Wienken, Eike Steffen

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.

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%.

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.

Using fuel cells in energy generation makes it possible to provide clean energy in line with the demand. Fuel cells offer a major advantage over other renewable energy sources whose generation is dependent on external influences. However, fuel cells cannot compete economically with conventional energy generation systems such as diesel generators. Such an economical constraint is partly due to the higher energy requirements of hydrogen storage. Metal hydride storage systems offer the possibility of reducing the energy intensity of storage due to low storage pressures. Heat is also required to operate such storage systems, which can be provided from the fuel cell's waste heat. To extract the heat from the fuel cell, a novel cooling circuit structure for large-scale applications is presented and simulated, considering the requirements of the metal hydride storage system regarding temperature (60 °C) and mass flow (60 kg/min). The architecture of the cooling concept consists of a primary and a secondary circuit, whereby the primary circuit is responsible for cooling the fuel cell and the secondary circuit for extracting the heat. Finally, simulation data are presented, which show the system behaviour in the event of changes in the fuel cell's electrical load and the heat consumer's thermal load. This coupling strategy shows that the cooling system is suitable for extracting the waste heat and keeping all essential parameters constant.