This research proposes an innovative solar thermal plant able to generate mechanical power through an optimized system of heliostats with Scheffler-type solar receivers coupled with screw-type steam expanders. Scheffler receivers appear to perform better than parabolic trough collectors due to the high compactness of the focal receiver, which minimizes convective and radiative heat losses even at high vaporization temperatures. At the same time, steam screw expanders are volumetric machines that can be used to produce mechanical power with satisfactory efficiency also by admitting two-phase mixtures and with further advantages compared to steam turbines: low working fluid velocities, low operating pressures, and avoidance of overheating. This study establishes a mathematical model to assess the energetic advantages of the planned solar thermal power system by evaluating the solar-to-electricity efficiency for different off-design working conditions. For this purpose, a numerical model on the Scheffler receiver is initially investigated, thus assessing all the energy losses which affect the heat transfer phase. A thermodynamic model is then developed to evaluate the energy losses and performance of the screw expander under real working conditions. Finally, parametric optimization of the solar energy conversion is performed in a wide range of operating conditions by establishing thermodynamic formulations related to the whole solar electricity generation system. Water condensation pressure and vaporization temperature are so optimized with respect to global energy conversion efficiency which, under the best operating conditions achieved in this research, rises from 10.9% to 14.4% with increasing solar irradiation intensity. Hence, the combined use of screw expanders and Scheffler receivers for solar thermal power system application can be a promising technology with advantages over parabolic dish concentrators. Novelty statement: This research proposes an innovative direct steam solar power plant based on an SRC, with water utilized as both heat transfer and working fluid, equipped with Scheffler solar receivers as a thermal source and screw expanders as work-producing devices. Technical studies and energy assessments of this kind of SEGS at part-load operation do not exist in scientific literature; after reviewing the literature, it was determined that volumetric expanders have been rarely combined with Scheffler receivers for solar thermal power system application. In effect, combined use of screw expanders and Scheffler-type solar concentrator in a direct steam solar power system represents a completely new plant configuration; however, as a promising DSG solar system, at present numerical model of this new sort of SEGS is lacked in literature and the optimum operating conditions have yet to be defined. For this reason, the chief objective of this paper is to define a first parametric optimization of all thermodynamic variables involved to maximize global efficiency of the proposed solar thermal power generation system for ordinary working conditions.