Performance comparison of basic and parallel double evaporator Organic Rankine Cycle integrated with

Abstract

Keywords: Performance comparison; ORC; PDORC; Solar Power Tower; Intercooled Cascade sCo2 Cycle

Introduction

Energy consumption used for the cooling and power generation increased drastically in recent years [1]. Consequently, fossil fuel consumption is continuously raising that leads to increase carbon emissions. However, fossil-fuel stocks are steadily decreasing. This leads to the challenge of finding clean and reliable energy resources [2]. There are currently numerous renewable energy resources being used to generate clean and ecofriendly power such as,

solar, wind, biomass, and geothermal energy. Because of its availability, low cost and noise free operation, solar energy is more suitable in power generation among these renewable energy resources [3]. Solar collectors are used to collect sunlight for cooling, heating, and power generation [4].Concentrated solar collectors like parabolic trough collectors, dish collectors, heliostats, etc. are used for high temperature applications. Kalogirou [5] recorded parabolic trough collector (60–500°C), dish collector (750–1000°C), and

*Corresponding author. *E-mail address: yuniskhan21@gmail.com This paper was recommended for publication in revised form by Regional Editor Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

heliostat (150–1500°C) working temperature range. Among other solar technologies, concentrated solar power (CSP) technology plays a key role in solving the current and future question of power generation in tropical countries through the use of solar heat that is readily accessible in nature [6]. The SPT is the latest technology amongst the various CSP technologies. SPT system consists of number of complex sub-systems such as receiver, 75-150 m high tower, thermal storage system (optional), heliostat field with per heliostat area of 50-150 m2 and power conversion system. Solar radiation is centered on receiver by heliostat field where it is used to produce high temperature heat for power generation through a high energy cycle or for industrial process supply [7]. Several studies were performed on the SPT driven cycles such as combined recompression sCO2 cycle and transcritical CO2 (carbon dioxide) cycle [8], sCO2 Brayton cycle [9], recompression sCO2 with and without main compressor intercooling [10], triple combined cycle [11], multi generation combine cycle [12]. The CO2 above its critical values (7.38 MPa, 30.9°C) is said to be sCO2. Further, sCO2 cycle is the cycle that can utilize the heat from various heat sources such as solar thermal energy, geothermal, coal power and natural gas [13]. A few researcher published articles on the basis of sCO2 such as Khan and Mishra [14] conducted a study on the solar parabolic trough collector integrated combined partial heating sCO2 cycle and ORC. They also investigated the effect of solar system on performance of the combined system. They came to the conclusion that adding ORC to the partial heating cycle increased thermal efficiency by 4.47%. The exergy efficiency of the combined system increased with the solar irradiation. Finally they also observed that R1233zd(E) is the best performing fluid among the other working fluids considered such as R1224 yd(Z), R1234ze(Z), R1234yf, R1234ze(E), and R1243zf. Also Khan and Mishra [15] compared standalone pre-compression sCO2 cycle with combined ORC integrated with the SPT. It was found that thermal efficiency and power output of the standalone pre-compression sCO2 cycle were improved by 4.51% and 4.52% respectively by addition of ORC as bottoming cycle. They also concluded that R227ea recovered highest heat among the considered working fluids. Besarati and Goswami [16] performed a comparative analysis of the various configurations of the sCO2 cycle such as recompression, simple recuperated, and partial cooling cycle. They first compared the systems standalone and used ORC as the bottom cycle thereafter to recover the waste heat from all three cycles and compared it again. They concluded that the combined cycle achieved more than 50% thermal efficiency as well as the recompression cycle being the best performing cycle among the cycles considered. Khan and Mishra [41] carried out exergoeconomic analysis of the SPT driven pre-compression sCO2 cycle and ORC. They observed R1336mzz(z) gave highest thermal efficiency,

exergy efficiency and power output at 950W/m2 of solar irradiation. Also in another study Khan and Mishra [42] performed thermal analysis of SPT driven recompression sCO2 cycle with main compression intercooling combined with bottoming ORC using low global warming potential fluids. They found that after the addition of the ORC thermal efficiency of the combined cycle improved by 7-8% with reference conditions. Kim et al [17] also performed a comparative analysis on 12 different sCO2 cycle configurations such as simple recuperated cycle, recompression, single heated cascade cycle, partial heating, dual cascade cycle, pre-compression, single heated cascade cycle with intercooler, intercooled dual heated cascade cycle, dual expansion cycle, dual heated and flow split cycles for triple heating cycle and partial recuperation cycle as land filling gas turbine bottoming. They found that pre-compression and recompression cycle had high thermal efficiency than other considered cycles. They also suggested for future research on combination of two cycles. Yu et al. [18] considered four configurations of the sCO2 cycles such as pre-compression, simple recuperated, recompression cycle, and split flow recompression cycle, for recovering the waste heat from the internal combustion engine. Further, thermal efficiency and waste heat recovery ratio of the four systems were determined. The maximum waste heat recovery ratio for pre-compression and sCO2 cycle recovery was found to be reached at 5.8 MPa and 7.65 MPa respectively. Furthermore, split flow cycle was found as best performing cycle with highest recovery ratio was 24.75%. Neises and Turchi [19] examined the cost, design, and performance of the three sCO2 cycle configurations, such as simple cycle, recompression cycle and partial cooling cycle, driven by the molten salt solar power tower. As a result, highest thermal efficiency was achieved in case of the recompression cycle followed by partial cooling and a simple cycle. Partial cooling was found to be more costly than other cycles due to the high demand for turbo machinery. One extra compressor and one intercooler are used in the cascade cycle with an intercooler [20] than in the single heated cascade cycle. Turbine work and compressor work are improved by the intercooling process, resulting in high thermal cycle efficiency compared to the basic cascade cycle for the same heat input [20]. As a result, due to the greater impact of increased thermal efficiency of the cycle than the decreased amount of absorbed heat, this cycle has 0.09MWe higher net produced work. This cycle, however, has not only more components than the partial heating cycle, but also comparable net work generated with the partial heating cycle [17]. Apart from this, ORC is the useful technology for the recovery of low temperature waste heat. ORC is used as a bottoming cycle in the current scenario. There are a few studies to support this comment such as Shaaban [21] carried out a solar integrated combined cycle study in which ORC and steam Rankine cycle were used as a bottoming

cycle for the recovery of waste heat. The author considered 15 working fluids in the ORC and found that R1234ze(Z) was the best fluid in terms of thermo-economic, environmental and safety considerations. Hoang [22] reviewed for application of the ORC as bottoming cycle to the exhaust waste heat recovery from diesel engines. They found that ORC can achieve up to 25% thermal efficiency, while combined system with diesel engine achieved up to 90% thermal efficiency. Song et al. [23] conducted a performance and optimization analysis of the combined cycle of sCO2 and ORC. The effects of topping cycle recuperative ratio, pressure ratio on the performance of the bottoming ORC were examined. They resulted in an improvement in the thermal efficiency of the existing sCO2 cycle by adding the ORC system. ORC system recovered completely residual heat. Apart from basic ORC, PDORC also suitable for recovering more waste heat as compared to basic ORC. PDORC produced more work than the basic ORC [24]. From the literature survey, it was observed that performance of the intercooled cascade sCO2 cycle can be enhanced further by incorporating a low temperature bottoming ORC. However, detailed thermodynamic analysis of SPT driven combined intercooled cascade sCO2 cycle and ORC is not present in the literature. Also no PDORC system was used as bottoming cycle in literature as waste heat recovery cycle. Therefore, present study deals with thermodynamic analysis of the combined cycle driven by solar power tower. Further, the performance of the standalone intercooled cascade sCO2

cycle was compared by incorporating the basic ORC (configuration-1) and PDORC (configuration-2) as bottoming cycle along with parametric analysis of the proposed combined cycles was also conducted and best configuration for waste heat recovery was found out. This demonstrates the novelty of the present research. A computer program in engineering equation solver software was made to simulate the system. Exergy, thermal efficiency, net output work were considered as performance parameters of the proposed systems. The effects of the system variable such as SPT design parameters (solar irradiation or direct normal irradiation (DNI), solar receiver emittance, concentration ratio), inlet temperature of the high temperature turbine, inlet temperature of compressors and inlet pressure of the compressor-1 on the system performance were investigated. At last waste heat recovery ratio for the both configurations was also investigated.

System Description

Current study deals with two configurations for comparison. First configuration consist three subsystems such as solar system, intercooled cascade sCO2 cycle [17] and bottoming basic ORC (configuration-1) as shown in Figure 1a. Another configuration also consist two subsystems same as first configuration but PDORC [24] as bottoming cycle (configuration-2) as shown in Figure 1b. Mixture of molten salt considered as the heat transfer fluid (HTF) in solar system. The sCO2 stream is flowing in the intercooled cascade

Figure 1a. Schematic diagram of SPT driven combined intercooled cascade sCO2and ORC (configuration-1).

Figure 1b.Schematic diagram of SPT driven combined intercooled cascade sCO2 with and PDORC (configuration-2) cycle. After taking the heat through the heat exchanger-1 (HEX1) sCO2 stream expanded in the high temperature turbine (HT). Expanded stream have much amount of heat therefore it is recuperated in the high temperature recuperator (HTR) then goes to the low temperature recuperator (LTR) after mixing with expanded stream coming from low temperature turbine (LT). After LTR some heat is still remain. This remaining heat is used by ORC through heat exchanger-2 (HEX2). Further, sCO2 stream compressed through the compressor-1 (Comp 1) then perfectly intercooled, after this stream again compressed in compressor-2 (Comp 2). A fraction of total mass of sCO2 stream goes to LTR and remaining goes to HEX1, this repeats again and again. In the configuration-2 instead of basic ORC, the PDORC is used for recovering the waste heat from HEX2 and intercooler simultaneously as shown in Figure 1b. After getting the heat through HEX2 (It is possible to call it evaporator-1) organic working fluid (HFO1234yf) stream is expanded in organic turbine-1 (OT1), expanded stream mixes with the stream that is coming out from the intercooler (It is possible to call it evaporator-2). Total mass of R1234yf stream is expanded in the organic turbine-2 (OT2). After the condenser HFO1234yf stream splits in two parts, one part goes to the intercooler through the pump-2 to recover remaining part of the waste heat. Another part goes to the HEX2 for recovering the waste heat. Thus PDORC completed and repeated again

and again. T-s diagrams of the intercooled cascade sCO2 cycle, basic ORC and PDORC corresponding to the state points are shown in Figures 2a, 2b, and 2c respectively.

Table 1. Input parameters for simulation of the proposed model Geometric and operating parameters for SPT

Thermodynamic Analysis

Assumptions Performance analysis of the SPT powered combined cycle was performed considering following assumptions to support the simulation; (1) All system components are in a steady state conditions. (2) Pressure and friction loss are neglected in each component and pipes. (3) All thermodynamic processes are polytropic. (4) Energy due to height and velocity of the each component is neglected

Pdorc OT1

balance Eq. (6) for each component after assuming no heat loss in the component [31]. After calculating the exergy destruction rate for each component, total exergy destruction rate for the combined cycle is calculated as; (17)

Furthermore, in this section exergy analysis of the combined system also to be discussed. Exergy destruction in each component is determined by applying the exergy

(23) (19) On the basis of the thermal modeling, numerous mathematical relations are used in the performance analysis of the solar power tower powered combined cycle have been discussed below; Combined cycle exergy efficiency is determined as [31,33]; (20)

(22) The combined cycle’s thermal efficiency can also be defined by the relation between thermal and exergy efficiency of the combined cycle [31];

Waste heat recovery ratio (WHRR) is a performance parameter is to be defined which represents the capacity to recover waste heat from the topping cycle. It is defined as the ratio of the net power output (net power output of ORC) to the maximum available waste heat to be recovered from waste heat source [34]. WHRR for the bottoming cycle is expressed as; (24) Where, h0 and hHEX2 are the enthalpy of waste heat of the topping cycle at environmental temperature and at the inlet of HEX2 respectively. is mass flow rate of the sCO2 flowing in topping cycle. Modeling equations of the SPT powered combined cycle were solved in engineering equation solver (EES) [35]. Validation of the proposed cycles In order to ensure the correct use of the modeling equation, previous studies were used to validate the current cycles. There is no availability of the literature on combined cascade sCO2 cycle with intercooling and ORC/ PDORC. Therefore, both toping and bottoming cycles

Table 4. Validation of different cycles Validation of toping cascade sCO2 cycle with intercooling

HT inlet pressure=27.46 MPa, HT inlet temperature = 494.17 °C, Compressor-1 inlet pressure = 6.57 MPa, Compressor-1 inlet temperature =36.8 °C, ηcomp 1 = 0.85, ηHT = 0.9

OT outlet pressure = 0.1515 MPa, ηOT = 0.6, ηPump = 0.5, OT inlet pressure = 3.395 MPa,

OT inlet temperature = 154.2 °C, OT outlet pressure= 0.2504 MPa, ηOT = 0.6, ηPump = 0.5

were validated separately with existing literature as listed in table 4. Thermal efficiency has been taken as the validation parameter for both the cycles. Cascade sCO2 with intercooling cycle was validated with the previous study Kim et al. [17]. Apart from this, bottoming ORC and PDORC were also validated with previous studies Clemente et al. [36] and Dai et al. [24] respectively at same baseline conditions respective to references. Thermal efficiency of the both topping and bottoming cycle was found nearly to the respective previous research as shown in table 4. However, present study was performed with input parameters that are different from the previous studies those were used for the validation purpose.

explanation behind this is that thermal efficiency is directly linked to exergy efficiency [31]. Increase in solar irradiation from 400 to 950 W/m2, the thermal efficiency of the standalone cycle, configuration-1 and configuration-2 were increased from 26.76% to 47.23%, 27.89% to 48.3% and 30.06% to 50.38% respectively.

Results And Discussion

Current study deals with the performance evaluation and comparison of the solar power tower driven cascade sCO2 cycle with intercooling with basic ORC and parallel double evaporator ORC (PDORC). Simulation of the current systems has been performed with the help of EES. Effects of the variable on the system performance have been investigated keeping constant all other variable as listed in table 1. System performance evaluation with solar irradiation The system’s thermal efficiency is also based on solar irradiation. Base condition of solar irradiation has been taken as 850 W/m2as per Indian climate at Mumbai. As the current combined model is powered by a solar power tower, the effects of solar irradiation on the efficiency of the system must therefore be investigated. With solar irradiation, the exergy efficiency of the combined cycle has been continuously increased. The explanation behind this is that the solar concentrator field efficiently utilizes increased solar irradiation. This corresponds to an increase in the inlet exergy of the combined cycle [33]. According to the left axis of Figure 3, maximum exergy efficiency for standalone cycle, configuration-1, and configuration-2 was 69.76%, 70.86%, and 72.98% for the 950 W/m2 of solar irradiation. At 950 W/m2 of solar irradiation, the exergy efficiency of the intercooled cascade sCO2 was enhanced by 1.57% and 4.61%, respectively, by integrating the basic ORC and the PDORC. It can be observed that improvement in the exergy efficiency with PDORC is more than the basic ORC. Because PDORC recovered more heat compared to the basic ORC due to the double evaporators. Additional heat was recovered from the intercooler by the incorporating the PDORC. With solar irradiation, net power output and thermal efficiency of the system have also increased. At 950 W/m2 of solar irradiation, configuration-2 achieved the highest thermal efficiency and net power output of 50.38% and 273.3 kW, respectively, as indicated in the right axis of Figures 3 and 4. The curve for thermal efficiency and power output has the same pattern as the curve for energy efficiency. The

Figure 3. Exergy efficiency and thermal efficiency variation with the solar irradiation.

Performance evaluation with solar emittance Solar emittance is the important parameter to be examined because it affects the receiver performance. It is seen in Figure 5, both exergy efficiency and thermal efficiency of the combined cycle decreases with the solar emittance. Receiver’s surface temperature is the function of the solar emittance. Receiver efficiency decreases with the solar emittance. That means more heat loss to the surrounding, consequently less heat available to the combined cycle. As a result, the combined cycle’s exergy efficiency and thermal efficiency are reduced. Increase in solar emittance from

0.05 to 0.2 reduces the exergy efficiency and thermal efficiency of configuration-2 by 3.57% and 6.28% respectively. As far as we know, all of the cycles operate under the identical input conditions, with the exception of bottoming cycles like ORC/PDORC. As a result, the curve pattern is consistent across all combinations. The similar effect of solar emittance was seen in the other two configurations as well.

Performance evaluation with HT inlet temperature The temperature of the molten salt has a direct impact on the HT inlet temperature (HTIT). HTIT rises in proportion to the molten salt’s temperature. The heat loss in the solar receiver will, however, increase as the temperature of the molten salt rises. This results in lower receiver efficiency [8]. However, in the present study, the parameters of the solar field are kept constant and shown in Table 1. The main objective is therefore to compare the thermal performance of the standalone cycle with configuration-1 and configuration-2. While investigating the effects of HTIT, other input parameters have been kept constant as shown in Table 1. The exergy efficiency, thermal efficiency and power output have increased with HTIT as can be seen in Figures 7 and 8 respectively. The reason behind increased efficiency and power output is that as HTIT increases the enthalpy difference across the HT, resulting in increased output power [15]. However, efficiency and output power have a different trend as HTIT increases, as shown in Figures 7 and 8.The highest thermal performance was achieved with configuration-2. At 800°C of

Figure 5. Exergy efficiency and thermal efficiency variation with solar emittance.

Performance evaluation with the concentration ratio Another receiver design parameter to consider is the concentration ratio, which has an impact on the combined system’s performance. As illustrated in Figure 6, increasing the concentration ratio increases combined cycle exergy efficiency and thermal efficiency. As the concentration ratio rises, the receiver efficiency rises, causing the HTF outlet temperature to rise, as the turbine inlet temperature is inversely proportional to the receiver outlet temperature. As a result, as the temperature of the turbine inlet grew, the combined cycle efficiency increased. The configuration-2 achieved the highest exergy efficiency and thermal efficiency once again. Exergy efficiency and thermal efficiency rise by 7.85% and 56.25%, respectively, when the concentration ratio is increased from 200 to 1400.

Figure 6. Exergy efficiency and thermal efficiency variation with the concentration ratio.

Figure 7. Exergy efficiency and thermal efficiency variation with inlet temperature of HT.

Figure 8. Power output variation with inlet temperature of HT.

HTIT, the highest exergy efficiency, thermal efficiency, and power production of configuration-2 were 71.22%, 51.43%, and 314.9 kW, respectively. Efficiency and output power are therefore highest with higher input thermodynamic variables. However, higher thermal performance cannot be achieved due to safety and material limitations. Although at same solar irradiation supercritical carbon dioxide cycle achieved higher efficiency than the superheated steam cycle at higher input turbine conditions [37,43].

at critical pressure, the highest density of CO2 results in decreased compression power [38]. As a result, the highest net power output and thus the best thermal and exergy efficiency are achieved. In the case of configuration-2, the best thermal performance was achieved. As shown in Figures 9 and 10, the configuration-2 achieved the highest exergy efficiency, thermal efficiency, and output power of 73%, 49.42%, and 269.4 kW at an optimum pressure of 7.74 MPa.

Performance evaluation with the compressor-1 inlet pressure It was observed that in the subcritical region where the pressure is below the critical value of 7.38 MPa the thermal and exergy efficiency increases with compressor-1 inlet pressure. Beyond the critical pressure of sCO2, thermal and exergy efficiency increase and thereafter decrease. This indicates that there is a pressure point at which net power output and efficiency are at their maximum values. Exergy efficiency, thermal efficiency, and output power of the system have a bell-shaped curve as can be seen in Figures 9 and 10 respectively. The explanation for this pattern is that

Performance evaluation with compressors inlet temperature The intercooling has been assumed to be perfect in this investigation. That is, following the compressor-1, the intercooler lowers the sCO2 stream temperature to the compressor-1’s inlet temperature. The compressor’s inlet temperature has an impact on system performance. The influence of the CsIT on system performance was investigated while the other input parameters were kept constant as shown in Table 1. The compressor-1 inlet pressure has been fixed at 7.5 MPa. The CsIT ranges from 32°C to 38°C. The thermal performances of the all configurations were decreased with the CsIT as shown in Figures 11 and 12. The

Figure 9. Exergy efficiency and thermal efficiency variation with compressor-1 inlet pressure.

Figure 11. Exergy efficiency and thermal efficiency variation with compressors inlet temperature.

Figure 10. Power output variation with compressor-1 inlet pressure.

Figure 12. Power output variation with compressors inlet temperature.

reduction in thermal performance of the system is due to the fact that as the temperature rises over the critical state of CO2, the CO2’s specific heat falls, resulting in a decreased enthalpy at the compressor-1 inlet [39]. This results in a greater enthalpy differential across the compressor, resulting in increased input power. As a result, the net power output is reduced. The heat available at the inlet to the combined cycle is known to be constant, depending on the SPT parameters [15]. As a result, the efficiency and net output of the combined cycle were reduced with the CsIT. It was also shown that the total decrease in thermal performance of the combined system was due to the standalone sCO2 cascade cycle. As seen due to increse in CsIT performance of bottoming ORC also sligtly decreased due to slight decrement in turbine output. Therefore, the performance of the bottoming ORC was not significantly affected by the variation of the CsIT. It is shown that configuration-2 perfomed well among other configurations. As a result, the configuration-2’s highest exergy efficiency, thermal efficiency, and power output were 70.17%, 47.51%, and 259 kW at 32°C of CsIT, respectively, as shown in Figures 11 and 12. Effects on waste heat recovery ratio Main objective of the current study is to compare the effect of basic ORC (configuration-1) and PDORC (configuration-2) to recover the waste heat. That means which system is better to recover the waste heat configuration-1 or configuration-2. WHRR is for the configuration-2 is more than the configuration-1. That means PDORC recovered more heat than basic ORC. At 0.95 effectiveness of HEX2, the highest WHRR for configuration-1 and configuration-2 was 0.1372 and 0.2016, respectively. As a result, the PDORC recovers 46.93% more heat than the ORC, as shown in Figure 13. With the effectiveness of HEX2, WHRR increased continuously. As the recovery of waste heat in HEX2 is increased with effectiveness, the increase in WHRR with HEX2 effectiveness can be clarified. As a result, more enthalpy leads to improved work output across

Table.5. Optimum major results calculated at 950 W/m2 of DNI Parameters

the ORC turbine [15]. As a result, according to the Eq. (24), WHRR increased. Effect of the effectiveness of the LTR on the performance of the ORC was also examined further in this section. WHRR was slightly increased with the effectiveness of the LTR as shown in Figure 14. The reason behind this is that when the effectiveness of the LTR is increased, more heat is recovered by the cold stream of the sCO2. This leads to a lower sCO2 temperature at inlet to HEX2. In other words, the low heat at the inlet to the HEX2 can be said. It reduces the inlet temperature of the ORC turbine. As a result, the lower inlet temperature of the ORC turbine increases the output power of the ORC turbine in case of organic working fluids [40]. On the other hand at inlet of HEX2 enthalpy also decreases because of lower heat at the HEX2 inlet. As a result, WHRR increased with the LTR effectiveness according to the Eq. (24). WHRR for the configuration-2 is more than the configuration-1. At 0.95 LTR effectiveness, the maximum WHRR for configuration-1 and configuration-2 was 0.1197 and 0.1775, respectively, as shown in Figure 14. Apart from all these, calculated major results from result ant discussion section are listed in the table 5 for better understanding.

Conclusions

From the result and discussions sections following conclusions were made. • Thermal performance of system increased with solar irradiation. The standalone cycle, configuration-1 and configuration-2 show maximum exergy efficiency of 69.76%, 70.86% and 72.98% respectively at the 950 W/m2 of solar irradiation. • By incorporating the basic ORC and the PDORC to the intercooled cascade sCO2 cycle, exergy efficiency were improved by 1.57% and 4.61% while thermal efficiency improved by 2.26% and 6.66% respectively at 950 W/m2 of solar irradiation. • System performance falls as solar emittance increases, but increases when the concentration ratio increases. As a result, in order to improve the combined cycle’s performance, the solar emittance must be reduced and the concentration ratio must be increased. • The highest exergy efficiency, thermal efficiency and output power of the configuration-2 were achieved as 73%, 49.42% and 269.4 kW at an optimum pressure of 7.74 MPa of compressor-1 inlet pressure. • At 0.95 of LTR effectiveness, the maximum WHRR for configuration-1 and configuration-2 was 0.1197 and 0.1775, respectively. • The PDORC recovered 48.28% more waste heat than the basic ORC. Therefore, configuration-2 is more suitable to recover the waste heat completely. • Finally, it was highly recommended to design the SPT system carefully to get better performance of the combined cycle.

Nomenclature

Heliostat area (m2) Direct normal irradiation (W/m2) Actual solar heat received by heliostat field (kW) Solar exergy (kW) Exergy rate (kW) Receiver’s view factor Coefficient for convection heat loss (W/ m2-K) specific entropy (kJ/kg-K) Number of heliostat Mass flow rate (kg/s) Solar heat received by heliostat field (kW) Power (kW) specific enthalpy (kJ/kg) Temperature (K) Specific heat (kJ/kg-K) Supercritical carbon dioxide Rate of exergy destruction (kW) Receiver surface temperature (K) Heat rate in (kW)

solar power tower Heliostat efficiency Thermal efficiency Fraction of mass of sCO2 Exergy efficiency Receiver thermal efficiency Heat loss from the receiver (kW) Heat received by central receiver (kW)

Abbreviations

Comp 1 Comp 2 Cond Conf 1 Conf 2 CR CsIT CV HEX1 HEX2 HT HTIT HTR LT LTR ORC OT OT1 OT2

Pdorc WHRR

Compressor-1 Compressor-2 condenser configuration-1 configuration-2 concentration ratio Compressors inlet temperature (°C) Control volume heat exchanger-1 heat exchanger-2 High temperature turbine High turbine inlet temperature (°C) High temperature recuperator Low temperature turbine Low temperature recuperator Organic Rankine cycle ORC turbine ORC turbine-1 ORC turbine-2 Parallel double evaporator ORC Waste heat recovery ratio

exit dead condition receiver heliostat inlet particular atate Sun molten salt

Efficiency Effectiveness Solar absorbance Change in property Sun’s subtended cone half angle(rad) Stephen Boltzmann constant (W/m)

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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