Optimization of the self-condensing CO2 transcritical power cycle using solar thermal energy

Highlights: • The novel CO₂ transcritical power cycle operates with conventional water cooling. • Theoretical study was executed on the relationships among the cycle's parameters. • The novel cycle simplifies the development of the pressurizing component. -- Abstract: Compared with the conventional Rankine cycle, the CO₂ transcritical power cycle gives a higher thermal efficiency because of its high average heat absorbing temperature and is suitable for driving a compact system. The self-condensing CO₂ transcritical power cycle can solve the problem that CO₂ is difficult to condense in a conventional CO₂ transcritical power cycle using conventional water cooling. Based on solar thermal energy, a theoretical analysis model was established to study the relationship between the cycle performance and the operating parameters. The results showed that the thermal efficiency increases with increasing the cooled pressure with a low final cooled temperature. By increasing the final cooled temperature, a peak appears on the thermal efficiency curve. The outlet temperature of the cooling water is affected by a shift of the pinch point position in the cooler. According to the variation of the outlet temperature of the cooling water and the proportion of the mass flow rate of CO₂ in the power sub-cycle and that in the whole cycle, it can be concluded that conditions with a very low cooled pressure are uncontrollable. In these conditions, the maximum thermal efficiency of the self-condensing CO₂ transcritical cycle is 0.3463, which is 0.0313 a little lower than that of the supercritical CO₂ Brayton cycle. However, the novel cycle simplifies the development of the pressurizing component and avoids the liquid hammer in the pressurizing process.