Elsevier

Renewable Energy

Volume 188, April 2022, Pages 372-383
Renewable Energy

Relationship between number of turns of serpentine structure with metal foam flow field and polymer electrolyte membrane fuel cell performance

https://doi.org/10.1016/j.renene.2022.02.001Get rights and content

Abstract

Metal foam flow field is applied on a polymer electrolyte membrane fuel cell (PEMFC) to improve its performance by enhancing mass transfer property. Generally, the metal foam is employed without any structure in the channel location, which results in the mainstream of reactants not flowing to the corner of the reaction area and instead of flowing straight from inlet to outlet. This causes an uneven reaction rate throughout the reaction area. To resolve the problem, the serpentine structure was devised on a metal foam flow field at the cathode to guide the reactant flow path to the corner of the reaction area. The number of turns of the serpentine structure was controlled as variables. With the increase in the number of turns, the reactant concentration at reaction sited increased, improving the PEMFC performance. At 0.5 V, PEMFC with metal foam and 2 turns serpentine structure shows 4.7% improved performance. However, due to the increased length of flow from the structure, the pressure drop that induced high parasitic loss became higher. As a result, the net power of PEMFC with serpentine structure considering parasitic loss improved 1.7% comparing to PEMFC with bulk metal foam.

Introduction

Environmental pollution is a great threat to the earth's ecosystem, including human society. The operation of energy conversion devices that use fossil fuel and emit byproducts like carbon dioxide, NOx, SOx cause global warming and climate changes. To resolve the problem caused by conventional energy conversion devices, research and development of various eco-friendly energy conversion devices have garnered significant attention. A fuel cell is among the most promising eco-friendly energy conversion devices. Since the fuel cell generates electricity via electrochemical reaction of hydrogen and oxygen directly, it is highly efficient and produces only water as a byproduct. In addition, it produces less vibration and noise compared to other conventional generators. The fuel cell types are classified according to the electrolyte materials used in fuel cells. Polymer electrolyte membrane fuel cell (PEMFC) is one such type, which uses polymer membrane as an electrolyte. The polymer membrane has proton conductivity only under the hydrated state; therefore, the PEMFC is operated at approximately 80 °C, which is below the boiling point of the water. Because of the low operating temperature and high power density, the PEMFC can be applied to various fields, including portable devices, vehicles, and stationary generations. However, there exist certain issues with PEMFC, such as high cost for manufacturing and difficulty of water management. To resolve the problem and improve the performance of PEMFC, researchers have attempted to develop new materials and novel manufacturing process, determine optimal operating conditions, or design an optimized flow field. The flow field optimization is a particularly important research area for even reactant gas distribution and elimination of water from reaction site without encountering dry condition of electrolyte. Through the flow field optimization, the reactant concentration at the reaction site increases, and the high reactant concentration reduces activation and concentration losses, which decide the performance of PEMFC. Traditionally, three kinds of channel which are serpentine, parallel and interdigitated channel were used widely as flow field. Each channel has different advantages and disadvantages. Parallel channel has low pressure drop as well as bad reactant supply and water discharge. While, interdigitated channel has good reactant supply, good water discharge and high pressure drop. The characteristics of serpentine channel is located between parallel channel and interdigitated channel. The channels were made by engraving on the bipolar plate, which make the producing process of channel wasteful of materials and expensive in terms of time. To solve the problems and limitation of conventional channels by develop new flow field, a lot of researches have conducted.

As the result of many previous studies, new types of flow fields such as 3D fine mesh and metal foam have been designed to improve the performance of PEMFC. 3D fine mesh has a complex geometry, which induces the inflow of reactant or outflow of product. It changes the momentum direction of the reactant at the bulk flow field to the direction of the gas diffusion layer (GDL) by blocking the path of the reactant or using a slope mechanism toward GDL. There is no typical shape of 3D fine mesh; therefore, researchers have attempted to determine the important factor of 3D fine mesh for PEMFC performance or the optimal shape of 3D fine mesh [[1], [2], [3], [4]]. Bao et al. [2] fabricated a 3D fine mesh similar to Mirai's 3D fine mesh and analyzed the dependence of its water discharge ability on velocity, droplet size, and baffle angle. Zhang et al. [3] also created a 3D fine mesh model and compared the 3D fine mesh PEMFC and parallel channel PEMFC. The 3D fine mesh model showed better performance at high current density than the parallel channel model. Niu et al. [4] applied a 3D flow field into a straight channel and analyzed the 3D flow field model using the volume of fluid method to observe the liquid water discharge. Similar to the previous studies, the 3D flow field model exhibited better performance than the straight channel, particularly in high current density regions. Though 3D fine mesh improves the performance of PEMFC, it is difficult to manufacture it and to determine its optimal geometry owing to its complex structure.

Metal foam is another new type of flow field, and it is quite different from the conventional flow fields and 3D fine mesh. Other flow fields have space for bulk flow of fluid referred to as a channel. where there is no solid component. However, as the name of the flow field suggests, in a metal foam flow field, the metal foam is located between the GDL and bipolar plate, where the channel was originally located, and the metal foam plays the main flow path instead of a channel. This filled metal foam promotes the reactant supply to the reaction site and reduces the weight of PEMFC while increasing or maintaining the thermal and electric conductivity [5]. In addition, the metal foam is easier to manufacture to suit specific needs and easier to apply than 3D fine mesh [6]. These advantages resulted in an increased focus on research in this regard. The previous research, which attempted to determine the effect of metal foam properties on PEMFC performance, have been conducted over a decade via experiments [[5], [6], [7], [8], [9]] and simulations [[10], [11], [12], [13], [14], [15]]. Certain studies using experiments [6,7] compared the performance of PEMFC with metal foam to that of PEMFC with a conventional channel. In the research by Park et al. [6], the metal foam flow field PEMFC exhibited 178% improved PEMFC performance compared to the conventional serpentine PEMFC. Kim et al. [7] also measured metal foam flow field PEMFC performance and the PEMFC performance with metal foam was 34% higher than the PEMFC performance with a parallel serpentine channel. Further, experimental studies [5,8,9] analyzed the effect of metal foam properties and conditions on PEMFC performance. Tseng et al. [5] reported the effect of metal foam compression ratio and surface treatment on PEMFC performance with metal foam. When the metal foam was highly compressed, the PEMFC performance was improved, and PTFE coating also enhanced PEMFC performance. Shin et al. [8] showed that 800 μm thickness of metal foam achieved the highest PEMFC performance among 450, 580, 800, 1200 μm of metal foam thickness cases. Awin et al. [9] analyzed the performance of PEMFC by varying the porosity of metal foam. They reported that the metal foam porosity at the cathode did not affect the PEMFC performance; however, the metal foam porosity at the anode side influenced the PEMFC performance and a high porosity resulted in high PEMFC performance. Similar to experimental studies, the simulation studies for PEMFC having metal foam [[10], [11], [12], [13]] also compared metal foam PEMFC and conventional channel PEMFC. Afshari et al. [10] showed that metal foam PEMFC exhibited 34% improved performance than straight channel PEMFC. Jo et al. [12] compared metal foam PEMFC and serpentine channel PEMFC under 1 and 2 bar conditions, where the metal foam PEMFC performance was better than serpentine channel PEMFC in both cases. Carton et al. [14] analyzed the correlation between pressure drop and the number of pores per inch. When the pores per inch increased, the pressure drop increased. Jo et al. [15] studied the effect of permeability and wettability of metal foam on liquid transfer at metal foam flow field. The relative wettability of metal foam for GDL determines the water discharge capability.

There exists another approach to understanding the effect of metal foam on PEMFC performance and involves the analysis of internal flow in a metal foam. Metal foam is porous media created using metal such as nickel, and it does not include a guide for a specific flow route of fluid for its random porosity. The fluid in metal foam flows in a manner that results in minimum pressure drop. In this case, because the reactants are not evenly distributed over the entire reaction site, in a part of the reaction site, there may be a lack of reactant or excessive current density, and partially high temperature may be caused in the other part. To resolve this issue of metal foam, several studies were conducted [[16], [17], [18], [19]]. Tseng et al. [16] applied a parallel flow field into the metal foam flow field to guide the flow and compared the result with a serpentine channel. The metal foam flow field PEMFC exhibited better performance than that of serpentine channel PEMFC. In the metal foam flow field, which causes a higher pressure drop than the void flow field, the parasitic loss due to pressure drop should be considered for a more meaningful result. However, the study did not describe the influence of pressure drop. Tsai et al. [17] analyzed four flow designs with metal foam. The flow field, which was divided into three parallel blocks, showed the highest output power and a net power of PEMFC. Thus, the previous studies demonstrated that controlling the internal flow of metal foam improves the PEMFC performance. Further, research on the combination of parallel structure and metal foam were conducted. However, the combination of serpentine structure, which achieves a higher output of PEMFC than parallel structure, with metal foam was not studied. The serpentine structure guides the reactant to the corner of the reaction area, and it is expected that the combination of serpentine structure and metal foam would improve the performance of PEMFC. Therefore, determining the effect of the combination of the serpentine structure and metal foam is worthwhile to achieve high-performance PEMFC.

In this study, the effect of the number of turns of serpentine structure in metal foam flow field on the cathode side on the PEMFC performance was analyzed. Four types of PEMFC models having different serpentine structures at metal foam flow fields and another PEMFC model having serpentine channel for comparison were created. The first model had bulk metal foam flow field without a serpentine structure as a reference case, and the other PEMFC models had two, four, and six turns of serpentine structure. The analyses were conducted via computational fluid dynamics. The performance of PEMFC was enhanced proportionally to the number of turns of serpentine structure. However, meanwhile, the number of turns of the serpentine structure increased pressure drop causing parasitic power loss. Thus, through this study, when attempting to achieve the high-performance PEMFC, how the metal foam flow field is applied on PEMFC can be suggested, and it is expected to accelerate the commercialization of PEMFC.

Section snippets

PEMFC model and the geometry

The PEMFC model for analysis had a reaction area of dimensions 100 mm × 100 mm (100 cm2 area), which is a practical reaction area level. The metal foam flow field with serpentine structure was applied only into the cathode flow. The flow field configurations are shown in Fig. 1. In the metal flow field, the width of the serpentine structure was much wider than that of the normal serpentine channel, which is ∼1 mm, and the width of two, four, and six turns were 30, 16, and 10 mm, respectively.

Results and discussions

Before studying effect of serpentine structure at metal foam flow field on the performance of PEMFC, the analysis of performance of PEMFC having cathode bulk metal foam flow field and cathode serpentine channel cae was conducted. In Fig. 3 the IV curves of PEMFC having bulk metal foam flow field and PEMFC having serpentine channel is displayed. At all voltage conditions, the performance of PEMFC having metal foam flow field was higher than the PEMFC having serpentine channel and the gap of

Conclusion

In this study, the serpentine structure was applied in a metal foam flow field, which did not have any flow guide that distributed the reactant evenly. The serpentine structure improved the performance of PEMFC with the metal foam flow field by upgrading the mass transfer characteristics. For the PEMFC case without the structure, the reactant was supplied into the reaction site only via diffusion, rather than convection, which provides more activated mass transfer. Accordingly, the bulk metal

CRediT authorship contribution statement

Jonghyun Son: Conceptualization, Methodology, Software, Validation, Formal analysis, Data curation, Writing – original draft, Visualization. Sukkee Um: Conceptualization, Methodology, Formal analysis, Resources, Supervision. Young-Beom Kim: Conceptualization, Formal analysis, Writing – review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20213030030190).

References (33)

Cited by (0)

View full text