Elsevier

Renewable Energy

Volume 41, May 2012, Pages 86-95
Renewable Energy

Simulation of an innovative flow-field design based on a bio inspired pattern for PEM fuel cells

https://doi.org/10.1016/j.renene.2011.10.008Get rights and content

Abstract

Proton exchange membrane (PEM) fuel cell performance is directly related to the bipolar plate design and their channels pattern. Power enhancements can be achieved by optimal design of the type, size, or patterns of the channels. It has been realized that the bipolar plate design has significant role on reactant transport as well as water management in a PEM Fuel cell. Present work concentrates on improvements in the fuel cell performance by optimization of flow-field design and channels configurations. A three-dimensional, multi-component numerical model of flow distribution based on Navier–Stokes equations using individual computer code is presented. The simulation results showed excellent agreement with the experimental data in the previous publications. In this paper, a new bipolar plate design inspired from the existed biological fluid flow patterns in the leaf is presented and analyzed. The main design criteria in this research are based on more uniform velocity distribution and more homogeneous molar spreading of species along the flow channels and also higher voltage and power density output in different current densities. By developing a numerical code it was found that the velocity and pressure profiles on catalyst surface are much more uniform, reactant concentration on catalyst surface is very more homogeneous and the power density is higher than parallel and serpentine flow channels up to 56% and 26% respectively.

Highlights

► Power enhancements can be achieved by optimal pattern design of the bipolar plates. ► Bio inspired (BI) patterns show more uniform species distribution along channels. ► Power density from BI Bipolar plate is higher than Conventional flow channels.

Introduction

Fuel cells are one of the most attractive technologies to supply the increasing energy demand in the world. This technology can produce heat and power from a variety of primary energy sources such as hydrogen, methanol or natural gas [1]. In PEM (Polymer Electrolyte Membrane) fuel cells, water, heat and electricity are the only products. Because of their specific advantages such as high efficiency, low-temperature operation and high power density, PEM fuel cells have special place among the other types of fuel cells.

One of the most important and effective elements in the improvement of efficiency and power density of fuel cells are the bipolar plates. These components supply fuel and oxidants, remove generated water, collect produced current and provide mechanical support for the brittle membrane electrode assembly in fuel cell stack. According to Li et al. [2], bipolar plates comprise about 60% of the weight and 30% of the total cost in a fuel cell stack. The channels design and its pattern considerably affect the effectiveness of mass transport as well as electrochemical reactions inside the cell. The optimal design of the channels dimension, shape, pattern and configuration will lead to an improved and enhanced bipolar plate.

During the recent years there were so many researches including experimental, analytical or numerical studies on different important parameters of fuel cells such as heat management water transport and humidity control [3], [4], [5], [6], [7], species transport studies [8], [9], fuel cell control systems [10], [11], [12], also research on characteristics of gas diffusion layer, catalyst layer or membrane including porosity, permeability and material properties as well as numerical approaches to ion and mass transport modeling, thickness or dimensional optimization [13], [14], [15], [16], [17], [18], [19], [20], [21], different experimental works on fuel cell performance parameters [22], [23], [24] and etc.

There are several research works which have been performed to investigate the effects of various bipolar plate patterns on fuel cell performance. For this purpose some bipolar plate configurations such as serpentine, interdigitated, parallel, spirals or even porous carbon and perforated stainless steel bipolar plates are typically considered in many open literatures. Carrette et al. [25], Yang et al. [26] and Ferng and Su [27] have investigated the effects of various bipolar plate designs and configurations on the performance of fuel cell. Barreras et al. [28] used both 2D numerical and experimental simulation techniques to analyze the flow distribution throughout a parallel flow channels design fuel cell. Indeed, Lozano et al. [29] studied the fluid dynamic performance in three bipolar plate types by numerical simulation and experimental methods. The proposed configurations that they considered were a set of parallel flow field, a branching cascade type, and a serpentine-parallel channel pattern. Zhou et al. [30] designed a new channels configuration to find a better flow field. There were many independent inlets and outlets in their new pattern. More two-dimensional simulations have been also used to examine different designs to optimize the uniformity of species spreading and to find a better pressure distribution along the channels. Several of these models have neglected the effects of the GDL, catalyst layer and the membrane or the influence of the ribs or walls of the bipolar plates. Some of them only reflect areas where cell efficiency is the largest; consequently overestimated the gained power [31], [32], [33], [34]. Hontanon et al. [35] analyzed gas flow distribution system using a three-dimensional numerical model. They concluded that the porous materials produced better flow distribution profiles and reactant gas dispersion in comparison with conventional grooved plates. Besides Hontanon, Hu et al. [36], Liu et al. [37] and Boddu et al. [38] have used a commercial CFD solver to develop models of some common flow channels configuration. Furthermore, Berning et al. [39] have developed a 3D CFD code to analyze the transport phenomena in a PEM fuel cell. In this outstanding work a single channel of bipolar plate has been modeled.

In this work a three-dimensional model has been developed to consider the effects of walls, ribs and different layers on the fluid flow and species distribution. To generate this model a computational fluid dynamic code has been applied to provide a flexible environment in order to model various complicated channels arrangement and also to use a single domain approach for different layers. Moreover this CFD code can provide the foundations for future developments of this work; adding the membrane or enhancing the catalyst layer model. Furthermore, a new flow channel configuration is introduced based on the existed patterns in the nature.

Bipolar plates not only provide high contact surface area to harvest electrons, but also they provide high performance reactant gas supply to achieve greater mass transport rates without increasing the pressure drop as well as removing water from reaction sites. The main criteria to design and fabricate flow fields are as follow:

  • Flow channel path configuration: the path that reactants move inside the channel is one of the important issues in flow-field design and fabrication. Different path configurations are considered to improve mass transport inside gas diffusion layer as well as reaction kinetic in the catalyst layer.

  • Material selection: alternative materials and manufacturing techniques for bipolar plates need to be compared and investigated. Properties including good electrical conductivity, corrosion resistance, ease of manufacturability, high electrical conductivity, and thin and lightweight plates, are the main criteria in material selection process.

  • Width of the ribs between the channels: conventional bipolar plates are made by grooving steel or graphite plates which lead to formation of channels and ribs. The width of the ribs between the channels has an important role in overall bipolar plate performance. Minimizing this factor improves mass transport in the gas diffusion layer generally in the regions adjacent to the ribs. However, the porosity of the gas diffusion layer may be reduced in the regions under the land area as a result of the assembly pressure.

  • Channel cross section: this factor affects flow velocity and pressure loss. An optimized cross section helps to remove water droplets and prevents creation of liquid water in channels.

  • Ratio of channel to land area: by increasing the channel dimensions the pressure drop would be declined and therefore a better performance is predicted, however, maximizing channel to land area ratio would create some problems for electron collection.

  • Channel length: to provide enough mass for electrochemical reactions on the catalyst layer it is necessary to spread channels on the electrode surface, however, long channels will cause more pressure losses in bipolar plates.

The objective of this work is to improve the performance of PEM fuel cells through optimization of flow channels design and configuration. The approach used in this research is based on computational fluid dynamic (CFD) analysis of several bipolar plate patterns. The effects of various channels/lands configuration in different patterns such as the parallel and one path parallel-serpentine have been simulated and compared. Since the bipolar plates have the dual role; providing reactant species for electrochemical reactions and collecting produced current, the fraction of land to channel area should be in a proper range. For design and optimization purposes a three-dimensional computational model based on finite volume method and SIMPLE algorithm is developed using the steady Navier–Stokes equations. The outcomes of the numerical simulations showed good agreement with previously published experimental results. This point confirms the validity of numerical model to study the design parameters such as channel patterns and configurations without the need of actually fabricating the plates. After performance comparison of various patterns in bipolar plats, an innovative bipolar plate configuration is presented and analyzed. This pattern is based on drafts which exist in the nature, patterns which have been improved during millions of years of evolution. We can see the simplest example of these patterns in the veins of plants and leafs and also the blood distribution system in the body. Some kinds of these patterns are exhibited in Fig. 1.

In short, the novelties of the present work compared to other similar works are as follows:

  • Develop an unique three-dimensional model based on finite volume approach

  • Using single domain approach in fluid flow simulation which one set of governing equations is valid for all the sub regions. Therefore no interfacial boundary condition is required to be specified for different regions

  • Modeling the conventional types of flow filed patterns (parallel and serpentine) and Comparison and evaluation of different flow filed types by obtaining their performance and power densities

  • Presenting and modeling a new innovative bio inspired flow-field design based on existed samples in the nature (veins of plants and leafs).

Section snippets

Mathematical modeling

The main reason of using the bipolar plates is to provide a uniform distribution of fuel (hydrogen) on the anode side or oxidizer (oxygen or simply air) on the cathode side of the PEMFC. In short, bipolar plate does its main tasks: (1) supplies reactant, (2) remove reactant products (water) and (3) harvest electrical current.

As mentioned before, the shape, size and pattern of flow structure can significantly affect the fuel cell performance. A wide variety of flow patterns is presented in many

Results and discussion

Because of the small dimensions of the gas flow channels and the small thickness of the gas diffuser layer, it is so difficult to determine the flow field and concentration field in the gas channel and gas diffuser. As a result, few papers can be found who addressed the quantifications of such fields or distributions. There are some papers which have shown these results in 2D geometry [28], [30], and also some few sources which have exhibited a single channel in 3D geometry [37], [39], however,

Conclusion

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of flow channels. This paper presents a three-dimensional, steady state, multi-component model to investigate different flow channel designs in bipolar plate. The approach used in this work, is based on CFD investigation of several flow-field topologies. After comparison of advantages and disadvantages of

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