An adaptive deign approach for a geothermal plant with changing reource characteritic M. Imroz Sohel 1,*, Mathieu Sellier 2, Suan Krumdieck 2 1 Scion, e Papa ipu Innovation Park, 49 Sala Street, Rotorua, New Zealand 2 Department of Mechanical Engineering, Univerity of Canterury, Private ag 4800, Chritchurch, New Zealand * Correponding author. el: +64 7 3435730; fax: +64 7 3435375; E-mail addre: mohammed.ohel@cionreearch.com Atract: Geothermal power plant are deigned for optimal utilization of geothermal reource. However, geothermal field typically undergo ignificant change in reource characteritic uch a preure, temperature and team quality over their life pan. With appropriate reervoir modelling it i poile to predict the future reource characteritic of a geothermal field to a reaonale degree of accuracy. We propoe a new adaptive deign approach that would allow geothermal power plant to take into account the change of reource characteritic that occur over a 30-40 year time horizon aed on the reult of reervoir modelling. Currently, it i difficult and expenive to modify or renovate an exiting plant due to pace contraint, piping arrangement, tranportation of machinery etc. he adaptive deign approach would allow cot effective modification in operation and equipment to adjut to change in reource characteritic in the future. A imple model for a typical comined cycle geothermal power plant i conidered a a tet cae for the adaptive deign approach. Simulation i carried out uing change in oth wellhead pecific enthalpy and ma flow rate. here are four cae tudie preented in thi paper that analyed variou poile option of the hypothetical power plant depending on the change in reource characteritic. aking into account the reult of the imulation, alternative plant deign are preented and improvement in performance are dicued. Although, the initial invetment cot might go up a a conequence of adaptive deign, over the life pan of the plant the total enefit may e greater. Keyword: Geothermal power, reource characteritic change, adaptive deign, low temperature power ource. 1. Introduction We are at a point of time when on one hand, the negative effect of anthropogenic atmopheric alteration are more evident than ever, and on the other, the demand for energy i ever increaing. Although it claimed that there exit a vat reerve of foil fuel, field y field petroleum production i decreaing [1]. he huge challenge of emiion reduction, growing energy demand and peak oil can e approached in two way. Firtly, y improving energy converion efficiency of traditional energy ource and econdly, witching to more and more renewale energy ource. Unfortunately, mot renewale energy ource are dependent on climatic variation and are not uitale for ae load operation. Geothermal energy, on the contrary, provide a clean, reliale ource of renewale energy. Energy concentration in geothermal ource i much higher than in many other renewale ource. Moreover, geothermal power plant are conidered to have ignificant lower CO 2 emiion than a tandard comined cycle power plant or a pulverized coal fired power plant [2]. Current reearch and development trend toward geothermal power generation, pecifically, low temperature power cycle are noticeale [3-12]. Geothermal power plant are generally deigned aed on contant reource characteritic. However, it ha een oerved in many plant that the reource characteritic change ignificantly throughout the lifetime of the plant [13]. Conequently, deterioration of plant performance and unplanned deign change occur. However, geothermal power plant are very capital intenive and it i not very eay to change a plant to adapt to reource characteritic different from the original deign. 1241
By appropriate reervoir modelling, it i poile to predict future reource characteritic depending on variou parameter including the rate of reource utilization, the percentage of rine reinjection etc [14]. In thi paper we propoe an adaptive deign approach where proviion are kept for a plant to adapt to reource characteritic change at the time of uilding which may ave a great deal of effort and money in the long run. We have preented everal cae tudie to demontrate the enefit of the adaptive deign approach. 2. Methodology We have taken a hypothetical comined cycle geothermal power plant for our tudy. he geothermal fluid i a mixture of team and rine. Steam i eparated from the rine in a uitale eparator then ued to power a team turine. he exhaut team from the team turine i ued to power ottoming organic Rankine cycle unit (). he eparated rine i alo ued in other organic Rankine cycle unit (BRN-ORC). After the heat recovery, oth condened team and geothermal rine are mixed together and reinjected to the reervoir. Pentane i ued a the working fluid in the inary cycle. Fig. 1 how a chematic of the hypothetical power plant. he ae cae conidered here ha four and two BRN- ORC a preented in Fig. 1. VEN SEAM URBINE SEAM BRINE BRN-ORC BRN-ORC Fig. 1. Schematic diagram of the comined cycle geothermal power plant. Reinjection 2.1. he component model Simple model have een ued for the analyi preented here. Independent component module are developed in Matla/Simulink [15] which can e connected later to develop a ytem model. he thermo-phyical propertie are calculated uing the REFPROP [16] dataae. he working fluid flow round the cycle and each proce may e analyed uing the energy conervation, ma conervation and entropy generation applied to a ytem oundary around each ytem component. Change in kinetic energy and potential energy may e neglected and equilirium condition can e aumed at the cro-ection of oth inlet and outlet. Detailed dicuion on the modelling of thee ORC i availale in our previou work [9]. 1242
2.2. he Reource Affected Performance Model A geothermal field pae through four different phae or period [13]: (1) development, (2) utainment, (3) decline and (4) renewale. During the lat phae, a geothermal reource approache the ideal of a utainale and renewale reource. o attain thi pahe require prudent management of the reource. In a Reource Affected Performance Model (RAPM) we change the geothermal reource characteritic and oerve the effect on plant performance. We aume that the reervoir modelling predict that the geothermal reource enthalpy will increae from aout 1400 kj/kg to 2000 kj/kg over the life time of the power plant. An adaptive deign approach i dicued here which keep proviion for thi change in reource characteritic. Applying conervation of ma at the well head m = m + m (1) where, m i the total ma flow rate at the well head, i the team ma flow rate. Dividing Eq. (1) with m yield m i the rine ma flow rate and m 1 = C + C (2) where, C i defined a rine content and C i defined a team content. It i advantageou to expre reource characteritic a team content ( C ). Applying energy alance at the well head m h = mh + m h (3) R where, h R i the reource enthalpy, h i the enthalpy of the rine (aturated liquid) and h i the enthalpy of the team (aturated vapour). he reinjection temperature i calculated from the energy alance of mixing of rine and condenate efore reinjecting to the geothermal field. m h = m h + m h (4) RNG c c where RNG tand for reinjection, tand for rine and c tand for condenate. From Eq. (2), if the team content of a geothermal field ( C ) increae, the rine content ( C ) mut e reduced and vice vera. If we want to keep m and h unchanged a C increae or decreae, we mut manipulate parameter of the left hand ide of Eq. (3). Since, h R i the parameter characteried y geothermal reource, we may not want to manipulate it. he only uitale olution would e to control the geothermal fluid flow rate ( m ). When C increae, we can keep m contant y uing condenate recirculation and increaed geothermal fluid flow rate ( m ). If we are intereted only on the contant heat tranfer in the vaporizer, the reinjection temperature (i.e. rine outlet temperature) can e lowered. he following aumption are made for the RAPM. 1243
1. Operating tate point of the geothermal power plant remain unchanged i.e., the change in ma flow rate in team and rine are only reponile for the change in overall heat tranfer coefficient. 2. o control the vaporizer team outlet condition, exce team i vented to the atmophere. 3. he off-deign well-head condition i alway within the wet-team zone i.e., there i no change in temperature at the well head and the geothermal fluid i a mixture of team and rine. 3. Reult here are four cae tudie preented which analyze adaptive deign approach to addre the change in geothermal reource characteritic. hee cae tudie preent four poile olution for the aumed future reource characteritic. 3.1. he ae cae Normally, each turine ha an operating limit and for the team turine it ha een fixed to 37 MW. For the pentane turine the maximum power i fixed at 7 MW. Fig. 2 how the plant output in the ae cae a the reource enthalpy increae. With increaing team content from aout 25% (1400 kj/kg) to aout 35%, the team turine reache it maximum and produce the ame power thereafter. Since the team turine i unale to utilize the exce team, the ottoming cycle i receiving condenate at an elevated ma flow rate. herefore, the power output of the increae and owing to a lack of rine, the BRN-ORC are producing much le than their capacity. 3.2. Cae tudy 1: increaed geothermal fluid flow rate he reduced rine flow prolem can e tackled in many way. If one ue exce geothermal rine to reheat the condenate collected from the, an increaed ma flow of rine can e enured for the BRN-ORC. Fig. 3 preent a chematic diagram of uch a deign. Here, more power i eing produced at the expene of more geothermal fluid, which mean the reource i eing utilized at a higher rate; not necearily enuring optimum utilization. Fig. 4 how a correponding improvement in plant performance y adopting thi approach. It i noticeale from Fig. 4 that the BRN-ORC produce gradually le power from 25% team content to 35% then it power production i independent of team content. Since, it i more efficient to directly expand team in a turine to produce power than in ottoming cycle, one hould utilize a much team a poile in the team turine within it manufacturing limit. By increaing the geothermal fluid flow rate, the rine reinjection temperature doe not change much. 3.3. Cae tudy 2: upgrading the team turine Fig. 5 how the performance of the geothermal power plant with increaing team content when the original team turine i replaced with a higher capacity. he rated capacity of the new turine i aumed 42 MW with the maximum power 47 MW. It i evident from the figure that uch an upgrade reult in ignificant improvement in power output. However, it i aociated with large capital invetment. 1244
Power (cale for total and team turine) [MW] Enthalpy [kj/kg] 1268 0.2 1459 0.3 1650 0.4 1841 0.5 2032 0.6 2223 0.7 80 70 60 50 40 otal S 30 BR 20 BO 8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 Power (cale for ottoming and rine unit) [MW] Fig. 2. heoretical power for the ae cae a a function of reource enthalpy VEN SEAM URBINE MIXER BRN-ORC SEAM BRINE SEPARAOR BRN-ORC PRODUCION Fig. 3. Adaptive deign for an increaed flow of geothermal fluid INJECION 3.4. Cae Study 3: contant flow of geothermal fluid and lowered reinjection temperature In cae 1, more geothermal fluid wa ued to overcome the prolem of reduced rine in BRN- ORC which reult in utilization of the reource at a higher rate. he reinjection temperature of the geothermal rine i not affected much. In the ae cae, the reinjection temperature i aout 125 C. he minimum recommended reinjection temperature of the ite i aout 80 ºC to prevent ilica formation. So there i a poiility of further extracting heat from the reinjected rine. he alternative deign would look the ame a Fig. 3. However, the geothermal reource i utilized at contant rate i.e. ma flow of geothermal fluid to the plant i the ame a the ae cae. he plant performance would look like the ame a Fig. 4 ut the reinjection temperature will change. Fig.6 how the correponding reduction in reinjection temperature. It i clear from Fig. 6 that it i poile to tailize the rine flow rate of the BRN-ORC and conequently power output y keeping the reinjection temperature within an acceptale limit (80 ºC). 1245
80 8 Power (cale for total and team turine) [MW] 70 60 50 40 30 BO otal 20 0 BR S 7 6 5 4 3 2 1 Power (cale for ottoming and rine unit) [MW] Fig. 4. heoretical power for ae cae with increaed ma flow of geothermal rine to keep the rine flow rate contant for the BRN-ORC a function of reource enthalpy Power (cale for total and team turine) [MW] 80 70 60 50 40 30 S otal 20 0 BR BO 7 6 5 4 3 2 1 Power (cale for ottoming and rine unit) [MW] Fig. 5. heoretical power of the geothermal power plant with a higher capacity team turine 150 Dicharge temperature [C] 140 130 120 110 100 90 Cae 1 Cae 3 Cae 4 80 Fig. 6. heoretical reinjection temperature for cae 1, cae 3 and cae 4 3.5. Cae tudy 4: contant flow of geothermal fluid with exce team (50/50) It wa tated earlier that the team turine ha a power producing limit. Beyond thi limit, the team turine cannot utilize the exce team and the conequence i a higher dicharge enthalpy. Another poile alternative i depicted in Fig. 7. he exce team can e ypaed and ued to reheat the condenate collected from the. he reult of mixing exce team (50%) and condenate (50%) are preented in Fig. 8. It i clear from Fig. 8 that 1246
the reheating of the condenate y exce team mitigate the reduced rine for the BRN- ORC. he reinjection temperature i not reduced y thi approach (Fig. 6). CONDENSAE CONDENSAE VEN SEAM URBINE SEAM MIXER BRINE SEAM SEPARAOR BRINE PRODUCION CONDENSAE INJECION Fig.7. Adaptive deign for a contant flow of geothermal fluid and regenerative heating Power (total and team turine) [MW] 70 60 50 40 30 BR BO otal 20 3 S 7 6.5 6 5.5 5 4.5 4 3.5 Power (ottoming and rine unit) Fig. 8. heoretical power of the geothermal power plant and contant ma flow of geothermal rine with regenerative heating of the rine y exce team 4. Dicuion and concluion hi paper ha introduced an alternative deign approach that take into account poile change in future reource characteritic. A geothermal power plant are very capital intenive and it i not very eay to change a plant to adapt to reource characteritic different from the original deign, keeping proviion for future reource characteritic can e very effective. Although, the initial invetment cot might go up a a conequence of adaptive deign, over the life pan of the plant the total enefit may e greater. A proper cot enefit analyi i neceary to identify the economic enefit. here are four cae tudie preented in thi paper that analyed variou poile option of the hypothetical power plant depending on the hypothetical change in reource characteritic. he reult how proviion that could e kept in the plant for future reource characteritic. he next phae i to do a cot enefit 1247
analyi of thee option and elect the optimum option. In thi paper we have only dicued adaptive deign approach for increaing reource enthalpy. Similarly, adaptive deign for a decreaing reource enthalpy can alo e carried out which will provide different proviion for the geothermal power plant. One uch proviion i that one or more of the can e deigned in uch a way that they can e ued a BRN-ORC when geothermal reource enthalpy reduce to utilize the increaed rine availale. Reference [1] IEA, World Energy Outlook. 2008: International Energy Agency. [2] Barier, E., Geothermal energy technology and current tatu: an overview. Renewale and Sutainale Energy Review, 2002. 6(1-2): p. 3-65. [3] DiPippo, R., Second Law aement of inary plant generating power from lowtemperature geothermal fluid. Geothermic, 2004. 33(5): p. 565-586. [4] Chen, H., D.Y. Gowami, and E.K. Stefanako, A review of thermodynamic cycle and working fluid for the converion of low-grade heat. Renewale and Sutainale Energy Review, 2010. DOI:10.1016/j.rer.2010.07.006 [5] Bomarda, P. and M. Gaia. Geothermal Binary Plant Utiliing an Innovative Non- Flammale Azeotropic Mixture a Working Fluid. in Proceeding 28th NZ Geothermal Workhop. 2006. [6] Madhawa Hettiarachchi, H.D., et al., Optimum deign criteria for an Organic Rankine cycle uing low-temperature geothermal heat ource. Energy, 2007. 32(9): p. 1698-1706. [7] DiPippo, R., Ideal thermal efficiency for geothermal inary plant. Geothermic, 2007. 36(3): p. 276-285. [8] Sohel, M.I. and M. Jack, Efficiency improvement y geothermal heat integration in a lignocelluloic iorefinery. Bioreource echnology, 2010. 101 p. 9342-9347. [9] Sohel, M.I., et al., An iterative method for modelling the air-cooled organic Rankine cycle geothermal power plant. International Journal of Energy Reearch, 2010. DOI: 10.1002/er.1706 [10] Sohel, M.I., et al., Dynamic Modelling and Simulation of an Organic Rankine Cycle Unit of a Geothermal Power Plant. Proceeding World Geothermal Congre 2010 Bali, Indoneia, 25-29 April 2010. [11] Atren, A.D., H. Gurgenci, and V. Rudolph, Electricity generation uing a caron-dioxide thermoiphon. Geothermic, 2010. 39(2): p. 161-169. [12] Atren, A.D., H. Gurgenci, and V. Rudolph, CO2 hermoiphon for Competitive Geothermal Power Generation. Energy & Fuel, 2008. 23(1): p. 553-557. [13] DiPippo, R., Geothermal Power Plant: Principle, Application and Cae Studie. 2005: Elevier Ltd. [14] RJVL, ROOKAWA GEOHERMAL DEVELOPMEN Reource Conent Application and Aement of Environmental Effect. 2007, Rotokawa Joint Venture Limited, C/- Mighty River Power Limited, 160 Peachgrove Road, PO Box 445, HAMILON, New Zealand. [15] MathWork, www.mathwork.com. 2008. [16] REFPROP. National Intitute of Standard and echnology (NIS), 2007. Availale from: http://www.nit.gov/. 1248