Comparison of a Single-screw and a Scroll Expander under Part-load Conditions for Low-grade Heat Recovery ORC Systems

Abstract

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 both expansion 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.