Identifying battery storage system to ensure stable grid operation – the GridBatt project

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The aim of the “GridBatt” project is to work out the special requirements when using a battery storage system to ensure stable grid operation, in order to adapt the storage system to the requirements already at the design stage, to optimally dimension it and to optimise its operation strategy. Only a holistic view of the cell chemistry, the interface to the system (usually the inverter), the system requirements and the respective feedbacks makes it possible to exploit the full potential of battery storage systems. A comparison of the particular requirements, which typically demand high power rate with low energy throughput and high fluctuation, with storage technologies currently in use shows that there is a lack of technical solutions.

The GridBatt project aims to establish the state of the art by characterising different power storage systems based on the dynamic requirements in the electrical grid. The examinations are carried out on cell level, but are derived from the system requirements of the grid. A further goal is therefore the formulation of transfer functions between the grid and the battery via the converter.
In order to evaluate the potential of Aluminium-ion-batteries (AIB) for high dynamic requirements, they are electrochemically characterized in the project and models are created to describe the dynamical current-voltage behaviour. On the basis of the developed cell model, an upscaling to system level is carried out in order to evaluate the behaviour of an AIB storage unit on the grid in the overall simulation.

For the consideration of the effects of the battery behaviour on the overall system (battery, inverter, grid), a model must be created for the battery that correctly represents the dynamic behaviour. Existing models have to be extended to represent the electrochemical and electrophysical behaviour in electrochemical limiting regions. Since data sheet specifications typically cannot represent the highly dynamic behaviour, measurement methods must be developed and implemented for various technologies to verify the grid-stabilising effect.
It is necessary to check how far the limiting current loads can be extended in terms of thermal, electrochemical and electrical behaviour without damaging the battery irreversible. Various theoretical and experimental approaches are known from the literature, but they deal exclusively with high discharge rates and at laboratory cell level. The transferability to the charging direction and to industrially relevant cell sizes is currently being tested with the help of constant current loads (so-called rate capability tests) and voltage jumps (chronoamperometry).
For further characterisation, impedance measurements and current pulse measurements are carried out. One aim of the investigations is to develop and validate a fast and generally applicable parameterisation method for high-power applications. To this end, measurements are currently being carried out on various lithium-ion and first aluminium-ion batteries. When determining the current rate-dependent capacity, a significant test shortening is achievable by applying CA, since quasi all constant current rates are included in the curve.
Therefore, our aim is to develop a standardized test specification by combining different techniques like current step and voltage step or chrono-amperometry to predict the maximum power respective the transient energy content in dependence of the state of charge. As a result, we expect a classification of different technologies referring to the short time power-energy-capability.

Acknowledgement
The project “GridBatt” on which this publication is based was funded by the Federal Ministry of Education and Research under the funding code 03XP0307 A-C. The authors are responsible for the content of this publication.

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