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

• Theoretical comparison on S-CO 2 /ORC and S-CO 2 /Kalina combined cycles were made. • Heat transfer characteristic of the evaporators in combined cycle was discussed. • Working medium and parameter optimization were conducted. • Reheating can only improve the ORC thermal efficiency of certain working mediums, but it can improve the overall performance of all mediums. • A detailed heat transfer coupling is performed for the Kalina cycle to recover afterheat near the critical point of CO 2 . The supercritical carbon dioxide (S-CO 2 ) Brayton cycle is considered to be one of the most promising cycles for the fourth-generation nuclear power system. In this paper, the S-CO 2 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-CO 2 Brayton combined cycles and traditional S-CO 2 Brayton reheating cycle are made. The comparison results show that R32 and Ammonia based S-CO 2 /ORC combined cycle obtain the best performance, with thermal efficiency enhanced by 2.015% and 2.036%, respectively. The S-CO 2 /Kalina combined cycle obtains an improvement by 1.79% with ammonia-water concentration at 0.85, which is slightly lower than S-CO 2 /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-CO 2 /Kalina combined cycle to better match the temperature changing characteristics of the heat source compared with ORC, and the slope of the CO 2 heat transfer line near the critical point of CO 2 is the key factor that limits the improvement of S-CO 2 /Kalina combined cycle efficiency.