Electrical Aircrafts – Status and future development

This article aims to provide an overview of current electrical aircraft projects and technologies, the influence of electrifying aviation on the Norwegian grid, development status of electric aircraft charging and future technologies related to electric aviation.

The report is written by Abdalla Abdellatif, research assistant with UiT, summer 2019.

Introduction

In Norway, the aviation industry is responsible for emitting an annual average of 3.5 million tons CO2 equivalents. By using electric airplanes for domestic routes, this number can be reduced according to a conservative estimate by 1.2 million tons. That is enough to meet Avinor’s target to reduce green house gas emissions by 50% on 2020.

The advantages of shifting to electric aviation are far beyond reduction in green house gases. The use of electric motors and drive systems is preferred over jet turbo engines due to the following reasons:

  • electric motors have much higher efficiency rates (more than 90%) compared to combustion engines (typically 20% – 25%)
  • electric motors are more reliable
  • less noisy and easier to maintain and control

Electric aircraft propulsion examples

The flexibility and scalability of electric motors have opened the door to integrating advanced aerodynamically efficient design concepts in several airplane projects.

Examples of such are:

  • Distributed Propulsion (DP) as in NASA X-57 Maxwell, where there are 12 small motors distributed on wings and operated during take-off to increase lift and reduce drag.
  • Vertical take-off and landing (VTOL) as in Airbus City Airbus, 8 motors arranged in 4 redundant sets
  • Propulsive fuselage concept(PFC) as in Centerline project, one electric motor propeller at the rear end of the airplane supplied by two turbo generators to compensate for fuselage aerodynamic drag.
  • Wing tip fans as in Eviation Alice, where two small electric motors are fitted in the far ends of wings

Electric aviation can be categorized based on range and passenger capacity to the following:

  • Urban Air: range < 80 Km, passengers 2 -4
  • Thin Regional Air: range <1200 Km, passengers 9+
  • Regional Air: range <1500 Km, passengers 30-150
  • Large commercial Air: range>1500 Km, passengers 150+.

Several electrical aircraft projects are currently ongoing with some recently completed their test flights and reached the certification phase. Table 1 shows an overview of some ongoing and recently launched models.

Table 1 – Example of ongoing and recently launched models

CategoryAirplane modelCapacity
[seats]
Max. range
[km]
Power train
architecture
Max powerBattery
[kWh]
Development
status
Thin Regional AirZunum Aero A-10121160Series-hybrid1 MW
(500 kW turbo gen.)
unknownFlight test scheduled in 2021
Thin Regional AirAlice91046All Electric780 kW900Flight test scheduled in 2019
Urban AirCity Airbus (helicopter)430All Electric140 kW x
4 batteries
110Flight tested may 2019 - under certification
Urban AirAmpaire EEL 33751200Parallel-hybrid311 kW
(154 kW engine)
unknownFlight tested june 2019 - under certification
TrainerAlpha Electro2200All Electric60 kW21Production

Electric aircraft power train architecture examples

Shortcomings related to current battery energy densities have led to development of turbo-electric propulsion for air crafts in order to increase their range and capacity. Power train architectures for ongoing projects are shown in Figure 1 and can be categorized as follows:

  • All Electric, where battery drives an electric motor connected to a propeller. Examples found in Alice, Alpha Electro and City Airbus.
  • Series Hybrid, a generator (range extender) kicks in providing electricity when the battery charge is close to depletion (normally at 20% charge), as in Zunum Aero.
  • Parallel Hybrid, where both turbo generator and battery are used simultaneously to supply power to the motor(s) as in Ampaire EEl 337.

Figure 1 – Electric aircraft power trains

Impact of electrifying air travel on the Norwegian grid

With the continuation of the current battery energy density improvement rate of 8% it’s expected that by 2030 fully electric or hybrid electric passenger planes will be sufficiently developed to take over short haul flights. Hybrid solutions with onboard generated energy restricted as a reserve or a “range extender” that won’t be used in many shorter routes. A target has been set by policy makers in Norway to electrify all short-haul flights by 2040.

According to Statnett, the electrification of all domestic flights will have a minor impact on the Norwegian electricity demand -as low as 2 TWh – subject to development of highly efficient electrical drive systems with high battery densities. Figure 2 shows an estimated total of 14 TWh consumption in transport section assuming the “extensive electrification” scenario which results from partial electrification using currently developed solutions.

Figure 2 – Energy use in TWh for the transport sector in 2017 from the Statistics Norway energy balance and with electrification (Source: Statnett)

Statnett also estimates a 50% saving in aviation energy consumption leading to a reduction from 10 TWh shown in Figure 2 to 5 TWh considering the “Fully electric with hydrogen” scenario. That is due to the much higher efficiency of electric systems. The main impact will be on the transmission and distribution grid where high power demand might be required.

Charging solutions for Lithium variant batteries

Charging typology might be different depending on the Aircraft category, available charging time and battery capacity. If we take City Airbus as an example of the future “Air taxi” model of urban transportation, the compatibility with the fast and ultra-fast charging infrastructure already developed for electric vehicles will be of a great advantage. For a series hybrid arrangement where batteries will be charged during flight through turbo generator a fast charging scheme might be sufficient. If we consider larger passenger aircrafts, then battery swap might be the best option where batteries are being replaced after each flight and charged in a dedicated station at the airport.

Little information is available on the chargers for the undergoing and recently developed prototypes. Pure Flight has developed combined charging stations for both cars and light aircrafts. Table 2 shows examples of different charging solution for electric aircraft

Table 2 – Example of electric aircraft chargers

ModelPower
[kW]
TypeArrangementLevelStandards
Pure flight
Nikola
30 - 100+Cars / airplanesOff board2,3CHAdeMO / CCS2 / ΦNIX AERO
Pure flight
Andre
10 - 20Cars / airplanesOff board2,3CHAdeMO / CCS2 / ΦNIX AERO
Pure flight
Jan
2.5 - 5AirplanesOn board2CHAdeMO / CCS2 / ΦNIX AERO
Pipistrel Sky
charger
2AirplanesOn board1,2Pipistrel
Pipistrel charger8 - 20AirplanesOff board2Pipistrel

There are also examples of battery swap concepts, where the Easyjet/Wright concept are designing their aircraft for quick replacement of batteries.

Future technology

Electroaerodynamic (EAD) propulsion

A research by MIT scholars to develop a system for flying aircrafts without incorporating any moving parts depending on air ionization where they managed to fly a heavier than air prototype, the concept is to generate and accelerate ions from air using  corona discharge by applying a constant high voltage across asymmetric electrodes those generated ions will collide with neutral molecules coupling the momentum of the accelerated ions with that of air resulting in a thrust force opposite to the ion flow. Figure 3 and 4 shows a picture of the airplane and the architecture of its high voltage power converter respectively.

Low overall efficiency and low thrust density are main challenges to such technology.

Figure 3 – MIT Electroaerodymanic propulsion prototype (MiT)

Figure 4 – Architecture of the ultra-high voltage (40 kV) on-board power converter (MiT)

Aquifer

Research project led by NASA to develop an integrated Nano Electro Fuel (NEF) battery with Rim Driven Motors (RIM) for aircrafts resulting in a range increase of 1.7 times over an all electric battery. The concept is to pump a liquid electrolyte formed by a positively and negatively charged iron infused water based liquids into a flow cell allowing for multiple charge and discharge cycles. Integrating the battery with motors will eliminate conductive EMI and weight of cable runs while providing liquid based cooling for motors. Figure 5 shows a picture of the system demonstrator. The project is at early stages of technical feasibility studies.

Figure 5 – AQUIFER demonstrator (NASA)

Cryogenic hydrogen system for electric aircraft

A research led by Illinois university to develop an electric aircraft platform that uses cryogenic liquid hydrogen for storing energy, which is later converted into electric energy through a series of fuel cells. The low temperature environment required for storing liquid hydrogen will be used to support superconductors and high-power motor systems. Figure 6 shows a concept sketch of the system.

Figure 6 – Concept sketch of cryogenic fuel cell powered aircraft (Illinois University)

Bibliography

  • Jan Otto Reimers, Green Future AS, Introduction of Electric Aircraft in Norway Feasibility study by Green Future AS, 2018
  • Dag Falk-Petersen, Avinor, The Future of Aviation Nordic EV Summit, 2018
  • Haofeng Xu et all, Flight of an aero plane with solid-state propulsion, DOI: https://doi.org/10.1038/s41586-018-0707-9 ,2018
  • Pipistrel Alpha Elctro, Pipistrel ALPHA ELECTRO Information Pack ,2017
  • Vegard Holmefjord, Anders Kringstad , Statnett, An electric Norway – from fossil to electricity, 2019
  • Cheryl L. Bowman et all, Turbo- and Hybrid-Electrified Aircraft Propulsion Concepts for Commercial Transport, 2018
  • Kurt Papathakis, Linda Taylor, Integration of Nano-Electrofuel (NEF) Flow-Cell Batteries with Rim-Driven Motors (RDM) for Improved Safety, Noise, Charging Time, and Range of Aircraft Electric Propulsion, 2018

Relevant web-pages for more information

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