This paper analyzes the performance of an Organic Rankine Cycle (ORC) system at part load operation. The objective is to understand its behavior from a thermodynamic perspective, identifying which elements are the most critical and which are the best operating points for each level of demanded power. This paper also compares two working fluids: R245fa and Solkatherm ES36 (SES36) for the same cycle specifications. The results have shown that the scroll isentropic efficiency has a great influence on the cycle performance and its thermal efficiency and that SES36 arises as a potential better fluid than R245fa. At the given maximum and minimum temperatures, the best operation point was determined. This allows reaching a maximum efficiency for each demanded level of power; depending on the required amount of power, the expander speed and the working pressure are adjusted. (C) 2014 Elsevier Ltd. All rights reserved.