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Thermodynamic analysis of an Organic Rankine Cycle (ORC) based on industrial data
Applied Thermal Engineering 91 (2015) 43e52
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Research paper
Thermodynamic analysis of an Organic Rankine Cycle (ORC) based on industrial data N. Filiz Tumen Ozdil*, M. Rıdvan Segmen, Atakan Tantekin Department 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
a b s t r a c t
Article history: Received 20 May 2015 Accepted 25 July 2015 Available online 10 August 2015
In this study, thermodynamic analysis of an Organic Rankine Cycle (ORC) is presented in a local power plant that is located southern of Turkey. The system that is analyzed includes an evaporator, a turbine, a condenser, a pump and a generator as components. System components are analyzed separately using actual plant data and performance cycle. The relationship between pinch point and exergy efficiency is observed. As the pinch point temperature decreases, the exergy efficiency increases due to low exergy destruction 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 water phase of the evaporator inlet, exergy destruction and exergy efficiencies of components and overall system 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 mixture form which has quality 0.7. Lastly, it is calculated as 39.95% for saturated vapor form. Moreover, exergy destruction rates of the system are 520.01 kW for saturated liquid form, 598.39 kW for water mixture form which has quality 0.3, 609.5 kW for water mixture form which has quality 0.7 and 614.63 kW for saturated vapor form. The analyses show that evaporator has important effect on the system efficiency in terms of exergy rate. The evaporator is investigated particularly in order to improve the performance of the overall system. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Organic Rankine Cycle Exergy Thermodynamic analysis Heat recovery
1. Introduction Energy and energy production systems have being the major topic of the thermodynamics in recent years. Energy can not be generated or consumed by itself so the energy production process is become the vital concept in the world. The waste heat recovery is the most suitable source for the energy production due to lack of
€g retmenler Bulvarı 46278 Sk. No:3, * Corresponding author. Yes¸iloba, Mah.O 01180 Seyhan/Adana, Turkey. Tel.: þ90 0 322 455 00 00/2055; fax: þ90 0 322 455 00 09. E-mail addresses: [email protected] (N.F. Tumen Ozdil), mrsegmen@ adanabtu.edu.tr (M.R. Segmen), [email protected] (A. Tantekin). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.079 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
the fossil fuels and global warming. Waste heat recovery process provides the energy conversation and decrement of the thermal pollution. Although the steam turbine is the most common technology in the energy production process, due to necessity of high operational temperature and pressure, it is not suitable for low temperature and pressure condition. Organic Rankine Cycle is generally preferred for the processes having low temperature like T < 150 C. This process named as Organic Rankine Cycle owing to usage of the organic fluid as working fluid instead of water and high pressure steam. The organic fluid that is used in ORC, has high molecular weight liquid with low boiling temperature than water. Exergy is the measurement of the maximum useful work that can be obtained in the system. Therefore, it has become more
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Nomenclature ex _ D Ex h _ m P Q_ s T _ W w
specific exergy (kJ/kg) exergy destruction (Kw) specific enthalpy (kJ/kg) mass flow rate (kg/s) pressure (bar) rate of heat transfer (kW) specific entropy (kJ/kg K) temperature (K) rate of work (kW) uncertainty
Subscripts Evp. evaporator Cond. condenser
important subject than energy in order to specify the useful work. Moreover, the exergy can be called as irreversibility in thermodynamic point of view. Because of the irreversibility, exergy can be consumed or destroyed in the processes. The consumption of the exergy rate in a process is directly related with the entropy generation. There are a lot of studies about Organic Rankine Cycle for power generation from waste heat recovery [1e4,6e17]. Based on the studies of Kas¸ka [1], energy and exergy analysis in a steel plant was performed using actual plant data. He concluded that the energy and 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 to lower as evaporator, turbine, condenser and pump, respectively. In addition, it was observed that evaporator pressure had an important effect on the energy and exergy efficiency in this paper. Gomez et al. [2] implemented energy and exergy analysis for the power plant. They modeled and simulated the system using EES (Engineering Equation Solver) to present the effects of key parameters on the efficiency. The effects of the temperature, pressure and compression ratio on the power plant were observed in their study. They concluded that lower compression pressure ratio (r) caused high thermal efficiency due to more effective regeneration process. Moreover, decrement of the helium temperature at the compressor inlet caused sharp drop in the compression work due to decrement of the specific volume. There are several studies regarding to the working fluid in literature [5,13,17,20]. As understood from these studies, the working fluid using in the ORC cycle has important effect on the performance of the power generation systems. Roy et al. [4] investigated the effect of different working fluid on the efficiency and irreversibility rate on the system. They demonstrated that R123 working fluid used in ORC system, had positive impact on efficiency in turbine. Moreover, R-123 working fluid was observed as the best working fluid among the other working fluid options in their study. Mohanraj et al. [5] presented both experimental and theoretical studies about environment friendly alternative working fluids. They concluded that HC mixtures and R152a were found to be better substitutes for R12 and R134a in domestic refrigeration systems. Furthermore, R290, R1270, R290/R152a, R744 and HC/HFC mixtures were found to be the best long-term alternatives for R22 in air conditioning and heat pump applications. R123 was found to be a convenient alternative to R11, R12 and R22 in chiller applications. R152a and HC mixtures were found to be the best option for
Cons. Sat. pp. rej Dest. Turb. Sgen Cyc. 0
consumed saturation pinch point rejection destruction Turbine entropy generation cycle reference state
Greek symbols temperature difference entropy difference enthalpy difference first law efficiency second law efficiency
DT Ds Dh hI hII
automobile air conditioners. Moreover, they showed that the usage of less harmful refrigerants like R290, R1270, R744 and HC/HFC in air conditioning and heat pump applications played a vital role in the developing countries for reducing the environmental impact of halogenated refrigerants. Long et al. [13] showed that the thermophysical properties of the working fluid had little impact on internal exergy efficiency. However they played an important role in external exergy efficiency. They concluded that the selection of the working fluid strictly depended on evaporation temperature. To achieve high overall exergy efficiency, working fluid that had lower critical temperature must be used in the ORC system. There are limited studies about pinch analysis in literature. Sue and Chuang [6] presented the exergy analyses for a steam cycle system. They predicted the system efficiency more precisely with the help of the pinch point analysis. Based on this study, increasing the pinch points decreased the efficiency of the combined cycle. Moreover, the 10 C pinch temperature increment in the HRSG affected the overall combined cycle efficiency by 0.3%. They noted that the pinch point design value had to be carefully evaluated based on anticipated operating factors in order to obtain an optimum design. Based on another studies about pinch analysis, Li et al. [7] examined the effect of the pinch point temperature difference and the evaporation temperature on the performance of Organic Rankine Cycle. They concluded that some organic working fluids could not reach the maximum net power output because of the low temperature corrosion. According to their results, as the pinch point temperature difference of the evaporator increased, the total heat transfer area decreased first and then increased. On the other hand, cost-effective performance demonstrated the opposite variation tendency. Moreover, the pinch point temperature difference of the evaporator was observed for the optimization cost-effective performance. They found that the results had similarity with different organic working fluids. Even though, there are several studies concerned with the thermodynamic analysis of the ORC systems in power generation plants, there are some details need to be discussed and discovered to improve the energy production from the ORC system in real industry. In this study, a detailed thermodynamic analysis was carried out for an Organic Rankine Cycle which generates electricity using waste heat recovery in a local steel plant. Based on the first and second law of thermodynamics, the energy and exergy analysis was performed for each component of the ORC system. The energy and exergy efficiencies, exergy destruction rate, entropy generation and the effect of the various parameters on the components were observed.
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2. System description In this study, Organic Rankine Cycle is investigated based on thermodynamic analysis which is placed in Adana located in southern of Turkey. The system has 260 kW capacity and specifications of the system components are demonstrated in Table 1. The system involves an evaporator, a condenser, a turbine and a pump as subsystems. The generator, heating and cooling water collectors are accepted as the auxiliary components. The Organic Rankine Cycle produces electricity using waste heat in low temperature in order to reduce the operating costs of company. The working fluid using in the ORC cycle is R245fa which has good thermodynamic properties such as low specific heat and viscosity, low toxicity, low ozone depletion potential, low flammability. Most of the ORC systems are using R245fa as the working fluid have moderate global warming potential of 950, power density, while its lower critical pressure at higher temperature allows for reasonable system thermal efficiency [20]. Owing to the above mentioned properties and the favorable economic conditions, R245fa is a convenient option as working fluid. The properties of the R245fa obtained 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 pressure fluid is compressed to high pressure fluid by a pump as can be seen in Fig. 6 (state 5 to 6). Then high pressure fluid enters and passes through the evaporator. In the evaporator, high pressure fluid [6] has become heated and pressurized vapor [3] using the heat capacity of inlet water (state 1 to 2). After that the heated and pressurized vapor enters in turbine. And it leaves from turbine as low pressure vapor and generates electricity (state 3 to 4). Lastly, the low pressure vapor goes through the condenser, and the working fluid leaves from condenser as saturated liquid (state 4 to 5) and the cycle continues. Some of data are measured on system and remaining data are read the computer aided control panel, directly. Dead state conditions of the working fluid (R245fa) and the water are accepted as 1 bar and 25 C. Before the starting analysis, first step is to measure the mass flow rate of the condenser cooling water [7]. Mass flow rate of the condenser cooling water is measured by GE-PT878 which is ultrasonic flowmeter equipment ranges from ½00 to 7.6 mm with ±1% accuracy. Mass flow rate of the R245fa is calculated from the mass and energy balance equations from condenser cooling water and the mass flow rate of the evaporator is estimated from the first law of thermodynamic. The water collectors on the system are placed in the evaporator and condenser inlet and outlet. Pressure and temperature measurement devices are put on collectors in order to measure the properties of water and the working fluid.
Fig. 1. Saturated liquid property deviation Ref. [19].
3. Thermodynamic analysis of the ORC and assumption The first law of thermodynamics is explained as the conservation of energy, thermodynamically. The total energy which is being constant in the processes can be converted or transferred. The first law of thermodynamics is based on the observations that are proven with experiments. The second law of thermodynamics is described as the quality of energy, thermodynamically. The second law refers the change of the quality of energy during the phase
Fig. 2. Saturated vapor property deviation Ref. [19].
change in the processes. The maximum useful work in a process is called as exergy or irreversibility. Entropy generation is the change of entropy due to irreversibility which causes the increment of the exergy destruction in the system. If the entropy generation is high
Table 1 Specifications of the system components. Components
Model/Type
Capacity
Dimensions
Operating pressure
Evaporator Condenser Turbine Pump
Shell and tube Shell and tube CARRIER PC-51 Grundfos/KB-G-A-E-HOBE
3161 kW (Max) 2885 kW (Max) 272 kW (Max) 267 GPM/50 kW motor
Ø1000 mm 6000 mm Ø900 mm 6000 mm e e
345 345 345 135
psi psi psi psi
(max) (max) (max) (max)
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Fig. 3. Identical properties for R245fa along the critical isotherm (T ¼ Tc) Ref. [19].
during the phase change, the exergy destruction rate is high that high amount of exergy destruction rate is undesirable in the power generation plants. The higher exergy destruction rate means lower useful energy production in the processes. The pinch point analysis is a major term to observe the performance of the ORC for power generation. In pinch point analysis, the feasible energy targets are calculated to minimize energy consumption. Operating conditions and energy supply methods can be formed and heat recovery systems can be optimized by pinch point analysis in order to minimize the energy consumption. In this study, analyses are performed for four different water phases of evaporator inlet in order to investigate the effect of water phase on efficiency. These conditions can be listed as;
Fig. 5. Pressureeentropy change for R245fa Ref. [19].
i. ii. iii. iv.
Saturated liquid, Water mixture (quality 0.3), Water mixture (quality 0.7), Saturated vapor phase of the evaporator inlet.
The data which are obtained and measured in the real operating system are given in Table 3, for first condition (saturated liquid). According to Table 3, mass flow rate of the cooling water through the condenser is measured as 110 kg/s. Mass flow rate of the working fluid is calculated as 10.63 kg/s and evaporator heating water is estimated as 13.48 kg/s with the help of energy balance equation based on first law of thermodynamics. Inlet and outlet temperatures of evaporator which are obtained on the control panel directly are 127 C and 83.9 C, respectively. On the other hand, the condenser cooling water inlet and outlet temperatures are 27.5 C and 32.4 C, respectively. The following assumptions are made in this study; Pressure drops, potential and kinetic energy changes are neglected 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;
X X _ in ¼ _ out Total Mass Inlet ¼ Total Mass Outlet / m m (1) General energy and exergy balance;
X _ ¼ _ f hf Total Energy Initial ¼ Total Energy Final /Q_ þ W m X _ in hin m (2) Fig. 4. Pressureeenthalpy change for R245fa Ref. [19].
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Table 2 The 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
Exergy transfer by heat at the temperature Ts is defined by Eq. (3).
_ _ /Ex heat ¼ Q ð1 T0 =Ts Þ
(3)
TotalExergy Initial ¼ Total Exergy FinalþTotal Exergy Consumed þ TotalExergy Destruction _ þ Ex _ cons þ Ex _ _ ¼ Ex /Ex i
f
D;total
_ ¼ mex _ /Ex
_ D and T0 are the exergy destruction and ambient where the Ex temperature, respectively. When we applied the first and second law of thermodynamic on the system we obtain the results for each component and entire cycle, as described below.
3.1. Energy and exergy balance of evaporator
(4)
Energy and exergy balance equations through the evaporator can be written as Eq. (8).
(5)
_ 1 h1 þ m _ 6 h6 ¼ m _ 2 h2 þ m _ 3 h3 /m
_ 1 and m _ 2 is the mass flow rate of the heating water of the m evaporator.
_ is exergy rate. where the “Ex”
/ex ¼ h h0 T0 ðs s0 Þ
(6)
_ D;evp _ 1 ex1 þ m _ 6 ex6 ¼ m _ 2 ex2 þ m _ 3 ex3 þ Ex /m
Entropy Generation
/Sgen
(8)
(9)
Entropy generation for evaporator can be given as Eq. (10).
. _ D T ¼ Ex 0
(7)
. _ D;evp T /Sgen;evp ¼ Ex 0
(10)
Fig. 6. Schematic representation of ORC system Ref. [21].
Table 3 The thermophysical data for saturated liquid of the evaporator inlet (Condition-1). 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Sat. Liq Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 13.48 13.48 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 533.6 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 1.603 1.119 1.789 1.802 1.135 1.134 0.402 0.467
e e 814.79 294.78 471.68 171.42 71.23 79.71 12.18 48.44
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The exergy efficiency eqn. of evaporator is given by Eq. (11).
_ 3 ðex3 ex6 Þ=m _ 1 ðex1 ex2 Þ /h2;evp ¼ m
(11)
Overall exergy destruction is occurred in the cycle, the overall exergy rate of the system is defined by Eq. (24)
_ _ _ _ _ _ _ /Ex cyc;in ¼ Exevp þ Exturb þ Excond þ Expump þ Excond;rej þ Wturb (24)
3.2. Energy and exergy balance of turbine
The exergy destruction rate based on heat rejection on the condenser using cooling water is calculated by Eq. (25)
Energy and exergy balance equations through the turbine which is assumed as adiabatic, can be written as Eq. (12).
_ _ _ _ _ _ _ /Ex cond;rej ¼ Excyc;in Exevp Exturb Excond Expump Wturb
_ _ 3 h3 ¼ m _ 4 h4 þ W /m turb
(25) (12)
_ 4 is the mass flow rate of the working fluid R245fa. _ 3 and m m
_ _ _ 3 ex3 ¼ m _ 4 ex4 þ W /m turb þ ExD;turb
(13)
Entropy generation for turbine can be given as Eq. (14).
. _ /Sgen;turb ¼ Ex D;turb T0 . _ _ Ex Ex 3 4
(15)
_ _ w;evp ðh1 h2 Þ T0 ðs1 s2 Þ /Ex cyc;in ¼ m
(26)
(27)
The overall cycle efficiency is the ratio of the net turbine power to the net heat transfer rate as seen below:
. _ net Q_ /h1;cyc ¼ W cyc;in
(28)
Q_ cyc;in which is the heat input on the evaporator, can be defined as Eq. (29);
3.3. Energy and exergy balance of condenser Energy and exergy balance equations through the condenser can be calculated with the help of Eq. (16).
_ 4 h4 þ m _ 7 h7 ¼ m _ 5 h5 þ m _ 8 h8 /m
. _ net Ex _ /h2;cyc ¼ W cyc;in The exergy rate of cycle is defined by Eq. (27)
(14)
The exergy efficiency equation of turbine is given by Eq. (15).
_ /h2;turb ¼ W turb
_ The Ex cond;rej; is called as outgoing exergy rate through the ambient. The overall exergy efficiency is defined by Eq. (26)
(16)
_ w;evp ðh1 h2 Þ /Q_ cyc;in ¼ m
(29)
Pinch point temperature is determined with respect to the first law of thermodynamic equations as follows [1],
_ 8 is the mass flow rate of the cooling water of the _ 7 and m m condenser.
_ w;evp h1 hpp ¼ m _ R245fa h3 hR245fa;sat /m
(30)
_ _ 4 ex4 þ m _ 7 ex7 ¼ m _ 5 ex5 þ m _ 8 ex8 þ Ex /m D;cond
_ w;evp hpp h3 ¼ m _ R245fa hR245fa;sat h6 /m
(31)
(17)
Entropy generation for condenser can be given as Eq. (18).
. _ /Sgen;cond ¼ Ex D;cond T0
(18)
The exergy efficiency eqn. of condenser is calculated by Eq. (19).
_ 7 ðex8 ex7 Þ=m _ 4 ðex4 ex5 Þ /h2;cond ¼ m
(19)
hR245fa,sat is called as saturated liquid enthalpy of the working fluid at 95.4 C evaporation temperature while hpp is the pinch point enthalpy of which water circulates in evaporator. According to the above equations, pinch point temperature is calculated as 101.3 C. TR245fa,sat and Tpp are vaporization temperature and pinch point temperature of the evaporator inlet, respectively. DTpp is the difference between TR245fa,sat and Tpp and therefore DTpp is found as 5.9 C.
3.4. Energy and exergy balance of pump
4. Results and discussions
Energy and exergy balance equations through the pump which is assumed as adiabatic, can be written as Eq. (20).
The Organic Rankine Cycle produces electricity using low temperature waste heat. In this study, the thermodynamic analysis is applied based on the first and second law of thermodynamics to calculate exergy destructions and exergy efficiency of the system. In addition, entropy generation is analyzed to demonstrate its effect on system performance directly. Moreover, effect of four different water phase of evaporator inlet on the system efficiency was investigated. The thermophysical properties of the different evaporator inlet phases are shown in Tables 3e6. When the water phase of the evaporator inlet is accepted as saturated liquid, the system produces 2464.7 kW heat from heat source and generates 259.4 kW gross power. Pump consumes 13.8 kW power in order to circulate the working fluid in the system and the net power is found as 245.6 kW. The energy and exergy efficiency of the system are calculated as 9.96% and 47.22%,
_ pump ¼ m _ 5 h5 þ W _ 6 h6 /m
(20)
_ pump ¼ m _ _ 6 ex6 þ Ex _ 5 ex5 þ W /m D;pump
(21)
Entropy generation for pump can be given as Eq. (22)
. _ D;pump T /Sgen;pump ¼ Ex 0
(22)
The exergy efficiency eqn. of pump is calculated by Eq. (23)
. _ _ pump _ Ex /h2;pump ¼ Ex W 5 6
(23)
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Table 4 The 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Water-Mix Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 2.96 2.96 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 1181.55 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 3.229 1.119 1.789 1.802 1.135 1.134 0.401 0.467
e e 663.26 64.86 471.68 171.42 71.23 79.71 12.18 48.44
Table 5 The 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Water-Mix Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 1.44 1.44 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 2056.9 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 5.428 1.119 1.789 1.802 1.135 1.134 0.401 0.467
641.09 31.58 471.68 171.42 71.23 79.71 12.18 48.44
respectively while exergy destruction of the cycle is calculated as 274.4 kW. The highest exergy destruction occurs in the evaporator for whole system with 128.06 kW. The results of Kas¸ka [1] also show same behavior. As illustrated in Fig. 7, exergy rejection at the condenser is calculated as 60.63 kW. Moreover, the exergy efficiencies of evaporator, turbine, condenser, and pump are calculated as 75.37%, 86.39%, 60.51%, and 61.4%, respectively (Fig. 8). When compared with the study of Kas¸ka [1], both results for distribution of the exergy efficiency rate are closer to each other. Water phase of evaporator inlet is assumed as water-mix of which quality is 0.3 at condition-2. The data are given in Table 4 for this condition. In condition 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 condition 1, energetic and exergetic efficiencies of evaporator and cycle are calculated as 65.5% and 41.04%, respectively in condition-2 as exergetic efficiencies are shown in Table 8. These results show that both energy and exergy efficiencies decrease compared with previous condition. As can be seen in Table 7, the exergy destruction in the evaporator for condition-2 is higher than that for condition-1 because of poor heat exchange between heating water and the working fluid through the evaporator.
Water phase of evaporator inlet is accepted as water-mix, of which quality is 0.7 at condition-3. The data are given in Table 5 for condition-3. According to Table 8, exergy destruction in the evaporator for condition-3 is higher than that for condition 1 and 2, hereby the exergy efficiency of this condition is less than that of previous conditions. Exergy destruction and exergy efficiency data
Fig. 7. Exergy destruction for each component at condition-1 (saturated water).
Table 6 The thermophysical data for superheated vapor of evaporator inlet (Condition-4). 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Water-Mix Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 1.04 1.04 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 2716.3 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 7.077 1.119 1.789 1.802 1.135 1.134 0.401 0.467
637.41 22.78 471.68 171.42 71.23 79.71 12.18 48.44
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Table 7 Exergy destruction of the system components for four different phases. Components
Exergy destruction1 (kW)
Exergy destruction2 (kW)
Exergy destruction3 (kW)
Exergy destruction4 (kW)
Evaporator Turbine Condenser Pump Rejection at cond.
128.06 40.85 39.55 5.325 60.63
206.42 40.85 39.55 5.325 60.63
217.53 40.85 39.55 5.325 60.63
222.66 40.85 39.55 5.325 60.63
Table 8 Exergy efficiency of system components and cycle for four different water phases. Components
Exergy efficiency1 (%)
Exergy efficiency2 (%)
Exergy efficiency3 (%)
Exergy efficiency4 (%)
Evaporator Turbine Condenser Pump Cycle
75.37 86.39 60.51 61.40 47.22
65.5 86.39 60.51 61.40 41.04
64.49 86.39 60.51 61.40 40.29
64.18 86.39 60.51 61.40 39.95
Fig. 8. Exergy efficiency for each component and entire cycle at condition-1 (saturated water).
for condition-4 are also demonstrated in Tables 7 and 8, respectively. At condition-4, water phase of evaporator inlet is assumed as saturated-vapor. When compared to the other conditions, maximum exergy destruction and minimum exergy efficiency occur in this condition. The effect of the water phase of the evaporator inlet on exergy destruction and exergy efficiency is illustrated in Figs. 9 and 10.
As the heating water phase in the evaporator inlet changes from liquid to vapor, internal energy of the heating water increases. Because of the higher internal energy of vapor, heat transfer rate from the heating water to R245fa is higher for vapor than that for liquid. As the heat transfer rate increases, exergy rate of the system also increases. Due to the increment of system exergy rate, exergy efficiency decreases.
Fig. 9. Distribution of exergy destruction on the evaporator for four different water phases.
Fig. 10. Distribution of exergy efficiency on the evaporator for four different water phases.
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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 water phases.
In the exergy efficiency formula as can be seen in Eq. (26), the increasing exergy rate of the system with constant net power causes decrement of the exergy efficiency of the system. As the heating water phase in the evaporator inlet changes from liquid to vapor, the exergy rate of the system increases due to increment of the enthalpy and entropy values of the heating water. As a result, using vapor as heating water means higher exergy rate results in higher internal energy, higher enthalpy and higher entropy values.
51
Therefore, the most efficient condition is observed as condition-1 in which water phase of the evaporator inlet is accepted as saturated liquid. Namely, the low exergy rate is observed for this condition compared with the others. Based on the first law of thermodynamics, mass flow rate of the water in the evaporator decreases, as the water phase changes liquid form to vapor form due to the increment of the enthalpy and entropy values during the phase change. Furthermore, entropy generation is investigated in order to obtain the performance of the components of the system. The entropy generation is demonstrated in Fig. 11. The highest entropy generation occurs in evaporator with 0.43 kW followed by turbine, condenser and pump with 0.14 kW, 0.13 kW and 0.02 kW respectively. Fig. 12 shows the entropy generation at different water phase of the evaporator inlet. The highest entropy generation rate occurs in the condition-4 and 3, 2 and 1 follow it. Moreover, Fig. 13 shows the effect of different pinch point temperature on the exergy efficiencies at constant pressure and net power output. When the DTpp reduces, the exergy efficiency increases results in increment of the net power-exergy rate ratio due to the decrement of the exergy rate in the cycle. These variations in the DTpp play important role for the effectiveness of the system. Meanwhile, if the DTpp reduce, the exergy loss of the evaporator will also reduce. It is consistent with results of Sue and Chuang [6]. The effect of the ambient temperature on the system efficiency is observed in the running system. It is observed that the variation of ambient temperature has minor effect on the system efficiency. There is no significant effect of the ambient temperature on the system efficiency, because the system, which is investigated in this article, is located in a closed area. Consequently, the system efficiency increases from 47.22% to 47.45% as the ambient temperature increases from 25 C to 40 C as can be seen in Table 9. Due to the minor increment in the exergy efficiency, the effect of the ambient temperature on the system efficiency is neglected in this study. However, the variation of the ambient temperature has major effect on the system efficiency in another investigation [1]. Moreover, the effect of the mass flow rate on the system efficiency is observed in the running system and exergy efficiency of the system is calculated using these dynamic values theoretically. The mass flow rate of the condenser cooling water is measured by ultrasonic flowmeter about 1 h in order to observe the range of the mass flow rate. The calculations are repeated based on dynamic real system. The mass flow rate ranges from 104 kg/s to 114 kg/s. Fig. 14 shows the effect of the mass flow rate variation of the condenser cooling water on the system efficiency. As the mass flow rate
Fig. 13. The effect of the pinch point temperature on the exergy efficiency at constant net power.
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N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52
Table 9 The effect of the ambient temperature on the system efficiency.
Exergy efficiency (%)
Winter (25 C)
Summer (40 C)
Total changes
47.22
47.45
0.48
temperature on the system efficiency is neglected in this paper. It is observed that the as the pinch point temperature decreases, the exergy rate of the system decreases. Furthermore, the decrement of the exergy rate causes the increment of the exergy efficiency of the system. In order to improve performance of the ORC system, it can be proposed; In the evaporator inlet, saturated liquid form should be used or the quality of the water should approach the zero. the difference between the pinch point temperature and evaporation temperature should be less than 5.9 C. different working fluid type which has better heat-power conversion capacity with less energy consumption, can be used in ORC in order to increase power output. the execution of the thermoeconomic analysis of the ORC cycle can be examined in the future.
References Fig. 14. The effect of the mass flow rate on the exergy efficiency of the system.
decreases, the exergy efficiency increases due to the decrement of the exergy rate with constant net power. An uncertainty analysis is applied for energy and exergy efficiencies of system, using below equations;
wh1 ¼ h1 wh2 ¼ h2
"
"
wW_ net _ net W
wW_ net _ net W
2 þ
2 þ
w 2 m_
_ m
w 2 m_
_ m
þ
þ
w
Dh
2
#1 2
(32)
Dh
w
Dh
Dh
2
þ
w 2 T
T
þ
w 2 Ds
#1 2
Ds (33)
where “w” is the uncertainty amount of dependent variable for the first and second law efficiency. The uncertainty results of energy and exergy efficiencies are calculated as 0.48% and 3% respectively. These results consistent with results of Ege and S¸ahin [18]. 5. Conclusions This study presents a detailed thermodynamic analysis of an ORC cycle in steel industry. The highest entropy generation is observed in the evaporator that is approximately 0.43 kW. The major exergy destruction occurs in the evaporator with about 128.06 kW and followed by turbine, condenser and pump, respectively. The main reasons of the inefficiency occurred in the evaporator are high heat input, water phase of the evaporator inlet. The second law efficiencies of the ORC system for four different conditions are 47.22%, 41.04%, 40.29% and 39.95%, respectively. The exergy efficiency significantly decreases when the water phase changes liquid form to vapor form due to the better heat exchange through the water and the working fluid R245fa in the evaporator. Thus, most efficient water phase of which the evaporator inlet is observed as the saturated liquid form. The effects of the various ambient temperatures and pinch point temperature on the exergy efficiency are also observed, in this study. The system efficiency increases from 47.22% to 47.45% as the ambient temperature increases. Due to the minor increment, the effect of the ambient
€ Kas¸ka, Energy and exergy analysis of an organic Rankine for power gen[1] O. eration from waste heat recovery in steel industry, Energy Convers. Manag. 77 (2014) 108e117. [2] M. Gomez, R. Garcia, J. Gomez, Thermodynamic analysis of a Brayton cycle and Rankine cycle arranged in series exploiting the cold exergy of LNG(liquefied natural gas), Energy 66 (2014) 927e937. [3] J. Bao, L. Zhao, Exergy analysis and parameter study on a novel auto-cascade Rankine cycle, Energy 48 (2012) 539e547. [4] J.P. Roy, M.K. Mishra, A. Misra, Performance analysis of an Organic Rankine Cycle with superheating under different heat source temperature conditions, Appl. Energy 88 (2011) 2995e3004. [5] M. Mohanraj, S. Jayaraj, C. Muraleedharan, Environment friendly alternatives to halogenated refrigerant e a review 3 (2009) 108e119. [6] D. Sue, C. Chuang, Engineering design and exergy analyses for combustion gas turbine based power generation system, Energy 29 (2004) 1183e1205. [7] Y.R. Li, J. Wang, M. Du, Influence of coupled pinch point temperature difference and evaporation temperature on performance of organic Rankine cycle 42 (2012) 503e509. _ Dincer, Exergy and exergoeconomic analyses and optimization [8] R. El-Emam, I. of geothermal organic Rankine cycle, Appl. Therm. Eng. 59 (2013) 435e444. [9] E. Ersayin, L. Ozgener, Performance analysis of combined cycle power plants: a case study, Renew. Sustain. Energy Rev. 43 (2015) 832e842. [10] A. Khaliq, S. Kaushik, Second law based thermodynamic analysis of Brayton/ Rankine combined power cycle with reheat, Appl. Energy 78 (2004) 179e197. [11] H. Yazıcı, R. Selbas¸, Buharlı güç santralinin enerji ve ekserji analizi, Enerji 10 (2011) 1. [12] B. Peris, J. Esbrí, F. Moles, R. Collado, A. Babiloni, Performance evaluation of an Organic Rankine Cycle (ORC) for power applications from low grade heat sources, Appl. Therm. Eng. 75 (2015) 763e769. [13] R. Long, Y.J. Bao, X.M. Huang, W. Liu, Exergy analysis and working fluid selection of organic Rankine cycle for low grade waste heat recovery, Energy 73 (2014) 475e483. [14] U. Lee, K. Park, Y.S. Jeong, S. Lee, C. Han, Design and analysis of a combined Rankine cycle for waste heat recovery of a coal power plant using LNG cryogenic exergy, Ind. Eng. Chem. Res. 53 (2014) 9812e9824. [15] Y. Zhu, Z. Hu, Y. Zhou, L. Jiang, L. Yu, Applicability of entropy, entrancy and exergy analyses yo the optimization of the Organic Rankine Cycle, Energy Convers. Manag. 88 (2014) 267e276. [16] J.L. Wang, L. Zhao, X.D. Wang, An experimental study on the recuperative low temperature solar Rankine cycle using R245fa, Appl. Energy 94 (2012) 34e40. [17] S. Aghahosseini, I. Dıncer, Comparative performance analysis of lowtemperature Organic Rankine Cycle (ORC) using pure and zeotropic working fluids, Appl. Therm. Eng. 54 (2013) 35e42. [18] A. Ege, H.M. S¸ahin, Determination of uncertainties in energy and exergy analysis of a power plant, Energy Convers. Manag. 85 (2014) 399e406. [19] Cool prop web-site. http://www.coolprop.org/v4/Fluids/R245fa.html (accessed June 2015). [20] B. Datla, J. Brasz, Comparing R1233zd and R245fa for low temperature ORC applications, in: International Refrigeration and Air Conditioning Conference, 2014, p. 1524. an, A. Kos¸an. Novel application of [21] A. Durmaz, R. Pugh, S¸. Yazıcı, K. Erdog Organic Rankine Cycle (ORC) technology for waste heat recovery from reheat furnace evaporative cooling system. https://www.tmeic.com/Repository/ Media/AIST_Toscelik_ORC_v9.pdf.
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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 on industrial data N. Filiz Tumen Ozdil*, M. Rıdvan Segmen, Atakan Tantekin Department 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
a b s t r a c t
Article history: Received 20 May 2015 Accepted 25 July 2015 Available online 10 August 2015
In this study, thermodynamic analysis of an Organic Rankine Cycle (ORC) is presented in a local power plant that is located southern of Turkey. The system that is analyzed includes an evaporator, a turbine, a condenser, a pump and a generator as components. System components are analyzed separately using actual plant data and performance cycle. The relationship between pinch point and exergy efficiency is observed. As the pinch point temperature decreases, the exergy efficiency increases due to low exergy destruction 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 water phase of the evaporator inlet, exergy destruction and exergy efficiencies of components and overall system 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 mixture form which has quality 0.7. Lastly, it is calculated as 39.95% for saturated vapor form. Moreover, exergy destruction rates of the system are 520.01 kW for saturated liquid form, 598.39 kW for water mixture form which has quality 0.3, 609.5 kW for water mixture form which has quality 0.7 and 614.63 kW for saturated vapor form. The analyses show that evaporator has important effect on the system efficiency in terms of exergy rate. The evaporator is investigated particularly in order to improve the performance of the overall system. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Organic Rankine Cycle Exergy Thermodynamic analysis Heat recovery
1. Introduction Energy and energy production systems have being the major topic of the thermodynamics in recent years. Energy can not be generated or consumed by itself so the energy production process is become the vital concept in the world. The waste heat recovery is the most suitable source for the energy production due to lack of
€g retmenler Bulvarı 46278 Sk. No:3, * Corresponding author. Yes¸iloba, Mah.O 01180 Seyhan/Adana, Turkey. Tel.: þ90 0 322 455 00 00/2055; fax: þ90 0 322 455 00 09. E-mail addresses: [email protected] (N.F. Tumen Ozdil), mrsegmen@ adanabtu.edu.tr (M.R. Segmen), [email protected] (A. Tantekin). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.079 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
the fossil fuels and global warming. Waste heat recovery process provides the energy conversation and decrement of the thermal pollution. Although the steam turbine is the most common technology in the energy production process, due to necessity of high operational temperature and pressure, it is not suitable for low temperature and pressure condition. Organic Rankine Cycle is generally preferred for the processes having low temperature like T < 150 C. This process named as Organic Rankine Cycle owing to usage of the organic fluid as working fluid instead of water and high pressure steam. The organic fluid that is used in ORC, has high molecular weight liquid with low boiling temperature than water. Exergy is the measurement of the maximum useful work that can be obtained in the system. Therefore, it has become more
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Nomenclature ex _ D Ex h _ m P Q_ s T _ W w
specific exergy (kJ/kg) exergy destruction (Kw) specific enthalpy (kJ/kg) mass flow rate (kg/s) pressure (bar) rate of heat transfer (kW) specific entropy (kJ/kg K) temperature (K) rate of work (kW) uncertainty
Subscripts Evp. evaporator Cond. condenser
important subject than energy in order to specify the useful work. Moreover, the exergy can be called as irreversibility in thermodynamic point of view. Because of the irreversibility, exergy can be consumed or destroyed in the processes. The consumption of the exergy rate in a process is directly related with the entropy generation. There are a lot of studies about Organic Rankine Cycle for power generation from waste heat recovery [1e4,6e17]. Based on the studies of Kas¸ka [1], energy and exergy analysis in a steel plant was performed using actual plant data. He concluded that the energy and 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 to lower as evaporator, turbine, condenser and pump, respectively. In addition, it was observed that evaporator pressure had an important effect on the energy and exergy efficiency in this paper. Gomez et al. [2] implemented energy and exergy analysis for the power plant. They modeled and simulated the system using EES (Engineering Equation Solver) to present the effects of key parameters on the efficiency. The effects of the temperature, pressure and compression ratio on the power plant were observed in their study. They concluded that lower compression pressure ratio (r) caused high thermal efficiency due to more effective regeneration process. Moreover, decrement of the helium temperature at the compressor inlet caused sharp drop in the compression work due to decrement of the specific volume. There are several studies regarding to the working fluid in literature [5,13,17,20]. As understood from these studies, the working fluid using in the ORC cycle has important effect on the performance of the power generation systems. Roy et al. [4] investigated the effect of different working fluid on the efficiency and irreversibility rate on the system. They demonstrated that R123 working fluid used in ORC system, had positive impact on efficiency in turbine. Moreover, R-123 working fluid was observed as the best working fluid among the other working fluid options in their study. Mohanraj et al. [5] presented both experimental and theoretical studies about environment friendly alternative working fluids. They concluded that HC mixtures and R152a were found to be better substitutes for R12 and R134a in domestic refrigeration systems. Furthermore, R290, R1270, R290/R152a, R744 and HC/HFC mixtures were found to be the best long-term alternatives for R22 in air conditioning and heat pump applications. R123 was found to be a convenient alternative to R11, R12 and R22 in chiller applications. R152a and HC mixtures were found to be the best option for
Cons. Sat. pp. rej Dest. Turb. Sgen Cyc. 0
consumed saturation pinch point rejection destruction Turbine entropy generation cycle reference state
Greek symbols temperature difference entropy difference enthalpy difference first law efficiency second law efficiency
DT Ds Dh hI hII
automobile air conditioners. Moreover, they showed that the usage of less harmful refrigerants like R290, R1270, R744 and HC/HFC in air conditioning and heat pump applications played a vital role in the developing countries for reducing the environmental impact of halogenated refrigerants. Long et al. [13] showed that the thermophysical properties of the working fluid had little impact on internal exergy efficiency. However they played an important role in external exergy efficiency. They concluded that the selection of the working fluid strictly depended on evaporation temperature. To achieve high overall exergy efficiency, working fluid that had lower critical temperature must be used in the ORC system. There are limited studies about pinch analysis in literature. Sue and Chuang [6] presented the exergy analyses for a steam cycle system. They predicted the system efficiency more precisely with the help of the pinch point analysis. Based on this study, increasing the pinch points decreased the efficiency of the combined cycle. Moreover, the 10 C pinch temperature increment in the HRSG affected the overall combined cycle efficiency by 0.3%. They noted that the pinch point design value had to be carefully evaluated based on anticipated operating factors in order to obtain an optimum design. Based on another studies about pinch analysis, Li et al. [7] examined the effect of the pinch point temperature difference and the evaporation temperature on the performance of Organic Rankine Cycle. They concluded that some organic working fluids could not reach the maximum net power output because of the low temperature corrosion. According to their results, as the pinch point temperature difference of the evaporator increased, the total heat transfer area decreased first and then increased. On the other hand, cost-effective performance demonstrated the opposite variation tendency. Moreover, the pinch point temperature difference of the evaporator was observed for the optimization cost-effective performance. They found that the results had similarity with different organic working fluids. Even though, there are several studies concerned with the thermodynamic analysis of the ORC systems in power generation plants, there are some details need to be discussed and discovered to improve the energy production from the ORC system in real industry. In this study, a detailed thermodynamic analysis was carried out for an Organic Rankine Cycle which generates electricity using waste heat recovery in a local steel plant. Based on the first and second law of thermodynamics, the energy and exergy analysis was performed for each component of the ORC system. The energy and exergy efficiencies, exergy destruction rate, entropy generation and the effect of the various parameters on the components were observed.
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45
2. System description In this study, Organic Rankine Cycle is investigated based on thermodynamic analysis which is placed in Adana located in southern of Turkey. The system has 260 kW capacity and specifications of the system components are demonstrated in Table 1. The system involves an evaporator, a condenser, a turbine and a pump as subsystems. The generator, heating and cooling water collectors are accepted as the auxiliary components. The Organic Rankine Cycle produces electricity using waste heat in low temperature in order to reduce the operating costs of company. The working fluid using in the ORC cycle is R245fa which has good thermodynamic properties such as low specific heat and viscosity, low toxicity, low ozone depletion potential, low flammability. Most of the ORC systems are using R245fa as the working fluid have moderate global warming potential of 950, power density, while its lower critical pressure at higher temperature allows for reasonable system thermal efficiency [20]. Owing to the above mentioned properties and the favorable economic conditions, R245fa is a convenient option as working fluid. The properties of the R245fa obtained 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 pressure fluid is compressed to high pressure fluid by a pump as can be seen in Fig. 6 (state 5 to 6). Then high pressure fluid enters and passes through the evaporator. In the evaporator, high pressure fluid [6] has become heated and pressurized vapor [3] using the heat capacity of inlet water (state 1 to 2). After that the heated and pressurized vapor enters in turbine. And it leaves from turbine as low pressure vapor and generates electricity (state 3 to 4). Lastly, the low pressure vapor goes through the condenser, and the working fluid leaves from condenser as saturated liquid (state 4 to 5) and the cycle continues. Some of data are measured on system and remaining data are read the computer aided control panel, directly. Dead state conditions of the working fluid (R245fa) and the water are accepted as 1 bar and 25 C. Before the starting analysis, first step is to measure the mass flow rate of the condenser cooling water [7]. Mass flow rate of the condenser cooling water is measured by GE-PT878 which is ultrasonic flowmeter equipment ranges from ½00 to 7.6 mm with ±1% accuracy. Mass flow rate of the R245fa is calculated from the mass and energy balance equations from condenser cooling water and the mass flow rate of the evaporator is estimated from the first law of thermodynamic. The water collectors on the system are placed in the evaporator and condenser inlet and outlet. Pressure and temperature measurement devices are put on collectors in order to measure the properties of water and the working fluid.
Fig. 1. Saturated liquid property deviation Ref. [19].
3. Thermodynamic analysis of the ORC and assumption The first law of thermodynamics is explained as the conservation of energy, thermodynamically. The total energy which is being constant in the processes can be converted or transferred. The first law of thermodynamics is based on the observations that are proven with experiments. The second law of thermodynamics is described as the quality of energy, thermodynamically. The second law refers the change of the quality of energy during the phase
Fig. 2. Saturated vapor property deviation Ref. [19].
change in the processes. The maximum useful work in a process is called as exergy or irreversibility. Entropy generation is the change of entropy due to irreversibility which causes the increment of the exergy destruction in the system. If the entropy generation is high
Table 1 Specifications of the system components. Components
Model/Type
Capacity
Dimensions
Operating pressure
Evaporator Condenser Turbine Pump
Shell and tube Shell and tube CARRIER PC-51 Grundfos/KB-G-A-E-HOBE
3161 kW (Max) 2885 kW (Max) 272 kW (Max) 267 GPM/50 kW motor
Ø1000 mm 6000 mm Ø900 mm 6000 mm e e
345 345 345 135
psi psi psi psi
(max) (max) (max) (max)
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Fig. 3. Identical properties for R245fa along the critical isotherm (T ¼ Tc) Ref. [19].
during the phase change, the exergy destruction rate is high that high amount of exergy destruction rate is undesirable in the power generation plants. The higher exergy destruction rate means lower useful energy production in the processes. The pinch point analysis is a major term to observe the performance of the ORC for power generation. In pinch point analysis, the feasible energy targets are calculated to minimize energy consumption. Operating conditions and energy supply methods can be formed and heat recovery systems can be optimized by pinch point analysis in order to minimize the energy consumption. In this study, analyses are performed for four different water phases of evaporator inlet in order to investigate the effect of water phase on efficiency. These conditions can be listed as;
Fig. 5. Pressureeentropy change for R245fa Ref. [19].
i. ii. iii. iv.
Saturated liquid, Water mixture (quality 0.3), Water mixture (quality 0.7), Saturated vapor phase of the evaporator inlet.
The data which are obtained and measured in the real operating system are given in Table 3, for first condition (saturated liquid). According to Table 3, mass flow rate of the cooling water through the condenser is measured as 110 kg/s. Mass flow rate of the working fluid is calculated as 10.63 kg/s and evaporator heating water is estimated as 13.48 kg/s with the help of energy balance equation based on first law of thermodynamics. Inlet and outlet temperatures of evaporator which are obtained on the control panel directly are 127 C and 83.9 C, respectively. On the other hand, the condenser cooling water inlet and outlet temperatures are 27.5 C and 32.4 C, respectively. The following assumptions are made in this study; Pressure drops, potential and kinetic energy changes are neglected 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;
X X _ in ¼ _ out Total Mass Inlet ¼ Total Mass Outlet / m m (1) General energy and exergy balance;
X _ ¼ _ f hf Total Energy Initial ¼ Total Energy Final /Q_ þ W m X _ in hin m (2) Fig. 4. Pressureeenthalpy change for R245fa Ref. [19].
N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52
47
Table 2 The 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
Exergy transfer by heat at the temperature Ts is defined by Eq. (3).
_ _ /Ex heat ¼ Q ð1 T0 =Ts Þ
(3)
TotalExergy Initial ¼ Total Exergy FinalþTotal Exergy Consumed þ TotalExergy Destruction _ þ Ex _ cons þ Ex _ _ ¼ Ex /Ex i
f
D;total
_ ¼ mex _ /Ex
_ D and T0 are the exergy destruction and ambient where the Ex temperature, respectively. When we applied the first and second law of thermodynamic on the system we obtain the results for each component and entire cycle, as described below.
3.1. Energy and exergy balance of evaporator
(4)
Energy and exergy balance equations through the evaporator can be written as Eq. (8).
(5)
_ 1 h1 þ m _ 6 h6 ¼ m _ 2 h2 þ m _ 3 h3 /m
_ 1 and m _ 2 is the mass flow rate of the heating water of the m evaporator.
_ is exergy rate. where the “Ex”
/ex ¼ h h0 T0 ðs s0 Þ
(6)
_ D;evp _ 1 ex1 þ m _ 6 ex6 ¼ m _ 2 ex2 þ m _ 3 ex3 þ Ex /m
Entropy Generation
/Sgen
(8)
(9)
Entropy generation for evaporator can be given as Eq. (10).
. _ D T ¼ Ex 0
(7)
. _ D;evp T /Sgen;evp ¼ Ex 0
(10)
Fig. 6. Schematic representation of ORC system Ref. [21].
Table 3 The thermophysical data for saturated liquid of the evaporator inlet (Condition-1). 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Sat. Liq Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 13.48 13.48 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 533.6 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 1.603 1.119 1.789 1.802 1.135 1.134 0.402 0.467
e e 814.79 294.78 471.68 171.42 71.23 79.71 12.18 48.44
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The exergy efficiency eqn. of evaporator is given by Eq. (11).
_ 3 ðex3 ex6 Þ=m _ 1 ðex1 ex2 Þ /h2;evp ¼ m
(11)
Overall exergy destruction is occurred in the cycle, the overall exergy rate of the system is defined by Eq. (24)
_ _ _ _ _ _ _ /Ex cyc;in ¼ Exevp þ Exturb þ Excond þ Expump þ Excond;rej þ Wturb (24)
3.2. Energy and exergy balance of turbine
The exergy destruction rate based on heat rejection on the condenser using cooling water is calculated by Eq. (25)
Energy and exergy balance equations through the turbine which is assumed as adiabatic, can be written as Eq. (12).
_ _ _ _ _ _ _ /Ex cond;rej ¼ Excyc;in Exevp Exturb Excond Expump Wturb
_ _ 3 h3 ¼ m _ 4 h4 þ W /m turb
(25) (12)
_ 4 is the mass flow rate of the working fluid R245fa. _ 3 and m m
_ _ _ 3 ex3 ¼ m _ 4 ex4 þ W /m turb þ ExD;turb
(13)
Entropy generation for turbine can be given as Eq. (14).
. _ /Sgen;turb ¼ Ex D;turb T0 . _ _ Ex Ex 3 4
(15)
_ _ w;evp ðh1 h2 Þ T0 ðs1 s2 Þ /Ex cyc;in ¼ m
(26)
(27)
The overall cycle efficiency is the ratio of the net turbine power to the net heat transfer rate as seen below:
. _ net Q_ /h1;cyc ¼ W cyc;in
(28)
Q_ cyc;in which is the heat input on the evaporator, can be defined as Eq. (29);
3.3. Energy and exergy balance of condenser Energy and exergy balance equations through the condenser can be calculated with the help of Eq. (16).
_ 4 h4 þ m _ 7 h7 ¼ m _ 5 h5 þ m _ 8 h8 /m
. _ net Ex _ /h2;cyc ¼ W cyc;in The exergy rate of cycle is defined by Eq. (27)
(14)
The exergy efficiency equation of turbine is given by Eq. (15).
_ /h2;turb ¼ W turb
_ The Ex cond;rej; is called as outgoing exergy rate through the ambient. The overall exergy efficiency is defined by Eq. (26)
(16)
_ w;evp ðh1 h2 Þ /Q_ cyc;in ¼ m
(29)
Pinch point temperature is determined with respect to the first law of thermodynamic equations as follows [1],
_ 8 is the mass flow rate of the cooling water of the _ 7 and m m condenser.
_ w;evp h1 hpp ¼ m _ R245fa h3 hR245fa;sat /m
(30)
_ _ 4 ex4 þ m _ 7 ex7 ¼ m _ 5 ex5 þ m _ 8 ex8 þ Ex /m D;cond
_ w;evp hpp h3 ¼ m _ R245fa hR245fa;sat h6 /m
(31)
(17)
Entropy generation for condenser can be given as Eq. (18).
. _ /Sgen;cond ¼ Ex D;cond T0
(18)
The exergy efficiency eqn. of condenser is calculated by Eq. (19).
_ 7 ðex8 ex7 Þ=m _ 4 ðex4 ex5 Þ /h2;cond ¼ m
(19)
hR245fa,sat is called as saturated liquid enthalpy of the working fluid at 95.4 C evaporation temperature while hpp is the pinch point enthalpy of which water circulates in evaporator. According to the above equations, pinch point temperature is calculated as 101.3 C. TR245fa,sat and Tpp are vaporization temperature and pinch point temperature of the evaporator inlet, respectively. DTpp is the difference between TR245fa,sat and Tpp and therefore DTpp is found as 5.9 C.
3.4. Energy and exergy balance of pump
4. Results and discussions
Energy and exergy balance equations through the pump which is assumed as adiabatic, can be written as Eq. (20).
The Organic Rankine Cycle produces electricity using low temperature waste heat. In this study, the thermodynamic analysis is applied based on the first and second law of thermodynamics to calculate exergy destructions and exergy efficiency of the system. In addition, entropy generation is analyzed to demonstrate its effect on system performance directly. Moreover, effect of four different water phase of evaporator inlet on the system efficiency was investigated. The thermophysical properties of the different evaporator inlet phases are shown in Tables 3e6. When the water phase of the evaporator inlet is accepted as saturated liquid, the system produces 2464.7 kW heat from heat source and generates 259.4 kW gross power. Pump consumes 13.8 kW power in order to circulate the working fluid in the system and the net power is found as 245.6 kW. The energy and exergy efficiency of the system are calculated as 9.96% and 47.22%,
_ pump ¼ m _ 5 h5 þ W _ 6 h6 /m
(20)
_ pump ¼ m _ _ 6 ex6 þ Ex _ 5 ex5 þ W /m D;pump
(21)
Entropy generation for pump can be given as Eq. (22)
. _ D;pump T /Sgen;pump ¼ Ex 0
(22)
The exergy efficiency eqn. of pump is calculated by Eq. (23)
. _ _ pump _ Ex /h2;pump ¼ Ex W 5 6
(23)
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49
Table 4 The 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Water-Mix Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 2.96 2.96 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 1181.55 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 3.229 1.119 1.789 1.802 1.135 1.134 0.401 0.467
e e 663.26 64.86 471.68 171.42 71.23 79.71 12.18 48.44
Table 5 The 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Water-Mix Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 1.44 1.44 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 2056.9 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 5.428 1.119 1.789 1.802 1.135 1.134 0.401 0.467
641.09 31.58 471.68 171.42 71.23 79.71 12.18 48.44
respectively while exergy destruction of the cycle is calculated as 274.4 kW. The highest exergy destruction occurs in the evaporator for whole system with 128.06 kW. The results of Kas¸ka [1] also show same behavior. As illustrated in Fig. 7, exergy rejection at the condenser is calculated as 60.63 kW. Moreover, the exergy efficiencies of evaporator, turbine, condenser, and pump are calculated as 75.37%, 86.39%, 60.51%, and 61.4%, respectively (Fig. 8). When compared with the study of Kas¸ka [1], both results for distribution of the exergy efficiency rate are closer to each other. Water phase of evaporator inlet is assumed as water-mix of which quality is 0.3 at condition-2. The data are given in Table 4 for this condition. In condition 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 condition 1, energetic and exergetic efficiencies of evaporator and cycle are calculated as 65.5% and 41.04%, respectively in condition-2 as exergetic efficiencies are shown in Table 8. These results show that both energy and exergy efficiencies decrease compared with previous condition. As can be seen in Table 7, the exergy destruction in the evaporator for condition-2 is higher than that for condition-1 because of poor heat exchange between heating water and the working fluid through the evaporator.
Water phase of evaporator inlet is accepted as water-mix, of which quality is 0.7 at condition-3. The data are given in Table 5 for condition-3. According to Table 8, exergy destruction in the evaporator for condition-3 is higher than that for condition 1 and 2, hereby the exergy efficiency of this condition is less than that of previous conditions. Exergy destruction and exergy efficiency data
Fig. 7. Exergy destruction for each component at condition-1 (saturated water).
Table 6 The thermophysical data for superheated vapor of evaporator inlet (Condition-4). 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 0 1 2 3 4 5 6 7 8
Water R245fa Water Water R245fa R245fa R245fa R245fa Water Water
Dead State Dead State Water-Mix Comp. Liq Sat-Vapor Sup-Vapor Sat-Liq Comp. Liq Comp. Liq Comp. Liq
e e 1.04 1.04 10.63 10.63 10.63 10.63 110 110
298 298 400 356.9 368.4 326 303 303.17 300.5 305.4
1 1 2.7 2.2 11.4 2.4 2.4 11.4 1.37 1.37
104.9 424.7 2716.3 350.8 471.5 447.1 238.9 239.4 115.1 135.2
0.367 1.781 7.077 1.119 1.789 1.802 1.135 1.134 0.401 0.467
637.41 22.78 471.68 171.42 71.23 79.71 12.18 48.44
50
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Table 7 Exergy destruction of the system components for four different phases. Components
Exergy destruction1 (kW)
Exergy destruction2 (kW)
Exergy destruction3 (kW)
Exergy destruction4 (kW)
Evaporator Turbine Condenser Pump Rejection at cond.
128.06 40.85 39.55 5.325 60.63
206.42 40.85 39.55 5.325 60.63
217.53 40.85 39.55 5.325 60.63
222.66 40.85 39.55 5.325 60.63
Table 8 Exergy efficiency of system components and cycle for four different water phases. Components
Exergy efficiency1 (%)
Exergy efficiency2 (%)
Exergy efficiency3 (%)
Exergy efficiency4 (%)
Evaporator Turbine Condenser Pump Cycle
75.37 86.39 60.51 61.40 47.22
65.5 86.39 60.51 61.40 41.04
64.49 86.39 60.51 61.40 40.29
64.18 86.39 60.51 61.40 39.95
Fig. 8. Exergy efficiency for each component and entire cycle at condition-1 (saturated water).
for condition-4 are also demonstrated in Tables 7 and 8, respectively. At condition-4, water phase of evaporator inlet is assumed as saturated-vapor. When compared to the other conditions, maximum exergy destruction and minimum exergy efficiency occur in this condition. The effect of the water phase of the evaporator inlet on exergy destruction and exergy efficiency is illustrated in Figs. 9 and 10.
As the heating water phase in the evaporator inlet changes from liquid to vapor, internal energy of the heating water increases. Because of the higher internal energy of vapor, heat transfer rate from the heating water to R245fa is higher for vapor than that for liquid. As the heat transfer rate increases, exergy rate of the system also increases. Due to the increment of system exergy rate, exergy efficiency decreases.
Fig. 9. Distribution of exergy destruction on the evaporator for four different water phases.
Fig. 10. Distribution of exergy efficiency on the evaporator for four different water phases.
N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52
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 water phases.
In the exergy efficiency formula as can be seen in Eq. (26), the increasing exergy rate of the system with constant net power causes decrement of the exergy efficiency of the system. As the heating water phase in the evaporator inlet changes from liquid to vapor, the exergy rate of the system increases due to increment of the enthalpy and entropy values of the heating water. As a result, using vapor as heating water means higher exergy rate results in higher internal energy, higher enthalpy and higher entropy values.
51
Therefore, the most efficient condition is observed as condition-1 in which water phase of the evaporator inlet is accepted as saturated liquid. Namely, the low exergy rate is observed for this condition compared with the others. Based on the first law of thermodynamics, mass flow rate of the water in the evaporator decreases, as the water phase changes liquid form to vapor form due to the increment of the enthalpy and entropy values during the phase change. Furthermore, entropy generation is investigated in order to obtain the performance of the components of the system. The entropy generation is demonstrated in Fig. 11. The highest entropy generation occurs in evaporator with 0.43 kW followed by turbine, condenser and pump with 0.14 kW, 0.13 kW and 0.02 kW respectively. Fig. 12 shows the entropy generation at different water phase of the evaporator inlet. The highest entropy generation rate occurs in the condition-4 and 3, 2 and 1 follow it. Moreover, Fig. 13 shows the effect of different pinch point temperature on the exergy efficiencies at constant pressure and net power output. When the DTpp reduces, the exergy efficiency increases results in increment of the net power-exergy rate ratio due to the decrement of the exergy rate in the cycle. These variations in the DTpp play important role for the effectiveness of the system. Meanwhile, if the DTpp reduce, the exergy loss of the evaporator will also reduce. It is consistent with results of Sue and Chuang [6]. The effect of the ambient temperature on the system efficiency is observed in the running system. It is observed that the variation of ambient temperature has minor effect on the system efficiency. There is no significant effect of the ambient temperature on the system efficiency, because the system, which is investigated in this article, is located in a closed area. Consequently, the system efficiency increases from 47.22% to 47.45% as the ambient temperature increases from 25 C to 40 C as can be seen in Table 9. Due to the minor increment in the exergy efficiency, the effect of the ambient temperature on the system efficiency is neglected in this study. However, the variation of the ambient temperature has major effect on the system efficiency in another investigation [1]. Moreover, the effect of the mass flow rate on the system efficiency is observed in the running system and exergy efficiency of the system is calculated using these dynamic values theoretically. The mass flow rate of the condenser cooling water is measured by ultrasonic flowmeter about 1 h in order to observe the range of the mass flow rate. The calculations are repeated based on dynamic real system. The mass flow rate ranges from 104 kg/s to 114 kg/s. Fig. 14 shows the effect of the mass flow rate variation of the condenser cooling water on the system efficiency. As the mass flow rate
Fig. 13. The effect of the pinch point temperature on the exergy efficiency at constant net power.
52
N.F. Tumen Ozdil et al. / Applied Thermal Engineering 91 (2015) 43e52
Table 9 The effect of the ambient temperature on the system efficiency.
Exergy efficiency (%)
Winter (25 C)
Summer (40 C)
Total changes
47.22
47.45
0.48
temperature on the system efficiency is neglected in this paper. It is observed that the as the pinch point temperature decreases, the exergy rate of the system decreases. Furthermore, the decrement of the exergy rate causes the increment of the exergy efficiency of the system. In order to improve performance of the ORC system, it can be proposed; In the evaporator inlet, saturated liquid form should be used or the quality of the water should approach the zero. the difference between the pinch point temperature and evaporation temperature should be less than 5.9 C. different working fluid type which has better heat-power conversion capacity with less energy consumption, can be used in ORC in order to increase power output. the execution of the thermoeconomic analysis of the ORC cycle can be examined in the future.
References Fig. 14. The effect of the mass flow rate on the exergy efficiency of the system.
decreases, the exergy efficiency increases due to the decrement of the exergy rate with constant net power. An uncertainty analysis is applied for energy and exergy efficiencies of system, using below equations;
wh1 ¼ h1 wh2 ¼ h2
"
"
wW_ net _ net W
wW_ net _ net W
2 þ
2 þ
w 2 m_
_ m
w 2 m_
_ m
þ
þ
w
Dh
2
#1 2
(32)
Dh
w
Dh
Dh
2
þ
w 2 T
T
þ
w 2 Ds
#1 2
Ds (33)
where “w” is the uncertainty amount of dependent variable for the first and second law efficiency. The uncertainty results of energy and exergy efficiencies are calculated as 0.48% and 3% respectively. These results consistent with results of Ege and S¸ahin [18]. 5. Conclusions This study presents a detailed thermodynamic analysis of an ORC cycle in steel industry. The highest entropy generation is observed in the evaporator that is approximately 0.43 kW. The major exergy destruction occurs in the evaporator with about 128.06 kW and followed by turbine, condenser and pump, respectively. The main reasons of the inefficiency occurred in the evaporator are high heat input, water phase of the evaporator inlet. The second law efficiencies of the ORC system for four different conditions are 47.22%, 41.04%, 40.29% and 39.95%, respectively. The exergy efficiency significantly decreases when the water phase changes liquid form to vapor form due to the better heat exchange through the water and the working fluid R245fa in the evaporator. Thus, most efficient water phase of which the evaporator inlet is observed as the saturated liquid form. The effects of the various ambient temperatures and pinch point temperature on the exergy efficiency are also observed, in this study. The system efficiency increases from 47.22% to 47.45% as the ambient temperature increases. Due to the minor increment, the effect of the ambient
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