Department of Environmental & Natural Resources Management
George N. Prodromidis & Frank A. Coutelieris
EXERGY describes the MAXIMUM USEFUL WORK
Energy = exergy + anergy
eph
physical exergy expresses the useful work obtainable when the
chemical species are brought from the state of the system to the environmental
state through physical processes involving only thermal interactions.
 T
 T CP
 P   
e ph  m   CP dT  T0  
dT  R ln    
T
 T0 T
 P0   
 0
ech
chemical exergy expresses the produced useful work when the
chemical species are brought in chemical equilibrium with their environment.

ech  mtot   xi  e0 i  R T0
 i

i

xi ln  xi  

FUEL CELL is a device that directly converts chemical energy of
fuel to electricity
H2
Air
H2
Electrolyte
Anode
c
O2
Cathode
Hydrogen
H2O
H2 + ½ O2 ---> H2O + Wel +Q
Wel
Hydrogen
Production
Hydrogen Storage
Fuel Cell
Q
energy
safety
Wel
Fuel Storage
Reforming
H2 Production
Fuel Cell
Q

To propose a detailed thermodynamic model
(THERMAS) on the optimization of SOFC-based power
plants.

To incorporate the fundamental exergy approach in a
mathematical simulation model.

To study the viability of such systems by using OPF
(OPtimization Factor).

To conclude on SOFC-based systems supplied by low
hydrocarbon content fuels.
BIOGAS is a multi-gas mixture produced under the eco-friendly way
of anaerobic digestion
Animal
wastes
Human
wastes
Anaerobic
digester
Biogas
• CH4
• CO2 (15%-45%)
• H2O (5%-15%)
Crops

The composition of the final biogas fuel depends on the organic source
and the duration of the biological process.
1.
Several biogas compositions were chosen as inlet fuel in a
specific SOFC-based power plant.
2.
The real life operation of this system was simulated, through
a thermodynamic model (THERMAS) based on energy and
exergy theory, in computational environment.
3.
Also this simulation tool integrates an extended parametric
analysis in each case study under real life conditions without
any theoretical restrictions.
4.
The optimization process was finalized in terms of an
innovative optimization factor (OPF) for each simulated fuel
scenario.
5.
Comparison among them examines the sustainable
exploitation of low hydrocarbon over purified fuels.
EXERGY BALANCE
is appropriate to be satisfied per device
 
 


e
Q
e
  ech  mtot T  xi CP i    e    ech  mtot T  xi CP i    I  Wel
i
i
  in
  out
 k 
 k 
 
 

eQ is an exergy term associated with the heat transfer.

I is the irreversibility rate associated
with the heat losses which describes the
exergy destruction.
Wel      H prod  T S prod    H react  T S react  
i

Wel is the electric load produced by SOFC.

H is the enthalpy and S the entropy terms.
OPTIMIZATION CRITERION
of THERMAS Modeling
OPF   nen  nex  100, with -100<OPF  100 and nen  0

OPF (OPtimization Factor) describes the difference between the entire
amount of produced energy and the useful one.

nen is the energy and nex the exergy efficiency terms.

Optimal systems can be characterized by an almost zero OPF.

The upper limit describes a heating system (i.e. a wood burning stove).

The lower limit represents a system with initial energy potential,
transformed totally into useful work.
•
The water vapor content = ~10%.
• The extension of electrochemical oxidization reaction >= ~ 15%.
99% CH4
80% CH4
70% CH4
60% CH4
40
Linear (60% CH4 )
 The influence of CO on
electricity production and
OPF value is of low
30
OPF
importance.
20
10
0
15
10
5
0
Extension of electrochemical oxidization of CO (%)
•
WGS reaction happens in the bulk phase of the reformer.
• The water vapor content = ~10%.
• The extension of WGS reaction >= ~95%.
99% CH4
80% CH4
70% CH4
60% CH4
40
 For almost pure CH4 while
the WGS extension drops from
95% up to 30% the OPF
value decreases by 6%.
30
OPF
Linear (60% CH4 )
20
 For non-purified fuels in
CH4 the OPF value slightly
decreases only by 2%.
10
0
95
70
50
Extension of WGS reaction (%)
30
•
•
•
Both reactions influence the OPF value under the same trend.
These reactions have a significant impact on the system overall
behavior.
By lowering separately each reaction’s extension, while the other
parameters of the system are kept as constant, OPF becomes
remarkably better.
40
99% CH4
OPF
30
 Pure methane scenario presents
80% CH4
an improved behavior on OPF
70% CH4
value at about 21%.
20
60% CH4
10
Linear
(60% CH4)
 By lowering the purification of
biogas in methane this improvement
on OPF is limited to approx. 13%.
0
90
70
50
30
Extension of reforming reaction (%)

Use optimized systems instead of over-sized!

Poor fuel compositions might be successfully used in
optimized plants.

Exergy analysis provides more reliable results,
compared with energy one.
Department of Environmental &
Natural Resources Management
Thank You
For Your
Attention