Since the 1950s, aerospace has made incredible progress in increasing both the power and efficiency of engines. Yet, recent studies suggest engine technology may be close to the thermodynamic limits on the amount of energy it can extract from carbon-based fuel at a reasonable investment. That means more travel and more aircraft will lead to increased fuel use and more greenhouse gas emissions unless a suitable substitute for fossil fuel is found.
Today, the global aviation industry accounts for 2.4 percent of total carbon-dioxide emissions and about 12 percent of the greenhouse gases released by the transportation industry. But those percentages are expected to increase as expanding demand for air travel, particularly in Asia, requires the addition of substantially more capacity. By 2028, revenue passenger kilometers are expected to increase by more than 60 percent to 12 trillion, and the size of the global fleet will expand by 43 percent to more than 39,000 aircraft. By 2050, the International Civil Aviation Organization (ICAO) predicts emissions from aviation could increase by more than 300 percent. It’s also likely its share of global GHG production will rise if no meaningful attempt is made to turn away from fossil fuels, given efforts by other carbon-emitting sectors, such as electricity generation.
Since a complete solution to the emissions problem—a commercial jet powered entirely by something other than fossil fuels—is likely to be decades away, a few pioneering aerospace companies are borrowing an idea from the automotive industry and creating hybrids. Just as they did for cars, these propulsion systems that are part internal combustion and part electric could represent an interim strategy to cut emissions and fossil fuel consumption until fully sustainable electric or hydrogen-powered aircraft are commercially developed. Given mounting evidence that the pace of climate change is accelerating, aviation can ill afford to wait another two decades to address its emissions problem.
While hybrid automobiles don’t reduce emissions as much as fully electric vehicles, they do almost cut them in half compared to gasoline-powered cars. For aviation, going hybrid would still mean overcoming many engineering challenges and would require regulatory approval, but this alternative would likely be realized sooner than a fully electric airliner.
A higher hurdle
Research into the electrification of transportation is part of the global effort to avert large-scale drought and significant sea level rise by reducing greenhouse gas emissions. While every nation and industry has pledged to do their part, carbon dioxide emissions continue to rise—up 2.7 percent last year, according to a report by the Global Carbon Project. Numbers from the International Air Transport Association show aviation emissions up 26 percent since 2013. With the pressure on to at least stabilize the situation, aviation needs to come up with immediate solutions in the next few years or face penalties under a United Nations-sponsored agreement called the Carbon Offsetting and Reduction Scheme for International Aviation that require airlines to cap emissions at 2020 levels.
While electric cars and light vehicles are expected to dominate auto sales by 2040, Emissions Analytics, a global testing and data specialist that measures real-world emissions and fuel efficiency, has argued that promoting hybrids in the short term may in fact be more effective at cutting emissions over the long run. Why would not the same be true for aviation, given how elusive complete electrification of air transport is proving?
Representing a step change for the industry toward lower emissions, hybrids are powered by both conventional kerosene-fed turbine engines and electric motors using “clean” power stored in batteries or produced by hydrogen fuel cells. This dual-technology propulsion system would be usable through all phases of flight, and besides lowering emissions, it would likely cut an aircraft’s consumption of jet fuel—the second biggest operating cost for airlines.
Working the problem
At least one small-scale attempt to assign some engine functions to electric motors has already been successful, but unfavorable economics stopped it from being adopted. In 2016, Safran Landing Systems and Honeywell abandoned an electric taxiing system that would have cut emissions and fuel use especially for airlines that focus on frequent, short-haul flights, because low fuel prices made the system less attractive to carriers.
There have also been test flights of small hybrid aircraft. Here are some examples: The University of Stuttgart’s e-Genius plane conducted at least two successful flights over the Alps using a relatively complex propulsion system consisting of an electric motor, batteries, a generator, and an internal combustion engine. Similarly, Diamond Aircraft Industries and Siemens announced the first flight of a multi-engine hybrid electric aircraft late last year.
More ambitious hybrid experiments are underway as well. Most recently, SAS and Airbus announced a collaboration to create a hybrid-electric aircraft for large-scale commercial use. At the Paris Air Show in late June, hybrid projects were unveiled by Airbus, Safran, Daher, and Eviation, an Israeli startup. Rolls Royce also announced its intention to acquire Siemens’ eAircraft business at Le Bourget, another indication of growing commitments to an electric future.
Battery versus fuel cell
Even with hybrids, there is a choice to be made about whether to go with lithium-ion batteries, now commonly used in electric vehicles and smartphones, or hydrogen fuel cells. Each has its advantages, and each its drawbacks.
The biggest hurdle facing batteries is the size required to provide enough power to run a commercial airliner, given current technology. One problem: Lithium-ion batteries have a relatively low energy density per unit of mass compared with current kerosene-based jet fuel. In a hybrid, of course, that disadvantage is offset by the fact that the battery is being backed up by a conventional internal combustion engine.
A next-generation battery also is needed to improve recharging time if it is to be used on aircraft that often have just 30 minutes between flights. While a rapid-charging option has been in favor for electric cars, researchers are still trying to develop battery-swapping capability, despite early problems with the technology. Tesla is quietly continuing to test it, and Chinese electric car maker Nio also is a proponent. If battery-swapping succeeds for cars—with some economic and environmental concerns to overcome as well—it would allow a fully charged battery to be substituted for a dead battery in the time it takes to fill up a fossil fuel-powered car. It would also remove one impediment for battery use by aircraft.
Lithium-ion batteries have other drawbacks, such as the fact that China controls 61 percent of global battery capacity. China also is the third largest miner of the raw material, lithium—often referred to as white petroleum because of its mounting economic importance—and has been buying up reserves in Chile, the second largest producer.
Hydrogen fuel cells are another alternative being researched. The most abundant element on earth, hydrogen has higher energy density per unit of mass than either kerosene-based jet fuel or batteries – some 33,300 watt-hours per kilogram versus 11,900 for conventional jet fuel and only a few hundred for batteries. Hydrogen fuel cells have been used on every manned US space flight, from Apollo through the space shuttle.
Three years ago, the HY4 – a four-seater using only an electric motor powered by a hydrogen fuel cell – successfully took off from the Stuttgart airport in Germany, staying airborne for 10 minutes. In Singapore, the world’s first regional hydrogen-electric passenger aircraft was unveiled in October 2018. Although considered aspirational by many in the industry, the company has reportedly already received inquiries from regional airlines.
Within these cells, hydrogen and oxygen are combined electrochemically to produce electricity. Their only byproducts are heat and water vapor. But today most hydrogen is produced by separating it from natural gas, a process that then produces methane, another greenhouse gas. Hydrogen also can be produced from water, but the process is relatively expensive. Finally, fuel cell technology in general is pricey—another disadvantage.
Many aerospace executives privately discuss liquid hydrogen as the potential endgame for aircraft, emulating the propulsion design that the National Aeronautics and Space Administration (NASA) has used for decades to power its rockets. Recently, NASA funded a program at the University of Illinois that aims to develop a fully electric aircraft platform that uses cryogenic liquid hydrogen as an energy storage method. But for several years, the agency also has been researching the development of hydrogen hybrid aircraft.
Clearly, more must be done, and one of the biggest benefits of a hybrid strategy is that it may buy the aerospace industry the time it needs to create a new kind of flying machine that doesn’t use fossil fuels at all. Given the absolute necessity to cut emissions, an industry hybrid strategy that focuses on incremental solutions raises the prospect of real progress on reducing emissions in the near term, even as air travel continues to expand
Date: Sep 3, 2019