2. HOLISTIC DESIGN APPROACH

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1 roceedings World Geotherml Congress 200 Bli, Indonesi, April 200 Holistic Design Approch for Geotherml Binry ower lnts with Optimized Net Electricity rovision Stephnie Frick, Stefn Krnz2 nd Ali Sdt3 Helmholtz Centre otsdm GFZ Germn Reserch Centre for Geosciences, Telegrfenberg, D-4473 otsdm, Germny Keywords: Net electricity, binry power plnts, power plnt optimiztion, power plnt design, holistic pproch binry power plnts cn significntly vry depending on site-specific conditions. ABSTRACT The contribution of geotherml binry power plnts to the energy system is bsed on the provision of net electricity. Tht is defined by the produced gross electricity from which the uxiliry power to run power consuming components in the different prts of the plnt must be deduced. In contrst to other therml power plnts the rtio of uxiliry power to gross electricity cn significntly vry in geotherml binry power plnts depending on sitespecific conditions. It typiclly lies between 30 to 50 % but cn be lso higher depending on the site-specific energetic effort to deliver the geotherml fluid from the reservoir or the effort for the recooling of the conversion cycle. In order to optimize the provision of net electricity t specific site it is hence importnt to consider the different chrcteristics of gross electricity production nd uxiliry power consumption. The pper will therefore introduce geotherml-specific, holistic design pproch in which not only prmeters, which chrcterize the qulity of single plnt components, but lso site-specific reservoir nd mbient conditions re considered. A cse study will show tht mximizing the instlled electricl cpcity, which is typicl in other power plnt pplictions, does not result in n optimum net electricity output. With the presented methodology, in contrst, it is possible to relize geotherml binry power plnts with higher net electricity output bsed on existing technology. Figure : Schemtic set-up of geotherml binry power plnt showing the different subsystems The power demnd for the geotherml fluid production, for exmple, is determined by the reservoir chrcteristics nd the delivered flow rte, e.g. Heidinger et l. (2006), Snyl et l. (2005), Legrth (2003). When dimensioning the geotherml fluid flow from specific reservoir, technicl restrictions, such s the mximum instlltion depth nd mximum pump cpcity, must be considered. Another importnt spect is the over-proportionl increse of the pumping effort with incresing fluid flow rte. Since higher flow rte results in lrger drw down of the fluid level in the production well, the production effort shows qudrtic dependence with respect to the flow rte. The increse of the fluid production effort with incresing geotherml fluid flow rte is stronger for lower reservoir productivities (Figure 2).. INTRODUCTION Binry power plnts re used to produce power from low to medium temperture het sources. Since the predominnt prt of the world wide geotherml potentil is bsed on temperture level between 00 nd 200 C, binry power plnts will ply more nd more importnt role for geotherml power genertion in the future. The role which different power plnt technologies or energy sources ply in the electricity mix is typiclly mesured by the totl instlled power cpcity. However, the rel contribution of power plnts is their net electricity output which is defined by the produced gross electricity from which the uxiliry power to run the power consuming components in plnt must be deduced. Figure 2: Exmple of fluid production effort s function of geotherml fluid flow for different reservoir productivities Geotherml binry power plnts contin not only the power unit on the surfce but lso the geotherml fluid loop to deliver the fluid from the reservoir (Figure ). Therefore, the power consuming components of such plnts re in the geotherml fluid loop (such s the downhole pump), in the binry power unit (such s the feed pump), nd the recooling system (such s cooling pumps nd fns). Existing experiences show tht, in contrst to other therml power plnts, the uxiliry power demnd in geotherml The uxiliry power consumed by the recooling system which hs considerble influence on the net power output due to reltively lrge wste het mounts - is significntly influenced by site preconditions nd mbient conditions, e.g. Moy nd Diippo (2007), Kröger (2004), IC (200), Klenke (970). The reltive mount of wste het thereby increses with decresing conversion efficiencies (Figure 3). For removing the wste het from the binry cycle, suitble het sink, such s surfce wter, wter

2 from groundwter wells or mbient ir is necessry. In conventionl power plnt engineering, wter cooling is usully preferred to ir cooling due to the bility to relize lower condensing tempertures nd therefore lrger power output. However, the precondition of sufficient supply of cooling wter cn t mny sites, if ny, only be met with dditionl technicl nd energetic effort. The design of the recooling system in geotherml binry power plnts will therefore oftentimes be compromise between technicl reliztion nd energetic spects. Figure 3: Specific wste het s function of the conversion efficiency Bsed on the considertions bove, geotherml binry power plnt design must focus on net electricity provision nd integrte site-specific reservoir chrcteristics, mbient conditions nd opertion chrcteristics in holistic design pproch. In the following, such n pproch will be introduced nd pplied in theoreticl cse study. It will be shown tht the presented methodology leds to geotherml binry power plnts with higher net electricity output bsed on existing nd proven technology. 2. HOLISTIC DESIGN AROACH THEORETICAL CONSIDERATIONS The design of the subsystems in geotherml binry power plnts depends on different site-specific influences nd prmeters. In order to design relible nd efficient geotherml binry power plnts it is therefore importnt to integrte these different chrcteristics in n overll or holistic design pproch. The most importnt spects or differences of holistic design pproch compred to the seprte design of ech subsystem re: n optimum geotherml fluid flow rte: The gross power which cn be generted from geotherml resource linerly increses with incresing geotherml fluid flow rte. However, the considertions bove hve shown tht incresing the flow rte t site lso results in n over-proportionl increse in power consumption of the fluid production system (Figure 2). This mens tht n optimum geotherml fluid flow rte exists for which the net power provision of geotherml binry power plnt reches its mximum. Assuming specific plnt set-up on the surfce, the optimum flow rte for site with lower reservoir productivity is therefore lower. Referring to different surfce plnts set-ups, the optimum flow rte is incresing with more efficient nd better utiliztion of the geotherml het such s with more efficient (nd relible) binry units or the supply of the residul het in the geotherml fluid fter the het trnsfer to the binry unit. n optimum working fluid: The selection of the working fluid enbles the dpttion of the binry conversion cycle to the chrcteristics of the geotherml het source. This is due to different shpes of the dewpoint curve nd different evportion chrcteristics which cn be relized with different medi nd mixtures. The choice of suitble working fluid is therefore n importnt spect in designing geotherml binry power plnts. A suitble working fluid must llow relible opertion (e.g. thermlly stble in the long-term, comptible with other mterils used in the binry cycle), high conversion efficiency nd good utiliztion (i.e. the cooling of the geotherml fluid) of the geotherml het. Due to the reltively lrge wste het mount in geotherml binry power plnts, lso selection of the working fluid ccording to the het sink nd opertion chrcteristics must be considered. n optimum evportion temperture: The evportion temperture contrrily influences the conversion cycle efficiency nd the utiliztion of the geotherml het. Hence, n optimum evportion temperture exists for which the power output reches mximum. Regrding the design of the evportion lso nnully vrying mbient conditions which might influence the condenstion temperture must be considered. If geotherml binry power plnt should lso supply het in seril connection to the binry power unit, the evportion temperture lso depends on the temperture which is required t the outlet of the binry unit in order to provide certin supply temperture. The outlet temperture of the geotherml fluid cn be incresed with higher evportion tempertures. Another possibility is internl het recupertion in cse dry working fluid is used. n optimum condenstion temperture: The gross power output of binry unit is incresing for decresing condenstion tempertures due to the incresing enthlpy difference in the expnsion mchine. However, lso the requirements for the recooling re incresing with decresing condenstion tempertures. This is becuse lower tempertures must either be relized by lower cooling sink tempertures such s in cse of once-through cooling systems. At mny sites, where once-through cooling is not n option, lower condenstion tempertures cn only be relized by lrger uxiliry power input to the fns of wet cooling towers or ir coolers. The correltion between condenstion temperture nd uxiliry power demnd is determined by the recooling system, its performnce nd the mbient conditions. Therefore, regrding the net electricity production, lso n optimum condenstion temperture does exist. It must be considered tht the mbient conditions cn significntly vry during the yer. Regrding the reltively lrge wste het mounts in geotherml binry power plnts, the technicl use of the wste het (e.g. conventionl cogenertion) cn lso reduce the demnd for recooling. 3. CASE STUDY The relevnce of holistic design pproch will be presented in the following in simplified cse study. The objective of this cse study is to mximize the net power output of geotherml binry power plnt t specific site. Two different pproches re thereby compred. Bsed on the implementtion of existing nd relible technology for both pproches, one pproch is iming t the mximiztion of the instlled cpcity or gross power nd the other one is directly mximizing the net power by mens of holistic plnt design. 2

3 3. Reference Cse The cse study is bsed on n verge geotherml low temperture site referring to reservoir nd site preconditions. The geotherml reservoir is ssessed with doublet. On the surfce, the het of the produced geotherml fluid is used in binry unit. The conversion unit is bsed on Rnkine Cycle with pure working fluid nd is cooled by forced-drught cooling tower. The reservoir nd the geotherml fluid re defined s follows: reservoir / fluid temperture 50 C reservoir depth 4,000 m productivity index 30 m 3 /(h M) injectivity index 30 m 3 /(h M) pore pressure grdient 0.7 br/00 m specific het cpcity fluid 3.5 kj/kg K fluid density.47 kg/m 3 The design will be mde for two different mbient conditions: mbient temperture 5 C, reltive ir humidity 75 % mbient temperture 20 C, reltive ir humidity 66 % The prmeters which re vried in the following in order to derive the optimum plnt design for the two different design pproches re: the working fluid, the evportion temperture T V (corresponding to certin reinjection temperture of the geotherml fluid T,out ), the condenstion temperture T C, nd the geotherml fluid flow rte m&. Generl considertions in plnt design lso refer to component-specific prmeters, such s efficiencies of turbines nd pumps, temperture differences in het exchngers or cooling tower chrcteristics. For the comprison of the different design pproches, these generl design considertions will not be explicitly ddressed. Following prmeters re therefore ssumed for both design pproches: Geotherml fluid loop efficiency downhole pump 0.75 well hed pressure production well 0 br intke pressure downhole pump 0 br reltive roughness riser tube reltive roughness csing dimeter riser tube 5 dimeter csing 8 ½ Binry conversion cycle het exchnger temperture differences 5 K pressure loss per het exchnger 0. br efficiency feed pump 0.8 isentropic efficiency turbine 0.75 mechnicl efficiency turbine 0.95 mechnicl efficiency genertor 0.95 Wet cooling tower pproch (to wet bulb temperture) 3 K cooling rnge 6 K cooling tower constnt 0.8 specific het cpcity wter 4.2 kj/kg K instlltion height cooling tower fill.5 m wter-sided pressure losses br pressures increse fns br cooling pump efficiency 0.8 cooling fn efficiency Net ower Clcultion The net power net is clculted bsed on the gross power gr from which the uxiliry power ux of the subsystems is deduced. net = () gr ux 3.2. Gross ower In order to clculte the gross power of the binry power unit the evportion nd condenstion temperture need to be defined for specific working fluid nd ccording to the temperture profile of the het source. The live vpor is ssumed to be sturted vpor, unless the exhust stem wetness in the turbine does not exceed n llowed limit. The working fluid mss flow cn be clculted from the energy blnce round the het input: m& c T T ) = m& ( h h ) (2) ( p,, in, out, pc, V where, c p,, T,in, T,out re mss flow, specific het cpcity, inlet temperture nd reinjection temperture of the geotherml fluid, nd m&, h,pc nd h,v re m& working fluid mss flow, enthlpy of the pressurized condenste nd live vpor enthlpy, respectively. The gross power is then clculted with the working fluid mss flow, the difference of live vpor nd exhust stem enthlpy (h,v h,e ), which ccounts the isentropic turbine efficiency, nd the turbine nd genertor efficiency T nd G, respectively: = m& ) (3) gr ( h h, V, E T G Auxiliry ower The uxiliry power demnd is the sum of the uxiliry power demnd in the geotherml fluid loop ux,, in the binry unit ux,bin nd in the recooling system ux,cool : + + ux ux, ux, bin ux, cool = (4) For the estimtion of the uxiliry power consumption in the geotherml fluid loop, the dynmic fluid level is clculted referring to top ground surfce: h DFL V& = h +, (5) SFL ρ g I where h DFL, h SFL, V &, ρ, g nd I re dynmic fluid level, sttic fluid level (depending on the pore pressure grdient, the reservoir depth nd the geotherml fluid flow rte nd density), grvity constnt nd productivity index of the reservoir. Assuming tht the pressure t the bottom of the injection well, resulting from the wter hed in the injection well nd its wellhed pressure re sufficient to overcome the reservoir pressure, the uxiliry power demnd of the geotherml fluid loop cn be clculted s follows: = & ρ p ) (6) wh V ( ( g h ) + p + ux, DFL fr where p fr, p wh nd D re friction losses in the production well clculted ccording to Legrth (2005), well hed pressure nd downhole-pump efficiency, respectively. D 3

4 The uxiliry power to run the feed pump in the binry unit depends on the working fluid flow rte nd density ρ, the pressure increse from the condenstion to the evportion pressure, p C nd p V, respectively, nd the efficiency of the feed pump F m = & p ) (7) C ( p ux, bin V ρ F The uxiliry power demnd of the open wet cooling tower results from the opertion of the cooling wter pump nd the fns to generte the forced drught: + ux, cool ux, C ux, fn = (8) where ux,c nd ux,fn re the power consumption of cooling wter pump nd fns respectively. The uxiliry power demnd of the cooling wter pump is: m = & ρ g h ) (9) fill ( p + ux, C ρ where m& C, p, ρ, h fill nd C re cooling wter flow rte, friction losses, cooling wter density, height of cooling tower fill nd cooling pump efficiency. The cooling wter mss flow is derived from the energy blnce round the condenser: m& c T T ) = m& ( h h ) (0) ( p,, w, c, E, C where c p,, T,w, T,c nd h,c re the specific het cpcity of the cooling wter, temperture of the heted nd cold cooling wter, nd enthlpy of the working fluid condenste t the feed pump inlet. The temperture difference (T,w - T,c ) is referred to s cooling rnge which depends on the cooling tower design. The type of the condenser is ssumed to be cross flow so tht the temperture of the heted cooling wter depends on the condensing temperture in the binry cycle T C nd the pinch point or temperture difference T C in the condenser: T = T T (), w C C The uxiliry power demnd of the fn in the cooling tower depends on the ir flow m&, the pressure increse in the fn p fn nd the fn efficiency fn : m = & p (2) fn zx, fn ρ fn where ρ is the ir density t mbient pressure. The ir flow rte is determined ccording to Klenke (970) by the cooling wter mss flow, the ir rtio λ nd the theoreticl minimum ir flow l min, which re both clculted from the mbient conditions, such s ir temperture nd reltive humidity nd the cooling tower performnce (e.g. pproch nd cooling tower constnt) for specific mode of opertion: m& = λ m& (3) l min 3.3 Results The results of the cse study re shown in Figure 4. It cn be seen tht under the conditions ssumed for the exmple site, the net power vries between 0.9 nd.3 MW nd tht geotherml fluid production nd recooling hve significnt uxiliry power demnd which rnges from 39 to 47 % nd 9 to 8 %, respectively. Compring the two design pproches, the mximiztion of the net power results in 2 % or 4 % higher net power output compred to the plnts which would be relized bsed on mximizing the gross power. The instlled cpcity or gross power (i.e. sum of net power nd uxiliry power in Figure 4) referring to the plnts with optimized net power is in contrst 5 % or 2 % lower. With net power mximiztion the rtio of net power output to gross power is therefore incresed by 33 nd 43 %, respectively. These improvements re bsed on the differently chosen design prmeters which especilly result in decrese of the uxiliry power demnd for recooling nd geotherml fluid production. From the following tble it cn be seen tht ll design criteri except the working fluid depend on the design objective. Gross power mximiztion Net power mximiztion Ambient conditions 5 C, 75 % 20 C, 66 % 5 C, 75 % 20 C, 66 % Working fluid isobutne isobutne T V [ C] T,out [ C] T C [ C] m& [kg/s] The optimum evportion temperture T V vries only by little between gross nd net power mximiztion nd between the ssumed mbient scenrios. Electricl power in kw 5,000 4,000 3,000 2,000,000 0 _net _ux, _ux,bin _ux,cool _net/_gr mbient: 5 C, 75% "gross "net power power mx" mx" mbient: 20 C, 66% "gross "net power power mx" mx" Figure 4: Comprison of net power output, uxiliry power consumption nd rtio of net power to gross power for different design pproches nd mbient conditions Regrding the optimum condenstion temperture, in contrst, the design pproch nd the mbient scenrio hve significnt influence. The working fluid in the binry cycle of the plnt designed for mximum gross power is condensed t the minimum condenstion temperture depending on the mbient conditions. This results in specific uxiliry power demnd for recooling of 7 to 9 kw el /MW th. When mximizing the net power in contrst, _net / _gr 4

5 higher condenstion temperture is chosen so tht the specific recooling effort is reduced to 0 to kw el /MW th which results in decrese of the uxiliry power demnd for recooling by 50 nd 53 %. A significnt influence of the design pproches cn lso be seen for the optimum flow rte. Mximizing the power plnt for gross power, the mximum flow rte with respect to technicl restrictions (e.g. instlltion depth, volume flow, pump cpcity) is produced from the reservoir. For the ssumed reservoir conditions the mximum flow rte is limited to 00 kg/s. Applying the holistic design pproch, lower flow rte of 82 nd 88 kg/s is produced. Figure 5 shows tht the optimum flow rte for specific reservoir depends on the gross power output but lso on the uxiliry power demnd for recooling. Under more fvorble mbient conditions more gross power is produced nd less uxiliry power consumed for recooling so tht the optimum flow rte reches higher vlue. roducing net power optimized flow rte, the uxiliry power consumption of the downhole pump is reduced by 24 nd 34 %, respectively. Electricl power in kw. 3,000 2,500 2,000,500, gr - ux,bin - ux,cool net mbient: 5 C, 75% 20 C, 66% mbient: mbient: 5 C, 20 C, 75% 66% optimum flow rte Therml wter flow rte in kg/s Figure 5: Influence of mbient conditions on optimum geotherml fluid flow rte nd mximized net power output In contrst to the downhole pump, the reduction of the uxiliry power consumption of the feed pump in the binry cycle is lower nd lies t 2 nd 7 %, respectively. This improvement is bsed on lower pressure increse in the feed pump due to the higher condensing pressure nd the lower working fluid flow rte. 4. CONCLUSION AND OUTLOOK Bsed on generl considertions nd by mens of cse study it could be shown tht geotherml binry power plnts re, t typicl sites, chrcterized by considerble uxiliry power demnd. This is especilly due to the energetic effort for the production of the geotherml fluid nd the recooling of the binry unit. In order to relize geotherml binry power plnts with optimized net electricity provision, lso resulting in n optimized contribution to the energy mix, holistic design pproch hs been presented. This pproch hs its focus on relible overll system nd defines plnt efficiency bsed on net insted of gross power output. The results of the cse study could show tht holistic design pproch cn led to considerble improvement of the net power output bsed on existing technology. From n economic viewpoint it is lso relevnt tht this improvement is chieved with smller instlled plnt cpcity. Holistic geotherml plnt design therefore cnnot only increse revenues but lso decrese cpitl investments. For ongoing studies on holistic design of geotherml binry power plnts, further spects, long with the design prmeters discussed in this pper, must be considered. A very importnt spect regrding net electricity provision is the plnt vilbility. In this context experience hs shown tht especilly the relibility of the geotherml fluid loop must be considered very crefully in the design of geotherml binry power plnts. roblems such s scling nd corrosion cn significntly impir the plnt performnce or my even result in temporl shut down of opertion. Also the relibility of the binry power unit on the surfce is crucil spect in geotherml binry plnt design. Another spect is tht in mny cses, binry power units will not lwys be operted t their design point so tht opertion chrcteristics nd prt lod behvior must be considered in plnt design. Regrding n nnul opertion period, prt lod opertion cn result from vrying mbient conditions nd therefore vrying recooling performnce. Also the dditionl provision of district het cn led to prt lod opertion of the binry unit becuse combined energy supply, for exmple power nd het, from low to medium temperture geotherml resources is typiclly relized in prllel or seril connection. Regrding the design of the binry unit with district het supply in seril, the cooling of the geotherml fluid might be limited t times of het demnd. In cse of prllel connection, the binry unit might be fed with vrible mss flow during the yer. Regrding the lifetime opertion of binry power unit chnges in the reservoir productivity or cooling of the reservoir cn hve n influence on the plnt opertion. Aprt from technicl considertions, lso non-technicl spects, such s vilbility of cooling wter, cooling wter tretment, noise emission thresholds or lnd use, re issues which cn be integrted in holistic pproch. In generl it cn be concluded tht holistic power plnt design must be site specific pproch becuse reservoir nd site preconditions but lso opertionl chrcteristics vry from site to site. It is importnt to note tht the success of holistic project development is bsed on interdisciplinry collbortion. ACKNOWLEDGEMENTS This study ws crried out within the Europen project LOW-BIN with the support by the Europen Commission (Dg Tren) in the Sixth Frmework rogrmme riority. REFERENCES Diippo, R.: Geotherml ower lnts, rinciples, Applictions, Cse Studies nd Environmentl Impct, second edition, Elsevier Ltd, Oxford, (2008), 493 pp. Heidinger,., Dornstädter, J., Fbritius, A.: HDR economic modelling: HDRec softwre, Geothermics 35, (2006), Integrted ollution revention nd Control (IC): Reference Document on the ppliction of Best Avilble Techniques in Industril Cooling Systems, Europen Commission (200). 5

6 Klenke, W.: Zur einheitlichen Beurteilung und Berechnung von Gegenstrom- und Kreuzstromkühltürmen, Kältetechnik-Klimtisierung 22. Jhrgng, Heft 0/970, (970), Köhler, S.: Geothermisch ngetriebene Dmpfkrftprozesse Anlyse und Vergleich binärer Krftwerke, Disserttion, Technische Universität Berlin, Germny, (2005), 84 pp. Kröger, D.: Air-cooled het exchngers nd cooling towers: therml flow performnce evlution nd design, Volume II, enn Well Corportion, (2004), 500 pp. Legrth, B.: Erschließung sedimentärer Speichergesteine für eine geothermische Stromerzeugung, Disserttion, Technische Universität Berlin, Berlin, Germny (2003). Moy,., Diippo, R.: Unit 5 bottoming binry plnt t Mirvelles geotherml field, Cost Ric: lnning, design, performnce nd impct, Geothermics 36, (2007), Sleh, B., Koglbuer, G., Wendlnd, M., Fischer, J.: Working fluids for low-temperture orgnic Rnkine cycles, Energy 32, (2007), Snyl, S., Kitz, K., Glspey, D.: Optimiztion of ower Genertion from Moderte Temperture Geotherml Systems A Cse History. roceedings World Geotherml Congress 2005, Antly, Turkey (2005). 6