Development of 200kW ClassSOFC Combined Cycle Systemand Future View
TADASHI GENGO*1

YOSHINORI KOBAYASHI*1

YOSHIMASA ANDO*1

NAGAO HISATOME*2

TATSUO KABATA*2

KENICHIRO KOSAKA*3

The SOFC (Solid Oxide Fuel Cell) is a fuel cell designed to work at high temperatures of 900 to 1000°C. A huge variety of fuels
can be used for the SOFC, including coal. Regardless of the scale, the SOFC achieves a high power generation efﬁciency even witha single unit, in comparison with a conventional thermal engine. Because the SOFC works at a high temperature, a combined
cycle with gas turbines can be adopted. The tubular-type SOFC developed by Mitsubishi Heavy Industries, Ltd. (MHI) is robustly
structured and suitable for systems with dynamic ﬂuctuations, such as combined cycle systems with gas turbines. It can generate
power with an efﬁciency of 70% or higher with natural gas fuel, and 60% or higher with coal fuel. With features such as theseworking in its favor, this SOFC may become a trump card for CO2 reduction.

1. Approach of MHIMHI has been developing cell materials, cell structures,
manufacturing technologies, and cell modules since in1984. As a first step towards a combined cycle with SOFCand gas turbines designed to take maximum advantage ofthe features of the SOFC from MHI, the New Energy andIndustrial Technology Development Organization (NEDO)
commissioned MHI to develop a “High-Efﬁciency Combined
Cycle System with a Tubular Type SOFC” in FY 2004. MHI
worked on the project, developing and demonstrating a cell
technology and control technology by the end of FY 2005.
MHI then began manufacturing a 200kW-Class Combined
Cycle System (Fig. 1) by combining the tubular type SOFCand a micro gas turbine (MGT), from FY 2006.

2. Features of combined cycle system

is installed upstream of the combustor of the gas turbine.
First , the f uel is fed into the SOFC a nd the chemicalenergy is directly converted into electric power. Next, allof the remaining fuel is fed to the gas turbine and used forpower generation. Air, meanwhile, is pumped up to a highpressure by the gas turbine, supplied to the SOFC for use
as an oxidizing agent, and sent to the gas turbine with hightemperature exhaust heat. Even the sensible heat andpressure of the hot pressurized air are used as part of theheat source in the gas turbine, and both are converted into
electric power. A high power-generation efficiency in thewhole system is thus realized.

System schematic diagram
SOFC

In the SOFC-MGT combined cycle system, the SOFC

City gas
Recirculation
blowerInverter
Micro gas turbine
Exhaust air
SOFC pressure vessel

Air

Exhaustfuel

Combustor
Microgas turbine
Recuperator

Fig. 1 200kW-class SOFC-MGT combined cycle system
The world’s largest-class pressurized combined cycle system
combined with a micro gas turbine.
*1 Power Systems Headquarters*2 Nagasaki Shipyard & Machinery Works*3 Nagasaki Research & Development Center, Technical Headquarters33
33

Exhaust gas

Fig. 2 Main systematic diagram of the 200kW-class SOFC-MGT
combined cycle system
Conﬁguration in MHI’s tubular-type SOFC system.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)

class SOFC-MGT combined cycle system. A power output
of 143 W was confirmed at the planned operation voltage(0.65 V/cell), and the maximum rated power was 151 W. The
performance did not deteriorate before or after the heat cycle
test (Fig. 4).

: Initial voltage: Voltage after heat cycle: Initial output: Output after heat cycle

Voltage/cell (V)

1.2

3. Development of the high-efﬁciency tubular-type
SOFC combined cycle system
3.1 Technology to realize the high-power cell tube
A high-power cell tube (Fig. 3) with improved cell tube
performance was developed for application to the 200kW-

180

1.0

150

0.8

1200.6900.4• Fuel : hydrogen• Temperature : 900ºC• Operating pressure : atmospheric• Length of cell tube : 1500 mm0.20.00200400600
800
Current density (mA/cm2)

60

Power output (W)In the tubular type SOFC from MHI, the exhaust fuelisn’t combusted in the SOFC module. Instead, the system
is conﬁgured to enable the separate removal of the exhaust
fuel and exhaust air (Fig. 2). In comparison with the system
in which the exhaust fuel is combusted in the module, thetemperature of the exhaust fuel and that of the exhaust air
are low at the SOFC module outlet, raising the temperatureof the gas turbine inlet derived from the combustion in thegas turbine combustor, and the gas turbine can be operated
under conditions of high efficiency and high power. The
separate removal of the exhaust fuel and exhaust air ismore beneficial, as high-temperature pipes impose major
restrictions in the design of large-capacity combined cycle
systems.

3001000

Fig. 4 Current/voltage characteristics of high-power cell tube
Veriﬁcation of maximum rated output of 151 W using a high-powercell tube.

Fig. 3 Appearance of the tubular type cell tube
Cell tube of 1500 mm length and 28 mm diameter.

Air supply to SOFC
Fuel supply to MGT
SOFC power generationMGT power generation

Power output (relative value to rated conditions (%))
Voltage (relative value to theoretical voltage (%))

110

MGT
generate

SOFC
generate

Fuel supply
to MGTAir supply
to SOFC

Fuel supply
to MGT

SOFC
generate

Trip

MGT
generate

100

Temperaturein the generation room

9060

1100
1000
900
800

80
70

Trip700

MGT
power output

50600
500

Voltage

400

4030300

SOFC power output

20200

10100

0

Jun.28 Jun.30 Jul.2

Jul. 4

Jul.6

Jul.8

Maximum
power outputof system:
75 kW
Temperature (ºC)

SOFC loadrun-back

Air supplyto SOFC

Jul.10 Jul. 12 Jul.14 Jul.16

0

Jul.18 Jul.20 Jul. 22Fig. 5 Veriﬁcation test for the SOFC-MGT combined operation
Stable power generation operation can be achieved in a state of combined cycle operation. A maximum combined cycle power
output of 75 kW has been veriﬁed.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)34

3.2 Low calorie fuel burning MGT
The gas turbine for the SOFC combined cycle requires
a technology to stably combust low-calorie fuel (any fuelwith a heating value of one-tenth of city gas, or less) after
consuming a major part of the chemical energy in the SOFC.
MHI designed and manufactured a low-calorie combustor
for the SOFC combined cycle by applying a technology for
combusting low-calorie gasses such as blast furnace gas,a technology cultivated for MHI’s industrial gas turbines,
to the combustor for the MGT. Operation tests with thiscombustor installed on the MGT confirmed that stable
operation can be achieved even with a low- calorie gas
simulating the SOFC exhaust fuel.

Sub-module voltage (V)

500

: #1
: #2
: #3: #4: #5400300200100020406080 100120 140 160 180 200

Sub-module current (A)Fig. 7 Single sub-module performance of the modiﬁed module
Adequate performance of each sub-module has been conﬁrmed.

3.3 Operation test for the SOFC-MGT combined
cycle system
An operation test on the 40kW-class module was carriedout with SOFC single body. Afterwards, the SOFC 40kWclass module that had been used for the operation test
was connected to the MGT on which the low-calorie gas
combustor was installed, to perform a verification test on
combined operation. Thus, we succeeded in the SOFC-gas
turbine combined cycle for the ﬁrst time ever in Japan. The
veriﬁcation operation conﬁrmed that power could be stably
generated during combined cycle operation, with maximum
combined generated power of 75 kW (Fig. 5).
3.4 Development of the modiﬁed sub-module
The cartridge is the minimum unit used to supply the fueland air, and to collect the electric power. Each cartridge ismade up of an assembly of 104 cell tubes, and four cartridges
make up one sub-module. The sub-modules are installed in
the pressure vessel, for the pressurized operation, makingup one module. The modules are conﬁgured to easily enable
increases in the unit capacity by allocating the sub-modules
in the longitudinal direction of the pressure vessel (Fig. 6).
We performed the modification design to homogenize
the temperature in the module generation room. T he
configuration adopted enables the fuel to flow downwards
inside the cell tube, and the air to ﬂow upwards outside thecell tube. Heat is exchanged between the hot exhaust air andcold fuel in the upper part of the cell tube, and between thecold air and hot exhaust fuel in the lower part of the cell tube.
Thus, the power-generation part can be maintained at a hightemperature. In the modiﬁed module, the thermal insulationof the upper part has been enhanced, and the amount ofheat dissipated from the generation room to the outside
has been suppressed. In parallel, the heat transfer surfaceof the upper part of the cell tube has been increased, andthe temperature of the upper part of the cell tube has beenraised. In the lower part of the cell tube, the air temperature
has been raised by increasing the amount of heat exchanged
from the hot exhaust air outside the generation room. The
temperature in the lower part of the cell tube has been raised.
In this way, the temperature of the upper part and the lower
part in the generation room have all been raised as well, andwe sought to homogenize the temperature distribution in thegeneration room.
We m a nu f a c t u r e d t h e m o d i f i e d s ub - m o du l e s fo r
application to the 200kW-class SOFC-MGT combined cycle
system, and carried out a single unit performance test
(Fig. 7). The sub-modules are conﬁrmed the effect of thehomogenized temperature distribution in the generation
room. They performed adequately.

Pressurevessel

Cell

Cartridge

Sub-module

ModuleFig. 6 Structure of module
The cartridge conﬁguration adopted for the module has excellent reliability
and expandability.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)35

plant, in the future. A net power generation efficiency ashigh as 70% (LHV) or more can be achieved with a natural
gas fired several-hundred-MW-class SOFC + gas turbine +steam turbine combined cycle system (see the ﬁgure at thebeginning of this report). It will even be possible to achievea net power generation efficiency as high as 60% (LHV) or
more with a several-hundred-MW-class coal gasiﬁer (IGCC)+ SOFC + gas turbine + steam turbine combined cyclesystem. Thus, power producers may be able to play this new
SOFC as a trump card for CO2 reduction.
As a market for the first practical application, MHI has
been considering a middle-capacity SOFC-MGT combinedcycle system (several hundred kW thru several MW ) for
application to the natural gas and partial topping system
in which a relatively small- capacit y SOFC is inst alled
upstream, with countermeasures for efﬁciency improvementof the existing gas turbine combined cycle system. We willcontinue to develop cell technologies in pursuit of improved
reliability and low cost, together with a technology to realize
the system, with an unf lagging commitment to realize
commercial products in the future.
This technology has been established under commissioned
research and joint research work. We would like to express
our thanks to the commissioning organization and joint
research parties.

SOFC pressure vessel

Micro gas turbine

Fig. 8 Appearance of the 200kW-class SOFC-MGT combined
cycle system
The 20 0 kW- class SOFC -MGT combine d cycle system was
manufactured

3.5 200kW-class SOFC-MGT combined cycle system
Based on the cell development , cont rol technolog ydevelopment, and performance plan until now, we designed
and manufactured the main body of the 200kW-class SOFCMGT combined cycle system (cell tubes, cartridges andmodules) a nd per ipheral components, a nd con f ir med
performance by a trial run (Fig. 8).
The net power generation efficiency of the total system
was 50% (LHV) at the planned operation point of the 200kWclass SOFC-MGT combined cycle system.

4. Approach for practical application
In planning out its product concept for the ultimate SOFC,
MHI seeks to develop an SOFC-gas turbine-steam turbine
combined cycle system consisting of an SOFC and gas
turbine-steam turbine combined cycle system in a large-scalecommercial thermal power generation plant. This system,
as MHI envisions it, will be fully capable of extracting thehigh-efficiency power generation provided by the SOFC,
and adoptable as an alternative to a large-scale thermal

Tadashi Gengo

Yoshinori Kobayashi

Yoshimasa Ando

Nagao Hisatome

Tatsuo Kabata

Kenichiro Kosaka

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)36