Design of steam condensation temperature for an innovative solar thermal power generation system using cascade Rankine cycle and two-stage accumulators

An innovative solar thermal power generation system using cascade steam-organic Rankine cycle (SORC) and two-stage accumulators has recently been proposed. This system offers a significantly higher heat storage capacity than conventional direct steam generation (DSG) solar power plants. The steam condensation temperature (T-2) in the proposed system is a crucial parameter because it affects the SORC efficiency (eta(SORC)) in normal operations and the power conversion of the bottoming organic Rankine cycle (ORC) in the unique heat discharge process. The present study develops a methodology for the design of T-2 with respect to a new indicator, that is, the equivalent heat-to-power efficiency (72,9). neg is a compromise between the efficiencies in different operation modes. The effects of main steam temperature (T-1), Baumann factor (a), mass of storage water (M-w), and ORC working fluid on T-2 are investigated. Results show that neg is a better indicator than eta(SORC). The optimum steam condensation temperature (T-2,T-opt) that corresponds to the maximum eta(eq) (eta(eq,max)) is generally higher than that based on the maximum eta(SORC), T-2,T-opt, reduces as T-1, a, and M-w decrease. eta(eq,max) rises with the increment of T-1 and the decrement of a and M-w. Pentane is a more preferable ORC fluid than benzene and R245fa. The T-2,T-opt and eta(eq,max) of pentane are, respectively, 139-190 degrees C and 20.93%-24.24%, provided that T-1 ranges between 250 degrees C and 270 degrees C, a varies from 0.5 to 1.5, and M-w changes from 500 ton to 1500 ton.