Thermo-economic assessment and retrofitting of an existing electrical power plant with solar energy under different operational modes and part load conditions

Mehrpooya, Mehdi; Taromi, Morteza; Ghorbani, Bahram Article Thermo-economic assessment and retrofitting of an existing electrical power plant with solar energy under different operational modes and part load conditions Energy Reports Provided in Cooperation with: Elsevier Suggested Citation: Mehrpooya, Mehdi; Taromi, Morteza; Ghorbani, Bahram (2019) : Thermoeconomic assessment and retrofitting of an existing electrical power plant with solar energy under different operational modes and part load conditions, Energy Reports, ISSN 2352-4847, Elsevier, Amsterdam, Vol. 5, pp. 1137-1150, https://doi.org/10.1016/j.egyr.2019.07.014 This Version is available at: https://hdl.handle.net/10419/243657 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. 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If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. https://creativecommons.org/licenses/by-nc-nd/4.0/ Energy Reports 5 (2019) 1137–1150 Contents lists available at ScienceDirect Energy Reports journal homepage: www.elsevier.com/locate/egyr Research paper Thermo-economic assessment and retrofitting of an existing electrical power plant with solar energy under different operational modes and part load conditions Mehdi Mehrpooya a,∗, Morteza Taromi b, Bahram Ghorbani c aDepartment of Renewable Energies and Environment, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran bDepartment of Energy Engineering, Faculty of Environment and Energy, Science and Research Branch, Islamic Azad University, Tehran, Iran cFaculty of Engineering Modern Technologies, Amol University of Special Modern Technologies, Amol, Iran article info Article history: Received 8 May 2019 Received in revised form 28 June 2019 Accepted 22 July 2019 Available online xxxx Keywords: Retrofitting Natural gas fired power plant Solar energy Parabolic trough collector abstract This paper investigates operational modification of 250 MW Rajaee natural gas fired electrical power plant by supplying a portion of the required heat load from the solar energy source. The base case and the introduced hybrid system, both are simulated in Thermoflow and MATLAB softwares. Simulation of parabolic collector solar field in both methods of power boosting and fuel saving is performed by MATLAB. An economic analysis is done and optimal solar contribution is calculated. The obtained results specify that in solar aided electrical power generation mode can reach higher thermal efficiency in comparison with the using natural gas as fuel. In this case, with utilizing the solar field (120,000 m2) the thermal efficiency extends from 37.0% to 39.1%. The electrical power generation by employing 7.00% of solar heat energy, up to 24.0 MW can be improved. In the fuel saving mode, the gross annual cutbacks of the fuel consumption and CO2emissions rates for a 12×104m2solar collector receiver are 35,125×103kg and 11,164×103kg; respectively. The electrical power generation costs and fuel consumption rate saving are 80.0 US$/kWh. Also, period of return for the electrical power generation mode is six years. ©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction It is indisputable fact that conventional coal or natural gas fired power plants are the superior method for electrical power generation in the world (Hou and Hughes,2001). However, the impact of pollutants emission on the economy, human health, and environment issues are increasingly being considered. In order to compensate for these effects, clean and sustainable energy source such as of using solar thermal energy for electrical power generation has been accepted as the efficient way (Moradi and Mehrpooya,2017;Mehrpooya et al.,2016b). Since solar energy is unstable as well as periodic, solar electrical power generation solely, is costly (Fernández-García et al.,2010;Ashouri et al.,2015;Mehrpooya et al.,2015). A come into view technology which is known as concentrating solar power (CSP) contains considerable potential for areas with permanent sun radiation and clear sky (Mehrpooya and Sharifzadeh,2017). The electrical power generation by CSPs covers the variable daily ∗Corresponding author at: Department of Renewable Energies and Environment, Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran. E-mail address: [email protected] (M. Mehrpooya). demand in places where air conditioning systems are mostly used (Hernández-Moro and Martínez-Duart,2013). International Energy Agency predicated that CSPs provide about 11.3% of the world’s total electrical power by 2050 (Achenbach and Riensche, 1994). From two decades ago, significant efforts have been done to integrate solar thermal energy with the fossil fuel electrical power generation plants (Mehrpooya et al.,2016a). These works along with long and short terms of sustainable development of the thermal electrical power plants which are utilizing solar thermal energy as heat source (Steinfeld and Palumbo,2001). In this regards, an electrical power plant which consists of the existing 44 MW coal-fired and 4 MW CSP was built in Colorado (2010) (Peng et al.,2014). By integrating solar thermal energy with fossil fuel, carbon dioxide (CO2) emission rates. Fuel costs and the sole solar system drawbacks can be decreased simultaneously (Costa,2011). Zhao et al. (2012) propose a hybrid electrical power plant which is consisting of medium temperature solar and coal thermal energy as fuel with advantages of CO2capturing. In this work, the principal idea is retrofitting of a conventional electrical power plant with solar thermal energy to substitute parts of the extracted steam in the regenerative Rankin cycle (Montes et al.,2011). Ying and Hu (1999) present the solar–coal hybrid electrical power plant and calculate the thermodynamic benefits https://doi.org/10.1016/j.egyr.2019.07.014 2352-4847/©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1138 M. Mehrpooya, M. Taromi and B. Ghorbani / Energy Reports 5 (2019) 1137–1150 Nomenclature a Aperture area (m2) B Day angle/ Surface tilt angle to horizontal (◦) CpSpecific heat capacity at constant pressure (kJ/kg.◦C) D Diameter (m) DNI Direct Normal Irradiance (W/m2) H Enthalpy (kJ) k Thermal conductivity (kW/m.◦C) L Length (m) ˙ mMass flow rate (kg/s) N Gross electrical generation (kW) Nu Nusselt number (-) p Pressure (bar) ˙ QHeat transfer rate (kW) S Absorbed heat by the solar collector (kW) T Temperature (◦C) W Width (m) ˙ W Electrical power (W) V Velocity (m/s) Greek letters ηEfficiency ΛLongitude angle ΩHour angle ∆Declination angle θAngle αAltitude angle ΦLatitude angle γazimuth angle ΘIncidence angle Subscripts 0 Atmospheric condition A Ambient el Electrical power i Component ‘‘i’’ O Optimum s Solar se Solar-to-electricity Abbreviations A Area B Bled steam CC Combustion chamber DEA Deaerator DSG Direct Steam Generation EC Economizer fw Feed water gen Generator H Efficiency HP High pressure HSPP Hybrid Solar Power Plant of a three stage regenerative Rankin cycle. Advantage of solar aided electrical power generation concept by energy and exergy analysis is investigated (Hu et al.,2010). The state of the working fluid is assumed unchanged with respect to indicating how working fluid can be used in different situations along with various temperatures (Kearney et al.,2003). The replacement of heaters with solar energy in different scenarios is analyzed (Popov,2011). In this work concluded that, high pressure economizer has better performance. Some cases with (energy, exergy, environmental and economic) analysis of hybrid solar thermal electrical power plant are proposed (Suresh et al.,2010). Also, it is discussed how different kinds of replacement can have different effects on whole system performance. A more efficient way to introduce the solar electrical power in a combined cycle is preheating the combustion air in the gas turbine (Kelly et al.,2001). In this case, solar heat energy can be demonstrated in efficient way to achieve a multi-level of utilizing solar energy into conventional electrical power plants (Yang et al.,2011). The conversion efficiency of solar thermal energy to the electricity for power plant using coal as base fuel that uses low (100 ◦C) or medium (260 ◦C) solar heat energy source is 36.6% (Lewis and Nocera,2006). The solar friction is the portion of solar thermal energy contributed towards the net electrical power output of hybrid solar power plant (HSPP) is limited. This is because of the additional cost which is caused in adapting the additional solar energy capacity. Also may not be applicable because of technical limitation of steam turbine is smaller than its specific cost (Horn et al.,2004). Thermo-economical assessment on central receiver system (CRS) integrated with hybrid solar gas turbine power plant is performed (Niknia and Yaghoubi, 2013). Similar studies on retrofitting new and existing coal-fired power plants with solar thermal energy are investigated (Niknia and Yaghoubi,2012;Van Sciver,2011). In the most of the proposed HSPP, effluent stream of gas turbine enters the recovery system which uses solar energy as additional source to provide required heat for bottoming cycle. The solar energy is used to achieve lower NOxemission targets and at the same time the plant is able to attain savings in coal consumption up to 900 tons per year (Muñoz et al.,2009;Spelling et al.,0000). It is anticipated that the project will increase the electrical power plant’s efficiency by up to 5.00% and will reduce CO2emissions by 2000 tons per year. A study is carried in direct steam generation (DSG) parabolic plant in Platform Solar de Almeria (PSA) in Spain. It is proved that, this type of plant reduces the electricity cost by 26.0% while having lower field pressure drop as compared to indirect solar systems at the same size. This would in turn, reduce the average field temperature and thermal losses which leads to higher solar field operating efficiency achieved. Unlike the DSG system, in order to reach higher operating temperature (about 600 ◦C), recently liquids such as molten salts are used as heat transfer fluid (HTF) have been developed. This type of HTF has greater stability against temperature in contrast with other types (Kalogirou,2009). Although ionic liquids have mentioned advantages, but they expensive. This investment cost should be compared with other operating costs which are receiver maintenance to determine their true cost effectiveness (Giostri et al.,2012). Detail list of solar CSP and hybrid solar power plants around the world can be found in Barlev et al. (2011). In addition to the stated advantages, in a DSG hybrid solar arrangement, the used feed water in the Rankine cycle is led into the solar field and is directly heated by the incoming solar irradiation without requiring an intermediate heat exchanger. It results noticeable financial advantages and efficiency gains due to reduction of thermal and exergy losses within the system. Even though, it is in its developmental stage, the effort to integrate DSG systems with storage of thermal energy can possibly be a way forward of the future during non-solar periods of operation (Khaled,2012). A various range of operating temperature from 60–300 ◦C, 100–500 ◦C and 150–2000 ◦C can be obtained by parabolic trough collector, M. Mehrpooya, M. Taromi and B. Ghorbani / Energy Reports 5 (2019) 1137–1150 1139 solar dishes and heliostat fields, respectively (Van Sciver,2011). The process components of the Rankine cycle are steam turbine, heat exchanger, condenser and feed water pump. The heat of vaporization of the working fluid affected the efficiency of the Rankine cycle. In general, steam turbine inlet temperature values is about of 550 ◦C. This temperature leads Carnot efficiency about of 63.0% is obtained (Ahmadi and Dincer,2010). The Rankine cycle can be consider as the competitive cycle for using the sun’s thermal energy for electrical power generation. Among the working fluids, CO2is one of the most common type of working fluid due to its properties which are non-explosive, non-flammable, and naturally abundant (Chen et al.,2006). Another approach for using solar thermal energy is cogeneration system (Raj et al., 2011). The thermal efficiency of the cogeneration systems are about of 40.0–50.0% (Ahmadi and Dincer,2010). According to the proposed description provided, using the solar thermal energy in the conventional electrical power plant has advantages in improving operating performance and decreasing costs. In this study, retrofitted design based on the solar energy as an alternative renewable energy source for existing steam electrical power plant is done. A parabolic trough solar collector system is designed and integrated to the process. All of the required data for modeling of the solar system are developed and presented. The solar irradiation variations based on the considered climate data is shown. Retrofitted electrical power plant efficiency is calculated and effects of the key parameters such as solar fraction on the hybrid system operating performance are investigated. 2. Plant description These days, most of the thermal electrical power plants operation is heavily dependent on fossil fuels for meeting the unceasing thirst for energy. Most of these plants, use coal as the main heat energy source for steam production in the Rankine cycle. A drastic step towards a completely renewable fuel based macro economy will certainly lead to major economic disturbance in every sect of the society. A smart decision would be the introduction of the renewable technologies in smaller doses in conjunction with the already existing fossil-based infrastructure to minimize drastic economic uncertainties. In order to widen the pre-existing knowledge base on the hybrid coal–solar schemes, a parabolic trough solar field is aided a 250MW coal fired power unit in Abyek, Qazvin, Iran (latitude 36.2◦N, longitude 50.3◦E). Qazvin is an area with high solar irradiation sources. The general geographical location data is shown in Table 1. The feed water heaters are numbered in a pressuredescending order, which is currently being used in electrical power plants. The plant is constructed on 3.43 km2stretch of land with extensive plot remaining for expansion if solar options are considered. Its vast plane terrain is best suited to parabolic collector plants which require flat topography for deployment. The electrical power plant is a base load type of sub-critical thermal plant. The design rate capacity will be reached to 1000 MW. In this case, it can be considered among the high-capacity power plants in Iran. The plant includes four units with the gross capacity of 250 MW at design rate conditions. Each of the units is equipped with separate generators. These generators have the ability to works up to 10.0% of the steam turbine capacity. General component specifications of electrical power plant are presented in Table 2 (Turchi and Heath,2013). 3. System description All the existing conventional electrical power plants which use coal or gas as fuel are the regenerative type of the Rankine steam cycle. In this kind of cycles, a part of the steam from the steam turbine is used for preheating the boiler feed water from about 70.0 ◦C (output stream from the condenser) to 300 ◦C (input stream to the economizer) (Hu et al.,2010). In this way, the Rankine steam cycle overall thermal efficiency is increased. But at the same time, the electrical power generation per unit of the input steam to the boiler is decreased. Consequently, the amount of saved steam can go through steam turbine to expands and generate more electrical power (Montes et al.,2009). As can be seen from Fig. 1, air is taken from atmosphere and pressurized by the inlet compressor. Then, it is heated by the effluent flue gas steam from economizer in the air preheater. The hot and pressurized air enters the furnace as combustion chamber (CC). In the CC, the coal as fuel is burned with the air with a ratio about of 1 to 20. The combustion product which is a hightemperature mixture of gasses is used in the evaporator as the heat source. The combusted mixture is subsequently led to the boiler evaporator section where it generates saturated vapor at 138 bar within the thick walled riser pipes without the need to return to a boiler drum. The flue gas mixture is then follows to the rest of the other heat exchangers to lose its thermal energy to the steam flowing through. The generated superheat steam with temperature and pressure of about 540 ◦C and bar passing through high-pressure turbine. The high pressure (HP) turbine effluent stream is divided into the two streams. A major portion of this stream enters heater at the pressure about of 33.4 bar while the other small parts are utilized in the second HP closed feed water heater as heat sources. The steam after reheating enters the medium pressure (MP) and low pressure (LP) turbines; respectively. Extractions from intermediate or exhaust pressures of each stage are made to increase the efficiency of the Rankine steam cycle. Apart from the mentioned extractions for heating in the open and closed feed water heaters, one seal steam extraction is made from the intermediate pressure turbine exhaust. The purpose of this extraction is to provide sealing for both the HP and LP ends of the turbine stages through pilot sealing so as to prevent the infiltration of low-temperature air at the lower pressure end while maintaining positive pressure at the highpressure end during start-up. Once the turbine load is increased at its ramp rate, the sealing steam can be shut off and sealing can be provided from the high-pressure end to the low-pressure side. The LP steam turbine exhaust, along with the rest of extraction returns from the heaters, is condensed with the help of cooling water from the cooling tower. Feed water pump subsequently raises the condenser exit pressure beyond its saturation pressure that matches the steam extraction pressure of the deaerator. In this plant, two different categories of the feed water heater are used. Tree low pressure (LP) feed water heater which is open type is utilized before deaerator. As well as, two high pressures (HP) open types of feed water heater which are implemented to reach the temperature of the water to desired value before entering the economizer. The drain lines in between the open feed water heaters are throttled to their next heater turbine steam extraction pressure for better heat exchange. The second feed water pump is utilized to prepare the required HP turbine inlet pressure and also overcome the pressure drops in the different heat transfer sections of the boiler. The schematic process diagram of the electrical power plant with its main operating parameters is shown in Fig. 1 and Table 3; respectively. 4. Process simulation Rankine cycles are the most used cycle in the solar aimed electrical power plant. The Rankine cycle consists of four different thermodynamic states. Heat addition and rejection at constant pressure and adiabatic reversible (isentropic) compression and 1140 M. Mehrpooya, M. Taromi and B. Ghorbani / Energy Reports 5 (2019) 1137–1150 Table 1 Rajaee electrical power plant site specific geographical and climatic data. Quantity Unit Value Site Name – Rajaee electrical power plants Latitude – 50 Longitude – 36 Average annual DNI kWh/m21300 Average annual dry bulb temperature ◦C 18 Elevation m 1360 Table 2 General specifications of Rajaee electrical power plant. General Rated Plant Capacity Firm Output Design efficiency at rated turbine output Average availability Working hours per day 1000 MW 1850 37% 93.37% 24 Turbines Manufacturer Type Generator output Speed Generator efficiency High pressure turbine isentropic efficiency Intermediate pressure turbine isentropic efficiency Low pressure turbine isentropic efficiency MHI Multi cylinder impulse reaction 312 MVA 3000 98% 85% 91% 85% Boiler Manufacturer Type Number IHI SR Single Drum Natural Circulation 4 Generators Manufacturer Rated capacity Total efficiency of generator MHI 312 MVA 98% Fig. 1. Schematic process diagram of 250 MW Rajaee electrical power plant. M. Mehrpooya, M. Taromi and B. Ghorbani / Energy Reports 5 (2019) 1137–1150 1141 Table 3 Main operating parameters of the electrical power plant. Stream number Mass flow rate (kg/hr) Temperature (◦C) Pressure (bar) Enthalpy (kcal/kg) (1) 629775 60.89 – 60.87 (2) 22256 86.45 0 .6239 629.3 (3) 629775 82.33 – 82.34 (4) 24851 148.8 1.45 661.8 (5) 629775 105.5 – 105.6 (6) 21544 209.1 2.61 689.2 (7) 629775 125 – 125.4 (8) 42754 317.1 7.40 739.2 (9) 779469 167.4 – 171.2 (10) 43553 434 17.3 794.4 (11) 779469 203.6 – 209.0 (12) 63387 349.1 37.3 740.6 (13) 779469 243.2 – 251.8 (14) 77902 538.01 140 819.3 (15) 694344 349 34.3 740.6 (16) 694344 538 34.3 844.9 expansion processes. During the heat transfer process, the working fluid phase is changed to prepare essential isothermal heat. Bu using regenerator in the Rankine cycle, the efficiency is increased due to the increasing the level of the heat transfer temperature. The reason is that the liquid is preheated before enters the vapor regenerator by using the turbine effluent stream heats. The regeneration process in the Rajaee electrical power plant is demonstrated by using some of the vapor that has partially expanded through the turbine. The extracted heat is used to preheating the pressurized liquid before entering the vapor generator. As shown in Fig. 2, in the Rajaee electrical power plant, two different types of the feed water heaters are utilized. In the open feed water heater, the pressurized liquid is preheated (2→3) by using extracted eat energy of the steam from the LP turbine at point ‘‘b’’. This extracted steam from the LP turbine has the same pressure with the pressurized liquid at the outlet of the pump #1. Increasing the pressure of the open feed water heater outlet liquid to the level of the vaporizer pressure is accomplished by using the pump #2 (3→4). The pressurized liquid goes through the closed feed water heater which exchanging heat is done at across the surface. In this heater, the heat of the extracted vapor from the HP turbine at point ‘‘a’’ is used to preheat the pressurized liquid (4→5). Unlike the prior, there is no requirement for extracted vapor and inlet pressurized liquid to being at the same pressure. The obtained liquid from the condensation of the vapor is fed back to the open feed water heater at lower pressure. Fig. 3 shows the integration of the parabolic trough collector (PTC) solar field with Rankine steam electrical power plant which uses coal as fuel. The flow rate of the produced steam depends on the solar irradiation energy from the parabolic trough collectors for heating the feed water at the economizer entrance. The economizer operates at temperatures 200.6 to 357.4 ◦C, so low freezing point and medium temperature operating mineral oils such as Therminol VP-1 and Hi-tech oil can be utilized as heat thermal fluid (HTF). For times of the day when the solar irradiation energy is not available, all the feed water goes through the steam cycle by the by-pass valve. One of the constraints in the hybrid cycles is that the quality of the produced steam by using the solar irradiation energy should meet the desired requirements. Also, choosing the temperature of heat source must be higher than of the feed water or steam. Since heat energy should not be gained from the steam cycle at any section of the heat exchangers. By implementing economizer at the hybrid cycle, the temperature of the solar field is set to be 393 ◦C. Also, the HTF temperatures when coming back to the solar field is 290 ◦C to meet the above-mentioned criteria. The hybrid electrical power plant performance is affected by the solar field. So, the amount of energy consumption and the resulting economic benefit after the integration need to be analyzed. To analyze the performance of the solar field model. Fig. 4 illustrates the logic flow diagram of the used computer program. 4.1. Solar field The PTC is a kind of solar collector that is straight in one dimension and curved as a parabola in the other two dimensions. The surface of the PTC is polished with a mirror which is fabricated by metal. The energy of solar irradiation goes through the parallel mirror symmetry planes and is focused along the focal line. The focal line is the place for heating the objects. The PTC position is aligned with the north to the south axis of the Earth and rotated to track the sun movements in the sky over the day. As another option, the PTC can be aligned on the East–west axis of the Earth. In this case, the alignment just needs to be changed during the seasons and the requirement to the tracking motors is eliminated. But, the overall efficiency of the PTC due to the cosine loss is decreased. Fig. 5 shows the considered PTC which single tracking axis. The PTC incidence angle should be corrected because a discrepancy occurs in the higher angular incidence displacement. The discrepancy is a result of reflection and absorption losses across the receiver glass envelope. The losses have a direct relationship with incidence angles. To overcome these incremental losses in case of increasing incidence angles, the modifier of the incidence angle (Giostri et al.,2012) is utilized to rectify the discrepancy in angular displacement. A precise calculation of extraterrestrial radiation is as follow (Lanhua and L.,2012): S=24 ×3600 ×Gpn/π ×cos d(fi×cos d(delta)×sin d(omigas) +2×π×omigas)/360◦×sin d(fi)×sin d(delta) (1) Where Sis the extraterrestrial solar radiations on a horizontal surface. omigas =acos d(tan d(delta)×(−tan d(fi))) (2) Where omigas is the sunset hour angle. delta =23.45◦×sin d(360◦×(284)/365) (3) where delta is the solar declination. Gon =Gsc ×(1 +0.033 ×cos d(360◦×n/365)) (4) Where Gon is the extraterrestrial radiation on the normal plane, fiis the local geographical latitude (equals to 35.69◦), Llog is the local geographical longitude (equals to 51.42◦), Gsc is the solar 1142 M. Mehrpooya, M. Taromi and B. Ghorbani / Energy Reports 5 (2019) 1137–1150 Fig. 2. Rankine cycle of the Rajaee power plant incorporating reheat and regeneration feed water heating. Fig. 3. Integration of the PTC solar field with the Rankine steam electrical power plant. constant (equals to 1300 W/m2) and is the number of day in the year. The LS-3 is the most utilized solar collector receiver in the design of hybrid solar electrical power plants and due to its proven performance (Hong et al.,2014). The maximum operating temperature of the HTF existing from solar collector receiver is 390 ◦C (Shahin et al.,2016). Tables 4 and 5illustrate the solar collector receiver geometric values and input parameters for the PTC. These parameters were utilized to calculate model variables which are the input useful energy to the Therminol VP-1 (HTF), the collector plane temperature, the gained solar thermal energy to the collector, and solar collector receiver thermal efficiency. The gained thermal energy (useful energy) from the PTC which depends on the absorbed solar radiation incident minus losses of the solar field to the atmosphere can be calculated as follow (Soteris,2009): Qu=FR×((S×Aa)−(Ar×UL×(Tro −To))) (5) Where Aais area of the receiver (equals to 70 m2), FRis the heat removal factor (-), S is the absorbed heat by the solar collector receiver (kW/m2), and ULis the overall heat loss coefficient of the solar collector receiver (kW/m2.◦C), The area of the solar collector receiver and cover plane can be calculated as follows (Soteris, 2009): Ar=π×Do×L(6) AG=π×DG×L(7) Aa=(W−DG)×L(8) M. Mehrpooya, M. Taromi and B. Ghorbani / Energy Reports 5 (2019) 1137–1150 1143 Fig. 4. Logic flow diagram of the solar field model for performance analysis. Table 4 The LS-3 type solar collector receiver geometric values. Parameter Symbol Value Single collector width W5.760 (m) Single collector length L12.27 (m) Receiver inner diameter Dr,i0.0661 (m) Receiver outer diameter Dr,o0.07 (m) Cover inner diameter Dc,i0.1153 (m) Cover outer diameter Dc,o0.1214 (m) Emittance of the cover εcv0.86 (-) Emittance of the receiver εr0.15 (-) Reflectance of the mirror ρc0.94 (-) Intercept factor γ0.93 (-) Transmittance of the glass cover τ0.96 (-) Absorbance of the receiver α0.96 (-) Table 5 Input parameters for the performance analysis of the PTC. Parameter Symbol Value Ambient temperature T0298.15 (K) Solar irradiation Gb1050 (W/m2) Therminol VP-1 (HTF) density ρc1060 (kg/m3) Thermal conductivity of air kair 0.024 (W/m.K) Thermal conductivity of HTF kr0.096 (W/m.K) Kinematic viscosity of HTF υHTF 9.9×10−7(m2/s) Receiver mass flow rate ˙ mr0.8 (kg/s) Temperature at receiver output Tro 546.3 (K) Temperature at receiver input Tri 493.3 (K) Where L,Wand Doare the collector length, width and the receiver cover plane outer diameters (m); respectively (Soteris, 2009). FR= ˙ mFR ×CP×(1−exp (AR×UL×F1 ˙ mFR×CP)) Ar×UL (9) Fig. 5. The angle of incidence at the PTC aperture area (Kalogirou,2013). Where ˙ mFR is the HTF mass flow rate (kg/s) in the solar collector receiver, CPis the HTF specific heat capacity at constant pressure energy which can be calculated by using the average temperature of the inlet and outlet HTF temperature in the collector receiver, and F1is the receiver efficiency factor (-) (Soteris,2009). UL=1 (AR (HCA+HRCA)×AG)+(1 HRRC ) (10) Where HRCA is the radiation heat coefficient (kW/m2.◦C) between ambient conditions and the receiver cover planes, and can be calculated as follows (Soteris,2009): HRCA =GCE ×5.67 ×10−8×(TG+TO)×(TG2+TO2)(11) 1144 M. Mehrpooya, M. Taromi and B. Ghorbani / Energy Reports 5 (2019) 1137–1150 Where GCE represents the emittance of the cover (-). The heat coefficient radiation among the collector receiver and the cover can be calculated as follows (Soteris,2009): HRRC =5.67 ×10−8×(TG+TR)×(TG2+Tr,av2) (1 RE )+((AR AG)×(( 1 GCE )−1))(12) Where RE is the receiver emittance (-) and Tr,avis the average temperature of the inlet and outlet HTF in the solar collector receiver. The receiver cover plane temperatures can be calculated as follows (Soteris,2009): Tg=(AR×HRRC ×TR)+(AG×To×(HRCA +HCA)) (AR×HRRC)+(AG×(HRCA +HCA)) (13) So, the reflected solar radiation (HRCA), (kW) upon the collector receiver which is input heat energy in the system can be calculated as follows (Soteris,2009): HRCA =Aa×FR×S×ColrCols(14) Where Colrand Colsare the total number of solar collector receiver in rows and in series, respectively. Fig. 6 compares the results of solar radiation simulation in this paper with two references (Moradi and Mehrpooya,2017;Sabziparvar,2008). 4.2. Boiler The boiler is one of the main equipment in a coal-fired power plant. In this work, the boiler type and structures are not discussed in detail and just related operating parameters such as temperature, pressure, enthalpy and mass flow rate of the working fluid are considered. The coal consumption rate can be calculated by using the energy balance as follows (Hong-juan et al., 2013): ˙ mcoal =˙ mms ×(hms −hmw)+˙ mrs ×(hro −hri) ˙ Qcoal ×ηb (15) where ˙ mcoal is the coal consumption rate (kg/h); ˙ mms and hms are the main stream mass flow rate (kg/h) and enthalpy (kJ/kg), respectively; hmwis inlet enthalpy to the boiler (kJ/kg); mmrs is the mass flow rate of the reheated steam (kg/h); hro and hri are the outlet and inlet enthalpy of the reheated steam (kJ/kg); qcoal is coal thermal energy (kJ/kg); ηbis the efficiency of boiler. 4.3. Turbine In the case of replacing the extracted steam with the solar thermal energy, the steam goes through the lower stage turbines which cause steam turbine is worked working at off design conditions (Montes et al.,2009). The turbine efficiency reduction rate can be calculated by using the stream flow ratio as follows (Hong-juan et al.,2013): Reduction (%) =0.191−0.409×(˙ m/˙ mref )+0.218×(˙ m/˙ mref )2(16) Where ˙ mis the at part load condition flow rates and ˙ mref is the design conditions flow rates. The variation of turbine efficiency is less than 1% even if the first stage extracted steam is totally replaced by the solar thermal energy. So, the steam turbine efficiency of the solar hybrid electrical power plant can be considered same as existing electrical power plant. 4.4. Feed water heaters Extraction steam elevates inlet feed water temperatures to the boiler. Therefore, the plant thermal efficiency is increased. The adopted model for the feed water heater is described as follows (Hong-juan et al.,2013): ˙ mfw,i×(hwo,i−hwi,i)=˙ mi×(hi,i−hd,i)+˙ md,i−1×(hd,i−1−hd,i) (17) where ˙ mfw,iis the ith heater feed water mass flow rate (kg/h); hwo,iand hwi,iare the ith inlet and outlet heater feed water enthalpy (kJ/kg), respectively; ˙ miand ˙ md,iare the ith extracted steam and drain water flow rate of the heater (kg/h)hi,iand hd,i are ith the extracted steam and drain water enthalpy of heater (kJ/kg). 4.5. Deaerator The deaerator helps purge oxygen from the feed water and controlling the corrosion. The heat balance equation is as follows (Hong-juan et al.,2013): ˙ mfw,o=˙ mcond +˙ mb+˙ mfw,i(18) 4.6. Model evaluation To evaluate the efficiency of the solar thermal energy utilization in the hybrid solar electrical power plant, the conversion solar heat energy to electricity efficiency (ηse) can be calculated as follows (Hong-juan et al.,2013): ηse =1000Ps ˙ QLd =1000 ×(PZ−˙ Qb.ηref ) ˙ QLd (19) Where Psis output electrical power by using solar thermal energy (kW), PZis total output power from the hybrid solar electrical power plant (kW), ˙ Qbis the heat load of the boiler (kW), ˙ QLd is the direct normal irradiance focused on the collector (kW) and ηref is the efficiency of reference electrical power plant which uses natural gas as fuel. The gross value of the electrical power generation in the hybrid solar power plant can be calculated as follows (Hong-juan et al.,2013): Nsolar =˙ Qsolar ×ηsteam (20) Where ηsteam is the thermal efficiency of the first stage extracted steam before usage, and ˙ Qsolar is solar thermal energy (kW). The solar thermal energy conversion to electricity index can be calculated as follows (Hong-juan et al.,2013): Nsolar =nsolar DNI ×S(21) Where DNI is the direct normal insolation (kW/m2), and Sis the aperture area of the solar collector recovers (m2). 5. Result and discussions The examined solar hybrid electrical power plant is situated in the northern area of Iran (Qazvin city). The electrical power plant nominal capacity is 250 MW. 225 MW of generated electrical power is sent to the grid network and the remaining 25 MW is used for the plant as the utility. As maximum capacity (rated load) of the electrical power plant is almost 250 MW, so the implemented scenarios do not impair the safety margins of the plant. The scenarios in which electrical power plant operates in fuel saving mode are safer due to this reason that the electrical power output is below the rated load value. The simulation results in the case of maximum solar field area for both case of electrical power boosting and fuel saving mode are presented. The maximum electrical power output (power