Thermodynamic Analysis for Combined Cycle of Power Plant

Combined cycle power plants (CCPPs) integrate a gas turbine (GT) and a steam turbine (ST), using a Heat Recovery Steam Generator (HRSG), which is a key component of modern, highly efficient power systems. In such plants, the gas turbine operates on the Brayton cycle, and its exhaust heat is recovered by the HRSG to produce the steam required by the steam turbine, which in turn operates on the Rankine cycle. Thus, the term "combined cycle" reflects the integration of these two thermodynamic processes [1-3]. In a standard combined cycle power plant, the hot exhaust gases from the gas turbine flow directly into the HRSG, where they produce superheated steam at the required pressure and temperature without the need for additional fuel. Fuel consumption is thus limited to the gas turbine's combustion chamber. Because the HRSG provides the thermal link between the GT and the ST, its design is critical to maximizing heat transfer and improving overall plant efficiency. An efficient HRSG design aims to optimize heat exchange while minimizing surface area, taking into account factors such as the heat transfer coefficients of the working fluids in the economizer and evaporator sections, relative to the temperature and flow rate of the gas turbine exhaust. An open cycle gas turbine consists of a compressor, a combustion chamber, and a turbine. The compressor compresses the air before it is mixed with fuel in the combustion chamber. The resulting high-temperature gases, typically between 900 and 1400 ℃, expand through the turbine to generate power, releasing flue gas [4]. Flue gas refers to the combustion products that exit through a chimney or duct, usually from boilers, furnaces, or other power generation equipment.

The combined cycle power plant concept is based on two fundamental thermodynamic cycles: the gas turbine cycle (Brighton) and the steam turbine cycle (Rankine), as illustrated in Figure 1 [5]. From this figure, the input temperature in a gas turbine cycle is much higher than the input temperature in a steam cycle.

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Figure 1. Gas-steam combined cycle. (a) Gas turbine cycle having excessive waste heat, (b) combined cycle using turbine exhaust to produce steam efficiently

As we stated earlier, the combined cycle integrates two thermodynamic processes: the Brayton cycle (gas turbine) and the Rankine cycle (steam turbine). High temperature flue gases from the gas turbine enter the HRSG, where they transfer heat to the water and generate steam, most of the required energy being the latent heat of vaporization. In the gas turbine, compressed air mixes with fuel and combusts at high temperature, driving the turbine and producing electricity. In a combined cycle plant, the waste heat from the gas turbine is recovered rather than discharged, improving overall efficiency. The HRSG captures this exhaust heat to produce steam for the steam turbine, which drives an additional generator and increases total power output, as shown in Figure 2.

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Figure 2. Schematic of a simple combined cycle power plant [6]

This design is more advanced than the simpler design shown in Figure 2, as it incorporates key thermodynamic components such as a superheater (SH), reheater (RH), condenser, and feedwater heaters, including a deaerator. Without additional heating, the steam exiting the drum remains saturated and condenses quickly; therefore, recovering its latent heat significantly improves thermal efficiency [6]. However, excessive condensation can pose a problem for turbine operation, as typical guidelines specify a moisture content of 10% in the downstream stages of a low pressure (LP) turbine [7]. The moisturize levels exceeding this limit can cause significant blade erosion and damage. Since the advent of steam generators, designers have been constantly improving the condenser, the main heat exchanger (HE) in the system, as thermodynamic analysis shows that efficient steam condensation can increase turbine output by about one-third [8]. As steam condenses in the condenser, a strong vacuum is generated, increasing the thrust through the LP turbine. However, heat loss due to waterside fouling, scaling, or steam air in-leakage can significantly reduce efficiency, potentially costing the plant five to six figure increases in annual fuel expenses [9]. The steam surface condenser is a central component of thermal power plants and a critical element of the water-steam cycle. Its performance directly affects the plant's power output, operating costs, and overall efficiency. Because condenser heat transfer governs the final stage of steam energy extraction, maintaining high heat exchange efficiency is essential for optimal plant performance [10]. Deficiencies in the condenser can arise from multiple sources, leading to reduced power output, increased fuel consumption and operating costs, poor environmental performance due to high carbon dioxide emissions, and ultimately reduced profit margins [11]. The performance of the condenser in a steam turbine system depends primarily on the effectiveness of heat transfer from the exhaust steam to the cooling water; the faster the heat removal rate, the faster the steam condenses. While many power plants have focused on improving heat consumption rates through combustion optimization, furnace adjustment, fuel modification, and turbine upgrades [12-14], the performance of the condenser and its influence on overall cycle efficiency has received comparatively less attention. This study addresses this gap by conducting a detailed thermodynamic analysis of a single pressure, unfired HRSG integrated into a CCPP. Unlike previous studies that have examined CCPP performance using simplified assumptions, this work provides a focused evaluation of HRSG heat transfer behavior through pinch point and approach point temperature analysis. The study quantifies the effects of turbine inlet temperature, exhaust gas temperature, and ambient air temperature (AAT) on heat recovery and combined cycle efficiency, and develops thermal balance relationships to determine optimal operating conditions that maximize steam generation and cycle performance. The findings offer a clearer understanding of how HRSG design parameters influence overall CCPP efficiency and provide practical guidance for enhancing heat transfer effectiveness in real power plant applications.

This section presents the thermodynamic behavior of the CCPP under different operating conditions, with emphasis on the influence of pinch point temperature, exhaust gas temperature, turbine inlet temperature, and AAT. The discussion focuses on why these variations occur based on cycle thermodynamics and heat transfer principles. The heat recovered from the flue gases can be balanced in two ways: from the exhaust side and from the water profile. The heat recovery system from the gas turbine is computed using Eq. (9), and the heat recovery system from the steam turbine is calculated using Eq. (10):

$\mathrm{Q}_{\text {recovery}}=\mathrm{m}_{\mathrm{gt}} \mathrm{C}_{\mathrm{pg}}\left(\mathrm{T}_{\mathrm{g} 1}-\mathrm{T}_{\mathrm{g} 4}\right)$           (9)

$\mathrm{Q}_{\text {recovery}}=\mathrm{m}_{\mathrm{st}}\left(\mathrm{h}_{\mathrm{sh}}-\mathrm{h}_{\mathrm{fw}}\right)$              (10)

Figure 4 depicts the temperature enthalpy profile of the HRSG that was executed based on temperature parameters as provided in Section 2.2. This figure illustrates heat transfer between the hot flue gases and the water steam path. The profile highlights the locations of the economizer, evaporator, and superheater and emphasizes the minimum temperature difference (pinch point) in the evaporator section. The shape of the curves demonstrates that the evaporator operates near the saturation temperature, where heat addition is dominated by latent heat. Because phase change occurs at nearly constant temperature, the driving temperature difference between the two streams becomes small. This explains why the pinch point typically appears in the evaporator rather than in the economizer or superheater.

The relationship between the temperature of hot gases and the pinch point temperature difference (T_pp) in the HRSG is depicted in Figure 5. The pinch point temperature difference is a crucial distinction that must be considered [18], which is the difference between the saturation temperature and the temperature of the gas at the evaporator exit. Based on observations, the values of T_pp grow as the temperature of hot gases rises. This occurs because higher flue gas temperatures increase the temperature gradient between the evaporator exit and the steam saturation line. Consequently, more heat is available for recovery, but the increasing temperature difference also indicates that additional surface area would be required to maintain effective heat transfer in practical HRSG designs. A higher pinch point temperature difference generally reduces HE effectiveness. Therefore, while high exhaust temperatures supply more recoverable energy, they must be balanced with an economically feasible heat transfer area.

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Figure 4. The temperature enthalpy (T-S) profile in the Heat Recovery Steam Generator (HRSG)

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Figure 5. The temperature of inlet hot gases over pinch point temperature difference

As illustrated in Figure 6, the thermal efficiency of a gas turbine increases as the temperature of hot gas rises. This trend follows the Brayton cycle principle: increasing the turbine inlet temperature raises the specific work output because the expansion ratio increases. With a larger temperature difference between the turbine inlet and outlet, more useful work is extracted per unit of fuel. Thus, increasing turbine inlet temperature directly improves the thermal efficiency of the topping cycle, which subsequently raises the amount of heat delivered to the HRSG and enhances steam production in the bottoming cycle.

Figure 7 illustrates the variation of heat supplied (Q) to the gas turbine (upper curve) and steam turbine (lower curve). As the temperature of the flue gases increases, the heat supplied to both turbines rises. For the gas turbine, this is due to the greater enthalpy of combustion gases entering the turbine stage. For the steam turbine, the elevated exhaust temperature from the gas turbine increases the available heat absorbed in the HRSG, generating more superheated steam. This demonstrates the thermodynamic coupling between the topping and bottoming cycles: improvements in gas-turbine conditions directly translate into enhanced steam cycle performance.

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Figure 6. The temperature of inlet hot gas over thermal efficiency of gas turbine

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Figure 7. The temperature of hot gases over heat supplied of gas and steam turbine

Figure 8 illustrates a comparison of the thermal efficiencies of the gas turbine, steam turbine, and combined cycle. As heat input increases, the combined cycle efficiency consistently remains higher than that of either cycle individually. This is expected; the CCPP recovers waste heat that would otherwise be lost to the environment in a simple gas turbine system. The increase in efficiency with higher heat input is attributed to the optimal utilization of the high temperature exhaust gases. When the turbine inlet temperature rises, the HRSG receives hotter gases and generates more steam at higher enthalpy. This reduces the amount of heat exhausted to the flue and increases the useful work produced by the steam turbine. Therefore, the combined cycle benefits more from higher turbine inlet temperature than either cycle individually.

The relationship between AAT and power is depicted in Figure 9. According to the findings from this figure, rising AAT reduces power output for both gas turbine and combined cycle systems. This behavior is attributed to the reduction in air density at higher temperatures. At the same volumetric flow (m³/s), the mass flow rate (kg/s) entering the compressor decreases as AAT increases [19, 20]. This decrease in mass flow reduces turbine output power and increases compressor workload, resulting in a performance drop. The combined cycle exhibits smaller losses than the gas turbine alone because the HRSG mitigates some of the performance degradation by utilizing the hotter exhaust gases. However, overall power output decreases as AAT increases, highlighting the sensitivity of CCPPs to environmental conditions.

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Figure 8. Heat supplied over thermal efficiencies of cycle temperature

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Figure 9. Relationship between the ambient air temperature (AAT) and turbine power output

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Figure 10. Relationship between stack temperature and Heat Recovery Steam Generator (HRSG) heat capacity

Figure 10 shows the relationship between stack temperature and HRSG heat capacity (MW), demonstrating that the HRSG heat capacity decreases as the stack temperature increases. A higher stack temperature indicates that more heat is being discharged to the environment instead of being recovered in the HRSG. This reduces the energy transferred to the water-steam path and consequently lowers steam production. Ideally, the stack temperature should be maintained as low as possible within practical limits to maximize heat recovery. However, maintaining excessively low stack temperatures requires a larger heat transfer area and may risk condensation of acidic components, highlighting the engineering trade-off between efficiency and cost.