A comparison of piston, screw and scroll expanders for small scale Rankine cycle systems

This paper aims at helping the designer of micro-scale Rankine Cycle heat engines to best select the expander among piston, screw and scroll machines.
The first part of the paper presents a state of the art of these three technologies of positive displacement machines. The technical constraints inherent to each machine (rotational speed, pressure ratios, maximum temperatures, volumetric expansion ratios, etc.) are listed and the performance mentioned in the open technical and scientific literature is presented.
The second part of the paper deals with the modeling of such expanders. Different simulation models are proposed: black-box, grey-box and white-box models. These three categories of modeling are specifically adapted to different purposes: design of the expander, design of the micro-CHP system, and dynamic simulation/control of the CHP unit.
The last part of the paper presents a graphical methodology of selection of expansion machines and working fluids based on operating maps. It is stressed that the selections of both the expansion machine and working fluid should be conducted simultaneously.