## 1 Introduction
### Policy Proposals and Trends in the Energy Storage Market in the European Union  
In the EU's Energy Union strategy, there are five closely mutually reinforcing and interrelated dimensions:  
* A fully integrated European energy market,
* Energy efficiency contributing to the moderation of demand,
* The decarbonization of the economy,
* Trust, energy security, and solidarity, and
* Competitiveness, research and innovation [72]. The energy storage systems have the potential to play an increasing role in achieving each of these dimensions. The need for new energy storage capacity is limited, and the economic perspective is currently not very favorable for new installations. However, in the longer term, more grid flexibility will be needed if higher shares of VRE sources are integrated. The European Union should invest more in research and development activities and product development into promising directions to become competitive in storage technologies, and it is also an important aspect to constantly analyze the most effective options. Storage innovations can generate opportunities and create new employment for the EU. Demand side management, flexible generation units, energy storage, and interconnections can all act as flexibility options. For the future, the development of capacity markets should be harmonized at the European level with clear guidelines to ensure neutrality in relation to technology choices for flexibility storage options and following an integrated energy market approach. The energy storage technologies in support of the transmission and distribution networks could provide greater reliability, stability, and resilience for the EU. Thereby, energy storage can help to reduce, defer, or even avoid investments in transmission and distribution infrastructure when it is a more economical choice. The use of energy storage technologies by national grid operators is very limited nowadays. According to some opinions, access should be provided to storage technologies with new business models. It would be necessary to allow grid operators to invest, exploit, and use energy storage services together with residential and industrial customers of the energy sector to strengthen the flexibility, resilience, and reliability of the grids [72]. Published in June 2019, Regulation (EU) 2019/943 on the internal market for electricity is considered as one of the last but most important documents of the Clean Energy Package with special regard to its provisions shaping the future of energy storage. The inclusion of consumers in the electricity market by enhancing the demand-side response and energy storage will be a significant step toward a more flexible electricity system that can keep up with the intense deployment of renewable energy technology and respond to the continuously growing energy demand [73]. The rapid deployment of renewable energy technologies is closely linked to the EU's long-term goal to achieve carbon neutrality by 2050. Such a shift toward clean energy urges increased electrification, entailing a growing need for upgrading energy storage solutions. Under the new regulation, balancing markets shall ensure non-discrimination between market actors, taking into account different technical capacities of sources of electricity generation, storage, and demand-side response. Moreover, the non-discrimination of energy storage facilities should also be considered in terms of access to electricity networks. Energy storage, as defined in the common rules for the internal electricity market, is one of the numerous novelties of the new regulation, including not only reconversion into electricity, but also into other energy carriers. The new policy framework is also setting up expectations toward distribution system operators (DSOs), highlighting their key role in the better integration of renewable energy sources, distributed power generation, and other resources embedded in the electricity network, such as energy storage solutions.