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Power Sharing: Non-Propulsive Energy project investigates future aircraft architectures for electric flight

Power Sharing Engine - APU
Power Sharing Engine - APU

Today's aircraft engines, in addition to propelling the aircraft along their route, also generate the electrical power required for many onboard functions – cockpit avionics, satellite communications, passenger cabin lighting and heating, galleys for warming food, and other functions. The checklist gets longer as aircraft replace hydraulic and pneumatic systems with electric alternatives. Clean Sky's Non-Propulsive Energy (NPE) project focuses on new architectures that enable the aircraft's Auxiliary Power Unit (APU) – which is usually used solely to provide power when the aircraft is on the ground at the airport gate – to serve the demands of the next generation of more electrical aircraft by generating power during flight, not just on the ground. By having greater symbiosis between the main engines and the APU throughout the entire flight envelope, energy savings are envisaged that will translate into environmental benefits for European mobility.

The shift towards electrification for future airliners and business jets means that aircraft electrical architectures will have to be conceived in a more integrated manner in order to realise energy efficiencies. Clean Sky's Non-Propulsive Energy project, which runs until 2023 and is led by Safran, aims to gain fuel burn advantages for engines ranging from business jet aircraft to large passenger aircrafts, by devising, studying, testing and validating various aircraft electrical energy architecture options.

‘One of the main challenges in Clean Sky's Non-Propulsive Energy project is to determine what kind of architecture is required in terms of power sharing during the different flight phases, because we still envisage using some non-propulsive energy produced by the engine, but the idea is really to alleviate, or to reduce close to zero, the amount of energy being filtered off from the engine for providing on-board electricity for the aircraft's non-propulsive functions,’ says Sébastien Dubois, Clean Sky Acting Head of Programme.

The NPE project anticipates the needs of aviation's 2030+ timescale, though it's too late for today's aircraft as the electrical architectures of existing airliners and bizjets are already frozen. The focus is on having an APU, or possibly even 2 APUs, that can supply the heavier electrical requirements for the next generation of aircraft, says Dubois:

‘We are talking about an APU in the power range of 200 to 300 kilowatts, and we're assessing what the architecture would look like. There are several elements at stake: power generation, power conversion and power distribution.

‘Additionally, we're focusing on different safety aspects. In the event of a fault developing we need to be able to rapidly disconnect the power from certain devices in order to avoid burning out the APU. We then need to simulate how all these elements perform during the different flight phases to determine what the energy demands are. This will allow us to define the power requirements, the handling of high-voltages, the quick disconnect functions, electrical distribution, thermal management, buffering capabilities, sizing and overall integration for the various elements. We intend to reach between TRL4/5, depending on the technologies which will be investigated.’

Left: LP/HP Power Extraction - Right: HP Power Extraction
Left: LP/HP Power Extraction - Right: HP Power Extraction

One of the overall opportunities in the project is the prospect of optimising engine performance by intelligent power sharing between the main engines and the APU. The non-propulsive energy extracted from today's main aircraft engines is mainly in the form of air pressure. This reduces the engine's propulsive potential – and it is also not the most efficient way of generating onboard side energy.

‘If we want to deliver onboard electrical power this brings issues for the main engine operability. And so this is the reason why we're trying to find the best compromise between the engine and the APU. This will enable us to save some fuel and reduce CO2,’ says Alain Garassino, R&T Control System Module Manager at Safran Aircraft Engines.

‘Another aspect is that as engine bypass ratios get higher, we have bigger engines, and this affects low idle speeds on the engine. By sharing power generation between the main engines and the APU it gives us the capability to reduce idle speed and save fuel, because the APU would be delivering electrical power for NPE onboard functions,’ says Garassino.

To achieve these ambitions, Safran Engines is working together with Safran Power Units and liasing closely with Airbus for the Large Passenger Aircraft application. They are also working with Dassault Aviation to find out whether this application could be used in business jets, because, says Safran's Diette, ‘there is a global balance that needs to be studied and validated. It is clearly a full loop that we need to do with the airframe – we cannot just propose things from our side without having an impact on the whole aircraft. 

‘Rethinking how an aircraft is designed today and optimising its components – taking into account each contributor and including weight and installation aspects – is a fully collaborative activity, and we need to have integration between each contributing player for all the systems on an aircraft. The idea was to bring together all the people from engines, APU and airframe and all the systems dealing with NPE – to see what could be the optimum configuration.’ 

To date, one NPE architecture is being assessed for use in bizjets and three architectures are being evaluated for Large Passenger Aircraft by means of three demonstrators: Demo 1 focuses on power sharing between the main engines and APU; Demo 2 evaluates a permanent magnet generator; and a third demo looks at an outlet guide vane (OGV) heat exchanger.

Ground Demo 1 aims to validate the feasibility of power sharing between an aircraft's main engines and its APU, by examining switching loads between the APU load bank and the main engines, and also investigating the stability of the control system and the incorporation of a buffer. Another key aspect of Demo 1 is the validation of the integration of various APU techno-bricks supporting the APU's performance, such as the starting capabilities on the ground, mechanical and thermal APU behaviour, control law validation for normal and emergency conditions, transient load behaviour, plus validation of the use of the APU in case of a main engine generator failure.

Ground Demo 2, which focuses on a permanent magnet generator (PMG) solution, is supported by three Clean Sky consortia: QUICK, led by Nottingham University, which is developing a set of equipment that is capable of rapidly disconnecting the mechanical power from the generator; IGNITE (also Nottingham) is looking at the integration of the overall aspects onto the iron bird test rig; and SPARTAN, which focuses around power modules for the power conditioning of multiple electrical sources, is managed by Edair. There will be design development at the equipment level, and once these sub-projects have been demonstrated and validated they will all be integrated on a test bench which will be capable of mimicking the overall aircraft systems behaviour using a partial iron bird.

Power Sharing Engine - APU
Power Sharing Engine - APU

The NPE project requires the contribution of many niche players. For the SMEs involved – especially those interested in bringing their expertise from outside of the aeronautics sector to strengthen European aviation – working on this Clean Sky initiative opens up new doors and promises new future opportunities. Three SMEs are involved in Demo 1, which focuses on power sharing between the main engines and the APU: ERNEO, BrightLoop, and AKIRA. 

ERNEO, a French company specialising in the design and manufacture of electric motors, electric generators and magnetic systems, compared different electrical machine topologies to find the best solution in terms of performances in relation to power to mass ratio, regarding the constraints of the application. They first made a trade-off study where they went deep enough into the design phase to compare different electrical machines topologies in terms of performances (mass, volume, efficiency), reliability and also TRL. With those results, they were able to select the most suitable topology for the application. 

ERNEO is currently at the study phase and has also started to build different experimental mock-ups to assess the main risks. They will stay in this phase until they reach the critical design review in May 2020.

Another beneficiary within the project, French SME BrightLoop, is in charge of Demo 1's power electronics, the technology that converts the AC power from the APU into the DC power required for the aircraft's electrical network.

The challenge BrightLoop is addressing is to make the conversion from AC to DC efficiently without generating too much heat, while taking care of the compatibility and reducing weight and space. 

‘The gas turbine at the heart of the APU project is running at a very high internal speed of 60,000 rpm, or even higher, and on the current systems we use mechanical reduction to go to 3000 rpm and then use a simple generator on the shaft to make electricity. The idea is to work at high speeds in order to have a smaller electric generator and then have a power electronics stage that will interface in an intelligent manner with the aircraft electrical network,’ says Florent Liffran, BrightLoop's CEO.   

This is really a new step in aeronautical electric architecture as high voltage DC networks in airliners or business jets do not yet exist. If the NPE team can crack this challenge it will be a significant step forward for the next generation of more electric aircraft.

‘We've identified certain technical points that need to be solved at component level in order to demonstrate that the overall concept is OK, and we are just finishing this phase,’ says Liffran, whose company is now entering the second phase where they will design the equipment that will be installed in the demo. The architecture for this is now frozen and BrightLoop is now developing the equipment. Afterwards there will be a third phase in which all the partners will deliver their components and these will all be integrated into the full demo in 2021.

AKIRA, the third SME that is involved with the NPE project, is a French company specialised in energy conversion systems and the development of special test benches. Within this Clean Sky 2 project, AKIRA is in charge of the integration of the whole system, through the design of a high-speed gearbox between the gas generator and the electrical machines, and by supplying support functions like cooling and lubrication. The main challenges are the high speed (needed to ensure the safe operation of the rotating shafts), the weight constraints, developing a concept for a system that is as representative as possible, and maintenance of the sub-systems in a fully-integrated design. Their portion of the project is currently in the critical design period, and within a few months they will start supplying the product, in order to be able to start settings and first tests before the year is out. 

According to the SMEs, Clean Sky gives small, innovative companies the chance to work on big industrial projects, and to better understand the needs and limitations of the aircraft industry. 

‘Clean Sky is a very nice opportunity for our company to be part of an industry and part of a story within a sector that is not usually structured to allow new and small players to be involved,’ says Liffran. ‘This is very important, and I think without this kind of programme, it would be very difficult for us to enter the aeronautics business. This project also enables us to innovate and to demonstrate actual achievements, which is also very important for European SMEs, to have products on the table – it's much more convincing than just ideas.’

‘Working in the context of Clean Sky allows us to develop innovative technologies that link in with the real future needs of important companies like Safran,’ says Dr. Ziegler, CEO of ERNEO. 

‘It enables us to carry out interesting development research that we can use for other aeronautical applications, and it helps us to better understand the constraints involved in aeronautics. It also helps us focus specifically on this complex aeronautical application and to better understand the long-term work that we have to do in order to complete the project. We know it's just a step along a long journey and even if we find a solution with good performances we will still have work to do.’

Despite the fact that Clean Sky's NPE project has another 2-3 years yet to run, there is still a lot of work to be accomplished, both in terms of resolving the complex electrical architecture to make power sharing between the aircraft's main engines and APU viable, but also from the broader perspective of working towards the next generation of airliners and bizjets for European mobility.

‘It's a long road towards electrification, but the aviation industry is now finding its route, especially in the context of decarbonisation, and in Clean Sky we have made a lot of progress over the last 10 years,’ says Clean Sky's Sébastien Dubois. ‘And now we expect to transform all these activities into concrete exploitation, as long as we can demonstrate the benefits of such an architecture and approach, which is really at the core of this Non-Propulsive Energy project. We see a lot of opportunities to amplify the effort in this field which is promising for the next period of European research.’