Screw expander characteristics for ORC system design

Dynamic expanders, axial or radial, used to be the common solution in designing ORC’s though the use of screw expanders is continuously increasing. In the power range of 5 kW to 1 MW a number of screw expander ORC’s are operating in the field. Dynamic expanders and screw expanders show some fundamental differences in characteristics which are important for understanding differences between, and potentials for, the ORC systems in which they are integrated. This paper describes typical characteristics of screw expanders and relate to design optimization opportunities for the ORC. Screw expander design functions, like multi-phase expansion, side-loading, stall speed and postcompression, are described and the corresponding opportunities for cycle-optimization are presented. Screw expander performance characteristics, and the conventional methodology for expressing screw expander performance, is explained as well as some related potentials for advanced ORC control strategies. Typical mechanical limitations of the screw expander and the implications on ORC system design freedom is discussed. Scope is limited to twin screw expanders with synchronized rotors though large parts of the explanations are also relevant for triple screw expanders and screw expanders with direct rotor contact. The major differences related to such technologies are commented. To explain performance characteristics properly novel efficiency data is presented based on testing of a commercially available twin screw expander.     «

Dynamic expanders, axial or radial, used to be the common solution in designing ORC’s though the use of screw expanders is continuously increasing. In the power range of 5 kW to 1 MW a number of screw expander ORC’s are operating in the field. Dynamic expanders and screw expanders show some fundamental differences in characteristics which are important for understanding differences between, and potentials for, the ORC systems in which they are integrated. This paper describes typical characteris...     »