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

Fuel Stream

Interconnect

Electrolyte

Anode

Interconnect

Air Stream

Cathode

2H2 2H2O

4e- + O2 2O2-

e-

O2 + 4e- 2O2- Load

• 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.

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

Generator

M

Stack

Fuel

Air

M

Com pressor Turbine

CompressedAir

PressurizedPreheated Air

Fuel CellExhaust and

Unutilized Fuel

ExhaustGases

PressurizedCombustion

Products

ExpandedCombustion

Products

HeatExchangers

Startup/PostCombustor

Steam

Reform eror

Gasifier

An

od

e

Ele

ctro

lyte

Cat

ho

de

Pow erConditionerM

• Benefits– High efficiency (η > 60%)

• Common combined cycle plants η ~ 50% maximum

– Lowered emissions for criteria pollutants

– Depending on fuel carbon dioxide can be eliminated or at least sequestered

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?

Heat Rejected

Size of Fuel Cell

Total Power

Turbine Power

Turbine Inlet Temp

Recuperator Heat Transfer

SOFC Power

Additional Power Potential

Influence Diagram

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 1 2 3Compressor Work Input (W) 1600000 1600000 1600000Brayton Heat Input (W) 5400000 5400000 5400000Turbine Inlet Enthalpy (J/kg) 1524140 1593100 1679310Turbine Work Output (W) 3384770 3514570 3676820Recuperator Heat Transfer (W) 100000 500000 1000000Recuperator Exit Enthalpy (J/kg) 923316 900936 872962Heat Rejection (W) 3615230 3485430 3323180Brayton Efficiency (%) 33.0513 35.455 38.4596Recuperator Exit Temp. (K) 891.2 871.1 846Turbine Inlet Temperature (K) 1407 1464 1535

SOFC/GT performance under uncertainty

• Mass flow rate dominates turbine output power

• Turbine output normally distributed

10095908580757065605550454035302520151050

m_Fuel

HeatTx

T_AnIn

64%

20%

16%

Value (W)

-4.5e+006-5e+006-5.5e+006-6e+006-6.5e+006-7e+006-7.5e+006-8e+006-8.5e+006-9e+006

Oc

cu

rre

nc

es

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

W_tVariable Mean Standard Dev.Turbine Power (W) -6837400 765854Fuel Cell Power (W) -1012300 20965By-product Heat (W) -6871890 1506870Recuperator Exit Enthalpy (J/kg) 1302640 193087Fuel Cell Exit Temperature (K) 1083.24 39.78

Main Effects: Turbine Power (W)

m_fuel

Heat_xfer

Anode_Temp

Turbine output distribution

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– 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

StandardBrayton Cycle

RecuperatedBrayton Cycle

Recuperator (built from CIM)

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