Multifunctional structural batteries – the promise of weight reduction in hybrid electric aircraft
Creators: name: Kühnelt, Helmut
Publisher: name: Zenodo
Year: 2023
Subjects: structural batteries
multifunctional energy storage
aircraft electrification

description: Abstract


The introduction of hybrid electric aircraft is one of the envisaged ways to innovate the market of civil aviation while striving for climate neutrality by 2050, as stipulated by the European Green Deal and ACARE’s updated European vision for sustainable aviation [1,2], as electrification of propulsion can enable alternative propulsion configurations to increase tank-to-thrust efficiency and to achieve synergies with other aircraft sub-systems.


Hybrid electric propulsion is nowadays considered mainly for aircraft up to regional size with two main options for the onboard electrical power source: mechanical or electrochemical conversion, with a generator or a (H2) fuel cell system, and/or storage in batteries. Either option will show a substantial increase of aircraft empty mass, even when utilizing upcoming ultra-high energy density lithium-ion batteries that are expected to reach the theoretical limits of this battery technology in the coming years [3]. As post-Li-ion batteries (e.g. metal-S or metal-O2) are still far from market, this poses significant challenges for electrifying the propulsion of larger aircraft, ranging from commuter over regional to short-medium range.


Multifunctional electrical energy storage, equivalently referred to as structural batteries, are capable of storing electrical energy while bearing mechanical loads, seamlessly allowing for storage capabilities at zero weight penalty. Over the last decades, many concepts of this technology have been investigated, exploring different degrees of multifunctionality, i.e., from multifunctional constituents and materials to multifunctional structures, and different cell designs have been demonstrated, including reinforced planar cells and coaxial wires.  However, multifunctional efficiency adequate for aeronautic applications has not been achieved so far and several gaps in research, technology development and, specifically to the aeronautic field, in airworthiness certification have never been tackled [4,5].


The CleanSky 2 project SOLIFLY (2021-2023, GA 101007577) and the HORIZON project MATISSE (2022-2025, GA 101056674) address the fundamentals of structural batteries, by combining research and technology development in the fields of: (a) structural electrochemistry, developing structural electrolytes for state-of-art high energy active materials (NMC, Si-graphite) that are performant, safe, mechanically capable and thermally stable; (b) integration of structural battery cells into carbon fibre composite laminate and sandwich structures, considering structural materials and manufacturing practices accepted by aeronautic industry; (c) on-cell sensing and monitoring micro-electronics for both energy storage and surrounding structure spanning a sensor network that will make aeronautic parts smart over their entire life, from manufacturing over use to second life and recycling; and (d) manufacturing and certification while exploring the potential of deploying such technology in multiple aircraft categories.


Within SOLIFLY and MATISSE, the feasibility of aeronautic structural batteries will be demonstrated first within a standard interior aircraft part (i.e. a stiffened composite panel of representative size, 2023), and later at full scale in a replaceable wingtip of a fully electric light aircraft (i.e. Pipistrel VELIS Electro, 2025), achieving TRL 4 by 2025.


This paper will present an overview of the current research performed and preliminary results achieved within SOLIFLY and MATISSE with focus on recent tests of functional structural battery coupons and an outlook on the upcoming demonstration of the SB panel. Furthermore, the steps necessary to mature further SB technology and enable industry uptake will be discussed.  


Acknowledgment


The authors are grateful to the European Commission for supporting the present work, performed within the SOLIFLY project, funded by the Clean Aviation Joint Undertaking under the European Union’s Horizon 2020 research and innovation program (Grant Agreement no. 101007577), and within the MATISSE project funded under the European Union’s Horizon Europe research and innovation program (Grant Agreement no. 101056674). This publication solely reflects the authors’ view and neither the European Union, nor the funding Agency can be held responsible for the information it contains.


References 


[1] EU Commission. COMMUNICATION FROM THE COMMISSION The European Green Deal. 2019.


[2] EU Commission and Directorate-General for Research and Innovation. Fly the Green Deal : Europe’s vision for sustainable aviation. Publications Office of the European Union; 2022. https://doi.org/doi/10.2777/732726.


[3] Frith JT, Lacey MJ, Ulissi U. A non-academic perspective on the future of lithium-based batteries. Nat Commun 2023;14:420. https://doi.org/10.1038/s41467-023-35933-2.


[4] Hopkins BJ, Long JW, Rolison DR, Parker JF. High-Performance Structural Batteries. Joule 2020;4:2240–3. https://doi.org/10.1016/j.joule.2020.07.027.


[5] Kühnelt H, Beutl A, Mastropierro F, Laurin F, Willrodt S, Bismarck A, et al. Structural Batteries for Aeronautic Applications—State of the Art, Research Gaps and Technology Development Needs. Aerospace 2022;9:7. https://doi.org/10.3390/aerospace9010007.