A new generation of low- and no-carbon fuels and aircraft technologies are in the pipeline, which will make aviation greener, quieter, faster and more efficient in the not too distant future. As a curtain-raiser, Jon Herbert looks at an unexpected breakthrough that is turning carbon dioxide (CO₂) into a sustainable fuel.

Aircraft and airports are almost synonymous with large carbon, noise and air pollution footprints. Change is unlikely to happen overnight; the dream of all-electric flight remains just that, a dream. Even so, a series of crucial innovations could transform almost every aspect of flight in the foreseeable future.

Progress is being made on three fronts. More climate-friendly fuels are the first. Increasingly, efficient aircraft designs are the second. Importantly, a revolution in aircraft manufacture is the third and producing new exotic, ultra-lightweight, fuel-saving components that were not possible before.

However, on a wider transportation front, a chance US development has created a modern and successful technical version of the once futile quest for the Philosopher’s Stone which alchemists of old believed had the power to turn base metals into gold.

In October 2016, researchers at the US Department of Energy in Tennessee reported how they have almost accidentally discovered the secret of turning atmospheric CO₂ into ethanol. Essentially, they have reversed the combustion process — a monumental change that almost literally did occur overnight.

They are now working on the implications of what was meant to be only the first in a complex series of experiments investigating whether new catalysts could be effective in producing methanol as a forerunner. To their surprise, the team’s first attempt delivered ethanol at the extremely high conversion level of 63% to 70%. Could a new generation of aviation fuels be down the line?

First time lucky

The process based on nanotechnology uses copper particles embedded in tiny carbon spikes to trigger a complex chemical reaction producing ethanol in one go when an electric current is applied. Because the materials involved are relatively cheap, the team believes fully-scaled up technology could be used to store excess energy generated by wind, solar, tidal and other renewable sources.

In 2015, world fossil fuels CO₂ emissions totalled circa 38.2 billion tonnes. If industrialised successfully, the new process could produce a fuel that can be used by the world’s existing car, lorry and bus fleet while removing carbon from the atmosphere until it is re-released as vehicle exhausts. A zero-sum cycle.

Emphasising the team’s achievement, lead author of a paper on the experiment, Dr Adam Rondinone explained, “We’re taking carbon dioxide, a waste product of combustion, and we’re pushing that combustion reaction backwards with very high selectivity to a useful fuel.”

Another advantage of the team’s work is that liquid fuels have a high-energy density and are easily transported. Being able to produce such fuels sustainably is a potential game-changer that is being pursued by many research teams, but not with the simplicity achieved in Tennessee. The ultimate grail is to use sunlight as the green energy source making the reaction possible.

Hydrogen-powered flight

Meanwhile, the world’s first commercial hydrogen-powered flight has flown at Stuttgart Airport. Developed primarily by Germany’s DLR Institute of Engineering Thermodynamics, the HY4 is a four-seater aircraft using a hydrogen fuel cell that has a 750km to 1500km range and maximum and cruising speeds of 200km/h and 145km/h.

If a renewable energy source is used to produce hydrogen through the electrolysis of water, the aircraft can in theory at least operate with zero emissions. The HY4’s power system involves a hydrogen storage unit linked to a low-temperature hydrogen fuel cell which produces electricity with only oxygen and water as by-products. A lithium battery supplies peak energy loads during take offs and landings.

Powering trans-world passenger long-haul flights with hydrogen still poses formidable challenges which make conventional aircraft propulsion almost inevitable for a long time to come. However, as an interim, hydrogen-powered regional flights are now feasible and as the technology is refined further could be seen as “electric air taxis” for shorter local hops.

Virgin in Concorde’s footsteps

Concorde introduced the concept of supersonic flight for the public in 1969. However, its service was always loss-making and ended in 2003. Only 14 aircrafts ever flew commercially and had seating for just 100 passengers. Air friction heat caused the fuselage to stretch up to 10 inches during flight.

Concorde’s record Atlantic crossing was made in just under three hours. Cruising speed was 1350mph, more than twice the speed of sound, at a maximum height of 11 miles (60,000ft). With a fuel capacity of 26,286 gallons, the aeroplane wasn’t very sustainable.

Even so, Concorde seems to hold one of the keys to contemporary supersonic flight — small aircraft carrying small numbers of passengers who consider the time savings worth the price.

NASA, Boeing, Airbus, Lockheed Martin, and as of November 2016 Denver-based Boom, are looking at a modern high-speed service. Boom is backed by Sir Richard Branson and envisages aircraft with about half of Concorde’s seat numbers costing roughly the same as today’s business class fares.

By using available and tested rather than ground-breaking technology, Boom plans to be in the air by 2023; Virgin Galactic’s manufacturing arm is providing engineering, test and operational services and support. Virgin also has an option to buy the first 10 planes off the production line. The market will prove what demand there is for this much greener than Concorde-form of flight.

Blue water technologies

The next step in achieving sustainable flight is quite large. It calls for integration of not only more environmentally-compliant propulsion systems but also the evolving design of airframes linked to new manufacturing processes, such as 3D printing.

An example cited is marrying the efficiencies that can be achieved with innovative high-speed wing designs with the practical inefficiencies of aircraft that still have to land at low speeds with the unavoidable need for landing gear which ruins the aerodynamics.

Iain Gray, former Airbus UK Managing Director and now Cranfield University’s Director of Aerospace has been quoted as saying, “… the biggest thing we can do is look at the integration of the power plant and the airframe itself”.

The de Havilland DH 106 Comet was the world’s first commercial jet which made its debut in 1952. What marked it out were the narrow diameter air intakes for its de Havilland Ghost turbojet engines embedded within the wing. In contrast, the powerful engines of modern wide-body airliners are mounted on pylons. However, this trend won’t necessarily last. Integrated engines are more aerodynamic and fuel-efficient.

Airbus and Rolls-Royce are working together on a very advanced concept study called E-Thrust. This hybrid electrical flight system would see an advanced gas power unit generating the energy needed to power clusters of electric fans providing thrust spaced along the wings. It would also charge main batteries. On descent, the fans revert to being wind turbines putting power back into the aircraft.

The design’s strengths centre on a complex series of efficiencies. Until battery weights are reduced substantially, hybrid systems may be inevitable. However, it will be decades before they take to the skies.

Rolls-Royce is also focusing on another approach to green air travel. Its Advance engine design will have 20% better fuel burn and carbon emissions characteristics than the first version of its Trent engine and use a carbon titanium fan to save weight — the equivalent of seven or eight passengers — thus making the core of the engine more efficient. Its next innovation is UltraFan which is a geared design with a variable pitch system that will cut emissions by at least 25% by 2025.

3D lightweight

Emerging manufacturing, design and operational technologies are converging too. An example is 3D printing which it is predicted will have a major impact on aircraft environmental footprints.

3D printing allows the design of light, strong, minimum-weight components that perform as required but could never have been manufactured by conventional techniques. Higher initial costs are balanced out by greater fuel efficiency. When thousands of parts are involved, carbon and cash savings are substantial.

3D technology offers a fundamental gain. It turns a powder into high-performance components. This is an attractive option compared to the conventional alternative of taking metal blocks and “turning 95% of them into swarf”, according to 3D printer-maker, Renishaw, a leading engineering and scientific technology company. Renishaw is part of the Airbus-led WINDY project developing 3D technology for wind components in high-stress environments.

An important outcome could be lighter and more functional safety-critical parts. An example cited is turbine blades complete with small ventilation grooves channelling protective cooling air which allows engines to work sustainably at higher temperatures.

Better flight management

There is a further area where planes could be operated more effectively — better ways to pack more planes into a limited airspace and land them safely and quickly.

Air traffic controllers have suggested that separating planes by time rather than distance could cut delays for passengers, carbon and noise at the end of long journeys, with Heathrow as a test-bed. This is based on the idea that headwinds can alter the safe distance between aircraft approaching to land, allowing them to be bunched closer together. Some 65 days a year are affected by wind; circa eight lost aircraft movements could be saved every hour.

Another futuristic suggestion for managing the large number of flights around the world is to group airliners flying in closely formatted squadrons.

Taking agonising decisions affecting real communities on the ground and the UK’s over-stretched strategic airport and air capacity expansion cannot be avoided. However, the technologies involved are not standing still.

Published by Croner-i on 4 January 2017