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)

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Fuel Cell Power System Scheme

Lesson IX

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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

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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.

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Rankine Cycle Fuel Cell Hybrid System (1/2)

Lesson IX

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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

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Combined Brayton-Rankine Cycle Fuel Cell

Hybrid System (2/3)

Lesson IX

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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.

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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

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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%.

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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).

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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

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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

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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

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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

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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

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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

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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

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SOFC Hybrid Systems (10/10)

Pressurised SOFC based Hybrid system combined with a STIG plant

Lesson IX