Dual intake rotary vane expander technology: Experimental and theoretical assessment

Rotary Vane Expander is an interesting solution for small-scale ORC power unit due to its reliability, flexibility and competitive cost. As demonstrated by the authors in previous works, the introduction of a secondary intake port leads to an increase of the aspirated mass flow rate and consequently of the mechanical power produced by the machine. In this paper, theoretical and experimental studies were conducted in order to assess the potential benefits in terms of efficiency introduced by the dual intake expander and the trade-off with the produced power for a given pressure-drop. The theoretical results showed that if the relative gain of mechanical power produced by the dual intake technology is higher than that of working fluid mass flow rate, the efficiency grows when the same machines operate at the same upstream and downstream pressures. Two expanders have been designated, built and tested giving the possibility to experimentally verify the performances of a single and a double intake machine. From measured data a mathematical model of the expander was validated, allowing to use it as a virtual platform for further machine optimization and improvement. It was observed that the efficiency gain introduced by the dual intake device depends on the OEM volumetric efficiency and on the pressure ratio. The global efficiency of the dual intake expander grows up to 30% if the volumetric efficiency is 50% and the pressure ratio is 2.3 while the efficiency benefit decreases to 5% if the volumetric efficiency is 70% and the pressure ratio is 3. Nevertheless, even if the global efficiency would be equal for the two machines, the dual intake technology always allows to increase the delivered mechanical power.