Abstract The current work presents a thermodynamic analysis of a Trilateral Flash Cycle (TFC) system for low grade heat to power conversion applications. Novel aspects of the research are the usage of rotary positive displacement expanders as prime movers of the TFC system as well as the reference to working fluids and their mixtures at the state of the art. In particular, the role of a correct built-in volume ratio of the expander with respect to the pressure ratio of the thermodynamic cycle is emphasized. In fact, a mismatching of these two quantities would lead to an isochoric expansion process which, in turn, might negatively affect the overall power recovery. With reference to a transcritical CO 2 stream at 100°C as heat source for the TFC system, parametric and screening studies were carried out using different expander built-in volume ratios and working fluids respectively. Among the fluids analyzed, results showed that pure substances such as R1234ze(E) and propane would provide a greater specific work but, on the other hand, would require built-in volume ratios (8 and 14) that are beyond the capabilities of rotary positive displacement expanders (5). The addition of CO 2 to the afore mentioned working fluids would ease the mismatching issue but, at the same time, would reduce the specific power output. Regarding the built-in volume ratio analysis, it was found that optimal values change in accordance to the working fluid and refer to an expansion process with a slight isochoric phase.