Fuel Cells vs. Competing Technologies

Investments of over $1 B have been made for Fuel Cell R&D over the past five decades, for space and terrestrial applications; the latter includes military, residential power and heating, transportation and remote and portable power. The types of fuel cells investigated for these applications are PEMFCs (proton exchange membrane fuel cells), AFCs (alkaline fuel cells), DMFCs (direct methanol fuel cells), PAFCs (phosphoric acid fuel cells), MCFCs (molten carbon fuel cells), SOFCs (solid oxide fuel cells). Cell structure, operating principles, and characteristics of each type of fuel cell is briefly compared. The performances of fuel cells vs. competing technologies are analyzed. The key issues are which of these energy conversion systems are technologically advanced and economically favorable and can meet the lifetime, reliability and safety requirements. This paper reviews fuel cells vs. competing technologies in each application category from a scientific and engineering point of view.

Oxygen: Aluminum Fuel Cells and Most Practical Fields of Their Applications

On the total combination of their power, electric, economic, operating and ecological characteristics fuel cells (FC) on the base of oxygen (air) - aluminum (AA) electrochemical system are one of the most effective fuel cells. On their power/mass performances they are worse only the oxygen-hydrogen FC and some types of FC with the lithium anode. It is easy to recharge AA FC by means of the mechanical replacement of the working components after their expenditure. AA FC storage period is not less then ten years without degradation of their characteristics. Thus it is a prospective current source with a repeated operation. It is distinguished by the high power performances, long storage period and ecological cleanness as during their exploitation, as during their producing and utilization of the waste FC and the products of the reactions. The authors developed fundamental researches of the processes which take place at the systems and assemblies of power plants (PP) with AA FC. These researches allowed to eliminate the basic defects of AA FC which blocked their practical application. Autonomous current sources on the AA FC base can be effectively applied for the different users power supply in the main supply absence conditions. The basic application fields: telecommunication systems, transport, rescue-emergency parties and so forth. We created the aluminum-air fuel cells power plants (PP) with the alkaline and salt electrolytes with the different additions and special developed anode alloys. They have the wide ranged capacities from the unites of W up to hundreds of kW.

Exergy Analysis of Polymer Electrolyte Fuel Cell Systems Using Methanol

An exergy (available energy) analysis has been conducted on a typical polymer electrolyte fuel cell (PEFC) system using methanol. The material balance and enthalpy balance were calculated for the PEFC system using methanol steam reforming, and the exergy flow was obtained. Based on these results, the exergy loss in each unit was obtained, and the difference between the enthalpy and exergy was discussed. The exergy loss in this system was calculated to be 178kJ/mole MeOH for the steam reforming process of methanol. Although the enthalpy efficiency approached unity as the recovery rate of the waste heat from the cell approached unity, the exergy efficiency remained around 0.45 since the cell’s operating temperature of 80°C is low. It was also found that the cell voltage should exceed 0.82V in order to obtain the exergy efficiency of 0.5 or higher. A direct methanol fuel cell (DMFC) was analyzed using the exergy and compared with the methanol reforming PEFC. In order to obtain the exergy efficiency higher than that of PEFC with steam reforming, the cell voltage of the DMFC should be 0.48V or greater at the current density of 600mA/cm2.

Heat and Mass Transfer and Two Phase Flow in Hydrogen Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells

Fuel cells are related to a number of scientific and engineering disciplines, which include electrochemistry, catalysis, membrane science and engineering, heat and mass transfer, thermodynamics and so on. Several thermophysical phenomena such as heat transfer, multicomponent transport and two phase flow play significant roles in hydrogen proton exchange membrane fuel cells and direct methanol fuel cells based on solid polymer electrolyte membrane. Some coupled thermophysical issues are bottleneck in process of scale-up of direct methanol fuel cells and hydrogen proton exchange membrane fuel cells. In present paper, experimental results of visualization of condensed water in fuel cell cathode microchannels are presented. The equivalent diameter of the rectangular channel is 0.8mm. Water droplets from the order of 0.08mm to 0.8mm were observed from several different locations in the channels. Several important problems, such as generation and change characteristics of water droplet and gas bubble, two phase flow under chemical reaction conditions, mass transfer enhancement of oxygen in the cathode porous media layer, heat transfer enhancement and high efficiency cooling system of proton exchange membrane fuel cells stack, etc., are discussed.

Gas Flow and Heat Transfer Analysis for an Anode Duct in Reduced Temperature SOFCs

In this study, a fully three-dimensional calculation method has been further developed to simulate and analyze various processes in a thick anode duct. The composite duct consists of a porous layer, the flow duct and solid current connector. The analysis takes the electrochemical reactions into account. Momentum and heat transport together with gas species equations have been solved by coupled source terms and variable thermo-physical properties (such as density, viscosity, specific heat, etc.) of the fuel gases mixture. The unique fuel cell conditions such as the combined thermal boundary conditions on solid walls, mass transfer (generation and consumption) associated with the electrochemical reaction and gas permeation to / from the porous electrode are applied in the analysis. Results from this study are presented for various governing parameters in order to identify the important factors on the fuel cell performance. It is found that gas species convection has a significant contribution to the gas species transport from / to the active reaction site; consequently characteristics of both gas flow and heat transfer vary widely due to big permeation to the porous layer in the entrance region and species mass concentration related diffusion after a certain distance downstream the inlet.

A Simplified Three-Dimensional CFD Approach for PEMFC and DMFC Design

This paper describes the fundamental theory, algorithm and computation methods to predict the performance of proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) using a simplified computational fluid dynamics (CFD) approach. Based on the common transport phenomenon inside both fuel cells, the mass, momentum, energy and species equations were derived. Darcy laws were employed to simplify the momentum equation and also to linearize the species equation. The mathematical model was solved in various flow channel designs and some membrane electrode assembly (MEA) options. The major concern is mainly on the cathode side, in the PEMFC case, that dominates the performance deterioration due to potential loss in the flow field. In the case of DMFCs, both anode and cathode sides are simulated. The methanol crossover effect is also included. This virtual performance test bench plays an important role in the prototype fuel cell design. The computer aided design tool is proved to be useful in configuration designs. Additionally, it provides the detailed transport phenomenon inside the fuel cell stack.

Fabrication and Performance Evaluation of Miniaturized Proton Exchange Membrane (PEM) Fuel Cells

Two types of miniaturized PEM fuel cells are designed and characterized in comparison with a compact commercial fuel cell device in this paper. One has Nafion® membrane electrolyte sandwiched by two brass bipolar plates with micromachined meander-like gas channels. The cross-sectional area of the gas flow channel is approximately 250 by 250 (μm). The other uses the same Nafion® membrane and anode structure, but in stead of the brass plate, a thin stainless steel plate with perforated round holes is used at cathode side. The new cathode structure is expected to allow oxygen (air) being supplied by free-convection mass transfer. The characteristic curves of the fuel cell devices are measured. The activation loss and ohmic loss of the fuel cells have been estimated using empirical equations. Critical issues such as flow arrangement, water removing and air feeding modes concerning the fuel cell performance are investigated in this research. The experimental results demonstrate that the miniaturized fuel cell with free air convection mode is a simple and reliable way for fuel cell operation that could be employed in potential applications although the maximum achievable current density is less favorable due to limited mass transfer of oxygen (air). The relation between the fuel cell dimensions and the maximum achievable current density is also discussed with respect to free-convection mode of air feeding.

An Analysis of Water Management for a PEM Fuel Cell System in Automotive Drive Cycles

Low-temperature operation of a Proton Exchange Membrane (PEM) fuel cell system requires humidification of the membrane. The amount of water produced electrochemically within the fuel cell system is directly related to the system power output. In a vehicular application where the power output may vary substantially over time, it is critical that water management be addressed in the fuel cell and vehicle system design. This paper introduces the integration of a detailed fuel cell system model within a hybrid electric vehicle system model. The newly integrated models provide the capability to better understand the impacts of a variety of fuel cell and vehicle design parameters on overall system performance. Ultimately, coupling these models leads to system optimization and increased vehicle efficiency. This paper presents the initial results of a parametric study to quantify the impacts of condenser size and cathode inlet relative humidity on system water balance under realistic drive cycles in a fuel cell hybrid electric sport utility vehicle. The vehicle simulations included operation under both hot and ambient start conditions. The study results demonstrate that ambient start or aggressive drive cycles require larger condensers or water reservoirs to maintain a neutral water balance than either hot start or less aggressive drive cycles.

A Silicon Microfabricated Direct Formic Acid Fuel Cell

A silicon-based microfabricated fuel cell running on formic acid has been developed to provide a high energy and power density power source on the millimeter size scale. A polymer electrolyte membrane fuel cell was fabricated utilizing the Nafion™112 membrane bonded between electrodes on silicon substrates. The cell was fueled by a concentrated formic acid-water solution and the catalyst used was Pt. The preliminary result shows that the microfabricated formic acid fuel cell may be a promising alternative for very small portable fuel cell applications.

Investigation of Transport Phenomena in Micro Flow Channels for Miniature Fuel Cells

This paper presents a study on the transport phenomena related to gas flow through fuel cell micro-channels, specifically the impact of dimensional scale on the order of 100 microns and below. Especially critical is the ability to experimentally verify model predictions, and this is made efficiently possible by the use of structural photopolymer (SU-8) to directly fabricate functional fuel cell micro-channels. The design and analysis components of this investigation apply 3-D multi-physics modeling to predict cell performance under micro-channel conditions. Interestingly, the model predicts that very small channels (specifically 100 microns and below) result in a significantly higher peak power density than larger counterparts. SU-8 micro-channels with different feature sizes have been integrated into fuel cell prototypes and tested for comparison against model predictions. The results not only demonstrate that the SU-8 channels with metal current collector show quite appreciable performance, but also provide experimental verification of the merits of channel miniaturization. As predicted, the performance in terms of peak power density increases as the feature size of the channel decreases, even though the pressure drop is higher in the more narrow channels. So it has been observed both theoretically and experimentally that cell performance shows an improving trend with micro-channels, and design optimization for miniature fuel cell provides a powerful method for increasing power density.