Performance Investigation of Cycle-Integrated Parallel Hybrid Turboshafts

Motivated by the long term target settings for research and innovation in Europe and in North America, initial investigations of parallel hybrid electric power plant systems have indicated significant fuel reduction potentials for short range air transport. While an electric motor assists the gas turbine in suppling mechanical power to the gas turbine within the classical parallel hybrid topology, in the present paper, a more sophisticated variant, namely the Cycle-Integrated Parallel Hybrid (CIPH) is considered. More specifically, the design and performance implications of a CIPH power plant architecture are investigated with regard to an advanced turboshaft (TS) engine application for helicopters. For this purpose, an array of compressor stages of the baseline TS power plant are decoupled from the turbine section, and are driven mechanically independently by means of electric motors. The baseline power plant of the investigated concept is derived for a 12-ton-helicopter accommodating 19 passengers on a 450nm mission. It consists of an axi-centrifugal compressor powered by the high pressure turbine as well as a free low pressure (power) turbine delivering a maximum shaft power of 3300 kW. For the presented CIPH concept, the axial compressor section is electrified with the help of linear electric motors mounted at the blade tips. Due to typical design characteristics of electric motors, counter rotating stages are considered most appropriate for the targeted TS power plant application. The electric motor power supply is realized through a Power Management And Distribution system featuring proper levels of redundancy. For the electrical energy storage, advanced battery technology is taken into account. Hybrid electric Energy and Propulsion Systems (EPS) can be characterized meaningfully by the degree of power hybridization, H_P, being defined as the ratio of the installed electric power to the total power. For the presented CIPH application, a best and balanced HP of 19.7% has been identified. In typical part load operation, this may lead to relative Power Specific Fuel Consumption (PSFC) improvements of up to 45% and overall efficiency has been almost doubled compared to the TS reference. With the implementation of electric power within the cycle, additional degrees of freedom for power plant operation and control can be established. At vehicular level, a retrofitted version of the reference helicopter equipped with the CIPH TS propulsion system faces more than 50% reduced range, but simultaneously reduces total energy consumption (fuel plus electrical) by 28% and CO₂ by 42% compared to the reference vehicle at identical reduced range.