Exergy analysis of a hybrid solar‐fossil fuel power plant

In this study, exergy analysis, energy analysis, and mathematical modeling are performed in a 35 MW solar‐fossil fuel power plant. The losses of exergy and energy in different components and also changes of the efficiency of exergy and energy are analyzed at a specific day, 20th June. The assumed power plant in this study is Solar Electric Generating Station VI (SEGS VI), located in California's Mojave Desert. A parametric study, under different working conditions, including different working pressures, temperatures, collector output temperature, steam flow rate, and heat transfer fluid (HTF) flow rate is studied and the effect of variation of parameters on the performance of the plant is investigated. Authors found that, the maximum exergy loss happens in the collector and the maximum energy loss occurs in the condenser. Energy analysis shows that 47% of the total loss energy in the cycle happens in the condenser, as the main component that wastes energy. From exergy analysis, the collector and then boiler are the main components wasting exergy where 68.32% of total exergy loss occurs in these two components in hybrid mode (solar‐fossil fuel). Exergy and Energy efficiency variations throughout the day show that minimum exergy efficiency (32.7%) and maximum energy efficiency (23%) occurs at 12 am. Exergy efficiency variation versus turbine inlet pressure shows that the maximum exergy efficiency (26%) accure at 95 bar. The changes of the absorbed heat and solar irradiation of the 20th of June shows a good agreement with the measured data in validated reference.


| INTRODUCTION
The energy received by the Earth from the sun in 1 day, provides energy for more than 20 years. Because the amount of energy that comes from the sun to Earth's surface is 120 × 10 5 . 1 Solar energy development can expand the level of energy security because it is an energy source that is independent and infinite. In addition, the use of solar energy reduces the environmental impacts. 2,3 Concentrating solar thermal power plant (CSP) is one of the best choices among clean energies. Also, it can be easily coupled with a fossil fuel boiler to solve variability problem. Wang et al 4 studied the future strategy for using concentrating solar power in China. Duan et al 5 Simulated a solar collector connected to an auxilliary fossil fuel heater to generate power and reduce fossil fuel consumption. Behar et al 6 developed a novel parabolic trough collector and they compared obtained results with experimental data. Bellos and Tzivanidis 7 studied | 147 VAKILABADI et AL. exergy destruction in a parabolic trough collector. They used air and therminol oil as heat transfer fluid (HTF) and concluded when air is HTF, maximum exergy efficiency is 25.62% while for therminol oil is 31.67%. Many advances in the last two decades were performed in the sense of CSP to be cost effective. 8,9 Alashkar et al 10 analyzed a solar power plant economically. The energy analysis obtained from the first thermodynamic law, gives quantitative evaluation of different energy losses taking place in all the components. Therefore, exergy analysis is needed to analyze energy losses qualitatively as well as quantitatively. 11, 12 Ibrahim and Rahman 13 studied a combined power plant with thermal analysis. Mehrpooya et al 14 investigated a solar chimney power plant with exergy analysis. They calculated the power output of the power plant throughout the year and concluded that exergy efficiency is varied from 3.5% to 93.3%. Abuelnuor et al 15 studied exergy analysis on a combined cycle power plant. Sukumaran 16 applied exergy analysis on a solar powered airport. Fontalvo et al 17 investigated a power plant that its power generation system is the same as present study and is intended as a hybrid power plant. Sheu and Mitsos 18 optimized a hybrid power plant. At this power plant, fossil fuel boiler and solar collector provide the heat needed for the power plant and is considered as a hybrid fossil-solar power plant. Peng et al 19 analyzed a hybrid fossil-solar power plant. In this study, fossil fuel boiler and solar collectors simultaneously provide the heat needed for the power plant. In 2016, Ahmadi and Toghraie 20 performed exergy and energy analysis on a thermal power plant. They used energy analysis and showed that 69.8% of the total lost energy occurs in the condenser. Also with exergy analysis, they concluded 85.66% of the total exergy is lost in boiler. Luyao et al 21 performed exergy analysis for a regenerative turbine in a power plant with a capacity of 1000 MW. They showed exergy destruction in the regenerative turbine system decreases with increasing output power of plant. Eboh et al 22 investigated a new model for calculating the chemical exergy of solid waste. Aljundi 23 studied exergy analysis for a steam power plant. The primary goal of this study was to analyze the various components to determine which component has the most exergy destruction. They concluded that the maximum energy dissipation happens in the condenser and the maximum exergy destruction occurs in the boiler. Ali et al 24 investigated a combined power plant with exergy analysis. They showed that the maximum exergy destruction occurs in the boiler and maximum heat loss happens in the condenser. Regulagadda 25 analyzed exergy destruction in the different components in a power plant. They found the maximum exergy destruction is in boiler. Pattanayak et al 26 performed exergy analysis on a combined power plant. They calculated exergy destruction in different components and concluded that boiler has the most exergy destruction. Also Sengupta et al 27 and Zhao et al 28 in separate studies, calculated exergy destruction in different components and showed that the maximum exergy destruction occurs in the boiler. Ameri et al 29 in 2008 Carried out an exergy analysis for a thermal power plant. In 2017, Ibrahim et al 30 modeled thermal operation of gas turbine power plant with exergy analysis. They calculated exergy destruction for the boiler, the gas turbine and the compressor. They showed that exergy efficiency of combustion chamber, air compressor and gas turbine are 67.5%, 94.9%, and 92%, respectively. In 2017, Sharma and Singh 31 studied the influence of different variables such as fin density on exergy analysis in HRSG. They investigated exergy losses in a low pressure evaporator and concluded that fin density has low effect on exergy efficiency in HRSG. Exergy analysis in shell and helically coiled tube heat exchangers was investigated by Alimoradi. 32 He considered water as a heat transfer fluid in shell and tubes. Geometrical variables effects on the exergy destruction were studied and the results showed that, with 50% increase of coil radius, it causes a 10.7% reduction in the exergy efficiency. In addition, a 50% increase in the coil length causes a 8.9% reduction in the exergy efficiency. Yildirim and Genc 33 applied the first and second laws of thermodynamic analysis for a milk powder manufacture process. The results showed that the first and second law efficiencies for the overall system are 85.4% and 57.45%, respectively. In 2017, Vandani et al 34 investigated exergoeconomic effect of an additional feed water heater after condensate pump, in a steam cycle. The results showed that increasing the feed water in this cycle, the final cost rate will decrease by 0.16% and the exergy efficiency increase by 0.33%. Mohtaram et al 35 investigated the effect of compressor pressure ratio on ammonia water combined cycle. Bellos et al 36 performed a parametric study on a gas turbine coupled with solar collectors. They analyzed gas turbine and solar collectors separately and results showed that the usage of collector in gas turbine leads to 64% fuel saving, but decreases by 2.8% electricity production. In 2016, Lior 37 studied the exergy and energy analysis of hydrofractured shale gas. The study calculated the amount of exergy loss in each component and showed that exergy loss increases with time. Also, in the same year, Hofmann and Tsatsaronis 38 performed exergy analysis on a binary Rankine cycle. In this study, exergy analysis was completed on a double Rankine cycle with the working fluids of Potassium and water. They concluded that the amount of efficiency of a Binary Rankine process is considerably higher than a conventional coal-fired. Neri et al 39 calculated the solar radiation exergy from the real radiation data. In this study, several methods were investigated in order to increase power efficiency with solar radiation. Srinivas and Reddy 40 optimized a cogeneration plant. In this study, the coupling of the Kalina cycle and Vapor absorption refrigerator system, a simultaneous cooling unit was developed. The plant characteristics were studied by changing the ratio of mass fraction to produce cooling or power generation; this study showed that the characteristics are optimal for a mass fraction ratio of 45%. Also, the characteristic power and characteristic cooling in these conditions are 62 and 72 kJ/kg, respectively. Gonca 41 studied exergy analysis of a gas turbine unit with two reheaters and two intercoolers. The effect of design parameters, turbine pressure ratio on the operation characteristics and exergy efficiency, has been studied. The research concluded that exergy efficiency rises with increasing the turbine pressure ratio. Hadizadeh et al 42 improved efficiency of steam power plants applying chillers. They concluded that the steam extraction of high pressure turbine approach is more efficient than other approaches. Khaliq and Kaushik 43 evaluated the second law of thermodynamics for the combined cycle of Rankine and Brayton. They found that exergy destruction in combustion chamber is over 50% of the total exergy destruction in the overall cycle. Reddy et al 44 studied the exergy analysis and performance evaluation on a power plant with 50 MW capacity. They studied the effects of pressure and plant location on the exergy efficiency and showed that exergy efficiency increases by 1.51% with increasing pressure from 90 to 105 bar. Singh and Kaushik 45 studied a fossil fuel steam power plant. They understand that maximum exergy destruction occurs in boiler, which is about 62.03%, and that it decreases with the reduction of the excess air combustion or reduction of the gas outlet temperature. Exergy analysis in a solar power plant is investigated by Al-Sulaiman. 46 He showed that more than 50% of the inlet exergy dissipates in collector, and 70% of the total exergy loss happens in collector. Velmurugan et al 47 studied energy and exergy analysis for solar heaters with different geometries and concluded that wire mesh dual-pass solar air heater has the higher efficiency. In 2016, Deniz 47 did an exergy and energy analysis on a desalination system with a flat solar collector. Its results showed that the maximum exergy and energy efficiencies for optimum flow rate values of 2.76% and 48.1%, respectively. In 2016, Zhu et al 48 performed studies on exergy analysis of a solar tower coupled with a fossil fuel power generator. In this study, exergy performance and exergy losses analysis were done for each component. Results showed that the boiler and solar tower have minimum exergy performances that together the exergy losses of these two components constitute 85% of the total exergy losses of all components. In 2017, Terhan and Comakli 49 studied energy and exergy analysis of boilers with gas as fuel. They found out that, the part with the most irreversibility in boilers is the combustion chamber. In 2015, Zheng et al 50 analyzed thermodynamically a thermal power plant with a solar tower. They studied the effects of various parameters on the exergy performance of a power plant. They concluded that with the rise of the operating temperature of the receiver, the exergy and energy performance would be increased till an optimum temperature and then after that it reduces. Said et al investigated using of nanoparticles in Heat Transfer Fluid (HTF).
They concluded with nanoparticles, the temperature difference between the HTF and the environment decreases and exergy destruction in the collector is reduced. 51 The aim of this research is to perform a dynamic simulation and Exergy analysis on SEGS VI power plant. This is a solar-fossil fuel power plant. In dynamic simulation, the amount of absorbed sun radiation, the effect of flow rate and temperature of oil on the power plant performance, effect of turbine inlet pressure on power plant output power is carried out. Due to the variability of the sun's radiation, in order to produce a constant power for the power plant, the variable boiler fuel consumption is considered. To improve the performance of the power plant, it is necessary to find the components with the highest exergy destruction. With Exergy analysis, exergy destruction in different components and their effects on power plant performance is analyzed. The effect of turbine inlet pressure, oil flow rate, and oil temperature on exergy efficiency is investigated. Figure 1 schematically shows the entire cycle of solar-fossil fuel power plant. This cycle includes several parabolic trough collectors, auxiliary heater (boiler) and steam power cycle. The existence of a boiler is essential, in order to assure sustainable working conditions in steam cycle and the supply of a stable output power throughout the day.

| Modeling of parabolic trough collector
In this study, the modeled collectors are LS-2 manufactured by LUZ Company (Albuquerque, NM, USA). In the previous study, Ashouri et al, 52 Stuetzle et al 53 and Fontalvo 17 used this type of collector in their research. Therminol oil VP-1 is considered as heat transfer fluid (HTF). Therminol oil VP-1 is a common fluid for transferring heat that extensively being used in industry. Its specific heat capacity is written as follows 54 : where in the Equation 1 T is in Celsius and C p is in J/kg k.
Flux of heat absorbed by collector (S b ) depends on four parameters: (a) the amount of direct normal radiation (Ib). (b) Optical efficiency from radiation characteristics (ϵ opt ). (c) Different amount of dirt on the mirrors (γ). (d) Incident angle (θ). Flux of heat absorbed by collector is calculated as follow 55 : The direct normal radiation (I b ), can be obtained from Typical Meteorological Year (TMY) file that is presented by National Solar Radiation Database. 56 γ is a factor that is different for each day and is depend on dirty of collector surface. In this study, γ is assumed to 0.95. F(θ) is incidence modifier function that describe dependence of θ to S b and is obtained from the expression below 55 : In the condition that cos(θ) > 0.9: And if cos(θ) < 0.9, the corresponding equation is as follows: ϵ opt is optical efficiency from radiation characteristics. In the annulus between the absorber tube and the glass envelope, the radiation, which is not absorbed by the absorber but instead reflected back to the glass envelope, is partially reflected at the glass envelope back to the absorber again where part of it may now be absorbed. That's why there is a slight increase of the absorbed solar beam radiation compared to the radiation from single absorption, accounted through the definition of the transmittance-absorptance product (τα). For most practical solar collectors, a reasonable approximation is as below 55 : That ρ, τ, and α are constants and are equal to 0.95, 0.915, and 0.94, respectively.
θ is incident angle. That is the angle between the line perpendicular to the collector and the solar beam radiation vector. Cosine of this angle is calculated as follow 55 : That in Equation 6, θ z , ω, and δ define the position of the sun throughout the day. θ z is Zenith angle. It is projection of incidence angle onto a horizontal plane. 55 ω is the angular displacement of the sun east or west of the local meridian due to rotation of the earth on its axis at 15° per hour. 55 (3) Schematic of a 35 MW solar-fossil fuel power plant δ is declination angle, the angular position of the sun when the sun is on the local meridian and is defined as follows. 55 In above equation, N is the day of year, is 1 for January 1st and is 365 for December 31st.
ϕ is the latitude angle, the location of SEGS VI plant is φ = 35°N. t s is solar time. Time based on the apparent angular motion of the sun across the sky. 57 L st and L loc are standard longitude and local longitude, respectively. The SEGS VI plant is located at the standard longitude and local longitude of 120°W and 117.022°W, respectively.
The parameter E t is the equation of time, which takes into account the perturbations in the earth's rate of rotation which affect the time the sun crosses the observer's meridian 57 : D is the day angle and is defined from the below equation 57 : In this plant, there are 50 collectors that are connected in parallel. Each collector is made up of 16 smaller collectors. These 16 collectors are connected in series. Due to the high variation in oil temperature from entering to the first collector till exiting from the 16th collector, there is a great deal of error if we want to obtain the heat loss of the oil using the average input and output temperature. Thus, we need to divide the total length of the absorber tube into smaller segments and obtain the heat loss by using the average input and output temperature of each segment and with the summation of heat loss in each segment; the output oil temperature from the 16th collector is obtained. With this method the error is very low. The received heat for each segment is: T 1 is the HTF temperature inside the absorbent tube and T 6 is the environment temperature.
A r and A a are defined as follow: That W a , D o , D i, and L c are aperture width of collector, outlet diameter of absorber, outlet diameter of envelope, and Length of element of absorber tube, respectively. In Equation 13, the first term is the absorbed heat by Absorber and the second term is the heat loss from that. F R is calculated as follows 58 : U L is calculated based on heat loss for each segment between absorber tube and environment. That: Now, for calculation of U L , T 3, and T 4 are needed. To obtain these temperatures, it is necessary to obtain temperature of different layers in Figure 2 that is calculated in the Appendix.
With energy balance equation, the useful absorbed heat by the oil is obtained as below: In other to calculate the transferred heat from absorber to the HTF, collector is divided into small elements. The heat loss to the environment is calculated with high precision.
In Figure 2, different heat transfer mechanisms, conduction, convection, and radiation, that occur in absorber tube are depicted. The requested characteristics of PTC have been mentioned in the table 1: The assumptions made in order to modeling the collector are given as follow: 1. The space between the absorber tube and the envelope is filled by air. 2. The glass envelope is assumed to have no radial temperature gradients. 3. γ is assumed to be 0.95. This is a factor varying from 1 day to the other. Different amount of dirt on the mirrors and the number of broken collectors influence this factor.

Ambient temperature and pressure is assumed to be 25°C
and 100 kPa, respectively.

| Steam cycle
In order to derive the thermodynamic characteristics of the steam cycle, energy, and mass balance is written for each component.
In Figure 3, a schematic of the steam section of the power plant is shown.
Some assumptions are considered in order to simplify the model: 1. It is assumed that the system performance is time independent. 2. The pressure loss is negligible in the pipes.
3. Heat loss is assumed to be zero in the pipes, pumps and turbines. 4. The output stream from the condenser is assumed as saturated liquid.
The power generated by turbine is calculated as follows: where h 1 , h 2 , h 3 , h 4 are calculated with regard to the high pressure and low pressure turbine efficiency as follows: And for pumps, the efficiency is defined as follow: The balance of energy for LPFH: The enthalpy h 23 can be estimated using the following energy balance: To calculate h 3 , the energy balance in the open feed water heater (dearator) can be rewritten as follows: HPFH is a closed feedwater heater and energy conservation is written as follow: The drain enthalpy, h 21 , is calculated from an effectiveness equation for HPFH: The incoming stream to the condenser, h 20 , is defined as Equation 52.
The amount of heat that is absorbed by the condenser of the cycle is calculated as follows: Isentropic efficiency of turbines and pumps are shown in the Table 2.
The mathematical model is carried out by MATLAB and the thermodynamic properties of the working fluids are obtained from REFPROP 8.0. 62

| Backup energy modeling of the power plant
All CSP power plants require a backup system. This backup system is used to initiate the system, or shut it down, or to be used in order to provide a constant output and increase the operational hours of the power plant. In this power plant, the backup system is a heater with a natural fuel gas. During the time that solar energy cannot provide the heat to supply superheated vapor, this system connects to the circuit in parallel. Hence, no heat storage energy is needed.
where ̇q Aux is the heat given to the fluid by the fossil-fuelfired furnace and T out is the fluid temperature after heat absorbing, T in is the fluid temperature before entering into the backup system.
In this solar-fossil power plant the existing boiler supplies the required energy conditions of insufficient solar energy.

| Energy analysis
In this analysis, the heat loss from different components will be studied and also for a fossil-solar power plant the amount of absorbed energy will be analyzed. Energy balance in an energy system is written as below:

Turbine Efficiency
High

| Exergy analysis
Exergy analysis calculates the amount of destruction of the useful work and the irreversibility of each component. In this paper, exergy analysis for a 35 MW solar fossil power plant, the amount of exergy loss in each component and also the amount of the total exergy performance at different hours are evaluated. For a single component, the balance of exergy is written as follows: That: In Equation 60, T r is the temperature of heat source. By neglecting the potential and kinetic exergy, the flow exergy is equal to the physical exergy.
Exergy in each stream defined as: Exergy performance in solar application is the total output work divided by exergy received from the sun: where ∑Ė in is the sum of the sun exergy and the auxiliary heater exergy. The sun exergy is calculated by the following equation 63 : In Equation 63, T sun is the temperature of sun surface that is considered 6000 K.
The rate of the exergy destruction for each component Y D is written as below 46 : The contribution of each component in total exergy destruction is defined as follow 46 : The potential to improve a component to be assessed as follows 46 :

| DISCUSSION
In this study, exergy analysis is performed on a SEGS VI solar-fossil power plant that to the authors' knowledge, there is no data available in the literature which evaluates exergy analysis of the SEGS VI power plant. The amount of exergy destruction is calculated in different components and the effect of various parameters such as turbine inlet pressure and HTF flow rate on exergy efficiency and output power is done. So it is a useful study to optimize the performance of the SEGS VI power plant. According to the author's knowledge, the amount of irreversibility in each component and effect of key parameters on the exergy efficiency has not been studied in any research (as shown in Figure 11). Stuetzle et al 53 studied automatic control for SEGS VI power plant. Saracoglu 64 modeled SEGS VI power plant with SAM (Solar Advisory Model) software. Rolim et al 65 performed an analytical model for SEGS VI power plant. To apply energy and exergy analyzes, the power plant was simulated using MATLAB and REFPROP softwares. Then, having the thermodynamic properties for each stream, the exergy and energy destruction was calculated in each component.
In Figure 4, the changes of the fuel flow rate, exergy efficiency and energy efficiency are shown. When an insufficient solar energy is available, an auxiliary heater with fossil fuel supplies the required energy. As it is shown, in the hours of the day that the solar energy is insufficient, fuel flow rate is higher. Also in Figure 4, energy performance of power plant on the 20th of June is shown. It is shown that solar mode energy efficiency of power plant is lower than fossil fuel mode. By increasing the amount of solar radiation, the amount of energy dissipation increases due to rising HTF temperature. Therefore, the energy efficiency is low in the hours with the highest radiation. In the hours that most of the required thermal energy is provided through the auxilliary heater, the energy efficiency is higher due to the proper insulation for the heater. The exergy efficiency is described as the output work of the power plant divided by the gained solar and fossil fuel energy of the plant. With the change of the absorbed energy by the parabolic trough collector and the fuel flow rate throughout the day, the power plant exergy efficiency would be changed. As seen in Figure 4, during daytime hours the amount of the absorbed solar energy is maximum, the exergy efficiency during those hours are minimal. This means that the existing collectors only absorb a specific amount of solar energy. Hence, in order to optimize the power plant at first, the amount of solar energy absorbed by the collectors must be increased. In the hours that the plant uses the boiler, as a result of high performance of fossil fuel, the amount of exergy performance of the power plant is high.
Exergy loss of each component is shown in Figure 5. As shown in Figure 5, the loss of exergy in the solar collector has the maximum value due to high temperature difference between HTF and environment. Hence, optimization of this component has the greatest impact on exergy efficiency and a good design of solar collector is needed to reduce the losses. After the collector, the greatest exergy loss happens in the heat exchanger, and the condenser, respectively. The large exergy destruction in the heat exchanger and the condenser is due to the high temperature difference and high energy dissipation in these components.
Exergy losses in pumps are negligible due to the generated pressure difference in the fluid.  In Figure 6 the exergy and energy performances for different components in solar mode are shown. The minimum exergy efficiency happens in the collector, because the oil is heated and its entropy increases. Therefore according to definition of exergy, its exergy efficiency decreases. The minimum energy efficiency occurs in the condenser, because vapor is converted to saturated water. Therefore it has the highest energy dissipation and thus the lowest energy efficiency.
In Table 3, the exergy loss in main components of a power plant is shown when an axillary boiler and solar energy are used simultaneously. During the simultaneous use of two heat sources, the maximum exergy loss happens in collector and boiler. The cause of this exergy loss in these two components is the high temperature gradient in these components. Therefore, for optimizing the power plant, first these two components should be thermodynamically optimized. Figure 7 shows the output power and efficiency of the second law of thermodynamics in terms of different inlet pressures of the high pressure turbine. In this diagram, when turbine inlet pressure increases, exergy efficiency increases initially and then decreases. As turbine input pressure increases, the input temperature to the turbine and the output power increases simultaneously. Exergy efficiency increases with increasing output power, but with the rising the entrance temperature to the turbine, entropy of the steam stream increases. So the efficiency of the exergy reduces. Thus, initially with increasing turbine inlet pressure, the increase in the amount of output power is dominated by the increase in the input temperature to the turbine. But after a pressure of about 95.8 MPa, the increase in turbine inlet temperature prevails over the output power. Therefore, the variation in exergy efficiency will be on the upside and then on the downside, based on the turbine input pressure.
In Figure 8 Changes of the output power and output temperature from collector at different hours are shown, during the 24 hours of the study time period when auxiliary heater is off. With rising the amount of sun radiation, the exergy rate of the exhaust stream from the collector increases, so the amount of useful energy that is given to the steam cycle rises. The increase in the useful energy provided to the steam cycle results in an increase in the exergy of the steam entering the turbine, which ultimately produces more power. It also appears in Figure 8, in the hours of the day that the radiation is high, the output temperature of the collector increases which is due to the large amount of radiation entering to the collector. In this figure, it is understood that in the period of time that the sun's radiation is the maximum value, one would have the maximum output power.
In Figure 9, the output power variation in term of HTF flow rate is shown. With increasing the flow rate of HTF, the amount of entrance exergy to the heat exchanger rises which gives more exergy to the steam cycle which leads to an increase in the output power. Also in Figure 10, the exergy efficiency is a linear function of the HTF flow rate that with the rise of the flow rare the exergy efficiency augments. By increasing the HTF flow rate, the exergy value of output HTF stream from the collector increases. Thus the total exergy efficiency, which is proportional to the exergy value of the output HTF stream from the collector, is rised. Also it is appears from Figure 10, in a constant HTF flow rate, exergy efficiency increases with rising HTF temperature. Because with increasing the HTF temperature, the amount of stream energy is rised and according to the definition of exergy, the amount of exergy efficiency increases.
In solar collectors, only a certain amount of the existed solar energy would be absorbed by the collector, and that  the absorbed energy varies along the day. In addition, the amount of the available solar energy will vary according to time. These changes are shown in Figure 11 and show a good agreement with the existing data.

| CONCLUSION
In this research, dynamic simulation and Exergy analysis of a solar-fossil fuel power plant is studied. In dynamic simulation, the amount of absorbed sun radiation, the effect of flow rate and temperature of oil on the power plant performance, effect of turbine inlet pressure on power plant output power is carried out. Due to the variability of the sun's radiation, in order to produce a constant power for the power plant, the boiler fuel consumption is also variable. The simulation results show that maximum boiler consumption is 1.8 kg/s. Exergy analysis, exergy destruction in different components and their effects on power plant performance are analyzed. To improve the performance of the power plant, it is necessary to find the components with the highest exergy destruction. The results show that collector has the highest exergy destruction among all of components. Therefore, in order to rise the performance of solar thermal power plants, one should focus on this component (solar collector). The steam cycle has little impact on the entire exergy loss, so by changing the physical and optical properties of the collector, the exergy efficiency and cycle thermal efficiency of the cycle will be increased. The effect of turbine inlet pressure on exergy efficiency shows that when this pressure is 95.8 bar, the exergy efficiency is maximum. Variations of Exergy efficiency in the length of day show that the minimum exergy efficiency (32.78%) occurs when we have the highest radiation levels. And when there is no solar radiation in the night, fossil fuel-fired auxiliary boiler, and exergy efficiency is in its highest value (44.94%). Also, variation of flow rate and temperature of oil on exergy efficiency shows that increasing oil flow rate and oil temperature, exergy efficiency increases linearly. The results have been compared with valid results, which have a good agreement.