The aim of the present work was the optimization of a small size Organic Rankine Cycle (ORC) system powered by a linear Parabolic Trough Collector (PTC) solar field by means of numerical model code developed on purpose. In the proposed scheme the solar energy is collected by a newly designed low cost PTC of 20m2 with a single tracking axis and it is concentrated on an opaque pipe collector in which flows as thermal fluid the Therminol® 66 oil. An oil-free scroll expander coupled with a 2 kW electrical generator is used to convert fluid energy into electrical energy. Cogeneration is performed by accumulating the waste in a double effect tank for domestic hot water (DHW) and heating. The use of cogeneration improves the global efficiency of the system from about 12% up to 30%. The numerical model implements the primary and secondary circuit as well as a detailed model of the PTC and the scroll expander. Real weather data of Brindisi (Italy) is considered to estimate and to optimize the performance of the plant. The system control and management have been optimized with the numerical model to increase the performance of the scroll expander with the development of energy-save strategies