EXERGY ANALYSIS OF A SOFC BASED COGENERATION SYSTEM FOR BUILDINGS

Transcription

1 EXERGY ANALYSIS OF A SOFC BASED COGENERATION SYSTEM FOR BUILDINGS Can Ozgur Colpan cocolpan@connect.carleton.ca Ibrahim Dincer, PhD Ibrahim.Dincer@uoit.ca Feridun Hamdullahpur, PhD Feridun_Hamdullahpur@carleton.ca 2008 ASHRAE Winter Meeting New York City, January 19-23

2 Introduction Cogeneration SOFC Outline Literature Survey Objective of this Study Proposed System Configuration SOFC Modeling Exergy Analysis Results Conclusions

3 Introduction - Cogeneration Also known as combined heat and power. Used to increase the fuel utilization efficiency. Classified according to the prime movers: Gas turbines Steam turbines Reciprocating engines Combined cycles Micro turbines Stirling engines Fuel cells: Molten Carbonate Fuel Cell (MCFC) Solid Oxide Fuel Cell (SOFC)

4 Cogeneration in Building Applications Some building applications suitable for cogeneration include: Hospitals Institutional buildings Hotels Office buildings Single and multi-family residential buildings Electrical load 1 MW

5 Introduction - SOFCs High temperature fuel cell ( C) Application areas: Stationary power and heat generation Transportation applications Portable applications Advantages: No need for precious metal electrocatalysts Fuel flexibility Internal reforming Good thermal integration with other systems Disadvantages: Degradation due to carbon deposition and sulphur poisoning Challenges with construction and durability

6 Introduction Operation of SOFCs H 2 H 2 Anode Electrolyt e Cathod e H 2 O H 2 O e - e - e - e - O 2- O 2- Load e - e - e - e - O 2 O 2

7 Introduction Classification of SOFCs Classification criteria Temperature level Cell and stack design Type of support Flow configuration Fuel reforming type Types Low temperature SOFC (LT-SOFC) (500 C 650 C) Intermediate temperature SOFC (IT-SOFC) (650 C 800 C) High temperature SOFC (HT-SOFC) (800 C C) Planar SOFC (Flat-planar, radial-planar) Tubular SOFC (Micro-tubular, tubular) Segmented-in-Series SOFC (or Integrated-planar SOFC) Monolithic SOFC Self-supporting (Anode-supported, cathode-supported, electrolytesupported) External-supporting (Interconnect supported, porous substrate supported) Co-flow Cross-flow Counter-flow External reforming SOFC (ER-SOFC) Direct internal reforming SOFC (DIR-SOFC) Indirect internal reforming SOFC (IIR-SOFC)

8 System Level Modeling Classical approach Energy analysis of the system A better approach Exergy analysis of the system Literature survey on modeling of SOFC based systems: Colpan et al. (2007): Thermodynamic modeling of a DIR-SOFC with anode recirculation and operating with syngas. The effect of recirculation ratio and fuel utilization ratio on cell voltage, power output, electrical efficiency and air utilization ratio are investigated. Granovskii et al. (2007): Exergy and energy analyses for the two SOFC gas turbine systems. Their efficiencies and capabilities to generate power at different rates of oxygen conductivity through the SOFC electrolyte (ion conductive membrane), as well as various efficiencies for natural gas conversion to electricity in the SOFC stack are determined.

9 Literature survey (Continued): System Level Modeling Ghosh and De (2003): The effect of pressure ratio and temperature on the exergy destructions and exergetic efficiencies of an integrated gasification combined cycle with a high-temperature pressurized SOFC in the topping cycle and a single-pressure, non-reheat steam in the bottoming cycle. Douvartzides et al. (2003): The effect of operation parameters on exergy destructions and losses within an ethanol-fueled SOFC system including an external steam reformer, an afterburner, a mixer and two heat exchangers. Calise et al. (2006): A full and partial load exergy analysis of a hybrid SOFC GT power plant.

10 Objective of this Study Analyze the exergetic performance of a SOFC based cogeneration system which may supply the heat and power demand of a neighbourhood. The following are calculated: Exergetic destructions in the components Exergy loss to the surroundings Exergetic efficiency of the system Effect of ambient temperature on the performance

11 System Configuration Fuel Compressor 1 Heat Recovery Steam Generator 9 A Combustor DS SOFC Stack 6 Recuperator 4 D Gas turbine C Air Compressor B 8 3

12 Fuel Input Data Methane Environmental temperature 25 C Environmental pressure Net electrical work output of the system SOFC 1 atm 1 MW Exit Temperature 1000 C Temperature difference between exit and inlet Pressure Operating voltage 100 C 15 atm 0.7 V Active surface area of a single cell 100 cm 2 Fuel utilization ratio 0.85 Thickness of anode 50 µm Thickness of electrolyte 150 µm Thickness of cathode 50 µm Thickness of interconnect 5 mm HRSG (Heat Recovery Steam Generator) Steam drum pressure 12 bar Pinch point 10 C Evaporator approach temperature 10 C Condensate return temperature 25 C Heat loss from HRSG 2% Pressure drop on the air side 5% Gas Turbine Pressure ratio 5:1 Isentropic efficiency 0.85 Electric generator efficiency 0.98 Isentropic efficiency of compressors 0.85

13 STEPS: 1. Derive the equilibrium exit and inlet gas compositions in terms of recirculation ratio and other cell parameters. 2. Solve the chemical equilibrium reaction equations together with the fuel cell related equations. 3. Considering ohmic, activation and concentration polarizations, calculate air utilization ratio by solving the energy balance enclosing the fuel cell. 4. Calculate cell voltage, power output and electrical efficiency of the cell. SOFC Model For more details, please see Colpan et al. (2007).

14 CH 4 2CO CO + H SOFC Model Carbon Deposition Carbon activities 2 C ( s ) CO 2 + 2H + C C ( s ) 2 ( s ) + H 2 O a a c5 c4 = = K K 5 4 x ( H ) x 2 2 x eq CH 4 eq ( CO x ) eq CO 2 eq 2 α c 1 Carbon formation is observed α c < 1 Carbon formation is thermodynamically impossible a c6 = K 6 x x CO eq H 2 O eq x H 2 eq For more details, please see Colpan et al. (2007).

15 Exergy Analysis-I Exergy destruction is due to irreversibilities within a system Friction Expansion Mixing Chemical Reactions Heat transfer through a finite temperature diff. Internal Irreversibilities External Irreversibilities Exergy loss is associated with heat rejection to the surroundings.

16 Exergy Analysis-II The steady state form of control volume exergy balance: T o 0 = 1 Q j W cv + m i exi m e exe Ex T j j i e Where PH KN PT ex = ex + ex + ex + ex CH D Exergetic efficiency of a component: Ex P Ex D + Ex ε = = 1 Ex Ex F F Exergetic efficiency of the overall system: ε = 1 y D y L L

17 Exergy Analysis-III The exergy destruction rate in a component may be compared to the exergy rate of the fuel provided to the overall system as follows: y D = Ex Ex D F The exergy destruction rate of a component may be compared to the total exergy destruction rate within the system giving the ratio. y Ex * D D = E xd,tot The exergy loss ratio is defined similarly by comparing the exergy loss rate to the exergy rate of the fuel provided to the overall system. y L Ex = Ex L F

18 Exergy Analysis-IV Fuel compressor Air compressor Recuperator SOFC (including combustor) Gas turbine HRSG

19 Recirculation ratio, r Carbon activity, α c Results-I 1. Minimum recirculation ratio that will prevent carbon deposition: 2. For r = 0.4, it is found that the air utilization ratio is 17% and the power output of a single cell is W. 3. Thermodynamic properies and exergy flow rate of each state is calculated. State m (kg/s) T ( C) P (kpa) E x ph (kw) E x (kw) E x (kw) ch

20 Results-II Exergy destructions and losses Control Volume (CV) E x D, E xl (kw) CV yd and y L 5% 7% CV2 CV CV CV CV CV Stream % 13% 6% 0% 7% CV3 CV4 CV5 CV6 Stream10 Utilized exergy Effect of ambient temperature on the system performance y * D 19% 1% 15% 0.8 CV2 FUE, ε FUE ε 24% CV3 CV4 CV5 CV % Ambient temperature ( C)

21 Conclusions A fuel utilization efficiency of 68% and an exergetic efficiency of 62% under main operating conditions. Control volume enclosing the SOFC and the combustor has the highest exergy destruction which accounts for 12.5% of the exergy of the fuel and 40.5% of the total exergy destructions. Exergy loss (exergy flow rate of the stack) accounts for 7% of the exergy of the fuel. Fuel utilization efficiency increases, whereas exergetic efficiency decreases, with an increase with the environmental temperature. A better thermodynamic performance compared to other conventional cogeneration systems.

22 Acknowledgements The financial and technical support of an Ontario Premier s Research Excellence Award, the Natural Sciences and Engineering Research Council of Canada, Carleton University and University of Ontario and Institute of Technology is gratefully acknowledged.

23 References Calise, F., Palombo, A., Vanoli, L Design and partial load exergy analysis of hybrid SOFC-GT power plant. Journal of Power Sources pp Colpan, C.O., Dincer, I., Hamdullahpur. F Thermodynamic modeling of direct internal reforming solid oxide fuel cells operating with syngas. International Journal of Hydrogen Energy. 32. pp Colpan, C.O., Dincer, I., Hamdullahpur. F. A review on macro-level modeling of planar solid oxide fuel cells. International Journal of Energy Research. In Press Douvartzides, S.L., Coutelieris, F.A., Tsiakaras, P.E On the systematic optimization of ethanol fed SOFC-based electricity generating systems in terms of energy and exergy. Journal of Power Sources. 114: Ghosh, D., De, S Thermodynamic performance study of an integrated gasification fuel cell combined cycle-an exergy analysis. Proceedings of the Institution of Mechanical Engineers-A. 217(6). pp Granovskii, M., Dincer, I., Rosen, M. A Performance comparison of two combined SOFC gas turbine systems. Journal of Power Sources pp