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Applied Thermal Engineering 91 (2015) 43e52

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Research paper

Thermodynamic analysis of an Organic Rankine Cycle (ORC) based onindustrial data

N. Filiz Tumen Ozdil*, M. Rıdvan Segmen, Atakan TantekinDepartment of Mechanical Engineering, Adana Science and Technology University, 01180 Adana, Turkey

h i g h l i g h t s

� Energy and exergy analysis of an Organic Rankine Cycle (ORC).� The main reasons of the irreversibility in the ORC.� Determination of exergy efficiency for the different water phases in the evaporator inlet.� Determination of the effect of the ambient temperature on ORC efficiency.

a r t i c l e i n f o

Article history:Received 20 May 2015Accepted 25 July 2015Available online 10 August 2015

Keywords:Organic Rankine CycleExergyThermodynamic analysisHeat recovery

* Corresponding author. Yesiloba, Mah.€O�gretmen01180 Seyhan/Adana, Turkey. Tel.: þ90 0 322 455 0000 09.

E-mail addresses: fozdil@adanabtu.edu.tr (N.F.adanabtu.edu.tr (M.R. Segmen), atantekin@gmail.com

http://dx.doi.org/10.1016/j.applthermaleng.2015.07.071359-4311/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this study, thermodynamic analysis of an Organic Rankine Cycle (ORC) is presented in a local powerplant that is located southern of Turkey. The system that is analyzed includes an evaporator, a turbine, acondenser, a pump and a generator as components. System components are analyzed separately usingactual plant data and performance cycle. The relationship between pinch point and exergy efficiency isobserved. As the pinch point temperature decreases, the exergy efficiency increases due to low exergydestruction rate. The energy and exergy efficiencies of the ORC are calculated as 9.96% and 47.22%,respectively for saturated liquid form which is the real condition. In order to show the effect of the waterphase of the evaporator inlet, exergy destruction and exergy efficiencies of components and overallsystem are calculated for different water phases. The exergy efficiency of the ORC is calculated as 41.04%for water mixture form which has quality 0.3. On the other hand, it is found as 40.29% for water mixtureform which has quality 0.7. Lastly, it is calculated as 39.95% for saturated vapor form. Moreover, exergydestruction rates of the system are 520.01 kW for saturated liquid form, 598.39 kW for water mixtureform which has quality 0.3, 609.5 kW for water mixture form which has quality 0.7 and 614.63 kW forsaturated vapor form. The analyses show that evaporator has important effect on the system efficiency interms of exergy rate. The evaporator is investigated particularly in order to improve the performance ofthe overall system.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Energy and energy production systems have being the majortopic of the thermodynamics in recent years. Energy can not begenerated or consumed by itself so the energy production process isbecome the vital concept in the world. The waste heat recovery isthe most suitable source for the energy production due to lack of

ler Bulvarı 46278 Sk. No:3,00/2055; fax: þ90 0 322 455

Tumen Ozdil), mrsegmen@(A. Tantekin).

9

the fossil fuels and global warming. Waste heat recovery processprovides the energy conversation and decrement of the thermalpollution. Although the steam turbine is the most common tech-nology in the energy production process, due to necessity of highoperational temperature and pressure, it is not suitable for lowtemperature and pressure condition. Organic Rankine Cycle isgenerally preferred for the processes having low temperature likeT < 150 �C. This process named as Organic Rankine Cycle owing tousage of the organic fluid as working fluid instead of water and highpressure steam. The organic fluid that is used in ORC, has highmolecular weight liquid with low boiling temperature than water.Exergy is the measurement of the maximum useful work that canbe obtained in the system. Therefore, it has become more

Nomenclature

ex specific exergy (kJ/kg)_ExD exergy destruction (Kw)h specific enthalpy (kJ/kg)_m mass flow rate (kg/s)P pressure (bar)_Q rate of heat transfer (kW)s specific entropy (kJ/kg K)T temperature (K)_W rate of work (kW)w uncertainty

SubscriptsEvp. evaporatorCond. condenser

Cons. consumedSat. saturationpp. pinch pointrej rejectionDest. destructionTurb. TurbineSgen entropy generationCyc. cycle0 reference state

Greek symbolsDT temperature differenceDs entropy differenceDh enthalpy differencehI first law efficiencyhII second law efficiency

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e5244

important subject than energy in order to specify the useful work.Moreover, the exergy can be called as irreversibility in thermody-namic point of view. Because of the irreversibility, exergy can beconsumed or destroyed in the processes. The consumption of theexergy rate in a process is directly related with the entropygeneration.

There are a lot of studies about Organic Rankine Cycle for powergeneration from waste heat recovery [1e4,6e17]. Based on thestudies of Kaska [1], energy and exergy analysis in a steel plant wasperformed using actual plant data. He concluded that the energyand exergy efficiencies of the system were calculated as 10.2%,48.5%, 8.8% and 42.2%, respectively for two different actual cases.Furthermore, the exergy destruction rate was listed from higher tolower as evaporator, turbine, condenser and pump, respectively. Inaddition, it was observed that evaporator pressure had an impor-tant effect on the energy and exergy efficiency in this paper. Gomezet al. [2] implemented energy and exergy analysis for the powerplant. They modeled and simulated the system using EES (Engi-neering Equation Solver) to present the effects of key parameterson the efficiency. The effects of the temperature, pressure andcompression ratio on the power plant were observed in their study.They concluded that lower compression pressure ratio (r) causedhigh thermal efficiency due to more effective regeneration process.Moreover, decrement of the helium temperature at the compressorinlet caused sharp drop in the compression work due to decrementof the specific volume.

There are several studies regarding to the working fluid inliterature [5,13,17,20]. As understood from these studies, theworking fluid using in the ORC cycle has important effect on theperformance of the power generation systems. Roy et al. [4]investigated the effect of different working fluid on the efficiencyand irreversibility rate on the system. They demonstrated that R-123 working fluid used in ORC system, had positive impact on ef-ficiency in turbine. Moreover, R-123 working fluid was observed asthe best working fluid among the other working fluid options intheir study. Mohanraj et al. [5] presented both experimental andtheoretical studies about environment friendly alternative workingfluids. They concluded that HC mixtures and R152a were found tobe better substitutes for R12 and R134a in domestic refrigerationsystems. Furthermore, R290, R1270, R290/R152a, R744 and HC/HFCmixtures were found to be the best long-term alternatives for R22in air conditioning and heat pump applications. R123 was found tobe a convenient alternative to R11, R12 and R22 in chiller applica-tions. R152a and HC mixtures were found to be the best option for

automobile air conditioners. Moreover, they showed that the usageof less harmful refrigerants like R290, R1270, R744 and HC/HFC inair conditioning and heat pump applications played a vital role inthe developing countries for reducing the environmental impact ofhalogenated refrigerants. Long et al. [13] showed that the ther-mophysical properties of the working fluid had little impact oninternal exergy efficiency. However they played an important rolein external exergy efficiency. They concluded that the selection oftheworking fluid strictly depended on evaporation temperature. Toachieve high overall exergy efficiency, working fluid that had lowercritical temperature must be used in the ORC system.

There are limited studies about pinch analysis in literature. Sueand Chuang [6] presented the exergy analyses for a steam cyclesystem. They predicted the system efficiency more precisely withthe help of the pinch point analysis. Based on this study, increasingthe pinch points decreased the efficiency of the combined cycle.Moreover, the 10 �C pinch temperature increment in the HRSGaffected the overall combined cycle efficiency by 0.3%. They notedthat the pinch point design value had to be carefully evaluatedbased on anticipated operating factors in order to obtain an opti-mum design. Based on another studies about pinch analysis, Li et al.[7] examined the effect of the pinch point temperature differenceand the evaporation temperature on the performance of OrganicRankine Cycle. They concluded that some organic working fluidscould not reach the maximum net power output because of the lowtemperature corrosion. According to their results, as the pinchpoint temperature difference of the evaporator increased, the totalheat transfer area decreased first and then increased. On the otherhand, cost-effective performance demonstrated the opposite vari-ation tendency. Moreover, the pinch point temperature differenceof the evaporator was observed for the optimization cost-effectiveperformance. They found that the results had similarity withdifferent organic working fluids.

Even though, there are several studies concerned with the ther-modynamic analysis of the ORC systems in power generation plants,there are some details need to be discussed and discovered toimprove the energy production from theORC system in real industry.In this study, a detailed thermodynamic analysis was carried out foran Organic Rankine Cycle which generates electricity using wasteheat recovery in a local steel plant. Based on the first and second lawof thermodynamics, the energy and exergy analysis was performedfor each component of the ORC system. The energy and exergy effi-ciencies, exergy destruction rate, entropygeneration and the effect ofthe various parameters on the components were observed.

Fig. 1. Saturated liquid property deviation Ref. [19].

Fig. 2. Saturated vapor property deviation Ref. [19].

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52 45

2. System description

In this study, Organic Rankine Cycle is investigated based onthermodynamic analysis which is placed in Adana located insouthern of Turkey. The system has 260 kW capacity and specifi-cations of the system components are demonstrated in Table 1.

The system involves an evaporator, a condenser, a turbine and apump as subsystems. The generator, heating and cooling watercollectors are accepted as the auxiliary components. The OrganicRankine Cycle produces electricity using waste heat in low tem-perature in order to reduce the operating costs of company. Theworking fluid using in the ORC cycle is R245fa which has goodthermodynamic properties such as low specific heat and viscosity,low toxicity, low ozone depletion potential, low flammability. Mostof the ORC systems are using R245fa as the working fluid havemoderate global warming potential of 950, power density, while itslower critical pressure at higher temperature allows for reasonablesystem thermal efficiency [20]. Owing to the above mentionedproperties and the favorable economic conditions, R245fa is aconvenient option as working fluid. The properties of the R245faobtained from Ref. [19] can be seen in Figs. 1e5 and Table 2.

The schematic diagram of the ORC system is illustrated in Fig. 6.In the system, working fluid is pumped, firstly. Namely, low pres-sure fluid is compressed to high pressure fluid by a pump as can beseen in Fig. 6 (state 5 to 6). Then high pressure fluid enters andpasses through the evaporator. In the evaporator, high pressurefluid [6] has become heated and pressurized vapor [3] using theheat capacity of inlet water (state 1 to 2). After that the heated andpressurized vapor enters in turbine. And it leaves from turbine aslow pressure vapor and generates electricity (state 3 to 4). Lastly,the low pressure vapor goes through the condenser, and theworking fluid leaves from condenser as saturated liquid (state 4 to5) and the cycle continues.

Some of data are measured on system and remaining data areread the computer aided control panel, directly. Dead state condi-tions of the working fluid (R245fa) and the water are accepted as1 bar and 25 �C. Before the starting analysis, first step is to measurethemassflow rate of the condenser coolingwater [7].Massflow rateof the condenser cooling water is measured by GE-PT878 which isultrasonicflowmeter equipment ranges from½00 to 7.6mmwith±1%accuracy. Mass flow rate of the R245fa is calculated from the massand energy balance equations fromcondenser coolingwater and themass flow rate of the evaporator is estimated from the first law ofthermodynamic. Thewater collectorson the systemareplaced in theevaporator and condenser inlet and outlet. Pressure and tempera-turemeasurement devices are put on collectors in order tomeasurethe properties of water and the working fluid.

3. Thermodynamic analysis of the ORC and assumption

The first law of thermodynamics is explained as the conserva-tion of energy, thermodynamically. The total energy which is beingconstant in the processes can be converted or transferred. The firstlaw of thermodynamics is based on the observations that areproven with experiments. The second law of thermodynamics isdescribed as the quality of energy, thermodynamically. The secondlaw refers the change of the quality of energy during the phase

Table 1Specifications of the system components.

Components Model/Type Capacity

Evaporator Shell and tube 3161 kW (Max)Condenser Shell and tube 2885 kW (Max)Turbine CARRIER PC-51 272 kW (Max)Pump Grundfos/KB-G-A-E-HOBE 267 GPM/50 kW

change in the processes. The maximum useful work in a process iscalled as exergy or irreversibility. Entropy generation is the changeof entropy due to irreversibility which causes the increment of theexergy destruction in the system. If the entropy generation is high

Dimensions Operating pressure

Ø1000 mm � 6000 mm 345 psi (max)Ø900 mm � 6000 mm 345 psi (max)e 345 psi (max)

motor e 135 psi (max)

Fig. 3. Identical properties for R245fa along the critical isotherm (T ¼ Tc) Ref. [19].

Fig. 5. Pressureeentropy change for R245fa Ref. [19].

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e5246

during the phase change, the exergy destruction rate is high thathigh amount of exergy destruction rate is undesirable in the powergeneration plants. The higher exergy destruction rate means loweruseful energy production in the processes. The pinch point analysisis a major term to observe the performance of the ORC for powergeneration. In pinch point analysis, the feasible energy targets arecalculated to minimize energy consumption. Operating conditionsand energy supply methods can be formed and heat recovery sys-tems can be optimized by pinch point analysis in order to minimizethe energy consumption.

In this study, analyses are performed for four different waterphases of evaporator inlet in order to investigate the effect of waterphase on efficiency. These conditions can be listed as;

Fig. 4. Pressureeenthalpy change for R245fa Ref. [19].

i. Saturated liquid,ii. Water mixture (quality 0.3),iii. Water mixture (quality 0.7),iv. Saturated vapor phase of the evaporator inlet.

The data which are obtained and measured in the real operatingsystem are given in Table 3, for first condition (saturated liquid).According to Table 3, mass flow rate of the cooling water throughthe condenser is measured as 110 kg/s. Mass flow rate of theworking fluid is calculated as 10.63 kg/s and evaporator heatingwater is estimated as 13.48 kg/s with the help of energy balanceequation based on first law of thermodynamics. Inlet and outlettemperatures of evaporator which are obtained on the controlpanel directly are 127 �C and 83.9 �C, respectively. On the otherhand, the condenser cooling water inlet and outlet temperaturesare 27.5 �C and 32.4 �C, respectively. The following assumptions aremade in this study;

Pressure drops, potential and kinetic energy changes areneglected on the system.The system operates in a continuous steady state flow process.The system is adiabatic which means there is no heat loss.

General definitions and equations are given as below.General mass balance;

Total Mass Inlet ¼ Total Mass Outlet�/

X_min ¼

X_mout

�(1)

General energy and exergy balance;

Total Energy Initial¼Total EnergyFinal�/ _Q þ _W¼

X_mfhf

�X

_minhin

�(2)

Table 2The properties of R245fa.

ASHRAE number Molecular formula Atmospheric lifetime (years) Net GWP 100-yr Molecular mass Critical Temp. �C Critical Pressure (absolute) kPa

R-245fa C3H3F5 7.6 950 134 154.05 3.640

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52 47

Exergy transfer by heat at the temperature Ts is defined by Eq.(3).

/ _Exheat ¼ _Qð1� T0=TsÞ (3)

TotalExergyInitial¼TotalExergyFinalþTotalExergyConsumedþ TotalExergyDestruction

��/ _Exi¼ _Exfþ _Exconsþ _ExD;total

�(4)

/ _Ex ¼ _mex (5)

where the “ _Ex” is exergy rate.

/ex ¼ h� h0 � T0ðs� s0Þ (6)

Entropy Generation

/Sgen ¼ _ExD.T0 (7)

Fig. 6. Schematic representatio

Table 3The thermophysical data for saturated liquid of the evaporator inlet (Condition-1).

State no Fluid type Phase Mass flow rate (kg/s) Temperature T (K) Pr

0 Water Dead State e 298 10 R245fa Dead State e 298 11 Water Sat. Liq 13.48 400 22 Water Comp. Liq 13.48 356.9 23 R245fa Sat-Vapor 10.63 368.4 114 R245fa Sup-Vapor 10.63 326 25 R245fa Sat-Liq 10.63 303 26 R245fa Comp. Liq 10.63 303.17 117 Water Comp. Liq 110 300.5 18 Water Comp. Liq 110 305.4 1

where the _ExD and T0 are the exergy destruction and ambienttemperature, respectively.

Whenwe applied the first and second law of thermodynamic onthe system we obtain the results for each component and entirecycle, as described below.

3.1. Energy and exergy balance of evaporator

Energy and exergy balance equations through the evaporatorcan be written as Eq. (8).

/ _m1h1 þ _m6h6 ¼ _m2h2 þ _m3h3 (8)

_m1 and _m2 is the mass flow rate of the heating water of theevaporator.

/ _m1ex1 þ _m6ex6 ¼ _m2ex2 þ _m3ex3 þ _ExD;evp (9)

Entropy generation for evaporator can be given as Eq. (10).

/Sgen;evp ¼ _ExD;evp.T0 (10)

n of ORC system Ref. [21].

essure P (bar) Enthalpy, h (kJ/kg) Entropy, s (kJ/kg K) Exergy rate Ex (kW)

104.9 0.367 e

424.7 1.781 e

.7 533.6 1.603 814.79

.2 350.8 1.119 294.78

.4 471.5 1.789 471.68

.4 447.1 1.802 171.42

.4 238.9 1.135 71.23

.4 239.4 1.134 79.71

.37 115.1 0.402 �12.18

.37 135.2 0.467 48.44

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e5248

The exergy efficiency eqn. of evaporator is given by Eq. (11).

/h2;evp ¼ _m3ðex3 � ex6Þ= _m1ðex1 � ex2Þ (11)

3.2. Energy and exergy balance of turbine

Energy and exergy balance equations through the turbine whichis assumed as adiabatic, can be written as Eq. (12).

/ _m3h3 ¼ _m4h4 þ _Wturb (12)

_m3 and _m4 is the mass flow rate of the working fluid R245fa.

/ _m3ex3 ¼ _m4ex4 þ _Wturb þ _ExD;turb (13)

Entropy generation for turbine can be given as Eq. (14).

/Sgen;turb ¼ _ExD;turb.T0 (14)

The exergy efficiency equation of turbine is given by Eq. (15).

/h2;turb ¼ _Wturb

.�_Ex3 � _Ex4

�(15)

3.3. Energy and exergy balance of condenser

Energy and exergy balance equations through the condenser canbe calculated with the help of Eq. (16).

/ _m4h4 þ _m7h7 ¼ _m5h5 þ _m8h8 (16)

_m7 and _m8 is the mass flow rate of the cooling water of thecondenser.

/ _m4ex4 þ _m7ex7 ¼ _m5ex5 þ _m8ex8 þ _ExD;cond (17)

Entropy generation for condenser can be given as Eq. (18).

/Sgen;cond ¼ _ExD;cond.T0 (18)

The exergy efficiency eqn. of condenser is calculated by Eq. (19).

/h2;cond ¼ _m7ðex8 � ex7Þ= _m4ðex4 � ex5Þ (19)

3.4. Energy and exergy balance of pump

Energy and exergy balance equations through the pump whichis assumed as adiabatic, can be written as Eq. (20).

/ _m5h5 þ _Wpump ¼ _m6h6 (20)

/ _m5ex5 þ _Wpump ¼ _m6ex6 þ _ExD;pump (21)

Entropy generation for pump can be given as Eq. (22)

/Sgen;pump ¼ _ExD;pump

.T0 (22)

The exergy efficiency eqn. of pump is calculated by Eq. (23)

/h2;pump ¼�_Ex6 � _Ex5

�._Wpump (23)

Overall exergy destruction is occurred in the cycle, the overallexergy rate of the system is defined by Eq. (24)

/ _Excyc;in ¼ _Exevpþ _Exturbþ _Excondþ _Expumpþ _Excond;rejþ _Wturb

(24)

The exergy destruction rate based on heat rejection on thecondenser using cooling water is calculated by Eq. (25)

/ _Excond;rej ¼ _Excyc;in� _Exevp� _Exturb� _Excond� _Expump� _Wturb

(25)

The _Excond;rej; is called as outgoing exergy rate through theambient.

The overall exergy efficiency is defined by Eq. (26)

/h2;cyc ¼ _Wnet

._Excyc;in (26)

The exergy rate of cycle is defined by Eq. (27)

/ _Excyc;in ¼ _mw;evpðh1 � h2Þ � T0ðs1 � s2Þ (27)

The overall cycle efficiency is the ratio of the net turbine powerto the net heat transfer rate as seen below:

/h1;cyc ¼ _Wnet

._Qcyc;in (28)

_Qcyc;in which is the heat input on the evaporator, can be definedas Eq. (29);

/ _Qcyc;in ¼ _mw;evpðh1 � h2Þ (29)

Pinch point temperature is determined with respect to the firstlaw of thermodynamic equations as follows [1],

/ _mw;evp�h1 � hpp

� ¼ _mR245fa

�h3 � hR245fa;sat

�(30)

/ _mw;evp�hpp � h3

� ¼ _mR245fa

�hR245fa;sat � h6

�(31)

hR245fa,sat is called as saturated liquid enthalpy of the workingfluid at 95.4 �C evaporation temperature while hpp is the pinchpoint enthalpy of which water circulates in evaporator. Accordingto the above equations, pinch point temperature is calculated as101.3 �C. TR245fa,sat and Tpp are vaporization temperature and pinchpoint temperature of the evaporator inlet, respectively. DTpp is thedifference between TR245fa,sat and Tpp and therefore DTpp is found as5.9 �C.

4. Results and discussions

The Organic Rankine Cycle produces electricity using low tem-perature waste heat. In this study, the thermodynamic analysis isapplied based on the first and second law of thermodynamics tocalculate exergy destructions and exergy efficiency of the system. Inaddition, entropy generation is analyzed to demonstrate its effecton system performance directly. Moreover, effect of four differentwater phase of evaporator inlet on the system efficiency wasinvestigated. The thermophysical properties of the different evap-orator inlet phases are shown in Tables 3e6.

When the water phase of the evaporator inlet is accepted assaturated liquid, the system produces 2464.7 kW heat from heatsource and generates 259.4 kW gross power. Pump consumes13.8 kW power in order to circulate the working fluid in the systemand the net power is found as 245.6 kW. The energy and exergyefficiency of the system are calculated as 9.96% and 47.22%,

Table 4The thermophysical data for water-mix of evaporator inlet (x ¼ 0.3) (Condition-2).

State no Fluid type Phase Mass flow rate (kg/s) Temperature T (K) Pressure P (bar) Enthalpy, h (kJ/kg) Entropy, s (kJ/kg K) Exergy rate Ex (kW)

0 Water Dead State e 298 1 104.9 0.367 e

0 R245fa Dead State e 298 1 424.7 1.781 e

1 Water Water-Mix 2.96 400 2.7 1181.55 3.229 663.262 Water Comp. Liq 2.96 356.9 2.2 350.8 1.119 64.863 R245fa Sat-Vapor 10.63 368.4 11.4 471.5 1.789 471.684 R245fa Sup-Vapor 10.63 326 2.4 447.1 1.802 171.425 R245fa Sat-Liq 10.63 303 2.4 238.9 1.135 71.236 R245fa Comp. Liq 10.63 303.17 11.4 239.4 1.134 79.717 Water Comp. Liq 110 300.5 1.37 115.1 0.401 �12.188 Water Comp. Liq 110 305.4 1.37 135.2 0.467 48.44

Table 5The thermophysical data for water-mix of evaporator inlet (x ¼ 0.7) (Condition-3).

State no Fluid type Phase Mass flow rate (kg/s) Temperature T (K) Pressure P (bar) Enthalpy, h (kJ/kg) Entropy, s (kJ/kg K) Exergy rate Ex (kW)

0 Water Dead State e 298 1 104.9 0.3670 R245fa Dead State e 298 1 424.7 1.7811 Water Water-Mix 1.44 400 2.7 2056.9 5.428 641.092 Water Comp. Liq 1.44 356.9 2.2 350.8 1.119 31.583 R245fa Sat-Vapor 10.63 368.4 11.4 471.5 1.789 471.684 R245fa Sup-Vapor 10.63 326 2.4 447.1 1.802 171.425 R245fa Sat-Liq 10.63 303 2.4 238.9 1.135 71.236 R245fa Comp. Liq 10.63 303.17 11.4 239.4 1.134 79.717 Water Comp. Liq 110 300.5 1.37 115.1 0.401 �12.188 Water Comp. Liq 110 305.4 1.37 135.2 0.467 48.44

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52 49

respectively while exergy destruction of the cycle is calculated as274.4 kW. The highest exergy destruction occurs in the evaporatorfor whole system with 128.06 kW. The results of Kaska [1] alsoshow same behavior. As illustrated in Fig. 7, exergy rejection at thecondenser is calculated as 60.63 kW. Moreover, the exergy effi-ciencies of evaporator, turbine, condenser, and pump are calculatedas 75.37%, 86.39%, 60.51%, and 61.4%, respectively (Fig. 8). Whencompared with the study of Kaska [1], both results for distributionof the exergy efficiency rate are closer to each other. Water phase ofevaporator inlet is assumed as water-mix of which quality is 0.3 atcondition-2. The data are given in Table 4 for this condition. Incondition 2, net and gross power is accepted same as condition-1.Furthermore, energetic and exergetic efficiency values of turbine,condenser and pump are the same as condition 1. Unlike the con-dition 1, energetic and exergetic efficiencies of evaporator and cycleare calculated as 65.5% and 41.04%, respectively in condition-2 asexergetic efficiencies are shown in Table 8. These results show thatboth energy and exergy efficiencies decrease compared with pre-vious condition. As can be seen in Table 7, the exergy destruction inthe evaporator for condition-2 is higher than that for condition-1because of poor heat exchange between heating water and theworking fluid through the evaporator.

Table 6The thermophysical data for superheated vapor of evaporator inlet (Condition-4).

State no Fluid type Phase Mass flow rate (kg/s) Temperature T (K) Pr

0 Water Dead State e 298 10 R245fa Dead State e 298 11 Water Water-Mix 1.04 400 22 Water Comp. Liq 1.04 356.9 23 R245fa Sat-Vapor 10.63 368.4 114 R245fa Sup-Vapor 10.63 326 25 R245fa Sat-Liq 10.63 303 26 R245fa Comp. Liq 10.63 303.17 117 Water Comp. Liq 110 300.5 18 Water Comp. Liq 110 305.4 1

Water phase of evaporator inlet is accepted as water-mix, ofwhich quality is 0.7 at condition-3. The data are given in Table 5 forcondition-3. According to Table 8, exergy destruction in the evap-orator for condition-3 is higher than that for condition 1 and 2,hereby the exergy efficiency of this condition is less than that ofprevious conditions. Exergy destruction and exergy efficiency data

essure P (bar) Enthalpy, h (kJ/kg) Entropy, s (kJ/kg K) Exergy rate Ex (kW)

104.9 0.367424.7 1.781

.7 2716.3 7.077 637.41

.2 350.8 1.119 22.78

.4 471.5 1.789 471.68

.4 447.1 1.802 171.42

.4 238.9 1.135 71.23

.4 239.4 1.134 79.71

.37 115.1 0.401 �12.18

.37 135.2 0.467 48.44

Fig. 7. Exergy destruction for each component at condition-1 (saturated water).

Table 7Exergy destruction of the system components for four different phases.

Components Exergy destruction1 (kW) Exergy destruction2 (kW) Exergy destruction3 (kW) Exergy destruction4 (kW)

Evaporator 128.06 206.42 217.53 222.66Turbine 40.85 40.85 40.85 40.85Condenser 39.55 39.55 39.55 39.55Pump 5.325 5.325 5.325 5.325Rejection at cond. 60.63 60.63 60.63 60.63

Table 8Exergy efficiency of system components and cycle for four different water phases.

Components Exergy efficiency1 (%) Exergy efficiency2 (%) Exergy efficiency3 (%) Exergy efficiency4 (%)

Evaporator 75.37 65.5 64.49 64.18Turbine 86.39 86.39 86.39 86.39Condenser 60.51 60.51 60.51 60.51Pump 61.40 61.40 61.40 61.40Cycle 47.22 41.04 40.29 39.95

Fig. 8. Exergy efficiency for each component and entire cycle at condition-1 (saturated water).

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e5250

for condition-4 are also demonstrated in Tables 7 and 8, respec-tively. At condition-4, water phase of evaporator inlet is assumed assaturated-vapor. When compared to the other conditions,maximum exergy destruction and minimum exergy efficiencyoccur in this condition. The effect of the water phase of the evap-orator inlet on exergy destruction and exergy efficiency is illus-trated in Figs. 9 and 10.

Fig. 9. Distribution of exergy destruction on the evaporator for four different waterphases.

As the heating water phase in the evaporator inlet changes fromliquid to vapor, internal energy of the heating water increases.Because of the higher internal energy of vapor, heat transfer ratefrom the heating water to R245fa is higher for vapor than that forliquid. As the heat transfer rate increases, exergy rate of the systemalso increases. Due to the increment of system exergy rate, exergyefficiency decreases.

Fig. 10. Distribution of exergy efficiency on the evaporator for four different waterphases.

Fig. 11. Distribution of entropy generation for system components at condition-1(saturated water).

Fig. 12. Distribution of entropy generation on the evaporator for four different waterphases.

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52 51

In the exergy efficiency formula as can be seen in Eq. (26), theincreasing exergy rate of the system with constant net powercauses decrement of the exergy efficiency of the system. As theheating water phase in the evaporator inlet changes from liquid tovapor, the exergy rate of the system increases due to increment ofthe enthalpy and entropy values of the heating water. As a result,using vapor as heating water means higher exergy rate results inhigher internal energy, higher enthalpy and higher entropy values.

Fig. 13. The effect of the pinch point temperature o

Therefore, themost efficient condition is observed as condition-1 inwhich water phase of the evaporator inlet is accepted as saturatedliquid. Namely, the low exergy rate is observed for this conditioncompared with the others. Based on the first law of thermody-namics, mass flow rate of the water in the evaporator decreases, asthe water phase changes liquid form to vapor form due to theincrement of the enthalpy and entropy values during the phasechange.

Furthermore, entropy generation is investigated in order toobtain the performance of the components of the system. The en-tropy generation is demonstrated in Fig. 11. The highest entropygeneration occurs in evaporator with 0.43 kW followed by turbine,condenser and pump with 0.14 kW, 0.13 kW and 0.02 kW respec-tively. Fig.12 shows the entropy generation at different water phaseof the evaporator inlet. The highest entropy generation rate occursin the condition-4 and 3, 2 and 1 follow it. Moreover, Fig. 13 showsthe effect of different pinch point temperature on the exergy effi-ciencies at constant pressure and net power output. When the DTppreduces, the exergy efficiency increases results in increment of thenet power-exergy rate ratio due to the decrement of the exergy ratein the cycle. These variations in the DTpp play important role for theeffectiveness of the system. Meanwhile, if the DTpp reduce, theexergy loss of the evaporator will also reduce. It is consistent withresults of Sue and Chuang [6].

The effect of the ambient temperature on the system efficiencyis observed in the running system. It is observed that the variationof ambient temperature has minor effect on the system efficiency.There is no significant effect of the ambient temperature on thesystem efficiency, because the system, which is investigated in thisarticle, is located in a closed area. Consequently, the system effi-ciency increases from 47.22% to 47.45% as the ambient temperatureincreases from 25 �C to 40 �C as can be seen in Table 9. Due to theminor increment in the exergy efficiency, the effect of the ambienttemperature on the system efficiency is neglected in this study.However, the variation of the ambient temperature hasmajor effecton the system efficiency in another investigation [1].

Moreover, the effect of the mass flow rate on the system effi-ciency is observed in the running system and exergy efficiency ofthe system is calculated using these dynamic values theoretically.The mass flow rate of the condenser cooling water is measured byultrasonic flowmeter about 1 h in order to observe the range of themass flow rate. The calculations are repeated based on dynamic realsystem. The mass flow rate ranges from 104 kg/s to 114 kg/s. Fig. 14shows the effect of the mass flow rate variation of the condensercooling water on the system efficiency. As the mass flow rate

n the exergy efficiency at constant net power.

Table 9The effect of the ambient temperature on the system efficiency.

Winter (25 �C) Summer (40 �C) Total changes

Exergy efficiency (%) 47.22 47.45 0.48

Fig. 14. The effect of the mass flow rate on the exergy efficiency of the system.

N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e5252

decreases, the exergy efficiency increases due to the decrement ofthe exergy rate with constant net power.

An uncertainty analysis is applied for energy and exergy effi-ciencies of system, using below equations;

wh1

h1¼

"�w _Wnet

_Wnet

�2

þ�w _m

_m

�2þ�wDh

Dh

�2#12

(32)

wh2

h2¼

"�w _Wnet

_Wnet

�2

þ�w _m

_m

�2þ�wDh

Dh

�2þ�wT

T

�2þ

�wDs

Ds

�2#12

(33)

where “w” is the uncertainty amount of dependent variable for thefirst and second law efficiency. The uncertainty results of energyand exergy efficiencies are calculated as 0.48% and 3% respectively.These results consistent with results of Ege and Sahin [18].

5. Conclusions

This study presents a detailed thermodynamic analysis of anORC cycle in steel industry. The highest entropy generation isobserved in the evaporator that is approximately 0.43 kW. Themajor exergy destruction occurs in the evaporator with about128.06 kW and followed by turbine, condenser and pump,respectively. The main reasons of the inefficiency occurred in theevaporator are high heat input, water phase of the evaporator inlet.The second law efficiencies of the ORC system for four differentconditions are 47.22%, 41.04%, 40.29% and 39.95%, respectively. Theexergy efficiency significantly decreases when the water phasechanges liquid form to vapor form due to the better heat exchangethrough the water and the working fluid R245fa in the evaporator.Thus, most efficient water phase of which the evaporator inlet isobserved as the saturated liquid form. The effects of the variousambient temperatures and pinch point temperature on the exergyefficiency are also observed, in this study. The system efficiencyincreases from 47.22% to 47.45% as the ambient temperature in-creases. Due to the minor increment, the effect of the ambient

temperature on the system efficiency is neglected in this paper. It isobserved that the as the pinch point temperature decreases, theexergy rate of the system decreases. Furthermore, the decrement ofthe exergy rate causes the increment of the exergy efficiency of thesystem.

In order to improve performance of the ORC system, it can beproposed;

� In the evaporator inlet, saturated liquid form should be used orthe quality of the water should approach the zero.

� the difference between the pinch point temperature and evap-oration temperature should be less than 5.9 �C.

� different working fluid type which has better heat-power con-version capacity with less energy consumption, can be used inORC in order to increase power output.

� the execution of the thermoeconomic analysis of the ORC cyclecan be examined in the future.

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