Werkstoffkunde

Effective hydrogen storage is vital for the widespread adoption of hydrogen in energy systems, as it enables flexibility across various sectors. However, assessing such energy storage systems' suitability in future energy system configurations presents several challenges. One such challenge is the identification of representative operational scenarios for experimental testing of storage systems. Against this background, this paper presents an approach to derive such operational scenarios with the help of energy system modelling and optimization. Using the open-source energy system model and data set of Europe, PyPSA-Eur, cost-optimal future energy system configurations are identified, allowing the derivation of operational scenarios for energy storage facilities from the operation of the overall energy system. For this purpose, the methodology provides a way to identify a representative storage system from the entirety of corresponding storages in the energy system. Further, it allows determining representative time series sections using a segment identification algorithm, providing a basis for experimental technology testing. For an exemplary application of this methodology, further post-processing is implemented to consider the feasibility limits of subsystem components. The results showcase the effectiveness of the approach, offering a transparent and reproducible framework for defining operational scenarios for storage testing aligned with future energy system requirements.

The Digi-HyPro (Digitalized Hydrogen Process Chain for the Energy Transition) project's conceptual development of the SET-Unit investigates and facilitates the connection between the electric, gas, and mobility grid. This application report describes the experimental design of the Smart Energy Transition unit (SET-Unit), contemplating the bottom-up and top-down approaches. For the bottom-up approach, the design of core devices such as metal hydride-based hydrogen storage (MHS) and compressor (MHC) systems are shown. The gas separation system (GSS) concept is based on a hybrid process composed of membrane and pressure swing adsorption (PSA) for the gas grid coupling. Commercial anion exchange membrane electrolyzer (AEM-EL) and polymer exchange membrane fuel cell (PEM-FC) are assembled for the power grid connection. For the top-down approach, the first experimental SET-Unit composed of AEM-EL–MHS–PEM-FC in the nominal power range between 5 and 10 kWel and its control strategy for the optimal hydrogen and heat coupling is presented. All experimental development is carried out in the facilities of the Helmholtz-Zentrum Hereon in the frame of a cooperation agreement with the Helmut Schmidt University/University of the Federal Armed Forces.

Das Kaltgasspritzen entwickelt sich zu einem Verfahren mit großem Potenzial für die Reparatur metallischer Bauteile, insbesondere für das Aufbringen von hitze- und oxidationsempfindlichen Materialien. In diesem Zusammenhang ermöglicht der Einsatz von Automatisierung und Robotik eine flexible Steuerung des Reparaturprozesses. Um einen optimalen Reparaturprozess zu gewährleisten, müssen die verschiedenen Anforderungen des robotergeführten Kaltgasspritzens bereits in der simulativen Planungsphase berücksichtigt werden. Herkömmliche Trajektorien zum Materialauftrag berücksichtigen jedoch oft nicht die bei Reparaturen zu beachtenden geometrischen Randbedingungen des Materialaufbaus, den effizienten Materialeinsatz und die zugrundeliegenden Einschränkungen der Roboterkinematik. In dieser Arbeit wird daher ein Konzept zur automatisierten Trajektorienplanung und anschließenden Trajektorienoptimierung zur Reparatur durch robotergeführtes Kaltgasspritzen beschrieben. Das Ziel ist es, eine optimierte Trajektorie zu erzeugen, die die Anforderungen des Kaltgasspritzens und der Roboterkinematik berücksichtigt, um eine qualitativ hochwertige Reparatur und einen effizienten Materialeinsatz zu gewährleisten. Dazu gehören die Minimierung des überschüssigen Materials und die Minimierung des Rucks bei der Roboterbewegung. Die Ergebnisse zeigen die erfolgreiche Anwendung der initialen Trajektorienplanung und der anschließenden Trajektorienoptimierung für die Bauteilreparatur durch Kaltgasspritzen.

Rotational symmetry parts are common and essential in industrial applications. Cold spray additive manufacturing (CSAM) is an attractive and rapidly developing solid-state material deposition process, providing an efficient and convenient method for producing such parts, as it allows for the rapid formation of high-quality, large-volume 3D objects. Since there is no highly reactive liquid phase involved in this process, the deposited material is free of oxides. As compared to conventional additive manufacturing methods, cold spraying enables to reduce the production costs and times. In this work, a general implementation method for CSAM of rotating symmetry casing parts is presented. Here, the developed application can handle rotational symmetry parts of arbitrary geometry in the form of CAD files to generate precise toolpaths. Robot offline programming allows for process simulation, analysis, and optimization. Additionally, modelling of robot kinematics is employed to evaluate the effect of the planned toolpaths on the spraying process, ensuring efficient and precise manufacturing processes.

With the integration of renewable energy production into grids, hydrogen storage is an effective solution for coping with the fluctuating nature of the resources and reliably providing energy demands. Metal hydride storage is seen as a key technology due to its low operating pressure and temperatures near ambient, while it has a significant volumetric capacity (for room temperature hydrides: 50-110 kg/m³) compared to pressurized (40 kg/m³ under 700 bar and room temperature) or even liquified hydrogen (70 kg/m³ at – 253 ºC and 1 bar). One potential application with metal hydride storage lies in the flexibilization of residential energy demand. Excess photovoltaic generation from a house can power an electrolyser to produce hydrogen, which is then stored in the metal hydride storage. When power and heat are needed in the building, the hydrogen is released into a fuel cell. This case study investigates the dispatch optimization of a metal hydride storage system within a residential household energy system. The interaction of the electrolyser, metal hydride storage, and fuel cell, all components of a container solution called Smart Energy Transform Unit, was studied during summer and winter. Results show that in an exemplary period in winter, from 21 December 2021 to 28 December 2021, the total electricity demand is 98% covered by supply from the grid due to the low photovoltaic generation, which also yields a low hydrogen production; the total heat demand is 90% covered by the heat pump and the thermal storage as a buffer. During an exemplary period in summer, from 20 June 2021 to 27 June 2021, the system is self-sufficient, as hydrogen was stored during the day due to the high yield of photovoltaic generation, and hydrogen is used in a fuel cell at night to provide energy demands. In addition, heat pump operation during summer is small due to the heat provided by the electrolyser, the fuel cell, and the thermal buffer storage. The PV system, together with the Smart Energy Transform Unit, covers 99% of the total electric demand during this period in summer, while for the total heat demand, a coverage of 85% is observed, and the heat pump covers 15%.

Material deposition in cold spraying occurs in solid state and thus avoids undesired effects of melting and solidification. However, residual stress conditions in cold sprayed coatings could limit possible part performance. The temperature distribution and thermal history of the cold sprayed components has significant influence on stress distribution and thus deposition and part quality. The present study investigates the effect of substrate material and nozzle traverse speed (as a secondary parameter) on effective temperatures and residual stress distributions of titanium-grade 1 deposits. The results demonstrate that substrate material properties and nozzle traverse speeds have significant influence on residual stresses of the cold spray deposit. It is understood that coefficient of thermal expansion (CTE) difference of the coating and substrate materials has significant effect on residual stress state. On the other hand, the residual stresses change from more compressive to more tensile state as the temperature of the components increases by decreasing the nozzle traverse speed. These findings indicate that thermal parameters affect residual stresses substantially. Thus, by adjusting the kinematic parameters and reducing maximum reached local temperatures within the part, more favorable stress states of the finished component can be obtained. The attained knowledge is essential for the development of high-quality deposits and the selection of the best strategies for repair and additive manufacturing applications.

Anthropogenic climate change is one of the biggest problems facing society today, and counteracting it requires replacing the fossil fuel-based infrastructure with renewable energy alternatives, such as green hydrogen. Photoelectrochemical cells (PECs) offer a sustainable way to perform solar water splitting, and thus to convert energy provided by the sun into chemical form through the generation of hydrogen. A main component of PECs are photoelectrodes, which are manufactured with semiconducting materials in order to absorb light, with generation of charge carriers, which contribute to drive the photoelectrochemical reactions at the interface to the electrolyte. The structure of photoelectrodes at the nanoscale determines their performance for light absorption, carrier generation, and carrier transport and transfer. Nanofabrication methods are a group of technologies that enable the construction of interfaces and devices with nanometer precision. In this thesis, nanofabrication methods, in particular atomic layer deposition (ALD), are explored for the manufacture of individual components for photoelectrodes. The resulting photoelectrode components are investigated in terms of performance as well as stability, which are the main challenges towards large scale implementation of PEC technology. Stability has the most significant impact on the longevity and hence industrial applicability of photoelectrochemical cells. A significant part of this work focuses on the synthesis and investigation of thin coatings to protect photoelectrodes from corrosion. Thin films of TiO2 were deposited by atomic layer deposition, and the influence of deposition conditions on crystallinity, and on optoelectronic and photoelectrochemical properties were investigated. The effect of film structure on stability under different photoelectrochemical operating conditions was directly quantified by ex-situ spectroscopic ellipsometry. An additional significant component of modern photoelectrodes are layers for selective charge transport, usually transparent conducting oxides, which help extract the generated charge carriers from the photoabsorber. In this work, a series of transparent conductive oxides (ZnO:Al, ZnO:Hf, ZnO:Ti) with different doping atoms at various doping concentrations were obtained by atomic layer deposition using supercycles, and their optoelectronic properties were characterized, compared, and optimized with respect to resistivity. A further step towards large-scale deployment of PECs requires the implementation of materials capable of optimally using the solar spectrum. In this work, selected earth-abundant photoabsorbers (Fe2O3, CuO, Cu2O) with suitable band gaps for solar water splitting, were synthesized by conformal and uniform atomic layer deposition, and approaches were developed towards the possible first time synthesis of the promising photoabsorber CuFeO2 by the atomic layer deposition. Finally, using the nanofabrication methods of direct write lithography and reactive ion etching, different reproducible geometric structures with varying aspect ratios and high specific surface areas were created in square and hexagonal arrangements in silicon wafers, in order to improve the studied absorption properties and photocurrent densities of deposited photoabsorbers by orthogonalization of the light absorption and charge separation processes.

In this thesis, the development of sustainable materials for hydrogen storage is studied. Four model types of hydrogen storage materials are included in this work, which are the complex metal hydride NaAlH₄, solid solution room temperature FeTi alloys, compositionally complex alloys (CCAs), and the reactive hydrides composite (RHC) 2NaBH₄ + MgH₂. The first part of this thesis focuses on developing a complex metal hydride of NaAlH₄ by using waste Al alloy as raw material. The synthesised less pure NaAlH₄ exhibits good reversible hydrogen capacity, whereas the pure NaAlH₄ is not reversible. The second part of this thesis is devoted to developing a FeTi-based metal hydride by using waste steel and Ti alloy scraps as raw materials. Astonishingly, at 50 °C and 100 bar of H₂, the hydrogen storage capacities measured for the FeTi alloys synthesised from recycled scraps are extremely close to the value measured for the pure FeTi. In the third part of this thesis, the hydrogen storage properties of some CCAs are investigated, which further helps in screening the suitable types of waste metal alloys to be used as raw materials. With the synthesised hydrogen storage materials, the main issues are kinetic and thermodynamic tuning. The selected waste steels, Ti alloys, Mg alloys, and Al alloys, are just examples of the vast variety of scrap materials potentially useful for synthesising sustainable hydrogen storage materials. In addition, the influence of single impurities cannot be distinguished. Therefore, the RHC system of 2NaBH₄ + MgH₂ was chosen as a model system to study the effects of well-defined additives on hydrogen storage performances, as can be seen in the last part of this thesis. This work shows that by using waste metal scraps instead of high-purity elements as raw materials, the carbon footprint and costs are tremendously reduced for producing hydrogen storage materials without deteriorating the hydrogen storage properties. This work opens a new path to the development of environmentally sustainable alloys for hydrogen storage purposes.