Geo-ocean Thermal Energy Conversion (geotec)

Renewable Energy 111 (2017) 372e380

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Geo-Ocean Thermal Energy Conversion (GeOTEC) power cycle/plant N.H. Mohd Idrus a, M.N. Musa a, W.J. Yahya b, *, A.M. Ithnin b a b

Ocean Thermal Energy Centre, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia Vehicle System Engineering, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 September 2016 Received in revised form 20 February 2017 Accepted 28 March 2017 Available online 8 April 2017

A new Rankine power cycle utilising a combination of ocean thermal energy and geothermal waste energy is proposed and thus called a GeOTEC (Geo-Ocean Thermal Energy Conversion) power cycle/ plant. The potential geothermal waste heat, which exists in the form of raw hot natural gas is continuously pumped from a shallow water Malaysia-Thailand Joint Authority (MTJA) gas production platform, and the supply data is estimated based on the output of the platform. A thermodynamic model derived from an energy balance calculation is used to simulate the proposed GeOTEC cycle with Matlab. A capital cost estimation is performed for the proposed GeOTEC based on the commercially available components and manufacturing practices. With higher superheated ammonia temperature, GeOTEC power plant efficiency increases, while the net power output decreases. A maximum net power produced by the proposed GeOTEC is 32.593 MW with estimated capital cost of USD4,489/kW. © 2017 Elsevier Ltd. All rights reserved.

Keywords: OTEC Geothermal waste heat Rankine cycle GeOTEC

1. Introduction Closed cycle systems can be divided into organic Rankine cycle (ORC) and absorption power cycle [1]. A research work by Wei et al. showed that ORC operates best in higher grade heat source of heating temperatures between 280
C and 300
C [2]. For a lower grade heat source, an ammonia-water absorption cycle exhibits a higher thermal efficiency compared to that of ORC [3]. However, the most efficient solution to improve the thermal efficiency of Rankine power generation cycle is by increasing the temperature of the heat source. Therefore, any efforts to increase the temperature difference between the hot source and cold sink would lead to a better plant performance and a higher cycle efficiency. OTEC technology generates electricity by harnessing natural thermal energy found within the ocean [4]. OTEC power cycle uses the temperature difference between warmer surface seawater and cold deep seawater to produce electricity. Surface sea water between 100 and 200 meters depth have a temperature range between 30 to 25
C: Cooler deep sea water found beyond 600 m depth ranges between 3 to 5
C, which yield a temperature difference of 10 to 25
C with higher differences occur in tropical and equatorial waters [5e10]. J. D'Arsenoval, a French scientist, developed OTEC technology in

* Corresponding author. E-mail address: [email protected] (W.J. Yahya). http://dx.doi.org/10.1016/j.renene.2017.03.086 0960-1481/© 2017 Elsevier Ltd. All rights reserved.

1881 [11]. In 1930, his student, G. Claude was the first to build a simple experimental OTEC power plant of 22 kW in Cuba [12]. From this significant effort, scientists began to show interests in OTEC, and some designs of OTEC power plants were developed and run experimentally. In the 1900s, a small plant of 100 kW net power output was operating successfully for six years in Hawaii. In 2002, a floating 1 MW OTEC plant known as Sagar Shakthi was built by the National Institute of Ocean Technology in corporation with Saga University Japan in India [11,13]. Lockheed Martin's has been working on 10 MW OTEC pilot plant in Hawaii, and the system would be a prospect for sizing up into a commercial plant in the future [13]. This evidence shows the world interest in evaluating OTEC implementation as a sustainable energy source, and extensive studies and effort are focused on OTEC development. Several studies on OTEC components which was done by Uehara et al. showed R717 or ammonia is the suitable working fluid for a closed-cycle OTEC plant [5,14,15]. Nevertheless, the small temperature difference between warm surface sea water and cold deep sea water resulted in a low performance of a closed Rankine cycle OTEC plant. A reversible efficiency for a perfect cycle of an OTEC system amounts only about 8% which implies that almost 92% of the ocean thermal energy source is rejected to the cold deep sea water during the power generation process. In 1985, A. Kalina [11] proposed an absorption cycle for OTEC power plant, by using a mixture of ammonia and water as its working fluid. This process required a separator and a regenerator.

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This so-called Kalina cycle was applied to a wide range of lowtemperature OTEC systems, and the thermal efficiency theoretically increased. However, the performance of the evaporator and condenser fell due to the use of the binary fluid [16]. About a decade later, a new OTEC cycle was developed by H. Uehara which provided an improvement to that of Kalina cycle with an even higher theoretical efficiency [11]. Uehara cycle used a liquid-vapour separator to separate dry vapour and liquid in the wet vapour mixture, ensuring exclusive entry of dry vapour into the turbine and consequently reduces the load of the condenser [17]. Another alternative and innovative way to improve the cycle efficiency has been proposed by Yamada et al. and Kim et al., by using an external heat source to increase the temperature difference. Yamada et al. proposed an addition of solar collector into the OTEC cycle [18]; a system called SOTEC, whereas Kim et al. used the waste heat from nuclear power plant condenser effluent to replace the seawater heat in OTEC system [19]. Kim et al. [19] studies showed an improvement in OTEC system efficiency by 2% by using condenser effluent. They also suggested that superheated vapour produced by an evaporator should prevent cavity, but cause little impact on system efficiency. Yamada et al. [18] concluded that the addition of external heat source by solar collector enhanced OTEC thermal efficiency by 2.7 times higher compared to that of conventional OTEC operation. In a study by Saitoh and Yamada [20] multiple Rankine cycle systems have been proposed to improve the cycle efficiency. The cycle used both solar thermal and ocean thermal energies. Straatman and Van Sark [21] described a conceptual design of a combined OTEC system and an offshore solar pond known as OTEC-OSP hybrid. In the other hand, Soto and Vergara [22] proposed a hybrid OTEC cycle where the flow that enters the evaporator is pumped from a thermal power plant discharge. Their effort contributes to extending OTEC potential to colder water areas. Furthermore, the simulation demonstrated the combination of OTEC plant to Punta Alcalde coal-fired power station, where the power plant efficiency improved by 1.3%. Recent work by Aydin et al. [23] simulates the addition of solar collector as a pre-heater or superheater into OTEC system. Both the preheating and superheating enhance OTEC power production by 20e25%, with superheating requires less collector area compared to the pre-heating system. The addition of solar superheater improved the efficiency of the existing system by 60%, signifying the superheating method is a better approach in OTEC [23]. All these efforts to increase the temperature of the heat source improves OTEC overall efficiency. Several of these studies also aim to achieve a low electricity cost, but the results emphasised more on the efficiency improvements rather than a quantitative cost analysis. In this study, we proposed a new concept of combining OTEC and offshore geothermal waste energy to increase the temperature difference between the hot and cold heat sources. However, the potential waste energy is estimated and limited to 27.49 MW. The temperature of the heat source is manipulated to determine the maximum potential output and system efficiency. Capital cost estimation is performed to evaluate the feasibility of the proposed system. 2. GeOTEC power system description The proposed GeOTEC power cycle utilises both energy sources from warm seawater and raw natural gas from offshore gas production platform. Warm seawater is pumped into the evaporator to evaporate ammonia as the working fluid into a saturated vapour. The geothermal waste energy is used in GeOTEC cycle to superheat the saturated ammonia vapour before it enters the turbine to generate electricity. The turbine exhaust gases enter the condenser

373

to be cooled by cold deep seawater and condensed back into liquid ammonia. The schematic of GeOTEC power cycle which includes geothermal water heating system (GWHS), heat exchanger, geothermal superheater, evaporator, turbine, condenser, and pumps is shown in Fig. 1. The T-s diagram in Fig. 2 shows all the state points in the GeOTEC closed cycle, where Tsh is the superheating temperature, Te is the evaporating temperature, Tc is the condensing temperature, Pe is the evaporating pressure, and Pc is the condensing pressure. 2.1. Geothermal base data of hot raw natural gas Table 1 lists the geothermal data used in the sizing exercise for the proposed GeOTEC power plant. As shown in Fig. 3, the potential of geothermal waste energy generation was estimated from the operating Malaysia Thailand Joint Authority (MTJA) offshore gas production platform. However, the actual potential of geothermal waste energy is more anticipated in deep sea oil and gas (O&G) platform operations. Data of raw natural gas obtained from the MTJA platform and the calculated potential waste heat are shown in Table 1, whereas the specific heat capacity of raw natural gas was calculated based on chemical composition obtained from the Terengganu's Bergading platform and shown in Table 2 using the following equation:




CPnat:gas ¼ CPCH  nCH4 þ CPC H  nC2 H6 þ CPC H  nC3 H8 4 2 6 3 8       þ CPCO2  nCO2 þ CPH2S  nH2 S þ CPH2O  nH2 O where CP signify the specific heat capacity for crude natural gas and its compositions, and n is the percentage of chemical composition, each specified by the subscript. The potential geothermal waste energy from the MTJA platform, Qnat:gas is estimated using the equation below:

_ nat:gas  CPnat:gas  DT  hHE  hSH Qnat:gas ¼ m _ nat:gas is the mass flow rate of raw natural gas given in where m Table 1, DT is the temperature difference of raw natural gas, hHE is GWHS heat exchanger efficiency and, hSH is geothermal super heater efficiency. The temperature difference of raw natural gas is given as:

DT ¼ Tinðnat:gasÞ  Toutðnat:gasÞ where Tinðnat:gasÞ is the temperature of raw natural gas entering the cooler and, Toutðnat:gasÞ is the temperature of raw natural gas leaving the cooler. 3. Thermodynamic design of GeOTEC In the GeOTEC analysis, ammonia has been established to be the best working fluid, justified by the reductions in heat exchanger size and piping cost [25,26]. The T-s diagram in Fig. 3 shows that constant pressure is assumed during evaporation and superheating, PE ¼ P2 ¼ P3 ¼ P4 : This is also assumed during heat extraction in the condenser, PC ¼ P5 ¼ P1 . 3.1. Geothermal Water Heating System (GWHS) GWHS is proposed to be used to capture the geothermal waste energy of the hot raw natural gas from a gas production platform into the GeOTEC power cycle. The waste energy is transferred from geothermal through GWHS which uses water as the heat carrier.

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Fig. 1. Schematic diagram of proposed GeOTEC power cycle.

The main components in GWHS are the pump and heat exchanger. From energy and mass balances GWHS pump work, WPGWHS is given as follows:

WPGWHS ¼

_ H2O  vH2O  DPGWHS m

hPGWHS

_ H2O is the mass flow rate of water, vH2O is the specific where m volume of water, DPGWHS is the total pressure difference of GWHS piping, and hPGWHS is the GWHS pump efficiency. The heat transferred in a GWHS heat exchanger is given as follows:

_ H2O  CPH2O  ðDTÞH20 QGWHS ¼ m

where CPH2O is the specific heat capacity of water, and ðDTÞH20 is the temperature change of water. The temperature difference of water is given by:

ðDTÞH20 ¼ ToutðH2OÞ  TinðH2OÞ where ToutðH2OÞ is the temperature of water leaving the heat exchanger and, TinðH2OÞ is the temperature of water entering the heat exchanger. 3.2. Warm and cold seawater pumping power A big portion of energy consumption for all pumps in GEOTEC is contributed by the water pumps, cold and warm. The following

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_ wf ðh3  h2 Þ QE ¼ m _ wf ðh4  h3 Þ QSH ¼ m where h2 and h3 represent enthalpy at each particular state point. While the rate of heat rejected at the condenser ðQC Þ is given by:

_ wf ðh5  h1 Þ QC ¼ m Where h1 is the enthalpy at state point 1. The power required by the fluid pump in the system is given as follows:

_ wf ðh2  h1 Þ WPwf ¼ m So, the total nett power produced by GeOTEC system can be written as:


Wnett ¼ WT  WPGWHS þ WPwf þ WPwsw þ WPcsw The cycle efficiency is defined as the total nett power produced by the system divided by the total rate of heat input to the system, as given by the following equation:

Fig. 2. T-s diagram of GeOTEC cycle.

Table 1 Data of raw natural gas from offshore gas production platform. Data

Symbol

Value

Units

Temperature of raw natural gas at inlet

Tinðnat:gasÞ

110




Temperature of raw natural gas at outlet Volume flow rate of raw natural gas Specific heat capacity of raw natural gas

Toutðnat:gasÞ vnat:gas CPnat:gas

40 1000 1.819




C mmscfd

Potential waste energy from MTJA platform

Qnat:gas

27.49

MW

kJ $K kg

_ wsw  DHwsw  g m

hPwsw

_ wsw is the mass flow rate of warm seawater, DHwsw is total where m head difference in warm sea water piping, g is the gravitational acceleration, and hPwsw is the warm seawater pump efficiency. While for cold sea water pump, the required power is estimated by:

WPcsw ¼

Wnett QE þ QSH

C

equations represent power (work performed per second) required by both pumps. For warm sea water pump, the required power is estimated by:

WPwsw ¼

hGeOTEC ¼

_ csw  DHcsw  g m

hPcsw

_ csw is the mass flow rate of cold seawater, DHcsw is total where m head difference in cold seawater piping, and hPcsw is the cold seawater pump efficiency.

3.3. Energy and power of ammonia Rankine cycle From energy and mass balance, turbine power, WT is given by:

_ wf ðh4  h5 ÞhT hG WT ¼ m _ wf is the mass flow rate of the working fluid, h4 is the Where m enthalpy at state point 4, h5 is the enthalpy at state point 5, hT is the turbine efficiency, and hG is the generator efficiency. Rate of heat transfers in the evaporator ðQE Þ, and superheater ðQSH Þ are obtained as follows:

3.4. Heat exchanger total heat transfer areas e evaporator, superheater, and condenser Heat exchangers are the most critical yet expensive components in OTEC power plant construction. In the early stage of OTEC development, titanium shell and tube heat exchangers were recommended and selected for OTEC power plants. However, it was soon found that the cost of shell and tube heat exchangers were estimated about 45.7% of the total construction cost of an OTEC power plant [27]. To reduce costs, plate heat exchangers are selected instead of shell and tube. The total heat transfer area of the evaporator is calculated as follows:

AE ¼

QE UE ðLMTDE Þ

where UE is the overall heat transfer coefficient of evaporator, and LMTDE is the log mean temperature difference of the evaporator. The area of the superheater is estimated by:

ASH ¼

QSH USH ðLMTDSH Þ

where USH is the overall heat transfer coefficient of superheater, and LMTDSH is the log mean temperature difference of super heater. The area of the condenser is estimated by:

AC ¼

QC UC ðLMTDC Þ

where UC is the overall heat transfer coefficient of super heater, and LMTDC is the log mean temperature difference of the super heater. 4. Calculation and result Table 3 summarises the assumptions and the setup conditions

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Fig. 3. Schematic diagram of MTJA gas production process with courtesy of MTJA.

Table 2 Raw natural gas composition provided by Bergading platform of Terengganu, Malaysia [24]. Chemical name

Chemical formula

Percentage, n (%)

Specific heat capacity, Cp (kJ/kg.K)

Methane Ethane Propane Carbon dioxide Hydrogen sulphide Water

CH4 C2H6 C3H8 CO2 H2S H2O

50 10 5 30 1 4

2.254 1.766 1.679 0.846 1.017 4.186

for calculation purpose. The calculation was performed with different superheated temperature values and different saturated mixture ratios referred as state point 3 (x3). The pressure in the evaporator, superheater, and condenser, together with the enthalpy values at each state point were generated in the simulation by Refprop software. The power plant efficiency was shown in Fig. 4 for different state point 3 and different superheated ammonia temperature. The data in Fig. 4 indicates that the highest efficiency obtained by different superheated temperature. It is obvious that the higher superheated temperature was set, the higher efficiency can be achieved, rendered by the higher temperature difference between heat source and heat sink [28]. However, the efficiency decreases with higher vapour ratio or higher value of x3. At the highest superheated temperature of 80
C, the cycle efficiency for x3 ¼ 1 is 4.53% while at x3 ¼ 0.5 is 4.61%. The difference between both values is 0.08 yielding only 1.74% efficiency difference between highest and lowest value of x3.

The net power output of GeOTEC is illustrated in Fig. 5 for various superheated temperatures. At higher superheated temperatures, the net power output appears to drop. For a reduction in the x3 values, the decrease rate of power output becomes less significant. Relatively higher net power output is obtained with x3 ¼ 1 compared to the rest of x3 values. By taking both the efficiency and net power output into account, system capacities given by the parameter x3 ¼ 1 is taken for capital cost analysis. This value is selected because larger power capacity can then be produced. Moreover, the efficiency decreases only slightly compared to other values of x3. Fig. 6 shows that the mass flow rate of ammonia decreases with the increase in superheated temperature. Since the proposed GeOTEC power plant maximises the utilisation of waste heat, any changes made to the superheated temperature affect the ammonia flow rate. This consecutively determines the heat compounded by the warm seawater. A significant difference can be seen between the superheated temperature of 40
C and 50
C. The mass flow rate

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Table 3 Condition for calculation of proposed GeOTEC cycle. Parameter

Symbol

Value

Units

Temperature of water at inlet

TinðH2OÞ

90




Temperature of water at outlet Specific volume of water Pressure difference in GWHS pump Efficiency of GWHS heat exchanger Efficiency of GWHS pump Temperature of warm sea water inlet

ToutðH2OÞ vH20 DPGWHS




TinðwswÞ

30 0.001 200 0.95 0.85 30

C m3/kg kPa e e
C

Temperature of warm sea water outlet Temperature of cold sea water inlet

ToutðwswÞ TinðcswÞ

27.1 5




Temperature of cold sea water outlet Evaporation temperature Condensation temperature Efficiency of turbine Efficiency of generator Efficiency of superheater Efficiency of evaporator Efficiency of condenser Efficiency of warm sea water pump

ToutðcswÞ TE TC

7.8 25.9 9.1 0.86 0.975 0.95 0.95 0.95 0.85




Efficiency of cold sea water pump Head differential of warm sea water pump Head differential of cold sea water pump Gravitational acceleration Superheated temperature of working fluid State point 3 (vapour quality) Overall heat transfer coefficient of superheater Overall heat transfer coefficient of evaporator Overall heat transfer coefficient of condenser

hHX hPGWHS

hT hG hSH hE hC hPðwswÞ hPðcswÞ DHwsw DHcsw g TSH x3 USH UE UC




C

C C

C C
C e e e e e e


0.85

e

2.62 4.49 9.81 40, 50, 60, 70, and 80 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 4000 4000 3500

m m m/s2
C e kW/m2.K kW/m2.K kW/m2.K

Fig. 6. Mass flow rate of ammonia for x3 ¼ 1.0. Fig. 4. GeOTEC efficiency at different superheated ammonia temperatures.

of ammonia can also be observed to decrease with the power plant capacities proportionally. The amount of heat transferred to the evaporator and superheater by warm seawater and waste heat and the heat rejected by condenser are all shown and compared in Fig. 7 for different system sizes. Apparently, at the highest capacity, a significant amount of heat input from warm seawater is required. The heat input from the raw natural gas is meant to increase heat source temperature and superheat the ammonia. From the results, it is evident that to gain higher GeOTEC capacity, one must involve the additional heat from warm seawater because the waste heat is limited. The power consumption of all four pumps in a GeOTEC plant is plotted in Fig. 8. The power requirements vary from 11.657 MW to 32.593 MW. The highest to lowest energy are respectively consumed by the cold seawater pump, warm seawater pump, ammonia pump, and GWHS pump. 5. Estimation of Total capital investment (TCI)

Fig. 5. Net power output obtained at different parameters.

Total capital investment (TCI) estimate of GeOTEC power plant

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Fig. 7. Heat transfer rate in the evaporator, superheater, and condenser.

Fig. 9. TCI breakdown [29].

Fig. 8. Pumps work of GeOTEC plant.

was calculated for capacities discussed in earlier calculations. The capital cost to purchase the principal components and equipment is referred as a fixed-capital investment (FCI). The total capital cost or total capital investment (TCI) is the sum of fixed capital investment, and other outlays as given by the following equation:

TCI ¼ FCI þ Other outlays The components to estimate TCI is listed in Fig. 9. FCI involves two main components, namely direct and indirect cost while other outlays are startup cost, working capital, licencing, R&D, and allowance for used during construction (AFUDC). Table 4 summarises the TCI estimation for five different GeOTEC capacities. All the main cost components of GeOTEC power plant were estimated as purchased equipment costs (PEC). All the cost estimates are based on commercially available components and manufacturing practices which have been obtained from suppliers' advice. Referring to suppliers advice, the available surface area of a titanium plate is 400 m2, which are much smaller than the required size. The cost of heat exchangers in this study was estimated by linearly scaling up the existing size and price to meet the required specification. Nickel-aluminium-bronze pumps were quoted by the supplier for OTEC application with the price of USD 214,000 for 6000 m3/h capacity. The same method applied to estimates for all pumps. Turbine cost for different capacities was obtained and estimated from www.gas-turbines.com/trader. Mooring cost estimation was obtained by totalling up the sub-components including steel anchors and chains, buoys, marine rope, and hydraulic winch. The vessel size and mooring were selected to fit each system within the capacity range. Each GeOTEC systems required 1120 m HDPE pipe for both warm and cold sea water flow with estimated price of

350 USD/m. For working fluid, a total of 6728 kg stainless steel pipe with the cost of 3.9 USD/kg is required by each GeOTEC systems. All installation costs are included in the prices. . The remaining direct and indirect costs were estimated based on the average PEC percentage for a typical thermal design project. The start-up cost for GeOTEC power plant was calculated as the sum of one month fixed O&M cost, plus 2% of FCI. Vega, [30] estimates a total of 17 labour positions are required to operate a floating OTEC plant at 24/7 shift. By using USA labour rates, USD 3.4 million was determined as the first year O&M cost for an OTEC plant of 10e100 MW [30]. Assuming a similar condition in a GeOTEC plant, one month fixed O&M cost is estimated as USD 0.618 million expressed in 2016 USD. Thus, the startup costs for GeOTEC can be expressed as follows:


Startup costs ¼ $ 0:618  106 þ ð0:02  total FCIÞ Working capital in other outlays is associated with the fund reserved for operation expenses before any electricity can be sold. According to the reference [31], working capital is estimated as the sum of two months of fuel and variable operating cost, 3 months of labour cost, and 25% contingencies of a total of the mention items. In a renewable energy plant, fuel costs nothing and in a GeOTEC power plant the variable operating costs are assumed to be negligible. Other than fuel, variable operating costs include catalysts, chemicals and waste materials which significantly are not applicable in GeOTEC green technology. Therefore, for GeOTEC TCI, the working capital is expressed as the sum of three months of operation cost, and 25% of contingencies of the operation cost, whereas labour cost is referred to as operation cost or fixed O&M cost.

i h
Working capital ¼ 3  $ 0:618  106 i h
þ 0:25  $ 0:618  106 ¼ $2; 008; 500

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Table 4 Summary of GeOTEC total capital investment estimation. Total Capital Investment/Power Plant Capacity

32.593 MW

20.598 MW

15.754 MW

13.196 MW

11.657 MW

1. Fixed capital investment A. Direct cost i) Onsite costs  Purchased equipment cost (PEC) - Heat exchangers - Pumps - Turbine-generator - Working fluid, tanks and pipes - Vessel and mooring  Purchased equipment installation (included in PEC)  Piping (included in PEC)  Instrumentation and control (20% of PEC)  Electrical equipment and materials (11% of PEC) ii) Offsite costs  Land  Civil, structural and architectural  Service facilities (35% of PEC)

20,912,000 19,181,220 12,600,000 408,050 5,402,200 0 0 11,700,690 6,435,380

13,076,000 11,410,200 11,000,000 408,050 5,402,200 0 0 8,259,290 4,542,610

9,460,000 8,402,200 10,000,000 408,050 5,402,200 0 0 6,734,490 3,703,970

7,352,000 6,657,000 8,000,000 408,050 5,402,200 0 0 5,563,850 3,060,118

6,246,000 5,584,000 4,800,000 408,050 5,402,200 0 0 4,488,050 2,468,428

0 0 20,476,208

0 0 14,453,758

0 0 11,785,358

0 0 9,736,738

0 0 7,854,088

7,769,258 14,567,359 3,350,492

5,484,169 14,567,359 2,365,048

4,471,701 8,384,440 1,928,421

3,694,397 6,926,993 1,593,209

2,980,065 5,587,622 1,285,153

3,074,057 2,008,500 18,420,426

2,351,683 2,008,500 13,002,621

2,031,617 2,008,500 10,602,125

1,785,891 2,008,500 8,759,183

1,560,073 2,008,500 7,065,548

Total (USD)

146,305,820

104,046,945

85,323,072

70,948,129

57,737,777

TCI estimation per unit kW (USD/kW)

4489

5051

5416

5376

4953

B. Indirect cost i) Engineering and supervision (8% of DC) ii) Construction cost and contractor's profit (15% of DC) iii) Contingencies (15% of the above sum) 2. Other outlays a. Start-up cost b. Working capital c. Cost of licensing and R&D, and AFUDC (15% of FCI)

Table 5 GeOTEC power plant TCI estimates. Superheated temperature, Tsh (
C)

Nett power, Wnett (MW)

Total power plant cost (USD)

Cost per kW (USD/kW)

40 50 60 70 80

32.593 20.598 15.754 13.196 11.657

146,305,820 104,046,945 85,323,072 70,948,129 57,737,777

4489 5051 5416 5376 4953

The cost of licensing, R&D, and AFUDC was estimated for GeOTEC using assumption by Bejan et al. [29] which expressed the sum of both costs equivalents to 15% of the FCI:

R&D þ AFUDC ¼ 0:15 FCI Table 5 summarises the total capital costs of GeOTEC with five different capacities in USD/kW expressed in 2016 USD. All the net output values were the result of a variation in the superheated ammonia temperatures. The trend of the cost estimation can be seen in Fig. 10 which illustrates that bigger power plant capacity relates to higher total estimated capital. It also shows that the increase in the capital costs flattens with the increase of capacity. The cost per kW, however, shows a different trend (Fig. 11). Starting at 11.657 MW GeOTEC plant, the cost is USD4,953/kW; it escalates up to USD5,416/kW for 15.754 MW plant. At 20.598 MW, its cost per kW is USD5,051 which is lower than that of lowest capacity; 13.196 MW. The most economical plant is that with a power output of 32.593 MW with $4489/kW. This cost is apparently cheaper than the cost of OTEC by Vega [30] which respectively costs18,600 $/kW for 10 MW and 12,000 $/kW for 35 MW. However, economic analysis is recommended to evaluate GeOTEC feasibility more accurately by indicating the unit costs of electricity produced by a GeOTEC plant.

6. Conclusion From the thermodynamic analysis of the proposed GeOTEC power plant, the maximum vapour to water ratio which indicates x3 ¼ 1 is concluded as the best among other values since it provides significantly higher net capacity. The maximum GeOTEC net power output that can be produced with a geothermal waste energy input of 27.49 MW is 32.593 MW estimated from the data of a shallow water gas production platform. A higher output with higher

Fig. 10. TCI estimation of GeOTEC power plant as function of plant size.

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N.H. Mohd Idrus et al. / Renewable Energy 111 (2017) 372e380

Fig. 11. TCI estimation expressed in unit $/kW.

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