Sustainable Energy

How Aviation Can Come Clean

Advanced technology won’t be enough for the industry to meet its own greenhouse-gas targets.

Last week the global aviation industry called on the United Nations to establish a single, worldwide policy for reducing aviation greenhouse-gas emissions, in an attempt to avoid a costly network of regional regulations. The industry proposed two primary goals–that by 2020 it should stop increasing its greenhouse emissions, and that by 2050 it should cut its emissions by 50 percent compared to 2005 levels.

Flying wing: The Boeing X-48B, an unmanned prototype with a 6.4-meter wingspan, has a blended-wing design that could one day replace that of today’s commercial planes.

These goals, while less stringent than the 80 percent reductions proposed for the rest of the world’s economy, may nevertheless prove too ambitious, some experts say. Furthermore, an array of potential technologies that could significantly reduce emissions will be difficult to deploy quickly in an industry that is reluctant to take on the cost and risk of radical innovation and that can take decades to replace old airplanes.

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Whichever new technologies do get implemented may not be enough to keep up with the industry’s growth. Each year the industry reduces fuel consumption by improving efficiency by 1.5 percent to 2 percent. But each year people fly more–the industry is expected to grow by 4 percent to 5 percent–overwhelming fuel savings from efficiency.

Part of the problem is that it takes the industry as long as 20 to 30 years to replace planes. This means that the efficiency improvements of planes introduced in 2010 won’t be seen throughout the fleet until 2025 or later. If things continue as they have been in recent years, by 2050 the industry will have to fly “three times as many airplanes with only half as many emissions,” says Ian Waitz, a professor of aeronautics and astronautics at MIT and director of the Partnership for Air Transportation Noise and Emissions Reduction. “It’s a tremendous challenge,” he says. The challenge is so great that climate-change policies may force a tradeoff–requirements to cut emissions may increase prices and slow the growth of the industry.

The aviation industry can limit its emissions in three basic ways–making airplanes more efficient, improving logistics to waste less fuel, and replacing fossil fuels with biofuels. But some potential technical improvements are limited because of the engineering requirements of airplanes. For example, it’s conceivable that batteries and electric motors could one day replace internal combustion engines in cars. But batteries don’t store enough energy to transport a commercial airliner across the Pacific.

With these limitations in mind, by 2020, new technologies could make aircraft about 20 percent to 35 percent more efficient, on average, than planes today. Fuselage coatings and adjustable wings, among other things, could reduce drag. Engines that run hotter and at higher pressures would use less fuel, as would engines that use gears to optimize the speeds of different parts of a turbine, and open-rotor designs that resemble and have some of the efficiency advantages of turboprops.

The weight of airplanes could be decreased by using composite materials, for example, or replacing heavy wiring with lighter fiber optics. Some of these advances have already been incorporated into new airplanes. Airbus’s A380 and Boeing’s 787, for example, are expected to reduce fuel consumption by something like 17 percent to 20 percent. Beyond 2020, the industry could employ more radical designs. It could abandon the ubiquitous tube-and-wing design of airplanes, for example, in favor of something called the blended wing. The entire airplane would essentially be a wing, increasing the amount of lift it generates and reducing fuel consumption by perhaps 25 percent. Such a design has been considered for many decades but has yet to be used for commercial aircraft, although a variant was used for the military’s B-2 bomber.

Improving flight logistics could shave another 8 percent off fuel consumption by 2020. With cooperation from governments, it may be possible for planes to fly more direct routes. Better air traffic control technologies could also reduce the amount of fuel planes waste idling on the runway or waiting to land.

Finally, advanced biofuels could decrease carbon emissions by about 5 percent by 2020, according to aviation industry estimates. The contribution from biofuels is highly uncertain, however, and existing biofuels–ethanol and biodiesel–won’t work in today’s airplanes for a variety of reasons. Ethanol simply doesn’t store enough energy, and it introduces safety concerns because it’s much easier to ignite than jet fuel. Biodiesel would require heating at cold temperatures, and, more important, it breaks down at high temperatures, says James Hileman, associate director of the Partnership for Air Transportation Noise and Emissions Reduction. That leaves only advanced biofuels, such as hydrocarbons that are almost identical to jet fuel and can be made by refining oils produced by algae. But so far these are very expensive and available only in small quantities. In the distant future, alternative fuels such as liquefied hydrogen might help, but large obstacles remain, including the difficulty of storing liquid hydrogen on an airplane.

There are many possibilities for reducing emissions, but even the aviation industry acknowledges they won’t be adequate to meet its goals. The industry will likely exceed its emissions cap by 90 million tons of carbon dioxide in 2025, says Quentin Browell, assistant director for environmental issues in aviation at the International Air Transport Association, the group that announced the emissions goals. To make up for this, it will have to purchase offsets–essentially paying other industries to reduce emissions.

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