What is a hybrid electric aircraft (HEA)?
A hybrid electric aircraft (HEA) uses a combination of traditional fossil fuel-powered engines and electric motors to provide propulsion. HEAs typically use electric motors for takeoff and landing, while conventional engines give power while in the air. Batteries or other energy storage systems, such as fuel cells, power electric motors.
HEA technology is still in the early stages of development. Still, there is growing interest in its potential to reduce the environmental impact of air travel, especially as businesses and consumers see the successes of electric vehicles.
How long does it currently take to charge a HEA?
Hybrid electric aircraft (HEA) technology is still in the early stages of development, but some figures are available from companies doing research and testing. That said, all HEAs are different based on size and structure.
In September 2022, Air Canada placed a purchase order for 30 hybrid-electric aircraft from Swedish manufacturer Heart Aerospace. Those planes are expected to take 30 to 50 minutes to charge. United and Mesa are also among the airlines that placed orders for the Heart ES-30.
In an earlier 2015 test, engineers from the University of Cambridge tested a plane with a “parallel hybrid-electric propulsion system,” suggesting that their design could also recharge its batteries during flight.
What is the range of a hybrid electric aircraft?
Like charge time, a HEA’s range depends on various factors, including the size and weight of the aircraft, the type and capacity of the energy storage system, and the specific operating conditions.
The Heart ES-30’s 30-50 minute charge time is estimated to give a 250-mile range with an onboard turbine generator running on sustainable aviation fuel. With a full charge, Heart says the HEA can move 30 passengers and their bags 125 miles without the turbine generator.
Lilium, City Airbus, Boeing Aurora eVTOL, and Bye Aerospace Sun Flyer 2 are examples of other projects with varying ranges. The aircraft under the business and regional aircraft category claim a longer flight range of close to 1000 km (621 miles) with a seating capacity of around ten.
How do hybrid electric aircraft work?
HEAs use both traditional fossil fuel-powered engines and electric motors to provide propulsion, relying on batteries or other energy storage systems.
What is the primary fuel source?
The primary fuel source for HEA varies based on design, but most use aviation fuel, including sustainable aviation fuel.
Electric motors, however, are commonly powered by batteries. Other energy storage systems, such as fuel cells, can be used, but batteries can be charged from various sources, so many options exist. These include renewable energy sources, such as solar or wind power, and conventional electricity from the grid.
Using electric motors and alternative fuel sources may reduce emissions compared to conventional fossil fuel-powered aircraft.
What are the main components?
- Traditional engines are used to provide primary propulsion during cruising.
- Electric motors are used for takeoff, landing, and as an auxiliary power source during cruising.
- An energy storage system stores and delivers electrical energy to the electric motors.
- The energy storage system can be batteries, fuel cells, or a combination of both.
- Electronics are used to manage and control the flow of electrical energy between the energy storage system, electric motors, and traditional engines.
- Control systems manage the operation of the hybrid electric aircraft and ensure that the traditional engines and electric motors work together efficiently and safely.
How does a hybrid electric aircraft create propulsion?
The specific propulsion system can vary depending on the design of the aircraft, based on using electric motors for takeoff and landing, while the traditional engines provide power during cruising.
During takeoff and climb, the electric motors provide additional power to the aircraft, reducing the need for the traditional engines to operate at full power. Once the aircraft reaches cruising altitude, the conventional engines take over, and the electric motors are either turned off or used to assist the engines if needed.
During descent and landing, the electric motors are used again to provide additional power and reduce noise pollution.
How does hybridizing reduce fuel burn?
Hybridizing an aircraft has the potential to improve the overall efficiency of the propulsion system. Using electric motors to provide additional power during takeoff and climb reduces the overall fuel consumption during these phases of flight, which are typically the most fuel-intensive.
Electric motors can help reduce the aircraft’s speed during descent and landing, reducing the required braking and fuel consumption. Compared to conventional aircraft, using HEAs reduces fuel burn by simply reducing the time a conventional engine is in action.
How is the tech behind hybrid electric aircraft constantly evolving?
Batteries are one of the key technologies to enable HEA to reach their potential. The development of lighter and more powerful batteries will help drive the adoption of HEAs. Advances in battery technology can increase HEAs’ range and allow longer flights without refueling.
Other aspects of HEAs are evolving too. Improved electric motor efficiency increases overall propulsion system efficiency, and alternative fuel sources like biofuels or hydrogen fuel cells further reduce the environmental impact of HEAs and increase their sustainability. Efficient power management systems optimize fuel efficiency and performance by balancing energy sources.
Materials and design matter too. Lightweight, high-strength materials like carbon fiber composites can reduce the aircraft’s weight for greater efficiency, as can new aircraft designs, such as distributed electric propulsion and tilt-wing or tilt-rotor configurations.
How does energy density factor in?
Energy density determines how much energy can be stored in the HEA’s battery system and how far the aircraft can travel on a single charge when using a battery as an energy source. Most HEAs rely on batteries to power their electric motors, so the energy density of these batteries directly affects the range and performance of the aircraft.
The energy density of a battery refers to the amount of energy that can be stored per unit of volume or mass. Batteries with higher volumetric energy-density can store more energy in a smaller or lighter package, which increases the range of the aircraft and reduces its weight, allowing for a more efficient takeoff.
As a result, innovations in higher energy-density batteries can significantly increase the viability of HEAs.
Is energy density still a limiting factor?
Though there have been improvements in lithium-ion batteries, energy density can be considered a limiting factor for the range and performance of hybrid electric aircraft. Right now, the energy density of even the most advanced batteries is comparatively lower than traditional aviation fuels like jet fuel.
Currently, a battery would need to be much larger and heavier than a comparable amount of fuel to provide the same amount of energy. This is a challenge for aviation, which requires lightweight battery technology to enable electrification of longer flights and larger aircraft.
HEAs can overcome this limitation using a combination of batteries and traditional combustion engines to provide power. Still, ongoing research and development in battery technology will help further improve the range and performance of HEAs.
What is the heaviest passenger load tested so far?
The previously mentioned Heart ES-30 is a 30-passenger electric aircraft, replacing the company’s earlier 19-seat design, the ES-19. However, it has yet to be thoroughly tested.
Heart’s engineering team conducted testing with a ground-based prototype of the ES-19’s complete electric propulsion system in 2021. As for the ES-30, a full-scale integrated test facility with a full-flight simulator of the ES-30 has been built. A proof-of-concept aircraft is expected to be rolled out in 2024, with flight testing planned to start in 2026.
In 2020, a modified Cessna Caravan 208B with just a seat installed for the pilot flew for 30 minutes solely powered by electricity. VoltAero’s Cassio, with options for four-, six- and nine-seat configurations, also completed a test flight. For now, companies are starting small and light.
What other challenges are there with developing/flying HEAs?
Designing and optimizing the hybrid propulsion system for the highest possible efficiency and performance is challenging. It requires integrating different power sources and managing their operation to minimize energy waste and maximize power output. However, there are even more challenges outside of batteries and propulsion.
Developing and certifying new aircraft designs is a complex and expensive process that involves meeting stringent safety and performance standards set by aviation authorities. HEAs are based on emerging technologies, so there is not often a clear regulatory framework. New infrastructure and supply chains are also needed, like charging stations.
This adds up to economic challenges with higher upfront costs compared to conventional aircraft. Like other novel technology, HEAs will require time and investment to take off.
How are they being addressed by engineers and researchers?
Researchers are developing more efficient electric motors, optimizing the integration of electric and traditional propulsion systems, and creating aerodynamic designs that can reduce drag and improve efficiency. Materials research and structural engineering are ongoing in the aviation and aerospace field, creating materials that can withstand high temperatures and pressures.
On the certification, regulation, and infrastructure side, regulatory bodies must research and develop new infrastructure, standards, and certification processes for HEAs.
What companies are working on them now?
Along with Heart’s ES-30, previously mentioned, other HEAs are in the works:
- Airbus created the E-Fan X, a hybrid electric aircraft demonstration developed in partnership with Rolls-Royce and Siemens. The project was canceled during the pandemic.
- Boeing is working on the development of a hybrid electric propulsion system for aircraft, modifying a test aircraft and supporting the flight testing of a megawatt-class hybrid-electric aircraft propulsion system being developed by aero-engine maker GE Aviation.
- Sikorsky Aircraft is developing a hybrid-electric vertical take-off and landing (VTOL) aircraft demonstrator vehicle. The Sikorsky Autonomy Research Aircraft (SARA) is a reconfigured S-76B.
- Earlier this year, ZeroAvia flew its Dornier 228 testbed aircraft with one turboprop replaced by a prototype hydrogen-electric powertrain in the cabin, consisting of two fuel cells and a lithium-ion battery.
- Several companies are working on developing HEAs or related technologies, creating new propulsion systems, retrofitting existing aircraft, and developing all-electric aircraft.
What do experts say about the future of hybrid electric aircraft?
Many experts believe that hybrid and electric aircraft will eventually be a reality. And they think that we will first see HEAs in use in smaller settings.
Airline leaders are first planning to use early hybrid or electric-only planes for short, urban commuter routes as an important first step toward scalable technologies.
Aerospace engineer and assistant professor Gökçin Çınar spoke to The Conversation about the future of electric planes, noting there are fuel burn benefits from batteries in larger jets by using hybrid propulsion systems. She mentions a 2030-2035 target for smaller regional aircraft.
She adds that using batteries as a power assist during takeoff and climb is a promising option. She sees hybridization as a mid-term option for larger jets but a near-term solution for regional aircraft.
What impact could the widespread use of HEAs have on air travel?
One of the most significant benefits of HEAs is their potential to reduce emissions and improve the environmental impact of air travel. There are growing concerns about climate change and air pollution worldwide and an interest in green technology. For people and companies worried about the impact of air travel — a significant contributor to climate change — HEAs represent a major technological shift.
HEAs could also lower airlines’ operating costs by reducing fuel consumption and maintenance costs. This could make air travel more affordable and accessible to more people. And, with advanced electric motors and energy storage systems, HEAs could result in faster and more reliable air travel, with fewer delays and cancellations.
With short-range, affordable travel and the addition of VTOL technology, HEAs could open up new routes and destinations not currently accessible by conventional aircraft. This could help to connect more communities and promote economic development in underserved areas.
Finally, HEAs are quieter than conventional aircraft, especially on takeoff and landing with battery assist, which could reduce noise pollution in and around airports.
Timeframe for a large passenger hybrid aircraft
Developing a large passenger HEA will require significant technological advances and regulatory approval. Still, some companies have these larger aircraft in sight.
For example, Wright Electric is developing a 186-seat commercial jet with an 800-mile range with a proposed launch of 2030 and an electric 100-seat plane that could take off in 2026 named the Wright Spirit, an electric-powered version of the BAe 146 regional jet originally manufactured by BAE Systems.
Photo credit: Heart Aerospace