Periodic Reporting for period 2 - SOLIFLY (Semi-SOlid-state LI-ion batteries FunctionalLY integrated in composite structures for next generation hybrid electric airliner)
Reporting period: 2022-07-01 to 2023-12-31
Shifting from fossil-fuel powered to increasingly electrical aircraft propulsion, progress is needed in aviation-grade battery technologies. Multifunctionality could enable to overcome the substantial weight penalties of conventional battery energy storage.
The aim of SOLIFLY was to develop aero-fit multifunctional composite structures with integrated semi-solid-state batteries, thus investigating the viability of structural battery (SB) technology for the next generation hybrid electric aircraft.
Within SOLIFLY, two different SB cell concepts based on an energy-dense structural electrochemistry were realized, as well as an approach for their integration into aeronautic solid CF composite structures was developed and tested. SOLIFLY has demonstrated for the first time an aeronautic-grade high-strength multifunctional composite stiffened panel with predictable mechanical properties and a high amount of functional SB cells at insignificant weight increase.
SOLIFLY has linked its technology developments to actual needs of the aviation industry by incorporated the expertise of aircraft manufacturers and suppliers and by evaluating aspects of airworthiness certification and manufacturing, and their potential at aircraft level. A technology roadmap and scale-up strategy ensure that SOLIFLY SB technology can be further matured and eventually industrialized.
The aim of SOLIFLY was to develop aero-fit multifunctional composite structures with integrated semi-solid-state batteries, thus investigating the viability of structural battery (SB) technology for the next generation hybrid electric aircraft.
Within SOLIFLY, two different SB cell concepts based on an energy-dense structural electrochemistry were realized, as well as an approach for their integration into aeronautic solid CF composite structures was developed and tested. SOLIFLY has demonstrated for the first time an aeronautic-grade high-strength multifunctional composite stiffened panel with predictable mechanical properties and a high amount of functional SB cells at insignificant weight increase.
SOLIFLY has linked its technology developments to actual needs of the aviation industry by incorporated the expertise of aircraft manufacturers and suppliers and by evaluating aspects of airworthiness certification and manufacturing, and their potential at aircraft level. A technology roadmap and scale-up strategy ensure that SOLIFLY SB technology can be further matured and eventually industrialized.
A state of art review of structural batteries and their aeronautic potential was published. Requirements for aeronautic SB were assessed with active support of the industrial AB (Piaggio Aerospace, Pipistrel Vertical Solutions, FACC, Dassault Aviation), analysing electrochemical/mechanical/structural perspectives, manufacturing constraints and airworthiness and safety aspects.
A thermoplastic polymer-ionic liquid structural electrolyte and energy-dense composite electrodes were developed and implemented in 2 SB cell concepts: UNIVIE’s Coated Carbon Fibers (CCF), showing suitable mechanical performance but lower maturity in electrical performance and reproducibility, and AIT’s Reinforced Multilayer Stack (RMS), achieving around 50 Wh/kg and a Young’s modulus of 10 GPa. Their scalable manufacturing was demonstrated by AIT delivering 40 RMS SB cells for the SOLIFLY demonstrator.
An SB integration concept was developed by ONERA with a numerical strategy for integrating thin SB cells in solid CF composite laminates, quantifying the impact of the battery insert on the mechanical performance with respect to its size, shape and position in the structure. For the first time, the damage and failure process of the SB laminate was studied, and recommendations were established. The mechanical behaviour and failure properties of the two battery cell concepts were characterized by ONERA with multi-instrumented measurements for the use in numerical design such multifunctional structures. A modified curing cycle was developed that preserves the SB electrical properties and composite mechanical properties.
For final demonstration, the SOLIFLY multifunctional demonstrator, a stiffened CF composite panel integrating 20 AIT RMS SB cells in its skin, increasing weight only by 2.6%, and a monofunctional reference were designed by simulation for sustaining high uniaxial compression loading while fitting to ONERA’s test facility and manufactured. The global stiffness of both panels was found very similar, what validates the proposed design, while the first buckling load was reduced due to initial delamination introduced the electrical insulation of the SB cells. Still, the multifunctional panel was able to carry more than 18 tons without failure which matches our initial goal of the SOLIFLY project to produce a high-strength multifunctional part. 80% of the SB cells were electrically functional after curing and none of the controlled cells failed due to the mechanical test. To our knowledge, the SOLIFLY demonstrator is the first multifunctional stiffened panel with high mechanical properties that has been successfully manufactured and tested.
SOLIFLY has assessed the manufacturability of the developed multifunctional technology (CustomCells), identified TRL step up and examined certification aspects including first discussions with EASA specialists (UNINA). The potential of SB integration was studied at aircraft level showing that a considerable amount of electrical energy could be stored and even could contribute to hybrid electric propulsion (CIRA). A workshop and public event presented the project outcomes to the AB and a wider public, discussing in detail how structural batteries could contribute to future climate neutral air transport.
A thermoplastic polymer-ionic liquid structural electrolyte and energy-dense composite electrodes were developed and implemented in 2 SB cell concepts: UNIVIE’s Coated Carbon Fibers (CCF), showing suitable mechanical performance but lower maturity in electrical performance and reproducibility, and AIT’s Reinforced Multilayer Stack (RMS), achieving around 50 Wh/kg and a Young’s modulus of 10 GPa. Their scalable manufacturing was demonstrated by AIT delivering 40 RMS SB cells for the SOLIFLY demonstrator.
An SB integration concept was developed by ONERA with a numerical strategy for integrating thin SB cells in solid CF composite laminates, quantifying the impact of the battery insert on the mechanical performance with respect to its size, shape and position in the structure. For the first time, the damage and failure process of the SB laminate was studied, and recommendations were established. The mechanical behaviour and failure properties of the two battery cell concepts were characterized by ONERA with multi-instrumented measurements for the use in numerical design such multifunctional structures. A modified curing cycle was developed that preserves the SB electrical properties and composite mechanical properties.
For final demonstration, the SOLIFLY multifunctional demonstrator, a stiffened CF composite panel integrating 20 AIT RMS SB cells in its skin, increasing weight only by 2.6%, and a monofunctional reference were designed by simulation for sustaining high uniaxial compression loading while fitting to ONERA’s test facility and manufactured. The global stiffness of both panels was found very similar, what validates the proposed design, while the first buckling load was reduced due to initial delamination introduced the electrical insulation of the SB cells. Still, the multifunctional panel was able to carry more than 18 tons without failure which matches our initial goal of the SOLIFLY project to produce a high-strength multifunctional part. 80% of the SB cells were electrically functional after curing and none of the controlled cells failed due to the mechanical test. To our knowledge, the SOLIFLY demonstrator is the first multifunctional stiffened panel with high mechanical properties that has been successfully manufactured and tested.
SOLIFLY has assessed the manufacturability of the developed multifunctional technology (CustomCells), identified TRL step up and examined certification aspects including first discussions with EASA specialists (UNINA). The potential of SB integration was studied at aircraft level showing that a considerable amount of electrical energy could be stored and even could contribute to hybrid electric propulsion (CIRA). A workshop and public event presented the project outcomes to the AB and a wider public, discussing in detail how structural batteries could contribute to future climate neutral air transport.
AIT’s first-generation RMS SB cell already exceeds the specific energy of other SotA while providing suitable mechanical properties and safety in handling and operation together with scalable manufacturing and compatibility with composite materials and manufacturing processes that are accepted by aeronautic industry.
SB integration into solid quasi-isotropic CF/epoxy UD laminates, which are widely used in aeronautical industries, was validated, achieving high mechanical properties of the multifunctional parts and gain in understanding the damage and failure process in presence of SB cells. The modified curing cycle is applicable to all epoxy matrices available on the market.
The SOLIFLY multifunctional demonstrator - the first multifunctional stiffened panel with high mechanical properties – has been successfully manufactured and tested, proving that electric energy storage can be integrated in high strength aeronautic composite structures without degrading their mechanical properties at minimum weight impact.
Integrating lightweight SOLIFLY SB cells in solid CF laminate, an effective energy density at integration level of 500+ Wh per kg of added weight can be obtained, indicating high weight saving potential of SB. At aircraft level, considering commuter (9-19 pax) and regional aircraft (40-110 pax), SB integrated in external aircraft surface areas (assuming SB cells with suitable structural capabilities and being operational under the aircraft’s harsh environmental conditions) could store between 50 and 500 kWh, in regional aircraft around 4 kWh per passenger with a weight reduction of around 6 kg per passenger. Considering also non-primary parts for SB integration, such as interior panels and liners or overhead bins, SB storage capacity could be significantly increased. The CO2 reduction potential of SB for future 90-pax turboprop regional aircraft with EiS in 2035 and 2050 was estimated between around 3% (SB power only non-propulsive loads) and around 6% (SB contribute to hybrid electric propulsion). Even if such assessments tend to oversimplify, nevertheless, they indicate the potential of multifunctional electric energy storage for future climate neutral air transport.
SB integration into solid quasi-isotropic CF/epoxy UD laminates, which are widely used in aeronautical industries, was validated, achieving high mechanical properties of the multifunctional parts and gain in understanding the damage and failure process in presence of SB cells. The modified curing cycle is applicable to all epoxy matrices available on the market.
The SOLIFLY multifunctional demonstrator - the first multifunctional stiffened panel with high mechanical properties – has been successfully manufactured and tested, proving that electric energy storage can be integrated in high strength aeronautic composite structures without degrading their mechanical properties at minimum weight impact.
Integrating lightweight SOLIFLY SB cells in solid CF laminate, an effective energy density at integration level of 500+ Wh per kg of added weight can be obtained, indicating high weight saving potential of SB. At aircraft level, considering commuter (9-19 pax) and regional aircraft (40-110 pax), SB integrated in external aircraft surface areas (assuming SB cells with suitable structural capabilities and being operational under the aircraft’s harsh environmental conditions) could store between 50 and 500 kWh, in regional aircraft around 4 kWh per passenger with a weight reduction of around 6 kg per passenger. Considering also non-primary parts for SB integration, such as interior panels and liners or overhead bins, SB storage capacity could be significantly increased. The CO2 reduction potential of SB for future 90-pax turboprop regional aircraft with EiS in 2035 and 2050 was estimated between around 3% (SB power only non-propulsive loads) and around 6% (SB contribute to hybrid electric propulsion). Even if such assessments tend to oversimplify, nevertheless, they indicate the potential of multifunctional electric energy storage for future climate neutral air transport.