Thermal analysis of a 1-kW hydrogen-fueled solid oxide fuel cell stack by three-dimensional numerical simulation

https://doi.org/10.1016/j.enconman.2020.113213Get rights and content

Highlights

  • Air flow determines the temperature distribution of a 1-kW planar SOFC stack.

  • A large temperature gradient is imposed on unit-cells and sealants near air inlets.

  • Continuous air heating throughout the stack affects such thermal characteristics.

  • Gaseous advection is a key pathway to release the heat produced from unit-cells.

  • Interconnect conduction between units is effective at the top and bottom of a stack.

Abstract

This study aims at elucidating the thermal characteristics and heat transfer mechanism of a commercial-scale, planar solid oxide fuel cell (SOFC) stack. The thermal management of a SOFC stack is key for its stable operation and market deployment. It is necessary to understand the internal thermal conditions and heat transfer pathways for satisfying such needs. In this regard, this study conducts three-dimensional numerical simulation accounting for an actual 1-kW stack geometry and its operating conditions. The thermal-flow conditions of the stack composed of 30 unit-cells are spatially resolved. Results show that temperature inside a stack changes in a way that it follows the air flow. As the air and fuel proceed to the top of the stack, the effect of the air flow is more prominent while showing a smaller temperature difference of unit-cells inside a repeating unit and a lower rate of its temperature increase. Given the temperature distribution, a large temperature gradient is imposed on unit-cells and sealants near the air inlet, especially in the lower repeating units. It is also shown that, in the direction of stack height, gaseous advection is a key pathway through which the heat released from a unit-cell is transferred, and conductive heat transfer through metallic interconnects contributes to the overall heat transfer rate at the top and bottom of the stack. As the gaseous advective cooling in the direction of stack height is reduced, the heat transfer rates towards the gas inlets and outlets within a repeating unit by interconnect conduction and gaseous advection, respectively, are also lowered, which is compensated by conductive heat transfer between repeating units. It can be inferred that controlling gas heating near the inlet manifold (especially, at the bottom of the stack), advection through the stack, and metallic conduction between repeating units may change the key heat transfer pathways and internal thermal conditions.

Introduction

Solid oxide fuel cell (SOFC) has been gaining attention as a next-generation power technology due to its various advantages (i.e., high efficiency, fuel flexibility, modularity) as well as environmental friendliness (i.e., no NOx/SOx emissions, small footprints). Its high operating temperature above 700 °C provides high rates of reaction kinetics and low polarization resistances, assuring a high efficiency of nearly 60%. The residual heat due to high temperature operation can be further recuperated for district heating or gas turbine operation [1], [2], [3], [4], [5], [6], [7]. The high temperature makes SOFC stack components to use chemically stable ceramic materials or ceramic-metal composite materials, providing high resistance to corrosion and allowing direct hydrocarbon fuel utilization [8], [9], [10]. Based on a stacked-cell structure, a typical SOFC stack available from manufacturers produces 1–5 kW electrical power, which can be further raised by forming a module [5], [11], [12]. Given these advantages, a large number of demonstrations have been conducted such that Bloom Energy in the United States has over 350 MW of cumulative installation, and the Ene-Farm program in Japan has installed 60,000 SOFC units (50 MW) since 2009 [13]. The efficiency as high as 61% and the degradation rate as low as 0.4%/1000 h have been obtained from the demonstrations.

Thermal management of a SOFC stack is essential to make this technology viable with sufficient reliability. A stack or its module operating at high temperature must overcome a wide range of problems arising from thermal imbalance or thermal stresses [14], [15], [16]. Simultaneous electrochemical (exothermic) and thermochemical (endothermic) reactions result in localized heat generation and consumption, thereby causing large temperature gradients inside a repeating unit of the stack [17]. Flow patterns developed by air and fuel flows (e.g., co-flow, counter-flow, cross-flow) provide uneven heat transfer pathways, which also makes non-uniform temperature distribution and its gradient [18]. Moreover, the gas manifold design (e.g., internal manifold, external manifold) influences the mass flow rate of gases to each repeating unit, determining their heat capacity and leading to a temperature gradient in the direction of stack height [19], [20]. A recent experimental study indeed shows that commercially available stacks have 100–150 °C temperature difference within the stack structure [21]. Such temperature gradients and non-uniform temperature may result in crack and fracture due to the mismatch of thermal expansion coefficients between stack components [22]. Thus, it is essential to provide effective thermal management for uniform temperature and lower temperature gradients inside a stack prior to its market deployment.

Identifying internal thermal conditions and elucidating heat transfer mechanism of a SOFC stack is critical in obtaining effective thermal management. A SOFC repeating unit composed of a unit-cell and metallic interconnects forms a complex thermal environment associated with simultaneous heat generation and its transfer [14], [17]. The net heat released from thermo-electrochemical reactions is highly influenced by the surrounding gas flows and their local thermodynamic states. In the meantime, the heat released is transferred to the surrounding air and fuel flow and metallic interconnects, changing their temperature. Such coupling can be more complicated when considering a stacked-cell structure similar to a multi-layer heat generator and exchanger. In this regard, it is necessary to elucidate the internal thermal characteristics (i.e., temperature distribution and its gradient) and heat transfer mechanism of a multi-scale, multi-component, multi-layer stack [14], [17], [23], [24]. However, it is difficult to investigate experimentally the internal thermal conditions and heat transfer mechanism of a SOFC stack due to its high temperature operation and hermetic sealing. Such conditions make it extremely difficult to use measurement probes penetrating into a stack structure and measure the internal local thermodynamic states [25], [26]. Recent experimental activities have only measured the stack surrounding surface temperature, and incoming and outgoing gas temperature [21]. To overcome such limitations, three-dimensional numerical simulations should be conducted while assuming the same geometry and operating conditions of an actual SOFC stack. Furthermore, to simplify the internal reacting environment and to focus on the thermal analysis, many of recent studies assumed hydrogen as a fuel [5], [16], [18], [20], [21]. Although hydrogen fuel supply may not fully account for actual SOFC stack operation typically supported by direct hydrocarbon fuel supply, it provides sufficient basis on which the detailed thermo-fluid analysis can be performed, thereby building up insight on the internal conditions of a multi-scale stack. Based on this, the effect of hydrocarbon fuel supply on the internal thermal conditions can be further elucidated.

Indeed, a number of numerical modeling studies on the thermal conditions of a hydrogen-fueled SOFC stack have been carried out, but they are limited to a small-scale model without considering actual geometry or detailing the heat transfer mechanism. The analyses at a unit-cell level were conducted considerably while investigating heat transfer, electrochemical reactions, and dynamic behavior in order to understand the internal thermal characteristics of a single repeating unit [14], [17], [23], [24], [27]. In contrast, a stack with multiple unit-cells stacked vertically have different characteristics, making it necessary to perform three-dimensional stack modeling and detailed thermal analysis [28]. In spite of such necessity, most of the three-dimensional modeling studies assumed a short-stack and virtual geometry, with a lack of discussion on heat transfer mechanism, as summarized in Table 1. In general, flow analyses were performed confirming the uneven distribution of mass flow in a stack [29], [30], [31], [32], and thermal studies were conducted investigating a temperature profile inside a unit-cell or a short-stack [18], [19], [28], [33], [34], [35], [36], [37] or a full-stack [20], [38]. The previous studies have typically assumed a stack or a short-stack with simplified flow channels and components and analyzed the effect of new design or configuration on the flow uniformity and temperature profile. These studies did not discuss in detail three-dimensional thermal distribution and heat transfer mechanism. Although this may capture some thermal-flow features of a stack, the investigation which does not account for actual geometry makes it difficult to elucidate the detailed three-dimensional thermal conditions and heat transfer mechanism of an actual, multi-layer, commercial-scale stack. To address this issue, a few studies have been performed for the thermal analysis of a stack while considering actual stack geometry [18], [33], [35], [36], [37]. They discussed the thermal distribution in a stack, but still assumed a short-stack with less than 10 layers or an active area about 25% of a unit-cell used in an actual stack. Such modeling limitations arise from extensive computational costs when accounting for a large-area cell and more than 30 stacked-cells (nearly 1-kW stack). To satisfy the needs for elucidating internal thermal conditions and heat transfer mechanism of an actual, commercial-scale (e.g., 1-kW) stack while reducing computational costs, it is necessary to construct a model resolving an actual stack geometry and focusing on the interactions between heat transfer and gas flows.

In this study, a 1-kW planar SOFC stack with 30 stacked cells is modeled and examined numerically in order to elucidate its internal thermal conditions and heat transfer mechanism. The three-dimensional model developed in this study resolves the actual geometry of the commercial scale stack and incorporates a hear source at each repeating unit estimated from thermo-electrochemical analysis [17]. The model is used for investigating the interactions between heat transfer and gas flows, which enables discussing temperature distribution and its gradients and elucidating heat transfer mechanism in the entire stack. To overcome the limitations of the previous studies, this study provides a model and discussion with the following novelty:

  • Three-dimensional thermal-flow simulation for an actual geometry of a 1-kW planar SOFC stack

  • Use of effective materials properties measured experimentally by using in-house fabricated stack components

  • Estimation of temperature distribution in the direction of stack height and width and sources of such variation

  • Investigation of temperature gradients imposed on stack components and sources of such gradients

  • Elucidating heat transfer mechanism in the entire stack for effective thermal management

Section snippets

Numerical modeling

To examine the internal thermal conditions and heat transfer mechanism of a 1-kW planar SOFC stack, a physical model is developed resolving the actual stack geometry and incorporating a heat source at each repeating unit. The schematic diagram of the 30 planar-cell stack considered in this study is shown in Fig. 1 (refer to Supplementary Table S1 for the key geometric parameters of the stack). The stack is composed of unit-cells that have an area of 144 cm2 (with an active area of 100 cm2) and

Results and discussion

To elucidate thermal conditions and heat transfer mechanism inside a SOFC stack, temperature distribution and its gradient of gases and each component including metallic interconnect, unit-cell, and sealant are detailed. Key thermal characteristics at each repeating unit and sources of their variation are discussed in detail. Based on this, primary heat transfer pathways through the entire stack are envisioned, which may enable suggesting a design approach for thermal management.

Conclusions

To elucidate the internal thermal conditions and heat transfer mechanism of a 1-kW planar solid oxide fuel cell (SOFC) stack, a physical model was developed by resolving the actual stack geometry and accounting for the heat released from unit-cells. The conventional SOFC stack available in South Korea is considered in this study, whose geometry is implemented in three-dimensional numerical simulation. The size, shape and thermal-flow properties of stack components also referred to actual

CRediT authorship contribution statement

Dong Hwan Kim: Conceptualization, Methodology, Software, Formal analysis, Writing - original draft. Yonggyun Bae: Methodology, Software. Sanghyeok Lee: Conceptualization, Methodology, Software. Ji-Won Son: Supervision. Joon Hyung Shim: Supervision, Funding acquisition. Jongsup Hong: Supervision, Writing - review & editing, Project administration, 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.

Acknowledgements

This research was financially supported by the Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (no. 20173010032170), the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT& Future Planning (2017M1A2A2044989).

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