Comparative analysis on design and off-design performance of novel cascade CO₂ combined cycles for gas turbine waste heat utilization

To provide a higher energy conversion efficiency for gas turbines (GT), this paper proposes a novel two-stage cascaded supercritical CO₂ and transcritical CO₂ (sCO₂-tCO₂) power cycle for waste heat recovery (WHR) of GT exhaust. A comparative study on system design and off-design performance is conducted between the typical GT-sCO₂ cycles and the novel GT-cascaded CO₂ (GT-CCO₂) cycles based on the detailed steady-state mathematical models and self-built simulation platform. The simple layout (SSBC) and recompression layout (RSBC) are chosen to represent the typical sCO₂ Brayton cycle configurations. The simulation results show that the SSBC-tCO₂ cycle is more suitable than the RSBC-tCO₂ cycle to work as the bottoming cycle for GT since the SSBC-tCO₂ cycle can produce higher power with a simpler configuration. Compared with the traditional GT-RSBC and GT-SSBC, the optimal GT-CCO₂ cycle (GT-SSBC-tCO₂) gains an improvement of 5.32% and 4.32% for the thermal efficiency, and a decrement by 4.08% and 2.42% for the Levelized cost of electricity respectively. The combined GT-CCO₂ cycles show superior performance with the variable inlet guide vane modulation during the off-design conditions because this control strategy increases the GT exhaust gas temperature and thus brings about a large improvement of the bottoming cycle output power. These findings could provide references for high efficiently utilizing gas turbine exhaust and verify the commercial viability of the CO₂ system.