Utilization of oil wells for electricity generation: Performance and economics

Utilization of oil wells for electricity generation: Performance and economics

Energy xxx (2015) 1e7 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Utilization of oil wells fo...

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Energy xxx (2015) 1e7

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Utilization of oil wells for electricity generation: Performance and economics Mohamad Kharseh a, *, Mohammed Al-Khawaja a, Ferri Hassani b a b

Qatar University, Mechanical & Industrial Engineering Department, Doha, Qatar McGill University, Department of Mining Metals and Materials Engineering, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 December 2014 Received in revised form 21 June 2015 Accepted 19 July 2015 Available online xxx

There is a general agreement that the climate change, which is the most important challenge facing humanity, is anthropogenic and attributed to fossil fuel consumption. Therefore, deploying more renewable energy resources is an urgent issue to be addressed. Geothermal refers to existing heat energy in deep rock and sedimentary basins. Traditionally, geothermal energy has been exploited in places with plentiful hot water at relatively shallow depth. Unfortunately, the high exploration and drilling costs of boreholes is the main barrier to the commerciality of geothermal worldwide. In oil producing countries, such problems can be overcome by utilizing oil or gas wells. The current study presents thermodynamic and economic analyses of a binary geothermal power generation system for commercial electricity generation. Two different source temperatures (100 and 120  C) and constant sink temperature (29  C) were considered. The optimal working fluid and optimal design that improve the performance of the plant are determined. For the current costs in Qatar, the economical analysis of 5 MW geothermal plant shows that the levelized cost of electricity for the plant varies from 5.6 to 5.2 ¢/kW. Whereas, the payback period of such plants lies between 5.8 and 4.8 years. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Geothermal energy Electricity generation Organic Rankine cycle Performance Economic viability

1. Introduction Owning to the awareness of the correlation between fossil fuel consumption and the ongoing climate changes, the future looks promising for renewable energy development. The main problem encountered by the developer is the fluctuation in the availability of renewable energy. To overcome this issue, the future energy systems, which exploit energy from different resources, must work together to even out the fluctuations in available sources of renewable energy. Geothermal refers to existing heat energy in deep rock and sedimentary basins. These formations can provide superheated steam or hot fluid that can be utilized to generate electricity using any work producing device. Depending on the condition of the geothermal fluid in the reservoir, different power producing cycles may be used, including direct steam, flash-steam (single and double-flash), binary, and combined flashebinary cycles. Although the direct steam cycle is the simplest geothermal cycle, binary

* Corresponding author. E-mail addresses: [email protected] (M. Kharseh), (M. Al-Khawaja), [email protected] (F. Hassani).

[email protected]

power plants have been proven to have greater efficiencies than flashing plants for liquid-dominated low-temperature geothermal resources in the range of 100  C and 170  C. Compared to other renewable energy resources, geothermal energy is a stable resource and independent of the climate. Therefore, geothermal energy is suitable for supplying base-load power. Geothermal energy is a mature technology market with total installed capacity worldwide about 10700 MW [1]. The annual electrical generation from such plants is 67.2 TWh at a capacity factor above 90% [2]. Nevertheless, the world's target by 2050 is to reach 1180 TWh of annual electricity generation from geothermal sources (about 6% of current global electricity consumption) [2]. Although the design of binary plants has been addressed in different studies, it is still an area of active research. To be exact, the low First Law efficiency (<10%) and low Second Law efficiency (25e45%) and the large required heat exchanger [1,3] means more research are required. Usually, geothermal plants are located in places characterized with the relatively rare combination of large reservoirs of hot water at shallow depth. Unfortunately, such conditions exist in relatively few places around the world. Therefore, according to the U.S. Energy Information Administration, geothermal resources have limited potential for growth. Instead, growth is expected from

http://dx.doi.org/10.1016/j.energy.2015.07.116 0360-5442/© 2015 Elsevier Ltd. All rights reserved.

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unconventional resources called EGS (enhanced geothermal systems). Fortunately, temperature increases with depth below the surface everywhere on the earth. In most hydrocarbon-producing areas, the gradient is usually in the range of 11e30  C per 1000 m of depth increase [4]. Consequently, the temperature at depths of 3e10 km below the surface is enough to be considered as promising future sources of geothermal energy. This fact creates a possibility of exploiting geothermal energy everywhere by means of binary power plants. In the light of significant improvements in binary ORC (organic Rankine cycles), harvesting energy from a low temperature source becomes possible. The analysis and the improvement of the performance of ORC were widely addressed in the literature [1,3,5e9]. Franco, for instance, investigated the exploitation geothermal energy of low temperature in the range 100e130  C [1]. In his study, Franco showed possibility to improve the ORC by using recuperated cycle configurations [1]. Another study showed the exergy analysis of a 12.4 MW existing binary geothermal power plant [3]. Kanoglu showed that about 70% of exergy of the geofluid can be utilized by ORC, while the corresponding thermal efficiency for the plant is 5.8% [3]. Thermal and utilization efficiencies were calculated for 35 MW geothermal power plant of high resource temperature (160e254  C) [6]. Due to the high resource temperature, DiPippo et al. showed that the thermal efficiency of 15.1%, while the Second Law efficiency of 37.2% [6]. However, high depth for such energy creates technical and financial problems. The economic viability of utilizing geothermal for power generation was addressed in literature capacity [2,10e17]. The perspectives of development of geothermal power plants of medium-low temperature were discussed by Franco [10]. Based on investment costs of 1250 V/kWel for the ORC, Preibinger et al. showed that the payback period can be reduced by 10% and the cash flow can be increased by 8% by using a transcritical ORC [17]. Still, the high initial capital cost of geothermal power plants, which is currently in the range of 2130e5200 US$/kW [2,11], is the main barrier to commerciality of geothermal energy. Borehole drilling accounts for up to 40% of the total cost of the project [2,11]. Considering the fact that oil wells are usually drilled to big depth and, consequently, the wells bottom is of high temperature. Hence, oil wells can be economically used as geothermal wells. This way, drilling cost can be eliminated from the initial cost of geothermal plants and, accordingly, the economic feasibility of geothermal power plants can be improved effectively. The current work discusses the thermodynamic and economical analyses of utilizing abandoned oil wells for power generation. Qatar was selected as a case study and, therefore, the source and sink temperatures were selected according to the local conditions. Thermodynamic and economic performances of Organic Rankine cycles are analyzed and optimal design criteria are defined. It is important to note that despite the fact that this study is limited to the selected working conditions, it is expected the approach can be successfully applied for other conditions. This study can be extended to any waste heat source other than oil well. 2. Synergy with oil industry The similarities between geothermal and oil extraction operations create the possibility of using the advanced technology and experience from the petroleum industry in geothermal operations [18]. This can facilitate the transition from natural resources to enhanced geothermal energy. Exploiting the oil wells represents the low hanging fruit in geothermal energy production in oil countries. Recently, more and more attention has been paid to geothermal power generation by utilizing hot fluids co-produced from oil and gas reservoirs. The geothermal gradient varies from

basin to basin. In most hydrocarbon-producing areas, the gradient is usually in the range of 11e30  C per 1000 m of depth increase. Since the oil and gas wells in many cases are drilled to a big depth below the ground surface, the temperature of produced geofluid is high enough to generate electricity. In addition, the water-cut of an oil well, which is very high and up to 98% of the flow rate of the well in some cases, increases with time [19]. The temperature of the produced geofluid, in the water-cut period of the well's life, increases with time as well. It is worth mention that the water-cut in many mature oil and gas fields, is usually considered a nuisance to oil and gas producers because they are required to dispose or reinject the water into the reservoirs. This process costs a lot and reduces the net profit of the oil and gas producers. Thus, the exploitation of oil wells for electricity generation gives a new opportunity to low yield oil and gas producers. In oil producing counties, there are many abandoned wells that can be used as sources for energy generation. Obviously, active wells can also be used as energy source for binary plants. In addition to eliminating of the drilling and exploration costs associated with deep boreholes, the use of oil wells implies that all required data for the geothermal plant are available. 3. Methodology Geothermal system design requires the determination of thermodynamic performance and economic viability. A computational model was built in order to simulate thermodynamic and economic performances of organic Rankine cycles. In Qatar, high-temperature wells have been successfully drilled into reservoirs where temperatures exceed 149  C [20]. To be more realistic the source temperature will be taken as 100 and 120  C [21]. The sink temperature was assumed to be 29  C, which is equal to the annual mean air temperature. From the thermodynamic analysis the optimal design of the geothermal power plant was obtained. Also, the economic analysis was carried out at current conditions in Qatar and resulted in determination of different figures of merit including NPV (net present value), IRR (internal rate of return), the PBT (payback time), and LCOE (levelized cost of electricity). 3.1. Thermodynamic analysis There are chemical and mechanical problems associated with the use of geothermal sources for electricity generation. From a chemical perspective, most geofluid of high temperature contain non-condensable gases, hazardous compounds, corrosive ions, and insoluble materials. From a mechanical viewpoint and in the case of a relatively low geothermal temperature, non-aqueous secondary fluids with low boiling points are needed. To overcome these problems, a binary cycle is considered. The schematic of the suggested unit is illustrated in Fig. 1. The geofluid, which is extracted from the well (state 8), passes through the heat exchange system where heat is transferred to the working fluid. After the heat exchanger system (state 11), the geofluid either is used in oil/gas process cycle or reinjected into the well again. The working fluid will be superheated vapor at turbine inlet (state 1). The working fluid is condensed upon exiting the condenser via exchanging heat with a coolant (state 4), and returns to the heater (state 6) by means of a feed pump to complete the cycle. The mechanical power extracted from the turbine is converted to electrical power in a generator, which is referred to as gross power. The following thermodynamic analysis is available in the literature [22e24]. Keep in mind that the current study is investigating the utilizing the abandoned oil well, a part of generated energy will be lost as parasitic power. In binary organic cycle, the parasitic

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Where, mf is the mass flow rate of the geothermal fluid; h8 and ho is the enthalpy of geofluid at geothermal resource temperature (T8) and ambient temperature (To); s8 and so is the entropy of geofluid at geothermal resource temperature and ambient temperature, respectively. In this study, the EES (Engineering Equation Solver)-base computational model was built to simulate the geothermal power plant operation. In this model, the heat exchanger used on the boiling side was simulated as three stages. The first one is used for preheating; the second one is used for evaporating, while the last stage is used for super-heating. A similar set is used on the condensing side, but with two stages only. The first one is used for precooling, while the second one is used for condensing. The EESbase model contains two kinds of parameters. The first kind of the parameters depend on the working conditions such as geofluid flow rate, geofluid temperature, and the temperature of the coolant at the inlet of the condenser. The latter depends on the location of the geothermal plant since the coolant can either be seawater or ambient air. The second kind of the parameters depends on designer selection such as the efficiencies of the turbine, generator, and the pump, as well as the effectiveness of the heat exchangers (i.e. boiler, condenser, cooler, heater, and super heater). Since the EES program is a built-in-property program, different working fluid can be selected. Briefly, the following variables are assumed to be specified and treated as input parameters to the EES-base model (see Fig. 1): 1. Geofluid temperature at the outlet of the well (source temperature), 100 or 120  C 2. The mass flow rate of geofluid, kg/s 3. Temperature of coolant in the condenser (sink temperature), 29  C 4. Isentropic efficiency of the turbine, his ¼ 90% [23]. 5. Turbine-generator efficiency, htg ¼ 80% [23]. 6. Mechanical efficiency of the feed pump, hp ¼ 90% [23].

Fig. 1. Schematic of plant and thermodynamic cycle on T-s diagram.

power, including feed pump, condenser fans, and auxiliaries, typically consumes about 25% of the gross power generated in an aircooled binary geothermal system [1,7]. In a water-cooled condenser the parasitic power stands also for 25% of the gross power since the power to pump the water (in water-cooled condenser) and the power to drive the fan (in air-cooled condenser) are almost identical. Thus, the actual net power output of the plant, say Pnet, is given by, see Fig. 1:

Pnet ¼ 0:75$htg $m_ w $ ðh1  h2 Þ

(1)

where, mw is the mass flow rate of working fluid; htg is the turbine-generator efficiency; h1 and h2 stand for the working fluid's enthalpy at the inlet and outlet of the turbine, respectively (see Fig. 1). The Second Law efficiency, or what so called utilization efficiency, is usually used to measure how much available work of resource will be converted into useful output [14]. To determine the utilization efficiency the exergy of the geofluid needs to be defined. Since the process involves only thermal interaction with the environment, then the exergy is defined as the maximum work when the stream of geofluid is brought from its initial state to the environmental state [25]. This way, Second Law efficiency of a geothermal plant is, see Fig. 1:

hu ¼

Pnet _ f $ðh8  ho  To ðs8  so ÞÞ m

(2)

The main objective of the thermodynamic analysis is to determine the optimal working fluid and designing of the ORC for working conditions of Qatar. The optimal working fluid is defined as the fluid that gives the best performance of the plant. From literature one can find that R32, R114, R134a, isobutene, isopentane, and n-pentane are good candidate working fluids commonly used in binary power plants [5]. To select the optimal working fluid at the working conditions of Qatar, EES-base model was run for each candidate working fluid. Now, the optimal design of the ORC is to determine the proper design of the heat exchangers used on the boiling and condensing for the best performance. Indeed, for specific working conditions (i.e. source and sink temperatures), the thermal effectiveness of the heat exchanger, which depends on its size, has a significant impact of on the cycle performance. As example, it has been shown that the major exergy losses in a binary Rankine cycle occur in the condenser [7,18,23,26e28]. To optimize the performance a range of thermal effectiveness of the heat exchangers are inserted as requested by the EES program when the optimizing option was used. The simulation was repeated for two different source temperatures, namely 100 or 120  C, and 29  C for sink temperature. 3.2. Economic viability of the investment The economic analysis leads to determination of the investment and operating costs, the net annual profit, the net present value, the levelized cost of electricity, and the payback time.

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3.2.1. Investment cost The investment costs are composed of four components: (1) exploration and resource confirmation; (2) drilling of wells; (3) surface facilities; and (4) the power plant [29]. Each component has its impact on the total cost of the project as follows: The first component represents 10e15%, the second component 20e35%, the third component 10e20% and finally, the fourth component 40e80% [2,11]. Indeed, the investment cost for typical binary geothermal plants decreases by increasing the temperature of heat recourse. Obviously, a lower temperature of heat resource required larger heat exchangers which increase the investment cost. Survey studies show that the investment cost for typical binary geothermal plants varies between USD from 2130 to 5200 per kW of electricity generation capacity [2,11e15]. Certainly, this cost is inversely proportional to the resource temperature. Using the costs of geothermal plants that are presented in Refs. [12], the total cost of a geothermal plant can be derived as following:

Cinv;total ¼ 8919$e0:008$Tg $Pnet

(4)

3.2.2. Operation and maintenance cost Annual O&M (operation and maintenance) costs of the plant represent expenses on equipment and services that occur after the system is installed. This cost is almost independent of the temperature of the geothermal source, while it is inversely proportional to the plant size [14]. The O&M costs in the U.S., for example, the average cost is 25 USD per MWh for binary plants [2,11]. It is worth mention that the O&M costs in Qatar are expected to be a little bit lower than those in USA, but there are no such data for Qatar. Thus, operation and maintenance costs of year ‘n’ (COM,n) becomes: n1

COM;n ¼ 0:025$ð1 þ erÞ

$Q n

(5)

Where er is the annual escalation rate of O&M costs; Qn (kWh) is the annual energy output of the power plant, which is given by:

Q n ¼ 8760$a$b$Pnet

RCOEgt ¼ 0:0924

$ kWh

(7)

3.2.4. Annual profit The income of the first year of operation (in USD), i.e. the net cash flow of the first year (Cp,n) due to installing 5 MW geothermal power plant is:

Cp;n ¼ 8760$a$b$Pnet $RCOEgt  COM;n

(8)

(3)

Where Pnet represents net power in kW (Eq. 1); Tg the temperature of heat resource. Indeed, in the case of utilizing existing oil well, exploration and drilling (i.e. about 40% of total investment cost) are zero. Thus, the investment cost, say Cinv, becomes:

Cinv ¼ 5351$e0:008$Tg $Pnet

fuel cost of the gas-fired turbine in Qatar is 0.046 $/kWh. Following the same approach used by Marafia [30], but with a nominal discount rate d ¼ 6.48% (real discount rate of 4.5% with inflation rate of 1.9% [31]), one can find that the real cost of electricity generation from gas turbine in Qatar is 0.0684 $/kWh. Taking into account the carbon dioxide emission cost, which is 0.0240 $/kWh [32], the real cost of electricity generated by gas turbine in Qatar becomes:

(6)

Where a represents the capacity factor, assumed to be 90% [2,6] and b represents the availability of the power plant, which is assumed to be 92% [6]. 3.2.3. Real cost of electricity in Qatar Certainly, electricity output of the geothermal power plant can be converted into a monetary value by multiplying the annual energy output of the plant by the RCOE (real cost of electricity) generated by a fossil-fuel-fired power plant. In 2001 RCOE was calculated for Qatar working conditions and found as 0.0573 $/kWh [30]. The estimation of RCOE in Ref. [30] was made based on the following assumptions. The capital cost is 275 $/kW, while the maintenance and fuel cost is 0.005 and 0.045 $/kWh, respectively. Taking into account an inflation rate of 1.9% [31], the current capital cost can be approximated at 351 $/kW, while current maintenance costs of 0.0064 $/kWh. In 2000 natural gas's price was 3.876 $ per standard cubic foot, while the current price is 3.947. Thus, current

Where Pnet is the net power or the useful output of the plant; RCOE is the real cost of electricity; CO&M is the annual cost which covers operation and maintenance.

3.2.5. Net present value of the project In finance, the NPV (net present value) of a project is used to help the decision maker distinguish between different investment opportunities. It is always better to invest in the project with a higher NPV. The NPV is defined as the sum of the investment costs (negative) and the present value of cash flows (positive) over the life of the project:

NPV ¼ Cinv þ

T X ð1 þ erÞn $Cp;n ð1 þ dÞn n¼1

(9)

Where ‘T’ is lifetime of the plants, in current study T ¼ 30 years; er is the escalation rate of electricity price; d is nominal discount rate.

3.2.6. Levelized cost of electricity The economic potential of a generation project is usually evaluated by estimating the LCOE (levelized cost of electricity). The LCOE is commonly used to compare the cost of energy generated by a renewable resource with that of a standard fossil-fueled generating unit. The latter is defined as the total cost of installing and operating a project expressed in dollars per kilowatt-hour of electricity generated by the system over its life cycle. In this case, the LCOE accounts for installation cost, operation and maintenance, and the quantity of electricity the system generates over its lifetime. By definition, the LCOE is given [33,34]:

P COM;n Cinv þ Tn¼1 ð1þdÞ n LCOE ¼ PT Qn

(10)

n¼1 ð1þdÞn

Where Q ¼ 8760∙a∙b∙Pnet is the energy output of the power plant; d is the nominal discount rate; T is analysis period.

3.2.7. Internal rate of return In order to prioritize different projects, the IRR (internal rate of return) can be used. IRR is defined as the value of the discount rate that makes the net present value (Eq. 9) equals zero. It is more desirable to select the project with higher internal rate of return. Mathematically, IRR is given by:

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Where ‘Cinv’ is the total PV plant system cost; ‘d’ is the nominal discount rate; ‘er’ is the escalation rate of electricity price; ‘Cp’ the income of the first year of operation. 4. Results and discussions For sake of simplicity, the flow rate of the geofluid was assumed 1 kg/s. In order to select the optimal working fluid that provides the best performance, for each candidate working fluid the EES-base model was run to optimize the performance of ORC. The simulations were carried out at the following specific conditions: the geofluid and sink temperatures are taken to be 100  C and 29  C, respectively. Fig. 2 shows R32 is the optimal working fluid at the considered working conditions. Thus, R32 will be selected as the working fluid in the following calculations (see Table 1). In order to determine the optimal designing of ORC, the EESbase model was run as mentioned above. The simulation was repeated for two different resource temperatures 100 or 120  C, and 29  C sink temperature. The results are illustrated in Fig. 3 and Table 1. As shown, increasing the source temperature leads to a considerable increase in electricity output of the plant. The utilization efficiency also increases with the source temperature, but at a lower rate than the plant's output. To perform the economic analysis a 5-MW geothermal power plant was selected taking into the consideration the current costs in Qatar. To fulfill this aim some assumptions were made as shown in Table 2. Taking these assumptions into account, the economic analysis of the geothermal power plant is performed for the two resource temperatures of 100 and 120  C. The results are tabulated in Table 3. The cash flow profile of each power plant scenario is illustrated in Figs 4 and 5. In order to calculate the payback time of the geothermal power plant, the cumulative cash flow was calculated based on Figs 4 and 5. The calculations show that the payback time of geothermal power plant is almost equal to 5.8 and 4.8 years for the resource temperatures of 100 and 120  C, respectively.

Fig. 2. The optimal performance of ORC of different working fluids for resource temperature of 100  C, and sink temperature of 29  C.

Fig. 3. Maximum output and utilization efficiency of geothermal power plant for 1 kg per second geofluid flow rate along with the resource temperature of 100 and 120  C and coolant temperature of 29  C.

0 ¼ Cinv þ

T X ð1 þ erÞn $Cp;n ð1 þ IRRÞn n¼1

(11) 5. Conclusions The current study investigates the utilization of existing oil wells as a geothermal source for electricity generation at the working conditions in Qatar. It is important to note that despite the fact that this study is limited to the selected working conditions, the approach can be successfully applied for other conditions. The working fluid that gives the best performance of the plant is found to be R32. The optimal design of organic Rankine cycle is

3.2.8. Payback time The viability of a renewable energy project is usually evaluated by estimation of the PBT (payback time) of the installation in years. The PBT is defined as the period necessary to recover the project cost of an investment while accounting for the time value of money. In other word, the PBT is the time required to make the accumulated present value of the net cash flows covers the initial investment cost [35]. In the current work, a mathematical expression of the payback time can be obtained by solving Eq. (16) in Ref. [36]:



2 ln41  PBT ¼

Cinv $

 1þer 1 1þd

Cp

Table 2 Data for evaluation of plant economics.

3 5

  ln 1þer 1þd

(12)

Power plant capacity

5-MW

Inflation rate and

1.9 [31]

Capacity factor [2,6] Availability factor [2,6] Escalation rate in O&M costs

90% 92% 1.9% [31]

Real discount rate Nominal discount rate Lifetime of the project

4.5% [31] 6.48% 30 years

Table 1 Optimal design of heat exchangers and output of ORC, R32 as a working fluid. Source temp.

Sink temp.

Effectiveness of heat exchangers Pre-cooler

Condenser

Heater

Boiler

Super-heater

Net electricity kW

Thermal efficiency

Utilization efficiency

T8 ( C)

T12 ( C)

εcoo

εc

εh

εb

εs

Pnet

hI (%)

hu (%)

100 120

29 29

80 80

30 30

80 80

75.6 70.0

65.8 70.0

6.8 13.2

4.2 5.9

22.4 28.4

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Table 3 Economic analysis results for geothermal plant in Qatar. Source temp. ( C)

Investment cost (M$)

First year O&M (M$)

First year cash flow (M$)

NPV (M$)

LCOE ¢/kWh

IRR (%)

100 120

12 10.2

0.91 0.91

2.4 2.4

27.1 28.8

5.6 5.2

22.1% 25.7

Cp,n d h mf mw n NPV Pnet Qn RCOEgt Fig. 4. Cash flow profile associated with a 5 MW geothermal power plant in Qatar for a source temperature of 100  C.

S Tpp

a b his hp htg hu

the net annual profit (i.e. cash flow) of the geothermal plant, ($) nominal discount rate, (dimensionless) working fluid's enthalpy, (kJ/kg) mass flow rate of the geothermal fluid, (kg/s) mass flow rate of working fluid, (kg/s) analysis period, (years) net present value of geothermal plant project, ($) useful output of the plant, (kW) annual energy output of the geothermal plant, (kWh) real cost of electricity generated by gas turbine in Qatar, ($/kWh) entropy, (kJ/kg.K) pinch-point temperature in the precooler, ( C) capacity factor of the geothermal plant, (dimensionless) availability of the geothermal plant, (dimensionless) isentropic efficiency of the turbine, (dimensionless) Mechanical efficiency of the feed pump, (dimensionless) turbine-generator efficiency, (dimensionless) exergy efficiency of a geothermal plant, (dimensionless)

References

Fig. 5. Cash flow profile associated with a 5 MW geothermal power plant in Qatar for a source temperature of 120  C.

determined at different resource temperatures (i.e. 100 and 120  C) and a constant sink temperature (i.e. 29  C). The calculations show that, for each kilogram per second of flow rate of geofluid, the optimal useful output of the plant varies from 6.8 to 21 kW for resource temperature of 100e120  C, respectively. While the optimal utilization efficiency changes from 22 to 27%. The economical analysis of a 5-MW geothermal plant in Qatar shows that the payback time and the levelized cost of electricity of the geothermal plant is less than 6 years and 6 ¢/kWh, respectively, even for the lowest selected resource temperature (i.e. 100  C). It is worth to mention that the current levelized cost of electricity for gas-fired turbines in Qatar is 9.2 ¢/kWh. Hence a geothermal plant that utilizes existing oil is economically feasible. Acknowledgments This work was made possible by a grant (NPRP 7e725e2-270) from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. Nomenclature Cinv Cinv,total COM,n

investment cost of geothermal plant excluding exploration and drilling costs, ($) total cost of a geothermal plant, ($) annual operation and maintenance costs, ($)

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Please cite this article in press as: Kharseh M, et al., Utilization of oil wells for electricity generation: Performance and economics, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.07.116