An organic Rankine cycle (ORC) is a power cycle employing an organic working fluid. The term ORC is also applied generally to any Rankine cycle with a low-grade heat source (80° – 300°C). Because ORC are often employed in small-scale applications, use of positive displacement equipment is favored over the centrifugal units used in large-scale power plants. A key feature of a positive displacement expander is its built-in volume ratio.

An ORC with a scroll-type expander is studied experimentally. Data for steady state tests of the ORC are presented according to a proposed steady state standard. It is shown that the adiabatic efficiency of the expander can be fully characterized by its filling factor and the expansion volume ratio imposed across it. In particular, the peak adiabatic efficiency occurs near a filling factor of unity and an expansion volume ratio near the built-in volume ratio of the expander. The influence of the expander’s performance on cycle efficiency is considered. A procedure is presented which allows prediction of cycle performance based on knowledge of the expander efficiency versus expansion volume ratio, cycle operating conditions, and working fluid. Using this procedure, an optimal expander can be chosen for a set of cycle operating conditions based on its peak efficiency and built-in volume ratio.


organic rankine cycle scroll expander low-grade waste heat optimal


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