Simulation and Performance Analysis of a Solar-Integrated Steam Power Cycle

Abstract

Fossil fuel-based power production faces challenges, particularly greenhouse gas emissions, that contribute to global warming. This paper explores retrofitting an existing 207.8 MW coal-fired steam power unit with a Concentrated Solar Power (CSP) tower system and Thermal Energy Storage (TES) systems to create a hybrid solar–coal power plant. The concept integrates a solar component and a two-tank TES system into the existing steam Rankine cycle. Thermodynamic modeling and balance calculations were performed using Ebsilon Professional software (version 16) to analyze the design. Three injection points for feedwater preheating were analyzed, with flow rates that varied from 10 to 100 kg/s. Thermodynamic simulations show that solar contributions of 16.0 MW (Variant 1), 27.6 MW (Variant 2), and 37.6 MW (Variant 3) increase net electricity output to 213.5 MW, 216.8 MW, and 219.3 MW, respectively. The corresponding thermal efficiencies rise from 42.6% to 43.8%, while the hybrid system’s total efficiency improves up to 29.6%. These results demonstrate that controlled feedwater diversion and solar integration can enhance performance, reduce coal dependency, and lower CO₂ emissions without compromising operational stability.

1. Introduction

Coal remains a critical energy source in the power sector, especially in developing countries where the existing power plant infrastructure heavily relies on its usage [1,2,3]. The growing challenges associated with fossil fuel-based power production, particularly the emission of greenhouse gases, have become a critical global concern due to their significant contributions to environmental degradation.

As the world transitions toward a more sustainable and environmentally friendly energy future, renewable energy sources (RESs) have emerged as pivotal alternatives. The reliance on fossil fuel resources has steadily declined due to advancements in renewable energy technologies and materials. The projections indicate that by 2040, renewable energy sources will account for 66% of global energy production [4]. Concentrated Solar Power (CSP) systems and thermal energy storage (TES) offer a promising solution for reliable power generation under varying solar radiation conditions [5]. This advantage positions CSP above other renewable technologies, such as photovoltaics and wind power, which are often regarded as leading alternatives to fossil fuels.

Despite these benefits, CSP systems face significant challenges, including high initial investment costs, lower efficiency compared to fossil fuel-based systems, and the technical complexities associated with large-scale TES implementation. These barriers limit the widespread adoption of CSP technology [6,7]. Integrating solar energy into coal-fired power plants, forming a solar-assisted coal-fired power system (SACP), presents a viable approach to address these issues. This hybrid system has substantial potential to reduce coal consumption while mitigating the limitations associated with standalone CSP plants [8]. One of the key advantages of CSP is its ability to provide baseload power when integrated with suitable TES systems. This integration ensures a steady and reliable power supply and enhances renewable energy systems’ overall efficiency and dispatchability [9]. The solar TES system is considered to be one of the attractive solutions for utilizing solar energy. It has been studied due to its high efficiency and capacity to supply energy (mainly heat and power) for domestic and industrial demands [10].

The study of energy storage materials and their applications for electricity and heat storage to mitigate peak demand–supply imbalances is a popular research area today. Heat accounts for the majority (70 to 80%) of daily energy demand in domestic settings, particularly for hot water and space heating needs [11]. Since the discovery of fire, fossil fuels have been predominantly used for heat production. However, burning fossil fuels is responsible for 40% of human-related CO₂ emissions, exacerbating global warming. Additionally, fossil fuels are finite resources, with projections indicating that oil reserves could be exhausted within the next 30 years, around 2050 [12].

Heat is central to the global energy chain, connecting primary and secondary energy sources. Its conversion, transfer, and storage processes account for 90% of the world’s total energy budget [13]. Therefore, TES methods can help balance heat production and consumption by addressing the heat demand–supply gap. The TES concept consists of storing cold or heat, which is determined according to the temperature range in a thermal battery (TES material) operational working for energy storage. The main benefit of using TES with a charging process is its ability to meet heat demand even after the primary energy source is unavailable. For example, solar energy collected during the day can be stored in a thermal medium and then utilized to provide heat at night. TES systems can serve short-term and long-term purposes, i.e., short-term attributes to storing heat for hours or days, and long-term or seasonal attributes of storing heat for several months to be utilized whenever demanded within the last two decades. TES systems have been incorporated into renewable energy technologies to balance heat supply and demand for both residential and power plant applications [11,14].

Studies have investigated the technical and economic implications of integrating solar-heated feedwater into steam power plants. Wu et al. (2018) [15] demonstrated that integrating solar TES into a 600 MW coal-fired power plant to preheat feedwater and reheated steam significantly improves efficiency. Using two solar fields, the system increased overall cycle efficiency by 1.91% and solar-to-electricity efficiency by 6.01%, with the solar contribution to energy input rising from 9.53% to 17.26%. The electricity output per unit of solar aperture area improved by 26.91%, highlighting the effectiveness of this dual solar integration in enhancing performance and renewable energy utilization [15]. Another study introduced a novel dual feedwater circuit for a parabolic trough solar power plant, employing two trains of heat exchangers. This design allowed for extended operating hours and reduced reliance on fossil fuels during nighttime by adjusting the feedwater flow through different circuits based on solar availability [16]. Thermodynamic modeling and balance calculations, performed using Ebsilon Professional software, demonstrate the feasibility and efficiency of this hybrid system [17,18].

Recently, Hu et al. developed a dual-time scale dispatch model for the dynamic operation of a CSP–photovoltaic (PV) hybrid plant to increase renewable energy utilization [19]. This model accounts for actual startup conditions and incorporates dynamic switching behaviors of key subsystems, including the receiver, thermal storage, and power cycle. By considering the unique dynamic operational features of the CSP-PV hybrid system, the model employs different time scales for forecasting during day-ahead and intraday periods, enhancing the plant’s operational efficiency and its ability to respond effectively to changing weather conditions. A solar dish-based CSP system with a triple-extraction Rankine cycle was proposed by Sarker et al. [20]. The hybrid configuration integrates thermal energy storage (TES) using molten salt, an organic Rankine cycle (ORC), and a natural gas–fired oxy-fuel combustion chamber to ensure stable power output during periods without solar input. Findings indicate that this design reduces fuel consumption by 74% compared to a conventional fossil fuel-based power plant.

Existing studies on integrating solar thermal energy with coal-fired power plants have primarily focused on large-scale feedwater preheating or the use of multiple solar fields. However, there is a lack of detailed investigations into retrofitting existing coal-fired steam units with hybrid solar–coal systems that utilize various solar feedwater injection locations and two-tank TES configurations. The influence of varying diverted feedwater flow rates on fuel-only efficiency, hybrid efficiency, and total system performance under constant boiler inlet temperatures has not been explored very much either. Moreover, comprehensive thermodynamic modeling of incremental solar contributions from CSP towers integrated with two-tank TES in hybrid coal plants remains limited, hindering the understanding of their effects on coal consumption and plant optimization. Unlike the case for CSP-PV hybrids, there is a notable lack of dynamic operational models for coal units that are retrofitted with CSP and TES, which constrains the development of effective strategies for hybrid operations and fossil fuel displacement.

The present work addresses these gaps by proposing a novel retrofit of existing coal-fired steam power plants with CSP and two-tank TES systems. Leveraging the established steam Rankine cycle, the Hybrid Solar–Coal Power Plant (HSCPP) design systematically evaluates three injection points before high-pressure feedwater preheaters in a 207.8 MW unit. It further investigates the effects of varying diverted feedwater flow rates (10–100 kg/s) on net power, fuel-only efficiency, hybrid efficiency, and overall performance at a fixed boiler inlet temperature of 221 °C. Finally, the study quantifies incremental solar contributions from the CSP tower with TES, analyzing their impact on coal use and plant efficiency using thermodynamic modeling framework in Ebsilon.

2. Analyzed System Overview

2.1. Steam Power Plant

The analyzed steam power plant is a power unit consist mainly of steam boiler, high-pressure steam turbine, low-pressure steam turbine, generator, condenser, three low-pressure feedwater pre- heaters, three high-pressure feedwater pre-heaters, feedwater pump, deaerator. The analyzed steam power plant operates to produce electricity; for this purpose, thermal energy comes from solid fuel combustion inside the pulverized coal-fired boiler (Figure 1).

The power output, efficiency and thermal energy input depends on the current operating conditions and can vary in the power plant load range. For selected conditions, the main parameters of analyzed steam power plant are presented in

Table 1. Main factors of steam power plant.

Parameter	Unit	Value
Power output	MW	207.8
Thermal power input	MW	501.26
Thermal efficiency	%	41.45%
Boiler feedwater inlet temperature	°C	221
Steam mass flow rate	t/h	717.5
Steam temperature	°C	537.8
Steam pressure	bar	104.2
Vapor pressure in condenser	bar	0.067

.

2.2. Concentrated Solar Power (CSP) Technology

Solar power collectors have proven to be an innovative and essential technology in the shift towards renewable energy. These devices harness sunlight and convert it into usable electricity, offering a sustainable alternative to traditional energy sources. Their growing popularity stems from several benefits, including the reduction in greenhouse gas emissions and the potential to lower electricity costs significantly. Utilizing solar energy, these systems contribute to global efforts to combat climate change, making them an attractive option for both residential and commercial use.

Concentrated solar power (CSP) is particularly attractive among various solar energy technologies due to its high efficiency, low operating cost, and good scaling potential [21,22]. Solar energy is converted into electricity by means of a CSP plant composed of four main elements: a concentrator, a high-temperature solar receiver, a fluid transport system, and a power generation block (e.g., Rankine cycle, Stirling cycle). This type of collector captures the radiation received on a relatively large surface. Parabolic troughs or flat mirrors on a smaller surface are used to concentrate these radiations. The objective of concentrating solar energy collectors is to focus the solar radiation received on a surface at a single point to obtain high temperatures. This thermal solar panel is useful for high- and very-temperature solar installations. In addition, these are used to generate water vapor with very high pressure, as seen in a conventional thermal power plant. More than 6 GW of CSP plants were installed in 2020 [23], and the largest TES from installed CSP can generate clean electricity for 24 h by reducing CO₂ emission. In the broader energy context, CSP is projected to play a meaningful role in global decarbonization pathways. In highly ambitious scenarios, new renewables such as CSP, wind, and geothermal could supply up to 83% of global electricity by 2040, with the potential to reach 100% renewable electricity by 2050 [24].

2.3. Solar Power Tower (SPT) Technology

The solar power tower (SPT) technology, chosen for this modeling and simulation, is a type of concentrated solar power (CSP) system that utilizes heliostats—sun-tracking mirrors—to focus sunlight onto a receiver atop a tower. The receiver’s heat transfer fluid (HTF) is heated to approximately 600 °C and used to generate steam, which drives a turbine and generator to produce electricity. The flexibility of SPT system allows for various configurations of heliostat fields, receiver designs, and heat transfer fluids. As depicted in Figure 2, CSP systems concentrate solar irradiation using heliostats to heat an HTF, which can either directly drive a turbine or transfer energy through a heat exchanger to generate steam in a secondary cycle. The heliostat mechanism tracks the sun’s movement, capturing sunlight efficiently even during winter when the sun is lower on the horizon. This adaptable configuration makes solar power towers a robust and efficient solution for clean energy production [21].

3. Materials and Methods

Section 3.1 outlines the modeling methodology for the power plant, detailing the system layout, integration of the solar field with the steam cycle, and key assumptions. The tools and techniques are described to simulate the hybrid solar–coal configuration under realistic conditions. Section 3.2 focuses on the thermodynamic simulation of the hybrid system, specifying input parameters for the solar field (e.g., solar irradiance, ambient temperature, and collector efficiency) and the steam cycle (e.g., turbine and condenser pressures). These inputs are tailored to the chosen location, accurately representing the system’s performance.

3.1. Modeling Description

3.1.1. Solar Tower Receiver and Heliostat Field

The sun angles and incident power calculations are essential for solar energy applications, particularly in Concentrated Solar Power (CSP) systems. Using the standard DIN 5034 [25], the solar angles, such as the solar altitude and azimuth, is calculated [26]. For solar irradiance, the Direct Normal Irradiance (DNI) is considered and its estimation using the Clear Sky model by Hottel [27] or Clearness Index originally developed by Liu and Jordan [26]. The Clear Sky model estimates DNI for cloudless conditions based on atmospheric parameters and solar geometry, while the Clearness Index considers cloud cover and atmospheric clarity. These parameters determine the usable solar irradiance and incident power on the receiver aperture.

The total usable solar irradiance $({\dot{Q}}_{usable})$ is calculated as in Equation (1):

$${\dot{Q}}_{usable}=DNI\cdot A_{heliostat}$$

(1)

where

$A_{heliostat}$ is the total aperture area of the heliostat field, m².

The incident power on the receiver aperture area is calculated as in Equation (2):

$${\dot{Q}}_{incident}=DNI\cdot \rho \cdot {\eta }_{field}\left({\gamma }_s,{\alpha }_s\right)\cdot {\eta }_{wind}$$

(2)

where

DNI—Direct Normal Irradiance, W/m²

$\rho$—the average field reflectivity, -

${\eta }_{field}\left({\gamma }_s,{\alpha }_s\right)$—solar field optical efficiency depends on the sun azimuth ${\gamma }_s$, and the sun elevation ${\alpha }_s$

${\eta }_{wind}$—wind correction

$${\dot{Q}}_{eff}={\dot{Q}}_{incident}-{\dot{Q}}_{loss}$$

(3)

$${\dot{Q}}_{loss}={\dot{Q}}_{loss,opt}+{\dot{Q}}_{loss,conv}+{\dot{Q}}_{loss,rad}$$

(4)

where

${\dot{Q}}_{eff}$—power of effective heat, kW

${\dot{Q}}_{eff}$—losses composed of optical ${\dot{Q}}_{loss,opt}$, convective ${\dot{Q}}_{loss,conv}$ and radiation losses ${\dot{Q}}_{loss,rad}$, kW.

3.1.2. Rankine Cycle

The Rankine cycle power unit is represented under the assumption of steady-state balances for energy, mass and pressure. The total efficiency of the presented cycle can be calculated after simulating each characterized scenario, using the main output variables, such as net power output and thermal energy input (heat rate to the cycle through the boiler). To capture the influence of variations in DNI, the solar and coal-fired section components of the plant (such as the boiler and superheater) are modeled using a time-series mode. During steam generation, saturation pressure is treated as constant, as well as the isentropic efficiencies of the turbomachinery, which are constant for each steam turbine stage. Additional key assumptions include neglecting conductive heat transfer, ignoring non-axial temperature gradients, and assuming the condenser operates at constant conditions.

3.1.3. Feedwater Extraction and Solar Integration

Integrating solar energy into conventional steam power plants by extracting a portion of the feedwater before the high-pressure (HP) heater and routing it through a solar-heated heat exchanger is a strategy explored to enhance efficiency and reduce fossil fuel consumption. This approach allows the feedwater to absorb thermal energy coming from solar field, thereby decreasing the steam extraction from the turbine for feedwater heating and increasing the overall power output. In this configuration, a specific fraction of the feedwater is diverted before entering the HP heater and directed through a solar heat exchanger. The flow rate of this extracted feedwater is a critical parameter, typically controlled within a range of 10 kg/s to 100 kg/s. Adjusting this flow rate is essential to optimize the solar field’s thermal energy absorption and maintain the power plant’s desired operating conditions.

3.2. Thermodynamic Simulation of Hybrid System

The thermodynamic simulation for this study is performed based on the location in the Kalahari Desert, Southern Africa. The Kalahari Desert, situated at coordinates 25.5920° S and 21.0937° E, is known for its high solar irradiance and extreme summer temperatures, which range between 43 °C and 46 °C. These conditions make it an ideal location for evaluating the performance of solar-assisted thermodynamic systems. The simulation was conducted for a specific date, 15 June 2021, during the peak summer season, and at 11:00 a.m. to capture the maximum solar intensity of the day. Oil was used as the thermal fluid medium to accurately model the system’s thermal behavior due to its high thermal stability and efficiency in absorbing and transferring heat within the CSP system.

The simulation employed Ebsilon Professional software, a robust tool for designing and optimizing thermodynamic systems, to model a hybrid solar–coal power plant. The system integrates solar thermal energy into a conventional Rankine cycle, with components such as solar collectors, heat exchangers, and turbines modeled to simulate their interactions and assess overall performance. The environmental parameters, including the desert’s extreme ambient temperature, were incorporated into the model to ensure realistic and accurate results. This methodology comprehensively evaluates the CSP systems performance under high-temperature, high-solar-irradiance conditions and offers valuable insights into integrating solar energy into hybrid solar–coal power plant designs. The schematic layout of the power plant concept is shown in Figure 3.

3.2.1. Solar Field Simulation

In the operation of a solar field within a CSP system, solar energy is captured, stored, and transported to enable electricity generation. The input data of Solar Field on 15 June 2021 is presented in

Table 2. Solar Field data on 15 June 2021.

Date and Time	Power Absorbed by the Fluid	Effective Receiver Temperature	Effective Field Efficiency
12:00:00 a.m.	0.00 MW	274.71 °C	0%
2:00:00 a.m.	0.00 MW	274.71 °C	0%
4:00:00 a.m.	0.00 MW	274.71 °C	0%
6:00:00 a.m.	46.44 MW	304.78 °C	35.72%
8:00:00 a.m.	75.25 MW	323.41 °C	57.25%
10:00:00 a.m.	83.91 MW	328.76 °C	63.72%
12:00:00 p.m.	87.37 MW	330.86 °C	66.30%
2:00:00 p.m.	83.89 MW	328.75 °C	63.70%
4:00:00 p.m.	75.22 MW	323.39 °C	57.22%
6:00:00 p.m.	46.44 MW	304.78 °C	35.72%
8:00:00 p.m.	0.00 MW	274.71 °C	0%
10:00:00 p.m.	0.00 MW	274.71 °C	0%

. The process begins with the collection of solar radiation, which is concentrated and transferred to a working fluid. This fluid serves as the medium for storing and transferring thermal energy within the system. The thermal energy is stored in designated units to ensure continuous operation, even during periods when solar radiation is unavailable, such as nighttime or cloudy weather.

The energy storage system plays a crucial role in maintaining the efficiency and reliability of the plant by enabling consistent energy availability. Fluid flow throughout the system is carefully managed using control mechanisms to optimize energy transfer and maintain operational efficiency (

Table 3. Input data for the solar field in Hybrid Solar–Coal Power Plant model.

Parameter	Value
Cold storage temperature	275.3 °C
Hot storage temperature	391.3 °C
Thermic fluid pressure inlet to the solar tower	15 bar
Mass flow rate of thermic fluid to solar tower	500 kg/s
Amount of fluid used in the circuit	15 t

). Once the thermal energy is adequately stored and regulated, it is sent to the next stage of the power generation process, where it is converted into electricity.

3.2.2. Steam Cycle Simulation

The steam Rankine cycle power plant operation simulation was designed using Ebsilon^® Professional software. The components of the steam cycle in Ebsilon are the steam turbine, steam generator, condenser, pump, deaerator, and preheaters (Figure 4). The three model variants were developed for the simulations, where part of the feedwater before each HP preheater is diverted to the heat exchanger (HE). The hot medium in the HP preheaters is the bleed from stages of HP turbine, and the cold medium is the feedwater from the deaerator to the boiler. The diverted feedwater to the heat exchanger is again mixed with the feedwater line before the boiler inlet, keeping boiler feedwater temperature at the same level. Input data for the steam cycle in hybrid Solar–Coal Power Plant model is provided in Table 3.

3.2.3. Hybrid Solar–Coal Power Plant Simulation

The simulation of a solar thermal power system that integrates a solar field with a steam cycle is designed to ensure accurate modeling under both standard operating conditions and varying scenarios. It involves multiple steps to represent the system’s behavior comprehensively. First, the simulation software sets up the solar field and steam cycle components. Key parameters, such as fluid properties, flow rates, and temperature settings, are defined to reflect realistic conditions. Enthalpy values at the inlet and outlet of each steam turbine stage are carefully specified to maintain precise energy balance across the system.

The simulation then calculates heat balance to model energy flows through the solar field, TES, and steam cycle. Turbine efficiency, fluid properties, and flow rates are meticulously modeled to align with design specifications and ensure efficient system operation. When simulating off-design conditions, adjustments are made to account for varying loads and changes in system behavior. For example, efficiency curves are defined for each steam turbine stage to capture variations in isentropic efficiency, and the boiler’s pressure-flow relationship is configured to model its performance accurately. Finally, the system’s electrical output is determined based on steam flow from the boiler, allowing the model to predict performance under fluctuating energy demands or environmental conditions. The modeling of the solar–coal power plant operation was designed using Ebsilon^® Professional software and presented in Figure 4.

4. Result and Discussion

Three different analyses based on the variants were classified according to the extraction of feed water. A part of the feedwater is extracted before the HP heater and flows through the heat exchanger to utilize solar energy. The flow rate of extracted feedwater is controlled from 10 kg/s to 100 kg/s. The simulated result data are presented and discussed below.

To evaluate the integration of solar thermal energy into the feedwater preheating process, three simulation variants were developed. In each variant, a portion of the feedwater was extracted before a specific high-pressure (HP) feedwater preheater and diverted through a solar heat exchanger, where it was preheated using thermal energy from the solar field. The point of extraction varied between variants, resulting in different outlet temperatures from the heat exchanger: 184 °C (Variant 1), 158 °C (Variant 2), and 134 °C (Variant 3). The solar-preheated feedwater was then mixed back into the main feedwater line before entering the boiler. This configuration maintained a constant boiler feedwater inlet temperature of 221 °C across all simulations, ensuring stable boiler operation while enabling analysis of solar energy’s contribution to system performance.

4.1. Energy Contribution from Solar and Coal System

In all three simulation scenarios, the temperature of the feedwater at the boiler inlet was held constant at 221 °C. This design choice ensured that the operational conditions of the steam boiler remained stable and unaffected by the integration of solar energy. Consequently, the thermal energy supplied by the boiler which is sourced from coal combustion remained consistent across all variants, even as the contribution of solar heat to the feedwater preheating process increased and led to higher levels of electrical power generation. This stability in boiler operation was crucial for isolating and accurately assessing the influence of solar thermal integration on the overall system efficiency and power output. Extraction points of the variant 1 (a), variant 2 (b) and variant 3 (c) before HP preheater are shown in Figure 5.

4.2. Efficiency Analysis

To investigate improvement of steam cycle performance by the use of additional heat source in the cycle, the power output and efficiency of the process were defined. The efficiency of the power plant can be calculated using the following equations:

$${\eta }_F={\displaystyle \frac{N_{el}}{{\dot{Q}}_F}}$$

(5)

$${\eta }_{FS}={\displaystyle \frac{N_{el}}{{\dot{Q}}_F+{\dot{Q}}_S}}$$

(6)

$${\eta }_{Total}={\eta }_F\cdot {\eta }_H$$

(7)

${\eta }_F$—steam cycle fuel-driven efficiency, %

${\eta }_{FS}$—steam cycle fuel and solar-driven efficiency, %

${\eta }_H$—heliostat field efficiency, %

$N_{el}$—power output, MW

${\dot{Q}}_F$—heat rate to the cycle through boiler (energy from fuel), MW

${\dot{Q}}_S$—heat rate to the cycle through solar heat exchanger (solar energy), MW.

It is worth to note that the heliostat field efficiency (${\eta }_H$) measures how effectively sunlight collected by heliostats is delivered to the central receiver in a solar tower system; compared to the solar energy incident on the mirrors, considering orientation, reflectivity, atmospheric losses, and geometric shading/blocking effects. Not all incident radiation reaches the receiver, as several losses occur along the way.

4.2.1. Variant 1

The hybrid solar-assisted steam power plant simulation demonstrates the successful integration of solar energy with a conventional coal-fired power system. Solar energy power contributes 16.045 MW, through preheating, reducing the dependency on coal. This reduces overall fuel consumption and improves the sustainability of the power plant. The simulated result for Variant 1 is presented in Figure 6. The coal system remains the main energy source, generating 501.258 MW of thermal energy, while the plant produces a net electricity production of 213.45 MW after accounting for losses. The system’s overall efficiency is 42.58%, which is relatively high for a coal power plant, highlighting the benefits of energy combination. The total efficiency of the Hybrid Solar–Coal Power Plant reached the value of 27.93 in the analyzed case scenario. The detailed total efficiencies for each mass flow at different solar heat input and power output are presented in

Table 4. Simulation results for variant 1.

Mass Flow	Solar Heat Input	Power Output	Fuel Efficiency	Fuel+ Solar Efficiency	Total Efficiency
kg/s	MW	MW	%	%	%
10	1.60	208.47	41.59	41.46	27.28
20	3.21	209.02	41.70	41.43	27.35
30	4.81	209.57	41.81	41.41	27.42
40	6.42	210.13	41.92	41.39	27.50
50	8.02	210.68	42.03	41.37	27.57
60	9.63	211.23	42.14	41.35	27.64
70	11.23	211.79	42.25	41.33	27.71
80	12.84	212.34	42.36	41.30	27.79
90	14.44	212.89	42.47	41.28	27.86
100	16.04	213.45	42.58	41.26	27.93

. If the external heat source of the Rankine cycle is assumed as the sum of coal and sun energy input, the efficiency is lower. In the case where the feedwater mass flow rate is 100 kg/s, the efficiency is the lowest, but the power output of the power plant is highest compared to the scenarios with lower feedwater mass flow rates.

4.2.2. Variant 2

The simulation results for Variant 2 presented in Figure 7 were built upon a previous configuration of the solar-assisted steam power plant, where the solar contribution was lower, at 16.045 MW, and the fuel-based thermal efficiency of the system was 42.58%, when the maximum total efficiency is equal to 27.93%. In the earlier configuration, the plant achieved a maximum net electricity output of 213.45 MW, with the coal-fired system providing the same consistent thermal input of 501.258 MW. While the solar input in the previous case positively impacted performance, its smaller contribution limited the extent of efficiency gains. The detailed total efficiencies of variant 2 are presented in

Table 5. Simulation results for variant 2.

Mass Flow	Solar Heat Input	Power Output	Fuel Efficiency	Fuel + Solar Efficiency	Total Efficiency
kg/s	MW	MW	%	%	%
10	2.76	208.80	41.66	41.43	28.05
20	5.51	209.69	41.83	41.38	28.17
30	8.27	210.58	42.01	41.33	28.29
40	11.03	211.47	42.19	41.28	28.41
50	13.79	212.36	42.36	41.23	28.53
60	16.54	213.24	42.54	41.18	28.65
70	19.30	214.13	42.72	41.14	28.77
80	22.06	215.02	42.90	41.09	28.89
90	24.82	215.91	43.07	41.04	29.01
100	27.57	216.80	43.25	41.00	29.13

.

In comparison, the current configuration demonstrates a notable improvement with a higher solar energy input of 27.57 MW, which directly enhances the plant’s thermal efficiency to 43.25% and increases the net electricity output to 216.8 MW. This progression underscores the impact of increasing solar integration on improving system performance and sustainability. The consistent coal input across both cases highlights the complementary role of solar energy in reducing fuel consumption without compromising base-load reliability. These findings collectively highlight the scalability and efficiency benefits of incorporating higher solar energy contributions into traditional coal-based power plants.

4.2.3. Variant 3

The simulated results for Variant 3 are shown in Figure 8. The solar system contributes up to 37.62 MW, which represents a substantial increase from prior configurations. This energy is utilized to preheat the working fluid or supplement steam production, reduce the load on the coal system, and improve the overall thermal fuel-driven efficiency of the plant. The coal-fired section still delivers a consistent thermal energy input of 501.258 MW, ensuring reliable base-load boiler operation. The integration of solar energy complements rather than replaces the coal energy input, thereby improving fuel efficiency and reducing specific coal consumption. The generator produces a maximum net electricity output of 219.31 MW, which is the highest among the analyzed simulations. The plant’s thermal efficiency improves further to 43.75%, reflecting a steady upward trend as solar energy input increases. The maximum total efficiency of the hybrid system reaches 29.63%. The detailed total efficiencies for each mass flow at different solar heat input and power output are presented in

Table 6. Simulation results for variant 3.

Mass Flow	Solar Heat Input	Power Output	Fuel Efficiency	Fuel + Solar Efficiency	Total Efficiency
kg/s	MW	MW	%	%	%
10	3.76	209.06	41.71	41.40	28.24
20	7.52	210.19	41.93	41.31	28.40
30	11.29	211.33	42.16	41.23	28.55
40	15.05	212.47	42.39	41.15	28.71
50	18.81	213.61	42.62	41.07	28.86
60	22.57	214.75	42.84	41.00	29.01
70	26.34	215.89	43.07	40.92	29.17
80	30.10	217.03	43.30	40.84	29.32
90	33.86	218.17	43.52	40.77	29.48
100	37.62	219.31	43.75	40.70	29.63

.

4.2.4. Results Comparison

The increase in solar energy input leads to a consistent improvement in thermal efficiency across the analyzed configurations. Compared to results from variants 1 and 2, with efficiencies of 42.58% and 43.25%, this simulated result from variant 3 achieves the highest efficiency of 43.75%. With a solar contribution of 37.62 MW, the coal consumption needed for generating electricity can be reduced. Although the coal input remains constant, the improved thermal efficiency indicates reduced fuel-specific energy requirements. The increased integration of solar energy reduces the environmental footprint of the power plant by lowering coal consumption and greenhouse gas emissions. This highlights the potential for hybrid systems to contribute to cleaner and more sustainable power generation. The relationship between mass flow rate and the efficiencies of solar-assisted steam power plant, solar contribution and electricity generation (power output) for three variants are illustrated in Figure 9 and Figure 10. Overall, the efficiency ${\eta }_F$ increases steadily as the mass flow rate rises. At lower mass flow rates (10–30 kg/s), all three variants show relatively low and similar efficiency levels, with Variant 1 performing the least efficiently. However, as the mass flow rate increases (50–100 kg/s), the differences between the variants become more pronounced. Variant 1 achieves an efficiency starting at 40.5% at 10 kg/s and peaks just below 43.0% at 100 kg/s. Variant 2 performs moderately better, reaching around 43.25% efficiency at the highest mass flow rate. Variant 3 outperforms both, starting at approximately 41.0% at 10 kg/s and reaching nearly 44.0% at 100 kg/s. In contrast to Figure 9, which shows increasing efficiency with higher mass flow rates, Figure 10 demonstrates a slight decline in efficiency across all three variants as the mass flow rate increases. At lower mass flow rates (10–30 kg/s), the efficiency is higher, with Variant 1 achieving the best performance at approximately 41.50%, followed by Variant 2 at 41.30% and Variant 3 at 41.10%. However, as the mass flow rate increases to 100 kg/s, efficiency decreases slightly, with Variant 1 maintaining the highest efficiency at 41.20%, followed by Variant 2 at 41.00%, and Variant 3 at 40.50%.

These results highlight the impact of increased solar energy integration on system performance. Variant 3, with the highest solar contribution, consistently achieves better efficiency by reducing reliance on coal and improving thermal energy utilization. Additionally, higher mass flow rates enhance the overall performance of all variants by improving heat transfer, steam generation and electricity generation. The efficiency gains are most notable at higher mass flows, where Variant 3 demonstrates the most significant improvements.

5. Conclusions

The heat absorbed by the fluid from the solar tower receiver varies significantly, ranging from 0 to 86.41 MW, depending on the intensity of solar radiation during operating hours. This variability highlights the intermittent nature of solar energy and the critical role of solar thermal systems in effectively capturing and utilizing solar heat. Despite this variability, the feedwater temperature at the boiler inlet remains constant at 221 °C, ensuring stable operation of the steam cycle and preventing thermal stresses within the boiler system. This stability is achieved by integrating the solar thermal component into the conventional Rankine cycle without disrupting the boiler’s operational parameters.

Interestingly, the energy consumed by the steam boiler remains constant across all three variants of the hybrid system, regardless of the increase in the power generation of the electric generator. This indicates that the additional power output is directly attributable to the solar contribution, which offsets the energy demand for preheating the feedwater. By reducing steam extraction for feedwater heating, the system channels more energy to the turbines, thereby increasing overall power generation efficiency. This integration strategy demonstrates the effective utilization of solar energy to supplement and enhance the performance of traditional coal-fired power plants without altering the boiler’s energy input, ensuring operational consistency. Such designs improve thermal efficiency and allow for increased electricity generation during peak solar hours, making the hybrid power plant a promising solution for reducing fossil fuel dependence and CO₂ emissions while maintaining reliable power output. Furthermore, the ability to sustain a constant feedwater temperature and boiler energy input under varying solar conditions underscores the robustness of the hybrid system, which is critical for achieving a cleaner energy transition.

Incorporating solar energy significantly enhances the efficiency of the plant while reducing emissions. Even with a modest solar contribution, the system achieves improved performance and lower environmental impact. The findings highlight the potential for further optimization by increasing the solar input, which could lead to greater reductions in coal dependency. In conclusion, the proposed Hybrid Solar–Coal Power Plant solution shows a promising path toward balancing sustainability and reliability. It leverages the renewable benefits of solar energy to enhance the performance of traditional coal systems, opening up the potential for cleaner and more efficient energy solutions.