The screw-type volumetric expander has great potential in distributed solar electric generating system (SEGS) applications regarding its ability of handling both steam and liquid. A parabolic trough collector (PTC)-coupled cascade thermodynamic system with the top screw expander (SE)-based steam Rankine cycle and the bottom turbine-based organic Rankine cycle (ORC) has the advantages of avoidance of superheater, and relatively low technical requirements in heat collection and storage. A significant characteristic of the solar cascade system is the highly off-design operation owing to the small built-in volume ratio (r(v,b)) of SE. However, at present model on the system part-load behavior is lacked and the optimum working condition has yet to be determined. In this paper, an approximate SE over-expansion model is established, which reveals variation of SE isentropic efficiency with operating pressure ratio. Then off-design behavior of the whole system is modeled. The solar power efficiency on different conditions is investigated. Optimization of the system is conducted. Results indicate that the optimum hot side temperature ranges from about 499 K to 543 K when beam solar radiation (G(b)) is 600-800 W/m(2). Maximum solar thermal power efficiency of 13.74-15.45% can be achieved in the situation of SE's rum of 5.0. The impact of low r(v,b) on power conversion is limited owing to SE good part-load behavior, and maximum efficiency of 13.12-15.11% is obtained when r(v,b) is 3.5. Annual optimization in Phoenix, Sacramento, Cape Town, Canberra, Lhasa and Barcelona is further implemented. The solar power output varies from 201.8 kWh/m(2) to 347 kWh/m(2) per year. (C) 2017 Elsevier Ltd. All rights reserved.