Electric aircraft journey begins with hybrid designs, short routes

Electrification, a new paradigm in aircraft propulsion systems, can profoundly affect the aviation industry like the jet engine did after World War II. Electric flight — the next revolution in power electronics — promises lower carbon footprint, reduced noise, and lower cost of aircraft operations.

The technological advancements in power electronics and energy density are evolving aviation electrification from a concept to reality. Electrification is transforming the automotive industry right now, and that helps create a technology roadmap for disrupting another large transportation segment: aerospace.

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Figure 1: Design engineers have their work cut out for honing the major building blocks of aircraft electrification.

Air travel creates a particularly large carbon footprint, and that makes the switch from kerosene-powered gas turbines and generators to battery power a key initiative to tackle global warming. The European Union data shows that aviation accounts for approximately 3% of Europe’s total greenhouse gas emissions and more than 2% of global emissions.

But where does electric aircraft stand today, and what’s the tangible progress it can make in the coming years? This article takes a closer look at the technological challenges that electric flights face. It also reviews the aircraft electrification progress in propulsion systems, energy storage, and thermal management.

Electric aircraft challenges

For a start, to make electric aviation a reality, batteries need to have greater energy density and thus longer service life. That leads to one of the most significant challenges for electrifying aviation: the weight-to-power ratio of batteries.

Batteries are bigger and heavier than fuel, and an aircraft will have to carry the full weight of the battery for the whole flight. So the key challenge will be to significantly improve the energy storage capacity per unit weight of batteries. Moreover, engineers will have to develop light and efficient electrical generators and motors and other power electronics components that can convert, condition, and switch to high-voltage power smoothly and efficiently.

Besides the power-to-weight ratio, the aviation industry takes safety incredibly seriously, so the safety standards for batteries in aircraft will be even more stringent than for passenger cars. That, in turn, calls for batteries with far more improved reliability. The safety of high-voltage systems is also a major concern.

Aviation electrification also demands a transformation of the existing hardware and software solutions. For instance, new power electronics systems are required to control and transfer the electric energy to electric motors. Electric aviation can’t rely on unproven power distribution systems and electric motors and generators.

The aerospace industry has started to tackle the twin challenges of energy storage capacity and powertrain performance. However, these solutions are targeted at “more electric” or hybrid aircraft rather than an all-electric alternative.

In 2017, for instance, Airbus joined hands with Rolls-Royce and Siemens to develop hybrid-electric propulsion for commercial aircraft. The outcome of this collaboration — E-Fan X hybrid-electric technology — replaced one of the aircraft’s four gas turbine engines with a 2-mW electric motor from Siemens. It supports takeoff and climbing while using 700-kW lithium-ion batteries.

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Figure 2: E-Fan X demonstrator aims to address design challenges related to high-power propulsion systems. 

These are still early days in aircraft electrification designs, and what’s required is a system engineering approach to propulsion systems, energy storage, and thermal management. That includes taking into account all the heating and cooling requirements as well as pumps, valves, and fans.

More electric aircraft

For now, batteries with the energy capacity to fly longer distances will be too heavy, and thermal concerns about battery packs still prevail. The aerospace industry will have to make a huge investment to take the battery chemistry to a new level while investigating new battery technologies such as sulfur-ion and lithium-air.

That leaves the aviation industry with a couple of choices. First, small planes can fly over short-haul routes and operate as air taxis to transport passengers to nearby cities. Second, more electric-hybrid versions can deliver tangible benefits through reduced emissions and enable the development of motors, generators, and other power electronics components to transition to fully electric operations.

In May 2020, a nine-seat Cessna Grand Caravan, powered by a 560-kW propulsion system, flew an all-electric test flight for 30 minutes over the state of Washington. Apparently, battery technology won’t be available for larger aircraft in the near future even for a hybrid design because energy storage to fly a larger aircraft for a long distance will be too heavy.

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Figure 3: magni500 is the electric propulsion system for the all-electric Cessna Grand Caravan. 

As a result, the major focus for electric aerospace will be small aircraft running on short routes. According to aviation data company OAG, about 45% of flights globally span less than 500 miles. Not surprisingly, therefore, Norway and its state-owned airport operator, Avinor, have committed to the electrification of all flights under 90 minutes by 2040.

One step at a time

In the commercial arena, Airbus A350 and Boeing 787 have started using battery power for several of their on-board systems. That encompasses the electrification of propulsion systems for takeoff and climb operations. At the same time, however, these aerospace giants are working on fully electric propulsion designs.

The strategy starts with hybrid designs first and then moves to an all-electric system, with batteries providing energy for propulsion. Meanwhile, the aerospace industry will have to solve the battery’s power-to-weight ratio conundrum. The battery industry observers believe that it won’t be possible until the 2030s.

Other design considerations for aircraft electrical systems include wiring routes, electrical interference, and thermal environment created by high-capacity batteries. Therefore, we are likely to see aircraft powered by electric-hybrid engines for short-haul flights in the coming years.

Even that’s a good start for reducing greenhouse emissions and lowering aircraft noise. Nevertheless, the journey toward aviation electrification is irreversible. It’s apparent from the French government’s €15 billion bailout package for its aviation sector, which includes €1.5 billion to be spent on R&D of alternative fuels.

 

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