Design and thermoeconomic analysis of a multi-effect desalination unit equipped with a cryogenic refrigeration system

Energy Conversion and Management 202 (2019) 112208

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Design and thermoeconomic analysis of a multi-effect desalination unit equipped with a cryogenic refrigeration system

T

⁎

Bahram Ghorbania, Reza Shirmohammadib, Majid Amidpourc,d, , Fabio Inzolid, Matteo Roccod a

Faculty of Engineering Modern Technologies, Amol University of Special Modern Technologies, Amol, Iran Department of Renewable Energy and Environment, Faculty of New Sciences & Technologies, University of Tehran, Tehran, Iran c Mechanical Engineering Faculty, Energy System Group, KNToosi University of Technology, Tehran, Iran d Energy Department, Polytechnic University of Milan, Milan, Italy b

A R T I C LE I N FO

A B S T R A C T

Keywords: Liquefied natural gas Absorption/mixed-refrigerant Concentrated solar power Desalination Exergy analysis Economic evaluation

In this paper, a hybrid tri-generation system is designed to produce LNG and freshwater simultaneously based on a multi-effect desalination system and analyzed using energy and exergy analyses. For natural gas liquefaction, the initially required cooling of the cycle is supplied by ammonia-water absorption system, and the required intermediate cooling load and liquefaction process are provided by a refrigeration system with mixed fluid. Two alternatives for providing power generation to the system and the required heat for driving the absorption cycle have been considered and compared. The first alternative relies on a natural-gas-fired power plant, while the second is based on steam-solar power plant with dish collectors. The former results in the total energy efficiency of 85.8% (LHV). The highest thermodynamic irreversibilities occur in the heat exchangers (61%), even if they have the highest exergy efficiency compared to the other plant equipment. On the other hand, using solar dish collectors enables to decrease carbon dioxide emissions by 40%, and increases freshwater and LNG production by 95% and 4.7% respectively. The power obtained per kg of LNG produced is about 0.19 kWh, which is less than or equal to the other similar patents. The results of the economic analysis with the annualized cost method of the system show that the products prime cost of both structures 1 and 2 are equal to 0.2580 and 0.1784 US$ per kg of LNG, respectively. A suitable strategy for modelling enhancement of system’s parameters is suggested by expanding of sensitivity analysis on the important parameters of the developed system.

1. Introduction Due to its high availability and low financial cost, natural gas (NG) is likely to pave the way for transport sector with substantial demand in the following decades [1]. In particular, the strategic relevance of liquefied natural gas (LNG) is nowadays widely recognized since it can be easily transported for long distances without relying on a static distribution infrastructure [2]. LNG For transportation is identified as a practical solution where the distance between reservoirs of NG to downstream markets is prolonged [3]. Surging in energy cost and demand lead to growing efficient energy consuming methods for floating LNG production application [4]. On the other hand, potable water and cleaner multi-generation systems are two vital segments for sustaining human life on earth [5]. Water desalination is a dependable technology for making of drinkable water, even if its usage is prevented due to the high economic costs, and high energy consumption [6]. In other words, LNG will be very important and strategic in the future, and energy-

⁎

saving and multi-generation systems will be critical as well. Several hybrid energy systems have been developed for natural gas liquefaction [7]. Mixed refrigerant refrigeration systems as low-temperature energyefficient applications have been used for natural gas liquefaction [8]. Exergy-based analyses are tools that aid the assessment of liquefied natural gas processes and disclose paths to improve them [9].Various energy sources can be connected in a hybrid energy system [10]. Development of integrated structure including solar dish collectors as well as Multi-Effect Desalination (MED) units has been increased because of their environmental-friendly nature [11]. In the following, some of the developed energy systems are reviewed. Shakouri et al. [12] developed an optimal model for the MED system integrated with gas turbine. Grass root design of desalting plant and feasibility of integration were investigated by retrofit viewpoint for Lavan Island. Mordai et al. [13] developed a hybrid power generation system equipped with a MED for simultaneous power generation and thermal waste recovery. Ghorbani et al. [14] developed a commercial

Corresponding author. E-mail address: [email protected] (M. Amidpour).

https://doi.org/10.1016/j.enconman.2019.112208 Received 30 August 2019; Received in revised form 16 October 2019; Accepted 17 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

COP MED Qevap Qgen Ppump Qs Is Aa Qr Qu Ql Qlk Qlc Qlr Nul Tw Ta Ac Aw Grl g L hc

K Kt Gon Gsc c H0 Hd Is Id It ΔT N Ts TN F B D x ms hs Llog D CP Q1 T' NEA PCF TCF Ra Eẋ ṁ T0

si ex h0 s0 ṅi rj oj

Coefficient of performance (–) Multi-effect desalination Evaporator heating load (kW) Generator heating load (kW) The power consumption of pump (kW) Power reached on the surface of the dish (W) Beam solar radiation reached to concentrator surface (W/ m2) The surface area of the concentrator (m2) Power reached to the receiver (W) Useful thermal power reached to the receiver (W) Power loss in the receiver (W) Power loss from the receiver through conduction (W) Power loss from the receiver through convection (W) Power loss from the receiver through radiation (W) Nusselt number (-) Receiver temperature (°C) Ambient Temperature (°C) The surface area of the receiver aperture (m2) Cavity internal area of the receiver (m2) Grashof number Gravitational acceleration (9.806 m/s2) Diameter (m) Convection heat transfer coefficient between the absorber and air and ambient air, W/m2K Thermal conductivity of the ambient air, W/m K Clearness index of a day Extraterrestrial radiation incident Solar constant (1367 W/m2) Geometrical concentration ratio (-) Extraterrestrial solar radiation Daily diffuse solar radiation Hourly global solar radiation Hourly diffuse solar radiation Hourly global solar radiation Temperature difference Number of the effects The temperature of heating steam (first effect) The temperature of the heating steam (final effect) Seawater mass flow rate (kg/s) Brine mass flow rate (kg/s) Desalinated water mass flow rate (kg/s) Salinity percent (%) Heating steam mass flow rate Specific enthalpy of the heating steam Local geographical longitude Distillated mass flow rate (kg/s) Specific heat capacity Steam heating is the first stage of desalination The temperature of the desalinated water from the previous effect Non-equilibrium allowance Pressure correction factor Temperature correction factor Entrainment ratio of the steam ejector Exergy rate (kW) Mass flow rate (kg/s) The temperature of the dead state (K)

R M C H O N S A in out Q̇ Ẇ h ηex

specific entropy (kJ/kg.K) Specific exergy (kJ/kg) Specific enthalpy at reference state (kJ/kg) Specific enthalpy at reference state (kJ/kg.K) Mole rate of the ith stream (mole/kg) Mole fraction of the j th component in the stream Standard chemical exergy of the component in the stream (kJ/mole) Universal gas constant (8.314 J/mol. K) Molar mass (kg/mole) Carbon composition wt% Hydrogen composition wt% Oxygen composition wt% Nitrogen composition wt% Sulphur composition wt% Ash composition wt% Input Output Rate of heat transfer (kW) Rate of shaft work (kW) Specific enthalpy Exergy Efficiency

Greek Letters

ρ λ τ α γ η φ1 β v δ ∅ ωs εeff εc σ ω

Reflectance Land factor of un-shading Transmittance Absorptance Intercept factor Efficiency (%) Tilt angle of the cavity (Radian) Coefficient of thermal expansion (1/°C) Kinematic viscosity (m2/s) Declination Latitude Sunset hour angle Effective infrared emittance of the cavity Cavity surface emittance Stefan–Boltzmann constant (5.67 × 10−8 W/m2 K4) Hour angle

Subscripts and superscripts

eff ev hs des ex phys chem v f b i ms o r c

2

Effective Entrained vapor Heating steam Destruction Exergy Physical Chemical Vapor Seawater Brine Desalinated water Motive steam Optical Receiver Collector

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thermal energy, a Kalina cycle and a desalination unit. Piadehrouhi et al. [32] introduced a novel poly-generation system including, ASU, oxyfuel power plant and CO2 capture with solar dish. In the developed hybrid system, absorption chiller, steam power cycle and ORC power generation were driven by the solar dish collectors. Shirmohammadi et al. [33] investigated an actual CO2 capture, integrated to the natural gas fire power plant, and utilized an ammonia absorption system driven from waste heat of stack. They applied technoeconomic evaluation to the aforementioned system. Mehrpooya et al. [34] developed a hybrid system consists of concentrated solar thermal plant, including parabolic dish collectors and steam turbine, equipped with desalination and ammonia absorption refrigeration systems for providing power, cooling and fresh water at the same time. Mehrpooya et al. [35] proposed biogas upgrading process with the flat plate solar collectors equipped with Kalina power cycle. They employed exergy and sensitivity analyses to evaluate the performance of the system. Niasar et al. [36] developed a hybrid system comprising gas to liquids, steam power plant, absorption refrigeration and desalination. They conducted exergy, economic and sensitivity analyses for the system above. Han et al. [37] developed a novel triple ORC process for waste heat and cold energy recovery of LNG-fueled vessels. They concluded that the developed system could provide the energy recovery requirements of LNG-fueled vessels more efficiently. Emadi et al. [38] developed a hybrid system in which LNG re-gasification process employed as the heat sink of a geothermal driven multi-generation system. They optimized the proposed system considering exergy efficiency, total cost rate, and production capacity of hydrogen, and the system as the objective function. The developed system obtained considerable enhancement in power generation, cooling and production of hydrogen. Ghaebi et al. [39] developed an innovative NH3-H2O combined cooling and power cycle in which waste heat was employed as the low-temperature heat source, and LNG cold energy was used as the heat sink. The system was then comprehensively analyzed using energy, exergy, and economic approaches. Ehyaei et al. [26] developed a solid oxide fuel cell, integrated with gas and steam trigeneration systems. Energy, exergy and economic analyses were then conducted on the system. A multi-objective optimization considering exergy efficiency and the electricity cost as the objective functions were also done. Habibi et al. [40] developed a new integrated system comprising of Partial Evaporation Rankine Cycle (PERC) integrated with Organic Rankine Cycle and LNG power system. The system is analyzed in terms of thermodynamic and economic points of view. They also applied a multi-objective optimization to obtain the best operational points of the system. Results depicted that LNG and PERC systems had an appropriate performance from energy and exergoeconomic approaches, respectively. The goal of the paper is to introduce a hybrid poly-generation system used in LNG and freshwater production system and analyze the configurations using energy and exergy analyses. Designing and comparing two alternative configurations for providing power generation and the required heat for driving of absorption system have been carried out. The first alternative relies on a natural-gas-fired power plant, while the second is based on steam-solar power plant with dish collectors. The required cooling of the cycle for liquefaction is provided by ammonia-water absorption system and mixed-refrigerant refrigeration system. Hot gas from the gas turbine is utilized to provide required heat in generator of ARS and produce steam for MED system. Solar collectors are used to provide heat for the power generation cycle to reduce the amount of natural gas used to produce fuel and reduce carbon dioxide emissions. The annualized cost method of the system is used to investigate economic feasibility of the two integrated structures. This study is an attempt to cover the research gap exiting in the field of hybrid energy systems. In the end, for validation of the integrated structure, each section of the system is compared separately with industrial process as well as the other sources, and its accuracy is validated.

steam power plant with two regenerative boilers in which one of them was replaced with parabolic solar dish collectors. The system saved the produced thermal energy by the phase change material (PCM) in a storage tank. The results show the necessity of the existence of an auxiliary fired‐gas boiler to provide constant load during the whole 24 h. Najafi et al. modelled a Solid Oxide Fuel Cell-Gas Turbine (SOFCGT) coupled with a multi-stage flash desalination unit. Exergetic, economic and environmental analyses were conducted to evaluate the hybrid system [15]. Bailera et al. [16] developed a novel cogeneration system combining solar photovoltaic, chemical storage through power to gas, and an oxy-fuel boiler. They employed a decision-making methodology to manage and size the aforementioned system. Ghaebi et al. [17] developed a system including a humidification-dehumidification desalination system, a Kalina cycle, an absorption refrigeration cycle, and a domestic water heater unit system. They analyzed the system thermoeconomically and optimized by means of evolutionary algorithm. Shakib et al. coupled multi-effect desalination system to a gas turbine plant considering heat recovery steam generator and analyzed the system thermodynamically [18]. Moaleman et al. developed a hybrid system including concentrating photovoltaic-thermal unit and ammonia absorption system. A concentrating PV-thermal system, coupled with a linear Fresnel collector, was used as the heat source. It was concluded that utilizing concentrating collector instead of conventional ones, the system was not able to provide sufficient thermal energy to drive the refrigeration system [19]. Sarabchi et al. [20] developed a novel poly-generation system including a PEM fuel cell and a Kalina cycle integrated with a solar methanol steam reformer. They analyzed the system above from environmental and thermoeconomic point of view. Kaniyal et al. [21] investigated operation and performance of a poly-generation system incorporating solar resource, and environmental and energy performance of process was compared for validation. Mehrpooya et al. [22] developed a Molten Carbonate Fuel Cell (MCFC) hybrid power generation process by solar parabolic dish. They analyzed the hybrid system by means of exergy approach and the overall exergy efficiency of the system was equal to 63.19%. Sensitivity analysis was also carried out for the abovementioned system. Ghorbani et al. [23] proposed a hybrid system including MCFC power plant and MED system. The exhaust heat of the power plant is employed to supply the required heat duty. The MED unit could provide about 118944 kg/h fresh water, and the absorption chiller could provide 32090 kW refrigeration. Zare et al. examined a hybrid system utilizing from outlet gas of gas turbine to generate power by two organic Rankine cycles and pure water [24]. Akbari et al. developed a new hybrid system, utilizing geothermal energy, a Kalina cycle, a lithium bromide-water heat transformer and desalination system for generating of power and pure water [25]. Ehyaei and Rosen [26] proposed a triple cycle including an auxiliary burner, a SOFC, gas and steam systems. Energy, exergy, and economic analyses along with optimization by a multi-objective genetic algorithm was conducted for the triple cycle. Sensitivity analyses were then performed for the critical variables of optimization. Khoshgoftar Manesh et al. [27] optimized coupling of multi-stage flash desalination and PWR power plant using thermo-economic and metaheuristic methods. Mohammadi and Mehrpooya [28] proposed a hybrid CCHP system by integrating geothermal flash, Kalina and RO systems. Montazerinejad et al. [29] developed a novel solar-CCHP system to supply cooling, heating and power. They used both conventional and advanced exergy analyses to evaluate the CCHP system. Meratizaman et al. [30] proposed a hybrid system in which the outlet gases of SOFC-GT power cycle was employed in HRSG to generate steam for desalination system. Ghorbani et al. [31] proposed a hybrid system including LNG production process integrated with CO2 separation and MED units for polygeneration purpose. They presented a cogeneration and freshwater production for a region in southern coast of Iran, i.e. Persian Gulf. The system utilizes from solar flat plate collectors for supplying required 3

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2. Novelty

processes of the LNG production cycle, the power generation cycle and the water and ammonia absorption refrigeration cycle system, the Hysys software is used. The multi-effect desalination system and solar dish collector are simulated by Matlab. TRANSYS software is used to provide meteorological information of Bushehr province. For exergy and economic analyses of the developed system, the simultaneous link among TRANSYS, Hysys and Matlab are employed. Somers and Michael showed that the Peng-Robinson equation of state (EOS) is the best choice for simulating the water and ammonia absorption refrigeration cycle [41]. Mehrpooya et al. [42] also used the Hysys simulator and Peng Robinson EOS to simulate the absorption refrigeration cycle. Poling et al. [43] proposed the Peng-Robinson EOS to predict the properties of non-polar pure materials near saturation conditions. Vidal [44] reported that the Peng-Robinson EOS is good for predicting the properties of pure nitrogen, methane, ethane, and propane. Chen et al. [45] calculated the average error value of the bubble point pressure, the percentage of the vapor composition of the mixture in vapor and liquid equilibrium, as well as the specific fluid volume for different hydrocarbon binary mixtures using the Peng-Robinson EOS. The values for the state above was equal to 4.12%, 2.08% and 3.41%, respectively. Remeljej and Hoadley [46], as well as Gong et al. [47], used the Peng-Robinson EOS for phase equilibrium analysis and prediction of enthalpy and entropy of mixed refrigerant in a natural gas liquefaction process involving a mixture of hydrocarbons (C1 to C5) and nitrogen. By analyzing and categorizing the points above, it can be concluded that what is in the literature to calculate the thermodynamic properties of the mixed refrigerant in this research is the use of the Peng-Robinson EOS for phase equilibrium or Lee-Kessler EOS to predict enthalpy and entropy. Therefore, since the Peng-Robinson EOS is simpler to use than the Lee-Kessler equation computationally, in this paper, in order to calculate the equilibrium vapor and liquid phases of the mixed refrigerant, and to predict enthalpy and entropy, the Peng-Robinson EOS is used.

One of the main problems of power plants in the cold season is a pressure drop. For this purpose, Mazut is used to supply required heat of boiler in power plants. The use of Mazut damages the environment and pollutes the atmosphere. LNG is produced and stored in many developed countries during the hot season, and in the peak of natural gas consumption (cold season), is used instead of Mazut as the fuel in boilers. On the other hand, to reduce power consumption in LNG production, absorption refrigeration cycle is used instead of the pre-cooling compression refrigeration cycle. Solar dish collectors along with power generation cycle are used to provide power and heat required in the LNG production system. Part of the heat of solar dish collectors is also used for producing freshwater. The integrated structure developed in this paper is investigated in Bushehr city with real climate condition during a year. Production of LNG with appropriate cost, high exergy efficiency, and low specific power of the developed integrated system compared to the other structures developed in industry and other articles are the main advantages of the developed structure. In retrospect, two fundamental problems of the water and energy crisis were solved using fossil fuels and groundwater resources. Nowadays, with the decline of underground resources and increase in carbon dioxide levels, humans are forced to use renewable energies. One of the ways to reduce carbon dioxide in the integrated structure is to use solar dish collectors instead of fossil fuels in boilers, which is another advantage of the developed structure. 3. Process description Fig. 1 displays the trigeneration hybrid system of freshwater and LNG using a multi-stage desalination system and a two-stage absorption-mixed refrigerant refrigeration system. Table 1 presents the composition of streams in the hybrid system. The natural gas stream is shown 1 in yellow in Fig. 1; the stream (line 100) at temperature of 26.85 °C and pressure of 65 bar by passing through HX3, HX2, and HX1 multi-stream heat exchangers reaches to −160.1 °C. Next, the stream’s pressure drops in the V1 expansion valve and the stream is delivered into the D1 separator at temperature of −166 °C and pressure of 1 bar, and the liquid LNG product is then departed from the bottom of the separator. The initially required cooling of the cycle is supplied by absorption system (Cycle 300), and the necessary intermediate cooling and liquefaction process is provided by mixed-refrigerant refrigeration system (Cycle 200). To simulate integrated structures, TRANSYS Version 16, Hysys Version 10, and Matlab Version 2013 are used. To simulate the

409

201

406

D2

HX9

V9

400

V9

V7

V10

Ejector

V8

619

HX15

`

610 608

605

612 609 606

HX16

616 613 611

615

622 618

617 621

620

501

HX19 418

417

C4

502

503

625

623 628

626

629 631

504

407

P102

614

HX14

25 ºC, 1 bar, 53.64 kg/s, Sea Water

Desalinated Water

Reactor

401

500

627

C2

213

C1 26.85 ºC, 65 bar, 2.877 kg/s, Natural Gas

603

632

D7

HX10

506

212

607

HX13

602

413

HX12

305

211

405

214

101

202

601

604 507

634

624

508

315 302

636

637

D6

310

308

Sea Water Storage

215

HX3

HX2 207

408

D3

313

P100

208

314

600

HX4

307

102

303

HX6

P101

210

203

635 522

Gas Turbine

209 V3

C3

HX7 305

D5

100

415

216

D4

206

311

The water-ammonia refrigeration system is consists of following devices 1- Generator or desorber: to separate water from ammonia, 2Purifier: to purify ammonia flow from residual droplets of water, 3Condenser: to liquidate purified ammonia stream, 4- Evaporator: to produce cooling load by evaporating ammonia, 5- Absorbent: to absorb ammonia into water, 6- Heat exchangers: to recover thermal energy of streams and increase cycle efficiency 7. Pump and expansion valve: to increase and decrease pressure to facilitate desorption and absorption processes. Several references have been used as design sources to

HX18

HX1

300 200

412

309

T100

103 204

205

306

404

HX8

V2

LNG -165 ºC, 1 bar, 31.70 kg/s, LNG

312

V4

V1

HX5

V5

104

26.85 ºC, 65 bar, 34.37 kg/s, Natural Gas

D1

HX11

410

105

3.1. Cycle 300

HX17

630

505

AIR

Fig. 1. An integrated trigeneration system for freshwater and LNG production using a MED system and a two-stage absorption/mixed-refrigerant refrigeration structure. 4

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Table 1 Specifications of the molar composition of streams in the trigeneration system. Stream

N2

CH4

C2H6

C3H8

i-C4H10

n-C4H10

O2

CO2

H2O

100 (Natural Gas), 102 LNG 200 202, 203 207 410 412 500 (Natural Gas) AIR 504 Stream 300 303 304 307, 308 315 317 600 601 604 605 610 611 623 630

0.0400 0.015 0.070 0.153 0.026 0 0 0.400 0.767 0.769 NaCl 0 0 0 0 0 0 0 0.039 0 0.081 0 0.071 0.019 0

0.875 0.893 0.418 0.641 0.299 0 0 0.875 0 0 H2O 0.739 0.001 0.934 0.0009 0.233 0.638 1 0.961 1 0.919 1 0.929 0.981 1

0.055 0.059 0.229 0.164 0.371 0 0 0.055 0 0 NH3 0.2607 0.999 0.066 0.9991 0.766 0.362 0 0 0 0 0 0 0 0

0.021 0.022 0.213 0.042 0.304 0 0 0.021 0 0

0.005 0.005 0 0 0 0 0 0.005 0 0

0.004 0.004 0 0 0 0 0 0.004 0 0

0 0 0 0 0 0 0 0 0.233 0.153

0 0 0 0 0 0 0 0 0 0.026

0 0 0 0 0 1 1 0 0 0.051

(HX7 heat exchanger) and is completely converted to liquid. In this paper, the cooling loads in the purifier and condenser are provided by the cooling water. The ammonia stream from the condenser (line 305) is cooled in the (HX8) by heat exchanging with the ammonia stream (line 306) from the evaporator (HX3) which is at the temperature of 24.45 °C and then enters into the expansion valve (V5). After the pressure and temperature of the stream are decreased up to the 1.2 bar and 29.55 °C for producing cooling load, it then enters the evaporator HX3. Information related to the simulation of the absorption refrigeration cycle in Hysys software has been adapted to the variations of the conditions outlined in the following reference [48].

simulate the primary water-ammonia cycle. Somers and Michael showed that the Peng-Robinson EOS is the best choice for simulating the water and ammonia absorption refrigeration cycle [41]. Mehrpooya et al. [42] also used the Hysys simulator and Peng Robinson EOS to simulate the absorption refrigeration cycle. Initially, using the data in references, the water and ammonia absorption refrigeration cycle is simulated by the Hysys software, and then the design of the refrigeration system with the required specifications of this study is investigated. According to the existed references [41,42], it is assumed that the pressure drop in the heat exchangers and distillation tower is negligible. To simulate the distillation tower in Aspen-HYSYS software, the following characteristics are considered as inputs: 1- Pressure drop along the tower 2- Number of theoretical trays 3- Feed tray location 4- Two other tower characteristics to determine flow rate, component composition and other characteristics of upstream and downstream flows. The water-ammonia absorption system, with its composition presented in Table 1 is the hottest cycle in the process, as indicated in Fig. 1 with purple colour. A mixture of water-ammonia with a molar percentage of 73.93% water and 26.07% ammonia at 1.2 bar and 31.91° C (line 300) flows into a P100 pump to reach to the pressure of 13 bar. The stream 301 passes through the T100 tower (generator) from the HX4 heat exchanger, utilized for heat recovery and surging in the cycle coefficient, and is preheated by heat exchange with dilute aqueous ammonia from the generator, and the stream (line 302) enters into the T100 tower. A portion of the ammonia in the rich mixture is evaporated by the heat of the generator and separated from the remainder of the dilute mixture of water and ammonia. The striped ammonia from the top of the tower (line 313) enters into the purifier (HX6) to separate the remaining amount of water from it. A condenser located in distillation column (HX6) is acted as a purifier and the column (T100) and its reboiler (HX12) work as a generator. The inlet stream entered into the tower by getting the heat in reboiler (HX12) and passing through the trays of the tower is separated into the top and bottom streams in the tower. A dilute stream (line 304) after exiting from the bottom of the generator, a significant amount of its thermal energy is transferred in the preheater (HX4) and during heat exchanging with input rich mixture entered into the tower, resulting in temperature increase of rich mixture. Having left with a pressure of 13 bar and temperature of 45.49 °C, the ammonia gas (line 303) enters the ammonia condenser

3.2. Cycle 200 Mixed-refrigerant cycle has the final task of intermediate cooling and liquefaction of natural gas. The composition properties of the components of this cycle are presented in Table 1. For better understanding, the mixed-refrigerant cycle is shown in green colour. Information on the simulation of a multi-component refrigerant cycle in Hysys software has been adapted to the changes in the conditions outlined in the following reference [49]. Stream 200 at the temperature of 31.85 °C and pressure of 48.60 bar is cooled in the HX1 up to −26.55 °C. It then flows through stream 201 to the D2 drum flash, and the outlet stream from the top of the drum flash enters the HX2 and its temperature is reduced to the temperature of −128.4 °C. The refrigerant flow is then cooled by passing through the HX4 to the temperature of −155 °C. The refrigerant flow returns to this heat exchanger, but before returning the refrigerant, outlet refrigerant 204 has to pass through the V2 expansion valve. The outlet of this valve is stream 205, which is reached to the pressure of 3 bar and temperature of −163.2 °C. This stream enters the HX4 and provides the required cooling, and the refrigerant flow outlet from the exchanger reaches to 138.8 °C. The outlet liquid from the D2 flash drum enters HX2 (line 207), and its temperature is reduced to −128.4. Then the stream 208 enters the V3 expansion valve, and its pressure is reduced by 3 bar. The outlet streams of 209 and 206 combine in the mixer, and as line 210 enters to the HX2 and provide the required cooling. Stream 211 enters the C1 compressor, and after compressing it up to 20.5 bar, the stream (line 212) at temperature of 92.10 °C enters the HX9 and its 5

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developed structure, MED is coupled with ejector because the energy consumed as well as the cost of construction and maintenance is lower than a compressor. Two flow lines are introduced in the multi-stage evaporative system; one is containing seawater for the steam and the other used in the system. The steam can be provided by several approaches. In the developed structure, outlet hot gas from the gas turbine is employed to produce steam for the MED system [51]. The initial temperature differences are constant between successive stages and by assuming that TS and TN are steam temperatures at 1st and last stages; respectively, following equations are derived:

temperature is declined up to 31.85° C. The cooled stream, i.e. line 213, enters the second cycle compressor (C2 compressor). The outlet of C2, line 214 in Fig. 1, reaches the pressure of 31.56 bar and the refrigerant temperature is increased to 62.09 °C because of the compression. Stream 214 is cooled by passing through the HX10 to temperature of 31.85 °C. This cooled stream is called 215 which then enters into the compressor C3 and after compression up to 63.11 °C bar as line 216 which is at temperature of 63.11 °C, enters into the HX11 and cools up to 31.85C, and thereby the cycle continues. 3.3. Multi-Effect desalination system

ΔT = Thermal desalination system is considered as the first and foremost method of pure water production from seawater, operating under vacuum and condense seawater vapors, in which an ejector is used for creating of vacuum [34,50]. The system is consist of a four-stage evaporating desalination system with parallel feeding and several evaporators and a condenser for condensing the steam produced in the last stage. Fig. 1 displays the schematic of aforementioned system. In the

Ts - TN N

(1)

T1 = Ts - ΔT Ti+1 = Ti − ΔT i= 2. ..N

(2)

Salt and water mass equilibrium in the 1st stage of desalination unit are given by Eqs. (3) and (4), and for the 2nd to Nth stages are given by Eqs. (5) and (6):

Table 2 Operational conditions of streams in the trigeneration system. Stream

Temperature (°C)

Pressure (kPa)

Mass flow (kg/s)

Stream

Temperature (°C)

Pressure (kPa)

Mass flow (kg/s)

100 101 102 103 104 105 LNG 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 400 401 402 403 404 405 406 407

26.85 −26.55 −128.45 −160.15 −166.0 −166.0 −166.0 31.85 −26.55 −26.55 −128.4 −154.9 −163.1 −138.7 −26.55 −128.4 −132.8 −133.6 −29.56 92.10 31.85 62.09 31.85 63.11 31.91 32.05 126.7 45.49 173.8 33.98 −24.45 −29.55 −29.45 −2.61 37.05 37.31 56.20 104.2 45.49 45.49 154.9 173.8 25.00 25.00 25.00 25.00 25.00 25.00 25.00 30.00

6500 6500 6500 6500 100.0 100.0 100.0 4860 4860 4860 4860 4860 300.0 300.0 4860 4860 300.0 300.0 300.0 2050 2050 3156 3156 4860 120.0 1300 1300 1300 1300 1300 1300 120.0 120.0 120.0 1300 120.0 120.0 1300 1300 1300 1300 1300 100.0 143.0 143.0 143.0 143.0 143.0 143.0 143.0

34.37 34.37 34.37 34.37 34.37 2.67 31.70 73.82 73.82 20.27 20.27 20.27 20.27 20.27 53.55 53.55 53.55 73.82 73.82 73.82 73.82 73.82 73.82 73.82 124.2 124.2 124.2 24.84 99.36 24.84 24.84 24.84 24.84 24.84 99.36 99.36 124.2 33.48 33.48 8.63 127.6 28.28 5297 5297 4343 951.7 274.5 233.7 443.4 443.4

408 409 410 411 412 413 414 415 416 417 418 500 501 502 503 504 506 507 508 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 629 630 631 632 633 634 635 636 637

30.00 30.00 25.00 25.00 25.00 30.00 30.00 30.00 30.00 28.73 30.00 26.85 −1.48 15.00 339.56 943.8 489.9 164.2 80.63 77.45 56.01 67.96 56.39 67.96 67.96 62.92 56.02 64.51 64.50 64.50 64.50 59.99 60.96 56.02 60.96 60.96 57.24 58.00 56.02 60.96 58.64 56.58 52.17 25.00 56.00 56.00 56.00 56.00 56.00 56.00

143.0 143.0 143.0 143.0 143.0 143.0 143.0 143.0 143.0 143.0 143.0 6500 1300 1300 1063 1063 101.3 101.3 101.3 18.22 29.93 25.93 15.22 25.93 25.93 22.93 26.12 22.12 22.12 22.12 22.12 19.12 18.73 22.73 18.73 18.73 15.73 16.73 20.73 18.73 16.73 15.22 12.22 101.3 97.32 97.32 97.32 97.32 97.32 97.32

233.7 274.5 2054 862.7 1426 1426 862.7 2054 951.7 19.03 19.03 2.88 2.88 2.53 172.2 174.7 174.7 174.7 174.7 6.22 13.41 13.41 6.22 6.11 7.30 6.11 13.41 13.41 20.71 5.99 14.72 5.99 13.41 13.41 28.13 5.98 5.98 13.41 13.41 22.15 35.56 18.15 18.15 53.64 53.64 53.64 13.41 13.41 13.41 13.41

6

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ΔT =

Ts - TN N

subscript s is steam in the 1st stage. The value of di can be derived as follows:

(3)

T1 = Ts - ΔT Ti+1 = Ti − ΔT i= 2. ..N

di = Di - 1 Cp

(4)

0.55 (Tvi − 1 − T″i ) 33(Ti - 1 - T) i Ti″ = Tvi + (NEA)i (NEA)i = λ vi Tvi

(9)

F = B1 + D1 x f F = xb1 B

(5)

F = Bi - Bi - 1 + Di x f F = xbi Bi - x bi - 1 Bi - 1

(6)

where Tν and T ″ are temperatures of steam and freshwater produced in the preceding stages and the NEA is non-equilibrium allowance. In Eq. (10), the value of Ra is steam ejector entrainment ratio.

PCF = 3 × 10−7 (Pms )2 − 9 × 10−4 (Pms ) + 1.6101 TCF = 2 × 10−8 (Tev )2 − 6 × 10−4 (Tev ) + 1.0047

where F, B, and D, are mass flow rate of the freshwater, wastewater and saline water in kg/s, x is the salinity percent and subscripts f, b, and i are related to saline water, wastewater and stage number; respectively. Heating steam of the first stage of the desalination system provided by a power plant is equal to the heat value of Q [52]. Therefore, the equilibrium energy equations in the first stage and stages 2 to N are as follows:

Q1 = ms (h sout - h sin ) = FCp (T1 − Tf ) + D1 λ v1

Ra =

mms mev

= 0.296

1.19 phs 1.04 pev

pms 0.015

( )( ) PCF TCF

pev

(10)

mev = mhs − mms mhs = mms (1 +

1 ) Ra

(11)

The thermal desalination system and water cooling system are shown in black and red in Fig. 1, respectively. Information on the desalination equations is derived from the following article [31]. Table 2 presents the operational condition of streams, and Table 3 shows the equipment specifications of the system. The power generation cycle by combustion of natural gas (line 502) and compressed air (line 503) in the reactor can produce 94.41 MW power in gas-turbine. 21.91 MW of generated power is consumed during the production of LNG, and 57.65 MW is employed for compression of intake air of the

(7)

Qi = (Di - 1 + di − 1 ) λ vi − 1 = FCp (Ti − Tf ) + Di λ vi + Bi − 1Cp (Ti − 1 − T) i i=2,. .,N (8) where h (enthalpy) in kJ/kg, di is the amount of created vapor in the flash compartment in kg/s, and the subscript d is related to the comprised vapor in each stage condensed in the following stage, and Table 3 Specifications of equipment in the trigeneration system. Pump & P100 P101 P102 Compressor C1 C2 C3 C4 Turbine GT1 Cold Box HX1 HX2 HX3 Shell and Tube exchanger HX4 HX5 HX6 HX7 HX8 HX9 HX10 HX11 HX12 HX13 HX14 HX15 HX16 HX17 HX18 HX19 Ejector Ejector

Adiabatic Eff. 75% 75% 75%

Power(kW) 224.2 0.1756 253.8

Δ P (kPa) 1180 17.78 43.00

Pressure head (m) 138.1 2.183 4.398

P ratio (-) 10.83 2.168 1.430

Capacity (m3/h) 513.1 26.68 19,130

Adiabatic Eff. 80% 80% 80% 80%

Power(kW) 15,138 3408 3158 57,659

Δ P (kPa) 1750 1106 1704 962.6

Operation Mode Centrifugal Centrifugal Centrifugal Centrifugal

Polytropic Eff. 82.92% 80.78% 80.33% 87.37%

P ratio (-) 6.833 1.540 1.540 10.50

Adiabatic Eff. 80%

Power (kW) 94,414

Δ P (kPa) 962.6

P ratio (-) 0.0952

Min. Approach (°C) 4.002 4.011 4.002

LMTD (°C) 15.12 16.73 19.65

Duty(kW) 4843 41,336 28,250

Hot Pinch Temp. (°C) −160.1 −26.55 −26.45

Min. Approach (°C) 10.00 9.610 20.48 10.14 10.01 6.850 6.850 6.850 9.314 6.373 9.456 6.542 6.335 6.567 24.24 15.00

LMTD (°C) 15.86 14.25 40.45 17.23 22.68 22.23 14.41 14.70 47.71 12.83 14.98 13.76 11.96 16.99 41.14 21.56

Duty(kW) 61,355 42,932 18,029 29,809 6924 9268 4883 5738 62,298 15,521 14,959 14,690 116 7254 15,352 101.2

Hot Pinch Temp. (°C) 37.05 31.91 45.48 34.14 −24.44 31.85 31.85 31.85 164.2 56.38 67.96 64.50 57.24 56.57 80.63 30.00

Size of Suction Nozzle (mm) 200

Motive Nozzle Size (mm) 250

Discharge Nozzle Size (mm) 300

Pressure Drop (kPa) 14.78

Number of stages 6

Feed stage 3

Tray/Packed Space (m) 0.5500

Internal Type Sieve

Column T100

7

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HX3

40

-40

20

Hot Composite Curve

Temperature (ºC)

Temperature (ºC)

30

HX2

10 0 -10

Hot Composite Curve

-60 -80 Cold Composite Curve

-100

Cold Composite Curve -20

-120 -30 -140

-40 0

5000

10000

15000

20000

25000

0

30000

10000

20000

Heat Flow (kW)

50000

Overall Composite Curve

40

HX1

10

-135

Temperature (ºC)

-130

Temperature (ºC)

40000

(b)

(a) -125

30000

Heat Flow (kW)

Hot Composite Curve

-140 -145 -150

Hot Composite Curve

-20 -50 -80

-110

-155 Cold Composite Curve

Cold Composite Curve

-140

-160 -165

-170 0

1000

2000

3000

4000

5000

0

20000

40000

Heat Flow (kW)

Heat Flow (kW)

(c)

(d)

60000

80000

Fig. 2. Composite curves of heat exchangers and developed integrated structure. Table 4 Fuel, product, and destroyed exergy of each equipment in the trigeneration system. Equipment Tag. HX1 HX2 HX3 HX4 HX5 HX6 HX7 HX8 HX9 HX10 HX11 HX12 HX13 HX14 HX15 HX16 HX17 HX18 HX19 C1 C2 C3 C4

̇ F (kW) EX 3,377,862 8,603,709 5,643,962 782,797 2,728,996 1,520,682 1,960,536 1,004,687 3,903,774 3,691,621 3,735,600 387,479 5404 5636 5439 5297 14,898 8816 159,716 3,454,116 3,453,978 3,456,195 58,650

̇ P (kW) EX 3,377,329 8,601,288 5,640,100 780,102 1,266,149 1,518,378 1,959,882 1,003,765 3,902,843 3,691,330 3,735,253 376,817 5702 5502 5305 5153 14,460 7206 159,707 3,451,577 3,453,368 3,455,632 53,659

Equipment Tag.

̇ D (kW) EX 533.3 2421 3861 2694 1,462,846 2303 654.0 921.9 931.0 291.1 346.6 10,662 151.2 133.7 133.9 143.8 438.2 1609 8.100 2538 610.1 563.2 4991

Turbine Pump100 Pump101 D1 D2 D3 D4 D5 D6 D7 Reactor V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 Ejector T100

8

̇ F (kW) EX 139,686 632,032 1108 1,695,963 3,457,185 638,561 4582 5966 7381 7370 193,956 1,697,213 843,756 2,627,674 138,159 505,796 140,874 2638 2638 2638 2638 3267 1,706,300

̇ P (kW) EX 132,786 632,000 1108 1,695,963 3,457,185 638,536 4582 5966 7381 7370 139,686 1,695,963 843,225 2,626,749 138,022 505,729 140,304 2637 2637 2637 2637 2766 837,900

̇ D (kW) EX 6899 31.96 0.040 0.0003 0.0005 25.67 0.0003 0.003 0.007 0.008 54,269 1249 530.3 925.2 136.6 66.87 570.3 1.109 1.081 1.031 0.9761 51.05 868,426

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Table 5 Exergy efficiency of each equipment in the trigeneration system. Components and exergy efficiency expression

Component identifier

Exergy efficiency (%)

Component identifier

Exergy efficiency (%)

Heat Exchangers [75]

HX1 HX2 HX3 HX4 HX5 HX6 HX7 HX8 HX9 HX10 C1 C2 P100

88.99 94.14 91.29 95.61 94.92 87.22 97.81 86.69 89.96 94.04 83.23 82.10 85.75

HX11 HX12 HX13 HX14 HX15 HX16 HX17 HX18 HX19

93.96 97.25 99.03 99.11 99.09 87.16 93.96 69.58 98.51

C3 C4 P101

82.17 91.34 77.47

V1 V2 V3

43.71 63.72 58.32

V6 V7 V8

51.01 43.24 67.12

V4

67.21

V9

66.17

V5

74.12

V10

73.12

n

ηex

m

⎧ ∑1 (ṁ Δe) ⎫ ⎧ ∑1 (ṁ Δe) ⎫ ⎤ =1− ⎡ ⎢ ⎨ ∑1n (ṁ Δh) ⎬ − ⎨ ∑1m (ṁ Δh) ⎬ ⎥ ⎩ ⎭ ⎩ ⎭c ⎦ h ⎣

Compressors [76]

ηex =

∑ (ṁ . e )i − ∑ (ṁ . e )o W

Pumps[76]

ηex =

∑ (ṁ . e )i − ∑ (ṁ . e )o W

Turbines[76]

ηex =

93.19

W ∑ (ṁ . e )i − ∑ (ṁ . e )o

Ejector

ηex =

98.44

∑ (ṁ . e )o ∑ (ṁ . e )i

Expansion valves [77] T0 T − T0 dh T T e Ph = e ΔT + e Δp

e ΔT = ∫

ηex =

eoΔT − eiΔT Δp Δp ei − eo

Column [78]

ηex =

49.11

Wmin Wmin + LW

Wmin =

∑

Ex −

Out of stream

∑

Ex

in to stream

LW = T0 ΔS irr = Lost Work Gibbs Reactor [64]

ηex =

72.02

∑ (ṁ . e )o ∑ (ṁ . e )i

Overall Exery efficiency

ηex = 1 −

90.43

Total irreversibility in cycle Total consumed power in cycle

transmitted more efficiently as the gas cooling curves and refrigerant heating curves are closer to each other. The lower the temperature difference of the heat exchanger, the higher the amount of heat transfer area, the overall volume of the heat exchanger, and design complexity. In heat exchanger No. 1, the cooling curve of gas and heating curve of refrigerant are with more space from each other because of using pure refrigerant.

0.283% 61%

0.358%

2.231% 0.144%

35.69%

4. Exergy analysis Cold Boxes

Exergy is the energy that is available to be used [53]. Supposing that the potential, kinetic, and the other forms of energy are unchanged or insignificant, Exergy, presented in the following equation, is divided into physical and chemical parts [54]:

Heat Exchangers Turbines

0.28% 0.002%

Compressors & Pumps Reactor Valves and Drums

e = e ph + e ch

Towers Ejector

(12)

Physical exergy is obtained by the following equation [54]:

Fig. 3. The rate of exergy destruction for each component of the trigeneration system.

e ph

= (h − h°) − T° (s − s°)

(13)

where ho and so are enthalpy and entropy in the standard state, respectively [54]. Exergy rate of the chemical in the mixture is obtained by [54]:

combustion chamber. The rest of generated power is consumed in the desalination unit and the refrigeration cycle. Fig. 2 shows the composite curves of each heat exchanger of the LNG unit and the entire process of the LNG unit. As shown in Fig. 2, heat exchangers have a relatively optimal performance in the system. All of the three heat exchangers have optimal designs, which is one of the reasons for the high efficiency of this process. Heat can be

e ch =

∑ xi e°i

(14)

For real mixture, intermolecular forces difference should be considered as follows [54]:

e ch = 9

∑ (xi e°i) + ΔGmix

(15)

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Table 6 Validation of the developed absorption refrigeration system with Mehrpooya et al.

Low pressure (kPa) High pressure (kPa) Condenser cooling load Qcond (kW) Absorption cooling load Qabs (kW) Heating load in generator Qgen (kW) Evaporator heating load Qevap (kW) Rectifier cooling load Qrec (kW) Cycle performance coefficient Concentration of ammonia in lean solution (mole %) Power consumption (kW) Concentration of ammonia in rich solution (mole %)

Mehrpooya et al. [42]

Relative Error

120 1300 7989 5301 10,838 5202 2744 0.4756 26.07 69.23 6.61

120 1300 7948 5293 10,400 5079 2643 0.4851 26.07 68.32 6.61

0 0 0.5007% 0.1509% 4.050% 2.345% 3.601% −0.0173% 0 1.285% 0

Salt Water Holding Tank

Power 55628 kW

Solar Dish Collectors

Natural Gas 26.85 ºC, 6500 kPa, 1.697 kg/s

Natural Gas 26.85 ºC, 6500 kPa, 34.37 kg/s

Desalinated Water 52.12 ºC, 12.19 kPa, 374.3 kg/s

Desalination System

Steam Power Plant Power 224.3 kW Power 21930 kW

Heat Duty 62298 kW

Pipe line gas

Sea Water 25 ºC, 101.3 kPa, 3982 kg/s

Heat Duty 316543 kW

Heat Duty 456207 kW

Boiler

Modelling in this article

Brine 58 ºC, 16.73 kPa, 731.7 kg/s

Absorption Refrigeration System

Mixed Refrigerant System

Natural Gas -165 ºC, 100 kPa, 31.70 kg/s

Fig. 4. Block diagram of using solar dish collectors instead of boilers in the trigeneration system.

308

305 201 406

HX9

212

HX10

506

614

HX14

508

603

619

HX15

`

610 608

605

612 609 606

613 611

615

622

618

617 621

620

625

623 628

626

629

504

407

P104

HX16

616

25 ºC, 1 bar, 53.64 kg/s, Sea Water

213

C1

602

627

C2

V7

V8

V9

V10 HX13

632

P103

211

405

413

HX12 409

D2

Ejector

D3

507

214

101 207

607

604

HX3

HX2

Sea Water Storage

600 315

302

P100

208

624 601

D7

102

215

P101

210

203

408

634

D6

V3

314

313

HX4 310

307

303

HX6

D5

100

415

636

637

D4

206

200

635 522

HX18

HX1

C3

305

311 300

216

HX7

306

T100

204103

412

309

Steam Turbine

205

404

HX8

V2

LNG -165 ºC, 1 bar, 31.70 kg/s, LNG

312

V4

V1

HX5

V5

104

26.85 ºC, 65 bar, 34.37 kg/s, Natural Gas

D1

HX11

410

105

631

Desalinated Water

HX17

630

P104

Heater P102

Solar Dish Collector

Fig. 5. Developing of an integrated solar power plant for producing LNG and freshwater.

Ex in + ExQi = Ex out + ExQout + Wshaft + I

Free energy of Gibbs is obtained by[54]:

ΔG mix = G −

∑ xi Gi

(17)

where I stands for the irreversibility of the system. Table 4 shows the amount of input exergy, exergy output, and exergy destruction in the system. Lost work and exergy efficiency

(16)

exergy balance is obtained by [54]: 10

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500

Table 8 Relationships used to calculate equipment prices in two integrated structures.

Hourly global solar power (MW)

450 400

Component

Purchased equipment cost functions

350

Steam turbine

CST = 3644.3(W)0.7-61.3(W)0.95 [71,77], Original year: 2003 CEx = Cost of Expander (k$) 0.85 CRiboler =8500 + 409 × ARe boler [71], Original year: 2003

300 General heat exchanger Condenser

250 200 150

Heat exchanger MED Steam ejector

100 50

Pump

0 0

730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760

Time (h)

(a) Hourly global boiler power (MW)

500 Drum

450 400 350 300

Boiler

250 200

Absorber

150 100 50 0 0

730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760

Time (h)

Solar parabolic dish

−1 dPt0.15dPs−0.15 [71], Orginal Ccondenser = 201.67 Q ΔTLMTD year: 2003

CMED

effects = 430

−1 dPt−0.01dPs−0.1 [71] × 0.582 Q ΔTLMTD

CEjector = 16.14 × 989 mvapor (Ti/Pi)0.05 (Pe)−0.75 [71] CE = Cost of Steam Ejector ($), Original year: 2004 CP = fMfTCb [5] Cb = 1.39exp[8.833–0.6019(lnQ(H)0.5) + 0.0519(lnQ (H)0.5)2], Q in gpm, H in ft head fM = Material Factor fT = exp[b1 + b2(lnQ(H)0.5) + b3(lnQ(H)0.5)2] b1 = 5.1029, b2 = -1.2217, b3 = 0.0771, Original year: 2003 CD = fmCb + Ca [5] Cb = 1.218exp[9.1–0.2889(lnW) + 0.04576(lnW)2], 5000 < W < 226000 lb shell weight Ca = 300D0.7396 L0.7066, 6 < D < 10, 12 < L < 20 ft fm = Material Factor, Original year: 2003 CBoiler = A (MSteam) + B [5] A = 0.249 Pboiler + 47.19 B = 3.29 Pboiler + 624.6 Original year: 2003 Cb = 1.128exp(6.629 + 0.1826 (logW) + 0.02297*(logW) 2) [5] Cp1 = 300 (D0.7395) (L0.7068) C1 = 1.218 [(1.7Cb + 23.9 V1 + Cp1) ] C2 = Cost of installed manholes, trays and nozzles C3 = Cost of Cooler C4 = Cost of Heater CAb = C1 + C2 + C3 + C4 CAb = Cost of Drum ($), Original year: 2003 CBoiler = 50 A , A = area (m2) [61]

(b) Fig. 6. Heat generated by solar dish collectors and use of auxiliary boiler during of day for Case2.

equipment. This issue shows that the performance of the equipment must be analyzed in terms of irreversibility and exergy efficiency. Therefore, the heat exchangers performance according to the thermodynamic second low in the system is acceptable. Table 6 compares the characteristics of the absorption system before surging in mass flow with the characteristics of the system presented by [42]. After comparison, for applying the system in the trigeneration unit, its flow rate is raised.

indicators should be checked simultaneously for each equipment to better compare the performance of different equipment of each process. To study the exergy efficiency of each equipment in the system, the equations presented in Table 5 are used. It is possible to decline the amounts of irreversibility and enhance energy efficiency by changing in operating conditions, replacing the equipment with more suitable items or changing the structure of the processes. Fig. 3 shows the rate of exergy destruction of the equipment in the system. Due to the configuration of system, the highest amount of exergy destruction has occurred in heat exchangers. On the other hand, these heat exchangers have the highest amount of exergy efficiency compared to other

5. Solar thermal dish collector cycle Solar parabolic dishes are designed to receive solar energy on two axes. Moreover, for reaching the temperature of 1200 K in the receiver, the structure of the used materials must be chosen meticulously.

Table 7 Main parameters of the trigeneration system in Case 2. Different parameters of the integrated structure

values

Generated thermal energy by solar dish collector for a year Auxiliary thermal energy by syngas in one year Number of collectors The power generated by power plant in one year The amount produced by freshwater The amount of consumed power in the natural gas liquefaction cycle Energy consumption in the LNG cycle Gain output ratio in MED

1352.2 GWh 3201.5 GWh 250,000 679.7 GWh 1350 m3/h 1300 m3/h 192.1 GWh 2.918 87.31%

Overall thermal efficiency (LHV Base)ηOverall, LHV =

̇ ̇ − WCompressors ̇ ṁ LNG × LHVLNG + Qequal Absorbtion + WST 1,2,3 + Ẇ Pumps1,2,3 ̇ (ṁ fuel × LHV fuel )Totall + Q̇Boiler + QSolar

Overall thermal efficiency (HHV Base)ηOverall, LHV =

̇ ̇ − WCompressors ̇ ṁ LNG × LHVLNG + Qequal Absorbtion + WST 1,2,3 + Ẇ Pumps1,2,3 ̇ (ṁ fuel × HHV fuel )Totall + Q̇Boiler + QSolar

11

88.28%

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100 85.82 87.31

90

Case 1 Case 2

90.43 91.12 79.84

80 70 60

55.62

50 40 30

24.94

21.96

20 10.35

10

14.96

14.57 6.21

0 Fuel-natural Overall Overall thermal Exergy gas (ton/h) efficiency efficiency (%) (LHV Base %)

Power produced (MW)

Power CO2 produced (ton/h) consumption (MW)

350

Case 1

307

Case 2

300 250 200 150

114.1 119.8

100

62.30 65.41

50 0.653

0

LNG produced (ton/h)

13.47

Desalinated water (ton*100/h)

18.03 18.93

15.35

Heat entering Heating load in Condenser the desalination generator Qgen cooling load (MW) (MW) Qcond (MW)

Fig. 7. Comparison between the main parameters of the two developed integrated structures of the present study. 460000

Number of Parabolic Dished

2200

Fresh Water Production

2100 2000 1900

360000

1800 1700

310000

1600 260000

1500 1400

210000

1300 1200

160000

Fresh Water Production (m3/h)

Number of Parabolic Dished

410000

1100 110000

1000 360

400

440

480

520

560

600

Solar Energy (MW)

Fig. 9. Changes in the number of solar dish collectors and the amount of produced freshwater compared to the solar energy changes required for the developed integrated Case 2.

Fig. 8. Comparison between the specific powers of the LNG system in the two developed integrated structures compared to those reported by [3,4,9,64–67]

12

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LNG Production

94

Overall thermal efficiency

460

92

LNG Production (m3/h)

440 90

420 400

88

380 86

360 340

84

320 82

300 280

η0 = λρταγ cos(θ) Overall thermal efficiency (LHV Base %)

480

θ is equal to the incident angle, and since the solar dishes follow the sun's movement during the day, the value of this angle is zero. η0 = λρταγ

400

440

480

520

560

(24)

The total amount of energy loss from the receiver of the solar dish is gained using the below equation in which, Qlk is the heat loss because of the conduction heat transfer in receiver, Qlc is equal to the heat loss due to the conduction heat transfer in receiver entrance, and Qlr is the heat loss because of the radiation heat transfer in receiver entrance [59]:

Ql = Qlk + Qlc + Qlr

(25)

The useful energy from a solar dish is calculated by the following relationships [19,60]:

80 360

(23)

600

Solar Energy (MW) Fig. 10. Changes in the LNG production and total thermal efficiency concerning the solar energy changes required for the system Case 2.

Nul = 0.106Grl1/3 (Tw / Ta )0.18 (4.256Ac / Aw ) sh (φ1)

(26)

Grl = gβ (Tw / Ta) L3 / v 2

(27)

h (φ1) = 1.1677 −

1.0762 sin(φ10.8324 )

(28)

hc = Nul K / L

(29)

Qlc = hc Aw (Tw − Ta)

(30)

The amount of thermal energy losses in the receiver can be calculated by equation (27) [55].

Resistant metals or alloys should be used for high temperatures up to 1473 K. The generated power of each dish with the area of 12.56 square meters can be obtained from the following equation [55]. (18)

A c = Aa / c

(33)

(34)

(19) 6. Case study Bushehr province is one of the southern provinces of Iran and the 17th largest province of the country in terms of the area located on the border of the Persian Gulf [62]. The centre of this province is the Bushehr port. The province is strategically and economically important because of its strategic location on the Gulf coast, the export and import of maritime, the fishing industry, the existence of oil and gas reserves (South and North Pars), agriculture and palm trees, and the existence of a nuclear power plant [63].

(20)

ηr is equal to the efficiency of receiver obtained from the ratio of the net obtained energy from the solar dish to the amount of energy reaching to the solar dish [58]. ηr = Qu/ Qr

(32)

The amount of ultraviolet radiation, total solar radiation and total hourly solar radiation on the horizontal surface can be extracted according to the equations of the referenced article [61]. Information about the wind blow, the daily temperature effect and the amount of solar radiation for a geographical point (Bushehr province in Iran as a case study) is derived from the TRNSYS software and its link to the Matlab software.

η0 is equal to the optical efficiency of the solar dish, obtained from the ratio Qr to Qs [57]. η0 = Qr / Qs

εeff = 1/[1 + (1/ εc − 1) Ac / Aw ]

Gon = Gsc [1 + 0.033 cos(360°n/365)]

where Qs is the received energy to the surface of the collector, Aa is the area of the dish, Is is total solar irradiation reaching to each collector from the sun. In steady-state, Qu is the utilized energy from the solar dish, obtained from the difference of Qr i.e. the energy reaching the receiver and Ql the losses energy in the receiver [56].

Qu = Qr − Ql

(31)

The value εeff is around 1. Since the value of εc is in the range of 0.8 to 1, and the value ( Aw / Ac ) is greater than 5; therefore, the receiver is considered as a black body. The obtained results of the main parameters of a solar dish used in the trigeneration system are obtained. Area of each solar dish and the internal cavity area of receiver are 12.56 and 0.0645 m2 , respectively. The land factor of un-shading and intercept factor of receiver are equal to 0.99. Transmittance–absorptance product and Cavity surface emittance are equivalent to 0.9. Dish reflectance and geometric concentration ratio are equal to 0.94 and 3000, respectively. Thermal conductivity, characteristic dimension and tilt angle of cavity are 0.025 W/mk, 0.254 m and π/2 radian, respectively.

Fig. 11. Comparison of the total global solar radiation obtained in the present work with those reported by [61,70]

Qs = Is Aa

Qlr = Ac εeff σ (Tw4 − Ta4 )

(21)

The thermal efficiency of the solar dish is derived by:

ηc = Qu/ Qs

7. Economic evaluation

(22)

Annualized Cost of System (ACS) is the selected method of economic

The optical efficiency of a solar dish is derived by: 13

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(a)

(b)

(c) Fig. 12. Comparison of the gain output ratio concerning compression ratio obtained in the present work with those reported by [71,72] 0.82

Current study

by Wu

analysis for the two integrated structures developed in this paper. Parameters of return on investment, product cost, and initial investment cost are influential parameters for choosing the appropriate integrated structure for cogeneration systems. In this method, all costs of a system are calculated over its estimated lifetime. These costs consist of Annualized Capital, Replacement, Maintenance, and Operating Costs with the following abbreviation (Cacap, Carep, Camain, Caope). Since the useful lifetime of the project is assumed to be twenty years, the cost of components substitution is neglected. Marshal and Swift Cost Index is employed for economic analysis of the equipment [49].

by Mehrpooya

Thermal efficiency in solar collector

0.815 0.81 0.805

0.8 0.795

Costreference year = Costoriginal year

0.79

Cost index reference cos t year Cost index original cos t year

(35)

0.785

8. Results and discussion 0.78 600

700

800

900

1000

1100

Fig. 4 shows a block diagram of the solar dish collector and an auxiliary boiler in the system. Fig. 5 shows the structure of the process of LNG and freshwater production using solar plate collectors. Fig. 6 shows the amount of power generation and the use of auxiliary heaters in different days of the year in terms of two constant parameters, i.e. the total solar collector area and the flow rate passing through the collectors. Since this system is usually able to use the solar collector from 8 am to 19 pm during the day, use of an auxiliary heater,

Average operating wall temperature in the cavity (°C) Fig. 13. Comparison of the thermal efficiency in solar collector versus average operating wall temperature in the cavity obtained in the present work with those reported by [56,61]

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Table 9 The process of performing economic analysis in the two integrated structures. Definition

Parameter

Annualized Cost of System

ACS = Cacap (Components) + Carep (Components) + Camain (Components) + Caope (Labor Cost + Fuel Cost + Insurance Cost) Ccap = Fixed capital investment + Others outlays = Direct costs + Indirect costs + Others Outlays

Annualized Capital Cost

Cacap = Ccap.CRF(i,Yproj) = Ccap. Annualized Replacement Cost

Annualized Maintenance Cost Annualized Operating Cost Operating Flow Cost

Net Present Value Cost1 = Cost of Total produced N2 (US$ per Year) Cost2 = Cost of total produced Freshwater (US$ per Year) NEW ACS = ACS- Cost1- Cost1 Levelized cost of Product Total Product in one Year (kg LNG) Prime Cost Summary Of Product Cost Annual Benefit Net Annual Benefit Period Of Return Rate Of Return Additive Value

j−f 1−f

j = 17, f = 20% [49]

j (1 + i)Yproj − 1

For Yproj = 20 , Camain = 0.05 of Capital Cost OFC= (Labor Cost + Fuel Cost + Insurance Cost + Utility) Labor no. = 500 , Labor Cost = 400 US$ per Month [48] Cost of Natural Gas = 2 (US$ per Million Btu) [79] Cost of Electrical Energy = 0.15 (US$ per kWh) Cost of Insurance = 0.02 of Capital Cost NPV = ACS/ CRF(i,Yproj) (Electrical Energy Price) = 0.15 (US$ per kWh) [49] (Freshwater Price) = 1.5 (US$ per m3) [80] LCOP = NEW ACS/ Total product per year VOP = Volume of Product , PC = OFC/VOP COP = Cost Of Product, SOPC = VOP. COP COP = 6 (US$ per Million Btu) [48] AB = SOPC- OFC NAB = AB.(1-Tax percent), Tax = 0.1(AB) POR = Ccap/NAB ROR = NAB/ Ccap AV = COP-PC

amount of emitted carbon dioxide decreases from 24.94 ton/h to 14.96 ton/h, and the amount of natural gas consumption decreases from 10.35 ton /h to 6.21 ton/h. One of the most effective parameters in the investigation of LNG cycles is the Specific Power, defined as the ratio of consumed power to liquefy natural gas into a produced LNG. In Fig. 8, a comparison of the specific power of the simulated LNG structure with the other LNG cycles, presented in similar cycles in other articles and industrial patents, is conducted according to the following references [3,4,9,64–67]. Specific power values of case 1 and 2 are about 0.19 which are reasonable value compared to the other works. In systems with many design variables, proper system modelling is conditional to both identify the critical parameters of the system and understand the behaviour of the system to their changes correctly [68]. Sensitivity analysis investigates normal behavior and continuity of essential parameters, while identifies sensitive parameters of system, which is one of the methods for verifying the design of process structures [69]. Fig. 9 depicts the variation in the number of dish collectors and the amount of produced freshwater compared to the increase of solar thermal energy in the system. Fig. 9 depicts that the amount of produced freshwater increases with the amount of solar heat entering into the system, due to the higher heat transferring into the heat exchangers of the desalination system, so the number of solar dish collectors should be increased to provide required heat. Fig. 10 shows the changes in the production of LNG and total thermal efficiency relative to the solar energy changes required for the system. By increasing the amount of generated solar heat, the amount of transferred heat to the generator of ARS increases, thus this issue leads to increasing of the amount of produced LNG. Furthermore, the overall efficiency of the system decreases with the amount of solar heat given into the system, which is due to the nature of the system. This is reflected in the increase in the amount of solar energy given to the system relative to the amount of LNG production. Fig. 11 compares obtained total global solar radiation of this research with similar works presented by Moradi and Mehrpooya [61] and Sabziparvar [70]. Comparison of the values of solar radiation shows the accuracy of the present study. Fig. 12 compares gain output

Table 10 Economic analysis results of two integrated structures.

Total capital cost (MMUS$) Electrical energy cost (MMUS$ / Year) Levelized cost of product (US$ / kg) Net annual benefit (MMUS$ / Year) Annualized operating cost (MMUS$ / Year) Annualized cost of system (MMUS$ / Year) Net present value (MMUS$ / Year) Period of return (Year) Rate of return (Percent) Insurance cost (MMUS$ / Year) Additive value (US$ / kg LNG) Prime cost of product (US$ / kg LNG)

i=

Crap = Ccap (In Base). (1 + i)Yproj Carep = Crap.FSF (I,Yproj) = Crap.

Parameter

i . (1 + i)Yproj (1 + i)Yproj − 1

Value Case1

Case2

501.4 104.6 0.1710 130.5 219.2 251.6 3898 3.842 26.02 10.02 0.0445 0.2580

989.4 29.12 0.1324 189.7 151.6 216.8 3361 5.334 18.74 20.37 0.1252 0.1784

that is, a boiler using natural gas for heating supplier is required. The amount of heat provided by solar collectors and auxiliary boiler is shown in Fig. 6 for different hours interval per year. Hot oil with molar composition of 24.62% BiPhenyl and 75.38% diPH-Ether is used to transfer heat in solar dish collector. Table 8 illustrates the main parameters of a solar dish used in the developed hybrid system. According to Table 7, the solar collector has a heat capacity of 1352.2 GWh per year, which is supplied by 250,000 collectors. This structure produces 679.7 GWh of power over a year, 192.1 GWh of which is the specific energy consumption of the LNG cycle over a year, and the rest being spent on domestic and industrial use. Gain output ratio of multi-effect desalination system is 2.918, which is the same for the similar industrial cases in the articles. Fig. 7 displays a comparison between the main parameters of the two developed structures. Based on this comparison with the placement of the solar dish collector, the amount of produced LNG increases from 114 ton/h to 119.8 ton/h, and the amount of produced freshwater increases from the 65.3 ton/h to 1347 ton/hr. On the other hand, the

15

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present work with those reported by Moradi and Mehrpooya [61] and Wu et al. [56]. The trend of reduction in thermal efficiency with average operating wall temperature in the cavity shows the accuracy of calculation in the present study. The prime Annualized Capital Cost consists of constant (direct and indirect) and other costs. Table 8 is used to calculate the cost of equipment in two integrated structures. Table 9 displays the process of economic analysis for the two integrated structures. Table 10 presents the economic outcomes of the two integrated structures. Due to the return on investment rate and the cost of the product in the two structures described in this paper, it can be justified under the economic conditions. The product price of both structures 1 and 2 are equal to 0.2580 and 0.1784 US$ per kg LNG, respectively. The value of the product price for LNG in the reference [49] is equivalent to 0.246 US$ per kg LNG and for reference [73] is at least equal to 0.26 US$ per kg LNG. The difference between these amounts in the articles and the value of product in the two structures is due to the type of configuration and composition of natural gas. Sensitivity analysis is conducted on economic parameters for evaluating the two structures in different economic conditions. Fig. 14 shows changes in period of return on capital cost in the two integrated structures presented concerning the changes in the price of electricity and LNG. The results illustrate that in system No. 1, the period of return is less sensitive to the price of electricity. The period of return decreases with electricity prices and LNG prices in the market. As for the LNG price 5.3 US$/MMBTU and more, the period of return in integrated structure of No. 1 is less than five years. The results show that the period of return to the price of electricity is more effective when LNG prices are low for the integrated structure of No. 2. Fig. 15 shows a comparison of the prime cost of product obtained in the present work with those reported by [49,73,74]. The prime cost of product represents the cost (US$) for producing one kilogram of LNG. As it is clear from the figure, case 1 and 2 have the prime cost of product of 0.258 and 0.178 US$ per kg LNG. The lowest amount of the value was reported by Wang et al. 2014 [73] with 0.224 US$ per kg LNG. However, the developed structure in the paper i.e. Case 2 has supremacy over the other structures in terms of prime cost of product. Fig. 16 shows sensitivity analysis of the integrated structures with respect to the key variables. The products of integrated structures are elecrical energy, liquified natural gas, and freshwater. The variations of these products' price are investigated in terms of main economic parameters such as period of return, prime cost, additive value, and net annual benefit. Fig. 16 (a) and (b) show variation of period of return and prime cost of products versus electrical energy price for cases 2 and 1, respectively. Unlinke prime cost, the period of return decreases with electrical energy price. Fig. 16 (c) and (d) show variation of period of return and additive value versus LNG cost of market for cases 2 and 1, respectively. According to these graphs, period of return decreases with LNG cost of market, while additive value increases with market LNG cost. Fig. 16 (e) and (f) show variation of period of return and net anuual benefit versus freshwater price for cases 2 and 1, respectively. In this case, the period of return decreases with the price of fress water, whereas net annual benefit increases with the price of freshwater. According to these detaied sensitivity analyses, the supremercy of case 2 over case 1 is proven in terms of economic parameters.

(a)

(b) Fig. 14. Changes in the period of return of the two integrated structures versus to variation in electricity and LNG prices.

9. Conclusion Fig. 15. Comparison of prime cost of the product obtained in the present work with those reported by [49,73,74]

Developing a hybrid poly-generation system is herein conducted for producing LNG and freshwater at the same time. The MED system is used for freshwater production, and ammonia-water absorption system (precooling) and multi-refrigerant refrigeration system (liquefaction) are employed for LNG production. The developed system for providing power and heat utilizes from a natural-gas-fired power plant. Hot gas from the gas turbine is used to provide required energy for absorption system and to generate steam for MED system (MED). The system has

ratio concerning compression ratio of the present study with the ones developed by Reyhani et al. [71] as well as Wang and Noam [72]. The comparison is conducted in terms of 4 effect, 6 effect and 8 effect. Fig. 13 shows comparison of the thermal efficiency in solar collector versus average operating wall temperature in the cavity obtained in the 16

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(a)

(b)

(c) (d)

(e) (f)

Fig. 16. Sensitivity analysis of the integrated structures with respect to the key variables.

system, the produced freshwater, the number of solar collectors and the production of LNG are increased by 75.77%, 182.2% and 56.67%; respectively, while the total thermal efficiency is decreased by 12.41%. The products' price of integrated structures i.e. elecrical energy, liquified natural gas, and freshwater are investigated with respect to period of return and the main ceonomic parameters. Resutls show that the period of return decreases with the price of the aformentioned products. The results of the economic analysis with the method of annualized system costs show that in structure 1, the cost of the product and the period of return are 0.2580 US$ per kg of LNG and 3.842 years. By adding solar dish collector instead of gas-fired boiler, the prime cost of product and the period of return are equal to 0.1784 US$ per kg LNG and 5.334 years. Besides, the cost of initial investment is increased by 49.32%, and the annual net profit and the cost of product are decreased by 13.7% and 30.85%, respectively.

total efficiency of 85.82% and exergy efficiency of 90.43%, producing freshwater by 65.3 ton/ h and LNG by 114.1 ton/h. For better comparison of different equipment of each process, the exergy efficiency and exergy destruction values are analyzed. The exergy analysis of the system depicts that the highest amount of exergy destruction belongs to the heat exchangers by 61%. Since the hybrid system consumes 10.35 ton/h of natural gas, it releases about 24.94 t/h of carbon dioxide to the atmosphere. To reduce both the consumption of natural gas and emission of carbon dioxide, solar dish collectors are employed for providing required heat for the steam power plant. In the second integrated structure, 1347 ton/h of freshwater and 119.8 ton/h of LNG are produced, respectively, and the overall efficiency and the exergy efficiency of the integrated structure are 87.31% and 91.12%, respectively. By adding solar collectors, the amount of carbon dioxide emission and natural gas consumption are reduced by 40% and 40.01%, respectively. The sensitivity analysis is also conducted and depicted that with an increase of 56.67% in the amount of solar energy entered into the 17

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Declaration of Competing Interest

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[58]

[59] [60]

[61]

[62]

[63]

[64]

[65]

[66]

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