Comprehensive model of a single screw expander for ORC-systems

The Organic Rankine Cycle (ORC) system is considered one of the leading technologies for waste heat recovery applications. In order to increase the overall efficiency, design improvements and optimization analyses are necessary. The expander is the key component to convert the thermal energy in useful work. Several volumetric and turbine expanders have been investigated. Among volumetric machines, scroll, twin-screw and vane-type expanders are the main technologies. Recently, a number of experimental studies on single screw expanders (SSE) have been carried out, demonstrating the feasibility of employing such machine as an ORC expander. The single screw configuration presents a number of advantages over twin screw including balanced loading of the main screw, long working life, high volumetric efficiency. In this paper, a detailed geometry-based multi-chamber model of a single screw expander is presented. The model is based on a rotation-dependent function that entirely describes the geometry of the engaging surface between the screw rotor and the starwheels. The computation of the swept volume at each angular step and the inlet conditions, enables the solution of the system of differential equations governing the thermodynamic problem. The level of details of the model, which includes heat transfer losses, oil-flooded mixing and leakage paths calculation, allows the investigation of the impact of design changes to the expander machine performance. The validation of the proposed model is realized with experimental data obtained from an ORC set up which employs a SSE and Solkatherm (SES36) as working fluid. Test results have shown a maximum isentropic efficiency of 67% at 3000 rpm. The model predicts the mass flow rate and the power output within ± 10% and ± 15% respectively.