Beatrice Lang
Electrified aircraft propulsion (EAP) is the next step in the evolution toward electric aircraft, with around 215 types of electric-powered aircraft currently being developed worldwide. However, electric propulsion for planes comes with several challenges. The biggest challenge is the low energy density of batteries, which is about 50 times lower than that of jet fuel. This makes electric propulsion impractical for long-range aircraft and may restrict its use to short-haul routes. Other challenges include wiring, cooling systems, power electronics, and advancements in electric motor technology. Researchers at NASA are working on innovative technologies, aircraft concepts, and test facilities to address these challenges and make electric propulsion a reality for the aviation industry.
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What You'll Learn
Electric propulsion may be restricted to short-haul routes
Electric propulsion for aircraft is an area of active research and development, with around 215 types of electric-powered aircraft being developed worldwide. Electrified Aircraft Propulsion (EAP) offers the possibility of reducing fuel and energy usage, emissions, and operating costs in aviation. However, there are several challenges that restrict the widespread adoption of electric propulsion, especially for medium and long-haul routes.
One of the primary challenges is the low energy density of batteries. Jet fuel has a much higher energy density than commercially available lithium-ion batteries, which results in a significant range advantage for traditional aircraft. The weight of the batteries and the need for cooling systems further reduce the efficiency of electric aircraft. While battery technology continues to improve, it has not yet achieved sufficient maturity to make commercial electric air transport viable for long-haul flights.
Another challenge is the high voltage requirements of electrified propulsion systems. Single-aisle airliners may require voltages of up to 3000 volts, and higher voltages can lead to hazardous issues such as partial discharge. New cabling designs and insulation systems must be developed to address these challenges and ensure safe operation at high altitudes.
Additionally, the electrification of aircraft propulsion systems requires advancements in various components, including motors, converters, circuit breakers, and cooling systems. These components need to be lightweight and efficient to minimize the weight and energy penalties associated with electric propulsion. While NASA and other organizations are actively working on these challenges, it may take time before viable solutions are found for medium and long-haul routes.
As a result of these challenges, electric propulsion may indeed be restricted to short-haul routes in the near future. However, this technology still has the potential to revolutionize regional air transport and make a significant impact on the aviation industry. Electric aircraft are expected to be more efficient, quieter, safer, and greener than traditional aircraft, even if they are initially limited to shorter routes.
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Low energy density of batteries
The low energy density of batteries is a significant challenge for electric propulsion in commercial aviation. Jet fuel has an energy density of about 12,000 Wh/kg, whereas commercially available lithium-ion batteries have a much lower energy density of around 250 Wh/kg at the cell level. This gap is substantial, and it makes electric propulsion impractical for long-range aircraft. Indeed, the specific energy of electricity storage is only a fraction of aviation fuel, at 2% in 2018.
The low energy density of batteries means that a 500 nmi (930 km) mission for a 12-passenger aircraft would require a six-fold increase in battery power density. This is a considerable challenge, and it may restrict electric propulsion to short-haul routes. However, pioneers in electric propulsion believe that small, short-range aircraft using available batteries are possible. These aircraft could have up to 19 seats and cover regional routes of less than 250 miles.
While battery technology is not yet mature enough for commercial electric air transport, advancements in this area are crucial for the future of electric aviation. Electric propulsion systems will also need to improve power densities and efficiencies beyond automotive levels to reduce weight and waste heat. Additionally, higher voltages in electric propulsion systems can lead to hazards such as partial discharge, requiring new cabling designs and insulation systems.
To address the challenges posed by low battery energy density, hybrid-electric aircraft will likely rely on batteries and other power sources, such as ultra-efficient generators or fuel cells, to improve range and safety. For example, Airbus is considering hybrid layouts that use hydrogen burned in a gas turbine or converted into electricity in fuel cells. During high-power phases like takeoff and climb, both the turbine and an electric motor would drive the propeller or fan, while the cruise flight would rely solely on the turbine.
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Power storage needs to be improved
Power storage is one of the biggest challenges to the use of electric propulsion in commercial aviation. The energy density of jet fuel is about 12,000 Wh/kg, while commercially available lithium-ion batteries have an energy density of about 250 Wh/kg at the cell level. This figure is typically 20% lower at the pack level, including the weight penalty for thermal-runaway containment and other safety features. The specific energy of electricity storage was only 2% of aviation fuel in 2018. This 1:50 ratio makes electric propulsion impractical for long-range aircraft, as a 500 nmi (930 km) mission for a 12-passenger aircraft would require a six-fold increase in battery power density.
While battery technology is still maturing, hybrid-electric aircraft will need both batteries and other power sources such as ultra-efficient generators or fuel cells to power the aircraft, recharge batteries, and improve aircraft safety, efficiency, and range. Advances in power densities and efficiencies of electric motors and power electronics beyond automotive levels are also needed. High power densities will reduce weight and volume, while higher efficiencies will reduce waste heat and the weight of the necessary cooling systems.
NASA has been working on new technologies, including motors, converters, circuit breakers, batteries, and cooling systems to keep components cool. Their High-Efficiency Megawatt Motor (HEMM) is being designed to meet the needs of electrified aircraft propulsion. HEMM has a target performance of 1.4 MW.
Airbus is exploring the use of superconducting technology in electrical machines and distribution cables. Superconductors are materials that have no electrical resistance when cooled to cryogenic temperatures, thereby increasing efficiency and reducing weight.
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The need for advances in power densities and efficiencies of electric motors
Electrified aircraft propulsion (EAP) is an innovative technology that offers new possibilities for improving efficiency and reducing energy consumption in aviation. However, one of the key challenges in the transition to electric propulsion is the need for advances in power densities and efficiencies of electric motors.
The energy density of current battery technology falls far short of that of jet fuel, with lithium-ion batteries having an energy density of around 250 Wh/kg compared to jet fuel's 12,000 Wh/kg. This discrepancy poses a significant challenge to the adoption of electric propulsion, as it necessitates a substantial increase in battery power density to enable long-range flights for electric aircraft.
To address this issue, researchers are working on improving power densities and efficiencies of electric motors. For instance, NASA's High-Efficiency Megawatt Motor (HEMM) aims to meet the needs of electrified aircraft propulsion by increasing power capability while minimizing weight and loss. Additionally, the use of lightweight materials and revolutionary superconducting technologies can help reduce weight and improve efficiency.
Advancements in power densities and efficiencies are crucial to ensuring that electric aircraft can achieve longer ranges and improved performance. By reducing weight and increasing power, electric propulsion systems can become more efficient and viable for a wider range of aircraft applications.
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High voltages at cruising altitudes could lead to hazardous partial discharge
The shift from traditional aircraft engines that run on fossil fuels to electric propulsion systems is a significant challenge for the aviation industry. While electric propulsion promises greater efficiency, quieter flights, improved safety, and reduced environmental impact, several technological hurdles must be overcome. One critical challenge is addressing the issue of high voltages at cruising altitudes, which can lead to a hazardous phenomenon known as partial discharge.
Aircraft have traditionally used voltages of around 28 volts for power distribution. However, with the advent of electric propulsion, these voltage requirements are set to increase significantly. The first all-electric aircraft are expected to utilize voltages of up to 500 volts. Moreover, designers exploring megawatt-class electrified propulsion systems for single-aisle airliners are considering voltages in the kilovolt range, upwards of 3,000 volts.
At cruising altitudes, where air pressure is reduced, operating at such high voltages can result in a challenge known as partial discharge. Partial discharge occurs when an electrical insulation system fails to contain the voltage within, leading to a discharge of electricity. This can have severe consequences for the aircraft's electrical systems and safety. Therefore, it is crucial to address this challenge before widespread adoption of electric propulsion systems in aviation.
To mitigate the risks associated with partial discharge, researchers and engineers are developing innovative cabling designs and insulation systems. These advancements aim to ensure that the electrical insulation can effectively contain the high voltages, preventing unintended discharges. Additionally, the exploration of superconducting technology in electrical machines and distribution cables is underway. Superconductors, when cooled to cryogenic temperatures, offer zero electrical resistance, enhancing efficiency and reducing weight.
While the challenge of high voltages and the risk of partial discharge are substantial, ongoing advancements in technology and insulation systems demonstrate promising progress toward safe and efficient electric propulsion for aircraft. These developments are crucial steps in the journey towards electrifying aviation and realizing its associated benefits.
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Frequently asked questions
Electric propulsion refers to a range of propulsion architectures that use electrically driven motors to provide thrust.
Electric propulsion can make aircraft more efficient, quieter, safer, and greener.
Electric propulsion requires advances in power densities and efficiencies of electric motors and power electronics beyond automotive levels. Additionally, the low energy density of batteries is a significant challenge.
Examples include the Siemens-modified Extra EA-300 acrobatic airplane and the NASA Electric Aircraft Testbed (NEAT).
Electric propulsion for planes is currently being developed and tested, with the goal of reducing fuel consumption, emissions, and operating costs. The technology is expected to be more widely adopted in the coming decades.
