For more than a century, aircraft have primarily been powered by carbon-based fuels such as kerosene or gasoline. These fuels contain a lot of energy for their weight and provide the power to lift the largest airliners. However, as a major contributor to CO2 emissions, and with dwindling oil resources, the future of aviation is dependent on finding an alternative power source.
The switch to electric propulsion in aircraft is one such area of investigation. The E-Fan X, for example, is a hybrid-electric aircraft, currently being jointly developed by Airbus, Rolls Royce, and Siemens. By replacing one of the turbofans used in a regular aircraft with a 2MW liquid-cooled electric motor, its developers hope to boost power for take-off and climb as well as facilitating an electric-only descent, which would significantly lower fuel burn and local atmospheric emissions. However, reduced emissions are not the only driver of electrification within aviation.
Electric aircraft could transform the way we travel
Vertical take-off and landing, combined with much quieter electric aircraft, could radically change the way we fly. With this reduction in sound pollution, and with no need for lengthy airport runways, I believe we’ll see many more inner-city airports, coupled with less flying restrictions.
Electric planes really do have the potential to transform the way we work, travel, and live. For instance, many people want to live in the countryside, but are prohibited by a lack of rural employment opportunities. Electrification would enable them to commute, via air, to local cities for work. And within cities themselves, it would facilitate short journeys using 4-6 seat taxi planes – making it easy to quickly travel from south to north London, or from San Francisco to Palo Alto, for example.
Of course, there is also a huge commercial opportunity – electrification can stand to make short-haul air travel cost-effective for providers, with additional revenue coming from urban travel and longer flying hours.
So, what’s limiting the trajectory of this air travel revolution?
High power batteries and motors add excess weight
A significant obstacle to all-electric propulsion is the power to weight ratio of the batteries and motors needed to supplant a traditional kerosene-powered engine. Various battery chemistries are under investigation, with Li-O2 currently out in front, but it’s safe to say that a fully electric system will require batteries with a far greater energy density than those currently available, in order to match power requirements without adding too much weight to the aircraft.
Current commercial batteries offer around 170Wh/kg of battery specific energy, yet according to research from NASA, a single-aisle 737 class aircraft, even in a hybrid electric configuration, would require a battery specific energy of 1,000Wh/kg or higher. To achieve this kind of power will necessitate new battery chemistries, but also new battery pack designs featuring greater integration and enhanced thermal management.
Electric motors are another major sticking point. High power density electric motors will be essential to convert the electrical energy provided by batteries into the thrust required for take-off and flight. Siemens has already developed a new 50kg electric motor that delivers a continuous output of around 260kW using a hybrid-electric propulsion system but considering a commercial aircraft may require anything from two to fifty megawatts – it’s clear there’s still a long way to go.
Increased operating voltages may lead to over-heating
A further challenge with adopting more-electric, and ultimately all-electric, propulsion technologies is that operating voltages will be significantly elevated, prompting a rise in system temperatures. A five to ten-megawatt system will generate hundreds of kilowatts of heat from multiple sources, including motors, batteries, power electronics, and cabling. These higher temperature systems create a pressing need to improve heat dissipation to prevent overheating and component failure.
Cooling technologies will play their part, but appropriate thermally-conductive insulation materials will also have a major role.
Thermally-conductive insulation solutions are not yet available
Today, insulation solutions which can support the extremely high voltages required to power fully-electric or hybrid-electric aircraft, do not exist. In power transmission and cabling systems, unscreened cable is currently the norm. Insulation thicknesses are selected to reduce the electric field to a point below which partial discharge occurs. This sparking phenomenon, which is common in equipment with a high electrical field, generates a short-duration plasma burst and causes rapid localized temperature spikes – all of which would be undesirable in an electric aircraft.
Continued use of unscreened cable would lead to prohibitively large insulation thicknesses at high voltages so a move to screened cables with an earthed outer layer may well be required to eliminate partial discharge between wires.
In motors and machines, the need to maximize power density also contradicts the need for robust, partial discharge-free insulation. Systems used at ground level, at voltages above 690V, are designed to withstand small amounts of partial discharge, but this voltage level will be lower at altitude. In fact, a combination of pressurization, insulation, and separation may well be required to avoid the corona effect of high altitude, high voltage arcing.
Research into new, thermally-conductive insulation technologies is vital, not least because insulation will be subject to different environmental conditions at altitude – namely, lower temperatures, lower pressures, higher levels of ozone, and regular variations in humidity.
High-performance PEEK insulation can realize vital advances
Zeus is currently collaborating with major players in electric propulsion to develop an enhanced, single layer PEEK insulated wire. PEEK’s characteristics as a lightweight, high temperature polymer with a high dielectric breakdown strength make it an ideal replacement for the Polyimide (Kapton®), film wrapping, and enamel coated wire that has been widely used in civil and military aircraft insulated wire applications. A high strength-to-weight ratio means PEEK can also be used in much thinner insulations, replacing thicker, heavier materials, yet providing similar or even improved performance.
In our work with leading aerospace industry partners, high-performance PEEK insulation has delivered several significant benefits, with robust performance and reduced defects contributing to system-wide cost savings. If we look at electric motor technology, for example, the ability to use PEEK in a thin layer provides a significant advantage. Utilising a thinner PEEK layer allows for more copper around the stator. This increases efficiency, which in turn allows the motor to either use less energy to deliver the same output or use the same energy with an increased output.
The ability to implement a thin, single-layer solution has other benefits too. Reduced delamination leads to less potential failures and longer product lifespans. And with fewer field failures, warranty costs will be lower. In fact, cost savings are generated in several ways. Our PEEK insulated wire is a 100% defect-free product. When you consider that magnet wire typically has no more than 3 defects per 5m, this can significantly reduce inspection costs. Manufacturing is also optimized with our abrasion resistant solution leading to lower conversion costs and lower yield loss since PEEK does not crack, chip, or flake.
Based on our experience of working with PEEK in many different forms over the last 25 years, Zeus is positive that this high-performance polymer can realize vital advances in next generation aerospace design.
The future of flying is electric planes
It’s clear that aircraft electrification will happen – it’s a question of when, rather than if, this transition occurs. Yes, commercial air travel on all-electric aircraft may be a long way in the future, but the market opportunities are immense for those involved in making this a reality. Being part of this evolution is immeasurably exciting, not just for me personally, but for the entire Zeus aerospace team.
I think it’s fair to say – the sky’s the limit.