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

Abstract

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 (T2) in the proposed system is a crucial parameter because it affects the SORC efficiency (ηSORC) 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 T2 with respect to a new indicator, that is, the equivalent heat-to-power efficiency (ηeq). ηeq is a compromise between the efficiencies in different operation modes. The effects of main steam temperature (T1), Baumann factor (a), mass of storage water (Mw), and ORC working fluid on T2 are investigated. Results show that ηeq is a better indicator than ηSORC. The optimum steam condensation temperature (T2,opt) that corresponds to the maximum ηeq (ηeq,max) is generally higher than that based on the maximum ηSORC. T2,opt reduces as T1, a, and Mw decrease. ηeq,max rises with the increment of T1 and the decrement of a and Mw. Pentane is a more preferable ORC fluid than benzene and R245fa. The T2,opt and ηeq,max of pentane are, respectively, 139–190 °C and 20.93%-24.24%, provided that T1 ranges between 250 °C and 270 °C, a varies from 0.5 to 1.5, and Mw changes from 500 ton to 1500 ton.