Series or parallel hybrid-electric aircraft propulsion systems? Case studies of the ATR42 and ATR72

Abstract

With the ever-increasing pressure on the environment, there is the political will to achieve air travel in a sustainable manner, while continuing to offer economic growth and serve society’s needs. In the European Union, this is driven by its ambitious strategy Flightpath 2050. More recently this has been renewed through the Green Deal which also includes sustainable aviation. This has promoted a substantial research effort into long-term goals such as the development of hydrogen powered aircraft. However, this paper argues that through careful design and optimisation, hybrid-electric propulsion systems (gas turbine with electric drives) can fill a gap in the short-to-medium term. This is particularly true for the short haul, regional aircraft. Such aircraft perform over 40% of all commercial flights and thus small fuel savings can immediately account to a significant impact on the environment. Although hybrid-electric aircraft would still produce some emissions, such propulsion systems could be effective to reduce emissions. The paper considers a case study of the ATR 42 and ATR 72 models (ATR - Avions de Transport Regional) and investigate their hybridization potential with increasing passenger capacity and expected improvement in battery energy density over the coming decades. The paper computes the in-flight CO2 emissions for the various degree of hybridization. The results show that parallel hybrid electric provides a slight edge in emission savings than series hybrid architecture. Due to the limitations in battery energy density, this potential decreases as the passenger carrying capacity increases, but improves as the energy density of batteries increases. Surprisingly, the results show that the assumption that smaller aircraft are easier to hybridize is often not fully true and requires careful consideration on a number of variables.