The components for energy generation and storage in self-sufficient energy supplies operate internally with direct current (DC). However, they are coupled by way of alternating current (AC). The necessary rectification and alternating direction incurs losses and is unnecessary. DC/DC converters equalise the level of the components far more efficiently and are now being developed by the Fraunhofer IEE project MarrakEsH.
A clear division between the sectors used to be in place: Some were consumers, while others generated energy. The dissolution of this rigid categorisation is one exciting change brought about by the All Electric Society. In future, energy will be stored, generated and consumed in all sectors – meaning that decentralised solutions are a keyword here. And work is currently underway in the building sector, for example.
Energy supply systems based on renewable energies usually consist of a photovoltaic system, a battery and a connection to the conventional grid. The latter supplies energy when the PV system is not delivering power, and the battery capacity is exhausted. In the case of self-sufficient systems based on renewable energies, it is expedient to replace the grid connection with a combination of an electrolyser for hydrogen production and a fuel cell for generating electricity from hydrogen.
The crux of all systems in this area is that the components operate internally with direct current. Until now, however, the systems have always been interlinked by way of alternating current connections.
This means that the electricity within the system must always be managed by way of inverters. In addition, each of these inverters is preceded by a DC/DC converter that brings the DC voltage to a suitable level.
As rectifiers and inverters each have a specific scope of efficiency, energy is lost. These components also contribute to the high price of such solutions. It would be much more efficient to link the components directly via DC/DC converters and only install a single inverter in the line to consumers. This means that the electrical energy can be exchanged between the components much more efficiently without conversion, while also saving on expensive components.
Self-sufficient energy supply: conventional/fossil (left) and MarrakEsH project approach (right) / Source: Fraunhofer IEE
The "Modular, regenerative and self-sufficient energy supply with H2technology project" (MarrakEsH), which is funded by the Federal Ministry of Economics and Climate Protection (BMWK) and involves six partners, is dedicated to this approach: GKN HYDROGEN GmbH, Proton Motor Fuel Cell GmbH, Würth Elektronik eiSos GmbH & Co. KG, Infineon Technologies AG, Bonn-Rhein-Sieg University of Applied Sciences (H-BRS) and the Fraunhofer Institute for Energy Economics and Energy System Technology (Fraunhofer IEE) have all teamed up.
The development of a DC/DC converter based on modern gallium nitride power semiconductors with a switching frequency of up to two MHz is one key focus of the collaboration.
The entire system is being developed at GKN HYDROGEN, which is additionally developing a new type of metal hydride-based hydrogen storage unit, which is designed to provide higher storage capacity at lower costs. Moreover, the metal hydride-based hydrogen storage system can be discharged more effectively at higher operating temperatures. The latest generation of fuel cells, which Proton Motor Fuel Cell is contributing to the project, is optimally adapted to this storage unit. The thermal losses of the fuel cell are used to dissolve the hydrogen from the metal hydride of the H2 storage tank.
In the meantime, the definition phase of the project has been completed at system level and the sub-systems have all been described. Here at the IEE we are now working together with the Rhein-Sieg University of Applied Sciences on the topology of our MMPU (modular multiport converter).
Daniel Haake
Fraunhofer Institute IEE, Department of Power Converters and Electric Drive Systems
H-BRS is involved in the project with two contributions: On the one hand, a flexible, intelligent energy management system is being developed that optimally controls the energy flows between electrical energy generators, storage systems and consumers and also ensures the efficient utilisation of system waste heat. Secondly, H-BRS is developing a scaled laboratory model of the DC/DC converter, connecting the energy generators and storage units. As Prof Dr Marco Jung, Professor of Electromobility and Electrical Infrastructure at Fraunhofer IEE, comments: "Thanks to the development and investigation of an initial scaled laboratory sample at an early juncture, we are able to analyse and evaluate the challenges and impacts that arise from switching frequencies in the MHz range. The results of these tests are incorporated directly into the corresponding demonstrator."
This demonstrator, in turn, is being developed by Fraunhofer IEE. Featuring a switching frequency of up to two MHz, it forms the interface for connecting the fuel cell and electrolyser to the self-sufficient energy supply system. The very high switching frequency aimed for makes it possible to realise very compact DC/DC converters.
Würth Elektronik eiSos is providing the necessary magnetic components. Infineon Technologies is coordinating the project and supplying the high-performance controller hardware required for the power electronic converters, as well as power transistors made of silicon and gallium nitride. Within the context of the project, the controller firmware is being developed and adapted so as to enable the converters to operate at switching frequencies of up to two MHz.
