EXPANSION MACHINE FOR A LOW POWER-OUTPUT STEAM RANKINE-CYCLE ENGINE

The performance of the expansion device in a Rankine-cycle engine is one of the major parameters dictating the engine's overall energy-conversion efficiency. In this paper, the screening process undertaken to choose the most suitable expansion machine for a steam Rankine-cycle engine, operating principally as a 'mini' combined heat-and-power unit, is described. In the low power-output range (i.e. 5-20 kW) envisaged rotary, positive-displacement machines offer many advantages compared with turbines and reciprocating-piston expanders. So rotary-vane, helical-screw and Wankel-type expansion devices were short listed. However further assessments, based upon operational problems and cost effectiveness, led finally to the choice of the Wankel-type expander for the proposed application. Nevertheless, for this machine to be commercially successful, existing designs need to be modified and optimised.