Variable volume ratio free-piston expander: Prototyping and experimental campaign

Abstract

This paper describes a dynamic model developed for control purposes of a novel free piston expander with a variable built-in volume ratio (BVR) for Organic Rankine Cycle (ORC) applications. In order to demonstrate the feasibility of the proposed concept and to validate the theoretical model, a test setup has been designed and built. An integrated two degree of freedom motor/alternator is substituted by a separate linear motor/alternator (LMA) and a rotary servo motor designed and made from standard industrial components. This approach gives a possibility to develop and to test the integrated alternator and the expander itself independently. The model has been validated by running the setup in compressor mode. The cogging force and the frictional force, which are difficult to obtain analytically, are identified and mapped. An estimation of the geometry and the mass of the piston of an integrated design has also been made. The obtained results are sufficient for the control algorithm development since all acting forces are calculated or estimated with a good accuracy.