If the various emerging avenues the aero-engine industry is determinedly pursuing – such as allelectric, hybrid and hydrogen propulsion succeed, air travel could change beyond recognition in a few decades
The commercial aviation industry, in general, and aero-engine manufacturers in particular, are a rather anxious lot nowadays. Their worry is not just because passenger numbers have plunged dramatically due to Covid-19 pandemic. After all, travellers will surely flock back to the ticketing counters once the pandemic subsides. The concern is more because of intensifying pressures from governments and international agencies to reduce the environmental impact of aviation.
In 2019, the world’s airlines carried 4.5 billion passengers and emitted 915 million tonnes of carbon dioxide (CO2). While this amounts to only two per cent of all human-induced CO2 emissions and 12 per cent of emissions from all transport sources, there is heightened concern on two counts. First, because aviation’s true environmental impact is probably much greater due to non-CO2 emissions such as Nitrogen Oxides (NOx), soot and contrails produced at high altitude. Second, because aviation is projected to be one of the fastest-growing sources of CO2 emissions and therefore, is squarely in the crosshairs of environmentalists.
The aviation industry knows it has significant capacity for expansion and utilisation of existing and emerging techniques to decrease its carbon footprint. It is committed to cooperate with global efforts to avert climate change by achieving net zero emissions by 2050.
FOCUS ON FUEL EFFICIENCY
In pursuance of the industry’s 2050 goal, the focus of manufacturers is to make aeroengines ever more fuel efficient. This not only reduces emissions, but usually has the effect of lowering operating costs – truly a win-win proposition. Noteworthy examples include CFM International’s LEAP-1 family and Pratt & Whitney’s PurePower range of high-bypass turbofans.
LEAP-1 uses advanced design techniques, lightweight composite materials, special coatings and advanced combustion and cooling technology to boost fuel efficiency. Consequently, it provides 15 per cent lower fuel burn compared with its predecessor engine, the CFM56-5B. However, there is a limit to how much more fuel efficient conventional engines can become.
The aviation industry is committed to cooperate with global efforts to avert climate change by achieving net zero emissions by 2050
That is why Pratt & Whitney chose to develop a different architecture for its PurePower range – the Geared Turbo Fan (GTF). Although PurePower engines experienced some teething troubles, the complex Fan Drive Gear System (FDGS) has performed flawlessly, belying the fears of industry experts that it could be a significant area of risk. The engine that already powers five families of airliners has proved its worth by reducing fuel burn and carbon emissions by up to 20 per cent and by dramatically reducing noise. The PurePower PW1100G engine powering the A320neo has achieved a world-class dispatch reliability rate of 99.98 per cent. In May, Pratt celebrated the delivery of the 1,000th aircraft powered by GTF engines.
According to a company release, GTF engines have saved more than 1.8 billion litres of fuel and avoided more than 4.7 million tonnes of carbon emissions, while accumulating more than 8.9 million engine flight hours of experience. And this is only the start, because Pratt has orders and commitments for another 10,000 GTF engines. Future refinements could include greater thermal efficiency, use of advanced materials and other engine enhancements. Pratt is also collaborating with NASA to design engines of up to 18:1 bypass ratio compared with the 12 to 13 ratio of GTF engines. Ultimately, other engine manufacturers may have to switch to the GTF or some other architecture to achieve comparable fuel efficiency.
Across the Atlantic, Rolls-Royce is making good progress with its UltraFan, which will be the world’s largest aero-engine. Ultra-Fan has the world’s largest rotor blades made from Carbon Fibre-Reinforced Polymer (CFRP). With variants intended for both narrow-body and wide-body aircraft, Rolls-Royce is predicting as much as 25 per cent fuel efficiency improvement compared with first-generation Trent engines. The company hopes to complete a prototype by the end of 2021. The icing on the cake is that the first test run will be on 100 per cent Sustainable Aviation Fuel (SAF). The aim is to demonstrate that aero-engines can operate with SAF alone as a true “drop-in” fuel.
SAF have become a reality in the past ten years or so. When manufactured appropriately, SAF can be produced without creating stress on agricultural lands or depleting food crops. By the end of 2020, over 3,00,000 flights had been operated using SAF. At present, it is certified for blends of up to 50 per cent with conventional jet fuel. Yet the global SAF use amounts to only 0.1 per cent of total aviation fuel, mainly due to high cost and limited availability. In fact, it costs as much as three to five times the price of jet fuel, a figure that should drop as its use increases and production consequently rises. SAF production would need to be scaled up dramatically to meet a substantial part of the 500 million tonnes a year projected to be required by commercial aviation by 2050. However, this figure could prove to be an overestimate if efforts to replace the gas-turbine engines of future shortand medium-haul aircraft with radical new power plants based on electricity and hydrogen, fructify.
Electricity is looking increasingly attractive as an emerging non-polluting option. That is why several aero-engine majors as well as start-ups are getting serious about it. In June 2020, Seattle-based MagniX succeeded in flying the biggest commercial battery-powered aircraft so far – a nine-seat Cessna Grand Caravan. Washington-based Eviation is getting ready for the first flight of Alice, an all-electric, nineseat luxury plane. Alice has a huge 820-kWh battery pack for an impressive 800 km range without recharging.
While fossil fuels are still the predominant power source, electricity and hydrogen are emerging as viable propulsion options
The main challenge currently is the low energy density of lithium-ion batteries – about 250 Watt-hours per kilogram (Wh/kg) against about 12,000 Wh/kg for jet fuel. While this makes electricity impracticable as an option for large and heavy planes, there are viable prospects for operating regional aircraft with up to 19 seats over distances of less than 400 km. Even if electric propulsion remains restricted to short-haul routes, it would have a tremendous impact on the airline industry.
Another option being actively pursued is hybrid propulsion. Jet engines would provide boost power for takeoff while batteries or fuel cells would provide steady power for cruise. General Electric expects to demonstrate a new hybrid-electric propulsion system for regional airliners by the mid-2020s.
HIGH ON HYDROGEN
Hydrogen propulsion for aviation poses a different set of problems. Hydrogen has thrice the energy density of aviation fuel, but occupies between three and four times the volume. It has to be stored at great pressure at minus 253 degrees centigrade – a tough prospect for an airliner. However, some reputed companies are trying to solve these problems. Germany’s MTU Aero Engines in partnership with DLR Aerospace Research Centre is converting a 19-seat Dornier 228 regional airliner using fuel cells that they are jointly developing. They hope to conduct the first flight of a technology demonstrator in 2026. One of the Dornier’s two turboprop engines will be replaced by a 500kW electric motor, powered by electricity produced by hydrogen fuel cells. In September 2020, Airbus also announced plans to bring hydrogen-powered airliners to the market by around 2035. One of the three options under consideration is a blended-wing airliner that could carry up to 200 passengers on flights of around 2,000nm.
Companies such as California-based Universal Hydrogen and ZeroAvia are also looking to convert existing 40- to 60-seat regional airliners such as the Bombardier Dash 8 or ATR 42 to hydrogen propulsion. If their efforts are successful, the planes could be ready for commercial service in the next five years or so.
LIFECYCLE SUSTAINABILITY – THE KEY
The aviation industry’s approach throughout its 120-year history has always been to refine existing aircraft and develop new and better ones. While fossil fuels are still the predominant power source, electricity and hydrogen are emerging as viable propulsion options.
However, a key consideration is that the electricity that powers all-electric aircraft, needs to come from environmentally friendly sources, no coal-fired or gas-fired generation plants, for instance. The same holds good for hydrogen propulsion where the method of hydrogen production is critical in assessing the full lifecycle sustainability of the fuel. Today, most hydrogen is designated as “brown” because it takes electricity to produce and that electricity often comes from an environmentally unfriendly source. “Green” hydrogen refers to hydrogen produced through electrolysis (separating water into hydrogen and oxygen) using electricity from renewable sources.
If the various emerging avenues the aero-engine industry is determinedly pursuing - such as all-electric, hybrid and hydrogen propulsion succeed, air travel could change beyond recognition in a few decades. And the industry would have done its bit to avert climate change.