Green skies ahead: charting a course to sustainable aviation

18 July 2014
By Edd Gent
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The E-Fan all-electric demonstrator flies above Farnborough International Airshow earlier this week

The E-Fan all-electric demonstrator flies above Farnborough International Airshow earlier this week

The E-Fan is powered by an array of lithium-ion batteries, which drive two ducted variable pitch fans

The E-Fan is powered by an array of lithium-ion batteries, which drive two ducted variable pitch fans

Airbus' E-thrust concept is for a plane powered by a hybrid distributed propulsion system

Airbus' E-thrust concept is for a plane powered by a hybrid distributed propulsion system

Distributed propulsion allows for a more aerodynamic airframe

Distributed propulsion allows for a more aerodynamic airframe

Rolls-Royce is one of the companies involved in investigating open rotor technology

Rolls-Royce is one of the companies involved in investigating open rotor technology

The TaxiBot robotic tug system could cut taxiing emissions by up to 84 per cent

The TaxiBot robotic tractor system could cut taxiing emissions by up to 84 per cent

While most industries find doing their part to combat climate change a burden, for aviation there is an enviable alignment between cutting costs and CO2. Better fuel efficiency reduces both emissions and the single largest cost to the industry, but the sector is still looking for a game changer that could realise the dream of cheap green air travel.

Worldwide, flights last year produced 705 million tonnes of CO2, according to industry coalition the Air Transport Action Group (ATAG), a figure that represents 2 per cent of all human CO2 emissions and 12 per cent from all transport sources. The group also estimates that the industry gets through roughly 1.5 billion barrels of fuel a year, and with prices currently hovering around $120 a barrel this represents a hefty bill.

Analysis by the United States Department of Transport has shown that the cost of labour was two-and-a-half times that of fuel in 2000, but by 2010 they had reached parity at roughly 25 percent. The view in industry is that this trend has continued and has seen fuel become the biggest single operating expense for carriers.

With this background it is unsurprising that the big headlines in aviation these days are generally centred on efficiency gains. Boeing’s 787 Dreamliner began operations at the end of 2011 to much fanfare, with its primarily composite airframe and electrical flight systems being praised for delivering 20 per cent greater fuel efficiency than the 767 it was designed to replace.

At this year’s Farnborough International Airshow, the big news was the unveiling of Airbus’ A330neo, an update of the older model and a direct challenge to the Dreamliner.  The design’s aerodynamic rework, new Rolls-Royce Trent 7000 engines and increased capacity, will achieve a 14 per fuel burn reduction compared to the older model but at a considerably lower cost than its American rival.

But while the figures look impressive, ATAG also note that modern jets are already 70 per cent more efficient than they were in the 1970s. As efficiency increases the impressive percentages actually represent diminishing returns in real terms. According to Eric Dautriat, executive director of the €1.6bn EU-backed aeronautical research program Clean Sky, the industry needs to be looking for “game changers”.

“There is still room for efficiency and incremental steps, which can also be part of such a program like clean sky,” he says. “But if we consider what should happen before 2050 to reach the goals set by the air transport community we should also consider major breakthroughs in the design of engines and aircraft.”

Breakthrough efficiency

Clean Sky was founded in 2008 by the European Commission and the European aerospace industry as a public-private partnership designed to be the main contributor to goals set out by the Advisory Council for Aeronautics Research in Europe in 2001 to reduce aviation emissions by 50 per cent by 2020 – goals which were amended in 2011 to a cut of 50 per cent below 2005 levels by 2050.

The scheme has been focussed on taking innovative ideas from the lab and getting them to the demonstrator phase, with the majority of the 20 or so projects being worked on expected to be ready in the next two years – among them highly aerodynamic wing designs that take advantage of laminar flow, all electric aircraft architectures, flight path optimisation technologies and diesel engines for helicopters.

The most “emblematic” of the scheme’s programmes, according to Dautriat, is the contra-rotating open rotor (CROR) engine – a cross between turboprop and turbofan technology that features two sets of uncased blades that rotate in opposite directions – which could cut fuel consumption by 30 per cent compared with current aircraft.

Manufacturers have been squeezing extra fuel efficiency out of their turbofans by increasing the diameter of the fan allowing for a greater bypass ratio – the proportion of air that passes just through the fan stage of the engine rather than the turbine at its core – which improves efficiency. But fan diameters have almost reached their limit as after a point the engine’s weight and drag outweigh the increase bypass ratio.

By removing the ducting around a traditional turbofan the open rotor design spins in open air allowing engineers to optimise the bypass ratio. The trade-off though is increased noise, a major problem in the face of increasingly strict regulation on this issue and one that will require a great deal of research to overcome.

Ground tests of the engine will begin in early 2016, with flight tests to follow in 2019, but the fact that the concept has been around since the 1980s is indicative of the challenge Dautriat says the industry faces in getting game changing technology to market.

“Everyone has known these things for decades and plenty of ideas have been proposed at concept level, but what is at stake is to make them pay. This is why Clean Sky is focussing on high technology readiness levels,” he says. “It’s really about de-risking new technology.”

Alternative propulsion

But what if rather than trying to increase the fuel efficiency of engines, you do away with fossil fuels altogether? Airbus Group is planning to introduce an all-composite electrically-powered two-seater training aircraft called E-Fan by 2017 after carrying out its first public flight earlier this year, followed by further demonstrations at this year’s Farnborough Airshow.

The zero-emission aircraft relies on an array of 250 lithium-ion batteries made by KOKAM to power two 60kW electric motors to drive two ducted variable pitch fans. So far the demonstrator has completed more than 13 hours of airborne testing, reaching an altitude of 2,000ft, a 160mph cruise speed and roughly 60mph take-off speed.

Range is a problem for the E-Fan – its longest flight so far has been 37 minutes and it is only designed for a flight time of a hour and a quarter – but Dr Detlef Müller-Wiesner, head of E-aircraft at Airbus, is confident the aircraft is more than just a vanity project. “There is really a market, especially for basic pilot training because the first 20 hours of the curriculum what you are doing is just take off, touch down, turn around and do it again, and this is ideal for that,” he says.

The fact that the plane sounds like a “flying hairdryer” could be particularly attractive for flying schools, which are subject to strict restrictions on flying times due to noise, and Müller-Wiesner claims the aircraft should provide a lower total cost of ownership. The company is planning a family of aircraft, including a hybrid four-seater, which combines the battery technology with a combustion Wankle rotary engine as a range extender.

While the company is confident of the commercial success of the E-Fan, they have bigger plans for their electric propulsion research programme. Airbus has teamed up with Rolls-Royce and Siemens to investigate the possibility of a hybrid distributed propulsion system for aircraft that could be ready by 2050 as a stepping stone to fully electric airlines.

Their E-Thrust concept would see a single large gas turbine positioned at the back of the plane and an energy storage system that would share responsibility for powering an array of electrically driven fans. As the fans would be smaller than conventional turbofan engines they could be integrated into the airframe, allowing a significantly more aerodynamic design.

In take-off and climb the two power systems would work in concert, in cruise the gas power unit would provide cruise power and replenish the energy storage system, in the early stages of descent power will be provided from just the energy storage system and in the final stages of descent the fans will windmill to replenish the energy storage system.

Battery energy densities will have to be greatly increased for the concepts energy storage system to be effective, and those behind the project are pinning their hopes on lithium-air batteries currently under development but not yet commercially available.  The commercial realisation of superconductivity will also be vital, as this is a key enabling technology for the cables, generators and motors that will distribute power form the turbine, but Müller-Wiesner is optimistic. “It’s not a dream like nuclear fusion,” he says. “It’s existing today you only have to work on the industrial application, how you construct a superconducting engine.”

Sustainable fuel

For others though, this is blue sky thinking and that there are easier wins to be had. Farnborough Airshow also saw the launch of a new report by Sustainable Aviation – a coalition of UK airlines, airports and manufacturers – that predicted that sustainable aviation fuels could reduce the UK aviation’s carbon emissions by 18 per cent by 2050, a doubling of the groups 2008 estimate of a 9 per cent reduction by 2050.

While current sustainable jet fuels are roughly three times the price of standard fuel, according to chair of sustainable aviation and British Airways head of environment Jonathan Counsell, the group’s increased optimism has been driven by rising costs of conventional fuel and progress on the process of developing sustainable alternatives.

The BA backed Solena Fuels waste-to-fuel plant in Essex is due to open in 2017 and the firm is backing it to prove the technology can be cost competitive. But while there are government incentives for biofuels used for ground transport, the same subsidies don’t exist for aviation fuels, something the group believes has to change

“All we’re asking for is a level playing field with ground transport fuels. Our message to government is that aviation doesn’t have an alternative to aviation fuel,” says Counsell. “The most critical period the industry faces in terms of the development of alternative fuels is now. The maximum risk is in the next 10 to 15 years because this is using essentially using new technology. These are first-of-a-kind plants so there is a massive risk. The biggest challenge we face is attracting the necessary investment to build these plants. Once we show the plants work the risk tails off and you can ramp up fairly quickly.”

Low hanging fruit

Calls for government intervention and funding are common in the green aviation sector as most solutions are R&D intensive and carry little short or even medium term benefit for manufacturers, but there is still some low hanging fruit waiting to be plucked.

A Boeing 747 burns a tonne of fuel every 17mins it is on the ground with its engines running, while an Airbus A380 gets through three in an hour, and for each tonne of fuel burned roughly 3.2 tonnes of CO2 is produced. Israel Aircraft Industries claims its TaxiBot system can cut 84 to 85 per cent of fuel burn during taxi by using a semi-robotic tug to get planes from the gate to the runway.

As of this month the system is certified for use with Boeing 737s in Europe and Israel, with US certification to follow shortly due to bilateral agreements between aviation regulators. The system should be certified for use with Airbus A320s by September which TaxiBot program director Ran Braier says should account of 70 per cent of the market.

A driver uses the tractor to push the aircraft back from the gate before handing control to the pilot who controls acceleration of the vehicle from the cockpit. The plane’s nose landing gear rests on a piston-like energy absorption mechanism that is coupled with a sensitive control system to detect when the aircraft breaks are applied, allowing the tractor’s electric motors to proportionally decelerate. Pilot control overcomes safety issues of shared control of the aircraft that scuppered experiments with tractors in the 70s and 80s, while using the aircraft breaks rather than breaks on the tractor prevents damage to the nose landing gear.

Braier believes the system could save the aviation industry $1bn and 5 million tonnes of CO2 a year. The certification tests were carried out at Frankfurt International Airport on a Lufthansa 737 and Gerhard Baumgarten, from the company’s LEOS ground support subsidiary, is equally enthusiastic. “Technology has reached a certain limit and it is very difficult to save more fuel, aircraft are very efficient now,” he says, “With TaxiBot it’s a huge step forward, a quantum leap from our point of view, to save fuel.”

There are other less obvious solutions for reducing aviation’s environmental impact, such as the Single European Sky ATM (air traffic management) Research project, which started in 2004 and aims to overhaul and standardise Europe’s air traffic management by 2020. While the primary goals are not sustainability, side effects of the system such as flight path optimisation, increased capacity and better airport planning will all reduce fuel burn and therefore emissions.

“I don’t think there is going to be one key technology. There’s no one place from which a major evolution shall come from,” says Clean Sky’s Dautriat. Just last week his organisation issued a call for proposals for Clean Sky 2, a second incarnation of the original project but this time with €4bn in funding.

The timescales for most of these projects to come to fruition will be decades into the future, but still, no matter how incremental the progress is Dautriat is at least glad to be operating in an industry where business success and sustainability are so closely aligned. “The objectives of Clean Sky 2 are twofold, environmental and competitive,” he says. “Fortunately they are not going against each other, they are working hand in hand.”

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