) Tmem(C) Illinois Institute of Technology 3

24 Ohmic Resistance Electrical Conductivity,, Increases with Humidity RH x w P P (T) sat x w = 0.35 Illinois Institute of Technology 4

25 Integration of Humidity Control RH sp PI T mem sp PI Q cool PEMFC T mem RH Power sp PI Air Flow Power Illinois Institute of Technology 5

26 Closed Loop Response with Humidity Controller (cont) Illinois Institute of Technology 6

27 Closed Loop Response with Humidity Controller RH sp = 90% Tmem(C) Tgas(C) Illinois Institute of Technology 7

28 Summary Water Management Critical to Cell Operation Identified Possible MV s and CV s System Unstable Without Temperature Control Suggested a Multilevel Feedback Structure Relative Humidity Control Loop Can Improve Efficiency Illinois Institute of Technology 8

29 Future Work Expand Model Anode Chamber with Non-Pure Hydrogen Longer Channel (similar to PFR) Investigate other MV s Operating Pressure and Voltage Inlet Temperature and Relative Humidity Multiple Input & Multiple Output Controller Model Based Constraint Handling Explicit Efficiency Calculations Illinois Institute of Technology 9

30 Acknowledgements Center of Electrochemical Science and Engineering at IIT. Graduate College, IIT Ayman Al-Qattan, ChEE Department, IIT. Illinois Institute of Technology 30