The manufacturers of both auxiliary power units (APUs) and ground power units (GPUs) are reacting to the needs of their customers, who are under enormous financial pressures from the rising cost of fuel and environmental legislation. Lowering fuel consumption is the number one priority for airlines and a number of solutions are being tried, as David Smith finds out
Fuel makes up around 40% of an airline’s direct operating costs, a figure that rises to 50% in some parts of the world. Moreover, the aviation industry is increasingly being targeted by governments demanding cuts in emissions. The concerns of legislators are understandable, given the growing quantity of emissions from aviation. According to the European Commission (EC), they rose by 87% in Europe between 1990 and 2006, and by 2020 global emissions will be 70% higher than in 2005. By 2050 they could increase by a further 300%, according to the International Civil Aviation Organization (ICAO). Clearly, there is pressure to use the minimal amount of fuel.
Possible alternatives when it comes to improving fuel consumption by means of alternative power sources range from improving existing APU products or using GPUs to take over APU functions, to other more drastic steps.
APUs are one of the major sources of fuel consumption, so they are a prime target for innovative thinking. Perhaps the most radical solution has been proposed by Honeywell – a global giant in the APU market – which has come up with the Electric Green Taxiing System (EGTS), an innovation that the company claims will mean reduced costs, emissions and noise. It was successfully trialled at the Paris Air Show this summer and should be on the market in 2016.
Honeywell teamed up with the high-technology group Safran to design the EGTS at 13 facilities around the world, a project that involved the work of no less than 200 engineers. The motivating factor behind the initiative was the high proportion of airline fuel costs spent on taxi operations, which represent up to 6% of fuel consumption for short-haul fleets at congested airports. A single-aisle aircraft operates an average 2.3 hours on the ground during eight to 10 daily rotations. Short-haul aircraft burn an estimated five million tonnes of fuel per year during taxi operations alone.
EGTS technology allows aircraft to avoid using their main engines during taxiing and instead to taxi autonomously using electrical power. It is similar to the system used in a hybrid car when the vehicle switches to electric power at slow speeds. EGTS uses the APU generator to power electric motors in the main landing gear. Each powered wheel of an EGTS-equipped aircraft is fitted with a control system, so pilots can manage speed and direction during taxi operations.
The development has been well received in the aviation industry. Michel Laudy, short and medium-haul fleet director, Air France-KLM, believes that green taxiing could be of significant value to Air France: “For us, the fuel savings using an electric solution would be significant,” he notes.
“At Paris Charles De Gaulle (CDG) International Airport we typically have around 70 single-aisle aircraft operating on routes (of) less than 500 nautical miles every day. Around 60% of these fly less than 400 nautical miles. The taxi-out time per aircraft averages around 17 minutes. At Orly, it is around 10 minutes. It (electric taxiing) would also give greater autonomy to the aircraft and cut down handling costs.”
Meanwhile, Milan Cornelsen, director fleet and asset management at German low-cost airline TUIfly, says that EGTS would bring greater independence to the company’s fleet: “The independence comes because we would not need any push-back cart at outstations and destinations. If the push-back cart is defective or occupied, or the driver is on strike, our passengers currently have to wait for their departure. With an electric taxi system we would just need the marshaller to help guide us,” he observes.
The energy savings are significant because an aircraft’s main turbofan engines are optimised for flying rather than taxiing, so they burn a disproportionate amount of fuel during ground operations. For an A320, for example, making a 500 nautical mile flight, it is estimated that EGTS will save up to 4% of block fuel costs, compared with standard taxiing procedures. The environmental benefits are also interesting. One aircraft using EGTS is the equivalent of removing 400 cars from Europe’s roads in terms of fuel use and carbon dioxide emissions, or 300 if you consider nitrogen oxide emissions.
Alain Chapgier, corporate strategy manager at airport operator Aéroports de Paris (AdP), considers that the system would work at any airport: “It could work both during the dispatch and landing phases. It could also be used to move aircraft to maintenance facilities or parking stands.
“This saves time and money for both the airline and the airport handling teams,” he goes on. “As our average airport taxi-out time is around 25 minutes, we think any solution must have certain minimum parameters, such as a taxi speed of at least 20 knots with good acceleration up to 10 knots. It must also be able to deal with slopes and have good integration with the aircraft’s existing systems.”
Improving existing GPUs
Other APU manufacturers have tried less radical solutions that involve upgrading existing APU systems to make them more efficient. Eileen Drake, vice president of Pratt & Whitney AeroPower, points out that the market trend is to provide higher power APUs that are eco-friendly and less costly to operate.
She explains: “Our new APUs are more powerful than ever (in order) to accommodate the increased electrical requirements of new generation aircraft. For instance, our APS5000 APU for the Boeing 787 produces 450kW of electrical power, which is five times greater than the 767 APU. We are also developing an advanced APU product for Embraer’s second-generation E-Jet – the APS2600[E] – which provides more electric power capability at a significantly higher operating altitude.”
Drake says that the company’s customers are also demanding lower noise levels. She notes that Pratt & Whitney has taken measures to reduce APU noise, including making design changes to the compressors and combustor, as well as developing better acoustic suppression materials inside the installation inlet/ducts where the APU is installed in the aircraft.
The final demand from today’s customers is, of course, to reduce emissions. Pratt and Whitney’s new APUs, Drake informs, are made to withstand higher temperatures, incorporating advanced materials for hot section parts, because hotter temperature operations result in lower fuel burn. “The APS5000 is the industry’s first all-electric APU for large commercial aircraft. It emits 10 per cent fewer emissions and is 50 per cent quieter than current Boeing 767 APUs,” she enthuses.
A further trend in the market, she continues, is the move towards offering more integrated support solutions on an hourly basis. Pratt & Whitney has started to offer all-inclusive support, which covers maintenance and spares, at an hourly rate based on APU operational hours. “This arrangement allows operators to accurately predict operating costs and save money. We also offer 24/7 worldwide customer support to ensure timely product support and issue resolution,” Drake adds.
Looking to the future, Pratt and Whitney believes alternative sources of power could play an increasingly important role and the company has been ploughing a lot of resources into studying both electric fuel cells and plant-based fuels. “The hydrogen-based electric fuel cells look promising because they have the potential to benefit airplane efficiency and lower emissions,” she says. “We’re also looking at using plant-based bio-jet fuels and synthetic fuels. We’ve conducted significant tests, and plan further tests because we’ve found that our APUs can operate well with bio-fuels.”
Developments in GPU technology
Manufacturers of GPUs have also had to pay attention to the green agenda. AXA Power, Hobart, Houchin and Trilectron are all well-known brands that belong to Illinois Tool Works (ITW), a manufacturer that has introduced what it calls its ‘Go Green On Ground’ strategy. This involves getting external 400Hz solid-state power units and pre-conditioned air units (PCAs) to take over the role of the aircraft APU while the aircraft is parked at the gate.
The benefits of a concept such as Go Green On Ground are regarded as increasingly beneficial because many airport operators have restricted the use of aircraft APUs in order to reduce carbon dioxide emissions and noise. It also means substantial savings on fuel and APU maintenance for airlines. Industry reports suggest savings of as much as £100,000 (US$150,000) per year for each gate.
Poul Elvstrøm, AXA vice president international sales, remarks: “Go Green on Ground means aircraft captains can shut down the on-board APU as soon as possible after docking at the gate, and APU carbon emissions can be reduced by as much as 85%. For airlines, this is crucial in helping them to avoid exceeding their emissions quotas. If that happens, they can be forced to buy special emission permits, or accept fines for every excess ton of carbon dioxide emissions.”
A major development in the GPU world has been the change from aircraft requiring a power factor ratio of 0.8 to newer aircraft, such as the A380 and the B787, that require a power factor ratio of 1.0. As a result, instead of delivering 72kW in the case of a 90kVA converter, the requirement is now 90kW. “Year-by-year, we notice an increase in the market demand for 90kVA power supplies for airport installations,” observes Vibeke Bo Hansen, marketing manager at AXA Power.
Hansen sees a promising future for the GPU market because of the APU restrictions that are already in place at half of Europe’s airports. “The restrictions are driving the market away from the use of APUs to GPUs. In the Middle East and Far East, APUs are still commonly used, and also diesel units are widely used. But we see steps towards bringing in APU bans in those markets which will increase the demands for solid-state equipment of all kinds – not just GPUs and PCAs,” she concludes.
