The International Air Transport Association (IATA) has set an ambitious goal of reducing climate-altering emissions from global aviation 50 percent by 2050. Meanwhile, air travel continues to grow rapidly and is expected to skyrocket from 2.1 billion passengers in 2006 to more than 3.3 billion by 2014. Among the options for reducing emissions are switching to alternative fuels and boosting aircraft efficiency. But are these options realistic, and if so, how soon can they happen?
One potential solution, cleaner-burning alternative fuels, is receiving considerable attention. Recent trials have confirmed the use of drop-in biofuels in aviation as technically feasible. Similarly, hydrogen has been used successfully to power light aircraft. But questions about the sustainability and scalability of these technologies remain, and it is unlikely that these fuels will provide significant CO2 reductions in the coming decade. Rather, more-efficient jet engines and the use of lighter composite materials hold the largest potential to reduce greenhouse gas emissions in the short-to-medium term.
Global aviation currently accounts for more than 2 percent of global carbon dioxide emissions, and rising demand for air travel, especially in Asia, will only increase emissions. In Europe, aviation-related emissions jumped 87 percent between 1992 and 2006, driven in part by the rise of low-fare airlines since the late 1990s. The Intergovernmental Panel on Climate Change (IPCC) estimates that the climate impact of aviation may be closer to 5 percent because emissions at cruising altitude are far more damaging than those at ground level.
U.S. standards currently include specifications for some types of aviation biofuels, which run the gamut from Biomass to Liquid (BTL) fuel, to Hydrogenated Renewable Jet (HRJ) fuel, to hydrogen. But how likely or desirable is large-scale use of these fuels, what technical barriers do they face, and how environmentally sustainable are they? Could the aviation industry achieve greater emission reduction by improving the efficiency of aircraft that use conventional fuels?
Several airlines are spearheading the search for alternative fuels. Richard Branson’s Virgin Atlantic completed a ballyhooed flight from London to Amsterdam in February 2008 running on a blend of 20 percent coconut and babassu methylester. Qatar Airways, Continental Airlines, and British Airways, among others, have since followed suit and tested various biofuels blends.
This enthusiasm for alternative fuels is not altruistic. Fuel prices have been on the rise, increasing more than 40 percent in the past year. Come January 2012, aviation also will be included in the European Emissions Trading Scheme (ETS), a move that the IATA estimates will result in the equivalent of a 19 percent increase in fuel expenses by 2020. Lufthansa Airlines expects costs in excess of $350 million per year. Biofuels could represent an opportunity for airlines to meet their emissions targets and reduce overall fuel costs.
Most biofuels, such as HRJ and BTL, are considered “drop-in” alternative fuels, meaning they require only minor modifications to jet engines. The technology required to produce the fuels is commercially available, and commercial-scale production of BTL fuels is planned for 2012. Similarly, small-scale production of HRJ fuel in existing biofuels plants began in 2010. Cost competitiveness is expected in 2–3 years for some kind of biofuels. Hence, the extent to which biofuels can be used to meet aviation emissions targets will depend less on technological feasibility and more on the sustainability of these fuels.
The degree to which biofuels could deliver lifecycle greenhouse gas savings compared with conventional kerosene depends heavily on the type of feedstock used. The savings could be up to 95 percent for BTL, 66–89 percent for new energy crops, and up to 98 percent for algae. Conventional biofuel crops, such as corn, involve higher emissions, however, and the use of energy-intensive fertilizers could eliminate 50–80 percent of emissions savings.
Lifecycle emissions savings are also lower if growing the biofuel feedstock results in direct or indirect changes in land use, such as the clearing of forests that absorb carbon. Hence, the level by which biofuels can reduce lifecycle emissions remains uncertain. The IATA assumes a 60–90 percent reduction in greenhouse gas emissions for BTL biofuels, but predicts up to 70 percent increase in emissions for HRJ fuels depending on the type of feedstock and where it is grown.
Even if biofuels could, in theory, compete with conventional jet fuels, there are questions about whether the limited supply of biomass should even be used in aviation. Virgin Atlantic, the United Kingdom’s second largest airline, burns 2 million tons of aviation fuel per year. Producing an equivalent amount of biofuels using soy and rapeseed would require one-third of the U.K.’s arable land, just to fuel Virgin’s fleet. Large-scale production of biofuels for aviation does not seem feasible unless the yield per hectare increases significantly, either by using higher-yielding crops such as jatropha, or by making technological breakthroughs in the use of algae for biofuel production.
Hydrogen represents another clean fuel alternative. Since 2008, the German Aerospace Center (DLR) has been conducting small-scale trials to prove the feasibility of the technology, yet significant technical barriers remain to using hydrogen in large jetliners. The energy density of hydrogen is one-quarter that of kerosene. This means that the fuel could no longer be stored exclusively in a plane’s wings and undercarriage, but would instead require huge externally mounted tanks that increase drag and reduce fuel efficiency. Producing hydrogen also requires large amounts of energy. If the electricity for this process came from coal-fired power plants, the lifecycle greenhouse gas emissions of hydrogen could exceed those of conventional aviation fuel.
So how can aviation clean up its act? The largest potential to reduce emissions in the short-to-medium term may well lie with increases in efficiency. According to the IATA, the aviation industry improved fuel efficiency 17 percent between 2001 and 2009 and aims to reduce fuel consumption by another 25 percent by 2020. By 2020, airlines will need to replace 5,500 planes, or 27 percent of the global fleet. Boeing and Airbus, with a combined marketshare well over 90 percent, have announced several new engine options for their most popular planes, the Boeing 737 and the Airbus A320.
Pratt & Whitney, CFM International, and Rolls Royce, the three largest engine manufacturers, have made significant strides in efficiency as well. Pratt & Whitney is betting on a “geared turbofan” design, which it claims is 10–15 percent more efficient than conventional jet engines. CFM International, a consortium of General Electric and the French manufacturer SNECMA, is developing a rival design called Leap-X.
In addition to increases in efficiency, airlines are eagerly awaiting redesigned versions of the 737 and A320 that use new and lighter composite materials. This could reduce fuel consumption by more than 25 percent. However, Boeing and Airbus are sitting on a backlog of 4,500 orders that will keep them busy well past 2015. As a result, neither manufacturer has much interest in spending billions of dollars to develop a completely new design.
In the short term, improvements in engine efficiency hold the largest potential to decrease aviation fuel consumption and reduce CO2 emissions. Lighter and more efficient types of aircraft are not likely to hit the market early in the next decade. Similarly, the use of biofuels or hydrogen in the industry will remain limited to small-scale demonstration projects for the foreseeable future. It appears that the aviation industry faces a difficult path ahead in achieving its goals for a cleaner, more sustainable future.