# Thermodynamic Analysis of Partitioned Combined Cycle using Simple Gases
## 1 Introduction
They optimized the cost and efficiency. They proposed to retrofit the turbine by using empirical optimization techniques, although the exchanger pressure drop is calculated and included in the model but waste heat recovery with a bottoming cycle is not evaluated. Moreover, the gas mixture was assumed as an ideal. New gas turbines offer high temperatures and pressures. Taking working fluid at these conditions as an ideal gas can seriously affect the conclusions. [PERSON] et al. discussed the recuperation at different levels of turbine stages for an aero Gas Turbine (GT) [39]. Tubular exchanger design as a recuperator was also discussed [40]. The heat recuperation concept was also applied on Combined Cycle Gas Turbine (CCGT) [41; 42; 43]. The double acting Stirling cycle has also been integrated, using a heat exchanger, with GT and reported to increase the efficiency for low wattage power generators [44]. [PERSON] proposed that the combustion chamber of a GT can be used as a hot reservoir for the Stirling cycle [45]. [PERSON] et al. modeled (using the Number of Transfer Units (NTU)-effectiveness method) a high temperature (>1123 K) ceramic heat exchanger for an externally fired gas turbine. In externally fired gas turbines, heat is transferred from an external combustion reaction to the working fluid in the Brayton cycle [46]. The same principle is applied for GTs, operating on nuclear energy. Printed circuit heat exchangers (PCHE) are also reported to exchange heat with working fluids [47]. PCHE allow gas-gas heat exchange with an effectiveness of more than 90% [48; 49; 50; 51]. However, the drawback lies in the huge pressure drop. Thus, PCHE are mostly suitable to the heavily pressurized systems like supercritical carbon dioxide or to power cycles with working fluid that offers less exergy loss due to pressure drop [52; 53]. The idea of heat recuperation is due to the Brayton cycle waste heat, which results because of:  
1. Non ideality of the working fluids, resulting in entropy loss (high exhaust temperature);
2. Adiabatic loss owing to moving machinery;A pressure ratio that cannot be optimized to recover all the heat present in the working fluid in the Brayton cycle;
4. High excess air to combustors because of the metallurgical temperature limit. The exergy analysis conducted on CCGT and on its variants optimized the loss because of second law. Therefore, the underlined principles of the first two points are well defined and waste heat cannot be avoided beyond a certain limit. In our previous publication [54], it was shown that pressure ratios are chosen as a function of turbine inlet temperature. There is an optimum pressure ratio for each combustion temperature in CCGT. Theoretically, a higher temperature in the combustion chamber always favors the thermal efficiency. The maximum temperature that can be achieved is adiabatic flame temperature, but the material constraints limit this temperature approach. High excess air, used for lowering the temperature, can be curtailed by using exhaust gas recirculation, but compressor exergy loss because of high exhaust temperature does not allow it beyond certain threshold. Furthermore, any additional fluid offering less thermodynamic loss than excess air cannot be used, as its recovery from the exhaust mixture poses a challenge. In this study, a novel approach is discussed to limit excess air and recover energy before it manifests in the form of waste heat. The objective is to maximize heat recovery from the flue gases in CCGT. This partially eliminates the need for recuperation. The advantage of the presented approach is that high temperature differences for the heat exchanger are ensured, which results in its size reduction. Moreover, the recuperation concepts are applied on relatively small wattage turbines. This article, on the other hand, focuses on a 755 MW CCGT. ASPEN Tech. process simulators are reported to imitate the CCGT (with added variations) process conditions within the acceptable range [55; 56]. The same was used to conduct this study.