Analysis of a novel solar electricity generation system using cascade Rankine cycle and steam screw expander

A novel solar electricity generation system (SEGS) using cascade cycle is proposed. The top and the bottom are steam Rankine cycle (SRC) and organic Rankine cycle (ORC). Particulary, screw expander (SE), which is characterized by good applicability in power conversion with steam-liquid mixture, is employed in the SRC. Steam is generated directly in the parabolic trough collectors (PTC) and expands in the SE. The heat released by steam condensation is used to drive the ORC. This type of SEGS has the advantages of avoidance of superheated steam, moderate operating temperature and pressure, low technical requirements in heat collection and storage, and suitableness for distributed power generation. Simulation of the system performance is conducted on the use of ten ORC fluids. Four hot/cold side temperatures of 473/313 K, 473/293 K, 523/313 K and 523/293 K are exemplified. The results indicate the ORC evaporation temperature corresponding to theoretical maximum solar power efficiency fails to provide a pressure ratio (PR) that matches the SE built-in PR. Off-design operation of the SE is recommended for the purpose of higher system efficiency and simpler ORC turbine. Efficiency of 13.68-15.62% for the proposed system can be achieved.