Power Recovery from Low Grade Heat by Means of Screw Expanders

Current concerns with climate change have resulted in greatly increased interest in power recovery from low grade heat sources. This includes both hot fluid streams which can be expanded directly to produce mechanical power and those which act as a source of heat to closed cycle power generation systems. Power recovery from low grate heat by means of screw expanders with a generalised overview of how best to recover power from such sources, based on thermodynamic considerations, which differs to the approach used in classical thermodynamics textbooks and which includes an introductory description of the types of working fluid that are used in systems used to recover power from such sources and the criteria that must be taken into account in their selection. This is followed by a description of the mathematical modelling of twin screw machine geometry. The modelling of the thermodynamics and fluid flow through such machines is then given, together with how this is used to predict their performance. Finally a detailed description is given of systems currently used or projected both for direct expansion of the source fluid and by recovery of heat from it, which includes those which are particularly suited to the use of screw expanders in place of turbines.

A novel generalised approach to the thermodynamics of power recovery from low grade heat systemsGives criteria for working fluid selectionProvides details of, and how to model, screw expander geometryDetails how to estimate screw expander performanceSurveys types of system used for power recovery from low grade heat and where this can be improved by the use of screw expanders.

Expanders for power recovery: Turbines; Positive displacement machines; Power plant thermodynamics: Maximum work; Some observations; Practical considerations; Working fluids other than water; Fluid properties; Screw expander rotors geometry and manufacture: Introduction; review of contemporary rotor profiles; Screw expander rotor geometry; Features of ‘N’ rotor profiles; Geometry of rotor manufacturing tools; Design of screw expander housings and choice of bearings; Modelling and performance calculation of screw expanders: The screw expander process and mathematical modelling; Equations governing the screw machine processes; Flow through the admission and discharge ports; Flow through the leakage paths; Injection of oil and other liquids; Solution procedure for the screw machine thermodynamics; Calculation of thermodynamic properties of working fluids; Calculation of machine performance parameters; Results of modelling and experimental investigations; Calculation of pressure forces acting on screw machine rotors; Radial, axial rotor loads, torque and bearing reactions; Rotor deflections; Recent advances in screw machine development; Applications for screw expanders: Screw expander lubrication; Systems; Cycles; Cycle optimisation; Bibliography.