In the last years one of the main research topics in energy field is represented by Organic Rankine Cycles (ORCs), due to their applicability in energy recovery from waste heat and in distributed combined heat and power (CHP) generation, particularly in small and micro scale systems. One of the key devices of the cycle is the expander: it must have a limited cost (like all the other components, in order to ensure the economic feasibility), but also a high efficiency, since the temperature of the heat source is often low and then the cycle efficiency is inherently scarce. In the first part of this paper a literature review on various positive-displacement expanders is presented, in order to outline their performances and their application field. Then, the numerical model of a volumetric reciprocating expander is implemented. This model, and another one previously developed to simulate scroll expanders, is combined with a thermodynamic model of the whole ORC system, so that a comparison between the two technologies can be carried out. The results confirm the possibility of realizing small scale energy recovery and cogeneration (CHP) systems with acceptable electrical efficiency also adopting low-cost components, directly derived from large scale industrial components.

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