Simulation and Thermodynamic Analysis of a Two-Stage Organic Rankine Cycle for Utilisation of Waste Heat at Medium and Low Temperature Levels

Abstract

A two-stage ORC process is proposed as a technical option for utilising waste heat from power generation processes in combustion engines, and specifically as an efficient means of increasing the total efficiency of energy conversion in existing and projected biogas plant. Due to process-related factors, the utilisation of waste heat in the exhaust gas heat exchanger and from the cooling system of the combustion engine is performed at different temperature levels and is realised in the ORC process in two stages by means of a micro-turbine and a screw engine. In the course of the research work conducted, a complex calculation and simulation program was designed and created with which thermodynamic analysis as well as, to some degree, operational analysis of the respective processes were carried out.The novelty of the concept developed here consists in adapting the framing conditions of a two-stage ORC process to the demands posed by utilizing the waste heat of biogas plants and to the practical application possibilities with respect to pressure ratios and temperature levels of expansion machines. The concept has been thermodynamically investigated and the results form the basis for constructing a test installation which shall enable the verification of these theoretical results. Key words: Low temperature energy use; Organic rankine cycle; Two-stage process