J. Cent. South Univ. (2013) 20: 457–465
DOI: 10.1007/s11771-013-1507-x
Performance analysis and improvement of geothermal binary cycle power plant in oilfield
LI Tai-lu(李太禄)1,2, ZHU Jia-ling(朱家玲) 1,2, ZHANG Wei(张伟) 1,2
1. Key Laboratory of Efficient Utilization of Low and Medium Grade Energy of Ministry of Education
(Tianjin University), Tianjin 300072, China;
2. Tianjin Geothermal Research and Training Center, School of Mechanical Engineering,
Tianjin University, Tianjin 300072, China
Central South University Press and Springer-Verlag Berlin Heidelberg 2013
Abstract: In order to improve the efficiency of a geothermal power plant, oil wells in the high water cut stage were used as geothermal wells, thereby improving the recovery ratio and economic benefit. A new function that reflects both the technical and economic performances was put forward and used as the objective function. An organic Rankine cycle (ORC) was analyzed through the energetic and exergetic analyses, and the reasons for low efficiency were pinpointed. Results indicate that geothermal water directly transferring heat to the working fluid reduces energy dissipation and increases cycle efficiencies. The net power output with an internal heat exchanger (IHE) is averagely 5.3% higher than that without an IHE. R601a and R601 can be used to replace R123 for geothermal water below 110 °C. Moreover, the modified ORC dramatically outperforms the actual one.
Key words: geothermal power generation; organic Rankine cycle; energetic and exergetic analyses; oilfield; internal heat exchanger
1 Introduction
The National Bureau of Statistics of China (NBSC) has estimated that China had a total energy consumption of 3.066 billion tons of standard coal in 2009, among which raw coal, crude oil, natural gas, and other forms account for 70.3%, 18.0%, 3.9%, and 7.8%, respectively [1]. The energy structure in China has been coal- dominated for a long time, thereby resulting in many serious environmental problems, such as global warming, ozone layer destruction and atmospheric pollution.
Among renewable energies, geothermal resources are the forms of energy being used today in various applications. The high temperature geothermal water is the most suitable for commercial production of electricity [2]. Geothermal binary power plants widen the scope of geothermal energy to generate electricity. For low and medium temperature geothermal water, binary cycle power plants can be used. Heat sources between 350 and 450 K reach a thermal efficiency ranging from 5% to 15% [3]. The main problem of low-temperature geothermal power generations is their poor economic efficiency because the drilling cost accounts for more than 50%.
Petroleum industry is one of the most significant energy industries. The average water cut of oilfields in China has reached 81.4% [4]. Oil wells with the water cut above 98% are considered to be unworthy further exploiting, but large quantities of oil are still unexploited. Lots of heat is needed for oil gathering and transportation, and oil boilers are mainly used as the heat source with a low efficiency, and the running costs increase with oil price [5]. Geothermal water over 85 °C can be used to replace oil boilers as the heat source of oil gathering and transportation heat tracing. However, geothermal water often exceeds 110 °C, and the stand-alone heat utilization will result in energy dissipation.
As mentioned above, the economic efficiency of oilfields in the high water cut stage is low. Traditional geothermal power plants show poor economic due to high drilling cost. Oil wells are used as geothermal wells, and the petroleum-associated water is used as the heat source. Geothermal water is first to generate electricity, and then it is used to heat up the backwater of oil gathering and transport the heat tracing. Besides power output and heating tracing, large quantities of crude oil can be recovered. Geothermal power plants in oilfield are much more competitive than the traditional ones, and the payback period can be greatly cut down. Geothermal power generations using organic fluids in recovering low and medium temperature energy sources have attracted much attention in recent years. Hettiarachchi et al [6] presented a design criterion for organic Rankine cycle (ORC) utilizing low temperature geothermal source. The result showed that ammonia was preferable. Borsukiewicz-Gozdur and NOWAK [7] returned the stream of geothermal water from downstream of the evaporator for a repeated passage through that heat exchanger to maximize power output. Working fluids studied were R227ea, RC318, R236fa, R600a and R245ca. Heberle and BRUGGEMANN [8] studied the series and parallel circuits of ORC and heat production for geothermal resources below 450 K by the second law analysis. The working fluids, isopentane, isobutene, R245fa and R227ea, were investigated. Saleh et al [9] used alkanes, fluorinated alkanes, ether and fluorinated ethers as working fluids in ORCs for geothermal power plants operating between 100 and 30 °C. Desai and Bandyopadhyay [10] found that regeneration and turbine bleeding could improve the thermal efficiency of the basic ORC. Kanoglu and BOLATTTURK [11] investigated the exergetic efficiency and loss of each component of a binary cycle system driven by a liquid-dominated geothermal resource of 160 °C. Yari [12–13] analyzed different geothermal cycles and several dry fluids for the ORC. Results showed that R123 with IHE had the highest efficiency. Guo et al [14] investigated different pure organic fluids for the transcritical ORC driven by low temperature geothermal sources (80–120 °C). They recommended R125 as the supercritical fluid. Hua et al [15] presented and analyzed a simplified dual-pressure ammonia-water cycle with 85–110 °C heat source. Wang et al [16] investigated several working fluids for low temperature heat recovery. The results showed that R123 was the best choice. Wang et al [17] established an optimized mathematical model to improve the performance of the power generation driven by low-temperature waste heat.
In this work, an actual geothermal power plant in an oilfield was analyzed based on the data of the power plant. The objective was to improve the efficiency of the plant. Firstly, the reasons for low efficiency were pinpointed. Then, the system performance was calculated numerically through the energetic and exergetic analyses. Finally, a new objective function was put forward and used to optimize the ORC system, and the most suitable working fluid was obtained.
2 System description and modeling
2.1 System description
Figure 1 shows the schematic diagram of an actual geothermal power plant in an oilfield. The system consists of a plate heat exchanger (PHE), a hot water pump, an evaporator, a turbine, a condenser, a feed pump, a cooling pump, and a cooling tower. Geothermal water transfers heat to water. The organic fluid absorbs heat from the water to generate high-pressure vapor in the evaporator, then the vapor flows into the turbine and its enthalpy is converted into work. The vapor exits the turbine and it is led to the condenser where it is liquefied by cooling water. The liquid available at the condenser outlet is pressurized and flows into the evaporator, then a new cycle begins. The ORC can be identified as 1→2→3→4→1. Figure 2 shows the actual ORC power generation.
The schematic diagram and the corresponding T–s diagram of the modified ORC system are shown in Figs. 2 and 3. For the modified power plant, geothermal water directly transfers heat to the working fluid without intermediate heat carrier, so the PHE and the hot water pump are dismantled whereas an IHE is employed. The modified ORC can be identified as 1→2a→2→3→4→4a→1.
The advantages of the modified ORC system are shown as follows: on one hand, direct heat transfer between geothermal water and working fluid can prevent additional heat loss and the temperature decrease of the heat source, and adding an internal heat exchanger (IHE) can reduce the heat addition to the working fluid and lead to a higher efficiency. On the other hand, the dismantling of the hot water pump can make full use of the excessive pressure of geothermal water, thereby decreasing the initial investment and operating cost.
Fig. 1 Schematic diagram of actual ORC geothermal power plant
Fig. 2 Schematic diagram of modified ORC geothermal power plant
Fig. 3 T–s diagram of modified ORC
2.2 Modeling
The energy and exergy analyses based on the first and second laws of thermodynamics were evaluated for the working fluids investigated. For simplicity, the following hypotheses were made:
(1) Geothermal power plants operate steadily.
(2) Saturated vapor is considered at the turbine inlet and saturated liquid at the condenser exit.
(3) The kinetic and potential energy changes are negligible.
(4) The temperature and friction losses are neglected.
The mathematical model is expressed by
Wse=m(h1–h2) (1)
where Wse is the mechanical work of the screw expander, m is the mass flow rate of the working fluid, and h1 and h2 are the incoming and outgoing flow enhalpies of the screw expander, respectively.
Ise=mT0(s2–s1) (2)
where Ise is the irreversibility for the screw expander, T0 is the ambient temperature, and s1 and s2 are the incoming and outgoing flow entropies of the screw expander, respectively.
Qc=m(h2–h3) (3)
Ic=mT0[(s3–s2)–(h3–h2)/TL] (4)
Wpc=m△pc/(ηpcρ) (5)
where Qc is the absolute value of the heat transfer in the condenser, h3 is the enthalpy at the condenser outlet, Ic is the irreversibility of the condenser, S3 is the entropy at the condenser outlet, TL is the temperature of heat carrying fluid in the condenser, Wpc is the power of the cooling pump, △pc is the pressure drop of the cooling water, ηpc is the isentropic efficiency of the cooling pump, and ρ is the density of water.
Wp=m(h4–h3) (6)
Ip=mT0(s4–s3) (7)
where Wp represents the power of the pump, Ip is the irreversibility for the pump, and h4 and s4 are the enthalpy and entropy at the outlet of the pump, respectively.
Qe=m(h1–h4) (8)
Ie=mT0[(s1–s4)–(h1–h4)/TH] (9)
Wpe=m△pe/(ηpeρ) (10)
where Qe represents the absolute value of the heat transfer in the evaporator, Ie is the irreversibility for the evaporator, TH is the temperature of the heat source in the evaporator, Wpe is the pump power of the hot water, △pe is the pressure drop of hot water, and ηpe is the isentropic efficiency of the hot water pump.
Itot=Ise+Ic+Ip+Ie (11)
Wnet=ηgWse–Wp–Wpc–Wpe (12)
ηth=Wnet/Qe (13)
ηex=Wnet/(Wnet+Itot) (14)
where Itot is the total irreversibility, Wnet is the net power output, ηg is the efficiency of the generator, and ηth and ηex are the thermal and exergetic efficiencies, respectively.
Numerical correlations were used to calculate the heat transfer coefficient in the evaporator as follows [18–21].
3 Validation
Numerical solution is validated with the data of Saleh et al [9] for various working fluids based ORC without regenerators in the same operating conditions. The results of present solutions show very good agreement with the results in Ref. [9] as given in Table 1. The differences mainly derive from the selection of equation of state (EOS) that the BACKONE EOS was adopted in Ref. [9] while the fundamental EOS was selected in this work.
4 Results and discussion
4.1 Analysis of basic ORC
Table 2 gives the parameters of an actual plant, which derive from an actual geothermal binary power plant in an oilfield.
Table 3 gives the thermal and exergetic efficiencies in references. It can be seen from Tables 2 and 3 that the efficiencies of the actual power plant are lower than those in the references.
Figure 4 shows the irreversibility of different components for the actual ORC. It can be seen that the irreversibility in the evaporator is the highest, reaching 238.1 kW, followed by the condenser. The irreversibility in the PHE is almost the same as in the turbine, about 135 kW. The irreversibility caused by the feed pump is the lowest, only 3.03 kW. The PHE results in heat dissipation. More seriously, it decreases the heat source temperature, which is extremely disadvantageous to low temperature geothermal resources. Therefore, the more efficient evaporator should be used to dismantle the PHE.
4.2 Selection of working fluids
The selection of working fluids has a significant effect on the system performance. The working fluids used to simulate the performance of ORC system are listed in Table 4. The normal boiling point ranges from –11.96 to 47.59 °C.
Table 1 Validation of numerical model with previous published data for various fluids-based ORC
Table 2 Parameters of actual ORC power plant in oilfield
Table 3 Thermal and exergetic efficiencies in References
4.3 Comparison of ORC with and without PHE
The relevant parameters used in the simulation were derived from the aforementioned actual binary geothermal power plant in oilfield in China, as listed in Table 5. The working fluid is considered saturated at the turbine inlet and saturated liquid at the condenser outlet.
Table 6 shows the optimal evaporating temperature for working fluids with and without PHE. It can be seen that the maximum net power output for the ORC without PHE is averagely 20% higher than that with PHE, For temperature ranges from 82 to 87°C and RC318 shows the highest net power output followed by R601a, whereas it ranges from 92 to 99°C for the cycle without a PHE, and R601a shows the highest net power output followed by RC318. R124 shows the lowest net power in both cases.
Fig. 4 Irreversibility of different components for actual ORC power plant
Therefore, the more efficient evaporator should be used to dismantle the PHE, thereby improving the cycle performance.
4.4 Comparison of ORC with and without IHE
Table 7 shows the net power output versus the evaporating temperature with and without an IHE. The net power output for the ORC with an IHE is averagely 5.30% higher than that without IHE, because the working fluid with high pressure absorbs a small fraction of condensation heat before entering the evaporator.
Table 4 Thermodynamic properties of working fluids
Table 5 Parameters used in simulation under saturated conditions
The working fluids show similar variation trends, R601a is taken as an example to simplify the analysis. The log mean temperature difference (LMTD) and the evaporator heat exchange area of the cycle without an IHE are lower than those with an IHE for the evaporating temperature lower than 65 °C, as opposed to the evaporating temperature higher than 65 °C. The irreversibility of the evaporator and the condenser for the cycle with an IHE is lower than that without IHE,because the IHE reduces the condensation heat load and decreases the mean temperature difference between geothermal water and working fluid in the evaporator. As for the pump and the turbine, the irreversibility for the cycle with an IHE is higher than that without IHE, because an IHE increases the mass flow rate of the working fluid. The irreversibility is proportional to the mass flow rate with other parameters remaining steady. Therefore, the ORC with an IHE effectively increases the system performance.
Table 6 Optimal evaporating temperature for working fluids with and without PHE
Table 7 Optimal evaporating temperature for working fluids with and without IHE
4.5 Optimization of ORC with IHE but without PHE
Figure 5 shows the LMTD versus the evaporating temperature for the ORC with an IHE but without a PHE. It can be seen that the LMTD decreases with the evaporating temperature. For the evaporating temperature lower than 85 °C, R141b has the highest LMTD followed by R113, and RC318 has the lowest LMTD. For the evaporating temperature higher than85 °C, RC318 has the highest LMTD whereas R141b has the lowest LMTD.
Fig. 5 LMTD versus evaporating temperature for ORC with IHE but without PHE
Figure 6 shows the heat exchange area of the evaporator versus the evaporating temperature for the ORC with an IHE but without a PHE. It can be seen that the heat transfer area of the evaporator increases with the evaporating temperature. For the evaporating temperature lower than 85 °C, RC318 has the highest heat transfer area followed by R236fa, and R141b has the lowest heat exchange area. For the evaporating temperature higher than 85 °C, R141b has the highest heat exchange area whereas RC318 has the lowest heat exchange area. It can be concluded from Figs. 5 and 6 that the variation trend is quite contrary, due to the fact that the evaporator heat exchange area is inversely proportional to the LMTD for a specific evaporating temperature.
Fig. 6 Evaporator heat transfer area versus evaporating temperature for ORC with IHE but without PHE
Figure 7 shows the net power output versus the evaporating temperature for the ORC with an IHE but without a PHE. It can be seen that the net power output first increases and then decreases after reaching the maximum. RC318 always shows the highest net power output except that R601a has the highest net power output for the evaporating temperature between 92 and 93 °C whereas R124 invariably exhibits the lowest net power output. There exists an optimal maximum evaporating temperature maximizing the net power output, which varies with working fluid. RC318 presents the maximum net power output of 504.3 kW with 1.69% higher net power output than R601a which shows the second highest net power output.
Fig. 7 Net power output versus evaporating temperature for ORC with IHE but without PHE
Figure 8 shows the ratio of the net power output to the heat transfer area of the evaporator versus the evaporating temperature for the ORC with an IHE but without a PHE. It can be seen that the ratio of the net power output to the heat transfer area of the evaporator first increases and then decreases. The slope suddenly decreases after the turning point and the ratio approximately linearly reduces. There exists an optimal evaporating temperature maximizing the ratio of the net power output to the heat transfer area of the evaporator. The ratio reaches its maximum with an evaporating temperature of about 81 °C, which is consistent for the working fluids investigated. The maximum ratio for RC318 is the highest, reaching 1.61 kW/m2 followed by R601a, R601, and R114. R114 presents the lowest ratio for the evaporating temperature lower than 96 °C whereas R141b is the lowest for the evaporating temperature higher than 96 °C.
Fig. 8 Ratio of net power output to evaporator heat transfer area versus evaporating temperature for ORC with IHE but without PHE
The efficiencies only reflect system technical performance but do not reflect system economic performance, whereas the ratio of the net power output to the evaporator heat exchange area is just the opposite, and both of them are partial. Taken the technical and economic performances into consideration, a new function fobj is put forward and used as the objective function shown as follows:
fobj=(Wnet/Wnet,o)·(ηth/ηth,o)·(Ao/A) (15)
where A is the heat transfer area in the evaporator, and subscribe “o” stands for the original actual geothermal power plant.
Figure 9 shows the objective function versus the evaporating temperature for the ORC with an IHE but without a PHE. It can be seen that the objective function first increases and then decreases after reaching the maximum. All the working fluids reach their maximum at the turning point except that R124 and RC318 get the maximum at the evaporating temperature lower than the turning point. RC318 shows the highest objective function within the scope of this work.
Fig. 9 Objective function versus evaporating temperature for ORC with IHE but without PHE
The maximum objective function and the corresponding parameters of the ORC with an IHE but without a PHE are shown in Table 8. RC318 has the highest objective function value reaching 4.04 followed by R601a and R601; whereas R124 presents the lowest objective function value.
Table 8 Maximum objective function and optimal evaporating temperature for ORC with IHE but without PHE
4.6 Overall analysis of working fluids
The previous analyses aim at increasing the tech-economic performance of the ORC plant with a view of the working fluid thermodynamic properties. Other parameters should also be considered, such as low ODP, low GWP, non-flammable, non-toxic and non-corrosive. Considering the environmental characteristics (ODP value less than 0.2, GWP value less than 1 500), R113 is now banned and R114 has now been phased out due to their high ODP values. RC318 and R236fa should be abandoned because of their high GWP values. In addition, R123 is now being phased out.
Finally, R601a followed by R601 can be used as the most suitable working fluids for low temperature binary cycle geothermal plants with geothermal water lower than 110 °C.
5 Conclusions
1) Direct heat transfer between geothermal water and the working fluid in the evaporator is preferable, improving the system efficiency by about 20.0% compared with indirect heat transfer.
2) The IHE recovering a portion of condensation heat not only lowers the heat load in the condenser but also reduces the heat absorption of unit mass working fluid in the evaporator, thereby increasing the net power output by about 5.3%.
3) R601a and R601 can be used to replace R123 as the working fluid of geothermal power generation for geothermal water below 110 °C.
4) The modified ORC plant outperforms the actual one evidently, which can be widely used in engineering applications.
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(Edited by HE Yun-bin)
Foundation item: Project(2012AA052804) supported by the National High Technology Research and Development Program of China
Received date: 2012–01–04; Accepted date: 2012–05–04
Corresponding author: ZHANG Wei, PhD; Tel: +86–22–27401830; E-mail: zhangweitju@163.com
