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Abstract Battery technologies based in multivalent charge carriers with ideally two or three electrons transferred per ion exchanged between the electrodes have large promises in raw performance numbers, most often expressed as high energy density, and are also ideally based on raw materials that are widely abundant and less expensive. Yet, these are still globally in their infancy, with some concepts (e.g. Mg metal) being more technologically mature. The challenges to address are derived on one side from the highly polarizing nature of multivalent ions when compared to single valent concepts such as Li+ or Na+ present in Li-ion or Na-ion batteries, and on the other, from the difficulties in achieving efficient metal plating/stripping (which remains the holy grail for lithium). Nonetheless, research performed to date has given some fruits and a clearer view of the challenges ahead. These include technological topics (production of thin and ductile metal foil anodes) but also chemical aspects (electrolytes with high conductivity enabling efficient plating/stripping) or high-capacity cathodes with suitable kinetics (better inorganic hosts for intercalation of such highly polarizable multivalent ions). This roadmap provides an extensive review by experts in the different technologies, which exhibit similarities but also striking differences, of the current state of the art in 2023 and the research directions and strategies currently underway to develop multivalent batteries. The aim is to provide an opinion with respect to the current challenges, potential bottlenecks, and also emerging opportunities for their practical deployment.

This article gives an overview of grid connected electrical energy storage systems worldwide, based on public available data. Technologies considered in this study are pumped hydroelectric energy storage (PHES), compressed air energy storage (CAES), sodium-sulfur batteries (NaS), lead-acid batteries, redox-flow batteries, nickel-cadmium batteries (NiCd) and lithium-ion batteries. As the research indicates, the worldwide installed capacity of grid connected electrical energy storage systems is approximately 154 GW. This corresponds to a share of 5.5 % of the worldwide installed generation capacity. Furthermore, the article gives an overview of the historical development of installed and used storage systems worldwide. Subsequently, the focus is on each considered technology concerning the current storage size, number of plants and location. In summary it can be stated, PHES is the most commonly used technology worldwide, whereas electrochemical technologies are increasingly gaining in importance. Regarding the distribution of grid connected storage systems reveals the share of installed storage capacity is in Europe and Eastern Asia twice as high as in North America. International Journal of Sustainable Energy Planning and Management, Vol 9 (2016)

The relevance of energy storages as important flexibility technology for the prospective energy supply system has been discussed in many projects and studies during the last years. In the context of the project Netzintegrierte Stromspeicher1 Fraunhofer UMSICHT has developed a deterministic optimization model for storage commitment, named GOMES Generic Optimization Model for Energy Storage. It has the purpose to compare and evaluate different storage applications both technical and economical. Due to its generic nature GOMES is applicable to different storage technologies as well as different markets. Moreover it possesses the particularity that a simultaneous participation in different markets is feasible. Its modular design allows an easy and fast modelling of new scenarios. GOMES ap plies a rolling horizon to restrict the perfect foresight to a predetermined timeframe. The optimization determines the most beneficial and economical storage commitment and delivers therefore information about the maximal reachable revenue of the examined storage application. This allows to calculate break-even capital costs and to determine the optimal storage plant design. In this case study GOMES is used to evaluate the profitability and the systemic effect of a hybrid power plant consisting of a wind farm (older than 20 years → post-feed-in-tariff) and an electric energy storage device. Exemplarily a redox flow battery was applied as electric energy storage. Two cases were examined: in the single-market operation mode the hybrid power plant only sells the generated wind energy at the spot market. In the multi-market operation mode the storage device is additionally allowed to trade at the spot market itself. In the single-market operation mode the optimal storage design from an economical point of view tends to be very small (storage power of 0.4 MW which equals 2% of the installed wind farm power, capacity of 6 full load hours). This has the consequence that the systemic effect is also very small. Wind energy is only shifted in 6.7% of the hours of the year. The calculated break-even capital costs for the redox-flow battery lie with 2 200 /kW in the lower range of todays capital costs for redox flow batteries. The multi-market operation mode has several consequences: the storage device performs more cycles and gets a little higher annual revenue. Neverthel ess the break-even capital costs slightly decrease to 2 100 /kW. This results from the shorter lifetime (17.2 instead of 19.2 years because the maximum numbers of cycles of 10 000 is reached earlier) and the therefore shorter amortization period. Simultaneously even less wind energy is shifted from low load to peak load hours. To conclude, selling the generated wind energy with the support of a storage device at the spot market could be an interesting alternative for wind farms that are no longer subject to the feed-in-tariff in the near- to midterm future. A single-market operation mode is both more economic for the operator of the hybrid power plant and more advantageous for the system (however on a low level). Nevertheless future studies should compare the here described strategy to other direct market strategies.

Battery energy storage systems have gained increasing interest for serving grid support in various application tasks. In particular, systems based on lithium-ion batteries have evolved rapidly with a wide range of cell technologies and system architectures available on the market. On the application side, different tasks for storage deployment demand distinct properties of the storage system. This review aims to serve as a guideline for best choice of battery technology, system design and operation for lithium-ion based storage systems to match a specific system application. Starting with an overview to lithium-ion battery technologies and their characteristics with respect to performance and aging, the storage system design is analyzed in detail based on an evaluation of real-world projects. Typical storage system applications are grouped and classified with respect to the challenges posed to the battery system. Publicly available modeling tools for technical and economic analysis are presented. A brief analysis of optimization approaches aims to point out challenges and potential solution techniques for system sizing, positioning and dispatch operation. For all areas reviewed herein, expected improvements and possible future developments are highlighted. In order to extract the full potential of stationary battery storage systems and to enable increased profitability of systems, future research should aim to a holistic system level approach combining not only performance tuning on a battery cell level and careful analysis of the application requirements, but also consider a proper selection of storage sub-components as well as an optimized system operation strategy.