Airliners now entering revenue service will be around for the next few decades, over which time forecasters expect the cost of kerosene to rise significantly. Higher oil extraction costs and likely carbon dioxide (CO2) emission limits will no doubt radically alter air transport economics. The industry will simultaneously have to drastically reduce CO2 emissions from aircraft engines and find alternative fuels for them.
Research scientists are already exploring ways to solve the economy-ecology equation. But no solution proposed to date sounds really promising. A forum organized in Toulouse last February by French aerospace science association Fedespace left attendees with little optimism.
Year after year, the volume of new oil reserves discovered does not match oil consumption. Production, after a peak, will decline inexorably. Pessimists see production peaking in just five years. Even the optimists give us only until 2050.
Meanwhile, oil extraction is becoming more technically challenging and, therefore, even more expensive. All this will certainly push up fuel prices to unprecedented levels, with recent industry projections showing rates as high as $100 per barrel (compared with today’s prices of around $50).
Aviation, along with many other industries, also needs to stop global warming by limiting greenhouse gas emissions. Some will say that air transport accounts for a mere 12 percent of all transport modes’ CO2 emissions. However, regulators will likely place air transport at the forefront of efforts to reduce CO2 output under the terms of the Kyoto agreement.
Since CO2 emission occurs proportionally to the volume of fuel burned, drastically reducing the fuel burn on commercial aircraft will prove vital.
Experts at France’s Académie de l’Air et de l’Espace already anticipate an impressive 30-percent improvement in aircraft efficiency during the 2002-2052 period. But the same experts also predict that traffic will grow by 6.5 times over the same time frame. Total fuel burn will therefore increase five-fold, should today’s paradigm remain unchanged.
The Advisory Council for Aeronautics Research in Europe has set a goal for a 50-percent reduction of aircraft CO2 emissions by 2020. This includes physical improvements to engines and airframes as well as air traffic management optimization.
Recently, the European Commission-funded CLEAN engine technology demonstrator completed a successful first series of tests. CLEAN aims to cut CO2 emissions by 20 percent. Among other new technologies, it features a heat exchanger and a lean premix prevaporized combustor.
According to Gilles Rollin, head of Snecma’s combustion department, improvements in engine efficiency can come from four sources. The first is to further increase the overall pressure ratio; the second to increase combustion temperatures; the third to boost the bypass ratio; and the fourth, to increase the aerodynamic efficiency of each turbomachinery component. The latter two options, however, have approached their limits, Rollin stressed.
Rationalizing air transport operations could also reduce fuel burn. For example, Marc Pélegrin, president of Fedespace, argued against ultra- long-range flights. He maintained that such flights are far from fuel efficient because, for a 8,000-nm leg, each pound of fuel in the airliner tanks requires another 0.7 pound just to carry it. Separately, he pleaded for less wasteful routes and air traffic patterns around airports.
He also identified fuel wastage during aircraft taxiing. “Some 330 to 380 pounds of fuel are burned for a 15-minute taxi,” Pélegrin pointed out. He advocated driving aircraft wheels with electric motors, arguing that auxiliary power units would burn only 68 pounds of fuel to produce the current needed for the same operation.
While few would argue against the need for alternative fuels, their production costs, world reserves, ease of use and energetic density can’t match those of today’s oil-based jet-A. Moreover, producing an alternative fuel must at least generate no more CO2 emissions than jet-A. Since producing an alternative fuel often requires a lot of energy, that goal will not be easy to attain.
Nonetheless, what alternative fuels could be available? Synthetic kerosene comes to mind first. One could obtain kerosene from hydrocarbons other than oil. According to Rollin, using synthetic fuels would require only minor adjustments to current engines.
The synthesis process is called Fischer-Tropsch. It can produce kerosene from natural gas, coal, oil industry refuse or biomass and could therefore present an option for air transport.
Of course, there are drawbacks. First, the process’ thermal efficiency at most can reach 50 percent. Second, it emits significant amounts of CO2. Third, the synthesized fuel costs at least twice the price of the hydrocarbon matter from which it is derived. At today’s conditions in France, it would cost $100 per barrel.
Hydrogen presents one possible alternative. However, hydrogen is an energy vector, not a source of energy. In other words, one has to make hydrogen by extracting it from water, then liquefied at very low temperatures.
As early as 1956, NASA flight-tested a Boeing 757 partially powered by hydrogen. Yet, some questions about hydrogen remain unanswered. We know little, for example, about the environmental effect of hydrogen-generated contrails.
More importantly, hydrogen production costs a lot. The liquefaction process itself consumes 35 percent of the hydrogen’s energy. And distributing hydrogen fuel requires heavy investment.
“Hydrogen can be considered as an alternative fuel in the long term only, assuming that a powerful and economical energy source is available,” Paul Kuentzmann, an energy specialist with French research agency Onera, concluded. He wonders whether nuclear energy could be that source.
Moreover, hydrogen is not as easy to use as jet-A. For example, fuel could no longer be stored in aircraft wings. In the event of an uncontained engine failure, the aircraft’s design must eliminate the risk of shrapnel perforating a fuel tank. Therefore, fuel needs to be stored in the forward and aft sections of the fuselage.
Nonetheless, Airbus has studied in depth the economics of a hydrogen-powered airliner. For a given payload, takeoff weights wouldn’t exceed those of its current aircraft. “The empty airframe would be heavier but the fuel is lighter,” Sébastien Rémy, an Airbus expert in propulsion technology, explained.
However, the higher volume of the fuel and the need to store it in the fuselage would require a bigger aircraft, increasing drag by 10 percent greater and doubling operating costs.
Of course, experts base their estimates on current market conditions, with prices at around $0.85 per gallon for jet-A and $5 per kilogram for liquid hydrogen. Kerosene costs could jump to $2 per gallon and technology may drive hydrogen costs down to $2 per kilogram, Rémy predicted. In that case, the operating costs would rise by only 15 percent.
Another cryogenic fuel, methane, could prove a suitable alternative to jet-A in many ways. Most notably, it emits 13 to 25 percent less CO2 than today’s kerosene for the same amount of energy. Using liquid methane is almost possible with current engines, Rollin said. “It would have little impact on the turbomachinery design,” he explained.
We can extract methane from natural gas–a relatively limited resource. It can also come from methane hydrates, found mainly in deep oceans, where abundant reserves exist. But it’s both difficult and risky to extract methane hydrates because the likely leaks would cause terrible greenhouse effects.
Organic fuels (or biofuels) have interesting qualities. They produce no more CO2 than the vegetables that absorb them during their growth. However, aircraft cannot use biodiesel (an organic fuel widely used in automobiles) because of its high freezing point: -15 degrees C (5 degrees F). Jet-A freezes at -40 degrees C (-40 degrees F). Nevertheless, Brazilian manufacturer Embraer has recently flown the Ipanema, a crop-dusting single-seater, with ethanol, which is produced from crops, such as corn, wheat and sugar.
Of course, the availability of arable land, or lack thereof, limits the amount of biofuel the world can produce. Replacing oil with biofuels would need more than twice the earth’s arable surface.
Engineers have also considered fuel cells, but most experts think they can’t work in aircraft. “Their weight would need to be divided by ten,” Rollin said. Smaller fuel cells still hold promise for onboard system power, however.
Considering all the alternatives, scientists seem to agree that synthetic kerosene presents the best alternative in the mid-term (30 to 40 years). At least it would allow for the continued use of conventional-technology aircraft. For the longer term, they put a cryogenic fuel (either methane or hydrogen) at the top of the list. “Providing its production has real benefits over that of synthetic kerosene, plus environmental advantages,” Kuentzmann stressed.