Energy, Exergy Analysis and Sustainability Assessment of a Thermal Power Plant Operating in Various Environmental Conditions Using Real Operational Data

Abstract

It is well known that fossil fuels, especially coal, are still intensively used when considering the distribution of the main energy demand for electricity generation. Efforts to increase and optimise the efficiency of energy production are accelerating as global demand for energy continues to rise. In meeting the world’s energy needs, thermal power plants have an essential role to play. However, it remains an ongoing concern to improve their performance and sustainability. In this study, based on real operating data at varying ambient temperatures, an exergy analysis and an exergy-based sustainability assessment of a 210 MW coal-fired thermal power plant in Turkey are presented. The results of the energy analysis show that 59.01% of the total energy destruction belongs to the boiler and 12.29% to the intermediate-pressure turbine. This means that these are the main components for energy analysis. According to the obtained results of the exergy analysis, the boiler is the main constituent with the maximum exergy destruction, with a rate of 71.00% among the other constituents at the reference temperature of 25 °C. In addition, the relative irreversibility values were calculated as 79.43% in the boiler, 5.42% in the intermediate-pressure turbine (IPT), and 4.22% in the low-pressure turbine (LPT). These are the components that cause the most intensive irreversibility among the other plant components. Moreover, the component that had the greatest exergy efficiency was the ejector, at 98.62%, followed by the high-pressure heater (HPH-3) at 96.00%, the low-pressure heater (LPH-2) at 88.16%, and the high-pressure turbine (HPT) at 86.12%. The sustainability efficiency indicator (SEI) and the exergetic ecological index (ECEI) for the thermal power plant were 2.50 and 0.245, respectively, according to the exergy-based sustainability indices. The boiler, the turbine group, and the condenser are especially significant for increasing plant efficiency due to their high potential for improvement.

1. Introduction

Meeting energy demands is a very important criterion for development. Today, dependence on energy has increased rapidly due to population growth, global industrialisation, urbanisation, and the evolution of technology. Nevertheless, there is an accelerated surge in energy usage, which is associated with emissions and global warming. Currently, around 80 percent of electricity is generated from fossil fuels, and just 20 percent from renewables [1,2]. The growing requirement for energy is boosted by the world’s growing population and global economic progress, creating a huge gap between energy availability and demand. Studies show that electricity demand accounts for about 6 percent of the average annual growth of the global economy. Projections also point to a continuous increase in electricity consumption over the coming decades [3,4]. Thermal power plants, which use a variety of fuel sources, play an important part in supplying the world’s energy demands. Despite calls to adopt more sustainable energy sources, coal-fired plants continue to play an important role in electricity generation, particularly in countries where coal is the main fuel [5]. It is crucial to understand the irreversible losses in these power plants to improve operational efficiency and sustainability. Energy and exergy appraisals provide a comprehensive framework for utilising thermal concepts to identify potential and enhance efficiency [6]. Exergy analysis is applied to find out the positions, amounts, factors, sources, and properties of energy losses. In designing energy-efficient systems and improving existing systems, this method has a key role to perform. Also, it is a powerful method that can be used to identify the cause of irreversibilities occurring in thermal systems [7]. It takes a prominent position in building, analysing, and enhancing thermal power plants [8]. As environmental concerns and the management of environmental impacts have come to the fore, the idea of sustainability has emerged as a major priority. Therefore, in this study, the exergetic sustainability index concept was used to determine the baseline sustainability level in the evaluation of the thermal power plant. There are a number of review articles and research papers in the literature on energy and exergy analysis in thermal power plants. Elwardany et al. [9] performed a detailed review of thermodynamic modelling and assessment of coal-fired power plants, highlighting the critical role of power plants in the transition to more sustainable thermal power generation. Si et al. [10] carried out an exergetic evaluation of a 1000 MW coal-fired ultra-supercritical power plant. In this study, it was reported that the exergy loss rate of the boiler was as high as 85%, followed by the high-pressure turbine and low-pressure turbines. The overall exergy efficiency was calculated to be 45.4%. Sairamkrishna et al. [11] conducted an energy and exergy evaluation of a 210 MW coal-fired power plant. The plant components were divided into three different sections, and calculations were performed. According to the results, the boiler contributed the most to the exergy losses (68.27%), whereas the condenser was found to lose the most energy (49.92%). With the mentioned analysis method, exergy destruction and losses in the system can only be shown as a coarse distribution. However, this is insufficient to reveal the potential of the individual components within the system to be developed separately and the sustainability interactions between different components. Rocha and Silva [12] analysed an ultra-supercritical coal-fired power plant from exergetic and environmental points of view. The total exergy destruction of the power plant was found to be 965.95 MW. Approximately 91% of this (876.8 MW) was reported to be due to the boiler. Furthermore, the power plant’s entire exergy efficiency was calculated at 41.54%. A detailed exergy-based study, including economic and sustainability analyses, was conducted by Kumar et al. [13] for a 210 MW coal-fuelled power plant. Exergy analysis outcomes indicated that the major critical piece of equipment in the whole facility (the boiler) had 88.91% exergy destruction. It was also stated that coal combustion and ash emission decreased with the increase in ambient temperature. Sustainability analysis revealed that a high depletion coefficient corresponds to a low sustainability coefficient. A study of the influence of enthalpy and pressure loss in pipelines on the efficiency of a coal-fired power plant was carried out by Geete [14]. At full load, it was stated that the boiler had a maximum 729.54 kW/kg entropy generation rate and 221,051.3 kW exergy destruction rate, whereas the exergy destruction (16.69 kW/K) and irreversibility (5059.25 kW) values of low-pressure turbines were minimal. Chen et al. [15] performed a comprehensive energy analysis for a S-CO₂ coal-fired power plant with single and double reheating applications. According to the research, the exergy losses due to pressure losses were split from the overall exergy losses for thermal processes in specific equipment. This method aimed to reveal the effects of mass transfer and heat transfer processes on exergy destruction. Yang et al. [16] discussed a coal-fuelled high-capacity plant for power generation in the framework of a comprehensive exergy analysis. It was revealed that the energy reduction possibilities, both for the whole system and for specific components, are incompatible with the exergy losses. According to the findings, most exergy destruction takes place in the boiler subsystem, although it was demonstrated that enhancing this component would not be sufficient to reduce fuel consumption. It was also reported that conventional and advanced exergy results differ significantly across different components. Gogoi et al. [17] performed the energy, exergy, and exergoeconomic (3E) evaluation of a CCPP. The 3E results showed that the proposed CCPP gives a net power of 54.22 MW at a specified operating condition, with energy and exergy efficiencies of 44.79% and 40.89%, respectively. The component-based analysis indicated that about 73% of the total exergy destruction takes place in the combustion chamber alone. Cehaji’c et al. [18] evaluated a high-capacity plant unit by means of exergy analysis and identified the most exergy-wasting regions. The boiler was reported to have the greatest exergy loss in nominal conditions, followed by the turbine, condensers 1 and 2. It was emphasised that, by revealing the impact of different performance requirements on the whole effectiveness of the cycle, the system can be optimised.

The performance of a solar thermal power plant was evaluated by Moreno-Gamboa et al. [19] from different points of view. A parametric evaluation was carried out with various working fluids, and energy and exergy analyses were applied. Based on the model, the most efficient working fluid for the system was found to be helium. Syah et al. [20] modelled a power plant integrated with a kind of fuel cell and a generator. They optimised the system with multiple decision-making mechanisms and exergy-based analysis methods. The exergy efficiency and exergy destruction values of the fuel cell and generator were calculated. A comparable investigation was also conducted to evaluate the influence of variations in running conditions and materials on system behaviour. Based on the information obtained from previous research, it has been found that there are various shortcomings in energy- and exergy-based analyses, leading to an inappropriate assessment of system performance. These shortcomings can be listed as follows:

• Energy/exergy analyses have been carried out on a limited number of components and considered very few exergetic variables in thermal power plants. The studies are therefore incomplete in terms of an exact assessment of the irreversibilities and inefficiencies that impact the whole performance of the plant. There has been no study on the assessment of exergy efficiency for a thermal power plant under different environmental conditions and considering different exergetic variables, like exergy efficiency, relative irreversibility, improvement potential rate, fuel depletion ratio, and exergy factor. In addition, no study has reported the relationship between the sustainability efficiency indicator (SEI), exergetic ecological index (EEI), and exergetic efficiency.
• The results of this study may enable more effective decisions to be made about the design of future thermal power plants for both investors and researchers. In addition, by integrating sustainability indices within the analysis, it provides an approach to decrease the negative environmental effects of energy production. Hence, based on the stated reasons, this study analyses a coal-fired thermal power plant to find a solution for the effective utilisation of energy and the reduction in energy consumption.
• A content analysis of the coal utilised in the plant was carried out prior to the thermodynamic analysis. These data were used to determine the combustion equation of the fuel. Then, the energy and exergy balances of all individual equipment were established along with the fuel and flue gases, thermodynamic properties of each node points were determined according to the data collected from the plant, and the energy and exergy analyses were performed for the thermal power plant under different environmental conditions. The exergetic ecological index and the sustainability efficiency indicator were used for the first time together with the comprehensive exergetic parameters to assess the plant at different ambient temperatures, ranging from 25 to 46 °C.

2. Materials and Methods

2.1. System Description

In this research, the actual operating data of a coal-fuelled power plant called the Yeniköy Thermal Power Plant, located in Muğla, Turkey, were utilised. Yeniköy Thermal Power Plant consists of two units, each of which has a power of 210 MW, and the plant has a total power of 420 MW. Within the scope of this study, only one of the units is discussed. In this unit, a turbine group, steam boiler, condenser, pump groups, heater groups, and an auxiliary group called the gland condenser and ejector are included. The turbine group comprises a low-pressure turbine (LPT), an intermediate-pressure turbine (IPT), and a high-pressure turbine (HPT). The heater groups consist of four low-pressure feedwater heaters (LPHs) and three high-pressure feedwater heaters (HPHs). There are a condenser extraction pump (CEP) group and boiler feed pump (BFP) group in the power plant unit. Figure 1 presents the general layout of the discussed energy generation plant, along with the numbering system used in the calculations. Each unit operates in accordance with the cycle of a steam power plant. The supply water passes into the boiler at state 13 and, after the combustion process in the boiler, steam becomes superheated and exits the boiler at high temperature and pressure at state 14. Then, superheated steam enters the high-pressure turbine; it expands as it passes through the turbine, and then the temperature and pressure drops, the low-energy steam expands again in the turbine, and part of it enters the high-pressure heaters, where the rest of the vapour is transferred to the boiler for reheating at state point 15 and to the intermediate-pressure turbine at state point 16. There is some steam expansion at state points 21, 22, 23, and 24. The remaining part of the vapour passes the low-pressure turbine at state point 17. After the condensation in the low-pressure turbine, the vapour passes to the low-pressure heater at point 25, while the rest is collected in the condenser.

A cooling tower is used to cool and reutilise the liquid collected in the condenser. The condenser extraction pump transfers the cooling circuit water to the main water supply line. The water in the main feed line then passes through the ejector, low-pressure heater 1, gland condenser, and low-pressure heaters 2, 3, and 4 to the deaerator. During this time, the main feedwater coming from the deaerator is combined with the bleed steam separated from the medium-pressure turbine and the high-pressure turbine and supplied to the boiler through the high-pressure heaters 1, 2, and 3 by the boiler feed pump. In this way, the cycle is repeated.

2.2. Methodology

Energy and exergy analysis methods are frequently used in the evaluation of thermal coal-fired power plants. While energy analysis provides a picture of the present condition of the system, exergy analysis can identify the position and extent of irreversibilities and the amount of potential useful energy for the plant. In this study, EES (Engineering Equation Solver) software (V12.011) was applied to determine the thermophysical properties and to conduct the energy and exergy analysis [21]. The principal steps in carrying out an energy, exergy, and sustainability assessment of a coal-fired power plant are as follows:

• Draw a process stream diagram of the cycle in the power plant and identify the inlet and outlet points of all components in the cycle.
• Consider the main system components of the power plant and measure the mass flow rate, temperature, and pressure at each inlet and outlet of these components.
• The thermophysical properties, such as enthalpy and entropy, required for each inlet and outlet point are determined by the EES software.
• Mass, energy, and exergy balances are established, and corresponding quantities of energy and exergy are calculated for each component.
• Carry out energy and exergy analyses for all components involved.
• Assess the thermal power plant and its components in terms of energetic efficiency, exergetic efficiency, exergy destruction, and exergetic parameters.
• Perform the sustainability indicators to determine the sustainability of the cycle and its components.

2.2.1. Component-Based Energy-Exergy Analysis

Energy and exergy analysis is used in thermal power plants to identify the current situation and then to determine the magnitude and locations of improvements required. In this study, the main components of a 210 MW coal-fired thermal power plant are investigated using energy and exergy analysis. The equations given are generally intended for the energetic and exergetic evaluation of a thermal power plant.

The conservation of energy, known as the first law of thermodynamics, is expressed by Equation (1) for an open steady-state flow system:

$$\dot{Q}-\dot{W}={\sum }_{\dot{m}}(h_i+{\displaystyle \frac{V_i^2}{2}}+\mathrm{g}{Z}_i)-{\sum }_{\dot{m}}(h_o+{\displaystyle \frac{V_o^2}{2}}+\mathrm{g}{Z}_o)$$

(1)

Exergy analysis can provide a detailed knowledge of a system’s thermodynamic performance; thus, optimisation of the system can be achieved. This method is a very effective way of identifying the particular component that requires enhancement. This results in savings in energy consumption, increased energy efficiency, and reduced carbon emissions. By providing a comprehensive view, exergy analysis enables engineers to identify the exergy destructions occurring at different components within a system, and to take action to reduce them. As a result, it offers significant advances in both cost savings and energy efficiency [19]. A summary of the key thermophysical features critical to our analysis is provided in

Table 1. Mass flows and thermodynamic features of node points.

State No	T (°C)	P (kPa)	$\dot{\mathit{m}}$ (kg/s)	h (kJ/kg)	s (kJ/kgK)	ex (kJ/kg)	$\dot{\mathit{E}\mathit{x}}$ (kW)
1	42	7.2	123.05	167.53	0.5724	1.52	186.99
2	46	1900	123.05	194.1	0.6509	4.68	576.49
3	45	1750	123.05	190.1	0.6377	4.62	568.57
4	58	1600	123.05	244.1	0.8052	8.68	1068.16
5	64	1300	123.05	269	0.8804	11.16	1373.22
6	95	1125	131.56	406	1.238	41.54	5465.13
7	120	800	136.36	504.44	1.527	53.82	7339.31
8	141	550	145.78	590.20	1.75	73.09	10,655.31
9	153	800	145.78	644.09	1.872	90.60	13,208.68
10	155	16,500	145.78	663.7	1.875	109.32	15,936.59
11	169	16,400	150.61	723.7	2.013	128.17	19,304.4
12	191	16,300	163	825.16	2.22	166.73	27,177.22
13	219	14,500	172	958.49	2.48	221.64	38,123.08
14	534	12,800	161.38	3430	6.56	1477.59	238,467.30
15	324	2400	140.10	3068	6.76	1056.26	147,989.40
16	535	2200	135.78	3543	7.48	1316.29	178,726.70
17	170	120	115	2814	7.61	547.34	62,944.56
18	63	7.26	111.38	2617	8.39	118.08	13,153.37
19	323	2650	12.22	3059	6.70	1064.55	13,011.23
20	403	3990	9.06	3400	6.78	1382.89	12,529.03
21	464	1210	5.27	3398	7.56	1145.95	6048.08
22	404	500	9.42	3600	7.80	1277.29	12,032.09
23	275	280	4.8	3300	7.64	1025.89	4924.27
24	183	130	8.5	2840	7.63	567.67	4825.27
25	87	27	3.61	2660	7.91	304.49	1099.56
26	27.8	180	8888.88	116.6	0.40	0.20	1793.60
27	34.6	120	8888.88	146.75	0.49	2.48	22,042.77
28	25	101.325	2.78	104.8	0.3669	0.059445	0.165257

.

Exergy is the expression for the highest available work that can be achieved during a reversible change in state of a thermodynamic system that brings it into equilibrium with its surroundings. Within certain boundaries, the most fundamental exergy expression can be formulated as given below [20]:

$$\sum (1-{\displaystyle \frac{T_0}{T_i}})\dot{Q}+{\sum }_{in}{\dot{Ex}}_{in}-{\sum }_{out}{\dot{Ex}}_{out}=\dot{W}+{\dot{Ex}}_D$$

(2)

The formula can be simplified as follows if work and heat are not transferred across the system boundaries:

$${\dot{Ex}}_D={\sum }_{in}{\dot{Ex}}_{in}-{\sum }_{out}{\dot{Ex}}_{out}$$

(3)

While exergy analysis of a system is conducted, exergy balance equations are formed at the component level for all system components. At the component level, the product exergy rate (${\dot{Ex}}_P$) can be expressed as the exergy output per unit time, and the fuel exergy rate (${\dot{Ex}}_F$) can be defined as the fuel required to achieve the desired exergy output per unit time.

With respect to the fuel exergy rate and the product exergy rate, the exergy balance of a component can be formulated as shown below:

$${\dot{Ex}}_D={\dot{Ex}}_F-{\dot{Ex}}_P$$

(4)

When the initial temperature ($T_i)$ and pressure ($P_i)$ values are balanced to the determined dead-state temperature ($T_0)$ and pressure ($P_0)$, if the kinetic exergy rate and the potential exergy rate are neglected, the sum of the exergy rates is given by the following equation:

$${\dot{Ex}}_{total}={\dot{Ex}}^{ph}+{\dot{Ex}}^{ch}$$

(5)

The physical exergy for steady-state open systems is expressed by the equation below. The maximum work that can be achieved is defined as physical exergy:

$${\dot{Ex}}^{ph}={\dot{m}}_i\left[\left(h_i-h_0\right)-T_0(s_i-s_0)\right]$$

(6)

In power plants, the calculation of chemical exergy is relevant due to processes that involve chemical reactions such as combustion and flue gas formation. Chemical exergy is the exergy of a flow of matter when the state of matter matches the condition of the surroundings. The specific exergy rates for the gas mixtures can be obtained as follows:

$$e_{ex}^{-ch}=\sum x_k{e}_k^{-0}+\overline{R}{T}_0\sum x_{k}In{x}_k$$

(7)

The fuel exergy rate released by coal combustion in the boiler can be calculated as follows [22]:

$${\dot{Ex}}_{fuel}={\dot{m}}_{fuel}\times {LHV}_f\times \left(1.0064+0.1519\times {\displaystyle \frac{H}{C}}+0.0616\times {\displaystyle \frac{O}{C}}+0.0429\times {\displaystyle \frac{N}{C}}\right)$$

(8)

where ${\dot{m}}_{fuel}$ is the mass flow rate of coal burning in the boiler. In this power plant, the mass flow rate of the coal is 81.2 kg/s. ${LHV}_f$ represents the lower heating value of the fuel, and it is 1859.3 kcal/kg for the coal used in this power plant. Equation (8) also shows the mass of products (oxygen, carbon, hydrogen, and nitrogen) in the fuel.

Table 2. Energy, exergy, and exergy efficiency equations for the plant components.

	Energy balance equation: ${\dot{E}}_{fuel}+{\dot{m}}_{13}{h}_{13}+{\dot{m}}_{15}{h}_{15}+{\dot{m}}_a{h}_a={\dot{m}}_{14}{h}_{14}+{\dot{m}}_{16}{h}_{16}+{\dot{m}}_g{h}_g+{\dot{Q}}_{boiler}$ Exergy balance equation: ${\dot{Ex}}_{D,boiler}{=\dot{Ex}}_{13}+{\dot{Ex}}_{15}+{\dot{Ex}}_{fuel}+{\dot{Ex}}_{air}-{\dot{Ex}}_{14}-{\dot{Ex}}_{16}-{\dot{Ex}}_g$ ${\epsilon }_{boiler}={\displaystyle \frac{\left({\dot{Ex}}_{14}+{\dot{Ex}}_{16}+{\dot{Ex}}_g\right)}{{\dot{Ex}}_{fuel}+{\dot{Ex}}_{13}+{\dot{Ex}}_{15}+{\dot{Ex}}_{air}}}$		Energy balance equation: ${\dot{m}}_{17}{h}_{17}={\dot{m}}_{25}{h}_{25}+{\dot{m}}_{18}{h}_{18}++{\dot{W}}_{LPT}+{\dot{Q}}_{LPT}$ Exergy balance equation: ${\dot{{\dot{Ex}}_{D,LPT}=Ex}}_{17}-{\dot{W}}_{LPT}-{\dot{Ex}}_{25}-{\dot{Ex}}_{18}$ ${\epsilon }_{LPT}={\displaystyle \frac{{\dot{W}}_{LPT}}{{\dot{Ex}}_{17}-{\dot{Ex}}_{25}-{\dot{Ex}}_{18}}}$
	Energy balance equation: ${\dot{m}}_{14}{h}_{14}={\dot{m}}_{15}{h}_{15}+{\dot{m}}_{19}{h}_{19}+{\dot{m}}_{20}{h}_{20}+{\dot{W}}_{HPT}+{\dot{Q}}_{HPT}$ Exergy balance equation: ${\dot{{\dot{Ex}}_{D,HPT}=Ex}}_{14}-{\dot{W}}_{HPT}-{\dot{Ex}}_{15}-{\dot{Ex}}_{19}-{\dot{Ex}}_{20}$ ${\epsilon }_{HPT}={\displaystyle \frac{{\dot{W}}_{HPT}}{{\dot{Ex}}_{14}-{\dot{Ex}}_{15}-{\dot{Ex}}_{19}-{\dot{Ex}}_{20}}}$		Energy balance equation: ${\dot{m}}_{18}{h}_{18}+{\dot{m}}_{28}{h}_{28}+{\dot{m}}_{26}{h}_{26}={\dot{m}}_{27}{h}_{27}+{\dot{m}}_1{h}_1+{\dot{Q}}_{COND.}$ Exergy balance equation: ${{\dot{Ex}}_{D,COND.}=\dot{Ex}}_{18}+{\dot{Ex}}_{28}+{\dot{Ex}}_{26}-{{\dot{Ex}}_{27}-\dot{Ex}}_1$ ${\epsilon }_{COND.}={\displaystyle \frac{{\dot{Ex}}_{27}-{\dot{Ex}}_{26}}{{\dot{(Ex}}_{18}+{\dot{Ex}}_{28})-{\dot{Ex}}_1}}$
	Energy balance equation: ${\dot{m}}_{16}{h}_{16}={\dot{m}}_{17}{h}_{17}+{\dot{m}}_{21}{h}_{21}+{\dot{m}}_{23}{h}_{23}+{\dot{m}}_{24}{h}_{24}+{\dot{W}}_{IPT}+{\dot{Q}}_{IPT}$ Exergy balance equation: ${\dot{{\dot{Ex}}_{D,IPT}=Ex}}_{16}-{\dot{W}}_{IPT}-{\dot{Ex}}_{17}-{\dot{Ex}}_{21}-{\dot{Ex}}_{22}-{\dot{Ex}}_{23}-{\dot{Ex}}_{24}$ ${\epsilon }_{IPT}={\displaystyle \frac{{\dot{W}}_{IPT}}{{\dot{Ex}}_{16}-{\dot{Ex}}_{17}-{\dot{Ex}}_{21}-{\dot{Ex}}_{22}-{\dot{Ex}}_{23}-{\dot{Ex}}_{24}}}$		Energy balance equation: ${\dot{m}}_1{h}_1+{\dot{W}}_{CEP}={\dot{m}}_2{h}_2+{\dot{Q}}_{CEP}$ Exergy balance equation: ${\dot{Ex}}_{D,CEP}={\dot{W}}_{CEP}-{\dot{(Ex}}_2-{\dot{Ex}}_1)$ ${\epsilon }_{CEP}={\displaystyle \frac{{\dot{Ex}}_2-{\dot{Ex}}_1}{{\dot{W}}_{CEP}}}$
	Energy balance equation: ${\dot{m}}_2{h}_2={\dot{m}}_2{h}_2+{\dot{Q}}_{EJEC.}$ Exergy balance equation: ${\dot{Ex}}_{D,EJEC.}={\dot{Ex}}_2-{\dot{Ex}}_3$ ${\epsilon }_{EJEC.}={\displaystyle \frac{{\dot{Ex}}_3}{{\dot{Ex}}_2}}$		Energy balance equation: ${\dot{m}}_8{h}_8={\dot{m}}_9{h}_9+{\dot{Q}}_{DEA.}$ Exergy balance equation: ${\dot{Ex}}_{D.DEA.}={\dot{Ex}}_8-{\dot{Ex}}_9$ ${\epsilon }_{DEA.}={\displaystyle \frac{{\dot{Ex}}_9}{{\dot{Ex}}_8}}$
	Energy balance equation: ${\dot{m}}_{25}{h}_{25}+{\dot{m}}_3{h}_3={\dot{m}}_4{h}_4+{\dot{Q}}_{LPH-1}$ Exergy balance equation: ${\dot{Ex}}_{D,LPH-1}={\dot{Ex}}_{25}+{\dot{Ex}}_3-{\dot{Ex}}_4$ ${\epsilon }_{LPH-1}={\displaystyle \frac{{\dot{Ex}}_4}{{\dot{Ex}}_{25}+{\dot{Ex}}_3}}$		Energy balance equation: ${\dot{m}}_{22}{h}_{22}+{\dot{m}}_7{h}_7={\dot{m}}_8{h}_8+{\dot{Q}}_{LPH-4}$ Exergy balance equation: ${\dot{Ex}}_{D,LPH-4}={\dot{Ex}}_{22}+{\dot{Ex}}_7-{\dot{Ex}}_8$ ${\epsilon }_{LPH-4}={\displaystyle \frac{{\dot{Ex}}_8}{{\dot{Ex}}_{22}+{\dot{Ex}}_7}}$
	Energy balance equation: ${\dot{m}}_4{h}_4+{\dot{m}}_{gs}{h}_{gs}={\dot{m}}_5{h}_5+{\dot{Q}}_{GC}$ Exergy balance equation: ${\dot{Ex}}_{D,GC}={\dot{Ex}}_4+{\dot{Ex}}_{gs}-{\dot{Ex}}_5$ ${\epsilon }_{GC}={\displaystyle \frac{{\dot{Ex}}_5}{{\dot{Ex}}_4+{\dot{Ex}}_{gs}}}$		Energy balance equation: ${\dot{m}}_{21}{h}_{21}+{\dot{m}}_{10}{h}_{10}={\dot{m}}_{11}{h}_{11}+{\dot{Q}}_{HPH-1}$ Exergy balance equation: ${\dot{Ex}}_{D,HPH-1}={\dot{Ex}}_{21}+{\dot{Ex}}_{10}-{\dot{Ex}}_{11}$ ${\epsilon }_{HPH-1}={\displaystyle \frac{{\dot{Ex}}_{11}}{{\dot{Ex}}_{21}+{\dot{Ex}}_{10}}}$
	Energy balance equation: ${\dot{m}}_{24}{h}_{24}+{\dot{m}}_5{h}_5={\dot{m}}_6{h}_6+{\dot{Q}}_{LPH-2}$ Exergy balance equation: ${\dot{Ex}}_{D,LPH-2}={\dot{Ex}}_{24}+{\dot{Ex}}_5-{\dot{Ex}}_6$ ${\epsilon }_{LPH-1}={\displaystyle \frac{{\dot{Ex}}_6}{{\dot{Ex}}_{24}+{\dot{Ex}}_5}}$		Energy balance equation: ${\dot{m}}_{11}{h}_{11}+{\dot{m}}_{19}{h}_{19}={\dot{m}}_{12}{h}_{12}+{\dot{Q}}_{HPH-2}$ Exergy balance equation: ${\dot{Ex}}_{D,HPH-2}={\dot{Ex}}_{19}+{\dot{Ex}}_{11}-{\dot{Ex}}_{12}$ ${\epsilon }_{HPH-2}={\displaystyle \frac{{\dot{Ex}}_{12}}{{\dot{Ex}}_{19}+{\dot{Ex}}_{11}}}$
	Energy balance equation: ${\dot{m}}_{23}{h}_{23}+{\dot{m}}_6{h}_6={\dot{m}}_7{h}_7+{\dot{Q}}_{LPH-3}$ Exergy balance equation: ${\dot{Ex}}_{D,LPH-3}={\dot{Ex}}_{23}+{\dot{Ex}}_6-{\dot{Ex}}_7$ ${\epsilon }_{LPH-3}={\displaystyle \frac{{\dot{Ex}}_7}{{\dot{Ex}}_{23}+{\dot{Ex}}_6}}$		Energy balance equation: ${\dot{m}}_{12}{h}_{12}+{\dot{m}}_{20}{h}_{20}={\dot{m}}_{13}{h}_{13}+{\dot{Q}}_{HPH-3}$ Exergy balance equation: ${\dot{Ex}}_{D,HPH-3}={\dot{Ex}}_{20}+{\dot{Ex}}_{12}-{\dot{Ex}}_{13}$ ${\epsilon }_{HPH-3}={\displaystyle \frac{{\dot{Ex}}_{13}}{{\dot{Ex}}_{20}+{\dot{Ex}}_{12}}}$

shows the component-based energy, exergy, and exergy efficiency balance equations. In exergy analysis, there are some exergetic parameters that enable effective evaluation of the whole system.

In accordance with the most general definition of efficiency, exergetic efficiency is the ratio of the desired product exergy rate to the amount of required fuel exergy rate. It is presented below:

$$\epsilon ={\displaystyle \frac{{\dot{Ex}}_{P,k}}{{\dot{Ex}}_{F,k}}}$$

(9)

The fuel depletion ratio is the ratio of the exergy destruction of each system component to the total amount of the fuel exergy rate supplied for the system. It can be formulated as follows:

$$\delta ={\displaystyle \frac{{\dot{Ex}}_{D,k}}{{\dot{Ex}}_{F,total}}}$$

(10)

Relative irreversibility is the fraction of exergy destroyed in the component concerned compared to the whole exergy destroyed in the whole system. This is defined as follows:

$$RI={\displaystyle \frac{{\dot{Ex}}_{D,k}}{{\dot{Ex}}_{D,total}}}$$

(11)

The exergetic factor, which indicates the share of the equipment’s fuel exergy rate in the plant’s total fuel exergy rate, is computed using the following formula:

$$f={\displaystyle \frac{{\dot{Ex}}_{F,k}}{{\dot{Ex}}_{F,total}}}$$

(12)

According to the purpose of the implementation of the exergy analysis, the improvement potential rate, developed to determine the improvement possibilities in the process or system to which it is applied, is shown in Equation (13):

$$IP=(1-{\epsilon }_k)\times ({\dot{Ex}}_{in,k}-{\dot{Ex}}_{out,k})$$

(13)

2.2.2. Exergetic Sustainability Performance Indicators

Indicators for assessing the sustainability of energy systems are essential for a planned sustainable energy society. Sustainability is the reconciliation of social, environmental, and economic parameters in the management of resources. In this respect, exergy analysis is becoming more accurate in the sustainability assessment of energy systems, as the exergy of a system includes environmental conditions as a reference.

Based on the results of the exergy analysis, the effectiveness of the presented plant was also assessed with the help of sustainability indicators. The sustainability efficiency indicator is related to exergy efficiency and has the following expression:

$$\mathrm{SEI}={\displaystyle \frac{1}{1-{\epsilon }_{\mathrm{k}}}}$$

(14)

The exergetic ecological index (ECEI) is a further exergy-based sustainability indicator, and it is a measure of exergy efficiency and is a function of the depletion ratio. The description of this index is as follows [22]:

$$\mathrm{E}\mathrm{C}\mathrm{E}\mathrm{I}=\epsilon -{\displaystyle \frac{{\dot{\mathrm{E}\mathrm{x}}}_{\mathrm{D}}}{{\dot{\mathrm{E}\mathrm{x}}}_{\mathrm{F}}}}$$

(15)

It is desirable that the indicated index is as high as possible. This is because a high index means that the total exergy resource transformed into utilisable exergy is greater than the consumed exergy resource for the respective system or component.

3. Results and Discussion

In this study, the performance evaluation of the 210 MW coal-fired thermal power plant in Yeniköy was carried out by applying energy and exergy analysis. In addition, different exergetic parameters and sustainability indices were also used to provide a comprehensive evaluation of the system components. For thermal power plants that consume large amounts of carbon-based fuels, evaluation methods related to exergy efficiency, sustainability, and environmental impact are also essential. Exergy-based indices are used in the evaluation of thermal systems by means of the results of exergy analysis. The concept of sustainability relates to resource management in social, natural, and economic terms. There is a direct relationship between exergy efficiency and sustainability. Increasing exergy efficiency can contribute to sustainability through fuel savings and maximising the use of available energy resources. Higher exergy efficiency generally results in less resource consumption, lower emissions, and minimised environmental impact. Due to the intensive use of fossil fuels in the present study, it is very useful to analyse and improve the existing system. For this reason, the sustainability efficiency indicator and the exergetic ecological index, which highlight the relationship among environmental impact, sustainability, and exergy efficiency, are used in this study.

For the purposes of the calculations, the reference temperature and pressure were taken to be 25 °C and 101.325 kPa, respectively.

Table 3. Elemental analysis of the coal as received.

C (%)	H (%)	N (%)	S (%)	O (%)	Moisture (%)	Ash (%)	LHV (kcal/kg)
30.51	2.93	1.1	3.4	9.3	23.39	29.37	1859.3

shows the content information obtained from the elemental analysis of the fuel used in the power plant.

Table 4. Results of exergy analysis and some exergetic parameters for system evaluation.

Components	${\dot{\mathit{E}\mathit{x}}}_{\mathit{F}}$ (kW)	${\dot{\mathit{E}\mathit{x}}}_{\mathit{P}}$ (kW)	${\dot{\mathit{E}\mathit{x}}}_{\mathit{D}}$ (kW)	ε (%)	δ (%)	RI (%)	IP (kW)	f (%)
Boiler	868,557.76	462,043.72	349,452.79	53.19	28.16	79.43	163,555.58	70.00
HPT	64,937.62	55,930	9007.62	86.12	0.73	2.05	1249.47	5.23
IPT	87,952.40	64,080	23,872.40	72.85	1.92	5.42	6479.55	7.08
LPT	48,691.62	30,005	18,686.61	61.62	1.91	4.22	7171.46	4.96
Condenser	12,966.54	5537.55	7428.98	42.70	0.60	1.68	4256.32	1.04
CEP	570.96	389.49	181.46	68.21	0.02	0.041	57.67	0.05
Ejector	576.49	568.56	7.92	98.62	0.00	0.00179	0.11	0.05
LPH-1	1668.13	1068.15	599.97	64.03	0.06	0.13	215.79	0.170
GC	1077.80	305.06	772.73	28.30	0.08	0.17	554.01	0.11
LPH-2	6198.50	5465.13	733.36	88.16	0.06	0.16	86.77	0.49
LPH-3	10,389.40	7339.30	3050.09	70.64	0.25	0.69	895.44	0.83
LPH-4	7569.36	10,655.31	8716.08	55.00	0.16	0.35	319.11	0.771
Deaerator	16,703.39	13,208.67	3494.72	79.07	0.28	0.79	731.17	1.35
BFP	7272	2727.91	4544.09	37.51	0.37	1.03	2839.49	0.58
HPH-1	21,984.67	19,304.39	2680.27	87.80	0.22	0.60	326.77	1.77
HPH-2	32,315.62	27,177.22	5138.40	84.09	0.41	1.16	817.04	3.46
HPH-3	39,706.25	38,123.08	1583.17	96.01	0.13	0.36	63.12	3.19

provides detailed information on the exergy destruction rates allocated to the individual constituents of the thermal power plant, as well as the other valuable exergetic parameters used to evaluate the efficiency of the plant. It is possible to conclude from the exergy analysis that the greatest rate of exergy depletion is experienced in the boiler (349,452.79 kW), whereas the lowest level of exergy degradation takes place in the ejector (7.92 kW). This outcome is consistent with earlier studies of a range of power plant configurations [4]. The existence of chemical reactions in the boiler, where the greatest amount of exergy destruction is achieved, leads to an augmentation of the total amount of exergy destruction. For all system components, the amount of exergy destruction varies in direct proportion to the dead-state temperature. It can be observed that, when the dead-state temperature changes from 25 °C to 46 °C, the exergy destruction amounts of the boiler and the cycle in particular are significantly affected.

Figure 2 shows the effect of the variation in the dead-state temperature on the amount of exergy destruction of the cycle and its components. At a dead-state temperature of 25 °C, the exergy destruction of the boiler was computed to be 349.45 MW, so it can be said that a substantial amount of useful energy could not be utilised in the boiler. The boiler reached a 360.05 MW exergy destruction rate when the dead-state temperature was brought up to 46 °C. This result shows that the irreversibilities that arise in the boiler become worse at higher ambient temperatures, and the plant operates more inefficiently.

When the turbine group was considered, the exergy destruction values were obtained as IPT 23.87 MW, LPT 18.68 MW, and HPT 9.00 MW at a 25 °C dead-state temperature. The largest exergy destruction occurred in the IPT, and the lowest exergy destruction occurred in the HPT. When the ambient temperature rose to 46 °C, the exergy destruction occurring in the IPT was 24.95 MW, in the HPT was 9.48 MW, and in the LPT was 20.00 MW. All three turbines become less efficient as the outside temperature rises.

Similarly, the graph shows that the exergy destruction value incurred in the cycle was 439.95 MW at a reference temperature of 25 °C, and the exergy destruction value increased to 460.00 MW as the environmental temperature was boosted from 25 °C to 46 °C. Thus, it can be concluded that the cycle is adversely affected by high-temperature conditions.

The effect of ambient temperature variation with regard to the exergy efficiency of the main components is shown in Figure 3. At a 25 °C reference temperature, the highest exergetic efficiency value belongs to the HPT, with 86.12%, followed by the IPT with 72.85%, LPT with 61.62%, and boiler with 53.19%. The exergetic efficiency value of the cycle at the specified reference temperature was calculated as 59.94%. Under these conditions, we can observe that the HPT works with high efficiency, while the boiler and the LPT consume more useful energy. On the other hand, it was noted that, as the ambient temperature increases, the exergy efficiencies tend to decrease for the principal constituents of the cycle. The exergy efficiencies of the high-pressure turbine, intermediate-pressure turbine, and low-pressure turbine were calculated as 85.25%, 71.63%, and 59.37%, respectively, assuming an outdoor temperature of 46 °C. The exergy efficiency of the boiler dropped to 50.00%, and the exergy efficiency of the cycle dropped to 56.00%. The exergetic efficiency values show that the boiler and the cycle in particular are more sensitive to changes in ambient temperature.

There are different levels of exergy and energy destruction in the components of the power plant. The energy and exergy destruction distributions for the plant components are shown in Figure 4.

According to Figure 4, 59% of the energy destruction in the cycle is caused by the boiler. The turbine group is responsible for 19.18% of the energy destruction, and the condenser for 5.33%. Figure 5 also illustrates the exergy destruction rates occurring in the system components. From Figure 5, it can be seen that the boiler accounts for 70% of the total exergy destruction, with 349.45 MW; the turbine group has an exergy destruction rate of 10%, with 51.56 MW; and the contribution of the condenser to the exergy destruction rate is only 2%, corresponding to 7.43 MW of the total exergy destruction. Heat losses and incomplete combustion of the fuel are the causes of inefficiency, especially in the boiler. The exergetic efficiencies of the components in the cycle and the temperature distribution of the working fluid are illustrated in Figure 6. The graph reveals that, as the working fluid temperature increases, the exergy efficiencies of the high-pressure heaters increase. It has been demonstrated that the inefficiencies in the boiler component, where radiative heat transfer is intense, are higher than those that occur in the turbine components by convection [16].

The interaction between the sustainability efficiency indicator and the exergy efficiency for the system components is shown in Figure 7. The relationship between the sustainability efficiency index and exergy efficiency follows a linear path. In other words, the inverse relationship between dead-state temperature and exergy efficiency can also be valid for the sustainability efficiency index. Fossil fuel power plants are more susceptible to the relationship between exergy efficiency and sustainability. The ejector (72.72) and HPH-3 (25.08) have the highest sustainability efficiency indicators, while the condenser (1.74) and the boiler (2.13) and have the lowest, as a rectilinear relationship exists with the sustainability efficiency indicator and exergy efficiency.

An essential parameter for the estimation of the performance of a thermal power plant is the exergy efficiency. The maximum exergy efficiency was found to be 98.62% for the ejector, 96.012% for HPH-3, and 86.12% for the HPT, whereas the lowest exergy efficiency was found to be 28.30% for the GC, 42.70% for the condenser, and 53.19% for the boiler, respectively.

The fuel depletion rate “δ” was found to be greatest for the boiler, at which the largest exergy destruction value arises, given as the proportion of the exergy depletion rate of the kth component to the overall exergy rate of the fuel supplied to the plant. Meanwhile, the minimum is reached in the ejector, where the least amount of exergy depletion occurs between the equipment. The fuel depletion rates of the boiler and the total turbine group were found to be 37.30% and 4.61%, respectively, which are greater compared the other plant equipment. The precedence of the components to be focalised is indicated by the relative irreversibility rate “$RI$”, which represents the impact of exergy dissipation in the components on the system as a whole. Therefore, this study shows that the component of the system with the highest rate of exergy destruction is the boiler, where the chemical combustion reaction takes place.

The turbine group emerges as the other region where the exergy destruction rate is concentrated. The highest relative irreversibility rates, as a function of the exergy depletion rates, are 79.43% for the boiler, 4.22% for the LPT, 5.42% for the IPT, and 2.05% for the HPT. The exergetic improvement potential rate $\mathrm{“}IP\mathrm{”}$ indicates the amount of exergy consumption that can be reduced with the enhancements in the related component. If a component has a high exergy destruction value and a low exergetic efficiency value, then the potential for exergetic improvement will be high. According to this information, the boiler, condenser, IPT, HPT, and LPT have the greatest potential for improvement compared to other components in the presented thermal power plant. The exergetic factor “f” is considered as the proportion of the exergy value of the fuel rate of the k-th component to the exergy value of the fuel existing in the overall system. In other words, the exergetic factor can be defined as a parameter by which we can identify the component that consumes the greatest useful exergy rate among the plant equipment. On the basis of the findings in Table 3, the components that consume the most exergy flow can be listed as follows: boiler, IPT, HPT, LPT, HPH-2, HPH-3, and HPH-1, is determined to be 70.00%, 7.08%, 5.23%, 4.965%, 3.465%, 3.199%, and 1.77%, respectively.

The ECEI values are presented in Figure 8. The characteristic of this index is that it can take negative values. The index defines the relationship between exergy efficiency and the fuel depletion ratio. In other words, it can be defined as the value obtained by deducting the irreversibilities from the desired useful output. For this reason, it is a useful index for comparing useful output and losses. Except for the GC, BFP, and condenser, the results indicate that the plant components generally have positive values. The ECEI value is desired to be as high as possible, which means that the useful output is high. The ECEI values of the ejector, HPH-3, LPH-2 HPH-1, and HPT were calculated as 0.97, 0.93, 0.76, 0.75, and 0.72, respectively, and these values are the highest values among the plant components.

Table 5. The comparison parameters for power plants.

Power Plant	Capacity (MW)	First Law Efficiency (%)	Second Law Efficiency (%)	Refs.
Coal-fired thermal power plant in India	210	34.43	37.27	[13]
Combined-cycle power plant in Assiut, Egypt	750	34.6	33.5	[23]
Circulating Fluidised Bed Power Plant (CFBPP) in Turkey	160	37.16	31.26	[24]
Ultra-supercritical power plant in China	660	-	41.4	[16]
Steam power plant in Serbia	348.5	39	35.77	[25]
Yeniköy Thermal Power Plant in Turkey	210	23.73	22.82	(Current study)

provides a comparable assessment of different power plants located in the varied geographic regions of the world in terms of energy and exergy efficiency [13,16,23,24,25]. Data from power plants in India, Turkey, China, Serbia, and Egypt are included in the table. The data indicate that considerable differences in efficiency levels exist between these different power plants. The steam power plant in Serbia has the highest energy efficiency, at a remarkable 39 percent [25]. Among the listed power plants, the plant in China [16] is characterised by the greatest exergy efficiency, at 41.4%. On the other hand, this research concentrates on Yeniköy Thermal Power Plant, revealing an energy efficiency of 23.73% and an exergy efficiency of 22.82%. Although these data are considerable, they are slightly lower than the values shown in the table for the other power plants. These differences in efficiency levels indicate that there will be a potential for improvement of the efficiency of the Yeniköy Thermal Power Plant. The low efficiency levels can be attributed to various factors, such as non-ideal design, inadequate maintenance and repair, or improper operating procedures in the coal selection and combustion process.

Consequently, by applying exergy analysis and sustainability indices, refinements in plant design, maintainability procedures, and operational approaches have the ability to improve the overall performance of the plant. Our findings highlight the importance of optimising construction and working methods to increase efficiency, reduce expenses, and eventually improve the electricity generation performance. This research emphasises the crucial impact of assessing both exergy analysis and sustainability indices while evaluating thermal power plants’ performance.

4. Conclusions

This study introduces a thermal power plant and provides information on the overall system and all of its individual components. In order to gain knowledge about the system and its constituent parts corresponding to the energy and exergy analysis, exergetic sustainability indices were obtained for a range of ambient temperatures, from 25 to 46 °C. Also, the exergy destruction rates in the components were determined, and additional exergetic parameters were proposed to optimise the system and its components.

Some essential outcomes can be presented from the energy-exergy analysis and exergetic sustainability indices of the thermal power plant, as follows:

• The boiler is the primary component to consider, because it has the maximum value of exergy destruction “${\dot{Ex}}_D$” (349,452.79 kW) and the maximum exergy improvement potential “IP” (163,555.58 kW). Moreover, the relative irreversibility ratio “RI” of this component is more than 79.43%. As expected, the boiler with the highest irreversibility ratio is the one where chemical reaction take place. In order to reduce the exergy losses in the boiler, the combustion process needs to be carried out efficiently. Some of the measures that can be taken to achieve this include proper mixing of the air and fuel ratio, ensuring that the fuel has a suitable oxygen content for combustion, providing adequate insulation in the relevant equipment, and adding a waste heat exchanger to the system to utilise the waste heat from the flue gases.
• In order to eliminate the irreversibilities occurring in the turbine group, the turbine inlet temperature should be increased. To achieve this high temperature, the boiler must be superheated, and the turbine material needs be made of a heat-resistant material.
• The energy analysis showed that the boiler is the largest energy-destroying component in the system, with 59.01% of the total energy loss. The boiler is followed by the IPT, with 12.29% energy loss, and the condenser, with 5.32% energy loss. Enhancement of these components should be prioritised to increase the performance of the plant.
• In addition, the SEI (2.136) and ECEI (0.130) values for the boiler are relatively lower than those of the other components. This indicates that the boiler is not a sustainable component and that it is not sufficient to convert the exergetic fuel rate into a useful exergy rate. The SEI values of the turbine group were found to be 7.20 for the HPT, 3.68 for the IPT, and 2.60 for the LPT when the sustainability indices SEI and ECEI were assessed for the plant components. The ECEI values were calculated as 0.722 for the HPT, 0.457 for the IPT, and 0.232 for the LPT. The HPT stands out as the most energy-efficient component among the other turbine types.
• The minimum exergy efficiency rates “ε” were computed at the GC (28.30%), condenser (42.70%), and boiler (53.19%). In addition, since the condenser and boiler have a high improvement potential rate, technological and physical enhancements in those components will increase the exergetic performance of the plant. Therefore, it would be beneficial to focus on these components.
• As the reference temperature varies, the exergy destruction and exergy efficiency also vary for the main plant components and for the whole cycle. The exergy efficiency of the cycle decreased by 6.52% as the reference temperature increased from 25 °C to 46 °C. The overall cycle exergy efficiency was found to be 59.94%.
• Since the exergy efficiencies of some components in the thermal power plant are quite high (ejector, deaerator, HPH-1, HPH-2, and HPH-3) and the exergy destruction rates are relatively low compared to other components (LPH-1, LPH-2, LPH-3, LPH-4, BFP), it would not be necessary to focus on their improvement.
• When the ECEI and SEI values, which are exergetic sustainability parameters, are analysed, it can be seen that the values belonging to the ejector, deaerator, HPT, and IPT have better values than the condenser, boiler, and GC.
• Advanced exergy and exergoeconomic analyses will be taken into consideration for future studies.