Ishihara T. (Ed.) Perovskite Oxide for Solid Oxide Fuel Cells (Fuel Cells and Hydrogen Energy)

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oxides are found to be suitable and widely used as cathode materials in a range

of intermediate- to high-temperature fuel cells.

9.2.3.1 Strontium-Doped Samarium Cobaltite

Strontium-doped samarium cobaltite (SSC) is selected as the air electrode

material of our standard cells. The slurry of Sm

0.5

CoO

3–d

powder is screen

printed onto the electrolyte disk with an already sintered anode. The final

sintering of the cathode is performed in air at a temperature range of

10008–12008C for 3 h. The average thickness of the porous cathode is about

30–50 mm. Figure 9.3(b) shows the electron microscope imag e of the air-fired

microstructure of the air electrode of our standard cell. It has been demon-

strated that the cathodic overpotential of the cell is very small [16].

9.2.3.2 Lanthanum-Doped Bar ium Cobaltite

The A site of barium cobaltite (BaCoO

) is partially substituted by lanthanum

ions to improve the catalytic and electrical properties of this particular oxide.

We have performed a series of performance measurements by varying the

lanthanum content between 30 at% and 50 at% and determined the optimal

value as 50 at%, that is, an equal amount to the barium content on the A site of

the barium cobaltite. Figure 9.5 shows a comparison between I-V-P character-

istics of cells made up of two different cathode materials, namely,

0.5

CoO

3–d

(BLC) and SSC measured at 7508C using pure hydrogen as

fuel. It is found that the cells made up of the BLC cathode have a very similar

performance to those utilizing SSC as a cathode material [17].

0.2

0.4

0.6

0.8

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Current Density (A/cm

)

Terminal Voltage (V)

0.1

0.2

0.3

0.4

0.5

0.6

Power Density (W/cm

)

BLC

SSC

Fig. 9.5 I-V-P

characteristics of single cells

using strontium-doped

samarium cobaltite (SSC)

and Ba

1–x

CoO

3–d

(BLC)

as cathode

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

189

9.3 Stack Development

In fuel cells, as in batteries, electrochemicall y active cells should be electrically

connected in series to produce usable power. Stacking is the term most com-

monly used for connecting cells in electrical series for fuel cells. Bas ically, the

stack de sign addresses the delivery of gaseous species to the cells together with

loss-free collection of the electrical current and management of heat. These, in

principle, have direct influence on the electrochemical performances of the

individual cells. Successful heat management, on the other hand, is critically

important for the structural integrity of the components used in a stack

assembly.

Compared to alternative s tacking configurations proposed for planar cells,

interconnecting cells without the need for using seals offer a great deal of

simplicity [18]. Our disk-type cell stacking concept exploits the sealless design

concept and consists of a number of repeat units, each of which is made up of

only cells, porous current collectors, and metallic separators. F igure 9.6

illustrates the conceptual drawing of the single cell stack unit. Air and fuel

gas are supplied to the unit through the channels inside the s eparators. The

openings at the center of separators serve as gas inlets to the porous current

collectors. The uniform flow regimen of gas along the radial direction over the

electrodes is maintained by t he help of the porous current collectors. The

thicknesses and the porosity values of the current collectors are carefully

optimized to obtain the highest rates of axial diffusion of gases. Ferritic

stainless steel is used as a material for the separator plates. As there are no

circumferential seals attached to the cells, the unutilized fuel and depleted air

are freely mixed and burned around the stack unit. The combustion heat is

further utilized for the heat management of the entire stack and the balance of

plant c omponents around it (steam genera tor, pre-reformer, and heat exchan-

gers, etc.).

Figure 9.7 shows the actua l cell stack unit assembly for the fourth-

gene ration 1- kW c lass module. The individual s eparator plates are designed

Metallic separator

Porous current collector

Cathode

Electrolyte

Anode

Air

Fuel

Fig. 9.6 Conceptual

illustration for the sealless

stack unit

190 T. Akbay

by using a rather unique concept of internal gas manifolding. Holes on

opposite corners serve as gas manifolds when separator plates are stacke d

by placi ng ceramic rings over them. Inne r gas channels in the separ ator

plate connect the gas manifolds to two openi ngs located at the center of

opposite sides of the separator. The fuel gas supplied from the central

opening flows through the anode-side current coll ector and undergoes

internal steam reforming and anode electr ode reaction. Meanwhile, the air

supplied from the opening on the opposite side of the separator flows

through the cathode-side current collector and takes part in the cathode

electrode reaction.

Apart from supplying gases to the cells and serving as e lectrical connec-

tion between individual cells, the separator plate in our stack design has an

additional functionality for isolating the compressive forces on the disk-

type cells placed at their center and the ceramic rings use d for the gas

manifolds. This unique functionality is accomplished by attaching flexible

arms to the separator plates. The manifold ends of the separator arms and

ceramic rings must be tightened by bolts and nuts to make hermetic seals.

On the other hand, the interconnec tion parts of the separators where cells

and current collectors are placed require certain levels of pressure to mini-

mize electric contact resistance between them. The milder load requirement

on the interconnection parts is mainly exerted by a wei ght at the top of the

cell stack.

The fourth- gener ation 1-kW cla ss stack s hown in Fig. 9.8 consists of

46 cells connected in electrical series. Electrically insulated clear hole

flanges are attac hed to the extreme ends of the stack and tightened by

using st ud bolts to fixate the entire assembly. The air inlet for the stack is

attached to the air manifold at the mid-height of the stack while the fuel

inlets are attached to top and bottom flang es that have built-in openi ngs to

the fuel manifolds. To enhance the heat exchange between the stack and

balance of plant components, an additional ra diator plate is insert ed at the

mid- height of the stack.

Porous current collector (air)

Cathode/Electrolyte/Anode

Porous current collector (fuel)

Metallic separator

Fig. 9.7 Cell stack unit

assembly for the fourth-

generation 1-kW class

module

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

191

9.4 Module Development

The previously mentioned sealless stacking concept is highly pertinent to elec-

trical power scaling by means of altering the number of cells. In other words, the

number of cells can easily be increased for higher power output per stack.

However, there is an intrinsic limit for the maximum number of standard-

sized (120 mm in diameter) cells that can actually be stacked due to the very

nature of the sealless stacking design. As can be imagined, the larger the number

of cells, the taller the stack, which exerts its own weight as increased amount of

pressure on the porous current colle ctors and cells toward the bottom of the

stack. To eliminate a possible mechanical failure in cells, we have decided to

build a generic-sized stack to be connected electrically in series for manufactur-

ing larger-sized SOFC power production units.

As illu strated in Fig. 9.9, we often use the term module for a DC power

generation unit composed of generic SOFC stack(s) together with all the rest of

the hot balance of plant components efficiently packed inside a thermally

insulated lining. The module utilizes desulfurized town gas and deionized

water for internal steam reforming. Air is preheated before entering to the

stack. The relative positions of the components inside the module are carefully

designed for optimal heat management.

9.4.1 A 1-kW Class Single-Stack Module

Single-stack modules capable of generating around 1 kW electrical power out-

put have been developed as test platforms and continuously evolved up to the

current design, named the fourth generat ion. In all generations, the internal

steam reforming concept is adopted for thermally self-sustained operation,

except for the first-generation module, which is designed for hydrogen fuel. In

each design iteration, the SOFC stack together with the balance of plant

Fuel inlet

Air inlet

Radiator plates

Fuel inlet

Fig. 9.8 CAD

representation of the fourth-

generation 1-kW class

SOFC stack

192 T. Akbay

components is optimized for obtaining higher electrical conversion efficiencies

and long-term stability.

A constant power output durability test of the fourth-generation 1-kW class

module is performed over 4200 h. The operating conditions are listed in Table 9.1.

The data recorded during the entire test period are plotted as a graph (shown in

Fig. 9.10). The operation was interrupted briefly at 1000 h to correct the erratic

behavior of the stack that started around 600 h. After the restart, the remaining

period of operation was trouble free. The electrical efficiency remained about

50% Higher Heating Value (HHV) throughout the test. The voltage degradation

after the restart was calculated as 0.5% per 1000 h.

A cyclic power output durability test is also conducted for more than 1000

cycles. The DC power output of the stack is alternately cycled between 100%

and 10% for the frequency of 6 cycles per hour (Fig. 9.11). The stack behavior

was rather stable, while the degradation rate for certain cells was slightly higher

than the constant power output test case.

Blower

Air

Water

Town gas

: BoP component

Burner or

Elect. heater

Exhaust heat

recovery unit

Deionizer

Desulfurizer

Fig. 9.9 Concept of the module

Table 9.1 Operating conditions for the durability test

of the fourth-generation 1-kW module

Fuel Town gas (13 A)

Total current 33.9 A

Current density 0.3 A/cm

Fuel utilization 71%

S/C 3.0

Maximum separator temperature 7908C

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

193

200

400

600

800

1000

1200

1400

1600

0 1000 2000 3000 4000 5000

Time (h)

Output Power (W), Power density(mW/cm

)

Electrical

Efficiency[LHV,HHV] (%),

Fuel

Uti

zati on (%),

Air Utilization (%),

Termainal Volta

e(V), Current (A)

Fuel Utilization (fixed)

Output Power

Electrical Efficiency [LHV]

Electrical Eficiency [HHV]

Terminal Voltage

Current (fixed)

Air Utilization

Power Density

Fig. 9.10 1-kW module durability test over 4200 h

100

120

140

8:20:00 8:30:00 8:40:00 8:50:00 9:00:00 9:10:00 9:20:00

Time

Output power [W × 0.1], Efficiency [%]

660

680

700

720

740

760

780

800

Tem

erature [°C ]

Efficiency (HHV)

Output Power Maximum Temperature

Maximum Temperature

Output power

Efficiency

Fig. 9.11 Cyclic durability test results for the 1-kW module

194 T. Akbay

9.4.2 A 10-kW Class Multi-Stack Module

Figure 9.12 depicts the conceptual drawing of the 10-kW class intermediate-

temperature SOFC module. The outer dimensions of the module are about 1 m

(W)  1 m (D)  2 m (H). A three- dimensional array of 16 generic stacks (2 

2  4) that are connected electrically in series is used to produce the output

power of 12.6 kW DC with a gross electrical efficiency of 50% (HHV) at an

operation temperature below 8008C. A flat-plate box-type steam reformer is

designed and positioned vertically between the stacks to keep its temperature as

high as possible. Fuel gas and air streams introduced to the module are passed

through dedicated plate-type heat exchangers before being distributed to the

SOFC stacks in the array. Start -up burners that utilize town gas are positioned

inside the insulator linings near the stacks to heat up the module. Table 9.2

summarizes the design specifications of the 10-kW class module.

Start-up burner

Heat exchanger

Cell stack

Reformer

Fig. 9.12 Conceptual view of a 10-kW class module

Table 9.2 Specifications of the 10-kW class module

Fuel Town gas (13 A)

Output power 12.6 kW DC

Electrical efficiency 50% HHV

Maximum separator temperature <8008C

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

195

A short-term performance test demonstrated that the 10-kW class module is

capable of meeting the design specifications for full load and partial load opera-

tions. Figure 9.13 shows the various data recorded during the test. The DC power

output of 12.7 kW is obtained under full load operation at 76% fuel utilization,

analogous to the electrical efficiency of 50% HHV. The operational character-

istics during the thermally self-sustained operation are listed in Table 9.3.

9.5 System Development

While demonstrating the 10-kW class module capability in meeting the DC

target specifications, a combined heat and power (CHP) system is developed for

utilizing the module within a co mplete system configuration [19, 20]. The

photograph shown in Fig. 9.14 is for the actual 10-kW class CHP system

installed at Rokko Testing facilities of The Kansai Electr ic Power Co., Inc.

The system consists of a 10-kW class SOFC module, a unit for gas and water

supply equipment, a unit for power electronics, a control unit, and an exhaust

heat recovery unit.

400

800

1200

1600

06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00

100

Partial load : 6.3kW

Partial

load:

3.2kW

Partial load : 2.0kW

50 HHV

Full load

12.7kW

Stack min.

temperature

Fuel utilization

Output power (DC)

Electrical

efficiency

400

800

1200

1600

06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00

Time

100

Electrical efficiency (%HHV)

Fuel utilization (%)

Partial load : 6.3kW

Partial

load:

3.2kW

Partial load : 2.0kW

50%HHV

Full load

12.7kW

Stack max. temperature

Stack min.

temperature

Fuel utilization

Output power (DC)

Electrical

efficiency

Output power (× 10W)

Stack temperature (°C)

Fig. 9.13 Operation data of the first 10-kW class module

Table 9.3 Typical performance of the 10-kW class module

DC output power (kW) 12.7

Average area-specific power density (W/cm

) 0.208

DC terminal voltage (V) 415

Efficiency at DC terminal (%HHV) 50

Fuel utilization (%) 76

Air utilization (%) 53

Average cell voltage (V) 0.77

Stack temperature (8C) 675–787

196 T. Akbay

The control unit of the system is specifically designed for automated start-up,

power generation, hot-standby, and scheduled or emergency shut-down pro-

cesses. For the power electronics, a DC/AC inverter is attached to the module

for grid connection. The heat recovery unit is attached to the module’s exhau st

to generate hot water at a temperature between 608 and 908C. The volumetric

capacity of the hot water tank is selected as 370 liters with a suitably sized water

pump. The condensed water recycling unit is attached to the system for possible

utilization of pure water for internal steam reforming.

An initial performance test is performed on the system to validate the design

specifications. The test results are summarized in Table 9.4. The AC power

output of the system is obtained as 10.1 kW wi th a corresponding AC electrical

conversion efficiency of 41% HHV. The overall syst em efficiency, on the other

hand, is recorded as 82% HHV when the module exhaust heat is recovered as

hot water at 608C. These results clearly demonstrate that an intermediate-

temperature SOFC CHP system of this size is perfectly capable of generating

electric power as well as quality heat with high efficiency figures.

Figure 9.15 shows the data obtained during the long-term stability test per-

formed over an accumulated period of 3400 h. The system is stably operated during

2.35m

SOFC

module

DC/AC

inverter

Heat

recovery

unit

Control

unit

2m1.5m

Fig. 9.14 The 10-kW class CHP system

Table 9.4 Test results of the 10-kW CHP system

Fuel Town gas (13 A)

AC output power 10.1 kW

AC electrical efficiency 41% HHV

Overall efficiency

82% HHV

Maximum separator temperature 7768C

CHP, combined heat and power.

Exhaust heat is recovered as hot water at 608C.

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

197

the comprehensive portion of the test period except for some unexpected electrical

trips caused by malfunctioning gas flow controllers and temperature sensing

system. The fluctuation in the overall system efficiency is attributed to a lag

between the heat recovery control and the module’s inlet water flow rate control.

9.6 Stack Modeling

As numerous parameters affect the operational characteristics of fuel cells, the

search for an optimum combination can be very difficult. In this respect, a

mathematical modeling app roach is widely used in fuel cell research and devel-

opment to perform various analyses ranging from estimation to design itera-

tion. Predictive models, however, are only useful when they are verified by

experimental evidence.

Among various levels of modeling, particular attention is paid to the cell and

stack models for supporting our stack development. In this section, the discus-

sion is be restricted to a standard computational fluid dynamics (CFD) analysis

coupled with rigorous elect rochemistry of the generic 34-cell stack. The stan-

dard approach includes solving the governing equations of mass and momen-

tum conservation (Navier–Stokes) together with energy, species, and charge

transport phenomena. Chemical and electrochemical reaction kinetics are also

included to evaluat e necessary source or sink terms for the transport equations.

A commercially available CFD package is used for solving the coupled set of

transport equations [21]. Detailed explanation of the multiphysics model, which

is out side the scope of this section, may be found elsewhere [22, 23].

100

200

300

400

500

600

700

800

900

Average Stack Temperature (°C), Voltage (V)

Overall Efficiency

AC Output Power

AC Efficiency

DC Output Power

DC Current

DC Voltage

DC Efficiency

Fuel Utilization

Ave. Stack Temp.

srh915srh055srh492srh7361

Output Power (kW), Efficiency (% HHV),

Fuel Utilization (%), Current (A)

0 500 1000 1500 2000 2500 3000 3500

Time (h)

Fig. 9.15 Long-term stability test of the 10-kW CHP system

198 T. Akbay