Feasibility Study of an Organic Rankine Cycle Waste Heat Recovery System for the Engines of a Partial-Turboelectric Aircraft

This study analyzes a novel combined-cycle turbofan (CC-TF) engine concept consisting of a turbofan engine and an organic Rankine cycle (ORC) bottoming unit. The aim of the work is to identify if benefits in terms of mission fuel consumption can be derived from the CC-TF engine if compared to a simple-cycle turbofan engine with entry into service in the year 2035 for the test case of a partial-turboelectric aircraft. Propulsion system designs for minimum fuel consumption are obtained by coupling an optimizer with a multidisciplinary simulation framework, incorporating models of the engine, the distributed propulsion system, the ORC unit, the heat exchangers, as well as a simplified model of the aircraft to perform mission analysis. The results suggest that fuel savings of around 4% are possible with the CC-TF concept if compared to the benchmark engine. The ORC system contributes approximately one-fifth of the required electrical power demand and features a mass-specific power of approximately 1 kW/kg. The estimated thrust-specific fuel consumption of the propulsion system adopting the CC-TF engine is 13.5 mg/Ns, which is approximately 6% lower than that of the benchmark. The CC-TF engine benefits from the ORC system in two ways. Firstly, the contribution of the ORC system to shaft power production reduces the required fuel mass flow rate. Secondly, the thermal energy transfer to the engine bypass stream from the ORC condensers increases both the engine propulsive efficiency and specific thrust.