 Open Access
 Total Downloads : 1297
 Authors : Mukesh Kumar Baliwal, Dr.A.Bhargava, Mr. S.N. Joshi, Sunil Kumar
 Paper ID : IJERTV2IS80832
 Volume & Issue : Volume 02, Issue 08 (August 2013)
 Published (First Online): 29082013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Modeling and Simulation of Solid Oxide Fuel Cell Based Distributed Generation System
1Mukesh Kumar Baliwal, 2Dr.A.Bhargava, 3Mr. S.N. Joshi,4Sunil kumar
1,4M.Tech Scholar (Power Systems), Dept. of Electrical Engineering, UCERTU Kota (Rajasthan)
2Associate Professor, Dept. of Electrical Engineering, UCERTU Kota (Rajasthan)
3Assistant Professor& HOD, Dept. of Electrical Engineering, GWEC, Ajmer (Rajasthan)
Abstract Due to ever increasing energy consumption, rising public awareness of environmental protection and steady progress in power deregulation alternative (i.e. renewable and fuel cell based) DG system has attracted increased interest. Fuel cell systems show great potential especially in the area of DG due to their fast technological development and merits, such as, high efficiency, zero or low emission (of pollutant gases) and flexible modular structure. This paper describes dynamic modeling and simulation results of SOFC based Distributed generation system. The SOFC is modeled individually and latterly integrated to the grid. Dynamic model of SOFC is developed with the help of MATLAB / SIMULINK software. Simulation studies have been carried out to verify the system performance under fault condition.
Keywords Fuel cell, Distributed generation, inverter,Solid oxide fuel cell (SOFC)

INTRODUCTION
Distributed generation is referred in general to small generators, starting from a few kW up to 10 MW, whether connected to the utility grid or used as stand alone at an isolated site. Normally small DGs, in the 5 250 kW range serve households to large buildings [2]. DG technologies can be categorized to renewable and nonrenewable DG. Fuel cells based DG system is considered an alternative to centralized power plants due to their nonpolluting nature, high efficiency, flexible modular structure, safety and reliability. At present, they are under extensive research investigation as the power source of the future, due to their characteristics. A fuel cell converts chemical energy directly to electrical energy through an electrochemical process. As opposed to a conventional storage cell, it can work as long as the fuel is supplied to it. There are many motivations in developing this method of energy generation and it needs further
development to have a realistic system analysis combining various subsystems and components [1].
The integration of the fuel cell system is to provide the continuous power supply to the load as per the demand. In the fuel cell energy system which is used for the distributed generation applications, the source is integrated with the DC DC boost converter to stabilize the voltage from the fuel cell. The output of the boost converter is then fed to the three phase PWM inverter to get the three phase ac voltage for the grid connected applications. The overall block diagram of the fuel cell energy system is shown in figure 1.[2]
Fig. 1: Block Diagram of Fuel cell energy System

SOFC
Solid oxide fuel cell is based on the concept of oxide ion migration through an oxygen ion conducting electrolyte from the oxidant electrode (cathode) to fuel electrode (anode) side. It operates at temperatures in the range of 600 1000ÂºC, which makes them highly efficient as well as fuel flexible. In case of SOFC the electrolyte is a dense solid that involves ceramic materials like Yttriumstabilized zircon dioxide whose function is to prevent electrons from crossing over while allowing passage to the charged oxygen ions [6].
Fig. 1: Schematic diagram of a SOFC.
The chemical reactions that take place inside the SOFC which are directly involved in the production of electricity are as follows [2]
Taking the Laplace transforms both side and gives the expression for partial pressure of hydrogen
1
At anode
k
H
H
p 2
qin

2k I
(10)
2H2
2O2 2H O 4e
(1)
H2 1 s H2
H
H
2
r fc
2
2
2
2
2CO 2O2 2CO
At cathode
4e
(2)
In similar way, the partial pressure of oxygen and water is given by
1
2
2
2
2
O 4e 2O
(3)
k
O
O
O
O
q
q
H2
H2
p 2 in
2
2
1O s

kr I fc
(11)
Overall cell reaction:
2H2 O2 2H2O
(4)
2
1
p kH2O 2k I
(12)
2
2
H2O
1
H Os
r fc


MODELING OF SOFC
The following assumptions are made in developing the mathematical model of fuel cell stack. The gases considered are ideal, that is, their chemical and physical properties are not corelated to the pressure. Nernst equation is applicable and fuel cell temperature is constant at all times. The ideal gas law is used to
Calculation of Stack Voltage
The expression for stack output voltage Vfc of a fuel cell can be obtained applying Nernsts equation and also taking into account the voltage losses such as the Ohmic, Activation and mass transportation (concentration) losses as:
calculate the partial pressure of all the gases. For
Vfc Efc Vact Vconc Vohmic
(13)
hydrogen is given by.
2 2
2 2
pH Van nH RT
(5)
The value of the Nernst voltage equation (Efc) is found from Nernst equation
Taking the derivative of the equation (5) w.r.t.
time
E N E0
RT p p0.5
H O
H O
2
2
ln 2 2
d p
d nH2 RT
fc 0
fc 0
(6)
2F
pH O
(14)
dt H2
dt
Van

Calculation of Voltage Losses
The hydrogen molar flow is further divided into three parts and their relationship can be expressed as follows

Activation voltage losses
The reason for this loss in SOFC is the sluggishness of chemical reaction that takes place
d p
RT
qin

qout qr
(7)
on the surface of electrodes. A certain amount of
H2 H2 H2 H2
dt Van
According to the electrochemical relationship, the quantity of hydrogen that react is given by.
voltage produced by fuel cell is lost in carrying the reaction forward that transfers the electrons to or from the electrode. Activation losses are estimated using Tafel equation [2].
q
q
r N0 I fc
H2 2F
2kr I fc
(8)
Vact B ln i


Concentration voltage losses
(15)
Substituting (8) in (7), the time derivative of hydrogen partial pressure can be expressed as
V
V
d RT
These losses are also known as mass transport losses and are caused due to the reduction in concentration of reactants in the region of electrode as the fuel is consumed. The consumption of reactants at respective electrodes, i.e. hydrogen at the anode and oxygen at
the cathode leads to a slight reduction in
p
qin

qout 2k I
(9)
concentrations of the reactants. Due to the reduction in
dt H2
H2 H2
an
r fc
concentrations, there is a drop in partial pressure of
gases which will result in a reduction of voltage that portion of the electrode can produce [2].
In this model, constant utilizatio mode is considered. The fuel utilization is defined as the ratio between fuel
Vconc m expni


Ohmic voltage losses
(16)
flow that reacts and the fuel flow injected to the stack and is expressed as:
U
U
qr
These losses in SOFCs are caused due to the resistance
both to flow of electrons through the electrodes and to the migration of ions through the electrolyte. In
H2
q
q
f in
H2
(18)
addition, the fuel cell interconnects or bipolar plates also contribute to the Ohmic losses. Ohmic loss is given by
V rI
The fuel utilization ranging from 0.8 to 0.9 yields
better performance and prevent overused and underused fuel conditions. The optimum utilization factor assumed for this model is 0.85.
ohmic fc
(17)


Implementation of SOFC Model in Simulink
/MATLAB
The SOFC model is based on the expression for partial pressures of hydrogen, oxygen, water, Nernsts voltage, Ohmic loss, activation loss and mass transportation loss. A comprehensive dynamic model of a SOFC has been developed and simulated in the MATLAB / Simulink as shown in Fig. 3 and V I characteristic of SOFC in fig.4.
Pref

SIMULATION MODEL OF SOFC BASED DG SYSTEM
SOFC one of the most developed fuel cells show great promise in stationary power generation applications. In isolated mode FCs based DG system can be used to supply power to remote areas or supply power during grid failure. The system may be supported by batteries or capacitors or other energy storage devices [8].
The complete model of SOFC based DG system with power electronics threephase resistive load is shown
ef
ef
Pref
Ir
1 Ifc
[Ifc]Goto
in Fig. 5. The individual component modeling of
Vfc Vfc
Divide
Te.s+1 Transfer Fcn
2 K
Product
2*Kr*If c
2*Kr*If c
1 qH2
1/KH2 TH2.s+1
Transfer Fcn2
PH2
f(u) Fcn
Product2
SOFC is given in this paper. The three phase inverter, pulse generator, PWM and AC filter available in
constant Gain Uopt
Uopt
Divide1
Tfc.s+1
Transfer Fcn1
rHO
.
Divide2
qO2
1/KO2 TO2.s+1
Transfer Fcn4 1/2 Gain1
PO2
F
R
2F
F
R
2F
Divide3
R
Add
E0
E0 N0
Product3
Scope
Simpower System of the MATLAB has been used.
1/KH2O PH2O TH2O.s+1
Transfer Fcn5
T Product1 T
i
N0
[Ifc]r From
r
Ohmic
Product4
Subtract
Discrete,
= 5.14e006
Discrete,
= 5.14e006
powergui
Pulses
current density B
B
Product5
Activation
eu
Concentration
L
+ i DC bus current
Inverter
n Product6 Math Function
Product7 –
g
n
m VDC+
m
Diode2
+ aA A a c
A
A
A c
46
45.5
45
Voltage (V)
Voltage (V)
44.5
44
Fig. 3: Simulink diagram of a dynamic model of SOFC
VI characteristic of SOFC
g
g
IDC
SOFC
VC
E
E
C
C
PWMIGBT
B bB
– C cC

b aB
C
C

c b
A B C
A B C
c
A
A
B
B
C
C
AC filters
43.5
43
42.5
10 15 20 25 30 35 40 45 50
Current (A)
Fig. 4: V I characteristic of SOFC
Fig. 5: Simulink model of SOFC based DG system
The output inverted voltage and current waveform of SOFC based distributed generation system are shown in fig. 6.
4
4
Va
Vb
Vc
Va
Vb
Vc
2 x 10
Voltage(V)
Voltage(V)
1
0
Output Voltage waveform of SOFC plant
Table 1: values of voltage and current without and with fuel cell plant under normal and faulty condition
Voltage
Without fuel cell plant
With fuel cell plant
V (nom.)
11.03Kv
11.04Kv
I (nom.)
941.9A
933.1A
V (fault)
7.813Kv
7.821Kv
I (fault)
7283.33
Amp.
7250.85
Amp.
Voltage
Without fuel cell plant
With fuel cell plant
V (nom.)
11.03Kv
11.04Kv
I (nom.)
941.9A
933.1A
V (fault)
7.813Kv
7.821Kv
I (fault)
7283.33
Amp.
7250.85
Amp.
1
20 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
time(sec)
0.5
Current(A)
Current(A)
0
0.5
Output Current waveform of SOFC plant
Ia Ib Ic
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Time(sec)
Fig. 6: Output voltage and current waveform of SOFC plant


Fuel cell Plant connected to the grid during LG Fault
Discrete,
= 5.14e006
pow ergui
L
–
–
+ i DC bus current
Inverter
g
Pulses
IDC
VDC+
VC
g
g
E
E
C
C
PWMIGBT
Diode2
+
A
v
v
+ B
– – C
VM2
aA A a
bB B b
c C C c

A

B
c A
c aB
b
A B C
A B C
c C
AC filters
Fig.8 shows voltage and current waveforms when with
SOFC
A a b

C
A B C
A B C
A B C
A B C
Display3
fuel cell plant to the grid during LG fault, respectively. Single line to ground fault takes place on he grid during time period t=0.1 to 0.3 Sec. During
A b
N B B b
C C c
A B C
A B C
A B C
A B C
bcA bc aB b
Display4
vgrid
ThreePhase Source1
A
A B C
A B C
B
fault we have analyzed the parameters such as voltage, current and checked the system stability. It is clear
from the above fig.8 that voltage profile is
ThreePhase Programmable Voltage Source1
ThreePhase Transformer (Two Windings)1
cC C
considerably improved after fuel cell plant interconnected with the grid. The various data of voltage and current are shown in table 1. After
ABC
ABC
Fig. 7: Simulink model with fuel cell plant during LG fault
connecting the fuel cell plant system to the existing system we can say that power system stability is being
4
x 10
Va
Vb
Vc
Va
Vb
Vc
1.5
Voltage waveform on the grid when fuel cell plant is connected during LG fault
improved.
1
Voltage(V)
Voltage(V)
0.5
0
0.5
1
1.5
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
time(sec)


CONCLUSION

This paper shows the impact of fuel cell power system on the stability of power system. The dynamic modeling and simulation results of a fuel
1
0.5
Current(A)
Current(A)
0
0.5
1
4
x 10
Current waveform on the grid when fuel cell plant is connected during LG fault
cell based power system which consists of solid oxide fuel cell (SOFC) for power generation. The SOFC modeled individually and latterly integrate in Matlab/Simulink software. The developed Simulink model of fuel cell system is then
Ia Ib 

Ic 

Ia Ib 

Ic 

1.5
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Time(sec)
Fig. 8: voltage and current waveform with fuel cell plant during LG fault
connected to 11Kv grid through an AC bus. . Simulation studies have been carried out to verify the system performance under faulty condition.
Simulation results show that after combining fuel cell system the systems stability is considerably improved as compared to using just fuel cell power.
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Mukesh kumar Baliwal MTech. scholar University College of Engg. Kota,MailID received B.Tech Degree from RTU, Kota currently pursuing M.tech and area of research in Renewable Energy Sources And Distributed generation.
Sunil Kumar MTech. scholar University College of Engg. Kota, MailID received B.Tech Degree from RTU, Kota currently pursuing M.tech and area of research in Renewable Energy Sources And Power quality.