Thermodynamic and economic analysis and optimization of power cycles for a medium temperature geothermal resource

Thermodynamic and economic analysis and optimization of power cycles for a medium temperature geothermal resource

Energy Conversion and Management 78 (2014) 39–49 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.el...

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Energy Conversion and Management 78 (2014) 39–49

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Thermodynamic and economic analysis and optimization of power cycles for a medium temperature geothermal resource Ahmet Coskun a,⇑, Ali Bolatturk a, Mehmet Kanoglu b a b

Department of Mechanical Engineering, Suleyman Demirel University, 32260 Isparta, Turkey Department of Mechanical Engineering, University of Gaziantep, 27310 Gaziantep, Turkey

a r t i c l e

i n f o

Article history: Received 22 July 2013 Accepted 16 October 2013

Keywords: Geothermal energy Geothermal power plant Kalina cycle Economic analysis Exergy Optimization

a b s t r a c t Geothermal power generation technologies are well established and there are numerous power plants operating worldwide. Turkey is rich in geothermal resources while most resources are not exploited for power production. In this study, we consider geothermal resources in Kutahya–Simav region having geothermal water at a temperature suitable for power generation. The study is aimed to yield the method of the most effective use of the geothermal resource and a rational thermodynamic and economic comparison of various cycles for a given resource. The cycles considered include double-flash, binary, combined flash/binary, and Kalina cycle. The selected cycles are optimized for the turbine inlet pressure that would generate maximum power output and energy and exergy efficiencies. The distribution of exergy in plant components and processes are shown using tables. Maximum first law efficiencies vary between 6.9% and 10.6% while the second law efficiencies vary between 38.5% and 59.3% depending on the cycle considered. The maximum power output, the first law, and the second law efficiencies are obtained for Kalina cycle followed by combined cycle and binary cycle. An economic analysis of four cycles considered indicates that the cost of producing a unit amount of electricity is 0.0116 $/kW h for double flash and Kalina cycles, 0.0165 $/kW h for combined cycle and 0.0202 $/kW h for binary cycle. Consequently, the payback period is 5.8 years for double flash and Kalina cycles while it is 8.3 years for combined cycle and 9 years for binary cycle. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The rising energy demand, the limited supply of fossil fuels and their detrimental environmental impacts (e.g. global warming) have intensified the worldwide search for cleaner sources of energy. Among renewable energy sources, geothermal energy has a special place largely because of its vast worldwide resources and its capacity to provide base-load electricity due to non-intermittent nature of geothermal energy [1]. Geothermal heat comes from beneath the earth surface with temperatures varying between 50 and 350 °C. It occurs mainly in the form of steam, mixtures of steam and water or just liquid water [2]. In literature, there are many studies related to analysis of geothermal power plants. Aneke et al. [2] investigated the IPSEpro model of the Chena Geothermal Organic Rankine Cycle (ORC) Power Plant and the results are validated using actual data. IPSEpro is modular-mode as well as equation-oriented steady state energy simulation software. The validated model was used to investigate the effect of variation in the geothermal source temperature on ⇑ Corresponding author. Tel.: +90 246 211 1253. E-mail address: [email protected] (A. Coskun). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.10.045

plant performance. The analysis showed that an increase in the geothermal source temperature above the design point increases the working fluid flow rate, decreases the working fluid degree of superheat at the inlet of the turbine (evaporator exit), increases the plant net power output, and reduces the efficiency. Kanoglu and Bolatturk [3] studied a binary geothermal power plant exergetically using actual plant data to assess the plant performance and pinpoint sites of primary exergy destruction. In this study, the energy and exergy efficiencies of the plant were obtained to be 4.5% and 21.7%, respectively. Also, the effects of turbine inlet pressure and temperature and the condenser pressure on the exergy and energy efficiencies, the net power output and the brine reinjection temperature are investigated and the trends are explained. Gabbrielli [4] proposed a novel approach for the design point selection of small scale ORC binary geothermal power plants. Four design points relative to different values of the brine temperature during geothermal well exploitation have been compared from the economic point of view using off-design simulations of the whole operating life. In particular, the large increase of the R134a mass flow rate and, consequently, of the highest pressure implies severe modifications of the expander outlet. Yari [5] investigated the different geothermal power plant concepts, based on the exergy anal-

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Nomenclature COE C Ccapital CCE Cesc Cnon-esc CO&M Cplant Csurf Ctotal Cunit e er E_ Ee Ep Eper f HTR h i i⁄ I_ Lf LTR _ m n

capital cost per unit energy ($/kW h) the total expenditure amount including escalation ($) constant annual capital cost ($) constant expenses ($/kW year) the amount of total escalation expenditure ($) the amount of non-escalation expenditure ($) annual operation and maintenance expense ($) the amount of physical construction ($) unit cost for surface equipment ($/kW) total cost (M$) unit cost of plant ($/kW) specific exergy (kJ/kg) escalation rate (%) exergy rate (kW) annual electrical energy production amount (MW h) unit price of electricity ($/kW h) percentage of annual expenditure during construction (%) average annual producer prices inflation (%) high temperature recuperator specific enthalpy (kJ/kg) annual interest rate (%) the interest rate including inflation rate (%) exergy destruction (kW) load factor (%) low temperature recuperator mass flow rate (kg/s) lifetime of the power plant (year)

ysis for high-temperature geothermal resources. In this study, the considered cycles are a binary geothermal power plant using a simple ORC, a binary geothermal power plant using an ORC with an internal heat exchanger (IHE), a binary cycle with a regenerative ORC, a binary cycle with a regenerative ORC with an IHE, a singleflash geothermal power plant, a double-flash geothermal power plant and a combined flash-binary power plant. With respect to each cycle, a thermodynamic model had to be developed. The performance of each cycle has been discussed in terms of the secondlaw efficiency, exergy destruction rate, and first-law efficiency. The maximum first-law efficiency was obtained to be 7.7% for the ORC with an IHE with R123 as the working fluid. The first-law efficiency based on the energy input to the ORC in binary cycle with the regenerative ORC with an IHE and R123 as the working fluid is 15.4%. The value for the flash-binary with R123 as the working fluid was 11.8%. Hettiarachchi et al. [6] investigated a cost-effective optimum design criterion for ORC utilizing low-temperature geothermal heat sources. The optimum cycle performance is evaluated and compared for working fluids that include ammonia, HCFC123 and n-Pentane. Ammonia has minimum objective function and maximum geothermal water utilization, but not necessarily maximum cycle efficiency. DiPippo [7] presented the second law assessment of binary plants generating power from low-temperature geothermal sources. The results show that binary plants can operate with high second law or exergetic efficiencies even when the motive fluids are low-temperature and low-exergy. Exergetic efficiencies of 40% or higher have been achieved in certain plants. The main design feature leading to a high second law efficiency lies in the design of the heat exchangers to minimize the loss of exergy during heat transfer processes. Shengjun et al. [8] investigated the

PWFO&M present value of annual operation and maintenance expenses ($) PWFcapital present value of the capital cost ($) ORC Organic Rankine cycle heat flow rate (kW) Q_ SPP payback periods (year) s specific entropy (kJ/kgK) T temperature (°C) t escalation period (year) _ net net power (kW) W gth thermal efficiency ge exergetic efficiency Subscripts 0 dead state Cond condenser f saturated liquid geo geothermal fluid HE heat exchanger in inlet p pump pp pinch point r reversible t turbine out outlet reinj reinjection

parameter optimization and performance comparison of the fluids in subcritical ORC and transcritical power cycle in low-temperature binary geothermal power system. The optimization procedure was conducted with a simulation program written in Matlab using five indicators: thermal efficiency, exergy efficiency, recovery efficiency, heat exchanger area per unit power output and the levelized energy cost. The analysis showed that the choice of working fluid varies the objective function and the value of the optimized operation parameters are not all the same for different indicators. R123 in subcritical ORC system yields the highest thermal efficiency and exergy efficiency of 11.1% and 54.1%, respectively. Although the thermal efficiency and exergy efficiency of R125 in transcritical cycle is 46.4% and 20% lower than that of R123 in subcritical ORC, it provides 20.7% larger recovery efficiency. The levelized energy cost value is relatively low. DiPippo [9] found that actual binary plants can achieve relative efficiencies as high as 85%. The paper discusses cycles using twophase expanders that in principle come close to the ideal triangular cycle. Franco and Villani [10] analyzed that the brine specific consumption, ranging from 20 to 120 kg/s for each net MW produced, and the efficiency of the plants, ranging from 20% to 45% in terms of second law efficiency, are dictated mainly by the combination of the brine inlet temperature, the brine rejection temperature and the energy conversion cycle being used. It is shown that optimization of the plant can yield improvements of up to 30–40% in terms of reduction of brine specific consumption compared to conventional design. Coskun [11] studied geothermal sources with low, medium and high temperatures that may be suitable for power generation in Turkey. Optimum plants chosen in terms of maximum net power, thermal and exergetic efficiency were selected according to properties of these sources. These plants are single flash, double flash,

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binary, combined, regenerative binary, regenerative binary with heat exchanger, binary with regenerator and Kalina cycle plants. Selected plants were optimized according to turbine inlet pressure by maximizing net power, energy and exergy efficiencies. An economic analysis (the costs per unit energy, payback periods, etc.) in which interest, inflation and escalation rates were included to the costs were also carried out. The changes of net power and specific costs of the plants with geothermal fluid temperatures were investigated, as well. In this study, we consider geothermal resources in Kutahya– Simav region of Turkey having geothermal water at 98–162 °C range. Kutahya–Simav region located in the west of Turkey. It is among the most significant 15 geothermal fields in Turkey. The study is aimed to select the most suitable plant (or plants) based on the characteristics of the resource. The cycles are to be optimized based on turbine inlet pressure. The purpose of selecting the cycle type and the optimization of operating conditions is to maximize the power production from the geothermal resource. Exergy losses are evaluated for all plant designs considered. Also, cost analysis of the plants is performed considering interest, inflation and escalation rates. This study is yield a realistic estimate of the power production potential from Kutahya–Simav geothermal region based on thermodynamic analysis and this information may be used for the academic and industrial area. Besides the exergy analyses of geothermal power plants have been studied by researchers, in this study it is determined the most suitable plant for a geothermal resource. Furthermore, it is possible to perform the thermodynamics and economic analyses of the geothermal power plants.

2. Geothermal power cycles In this study, the thermodynamic cycles shown in Figs. 1–4 (double flash, binary, combined flash/binary, Kalina) are examined for the considered resource. Flash steam plants are used to generate power from liquid-dominated resources that are hot enough to flash a significant proportion of the water to steam in surface equipment, either at single-flash or double-flash plants. Certain percentage of geothermal fluid evaporates during the flashing process (a throttling process) during which pressure drops while enthalpy remains constant. Binary cycle plants use the geothermal brine from liquid-dominated resources usually at relatively low temperature. These plants operate with a binary working fluid (isobutane, R-114, isopentane, etc.) that has a low boiling temperature in a Rankine cycle. The working fluid is completely vaporized and usually superheated by the geothermal heat in the vaporizer. The vapor expands in the turbine. It is then condensed in a water-cooled condenser before being pumped back to vaporizer to complete the cycle (Fig. 2).

Fig. 2. Binary cycle.

The combined flash/binary design found its way into the practice to take advantage of the benefits associated with both the flash and binary designs. Combined cycles are suited for high temperature geothermal resources. In the combined cycles, brine is flashed to obtain some vapor, which is directed to a steam turbine to produce work. The geothermal fluid leaving the separator is used to vaporize a binary working fluid in a heat exchanger before being reinjected back to the ground. Binary vapor leaving the heat exchanger is directed to a separate turbine to produce additional work (Fig. 3). The schematic diagram of the Kalina cycle is shown in Fig. 4. The ammonia-rich vapor is separated from the liquid phase in a separator. The vapor and liquid phases are condensed in the condenser. Condensing vapor is pumped back to low temperature recuperator. The vapor and liquid phases are merged together in the low temperature recuperator. Then, the ammonia-water mixture is heated in the high temperature recuperator. This mixture is superheated by the geothermal heat in the heat exchanger. 3. Thermodynamic and economic analysis of cycles Mass, energy and exergy balances for any control volume at steady state with negligible kinetic and potential energy changes can be expressed, respectively, by

X

_ in ¼ m

_ ¼ Q_ þ W

X X

_ ¼ E_ heat þ W

_ out m

ð1Þ

_ out hout  m

X

E_ out 

X

X

_ in hin m

E_ in þ I_

ð2Þ ð3Þ

where the subscriptions in and out represent the inlet and exit _ the net heat and work inputs, m _ is the mass flow states, Q_ and W rate, h is the enthalpy, and I_ is the rate of irreversibility. The sub-

Fig. 1. Double flash cycle.

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Cooled 12 13 water

1

Condenser 1

Power Turbine

Geothermal fluid

10

14 15

2

Separator Flash 11

7 Power

Cooled water

3

Condenser 2

Turbine

4 6

8

Pump

5 9

Heat exchanger

Reinjection Fig. 3. Combined cycle.

NH3+H2O Seperator

NH3

4 Power

3

Geothermal fluid

Turbine

Heat exchanger 1

10 H2O 5 9

8

HTR

2

Condenser

12

LTR

Cooled water

13

11 6

7

NH3+H2O

Pump

Q

Reinjection Fig. 4. Kalina cycle.

script 0 stands for the restricted dead state. Also, E_ heat is the net exergy transfer by heat at the temperature T, which is given by

X

E_ heat ¼

ð1  T 0 =TÞQ_

ð8Þ

ð4Þ

_ p;r is the reversible pump power, which is equal to W _ p  I. _ where W In the binary cycles, an energy balance can be written for the heat exchanger as:

ð5Þ

_ geo ðhgeo  hpp Þ ¼ m _ binary ðht;in  hf;binary Þ m

The specific flow exergy is given by

e ¼ h  h0  T 0 ðs  s0 Þ

_ W

gp;e ¼ _ p;r Wp

ð9Þ

and Multiplying specific exergy by the mass flow rate of the fluid gives the exergy rate

_ E_ ¼ me

ð6Þ

The exergetic efficiency of a turbine is defined as a measure of how well the stream exergy of the fluid is converted into actual turbine output. Then,

_ W

gt;e ¼ _ t W t;r

ð7Þ

_ t is the actual turbine power and W _ t;r is the reversible turwhere W _ t þ I. _ The exergy efficiency of the bine power, which is equal to W pump is defined similarly as

_ geo ðhpp  hreinj Þ ¼ m _ binary ðhf;binary  hHE;in Þ m

ð10Þ

_ geo and m _ binary are the mass flow rate of geothermal brine where m and binary fluid, respectively. hf is the saturated liquid enthalpy of binary fluid at the saturated (vaporization) temperature. Also, hpp is the enthalpy of the brine at the pinch-point temperature of the brine. The exergetic efficiencies of a heat exchanger may be measured by increase in the exergy of the cold stream divided by the decrease in the exergy of the hot stream [12]. Applying this definition to heat exchanger, we obtain

ðE_

 E_ Þ

ge;HE;Cond ¼ _out _ in cold ðEin  Eout Þhot

ð11Þ

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where the subscripts cold and hot represent the cold stream and the hot stream, respectively. The difference between the numerator and denominator of Eq. (11) is the exergy destruction in the heat exchanger or condenser. One may take all the exergy given up by the hot fluid in the condenser as part of the exergy destruction for the power plant. Following Kestin [13] and DiPippo and Marcille [14], the energy efficiency of a geothermal power plant may be expressed as

_ net W gth ¼ _ mgeo ðhgeo  h0 Þ

_ W

ð13Þ

We believe that using energy and exergy content of geothermal water relative to dead state provides a common base for the comparison of various cycles studied in this paper. Then for a given geothermal resource, the energy input and exergy input values become the same for different cycles. Note that Eqs. (12) and (13) are analogous to using heating value of the fuel and exergy of the fuel in the energy and exergy efficiencies of a conventional steam power plant, respectively. The investment cost of geothermal power plants is divided into surface costs (plant equipment and construction) and subsurface costs (reservoir exploration and drilling). Surface costs can be estimated with relative exactitude for a specific location and reservoir characteristics; however, a higher uncertainty is associated with subsurface costs when reservoir characteristics are not well known [15]. The costs associated with building and operating a geothermal power plant vary widely and depend on such factors [16] as        

_  5ÞÞ C surf ¼ 2000 expð0:0045ðW

ð14Þ

ð12Þ

where the expression in the denominator is the energy input to the power plant with respect to the environmental state. Using the exergy of geothermal water with respect to dead state as the exergy input to the plant, the exergy efficiency of a geothermal power plant can be expressed as

ge ¼ _ net Ein

kW. The next three items determine the capital cost of the energy conversion system; whereas, the last two affect the cost of running the plant (i.e., debt service, and operating and maintenance) [16]. We will consider, for surface costs, a unit cost exponentially decreasing from 2000 $/kW for a 5 MW plant to 1000 $/kW for a 150 MW plant. Unit cost for surface equipment ($/kW) is defined as a function of installed power capacity (MW) [20],

Resource type (steam or hot water). Resource temperature. Reservoir productivity. Power plant size. Power plant type (single-flash, binary, etc.). Environmental regulations. Cost of capital. Cost of labor.

The temperature of the resource is an essential parameter which influences the cost of power plants. Each power plant is designed to optimize the use of the heat supplied by the geothermal fluid. The size and thus cost of various components (e.g. heat exchangers) is determined by the resource temperature. As the temperature of the resource increases, the efficiency of the power system increases and the specific cost of equipment decreases. The temperature of the resource is a main criterion for selecting the cycle [17]. The evaluation of the plant cost requires a knowledge of the size of every major component [18]. Plant size is a significant cost factor, especially in the case of single unit condensing steam turbines, but excluding ORC plants which are typically modular. Other factors to consider are the optimization of condenser pressure, means of gas extraction, and the use of standard power units [19]. The first three factors influence the number of wells that must be drilled for a given plant capacity. Using typical costs and power potential for production wells, a single well can cost 100–400 $/

Operating and maintenance costs (O&M), both fixed and variable, are assumed equal for steam and ORC section. If both sections are present, fixed maintenance costs will therefore double with respect to simple binary or flash cycle [18]. The variable portion of the annual O&M costs vary with the level of generation, such as costs of supplies, and consumables, [21]. Operating and maintenance cost per year is assumed to be proportional to the energy production. Unit O&M cost also has an exponential decline with increasing plant energy production [21]. We will consider a unit cost for O&M that exponentially decreases from 20 $/MW h for a 5 MW plant to 14 $/MW h for a 150 MW plant. Operating and maintenance costs can be expressed as a function of the installed power capacity (MW) [15],

_  5ÞÞ C O&M ¼ 20 expð0:0025ðW

ð15Þ

The amount of physical construction of a geothermal power plant is given by

_ net C plant ¼ C unit W

ð16Þ

_ net is the net power of where Cunit is the unit cost of plant in $/kW, W plant in kW. 3.1. Escalation load Escalation is the price rise of the material, equipment and labor used during construction of power plants. Therefore, due to escalation of expenditures made during the plant construction the direct construction cost will be higher. The amount of non-escalation expenditure for any year is calculated from

C nonesc ¼ C plant Eper

ð17Þ

where Eper designates percentage of annual expenditure during construction. The amount of total escalation expenditure during the construction of plants is expressed as

C esc ¼

t X C nonesc ð1 þ er Þt

ð18Þ

t¼1

where er is the escalation rate and t is the escalation period. 3.2. Interest and inflation load When a loan is used for expenditures during construction of the power plant, it is also necessary to repay interest load in addition to the expenses with escalation. At the end of the construction of the power plant, the total amount that has to be repaid is defined by  bt

CðtÞ ¼ C esc ð1 þ i Þ

ð19Þ *

where b is the construction time in year, i is the annual accumulated interest rate including annual producer prices inflation. The interest rate including inflation rate is calculated from [22] 

i ¼

1þi 1 1þf

ð20Þ

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where i annual interest rate and f is average annual producer prices inflation. The total expenditure amount including escalation, interest and inflation rates can be calculated from



b X CðtÞ

ð21Þ

t¼1

3.3. Constant annual capital cost of enterprise Considering that the starting date of repayment of the credit used due to the expenditures made at the construction of the power plant is the delivery date of the power plant, constant annual capital cost to repay during the lifetime of the power plant is calculated as 

C capital ¼ C

 n

i ð1 þ i Þ  n ð1 þ i Þ  1

ð22Þ

where n designates the lifetime of the power plant. Annual amount of electrical energy production in the power plant (MW h) is given by

_ net Lf Ee ¼ 8760 W

ð23Þ

where Lf is load factor which is taken as 0.90 in geothermal power plants. 3.4. Annual operating and maintenance expenses In geothermal power plants annual operating and maintenance expenses are taken as 3% or 4% of the investment costs of power plants [23]. Annual operating and maintenance expense is defined as

_ net C O&M ¼ C CE W

ð24Þ

where CCE is constant expenses denominated by $/kW year. 3.5. Reduction of future expenses to today The present value of the capital cost at the end of n period can be defined as

PWF capital ¼

n X  n C capital ðnÞ  ð1 þ i Þ

ð25Þ

n¼1

The present value of annual operation and maintenance expenses can be calculated from

PWF O&M ¼

n X  n C O&M ðnÞ  ð1 þ i Þ

ð26Þ

n¼1

By the proportion of today’s totals of the all expenses made throughout construction and operating life of the power plant to the revenue of energy the power plant will produce during its lifetime, the following equation is obtained

COE ¼

PWF capital þ PWF O&M Ee

ð27Þ

Here, COE designates the capital cost per unit energy in $/kW h The simple payback periods of power plants (year) are defined as

SPP ¼

PWF capital þ PWF O&M Ee Ep

where Ep is unit price of electricity, in $/kW h. In this study, the following assumptions are made:

ð28Þ

 The heat source for the cycles are the flow of geothermal water (brine) entering the plant at 162 °C with a mass flow rate of 100 kg/s [24].  Dead state temperature and pressure for the cycles are 25 °C and 100 kPa, respectively.  Turbine and pump isentropic efficiencies are 85%.  In binary cycle and combined cycle, the binary working fluid is isobutane. Note that isobutane is commonly used as the working fluid for binary geothermal plants using liquid, low-to-medium temperature resources.  In Kalina cycle, the working fluid is NH3–H2O.  The pinch-point temperature difference (DTpp) that exists in the heat exchanger of binary and Kalina cycles is taken as 5 °C.  Ten percent of the power produced is internally consumed in power plants [25]. Internal power consumption is typically higher than this (up to 25% or more) in binary plants particularly when condensers are air cooled. However, for uniform treatment, we take this 10% for all cycles but the effect of this uncertainty should be kept in mind when interpreting the results.  Effectiveness of heat exchangers for binary cycles is 80%.  Lifetime of plants is 25 years.  Construction time of double flash, combined and Kalina plants are 5 years and binary plant is 2 years.  Interest rate is 7.75% [26]. Escalation rate is 5%.  Annual inflation rate is 2.2% [27].  The unit price of electricity is 0.05 $/kW h.  Load factor is 0.90.

4. Results and discussion Using the formulation, cycle data, and assumptions, the cycles are simulated in a computer program (EES) with built-in thermodynamic property functions [28]. Parametric studies are performed to study effect of certain operating parameters (such as resource temperature and turbine inlet pressure) on the thermodynamic and economic results (such as unit cost and net power) for different cycles considered in this paper. The resource considered in this study is a liquid resource at 162 °C. There are other geothermal resources in western Turkey at a temperature close to this value. The parametric analysis is performed at various temperature ranges suitable for the considered cycles and these ranges also include 162 °C. The assumptions and values given at the end of the previous section are used in this analysis. Plant operation is simulated using EES program under the stated operating conditions and assumptions and the results are obtained at different operating points. The investigation indicates an optimum point of operation at a certain turbine inlet pressure for which the net power output is a maximum. As a result, thermodynamic and economic results not only apply to the specific resource considered in this study but also applies to similar liquid resources at the given temperature ranges. Properties are calculated at all state points for various cycles and results are given in Tables 1–4. Note that the pressures in the second flash chamber and condenser are subatmospheric. The condenser pressure in a double-flash cycle is typically very low. Having a higher but subatmospheric turbine inlet pressure allows power output from the turbine. This flash pressure is obtained from a thermodynamic analysis but one can use a pressure slightly above the atmospheric in the flash chamber to eliminate practical problems such as air leakage and pump needed to extract liquid. The total power output from the plant will not be affected much due to such a modification. The values corresponding to optimum values of power cycles considered for this resource are given in Table 5.

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A. Coskun et al. / Energy Conversion and Management 78 (2014) 39–49 Table 1 Properties and exergy rates for double flash cycle. State no.

Fluid

Phase

Temperature (°C)

Pressure (kPa)

Enthalpy (kJ/kg)

Entropy (kJ/kg °C)

Mass flow rate (kg/s)

Exergy rate (kW)

0 00 1 2 3 4 5 6 7 8 9

Brine Water Brine Brine Brine Brine Water Water Brine Brine Brine

Dead state Dead state Sat. vapor Sat. vapor Liquid–vapor Sat. liquid Liquid Liquid Liquid Liquid–vapor Liquid–vapor

25 25 121 82.6 45.8 45.8 18 28 162 121 82.6

100 100 205.3 52.6 10 10 2.1 3.8 650.3 205.3 52.6

104.8 104.8 2707 2647 2374 191.8 75.5 117.4 684.2 684.2 508.2

0.37 0.37 7.12 7.58 7.49 0.65 0.27 0.41 1.96 1.99 1.56

– – 8 6.5 14.5 14.5 756 756 100 100 92

– – 4719 2552 2105 42.1 263.2 47.9 10,371 9687 4324

Table 2 Properties and exergy rates for binary cycle. State no.

Fluid

Phase

Temperature (°C)

Pressure (kPa)

Enthalpy (kJ/kg)

Entropy (kJ/kg °C)

Mass flow rate (kg/s)

Exergy rate (kW)

0 00 000 1 2 3 4 5 6 7 8

Brine Isobutane Water Isobutane Isobutane Isobutane Isobutane Brine Brine Water Water

Dead state Dead state Dead state Sat. liquid Comp. liquid Sup. vapor Sup. vapor Liquid Liquid Liquid Liquid

25 25 25 29.6 30.9 152 99.9 162 78.5 18 28

100 100 100 400 2316 2316 400 650.3 44.7 2.1 3.8

104.8 599 104.8 270.8 275 805.4 731.4 684.2 328.8 75.5 117.4

0.37 2.52 0.37 1.25 1.25 2.68 2.72 1.96 1.06 0.27 0.41

– – – 67 67 67 67 100 100 737.6 737.6

– – – 3401 3637 10,457 4795 10,371 1803 256.8 46.7

Table 3 Properties and exergy rates for combined cycle. State no.

Fluid

Phase

Temperature (°C)

Pressure (kPa)

Enthalpy (kJ/kg)

Entropy (kJ/kg °C)

Mass flow rate (kg/s)

Exergy rate (kW)

0 00 000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Brine Isobutane Water Brine Brine Brine Isobutane Isobutane Isobutane Isobutane Brine Brine Brine Brine Water Water Water Water

Dead state Dead state Dead state Sat. vapor Liquid–vapor Sat. liquid Sat. liquid Comp. liquid Sat. vapor Sat. vapor Sat. liquid Comp. liquid Sat. liquid Liquid–vapor Liquid Liquid Liquid Liquid

25 25 25 129.6 45.8 45.8 29.6 30.1 119.6 83.9 129.6 69.4 162 129.6 18 28 18 28

100 100 100 266.7 10 10 400 1400 1400 400 266.7 266.7 650.3 266.7 2.1 3.8 2.1 3.8

104.8 599 104.8 2719 2301 191.8 270.4 272.6 749.4 698.8 544.5 290.5 684.2 684.2 75.5 117.4 75.5 117.4

0.37 2.52 0.37 7.03 7.26 0.65 1.24 1.24 2.61 2.63 1.63 0.95 1.96 1.98 0.27 0.41 0.27 0.41

– – – 6.4 6.4 6.4 53.3 53.3 53.3 53.3 93.6 93.6 100 100 323.8 323.8 545 545

– – – 4032 902.3 18.7 2703 2801 6586 3494 5912 1201 10,371 9944 112.7 20.5 189.8 34.5

The cost of energy produced in geothermal power cycles and the resulting payback periods are given in Table 6. The costs of plants are taken as 1294 $/kW for double flash cycle, 2259.2 $/ kW for the binary cycle [17], 1850 $/kW for combined cycle, and 1300 $/kW for Kalina cycle [29]. The total cost turns out to be the highest for the binary cycle and the lowest for the double flash cycle. The cost of producing a unit amount of electricity is calculated to be 0.0116 $/kW h for double flash and Kalina cycles, 0.0165 $/kW h for combined cycle and 0.0202 $/kW h for binary cycle, indicating that double flash and Kalina can provide higher potential revenues. Consequently, the payback period is 5.8 years for double flash and Kalina cycles while it is 8.3 years for combined cycle and 9 years for binary cycle. Sener and Aksoy [30] found the cost of electricity from geothermal resources in Turkey to be around $0.057/kW h, with a payback time of 7–8 years for specific types of investment. These payback periods are typical for new

geothermal power plants in Turkey. Considering that a typical agreement period for buying geothermal electricity is 30 years, geothermal power investments remain attractive in most cases. The change of total cost of the cycles with the net power is shown in Fig. 5. As seen from this figure, the total cost of the cycles increases with the increase in the net power. Unit costs of the double flash cycle and Kalina cycle are close to each other, and these cycles are distinguished from the others as the most economical plant types in terms of the total costs. Variation of net power and specific cost of the plants with the temperature of geothermal fluid is presented in Figs. 6–9. As the resource temperature increases, the net power increases and the specific cost decreases. Effect of interest rate on total cost and payback period of double flash cycle is shown in Fig. 10. As the interest rate increases, both the total cost and payback period increase, as expected. This trend

46

A. Coskun et al. / Energy Conversion and Management 78 (2014) 39–49

Table 4 Properties and exergy rates for Kalina cycle. State no.

Fluid

Phase

Temperature (°C)

Pressure (kPa)

Enthalpy (kJ/kg)

Entropy (kJ/kg °C)

Mass flow rate (kg/s)

Exergy rate (kW)

0 00 000 000 0 1 2 3 4 5 6 7 8 9 10 11 12 13

Brine NH3–H2O NH3 Water Brine Brine NH3–H2O NH3 NH3 NH3 NH3 NH3–H2O NH3–H2O H2O H2O Water Water

Dead state Dead state Dead state Dead state Sat. liquid Sat. liquid Sup. vapor Sup. vapor Liquid–vapor Sat. liquid Comp. liquid Comp. liquid Comp. liquid Comp. liquid Comp. liquid Liquid Liquid

25 25 25 25 162 103.5 152 152 29.5 29.5 33.1 50 69.3 152 80 18 28

100 100 100 100 650.3 114.9 12,222 12,222 1150 1150 12,222 12,222 12,222 12,222 12,222 2.1 3.8

104.8 599 1547 104.8 684.2 434.1 577.3 1453 1244 339.2 360.9 44.5 135.5 648.1 344.7 75.5 117.4

0.37 2.52 6.61 0.37 1.96 1.35 2.03 4.35 4.47 1.48 1.49 0.61 0.88 1.85 1.07 0.27 0.41

– – – – 100 100 56.6 39.6 39.6 39.6 39.6 56.6 56.6 17 17 857.6 857.6

– – – – 10,361 3705 13,061 22,957 13,277 12,743 13,479 14,025 7474 1720 525.6 298.6 54.3

Table 5 Net power and energy and exergy efficiencies of cycles for Kutahya–Simav region based on optimum turbine inlet pressure. Plants

Optimum turbine inlet pressure (kPa)

Net power (kW)

Energy efficiency (%)

Exergy efficiency (%)

Double flash cycle Binary cycle Combined cycle Kalina cycle

205.3 2316 266.7 12,222

3991 4184 4726 6148

6.9 7.2 8.2 10.6

38.5 40.3 45.6 59.3

Table 6 Costs and payback periods of cycles for Kutahya–Simav region. Plants

Total costs (M$)

Cost per unit energy ($/kW h)

Payback periods (Year)

Double flash cycle Binary cycle Combined cycle Kalina cycle

9.00 14.91 15.40 14.08

0.0116 0.0181 0.0165 0.0116

5.8 9 8.3 5.8

should be similar in other cycles. It is clear that economic feasibility of a geothermal plant investment is greatly influenced by interest rate. The effect of turbine inlet pressure on the net power output for double-flash, binary, and combined cycles are given in Fig. 11. The analysis is performed fora geothermal resource at 162 °C. Under the given values and assumptions as specified in Section 3, the turbine inlet pressure is varied for the net power

output. In flash cycles, as the flash pressure (i.e., turbine inlet pressure) increases the amount of resulting vapor decreases but its temperature and specific enthalpy increases. Therefore, there must be an optimum point for flash pressure to yield a maximum total enthalpy. This point also represents the maximum power for the cycle. The optimum pressure for double flash cycle is determined to be 205 kPa (see Table 5). The analysis of an existing single-flash geothermal power plant by Kanoglu and Cengel [31] give a similar trend for the effect of flash pressure on the net power output. The optimum pressure is determined to be 267 kPa in combined cycle. In binary cycle, there is no flashing process. The power output from the cycle is determined at different heat exchanger (i.e., turbine inlet pressure). It turns out that the net power is maximized at a pressure of 2316 kPa. This is the pressure of binary fluid (working fluid) of the cycle. In the analysis of the binary cycle, the turbine inlet temperature is fixed and only pressure is varied. Increasing pressure also increases the pumping power but the enthalpy of the fluid at the turbine inlet also increases. As a result,

26

Binary cycle

3000

24

Combined cycle Double flash cycle

2750

20

Kalina cycle

18 16 14 12 10

3500 2500 2250

3000

2000 2500

1750

Net power (kW)

22

4000

Double flash cycle Unit cost, Cunit ($/kW)

Total cost, C total (M$)

28

1500 2000

8 1250

6 4 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Net power, Wnet (kW) Fig. 5. Net power vs. total cost for cycles.

1000 120

125

130

135

140

145

150

155

1500 160

Resource temperature, Tgeo (°C) Fig. 6. Resource temperature vs. unit cost and net power for double flash cycle.

47

A. Coskun et al. / Energy Conversion and Management 78 (2014) 39–49 4500

Double flash cycle

4000 3500

4500 4000

3000

3500

2500

Net power (kW)

5000

3000 2000

2500

Payback period, SPP (year)

20.5

5500

Unit cost, Cunit ($/kW)

35

22.5

Binary cycle

30

18.5 16.5

25

14.5 20

12.5 10.5

15

8.5 6.5

Total cost, C total (M$)

6000

10

4.5 2000 120

125

130

135

140

145

150

155

1500 160

2.5

5

7.5

10

12.5

15

17.5

20

22.5

5 25

Resource temperature, Tgeo (°C)

Interest rate, i* (%)

Fig. 7. Resource temperature vs. unit cost and net power for binary cycle.

Fig. 10. Interest rate vs. payback period and total cost for double flash cycle.

4000

7500 Combined cycle

5000 7000

6000

2500

5500 5000

2000

4500 1500 1000 150

Net power (kW)

3000

4000

155

160

165

170

175

180

185

Combined cycle Binary cycle Double flash cycle

4500

6500

Net power (kW)

Unit cost, Cunit ($/kW)

3500

4000 3500 3000 2500

3500 190

2000

Resource temperature, Tgeo (°C)

0

500

1000

1500

2000

2500

3000

3500

Turbine inlet pressure (kPa) Fig. 8. Resource temperature vs. unit cost and net power for combined cycle. Fig. 11. Turbine inlet pressure vs. net power for double flash, binary, and combined cycles. 6300

1400

Kalina cycle 6250

1380 6200 1370 6150

1360 1350

6100

1340

6050

1330

Net power (kW)

Unit cost, Cunit ($/kW)

1390

6000 1320 5950

1310 1300 140

145

150

155

160

165

170

175

5900 180

optimization of the operating conditions of the power cycles may lead to an increase in the performance of the designed cycles for Kutahya–Simav region. These plants were optimized according to turbine inlet pressure maximizing net power, energy and exergy efficiencies. Especially in double flash and combined plants flashing pressures and turbine inlet pressures were separately optimized. Turbine outlet pressure was optimized according to the outlet temperature of cooling water used in the condenser. Thermodynamic analysis and optimization results as summarized in Table 5 indicate that Kalina cycle provides the maximum power output for this resource. The power output in the Kalina cycle is

Resource temperature, Tgeo (°C) Fig. 9. Resource temperature vs. unit cost and net power for Kalina cycle.

7000 Kalina Cycle

there must be an optimum pressure yielding a maximum net power for the cycle. A similar study for the Kalina cycle gives an optimum ammonia pressure of 12.2 MPa (Fig. 12 and Table 5). Thermodynamic explanation of the process to yield an optimum pressure is analogous to a flash cycle. Optimum plants in terms of maximum net power, thermal and exergetic efficiency were selected according to properties of Kutahya–Simav geothermal region at medium temperature. These plants were optimized according to various parameters. The optimization process was performed to minimize the exergy losses of the units of the geothermal power plants. Optimal operating conditions were determined for the net power of these cycles. The

Net power (kW)

6500

6000

5500

5000

4500

4000 10000

11000

12000

13000

14000

15000

Turbine inlet pressure (kPa) Fig. 12. Turbine inlet pressure vs. net power for Kalina cycle.

48

A. Coskun et al. / Energy Conversion and Management 78 (2014) 39–49

about 50% greater than that in the double flash cycle. In Kalina cycle, the ammonia–water mixture has a varying boiling and condensing temperatures. This leads to a higher optimum turbine inlet pressure. The net power, thermal efficiency and exergetic efficiency of the cycle is higher in Kalina cycle compared to other cycles. This is partly due to high values of enthalpy for ammonia at high pressure. The power outputs in binary and double-flash cycles are close but much smaller than Kalina cycle. DiPippo [7] also points out that Kalina cycle can theoretically produce 50% more power than binary (and double flash) cycles but this is not the case in actual operations. Kalina cycle produces more power for the same resource by decreasing the geothermal temperature to lower values at the plant outlet. Kalina cycle is a new technology compared to flash and binary cycles and technologic developments may allow this performance to be approached in the future. The results indicate that at optimum pressures, the power output in combined cycle is 18% greater than that in double flash cycle and 13% greater than that in binary cycle (Table 5). The power output comparison of these four cycles also holds for the energy and exergy efficiencies since the energy and exergy input to all cycles are the same. The first law and second law efficiencies of Kalina cycle are determined to be 10.6% and 59.3%, respectively. These efficiencies are 8.2% and 45.6% for combined cycle, 7.2% and 40.3% for binary cycle and 6.9% and 38.5% for double flash cycle (Table 5). The study also includes the effect of ammonia concentration on the net power. The net power decreases to a certain value, and then increases again as the mass percentage of ammonia increases, as shown in Fig. 13. Exergy analysis can be used for selecting the most suitable geothermal cycle and optimization of cycle operation. It also allows determination of losses for the system components and the entire cycle. The minimization of losses allows the system to perform better and consequently power output is maximized. Exergy analysis

6500

Kalina Cycle

Net power (kW)

6000

5500

5000

4500

4000 0.70

0.73

0.75

0.78

0.80

0.83

0.85

0.88

0.90

Percentage by weight of ammonia (%) Fig. 13. Percentage by weight of ammonia vs. net power for Kalina cycle.

indicates that work potential is lost in turbines, condensers, heat exchangers, and by reinjection, as shown in Table 7. In Kalina cycle, condenser exergy losses represents only 2% of the exergy input, which is one reason for high performance of this cycle. The most exergy losses are due to high temperature recuperator. In double flash cycle, reinjection of geothermal water represents 20% of exergy input to the cycle. Most exergy are lost in the condenser with a rate of 23%. In all geothermal cycles, a significant fraction of the input geothermal exergy to the plant is lost in the reinjection process. The analysis here also confirms this finding. 5. Conclusions Optimum first law efficiencies vary between 6.9% and 10.6% while the optimum second law efficiencies vary between 38.5% and 59.3% for the geothermal power cycles. Kalina cycle is followed by combined cycle and binary cycle in terms of the maximum power output, the first law, and the second law efficiencies. The cost of producing a unit amount of electricity is calculated to be 0.0116 $/kW h for double flash and Kalina cycles, 0.0165 $/kW h for combined cycle and 0.0202 $/kW h for binary cycle. The payback periods of Kalina cycle and double flash cycle are determined to be 5.8 years while it is 8.3 years for combined cycle and 9 years for binary cycle. The purpose of the study is to investigate the determination of the optimum electricity production by double flash, binary, combined flash/binary, Kalina cycle plants. The study is determined to the optimum operating conditions of the selected cycles for the Kutahya–Simav region. Especially, in double flash and combined plants flashing pressures and turbine inlet pressures are separately optimized. Furthermore, Kalina cycle is also optimized for the geothermal resource as the Kalina cycle is a new technology compared to flash and binary cycles and technologic developments may allow us to approach this performance in the future. Besides energy and exergy analysis of the plants, economic analysis (the costs per unit energy, payback periods, etc.) in which interest, inflation and escalation rates which have been included to the costs are also carried out. The cost to the operators is an important factor due to economic analyses are very important for designer. Therefore, the cost of energy produced in geothermal power cycles and the resulting payback periods for the geothermal resource are presented. The selection of best geothermal power plant design for a given resource depends on power generation potential and cost results among other practical considerations. As a result, these analyses are expected to be very useful for the designers who want to install a power plant into this region. Thermodynamic and economic analysis conducted in this study for a particular geothermal resource indicates that Kalina cycle presents a viable choice for both thermodynamically and economically.

Table 7 Exergy losses of cycle components for Kutahya–Simav region, in kW. Plants Double flash cycle Binary cycle Combined cycle Kalina cycle

Total exergy 9042.9 10370.4 9944.4 34556.4

Net power

Turbine

Condenser

Reinjection

Parasitic power

Heat exchanger

Pump

Condenser 1

Condenser 2

HTR

3991 (44%) 4184 (41%) 4726 (49%) 6148 (18%)

731.5 (8%) 705.2 (7%) 842.4 (8%) 1434 (4%)

2063 (23%) 1394 (13%) –

1814 (20%)

443.4 (5%)











1803 (17%)

495.7 (5%)

1747 (17%)







1222 (12%)

537.9 (5%)

924.7 (9%)

883.7 (9%)

790.6 (8%)



533.5 (2%)

3705 (11%)

1237 (4%)

1069 (3%)

41.5 (0%) 17.1 (0%) 125.9 (0%)





20,304 (58%)

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