Thermodynamic Modeling of Solid Oxide Fuel Cell-Gas Turbine Combined Cycle Power Plant

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1 Research Article International Journal o Current Engineering and Technology E-ISSN , P-ISSN INPRESSCO, All Rights Reserved Available at Thermodynamic Modeling o Solid Oxide Fuel Cell-Gas Turbine Combined Cycle Power Plant M. Sreeramulu Ȧ* and K. Deepak Ḃ Ȧ Department o Mechanical Engineering, Maheshwara Engineering College, Hyderabad , Andhra Pradesh, India. Ḃ Department o Mechanical Engineering, Vardhaman College o Engineering, Shamshabad , Andhra Pradesh, India Accepted 10 January 2014, Available online 01 February 2014, Special Issue-2, (February 2014) Abstract Fuel cells oer a more eicient and uel lexible technology that produces heat and power with lower emissions. It is observed that by integrating uel cells with the gas turbines, more than %thermal eiciency can be achieved rom the combined cycle. This proves to be a better exergetic perormance when compared to the conventional gas turbine plants. A Solid Oxide Fuel Cell-Gas Turbine combined cycle power plant is modeled and parametric exergy analysis is done to evaluate the energy eiciency, exergy eiciency and the exergy destruction o each component in the system. The eect o pressure ratio, turbine inlet temperature and ambient temperature on the perormance o the system is investigated by using diesel and gasoline as uels. The outcome o the modeled system reveals that SOFC and combustion chamber are the main sources o exergy destruction. At optimum pressure ratio, the total thermal eiciency using gasoline and diesel is ound to be 71.36% and.% while the exergy eiciency is ound to be.53% and.76% respectively. Keywords: SOFC, Gas turbine, Exergy, Exergy destruction, Combined cycle. 1. Introduction 1 High temperature uel cells like Solid Oxide Fuel Cell and Molten Carbonate Fuel Cell have more potential to achieve higher eiciency or electricity production and are being used in USA (Williams, 2004).It is observed that pressurized SOFC increases the perormance with an increase in output voltage. However the improved perormance allows the integration o the SOFC with gas turbine and needs the low o hot pressurized gas to operate. Since the SOFC stack operates at 1000 C it produces a high temperature exhaust gas. At elevated pressure, the hot pressurized exhaust gas can be used to drive a turbine. In SOFC-GT combined system, the compressed air needed by the uel cell is supplied rom the compressor o the gas turbine plant. The SOFC perorms as combustor and the exhaust rom the SOFC through combustion chamber drives the turbine and generator. Electrical Power is thus generated by the SOFC (DC) and the generator (AC) using the same uel and air low. While the thermal eiciency o a conventional gas turbine plant is around 40%, the SOFC / MCFC integrated gas turbines will have an eiciency o 56%-76% with varied conigurations (Haseli, et al, 2008). The parametric exergy analysis o each component o the hybrid system is revealed rom the literature survey (Cocco, et al, 2007).Siemens-Westinghouse power company developed the tubular SOFCs or dierent applications o stationary power generation. By integrating the SOFC stacks with *Corresponding author: M. Sreeramulu gas turbine and pressuring the system (PSOFC/GT),the eiciencies as high as -75% could be obtained (Casanova, 1988). An internal reorming hybrid SOFC/GT system could get an electrical eiciency more than 60 %(Chan, et al,2002).the SOFC/GT hybrid system can be extended to multi MW power generation system depending upon commercially available gas turbines(song, et al, 2006). The SOFC systems were also analyzed by Calise, et al (Calise, et al, 2006). Zhang et al have presented the integration strategies or SOFCs (Zhang, et al, 2010).Douvartzideset al revealed an energyexergy analyses to optimize the operation conditions o a SOFC/GT power plant considering only hydrogen oxidation within the uel cell and rejecting the eect o the cell losses instead o methane reorming and carbon monoxide conversion (Douvartzides, et al, 2004).Granovskii et al, have compared the perormance o two combined SOFC-GT systems (Granovskii, et al, 2007). A thermodynamic exergy analysis o a combined gas turbine power system with a solid oxide uel cell was carried by Haseli et al(haseli, et al, 2008).The application o Yttria-stabilised zirconia cermet anodes in SOFC systems allows the conversion o methane into hydrogen and corbon monoxide on their suraces. This consists o two simultaneous processes; conversion o methane to synthesis gas (internal reorming) and electricity generation via oxidation o the synthesized gas(cocco, et al, 2007). The cogeneration eiciencies o uel cell systems have also been evaluated and compared with exergy methods(granovskii, et al, 2008).Zhu and Kee have revealed the impact o uel utilization actor on DOI: International Conerence on Advances in Mechanical Sciences 2014

2 M. Sreeramulu et al International Journal o Current Engineering and Technology, Special Issue-2 (Feb 2014) SOFC eiciency using a detailed electrochemical model and generated eiciency maps which give the range o methane-steam mixtures or maximum eiciency (Zhu and Kee, 2006). The exergy analysis o combined cycle power generation unit gives the details o exergy eiciency, exergy loss and exergy destruction or individual components in the plant and also or the overall plant (Reddy and Mohammed, 2007).The pressure ratio and the turbine inlet temperature are considered as the key parameters in the analysis o combined cycle power plant (Srinivas, et al, 2006).The main objective o the present work is to investigate the perormance o SOFC-GT plant using diesel and gasoline as uels and to evaluate the exergy losses in each component o the plant. The results may be utilized or the development o eicient operating conditions o the plant. Fig. 1 Schematic low diagram o the SOFC- GT combined cycle power generating system; REC: recuperator, FC: uel cell, CC: combustion chamber, COM: compressor, TUR: turbine, GEN: Generator. 2. Thermodynamic modeling and analysis Assumptions made: Cell area, A c = 834 cm 2 Cell voltage, V = V Current density, j = 0.3 A/cm 2 HHV o Diesel = kj/kg LHV o gasoline = kj/kg Air utilization actor, U a = 25 % Fuel utilization actor, U =85% SOFC stack temp, T stack = 1273K DC AC Inverter Eiciency, η inverter = 0.89 DC power output rom uel cell stack = 2000 kw Speciic chemical exergy o Diesel = 430J/kg Speciic chemical exergy o Gasoline= kj/kg The schematic low diagram o the SOFC-GT combined cycle power generating system is shown in Fig. 1. The uel is supplied to the SOFC and combustion chamber. The air is pressurized in the compressor, preheated in the recuperator and is supplied to the cathode o the uel cell. The outlet air rom cathode is used to burn the residual hydrogen, carbon oxide and uel in the anode outlet gas. As the products o the chemical reaction are lean, additional uel is injected into the combustion chamber to stabilize the combustion. The extra uel supplied is not intended or increasing the turbine inlet temperature. The high pressure lue gas rom combustion chamber is expanded in the turbine. The exhaust gas rom the turbine is utilized to preheat the compressor outlet air in the recuperator. A computer program is developed to perorm the analysis o the plant which consists o several control loops to calculate the thermodynamic properties o the luid and exergy values at various state points. The eects o system parameters viz. compressor pressure ratio, turbine inlet temperature, air uel ratio and ambient temperature on the plant perormance are depicted. The ollowing are the chemical reactions that take place generally in SOFC during power generation (Rao, et al, 2003). 2 Anode : H 2 O H 2O 2e (1) 2 CO O CO2 2e 2 Cathode : O2 4e 2 O (3) In the current analysis, it is assumed that the uel reacts with H 2 O and releases H 2 and CO. CO again reacts with H 2 O in shit and produces H 2. The heat required or reormer is supplied by the SOFC. Ideal voltage values or an intermediate temperature SOFC operating at C and C are 0.99V and 0.91V respectively (Uechi, et al,2004).the thermodynamic perormances o all the components o the system are analyzed using the ollowing governing equations. 2.1 SOFC The rate o heat production due to irreversibilities (Zhang, et al, 2010) Q,, gen Fc Pele DC 1 10 Vc, kw (4) Mass low rate o air P 7 ele, DC ma, FC Vc, kg/s (5) Physical exergy T P Ex, phy mc p T T0 T0 ln RT0 ln T0 P0 (6) Chemical exergy xi E x, chemical m R T0 xi ln yi (7) Irreversibility o the combustion reaction I T S S S S S FC 0 p 4 p 0 a 3 a 0 reaction 0 (8) T0 S rxn m c LHV 1 (9) by energy balance(calise, et al, 2007) m h m U LHV m 1 U h, in P m h c c ele, DC 4 4 By exergy balance (Calise, et al, 2007) Pele, DC ex, FC m m U (2) (10) m m c m 2.2 Combustion Chamber by energy balance t c ch, (11) 511 International Conerence on Advances in Mechanical Sciences 2014

3 Power Developed, kw Exergy Eiciency, % Thermal Eiciency, % M. Sreeramulu et al International Journal o Current Engineering and Technology, Special Issue-2 (Feb 2014) m U m h Q m h Q 0 3 c 4 combustion 5 5 loss (12) where Q m U m LHV combustion c 1 c (13) Q m U m LHV loss c 1 c 1 combustion (14) By exergy balance irreversibility in combustion chamber I S S S S S CC p 5 p 0 a 4 a 0 rxn 0 (15) Exergy eiciency m55 m44 ex, CC 100 m c 1 U ch, m cc ph ch (16) 2.3 Compressor The variation o cycle and exergy eiciency with the variation o pressure ratio is depicted in Fig. 2 and Fig Fig. 2 Variation o cycle thermal eiciency with pressure ratio Irreversibility in the compressor T S I compressor 0 2 S1 Exergy eiciency m1 compressor m 1 2 h 2 1 h 2.4 Recuperator Irreversibility in the recuperator Exergy eiciency m Ψ ηex,rec m Ψ Gas Turbine Ψ Ψ Rate o exergy loss in the gas turbine I m T S gasturbine Exergy eiciency W m gasturbine S6 5 gasturbine Perormance o the plant Total net power develop by the system (17) (18) (19) (20) (21) (22) Fig.3Variation o cycle exergy eiciency with pressure ratio The thermal and exergy eiciencies increase with the pressure ratio up to 9 and then decrease. Using gasoline as uel, at optimum pressure ratio9the total thermal eiciency and the exergy eiciency is ound to be 71.36% and.53%. Similarly using diesel as uel, at the optimum pressure ratio 9 the total thermal and exergy eiciencies are ound to be.% and.76% respectively. Fig. 4 presents the variation o net power output rom the gas turbine with the pressure ratio. 900 P net PFC, AC Pgen (23) The total thermal eiciency o the cycle Pnet th, cycle 100 Q tot The exergy eiciency o the cycle Pnet ex cycle m, m t, ch 3. Results and discussion 100 (24) (25) Fig.4 Eect o the pressure ratio on net power developed by the generator 512 International Conerence on Advances in Mechanical Sciences 2014

4 Thermal Eiciency, % Exergy Destruction, kw Net Power Developed, kw Exergy Destruction, kw Exergy Eiciency, % M. Sreeramulu et al International Journal o Current Engineering and Technology, Special Issue-2 (Feb 2014) The increase in power output is due to the higher cell e. m.. The maximum net power output is kw at the optimum pressure ratio 9. The dierence between enthalpy o reaction and cell electrical output is the heat energy available or raising the temperatures o input uel gas and air in uel cell up to the cell operating temperature. The greater the heat or raising the temperature o gas and air streams, the more is the exergy loss Fig.5 Eect o the pressure ratio on exergy destruction o the system With higher pressure ratio, this heat decreases as the dierence between uel cell operating temperature and the temperature o air to the uel cell decreases. Hence, the exergy loss decreases and exergetic eiciency increases. Fig. 5 depicts the variation o exergy destruction with the increase in pressure ratio due to increase irreversibilities in all the components particularly in combustion chamber and uel cell. At higher pressure ratio, the electrochemical reaction in the uel cell increases resulting in less combustibles or combustion in combustion chamber. Hence, the exergy destruction due to chemical reaction in combustion chamber decreases at higher pressure ratio. As a result, the exergy loss decreases and exergetic eiciency increases or combustion chamber with higher pressure ratio. With increase in pressure ratio, the temperature o the exhaust gas orm the gas turbine decreases. Hence, at high pressure ratio, the heat transer in the recuperator occurs at low temperature dierence. This gives lower exergy loss at high pressure ratio in the recuperator. Fig.6 and 7 show the variation o thermal and exergy eiciency with the turbine inlet temperature. The eiciencies increase with increase in turbine inlet temperature It is due to the utilization o exergy o unburnt uel rom the uel cell and the additional supply o uel in the combustion chamber. In Fig.8 the variation o net power developed by the gas turbine with the turbine inlet temperature is presented. The increase in power output requires higher uel low rate which leads to the increase in total exergy destruction. So power output rom turbine can be controlled by adjusting air uel ratio Fig.7 Eect o the turbine inlet temperature on exergy eiciency o the system Fig. 8: Eect o the turbine inlet temperature on net power developed by the generator. The variation o exergy destruction with the turbine inlet temperature is presented in Fig. 9.Both the exergy destruction and net power output rom the turbine as well are ound to increase with increase in turbine inlet temperature. This is due to the act that the temperature o the working luid in the gas turbine is high. Power output o about 30 to50% o uel cell power can be obtained rom the generator. The eect o increase in ambient temperature on the variation o eiciency is depicted in the Fig Fig.6 Eect o the turbine inlet temperature on thermal eiciency o the system International Conerence on Advances in Mechanical Sciences 2014

5 Net Power Developed, kw Eiciency, % M. Sreeramulu et al International Journal o Current Engineering and Technology, Special Issue-2 (Feb 2014) Fig.9 Eecto Turbine inlet temperature on exergy destruction o the system Fig.10 Eecto the ambient temperature o air on thermal and exergy eiciency o the system Thermal Eiciency Exergy Eiciency Ambient Temperature, K Ambient Temperature, K Fig.11 Eect o the ambient temperature o air on net power developed by the generator. Due to higher power requirements, the cycle eiciency decreases with an increase in ambient temperature.fig.11 presents the variation o net power developed by the gas turbine with the ambient temperature o air. The net power output rom the gas turbine is decreases by 106 kw, or every 20 C increase in the ambient temperature. The maximum exergy destruction occurs in the SOFC and combustion chamber irrespective o the type o uel used. This is due to the high temperature o the working luid, chemical reactions and a small pressure drop. At the optimum pressure ratio o 9, the maximum exergy destruction is ound to be 1622 kw or gasoline and 1594 kw or diesel. Fig.12 shows the exergy o working luid in each component o the system. Fig.12 Exergy o working luid in each component o the system. Table 1Comparison o perormance o SOFC-GT power generating system with diesel and gasoline uels Fuel used Particulars Diesel Gasoline Thermal eiciency. % 71.36% Exergy eiciency.76 %.53% Exergy Destruction 1594 kw 1622kW Conclusion 1) Combined cycle power plants have higher eiciency than those o recuperated conventional gas turbine plants because the exergy losses o combustion are minimized. 2) An increase in pressure ratio results in lower rate o exergy destruction o the plant, which increases the total thermal and exergy eiciency o the system. 3) The increase in turbine inlet temperature and ambient temperature yields higher rate o exergy destruction o the plant. 4) The generating eiciency peaks at a particular pressure ratio, but the variation o pressure ratio near the optimum point does not lead to a remarkable dierence. 5) The exergy destruction in uel cell is highest compared to other components in the system, though it is an eicient device. 6) Exergy analysis o each component provides better understanding o losses at various states o the system. 7) Gasoline gives higher energy and exergy eiciencies compared to diesel. 8) Diesel ueled SOFC-GT hybrid systems are more clean and eicient energy solutions compared to internal combustion engines or electrical power generation. Nomenclature Ψ I HHV h I j LHV m P Q Q Gen, FC speciic exergy uel (kj/kg) exergy destruction rate (kw) higher heating value (kj/kg) enthalpy (kj/kg) current (ma) current density (ma/cm2) lower heating value (kj/kg) mass low rate (kg/s) pressure (kpa) heat transer rate (kw) heat generation rate within the cell stack (kw) R universal gas constant (8.314 J/mole K) T temperature (K) To reerence temperature (K) W power (kw) W FC,dc DC power output o the cell stack (kw) Wnet net power output o the plant (kw) λ Stoichiometric constant 514 International Conerence on Advances in Mechanical Sciences 2014

6 M. Sreeramulu et al International Journal o Current Engineering and Technology, Special Issue-2 (Feb 2014) Subscript act cyc c Comb Conc ch, Ex FC m Gen In Ohm Out Ph, th, th Reerences activation Cycle Cell combustion chamber concentration chemical exergy o uel (kj/kg) exergy uel cell mechanical exergy o uel (kj/kg) generator inlet ohmic outlet physical exergy o uel (kj/kg) thermal exergy o uel (kj/kg) thermal M.C.Williams,(2004), Fuel Cell Handbook, 7th edition, EG&G Technical Services, Inc. Morgantown, West Virginia. Y.Haseli, I.Dincer, G.F.Naterer,(2008), Thermodynamic modeling o a gas turbine cycle combined with a solid oxide uel cell, International Journal o Hydrogen Energy 33, pp D.Cocco and V.Tola,(2007), Comparative perormance analysis o internal and external reorming o methanol in SOFC MGT hybrid power plants, ASME Journal o Engineering or Gas Turbines and Power 129, 2, pp A. D. Rao and G.S.Samuelsen,(2003), A thermodynamic analysis o tubular solid oxide uel cell based hybrid systems, transactions o the ASME, J.Engg.Gas Turbines and Power 125, 1, pp.59-. Casanova,A.,(1998), A consortium approach to commercialized westing house solid oxide uel cell Technology, Journal o Power Sources,71, pp.65-. Chan,S.H.,H.K.Ho and Y.Tian,(2002), Modelling o simple hybrid solid oxide uel cell and gas turbine power plant, J.Power Sources,109, pp Song,T.W., J.L.Sohn, T.S.Kim, and S.T.Ro,(2006), Perormance characteristics o a MW-Class SOFC/GT hybrid system based on a commercially available gas turbine, J.Power Sources,158, pp Calise,F., M.D.Accadia, M.L.Vanoli and M.R.VonSpakovsky,(2006), Single-level optimization o a hybrid SOFC/GT power plant, Journal o Power Sources,159, pp Calise,F., M.D.Accadia, M.L.Vanoli and M.R.VonSpakovsky,(2007), Full load synthesis/design optimization o a hybrid SOFC/GT power plant. Energy, 31, pp Zhang, X,S.H.Chan,G.Li,H.K.Ho,J.Li and Z.Feng,(2010). A review o integration strategies or solid oxide uel cells J.Power Sources,195, pp.5-2. Douvarttzides,S., Coutelieris,F.,Tsiakaras,P., (2004), Exergy analysis o a solid oxide uel cell power plant ed by either ethanol or methane, Journal o Power Sources,131, pp Granovskii, M.,I. Dincer and M.A.Rosen, (2007). Perormance comparison o two combined SOFC-Gas turbine systems, J.PowerSources, 165, pp Haseli.Y.,I.Dincer and G.F.Naterer,(2008), Thermodynamic analysis o combined gas turbine power system with a solid oxide uel cell through exergy.thermochimicaacta, 480, 1-9 Companari,S., (2001), Thermodynamic model and parametric analysis o a tubular soc module, Journal o Power Sources, 92, pp Granovskii,M.,I.Dincer and M.ARosen, (2008),Exergy analysis o gas turbine cycle with steam generation or methane conversion within solid oxide uel cells, ASME J.Fuel Cell Sci.Technol., 5(3),p.03, 1005 Zhu,H., and Kee,R.J., (2006),Thermodynamics o SOFC eiciency and uel utilization as unction o uel mixtures and operating conditions, Journal o Power Sources,161, pp Reddy,B.V.and Mohammed, K., (2007), Exergy analysis o a natural gas ired combined cycle power generation unit, Int.J.Exergy,Vol.4,No.2, pp Srinivas, T., Gupta, A.V.S.S.K.S., Reddy, B.V.,and Nag,P.K., (2006), Parametric analysis o a coal based combined cycle power plant. Int.J.Energy Res.,30(1), pp H.Uechi, Sh.Kimijima, N.Kasagi, (2004), Cycle analysis o gas turbine uel cell cycle hybrid micro generation system, ASME Journal o Engineering or Gas Turbines and Power 126, 4, pp International Conerence on Advances in Mechanical Sciences 2014