El.fly - Glimrende dybdeartikkel i AW&ST

Etter å ha lest dette er det åpenbart for undertegnede at elektrifisering av det norske kortbanenettet i 2030 er prematurt. Det er en spennende utvikling på gang, men det kan være at naturen selv legger hindringer i veien a la elektriske utladninger og lyn, uansett om flyskrog er av kompositter eller aluminium. Det norske kortbanenettet er som vi vet, i et område av verden hvor været kan- og vil skape forstyrrelser for dagens trafikk. Det vil altså ta veldig mange år før elektriske fly vil kunne takle dette. Jeg tror at fremtiden ligger i hybride løsninger. (Red.) Electrification Will Take Different Forms Across Aviation Sectors Graham Warwick Will aviation be electrified? The answer among industry executives is an overwhelming “yes,” but in different ways, on different time frames, in different markets. As the technologies for electric propulsion advance, it is becoming clear one size will not fit all—and that may be a good thing. Civil-aircraft propulsion today is stratified broadly into piston engines for short-range general aviation (GA), turboprops and small turbofans for regional and business aviation, and large turbofans for medium- and long-haul commercial air transport. Electric propulsion of the future promises to be much more varied, ranging from all-electric through increasing degrees of hybridization to fully turboelectric. Hybrid-electric propulsion looks viable in regional market Megawatt-class electric technology could have wider use Electrification is expected to affect all classes of aircraft Reasons are not just the readiness of technologies for electrification, but their performance potential relative to conventional propulsion. Large turbofans powering long-range airliners are already highly efficient, making it hard for electrification to buy its way onto aircraft. But clean, quiet, reliable electric propulsion could find a place relatively easily in short-range GA. And the middle ground of regional aviation could prove fertile for hybrid-electric propulsion. As the technologies mature, industry is beginning to make these distinctions as it comes to grips with the promises and challenges of electric propulsion. The promises include lower energy costs, emissions and noise as well as configurations in which distributed electric propulsion improves aerodynamic efficiency or enables novel capabilities. The challenges include the energy density of batteries; the power density of generators, inverters and motors; thermal management; and the risks of high voltages at high altitudes. While electrification of aviation may have begun with light sport aircraft and entrepreneurial startups, the fact that established aircraft and engine manufacturers are taking the trend seriously was made clear at the American Institute of Aeronautics and Astronautics’ Aviation 2018 conference in Atlanta and Propulsion & Energy conference in Cincinnati in July. “For us, electrification is real,” says Mike Mekhiche, global head of Rolls-Royce’s electric team. “Drivers can vary from capability on the military side to efficiency, environmental reasons, as well as the value proposition around time-saving.” And he is not talking about the distant future. “The technology available today allows us to deliver flying platforms that are either all-electric or hybrid electric,” he says. “It is a question of timing, and it goes by segment,” says Alexander Simpson, leader of GE Aviation’s hybrid-electric propulsion team. “It’s not just a question of technology. It’s also a question of market, business case, public perception and regulatory readiness. This transition is to some degree already underway,” he says, citing the more-electric Boeing 787 and Lockheed Martin F-35. “But now we are talking about distributing energy for propulsion, which is a different proposition.” Industry thinks in classes of aircraft, from short-range urban air taxis through short-haul regional aircraft to long-range widebodies requiring many orders of magnitude more energy, says Michael Winter, senior fellow for advanced technology at Pratt & Whitney. “We are not going to use the same propulsion architecture for each of these,” he says. But the same technologies can be used in different ways.

Rolls-Royce is studying a variety of hybrid-electric regional airliner configurations, looking for efficiency and other benefits. Credit: Rolls-Royce

“If I had about 0.5-1 megawatt today, I could do urban air mobility. If I use that in a serial hybrid configuration, with a gas turbine to generate electricity combined with batteries and an electric propulsor, then I can address the middle space in the market. Then I can take advantage of a parallel hybrid architecture to bring that to bear on a large aircraft,” he says. “There is a sweet spot here, at 0.5-1 megawatts . . . and it is not a big stretch from where we are today in technology,” says Winter. “The technology is reaching maturity quickly, and it becomes enabling from the very small to the very large.” Engine manufacturers are already researching electric propulsion. Under a NASA contract, Pratt’s sister organization United Technology Research Center has studied using battery assist for takeoff and climb and reoptimizing the turbofan in cruise for an overall energy reduction of 5%. Rolls has developed the Electrically Variable Engine concept in which battery use is managed over a range of missions, for reductions of up to 20% in fuel burn and 10% in energy use over flights shorter than 1,000 nm. GE has demonstrated dual-spool power extraction using a military turbine engine, taking 250 kW off the high-pressure spool and 750 kW off the low-pressure spool in altitude testing at NASA Glenn Research Center. GE has also tested a 1-megawatt motor/generator driving an 11-ft.-dia. Dowty propeller. Safran has signed up to supply the hybrid-electric distributed propulsion system for Bell’s electric vertical-takeoff-and-landing (eVTOL) air taxi demonstrator and has conducted initial ground runs of the turbogenerator-based system. Siemens has electric motors flying or in development for several aircraft types and is working with Airbus to power its CityAirbus four-seat eVTOL demonstrator.

Safran has begun ground tests as it develops the hybrid-electric distributed propulsion system for Bell’s eVTOL demonstrator. Credit: Safran

“Airbus believes in electrification,” says Rudiger Thomas, hybrid-electric technology roadmap owner in the Airbus Corporate Technology Office. “Electrification opens a new dimension for energy management, propulsive and non-propulsive. And it does not suffer the performance and cost penalties of small turbines: Electric motors scale and are less costly.” Airbus has made significant investments in electrification. These include the E-Aircraft Systems House being built with Siemens to test electric and hybrid-electric power systems up to 20 megawatts. The facility is planned to be operational in 2019. “A new single-aisle serial hybrid would need two 20-megawatt engines. We are not saying it makes sense, but we have sized the facility to prepare for the possibility that we will go down that road,” Thomas says. Airbus, with Siemens and Rolls, also plans to fly the E-Fan X hybrid-electric regional aircraft demonstrator in 2020. E-Fan X is planned to be an Avro RJ modified with a 2-megawatt-class system comprising a Rolls turbogenerator and Airbus batteries powering a Siemens motor driving a Rolls fan replacing one of the regional jet’s four turbofan engines. “E-Fan X is a technology demonstrator. We will run electric propulsion with the turbogenerator or batteries and switch between them. We will test ultra-high-voltage, high-altitude operations,” Thomas says. Airframers are asking the question “How does electric propulsion buy its way onto aircraft?” says Brian Yutko, vice president for research and development at Boeing subsidiary Aurora Flight Sciences. “We look at it in three ways. Performance through integration: taking aircraft that are performing existing missions and extracting more efficiency and lower cost. Unconventional missions: things that aircraft can’t do today. And market potential: opening new markets, primarily through cost-efficiency.” Unconventional missions include urban air mobility, both for passengers and cargo. Potential new markets include short-range or thin-haul regional transport. This is the niche being pursued by Boeing-backed startup Zunum Aero with the 12-seat hybrid-electric aircraft it plans to bring to market in 2022. The megawatt-class aircraft will be heavier than competing turboprops, with a much shorter range of 700 nm. But with operating costs 40-80% lower, Zunum is hoping to revitalize the regional market. “In our mind, regional- and business-jet type of aircraft [flying 300-1,000 nm and carrying 1-4 or 20-100 passengers] represent the first opportunity to implement hybrid-electric propulsion systems,” says Mekhiche. “There is a strong value proposition driving electrification and a need in the marketplace for this capability.” Benefits include lower infrastructure costs for vertiports and small airports, he adds. Regional jets make up 60% of Embraer’s business. “We see a disruption coming to this market, and we want to be part of the disruption, not the disrupted,” says Daniel Moczydlower, vice president of technology development at the Brazilian aircraft manufacturer. “It will be a step-by-step process,” he says, noting that Embraer is already engaged with Uber on fully electric VTOL. The next step could be a short-range commuter aircraft. “We are targeting when to hit the market with some degree of hybridization under Part 23. The big question is when it will hit under Part 25,” Moczydlower says. Part 23 governs certification of aircraft weighing 12,500 lb. or less, and the recent rewrite of FAA rules, and Europe’s equivalent, enables industry standards to be used to demonstrate compliance. This change underpins the explosion of interest in developing small electric aircraft, from eVTOLs to commuters. Part 25 governs larger transport-category aircraft and has not yet been rewritten. “Airbus and other players are getting prepared to support [standards developer] SAE International in setting up a new committee for hybrid-electric propulsion for Part 25 aircraft in the near term,” says Thomas. “Standardization is the first step, and Part 23 is much further forward than Part 25.” Embraer wants to be prepared. “We really believe it will happen. It is too risky to assume it will not happen,” Moczydlower says. “Within Embraer, we are having an interesting discussion between the technology group and the market intelligence unit. Is there a market? It is difficult to tell. We do see it, but the question is when do we come up with a product and at what size?”

Bauhaus Luftfahrt is leading the European-funded Centerlineproject to mature an electrically powered “propulsive fuselage.” Credit: Bauhaus Luftfahrt

The size question is critical, as there are four basic approaches to electrifying aircraft propulsion. An all-electric system using batteries or fuel cells for energy storage is suited only to small, short-range aircraft at foreseeable levels of technology. Serial or parallel hybrid-electric power using an internal-combustion or turbine engine looks viable for regional aircraft. Turboelectric propulsion using jet fuel for energy storage looks to be the only feasible approach for large commercial aircraft. Uber wants all-electric propulsion for its eVTOL urban air taxis to minimize noise, emissions and infrastructure requirements. But the low energy density of lithium-ion batteries compared with aviation fuel limits range, even with expected improvements. A four-passenger eVTOL could fly about 50 km (30 mi.) on today’s batteries, and by 2025 fly 90-100 km based on projected improvements, says Yutko. “If you take today’s battery energy density of 150-200 Wh/kg at the pack level and ramp that up at 8% a year, it’s still not a viable path for large-scale aircraft for the foreseeable future,” says Simpson. “That doesn’t mean it doesn’t have a place on those aircraft, or that it doesn’t have a place on small-scale and short-range aircraft. We do view that as a path worth pursuing going forward.” New, more energy-dense battery chemistries are being commercialized, but initially they will be expensive and have limited lives in an urban-eVTOL duty cycle that demands high charging and discharging rates. Electric aircraft for longer-range suburban and regional services will have to be hybrid at first, but hybridization adds the complexity and weight of motors, generators, batteries, electronics and wiring. “You get a pretty severe degradation in mission performance as you electrify,” says Yutko. Analyses suggest reduced-capability hybrids can have lower energy costs than traditional aircraft, though. “There is some potential on certain missions to beat conventionally powered aircraft on an energy-only cost basis,” he says. “If you believe there is a market in 500 mi. and below, then you are talking about something interesting.” In serial hybrid propulsion, the engine drives a generator to produce electricity and charges the battery that powers the motor and propulsor. In parallel hybrid propulsion, the propulsor is driven by either or both the engine and the battery. A serial architecture enables distributed electric propulsion, in which propulsors are distributed around the airframe to improve aerodynamics and increase redundancy. “Hybrid-electric is not one solution. It fundamentally depends on aircraft configuration,” says Thomas. GE’s early studies have focused on the narrowbody airliner market, because of its large established market share with CFM56 and Leap 1 engines produced jointly with Safran. “We understand the technologies that are needed, and some of them are very challenging,” says Simpson. One promising configuration studied by GE as well as NASA and the other engine manufacturers takes electrical power from the underwing engines and distributes it to a fan mounted in the aft fuselage. The fan ingests and reenergizes the slow-moving boundary layer over the fuselage, filling in the aircraft’s wake and reducing drag and fuel burn. “This is a relatively simple version of a distributed propulsion system,” says Simpson. But he cautions: “This is not child’s play. This is significant amounts of power, megawatt-class from each engine, distributed through the airframe, likely at higher voltages than we are used to.” Distribution voltages in the kilovolt range will be needed to minimize cable weight, requiring solutions to arcing issues at high altitude. Initial studies of the concept by NASA, which calls it STARC-ABL (for Single-aisle Turboelectric Aircraft with Aft Boundary Layer Propulsor), indicated block fuel-burn reductions of around 10%. But another, higher-fidelity look at the configuration, correcting a mistake in the original analysis and updating the technology assumptions, has reduced the saving to a less promising 3.5%. The revised result has been confirmed by an external audit of STARC-ABL conducted by Aurora, says Yutko. The result underlines how hard it is for electric propulsion to show a benefit with foreseeable technology, particularly in large aircraft. “We didn’t expect a benefit from partial turboelectric, so we were excited when we saw 10%. We would have been excited with 3%,” says Cheryl Bowman, technical co-lead for NASA’s hybrid gas-electric project. STARC-ABL is aimed at a 2035 entry into service, and the revisit included an increase in the performance projected for advanced turbofans expected to be available in that time frame. “The better the turbofan, the longer it takes hybrid-electric to show a benefit,” says Bowman. That is a key reason why electric propulsion looks less attractive on large aircraft, with their 55%-efficient high-bypass turbofans, than it does on regional aircraft with 30%-efficient gas turbines—a difference that is a result of both their smaller size and the lower investment in that market sector. Electric systems are fundamentally more efficient than turbines, but in STARC-ABL’s partially turboelectric architecture, the inefficiencies in the generator, distribution, electronics and motor stack up to erode the aerodynamic benefit from boundary-layer ingestion. And those inefficiencies generate kilowatts of heat that must be used, driving up the weight for thermal management. There is potential for improvements in component efficiencies and in optimization of the thruster, but the agency also has one-year studies underway on two other potentially viable single-aisle concepts, says Bowman. The megawatt-class STARC-ABL propulsion system will still be tested in the NASA Electric Aircraft Testbed at Plum Brooke, Ohio. This will include running the system at simulated high altitude. As industry eyes electrification across a range of markets, “We are going to need a range of technologies; it’s not one size fits all,” says Simpson. “We are going to have to prepare for a range of voltages, into the kilovolt range. And we are going to need altitude-ready components—motors, generators, inverters, etc. The airframe and propulsion system will be integrated as never before, and the components in that system are going to be highly integrated to achieve the power densities we need.” While pouring cold water on claims made for the potential on large aircraft of power regeneration, noise reduction and new configurations, Winter says a parallel-hybrid approach could have value. “If I had a megawatt motor/generator on the low spool, there is no doubt I could operate the engine at its sweet spot for efficiency for more of the mission. I could reduce core temperatures and take advantage of that in either higher overall pressure ratio and thermal efficiency or lower maintenance cost,” he says. If a second, 0.5-megawatt motor/generator is added to the high spool, “I can have an adaptive engine where I can trade power between the spools. [But] this is really pushing the edges of cyber-physical control and the systems modeling required to optimize the system,” Winter says. A megawatt of power generation from the low spool could also allow more extreme electrification of aircraft subsystems than in the Boeing 787. “We do recognize a value proposition there,” he adds. Industry is also aware of the potential for disruption that comes with electrification. Airbus has moved aggressively into urban air mobility in part because of the risk to its helicopter business. Eliminating the need to develop a gearbox significantly lowers the barrier to entry to vertical flight. Engine-makers see the potential for disruption of their traditional worksplit with airframers as well. New players are entering the propulsion market based on electric technology, among them Siemens and startups such as H55, MagniX and VerdeGo. On the E-Fan X, Rolls shares the propulsion system with Airbus and Siemens—a possible sign of things to come. “We have to understand what that [supply scope] impact looks like and develop a business model that continues to ensure we provide the value the market expects from us,” says Mekhiche.