Experimental campaign and modeling of a low-capacity waste heat recovery system based on a single screw expander

摘要

In recent years, due to the increasing concern over energy shortage and global warming, the interest in low grade heat recovery from industrial processes has grown dramatically. Several studies have underlined the potential of small-capacity Organic Rankine Cycle (ORC) power plants for Waste Heat Recovery (WHR) applications. For such systems, accurate models based on actual experimental data represent an important tool, in particular when control issues are considered. This paper presents an experimental campaign on a 10-kWe WHR ORC unit carried out both in steady-state and in transient conditions. The unit presents a regenerative architecture, uses Solkatherm (SES36) as working fluid and a single screw expander as expansion machine. The dynamic response of the unit is analyzed by applying a step function to the pump rotational speed while keeping the expander rotational speed constant. During the experiments, the control of the external heating circuit ensures a steady heat source temperature at the inlet of the evaporator. It has been ensured that the power plant was operating in steady-state condition before any change is imposed to the system. The test rig exhibits a good first law efficiency (about 11% efficiency for a heat source of 125*C) and allows measuring a wide range of operating conditions. Maximum isentropic efficiency of the screw expander is about 60%. Following the experimental campaign, steady state performance curves of the screw expander are developed and calibrated with the experimental data. Based on these curves, a complete dynamic model of the ORC power system is then built using the ThermoCycle library, an open-source Modelica library developed at the Energy Systems Research Unit at the University of Liege. The Solkatherm properties are computed with the external program CoolProp, linked to Modelica through an appropriate package. This dynamic model is also calibrated and validated with experimental data.