While electric propulsion is more efficient, but it generates far less thrust, which is why electric planes tend to be slow. Airbus’ two-seat electric plane could only go a maximum speed of about 136 miles per hour. A solar-powered plane that completed an around-the-world journey this summer had an average airspeed of 47 miles per hour. The plane, called Solar Impulse 2, had more than 17,000 solar cells that powered four electric motors.
Industries are now exploring Hybrid-electric propulsion systems that employ two or more distinct types of power, for example, power from an internal combustion engine (in the case of an aircraft, this would be using kerosene) and an electric motor. By taking advantage of both electric motor and internal combustion engines, hybrid-electric propulsion systems provide not only a benefit in fuel-saving but also a reduction in takeoff noise and emission levels.
Some of the main advantages of HEP compared with the traditional propulsion are: (a) increasing the global aircraft efficiency; (b) increasing aircraft reliability, power distribution/quality, and flight range; (c) emissions and noise reduction; (d) capacity of extending the market to smaller airports.
HEP systems combine fuel-burning engines with electric motors and batteries and are driven more by environmental concerns of reducing carbon-dioxide emissions. Internal United Technologies Corporation studies indicate that commercial hybrid-electric and electric propulsion could: Reduce aircraft noise up to 85 percent, Improve fuel consumption up to 40 percent, reduce carbon dioxide emissions by more than 20 percent and Reduce airline operating and maintenance costs up to 20 percent. Hybrid-electric propulsion architectures could have broad applicability across different aircraft types, with potential fuel savings ranging from 5 percent in large commercial aircraft, to 30 percent for regional commuter aircraft. These advances in efficiency will also support the growing adoption of sustainable aviation fuels.
The HEPS is an evolution where small planes can use airfields with smaller runways for commercial applications. Electric motors have higher torque at low speeds, making the aircraft’s acceleration during takeoff to be higher. These new aircraft concepts with architectures that increase propelling torque and power, for shorter takeoff distances, can enable future devices to utilize smaller airports . Therefore, until electric energy storage systems are ready to allow fully electric aircraft, the combination of combustion engine and electric motor as a hybrid-electric propulsion system seems to be a promising intermediate solution.
Hybrid Electric Vehicle Configurations
Promising fuel savings between 5% to 30% in commercial aircraft, a HEP has six different subtypes, comprising various combinations and pairings of batteries, motors, propellers, and fuel-burning turbofan and turbojet engines.
The idea of electrifying the propulsion system yielded to a realm of propulsion system configurations, falls into three primary domains: 1—fully electric, 2—turboelectric, and 3—hybrid electric. The configurations are characterized as per the extent of usage of the electrical energy source and based on the electrical powertrain arrangement. A fully electric system relies upon a battery or some other means of electrical energy source as a sole means to power the propulsion system. Such design features the advantages of a highly efficient conversion system and is the only configuration which has the potential for zero inflight emission and is much quieter in operation. A few publicized conceptualized designs in the fully electric category are: NASA’s SCEPTOR X-57, in the general aviation sector, Bauhaus Luftfahrt’s VoltAir, and Ce-Liner designs in the regional and single-aisle segments, respectively. The majority of the aircrafts in this category are designed for smaller size, with a few exceptions.
Turboelectric configuration retains fuel as main source of energy and converts the chemical energy available in the fuel into electrical power either fully or partially to drive the propulsor. A turboelectric configuration features lower efficiency due to the additional losses in the electrical drive system for the conversion and transmission of mechanical power to electrical power and again from electrical to propulsive power. However, on the positive side, the concepts are well poised for implementing the novel concepts such as distributed propulsion arrangement with or without a boundary layer ingesting system. A further variant to the turboelectric is a partially turboelectric configuration, wherein the propulsive thrust is produced by both gas generator driven propulsors and the turbofans. Given a significant reliance on gas turbine technology, these configurations are deemed to be viable with modest beyond state of the art advancement in electrical component technology.
Full and partial turboelectric configurations do not rely on batteries for propulsion energy during any phase of flight. Rather, they use gas turbines to drive electric generators, which power inverters and eventually individual direct current (DC) motors that drive the individual distributed electric fans. A partial turboelectric system is a variant of the full turboelectric system that uses electric propulsion to provide part of the propulsive power advances beyond the state of the art than are required for a full turboelectric system. Because it is relatively easy to transmit power electrically to multiple widely spaced motors, turboelectric and other electric propulsion concepts are well-suited to distributed propulsion for higher bypass ratios, and they provide aircraft design options for maximizing the benefits of boundary layer ingestion (BLI) in the fans. Turboelectric propulsion research is high-priority approaches for developing advanced propulsion and energy system technologies that could be introduced into service during the next 10 to 30 years to reduce CO2 emissions.
Moreover, it is established that successful application of the aircraft based on turboelectric/partially turboelectric configurations has a high reliance on superiority of the electrical drivetrain components’ technologies . A drivetrain is the collection of components that deliver power from a vehicle’s engine or motor to the vehicle’s wheels. The system level benefits and challenges of fully/partially turboelectric configurations were studied under NASA’s N3-X , the single-aisle partially turboelectric aircraft with an aft boundary layer propulsor (STARC-ABL) design, Boeing’s Subsonic Ultra Green Aircraft Research (SUGAR) Freeze design, and Empirical Systems Aerospace Inc. (ESAero)’s Environmentally Conscious 150 (ECO-150) design.
The term hybrid electric is coined for the options wherein the propulsion system utilizes more than one type of energy sources, such as fuel/chemical and battery/electrochemical system. The hybrid electric configuration further branches off to series and parallel types, which are characterized based on the nature of the node connecting the two constituent energy sources.
Various Hybrid Electric Vehicles Configurations:
- Parallel Hybrids:
The parallel hybrids incorporate a mechanical connection between engine and propeller, allowing the operator to run the engine drawing power either from the battery exclusively or via the turbine engine. The internal combustion engine of various parallel hybrids can also act as a generator for supplemental recharging. Currently, these commercialized hybrids use a full size combustion engine with a single, small electric motor designed to adjunct the main engine, not to be the sole source of motive power from launch. Parallel hybrids are more efficient than their non-hybrid counterpart vehicles especially during urban stop-and-go conditions.
With a benefit of requiring less numbers of components, the configuration enjoys advantages related to weight saving. Nonetheless, on the downside, it involves mechanical coupling which causes operational and control complexity. Another limiter for the parallel hybrid electric operation is the impact from the imposed operating conditions, that is not optimum for the performance of turbomachinery components. Hybridized condition entails reduced power demand from the engine core, under a fixed thrust requirement. In order to provide a lower shaft power output, the high-pressure shaft adopts to a reduced operating speed while the mass flow and the fan speed remain the same.
- Series Hybrids:
In series hybrids only the electric motor drives the drive train, and a smaller internal combustion engine works as a generator to power the electric motor or to charge the batteries. They also usually have a larger battery pack than parallel hybrids, making them more expensive. When the batteries are low, the small IC engine can generate power, making them more efficient.
In a serial hybrid configuration, the propulsors are supplied electrically either from a gas turbine driven generator or from an electrical energy source. This arrangement enables decoupling of the gas turbine system from the propulsors by an electrical conversion and the transmission system. The inherent advantage of serial hybrid electric configuration is the flexibility to operate the gas turbine independent of the fan speed, thus making it feasible to operate at its maximum efficiency. Both serial and parallel hybrid electric architectures can store the electrical power in a battery to supplement the propulsive power in different operational segments.
The gas turbine is used to drive an electrical generator, the output of which drives the motors and/or charges the batteries. Series hybrid systems are compatible with distributed propulsion concepts, which electrical components for a partial turboelectric system can be developed with use multiple relatively small motors and fans.
A serial/parallel hybrid electric configuration represents co-existence of both serial and parallel operation, meaning, it is arranged to drive the propulsors, both electrically, from power sources—a gas turbine driven generator or a battery and mechanically from the gas turbine. The decoupling of the propulsors from the power generation source in a serial/parallel hybrid electric configuration relieves the constraint of sizing or operating the engine to meet propulsor power requirement and thus, gives further opportunity for optimizing the engine performance in different operating segments. The series/parallel partial hybrid system has one or more fans that can be driven directly by a gas turbine as well as other fans that are driven exclusively by electrical motors; these motors can be powered by a battery or by a turbine-driven generator.
U of Illinois team models capabilities of hybrid-electric propulsion systems for general aviation aircraft
Researchers at the University of Illinois at Urbana-Champaign have utilized a series of simulations to model the performance of twin-engine hybrid-electric general-aviation propulsion systems. Their paper appears in the Journal of Aircraft. They created a flight-performance simulator to represent accurately the true flight performance of a Tecnam P2006T on a general mission to include take off, climb, cruise, descent, and landing, along with sufficient reserves to meet FAA regulations.
They found that current technology allows a parallel hybrid configuration to achieve a maximum theoretical range of approximately 175 n mile. The results also indicated that parallel hybrid architectures will offer an effective near-term configuration, by offering greater range performance than a series hybrid with incremental future advancements in battery specific energy density and electric motor power density. However, distant future advancements in these technologies will allow series-hybrid architectures to produce similar range capabilities with improved fuel economy over parallel-hybrid architectures.
Ansell said adding more batteries to fly farther may seem logical, but it works against the goal to make an aircraft as lightweight as possible. Ansell said that, overall, a hybrid-electric drivetrain can lead to substantial improvements in fuel efficiency of a given aircraft configuration, though these gains depend strongly on the coupled variations in the degree of drivetrain electrification and the required mission range. Both of these factors influence the weight allocation of battery and fuel systems, as well as the weight scaling imposed by internal combustion engine and electrical motor components. In general, to obtain the greatest fuel efficiency a hybrid architecture should be used with as much electrification in the drivetrain as is permissible within a given range requirement.
Despite its advantages, several goals must be achieved to make the technology viable. High-power and high-voltage applications at altitude come with varying challenges, including materials, design, and testing restrictions. Coupled with operation in a harsh environment and the need to minimize failure rates during flight, novel architectures, including fault-tolerant designs, and novel processes, such as arc detection and HV insulation testing, need to be investigated and developed.
Thermal management optimization of hybrid-electric propulsion systems will need to be understood in detail to reduce the overall system weight and maximize fuel savings. System integration understanding will also be critical to reducing cost and improving product manufacturability. Taking these technologies to certifiable standards will require further understanding of end-application requirements, which continue to evolve in the emerging hybrid-electric aviation market space.
NASA’s Subsonic Fixed Wing project identified four “corners” to be overcome: noise, emissions, aircraft fuel burn, and field length. The hybrid-electric propulsion offers operational flexibility, due to the greater number of components. Fuel and battery sources allow more possibilities for managing of the propulsion system in the various stages of a mission, and reduce the energy consumption, compared with traditional. However, it implies an increase in the necessary load during the design phase, and greater complexity in the operation. Proper management of the electrical components and combustion is mandatory to meet the environmental requirements and reduce the fuel consumption
There is not a unique solution for aircraft HEP. Some studies claim that actual technology HEPS in parallel architecture is best suited for aircraft with fluctuating power requirements, this means short duration of high-power requirements (Finger et al. 2020).
HEP offers operational flexibility due to the variety of components and possibilities, but this may imply an increasing in weight. Consequently, it is necessary increasing in thrust during takeoff, which must be considered in the aircraft design phase. HEP increases the complexity in operation and design, but with a management system operating the engines close to their maximum efficiency operation conditions, and with proper energy storage control, environmental requirements and reductions in energy consumption may be achieved
Hybrid Electric Aircraft Technologies
In addition to generating more power, we also need to be able to control, protect and manage the power and thermal environment. Managing significant amounts of electrical power at high altitudes is not easy to do and real expertise is needed. Highly efficient power distribution and conversion is required to maximize the use of available power and minimize the thermal management system. High voltage systems will be required for commercial aircraft. However, isolating high voltage at high altitudes is challenging. Additional spacing and insulation systems are required, and that impacts weight. Also the safe use and management of electricity on an aircraft—the system design—is critically important and another crucial challenge.
The largest share of aircraft is operating within the short and mid-range distance, covering distances about 1000–3000 nautical miles (nm) and up to 240 passengers (PAX). However, to create an advantage by applying hybrid-electric propulsion systems to such aircraft requires low masses and high efficiencies of the electric components that are very challenging compared to the state of the art . The use of superconducting and cryogenic-cooled components could potentially overcome these limitations. Firstly, due to the enormous current carrying capabilities of superconductors, using them in electric machines or cables can reduce the masses and required voltage levels of such components.
Secondly, when cooling these components with liquid hydrogen, the evaporated hydrogen can be used further as a fuel for power generation instead of requiring energy densities for batteries that are orders of magnitude higher than the state of the art. Power- and thrust generation of aircraft by combustion of (liquid-) hydrogen instead of kerosene have been part of numerous investigations during the past. Investigations showed that using hydrogen reduces the CO2 -emissions while increasing the efficiency of the engine. However, due to its low volumetric density, the volume to store the required amount of liquid hydrogen in the aircraft removes this advantage on an aircraft-level. It should be noted, that the Soviet aircraft TU-155 successfully passed experimental flights between Moscow and Kiev with hydrogen powered engines in 1988.
Collins Aerospace is working with sister business Pratt & Whitney Canada to advance sustainable hybrid-electric propulsion technology for the aviation industry. In collaboration with De Havilland Aircraft of Canada, Collins and Pratt & Whitney are integrating a new hybrid-electric propulsion system into a De Havilland Dash 8-100 flight demonstrator. The demonstrator will be re-engined on one side with a 2 megawatt-class propulsion system that combines a fuel-burning engine from Pratt & Whitney with a battery-powered electric motor from Collins in a parallel hybrid configuration.
Both the engine and the motor will each generate about 1 megawatt of power for a 50/50 power split. The engine will be optimized for cruise efficiency, while the electric motor will provide extra power during take-off and climb, demonstrating around 30 percent better fuel efficiency, compared to existing turboprop engines.
Although this technology is still in development, progress is quickly being made. For the Dash 8 demonstrator, Pratt & Whitney and Collins are targeting ground testing in 2022, followed by flight testing in 2024.
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