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TECHNICAL PAPERS

# Numerical Analysis of Thermal Behavior of Small Solid Oxide Fuel Cell Systems

[+] Author and Article Information

Department of Electrical Engineering, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japanshimada.takanobu@jaxa.jp

Tohru Kato, Yohei Tanaka

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan

J. Fuel Cell Sci. Technol 4(3), 299-307 (Jul 31, 2006) (9 pages) doi:10.1115/1.2744049 History: Received December 05, 2005; Revised July 31, 2006

## Abstract

Recently, small solid oxide fuel cell (SOFC) systems have been developed for various applications because of their high performance. In such small generation systems, quick and frequent start-stops are often required. However, it is generally considered that these start-stops with SOFC systems are not preferable because SOFC systems are operated at high temperature. Also, quantitative studies on the thermal behavior of small SOFC systems are limited. The purpose of this paper is to obtain insight into the possibility of using small SOFC systems with quick and frequent start-stops. A simple two-dimensional numerical model for $1kW$-class SOFC systems was fabricated to study this problem. The model consists of a cylindrical SOFC stack, a prereformer on the stack, a heat exchanger for exhaust gas, and a thermal insulator that covers the stack and the prereformer. Using this model, first, the characteristics of the power generation efficiency were estimated under various operating conditions. In addition, the validity of the modeling was verified. Next, the start-up dependence on their structure and operating conditions was investigated. Finally, for the cyclic daily start-up and shutdown (DSS) procedure, the total efficiency during a day was calculated when the energy loss during start-stops is considered. As a result of the analysis, the following points were found. First, the validity and accuracy of the modeling was established, and their efficiency under the rated condition becomes 60% (DC/HHV) at a steam-carbon $ratio=2.5$ and an oxygen $utilization=50%$. Next, the thickness of the thermal insulator $(0.03W∕m∕K)$ is required to be more than $6cm$ to reduce the heat loss from the outer surface of the thermal insulator to $<5%$ of the provided fuel energy $(2kW)$ under the rated condition. In this case, it takes ca. $150min$ to start, if the fuel (methane) flow rate is $3.02NL∕min$, which is equivalent to $2kW$ of heat flow. Finally, for the DSS operation, consisting of repetition of a $16h$ operation and an $8h$ stop in a day, the total efficiency decreases by ca. 1.5% from the rated power generation efficiency. Therefore, it is clarified that $1kW$-class SOFC systems can be quite suitable even in the case where quick and frequent start-stops are required.

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## Figures

Figure 1

Schematic structure of a small SOFC system

Figure 2

Schematic diagram of gas flow in small SOFC systems

Figure 3

A sample of temperature distributions in SOFC stack, prereformer, and thermal insulator

Figure 4

Steady-state outline flowchart of the SOFC system when P=1kW, Uf=0.75, Uox=0.25, S∕C=3, ηhex=0.7

Figure 5

Steady-state outline flowchart of 1kW-class module

Figure 6

Variations in efficiency ηdc with oxygen utilization Uox from 20% to 50% or steam-carbon ratio S∕C from 2 to 4

Figure 7

Dependence of thickness of the thermal insulator on start-up time and heat loss at the operating temperature

Figure 8

Dependence of thermal conductivity of the thermal insulator on start-up time and heat loss at the operating temperature

Figure 9

Dependence of fuel flow rate on start-up time and fuel consumption during start-up

Figure 10

Dependence of heat capacity of the SOFC stack on start-up time and fuel consumption during start-up

Figure 11

Dependence of temperature effectiveness of heat exchanger on start-up time and exchanged heat during start-up

Figure 12

Temperature variation in the SOFC stack for 24h after shutdown with thickness and performance of the thermal insulator

Figure 13

Dependence of operating time on total daily efficiency during start-up

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