Design and Thermodynamic Analysis of the Next-Generation Gas Turbine-Transcritical CO2-Combined Cycle

The development of high-efficiency, low-pollution, zero-emission, flexible-fuel, and low-cost gas turbine (GT) generator sets is a crucial strategy to achieve carbon peaking and carbon neutrality goals. This research utilizes the reheat GT cycle (RGTC) as the top cycle and constructs four RGTC-supercritical/transcritical carbon dioxide (s/tCO₂) combined cycle systems with progressively enhanced configurations of the CO₂ bottom cycle. Modeling and thermodynamic analysis are conducted using next-generation GT parameters. When the supercritical CO₂ (sCO₂) dual recuperated bottom cycle (sCO₂DRBC) is used as the bottom cycle, the high temperature at the CO₂ compressor outlet results in gas exhaust temperatures exceeding 100°C, indicating insufficient waste heat utilization. Consequently, the operating condition of the CO₂ bottom cycle is changed from supercritical to transcritical, utilizing the low-temperature CO₂ fluid at the pump outlet to further absorb waste heat from the gas. This modification results in a final gas exhaust temperature of ~70°C and a 2.84% increase in combined cycle energy efficiency. Additionally, considering the substantial latent heat of vaporization in the water within the gas, its effective utilization can further enhance cycle energy efficiency. Therefore, the transcritical CO₂ (tCO₂)DRBC is further refined by incorporating a medium-temperature turbine (MT), medium-temperature recuperator (MTR), and phase change heater (PH) to form the modified tCO₂ bottom cycle I (tCO₂MBC-I). The RGTC-tCO₂MBC-I combined cycle achieves a 69.56% combined cycle energy efficiency with the absorption of some gas latent heat, demonstrating a significant improvement in energy efficiency. Further analysis reveals that the flow rate of the bottom CO₂ cycle is increased by the enhanced design due to the need to absorb additional gas latent heat, exacerbating losses at the bottom cycle precooler. Consequently, an expander (positioned before the precooler) is integrated into the tCO₂MBC-I to form the modified tCO₂ bottom cycle II (tCO₂MBC-II). Calculated results indicate an energy efficiency of 69.91% for the RGTC-tCO₂MBC-II combined cycle. This paper presents three modifications to the previously studied sCO₂DRBC, with the final RGTC-tCO₂MBC-II combined cycle demonstrating considerable energy efficiency advantages. The findings suggest its potential to become the next generation of GT combined cycle units.