Fuel Cell Array Research Articles

Micro fuel cell array with ‘flip-flop’ interconnection

Micro fuel cell array with ‘flip-flop’ interconnection

Study of multistage oxidation by flowsheet calculations on a combined heat and power molten carbonate fuel cell plant

The multistage oxidation configuration consists of a set of serially connected fuel cell stacks. By connecting the stacks serially, more homogenous current distribution over the cell surface can be achieved resulting in lower irreversible losses. This article presents a detailed assessment of multistage oxidation by flowsheet calculations in which the influence of operating temperature and gas composition on the fuel cell performance is incorporated. A 250 kW molten carbonate fuel cell (MCFC) combined heat and power (CHP) plant is used as reference and the fuel cell stack unit is substituted by two serially connected units ( N=2). Two multistage configurations are examined: (A) both anode and cathode flows are serially connected; (B) only the anode flow is serially connected while the cathode flow is parallel connected. For all systems, the total cell active area, cell current density, overall fuel utilization and gas temperature at the inlet and outlet of the fuel cell array are kept constant. Fuel cell performance at the operating conditions is calculated using a numerical model of the flowsheeting program. Influences of operating temperature and gas composition on the cell performance are incorporated using empirical relations that describe irreversible losses of the cell as function of these parameters. System performances are compared in order to assess the benefits of the multistage oxidation configurations. Differences in performance between the two multistage oxidation configurations are studied by analyzing the difference in exergy loss of stacks, stack power output, cooling requirement and cathode gas massflow and composition. Detailed flowsheet calculations show that the improvement in efficiency is about 0.6% for configuration A, and 0.8% for configuration B. Improvements are obtained by the enhanced fuel cell power output while the expander power output is slightly reduced. Heat output is slightly reduced due to the improved fuel cell conversion. Analysis of stack output revealed a intricate interaction between stack and the rest of the fuel cell system. Their mutual influences are examined and the results explain differences in results between configuration A and B.

Lateral Ionic Conduction in Planar Array Fuel Cells

A performance degradation phenomenon is observed in planar array fuel cells. This effect occurs when multiple cells sharing the same electrolyte membrane are connected in series to build voltage. The open circuit voltage (OCV) and low current behavior of such a series connected planar stack is lower than should be expected. The flow of ionic cross currents between cells in the array, dubbed membrane cross-conduction, is proposed as the likely cause for this loss phenomenon. This hypothesis is confirmed by experimental observations. An equivalent circuit model for a planar double cell is developed which takes into account membrane cross conduction. This model is shown to predict the observed current-voltage behavior of an experimental planar double cell while a simple series model does not. The validated model is used to investigate the impact of various fuel cell parameters on the membrane cross-conduction effect. Design rules are extracted to minimize membrane cross-conduction losses for a linear fuel cell array. It is concluded that the membrane cross-conduction phenomenon primarily affects the OCV and low current density behavior of planar fuel cell arrays. Losses due to membrane cross conduction are minimal for conservative cell spacing, but can be significant for densely packed fuel cell arrays. © 2003 The Electrochemical Society. All rights reserved.

Design and fabrication of a micro fuel cell array with “flip-flop” interconnection

A design configuration is presented for integrated series connection of polymer electrolyte fuel cells in a planar array. The design is particularly favorable for miniature fuel cells and has been prototyped using a variety of etch and deposition techniques adopted from microfabrication. The series path is oriented in a “flip-flop” configuration, presenting the unique advantage of a fully continuous electrolyte requiring absolutely no interconnecting bridges across or around the membrane. Electrical interconnections are made by thin-film metal layers that coat etched flow channels patterned on an insulating substrate. Both two-cell and four-cell prototypes have successfully demonstrated the expected additive performance of the integrated series, and peak power in a four-cell silicon assembly with hydrogen and oxygen has exceeded 40 mW/cm 2. Factorial experimentation has been applied to investigate the adequacy of metal film conduction over etched topology, and results conclude that film thickness dominates over other design parameters. The design effort and subsequent testing has uncovered new topics for extended study, including the possibility of lateral ionic conduction within the membrane as well as the effects of non-uniform reactant distribution.