Thermodynamic assessment of combined supercritical CO₂ cycle power systems with organic Rankine cycle or Kalina cycle

The supercritical carbon dioxide (S-CO₂) Brayton cycle is considered to be one of the most promising cycles for the fourth-generation nuclear power system. In this paper, the S-CO₂ Brayton combined cycles composed of organic Rankine cycle (ORC) and Kalina cycle as the bottom cycle are proposed to improve the cycle performance. A new system performance parameter η/Q_v,max is introduced for the first time to evaluate the economy and compactness balance of the combined cycle. Parametric study based on different working fluids for bottom cycle is conducted, comparison study between the S-CO₂ Brayton combined cycles and traditional S-CO₂ Brayton reheating cycle are made. The comparison results show that R32 and Ammonia based S-CO₂/ORC combined cycle obtain the best performance, with thermal efficiency enhanced by 2.015% and 2.036%, respectively. The S-CO₂/Kalina combined cycle obtains an improvement by 1.79% with ammonia-water concentration at 0.85, which is slightly lower than S-CO₂/ORC combined cycle. Superheating the organic working fluids can improve the exergy efficiency of the evaporator and increase η_orc/Q_v,max, but the efficiency of the cycle is not necessarily improved. The temperature slip contributes in reducing the temperature difference in the evaporator, and this helps the S-CO₂/Kalina combined cycle to better match the temperature changing characteristics of the heat source compared with ORC, and the slope of the CO₂ heat transfer line near the critical point of CO₂ is the key factor that limits the improvement of S-CO₂/Kalina combined cycle efficiency.