system improvement and thermodynamic optimization of...
TRANSCRIPT
GRC Transactions, Vol. 38, 2014
797
System Improvement and Thermodynamic Optimization of an Organic rankine Cycle based Geo-Plant in an Oilfield in China
Tailu Li1, Qiulin Wang2, Jialing Zhu2, and Wencheng Fu3
1School of Energy and Safety Engineering, Tianjin Chengjian University, Tianjin, Pr China 2Key Laboratory of Efficient Utilization of Low and Medium Grade Energy,
MOE, Tianjin University, Tianjin, Pr China3School of Automation, Tianjin University of Technology, Tianjin, Pr China
KeywordsGeo-plant, Organic Rankine cycle, Energy and exergy analy-ses, Oilfield, Internal heat exchanger
AbSTrACT
With respect to an actual geo-plant in China, the authors have identified four critical measures. They are, removing the plate heat exchangers (PHEs), installing an internal heat exchanger (IHE), adding a preheater, and replacing the working fluid, are proposed, aiming to improve the system performance. Starting from the original system, the four measures are implemented in a step-wise manner. The modified system is compared with the original one by performing the energy and exergy analyses. The results indicate that the four measures are effective to the system improvement, and the detailed findings are as follows: (1) Heat source directly transferring heat to the working fluid is preferable, especially for the low- and medium-grade geothermal water. (2) Installing an IHE reduces the power consumption of the cooling pump. (3) Installing an IHE or adding a preheater can enhance the specific enthalpy at the evaporator inlet, thereby improving the system performance. (3) Isopentane exhibits the most attractive performance and its flammability can be suppressed by mixing it with R245fa. (4) The optimal R245fa mass fraction recommended in the mixture is 0.45 in engineering applications. Overall, the modified system is shown to be able to improve the net power output from 270 to 458.3kW (an increase of about 70%). The conclusions made in this study are not limited to the specific geo-plant considered, and might be extended to the design and operation of other geo-plants alike.
1 Introduction
Energy consumption in China continues to rise and the total energy consumption in 2009, 2010, and 2011 was about 3.07, 3.25, and 3.48 billion tons of standard coal, whereas the total energy production was only about 2.75, 2.97, and 3.18 billion tons [1]. The enormous gap relies heavily on import, however,
the unpredictable international political situation brings great danger to the country, and the nation has taken steps to achieve the self-sufficiency in energy. The geothermal energy, solar radiation, biomass combustion, and industrial waste heat have already attracted attention. Due to the low specific enthalpy of such heat sources, the organic Rankine cycle (ORC) has been proven as an effective way of converting low-grade thermal heat into electricity [2-4].
During the past few decades, researchers have done a lot of work in order to increase the thermal efficiency of the ORC. The corresponding studies focusing on ORC can be summarized into five categories:
(1) Modifying the system configuration. Kanoglu [5] ana-lyzed a dual-level binary geothermal plant. Gnutek and Bryszewska-Mazurek [6] proposed a multi-cycle ORC engine. Mago et al. [7], and Roy and Misra [8] analyzed a regenerative ORC producing a higher efficiency compared with the basic ORC. Clemente et al. [9] investigated scroll expanders derived from the compressors in the HVAC field to recover heat from an internal combustion engine. Borsukiewicz-Gozdur and Nowak [10] maximized the working fluid flow as a way of increasing power output of geothermal power plant. Yari [11] compared a simple ORC, an ORC with an IHE, a regenerative ORC, and a regenerative ORC with an IHE. Zhu and Zhang [12], Douglas and Gregory [13], and Lin and Luo [14] installed an IHE to improve the system thermal efficiency. Wang et al. [15] analyzed a novel system combining a dual loop ORC with a gasoline engine. Chacartegui et al. [16] pro-posed an alternative ORC bottoming cycles for combined cycle power plants.
(2) Screening the suitable working fluid. Badr et al. [17] considered thermodynamic and thermophysical proper-ties of organic working fluids for Rankine-cycle engines. Saleh et al. [18] showed that fluids with relatively low critical temperatures are to be preferred. Desai et al. [19] presented that dry fluids were preferable. Hung et al. [20]
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showed that wet fluids with very steep saturated vapor curves in the T-s diagram had a better overall performance in energy conversion efficiencies. Hettiarachchi et al. [21] showed that ammonia was to be preferred. Roy et al. [22], Yamamoto et al. [23], and Yari [11, 24] proposed that R123 was preferable. Chen et al. [25] indicated that zeotropic mixtures had a higher efficiency. Tchanche et al. [26] found that R134a appeared to be the most suitable for small-scale solar applications. Lakew et al. [27] found that R227ea gave the highest power for the heat source temperature range of 80-160℃ and R245fa produced the highest in the range of 160-200℃. Moreover, Borsukiewicz-Gozdur and Nowak [10, 28], Guo et al. [29], Quoilin et al. [30] and Li et al. [31] studied different working fluids in different heat source temperature ranges. Wang et al. [32] did an experimental study on the recuperative low temperature solar Rankine cycle using R245fa. Wei et al. [33] did a system performance analysis and optimized an ORC system using R245fa.
(3) Optimizing the cycle parameters. Mago et al. [7] indicated that superheating the working fluid led to an increase of irreversibilities in exchangers and a decrease in efficiency. Liu et al. [34] found that the thermal efficiency showed a weak function of the critical temperature and that the maximum total heat recovery occured at the appropriate evaporating temperature. Gu and Sato [35], Schuster et al. [36], and Chen et al. [25] investigated the supercriti-cal ORC using different working fluids. Chen et al. [37], Baik et al. [38-40], Chen et al. [41], Shengjun et al. [42], Vélez et al. [43], and Cayer et al. [44-45] studied the transcritical ORC. Li et al. [46] did an optimization of low temperature solar thermal electric generation with ORC in different areas.
(4) Investigating and optimizing the cogeneration system. Ko-sugi et al. [47] presented an economic feasibility study for a natural gas-fired combined heat and power (CHP) facil-ity in a Chinese industrial area and developed a model to optimize the installation capacity. Khan et al. [48] studied a cogeneration system with thermal energy storage both from technical and economical points of view. Maid-ment and Tozer [49] described the cooling/heating/power requirments of a typical supermarket. Nafety and Sharaf [50] proposed a system combined solar ORC with reverse osmosis desalination process. Heberle and Brüggemann [51] considered the option of CHP for geothermal resources below 450K. Tempesti et al. [52] analyzed a micro CHP plant operating through an ORC using renewable energy.
(5) Studying the system techo-economic performance. Sahoo [53] did an exergoeconomic analysis and optimization of a cogeneration system using evolutionary programming. Arslan and Kose [54] conducted an exergoeconomic op-timization of integrated geothermal system. Sharaf et al. [55] evaluated a thermo-economic analysis of a combined solar ORC with multi-effect distillation desalination pro-cess. Li et al. [56] carried out an exergoeconomic analysis and performance optimization of a condenser for a binary mixture of vapors in the ORC system.
The present study analyzes the reason for the low efficiency of an actual geo-plant in an oilfield in China and attempts to pro-pose some schemes to improve the system. R123, R245fa, and isopentane are selected as the working fluid. Moreover, R245fa is employed to mix with isopentane to suppress the flammability of isopentane. The cycle performance of the modified system is compared with that of the original one.
2 System Modeling2.1 System Description
Fig. 1 shows the schematic diagram of the original and modi-fied geo-plants. The original system consists of a production well, two PHEs, a hot water pump, an evaporator, a screw expander, a generator, a condenser, a pump, a cooling pump, and a cooling tower. Four fluids in the system can be identified as follows:
a) Geothermal water: a→b.
b) Hot water: a’→b’→a’.
c) Working fluid: 1→2→3→4→1.
d) Cooling water: e→f→e.
Due to the defects of the original power plant, some improve-ment schemes are proposed to improve the system performance of the power plant, and the details are as follows:
(a) Removing the PHEsFor the original system, indirect heat transfer between geother-
mal water and the working fluid was adopted, that is, geothermal water→hot water→working fluid. The temperature difference between geothermal water (110-90°C) and hot water (80-100°C) is 10°C. The PHEs decrease the available temperature of the heat source, thereby reducing the net power output. It should be pointed out that direct heat transfer between the geothermal water and the working fluid is the most desirable.
(b) Installing an IHEThe working fluid at the screw expander outlet has a higher
temperature than that at the condenser outlet. Installing an IHE can recover a portion of heat that should be liquefied by cooling water, thereby reducing the power consumption caused by the cooling tower and the cooling pump. On the other hand, installing an IHE can enhance the temperature of the working fluid at the inlet of the evaporator.
(c) Adding a PreheaterCascade utilization of geothermal water was adopted,
namely, geothermal water is first used to generate electricity, and then it goes into an oil gathering and transportation heat tracing station (OGTHTS), and finally it is reinjected. Tail wa-ter at the outlet of the OGTHTS is about 55℃, which is much higher than the temperature of working fluid at the inlet of the evaporator. The advantage of installing a preheater is that the heat absorption per unit mass will be reduced, thereby increas-ing the mass flow rate of the working fluid. More net power output can be generated.
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(d) Using Zeotropic MixturesThe original plant employs R123 as the working fluid,
the data under the design and operating conditions indi-cate that thermodynamic properties of R123 is not good as other substances. So other available fluids, especially the zeotropic mixtures, can be used in order to increase the system performance.
The modified system consists of a production well, a preheater, an evaporator, a screw expander, a generator, a condenser, a pump, a cooling pump, an IHE and a cooling tower. The geothermal water and the cooling water are the same as the original plant. The tail water and working fluid for the modified system can be identified as follows:
a) Tail water: c→d.b) Working fluid: 1→2a→2→3→4a→4b→4→1.Compared with the original system, the modified one
removes the PHEs but adds an IHE and a preheater. The T-s diagram of the original and modified ORC systems are shown in Fig. 2.
The advantages of the modified system include: geothermal water directly transfers heat to the working fluid, thereby preventing additional heat loss and the temperature decrease. On the other hand, adding an IHE reduces the heat load of the condenser, thereby lowering the power consumption of the cooling tower and the cool-ing pump. Adding a preheater makes a better use of the tail water and increases the temperature of the working fluid at the inlet of the evaporator, thereby increasing the system technical performance. The IHE and the preheater in the modified system have almost the same area as the PHE in the original one. The initial investments of the two systems can be regarded as nearly equaled. After removing the hot water pump,
the operating cost for the modified system will be lower than the original one.
2.2 Governing EquationsThe energetic and exergetic analyses based on the first and second laws of thermodynamics are performed for the working fluids investigated. For simplicity, the following assumptions are made:
(1) Geo-plants are operated in a steady state.
(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 thermal loss and the friction loss in the pipes are neglected. There are only two pressures: an evaporating pressure pe, and a condensing pressure pc.
(5) The isentropic efficiency of the screw expander ηse, the pumpηp, the hot water pumpηp,hw, and the cooling water pump ηp,cw is set to be 0.6, 0.6, 0.75, and 0.75, respectively.
(6) Electrical generator efficiency is taken as 0.90.
(7) Atmospheric condition is taken as 0.101325MPa and 25°C.
(8) Pinch point (PP) in the evaporator in 5°C.
(9) The temperature at the screw expander outlet (t2) and the condenser outlet (t3) is 45, and 35℃, respectively.
PHEEvaporator
Condenser
Screwexpander
Generator
Cooling tower
Pump Cooling pump
2
3 4
1
1
2
3 4a
2a 4b 4
Evaporator
IHE
Generator
Pump Cooling pump
Cooling towerCondenser
(a) The original geo-plant
Hot water pump
f
e
f
e
a
b
a
b
c
a'
b'
d
Production well
Production well
(b)The modified geo-plant
Preheater
c' e d
d'
Screwexpander
Figure 1. Schematic diagram of the geo-plant.
s (kJ/kg-K)
T(°
C)
Pc
1
2
3
4
Geothermal water
Cooling water
ab
ef
Pe
Hot water
a'
b'
(a)The original geo-plant
s (kJ/kg-K)
T(°
C)
Pc
1
2a
3
4b
Cooling water
ab
ef
PeTail waterd
c
(b)The modified geo-plant
24a
4
Figure 2. T-s diagram of the geo-plant.
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2.2.1 The Original System
PHE
mgw(ha-hb)= mhw(ha’-hb’) (1)IPHE=mgw[(ha-hb)-T0(sa-sb)]-mhw[(ha’-hb’)-T0(sa’-sb’)] (2)
Hot Water Pump
Wp,hw=mhwphw/(ηp,hwρhw) (3)Ihw=mhwT0(sb’-sc’) (4)
Evaporator
Qe=mwf(h1-h4) (5)Ie=mhw[(ha’-hc’)-T0(sa’-sc’)]-mwf[(h1-h4)-T0(s1-s4)] (6)
Screw Expander
ηse=(h1-h2)/ (h1-h2s) (7)Wse=mwf(h1-h2s)ηse=mwf(h1-h2) (8)
Ise=mwfT0(s2-s1) (9)
Condenser
Qc=mwf(h2-h3) (10)Ic=mwf[(h3-h2)-T0(s3-s2)]-mcw[(hf-hd)-T0(sf-sd)] (11)
Pump
Wp=mwf(pe-pc)/(ηpρwf) (12)Ip=mwfT0(s4-s3) (13)
Cooling Pump
Wp,cw=mcwΔpcw/(ηp,cwρcw) (14)Icw=mcwT0(sd-se) (15)
Total Irreversibility
Itot=IPHE+Ip,hw+Ise+Ic+Ip+Ie+Ip,cw (16)
Net Power Output
Wnet=ηgWse-Wp-Wp,cw-Wp,gw (17)
Thermal Efficiency
ηth=Wnet/Qe (18)
Exergetic Efficiency
Exa=mgw[(ha-h0)-T0(sa-s0)] (19)Exb=mgw[(hb-h0)- T0(sb-s0)] (20)ηex=Wnet/(Exa-Exb) (21)
2.2.2 The Modified SystemPreheater
mgw(hc-hd)= mwf(h4-h4b) (22)IPreheater=mgw[(hc-hd)-T0(sc-sd)]-mwf[(h4-h4b)-T0(s4-s4b)] (23)
Evaporator
mgw(ha-hb)=mwf(h1-h4) (24)Ie=mgw[(ha-hb)-T0(sa-sb)]-mwf[(h1-h4)-T0(s1-s4)] (25)
Screw Expander
ηse=(h1-h2a)/ (h1-h2a,s) (26)Wse=mwf(h1-h2a,s)ηse=mwf(h1-h2a) (27)
Ise=mwfT0(s2a-s1) (28)
IHE
h2a-h2=h4b-h4a (29)IIHE=mgwT0(s2+s4a-s2a-s4b) (30)
Condenser
Qc=mwf(h2-h3) (31)Ic=mwf[(h3-h2)-T0(s3-s2)]-mcw[(hf-hd’)-T0(sf-sd’)] (32)
Pump
Wp=mwf(pe-pc)/(ηpρwf) (33)Ip=mwfT0(s4a-s3) (34)
Cooling Pump
Wp,cw=mcw∆pcw/(ηp,cwρcw) (35)Icw=mcwT0(sd’-se) (36)
Total Irreversibility
Itot=IPreheater+Ie+Ise+Ic+Ip+IIHE+Icw (37)
Net Power Output
Wnet=ηgWse-Wp-Wp,cw-Wp,gw (38)
Thermal Efficiency
ηth=Wnet/Qe (39)
Exergetic Efficiency
Exa=mgw[(ha-h0)-T0(sa-s0)] (40)Exb=mgw[(hb-h0)- T0(sb-s0)] (41)Exc=mgw[(hc-h0)-T0(sc-s0)] (42)Exd=mgw[(hd-h0)- T0(sd-s0)] (43)ηex=Wnet/(Exa-Exb+Exc-Exd) (44)
3 Validation
The results are validated by the data of Saleh et al. [18] for various working fluids-based ORC without regenerator and for the same operating conditions. The results of present solutions show a very good agreement with the results given in the Ref. [18], as shown in Table 1. The differences mainly come from the selection of equation of state (EOS) that the BACKONE EOS was adopted in the Reference, while the fundamental EOS was used in this paper.
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4 results and Discussion4.1 Original Geo-Plant
Table 2 shows the system parameter of the original geo-plant in an oilfield in China. Fig. 3 shows the design and operating efficiency of the original geo-plant. The original plant operates intermittently, with only 240 days of operating data available. Among the 240 days, the system efficiency reached or exceeded the design value only 92 days, the remaining 148 days lower than the design value, with the lowest efficiency of 2.21%. The reasons are mainly as follows:
(1) The environment temperature is high and the OGHHTS needs less heat, thereby lowering the temperature at the evaporator outlet and the average temperature of geothermal water in the evaporator.
(2) While the environment temperature is high, especially in sum-mer, the temperature of the cooling water at the condenser inlet will be higher, thereby resulting in the increase of the tempera-ture at the screw expander outlet. The net power output will be reduced, and the thermal efficiency shows the similar trend.
Table 2. System parameters of the original geo-plant in an oilfield in China.
Parameters SymbolValue/Type
Geothermal water inlet temperature ta (°C) 110Geothermal water outlet temperature tb (°C) 90Geothermal water mass flow rate mgw (kg/s) 69.44Intermediate hot water inlet temperature ta’ (°C) 100Intermediate hot water outlet temperature tb’ (v) 80Intermediate hot water mass flow rate mhw (kg/s) 69.44Turbine inlet temperature t1 (°C) 82Turbine inlet pressure p1 (°C) 0.52Turbine exhaust temperature t2 (°C) 42Turbine exhaust pressure p2 (°C) 0.17Working fluid R123Working fluid mass flow rate mwf (kg/s) 28.4Cooling water inlet temperature td (°C) 30Cooling water outlet temperature tf (°C) 40Net power output Wnet (kW) 270Thermal efficiency ηth (%) 4.65
Exergetic efficiency ηex (%) 25.76
Table 3 shows the thermal and exergetic efficiencies in Refer-ences [5, 11, 57-58]. From Tables 2 and 3, it can be seen that the thermal and exergetic efficiencies of the original power plant are
much lower than the plants in the References. The actual thermal efficiency under the operating conditions ranges from about 2.2 to 5.8%, which is also lower than those in the References.
Table 3. Thermal and exergetic efficiencies in References [5, 11, 57-58].
Item ηth /% ηex /% SourceDual-level of geothermal power plant 8.9 29.1 [5]Simple ORC of R123 7.65 38.76 [11]Simple ORC of n-pentane 7.376 37.37 [11]R141b in b2 cycle 17 — [54]R600a in o3 cycle 15.1 — [54]R601a in o2 cycle 15.9 — [54]Otake pilot binary geothermal power plant
12.9 53.9 [57]
Heber SIGC geothermal power plant 13.2 50.7 [57]ORC 20.6 35.5 [58]
4.2 Modified Geo-Plant
Step 1: Removing the PHEs
Table 4 shows the thermodynamic properties of the working fluids investigated. Fig. 4 shows the net power output for differ-ent working fluids. The net power output first increases and then decreases with the evaporating temperature after reaching the maximum value, which is consistent for the power plant with and without the PHEs. The net power output depends on the product of the specific power output and the mass flow rate of the working fluid. The specific power output increases with the evaporating temperature, for the mass flow rate of the working fluid the reverse is the case. The difference between the geo-plant with and without the IHE is the available range of the evaporating temperature. Since the temperature difference at the pinch point is 3℃, the evaporating temperature ranges from 77 to 96°C for the system with the PHEs, whereas it is between 87 and 106°C for the system without the PHEs. It can be easily seen from Fig. 4 that isopentane demonstrates the highest net power output in both cases. However, R123 and R245fa show different variation
Figure 3. Design and operating efficiency of the original geo-plant.
Table 1. Comparison of the numerical results with previous published data.
Sub-stance
tcri / °C
Pcri / MPa
t1 / °C
t3 / °C
Pe /MPa
Pc / MPa
m /kg×s-1
°Cth /% Source
R125 66.18 3.630 40.06 30.00 2.000 1.564 400.37 2.32 [18]
R125 66.18 3.630 40.06 30.00 2.000 1.564 400.37 2.35 Present
R290 96.65 4.250 57.14 30.00 2.000 1.079 48.776 5.91 [18]
R290 96.65 4.250 57.14 30.00 2.000 1.079 48.776 5.81 Present
R134a 101.03 4.056 67.75 30.00 2.000 0.772 68.55 7.74 [18]
R134a 101.03 4.056 67.75 30.00 2.000 0.772 68.55 7.48 Present
0 50 100 150 200
3.0
3.5
4.0
4.5
5.0
5.5
Operating Design
Ther
mal
eff
icie
ncy (%)
Running time (Day)
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trends. R123 outperforms R245fa for lower evaporating tempera-tures, for higher evaporating temperatures the reverse is the case. The turning points for the system with and without the PHEs are 82 and 93℃, respectively. The net power output for all the three fluids sharply decreases with the evaporating temperature and tends to 0kW, indicating that no net electricity will be generated.
Table 5 shows the net power output at the optimal evaporating temperature with and without the PHEs. The optimal evaporat-ing temperature makes the maximum net power output. After removing the PHEs, the thermal efficiency for R123, R245fa, and isopentane are increased by 28.5, 25.8, and 28.7%, respectively. Moreover, it should be pointed out that the optimal net power output for the system with the PHEs calculated by the numerical model in this paper is 277.2kW, which is close to the net power output under the design condition. This is another evidence of the agreement between the numerical results and the actual operat-ing data.
Table 5. Net power output at the optimal evaporating temperature with and without the PHEs.
Substance Item te,opt (℃) Wnet,opt (kW)
R123PHEs+ 82 277.2
PHEs- 93 356.3
R245faPHEs+ 83 278.1
PHEs- 94 349.9
isopentanePHEs+ 83 349.6
PHEs- 94 449.9
Fig. 5 shows the irreversible loss of the cycle components of the geothermal power plant with and without the PHEs. R123 is used to simplify the analysis, and similar results can be obtained for R245fa and isopentane. The sequences of the irreversible loss for the cycle components are as follows:
(a) System with the PHEs: Ie>Ic>Ise>IPHE>Ip.(b) System without the PHEs: Ie>Ic>Ise>Ip.(c) The evaporator generates the largest irreversible loss and the pump the lowest, which is consistent for the system with and without the PHEs. The total irreversible loss for the system with and without the PHEs is 721.1 and 604.2kW, respectively. Removing the PHEs reduces the total irreversible loss by about 116.8kW. However, the irreversible loss caused by
the PHEs is 133.4kW. It can be obtained that the decrease of the total irreversible loss is less than the irreversible loss caused by the PHEs. The evaporator and condenser increase their irreversible losses, and the reason is that removing the PHEs increases the available temperature of the geothermal water, from 100-80°C to 110-90°C. However, the turbine and the pump decrease their irreversible losses, and this is because removing the PHEs decreases the mass flow rate of the working fluid, from 29.73 to 28.83kg/s. After removing the PHEs, the net power output increases from 277.2 to 356.3kW.
Through the above analysis, removing the PHEs increases the system performance, which is preferable.
Step 2: Installing an IHE
Based on Sec -tion 4.2.1, an IHE is further added, tend-ing to decrease the power consumption of the cooling pump. Fig. 6 shows the spe-cific power output of R123, R245fa, and isopentane for the geo-plant with an IHE. The specific power output monotonically increases with the
evaporating temperature, which is consistent for R123, R245fa, and isopentane. The sequence of the growth rate of the specific power output the specific power output is at the same evaporat-ing temperature as follows: rwt,isopentane>rwt,R245fa>rwt,R123, so the difference among them becomes larger and larger with the evaporating temperature.
Fig. 7 shows the mass flow rate of R123, R245fa, and isopen-tane with the evaporating temperature for the system with and without an IHE. The mass flow rate decreases with the evaporat-
Table 4. Thermodynamic properties of the working fluids.
Substances
Physical data Environmental data
SourcesType of Fluids
M(g/ mol)
Tb (°C) Tcri(°C)
Pcri (MPa)
ALT (yr) ODP
GWP (100yr)
1 R123 152.93 27.82 183.68 3.662 1.3 0.02 77 [59] Isentropic2 R245fa 134.05 14.90 154.05 3.640 7.6 0 1030 [60] Isentropic3 isopentane 72.15 27.8 187.2 3.380 0.01 0 ~20 [61] Isentropic
75 80 85 90 95 100 1050
100
200
300
400
Without PHEs
Net
pow
er o
utpu
t (k
W)
Evaporating temperature ( oC)
R123 R245fa isopentane R123 R245fa isopentane
With PHEs
Figure 4. Net power output for different working fluids.
(b) Without the PHEs
Ip=5.29kWrp=0.88%
Ise=152.6kWrse=25.26%
Ie=243.2kWre=40.25%
Ic=203.1kWrc=33.62%
(b) Without the PHEs
Ip=5.29kWrp=0.88%
Ise=152.6kWrse=25.26%
Ie=243.2kWre=40.25%
Ic=203.1kWrc=33.62%
Figure 5. Irreversible loss of the geo-plant using R123 as the working fluid.
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ing temperature, and the reduction rate for te<te,opt is evidently lower than that for te>te,opt. The mass flow rate for the system with an IHE is higher than that without an IHE, and this is because the IHE enhances the specific enthalpy of the working fluid and decreases the heat absorption per unit mass of the working fluid from the evaporator, thereby adding the mass flow rate of the working fluid. The difference between the system with and with-out an IHE is decreased with the evaporating temperature. For a constant evaporating temperature, R123 exhibits the highest mass flow rate, whereas isopentane is the lowest.
Fig. 8 shows the power consumed by the cooling pump with the evaporating temperature with and without an IHE. It can be clearly seen that the power consumed by the cooling pump decreases with the evaporating temperature. Installing an IHE reduces the power consumed by the cooling pump, and difference between the
systems with and without an IHE first increases and then decreases with the evaporating temperature after reaching its maximum, which is consistent for all the working fluids investigated. The maximum reduction between the systems with and without an IHE for R123, R245fa, and isopentane is 1.08, 1.27, and 0.44kW. Installing an IHE can evidently decrease the power consumed by the cooling pump, thereby enhancing the system performance.
Fig. 9 shows the net power output of R123, R245fa, and iso-pentane with and without an IHE. From the figure, it can be seen that the isopentane apparently outperforms R123 and R245fa in the net power output. Moreover, installing an IHE enhances the net power output, which is consistent for all the three fluids. Table 6 shows the net power output at the optimal evaporating tempera-ture with and without an IHE. The growth rate of the maximum net power output for R123, R245fa, and isopentane is 2.47, 4.34, and 3.04%, respectively.
Figure 6. Specific power output of the geo-plant with an IHE but without the PHEs.
88 90 92 94 96141516171819
363840424446
R123
R245fa
Spe
cific
pow
er o
utpu
t (k
W/k
g)
Evaporating temperature ( oC)
isopentane
86 88 90 92 94 96 98 100 102 104 1060
5
10
15
20
25
30 Without an IHE：
With an IHE：
Mas
s flo
w r
ate
(kg/
s)
Evaporating temperature ( oC)
isopentane
R245fa
R123
Figure 7. Mass flow rate with and without an IHE.
86 88 90 92 94 9626
28
30
32
34
Pow
er c
onsu
mpt
ion
of c
oolin
g pu
mp(
k
Evaporating temperature ( oC)
Without an IHE： R123 R245fa isopentaneWith an IHE： R123 R245fa isopentane
Figure 8. Power consumption of the cooling pump with and without an IHE.
86 88 90 92 94 96 98 100 102 104 1060
100
200
300
400
500
Net
pow
er o
utpu
t (k
W)
Evaporating temperature ( oC)
Without an IHE： R123 R245fa isopentaneWith an IHE： R123 R245fa isopentane
Figure 9. Net power output with and without an IHE.
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Table 6. Net power output at the optimal evaporating temperature with and without an IHE.
Substance Item te,opt (°C) Wnet,opt (kW)
R123IHEs+ 93 356.3
IHEs- 93 365.1
R245faIHEs+ 94 349.9
IHEs- 94 365.1
isopentaneIHEs+ 94 449.9
IHEs- 93 463.6
Step 3: Adding a Preheater
Based on Sections 4.2.1-4.2.2, a preheater using the tail wa-ter from OGTHTS is further added to preheat the working fluid before entering the evaporator, tending to increase the system performance.
Fig. 10 shows the mass flow rate with and without a preheater. It can be seen from Fig. 10 that the variation trend is similar to that shown in Fig. 7. The preheater increases the mass flow rate of the working fluid, and the growth rate decreases with the evaporating temperature for te<te,opt. However, the mass flow rates of a specific working fluid for the system with and without a preheater are always the same for te>te,opt.
Fig. 11 shows the irreversible loss of the evaporator with the evaporating temperature for the geo-plant with and without a preheater. The irreversible loss of the evaporator first decreases and then increases with the evaporating temperature, and there is a minimum value for te=te,opt. The minimum irreversible loss caused by the evaporator follows the following sequence:
(a) System without a preheater: Ie,isopentane>Ie,R245fa>Ie,R123.(b) System with a preheater: Ie,isopentane>Ie,R245fa>Ie,R123.For both cases, isopentane generates the highest irreversible
loss caused by the evaporator, for R123 the lowest.
Fig. 12 shows the net power output with the evaporating temperature for the geo-plant with and without a preheater. It can be seen from Fig. 12 that the variation trend is similar to that shown in Fig. 9. Table 7 shows the net power output at the optimal
evaporating temperature with and without a preheater. The growth rates of the maximum net power output for R123, R245fa, and isopentane are 4.44, 4.14, and 3.47%, respectively.
Table 7. Net power output at the optimal evaporating temperature with and without a preheater.
Substance Item te,opt (°C) Wnet,opt (kW)
R123Preheater+ 93 365.1Preheater - 92 381.3
R245faPreheater+ 94 365.1Preheater - 93 380.2
isopentanePreheater+ 93 463.6Preheater - 92 479.7
Substance Item te,opt ( ) Wnet,opt (kW)
R123 IHEs+ 93 356.3
IHEs- 93 365.1
R245fa IHEs+ 94 349.9
IHEs- 94 365.1
isopentane IHEs+ 94 449.9
IHEs- 93 463.6
86 88 90 92 94 96 98 100 102 104 1060
5
10
15
20
25
30
Mas
s flo
w r
ate
(kg/
s)
Evaporating temperature ( oC)
Without a preheater： R123 R245fa isopentaneWith a preheater： R123 R245fa isopentane
Figure 10. Mass flow rate with and without a preheater.
86 88 90 92 94 96 98
200
300
400
500
Irre
vers
ible
loss
(kW
)
Evaporating temperature ( oC)
Without a preheater: R123 R245fa isopentaneWith a preheater: R123 R245fa isopentane
Figure 11. Irreversible loss of the evaporator with and without a preheater.
86 88 90 92 94 96 98 100 102 104 1060
100
200
300
400
500
Net
pow
er o
utpu
t (k
W)
Evaporating temperature ( oC)
Without a preheater： R123 R245fa isopentaneWith a preheater： R123 R245fa isopentane
Figure 12. Net power output with and without a preheater.
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Step 4: Using Zeotropic Mixture of R245fa and Isopentane
From Sections 4.2.1-4.4.3, it can be concluded that isopentane exhibits excellent thermodynamic performances among the fluids investigated. However, one of the most important problems with using isopentane as the working fluid is its flammability. Once this problem is solved, it could be widely applied in engineering application. One possible way to suppress the flammability of isopentane is to mix it with one or more suitable additives, which has been studied by Oellrich et al. [62]. Garg et al. [63] mixed isopentane with R245fa to restrain the flammability of isopentane. They took a conservative approach to ensure the safety of the utilization of isopentane, and a 30% mole fraction of R245fa in the mixture of R245fa and isopentane was set to be the minimum limit. Moreover, a 30% mole fraction of R245fa in the mixture of R245fa and isopentane corresponds to about 45 % mass fraction of R245fa in the mixture of R245fa and isopentane.
Fig. 13 shows the bubble point and the dew point temperatures of the zeotropic mixture of R245fa and isopentane with the mass fraction of R245fa. The bubble point temperature and the dew point temperature of the zeotropic mixture are first decreased with the mass fraction of R245fa until reaching the minimum value while the mass fraction of R245fa is 0.9, and after that both increase a little. It can be clearly seen that the bubble point temperature and the dew point temperature of the mixture depend on the pressure, that is, both of the two temperatures vary with the pressure. This variation trend is consistent with the mass fraction of R245fa ranging from 0 to 1.
Fig. 14 shows the temperature glide of the zeotropic mixture of R245fa and isopentane with R245fa mass fraction. The temperature glide initially increases and reaches a maximum value while the mass fraction of R245fa is about 0.4, and then decreases with the mass fraction of R245fa, which is accordance with the mass frac-tion of R245fa of 0-0.92. For the mass fraction of R245fa of 0.92-1, the temperature glide slightly increases and then decreases with the mass fraction of R245fa, and there exists a maximum value while the mass fraction of R245fa is about 0.92. For a specific mass fraction
of R245fa, the value of the temperature glide rests with the pres-sure, i.e., the higher the pressure, the larger the temperature glide.
Fig. 15 shows the contour of the thermal efficiency as a func-tion of the R245fa mass fraction and the evaporating temperature. The scope of the R245fa mass fraction ranges from 0.45 to 0.95. It can be seen from Fig.15 that the thermal efficiency decreases with the increment of the R245fa mass fraction for a specific evaporating temperature. However, for a constant R245fa mass fraction, the thermal efficiency initially increases until reaching a maximum value and then decreases with the R245fa mass fraction.
In order to get a higher thermal efficiency, the R245fa mass fraction should be as low as possible. A mass fraction of R245fa of 0.45 in the mixture of R245fa and isopentane is advised in engineering practice. The optimal net power output for the mass fraction of R245fa of 0.45 is 458.3kW, whereas the optimal net power output for isopentane is 484.0kW. The optimal net power output is only decreased by about 5.31% to solve the flammabil-ity of isopentane.
0.0 0.2 0.4 0.6 0.8 1.090
100110120130140150160170180
P=1.0MPa
P=1.45MPa
Tem
pera
ture
(oC)
Mass fraction of R245fa
P=3.0MPa
□ Bubble point △ Dew point
Figure 13. Bubble point and dew point temperatures of the zeotropic mixture of R245fa and isopentane.
0.0 0.2 0.4 0.6 0.8 1.00
1
2
3
4
5
6
Tem
pera
ture
glid
e (o
C)
Mass fraction of R245fa
P=1MPa P=2MPa P=3MPa
33.85
31.38
28.90
26.43
23.95
36.33
31.38 21.48
88 90 92 94 96 98 100
0.5
0.6
0.7
0.8
0.9 ηex(%)
Evaporating temperature ( oC)
R24
5fa m
ass
frac
tion
19.00
21.48
23.95
26.43
28.90
31.38
33.85
36.33
38.80
Figure 14. Temperature glide of the zeotropic mixture with R245fa mass fraction.
Figure 15. Contour of exergetic efficiency as a function of the R245fa mass fraction and the evaporating temperature.
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5 Conclusions
Four measures were proposed to improve the system perfor-mance of an actual geo-plant in China. The results from energy and exergy analyses validated the effectiveness of the four mea-sures. The main conclusions drawn from the present study can be summarized as follows:
(1) The heat source directly transfering heat to the working fluid is advisable, especially for the low and medium grade geothermal water.
(2) Installing an IHE can not only reduces the power consump-tion of the cooling pump but also increases the specific enthalpy at the evaporator inlet, thereby enhancing the system performance.
(3) Adding a preheater recovers a portion of the waste heat from the OGTHTS and improves the net power output.
(4) Removing the PHEs and installing a preheater and an IHE greatly increases the net power output.
(5) Mixing additives with isopentane can suppress the flam-mability of isopentane. In order to get a higher thermal efficiency, the R245fa mass fraction should be as low as possible. R245fa mass fraction of 0.45 in the mixture is advised in engineering applications, with the net power output decreased by only 5.31% compared with the origi-nal geo-plant.
Acknowledgement
The authors gratefully acknowledge the financial sup-port provided by the National High Technology Research and Development Program of China (863 Program) (Grant No. 2012AA052804).
Nomenclature
Ex Exergy (kW)h specific enthalpy (kJ/kg)I irreversibility rate (kW)M molar mass (kg/kmol)m mass flow rate (kg/s)p pressure (MPa)Q heat transfer rate (kW)s specific entropy (kJ/(kg·℃))T temperature (K)t temperature (℃)W power (kW)ΔP pressure difference (Pa)
Greek symbolsη efficiency (%)ρ density (kg/m3)
Subscriptsc condensercri critical
cw cooling watere evaporatorex exergeticg generatorgw geothermal waterhw hot waternet netopt optimalp pump, pinchpp pinch points isentropicse screw expanderth thermaltot totalwf working fluid0 environment1, 2, 3, 4, 2a, 4a, 4b, a, b, c, d, e, f, a’, b’, c’,d’
state points
Acronyms ALT atmosphere life time (yr)CHP combined heat and powerGWP global warming potentialIHE inner heat exchangerNBSC National Bureau of Statistics of ChinaODP ozone deletion potentialORC organic Rankine cycleOGTHTS oil gathering and transportation heat tracing stationPHE plate heat exchanger
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