(microsoft powerpoint - lesson ix [modalit\340 compatibilit\340])
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A.A. 2011-2012
Sustainable Energy
Mod.1: Fuel Cells & Distributed
Generation Systems
Thermochemical Power Group (TPG) - DiMSET – University of Genoa, Italy
Dr. Ing. Mario L. Ferrari
A.A. 2011-2012
Lesson IX
Lesson IX: fuel cell systems (hybrid systems)
A.A. 2011-2012
Fuel Cell Power System Scheme
Lesson IX
A.A. 2011-2012
Hybrid System Aspects�Fuel cells generate high temperature exhausts.
�Especially high temperature fuel cells generate exhausts at high exergy
content (exhaust temperature: ~650°C for MCFC, up to 1000°C for SOFC.
�Gas turbine highest temperature (expander inlet): 900°C-1450°C.
�Steam plant highest temperature (expander inlet): 500°C-600°C.
�System efficiency increase possible with power system coupling.
Lesson IX
A.A. 2011-2012
Regenerative Brayton Cycle Fuel Cell Hybrid
System (1/3)
Lesson IX
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Regenerative Brayton Cycle Fuel Cell Hybrid
System (2/3)
Lesson IX
Fuel Cell/Regenerative Brayton Cycle Advantages
�Fuel cells generate high temperature exhausts.
�Simple cycle arrangement, minimum number of components.
�Relatively low compressor and turbine pressure ratio, simple machines.
�Relatively low fuel cell operating pressure, avoiding the problems
caused by anode/cathode
�Pressure differential and high pressure housing and piping.
�Relatively low turbine inlet temperatures, perhaps 1950°F for SOFC and
1450°F for MCFC systems. Turbine rotor blade cooling not required.
�Relatively simple heat removal arrangements in fuel cells, accomplished
by excess air flow.
�No internal heat transfer surface required for heat removal.
�Fuel conversion in cells maximized, taking full advantage of fuel cell
efficiency.
�Adaptability to small scale power generation systems.
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Regenerative Brayton Cycle Fuel Cell Hybrid
System (3/3)
Lesson IX
Fuel Cell/Regenerative Brayton Cycle Disadvantages
�Tailoring of compressor and turbine equipment to fuel cell temperature
and cycle operating pressure required (it is not clear to what extent available
engine supercharging and industrial compressor and turbine equipment can
be adapted to this application.)
�Large gas to gas heat exchanger for high temperature heat recuperation
required.
�Efficiency and work output of the cycle sensitive to cell, compressor, and
turbine efficiencies; pressure losses; and temperature differentials.
A.A. 2011-2012
Rankine Cycle Fuel Cell Hybrid System (1/2)
Lesson IX
A.A. 2011-2012
Rankine Cycle Fuel Cell Hybrid System (2/2)
Lesson IX
Fuel Cell/Rankine Cycle Advantages
�Ambient pressure operation within the fuel cell.
�Heat recovery in a boiler, avoiding the high temperature gas to gas
exchanger of a regenerative Brayton cycle.
�No gas turbine required, only fans for air and exhaust product gas flow.
�Steam available for cogeneration applications requiring heat.
Fuel Cell/Rankine Cycle Disadvantages
�Fuel cell big size required (>10-50 MW).
�Not available for distributed generation.
�Inherently lower efficiency than regenerative Brayton and combined
Brayton-Rankine cycles.
�requirement for cooling and feed water.
�greater complexity than regenerative Brayton cycle arrangement.
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Combined Brayton-Rankine Cycle Fuel Cell
Hybrid System (1/2)
Lesson IX
A.A. 2011-2012
Combined Brayton-Rankine Cycle Fuel Cell
Hybrid System (2/3)
Lesson IX
A.A. 2011-2012
Combined Brayton-Rankine Cycle Fuel Cell
Hybrid System (3/3)
Lesson IX
Fuel Cell/Combined Brayton-Rankine Cycle Advantages
�integrated plant and equipment available for adaptation to fuel cell heat
recovery.
�high efficiency system for heat recovery.
Fuel Cell/Combined Brayton-Rankine Cycle Disadvantages
�Fuel cell big size required (>10-50 MW).
�Not available for distributed generation.
�Complex, multi component, large scale system for heat recovery.
�Adaptation of existing gas turbine required to provide for air take off and
return of hot depleted air and partially burned fuel.
�High pressure operation of the bulky fuel cell system required.
�Precise balancing of anode and cathode pressures required to prevent
rupture of fuel cell electrolyte.
�Indirect heat removal required from fuel cells with compressed air, initially
at low temperature, to enable significant conversion of the fuel flow in the
cells.
A.A. 2011-2012
Hybrid System Comparison
Lesson IX
�Efficiency values not realistic: machine and cell efficiency for MW-size
power plants, no thermal losses, no electrical losses, direct use of CH4 (no
reforming), no auxiliary losses.
�Realistic efficiency values: 60%-68%.
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PAFC Hybrid Systems (1/3)
Lesson IX
A.A. 2011-2012
PAFC Hybrid Systems (2/3)
Lesson IX
�This cycle is very similar to the 11 MW IFC PAFC cycle that went into
operation in 1991 in the Tokyo Electric Power Company system at the Goi
Thermal Station (V = 750 mV and Uf=80%).
�This is a 12 MW plant with V = 760 mV and Uf=86%.
A.A. 2011-2012
PAFC Hybrid Systems (3/3)
Lesson IX
�Ambient air (stream 200) is compressed in a two-stage compressor with
intercooling to conditions of 193ºC (380ºF) and 8.33 atm.
�The majority of the compressed air (stream 203) is utilized in the fuel cell
cathode; however, a small amount of air is split off (stream 210) for use in
the reformer burner.
�The spent oxidant (stream 205) enters a recuperative heat exchange before
entering a cathode exhaust contact cooler, which removes moisture.
�The cycle exhaust (stream 304) is at approximately 177ºC (350ºF).
A.A. 2011-2012
MCFC Hybrid Systems (1/15)
Lesson IX
Athmospheric MCFC based Hybrid system (1/2)
Plant theoretically
analyzed by National
Fuel Cell Research
Center (NFCRC) and
National Energy
Technology Laboratory
(NETL)
M
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MCFC Hybrid Systems (2/15)
Lesson IX
Athmospheric MCFC based Hybrid system (2/2)
Fuel cell parameters
Gas turbine parameters
On design performance
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MCFC Hybrid Systems (3/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (1/9)
Fuel cell layout (1/2)
SHR: sensible heat reformer
ECB: exhaust catalytic burner
CCB: cathode catalytic burner
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MCFC Hybrid Systems (4/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (2/9)
Fuel cell layout (2/2)
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MCFC Hybrid Systems (5/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (3/9)
Fuel cell data
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MCFC Hybrid Systems (6/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (4/9)
Pressurization by auxiliary compressors
•Taking into account the power for the two auxiliary compressors the stack
system efficiency is reduced to 40.1%
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MCFC Hybrid Systems (7/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (5/9)
Pressurization by turbocharger
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MCFC Hybrid Systems (8/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (6/9)
MCFC-mGT hybrid system (using a simple cycle gas turbine)
•Cell exhaust gas enters the
gas turbine expander at about
3.5 bar and 700°C.
•Possibility of post-
combustion in the ECB.
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MCFC Hybrid Systems (9/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (7/9)
•Elimination of cathodic
recycle.
•CCB outlet temperature is
obtained with additional
fuel (18% of the total fuel
flow).
CFC-mGT hybrid system (using a regenerated cycle gas turbine) – Scheme 1
A.A. 2011-2012
MCFC Hybrid Systems (10/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (8/9)
•Elimination of cathodic recycle.
•Elimination of fuel injection in
the CCB.
•CCB outlet temperature is
obtained with additional fuel in
the ECB (21% of the total fuel
flow).
•With this approach machine
efficiency is increased.
CFC-mGT hybrid system (using a regenerated cycle gas turbine) – Scheme 2
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MCFC Hybrid Systems (11/15)
Lesson IX
Pressurised MCFC based Hybrid system by ANSALDO Fuel Cells (9/9)
Performance comparison
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MCFC Hybrid Systems (12/15)
Lesson IX
Pressurised MCFC based Hybrid system by TOYOTA (1/4)
Fuel cell data
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MCFC Hybrid Systems (13/15)
Lesson IX
Pressurised MCFC based Hybrid system by TOYOTA (2/4)
MCFC/mGT hybrid system layout
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MCFC Hybrid Systems (14/15)
Lesson IX
Pressurised MCFC based Hybrid system by TOYOTA (3/4)
MCFC/mGT hybrid system: performance estimation
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MCFC Hybrid Systems (15/15)
Lesson IX
Pressurised MCFC based Hybrid system by TOYOTA (4/4)
MCFC/mGT hybrid system: 2005 World EXPO test results
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SOFC Hybrid Systems (1/10)
Lesson IX
Athmospheric SOFC based Hybrid system (1/2)
Critical component
(T7=1000°C)FC – Fuel compressor
SOFC- Fuel Cell
B- Burner
C – Compressor
E – Expander
REC- Recuperator
FP – Fuel pre-heating
β= 4; β= 4; β= 4; β= 4; PFC ac=1198 kW; PGT ac=422 kW; PHS=1550 kW; ηηηηFC=0.44; ηηηηHS=0.55
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SOFC Hybrid Systems (2/10)
Lesson IX
Athmospheric SOFC based Hybrid system (2/2)
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SOFC Hybrid Systems (3/10)
Lesson IX
Pressurised SOFC based Hybrid system (1/6)
Critical component FC – Fuel compressor
SOFC- Fuel Cell
B- Burner
C – Compressor
E – Expander
REC- Recuperator
FP – Fuel pre-heating
M_fuel = 0.018 M_air =1.05 PFC/PHS = 83% β = 4Fuel Cell Power = 474.8 kW Hybrid System Power =565.8 kW
Fuel Cell Efficiency = 0.53 Hybrid System Efficiency =0.62
A.A. 2011-2012
Theory of Pressurisation 1• Ideal cell voltage is a function of pressure (Nernst Equation)
( )
⋅
⋅
⋅+
⋅
∆−=
OH
HO
e
S
e
idealp
pp
Fn
TR
Fn
GE
2
22
210
ln
Cathode O2 partial pressure
Anode partial pressure ratio
remains constant
Ideal cell voltage increases with increasing pressure
SOFC Hybrid Systems (4/10)
Pressurised SOFC based Hybrid system (2/6)
Lesson IX
A.A. 2011-2012
Theory of Pressurisation 2
• Cell voltage = Ideal voltage - Losses
diffusionohmicactivationidealcell EE ηηη −−−=
Electrode process losses
reduce with increasing
pressure
Mass transport losses
reduce with increasing
pressure
At constant current, cell voltage and hence
power increase with pressure
SOFC Hybrid Systems (5/10)
Pressurised SOFC based Hybrid system (3/6)
Lesson IX
A.A. 2011-2012
0,55
0,6
0,65
0,7
0,75
0,8
0,85
0,9
0,95
1
1,05
0 2 4 6 8 10 12 14
p [atm]
V [
V]
Nernst
Real
SOFC Hybrid Systems (6/10)
Pressurised SOFC based Hybrid system (4/6)
Lesson IX
A.A. 2011-2012
•Pressurisation reduces pressure
drops caused by flows
•Pressurisation reduces pumping
work required to overcome pressure
drops: allows greater power density
through reduction in passage size;
reduction in stack volume big driver
for overall system cost.
•Reduces area and cost of heat
exchangers
•Increases cell performance: 50%
stack efficiency; translates to €/kW.
DIRECT BENEFITS OF PRESSURE
SOFC Hybrid Systems (7/10)
Pressurised SOFC based Hybrid system (5/6)
Lesson IX
A.A. 2011-2012
Pressurised and atmospheric HS compared
Identical stack in pressurised and
atmospheric configurations
• Near term SOFC stack
• Underlying stack efficiency 50%
• System efficiency exceeds stack
efficiency for pressurised case
•Atmospheric recuperator must be
done with exotic material
•Pressurised recuperator can be
stainless steel
SOFC Hybrid Systems (8/10)
Pressurised SOFC based Hybrid system (6/6)
Lesson IX
A.A. 2011-2012
7
Water
3ST
Cooling Air
8HT
11S
10S
6
9
5
2
EE
4
C
1
1ST
DCAC
DCAC
3F
RR
BRRR B
2F
10
CO
4F1F
FFC FP
HTCERAMIC
HX
3HT
CERAMICHX
HRSG
ST
2ST
4ST
11ALSTOM Turbine (7.9 ALSTOM Turbine (7.9 MWeMWe))
1422 K288 KInlet Temperature
0.876
Is. Blading
0.851
Is. BladingIsentropic Efficiency
12.4214.13Pressure Ratio
26.81 kg/s in
29.74 kg/s out
29.47 kg/sMass Flow
TurbineCompressor
1422 K288 KInlet Temperature
0.876
Is. Blading
0.851
Is. BladingIsentropic Efficiency
12.4214.13Pressure Ratio
26.81 kg/s in
29.74 kg/s out
29.47 kg/sMass Flow
TurbineCompressor
Fuel Cell Power = 19.2 MW
GT+SOFC Power = 23.51 MW
Steam Turbine Power = 2.4 MW
Fuel Cell Efficiency = 0.635
GT+SOFC Efficiency = 0.78
Plant Efficiency =0.85
Plant Power= 25.82 MW
PFC/PHS= 81.5%
MFUEL/MAIR=0.0205
TITcalc=1174 K
SOFC Hybrid Systems (9/10)
Pressurised SOFC based Hybrid system combined with a steam plant
•• No No RecuperatorRecuperator
Lesson IX
A.A. 2011-2012
SOFC Hybrid Systems (10/10)
Pressurised SOFC based Hybrid system combined with a STIG plant
Lesson IX