Geometry-based Modeling of Single Screw Expander for Organic Rankine Cycle Systems in Low-grade Heat Recovery

The constantly rising energy demand and the depletion of fossil fuel resources currently represent the main concerns for global energy sustainability, alongside environmental issues. Low-grade energy sources have moved into the limelight and Organic Rankine Cycle (ORC) technology is well known for its suitability to recover waste heat.The optimization of the expansion process of an ORC system is crucial in order to maximize the power output, to increase the overall efficiency, and to obtain an economically feasible machine. Scroll, reciprocating, single screw, twin-screw and vane-type expanders are the main technologies. Screw expanders are particularly suited for low-grade heat recovery. The advantages of single screw technology over twin-screw include the balanced loading of the main screw, long working life, high volumetric efficiency, low leakage, low noise, low vibration, and a simplified configuration.In this paper, a detailed geometry-based model of a single screw expander is presented. The model is based on a rotation-dependent function that completely describes the geometry of the screw rotor and the engaging surfaces of the star wheels. A computation of the swept volume at each angular step and of the inlet conditions leads to a solution of the system of differential equations governing the thermodynamics of the expansion process. The model considers heat losses, oil-flooded mixing and leakage paths and permits the investigation of the impact of design modifications on the expander performance.Furthermore, the validation of the proposed model against the available literature is presented.