The Airplane - Jay Spenser [135]
Aviation’s future is inextricably combined with our perspective on and attention to the global environment. Ultimately, the world may need to fly less. In the meantime, aviation must “clean up its act” even as the rest of the world does.
Jetliners burn kerosene, which is the traditional fuel of turbine-powered aircraft.1 Since the jet age began, the world has learned how to build jetliners that use 70 percent less of the stuff per passenger seat. This progress continues with the 787 Dreamliner and the new engines developed for it.
Despite this trend, growth in the number of jetliners in service around the world threatens to significantly increase aviation’s total contributions to global warming in the coming years. Consequently, the industry is taking bold steps to further improve its environmental performance.
Like other transportation sectors, aviation is looking to biofuels to reduce emissions. Because the carbon dioxide in biofuels was pulled from the atmosphere when the fuel’s feedstock grew, it does not represent a net addition of CO2 to the atmosphere, in contrast to the carbon released by petroleum-based fuels, which was formerly underground.
Aviation is particularly challenging because jetliners need a fuel that packs a very high energy density and has inherent resistance to low temperatures. This eliminates ethanol as a candidate because it has a low energy density; an equivalent amount of ethanol would take a jetliner only half as far. It also eliminates biodiesel, which offers 80 percent of the energy density of kerosene but would solidify at cruise altitude, where temperatures typically hover around-60°F (-50°C).
Fortunately, energy experts are now evaluating a promising alternative: microscopic algae that have the ability to produce lipids convertible into a fuel closely resembling kerosene. These microalgae need only bright sunlight and CO2. They thrive in brackish water unfit for other use, making harsh desert environments ideal for their cultivation. Consequently, large-scale production of bio-jetfuel would not require arable land, thus avoiding the downside of biofuels that pit the world’s desire for energy against its need for food and forests.
Algae are far more productive than terrestrial plants grown for energy. Algal production in shallow ponds produces oil yields up to fifty times higher than oilseed crops grown on an equivalent area of farmland. Moreover, these algal cultures draw carbon out of the atmosphere so aggressively that their use is also being explored for scrubbing CO2 out of the industrial effluents of refineries, power plants, and other existing point sources of industrial pollution.
Intensive cultivation in enclosed plastic tubes called reactors holds out the promise of large-volume production as well as significant economies of scale. The aviation industry is working with other interested parties to see whether long parallel rows of these reactors, or other cultivation methods, might supply aviation’s future needs. Other potential biofuels are also being evaluated, as is the use of bacteria or algae that are genetically engineered to produce hydrocarbon chains. These might be the basis for a range of synthetic fuels that do not add new CO2 to the atmosphere.
The effects of other jetliner emissions aside from CO2 are also being addressed. Oxides of nitrogen (NOx) are the chief focus of concern. These emissions create ozone (O3), a potent greenhouse gas when released high in the atmosphere.2 Broad efforts are in progress to reduce NOx formation in jet engines. Water vapor, another emission, is only a mild greenhouse gas, but it freezes into long white contrails that promote the formation of cirrus clouds. The impact on the global environment of this cloud creation is another focus of scientific study.
Human inventiveness and technology translation will benefit aviation in many ways. One example that will soon be available is the fuel-cell auxiliary power unit, whose developers include Airbus and Boeing.
Fuel cells are devices that convert a fuel into electricity