Aviation cracking environmental challenges

Aviation International News » June 2009
June 4, 2009, 7:53 AM

With oil prices and financial markets so unstable, one could easily assume that global warming and alternative fuels are far from the minds of most aircraft operators. However, achieving sustainable growth in the aviation industry was the focus of a recent “Greener by Design” lecture hosted by London’s Royal Aeronautical Society (RAeS).

Many of the current environmental initiatives were under way long before the economic crisis started and they will continue well after the dust has settled. However, there is growing realization in aviation that the industry must do more than address environmental issues; it must also communicate to the general public the changes that it is making.

David Rowland, president of the RAeS at the time of the meeting and a former Concorde pilot, summed up the problem this way: “We are misunderstood as an industry…we need to win the soundbite battle with the young and the media.” He then pointed out that airplanes fly only because people want them to, noting, “We don’t just put aircraft on routes for the sake of it; we do it because people want it.”

One of the challenges of addressing environmental issues is communicating to the public the nature of the changes the industry has made and is in the process
of making. Aviation has struggled against environmental lobbyists, who have often won the public relations battle, and decades of advancing aerospace technology has been under-represented.

It was therefore enlightening to hear Carl Hodges, something of an environmental guru for the past few decades and now chairman of Global Seawater, an environmental organization, single out aviation as the industry that is doing more than any other to address environmental challenges. “Aviation is doing everything right, from everything I’ve seen,” he said.

Costs versus Benefits
Atmospheric scientists are making great efforts to better understand the effects of jet emissions on the environment and to put everything in context. A key aspect is that engine designers are often forced to make tradeoffs; for example, as they reduce fuel burn, with hotter combustors, NOx emissions increase.

Professor Keith Shine of the Department of Meteorology at the UK’s Reading University told the conference that below 60,000 feet NOx forms ozone but destroys methane, by forming the hydroxyl radical–“the detergent of the atmosphere.” Therefore it has both bad and good effects. “But the impact depends on both height and location; the ozone effect is local but the methane effect is global because it has a far longer life.”

Engine designers have been attempting to reduce fuel burn, and thus CO2, while minimizing increases in NOx emissions. John Moran, corporate specialist for combustion with Rolls-Royce, said that airplane engine designers have been reducing the fuel burn of engines at an average rate of 1 percent per annum. But with rich-burn combustors now highly optimized, Rolls-Royce has been developing lean-burn technology. “With rich-burn we are looking at better fuel injectors, cooling, everything–to break the CO2/NOx seesaw…but there is a limit. So we have been researching burning on the lean side for about 10 years. Land-based plants can put out about 10 percent of the NOx [of an equivalent aero engine],” he added.

John Green, a founder of Greener by Design, noted that a small increase in fuel burn leads to a large drop in NOx. In fact, if the temperature is increased from current levels to reduce fuel burn by 1 percent, the NOx increases by 15 percent. He added that flying lower and slower would improve efficiency, “but there is the question of engine pressure ratio and then cruise Mach number. These are not easy questions. What if you told airlines to start flying around at Mach 0.7, and then had new, slower aircraft mixing with the existing fleet of faster aircraft?”

The focus with NOx, however, as Moran pointed out, has been on emissions around airports (although as Roger Wiltshire, secretary general of the British Air Transport Association, said, the NOx issue at low level could largely go away once ground transport is no longer emitting it).

The best way to reduce NOx, according to Moran, would be to inject water into the engine exhaust; it would require about one metric ton per flight for a 747. But this would mean greater complexity and weight, which affects fuel burn.

Dr. Mark Taylor, senior project engineer with Rolls-Royce, said that the company is making good progress with open-rotor engines, which offer a far more efficient solution for the next generation of narrowbodies. Between Mach 0.7 and 0.8 they have a large fuel-burn advantage (of the order of 20 percent) “because they remove the drag and weight of the nacelle and allow a bigger fan for the same core.” Noise is an issue, but high oil prices might justify taking a closer look at open-rotor engines–last evaluated when oil prices spiked in the 1980s.

This is despite the many challenges, continued Taylor, such as “certification, transmission heat, prop construction, reliable pitch-change mechanisms and integrating 14-foot props onto the aircraft.” Rolls-Royce has optimized blade numbers for the front and back fans of an experimental propfan, and although there are different numbers of blades Taylor declined to reveal the numbers, or the spacing, which is critical to minimizing interference noise.

Tests have been carried out as part of the EU 7th Framework Dream project with open-rotor noise testing at DNW (the now-merged German and Dutch wind-tunnel research bodies) on a one-sixth-scale, 28-inch-diameter rig. “The big news is that test results are in line with predictions and Rolls-Royce believes it could produce a viable open-rotor engine. The question now is whether to go for a geared turbofan, advanced turbofan with low noise or open rotor with low emissions,” Taylor said. Testing will now continue at ARA Bedford in England, he said.

Open rotors (or “propfans”) would enable the fabled Acare targets (see box below) to be met far more easily, while it will be “difficult for existing engines to meet them,” believes Taylor. For the 100- to 200-seat sector, on current fleet growth projections there will be a fourfold increase in CO2 emissions by 2040, but if open rotors were introduced in 2018 the 2040 level would be “half that…and equate to a $3 million fuel saving per aircraft per year,” added Taylor.

More efficient aircraft also offer benefits in terms of emissions trading schemes– important in the context of the European emissions trading scheme (ETS), due to be introduced in 2012, and a global scheme possible farther into the future. To this end, Wiltshire of BATA reported that ICAO is moving ahead with its new Group on International Aviation and Climate Change, which has had four meetings so far.

The challenges to implementing a global ETS are huge, he pointed out. First, various countries can block progress, and international emissions are “economically sensitive”–especially over the respective contributions of developed and developing nations. Meanwhile, the European ETS will have to be modified to be more widely acceptable.

Of the international climate control conferences linked to the Kyoto Protocol and its replacement after 2012, “Let’s see if the road from Bali 2007 to Copenhagen 2009 takes us further forward,” said Wiltshire. The United Nations Climate Change Conference will take place in Copenhagen from December 7 to 18.

Contrail Cirrus
Dr. Hermann Mannstein, a scientist at the German Aerospace Centre (DLR), told the conference that the warming effect of contrail cirrus (rather than just linear contrails) could be far greater than currently perceived, but that “because the uncertainty is so large it has been left out of the total contribution [of aviation to climate change].” The radiative forcing contribution has been estimated at around 30 mW/m2 but with an error band from 10 to 80 mW/m2, said Mannstein.

“The large uncertainty is due to poor statistics. Contrail images [from satellites] are the smoking gun and with increased efficiency of aircraft there is an increasing probability of forming contrails.” Flight planning could help, he suggested, as the “super-saturated regions of the atmosphere are usually shallow and limited in size, so could be avoided, but good forecasting of moisture content would be necessary.”

Mannstein told delegates that during the day the reflection from contrails negates the warming effect, so the real net warming effect occurs at night. He hypothesized that there could be a “penalty on contrail production,” but pointed out that this could be impractical “because surveillance is extremely hard.” In addition, he said, “you would have to weigh the use of different flight routes [against] the additional fuel used.”

While Acare has not really considered contrails or contrail cirrus, Mannstein said that by 2012 the EU-IP Quantify and UK Omega programs should have produced some results, so that the contribution of contrails can be better understood.

Increasing management efficiency is perhaps the most tangible way of reducing aviation emissions. Eurocontrol head of environment Andrew Watt, having seen the aircraft-induced cloud cover on satellite images, agreed that ATC “might be able to help. We are just integrating climate change into our work but contrail cirrus is still a bit esoteric,” he said.

With the less esoteric aspects, Eurocontrol has developed Tesa (Toolset for Environmental Sustainability Assessment). “We are gradually building emissions issues into our planning, and for the first time last year the global aircraft emissions model [part of Tesa] was integrated into the planning suite, saving 10,000 nm distance flown per day–equivalent to around 20,000 tons of fuel per year.”

However, Watt pointed out how important it is to have a good understanding of the impact policy decisions could have. Eurocontrol is collecting vast amounts of data, which it archives daily. “We’ve already put emissions algorithms in there, and ran a study which showed that if you cut out 30 percent of the traffic it would remove only 4 percent of the fuel. This showed– in prototype form–that we will be able to assess the real impact of policy choices.”

“The development of functional airspace blocks [FABs] is [also] very important,” added Watt, “so there is less of a sovereign and more of a cooperative approach.” The goal is to phase in these airspace blocks over a five- to 10-year period beginning in 2012. This will mean fewer ATC authorities will be responsible for any one region. “For example, the English Channel will be covered by just two FABs, and you won’t have lots of different countries dealing with it.”

Cleaner Fuels
In addition to burning less fuel, the other major environmental objective for aviation is replacing existing fuels with cleaner ones that can do the job just as well. This is a tall order, delegates heard, but a surprising amount of progress is being made. Graham Ellis, business manager for renewable energy with Honeywell’s UOP subsidiary, said, “halophytes [salt-tolerant plants] are the crude oil of the future…. Now we just need to turn them into jet fuel.”

UOP is developing the necessary processes for use by the refining industry. This will take plant oil through several steps, said Ellis, including deoxygenation, selective cracking/ isomerization, and the addition of aromatics to reach something that can meet the strict jet-A1 specification.

Ellis emphasized that a thorough understanding of the environmental benefit requires a look at the entire energy cycle, not only the burning and emissions aspects. “Today we waste 25 percent of the energy actually making kerosene…oils such as jatropha are less efficient fuels but there is much less energy put in [to producing them] and they are far cleaner when you burn them.”

Chris Lewis, specialist for fuels with Rolls-Royce, said that Rolls is looking at sustainable alternative fuels and that “the challenge is to make the engine perform still, as there are lots of demands placed on the fuel. It needs high energy density and it is also a working fluid that must withstand high temperatures without coking up the engine.” Lewis said that Rolls is developing a test rig with England’s University of Sheffield to evaluate candidate fuels as they are developed.

In addition, Rolls-Royce, Boeing and Air New Zealand recently flew a 747 with one engine (RB211-524) powered by 50 percent biodiesel, and Rolls and British Airways plan to evaluate five candidate fuels, also using an RB211-524. “There will be jet-A1 as the base and four novel fuels,” he said. “BA has put out a request for alternative fuel candidates and we will downselect from that.

“The industry focus at present is on a ‘drop-in’ fuel [for example, synthetic and not necessarily sustainable] but rapidly emerging are biomass to liquid, hydrogenated fats and vegetable oils, algae, jatropha and so on.”

Sian Foster, corporate sustainability and responsibility manager with Virgin Atlantic Airways, told the conference that its much-publicized flight of a Boeing 747 in February last year with one engine powered by 20 percent biofuel was only the start for the airline, which has set itself a target of 2020 for improving fuel efficiency per revenue tonne kilometer by 30 percent over 2007.

“That was just the start of it. Second- and third-generation biofuels are looking much more promising. Virgin anticipates up to 5 percent of its fuel uplift being from biofuel by 2015, and at least 20 percent by 2020, but it is difficult to act unilaterally; we need to achieve critical mass within the industry,” said Foster. This, she added, was the reason for launching the Sustainable Aviation Fuel Users Group in September. (UOP is a member, as is Boeing).

The conference saw other proposals such as air-to-air refueling, an idea being developed by Dr. Raj Nangia of Nangia Aero Research Associates. As Greener by Design’s John Green has often pointed out, stopping halfway along a long-haul route to refuel could save fuel, because the aircraft is not carrying the fuel for the second leg on the first. Nangia has taken this a step further, and told delegates how net fuel burn could be slashed by 30 to 40 percent by using in-flight refueling (compared with only 5 percent for interim stops). The savings, he claimed, would dwarf the cost of the refueling fleet.   

Advisory Council Targets

Acare (the Advisory Council for Aeronautics Research in Europe) was launched at the 2001 Paris Air Show to support the January 2001 paper, European Aeronautics: A Vision for 2020, from the so-called ‘group of personalities’ in European aviation.

The strategic research agenda set a target of 2020 for engine advances to contribute a 20-percent reduction in CO2 per passenger-kilometer, an 80-percent reduction in NOx emissions and a 10-dB reduction in noise per certification point.  

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