ICA C "Energy Storage and Conversion"

While new energy storage and conversion technologies are entering the automotive sector and e-mobility is predicted to restructure this sector within the next decade, present aircraft technologies still solely rely on combustion of fossil fuel for propulsion. Society therefore requires new, substantially more sustainable and efficient-energy supplies that meet its need for mobility. ICA C will explore the most promising conversion and storage technologies and their interactions, including chemical and electrochemical energy storage, electrochemical, chemical and mechanical energy conversion and electrification, focussing on increasing the specific energy, specific power, and efficiency while reducing emissions.

Second Project Phase (2023-2025)

Focus of this project is on the design of future aircraft propulsion and energy systems. Such concepts gain in importance for the transformation of propulsion systems utilising conventional fuels towards regenerative energies. The design of such systems is a challenging task. On the one hand the operation strategy/energy management has to be considered in many cases from the beginning on for decision making and component sizing. On the other hand, and in contrast to stationary systems, space and weight limitations have to be taken into account, which requires typically an iterative or concurrent design of the energy system and aircraft. In addition to the thrust generating and electrical sub-systems, which have been addresses in the first funding phase, special focus is now given to the consideration of all sub-systems and its components being responsible for the thermal management, the air supply and other material feeds.

C1.1

In the previous phase of the SE²A cluster, two different approaches were used for the development of solid, structurally integrable electrolytes. Firstly, a lithium pouch cell was integrated as an interlayer in a composite structure. Secondly, a high-strength structural electrolyte was created by embedding sodium-based polymer electrolytes in glass fibre. In this phase of the project, different integration routes will be investigated with a focus on feasibility, weight reduction and maximum energy storage capacity. Both the development of an electrode based on conventional carbon fibre semi-finished products and the development of a conventional battery electrode applied to the surface of carbon fibres will be considered. Small prototypes are built, which are tested mechanically and electrically. After that, a demonstrator will be built.

C2.2

This project aims to implement a solid state lithium sulfur battery (SSLSB) at coin‐cell level with the potential to build multilayer pouch‐cells with high specific energy and high cycling stability, through a detailed understanding of degradation mechanisms and kinetics by deriving pathways for material, electrolyte, electrode and battery optimization for aviation specifications.

C2.3

In the first phase of SE2A, Project C3.1 explored the potential of morphing high pressure compressor blade designs based on titanium alloys and composite actuators made of active materials. To further expand and enhance the morphing capabilities of engine blades, the subsequent project focuses on state-of-the-art UHBR fan blading. By applying 3D-design measures, such as sweep and dihedral as well as alternative materials and advanced actuation concepts, this research aims to boost sustainability of future aircraft engine concepts, independent of the respective propulsion architecture. In the design of UHBR fan blading, the focus especially lies on a composite blade body to increasingly tailor the achievable morphing magnitudes and shapes. Through structural morphing simulations and aerodynamic performance evaluations a suitable blade prototype is identified and manufactured. The composite blade prototype and its morphing behavior is then experimentally evaluated under realistic but static load conditions as a preparation for a future application in a rotating environment.

C3.1

In the project "Synthetic Fuel Combustion for Aviation Application", eFuels are being investigated for their suitability for the LPP combustion concept. In this way, in addition to CO2 savings, a significant reduction in soot and NOx emissions is to be achieved, bringing aviation closer to climate neutrality.
With the help of experimental and numerical methods, various eFuel candidates will be investigated for flame stability limits and ignition delay times at engine relevant conditions.

C3.3

To fight climate change, the reduction of greenhouse gas emissions and the transition to carbon-neutral technologies in aviation are extremely urgent. This requires innovative approaches such as alternative fuels including hydrogen and sustainable aviation fuels (SAF) or electric propulsion. While electric propulsion systems are suitable for short-haul flights, gas turbines fueled by jet fuels remain important for medium- and long-haul flights. SAFs are considered a promising alternative, but they can still generate soot particles. This project aims to develop and apply high-performance computing numerical models to gain a comprehensive understanding of the emission characteristics of sustainable aviation fuels, particularly on non-volatile soot particle formation.

C3.5

The goal of this project is to accelerate the turbomachinery design process by developing fast and accurate AI-enhanced prediction methods. Derived from this goal, the two objectives of this project are to provide an AI-enhanced meanline-based prediction tool, and an AI-enhanced 3D-based flow-field prediction tool, which improve different stages of the compressor design process.

C3.6

The research project investigates solutions for electric distribution and conversion in both conventionally and innovatively cryogenically-cooled systems for aviation applications. With aircraft-specific goals and challenges such as power density (gravimetric and volumetric), efficiency, and reliability, the project team is tackling an optimization problem that requires a holistic approach. A significant challenge lies in navigating the wide range of topologies available for the distribution system, converters, inverters, and electric machines. To address these challenges, the team is conducting FEM calculations and experiments to gather models and refine existing ones, aiming to enhance the overall understanding and effectiveness of the systems under investigation.

C4.1

The design of an electric propulsion system for civil aircraft has to pursue a multitude of objectives. In first place, a high reliability must be achieved according to aircraft safety requirements. Additionally, low weight and high efficiency are crucial to achieve as low as possible take-off weight. While weight transforms directly into power density of the propulsion drive system, efficiency is of high importance because of the associated additional energy storage weight to cover the losses. Depending on the mission, this can be of higher importance than the weight of the propulsion system itself. This project aims to investigate the trade-offs between reliability, efficiency and power density. Furthermore, the theoretical findings will be underlain with experimental investigations, also embedded in the “decentral” Electric Aircraft Ground Lab Environment (E²AGLE). Finally, based on these investigations, a methodology for the holistic design of an electric propulsion system for civil aircraft will be provided.

C4.2

The objective of this project is to develop a methodology for the design of highly integrated thermal management systems in aircraft for all possible propulsion system solutions, including the environmental control system, and exploiting advanced cooling technologies and possibly energy harvesting. Future aircraft will have an increased number of low temperature heat sources below 100 °C that have specific thermal requests for safe operation. These mostly electric devices are located in different parts of the aircraft and need to be integrated into a thermal management system that does not only guarantee heat rejection to ambient air but also has a high power density. Both challenges, the integration of many distributed heat sources and the design of a lightweight system, are addressed with a holistic approach by developing a modular simulation framework in the programming language Modelica supported by exemplary experiments.

C5.1

This project has the ambition to develop a multi-disciplinary/multi-fidelity design methodology and novel diffuser concepts for highly compact and efficient organic Rankine cycle turbines, to be adopted in waste heat recovery units of hybrid-electric propulsion and aircraft thermal management systems. The ultimate goal is to develop validated design guidelines and scaling principles that can enable the selection of the best turbine configuration and its optimal integrated design into ORC systems. To this end, the conceived design methodology will target the concurrent design of the ORC turbine and the adjacent components (volute, diffuser) in order to achieve substantial weight reduction, by also taking advantage of components interaction. Further weight reductions will be attempted by integrating the recuperator of the ORC system into a novel regenerative diffuser. The methods developed in the project shall be also applicable to the optimal design of turboshaft engines for onboard power generation as well as to high-speed turbomachinery for fuel-cell based propulsion systems.

C5.2

Low-temperature polymer electrolyte fuel cell systems (FCSs) in comparison to combustion engines need to transfer comparatively large amounts of heat at comparatively low temperature to ambient via their cooling system. Often a mobile FCS’s cooling capacity limits its electrical power output. Cryogenic hydrogen is therefore typically utilized as FCS heat sink, even though such heat transfer destroys most hydrogen exergy. This project therefore investigates cryogenic exergy utilisation systems (CEUS) which convert a fraction of FCS waste heat partly to electrical or mechanical energy and use the remaining part for hydrogen conditioning. By using a CEUS, less heat has to be transferred to ambient and more energy for propulsion becomes available.

C5.3

How and why do fuel cells age in airplanes, and what can be done about it? These questions are being explored in the project "C6.1: Data-driven understanding of aviation PEM fuel cells under reliability aspects." Initially, the dominant aging mechanisms are identified and replicated in the laboratory. The generated data is then used to make predictions about fuel cell aging using machine learning. This enables the determination of strategies for extending the lifetime and increasing the reliability of fuel cell systems in aviation applications, such as passenger aircraft.

C6.1

For the broad commercialization of polymer electrolyte membrane fuel cells (PEMFCs) for aviation applications, one of the major challenges needs to be overcome: enabling low platinum group metals (PGM) loading while maintaining high performance and long-term durability. PGM-based alloy cathode materials have the potential to significantly decrease the stack cost. However, when the cathode PGM loading goes down to below 0.1 mg/cm² to meet the cost targets, the specific power density output of the PEMFC decreases dramatically due to the high oxygen transport resistance between the interface of ionomer and PGM nanoparticle surface. In this project, the groups of Oezaslan and Raabe are linking experiments with molecular dynamics (MD) simulations to get fundamental insights into the relevant key parameters to increase the O₂ permeation and diffusivity through the thin ionomer film to the catalytically active PGM-based surface. It is aimed at deducing information on both optimal ionomer and catalyst compositions as well as on most suitable PEMFC operation conditions to minimize the O₂ transport resistance. Combining this knowledge with experimental results on the stability and ionomer degradation allows predicting, designing and (nano)engineering cathode electrode material properties for high performance, reliability and durability in aviation.

C6.2

This project focuses on the design of an optimized air management system for a fuel cell powered medium range aircraft. The objective of this project is finding the most promising combination of all components of the fuel cell system in on- and off-design operating points. Boundary conditions like redundancy, low system weight and volume, and the required operating environment for the fuel cell stack will be considered for every operating point. The followed approach is to explore the available design space for the air management system of a fuel cell powered commercial medium range passenger aircraft. A key aspect of this study is the constraints imposed by the off-design operation of all components. These include system-level constraints such as maintaining a positive water balance or not overheating the system.

C6.3

Hydrogen fuel cells with high energy and power densities offer significant potential for future zero-emission aircraft. However, current fuel cells do not yet achieve the energy-density levels needed in aircraft. Against this background, we aim to obtain designs of robust and highly performant fuel cell systems. The project focuses on the characterization of fuel-cell applications in aircraft, modeling and simulations of fuel-cell components for better water management, as well as system designs regarding specific aircraft relevant operating conditions.

C6.4

First Project Phase (2019-2022)

Sustainable aviation aims for carbon-neutral and pollution-free, which makes green hydrogen powered fuel cells a very promising power conversion solution. The power conversion process is also intrinsically quiet, which helps reducing the aircraft noise emissions. So far, aviation-specific requirements on fuel cell system designs remain largely unaddressed. Among them, drastic improvements on specific energy, reliability, lifetime, and flexibilities in operation and in airframe integration are of immediate interests. These challenges originated the Junior Research Group (JRG) “Fuel cells for aviation”.

The ultimate contribution of this JRG is in a solution pool of optimal thermal fluid designs of fuel cell systems propelling a future aircraft concept. We are working on interpreting the macro-, meso-, micro- processes and their interplays numerically and experimentally, via robust design, via multi-physics modeling, via multiscale analysis and multi-criterion optimization, and via experimental verification.

More importantly, we practice the knowledge and conduct control and integration of these processes for the systems. It is our best intention that by pushing to the limit in fulfilling the stringent aviation requirements, we can boost the popularization of fuel cells in the transport sector, and in general to contribute to the hydrogen era.

Within this project a systematic methodological approach is to be developed for selecting and dimensioning an appropriate on-board energy supply concept for any given specific propulsion configuration. Its development comprises a detailed model-description of energy consumers and suppliers within the system under consideration. From these, physically-motivated reduced order models are derived. An adequate super-structure framework is used to interlink the single components. Uncertainties are incorporated by stochastic analysis and surrogate models, enabled by applying ideas from the arbitrary Polynomial Chaos expansion. This at hand, global sensitivities of the system can be analyzed and robustness and reliability measures can be computed. Such a comprehensive and fundamental rethinking of energy supply concepts and technologies shall lift the potential for a considerable increase in efficiency, specific energy and power, and sustainability – not only for the energy supply system itself, but also for the single components. Additionally, the energy supply system model is made available for overall aircraft design studies in ICA-B and on-ground supply concepts in ICA-A and helps to identify promising concepts as well as to properly select the energy system components and organize them in an appropriate structure.

C1.1

The objective of this research project is to lay the scientific foundation for the production of aviation fuels via electrosynthesis. To this end, chemical structures and compounds suitable for an energy efficient and low-emission lean-premixed-prevaporized (LPP) combustion will be identified and proposed to the collaboration partners in ICA-C3. Electrochemical and bioelectrochemical synthesis routes for these compounds, based on renewable precursors (biogenic compounds and CO2) will be developed and investigated in detail. Key processes like the electrochemical hydrogenation will be studied in-depth to quantify underlying reaction mechanisms and kinetics, the energy efficiency of the reaction, and product selectivity. Relationships between reaction parameters, the structural elements of the precursor
molecules, and the selectivity and efficiency of the electrosynthetic reactions will be established. The experimental electrochemical and bioelectrochemical research will be complemented by molecular simulation approaches to develop a knowledge-based toolbox for an energy-efficient aviation fuel synthesis.

C2.1

Energy storage devices are constantly a great challenge for electrification in aircraft. It demands that these devices do not only possess high specific energy and excellent durability, but also the possibility to integrate them into the system without increasing system weight and volume. One of the most promising approaches is to consider multifunctional materials, in which the different functionalities can be optimized in one packaging. By this the system weight and volume can be strongly reduced, meanwhile the system performance improves. In our approach the fiber-reinforced composites used for light weight structure processing can be integrated with novel types of all-solid-state battery materials in micro- and nano-scale resulting in multifunctional batteries which are able to act as structures for the aircrafts. Focus of the present project is developing new types of battery structures and the correlated processing of the battery materials to structurally integrate them into the fiber reinforced composites with
minimal reduction of their mechanical performance. The integration of these multifunctional energy storage composite systems into the aircraft structures will largely save the weight and volume of the system and bring about a great aspect of cost efficiency.

C2.2

The components of batteries for aviation must allow high power and hence high current densities while at the same time providing sufficient specific energy (also called gravimetric energy density). Probably the most promising battery type to fulfil these requirements is an all-solid-state lithium sulfur battery. However, performance of present cells is far away from their thermodynamic optimum and their specific energy and power are not sufficient yet for aviation. The stated performance improvements can only be achieved by a detailed understanding of the effects of the most promising materials and material combinations, their interfaces and of their micro-/meso-structural characteristics on the electrochemical performance. Thus, the challenge is a multiscale understanding and tailoring on surface, electrode and cell level, requiring a combined experimental and model-based approach. In detail, experimental works on the targeted design and an understanding of the influence of processing on final performance needs to be coupled with mechanistic modelling of the processes, structures and interfaces in the electrodes using surrogate and kinetic modelling.
In this project, we focus our work on the composite cathode, in which the interface between the solid electrolyte and the other cathode materials, i.e. active material, conductive additive and binder, will be a key aspect of our research. New material and processing concepts for the preparation of components for all-solid-state lithium sulfur cells will be investigated in close interaction between theory and experiment. These will include the minimisation of passive materials, carbon-sulfur composites with solid electrolyte on the cathode side, composite solid ion-conductors with specific micro-/meso-structure, a tailored internal interface design between the electrolyte and the active material composites, and joint assessment of sustainability of the materials with ICA-A. This will be an important step towards enabling sustainable cells with specific energy and power suitable for the application in aviation. Another crucial challenge is the development of structured lithium metal anodes providing a stable interface to the electrolyte, which will be addressed in future works in the cluster.

C2.3

In order to allow future compressors and their individual stages to operate at high efficiency working lines in the gas turbine, measures to adapt the compressor aerodynamics are required. On the other hand, all foreseen propulsion technologies – regardless of their individual shaft power source – will require high propulsion efficiencies. Especially for short range mission, the off-design efficiency of the fan does have significant influence on the overall performance as block fuel and thus DOC of the aircraft. The requirement is additionally amplified considering the reduced fan pressure ratios of UHBR-fans which do typically not lead to a chocked nozzle flow condition and therefore allows variable backpressure of the fan. DLR-FA and IFAS have previously investigated fundamental means of active blade shaping while the research at TFD at Leibniz University in Hannover is focussing on active flow control using blowing or suction technologies. The proposed project shall focus on detailed aerodynamic and structural analysis of the potential of active blade shaping for increased operating ranges of compressors and fans without an efficiency penalty. The target is a stagger angle control in the range of ±1…2 ° for the blade outer span, which approximately corresponds to a mass flow displacement of about ±5% (for typical fan design conditions). This shall be realized by a coupling of the aerodynamic and structural blade design methods for 2D and 3D geometries. For maximising the blade actuation using piezoceramic actuators blade materials, fibre matrix orientation, blade as well as integration geometries have to be investigated. Based on a manufactured full scale test blade, the real geometries for the actuated blade will be measured and used for off-design RANS calculation.

C3.1

This project focuses on the design of an innovative multi-stage axial compressor for sustainable aircraft propulsion by means of higher power density, higher efficiency, and a wider operating range. Current compressor technologies achieve high levels of efficiency when operating at aerodynamic design parameters but tend to be less efficient and even unstable at off-design conditions. For new aircraft propulsion systems, whether for gas-turbine or for fuel-cell based generators, more flexible compressor technologies are necessary. By combining active flow control methods like fluid injection and aspiration, together with shape adaptive compressor blading developed within SE²A by IFAS, power density can be increased and wider stable operation ranges can be achieved.
The fundamentals and positive effects of active flow control using aspiration and injection in compressors have already been successfully demonstrated in the PI’s previous work for fundamental setups. Based on these results, a new aerodynamic multistage compressor design will be developed and implemented in the axial compressor test rig at the Institute of Turbomachinery and Fluid Dynamics. Numerical calculations, validated by measurements, will be used to gain deeper understanding about the potential and technological limits of the combination of these active flow control systems to create adaptive high-speed compressors.

C3.2

One path to sustainable aviation is expected to be based on the utilization of synthetic jet fuels, which are produced using renewable electric energy. In this combination such fuels are called as electrofuels. One approach is to synthesize electrofuels with mostly similar properties to the current fuels like kerosene, being named as ‘drop in’ fuels, where the combustion processes are quite similar to the current ones, but CO2 neutrality is assured from the fuel production step based on renewable energy. Another approach is to synthesize the fuel in such a way that the combustion properties are improved, especially with respect to emissions like soot and nitrogen oxides. Here, new synthetic fuels will be seen in the focus with "tailor-made" properties, to prevent CO2 emissions and pollutant emissions at the same time. The planned research project is focused on this second approach. For the purpose of soot and NOx reduction the lean prevaporized premixed (LPP) burner concept is seen as research vision, as prevaporization of liquid fuels and premixing with the combustion air allows to prevent any soot formation and for lean mixtures reduces the NOx emission significantly. In order to realize such concepts the fuel needs to have such properties that neither pre-ignition nor flashback can happen under the operation conditions of jet engines. Within this project fundamental studies will be undertaken. For experiments, a very flexible mixing and burner arrangement will be built up at ITV Hannover where the processes of prevaporization, premixing, pre-ignition and flashback can be investigated for very different fuels with quantitative variation of the fuel-air mixing ratio and the
preheating temperature. At PTB Braunschweig the relevant properties of the different new tailor-made fuels will be investigated, in conjunction with the development of such fuels from the partners at TU Braunschweig (groups of Schröder, Spieß, Raabe, Project ICA-C2.1). The combustion properties, e.g., ignition delay and speciation of the target fuels will be experimentally determined. Additionally, chemical kinetic mechanisms will be developed and validated to construct predictive CFD model. The combustion and flame stabilization properties of several different liquid electrofuels will be investigated, which in cooperation with the ICA-C2.1 group will be analyzed such that both the synthetic production possibilities and the combustion properties are related together and that structure-property relationships can be established. According to the current knowledge the electrofuels will contain for instance aromatic and cyclic hydrocarbons, alcohols like propanol and butanol, furan derivatives (e.g., furan, methyl and dimethylfuran).

C3.3

This project targets electric propulsion drive systems – including power electronics - for future electrified aircraft. The key objective is to increase the specific power of the propulsion motor and the corresponding power conversion system by a significant factor. Since current density is one of the important parameters, different options for the cooling system are designed and investigated with a focus on e-motor cooling. For motors with highest specific power, the impact of superconducting elements will be evaluated in simulations and experiments. Another key research hypothesis is that designing an e-motor not for continuous duty but according to the requirements of a (worst case) mission making use of thermal capacities and phase-change cooling methods will allow to further reduce the motor’s size and weight.
In the context of future aircraft, cooling with low temperature will be available to some extent (gas expansion from fuel cell H2 tanks). Therefore, the behavior of power electronics at low temperatures will be investigated experimentally; the results will be used in the design of power electronics for propulsion, but also in a parallel project ‘Power Supply System for All Electric Aircrafts’ in C4.2. The various options for the architecture of the power electronic conversion system are studied and evaluated in order to prepare design decisions on system level. Here, different approaches depending on the electric power system topology (central DC rail or decentral energy storage) are possible.
The research on power electronics and electrical machines is supported by the three PIs in particular by the exchange of component models and the ongoing comparison and evaluation of models to describe and evaluate the energy conversion chain continuously during the project.

C4.1

This project covers the investigation of a 5MW all electric aircraft power supply system with fuel cells and batteries as DC sources. The main focus is on the High Voltage (HV) DC/DC converters for the connection of the sources and loads to the high voltage DC bus and on the safety and protection concepts of the DC system. The realization with significant specific weight reduction of the power supply systems will be carried out by means of an overall system optimization approach where component models will be provided. The electrical analysis of sources and loads under different operation modes of the aircraft will be conducted, while respecting the safety requirements. A strong cooperation with other projects in the SE²A cluster enables a substantial contribution to the targets of the cluster. Besides the investigations on the focus topics of each applicant, the E²AGLE laboratory setup will be supported.

C4.2

The primary objective of this project is to devise a fast and accurate prediction method for unsteady compressor flows with variable, diabatic boundary conditions. Such a novel method will provide a framework for calculating the mass, energy, and heat fluxes relevant for the system design as well as typical compressor performance parameters, such as the pressure ratio, efficency, and total-pressure losses.

JRP