AN INTEGRATED APPROACH FOR MODELING OF WATER AND THERMAL TRANSPORT IN A POLYMER ELECTROLYTE FUEL CELL AND STACK

Open Access
Author:
Khandelwal, Manish
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 13, 2008
Committee Members:
  • Clinton Matthew Mench, Dissertation Advisor
  • Matthew M Mench, Committee Chair
  • Thomas Litzinger, Committee Member
  • Daniel Connell Haworth, Committee Member
  • Turgay Ertekin, Committee Member
Keywords:
  • Fuel Cell
  • Modeling
  • Stack
  • two-phase
  • Thermo-osmosis
Abstract:
To enhance durability and cold start performance of PEFCs, residual water in the fuel cell components must be minimized during operation and after shutdown. In this work, an integrated approach is developed and adopted to investigate the non-parasitic modes of water transport in the fuel cell components, and then identify the key controlling parameters to maximize water drainage during shutdown. A complete stack thermal model and two-phase single cell model has been developed to achieve this goal. A computational stack thermal model was developed to investigate the cold start behavior, and to estimate detailed spatial temperature distribution across the cells. The model was experimentally validated using measured data for a 20 cell stack. A parametric study was also conducted to determine the governing parameters, relative impact of the thermal mass of each stack component and ice, internal and external heating mechanism, and anticipated temperature distribution in the stack at start-up for various operating conditions. This model was also used to estimate the end cell temperature during PEFC stack shutdown, which can be used as a transient boundary condition for the unit cell model. The result of this work can be directly incorporated into fuel cell stack design and material selection. A two-phase unit fuel cell model was also developed to investigate the water and thermal transport in the PEFC components after shutdown, which for the first time includes thermo-osmotic flow in the membrane. The model accounts for capillary and phase-change induced flow in the porous media, thermo-osmotic and diffusive flow in the polymer membrane. Results conclusively demonstrates that during shutdown to the frozen state, residual water at the cathode can be controlled, and freeze damage can be avoided by balancing the phase-change induced flux in the diffusion media with the net balance of thermo-osmosis and diffusion flux in membrane. The concept of using controlled temperature gradients to non-parasitically remove excess water during shutdown can be used as a design solution to improve the anode end cell cold start performance in a PEFC stack. In practical applications, accelerated damage along the end plates of stack has been observed. To investigate the effect of stack design on the end cell water transport, both the stack and single cell model were integrated. The single cell model was used to estimate the local water distribution with land/channel boundary condition, and the experimentally validated stack thermal model provided the temperature boundary condition to simulate the end cells. Two different stack designs that exist in the patent literature but not have been explained theoretically were investigated for water drainage. For the first time, a complete physical explanation for the observed effects has been presented. The results of this work can be potentially used to improve stack design and develop shutdown protocols to minimize the residual water.