Transcript Slide 1

Application Domain
• The Energy Problem: Growing world demand and diminishing supply
– Efficient, large scale (> 1MW) power production is a necessity
– Environmentally responsible solutions are also a necessity.
• Potential Solutions
– Renewable resources and technologies (wind, solar, bio-mass, etc.)
– Efficiency/conservation measures
• Demand Side: End use conservation
• Supply Side: Exploitation of by-product heat
– Advanced power cycles
•
•
•
•
Cogeneration of Steam (by-product heat used for process heating)
Combined Cycle (gas turbine topping cycle, steam bottoming cycle)
Integrated Gasification Combined Cycle
Solid Oxide Fuel Cell/Gas Turbine (SOFC/GT) Hybrids
SOFC Basics
• SOFC Operation: Electrochemical oxidation of hydrogen and reduction
of oxygen generates electrical current for an external load.
• SOFC General Benefits
– Direct conversion of chemical energy to electrical
– High temperature operation (800-1000°C)
• High quality by-product heat, and enhanced chemical kinetics
• Reduces the need for expensive catalysts.
– Reduced greenhouse gas emissions and criteria pollutants (e.g. NOx or SOx)
– Internal reformation at high temperatures allows for broader fuel options.
e-
Fuel Stream
2H2O
Load
O2 + 4e-
2H2
Interconnect
Anode
2O24e- + O2
2O2-
Air Stream
Electrolyte
Cathode
Interconnect
SOFC/GT Hybrids
• Operational Basics
– Air stream to SOFC pressurized by compressor and preheated by recuperative
heat exchanger
– High temperature SOFC exhaust expanded through turbine for power generation
– Combustion of unutilized fuel in exhaust can boost power produced by turbine
Stack
• Benefits
– High efficiency (η > 60%)
Exhaust
Gases
Heat
Exchangers
Compressor
Air
Turbine
Anode
Cathode
M
Expanded
Combustion
Products
Electrolyte
Compressed
Air
Steam
Reformer
or
Gasifier
Pressurized
Preheated Air
M
• Common combined
cycle plants η ~ 50%
maximum
– Lowered emissions for
criteria pollutants
– Depending on fuel carbon
dioxide can be eliminated
or at least sequestered
Power
Conditioner
M
Fuel
Generator
Pressurized
Combustion
Products
Fuel Cell
Exhaust and
Unutilized Fuel
Startup/Post
Combustor
Design Decision
• By-product heat provides cogeneration/bottoming cycle opportunities
• Recuperative heat exchanger enhances SOFC/GT cycle performance
• The Catch: Increasing recuperator heat transfer decreases the quantity
and quality of by-product heat.
– Quality is used in the thermodynamic sense, i.e. the “usefulness” of heat.
• Primary Questions
– How much recuperator heat transfer?
– How large of a fuel cell?
– What are the priorities? Total power? Cogeneration?
Influence Diagram
Recuperator Heat
Transfer
Size of Fuel Cell
Turbine
Power
SOFC
Power
Turbine Inlet
Temp
Heat
Rejected
Additional Power
Potential
Total
Power
SOFC/GT Dymola Model
Brayton Cycle Performance
• Results of increasing heat exchanger heat transfer
– Higher turbine work output
– Lower recuperator exit enthalpy, i.e. lower quality heat
– Lower heat rejection
• Trade-off between SOFC/GT power and cogeneration
Case
Compressor Work Input (W)
Brayton Heat Input (W)
Turbine Inlet Enthalpy (J/kg)
Turbine Work Output (W)
Recuperator Heat Transfer (W)
Recuperator Exit Enthalpy (J/kg)
Heat Rejection (W)
Brayton Efficiency (%)
Recuperator Exit Temp. (K)
Turbine Inlet Temperature (K)
1
1600000
5400000
1524140
3384770
100000
923316
3615230
33.0513
891.2
1407
2
1600000
5400000
1593100
3514570
500000
900936
3485430
35.455
871.1
1464
3
1600000
5400000
1679310
3676820
1000000
872962
3323180
38.4596
846
1535
SOFC/GT performance under
uncertainty
• Mass flow rate dominates
turbine output power
• Turbine output normally
distributed
m_fuel
Heat_xfer
Anode_Temp
m_Fuel
64%
HeatTx
20%
T_AnIn
16%
0
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Main Effects: Turbine Power (W)
W_t
80
75
70
65
60
55
Occurrences
Variable
Mean
Standard Dev.
Turbine Power (W)
-6837400
765854
Fuel Cell Power (W) -1012300
20965
By-product Heat (W) -6871890
1506870
Recuperator Exit
Enthalpy (J/kg)
1302640
193087
Fuel Cell Exit
Temperature (K)
1083.24
39.78
5
50
45
40
35
30
25
20
15
10
5
0
-9e+006
-8.5e+006
-8e+006
-7.5e+006
-7e+006
-6.5e+006
-6e+006
-5.5e+006
-5e+006
Turbine output distribution
Value (W)
-4.5e+006
Challenges
• Dymola
– Understanding ThermoTech files
– Building components
• Building the model
– High Level doesn’t work
– Use of Examples
• Model Center
– Arena
– Maximum Estimation Likelihood
Dymola
• TechThermo
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–
–
–
Not completely developed
Doesn’t follow exact thermodynamic properties
Thermodynamic logic of library convoluted
Lots of Component-Icon-Models (CIM)
• Empty containers
• Can require extensive coding
Dymola
• Building Components
– Finding relevant equations
– Learning the code
– Debugging
Model Building
• Started at a High Level
– Too much too fast
– Singularity problems
– Needed to target specific
areas
Model Building
• Success
–
–
–
–
Started small
Evaluated each individual component
Combined smaller “blocks”
Built components as needed
Recuperator
(built from CIM)
Standard
Brayton Cycle
Recuperated
Brayton Cycle
Model Center
• Arena
– Limited knowledge of software
– Not sure how to fit it in
• Elicitation of Beliefs
– Hard to grasp the mathematical concept
– ZunZun to the rescue