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Experimental study on the net efficiency of an Organic Rankine Cycle with single screw expander in different seasons
Energy 165 (2018) 769e775
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Experimental study on the net efficiency of an Organic Rankine Cycle with single screw expander in different seasons Ying-Kun Zhao, Biao Lei*, Yu-Ting Wu, Rui-Ping Zhi, Wei Wang, Hang Guo, Chong-Fang Ma MOE Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Beijing Key Laboratory of Heat Transfer and Energy Conversion, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2018 Received in revised form 1 August 2018 Accepted 4 September 2018 Available online 4 October 2018
The present paper experimentally investigates the thermal performances of an Organic Rankine Cycle (ORC) using a self-developed single screw expander. Experiments were conducted to study the net efficiency of the ORC under different cooling water flows in different seasons. Taking the power consumed by the circulation pump, cooling water pump, cooling tower fan and lubricant oil pump into account, the experimental results showed that the maximum value of produced net power and net efficiency achieved 3.27 kW and 3.04%, respectively, obtained at a cooling water flow of 12 m3 h1 in winter. The experimental results also reported that the shaft power and shaft efficiency of the expander increased gradually with growth of cooling water flow. Under the same evaporating temperature, the performances of the ORC system deteriorated with the increase of ambient temperature, and the net efficiency of the system in summer was decreased by more than 16.45% than that in winter. In addition, the power consumption by the cooling system was the largest factor restricting the net efficiency than the power consumption by other auxiliary machines. Therefore, reducing the power consumption by the cooling system can effectively enhance the net efficiency of the ORC system. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Organic Rankine cycle Net efficiency Single screw expander Cooling system
1. Introduction At present, in order to recover low temperature waste heat, the thermoelectric microgeneratorshe [1], thermochemical recuperations [2] and ORCs (Organic Rankine Cycle) are three important technologies. Among them, ORC is a promising technology because it can effectively convert low temperature heat into power with relatively high efficiency, simple configurations and suitable working pressures. The purpose of an ORC system is producing power. However, the cooling system (condenser fans or water pump and cooling tower fan), lubricant oil system (lubricant oil pump) and the circulation pump always consume a portion of the expander power. Meanwhile, due to the low evaporation temperature and the resulting low thermodynamic efficiency, the heat released by the working fluids in the condenser for per unit of expander power is several times larger than that in traditional steam Rankine cycles. Therefore, the power consumption by the cooling system is also much larger, and it should be given much attention in the study of ORC system. In addition to the cooling system, the circulation pump and
* Corresponding author. E-mail address: [email protected] (B. Lei). https://doi.org/10.1016/j.energy.2018.09.013 0360-5442/© 2018 Elsevier Ltd. All rights reserved.
the lubricant oil pump, which is widely used for lubricating positive-displacement expander, also consume the expander power. Obviously, the net power produced by an ORC should be defined as the expander power minus the power consumed by all auxiliary machines, and the system net efficiency should be also defined as the ratio of the net power to the evaporator load. Recently, scholars have done a lot of works on ORC in which the main concentration is the selection of working fluids [3e5], key components [6e8] or thermal analyses [9e11]. There is very few work concentrating the net efficiency of ORC system. Lots of theoretical works have been carried out to study the ORC. Song et al. [12] built a oneedimensional analysis method of the ORC for industrial waste heat recovery. In their work, only the power consumption by the circulation pump was considered, and the maximum net power and thermodynamic efficiency of the ORC system were 534 kW and 13.5% respectively. Dong et al. [13] analyzed the effects of the heat sink temperature on the performances of an ORC system which uses low-grade heat below 80 C. They also only took the power consumption by the circulation pump into account and they reported that the produced net power and net efficiency of the ORC system were observed to decrease with increasing heat sink temperature. Li et al. [14] also neglected the power consumption by auxiliary machines except the
770
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2 3 4 is sh P W C oil fan evap cond V m
Nomenclature P h p q N n Q
h ε k
power [W, kW] enthalpy [kJ$kg1] pressure [MPa] flow rate [m3$h1, kg$h1] torque [N$m] rotational speed [r$min1] heat [kJ] efficiency[%] expansion ratio power consumption index
Subscripts 1 inlet state of expander
circulation pump and they had come to the similar conclusion with Dong et al. In general, most of the theoretical works noticed the power consumption by the circulation pump, while neglected that consumed by other auxiliary machines. As for experimental investigations, the power consumption by the auxiliary machines did not draw enough attention. Table 1 summarizes recent experimental results which focus on the efficiency of ORC. The table reported that only some of the investigators took the power consumption by the circulation pump into account, and the power consumption by other machines in cooling system was not considered in all the literatures. It is worth mentioning that a few investigators have noticed that the cooling system has large influences on the performances of ORC system and paid special attention to the cooling system. He et al. [31] theoretically examined the effects of ambient temperatures on operational conditions of an air-cooled ORC, and they reported that in order to maintain a turbine back pressure, the heat transfer area of condenser should be increased by 42% when the ambient temperature changes from 304 K to 308 K. Shao et al. [18] presented some experimental results about a water-cooled ORC with different cooling water flows. They reported that the produced power increased from 889.47 W to 1242.67 W with the growth of cooling water flow, and the power consumption by the cooling system was also neglected. Usman et al. [32] made a thermo-economic comparison of air-cooled and cooling tower based ORC for different geographical climate conditions. In general, although a few investigators noticed the importance of cooling system for ORC, there
outlet state of expander outlet state of condenser outlet state of circulating pump isentropic shaft circulating pump water pump cooling system lubricant oil pump cooling tower fan evaporator condenser volume Mass
is still lack of experimental results about the cooling system in ORC, especially those results which took the power consumptions by the cooling system into account. It is even rarer that the power consumptions by auxiliary machines were all considered. In the present work, an experiment on the net efficiency of ORC with varied cooling water flow has been carried out in winter and summer respectively, and the power consumed by the working fluid system (circulation pump), cooling system (water pump and cooling tower fan) and lubricant oil system (lubricant oil pump) were all taken into account. Therefore, the real values of ORC net efficiency were experimentally obtained. 2. Descriptions of the ORC system Fig. 1 gives a schematic diagram of the ORC system which mainly includes four circuits: working fluid circuit (indicated by solid lines), heat source circuit (indicated by dash lines), cold source circuit (indicated by center lines) and lubricant oil circuit (indicated by dash dot lines). A picture of the experimental facilities is presented in Fig. 2, which includes single screw expander, oil separator, evaporator, condenser, circulation pump, conductive oil boiler, water pump and cooling tower. The main parameters of the single screw expander, oil separator, evaporator, circulation pump, conductive oil boiler were provided in Reference [10]. The parameters of the condenser, water pump and cooling tower are presented in Tables 2e4, respectively. R123 was used as the working fluid. The lubricant oil system and
Table 1 List of the experimental study on Net efficiency of ORC in literatures. Authors
Working fluids
Cooling mediums
Expander power
cycle efficiency
Comments
Zheng et al. [15] Twomey et al. [16] Hsieh et al. [17] Shao et al. [18] Li et al. [19] Yamamoto et al. [20] MIAO et al. [21] Landelle et al. [22] Pang et al. [23] Pu et al. [24] Yang et al. [25] Sun et al. [26] Suankramdee et al. [27] Unamba et al. [28] Liu et al. [29] Peris et al. [30]
R245fa R134a R218 R123 R245fa/R601a R123 R123 R134a R245fa/R123 R245fa R245fa R245fa R141b R245fa R123 R245fa
Water Water Water water water Water Water Water Water Water Water Water Water Water Water Air
0.35 kW 0.676 kW e e 0.55 kW 0.15 kW 2.645 kW 6 kW e 1.979 kW 2.64 kW 1.9 kW 0.185 kW 0.6 kW 0.76 kW 15.93 kW
5% 3.47% 5.7% 5.2% 4.45% 1.25% 5.64% 1% 4.4% 4.01% 5.92% 3.01% 1.57% 6% 2.9% 10.88%
Not considering any auxiliary power consumption Not considering any auxiliary power consumption Only the circulation pump consumption considered Only the circulation pump consumption considered Not considering any auxiliary power consumption Only the circulation pump consumption considered Only the circulation pump consumption considered Only the circulation pump consumption considered Only the circulation pump consumption considered Not considering any auxiliary power consumption Only the circulation pump consumption considered Only the circulation pump consumption considered Not considering any auxiliary power consumption Only the circulation pump consumption considered e Only the circulation pump consumption considered
Y.-K. Zhao et al. / Energy 165 (2018) 769e775
771
Fig. 1. Schematic diagram of the ORC system.
Table 3 Parameters of the cooling water pump. Items
Values
Model Rotational speed Flow rate Delivery head Stages Power
TP50-190/2 2870 r$min1 22.2 m3 h1 15.5 m 2 1.5 kW
Table 4 Parameters of the cooling tower.
Fig. 2. A photograph of the experimental ORC system.
pressure losses in the oil separator, condenser, evaporator and pipes were optimized by using the theories developed by Lei et al. [33,34]. The pressures and temperatures at key state points were Table 2 Parameters of the condenser. Items
Values
Number of tube side Number of shell side Heat transfer area Maximum pressure of shell side Maximum pressure of tube side
5 2 21.6 m2 1.6 MPa 2.5 MPa
Items
Values
Model Flow rate Temperature drop Power
DLT-20 20 m3 h1 8 C 0.55 kW
measured by the temperature and pressure sensors, and the data were collected by an Agilent data acquisition instrument and a FC2022 multi-channel data acquisition instrument. The parameters of the sensor are shown in Table 5. 3. Thermodynamic analyses Fig. 3 describes the T-s diagram of the ORC system. The enthalpy, entropy and density of R123 at key state points were calculated by using Refprop 9.0 software developed by NIST. The handling method of experimental data was as follows. The condenser load is obtained by
Qcond ¼ qm ðh2 h3 Þ
(1)
Here qm is the mass flow of R123. h2 and h3 are the enthalpy of R123 at the condenser inlet and outlet, respectively. The condenser
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Table 5 Parameters of the sensors in ORC system.
hsh ¼
Measured Parameter
Measuring range
Accuracy
t1 t2 t3, t5, t6 p1 p2 p3 p4 Mass flow meter of R123 Volume flow meter of water Rotational speed sensor Torque sensor Power meter of circulation pump Power meter of water pump
50e150 C 0e200 C 50e100 C 0e2 MPa 0.1e0.5 MPa 0e1.5 MPa 0.1e1.6 MPa 0e3000 kg h1 4e40m3 h1 0e8000r$min1 0e200 N$m 0e5 kW 0e10 kW
±1 C ±1 C ±0.75 C ±0.25% ±0.5% ±0.1% ±0.5% ±0.2% ±0.5% ±0.5% ±0.5% ±0.5% ±0.5%
Psh qm ðh1 h2s Þ
Here h2s is the enthalpy of R123 at the single screw expander outlet when the R123 is isentropically expanding. The power consumption index k is defined as the ratio of the power consumption by the subsystem to the expander power, respectively. Therefore it is obtained by
ki ¼
Pi Psh
(6)
Here Pi is the power consumed by the R123 circuit (circulation pump), cold source circuit (water pump and cooling tower fan) and lubricant oil circuit (lubricant oil pump). The net power of the ORC system is obtained by
Pnet ¼ Psh PP PC Poil
T
(5)
(7)
The net efficiency of the ORC system is obtained by
luid ce f our s t a He
hnet ¼
Pnet Qevap
(8)
1
4. Experimental results 4s
4
6
4.1. Experimental methodology
2s 2
3
fluid Cold source
5
s
O Fig. 3. T-s diagram of ORC.
is of tube and shell side, and the organic fluids flows in the shell side. In the bottom of the shell side, there is a tank collecting the dropped organic liquid. Therefore, the organic fluid in the condenser is mixture of saturated liquid and saturated gas, and the state of organic fluid at the condenser outlet which is discharged from the bottom of the tank can be considered to be saturated. The evaporator load is obtained by
Qevap ¼ qm ðh1 h4 Þ
(2)
Here h1 and h4 are the enthalpy of R123 at the evaporator outlet and inlet, respectively. The expansion ratio is obtained by
εp ¼
p1 p2
(3)
Here p1 and p2 are the pressure of R123 at the single screw expander inlet and outlet, respectively. The produced power by the single screw expander is obtained by
Psh ¼
N$n 9:549
(4)
Here N and n are the output torque and rotational speed of the single screw expander, respectively. They can be obtained by the sensors located at the shaft between the single screw expander and the AC electrical dynamometer. The shaft efficiency is obtained by
During the experiments, the temperature and flow of the heat source were operated at 180 ± 5 C and 5410 kg h1, respectively. The superheat degree of R123 at the single screw expander inlet was controlled between 2 and 4 C by adjusting the rotational speed of the circulation pump. The single screw expander was operated at 3000 ± 20r$min1 by regulating the electric current through the AC electrical dynamometer. Working with above parameters, the ORC system can work stable with an evaporating pressure of 1.2 ± 0.03 MPa. The cooling water flow was controlled between 8 and 19 m3$h1 by adjusting the rotational speed of the water pump. The cooling tower fan and the lubricant oil pump worked in stable states without changing their speeds. The electric energy consumptions by the cooling tower fan, lubricant oil pump, water pump, circulation pump were all measured. In addition, the output torque and rotation speed of the single screw expander, flow rates of R123 and water, pressures and temperatures at key state points were measured and recorded. The experiments were divided into two groups. One was conducted in summer when the ambient temperature was 32e34 C, and the other one was conducted in winter when the ambient temperature was 0e5 C. 4.2. Experimental results of the heat exchangers The condensation temperatures of R123 with increasing cooling water flow in different seasons are depicted in Fig. 4. The figure presents that the condensation temperature of the ORC system can be decreased by increasing cooling water flow or reducing ambient temperature. The reason of this is obviously the enhanced heat transfer between organic fluid and the ambient with the rise of water flow which worked as a heat transfer medium between them. Therefore, the temperature difference for heat transfer between R123 and the ambient was reduced, and it reduced the condensation temperature. Hoverer, the decline rate of condensation temperature got very slowly after the cooling water flow reached 12 m3 h1. In winter, the condensation temperature decreased from 35.90 C to 27.59 C with the rise of cooling water
Y.-K. Zhao et al. / Energy 165 (2018) 769e775
55 50
tcond
45
summer winter
40 35 30 25
8
10
12
14 3
16
18
20
-1
qV (m ·h ) Fig. 4. Influences of cooling water flow on condensation temperature in different seasons.
flow, and the values were from 55.18 C to 51.76 C in summer. This means the condensation temperature of R123 was 19.28e24.17 C higher in summer than that in winter. It showed that lower ambient temperature was more significant than adjusting the cooling water flow to reduce the condensation temperature. Fig. 5 presents the condenser load and evaporator load as functions of the cooling water flow in different seasons. The figure shows that the condenser load and evaporator load were improved by increasing the cooling water flow or reducing ambient temperature. After the cooling water flow reached 12 m3 h1, the condenser load and evaporator load kept a slow and gradual increase. With the growth of cooling water flow, in the scope of experiments the condenser load and evaporator load increased by 3.51 kW, 3.17 kW in summer, and increased by 9.17 kW and 10.27 kW in winter.
773
the single screw expander in this condition. The single screw expander is very sensitive to the back pressure [35], which was directly influenced by the condensation temperature. In summer, the expansion ratios varied from 4.09 to 4.44, and in winter, the expansion ratios varied from 5.41 to 6.38. Fig. 7 illustrates the shaft power and shaft efficiency of the single screw expander as functions of cooling water flow in different seasons. The figure shows that the shaft power and shaft efficiency were improved by increasing the cooling water flow or reducing ambient temperature. However, the growth rate of the shaft power and shaft efficiency gradually decreased with increasing cooling water flow. With increasing cooling water flow, the performances of the expander was improved, and the maximum values of the expander shaft power and shaft efficiency achieved 6.58 kW and 42.53%, respectively when cooling water flow was 19 m3 h1 in winter. The values were 1.45 kW and 0.62% point higher than those in summer.
4.4. Experimental results of the ORC net efficiency Fig. 8 depicts the power consumed by the working fluid circuit (circulation pump), cold source circuit (water pump and cooling tower fan) and lubricant oil circuit (lubricant oil pump) as functions of the cooling water flow in different seasons. The figure shows that
4.3. Experimental results of the single screw expander Fig. 6 presents the expansion ratios of the single screw expander as a function of the cooling water flow in different seasons. It reports that the expansion ratio was increased with the rise of cooling water flow. The reason is obviously the decreasing back pressure of Fig. 6. Influences of cooling water flow on expansion ratios in different seasons.
Fig. 5. Influences of cooling water flow on condenser load and evaporator load in different seasons.
Fig. 7. Influences of cooling water flow on shaft power and shaft efficiency in different seasons.
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Y.-K. Zhao et al. / Energy 165 (2018) 769e775
Fig. 8. Influences of cooling water flow on power consumped by the subsystems in different seasons.
the power consumed by the circulation pump had a slight growth, while the power consumed by the cooling system showed a parabolic growth with increasing cooling water flow rate. As for the lubricant oil pump and the cooling tower fan, they ran in very stable states, and their rotational speeds were also not changed. Therefore, with the changes of cooling water flow, the power consumed by them almost kept constant. Fig. 9 presents the power consumption index as functions of the cooling water flow in different seasons. The figure shows that the kc had a sharp rise with increasing cooling water flow rate, while the other parameters had a very slow reduction tendency. Moreover, it can be seen that the R123 circuit, cold source circuit and lubricant oil circuit consumed 20.44%e23.23%, 14.77%e41.58% and 4.20%e 4.82% of the expander power in winter, respectively, and consumed 27.33%e31.45%, 21.83%e39.96% and 5.38%e6.28% of the expander power in summer, respectively. This means that the cold source circuit consumed the largest portion of the expander power, and it should get much attention to be paid. Fig. 10 shows the variations of the net power and net efficiency of the ORC system as functions of the cooling water flow in different seasons. The figure presents that the net power and net efficiency of ORC system had a parabolic tendency which increased firstly then
Fig. 10. Influences of cooling water flow on the system net power and net efficiency in different seasons.
decreased with increasing cooling water flow. When the cooling water flow was small, Fig. 7 reported a relatively sharp rise of expander power with the increase of cooling water flow, and Fig. 8 reported the power consumed by the auxiliary machines was quite small. According to Eq. (7) and Eq. (8), the produced net power and net efficiency of the ORC system was improved. However, when the cooling water reached 12 m3 h1, the growth rate of expander power got very small, while the power consumed by the water pump got a sharp rise. This resulted in the reduction of the net power and net efficiency. Therefore, the net power and net efficiency of ORC system increased firstly then decreased. The maximum value of the net power and net efficiency of ORC system were 3.27 kW and 3.04%, respectively, obtained at the cooling water flow of 12 m3 h1 in winter. In terms of net efficiency of the system, it deteriorated by more than 16.45% in summer than in winter under the optimal flow of cooling water. From Figs. 7 and 9, it was known that the shaft power of the single screw expander were 6.39 kW when the cooling water flow was 12 m3 h1 in winter. Meanwhile, the kp, kc and koil were 0.208, 0.237 and 0.043, respectively. This illustrated the power consumption by the cooling system was the largest factor that restricting the cycle net efficiency, and the power consumption by the circulation pump was the second one. Therefore, reducing the power consumption by the cooling system is the most effective way to enhance the net efficiency of ORC. 4.5. Uncertainty analyses According to the theory of error propagation, the uncertainties are calculated using the root-sum-square method. Provided measurements are uncorrelated and random, the uncertainty DY on the variable Y is calculated as function of the uncertainties DXi on each measured variables Xi by Eq. (9).
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 uX vY DY ¼ t DXi vXi i
(9)
It can be obtained that the maximum absolute error of expander power was ±0.19 kW, and the maximum absolute error of net efficiency was ±0.25%。 5. Conclusions Fig. 9. Influences of cooling water flow on power consumption index in different seasons.
An experiment on the net efficiency of ORC with different
Y.-K. Zhao et al. / Energy 165 (2018) 769e775
cooling water flows were carried out in different seasons, and the power consumptions by the circulation pump, water pump, cooling tower fan and lubricant oil pump were all taken into account. Some conclusions can be drawn as follows: (1) Increasing the cooling water flow can reduce the condensation pressure; improve the shaft power and the shaft efficiency of the single screw expander in the scope of experiments. (2) The performances of the ORC system deteriorated with the increase of ambient temperature. In terms of the net efficiency of the system, it deteriorated by more than 16.45% in summer than in winter under the optimal flow of cooling water. (3) There was an optimal value of cooling water flow for getting the best performances of the ORC system, and the value was 12 m3 h1, at which the net power and net efficiency of ORC system achieved 3.27 kW and 3.04%, respectively. (4) The power consumption by the cooling system was the largest factor that restricting the cycle net efficiency, and the power consumption by the circulation pump was the second one. Therefore, reducing the power consumption by the cooling system is the most effective way to enhance the net efficiency of ORC. Acknowledgment This work was supported by the National Natural Science Foundation of China (NO. 51706004). And a part of this work was supported by the National Key R&D Program of China (NO. 2016YFE0124900). References [1] Ando Junior OH, Maran ALO, Henao NC. A review of the development and applications of thermoelectric microgenerators for energy harvesting. Renew Sustain Energy Rev 2018;91:376e93. [2] Tartakovsky L, Sheintuch M. Fuel reforming in internal combustion engines. Prog Energy Combust Sci 2018;67:88e114. [3] Hung TC, Shai TY, Wang SK. A review of organic rankine cycles (ORCs) for the recovery of low-grade waste heat. Energy 1997;22(7):661e7. [4] Maizza V, Maizza A. Unconventional working fluids in organic Rankine-cycles for waste energy recovery systems. Appl Therm Eng 2001;21(3):381e90. [5] Papadopoulos AI, Stijepovic M, Linke P. On the systematic design and selection of optimal working fluids for Organic Rankine Cycles. Appl Therm Eng 2010;30(6e7):760e9. [6] Hajabdollahi H, Ganjehkaviri A, Jaafar MNM. Thermo-economic optimization of RSORC (regenerative solar organic Rankine cycle) considering hourly analysis. Energy 2015;87:369e80. [7] Lecompte S, Huisseune H, Broek MVD, et al. Part load based thermo-economic optimization of the Organic Rankine Cycle (ORC) applied to a combined heat and power (CHP) system. Appl Energy 2013;111(11):871e81. [8] Quoilin S, Declaye S, Tchanche BF, et al. Thermo-economic optimization of waste heat recovery Organic Rankine Cycles. Appl Therm Eng 2011;31(14e15):2885e93. [9] Shen LL, Wang W, Wu YT, et al. A study of clearance height on the performance of single-screw expanders in small-scale organic Rankine cycles. Energy 2018;153:45e55. [10] Lei B, Wang W, Wu YT, et al. Development and experimental study on a single screw expander integrated into an Organic Rankine Cycle. Energy 2016;116:
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Experimental study on the net efficiency of an Organic Rankine Cycle with single screw expander in different seasons Ying-Kun Zhao, Biao Lei*, Yu-Ting Wu, Rui-Ping Zhi, Wei Wang, Hang Guo, Chong-Fang Ma MOE Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Beijing Key Laboratory of Heat Transfer and Energy Conversion, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2018 Received in revised form 1 August 2018 Accepted 4 September 2018 Available online 4 October 2018
The present paper experimentally investigates the thermal performances of an Organic Rankine Cycle (ORC) using a self-developed single screw expander. Experiments were conducted to study the net efficiency of the ORC under different cooling water flows in different seasons. Taking the power consumed by the circulation pump, cooling water pump, cooling tower fan and lubricant oil pump into account, the experimental results showed that the maximum value of produced net power and net efficiency achieved 3.27 kW and 3.04%, respectively, obtained at a cooling water flow of 12 m3 h1 in winter. The experimental results also reported that the shaft power and shaft efficiency of the expander increased gradually with growth of cooling water flow. Under the same evaporating temperature, the performances of the ORC system deteriorated with the increase of ambient temperature, and the net efficiency of the system in summer was decreased by more than 16.45% than that in winter. In addition, the power consumption by the cooling system was the largest factor restricting the net efficiency than the power consumption by other auxiliary machines. Therefore, reducing the power consumption by the cooling system can effectively enhance the net efficiency of the ORC system. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Organic Rankine cycle Net efficiency Single screw expander Cooling system
1. Introduction At present, in order to recover low temperature waste heat, the thermoelectric microgeneratorshe [1], thermochemical recuperations [2] and ORCs (Organic Rankine Cycle) are three important technologies. Among them, ORC is a promising technology because it can effectively convert low temperature heat into power with relatively high efficiency, simple configurations and suitable working pressures. The purpose of an ORC system is producing power. However, the cooling system (condenser fans or water pump and cooling tower fan), lubricant oil system (lubricant oil pump) and the circulation pump always consume a portion of the expander power. Meanwhile, due to the low evaporation temperature and the resulting low thermodynamic efficiency, the heat released by the working fluids in the condenser for per unit of expander power is several times larger than that in traditional steam Rankine cycles. Therefore, the power consumption by the cooling system is also much larger, and it should be given much attention in the study of ORC system. In addition to the cooling system, the circulation pump and
* Corresponding author. E-mail address: [email protected] (B. Lei). https://doi.org/10.1016/j.energy.2018.09.013 0360-5442/© 2018 Elsevier Ltd. All rights reserved.
the lubricant oil pump, which is widely used for lubricating positive-displacement expander, also consume the expander power. Obviously, the net power produced by an ORC should be defined as the expander power minus the power consumed by all auxiliary machines, and the system net efficiency should be also defined as the ratio of the net power to the evaporator load. Recently, scholars have done a lot of works on ORC in which the main concentration is the selection of working fluids [3e5], key components [6e8] or thermal analyses [9e11]. There is very few work concentrating the net efficiency of ORC system. Lots of theoretical works have been carried out to study the ORC. Song et al. [12] built a oneedimensional analysis method of the ORC for industrial waste heat recovery. In their work, only the power consumption by the circulation pump was considered, and the maximum net power and thermodynamic efficiency of the ORC system were 534 kW and 13.5% respectively. Dong et al. [13] analyzed the effects of the heat sink temperature on the performances of an ORC system which uses low-grade heat below 80 C. They also only took the power consumption by the circulation pump into account and they reported that the produced net power and net efficiency of the ORC system were observed to decrease with increasing heat sink temperature. Li et al. [14] also neglected the power consumption by auxiliary machines except the
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2 3 4 is sh P W C oil fan evap cond V m
Nomenclature P h p q N n Q
h ε k
power [W, kW] enthalpy [kJ$kg1] pressure [MPa] flow rate [m3$h1, kg$h1] torque [N$m] rotational speed [r$min1] heat [kJ] efficiency[%] expansion ratio power consumption index
Subscripts 1 inlet state of expander
circulation pump and they had come to the similar conclusion with Dong et al. In general, most of the theoretical works noticed the power consumption by the circulation pump, while neglected that consumed by other auxiliary machines. As for experimental investigations, the power consumption by the auxiliary machines did not draw enough attention. Table 1 summarizes recent experimental results which focus on the efficiency of ORC. The table reported that only some of the investigators took the power consumption by the circulation pump into account, and the power consumption by other machines in cooling system was not considered in all the literatures. It is worth mentioning that a few investigators have noticed that the cooling system has large influences on the performances of ORC system and paid special attention to the cooling system. He et al. [31] theoretically examined the effects of ambient temperatures on operational conditions of an air-cooled ORC, and they reported that in order to maintain a turbine back pressure, the heat transfer area of condenser should be increased by 42% when the ambient temperature changes from 304 K to 308 K. Shao et al. [18] presented some experimental results about a water-cooled ORC with different cooling water flows. They reported that the produced power increased from 889.47 W to 1242.67 W with the growth of cooling water flow, and the power consumption by the cooling system was also neglected. Usman et al. [32] made a thermo-economic comparison of air-cooled and cooling tower based ORC for different geographical climate conditions. In general, although a few investigators noticed the importance of cooling system for ORC, there
outlet state of expander outlet state of condenser outlet state of circulating pump isentropic shaft circulating pump water pump cooling system lubricant oil pump cooling tower fan evaporator condenser volume Mass
is still lack of experimental results about the cooling system in ORC, especially those results which took the power consumptions by the cooling system into account. It is even rarer that the power consumptions by auxiliary machines were all considered. In the present work, an experiment on the net efficiency of ORC with varied cooling water flow has been carried out in winter and summer respectively, and the power consumed by the working fluid system (circulation pump), cooling system (water pump and cooling tower fan) and lubricant oil system (lubricant oil pump) were all taken into account. Therefore, the real values of ORC net efficiency were experimentally obtained. 2. Descriptions of the ORC system Fig. 1 gives a schematic diagram of the ORC system which mainly includes four circuits: working fluid circuit (indicated by solid lines), heat source circuit (indicated by dash lines), cold source circuit (indicated by center lines) and lubricant oil circuit (indicated by dash dot lines). A picture of the experimental facilities is presented in Fig. 2, which includes single screw expander, oil separator, evaporator, condenser, circulation pump, conductive oil boiler, water pump and cooling tower. The main parameters of the single screw expander, oil separator, evaporator, circulation pump, conductive oil boiler were provided in Reference [10]. The parameters of the condenser, water pump and cooling tower are presented in Tables 2e4, respectively. R123 was used as the working fluid. The lubricant oil system and
Table 1 List of the experimental study on Net efficiency of ORC in literatures. Authors
Working fluids
Cooling mediums
Expander power
cycle efficiency
Comments
Zheng et al. [15] Twomey et al. [16] Hsieh et al. [17] Shao et al. [18] Li et al. [19] Yamamoto et al. [20] MIAO et al. [21] Landelle et al. [22] Pang et al. [23] Pu et al. [24] Yang et al. [25] Sun et al. [26] Suankramdee et al. [27] Unamba et al. [28] Liu et al. [29] Peris et al. [30]
R245fa R134a R218 R123 R245fa/R601a R123 R123 R134a R245fa/R123 R245fa R245fa R245fa R141b R245fa R123 R245fa
Water Water Water water water Water Water Water Water Water Water Water Water Water Water Air
0.35 kW 0.676 kW e e 0.55 kW 0.15 kW 2.645 kW 6 kW e 1.979 kW 2.64 kW 1.9 kW 0.185 kW 0.6 kW 0.76 kW 15.93 kW
5% 3.47% 5.7% 5.2% 4.45% 1.25% 5.64% 1% 4.4% 4.01% 5.92% 3.01% 1.57% 6% 2.9% 10.88%
Not considering any auxiliary power consumption Not considering any auxiliary power consumption Only the circulation pump consumption considered Only the circulation pump consumption considered Not considering any auxiliary power consumption Only the circulation pump consumption considered Only the circulation pump consumption considered Only the circulation pump consumption considered Only the circulation pump consumption considered Not considering any auxiliary power consumption Only the circulation pump consumption considered Only the circulation pump consumption considered Not considering any auxiliary power consumption Only the circulation pump consumption considered e Only the circulation pump consumption considered
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771
Fig. 1. Schematic diagram of the ORC system.
Table 3 Parameters of the cooling water pump. Items
Values
Model Rotational speed Flow rate Delivery head Stages Power
TP50-190/2 2870 r$min1 22.2 m3 h1 15.5 m 2 1.5 kW
Table 4 Parameters of the cooling tower.
Fig. 2. A photograph of the experimental ORC system.
pressure losses in the oil separator, condenser, evaporator and pipes were optimized by using the theories developed by Lei et al. [33,34]. The pressures and temperatures at key state points were Table 2 Parameters of the condenser. Items
Values
Number of tube side Number of shell side Heat transfer area Maximum pressure of shell side Maximum pressure of tube side
5 2 21.6 m2 1.6 MPa 2.5 MPa
Items
Values
Model Flow rate Temperature drop Power
DLT-20 20 m3 h1 8 C 0.55 kW
measured by the temperature and pressure sensors, and the data were collected by an Agilent data acquisition instrument and a FC2022 multi-channel data acquisition instrument. The parameters of the sensor are shown in Table 5. 3. Thermodynamic analyses Fig. 3 describes the T-s diagram of the ORC system. The enthalpy, entropy and density of R123 at key state points were calculated by using Refprop 9.0 software developed by NIST. The handling method of experimental data was as follows. The condenser load is obtained by
Qcond ¼ qm ðh2 h3 Þ
(1)
Here qm is the mass flow of R123. h2 and h3 are the enthalpy of R123 at the condenser inlet and outlet, respectively. The condenser
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Table 5 Parameters of the sensors in ORC system.
hsh ¼
Measured Parameter
Measuring range
Accuracy
t1 t2 t3, t5, t6 p1 p2 p3 p4 Mass flow meter of R123 Volume flow meter of water Rotational speed sensor Torque sensor Power meter of circulation pump Power meter of water pump
50e150 C 0e200 C 50e100 C 0e2 MPa 0.1e0.5 MPa 0e1.5 MPa 0.1e1.6 MPa 0e3000 kg h1 4e40m3 h1 0e8000r$min1 0e200 N$m 0e5 kW 0e10 kW
±1 C ±1 C ±0.75 C ±0.25% ±0.5% ±0.1% ±0.5% ±0.2% ±0.5% ±0.5% ±0.5% ±0.5% ±0.5%
Psh qm ðh1 h2s Þ
Here h2s is the enthalpy of R123 at the single screw expander outlet when the R123 is isentropically expanding. The power consumption index k is defined as the ratio of the power consumption by the subsystem to the expander power, respectively. Therefore it is obtained by
ki ¼
Pi Psh
(6)
Here Pi is the power consumed by the R123 circuit (circulation pump), cold source circuit (water pump and cooling tower fan) and lubricant oil circuit (lubricant oil pump). The net power of the ORC system is obtained by
Pnet ¼ Psh PP PC Poil
T
(5)
(7)
The net efficiency of the ORC system is obtained by
luid ce f our s t a He
hnet ¼
Pnet Qevap
(8)
1
4. Experimental results 4s
4
6
4.1. Experimental methodology
2s 2
3
fluid Cold source
5
s
O Fig. 3. T-s diagram of ORC.
is of tube and shell side, and the organic fluids flows in the shell side. In the bottom of the shell side, there is a tank collecting the dropped organic liquid. Therefore, the organic fluid in the condenser is mixture of saturated liquid and saturated gas, and the state of organic fluid at the condenser outlet which is discharged from the bottom of the tank can be considered to be saturated. The evaporator load is obtained by
Qevap ¼ qm ðh1 h4 Þ
(2)
Here h1 and h4 are the enthalpy of R123 at the evaporator outlet and inlet, respectively. The expansion ratio is obtained by
εp ¼
p1 p2
(3)
Here p1 and p2 are the pressure of R123 at the single screw expander inlet and outlet, respectively. The produced power by the single screw expander is obtained by
Psh ¼
N$n 9:549
(4)
Here N and n are the output torque and rotational speed of the single screw expander, respectively. They can be obtained by the sensors located at the shaft between the single screw expander and the AC electrical dynamometer. The shaft efficiency is obtained by
During the experiments, the temperature and flow of the heat source were operated at 180 ± 5 C and 5410 kg h1, respectively. The superheat degree of R123 at the single screw expander inlet was controlled between 2 and 4 C by adjusting the rotational speed of the circulation pump. The single screw expander was operated at 3000 ± 20r$min1 by regulating the electric current through the AC electrical dynamometer. Working with above parameters, the ORC system can work stable with an evaporating pressure of 1.2 ± 0.03 MPa. The cooling water flow was controlled between 8 and 19 m3$h1 by adjusting the rotational speed of the water pump. The cooling tower fan and the lubricant oil pump worked in stable states without changing their speeds. The electric energy consumptions by the cooling tower fan, lubricant oil pump, water pump, circulation pump were all measured. In addition, the output torque and rotation speed of the single screw expander, flow rates of R123 and water, pressures and temperatures at key state points were measured and recorded. The experiments were divided into two groups. One was conducted in summer when the ambient temperature was 32e34 C, and the other one was conducted in winter when the ambient temperature was 0e5 C. 4.2. Experimental results of the heat exchangers The condensation temperatures of R123 with increasing cooling water flow in different seasons are depicted in Fig. 4. The figure presents that the condensation temperature of the ORC system can be decreased by increasing cooling water flow or reducing ambient temperature. The reason of this is obviously the enhanced heat transfer between organic fluid and the ambient with the rise of water flow which worked as a heat transfer medium between them. Therefore, the temperature difference for heat transfer between R123 and the ambient was reduced, and it reduced the condensation temperature. Hoverer, the decline rate of condensation temperature got very slowly after the cooling water flow reached 12 m3 h1. In winter, the condensation temperature decreased from 35.90 C to 27.59 C with the rise of cooling water
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55 50
tcond
45
summer winter
40 35 30 25
8
10
12
14 3
16
18
20
-1
qV (m ·h ) Fig. 4. Influences of cooling water flow on condensation temperature in different seasons.
flow, and the values were from 55.18 C to 51.76 C in summer. This means the condensation temperature of R123 was 19.28e24.17 C higher in summer than that in winter. It showed that lower ambient temperature was more significant than adjusting the cooling water flow to reduce the condensation temperature. Fig. 5 presents the condenser load and evaporator load as functions of the cooling water flow in different seasons. The figure shows that the condenser load and evaporator load were improved by increasing the cooling water flow or reducing ambient temperature. After the cooling water flow reached 12 m3 h1, the condenser load and evaporator load kept a slow and gradual increase. With the growth of cooling water flow, in the scope of experiments the condenser load and evaporator load increased by 3.51 kW, 3.17 kW in summer, and increased by 9.17 kW and 10.27 kW in winter.
773
the single screw expander in this condition. The single screw expander is very sensitive to the back pressure [35], which was directly influenced by the condensation temperature. In summer, the expansion ratios varied from 4.09 to 4.44, and in winter, the expansion ratios varied from 5.41 to 6.38. Fig. 7 illustrates the shaft power and shaft efficiency of the single screw expander as functions of cooling water flow in different seasons. The figure shows that the shaft power and shaft efficiency were improved by increasing the cooling water flow or reducing ambient temperature. However, the growth rate of the shaft power and shaft efficiency gradually decreased with increasing cooling water flow. With increasing cooling water flow, the performances of the expander was improved, and the maximum values of the expander shaft power and shaft efficiency achieved 6.58 kW and 42.53%, respectively when cooling water flow was 19 m3 h1 in winter. The values were 1.45 kW and 0.62% point higher than those in summer.
4.4. Experimental results of the ORC net efficiency Fig. 8 depicts the power consumed by the working fluid circuit (circulation pump), cold source circuit (water pump and cooling tower fan) and lubricant oil circuit (lubricant oil pump) as functions of the cooling water flow in different seasons. The figure shows that
4.3. Experimental results of the single screw expander Fig. 6 presents the expansion ratios of the single screw expander as a function of the cooling water flow in different seasons. It reports that the expansion ratio was increased with the rise of cooling water flow. The reason is obviously the decreasing back pressure of Fig. 6. Influences of cooling water flow on expansion ratios in different seasons.
Fig. 5. Influences of cooling water flow on condenser load and evaporator load in different seasons.
Fig. 7. Influences of cooling water flow on shaft power and shaft efficiency in different seasons.
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Fig. 8. Influences of cooling water flow on power consumped by the subsystems in different seasons.
the power consumed by the circulation pump had a slight growth, while the power consumed by the cooling system showed a parabolic growth with increasing cooling water flow rate. As for the lubricant oil pump and the cooling tower fan, they ran in very stable states, and their rotational speeds were also not changed. Therefore, with the changes of cooling water flow, the power consumed by them almost kept constant. Fig. 9 presents the power consumption index as functions of the cooling water flow in different seasons. The figure shows that the kc had a sharp rise with increasing cooling water flow rate, while the other parameters had a very slow reduction tendency. Moreover, it can be seen that the R123 circuit, cold source circuit and lubricant oil circuit consumed 20.44%e23.23%, 14.77%e41.58% and 4.20%e 4.82% of the expander power in winter, respectively, and consumed 27.33%e31.45%, 21.83%e39.96% and 5.38%e6.28% of the expander power in summer, respectively. This means that the cold source circuit consumed the largest portion of the expander power, and it should get much attention to be paid. Fig. 10 shows the variations of the net power and net efficiency of the ORC system as functions of the cooling water flow in different seasons. The figure presents that the net power and net efficiency of ORC system had a parabolic tendency which increased firstly then
Fig. 10. Influences of cooling water flow on the system net power and net efficiency in different seasons.
decreased with increasing cooling water flow. When the cooling water flow was small, Fig. 7 reported a relatively sharp rise of expander power with the increase of cooling water flow, and Fig. 8 reported the power consumed by the auxiliary machines was quite small. According to Eq. (7) and Eq. (8), the produced net power and net efficiency of the ORC system was improved. However, when the cooling water reached 12 m3 h1, the growth rate of expander power got very small, while the power consumed by the water pump got a sharp rise. This resulted in the reduction of the net power and net efficiency. Therefore, the net power and net efficiency of ORC system increased firstly then decreased. The maximum value of the net power and net efficiency of ORC system were 3.27 kW and 3.04%, respectively, obtained at the cooling water flow of 12 m3 h1 in winter. In terms of net efficiency of the system, it deteriorated by more than 16.45% in summer than in winter under the optimal flow of cooling water. From Figs. 7 and 9, it was known that the shaft power of the single screw expander were 6.39 kW when the cooling water flow was 12 m3 h1 in winter. Meanwhile, the kp, kc and koil were 0.208, 0.237 and 0.043, respectively. This illustrated the power consumption by the cooling system was the largest factor that restricting the cycle net efficiency, and the power consumption by the circulation pump was the second one. Therefore, reducing the power consumption by the cooling system is the most effective way to enhance the net efficiency of ORC. 4.5. Uncertainty analyses According to the theory of error propagation, the uncertainties are calculated using the root-sum-square method. Provided measurements are uncorrelated and random, the uncertainty DY on the variable Y is calculated as function of the uncertainties DXi on each measured variables Xi by Eq. (9).
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 uX vY DY ¼ t DXi vXi i
(9)
It can be obtained that the maximum absolute error of expander power was ±0.19 kW, and the maximum absolute error of net efficiency was ±0.25%。 5. Conclusions Fig. 9. Influences of cooling water flow on power consumption index in different seasons.
An experiment on the net efficiency of ORC with different
Y.-K. Zhao et al. / Energy 165 (2018) 769e775
cooling water flows were carried out in different seasons, and the power consumptions by the circulation pump, water pump, cooling tower fan and lubricant oil pump were all taken into account. Some conclusions can be drawn as follows: (1) Increasing the cooling water flow can reduce the condensation pressure; improve the shaft power and the shaft efficiency of the single screw expander in the scope of experiments. (2) The performances of the ORC system deteriorated with the increase of ambient temperature. In terms of the net efficiency of the system, it deteriorated by more than 16.45% in summer than in winter under the optimal flow of cooling water. (3) There was an optimal value of cooling water flow for getting the best performances of the ORC system, and the value was 12 m3 h1, at which the net power and net efficiency of ORC system achieved 3.27 kW and 3.04%, respectively. (4) The power consumption by the cooling system was the largest factor that restricting the cycle net efficiency, and the power consumption by the circulation pump was the second one. Therefore, reducing the power consumption by the cooling system is the most effective way to enhance the net efficiency of ORC. Acknowledgment This work was supported by the National Natural Science Foundation of China (NO. 51706004). And a part of this work was supported by the National Key R&D Program of China (NO. 2016YFE0124900). References [1] Ando Junior OH, Maran ALO, Henao NC. A review of the development and applications of thermoelectric microgenerators for energy harvesting. Renew Sustain Energy Rev 2018;91:376e93. [2] Tartakovsky L, Sheintuch M. Fuel reforming in internal combustion engines. Prog Energy Combust Sci 2018;67:88e114. [3] Hung TC, Shai TY, Wang SK. A review of organic rankine cycles (ORCs) for the recovery of low-grade waste heat. Energy 1997;22(7):661e7. [4] Maizza V, Maizza A. Unconventional working fluids in organic Rankine-cycles for waste energy recovery systems. Appl Therm Eng 2001;21(3):381e90. [5] Papadopoulos AI, Stijepovic M, Linke P. On the systematic design and selection of optimal working fluids for Organic Rankine Cycles. Appl Therm Eng 2010;30(6e7):760e9. [6] Hajabdollahi H, Ganjehkaviri A, Jaafar MNM. Thermo-economic optimization of RSORC (regenerative solar organic Rankine cycle) considering hourly analysis. Energy 2015;87:369e80. [7] Lecompte S, Huisseune H, Broek MVD, et al. Part load based thermo-economic optimization of the Organic Rankine Cycle (ORC) applied to a combined heat and power (CHP) system. Appl Energy 2013;111(11):871e81. [8] Quoilin S, Declaye S, Tchanche BF, et al. Thermo-economic optimization of waste heat recovery Organic Rankine Cycles. Appl Therm Eng 2011;31(14e15):2885e93. [9] Shen LL, Wang W, Wu YT, et al. A study of clearance height on the performance of single-screw expanders in small-scale organic Rankine cycles. Energy 2018;153:45e55. [10] Lei B, Wang W, Wu YT, et al. Development and experimental study on a single screw expander integrated into an Organic Rankine Cycle. Energy 2016;116:
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