CEAS Aeronaut J (2018) 9:15–25 https://doi.org/10.1007/s13272-017-0272-1 ORIGINAL PAPER Analysis and design of hybrid electric regional turboprop aircraft 1 1 1 Mark Voskuijl • Joris van Bogaert • Arvind G. Rao Received: 10 April 2017 / Revised: 22 September 2017 / Accepted: 10 October 2017 / Published online: 31 October 2017 Ó The Author(s) 2017. This article is an open access publication Abstract The potential environmental benefits of hybrid Pa Power available [J/s] electric regional turboprop aircraft in terms of fuel con- Pbr Shaft power [J/s] sumption are investigated. Lithium–air batteries are used as R Range [m] energy source in combination with conventional fuel. A S Power split [–] validated design and analysis framework is extended with t Time [s] sizing and analysis modules for hybrid electric propulsion T Thrust [N] system components. In addition, a modified Bre´guet range V Airspeed [m/s] equation, suitable for hybrid electric aircraft, is introduced. W Weight [N] The results quantify the limits in range and performance for g Efficiency [–] this type of aircraft as a function of battery technology U Supplied power ratio [–] level. A typical design for 70 passengers with a design w Hybridization factor [–] range of 1528 km, based on batteries with a specific energy of 1000 Wh/kg, providing 34% of the shaft power Indices and subscripts throughout the mission, yields a reduction in emissions by bat Batteries 28%. cryo Cryocooler em Electric motor Keywords Hybrid electric propulsion Á Range equation Á inv Inverter Aircraft design Á Mission analysis prop Propeller List of symbols Abbreviation CL Lift coefficient [–] MTOM Maximum take-off mass CD Drag coefficient [–] -1 cP Power-specific fuel consumption [m ] -1 cT Thrust-specific fuel consumption [s ] E Energy [J] 1 Introduction g Gravitational acceleration [m/s2] H Energy per unit fuel or battery [J/kg] The potential for hybrid electric or even full electric p Power density [kW/kg] commercial aircraft has received a great deal of attention P Power [J/s] over the past years. The industry is currently in the process of adopting the ‘more-electric’ aircraft in which various aircraft systems (pneumatic and hydraulic) are replaced by & Mark Voskuijl electric systems [1–5]. Various recent studies highlight the [email protected] potential of (partly) using electric power for propulsion. 1 Delft University of Technology, Kluyverweg 1, This opens a realm of new propulsion system architectures 2629 HS Delft, The Netherlands and even new aircraft configurations. Three basic 123 16 M. Voskuijl et al. approaches to the use of electric power for propulsion can with a design range of 1667 km if batteries with a specific be distinguished. energy of 750 Wh/kg are available. The VOLTAIR design, however, is also based on the beneficial effects of other • Batteries as energy source to (partly) replace fossil technologies; (1) a propulsive fuselage configuration fea- fuels. turing boundary layer ingestion, (2) a low slenderness • Provide electric power at specific flight phases to limit fuselage with reduced structural weight and (3) a 60% the variation in the gas turbine operability. natural laminar flow wing. These combined technologies • Novel aircraft configurations (e.g., distributed are claimed to yield a 25% improvement in energy effi- propulsion). ciency. The VOLTAIR aircraft design does not converge First of all, batteries can be used as an energy source just for longer range missions or larger payloads, indicating a like conventional fuel. The fundamental challenge in using physical limit in size for full electric regional aircraft. The batteries as an energy source for propulsion is the low studies described above highlight that there is a limit in specific energy of batteries compared to the high specific aircraft size even when significant improvements in battery energy of conventional jet fuel. The specific energies of technology are achieved. The use of batteries as energy state-of-the-art batteries are in the order of 100–200 Wh/kg source alongside conventional fuel therefore shows the at cell level, whereas gasoline has a specific energy of most promise for the regional aircraft segment. Further- approximately 13,000 Wh/kg. Lithium–air batteries, how- more, the vast majority of air travel is conducted by ever, show great promise with a theoretical specific energy regional aircraft and short-to-medium-range single-aisle of 11,680 Wh/kg [6]. Many significant challenges have to jets [12]. At present, however, there are no research studies be overcome before even a small part of this theoretical on the use of batteries as energy source for regional tur- value can be achieved in reality. Consequentially, the use boprop aircraft with a conventional configuration. A sum- of batteries as energy source is at the moment only feasible mary of current and future battery technology at both cell for small aircraft, which have a low weight and limited and system level is shown in Fig. 1. range. Currently, there are already various flying demon- A second approach to the use of batteries for propulsion strators and production versions of manned full electric is to provide additional power in specific mission phases general aviation aircraft. Several recent small electric air- such as take-off and climb. By doing so, the conventional craft initiatives are described by Wells [7]. A realistic gas turbines can be optimized for a single flight condition design procedure for small full electric aircraft was (cruise). As a result, they can be smaller (lower weight) and recently introduced by Riboldi and Gualdoni [8]. Studies more efficient. It was demonstrated by Perullo and Mavris for larger aircraft are usually based on estimates of the [13] that it is important to combine the mission optimiza- specific energy of future batteries. Pornet et al. [9] inves- tion, aimed at finding the optimum transient power split, tigated the use of batteries as energy source alongside with the aircraft design optimization. Ang et al. [14] conventional jet fuel as a retrofit for short-to-medium- investigated the benefits of this approach for a hybrid range single-aisle turbofan aircraft. They conclude that the electric version of an Airbus A320. It is demonstrated that use of batteries with an energy density of 1500 Wh/kg as an for short-range missions of 1000 km a fuel burn saving of energy source can provide a large block fuel reduction 7.5% can be achieved when the engine is scaled to 90% (- 16%) on short-range missions in case the energy ratio of fuel and batteries is 82–18%. Further analysis by Pornet 1100 et al. demonstrates that batteries with an energy density 1000 below 1000 Wh/kg provide no significant fuel savings at 900 all. A similar study was performed by Friedrich and 800 Robertson [10] in which a Boeing 737–800 was retrofitted 700 with a hybrid electric propulsion system. Assuming a 600 specific energy of 750 Wh/kg, a 10.4% fuel saving was Current Li-ion (cell) computed on a 2 h mission. The energy stored in the bat- 500 Current Li-ion (system) Optimistic Li-ion* (cell) 400 teries is 6% of the total energy (fuel and batteries). This Optimistic Li-ion* (system) comes at the expense of an increase in aircraft weight of 300 Optimistic Li-Sulfur* (cell) Optimistic Li-Sulfur* (system) 200 approximately 10,000 kg. This added weight was, how- Optimistic Li-Air* (cell) Specific Energy Density [Wh(total)/liter] * ever, not accounted for by a structural redesign of the 100 Optimistic Li-Air (system) * Assumes Li-Metal negative aircraft. The feasibility of a full electric regional aircraft 0 concept designated VOLTAIR was investigated by Stu¨ckl 0 100 200 300 400 500 600 700 800 Specific Energy Density [Wh(total)/kg] et al. [11]. Their calculations demonstrate that it is feasible to design a full electric regional aircraft for 70 passengers Fig. 1 High energy batteries cell and system level comparison 123 Analysis and design of hybrid electric regional turboprop aircraft 17 and electric power is used to assist the take-off and climb The structure of this paper is as follows. First, an phases (with 25 and 14% electric power, respectively). existing regional turboprop aircraft will be selected as a These estimates were based on batteries with a specific reference. Next, a hybrid electric propulsion system energy of 600 Wh/kg. architecture will be defined for this aircraft. This includes a Finally, having a hybrid electric propulsion system description of the hybrid electric propulsion system makes it possible to separate the engine from the propulsor parameters. The overall design methodology will be dis- and thereby new aircraft configurations become feasible. cussed subsequently. For validation purposes, this design An example is the NASA N3-X design of a hybrid wing method will first be used to design a conventional turbo- body aircraft with a turboelectric distributed propulsion prop aircraft with the same top-level aircraft requirements system [15]. Distributed propulsion can be used to enhance as the reference aircraft. After validation, the design the efficiency by wake-filling and boundary layer ingestion. method will be used to explore the design space ranging In addition, a higher integration of the propulsion system from 100% conventional fuel to full electric propulsion. and airframe can lead to a lower structural weight. Fur- This will be done for various design ranges and battery thermore, in case a single core engine is used to power technology levels. A single hybrid electric aircraft design multiple electrically driven fans, a higher effective bypass which shows most promise in terms of fuel savings and ratio can be achieved [16]. This is applied for example in practical feasibility will finally be selected and investigated the Airbus-Rolls Royce E-Thrust concept [17].