Preparing Your Airport for Electric Aircraft and Hydrogen Technologies (2022)

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10 What Are Electric Aircraft? 2.1 Electric Aircraft Concepts Around 100 electric aircraft designs are under development worldwide, each somewhat unique in their configurations and capabilities and reflecting the assumptions made by their designers and the different markets they target. These designs can be best understood by grouping them based on their configuration, capabilities, and missions (Figure 3). The ACRP Project 03-51 research effort focused on fixed-wing manned aircraft used for private and recreational flights, training purposes, air taxi services, small commuter flights, and regional aviation. It does not cover small UAS and eVTOL vehicles even though they are considered electric aircraft, per se. Although the current state of the technology does not yet meet their mission requirements, larger commercial service electric aircraft are also discussed. Long-endurance, unmanned, aerial electric vehicles primarily used by the Department of Defense, law enforcement, and federal and state agencies are not specifically addressed in this report. Their needs are mostly similar to those of manned aircraft with regard to the airport operational challenges and the electricity demand. The lower gravimetric energy density of batteries compared to jet fuel limits the capabilities of pure electric vehicles, and in particular, their range and passenger capacity. Larger aircraft may have to be equipped with hybrid-electric powertrains (either parallel or turboelectric) to increase range and passenger capacity. As a result, many of the electric aircraft that are under develop- ment are smaller and have lower capacity. These vehicles will fly shorter missions, whether urban, suburban, or regional air mobility. These emerging aviation markets operated with new generations of electric aircraft are also known as advanced air mobility (AAM). Urban air mobility (UAM) missions typically require a vertical takeoff and landing (VTOL) capability owing to the limited ground footprint available to design and build vertiports over valuable land close to city centers. These vehicles provide connectivity within a dense metropolitan area, and with other modes of transportation including existing airports. São Paulo, Brazil, is one of the few cities in the world with an effective UAM system, operated with conventional helicopters. Its downtown accommodates more than 400,000 operations per year. Use of eVTOLs might facilitate the implementation of UAM at many more large cities over the coming decades since they are significantly quieter, greener, and potentially cheaper to operate than conventional VTOLs (helicopters). Some short-haul inter-urban connections—suburban or community air mobility—might use constrained facilities with runways shorter than 5,000 feet (1500 meters) that cannot be expanded because of encroachment or other physical limitations. On-demand and scheduled flights could be performed by short takeoff and landing (STOL) capable aircraft. Most of these aircraft should have a slightly higher capacity and range than VTOLs. Billy Bishop Toronto City Airport in Canada and London City Airport in the United Kingdom are the C H A P T E R   2

What Are Electric Aircraft? 11   busiest urban STOLports in the world. As for UAM, electric aircraft can provide more socially acceptable operations than conventional aircraft and revive the interest for STOL facilities. Regional air mobility missions—at least, initially—are flown by horizontal (or “conventional”) takeoff and landing vehicles (CTOL), owing to the longer-range requirements and the resulting need for greater energy efficiency. On-demand and scheduled flights should be offered from smaller airports to other community airports or larger aviation facilities. The current operations of air carriers such as Cape Air in the northeast region and Harbour Air in the Puget Sound (Seattle-Vancouver area) have regional air mobility features. Cape Air operates from small facilities (e.g., Nantucket Memorial Airport) and large hub airports as in Boston Logan International Airport (BOS) and St. Louis Lambert International Airport (STL). This will depend on local geographical constraints, local regulatory constraints, and market demand. Consequently, a diverse subset of configurations has been explored by the industry, in addition to traditional fixed-wing aircraft designs. Figure 4 highlights a subset of designs that are capable of VTOLs. Each configuration has a different level of energy efficiency, which affects both the type of mission that it is optimized for and the nature of the infrastructure required to support operations. Figure 5 details the relative efficiency of various configurations of VTOL vehicles. For these configurations and for comparison, the maximum takeoff weight (MTOW) is set at 5,000 pounds, the cruise speed is set at 150 miles per hour (mph), and the mission length is set at 24 miles. An exhaustive review of each of the 170 aircraft designs is beyond the scope of this report. Instead, the various electric-vehicle designs are characterized according to their overall con- figuration and overall size, classified into eight primary categories. These include the fixed-wing aircraft category, which is further divided according to passenger capacity into the 1- to 4-seat, the 6- to 9-seat, and the 40- to 80-seat categories, as well as the helicopters, the tilt rotors, the tilt wing, and the lift plus cruise. Figure 3. Electric aircraft market segments.

12 Preparing Your Airport for Electric Aircraft and Hydrogen Technologies Note: United Technologies Corporation (UTC) Project 804 Concept in terms of regional aviation. Figure 4. Examples of electric aircraft per mission. Figure 5. Energy efciency in vertical ight of various aircraft congurations.

What Are Electric Aircraft? 13   Figure 6 and Figure 7 show the evolution of the engine power and battery capacity for electric aircraft per their year of effective or expected first flight. Meanwhile, high-level characteristics are shown in Table 1, and the baseline aircraft retained for the study of six use-case scenarios that will be defined in the subsequent section are found in Table 2. 2.2 Electric Aircraft Energy Efficiency With the higher energy efficiency of electric powertrains (Figure 8) and the relatively stable energy cost of electricity against jet fuel (Figure 9), several aerospace industry groups have implemented sizable development programs to further advance electrified aircraft platforms for larger scale, commercial applications. Several key challenges still exist, however, when moving toward these commercial aircraft platforms. The following top priorities to enable future electrified aircraft platforms remain: • Required improvements in areas related to specific energy (weight reduction) of electrical energy storage systems; • Increased specific and rated power; • Aviation-compatible packaging of electric machines; • Strategic approaches to thermal management; and • Integration impact assessments. Assuming success is achieved in these key areas, many aircraft developers are targeting entry into service date ranges of approximately 2030 to 2035 for electrified variants of regional aircraft (fewer than 30 to 80 passengers) and dates closer to 2050 for electrified variants of single-aisle aircraft (130 to 180 passengers). Figure 6. Evolution of the electric aircraft engine power.

14 Preparing Your Airport for Electric Aircraft and Hydrogen Technologies Figure 7. Evolution of electric aircraft battery capacity. Table 1. Configuration and classes of electric vehicles.

What Are Electric Aircraft? 15   Table 2. Baseline aircraft concepts for use cases. Figure 8. Efficiency of traditional and electrified powertrains.

16 Preparing Your Airport for Electric Aircraft and Hydrogen Technologies Successfully implementing electrified aircraft hinges on not only the technical feasibility and production capability of the aircraft in isolation, but also on broader support infrastructure for these aircraft to be implemented at scale. Given transportation’s large energy requirements, broadly, growth in the capacity of electricity generation is needed, either on-site or across a national grid. Additional factors also include several items within airport planning related to space, ground sup- port equipment (GSE), service personnel, and technicians, and power and energy delivery capacities. These considerations must also be further developed to truly realize growth in electrified aircraft. 2.3 Integral Electric Aircraft Components Energy Storage Energy storage technologies, such as batteries, are key components of all-electric vehicles. A battery is an individual cell that converts chemical energy to electrical energy through electro- chemical reactions. When a current is sent into the battery, this chemical reaction can be reversed. A battery pack is a collection of two or more cells connected in a series or parallel configuration. The design and performance of battery packs are crucial components of electric vehicles. In these types of vehicles, battery packs are the main source of energy and contribute significantly to the overall vehicle weight. Common parameters to describe the performance of the many types of batteries are as follows: • Specific Energy (Watt-hours per kilogram or Wh/kg) is the most commonly used battery performance parameter and is used in estimating electric-vehicle endurance. It describes the amount of electrical energy stored per unit of battery mass. • Specific Power (Watts per kilogram or W/kg) is the amount of power delivered per unit mass of the battery. This parameter can be used to describe how fast a battery can take in energy and release it. These two parameters are most commonly used when comparing battery technologies. A bat- tery’s capacity decreases as the discharge time increases. A vehicle that operates at a high-power setting will quickly deplete its batteries. Likewise, a vehicle with high specific energy batteries will have low specific power characteristics. Charge capacity is another important battery parameter that affects the performance of electric aircraft. How quickly a battery is discharged will affect the total energy the battery can provide. The slower the current draw is, the more energy can be extracted from the battery. Figure 9. Propulsion equivalent energy costs for jet fuel and electricity.

What Are Electric Aircraft? 17   Additional battery considerations that affect the performance and operations of electric vehicles include the following: • Specific Power: 1 kW/kg for most applications, although some applications might require 2 to 3 kW/kg. • Cycle Life: The number of discharge charge cycles the battery device can experience before it fails to meet specified performance criteria. The number of battery cycles depends on the type of operations. • Cold Weather Performance: Lithium-ion batteries show faster degradation outside accept- able temperature ranges. Rechargeable battery technology has a relatively low energy density compared to fossil fuels (Figure 10), which limits the range and scope of operations for electric aircraft. However, energy density of batteries is increasing and has grown by 8 percent each year in recent years (Figure 11). Battery technology performance is projected to continue to grow. A 2017 NASA Battery Technologies Workshop provided the following industry projections on battery technologies: • Specific energy is expected to increase by 5 to 8 percent per year. • Implementation of innovative concepts that can improve packing and integration, such as the following: – Lightweight container structure (e.g., cellular, lattice block); – Multifunctional structures with load-carrying capability for packaging materials; – Advanced thermal management techniques (e.g., phase change materials if the cost is not a factor, high conductivity materials); – Integrated thermal management system to cool battery packs; – Polymer heat exchangers; and – Larger cells. Figure 10. Volume and mass-specific energy characteristics of different energy storage systems.

18 Preparing Your Airport for Electric Aircraft and Hydrogen Technologies • Innovation in battery health management, such as moving to a software-based battery health management system for state-of-charge and state-of-health estimation. The U.S. Department of Energy, the battery industry, academia, and National Labs have raised their performance expectations to 300 Wh/kg at pack level (approximate 400 Wh/kg at cell level) for automotive and industrial applications but have not focused on electric aircraft applications. Electric aircraft will need to verify their performance, safety, and integration to bolster battery-related research. Beyond this approximate 400 Wh/kg capability at the cell level, the aeronautics community can focus on developing batteries with 600 Wh/kg specific energy at the cell level (400 to 500 Wh/kg at pack level), which is believed to be an achievable target. Higher levels of specific energy, on the order of greater than 700 Wh/kg at the pack level, are almost impossible to achieve at the short- term horizon, assuming current technologies (Figure 12). Figure 11. Roadmap for lithium-ion battery technology. Figure 12. Battery requirements for different classes of vehicles.

What Are Electric Aircraft? 19   Powertrain Architectures There are many variants of electric and hybrid-electric propulsion architectures. One way to categorize aircraft propulsion architectures is by their degree of hybridization, which describes how much of their energy source and power comes from batteries and electric motors. Figure 13 illustrates the typical powertrain architectures. Hybrid-electric architectures, for which there are various possible configurations, provide additional power to an existing propulsion system such as a turbofan by including an electric motor. The energy source for the motor can be a battery or a generator. Hybrid systems are less efficient at energy storage but are more efficient at energy conversion. Turboelectric architectures use kinetic energy from a fuel-burning turboshaft engine to drive a generator, which produces the energy to drive the electric motor. Finally, all-electric power- train architectures have motors that rely entirely on batteries as the only source of energy. These systems are highly efficient compared to traditional combustion. The main electric components of an electric aircraft powertrain are listed in Figure 14. Note: “Fuel” can be hydrocarbon fuel (e.g., Jet A1) or (in the near future) hydrogen. Figure 13. Electrical propulsion architectures.

20 Preparing Your Airport for Electric Aircraft and Hydrogen Technologies Figure 14. Main electric aircraft components.