Tech Briefs

The electric taxiing system does not require the use of jet engines or the auxiliary power unit.

With airlines increasingly directing their attention to operating costs and environmental initiatives, the More Electric Architecture for Aircraft and Propulsion (MEAAP) is emerging as a viable solution for improved performance and ecofriendly aircraft operations. An electric taxiing system was developed that does not require the use of jet engines or the auxiliary power unit (APU) during taxiing, either from the departure gate to takeoff, or from landing to the arrival gate.

Power system block diagram of an Electric Taxiing System.
Cutting engine operating times during taxiing, including wait and standby times, would require an electric power management system that shuts down the main power supply, including the engines and APU. Clearly, the alternative electric power source would simultaneously need to supply power not just for taxiing propulsion, but for all aircraft electric and electronics systems.

The MEAAP concept offers the promise of greater power management efficiency compared to conventional aircraft systems. Researchers have developed several cornerstone technologies, including high-voltage drivers, fault tolerance systems, and electrical-mechanical engineering technologies, all with the goal of building more efficient electric systems for aerospace applications.

This work targets practical applications of energy management technology, focusing primarily on electric power systems that take over the roles of conventional aircraft systems. This system must address two issues: safety and reliability, and power source and storage. The autonomous taxiing approach must incorporate an electrically driven wheel system. A more eco-friendly approach requires the replacement of devices that run on jet fuel. Instead of an engine or APU generator, a smaller primary power source based on a fuel cell or other battery system can be used. Comprised of a common electric motor and inverter, the electric taxiing system drives the front gear or main gear wheels. Each of the aircraft's electric-powered wheels is controlled by state-of-the-art power electronics and controllers. This system gives pilots total control of aircraft speed, direction, and braking while on the loading apron and taxiway.

This electric system requires a wholly new electric power source. Before taxiing, the aircraft avionics system must be activated, while the environmental control system (ECS), including air conditioning, must work continuously. The ECS must draw its power from another power source, since we cannot draw on conventional engines or APU power. To shift from the engine and APU to a substitute power source at the airport, aircraft systems must be designed to reduce power requirements, since the substitute power sources will have lower energy and power densities than gas turbine generators.

After leaving the loading apron, the aircraft operates on a taxiway. Smooth runway performance in real-world systems will likely require an electrical taxiing system capable of speeds of 30 to 40 km/h. A single-aisle aircraft requires a 6-kW power source to run at 4 km/h. Based on the taxiing speed goal, approximately 60 kW power will be required when running at maximum speed (40 km/h).

As it moves on the taxiway, the aircraft will repeatedly stop and go while lining up and waiting for takeoff. To meet the significant peak power requirements for heavy aircraft, a temporary power energy storage system can augment the primary power source. In addition to accommodating high-density acceleration and bump climbing during the typical taxiway traffic cycle, the power energy storage system can provide energy to the electrically driven wheels with the higher power levels required for relatively short durations.

When the power required to propel the aircraft is less than the power produced by the primary power source, the excess energy can be stored in the power energy storage system for later use, thereby achieving eco friendly power/energy control. General electrical specifications for aircraft systems do not permit regeneration to the electrical system. Regenerative current causes distortion on the power buses. A power energy storage system capable of absorbing high regenerated power during regenerative braking will boost system efficiency. Assuming regenerative power is equivalent to the power required for acceleration, regenerative energy storage must be capable of providing 80 kW over 10 seconds. Several available storage devices can meet these regenerative power needs, including lithium-ion batteries, supercapacitors, and flywheel batteries.

To overcome the issues posed by conventional fuel cell systems, a regenerative fuel cell (RFC) system is one solution for FC fuel supply and onboard storage. An RFC system requires mere water as fuel, since the RFC, which incorporates a water electrolyzer, is an autonomous system that produces oxygen and hydrogen. The RFC consists of proton exchange membrane fuel cells (FC), electrolyte cells (EC) linked to the main power bus, and fuel storage tanks. Some of the main bus power is supplied to the EC to produce hydrogen and oxygen from water for FC operations. The power generated from the main engine generators during climb, cruise, and descent is transmitted to the EC for hydrogen and oxygen refueling, as well as for supplying electricity for all aircraft systems. Hydrogen- and oxygen- fueled FC power is used for electric power generation when needed.

Once the aircraft arrives at the gate, the power supply from an internal source shuts down. Aircraft electricity is provided from the external power supply — the Ground Power Unit (GPU).

The GPU provides the power needed to generate hydrogen and oxygen in the RFC system to prepare for the subsequent flight and operations.

This work was done by Hitoshi Oyori of IHI Aerospace Co. Ltd. and Noriko Morioka of IHI Corporation. The full technical paper on this technology is available for purchase through SAE International at http://papers.sae.org/2013-01-2106.