ICA C "Energy Storage and Conversion"

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.

JRG-C3

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, therefore, 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