Structural battery cells corresponding to the two concepts studied in SOLIFLY. Reinforced AIT laminate (RMS, left) and UNIVIE coated carbon fibers (CCF, right). Source | Alexander Boytle (left) and Qixiang Jiang (right)

What if you could combine electric battery technology with aircraft designs to reduce weight? This was the premise of the Clean Sky 2 project “Semi-solid-state lithium-ion batteries functionally integrated into composite structures” (SOLIFLY), which ran from January 2021 to December 2023. A recently completed three-year project successfully explored the feasibility of multinational “structural batteries” that could be used in aircraft structures to transfer mechanical loads while simultaneously storing electrical energy. The partners demonstrated such capabilities by producing a high-strength, multi-functional, aircraft-grade composite stiffening panel that includes 20 integrated structural battery cells.

“Integrating batteries into an aircraft design increases overall power density while saving weight and therefore reducing emissions,” says Jimmy Chen, Clean Aviation Project Manager.

To achieve the goals of SOLIFLY, project coordinator AIT Austrian Institute of Technology GmbH (Vienna) in collaboration with the University of Vienna (UNIVIE) has developed a new structural electrochemistry formula based on high-energy materials in combination with a safe semi-solid structural electrolyte. This electrochemical formulation is reported to be scalable and compatible with aircraft composite materials and manufacturing processes, including autoclave curing. Two cell concepts with different degrees of integration were developed (image above).

The concept proposed by UNIVIE uses carbon fibers as current collectors, while the AIT concept mechanically strengthens the active components of the cells and is based on well-established battery cell manufacturing processes.

In addition, ONERA, the French Aerospace Laboratory (Palaiseau), has developed a concept for the integration of solid composite laminates using a numerical framework that not only evaluates and minimizes the influence of the structural cell on the mechanical properties of a multifunctional structure, but also evaluates the occurrence of damage; this is an important aspect when it comes to their certification. The production of multifunctional composites was also discussed, including off-lab processing of battery cells and production of fully cured composites with still functional battery cells. A reinforced panel was chosen to demonstrate SOLIFLY technology because it is a standard high-strength aircraft component. The SOLIFLY multi-functional demonstrator, comprising 20 structural battery cells, was tested at ONERA using multi-instrumentation.

SOLIFLY is a demonstrator of an aircraft-grade, high-strength, multi-functional rigid panel housing 20 structural battery cells. Source | Frederic Lauren

CustomCells Itzehoe GmbH (Itzehoe, Germany), an SME specialized in energy storage, UNINA (University degli Studi di Napoli Federico II, Naples, Italy) and CIRA, Italian Center for Aerospace Research (Capua, Utalia) assessed the techno-economic aspects of this technology . production, certification and potential upgrades at the aircraft level. At the same time, an industrial advisory board consisting of experts from Piaggio Aerospace, Pipistrel, FACC and Dassault Aviation provided input and advice, monitoring the progress of the project.

One of the key tasks in the development of structural electrochemistry was the need to create a structural electrolyte based on thermoplastics. The originally conceived epoxy resin system demonstrated fundamental problems that could not be resolved within the project’s timeline. To produce multifunctional structural carbon fiber battery composites in an autoclave, the curing cycle needed to be adapted to preserve the functionality of the battery cells while ensuring that the carbon fiber was still fully cured.

The SOLIFLY results, although still at a low technology readiness level (TRL), indicate that several hundred kilowatt-hours of electricity could potentially be stored using structural batteries in future regional and short-to-medium-haul aircraft. These structural batteries can power secondary loads such as passenger infotainment systems, cabin avionics, or more power-hungry systems including electronic taxiing (moving an aircraft between a terminal gate or parking lot and the runway without the use of tug or engine thrust). ).

According to the partners, SOLIFLY could even facilitate hybrid-electric propulsion in smaller aircraft at reduced weight without affecting the aircraft’s center of gravity or reducing useful volume. The introduction of hybrid-electric propulsion and electronic taxiing has the potential to reduce the environmental impact of aircraft by reducing fuel consumption. In addition, electronic taxiing has the added benefit of reducing noise on the ground, thereby improving local air quality and social acceptance in and around airports.

“We were able to demonstrate that multifunctional energy storage is possible without compromising the structural requirements of the aerospace industry,” says Dr. Helmut Kühnelt, Senior Research Engineer for Electric Vehicle Technologies at AIT. “We now have a much better understanding of the potential of this technology, as well as the research needs and challenges that lie ahead. But they seem more surmountable than before.”

SOLIFLY paved the way for the subsequent project MATISSE (also coordinated by AIT), funded under Horizon Europe, which started in September 2022 and will run until August 2025. MATISSE will improve the multifunctional performance of structural batteries, expand their structural integration for sandwich composites, make them smart by adding integrated sensing capabilities, and study the impact loading of multifunctional composite structures. The project will also demonstrate the technology at full scale in a multifunctional wingtip for Pipistrel. Velis Electro light aircraft with the goal of achieving TRL4 by the end of the project.

In addition, AIT received a research grant from the U.S. Air Force European Aerospace Research and Development Office to improve the multi-functional performance and service life of aircraft structural batteries. The project continues until November 2025.