Comparison of a Single-Screw and a Scroll Expander under Part-Load Conditions for Low-Grade Heat Recovery ORC Systems

Increasing energy-demand and environmental concerns are leading to a growing interest in alternative and sustainable energy solutions. Low-grade waste heat recovery through ORC systems is considered to be one of the most feasible eco-friendly energy conversion technologies. In this paper, a comprehensive model of an organic Rankine cycle (ORC) system for micro-CHP applications is presented in order to compare the behavior under part-load conditions of two types of positive displacement expanders, a single-screw expander and a scroll expander. The ORC model is implemented in the AMESim environment, which allows system modeling both for steady-state and transient analysis. Typical residential electrical and heating loads are considered to study the performances of a suitable small-scale system. By means of a detailed quasi-1D model for a single-screw expander and a Reverse Engineering (RE)-Computational Fluid Dynamics (CFD) approach for a scroll expander, the performance characteristic maps of bothexpansion machines, in terms of isentropic efficiency as function of expansion ratio and rotational speed, have been determined and implemented in the AMESim® model. A comparison between the selected expanders is presented to address the influence of different positive displacement-type expanders on the overall ORC performances at off-design conditions. The capability of the system to meet the requested loads is also discussed.