Performance Improvements of Scroll and Sliding Vane Expanders Via a Double Intake Port Technology for ORC-Based Power Units

Sliding Vane Rotary (SVRE) and Scroll machines are the most referenced Expanders for small scale Organic Rankine Cycle (ORC)-based power units. Indeed, volumetric expanders are generally preferred to the dynamic ones as they better address severe off-design conditions. Nevertheless, they present some intrinsic limitations related to friction, volumetric performance and geometrical constraints. Moreover, these machines behave like an “equivalent rotating valve” and their permeability (relationship between flow rate and pressure drop) primarily depends on the in taken mass flow rate. In this paper a Double Intake Port (DIP) technology is considered to achieve important benefits in terms of expander performance. DIP involves that after the closure of the main intake port, an additional port is opened fed by the working fluid at the same thermodynamic conditions of the first port. Thanks to this new aspiration, the pressure inside the vanes increases and, therefore, the indicated power as well as the capability of the machine to aspirate a greater quantity of fluid. A lumped effect of the DIP technology is the increase of the permeability of the expander able to elaborate a higher mass flow rate for a given pressure difference. The efficiency of the original and DIP machines is discussed as well as the effects on the on the power produced. To perform this analysis, comprehensive theoretical models of both expanders were carried out and experimentally validated. Subsequently, the models were used as a software platform to assess a best design of the DIP expanders in terms of performances.