# Thermodynamic Analysis of Partitioned Combined Cycle using Simple Gases
## 1 Introduction  
In stationary combined cycle gas turbines (CCGT), the Rankine cycle is used commercially as a bottoming cycle. The high exhaust temperature in the Brayton cycle instigates immense energy loss. This heat is partially recovered in the Rankine cycle. The efficiency of the modern Brayton cycle is higher than the Rankine cycle (fluid exergy loss is less in the Brayton cycle than the Rankine cycle as the latter requires latent heat at low pressures). Therefore, empirically high heat reclamation to work in the Brayton cycle enhances the overall efficiency. Due to some associated Rankine cycle limitations, several variants have been proposed to enhance the overall efficiency of the gas turbine (GT). One effective approach is the regeneration cycle (RG). Direct or indirect methods can be used to recover heat before rejection of flue gases to the bottoming cycle (if one is available). Direct methods employ a simple heat exchange concept and recover heat, where temperature difference and component efficiencies are favorable. Thermodynamically, it is most pragmatic to exchange heat after compression and before the combustion chamber in a cycle operating above atmospheric pressure. Indirect methods usually involve steam generation/injection or endothermal chemical reactions. Steam injection is usually employed to reduce oxides of nitrogen (NOx), although it has been reported to augment the power output [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11] but it results in latent heat and entropy loss associated withworking fluid. Any efficiency enhancement by this technique is only reported for the simple Brayton cycle. Moreover, combustors should be modified (larger design is required) before steam injection utilization [12; 13; 14]. Furthermore, a closed cycle (like the Rankine cycle) eclipses it because of water loss, as downstream water recovery from flue gases is not a convenient option. Although, condenser addition is suggested, but the effects of acid gases condensation are not accounted [15]. Any chemical reaction intrinsically involves the entropy loss and lowers the advantages of heat recuperation (compared to direct heat recovery). [PERSON] et al. used exergy rates to explain this aspect [16]. Mostly, recuperation involves reforming reactions [17; 18; 19] or gasification reactions [20; 21; 22; 23; 24]. [PERSON] et al. studied the benefits of integrating both reforming and gasification along with CO\({}_{2}\) capture [25]. The incorporation of fuel cells is also reported to enhance GT efficiency [26; 27; 28; 29]. [PERSON] concluded, based on his characteristic plane technique, that simple indirect heat recovery techniques are not adequate. Thus, indirect methods must be used in combination to harness any benefit. Additionally, he provides different ranges for heat recuperation usage for retrofitting the Brayton cycle to become comparable with CCGT [30]. Several previous investigations signify direct heat recuperation as a commercially viable approach, provided an efficient heat exchanger is present. Direct heat exchange with a combustion chamber has not been reported but preheating of the combustion fuel and air with exhaust has been extensively studied for GT. Excessive pressure drop in the heat exchanger is specifically important [31]. Practically, transferring heat involves extensive pressure drops. Therefore, earlier studies showed that direct heat recuperation requires pressure ratios optimization [32]. Pressure ratios also manifest an effect on the compressor discharge temperature. Furthermore, intercooling of the compressor enhances the heat regeneration gain (mainly because of high temperature gradients in the downstream heat exchanger) [33; 34]. [PERSON] et al. reviewed recuperators for micro GTs and reported that an effective exchanger should have high effectiveness (>90%) and less pressure drop (<3%) [35]. [PERSON] et al. [36; 37], and [PERSON] et al. [38] applied the direct energy recuperation on a 60 MW Siemen's gas turbine (Brayton cycle) and 106 MW using a tubular exchanger.