Proposal and application of supercritical steam Rankine cycle using supercritical reheating regeneration process and its comparison between S-CO2 Brayton cycle

Abstract

Compared with the steam Rankine cycle, high efficiency is one of the core advantages of the supercritical carbon dioxide (S-CO2) Brayton cycle. However, the steam Rankine cycle still has the possibility of breaking its efficiency limit. This paper originally proposes the supercritical reheating regeneration process and constructed the supercritical regenerative Rankine cycle. It's found that the supercritical regeneration process can not only heat the feedwater, but also heat the reheated steam. then the thermal efficiency of the Rankine cycle can be further increased. Meanwhile, under the framework of the Carnot cycle, the average temperature at which heat is added (Tave,h) and the average temperature at which heat is rejected (Tave,c) are set as the optimization value, to perform a comparative study between the S-CO2 Brayton cycle and the supercritical steam Rankine cycle. Through analysis, by applying the supercritical regeneration process, Tave,h of the Rankine cycle is significantly increased. It can reach or exceed Tave,h of the S-CO2 cycle. Meanwhile, the heat-rejection process in the condenser of the Rankine cycle is the isothermal process, and its Tave,c is significantly lower than that of the Brayton cycle, which is a non-isothermal heat rejection process. Thus, the thermal efficiency of the steam Rankine cycle can surpass the S-CO2 cycle. When the turbine inlet parameter is 30 MPa/620 °C, the thermal efficiency and average temperature at which heat is added are 55.65 % and 573.6 °C for RRRC + RH and 54.72 % and 568.8 °C for TC + RH, respectively. Based on the supercritical regeneration process, the Rankine cycle overcomes the existing efficiency bottleneck, and has the potential to surpass the thermal efficiency of the S-CO2 cycle.