TECHNICAL FEASIBILITY OF A NOVEL OIL-FREE CO-ROTATING SCROLL EXPANDER FOR A KALINA CYCLE

摘要

Kalina cycle is of great interest since it uses low temperature energy resources and yields better performancethan typical power cycles based on Rankine technology. Moreover, the use of natural working fluids likeammonia and water have no ozone depletion potential (ODP=0) and no greenhouse effect (GWP=0).However, Kalina cycles have not been studied in depth for low power range because of the lack of suitableexpansion devices that can work in presence of ammonia and water. The main requirements for theexpansion device are material compatibility with ammonia (stainless steel or aluminium) and complete oilfreenessto avoid the occurrence of water-oil emulsions. A novel scroll machinery based on co-rotatingtechnology (currently in a prototyping stage) allows to work without internal lubrication in expansion mode.In order to demonstrate the expander’s working principle and characterize its preliminary performance anexperimental investigation has been performed. A simplified semi-empirical reduced order model isproposed to predict its performance. Finally, a technical feasibility study about the novel oil-free co-rotatingscroll machine working as expander in a Kalina cycle is carried out. The calculations show that the corotatingscroll expander working in a Kalina cycle could achieve overall isentropic efficiencies in the range54% - 60% and that the Kalina cycle could achieve net thermal efficiencies in the range of 6.3% - 8.7% andexergy efficiencies between 23.8% and 33.6% depending on the sink water temperature.