1. 1 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply

2. 2 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Contents 1.Introduction 2.Heat transfer basics 3.Heat exchanger types 4.SOFC system application 5.Air supply challenges in SOFC systems 6.Blower fundamentals 7.Control of air supply

3. 3 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Introduction

4. 4 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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)

5. 5 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Heat transfer basics

6. 6 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply List of symbols Heat transfer basics

7. 7 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

8. 8 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Heat transfer equations Heat transfer basics  Total heat transfer rate between hot and cold fluids  T mean... mean logarithmic temperature difference  Heat transfer coefficient for single plane wall h i... inner heat transfer coefficient h o... outer heat transfer coefficient s... wall thickness R S... heat resistance (analogy to electrical systems)  Not considered: radiation Homework: 1)Calculate “U A” for s=1 mm 2)Check impact of k, h i and h o 3)Investigate formulation for pipe flow

9. 9 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Heat exchanger modelling 0D modelling  Log Mean Temperature Difference (LMTD)  NTU procedure   -NTU procedure 1D modelling  Cell method 2D/3D modelling  CFD analysis

10. 10 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Logarithmic mean temperature difference (LMTD) Co-flow (parallel flow)Counter-flow Co-flow (parallel flow) Counter-flow

11. 11 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

12. 12 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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 h x 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

13. 13 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Heat transfer coefficient Heat transfer basics Typical orders of magnitude for heat transfer coefficients

14. 14 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Characteristic units Heat transfer basics  Reynolds number Ratio of inert forces to viscous forces in flows  Nusselt number R atio of convective to conductive heat transfer  Péclet number Ratio of advection of flow and thermal diffusion  Prandtl number Ratio of momentum diffusivity and thermal diffusivity Hydraulic diameter D h =4*A/P A... area; P... wetted perimeter GeometryDhDh PipeD Duct2a*b/a+b Parallel plates2*H Flate plateL

15. 15 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Flow characteristics Heat transfer basics Laminar flowTurbulent flow Osborne Reynolds Laminar and turbulent boundary layers over a flat plate Thermal boundary layer Re C = 2100-2300 Critical Reynolds number of transition from laminar to turbulent flow in pipes, ducts and between parallel plates

16. 16 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

17. 17 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Nusselt number correlations Heat transfer basics Kays & Crawford, 1980 Laminar flow correlations

18. 18 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

19. 19 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Heat exchanger types

20. 20 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Flow directions x/L T 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

21. 21 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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)

22. 22 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

23. 23 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply SOFC system application

24. 24 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Typical SOFC system Reformer Evaporator Afterburner SOFC Heat recovery unit Air preheater Fuel preheater Desulphurizer stack Inverter Control system Blower Flow sensor Water pump Flow sensor Deionization Gas valve Flow sensor

25. 25 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

26. 26 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Design criteria §1Maximize heat transfer rates  Optimal system layout “Pinch Point Analysis”  Heat exchanger design for maximal temperature differences (co- /cross or counter-flow) §2Minimize 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 §4Minimize costs SOFC application

27. 27 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Design criteria SOFC application

28. 28 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Heat transfer modelling Sieder & Tate:  Standard calculations of heat transfer coefficients fail  Internal heat conduction has to be considered SOFC application Design of high-temperature gas/gas heat exchangers

29. 29 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

30. 30 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Chromium evaporation Source: Behr

31. 31 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Chemical stability: corrosion Source: Behr

32. 32 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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.

33. 33 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Air supply challenges in SOFC systems

34. 34 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Motivation Effect of pressure loss and blower efficiency on performance of a steam reforming system with partly internal reforming 100 mbar,  Bl =20% 100 mbar,  Bl =30% 50 mbar,  Bl =30% 50 mbar,  Bl =20% 100 mbar,  Bl =20% 100 mbar,  Bl =30% 50 mbar,  Bl =30% 50 mbar,  Bl =20% System efficiency Blower power Heat demand for 1.5 kW DC system: cathode air versus stack inlet temperature

35. 35 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

36. 36 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Blowers in SOFC systems SOFC applications Flow rates / kWDp / T Availability Cathode air blower 70... 200 Nl/min 20... 200 mbar 20 °C Standard products Burner air blower Start-up 100... 250 Nl/min Normal operation 30... 80 Nl/min 10... 50 mbar 20 °C Standard products Gas blower 3... 8 Nl/min 20... 50 mbar 20 °C Safety critical Biogas industry Anode off-gas recirculation 30... 80 Nl/min 10... 50 mbar 100... 800 °C Safety critical Hardly available Cathode air recirculation 70... 200 Nl/min10... 50 mbar 600... 800 °C Hardly available

37. 37 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Cathode air supply options 3) Cathode air recirculation 1) Cathode air/exhaust gas heat exchanger 2) Cathode air/air heat exchanger Off-gas burner SOFC stack Cathode air 900-1100 °C Reformate 700 °C 800 °C Cathode air 900-1100 °C 700 °C 800 °C Reformate 700 °C 800 °C 20 °C Cathode air 900-1100 °C Reformate

38. 38 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

39. 39 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Blower fundamentals

40. 40 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Classification Introduction Discharge Pressure / Suction Pressure = Compression Ratio  = p D /p S Turbo machinery Fan 1 < p D /p S < 1.1 Blower 1.1 < p D /p S < 3.0 Compressor p D /p S > 3.0 pp 50 mbar100 mbar200 mbar Compression ratio p D /p S = 1.05 p D /p S = 1.1 p D /p S = 1.2 TypeFanFan/blowerBlower

41. 41 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

42. 42 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

43. 43 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

44. 44 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Blower power demand Specific values  Theoretical power  Total pressure increase  Dynamic pressure  Shaft Power  Motor power

45. 45 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Blower efficiencies Specific values  Shaft power efficiency V loss … Flow losses in tip of rotor  p loss … Sum of internal pressure losses P m … Mechanical friction losses  Total efficiency  Typical efficiencies0.6 … 0.8 large blowers 0.5 … 0.6 middle sized blowers 0.3 … 0.5 small blowers 0.25 … 0.3 side channel blowers

46. 46 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Some rules of thumb... Specific values  Temperature increase  If equipment characteristics is a quadratic parabola: - Volume flow ~ Speed - Pressure ~ (Speed)² - P M ~ (Speed)³

47. 47 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Performance maps Characteristics Source: R&D Dynamics Characteristic diagramEfficiency diagram

48. 48 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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

49. 49 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Example: Ametek Nautilair Characteristics  =0.41  =0.52  =0.3  =0.41

50. 50 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Example: Elmo-Rietschle G-200 Characteristics

51. 51 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Control of air supply

52. 52 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and 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

53. 53 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply 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)

54. 54 Oliver Posdziech - Staxera/sunfire GmbH Heat Exchangers and Air Supply Thank you for your attention