DESIGN, SIMULATION AND EVALUATION OF DOMESTIC MICRO-CHP UNITS

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1 IGEC-7, June 17-1, Västerås, Sweden DESIGN, SIMULATION AND EVALUATION OF DOMESTIC MICRO-CHP UNITS Michel Mlynski, Frncesco Turoni, Alexnder Hlwenk, Michel Schreiber EUtech Scientific Engineering Dennewrtstrsse 5-7, 568 Achen, Gerny ABSTRACT The liittion of the greenhouse effect nd globl wring will be the ost iportnt nd chllenging tsk of the next decdes. One contribution being discussed is the introduction of new technologies in the doin of doestic energy supply for hoes. Cogenertion units, lso known s icro cobined het nd power units (icro-chp), cli to significnt overll-increse in efficiency copred to conventionl heting instlltions. However, s of now only few icro-chp technologies hve reched the product level resulting in liited instlled units. As result, experience nd results fro ctul instlltions re not widely vilble. We hve developed siultion tools tht llow us to clculte precisely the costs, vilbility, efficiency nd CO -svings of vrious icro-chp technologies, including fuel cell icro-chp units. The results cn then be copred with one nother, s well s with dt fro conventionl heting instlltions, king it possible to identify the benefits nd shortcoings of ech technology. Further ore, the results of siultions cn lso be used to identify res with high potentil for optiiztion nd to guide future developent. INTRODUCTION The issue of environentl protection continues to gin in iportnce. While few pollution probles, such s NO -bsed strtospheric ozone depletion see to hve becoe less drtic (EPA, 3) others, such s crbon dioxide eissions nd the relted greenhouse effect (Alley et l., 7 / EPA, 6) dend urgent ction. One widely discussed wy to contribute to environentl protection nd pollution reduction is to introduce new technologies in the doin of doestic energy supply for hoes. Cogenertion units, lso known s icro cobined het nd power units (icro-chp), cli to be ble to increse overll efficiency nd to crete significnt CO svings copred to conventionl heting instlltions (Hrrison, / Frei-Hrdt, 5). Typicl icro-chp heting units re bsed on Stirling engines, Rnkine cycles, internl cobustion engines or fuel cells (FCs). In the UK, icro-chp hd been identified s key eleent of the Governent's energy strtegy (DTI, 3). While soe icro-chp technologies hve reched the rediness for rketing (Hrrison, ), the overll nuber of instlled units is still quite low in ny countries (DTI, 5 / Pfeifer, 5). There re therefore liited experience nd results fro ctul instlltions vilble. Coputer-ided siultions provide n lterntive pproch to investigting nd nlyzing the syste chrcteristics of icro-chps. While siultions on the cheicl nd physicl levels re necessry for the design nd construction of single coponents or eleents of coponents, odeling on the therodynic nd syste levels is required in order to odel, nlyze nd optiize n entire unit. In this pper, we present siultion tools tht hve been developed to ccoplish ultiple types of syste siultions, with the focus on fuel cell cogenertion units. In the second section, Therodynic Modeling of fuel cell icro-chp Units, we discuss techniques for odeling coplete unit s coposition of ultiple coponents. Principles of physicl nd therodynic conservtion, edi coposition nd representtion, equilibriu cheistry, therodynic sttes nd properties, phse chnges nd blncing re ddressed. In the third section, Syste Siultions, we present ethods for perforing rpid, yet precise siultions of entire systes, e.g. house including its heting instlltion. Such siultions ke it possible to clculte costs, vilbility, overll efficiency nd CO -svings, s well s to copre vrious heting syste technologies, including icro-chp units versus conventionl heting instlltions. In section four, Results nd Discussion, we present selection of results we hve generted. Working fro such results, res with high potentils for optiiztion cn be esily identified nd guide future developent. THERMODYNAMIC MODELING OF FUEL CELL MICRO-CHP UNITS There re t present ny siultion tools vilble for cheicl nd physicl odeling of hydrulic nd therodynic coponents. Soe typicl exples include FEMLAB, Fluent, Modelic, Dyol nd ASPEN. Such tools llow highly detiled odeling of cheicl nd physicl processes. However, odeling Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

2 IGEC-7, June 17-1, Västerås, Sweden n entire unit, coposed of dozens of prts nd coponents, shrply increses the effort involved in ipleenttion s well s the siultion tie. Since it is necessry to conduct therodynic odeling on the syste level in order to odel, nlyze nd optiize n entire unit precisely, other siultion tools were needed. An dditionl requireent ws preferbly to design nd evlute the coplete control ppliction for such icro-chps in the se developent environent s the siultion. The decision ws de to use MATLAB/Siulink, which llows the user to conduct odeling, siultion nd controller design on the se pltfor. One drwbck of Siulink is tht therodynic odeling is brely supported. Therefore, the therodynic Siulink librry FClib ws developed, providing siultion blocks for representtion nd blncing of flow sses, therodynic nd conservtive clcultions, equilibriu nd rection cheistry, typicl coponents nd sseblies, nd djustent of species used. Modules for therodynic siultions The librry we developed provides collection of odules such s pups, fns, copressors, vlves, splitters, ixers, het exchngers, huidifiers nd huidity exchngers, dryers, burners, cheicl rectors, nd lst but not lest fuel cells nd inverters, s Fig..1 shows. These odules re used to esily set up coplete icro-chp unit siply by selecting nd connecting the required odules. Medi flows re represented by objects, encpsulting the following edi properties: olr flow teperture pressure enthlpy flow entropy flow free Gibbs enthlpy flow het cpcity flow coposition of flow edi gseous frctions edi olr frctions All coponent odules s well s the flow objects re bsed on therodynic property nd stte clcultions, which re discussed in the following section. Fig..1: Extrction of odules of the siultion librry Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

3 IGEC-7, June 17-1, Västerås, Sweden Therodynic fundentls Therodynic sttes nd trnsfortions re bsiclly represented by their enthlpy, entropy, free Gibbs enthlpy nd het cpcity. These re clculted bsed on the JANAF-thero physicl tbles, using the NASA polynoil representtion (Gordon, 1971). Eq. (.1) to (.) give the clcultions for gseous phses t tepertures between K nd 6 K. 3 B T C T D T E T F H /( RT ) = A (.1) 3 5 T 3 C T D T E T S / R = A ln( T ) + B T G 3 G = H S T (.3) (.) C p / R T 3 = A + B T + C T + D T + E (.) For liquid phses the het cpcity C p is typiclly ssued to be constnt. In this cse enthlpy, entropy nd free Gibbs enthlpy re given by Eq. (.5) to (.7). H = H + C ( T 98 ) (.5) K, 98[ K ] p T S = S + C ln, 98[ K ] p 98K (.6) G = H S T (.7) Cheicl rections, e.g. within rector block such s CH-Reforer or Shift Rector, re described by generl rection equtions ccording to Eq. (.1). v A + v B v C v D (.1) A B C + D The corresponding cheicl equilibriu reltion is given by ln vc C va A vd D vb B G = RT, (.11) clculting the ole frctions of the outgoing flows (Sonntg, 3), with y P i = (.1) P i i i being the ctivity coefficients for the given coponents ole frctions y i. G is given for constnt tepertures by G = H T S. (.13) The cheicl equilibriu eqution, Eq. (.11), is solved using Eq. (.13). In this wy, we obtin the ole frctions of the equilibriu ixture. Siultions Coplete icro-chp units re odeled by selecting nd interconnecting odules, where ech odule represents coponent of the rel process. This kes it possible to set up the siultion odels in wy siilr to the process flow-chrts. Different connections nd piping cn be evluted by siply chnging the signl connections. The se holds true for nlyzing different process technologies, e.g. preferentil oxidtion (PROX) copred to ethnizing rector: by exchnging the corresponding odules, ny process technology cn be Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

4 IGEC-7, June 17-1, Västerås, Sweden Fig..: Model (extrction) of the gs production of fuel cell icro-chp unit evluted s well s optiized in conjunction with the whole unit. Fig.. shows n exple of the gs production of fuel cell icro-chp unit. Since the siultion environent Siulink kes it possible to design nd evlute coplete control pplictions, it is possible to introduce softwre developent techniques such s odel bsed design nd Rpid Control Prototyping (RCP) (Mlynski, / Turoni, 5) nd include the into the design of the entire icro-chp. Controller pplictions re ndtory for icro-chp nd will be discussed in the section Results nd Discussion. SYSTEM SIMULATIONS As discussed in the previous section, detiled therodynic siultions re perfored in order to design nd evlute heting instlltions. However, siultions on syste level re perfored to gther infortion bout the syste s whole, e.g. house including its heting instlltion. Siultion results re typiclly used to inspect the syste s behvior nd perfornce, clculte costs, vilbility, efficiency, CO -svings, etc., copre different heting instlltions, identify res with high optiiztion potentil nd guide future developent. Requireents for syste siultions To ccoplish the gols entioned bove, severl requireents ust be et. Upon developent of the siultion tools discussed in this pper, jor focus hs been set for these requireents. First of ll, consuption dt re required tht represent the house s energy consuption used in the siultion. Such consuption dt coprise the therl energy for heting nd hot wter supply s well s wter nd electricl energy consuption. There re two different well estblished pproches for collecting consuption dt. One is to develop odel of the house nd to siulte the household consuption depending on the seson, the outside teperture, the insultion, the nuber of people residing in the house, their hbits, nd so on. An lterntive pproch is to esure rel dt in the field nd enter it into the siultion. Though the second pproch requires esuring lrge sets of dt, these dt re copletely relistic, since they re bsed on ctul esureents. The siultion discussed in this pper utilizes the second pproch s shown in Fig. 3.1: ll of the consuption dt esured in the field re used s input dt for the siultion. The heting instlltion is siulted to deliver energy in ccordnce with these input dt. The siultion results re the operting Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

5 IGEC-7, June 17-1, Västerås, Sweden power of the instlltion t ech siultion step, the gs consuption, the buffer filling degree, the net blnce, the costs nd CO eission, ong other fctors. The second requireent on syste siultions is tht they possess the bility to perfor siultions over extended periods. Siulting just dy or week typiclly does not give enough infortion on the syste s perfornce, nor on its overll costs, efficiency, etc. Setting up four different weeks, representing the four sesons of the yer, iplies to ny erroneousness when projecting the results to coplete yer. Siulting longer periods, i.e. onths up to severl yers, is thus inevitble. This requireent, siulting over onths or yers, rise nother necessity: besides being bsolutely precise, siultions ust lso be fst. If one were to set up odel of new heting instlltion nd then conduct siultion of one yer, it would not tter whether tht siultion took n hour or hlf dy. Yet s we will show, siultions of heting instlltions include dozens of preters. These preters ust then be ltered upon evlution nd optiiztion. Since different cobintions of preter vlues hve different effects, ultiple siultion runs hve to be conducted in order to evlute the syste copletely. As result, siultions not only hve to be very precise, but extreely fst, too. Lst but not lest, tools for optiiztion, e.g. utotic deterintion of optiized preters, re necessry. Such tools include the definition of nd iniiztion of cost functions or heuristic serch lgoriths. In cobintion with the previously entioned rpid siultion, such tools ke it possible to perfor coprehensive nlysis of n entire syste within few hours. Siultion set-ups To siulte syste, the odeled heting instlltions re set up in the syste siultion environent. The siultion environent includes the odeling of dditionl coponents such s hot wter tnks nd pek lod heters (PLH), which re used in prticulr in fuel cell icro-chps (FC-CHP). Fig. 3. shows the block digr of the FC-CHP discussed in this pper. For ech syste, round 5 preters ust be set in order to dpt the siultion to n existing heting instlltion: Hot wter tnk Volue, surrounding teperture, het loss due to rdition nd convection, teperture set point, etc. Fuel cell Miniu/xiu therl nd electricl power, efficiency, strtup nd shut down ties, blocking ties, controller preteriztion, etc. Pek lod heter Miniu/xiu therl power, efficiency, strtup nd shut down ties, blocking ties, controller preteriztion, etc. Siultions re perfored with vrible step size solvers to chieve two gols: xiu siultion resolution, especilly in res with dynic trnsitions, nd xiu siultion speed. The siultion environent discussed in this pper llows siultions tht re typiclly 15, ties fster thn rel tie, king it possible to siulte coplete nnul cycle with resolution of one inute in pproxitely four inutes on conventionl.8 GHz Pentiu processor. CO therl losses esured dt het consuption wter consuption electricl consuption outdoor teperture dte nd tie siultion results operting power gs consuption buffer filling degree net blnce CO eission econoic blnce electricity nturl gs wter supply pek lod heter PEM FC CHP hot wter tnk flow return hot wter rditors nd floor heting hot wter consuption electricity nturl gs wter heting instlltion hot wter tnk electricl power electricl power consuption Fig 3.1: Inputs nd outputs of syste siultions Fig 3.: Block digr of FC-CHP unit Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

6 IGEC-7, June 17-1, Västerås, Sweden RESULTS AND DISCUSSION Severl FC-CHP units hve been odeled nd siulted. After designing these odels, we conducted n nlysis of the. Fig..1 shows the isobr vporiztions of edi flow with phse chnges t different pressures (T-s digr). A focus ws set on proton exchnge ebrne (PEM) fuel cells. These fuel cells cn be driven with pure hydrogen only. Therefore, PEM FC-CHP hve gs production on bord, reforing nturl gs to hydrogen (Pukrushpn, ). The first production step is to refor ethne (CH ) within the so-clled ste reforer rector. According to Eq. (.1), the two rections tht tke plce in the rector re given by Eq. (.1) nd (.). CH + H O 3 H + CO (.1) CO + H + (.) O CO H The corresponding cheicl equilibriu reltions re given by Eq. (.3) nd (.). ln ln 3 ( yh ph ) ( y p ) CO CO G = ( y p ) ( y p ) p RT CH CH H O H O ( yco pco ) ( y p ) H H G = ( y p ) ( y p ) RT CO CO H O H O The first rection, ccording to Eq. (.1), is strongly endotheric. The het energy required is supplied by lbd burner, s shown in Fig... Fig.. shows the olr concentrtions t the output of the ste reforing rector for ste-to-crbon (S/C) rte of 3. (.3) (.) Fig..1: T-s digr for isobr vporiztions Fig..: Molr concentrtions of ste reforing After the odeling nd design of FC-CHPs few syste siultions were conducted nd copred to the siultion of conventionl heting instlltion. Tble.1 shows the ost iportnt preters of the FC- CHP (FC: fuel cell, PLH: dditionl pek lod heter). Tble. shows the settings for the conventionl heting instlltion. Tble.3 shows the prices used in the siultions to be discussed next. The totl efficiency of the fuel cell is 8%, which is quite low copred to other publictions (Hrrison, / Wllrk et l., ). Nevertheless, syste siultions show the dvntges of the FC-CHP copred to conventionl heting instlltion. This vlidtes this technology s dvntges over conventionl heting instlltions, even for systes tht re not fully developed. Fig..3 shows the costs nd CO eissions for coplete yer, split into spring, suer, fll nd winter nd with differentition between weekdys (WD) nd weekends (WE), s well s for the whole yer. The results show tht FC-CHP y sve up to 7.5% of costs (opertionl costs, not tking possible higher intennce costs into considertion) nd up to 3.3% of CO eissions per yer, copred to conventionl heting instlltion. Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

7 IGEC-7, June 17-1, Västerås, Sweden Tble -1: Fuel cell icro-chp Tble -: Conventionl heting instlltion Preter Vlue Preter Vlue FC: in. therl power 1. W Min. therl power. W FC: x. therl power 6. W Mx. therl power. W FC: therl efficiency 6% Therl efficiency 85% FC: electricl efficiency % Miniu on-tie 6 s FC: iniu on-tie 18 s Miniu off-tie s FC: iniu off-tie 3 s Hot Wter tnk: volue 5 l PLH: in. therl power 1. W PLH: x. therl power 1. W Tble -3: Prices PLH: therl efficiency 85% Preter Vlue PLH: iniu on-tie 6 s Gs,53 / kwh PLH: iniu off-tie s Electricity (drwn),16785 / kwh Hot Wter tnk: volue 75 l Electricity (feed bck into grid) -,9 / kwh Entire yer FC-CHP: Costs: CO : 7.9 kg Conv. heting instlltion: Costs: 31.5 CO : kg Coents Opertionl costs only (intennce costs excluded) Gs price for ll systes without ny petroleu tx reduction Abbrevition key WD: weekdy (Mon Fri) WE: weekend (St, Sun) Fig..3: Costs nd CO eissions of FC-CHP nd conventionl heting instlltion As discussed bove, n iportnt issue concerns identifying res with high potentil for optiiztion to guide future developent. By vrying preters nd re-running syste siultions, we cn deonstrte the influence of preters such s efficiency, xiu power, degree of odultion (lowest possible power for given xiu power) s well s the control strtegy (lso denoted by energy ngeent). The durbility of current fuel cells is still liited nd is highly ffected by the nuber of ties they re strted up nd shut down (strt-ups nd shut-downs). A gret del of reserch ctivity therefore focuses on optiizing the fuel cell life-cycle. Additionlly, it is worth-while to concentrte on the energy ngeent of FC-CHPs, since this considerbly ffects the nuber of strt-ups nd shut-downs, respectively. Fig.. shows the coprison of FC-CHP with siple energy ngeent (FC-CHP 1: controlled by het dend only) nd n intelligent energy ngeent (FC-CHP : considering the seson, outdoor teperture, current het dend, predicted upcoing energy dend, etc.). The results show tht introducing n intelligent energy ngeent cn reduce the costs nd CO eissions slightly (3.9% nd 3.7%, respectively, per yer). Siultneously, the nuber of strt-ups nd shut-downs of the fuel cell cn be reduced significntly fro 318 per yer down to 19 per yer, which is reduction of 9.% nd will considerbly extend the life spn of the fuel cell. Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

8 IGEC-7, June 17-1, Västerås, Sweden Entire yer FC-CHP 1: Costs: CO : kg FC-CHP : Costs: CO : 7.9 kg Coents Opertionl costs only (intennce costs excluded) Gs price without ny petroleu tx reduction Abbrevition key WD: weekdy (Mon Fri) WE: weekend (St, Sun) Fig..: Nuber of strtups of FC-CHP with siple nd intelligent energy ngeent CONCLUSIONS Therodynic nd syste siultions cn significntly speed up the design, evlution, nlysis nd optiiztion of icro-chps. Siultion is thus n iportnt tool for engineering nd developent nd cn contribute to the iproveent of icro-chps. In this pper we hve presented the siultion tools being developed nd hve discussed the results obtined fro investigting fuel cell icro-chps. The results not only confir the dvntges of this technology over conventionl heting instlltions, but lso serve to identify the benefits nd shortcoings of ech technology. Further-ore, we hve shown tht siultions cn be used for rpid nd siple nlysis nd optiiztion of icro-chps nd guide future developent. REFERENCES R. Alley, et l. 7. Clite Chnge 7: The Physicl Science Bsis. WMO, UNEP, Working Group I of the PICC, Pris, Februry 7, DTI. 3. Energy White Pper; Our Energy Future. 3, TSO ISBN DTI. 5. Potentil of icrogenertion for the UK, Study nd Anlysis. Finl Report, 1 th Noveber 5, EPA. 3. Ntionl Air Qulity nd Eission Trends Report, 3 Specil Studies Edition. U.S. Environentl Protection Agency, EPA 5/R-3-5, Septeber 3 Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden

9 IGEC-7, June 17-1, Västerås, Sweden EPA. 6. Inventory of U.S. Greenhouse Gs Eissions nd Sinks: 199. U.S. Environentl Protection Agency, EPA 3/R-6-, April 6 onsinventory6.htl S. Gordon, B.J. McBride Coputer Progr for Clcultion of Coplex Cheicl Equilibriu Coposition, Rocket Perfornce, Incident nd Reflected Shocks nd Chpn-Jouguet Detontions. NASA SP-73 (1971) Mrkus Frei-Hrdt. 5. MicroCHP Technologies - The Role of MTS, Mrket Opportunity nd Threts. EFC5, 1st Europen Fuel Cell Technology nd Applictions Conference, , Ro, Itly M. Mlynski, M. Schreiber.. Optiierung des Entwicklungsprozesses durch den Einstz von Siultionswerkzeugen. 3. Interntionler Brennstoffzellen-Technologietg, 1.11., Achen, Gerny Jerey Hrrison.. Micro Cobined Het & Power (CHP) for housing. 3rd Interntionl Conference on Sustinble Energy Technologies, Nottingh, UK, 8-3 June T. Pfeifer, K. Rühling, P. Nehter. 5. Entwicklungstendenzen bei sttionären Brennstoffzellennlgen. XXXVII. Krftwerkstechnisches Kolloquiu, Oktober 5, Dresden, Gerny; Tgungsbnd II, V39 Jy T. Pukrushpn, Ann G. Stefnopoulou, Huei Peng.. Control of Fuel Cell Power Systes. Springer.. Richrd E. Sonntg, Clus Borgnkke, Gordon J. Vn Wylen. 3. Fundentls of Therodynics. John Wiley & Sons. 3. F. Turoni, M. Mlynski, A. Sdtskk, M. Schreiber. 5. Model bsed design of controller for fuel cell systes. Proceedings of the MBDC5, Model-Bsed Design Conference, June 8-9, 5, Munich, Gerny. Shker Verlg Achen, ISBN Cecili Wllrk, Per Alvfors.. Design of sttionry PEFC syste configurtions to eet het nd power dends. Journl of Power Sources 16,, pges 83-9 Fro: Conference Proceedings of the 3rd Interntionl Green Energy Conference, June 17-1, 7, Västers, Sweden