Exergoenvironmental Analysis of Heat Recovery from Solid Oxide Fuel Cell System for Cooling Applications

Transcription

1 2203 A publcaton of CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 2015 Chef Edtors: Sauro Perucc, Jří J. Klemeš Copyrght 2015, AIDIC Servz S.r.l., ISBN ; ISSN The Italan Assocaton of Chemcal Engneerng Onlne at DOI: /CET Exergoenvronmental Analyss of Heat Recovery from Sold Oxde Fuel Cell System for Coolng Applcatons Phancha Tppawan a,b, Amorncha Arpornwchanop* a, Ibrahm Dncer b a Computatonal Process Engneerng Research Unt, Department of Chemcal Engneerng, Facuty of Engneerng, Chulalongkorn Unversty, Bangkok 10330, Thaland b Clean Energy Research Laboratory (CERL), Faculty of Engneerng and Appled Scence, Unversty of Ontaro Insttute of Technology (UOIT), Ontaro L1H 7K4, Canada Amormcha.a@chula.ac.th A recovery of hgh-qualty by-product heat from a sold oxde fuel cell (SOFC) for steam producton appears to be an nterestng alternatve to mprove the SOFC performance. The produced steam can be used for chlled water generaton va an absorpton chller. However, the man challenge n desgnng ths combned cycle s the proper use of the SOFC exhaust gas n order to optmze the chlled water generaton. Exergy analyss plays an mportant role n developng strateges and n provdng gudelnes for more effectve use of exhaust heat n exstng systems. In ths study, a combned analyss of exergy and envronmental mpact (through greenhouse gas emssons) of the chlled water producton process usng the SOFC exhaust heat are performed under exergoenvronmental analyss. The study determnes energy and exergy effcences, exergy destructons and CO2 emssons as well as sustanablty ndexes (SI). 1. Introducton Electrcty s today's most mportant energy form n our whole way of lfe. Its producton, converson and use closely affect the envronment and sustanable development. That s, globally, fossl fueled power statons are stll major emtters of CO2, and concern about these CO2 emssons requres low carbon solutons. Currently, developng more effcent energy systems whch can be drven by renewable resources s one of the most appealng solutons to overcome envronmental ssues. Sold oxde fuel cells (SOFCs) have been recevng ntense attenton to serve as a potental opton for power generaton because they offer hgh electrcal effcency and nearly zero pollutant emsson, dependng on the fuel used. Moreover, SOFCs produce hgh qualty by-product heat that can be recovered not only for heatng by drectly feedng to heat-demandng devces but also for coolng by drvng an absorpton chller for potentally trgeneraton applcatons. Heat recovery from the SOFC exhaust gas for coolng applcatons faces a crucal challenge n process desgn. Thermodynamcally, transferrng heat through dfferent temperatures, mxng of two or more dfferent substances, occurrng spontaneous chemcal reactons and flowng of electrc current n the system are all the man causes of rreversblty. The exhaust gas leavng the fuel cell under regular operatng condtons has too hgh temperature to be used drectly for an absorpton coolng system. Developng strateges and provdng gudelnes for more effectve use of exhaust heat for ths combned cycle are needed. Exergy analyss has become a very mportant tool for mprovng the effcency of energy-resource usage, as t quantfes the locatons, types and magntudes of wastes and losses. As a multdscplnary concept, exergy analyss can be appled va varous technques to assessng and mprovng the effcency of processes, devces and systems. Combned Pnch and exergy analyss for the refrgeraton system was studed by Rae (2011). The total ste analyss (TSA) and exergy analyss are also subjects of nterest n ndustral clusters (Hackl and Harvey, 2012). Many prevous studes have examned the feasblty of usng absorpton chllers based on SOFCs for trgeneraton plants (Fong and Lee, 2014). Most of these studes have concluded that the SOFC trgeneraton system s an attractve technology for energy generaton n comparson to other Please cte ths artcle as: Tppawan P., Arpornwchanop A., Dncer I., 2015, Exergoenvronmental analyss of heat recovery from sold oxde fuel cell system for coolng applcatons, Chemcal Engneerng Transactons, 43, DOI: /CET

2 2204 technologes. However, the study on SOFCs fed by renewable fuels as a prme mover of the trgeneraton plants through exergoenvronmental analyss s qute lmted. The am of ths study s focused on a combned analyss of exergy and envronmental mpacts of the chlled water producton process usng the SOFC exhaust heat. Ethanol s chosen to be a prmary fuel n the SOFC system, because t s clean, affordable and low-carbon bofuel. The system s desgned to determne energy and exergy effcences, exergy destructons and CO2 emssons of each component n the system. A sustanablty ndex (SI) evaluaton s carred out and dscussed. 2. System descrpton A schematc dagram of the ethanol fuelled-sofc system ntegrated wth an absorpton coolng system s shown n Fgure 1. The system s dvded nto four man sectons: (1) an ethanol steam reformer to convert ethanol nto H2-rch gas va the steam reformng process, (2) the SOFC stack to generate electrcty from the electrochemcal reacton of hydrogen and oxygen, (3) heatng unts whch comprsed of an afterburner, an ar pre-heater and a heat recovery steam generator (HRSG), and (4) coolng system; a double-effect LBr/water absorpton chller, to produce chlled water for coolng applcatons. 2.1 Ethanol steam reformer The ethanol steam reformer s a fuel processor to convert ethanol nto hydrogen-rch gas by usng steam as a reformng agent. It s performed at a temperature of 980 K and the steam-to-ethanol rato s lmted to 1.8 (Tppawan and Arpornwchanop, 2014). The equlbrum composton of the reformng products s determned by the mnmzaton of a total Gbbs free energy. It s assumed that heat requred for the reformng process s suppled by an anode exhaust gas and there are no heat losses to the surroundng. In these examnatons, the mole fractons of hydrogen-rch gas from the ethanol steam reformng process are 0.60 H2, 0.2 CO, 0.1 H2O, 0.06 CO2 and 0.04 CH4. Fgure 1: Schematc dagram of the ethanol-fuelled SOFC system ntegrated wth a double-effect absorpton coolng system

3 Sold oxde fuel cell (SOFC) The SOFC s a power generator devce that converts the chemcal energy from ethanol reformate nto electrc power va an electrochemcal reacton. The electrcty generated by SOFC s drect current (DC), whch s coverted to alternatng current (AC) through an nverter. The electrochemcal model of SOFC proposed by Aguar et al. (2004) s used n ths study. The nternal reformng of CO and CH4 can proceed nsde the cell stack whle the electrochemcal reacton of hydrogen s only present. The recyclng of the anode exhaust gas n ths ntegrated SOFC system s consdered to mprove ts performance. 2.3 Heatng unts In general, the SOFC cannot electrochemcally utlze 100% of the fuel. The resdual fuel and the excess ar are combusted n an afterburner to obtan extra useful heat at hgh temperatures. The equlbrum composton of the combuston process s determned by a stochometrc approach. The hot flue gas from the afterburner flows through an ar pre-heater to exchange the heat for ar preheatng n the heat exchanger and then passes the HRSG, where t transfers the heat to the feed water, whch s heated and converted nto a superheated steam whch s used to drve a double-effect absorpton chller. 2.4 Coolng system The key components of the coolng system are an evaporator, an absorber, a low pressure generator (GE,LP), a hgh pressure generator (GE,HP), a condenser, two soluton heat exchangers (SHE), a pump and throttlng valves, as shown n Fgure 1. The heat n superheated steam from HRSG s suppled to an absorpton chller to generate chlled water. Smulaton of a double-effect absorpton chller s based on the model developed by Herold et al. (1996). The chlled water return temperature s set at 285 K, but the chlled water supply temperature s vared accordng to the amount and performance of heat recovery. 3. Modellng and assessment 3.1 Energy and exergy analyses The frst and second laws of thermodynamcs are appled to each component n the ntegrated system. Consderng each component as a control volume under the steady-state condton, the conservaton of energy balance equaton around the components of the system n Fgure 1 can be wrtten as follows: Q cv -W cv = nh - nh (1) out n where n and h represent the molar flow rate and specfc molar enthalpy, respectvely, of the flow stream nto and out of each component n the system. Unlke energy, exergy s not conserved. Exergy s defned as the maxmum work that can be extracted from a system nteractng wth a reference envronment (Dncer and Rosen, 2013). Exergy destructon s an mportant parameter n an exergy analyss. It s formulated as the lost of potental work due to ts rreversblty. The exergy destructon rate of a control volume at a steady state s defned as T0 Ex d = 1- Q cv-w cv+ nex - nex T (2) n out where ex s the specfc molar flow exergy for each component of the nlet and outlet flow streams. The subscrpt s the property value at state, and the subscrpt 0 s the value of a property at the surroundngs. Ths exergy can be defned by neglectng the knetc and potental energy changes as follows: ex = ( h - h )- T ( s - s ) + ex (3) ch The specfc chemcal exergy ( ex ch ch = ch, + 0 ln ) of dfferent speces n a gas mxture s defned as: ex y ex RT y y (4) where ex s the specfc molar chemcal exergy of the flow streams and y ch, s the molar fracton of the gas speces n the gas mxture. The standard molar chemcal exerges of relevant substances are reported by

4 2206 Kotas (1995). The enthalpy and entropy for each gaseous component are calculated usng thermophyscal property functons n the EES (Engneerng Equaton Solver) as a functon of the temperature and/or pressure. The equatons used to calculate the energy and exergy effcences for the trgeneraton system can be expressed as follows: W SOFC + Q h + Q c ηtr = Q fuel, n (5) ψ tr W + Ex + Ex = Ex SOFC h c fuel,n (6) where Q and Ex are the energy and exergy rates, respectvely, the subscrpts h and c ndcate the heatng and coolng, respectvely and 3.2 Exergoenvronmental analyss W s the output power of SOFC. SOFC Energy converson processes should be desgned by reducng an envronmental mpact, resultng n the mproved process performance. Because CO2 emsson s a sgnfcant ssue, a reducton n ths green house gas n the afterburner of the SOFC system can lead to the mprovement of the cycle. The normalzed CO2 emssons of the trgeneraton system can be expressed as follows: m CO Em(CO 2 2,tr) = (5) ( W SOFC + Q h + Q c) To mprove envronmental sustanablty, a sustanablty ndex SI s used here to relate the exergy wth envronmental mpact as: 1 SI = (6) DP where D P s the depleton number, whch s defned as the exergy destructon/nput exergy. Ths relaton demonstrates how reducng a system s envronmental mpact can be acheved by reducng ts exergy destructon (Dncer and Naterer, 2010). 4. Results and dscusson The process equpment data and model, as mentoned n the prevous secton, are used to determne the energy and exergy effcences and exergy destructon rate, and to estmate the envronmental mpacts for the trgeneraton system. The analyses n the effect of current densty and SOFC temperatures are shown n Fgures 2-5. Fgure 2 shows that ncreasng the current densty causes a decrease n the effcences, but an ncrease n the exergy destructons. Gven the constant fuel utlzaton, an ncrease of current densty means an ncrease of the ethanol fuel flow fed nto the system. Ths leads to the hgher nputs of the system and hgher exergy destructon. However, the sustanablty ndex decreases wth ncreased current densty as shown n Fgure 3. It mples that the destructon of exergy manly domnates the ncrease of exergy from nput. Fgure 3 also shows that ncreasng current densty ncreases CO2 emsson due to the ncrease of the mass flow rate njected from the afterburner. On the other hand, ncreasng the SOFC temperature enhances the effcences and the exergy destructon decreases as presented n Fgure 4. Ths s attrbuted to the fact that the ncrease of temperature promotes the electrochemcal reacton n the SOFC stack to generate more electrcal power. The optmum values of the SOFC temperature are shown n Fgures 4-5. At the optmum temperature, a decrease n the exergy destructon of SOFC s hgher than an ncrease n the exergy destructon of afterburner. Consequently, the total exergy destructon s mnmzed and sustanablty ndex reaches the maxmum value. The exergy destructon n system components s shown n Fgure 6. The hghest exergy destructon occurs n the afterburner manly due to the rreversbltes assocated wth the combuston reacton and the large temperature dfference between the gases enterng the afterburner and the flame temperature. Sgnfcant exergy destructon can also be notced n the SOFC, HRSG and the ar pre-heater. To provde envronmental nsghts, Fgure 7 shows that the trgeneraton system (ncludng coolng applcaton) has less CO2 emsson than the SOFC and the combned-heat and power cycles, provdng a sgnfcant motvaton for the use of heat from SOFC for coolng applcaton as well. It s also observed that the present trgeneraton system provdes hgher power output, compared to other types of conventonal power plants.

5 2207 Fgure 2: Effect of current densty on the energy and exergy effcences and the total exergy destructon rate of the trgeneraton system Fgure 3: Effect of current densty on CO2 emsson and sustanablty ndex Fgure 4: Effect of SOFC temperature on the energy and exergy effcences and the total exergy destructon rate of the trgeneraton system Fgure 5: Effect of SOFC temperature on CO2 emsson and sustanablty ndex Fgure 6: Exergy destructon n the system components Fgure 7: Comparson of CO2 emsson and power output of dfferent types of plants

6 Conclusons An exergoenvronmental analyss of sold oxde fuel cell systems for heatng, coolng and electrcty generaton has been performed to provde useful nsghts n terms of heat recovery and envronmental aspects. The results show that system effcences, exergy destructons, the amount of CO2 emsson and sustanablty ndex are notably affected by operatng current densty and SOFC temperature. The afterburner and SOFC are the two man components wth the hgh exergy destructon whle the use of absorpton chllers offers effectve heat recovery wth the lowest exergy destructon of the whole system. In addton, chlled water generaton from absorpton chllers n the trgeneraton system s one of the best heat recovery strateges that exhbt lesser CO2 emssons than other types of conventonal power plants. Acknowledgements The support from the Ratchadaphseksomphot Endowment Fund of Chulalongkorn Unversty (RES EN) and the Thaland Research Fund s gratefully acknowledged. P. Tppawan would lke to thank the Dutsadphphat Scholarshp, Chulalongkorn Unversty. References Aguar P., Adjman, C.S., Brandon, N.P., 2004, Anode-supported ntermedate temperature drect nternal reformng sold oxde fuel cell. I: model-based steady-state performance, J. Power Sources, 138, Dncer I., Naterer G.F., 2010, Assessment of exergy effcency and Sustanablty Index of an ar water heat pump. Internatonal Journal of Exergy, 7 (1), Dncer I., Rosen MA. Exergy: energy, envronment and sustanable development. 2nd edton, Elsever; Fong K.F., Lee C.K., 2014, Investgaton on zero grd-electrcty desgn strateges of sold oxde fuel cell trgeneraton system for hgh-rse buldng n hot and humd clmate. Appled Energy, 114, Hackl R. And Harvay S., 2012, Total ste analyss (tsa) and exergy analyss for shaft work and assocated steam and electrcty savngs n low temperature processes n ndustral clusters, Chemcal Engneerng Transactons, 29, 73-78, DOI: /CET Herold K.E., Radermacher R., Klen S.A., 1996, Absorpton chllers and heat pumps. CRC press. Kotas T.J., 1995, The exergy method of thermal plant analyss. Florda: Kreger Publshng Co. Rae B., 2011, Optmzaton n energy usage for refrgeraton systems usng conbned pnch and exergy analyss, Chemcal Engneerng Transactons, 25, , DOI: /CET Tppawan P., Arpornwchanop A., 2014, Energy and exergy analyss of an ethanol reformng process for sold oxde fuel cell applcatons. Boresource Technology, 157,