Multi-Vane expanders as prime movers for low-grade energy organic Rankine-cycle engines

The behaviour of the prime mover in a low-grade energy Rankine-cycle engine is one of the most important factors affecting the overall system performance. A survey of published data concerning various types of expander in use in Rankine engines and an analysis based on the concept of similarity shows that, for the low power outputs, positive-displacement expanders have potential advantages when compared with turbines. However, the multi-vane expander (i.e. the MVE) offers considerable promise as the most appropriate prime mover for organic Rankine-cycle engines utilizing solar energy or waste heat as energy inputs, especially in the developing countries. The features and characteristics of the appropriate MVE's are discussed, together with the operational problems remaining to be solved.