- Email: [email protected]
Conventional and advanced exergy analyses of a geothermal driven dual fluid organic Rankine cycle (ORC)
Accepted Manuscript Research Paper Conventional and advanced exergy analyses of a geothermal driven dual fluid organic Rankine cycle (ORC) Hossein Nami, Arash Nemati, Farshad Jabbari Fard PII: DOI: Reference:
S1359-4311(17)31712-X http://dx.doi.org/10.1016/j.applthermaleng.2017.05.011 ATE 10322
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
13 March 2017 22 April 2017 4 May 2017
Please cite this article as: H. Nami, A. Nemati, F. Jabbari Fard, Conventional and advanced exergy analyses of a geothermal driven dual fluid organic Rankine cycle (ORC), Applied Thermal Engineering (2017), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2017.05.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Conventional and advanced exergy analyses of a geothermal driven dual fluid organic Rankine cycle (ORC) Authors Hossein Nami*, Arash Nemati*, Farshad Jabbari Fard Faculty of Mechanical Engineering, University of Tabriz, 29th Bahman Blvd., Tabriz, Iran *Corresponding authors: Email address: [email protected], [email protected] Phone number: +98 9367848823 Abstract A study on geothermal driven dual fluid organic Rankine cycle is presented in this paper using conventional and advanced exergy analysis methods to provide information about system components interactions. Special operating conditions are performed for system components in order to obtain real, unavoidable and ideal performance. The exergy destruction rate in each component is divided into endogenous, exogenous, avoidable and unavoidable parts to reveal more detailed information about effects of components inefficiency on each other exergy destruction and the real potential of the system for improvement. The conventional exergy analysis reveals that, low pressure vapor generator (LPVG), high pressure vapor generator (HPVG) and condenser (COND) are the most important component by 38.11, 29.98 and 15.93% of the total exergy destruction rate, respectively. Despite the conventional exergy analysis results, advanced exergy shows that only 15% of the COND exergy destruction is avoidable which includes 7% of system avoidable exergy destruction rate. Also, LPVG, low pressure 1
turbine (LPT) and HPVG are the most important components from the viewpoint of advanced exergy because of their considerable endogenous avoidable exergy destruction rates. Keywords: Advanced exergy, Conventional exergy, Dual fluid ORC, Geothermal heat source 1. Introduction Developing technology has urged investigators and engineers to design more efficient energy conversion systems. This duty becomes even more important if we consider the increasing demand for energy by the developed countries in spite of the limited resources of fossil fuel [1]. Ozone layer destruction and electricity price raising, motivate governments to use renewable sources for power generation [2]. Low grade heat power generation systems like organic Rankine cycle (ORC), Kalina cycle (KC) and the other ones may be solutions for these problems. Because of the ORCs simplicity of the technology, availability of components, reliability, and flexibility, they are considered as a practical solution [3]. Besides waste heat, geothermal, solar and ocean thermal energy sources is available to provide a clean alternative to fossil fuel combustion in ORCs. Many researches have been done on geothermal energy as a low-grade heat source for ORCs from the perspective of conventional energy and exergy analysis, recently. Heberle and Brüggemann [4] presented an exergy based analysis for working fluid selection in a geothermal organic Rankine cycle. They concluded that, fluids with lower critical temperatures, like R227ea and isobutane are more preferable for power generation in ORC. Yari [5] studied various types of geothermal ORC power plants from the perspective of exergy and concluded that, combined flash-binary with R123 as working fluid had the highest exergy efficiency. Heberle et. al [6] investigated zeotropic mixtures as working fluid in ORCs for geothermal low-
2
grade energy. They reported that, the temperature glides between saturated vapor of zeotropic mixtures and saturated liquid at a particular pressure, increases the exergy efficiency in comparison to pure fluids. Effect of working fluid on geothermal ORC performance has been investigated by Liu et al. [7]. They reported that, R600a as working fluid had the maximum net output power followed by R245fa and R600 for geothermal heat sources with temperature higher than 120 ˚C and reinjection temperatures higher than 70 ˚C. Zhai et al. [8] compared HFC (hydro fluorine carbon) and HC (hydro carbon) fluids as working fluid in geothermal ORC systems and concluded that, the amount of energy which is absorbed from the heat source has an important effect on ORC output power. They also stated that, working fluids with higher ratio of specific heat capacity to latent heat of evaporation absorb more energy and produce more power. Guzovic et al. [9] compared basic and a dual-pressure ORC in a geothermal power plant and resulted that, the dual-pressure ORC has higher both efficiency (65% vs. 52%) and net power (6371 kW vs. 5270 kW). The influence of zeotropic mixtures in a low-medium temperature geothermal ORC has been analyzed by Kang et al. [10]. They found that evaporating temperature and zeotropic mixture ratio that gives the highest temperature glide has a significant effect on the ORC power generation and they suggested that R245fa/R600a (0.9/0.1) is the most preferable mixture. Sadeghi et al. [11] studied different zeotropic working fluids in various ORC configurations using geothermal energy as a heat source. They reported that, the main source of exergy destruction in ORCs are evaporators and using two series evaporators and zeotropic working fluid lead to a significant reduction in the total exergy destruction. Yang and Yeh [12] carried out an economic study on geothermal ORC performance and they reported that the importance of the pinch point temperature differences in the evaporator is much more than those in the condenser.
3
The conventional exergy analysis plays an important role in the exergy determination in the mentioned researches. Conventional exergy claims that, the most important component is the one with the highest exergy destruction and suggests more attention to the mentioned component for system performance improvement. But there is no hint that how much of the exergy destruction is unavoidable and how much is because of the effect of irreversibilities of other components. To overcome these shortcomings, the concept of the advanced exergy analysis has been proposed by researchers in recent years. Advanced exergy reveals interactions between the components of a system to estimate the real potential for development. In this approach, exergy destruction in each component is divided into avoidable part and unavoidable part and also into endogenous and exogenous exergy destruction [13]. Recently, more and more researchers use advanced exergy analysis method to investigate the exergy destruction in various energy conversion systems. The most important systems are natural gas liquefaction systems [14, 15], gas turbine power systems [16-18], Kalina cycle [19], biomass gasification units [20], geothermal district heating systems [21, 22], gas engine heat pumps for food drying processes [23, 24], cogeneration and tri-generation systems [25-27] and some other refrigerant systems. Based on the most recent published papers, the dual fluid ORC has higher power and efficiency than that of simple ORC. Furthermore, the possibility of choosing various working fluids for each loop of cycle can help designers to achieve the favorable operating conditions. To the best of the author's knowledge and by surveying the mentioned literatures, there is no research in which the advanced exergy analysis has been applied to the dual fluid ORC power plant with geothermal heat source. In order to cover the existing shortcomings in the literature, the performance of a geothermal driven dual fluid ORC power plant is thermodynamically
4
modeled. Then, to reveal more detailed information about components inefficiency influences on each other, exergy destruction and the real potential of the system for improvement, the advanced exergy methodology is adopted. Splitting the exergy destruction rates within each system component is schematically described and interpreted. The results are expected to help in geothermal driven dual fluid ORC system performance enhancement, considering the main sources of exergy destruction which is avoidable by better designing of the system. These results cannot be attained from the conventional exergy analysis and reported in this study for the first time. 2. System description and assumptions The schematic diagram of dual fluid ORC which is used in the geothermal power plant is shown in Fig. 1. The main purpose of this system is to produce power from the geothermal water as a heat source. As seen in the figure, a dual fluid ORC consists of nine different parts: a high pressure evaporator (HPE), a high pressure preheater (HPPH), a low pressure evaporator (LPE), a low pressure preheater (LPPH), a high pressure turbine (HPT), a low pressure turbine (LPT), a condenser (COND), a high pressure feed pump (HPFP) and a low pressure feed pump (LPFP). The working fluid of high pressure ORC preheats at the HPPH and enters the HPE at the saturated liquid condition. Having passed the HPE, the working fluid enters the HPT as saturated vapor and the HPT exit flow heats the organic fluid of low pressure ORC. Heated low pressure working fluid flows to the LPT at the saturated vapor condition where it expands and produces power. The expanded working fluid at the LPT enters the condenser and cools down to condenser temperature. HPPH exit geothermal fluid preheats the low pressure organic fluid in the LPPH and then heated fluid flows to the LPE. Fig. 1
5
The following assumptions are made in this work:
Mass flow rate, temperature and pressure of the geothermal water are considered to be 83 kg/s, 175 ˚C and 7 MPa, respectively [9].
The isentropic efficiency is considered for the turbines and pumps.
Ambient temperature and pressure are 25 ˚C and 1 bar, respectively.
The system operates under steady state condition [28].
The cooling water enters the condenser at the ambient condition.
Isopentane and Isobutane are considered as the working fluids of high and low pressure cycles, respectively.
External cycleʼs performances such as the cooling water circulation pumps of the condenser and geothermal water pumps are not taken into account, because the conventional and advanced exergy performance of the dual fluid ORC is independent from the geothermal water pumps, while the performances of the cooling water circulation pumps are so low and can be neglected.
3. Thermodynamic analysis 3.1.Conventional exergy analysis Considering each component as control volume, the mass and energy conservation equations as long as exergy balance equations are applied to the system components as follows [29, 30]:
m m
(1)
Q m i hi W m e he
(2)
i
e
6
E Q m i ei EW m e ee E D
(3)
where, EW W
(4)
T E Q Q (1 0 ) T
(5)
In these equations, subscripts i and e symbolize the component inlet and exit, respectively. Also,
Q is the rate of heat transfer, E W is the exergy rate of mechanical power, E Q is the exergy rate associated with the heat transfer, and E D is the exergy destruction rate. Moreover, T 0 is the ambient temperature. The specific exergy is the sum of specific physical and chemical exergy: e e ph ech
(6)
Neglecting the Potential and Kinetic exergies, the specific physical exergy can be expressed as follows [31]: e ph h h0 T0 (s s0 )
(7)
Also, the specific chemical exergy for a mixture of ideal gas can be written as follows [32-33]: n
n
i 1
i 1
echmix X i echi R T0 X i ln( X i )
(8)
7
here, X i is the molar fraction of a mixture component. Finally, the exergetic efficiency of dual fluid ORC as an energy system can be defined as follows [34, 35]:
W
net Ein
(9)
For the case of dual fluid ORC, W net is the net produced power and E in is the total entrance exergy rate provided by the geothermal fluid: W net W HPT W LPT W HPFP W LPFP
(10)
E in E gw,in E gw,out
(11)
The exergy balance equations for each component of dual fluid ORC are outlined in Table 1. Table 1
3.2.Advanced exergy analysis The advanced exergy analysis considers the interactions among system components and splits the exergy destruction rate in the kth component of the system into two parts, named as exogenous and endogenous exergy destructions:
E D,k E DEX, K E DEN, K
(12)
The exogenous exergy destruction rate in a component ( E DEX, K ) is due to the irreversibilities taking place in remaining components where the objective component operates under the ideal condition. Also, the endogenous exergy destruction rate in a component ( E DEN,K ) is associated to the irreversibility of the component itself, which operates under real conditions and others 8
operate under ideal conditions. To analyze the real locations of irreversibilities in the energy systems, splitting destroyed exergy into exogenous and endogenous parts provides information not obtainable by the conventional exergy analysis. So that, one can decide where to pay more attention in improving the system exergetic performance. Also, the rate of exergy destruction in kth component of an energy converting system can be split into avoidable and unavoidable parts [13]:
E D,k E DAV, K E DUN, K
(13)
In fact, the unavoidable part of destroyed exergy within a component is due to technical limitations (such as availability and cost of materials) which cannot be reduced [36]. The remaining exergy destruction represents the avoidable exergy destruction rate, which can be reduced or optimized by optimizing processes. Therefore, splitting the exergy destruction rate within a component into its unavoidable and avoidable parts highlights the potential for improving exergy efficiency of each system component. In addition, the combination of these two mentioned splitting concepts provides useful definitions of exergy destruction. The results of combination of endogenous, exogenous, avoidable and unavoidable exergy destruction rate are endogenous-avoidable, exogenousavoidable, endogenous-unavoidable and exogenous-unavoidable. The endogenous-avoidable exergy destruction rate is because of system components itself, which can be reduced by improving the component efficiency while the exogenous-avoidable represents the exergy destruction rate due to irreversibility of other components which can be controlled by improving the exergetic performance of the overall system. Detailed exergy destructions can be calculated as follows [37]: 9
ED E DEN,K,UN E pEN ,k EP
UN
k
(14)
EN ,UN E DEX,K,UN E UN D ,K E D ,K
(15)
E DEN,K,AV E DEN,K E DEN,K,UN
(16)
E DEX,K,AV E DEX,K E DEX,K,UN
(17)
Several methods have been provided by researchers for the advanced exergy analysis. Thermodynamic cycle method, exergy balance method, engineering method, equivalent components method and structural theory method are the known advanced exergy approaches. Among the all, thermodynamic method is the most convenient method which offers the best solution results for thermodynamic systems. The second one is engineering method which provides acceptable results. On the other hand, the structural theory method is not suitable to be applied to systems with higher values of endogenous exergy destruction. Also, the exergy balance method is used for more complex cycles. More details about advanced exergy approaches can be found in literature [16]. The thermodynamic cycle method is used for the present study due to its higher prediction accuracy. In the present work, to identify different parts of exergy destruction rates, models such as ideal, hybrid and real cycles are considered. It is worth mentioning that, in the ideal cycle, all components operate under the reversible conditions and also all processes considered to be irreversible in the real cycle. In order to calculate the endogenous exergy destruction in kth component of the hybrid cycle, the process occurring within the component is considered to be irreversible and all processes occurring in remaining components are considered reversible. Also, 10
it is valuable to note that, the number of hybrid cycles in the advanced exergy analysis of a system is equal to the number of system components. Table 2 represents the main points considered for the dual fluid ORC under ideal, real and unavoidable conditions. Table 2
4. Results and discussions 4.1.Results of the conventional exergy analysis Thermodynamic properties and mass flow rates of investigated dual fluid ORC at different state points under real, ideal and unavoidable conditions are outlined in Tables 3-5, respectively. Furthermore, the obtained values for the exergy efficiency, produced power by the HPT, LPT and net produced power are outlined in these tables. In order to analyze the ideal and unavoidable systems, the net produced power is considered as equal to the real systems’ net produced power (3443 kW). Regarding to the different operating conditions of the real, ideal and unavoidable, in order to achieve the same value of net produced power, various amounts of working fluid mass flow rates are calculated. Therefore, a reduction in mass flow rate of geothermal water is observed for the ideal and unavoidable system which leads to a decrease in working fluid mass flow rate. In fact, lower values of workings fluid mass flow rates in the ideal and unavoidable systems are due to irreversibility reduction in compare to the real system. It should be noted that, the exergy delivered by the geothermal water to the system working fluid is the exergy difference between the incoming and outgoing geothermal water. Also, the net produced power is considered as the total product exergy. Referring to Tables 3-5, the second law efficiency for real, unavoidable and ideal cycles are achieved 47.03%, 61.5% and 69.85%, 11
respectively. Therefore, technical limitations reduce the exergy efficiency of the system by about 8.35% in comparison to the ideal condition. Table 3 Table 4 Table 5
The results of the conventional exergy analysis for the real, ideal and unavoidable conditions are presented in Tables 6-8. In Tables 6, 7 and 8, all the components operate under the real, ideal and unavoidable conditions, respectively. These tables include some important exergetic parameters such as fuel, product and destroyed exergy rates, exergy efficiency, the ratio of the exergy destruction rate to total entering exergy rate and the ratio of the exergy destruction rate to the total exergy destruction rate for each component of the systems. According to Table 6, in real conditions, highest exergy destruction belongs to low pressure vapor generator (LPVG) followed by high pressure vapor generator (HPVG) and condenser (COND). High values of exergy destruction are mainly due to temperature mismatching between the hot and cold streams in these heat exchangers. Conventional exergy analysis proposes to focus on components with high values of exergy destruction. Therefore, from the viewpoint of conventional exergy analysis, designers should pay more attention to the mentioned components to improve the exergetic performance of the system. According to Tables 6-8, to produce the same value of net output power, real and unavoidable systems have about 204 and 60% more exergy destruction in comparison to the ideal system. Table 6 Table 7 Table 8
4.2.Results of the advanced exergy analysis 12
The obtained exergy destruction in the previous section can be explained in detail by the advanced exergy analysis. As mentioned before, destroyed exergy by each component can be divided into two parts, namely endogenous (the irreversibility due to internal inefficiencies of component) and exogenous (the irreversibility due to other components inefficiencies). By the fact, the advanced exergy analysis highlights the effects of technical limitations and different components interaction on the exergetic performance of the system. For calculation of the detailed parts of exergy destruction in each component, the endogenous exergy destruction rate, E EN D ,K , is obtained first. Then, using Eq. 12, the exogenous exergy destruction rate, E EX D ,K , is calculated. To find the value of the unavoidable exergy destruction rate in kth component, E UN D ,K , the unavoidable condition is considered instead of the real condition for the kth component, while others operate under the real condition. Also, avoidable EN ,UN exergy destruction rate, E AV is D ,K , is calculated using Eq. 13. Furthermore, the value of E D ,K
obtained using unavoidable condition instead of the real condition for the kth component in E EN D ,K ,UN ,AV . By introducing the value of E EN in Eq. 16, the amount of E EN is calculated. Finally, D ,K D ,K ,UN ,AV with the known values of E EN and E EN and using Eqs. 15 and 17, the amounts of D ,K D ,K
,AV E EX D ,K
,UN and E EX are determined. D ,K
Table 9 outlines the results of above mentioned calculation for different components of the system. Referring to Table 9, the value of the endogenous exergy destruction rate is higher than the value of the exogenous exergy destruction rate for all system components. Therefore, it can be concluded that the main part of exergy destruction in each component is due to its irreversibility itself. This means in order to improve the system performance, more attention should be paid on the internal irreversibilities of the components. As it is obvious in Table 9, the 13
greatest amounts of exogenous exergy destruction rate belong to LPVG, HPVG and condenser, respectively. So, it can be suggested that an improvement in other component's exegetic performance makes a reduction in the exogenous exergy destruction of the above mentioned components. It is worth mentioning that, the avoidable part of exergy destruction can only be controlled. Referring to Table 9, the highest value of the avoidable exergy destruction rate relates to LPVG, HPVG, HPT and LPT, respectively. It should be noted that, COND has one the most highest amounts of exergy destruction from the viewpoint of conventional exergy analysis while based on the advanced exergy analysis, it is not an effective component (due to its low value of avoidable exergy destruction). This clarifies the advantage of the advanced exergy analysis in comparison to the conventional exergy analysis. Furthermore, based on the Table 9, the total unavoidable exergy destruction rate is about two times more than avoidable part which point out only about 32% of destroyed exergy can be declined. Table 9 provides some important information for the system performance enhancement by the exogenous avoidable and endogenous avoidable exergy destruction rates. Since, a reduction in endogenous avoidable part (which can be reduced by enhancing the exergetic performance of the kth component only) is more attainable than the exogenous avoidable portion; the main attention should be paid on the endogenous avoidable part reduction. Focusing on the exogenous avoidable part (which can be decreased by improving the exergetic performance of the remaining components) should be on the next step [43]. As can be seen in Table 9, obtained endogenous avoidable exergy destruction rate for all components is higher than the exogenous avoidable part. Referring to Table 9, except for the HPVG, LPVG and COND, the endogenous avoidable exergy destruction is higher than the endogenous unavoidable. Among the all, the condenser has
14
the worst performance from the perspective of advanced exergy; because, 353.40 kW of the 416.20 endogenous exergy destruction rate is unavoidable. In other words, preventing 84.9% of the endogenous exergy destruction within the COND is not possible in spite of technological development. On the other hand, LPT and HPT are promising components from the viewpoint of advanced exergy analysis, which about 68.01 and 68.45% of their endogenous exergy destruction rates are avoidable, respectively. But in general, only 949.63 kW of the total 2643.22 kW endogenous exergy destruction rate is avoidable which means about 64.07% of system overall endogenous exergy destruction is not preventable. Table 9 clearly shows that the most part of the exogenous exergy destruction rate is unavoidable, which means that an improvement in exergetic performance of components do not guarantee a reduction in exogenous destroyed exergy. Moreover, the HPT has the most improvable performance, which is obtainable by improvement of other component's performance; because about 70.13% of its exogenous exergy destruction is avoidable which has the highest percentage of exogenous avoidable exergy destruction among all the components. For the case of LPT, the ,AV ,AV value of E EN is much higher than E EX . As a result, an optimizing the system exergetic D ,K D ,K
performance can be obtained by improving this component efficiency. Table 9
To give a clear imagination of the advanced exergy analysis, the exergy destruction rates of the system components are splitted into avoidable, unavoidable, endogenous, exogenous, exogenous avoidable, exogenous-unavoidable, endogenous-avoidable and endogenous-unavoidable parts which are indicated in Fig. 2. According to Fig. 2, HPT, LPT, HPFP and LPFP have a higher percentage of avoidable exergy destruction rates and HPVG, LPVG and COND have a higher percentage of unavoidable exergy
15
destruction rate. For the components with a higher percentage of avoidable exergy destruction, technical modifications can improve their exergetic performance which leads to system efficiency enhancement. Also, the highest percentage (67%) of endogenous avoidable exergy destruction belongs to the LPT and LPFP among all components. Fig.2
Fig. 3 illustrates contributions of the system components in total endogenous, exogenous, avoidable, unavoidable, endogenous avoidable, endogenous unavoidable, exogenous avoidable and exogenous unavoidable exergy destruction rates of the system. Also, good results can be obtained by comparing the conventional and advanced exergy analysis which is shown in Fig. 4. It is obvious from Figs. 3 and 4; the priority order for system components from the viewpoint of conventional exergy is LPVG followed by HPVG, COND, HPT, LPT, HPFP and then LPFP. But advanced exergy analysis reveals new aspects about exergy destruction. From the advanced exergy perspective, designers should focus first on the HPT and LPT more than COND due to higher avoidable exergy destruction rates. Moreover, the second highest endogenous exergy destruction rate (19.3%) belongs to LPT which indicates that this component’s performance is much improvable than HPVG, COND and HPT by technical modifications. On the other hand, HPT has a high percentage of exogenous available exergy destruction rate (23%) that can be eliminated by the other components performance enhancement. In addition, in spite of conventional exergy analysis results, for the case of COND, which has the third place between the components, the results of advanced exergy analysis don’t suggest any considerable improvement in this component. Fig.3 Fig. 4
16
5. Conclusion In the present study, conventional and advanced exergy analyses are applied to a geothermal heat driven dual fluid organic Rankine cycle. The main conclusions that can be obtained from the present work are listed as follows:
Based on the conventional exergy analysis results, LPVG and HPVG are the most important components by 38.11 and 29.98% of the total exergy destruction rate, respectively. Also, LPFP and HPFP are components with negligible exergy destruction. Moreover, COND with 15.93% of the total exergy destruction rate is the third effective component from the viewpoint of conventional exergy analysis.
From the perspective of the advanced exergy analysis, about 72% of the total exergy destruction rate is endogenous exergy destruction rate and for all of the system components this part is higher than exogenous exergy destruction.
In all the system components, except the HPVG, LPVG and COND, the endogenous avoidable exergy destruction is higher than the endogenous unavoidable. Also, LPT by 19.3% of the total endogenous avoidable exergy destruction rate has the second highest potential for performance improvement by technical modifications.
Only 15% of COND exergy destruction is avoidable which includes 7% of total avoidable exergy destruction rate. Therefore, despite the conventional exergy analysis results, COND is not important from the viewpoint of advanced exergy.
An improvement in the other component efficiencies improves HPT exergetic performance more than others because 23% of the total exogenous available exergy destruction rate belongs to HPT which is 17% of the HPT exergy destruction rate.
17
Nomenclature e
specific exergy (kJ/kg)
E
exergy rate (kW)
E D
exergy destruction rate
h
enthalpy (kJ/kg)
m
mass flow rate (kg/s)
P
pressure (kPa)
R
global gas constant (kJ/kg.K)
T
temperature ( C or K)
W
power (kW)
Q
heat transfer rate (kW)
s
entropy (kJ/kg.K)
Acronyms COND
condenser
HPE
high pressure evaporator
HPT
high pressure turbine
18
HPFP
high pressure feed pump
HPPH
high pressure preheater
HPVG
high pressure vapor generator
LPE
low pressure evaporator
LPT
low pressure turbine
LPFP
low pressure feed pump
LPPH
low pressure preheater
LPVG
low pressure vapor generator
ORC
organic Rankine cycle
Greek letters ɛ
exergy efficiency
Subscripts 0
ambient condition
ch
chemical
e
exit
D
destroyed
gw
geothermal water
19
i
inlet
in
input
k
kth component
min
minimum
ph
physical
Q
heat
W
work
Upscripts AV
avoidable
EN
endogenous
EX
exogenous
mix
mixture
UN
unavoidable
References [1] Ameri M, Ahmadi P, Hamidi A. Energy, exergy and exergoeconomic analysis of a steam power plant: a case study. International Journal of Energy Research. 2009 Apr 1;33(5):499-512. [2]
Eurostat.
Electricity
prices
for
household
eurostat.ec.europa.eu>.
20
consumers;
2013.
[3] Bianchi M, De Pascale A. Bottoming cycles for electric energy generation: parametric investigation of available and innovative solutions for the exploitation of low and medium temperature heat sources. Applied Energy. 2011 May 31;88(5):1500-9. [4] Heberle F, Brüggemann D. Exergy based fluid selection for a geothermal Organic Rankine Cycle for combined heat and power generation. Applied Thermal Engineering. 2010 Aug 31;30(11):1326-32. [5] Yari M. Exergetic analysis of various types of geothermal power plants. Renewable Energy. 2010 Jan 31;35(1):112-21. [6] Heberle F, Preißinger M, Brüggemann D. Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resources. Renewable Energy. 2012 Jan 31;37(1):364-70. [7] Liu Q, Duan Y, Yang Z. Performance analyses of geothermal organic Rankine cycles with selected hydrocarbon working fluids. Energy. 2013 Dec 15;63:123-32. [8] Zhai H, Shi L, An Q. Influence of working fluid properties on system performance and screen evaluation indicators for geothermal ORC (organic Rankine cycle) system. Energy. 2014 Sep 1;74:2-11. [9] Guzović Z, Rašković P, Blatarić Z. The comparision of a basic and a dual-pressure ORC (Organic Rankine Cycle): Geothermal Power Plant Velika Ciglena case study. Energy. 2014 Nov 1;76:175-86. [10] Kang Z, Zhu J, Lu X, Li T, Wu X. Parametric optimization and performance analysis of zeotropic mixtures for an organic Rankine cycle driven by low-medium temperature geothermal fluids. Applied Thermal Engineering. 2015 Oct 5;89:323-31.
21
[11] Sadeghi M, Nemati A, Yari M. Thermodynamic analysis and multi-objective optimization of various ORC (organic Rankine cycle) configurations using zeotropic mixtures. Energy. 2016 Aug 15;109:791-802. [12] Yang MH, Yeh RH. Economic performances optimization of an organic Rankine cycle system with lower global warming potential working fluids in geothermal application. Renewable Energy. 2016 Jan 31;85:1201-13. [13] Tsatsaronis G, Park MH. On avoidable and unavoidable exergy destructions and investment costs in thermal systems. Energy Conversion and Management. 2002 Aug 31;43(9):1259-70. [14] Vatani A, Mehrpooya M, Palizdar A. Advanced exergetic analysis of five natural gas liquefaction processes. Energy Conversion and Management. 2014 Feb 28;78:720-37. [15] Tsatsaronis G, Morosuk T. Advanced exergetic analysis of a refrigeration system for liquefaction of natural gas. International Journal of Energy and Environmental Engineering. 2010 Jan 1;1(1):1-8. [16] Kelly S, Tsatsaronis G, Morosuk T. Advanced exergetic analysis: Approaches for splitting the exergy destruction into endogenous and exogenous parts. Energy. 2009 Mar 31;34(3):384-91. [17] Morosuk T, Tsatsaronis G. A new approach to the exergy analysis of absorption refrigeration machines. Energy. 2008 Jun 30;33(6):890-907. [18] Petrakopoulou F, Tsatsaronis G, Morosuk T, Carassai A. Advanced exergoeconomic analysis applied to a complex energy conversion system. Journal of Engineering for Gas Turbines and Power. 2012 Mar 1;134(3):031801. [19] Fallah M, Mahmoudi SM, Yari M, Ghiasi RA. Advanced exergy analysis of the Kalina cycle applied for low temperature enhanced geothermal system. Energy Conversion and Management. 2016 Jan 15;108:190-201.
22
[20] Soltani S, Yari M, Mahmoudi SM, Morosuk T, Rosen MA. Advanced exergy analysis applied to an externally-fired combined-cycle power plant integrated with a biomass gasification unit. Energy. 2013 Sep 15;59:775-80. [21] Hepbasli A, Keçebaş A. A comparative study on conventional and advanced exergetic analyses of geothermal district heating systems based on actual operational data. Energy and Buildings. 2013 Jun 30;61:193-201. [22] Keçebaş A, Hepbasli A. Conventional and advanced exergoeconomic analyses of geothermal district heating systems. Energy and Buildings. 2014 Feb 28;69:434-41. [23] Gungor A, Erbay Z, Hepbasli A, Gunerhan H. Splitting the exergy destruction into avoidable and unavoidable parts of a gas engine heat pump (GEHP) for food drying processes based on experimental values. Energy Conversion and Management. 2013 Sep 30;73:309-16. [24] Gungor A, Tsatsaronis G, Gunerhan H, Hepbasli A. Advanced exergoeconomic analysis of a gas engine heat pump (GEHP) for food drying processes. Energy Conversion and Management. 2015 Feb 28;91:132-9. [25] Açıkkalp E, Aras H, Hepbasli A. Advanced exergoeconomic analysis of an electricitygenerating facility that operates with natural gas. Energy Conversion and Management. 2014 Feb 28;78:452-60. [26] Anvari S, Saray RK, Bahlouli K. Conventional and advanced exergetic and exergoeconomic analyses applied to a tri-generation cycle for heat, cold and power production. Energy. 2015 Nov 30;91:925-39. [27] Açıkkalp E, Aras H, Hepbasli A. Advanced exergoeconomic analysis of a trigeneration system using a diesel-gas engine. Applied Thermal Engineering. 2014 Jun 30;67(1):388-95.
23
[28] Nami H, Akrami E, Ranjbar F. Hydrogen production using the waste heat of Benchmark pressurized Molten carbonate fuel cell system via combination of organic Rankine cycle and proton exchange membrane (PEM) electrolysis. Applied Thermal Engineering. 2017 Mar 5;114:631-8. [29] Nemati A, Barzegar R, Khalil AS, Khatamnezhad H. Decreasing the emissions of a partially premixed gasoline fueled compression ignition engine by means of injection characteristics and EGR. Thermal Science. 2011;15(4):939-52. [30] Nami H, Ranjbar F, Yari M, Saeidi S. Thermodynamic analysis of a modified oxy-fuel cycle, high steam content Graz cycle with a dual-pressure heat recovery steam generator. International Journal of Exergy. 2016;21(3):331-46. [31] Nemati A, Barzegar R, Khalilarya S. The effects of injected fuel temperature on exergy balance under the various operating loads in a DI diesel engine. International Journal of Exergy. 2015;17(1):35-53. [32] Nami H, Mohammadkhani F, Ranjbar F. Utilization of waste heat from GTMHR for hydrogen generation via combination of organic Rankine cycles and PEM electrolysis. Energy Conversion and Management. 2016 Nov 1;127:589-98. [33] Nami H, Mahmoudi SM, Nemati A. Exergy, economic and environmental impact assessment and optimization of a novel cogeneration system including a gas turbine, a supercritical CO2 and an organic Rankine cycle (GT-HRSG/SCO2). Applied Thermal Engineering. 2017 Jan 5;110:1315-30.
24
[34] Nemati A, Nami H, Yari M, Ranjbar F, Kolvir HR. Development of an exergoeconomic model for analysis and multi-objective optimization of a thermoelectric heat pump. Energy Conversion and Management. 2016 Dec 15;130:1-3. [35] Nemati A, Nami H, Ranjbar F, Yari M. A comparative thermodynamic analysis of ORC and Kalina cycles for waste heat recovery: A case study for CGAM cogeneration system. Case Studies in Thermal Engineering. 2017 Mar 31;9:1-3. [36] Morosuk T, Tsatsaronis G. Advanced exergetic evaluation of refrigeration machines using different working fluids. Energy. 2009 Dec 31;34(12):2248-58. [37] Tsatsaronis G, Morosuk T. A General Exergy-Based Method for Combining a Cost Analysis With an Environmental Impact Analysis: Part II—Application to a Cogeneration System. InASME 2008 International Mechanical Engineering Congress and Exposition 2008 Jan 1 (pp. 463-469). American Society of Mechanical Engineers. [38] Rodríguez CE, Palacio JC, Venturini OJ, Lora EE, Cobas VM, dos Santos DM, Dotto FR, Gialluca V. Exergetic and economic comparison of ORC and Kalina cycle for low temperature enhanced geothermal system in Brazil. Applied Thermal Engineering. 2013 Apr 5;52(1):109-19. [39] Petrakopoulou F. Comparative evaluation of power plants with CO 2 capture: thermodynamic, economic and environmental performance. [40] Zare V. A comparative exergoeconomic analysis of different ORC configurations for binary geothermal power plants. Energy Conversion and Management. 2015 Nov 15;105:127-38.
25
[41] Zare V, Mahmoudi SM. A thermodynamic comparison between organic Rankine and Kalina cycles for waste heat recovery from the Gas Turbine-Modular Helium Reactor. Energy. 2015 Jan 1;79:398-406. [42] Boyaghchi FA, Molaie H. Investigating the effect of duct burner fuel mass flow rate on exergy destruction of a real combined cycle power plant components based on advanced exergy analysis. Energy Conversion and Management. 2015 Oct 31;103:827-35. [43] Tsatsaronis G, Morosuk T. Advanced exergetic analysis of a novel system for generating electricity and vaporizing liquefied natural gas. Energy. 2010 Feb 28;35(2):820-9.
26
Figures:
Fig. 1 Schematic diagram of the dual fluid ORC
27
HPT
LPT
HPVG (HPPH + HPE)
LPVG (LPPH + LPE)
28
COND
HPFP
LPFP
29
Fig.2 Splitting exergy destruction rates of dual fluid ORC components into endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable, endogenous-unavoidable, exogenous-avoidable and exogenous-unavoidable parts.
30
EN
EX
LPT LPFP 0% 0% HPT 8%
HPE+ HPPH 29%
HPT 7%
LPFP 1% HPFP 0%
LPT 10%
COND 16%
HPE+ HPPH 43%
COND 16% LPE+ LPPH 36%
HPFP 1%
LPE+ LPPH 33%
AV
UN
HPT LPT 4% 4% LPFP 0%HPFP 0%
HPT 17% HPE+ HPPH 35%
HPE+ HPPH 39%
LPT 16%
LPE+ LPPH 33%
LPFP 1%
LPE+ LPPH 23%
COND 7%
COND 20%
HPFP 1%
EN_AV
EN_UN
31
HPT 4%
HPT 16% HPE+ HPPH 38%
HPE+ HPPH 36%
LPT 19%
LPE+ LPPH 19%
LPT LPFP 5% 0% HPFP 0%
LPFP 1% HPFP COND 1% 6%
COND 21%
LPE+ LPPH 34%
EX_AV
EX_UN
HPT LPT LPFP 2% 0% 0% HPE+ HPPH 24%
HPT 23%
HPFP 0%
LPT 1%
COND 18%
LPFP 0% LPE+ LPPH 39%
COND 12%
HPE+ HPPH 48%
HPFP 1%
LPE+ LPPH 32%
Fig.3 Contributions of dual fluid ORC components in the total endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable, endogenous-unavoidable, exogenous-avoidable and exogenous-unavoidable exergy destruction rates of the system.
32
Fig. 4 Contribution of each component on cycle overall exergy destruction rate obtained from conventional and advanced exergy analyses (the contributions of the HPFP and LPFP are zoomed for clarification).
33
Tables: Table 1 Exergy balance equations for different components of dual fluid ORC. Components
Exergy destruction
Exergy efficiency
HPT
E1 E 2 W HPT
W HPT E1 E 2
LPT
E 6 E 7 W LPT
W LPT E 6 E 7
HPE+ HPPH
E gw ,1 E gw ,3 E 4 E1
E1 E 4 E gw ,1 E gw ,3
LPE+ LPPH
E 9 E 2 E gw ,3 E 6 E 3 E gw ,4
E 6 E 9 (E 2 E 3 E gw ,3 E gw ,4 )
HPFP
E 3 W HPFP E 4
E 4 E 3 W HPFP
LPFP
E 8 W LPFP E 9
E 9 E 8 W LPFP
COND
E cw,1 E 7 E cw,2 E 8
E cw,2 E cw,1 E 7 E 8
34
Table 2 The main assumptions for the dual fluid ORC under real, ideal and unavoidable conditions.
Component HPT
Real
Idealb
Unavoidable
s 0.85 a
s 1
s 0.95 c
T min 10Kd
T min 0K
T min 3Kf
HPVG
P 0%
PHPE 2%e
(HPE+ HPPH)
P 1%f
PHPPH 3%b HPFP
s 0.8 a
s 1
s 0.95 c, f
LPT
s 0.85 a
s 1
s 0.95 c
T min 10Kd
T min 0K
T min 3Kf
LPVG
P 0%
PLPE 2%e
(LPE+ LPPH)
P 1% f
PLPPH 3%b LPFP
s 0.8 a
s 1
s 0.95 c, f
T min 5Kd
T min 0K
T min 3Kb ,f
COND
P 0%
P 1% g
a
Ref. [38].
b
Ref. [19].
c
Ref. [39].
d
Ref. [40].
35
e
Ref. [41].
P 0.5% b, f
f
Ref. [42].
g
Ref. [20].
Table 3 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under real conditions.
Fluid
P(kPa)
T (˚C)
m (kg/s)
h (kJ/kg)
s (kJ/kg k)
E (kW)
1
Isopentane
1236
126.8
56.88
150.4
-0.3482
5708
2
Isopentane
400
95.75
56.88
115.1
-0.3313
3414
3
Isopentane
392
73.8
56.88
-232.7
-1.328
528.5
4
Isopentane
1300
74.38
56.88
-230.8
-1.327
622.7
5
Isopentane
1261
127.9
56.88
-82.91
-0.9316
2332
6
Isobutane
928.1
62.91
71.22
637.8
2.347
6305
7
Isobutane
469.1
42
71.22
615
2.36
4408
8
Isobutane
464.5
35
71.22
284.3
1.288
3608
9
Isobutane
976.3
35.36
71.22
285.5
1.289
3676
10
Isobutane
947
63.8
71.22
360.1
1.52
4077
1gw
Water
7000
175
83
744.5
2.083
10621
2gw
Water
7000
137.9
83
584.6
1.711
6565
3gw
Water
7000
114
83
483.3
1.457
4439
4gw
Water
7000
98.81
83
419.3
1.288
3300
1cw
Water
100
25
1009
104.8
0.3669
0
2cw
Water
100
30.58
1009
128.2
0.4445
216.3
State no.
ε = 47.03%, W HPT 2007 (kW), W LPT 1624 (kW), W net 3443 (kW)
36
Table 4 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under ideal conditions.
Fluid
P(kPa)
T (˚C)
m (kg/s)
h (kJ/kg)
s (kJ/kg k)
E (kW)
1
Isopentane
1300
129.5
38.75
154.3
-0.3428
3977
2
Isopentane
400
93.74
38.75
110.9
-0.3428
2294
3
Isopentane
400
74.6
38.75
-230.6
-1.322
371.8
4
Isopentane
1300
75.07
38.75
-229
-1.322
434
5
Isopentane
1300
129.5
38.75
-77.95
-0.9195
1641
6
Isobutane
1199
74.6
50.58
651.8
2.359
5002
7
Isobutane
464.5
41.62
50.58
614.4
2.359
3110
8
Isobutane
464.5
35
50.58
284.3
1.288
2562
9
Isobutane
1199
35.42
50.58
285.7
1.288
2631
10
Isobutane
1199
74.6
50.58
390.3
1.607
3115
1gw
Water
7000
175
46.04
744.5
2.083
5891
2gw
Water
7000
129.5
46.04
549
1.623
3204
3gw
Water
7000
99.44
46.04
421.9
1.295
1854
4gw
Water
7000
71.99
46.04
307
0.975
961.8
1cw
Water
100
25
383.9
104.8
0.3669
0
2cw
Water
100
35.4
383.9
148.3
0.5103
284
State no.
ε = 69.85%, W HPT 1682 (kW), W LPT 1892 (kW), W net 3443 (kW)
37
Table 5 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under unavoidable conditions.
Fluid
P(kPa)
T (˚C)
m (kg/s)
h (kJ/kg)
s (kJ/kg k)
E (kW)
1
Isopentane
1274
128.5
43.74
152.7
-0.345
4450
2
Isopentane
400
94.38
43.74
112.2
-0.3392
2601
3
Isopentane
396
74.2
43.74
-231.6
-1.325
413.1
4
Isopentane
1300
74.7
43.74
-229.9
-1.324
484
5
Isopentane
1287
129
43.74
-79.59
-0.9235
1833
6
Isobutane
1104
70.74
56.4
647.3
2.355
5389
7
Isobutane
466.8
42
56.4
615.1
2.361
3480
8
Isobutane
464.5
35
56.4
284.3
1.288
2857
9
Isobutane
1126
35.41
56.4
285.6
1.289
2926
10
Isobutane
1115
71.2
56.4
380.6
1.58
3390
1gw
Water
7000
175
54.93
744.5
2.083
7029
2gw
Water
7000
132
54.93
559.5
1.649
3973
3gw
Water
7000
103.7
54.93
439.7
1.343
2413
4gw
Water
7000
80.42
54.93
342.2
1.076
1431
1cw
Water
100
25
596.4
104.8
0.3669
0
2cw
Water
100
32.48
596.4
136.1
0.4705
229.3
State no.
ε = 61.5%, W HPT 1773 (kW), W LPT 1818 (kW), W net 3443 (kW)
38
Table 6 Results of the exergy analysis for dual fluid ORC under real conditions.
y
ED (%) E in ,tot
y*
ED (%) E D ,tot
E F (kW)
E P (kW)
E D (kW)
(%)
HPT
2294
2007
287
87.49
3.83
7.84
LPT
1896
1624
272
85.65
3.63
7.43
6181
5085
1096
82.27
14.63
29.96
4024
2629
1395
65.33
18.61
38.13
800.6
217.7
582.9
27.19
7.78
15.93
108.2
94.16
14.04
87.02
0.19
0.38
79.9
68.28
11.62
85.46
0.16
0.32
7321
3443
3658.56
47.03
48.83
100.00
Component
HPVG (HPE+ HPPH) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
39
Table 7 Results of the exergy analysis for dual fluid ORC under ideal conditions.
y
ED (%) E in ,tot
y*
ED (%) E D ,tot
E F (kW)
E P (kW)
E D (kW)
(%)
HPT
1682
1682
0
100.00
0.00
0.00
LPT
1892
1892
0
100.00
0.00
0.00
4037
3543
494
87.76
6.59
41.11
2815
2371
444
84.23
5.92
36.95
548
284.5
263.5
51.92
3.52
21.93
62.21
62.13
0.08
99.87
0.00
0.01
69.23
69.15
0.08
99.88
0.00
0.01
4929
3443
1201.66
69.85
24.38
100.00
Component
HPVG (HPE+ HPPH) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
40
Table 8 Results of the exergy analysis for dual fluid ORC under unavoidable conditions.
y
ED (%) E in ,tot
y*
ED (%) E D ,tot
E F (kW)
E P (kW)
E D (kW)
(%)
HPT
1849
1773
76
95.89
1.01
3.95
LPT
1908
1818
90
95.28
1.20
4.68
4616
3966
650
85.92
8.67
33.78
3170
2462
708
77.67
9.45
36.79
623.6
230.1
393.5
36.90
5.25
20.45
74.19
70.92
3.27
95.59
0.04
0.17
73.2
69.59
3.61
95.07
0.05
0.19
5598
3443
1924.38
61.50
34.38
100.00
Component
HPVG (HPE+ HPPH) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
41
Table 9 Results of the advanced exergy analysis for dual fluid ORC.
ED
E EN D ,K
E EX D ,K
E AV D ,K
E UN D ,K
,AV E EN D ,K
,UN E EN D ,K
,AV E EX D ,K
,UN E EX D ,K
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
287.00
218.00
69.00
196.66
90.34
148.27
69.73
48.39
20.61
272.00
267.60
4.40
185.88
86.12
183.18
84.42
2.70
1.70
1097.00
759.60
337.40
263.60
833.40
180.50
579.10
83.10
254.30
1395.00
961.20
433.80
410.80
984.20
360.50
600.70
50.30
383.50
582.90
416.20
166.70
88.10
494.80
62.80
353.40
25.30
141.40
14.04
9.50
4.54
9.79
4.25
6.62
2.88
3.17
1.37
11.62
11.12
0.50
8.12
3.50
7.76
3.36
0.36
0.14
3659.56
2643.22
1016.34
1162.95
2496.61
949.63
1693.59
213.32
803.02
Component
HPT LPT HPVG (HPPH + HPE) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
42
Figure captions Fig. 1 Schematic diagram of the dual fluid ORC Fig.2 Splitting exergy destruction rates of dual fluid ORC components into endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable, endogenous-unavoidable, exogenous-avoidable and exogenous-unavoidable parts. Fig.3 Contributions of dual fluid ORC components in the total endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable,
endogenous-unavoidable,
exogenous-avoidable
and
exogenous-unavoidable
exergy
destruction rates of the system. Fig. 4 Contribution of each component on cycle overall exergy destruction rate obtained from conventional and advanced exergy analyses (the contributions of the HPFP and LPFP are zoomed for clarification).
43
Table Captions: Table 1 Exergy balance equations for different components of dual fluid ORC. Table 2 The main assumptions for the dual fluid ORC under real, ideal and unavoidable conditions. Table 3 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under real conditions. Table 4 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under ideal conditions. Table 5 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under unavoidable conditions.
Table 6 Results of the exergy analysis for dual fluid ORC under real conditions. Table 7 Results of the exergy analysis for dual fluid ORC under ideal conditions. Table 8 Results of the exergy analysis for dual fluid ORC under unavoidable conditions. Table 9 Results of the advanced exergy analysis for dual fluid ORC.
Highlights
The advanced exergy analysis has been applied to the geothermal driven dual fluid ORC.
Exergy destruction unavoidable/avoidable.
The interactions between the components are found to be weak. The advanced exergy analysis presents different and more pragmatic results.
analyzed
44
as
endogenous/exogenous
and
45
S1359-4311(17)31712-X http://dx.doi.org/10.1016/j.applthermaleng.2017.05.011 ATE 10322
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
13 March 2017 22 April 2017 4 May 2017
Please cite this article as: H. Nami, A. Nemati, F. Jabbari Fard, Conventional and advanced exergy analyses of a geothermal driven dual fluid organic Rankine cycle (ORC), Applied Thermal Engineering (2017), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2017.05.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Conventional and advanced exergy analyses of a geothermal driven dual fluid organic Rankine cycle (ORC) Authors Hossein Nami*, Arash Nemati*, Farshad Jabbari Fard Faculty of Mechanical Engineering, University of Tabriz, 29th Bahman Blvd., Tabriz, Iran *Corresponding authors: Email address: [email protected], [email protected] Phone number: +98 9367848823 Abstract A study on geothermal driven dual fluid organic Rankine cycle is presented in this paper using conventional and advanced exergy analysis methods to provide information about system components interactions. Special operating conditions are performed for system components in order to obtain real, unavoidable and ideal performance. The exergy destruction rate in each component is divided into endogenous, exogenous, avoidable and unavoidable parts to reveal more detailed information about effects of components inefficiency on each other exergy destruction and the real potential of the system for improvement. The conventional exergy analysis reveals that, low pressure vapor generator (LPVG), high pressure vapor generator (HPVG) and condenser (COND) are the most important component by 38.11, 29.98 and 15.93% of the total exergy destruction rate, respectively. Despite the conventional exergy analysis results, advanced exergy shows that only 15% of the COND exergy destruction is avoidable which includes 7% of system avoidable exergy destruction rate. Also, LPVG, low pressure 1
turbine (LPT) and HPVG are the most important components from the viewpoint of advanced exergy because of their considerable endogenous avoidable exergy destruction rates. Keywords: Advanced exergy, Conventional exergy, Dual fluid ORC, Geothermal heat source 1. Introduction Developing technology has urged investigators and engineers to design more efficient energy conversion systems. This duty becomes even more important if we consider the increasing demand for energy by the developed countries in spite of the limited resources of fossil fuel [1]. Ozone layer destruction and electricity price raising, motivate governments to use renewable sources for power generation [2]. Low grade heat power generation systems like organic Rankine cycle (ORC), Kalina cycle (KC) and the other ones may be solutions for these problems. Because of the ORCs simplicity of the technology, availability of components, reliability, and flexibility, they are considered as a practical solution [3]. Besides waste heat, geothermal, solar and ocean thermal energy sources is available to provide a clean alternative to fossil fuel combustion in ORCs. Many researches have been done on geothermal energy as a low-grade heat source for ORCs from the perspective of conventional energy and exergy analysis, recently. Heberle and Brüggemann [4] presented an exergy based analysis for working fluid selection in a geothermal organic Rankine cycle. They concluded that, fluids with lower critical temperatures, like R227ea and isobutane are more preferable for power generation in ORC. Yari [5] studied various types of geothermal ORC power plants from the perspective of exergy and concluded that, combined flash-binary with R123 as working fluid had the highest exergy efficiency. Heberle et. al [6] investigated zeotropic mixtures as working fluid in ORCs for geothermal low-
2
grade energy. They reported that, the temperature glides between saturated vapor of zeotropic mixtures and saturated liquid at a particular pressure, increases the exergy efficiency in comparison to pure fluids. Effect of working fluid on geothermal ORC performance has been investigated by Liu et al. [7]. They reported that, R600a as working fluid had the maximum net output power followed by R245fa and R600 for geothermal heat sources with temperature higher than 120 ˚C and reinjection temperatures higher than 70 ˚C. Zhai et al. [8] compared HFC (hydro fluorine carbon) and HC (hydro carbon) fluids as working fluid in geothermal ORC systems and concluded that, the amount of energy which is absorbed from the heat source has an important effect on ORC output power. They also stated that, working fluids with higher ratio of specific heat capacity to latent heat of evaporation absorb more energy and produce more power. Guzovic et al. [9] compared basic and a dual-pressure ORC in a geothermal power plant and resulted that, the dual-pressure ORC has higher both efficiency (65% vs. 52%) and net power (6371 kW vs. 5270 kW). The influence of zeotropic mixtures in a low-medium temperature geothermal ORC has been analyzed by Kang et al. [10]. They found that evaporating temperature and zeotropic mixture ratio that gives the highest temperature glide has a significant effect on the ORC power generation and they suggested that R245fa/R600a (0.9/0.1) is the most preferable mixture. Sadeghi et al. [11] studied different zeotropic working fluids in various ORC configurations using geothermal energy as a heat source. They reported that, the main source of exergy destruction in ORCs are evaporators and using two series evaporators and zeotropic working fluid lead to a significant reduction in the total exergy destruction. Yang and Yeh [12] carried out an economic study on geothermal ORC performance and they reported that the importance of the pinch point temperature differences in the evaporator is much more than those in the condenser.
3
The conventional exergy analysis plays an important role in the exergy determination in the mentioned researches. Conventional exergy claims that, the most important component is the one with the highest exergy destruction and suggests more attention to the mentioned component for system performance improvement. But there is no hint that how much of the exergy destruction is unavoidable and how much is because of the effect of irreversibilities of other components. To overcome these shortcomings, the concept of the advanced exergy analysis has been proposed by researchers in recent years. Advanced exergy reveals interactions between the components of a system to estimate the real potential for development. In this approach, exergy destruction in each component is divided into avoidable part and unavoidable part and also into endogenous and exogenous exergy destruction [13]. Recently, more and more researchers use advanced exergy analysis method to investigate the exergy destruction in various energy conversion systems. The most important systems are natural gas liquefaction systems [14, 15], gas turbine power systems [16-18], Kalina cycle [19], biomass gasification units [20], geothermal district heating systems [21, 22], gas engine heat pumps for food drying processes [23, 24], cogeneration and tri-generation systems [25-27] and some other refrigerant systems. Based on the most recent published papers, the dual fluid ORC has higher power and efficiency than that of simple ORC. Furthermore, the possibility of choosing various working fluids for each loop of cycle can help designers to achieve the favorable operating conditions. To the best of the author's knowledge and by surveying the mentioned literatures, there is no research in which the advanced exergy analysis has been applied to the dual fluid ORC power plant with geothermal heat source. In order to cover the existing shortcomings in the literature, the performance of a geothermal driven dual fluid ORC power plant is thermodynamically
4
modeled. Then, to reveal more detailed information about components inefficiency influences on each other, exergy destruction and the real potential of the system for improvement, the advanced exergy methodology is adopted. Splitting the exergy destruction rates within each system component is schematically described and interpreted. The results are expected to help in geothermal driven dual fluid ORC system performance enhancement, considering the main sources of exergy destruction which is avoidable by better designing of the system. These results cannot be attained from the conventional exergy analysis and reported in this study for the first time. 2. System description and assumptions The schematic diagram of dual fluid ORC which is used in the geothermal power plant is shown in Fig. 1. The main purpose of this system is to produce power from the geothermal water as a heat source. As seen in the figure, a dual fluid ORC consists of nine different parts: a high pressure evaporator (HPE), a high pressure preheater (HPPH), a low pressure evaporator (LPE), a low pressure preheater (LPPH), a high pressure turbine (HPT), a low pressure turbine (LPT), a condenser (COND), a high pressure feed pump (HPFP) and a low pressure feed pump (LPFP). The working fluid of high pressure ORC preheats at the HPPH and enters the HPE at the saturated liquid condition. Having passed the HPE, the working fluid enters the HPT as saturated vapor and the HPT exit flow heats the organic fluid of low pressure ORC. Heated low pressure working fluid flows to the LPT at the saturated vapor condition where it expands and produces power. The expanded working fluid at the LPT enters the condenser and cools down to condenser temperature. HPPH exit geothermal fluid preheats the low pressure organic fluid in the LPPH and then heated fluid flows to the LPE. Fig. 1
5
The following assumptions are made in this work:
Mass flow rate, temperature and pressure of the geothermal water are considered to be 83 kg/s, 175 ˚C and 7 MPa, respectively [9].
The isentropic efficiency is considered for the turbines and pumps.
Ambient temperature and pressure are 25 ˚C and 1 bar, respectively.
The system operates under steady state condition [28].
The cooling water enters the condenser at the ambient condition.
Isopentane and Isobutane are considered as the working fluids of high and low pressure cycles, respectively.
External cycleʼs performances such as the cooling water circulation pumps of the condenser and geothermal water pumps are not taken into account, because the conventional and advanced exergy performance of the dual fluid ORC is independent from the geothermal water pumps, while the performances of the cooling water circulation pumps are so low and can be neglected.
3. Thermodynamic analysis 3.1.Conventional exergy analysis Considering each component as control volume, the mass and energy conservation equations as long as exergy balance equations are applied to the system components as follows [29, 30]:
m m
(1)
Q m i hi W m e he
(2)
i
e
6
E Q m i ei EW m e ee E D
(3)
where, EW W
(4)
T E Q Q (1 0 ) T
(5)
In these equations, subscripts i and e symbolize the component inlet and exit, respectively. Also,
Q is the rate of heat transfer, E W is the exergy rate of mechanical power, E Q is the exergy rate associated with the heat transfer, and E D is the exergy destruction rate. Moreover, T 0 is the ambient temperature. The specific exergy is the sum of specific physical and chemical exergy: e e ph ech
(6)
Neglecting the Potential and Kinetic exergies, the specific physical exergy can be expressed as follows [31]: e ph h h0 T0 (s s0 )
(7)
Also, the specific chemical exergy for a mixture of ideal gas can be written as follows [32-33]: n
n
i 1
i 1
echmix X i echi R T0 X i ln( X i )
(8)
7
here, X i is the molar fraction of a mixture component. Finally, the exergetic efficiency of dual fluid ORC as an energy system can be defined as follows [34, 35]:
W
net Ein
(9)
For the case of dual fluid ORC, W net is the net produced power and E in is the total entrance exergy rate provided by the geothermal fluid: W net W HPT W LPT W HPFP W LPFP
(10)
E in E gw,in E gw,out
(11)
The exergy balance equations for each component of dual fluid ORC are outlined in Table 1. Table 1
3.2.Advanced exergy analysis The advanced exergy analysis considers the interactions among system components and splits the exergy destruction rate in the kth component of the system into two parts, named as exogenous and endogenous exergy destructions:
E D,k E DEX, K E DEN, K
(12)
The exogenous exergy destruction rate in a component ( E DEX, K ) is due to the irreversibilities taking place in remaining components where the objective component operates under the ideal condition. Also, the endogenous exergy destruction rate in a component ( E DEN,K ) is associated to the irreversibility of the component itself, which operates under real conditions and others 8
operate under ideal conditions. To analyze the real locations of irreversibilities in the energy systems, splitting destroyed exergy into exogenous and endogenous parts provides information not obtainable by the conventional exergy analysis. So that, one can decide where to pay more attention in improving the system exergetic performance. Also, the rate of exergy destruction in kth component of an energy converting system can be split into avoidable and unavoidable parts [13]:
E D,k E DAV, K E DUN, K
(13)
In fact, the unavoidable part of destroyed exergy within a component is due to technical limitations (such as availability and cost of materials) which cannot be reduced [36]. The remaining exergy destruction represents the avoidable exergy destruction rate, which can be reduced or optimized by optimizing processes. Therefore, splitting the exergy destruction rate within a component into its unavoidable and avoidable parts highlights the potential for improving exergy efficiency of each system component. In addition, the combination of these two mentioned splitting concepts provides useful definitions of exergy destruction. The results of combination of endogenous, exogenous, avoidable and unavoidable exergy destruction rate are endogenous-avoidable, exogenousavoidable, endogenous-unavoidable and exogenous-unavoidable. The endogenous-avoidable exergy destruction rate is because of system components itself, which can be reduced by improving the component efficiency while the exogenous-avoidable represents the exergy destruction rate due to irreversibility of other components which can be controlled by improving the exergetic performance of the overall system. Detailed exergy destructions can be calculated as follows [37]: 9
ED E DEN,K,UN E pEN ,k EP
UN
k
(14)
EN ,UN E DEX,K,UN E UN D ,K E D ,K
(15)
E DEN,K,AV E DEN,K E DEN,K,UN
(16)
E DEX,K,AV E DEX,K E DEX,K,UN
(17)
Several methods have been provided by researchers for the advanced exergy analysis. Thermodynamic cycle method, exergy balance method, engineering method, equivalent components method and structural theory method are the known advanced exergy approaches. Among the all, thermodynamic method is the most convenient method which offers the best solution results for thermodynamic systems. The second one is engineering method which provides acceptable results. On the other hand, the structural theory method is not suitable to be applied to systems with higher values of endogenous exergy destruction. Also, the exergy balance method is used for more complex cycles. More details about advanced exergy approaches can be found in literature [16]. The thermodynamic cycle method is used for the present study due to its higher prediction accuracy. In the present work, to identify different parts of exergy destruction rates, models such as ideal, hybrid and real cycles are considered. It is worth mentioning that, in the ideal cycle, all components operate under the reversible conditions and also all processes considered to be irreversible in the real cycle. In order to calculate the endogenous exergy destruction in kth component of the hybrid cycle, the process occurring within the component is considered to be irreversible and all processes occurring in remaining components are considered reversible. Also, 10
it is valuable to note that, the number of hybrid cycles in the advanced exergy analysis of a system is equal to the number of system components. Table 2 represents the main points considered for the dual fluid ORC under ideal, real and unavoidable conditions. Table 2
4. Results and discussions 4.1.Results of the conventional exergy analysis Thermodynamic properties and mass flow rates of investigated dual fluid ORC at different state points under real, ideal and unavoidable conditions are outlined in Tables 3-5, respectively. Furthermore, the obtained values for the exergy efficiency, produced power by the HPT, LPT and net produced power are outlined in these tables. In order to analyze the ideal and unavoidable systems, the net produced power is considered as equal to the real systems’ net produced power (3443 kW). Regarding to the different operating conditions of the real, ideal and unavoidable, in order to achieve the same value of net produced power, various amounts of working fluid mass flow rates are calculated. Therefore, a reduction in mass flow rate of geothermal water is observed for the ideal and unavoidable system which leads to a decrease in working fluid mass flow rate. In fact, lower values of workings fluid mass flow rates in the ideal and unavoidable systems are due to irreversibility reduction in compare to the real system. It should be noted that, the exergy delivered by the geothermal water to the system working fluid is the exergy difference between the incoming and outgoing geothermal water. Also, the net produced power is considered as the total product exergy. Referring to Tables 3-5, the second law efficiency for real, unavoidable and ideal cycles are achieved 47.03%, 61.5% and 69.85%, 11
respectively. Therefore, technical limitations reduce the exergy efficiency of the system by about 8.35% in comparison to the ideal condition. Table 3 Table 4 Table 5
The results of the conventional exergy analysis for the real, ideal and unavoidable conditions are presented in Tables 6-8. In Tables 6, 7 and 8, all the components operate under the real, ideal and unavoidable conditions, respectively. These tables include some important exergetic parameters such as fuel, product and destroyed exergy rates, exergy efficiency, the ratio of the exergy destruction rate to total entering exergy rate and the ratio of the exergy destruction rate to the total exergy destruction rate for each component of the systems. According to Table 6, in real conditions, highest exergy destruction belongs to low pressure vapor generator (LPVG) followed by high pressure vapor generator (HPVG) and condenser (COND). High values of exergy destruction are mainly due to temperature mismatching between the hot and cold streams in these heat exchangers. Conventional exergy analysis proposes to focus on components with high values of exergy destruction. Therefore, from the viewpoint of conventional exergy analysis, designers should pay more attention to the mentioned components to improve the exergetic performance of the system. According to Tables 6-8, to produce the same value of net output power, real and unavoidable systems have about 204 and 60% more exergy destruction in comparison to the ideal system. Table 6 Table 7 Table 8
4.2.Results of the advanced exergy analysis 12
The obtained exergy destruction in the previous section can be explained in detail by the advanced exergy analysis. As mentioned before, destroyed exergy by each component can be divided into two parts, namely endogenous (the irreversibility due to internal inefficiencies of component) and exogenous (the irreversibility due to other components inefficiencies). By the fact, the advanced exergy analysis highlights the effects of technical limitations and different components interaction on the exergetic performance of the system. For calculation of the detailed parts of exergy destruction in each component, the endogenous exergy destruction rate, E EN D ,K , is obtained first. Then, using Eq. 12, the exogenous exergy destruction rate, E EX D ,K , is calculated. To find the value of the unavoidable exergy destruction rate in kth component, E UN D ,K , the unavoidable condition is considered instead of the real condition for the kth component, while others operate under the real condition. Also, avoidable EN ,UN exergy destruction rate, E AV is D ,K , is calculated using Eq. 13. Furthermore, the value of E D ,K
obtained using unavoidable condition instead of the real condition for the kth component in E EN D ,K ,UN ,AV . By introducing the value of E EN in Eq. 16, the amount of E EN is calculated. Finally, D ,K D ,K ,UN ,AV with the known values of E EN and E EN and using Eqs. 15 and 17, the amounts of D ,K D ,K
,AV E EX D ,K
,UN and E EX are determined. D ,K
Table 9 outlines the results of above mentioned calculation for different components of the system. Referring to Table 9, the value of the endogenous exergy destruction rate is higher than the value of the exogenous exergy destruction rate for all system components. Therefore, it can be concluded that the main part of exergy destruction in each component is due to its irreversibility itself. This means in order to improve the system performance, more attention should be paid on the internal irreversibilities of the components. As it is obvious in Table 9, the 13
greatest amounts of exogenous exergy destruction rate belong to LPVG, HPVG and condenser, respectively. So, it can be suggested that an improvement in other component's exegetic performance makes a reduction in the exogenous exergy destruction of the above mentioned components. It is worth mentioning that, the avoidable part of exergy destruction can only be controlled. Referring to Table 9, the highest value of the avoidable exergy destruction rate relates to LPVG, HPVG, HPT and LPT, respectively. It should be noted that, COND has one the most highest amounts of exergy destruction from the viewpoint of conventional exergy analysis while based on the advanced exergy analysis, it is not an effective component (due to its low value of avoidable exergy destruction). This clarifies the advantage of the advanced exergy analysis in comparison to the conventional exergy analysis. Furthermore, based on the Table 9, the total unavoidable exergy destruction rate is about two times more than avoidable part which point out only about 32% of destroyed exergy can be declined. Table 9 provides some important information for the system performance enhancement by the exogenous avoidable and endogenous avoidable exergy destruction rates. Since, a reduction in endogenous avoidable part (which can be reduced by enhancing the exergetic performance of the kth component only) is more attainable than the exogenous avoidable portion; the main attention should be paid on the endogenous avoidable part reduction. Focusing on the exogenous avoidable part (which can be decreased by improving the exergetic performance of the remaining components) should be on the next step [43]. As can be seen in Table 9, obtained endogenous avoidable exergy destruction rate for all components is higher than the exogenous avoidable part. Referring to Table 9, except for the HPVG, LPVG and COND, the endogenous avoidable exergy destruction is higher than the endogenous unavoidable. Among the all, the condenser has
14
the worst performance from the perspective of advanced exergy; because, 353.40 kW of the 416.20 endogenous exergy destruction rate is unavoidable. In other words, preventing 84.9% of the endogenous exergy destruction within the COND is not possible in spite of technological development. On the other hand, LPT and HPT are promising components from the viewpoint of advanced exergy analysis, which about 68.01 and 68.45% of their endogenous exergy destruction rates are avoidable, respectively. But in general, only 949.63 kW of the total 2643.22 kW endogenous exergy destruction rate is avoidable which means about 64.07% of system overall endogenous exergy destruction is not preventable. Table 9 clearly shows that the most part of the exogenous exergy destruction rate is unavoidable, which means that an improvement in exergetic performance of components do not guarantee a reduction in exogenous destroyed exergy. Moreover, the HPT has the most improvable performance, which is obtainable by improvement of other component's performance; because about 70.13% of its exogenous exergy destruction is avoidable which has the highest percentage of exogenous avoidable exergy destruction among all the components. For the case of LPT, the ,AV ,AV value of E EN is much higher than E EX . As a result, an optimizing the system exergetic D ,K D ,K
performance can be obtained by improving this component efficiency. Table 9
To give a clear imagination of the advanced exergy analysis, the exergy destruction rates of the system components are splitted into avoidable, unavoidable, endogenous, exogenous, exogenous avoidable, exogenous-unavoidable, endogenous-avoidable and endogenous-unavoidable parts which are indicated in Fig. 2. According to Fig. 2, HPT, LPT, HPFP and LPFP have a higher percentage of avoidable exergy destruction rates and HPVG, LPVG and COND have a higher percentage of unavoidable exergy
15
destruction rate. For the components with a higher percentage of avoidable exergy destruction, technical modifications can improve their exergetic performance which leads to system efficiency enhancement. Also, the highest percentage (67%) of endogenous avoidable exergy destruction belongs to the LPT and LPFP among all components. Fig.2
Fig. 3 illustrates contributions of the system components in total endogenous, exogenous, avoidable, unavoidable, endogenous avoidable, endogenous unavoidable, exogenous avoidable and exogenous unavoidable exergy destruction rates of the system. Also, good results can be obtained by comparing the conventional and advanced exergy analysis which is shown in Fig. 4. It is obvious from Figs. 3 and 4; the priority order for system components from the viewpoint of conventional exergy is LPVG followed by HPVG, COND, HPT, LPT, HPFP and then LPFP. But advanced exergy analysis reveals new aspects about exergy destruction. From the advanced exergy perspective, designers should focus first on the HPT and LPT more than COND due to higher avoidable exergy destruction rates. Moreover, the second highest endogenous exergy destruction rate (19.3%) belongs to LPT which indicates that this component’s performance is much improvable than HPVG, COND and HPT by technical modifications. On the other hand, HPT has a high percentage of exogenous available exergy destruction rate (23%) that can be eliminated by the other components performance enhancement. In addition, in spite of conventional exergy analysis results, for the case of COND, which has the third place between the components, the results of advanced exergy analysis don’t suggest any considerable improvement in this component. Fig.3 Fig. 4
16
5. Conclusion In the present study, conventional and advanced exergy analyses are applied to a geothermal heat driven dual fluid organic Rankine cycle. The main conclusions that can be obtained from the present work are listed as follows:
Based on the conventional exergy analysis results, LPVG and HPVG are the most important components by 38.11 and 29.98% of the total exergy destruction rate, respectively. Also, LPFP and HPFP are components with negligible exergy destruction. Moreover, COND with 15.93% of the total exergy destruction rate is the third effective component from the viewpoint of conventional exergy analysis.
From the perspective of the advanced exergy analysis, about 72% of the total exergy destruction rate is endogenous exergy destruction rate and for all of the system components this part is higher than exogenous exergy destruction.
In all the system components, except the HPVG, LPVG and COND, the endogenous avoidable exergy destruction is higher than the endogenous unavoidable. Also, LPT by 19.3% of the total endogenous avoidable exergy destruction rate has the second highest potential for performance improvement by technical modifications.
Only 15% of COND exergy destruction is avoidable which includes 7% of total avoidable exergy destruction rate. Therefore, despite the conventional exergy analysis results, COND is not important from the viewpoint of advanced exergy.
An improvement in the other component efficiencies improves HPT exergetic performance more than others because 23% of the total exogenous available exergy destruction rate belongs to HPT which is 17% of the HPT exergy destruction rate.
17
Nomenclature e
specific exergy (kJ/kg)
E
exergy rate (kW)
E D
exergy destruction rate
h
enthalpy (kJ/kg)
m
mass flow rate (kg/s)
P
pressure (kPa)
R
global gas constant (kJ/kg.K)
T
temperature ( C or K)
W
power (kW)
Q
heat transfer rate (kW)
s
entropy (kJ/kg.K)
Acronyms COND
condenser
HPE
high pressure evaporator
HPT
high pressure turbine
18
HPFP
high pressure feed pump
HPPH
high pressure preheater
HPVG
high pressure vapor generator
LPE
low pressure evaporator
LPT
low pressure turbine
LPFP
low pressure feed pump
LPPH
low pressure preheater
LPVG
low pressure vapor generator
ORC
organic Rankine cycle
Greek letters ɛ
exergy efficiency
Subscripts 0
ambient condition
ch
chemical
e
exit
D
destroyed
gw
geothermal water
19
i
inlet
in
input
k
kth component
min
minimum
ph
physical
Q
heat
W
work
Upscripts AV
avoidable
EN
endogenous
EX
exogenous
mix
mixture
UN
unavoidable
References [1] Ameri M, Ahmadi P, Hamidi A. Energy, exergy and exergoeconomic analysis of a steam power plant: a case study. International Journal of Energy Research. 2009 Apr 1;33(5):499-512. [2]
Eurostat.
Electricity
prices
for
household
eurostat.ec.europa.eu>.
20
consumers;
2013.
[3] Bianchi M, De Pascale A. Bottoming cycles for electric energy generation: parametric investigation of available and innovative solutions for the exploitation of low and medium temperature heat sources. Applied Energy. 2011 May 31;88(5):1500-9. [4] Heberle F, Brüggemann D. Exergy based fluid selection for a geothermal Organic Rankine Cycle for combined heat and power generation. Applied Thermal Engineering. 2010 Aug 31;30(11):1326-32. [5] Yari M. Exergetic analysis of various types of geothermal power plants. Renewable Energy. 2010 Jan 31;35(1):112-21. [6] Heberle F, Preißinger M, Brüggemann D. Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resources. Renewable Energy. 2012 Jan 31;37(1):364-70. [7] Liu Q, Duan Y, Yang Z. Performance analyses of geothermal organic Rankine cycles with selected hydrocarbon working fluids. Energy. 2013 Dec 15;63:123-32. [8] Zhai H, Shi L, An Q. Influence of working fluid properties on system performance and screen evaluation indicators for geothermal ORC (organic Rankine cycle) system. Energy. 2014 Sep 1;74:2-11. [9] Guzović Z, Rašković P, Blatarić Z. The comparision of a basic and a dual-pressure ORC (Organic Rankine Cycle): Geothermal Power Plant Velika Ciglena case study. Energy. 2014 Nov 1;76:175-86. [10] Kang Z, Zhu J, Lu X, Li T, Wu X. Parametric optimization and performance analysis of zeotropic mixtures for an organic Rankine cycle driven by low-medium temperature geothermal fluids. Applied Thermal Engineering. 2015 Oct 5;89:323-31.
21
[11] Sadeghi M, Nemati A, Yari M. Thermodynamic analysis and multi-objective optimization of various ORC (organic Rankine cycle) configurations using zeotropic mixtures. Energy. 2016 Aug 15;109:791-802. [12] Yang MH, Yeh RH. Economic performances optimization of an organic Rankine cycle system with lower global warming potential working fluids in geothermal application. Renewable Energy. 2016 Jan 31;85:1201-13. [13] Tsatsaronis G, Park MH. On avoidable and unavoidable exergy destructions and investment costs in thermal systems. Energy Conversion and Management. 2002 Aug 31;43(9):1259-70. [14] Vatani A, Mehrpooya M, Palizdar A. Advanced exergetic analysis of five natural gas liquefaction processes. Energy Conversion and Management. 2014 Feb 28;78:720-37. [15] Tsatsaronis G, Morosuk T. Advanced exergetic analysis of a refrigeration system for liquefaction of natural gas. International Journal of Energy and Environmental Engineering. 2010 Jan 1;1(1):1-8. [16] Kelly S, Tsatsaronis G, Morosuk T. Advanced exergetic analysis: Approaches for splitting the exergy destruction into endogenous and exogenous parts. Energy. 2009 Mar 31;34(3):384-91. [17] Morosuk T, Tsatsaronis G. A new approach to the exergy analysis of absorption refrigeration machines. Energy. 2008 Jun 30;33(6):890-907. [18] Petrakopoulou F, Tsatsaronis G, Morosuk T, Carassai A. Advanced exergoeconomic analysis applied to a complex energy conversion system. Journal of Engineering for Gas Turbines and Power. 2012 Mar 1;134(3):031801. [19] Fallah M, Mahmoudi SM, Yari M, Ghiasi RA. Advanced exergy analysis of the Kalina cycle applied for low temperature enhanced geothermal system. Energy Conversion and Management. 2016 Jan 15;108:190-201.
22
[20] Soltani S, Yari M, Mahmoudi SM, Morosuk T, Rosen MA. Advanced exergy analysis applied to an externally-fired combined-cycle power plant integrated with a biomass gasification unit. Energy. 2013 Sep 15;59:775-80. [21] Hepbasli A, Keçebaş A. A comparative study on conventional and advanced exergetic analyses of geothermal district heating systems based on actual operational data. Energy and Buildings. 2013 Jun 30;61:193-201. [22] Keçebaş A, Hepbasli A. Conventional and advanced exergoeconomic analyses of geothermal district heating systems. Energy and Buildings. 2014 Feb 28;69:434-41. [23] Gungor A, Erbay Z, Hepbasli A, Gunerhan H. Splitting the exergy destruction into avoidable and unavoidable parts of a gas engine heat pump (GEHP) for food drying processes based on experimental values. Energy Conversion and Management. 2013 Sep 30;73:309-16. [24] Gungor A, Tsatsaronis G, Gunerhan H, Hepbasli A. Advanced exergoeconomic analysis of a gas engine heat pump (GEHP) for food drying processes. Energy Conversion and Management. 2015 Feb 28;91:132-9. [25] Açıkkalp E, Aras H, Hepbasli A. Advanced exergoeconomic analysis of an electricitygenerating facility that operates with natural gas. Energy Conversion and Management. 2014 Feb 28;78:452-60. [26] Anvari S, Saray RK, Bahlouli K. Conventional and advanced exergetic and exergoeconomic analyses applied to a tri-generation cycle for heat, cold and power production. Energy. 2015 Nov 30;91:925-39. [27] Açıkkalp E, Aras H, Hepbasli A. Advanced exergoeconomic analysis of a trigeneration system using a diesel-gas engine. Applied Thermal Engineering. 2014 Jun 30;67(1):388-95.
23
[28] Nami H, Akrami E, Ranjbar F. Hydrogen production using the waste heat of Benchmark pressurized Molten carbonate fuel cell system via combination of organic Rankine cycle and proton exchange membrane (PEM) electrolysis. Applied Thermal Engineering. 2017 Mar 5;114:631-8. [29] Nemati A, Barzegar R, Khalil AS, Khatamnezhad H. Decreasing the emissions of a partially premixed gasoline fueled compression ignition engine by means of injection characteristics and EGR. Thermal Science. 2011;15(4):939-52. [30] Nami H, Ranjbar F, Yari M, Saeidi S. Thermodynamic analysis of a modified oxy-fuel cycle, high steam content Graz cycle with a dual-pressure heat recovery steam generator. International Journal of Exergy. 2016;21(3):331-46. [31] Nemati A, Barzegar R, Khalilarya S. The effects of injected fuel temperature on exergy balance under the various operating loads in a DI diesel engine. International Journal of Exergy. 2015;17(1):35-53. [32] Nami H, Mohammadkhani F, Ranjbar F. Utilization of waste heat from GTMHR for hydrogen generation via combination of organic Rankine cycles and PEM electrolysis. Energy Conversion and Management. 2016 Nov 1;127:589-98. [33] Nami H, Mahmoudi SM, Nemati A. Exergy, economic and environmental impact assessment and optimization of a novel cogeneration system including a gas turbine, a supercritical CO2 and an organic Rankine cycle (GT-HRSG/SCO2). Applied Thermal Engineering. 2017 Jan 5;110:1315-30.
24
[34] Nemati A, Nami H, Yari M, Ranjbar F, Kolvir HR. Development of an exergoeconomic model for analysis and multi-objective optimization of a thermoelectric heat pump. Energy Conversion and Management. 2016 Dec 15;130:1-3. [35] Nemati A, Nami H, Ranjbar F, Yari M. A comparative thermodynamic analysis of ORC and Kalina cycles for waste heat recovery: A case study for CGAM cogeneration system. Case Studies in Thermal Engineering. 2017 Mar 31;9:1-3. [36] Morosuk T, Tsatsaronis G. Advanced exergetic evaluation of refrigeration machines using different working fluids. Energy. 2009 Dec 31;34(12):2248-58. [37] Tsatsaronis G, Morosuk T. A General Exergy-Based Method for Combining a Cost Analysis With an Environmental Impact Analysis: Part II—Application to a Cogeneration System. InASME 2008 International Mechanical Engineering Congress and Exposition 2008 Jan 1 (pp. 463-469). American Society of Mechanical Engineers. [38] Rodríguez CE, Palacio JC, Venturini OJ, Lora EE, Cobas VM, dos Santos DM, Dotto FR, Gialluca V. Exergetic and economic comparison of ORC and Kalina cycle for low temperature enhanced geothermal system in Brazil. Applied Thermal Engineering. 2013 Apr 5;52(1):109-19. [39] Petrakopoulou F. Comparative evaluation of power plants with CO 2 capture: thermodynamic, economic and environmental performance. [40] Zare V. A comparative exergoeconomic analysis of different ORC configurations for binary geothermal power plants. Energy Conversion and Management. 2015 Nov 15;105:127-38.
25
[41] Zare V, Mahmoudi SM. A thermodynamic comparison between organic Rankine and Kalina cycles for waste heat recovery from the Gas Turbine-Modular Helium Reactor. Energy. 2015 Jan 1;79:398-406. [42] Boyaghchi FA, Molaie H. Investigating the effect of duct burner fuel mass flow rate on exergy destruction of a real combined cycle power plant components based on advanced exergy analysis. Energy Conversion and Management. 2015 Oct 31;103:827-35. [43] Tsatsaronis G, Morosuk T. Advanced exergetic analysis of a novel system for generating electricity and vaporizing liquefied natural gas. Energy. 2010 Feb 28;35(2):820-9.
26
Figures:
Fig. 1 Schematic diagram of the dual fluid ORC
27
HPT
LPT
HPVG (HPPH + HPE)
LPVG (LPPH + LPE)
28
COND
HPFP
LPFP
29
Fig.2 Splitting exergy destruction rates of dual fluid ORC components into endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable, endogenous-unavoidable, exogenous-avoidable and exogenous-unavoidable parts.
30
EN
EX
LPT LPFP 0% 0% HPT 8%
HPE+ HPPH 29%
HPT 7%
LPFP 1% HPFP 0%
LPT 10%
COND 16%
HPE+ HPPH 43%
COND 16% LPE+ LPPH 36%
HPFP 1%
LPE+ LPPH 33%
AV
UN
HPT LPT 4% 4% LPFP 0%HPFP 0%
HPT 17% HPE+ HPPH 35%
HPE+ HPPH 39%
LPT 16%
LPE+ LPPH 33%
LPFP 1%
LPE+ LPPH 23%
COND 7%
COND 20%
HPFP 1%
EN_AV
EN_UN
31
HPT 4%
HPT 16% HPE+ HPPH 38%
HPE+ HPPH 36%
LPT 19%
LPE+ LPPH 19%
LPT LPFP 5% 0% HPFP 0%
LPFP 1% HPFP COND 1% 6%
COND 21%
LPE+ LPPH 34%
EX_AV
EX_UN
HPT LPT LPFP 2% 0% 0% HPE+ HPPH 24%
HPT 23%
HPFP 0%
LPT 1%
COND 18%
LPFP 0% LPE+ LPPH 39%
COND 12%
HPE+ HPPH 48%
HPFP 1%
LPE+ LPPH 32%
Fig.3 Contributions of dual fluid ORC components in the total endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable, endogenous-unavoidable, exogenous-avoidable and exogenous-unavoidable exergy destruction rates of the system.
32
Fig. 4 Contribution of each component on cycle overall exergy destruction rate obtained from conventional and advanced exergy analyses (the contributions of the HPFP and LPFP are zoomed for clarification).
33
Tables: Table 1 Exergy balance equations for different components of dual fluid ORC. Components
Exergy destruction
Exergy efficiency
HPT
E1 E 2 W HPT
W HPT E1 E 2
LPT
E 6 E 7 W LPT
W LPT E 6 E 7
HPE+ HPPH
E gw ,1 E gw ,3 E 4 E1
E1 E 4 E gw ,1 E gw ,3
LPE+ LPPH
E 9 E 2 E gw ,3 E 6 E 3 E gw ,4
E 6 E 9 (E 2 E 3 E gw ,3 E gw ,4 )
HPFP
E 3 W HPFP E 4
E 4 E 3 W HPFP
LPFP
E 8 W LPFP E 9
E 9 E 8 W LPFP
COND
E cw,1 E 7 E cw,2 E 8
E cw,2 E cw,1 E 7 E 8
34
Table 2 The main assumptions for the dual fluid ORC under real, ideal and unavoidable conditions.
Component HPT
Real
Idealb
Unavoidable
s 0.85 a
s 1
s 0.95 c
T min 10Kd
T min 0K
T min 3Kf
HPVG
P 0%
PHPE 2%e
(HPE+ HPPH)
P 1%f
PHPPH 3%b HPFP
s 0.8 a
s 1
s 0.95 c, f
LPT
s 0.85 a
s 1
s 0.95 c
T min 10Kd
T min 0K
T min 3Kf
LPVG
P 0%
PLPE 2%e
(LPE+ LPPH)
P 1% f
PLPPH 3%b LPFP
s 0.8 a
s 1
s 0.95 c, f
T min 5Kd
T min 0K
T min 3Kb ,f
COND
P 0%
P 1% g
a
Ref. [38].
b
Ref. [19].
c
Ref. [39].
d
Ref. [40].
35
e
Ref. [41].
P 0.5% b, f
f
Ref. [42].
g
Ref. [20].
Table 3 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under real conditions.
Fluid
P(kPa)
T (˚C)
m (kg/s)
h (kJ/kg)
s (kJ/kg k)
E (kW)
1
Isopentane
1236
126.8
56.88
150.4
-0.3482
5708
2
Isopentane
400
95.75
56.88
115.1
-0.3313
3414
3
Isopentane
392
73.8
56.88
-232.7
-1.328
528.5
4
Isopentane
1300
74.38
56.88
-230.8
-1.327
622.7
5
Isopentane
1261
127.9
56.88
-82.91
-0.9316
2332
6
Isobutane
928.1
62.91
71.22
637.8
2.347
6305
7
Isobutane
469.1
42
71.22
615
2.36
4408
8
Isobutane
464.5
35
71.22
284.3
1.288
3608
9
Isobutane
976.3
35.36
71.22
285.5
1.289
3676
10
Isobutane
947
63.8
71.22
360.1
1.52
4077
1gw
Water
7000
175
83
744.5
2.083
10621
2gw
Water
7000
137.9
83
584.6
1.711
6565
3gw
Water
7000
114
83
483.3
1.457
4439
4gw
Water
7000
98.81
83
419.3
1.288
3300
1cw
Water
100
25
1009
104.8
0.3669
0
2cw
Water
100
30.58
1009
128.2
0.4445
216.3
State no.
ε = 47.03%, W HPT 2007 (kW), W LPT 1624 (kW), W net 3443 (kW)
36
Table 4 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under ideal conditions.
Fluid
P(kPa)
T (˚C)
m (kg/s)
h (kJ/kg)
s (kJ/kg k)
E (kW)
1
Isopentane
1300
129.5
38.75
154.3
-0.3428
3977
2
Isopentane
400
93.74
38.75
110.9
-0.3428
2294
3
Isopentane
400
74.6
38.75
-230.6
-1.322
371.8
4
Isopentane
1300
75.07
38.75
-229
-1.322
434
5
Isopentane
1300
129.5
38.75
-77.95
-0.9195
1641
6
Isobutane
1199
74.6
50.58
651.8
2.359
5002
7
Isobutane
464.5
41.62
50.58
614.4
2.359
3110
8
Isobutane
464.5
35
50.58
284.3
1.288
2562
9
Isobutane
1199
35.42
50.58
285.7
1.288
2631
10
Isobutane
1199
74.6
50.58
390.3
1.607
3115
1gw
Water
7000
175
46.04
744.5
2.083
5891
2gw
Water
7000
129.5
46.04
549
1.623
3204
3gw
Water
7000
99.44
46.04
421.9
1.295
1854
4gw
Water
7000
71.99
46.04
307
0.975
961.8
1cw
Water
100
25
383.9
104.8
0.3669
0
2cw
Water
100
35.4
383.9
148.3
0.5103
284
State no.
ε = 69.85%, W HPT 1682 (kW), W LPT 1892 (kW), W net 3443 (kW)
37
Table 5 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under unavoidable conditions.
Fluid
P(kPa)
T (˚C)
m (kg/s)
h (kJ/kg)
s (kJ/kg k)
E (kW)
1
Isopentane
1274
128.5
43.74
152.7
-0.345
4450
2
Isopentane
400
94.38
43.74
112.2
-0.3392
2601
3
Isopentane
396
74.2
43.74
-231.6
-1.325
413.1
4
Isopentane
1300
74.7
43.74
-229.9
-1.324
484
5
Isopentane
1287
129
43.74
-79.59
-0.9235
1833
6
Isobutane
1104
70.74
56.4
647.3
2.355
5389
7
Isobutane
466.8
42
56.4
615.1
2.361
3480
8
Isobutane
464.5
35
56.4
284.3
1.288
2857
9
Isobutane
1126
35.41
56.4
285.6
1.289
2926
10
Isobutane
1115
71.2
56.4
380.6
1.58
3390
1gw
Water
7000
175
54.93
744.5
2.083
7029
2gw
Water
7000
132
54.93
559.5
1.649
3973
3gw
Water
7000
103.7
54.93
439.7
1.343
2413
4gw
Water
7000
80.42
54.93
342.2
1.076
1431
1cw
Water
100
25
596.4
104.8
0.3669
0
2cw
Water
100
32.48
596.4
136.1
0.4705
229.3
State no.
ε = 61.5%, W HPT 1773 (kW), W LPT 1818 (kW), W net 3443 (kW)
38
Table 6 Results of the exergy analysis for dual fluid ORC under real conditions.
y
ED (%) E in ,tot
y*
ED (%) E D ,tot
E F (kW)
E P (kW)
E D (kW)
(%)
HPT
2294
2007
287
87.49
3.83
7.84
LPT
1896
1624
272
85.65
3.63
7.43
6181
5085
1096
82.27
14.63
29.96
4024
2629
1395
65.33
18.61
38.13
800.6
217.7
582.9
27.19
7.78
15.93
108.2
94.16
14.04
87.02
0.19
0.38
79.9
68.28
11.62
85.46
0.16
0.32
7321
3443
3658.56
47.03
48.83
100.00
Component
HPVG (HPE+ HPPH) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
39
Table 7 Results of the exergy analysis for dual fluid ORC under ideal conditions.
y
ED (%) E in ,tot
y*
ED (%) E D ,tot
E F (kW)
E P (kW)
E D (kW)
(%)
HPT
1682
1682
0
100.00
0.00
0.00
LPT
1892
1892
0
100.00
0.00
0.00
4037
3543
494
87.76
6.59
41.11
2815
2371
444
84.23
5.92
36.95
548
284.5
263.5
51.92
3.52
21.93
62.21
62.13
0.08
99.87
0.00
0.01
69.23
69.15
0.08
99.88
0.00
0.01
4929
3443
1201.66
69.85
24.38
100.00
Component
HPVG (HPE+ HPPH) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
40
Table 8 Results of the exergy analysis for dual fluid ORC under unavoidable conditions.
y
ED (%) E in ,tot
y*
ED (%) E D ,tot
E F (kW)
E P (kW)
E D (kW)
(%)
HPT
1849
1773
76
95.89
1.01
3.95
LPT
1908
1818
90
95.28
1.20
4.68
4616
3966
650
85.92
8.67
33.78
3170
2462
708
77.67
9.45
36.79
623.6
230.1
393.5
36.90
5.25
20.45
74.19
70.92
3.27
95.59
0.04
0.17
73.2
69.59
3.61
95.07
0.05
0.19
5598
3443
1924.38
61.50
34.38
100.00
Component
HPVG (HPE+ HPPH) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
41
Table 9 Results of the advanced exergy analysis for dual fluid ORC.
ED
E EN D ,K
E EX D ,K
E AV D ,K
E UN D ,K
,AV E EN D ,K
,UN E EN D ,K
,AV E EX D ,K
,UN E EX D ,K
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
(kW)
287.00
218.00
69.00
196.66
90.34
148.27
69.73
48.39
20.61
272.00
267.60
4.40
185.88
86.12
183.18
84.42
2.70
1.70
1097.00
759.60
337.40
263.60
833.40
180.50
579.10
83.10
254.30
1395.00
961.20
433.80
410.80
984.20
360.50
600.70
50.30
383.50
582.90
416.20
166.70
88.10
494.80
62.80
353.40
25.30
141.40
14.04
9.50
4.54
9.79
4.25
6.62
2.88
3.17
1.37
11.62
11.12
0.50
8.12
3.50
7.76
3.36
0.36
0.14
3659.56
2643.22
1016.34
1162.95
2496.61
949.63
1693.59
213.32
803.02
Component
HPT LPT HPVG (HPPH + HPE) LPVG (LPE+ LPPH) COND HPFP LPFP Overall
42
Figure captions Fig. 1 Schematic diagram of the dual fluid ORC Fig.2 Splitting exergy destruction rates of dual fluid ORC components into endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable, endogenous-unavoidable, exogenous-avoidable and exogenous-unavoidable parts. Fig.3 Contributions of dual fluid ORC components in the total endogenous, exogenous, avoidable, unavoidable, endogenous-avoidable,
endogenous-unavoidable,
exogenous-avoidable
and
exogenous-unavoidable
exergy
destruction rates of the system. Fig. 4 Contribution of each component on cycle overall exergy destruction rate obtained from conventional and advanced exergy analyses (the contributions of the HPFP and LPFP are zoomed for clarification).
43
Table Captions: Table 1 Exergy balance equations for different components of dual fluid ORC. Table 2 The main assumptions for the dual fluid ORC under real, ideal and unavoidable conditions. Table 3 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under real conditions. Table 4 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under ideal conditions. Table 5 Thermodynamic properties and mass flow rates at different state points of dual fluid ORC under unavoidable conditions.
Table 6 Results of the exergy analysis for dual fluid ORC under real conditions. Table 7 Results of the exergy analysis for dual fluid ORC under ideal conditions. Table 8 Results of the exergy analysis for dual fluid ORC under unavoidable conditions. Table 9 Results of the advanced exergy analysis for dual fluid ORC.
Highlights
The advanced exergy analysis has been applied to the geothermal driven dual fluid ORC.
Exergy destruction unavoidable/avoidable.
The interactions between the components are found to be weak. The advanced exergy analysis presents different and more pragmatic results.
analyzed
44
as
endogenous/exogenous
and
45