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

[+] Author and Article Information
Takanobu Shimada

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

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 (0.03WmK) 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 3.02NLmin, 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.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Schematic structure of a small SOFC system

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Figure 2

Schematic diagram of gas flow in small SOFC systems

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Figure 3

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

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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

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Figure 5

Steady-state outline flowchart of 1kW-class module

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Figure 6

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

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Figure 7

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

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Figure 8

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

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Figure 9

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

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Figure 10

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

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Figure 11

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

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Figure 12

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

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Figure 13

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



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