Energy, Exergic and Economic Analyses of a Novel Hybrid Solar–Gas System for Producing Electrical Power and Cooling

Abstract

This paper aims to evaluate the feasibility and performances of a novel hybrid solar–gas system, which provides electric power and cooling. By using Ebsilon (V15.0) software, the operation, advanced exergic and economic analyses of this hybrid system are conducted. The analysis results show that the total electric power and energy efficiency of the hybrid system are 96.0 MW and 45.8%. The solar energy system contributes an electric power of 9.0 MW. The maximum cooling load is 69.66 MW. The exergic loss and exergic efficiency of the whole hybrid system are 119.1 MW and 44.6%. The combustion chamber (CC) has the maximum exergic loss (56.5 MW). The exergic loss and exergic efficiency of the solar direct steam generator (SDSG) are 28.5 MW and 36.2%. For the air compressor (AC), CC, heat recovery steam generator (HRSG) and refrigeration system (CSS), a considerable part of the exergic loss is exogenous. The avoidable exergic loss of the CC is 11.69 MW. For the SDSG, there is almost no avoidable exergic loss. Economic analysis shows that for the hybrid system, the levelized cost of energy is 0.08125 USD/kWh, and the dynamic recycling cycle is 5.8 years, revealing certain economic feasibility. The results of this paper will contribute to the future research and development of solar–gas hybrid utilization technology to a certain extent.

1. Introduction

To meet the needs of the energy transition, solar energy [1,2,3,4,5] and solar-based multi-energy systems [6,7,8,9,10] have been well studied and developed. By contrast, the integrated solar–gas combined cycle (ISCC) technology is relatively more mature as many related theoretical, simulation and experimental studies have been launched, and thus some different kinds of ISCC systems have been proposed.

The first law of thermodynamics pays more attention to the quantity of energy, the second law of thermodynamics pays more attention to the quality of energy while the exergic analysis can present both the quantity and quality of energy simultaneously. By using the exergic analysis, the exergic destruction and exergic efficiency of every component of a system can be obtained. Therefore, the exergic analysis approach becomes a useful thermodynamic analysis method [11].

The traditional exergic analysis cannot provide information about the mutual influence relationship between system components, and thus it is difficult to identify the real reason for the increase in the irreversible losses of each component. The exergic destruction of a component is not only related to the efficiency of this component, but also affected by other components of the system. Hence, a reasonable exergic analysis process should consider the interaction between the components in the entire system rather than consider a certain component independently. The advanced exergic analysis is developed on the basis of the traditional exergic analysis and considers the actual conditions and unavoidable conditions of different components in the system. It can split the exergic destruction into the endogenous and exogenous parts, or into the avoidable and unavoidable parts. That will reveal the energy-saving potential of each component and provide a reference for improving the system performance. Tsatsaronis, Cziesla, Kelly and some other researchers have developed the methods of splitting exergic destruction into the endogenous and exogenous parts or into avoidable and unavoidable parts [12,13,14,15], which is an important supplement to the traditional exergic analysis method.

Many traditional exergic analysis and advanced exergic analysis studies have been conducted. Ameri et al. [16] carried out the exergic analysis of an ISCC system. They found that the ISCC system had great advantages in thermal performance. Tsatsaronis et al. launched the advanced exergic analyses of gas–steam systems [17], a refrigeration system [18] and some other energy conversion systems [19]. Acikkalp et al. [20] estimated the thermal behavior of a gas-fired power system by using the advanced exergic analysis and exergo-economic analysis methods. Petrakopoulou et al. [21] carried out an advanced exergic analysis of a power plant with chemical looping combustion.

There are also many existing studies on ISCC systems. For instance, Spelling and Laumert [22] conducted the economic analysis of an ISCC system with considering the environmental protection effect. The impact of CO₂ emission reduction brought by the usage of solar energy on the economic performance of the ISCC system was evaluated. Achour et al. [23] carried out a performance evaluation of the first ISCC system in Algeria. The results revealed the relatively better operation performance of the evaluated ISCC system compared with a traditional gas–steam turbine combined cycle. Bellos et al. [24] conducted a study on an ISCC system with parabolic trough collectors (PTCs). The results showed that by using solar energy, the ISCC system with the output power of 14.81 MW could reduce the annual fuel consumption by 64.0%.

According to the literature review, though researchers have conducted many studies on design and performance analysis of ISCC systems, advanced exergic analysis studies on ISCC systems are still relatively low, and need further work. This paper presents a novel hybrid solar–gas system. It is designed for power generation and cooling supply. By using the grey-box model and Ebsilon (V15.0) software, the operation, advanced exergic and economic analyses of the hybrid solar–gas system are carried out. The operational performance, exergic and economic parameters of the proposed hybrid solar–gas system are calculated and analyzed. The results of this paper may contribute to the future research and development of hybrid solar–gas systems.

2. Design of the Solar–Gas System

The diagram and working flow of the hybrid solar–gas system are presented in Figure 1. The hybrid solar–gas system is mainly comprised by the solar direct steam generator (SDSG), gas-steam turbine cycle and cooling supply system (CSS). The rated electric power of the hybrid solar–gas system is designed to be 96.0 MW. The SDSG employs PTCs.

In Figure 1, AC, CC and GT stand for the air compressor, combustion chamber and gas turbine. G represents the power generator. HWH, CH, LPEV, LPSH, HPEC, HPEV and HPSH are the hot water heater, condensate heater, low-pressure evaporator, low-pressure super-heater, high-pressure economizer, high-pressure evaporator and high-pressure super-heater, respectively. HPFP is the high-pressure feedwater pump. CP stands for the condensate pump. HPD and LPD are the high-pressure and low-pressure drums. HS and LS are the high-pressure and low-pressure stages of the steam turbine (ST). In this solar–gas system, the super-heated steam generated by the SDSG goes into the low-pressure steam supply pipe (LPSSP) and then drives the LS to generate electric power together with the steam coming from the heat recovery steam generator (HRSG). A stream of super-heated steam is extracted from the LPSSP and then sent to the CSS.

3. Research Methods

3.1. Modeling Method

According to the working principle and assumed parameters, the simulation model of the hybrid system is built with Ebsilon (V15.0) software. This software is extensively employed in research works related to modeling, simulation and thermodynamics analysis of different power systems. Its serviceability in simulation and analytical works of power systems have been reported by many references.

The simulation model of the hybrid system in this work is presented in Figure 2. The subsequent operation, exergic and economic analyses of the hybrid system are all based on this model.

To validate the ability of the simulation model and Ebsilon (V15.0) software, simulation results of a gas–steam turbine cycle reported by a reference are selected [25]. A gas–steam turbine cycle is newly established by using Ebsilon in this paper and its operation is simulated. The newly obtained simulation results and the referenced results are shown in

Table 1. Validation results of the simulation model and Ebsilon (V15.0) software.

Parameters	Referenced Values	Results in This Study
Total electric power/MW	87.0	86.98
Electric power of GT/MW	31.0	30.95
Electric power of ST/MW	25.0	25.10
Exhaust gas temperature/°C	546.0	546.35
High-pressure steam mass flow rate/t/h	80.0	79.75
High-pressure steam temperature/°C	500.0	500.0
High-pressure steam pressure/MPa	5.0	4.95
Low-pressure steam mass flow rate/t/h	17.0	17.26
Low-pressure steam temperature/°C	200.0	199.65
Low-pressure steam pressure/MPa	0.5	0.5

. It can be seen from Table 1 that the simulation results and referenced values have relatively good agreement.

3.2. Traditional Exergic Analysis Equations

By establishing the exergic analysis model and then listing the exergy balance equations, the traditional exergic analysis of a system can be conducted. The grey-box mode approach is employed to carry out the traditional exergic analysis of the solar–gas system. This approach considers all components of a system as grey-boxes and connects different grey-boxes with exergy flow lines. There are three exergy variables in an assumed grey-box unit, which are exergy supply E_x+, effective exergy E_x− and exergic loss E_d. Every component of the solar–gas system can be considered as this kind of grey-box unit.

According to the grey-box model, the exergy balance formula is

$$E_{\mathrm{d}}=E_{\mathrm{x}+}-E_{\mathrm{x}-}$$

(1)

and the exergic efficiency is

$${\eta }_{\mathrm{x}}=E_{\mathrm{x}-}/E_{\mathrm{x}+}=1-E_{\mathrm{d}}/E_{\mathrm{x}+}$$

(2)

Based on the two formulas and Figure 1, the exergic analysis (EA) equations for different components of the solar–gas system are listed below.

For the AC, the two EA equations are

$$E_{\mathrm{d},\mathrm{ac}}=W_{\mathrm{ac}}-E_{\mathrm{x}2}+E_{\mathrm{x}1}$$

(3)

$${\eta }_{\mathrm{ac}}=E_{\mathrm{x}2}/\left(W_{\mathrm{ac}}+E_{\mathrm{x}1}\right)$$

(4)

where W_ac is the energy consumption of the AC. E_x2 and E_x1 are the outlet and inlet exergy values of the AC.

The EA formulas for the CC are

$$E_{\mathrm{d},\mathrm{cc}}=E_{\mathrm{x}2}+E_{\mathrm{fuel}}-E_{\mathrm{x}3}$$

(5)

$${\eta }_{\mathrm{cc}}=E_{\mathrm{x}3}/\left(E_{\mathrm{x}2}+E_{\mathrm{fuel}}\right)$$

(6)

where E_fuel is the exergy of the inlet gas of the CC. E_x3 is the inlet exergy of the ST.

The EA formulas for the GT are

$$E_{\mathrm{d},\mathrm{gt}}=E_{\mathrm{x}3}-E_{\mathrm{x}4}-W_{\mathrm{gt},\mathrm{out}}$$

(7)

$${\eta }_{\mathrm{gt}}=\left(W_{\mathrm{gt},\mathrm{out}}+E_{\mathrm{x}4}\right)/E_{\mathrm{x}3}$$

(8)

where W_gt,out is the output power of the GT. E_x4 is the outlet gas exergy of the GT.

For the HRSG and ST, the EA equations are

$$E_{\mathrm{d},\mathrm{hrsg}}=E_{\mathrm{x}4}-E_{\mathrm{x}5}-(E_{\mathrm{x}6}-E_{\mathrm{x}9})$$

(9)

$${\eta }_{\mathrm{hrsg}}=\left(E_{\mathrm{x}5}+E_{\mathrm{x}6}\right)/\left(E_{\mathrm{x}4}+E_{\mathrm{x}9}\right)$$

(10)

$$E_{\mathrm{d},\mathrm{st}}=E_{\mathrm{x}6}+E_{\mathrm{x}5}+E_{\mathrm{x}12}-E_{\mathrm{x}7}-W_{\mathrm{st},\mathrm{out}}$$

(11)

$${\eta }_{\mathrm{st}}=\left(E_{\mathrm{x}7}+W_{\mathrm{st},\mathrm{out}}\right)/\left(E_{\mathrm{x}6}+E_{\mathrm{x}5}+E_{\mathrm{x}12}\right)$$

(12)

where E_x5 is the inlet steam exergy of the HS. E_x6 is the inlet steam exergy of the LS. E_x9 is the inlet feedwater exergy of the HRSG. E_x12 is the exergy of steam coming from the SDSG, which enters the LS. E_x7 is the exhaust gas exergy of the LS. W_st,out is the output power of the ST.

The EA formulas for the condenser and CP are

$$E_{\mathrm{d},\mathrm{cond}}=\left(E_{\mathrm{x}7}+E_{\mathrm{x}10}\right)-\left(E_{\mathrm{x}8}+E_{\mathrm{x}11}\right)$$

(13)

$${\eta }_{\mathrm{cond}}=\left(E_{\mathrm{x}8}+E_{\mathrm{x}11}\right)/\left(E_{\mathrm{x}7}+E_{\mathrm{x}10}\right)$$

(14)

$$E_{\mathrm{d},\mathrm{cp}}=W_{\mathrm{cp},\mathrm{con}}-\left(E_{\mathrm{x}9}-E_{\mathrm{x}8}\right)$$

(15)

$${\eta }_{\mathrm{cp}}=E_{\mathrm{x}9}/\left(W_{\mathrm{cp},\mathrm{con}}+E_{\mathrm{x}8}\right)$$

(16)

where W_cp,con is the power consumption of the CP. E_x8, E_x10 and E_x11 are the exergy values of outlet condensate water, inlet circulating water and outlet circulating water of the condenser.

The EA equations of the SDSG are

$$E_{\mathrm{d},\mathrm{sdsg}}=E_{\mathrm{x}13}+E_{\mathrm{x},\mathrm{solar}}-E_{\mathrm{x}12}$$

(17)

$${\eta }_{\mathrm{sdsg}}=E_{\mathrm{x}12}/\left(E_{\mathrm{x}13}+E_{\mathrm{x},\mathrm{solar}}\right)$$

(18)

where E_x13 is the inlet water exergy of the SDSG. E_x,solar is the solar radiation exergy input of the SDSG, and can be expressed as [26]

$$E_{\mathrm{x},\mathrm{solar}}=1-{\displaystyle \frac{4}{3}}{\displaystyle \frac{T_0}{T_{\mathrm{s}}}}+{\displaystyle \frac{1}{3}}{\left({\displaystyle \frac{T_0}{T_{\mathrm{s}}}}\right)}^4$$

(19)

where T₀ is the ambient temperature, and T_s is the equivalent temperature of the sun as a black body.

For the HPFP, the EA formulas are

$$E_{\mathrm{d},\mathrm{hpfp}}=W_{\mathrm{hpfp},\mathrm{con}}+E_{\mathrm{x}15}-E_{\mathrm{x}16}$$

(20)

$${\eta }_{\mathrm{hpfp}}=E_{\mathrm{x}16}/\left(W_{\mathrm{hpfp},\mathrm{con}}+E_{\mathrm{x}15}\right)$$

(21)

where W_hpfp,con is the power consumption of the HPFP. E_x15 and E_x16 are the exergy values of inlet and outlet water of the HPFP.

3.3. Advanced Exergic Analysis Equations

3.3.1. Exergic Loss Splitting

The main target of the traditional exergic analysis is to find the components with relatively large exergic losses or low exergy efficiencies. However, by using the traditional exergic analysis, it is hard to reveal the interactions among the components as well as the optimization potential of the relevant components. For a thermal system, on one hand, the exergic loss cannot exactly describe the performance of a certain component as part of the exergic loss may be related to other components. To deal with this problem, from the perspective of thermodynamics, the advanced exergic analysis approach splits the total exergic loss of a certain component into the endogenous and exogenous parts. On the other hand, due to some unavoidable factors (e.g., technical, material, economic limitations), a component has a part of exergic loss which cannot be optimized and decreased. To reflect the potential of energy saving of this component, from the perspective of engineering, the advanced exergic analysis approach splits the total exergic loss of a certain component into the avoidable and unavoidable parts.

The exergic loss splitting of a certain component is illustrated in Figure 3. E_d is the total exergic loss. E_d,en and E_d,ex are the endogenous and exogenous exergic losses. E_d,av and E_d,un are the avoidable and unavoidable exergic losses.

3.3.2. Equation of Endogenous and Exogenous Exergic Losses

For a certain component, its endogenous exergic loss is related to its own performance and operation state, while its exogenous exergic loss is caused by the inefficient operations of some other components. The total exergic loss of the component is the sum of the endogenous and exogenous exergic losses. Hence, there is

$$E_{\mathrm{d}}=E_{\mathrm{d},\mathrm{en}}+E_{\mathrm{d},\mathrm{ex}}$$

(22)

In this study, the endogenous and exogenous exergic losses of different components of the solar–gas combined system are calculated by using the exergy balance approach [27]. When the exergy balance approach is used for calculating the endogenous and exogenous exergic losses, a simulation model should be established. In this study, Ebsilon is utilized to establish the simulation models of the components of the solar–gas combined system. When the advanced exergic analysis of a certain component is conducted, the exergy balance approach needs to idealize other components firstly, and then the component’s isentropic efficiency, pressure drop and some other parameters should be changed. The detailed implementation steps of the exergy balance approach can be seen in the relevant references [27].

3.3.3. Equation of Avoidable and Unavoidable Exergic Losses

The analysis of avoidable exergic loss is beneficial to improving the performance of a thermal system. The unavoidable exergic loss is defined as the smallest exergic loss due to the limitations of technology, materials, manufacturing processes and economic conditions. The avoidable exergic loss is the difference between the total exergic loss and unavoidable part, and there is

$$E_{\mathrm{d}}=E_{\mathrm{d},\mathrm{av}}+E_{\mathrm{d},\mathrm{un}}$$

(23)

To calculate the unavoidable exergic loss of a certain component, the component should be isolated from other components of the system firstly, and then be artificially given a relatively reasonable and achievable efficiency according to the existing operation experience. After calculating the ratio of exergic loss and exergy production under the unavoidable condition, the avoidable exergic loss can be obtained. The parameter settings for typical components under the ideal and unavoidable conditions are shown in

Table 2. Parameters for hybrid solar–gas system components under different conditions.

Component	Ideal Condition	Unavoidable Condition
AC	Isentropic efficiency = 100.0% Mechanical efficiency = 100.0%	Isentropic efficiency = 96.0% Mechanical efficiency = 99.6%
CC	Combustion efficiency = 1.0 Pressure loss = 0.0 bar ALAM = 2.617	Combustion efficiency = 0.998 Pressure loss = 0.1 bar ALAM = 1.57
GT	Isentropic efficiency = 100.0% Mechanical efficiency = 100.0%	Isentropic efficiency = 98.0% Mechanical efficiency = 99.6%
Super-heater of HRSG	Efficiency = 100.0% Pressure drop = 0.0 bar	Efficiency = 90.0% Pressure drops at hot end/cold end = 0.5 bar/0.01 bar
Evaporator of HRSG	Pinch-point temperature difference = 0.01 K Pressure drop = 0.0 bar	Pinch-point temperature difference = 3.0 K Pressure drops at hot end/cold end = 0.5 bar/0.01 bar
Economizer of HRSG	Pinch-point temperature difference = 0.01 K Pressure drop = 0.0 bar	Pinch-point temperature difference = 3.0 K Pressure drops at hot end/cold end = 0.5 bar/0.01 bar
ST	Isentropic efficiency = 100.0% Mechanical efficiency = 100.0%	Isentropic efficiency = 96.0% Mechanical efficiency = 99.5%
COND	Terminal temperature difference = 0.0 K	Terminal temperature difference = 3.0 K
GEN	Efficiency = 100.0%	Efficiency = 99.5%
SDSG	Optical efficiency = 100.0% Heat loss coefficient = 0.0 W/(m2K) Efficiency of solar receiver tubes = 100.0%	Optical efficiency = 79.9% Heat loss coefficient = 10.0 W/(m2K) Efficiency of solar receiver tubes = 85.0%
CSS	Efficiency of generator = 100.0% Pressure drop of condenser = 0.0 bar Efficiency of absorber = 100.0% Terminal temperature difference of evaporator = 0.1 K	Efficiency of generator = 98.0% Pressure drop of condenser =0.05 bar Efficiency of absorber = 96.0% Terminal temperature difference of evaporator = 0.5 K

[28,29]. The parameters in Table 2 are used for the calculations and analyses of avoidable and unavoidable exergic losses, and not for the operation performance simulation of the hybrid system.

3.4. Economic Analysis Equations

For the hybrid system, its levelized cost of energy (LCOE) is [30]

$$LCOE={\displaystyle \frac{\left(r_{\mathrm{dis}}\cdot C_{\mathrm{tot}}+FOMC+C_{\mathrm{y},\mathrm{fuel}}\right)-C_{\mathrm{carbon}}}{M_{\mathrm{y}}}}+VOMC$$

(24)

where C_tot is the total construction cost of the hybrid system. FOMC and VOMC are the fixed and varying operation and maintenance costs. C_y,fuel and M_y are the annual fuel cost and annual electric power generation of the hybrid system. C_carbon is the income brought by the CO₂ emission reduction. r_dis is the discount rate. The annual income (IN_y) of the hybrid system is

$$I{N}_{\mathrm{y}}=M_{\mathrm{y}}\times e_{\mathrm{grid}}$$

(25)

where e_gird is the on-grid electricity price. The net present value of the hybrid system is

$$C_{\mathrm{net}}={\displaystyle \sum _{t=0}^n\left(I{N}_{\mathrm{y}}-FOMC\right)}{\left(1+r_{\mathrm{dis}}\right)}^{-t}$$

(26)

where n is the lifetime of the hybrid system.

4. Results and Discussion

4.1. Operation Performance

Setting the incident solar intensity as 900.0 W/m², the operation process of the hybrid system is simulated by using Ebsilon (V15.0) software. The main operation parameters of the solar–gas system are shown in

Table 3. Operation performance of the hybrid solar–gas system.

Parameter	Value
Electric power of solar–gas system	96.0 MW
Electric power of gas–steam cycle	87.0 MW
Maximum cooling load	69.66 MW
Exhaust gas temperature of GT	546.0 °C
Inlet steam temperature/pressure of ST	500.0 °C/5.0 MPa
Electric power contributed by SDSG	9.0 MW
Optical efficiency of SDSG	0.75
Output steam temperature/pressure of SDSG	220.0 °C/0.5 MPa
Energy conversion efficiency of solar–gas system	45.8%
Energy efficiency of GT	37.4%
Energy efficiency of HRSG	82.7%
Solar energy proportion	9.4%

. In Table 3, the initial parameters for operation simulations are obtained from the relevant reference [25], and operation performance parameters of the hybrid system are obtained from Ebsilon-based simulations in this study. The results show that the electric power and energy conversion efficiency of the hybrid system are 96.0 MW and 45.8%. The electric power of gas–steam cycle is 87.0 MW, and that contributed by the SDSG system is 9.0 MW. Therefore, the solar energy proportion of the hybrid system is 9.4%. The maximum cooling load of the hybrid system is 69.66 MW.

Effects of some key parameters on the operation performance of the hybrid solar–gas system are analyzed. Figure 4 presents the output power and energy conversion efficiency variations of the hybrid system when the incident solar intensity changes. As shown in Figure 4, when the incident solar intensity increased from 300.0 W/m² to 1000.0 W/m², the output power of the hybrid system increased from 89.6 MW to 97.1 MW, but the energy conversion efficiency decreased from 49.9% to 45.2%. That is mainly because the increase in the incident solar intensity improves the electric power contributed by solar energy, but the thermal-to-electric efficiency of solar energy is lower than the electric efficiency of the gas–steam turbine combined cycle.

Figure 5 presents the output power and energy conversion efficiency variations of the hybrid system when the ambient temperature changes. The results show that when the ambient temperature increased from −5.0 °C to 30.0 °C, the output power and energy conversion efficiency of the hybrid system both increase first and then decrease. When the ambient temperature is 10.0 °C, the hybrid system has both the maximum output power and maximum energy conversion efficiency (96.6 MW and 46.5%). This is mainly because 10.0 °C is the optimal ambient temperature value for the AC and CC. That is due to the operation characteristics of the gas–steam turbine combined cycle of the hybrid system.

In addition, the analysis results show that when the efficiency of the CC is low, the combustion of fuel will be insufficient. This will reduce the power output of the GT, decrease the exhaust temperature of the GT and thus reduce the heat absorption of the HRSG. Finally, the cooling capacity of the refrigeration system will decrease. Under the same refrigeration condition, if the efficiency of the refrigeration system decreases, the demand for steam in the refrigeration system will increase. In this way, the steam sent to the LS will be reduced, thereby making the output power of the hybrid system decrease.

Figure 6 shows the seven-day continuous operation simulation results of the hybrid system when the incident solar intensity changes. The results indicate that the SDSG system and gas–steam cycle can effectively work together. The coordinated operation of the SDSG system and gas–steam cycle can be realized, showing the technical feasibility of the hybrid system to a certain extent.

4.2. Traditional Exergic Analysis

The traditional exergic analysis results of the hybrid solar–gas system are presented in

Table 4. Traditional exergic analysis results of the solar–gas system.

Component	Ed (MW)	ηx
AC	3.86	92.29%
CC	56.45	61.98%
GT	2.97	91.25%
HPSH	0.591	87.42%
HPEV	1.705	85.18%
HPEC	0.516	78.39%
LPSH	0.036	72.72%
LPEV	0.256	88.07%
CH	0.507	68.58%
HWH	0.103	68.67%
HPFP	0.025	73.96%
ST	6.287	88.48%
COND	4.045	76.75%
CP	0.019	70.46%
SDSG	28.516	36.24%
CSS	8.27	24.61%
GEN	1.022	97.36%

. In Table 4, COND and GEN stand for the condenser and power generator, and other abbreviations can be seen in the Nomenclature section. According to the traditional exergic analysis results, the exergic loss and exergic efficiency of the whole hybrid solar–gas system are 119.1 MW and 44.6%, respectively.

Among all components of the hybrid system, the CC has the maximum exergic loss, which is 56.5 MW. The exergic efficiency of the CC is about 62.0%. Another component with large exergic loss is the SDSG, and its exergic loss and exergic efficiency are 28.5 MW and 36.2%. According to these results, it can be concluded that the CC and SDSG both have certain optimization potential. The CSS has the lowest exergic efficiency, which is 24.6%. The exergic loss of the CSS is 8.3 MW. However, as the main function of the CSS is to use the low-grade thermal energy to provide cooling, the economy of the CSS cannot simply be estimated by using the exergic analysis results.

4.3. Advanced Exergic Analysis

4.3.1. Endogenous and Exogenous Exergic Losses

This section presents the analysis results of endogenous and exogenous exergic losses of the solar–gas system. The exergy balance approach is employed and the ideal state parameters of different components for the calculations of endogenous and exogenous exergic losses are presented in

Table 5. Ideal state parameters of different components for the advanced exergic analysis.

Node	Temperature (°C)	Pressure (MPa)	Specific Enthalpy (kJ/kg)
1	25.0	0.103	25.07
2	510.0	1.02	560.0
3	1500.0	0.12	1213.0
4	546.0	1.45	597.0
5	504.5	4.995	3444.8
6	199.5	0.5	2854.0
7	45.0	0.01	2342.0
9	128.0	6.0	545.0
12	220.0	0.49	2898.6
13	80.0	0.6	333.4
Fuel	60.0	3.58	130.96

. The nodes in Table 5 are corresponding to those presented in Figure 1.

For the endogenous and exogenous exergic loss calculations of different components, some assumptions are adopted. For instance, the isentropic efficiency and mechanical efficiency of the AC are set to be 1.0. To minimize the exergic loss of the HRSG, the pressure drops of all heat exchangers in the HRSG are set to be 0.0. For the ST, GT and power generators, their isentropic efficiency and mechanical efficiency are all set to be 1.0. For the CC, its ideal state is difficult to be defined as chemical reactions happened in it. This study adopts the ideal assumptions of the CC presented in Ref. [31]. It is assumed that under the ideal state, the composition of the outlet gas of the CC is consistent with that under the actual state, the combustion efficiency of the CC is 1.0, the pressure drop of the CC is 0.0 and the excess air coefficient is the same as that under the actual state. The advanced exergic analysis results of different components are provided in Figure 7 and

Table 6. Calculation results of endogenous and exogenous exergic losses.

Component	Ed (MW)	Ed,en (MW)	Ed,ex (MW)	ken
AC	3.86	2.166	1.694	0.561
CC	56.45	37.2	19.25	0.659
GT	2.97	2.405	0.565	0.810
HRSG	7.605	4.357	3.248	0.573
ST	6.287	5.54	0.747	0.881
GEN	0.514	0.412	0.102	0.802
COND	4.165	2.886	1.279	0.693
SDSG	28.516	27.235	1.281	0.955
CSS	8.27	5.62	2.65	0.680

.

In Table 6, k_en is the ratio of the endogenous exergic loss to the total exergic loss. According to Figure 7 and Table 6, the endogenous exergic losses of the AC, CC, HRSG and CSS are 2.166 MW, 37.2 MW, 4.357 MW and 5.62 MW, and the corresponding exogenous exergic losses are 1.694 MW, 19.25 MW, 3.248 MW and 2.65 MW, respectively. For the four components, a considerable part of the exergic loss is exogenous. Their endogenous exergic loss proportions are 0.561, 0.659, 0.573 and 0.680. For the SDSG, 95.5% of the total exergic loss is endogenous. That is because the SDSG is relatively independent and slightly affected by other components of the solar–gas system.

4.3.2. Avoidable and Unavoidable Exergic Losses

For the calculation of avoidable and unavoidable exergic losses of different components of the solar–gas system, the parameter settings for typical components under the ideal and unavoidable conditions are shown in Table 2 (see Section 3.3.3). The calculation results of avoidable and unavoidable exergic losses of typical components of the solar–gas system are presented in Figure 8 and

Table 7. Calculation results of avoidable and unavoidable exergic losses.

Component	Ed (MW)	Ed,av (MW)	Ed,un (MW)	kav
AC	3.86	1.887	1.973	0.489
CC	56.45	11.685	44.764	0.207
GT	2.97	2.44	0.528	0.822
HRSG	7.605	4.215	3.39	0.554
ST	6.287	3.584	2.703	0.570
GEN	0.514	0.329	0.175	0.640
COND	4.165	0.274	3.891	0.066
SDSG	28.516	0.095	28.611	0.003
CSS	8.27	1.715	6.555	0.207

.

As shown in Figure 8 and Table 7, the avoidable exergic loss proportion of the SDSG is the lowest, which is 0.003. That means there is almost no avoidable exergic loss in the SDSG under the current condition. For the AC, CC, GT, HRSG, ST, GEN and CSS, a considerable part of the exergic loss is avoidable. The avoidable exergic loss of the CC is the maximum (about 11.69 MW). The avoidable exergic losses of the AC, GT, HRSG and ST are 1.89 MW, 2.44 MW, 4.22 MW and 3.58 MW. For these components (especially for the CC), there is still certain optimization potential according to the advanced exergic analysis results.

Advanced exergic analysis results have practical implications for the studied hybrid solar–gas system. The above advanced exergic analysis results have demonstrated endogenous, exogenous, avoidable and unavoidable exergic losses for different components. Based on these results, in the design stage of the hybrid system, it is possible to identify components with avoidable exergic loss and optimize their design accordingly. For the components with very small avoidable exergic loss, more concern should be given on their materials and manufacturing processes. That can improve the efficiency of practical design optimization work of the hybrid system. Taking the SDSG as the example, the SDSG has large exergic loss. However, most of the exergic loss of the SDSG is unavoidable. Therefore, to reduce the exergic loss of the SDSG, material selection improvement of solar collectors should be conducted. For instance, reflector materials with higher reflectivity should be used, and receiver tube surface-coating materials with higher absorptivity and lower emissivity should be explored.

4.4. Economic Analysis

For the economic analysis, the annual operation of the hybrid solar–gas system is simulated by using the solar radiation data of a typical city for a typical year. The exchange rate between the US dollar (USD) and the Chinese yuan (CNY) is 7.2. For the hybrid solar–gas system, the annual electric power generation is 489,600.0 MWh, and this value is obtained from the annual operation simulation of the hybrid system. The discount rate is chosen to be 0.1 [32]. Based on Ref. [33], the total construction cost of the hybrid system is USD 44,084,166.7, and the annual operation and maintenance cost is USD 1,445,972.2. According to Ref. [25], the annual fuel cost is USD 24,329,861.1, and the income brought by the CO₂ emission reduction is USD 1,165,555.6. The lifetime of the hybrid system is 30.0 years, and the on-grid electricity price is 0.10417 USD/kWh. The economic analysis results of the hybrid system are presented in

Table 8. Economic analysis results of the hybrid system.

Parameter	Result
LCOE	0.08125 USD/kWh
Dynamic recycling cycle	5.8 years
Net present value	USD 61,475,694.4
Internal rate of return	18.3%

.

The results indicate that for the hybrid solar–gas system, the levelized cost of energy is 0.08125 USD/kWh. The dynamic recycling cycle is 5.8 years. The net present value is about USD 61.48 million, and the internal rate of return is 18.3%. These results reveal the relatively presentable economic feasibility of the hybrid system. Figure 9 presents the variation of LCOE of the hybrid system when the annual fuel cost changes. The results show that with the annual fuel cost increased from 2.15 × 10⁷ USD to 2.71 × 10⁷ USD, LCOE of the hybrid system increases from 0.07553 USD/kWh to 0.08687 USD/kWh.

Table 9. Comparison of several typical ISCC systems.

Reference	Solar Field Type	Total Output Power (MW)	Output Power Provided by Solar Energy (MW)	Solar Proportion	Exergic Loss of the CC (MW)	Total Exergic Loss (MW)	LCOE (USD/kWh)
This work	PTC	96.0	9.0	9.4%	56.5	119.1	0.08125
Ref. [32]	PTC	470.3	39.8	8.5%	297.5	498.4	0.067
Ref. [34]	PTC	402.0	61.7	15.4%	136.8	278.9	0.1042
Ref. [35]	PTC	467.0	17.0	3.6%	260.9	432.3	--
Ref. [36]	PTC	160.0	22.0	13.8%	75.0	336.7	0.11

presents a brief comparison between the hybrid solar–gas system of this study and several other typical ISCC systems. As shown in Table 9, different ISCC systems may have different operation, exergic and economic performances due to their different system compositions and different investment and operation cost conditions. However, the mostly used solar field type in ISCC systems is PTC.

5. Conclusions

In this study, a novel hybrid solar–gas system for electricity production and cooling supply is proposed. Its electric power and energy conversion efficiency are 96.0 MW and 45.8%. The maximum cooling load is 69.66 MW. The solar energy proportion in the hybrid system is 9.4%. The advanced exergic analysis of the hybrid system is conducted. The results show that the total exergic loss and exergic efficiency of the hybrid solar–gas system are 119.1 MW and 44.6%. In the hybrid system, the CC has the maximum exergic loss (56.5 MW). The exergic loss and exergic efficiency of the SDSG are 28.5 MW and 36.2%. However, there is almost no avoidable exergic loss in the SDSG. For the AC, CC, GT, HRSG, ST, GEN and CSS, a considerable part of the exergic loss is avoidable. Especially, the avoidable exergic loss of the CC is the maximum (11.69 MW). For these components, there is still certain optimization potential. The advanced exergic analysis results make the thermodynamic performance of the hybrid system deeply understood. The advanced exergic analysis provides more specific references for the practical component optimizations of the hybrid system. The economic analysis results show that for the hybrid system, the levelized cost of energy is 0.08125 USD/kWh, the dynamic recycling cycle is 5.8 years, and the net present value is USD 61.48 million.

There are still some limitations and further works for this study. For instance, designs and performance comparison works on the hybrid solar–gas system with different types of solar collectors will be meaningful. In addition, the multi-objective optimization comprehensively considering operation performance and economic parameters on the hybrid system should be conducted in the next step.