Thermodynamic Modeling and Exergy Analysis of a Heat Recovery System in Meshkinshahr Geothermal Power Plant, Iran

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1 Proceedings orld Geotermal Congress 2010 Bali, Indonesia, April 2010 Termodynamic Modeling and xergy Analysis of a Heat Recovery System in Meskinsar Geotermal Power Plant, Iran Benam Radmer 1, Sajjad Radmer 2, Kosrow Baktari 1 and Hooman Farzane 1 1 Science and researc branc of Islamic Azad University, nvironment and nergy Faculty, Teran, Iran 2 University of Tabriz, faculty of mecanical engineering, Tabriz, Iran radmerbe@yaoo.com, sajjad.radmer@gmail.com, oomanfa@yaoo.com Keywords: Single flas cycle, ORC, exergy, efficiency ABSTRACT A 55 M geotermal power plant is projected to be installed in te Sabalan geotermal field. Te drilling of wells to support te 55 M power plant started in June ile in te drilling pase of te project, SUNA as considered installing a demonstration pilot 4 M power plant using steam condensing turbine in a single flas cycle. Te steam fraction of te two-pase fluid produced from geotermal wells in Sabalan is less tan 20 percent. In a single flas cycle te brine pase is discarded still contains a uge volume of energy. Te main objectives of tis study are: to develop termodynamic model and exergy analysis of a eat recovery system to produce electricity in an organic rankine cycle (ORC), parallel wit single flas system using energy content of separated brine; pinpoint locations and quantities of exergy losses and wastes; and to determine te optimum performance parameters of te main components. Te first and second law efficiencies and te productivity of geotermal fluid will be determined for single flas as well as for combined system. Te ngineering quation Solver (S) software was used for developing and analyzing matematical models of energy and exergy flows. Termoeconomic analysis using obtained results can be considered for furter work to elp decision makers. 1. INTRODUCTION Currently, te Sabalan Geotermal Field as four production wells and new wells are being dilled. A 4 M demonstration pilot power plant is considered to be installed on well NS-4 at tis field. Data obtained from discarged well test sows tat well NS-4 produces two-pase geotermal fluid wit mass flow of 56 kg/s and entalpy of 954 kj/kg at well ead pressure of 5.5 bar (SKM, 2005). Single flas cycle wit steam condensing turbine will be used as energy conversion system at te power plant. Using a single flas system will result in discarding a significant volume of energy in form of brine from separator, due to low quality of produced two-pase fluid. However, te double flas cycle is not feasible due to low wellead pressure. Using te energy content of saturated water discarded from separator in a binary ORC cycle paralleled wit single flas is considered in tis study. Figure 1 sows te process flow diagram of proposed system, te production fluid is separated to steam and brine troug te separator, steam is directed to steam condensing turbine and brine is led to te vaporizer in binary ORC plant. Te energy performance of power generation systems is usually evaluated based on te first law of termodynamics. However, compared to energy analysis, te exergy analysis 1 can better and accurately sow te location of inefficiencies. Integration of energy and exergy analysis can present a wole picture of te system performance. 2. SUMMARY OF THRMAL DSIGN OF TH PROPOSD SYSTM Te termodynamic design of te proposed system as been establised in S software. 2.1 Single Flas Cycle Te following plant operating parameters are used for te termal design: p sep 5 Separator pressure - [bar-a] p cond 01. Condenser Pressure - [bar-a] turb 80 Turbine isentropic efficiency- [%] pump 50 Pump isentropic efficiency - [%] T db 10 et-bulb temperature- [ C] All pressure and eat transfer losses are neglected. Te subscript numbers refer to state locations on figure 1. Te fraction and flow rate of te steam and brine can be defined by mass and eat balance of te separator as follows: m1x1 m2x2 m3x3 + (1) m11 m22 m33 + (2) ere te m and are te mass flow and entalpy of te stream at teir specified state on te system. Te subscript numbers denote te state position of stream at figure 1. Te turbine power production is: turb m2( 2 4) turb (3) s ere 4 and 4s are te entalpy values at te turbine exit states for actual and isentropic processes, respectively. (4)

2 Radmer et al. Figure1: Te process flow diagram of proposed combined system For te condenser, Q cond te eat rejected by cooling water is: cond Q m ( ) (5) Te mass flow of cooling water is defined by: Q m cond C ( 8 7) ere 7 and 8 are te entalpies of cooling water at te inlet and outlet of condenser. (6) Te power consumed by cooling water pump, calculated by: cwp cwp, isentropic. pump (7) cwp, isentropic νcw ppump (8) cwp is Te energy conversion (or first law) efficiency for a eat engine operating cyclically between two termal energy reservoirs is (Kotas, 1985): 1law ere net (9) Q in net is te power delivered to te network and Q in is te corresponding eat transfer to te engine per cycle. 2.2 Organic Rankine Cycle (ORC) Te working fluid operates in a contained, closed-loop cycle and is completely segregated from te eat source fluid. Tere are a number of possible variants of te cycle, in terms of eat excange configuration, turbine configuration, etc, wic may be selected as appropriate to te temperature and Pysical state(s) of eat source fluid. In tis case, te working fluid absorbs eat from a eat source (geotermal brine), via one sell and tube eat excangers. Tis eat causes te working fluid to evaporate; producing te igpressure vapor tat is ten expanded troug a turbinegenerator. Te low pressure turbine exaust vapor is ten led to te regenerator. In tis eat excanger, residual sensible eat in te low-pressure turbine exaust stream is used for initial preeating of te cold liquid from te motive fluid pump, tus increasing te cycle efficiency. Te cooled regenerator exaust vapor is ten condensed, using watercooled, sell-and-tube condenser. From te condenser, te liquid working fluid is pumped to ig pressure and returned to te regenerator to close te cycle. Te selection of te working fluid as great implications for te performance of a binary plant. ile tere are many coices available for working fluids, tere are also many constraints on tat selection tat relate to te termodynamic properties of te fluids as well as considerations of ealt, safety, and environmental impact (Dippipo, 2007). Te plant uses isopentane as te working (binary) fluid but te results can be compared wit tose using oter organic fluids suc as n- pentane, isobutane, n-butane, etc. Te geotermal brine leaving te vaporizer is directed to te reinjection well were it is reinjected back into te ground. Te following plant operating parameters are used for te termal design:, 10 T pp vap Temperature difference at vaporizer pinc point-[ C] 2

3 Radmer et al. T pp, regen 10 Temperature difference at regenerator pinc point-[ C] T pp, cond 10 Temperature difference at condenser pinc point-[ C] T Brine temperature at vaporizer outlet- [ C] T cw, low 20 Cooling water temperature at inlet of condenser-[ C] T cw, ig 30 Cooling water temperature at outlet of condenser-[ C] Te motive pump power is defined as: pump pump, isentropic. pump pump,isentropic isopentane ig low ν.( P P ) 100 Pig Te pressure on ig pressure side ( (16) (17) ) can be found as: P Hig P( isopentane,x 0,T T 10 ) (18) T ere x is te quality of isopentane and 10 temperature of isopentane in vaporizer. is te boiling turb 80 Turbine isentropic efficiency- [%] P Te pressure on low pressure side ( low ) is defined as: pump 50 Pump isentropic efficiency - [%] Te eat amount delivered from te geotermal water is determined by: Q m C T T (10) in, ORC 3. pater,.( 3 16) ere m 3 is te mass flow of geotermal brine in te vaporizer, C P,water is specific eat of water at constant pressure and T 3 and T16 are te temperature values of geotermal brine at inlet and outlet of vaporizer, respectively. Te mass flow of isopentane in binary cycle ( m isopentan e ) can be calculated as: Q in,orc m isopen tan e ( 11 9) (11) 9 and 11 are te entalpy values of isopentane at inlet and outlet of vaporizer, respectively. Te power production of turbine in ORC is: turb,orc m isopentane ( 11 12) turb, ORC 11 12s s (12) (13) is te entalpy of te ORC turbine inlet and 12 and are te entalpy values at te turbine exit state for actual and isentropic processes, respectively. Te eat excange taking place in te regenerator can be determined as: Q m ( ) (14) regen isopentane Plow P( isopen tan e,x 0,T T cw,ig ) T cw,hig (19) is te temperature of cooling water at outlet of condenser. Te first law efficiency of ORC is determined by: 1Law,ORC net,orc Q in,orc (20) 2.3 Single flas Combined wit ORC Te first law efficiency of combined system is defined as: combined 1law,combined Qin,combined (21) 3. XRGY ANALYSIS Te concept of exergy (sometimes called available work) relates to te maximum work (or power) output tat could teoretically be obtained from any system relative to given surroundings. e often refer to te state of te surroundings as te dead state because wen fluids are in termodynamic equilibrium wit te surroundings tere is no potential for doing work, and te fluid may be considered dead. (R.Dippo, 2004). Disregarding kinetic and potential energy canges, te specific flow exergy of geotermal fluid at any state (plant location) can be calculated from m( T ( s s )) (22) ere T 0 is te environment (dead state) temperature, and s are te entalpy and te entropy of te geotermal fluid at te specified state, and 0 and s 0 are te corresponding properties at te restricted dead state (Kanoglu, 2002). Q Te eat rejected in condenser cond,orc, is: Q m ( ) cond,orc isopentane 13 4 (15) xergetic fficiencies Te first law efficiency alone is not a realistic measure of te performance of plant, tus te second law of efficiency needs to be defined. Tere are various definitions for te exergetic efficiency in te literature. Te definition is used in tis study is called rational efficiency and defined by

4 Radmer et al Kotas (1995) as te ratio of te exergy recovered (or te desired product) to te exergy supplied to te system or process: divided by te decrease in te exergy of te ot stream (ark, 1995). Applying tis definition to te condenser in single flas cycle, we obtain desired e (23) input ε _ cond, SF (30) input output + destroyed (24) output desired + aste (25) ere desired Sum of desired exergy outputs (net positive work by te system); destroyed xergy rate lost in te system as a result of irreversibilities; waste xergy exiting te system wic still as capacity to do work. en tis concept is applied to a power plant as a wole, te overall exergetic efficiency reduces to a very simple formula, namely, te ratio of te net power output to te exergy of te motive fluid serving as te energy source for te plant (DiPippo and Marcille, 1984). Tus; te exergetic efficiency of te single flas cycle based on te two pase fluid exergy input to te plant can be calculated as: net, SF ε _ overall _ SF (26) 1, ere te net SF is te net power output of single flas cycle and 1 is te exergy rate of two pase geotermal fluid in state 1. Te overall exergetic efficiency for te combined system will be calculated as: Combined ε, Combined (27) 1 Te exergetic efficiency of a turbine is defined as a measure of ow well te stream exergy of te fluid is converted into actual turbine work output. Applying tis to te single flas condensing turbine, we obtain ε, SF Turb SF, Turb 2 4 (28) Te difference between te numerator and denominator in q. (28) is simply te exergy destruction in te turbine. I SF Turb ( 2 4 ) SF, Turb (29) Vaporizer, regenerator and condensers in te plant are essentially eat excangers designed to perform different tasks. Te exergetic efficiency of a eat excanger may be measured by te increase in te exergy of te cold stream ere te exergy rates are given in Table 1. Te difference between te numerator and denominator in q. (30) is te exergy destruction in te condenser. Tat is, I cond, sf ( ) ( ) (31) However te exergy drop of te working fluid across te condenser can be expressed as te exergy destruction in te condenser. Tat is, te exergy gained by te cooling water is not considered (Kanoglu,2002). Te exergetic efficiency of te eat excangers in te binary cycle is calculated as below: ε,vap ε,regen (32) (33) C, Hig C, Low ε, Condens (34) Te exergetic efficiency and exergy destruction for te motive pump in binary cycle are calculated from te following relations: ε,pump (35) pump pump pump I ( ) (36) Te exergy balance for te single flas cycle, ORC and combined system can be written as: net, SF ΣISF, processes 3 C, Low 16 ORC C, Hig ORC (37) Σ I (38) C, law combined (39) + C, ig + ΣI Combined RSULTS AND DISCUSSION Te termodynamic design and exergy analyses of te proposed system ave been establised in S software. In Table 1, temperature, pressure, mass flow, entalpy, entropy and exergy rate data for geotermal fluid, working fluid, and cooling water are given according to teir state numbers specified in Fig. 1. 4

5 Table 1. Te xergy Rates and Oter Properties at Various Plant Locations. State Numbers Refer to Fig 1. Radmer et al. STAT No. FLUID PHAS m ( kg / s ) T ( o C ) P ( bar ) ( kj/ kg) s ( kj/ kg K ) (k) 0 geotermal ' isopentane geotermal two pase geotermal steam geotermal liquid geotermal steam geotermal liquid Geoterma; liquid water liquid water liquid isopentane liquid isopentane liquid isopentane vapor isopentane vapor isopentane vapor isopentane liquid isopentane liquid geotermal liquid C,ig water liquid C,low water liquid State 0 and 0' are te restricted dead states for te geotermal and working fluids, respectively. Tey correspond to an environment temperature of 10 C and an atmosperic pressure of 0.75 bar-a, wic are te annual mean values measured at te plant area. For geotermal fluid, te termodynamic properties of water are used. Te termodynamic properties of working fluid, isopentane, are obtained from S software. Te temperature of brine at vaporizer outlet is restricted by te silica amorpous saturation point and sould not be lower tan 70 C to prevent te silica deposition inside te vaporizer. Te data listed in Table 1 was created by te termodynamic design model wen running it in S software. Te summarized results of te exergy analysis for te single flas cycle are presented in Table 2. Te results sow tat te total available exergy in two pase geotermal fluid produced by te well is k. Of tis available exergy, 5441 k exergy exists in te brine wic is disposed of at te separator; 6815 k is contained in te steam and connected to te condensing turbine of te single flas cycle. Te gross work developed by te turbine is 4166 k. Te exergetic efficiency of te turbine is defined as te ratio of te desired exergy output to te input exergy of te turbine and calculated to be 61%. Te total exergy in te steam exausted into te condenser is 2648 k. Te exergy destruction troug te condenser is defined as te exergy drop of te steam across te condenser and was found to be 2576 k. Tis value is 38% of te steam exergy input to te single flas cycle and is discarded to te cooling water. 5 Te overall exergy efficiency of te single flas cycle was found to be 33% wit reference to te total exergy from te connected well. For comparison, te overall energy efficiency for te single flas cycle was found to be 7.5%. Te large difference in te efficiencies sows tat most of te energy received at te wells, exits te plant wile still containing substantial exergy. Te turbines sowed ig exergy efficiencies because most of te exergy is exausted into te condensers and not consumed or destroyed. Tis means tat an improvement of te turbines will enable tem to extract more work from te fluids or alternatively, devise oter ways of using te exergy from te fluids exiting te system (Kwambai, 2005).Te greatest exergy losses occur in te condensers were most of te exergy is rejected and destroyed. Te exergy in te brine is significantly ig (44%) and is te main objective of tis study to be used to do more work wit a binary system. Te exergy content of te brine wic is led to te binary plant vaporizer is 5438 k. Of tis available exergy, 1484 k exits te vaporizer to re-inject to te re-injection well. Te outlet temperature of te brine in te vaporizer is restricted due to amorpous silica deposition temperature and is set at 70 C. Tis exergy wit te temperature of 70 C is significant and can be used by directly in open systems. Te exergy analysis results for te binary ORC plant are summarized in table 3. Te results sow tat te second Law

6 Radmer et al efficiency of te ORC plant is calculated to be 33% based on te exergy input to te plant and 45.3% based on te exergy input to te isopentane Rankine cycles. Tis means tat more tan 27% of te exergy of te brine is discarded as waste eat. Te exergy input to te isopentane Rankine cycle is 3954 k. Of tis exergy, 1792 k is converted to power and 2162 k is destroyed in te plant. Te causes of exergy destruction in te ORC plant include vaporizerregenerator losses, turbine-pump losses and te exergy of isopentane lost in te condenser. Te main exergy destruction takes place in vaporizer and condenser. Te summary of te exergy analysis for combined system of single flas cycle wit ORC binary plant is presented in table 4. Te results sow tat te desired exergy output from combined system in form of electricity is 5840 k wic as a 44% increase in comparison wit single flas cycle k of te exergy in te form of brine is discarded as waste eat wic is 72% less tan tat from single flas cycle. Te exergy destruction in te combined plant was found to be 4885 k. Te exergy flow diagram of combined system includes te exergy destruction percentages in te components, is sown in figure 2. Te substantial exergy loss takes place in condensers wit te percentage of 25.7%. Te first law and exergy efficiencies of te single flas cycle, ORC and combined system and te productivity of te geotermal fluids in te various cases are presented in table 5. Te first law efficiency of combined system is 11% and as an increase of 3.5% in comparison wit single flas cycle, were te second law efficiency of combined system sows te 14.7% increase compared to single flas. Te productivity of te geotermal fluid produced by te well is increased from 20 to 29 k/ton in combined system related to single flas. Table 2. Summary Results for te xergy Analysis of te Single Flas Cycle. Process or System Desired xergy xergetic xergy Input aste xergy xergy Destroyed fficiency (k) Output (k) output (k) (k) (%) ellead & separator Turbine Condenser pump Overall single Flas cycle (1) Overall single Flas Cycle (2) (1) based on te exergy of two pase fluid (2) based on te exergy of te steam Table 3. Summary Results for te xergy Analysis of te ORC. component xergy Destruction (k) fficiency (%) Vaporizer Turbine Regenerator Condenser Pump Overall ORC (1) Overall ORC (2) (1) based on te exergy available in Brine (2) based on te exergy input to ORC Description Table 4. Summary Results for te xergy Analysis of te Combined System. Value xergy Input To te Sysytem (k) Desired xergy Output (k) 5863 aste xergy Or Undesired Output (k) 1189 xergy Destruction (k) 5299 (%) xergetic fficiency

7 Table 5. First Law and xergetic fficiencies of te Single Flas Cycle, ORC and Combined System Plants. System First law fficiency (%) xergetic efficiency (%) Single Flas ORC Combined Radmer et al. Figure 2: xergy flow diagram of combined system, given as te percentages of two pase fluid exergy input CONCLUSION A significant volume of energy will be discarded as eat waste in form of re-injected brine by using a single flas cycle in Meskin sar geotermal power plant in Iran. Termodynamic modeling of a combined system of single flas cycle wit binary ORC plant and exergy analysis of te combined system was carried out and te locations and quantities of exergy losses, wastes and destructions in te different processes of te plant were pinpointed. In addition, te exergy analysis enabled te degree of termodynamic imperfections for te processes to be determined. S software was used for developing and analyzing of matematical models of energy and exergy flows. From te results, te following conclusions ave been drawn: 1. Te total exergy available from production well was calculated to be k. Te total exergy contained in te steam pase and connected to te single flas cycle was found to be 6814 k and te exergy contained in te waste brine is significantly ig to be discarded and can be utilized to obtain more work in a binary plant. 2. Te overall exergy efficiency for te single flas cycle plant is 33% and te overall energy efficiency is 7.5%, in bot cases wit respect to te fluid from te connected wells. Te productivity of te two pase fluid in single flas cycle was found to be 20 k/ton. 3. Te overall exergy efficiency for te ORC binary plant is 33% and te overall energy efficiency is 14.8%, in bot cases based on te brine led to te Binary plant. Te productivity of te brine in binary plant was found to be 10.5 k/ton. 4. Te overall exergy efficiency for te combined system is 47.7% and te overall energy efficiency is 11%, in bot cases wit respect to te fluid from te connected well. Te productivity of te two pase fluid in combined system was found to be 20 k/ton. 5. Te desired exergy output from combined system in form of electricity is 5840 k wic as a 44% increase in comparison wit single flas cycle. 6. xergy losses occurred in: brine re-injection (1556 k), turbines and pumps (558 k), vaporizer and regenerator (961 k) and te condensers (3183 k) wit te percentages of 12.6%, 4.5%, 7.7% and 25.7%, respectively based on te exergy available in two pase fluid produced by te well. A substantial amount of exergy is exausted to te condensers because of te big difference between te cooling water and operating fluid temperatures. Te wasted brine needs to be investigated regarding te silica deposition issue but can furter be utilized for direct uses suc as ot spas and medicinal uses wic can generate a lot of revenue. RFRNCS DiPippo, R., Marcille, D.F: xergy analysis of geotermal power plants. Geotermal Resources Council Transactions 8, (1984), DiPippo, R.: Second Law assessment of binary plants generating power from low-temperature geotermal fluids, Geotermics, 33, (2004), Kanoglu, M.: xergy analysis of a dual-level binary geotermal power plant, Geotermics, 31, (2002), Kotas, T.J.: Te exergy metod of termal plant analysis. Ancor Brendon Ltd, Tiptree, ssex, Great Britain, (1984), 293 pp. Kwambai, C.B.: xergy analysis of olkaria I power plant, Kenya. Report 5 in: Geotermal Training in Iceland UNU-GTP, Iceland, (2005). SKM (Sinclair Knigt Merz). : N-Sabalan geotermal project feasibility study, SUNA unpublised internal report (2005). 95 pp. ark, K.J.: Advanced Termodynamics for ngineers. McGraw-Hill, New York, (1995). 7