The need to reduce gas emissions, optimize aircraft performance, decrease operating and maintenance costs, is pushing aircraft industry to progress towards more electric aircraft (MEA), and ultimately an All Electric Aircraft. Most electric aircraft efforts can be categorized and subdivided by size, range, configuration, and target market.
While the vast majority of flights take place on regional and narrow-body aircraft, most of the carbon emissions from aircraft come from the wide-body category. For transport aircraft, specific energy of the battery is a considerable barrier, leading to consideration of hybrid combustion-electric systems. Smaller aircraft, though they consume less energy and emit less carbon, are more conducive to electrification,some aircraft in this category have already been electrified. 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).
While we have been able to modify cars, trains, trams, and boats Electric we are not yet seeing large electric aircraft. The main reason is that Land vehicles can easily cope with the extra mass from electricity storage or electrical propulsion systems, but aircraft are much more sensitive. 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 commercial aircraft can be divided into three categories: regional, narrow-body, and wide-body. 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 have found that the major factor in determining the specific energy required of aircraft is the range that the class of aircraft typically flies, meaning that smaller, short-range aircraft will require less demanding battery performance metrics than larger, longer-range aircraft. Aircraft must store onboard all the energy needed to move its mass for each flight (unlike a train connected to an electrical grid). Using a heavy energy source thus means more energy is needed for a flight, which leads to extra mass, and so on and on.
Today’s best available lithium ion battery packs provide around 200 watt -hours (Wh) per kilogram, about 60 times less than current aircraft fuel. This type of battery can power small electric air taxis with up to four passengers over a distance of around 100km. For longer trips, more energy-dense cells are needed.
The most significant limiting factor at this point is not the weight of the engines, or the design of the aircraft, but rather it is the batteries. Batteries at this point cannot provide the power-to-weight ratio needed for electric aviation to be feasible. 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.
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.
Electric propulsion can be powered by rechargeable batteries, fuel cells, or solar energy. As rechargeable Li-ion batteries have reached technological maturity, with an increase in performance metrics (Wh/kg, Wh/L, W/kg, and W/L) and a drop in price ($/kWh), they have enabled the electrification of multiple modes of transportation, recently including electric aircraft. Charge and discharge rates of these batteries have also improved. While battery performance metrics and adoption have increased, the incidence of safety-related issues has also increased.
“Lithium (Li) ion batteries are the current state-of-the-art stored energy technology for aviation,” said Dr. Ajay Misra, deputy director, Research and Engineering Directorate, NASA’s Glenn Research Center, “the same type of technology used in cells phones and electric cars.” For aviation, Misra added, the relevant measure of battery suitability is called pack-specific energy, also referred to as pack gravimetric energy density. A pack contains thousands of battery cells. The units of pack-specific energy are watt-hours per kilogram (Wh/kg). According to Misra, the pack-specific energy of the most commonly used Li-ion batteries today is about 150 to 170 Wh/kg. By contrast, the specific energy of jet fuel is nearly 12,000 Wh/kg, or about 70 to 80 times more energy-dense than Li-ion batteries.
Hence fully electric large aircraft require a major, yet-to-be-invented shift in energy storage. 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.
Because a battery is an energy storage device, by definition a good battery contains large amounts of energy in a confined space. Relying on flammable and combustible materials, current Li-ion batteries (LIBs) are susceptible to catastrophic fire and explosion incidents — most of which arrive without any discernable warning. Notably, Samsung Galaxy7 phones were banned from airlines because of this danger, and the Navy’s prohibition of e-cigarettes on ships and submarines is a direct response to the obvious need to reduce the flammability of the power source of choice for portable electronics, electric vehicles, and more.
These safety issues should be examined in further detail in the context of electric aviation, given the greater risks and unique failure modes present in an airborne environment. One of the most publicized battery-related fire events in aviation was the grounding of the Boeing 787 Dreamliner fleet after a controlled fire event in 2013. The Li-ion batteries involved in this incident were part of the auxiliary power unit and not used for propulsion.
Modern aircraft designs for “more electric” and “fully electric” aircraft have large battery packs ranging from tens of kWh for urban aviation to hundreds or thousands of kWh for commercial aviation. Such large battery packs require careful consideration of the safety concerns unique to aviation. The majority of these reported incidents are related to the failure mode of thermal runaway, either with or without internal shorts, where an exothermic reaction and ignition in one cell cascade into similar exothermic reactions in neighboring cells and eventually a critical portion of the battery pack 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.
Cell design has a strong influence on the thermal behavior, heating, and heat transfer in a Li-ion battery. Generally, most Li-ion batteries are designed as cylindrical, prismatic, or pouch cells. Most electric vehicles currently use either cylindrical cells or prismatic cells. The materials used for the cell casing, terminals and other packing artifacts are different between the different cell designs. The casing materials control the heat transfer between the cell and the surrounding environment and hence plays a crucial role in events of overheating. Cell overheating is one of the initial conditions required for thermal runaway.
Cell design also dictates the manner in which venting of gaseous products occurs during exothermic or parasitic reactions. The mechanical strength of the casing materials is strongly correlated to the likelihood of shorts forming due to external stress or point loads. Electric X-planes like the NASA Maxwell X-57 have used commercial-off-the-shelf (COTS) cylindrical cells due to their superior safety characteristics such as stipulated venting mechanisms and greater strength of the casing materials.
Battery pack design also plays a significant role in both thermal and functional safety and risk profile of an electric propulsion system. Pack designs control the risk of failure cascading from one cell to more cells within the battery pack. The arrangement of cells within the pack and the thermal management systems are crucial in the mitigation of excess heat generation from certain cells. The X-57 battery pack was tested with trigger cells and reportedly the fire from one cell did not propagate to other cells. Pack design and by extension the battery management system affects the ability to monitor the state-of-charge (SoC) and state-of-health (SoH) of individual cells. The SoC and SoH of individual cells determines the extent to which functional safety such as a sufficient and reliable supply of power can be controlled.
Battery safety mitigation strategies for aircraft
There are three stages to thermal runaway (1) onset of overheating, (2) heat accumulation and gas release process, and (3) combustion and explosion. Flaws or defects in manufacturing, internal shorts (due to separator issues, dendrites, and mechanical stresses) or other functional issues can cause Stage 1 resulting in the onset of overheating. If the overheating is mitigated in Stage 1 itself, thermal runaway could be completely avoided. However, an important point to note for electric aircraft is that once Stage 1 occurs, functional safety cannot be guaranteed since Stage 1 signifies that the battery has transitioned from normal to abnormal behavior
Among the mitigation strategies for Stage 1, airworthy batteries, as a first step, could require much higher quality control standards compared to batteries manufactured for electric vehicles or other applications thereby minimizing the incidence of manufacturing defects. Cell design decisions are instrumental in determining the possibility of Stage 1 occurring. For example, in the widely publicized Samsung Galaxy Note 7 fires, extremely thin separators were identified as one of the primary culprits in causing an onset of overheating.
For electric aircraft, mandated compliance with a stipulated set of minimum cell design metrics such as minimum separator thickness, electrode porosity, and heat capacity of the cell stack could avoid the use of cells that are a result of safety-performance tradeoffs.
Currently, aviation regulatory bodies rely on a standard published by RTCA Inc. (formerly known as Radio Technical Commission for Aeronautics) in 2017, called DO-311A to specify the mandatory testing and compliance requirements for rechargeable batteries used in aircraft. One of the key characteristics that DO-311A attempts to determine is the “airworthiness” of a battery, which implies the “compliance of a battery with all the requirements for safe operation in an airborne environment.”
One of the important considerations for Stage 1 in an airborne environment is the effect of low pressure and rarer air on the heating or burning rate of cells. As reported by Xie et al., for two cells operated at the same rate, the cell at 95 kPa pressure exhibited a burning rate that is 3.9 times higher than one at 20 kPa. In addition, it is reported that the time taken to reach thermal runaway is more sensitive to changes in charge/discharge rates at 20 kPa compared to 95 kPa. These observations imply that apart from possibly more stringent cell design and composition requirements for airworthy batteries, the unique heating characteristics of airborne batteries must be accounted for Stage 2, which involves the decomposition of certain cell components accompanied by an increase in temperature due to heat accumulation.
One of the popular fail-safe mechanisms to handle Stage 2 is the use of cell-venting mechanisms. A cell vent, once activated, releases all the gaseous products in a controlled manner into the surrounding environment. The release of gases simultaneously balances the heat accumulated within the cell. For airworthy batteries, the cell-venting needs to account for the possibility of a low-pressure environment in an aircraft. Low pressure presents a significant challenge to controlling the gas release process from a cell-venting event and poses the risk of causing an uncontrolled explosion of gases and heat during venting.
If Stage 2 is not controlled, the cell inevitably goes to Stage 3 where the organic liquid electrolyte forms the primary fuel for combustion aided by accumulated heat, gaseous decomposition products and oxygen from the cathode. The priority at Stage 3 is to prevent propagation of the fire, thermal runaway and system-failure.
High safety, high reliability packs, specifically targeting the prevention of Stage 3 from cascading have been traditionally developed for NASA’s manned missions. One of the suggested method is using heat-absorbing plates between cells was an obvious way to prevent cascading with thick plates made of copper or aluminum completely preventing the cascading. For an electric aircraft, however, such plates would significantly affect the specific energy of the battery pack which is a critical design parameter. One of the popular studies on strategies to prevent thermal runaway in high specific energy Li-ion battery systems has been by Darcy, who proposed five design rules for modules that achieves more than 190 Wh/kg used in space suits. The five rules covered: (1) risk of side-wall rupture, (2) cell spacing and heat dissipation, (3) fusing of parallel cells, (4) protecting adjacent cells from “hot ejecta” from exploding cells, and (5) preventing sparks from leaving the enclosure.
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