23rd World Gas Conference, Amsterdam 2006

SOFC DEVELOPMENT by Tokyo Gas, Kyocera, Rinnai and Gastar

Main author

T.ISHIKAWA

JAPAN

ABSTRACT

This paper reports SOFC development by Tokyo Gas, Kyocera, Rinnai and Gastar. The subjects are as follows:

• SOFC development of Tokyo Gas • SOFC system development using of the segment in-series cell stack • Progress of development

Development is conducted on these subjects and the results are included in this paper.

TABLE OF CONTENTS

1. Abstract

2. Body of Paper

3. Reference

4. List Table

Table 1 Types of fuel cells

Table 2 Comparison of SOFC cell stacks

Table 3 Target specifications

5. List of Figures

Fig.1 Efficiency vs. rated power of on-site co-generation systems

Fig.2 Flat-tubular segmented in-series SOFC cell stack

Fig.3 Design of flat-tubular segmented in-series SOFC cell stack

Fig.4 System diagrams for SOFC and PEFC

Fig.5 I-V characteristics of the cell stack

Fig.6 Fuel utilization dependence of the cell stack voltage with contour lines of electric

conversion efficiency

Fig.7 Time dependence of the voltage of a cell stack

Fig.8 Performance of the bundle (34 cell stacks)

Fig.9 SOFC thermally self-sustainable module (3kW)

Fig.10 SOFC module performance

SOFC DEVELOPMENT by Tokyo Gas, Kyocera, Rinnai and Gastar

Tadaaki Ishikawa, Tokyo Gas Co., Ltd.

Shoji Yamashita, Kyocera Corporation

Tsutomu Sobue, Rinnai Co., Ltd.

Koji Hase, Gastar Co., Ltd.

Introduction

For human beings, the 21st century is tremendously important from the viewpoint of the global

environment. To prevent further global warming, there needs to be immediate action on saving energy and

reducing greenhouse gas emissions. Japan declared in the Kyoto Protocol that it would reduce greenhouse

gas emissions to 6% of 1990 levels between 2008 and 2012. As part of the effort to achieve this target, a

shift of the primary energy source from oil to natural gas was stipulated by the Japan Energy Fundamental

Plan which controls long-term domestic energy policy in Japan. In addition, it is estimated that on-site

power supply will account for around 20% of total electric power generation by 2030. Under these

circumstances, natural gas fueled co-generation systems (CGS) with highly efficient fuel cells are expected

to play an important role in environmental and energy issues. In accordance with national policy, Tokyo Gas

intends to contribute to energy saving and environmental load reduction by promoting the adoption of

on-site power generators which use natural gas as an alternative to electric power depending on

conventional centralized power supply systems.

For example, we promote energy service business utilizing co-generation systems of the internal

combustion type fired by natural gas, such as the gas engine, and we marketed the world's first domestic

use fuel cell co-generation system with polymer electrolyte fuel cells (PEFC).

To contribute to preservation of the global environment, it is indispensable to develop more efficient

co-generation systems. Based on this recognition, in 2003 Tokyo Gas, Kyocera Corporation, Rinnai and

Gastar begun joint development of the solid oxide fuel cell (SOFC), the type of fuel cell that achieves the

highest electrical efficiency, with the aim of using it to produce generators that are friendly to the

environment.

This paper provides an overview of our present results, including the successful running of the module,

and describes objectives for producing practical SOFC systems.

Overview of SOFC

Table 1 compares the characteristics of all major types of fuel cells, including solid oxide fuel cells

(SOFC).

Of the fuel cells listed, the phosphoric acid fuel cell (PAFC) and polymer electrolyte fuel cell (PEFC) are

already in practical use.

SOFCs in particular, have many advantages, including high efficiency, low cost of manufacturing, and

availability for on-site power generation systems with rated power of between 1 kW and several MW. A

SOFC system and PEFC system are compared in Fig.1. SOFCs differ from PEFCs in that power is

generating at above 700 decrees Celsius (973K) using ceramic electrolytes, with O2- ions being transferred

through the electrolyte instead of H+ ions, such that H2O is formed in anode side. They also differ in that

CH4 is used because of the reforming process in the cell stack. Fig.2 maps capacity and electrical

efficiency of on-site power generators. The R&D target was to achieve generating efficiency of 45%HHV at

the several-kW class. This would enable application to new uses that were not possible with earlier

electrical generating systems.

Table 1 Types of fuel cells

Polymer Electrolyte Fuel Cell (PEFC)

Phosphoric Acid Fuel Cell (PAFC)

Molten Carbonate Fuel Cell (MCFC)

Solid Oxide Fuel Cell (SOFC)

Electrolyte Proton exchange

membrane Phosphoric acid

Molten alkaline

carbonate

Oxide (Zirconium)

Operating Temp. Room Temp. to 373K 473K 923K up to 1,273K

Charge Carrier H+ H+ O2- O2-

Fuel H2 H2 H2, CO H2, CO

Efficiency (HHV) up to 35% up to 40% up to 45% up to 45%

Applications Automobile、

Residential CGS CGS CGS, Power station CGS, Power station

Turbine Combined System

No No Yes Yes

Reformer for CH4 Fuel Necessary Necessary Unnecessary Unnecessary

Specifications Easy start-up, Practical use

Practical use Large scale power

plant use Low cost

manufacturing

Fig.1 Efficiency vs. rated power of on-site co-generation systems

SOFC development at Tokyo Gas

Tokyo Gas began R&D of SOFCs in 1989. In the first stage, we started to develop high temperature driven

planar SOFC stacks, and succeeded in developing the world's first kW-class stack generating power with

internal reforming, and we accumulated advanced elemental technology that gained worldwide recognition.

However, mechanical reliability was too low for practical use, so in 2001 we switched to developing

anode-supported planar SOFCs with reduced temperature operation. The development process was

supported by NEDO, and led to the establishment of technology for lower temperature operation in the

range from 1000 degrees Celsius down to 750 degrees Celsius, and also to the establishment of elemental

technology for high efficiency generation to achieve electrical efficiency of 53%HHVDC using this cell stack.

We successfully improved mechanical reliability by utilizing a thin film for the electrolyte, which is the most

fragile part of a cell stack, and supporting it in the anode pole. However, development demonstrated

barriers to practical application due to issues with the long term stability of cell stacks using metallic

interconnectors.

In 2003, we started to develop segmented in-series cell stacks operating at lower temperatures to apply

the reduced temperature operation technology and a unique interconnector technology that had already

been developed, and in 2004 Tokyo Gas, Kyocera, Rinnai and Gastar launched development of SOFC

systems using this type of cell stack. This cell stack has an innovative structure, achieving lower cost in

mass production and high-voltage / low-current generation by using a structure where multiple cells are

connected in series on a sintered ceramic substrate.

The joint development by the four companies is described below.

SOFC system development

Tokyo Gas, Kyocera, Rinnai and Gastar are jointly engaged in the development of SOFC based systems.

The development consists of two parts; DC power generator development, concerned with topics such as

cell stacks and bundles, and module and system development, which builds on the DC part.

Overview of cell stack development

The joint project group adopted a low-temperature operable segment in-series type flat tube cell stack

(as shown in Fig.2), developed by Tokyo Gas and Kyocera. The cell stack, designed by Tokyo Gas, has a

segmented in-series electrode structure that can be operated in low temperatures. This structure was

achieved using Kyocera’s flat tube production technology. Both companies have obtained the expected

electrical generation performance with this cell stack at an operating temperature of 750 degrees Celsius.

Fig.3 shows how segmented in-series cell stacks operate. Fuel (H2, CO, CH4), flows in several internal

tubes which go through the insulating substrate along the length of the unit, and air (O2) passes along the

fuel cell stack surface. Gas sealing, which was one of the problems for planar cell stacks, becomes easy for

fuel flow inside the insulation. This benefit is one of the features of this segmented in-series fuel cell stack.

On the rectangular insulation, 8-10 single cells are made on each side and compose one stack, and about

16-20 single cells are mounted on the front and back of each cell stack. Each single cell is constructed with

an anode and a cathode in the electrolyte like an ordinary fuel cell. Fuel is supplied to the anode from

inside the fuel cell stack through insulation substrates, and oxygen is supplied to the cathode from the cell

stack surface, generating power in each of the single cells.

The cell stack output is about 10 W per sheet of fuel cell stacks with about 16-20 single cells connected

in series electrically with the interconnector. High voltages and the high power levels are achievable by

connecting tens of fuel cell stacks in series and bundling. Piled up cell stack SOFC (planar) units are

compared with lined up (segmented in-series) units in Table 2.

Fig.2 Flat-tubular segmented in-series SOFC cell stack

Fig.3 Design of flat-tubular segmented in-series SOFC cell stack

Table 2 Comparison of SOFC cell stacks Features of piled-up SOFC units

(Example: planar type) Features of lined-up SOFC units (Example: segmented in-series type)

- Large electric current - High power density (possible) - Simple cell stack structure - Lower temperature operation is possible. - Problems of alloy interconnector (if used) - Developed in many R&D labs.

- High voltage - Advantage for sealing fuel (in the case of tubular type) - Complicated cell stack structure - Lower temperature operation is mainly determined by interconnector performance. - Non-conducting substrate contributes to cost. - Developed in few R&D labs.

Overview of module system development

The system diagram for a SOFC is shown in Fig.4, together with a PEFC system diagram for reference.

The characteristics of the SOFC system are as follows.

- Internally reforming is possible, so a reformer for H2 is unnecessary

- Ancillary cooling system is unnecessary

- Exhausted gas reaches 200-300 degrees Celsius

Development of the system has been led by Tokyo Gas, Rinnai and Gastar, and by combining the new cell

stack structure and the system construction technologies that Rinnai and Gastar have cultivated in the

process of developing and manufacturing gas appliances, the joint project group has started to develop a

SOFC power generating system with highly efficient electrical generation.

The development procedure adopted is to first concentrate on thermally self-sustainable module

development and after that to manufacture SOFC systems as applications of the module.

Fig.4 System diagrams for SOFC and PEFC

Progress of development

At the end of FY2004 (March, 2005) development had produced the following results.

Cell stacks and bundles

(a) The performance of the segment in-series cell stack is shown in Fig.5 and Fig.6.

- DC Electrical efficiency of the cell stack is 52.9%HHV at 750 degrees Celsius, and 48.4%HHV at 700

degrees Celsius.

- A durability test was conducted with a cell stack at 0.2 A/cm2 at 700 degrees Celsius (973K). As shown in

Fig.7, stable operation of a cell stack for 2,000 hours was achieved, with a degradation rate of about

3.5%/1000h. The cell stack has 14 single cells on each side, and the voltage of each side (one side has 7

single cells) was monitored. One side showed no degradation while the other side had degradation, which

is attributed to some fault in the cell fabrication process. Thus, the cell stack is expected to have

essentially high durability performance.

(b) The performance of a bundle of 34 cell stacks is shown in Fig.8.

- We confirmed that the output of the bundle with 34 cell stacks exceeds 300 W in the electric furnace. The

electrical efficiency of the case was 49%HHVDC (tested with H2 fuel and calibrated assuming the same

performance using methane).

FFig.7 Time dependence of the voltage of a cell stack.

Fig.5 I-V characteristics of the cell stack.

Fig.6 Fuel utilization dependence of the cell stack voltage with contour lines of electric conversion efficiency.

UH2(%)

Vdc

Fig.8 Performance of the bundle (34 cell stacks).

Fig.9 SOFC thermally self sustainable module (3 kW).

Fig.10 SOFC module performance

Module and system

Fig.9 shows a thermally self-sustainable SOFC module (10 bundles). Its rated output is 3 kW. The 10

bundles were mounted inside the module.

- We confirmed that the thermally self-sustainable generation module simultaneously achieved

generation of 3 kW of electricity and generative efficiency of 46.2% DC (HHV) (tested with H2 fuel and

calibrated assuming the same performance using methane). In addition, generative efficiency of 44.5%

DC (HHV) at 2.4 kW generation was confirmed with CH4 fuel (Fig.10).

Future plans

The next step is to perfect the technology to the extent that it becomes possible to build a practical SOFC

system incorporating two sets of modules and a power conditioner unit for AC power output, together with

automatic control. There are high expectations for SOFC application in co-generation systems that can be

utilized by low heat-electric ratio users and in on-site generators using natural gas because of the high

electrical efficiency achieved. Continuing development to enhance durability and reduce cost, there are

plans to begin field testing in 2007. The target system specifications are shown in Table 3.

Table3 Target specifications

Generation capacity 5 kW

Generation efficiency 45%HHV AC

Conclusion

Since 2003, Tokyo Gas, Kyocera, Rinnai and Gastar have been working on a joint program for the

development of a high-efficiency SOFC generation system for domestic and business use that is expected

to achieve the highest generation efficiency of all fuel cell types. Development has now progressed to the

extent that the module generates power with 44.5% electrical efficiency.

A preliminary system will be introduced in field tests starting 2007.