Thermal performance and economic analysis of supercritical Carbon Dioxide cycles in combined cycle power plant

A closed-loop, indirect, supercritical Carbon Dioxide (sCO 2 ) power cycle is attractive for fossil-fuel, solar thermal and nuclear applications owing to its ability to achieve higher efficiency, and compactness. Commercial Gas Turbines (GT’s) are optimised to yield maximum performance with a conventional steam Rankine cycle. In order to explore the full potential of a sCO 2 cycle the whole plant performance needs to be considered. This study analyses the maximum performance and cost of electricity for five sCO 2 cascaded cycles. The plant performance is improved when the GT pressure ratio is considered as a design variable to a GT to optimise the whole plant performance. Results also indicate that each sCO 2 Brayton cycle considered, attained maximum plant efficiency at a different GT pressure ratio. The optimum GT pressure ratio to realise the maximum cost reduction in sCO 2 cycle was higher than the equivalent steam Rankine cycle. Performance maps were developed for four high efficient cascaded sCO 2 cycles to estimate the specific power and net efficiency as a function of GT turbine inlet temperature and pressure ratio. The result of multi-objective optimisation in the thermal and cost (c$/kWh) domains and the Pareto fronts of the different sCO 2 cycles are presented and compared. A novel sCO 2 cycle configuration is proposed that provides ideal-temperature glide at the bottoming cycle heat exchangers and the efficiency of this cycle, integrated with a commercial SGT5-4000F machine in lieu of a triple-pressure steam Rankine cycle, is higher by 1.4 percentage point.