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Session: Session 11: Screw expanders
Session starts: Tuesday 13 October, 10:30
Presentation starts: 10:30
Room: 1B Europe

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Henrik Ohman (KTH, Royal Institute of Technology)
Per Lundqvist (KTH, Royal Institute of Technology)

Abstract:
Performance of Organic Rankine Cycles is sensitive not only to the entry temperature ratio between heat source and heat sink, but also to the temperature degradation of the heat source flow caused by the heat transfer to the process in pre-heater, evaporator and super heater. In order to adopt the cycle to the great variety of heat sources a multitude of fluids are required. Alternatively fluid mixtures, trans-critical or supercritical fluid conditions can be used to match the process temperatures with the heat source. Screw expanders offer an alternative approach to the matching problem of ORC’s as they allow for multi-phase expansion. Hereby the vapour fraction of the ORC evaporator exit can be used to partially match the temperatures of the process to a particular heat source. To provide a perspective on the use of screw expanders in ORC-systems previous experimental and commercial experience have been reviewed and discussed as well as some of the fundamental challenges in understanding screw expander characteristics. Screw expanders are versatile machines used for the production of mechanical work in power ranges from 3kW to 1.5MW. As the functional characteristics differ significantly from dynamic expanders the explanatory models used to generalise results are different. For dynamic expanders analysis plenty of academic research has resulted in a wide flora of publications and well generalized explanatory models. For screw expanders similar explanatory models exist mainly in commercially confidential environments. A few public sources disclose test data, often analysed using thermodynamic models suitable for dry gas expansion. With minor adaptations to the specifics of the screw expander such analysis typically gives reasonable result, though seldom suitable for detailed understanding of the process. In applications with 2-phase expansion theory used to simulate functional characteristics is entirely insufficient. This is mainly due to the fact that common dynamic expanders are seldom used for multi-phase expansion. This paper describes a review of multi-phase screw expander experiences and explains why a unique theory is required to model its characteristics. In the absence of such a unique theory a correlation is presented, allowing estimation of screw expander efficiency in multi-phase conditions, based on empirical data. By using this correlation measured dry expansion efficiency, or such efficiency simulated, can be used to estimate adiabatic efficiency with expansion entry vapour fractions ranging from 0 to 1. Hence estimating expansion efficiency during multi-phase expansion is simplified, allowing for better optimisation of the ORC-systems.