This paper explains a numerical optimization of a novel screw expander-based solar thermal electricity plant to evaluate the energetic benefits in specific case studies. In the proposed solar electricity generation system, which is based on the steam Rankine cycle, water is used as working fluid and storage, parabolic trough collectors as a thermal source and screw expander as power machine. Such solar system offers major advantages over conventional power plants adopting steam turbines: low operating pressures, good exploitation of low temperature heat sources, acceptable efficiency in energy conversion with steam-liquid mixtures and reduced size. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, the chief purpose of the present study is to develop a thermodynamic model to analyse the energy performance of the planned solar power system when off-design operating conditions befall. To assess maximum efficiency of the whole power plant at part-load operating conditions, numerical optimization is then performed in a specific range of fluctuating evaporation temperatures under fixed condensation pressures. Keywords: Steam screw expander, Solar thermal power efficiency, Direct steam generation, Part-load behavior, Polytropic expansion phase