Pre-Design Investigation of a Propulsion System with Compressor Electric Drive

With rising global temperatures, the aviation sector is actively seeking for new solutions to reduce its climate footprint. Highly advanced propulsion systems that are currently in development have started to utilize hybrid-electric technologies in order to enable the further optimisation of engine operational performance (e.g. CFM RISE concept (e.g. CFM RISE concept [1]). A significant step beyond this could be made by strategically introducing electric power in the compression section of a gas turbine engine through driving individual rotor stages of the compressor independently from the turbine shaft [2]. Limited by the on-board storage capacity of electric energy in batteries, initial application scenarios of advanced propulsion systems incorporating compressor electric drive will focus on regional turboprops and mid-size helicopters. In order to propose a feasible design, the present study analyses the particular concept of a parallel hybrid-electric propulsion system, in which electric power from advanced batteries is added to the cycle by two means: firstly, by directly driving a set of counter-rotating compressor stages, and secondly, by providing electrical assistance to the high pressure spool of the engine. This propulsion system concept is currently investigated as part of the LuFo VI-2 "HybVer" project, targeting a technology readiness level of TRL 2. In order to analyse the proposed propulsion system concept, a performance synthesis model using the BHL in-house framework Aircraft Propulsion System Simulation (APSS) has been created. The underlying design methodology, including the integration of the electric system data and the representation of the partially-electrified compressor is presented. Based on initially estimated power requirements of a regional turboprop (REG) and a mid-size helicopter (HEL), propulsion system sizing is performed by optimizing key thermodynamic cycle parameters. Additionally, operational behaviour for different power and hybridization settings is analysed. The results show that for the HEL propulsion system, a maximum hybridization degree of 40% during cruise conditions could be achieved. For the REG propulsion system, a maximum hybridization degree of 31% is possible. Compared to the design mission, this would enable significant PSFC reductions during shorter missions, when the installed battery capacity is fully utilized. The results of this paper serve as a starting point for the future investigations of the proposed propulsion system on aircraft level.