The civil global aviation market has experienced considerable economic growth in recent years and will keep increasing. It is estimated that around 1300 new international airports will be required, and the commercial aircraft fleet will double by 2050, with a projected passenger throughput of 7.2 billion in 2035.
This growth is desirable from an economic standpoint, but aviation is predicated on fossil fuels, which increase greenhouse gas and air pollutant emission.
Presently, Aviation Industry generates 2-3 percent of the world’s human-generated carbon dioxide emissions and 12 percent of the CO2 emissions from all transportation sources. It is projected to rise to 11% if new technology is not advanced in the next two decades to cope with the projected annual growth of the aviation industry.
To reduce its impact on the environment and improve the sustainability of its operations, the commercial aviation industry has committed to achieve net-zero air transport emissions by 2050.
Achieving these goals requires disruptive innovation in the propulsion system of aircraft. The need to optimize aircraft performance, decrease operating and maintenance costs, and reduce gas emissions is pushing aircraft industry to explore new concepts including more electric aircraft (MEA), and ultimately an All-Electric Aircraft. Electric propulsion can be powered by rechargeable batteries, fuel cells, or solar energy. Electric plane power is much simpler — batteries power an electric motor that spins a propeller.
Today, we have two serious challenges to electric propulsion. If we installed today’s electric batteries that could power a commercial air transport aircraft, the aircraft would be so heavy, that it would be aerodynamically, operationally and economically unfeasible. Even smaller, regional electric aircraft would have a range of less than 500 nautical miles. And, again, it would be economically unsustainable.
The reason is a function of the ability to store energy (expressed as energy density in kilowatt-hours per kilogram), and the ability to convert that energy into power (expressed as power density in kilowatts per kilogram). The task ahead of us is evident when you consider that jet fuel has 50 times the energy density of today’s batteries, and a typical jet engine has three times the power density of today’s electric engines. Further, as a traditional aircraft burns fuel, the aircraft gets lighter.
Despite the promising benefits, the insufficient energy densities and specific energies of electrical storage devices are major challenges as they induce severe weight and volume penalties. The greatest limitation to using battery-powered aircraft is weight. Dr. Anderson explained, “If the lithium-ion batteries that are used in cars today were converted for aircraft, the weight comparison for a Boeing 787 Dreamliner would be 223,000 pounds of jet fuel vs. 4.5 million pounds of battery. “Unless there is a cosmic change in the battery, it’s just not going to work for bigger, faster airplanes,” he said. “It’s going to be a really long time before batteries weigh less than liquid fuel.” As battery technologies advance, electric propulsion concepts are on the edge of disrupting aviation markets.
For more information about the technology and applications of Electric Aircraft please visit: Electric Skies: How Electric Airplanes are Revolutionizing the Future of Air Travel
While electric propulsion is more efficient, but it generates far less thrust, which is why electric planes tend to be slow. Airbus’ two-seat electric plane could only go a maximum speed of about 136 miles per hour. A solar-powered plane that completed an around-the-world journey this summer had an average airspeed of 47 miles per hour. The plane, called Solar Impulse 2, had more than 17,000 solar cells that powered four electric motors.
Industries are now exploring Hybrid-electric propulsion systems that employ two or more distinct types of power, for example, power from an internal combustion engine and with electric propulsion systems in aircraft. These systems typically use a combination of electric motors and batteries, along with a gas turbine engine or another type of internal combustion engine.
One of the main benefits of hybrid-electric propulsion systems is that they can significantly reduce fuel consumption and emissions, particularly during the most energy-intensive phases of flight. Batteries can then, for instance, provide clean take-off and landing to reduce emissions near airports. They can also provide a backup power source in the event of an engine failure or other emergency situations.
Internal United Technologies Corporation studies indicate that commercial hybrid-electric and electric propulsion could: Reduce aircraft noise up to 85 percent, Improve fuel consumption up to 40 percent, reduce carbon dioxide emissions by more than 20 percent and Reduce airline operating and maintenance costs up to 20 percent.
The HEPS is an evolution where small planes can use airfields with smaller runways for commercial applications. Electric motors have higher torque at low speeds, making the aircraft’s acceleration during takeoff to be higher. These new aircraft concepts with architectures that increase propelling torque and power, for shorter takeoff distances, can enable future devices to utilize smaller airports . Therefore, until electric energy storage systems are ready to allow fully electric aircraft, the combination of combustion engine and electric motor as a hybrid-electric propulsion system seems to be a promising intermediate solution.
Types of hybrid electric propulsion systems for aircraft.
There are several types of hybrid electric propulsion systems for aircraft. Here are some of the most common ones:
- Parallel Hybrid Electric Propulsion System: In this system, both the electric motor and the internal combustion engine are connected to the propeller or fan, allowing for either power source to drive the aircraft. The electric motor can be used to provide additional power during takeoff or climb, and the internal combustion engine can be used for cruising.
- Series Hybrid Electric Propulsion System: In this system, the internal combustion engine is used to generate electricity, which is then used to power the electric motor that drives the propeller or fan. The electric motor cannot directly drive the aircraft and is only used as a propulsion system.
- Turboelectric Propulsion System: This system uses a gas turbine engine to drive a generator, which produces electricity to power the electric motor that drives the propeller or fan. The electric motor can also be used as a generator during descent or landing, regenerating energy to recharge the battery.
- Distributed Electric Propulsion System: In this system, several small electric motors are distributed around the aircraft’s wings or body, each driving a small propeller or fan. This provides greater control and maneuverability for the aircraft, as well as reducing noise and emissions.
- Integrated Electric Propulsion System: This system integrates the electric motor and internal combustion engine into a single power source, allowing for greater efficiency and flexibility in power management. The electric motor can be used to supplement the internal combustion engine during high-power demand, such as takeoff or climb, and the internal combustion engine can be used for cruising or recharging the battery.
There is not a unique solution for aircraft HEP. Some studies claim that actual technology HEPS in parallel architecture is best suited for aircraft with fluctuating power requirements, this means short duration of high-power requirements (Finger et al. 2020).
Hybrid-electric propulsion architectures could have broad applicability across different aircraft types, with potential fuel savings ranging from 5 percent in large commercial aircraft, to 30 percent for regional commuter aircraft. These advances in efficiency will also support the growing adoption of sustainable aviation fuels.
Despite its advantages, several goals must be achieved to make the technology viable. High-power and high-voltage applications at altitude come with varying challenges, including materials, design, and testing restrictions. Coupled with operation in a harsh environment and the need to minimize failure rates during flight, novel architectures, including fault-tolerant designs, and novel processes, such as arc detection and HV insulation testing, need to be investigated and developed.
Thermal management optimization of hybrid-electric propulsion systems will need to be understood in detail to reduce the overall system weight and maximize fuel savings. System integration understanding will also be critical to reducing cost and improving product manufacturability. Taking these technologies to certifiable standards will require further understanding of end-application requirements, which continue to evolve in the emerging hybrid-electric aviation market space.
NASA’s Subsonic Fixed Wing project identified four “corners” to be overcome: noise, emissions, aircraft fuel burn, and field length. The hybrid-electric propulsion offers operational flexibility, due to the greater number of components. Fuel and battery sources allow more possibilities for managing of the propulsion system in the various stages of a mission, and reduce the energy consumption, compared with traditional. However, it implies an increase in the necessary load during the design phase, and greater complexity in the operation. Proper management of the electrical components and combustion is mandatory to meet the environmental requirements and reduce the fuel consumption
HEP offers operational flexibility due to the variety of components and possibilities, but this may imply an increasing in weight. Consequently, it is necessary increasing in thrust during takeoff, which must be considered in the aircraft design phase. HEP increases the complexity in operation and design, but with a management system operating the engines close to their maximum efficiency operation conditions, and with proper energy storage control, environmental requirements and reductions in energy consumption may be achieved
Hybrid Electric Aircraft Technologies
In addition to generating more power, we also need to be able to control, protect and manage the power and thermal environment. Managing significant amounts of electrical power at high altitudes is not easy to do and real expertise is needed. Highly efficient power distribution and conversion is required to maximize the use of available power and minimize the thermal management system. High voltage systems will be required for commercial aircraft. However, isolating high voltage at high altitudes is challenging. Additional spacing and insulation systems are required, and that impacts weight. Also the safe use and management of electricity on an aircraft—the system design—is critically important and another crucial challenge.
The largest share of aircraft is operating within the short and mid-range distance, covering distances about 1000–3000 nautical miles (nm) and up to 240 passengers (PAX). However, to create an advantage by applying hybrid-electric propulsion systems to such aircraft requires low masses and high efficiencies of the electric components that are very challenging compared to the state of the art . The use of superconducting and cryogenic-cooled components could potentially overcome these limitations. Firstly, due to the enormous current carrying capabilities of superconductors, using them in electric machines or cables can reduce the masses and required voltage levels of such components.
Secondly, when cooling these components with liquid hydrogen, the evaporated hydrogen can be used further as a fuel for power generation instead of requiring energy densities for batteries that are orders of magnitude higher than the state of the art. Power- and thrust generation of aircraft by combustion of (liquid-) hydrogen instead of kerosene have been part of numerous investigations during the past. Investigations showed that using hydrogen reduces the CO2 -emissions while increasing the efficiency of the engine. However, due to its low volumetric density, the volume to store the required amount of liquid hydrogen in the aircraft removes this advantage on an aircraft-level. It should be noted, that the Soviet aircraft TU-155 successfully passed experimental flights between Moscow and Kiev with hydrogen powered engines in 1988.
Collins Aerospace is working with sister business Pratt & Whitney Canada to advance sustainable hybrid-electric propulsion technology for the aviation industry. In collaboration with De Havilland Aircraft of Canada, Collins and Pratt & Whitney are integrating a new hybrid-electric propulsion system into a De Havilland Dash 8-100 flight demonstrator. The demonstrator will be re-engined on one side with a 2 megawatt-class propulsion system that combines a fuel-burning engine from Pratt & Whitney with a battery-powered electric motor from Collins in a parallel hybrid configuration.
Both the engine and the motor will each generate about 1 megawatt of power for a 50/50 power split. The engine will be optimized for cruise efficiency, while the electric motor will provide extra power during take-off and climb, demonstrating around 30 percent better fuel efficiency, compared to existing turboprop engines.
Although this technology is still in development, progress is quickly being made. For the Dash 8 demonstrator, Pratt & Whitney and Collins are targeting ground testing in 2022, followed by flight testing in 2024.
Hybrid Propulsion advancements and demonstrations
Urban Air Mobility (UAM) is a safe and efficient system for air passenger and cargo transportation within an urban area, aiming to decongest the road traffic, improve mobility, reduce transport time and decrease pollution. Research projects had received high investments, most of them for all-electric VTOLs (eVTOL). However, to become a reality this technology must face challenges such as approving regulations and standards, obtaining redundancy certifications, dealing with the weather conditions, and installing the urban infrastructure. Another issue is that VTOLs have high power demand at the beginning and end of a flight. This technology still holds high certification risk and failure modes, since it doesn’t have actuator redundancy or glide capability.
Many start-ups and manufacturers are developing UAVE designs. Volocopter is a two seats, VTOL multi-copter, able to fly up to 27 km at 70 km/h (Lenton 2018). Lilium Jet works on a eVTOL for two persons and an autonomy up to 300 km. Boeing NeXt program developed the passenger air vehicle prototype, an eVTOL with a range up to 80 km (Hilfer 2019). An eight-propeller aircraft concept with eVTOL is part of the EmbraerX approach. HEP is considered an effective substitute for conventional short and medium-range aircraft, and companies as Airbus, Siemens, Rolls-Royce and Boeing are strongly investing in this technology
Researchers from the University of Cambridge, in association with Boeing, have successfully tested the first aircraft to be powered by a parallel hybrid-electric system. An electric motor and petrol engine work together to drive the propeller. The demonstrator aircraft uses up to 30% less fuel than a comparable plane with a petrol-only engine. The aircraft is also able to recharge its batteries in flight, the first time this has been achieved.
During take-off and climb, when maximum power is required, the engine and motor work together to power the plane, but once cruising height is reached, the electric motor can be switched into generator mode to recharge the batteries or used in motor assist mode to minimise fuel consumption.
“Until recently, batteries have been too heavy and didn’t have enough energy capacity. But with the advent of improved lithium-polymer batteries, similar to what you’d find in a laptop computer, hybrid aircraft – albeit at a small scale – are now starting to become viable,” said Dr Paul Robertson of Cambridge’s Department of Engineering, who led the project.
In March 2017, the Extra 330LE aerobatic plane, powered by an electric propulsion system from Siemens, set two new speed records. At the Dinslaken Schwarze Heide airfield in Germany, the electric aircraft reached a top speed of 337.50kph over a distance of 3km, 13.48kph faster than the previous record set in 2013. Hybrid propulsion systems should contribute to the emergence of new VTOL (vertical takeoff and landing) and STOL (short takeoff and landing) aircraft, by enhancing their flight capabilities and expanding their range of missions.
Zunum Aero has revealed its plans to fly a hybrid electric aircraft 2020. The aircraft would have a range of around 700 miles at launch which indicates that the aircraft will be suitable for regional air transport, and the plan is to extend the flying range to 1,000 miles by 2030 which will make these aircraft commercially viable across the world.
With a view to providing clean powertrains for future aircraft, Roll-Royce is developing a 2.5-megawatt hybrid-electric propulsion system, billed as the most powerful of its kind. In Dec 2021, Engineers are already celebrating a notable milestone, with the system delivering more than a megawatt of power for the first time.
Rolls-Royce’s demonstrator Power Generation System 1 (PGS1) is designed as a versatile solution for next-generation aircraft, designed primarily with hybrid planes in mind but with a generator than can be adapted for those relying more heavily on electric systems. The PGS1 includes a thermal management system, purpose-made controls, a keg-sized generator and an AE2100 turbo-shaft engine to turn it.
General Atomics (GA) announced a “completely disruptive” “hybrid electric propulsion” for its concept MQ-Next drone in October 2022. This was preced2d by Russia’s United Engine Corporation (UEC) declaring in July last year research and development (R&D) efforts in “combined electric technology.”
GA’s MQ-Next displayed as a concept art at the company’s boot,h has new propulsion that could be a “completely disruptive technology,” according to its senior director of advanced programs, Mike Atwood, in a report on Breaking Defense. “It uses a hybrid electric system where a Tesla Model S and an RQ-170 got together, and you have a fully electric aircraft” capable of traveling greater distances,” Atwood said.
Atwood said they were working on “hybrid electric propulsion…to pioneer a novel way to propel airborne air-breathing vehicles.” GA’s president of aeronautical systems, Dave Alexander, touched upon the technical configuration of the HE system, which would consist of a “heavy fuel engine driving very efficient generators and motors (to get) fairly low (fan) speeds…and efficiency.” He added that getting the thrust out of the low-pressure ratio fan would be tricky.
He referred to the western Pacific theatre as the driving concern, where the technology, if successful, would “help cover the South China Sea.” Moreover, it is hoped to have other capabilities, like allowing an aircraft to take off from a small 3,000-foot runway, which also complements the US Air Force’s Agile Combat Deployment (AGE) concept.
Smaller air bases spread throughout the first and second island chains in the SCS would make it more difficult for the Chinese to locate them and present a targeting dilemma. The EurAsian Times had reported about US Marine Corps Assistant Commandant Eric Smith admitting “logistics” being the “pacing, wicked challenge in the South China Sea.” Smith had made the remarks during an online interaction with the Stimson Center.
Meanwhile, Russia’s under-development HE systems have been officially classified as a “sixth generation” capability for “combat aircraft.” “Now work is starting in several promising areas: the technologies of the sixth-generation engine, a combined powerplant, and the more electric engine technology.
The R&D effort on the sixth-generation demonstrator engine technology has been included in the long-term work schedule of the United Engine Corporation and our applications for state program financing,” Mikhail Reznikov, deputy CEO of the UEC, was quoted by TASS last year.
Russia also has a parallel HE development project meant for civilian aircraft. The Baranov Central Institute of Aviation Motors announced in February this year, “creating a line of hybrid power and electric power plants for aircraft with a capacity of one to one hundred seats, including convertiplanes and air taxis.”
An HE powerplant was tested in February 2022 on the Yak-40LL flying testbed. The project then envisages replacing the electrical component with a hydrogen fuel-cell battery, increasing flight duration by 3-4 times.
In the US, the civilian segment is targeted by Collins Aerospace and Pratt and Whitney, who announced a collaborative hybrid-electric technology demonstrator program. “Together, they are targeting propulsion systems for future advanced air mobility vehicles and – potentially – small and medium-sized regional aircraft,” said Aviation Week.
US Air Force Looks to Hybrid Electric Solution for eVTOL and UAS Energy Concerns
The Air Force is investing in LiquidPiston’s X-Engine technology to create a hybrid-electric propulsion system to power emerging technologies like unmanned aircraft systems (UAS) and orbs, the company announced in March 2021. The Small Business Technology Transfer (STTR) contract worth $150,000 was awarded through AFWERX to support Agility Prime, a program developing electric vertical take-off and landing (eVTOL) aircraft for commercial and military use.
UAS and eVTOLs are being developed with battery-powered propulsion systems which have limited their range of flight. The X-Engine technology would use fuel to power a generator and charge the aircraft’s batteries extending its flight time and range, according to the company. “Today’s solutions for power and energy are held back by a lack of technological innovation; gasoline engines are inefficient, diesel engines are big and heavy, and while the world wants to go electric, batteries lack significantly compared to the energy density of fuel,” Alec Shkolnik, CEO and co-founder of LiquidPiston, said in a statement. “The X-Engine solves these challenges, and with this contract, we look forward to showcasing the value a hybrid-electric configuration can bring to unmanned flight.”
The X-Engine runs on JP-8, diesel, and other heavy fuels but is 30 percent more fuel-efficient than a diesel engine, according to LiquidPiston. It is also five to 10 times smaller and lighter than a diesel engine and is two to four times more fuel-efficient than a small turbine. The Army also awarded LiquidPiston a Small Business Innovation Research (SBIR) contract in December 2020 to develop the X-Engine platform for small tactical generators. “Our work with the Air Force demonstrates the versatility and utility of our X-Engine across the Department of Defense including our ongoing work with the US Army,” Shkolnik said.
Collins Aerospace develops electric power systems lab
In April 2019, Collins Aerospace confirmed its commitment to hybrid-electric and electric propulsion technology by introducing The Grid: a 25,000-square-foot, next-generation, electric systems integration facility in Rockford, Illinois. The Grid will be the aviation industry’s most advanced electric power systems lab, and will be the test platform for the development of new products and systems for electric aircraft.
At The Grid, Collins will test high-powered generators, distribution systems, and motors, as well as install and test connected systems, such as actuation, air management and turbo machinery. The Grid will bring aircraft architecture integration testing to another level. The $50 million investment in The Grid is part of a larger $150 million total investment that Collins Aerospace expects to make in electric systems over the next three years and builds on the $3 billion the company has invested over the past decade.
The Grid will also serve as the research and development home for key pieces of United Technologies’ Project 804, a regional-size, hybrid-electric demonstrator aircraft that we are supporting along with Pratt & Whitney. The demonstrator will consist of an engine optimized for cruise efficiency and augmented by a battery-powered electric motor. Project 804’s hybrid-electric propulsion system is expected to yield an average fuel savings of 30 percent. In addition to its key focus on hybrid-electric propulsion, Project 804 is expected to advance other technologies as well, such as more power-dense electronics, lightly-hybridized larger engines, and hybrid supplemental power units.
We believe that to power a regional, hybrid-electric aircraft with usable range and less than 50 passengers, the energy and power density of current batteries will need to double. We believe there is a path to achieve this in the next few years, and that a hybrid-electric passenger aircraft with 50 passengers or fewer and a range of less than 500 miles will be certified within the next 10 to 15 years. Looking further ahead, to make a single-aisle, 100-seat hybrid-electric aircraft viable will require that densities double yet again. This capability is probably at least an additional 10 years beyond the regional case. This is a 4x density improvement over 15 years, according to Collins Aerospace.
Researchers at the University of Illinois study array of electrically powered ducted fans
Researchers at the University of Illinois gained new understanding in how the fans and especially their precise placement on the aircraft can affect the cross-conversation between propulsion and the airflow around the wing. In most commercial aircraft, the engines are isolated from the rest of the wing system. Instead of being embedded in the wing or mounted more closely to that surface, they hang out from underneath the wings. This is done, in part, to try to reduce the influence in cross coupling–the cross-communication between the engine’s RPM and the airflow characteristics about the airplane wing.
“If we allow those two systems to talk to each other, there is a lot of increased complexity in the flow field over the wing and into the propulsor–which also substantially alters the performance,” said Phillip Ansell, assistant professor in the Department of Aerospace Engineering in the College of Engineering at the University of Illinois. “We’ve taken two subsystems – propulsion and aerodynamics–and we’ve said that these are not isolated subsystems. These are now one thing.” “If we integrate the propulsors, which in this case are fans, into the wing, we can improve the aircraft’s propulsive efficiency by ingesting the low-speed air across wing surface into the propulsor. But it’s challenging to figure out how to do it in a smart way.”
This research project was conducted experimentally using a 3D printed model of an airfoil, which is a cross-section of a wing, mounted inside a subsonic wind tunnel. “We had a model with ducted fans mounted over the trailing edge of the airfoil. The flow goes across the upper surface and then into the fan,” Ansell said. He said that the manipulation of the throttle of the ducted fan mounted on top of the wing provided large changes in the aerodynamic behavior of the airfoil.
“We can adjust the throttle to make the fan spin faster or slower, so that I now have a high-speed jet that’s coming out the back end and acts to substantially lift the aircraft through a phenomenon known as supercirculation. It also changes the flow across the surface,” he said. “I have little regions of the flow on the surface called boundary layers. Whenever I ramp up the throttle and start pulling air into that propulsor, it thins out the boundary layer. It modifies the distribution of the pressure across the airfoil itself. There are some complex things happening. That fan RPM talking to the aerodynamics of the larger airfoil is substantial.”
Ansell said the study provides a new way to understand the dialogue between a full aircraft system and a propulsion system. It’s not just about increasing the throttle to create a larger thrust and produce a force that goes through the axis of the orientation of the fan. “It’s not that simple because it also changes the air flow over the wing,” Ansell said. “The different orientations of the end of the fan changes the performance of the wing section as well as the pressure distribution because it changes the local flow quality characteristics. We have now quantified that and can understand some aspects of what that looks like.
“We were able to take measurements to better understand what those variations in coupling characteristics are. Previously we knew that if we ramp up the throttle on this fan, the result is a thrust vector pointed in a certain direction. Now we know that it will also modify my local wing aerodynamics.”