Energy 36 (2011) 3791e3803. Contents lists available at ScienceDirect. Energy. journal homepage:

Energy 36 (2011) 3791e3803 Contents lists vilble t ScienceDirect Energy journl homepge: www.elsevier.com/locte/energy Proposl nd nlysis of dul-purpose system integrting chemiclly recuperted gs turbine cycle with therml sewter deslintion Chending Luo,b, *, N Zhng, Nom Lior c, Hu Lin,b Institute of Engineering Thermophysics, Chinese Acdemy of Sciences, PO Box 2706, Beijing 100190, PR Chin b Grdute School of the Chinese Acdemy of Sciences, PO Box 2706, Beijing 100190, PR Chin c Deprtment of Mechnicl Engineering nd Applied Mechnics, University of Pennsylvni, Phildelphi, PA 19104-6315, USA rticle info bstrct Article history: Received 28 Februry 2010 Received in revised form 5 October 2010 Accepted 17 November 2010 Avilble online 12 Jnury 2011 Keywords: Chemiclly recuperted gs turbine (CRGT) MED-TVC Sewter deslintion Power nd wter cogenertion Dul purpose plnts A novel cogenertion system is proposed for power genertion nd sewter deslintion. It combines the CRGT (chemiclly recuperted gs turbine) with the MED-TVC (multi-effect therml vpor compression deslintion) system. The CRGT contins MSR (methne-stem reformer). The produced syngs includes plenty of stem nd hydrogen, so the working medium flow increses nd NO x emissions cn chieve 1 ppm low. However, the wter consumption is lrge, 23 t/d wter per MW power output. To solve this problem nd produce wter for sle, MED-TVC is introduced, driven by exhust het. Such dul-purpose plnt ws nlyzed to investigte its performnce nd prmeter selection, nd compred with four conventionl cogenertion systems with the sme methne input. Some min results re following: In the bse cse of the CRGT with TITof 1308 C nd compression rtio of 15, the MED-TVC with 9 effects, the specific work output, performnce rtio nd CRGT-consumed wter rtio re 491.5 kj/kg, 11.3 nd 18.2%, respectively. Compred with the bckpressure ST (stem turbine)/cc (combined cycle) plus MED/MSF (multistge flsh), the CRGT þ MED hs better therml performnce, lower product costnd shorter pybck period, which indictes the CRGT þ MED dul-purpose system is fesible nd ttrctive choice for power nd wter cogenertion. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction CRGT (Chemiclly recuperted gs turbine) cycles combine nturl gs fired gs-turbine cycle with chemicl recupertion process [1e6]. In such cycles turbine exhust het is recovered in HRSG (het recovery stem genertor) nd the superheter is replced by MSR (methne stem reformer) to produce syngs, in which the following rections occur [7,8]: CH 4 þ H 2 O4CO þ 3H 2 DH ¼ 206:11kJ = ðmol CH 4 CO þ H 2 O4CO 2 þ H 2 C n H m þ nh 2 O4nCO þðm=2 þ nþh 2 DH ¼ 41:17kJ = ðmol COÞ The exhust het is thus recuperted chemiclly by the methne conversion to H 2 nd CO. Compring the syngs to the methne, the fuel heting vlue is rised [9]. It shows tht low pressure, high temperture nd high stem consumption help to increse the * Corresponding uthor. Grdute School of the Chinese Acdemy of Sciences, PO Box 2706, Beijing 100190, PR Chin. Tel.: þ86 10 82543030; fx: þ86 10 82543019. E-mil ddress: lcd866@hotmil.com (C. Luo). reforming conversion rte (the rtio between the converted methne to the methne input) [10]. Even though the fuel conversion is bsed on the vilble gs turbine exhust nd reches only moderte level, the bsic chemiclly recuperted cycle (without inter-cooling or rehet) simulted by Kesser [2] still chieved therml efficiency of 48.8%, higher thn tht of the STIG (stem injected gs turbine). Owe to the presence of significnt mount of stem nd hydrogen in the reformed gs, the NO x emissions hs been estimted to be s low s 1 ppm [2] nd the specific power output becomes higher thn tht of dry gs turbine cycle [11]. Such cycle lso hs, however, lrge wter consumption, bout 23 t/d wter per MW power output, which restricts the ppliction of the plnt; especilly in wter-short res [2]. A preliminry economic evlution of the CRGT system [7] indicted tht it is economiclly fesible only if low-cost source of wter is vilble. However, compred to the CC (Combined Cycle, composed of gs-turbine cycle nd stem turbine cycle, usully hving therml efficiency of 51e58%), the little lower therml efficiency, much simpler configurtion (n MSR insted of n entire stem turbine cycle configurtion) nd ultr low NO x emissions still mke CRGT quite ttrctive. Sewter deslintion is widely used commercilly to produce fresh wter [12]. LT-MED (Low temperture multi-effect deslintion) is one of the commonly used het-driven deslintion methods [13e24]. MED systems often hve 4 to 12 effects. For exmple, in 0360-5442/$ e see front mtter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.11.029

3792 C. Luo et l. / Energy 36 (2011) 3791e3803 Nomenclture A Totl het trnsfer re [m 2 ] Specific het trnsfer re [m 2 /(kg/s)] C Cost [k$] CR Concentrtion rtio COE Cost of electricity [$/kwh] COW Cost of wter [$/t] E Exergy [kw] e Specific exergy [kw/kg] H Annul running hours [h] h Enthlpy [kj/kg] i Discount rte m Mss flow [kg/s] n Number of the effects or plnt life [y] PBP Pybck period [y] PR Performnce rtio Q Therml energy [kw] R Annul revenue of the plnt [k$] R NC Methne conversion rte R pw Power-to-wter rtio R SN Stem-NG mole rtio T Temperture [K] t Temperture [ C] TBT Top brine temperture [ C] TIT Turbine inlet temperture [ C] TPC Totl plnt cost [k$] U Het-trnsfer coefficient W Work [kw] x Stem-ir mss rtio b Annul verge investment coefficient p Compression rtio 3 Exergy efficiency h Therml efficiency h isn Isentropic efficiency d CRGT-consumed wter rtio DT p Minimum het trnsfer temperture difference [ C] DT eq Chemicl equilibrium pproch temperture difference [ C] Subscripts 0 Bse cycle Ambient stte b Boiler ir Inlet ir of the CRGT cycle C Condenser cm Condenste of the motive stem COM Compressor con Consumption E Evportor en Entrined stem ex Exhust gs f Flue gs fu Fuel h Heting stem in Input m Motive stem net Net vlue om Opertion nd mintennce P Pump PHE Preheter pow Power subcycle s Inlet stem of the MSR syn Syngs T Turbine w Wter production w,tol Totl wter produced by MED Ref. [14], two MED operting modes (prllel nd prllel/cross) were studied, with the prllel/cross flow system hving the better performnce. Although the TBT (top brine temperture) in MED system is lower thn 70 C, the motive stem is often with pressure round 3 br ( 134 C) extrcted from stem turbines or rnging from 20 to 30 br (212e234 C) supplied directly from boiler [15,16].From the second lw of thermodynmics viewpoint, the big temperture difference cuses much exergy loss in the het exchnge process, so the TVC (therml vpor compressor) is introduced to improve the system therml performnce: the motive stem firstly entrins nd compresses frction of the vpor produced in n effect of the MED, nd then the mixture dischrges s the heting stem t temperture of bout 70 C [17,18]. Compred with the MED plnt without vpor compression, such MED-TVC rrngement meets the deslintion temperture requirements better nd chieves higher performnce rtio PR, which is defined s the rtio between the mss flow rtes of the produced fresh wter m w to tht of the consumed motive stem m m, PR ¼ m w =m m (1) Menwhile, the needs of cooling wter nd pumping power re lso decresed [13]. In MED with mechnicl vpor compression system, the MVC (mechnicl vpor compressor) cn improve the performnce of the MED s well s the TVC, but the TVC is dopted in this pper rther thn the MVC for its effectiveness, esier opertion nd mintennce, nd good economic chrcteristics [19]. Integrtion of the CRGT plnt with MED-TVC deslintion system llows it to be supplied with its needed fresh wter without depending on other wter supplies. In this cogenertion system, the low temperture motive stem for driving the MED-TVC is generted by the CRGT turbine exhust het recovery tht voids gret wste of exergy if such low-grde therml energy were provided directly by burning fuel in boiler. It s obvious tht the synergy of the power nd wter dul-purpose plnts hs significnt energy, economy nd environment benefits [12,13,20]. The min objective of this pper is to propose nd nlyze n integrted cogenertion system composed of CRGT (s the prime mover) nd MED-TVC (s the bottom cycle). To investigte the performnce nd prmeter selection of the energy, exergy nd wter production of the integrted cycle, the cogenertion system ws modeled, nlyzed nd compred to some typicl conventionl power nd wter cogenertion systems with the sme input. An economic perspective ws held to evlute the costs of electricity nd fresh wter nd the pybck periods of the different dul-purpose plnts. A prmetric sensitivity nlysis ws lso performed to exmine the influence of three importnt prmeters, the stem-ir mss flow rte rtio x of the CRGT, the sturtion temperture of the motive stem t m nd the number of MED effects n. The results were discussed to further clrify the synergy of the integrted system. 2. The cogenertion system configurtion The flow sheet of the cogenertion system is given in Fig.1 nd b. As shown in Fig.1, the key process of the CRGTcycle is the reforming

C. Luo et l. / Energy 36 (2011) 3791e3803 3793 Fig. 1.. Flow sheet of the CRGTþMED dul-purpose system, b. Flow sheet of the MED-TVC section in the dul-purpose system. between the superheted stem nd compressed NG (nturl gs) in the MSR, which represents recovering of the turbine exhust het both thermlly nd chemiclly. The integrtion of the CRGTnd MED- TVC is embodied in the motive stem (16) genertion in the HRSG nd the fresh wter (6) supply to the CRGT. The extrcted sturted stem (16) is sent into the TVC to run the MED-TVC bottom cycle. Fig. 1b illustrted the MED-TVC system. The sewter is fed into the nine effects in prllel. Using the TVC (therml vpor compressor), the motive stem (16) compresses prt (23) of the vpor generted in the effect 5 (E5), t medium temperture. In the TVC the expnsion of the motive stem compresses the entrined vpor, nd their mixture is dischrged from the TVC nd used s the heting stem (22) for the distilltion process. It is thereby condensed nd provides het for sewter evportion in the first effect (E1). Prt of the condenste (14) is returned into the HRSG, nd the reminder (27) is introduced into flsh chmber 1 (FLA1), where smll mount of vpor (28) flshes off becuse of pressure drop. The vpor (25) evported from the sewter in E1 is mixed with the vpor (28) nd the mixture vpor psses through the PHE1 (preheter 1) before routed into E2 (effect 2) to serve s the het source together with the brine from E1 (26). The condenste of the vpor flows into FLA2 (flsh chmber 2). This process is repeted in ll effects except for E5 nd E9. In E5, prt of the vpor is entrined by the TVC (23) (reserch shows tht entrining vpor from n intermedite effect is better thn from the lst effect for enhncing the performnce rtio PR [21]; only the reminder (29) is introduced into the PHE5 (preheter 5). While in E9, the vpor (30) is sent into the end CON (condenser) to prehet the sewter (19) before it flows into the DST. Prt of the preheted sewter is used s the feed of the nine effects (24), nd the blnce (21) is rejected bck to the se, nd so is the brine (concentrted sewter) outflow from E9 (20). 3. The cogenertion system simultion 3.1. Computtion model nd ssumptions Some properties of the feed stems re reported in Tble 1. The proposed systems hve ll been modeled with the ASPEN PLUS softwre [25], inwhich the component simultion is bsed on energy, mss nd species blnces, with the defult reltive convergence

3794 C. Luo et l. / Energy 36 (2011) 3791e3803 Tble 1 Composition nd some properties of feed strems. Nturl gs Air Sewter CH 4 (mol%) 100 e e N 2 (mol%) e 79 e O 2 (mol%) e 21 e H 2 O (mss%) e e 96.5 NCl (mss%) e e 3.5 Temperture ( C) 25 25 25 Pressure (br) 1.013 1.013 1.013 error (the reltive difference between the itertion used nd the one before) tolernce of 0.01%. For vlidtion, the model ws used to simulte the performnce of n MED-TVC nd of CRGT, seprtely, nd s shown in Sections 3.2 nd 3.3 the simultion results compred very well to vilble dt. The RK-SOAVE, STEAM-TA nd ELECNRTL physicl properties (vilble in ASPEN PLUS) re selected for deling with the processes where the working medi re gs, wter nd sline wter respectively (ccording to the instruction of ASPEN PLUS), nd the min ssumptions for cogenertion system simultion re summrized in Tble 2. 3.2. CRGT model vlidtion A turbine blde-cooling model presented in Ref. [7] ws incorported into the simultion model. The MSR is modeled s Gibbs rector, which determines the equilibrium conditions by minimizing Gibbs free energy, while the chemicl nonequilibrium effects due to rection kinetics re modeled using the chemicl pproch temperture difference DT eq. [10], which cn either be specified or clculted from the following equtions for typicl reformer using nickel-bsed ctlyst [2]: DT eq ¼ 0 if T syn 923 K (2) DT eq ¼ 43:33 1 T syn 273 =650 if T syn < 923 K (3) where T syn is the temperture of the produced syngs of the reforming rection. The therml efficiency is defined s: h ¼ W net =Q in (4) where Q in is the energy of the input fuel nd W net is the network output of the power cycle. Kesser et l. hve reported the temperture, pressure nd mss flow rtes of bsic CRGT configurtion [2]. To vlidte the simultion method used in this pper, bsic CRGT cycle ws simulted with the sme ssumptions nd the results were compred with those given in Kesser et l., s shown in Tble 3. The comprison shows tht the results gree quite well, with reltive differences of the key cycle prmeters within 3%. The stem-ir mss rtio x is defined s the rtion between mss flow rte rtio of the stem sent into the MSR (strem 7 in Fig. 1) nd the inlet ir of the CRGT (strem 1 in Fig. 1): x ¼ m s =m ir ¼ m 7 =m 1 (5) It directly ffects the methne conversion rte in the MSR nd the fuel demnd of the CRGT. Ref. [2] shows tht it hs significnt influence on the therml performnce of CRGT cycle; when the inlet ir of CRGT is fixed, within certin rnge(0w0.15), lrger x mens tht more stem is dded into the MSR, nd the endothermic rection of stem nd methne is strengthened, so more het energy of flue gs is recovered, resulting in higher therml efficiency while more wter consumption (h increses bout 0.8 %-points per 0.01 x dded). 3.3. MED-TVC model vlidtion The mss flow fed to ech effect depends on the energy blnce nd the minimum temperture difference llowed on ech effect, 2.5e2.8 C. The energy nd exergy blnces re derived with the following ssumptions: (1) equl temperture difference cross ech effect; (2) equl boiling point elevtion for ll effects; The mixture of the vpor flshed from both ccumulted distillte nd the brine prehets the sewter fed into ech effect. This gives decrese in temperture cross the preheters, which equl to the temperture drop between the effects [16]. The performnce of the TVC is tken from Ref. [17]. In the modeling nd simultion, the distillte produced in ech effect is considered to be slt free (ctul slt concentrtions re bout 10 ppm, negligible for the purposes of the conducted simultion nlyses). Tble 2 Min ssumptions for the simultion. Configurtions Prmeters Vlue Source MSR Pressure drop (% of inlet pressure) 10 Kesser K.F. et l., 1994[2] Miniml het trnsfer temperture difference gs/gs ( DT p,msr ) 20 C Kesser K.F. et l., 1994[2] Turbine Turbine inlet temperture (TIT) 1308 C Kesser K.F. et l., 1994[2] Isentropic efficiency (h isn,t ) 88% Kesser K.F. et l., 1994[2] HRSG Pressure drop (% of inlet pressure) 3 Kesser K.F. et l., 1994[2] Minimum het trnsfer temperture difference (DT p,hrsg ) 20 C (15 C) gs/gs(liquid) Miniml outlet flue gs temperture (t f ) 90 C Kesser K.F. et l., 1994[2] Compressors Isentropic efficiency (h isn,c ) 89% Kesser K.F. et l., 1994[2] Compression rtio (p) 15 Kesser K.F. et l., 1994[2] Combustor Pressure loss (% of inlet pressure) 3 Kesser K.F. et l., 1994[2] Pump Efficiency (h P ) 85% MED-TVC Number of effects (n) 9 Temperture drop/effect (DT) 3.8 C Alsfour F.N. et l., 2005[16] Boiling point elevtion (BPE) 0.8 C Alsfour F.N. et l., 2005[16] Top brine temperture (TBT) 65.6 C Alsfour F.N. et l., 2005[16] Motive stem temperture (t m ) 140 C (Sturted, 3.61 br) Drwish M. A. et l., 2003[15] Entrined stem temperture (t en ) 49.0 C (Sturted) Alsfour F.N. et l., 2005[16] Heting stem temperture (t h ) 69 C (Sturted) Alsfour F.N. et l., 2005[16] Ambient stte Temperture (t ) 25 C Pressure(P ) 1.013 br

C. Luo et l. / Energy 36 (2011) 3791e3803 3795 Tble 3 Dt summry for our simultion CRGT vlidtion (The stte point numbers refer to Fig. 1). CRGT Prmeters Ref. [2] ASPEN PLUS Air compressor inlet ir m 1 /m ir 1 1 stte (Stte point 1) t 1 15 C 15 C P 1 0.987 tm 0.987 tm CH 4 compressor inlet m 3 /m ir 0.021 0.021 CH 4 stte (Stte point 3) t 3 15 C 15 C P 3 4.93 tm 4.93 tm HRSG inlet wter stte m 5 /m ir 0.144 0.144 (Stte point 5) t 5 15 C 15 C P 5 1.97 tm 1.97 tm MSR (Stte point 9) Outlet syngs temperture (T syn ) 576 C 569 C HRSG outlet flue gs stte (Stte point 13) HRSG Turbine outlet exhust gs stte (Stte point 11) Overll cycle prmeters Input vribles. m 13 /m ir 1.155 1.155 t 13 140 C 140 C P 13 1.00 tm 1.00 tm Miniml het trnsfer 15.7 C 14.4 C temperture difference (DT p,hrsg ) m 11 /m ir 1.155 1.155 t 11 596 C 589 C P 11 1.04 tm 1.04 tm Stem-Methne 6.1 6.1 mole rtio (R SN ) Specific work output (w) 516 kj/kg 503 kj/kg Therml efficiency (h) 48.8% 47.8% To chrcterize the performnce of the MED-TVC, the performnce rtio PR hs been defined s Eq. (1). The specific het trnsfer re,, is defined s the het trnsfer re needed to produce 1 kg/s fresh wter: ¼ A=m w (6) where A is the totl het trnsfer re of the deslintion unit, composed of the re of the effects A E, the condenstion re of the end condenser A C nd the re of the preheters A PHE. To clculte their vlues, the het trnsfer coefficients U of the evportors, condensers, nd preheters re tken from Ref. [26] (see Appendix A), the temperture pproches ΔT (tken s the logrithmic temperture differences) hve been designed, nd the het duty Q is gotten from the simultion cse with ASPEN PLUS. The res cn be figured out by: A ¼ Q=ðU$DTÞ (7) The specific exergy consumption, e con,isdefined s the exergy consumed for producing 1 kg fresh wter: e con ¼ m m ðe m e cm Þ=m w ¼ m m ðe 16 e 14 Þ=m w (8) where e m is the specific exergy of the motive stem, nd e cm is tht of the condenste of the motive stem. Similrly, the specific energy consumption, q con,isdefined s the energy consumed for producing 1 kg fresh wter: q con ¼ m m ðh m h cm Þ=m w ¼ m m ðh 16 h 14 Þ=m w (9) where h m is the specific enthlpy of the motive stem, nd h cm is tht of the condenste of the motive stem. Using the model developed by the uthors, PR, E nd e con re clculted under the sme conditions s those previously given in Ref. [16], in which the simulted configurtions hve the sme operting conditions of n existing plnt in the United Arb Emirtes (the Umm A1-Nr plnt). The results re shown intble 4. It cn be seen tht the reltive differences re no more thn 3.6%, proving tht the model predictions compred well with the dt in the reference literture. Tble 4 Dt summry for the ASPEN PLUS MED-TVC check cse. MED-TVC Prmeters Ref. [16] ASPEN PLUS Number of effects (n) 6 6 Top brine temperture (TBT) 61.8 C 61.8 C Temperture drop/effect (DT) 3.8 C 3.8 C Boiling point elevtion (BPE) 0.8 C 0.8 C Motive stem pressure (P m ) 25 br 25 br Heting stem temperture (t h ) 65 C 65 C Feed sewter temperture (t feed ) 40 C 40 C Cooling sewter temperture (t se ) 30 C 30 C Motive stem/entrined stem (mss flow) 1.36 1.36 Performncertio (PR) 10.05 10.04 Specific het trnsfer re of the effects ( E ) 326.2 m 2 /(kg/s) 338.1 m 2 /(kg/s) Specific exergy consumption(e con ) 87.91 kj/kg 88.23 kj/kg Specific energy consumption(q con ) 252.87 kj/kg 253.02 kj/kg Input vribles. 4. The cogenertion system performnce nd discussion 4.1. Evlution criteri The system hs two useful products: power nd fresh wter. To chrcterize the cogenertion performnce, the power-to-wter rtio R pw is introduced. It is defined s the net generted power W net divided by the mss flow rte of the produced wter m w : W net ¼ W T W COM W P (10) R pw ¼ W net =m w (11) where W T is the work output of the turbine, nd the W COM nd W P is work consumed by compressors nd pumps, respectively. The exergy efficiency of the dul-purpose system 3 is clculted s the exergy output divided by the exergy input: 3 ¼ ðw net þ E w Þ=E in (12) where E in represents the exergy of the input NG, nd E w is the exergy of the wter production, which is given s the miniml work needed in reversible seprtion process for producing the sme mount of fresh wter s in the cogenertion system [13]. The rtio of wter used for the reformer (m s, strem 7 in Fig. 1) to the totl wter produced by the MED (m w,tol, strem 17 in Fig. 1) is defined s CRGT-consumed wter rtio d: d ¼ m s =m w;tol ¼ m 7 =m 17 (13) 4.2. Cogenertion system performnce Minstrem sttes of the cogenertion system including temperture, pressure, mss flow rte, vpor frction nd chemicl composition re presented in Tble 5. The performnce results re reported in Tble 6. The therml efficiency h of top cycle CRGT is 47.0%, nd the performnce rtio PR of the button cycle MED-TVC is 11.3. For the integrted system, the power-to-wter rtio R pw is 745 kj/kg nd the exergy efficiency 3 is 45.7%. Tble 7 shows the exergy destruction of different components in the cogenertion system in detil. The component exergy chnge is defined s the chnge in exergy between the entry stte nd the exhust stte of ech process. If the HRSG did not generte the motive stem, its exergy destruction would decline from 61.9 MW to 58.4 MW; however, the exergy loss of the flue gs would increse from 50.0 MW to 102.9 MW. The integrtion of the MED-TVC into the CRGT cycle is of gret benefit to decrese the exergy loss of the flue gs nd recover the exhust het to produce lrge mounts of fresh wter.

3796 C. Luo et l. / Energy 36 (2011) 3791e3803 Tble 5 Minstrem sttes of the CRGTþMED dul-purpose system. No. t ( C) p (br) m (kg/s) Vpor frction Molr composition N 2 O 2 CH 4 CO H 2 CO 2 H 2 O NCl 1 25 1.01 1000 1 0.79 0.21 2 399 14.9 777 1 0.79 0.21 3 25 1.01 20.7 1 1 4 140 21.3 20.7 1 1 5 25 3.0 120 0 1 6 25.1 22.5 120 0 1 7 474 21.4 120 1 1 8 407 21.3 141 1 0.162 0.838 9 567 19.2 141 1 0.101 0.004 0.183 0.043 0.669 10 1308 14.6 918 1 0.610 0.088 0.037 0.265 11 587 1.05 1131 1 0.641 0.109 0.031 0.219 12 494 1.03 1131 1 0.641 0.109 0.031 0.219 13 96.8 1.01 1131 1 0.641 0.109 0.031 0.219 14 65.6 0.3 58.4 0 1 15 65.6 3.75 58.4 0 1 16 140 3.61 58.4 1 1 17 25 3.0 660 0 1 18 25 3.0 540 0 1 20 35.1 3.0 144 0 0.934 0.066 22 69 0.3 89.6 1 1 23 49 0.12 31.2 1 1 24 32.4 3.0 804 0 0.989 0.011 Fig. 2 is the t-q digrm of the exhust het recovery process. The turbine exhust het is recovered in high-to-low temperture cscde. The horizontl-line segments () nd (b) represent the isotherml evportion processes of the wter strems (6) nd (15), respectively. The strem (6) is the wter supply for CRGTcycle nd the strem (15) is heted to be the motive stem to drive MED-TVC. It cn be seen tht owing to the integrtion of MED-TVC into the CRGT, the het recovery in HRSG is enhnced by strem (15) nd the therml mtch between the heting nd heted strems gets better. However, the temperture difference long () nd to the left is still lrge nd leding to exergy losses, which needs further improvement. The exergy nlyze in Tble 7 lso provides some guidnce for system performnce improvement. The combustion-ssocited exergy chnge is s usul the biggest item. This destruction cn be strightforwrdly decresed by enhncing the reforming rection (elevting the fuel heting vlue) or incresing the inlet temperture of gs turbine beyond the ssumed 1308 C. For exmple, the former cn be relized by incresing the stem led into the MSR. Tble 6 CRGTþMED dul-purpose system performnce summry. CRGT section Compressor inlet ir mss flow rte 1000 kg/s Stem-ir mss rtio (x) 0.12 Stem-NG mole rtio (R SN ) 5.16 NG conversion rte (R NC ) 31.7% Work output (w) 494.1 MW Therml efficiency of the top cycle (h) 47.0% MED-TVC section Performnce rtio (PR) 11.3 Specific het trnsfer re () 337 m 2 /(kg/s) Specific exergy consumption (e con ) 54.0 kj/kg Specific energy consumption (q con ) 216 kj/kg Wter production (m w,tol ) 660 kg/s Wter sent to CRGT (m s ) 120 kg/s Cogenertion system Fuel energy input (Q in ) 1051 MW Fuel exergy input (E in ) 1078 MW Work consumption for MED-TVC 2.61 MW Net work output (W net ) 491.5 MW Net wter production (m w ) 540 kg/s CRGT-consumed wter rtio (d) 18.2% Wter exergy (E w ) 1.50 MW Power-to-wter rtio (R pw ) 745 kj/kg Exergy efficiency (3) 45.7% Input vribles. When the mss flow of ir is fixed, the increse of stem-ir rtio x will result in the elevtion of output network (see Section 6.1). The ltter would be possible if more dvnced turbine is used. The turbine expnsion progress cuses the next lrgest exergy destruction, which cn be reduced by using more efficient turbine s well. The exergy destruction of HRSG is bit less thn tht of gs turbine. This destruction cn be wekened by decresing the temperture differences between the het exchnging strems, but this would obviously require lrger or/nd more complex het exchngers [10]. 5. Comprison with other cogenertion systems To determine whether the proposed dul-purpose system, CRGT integrted with MED-TVC (CRGT þ MED), hs n dvntge over the existing commonly used dul-purpose systems, we hve performed performnce comprison with four cogenertion systems, including the older, widely used Rnkine power genertion system hving bckpressure ST) with n MSF deslintion plnt (ST þ MSF) [27e31], the ST plus MED (ST þ MED), the more modern Tble 7 Exergy nlysis of the CRGTþMED dul-purpose system. Configurtion Amount(MW) Percentge Exergy input Nturl gs 1078.0 100.00% Exergy output Net power output 491.5 45.59% Fresh wter 1.5 0.14% Exergy destruction Combustor 317.9 29.49% Methne stem reformer (MSR) 10.0 0.93% Compressors nd pumps 28.8 2.68% Gs turbine 63.9 5.93% HRSG 61.9 5.74% Mixer 11.1 1.03% Therml vpor compressor (TVC) 15.4 1.43% Multi-effect deslintion (MED) 21.1 1.96% Flue gs 50.0 4.63% Brine 0.7 0.07% Mechnicl nd genertor losses 4.3 0.39% Exergy efficiency e 45.7%

C. Luo et l. / Energy 36 (2011) 3791e3803 3797 700 600 500 400 300 200 100 0 0 100 200 300 400 500 600 700 Fig. 2. The het recupertion t-q digrm of the CRGTþMED dul-purpose system. highest efficiency dul-purpose system comprised of CC with bckpressure stem turbine nd n MSF deslintion plnt (CC þ MSF) [15], nd the CC plus MED (CC þ MED) [12]. In the comprison nlysis ll systems re ssumed to consume methne t the sme flow rte for comprison bse. The reference MSF system considered here hs the chrcteristics given in Tble 8 [29]. It is operted t mximum brine temperture of 90 C, heting stem temperture of 100 C nd PR of 8. For the wter production s lrge s possible, ll stem of the ST/ CC topping cycle is extrcted to drive the MED/MSF bottom cycle. 5.1. Comprison with ST þ MED nd ST þ MSF In the ST plnt, the prmeters of the min stem re set to be 535 C, 161.8 br. The therml efficiency of the boiler is 98%, nd the therml efficiency of the whole system is 37% (the common prmeters of subcriticl power units in Chin). When the methne flow rte is 20.7 kg/s (s the sme s tht in CRGT), the min-stem flow rte is clculted to be 315 kg/s by energy blnce. As the heting stem of the MSF directly comes from the stem turbine fter the expnsion process, the bckpressure rises from originl 0.085 bre1.012 br (100 C), which is chosen for being equl to the heting stem temperture in Tble 8. By simulting the turbine model with n isentropic efficiency of 80% in ASPEN PLUS, the work output drops from 388.7 MW to 309.9 MW. As result, the therml efficiency of the Rnkine stem power cycle decreses from 37% to 29.5% (Tble 9), nd the mss flow rte of the heting stem in MSF is 315 kg/s. Similrly, when the bckpressure ST is integrted with MED, the bckpressure rises from originl 0.085 bre3.61 br (140 C), which re the prmeters of the motive stem in MED (Tble 2). After simultion with ASPEN PLUS, the turbine power output reduces Tble 8 Dt of the reference MSF plnt. MSF Prmeters Ref. [19] Top brine temperture (TBT) 90 C Heting stem temperture (t h ) 100 C Distillte output 313.25 kg/s Number of stges (recovery þ rejection) 21 þ 3 Performncertio (PR) 8 Specific het trnsfer re of the effects ( E ) 292 m 2 /(kg/s of wter) Specific exergy consumption(e con ) 60.84 kj/kg Mechnicl pumping energy (e P ) 3824 kw b from 388.7 MW to 260.5 MW. Eventully, the therml efficiency of the Rnkine stem power cycle decreses from 37% to 24.8% (Tble 9), nd the motive stem of MED is lso 315 kg/s. The comprison results of the CRGT þ MED, ST þ MED nd ST þ MSF systems re exhibited in Tble 9. Compred to the ST þ MSF system, the CRGT þ MED system hs 76% higher power output nd 79% less wter production with the sme energy input. The reson is tht the CRGT is much more efficient thn the Rnkine stem power cycle; lthough the performnce rtio PR of MED is lrger thn tht of MSF, the flow rte of the motive stem sent into MED (140 C stem, 58.4 kg/s) is smller thn tht fed into MSF (100 C stem, 315 kg/s), limited by exhust het recupertion in CRGT, leding the smller m w of the MED. As the exergy of the power is much lrger thn tht of the wter production, the exergy efficiency minly depends on the power output. Hence the exergy efficiency of the CRGT þ MED cogenertion system is higher thn tht of the ST þ MSF one. The power output of the ST þ MED system is the lowest in the three cogenertion systems, nd hereby its exergy efficiency is the worst. However, its fresh wter production is the highest, for it hs the PR of 11.3 (lrger thn 8 of MSF), the motive stem mss flow rte of 315 kg/s (lrger thn 58.4 kg/s of CRGT- cogenertion systems), nd no wter supply for the top cycle (like the CRGT). It is noteworthy tht compring the CRGT þ MED dul-purpose system with the ST- cogenertion systems, the MED bottom cycle did not decrese the efficiency of the CRGT top cycle; it just recovered the surplus exhust gs het fter the reforming stem recuperted its needed flue gs wste het. Although the wter production of CRGT þ MED is the lest, its electricity output is the most nd the exergy efficiency is the highest. Wht is more, the MED subsystem in CRGT þ MED hs the lest totl energy consumption nd het trnsfer re mong the dul-purpose units. In ddition, in the industry the MED system generlly requires less het trnsfer re thn MSF, but in this reserch the MED hs lrger specific het trnsfer re thn tht of MSF. The explntion is s following. Assume the wter production of the two systems is just the sme, 1 kg/s. As the specific energy consumption of MED is lower (54.0 kj/kg VS 57.7 kj/kg in Tble 9), the totl energy consumption Q of MED is smller. Since the MED contins the phse-chnge het trnsfer, the het trnsfer coefficients U of MED re usully lrger thn those of MSF. However, in MSF the temperture difference between the heting stem nd brine (TBT) is10 C(100 Ce90 C, ssumptions in Tble 8), while 3.4 C(69 Ce65.6 C, ssumptions in Tble 2) in MED, which cuses the het trnsfer temperture differences DT of MED is quite smller; nd hence through Eq. (7) the specific het trnsfer re A of MED becomes lrger. 5.2. Comprison with CC þ MED nd CC þ MSF With reference to the CC specifictions of the S109FA model (GE compny, 50 Hz, including MS9001FA gs turbine nd triplepressure-rehet stem cycle) [32e35], the CC in this pper ws simulted with ASPEN PLUS softwre. The min ssumptions of the gs turbine prt re shown in Tble 2 (TIT ¼ 1308 C nd p ¼ 15). In the stem cycle, three rehet pressure levels of the HRSG re 140/30/4 br, the condenstion pressure is 0.05 br, nd the stem temperture t turbine dmission is 562 C. Finlly, the gs turbine hs n exhust temperture of 587 C nd efficiency of 35.2%; the turbine outlet stem flow rte is 164 kg/s; the CC hs therml efficiency of 56%, nd the system net power output is 588.4 MW. To provide the 100 C heting stem for MSF (see Tble 8), the bckpressure of the stem turbine in the bottom cycle is rised from 0.05 br to 1.012 br/100 C. Therefore, the work output of the bove-described CC drops from 588.4 MW to 525.3 MW, nd the therml efficiency decreses to 50.0% (Tble 10).

3798 C. Luo et l. / Energy 36 (2011) 3791e3803 Tble 9 Comprison mong the CRGT þ MED, ST þ MED nd ST þ MSF systems. System performnce CRGT þ MED ST þ MED ST þ MSF Power section Fuel energy input (Q in ) 1051 MW 1051 MW 1051 MW Fuel exergy input (E in ) 1078 MW 1078 MW 1078 MW Work output 494.1 MW 260.5 MW 309.9 MW Therml efficiency of the top cycle (h) 47.0% 24.8% 29.5% Deslintion section Performnce rtio (PR) 11.3 11.3 8 Gross wter production 660 kg/s 3560 kg/s 2520 kg/s Specific het trnsfer re () 337 m 2 /(kg/s) 337 m 2 /(kg/s) 292 m 2 /(kg/s) Totl het trnsfer re (A) 222,000 m 2 1199550 m 2 735840 m 2 Specific energy consumption (q con ) 216 kj/kg 216 kj/kg 301 kj/kg Totl energy consumption (Q con ) 143 MW 768.9 MW 758.5 MW Specific exergy consumption (e con ) 54.0 kj/kg 54.0 kj/kg 57.7 kj/kg Totl exergy consumption (E con ) 33.9 MW 192.2 MW 145.4 MW Cogenertion system Work consumption for deslintion 2.61 MW 14.1 MW 30.8 MW Net work output (W net ) 491.5 MW 246.5 MW 279.2 MW Net wter production (m w ) 540 kg/s 3560 kg/s 2520 kg/s Top-cycle-consumed wter rtio (d) 18.2% 0 0 Wter exergy (W e ) 1.5 MW 9.9 MW 7.0 MW Power-to-wter rtio (R pw ) 745 kj/kg 69.2 kj/kg 111 kj/kg Exergy efficiency (3) 45.7% 23.8% 26.5% Similrly, when the bckpressure CC is integrted with MED, the bckpressure of the stem turbine rises from the originl 0.05 bre3.61 br (140 C), the motive stem prmeters of MED (Tble 2). In the simultion, the CC power output drops from 588.4 MW to 481.1 MW. Hence the therml efficiency of the CC decreses from 56% to 45.8% (Tble 10), nd the motive stem of MED is 164 kg/s. The comprison results of the CRGT þ MED, CC þ MED nd CC þ MSF systems re summrized in Tble 10. Both the power output nd wter production of CC þ MSF re lrger thn those of CRGT þ MED. Not only the therml efficiency of CC is higher thn tht of CRGT, but lso the motive stem mss flow rte of CC þ MSF (164 kg/s) is much bigger thn tht of CRGT þ MED (58.4 kg/s). As result, the therml performnce of the CC þ MSF system is better thn tht of CRGT þ MED. For CC þ MED system, the elevtion of the stem turbine bckpressure mkes the therml efficiency of the CC subcycle decline by 10.2%-points, so the work output of CC þ MED become less thn tht of CRGT þ MED nd the exergy efficiency is lso lower. In the three cogenertion systems, the power output of CC þ MED is the lowest, but its wter production is the highest. The reson is s the sme s tht of the ST þ MED system compred to the CRGT þ MED nd ST þ MSF systems in Section 5.1. From Sections 5.1 nd 5.2, it cn be seen tht with the sme energy input, the power nd wter productions of the five cogenertion systems re quite different from ech other. The CRGT þ MED system hs the second highest work output, the lowest wter production, nd the lest totl het trnsfer re. As the exergy of the fresh wter is too smll, the exergy efficiency 3 minly emphsizes the power output of the dul-purpose system; so the 3 is not enough to describe the dvntges nd disdvntges of the cogenertion systems. For exmple, the ST þ MED system hs the lowest 3, but its wter production is the lrgest. To compre the performnces of the cogenertion systems completely, the further economic study ws crried on. 5.3. Economic perspective The economic nlysis ws bsed on the following ssumptions: The price of NG is 4.24$/MMBtu (0.14$/Nm 3 ) for power genertion [36], ssumed to be constnt over the life of the system. The nnul running time H is 7000 h per yer, nd the plnt life n is 20 yers [30]. The discount rte i is 8% [31]. No lon is mde for the totl plnt investment. Tble 10 Comprison mong the CRGT þ MED, CC þ MED nd CC þ MSF systems. System performnce CRGT þ MED CC þ MED CC þ MSF Power section Fuel energy input (Q in ) 1051 MW 1051 MW 1051 MW Fuel exergy input (E in ) 1078 MW 1078 MW 1078 MW Work output 494.1 MW 481.1 MW 525.3 MW Therml efficiency of the top cycle (h) 47.0% 45.8% 50.0% Deslintion section Performnce rtio (PR) 11.3 11.3 8 Gross wter production 660 kg/s 1850 kg/s 1310 kg/s Specific het trnsfer re () 337 m 2 /(kg/s) 337 m 2 /kg 292 m 2 /(kg/s) Totl het trnsfer re (A) 222,000 m 2 623450 m 2 382520 m 2 Specific energy consumption (q con ) 216 kj/kg 216 kj/kg 301 kj/kg Totl energy consumption (Q con ) 143 MW 400 MW 394 MW Specific exergy consumption (e con ) 54.0 kj/kg 54.0 kj/kg 57.7 kj/kg Totl exergy consumption (E con ) 33.9 MW 100 MW 75.6 MW Cogenertion system Work consumption for deslintion 2.61 MW 7.33 MW 16.0 MW Net work output (W net ) 491.5 MW 473.7 MW 509.3 MW Net wter production (m w ) 540 kg/s 1850 kg/s 1310 kg/s Top-cycle-consumed wter rtio (d) 18.2% 0 0 Wter exergy (E w ) 1.5 MW 5.1 MW 3.6 MW Power-to-wter rtio (R pw ) 745 kj/kg 256 kj/kg 389 kj/kg Exergy efficiency (3) 45.7% 44.4% 47.6%

C. Luo et l. / Energy 36 (2011) 3791e3803 3799 The price of electricity is 0.08$/kWh [37]. The price of wter is 0.7$/m 3 [38]. BOP (Blnce of plnt) consists of the remining systems, components, nd structures tht comprise complete power plnt or energy system tht is not included in the prime mover. For conventionl power genertion system, the BOP is usully ssumed to be 15% of the known components cost [39]. The term O&M is the cost of operting nd mintennce, ssumed to be 4% of the first cost of the system [40]. However, due to the upkeep nd mintennce of the reformer (including the ctlyst), the BOP of the CRGT is ssumed to be 20% of the min components cost nd the O&M cost increses to be 10% of the first cost [33,40,41]. Txes nd insurnce re not considered in this evlution. The investment estimtion of the five dul-purpose systems is listed in Tble 11. With the sme mss flow of CH 4 input, we cn Tble 11 Investment cost of the dul-purpose systems. Plnt configurtion Price Cpcity Investment cost[10 3 $] CRGT þ MED Simple cycle section 212$/kW[32] 494.1 MW 104,758 MSR b 29$/kW[33,40,41] 133.2 MW 3864 HRSG c 244$/m 2 [39,40,42] 81,406 m 2 19,863 CRGT section e e 128485 MED-TVC 1520$/(m 3 /dy)[15,24] 660 kg/s 86,676 Totl plnt cost e e 215,161 ST þ MED Boiler d 20$/kW 1031 MW 20,322 Stem turbine nd e 388.7 MW 17,608 genertor e Blnce of plnt f e e 5689 ST section e e 43,619 MED-TVC 1520$/(m 3 /dy) [15,24] 3560 kg/s 467,462 Totl plnt cost e e 511,081 ST þ MSF Boiler 20$/kW 1031 MW 20,322 Stem turbine nd e 388.7 MW 17,608 genertor BOP e e 5689 ST section e e 43,619 MSF 1615$/(m 3 /dy)[30,31] 2520 kg/s 351,631 Totl plnt cost e e 395,250 CC þ MED CC section g 501$/kW[32] 588.4 MW 294,765 MED-TVC 1520$/(m 3 /dy) [15,24] 1850 kg/s 242,957 Totl plnt cost e e 537,722 CC þ MSF CC section 501$/kW[32] 588.4 MW 294,765 MSF 1615$/(m 3 /dy) [30,31] 1310 kg/s 182,792 Totl plnt cost e e 477,557 Combustor, gs turbine, genertor, compressors, nd BOP re included. The unit cost is tken from the simple cycle specifictions of the PG9351FA model (GE compny, 50 Hz) [32]. b Ni-bsed ctlyst is set inside. As the reforming rection is in the modertetemperture rnge (407e567 C), the unit cost of MSR is quite lower thn tht of the high temperture reforming rector in trditionl hydrogen-producing process [33,40,41]. c The het trnsfer re is clculted by Eq. (7). The het duty Q nd temperture pproches DT (tken s the men temperture difference) re gotten from the simultion cse with ASPEN PLUS. The het trnsfer coefficients U, 99 W/(m 2 C), nd unit cost re tken from the reserch in Ref. [42]. d Shnghi Boiler Fctory ws consulted for the unit cost of the boilers. The exchnge rte of conversion from RMB to US dollrs is 7. e Hngzhou Stem Turbine Fctory ws consulted for the investment cost, ccording to the cpcity of the stem turbine in the ST þ MED/MSF cses. The RMB/ USD foreign exchnge rte is 7. f As the ST system is just conventionl power genertion system, we ssumed the BOP ccounts for 15% of the known component cost of the system [37,43]. g Combustor, gs turbine, HRSG, stem turbine, genertor, compressors, nd BOP re included. The unit cost is tken from the CC specifictions of the S109FA model (GE compny, 50 Hz, including MS9001FA gs turbine nd triple-pressure-rehet stem cycle) [32]. find tht the cpitl cost of CRGT þ MED system is the lowest; however, the quite different cpcities of the systems (especilly the sewter deslintion subsystems) hve significnt influence on the totl plnt cost. For instnce, the MED-TVC in CRGT þ MED only costs 86676k$ (the lest in the deslintion subsystems), but its wter product is lso the lowest, just 660 kg/s. To compre the economic performnces of the cogenertion systems, the COE (electricity cost), COW (fresh wter cost), nd PBP (pybck period) of the systems should be nlyzed. The COE of the dul-purpose systems is clculted s:. H$Wnet;pow COE ¼ b$tpcpow þ C om;pow þ C fu;pow (14) The numertor is the nnul verge electricity cost. TPC pow is the totl plnt cost of the power subsystems (CRGT, ST nd CC). b is the nnul verge investment coefficient, function of the discount rte i nd plnt life n: h b ¼ i= 1 ð1 þ iþ ni (15) C om,pow is the nnul O&M cost of the power subsystems. It should be noted tht C fu,pow is the nnul fuel cost of the cogenertion systems, while W net,pow is the hypotheticl power output of the power subsystems without stem extrction or pump-work supply for deslintion plnts (W net,pow of CRGT, ST nd CC re 494.1 MW, 388.7 MWnd 588.4 MW respectively, see Section 4.1, 5.1 nd 5.2). In other words COE distributed in this wy is just the electricity cost of the power seprte genertion systems with the sme configurtion s the power subsystems in the cogenertion systems. By deducting the electricity cost from the whole product cost, the cost of the fresh wter COW is clculted s: COW ¼ b$tpc þ Com þ C fu H$W net $COE. ðh$m W Þ (16) TPC, C om nd C fu re the totl cost, nnul O&M cost nd fuel cost of the cogenertion systems respectively. W net nd m W re the net product outputs of the cogenertion systems. The pybck period PBP is the time by which ll the revenue of the plnt will hve become equl to the investment TPC [42,43]: h i.h R$ ð1 þ iþ PBP 1 ið1 þ iþ PBPi ¼ TPC (17) R is the nnul net revenue of the plnt: R ¼ R e þ R w C fu C om (18) R e nd R W re the nnul revenue of the net power nd wter product, defined s the output multiplied by the corresponding price. Tble 12 presents the comprison results. It is shown tht the COE () of the CRGT- cogenertion system is higher thn tht of CC- ones (0.0419$/kWh vs. 0.0400$/kWh), while both re lower thn tht of ST- systems (0.0426$/kWh). Becuse the efficiency of CC (56%) is higher thn tht of CRGT (47%), lthough the former configurtion is fr more complicted, which costs much more cpitl investment, its COE chieves the lowest yet. For ll the power subsystems, the nnul fuel cost C fu,pow occupies the most of the nnul verge electricity cost (64%w90%), nd followed by the nnul verge investment b TPC pow (4%w18%) nd nnul O&M cost C om,pow (6%~18%). The COW of the CRGT þ MED is the lowest in the five dulpurpose systems (1.28$/t). Tht is becuse the genertion of the motive stem just uses the surplus exhust gs het, so s not to reduce the power output of the CRGT power cycle; the deducted electricity cost from the totl product cost is quite lrge nd the wter cost prt is reltively smll. On the contrry, due to the stem

3800 C. Luo et l. / Energy 36 (2011) 3791e3803 Tble 12 Comprison of electricity nd fresh wter cost nd pybck period. Items Investment (10 3 $) Work output (MW) COE ($/kwh) Wter output (kg/s) CRGT þ MED 215,161 491.5 0.0419 540 1.28 81,577 3.9 ST þ MED 511,081 246.5 0.0426 3560 1.50 61,896 14.0 ST þ MSF 395,250 279.2 0.0426 2520 1.60 75,753 7.0 CC þ MED 537,722 473.7 0.0400 1850 1.74 105,937 6.8 CC þ MSF 477,557 509.3 0.0400 1310 1.79 119,839 4.9 COW ($/t) R (10 3 $) PBP (y) extrction for running MED/MSF, the network output of ST/CC power subsystems decreses gretly; the wter cost hs high proportion of the totl cost. Besides, the less TPC of CRGT-MED system is lso in fvor of the lower COW. Compred with the CC- cogenertion systems, the fresh wter of the ST- ones cost little less; the min resons re the totl plnt cost is reltively low nd the wter output is pretty lrge. The low TPC nd high revenue of CRGT þ MED system result in its shortest PBP in the five cogenertion systems (3.9 yers). The revenue of CC þ MED/MSF system is remrkble, but its high TPC mkes the PBP (6.8/4.9 yers) longer thn tht of CRGT þ MED; however, it is still shorter thn tht of ST þ MED/MSF (14.0/7.0 yers). Compred to the ST þ MED/MSF nd CC þ MED/MSF cogenertion systems, the CRGT þ MED system hs lower product cost nd 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Q in /Q in,0 W net /W net,0 m W /m W,0 0.0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 b 0.5 0.4 0.3 0.2 0.1 ε,δ ε δ R pw x Q in /Q in,0 W net /W net,0 m W /m W,0 R pw 0.0 0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 x 3500 3000 2500 2000 1500 1000 Fig. 3.. The effect of the stem-ir rtio x on normlized energy input Q in /Q in,0, power output W net /W net,0 nd wter production m w /m w,0, b. The effect of the stem-ir rtio x on exergy efficiency 3, CRGT-consumed wter rtio d nd power-to-wter rtio R pw. 500 shorter pybck period. Hence the CRGT þ MED dul-purpose system is considered to be fesible nd ttrctive for power nd wter cogenertion. 6. Prmetric sensitivity nlysis of the cogenertion system As the stem-ir mss rtio x hs strong effect on the therml performnce of CRGT, nd both the temperture of the motive stem t m nd the number of effects n directly ffect the PR of the MED-TVC, prmetric nlysis ws crried out to investigte their influence on the performnce of the cogenertion system. In the Fig. 3(), 4() nd 5(), the input fuel energy Q in, net work output W net nd mss flow rte of the produced wter m w re normlized by the corresponding vlues of the bse cycle (see 1.2 Q in /Q in,0 W net /W net,0 m W /m W,0 1.0 0.8 0.6 Q in /Q in,0 W net /W net,0 m W /m W,0 PR 0.4 10 100 120 140 160 180 200 220 240 t m b 0.5 0.4 0.3 0.2 0.1 ε,δ ε δ R pw PR R pw 0.0 100 120 140 160 180 200 220 240 t m 14 13 12 11 1800 1600 1400 1200 1000 800 Fig. 4.. The effect of the temperture of the motive stem t m on normlized energy input Q in /Q in,0, power output W net /W net,0, wter production m w /m w,0, nd performnce rtio PR b. The effect of the temperture of the motive stem t m on exergy efficiency 3, CRGT-consumed wter rtio d nd power-to-wter rtio R pw.

C. Luo et l. / Energy 36 (2011) 3791e3803 3801 2.0 1.5 1.0 Q in /Q in,0 W net /W net,0 m W /m W,0 Q in / Q in,0 W net / W net,0 m W / m W,0 PR PR 12 11 10 Since the exergy of the wter only ccounts for smll portion of the totl production exergy (W net þ E W ), the exergy efficiency 3 minly depends on W net nd Q in.asw net grows fster thn Q in, higher exergy efficiency of the CRGT section is chieved. It is noted tht the mximum stem-ir rtio is limited by the constrint on the minimum pinch point temperture difference of 15 C in HRSG [2]. From Fig. 3, we cn see tht when x gets close to 0.15, ll the exhust het is used for generting stem needed for reforming, none is left for motive stem genertion. Hrdly ny fresh wter would thus be produced. 0.5 9 6.2. Influence of the temperture of the motive stem t m 0.0 b 0.5 ε, 0.4 0.3 0.2 0.1 0.0 5 6 7 8 9 n δ 5 6 7 8 9 n section 4.2), so tht the chnge trend of them cn be exhibited more clerly in one figure. 6.1. Influence of the stem-ir rtio x Fig. 3 shows the influence of x on Q in, W net, m w, 3, d nd R pw when the inlet ir m ir is fixed. An incresing x implies more stem sent into the MSR. Since the TIT, the minimum het trnsfer temperture differences in MSR, HRSG nd MED re kept constnt, incresing stem rectnt cuses n increse of input fuel energy Q in. Due to the increse of the working fluid flowing through the turbine, W net is incresed. As more stem is introduced to MSR, the stem-ng mole rtio (R SN ) is enhnced, the endothermic rection of stem nd methne is strengthened, more het energy of flue gs is recovered nd the methne conversion recovers more exhust het for reforming, which results in less energy left for the motive stem genertion in HRSG. With fixed MED-TVC configurtion nd t m, decresing production of the motive stem cuses the decline of the wter production m w. The decrese in m w nd increse in W net mke the power-towter rtio R pw go up. Menwhile, the incresing stem sent into MSR nd decresing wter production cuse the CRGT-consumed wter rtio d move up rpidly. ε δ R pw R pw 8 1600 1400 1200 1000 Fig. 5.. The influence of the number of the MED effects n on normlized energy input Q in /Q in,0, power output W net /W net,0 wter production W net /W net,0 nd performnce rtio PR b. The influence of the number of the MED effects n on exergy efficiency 3, CRGTconsumed wter rtio d nd power-to-wter rtio R pw. 800 The motive stem is generted by the surplus exhust het fter the reforming stem recupertes its needed flue gs wste het. Since the compressor inlet ir mss flow rte, stem-ir mss rtio x, TIT, minimum het trnsfer temperture differences in MSR, HRSG nd MED re fixed, the stem sent into the MSR, surplus exhust het for motive stem genertion, the work output of the CRGT cycle nd Q in re kept the sme (Fig. 4). Hence if the motive stem temperture is rised, its flow rte will come down. An incresing t m implies less motive stem nd higher performnce rtio PR (Fig. 4). The increse of PR nd drop of m m hve opposite effects on the wter production. As result, the m w increses nd reches mximum, t t m ¼ 140 C mong the clculted points nd decreses fterwrds. Although the chnge of the pumping work of MED hs the sme trend s tht of m w, for the pumping work is rther smll compred to the work output of the CRGT, W net shows little chnge in Fig. 4. As result of the chnges of the W net nd m m, R pw becomes miniml nd the 3 mximl when t m is 140 C(Fig. 4b). When t m is 140 C, the CRGT-consumed wter rtio d lso reches the minimum due to the trend of m w nd the invribility of stem sent into MSR (Fig. 4b). 6.3. Influence of the number of MED effects n The number of effects n does not ffect the therml performnces of the top CRGT cycle. When n is incresed, the motive stem genertion in HRSG, the work output of the CRGT nd Q in (Fig. 5) re chngeless. While the n is incresing, the PR moves up, nd the m w rises (Fig. 5). Although the power consumption of pumps goes up in MED, it is still much less thn the work output of the CRGT. So the decrese of W net is quite inconspicuous (Fig. 5). Due to the chnge trends of the W net nd m w, the power-towter rtio R pw decreses. The CRGT-consumed wter rtio d lso declines becuse the stem sent into MSR keeps the sme but the m w rises. Since the increse of the exergy of the wter production is not remrkbly nd the W net hs little decrese, the increse of the exergy efficiency of the cogenertion system is quite restricted (Fig. 5b), 0.012% per effect dded. It cn be seen tht n minly ffects the performnce of the MED prt in the cogenertion system. The cost of wter COW ws figured out to decrese 0.02w0.03$ per effect dded, for the wter output grows fster thn the investment of the MED subsystem while n incresing. 7. Conclusions This pper proposed nd nlyzed novel cogenertion dulpurpose system, which integrtes CRGT with the low temperture (65.6 C) MED-TVC system. The turbine exhust het ws recovered for generting motive stem to run the MED-TVC, which in turn provided the reforming process with its needed wter. The min conclusions include: