THERMODYNAMIC ANALYSIS AND PERFORMANCE OPTIMIZATION OF ORGANIC RANKINE CYCLES FOR THE CONVERSION OF LOW-TO-MODERATE GRADE GEOTHERMAL HEAT

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1 THERMODYNAMIC ANALYSIS AND PERFORMANCE OPTIMIZATION OF ORGANIC RANKINE CYCLES FOR THE CONVERSION OF LOW-TO-MODERATE GRADE GEOTHERMAL HEAT Yekladi, P.J., Bell-Ochende, T. and Meyer J.P. Department f Mechanical and Aernautical Engineering, University f Pretria, Pretria Private Bag X20, Hatfield 0028, Suth Africa. Abstract The present study cnsiders a thermdynamic analysis and perfrmance ptimizatin f small binary-cycle thermal pwer plants perating with mderately lw-temperature and liquid-dminated thermal resurces in the range f 110 C t 160 C. The paper cnsists f an analytical and numerical thermdynamic ptimizatin f selected Organic Rankine Cycles (ORC) t maximize the cycle pwer utput. The ptimizatin prcess and Entrpy Generatin Minimizatin (EGM) analysis were perfrmed t minimize the exergy lss f the pwer plant. Optimal perating cnditins were determined fr maximum cycle pwer utput per unit mass flw rate f the thermal fluid. The maximum cycle pwer utput was bserved t increase expnentially with the thermal resurce temperature, whereas the ptimal turbine inlet temperature increased almst linearly with the increase in the thermal heat surce. In additin, a perfrmance analysis f selected rganic wrking fluids, namely refrigerants R123, R152a, isbutane and n-pentane, was cnducted under saturatin temperature and subcritical pressure perating cnditins f the turbine. Organic fluids with higher biling pint temperature, such as n-pentane, were recmmended fr the basic type f ORCs, whereas thse with lwer vapur specific heat capacity, such as butane, were mre suitable fr the regenerative ORCs. Keywrds: Gethermal energy, Organic Rankine Cycles, Optimizatin, Exergy analysis, binary cycle. Nmenclature Alphabetic symbls Specific heat capacity, J/kg.K Exergy rate, W Enthalpy, kj/kg Exergy destructin, W Mass flw rate, kg/s Pressure, Pa Heat transfer rate, W Specific entrpy, J/kg.K Temperature, C Pwer utput, W Abbreviatins Hydrcarbns Hydrchlrflurcarbns Hydrflurcarbns Greek symbls ƞ First Law efficiency, % ƞ Secnd Law efficiency, % Effectiveness, % Vapur ψ Specific exergy, W/kg Subscripts 0 Reference state 1 15 Thermdynamic states Cndenser Critical value Cling system Destructin Evapratr Gethermal fluid Heat exchanger Inlet Maximum Minimum Optimum Outlet Circulatin pump Pinch-pint Reinjectin Isentrpic Turbine Thermal

2 1. Intrductin Fr decades, diverse studies have been cnducted t develp renewable and sustainable energies while reducing the envirnment defects f glbal warming, greenhuse effect, air pllutin and waste f natural resurces. Amng a diversity f energy-efficient and envirnmental friendly technlgies identified fr pwer generatin, the thermal energy has prved t be an alternative energy surce fr electric pwer generatin due t its ecnmic cmpetitiveness, the peratinal reliability f its pwer plants, and its envirnmentally friendly nature [1]. Current research activities undertaken wrldwide have aimed at reducing the cst f thermal electricity prductin either in resurce explratin r extractin, reservir stimulatin, drilling techniques, r energy cnversin systems. The present study cnsiders a thermdynamic analysis and perfrmance ptimizatin f fur energy cnversin systems utilized in small binary-cycle thermal pwer plants perating with mderately lw-temperature and liquid-dminated thermal resurces in the range f 110 C t 160 C. Varius studies were cnducted by diverse authrs prpsing innvative methds t imprve the perfrmance f the binary-cycle thermal pwer plant perating with mderately lw-temperature thermal resurces. Amng thers, we may acknwledge Gu and Sat [2] wh studied the supercritical cycles. Kanglu [3] discussed dual-level binary thermal pwer plant, and DiPipp [4] prpsed bth a recvery heat exchanger (RHE) with a cascade f evapratrs with bth high- and lw-pressure turbines perating in a Kalina cycle. Desai and Bandypadhyay [5] recmmended incrprating bth regeneratin and turbine bleeding t the basic rganic Rankine cycles, whereas Gnutek and Bryszewska-Mazurek [6] suggested multicycle with different thermdynamic prperties. An investigatin n the ptimal design f the binary cycle pwer plants fr maximum cycle pwer utput, the sle bjective f this study, was discussed by Brsukiewicz-Gzdur and Nvak [7] wh maximized the wrking fluid flw t increase the pwer utput f the thermal pwer plant by repeatedly returning a fractin f the fluid dwnstream f the evapratr t cmpletely vaprize the wrking fluid prir expanding in the turbine. Madhawa Hettiarachchi et al [8] presented a cst-effective ptimum design criterin based n the rati f ttal heat transfer area t the net cycle pwer utput as the bjective functin, fr the simple ORC emplying lw temperature thermal resurces. In mst f the studies mentined abve, the minimizatin f the thermal fluid flw rate (r specific brine cnsumptin) fr a given cycle pwer utput was addressed as the bjective functin fr the ptimum design f the ORCs. The present study, hwever, fcuses n maximizing the cycle pwer utput fr a given thermal fluid flw rate while minimizing the thermal plant exergy destructin (r irreversibility) with careful design f the heat exchangers utilized in the thermal pwer systems. The paper cnsists f an analytical and numerical thermdynamic ptimizatin f the selected ORCs t maximize the cycle pwer utput. The ptimizatin prcess and Entrpy Generatin Minimizatin (EGM) analysis were perfrmed t minimize the exergy lss f the pwer plant. Optimal perating cnditins were determined fr maximum cycle pwer utput per unit mass flw rate f the thermal fluid. In additin, a perfrmance analysis f the selected rganic wrking fluids, namely refrigerants R123, R152a, isbutane and n-pentane, was cnducted t demnstrate the extent at which they d affect the design and peratin f the binary thermal pwer plants under saturatin temperature and subcritical pressure perating cnditins f the turbine. 2. Prpsed mdel Small binary cycle thermal pwer plants perating with mderate lw-grade and liquiddminated thermal resurces in the range f 110 C t 160 C are cnsidered. The lwgrade thermal heat can suitably be recvered by an ORC r Kalina cycle. Fr the purpse f this study, the ORC was preferred cnsidering its widely use in thermal pwer generatin, the simplicity f its pwer cycle, and the ease f maintenance [9].

3 In the literature, mre than 50 pure and mixtures f rganic cmpunds fr ORC have been cnsidered, and classified as wet, dry r isentrpic rganic fluids accrding t the slpe f its saturated-vapur line [9]. This study cnsiders refrigerants R123, R152a, isbutane and n-pentane as binary wrking fluids fr the cnversin f the lw-t-mderate grade thermal heat. Refrigerant R123 is an isentrpic rganic fluid with a near-vertical saturated vapur-phase line, thus a nearly infinitely large slpe f the saturated-vapur line. Refrigerant R152a belngs t the wet type, thus having a negative slpe f the saturatedvapur line. Isbutane and n-pentane represent dry rganic cmpunds characterized by a psitive slpe f the saturated-vapur line. The thermdynamic phases f the selected wrking fluids are illustrated n a Temperature vs. Entrpy diagram in Fig. 1. In Table 1, the main therm-physical prperties f the selected binary wrking fluids are listed, as btained frm EES (Engineering Equatin Slver) sftware [10]. Figure 1: T-s diagram f selected binary fluids fr ORC Wrking fluid R123 R152a R600a R601 Name 2,2-Dichlr-1,1,1-1,1- triflurethane Diflurethane Isbutane n-pentane Chemical frmula Type HCFC HFC HC HC Organic type Isentrpic Wet Dry Dry Therm-physical prperties Mlecular weight / / Table 1: Thermdynamic prperties f selected binary fluids fr [8,11] Fur ORCs were analysed analytically and numerically, and their perfrmance ptimized t maximum the cycle pwer utput. The selected ORCs are illustrated in Fig. 2. In Fig. 2a, a simple ORC type is shwn. The primary heat transfer medium is pumped at high pressure and cntinuusly circulated thrugh the earth in a clsed pipe system [12-13]. The fluid is thus heated by the linearly increasing undergrund temperature with depth, as it flws dwn the well. A secndary r binary fluid with a lwer biling pint and higher vapur pressure is therefre cmpletely vaprized and usually superheated by the primary fluid thrugh a clsed pipe system heat exchanger, t expand in the turbine and then cndense either in an air-cled r water-cled cndenser prir returning t the vaprizer and thus cmpleting

4 the Rankine cycle [14]. If the expansin prcess in the turbine terminates in the superheated regin, a heat recuperatr (r Internal Heat Exchanger, IHE) can be advantaus t preheat the binary wrking fluid prir evaprating in the heat exchanger t reduce the evapratr lad, and hence imprve the thermal efficiency f the cycle (Fig. 2b) [15-16]. Further imprvement f the heat exchange perfrmance and the Rankine cycle verall efficiency can be achieved with the additin f a tw-phase regenerative cycle [5,13], utilizing an pen feed-heater t preheat the binary wrking fluid prir evaprating in the heat exchanger, with the extracted fluid frm the turbine expanded vapur (Fig. 2c). A cmbinatin f regeneratr and recuperatr can als be emplyed t imprve the perfrmance f heat exchanger prcess (Fig. 2d) [13]. Figure 2: Schematic diagrams f the binary-cycle thermal pwer plants The cycles Temperature vs. Entrpy diagrams are illustrated in Fig. 3. Fr the simple ORC (Fig. 3a), prcesses 1-2 and 4-5 refers t reversible adiabatic pumping and expansin prcesses, respectively; whereas prcess 2-3 and 5-1 represent cnstant-pressure heat additin and rejectin, respectively. The additin f an IHE t the simple ORC is represented by states 3 and 7 n the cycle T-s diagram shwn in Fig. 3b. In cntrast t the basic ORC s, the regenerative cycles cnsist f three cnstant-pressure heat transfer prcesses (Fig. 3c). Ideally, the mixture f the turbine bleeding and the cndensate at the exit f the pen feed-rganic heater is assumed at saturated liquid cnditin and at the evapratr pressure [17]. The additin f an IHE t the regenerative ORC is illustrated by states 3 and 10 n the cycle T-s diagram shwn in Fig. 3d.

5 Figure 3: T-s diagrams f the binary-cycle thermal pwer plants Many ther pwer cycle designs have been prpsed and studied in the literature fr the cnversin f lw-t-mderate grade heat resurces, and aiming at imprving the perfrmance f the binary-cycle pwer plant. Fr instance, a heat recvery exchanger with a cascade f evapratrs emplyed in a Kalina cycle [18], a heat recvery cycle with a high and lw-pressure turbine [3] r multiple pressure levels [3], the Gswami cycle [11], a supercritical Rankine cycle [2], a trilateral flash cycle [11], etc. 3. Research methdlgy 3.1. Energy and exergy analysis Mass, energy, and exergy balances fr any cntrl vlume at steady state with negligible ptential and kinetic energy changes can be expressed, respectively, by [13-14,16] m & in = m& ut (1) Q & W& = m& uthut m& inhin (2) E & x W& + m& ψ m& ψ = I& (3) heat in in ut The heat exergy at temperature ut T j is given by [13,16] T E & xheat Q& j T = 1 (4) j And the flw (specific) exergy is ψ = h h T s s (5) ( ) ( )

6 The cycle pwer utput is determined by, [16] W & = W& + W& (6) net t p And the ttal exergy lst in the cycle and plant are given respectively by [14,16] I & cycle = all cmpnents I& i = I& p + I& HEs + I& + t I& c (7) I & = I& + I& + I& = Ex & W& (8) plant cycle rej CS in net Where the ttal exergy inputs t the ORC is determined by [3, 9,14,18] E & x = m& h h T s s (9) in [( ) ( )] 3.2. Perfrmance analysis The First- and Secnd-law efficiencies, based n the thermal fluid state at the inlet f the primary heat exchanger and with respect t the reference temperature, are defined respectively as [3,14,18] W& net η I = = (10) m & ( h h ) W& net η II = = (11) m & h h T s s [( ) ( )] Based n the heat transfer r energy input t the cycle, the First- and Secnd-law efficiency are given by [3,14,18] W& net W& net η I,2 == = (12) m& ( h hrej ) m& wf ( hwf, ut hwf, in ) W& net η II,2 = (13) m & h h T s s [( ) ( )] rej rej The perfrmance f a binary-cycle thermal pwer plant can als be evaluated using the cycle effectiveness, which represents the effectiveness f heat transfer t the cycle frm the thermal fluid, as [3,9,14,18] W& net ε = (14) m & h h T s s wf [( ) ( )] wf, ut wf, in wf, ut wf, in As discussed by Subbiah and Natarajan [9], the First-law efficiency is a quantitative measure f the effectiveness f the cnversin f the available thermal energy int useful wrk. The cycle effectiveness measures bth quantitatively and qualitatively the amunt f available energy t be transferred, and the Secnd-law efficiency accunts fr the verall exergy inputs t the cycle between the thermal fluid temperature at the utlet f the resurce well and the reference temperature. The perfrmance analysis f individual cmpnent f the cycle was evaluated using the fuel depletin rati, which is defined by [13,19]: I& i δ i = (15) Ex & in 3.3. Irreversibility analysis In Fig. 4a, the lss f exergy (irreversibility) generated during the heat transfer prcess ccurring in the Evapratr-Preheater unit is represented by the marked area f the temperature vs. heat transfer diagram, assuming linearity f the fluid cling curve. This significance lss f exergy is a cnsequence f the large difference in enthalpy r temperature between the thermal and the binary fluids [20]. The additin f an IHE t the simple ORC is demnstrated t reduce the irreversibility f the heat transfer prcess as the

7 wrking fluid was preheated prir entering the preheater (Fig. 4b). A decrease in irreversibility can als be achieved while utilizing a regenerative Rankine cycle t imprve the heat exchange perfrmance (Fig. 4c). Further reductin in irreversibility is pssible with a cmbinatin f a regeneratr and recuperatr (Fig. 4d). Figure 4: T-Q diagrams f the heat exchange prcess in the Evapratr-Preheater unit 3.4. Mdel validatin T validate the simulatin, the thermdynamic perfrmance f the selected ORCs was analysed using EES sftware [10]. The numerical data were validated with the wrk f Yari [13] fr refrigerant R123, at the perating cnditins listed in Table 2. Parameters P [kpa] P ext [kpa] Value * 581** T [ C] T c [ C] T E [ C] T [ C] T pp [ C] Ƞ p [%] Ƞ t [%] ε IHE [%] Table 2: Operating parameters used in the validatin f results * Fr the regenerative ORC ** Fr the regenerative ORC with an IHE The cmparisn shwn in Table 3 illustrates a very gd agreement between the present wrk and the results f Yari [13].

8 Perfrmance parameters Simple ORC Present wrk [13] ORC with IHE Present wrk [13] Regenerative ORC Present [13] wrk Regenerative ORC with IHE Present [13] wrk [kj/kg] [kj/kg] ƞ [%] ƞ, [%] ƞ [%] ƞ, [%] [%] Table 3: Validatin f the numerical mdel with a previusly published data [13] 3.5. Optimizatin mdel The paper cnsists f an analytical and numerical thermdynamic ptimizatin t maximize the cycle pwer utput. The ptimizatin prcess and Entrpy Generatin Minimizatin (EGM) analysis were perfrmed t minimize the exergy lss f the pwer plant. Fr a given cmbinatin f the thermdynamic cycle and wrking fluid, the ptimal perating cnditins, i.e. evaprative and cndensing temperatures, were determined fr maximum cycle pwer utput per unit mass flw rate f the thermal fluid, as illustrated by the simulatin flw chart shwn in Fig Results Figure 5: Flw chart f the simulatin prcedure 4.1. Thermdynamic perfrmance f the selected rganic binary fluids A thermdynamic perfrmance f the selected rganic binary fluids is cnsidered fr the simple and regenerative ORCs. The pinch-pint and cndensing temperatures were fixed at

9 5 C and 40 C respectively, while the turbine inlet temperature was varied frm the limiting temperature f cndensatin t the fluid input temperature. In Fig. 6, the variatin f the cycle pwer utput per unit mass flw rate f the fluid is pltted fr bth ORCs at subcritical pressure perating cnditins. Fr the simple ORC, a nearly identical maximum cycle pwer utput per unit mass flw f the thermal fluid was btained at abut similar ptimal turbine inlet temperature, irrespective f the type f rganic binary fluids (Fig. 6a). Fr the regenerative ORC, hwever, the ptimal turbine inlet temperature and maximum cycle pwer utput per unit mass flw f the thermal fluid differed significantly fr all the wrking fluids (Fig. 6b). A brief cmparisn f Figs. 6a and 6b has shwn nearly identical thermdynamic perfrmance fr isbutane, whereas the additin f an OFOH t the binary cycle utilizing R152a, R123 r n-pentane as wrking fluid resulted t a substantial reductin in the cycle pwer utput by as much as 15%, 26% and 42%, respectively. Figure 6: Cycle pwer utput per kg fluid as a functin f the turbine inlet temperature fr thermal resurce temperature f 110 C (a) Simple ORC and (b) Regenerative ORC In the studied range f heat surce temperature, the lwer the biling pint temperature f the rganic fluid, the higher the evaprating temperature fr its simple ORC (Fig. 7a). On the ther hand, the supremacy f rganic fluids with lw vapur specicific heat capacity, such as isbutane, t cnvert lw-t-mderate thermal resurce temperature at relatively lw evaprating temperature is remarkably demnstrated fr the regenerative ORC (Fig. 7b). Hence, fr the cnversin f lw-t-mderate grade thermal heat, rganic fluids with higher biling pint temperature, such as n-pentane, wuld be recmmended fr the simple ORC as discussed by Mag et al. [17], whereas rganic fluids with lwer vapur specific heat capacity, such as butane, wuld be mre suitable fr the regenerative ORC. Figure 7: Effect f fluid s (a) biling pint temperature, and (b) vapur specific heat capacity, n the ptimal turbine inlet temperature fr thermal resurce temperature f 130 C

10 4.2. Perfrmance analysis f the Organic Rankine Cycles A perfrmance analysis f the selected binary-cycles was cnducted using n-pentane as the rganic binary fluid. The cycle pwer utput per unit mass flw rate f the thermal fluid is pltted against the turbine inlet temperature fr the thermal resurce temperatures f 110 C and 160 C (Fig. 8). As discussed by Lakew and Blland [21], the increase in the turbine inlet temperature resulted in an increase f the enthalpy f the inlet fluid t the turbine and decrease in the flw rate f the wrking fluid. Cnsequently, fr each type f ORC, a maximum cycle pwer utput per unit mass flw rate f the fluid was btained fr an ptimal turbine inlet temperature. Mrever, fr the given perating cnditins f the ORCs, ne can cnclude that the additin f an IHE did nt really impact n the thermdynamic perfrmance f the cycle, whereas the regenerative system reduced significantly the cycle perfrmance. Figure 8: Cycle pwer utput per kg fluid as a functin f the turbine inlet temperature fr thermal resurce temperature f (a) 110 C and (b) 160 C The First- and Secnd-law efficiencies, based n the thermal fluid state at the inlet f the primary heat exchanger, and with respect t the reference temperature, are illustrated by Figs. 9 and 10 respectively, fr the thermal resurce temperatures f 110 C and 160 C. Bth efficiencies are bserved t increase with the turbine inlet temperature up t the same ptimal turbine inlet temperature, which als prduced maximum cycle pwer utput. Clearly, based n the effectiveness f the cnversin f the available thermal energy and exergy int useful wrk, the regenerative cycles have been less efficient and less perfrming cmpared t the basic ORCs. Figure 9: First-law efficiency at the primary heat exchanger inlet as a functin f the turbine inlet temperature fr thermal resurce temperature f (a) 110 C and (b) 160 C

11 Figure 10: Secnd-law efficiency at the primary heat exchanger inlet as a functin f the turbine inlet temperature fr thermal resurce temperature f (a) 110 C and (b) 160 C Based n the energy input t the cycle, the First- and Secnd-law efficiencies are represented in Fig. 11. At lw turbine inlet temperatures, the basic ORCs have been mre efficient than the regenerative ORCs. As the turbine inlet temperature increased, the regenerative ORC with an IHE became the mst efficient whereas the simple ORC shwed a pr perfrmance. This culd be attributed t the ability f the regenerative cycles t minimize the exergy lss (irreversibility) during the heat transfer prcess. The chice f the apprpriate ORC fr the cnversin f lw-t-mderate grade thermal heat in the given range f temperatures and based n the energy input t the ORC, is highly reliant n the turbine inlet cnditins required. Figure 11: (a) First- and (b) Secnd-law efficiency n heat transfer input t the ORC as a functin f the turbine inlet temperature The cycle effectiveness, which measures bth quantitatively and qualitatively the amunt f available energy t be transferred frm the thermal resurce t the rganic wrking fluid is pltted in Fig. 12, as a functin f the turbine inlet temperature. At high turbine inlet temperatures, the curves f the cycle effectiveness fr the different ORC types are bserved t flatten. Nevertheless, ne culd cnclude that the ORC with IHE enabled maximum cnversin f the available energy frm the thermal resurce t the rganic wrking fluid.

12 Figure 12: Cycle effectiveness as a functin f the turbine inlet temperature 4.3. Irreversibility analysis As illustrated by Fig. 13, the additin f an IHE t the binary cycle has substantially reduced the exergy destructin in the Evapratr-preheater, cndenser and cling system, by abut 40-70%, 20-30% and 5-15% respectively. The cycle pwer utput increased nly marginally with the regenerative ORC by less than 5% fr a given cmbinatin f the thermal fluid, evapratr and cndenser temperatures. Adding an OFOH t the binary cycle, n the ther hand, resulted in a remarkable reductin f the exergy destructin in all individual cmpnents f the binary cycle, typically 80-90% fr the Evapratr-preheater unit, 25-35% fr bth the cndenser and cling system, 20-30% fr the turbine, and 10-20% fr the pumping system. A significant reductin f 15-25% in cycle pwer utput was, hwever, bserved. The majr drawback with the additin f an IHE r/and OFOH lies in the increase in rejectin exergy destructin, 0-20% with the additin f an IHE alne, 20-35% while emplying an OFOH and up t 40% fr bth IHE and OFOH added t the binary-cycle. Figure 13: Variatin f Fuel depletin rati with T, T E and T c respectively (given in C) Frm Fig. 13, a sensitivity analysis is discussed fr a change in perating evapratin temperature, decrease in cndensing temperature and variatin in the temperature f the fluid resurce:

13 Fr a given thermal fluid and cndensing temperatures, an increment f 10 C in the evaprating temperature resulted t a substantial increase in the rejectin exergy destructin, back t the explitatin reservir, at apprximately 16-27%, whereas the exergy destructin f bth the evaprative and cndensatin prcesses decreased by 20-40% and 20-25% respectively. In additin, the ability t cnvert the ttal exergy input t useful wrk utput als drpped by apprximately 15%. Fr a given thermal fluid and evaprating temperatures, a decrease in cndensing temperature f 10 C yielded a decrease f rughly 71% in exergy destructin f the cndenser itself fr cycles nt using an IHE and nearly 92% fr thse with an IHE. Mrever, the cycle pwer utput was increased by 10-15%. Hence, the advantage f using an IHE is demnstrated t reduce significantly the cndensing lad; and the ptimal cndensing temperature t maximize the cycle pwer utput. As the temperature f the fluid resurce is reduced by 10 C, the cycle pwer utput is reduced by apprximately 18%. In shrt, a substantial decrease in wrk utput can result frm a small decrease in the thermal resurce temperature. In Fig. 14, the verall plant irreversibility is pltted against the turbine inlet temperature fr the thermal resurce temperatures f 110 C and 160 C. An ptimal turbine inlet temperature is bserved t yield minimum verall plant irreversibility, which als prduced maximum cycle pwer utput. Cnsequently, minimizing the lss f exergy in each cmpnents f the cycle, thus the verall plant irreversibility, wuld als maximize the cycle pwer utput. Figure 14: Overall plant irreversibility as a functin f the turbine inlet temperature fr thermal resurce temperature f (a) 110 C and (b) 160 C 4.4. Optimized slutin A parametric ptimizatin with n-pentane as wrking fluid fr the selected cycles was cnducted fr a thermal resurce temperature in the range f 110 C and 160 C. The ptimal perating cnditins were determined fr maximum cycle pwer utput per unit mass flw rate f the thermal fluid, as well as minimum verall plant irreversibility. As illustrated by Fig. 15a, the ptimal turbine inlet temperature is seen t increase almst linearly with the increase in the thermal resurce temperature. The additin f an IHE t the binary cycle has merely impacted n the ptimum perating cnditins f the ORCs, whereas adding an OFOH has required high ptimal turbine inlet temperatures, apprximately 10 C as cmpare t the basic Rankine ORCs, fr a given thermal resurce temperature.

14 In Fig. 15b, the maximum cycle pwer utput t be prduced per unit mass flw rate f the thermal fluid is pltted. At lw thermal resurce temperatures, belw 120 C, the basic ORCs generate nearly twice as much pwer utput than the regenerative ORCs. Clearly, the maximum cycle pwer utput per unit mass flw rate f the thermal fluid increases expnentially with the thermal resurce temperature. Hence, a substantial increase in cycle pwer utput is expected with a slight increase in the thermal resurce temperature [9]. Figure 15: (a) Optimal turbine inlet temperature, and (b) Maximum cycle pwer utput per kg fluid An ptimal First- and Secnd-law efficiency, at the primary heat exchanger inlet, is illustrated in Fig. 16. Based n the thermal fluid state at the primary heat exchanger inlet, the First and Secnd Law efficiencies are in the range f 4-9% and 37-47% respectively fr the basic ORCs; 2-6% and 19-33% respectively fr the regenerative ORCs. Figure 16: Optimal (a) First- and (b) Secnd-law efficiency at primary heat exchanger inlet Based n the energy input t the cycle, the First- and Secnd-law efficiencies as well as the cycle effectiveness are illustrated by Figs Frm Fig. 17a, the First-law efficiency fr the ptimum perating cnditins is in the range f 8-15% fr all ORCs cnsidered in this study. The nticeable lwer First-law efficiency is attributed t the mderately lwtemperature f the thermal resurces [14]. In Fig. 17b, the advantage f adding an IHE t the binary cycle t imprve the cycle Secnd-law efficiency is evident, and particularly at thermal resurce temperatures abve 130 C. Fr the ptimum perating cnditins, a maximum f 56% in Secnd Law efficiency is reached fr the ORCs with an IHE. This is apprximately 2-3% higher as cmpared t the ORCs withut an IHE fr the studied range f the thermal resurce temperature. A lk at the cycle effectiveness (Fig. 18), n the

15 ther hand, shwed better capability f transfer f the available energy t the wrking fluid fr the basic ORCs at 70-74%, as cmpared t 56-69% fr the regenerative ORCs. Here, the high sensitivity f the regenerative ORCs t variatins in the thermal resurce temperatures is demnstrated, as discussed by Franc and Villani [22]. Figure 17: (a) First- and (b) Secnd-law efficiency n heat transfer input t the ORC at the ptimum perating cnditins Figure 18: Cycle effectiveness at the ptimum perating cnditins 5. Cnclusins and recmmendatins A thermdynamic analysis and perfrmance ptimizatin f small binary cycle thermal pwer plants perating with mderately lw-temperature and liquid-dminated thermal resurces in the range f 110 C t 160 C, was cnsidered. Optimal perating cnditins were determined fr maximum cycle pwer utput per unit mass flw rate f the thermal fluid. The maximum cycle pwer utput was bserved t increase expnentially with the thermal resurce temperature, whereas the ptimal turbine inlet temperature increased almst linearly with the increase in the thermal heat surce. The additin f an IHE and/r an OFOH has been very prlific in imprving the effectiveness f the cnversin f the available thermal energy int useful wrk. Hwever, t avid a susceptible thermal pllutin f the envirnment caused by the fluid being discarded as waste heat at relatively high temperature [23], a cmbined pwer generatin and direct use in prcess r district heating applicatins as a cgeneratin system, can be an additinal ptin t imprve the energy utilizatin [14,23]. In additin, a perfrmance analysis f selected rganic wrking fluids, namely refrigerants R123, R152a, isbutane and n-pentane, was cnducted under saturatin temperature and subcritical pressure perating cnditins f the turbine. Organic fluids with higher biling pint temperature, such as n-pentane, were

16 recmmended fr the basic type f ORCs, whereas thse with lwer vapur specific heat capacity, such as butane, were mre suitable fr the regenerative ORCs. Althugh the present study limited itself t the thermdynamic perfrmance f the selected rganic fluids based n their thermdynamic prperties, the selectin f the ptimal rganic fluid is als subject t the chemical stability and cmpatibility with materials, the envirnmental impacts, the safety cncerns, and the ecnmical peratin f the wrking fluids [24-27]. References [1] DiPipp, R 1998, Gethermal Pwer Systems, in Ellitt, TC, Chen, K & Swanekamp, RC, Standard handbk f pwer plant engineering, 2 nd edn, MacGraw-Hill, New Yrk, pp [2] Gu, Z & Sat, H 2001, Optimizatin f cyclic parameters f a supercritical cycle fr thermal pwer generatin, Energy Cnversin and Management, vl. 42, n. 1, pp [3] Kanglu, M 2002, Exergy analysis f a dual-level binary thermal pwer plant, Gethermics, vl. 31, n. 1, pp [4] DiPipp, R 2008, Gethermal pwer plants principles, applicatins, case studies and envirnmental impact, Butterwrth-Heinemann, Lndn. [5] Desai, N & Bandypadhyay, S 2009, Prcess integratin f rganic Rankine cycle, Energy, vl. 34, n. 1, pp [6] Gnutek, Z & Bryszewska-Mazurek, A 2001, The thermdynamic analysis f multicycle ORC engine, Energy, vl. 26, n. 1, pp [7] Brsukiewicz-Gzdur, A & Nwak, W 2007, Maximising the wrking fluid flw as a way f increasing pwer utput f thermal pwer plant, Applied Thermal Engineering, vl. 27, pp [8] Calm, JM & Hurahan, GC, 2001, Refrigerant Data Summary, Engineered Systems, vl. 18, n. 11, pp [9] Subbiah, S & Natarajan, R 1988, Thermdynamic analysis f binary-fluid rankine cycles fr thermal pwer plants, Energy Cnversin and Management, vl. 28, n. 1, pp [10] Klein, SA 2012, Engineering Equatin Slver EES Academic Cmmercial V7.933, McGraw Hill. See als [11] Chen, H, Gswami, DY & Stefanaks, EK 2010, A review f thermdynamic cycles and wrking fluids fr the cnversin f lw-grade heat, Renewable and sustainable energy reviews, vl. 14, n. 1, pp [12] Bejan, A 1993, Heat transfer, Wiley, New Yrk. [13] Yari, M 2010, Exergetic analysis f varius types f thermal pwer plants, Renewable Energy, vl. 35, n. 1, pp [14] Kanglu, M & Blatturk, A 2008, Perfrmance and parametric investigatin f a binary thermal pwer plant by exergy, Renewable Energy, vl. 33, n. 1, pp [15] Demuth, OJ & Kchan, RJ 1981, Analyses f mixed hydrcarbn binary thermdynamic cycles fr mderate temperature thermal resurces using regeneratin techniques, INEL Rep. EGG-GTH-05710, Idah Falls, ID. [16] Aljundi, IH 2011, Effect f dry hydrcarbns and critical pint temperature n the efficiencies f rganic Rankine cycle, Renewable Energy, vl. 36, n.1, pp [17] Mag, PJ, Chamra, LM, Srinivasan, K & Smayaji, C, 2008, An examinatin f regenerative rganic Rankine cycles using dry fluids, Applied Thermal Engineering, vl. 28, n. 1, pp [18] DiPipp, R 2004, Secnd Law assessment f binary plants generating pwer frm lwtemperature thermal fluids, Gethermics, vl. 33, n.1, pp [19] Hepbasli, A 2008, A key review n exergetic analysis and assessment f renewable energy resurces fr a sustainable future, Renewable and Sustainable Energy Reviews, vl. 12, pp [20] Kaplan, U 2007, Advanced rganic Rankine cycles in binary thermal pwer plants, Wrld Energy Cuncil, Ormat Technlgies, inc.

17 [21] Lakew, AA & Blland, O 2010, Wrking fluids fr lw-temperature heat surce, Applied Thermal Engineering, vl. 30, n. 1, pp [22] Franc, A & Villani, M 2009, Optimum design f binary cycle pwer plants fr waterdminated, medium-temperature thermal fields, Gethermics, vl. 38, n.1, pp [23] Gu, T, Wang, HX & Zhang, SJ 2011, Fluids and parameters ptimizatin fr a nvel cgeneratin system driven by lw-temperature thermal surces, Energy, vl. 36, n. 1, pp [24] Maizza, V & Maizza, A 1996, Wrking fluids in nn-steady flws fr waste energy recvery systems, Applied Thermal Engineering, vl. 16, n. 1, pp [25] Tchanche, BF, Lambrins, Gr, Frangudakis, A & Papadakis G 2011, Lw-grade heat cnversin int pwer using rganic Rankine cycles- A review f varius applicatins, Renewable and Sustainable Energy Review, vl. 15, n.1, pp [26] Saleh, B, Kglbauer, G, Wendland M & Fischer, J 2007, Wrking fluids fr lwtemperature rganic Rankine cycles, Energy, vl. 32, n. 7, pp [27] DiPipp, R 1999, Small thermal pwer plants: Design, perfrmance and ecnmics, Gethermal Resurces Cuncil BULLETIN, pp. 1-8.