PERFORMANCE PREDICTION AND DESIGN OPTIMIZATION OF A kW-SIZE RECIPROCATING PISTON EXPANDER WORKING WITH LOW-GWP FLUIDS

Micro-ORC systems represent a promising technology in the field of the energy conversion from low-grade temperature sources. However, nowadays the working efficiency are still relatively low, resulting from the lack of appropriate expander machines but also from the need of optimal working fluid. The ideal working fluid should maximize the performance of the system for given operating conditions (as the hot source temperature) and, at the same time, it must respect the environmental impact restrictions, linked to the fluid ozone depletion potential (ODP) and global warming potential (GWP). In this study, low-GWP fluids, as R1234yf and R1234ze(E) have been compared with R134a, as working fluid of a kW-size reciprocating piston expander and the optimization of the built-in volume ratio has been performed for each analyzed fluid in design conditions. To this purpose, a previously calibrated and validated semi-empirical model of the expander has been integrated with a new gear pump model, in order to simulate the volumetric machines into the real operation of the ORC system. The comprehensive model is conceived to accommodate the change of the working fluid: model parameters taking into account the thermo-fluid-dynamic properties of the fluid are updated compared to the original values calibrated over R134a by means of an extensive experimental campaign.