1 oliver posdziech - staxera/sunfire gmbh heat exchangers and air supply

1 Oliver Posdziech - Staxera/sunfire GmbHHeat Exchangers and Air Supply

Heat Exchangers and Air Supply


Contents

1. Introduction2. Heat transfer basics3. Heat exchanger types4. SOFC system application5. Air supply challenges in SOFC systems6. Blower fundamentals 7. Control of air supply


Introduction


Heat exchangers and air supply

Heat exchanger applications in fuel cell systems Preheating of cathode air Preheating of gases for fuel processing:

- Gas preheater - CPOx air preheater

Reformer heat exchanger Evaporator for steam supply Cooling down of exhaust (off-) gases Condensator for water recovery Considerable cost factor for SOFC systems

Air supply systems in fuel cells Supply and preheating of cathode air Air supply to CPOX reactors Main consumer of auxiliary power

Gas/gas heat exchanger

Air blower (EBM Papst)


Heat transfer basics


List of symbols Heat transfer basics

viscositydynamic

viscositykinematic

tyconductivithermal

tcoefficientransferheatoverall

flowheatspecific

tyconductiviheat

enthalpyspecific

tcoefficientransferheat

pressureconstantatheatspecificcp

U

q

k

h

h


Heat transfer equations Heat transfer basics

First law of thermodynamics: change of enthalpy for steady state conditions

Hot side enthalpy balance

Cold side enthalpy balance

Fourier’s law for 1-D heat conduction

Newton’s law of heat transfer

T

p dTcmhdmQd

12 CCCpCC TTcmQ

21 HHHpHH TTcmQ

dx

dTkqd

WF TThq


Heat transfer equations Heat transfer basics

Total heat transfer rate between hot and cold fluidsDTmean ... mean logarithmic

temperature difference Heat transfer coefficient

for single plane wallhi ... inner heat transfer coefficient

ho ... outer heat transfer coefficient

s ... wall thicknessRS ... heat resistance (analogy to

electrical systems)

Not considered: radiation

meanTAUQ

AhAks

AhR

AU

oi

s11

11

Homework: 1) Calculate “U A” for s=1 mm2) Check impact of k, hi and ho

3) Investigate formulation for pipe flow


Heat exchanger modelling

0D modelling Log Mean Temperature Difference (LMTD) NTU procedure e-NTU procedure

1D modelling Cell method

2D/3D modelling CFD analysis


Logarithmic mean temperature difference (LMTD)

b

a

bamean

T

T

TTT

ln

Co-flow (parallel flow) Counter-flow

Co-flow (parallel flow)

Counter-flow

inout

outin

CHb

CHa

TTT

TTT

inin

outout

CHb

CHa

TTT

TTT


Logarithmic mean temperature difference (LMTD) Limitations of LMTD method:

- Only mean fluid properties and heat transfer coefficients- Mainly used if apparatus is already designed- Simple flow configurations (parallel flow)

In practice: cross-flow and multipass-flow heat exchangers

Heat transfer calculations with NTU (Number of Transfer Units) concept: correction factors for different configurations from diagrams

See literature for details


Heat transfer coefficient Heat transfer basics

Heat transfer coefficient h is function of:- Geometry- Flow type (laminar/turbulent)- Temperature - Fluid phase and phase changes- Flow velocity- History of flow (developing or fully developed velocity and thermal profiles)

Local heat transfer coefficient hx and mean coefficient h

Calculation with 3D CFD or empirical correlations Large number of experimental values for heat transfer and flow friction are

available for single phase flows Empirical correlations are based on non-dimensionalisation

characteristic units


Heat transfer coefficient Heat transfer basics

Typical orders of magnitude for heat transfer coefficients


Characteristic units Heat transfer basics

Reynolds numberRatio of inert forces to viscous forces in flows

Nusselt numberRatio of convective to conductive heat transfer

Péclet numberRatio of advection of flow and thermal diffusion

Prandtl numberRatio of momentum diffusivity and thermal diffusivity

hDV

Re

k

Dh hNu

PrRePe

k

Vcp

k

cp

Pr

Hydraulic diameter Dh=4*A/P

A ... area; P ... wetted perimeter

Geometry Dh

Pipe D

Duct 2a*b/a+b

Parallel plates 2*H

Flate plate L


Flow characteristics Heat transfer basics

Laminar flow Turbulent flow

Osborne Reynolds

Laminar and turbulent boundary layers over a flat plate

Thermal boundary layer

ReC = 2100-2300Critical Reynolds number of transition from laminar to turbulent flow in pipes, ducts and between parallel plates


Nusselt number correlations Heat transfer basics

Nusselt number correlations (local and mean Nu) are available for different cases

Critical is the choice of the correct empirical law:

- Laminar or turbulent flow (Reynolds number)- Gas or liquid, Prandtl number- Hydrodynamically and thermally developed or developing flow- Temperature range- Constant temperature or constant heat flux- Phase changes

Example (Schündler, 1972)

Details see literature

31

PrRe61.166.3 33

L

DNu h


Nusselt number correlations Heat transfer basics

Kays & Crawford, 1980

Laminar flow correlations


Literature

/1/ VDI Heat Atlas

/2/ S. Kakac, H. Liu: Heat Exchangers – Selection, Ratings and Thermal Design, CRC Press

/3/ J. Lienhard: A Heat Transfer Textbook, Phlogiston Press

/4/ W.M. Kays, M.E. Crawford, M.E., Convection Heat and Mass Transfer, McGraw-Hill


Heat exchanger types


Flow directions

x/L

T

x/L

T

Heat exchanger types

Counter flow:

Highest outlet temperature

Low DT higher area needed

Parallel (co-) flow:

Limited outlet temperature

High DT lower area needed

Cross flow:

Limited outlet temperature

Easiest flow distribution (low pressure losses!)

Mixed types:

Multiple passes with combinations of counter/ parallel and cross flow Heat exchanger with 2 shell

passes and 4 tube passes


Construction types Heat exchanger types Double tube

+ Simple, cheap+ High pressures, high temperatures - Low heat transfer surface

Flat plate+ High heat transfer areas+ Compact , efficient- Limited in temperatures and pressures

Spiral flow+ Simple, cheap- Small temperatures differences only

Shell-and-tube+ High pressures, high temperatures+ Large heat transfer areas- Large volume - High manufacturing efforts

Plate-fin / tube-fin + Fluids with different heat transfer coefficients (gas/water)


Types of fabrication

Sealed heat exchangers Only for lower temperatures

Continuous processes Laser beam welding Electron beam welding Tungsten inert gas (TIG) welding

Batch processes Diffusion welding Vacuum brazing

SOFC application


SOFC system application


Typical SOFC system

Reformer

Evaporator

AfterburnerSOFC

Heat recovery unit

Air preheater

Fuel preheater

Desulphurizer

stack

InverterControl system

BlowerFlow sensor

Water pumpFlow sensorDeionization

Gas valveFlow sensor


Availability and suppliers

Challenges High temperatures Low pressure losses Stack will suffer from material corrosions High thermal stresses Very ambitious cost targets Long lifetime (> 40,000 … 80,000 h)

Suppliers Standard components are not available Specialized suppliers like Kaori, SWSW, Dunlop, HeatInc, INNOWILL, Exergy or

sunfire, but mostly for small-scale applications Automotive suppliers like Behr, Modine, Bosal, ... Standard components from heating/climatisation for exhaust gas recovery

Source: Behr


Design criteria

§1 Maximize heat transfer rates Optimal system layout “Pinch Point Analysis” Heat exchanger design for maximal temperature

differences (co- /cross or counter-flow)

§2 Minimize pressure losses Power demand of blowers decreases,

system efficiency increases SOFC stacks are not gas tight reduce differential pressure between cathode and

anode as well as anode against ambient

Establish laminar flow conditions

§3 Minimize heat losses High temperatures cause high heat losses compact heat exchangers Integrate heat exchangers in hot areas of the system

§4 Minimize costs

SOFC application


Design criteria SOFC application

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50

u / [m/s]

Del

ta P

/ [m

bar

]

0

50

100

150

200

250

3000 1000 2000 3000 4000 5000 6000 7000

Reynolds number

Hea

t tra

nsf

er c

oeffi

cien

t

DP laminar [mbar]DP turbulent [mbar]Alpha laminarAlpha turbulent


Heat transfer modelling

Sieder & Tate:31

861

L

DNu e PrRe

.0

5

10

15

20

25

30

35

0 50 100 150 200 250 300

Vdot [Nl/min]

U [

W/m

²*K

]

Tin=400°C

Tin=760°C

Sieder & Tate (1936)

Standard calculations of heat transfer coefficients fail

Internal heat conduction has to be considered

SOFC application

Design of high-temperature gas/gas heat exchangers


Materials

Materials have to withstand up to 1100°C (off-gases from afterburner)

Ceramics or high-grade alloys/stainless steels are used high-temperature resistant stainless steels preferred due to material and fabrication costs

Critical issues: Corrosion, tinder formation and chromium evaporation, metal dusting, damage of welding or brazing connections

Stainless steels that form CrO as protection layers contribute very likely to the poisoning of the cathode

Material probe after 5000 h operation with reformate

Material probe after 5000 h operation with cathode air

SOFC application


Chromium evaporation

Source: Behr


Chemical stability: corrosion

Source: Behr


Strength of materials SOFC application

Only nickel based alloys have sufficient strength of materials high material costs

Strength of material is critical for high inlet temperatures (> 900 °C) downstream of afterburner.


Air supply challenges in SOFC systems


Motivation

Effect of pressure loss and blower efficiency on performance of a steam reforming system with partly internal reforming

100 mbar, hBl=20%

100 mbar, hBl=30%

50 mbar, hBl=30%

50 mbar, hBl=20%

100 mbar, hBl=20%

100 mbar, hBl=30%

50 mbar, hBl=30%

50 mbar, hBl=20%System efficiency

Blower power

Heat demand for 1.5 kWDC system: cathode air versus stack inlet temperature


Requirements of air supply system

Air supply system is the main consumer of electricity

Pressure drop needs to be low

High blower efficiency Large SOFC systems in the range from several kW to MW have to be designed

for several 1000 hours of continuous operation

Blowers have to be reliable and long-term stable Systems are expected to be profitable at costs < 1500 €/kW

Blowers have to cheap The cathode air heat demand is up to three times higher than the stack power

output

Efficient heat transfer required

Introduction


Blowers in SOFC systems SOFC applications

Flow rates / kW Dp / T Availability

Cathode air blower

70 ... 200 Nl/min 20 ... 200 mbar20 °C

Standard products

Burner air blower

Start-up100 ... 250 Nl/minNormal operation 30 ... 80 Nl/min

10 ... 50 mbar20 °C

Standard products

Gas blower 3 ... 8 Nl/min 20 ... 50 mbar20 °C

Safety criticalBiogas industry

Anode off-gas recirculation

30 ... 80 Nl/min 10 ... 50 mbar100 ... 800 °C

Safety criticalHardly available

Cathode air recirculation

70 ... 200 Nl/min 10 ... 50 mbar600 ... 800 °C

Hardly available


Cathode air supply options

3) Cathode air recirculation

1) Cathode air/exhaust gas heat exchanger

2) Cathode air/air heat exchanger

Off-gas burnerSOFC stackCathodeair

900-1100 °CReformate

700 °C800 °C

800 °C

Cathodeair

900-1100 °C700 °C 800 °C

Reformate

700 °C 800 °C

800 °C20 °C

Cathodeair

900-1100 °C

Reformate


Cathode air supply options

Option 1: Cathode air/exhaust gas

Option 2: Cathode air/cathode air

Option 3: Cathode air recirculation

Application µCHP, smallCHP, off-grid

µCHP, smallCHP smallCHP

Advantages High temp. diff. smaller HEX area

Fast system heat up via burner

Diffusion burner can be used

Separated air and gas supplies

Lower HEX temperatures lower chromium evaporation rates

No cathode air HEX required (most costly component)

Disadvantages High HEX inlet temperature (material strenght)

Complicated heat up procedure

Premix burner necessary

Availability of blower

Electr. power demand

Rotating part


Blower fundamentals


Classification Introduction

Discharge Pressure / Suction Pressure = Compression RatioP = pD/pS

Turbo machinery

Fan1 < pD/pS < 1.1

Blower1.1 < pD/pS < 3.0

CompressorpD/pS > 3.0

Dp 50 mbar 100 mbar 200 mbar

Compression ratio

pD/pS = 1.05 pD/pS = 1.1 pD/pS = 1.2

Type Fan Fan/blower Blower


Potential blower types Blowers types

Axial machine Complete axial flow direction

Pressure

Flow rate

Centrifugal machine

Axial flow entering, centrifugal discharge: 90° turning of gas

Pressure

Flow rate

Side-channel machine

Ring-shaped divided housing with paddle wheel that turns inside housing

Pressure

Flow rate


Manufacturers

Side channel blowers (regenerative blowers) Available in all flow rates and with pressures up to

1 bar(g) Costs normally high (low numbers of pieces) Manufacturers: Rico, Becker, Gardner Denver (Elmo Rietschle),

Elektror, Ziehl-Abegg, Vairex, Ametek, Mapro

Centrifugal blowers Some low-cost products from heating industry available Limited pressures (200 mbar @ very high motor speeds) Manufacturers: EBM-Papst, Ametek, Torin-Sifan, Domel, R&D

Dynamics

Blower types


Flow rate calculations Specific values

Volume flow Incompressible flow

Compressible flow

Volume flow at normal

conditions (0°C, 101.325 kPa)

Note: Air flow @ low pressures can be considered as incompressible

D

N

N

DDN

DSSDDS

T

T

p

pVV

TpTpVV

mV

)/(


Blower power demand Specific values

Theoretical power

Total pressure increase

Dynamic pressure

Shaft Power

Motor power

WpVP tth

][WpV

PSP

tSP

WpV

PMSP

tM

2

2vpdyn

Pappp dynstt


Blower efficiencies Specific values

Shaft power efficiency mlosstloss

tSP PppVV

pV

)(

MotorSPt

Vloss … Flow losses in tip of rotor

Dploss … Sum of internal pressure losses

Pm … Mechanical friction losses

Total efficiency

Typical efficiencies 0.6 … 0.8 large blowers

0.5 … 0.6 middle sized blowers

0.3 … 0.5 small blowers

0.25 … 0.3 side channel blowers


Some rules of thumb ... Specific values

Temperature increase Kin1200 t

tpT

If equipment characteristics is a quadratic parabola:

- Volume flow ~ Speed

- Pressure ~ (Speed)²

- PM ~ (Speed)³


Performance maps Characteristics

Source: R&D Dynamics

Characteristic diagram Efficiency diagram


Design point of blower: Matching of blower and

equipment (here: duct)

characteristics Increasing the volume flow

by 30 % doubles the

pressure loss in duct!

Performance maps Characteristics

Differential pressure or compression ratio versus flow rate


Example: Ametek Nautilair Characteristics

h=0.41h=0.52

h=0.3

h=0.41


Example: Elmo-Rietschle G-200 Characteristics


Control of air supply


Air flow measurement & control Blower control

1) Operation at constant speed Accurate flow control, but limited control range Simple blower electronics Low efficiency due to additional pressure loss Control by throttles or bypass

2) Speed control of blower Higher blower efficiency Complex electronic Electronic commutation (cheapest version) Frequency inverter Phase cutting

3) Air flow measurement Sensors with low costs and low pressure drops available (automotive

industry) Bosch: air mass sensor, Pierburg, VDO/Continental


Air control cycles

CPOX air flow rate Two control options: 1) control of air ratio, 2) control of CPOX temperature Option 1 requires long-term stable flow sensors for air and gas (changing gas

qualities to be taken into account), but: very fast load changes possible Option 2 works with different gas qualities, no flow sensor required, but:

overheating or soot formation (at low temperatures) during load changes possible

Cathode air flow rate Mostly used to control the stack temperature closed control loop Minimum oxygen utilization to be considered (40…50 %), otherwise drop of stack

performance and disturbance of control cycle Flow sensor, pressure sensors or pressure switches as flow safeguards

recommended In heat up mode, control of heat up rates via cathode air flow (no sensor needed)


Thank you for your attention

1 oliver posdziech - staxera/sunfire gmbh heat exchangers and air supply

Documents

air supplyheat exchangers

cathode air air supply

air supplylogarithmic

air supplynr

air supplyintroductionnr

details nr

mean fluid properties

enthalpy balancefouriers