An innovative solar power tower system coupling double-reheaters and subcritical cascade Rankine cycle

Double-reheaters are favorably employed in supercritical and ultra-supercritical Rankine cycle-driven units, but their application in subcritical cycles is rare. This paper proposes an innovative solar power tower system characterized by coupling double-reheaters and subcritical cascade Rankine cycle. The top cycle uses a near-azeotropic mixture biphenyl-diphenyl oxide (BDO), which vaporizes at 400 °C to drive a top turbine. Then the outlet BDO preheats and evaporates the water in the bottom cycle. The hot molten salts are divided into three branches to provide heat for steam superheating and reheating, as well as for BDO preheating and evaporation. The exhaust superheated steam from the high- and intermediate-pressure turbine outlets is reheated to 540 °C and respectively enters the intermediate- and low-pressure turbines. By simultaneously optimizing the water evaporation temperature and six-stage extraction pressures, the maximum thermal efficiency reaches 45.80 %, which is about 3.5 % more efficient than conventional solar tower systems. Meanwhile, the main steam pressure drops to 6.42 MPa, which is beneficial for high-pressure turbine design and operation. The equivalent payback period due to the additional investment is merely 6.15 years. The heat storage duration and capacity can also be increased appreciably by optimizing the extraction arrangement and molten salt flow paths.