EXERGY ANALYSIS OF A SOFC BASED COGENERATION SYSTEM FOR BUILDINGS

Transcription

1 EXERGY ANALYSIS OF A SOFC BASE COGENERATION SYSTEM FOR BUILINGS C. Ozgur Clpan 1, Ibrahim incer, Feridun Hamdullahpur 3 1, 3 epartment f Mechanical and Aerspace Engineering epartment, Carletn University, 115 Clnel by rive, Ottawa, Ontari, Canada K1S 5B6 Faculty f Engineering and Applied Science, University f Ontari Institute f Technlgy, 000 Simce Street Nrth, Oshawa, Ontari L1H 7K4, Canada s: cclpan@cnnect.carletn.ca, ibrahim.dincer@uit.ca, feridun_hamdullahpur@carletn.ca Abstract In this paper, eergy analysis f a slid ide fuel cell (SOFC) based cgeneratin system fr buildings is cnducted t investigate the eergetic perfrmance and determine the true lcatins and magnitudes f the eergy destructins/lsses within the system. First, the thermdynamic prperties and chemical gas cmpsitin f the inlet and eit f a direct internal refrming, high temperature SOFC are determined using the given input data. Secnd, an energy analysis is dne fr the ther cmpnents f the system t find all the thermdynamic data related t the plant. Third, the system is divided int several cntrl vlumes and an eergy analysis is applied t each f them t calculate the eergy destructin rates. The results f this study shw that the plant has a fuel utilizatin efficiency f 68%, whereas its eergetic efficiency is 6%. The cmpnent which destructs the mst eergy is the SOFC including the cmbustr which is mainly due t the cmbustin prcess and it accunts fr the 1.5% f the eergy f the fuel and 40.5% f the ttal eergy destructin f the system. Keywrds SOFC, fuel cell, cgeneratin, building, eergy, energy, carbn depsitin. 1. Intrductin uring the past tw decades, there has been increased interest in integrating systems fr cgeneratin, therwise knwn as cmbined heat and pwer, and district heating and cling. Crrespnding initiatives are acknwledged as a key cmpnent f the effrts f many cuntries t respnd t the challenge f climate change and t achieve secure, diverse and sustainable supplies f energy at cmpetitive prices. In cnventinal electricity generatin, nly a little prtin f fuel energy is cnverted int electricity and the remaining is lst as waste heat. Cgeneratin reduces this lss by prducing useful heat. Cgeneratin systems are generally classified accrding t their prime mvers. Currently; gas turbines, steam turbines, reciprcating engines and cmbined cycles are used. There are als new technlgies which are epected t cmpete with the current nes in the fllwing decades. These include fuel cells, micr turbines and Stirling engines. Amng different types f fuel cells, the nes perating at high temperatures have the chance t be used in cgeneratin systems; which are mlten carbnate fuel cell perating between C; and SOFC perating between C. SOFC is an energy cnversin device that cntains an ide in-cnducting electrlyte made frm a ceramic material. The main applicatin area f SOFC is statinary pwer and heat generatin, but smaller sizes f them may be used in transprtatin and prtable applicatins. They have many advantages ver ther fuel cell types: simpler in cncept since nly slid and gas phases eist, n electrlyte management issues, n need fr precius metal electrcatalysts, internal 1

2 refrming f gas mitures including hydrcarbns, and ability t use carbn mnide as fuel. SOFCs may be designed t perate in different temperature levels. High temperature SOFC (HT-SOFC) perating between C is the mst advantageus type in terms f thermal integratin with bttming cycles. In additin t this advantage, high temperature enables lwer hmic and activatin plarizatins; which in turn increase the perating cell vltage. Hwever, startup time increases and the structural integrity becme weaker. SOFCs may be designed as tubular r planar. Planar type is mre cmpact since cells can be stacked withut giving large vids like in the case f tubular design. Additinally, the current path is shrter, hence hmic lsses are lwer. Hwever, there is a need fr gas-tight sealing in planar design, whereas in tubular design, the cells may epand and cntract withut any cnstraints. A schematic f the part f a single planar cell is shwn in Figure 1. The PEN (Psitive/Electrlyte/Negative) structure cnsists f ande, electrlyte and cathde. Many cells shuld be brught tgether t btain meaningful pwer utput, which is als called stacking. Eergy analysis prvides mre insight cmpared t traditinal energy analysis. There are many studies in literature n the eergy analysis f different thermal systems [1-5]. Eergy analysis n SOFC based systems has als increased recently. In these systems, gas turbine, steam turbine and/r gasificatin system are generally integrated with SOFC. Ghsh and e [6] studied the thermdynamic analysis f an integrated gasificatin cmbined cycle with a high-temperature pressurized SOFC in the tpping cycle and a single-pressure, nnreheat steam in the bttming cycle. In their study, they assumed a cnstant temperature fr the SOFC. They neglected plarizatin lsses. The results f their study shw that an verall efficiency abve 54% is achievable fr the cmbined cycle. The same authrs studied the eergy analysis f the same system in their fllwing paper [7]. They discuss the effect f pressure rati and temperature n the eergy destructins and eergetic efficiencies at the system s cmpnents. uvartzides et al. [8] studied the effect f peratin parameters n eergy destructins and lsses within an ethanl-fueled SOFC system including an eternal steam refrmer, an afterburner, a mier and tw heat echangers. The paper by Calise et al. [9] presents a full and partial lad eergy analysis f a hybrid SOFC GT pwer plant which cnsists f: an air cmpressr, a fuel cmpressr, several heat echangers, a radial gas turbine, miers, a catalytic burner, an internal refrming tubular slid ide fuel cell stack, bypass valves, an electrical generatr and an inverter. The plant is simulated at full-lad and part-lad peratin, shwing energy and eergy flws thrugh all its cmpnents and thermdynamic prperties at each key-pint. The primary bjective f this study is t prpse a new cnceptual SOFC based cgeneratin system fr buildings and analyze this system thrugh eergy and its perfrmance thrugh eergy efficiency. Fr this purpse, the mdel develped by the authrs [10, 11] is used fr finding the thermdynamic prperties f SOFC. After finding all the thermdynamic data fr the system, eergy flw rates, eergy destructins and eergy lsses within the system are calculated.. Eergy Analysis Eergy analysis is a methd that uses the cnservatin f mass and cnservatin f energy principles tgether with the secnd law f thermdynamics fr the analysis, design and imprvement f energy systems. The eergy methd is a useful tl fr furthering the gal f mre efficient energy-resurce use, fr it enables the lcatins, types, and true magnitudes f wastes and lsses t be determined. Many engineers and scientists suggest that the thermdynamic perfrmance f a prcess is best evaluated by perfrming an eergy analysis in additin t r in place f cnventinal energy analysis because eergy analysis appears t prvide mre insights and t be mre useful in efficiency imprvement effrts than energy

3 analysis [1]. This paper will als reiterate sme f these aspects f eergy analysis fr equipment thrugh its applicatin n the new prpsed cncept fr a SOFC based cgeneratin system fr buildings. Eergy is the maimum wrk that may be achieved by bringing a system int equilibrium with its envirnment. If we neglect the magnetic, electrical, nuclear, kinetic and ptential effects, there are mainly tw types f eergy: pyhsical and chemical. The first ne measures the amunt f wrk when the system cmes int thermal (T = T ) and mechanical (P = P ) equilibrium. This cnditin is called as restricted dead state. Chemical eergy gives the amunt f wrk when the system is brught frm restricted dead state t dead state. At dead state, in additin t the thermal and mechanical equilibrium, the system is als at chemical equilibrium (µ=µ ). The physical flw eergy fr simple, cmpressible pure substances is given as e PH (h h ) T (s s ) (1) Chemical eergy may be calculated using the tables available in the literature [13,14] r using the fllwing frmulas. Fr water, e CH P (T ) g R T ln (),HO( g ) P Fr an ideal gas miture (the case when all the gas species appear in the envirnment), CH,k R T k ln (3) k e Fr an ideal gas miture (the general case), e CH k CH k e R T ln (4) k k Fr a hydrcarbn fuel, C a H b, e CH h T F s R T (a F b 4 (a ln ( ) h O b 4,CO a h ) s ( ) O,O a ( CO a s ) ab/4,h O( b h CO g ) H O(g) b s b/ ) T,P HO(g) T,P (5) The steady frm f cntrl vlume eergy balance is 0 T 1 Q j W cv m i ei m e ee E T (6) j j i e Unlike energy, eergy is nt generally cnserved but destryed by irreversibilities within a system. These irreversibilities may be classified as internal and eternal irreversibilities. Main surces f internal irreversibilities are frictin, epansin, miing and chemical reactin. 3

4 Eternal irreversibilities arise due t heat transfer thrugh a finite temperature difference. Eergy is lst when the energy assciated with a material r energy stream is rejected t the envirnment. The eergy destructin rate in a cmpnent may be cmpared t the eergy rate f the fuel prvided t the verall system as fllws; y E E (7) F The eergy destructin rate f a cmpnent may be cmpared t the ttal eergy destructin rate within the system giving the rati. y E * E,tt (8) The eergy lss rati is defined similarly by cmparing the eergy lss rate t the eergy rate f the fuel prvided t the verall system. y L E E L (9) F 3. Perfrmance Assessment Parameters There are tw imprtant parameters that are used t assess the perfrmance f a cgeneratin system. The first ne is fuel utilizatin efficiency, FUE, in which nly energy accunting is cnsidered. Its definitin may be given as (W FUE net ) m plant fuel H LHV prcess (10) where LHV is lwer heating value f the fuel. In defining the eergetic efficiency, it is necessary t identify bth the prduct and the fuel fr the system. The prduct represents the desired utputs generated by the system. The fuel represents the resurces epended t prduce the desired result. Eergetic efficiency f the system may be given as E E E P L 1 1 y y L (11) E E F F 4. Energy analysis f the system The system analyzed is shwn in Figure. Fuel and air cmpressrs increase the pressure f fuel and air, respectively, accrding t the perating pressure level f SOFC. There is always an amunt f unutilized fuel in the SOFC eit which depends n the peratin variables f the cell and it is burned in an afterburner t increase the temperature f the fuel cell eit. The cmbusted gas miture enters the gas turbine t generate pwer fr cmpensating the pwer requirement f the cmpressrs. The epanded gas prvides the heat fr increasing the temperature f the fuel and air cmpressr eits accrding t the SOFC inlet temperature requirement. The remaining enthalpy f the gas miture is used t prvide the heat t generate steam. The input data used in energy and eergy analysis f the system is given in Table 1. 4

5 Table 1. Input data f the system Fuel Methane Envirnmental temperature 5 C Envirnmental pressure 1 atm Net electrical wrk utput f the system 1 MW SOFC Eit Temperature 1000 C Temperature difference between 100 C eit and inlet Pressure 15 atm Operating vltage 0.7 V Active surface area f a single cell 100 cm Fuel utilizatin rati 0.85 Thickness f ande 50 µm Thickness f electrlyte 150 µm Thickness f cathde 50 µm Thickness f intercnnect 5 mm HRSG (Heat Recvery Steam Generatr) Steam drum pressure 1 bar Pinch pint 10 C Evapratr apprach temperature 10 C Cndensate return temperature 5 C Heat lss frm HRSG % Pressure drp n the air side 5% Gas Turbine Pressure rati 5:1 Isentrpic efficiency 0.85 Electric generatr efficiency 0.98 Isentrpic efficiency f cmpressrs 0.85 The main assumptins made in the analyses are given belw: The system perates at steady state. Kinetic and ptential energy effects are ignred. Ideal gas principles apply fr the gases. Cmplete cmbustin ccurs in the cmbustr. All the steam eprt frm the system returns as cndensate. Blw dwn requirements and deaeratr vent flws f HRSG are nt taken int accunt. Heat lsses t the envirnment frm the cmpnents are ignred ecept HRSG. Pressure drps alng the cmpnents are ignred ecept HRSG. Gas miture at the fuel channel eit is at chemical equilibrium. Thermdynamic mdeling f direct internal refrming SOFC with ande recirculatin is given in the paper by Clpan et al. [10]. In the first part f the mdel, using principles f thermdynamics, mathematical manipulatins and definitins f sme fuel cell related parameters; eit gas cmpsitin f the ande sectin is derived in terms f etents f the reactins (1)-(14) and mlar flw rate f gas species at the fuel channel inlet. Then using chemical equilibrium equatins and the relatin between the electric current and the mlar flw rate f hydrgen that is utilized, gas cmpsitin f the fuel channel eit is calculated. 5

6 H 1 O H O (1) CH H O 4 CO H O H CO 3H (13) CO (14) In the secnd part f the mdel, air utilizatin rati which measures the amunt f air that shuld be sent thrugh the air channel t carry away the unused heat is calculated fr an insulated fuel cell. Hence, air channel eit gas cmpsitin, plarizatins and wrk utput f the fuel cell are derived in terms f air utilizatin rati. Using the first law f thermdynamics fr the cntrl vlume enclsing the fuel cell, air utilizatin rati is fund. Then, fuel cell utput parameters are calculated using this rati. The prcedure is repeated fr each current density and the plarizatin curve is frmed. Accrding t a given peratin pint, the cell vltage and current density f the cell are determined frm this curve. The recirculatin rati shuld be taken lw enugh t prevent carbn depsitin pssibility. If we take methane as the fuel, the fllwing three reactins are the mst pssible nes fr the frmatin f carbn: CH 4 ( s ) H C (15) CO C ( s ) CO (16) CO H C ( s ) H O (17) The carbn activities f these reactins can be calculated using the fllwing equatins. If the carbn activity is less than unity 1, it means that there is n pssibility f carbn depsitin in the viewpint f thermdynamics. a a a c15 c16 c17 CH K 4 15 eq (18) H eq CO K 16 eq (19) CO eq CO K H 17 eq eq (0) HO eq where K 15, K 16, K 17 are the chemical equilibrium cnstants f Eqs. (15-17). This mdel gives the utput parameters fr a single cell with a given active surface area. Using this mdel, the input and utput pwer f all the devices may be calculated fr a single cell. Accrding t the desired net electrical pwer utput f the system, the required number f cells is cmputed. Then, the mass flw rate depended thermdynamic prperties f the system are fund. 5. Eergy analysis f the system Eergy analysis f a thermdynamic system cnsists f several steps: calculating the physical and chemical eergy flw rates, dividing the system int several cntrl vlumes calculating the eergy destructin rates fr each cntrl vlume calculating the eergy lsses 6

7 calculating the ratis related t the eergy destructin and lss finding the eergetic efficiency f the system Selectin f the cntrl vlumes and eergy balances fr them is shwn in Table. Table. Eergy balances fr the cntrl vlumes f the system N Cntrl Vlume Eergy Balance 1 1 A E EST E 1 E E A 3 4 B E EST E 3 E 4 E B 5 3 EST E E 5 E 4 E 6 E 8 E E E EST E 5 E 6 E 7 E 6 C c E EST E 7 E 8 E EST E 9 E 10 E 1 E 1 11 E Results Using the mdeling technique mentined abve and the input data given in Table 1, the calculatins are dne. First, the recirculatin rati needed t prevent the carbn depsitin is fund. Fr this purpse, an initial recirculatin rati f 0.1 is taken and it is increased by 0.1 since the carbn activity becmes less than 1. The variatin f carbn activity with recirculatin rati is shwn in Table 3. It may be seen that r=0.4 is the minimum recirculatin rati needed. Table 3. Carbn activity fr different recirculatin ratis Recirculatin rati, r Carbn activity, α c Fr r = 0.4, it is fund that the air utilizatin rati is 17% and the pwer utput f a single cell is W. The remaining thermdynamic prperties, physical and chemical eergy flw rates and ttal eergy flw rates f the states f the system are shwn in Table 4. 7

8 After applying the eergy balances, the eergy destructin rates and eergy lss rates are calculated tgether with their relevant ratis. These results are shwn in Table 5 and Figure 3. The fuel utilizatin rati and eergetic efficiency f the fuel cell are fund using Eqs. (10) and (11). The first ne is fund t be 68%, whereas the latter ne is 6%. Table 4. Thermdynamic prperties and eergy flw rates f the states State m (kg/s) T ( C) P (kpa) E ph (kw) E ch (kw) E (kw) Table 5. Eergy destructins and lsses Cntrl Vlume (CV) E, E L (kw) CV CV CV CV CV CV Stream The effect f ambient temperature n the system perfrmance parameters is als investigated and the results are shwn in Figure iscussin In this study, a cgeneratin system based n a pressurized, high temperature, direct internal refrming SOFC is analyzed. The cnfiguratin is selected suitable fr a system at which the pwer generatin is the main reasn f perating the system. In such systems, pressurizing the fuel cell is a necessity since the cell vltage r pwer utput f the cell increases with pressure. Additinally, the enthalpy f the HRSG inlet increases since the temperature difference alng the recuperatr fr the air and fuel flw side decreases; hence, the enthalpy difference rate, fuel utilizatin rati and eergetic efficiency f the system becmes higher. Frm Table 4, it may be interpreted that state 7 which is the eit f the cmbustr has the highest energy quality since its ttal flw eergy rate is higher than ther states. If we invent a device and bring this state int equilibrium with its envirnment, we may achieve.7 MW wrk utput. Frm Table 5, it may be seen that the cntrl vlume enclsing the SOFC and the cmbustr has the highest eergy destructin which is mainly due t the cmbustin f the depleted air and fuel streams f the fuel cell. They accunt fr the 1.5% f the eergy 8

9 f the fuel and 40.5% f the ttal eergy destructins. The nly eergy lss t the envirnment is the eergy flw rate f the stack, which is 7% f the eergy f the fuel. Additinally, the eergetic efficiency f the system is fund t be 6%. In cnventinal cgeneratin systems, a gas turbine is used as the electricity prductin device and its ehaust heat is recvered and utilized t prduce steam. In the bk by Bejan et al. [15], a gas turbine based cgeneratin system is analyzed and it is fund that this system has 50% eergetic efficiency. Hence, this paper shws that fuel cell based cgeneratin systems are very prmising t btain higher efficiencies. Ambient temperature affects the perfrmance f the system analyzed as shwn in Figure 4. A decrease in ambient temperature causes an increase in net electrical pwer utput f the system due t the decrease in the pwer input t the cmpressrs; but the inlet temperature f HRSG reduces which in turn decreases the amunt f steam prduced in the HRSG. When the perfrmance assessment parameters are calculated, it is fund that fuel utilizatin efficiency increases whereas eergetic efficiency decreases with an increase with the envirnmental temperature. As it can be fllwed frm this figure, there are nly a few percentage differences between these efficiencies. Hwever, since the eergetic efficiency gives mre meaningful values cmpared t fuel utilizatin efficiency, it may be suggested that the reader shuld cnsider the values f this parameter fr the perfrmance f the system. 8. Cnclusins Eergy analysis is applied t a new cnceptual SOFC based cgeneratin system fr buildings and eergetic perfrmance f the system is investigated fr a given input data set. This analysis includes the calculatins f eergetic destructins within the system, eergetic lsses t the envirnment and eergetic efficiency f the system. The results shw that the system has a 68% fuel utilizatin efficiency and 6% eergetic efficiency. Cmpared t ther eisting systems, it is als shwn that this system has a better thermdynamic perfrmance. In this study, the results f an O- mdeled SOFC are used fr finding the necessary thermdynamic data f the fuel cell. A mdel with a - r 3- apprach gives better results fr estimating the mass flw rate f the inlet f the fuel cell, and temperature and chemical gas cmpsitin f the eit streams f the fuel cell; hence the perfrmance f the system may be determined mre accurately. The results f this study may als be used as a basis fr a thermecnmic analysis study in which cst flw rates and csts f the prducts are calculated. It shuld be nted that the current price f SOFCs are cnsiderably high; hwever it is epected that the cst will decrease drastically in the near future due t sme technlgical imprvements, use f less epensive materials fr sme key cmpnents (e.g., catalysts, membranes), mass prductin, etc. Using the findings f this study, it may be claimed that this kind f a system shuld replace eisting technlgies when the prices f SOFCs reduce t acceptable values since its perfrmance is better thermdynamically. Additinally, due t the increase n cncerns n glbal warming, the necessity t this kind f a system, which has less pllutant, will be mre bvius in the future fr envirnmentally sustainable buildings. Acknwledgements The financial and technical supprt f an Ontari Premier s Research Ecellence Award, the Natural Sciences and Engineering Research Cuncil f Canada, Carletn University and University f Ontari and Institute f Technlgy is gratefully acknwledged. 9

10 References [1] Clpan, C.O., Yesin, T Energetic, eergetic and thermecnmic analysis f Bilkent cmbined cycle cgeneratin plant. Internatinal Jurnal f Energy Research. 30: [] Clpan, C.O., Yesin, T Thermdynamic and thermecnmic cmparisn f cmbined cycle cgeneratin systems. Internatinal Jurnal f Eergy. 3(3):7-90 [3] Syahrul S, Hamdullahpur F, incer I. 00. Eergy analysis f fluidized bed drying f mist particles. Eergy, An Internatinal Jurnal. : [4] Rsen, M.A., Pedinelli, N., incer, I Energy and eergy analyses f cld thermal strage systems. Internatinal Jurnal f Energy Research. 3(1): [5] incer, I. 00. On energetic, eergetic and envirnmental aspects f drying systems. Internatinal Jurnal f Energy Research. 6(8): [6] Ghsh,., e, S Thermdynamic perfrmance study f an integrated gasificatin fuel cell cmbined cycle-an energy analysis. Prceedings f the Institutin f Mechanical Engineers-A. 17(): [7] Ghsh,., e, S Thermdynamic perfrmance study f an integrated gasificatin fuel cell cmbined cycle-an eergy analysis. Prceedings f the Institutin f Mechanical Engineers-A. 17(6): [8] uvartzides, S.L., Cutelieris, F.A., Tsiakaras, P.E On the systematic ptimizatin f ethanl fed SOFC-based electricity generating systems in terms f energy and eergy. Jurnal f Pwer Surces. 114:03-1 [9] Calise, F., Palmb, A., Vanli, L esign and partial lad eergy analysis f hybrid SOFC-GT pwer plant. Jurnal f Pwer Surces. 158:5-44 [10] Clpan, C.O., incer, I., Hamdullahpur. F Thermdynamic mdeling f direct internal refrming slid ide fuel cells perating with syngas. Internatinal Jurnal f Hydrgen Energy. 3: [11] Clpan, C.O., incer, I. Hamdullahpur, F Effect f recirculatin n carbn depsitin fr direct internal refrming slid ide fuel cells. Prceedings f 3 rd Internatinal Energy, Eergy and Envirnment Sympsium. Evra, Prtugal. [1] incer, I. 00. The rle f eergy in energy plicy making. Energy Plicy. 30: [13] Ahrendts, J ie eergie chemisch reaktinsfahiger systeme. VI-Frschungsheft VI-Verlag, usseldrf:6-33 [14] Szargut, J., Mrris,.R., Steward, F.R Eergy analysis f thermal, chemical, and metallurgical prcesses. Hemisphere. New Yrk: [15] Bejan, A., Tsatsarnis G., Mran M Thermal design and ptimizatin, Jhn Wiley and Sns Inc., U.S.A. 10

11 Intercnnect Fuel channel PEN Structure Air channel Figure 1. Planar SOFC HRSG 9 Fuel Cmpressr 1 A 7 Cmbustr SOFC S 5 6 Recuperatr 4 Gas turbine C Air Cmpressr B 8 3 Figure. SOFC based cgeneratin system 6% 5% 7% 13% 6% 0% 7% CV CV3 CV4 CV5 CV6 Stream10 Utilized eergy 19% 41% 1% 15% 4% CV CV3 CV4 CV5 CV6 Figure 3. (a) Eergy destructins and lsses cmpared t the eergy f the fuel, (b) Eergy destructins f the cmpnents cmpared t the ttal eergy destructin 11

12 0.8 FUE, ε FUE ε Ambient temperature ( C) Figure 4. Effect f ambient temperature n the fuel utilizatin efficiency and eergetic efficiency f the system 1