Development and numerical modelling of a supercharging technique for positive displacement expanders

This study presents a novel strategy to enhance the recovery performance of any positive displacement expander technology which aims at the maximization of the power output rather than solely its efficiency. The approach is based on an auxiliary injection of fluid under the same suction conditions as the main intake but during the closed volume expansion phase. The operating principle of the supercharging technology is firstly outlined in theoretical terms, while the benefits over a conventional configuration are numerically assessed with reference to a sliding vane expander for applications based on Organic Rankine Cycles (ORC). The holistic modelling platform used for the benchmarking is preliminarily validated over an experimental campaign in which the vane expander was installed in a heavy-duty automotive ORC system and generated up to 1.9 kW (3% of the engine mechanical power) with an overall efficiency of 51.2%. After the simulation platform is validated, the auxiliary intake line is parameterized in terms of four geometrical quantities and the effects of the supercharging with respect to baseline angular pressure trace are shown. An optimization based on a genetic algorithm is eventually performed and the resulting optimized design led to an average mechanical power increase of 50.6%.