1、108 2008 ASHRAE ABSTRACTIn this paper, exergy analysis of a solid oxide fuel cell(SOFC) based cogeneration system for buildings is conductedto investigate the exergetic performance and determine thetrue locations and magnitudes of the exergy destructions/losses within the system. First, the thermody
2、namic propertiesand chemical gas composition of the inlet and exit of a directinternal reforming, high temperature SOFC are determinedusing the given input data. Second, an energy analysis is donefor the other components of the system to find all the thermo-dynamic data related to the plant. Third,
3、the system is dividedinto several control volumes and an exergy analysis is appliedto each of them to calculate the exergy destruction rates. Theresults of this study show that the plant has a fuel utilizationefficiency of 68%, whereas its exergetic efficiency is 62%. Thecomponent which destructs th
4、e most exergy is the SOFCincluding the combustor which is mainly due to the combustionprocess and it accounts for the 12.5% of the exergy of the fueland 40.5% of the total exergy destruction of the system.INTRODUCTIONDuring the past two decades, there has been increasedinterest in integrating system
5、s for cogeneration, otherwiseknown as combined heat and power, and district heating andcooling. Corresponding initiatives are acknowledged as a keycomponent of the efforts of many countries to respond to thechallenge of climate change and to achieve secure, diverse andsustainable supplies of energy
6、at competitive prices. In conventional electricity generation, only a little portionof fuel energy is converted into electricity and the remainingis lost as waste heat. Cogeneration reduces this loss by produc-ing useful heat. Cogeneration systems are generally classifiedaccording to their prime mov
7、ers. Currently; gas turbines,steam turbines, reciprocating engines and combined cycles areused. There are also new technologies which are expected tocompete with the current ones in the following decades. Theseinclude fuel cells, micro turbines and Stirling engines.Among different types of fuel cell
8、s, the ones operating athigh temperatures have the chance to be used in cogenerationsystems; which are molten carbonate fuel cell operatingbetween 600-700C; and SOFC operating between 500-1000C. SOFC is an energy conversion device that contains anoxide ion-conducting electrolyte made from a ceramic
9、mate-rial. The main application area of SOFC is stationary powerand heat generation, but smaller sizes of them may be used intransportation and portable applications. They have manyadvantages over other fuel cell types: simpler in concept sinceonly solid and gas phases exist, no electrolyte manageme
10、ntissues, no need for precious metal electrocatalysts, internalreforming of gas mixtures including hydrocarbons, and abilityto use carbon monoxide as fuel. SOFCs may be designed tooperate in different temperature levels. High temperatureSOFC (HT-SOFC) operating between 850-1000 C is the mostadvantag
11、eous type in terms of thermal integration withbottoming cycles. In addition to this advantage, high temper-ature enables lower ohmic and activation polarizations; whichin turn increase the operating cell voltage. However, startuptime increases and the structural integrity become weaker. SOFCs may be
12、 designed as tubular or planar. Planar typeis more compact since cells can be stacked without givinglarge voids like in the case of tubular design. Additionally, thecurrent path is shorter, hence ohmic losses are lower. However,there is a need for gas-tight sealing in planar design, whereasin tubula
13、r design, the cells may expand and contract withoutExergy Analysis of a SOFC-BasedCogeneration System for BuildingsC. Ozgur Colpan Ibrahim Dincer Feridun HamdullahpurStudent Member ASHRAE Member ASHRAEC. Ozgur Colpan is a doctoral student and Feridun Hamdullahpur is a professor in the Mechanical and
14、 Aerospace Engineering Departmentand Ibrahim Dincer is a professor of Mechanical Engineering at the University of Ontario Institute of Technology, Oshawa, Canada.NY-08-0162008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Trans
15、actions, Volume 114, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 109any constraints. A schematic of the part of a single planar cellis shown in Figu
16、re 1. The PEN (Positive/Electrolyte/Negative)structure consists of anode, electrolyte and cathode. Manycells should be brought together to obtain meaningful poweroutput, which is also called stacking.Exergy analysis provides more insight compared to tradi-tional energy analysis. There are many studi
17、es in literature onthe exergy analysis of different thermal systems 1-5. Exergyanalysis on SOFC based systems has also increased recently.In these systems, gas turbine, steam turbine and/or gasificationsystem are generally integrated with SOFC. Ghosh and De 6studied the thermodynamic analysis of an
18、integrated gasifica-tion combined cycle with a high-temperature pressurizedSOFC in the topping cycle and a single-pressure, non-reheatsteam in the bottoming cycle. In their study, they assumed aconstant temperature for the SOFC. They neglected polariza-tion losses. The results of their study show th
19、at an overall effi-ciency above 54% is achievable for the combined cycle. Thesame authors studied the exergy analysis of the same systemin their following paper 7. They discuss the effect of pressureratio and temperature on the exergy destructions and exergeticefficiencies at the systems components.
20、 Douvartzides et al.8 studied the effect of operation parameters on exergydestructions and losses within an ethanol-fueled SOFC systemincluding an external steam reformer, an afterburner, a mixerand two heat exchangers. The paper by Calise et al. 9 pres-ents a full and partial load exergy analysis o
21、f a hybrid SOFCGT power plant which consists of: an air compressor, a fuelcompressor, several heat exchangers, a radial gas turbine,mixers, a catalytic burner, an internal reforming tubular solidoxide fuel cell stack, bypass valves, an electrical generator andan inverter. The plant is simulated at f
22、ull-load and part-loadoperation, showing energy and exergy flows through all itscomponents and thermodynamic properties at each key-point.The primary objective of this study is to propose a newconceptual SOFC based cogeneration system for buildingsand analyze this system through exergy and its perfo
23、rmancethrough exergy efficiency. For this purpose, the model devel-oped by the authors 10, 11 is used for finding the thermody-namic properties of SOFC. After finding all thethermodynamic data for the system, exergy flow rates, exergydestructions and exergy losses within the system are calcu-lated.
24、EXERGY ANALYSISExergy analysis is a method that uses the conservation ofmass and conservation of energy principles together with thesecond law of thermodynamics for the analysis, design andimprovement of energy systems. The exergy method is auseful tool for furthering the goal of more efficient ener
25、gy-resource use, for it enables the locations, types, and truemagnitudes of wastes and losses to be determined. Many engi-neers and scientists suggest that the thermodynamic perfor-mance of a process is best evaluated by performing an exergyanalysis in addition to or in place of conventional energy
26、anal-ysis because exergy analysis appears to provide more insightsand to be more useful in efficiency improvement efforts thanenergy analysis 12. This paper will also reiterate some ofthese aspects of exergy analysis for equipment through itsapplication on the new proposed concept for a SOFC basedco
27、generation system for buildings.Exergy is the maximum work that may be achieved bybringing a system into equilibrium with its environment. If weneglect the magnetic, electrical, nuclear, kinetic and potentialeffects, there are mainly two types of exergy: physical andchemical. The first one measures
28、the amount of work when thesystem comes into thermal (T = To) and mechanical (P = Po)equilibrium. This condition is called as restricted dead state.Chemical exergy gives the amount of work when the systemis brought from restricted dead state to dead state. At deadstate, in addition to the thermal an
29、d mechanical equilibrium,the system is also at chemical equilibrium (=o). The physical flow exergy for simple, compressible puresubstances is given as(1)Chemical exergy may be calculated using the tables availablein the literature 13,14 or using the following formulas. Forwater,(2)For an ideal gas m
30、ixture (the case when all the gas speciesappear in the environment),(3)For an ideal gas mixture (the general case),(4)For a hydrocarbon fuel, CaHb,exPHhho()Tosso()=exCHRToPgTo()xoH2Og(),Po-ln=exCHRToxkxok,xk-ln=exCHxkexkCHRToxkxkln+=Figure 1 Planar SOFC.110 ASHRAE Transactions(5)The steady form of c
31、ontrol volume exergy balance is(6)Unlike energy, exergy is not generally conserved butdestroyed by irreversibilities within a system. These irrevers-ibilities may be classified as internal and external irreversibil-ities. Main sources of internal irreversibilities are friction,expansion, mixing and
32、chemical reaction. External irrevers-ibilities arise due to heat transfer through a finite temperaturedifference. Exergy is lost when the energy associated with amaterial or energy stream is rejected to the environment.The exergy destruction rate in a component may becompared to the exergy rate of t
33、he fuel provided to the overallsystem as follows;(7)The exergy destruction rate of a component may becompared to the total exergy destruction rate within the systemgiving the ratio.(8)The exergy loss ratio is defined similarly by comparingthe exergy loss rate to the exergy rate of the fuel provided
34、tothe overall system.(9)PERFORMANCE ASSESSMENT PARAMETERSThere are two important parameters that are used to assessthe performance of a cogeneration system. The first one is fuelutilization efficiency, FUE, in which only energy accountingis considered. Its definition may be given as(10)where LHV = l
35、ower heating value of the fuel.In defining the exergetic efficiency, it is necessary to iden-tify both the product and the fuel for the system. The productrepresents the desired outputs generated by the system. Thefuel represents the resources expended to produce the desiredresult. Exergetic efficie
36、ncy of the system may be given as(11)ENERGY ANALYSIS OF THE SYSTEMThe system analyzed is shown in Figure 2. Fuel and aircompressors increase the pressure of fuel and air, respectively,according to the operating pressure level of SOFC. There isalways an amount of unutilized fuel in the SOFC exit whic
37、hdepends on the operation variables of the cell and it is burnedin an afterburner to increase the temperature of the fuel cellexit. The combusted gas mixture enters the gas turbine togenerate power for compensating the power requirement ofthe compressors. The expanded gas provides the heat forincrea
38、sing the temperature of the fuel and air compressor exitsaccording to the SOFC inlet temperature requirement. Theremaining enthalpy of the gas mixture is used to provide theheat to generate steam. The input data used in energy andexergy analysis of the system is given in Table 1.The main assumptions
39、 made in the analyses are givenbelow:The system operates at steady state.Kinetic and potential energy effects are ignored.Ideal gas principles apply for the gases.Complete combustion occurs in the combustor.All the steam export from the system returns as conden-sate.Blow down requirements and deaera
40、tor vent flows ofHRSG are not taken into account.Heat losses to the environment from the components areignored except HRSG.Pressure drops along the components are ignored exceptHRSG.Gas mixture at the fuel channel exit is at chemical equi-librium.Thermodynamic modeling of direct internal reformingSO
41、FC with anode recirculation is given in the paper by Colpanet al. 10. In the first part of the model, using principles ofthermodynamics, mathematical manipulations and definitionsof some fuel cell related parameters; exit gas composition ofthe anode section is derived in terms of extents of the reac
42、tions(12)-(14) and molar flow rate of gas species at the fuel channelinlet. Then using chemical equilibrium equations and the rela-tion between the electric current and the molar flow rate ofhydrogen that is utilized, gas composition of the fuel channelexit is calculated.exCHhFab4-+hO2ahCO2b2- hH2Og
43、()+ ToPo,()=T0sFab4-+sO2asCO2b2- sH2Og()+ ToPo,()+ RT0x0 O2,()ab4+x0 CO2,()ax0 H2Og(),()b 2-ln01T0Tj-QjWcv mieximeexeExDei+j=yDExDExF-=yD*ExDExDtot,-=yLExLExF-=FUEWnet()plantHprocess+mfuelLHV-=ExPExF- 1ExDExL+ExF-1yDyL= =ASHRAE Transactions 111(12)(13)(14)In the second part of the model, air utiliza
44、tion ratio whichmeasures the amount of air that should be sent through the airchannel to carry away the unused heat is calculated for aninsulated fuel cell. Hence, air channel exit gas composition,polarizations and work output of the fuel cell are derived interms of air utilization ratio. Using the
45、first law of thermody-namics for the control volume enclosing the fuel cell, air utili-zation ratio is found. Then, fuel cell output parameters arecalculated using this ratio. The procedure is repeated for eachcurrent density and the polarization curve is formed. Accord-ing to a given operation poin
46、t, the cell voltage and currentdensity of the cell are determined from this curve.The recirculation ratio should be taken low enough toprevent carbon deposition possibility. If we take methane asthe fuel, the following three reactions are the most possibleones for the formation of carbon:(15)(16)(17
47、)The carbon activities of these reactions can be calculatedusing the following equations. If the carbon activity is lessthan unity “1”, it means that there is no possibility of carbondeposition in the viewpoint of thermodynamics.Table 1. Input Data of the SystemFuel MethaneEnvironmental temperature
48、25 CEnvironmental pressure 1 atmNet electrical work output of the system 1 MWSOFCExit Temperature 1000 CTemperature difference between exit and inlet 100 CPressure 15 atmOperating voltage 0.7 VActive surface area of a single cell100 cm2Fuel utilization ratio 0.85Thickness of anode 50 mThickness of electrolyte 150 mThickness of cathode 50 mThickness of interconnect 5 mmHRSG (Heat Recovery Steam Generator) Steam drum pressure 12 barPinch point 10 CEvaporator approach temperature 10 CCondensate return temperature 25 CHeat los
