Screw expanders in ORC applications, review and a new perspective
2015 (English)In: Proceedings of the 3rd International Seminar on ORC Power Systems, 2015Conference paper (Refereed)
Performance of Organic Rankine Cycles is sensitive not only to the entry temperature ratio betweenheat source and heat sink but also to the temperature degradation of the heat source flow, caused bythe heat transfer to the process in pre-heater, evaporator and super heater. In order to adopt the cycleto 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 theheat source. Screw expanders offer an alternative, new approach to the matching problem of ORC’s asthey allow for flexible multi-phase expansion. Hereby the vapour fraction at the expander entry can beused to partially match the temperatures of the process to a particular heat source. To provide aperspective on the use of such screw expanders in ORC-systems previous experimental andcommercial experience have been reviewed and discussed.Screw expanders are versatile machines used for the production of mechanical work in power rangesfrom 3kW to 1.5MW. As the functional characteristics differ significantly from dynamic expandersthe explanatory models used to generalise results are different. Plenty of research has resulted in wellgeneralized explanatory models for dynamic expander analysis. For screw expanders similarexplanatory models exist mainly in commercially confidential environments. A few public sourcesdisclose test data. In the few cases data has been investigated the analyses tend to rely onthermodynamic models suitable for dry gas expansion. Typically that leads to reasonable replicationof test results but seldom to models suitable for detailed understanding of the process. In applicationswith 2-phase expansion the theories used to simulate functional characteristics is entirely insufficient.The main reason for the scarcity of work in this field is probably the empirical difficulties in obtaininggood measuring data in multi-phase conditions.This paper describes a review of multi-phase screw expander experiences and explains why a uniquetheory is required to model its characteristics. In the absence of such a unique theory a correlationbased in empirical data is presented. This allow for estimations of screw expander efficiency in multi-phase conditions. Measured efficiency with dry expansion, or such efficiency simulated, can be usedto estimate adiabatic efficiency with expansion entry vapor fractions ranging from 0 to 1 by using thiscorrelation. Hence estimating expansion efficiency during multi-phase expansion is simplified,allowing for better optimisation of the ORC-systems. This way a new perspective of screw expanderpotential in ORC system integration can be presented.
Place, publisher, year, edition, pages
Research subject Energy Technology
IdentifiersURN: urn:nbn:se:kth:diva-188018DOI: 10.13140/2.1.3137.3523ISBN: 978-2-9600059-2-9OAI: oai:DiVA.org:kth-188018DiVA: diva2:933117
3rd International Seminar on ORC Power Systems, October 12-14, 2015, Brussels, Belgium
QC 201606072016-06-032016-06-032016-06-09Bibliographically approved