Thermo-economic optimization and part-load analysis of the combined supercritical CO₂ and Kalina cycle

The supercritical CO₂ (sCO₂) power cycle has emerged as a promising technology for the use of nuclear energy. However, a large amount of cooling heat is wasted in the sCO₂ gas cooler. In this paper, two typical sCO₂ Brayton cycles including simple sCO₂ cycle (SSC) and recompression sCO₂ cycle (RSC) are integrated with Kalina power cycles to provide a higher energy conversion efficiency for nuclear power plants. A comparison study of the SSC-Kalina cycle and the RSC-Kalina cycle is conducted from the viewpoints of thermodynamics and economics, and multi-objective optimization using genetic algorithms is carried out to gain maximum exergetic efficiency (η_ex) and minimum levelized cost of electricity (LCOE) at the design stage. To meet the requirement of wide-range load adjustment, the valve control strategy and variable-speed compressor control strategy are proposed for the topping sCO₂ cycle, while the sliding pressure control strategy is adopted for the bottoming Kalina cycle. The part-load performance of the combined cycle under different control strategies is analyzed and compared based on the preliminary design parameters for the main system components. Results show that the RSC-Kalina cycle always has better performance than the SSC-Kalina cycle, and they can gain an improvement by 6.37% and 7.53% for the η_ex, and 3.51% and 5.84% for the LCOE compared with the stand-alone RSC and SSC, respectively. Compared with the valve control strategy, the variable-speed compressor control strategy enables the RSC-Kalina cycle to obtain higher efficiency under partial plant loads, and the η_ex ranges from 29.67% to 58.24% under 10–100% relative plant load. The bottoming Kalina cycle can be well adapted to the parameter variations of the topping cycle by using the sliding pressure control strategy no matter which sCO₂ control strategy is adopted.