Research Paper
Studies on the effect of flow configuration on the temperature distribution and performance in a high current density region of solid oxide fuel cell

https://doi.org/10.1016/j.applthermaleng.2022.118120Get rights and content

Highlights

  • Three different flow configurations of solid oxide fuel cell have been compared.

  • The temperature distribution at 6000 A/m2 were analyzed.

  • In counter-flow, the most uniform temperature distribution was formed.

  • The power density is 5% higher in counter-flow than the other configurations.

  • Counter-flow presents the best efficiency of up to 3.35%.

Abstract

The effect of temperature distribution on the performance of solid oxide fuel cells (SOFCs) is analyzed for various channel designs in a high current density region. By considering heat accumulation due to local electrochemical reactions and cooling due to flow patterns, the heat distributions in three flow configurations (co-flow, counter-flow, and cross-flow) are analyzed, with the most appropriate channel in the high power region selected. The average operating temperature of the cell is the highest for the counter-flow configuration and the lowest for co-flow. The temperature of the counter-flow configuration is approximately 10 K higher than that of co-flow. The temperature distribution of counter-flow configuration, however, is the most uniform among different flow patterns. The maximum temperature difference in the counter-flow channel is approximately 8 K, but that in co-flow is approximately 22 K. Furthermore, the performance of the cell using the counter-flow configuration is the best in that it shows 5% higher power density and 3.35% higher system efficiency than the other flow configurations. Therefore, the counter-flow configuration is superior at high power because it has the lowest temperature gradient and the best cell performance. This paper contributes to the commercialization of the fuel cell by presenting appropriate parameters for temperature regulation and suitable flow configuration for operation at high power.

Introduction

A solid oxide fuel cell (SOFC) converts chemical energy into electrical energy through electrochemical reactions that uses solid oxide as an electrolyte. It is drawing attention as an eco-friendly energy device in that it does not directly emit pollutants [1]. An SOFC operates at relatively higher temperatures (700–1000 °C) than other fuel cells and both cells’ voltage and performance increase as the operating temperature rises [2], [3], [4]. The SOFC’s high operating temperature enables the cell to use hydrocarbons substances in a reforming reaction and several other fuels, such as biogas [5], [6]. Despite their advantages of high operating temperature, SOFCs’ extreme high temperature has become an obstacle to commercialization [7].

Another fuel cell which has high operating temperature is a molten carbonate fuel cell (MCFC). MCFC uses molten carbonate salt mixture as electrolyte and is used for industrial, and military applications. Since the operating temperature of MCFC is 600 °C or higher, same as SOFC, research on thermal management is underway to improve the performance [8], [9]. Elisabetta Arato et al. [10] developed a code and analyzed temperature distribution of the cell. A method of forming a more even temperature distribution according to thermal conductivity (K) and convection phenomenon (Nu) was studied. Murat Baranak et al. [11] compared the performance of the cell with different type of electrolyte and channel shape and analyzed the operating pressure dependency on cell potential. In MCFC, thermal management of fuel cells has been important, and studies such as comparing performance according to different flow configurations are also playing an important role. Therefore, research on SOFC, which is commercialized at higher temperature than MCFC, related to thermal management and performance comparison is also actively underway.

Excessive heat generated from electrochemical reactions of an SOFC results in the lower the cell performance and a larger drop in the corresponding voltage [12]. Therefore, previous studies have examined the efficient heat management of an SOFCs. In the most cases, the temperature of the heated cell is controlled by the ratio of cathode gas to anode gas (the air to fuel (AF) ratio). Companari et al. [13] confirmed that fuel cell effectiveness is positively affected when the air ratio is 3–4; the average cell temperature decreased from 54.7% to 50.4% at cross-flow and from 58.3% to 56.8% at counter-flow. Anderson et al. [14] also found that the rate of temperature increase is controlled by the degree of excess air; 45% of the heat generated can be used by the steam reforming reaction. Vincenzo Liso et al. [15] performed natural gas reform inside the SOFC through endothermic cooling with gas recirculation in the anode and cathode, resulting in higher electrical efficiency (85–95%) and a reduction in the stack fuel utilization from 80% to 60%. Zetting Yu et al. [16] analyzed the effects of the fuel utilization factor. They determined that when the fuel utilization factor increased, the cooling efficiency decreased from 30% to 20%, but electrical efficiency increased from 65% to 80%. Although previous studies can explain the changes in temperature according to inlet conditions, we must also need to understand the effect of temperature distribution on SOFC performance to manage heat properly.

Many numerical studies have been conducted to understand temperature distribution in an SOFC. They primarily analyzed the effect of flow configuration on the heat balance of the cell. Xia et al. [17] found through a comparison of co-flow and counter-flow that a more uniform temperature gradient in counter-flow was presented and counter-flow can produce 80 W/m2 larger power density than in the case of co-flow. Jakub Kupeki et al. [18] compared the temperature distributions of co-, counter-, and cross-flow and presented that the temperature gradient is the smallest in co-flow. However, the model was relatively simple, and the temperature gradient was confirmed over time. Tushar Choudary et al. [19] compared co-flow with counter-flow to confirm that the cell power density of counter-flow is 49.06% greater than that of co-flow. Moreover, the overall cell potential is 3–4% higher for co-flow than counter-flow. However, the simulation results were an inconsistent with the experimental results as the current density increased. Despite enormous efforts, few studies validated their models, failing to explain complex interactions among reactions in an SOFC due to model simplicity. Therefore, it is necessary to analyze the effect of various flow configurations on the temperature distribution of an SOFC with sufficient experimental results.

A study on temperature distribution at a high current density is essential to achieve SOFC commercialization, but existing studies have been conducted predominantly at low current densities. Few studies have examined SOFC performance in a high current density region. Girona et al. [20] developed modeling based on an experiment, but the maximum current density was 4500 A/m2. Monder et al. [21] analyzed the behavior of an SOFC after designing the model directly, but the maximum current density of the experiment was 2000 A/m2, achieved at a slightly low power output. Therefore, research on SOFCs at a high current density of 5000 A/m2 or higher is demanded to overcome the challenge of its success. However, a lack of high power output studies is evident.

In this study, the optimal channel design for generating high output was examined by investigating the thermal equilibrium among various reactions according to the flow configuration. OpenFOAM freeware was used to analyze the heat distribution characteristics at high power (6000 A/m2) according to each flow configurations of the cell. We first validated the developed model with experimental data and analyzed the effect of the AF ratio and current density (0 A/m2 to 6000 A/m2) on the operating temperature of the three flow configurations (co-flow, counter-flow, and cross-flow). Next, the temperature distribution of the cells according to the three flow configurations at high power was analyzed to determine which configuration is suitable for high power. Finally, the optimal types of channel design in a high current density region are suggested. This study addresses problems related to high temperature and uneven temperature distribution of the cell at high power and contributes to the commercialization of SOFCs.

Section snippets

Model geometry and component description

A model for a planar-type anode-supported SOFC is developed using OpenFOAM, as depicted in Fig. 1. The cell is primarily consisted of three parts: interconnections, gas channels, and a positive-electrolyte-negative (PEN) electrode assembly. The PEN components are divided into five layers: anode substrate layer (ASL), anode function layer (AFL), electrolyte, cathode function layer (CFL), and cathode current integration layer (CCL). The fuel and oxidant, essential for the reaction, are provided

Model validation

Based on the experiment results, the accuracy of the developed model is ensured. The results of the experiment and performance of the anode-supported SOFC are compared with a current–voltage (I–V) curve, as depicted in Fig. 5. The model used is the same cross-flow configuration as in the experiment and the same ratio as the AF ratio used in the experiment is applied. Validation is conducted in one cell unit, assuming that the stack of the experiment is a serial connection of the cells.

The

Conclusion

This study examined flow configurations for successfully operating a solid oxide fuel cell in a high current density region. We can manage operating temperature and distribute temperature evenly by selecting the best types of channels.

  • (1)

    As the current density increases, the temperatures of three flow configurations of the cell increases. When the air–fuel ratio increases from 0.233 to 0.699, the temperature decreases by at least 0.9% and up to 2.9% for all flows. Because co-flow has the greatest

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.

Acknowledgements

This research was supported by the Chung‐Ang university Research Scholarship Grants in 2021. This work was supported by the Industrial Strategic Technology Development Program (10082569, Development of design and package and prototype for commercial 5-kW-class SOFC-engine hybrid system).

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