Influence of Pressure Drop in PEM Fuel Cell Stack on the Heat and Mass Balances in 100 kw Systems

Transcription

1 Influence of Pressure Drop in PEM Fuel Cell Stack on the Heat and Mass Balances in 100 kw Systems

2 Outline Introduction Motivation Methodof Analysis Results and Discussion Conclusions

3 PEM Fuel Cell Introduction (0.2 to 0.5 mm) (0.03 mm) (membrane 0.05 to 0.1 mm) (0.03 mm) (0.2 to 0.5 mm) Compactness with high power density (0.7 V/ 1 A/cm 2 ) Low operating temperature (Lower than 100 o C) Electrolyte is in solid phase.

4 PEM Fuel Cell Introduction 2e - 2e - 2e - 2e - + 2H + 2H + +(nh 2 O) + 2H + + Oxidant (O 2 ) Anode: Cathode: Overall: Fuel (H 2 +H 2 O) H 2 Anode Electrolyte H 2 2H 2e 1 2 O2 2e H 2 1 H 2 O2 H 2O 2 H 2 O 1/2O 2 H 2 O Cathode Water is produced and migrated in cathode side.

5 Introduction PEM Fuel Cell Stack

6 Introduction PEM Fuel Cell Systems Selection of components depends on the application Fuel tank Methanol, Natural gas, etc. Fuel Fuel reformer Fuel reforming to hydrogen rich gas. Hydrogen rich fuel Controller Fuel cell stack Direct current Inverter or converter Alternating current Output Air (O 2 )

7 Introduction PEM Fuel Cell Systems Portable Application Stationary Application Automotive Application

8 Heat Exchangers Introduction

9 Motivation Design of bipolar plate Maldistribution and pressure drop in the stack depend on the design of bipolar plate.

10 Motivation Cell (-) Stoichiometry (-) Mass Mass flow flow fraction of the liquid water water (-) (-) Pressure (kpa) Liquid water Cathode 2.5 Liquid water 0.12 Cathode Anode 1.5 Anode Cell (-) Cell (-) a) Stoichiometry b) Pressure Maldistribution and pressure drop on the cathode side are large due to the large mass flow rate.

11 Motivation Heat generation in the stack Previously, it was assumed that each unit cell in the stack had the same performance in system studies. Q unit cell =IA unit cell (1.25-V unit cell ) Q stack = number of cells * Q unit cell Due to the effect of maldistribution and pressure drop, the performance of the unit cells is not identical.

12 Motivation The influence of stack design on the heat and mass balances in the system has not been deeply investigated. Objective of the Lecture Show the influence of the pressure drop and maldistribution in a PEM fuel cell stack on the heat and mass balances in 100 kw systems.

13 Methods of Analysis PEM Fuel Cell Stack The unit cell model was based on information in the literature.] PEM Fuel Cell System A commercial software, Integrated Process Simulation Environment (IPSEpro), is employed.

14 System and Operating Data Radiator Hydrogen Anode Cathode Condenser Compressor Air Intercooler or heater Recovered water Coolant loop T in ( o C) 80 P in (kpa) 200 Stoichiometry (anode/cathode) 1.2/2 Relative humidity (anode/cathode, %) 100/80 Reactant composition (anode/cathode) Hydrogen/air Current density (A/cm 2 ) 0.50

15 Results and Discussions dpstack/pstack,in dpstack/pstack,in (-) Current density (A/cm 2 ) Temperature ( o C) Pressure*10-2 (kpa) At high temperature, low pressure and high current density, the pressure drop in the stack is large.

16 Results and Discussions Without pressure drop With pressure drop Heat and Power (kw) Radiator 100 Power 50 0 Condenser Heater for humidification Heat and Power (kw) Radiator Power Condenser Heater for humidification Temperature ( o C) Pressure*10-2 (kpa) Heat load in the radiator is reduced, that in the condenser is increased by increasing the temperature. At high pressure the amount of water condensation in the stack becomes small.

17 Results and Discussions Without pressure drop With pressure drop Qradiator (kw) Temperature ( o C) Current density (A/cm 2 ) Qcondenser (kw) Temperature ( o C) Current density (A/cm 2 ) a) Radiator b) Condenser Increasing the temperature and current density, affects the heat load in the radiator and condenser significantly.

18 Results and Discussions Without pressure drop With pressure drop 300 Current density (A/cm 2 ) 80 Current density (A/cm 2 ) Qradiator (kw) Pressure 10* -2 (kpa) Qcondenser (kw) Pressure*10-2 (kpa) a) Radiator b) Condenser The current density affects the heat load in the radiator and condenser while the pressure affects mainly the the condenser

19 Results and Discussions Case 1 Case 2 Base case Number of flow channels n c Number of turns of the channel n t dpstack/pstack,in (-) *19 5*11 8* W w m Area of manifold (cm 2 ) Large pressure drop with small manifold and less number of flow channels in the unit cells.

20 Results and Discussions Without pressure drop With pressure drop Qradiator (kw) *7 5*11 3*19 Qcondenser (kw) *19 5*11 8* Area of manifold (cm 2 ) Area of manifold (cm 2 ) a) Radiator b) Condenser Influence on the heat load in the radiator and condenser by low number of flow channels due to large pressure drop.

21 Conclusions The heat load in the radiator is reduced by the pressure drop in the stack because the reduction of water condensation. The amount of heat, which is reduced in the radiator, is mostly shifted to the condenser for the water recovery. The reduced heat load in the radiator is about 5%. It corresponds to about 10 % increase in the condenser. The design of the radiator and condenser must consider this shift in heat load due to the pressure drop in the stack.