Theoretical and experimental characterization of single-screw expanders for ORC applications

Medium to low grade waste heat recovery from industrial processes is of particular interest to address both environmental concerns and the need of more efficient thermal systems. Organic Rankine cycles (ORCs) have been demonstrated to be a viable solution to convert waste heat into valuable electric power. In an ORC, the conversion of thermal energy into mechanical energy and, successively, into electrical energy occurs by means of an expander. In this work, a single-screw expander is investigated as a potential technology in the medium to low power output range due to its symmetric rotor configuration. At first, a generalized framework for the simulation of positive dispalcement compressors and expanders (PDSim) has been enhanced and applied to model both conventional and novel compressor types. A Z-compressor has been used as example to show how to handle different numerical challenges. Furthermore, such tool has been employed to develop a comprehensive mechanistic model of a single-screw machine. Secondly, a small scale ORC test rig has been used to characterized the performance of the expander with two working fluids, i.e. R245fa and SES36. This work resulted in numerical and experimental assessments of such machine to improve its design