SYSTEMATIC FLUID-SELECTION IN EARLY STAGES OF ORC DESIGN – A PRACTICAL ENGINEERING APPROACH

asme-orc2015 Tracking Number 196

The selection of a working fluid plays a major role in the design phase of organic Rankine cycles (ORC). Therefore, a suitable working fluid builds the foundation for a high power production in an ORC, which also leads to high cycle efficiencies and is hence one of the most significant selling arguments. But the selection of a working fluid should not only depend on the power output of the ORC. There are technical rules for example given by the legislator, which also has to be respected with high priority. For a detailed research of potential working fluids a selection-procedure was developed. The method uses 22 criteria out of six main categories (thermodynamical, procedural/thermodynamical, safety, environmental, economical and chemical), which should be considered with adequate priority related to the implementation of the ORC. During the first tests, not all of the 22 criteria were evaluated, therefore, the process was improved with an accurate elaboration of the omitted criteria and by the integration of the working fluid mass flow directly into the selection process. The procedure itself is divided into four main steps, the pre-selection, the elimination process, the ranking process and the final fluid choice. Such a proceeding requires the subdivision of the 22 criteria into elimination criteria (EC) and tolerance criteria (TC). An advantage of this disposition into a structured processing in the early design phase of the ORC is the direct elimination of working fluids in the pre-selection- and elimination-phase. The application of the selection-procedure into the design process leads also to a saving in development time and helps to make an educated decision. This contribution deals with a comprehensive compilation of all 22 criteria deemed as relevant for the fluid selection and shows examples of fluid selections.