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Fuel Processing for Fuel Cell Power Systems

Anna Stefanopoulou and Jing Sun

University of Michigan, Ann Arbor

Prepared for the 2006 American Control Conference Workshop on “Fuel Cell System Modeling and Control”

June 12, 2006

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OUTLINE

Introduction: fuel processing systems for fuel cells

Modeling and control of fuel processing systems

Integrated fuel cell and fuel processing systems

Fuel cell based CHP systems

Conclusion

Outline

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Hydrogen Generation for FCsDirect H2

Electrolyser

Solar

Regenerative

Fuel Processor

Source: Nature 414, 2001

-- Electrolysers (Convert water to H2 with solar, wind, and other renewable energy)-- Regeneration (Convert water to H2 using electricity, diesel engines, or gas turbines)-- Fuel processors (Convert hydrocarbon fuel, such as methanol or gasoline, to a H2 rich gas.)

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OUTLINEFueling of fuel cells

Fuel CellH2 tank Fuel

CellFuel tankFuelProc

Direct hydrogen On-board reforming

vs

• Centralized hydrogen production• Major infrastructure challenges

for H2 storage & distribution

• Able to use existing fuel storage and distribution infrastructure

• System complexity and added cost due to the fuel processing system

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OUTLINEDirect hydrogen: storage options• Cryogenic (liquid) hydrogen

• The energy to liquefy and store H2 takes 30-40% of its energy content

• Pressurized (gaseous) hydrogen• The energy needed to compress H2

to 5000 psi takes 4-8% of its energy.• Metal hydride or carbon nanofiber

storage• Manufacturing, low density, release

rates• Aqueous hydrides such as alkali-

stabilized sodium borohydride• Manufacturing, recycling, and

disposing

Adams et al., “The Development of Ford's P2000Fuel Cell Vehicle,” SAE 2000-01-1061

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OUTLINEDirect hydrogen: the cost of production and distribution

Centralized production, pipeline distributionProduction Distribution Total Cost Efficiency

H2 reformed fromnatural gas: $1.03 /kg $0.96 /kg $1.99 /kg 72.0%

+CO2 capture $1.22 /kg $0.96 /kg $2.17 /kg 61.0%

Gasoline (for reference): $0.93 /gal $0.19 /gal $1.12 /gal 79.5%

US. National Research Council and US. National Academy of EngineeringAug. 2004, ISBN 0-309-09163-2

On-board reforming: a critical enabling technology in the near term

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OUTLINEOn-board reformingConvert hydrocarbon feedstock to hydrogen rich reformate• Avoid sulfur poisoning and coke formation• Achieve high efficiency and thermal integration• Manage start up and load following transients• Deal with aromatic compounds and more complex fuel (such as

diesel)

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OUTLINEFuel Processing Systems (FPS)

Key fuel processing system functions• Reformer: thermochemically reforms a suitable hydrocarbon

feedstock in the presence of catalyst• Shift reactors: increase the H2 content through the water gas shift

reaction• Purifier: further reduce CO concentration and remove other

impurities in the reformate

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OUTLINEReforming technologies• Steam reforming:

• High efficiency• Large size and long start-up time

• Catalytic partial oxidation

• Fast response• Potential for carbon deposition

• Autothermal reforming

molkJhHCOOHCH /16.2063 224 =Δ+⇔+

molkJhHCOOCH /36221

224 −=Δ+→+

molkJhHCOOCH

molkJhHCOOHCH

/36221

/16.2063

224

224

−=Δ+→+

=Δ+→+81.281.281.2

85.5

83.2

SR

EnergyEfficiency* (%)

54.9Jet fuel55.7Diesel55.8Gasoline

77.5Natural Gas

Data n/aMethanol

POXFuel

Source: Brown, Journal of Hydrogen Energy, (26)4 , 2001

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OUTLINEShift conversions• Increase the H2 content by converting

CO and H2O to H2 and CO2 through water gas shift reaction:

• Two stage shift reactors:• HTS: high temperature shift: • LTS: low temperature shift:

molCOkJhHCOOHCO

/14.41222

−=Δ+⇔+ HTS LTS~15%CO ~3%CO ~0.4%CO

350oC-475oC 180oC-250oC

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OUTLINEH2 Purification• PEM fuel cells require pure hydrogen

as fuel• CO poisons the FC catalyst and

degrades performance• Different purification options:

• Preferential oxidation

• Separation membrane

molCOkJhCOOCO

/2802/1 22

−=Δ→+

WHPIN WHP

WLPOUT

WTH

HP Side

LP Side

PHP, PHP, XH2H2

PLP

OUT

H2 separator

H2

(H2), CO, N2, CO2H2, CO, N2, CO2

2

HP 0.6 LP 0.6TH HW K A (P P )= ⋅ ⋅ −

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OUTLINEFuel requirements for FC operations

Source: Fuel Cell Explained: James Larminie and Andrew Dicks

• Desulphurization: A major issue• Thermal integration, heat exchangers: will define total

system efficiency

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Natural Gas Fuel Processor System (FPS)

(BLO)Blower

(HDS)Hydro-

desulfurizer

(WGS)Water Gas Shift

222 HCOOHCO(WGS) +⇔+

(PROX)Preferential Oxidation

22 CO2OCO2(PROX) ⇒+

(MIX)Mixer

CH4I

(CPOX)Catalytic

Partial Oxidation OH2COO2CH(TOX)

H2COO21CH(POX)

2224

224

+⇒+

+⇒+ + heat

+ heat

Fuel Valve CO <0.001%

(10ppm)

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Controlling Hydrogen Generation

HDSTANKMIX

Fuel ValveFuel Valve

BlowerBlower

Excess HExcess H2 1 1 = -----------= ----------- Utilization Utilization

H2 GenerationH2 Generation

CPO

x

WGS+PROX

vfuel vBL

HydrogenControl

pa

wai

wao

pC

wb

wco

yH2FPS

V

I

Anode(a)Cathode(c)

CO RemovalCO RemovalSulfur RemovalSulfur Removal

H2 generation from Catalytic Partial OXidation (CPOX) Partial Oxidation: CH4 + 0.5O2 = CO +2 H2 + Heat (at 700o )Total Oxidation: CH4 + 2O2 = CO2 +2 H2O+ more Heat

TOXPOX

CO 2λ21 2

High oxygenLow oxygen

waste CH4

produce H2 butnot enough heat

create heat butnot enough H2

too muchheat

λO2C=O2/CH4 (mole/mole)

(PROX)Preferential Oxidation

(CPOX)Catalytic

Partial Oxidation

(WGS)Water Gas Shift

(HDS)Hydro-desulfurizer

Control ObjectivesCPOX temperature~700oC• Avoid catalyst overheat• High fuel conversion • No CH4 slipAnode H2 concentration• High fuel utilization• Avoid anode starvation

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State Equations for Transient Model

⎟⎠⎞

⎜⎝⎛ −= blo

blo

b

blo udt

d ωωτ

ω0100

1

BLO

( )hexblo

hexair

hexhex

WWVM

RTdt

dp−=

HEX

( )hdsfuel

hdsCH

hdshds

WWVM

RTdt

dp−=

4

HDS ( )cpoxmixCH

hds

mixCH

mixmixCH WxW

VMRT

dtdp

4

4

4 −=

( )cpoxmixair

hex

mixair

mixmixair WxW

VMRT

dtdp

−=

MIX

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( )reactHanan

Hwroxwrox

HanH

ananH WWxWx

VMRT

dtdp

,222

2

2 −−=

( )reactHanwrox

anan

anan

WWWVM

RTdt

dp,2

−−=

AN

State Equations for Transient Model (cont.)

⎥⎦

⎤⎢⎣

⎡+⎥

⎦

⎤⎢⎣

⎡−⎥

⎦

⎤⎢⎣

⎡=

reactionsfromheat

flowenthalpyoutlet

flowenthalpyinlet

, dtdT

Cm cpoxcpoxbedP

cpoxbed

CPOX

( )proxair

wgsOH

wroxcpox

wroxwrox

wroxwrox

WWWWVM

RTdt

dp++−=

2

( )wroxwroxH

cpoxHwrox

wroxH

wroxwroxH WxW

VMRT

dtdp

22

2

2 )1( −+= η

WROX

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λO2C

Model of CPOX Conversion

OH2COO2CH(TOX)

H2COO21CH(POX)

2224

224

+⇒+

+⇒+

22

222

CO2OCO2(COX)OH2OH2(HOX)

⇒+⇒+

CH4

O2

N2

H2O

CH4

O2

N2

H2OH2

COCO2

Τcpox

CPOX reaction products

A set of equations is developed to calculate the species conversion in CPOX based on these four reactions.

The conversion depends on time varying O2 to CH4 ratio, and CPOX temperature

Out

In

High TemperatureLow Temperature

O2 to CH4 ratio

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FPS Control Problem

Objectives: Regulating CPOX temperature, Tcpox and anode hydrogen mole fraction, yH2

Tcpox VH2

uvlv

ublo

HDS

HEX

blo

BLO

ω cpoxT

TANK

ANODE

CATHODE

MIX

CPOXWROX

Feedforward(Look-upTable)

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FPS Control Problem

Fedforward and Feedback is necessary to speed up the dynamics (overshoot actuators and force more fuel and air fast in the FPS!!)

Static FeedforwardStep on Ist

Tcpox

yH2

Pukrushpan et al, ACC 2003

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Decentralized PI control

160 A 90 A

10%1.33 sec12%6.58 secd

6%3.95 sec7%1.0 secc

10%1.33 sec7%1.0 secb

6%3.95 sec11%3.14 seca

yH2 - uvalveTcpox - ublo

Rise time and overshoot (PI Controllers)

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Decentralized Control Analysis

⎥⎦

⎤⎢⎣

⎡=⎥

⎦

⎤⎢⎣

⎡

valve

blozu

H

cpox

uu

Gy

T

2

|RG

A 11|

-|RG

A 12|

Large interactions at high frequencyLarge interactionsat low load

TGG )(RGA(G) 1−×=Relative Gain Array (*)

(*) see Skogestad and Postlethwaite, Multivariable Feedback Control Analysis and Design

Relative Gain Array is used to assess appropriate input/output pairing and to measure system interactions

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Multivariable Control (Model-Based Estimator & Feedback)

Linear optimal regulator and state estimator

Integrators⎥⎥⎦

⎤

⎢⎢⎣

⎡

−−

==22 H

refH

cpoxref

cpox

yyTT

zq&

Minimize ( )∫∞

++=0

dtqQqRuuzQzJ ITT

zT

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Controlled system performance (nominal power 60%)

Nonlinear Simulation.. Static Feedforward- Decentralized PI Controller-- MIMO Controller

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Controlled system performance (nominal power 30%)

Nonlinear Simulation.. Static Feedforward- Decentralized PI Controller-- MIMO Controller

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Tcpox yH2

Ist

uval

ublo

Control Design System Re-design

HDS

HEX

blo

BLO

ω cpoxT

TANK

ANODE

CATHODE

MIX

CPOXWROX

Key findings:1. Decoupler versus faster blower for Tcpox

control (modeling cost versus hardware cost)

2. Smaller HDS volume for faster H2

yH2

Excess hydrogen

Pukrushpan et al., IEEE T-CST Jan 2005

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Low temperature PEMFC based CHP systemPEMFCCPOX reformer for natural gasCatalytic burner for anode waste recirculationHeat exchanger for inlet preheating

OUTLINEThermal integration of FPS & FC systems

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IN OUTC / HC / H C / H

dm W Wdt

= −

C / HC / H C / H C / H

C / H

mP V R TMW

⋅ = ⋅ ⋅

( ) ( )IN IN IN OUT OUT OUTC / H C / H V C / H ref C / H V C / H refQ W c T T W c T T UA (LMTD)= ⋅ − − ⋅ − ± ⋅

( ) ( ) ( )IN IN OUT OUTHOT COLD HOT COLD

IN INHOT COLDOUT OUTHOT COLD

T T T TLMTD

T TlnT T

− − −=

⎛ ⎞−⎜ ⎟−⎝ ⎠

as defined for a co-flow HEX arrangement

( ) ( )UA Heat Transfer C. Area= ⋅

Heat Exchanger Model

Cold in Cold out

Hot in

Hot out

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CHP System – Modeling the CB

IN IN OUTBAnode Air Burner

dm W W Wdt

= + − BB B B

B

mP V R TMW

⋅ = ⋅ ⋅

( ) ( )

( )2

2

B IN An IN AirBB P Anode p Anode Ref Air p Air Ref

OUT OUTBurner p B Ref H

H

dTm c W c T T W c T Tdt

1W c T T HR WMW

⋅ ⋅ = ⋅ − + ⋅ − −

− ⋅ − − λ ⋅ ⋅ ⋅

2

Air

H

Wmin( ,1)34.2 W

λ =⋅

Catalytic Burner temperature map

H2 flow

Air (kg/s)

T bu

rner

(K)

Operate lean so no hydrogen exits the CB

When a down step is applied to the FC the amount of air should be adequate to burn the excess hydrogen

A bypass valve (or active air control) required when FC system failure occurs

Catalytic Burner Model

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Fuel and Air enter the system

Inlet flows are preheated...

...and mixed

fuel is desulfurized

PEMFC/FPS System overview

Heat

Power

Natural gas rich in methane (CH4) is considered as fuelProton Exchange Membrane (PEM) fuel cell stackRated power 250KWReforming CH4 through Catalytic Partial Oxidation (CPOX)Low pressure system

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Needs external heating source to provide the pre-heating energy

Operates at 75-90% fuel utilization to provide the load following capability

Excess fuel in the anode exhaust that can potentially have environmental impact

Motivations for tight thermal integration

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FPS & FC system with internal heatingUse excess hydrogen in the anode flow to preheat inlet flows

Catalytically (with no flame) burn hydrogen with air in the CB

No external fuel input to CB and external heat input to the HEX

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CHP System - Issues

Balance the heat generation and H2

generation in CPOX (how to choose the operating λO2C?)

Balance the fuel utilization in the FC (power generation) and heat generation in the CB so that the system is self sustainable (how to choose the fuel utilization and CPOX operating temperature?)

Deal with transient issues when power demand changes

Potential Issues

CPOX MIX Re actionsT f (T ,T )=

+ → +

= − ⋅

+ → +

= − ⋅

4 2 2

6POX

4 2 2 2

6TOX

Partial Oxidation :1CH O CO 2H2

JH 0.036 10 mol

Total Oxidation :CH 2O CO 2H O

JH 0.8026 10 mol

Δ

Δ

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CHP System - ModelingA control oriented, non-linear, dynamic model of the CHP system was developed based on thermodynamic principles and system identification (19 states).

Ideal gas law, gas mix law and mass/energy conservation principles were used to capture temperature and pressure dynamics in each volume

“Modeling and Dynamics of a Fuel Cell Combined Heat Power System”, V. Tsourapas et al, IASME Transactions, April 2004, v 1, issue 2, p288-294

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Optimization

Objective Function:

subject to

4CHusedW without CB

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System Efficiency Map

Effi

cien

cy (%

)

Air Setpoint (%) Fuel Setpoint (%)

100A FC Load

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Optimal Setpoints at Different LoadI=100A I=140A I=180A

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Optimal Operating Conditions

FC Load (A)38

Component vs. System Optimization

Component optimal setpoint CHP overall optimal

setpoint The resulting optimal setpoint reflects the balance between the heat generated internally with TOX inside the CPOX and externally through CB

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Performance Analysis – Steady State

Fuel Consumption decreases by 16%

FPS efficiency increase by 12%

Performance Analysis– Steady State

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CHP system

Performance Analysis – TransientH2 flow when there is a step change

in the load (current) step 90 160Amps. Fuel and air are changed according to look-up tables corresponding to the optimal setpoints

Similar transient response for both the systems with and without the CB recirculation

Fuel saving achieved without sacrificing performance!

Conventional system

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Transient Performance – H2 Starvation Analysis

CB temperature large time constant

About 3min required to reach new steady state temperature

The slow dynamics prevent a large temperature drop as well

Burner H2 starvation

Thermal inertia

CB

Tem

pera

ture

(K)

42

PHDS

PMIXPHDS-PMIX

WH2IN

λO2C

λ λ

Transient Performance – H2 Starvation Analysis

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What roles did the CB recirculation play

Improved overall fuel utilizationProvided a sensor for H2 starvation detectionCB temperature variations are small and do not affect transient performance due to the relatively slow time constant Hydrogen starvation problem for the CHP system is mainly attributed to:

Optimization yields the setpoints that are close to the operating boundary of the system Different dynamics in the fuel and air path (due to the different size of the HDS and HEX) cause λO2C overshoot, which further leads to CPOX temperature overshoot and hydrogen undershoot

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Feedback Control of the CHP system

Dynamic feedback, in combination with a state estimator, is designed to mitigate the transient problemThe temperatures for the burner and the CPOX are the only measured variables used for feedbackThe overall load following response is improved by 8-folds (from 5A/second to 40A/second) with the feedback control

Control Law:

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Control Problem FormulationSteady state operating points for variable λO2C (Optimum λO2C=0.69)

Directly controls TCPOX and TCB

Different air and fuel control authorities for TCPOX and TCB

TCB is a measure of the H2leaving the anode (i.e., an alternative H2 sensor) !!

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Controller Design

Linearized model:

where:

Balanced Realization and Truncation:

* “Control Design and Analysis for a Fuel Cell-Fuel Processor Combined Heat Power Plant”, Tsourapas et al, to be presented in ACC 05’

Only 5 states needed to observe z

19 states with cond(Ap)=8.4313x1015

5 states with cond(AM)=73.808

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Performance of the controlled system

* “Control Design and Analysis for a Fuel Cell-Fuel Processor Combined Heat Power Plant”, Tsourapas et al, to be presented in ACC 05’

OL :Open Loop (Feedforward control with the optimal map)

Est FB: Estimator +Feedback Controller

OL Performance exhibits H2starvation and large temperature overshoot during a load transition

Est FB Performance mitigates those problems and speeds up H2 production

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Performance Summary

w/ Rate limiter (40A/s)w/ Rate limiter (5A/s)

H2

flow

(kg/

s)

8x faster response

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Robustness to CPOX clogging

Fig.: CPOX clogging due to carbon residue and deforming

Source: SOFCo-EFS

Simulate CPOX clogging by reducing CPOX outlet valve

CPOX clogging deteriorates performance due to the low P system

Causes increase in H2 starvation and Tcpox

FB Control schemes designed are robust to CPOX clogging

Starvation Period

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Robustness to fuel variations

OL CL• It was verified that the CPOX product maps remain unchanged

• The model was modified to accept different fuel compositions

• Three fuels with different composition were examined

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OUTLINE

On-site co-generation of electricity and heat from a single source of fuel

Conventional CHP systems consist ofPrime mover, such as diesel engine, gas turbine, etc., combined with generator set

Waste heat recovery system to capture heat from the exhaust and cooling water jacket.

The ratio of heat to power often dictates the configuration of the system

The combined efficiency is very high (some claim to be as high as nearly 90%)

Combined Heat and Power (CHP) Systems

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OUTLINEFuel Cell Based CHP Systems

Involves both power and heat generationRequires pre-heating energy for fuel reformingRecuperates waste heat will increase the overall system efficiency and make the system self sufficientIf integrated with other conventional power systems (such as gas turbine), it will have synergetic effects on system performanceMost promising for distributed power generation and marine/heavy duty vehicle applications

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Low temperature PEMFC based CHP systemPEMFCCPOX reformer for natural gasCatalytic burner for anode waste recirculationHeat exchanger for inlet preheating

OUTLINEFuel cell based CHP system configurations

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High temperature SOFC based CHP systemDirect internal reforming SOFCTurbine/generator setTransient/start-up burnerHeat exchangers

OUTLINEFuel cell based CHP system configurations

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Siemens 100kW SOFC CHP systemSiemens 250kW CHP proof-of-concept demonstration220kW SOFC/GT hybrid system

OUTLINEExamples of fuel cell based CHP system

Source: http://www.siemenswestinghouse.com/en/fuelcells/demonstrations/index.cfm

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Multiple and heterogeneous power/heat plants involved

High efficiency and (intended for) self-sustaining

Close thermal, chemical, mechanical and electrical couplings among subsystems

More complex control, optimization and integration tasks

Control challenges: How to synergistically integrate the subsystems to achieve high system efficiency and fast load following capability

High efficiency system often operates on or close to the boundary of admissible state and input sets

Mobile requirements require fast load following capability and sufficient power reserve and safety margin

Characteristics of FC-based CHP Systems

⇒

⇒

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Baseline: Integrated FPS & FC system with internal heatingCHP system: with an addition of heat loadTotal system efficiency increased over all current loadsSystem efficiency increased at most power loads

Except high Pe/low heat loadMax electrical power available limited

Efficiency improvement of CHP systems

Total Efficiency vs. Current

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30

32

34

36

38

40

60 80 100 120 140 160 180 200 220 240Current Load (A)

Effic

ienc

y (%

)

Total Efficiency vs. Electrical Power

28303234363840

60 80 100 120 140 160 180 200Electrical Power (kW)

Effic

ienc

y (%

)

BaselineBP LowBP MidBP High

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ConclusionsFuel cell based CHP system is a clean and efficiency power solution for distributed generation and potentially for mobile applications

FC based CHP systems are complex due to the strong thermal, chemical, mechanical, and electrical couplings

Overall system optimization is necessary to assure synergetic integration of subsystems

For mobile applications, load following and dynamic response are very important. Performance will be critically depend on the control system