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Introduction:
The aviation industry is undergoing a transformative shift towards electric propulsion systems, driven by the imperative to reduce gas emissions, optimize aircraft performance, and decrease operating and maintenance costs. This progression has led to the emergence of More Electric Aircraft (MEA) and the eventual vision of All Electric Aircraft.
This shift towards electric aviation is driven by the need for more sustainable and environmentally friendly solutions. One of the key components enabling this transition is advanced battery technology. This article explores the critical role of batteries in this evolution, categorizing and dissecting the various facets of electric aircraft development based on size, range, configuration, and target market.
Weight Sensitivity and Energy Storage:
While we have been able to modify cars, trains, trams, and boats Electric we are not yet seeing large electric aircraft. Unlike land vehicles, aircraft are highly sensitive to increases in mass. The energy required for flight is directly proportional to the aircraft’s mass, posing a challenge for electric aviation.
For instance, increasing the mass of a car by 35% leads to an increase in energy use of 13-20%. But for a plane mass is crucial, energy use is directly proportional to mass: increasing its mass by 35% means it needs 35% more energy (all other things being equal).
The weight of batteries is a crucial factor, as heavier batteries result in increased energy consumption and operational challenges. Achieving the delicate balance between energy density and weight is a primary consideration in aircraft design.
Size, Range, and Configuration Dynamics:
Despite the majority of flights occurring on regional and narrow-body aircraft, it is the wide-body category that contributes most significantly to carbon emissions. This disparity necessitates different approaches to electrification based on the size and mission range of the aircraft.
The commercial aircraft landscape is divided into regional, narrow-body, and wide-body categories, each with distinct characteristics in terms of passenger load and mission range. Regional aircraft typically fly short missions, about 500 nautical miles (nmi) and carry low passenger loads (30–75), while wide-body aircraft carry high passenger loads (200–400) and fly much longer missions (>2000 nmi). Narrow-body aircraft fall in between, carrying medium passenger loads and flying ranges of ∼1000 nmi.
Researchers highlight that the specific energy required for an aircraft is strongly influenced by its typical flying range. Smaller, short-range aircraft demand less stringent battery performance metrics compared to their larger, long-range counterparts, underscoring the need for tailored solutions in electric aviation. While smaller aircraft are more conducive to electrification due to their lower energy consumption, larger aircraft often explore hybrid combustion-electric systems to address specific energy challenges.
Short-range electric commuter aircraft that carry up to 30 people for less than 800km, for instance, specifically require between 750 and 2,000Wh/kg, which is some 6-17% of kerosene-based jet fuel’s energy content. Even larger aircraft require increasingly lighter batteries. For example, a plane carrying 140 passengers for 1,500km consumes about 30kg of kerosene per passenger. With current battery technology, almost 1,000kg of batteries is needed per passenger. To make regional commuter aircraft fully electric requires a four- to tenfold reduction in battery weight. The long-term historical rate of improvement in battery energy has been around 3-4% per year, doubling roughly every two decades. Based on a continuation of this historical trend, the fourfold improvement needed for a fully electric commuter aircraft could potentially be reached around mid-century.
One of the most prolific markets for small aircraft is urban air mobility (UAM), which aims to carry two to five passengers or conduct last-mile parcel delivery over short distances (< 100 miles).
Current Battery Technology Limitations:
The most fundamental consideration in electric aircraft design is the energy density of the onboard batteries.For aircraft, where weight is a critical factor, achieving high energy density is paramount for extending flight range. Currently, jet fuel yields about 70 to 80 times more energy than an equivalent mass of the battery. In addition, Batteries have a higher maintenance cost than gas turbines, and on top of that require replacement after only 1,500 charge cycles.
The challenge lies in striking the right balance between energy density and overall weight, as heavier batteries can compromise the aircraft’s efficiency. The power-to-weight ratio required for large electric aircraft remains elusive, with jet fuel providing 70 to 80 times more energy than an equivalent mass of batteries. Furthermore, the maintenance cost and limited cycle life of batteries pose additional challenges.
Energy density refers to the amount of energy stored per unit volume or weight. Presently, the best lithium-ion batteries for aviation boast an energy density of approximately 250 watt-hours per kilogram (Wh/kg). While suitable for smaller aircraft, this level of power density falls short when it comes to powering larger passenger transports. However, Wright Electric’s CEO, Jeff Engler, envisions new battery technologies, such as high-temperature “molten batteries,” capable of reaching up to 1,000Wh/kg.
Exploring New Battery Technologies:
The pursuit of fully electric large aircraft requires a revolutionary leap in energy storage capabilities. Several other cathode materials such as lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), and lithium iron phosphate (LFP) have been commercialized along with numerous improvements to the graphitic anode leading to approximately a threefold increase in the specific energy of the Li-ion cells. Only next-generation chemistries, like Li–air or Li–CFx, may be able to meet some of the requirements needed for electric commercial aircraft to achieve the range and payloads required for adoption.
From the perspective of safety, the available chemical energy for exothermic reactions has increased by an equivalent amount. The quest for lighter, more efficient batteries is essential for the long-term viability of electric aviation. Engler expresses confidence in the potential of emerging battery chemistries that have been overlooked by traditional battery industries. He cites examples of cutting-edge developers achieving the 500Wh/kg threshold, a significant leap towards the target of 1,000Wh/kg. Engler emphasizes the need for batteries with such high energy density to facilitate widespread decarbonization in the aviation industry.
High-Temperature “Molten Batteries”:
Wright Electric’s focus on high-temperature battery technologies revolves around operating at elevated temperatures and adopting unique form factors. Engler explains that these batteries, akin to “molten batteries,” can have a higher fraction of active materials, ultimately reducing overall weight. Despite the engineering challenges associated with heating batteries to molten temperatures, the payoff lies in achieving both high power density and energy density.
Battery Safety in Aviation:
Safety is a top priority in aviation, and the same holds true for electric aircraft. The batteries must meet stringent safety standards to ensure the protection of passengers, crew, and the aircraft itself.
The most pertinent safety concerns related to batteries can be categorized into two broad areas: exothermic heat related events (thermal issues) and partial or complete loss of safety–critical power supply (functional issues). Degradation during operation of a battery can contribute to capacity fade, increased internal resistance, power fade, and internal short circuits, which lead to the loss of or decrease in propulsive power. When batteries are the primary source of onboard power and energy, it is crucial to be able to estimate their state-of-health in terms of capacity and power capability. Internal short circuits and other sources of excessive heat generation can lead to high temperatures within the cells of a battery pack leading to safety concerns and thermal events.
Lithium-ion batteries, widely used in aviation, have safety concerns related to thermal runaway and functional issues. Battery safety mitigation strategies encompass strict quality control standards, compliance with design metrics, and fail-safe mechanisms to prevent and control thermal events. This involves robust thermal management systems to prevent overheating, fire-resistant materials, and fail-safe mechanisms that can isolate faulty cells to prevent cascading failures.
In the context of electric aircraft, addressing thermal runaway – a three-stage process involving overheating, heat accumulation and gas release, and combustion – is crucial for ensuring battery safety. Early detection and mitigation during Stage 1 are vital, as functional safety cannot be guaranteed once this stage is reached, signifying a transition to abnormal behavior.
To enhance safety during Stage 1, airworthy batteries may necessitate higher quality control standards, minimizing manufacturing defects. Cell design decisions, including parameters like separator thickness and electrode porosity, play a pivotal role. Compliance with minimum cell design metrics, mandated by aviation regulatory bodies such as RTCA Inc.’s DO-311A, is essential for airworthiness, ensuring adherence to safety requirements in an airborne environment.
Considering the airborne environment’s low pressure and its impact on heating rates, stringent cell design and composition requirements for airworthy batteries are crucial for Stage 2. Fail-safe mechanisms, such as cell-venting, help control gas release during Stage 2, but the challenge lies in adapting to low-pressure conditions in aircraft, avoiding uncontrolled explosions.
If Stage 2 is not controlled, the battery enters Stage 3, where the organic liquid electrolyte becomes fuel for combustion. Prevention of fire propagation, thermal runaway, and system failure is paramount during Stage 3. Traditional strategies, developed for NASA’s manned missions, involve heat-absorbing plates between cells, but their use in electric aircraft may impact the battery pack’s specific energy, a critical design parameter.
Battery designs must incorporate robust safety features to prevent fires, explosions, or other hazardous events that could compromise the safety of the aircraft and its occupants. Battery management systems (BMS) play a critical role in ensuring battery safety by monitoring the battery’s health and performance, detecting potential faults, and taking corrective actions to prevent accidents. Advanced BMS technologies incorporate data analytics and machine learning to predict potential hazards and proactively prevent safety incidents.
Thermal Management
Battery operation generates heat, which can significantly impact its performance and longevity. In aircraft applications, thermal management is particularly challenging due to the limited space for cooling systems and the potential for heat buildup during high-power operation.
Effective thermal management is crucial to maintain optimal battery performance, prevent overheating, and extend battery lifespan. Engineers are developing innovative cooling techniques, such as liquid cooling systems and phase-change materials, to manage battery heat effectively in aircraft applications.
Battery Design and Aviation Safety:
The design of battery cells, whether cylindrical, prismatic, or pouch, significantly influences thermal behavior and safety. Considerations such as venting mechanisms, cell-venting for controlled gas release, and pack design play vital roles in mitigating safety risks.
The design of individual battery cells plays a crucial role in thermal behavior and safety. Li-ion batteries are commonly designed as cylindrical, prismatic, or pouch cells, each with unique thermal characteristics. Cell design influences the venting of gaseous products during reactions, and the mechanical strength of casing materials is linked to the likelihood of shorts caused by external stress. Electric X-planes, like NASA’s Maxwell X-57, opt for commercial-off-the-shelf cylindrical cells due to their superior safety features, including stipulated venting mechanisms.
Battery pack design is equally significant for thermal and functional safety in electric propulsion systems. It controls the risk of failure cascading within the battery pack, necessitating careful arrangement of cells and efficient thermal management systems. For instance, the X-57 battery pack underwent testing with trigger cells, demonstrating that a fire from one cell did not propagate to others. The overall pack design and battery management system directly impact the ability to monitor individual cell state-of-charge (SoC) and state-of-health (SoH), crucial for ensuring a reliable power supply.
Aviation regulatory bodies, guided by standards like DO-311A, focus on ensuring the airworthiness of batteries in the challenging airborne environment.
Weight Distribution:
The placement of batteries within the aircraft plays a crucial role in maintaining proper weight distribution. As batteries are typically heavy, their strategic placement is vital for maintaining stability and control during flight. Engineers must carefully design the aircraft’s structure to accommodate the weight of the batteries without compromising performance.
Lifecycle and Sustainability:
The lifecycle of batteries is a critical factor in assessing the overall sustainability of electric aircraft. The environmental impact of battery production, usage, and disposal must be carefully considered. Advancements in battery technology, such as recyclability and the use of environmentally friendly materials, are crucial for mitigating the environmental footprint of electric aviation.
Temperature Sensitivity:
Batteries are sensitive to temperature fluctuations, and this sensitivity is particularly pronounced in aircraft, where exposure to extreme temperatures during flight is common. Efficient thermal management systems are essential to ensure that batteries operate within their optimal temperature range, maximizing performance and longevity.
Charging Infrastructure:
The availability of charging infrastructure is essential for the widespread adoption of electric aircraft. Unlike ground-based electric vehicles, which can rely on a vast network of charging stations, aircraft require dedicated charging facilities at airports.
The development of efficient and rapid charging technologies is crucial to reduce ground times and enable the commercial viability of electric aircraft. Researchers are exploring various charging methods, including high-power charging systems and wireless charging technologies, to meet the specific needs of electric aircraft.
Innovation and Future Prospects:
Continuous innovation in battery technology is a driving force behind the evolution of electric aircraft. Researchers and engineers are exploring new materials, designs, and technologies to enhance energy storage capabilities and address the current limitations of batteries. Solid-state batteries, for example, hold promise for increased energy density and improved safety.
Conclusion:
The integration of electric propulsion systems in aircraft represents a paradigm shift in the aviation industry. Batteries are the linchpin of this transition, influencing everything from range and safety to charging infrastructure and sustainability.
The integration of batteries into aircraft presents a complex set of challenges and considerations. However, the potential benefits of electric propulsion in terms of environmental sustainability, noise reduction, and energy efficiency make it a compelling alternative to conventional fossil fuel-powered engines.
Overcoming the challenges of weight sensitivity, energy density, and safety concerns is imperative for the successful integration of electric aircraft into commercial aviation. The ongoing advancements in battery technology, coupled with stringent safety standards, will determine the pace and feasibility of this electrifying journey through the skies.
As technology advances, the aviation industry must navigate the challenges and seize the opportunities presented by the electrification of flight. With careful consideration of battery technologies and their implications, a future where electric aircraft soar through the skies efficiently and sustainably is on the horizon.
References and Resources also include:
https://link.springer.com/article/10.1557/s43577-021-00097-1
