Summary: The gravimetric and volumetric challenges of using batteries for aviation are significant, with batteries being far heavier than traditional jet fuel for the same energy output. A case study comparing jet fuel and lithium-ion batteries for a long-haul flight from Los Angeles to Sydney illustrates the substantial weight difference, making batteries impractical for such distances. However, short-haul flights and urban air mobility, such as eVTOLs, are more feasible for early adoption of battery technology. Advancements in battery types, like Lithium-air and Lithium-sulfur, offer promise for the future, but a multifaceted approach, including alternative sustainable fuels, is necessary for achieving aviation’s environmental goals.
Author: Matthew Fischbacher is an Airline Captain with a wealth of experience in aviation. He brings a unique perspective on Sustainable Aviation Fuel, combining practical expertise with insights into the future of the industry.
GRAVIMETRIC AND VOLUMETRIC CHALLENGES
The fundamental challenge for flight is overcoming gravity, necessitating an energy-dense fuel. Understanding battery technology in aviation requires a comprehension of both gravimetric (weight-based) and volumetric (volume-based) energy densities. To illustrate, let’s analyze the use of batteries in a long-haul flight scenario.
Aircraft traditionally rely on jet fuel with high gravimetric energy density. As jet fuel burns off during a flight, the aircraft becomes lighter, enhancing fuel efficiency. In contrast, batteries maintain their weight throughout a flight, highlighting the significance of their energy-to-weight ratio.
LONG-HAUL BATTERY FLIGHT ANALYSIS: LA TO SYDNEY CASE STUDY
Consider a hypothetical flight from Los Angeles to Sydney, mirroring the specifications of an Airbus A380, comparing traditional jet fuel propulsion with lithium-ion batteries.
Jet engines, with an efficiency between 35% and 40%, contrast with electric motors, potentially used in battery-driven aviation, which can achieve efficiencies over 90%.
KEY ASSUMPTIONS:
- Jet Fuel’s Gravimetric Energy Density: 43 MJ/kg.
- Lithium-ion Batteries’ Gravimetric Energy Density: 250 Wh/kg (0.9 MJ/kg).
JET FUEL-POWERED A380 METRICS:
- Fuel Consumption: 12 metric tons/hour for a 15-hour journey totals 180 metric tons.
- Total Energy from Jet Fuel: 180,000 kg × 43 MJ/kg = 7,740,000 MJ.
- Usable Energy (considering engine efficiency): 3,096,000 MJ.
BATTERY-POWERED EQUIVALENT:
- Required energy, adjusted for electric motor efficiency: 3,440,000 MJ.
- Necessary battery weight: 3,822,222 kg (3,822 metric tons)
The comparison reveals that the battery weight required is approximately 21 times that of jet fuel, even after adjusting for electric motor efficiency. This affirms the significant challenges in transitioning to battery power for aviation, particularly for long-haul flights.
“The comparison reveals that the battery weight required is approximately 21 times that of jet fuel, even after adjusting for electric motor efficiency.”
While this analysis provides a basic comparison, it simplifies the dynamic nature of flight. A more accurate model would consider the constant weight of batteries versus the decreasing weight of burning jet fuel, which would increase energy demands. Additionally, an iterative model that accounts for the extra energy needed to carry the increased battery weight would be more precise, further highlighting the technological challenges of long-haul, battery-powered aviation.
ENVISIONING A BATTERY-DRIVEN LONG-HAUL FLIGHT
Imagine stepping onto a conceptual aircraft modeled after the A380, and instead of the familiar hum of jet engines, there’s a remarkable silence. As you enter, the scene is strikingly different: the usual arrangement of seats and overhead compartments is replaced by an array of batteries, towering from floor to ceiling.
Sitting among these batteries, you realize that the entire 1,459,000-liter capacity of the aircraft, traditionally allocated for passengers, cargo, and fuel, is now occupied by this immense power source. However, for our hypothetical flight from Los Angeles to Sydney, even this colossal space falls short. The estimated requirement for battery volume stands at 1,472,308 liters, slightly exceeding the A380’s capacity. Gazing out the window, you notice additional battery storage pods attached beneath the wings.
This visualization underscores the significant challenges of powering long-haul flights with batteries. Even with a theoretical battery capacity of 650 Wh/L and utilizing every inch of one of the largest commercial airliners, the demands of such a journey present considerable obstacles. While the prospect of battery-driven aviation is tantalizing, it also highlights the enormity of the task at hand. Despite these challenges, there remains a sense of techno-optimism, fostering hope for a future where battery-powered aircraft become a reality in our skies.
SHORT-HAUL BATTERY FLIGHT ANALYSIS: VANCOUVER TO VICTORIA HARBOUR CASE STUDY
In the realm of short-haul flights, Canadian airline Harbour Air is pioneering the electrification of its fleet, particularly focusing on the de Havilland DHC-2 Beaver. Traditionally powered by a Pratt & Whitney radial engine, this case study examines a hypothetical 60-mile flight from Downtown Vancouver to Victoria Harbour, which would typically consume about 50 liters of avgas*.
KEY ASSUMPTIONS:
- Avgas Density: Gravimetric energy density of 43 MJ/kg and volumetric energy density of 33 MJ/liter.
- Contemporary lithium-ion batteries: Gravimetric energy density of 250 Wh/kg (0.9 MJ/kg).
AVGAS FUEL-POWERED DHC-2 BEAVER METRICS:
- Fuel Consumption: Assuming 50 liters per hour, the flight would require 50 liters of avgas.
- Energy from Avgas: 50 liters x 33 MJ/liter = 1,650 MJ.
- Usable Energy (considering piston engine efficiency): 5 MJ.
BATTERY-POWERED EQUIVALENT:
- Energy Needed from Battery: 412.5 MJ / 0.9 = 458.33 MJ.
- Required Battery Weight: 509.25 kg.
Comparing the fuel types, avgas for this journey weighs about 35.5 kg (density 0.71 kg/liter), while batteries would be over 14 times heavier. This stark weight difference highlights the challenges for battery-powered aviation, particularly in terms of the Beaver’s 900 kg useful load capacity.
PROSPECTS FOR SHORT-HAUL ELECTRIC FLIGHTS
Despite these challenges, the prospects for short-haul electric flights are promising. The benefits include reduced operating costs, quieter operations, and zero emissions. Additionally, the simplicity of electric powertrains could mean lower maintenance requirements.
The feasibility contrast between short and long-haul electric flights underscores the need for route-specific solutions. As battery technology advances, short-haul routes may lead the transition to electrification, while long-haul segments may rely on alternative fuels or hybrid systems for the foreseeable future.
MEDIUM-HAUL BATTERY FLIGHT CONSIDERATIONS
Exploring the transition to battery power for medium-haul flights requires reevaluating energy needs. This analysis focuses on the feasibility of replacing traditional jet fuel with battery technology.
KEY ASSUMPTIONS:
- Jet Fuel Energy Density: Jet A-1 fuel has an energy density of approximately 35.5 MJ/kg, equating to about 9,861 Wh/kg.
- Battery Energy Density: Current advanced lithium-ion batteries have an energy density ranging from 250 to 300 Wh/kg.
- Efficiency Metrics: Conventional jet engines operate with efficiencies between 30-40%, whereas electric motors can exceed 90% efficiency. This disparity suggests that electric aircraft could significantly reduce the energy footprint compared to jet-fueled counterparts.
- Operational Nuances: Electric motors maintain consistent efficiency across different power outputs, unlike internal combustion engines. Additionally, the energy reserves required for battery-driven aircraft could be less than for jet-fueled ones, offering weight savings.
Given that the high efficiency of electric aircraft could halve energy requirements per kilometer, to match the range offered by jet fuel, batteries would need an energy density close to 4,500 Wh/kg. This figure serves as an initial estimate, with actual requirements varying based on aircraft design, aerodynamics, weight, and other factors.
To transition medium-haul, regional jets away from jet fuel, battery technology needs to advance significantly, reaching energy densities near or above 4,500 Wh/kg. Future advancements in aerodynamics and other aviation technologies could potentially reduce this stringent energy density requirement. Though the path is laden with challenges, the potential for a future dominated by battery-powered medium-haul flights remains a promising and transformative goal for the industry.
BATTERY EVOLUTION AND FUTURE
Battery technology has evolved dramatically, transitioning from basic energy storage to today’s advanced solutions. This evolution is far from complete, with significant strides still on the horizon.
Historical Overview: Initially, lithium-ion batteries offered modest energy densities of 100-150 Wh/kg. However, a turning point came in 2008 with Tesla’s Roadster and Panasonic’s 18650 lithium cobalt oxide cells, reaching 150-200 Wh/kg. This milestone demonstrated the potential of combining advanced cell technology with innovative battery management and design.
Modern-day Landscape: Tesla’s 4680-type cell, a successor to the 2170-type used in the Model 3 and Model Y, now boasts densities of 272-296 Wh/kg. In 2022, Amprius Technologies introduced cells with an impressive 450 Wh/kg, followed by CATL’s “condensed battery” in 2023, setting a new benchmark at 500 Wh/kg.
Peering into the Future: While current advancements are approaching 400 Wh/kg, future projections suggest potential densities nearing 750 Wh/kg. However, for ambitious applications like long-haul aviation, even higher targets are pursued:
- Lithium-air (Li-air) Batteries: Theoretically reaching 3,500 Wh/kg, practical expectations are around 1,230 Wh/kg. Despite their promise, challenges in efficiency, longevity, and safety need to be addressed, with commercialization expected in the 2030s.
- Lithium-sulfur (Li-S) Batteries: Offering potentials up to 2,500 Wh/kg, they face hurdles in cycle stability before hitting the commercial market.
The advances in battery technology have also sparked the growth of urban air mobility, particularly eVTOLs (electric Vertical Take-Off and Landing aircraft), ideal for short urban trips. This sector’s development could further spur battery research, potentially hastening the arrival of batteries suitable for longer aviation applications.
ASSESSING ELECTRIC AVIATION PROSPECTS
In aviation, the integration of battery technology presents a blend of limitations and emerging opportunities. Currently, the energy density of batteries lags behind traditional aviation fuels, posing a barrier for long-haul flights. However, advancements in battery technologies, particularly Lithium-air (Li-air) and Lithium-sulfur (Li-S) batteries, signal promising future improvements.
Short-haul flights appear more viable for early adoption of electric flight technologies, potentially serving as pioneers for broader advancements. Urban air mobility, especially in the form of electric Vertical Takeoff and Landing (eVTOL) platforms, is poised to benefit significantly from current battery developments.
The active participation of industry leaders and regulatory bodies is pivotal in this transition towards electric aviation. Their continued engagement and openness to innovation are gradually shifting battery-powered flights from a distant vision to a more immediate possibility.
Nonetheless, relying solely on battery technology to address all environmental challenges in aviation may be overly optimistic. The current limitations of batteries necessitate a multifaceted approach, including exploring alternative sustainable fuels and technological innovations, to achieve the industry’s environmental objectives.
POLICY AND INDUSTRY OUTLOOK
Key indicators of an impending aviation battery revolution include:
- Breakthroughs in battery technology, achieving capacities beyond 4,000 Wh/kg.
- The ability to produce and supply these advanced batteries cost-effectively.
In conclusion, aviation’s relationship with battery technology is marked by both hurdles and potential. From the initial limitations of current batteries to future technologies that could rival traditional fuels, progress is unfolding. While short-haul flights are likely to benefit soon, long-haul flights face more complex challenges. Innovations in battery types and urban air mobility, such as eVTOLs, offer optimism. With industry and policymakers focused on emerging developments, the concept of battery-powered flights is drawing closer to reality. However, considering current battery constraints, a diverse strategy is essential for reducing aviation emissions in the short term.
*Daytime visual flights must carry enough fuel to fly to the first point of intended landing and then for an additional reserve for 30 minutes at normal cruising speed. This example thus includes 30 minutes of fuel for the flight and an additional 30 minutes for reserve. This consideration was disregarded in our previous long-haul example