EXERGY ANALYSIS OF A SOFC BASED COGENERATION SYSTEM FOR BUILDINGS
Can Ozgur [email protected]
Ibrahim Dincer, [email protected]
Feridun Hamdullahpur, [email protected]
2008 ASHRAE Winter Meeting New York City, January 19-23
Outline Introduction Cogeneration SOFC
Literature Survey Objective of this Study Proposed System Configuration SOFC Modeling Exergy Analysis Results Conclusions
Introduction - Cogeneration Also known as combined heat and power. Used to increase the fuel utilization efficiency. Classified according to the prime movers: Gas turbines Steam turbines Reciprocating engines Combined cycles Micro turbines Stirling engines Fuel cells: Molten Carbonate Fuel Cell (MCFC) Solid Oxide Fuel Cell (SOFC)
Cogeneration in Building Applications Some building applications suitable for cogeneration include: Hospitals Institutional buildings Hotels Office buildings Single and multi-family residential buildings
Electrical load 1 MW≥
Introduction - SOFCs High temperature fuel cell (500-1000 C) Application areas: Stationary power and heat generation Transportation applications Portable applications
Advantages: No need for precious metal electrocatalysts Fuel flexibility Internal reforming Good thermal integration with other systems
Disadvantages: Degradation due to carbon deposition and sulphur poisoning Challenges with construction and durability
Introduction – Operation of SOFCs
Load
H2 H2
O2- O2-
H2O H2O e- e- e- e-
O2 O2
e- e- e- e-
Anode
ElectrolyteCathode
Introduction – Classification of SOFCsClassification criteria Types
Temperature levelLow temperature SOFC (LT-SOFC) (500 C – 650 C)Intermediate temperature SOFC (IT-SOFC) (650 C – 800 C)High temperature SOFC (HT-SOFC) (800 C - 1000 C)
Cell and stack design
Planar SOFC (Flat-planar, radial-planar)Tubular SOFC (Micro-tubular, tubular)Segmented-in-Series SOFC (or Integrated-planar SOFC)Monolithic SOFC
Type of support
Self-supporting (Anode-supported, cathode-supported, electrolyte-supported)External-supporting (Interconnect supported, porous substrate supported)
Flow configurationCo-flowCross-flowCounter-flow
Fuel reforming typeExternal reforming SOFC (ER-SOFC)Direct internal reforming SOFC (DIR-SOFC)Indirect internal reforming SOFC (IIR-SOFC)
System Level Modeling Classical approach Energy analysis of the system A better approach Exergy analysis of the system Literature survey on modeling of SOFC based systems: Colpan et al. (2007): Thermodynamic modeling of a DIR-SOFC with
anode recirculation and operating with syngas. The effect of recirculation ratio and fuel utilization ratio on cell voltage, power output, electrical efficiency and air utilization ratio are investigated.
Granovskii et al. (2007): Exergy and energy analyses for the two SOFC–gas turbine systems. Their efficiencies and capabilities to generate power at different rates of oxygen conductivity through the SOFC electrolyte (ion conductive membrane), as well as various efficiencies for natural gas conversion to electricity in the SOFC stack are determined.
System Level Modeling Literature survey (Continued): Ghosh and De (2003): The effect of pressure ratio and temperature
on the exergy destructions and exergetic efficiencies of an integrated gasification combined cycle with a high-temperature pressurized SOFC in the topping cycle and a single-pressure, non-reheat steam in the bottoming cycle.
Douvartzides et al. (2003): The effect of operation parameters on exergy destructions and losses within an ethanol-fueled SOFC system including an external steam reformer, an afterburner, a mixer and two heat exchangers.
Calise et al. (2006): A full and partial load exergy analysis of a hybrid SOFC–GT power plant.
Objective of this Study Analyze the exergetic performance of a SOFC based
cogeneration system which may supply the heat and power demand of a neighbourhood. The following are calculated: Exergetic destructions in the components Exergy loss to the surroundings Exergetic efficiency of the system Effect of ambient temperature on the performance
System Configuration
Fuel Compressor
Air Compressor
1
2
3
4
5
6
8
7
9
SOFCStack
Combustor
Gasturbine
Recuperator
DS
10
12
11
A
BDC
Heat Recovery Steam Generator
Input DataFuel MethaneEnvironmental temperature 25 °CEnvironmental pressure 1 atmNet electrical work output of the system 1 MWSOFC
Exit Temperature 1000 °CTemperature difference between exit and inlet
100 °C
Pressure 15 atmOperating voltage 0.7 VActive surface area of a single cell 100 cm2
Fuel utilization ratio 0.85Thickness of anode 50 µmThickness of electrolyte 150 µmThickness of cathode 50 µmThickness of interconnect 5 mm
HRSG (Heat Recovery Steam Generator)Steam drum pressure 12 barPinch point 10 °CEvaporator approach temperature 10 °CCondensate return temperature 25 °CHeat loss from HRSG 2%Pressure drop on the air side 5%
Gas Turbine Pressure ratio 5:1Isentropic efficiency 0.85Electric generator efficiency 0.98
Isentropic efficiency of compressors 0.85
SOFC Model STEPS:1. Derive the equilibrium exit and
inlet gas compositions in terms of recirculation ratio and other cell parameters.
2. Solve the chemical equilibrium reaction equations together with the fuel cell related equations.
3. Considering ohmic, activation and concentration polarizations, calculate air utilization ratio by solving the energy balance enclosing the fuel cell.
4. Calculate cell voltage, power output and electrical efficiency of the cell.
For more details, please see Colpan et al. (2007).
SOFC Model – Carbon Deposition
OHCHCO
CCOCO2
H2CCH
2)s(2
)s(2
2)s(4
+⇔+
+⇔
+⇔
( )22Heq
4CHeq4
4cx
xKa
⋅=
( )2CO
eq
2COeq5
5c x
xKa
⋅=
O2Heq
2Heq
COeq6
6c x
xxKa
⋅⋅=
αc 1 Carbon formation is observed
αc < 1 Carbon formation is thermodynamically impossible
≥
Carbon activities
For more details, please see Colpan et al. (2007).
Exergy Analysis-I• Exergy destruction is due to irreversibilities within a system
• Friction
•Expansion
•Mixing
•Chemical Reactions
•Heat transfer through a finite temperature diff.
Internal
Irreversibilities
External
Irreversibilities
• Exergy loss is associated with heat rejection to thesurroundings.
Exergy Analysis-IIThe steady state form of control volume exergy balance:
Where
Exergetic efficiency of a component:
Exergetic efficiency of the overall system:
De
eej i
iicvjj
o xEexmexmWQTT10 −⋅−⋅+−⋅
−= ∑∑ ∑
CHPTKNPH exexexexex +++=
F
LD
F
P
xExExE
xExEε
+−== 1
∑ ∑−−=ε LD yy1
Exergy Analysis-III The exergy destruction rate in a component may be compared to the
exergy rate of the fuel provided to the overall system as follows:
The exergy destruction rate of a component may be compared to the total exergy destruction rate within the system giving the ratio.
The exergy loss ratio is defined similarly by comparing the exergy loss rate to the exergy rate of the fuel provided to the overall system.
F
DD xE
xEy
=
tot,D
D*D xE
xEy
=
F
LL xE
xEy
=
Exergy Analysis-IV
Fuel compressor
Air compressor
Recuperator
SOFC (including combustor)
Gas turbine
HRSG
Results-IRecirculation ratio, r Carbon activity, αc
0.1 10.14
0.2 2.65
0.3 1.16
0.4 0.59
1. Minimum recirculation ratio that will prevent carbon deposition:
2. For r = 0.4, it is found that the air utilization ratio is 17% and the power output of a single cell is 46.17 W.
3. Thermodynamic properies and exergy flow rate of each state is calculated.State (kg/s) T (°C) P (kPa) (kW) (kW) (kW)
1 0.032318 25.0 101.3 0.000 1660.600 1660.600
2 0.032318 277.6 1519.9 19.546 1660.600 1680.146
3 2.815823 25.0 101.3 0.000 0.000 0.000
4 2.815823 420.3 1519.9 1079.644 0.000 1079.644
5 0.032318 782.3 1519.9 59.292 1660.600 1719.892
6 2.815823 900.0 1519.9 2113.568 0.000 2113.568
7 2.848142 1093.4 1519.9 2704.008 3.817 2707.825
8 2.848142 722.1 304.0 1347.052 3.817 1350.869
9 2.848142 223.0 106.4 150.848 3.817 154.666
10 2.848142 190.0 101.3 113.163 3.817 116.980
11 0.036413 188.0 1200.0 30.786 0.091 30.877
12 0.036413 25.0 1200.0 0.000 0.091 0.091
m phxE chxE xE
Results-II
5% 7%
13%
6%
0%
7%
62%
CV2
CV3
CV4
CV5
CV6
Stream10
Utilized exergy
15%
24%
41%
19%1%
CV2
CV3
CV4
CV5
CV6
Control Volume (CV) , (kW)
CV1 1.785
CV2 77.887
CV3 122.534
CV4 207.601
CV5 96.126
CV6 6.900
Stream10 116.980
Exergy destructions and lossesLD y and y
*Dy
0.5
0.55
0.6
0.65
0.7
0.75
0.8
15 20 25 30 35 40
Ambient temperature (°C)
FUE,
ε FUE
ε
Effect of ambient temperature on the system performance
DxE LxE
Conclusions A fuel utilization efficiency of 68% and an exergetic efficiency
of 62% under main operating conditions. Control volume enclosing the SOFC and the combustor has
the highest exergy destruction which accounts for 12.5% of the exergy of the fuel and 40.5% of the total exergy destructions.
Exergy loss (exergy flow rate of the stack) accounts for 7% of the exergy of the fuel.
Fuel utilization efficiency increases, whereas exergetic efficiency decreases, with an increase with the environmental temperature.
A better thermodynamic performance compared to other conventional cogeneration systems.
Acknowledgements The financial and technical support of an Ontario Premier’s
Research Excellence Award, the Natural Sciences and Engineering Research Council of Canada, Carleton University and University of Ontario and Institute of Technology is gratefully acknowledged.
References Calise, F., Palombo, A., Vanoli, L. 2006. Design and partial load exergy analysis of
hybrid SOFC-GT power plant. Journal of Power Sources. 158. pp. 225-244 Colpan, C.O., Dincer, I., Hamdullahpur. F. 2007. Thermodynamic modeling of direct
internal reforming solid oxide fuel cells operating with syngas. International Journal of Hydrogen Energy. 32. pp. 787-795.
Colpan, C.O., Dincer, I., Hamdullahpur. F. A review on macro-level modeling of planar solid oxide fuel cells. International Journal of Energy Research. In Press
Douvartzides, S.L., Coutelieris, F.A., Tsiakaras, P.E. 2003. On the systematic optimization of ethanol fed SOFC-based electricity generating systems in terms of energy and exergy. Journal of Power Sources. 114:203-212.
Ghosh, D., De, S. 2003. Thermodynamic performance study of an integrated gasification fuel cell combined cycle-an exergy analysis. Proceedings of the Institution of Mechanical Engineers-A. 217(6). pp. 575-581.
Granovskii, M., Dincer, I., Rosen, M. A. 2007. Performance comparison of two combined SOFC–gas turbine systems. Journal of Power Sources. 165. pp. 307-314.
